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Wood : a self-renewing raw material |
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Wood provides the primary raw material for the European
Pulp, Paper & Board Industry. Recovered paper & board are considered
as a secondary raw material. Wood consists of cellulose
fibres that are bound together by a material called lignin.
Wood is one of the very few raw materials used by a major industry, which
is infinitely self-renewing.
The pulp & paper industry primarily uses either sawmill
residues (wood chips) or trees that are too thin (so-called thinnings)
to be processed economically in sawmills for other purposes.
The industry used to be based almost entirely on softwoods
such as spruce, pine, larch, fir and cedar. More recently, eucalyptus,
birch, aspen and other hardwoods growing in temperate
climates, have been introduced as a raw material as they provide improvements
in some specific properties of printing and writing papers, fluting for
corrugated cases and other grades of papers. Nevertheless, softwood provides
longer fibres (average 3mm compared with 1mm for hardwoods) and continue
to be used for papers required to have the highest strength characteristics.
Paper & board may also be produced with de-inked
pulps (DIP), i.e. pulp made from recovered paper from which inks
and other contaminants have been removed.
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The fibre "production" |
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Fibres for paper and board are produced in a pulp mill. The fibres are
separated from one another into a mass of individual fibres. This is called
woodpulp. The separation can be undertaken by a mechanical
process, where the fibres are teased apart, or by chemical
means, where the lignin binding the fibers together is dissolved
away by cooking the woodchips in suitable chemicals. After separation,
the fibres are washed and screened to remove any remaining fibre bundles.
Generally, the chemical pulp produced in the world today
is based on one of two processes: sulphate or sulphite.
Most chemical pulp is made by the alkaline sulphate or kraft process,
which uses caustic soda and sodium sulphate to "cook" the woodchips.
In the unbleached stage, a dark brown, but very strong pulp results, which
can be bleached to a high brightness if required. The acid or neutral
sulphite mills produce pulps which are easily bleached, usually with hydrogen
peroxide. These pulps fulfil today's demand for "chlorine free"
raw materials.
Thermo-Mechanical Pulp (TMP) and Chemi-Thermo-Mechanical-Pulp
(CTMP) are produced by a combination of the mechanical and chemical processes.
Quality papers require a pulp that does not discolour during
storage or go yellow when exposed to sunlight and which retains its strength.
One of the most effective ways of achieving all three requirements is
by bleaching. This has the additional advantages of improving
absorption capacity, removing any small pieces of bark or wood left behind
as well as giving a high level of purity.
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The various pulp grades |
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Mechanical pulps
Stone groundwood: pulp produced by grinding wood into relatively short
fibres. This pulp is used mainly in newsprint and wood-containing papers,
like LWC and SC papers. Thermo-mechanical (TMP): pulp produced in a thermomechanical
process where wood chips are softened by steam before entering a pressurised
refiner. TMP has mainly the same end-uses as stone groundwood. Variants
of the above two processes produce pressurised stone groundwood pulp and
refiner mechanical pulp.
Semi-chemical pulps
Semi-chemical: pulp produced in a two-stage process, which involves partial
digestion with chemicals, followed by mechanical treatment in a disc refiner.
The pulp is mainly used in production of fluting medium for corrugated
board.
Chemi-thermomechanical (CTMP): pulp produced in a similar way to TMP,
but the wood chips are chemically treated before entering the refiner.
This pulp has properties suited to tissue manufacture. Some CTMP pulp
is used in printing & writing paper grades. CTMP pulp is classified
under semi-chemical pulps in the Harmonised System of the Customs co-operation
council. In FAO, as well as other industry statistics, such chemi-thermomechanical
pulps are grouped with mechanical pulp.
Chemical pulps
Sulphite: pulp produced by cooking wood chips in a pressure vessel in
the presence of bisulphite liquor. End-uses range from newsprint, printing
& writing papers, tissue and sanitary papers. Sulphite can be either
bleached or unbleached.
Sulphate (or kraft): pulp produced by cooking wood chips in pressure vessels
in the presence of a sodium hydroxide (soda) liquor. The pulp may be unbleached
or bleached. End-uses are widespread, with bleached pulp particularly
used for graphic papers, tissue and carton boards. Unbleached pulp is
commonly used in liner for corrugated board, wrappings, sack and bag papers,
envelopes and other unbleached speciality papers.
Deinked-pulps
Pulp made from recovered paper from which inks and other contaminants
have been removed.
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The paper & board production stage |
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Woodpulp produced in a pulp mill may be fed directly to a paper machine
in an "integrated paper mill" or dried and pressed into bales
to be used as a raw material by papermills worldwide. Integrated
pulp is pulp that is produced for use as raw material in production
of paper & board at the same mill, or for shipment by a producing
mill to other mills, which it owns, controls or with which is affiliated
within the same country. Market pulp is pulp that is
sold in open competition with that of other producers. All pulp exported
from the producing country is considered to be market pulp.
The paper making process is also highly automated,
involving special knowledge and expertise. Production processes are optimised
for each grade of paper & board (newsprint, printing & writing
papers, packaging, hygiene papers...). Variables are many: raw material
composition (mixture of chemical softwood and hardwood pulp, mechanical
pulp, recovered paper, fillers, pigments, additives, etc.), machine size
(width, speed), type of production equipment, and automation level.
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