Kinetics of Silica Deposition From Simulated Geothermal Brines

Abstract

Abstract In our experiments, supersaturated brines were passed through columns packed with several forms passed through columns packed with several forms of silica (crystalline -quartz, polycrystalline quartz, and porous Vycor). Also, silica deposition on ThO2 microspheres and titanium powder was studied under controlled conditions of supersaturation, pH, temperature, and salinity. The residence time was varied by adjustments of flow rate and column length. The silica contents of the input and effluent solutions were determined colorimetrically by a molybdate method that does not include polymers without special pretreatment.The following observations have been made.1. Essentially identical deposition behavior was observed once the substrate was coated thoroughly with amorphous silica and the Brunauer-Emmett-Teller (BET) surface area of the coated particles was taken into account.2. The reaction rate is not diffusion-limited in the columns.3. The silica deposition is a function of the monomeric [Si(OH)4] concentration in the brine.4. The deposition on all surfaces examined was nucleated spontaneously.5. The dependence on the supersaturation concentration, hydroxide ion concentration, surface area, temperature, and salinity were examined. Fluoride was shown to have no effect at pH 5.94 and low salinity.A cursory study of the effect of salinity showed little difference for 0.09 and 1.0 molal NaCl solutions; however, increasing the concentration to 4.0 molal increased the deposition rate by more than one order of magnitude. The empirical rate equation that describes our data in 1 molal NaCl in the pH ranges to 8 and temperatures from 60 to 120 deg. C is ........(1) where A is the amorphous SiO2 surface area in square centimeters per kilogram of water in column voids; t is minutes; and the concentrations are in molal units. Hydroxide concentration was derived from the measured pH and the ionization quotient for water. In the expression given above, the rate constant is essentially independent of temperature over the range 60 to 120 deg. C. Introduction This paper reports on work conducted at the Oak Ridge National Laboratory (ORNL) to examine silica deposition behavior in dynamic geothermal systems from hydrothermal brines found in the western U.S., with particular emphasis on the factors affecting the kinetics of deposition impact columns. We begin this report with a general discussion on the behavior of silica. Behavior of Silica In dilute aqueous solutions, silica generally occurs in the acidic-to-neutral pH range as Si(OH)4. In basic solutions, the anionic species SiO(OH)3-, SiO2(OH)2/2-, and Si4(OH)2/18 have been observed in two potentiometric studies. The equilibrium reactions among these anions and the neutral silicic acid have been studies in detail in sodium chloride solutions to 300 deg. C. Also, the fluoride ion interacts with silicic acid in relatively acidic solutions, producing principally the SiF2/6- complex.* The producing principally the SiF2/6- complex.* The stability of the complex decreases as the temperature increases. SPEJ P. 239