Study of Silica Scaling from Geothermal Brines. Final Report, 15 November 1974--30 April 1977

摘要

Condensation of silica from brines supersaturate in silicic acid was studied over a range of pH (4.5 to 6.5), temperature (75 to 105 exp 0 C), salinity, and silicic acid concentration (700 to 1200 ppM as SiO sub 2 ). The experimental technique involved analyses for molybdate-reactive silicic acid as a function of time after a supersaturated solution of Si(OH) sub 4 was prepared by mixing aliquots of a stock silicate solution with buffered brine. The isothermal rate of SiO sub 2 condensation is a strong function of supersaturation (C/Ce), pH, and salinity. The overall kinetics follow what is expected from the general theory of phase transitions. The kinetic results suggest that growth of nucleated particles is activation controlled, at least initially. The increase of condensation rate with salt content is attributed to a decrease in equilibrium solubility of SiO sub 2 . In effect, higher salt concentrations increase supersaturation and lead to faster nucleation. Higher nucleation and growth rates are observed at higher pH's. An increase of 1 pH unit increases rates by a factor of about 10. The pH dependence is consistent with the hypothesis that SiO(OH) sub 3- is one of the reacting species. Temperature has little effect on condensation rate at a fixed initial silicic acid concentration. This result suggests that the increase in rate of activated steps brought about by increased temperature is essentially counterbalanced by a decrease in supersaturation (because of higher equilibrium solubility at higher temperatures). The kinetic results yield an activation energy of 17 kcal/mol for addition of silicic acid to a nucleated particle and 45 ergs/cm exp 2 for the interfacial energy between precipitated silica and the brine. NaCl, KCl, CaCl sub 2 and MgCl sub 2 had no specific effect on condensation rate. However, NaF had a large effect on rate; a few ppM (15 to 100) of fluoride ion accelerate condensation substantially. (ERA citation 04:003006)