Pulp is a lignocellulosic fibrous material prepared by chemically or mechanically separating
cellulose fibres from
wood,
fibre crops or
waste paper. Wood pulp is the most common raw material in
papermaking.
History
Using wood to make
paper is a fairly recent innovation. In the 1800s,
fibre 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,
[1] and by the Canadian inventor
Charles Fenerty in
Nova Scotia.
[2] Chemical processes quickly followed, first with
J. Roth's use of
sulfurous acid to treat wood, followed by
B. Tilghman's
U.S. patent on the use of
calcium bisulfite, Ca(HSO
3)
2, to pulp wood in 1867.
[3] 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.
[3] The invention of the
recovery boiler by
G. H. Tomlinson in the early 1930s
[1] 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 fibres
[4] made the kraft process the dominant pulping process starting in the 1940s.
[3]
Global production of wood pulp in 2006 was 160 million tonnes (175 million tons).
[5] 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
United States at 16%. Chemical pulp made up 93% of market pulp.
[6]
Manufacture of wood pulp
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.
A pulp mill is a manufacturing facility that converts wood chips or other plant fibre source into a thick fibre board 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 fibres (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 fibers without depolymerizing the cellulose fibres (chemically depolymerizing the cellulose weakens the fibres). The various mechanical pulping methods, such as groundwood (GW) and refiner mechanical (RMP) 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.
Harvesting trees
All kinds of paper are made out of 100% wood with nothing else mixed into them (with some exceptions, like fancy resume paper, which may include cotton). This includes newspaper, magazines and even toilet paper. 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.
[7] 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.
[8]
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 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.
[9]
There are roughly 2 short tons in a cord of wood.
[10]
Preparation for pulping
Main article:
WoodchippingWoodchipping 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 fibres 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 fibres:
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 fibres. Mechanical pulps are used for products that require less strength, such as
newsprint and
paperboards.
Thermomechanical pulp
Mechanical pulping process.
[11]Thermomechanical pulp is pulp produced by processing
wood chips using
heat (thus
thermo) and a mechanical refining movement (thus mechanical). 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% and 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 (around 95%) and as the lignin has not been removed, the fibres are hard and rigid.
[11]
Chemithermomechanical pulp
Wood chips can be pretreated with
sodium carbonate,
sodium hydroxide,
sodium sulfite 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 fibres 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 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
sulfite process being 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 freed 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% 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.
Alternative pulping methods
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. 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.
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
Today, some people and groups advocate using
field crop fibre or agricultural residues instead of wood fibre as being more
sustainable.
[citation needed] However, wood is also a renewable resource, with about 90% of pulp coming from plantations or reforested areas.
[7] Non-wood fibre 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.
[6][12] Non-wood pulp processing requires a high use of water and energy.
[13]
Nonwovens are in some applications alternatives to paper made from wood pulp, like
filter paper or
tea bags.
Comparison of typical feedstocks used in pulping[14]
| Component | Wood | Nonwood |
| Carbohydrates | 65-80 % | 50-80 % |
- Cellulose
| 40-45 % | 30-45 % |
- Hemicellulose
| 23-35 % | 20-35 % |
| Lignin | 20-30 % | 10-25 % |
| Extractives | 2-5 % | 5-15 % |
| Proteins | < 0.5 % | 5-10 % |
| Inorganics | 0.1-1 % | 0.5-10 % |
- SiO2
| < 0.1 % | 0.5-7 % |
Market pulp
Market pulp is any variety of pulp that is produced in one location, dried and shipped to another location for further processing.
[15] Important quality parameters for pulp not directly related to the fibres are
brightness, dirt levels, viscosity and ash content. In 2004 about it was produced about 55 milion
metric tons of market pulp.
[16]
Air dry pulp
Air dry pulp is the most common form to sell pulp. This is pulp dried to about 10 % moisture content. It is normally delivered as sheeted bales of 250 kg. The reason to leave 10 % moisture in the pulp is that this minimizes the fibre to fibre bonding and makes it easier to disperse the pulp in water for further processing to
paper.
[15]
Roll pulp
Roll pulp or
reel pulp is the most common delivery form of pulp to non tradtitional pulp markets.
