Stock preparation

Stock preparation

1 | Stoffaufbereitung | Sappi Fine Paper Europe

Contents
1. Stock preparation – Process description 3
1.1. Purpose 4
1.2. Process 5

2. Pulper 6

3. Refining 7
3.1. Hollander 8
3.2. Refiner 9
3.3. Purpose 12
3.4. Refining types 13
3.5. Measuring units 15

4. Components 16
4.1. Cellulose 17
4.2. Fillers 18
4.3. Starch sizing 19
4.4. Whiteness and brightness 21
4.5. Retention agents 24


1. Stock preparation – Process description

In stock preparation, all components used for the production of paper
are brought together and processed according to a specific recipe.

Ingredients Processing
Water Pulper
Cellulose Deflaker
Pulp Refiner
Recycled paper Drums
Fillers Conduits
Additives Sorting and cleaning


1.1. Purpose
Rules

Making sure that stock is ready at the right moment, in the required
quantity and in the appropriate mixture

Key rules
– Paper has to be suited for the intended purpose
 Every paper type requires a specific mix of ingredients
 The same is true for different grammages

– Mixture proportion in stock preparation
 5% solids – 95% water

– Mixture proportion in headbox / paper machine
 1% solids – 99% water


1.2. Process in time and space

• Liquid cellulose is stored in stacked containers
• Solid cellulose, delivered in bales, is dissolved in pulpers
• Dissolved cellulose is fed into a deflaker
– Flakes = small fibre lumps

• Conduits transport the stock during stock preparation
• Refiners are used to grind the cellulose
• Drums are used for mixing = pulp …
… and additives

Pulp
+ Additives
=
Paper stock

• When ready, the stock is transferred to the konstanten Teil of the paper machine


2. Pulper
1. Drum is filled with water
2. Bales of solid cellulose are added
3. Propellers turn the watery mass in
which the bales now dissolve
4. The result is a watery suspension

• Discontinuous pulper
 One mixing process at a time -
when ready, the suspension is
pumped off

• Continuous pulper
 Bales of cellulose are added
continuously and suspension
is pumped off continuously


3. Refining
Stock preparation installations

Past: Hollander Present: Refiner


3.1. Hollander Grindstones
• Historic refining device
– Origin: Holland, 17th century
– Formation of paper characteristics

“The Hollander is the actual paper
making machine.”

• Suspension is ground in a
water filled drum

Drum


3.2. Refiner

Stock output
Stock input

Engine
Grinder

Rotor

Stator


3.2. Refiner – the refining line

Conduits

Refiner Refiner


3.2. Refiner

Unrefined Refined


3.3 Purpose
Formation of paper characteristics

• Single fibres are treated in such a way that flexibility increases and
specific surface expands
– Helps binding of individual fibres
– Adds consistency
– Whiteness and opacity, however, decrease

• Controlled characteristics:
– Consistency
– Volume
– Transparancy (vs. opacity)
– Absorptivity
– Air permeability


3.4. Refining types – Free beating

• In free beating, the knives are at a steep angle to one another

• The fibres are cut according to refining method
– Hardly any fibrillation (length cuts), only shortening
– Resulting paper has good absorptive qualities

• Effects
– Fast de-watering on the paper machine
– High volume, e.g. concept, filter- and blotting paper


3.4. Refining – Wet beating

• In wet beating, the knives are relatively far apart

• Fibres are crushed, not cut

• Result is a bloated, slippery, slimy fibre suspension

• Slow de-watering on the paper machine
– Resulting paper is very dense, but has low opacity
– Glassy, transparent paper


3.5. Measuring units

Refining degree is measured in Schopper-Riegler (SR)

• Unrefined 13 - 17 SR
• Low refined 20 - 25 SR
• Medium refined 30 - 40 SR
• High refined 50 - 60 SR
• Very high refined 80 - 90 SR


4. Components
Rules

• All components must be adapted to the intended use of the paper
– „Retrofitting “
• The same components are also used in coating
• Components are exchangeable
• Production waste is re-usable by way of components,
but subject to Rule 1


4.1. Cellulose

• Softwood (short fibres)  strong and flexible
– NBSK (Northern Bleached Softwood Kraft)
 Spruce and beech sulphite cellulose
 Pine
 Birch, cedar, larch
– SBSK (Southern Bleached Softwood Kraft)
 Pine

• Hardwood (long fibres)  bulk and opacity
– BHKP (Bleached Hardwood Kraft Pulp)
 Primarily eucalypt (gum tree)
– Mixed hardwood pulp
 Mixed hardwood


4.2. Fillers

• Key determining factor is the intended use of the paper
– Fine paper can contain up to 25 % fillers

• Less expensive than cellulose
– Opacity, whiteness and flexibility

• Fillers acting as fibre binders
– Calcium carbonate (CaCO3 - clay)
 Main ingredient – natural substance and chemical compound
 Also suited as coating pigment
– Kaolin (China clay)
 China Clay – Kaolin from Cornwall, UK
– Talc
 Prevents porosity, stimulates fibre closure
– Titanium dioxide
 For enhanced opacity and brightness


4.3. Starch sizing
In stock preparation: bulk sizing

• Ink should settle on the paper, not be absorbed by it
– In contrast to, for instance, tissue paper where absorption is the primary function

• Key determining factor is the intended use of the paper
– Bulk sizing = in stock preparation
– Surface sizing = on the paper machine
– Bulk sizing + surface sizing = combined sizing

• Vegetable products
– Potato starch
– Wheat starch
– Corn starch

• Synthetic starch and latex (polymere compounds)

• In the past, alum and aluminium sulphates were also used – „resin sizing“
– Book damage: in moist conditions, hydrolysis turns alum into sulphuric acid


4.3. Measuring sizing degree

• The Cobb value is an expression of the absorptive power of paper
and solid or corrugated board

• Board and folding boxes:
 Key indicator of potential stability

• Paper:
 Key indicator of writability and printability
 Only papers with the appropriate absorption characteristics are suited for
writing and printing

• Low Cobb values indicate low absorptive power


4.4. Whiteness and brightness

• Premise:
 Neutral white = „dirty“ white for the human eye

• Purpose:
 „Whitest “ visual white

• Conclusion:
 The eye must be cheated by an optical illusion

• Execution:
 White with a bluish tone
 Brightness is enhanced through conversion of UV light into white light
 White pigments (Titanium dioxide)
 Optical brighteners (fluorescent substance)



• Measuring whiteness / brightness
– ISO Brightness: restricted to the blue range of the visible spectrum
 The majority of white papers have a total reflection level of 80 - 100 %

– CIE Whiteness: reflection in the full light range
 L* value represents brightness  black = 0 and white = 100
 a* value represents green/red proportion  green = -150 and red =100
 b* value represents blue/yellow proportion  blue = -100 and yellow = 150


Optical brighteners

• Invisible light is converted into blue visible light
– Brightness L* is enhanced
– Colour shift to blue, i.e. b* value becomes more negative

• Blankophor optical brighteners – barium sulphate – white pigment
 UV light in „disco“


4.5. Retention agents
Retention = lat.: retinere = „retain, hold “

• Premise:
– 1% solids and 99% water
– Pigments and fillers have low affinity to fibres

• Purpose:
– Solid components are left on the screen, water passes through

• Execution:
+ Pole (anode)
 Retention agents transmit positive charge to fillers (= cationic polymeres)
- Pole (cathode)
 Pulp has a negative charge (= anionic)

• Basic rule:
– Think of a magnet with positive and negative charge


Thank you
for your attention

Jörg Abelmann


Stock preparation

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Stock preparation