Condensation of silica from supersaturated silicic acid solutions

Abstract

Condensation of silica from brines supersaturated in silicic acid was studied over a range of pH (4.5–6.5), temperature (75–105°C), salinity, and silicic acid concentration (700 to 1200 ppm as SiO 2). The course of condensation was followed by analyzing for molybdate-reactive silicic acid. The isothermal rate of SiO 2 condensation is a function of supersaturation ( C C e ), pH, and salinity. At supersaturations less than about 3, there is an apparent transition period during which the rate of removal of silicic acid from solution is below the detection limit of the analytical technique. This period amounts to several hundred minutes at supersaturations of about 2.0. Growth of nucleated particles is activation controlled, at least initially. The primary nuclei have radii of the order of a few angstroms. Particles of several hundred angstroms are produced eventually. Nucleation and growth rates increase with increasing pH approximately by a factor of 10 for each pH unit. The pH dependence is consistent with the hypothesis that SiO(OH) 3 − is one of the reacting species. The rate of condensation increases salt content. In effect, higher salt concentrations increase supersaturation and lead to faster nucleation. Temperature (75–105°C) has little effect on condensation rate at a fixed initial silicic acid concentration. This result suggests that the increase in rate at higher temperatures is counterbalanced by a decrease in supersaturation. The activation energy for addition of silicic acid to a nucleated particle of SiO 2 is about 17 kcal/mole. The interfacial energy between precipitated silica and the brine is about 45 erg/cm 2. NaCl, KCl, and MgCl 2 have no specific effect on condensation rate. NaF has a large effect on rate; 15 to 100 ppm of fluoride ion accelerate condensation substantially.