GB848617A - Improvements in or relating to the preparation of anhydrite - Google Patents

Abstract

Anhydrite, having a particle size of 0.4 to 0.8 microns, is prepared by forming a slurry of gypsum of size 200 mesh (United States Standard) or less, and containing from 20-50% solids, adding between 5 and 50% of the slurry with rapid agitation, to a sulphuric acid of 74-80% concentration at a starting temperature below 70 DEG C., the slurry being added to the acid at a rate of from 0.0125 to 1.0 part calculated as CaSO4 for each part of sulphuric acid calculated as 60 degrees Baum<\>e acid per minute, and maintaining the temperature between 20 DEG C. and 80 DEG C.; adding the remainder of the slurry, and heating to between 80 DEG C. and boiling to convert all the gypsum to anhydrite. Preferably the acid concentration at the end of the first addition is between 20 and 50%. The gypsum slurry may be prepared by treating lime or limestone of less than 200 mesh, which preferably contains less than 0.1% of colour-degrading impurities, with waste sulphuric acid, which may contain between 0.1 and 2.0 grams of suspended titanium dioxide per litre, to give a slurry with an acid concentration of between 1 and 100 grams of sulphuric acid per litre. The anhydrite product may be used in the preparation of anhydrite-titanium dioxide pigments, for example by the coprecipitation method.