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SUMMER IN-PLANT TRAINING REPORT




      Shree Cement Ltd., Beawar(Rajasthan)
                   Starting From 7th May 2012-1st July 2012

                               Project Title:-
            How to Control Kiln Shell Corrosion

Submitted to: -                                          Submitted by:-
Shri Sanjay Jain                                         Ankit Karwa
HOD,Mechanical Dept.                                     4th year UG, MSE
Shree Cement Ltd.,Beawar                                 IIT Kanpur

How to control kiln shell corrosion, SCL Beawar (Raj.)                    Page 1
Acknowledgment
I take immense pleasure in thanking Shri Vinay Saxena, Plant head, Shri Sanjay Jain, HOD
Mechanical Department, and Shri Atul Sharma for having permitted me to carry out this
project work.
I wish to express my deep sense of gratitude to Shri R.P.Pareek and Shri S.Hawa for their
able guidance, useful suggestions and providing me necessary data which helped me in
completing the project work, in time.
Needless to mention Shri Manoj Sharma, who has been a source of inspiration and for his
timely guidance in the conduct of project work. I would also like to thank Shri sanjay
Baldwa, Shri Harshwardhan, Shri Manish Purohit for all their valuable and timely assistance
in the project work.
I’d also like to express my gratitude towards the Mechanical Library and Shri Pankaj
Sharma, Librarian for helping me to make available different references during the project.
Words are inadequate in offering my thanks to Shri Gopal Tripathi, Sr. Manager HRD for
providing us such good facilities during the project.
I would like to express my heartfelt thanks to Shri Vijay Vyas, Officer HRD, for his sincere
helps throughout the project without which it seemed impossible to complete the project.
Finally, yet importantly, I would like to express my heartfelt thanks to my institute, INDIAN
INSTITUTE OF TECHNOLOGY, KANPUR and its Material Science and Engineering
Department for providing me this opportunity to interact with this organization and
understand the intricacies of the corporate world.


                                                          Ankit Karwa,
                                                          4th year under graduate,
                                                          Material science and Engineering,
                                                          IIT Kanpur




How to control kiln shell corrosion, SCL Beawar (Raj.)                                 Page 2
Index
1)   Acknowledgment                                                 ………..2
2)   Abstract                                                       ………..4
3)   Introduction                                                   ………..5
4)   About Shree Cement                                             ………..6
5)   Manufacturing Process                                          ………14
6)   General Chemistry of Cement Manufacturing                      ………19
7)   The Kiln System                                                ………22
8)   Literature Review                                              ………30
     A) What is Corrosion                                           ………30
     B) Types of Corrosion                                          ………31
     C) What are Refractories                                       ………38
     D) Types of Refractories                                       ………41
9) Type and Composition of refractory used at SCL, Beawar           ………45
10) Full details of Refractory linings, Coating and SS Plate Used   ………48
11) Corrosion of Kiln shell                                         ………51
   A) Introduction                                                  ………51
   B) Corrosion of Cement Kiln                                      ………52
   C) Mechanism of Cement Kiln shell Corrosion                      ………53
   D) Role of Refractories in tackling shell corrosion              ………55
   E) Role of Process Parameters on Shell Corrosion                 ………60
12)Recommendations                                                  ………62
13)References                                                       ………72




How to control kiln shell corrosion, SCL Beawar (Raj.)                       Page 3
Abstract
Corrosion damage is a major issue in cement plants. The serious consequences of the
corrosion process in cement plants have become a problem of worldwide significance.
Corrosion causes plant shutdowns, waste of valuable resources, loss or contamination of
product, reduction in efficiency, costly maintenance, and expenses over design can
jeopardise safety.
Typically, once a plant or any piece of equipment is put into service, maintenance is
required to keep it operating safely and efficiently. This is particularly true for aging systems
and structures, many of which may operate beyond the original design life.
The type of corrosion mechanism and its rate of attack depend on the nature of the
atmosphere in which corrosion takes place. The first step in preventing corrosion is to
understand its specific mechanism. The second and most important as well as most difficult
step is to design an effective type of protection mechanism.
The Cement kilns are operating at higher thermal and volumetric loadings and utilizing
alternate raw materials and fuels which are rich in volatiles creating thereby severe service
conditions inside the rotary kiln. Such conditions cause the corrosion of the rotary kiln shell
to take place in hot running conditions. Therefore investigations reported in the paper were
unique in nature with a very specific target to understand the mechanism of such corrosion,
the role of various service conditions and process parameters on corrosion phenomena and
establish such remedial measures which could impede / reduce corrosion of rotary kiln shell
in running conditions.




How to control kiln shell corrosion, SCL Beawar (Raj.)                                   Page 4
Introduction:-
Cement is a common construction material; a binder in mortars and concretes that hardens
in the presence of water. Cement is called hydraulic, when the hardened product is stable
in an aqueous environment. The most widespread hydraulic cement today is portland
cement – a finely ground gray-to-white powder composed primarily of calcium silicates,
calcium aluminates, and calcium ferrites, derived from mineral ingredients (Figure below).




                                    Fig 1:Cement Powder

Cement is made by heating limestone (calcium carbonate) with some other materials such
as clay to about 1400°C in a kiln, where a molecule of carbon dioxide is liberated calcium
oxide, or quicklime, is formed, which is then blended with the other materials that have
been included in the mix. The resulting hard substance, called 'clinker', is then ground with
a small amount of gypsum into a powder to make 'Ordinary Portland Cement' or Portland
Cement, often referred to as OPC, the most commonly used type of cement in the world.
Portland cement or clinker can be blended or interground with other materials to achieve
certain properties. There are five classes of blended cement commonly used. They are as
follows:-
   • Portland blast-furnace slag cement
   • Portland-Pozzolan Cement(PPC)
   • Pozzolan-modified Cement

How to control kiln shell corrosion, SCL Beawar (Raj.)                                 Page 5
• Slag Cement
   • Slag-modified Portland

   The blended cement are gaining popularity because they require less energy to
   manufacture, they can be made with by-product materials that would normally be
   disposed in a landfill, thus reducing solid waste, and offer performance benefits for
   certain applications.
   When mixed with water, portland cement sets (stiffens) in a few hours and hardens over
   a period of weeks and months. These phenomena are caused by chemical reactions
   associated with hydration between the components of cement and water. The most
   common use of portland cement is in the production of concrete. Concrete is a
   composite consisting of aggregate (gravel and sand), cement, and water. As a
   construction material, concrete can be cast in almost any shape desired, and once
   hardened, can become a structural (load bearing) element. Portland cement is also used
   in mortars (with sand and water only) for plasters and screeds, and in grouts (cement
   water mixes) placed into gaps to consolidate brick walls, foundations, etc.


About Shree Cement:-
Cement industry falls in the category of manufacturing industry. With the growth of
economy, cement industry is also taking substantial leaps. One amongst the companies,
helping the cement industry to achieve its fast growth, is Shree Cement Ltd. It is located in
central Rajasthan, catering to the entire Rajasthan market with the most economic logistics
cost.
An ISO 9001:2000 Company, established in the Year 1984 & the Commercial Production of
Unit-I started in the Year 1985. Shree Cement Limited has its registered office located at
Beawar (Raj) & Corporate Office at Kolkata (W.B.).
                                  “JO SOCHE VOH PAAVE”
SHREE CEMENT LIMITED is an energy conscious & environment friendly business
organization. Having ten Directors on its board under the chairmanship of Shri B.G. Bangur,
the policy decisions are taken under the guidance of Shri H.M. Bangur, Managing Director.
Shri M.K. Singhi, Executive Director of the company, is looking after all day to day affairs.
The company is managed by qualified professionals with broad vision who are committed
to maintain high standards of quality & leadership to serve the customers to their fullest
satisfaction.

How to control kiln shell corrosion, SCL Beawar (Raj.)                                Page 6
The largest cement manufacturing plant at a single location in Northern India, under the
flagship of Executive Chairman Shri B.G. Bangur & Managing Director Shri H.M. Bangur. The
Company is aiming for 20 Million Ton Annual production by the year 2015.
The plant is located near the city of Beawar, Dist. Ajmer in Central Rajasthan, sate ling the
Beawar city at radius of 10 Kms. However the Beawar subdivision is will connected through
Rail and Road both, situated on National Highway No. 8.




                        Fig 2: Shree Cement Unit I & II, Beawar(Raj.)
Shree Cement is manufacturing 33, 43, 53 & Shree ultra Red Oxide grade Cement in
Ordinary Portland Cement (OPC) and Portland Pozzolana Cement (PPC). Pozzolana used in
the manufacture of Portland cement is burnt clay of fly-ash generated at thermal power
plants. While Ordinary Portland cement is grey fine powder which is the result from
crushing a dry mix made of clinker and gypsum. Shree cement manufactures both kinds of
cement.


How to control kiln shell corrosion, SCL Beawar (Raj.)                                Page 7
Cement Industry in India poised for healthy growth supported by following factors:-
   • Growth of the Indian Cement industry directly influenced by GDP which is expected
     to grow at the rate of 7%.
   • With various infrastructure projects like roads and highways, railways, port power
     projected and real estate being implemented, the demand for cement is expected to
     grow at a fast rate.
   • Tax relief against interest on housing loans, stable interest rates and increasing
     competition in housing finance would significantly help in growth of this sector.
   • 16% Share in Rajasthan Cement Production.
     Some of another plant located in RAS. The RAS plants are far from Beawar at least 35
     kms. All the same activities are doing in this plant & this plant is uses High grade
     material our compression in BEAWAR plant and this material send in BEAWAR plant
     & mix both the Material & get the superior quality product.

      One of newly grinding unit started in KHUSHKHERA –Distt-Bhiwadi, the plant is far
      from the BEAWAR plant almost 500 KM.


UNIT -1 at Beawar Distt: Ajmer                  UNIT -2 at Beawar Distt: Ajmer
Incorporated in 1979,                           Put up in 1997
                                                Cement Production -2.10 million tones
Established in 1985 Cement Production
(Expected)-1.20 million tones
UNIT -3 at Ras Distt: Pali                     UNIT -4 at Ras Distt: Pali
Incorporated in 2005                           Cement Production - 1.0 million tones

Cement Production - 1.0 million tones
UNIT -5,6,7 & 8 at Ras Distt: Pali             Khuskheda Grinding unit (Distt. Alwar)
Production – 1.00 million tons each            Suratgarh , Jaipur & Roorkee Grinding
                                               units: one grinding unit each
(also having a world record of 367 days)
Green Power Project of 43 MW at Beawar & RAS, Thermal Power Plant of 100 MW at RAS


How to control kiln shell corrosion, SCL Beawar (Raj.)                                Page 8
Production with Efficiency
 It has a track record of over 100% capacity utilization in the 18th yr of its existence. Against
 the national industry average of 84% it has registered the highest record production of
 3.02MT with 116% of capacity utilization.
 The commercial production of Unit I started in the year 1985. The production augmented
 exponentially from the capacity of 0.6 MTPA in 1985 to around 4.8 million ton presently
 through modification, capacity enhancement and continuous improvement. The revised
 target is 25 MTPA by 2015.


Innovative & Cost Conscious Management
 •   Leadership in the use of alternative waste fuel.
 •   Initiatives for Global warming reduction.
 •   Partial utilization of waste heat.
 •   Initiator in the use of pet coke for power generation in India.


PROGRAMES:
 •  Celebration of National Safety Day.
 •  Celebration of National Safety Week.
 •  Staff Training in the subject of Safety.
 •  Environment Safety Day.




How to control kiln shell corrosion, SCL Beawar (Raj.)                                   Page 9
Policies of the Company




How to control kiln shell corrosion, SCL Beawar (Raj.)   Page 10
How to control kiln shell corrosion, SCL Beawar (Raj.)   Page 11
How to control kiln shell corrosion, SCL Beawar (Raj.)   Page 12
How to control kiln shell corrosion, SCL Beawar (Raj.)                    Page 13
                  Fig 4: Typical process flow diagram of a cement plant
Manufacturing Process:
Minning
Limestone is the main raw material of cement and is obtained from the mines. Bore holes
are made in the mines at various locations, and samples are collected to test the CC%
(CaCO3 percentage) in the mines. Limestone is retrieved by blasting the mines and then the
rock material obtained from the mines is crushed. This crushed limestone (raw material) is
sent to the plant with the help of conveyer belts and/or transportation.
Stacking and Reclaiming
A stacker is a large machine used in bulk material handling applications. It is mainly used to
arrange the incoming feed in piles. It is important to maintain the homogenous and
uniformity before discharging to further process. A stacker usually operates on a rail-like
structure with movable wheels, but the main operation is performed on a fixed place. The
main function of a Reclaimer is to recover the material and at the same time maintain
uniformity. At this stage the material is collected in hoppers via conveyer belts. Reclaimers
are volumetric machines and are rated in m3/h (cubic meters per hour) for capacity, which
is often converted to t/h (tonnes per hour) based on the average bulk density of the
material being reclaimed. Reclaimers normally travel on a rail between stockpiles in the
stockyard and are generally electrically powered by means of a trailing cable.
Pile of limestone is made by horizontal stacking of different CC% limestone to get the
required CC%. To obtain homogenized limestone for cement production, vertical reclaiming
is done.
Raw Material Grinding
Raw mill (Vertical Roller Mill) is a grinding equipment which is used to grind the incoming
feed fed through hoppers. There are three hoppers in each unit. The first two carrying
limestone and third hopper containing Laterite (zinc slag i.e. molten residue at the bottom
of zinc smelter). Laterite has a high percentage of iron. Raw mill is tall unit containing a
horizontally movable disc and three vertical disc, assembled a way to stand the vibrations
with the virtue of the state of art suspension system. Raw mill grinds the feed to very small
size and the output is sucked through the provided vents. The output is generally known as
Raw Meal. This raw meal is stored in Silos.




How to control kiln shell corrosion, SCL Beawar (Raj.)                                Page 14
Coal Grinding
The heat required for heating and conversion is obtained by burning of grinded pet coke.
Pet coke is the residue of petroleum refining and is grinded again in a VRM (Coal mill). This
grinded pet coke is stored in a storage bin.
Clinkerisation
The raw material powder is fed into a preheater in which suction is created with the help of
suction gas pump. This provides a larger residence time in the cyclonic preheaters, which
are placed in a zigzag manner. The material reaches approximately 880oC at the end of the
preheater. In the preheater major reaction taking place is the decomposition of CaCO3 to
produce CaO. This material then enters the rotary kiln, in which hot air is blasted from the
opposite direction at a temp of about 1500oC. CaO (C), Al2O3 (A), SiO2 (S), Fe2O3 (F) react
in various parts of the kiln to produce C3A (10%), C4AF (10%), C2S (30%), C3S (50%). This
mixture is called as clinker. Clinker formed above is cooled with the help of the air in Grate
coolers, and then stored in clinker silos.
The key component of the gas-suspension pre-heater is the cyclone. A cyclone is a conical
vessel into which a dust-bearing gas-stream is passed tangentially. This produces a vortex
within the vessel. The gas leaves the vessel through a co-axial "vortex-finder". The solids are
thrown to the outside edge of the vessel by centrifugal action, and leave through a valve in
the vertex of the cone. Cyclones were originally used to clean up the dust-laden gases
leaving simple dry process kilns. If, instead, the entire feed of raw meal is encouraged to
pass through the cyclone, it is found that a very efficient heat exchange takes place: the gas
is efficiently cooled, hence producing less waste of heat to the atmosphere, and the raw
meal is efficiently heated. This efficiency is further increased if a number of cyclones are
connected in series.
The number of cyclones stages used in practice varies from 1 to 6. Energy, in the form of
fan-power, is required to draw the gases through the string of cyclones, and at a string of 6
cyclones, the cost of the added fan-power needed for an extra cyclone exceeds the
efficiency advantage gained. It is normal to use the warm exhaust gas to dry the raw
materials in the raw mill. The hot feed that leaves the base of the pre-heater string is
typically 20% calcined, so the kiln has less subsequent processing to do, and can therefore
achieve a higher specific output.




How to control kiln shell corrosion, SCL Beawar (Raj.)                                Page 15
Fig 6:Pyro Process
Cement kilns are used for the pyro-processing stage of manufacture of cement, in which
calcium carbonate reacts with silica-bearing minerals to form a mixture of calcium silicates.
Cement kilns are the heart of the cement production process: their capacity usually defines
the capacity of the cement plant. As the main energy-consuming and greenhouse-gas–
emitting stage of cement manufacture, improvement of their efficiency has been the
central concern of cement manufacturing technology.
The rotary kiln consists of a tube made from steel plate, and lined with firebrick. The tube
slopes slightly (1–4°) and slowly rotates on its axis at between 30 and 250 revolutions per
hour. Raw mix is fed in at the upper end, and the rotation of the kiln causes it gradually to
move downhill to the other end of the kiln. At the other end fuel, in the form of gas, oil, or
pulverized solid fuel, is blown in through the "burner pipe", producing a large concentric

How to control kiln shell corrosion, SCL Beawar (Raj.)                                 Page 16
flame in the lower part of the kiln tube. As material moves under the flame, it reaches its
peak temperature,before dropping out of the kiln tube into the cooler. Air is drawn first
through the cooler and then through the kiln for combustion of the fuel. In the cooler the
air is heated by the cooling clinker, so that it may be 400 to 800 °C before it enters the kiln,
thus causing intense and rapid combustion of the fuel.




                                   Fig 7: Cement Rotary kiln
Note: - The details of Cement Rotary kiln are discussed in detail in later part of the report.
Cement grinding
The clinker leaving the cooler has a variable particle size, typically of diameter 3-25 mm.
Thus it is necessary to grind the clinker to a more uniform particle size. This can either be
done directly after the cooler, or the clinker can be transferred to a silo until it is needed,
see Figure 2-1. Cement clinker is, however, not a stable material: It reacts readily with
water/water vapour and CO2 from the air. This will result in pre-hydration and carbonation.
Extended storage periods may therefore have an effect on the cement properties.
In connection with the clinker grinding, 3-6 wt-% gypsum (CaSO4󲐀2H2O) is added to the
clinker. The gypsum has a marked effect on both the strength and the setting of cement.
Additives such as coal fly ash, sand or raw material may also be added at this point in order
to contribute positively to the strength-giving properties of the cement. Some of these
Additives can replace a significant fraction of the clinker, thereby saving energy needed for
calcinations and clinker reactions. Thus the additives have a great potential to reduce
energy consumption and CO2-emissions per kg cement produced. For this reason, many
cement companies seek to minimize their environmental impact by exploring the use of
new Supplementary Cementitious Materials (SCM), examples being volcanic ashes or
kaolinite clays.
The grinding of clinker and additives can be performed with several different mill systems.
When a sufficient fineness is reached, the final Portland cement product is transferred to a
cement silo. It is stored in this silo until it is being packed and shipped to the end users.


