GB577970A - Improvements relating to the purification of calcium sulphate - Google Patents

Abstract

577,970. Calcium sulphate. KUBELKA, P. Dec. 29, 1938, No. 37789. [A Specification was laid open to inspection under Sect. 91 of the Acts, June 30, 1939]. [Class 1 (iii)] Calcium sulphate of natural or artificial origin is purified by dehydration to as far as the hemihydrate at least, and subjection in contact with a solution of electrolytes which accelerate catalytically the mutual transformation of the various forms of calcium sulphate, to slow crystallisation to the d:hydrate, whereupon the impurities precipitated by the crystallisation are mechanically separated from the pure gypsum. The initial dehydration may be carried out by dry methods but is preferably carried out in the solution of the electrolyte used for crystallisation. The hemihydrate may be produced by boiling crude gypsum in the solution whilst insoluble anhydrite may be produced by heating crude gypsum in the form of soluble anhydrite or partially dehydrated gypsum with seed material of insoluble anhydrite or hemihydrate, in the solution of the electrolyte at relatively high temperatures preferably above 80‹C. The suspension of hemihydrate is allowed to stand but that of insoluble anhydrite is rapidly cooled to about 40‹C. for crystallisation. Crystallisation normally takes 12 to 24 hours part:cularly in the presence of previously formed coarse dihydrate or plaster of paris. If the solution is disturbed the period is greatly increased with the formation of fine needle shaped crystals which are also formed when crystallisation is too rapid. These crystals may be transfored to the desired coarse form by the addition of ammonium salts to the solution. Suitable solutions for dehydration and crystallisation are 1 to 10 per cent. or more solutions of, sulphuric, hydrochloric or other mineral acids, alkali sulphates, ammonium chloride or sulphate, or sodium chloride. The crude gypsum may be freed from coarse impurities such as sand by a preliminary washing.