Removing silica from untreated sea or brackish water - by treating with base to ppte. calcium carbonate and silicate but not magnesium hydroxide

Removal of silica from untreated water (contg. brackish or sea water) comprises treatment with an amt. of alkalising agent (A) calculated so that at least CaCO3 and a silicate are pptd. selectively, without co-pptn. of a significant amt. of Mg(OH)2 dissolved in the water. The reaction time of the alkalising process and decantation of the ppte. is controlled so that selective pptn. is completed before most of the dissolved Mg(OH)2 is converted to solid phase. The silicate is the Mg and/or Na-Ca cpds. and pptn. carried out at a pH (up to 10.5) and Mg concn. at which the solubility prod. of Mg(OH)2 is significantly exceeded. USE/ADVANTAGE - Silica is removed from water without sepn. of large amts. of solid phase, and the treated prod. can be used as recirculating cooling water without problems of Mg silicate scale deposition. Water contg. over 3 (esp. 5-50) kg salt cu.m; partic. that from volcanic regions, can be treated. Only small amts. of (A) are used and the process can be done at room temp.