Kraft process

Sulphate process or kraft cooking ( so called because of the strength of the paper produced in this way, see Kraft ) is called a chemical- industrial process for the production of cellulose from the wood of trees or from annual plants such as reed, cereals ( straw), sugar cane ( bagasse ), maize or sunflower ( stalks ). The cell walls are digested there and the lignin contained in the plant material and polyoses are separated thereby. The resulting pulps may serve well as reinforcing fibers. The sulphate process, the wood chips are cooked in several hours in sodium hydroxide solution, is the most common process for production of paper pulp.

Method

The sulfate or kraft process chips or pulverized plant stems in pressure vessels are heated for three to six hours at elevated pressure ( 7-10 bar) with essentially caustic soda, sodium sulphide and sodium sulphate. In this case the lignin is a nucleophilic attack by the sulphide anion and is split into so-called black liquor ( soluble alkali lignin ) above, which is then separated by means of cell filters of the remaining pulp. When boiling and evaporating the lye malodorous thiols develop.

Preliminary processes

By the kraft digestion upstream hydrolysis, the proportion of polyoses can be reduced and a pulp are obtained with very high cellulose content.

Secondary processes

From the waste liquors of sulfate process itself can sometimes even more useful chemicals and by-products from the waste liquors win as the competing sulfite. In the processing of resinous wood (eg pine) falling per 1,000 kg of pulp at 30 kg tall oil and turpentine sulfate and various wood sugar. The resins may be processed into paper sizing agent to phenolic resins, dispersing agents and so on. From the fatty acids themselves alkyd resins, surfactants or plasticizers can be produced.

The by-product sodium lignin can be used for the production of plastics, and black liquor residues are processed into fertilizers. Therefore, the sulfate process offers economic advantages.

Process improvements

By skillful combination of process steps (eg burning of sulfate waste liquor ), the chemicals used can be largely win again. The addition of anthraquinone, which accelerates the cleavage of lignin as a catalyst can improve the energy balance of the process.

Properties and Areas

With a low use of sulfur, it is possible with this method, of inferior, resinous wood or of residues of annuals a pulp with good technical properties of paper manufacture. Both recycled fibers and wood fibers from the sulfite process is usually enough not to achieve increased paper strength, therefore sulphate pulp is used with a reinforcing fiber material, for example to produce paper bags, paperboard, technical abrading sheets, kraft paper and Erntebindegarn.

The information only about half of the wood raw material employed is obtained as the major product directly in the form of the pulp. The efficiency of the method is increased if the by-products are recycled.

Market significance

In the production of pulp for paper manufacture is worldwide ( 2009) mainly use this method. Around 85 % of electricity consumed in Germany pulp is obtained in the sulfate process, this in 2008 were 3.7 million tonnes of kraft pulp. The sulfite is used for only about 15% of the electricity consumed in Germany pulp. However, in Germany more sulfite pulp is produced. This is because the sulphate process earlier brought a very intense odor with it and usually drew the short straw in Germany due to strict conditions.