Process for producing alkali metal bisulphite or alkali metal sulphite solutions from waste liquors obtained by digesting pulp with alkali-sulphur compounds

摘要

Solutions of alkali metal sulphites or bisulphites are obtained from waste liquors resulting from the digestion of pulp with sulphate or sulphite lyes. Organic compounds are first removed from the liquors by combustion and sulphides and thiosulphates removed in known manner as by roasting. The residue, comprising sodium carbonate, sulphite, and sulphate is dissolved in water and divided into two predetermined parts. To the one part is added sulphurous acid and calcium carbonate sufficient to convert all alkali metal salts into bisulphites. The other part of the solution is then added to precipitate soluble calcium salts from the treated solution with simultaneous conversion of its own alkali metal content to sulphite or bisulphite. An example is furnished. The final solution obtained, after dilution if necessary, may be used for digesting pulp.