Recovery of silica from saline geothermal brine at the Yamagawa Power Plant for inhibition of scale formation by batch- and circulation-tests using cationic flocculant and quicklime

To suppress silica scale deposition from supersaturated silica, two types of field tests, i.e., batch- and circulation-tests were conducted at the Yamagawa Power Plant by adding cationic flocculant and quicklime. In the flow test, an improved seed circulation system was used and the flow rate of brine to be treated was about 1.2 tons/hour. The silicic acid concentration in the brine after steam–brine separation under atmospheric pressure was 630 mg/L. In the batch test, the reagent was mixed with the brine after a delay of zero minutes (no retention time; NRT) or 15–60 min (retention time for 15–60 min; 15 RT to 60 RT) at 90 °C. When a cationic flocculant (DADMAC–No.6) was added at concentrations ranging from 10 to 50 mg/L, the silicic acid concentration decreased to 300 mg/L with increasing retention time, regardless of the reagent concentration. In contrast, in the case of CaO addition, silica recovery rates increased with the amount of CaO added. The silicic acid concentration decreased to approximately 50 mg/L at NRT when 1 g/L CaO was added. In the flow test, DADMAC–No. 6 or CaO was added continuously under various conditions. The silicic acid concentration in the treated brine was reduced to approximately 400 mg/L when DADMAC–No. 6 was used. The turbidity of the treated brine was more than 10 NTU. It needs to be reduced to less than 1 NTU for underground reinjection. In contrast, the silicic acid concentration was approximately 100 mg/L when CaO was used, and the turbidity was approximately 0 NTU. These results indicate that addition of CaO is the best option for silica recovery from geothermal brine at the Yamagawa Power Plant. A practical-scale brine treatment plant has a treatment flow rate of 150 t/h. The estimated treatment cost at this time is 0.86–1.0 USD/ton of brine when DADMAC–No. 6 was used and 0.83–1.48 USD when CaO was used.