Papermaking
SCA Publication Papers Technical Support
Raw material
Introduction
The purpose of this guide is to provide the reader with an understanding
of the materials used to manufacture paper, and the processes employed
in a modern paper mill, to produce high quality products for both offset
and rotogravure printing.
Raw material
There are many species of trees which can be used in the manufacture of
paper products. However, in this guide, it is the use of mainly coniferous trees and recovered fibre from post consumer waste which will be
described in the manufacturing processes.
Fresh forest fibre as a source material
Wood from coniferous trees has the benefit of long, strong fibres, which
help to form a complex interwoven mesh on the paper machine to produce a good formation in the paper.
In Europe and the Nordic countries, it is spruce and pine trees which are
principally used, as these give the most advantageous properties from
their fibres in the papermaking processes.
Recovered fibre as a source material
In areas of high population, such as the UK and Central Europe, it
makes both economic sense and environmental good practice, to utilise
fibre from post consumer waste. The material collected, is a good source
of fibre and some useful minerals. These materials can be used exclusively to produce paper again, or to supplement fresh forest fibre in the
production of higher quality papers.
The SCA Group is well balanced in its
use of recovered and fresh fibre. Each
year we utilise 4.4 million tonnes of
recycled fibre and 4.2 million tonnes
fresh fibre. We are the largest converter
of recovered fibre in Europe, with 1.6
million tonnes of waste paper collected
annually by our own organisation.
Pulp production
Pulp Production from Forest Fibre
The big logs in the forest are used for sawn timber. The pulp and
paper industry is using left overs from sawmills, branches and tops
and the materials that remains from thinnings. The wood harvested
from the forest has two distinct parts. The inner, consisting of the
useful cellulose fibre, and the outer layer of bark. The bark would
detract from the paper quality if left in the pulping process and is
therefore removed before pulping takes place. Bark is an important
bio-fuel.
The debarked wood can then take a variety of routes to produce
pulp for papermaking. Mechanical pulp can be produced by different methods. However, the methods utilise approx. 98% of the
wood volume to produce a pulp with high opacity. Chemical pulp is
produced by removing Lignin from the wood. Lignin is the material
which acts as the binding agent for the fibres in the growing tree.
By removing the Lignin, the yield from the wood volume drops to
50%, producing fibre with good strength, but with less opacity.
The pulp
processes
Mechanical pulp
Yield of
90-98%
Thermo
Mechanical pulp
(TMP)
Yield of
90-98%
Chemi-Thermo
Mechanical pulp
(CTMP)
Yield of
85-90%
Chemical pulp
Sulphate
process
Yield of
43-52%
A TMP-refiner produces about 300 tonnes
pulp per day.
Mechanical Pulp – Ground Wood
This is the oldest pulping method, invented around
1840. The debarked logs are pressed against a rotating grindstone, which separates the individual fibres
through a shearing action. Water is added to aid the
process and also keep the grindstone cool under the
intense friction of the logs pressed against the stone.
The pulp passes through fine screens, so only individual
fibres can progress. The pulp is also cleaned of any
foreign materials such as sand and grit.
Pulp
The pulp process
Mechanical Pulp: Thermo – Mechanical Pulp
(TMP)
In this method of production, the logs are chopped into chips. The
chips are washed to remove any sand and grit which may cause
wear and tear on the processing equipment. The chips are heated
with steam to soften them and they are fed with pressurised water
into the refiner. The refiner consists of two counter-rotating discs,
with each disc having channels radiating from a central point to the
outer edge. These channels become narrower as they near the outer
edge of the disc. The softened chips are then fed in at the centre and
by the action of the discs, are broken down into individual fibres by
the time they reach the outer edge of the discs. Fibres that have not
fully separated are rejected at the screening stage and sent to a reject
refiner for further treatment.
Singel disc refiner.
Chemical Pulp: Also called Kraft Pulp –
Sulphate Process
As with TMP, the debarked logs are chipped and washed before
moving to the pulping stage. The chips are fed into a large cooking vessel, called a digester. Chemicals are introduced which will
dissolve the lignin binding the fibres, so releasing them from each
other. The process is helped by raising the temperature in the digester about 150-200° C. The pulp is then screened screened to remove
bundles of fibres which have not separated, then washed to remove
any traces of chemicals, sand and grit. The spent chemicals are then
recycled to be further reused in the process.
