Wood and pulp

Wood and pulp
1 | Wood and Pulp | Sappi Fine Paper Europe

Contents

1. Wood types 3
1.1. Wood structure 6
1.2. Wood growth 7
1.3. Cellulose formation 8
1.4. Chemical composition 9

2. Pulp production 10
2.1. Woodyard 11
2.2. Debarking 12
2.3. Chipping 13
2.4. Defibring 14
2.4.1. Mechanical vs. chemical pulp 15
2.4.2. Mechanical pulp 16
2.4.3. Chemical pulp 19
2.5. Measurement data 24
2.6. Bleaching 25
2.6.1. Bleaching methods 27

3. Overview pulp mill Sappi Gratkorn 28


1. Wood types
Typed

Wood
types

Deciduous Coniferous
(Hardwood) (Softwood)

Beech Spruce

Birch Pine

Poplar Fir


1. Wood types
• Structure

Hardwood Softwood
Short fibres Long fibres


1. Wood types
Characteristics for paper mixture (pulp)

Deciduous woord Nconiferous woodz
Short fibre Long fibre
Firmness Medium High
Flexibility Medium High
Bulk/Volume High Medium
Opacity High Medium
Suited for High g/m² Low g/m²
Fibre length 1 mm 2-3 mm
Fibre width 15-25 µm 35-45 µm

Basics: The magic is in the mixture


1.1. Wood structure
• Bark:
– Outer layer consisting of dead cork cells
– Protection against damage, loss of water, weather, bacteria
– Bark cells do not grow, they burst
• Cork cambium:
– Builds cork cells and thus, the bark
– Closes tears in the bark coat
• Phloem (Inner bark)
– Storage and transport of substances for the process of assimilation
Assimilation = Conversion of substance and energy
• Cambium:
– The living, growing part of the tree
• Sapwood:
– Transport of water and nutrients, strengthening and assimilation substance storage
– One new ring is formed every year (in seasonally affected phases)
– Storage of lignin (gum)
• Heartwood:
– Supporting column, which usually starts to grow at a tree age of 20-40 years
• Pith:
– Dead fibre tissue, the core of the tree trunk


1.2. Wood growth

Cambium splits
Spring off thin-walled
cells: spring
Broad growth
rings
wood

Cambium splits
Summer off thick-walled
cells: summer
Narrow growth
rings
wood

Winter No growth
No ring
formation

• Growth periods depending on climate and location


1.3. Cellulose formation

– Grape sugar (glucose) is converted to starch by shedding water
– As it multiplies, cellulose is formed


1.4. Chemical composition

• Cellulose:
– Primary cell wall substance
– Long, fibrous polysaccharide chain of glucose molecules
– Hydrogen bonding builds a crystalline structure

• Hemicellulose:
– Sealants and flexibilisators
– Short polysaccharide chain consisting of various sugars

• Lignin:
– Wood glue
– Filling substance in the cellulose frame
– Wood formation initiator
– Absorbs pressure, protects agains attacks


2. Pulp production
Process


2.1. Wood yard
Step 1


2.2. Debarking
Step 2: Debarking drum


2.3. Chipping
Step 3


2.4. Defibring
Step 4


2.4.1. Mechanical vs. chemical pulp

Mechanical pulp Chemical pulp

Cost Low High

Yield High: approx. 90% Low: approx. 50%

Chemicals Few Many (re-usable >99%)

Energy Intensive Energy generation

Brightness Low High
Yellowing High Low
Opacity High Low
Stiffness Low High


2.4.2. Mechanical pulp
Wood containing

• Stone groundwood (SGW)
– Developed1843 by F. G. Keller
– No pre-processing
– Processing under atmospheric pressure and temperature
– Yield: 90-95%  All wood components are fully maintained

• Brown groundwood
– Vaporization before grinding
– Fibres have brownish colour
– Yield: <90%  But better stiffness

• Pressurized groundwood (PGW)
– Processing under pressure (2 to 4.5 bar) and high temperature (110-135°C)

