Thermodynamic Analysis of Silica-Based Scale Precipitation Induced by Magnesium Ion

Abstract

The efficiency and maintenance of small geothermal power plants that use low-temperature hot spring water are affected by the adhesion scale. The precipitated scale samples from the binary power plant at Obama Hot Spring (Kyusyu, Japan) was analyzed and found to contain aragonite, quartz, and amorphous silica-based phases. Thermodynamic analysis revealed that aragonite, calcite, talc, sepiolite, and amorphous magnesium silicate were in supersaturated state in the geothermal water. Meanwhile, magnesium carbonate and amorphous silica were unsaturated. Mg²⁺ does not affect the precipitation of carbonate scale, and that of amorphous silicabased scale could be predicted thermodynamically after considering the influence of Mg²⁺. Based on the chemical composition of the geothermal water supersaturated with amorphous magnesium silicate, we extended a simple method to predict the precipitation of amorphous magnesium silicate. The saturation indexes of both components are decreased by decreasing the pH and the temperature; hence, this could be used to reduce the adhesion scale at Obama Hot Spring. At the pH is ≥ 9, the precipitation of amorphous magnesium silicate should be considered, even if amorphous silica is unsaturated in the hot spring water (assuming ionic strength I = 0.1–0.2). With regard to the temperature, the precipitation risk of amorphous magnesium silicate is high at around 100 ˚C, but greatly reduced by operating the geothermal water at around 75 ˚C.