Recovery of silica from aqueous silicate solutions via trialkyl or tetraalkylammonium silicomolybdate

Reaction of aqueous silicate with ammonium molybdate at pH 1.5 affords the β form of silicomolybdic acid. The latter can be precipitated by the addition of a variety of quaternary ammonium salts. The precipitated silicomolybdate can then be decomposed to silica in a two-step combustion process (at 350 and 1000 °C). Recoveries of Si are nearly complete, but the starting silicate solution must be made strongly basic prior to the reaction with molybdate. The overall process can be utilized for the recovery of silicon isotopes and as a method of silica purification. Key words: silica, silicate, silicomolybdate, silicon-29.