Wood pulp

Wood pulp is a dry fibrous material prepared by chemically or mechanically separating the fibers which make up wood.

Pulp can be either fluffy or formed into thick sheets. The latter form is used if the pulp must be transported from the pulp mill to a paper mill. Pulp which is shipped and sold as pulp (not processed into paper in the same facility) is referred to as market pulp. When suspended in water the fibers disperse and become more pliable. This pulp suspension can be laid down on a screen to form a sheet of paper, and this is the primary use for wood pulp. Wood pulp is the most common material used to make paper. The timber resources used to make wood pulp are referred to as pulpwood. Wood pulp comes from softwood trees such as spruce, pine, fir, larch and hemlock, and hardwoods such as eucalyptus, aspen and birch.


Using wood to make paper is a fairly recent innovation. In the 1800s, fiber crops such as linen fibres were the primary material source, and paper was a relatively expensive commodity. The use of wood to make pulp for paper began with the development of mechanical pulping in Germany by F.G. Keller in the 1840s. Chemical processes quickly followed, first with J. Roth's use of sulfurous acid to treat wood, followed by B. Tilghman's US patent on the use of calcium bisulfite, Ca(HSO3)2, to pulp wood in 1867. Almost a decade later the first commercial sulfite pulp mill was built in Sweden. It used magnesium as the counter ion and was based on work by Carl Daniel Ekman. By 1900 sulfite pulping had become the dominant means of producing wood pulp, surpassing mechanical pulping methods. The competing chemical pulping process, the sulfate or kraft process was developed by Carl F. Dahl in 1879 and the first kraft mill started (in Sweden) in 1890. The invention of the recovery boiler by G.H. Tomlinson in the early 1930s allowed kraft mills to recycle almost all of their pulping chemicals. This, along with the ability of the kraft process to accept a wider variety of types of wood and produce stronger fibers made the kraft process the dominant pulping process starting in the 1940s.

Global production of wood pulp in 2006 was 160 million tonnes (175 million tons). In the previous year, 57 million tonnes (63 million tons) of market pulp (not made into paper in the same facility) was sold, with Canada being the largest source at 21% of the total, followed by the US at 16%. Chemical pulp made up 93% of market pulp.

Manufacture of wood pulp

Harvesting trees

Most pulp mills use good forest management practices in harvesting trees to ensure that they have a sustainable source of raw materials. One of the major complaints about harvesting wood for pulp mills is that it reduces the biodiversity of the harvested forest. Trees raised specifically for pulp production account for 16 percent of world pulp production, old growth forests account for 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) certifies paper made from trees harvested according to guidelines meant to ensure good forestry practices.

The number of trees consumed depends whether mechanical processes or chemical processes are used. It has been estimated that based on a mixture of softwoods and hardwoods 12 meters (40 ft) tall and 15-20 centimeters (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 fiber therefore it takes about 12 trees to make 0.9 tonne (1 ton) of mechanical pulp or newsprint.

Preparation for pulping

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.


There are a number of different processes which can be used to separate the wood fibers:

Mechanical pulp

Manufactured grindstones with embedded silicon carbide or aluminum oxide can be used to grind small wood logs called "bolts" to make "stone groundwood" pulp (SGW). If the wood is steamed prior to grinding it is known as "pressure groundwood" 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 fibers. Mechanical pulps are used for products that require less strength, such as newsprint and paperboards.

Chemithermomechanical pulp

Wood chips can be pretreated with sodium carbonate, sodium hydroxide, sodium sulfite and other chemical 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 chemithermomechanical pulps (CTMP).

Chemical pulp

Chemical pulp is produced by combining wood chips and chemicals in large vessels known as digesters where heat and the chemicals break down the lignin, which binds the cellulose fibers together, without seriously degrading the cellulose fibers. 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 sulfite process being second.

Recycled pulp

Pulp can also be made out of waste paper and paperboard. Recycled pulp is most often used to make paperboard, newsprint or sanitary paper.


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.

Environmental concerns

The major environmental impacts of producing wood pulp come from its impact on forest sources and from its waste products.

Forest resources

The impact of logging to provide the raw material for wood pulp is an area of intense debate. Modern logging practices, using forest management seeks to provide a reliable, renewable source of raw materials for pulp mills. The practice of clear cutting is a particularly sensitive issue since it is a very visible effect of logging. Reforestation, the planting of tree seedlings on logged areas, has also been criticized for decreasing biodiversity because reforested areas are monocultures. Proponents of reforestation and plantations argue that in this respect trees are no different from any other agricultural crop. Logging of old growth forests accounts for less than 10% of wood pulp, but is one of the most controversial issues.

Effluents from pulp mills

Pulp mills are almost always located near large bodies of water because of they require substantial quantities of water for their processes. Delignification of chemical pulps releases considerable amounts of organic material into the environment, particularly into rivers or lakes. The wastewater effluent can also be a major source of pollution, containing lignins from the trees, high biological oxygen demand (BOD) and dissolved organic carbon (DOC), along with alcohols, chlorates, heavy metals, and chelating agents. Reducing the environmental impact of this effluent is accomplished by closing the loop and recycling the effluent (see black liquor) where possible, as well as employing less damaging agents in the pulping and bleaching processes.

Mechanical pulp is not a major cause for environmental concern since most of the organic material is retained in the pulp, and the chemicals used (hydrogen peroxide and sodium dithionite) produce benign byproducts (water and sodium sulfate (finally), respectively).

Bleaching with chlorine produces large amounts of organochlorine compounds, including dioxins. Increased public awareness of environmental issues, as evidenced by the formation of organizations like Greenpeace, influenced the pulping industry and governments to address the release of these materials into the environment . The amount of dioxin has been reduced dramatically by replacing some of all of the chlorine with chlorine dioxide. The use of elemental chlorine has declined significantly and as of 2005 was used to bleach 19-20% of all kraft pulp. EFC (elemental chlorine-free) pulping using chlorine dioxide is now the dominant technology worldwide (with the exception of Finland and Sweden where TCF is very important), accounting for 75% of bleached kraft pulp globally

Chemical pulp mills, especially kraft mills, are energy self-sufficient and very nearly closed cycle with respect to inorganic chemicals.


Today, some people and groups advocate using field crop fiber or agricultural residues instead of wood fiber as being more sustainable. However, wood is also a renewable resource, with about 90% of pulp coming from plantations or reforested areas. Non-wood fiber sources account for about 5-10% of global pulp production, for a variety of reasons, including seasonal availability, problems with chemical recovery, brightness of the pulp etc.

Research is under way to develop 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. This could have major environmental benefits in reducing the pollution associated with chemical pulping.

See also


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