PROCESS FOR RECOVERING PIGMENTARY TITANIUM DIOXIDE FROM NONPIGMENTARY WASTE TITANIUM DIOXIDE

1,140,029. Titanium dioxide. PPG INDUSTRIES Inc. 28 Feb., 1966 [18 March, 1965], No. 8601/66. Heading C1A. The tinting strength of non-pigmentary TiO 2 produced by vapour phase oxidation of Ti halide is increased by screening a slurry of the TiO 2 having a pH of 6-12 to remove particles greater than 90 Á in diameter, milling the slurry until the particles are all less than 10Á in diameter, and hydrocycloning the slurry. The slurry may have a solids content of 30-65% by wt. and a viscosity of 100-2500cp., and may contain 0À05-0À5% by wt. based on the wt. of oxide, of a dispersant, e.g. Na, K, Li, or NH 4 hypophosphate, hypophosphite, orthophos - phates, metaphosphates, pyrophosphate, or tripolyphosphate, NaNH 4 phosphate, ethylene diamine, ethylamine, diethanolamine, diethylene triamine, pyridine, triethylamine, or benzyl trimethyl ammonium hydroxide. The pH of the slurry may be adjusted by adding NaOH, KOH, LiOH, NH 4 OH, Ca(OH) 2 , BaCO 3 , K 2 CO 3 , Na 2 CO 3 , KHCO 3 ,NaHCO 3 , or NaOH + Na 2 CO 3 . The slurry may be ball-milled for 5 mins-36 hrs, and then diluted to a solids content of 15-25% by wt and hydroclassified to give a fine fraction having a particle size of less than 0À4Á. The non-pigmentary TiO 2 has a tinting strength of less than 1200 and is the material scraped from the walls of the oxidation reactor or removed from the bed particles in a fluidized bed process by milling. It may contain 0À1-10% by wt of another oxide, preferably Al 2 O 3 (an exhaustive list of other oxides is given).