The Solubility of Amorphous Aluminous Silica Between 100 - 350°C: Implications for Scaling in Geothermal Power Stations

Abstract

Amorphous, aluminous silica (AAS) scale samples from the Wairakei and Ohaaki power stations, New Zealand, have been characterised chemically and mineralogically. These analyses show that Al is present in the amorphous phase and not in a distinct aluminosilicate mineral. SEM and EMPA analyses show chemical zonation of increased concentrations of Al, which correlates with the concentrations of Na, K and Ca. The presence of Al makes the solubility of the scale dependent on pH and Al concentration and preliminary calculations show that amorphous aluminous silica can precipitate at temperatures as much as 25°C higher than of pure amorphous silica from geothermal brine. One of the studied scales (SP3 2011) was used in a flowthrough experiment at 100 - 200°C and pressures slightly above saturated water vapour pressure, using distilled water. Preliminary solubility results show concentrations below pure amorphous silica saturation; however, it is not believed that saturation was reached in the experiments, at least at temperatures below 200oC. This conclusion is supported by the Al/SiO2 ratio in the fluid.