2002 Volume 24 Issue 3 Pages 221-236
In the Onuma geothermal field, brines contain SiO2-concentrations of about 514 mg SiO2 per liter brine. The pH value of the brines is 7.2±0.1 at a temperature of 90°C. The dissolved SiO2 precipitates during flashing of the brines and causes severe problems by forming scales in the reinjection wells. Therefore, an experimental study was designed to explore how SiO2 can be removed from the brines most efficiently and in a controlled way. First, we performed tests with various reagents, i.e., high early-strength portland cement, CaCO3 (calcite), CaO, Ca (OH)2, and CaMg (CO3)2 (dolomite) to explore the efficiency of the reagents in removing silica from the brine. Our test showed that CaO is the most efficient reagent in removing the excess silica from the brine. Therefore, we used CaO and the following method to conduct all subsequent experiments : In a first step, CaO is added continuously to the brine. The CaO-reagent causes the precipitation of amorphous Ca-silicate. Then, in a second step, we reinject the precipitate to fresh brine to cause additional precipitation of Ca-silicate. We conducted all experiments at two different conditions: addition of 0.6g CaO/l and 1.5g CaO/l. The main results are an addition of 0.6g CaO/l reduced the silica concentration to 223 mg/l and increased the pH of the brine to 9.9. An addition of 1.5 g CaO/l caused a significantly stronger reduction in silica concentration of 4 mg/l and raised the pH value to 11.7 at the same time.