A close up of a segment.
SCA Östrand produces totally chlorine free (TCF) bleached kraft
pulp at our plant in Sweden. About half of the plant’s production
is used for SCA’s own manufacturing of publication papers and
hygiene products. The plant also produces chemical thermomechanical pulp for hygiene, packaging and other products.
The digester at SCA Östrand pulp mill.
Bleaching
1. Pulp after
cooking.
Bleaching
Bleaching is an absolute requirement for high quality. The
pulp produced from any of the pulping methods, has a
somewhat brown appearance. All pulps can be bleached
to higher brightness. Bleaching is an absolute demand
for high qualithy printing paper, because of better colour
reproduction.
2. Pulp after
screening.
3. After oxygen
delignification.
Although Chlorine gas and Chlorine Dioxide are extremely
efficient at bleaching wood fibres, environmental concerns
have meant the gradual removal of these chemicals from
the bleaching process. Compounds of Chlorine cannot
be fully neutralised by the Mill effluent plant. Therefore,
treated water discharged into rivers or the sea, still containing residues of chlorine compounds, e.g. dioxin, will
deplete oxygen and destroy aquatic habitats.
Pulp which is bleached without the use of these chemicals
is called Totally Chlorine Free (TCF). Chemicals typically
used for bleaching are:
Oxygen (O2), Ozone (O3), Hydrogen Peroxide (H2O2).
For mechanical pulps peroxide bleaching is the most common for higher brightness grades.
All of the residues left from these compounds can be more
easily treated in the Mill effluent plant and the final discharge water has no harmful effects on aquatic life.
4. Peroxide
bleaching,
step 1.
5. After ozone
bleaching.
6. Peroxide
bleaching,
step 2.
At SCA Östrand the production
has been chlorine free since
1996 when the old bleaching plant was replaced by
the modern facility. Operating
totally chlorine-free means that
no chemicals containing chlorine
are used in bleaching. At SCA
Ortviken the mechanical pulping
process is totally chlorine-free.
Recovered fibres
To make a quality product the recovered newspapers and magazines must
be fresh, clean and dry.
Pulp production from recovered fibres
Collection of old newspapers, magazines, publisher returns
and print over-runs form the basis of the pulp made from
recovered fibre. Collections require significant investment
to provide bespoke recycling bins in easily accessible locations for use by the public. These sit alongside commercial
collections from Publishers and Printers to provide the volume of raw material needed to make a significant contribution to paper manufacturing.
The first stage of production combines a measured amount
of newspapers and magazines, fatty acid soap solution,
plus large volumes of warm water and rotates them in a
large pulping vessel. This action breaks down the bonds
between the fibres and starts the process of de-inking, by
loosening the bonds of the ink from the fibres. This stage
also segregates most of the “heavy” unwanted material
which accompanies the newspapers and magazines. These
include the binding staples, “tip-on” advertising material
in magazines, CD cases, plastic wrapping and other foreign
objects. Further unwanted materials are taken out with
centrifugal cleaners and by screening the pulp.
The pulp is now cleaned in a multi-stage washing process
that systematically removes more than 99% of the ink
Aylesford Newsprint in UK produces 400,000 tonnes per
annum of 100% recycled newsprint, which is 1% of the
total world production of nearly 40 million tonnes and 4%
of European production of 9.2 million tonnes per annum.
In the process more than 500,000 tonnes of recovered
Newspapers and Magazines are used each year.
Papermaking
Deinking plant in Aylesford.
adhering to the fibres. Fatty acid soap is introduced in a
large vessel containing warm water and the “dirty” pulp.
The soap loosens the binding of the ink from the fibres.
Compressed air is passed from the bottom of the vessel
to the surface. In doing so, soap bubbles are produced
which attract the released ink particles. The bubbles, with
the ink attached, rise to the surface of the water to form a
dirty scum. This scum is then skimmed away in the waste
water. This is repeated using multiple vessels, until the
pulp is completely clean. Some bleaching of the pulp may
be required, to stabilise the brightness to a uniform and
consistent level.
Lifespan of recovered fibres
Fresh fibre is constantly required to sustain the recycling
process, since fibres only can be recovered up to seven
times. Only about 80% of recycled fibre can be recovered
in the deinking process. Repeated recycling gradually
results in shorter and weaker fibres that must eventually
be screened out during the recovery process. These spent
fibres can then be incinerated to produce energy.
