Process for producing durable titanium dioxide pigment

Claim 1

A batch process for preparing a coated titanium dioxide pigment having deposited thereon a dense amorphous inner coating of silica and an outer coating of alumina comprising: forming an aqueous slurry of a noncoated titanium dioxide pigment, said slurry having a pH adjusted to a value of at least about 9.8 and heating said slurry to an elevated temperature ranging from about 75° C. to about 90° C.; adding a water soluble silicate compound to said heated slurry while maintaining said heated slurry at said elevated temperature to initiate a deposition and cure of said dense amorphous inner coating of silica onto said pigment wherein said water soluble silicate compound is added to said heated slurry in an amount sufficient to provide from about 0.5 to about 5.0 weight percent, based on the total weight of the coated titanium dioxide pigment, of silica as a dense amorphous coating upon said pigment; adjusting the pH of said heated slurry consisting of two steps: (1) adjusting rapidly in a single adjusting step the pH of said heated slurry to a value of from 9.2 to about 9.4 while maintaining said heated slurry at said elevated temperature to complete the deposition and cure of said inner coating of silica; and (2) adjusting rapidly in a single adjusting step of pH of said heated slurry to a value of from about 2.8 to about 3.2 and thereafter commencing addition of an amount of a water soluble aluminum-containing compound sufficient to raise the pH of said heated slurry to a value of from about 5.5 to about 6.5 and to initiate a deposition of an outer coating of alumina on said silica coated pigment; continuing the addition of said water soluble aluminum-containing to said heated slurry in an amount sufficient to provide an outer coating of said alumina of a predetermined weight on said silica coated pigment while maintaining the pH of said heated slurry at said value of from about 5.5 to about 6.5; and adjusting the pH of said heated slurry to a value of from about 6.5 to about 8.5 and recovering said coated titanium dioxide pigment substantially as produced.