Fluff pulp is normally shipped on rolls (reels). This pulp is dried to 5 - 6 % moisture content. At the customer this is going to a comminution process to prepare for further processing.
[15]
Flash dried pulp
Some pulps are flash dried. This is done by pressing the pulp to about 50 % moisture content and then let it fall trough
silos that are 15 -17 m high. Gas fired hot air is the normal heat source. The temperature is well above the
char point of
cellulose, but large amount of moisture in the
fibre wall and
lumen prevents the fibres from being incinerated. It is often not dried down to 10 % moisture (air dry). The bales are not as densely packed as air dry pulp.
[15]
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 seek 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. Logging of
old growth forests accounts for less than 10% of wood pulp,
[7] but is one of the most controversial issues.
Effluents from pulp mills
The process effluents are treated in a biological
effluent treatment plant, which guarantees that the effluents are not toxic in the recipient.
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).
Chemical pulp mills, especially kraft mills, are energy self-sufficient and very nearly closed cycle with respect to inorganic chemicals.
Bleaching with chlorine produces large amounts of
organochlorine compounds, including
dioxins.
[17]
Odor problems
The kraft pulping reaction in particular releases foul-smelling compounds. The hydrogen sulfide reagent that degrades lignin structure also causes some demethylation to produce
methanethiol,
dimethyl sulfide and
dimethyl disulfide. These compounds have extremely low odor thresholds and disagreeable smells. The same compounds are released in microbial decay, or into e.g.
Camembert cheese, although the kraft process is a chemical one and does not involve any microbial degradation.
Paper production
The Fourdrinier Machine is the basis for most modern
papermaking, and it has been used in some variation since its conception. It accomplishes all the steps needed to transform a source of wood pulp into a final
paper product.
Economics
In 2009,
NBSK pulp sold for $650 /ton in the United States. The market had experienced a drop in price due to falling demand when newspapers reduced their size, in part, as a result of the recession.
[18]
See also
References
- ^ a b E. Sjöström (1993). Wood Chemistry: Fundamentals and Applications. Academic Press.
- ^ Burger, Peter. Charles Fenerty and his Paper Invention. Toronto: Peter Burger, 2007. ISBN 978-0-9783318-1-8 pp.25-30
- ^ a b c Biermann, Christopher J. (1993). Handbook of Pulping and Papermaking. San Diego: Academic Press. ISBN 0-12-097360-X.
- ^ "History of Paper". Retrieved 2007-10-08.
- ^ "Pulp production growing in new areas (Global production)". Metso Corporation. September 5, 2006. Archived from the original on 2007-10-23. Retrieved 2007-10-13.
- ^ a b "Overview of the Wood Pulp Industry". Market Pulp Association. 2007. Retrieved 2007-10-13.
- ^ a b c Martin, Sam (2004). "Paper Chase". Ecology Communications, Inc.. Archived from the original on 2007-06-19. Retrieved 2007-09-21.
- ^ "Certification Tracking products from the forest to the shelf". Retrieved 2007-09-21.
- ^ Conservatree.com
- ^ DOA.state.wi.us
- ^ a b Iggesund Paperboard AB (2008). Paperboard the Iggesund Way. p. 15.
- ^ Judt, Manfred (Oct-Dec 2001). "Nonwoody Plant Fibre Pulps". Inpaper International. Retrieved 2007-10-07.
- ^ News.southcn.com
- ^ Stenius, Per (2000) "1" PForest Products Chemistry Papermaking Science and Technology 3 Finland: Fapet Oy ISBN 952-5216-03-9 page 29
- ^ a b c d Nanko, Hirko; Button, Allan; Hillman, Dave (2005). The World of Market Pulp. Appleton, WI, USA: WOMP, LLC. pp. 2–3. ISBN 0-615-13013-5.
- ^ Nanko, Hirko; Button, Allan; Hillman, Dave (2005). The World of Market Pulp. Appleton, WI, USA: WOMP, LLC. p. 4. ISBN 0-615-13013-5.
- ^ "Effluents from Pulp Mills using Bleaching - PSL1". ISBN 0-662-18734-2 DSS. Health Canada. 1991. Retrieved 2007-09-21.
- ^ Lefebrvre, Paul (February 4, 2009). Wood products market looks soft. the Chronicle.