Aylesford Newsprint in UK produces newsprint from 100% recovered fibres.
Papermaking
The furnish
Once the pulp has been made to the correct brightness, dye
can be added to stabilise the exact shade. The human eye is
very perceptive to differences in shade. Depending on the
final product, certain other additives and process materials
may be introduced into the pulp. Large volumes of water
are added before moving the furnish to the headbox.
The headbox
In the headbox, the consistency of the furnish is 99%
water and process materials and 1% fibre. This volume
of water is required to prevent flocculation. Flocculation
is the tendency of the fibres to bunch together. If this was
allowed to happen, a poor sheet formation would result.
To help prevent flocculation, turbulence is created in the
headbox. The headbox distributes a controlled, even flow
of the furnish onto the next part of the paper machine to
begin forming the sheet of paper.
The headbox at PM 1 SCA Ortviken.
Picture of a Fourdrinier paper machine, PM 1 SCA Ortviken.
Fourdrinier or Twin-Wire Paper Machine
The pulp suspension exits the headbox in a controlled,
consistent flow onto the wire. The wire is a matrix or
screen with fine holes to allow the suspension to start the
drainage process and allow the fibres to form an interwoven mat. The wire is travelling at approximately the
same speed as the jet flow of the suspension. This is called
the jet-to-wire ratio, and this determines the formation of
the fibres into an inter-woven, cohesive layer, as the water
in the suspension begins to drain away. The majority of
the fibres orientate themselves in the direction of travel on
the wire. This produces paper with its greater strength in
the machine direction, compared with the cross direction.
If the drainage process were to rely on gravity alone, the
resulting paper sheet would have two distinct surfaces.
To prevent the production of two-sided paper, a second
wire meets the top of the suspension as it travels on the
bottom wire. The use of hydrofoils and suction boxes
allows drainage from the top side of the suspension, thus
evening out the distribution of the fine fibres and reducing
two-sidedness. The wire section increase the dry content
from 1% to 16-19%.
Papermaking
DuoFormer TQv
Picture of a Gap-Former machine, PM 11 at SCA Graphic Laakirchen.
Gap-Former Paper machine
A more modern development in paper technology has seen
the production of gap-formers, used in high speed paper
machines. This technology takes the pulp suspension from
the headbox and injects it, via individual nozzles across the
width of the paper machine, directly between two wires.
This allows drainage to take place from both sides simultaneously, thus producing a more uniform structure in sheet
formation.
DuoFormer TQv
In a gap former headbox, the furnish is injected between the
wires to begin the formation of the paper sheet.
In a gap former, the formation of the sheet and dewatering
starts, immediately the furnish enters the wires.
Papermaking
Picture of the press section.
Press Section
Once the formation of the paper sheet has been established, further removal of water takes place in the press
section of the paper machine. The paper sheet, which
still has a high water content, passes between a series of
large steel rollers which compress the sheet to squeeze
out further amounts of water. The paper web is held in a
“sandwich” of absorbent felt material as it passes between
the steel compression rollers. The felt material, in the form
of endless belts, act like blotting paper to absorb the water,
and vacuum boxes extract the water from the felts, before
they meet the paper sheet on the next revolution. At the
end of the press section, the dry content has increased to
40-50%. The sheet of paper can now support itself.
A Voith “shoe press” consolidates the formation of the sheet
during dewatering.
Papermaking
In the drying section the paper is dried by steam hearted steel cylinders.
Dryer Section
To establish the final moisture content of the paper, further
amounts of water are removed by evaporation. The dryer
section consists of a series of steam heated cylinders, over
which the paper web passes. The cylinders are arranged
in such a way that the paper web contacts first one side of
the paper, then the other to ensure even de-watering. The
paper web can be supported during this phase or it can be
self supporting, depending on the design of the equipment.
The support enhances the contact and heat transfer as well
as support high speed performance.
steel rollers which contact each side of the paper to smooth
out the top surface fibres. Some paper machines complete
the process with a soft calender. A soft calender has two
pairs of steel rollers. One roller in each pair is covered with
a soft plastic material and each pair is arranged in such
a way so contact is made on both sides of the paper with
each of the rollers. The pairing of the soft roller with a
hard roller produces different frictional forces on the paper
and imparts a slight glazing effect when smoothing the
paper fibres.