How to control kiln shell corrosion, SCL Beawar (Raj.)                                  Page 17
Fig 8: Detail flow sheet of whole process




How to control kiln shell corrosion, SCL Beawar (Raj.)              Page 18
General Chemistry of cement manufacturing
The Portland cement is made by heating a mixture of limestone and clay, or other materials
of similar bulk composition and sufficient reactivity, ultimately to a temperature of about
1450oC. Partial fusion occurs, and nodules of clinker are produced. The clinker is mixed
with a few per cent of calcium sulphate and finely ground, to make cement. The calcium
sulphate controls the rate of set and influences the rate of strength development. It is
commonly described as gypsum, but this may be partly or wholly replaced by the other
forms of calcium sulphate. Some specifications allow the addition of other materials at the
grinding stage. The clinker typically has a composition in the region of 67% CaO, 22% SiO2,
5% Al2O3, 3% Fe2O3 and 3% other components, and normally contains 4 major phases:
   1. Alite

   2. Belite

   3. Aluminate

   4. Ferrite


Alite:
Alite is the most important constituent of all normal Portland cement clinkers, of which it
constitutes 50-70%. It is tricalcium silicate (Ca3SiO5) modified in composition and crystal
structure by ionic substitutions. Reaction of this phase with water is very quick and in
normal Portland cements it is the most important of the constituent’s phases for strength
development; it ages up to 28 days, it is by far the most important.
Belite:
Belite constitutes 15-30% of normal Portland cement clinkers. It is di calcium silicate
(Ca2SiO4) modified by ionic substitutions and normally present wholly or largely as the β
polymorph. It reacts slowly with water, thus contributing little to the strength during the
first 28 days, but substantially to the further increase in strength that occurs at later ages.
By one year, the strengths obtainable from pure alite and pure belite are about the same
under comparable conditions.




How to control kiln shell corrosion, SCL Beawar (Raj.)                                  Page 19
Aluminate:
Aluminate constitutes 5-10% of most normal Portland cement clinkers. Basically it is tri
calcium aluminate (Ca3Al2O6) substantially modified in composition and sometimes also in
structure by ionic substitutions. It has a rapid reaction with water. This phase causes an
undesirable rapid setting so we need a controlling agent for this phase (like Gypsum).
Ferrite:
It is tetra calcium alumina ferrate i.e. Ca2AlFeO5, substantially modified in composition by
variation in Al/Fe ratio and ionic substitution. It has variable reaction rate with
water.Initially it reacts rapidly with water but reaction goes on slowing down with time
which is due to the difference in the compositions or other characteristics.
Clinker consists of MgO up to 4-5 %. If there is an excess of only 2% then it can cause
expansion of hardened concrete on reaction with water. This excess can occur as periclase
(MgO). Same problem might happen with SO3 if it is not under the limit of 3.5 % .
To produce white cement we increase the ratio of Al2O3 to Fe2O3. Dark color of the cement
is due to ferrite.
Reaction of cement with water is exothermic. This total heat can be reduced by lowering
alite and aluminate content in coarser grinding. This can also be done by the addition of fly
ash.
Clinker Reactions
After having passed the calciner, the calcined raw meal is admitted to the rotary kiln where
the remaining cement clinker reactions take place. Calcined raw meal is also called hot meal
in cement terminology in order to distinguish it from raw meal not yet calcined.
The names, chemical compositions and abbreviations used in cement nomenclature for the
four main constituents of Portland cement clinker are shown in Table 1. Belite and alite are
the strength-giving minerals. Alite reacts fast with water during hydration and accounts for
the early strengths while belite reacts slower and gives the cement its late strengths.
Ferrite and aluminate do not contribute to the strengths of the cement. But they possess
important properties when burning clinker. These properties of ferrite and aluminate will
be explained later in this section.
Name                           Chemical Composition             Nomenclature
Belite                         2Cao.SiO2                        C2S
Alite                          3Cao.SiO2                        C3S
Aluminate                      3CaO.Al2O3                       C3A
Ferrite                        4CaO.Al2O3.Fe2O3                 C4AF
Table 1: Main Constituents of Portland Cement

How to control kiln shell corrosion, SCL Beawar (Raj.)                                Page 20
Free lime (CaO), free periclase (MgO), earth alkali sulfates (CaSO4, MgSO4), Alkali sulfates
(Na2SO4, K2SO4) and other minor components may also be found in the clinker.
The requirements for the Portland cement clinker are:
   a) > 67 wt.-% calcium silicates (C2S and C3S). The remainder must mainly be Fe2O3,
      Al2O3 and other oxides.
   b) < 5 wt.-% MgO
   c) The CaO/SiO2 ratio by mass shall not be less than 2.0.
Clinker reactions occur at temperatures between 700-1450°C, see Figure 9. The reactions
forming the final clinker involve intermediate compounds, and the clinker reactions may be
affected by minor compounds. The overall clinker reactions are described, while a detailed
overview of the chemistry, phase relations etc.




                    Fig 9: Phase Diagram for cement clinker production




How to control kiln shell corrosion, SCL Beawar (Raj.)                               Page 21
The kiln system
The Production of cement may be divided into three parts: 1) Preparation of raw
materials,2)pyro-processing and 3)clinker processing, storage and shipment. Pyro-
processing covers the thermal treatment of the raw materials necessary to obtain the
cement clinker. Pyro-processing takes place in the preheater, calciner, rotary kiln and
cooler. These sections are commonly referred to as the kiln system.
Rotary kiln
The rotary kiln is often referred as heart of cement plant. This is where the chemical clinker
formation reactions take place. The rotary kiln is simply a long, cylindrical tube consisting of
an outer shell and an inner refractory lining. Typical lengths and diameters for modern
rotary kilns are between 40-100 m, and 3-6m, respectively. Rotary kilns are inclined 1-4°
and rotate 1-5rpm in order to facilitate mass transport and ensure clinker forming
processes such as nodulization. Production capacity is typically 2000-4000 tonnes of clinker
per day (tpd), but may be as high as 12000 tpd.

In the material outlet, the rotary kiln is equipped with a burner. The burner’s main function
is to form a flame to provide energy for clinker reaction to takes place. The flame of the
rotary kiln burner should be short, narrow and strongly radiant in order to achieve a good
heat transfer from the flame to materials in the bed.

Modern rotary kiln burners are often designed to burn a variety of fuels. This is today
achieved by using multi-channel burners with separate channels for fuels and primary air
which make it possible to adjust primary air amounts, injection velocities and momentum
independently of the fuel flows. Swirl may be used to enhance mixing and stabilize the
flame. The recent trend in multiple fuel burners is to use a single common fuel channel
which allows more flexibity towards fuel particle size and type.

                                                                               Fig 10:
                                                                               Left: Outer
                                                                               view of rotary
                                                                               kiln seen from
                                                                               above.
                                                                               Right: Inner
                                                                               view seen from
                                                                               burner end.




How to control kiln shell corrosion, SCL Beawar (Raj.)                                 Page 22
In early cement plants, the kiln system consisted only of a rotary kiln: Raw materials were
dried, preheated, calcined and burned to clinker on their way through the rotary kiln. This
process required very long rotary kilns, often significantly longer than 100 m. The raw
materials were either introduced as dry raw meal or as water/raw material slurry, and this
type of plants were therefore commonly referred to as dry long kilns or wet long kilns,
respectively. Due to a low energy efficiency, this type of cement plants is very expensive to
operate, and are rarely constructed today.

The rotary kiln consists of an outer steel shell and an inner refractory lining for thermal
insulation, in order to maintain and resist the high process temperatures. In a rotary kiln,
the refractory usually consists of bricks of special composition and sizes, able to withstand
high temperatures. However, the refractory may also be a cast lining of concrete. The
refractory lining is subject to a wide range of destructive influences through the mechanical
dynamics of the rotary kiln, the chemistry of the cement clinker process and the type of
fuels used. The intensity of these stresses varies according to the operating conditions and
kiln sections. The rotary kiln is therefore equipped with a range of refractory bricks with
different properties to ensure appropriate kiln zone lining.

The burning zone refractory lining usually suffers the greatest wear due to the higher
temperatures in the burning zone. However, the burning zone lining is protected by a
coating layer which prolong the lifetime of the refractory lining. The coating is a mass of
clinker or dust particles that adheres to the wall of the kiln, having changed from a liquid or
semiliquid to a solidified state.
Generally, the burning zone refractory lifetime is 9 to 12 months depending on the specific
kiln type and operating conditions. The refractory lifetime of the colder rotary kiln material
inlet zone is typically 12 to 48 months. Thus, these different rotary kiln zones do not have to
be replaced quite as frequently.

The most used brick types today are chromium-free magnesia-alumina-spinel bricks with
MgO content of 80-95 wt.-% and Al2O3 content of 3-18 wt.-%. Minor compounds are
typically Fe2O3, Mn2O3, SiO2, CaO and ZrO2. These brick types are regarded as having the
longest service life and the best price/performance ratio.

The increased use of alternative fuels in the cement industry may lead to higher levels of
recirculating alkali metals and sulfur within the kiln system.Recirculating alkali metals and
sulfur cause large quantities of salts to condense in and on the refractory lining,
predominantly in the temperature window 750°C to 1,100°C . Salt compounds enter into


How to control kiln shell corrosion, SCL Beawar (Raj.)                                Page 23
reactions with refractory bricks that contain alumina and the bricks can be destroyed by salt
crystallization and alkali spalling. Sulfur oxides make the reactions even worse, by
formation of alkali sulfate salt.

Various strategies are used by the refractories industry to counteract these wear processes.
These include use of additives that produce low gas permeability and reduce the
infiltration tendency of alkalis. Other solutions are sealing or impregnating the refractory
material to form a protective zone.

One of the most successful and widespread solutions is addition of 3-6 wt.-%
Silicon carbide, SiC, which leads to an appreciable resistance to alkali attack. The addition
of SiC leads in situ to formation of liquid phases, which seal the refractory surface and
protect against alkali infiltration.

Increased circulation of inorganic volatiles such as sulfur and chlorine in the kiln system,
due to increased alternative fuel utilization also entails a higher risk of kiln shell corrosion
(Jøns and Østergaard, 2001). Efforts have been made to identify suitable refractory steels
for cement rotary kilns, with characteristics that are a compromise between good creep
resistance, high corrosion resistance in the presence of chlorine and sulfur, and strong
resistance to abrasion when hot.


Thermal profile and kiln subdivisions


The rotary kiln thermal profile varies throughout its length, depending on the temperature
and chemical reactions involved during the process (see in Table 2).
The rotary kiln can be subdivided into several zones or regions that are exposed not only to
thermal and chemical wear but also to mechanical stresses. The influence of one or several
of these factors, to minor or greater proportion determines the refractory lining type
required for each zone:
   A. Decarbonation zone: from 300ºC to 1000°C (+)
This stage can occur either inside of the old wet process rotary kilns or in the preheater
tower of modern units consisting of two steps: Firstly, between 300°C and 650°C where the
raw meal heating occurs, accompanied by a dehydration reaction; Secondly, between 650°C
and 1000°C, when the limestone decarbonation takes place generating CO2 and CaO.
The first step is characterized by the following aspects:


How to control kiln shell corrosion, SCL Beawar (Raj.)                                  Page 24
   • Presence of raw meal (there are no new mineral phases development);
•   Erosion (due to raw meal flow at high velocities);
     •   low temperature;
     •   Evaporation and dehydration (of water) chemically bonded to the raw material.


In this zone it is very important that the refractory products have the capability to protect
the rotary kiln drive (good insulation degree) and good resistance to impacts of anomalous
build-ups. Bricks with less than 45% Al2O3 content are suitable. Besides that, when alkaline
salts are present, a vitreous glassy layer can develop with the alkali on the brick surface,
preventing its propagation or later infiltration.


In the second stage of these reactions, the development of new mineralogical phases
occurs:
     •   Formation of CaO and CO2;
     •   Formation of CA, C12A7 and C2S;
     •   Temperature variation;
     •   Alkali attack.
Usually, the use of bricks with a 70% Al2O3 content is recommended, which offers a high
mechanical resistance, low porosity, and low thermal conductivity. However, the risk of
eutectic reactions formations on the Al2O3-CaO- SiO2 , system and alkali resistance is a
limiting factor.


B.       Upper transition zone: from 1000ºC to 1238°C (+)
It is the most unstable and difficult area for refractory specification. Although the
temperature range varies from 1000°C to 1338°C, incidences of thermal overloads are
frequent. This fact is linked on the flame shape, to the fuel type and to the design of the kiln
main burner. Therefore, it is in this area where coating starts to develop as soon as first
drops of liquid phase appear. Coating becomes very unstable if the operational conditions
present high variability.




How to control kiln shell corrosion, SCL Beawar (Raj.)                                 Page 25
Table 2
C. Sintering zone: from 1338ºC to 1450°C (+)
In this area a full development of coating at 1450ºC(+) is expected. The presence of some
liquid phase facilitates the dissolution of C2S in the same what promotes the reaction that
generates C3S. The highest temperature in the kiln is reached at this area. Usually it should
be around 1450ºC for ordinary Portland Cements. Liquid phase is also around 25% at
1450ºC. If process is under control, coating will be stable and able to protect the lining
during the whole campaign. However, if there is a big variability at ram meal control
parameters or uneven fuels types shifting, coating will be unstable and refractories
submitted to an enormous thermo-chemical wear. The refractory products must resist high
temperatures, infiltration of molten liquid calcium silicates, and/or alkaline sulfates, and be
able to hold a stable coating.
Usually at this kiln zone it is possible to find:
   • Presence of incipient liquid phase from 18 to 32%, free lime and C2S;
   • Development of C3S by the reaction of CaO and C2S.
   • Clinker liquid phase infiltration and coating formation;
   • Chemical attacks by alkaline sulfates;
   • High operational temperature.




How to control kiln shell corrosion, SCL Beawar (Raj.)                                 Page 26
D. Lower transition zone from: 1400ºC to 1200°C (+)
This area usually operates between 1400°C and 1200°C. Around 1200ºC begins the
crystallization of the clinker the mineral phases, but not. Although the liquid phase can still
be present, it is a stage of low chemical activity, considering that most of C3S has already
been formed with a remaining amount of free lime around 1%. Nevertheless, it is a zone
submitted to temperature variations since it is right under the influence of the secondary
air temperature coming from the cooler.
This area is characterized by the following aspects:
   • Presence of the clinker liquid phase;
   • Chemical attacks by alkaline sulfates;
   • Frequent temperature variations when flame impinges over the brick;
   • Continuous thermal shock;
   • Redox atmosphere when using alternative fuels with poorly designed burner;
   • Mechanical stress imposed by the tire station and kiln shell ovality.
In order to support the temperature variations under mechanical stress, this part of the
process requires the use of basic bricks with high structural flexibility, low permeability to
gas, high hot modules of rupture and abrasion resistance.
E. Pre-cooling zone from: 1200ºC to 1000°C (+)
Originally, many kilns have been designed to promote the end of freezing and
crystallization of the just developed clinker phases. However, nowadays, the existence of
this zone into the kiln depends of the clinker cooler type and the secondary air
temperature entering into the kiln. With old grate coolers it was around 700ºC, and for the
modern high efficiency ones from 1150°C to 1100°C. In this zone at that temperature
range, there is high abrasion (clinker nodules), accentuated discharge erosion (by the
clinker dust carried by secondary and tertiary airs) and mechanic stresses (nose ring plates
and retention ring for refractory products).
The main characteristics of this kiln zone are:
    • High abrasion / erosion;
    • Frequent thermal shocks;
    • High mechanical stresses (compression/traction).
In most of the modern furnaces equipped with high efficiency coolers, this zone is not
inside the rotary kiln but in the first cooling compartment.


How to control kiln shell corrosion, SCL Beawar (Raj.)                                 Page 27
Wet Process
The original rotary cement kilns were called 'wet process' kilns. In their basic form they
were relatively simple compared with modern developments. The raw meal was supplied at
ambient temperature in the form of slurry.
A wet process kiln may be up to 200m long and 6m in diameter. It has to be long because a
lot of water has to be evaporated and the process of heat transfer is not very efficient.
The slurry may contain about 40% water. This takes a lot of energy to evaporate and
various developments of the wet process were aimed at reducing the water content of the
raw meal. An example of this is the 'filter press' (imagine a musical accordion 10-20 metres
long and several metres across) - such adaptions were described as 'semi-wet' processes.
The wet process has survived for over a century because many raw materials are suited to
blending as slurry. Also, for many years, it was technically difficult to get dry powders to
blend adequately.
Quite a few wet process kilns are still in operation, usually now with higher-tech bits bolted
on. However, new cement kilns are of the 'dry process' type.


Dry Process
In a modern works, the blended raw material enters the kiln via the pre-heater tower. Here,
hot gases from the kiln, and probably the cooled clinker at the far end of the kiln, are used
to heat the raw meal. As a result, the raw meal is already hot before it enters the kiln.
The dry process is much more thermally efficient than the wet process.
Firstly, and most obviously, this is because the meal is a dry powder and there is little or no
water that has to be evaporated.
Secondly, and less obviously, the process of transferring heat is much more efficient in a dry
process kiln.
An integral part of the process is a heat exchanger called a 'suspension preheater'. This is a
tower with a series of cyclones in which fast-moving hot gases keep the meal powder
suspended in air. All the time, the meal gets hotter and the gas gets cooler until the meal is
at almost the same temperature as the gas.
The basic dry process system consists of the kiln and a suspension preheater. The raw


How to control kiln shell corrosion, SCL Beawar (Raj.)                                 Page 28
materials, limestone and shale for example, are ground finely and blended to produce the
raw meal. The raw meal is fed in at the top of the preheater tower and passes through the
series of cyclones in the tower. Hot gas from the kiln and, often, hot air from the clinker
cooler are blown through the cyclones. Heat is transferred efficiently from the hot gases to
the raw meal.
The heating process is efficient because the meal particles have a very high surface area in
relation to their size and because of the large difference in temperature between the hot
gas and the cooler meal. Typically, 30%-40% of the meal is decarbonated before entering
the kiln.
A development of this process is the 'precalciner' kiln. Most new cement plant is of this
type. The principle is similar to that of the dry process preheater system but with the major
addition of another burner, or precalciner. With the additional heat, about 85%-95% of the
meal is decarbonated before it enters the kiln.




                                                                                 Fig 11:
                                                                               Preheater
                                                                             Precalciner Kiln




How to control kiln shell corrosion, SCL Beawar (Raj.)                               Page 29
Since meal enters the kiln at about 900 C, (compared with about 20 C in the wet process),
the kiln can be shorter and of smaller diameter for the same output. This reduces the
capital costs of a new cement plant. A dry process kiln might be only 70m long and 6m wide
but produce a similar quantity of clinker (usually measured in tonnes per day) as a wet
process kiln of the same diameter but 200m in length. For the same output, a dry process
kiln without a precalciner would be shorter than a wet process kiln but longer than a dry
process kiln with a precalciner.


Literature Review

Corrosion
   • What is corrosion
   • Types of corrosion

Refractories
   • What is refractory
   • Types of refractories
   • What is the composition of refractories used at SCL,Beawar



What is corrosion

Corrosion is the gradual destruction of material, usually metals, by chemical reaction with
its environment. In the most common use of the word, this means electro-
chemical oxidation of metals in reaction with an oxidant such as oxygen. Rusting, the
formation of iron oxides, is a well-known example of electrochemical corrosion. This type of
damage typically produces oxide(s) or salt(s) of the original metal. Corrosion can also occur
in materials other than metals, such as ceramics or polymers, although in this context, the
term degradation is more common. Corrosion degrades the useful properties of materials
and structures including strength, appearance and ability to contain a vessel's contents.
Many structural alloys corrode merely from exposure to moisture in the air, but the process
can be strongly affected by exposure to certain substances. Corrosion can be concentrated
locally to form a pit or crack, or it can extend across a wide area more or less uniformly
corroding the surface. Because corrosion is a diffusion controlled process, it occurs on


How to control kiln shell corrosion, SCL Beawar (Raj.)                              Page 30
exposed surfaces. As a result, methods to reduce the activity of the exposed surface, such
as passivation and chromate-conversion, can increase a material's corrosion resistance.
However, some corrosion mechanisms are less visible and less predictable.




