| 摘要 |
A titanium dioxide pigment produced by a continuous process and having a photostability of greater than about -4.5 is provided. The pigment includes a coating of amorphous dense silica in an amount between about 0.6% and about 10% and a coating of alumina in an amount between about 1.5% and about 10% by weight of the total pigment. The pigment of the present invention is produced in a continuous process wherein a soluble silicate is initially added to a titanium dioxide slurry at a temperature of at least about 85 DEG C and a pH of between 9.8 and 10.1. The soluble silicate is added in an amount sufficient to achieve a dense silica level on the treated pigment of between about 0.6% and about 10% calculated as SiO2, by weight based on the weight of the treated pigment. The slurry is then be neutralized in at least three neutralization steps by the addition of amounts sufficient of an acid to achieve pH levels of between about 9.6 and about 9.8, about 9.2 and about 9.4, and about 3 and about 4 during each of said neutralization steps. At a pH of between about 5 and about 6.5, an aluminium-containing material is then added to the slurry in an mount sufficient to achieve an alumina level of between about 1.5% and about 10% of the total pigment. The pigment is then neutralized, filtered, washed and dried. |
