Pulping, alkaline sulfate process

The process of pulping, degrading the lignin to a more soluble form so the cellulose fibers can be separated from it, involves some interesting chemistry. The kraft or alkaline sulfate process dominates this part of the industry. Approximately 78% of all pulp is made by the kraft process, 3%... 

After the crude pulp is obtained from the alkaline sulfate process, it must be bleached in stages with elemental chlorine, extracted with sodium hydroxide, and oxidized with calcium hypochlorite, chlorine dioxide, and... 

The recent studies of the Finnish group further emphasize the complexity of the xylan in wood cellulose. These workers have shown that the properties of the xylan in wood pulps depend not only on the method of pulping but also on the type of raw material. It was observed that, when pulps are prepared by the (acidic) sulfite process from the softwoods, pine and spruce, a substantial part of the xylan is of the acidic type (see Fig. 2, p. 303). Even when these sulfite pulps are refined further with hot alkali, some acidic xylan remains. However, when pulps are prepared from pine and spruce by the (alkaline) sulfate process, the acidic xylan is removed and only neutral xylan remains. 

The alkaline sulfate process uses sodium sulfide and sodium hydroxide as pulping chemicals. The acidic sulfite process is based on calcium, magnesium, and sodium or ammonium bisulfide. Worldwide, the sulfate process is used in up to 90% of cases. 

The pulp and paper industry and potable and wastewater treatment industry are the principal markets for aluminum sulfate. Over half of the U.S. aluminum sulfate produced is employed by the pulp and paper industry. About 37% is used to precipitate and fix rosin size on paper fibers, set dyes, and control slurry pH. Another 16% is utilized to clarify process waters. The alum sold for these purposes is usually Hquid alum. It is frequendy acidic as a result of a slight excess of H2SO4. Aluminum sulfate consumption by the pulp and paper industry is projected to remain constant or decline slightly in the near term because of more efficient use of the alum and an increased use of alkaline sizing processes (13). 

Kraft Process. The dominant chemical wood pulping process is the kraft or sulfate process. The alkaline pulping Hquor or digesting solution contains about a three-to-one ratio of sodium hydroxide, NaOH, and sodium sulfide, Na2S. The name kraft, which means strength in German,... 

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 S02 and NO 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.14 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. 

Schoettler has studied the alkali resistance of the xylan in pulps made from aspen wood. Verifying and extending previous work by March, he" observed a higher content of xylan resistant to 17.5% sodium hydroxide in aspen alkaline pulps (sulfate or soda process) than in aspen, chlorite holo-cellulose. Further chlorite treatment of an alkaline pulp did not reduce its xylan or resistant-xylan content. Alkaline (sulfate) cooking of the holo-cellulose did not increase the resistant-xylan content. These results are summarized in Table X. 

When certain softwoods are pulped by the sulfite process, the resulting pulp usually contains a considerable proportion of uronic acid anhydride which cannot be removed entirely in subsequent alkaline refining. When the sulfate process is used on softwoods, a pulp which is free from uronic acid anhydride may be obtained. This important difference between the sulfite and sulfate pulps from softwoods does not apply to hardwoods, which, when pulped by either process, still retain a considerable proportion of uronic acid anhydride in the final product. 

Chem. Descrip. Phosphate-based high activity multicomponent formulation with acrylate dispersants and proprietary sequestrants Uses Scale control agent, dispersant for alkaline deinking systems in papermaking keeps washer and extractor surfaces clean in pulping and bleaching processes and in heal exchangers and scrubber systems Features Provides protection from scale in the presence of both temp, and pH shock effective on calcium carbonate and calcium sulfate scale Properties Liq. 

Chemical pulping has been performed or proposed with a wide variety of reactants. Today the dominant chemicals used in pulping are sulfur based, although numerous sulfur-free processes have been proposed. The processes available currently include sulfate or Kraft pulping, acid and alkaline sulfite pulping, neutral sulfite semichemical... 

Himberg K, Pyysalo H, Paallysaho A, et al. 1987. Composition of sulfiirous exhaust gases from sulfate and semi-alkaline pulping (SAP) processes of a pulp mill. Atmos Environ 21 1671 -1674. 

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