The paper has now been produced to the correct specification and can take various paths for further processing.
For newsprint and newsprint based products, a machine
calender may be all the paper requires to produce the finished product. A machine calender consists of a number of
Paper which requires further processes to produce a final
product is wound onto a steel spool to produce “jumbo
reels”, also called “tambours”. Finally the dry content is
90-95%, depending on the type of product produced.
The dryer section behind closed doors at PM 11 in Laakirchen.
Papermaking
Coating
For the production of paper
requiring high quality and
high brightness, as well as
a longer “shelf life” and a
superior surface on which
to print, a coating layer can
be added to the base paper
produced on the paper
machine. The coating layer
consists mainly of Kaolin
(China Clay), Calcium
Carbonate (CaCo3). Binding agents are required to
ensure the fine, powdery
material of the kaolin and
calcium carbonate adhere
to the base paper to form a
cohesive layer.
Other materials, such as
optical brighteners may also
be added to enhance the
appearance of the paper.
These work by converting
ultraviolet light into the
visible spectrum, adding a
blue-white appearance to
the paper.
The final appearance of the
paper may have a gloss or
matt finish, depending on
further processes.
In a blade coater, the coating thickness is regulated by a steel blade scraping off the excess
coating and returning it to a reservoir.
Coating machine
In most publication paper production processes, the coating can be applied in one of two
ways, or in a combination of the two methods. Film coating applies a regulated volume
thickness onto the base paper. The coating layer follows the contours of the base paper,
producing a constant thickness layer of coating. Blade coating applies a coating layer in
excess of the final coating volume required. The excess coating is skimmed off the base
paper by a blade, leaving a smooth coating layer on the surface.
In both methods, first one side of the paper is coated and dried, and then the second
side follows in the same way. Drying is achieved by infra-red and air floatation dryers,
dependant on the manufacturer of the equipment.
Coating
Base paper
Film coating.
Coating
Base paper
Blade coating.
SCA Ortviken has two LWC papermachines producing 500,000
tonnes per year of heatset web offset. About 25% of the total volume
is labelled with the FSC certification. The mark for responsible forestry
management.
Laakirchen PM 11 - Janus Kalander
Papermaking
Calendering
To give the paper its final finish, the paper is calendered.
The job of the calender is to correct any slight anomalies
in the structure of the paper and add a final finish to the
paper. For coated paper, the calender polishes the coating surface to give smoothness and a gloss finish to the
required specification.
For SC (Super Calendered) paper, the same Kaolin and
Calcium Carbonate used to make coating, are integrated
with the pulp at the mixing stage (the furnish), prior to
arrival at the headbox of the paper machine. During the
formation of the paper sheet, these minerals are distributed throughout the structure of the paper, with a greater
amount migrating towards the outer surfaces of the paper.
These minerals, along with paper fibres on the surface of
the paper, are “plasticised and oriented” into a consolidated structure by pressure and heat. The result is a uniform
glossy paper surface, suitable for both heatset web offset
and rotogravure printing.
In the calander the glossy surface is created by friction between
soft and hard rolls.
Calanders
Calanders come in various forms including a machine
calender, as described earlier in this brochure, soft calenders and super calenders. In each case they perform tasks
which “finish” the paper surface to its final specification.
Calanders are made up of steel roller sets, between which
the paper passes. The rollers apply heat and pressure to
the paper, which glaze or polish the surface to the required
gloss level. Each alternate roller can be covered in a softer
plastic material which aids the polishing effect by a slight
speed differential between the steel roller and the plastic
covered roller. Steam heat passed through the hollow steel
roller also helps the process.
Janus calander system in SCA Graphic Laakirchen.
SCA Graphic Laakirchen is producing 510 000 tonnes
SC paper per year, including both heatset web offset
and rotogravure. The mill is situated in the central of
Europe and can in most cases reach their customers
within 24 hours.
Papermaking
Slitting and Winding
After final control of the paper via laboratory analyses, the
accepted paper pass on target to the winder. Which ever
grade is produced, it is still in the form of a Jumbo Reel
or Tambour. The paper now has to be cut and wound to
the requirements of the end user. Computer programs are
used to plan the cutting of the Jumbo reel, to maximise the
available paper, with minimum paper loss from the edges
of the reel.
