A Raman study on silica speciation in solutions related to geothermal brines

Abstract

Silica scaling is a problem that plagues geothermal powerplants and has a large influence on the efficiency of geothermal energy production. Silica scaling can reduce the output performance of a powerplant by 50% per year yet might also be a useful tool in extracting valuable materials from geothermal brines. Silica scaling occurs as silica polymers in solutions form colloidal particles that stick together and are cemented as monomeric silica is deposited on their surfaces. Understanding the influence of brine composition on silica speciation could lead to insights into controlling the precipitation of silica from geothermal brines. The effects of dissolved monovalent salts on silica speciation in solutions with chemical compositions like those of geothermal brines was studied using Raman spectroscopy. This was achieved by creating a database containing the chemical compositions of geothermal brines from regions where silica scaling is known to affect geothermal powerplants. The studied brines had a pH ranging from 6 to 10 and silica concentrations between 0 ppm and 1500 ppm. The sodium and potassium concentrations could reach 70,000 ppm and 20,000 ppm respectively, however most samples only contained up to 5,000 ppm and 500 ppm respectively. Based on this information the range of 0 ppm to 5,000 ppm of alkali cation concentrations was chosen to be analysed. The results from the Raman spectra showed that regardless of alkali cation concentration, the silica at a concentration of 8,500 and 7,000 ppm remained highly polymerized. The high silica concentration prevented Raman analysis at the observed pH values for geothermal brines as the solutions turned to a gel or gel like suspension of precipitate when the pH was lowered to these values. The gathered data varied greatly between measurements and fitted poorly to any observed trend, but the overall data indicates little effect of the added salt concentration on silica speciation. However, it is shown that the silica speciation at the studied concentrations is independent of the water activity which changes as ion concentration increases. As the silica concentrations detectable using Raman spectroscopy are significantly higher than those found in geothermal brines, there may be other effects at lower silica concentrations. Further analysis at lower silica concentrations, higher silica to cation ratios, and pH is required to gain a better understanding of the effect of monovalent salts on silica speciation in geothermal brines.