Types of corrosion

1) Uniform Corrosion or General Corrosion
This type of corrosion is chemical or electrochemical in nature. However, there are no
discrete anode or cathode areas. This form of corrosion is uniform over the surface of the
metal exposed to the environment. The metal gradually becomes thinner and eventually
fails.


The energy state of the metal is basically what causes this reaction. Referred to as the
“dust-to-dust” process, high levels of energy are added to the raw mmaterial to produce
the metal. This high energy level causes an unnaturally high electrical potential. One law of
chemistry is that all materials will tend to revert to its lowest energy level, or its natural
state. After high levels of energy are added to the metal, when it is exposed to the
environment (an electrolyte), it will tend to revert to its natural state. This process is
normally extremely slow, and is dependent on the ion concentration of the electrolyte that
it is exposed to. Only under very extreme conditions (acidic electrolyte) can this form of
corrosion be significant. The corrosion rate for steel climbs drastically at a pH below 4, and
at a pH of about 3 , the steel will dissolve.


General corrosion tends to slow down over time because the potential gradually becomes
lower. Failures of pipelines or tanks would not quickly occur from this type of corrosion
since no pitting or penetration of the structure occurs, just a general corrosion over the
entire surface (except under very extreme circumstances where the metal could dissolve in
an acid electrolyte). However, in nature, the metal is not completely uniform and the
electrolyte is not completely homogeneous, resulting in electrochemical corrosion cells that
greatly overshadow this mild form of corrosion.




How to control kiln shell corrosion, SCL Beawar (Raj.)                                Page 31
Fig 12: Uniform corrosion



2) Concentration Cell Corrosion
This type of corrosion is caused by an electrochemical corrosion cell. The potential
difference (electromotive force) is caused by a difference in concentration of some
component in the electrolyte. Any difference in the electrolyte contacting the metal forms
discrete anode and cathode regions in the metal. Any metal exposed to an electrolyte
exhibits a measurable potential or voltage. The same metal has a different electrical
potential in different electrolytes, or electrolytes with different concentrations of any
component. This potential difference forces the metal to develop anodic and cathodic
regions. When there is also an electrolyte and a metallic path, the circuit is complete,
current flows, and electrochemical corrosion will occur. Soil is a combination of many
different materials. There are also many different types of soil, and even the same type of
soil varies greatly in the concentration of its constituents. Therefore, there is no such thing
as truly homogeneous soil.


These soil variations cause potential differences (electromotive force) on the metal surface
resulting in electrochemical corrosion cells. Liquids tend to be more uniform, but can vary
in the concentration of some components such as




How to control kiln shell corrosion, SCL Beawar (Raj.)                                 Page 32
Fig 13: Concentration Cell Caused by Different Environments
oxygen varies by depth and flow rates. Biological organisms are present in virtually all-
natural aqueous environments, these organisms tend to attach to and grow on the surface
of structural materials, resulting in the formation of a biological film, or biofilm. These films
are different from the surrounding electrolyte and have many adverse effects. Following
are examples of common forms of concentration cell corrosion.


   I.   Dissimilar Environment
  II.   Oxygen Concentration
 III.   Moist/Dry Electrolyte
IV.     Non-Homogeneous Soil
  V.    Concrete / Soil Interface
VI.     Backfill Impurities
VII.    Biological Effects


3) Galvanic Corrosion
This type of corrosion is caused by an electrochemical corrosion cell developed by a
potential difference in the metal that makes one part of the cell an anode, and the other
part of the cell the cathode. Different metals have different potentials in the same
electrolyte. This potential difference is the driving force, or the voltage, of the cell. As with
any electrochemical corrosion cell, if the electrolyte is continuous from the anode to the
cathode and there is a metallic path present for the electron, the circuit is completed and
current will flow and electrochemical corrosion will occur.




How to control kiln shell corrosion, SCL Beawar (Raj.)                                    Page 33
I.      Dissimilar Metals
      The most obvious form of this type of corrosion is when two different kinds of metal are
      in the electrolyte and metallically bonded or shorted in some manner. All metals exhibit
      an electrical potential; each metal has its distinctive potential or voltage (paragraph 2-4).
      When two different metals are connected, the metal with the most negative potential is
      the anode; the less negative metal is the cathode. An “active” metal is a metal with a
      high negative potential, which also means it is anodic when compared to most other
      metals. A “noble” metal is a metal with a low negative potential, which also means it is
      cathodic when compared to most other metals.
      Dissimilar metal corrosion is most severe when the potential difference between the
      two metals, or “driving voltage,” is the greatest.




                                            Fig 14: Galvanic Corrosion Cell Caused by Different
         Metals

         Examples of active metals are new steel, aluminum, stainless steel (in the active
         state), zinc, and magnesium. Examples of noble metals are corroded steel, stainless
         steel (in the passivated state), copper, bronze, carbon, gold, and platinum.
         One example of this type of corrosion occurs when coated steel pipelines are
         metallically connected to bare copper grounding systems or other copper pipelines
         (usually water lines)


 II.     Dissimilar Alloys
         The most obvious example of this type of corrosion is different metal alloys. For
         example, there are over 200 different alloys of stainless steel.
         Also, metals are not 100 percent pure. They normally contain small percentages of
         other types of metals. Different batches of a metal vary in content of these other
         metals. Different manufacturers may use different raw materials and even the same
         manufacturer may use raw materials from different sources. Each batch of metal may

How to control kiln shell corrosion, SCL Beawar (Raj.)                                    Page 34
         be slightly different in electrical potential. Even in the same batch of metal, the
concentration of these other materials may vary slightly throughout the finished
       product. All these differences will produce the electromotive force for this type of
       corrosion to occur.

III.    Impurities in Metal
       No manufacturing process is perfect. Small impurities may be mixed into the metal as
       it is produced or cooled. Impurities at the surface of the metal may become part of
       the electrolyte causing concentration cell corrosion, or if metallic, they may be
       anodic (corrodes and leaves a pit behind), or cathodic (corroding surrounding metal).

IV.    Temperature
       Metal that is at an elevated temperature becomes anodic to the same metal at a
       lower temperature. As previously discussed, a more active metal is anodic to a more
       noble metal. Since elevated temperature makes a metal more active, it becomes
       anodic to the rest of the metal. This electrochemical corrosion cell may cause
       accelerated corrosion on metals that are at elevated temperatures.



4) Stray Current Corrosion
This type of electrochemical corrosion cell is caused by an electromotive force from an
external source affecting the structure by developing a potential gradient in the electrolyte
or by inducing a current in the metal, which forces part of the structure to become an
anode and another part a cathode. This pickup and discharge of current occurs when a
metallic structure offers a path of lower resistance for current flowing in the electrolyte.
This type of corrosion can be extremely severe because of very high voltages that can be
forced into the earth by various sources.


The potential gradient in the electrolyte forces one part of the structure to pick up current
(become a cathode) and another part of the structure to discharge current (become an
anode).


Stray current corrosion occurs where the current from the external source leaves the metal
structure and enters back into the electrolyte, normally near the external power source
cathode. The external power source is the driving force, or the voltage, of the cell. Stray
current corrosion is different from natural corrosion because it is caused by an externally
induced electrical current and is basically independent of such environmental factors as
concentration cells, resistivity, pH and galvanic cells. The amount of current (corrosion)

How to control kiln shell corrosion, SCL Beawar (Raj.)                                Page 35
depends on the external power source, and the resistance of the path through the metallic
structure compared to the resistance of the path between the external source’s anode and
cathode.




                                      Fig 15: Stray Current Corrosion Cell Caused by
External Anode and Cathode

An example of stray current corrosion is caused by impressed current cathodic protection
systems, where a “foreign” electrically continuous structure passes near the protected
structures anodes and then crosses the protected structure (cathode). This corrosion is
usually found after failures in the foreign structure occur. Stray current corrosion is the
most severe form of corrosion because the metallic structure is forced to become an anode
and the amount of current translates directly into metal loss. If the amount of current
leaving a structure to enter the electrolyte can be measured, this can be directly translated
into metallic weight loss. Different metals have specific amounts of weight loss when
exposed to current discharge. This weight loss is normally measured in pounds (or
kilograms) of metal lost due to a current of one amp for a period of one year (one amp-
year). For example, if a stray current of just two amps were present on a steel pipeline, the
result would be a loss of 18.2 kilo grams (40.2 pounds) of steel in one year. For a coated
pipeline, this could result in a penetration at a defect in the coating in an extremely short
period of time, sometimes only a few days.



5) Crevice Corrosion
Crevice Corrosion refers to the localized attack on a metal surface at, or immediately
adjacent to, the gap or crevice between two joining surfaces. The gap or crevice can be
formed between two metals or a metal and non-metallic material. Outside the gap or
without the gap, both metals are resistant to corrosion.
The damage caused by crevice corrosion is normally confined to one metal at localized area
within or close to the joining surfaces.



How to control kiln shell corrosion, SCL Beawar (Raj.)                                 Page 36
Fig 16: a type 316 stainless steel tube and tube sheet
 from a heat exchanger in a seawater reverse osmosis (SWRO) desalination plant suffered
 crevice corrosion due to the presence of crevice (gap) between the tube and tube sheet.


6) Pitting Corrosion
Certain conditions, such as low concentrations of oxygen or high concentrations of species
such as chloride which complete as anions, can interfere with a given alloy's ability to re-
form a passivating film. In the worst case, almost all of the surface will remain protected,
but tiny local fluctuations will degrade the oxide film in a few critical points. Corrosion at
these points will be greatly amplified, and can cause corrosion pits of several types,
depending upon conditions. While the corrosion pits only nucleate under fairly extreme
circumstances, they can continue to grow even when conditions return to normal, since the
interior of a pit is naturally deprived of oxygen and locally the pH decreases to very low
values and the corrosion rate increases due to an auto-catalytic process. In extreme cases,
the sharp tips of extremely long and narrow corrosion pits can cause stress concentration
to the point that otherwise tough alloys can shatter; a thin film pierced by an invisibly small
hole can hide a thumb sized pit from view. These problems are especially dangerous
because they are difficult to detect before a part or structure fails. Pitting remains among
the most common and damaging forms of corrosion in passivated alloys, but it can be
prevented by control of the alloy's environment.




                             Fig 17: Scheme of pitting corrosion

How to control kiln shell corrosion, SCL Beawar (Raj.)                                Page 37
What are Refractories
Any material can be described as a ‘refractory,’ if it can withstand the action of abrasive or
corrosive solids, liquids or gases at high temperatures. The various combinations of
operating conditions, in which refractories are used, make it necessary to manufacture a
range of refractory materials with different properties. Refractory materials are made in
varying combinations and shapes depending on their applications. General requirements of
a refractory material are:


   •   Withstand high temperatures
   •   Withstand sudden changes of temperatures
   •   Withstand action of molten metal slag, glass, hot gases, etc
   •   Withstand load at service conditions
   •   Withstand load and abrasive forces
   •   Conserve heat
   •   Have low coefficient of thermal expansion
   •   Should not contaminate the material with which it comes into contact


Depending on the area of application such as boilers, furnaces, kilns, ovens etc,
temperatures and atmospheres encountered different types of refractories are used.
Some of the important properties of refractories are:
a) Melting Point
Pure substances melt instantly at a specific temperature. Most refractory materials consist
of particles bonded together that have high melting temperatures. At high temperatures,
these particles melt and form slag. The melting point of the refractory is the temperature at
which a test pyramid (cone) fails to support its own weight.
b) Size
The size and shape of the refractories is a part of the design of the furnace, since it affects
the stability of the furnace structure. Accurate size is extremely important to properly fit
the refractory shape inside the furnace and to minimize space between construction joints.
c) Bulk Density
The bulk density is useful property of refractories, which is the amount of refractory
material within a volume (kg/m3). An increase in bulk density of a given refractory
increases its volume stability, heat capacity and resistance to slag penetration.


How to control kiln shell corrosion, SCL Beawar (Raj.)                                 Page 38
d) Porosity
The apparent porosity is the volume of the open pores, into which a liquid can penetrate, as
a percentage of the total volume of the refractory. This property is important when the
refractory is in contact with molten charge and slag. A low apparent porosity prevents
molten material from penetrating into the refractory. A large number of small pores is
generally preferred to a small number of large pores.
e) Cold Crushing Strength
The cold crushing strength is the resistance of the refractory to crushing, which mostly
happens during transport. It only has an indirect relevance to refractory performance, and
is used as one of the indicators of abrasion resistance. Other indicators used are bulk
density and porosity.
f) Pyrometric cones and Pyrometric cones equivalent (PCE)
The ‘refractoriness’ of (refractory) bricks is the temperature at which the refractory bends
because it can no longer support its own weight. Pyrometric cones are used in ceramic
industries to test the refractoriness of the (refractory) bricks. They consist of a mixture of
oxides that are known to melt at a specific narrow temperature range. Cones with different
oxide composition are placed in sequence of their melting temperature alongside a row of
refractory bricks in a furnace. The furnace is fired and the temperature rises. One cone will
bends together with the refractory brick. This is the temperature range in oC above which
the refractory cannot be used. This is known as Pyrometric Cone Equivalent temperatures.




                                                       Fig 17: Pyrometric cones
g) Creep at high Temperature
Creep is a time dependent property, which determines the deformation in a given time and
at a given temperature by a refractory material under stress.



How to control kiln shell corrosion, SCL Beawar (Raj.)                                Page 39
h) Volume stability, expansion, and shrinkage at high temperatures
The contraction or expansion of the refractories can take place during service life. Such
permanent changes in dimensions may be due to:
     •   The changes in the allotropic forms, which cause a change in specific gravity
     •   A chemical reaction, which produces a new material of altered specific gravity
     •   The formation of liquid phase
     •   Sintering reactions
     •   Fusion dust and slag or by the action of alkalies on fireclay refractories, to form
         alkali-alumina silicates. This is generally observed in blast furnaces
i) Reversible thermal expansion
Any material expands when heated, and contracts when cooled. The reversible thermal
expansion is a reflection on the phase transformations that occur during heating and
cooling.
j) Thermal Conductivity
Thermal conductivity depends on the chemical and mineralogical composition and silica
content of the refractory and on the application temperature. The conductivity usually
changes with rising temperature. High thermal conductivity of a refractory is desirable
when heat transfer though brickwork is required, for example in recuperators,
regenerators, muffles, etc. Low thermal conductivity is desirable for conservation of heat,
as the refractory acts as an insulator. Additional insulation conserves heat but at the same
time increases the hot face temperature and hence a better quality refractory is required.
Because of this, the outside roofs of open-hearth furnaces are normally not insulated, as
this could cause the roof to collapse. Lightweight refractories of low thermal conductivity
find wider applications in low temperature heat treatment furnaces, for example in batch
type furnaces where the low heat capacity of the refractory structure minimizes the heat
stored during the intermittent heating and cooling cycles. Insulating refractories have very
low thermal conductivity. This is usually achieved by trapping a higher proportion of air into
the structure. Some examples are:
   • Naturally occurring materials like asbestos are good insulators but are not
     particularly good refractories
   • Mineral wools are available which combine good insulating properties with good
     resistance to heat but these are not rigid
   • Porous bricks are rigid at high temperatures and have a reasonably low thermal
     conductivity.



How to control kiln shell corrosion, SCL Beawar (Raj.)                                 Page 40
Type of Refractories
Refractories can be classified on the basis of chemical composition, end use and methods of
manufacture as shown below.
Classification method                            Example
Chemical Composition
ACID, which readily combines with bases          Silica, Semisilica, Aluminosilicate

BASIC, which consists mainly of metallic         Magnesite, Chrome-magnesite, Magnesite-
oxides that resist the action of bases           chromite, Dolomite

NEUTRAL, which does not combine with             Fireclay bricks, Chrome, Pure Alumina
acids nor bases

Special                                          Carbon, Silicon Carbide, Zirconia

End use                                          Blast furnace casting pit

Method of manufacture                            Dry press process, fused cast, hand
                                                 moulded, formed normal, fired or chemically
                                                 bonded, unformed (monolithics, plastics,
                                                 ramming mass, gunning castable, spraying)
           Table 3: Classification of refractories based on chemical composition


 a. Fireclay refractories
Firebrick is the most common form of refractory material. It is used extensively in the iron
and steel industry, nonferrous metallurgy, glass industry, pottery kilns, cement industry,
and many others.
Fireclay refractories, such as firebricks, siliceous fireclays and aluminous clay refractories
consist of aluminum silicates with varying silica (SiO ) content of up to 78 percent and
                                                      2
Al O content of up to 44 percent. Table 4 shows that the melting point (PCE) of fireclay
  2   3
brick decreases with increasing impurity and decreasing Al O . This material is often used in
                                                             2   3
furnaces, kilns and stoves because the materials are widely available and relatively
inexpensive.



How to control kiln shell corrosion, SCL Beawar (Raj.)                                  Page 41
Brick Type         Percentage          Percentage         Percentage other        PCE °C
                   SiO2                Al2O3              constituents
Super Duty         49-53               40-44              5-7                     1745-1760

High Duty          50-80               35-40              5-9                     1690-1745

Intermediate       60-70               26-36              5-9                     1640-1680

High Duty          65-80               18-30              3-8                     1620-1680
Siliceous
Low Duty           60-70               23-33              6-10                    1520-1595

                            Table 4: Properties of fireclay Bricks
b. High alumina refractories
Alumina silicate refractories containing more than 45 percent alumina are generally termed
as high alumina materials. The alumina concentration ranges from 45 to 100 percent. The
refractoriness of high alumina refractories increases with increase in alumina percentage.
The applications of high alumina refractories include the hearth and shaft of blast furnaces,
ceramic kilns, cement kilns, glass tanks and crucibles for melting a wide range of metals.
c. Silica Brick
Silica brick (or Dinas) is a refractory that contains at least 93 percent SiO2. The raw material
is quality rocks. Various grades of silica brick have found extensive use in the iron and steel
melting furnaces and the glass industry. In addition to high fusion point multi-type
refractories, other important properties are their high resistance to thermal shock (spalling)
and their high refractoriness. The outstanding property of silica brick is that it does not
begin to soften under high loads until its fusion point is approached. This behavior contrasts
with that of many other refractories, for example alumina silicate materials, which begin to
fuse and creep at temperatures considerably lower than their fusion points. Other
advantages are flux and stag resistance, volume stability and high spalling resistance.
d. Magnesite
Magnesite refractories are chemically basic materials, containing at least 85 percent
magnesium oxide. They are made from naturally occurring magnesite (MgCO3). The
properties of magnesite refractories depend on the concentration of silicate bond at the
operating temperatures. Good quality magnesite usually results from a CaO-SiO2 ratio of
less than two with a minimum ferrite concentration, particularly if the furnaces lined with
the refractory operate in oxidizing and reducing conditions. The slag resistance is very high
particularly to lime and iron rich slags.

