To prevent the formation of silica scales at the geothermal power plants, a simultaneous precipitation test of silicic acid and aluminum by addition of cethyltrimethylammonium (CTA+) bromide (Br–), which is an organic ammonium salt, was carried out using a sodium silicate solution (400 ppm Si) containing aluminum (5 ppm Al). The concentration of CTA+ added was in the range of 10–6 ～ 10–3 mol dm–3. During the polymerization of silicic acid, aluminum combined to polysilicic acid particle. The negative charge density for the polysilicic acid particle with aluminum was higher than that for pure polysilicic acid particle, suggesting that Al3+ may be substituted with Si4+ in the pure polysilicic acid. It was clearly found that silicic acid and aluminum can be simultaneously precipitated from aqueous solution by the electrostatic interaction between CTA+ and polysilicic acid particle with aluminum. However, the precipitation of polysilicic acid particle significantly depended on the aging effect (time after the polymerization started) of polysilicic acid particles. In spite of the CTA+ concentration, more than 90 % of the aluminum was precipitated even at 10–6 mol dm–3.
