Treatment of Geothermal Waste Water to Prevent Silica Scaling

Silica scaling potential from geothermal waters puts limits on to what extent the heat from the water can be extracted. Once the waters become over-saturated with respect to amorphous silica, the handling of such waters becomes problematic. Many methods have been proposed to address this problem. One of them is aging the geothermal waste water allowing monomeric silica in excess of amorphous silica solubility to form polymeric silica thus reducing amorphous silica over-saturation. In geothermal water of low ionic strength polymeric silica has lesser tendency to precipitate than monomeric silica. If geothermal waste water is to be aged in a retention tank, or a pond, a good knowledge of polymerization rates are necessary. In this contribution we study the time required to lower the monomeric silica concentration in solutions initially containing 800 mg/kg silica to silica concentrations close to equilibrium with amorphous silica. We present experimental data on silica polymerization in the pH range 2.3-9.0 at 80°C and in the ionic strength range 0.024-0.50 at pH 7.13. The concentration of molybdate active silica is most reduced in solutions with pH between 7-8. At higher pH the solubility of amorphous silica increases and silica polymerization does not proceed as far as between pH 7 and 8. After one hour of polymerization solutions at pH 7-9 are close to equilibrium with respect to amorphous silica and very little is gained by aging the solution longer. Increased ionic strength speeds silica polymerization but is also enhances the rate of silica deposition.