Structure and properties of metallurgical slag ss

D E P A R T M E NT O F
Metallurgical and Materials
Engineering
National Institute of Technology, Rourkela

Structure and Properties
of Metallurgical Slag

By

Dr. S.Sarkar
Associate Professor
Dept. of Metallurgical and Materials Engg.
National institute of Technology, Rourkela

A short term program on process metallurgy of iron and steel making 1


Plan of Presentation Metallurgical and Materials
Engineering

• Introduction to metallurgical slag
• Structure of pure oxide
• Role of ionic radii
• Metal – oxygen bond
• Structure of slag
• Properties of slag
• Basicity
• Oxidising power
• Sulphide capacity
• Electrical and thermal conductivity
• Viscosity
• Surface tension
• Constitution of slag


Introduction – D E P A R T M E NT O F

Metallurgical Slag
Engineering

• The slag comprising of simple and/or complex compounds
consists of solutions of oxides from gangue minerals,
sulphides from the charge or fuel and in some cases halides
added as flux.

• Slag cover protects the metal and from oxidation and
prevents heat losses due to its poor thermal conductivity.

• It protects the melt from contamination from the furnace
atmosphere and from the combustion products of the fuel

• In primary extraction, slags accept gangue and unreduced
oxides, whereas in refining they act as reservoir of chemical
reactant(s) and absorber of extracted impurities.

Introduction – D E P A R T M E NT O F

Metallurgical Slag (cont.)
Engineering

• In order to achieve these objectives, slag must possess
certain optimum level of physical properties:
• Low melting point,
• Low viscosity,
• Low surface tension,
• High diffusivity
and chemical Properties:
• Basicity,
• Oxidation potential and
• Thermodynamic properties
• The required properties of slags are controlled by the
composition and structure.

Structure of Pure Oxides – D E P A R T M E NT O F

Role of ionic radii
Engineering

• Relative dimensions of cations and anions and type of bonds
between them are important factors in controlling the
structure of pure oxides.

• Table I: Radii of common cations, Rc and anions, Ra
Cations K+ Ca2+ Mn2+ Fe2+ Fe3+ Mg2+ Cr3+ Al3+ Si4+ P5+

Rc (nm) 0.133 0.099 0.08 0.074 0.061 0.066 0.063 0.051 0.042 0.035

Anions I- S2- Cl- O2- F-

Ra (nm) 0.220 0.184 0.181 0.140 0.133



Role of ionic radii (cont.) Metallurgical and Materials
Engineering

Coordination number, Rc/Ra ratio and structure of solid oxides
Structure Coordination number Ra/Rc Examples
Cubic 8 1 – 0.732 --

Octohedral 6 0.732 – 0.414 CaO, MgO, MnO, FeO

Tetrahedral 4 0.414 – 0.225 SiO2, P2 O5

Triangular 3 0.225 –0.155 --



Ionic radii (cont.) Metallurgical and Materials
Engineering

• In case of SiO2 four O2- ions
provide the frame of the
tetrahedron and the smaller Si4+
ion is situated within the frame
as shown in Fig.

• Since the neighbouring cations Structure of SiO2
(Si4+) are mutually repellent,
according to the Pauling’s second
law the interval between two Si4+
ions should be maximum.



Ionic radii (cont.) Metallurgical and Materials
Engineering

Structure of silica (a) solid (b) liquid



Metal –Oxygen bonds
Engineering

• There are two principal types of bonds found in
crystals: electrovalent and covalent.
• Electrovalent bond strength is lower than the
covalent bond. High temperature is required to
destroy the covalent bond.
• However, oxides exhibit varying proportion of both
ionic and covalent bonding in slag.
• Ionic bond fraction indicates the tendency to
dissociate in liquid state.



Engineering

• TiO2, SiO2 and P2O5, bonding is mainly covalent and
the electrovalent proportion is strong due to small
cations carrying higher charge with a coordination
number of 4.
• These simple ions combine to form complex anions
such as SiO4-4 and PO3-4 leading to the formation of
stable hexagonal network in slag systems.
• Hence they are classified as ‘network formers’ or
“acidic oxides”. For example
• SiO2 + 2O2- = SiO4-4
• P2O5 + 3O2- = 2(PO3-4)


Engineering

• The oxides with high ionic fraction form simple ions
on heating beyond the melting point or when
incorporated into a liquid silicate slag. For example :
CaO→Ca2+ + O2-
Na2O → 2Na+ + O2-

• As they destroy the hexagonal network of silica by
breaking the bond they are called ‘network
breakers’or‘basic oxides.



