The Sulfite Process

The sequence is as follows, (a) Sulfur dioxide, made by burning sulfur or roasting pyrite (FeS2), is passed over wet limestone (CaCOs) until a total of 7% SO2 has dissolved  

Sulfite paper has a relatively short life span, since residual acid will continue to hydrolyze the cellulose and cause embrittlement. Further sources of acid include aluminum sulfate (which is added together with resin to suppress bleeding or feathering of ink into the paper) and SO2 and NOa, from the atmosphere. Much of the world s library collections and archives will soon be lost as the paper crumbles. Various deacidification treatments (e.g., with ammonia, morpholine, cyclohexylamine carbamate, or diethyl-zinc) have been proposed and tried, but at best they can only halt the process of embrittlement and cannot reverse it. With the move to kraft pulping, alkaline peroxide bleaching, and increasing use of precipitated calcium carbonate as a filler, the high quality papers produced today are intrinsically acid free and should also resist subsequent acidification by S02-polluted air fairly well. 

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At room temperature, the bisulfite pH inflection poiat occurs at pH 4.5 and the monosulfite at pH 9. Analogous equations can be written for magnesium, calcium, and ammonia. The starting raw materials, ia addition to sulfur, are sodium hydroxide, magnesium oxide, calcium carbonate, or ammonia, depending on the base used. The four commercial bases used ia the sulfite process are compared ia Table 4. 

Paper Products. Paper (qv) products account for about 2% of sulfur demand. The largest single segment of demand is in the manufacture of wood pulp by the sulfite process (see Pulp). In this process, the main sulfur intermediate is sulfur dioxide, which is generally produced at the plant site by burning elemental sulfur. Some sulfur dioxide, however, is produced as a by-product at smelter operations, purified andUquefied, and shipped to the pulp mills. The sulfur dioxide is converted to sulfurous acid, and the salt of this acid is a principal component of the cooking Hquor for the sulfite process. 

Laugemittel, n. leaching agent, laugen, .t. lye, steep in l.ye buck leach, lixiviate. — gelaugte Saure, (Paper) tower liquor to which has been added liquor from the cookers (in the sulfite process). 

Sulfite pulps have less color than kraft pulps and can be bleached more easily however, they are not as strong. The efficiency and effectiveness of the sulfite process is also dependent on the type of wood furnish and the absence of bark. For these reasons, the use of sulfite pulping has declined in comparison to kraft pulping over time. 

Attisholz A process for recovering valuable waste products from the sulfite process for making wood pulp. This includes methanol, cymenes, and furfural. Developed in Switzerland. 

SCA-Billerud A variation of the sulfite process for making paper from wood, in which the waste hydrogen sulfide is burnt to sulfur dioxide and used to make sulfuric acid. 

Writing paper was first used in Egypt as far back as 2500-2000 BC, made from the papyrus reed. Paper manufacture began in China about AD 105. In 1690 the first American paper mill began its operation. Two recent dates of importance to modem paper technology are 1867, when Tilghman in the U.S. developed the sulfite process, and 1884, when Dahl in Germany discovered the kraft or sulfate process. 

The sulfite process for delignification of wood in the paper industry involves heating with acidic sulfite solutions at elevated temperatures under pressure. Under such conditions, oxidation occurs to a certain extent, and presumably, the sulfite is partly reduced to sulfide. The composition of the liquors resulting from the treatment of cellulose and hemicelluloses, and alditols with sulfite in acid medium has been described (Ref. 1, p. 1150). 

Extractives sometimes prevent or inhibit the utilization of wood. For example, woods containing phenolic type extractives cannot be pulped via the sulfite process. The so-called "pitch troubles" in the pulp and paper industry result from the tendency of the resin type extractives to coagulate and adhere to metal and fibrous surfaces. Also the presence of extractives result in a higher consumption of pulping chemicals and in lower pulp yields. 

There are many processes and variations of basic processes that can be used for making pulp from wood. The major processes are the kraft process (also known as the sulfate process), the groundwood and thermomechanical process, semichemical process, and the sulfite process. The kraft process remains dominant. 

Sulfur dioxide is used for refrigeration and also serves as raw material for the production of sulfuric acid. It is also used as a bleaching agent in the textile and food industries. It is an effective disinfectant and is employed as such for wooden kegs and barrels and brewery apparatus and for the prevention of mold in the drying of fruits. Sulfur dioxide efficiently controls fermentation in the making of wine. It is used in the sulfite process for paper pulp, as a liquid solvent in petroleum refining, and as a raw material in many plants in place of sulfites, bisulfites, or hydrosulfites. 

The sulfite process for paper-making produces a certain amount of wood sugars which cannot be sewered because of pollution laws. Fermentation permits recovery of the sugar as alcohol and single cell protein. A commercial plant is in operation. 

Because calcium was long used almost exclusively in the sulfite process as the base, no need existed for the recovery of this comparatively cheap chemical. Interest was therefore only directed toward the relatively simple recovery of excess sulfur dioxide. However, the introduction of the more expensive soluble bases, together with more stringent environmental requirements, stimulated the development of methods for the recovery of both heat and inorganic chemicals (base and sulfur). 

The kinetics of the delignification are of importance especially when considering the control of the pulping process. Since kraft pulping follows simpler kinetics than the sulfite processes, more applications have been adopted for this case. Because of the heterogenity of the system, however, pulping reactions are complicated and can therefore not be treated in the same fashion as homogeneous reactions in solution. 

Sulfite Process. The sulfite process uses a cooking liquor of sulfurous acid and a salt of the acid. Although calcium was the most widely used base at one time, it has been supplanted by sodium, magnesium, and ammonia. Lignin will react with the bisulfite ion (HSO3") under acidic conditions to form lignosulfonates that are soluble in water. For many years this was the preferred process because it produced pulps of light color that could easily be bleached, it used cheap chemicals in fairly limited amounts so that no recovery was necessary, and it was a relatively simple process to operate. 

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