Abstract: The application is concerned with a process for the manufacture of a titanium dioxide pigment from a titanium sulfate solution by hydrolysis in the presence of nuclei. The latter may be formed externally from any suitable source of titanium sulfate solution and preferably, but not exclusively, from a portion of the titanium sulfate solution used in forming the hydrous TiO.sub.2 ; the nuclei being prepared by mixing the titanium sulfate solution, from whatever source, simultaneously with an alkaline reacting solution and subsequently curing the mixture at elevated temperature. In the present application "titanium sulfate solution" is understood to be a solution which may also contain other substances besides titanium sulfate; and may be obtained, for example, by digestion of titaniferous ores or slags using sulfuric acid, followed by dissolution of the resulting digestion cake.

Abstract: Titanium sulphate solutions obtained in the sulphate process from titaniferous materials are hydrolized by contacting at 88.degree.-98.degree. C water with a first titanium sulphate solution having a TiO.sub.2 titer of 230-260 g/l and an acid factor of 1.75-1.85, adding at 88.degree.-98.degree. C to the mixture a second titanium sulphate solution having a TiO.sub.2 titer of 230-260 g/l and an acid factor of 2.1-2.4 and heating the resulting mixture at boiling point. A volumetric ratio of from 3:100 to 12:100 is maintained between the first and the second solution.

Abstract: In the recovery of titanium dioxide by subjecting an aqueous titanium dioxide hydrate suspension to at least one stage of filtering and washing to form a washed filter cake, adjusting the composition and calcining the adjusted and washed filter cake, the improvement which comprises filtering the aqueous titanium dioxide hydrate suspension under a pressure of about 1 bar to 16 bars to form a homogeneous substantially crack-free filter cake of a thickness of about 10 to 25 mm. Advantageously a membrane filter is employed, and washing is effected under a pressure of 1 to 16 bars and a liquid flow rate of 0.1 to 1 m/h to produce a filter cake having a solids content of at least 45 % by weight.