Engineering


Oxide z/(Rc+Ra) Ionic fraction Coordination Nature of the Oxide D E P A R T M E NT O F
of bond number Metallurgical and Materials
Engineering
Solid- -Liquid National Institute of Technology, Rourkela

Na2O 0.18 0.65 6 6 to 8

BaO 0.27 0.65 8 8 to 12
SrO 0.32 0.61 8 Network breakers
CaO 0.35 0.61 6 or
MnO 0.42 0.47 6 6 to 8 Basic oxides
FeO 0.44 0.38 6 6
ZnO 0.44 0.44 6
Mgo 0.48 0.54 6 Oxides like Fe2O3, Cr2O3 and
BeO 0.69 0.44 4 Al2O3 are known to be
…………. ……………... ……………... …… ……... …………………...
amphoteric due to their dual
Cr2O3 0.72 0.41 4
characteristics because they
Fe2O3 0.75 0.36 4 Amphoteric oxides behave like acids in basic slag
Al2O3 0.83 0.44 6 4 to 6 and as bases in acidic slag.
…………. ……………... …………….. …….. ………. …………………...
TiO2 0.93 0.41 4 Network formers

SiO2 1.22 0.36 4 4 or

P2O5 1.66 0.28 4 4 Acidic oxides



Structure of Slag Metallurgical and Materials
Engineering

• It is well known that most of the slags are silicates. When a
basic oxide is incorporated in to the hexagonal network of
silica it forms two simple ions.
• The fraction of basic oxide, expressed as O/Si ratio plays an
important role in destroying the number of Si-O joints.
O/Si Formula Structure
2/1 Si O2 Silica tetrahedra form a perfect three
dimensional hexagonal network
5/2 MO.2 SiO2 One vertex joint in each tetrahedron breaks to
produce two-dimensional lamellar structure.
3/1 MO. Si O2 Two vertex joints in each tetrahedron break to
produce a fibrous structure

7/2 3MO. 2SiO2 Three vertex joints in each tetrahedron break
4/1 2MO.SiO2 All the four joints break



Structure of Slag (cont.) Metallurgical and Materials
Engineering

   
O O O O
   
− 2+ −
O  Si  O  Si  O  + (CaO ) ⇔  O  Si  O + Ca + O  Si  O 
   
O O O O
   

   
O O O O
   
− + + −
O  Si  O  Si  O  + ( Na 2 O) ⇔  O  Si  O + Na + Na + O  Si  O 
   
O O O O
   

Fibrous structure of a pyroxene


Structure of fayalite2 (a) solid (b) liquid D E P A R T M E NT O F

Structure of Slag (cont.) Metallurgical and Materials
Engineering

Structure of fayalite (a) solid (b) liquid



Properties of Slag Metallurgical and Materials
Engineering

A knowledge of various chemical and physical
properties of slag is essential in order to adjust them
according to the need of extraction and refining
processes.
1. Basicity of Slags
• In slag systems, a basic oxide generates O2- anion
while an acidic oxide forms a complex by accepting
one or more O2 anions:
Base ↔ acid + O2-


Properties of Slag – D E P A R T M E NT O F

Basicity
Engineering

• For example, SiO2, P2O5, CO2, SO3 etc are acidic oxides
because they accept O2- anions as per the reaction:
(SiO2) + 2 (O2-) = SiO44-
• On the other hand basic oxides like CaO, Na2O, MnO
etc. generate O2- anions:
(CaO) ↔ Ca2+ +O2-
• The amphoteric oxides like Al2O3, Cr2O3 Fe2O3 behave as
bases in the presence of acid (s) or as acids in
presence of base (s):
(Al2O3) + (O2-) = 2 (Al O2-) or (Al2 O4 2- )
• (Al2O3) = 2(Al3+) + 3(O2-)


Basicity
Engineering

• In a binary slag viz. CaO-SiO2 the basicity index (I) is
given as:
I = wt % CaO / wt % SiO2
• For example a complex slag consisting of CaO, MgO,
SiO2 and P2O5 employed in dephosphorisation of
steel, basicity index2 is estimated as follows:
wt%CaO + 2 3 wt%MgO
I=
wt%SiO 2 + wt%P2 O 5



oxidising power
Engineering

• Oxidizing power means the ability of the slag to
take part in smooth transfer of oxygen from and to
the metallic bath.
• The oxidizing power of the slag depends on the
activity of the iron oxide present in the slag.
• The equilibrium between iron oxide in slag and
oxygen dissolved in metal is represented as:
• (FeO) = [ Fe ] + [ O ]
[ a ][ a ]
Thus [ a O ] ∝ ( a FeO )
Fe O
K=
(a ) FeO



Sulphide Capacity of Slag
Engineering

• Since slags are employed to remove sulphur from
metal, chemistry of sulphur in silicate slags
becomes interesting.
• Sulphide is soluble in silicate melts but elemental
sulphur does not dissolve to any appreciable extent.

