Sulfite process

The sulfite is developed in the 19th century, chemical- industrial process for the production of cellulose from wood. It is suitable only for resin-and silica- poor wood.

Method

When sulfite, depending on the cation used ( calcium, magnesium, sodium, ammonium ), the wood in the acidic or neutral medium are digested ( pulping ). In the process, the lignin is subjected to reduction to sulfonation molecule and is thereby converted into a water soluble salt of lignosulfonic acid, which can be removed easily from the fiber. Depending on the pH value present in the wood hemicelluloses either by acid hydrolysis to sugars ( such as D- mannose, D-glucose, D-xylose, L- arabinose, D-galactose, L- rhamnose ), and go to be converted into the aqueous phase above, or remain with the cellulose in the fiber. The thus prepared, very bright pulps are now used almost exclusively for the production of chemical cellulose or paper. The lignosulfonates formed as a by- product can be either recycled or burned commercially ( recovery).

To produce 1000 kg pulp takes about five cubic meters of wood and 90 kg sulfur.

Ca bisulphite method

The debarked, mechanically shredded into wood chips wood is heated in pressure cookers with Calciumhydrogensulfit seven to 15 hours at elevated pressure ( five to seven bar). The wood chips are soft, so they can crush easily when cooking. Then they are reduced to two to four millimeters long fibers, washed repeatedly, and bleached if necessary, before they are dried in the form of thicker paperboard. The calcium bisulfite was formerly obtained by means of an acid tower. It filled up limestone in the tube and sprinkled him with water, while sulfur dioxide is attached below.

Pulps are prepared by the calcium bisulfite method are particularly pure and are therefore preferably used for chemical applications. As the base can not be recycled, in contrast to other methods in the Ca bisulphite method, this is not very economical, as the lignin is not utilized. Today there is very little used for Sulfitzellstoffherstellung.

Magnesium bisulfite process

The magnesium bisulfite works similar to that of calcium, however, it is continuously applied. The base can be easily decomposed into magnesium oxide and sulfur dioxide, and therefore provides a good chemical recovery. Accordingly, the resulting magnesium lignosulfonates are primarily burned ( energy plus chemical recycling). Only in the absence of incineration capacity they are otherwise used. Often, however, a small part of the raw liquor is discharged to an accumulation of impurities which come out of the wood (eg heavy metal ions) to limit the process.

Secondary processes

Frequently, the wastewater is with its high wooden sugar content for the production of ethanol used ( liquor or brandy Sulfitsprit ). This is especially possible in the pulping of softwoods. Often today productions for the treatment and recycling of lignosulfonate are connected.

History

The sulphite process was invented in 1866 by the American Benjamin Tilghman (U.S. Patent 1867). 1874 there was a dispute as Alexander Mitscherlich claimed to have invented this technique in 1874 and placed on an industrially usable framework. However, the Supreme Court did not base this claim in 1884. 1879 was the first in Löhnbergerhütte operating according to the method of Mitscherlich factory; almost simultaneously established the inventor himself, together with his brother a factory in Hann Munden. One also formed at the end of the 1870s factory in Sweden was based on the use of Magnesiumsulfitlauge as Carl Daniel Ekman had described. In the manufacture of paper has come up with the sulfate process a more competitive pulping process developed. About 15 percent of the electricity consumed in Germany pulp obtained in the sulphite process, these were in 2008, 723,000 tons of sulphite pulp.

Relevance

In particular, from spruce wood is predominantly produced dissolving pulp with this method to date, arising from the viscose and cellulose derivatives such as cellulose ethers and esters. The significance of this method is mainly due to the very efficient delignification which readily bleachable, the high reactivity during further processing (with the same degree of purity ) and low investment costs. Sulfite pulps can be produced in high purity levels when subsequently process steps are carried out in the basic range ( hot and cold alkali refining ).