Acid sulfite process

Acid soak cleaners, for electroplating, 9 783 Acid soap, 22 728 Acid soluble dyes, 7 373t Acid solutions hydrazine, 13 587 in leaching chemistry, 16 152 Acid sulfite process, 23 540-541 Acid sulfite pulping, 21 22-23, 26 Acid treatment, in petroleum refining,... 

Figure 3.10 Relative amounts of lignin and carbohydrate removed during the Kraft and acid sulfite processes.

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... 

Generally, no cellulose is lost in the acid sulfite process. The losses of hemicelluloses are higher for hardwood than for softwood. Typical yield values for various wood constituents have been collected in Table 7-3 in which comparative data for the kraft process are also shown. 

For calcium-base liquor, the gas is passed through towers packed with limestone with water flowing down through the tower. Because of the limited solubility of calcium bisulfite [Ca(HS03)2], the pH of the liquor is very low (about 2), and free sulfurous acid is present. This usually is called the acid sulfite process. As mentioned before, calcium-base mills have essentially disappeared in the United States. 

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. 

Wood Pulps. Black spruce (Picea mariana) shavings were extracted with alcohol-benzene and a small portion ground in a Wiley mill to pass through a 40-mesh screen. This represented the 100% yield sample. The remaining shavings were cooked in one liter bombs by the acid sulfite process (sodium base, 1% combined and 5% total S02) at a liquor-to-wood ratio of 5 1. The time at the cooking temperature of 135 °C. was varied from 0 to 300 minutes, the contents removed, fiberized briefly in a Waring Blendor and washed with distilled water. Half of each sample... 

Similar difficulties would be encountered using the acid sulfite process on other woods containing stilbenes or polyphenols such as proanthocyanidins containing resorcinol of phloroglucinol moieties. For example, dihydroquercetin has been identified as the cause of inhibition of calcium base sulfite pulping of Pseudo-tsuga menziesii (138). 

Spent or black pulping liquors contain 10<7o to 20 o solids and must be concentrated to a solids content exceeding 50% in order to maintain burning in recovery furnaces. The liquor from the acid sulfite process requires equipment that will resist corrosion and be unaffected by scaling, but extractives in the liquor do not affect concentration or burning. 

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. 

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