Wood digestion

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When pulping the mechanical or chemical decomposition is of wood referred to in wood fibers. Mechanical pulping of the wood is used to produce wood pulp , and chemical pulping processes are used to produce pulp . These fibers are processed by the paper industry into paper , cardboard and cardboard . To a lesser extent, particularly pure pulp ( chemical pulp ) is further processed by the chemical industry.

Basics

Wood essentially consists of cellulose fibers arranged in bundles , which are embedded in a matrix of lignin molecules . As a support material and hardened polymer, the lignin is essentially responsible for the compressive strength of the fabric, while the embedded cellulose fibers ensure the tensile strength . So it is a penetration of tear-resistant, flexible fibers (cellulose) with a dense and rigid polymer as filling material (lignin). As analogies, technical materials such as reinforced concrete or natural fiber-reinforced plastics are also structured accordingly. Lignin also serves as protection against the penetration of water into the cell wall material and thus keeps it in the vessels ( xylem and phloem ) as well as inside the cells. There is a further protective effect against UV light as well as mechanical damage and the penetration of pests. After all, lignin is difficult to break down by bacteria or fungi and as a result inhibits the growth of pathogenic microorganisms.

In order to use cellulose as pulp in the paper and pulp industry, the lignin matrix must be dissolved and the cellulose exposed accordingly. Chemical wood pulping processes are used to release the pulp.

Mechanical wood pulping

Mechanical pulping produces wood pulp and other wood pulp for so-called wood-containing paper. A distinction is made as to whether the individual wood fibers are extracted from the log or from wood chips.

Grinding process

Wood grinder scheme

The grinding process is used to produce wood pulp that is obtained directly from the round wood . This is done with the help of a rotating grindstone, which gradually removes the fibers from the wood. A distinction is made between sanding at normal pressure or at overpressure. The former is referred to as stone groundwood pulp , the latter as pressure groundwood pulp .

The processes involved in breaking down the wood fibers from the round timber can be classified into three parts, with the individual influences overlapping. First, the middle lamella is softened . This is due to the high temperatures that result from the strong alternating pressure loads that act on the wood during the grinding process. Because not only a single log is being sanded at the moment, but there are always several logs in the vicinity of the grindstone, the heat generated by the friction can not escape, which is why it accumulates a few millimeters above the sanding zone. This leads to the plasticization of the lignin and the hemicelluloses before they come into contact with the grindstone. However, the softening temperature depends on the water content of the wood. The higher the water content of the wood, the lower the plasticizing temperature. While dry lignin softens between 135 ° C and 235 ° C, it plasticizes between 90 ° C and 100 ° C with a water content in the range of 30 to 40%. Hemicelluloses soften with a water content at this level already at 85 ° C, instead of 167 ° C to 217 ° C as a dry substrate. Then the fibers are separated. The alternating pressure stresses caused by the grinding stone initially lead to the deformation of the fiber structure and finally to the breakage of the bonds between and within the fibers. The resulting deformations facilitate the water absorption of the lignin, which in turn favors the further fibrillation of the detached fibers. Finally, the fibers come out. Depending on how sharp the surface of the grindstone is, a sharpened stone leads to a scratching or cutting effect on the fibers. If the grindstone is rather dull, it has a fibrillating or squeezing effect on the starting material.

The decisive property for the surface structure of the wood fibers is wood moisture in the stone process . A higher wood moisture content leads to a fiber material with better strength properties, lower drainage resistance, a higher degree of whiteness and a larger proportion of long fiber with higher quality fines. The moisture content should not be below 33 percent, as the fiber saturation of the wood must be guaranteed. Depending on the type of wood , this is between 28 and 32 percent. However, a moisture content of 50 to 60 percent has the best properties. Furthermore, better results can be achieved when grinding under positive pressure. This can be explained by the fact that the saturation temperature of the water drops with an increased pressure, which is why it is possible to work with higher temperatures in the grinding zone. In addition, no water can escape from the system because the entire grinding zone is tightly sealed with pressure grinders. Thus, the entire amount of water can be used for the plasticization of lignin and the hemicelluloses. In this way, fiber materials with better properties than stone grinding can be achieved under normal pressure.

