The
timber resources used to make wood pulp are referred to as
pulpwood. While in theory any tree can be used for pulp-making,
coniferous trees are preferred because the cellulose fibers in the pulp of these species are longer, and therefore make stronger paper. Some of the most commonly used trees for paper making include
softwoods such as
spruce,
pine,
fir,
larch and
hemlock, and
hardwoods such as
eucalyptus,
aspen and
birch. There is also increasing interest in
genetically modified tree species (such as GM
eucalyptus and GM
poplar) because of several major benefits these can provide, such as increased ease of breaking down lignin and increased growth rate. A
pulp mill is a manufacturing facility that converts wood chips or other plant fibre source into a thick fiberboard which can be shipped to a
paper mill for further processing. Pulp can be manufactured using mechanical, semi-chemical or fully chemical methods (kraft and sulfite processes). The finished product may be either
bleached or non-bleached, depending on the customer requirements. Wood and other plant materials used to make pulp contain three main components (apart from water): cellulose fibers (desired for papermaking),
lignin (a three-dimensional polymer that binds the cellulose fibres together) and
hemicelluloses (shorter branched
carbohydrate polymers). The aim of pulping is to break down the bulk structure of the fibre source, be it chips, stems or other plant parts, into the constituent fibres. Chemical pulping achieves this by degrading the lignin and hemicellulose into small, water-soluble molecules which can be washed away from the cellulose fibres without
depolymerizing the cellulose fibres (chemically depolymerizing the cellulose weakens the fibres). The various mechanical pulping methods, such as groundwood (GW) and refiner mechanical pulping (RMP), physically tear the cellulose fibres one from another. Much of the lignin remains adhering to the fibres. Strength is impaired because the fibres may be cut. There are a number of related hybrid pulping methods that use a combination of chemical and thermal treatment to begin an abbreviated chemical pulping process, followed immediately by a mechanical treatment to separate the fibres. These hybrid methods include thermomechanical pulping, also known as TMP, and chemithermomechanical pulping, also known as CTMP. The chemical and thermal treatments reduce the amount of energy subsequently required by the mechanical treatment, and also reduce the amount of strength loss suffered by the fibres.
Harvesting trees Creating pulp begins with harvesting wood from trees, and
forest management is necessary to ensure a continued supply. Pulp
tree plantations account for 16 percent of world pulp production,
old-growth forests 9 percent, and second- and third- and more generation forests account for the rest.
Reforestation is practiced in most areas, so trees are a
renewable resource. The FSC (
Forest Stewardship Council), SFI (
Sustainable Forestry Initiative), PEFC (
Programme for the Endorsement of Forest Certification), and other bodies certify paper made from trees harvested according to guidelines meant to ensure good forestry practices. The number of trees consumed depends on whether mechanical processes or chemical processes are used. It has been estimated that based on a mixture of
softwoods and
hardwoods 12 metres (40 ft) tall and 15–20 centimetres (6–8 in) in diameter, it would take an average of 24 trees to produce 0.9 tonne (1 ton) of printing and writing paper, using the
kraft process (chemical pulping). Mechanical pulping is about twice as efficient in using trees, since almost all of the wood is used to make fibre, therefore it takes about 12 trees to make 0.9 tonne (1 ton) of mechanical pulp or
newsprint. There are roughly two short tons in a
cord of wood.
Preparation for pulping Woodchipping is the act and industry of chipping wood for pulp, but also for other
processed wood products and
mulch. Only the
heartwood and
sapwood are useful for making pulp.
Bark contains relatively few useful fibers and is removed and used as fuel to provide steam for use in the pulp mill. Most pulping processes require that the wood be chipped and screened to provide uniform sized chips.
Pulping There are a number of different processes which can be used to separate the wood fiber:
Mechanical pulp Manufactured
grindstones with embedded
silicon carbide or
aluminum oxide can be used to grind small wood logs called "bolts" to make stone pulp (SGW). If the wood is steamed prior to grinding it is known as pressure ground wood pulp (PGW). Most modern mills use chips rather than logs and ridged metal discs called refiner plates instead of grindstones. If the chips are just ground up with the plates, the pulp is called refiner mechanical pulp (RMP) and if the chips are steamed while being refined the pulp is called thermomechanical pulp (TMP). Steam treatment significantly reduces the total energy needed to make the pulp and decreases the damage (cutting) to fibres. Mechanical pulps are used for products that require less strength, such as
newsprint and
paperboards.
Thermomechanical pulp ,
sodium hydroxide,
sodium sulfate and other chemicals prior to refining with equipment similar to a mechanical mill. The conditions of the chemical treatment are much less vigorous (lower temperature, shorter time, less extreme
pH) than in a chemical pulping process since the goal is to make the fibers easier to refine, not to remove lignin as in a fully chemical process. Pulps made using these hybrid processes are known as chemi-thermomechanical pulps (CTMP).
Chemical pulp Company, a pulp mill that makes
fluff pulp for use in
absorbent products with the
Kraft process Chemical pulp is produced by combining wood chips and chemicals in large vessels called digesters. There, heat and chemicals break down lignin, which binds
cellulose fibres together, without seriously degrading the
cellulose fibres. Chemical pulp is used for materials that need to be stronger or combined with mechanical pulps to give a product different characteristics. The
kraft process is the dominant chemical pulping method, with the
sulfite process second. Historically
soda pulping was the first successful chemical pulping method.
Recycled pulp Recycled pulp is also called
deinked pulp (DIP). DIP is
recycled paper which has been processed by chemicals, thus removing
printing inks and other unwanted elements and freeing the paper fibres. The process is called
deinking. DIP is used as raw material in
papermaking. Many
newsprint,
toilet paper and
facial tissue grades commonly contain 100 percent deinked pulp and in many other grades, such as lightweight coated for offset and printing and writing papers for office and home use, DIP makes up a substantial proportion of the furnish.
Organosolv pulping Organosolv pulping uses organic solvents at temperatures above 140 °C to break down lignin and hemicellulose into soluble fragments. The pulping liquor is easily recovered by distillation. The reason for using a solvent is to make the
lignin more soluble in the cooking liquor. Most common used solvents are
methanol,
ethanol,
formic acid and
acetic acid often in combination with
water.
Alternative pulping methods Research is under way to develop biopulping (biological pulping), similar to chemical pulping but using certain species of
fungi that are able to break down the unwanted lignin, but not the cellulose fibres. In the biopulping process, the fungal enzyme
lignin peroxidase selectively digests lignin to leave remaining cellulose fibres. This could have major
environmental benefits in reducing the pollution associated with chemical pulping. The pulp is bleached using
chlorine dioxide stage followed by neutralization and
calcium hypochlorite. The
oxidizing agent in either case oxidizes and destroys the dyes formed from the
tannins of the wood and accentuated (reinforced) by
sulfides present in it.
Steam exploded fibre is a pulping and extraction technique that has been applied to wood and other fibrous organic material.
Bleaching The pulp produced up to this point in the process can be
bleached to produce a
white paper product. The chemicals used to bleach pulp have been a source of environmental concern, and recently the pulp industry has been using alternatives to
chlorine, such as
chlorine dioxide,
oxygen,
ozone and
hydrogen peroxide. == Alternatives to wood pulp ==