Pulping, alkaline sulfite process

Wood chips are processed in mild chemical liquor and subjected to mechanical refining using disc refiners. Semichemical pulping liquors have variable composition ranging from sodium hydroxide alone, alkaline sulfite (sodium sulfite - - sodium carbonate), mixtures of sodium hydroxide and sodium carbonate, to Kraft green or white liquors [3]. Sodium sulfite/ sodium carbonate liquor is most commonly used and the pulp product obtained thereafter is referred to as neutral sulfite semichemical (NSSC) pulp. 

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. 

In contrast to the highly alkaline (pH 11-13) krafl processes, sulfite pulping processes are acidic in nature and are of two general types (1) the acid sulfite processes utilize caldnm, sodium, magnesium, or ammonium bisulfite in combination with tree or excess sulfur dioxide as... 

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... 

Black, N. R, ASAM Alkaline sulfite pulping process shows potential for large-scale application. Tappi J1991, 74 (4), 87-93. 

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. 

Sodium hydroxide appears to be the most important alkali employed in commercial operations, and two general methods for its use are of importance. One method employs dilute, sodium hydroxide solutions at elevated temperatures, and the other uses concentrated solutions at low temperatures. In the hot, alkaline refining process, temperature, time, concentration of alkali, and consistency of pulp may be varied over wide limits. The literature contains only a limited amount of data showing the effect of alkaline treatment on sulfite-pulp composition. However, from the available data, certain principles are apparent. 

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. 

Sulfite pulping is a process that uses mildly acidic sulfite solutions instead of the very alkaline solutions used in kraft pulping. Murarka et al. [201] published results of corrosion tests in acidic condensates in vapor phase sulfite pulping using a batch type laboratory reactor in which acid was condensed and collected. He devised a means to prevent corrosion due to the acid condensation. 

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. 

Solvent-based pulping (Organosolv) is still in a very early stage of industrial development. In this chemical pulping method the dehgnification of the biomass (usually wood) is done in an organic solvent or solvent plus water system. There are a variety of processes, which use different solvents e. g. ethanol, methanol, acetic acid, formic acid, often in combination with sodium hydroxide, alkaline sulfite, and/or anthraquinone. 

The two main procedures are the Kraft process and the sulfite process. Kraft cooking (alkaline) is the parent chemical pulping method globally. Sulfite process (acidic) makes the lignin water-soluble. Chemical pulp fibers are more flexible than mechanical pulp fibers. Chemical pulps give paper good strength qualities. 

Various permutations of the black liquor recovery boiler design are used to combust soda liquors in the soda pulping process and alkaline earth liquors and ammonium liquor (red liquor) in the sulfite pulping process. 

Although acidic pulping methods have largely been displaced over the past 50 years by neutral and alkaline processes, there is still a significant amount carried out. Acid sulfite pulping uses combinations of sulfur dioxide and water at high temperatures and pressures. An appropriate base is used to control the pH and, although usually acidic, it is possible to perform these reactions at neutral or even alkaline pH. The most active nucleophile present is the bisulfite ion,... 

There are a number of full-scale activated sludge plants that are in operation in countries such as the United States, Canada, and Finland, which treat effluents from Kraft, sulfite, TMP, CTMP, and newsprint mills [22]. SchneU et al. [70] reported the effectiveness of a conventional activated sludge process operating at an alkaline-peroxide mechanical pulping (APMP) plant at Malette Quebec, Canada. The full-scale plant achieved 74% reduction in filterable COD and nearly complete elimination of BOD5, resin acids, and fatty acids in the whole mill effluent. The treated effluent tested nontoxic as measured by a Microtox assay. Saunamaki [71] reported... 

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