Another important parameter is the increase in the grinding pressure, which is achieved by increasing the wood feed. This results in a Groundwood fiber material with better drainability, but this also leads to poorer strength properties. Furthermore, the peripheral speed of the stone is also of significant importance for the fiber properties. Increasing the speed, for example, leads to short-fiber and difficult to dewater fiber. Usually, to increase productivity in practice, the increase in the wood feed rate and the stone peripheral speed are increased in combination.

The grinding zone temperature is another property that affects the fiber material. However, this is difficult to determine in practice. Therefore, in order to be able to estimate the value, the temperature of the trough material, the spray water and the temperature conditions at the grinding zone outlet are determined. Normally, production takes place at trough temperatures between 60 ° C and 75 ° C, as this leads to a solid and long-fibred wood pulp, which can be drained well. Furthermore, the profile of the grindstone is relevant for the product. While sharpening can influence the macrostructure of the stone, the microstructure of the stone is created by the grain material stone. The influence of the structure of the whetstone on the log is determined with the help of the specific sanding work requirement. It serves both to describe the energy requirement and the quality of the Groundwood pulp, as well as for process regulation and control. Usually 90 to 150 kWh / 100 kg is used. A high specific grinding work requirement results in a finer and stronger wood pulp. The following diagram demonstrates its influence on the strength properties of the pulp.

Refiner process


Active pairs of a refiner: ➀ edge against edge
➁ face against face
➂ knife cell against knife cell

The refiner process is used to produce wood pulp from wood chips. Depending on the pretreatment, a distinction is made between TMP (thermo-mechanical pulp) and CTMP fibers (chemo-thermo-mechanical pulp) . While TMP fibers are only thermally pretreated, CTMP fibers are also pretreated with chemicals for digestion. Here, for softwoods preferred sodium used for hardwoods , however, sodium hydroxide .

The breakdown of the wood fibers in the refiner process also takes place through the plasticization of the lignin. This takes place both through mechanical and thermal stress during the grinding process, as well as through chemical pretreatment of CTMP fibers. The procurement of the resulting fiber depends on several factors. As with the Stein procedure, the defibration temperature is of great importance. In order for the lignin to soften, a temperature between 120 ° C and 135 ° C is required in the refiner. This should also be adhered to as far as possible, since only in this way a large part of the fibers can be removed undamaged from the fiber structure. Even a slight increase has negative effects on the quality of the fiber. If the defibration temperature is 140 ° C, this leads to a very coarse product. In addition, the softened lignin solidifies after cooling on the fiber surface, which leads to a hard fiber material which is completely unsuitable for paper production . If, on the other hand, the softening temperature is not reached during the defibration, the result is an equally coarse fiber material with only low strength. According to Wenderdel and Krug, lignin condensation even occurs at a temperature of over 170 ° C, which means that the resulting fiber is also unsuitable for the production of paper.

In addition, the distance between the grinding disks has an impact on the fiber morphology. However, this is difficult to control due to the changing pressure and temperature conditions during grinding, which is why frequent readjustments are necessary. This is because the smaller the distance between the grinding disks means that stronger physical forces act on the wood chips, which means that the fibers tend to break out of the fiber composite instead of being fibrillated.

TMP and CTMP fibrous materials differ in terms of their properties in some features. The main difference lies in the mean fiber length of the two wood pulps. CTMP fibers have a higher long fiber content and a lower fines content, which is due to the chemical pretreatment. Depending on the type of wood, only sodium sulphite or a mixture of sodium hydroxide and sodium sulphite is used. This process is known as sulfonation . It has the effect that the fibers on the middle lamella can be separated easily, as the sulfur interacts with the lignin that is mainly present there. Furthermore, the wood fibers are more easily accessible to water molecules. The reaction product is lignin sulfonic acid , which is less of a problem in papermaking than lignin. In addition, chemically pretreated fiber materials are more flexible and more readily bond than those that have only been thermally pretreated.

