Silica recovery from Sumikawa and Ohnuma geothermal brines (Japan) by addition of CaO and cationic precipitants in a newly developed seed circulation device

With the objective of preventing silica scaling, several types of chemical compounds such as silica gels, clay minerals, chlorides of Ca, Mg, Al and Fe, and calcium compounds were first examined to remove the excess silica in the Ohnuma geothermal brines. The total silica concentrations of silica in the brines after reaction by addition of these chemicals were observed to decrease linearly with an increase in the reagent concentration. Quicklime (CaO), added at dosages of 0.5 and 1.0 g/l, can remove excess and most silica at 90 °C, respectively. These results indicate that the silica concentration of Ohnuma brines (520 mg/l initial concentration) can be controlled by the amount of quicklime added, and that low-temperature resources can be used without scaling. Continuous flow tests were then performed at Ohnuma and Sumikawa fields by addition of CaO or cationic precipitants in a newly developed seed circulation device (flow rate of 10 l brine/min). At Ohnuma, the total silica concentrations of brines after reaction with CaO showed almost constant values: 10, 200 and 300 mg/l for additions of 1.0, 0.65 and 0.50 g CaO/l, respectively. At Sumikawa, 11 experiments were run, adding cationic precipitants. The total silica concentrations decrease with the increase in reagent concentrations (25-100 mg/l) and with an increase in the retaining time (15-120 min). With the addition of CaO (2 g/l) to Sumikawa geothermal brines with a retaining time of 120 min at 90 °C, the total silica concentration was almost non-detectable after 10 h from the start of the reaction. After reaction for 118 h, addition of CaO was suspended and only silica deposits were reused through the circulation pipe to desilicate brine. After this treatment, the total silica concentration quickly increased to 400 mg/l and remained constant for 210 h before increasing to 650 mg/l after 265 h. There are good correlations between the zeta potentials, a measure of colloidal stability, and the Si/Ca atomic ratios of the silica deposits and amounts of silica removed from brines at Ohnuma and Sumikawa. The CaO addition and circulation method (SACS method) is a very useful way of removing excess silica in brines for silica scale prevention.