Sulphate process (paper production)
Sulphate process or Kraft digestion (so called because of the strength of the paper produced in this way, see Kraft paper ) is a chemical-industrial process for the production of cellulose from the wood of trees or from annual plants such as reeds , grain (straw), sugar cane ( bagasse) ), Corn or sunflower (stems). The cell walls are opened up and the lignin and polyoses contained in the plant material are separated off. The resulting pulps can be used as reinforcing fibers . The sulphate process, in which wood chips are cooked in caustic soda for several hours, is the most common process used to make paper pulp.
It was invented in 1879 by Carl Ferdinand Dahl (Danzig), who also gave it the name (Kraft), and filed for a US patent in 1884. In 1890 the first factory used the process in Sweden.
Procedure
In the sulphate or Kraft process, wood chips or crushed plant stems are heated in pressure vessels for three to six hours at elevated pressure (7 to 10 bar ) with mainly sodium hydroxide , sodium sulphide and sodium sulphate . The lignin is split by a nucleophilic attack by the sulfide anion and turns into so-called black liquor (soluble alkali lignin), which is then separated from the pulp that remains with the help of cell filters. When the lye is boiled and evaporated, malodorous thiols develop .
Pre-litigation
A hydrolysis upstream of the Kraft digestion can reduce the proportion of polyoses and produce a pulp with a very high cellulose content.
Side processes
From the liquor from the sulphate process z. Sometimes even more useful chemicals and by-products are obtained than from the liquors from the competing sulphite process . In the processing of resin-rich woods (. Eg pine ) 30 kg fall to 1000 kg of pulp tall oil and sulfate - turpentine and various wood sugars on. The resins can also be processed into paper sizing agents , phenolic resins , dispersing agents , etc. Alkyd resins , surfactants or plasticizers can be produced from the fatty acids .
The by-product sodium lignin can be used in the production of plastics and black liquor residues can be processed into fertilizers . The sulphate process therefore offers economic advantages.
Process improvements
The chemicals used can largely be recovered through a clever combination of process steps (e.g. burning the sulphate waste liquor). The addition of anthraquinone , which acts as a catalyst and accelerates the cleavage of the lignin, can improve the energy balance of the process.
Properties and areas of application
With a very small amounts of sulfur it is possible with this method, from low grade, high-resin woods or from the remains of annual plants a pulp with good paper technology to produce properties. Both recycled fibers and wood fibers from the sulphite process are usually not sufficient to achieve increased paper strength, which is why sulphate cellulose is also used as a reinforcing fiber in order to produce, for example , paper sacks , cardboard , technical emery paper , kraft paper and harvesting twine.
During digestion, less than half of the raw wood used is obtained in the form of pulp. The intensity of the digestion cannot be increased indefinitely, since the selectivity decreases sharply with increasing use of chemicals or higher temperatures or cooking times. Under the strongly alkaline conditions there is an increasing degradation of the cellulose polymer from its ends (peeling reaction). The typical remaining lignin content before bleaching is therefore, expressed as the kappa number , below 30 for softwood sulphate pulp and below 20 for hardwood sulphate pulp. Lower degrees of digestion are usual for viscose pulp, with a correspondingly negative effect on the yield, but purer cellulose. For cellulose for packaging material that is not bleached, a kappa number of> 60 and a yield of> 50% is possible. Mechanical defibration of the hardly overcooked wood chips may then be necessary. The economy of the process is increased if the by-products are recycled.
Market importance
This process is mainly used worldwide (2009) in the production of pulp for paper manufacture. Around 85% 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% of the pulp used in Germany. However, more sulphite pulp is produced in Germany. This is because the sulphate process has a very unpleasant smell because of the thiols formed and a great deal of effort is required for odor control, such as suction and return of all gaseous emissions to the combustion boiler.
Web links
- US Federal Environment Agency on the sulfate process (PDF file; 264 kB)
- Available techniques in paper and pulp production ( Memento of July 15, 2012 in the Internet Archive ) (European Commission, 2001)
literature
- Entry to cellulose. In: Römpp Online . Georg Thieme Verlag, accessed on June 20, 2014.
- Erich Gruber: Paper chemistry - chemical basics of pulp and paper production
- T. McDonough: Kraft Pulp Yield Basics, 1998 Breaking the Pulp Yield Barrier Symposium Proceedings . In: HU Suess (Ed.): Pulp Bleaching Today . de Gruyter, Berlin / New York 2010, ISBN 978-3-11-020737-8 , pp. 48 .
Individual evidence
- ↑ Klaus Beneke: Benjamin Chew Tilghman and on the history of paper and its raw materials, University of Kiel, pdf
- ↑ Pulp and Paper Production: the Kraft process overview, Chemical Engineering Guide ( Memento of the original from April 22, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ 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 this notice . )
