| Abstract |
At open canal system in the Dieng geothermal power plant, Central Java, Indonesia, the polymerization of silicic acid in geothermal water and the precipitation of silica scales were investigated. The polymerization was examined by spectrophotometrically measured total silicic acid and monosilicic acid concentrations. Silica scale samples were obtained by immersing copper plates in geothermal water. The obtained silica scale samples on copper plates were characterized by XRD. Sulfuric acid is added into geothermal water to lower pH and to retard the deposition of silica scales at Dieng geothermal power plant. Under neutral pH condition (before the acidification), monosilicic (SiO2-M) and total silicic acid (SiO2-T) concentrations decreased simultaneously, suggesting that the rapid polymerization of silicic acid and the formation of polysilicic acid (SiO2-P) with particle size larger than 0.45 μm proceeded at the same time. Under acidic pH condition (after the acidification), SiO2-T concentration was constant (1300 ppm SiO2), however SiO2-M concentration decreased, suggesting that although polymerization of silicic acid occurred, the growth of the particles was retarded. Iron is well known to affect the silica scale formation. In the open canal system, with the decreasing of total silicic acid concentration under neutral condition, iron concentration in geothermal water was also decreased, suggesting that iron may be incorporated into polysilicic acid during the polymerization. In addition, iron may accelerate the reaction among monosilicic acids during the early polymerization stage. On the other hand, decreasing of iron concentration in geothermal water was retarded under acidic condition in accordance with total silicic acid concentration. It is possible that acidification by sulfuric acid may also limit the interaction between iron and monosilicic acid. Silica scale formed under acidic condition contained higher contents of iron than those formed under neutral condition. According to the XRD results, silica scale formed on the surface of copper plate mainly consists of amorphous silica. Some iron minerals were detected in the silica scale formed under acidic condition (e.g. magnetite). In addition, silicic acid adsorption experiment on silica gel suggests that silicic acid can be adsorbed on silica scale (represented by silica gel) even under acidified condition and without incorporation of iron. |