Pulp (paper)

Pulp is a fibrous lignocellulosic material prepared by chemically, semi-chemically, or mechanically isolating the cellulosic fibers of wood, fiber crops, waste paper, or rags. Mixed with water and other chemicals or plant-based additives, pulp is the major raw material used in papermaking and the industrial production of other paper products.

History

Before the widely acknowledged invention of papermaking by Cai Lun in China around AD 105, paper-like writing materials such as papyrus and amate were produced by ancient civilizations using plant materials which were largely unprocessed. Strips of bark or bast material were woven together, beaten into rough sheets, dried, and polished by hand. Pulp used in modern and traditional papermaking is distinguished by the process which produces a finer, more regular slurry of cellulose fibers which are pulled out of solution by a screen and dried to form sheets or rolls. The earliest paper produced in China consisted of bast fibers from the paper mulberry plant along with hemp rag and net scraps. By the 6th century, the mulberry tree was domesticated by farmers in China specifically for the purpose of producing pulp to be used in the papermaking process. In addition to mulberry, pulp was also made from bamboo, hibiscus bark, blue sandalwood, straw, and cotton. Papermaking using pulp made from hemp and linen fibers from tattered clothing, fishing nets and fabric bags spread to Europe in the 13th century, with an ever-increasing use of rags being central to the manufacture and affordability of rag paper, a factor in the development of printing. By the 1800s, production demands on the newly industrialized papermaking and printing industries led to a shift in raw materials, most notably the use of pulpwood and other tree products which today make up more than 95% of global pulp production.
The use of wood pulp and the invention of automatic paper machines in the late 18th- and early 19th-century contributed to paper's status as an inexpensive commodity in modern times. While some of the earliest examples of paper made from wood pulp include works published by Jacob Christian Schäffer in 1765 and Matthias Koops in 1800, large-scale wood paper production began in the 1840s with unique, simultaneous developments in mechanical pulping made by Friedrich Gottlob Keller in Germany and by Charles Fenerty in Nova Scotia. Chemical processes quickly followed, first with J. Roth's use of sulfurous acid to treat wood, then by Benjamin Tilghman's U.S. patent on the use of calcium bisulfite, Ca₂, 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; 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 to produce stronger fibres, made the kraft process the dominant pulping process, starting in the 1940s.
Global production of wood pulp in 2006 was 175 million tons. In the previous year, 63 million tons of market pulp was sold, with Canada being the largest source at 21 percent of the total, followed by the United States at 16 percent. The wood fiber sources required for pulping are "45% sawmill residue, 21% logs and chips, and 34% recycled paper". Chemical pulp made up 93% of market pulp.

Wood pulp

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 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. 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 : cellulose fibers, lignin and hemicelluloses. 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. The various mechanical pulping methods, such as groundwood and refiner mechanical pulping, 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.

Pulp category	Production
Chemical	131.2
Kraft	117.0
Sulfite	7.0
Semichemical	7.2
Mechanical	37.8
Nonwood	18.0
Total virgin fibres	187.0
Recovered fibres	147.0
Total pulp	334.0'''

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, SFI, PEFC, 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 tall and 15–20 centimetres in diameter, it would take an average of 24 trees to produce 0.9 tonne of printing and writing paper, using the kraft process. 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 of mechanical pulp or newsprint.
There are roughly two short tons in a cord of wood.

Preparation for pulping

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. If the wood is steamed prior to grinding it is known as pressure ground wood pulp. 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 and if the chips are steamed while being refined the pulp is called thermomechanical pulp. Steam treatment significantly reduces the total energy needed to make the pulp and decreases the damage to fibres. Mechanical pulps are used for products that require less strength, such as newsprint and paperboards.

Thermomechanical pulp

Thermomechanical pulp is pulp produced by processing wood chips using heat and a mechanical refining movement. It is a two-stage process where the logs are first stripped of their bark and converted into small chips. These chips have a moisture content of around 25–30 percent. A mechanical force is applied to the wood chips in a crushing or grinding action which generates heat and water vapour and softens the lignin thus separating the individual fibres. The pulp is then screened and cleaned, any clumps of fibre are reprocessed. This process gives a high yield of fibre from the timber and as the lignin has not been removed, the fibres are hard and rigid.

Chemi-thermomechanical pulp

Wood chips can be pre-treated with sodium carbonate, 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 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.