1 1
S 2 ( g ) + (O 2 − ) = O2 ( g ) + ( S 2 − ) (18)
2 2

(a )  p
S 2−


1
2 x
S 2−
.γ
S 2−
 p O2



1
2

(a )  p
O2
K= = (19)
 x  pS 
O 2− H2  O 2−  2 


Properties of slag – D E P A R T M E NT O F

Sulphide capacity of slag
Engineering

• The sulphur affinity of a slag, presented as molar sulphide
capacity is defined by the equation:
1
 pO  2  x 2− 
′ = x 2−  2
CS  = K O  (20)
S  pS   γ 2− 
 2   S 
• or a more useful term wt % sulphide capacity5 for
technologist is defined as
1
 p O2  2

C S = (wt% S)   (21)
 pS 
 2 
• Thus under similar conditions a slag with a high Cs will
definitely hold sulphur more strongly than the other with a
low Cs and hence will prove to be a better desulphuriser in a
metallurgical process.


Electrical and thermal D E P A R T M E NT O F

conductivity
Engineering

• Molten silica is a poor electrical conductor3. However its
conductivity increases to a great extent by addition of basic
oxides e.g. CaO, FeO or MnO as flux.
• This increase is due to the formation of ions.
• The conductivity values serve as a measure of degree of
ionization of the slag. The electrical conductivity of slags
depends on the number of ions present and the viscosity of
liquid slag in which they are present.
• Thus conductivity will be greater in liquid state and further
increases with the temperature.
• In general thermal conductivity of slag is very low but heat
losses are much higher due to convection.



Viscosity Metallurgical and Materials
Engineering

• Viscosity of slags are controlled by composition and
temperature. The viscosity , of a slag of a given
composition decreases exponentially with increase
of temperature according to the Arrhenius
equation:

η = A exp (E η/ RT)

• Basic oxides or halides with large ionic bond
fraction are more effective in reducing viscosity
than those with smaller bond fraction by breaking
bonds between the silica tetrahedra.


Engineering

Effect of addition of flux on activation energy



Engineering

• Viscosity decreases rapidly with temperature for both basic as well as
acid slags.
• But basic slags with higher melting points are more sensitive to
temperature.
• This indicates that activation energy for viscous flow of basic slags is
much lower than for acid slags.



Engineering

• Use of CaF2 as flux is more effective in reducing viscosity of
basic slags than that of acidic slags.
• This may be due to ability of F- ions to break the hexagonal
network of silica and the low melting point of undissociated
CaF2.
27
A short term program on process metallurgy of iron and steel making


Engineering

• Figure shows that addition
of Al2O3 to a basic slag
increases viscosity by
acting as network former.
• Addition of Al2O3 to an
acidic slag reduces
viscosity because it now
acts as network breaker.



Surface tension Metallurgical and Materials
Engineering

• The high rates of reaction in basic oxygen converters is due
to the physical conditions of the metal, slag and gaseous
phases in the converter.

• The theories regarding rapid reaction rates rely heavily on
the formation of slag – metal emulsion and slag foams
leading to creation of the large required reaction surface.

• The most important feature of emulsion and foam is the
considerable increase of the interfacial area between the
two phases leading to the high rate of reaction.



Engineering

• As surface tension is the work required to create unit area of
the new surface, the necessary energy for emulsifying a
liquid or a gas in another liquid increases with increasing
surface tension value.

• In a similar manner energy is liberated when interfacial area
decreases.

• Hence a low interfacial tension favors both formation and
retention of emulsion.



Engineering

• On this basis slag / metal and slag /gas systems are not
suitable for emulsification because of the high equilibrium
slag/metal interfacial tension.

• However the slag/metal interfacial tension is considerably
lowered to 1/100 of the equilibrium value due to mass
transfer.

• Addition of SiO2 or P2O5 to a basic oxide lowers3 the surface
tension due to the absorption of a thin layer of anions, viz.
SiO44- , PO43- on the surface.

• It has been reported that lowering of surface tension of FeO
by excess oxygen.

Constitution of MetallurgicalMetallurgical and Materials

Slag
Engineering

• The major constituents of
iron blast furnace slags can
be represented by a ternary
system: SiO2 – CaO – Al2O3.

• On the other hand all the
steelmaking and many
nonferrous slags are
represented by the ternary
system: SiO2- CaO – FeO.



Slag
Engineering

1.Basic open hearth steel furnace
2.Acid open hearth steel furnace
3.Basic oxygen converter
4.Copper reverberatory
5.Copper oxide blast furnace
6.Lead blast furnace
7.Tin smelting



Slag
Engineering


Structure and properties of metallurgical slag ss

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