How to control kiln shell corrosion, SCL Beawar (Raj.)                                 Page 42
e. Chromite Refractories
Two types of chromite refractories are distinguished:
   • Chrome-magnesite refractories, which usually contain 15-35 percent Cr O and 42-50
                                                                                 2   3
     percent MgO. They are made in a wide range of qualities and are used for building
     the critical parts of high temperature furnaces. These materials can withstand
     corrosive slags and gases and have high refractoriness.
   • Magnesite-chromite refractories, which contain at least 60 percent MgO and 8-18
     percent Cr O . They are suitable for service at the highest temperatures and for
                 2   3
      contact with the most basic slags used in steel melting. Magnesite-chromite usually
      has a better spalling resistance than chrome-magnesite.
f. Zirconia Refractories
Zirconium dioxide (ZrO2) is a polymorphic material. It is essential to stabilize it before
application as a refractory, which is achieved by incorporating small quantities of calcium,
magnesium and cerium oxide, etc. Its properties depend mainly on the degree of
stabilization, quantity of stabilizer and quality of the original raw material. Zirconia
refractories have a very high strength at room temperature, which is maintained up to
temperatures as high as 15000C. They are therefore useful as high temperature
construction materials in furnaces and kilns. The thermal conductivity of zirconium dioxide
is much lower than that of most other refractories and the material is therefore used as a
high temperature insulating refractory. Zirconia exhibits very low thermal losses and does
not react readily with liquid metals, and is particularly useful for making refractory crucibles
and other vessels for metallurgical purposes. Glass furnaces use zirconia because it is not
easily wetted by molten glasses and does not react easily with glass.
g. Oxide Refractories(Alumina)
Alumina refractory materials that consist of aluminium oxide with little traces of impurities
are known as pure alumina. Alumina is one of the most chemically stable oxides known. It is
mechanically very strong, insoluble in water, super heated steam, and most inorganic acids
and alkalies. Its properties make it suitable for the shaping of crucibles for fusing sodium
carbonate, sodium hydroxide and sodium peroxide. It has a high resistance in oxidizing and
reducing atmosphere. Alumina is extensively used in heat processing industries. Highly
porous alumina is used for lining furnaces operating up to 1850oC.
h. Monolithics
Monolithic refractories are single piece casts in the shape of equipment, such as a ladle as
shown in Figure 18. They are rapidly replacing the conventional type fired refractories in
many applications including industrial furnaces. The main advantages of monolithics are:


How to control kiln shell corrosion, SCL Beawar (Raj.)                                   Page 43
•   Elimination of joints which is an inherent weakness
   •   Faster application method
   •   Special skill for installation not required
   •   Ease of transportation and handling
   •   Better scope to reduce downtime for repairs
   •   Considerable scope to reduce inventory and eliminate special shapes
   •   Heat savings
   •   Better spalling resistance
   •   Greater volume stability

Monolithics are put into place using various methods, such as ramming, casting, gunniting,
spraying, and sand slinging. Ramming requires proper tools and is mostly used in cold
applications where proper consolidation of the material is important. Ramming is also used
for air setting and heat setting materials. Because calcium aluminate cement is the binder,
it will have to be stored properly to prevent moisture absorption. Its strength starts
deteriorating after 6 to 12 months.




                            Fig 18: A monolithic lining for Ladel




How to control kiln shell corrosion, SCL Beawar (Raj.)                             Page 44
Type and Composition of refractory used at SCL, Beawar (kiln 2)
Brick Type            TOPMAG A1        ALMAG 85          FERROMA              PERILEX 83
                                                         G 90
                      Magnesia-        Magnesia-         Magnesia-            Magnesia-
                      Fused Spinel     Fused Spinel      Hercynite            Chromite
Characteristi         77-81 MgO        85-89MgO          87-92MgO             81-85MgO
c Component
in %
bulk density          2.9-3.05         2.85-3            2.85-3               2.9-3.05
(g/cm3)
Apparent              15-17            16-18             16-18                17-19
porosity %
Cold                  65               50                50                   55
Crushing
Strength
N/mm2
Seger Cone            >42              >42               >42                  42
Thermal
expansion     400°C   0.3              0.4               0.4                  0.4
Lin. % at     800°C   0.9              0.8               0.9                  1.1
              1200°C  1.4              1.4               1.5                  1.7
Thermal               100              100               100                  80
Shock
resistance at
950°C/air
Thermal
Conductivity 300°C    3.9              4                 3.7                  4
W/m.K at      700°C   2.9              3                 3                    3
              1000°C  2.6              2.7               2.6                  2.8
Typical field         upper transition upper and lower burning                burning Zones
of                    zone, tyre       transition zones zone and              as well as upper
application           section subject  subject to severe upper                and lower
                         to high           service             transition     transition Zones
                         mechanical load   conditions with     zone, good     subject to
                         with redox        alkali attack and   coatability,   severe service
                         conditions and    redox               chrome ore     conditions,low
                         extreme alkali    conditions,         free           Chrome Content
                         attack, chrome    chrome ore free
                         ore free

How to control kiln shell corrosion, SCL Beawar (Raj.)                                Page 45
Above mentioned refractories are imported bricks. These bricks are basic bricks. Currently
these bricks are using in Kiln-2(at outlet where temperature is very high) with addition of
high Alumina bricks at kiln inlet where temperature is not so high. Where as in kiln-1 we are
using only High Alumina bricks.
Currently High Alumina Bricks (made in India) using in Kiln-1 and in inlet of kiln-2.
Product     Maximum     C.C.S.              Chemical analysis     Refractoriness        Material
name        Recommended (Kg/Cm2)                                  Pyrometric            Req.
            Temperature                                           Cone.orton
            (°C)
                                            Al2O3       Fe2O3
AC 40S      1420              395           40.5        2.15      30                    2.21
AC 60S      1480              545           59.2        2.5       35                    2.42
AC 70S      1460              720           69.8        3         36                    2.61




How to control kiln shell corrosion, SCL Beawar (Raj.)                                   Page 46
Full details of Refractory linings, Coating and SS Plate Used:
For Kiln-1
Fig 19: Refractory lining of kiln-1




How to control kiln shell corrosion, SCL Beawar (Raj.)                         Page 47
How to control kiln shell corrosion, SCL Beawar (Raj.)   Page 48
For Kiln-2
Fig 20: Refractory lining of Kiln-2




How to control kiln shell corrosion, SCL Beawar (Raj.)   Page 49
How to control kiln shell corrosion, SCL Beawar (Raj.)   Page 50
Corrosion of Kiln Shell
Introduction


With the ever increasing demand of cement due to the exponential growth of construction
industry, Indian Cement Industry has been put to perform at its best than ever before. With
the advances in understanding the cement chemistry and material behavior in side rotary
kiln, lot many alternate raw materials and fuels have been either in use or being
investigated for their suitability. While these alternate raw materials and fuels proved to be
beneficial in terms of financial aspects associated with them, but the presence of
deleterious volatile compounds posed equally serious threats to cause problems such as
kiln shell corrosion, build ups and rings besides attacking the refractory lining and reducing
their campaign lives. Amongst these, kiln shell corrosion is most serious problem as it acts
silently and reduces the shell thickness to below critical structural and mechanical limits of
stability of kiln shell.
Corrosion can be defined as the destruction or deterioration of material due to the reaction
with its environment. The grades of steel used for kiln shell range from general engineering
steel to low alloy steels. The minimum shell plate thickness is around 20 mm for the shell
section between two supports and those under the tyres (riding rings) may be between 60-
80 mm thick. A reduction of the shell thickness due to corrosion can be presumed to be
critical when the thickness of the shell becomes 15 mm or so. To overcome the problem of
kiln shell corrosion, the best way could be to prevent the volatiles to reach upto kiln shell.
To prevent the passage of volatiles, refractories play a vital role and the selection of proper
quality, adequate installation measures and highly oxidizing conditions in hottest zone of
the kiln, stable kiln operation at high speed proved as the key to reduce / minimize the
extent of kiln shell corrosion. Here I will try to bring out the role of the refractories and
process conditions to reduce / minimize the kiln shell corrosion based upon the studies
carried out in the past.




How to control kiln shell corrosion, SCL Beawar (Raj.)                                Page 51
Corrosion of Cement Kiln


Corrosion of cement kiln shell is influenced by a number of factors such as composition of
the metallic shell and its environment, temperature of the shell, cleanliness or roughness
of the shell surface, its contact with other materials and severe process conditions.
Further, it is determined largely by the degree to which the scale formed under particular
condition blocks further action between the shell and environment. Each steel or alloy
behaves more or less individually and forms its own characteristic type of scale whose
composition and imperviousness are specific to the given alloy, atmosphere, temperature
and duration of exposure. Consequently, even a slight difference in composition of steel or
atmosphere for instance, the presence of sulphur may have a substantial influence upon
the type and progress of corrosion.
It has been reported that carbon dioxide and sulphur dioxide are active scaling agents of
iron and steel, carbon dioxide being less deterrent. The presence of sulphur dioxide
increases rate of scaling and often results in deep intergranular penetration of the steel
through the formation of a liquid iron oxide – iron sulphide eutectic. The deleterious effects
of sulphur dioxide can be offset by providing excess oxygen. Alloying elements such as
chromium, aluminium and silicon present in steel may greatly affect the rate of scaling.
When present in significant concentration, they oxidize rapidly yielding a relatively
impervious film which retards the rate of further attack on the underlying metal. On the
other hand a high concentration of sulphur increases the rate of attack just as does the
sulphur in the atmosphere. The influence of carbon is relatively small. The main reason of
shell corrosion can be attributed to alternate oxidation at high temperature and acidic
reaction at low temperatures when the kiln is stopped for repairs. The corrosion
phenomenon takes place mainly due to presence of oxides, chlorides and sulphide at high
temperature. Various types of corrosion that affect the kiln shell are:
(i) Corrosion due to oxidation under high temperature
(ii) Corrosion due to sulphide under high temperature
(iii) Corrosion due to chloride under high temperature
(iv) Corrosion due to hygroscopic material
The rate of corrosion depends on the material, the surface condition, the corrosion
medium, the time available and the temperature. The resistance to scaling of steels also
diminishes in consequence of frequent temperature changes. Investigations have shown
that in the majority of cases the corrosive attack is intensified by frequent changes in

How to control kiln shell corrosion, SCL Beawar (Raj.)                               Page 52
temperature. In most cases, the damage due to high temperature corrosion manifests
itself in general removal of material or in superficial cracking. Oxidation in the kiln
atmosphere is possible due to the presence of O2, H2O/ H2, CO2/CO.

The occurrence of high temperature corrosion under surface deposits is also an important
factor. The damage due to corrosion is always intensified if the surface of the affected
portion is covered with such deposits. During fairly long kiln shutdown for repairs, a rusting
process is also presumed to be superimposed upon the high temperature corrosion
(scaling) that has occurred during kiln operation. The deposit of salts containing potassium
chloride in particular on the shell becomes active because being hygroscopic it absorbs
atmospheric moisture. It is observed that the chloride can reach the kiln shell in the form
of gases, the same is not the case for alkali oxides.

Alkalis can only penetrate the lining as a part of liquid potassium and / or Sodium salt
melts. If the corrosion products therefore contain substantial quantities of Potassium or
Sodium, the form of corrosion is termed as Hot Corrosion indicating that liquid phase takes
part in the corrosion reactions.
 Mechanism of kiln shell corrosion
The reactions inside the kiln are different from reactions on the kiln shell surface since both
the temperature and atmosphere are different. One of the most important reactions in the
lining is the oxygen consumption where SO2 consumes oxygen and condenses as SO3:
                                   2 SO2 (g) + O2 = 2 SO3 (↓)
The SO3 formed condenses as calcium or magnesium salts. The result can be that an
oxidizing environment inside the kiln turns into a reducing environment at the kiln shell.
Oxidation
In an oxidizing atmosphere, the iron from the steel shell will react with oxygen to form an
oxide scale. Generally, this oxide scale is formed by more or less firm layers of different iron
oxides, the compound with the highest oxygen content, Fe2O3, being found at the scale-
brick interface, and the compound with the highest iron content, FeO at the metal-scale
interface. At normal kiln operating temperatures, the outer layer becomes relatively firm.
Sulphidization
When no oxygen is present, SO2 takes over as the oxygen donor and a different reaction
occurs. The reaction may be written as follows:
                                4 Fe + 2 SO2 (g) = Fe3O4 + FeS2


How to control kiln shell corrosion, SCL Beawar (Raj.)                                    Page 53
Accordingly, a sulphidization reaction can be identified by the occurrence of either pyrite
(FeS2) or pyrrhotite (FeS) in the corrosion products. The oxidation by O2 and by SO2
alternates. As sulphide layers are more porous than oxide layers, the corrosion rate of the
shell will increase. However, experience from different plants shows that, as long as
chlorides are not present, the corrosion rate stays at an acceptably low level.
Sulphidization is enhanced by the presence of chlorides, mainly because they affect the
morphology of the corrosion scale, hindering the formation of a strong, protective oxide
layer. The total reaction is a chain process taking place at different temperatures. A
temperature gradient between the kiln atmosphere and the kiln shell is created by the
porous deposit and the refractory lining. The first reaction of the chain process takes place
in the kiln and can be described as high temperature hydrolysis of the thermally unstable
alkali chlorides to form the more stable sulphates. This reaction step is followed by re-
oxidation of hydrogen chloride gas (by oxygen or SO2) at lower temperatures to produce
elemental chlorine, which attacks the kiln shell. The basic reactions (with potassium as
alkali) are:
             2 KCl (g) + H2O (g) + SO2 (g) + ½ O2 (g) = K2SO4 + 2 HCl (g) (T > 900oC)
                         2 HCl (g) + ½ O2 (g) = Cl2 (g) + H2O (T < 400oC)
The formation of ‘free’ hydrochloric acid (HCl) gas in cement kilns is thus accompanied by
formation of alkali sulphates. When this is the case, the formation is restricted to a quite
narrow temperature between 1100 oC and 1300oC. The formation of hydrochloric acid is a
consequence of the thermal instability of calcium sulphate and the thermal stability of
potassium sulphate. The evaporation of alkali chlorides cannot begin until these
temperatures are reached since gas and material move counter-current in the kiln.
If most of the KCl in kiln feed can evaporate to KCl (g) at temperatures below 1000 to
1150 oC, the formation of HCl (g) will be quite limited because the tendency of KCl (g) to
hydrolyse at such temperatures is low. The low temperature evaporation of chlorides
explains why normal preheater kilns are less vulnerable to chloride-enhanced
sulphidization.
While, in most cases, chlorides in preheater kilns evaporate during or shortly after
calcinations without substantial formation of hydrogen chloride gas, the case is different for
kilns with tertiary air duct. Such kilns will show delayed alkali chloride evaporation and,
consequently, evaporation will be followed by more extensive hydrolysis of the chlorides.
Once Cl2 (g) is formed, it can reach the kiln shell through the refractory bricks or through
the gaps/joints within and between rings and will react with either the oxide-sulphide


How to control kiln shell corrosion, SCL Beawar (Raj.)                                  Page 54
layers or, most likely, directly with the kiln shell according to the following reactions
resulting in the corrosion of the kiln shell:
          • reaction with the oxide-sulphide layers:

                                   FeS + Fe3O4 + 4 Cl2 = 4 FeCl2 + SO2 + O2

          • reaction with the kiln shell:
                                         Cl2 + Fe = FeCl2

Role of Refractories in tackling shell corrosion
The role of refractories in cement kiln is primarily to protect the steel shell from the direct
attack of deleterious gases and clinker melt and to reduce the shell temperature so that
steel of the shell does not loose its properties. The reduction in shell temperature also leads
to energy conservation besides providing a workable condition near kiln shell. The entire
CRK system including, preheater, precalciner, rotary kiln and cooler is lined with suitable
size and quality of refractories to achieve the above mentioned advantages. Amongst all
the sections as mentioned above, the service conditions inside the rotary kiln are most
severe thereby requiring special attention for the shape, type and quality of refractories to
be used and installation practices to be employed.
Passage of Volatiles through Bricks
The various studies carried out by NCB have established that volatile pass through the body
of the refractory bricks and reach up to kiln shell. The samples of worn out refractory bricks
as collected during visits were cut into three sections, top, middle and bottom and these
were subjected to chemical and mineralogical investigations to find out the mineral phases
present and formation of new phases. The results of chemical analysis and XRD
investigations are given in Table 5 and Table 6 respectively for all the cases.




How to control kiln shell corrosion, SCL Beawar (Raj.)                                  Page 55
Brick Area                               Chemical Constituents %
               Al2O3 Fe2O3         SiO2      SO3         Na2O       K2O           Cl
Fresh Brick    72.35 2.49          17.85     0.07        0.16       0.38          0.01
Worn out Brick-Top Layer
Case I         60.57 3.56        15.08        0.76        0.17      6.75          1.00
Case II        69.40 2.33        16.39        0.96        0.29      3.35          0.41
Case III       62.10 4.00        19.81        0.37        0.44      5.20          0.35
Worn out Brick-Middle Layer
Case I         65.86 3.47        14.21        0.24        0.19      6.86          1.30
Case II        70.70 3.37        14.31        0.52        0.55      2.21          0.46
Case III       63.20 3.77        19.48        0.21        0.22      3.70          0.24
Worn out Brick-Bottom Layer
Case I         74.40 5.31        16.32        0.22        0.18      1.49          0.06
Case II        70.18 3.10        15.64        0.91        0.40      0.81          0.12
Case III       65.22 5.60        18.60        0.30        0.31      0.90          0.32
            Table 5: Chemical Analysis of Different Layers of Worn Out Bricks
The results of chemical analysis of the top layer indicate that bricks have undergone very
severe chemical attack, which has resulted in decrease of Alumina content. The
concentration of SO3, Na2O and K2O was in the range of 0.37 – 0.96, 0.17-0.44 and 3.35-6.75
percent respectively. The concentration of chloride was in the range of 0.35-1.00 percent.
The results of chemical analysis of middle layer indicate that the volatiles have traveled
through the bricks and have reached upto the middle layer of the bricks. Concentration of
SO3, Na2O and K2O are in the range of 0.21-0.52, 0.19-0.55 and 2.21-6.86 percent
respectively. The concentration of chloride was in the range of 0.24-1.30 percent.




How to control kiln shell corrosion, SCL Beawar (Raj.)                             Page 56
S. Plants                                    Mineral compounds present in
No              Top layer                            Middle layer      Bottom layer
1 Unused bricks Al2O3, Al6 Si2O13                    Al2O3, Al6 Si2O13 Al2O3, Al6 Si2O13

2    Case I             Al2O3, KAlSi3O8, Al6 Si2O13,   Al2O3, KAlSi3O8, Al2O3, Al6 Si2O13, ,KCl
                        KAlSiO4, Ca2.Al2SiO7 , KCl     Al6 Si2O13,
                                                       KAlSiO4,
                                                       Ca2.Al2SiO7 ,KCl

3    Case II            Al2O3, KAlSi3O8, Al2 SiO5,     Al2O3, KAlSi3O8, Al2O3, KAlSi3O8, Al2 SiO5,
                        Na2SO4, Ca2.Al2SiO7 KCl        Al2 SiO5,          Na2SO4,
                                                       Ca2.Al2SiO7        Ca3.Al6O12.CaSO4, KCl
                                                       NaCl

4    Case III           Al2O3, Al6Si2O13,              Al2O3,             Al2O3, Al6Si2O13, Al2O3
                        KAlSi2O6,Al2 SiO5,             Al6Si2O13, KCl,    SiO2, SiO2 Fe2O3, KCl,
                        Ca.Al2Si2O8 , KCl              Al2O3SiO2          NaCl
                                                       KAl.Si2O6 ,
                                                       ,
                                                       K2SO4, NaCl

                Table 6: XRD Investigations of different layers of Refractory Bricks


The results of chemical analysis of bottom layer of refractory bricks indicate that the
concentration and reactivity of these volatiles is so high that these are able to travel upto
the bottom of the bricks thereby reaching upto the kiln shell. The concentration of SO3,
Na2O and K2O are in the range of 0.22-0.91, 0.18-0.40 and 0.81-1.49 percent respectively.
The concentration of chloride was in the range of 0.06-0.32 percent. The comparison with
the top and middle layer indicates that the concentration of volatiles has decreased.


