FORMATION MECHANISM OF SILICA SCALE IN DIENG GEOTHERMAL POWER PLANT, INDONESIA

Abstract

Severe silica scale problem has occurred in Dieng geothermal power plant since the start of its operation. Silica scale is formed in surface pipeline systems, and this requires acidification of the brine using sulfuric acid to address the problem. In order to understand the formation mechanism of silica scale inside the brine pipeline, the polymerization behavior of silicic acid in geothermal water, and the precipitation of scale and its characteristics, were investigated in an on-site batch experiment.
The polymerization of silicic acid was investigated by spectrophotometry to determine monosilicic acid (SiO2(M)) and by ICP-OES to determine total silicic acid (SiO2(T)). Under neutral pH condition, SiO2(M) concentration decreased rapidly from 1,000 to 350 ppm, while SiO2(T) decreased moderately from 1,000 to 350 ppm. This suggests that silica precipitation follows the rapid growth of polysilicic acid. Even under acidic pH condition, polymerization proceeds: SiO2(M) concentration decreased from 1,000 to 600 ppm. However, SiO2(T) concentration was kept almost constant. This suggests that addition of sulfuric acid was not able to completely stop the polymerization of monosilicic acid, but only retarded the growth of particles (reaction between polysilicic acids).
Trace metal concentrations such as iron (Fe) and/or aluminum (Al), reported to promote the formation of silica scale, were determined by ICP-OES. Under neutral pH condition, a decrease in Fe concentration was observed coinciding with the decreasing SiO2(T) concentration. This suggests that Fe concentration in the brine is playing a role in the deposition of silica under neutral pH condition by accelerating the growth of polysilicic acids. On the other hand, both the Fe and SiO2(T) concentration in the brine remained constant after acidification by sulfuric acid. This suggests that Fe ions were prevented from binding into the polysilicic acid under acidic condition. Furthermore, the XRD and XRF analyses show that the scale is mainly composed of amorphous silica with high concentration of Fe.