Preparation of modified satin white pigment comprising zinc cation

摘要

Process for the preparation of a needle shaped pigment, of mean particle size of from 0.2 to 0.4 microns for coating paper, comprises: (i) continuous admixture of respective aqueous solutions of aluminium sulphate and zinc sulphate to an aqueous suspension of calcium hydroxide, wherein: (I) said solutions and said suspension have a concentration of from 5 to 9% by weight (wherein the concentration of aluminium sulphate is based on alumina) and are at a temperature of from 35 to 45 degrees C.; (II) the mole ratio of zinc oxide to calcium oxide is from 0.04:1 to 0.25:1; (III) the mole ratio of zinc oxide plus calcium oxide to alumina is from 6.5:1 to 7.5:1; (ii) ageing the resulting suspension, preferably for from 2 to 8 hours at from 57 to 63 degrees C. The aged suspension may be dehydrated to produce a cake, to which polyacrylic acid is added, which cake is then dried and pulverised.