Kinetics of Silica Deposition From Simulated Geothermal Brines

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Abstract
In our experiments, supersaturated brines were passed through columns packed with several forms of silica (crystalline α-quartz, polycrystalline α-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. 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. The reaction rate is not diffusion-limited in the columns. The silica deposition is a function of the monomeric [Si(OH)4] concentration in the brine. The deposition on all surfaces examined was nucleated spontaneously. 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 range 5 to 8 and temperatures from 60 to 120°C is (1) − d [ Si ( OH ) 4 ] d t = 0.12 A { [ Si ( OH ) 4 ] − [ Si ( OH ) 4 ] e } 2 [ OH − ] 0.7 , 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°C.