The results of XRD investigations of corresponding samples also indicate the interaction of
bricks with deleterious volatile oxides leading to formation of feldsphatic compounds like
Al6Si2O13, KAl.Si2O6 , Ca.Al2Si2O8, besides formation of most detrimental oxide i.e. KCl
(sylvite). Formation of these compounds in the brick matrix led to volume expansion and


How to control kiln shell corrosion, SCL Beawar (Raj.)                                     Page 57
breaking of ceramic bonds ultimately leading to breaking or loosening of bricks.
The above investigations of worn out refractory bricks have provided very useful
information for the possible causes of kiln shell corrosion. The chemical analysis of top
layers of bricks indicate that these bricks have undergone severe interaction with calcined
kiln feed or clinker and have absorbed volatiles through the open pores in the bricks. The
bricks have three types of pores namely:
         • Through pores
         • Closed pores within brick body
         • One side open and one side closed pores
Out of all the three types of pores, one side open and one side closed pores basically create
problem for brick failure and does not directly affect the phenomena of kiln shell corrosion.
The closed pores are the ones which neither damage brick structure by way of interaction
nor help in kiln shell corrosion.


The third types of pores are very dangerous from the point of view of damaging the brick
structure as well as causing kiln shell corrosion. Because these thorough pores provide
passage to volatiles to reach up to kiln shell. The best approach would be to check the
permeability of these bricks before dispatch from the site of manufacturers. The
permeability of bricks gives a very clear picture of through pores present in the bricks.
Passage of Volatiles through joints/gaps
The volatile rich kiln gases and clinker liquid are very prone to reach upto to kiln shell
through the gaps between joints within individual rings or ring to ring joints as well. These
gaps may occur due to various reasons such as:
   • Individual brick behavior at working temperature
   • Inadequate installation due to poor workmanship
   • Actual service conditions prevailing inside the kiln
   In order to confirm the possibility of volatiles traveling through the gaps within and
   between rings and reaching at the interface of bricks and kiln shell, another exercise was
   carried out. While collecting brick samples during kiln stoppage, some of the rings have
   shown shifting and gaps created between rings. Samples of worn out bricks were
   carefully taken out ensuring the coatings on the side surfaces of bricks to remain intact.
   These coatings were scrapped carefully and analyzed for their chemical constituents.
   The results of analysis indicate that two types of reactions are taking place.


How to control kiln shell corrosion, SCL Beawar (Raj.)                                Page 58
How to Control Kiln Shell Corrosion Report
How to Control Kiln Shell Corrosion Report
How to Control Kiln Shell Corrosion Report
How to Control Kiln Shell Corrosion Report
How to Control Kiln Shell Corrosion Report
How to Control Kiln Shell Corrosion Report
How to Control Kiln Shell Corrosion Report
How to Control Kiln Shell Corrosion Report
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How to Control Kiln Shell Corrosion Report
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How to Control Kiln Shell Corrosion Report

