Sodium Hydroxide is used in geothermal power plants to regulate the pH of sulfur.
One option described for precipitating heavy metals naturally present in geothermal brines involves adding a base, such as ammonia (preferred), sodium hydroxide, calcium hydroxide, sodium sulfide, or sodium polysulfide, to the silica removal process. The added base increases the pH of the brine-base mixture (to between about 6.2 and 6.6) and reacts with at least some of the heavy metals naturally contained in the brine, notably iron and/or lead. Finely divided, relatively insoluble heavy metal compounds precipitate from the brine, forming seed material onto which the supersaturated amounts of silica deposit. When ammonia is used, the base process preferably includes a further combining of the brine-base mixture with a flocculating agent (e.g., cationic polymer) so as to enhance flocculation of the insoluble silicious material.