Kinetic Evaluation of pH and Temperature Effects on Silica Polymerization in the Presence of Mg, Al and Fe

Abstract

Silica scale can cause pipe obstructions in the operation of geothermal power plants, and no effective countermea-sure has been established. In this study, we experimentally evaluated the effect of pH (3, 6, 9), temperature (298‒353 K), and the presence of metal elements (Mg, Al, and Fe) on silica polymerization and discussed an effective and practical countermeasure against silica scale formation. In our experiments, the behavior of silica and the metal elements varied greatly depending on pH and temperature. At pH 3, no silica polymerization or decrease of the metal elements was ob-served. At pH 6 and pH 9, however, silica polymerization proceeded. The nucleus growth rate of amorphous silica was lower at pH 6 than at pH 9 and higher in the presence of coexisting elements than in their absence. Furthermore, no induction period was observed in the presence of the metal elements. These results indicated that the metal elements promote the nucleation and growth of silica scale. On the basis of the observed silica scale formation rate, we suggest reducing the residence time (from pumping to re-injection) of hydrothermal fluid under neutral pH conditions in order to suppress silica scale formation, because the nucleus growth rate is relatively low at pH 6. For example, if the geothermal water is re-injected at 353 K, we can confine the decrease of silica concentration to less than 100 ppm by returning the fluid within 4 h.