Spent sulfite liquor

The remaining pair of acidolysis products isolated from Sephadex fraction B, as already mentioned, consisted of 4,4 -dihydroxy-3,3 -dimethoxystilbene (XXV) and 1,2-bisguaiacylpropanone (XXVI). The stilbene (XXV) was isolated several years ago from sulfite-spent liquor which had been heated with alkali (37) as well as from alkali-heated wood (J7, J9, 40) it has also been detected in kraft liquor (10). The structure of the previously unknown ketone (XXVI) (m.p., 121-122°C.) was derived from analytical and spectral data and established by synthesis (30). 

Since the cost study of the process using a two-stage desulfonated, de-ashed lignin showed that this lignin was too expensive, we decided to use a less costly lignin starting material. We decided to use either a concentrated, desugared sulfite-spent liquor or the dried solids from such a liquor. 

In addition to lignosulfonates and hemicelluloses and their degradation products, sulfite spent liquors contain small amounts of uronic acids, methyl... 

TABLE 7-5. Typical Composition of the Sulfite Spent Liquor Resulting from the Acid Sulfite Pulping of Norway Spruce... 

A furnace similar to the Tomlinson kraft recovery furnace is used for the combustion of magnesium-based sulfite spent liquors. In this case, however, no smelt is obtained instead the base is completely recovered as magnesium oxide in dust collectors the sulfur escapes as sulfur dioxide and is absorbed from the combustion gases in scrubber towers. However, because magnesium hydroxide has a very low solubility in water, a complete recovery of sulfur dioxide meets difficulties. 

Ammonium-based sulfite spent liquors can be burned in the same type of furnace as the calcium-based liquors. However, during combustion the base is decomposed to form nitrogen and water and the problems with fly ash are thus eliminated. All sulfur escapes to the combustion gases as sulfur dioxide which can be partly absorbed in an ammonia solution. 

The majority of the material in sulfite spent liquors originates from lignin (lignosulfonates) and hemicelluloses (Tables 7-5 and 10-1). A variety of useful products can be isolated and prepared from these liquors (Table 10.-2)... 

TABLE 10-2. Main By-Products Based on the Sulfite Spent Liquor and Condensates... 

In principle, the same carbohydrates and their degradation products formed after hydrolysis of wood can be recovered from sulfite spent liquors. However, this requires complicated and expensive separation techniques. The industrial use of sulfite spent liquor components is mainly limited to fermentation processes. The most common product is ethyl alcohol which is produced from hexose sugars by yeast (Saccharomyces cerevisae) and separated from the mixture by distillation. Even the carbon dioxide formed in the process can be recovered. Other fermentation products, including acetone, n-butanol, and lactic acid, can be produced by certain microorganisms. Because some contaminants, for example, sulfur dioxide, inhibit the growth of the yeast, they must be removed from the liquor prior to the fermentation. 

In principle, monosaccharides and their conversion products including furfural can be isolated from sulfite spent liquors. Because of the complicated separation technique needed and since alternative raw material sources, such as wood and agricultural wastes are available, these processes have so far been of very limited practical interest. Because of their carbohydrate content, sulfite spent liquors find use either directly or after some fractionation as a feed component for cattle. 

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