  • 1. SUMMER IN-PLANT TRAINING REPORT Shree Cement Ltd., Beawar(Rajasthan) Starting From 7th May 2012-1st July 2012 Project Title:- How to Control Kiln Shell Corrosion Submitted to: - Submitted by:- Shri Sanjay Jain Ankit Karwa HOD,Mechanical Dept. 4th year UG, MSE Shree Cement Ltd.,Beawar IIT Kanpur How to control kiln shell corrosion, SCL Beawar (Raj.) Page 1
  • 2. Acknowledgment I take immense pleasure in thanking Shri Vinay Saxena, Plant head, Shri Sanjay Jain, HOD Mechanical Department, and Shri Atul Sharma for having permitted me to carry out this project work. I wish to express my deep sense of gratitude to Shri R.P.Pareek and Shri S.Hawa for their able guidance, useful suggestions and providing me necessary data which helped me in completing the project work, in time. Needless to mention Shri Manoj Sharma, who has been a source of inspiration and for his timely guidance in the conduct of project work. I would also like to thank Shri sanjay Baldwa, Shri Harshwardhan, Shri Manish Purohit for all their valuable and timely assistance in the project work. I’d also like to express my gratitude towards the Mechanical Library and Shri Pankaj Sharma, Librarian for helping me to make available different references during the project. Words are inadequate in offering my thanks to Shri Gopal Tripathi, Sr. Manager HRD for providing us such good facilities during the project. I would like to express my heartfelt thanks to Shri Vijay Vyas, Officer HRD, for his sincere helps throughout the project without which it seemed impossible to complete the project. Finally, yet importantly, I would like to express my heartfelt thanks to my institute, INDIAN INSTITUTE OF TECHNOLOGY, KANPUR and its Material Science and Engineering Department for providing me this opportunity to interact with this organization and understand the intricacies of the corporate world. Ankit Karwa, 4th year under graduate, Material science and Engineering, IIT Kanpur How to control kiln shell corrosion, SCL Beawar (Raj.) Page 2
  • 3. Index 1) Acknowledgment ………..2 2) Abstract ………..4 3) Introduction ………..5 4) About Shree Cement ………..6 5) Manufacturing Process ………14 6) General Chemistry of Cement Manufacturing ………19 7) The Kiln System ………22 8) Literature Review ………30 A) What is Corrosion ………30 B) Types of Corrosion ………31 C) What are Refractories ………38 D) Types of Refractories ………41 9) Type and Composition of refractory used at SCL, Beawar ………45 10) Full details of Refractory linings, Coating and SS Plate Used ………48 11) Corrosion of Kiln shell ………51 A) Introduction ………51 B) Corrosion of Cement Kiln ………52 C) Mechanism of Cement Kiln shell Corrosion ………53 D) Role of Refractories in tackling shell corrosion ………55 E) Role of Process Parameters on Shell Corrosion ………60 12)Recommendations ………62 13)References ………72 How to control kiln shell corrosion, SCL Beawar (Raj.) Page 3
  • 4. Abstract Corrosion damage is a major issue in cement plants. The serious consequences of the corrosion process in cement plants have become a problem of worldwide significance. Corrosion causes plant shutdowns, waste of valuable resources, loss or contamination of product, reduction in efficiency, costly maintenance, and expenses over design can jeopardise safety. Typically, once a plant or any piece of equipment is put into service, maintenance is required to keep it operating safely and efficiently. This is particularly true for aging systems and structures, many of which may operate beyond the original design life. The type of corrosion mechanism and its rate of attack depend on the nature of the atmosphere in which corrosion takes place. The first step in preventing corrosion is to understand its specific mechanism. The second and most important as well as most difficult step is to design an effective type of protection mechanism. The Cement kilns are operating at higher thermal and volumetric loadings and utilizing alternate raw materials and fuels which are rich in volatiles creating thereby severe service conditions inside the rotary kiln. Such conditions cause the corrosion of the rotary kiln shell to take place in hot running conditions. Therefore investigations reported in the paper were unique in nature with a very specific target to understand the mechanism of such corrosion, the role of various service conditions and process parameters on corrosion phenomena and establish such remedial measures which could impede / reduce corrosion of rotary kiln shell in running conditions. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 4
  • 5. Introduction:- Cement is a common construction material; a binder in mortars and concretes that hardens in the presence of water. Cement is called hydraulic, when the hardened product is stable in an aqueous environment. The most widespread hydraulic cement today is portland cement – a finely ground gray-to-white powder composed primarily of calcium silicates, calcium aluminates, and calcium ferrites, derived from mineral ingredients (Figure below). Fig 1:Cement Powder Cement is made by heating limestone (calcium carbonate) with some other materials such as clay to about 1400°C in a kiln, where a molecule of carbon dioxide is liberated calcium oxide, or quicklime, is formed, which is then blended with the other materials that have been included in the mix. The resulting hard substance, called 'clinker', is then ground with a small amount of gypsum into a powder to make 'Ordinary Portland Cement' or Portland Cement, often referred to as OPC, the most commonly used type of cement in the world. Portland cement or clinker can be blended or interground with other materials to achieve certain properties. There are five classes of blended cement commonly used. They are as follows:- • Portland blast-furnace slag cement • Portland-Pozzolan Cement(PPC) • Pozzolan-modified Cement How to control kiln shell corrosion, SCL Beawar (Raj.) Page 5
  • 6. • Slag Cement • Slag-modified Portland The blended cement are gaining popularity because they require less energy to manufacture, they can be made with by-product materials that would normally be disposed in a landfill, thus reducing solid waste, and offer performance benefits for certain applications. When mixed with water, portland cement sets (stiffens) in a few hours and hardens over a period of weeks and months. These phenomena are caused by chemical reactions associated with hydration between the components of cement and water. The most common use of portland cement is in the production of concrete. Concrete is a composite consisting of aggregate (gravel and sand), cement, and water. As a construction material, concrete can be cast in almost any shape desired, and once hardened, can become a structural (load bearing) element. Portland cement is also used in mortars (with sand and water only) for plasters and screeds, and in grouts (cement water mixes) placed into gaps to consolidate brick walls, foundations, etc. About Shree Cement:- Cement industry falls in the category of manufacturing industry. With the growth of economy, cement industry is also taking substantial leaps. One amongst the companies, helping the cement industry to achieve its fast growth, is Shree Cement Ltd. It is located in central Rajasthan, catering to the entire Rajasthan market with the most economic logistics cost. An ISO 9001:2000 Company, established in the Year 1984 & the Commercial Production of Unit-I started in the Year 1985. Shree Cement Limited has its registered office located at Beawar (Raj) & Corporate Office at Kolkata (W.B.). “JO SOCHE VOH PAAVE” SHREE CEMENT LIMITED is an energy conscious & environment friendly business organization. Having ten Directors on its board under the chairmanship of Shri B.G. Bangur, the policy decisions are taken under the guidance of Shri H.M. Bangur, Managing Director. Shri M.K. Singhi, Executive Director of the company, is looking after all day to day affairs. The company is managed by qualified professionals with broad vision who are committed to maintain high standards of quality & leadership to serve the customers to their fullest satisfaction. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 6
  • 7. The largest cement manufacturing plant at a single location in Northern India, under the flagship of Executive Chairman Shri B.G. Bangur & Managing Director Shri H.M. Bangur. The Company is aiming for 20 Million Ton Annual production by the year 2015. The plant is located near the city of Beawar, Dist. Ajmer in Central Rajasthan, sate ling the Beawar city at radius of 10 Kms. However the Beawar subdivision is will connected through Rail and Road both, situated on National Highway No. 8. Fig 2: Shree Cement Unit I & II, Beawar(Raj.) Shree Cement is manufacturing 33, 43, 53 & Shree ultra Red Oxide grade Cement in Ordinary Portland Cement (OPC) and Portland Pozzolana Cement (PPC). Pozzolana used in the manufacture of Portland cement is burnt clay of fly-ash generated at thermal power plants. While Ordinary Portland cement is grey fine powder which is the result from crushing a dry mix made of clinker and gypsum. Shree cement manufactures both kinds of cement. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 7
  • 8. Cement Industry in India poised for healthy growth supported by following factors:- • Growth of the Indian Cement industry directly influenced by GDP which is expected to grow at the rate of 7%. • With various infrastructure projects like roads and highways, railways, port power projected and real estate being implemented, the demand for cement is expected to grow at a fast rate. • Tax relief against interest on housing loans, stable interest rates and increasing competition in housing finance would significantly help in growth of this sector. • 16% Share in Rajasthan Cement Production. Some of another plant located in RAS. The RAS plants are far from Beawar at least 35 kms. All the same activities are doing in this plant & this plant is uses High grade material our compression in BEAWAR plant and this material send in BEAWAR plant & mix both the Material & get the superior quality product. One of newly grinding unit started in KHUSHKHERA –Distt-Bhiwadi, the plant is far from the BEAWAR plant almost 500 KM. UNIT -1 at Beawar Distt: Ajmer UNIT -2 at Beawar Distt: Ajmer Incorporated in 1979, Put up in 1997 Cement Production -2.10 million tones Established in 1985 Cement Production (Expected)-1.20 million tones UNIT -3 at Ras Distt: Pali UNIT -4 at Ras Distt: Pali Incorporated in 2005 Cement Production - 1.0 million tones Cement Production - 1.0 million tones UNIT -5,6,7 & 8 at Ras Distt: Pali Khuskheda Grinding unit (Distt. Alwar) Production – 1.00 million tons each Suratgarh , Jaipur & Roorkee Grinding units: one grinding unit each (also having a world record of 367 days) Green Power Project of 43 MW at Beawar & RAS, Thermal Power Plant of 100 MW at RAS How to control kiln shell corrosion, SCL Beawar (Raj.) Page 8
  • 9. Production with Efficiency It has a track record of over 100% capacity utilization in the 18th yr of its existence. Against the national industry average of 84% it has registered the highest record production of 3.02MT with 116% of capacity utilization. The commercial production of Unit I started in the year 1985. The production augmented exponentially from the capacity of 0.6 MTPA in 1985 to around 4.8 million ton presently through modification, capacity enhancement and continuous improvement. The revised target is 25 MTPA by 2015. Innovative & Cost Conscious Management • Leadership in the use of alternative waste fuel. • Initiatives for Global warming reduction. • Partial utilization of waste heat. • Initiator in the use of pet coke for power generation in India. PROGRAMES: • Celebration of National Safety Day. • Celebration of National Safety Week. • Staff Training in the subject of Safety. • Environment Safety Day. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 9
  • 10. Policies of the Company How to control kiln shell corrosion, SCL Beawar (Raj.) Page 10
  • 11. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 11
  • 12. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 12
  • 13. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 13 Fig 4: Typical process flow diagram of a cement plant
  • 14. Manufacturing Process: Minning Limestone is the main raw material of cement and is obtained from the mines. Bore holes are made in the mines at various locations, and samples are collected to test the CC% (CaCO3 percentage) in the mines. Limestone is retrieved by blasting the mines and then the rock material obtained from the mines is crushed. This crushed limestone (raw material) is sent to the plant with the help of conveyer belts and/or transportation. Stacking and Reclaiming A stacker is a large machine used in bulk material handling applications. It is mainly used to arrange the incoming feed in piles. It is important to maintain the homogenous and uniformity before discharging to further process. A stacker usually operates on a rail-like structure with movable wheels, but the main operation is performed on a fixed place. The main function of a Reclaimer is to recover the material and at the same time maintain uniformity. At this stage the material is collected in hoppers via conveyer belts. Reclaimers are volumetric machines and are rated in m3/h (cubic meters per hour) for capacity, which is often converted to t/h (tonnes per hour) based on the average bulk density of the material being reclaimed. Reclaimers normally travel on a rail between stockpiles in the stockyard and are generally electrically powered by means of a trailing cable. Pile of limestone is made by horizontal stacking of different CC% limestone to get the required CC%. To obtain homogenized limestone for cement production, vertical reclaiming is done. Raw Material Grinding Raw mill (Vertical Roller Mill) is a grinding equipment which is used to grind the incoming feed fed through hoppers. There are three hoppers in each unit. The first two carrying limestone and third hopper containing Laterite (zinc slag i.e. molten residue at the bottom of zinc smelter). Laterite has a high percentage of iron. Raw mill is tall unit containing a horizontally movable disc and three vertical disc, assembled a way to stand the vibrations with the virtue of the state of art suspension system. Raw mill grinds the feed to very small size and the output is sucked through the provided vents. The output is generally known as Raw Meal. This raw meal is stored in Silos. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 14
  • 15. Coal Grinding The heat required for heating and conversion is obtained by burning of grinded pet coke. Pet coke is the residue of petroleum refining and is grinded again in a VRM (Coal mill). This grinded pet coke is stored in a storage bin. Clinkerisation The raw material powder is fed into a preheater in which suction is created with the help of suction gas pump. This provides a larger residence time in the cyclonic preheaters, which are placed in a zigzag manner. The material reaches approximately 880oC at the end of the preheater. In the preheater major reaction taking place is the decomposition of CaCO3 to produce CaO. This material then enters the rotary kiln, in which hot air is blasted from the opposite direction at a temp of about 1500oC. CaO (C), Al2O3 (A), SiO2 (S), Fe2O3 (F) react in various parts of the kiln to produce C3A (10%), C4AF (10%), C2S (30%), C3S (50%). This mixture is called as clinker. Clinker formed above is cooled with the help of the air in Grate coolers, and then stored in clinker silos. The key component of the gas-suspension pre-heater is the cyclone. A cyclone is a conical vessel into which a dust-bearing gas-stream is passed tangentially. This produces a vortex within the vessel. The gas leaves the vessel through a co-axial "vortex-finder". The solids are thrown to the outside edge of the vessel by centrifugal action, and leave through a valve in the vertex of the cone. Cyclones were originally used to clean up the dust-laden gases leaving simple dry process kilns. If, instead, the entire feed of raw meal is encouraged to pass through the cyclone, it is found that a very efficient heat exchange takes place: the gas is efficiently cooled, hence producing less waste of heat to the atmosphere, and the raw meal is efficiently heated. This efficiency is further increased if a number of cyclones are connected in series. The number of cyclones stages used in practice varies from 1 to 6. Energy, in the form of fan-power, is required to draw the gases through the string of cyclones, and at a string of 6 cyclones, the cost of the added fan-power needed for an extra cyclone exceeds the efficiency advantage gained. It is normal to use the warm exhaust gas to dry the raw materials in the raw mill. The hot feed that leaves the base of the pre-heater string is typically 20% calcined, so the kiln has less subsequent processing to do, and can therefore achieve a higher specific output. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 15
  • 16. Fig 6:Pyro Process Cement kilns are used for the pyro-processing stage of manufacture of cement, in which calcium carbonate reacts with silica-bearing minerals to form a mixture of calcium silicates. Cement kilns are the heart of the cement production process: their capacity usually defines the capacity of the cement plant. As the main energy-consuming and greenhouse-gas– emitting stage of cement manufacture, improvement of their efficiency has been the central concern of cement manufacturing technology. The rotary kiln consists of a tube made from steel plate, and lined with firebrick. The tube slopes slightly (1–4°) and slowly rotates on its axis at between 30 and 250 revolutions per hour. Raw mix is fed in at the upper end, and the rotation of the kiln causes it gradually to move downhill to the other end of the kiln. At the other end fuel, in the form of gas, oil, or pulverized solid fuel, is blown in through the "burner pipe", producing a large concentric How to control kiln shell corrosion, SCL Beawar (Raj.) Page 16
  • 17. flame in the lower part of the kiln tube. As material moves under the flame, it reaches its peak temperature,before dropping out of the kiln tube into the cooler. Air is drawn first through the cooler and then through the kiln for combustion of the fuel. In the cooler the air is heated by the cooling clinker, so that it may be 400 to 800 °C before it enters the kiln, thus causing intense and rapid combustion of the fuel. Fig 7: Cement Rotary kiln Note: - The details of Cement Rotary kiln are discussed in detail in later part of the report. Cement grinding The clinker leaving the cooler has a variable particle size, typically of diameter 3-25 mm. Thus it is necessary to grind the clinker to a more uniform particle size. This can either be done directly after the cooler, or the clinker can be transferred to a silo until it is needed, see Figure 2-1. Cement clinker is, however, not a stable material: It reacts readily with water/water vapour and CO2 from the air. This will result in pre-hydration and carbonation. Extended storage periods may therefore have an effect on the cement properties. In connection with the clinker grinding, 3-6 wt-% gypsum (CaSO4󲐀2H2O) is added to the clinker. The gypsum has a marked effect on both the strength and the setting of cement. Additives such as coal fly ash, sand or raw material may also be added at this point in order to contribute positively to the strength-giving properties of the cement. Some of these Additives can replace a significant fraction of the clinker, thereby saving energy needed for calcinations and clinker reactions. Thus the additives have a great potential to reduce energy consumption and CO2-emissions per kg cement produced. For this reason, many cement companies seek to minimize their environmental impact by exploring the use of new Supplementary Cementitious Materials (SCM), examples being volcanic ashes or kaolinite clays. The grinding of clinker and additives can be performed with several different mill systems. When a sufficient fineness is reached, the final Portland cement product is transferred to a cement silo. It is stored in this silo until it is being packed and shipped to the end users. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 17
  • 18. Fig 8: Detail flow sheet of whole process How to control kiln shell corrosion, SCL Beawar (Raj.) Page 18
  • 19. General Chemistry of cement manufacturing The Portland cement is made by heating a mixture of limestone and clay, or other materials of similar bulk composition and sufficient reactivity, ultimately to a temperature of about 1450oC. Partial fusion occurs, and nodules of clinker are produced. The clinker is mixed with a few per cent of calcium sulphate and finely ground, to make cement. The calcium sulphate controls the rate of set and influences the rate of strength development. It is commonly described as gypsum, but this may be partly or wholly replaced by the other forms of calcium sulphate. Some specifications allow the addition of other materials at the grinding stage. The clinker typically has a composition in the region of 67% CaO, 22% SiO2, 5% Al2O3, 3% Fe2O3 and 3% other components, and normally contains 4 major phases: 1. Alite 2. Belite 3. Aluminate 4. Ferrite Alite: Alite is the most important constituent of all normal Portland cement clinkers, of which it constitutes 50-70%. It is tricalcium silicate (Ca3SiO5) modified in composition and crystal structure by ionic substitutions. Reaction of this phase with water is very quick and in normal Portland cements it is the most important of the constituent’s phases for strength development; it ages up to 28 days, it is by far the most important. Belite: Belite constitutes 15-30% of normal Portland cement clinkers. It is di calcium silicate (Ca2SiO4) modified by ionic substitutions and normally present wholly or largely as the β polymorph. It reacts slowly with water, thus contributing little to the strength during the first 28 days, but substantially to the further increase in strength that occurs at later ages. By one year, the strengths obtainable from pure alite and pure belite are about the same under comparable conditions. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 19
  • 20. Aluminate: Aluminate constitutes 5-10% of most normal Portland cement clinkers. Basically it is tri calcium aluminate (Ca3Al2O6) substantially modified in composition and sometimes also in structure by ionic substitutions. It has a rapid reaction with water. This phase causes an undesirable rapid setting so we need a controlling agent for this phase (like Gypsum). Ferrite: It is tetra calcium alumina ferrate i.e. Ca2AlFeO5, substantially modified in composition by variation in Al/Fe ratio and ionic substitution. It has variable reaction rate with water.Initially it reacts rapidly with water but reaction goes on slowing down with time which is due to the difference in the compositions or other characteristics. Clinker consists of MgO up to 4-5 %. If there is an excess of only 2% then it can cause expansion of hardened concrete on reaction with water. This excess can occur as periclase (MgO). Same problem might happen with SO3 if it is not under the limit of 3.5 % . To produce white cement we increase the ratio of Al2O3 to Fe2O3. Dark color of the cement is due to ferrite. Reaction of cement with water is exothermic. This total heat can be reduced by lowering alite and aluminate content in coarser grinding. This can also be done by the addition of fly ash. Clinker Reactions After having passed the calciner, the calcined raw meal is admitted to the rotary kiln where the remaining cement clinker reactions take place. Calcined raw meal is also called hot meal in cement terminology in order to distinguish it from raw meal not yet calcined. The names, chemical compositions and abbreviations used in cement nomenclature for the four main constituents of Portland cement clinker are shown in Table 1. Belite and alite are the strength-giving minerals. Alite reacts fast with water during hydration and accounts for the early strengths while belite reacts slower and gives the cement its late strengths. Ferrite and aluminate do not contribute to the strengths of the cement. But they possess important properties when burning clinker. These properties of ferrite and aluminate will be explained later in this section. Name Chemical Composition Nomenclature Belite 2Cao.SiO2 C2S Alite 3Cao.SiO2 C3S Aluminate 3CaO.Al2O3 C3A Ferrite 4CaO.Al2O3.Fe2O3 C4AF Table 1: Main Constituents of Portland Cement How to control kiln shell corrosion, SCL Beawar (Raj.) Page 20
  • 21. Free lime (CaO), free periclase (MgO), earth alkali sulfates (CaSO4, MgSO4), Alkali sulfates (Na2SO4, K2SO4) and other minor components may also be found in the clinker. The requirements for the Portland cement clinker are: a) > 67 wt.-% calcium silicates (C2S and C3S). The remainder must mainly be Fe2O3, Al2O3 and other oxides. b) < 5 wt.-% MgO c) The CaO/SiO2 ratio by mass shall not be less than 2.0. Clinker reactions occur at temperatures between 700-1450°C, see Figure 9. The reactions forming the final clinker involve intermediate compounds, and the clinker reactions may be affected by minor compounds. The overall clinker reactions are described, while a detailed overview of the chemistry, phase relations etc. Fig 9: Phase Diagram for cement clinker production How to control kiln shell corrosion, SCL Beawar (Raj.) Page 21
  • 22. The kiln system The Production of cement may be divided into three parts: 1) Preparation of raw materials,2)pyro-processing and 3)clinker processing, storage and shipment. Pyro- processing covers the thermal treatment of the raw materials necessary to obtain the cement clinker. Pyro-processing takes place in the preheater, calciner, rotary kiln and cooler. These sections are commonly referred to as the kiln system. Rotary kiln The rotary kiln is often referred as heart of cement plant. This is where the chemical clinker formation reactions take place. The rotary kiln is simply a long, cylindrical tube consisting of an outer shell and an inner refractory lining. Typical lengths and diameters for modern rotary kilns are between 40-100 m, and 3-6m, respectively. Rotary kilns are inclined 1-4° and rotate 1-5rpm in order to facilitate mass transport and ensure clinker forming processes such as nodulization. Production capacity is typically 2000-4000 tonnes of clinker per day (tpd), but may be as high as 12000 tpd. In the material outlet, the rotary kiln is equipped with a burner. The burner’s main function is to form a flame to provide energy for clinker reaction to takes place. The flame of the rotary kiln burner should be short, narrow and strongly radiant in order to achieve a good heat transfer from the flame to materials in the bed. Modern rotary kiln burners are often designed to burn a variety of fuels. This is today achieved by using multi-channel burners with separate channels for fuels and primary air which make it possible to adjust primary air amounts, injection velocities and momentum independently of the fuel flows. Swirl may be used to enhance mixing and stabilize the flame. The recent trend in multiple fuel burners is to use a single common fuel channel which allows more flexibity towards fuel particle size and type. Fig 10: Left: Outer view of rotary kiln seen from above. Right: Inner view seen from burner end. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 22
  • 23. In early cement plants, the kiln system consisted only of a rotary kiln: Raw materials were dried, preheated, calcined and burned to clinker on their way through the rotary kiln. This process required very long rotary kilns, often significantly longer than 100 m. The raw materials were either introduced as dry raw meal or as water/raw material slurry, and this type of plants were therefore commonly referred to as dry long kilns or wet long kilns, respectively. Due to a low energy efficiency, this type of cement plants is very expensive to operate, and are rarely constructed today. The rotary kiln consists of an outer steel shell and an inner refractory lining for thermal insulation, in order to maintain and resist the high process temperatures. In a rotary kiln, the refractory usually consists of bricks of special composition and sizes, able to withstand high temperatures. However, the refractory may also be a cast lining of concrete. The refractory lining is subject to a wide range of destructive influences through the mechanical dynamics of the rotary kiln, the chemistry of the cement clinker process and the type of fuels used. The intensity of these stresses varies according to the operating conditions and kiln sections. The rotary kiln is therefore equipped with a range of refractory bricks with different properties to ensure appropriate kiln zone lining. The burning zone refractory lining usually suffers the greatest wear due to the higher temperatures in the burning zone. However, the burning zone lining is protected by a coating layer which prolong the lifetime of the refractory lining. The coating is a mass of clinker or dust particles that adheres to the wall of the kiln, having changed from a liquid or semiliquid to a solidified state. Generally, the burning zone refractory lifetime is 9 to 12 months depending on the specific kiln type and operating conditions. The refractory lifetime of the colder rotary kiln material inlet zone is typically 12 to 48 months. Thus, these different rotary kiln zones do not have to be replaced quite as frequently. The most used brick types today are chromium-free magnesia-alumina-spinel bricks with MgO content of 80-95 wt.-% and Al2O3 content of 3-18 wt.-%. Minor compounds are typically Fe2O3, Mn2O3, SiO2, CaO and ZrO2. These brick types are regarded as having the longest service life and the best price/performance ratio. The increased use of alternative fuels in the cement industry may lead to higher levels of recirculating alkali metals and sulfur within the kiln system.Recirculating alkali metals and sulfur cause large quantities of salts to condense in and on the refractory lining, predominantly in the temperature window 750°C to 1,100°C . Salt compounds enter into How to control kiln shell corrosion, SCL Beawar (Raj.) Page 23 reactions with refractory bricks that contain alumina and the bricks can be destroyed by salt