• Chemical groundwood (CGW)
– Chemical pre-processing
– Lignin is partially removed  Low yield


(Chemi) Thermo Mechanical Pulp

• TMP
– Wood chips are vaporized
– Chips are ground down to single fibres in refiners, under vapour pressure

• CTMP
– Wood chips are impregnated (mixed with a chemical substance)
– Cooked
– Ground
– Bleached


(Chemi) Thermo Mechanical Pulp

T: light cooking of
C: chemical woodchips
impregnation
of woodchips

P: pulp
ready for
bleaching
M: mechanical refining


2.4.3. Chemical pulp

• Purpose:
– Stripping and dissolving lignin
 Lignin is what holds the individual fibres together
– Maintaining the cellulose fibre structure

• In practice:
– Only residual lignin remains
– Hemicelullose and cellulose are also partially removed
– Yield is approx. 50% (mechanical pulp approx. 90%)

• Main types:
– Sulphate pulp
– Sulphite pulp



Sulphate pulp Sulphite pulp
(kraft pulp)

Cooking time 1-2 hours 3-8 hours

Temperature 170-180°C 130-140°C

pH-value 13+ (alkaline) 1-2 (acidic)

Yield 45-50% 50-55%

Stiffness High Low
Fibres assailed from outside Fibres assailed from within

Raw materials Flexible Limited
Cost High Low
Odour Heavy Neutral


Sulphate pulp

• Developed 1879
– 80 – 85 % of all pulp production worldwide

• Basic materials:
– Roundwood and wood chips

• Alkaline cooking process: pH value 11

• Special features:
– Suitable for all wood types, including annuals
– Obligatory recycling of used chemicals
– Energy generation from burnt waste


Sulphite pulp

• Developed 1876 - 1878
– 15 – 20 % of all pulp production worldwide

• Basic materials:
– Roundwood and wood chips

• Acidic cooking process: pH value 4
– Cooking time 3-8 hours in sulphurous acids (magnesium bisulphite)
– Lignin is washed out in acidic environment
– Defibring under pressure at 130-145°C
– Requires high quality wood
– Yield approx. 55%

• Special features:
– Not suitable for resinous wood types (pine)
– Recent regulations make recycling of used chemicals obligatory

Sulphite pulp

Advantages Disadvantages
Cost-effective use of small units from Limited raw material base
 Sulphite: 100,000 tons per year
 Sulphate: 500,000 tons per year

Good yield Bark sensitive

Good bleaching characteristics Low stiffness

High basic whiteness High burden on waste water
 Sulphite: greyish white / Sulphate: brown


2.5. Measurement data

Pulp hardness Kappa Residual lignin
Very soft 0.5 – 1 1.5%
Soft 11 – 15 1.5 – 2.5%
Normal 23 – 31 3.0 – 4.0%
Hard 46 – 54 6.0 – 7.0%
Very hard 54 – 69 7.0 – 9.0%


2.6. Bleaching

• Modification of coloured substances in the pulp
– Coloured substances lose the ability to fully absorb light
– Residual lignin and other undesired substances are removed

Cooking O₂ Bleaching


2.6. Bleaching

• Intermediate washing phases during chemical treatment

First washing phase Last washing phase


2.6.1. Bleaching methods

• Pulp can be bleached with:
– Elemental chlorine and chlorine dioxide
 In the past  Highly negative effects on the environment
 Modern ECF/TCF pulp  Elemental Chlorine Free/Totally Chlorine Free

– Ozone (O3) and oxygen (O₂) in various forms
 Resulting paper is virtually odour free
 Sappi was the first paper producer to use the „Sapoxal oxygen bleach process“

– Hydrogen peroxide (H₂O₂)
 Components: caustic soda, water glass, sulphuric acid
 Oxidative process: metal + oxygen is converted to metal oxide

– Natriumdithionit (NaS₂O4)
 Reductive process: metal oxide is dissolved into metal + oxygen


3. Overview pulp factory Sappi Gratkorn


Thank you
for your attention

Jörg Abelmann


Wood and pulp

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Wood and pulp