Silicic acid ionization and calculation of silica solubility at elevated temperature and pH application to geothermal fluid processing and reinjection

Abstract

Utilization of geothermal liquids is often complicated by the deposition in process equipment of dissolved salts and minerals, particularly amorphous silica. Herein we compute the solubility of silica as a function of temperature and pH in vapor saturated liquid water to 300°C and pH 12. Ionization of dissolved silica (silicic acid) at alkaline pH enhances the solubility. The ionization of silicic acid is commonly expressed: (A) H 4SiO 4 = H 3SiO 4 − + H +, for which reaction the enthalpy of ionization L A is highly temperature dependent and changes sign along the vapor saturated curve of liquid water. We advocate the alternative neutralization reaction (B) H 4 SiO 4 + OH − = H 3 SiO 4 − + H 2 O, which results in a linear relationship between log K B and T −1. The equilibrium constant for this reaction in vapor saturated liquid water from 0 to 300°C is given by log k B = 1479/ T − 0·6496 based on selected literature data. An iterative algorithm is developed to compute solubility of silica polymorphs as a function of temperature and pH in circumstances where literature data are lacking, using the neutralization reaction and neutral-pH solubility data. Conditions are specified under which certain approximations which greatly simplify the computational problem are valid. The mean ionic activity coefficient for H 3SiO 4 − in saturated silica solutions is estimated and shows excellent agreement with the Debye - Huckel theory for conditions where the latter is valid. Determination of this activity coefficient allows prediction of molal solubility from true thermodynamic equilibrium constants for the several participating reactions by combining our equation (7) and Table 1. This bypasses use of the more cumbersome molal concentration products. The results of computation compare favorably with the few data available in the literature. Application of these results to the problems of scaling control in process equipment and to reinjection of processed geothermal fluids suggests that alkalization receive consideration as a chemical control on scaling. This procedure would simultaneously facilitate fluid reinjection, and may be a superior alternative to currently proposed acidification procedures for some cases.