Recovery of Silica from the Sumikawa Geothermal Fluids by Addition of Cationic Nitrogen-Bearing Reagents

Abstract

In order to recover excess silica in fluids for prevention of silica scale deposition, two types (aril and estel) of cationic nitrogen-bearing reagents were examined at the Sumikawa geothermal plant, Japan. The total silica concentration in the fluid studied is 0.76kg/m3 after flashing under an atmospheric pressure where that of dissolved silica decreased quickly(within 15 minutes) from 0.76 to 0.39 kg/m3 due to polymerization. From this, two different procedures were examined; The cationic reagents were mixed into fluids at 90oC with no residence time(NRT; immediately after flashing or mixed into fluids with residence time of 15 minutes(RT; keeping at 90oC). The concentrations of total and dissolved silica in the fluids with both RT and NRT quickly decrease with increasing reaction time (5 minutes) after addition of the reagents. The degree of silica recovery is higher in the fluids with RT than fluids with NRT. These results mean that the reagent mainly reacts with the polymerized silica. The suitable concentration of the reagent added is around 0.05kg/m3 where the total concentration becomes almost constant to be 0.34kg/m3 similar to the solubility of amorphous silica at 90oC. There are no clear differences in the two types of reagents as for their degrees of silica recovery. After the reaction, the fluids become transparent. The deposited silica is amorphous and is composed of SiO2 and H2O with a minor amount of other elements. From their zeta potential data, one atom of cationic nitrogen seems to react with 30 atoms of silica.