| 摘要 |
The composite particles of titania and silica are provided, which are useful as a filler in paper making and impart excellent brightness and opacity to the resultant paper. The composite particles comprise the secondary silica particles used as a core and 2 to 25 % by weight, based on silica, of crystalline titanium dioxide carried by the surface of the secondary silica particles. The average particle diameter of the composite particles is 3 to 25 .mu. m and at least 80 % by weight of the particles have a diameter of 1 to 30 .mu. m. The composite particles are obtained by (1) adding a mineral acid to an aqueuos alkali silicate solution at 20 to 80.degree.C so that said mineral acid is added in an amount of 30 to 70 % by weight of the total amount of said mineral acid necessitated for neutralizing said aqueous alkali silicate solution, and increasing the temperature of the resultant mixture to 60 to 100 .degree.C in case where the temperature of the mixture is below 60 .degree.C , so as to precipitate secondary silica particles in said mixture; (2) adding crystalline titanium dioxide to said secondary silica particles at a pH of 7 or higher, said titanium dioxide being used in an amount of 2 to 25 % by weight of the total silica contained in said aqueous alkali silicate solution, to form a slurry; and (3) adding a mineral acid to the slurry to control the pH of the slurry to 3 to 6.5.
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