GB755948A - Improvements in or relating to the crystallization of calcium sulfate - Google Patents

Abstract

Waste sulphite liquor is subjected to alkaline hydrolysis with an alkali metal hydroxide solution in the presence of carbon dioxide, or with alkali metal carbonate solution to precipitate calcium carbonate and form alkali metal lignin sulphonate which is treated with an alkali metal hydroxide solution at a pH of 13 by heating under superatmospheric pressure for a time sufficient to effect hydrolysis of some sulphonic acid groups and methoxy-substituted aromatic groups of the lignin residue and also to effect carboxylation along the aliphatic chain. The latter effect may be augmented by introducing air during hydrolysis. The sodium salt is preferred but potassium, lithium, rubidium and c sium salts are referred to. The products are employed in the prevention of calcium sulphate scale deposition (see Groups III and XVIII).ALSO:The crystal habit of calcium sulphate, crystallized from concentrated aqueous solutions of calcium chloride which may contain sodium or ammonium chloride and which are substantially saturated with calcium sulphate, is altered and scale formation prevented by adding to the solutions a small amount, preferably 10 to 50 p.p.m., of an alkali metal salt of a hydrolysate obtained by the alkaline hydrolysis of lignin sulphonic acid, the hydrolysate having carboxyl groups and at least some residual sulphonic groups in the molecules thereof, and thereafter subjecting the aqueous solutions to agitation. The process is particularly applicable to the prevention of calcium sulphate scale deposited from concentrated calcium chloride solutions in the ammonia soda process for the production of sodium carbonate or in the production of magnesia and calcium chloride by reacting calcined dolomite with aqueous ammonium chloride solution. The sodium salt of the hydrolysate is preferred, and may be obtained by the alkaline hydrolysis of waste sulphite liquor (see Group II). According to examples, (i) burnt dolomite containing less than 5 per cent. CO2 was reacted with ammonium chloride solution and the resulting calcium chloride solution separated from the magnesia. The sodium salt of the hydrolysate was added to the discharge line from the reaction vessel in a concentration of 15-30 p.p.m., and the solution passed through a pipe to an ammonia distiller at 40 to 45 DEG C.; no deposition of calcium sulphate occurred in the line or the still. (iii) in the ammonia soda process wherein the ammonium chloride solution resulting from the reaction is passed to a pre-limer wherein milk of lime is added to evolve ammonia and form calcium chloride and the resultant liquor is passed to ammonia still, where it is heated at 75 to 100 DEG C., the sodium salt of hydrolysate is added to the discharge pipe from the pre-limer in a concentration of 20-35 p.p.m. Scale deposition in the discharge line and the ammonia still is thereby prevented. Solutions discharged from the pre-limer normally contain a 100 to 130 gms. per litre of calcium chloride with additional amounts of sodium chloride, and have a sulphate ion concentration of 0.9 to 1.8 grams per litre, that is approximately the saturation amount of calcium sulphate in the solutions concerned.