Process for removing silica from silica-rich geothermal brine

Abstract

A process is provided for reducing the silica content in silica-rich geothermal brine having dissolved heavy metals, particularly from brine which is flashed from a high natural pressure to a reduced pressure to convert some of the brine into steam, the flashing causing the brine to become supersaturated in silica. The process comprises combining a base material with the brine, in the brine flashing stage, to increase the brine pH from its natural range of about 5 to 5.5 to a range of between about 6.2 and about 6.6, with a pH of about 6.4 being preferred. The added base, which is preferably selected from ammonia, sodium or calcium hydroxide and sodium sulfide or polysulfide, reacts with heavy metals, notably iron, copper and lead in the brine to form a finely divided, insoluble compound or compounds which function as seed crystals onto which supersaturated amounts of silica are precipitated from the brine. The base may be added to the brine in an amount of between about 200 and about 400 ppm by weight. A contact time between the brine and the immediately-formed seed crystals of between about 2 and about 10 minutes is provided, with about 6 to about 8 minutes being preferred to provide sufficient time for the silica to precipitate onto the seed crystals. Thereafter the silicious insoluble material is separated from the brine. If ammonia is used as the base, a flocculating agent, for example, a cationic polymer such as polyacrylamide, is added at a rate of about 1 ppm by weight to flocculate the insoluble silicious material and thereby facilitate separation thereof from the brine. The brine may thereafter be reinjected into the ground for disposal.