Investigation of pH Control Method to Avoid Silica Scaling in the Sumikawa Geothermal Field

To avoid silica scaling, three different pH conditions (5.0, 6.5 and 7.3) were examined by passing Sumikawa geothermal fluids (SiO₂ concentration ; 775 mg/L) through columns (50 mmø, 55 cm in length) packed with alumina beads (l or 2 mmø). The flow rate decreased from 5 to 2~3 L/min with time (within 72 hours) in all pH conditions. Silica was found to deposit mainly around the top of the column (within 20 cm from the top), where Al concentration in the silica deposits increased with time. The total amount of silica deposited during experiments increased drastically with increasing amount of fluid after some induction periods where no significant amount of silica was deposited. In the most acidic condition (pH 5.0), silica deposition was also observed instead of an iron material such as SS400 was corroded. These results indicate that around pH 6.5 is a better condition to avoid silica scaling instead of a small amount of silica is still deposited for fluids with high SiO₂ concentration (< 700 mg/L).