The Jumbo reel is located on one side of the winder and
individual strawboard cores of the correct length are
placed in the arms of the receiving stations. When the
paper leads are secured to the strawboard cores, the paper
is tensioned and circular knives cut the paper as the Jumbo
reel unwinds and paper builds on the new cores.
Wrapping & Labelling
The reels are now the correct grade of paper, in a width,
diameter and grammage ordered by the customer/end user.
To protect the reels throughout the transportation chain,
the reels are wrapped in moisture proof wrapping and
labelled with the information required for easy identification. Inner end shields, belly wrapping, outer end shields
and labelling are all accomplished using a fully automated
system to complete the tasks. The reels are now ready to
be moved to a storage facility or directly to the customer.
Process Control
Throughout each stage of pulp and papermaking, strict
control of each stage is undertaken by highly skilled workers. Along with their technical knowledge, sophisticated
computer based control systems monitor and adjust critical parameters in each stage of the manufacturing processes. These are backed up by laboratory based systems
as a “double check” to the online control systems in the
manufacturing processes. These systems ensure that the
paper produced, consistently meets the technical specifications for the grade and will perform to the highest standards in each of the printing processes designated to the
grade produced.
Papermaiking
Environment
Renewable raw material – forestry
Whether obtained fresh from the forest or recovered,
wood fibre is the base for paper products. Wood fibre
is a renewable resource. It is recyclable, and when it is
finally worn out, it can be used for energy production. The
residual product, carbon dioxide, is reabsorbed by growing trees. There are systems for certification of sustainable
forest management, like PEFC and FSC. Their mission is
to achieve environmentally-sound, socially-beneficial and
economically-viable forestry world-wide.
The mark of responsible forestry
Water and energy
Water is used to dissolve the raw materials used for paper
production and to dissolve additives and chemicals used in
the production. Water transport the dissolved raw materials from the plants where they are produced to the paper
machine. Water is also used for cooling or sanitary purposes. The fresh water need has constantly been reduced
during a long period due to energy efficiency reasons and
careful usage. The waste water is treated in three steps;
mechanical, biological and chemical.
Energy usage is based, to large degree, on biofuels and is
primarily a combination of electricity and heating production. The mix of energy sources varies according to the
location of the mill and the products manufactured. Electricity is primarily used to produce mechanical pulp and to
drive motors/pumps. In the TMP process about 60% of the
electrical energy is recovered as low pressure steam that is
mainly used for drying of the paper in the paper machine.
Heat is obtained from a number of sources: the combustion of biofuel i.e. branches and tree tops, the combustion
of e.g. natural gas, often in combination of electricity production and from the recovery of energy in the mechanical
pulping process. The drying section of the paper machine
is the largest consumer of heat, in the form of steam. By
continuously improving and developing the manufacturing
processes, the paper industry has minimized emissions into
the air and water.
SCA is one of Europe’s largest private forest owner, with
some two million hectares of forest land. At SCA’s forest tree
nursery in Sweden, 85 million seedlings are raised each
year to produce new forest. We replace every tree we harvest with three new ones, either through natural regeneration or planting. SCA’s Swedish forests were certified under
the FSC scheme in January 1999. From one tree trunk
from SCA’s forest, a full 95% is used, either for products
or energy. Since SCA owns sawmills, pulp and paper mills,
and manufactures forest-based biofuel, all interact in one
efficient system. All grades of wood are used and every part
of a tree can be utilized.
The manufacturing processes used are efficient. Wood
losses are minimal. Wood chips from sawmills become raw
material for the pulp mill, while bark and shavings are used
as fuel. Residual products from the pulp mills, primarily bark
and liquors, also become fuel and are responsible for a
large portion of the mill’s own energy requirements.
SCA’s environmental policy is that we shall manufacture our
products in a way that continually reduces their impact on
the environment. Conservation of resources will be our guiding principle. Our long-term goal is closed processes.
Photo: Per-Anders Sjöquist. Illustrations: Voith AG.
Printer: Tryckeribolaget, M 2010. FSC nr SGS-COC-00332
Both the paper and the printer are FSC-certified.
publicationpapers.sca.com