Calcium-based spent liquors

Ammonia and ammonium persulfate can also be used as reagents (21). However, as pointed out by Nimz (48) and other authors, owing to the non-uniformity of the product, only 15% of urea-formaldehyde and 25% phenol-formaldehyde resin binder may be replaced by calcium base spent sulfite liquors. 

Substitution of potassium ferricyanide for 1% of sulfur dioxide dissolved in calcium-base spent sulfite liquor was possible at a pH of about 2.0. An increase in pH stabilized the cross-linking mixture unless 4% of ammonium chloride (dry wood basis) was added. In this case particle board could be produced at a pH of 4.5, with properties equivalent to those produced at a pH of 2.0, i.e., with IB values of up to 82.0 psi and acceptable water resistance (JOS, 109). 

Many other recovery alternatives have been proposed that iaclude ion exchange (qv), pyrolysis, and wet combustion. However, these have not gained general acceptance. A limited number of calcium-based mills are able to utilize their spent pulpiag liquors to produce by-products such as lignosulfates for oil-weU drilling muds, vanillin, yeast, and ethyl alcohol (see PETROLEUM Vanillin). 

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 requirements for water pollution control have been forcing a shift to concentrating spent sulfite pulping liquor by evaporation, followed by incineration with heat recovery (102). When calcium-base liquor is burned, the sulfur emerges as calcium sulfate and is not available for recycle to the pulping process. The flue gas from such furnaces in Sweden is reported to contain 0.2—0.3% sulfur dioxide, and in one Swedish mill a Bahco wet limestone scrubber is used to treat the gases (115). 

Black Liquor, Spent Sulfite, Calcium Base... 

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