Mechanical pulping

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Mechanical pulping is the process in which wood is separated or defibrated mechanically into pulp for the paper industry.

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The mechanical pulping processes use wood in the form of logs or chips that are mechanically processes, by grinding stones (from logs) or in refiners (from chips), to separate the fibers. Mechanical pulp can also be bleached to increase the brightness, but retaining the lignin in the pulp. [1]

Industrial mechanical pulping started in the 1840s with groundwood pulping, producing the pulp from grinding. This made wood fibers the main raw material in paper, instead of textile fibers. Later the chemical pulping processes started dominating for many paper types. [2] Today the groundwood pulping mills are few, but the mechanical pulping processes employing refiners are still important in the Pulp and paper industry. The mechanical pulps are primarily used in newspaper and magazine paper and the chemimechanical pulps for cardboard and soft paper.

Mechanical pulps in the paper industry

Wet pulp is loaded in the port in Umea, Sweden in 1967. Umea stadsarkiv-1967-lastning av vatmassa.jpg
Wet pulp is loaded in the port in Umeå, Sweden in 1967.

Mechanical pulp is very different from the pulp produced in the chemical processes (the sulfite process and the Kraft process). The chemical methods gives paper with higher strength and pulp that can be bleached further than the mechanical pulps. A benefit from the mechanical processes is that they have a high yield, usually between 90 and 98%, which means that nearly all the wood is used. The yield of the Kraft process is only about 50%, which makes the demand of wood twice as high for the same amount of produced pulp [3]

Another benefit of the mechanical pulps is the excellent printing properties, making them very useful in newsprint and magazines. This is because of their good properties of absorbing the ink and also their high opacities and low area density. Chemimechanical pulps can give paper with high stiffness, making them suitable for paper board and as a stiff layer in paper packaging. [1]

The mechanical pulping processes have a high yield, which at the same time means that all the lignin is still present in the pulp and paper. Lignin is modified by sunlight, making paper produced from mechanical pups susceptible to yellowing and becoming brittle. [3] Mechanical pulps can be bleached with hydrogen peroxide or sodium dithionite, [1] but the brightness is only increased slightly and the yellowing cannot be hindered as lignin is still present.

Mechanical pulping is primarily used in paper products with relatively short life span, such as news paper, books or brochures, but also in coated paper grades for magazines and catalogs. It is also used as the middle layer in cardboard because of its stiffness. These products are used to a high degree, which makes mechanical pulps common in recycled paper and as recycled fibers for paper production. The recycled fibers are mostly used in simpler cardboard, newspaper and tissue paper. [1]

A drawback with the mechanical pulping processes are that they require a high amount of electrical power. A typical refiner pulp can require 2000 kWh/mass ton pulp. [1] A larger mechanical pulp and paper mill can, including the paper production, consume 200-300 MW electricity. The chemical pulping processes can often generate enough energy (steam and electricity) to make the mill energy self-sufficient. This energy is generated from wood leftovers (bark and chips) and from black liquor, which in principle is 50% of the incoming wood (as the yield is less than 50% and a majority of the rest ends up in the black liquor). [4] In the mechanical pulp mills the majority of the electricity is converted to heat, creating steam which is utilized in the paper machines.

Mechanical pulping processes

A number of different mechanical processes exist: [5]

See also

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<span class="mw-page-title-main">Pulp (paper)</span> Fibrous material used notably in papermaking

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<span class="mw-page-title-main">Pulpwood</span> Timber intended for processing into wood pulp for paper production

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<span class="mw-page-title-main">Sodium sulfite</span> Chemical compound

Sodium sulfite (sodium sulphite) is the inorganic compound with the chemical formula Na2SO3. A white, water-soluble solid, it is used commercially as an antioxidant and preservative. It is also suitable for the softening of lignin in the pulping and refining processes of wood and lignocellulosic materials. A heptahydrate is also known but it is less useful because of its greater susceptibility toward oxidation by air.

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A pulp mill is a manufacturing facility that converts wood chips or other plant fiber sources into a thick fiber board 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.

<span class="mw-page-title-main">Black liquor</span> Industrial by-product

In industrial chemistry, black liquor is the by-product from the kraft process when digesting pulpwood into paper pulp removing lignin, hemicelluloses and other extractives from the wood to free the cellulose fibers.

