| 摘要 |
Finely divided hydrated silica having an average ultimate particle size of 0.015-0.1 micron is purified by treating an aqueous slurry thereof with an anion or cation exchage resin of larger particle size than the silica and then separating the silica from the resin by filtration, decantation or centrifugation. Incomplete separation may be effected, the remaining silica being washed from the resin. Filtration may be effected by passing the slurry through a sieve of 10-100 mesh (Tyler) and preventing accumulation of a layer of resin on the sieve of more than 1 in. thick. Partial decantation followed by centrifugation may be employed. The silica is stabilized before treatment, e.g. as described in Specification 745,890, or by drying at a temperature of at least 100 DEG C. and then redispersing in water. One or more pounds of ion exchanger may be employed per pound of silica in the slurry. In order to effectively remove cations, the pH should be reduced to below 5, preferably to 2-4, by the use of a cation resin alone or together with an aqueous acid. In the latter case an anion exchange resin may be subsequently employed to remove the anions introduced by the acid. According to a modification a cation exchange resin, alone or together with an anion exchange resin or an anion exchange resin in conjunction with a water soluble acid, may be added to a silica filter cake so as to produce the effect disclosed in Specification 774,585, i.e. a thinning out of the cake to produce a slurry. Specification 745,822 also is referred |