  • 24. crystallization and alkali spalling. Sulfur oxides make the reactions even worse, by formation of alkali sulfate salt. Various strategies are used by the refractories industry to counteract these wear processes. These include use of additives that produce low gas permeability and reduce the infiltration tendency of alkalis. Other solutions are sealing or impregnating the refractory material to form a protective zone. One of the most successful and widespread solutions is addition of 3-6 wt.-% Silicon carbide, SiC, which leads to an appreciable resistance to alkali attack. The addition of SiC leads in situ to formation of liquid phases, which seal the refractory surface and protect against alkali infiltration. Increased circulation of inorganic volatiles such as sulfur and chlorine in the kiln system, due to increased alternative fuel utilization also entails a higher risk of kiln shell corrosion (Jøns and Østergaard, 2001). Efforts have been made to identify suitable refractory steels for cement rotary kilns, with characteristics that are a compromise between good creep resistance, high corrosion resistance in the presence of chlorine and sulfur, and strong resistance to abrasion when hot. Thermal profile and kiln subdivisions The rotary kiln thermal profile varies throughout its length, depending on the temperature and chemical reactions involved during the process (see in Table 2). The rotary kiln can be subdivided into several zones or regions that are exposed not only to thermal and chemical wear but also to mechanical stresses. The influence of one or several of these factors, to minor or greater proportion determines the refractory lining type required for each zone: A. Decarbonation zone: from 300ºC to 1000°C (+) This stage can occur either inside of the old wet process rotary kilns or in the preheater tower of modern units consisting of two steps: Firstly, between 300°C and 650°C where the raw meal heating occurs, accompanied by a dehydration reaction; Secondly, between 650°C and 1000°C, when the limestone decarbonation takes place generating CO2 and CaO. The first step is characterized by the following aspects: How to control kiln shell corrosion, SCL Beawar (Raj.) Page 24 • Presence of raw meal (there are no new mineral phases development);
  • 25. Erosion (due to raw meal flow at high velocities); • low temperature; • Evaporation and dehydration (of water) chemically bonded to the raw material. In this zone it is very important that the refractory products have the capability to protect the rotary kiln drive (good insulation degree) and good resistance to impacts of anomalous build-ups. Bricks with less than 45% Al2O3 content are suitable. Besides that, when alkaline salts are present, a vitreous glassy layer can develop with the alkali on the brick surface, preventing its propagation or later infiltration. In the second stage of these reactions, the development of new mineralogical phases occurs: • Formation of CaO and CO2; • Formation of CA, C12A7 and C2S; • Temperature variation; • Alkali attack. Usually, the use of bricks with a 70% Al2O3 content is recommended, which offers a high mechanical resistance, low porosity, and low thermal conductivity. However, the risk of eutectic reactions formations on the Al2O3-CaO- SiO2 , system and alkali resistance is a limiting factor. B. Upper transition zone: from 1000ºC to 1238°C (+) It is the most unstable and difficult area for refractory specification. Although the temperature range varies from 1000°C to 1338°C, incidences of thermal overloads are frequent. This fact is linked on the flame shape, to the fuel type and to the design of the kiln main burner. Therefore, it is in this area where coating starts to develop as soon as first drops of liquid phase appear. Coating becomes very unstable if the operational conditions present high variability. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 25
  • 26. Table 2 C. Sintering zone: from 1338ºC to 1450°C (+) In this area a full development of coating at 1450ºC(+) is expected. The presence of some liquid phase facilitates the dissolution of C2S in the same what promotes the reaction that generates C3S. The highest temperature in the kiln is reached at this area. Usually it should be around 1450ºC for ordinary Portland Cements. Liquid phase is also around 25% at 1450ºC. If process is under control, coating will be stable and able to protect the lining during the whole campaign. However, if there is a big variability at ram meal control parameters or uneven fuels types shifting, coating will be unstable and refractories submitted to an enormous thermo-chemical wear. The refractory products must resist high temperatures, infiltration of molten liquid calcium silicates, and/or alkaline sulfates, and be able to hold a stable coating. Usually at this kiln zone it is possible to find: • Presence of incipient liquid phase from 18 to 32%, free lime and C2S; • Development of C3S by the reaction of CaO and C2S. • Clinker liquid phase infiltration and coating formation; • Chemical attacks by alkaline sulfates; • High operational temperature. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 26
  • 27. D. Lower transition zone from: 1400ºC to 1200°C (+) This area usually operates between 1400°C and 1200°C. Around 1200ºC begins the crystallization of the clinker the mineral phases, but not. Although the liquid phase can still be present, it is a stage of low chemical activity, considering that most of C3S has already been formed with a remaining amount of free lime around 1%. Nevertheless, it is a zone submitted to temperature variations since it is right under the influence of the secondary air temperature coming from the cooler. This area is characterized by the following aspects: • Presence of the clinker liquid phase; • Chemical attacks by alkaline sulfates; • Frequent temperature variations when flame impinges over the brick; • Continuous thermal shock; • Redox atmosphere when using alternative fuels with poorly designed burner; • Mechanical stress imposed by the tire station and kiln shell ovality. In order to support the temperature variations under mechanical stress, this part of the process requires the use of basic bricks with high structural flexibility, low permeability to gas, high hot modules of rupture and abrasion resistance. E. Pre-cooling zone from: 1200ºC to 1000°C (+) Originally, many kilns have been designed to promote the end of freezing and crystallization of the just developed clinker phases. However, nowadays, the existence of this zone into the kiln depends of the clinker cooler type and the secondary air temperature entering into the kiln. With old grate coolers it was around 700ºC, and for the modern high efficiency ones from 1150°C to 1100°C. In this zone at that temperature range, there is high abrasion (clinker nodules), accentuated discharge erosion (by the clinker dust carried by secondary and tertiary airs) and mechanic stresses (nose ring plates and retention ring for refractory products). The main characteristics of this kiln zone are: • High abrasion / erosion; • Frequent thermal shocks; • High mechanical stresses (compression/traction). In most of the modern furnaces equipped with high efficiency coolers, this zone is not inside the rotary kiln but in the first cooling compartment. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 27
  • 28. Wet Process The original rotary cement kilns were called 'wet process' kilns. In their basic form they were relatively simple compared with modern developments. The raw meal was supplied at ambient temperature in the form of slurry. A wet process kiln may be up to 200m long and 6m in diameter. It has to be long because a lot of water has to be evaporated and the process of heat transfer is not very efficient. The slurry may contain about 40% water. This takes a lot of energy to evaporate and various developments of the wet process were aimed at reducing the water content of the raw meal. An example of this is the 'filter press' (imagine a musical accordion 10-20 metres long and several metres across) - such adaptions were described as 'semi-wet' processes. The wet process has survived for over a century because many raw materials are suited to blending as slurry. Also, for many years, it was technically difficult to get dry powders to blend adequately. Quite a few wet process kilns are still in operation, usually now with higher-tech bits bolted on. However, new cement kilns are of the 'dry process' type. Dry Process In a modern works, the blended raw material enters the kiln via the pre-heater tower. Here, hot gases from the kiln, and probably the cooled clinker at the far end of the kiln, are used to heat the raw meal. As a result, the raw meal is already hot before it enters the kiln. The dry process is much more thermally efficient than the wet process. Firstly, and most obviously, this is because the meal is a dry powder and there is little or no water that has to be evaporated. Secondly, and less obviously, the process of transferring heat is much more efficient in a dry process kiln. An integral part of the process is a heat exchanger called a 'suspension preheater'. This is a tower with a series of cyclones in which fast-moving hot gases keep the meal powder suspended in air. All the time, the meal gets hotter and the gas gets cooler until the meal is at almost the same temperature as the gas. The basic dry process system consists of the kiln and a suspension preheater. The raw How to control kiln shell corrosion, SCL Beawar (Raj.) Page 28 materials, limestone and shale for example, are ground finely and blended to produce the
  • 29. raw meal. The raw meal is fed in at the top of the preheater tower and passes through the series of cyclones in the tower. Hot gas from the kiln and, often, hot air from the clinker cooler are blown through the cyclones. Heat is transferred efficiently from the hot gases to the raw meal. The heating process is efficient because the meal particles have a very high surface area in relation to their size and because of the large difference in temperature between the hot gas and the cooler meal. Typically, 30%-40% of the meal is decarbonated before entering the kiln. A development of this process is the 'precalciner' kiln. Most new cement plant is of this type. The principle is similar to that of the dry process preheater system but with the major addition of another burner, or precalciner. With the additional heat, about 85%-95% of the meal is decarbonated before it enters the kiln. Fig 11: Preheater Precalciner Kiln How to control kiln shell corrosion, SCL Beawar (Raj.) Page 29
  • 30. Since meal enters the kiln at about 900 C, (compared with about 20 C in the wet process), the kiln can be shorter and of smaller diameter for the same output. This reduces the capital costs of a new cement plant. A dry process kiln might be only 70m long and 6m wide but produce a similar quantity of clinker (usually measured in tonnes per day) as a wet process kiln of the same diameter but 200m in length. For the same output, a dry process kiln without a precalciner would be shorter than a wet process kiln but longer than a dry process kiln with a precalciner. Literature Review Corrosion • What is corrosion • Types of corrosion Refractories • What is refractory • Types of refractories • What is the composition of refractories used at SCL,Beawar What is corrosion Corrosion is the gradual destruction of material, usually metals, by chemical reaction with its environment. In the most common use of the word, this means electro- chemical oxidation of metals in reaction with an oxidant such as oxygen. Rusting, the formation of iron oxides, is a well-known example of electrochemical corrosion. This type of damage typically produces oxide(s) or salt(s) of the original metal. Corrosion can also occur in materials other than metals, such as ceramics or polymers, although in this context, the term degradation is more common. Corrosion degrades the useful properties of materials and structures including strength, appearance and ability to contain a vessel's contents. Many structural alloys corrode merely from exposure to moisture in the air, but the process can be strongly affected by exposure to certain substances. Corrosion can be concentrated locally to form a pit or crack, or it can extend across a wide area more or less uniformly corroding the surface. Because corrosion is a diffusion controlled process, it occurs on How to control kiln shell corrosion, SCL Beawar (Raj.) Page 30 exposed surfaces. As a result, methods to reduce the activity of the exposed surface, such
  • 31. as passivation and chromate-conversion, can increase a material's corrosion resistance. However, some corrosion mechanisms are less visible and less predictable. Types of corrosion 1) Uniform Corrosion or General Corrosion This type of corrosion is chemical or electrochemical in nature. However, there are no discrete anode or cathode areas. This form of corrosion is uniform over the surface of the metal exposed to the environment. The metal gradually becomes thinner and eventually fails. The energy state of the metal is basically what causes this reaction. Referred to as the “dust-to-dust” process, high levels of energy are added to the raw mmaterial to produce the metal. This high energy level causes an unnaturally high electrical potential. One law of chemistry is that all materials will tend to revert to its lowest energy level, or its natural state. After high levels of energy are added to the metal, when it is exposed to the environment (an electrolyte), it will tend to revert to its natural state. This process is normally extremely slow, and is dependent on the ion concentration of the electrolyte that it is exposed to. Only under very extreme conditions (acidic electrolyte) can this form of corrosion be significant. The corrosion rate for steel climbs drastically at a pH below 4, and at a pH of about 3 , the steel will dissolve. General corrosion tends to slow down over time because the potential gradually becomes lower. Failures of pipelines or tanks would not quickly occur from this type of corrosion since no pitting or penetration of the structure occurs, just a general corrosion over the entire surface (except under very extreme circumstances where the metal could dissolve in an acid electrolyte). However, in nature, the metal is not completely uniform and the electrolyte is not completely homogeneous, resulting in electrochemical corrosion cells that greatly overshadow this mild form of corrosion. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 31
  • 32. Fig 12: Uniform corrosion 2) Concentration Cell Corrosion This type of corrosion is caused by an electrochemical corrosion cell. The potential difference (electromotive force) is caused by a difference in concentration of some component in the electrolyte. Any difference in the electrolyte contacting the metal forms discrete anode and cathode regions in the metal. Any metal exposed to an electrolyte exhibits a measurable potential or voltage. The same metal has a different electrical potential in different electrolytes, or electrolytes with different concentrations of any component. This potential difference forces the metal to develop anodic and cathodic regions. When there is also an electrolyte and a metallic path, the circuit is complete, current flows, and electrochemical corrosion will occur. Soil is a combination of many different materials. There are also many different types of soil, and even the same type of soil varies greatly in the concentration of its constituents. Therefore, there is no such thing as truly homogeneous soil. These soil variations cause potential differences (electromotive force) on the metal surface resulting in electrochemical corrosion cells. Liquids tend to be more uniform, but can vary in the concentration of some components such as How to control kiln shell corrosion, SCL Beawar (Raj.) Page 32
  • 33. Fig 13: Concentration Cell Caused by Different Environments oxygen varies by depth and flow rates. Biological organisms are present in virtually all- natural aqueous environments, these organisms tend to attach to and grow on the surface of structural materials, resulting in the formation of a biological film, or biofilm. These films are different from the surrounding electrolyte and have many adverse effects. Following are examples of common forms of concentration cell corrosion. I. Dissimilar Environment II. Oxygen Concentration III. Moist/Dry Electrolyte IV. Non-Homogeneous Soil V. Concrete / Soil Interface VI. Backfill Impurities VII. Biological Effects 3) Galvanic Corrosion This type of corrosion is caused by an electrochemical corrosion cell developed by a potential difference in the metal that makes one part of the cell an anode, and the other part of the cell the cathode. Different metals have different potentials in the same electrolyte. This potential difference is the driving force, or the voltage, of the cell. As with any electrochemical corrosion cell, if the electrolyte is continuous from the anode to the cathode and there is a metallic path present for the electron, the circuit is completed and current will flow and electrochemical corrosion will occur. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 33
  • 34. I. Dissimilar Metals The most obvious form of this type of corrosion is when two different kinds of metal are in the electrolyte and metallically bonded or shorted in some manner. All metals exhibit an electrical potential; each metal has its distinctive potential or voltage (paragraph 2-4). When two different metals are connected, the metal with the most negative potential is the anode; the less negative metal is the cathode. An “active” metal is a metal with a high negative potential, which also means it is anodic when compared to most other metals. A “noble” metal is a metal with a low negative potential, which also means it is cathodic when compared to most other metals. Dissimilar metal corrosion is most severe when the potential difference between the two metals, or “driving voltage,” is the greatest. Fig 14: Galvanic Corrosion Cell Caused by Different Metals Examples of active metals are new steel, aluminum, stainless steel (in the active state), zinc, and magnesium. Examples of noble metals are corroded steel, stainless steel (in the passivated state), copper, bronze, carbon, gold, and platinum. One example of this type of corrosion occurs when coated steel pipelines are metallically connected to bare copper grounding systems or other copper pipelines (usually water lines) II. Dissimilar Alloys The most obvious example of this type of corrosion is different metal alloys. For example, there are over 200 different alloys of stainless steel. Also, metals are not 100 percent pure. They normally contain small percentages of other types of metals. Different batches of a metal vary in content of these other metals. Different manufacturers may use different raw materials and even the same manufacturer may use raw materials from different sources. Each batch of metal may How to control kiln shell corrosion, SCL Beawar (Raj.) Page 34 be slightly different in electrical potential. Even in the same batch of metal, the
  • 35. concentration of these other materials may vary slightly throughout the finished product. All these differences will produce the electromotive force for this type of corrosion to occur. III. Impurities in Metal No manufacturing process is perfect. Small impurities may be mixed into the metal as it is produced or cooled. Impurities at the surface of the metal may become part of the electrolyte causing concentration cell corrosion, or if metallic, they may be anodic (corrodes and leaves a pit behind), or cathodic (corroding surrounding metal). IV. Temperature Metal that is at an elevated temperature becomes anodic to the same metal at a lower temperature. As previously discussed, a more active metal is anodic to a more noble metal. Since elevated temperature makes a metal more active, it becomes anodic to the rest of the metal. This electrochemical corrosion cell may cause accelerated corrosion on metals that are at elevated temperatures. 4) Stray Current Corrosion This type of electrochemical corrosion cell is caused by an electromotive force from an external source affecting the structure by developing a potential gradient in the electrolyte or by inducing a current in the metal, which forces part of the structure to become an anode and another part a cathode. This pickup and discharge of current occurs when a metallic structure offers a path of lower resistance for current flowing in the electrolyte. This type of corrosion can be extremely severe because of very high voltages that can be forced into the earth by various sources. The potential gradient in the electrolyte forces one part of the structure to pick up current (become a cathode) and another part of the structure to discharge current (become an anode). Stray current corrosion occurs where the current from the external source leaves the metal structure and enters back into the electrolyte, normally near the external power source cathode. The external power source is the driving force, or the voltage, of the cell. Stray current corrosion is different from natural corrosion because it is caused by an externally induced electrical current and is basically independent of such environmental factors as concentration cells, resistivity, pH and galvanic cells. The amount of current (corrosion) How to control kiln shell corrosion, SCL Beawar (Raj.) Page 35
  • 36. depends on the external power source, and the resistance of the path through the metallic structure compared to the resistance of the path between the external source’s anode and cathode. Fig 15: Stray Current Corrosion Cell Caused by External Anode and Cathode An example of stray current corrosion is caused by impressed current cathodic protection systems, where a “foreign” electrically continuous structure passes near the protected structures anodes and then crosses the protected structure (cathode). This corrosion is usually found after failures in the foreign structure occur. Stray current corrosion is the most severe form of corrosion because the metallic structure is forced to become an anode and the amount of current translates directly into metal loss. If the amount of current leaving a structure to enter the electrolyte can be measured, this can be directly translated into metallic weight loss. Different metals have specific amounts of weight loss when exposed to current discharge. This weight loss is normally measured in pounds (or kilograms) of metal lost due to a current of one amp for a period of one year (one amp- year). For example, if a stray current of just two amps were present on a steel pipeline, the result would be a loss of 18.2 kilo grams (40.2 pounds) of steel in one year. For a coated pipeline, this could result in a penetration at a defect in the coating in an extremely short period of time, sometimes only a few days. 5) Crevice Corrosion Crevice Corrosion refers to the localized attack on a metal surface at, or immediately adjacent to, the gap or crevice between two joining surfaces. The gap or crevice can be formed between two metals or a metal and non-metallic material. Outside the gap or without the gap, both metals are resistant to corrosion. The damage caused by crevice corrosion is normally confined to one metal at localized area within or close to the joining surfaces. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 36
  • 37. Fig 16: a type 316 stainless steel tube and tube sheet from a heat exchanger in a seawater reverse osmosis (SWRO) desalination plant suffered crevice corrosion due to the presence of crevice (gap) between the tube and tube sheet. 6) Pitting Corrosion Certain conditions, such as low concentrations of oxygen or high concentrations of species such as chloride which complete as anions, can interfere with a given alloy's ability to re- form a passivating film. In the worst case, almost all of the surface will remain protected, but tiny local fluctuations will degrade the oxide film in a few critical points. Corrosion at these points will be greatly amplified, and can cause corrosion pits of several types, depending upon conditions. While the corrosion pits only nucleate under fairly extreme circumstances, they can continue to grow even when conditions return to normal, since the interior of a pit is naturally deprived of oxygen and locally the pH decreases to very low values and the corrosion rate increases due to an auto-catalytic process. In extreme cases, the sharp tips of extremely long and narrow corrosion pits can cause stress concentration to the point that otherwise tough alloys can shatter; a thin film pierced by an invisibly small hole can hide a thumb sized pit from view. These problems are especially dangerous because they are difficult to detect before a part or structure fails. Pitting remains among the most common and damaging forms of corrosion in passivated alloys, but it can be prevented by control of the alloy's environment. Fig 17: Scheme of pitting corrosion How to control kiln shell corrosion, SCL Beawar (Raj.) Page 37
  • 38. What are Refractories Any material can be described as a ‘refractory,’ if it can withstand the action of abrasive or corrosive solids, liquids or gases at high temperatures. The various combinations of operating conditions, in which refractories are used, make it necessary to manufacture a range of refractory materials with different properties. Refractory materials are made in varying combinations and shapes depending on their applications. General requirements of a refractory material are: • Withstand high temperatures • Withstand sudden changes of temperatures • Withstand action of molten metal slag, glass, hot gases, etc • Withstand load at service conditions • Withstand load and abrasive forces • Conserve heat • Have low coefficient of thermal expansion • Should not contaminate the material with which it comes into contact Depending on the area of application such as boilers, furnaces, kilns, ovens etc, temperatures and atmospheres encountered different types of refractories are used. Some of the important properties of refractories are: a) Melting Point Pure substances melt instantly at a specific temperature. Most refractory materials consist of particles bonded together that have high melting temperatures. At high temperatures, these particles melt and form slag. The melting point of the refractory is the temperature at which a test pyramid (cone) fails to support its own weight. b) Size The size and shape of the refractories is a part of the design of the furnace, since it affects the stability of the furnace structure. Accurate size is extremely important to properly fit the refractory shape inside the furnace and to minimize space between construction joints. c) Bulk Density The bulk density is useful property of refractories, which is the amount of refractory material within a volume (kg/m3). An increase in bulk density of a given refractory increases its volume stability, heat capacity and resistance to slag penetration. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 38
  • 39. d) Porosity The apparent porosity is the volume of the open pores, into which a liquid can penetrate, as a percentage of the total volume of the refractory. This property is important when the refractory is in contact with molten charge and slag. A low apparent porosity prevents molten material from penetrating into the refractory. A large number of small pores is generally preferred to a small number of large pores. e) Cold Crushing Strength The cold crushing strength is the resistance of the refractory to crushing, which mostly happens during transport. It only has an indirect relevance to refractory performance, and is used as one of the indicators of abrasion resistance. Other indicators used are bulk density and porosity. f) Pyrometric cones and Pyrometric cones equivalent (PCE) The ‘refractoriness’ of (refractory) bricks is the temperature at which the refractory bends because it can no longer support its own weight. Pyrometric cones are used in ceramic industries to test the refractoriness of the (refractory) bricks. They consist of a mixture of oxides that are known to melt at a specific narrow temperature range. Cones with different oxide composition are placed in sequence of their melting temperature alongside a row of refractory bricks in a furnace. The furnace is fired and the temperature rises. One cone will bends together with the refractory brick. This is the temperature range in oC above which the refractory cannot be used. This is known as Pyrometric Cone Equivalent temperatures. Fig 17: Pyrometric cones g) Creep at high Temperature Creep is a time dependent property, which determines the deformation in a given time and at a given temperature by a refractory material under stress. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 39