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Bleaching of wood pulp is the chemical processing of wood pulp to lighten its color and whiten the pulp. The primary product of wood pulp is paper, for which whiteness is an important characteristic. These processes and chemistry are also applicable to the bleaching of non-wood pulps, such as those made from bamboo or kenaf.

The sulfite process produces wood pulp that is almost pure cellulose fibers by treating wood chips with solutions of sulfite and bisulfite ions. These chemicals cleave the bonds between the cellulose and lignin components of the lignocellulose. A variety of sulfite/bisulfite salts are used, including sodium (Na+), calcium (Ca2+), potassium (K+), magnesium (Mg2+), and ammonium (NH4+). The lignin is converted to lignosulfonates, which are soluble and can be separated from the cellulose fibers. For the production of cellulose, the sulfite process competes with the Kraft process which produces stronger fibers and is less environmentally costly.

<span class="mw-page-title-main">Paper</span> Material for writing, printing, etc.

Paper is a thin sheet material produced by mechanically or chemically processing cellulose fibres derived from wood, rags, grasses, or other vegetable sources in water, draining the water through a fine mesh leaving the fibre evenly distributed on the surface, followed by pressing and drying. Although paper was originally made in single sheets by hand, almost all is now made on large machines—some making reels 10 metres wide, running at 2,000 metres per minute and up to 600,000 tonnes a year. It is a versatile material with many uses, including printing, painting, graphics, signage, design, packaging, decorating, writing, and cleaning. It may also be used as filter paper, wallpaper, book endpaper, conservation paper, laminated worktops, toilet tissue, currency, and security paper, or in a number of industrial and construction processes.

Dissolving pulp, also called dissolving cellulose, is bleached wood pulp or cotton linters that has a high cellulose content. It has special properties including a high level of brightness and uniform molecular-weight distribution. This pulp is manufactured for uses that require a high chemical purity, and particularly low hemicellulose content, since the chemically similar hemicellulose can interfere with subsequent processes. Dissolving pulp is so named because it is not made into paper, but dissolved either in a solvent or by derivatization into a homogeneous solution, which makes it completely chemically accessible and removes any remaining fibrous structure. Once dissolved, it can be spun into textile fibers, or chemically reacted to produce derivatized celluloses, such cellulose triacetate, a plastic-like material formed into fibers or films, or cellulose ethers such as methyl cellulose, used as a thickener.

In industrial paper-making processes, organosolv is a pulping technique that uses an organic solvent to solubilise lignin and hemicellulose. It has been considered in the context of both pulp and paper manufacture and biorefining for subsequent conversion of cellulose to fuel ethanol. The process was invented by Theodor Kleinert in 1968 as an environmentally benign alternative to kraft pulping.

<span class="mw-page-title-main">Paper chemicals</span> Chemicals used in paper manufacturing

Paper chemicals designate a group of chemicals that are used for paper manufacturing, or modify the properties of paper. These chemicals can be used to alter the paper in many ways, including changing its color and brightness, or by increasing its strength and resistance to water. The chemicals can be defined on basis of their usage in the process.

Bleached Chemi-ThermoMechanical Pulp (BCTMP) is a pulp product used by the paper industry as a less-expensive alternative for Northern Bleached Softwood Kraft (NBSK).

References

  1. 1 2 3 4 5 Pulp and paper chemistry and technology. Volume 2, Pulping chemistry and technology. Ek, Monica., Gellerstedt, Göran., Henriksson, Gunnar. Berlin: Walter de Gruyter. 2009. ISBN   9783110213423. OCLC   607553694.{{cite book}}: CS1 maint: others (link)
  2. ↑ Müller, Lothar, 1954- (2015-02-02). White magic : the age of paper. Spengler, Jessica. Cambridge, UK. ISBN   9780745681832. OCLC   902846600.{{cite book}}: CS1 maint: location missing publisher (link) CS1 maint: multiple names: authors list (link) CS1 maint: numeric names: authors list (link)
  3. 1 2 Biermann, Christopher J. (1996). Handbook of pulping and papermaking (2nd ed.). San Diego: Academic Press. ISBN   9780080533681. OCLC   162129067.
  4. ↑ "Types of Pulping Processes | CEPI - CONFEDERATION OF EUROPEAN PAPER INDUSTRIES". www.cepi.org. Retrieved 2019-06-11.
  5. ↑ Walker, J. C. F. (2006). Primary wood processing : principles and practice (2nd ed.). Dordrecht: Springer. ISBN   9781402043932. OCLC   209934701.
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