Chemical wood digestion

Dominant procedure

Sulphite factory in Köpmanholmen (Sweden) around 1900 with the sulphite tower characteristic of the sulphite process

The wood is chemically digested to produce cellulose . The paper industry mainly uses the following processes:

The sulphate process is currently (2009) mainly used in the production of pulp for paper manufacture. Around 85 percent of the pulp used in Germany is obtained using the sulphate process; in 2008 this was 3.7 million tons of sulphate pulp. The sulphite process is used for only about 15 percent or 723,000 tons of the pulp used in Germany. Around the world, around 80 percent of pulp comes from plants with a conventional or modified sulphate process, while only around 6 percent comes from the sulphite process (mainly chemical pulp). The remaining 14 percent is produced using the soda process or other digestion processes, mainly from annual crops ( bamboo, etc.).

Alternative procedures

A number of other digestion processes are designed to remove the binding lignin from the wood with the help of solvents , acids or enzymes and thus release the pulp. These procedures include the following:

supporting documents

  1. ↑ Secondary walls of fiber and wood cells. In: Peter Sitte, Elmar Weiler, Joachim W. Kadereit, Andreas Bresinsky, Christian Körner: Strasburger - textbook of botany for universities. 35th edition. Spektrum Akademischer Verlag, Heidelberg 2002, ISBN 3-8274-1010-X . Pp. 95-96.
  2. ↑ Hans W. Heldt and Birgit Piechulla: Plant biochemistry . 4th edition. Spektrum Akademischer Verlag, 2008, ISBN 978-3-8274-1961-3 , p. 420-422 .
  3. ↑ a b c d e Blechschmidt . 2010, p. 67-79 .
  4. ↑ Blechschmidt . 2010, p. 84 .
  5. ↑ a b Wenderdel et al. 2012, p. 88 .
  6. ↑ Roffael 1994 . S. 243-244 .
  7. ↑ a b Verband deutscher Papierfabriken eV: Papierkompass 2009 ( page no longer available , search in web archives: PDF  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove it this notice. ).@1@ 2Template: Dead Link / www.vdp-online.de@1@ 2Template: Dead Link / www.vdp-online.de  
  8. ↑ Akonen et al. 1997; P. 34.
  9. ↑ E. Gruber: Alternative digestion process , module 14 from the lecture notes “Basics of Pulp Technology” in the “Paper Technology” course at the Karlsruhe University of Cooperative Education, as of 2011 (PDF). With detailed information on the individual procedures.
  10. ↑ a b c d e Patent DE19962411A1 from July 5, 2001, information on the state of the art at that time under description .

literature

  • Aki Ahonen, Ludwig Lehner: Environmentally compatible wood pulping processes. Scientific study on the subject of "Implementation of the newly developed environmentally friendly wood digestion process". Landwirtschaftsverlag, Münster 1997, ISBN 3-7843-2877-6 ( series of renewable raw materials 8).
  • J. Blechschmidt: Pocket book of paper technology. Carl Hanser Verlag, Munich 2010.
  • E. Roffael, B. Dix, G. Bär, R. Bayer: About the suitability of thermo-mechanical and chemo-thermo-mechanical wood pulp (TMP and CTMP) from beech and pine wood for the production of medium-density fibreboard (MDF) . In: Wood as a raw material . tape 52 , no. 4 , 1994, pp. 239-246 , doi : 10.1007 / BF02619102 .
  • C. Wenderdel, D. Krug: Investigation of the influence of digestion conditions on the morphological characteristics of TMP pulp made from pine wood . In: European Journal of Wood and Wood Products . tape 70 , no. 1–3 , 2012, pp. 85-89 , doi : 10.1007 / s00107-010-0487-x .