  • 40. h) Volume stability, expansion, and shrinkage at high temperatures The contraction or expansion of the refractories can take place during service life. Such permanent changes in dimensions may be due to: • The changes in the allotropic forms, which cause a change in specific gravity • A chemical reaction, which produces a new material of altered specific gravity • The formation of liquid phase • Sintering reactions • Fusion dust and slag or by the action of alkalies on fireclay refractories, to form alkali-alumina silicates. This is generally observed in blast furnaces i) Reversible thermal expansion Any material expands when heated, and contracts when cooled. The reversible thermal expansion is a reflection on the phase transformations that occur during heating and cooling. j) Thermal Conductivity Thermal conductivity depends on the chemical and mineralogical composition and silica content of the refractory and on the application temperature. The conductivity usually changes with rising temperature. High thermal conductivity of a refractory is desirable when heat transfer though brickwork is required, for example in recuperators, regenerators, muffles, etc. Low thermal conductivity is desirable for conservation of heat, as the refractory acts as an insulator. Additional insulation conserves heat but at the same time increases the hot face temperature and hence a better quality refractory is required. Because of this, the outside roofs of open-hearth furnaces are normally not insulated, as this could cause the roof to collapse. Lightweight refractories of low thermal conductivity find wider applications in low temperature heat treatment furnaces, for example in batch type furnaces where the low heat capacity of the refractory structure minimizes the heat stored during the intermittent heating and cooling cycles. Insulating refractories have very low thermal conductivity. This is usually achieved by trapping a higher proportion of air into the structure. Some examples are: • Naturally occurring materials like asbestos are good insulators but are not particularly good refractories • Mineral wools are available which combine good insulating properties with good resistance to heat but these are not rigid • Porous bricks are rigid at high temperatures and have a reasonably low thermal conductivity. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 40
  • 41. Type of Refractories Refractories can be classified on the basis of chemical composition, end use and methods of manufacture as shown below. Classification method Example Chemical Composition ACID, which readily combines with bases Silica, Semisilica, Aluminosilicate BASIC, which consists mainly of metallic Magnesite, Chrome-magnesite, Magnesite- oxides that resist the action of bases chromite, Dolomite NEUTRAL, which does not combine with Fireclay bricks, Chrome, Pure Alumina acids nor bases Special Carbon, Silicon Carbide, Zirconia End use Blast furnace casting pit Method of manufacture Dry press process, fused cast, hand moulded, formed normal, fired or chemically bonded, unformed (monolithics, plastics, ramming mass, gunning castable, spraying) Table 3: Classification of refractories based on chemical composition a. Fireclay refractories Firebrick is the most common form of refractory material. It is used extensively in the iron and steel industry, nonferrous metallurgy, glass industry, pottery kilns, cement industry, and many others. Fireclay refractories, such as firebricks, siliceous fireclays and aluminous clay refractories consist of aluminum silicates with varying silica (SiO ) content of up to 78 percent and 2 Al O content of up to 44 percent. Table 4 shows that the melting point (PCE) of fireclay 2 3 brick decreases with increasing impurity and decreasing Al O . This material is often used in 2 3 furnaces, kilns and stoves because the materials are widely available and relatively inexpensive. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 41
  • 42. Brick Type Percentage Percentage Percentage other PCE °C SiO2 Al2O3 constituents Super Duty 49-53 40-44 5-7 1745-1760 High Duty 50-80 35-40 5-9 1690-1745 Intermediate 60-70 26-36 5-9 1640-1680 High Duty 65-80 18-30 3-8 1620-1680 Siliceous Low Duty 60-70 23-33 6-10 1520-1595 Table 4: Properties of fireclay Bricks b. High alumina refractories Alumina silicate refractories containing more than 45 percent alumina are generally termed as high alumina materials. The alumina concentration ranges from 45 to 100 percent. The refractoriness of high alumina refractories increases with increase in alumina percentage. The applications of high alumina refractories include the hearth and shaft of blast furnaces, ceramic kilns, cement kilns, glass tanks and crucibles for melting a wide range of metals. c. Silica Brick Silica brick (or Dinas) is a refractory that contains at least 93 percent SiO2. The raw material is quality rocks. Various grades of silica brick have found extensive use in the iron and steel melting furnaces and the glass industry. In addition to high fusion point multi-type refractories, other important properties are their high resistance to thermal shock (spalling) and their high refractoriness. The outstanding property of silica brick is that it does not begin to soften under high loads until its fusion point is approached. This behavior contrasts with that of many other refractories, for example alumina silicate materials, which begin to fuse and creep at temperatures considerably lower than their fusion points. Other advantages are flux and stag resistance, volume stability and high spalling resistance. d. Magnesite Magnesite refractories are chemically basic materials, containing at least 85 percent magnesium oxide. They are made from naturally occurring magnesite (MgCO3). The properties of magnesite refractories depend on the concentration of silicate bond at the operating temperatures. Good quality magnesite usually results from a CaO-SiO2 ratio of less than two with a minimum ferrite concentration, particularly if the furnaces lined with the refractory operate in oxidizing and reducing conditions. The slag resistance is very high particularly to lime and iron rich slags. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 42
  • 43. e. Chromite Refractories Two types of chromite refractories are distinguished: • Chrome-magnesite refractories, which usually contain 15-35 percent Cr O and 42-50 2 3 percent MgO. They are made in a wide range of qualities and are used for building the critical parts of high temperature furnaces. These materials can withstand corrosive slags and gases and have high refractoriness. • Magnesite-chromite refractories, which contain at least 60 percent MgO and 8-18 percent Cr O . They are suitable for service at the highest temperatures and for 2 3 contact with the most basic slags used in steel melting. Magnesite-chromite usually has a better spalling resistance than chrome-magnesite. f. Zirconia Refractories Zirconium dioxide (ZrO2) is a polymorphic material. It is essential to stabilize it before application as a refractory, which is achieved by incorporating small quantities of calcium, magnesium and cerium oxide, etc. Its properties depend mainly on the degree of stabilization, quantity of stabilizer and quality of the original raw material. Zirconia refractories have a very high strength at room temperature, which is maintained up to temperatures as high as 15000C. They are therefore useful as high temperature construction materials in furnaces and kilns. The thermal conductivity of zirconium dioxide is much lower than that of most other refractories and the material is therefore used as a high temperature insulating refractory. Zirconia exhibits very low thermal losses and does not react readily with liquid metals, and is particularly useful for making refractory crucibles and other vessels for metallurgical purposes. Glass furnaces use zirconia because it is not easily wetted by molten glasses and does not react easily with glass. g. Oxide Refractories(Alumina) Alumina refractory materials that consist of aluminium oxide with little traces of impurities are known as pure alumina. Alumina is one of the most chemically stable oxides known. It is mechanically very strong, insoluble in water, super heated steam, and most inorganic acids and alkalies. Its properties make it suitable for the shaping of crucibles for fusing sodium carbonate, sodium hydroxide and sodium peroxide. It has a high resistance in oxidizing and reducing atmosphere. Alumina is extensively used in heat processing industries. Highly porous alumina is used for lining furnaces operating up to 1850oC. h. Monolithics Monolithic refractories are single piece casts in the shape of equipment, such as a ladle as shown in Figure 18. They are rapidly replacing the conventional type fired refractories in many applications including industrial furnaces. The main advantages of monolithics are: How to control kiln shell corrosion, SCL Beawar (Raj.) Page 43
  • 44. Elimination of joints which is an inherent weakness • Faster application method • Special skill for installation not required • Ease of transportation and handling • Better scope to reduce downtime for repairs • Considerable scope to reduce inventory and eliminate special shapes • Heat savings • Better spalling resistance • Greater volume stability Monolithics are put into place using various methods, such as ramming, casting, gunniting, spraying, and sand slinging. Ramming requires proper tools and is mostly used in cold applications where proper consolidation of the material is important. Ramming is also used for air setting and heat setting materials. Because calcium aluminate cement is the binder, it will have to be stored properly to prevent moisture absorption. Its strength starts deteriorating after 6 to 12 months. Fig 18: A monolithic lining for Ladel How to control kiln shell corrosion, SCL Beawar (Raj.) Page 44
  • 45. Type and Composition of refractory used at SCL, Beawar (kiln 2) Brick Type TOPMAG A1 ALMAG 85 FERROMA PERILEX 83 G 90 Magnesia- Magnesia- Magnesia- Magnesia- Fused Spinel Fused Spinel Hercynite Chromite Characteristi 77-81 MgO 85-89MgO 87-92MgO 81-85MgO c Component in % bulk density 2.9-3.05 2.85-3 2.85-3 2.9-3.05 (g/cm3) Apparent 15-17 16-18 16-18 17-19 porosity % Cold 65 50 50 55 Crushing Strength N/mm2 Seger Cone >42 >42 >42 42 Thermal expansion 400°C 0.3 0.4 0.4 0.4 Lin. % at 800°C 0.9 0.8 0.9 1.1 1200°C 1.4 1.4 1.5 1.7 Thermal 100 100 100 80 Shock resistance at 950°C/air Thermal Conductivity 300°C 3.9 4 3.7 4 W/m.K at 700°C 2.9 3 3 3 1000°C 2.6 2.7 2.6 2.8 Typical field upper transition upper and lower burning burning Zones of zone, tyre transition zones zone and as well as upper application section subject subject to severe upper and lower to high service transition transition Zones mechanical load conditions with zone, good subject to with redox alkali attack and coatability, severe service conditions and redox chrome ore conditions,low extreme alkali conditions, free Chrome Content attack, chrome chrome ore free ore free How to control kiln shell corrosion, SCL Beawar (Raj.) Page 45
  • 46. Above mentioned refractories are imported bricks. These bricks are basic bricks. Currently these bricks are using in Kiln-2(at outlet where temperature is very high) with addition of high Alumina bricks at kiln inlet where temperature is not so high. Where as in kiln-1 we are using only High Alumina bricks. Currently High Alumina Bricks (made in India) using in Kiln-1 and in inlet of kiln-2. Product Maximum C.C.S. Chemical analysis Refractoriness Material name Recommended (Kg/Cm2) Pyrometric Req. Temperature Cone.orton (°C) Al2O3 Fe2O3 AC 40S 1420 395 40.5 2.15 30 2.21 AC 60S 1480 545 59.2 2.5 35 2.42 AC 70S 1460 720 69.8 3 36 2.61 How to control kiln shell corrosion, SCL Beawar (Raj.) Page 46
  • 47. Full details of Refractory linings, Coating and SS Plate Used: For Kiln-1 Fig 19: Refractory lining of kiln-1 How to control kiln shell corrosion, SCL Beawar (Raj.) Page 47
  • 48. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 48
  • 49. For Kiln-2 Fig 20: Refractory lining of Kiln-2 How to control kiln shell corrosion, SCL Beawar (Raj.) Page 49
  • 50. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 50
  • 51. Corrosion of Kiln Shell Introduction With the ever increasing demand of cement due to the exponential growth of construction industry, Indian Cement Industry has been put to perform at its best than ever before. With the advances in understanding the cement chemistry and material behavior in side rotary kiln, lot many alternate raw materials and fuels have been either in use or being investigated for their suitability. While these alternate raw materials and fuels proved to be beneficial in terms of financial aspects associated with them, but the presence of deleterious volatile compounds posed equally serious threats to cause problems such as kiln shell corrosion, build ups and rings besides attacking the refractory lining and reducing their campaign lives. Amongst these, kiln shell corrosion is most serious problem as it acts silently and reduces the shell thickness to below critical structural and mechanical limits of stability of kiln shell. Corrosion can be defined as the destruction or deterioration of material due to the reaction with its environment. The grades of steel used for kiln shell range from general engineering steel to low alloy steels. The minimum shell plate thickness is around 20 mm for the shell section between two supports and those under the tyres (riding rings) may be between 60- 80 mm thick. A reduction of the shell thickness due to corrosion can be presumed to be critical when the thickness of the shell becomes 15 mm or so. To overcome the problem of kiln shell corrosion, the best way could be to prevent the volatiles to reach upto kiln shell. To prevent the passage of volatiles, refractories play a vital role and the selection of proper quality, adequate installation measures and highly oxidizing conditions in hottest zone of the kiln, stable kiln operation at high speed proved as the key to reduce / minimize the extent of kiln shell corrosion. Here I will try to bring out the role of the refractories and process conditions to reduce / minimize the kiln shell corrosion based upon the studies carried out in the past. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 51
  • 52. Corrosion of Cement Kiln Corrosion of cement kiln shell is influenced by a number of factors such as composition of the metallic shell and its environment, temperature of the shell, cleanliness or roughness of the shell surface, its contact with other materials and severe process conditions. Further, it is determined largely by the degree to which the scale formed under particular condition blocks further action between the shell and environment. Each steel or alloy behaves more or less individually and forms its own characteristic type of scale whose composition and imperviousness are specific to the given alloy, atmosphere, temperature and duration of exposure. Consequently, even a slight difference in composition of steel or atmosphere for instance, the presence of sulphur may have a substantial influence upon the type and progress of corrosion. It has been reported that carbon dioxide and sulphur dioxide are active scaling agents of iron and steel, carbon dioxide being less deterrent. The presence of sulphur dioxide increases rate of scaling and often results in deep intergranular penetration of the steel through the formation of a liquid iron oxide – iron sulphide eutectic. The deleterious effects of sulphur dioxide can be offset by providing excess oxygen. Alloying elements such as chromium, aluminium and silicon present in steel may greatly affect the rate of scaling. When present in significant concentration, they oxidize rapidly yielding a relatively impervious film which retards the rate of further attack on the underlying metal. On the other hand a high concentration of sulphur increases the rate of attack just as does the sulphur in the atmosphere. The influence of carbon is relatively small. The main reason of shell corrosion can be attributed to alternate oxidation at high temperature and acidic reaction at low temperatures when the kiln is stopped for repairs. The corrosion phenomenon takes place mainly due to presence of oxides, chlorides and sulphide at high temperature. Various types of corrosion that affect the kiln shell are: (i) Corrosion due to oxidation under high temperature (ii) Corrosion due to sulphide under high temperature (iii) Corrosion due to chloride under high temperature (iv) Corrosion due to hygroscopic material The rate of corrosion depends on the material, the surface condition, the corrosion medium, the time available and the temperature. The resistance to scaling of steels also diminishes in consequence of frequent temperature changes. Investigations have shown that in the majority of cases the corrosive attack is intensified by frequent changes in How to control kiln shell corrosion, SCL Beawar (Raj.) Page 52 temperature. In most cases, the damage due to high temperature corrosion manifests
  • 53. itself in general removal of material or in superficial cracking. Oxidation in the kiln atmosphere is possible due to the presence of O2, H2O/ H2, CO2/CO. The occurrence of high temperature corrosion under surface deposits is also an important factor. The damage due to corrosion is always intensified if the surface of the affected portion is covered with such deposits. During fairly long kiln shutdown for repairs, a rusting process is also presumed to be superimposed upon the high temperature corrosion (scaling) that has occurred during kiln operation. The deposit of salts containing potassium chloride in particular on the shell becomes active because being hygroscopic it absorbs atmospheric moisture. It is observed that the chloride can reach the kiln shell in the form of gases, the same is not the case for alkali oxides. Alkalis can only penetrate the lining as a part of liquid potassium and / or Sodium salt melts. If the corrosion products therefore contain substantial quantities of Potassium or Sodium, the form of corrosion is termed as Hot Corrosion indicating that liquid phase takes part in the corrosion reactions. Mechanism of kiln shell corrosion The reactions inside the kiln are different from reactions on the kiln shell surface since both the temperature and atmosphere are different. One of the most important reactions in the lining is the oxygen consumption where SO2 consumes oxygen and condenses as SO3: 2 SO2 (g) + O2 = 2 SO3 (↓) The SO3 formed condenses as calcium or magnesium salts. The result can be that an oxidizing environment inside the kiln turns into a reducing environment at the kiln shell. Oxidation In an oxidizing atmosphere, the iron from the steel shell will react with oxygen to form an oxide scale. Generally, this oxide scale is formed by more or less firm layers of different iron oxides, the compound with the highest oxygen content, Fe2O3, being found at the scale- brick interface, and the compound with the highest iron content, FeO at the metal-scale interface. At normal kiln operating temperatures, the outer layer becomes relatively firm. Sulphidization When no oxygen is present, SO2 takes over as the oxygen donor and a different reaction occurs. The reaction may be written as follows: 4 Fe + 2 SO2 (g) = Fe3O4 + FeS2 How to control kiln shell corrosion, SCL Beawar (Raj.) Page 53
  • 54. Accordingly, a sulphidization reaction can be identified by the occurrence of either pyrite (FeS2) or pyrrhotite (FeS) in the corrosion products. The oxidation by O2 and by SO2 alternates. As sulphide layers are more porous than oxide layers, the corrosion rate of the shell will increase. However, experience from different plants shows that, as long as chlorides are not present, the corrosion rate stays at an acceptably low level. Sulphidization is enhanced by the presence of chlorides, mainly because they affect the morphology of the corrosion scale, hindering the formation of a strong, protective oxide layer. The total reaction is a chain process taking place at different temperatures. A temperature gradient between the kiln atmosphere and the kiln shell is created by the porous deposit and the refractory lining. The first reaction of the chain process takes place in the kiln and can be described as high temperature hydrolysis of the thermally unstable alkali chlorides to form the more stable sulphates. This reaction step is followed by re- oxidation of hydrogen chloride gas (by oxygen or SO2) at lower temperatures to produce elemental chlorine, which attacks the kiln shell. The basic reactions (with potassium as alkali) are: 2 KCl (g) + H2O (g) + SO2 (g) + ½ O2 (g) = K2SO4 + 2 HCl (g) (T > 900oC) 2 HCl (g) + ½ O2 (g) = Cl2 (g) + H2O (T < 400oC) The formation of ‘free’ hydrochloric acid (HCl) gas in cement kilns is thus accompanied by formation of alkali sulphates. When this is the case, the formation is restricted to a quite narrow temperature between 1100 oC and 1300oC. The formation of hydrochloric acid is a consequence of the thermal instability of calcium sulphate and the thermal stability of potassium sulphate. The evaporation of alkali chlorides cannot begin until these temperatures are reached since gas and material move counter-current in the kiln. If most of the KCl in kiln feed can evaporate to KCl (g) at temperatures below 1000 to 1150 oC, the formation of HCl (g) will be quite limited because the tendency of KCl (g) to hydrolyse at such temperatures is low. The low temperature evaporation of chlorides explains why normal preheater kilns are less vulnerable to chloride-enhanced sulphidization. While, in most cases, chlorides in preheater kilns evaporate during or shortly after calcinations without substantial formation of hydrogen chloride gas, the case is different for kilns with tertiary air duct. Such kilns will show delayed alkali chloride evaporation and, consequently, evaporation will be followed by more extensive hydrolysis of the chlorides. Once Cl2 (g) is formed, it can reach the kiln shell through the refractory bricks or through the gaps/joints within and between rings and will react with either the oxide-sulphide How to control kiln shell corrosion, SCL Beawar (Raj.) Page 54
  • 55. layers or, most likely, directly with the kiln shell according to the following reactions resulting in the corrosion of the kiln shell: • reaction with the oxide-sulphide layers: FeS + Fe3O4 + 4 Cl2 = 4 FeCl2 + SO2 + O2 • reaction with the kiln shell: Cl2 + Fe = FeCl2 Role of Refractories in tackling shell corrosion The role of refractories in cement kiln is primarily to protect the steel shell from the direct attack of deleterious gases and clinker melt and to reduce the shell temperature so that steel of the shell does not loose its properties. The reduction in shell temperature also leads to energy conservation besides providing a workable condition near kiln shell. The entire CRK system including, preheater, precalciner, rotary kiln and cooler is lined with suitable size and quality of refractories to achieve the above mentioned advantages. Amongst all the sections as mentioned above, the service conditions inside the rotary kiln are most severe thereby requiring special attention for the shape, type and quality of refractories to be used and installation practices to be employed. Passage of Volatiles through Bricks The various studies carried out by NCB have established that volatile pass through the body of the refractory bricks and reach up to kiln shell. The samples of worn out refractory bricks as collected during visits were cut into three sections, top, middle and bottom and these were subjected to chemical and mineralogical investigations to find out the mineral phases present and formation of new phases. The results of chemical analysis and XRD investigations are given in Table 5 and Table 6 respectively for all the cases. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 55
  • 56. Brick Area Chemical Constituents % Al2O3 Fe2O3 SiO2 SO3 Na2O K2O Cl Fresh Brick 72.35 2.49 17.85 0.07 0.16 0.38 0.01 Worn out Brick-Top Layer Case I 60.57 3.56 15.08 0.76 0.17 6.75 1.00 Case II 69.40 2.33 16.39 0.96 0.29 3.35 0.41 Case III 62.10 4.00 19.81 0.37 0.44 5.20 0.35 Worn out Brick-Middle Layer Case I 65.86 3.47 14.21 0.24 0.19 6.86 1.30 Case II 70.70 3.37 14.31 0.52 0.55 2.21 0.46 Case III 63.20 3.77 19.48 0.21 0.22 3.70 0.24 Worn out Brick-Bottom Layer Case I 74.40 5.31 16.32 0.22 0.18 1.49 0.06 Case II 70.18 3.10 15.64 0.91 0.40 0.81 0.12 Case III 65.22 5.60 18.60 0.30 0.31 0.90 0.32 Table 5: Chemical Analysis of Different Layers of Worn Out Bricks The results of chemical analysis of the top layer indicate that bricks have undergone very severe chemical attack, which has resulted in decrease of Alumina content. The concentration of SO3, Na2O and K2O was in the range of 0.37 – 0.96, 0.17-0.44 and 3.35-6.75 percent respectively. The concentration of chloride was in the range of 0.35-1.00 percent. The results of chemical analysis of middle layer indicate that the volatiles have traveled through the bricks and have reached upto the middle layer of the bricks. Concentration of SO3, Na2O and K2O are in the range of 0.21-0.52, 0.19-0.55 and 2.21-6.86 percent respectively. The concentration of chloride was in the range of 0.24-1.30 percent. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 56
  • 57. S. Plants Mineral compounds present in No Top layer Middle layer Bottom layer 1 Unused bricks Al2O3, Al6 Si2O13 Al2O3, Al6 Si2O13 Al2O3, Al6 Si2O13 2 Case I Al2O3, KAlSi3O8, Al6 Si2O13, Al2O3, KAlSi3O8, Al2O3, Al6 Si2O13, ,KCl KAlSiO4, Ca2.Al2SiO7 , KCl Al6 Si2O13, KAlSiO4, Ca2.Al2SiO7 ,KCl 3 Case II Al2O3, KAlSi3O8, Al2 SiO5, Al2O3, KAlSi3O8, Al2O3, KAlSi3O8, Al2 SiO5, Na2SO4, Ca2.Al2SiO7 KCl Al2 SiO5, Na2SO4, Ca2.Al2SiO7 Ca3.Al6O12.CaSO4, KCl NaCl 4 Case III Al2O3, Al6Si2O13, Al2O3, Al2O3, Al6Si2O13, Al2O3 KAlSi2O6,Al2 SiO5, Al6Si2O13, KCl, SiO2, SiO2 Fe2O3, KCl, Ca.Al2Si2O8 , KCl Al2O3SiO2 NaCl KAl.Si2O6 , , K2SO4, NaCl Table 6: XRD Investigations of different layers of Refractory Bricks The results of chemical analysis of bottom layer of refractory bricks indicate that the concentration and reactivity of these volatiles is so high that these are able to travel upto the bottom of the bricks thereby reaching upto the kiln shell. The concentration of SO3, Na2O and K2O are in the range of 0.22-0.91, 0.18-0.40 and 0.81-1.49 percent respectively. The concentration of chloride was in the range of 0.06-0.32 percent. The comparison with the top and middle layer indicates that the concentration of volatiles has decreased. The results of XRD investigations of corresponding samples also indicate the interaction of bricks with deleterious volatile oxides leading to formation of feldsphatic compounds like Al6Si2O13, KAl.Si2O6 , Ca.Al2Si2O8, besides formation of most detrimental oxide i.e. KCl (sylvite). Formation of these compounds in the brick matrix led to volume expansion and How to control kiln shell corrosion, SCL Beawar (Raj.) Page 57 breaking of ceramic bonds ultimately leading to breaking or loosening of bricks.
  • 58. The above investigations of worn out refractory bricks have provided very useful information for the possible causes of kiln shell corrosion. The chemical analysis of top layers of bricks indicate that these bricks have undergone severe interaction with calcined kiln feed or clinker and have absorbed volatiles through the open pores in the bricks. The bricks have three types of pores namely: • Through pores • Closed pores within brick body • One side open and one side closed pores Out of all the three types of pores, one side open and one side closed pores basically create problem for brick failure and does not directly affect the phenomena of kiln shell corrosion. The closed pores are the ones which neither damage brick structure by way of interaction nor help in kiln shell corrosion. The third types of pores are very dangerous from the point of view of damaging the brick structure as well as causing kiln shell corrosion. Because these thorough pores provide passage to volatiles to reach up to kiln shell. The best approach would be to check the permeability of these bricks before dispatch from the site of manufacturers. The permeability of bricks gives a very clear picture of through pores present in the bricks. Passage of Volatiles through joints/gaps The volatile rich kiln gases and clinker liquid are very prone to reach upto to kiln shell through the gaps between joints within individual rings or ring to ring joints as well. These gaps may occur due to various reasons such as: • Individual brick behavior at working temperature • Inadequate installation due to poor workmanship • Actual service conditions prevailing inside the kiln In order to confirm the possibility of volatiles traveling through the gaps within and between rings and reaching at the interface of bricks and kiln shell, another exercise was carried out. While collecting brick samples during kiln stoppage, some of the rings have shown shifting and gaps created between rings. Samples of worn out bricks were carefully taken out ensuring the coatings on the side surfaces of bricks to remain intact. These coatings were scrapped carefully and analyzed for their chemical constituents. The results of analysis indicate that two types of reactions are taking place. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 58