Evaluation of Silica Scale Inhibitors

Abstract

Silica scale is a serious problem for many geothermal power plants. Deposition of silica occurs when the temperature drops, causing the silica saturation index to decrease. Once formed, silica scale will reduce the efficiency of heat exchangers and reduce the reinjection capacity of reinjection wells. Further complicating things, sometimes silica precipitates as amorphous silica, sometimes it precipitates as a aluminosilicate or some other type of silicate. Laboratory experiments to mitigate both types of scale have been initiated. We evaluated a variety of inhibitors at pH = 7.0 using the molybdate active silica test. The best inhibitors prevented the polymerization of silica over 4 hours, even at low doses. Silica polymerization was prevented with as little as 7 ppm of inhibitor. Leading candidates were further tested in the presence of poisons such as aluminum silicate. Several of the most effective inhibitors of amorphous silica polymerization were ineffective in the presence of aluminum silicate. This work describes the progress towards the development of scale inhibitors which prevent both amorphous silica and aluminum silicate scale.