Mineral recovery through geothermal desalination

A recent review of mineral content in the geothermal waters of the Taupo Volcanic Zone has estimated gold content at over 20 parts per billion and silver content at 2,000 ppb (Simmons et al., 2016). Wells at Mokai and Rotokawa are estimated to be extremely rich and could produce 680-7500 kg Au/year. Lithium production from geothermal and petroleum brines are already accepted technologies but the current production techniques are not integrated seamlessly into a geothermal energy cascade. We propose to use a variant of multi-effect-distillation (MED) technology to close this gap. We present a setup that is currently on trial for caustic soda recycling from spent liquor recovered in the Bayer process for alumina production (Rahimi et al., 2016).We propose to use the same technology to concentrate the brines for mineral harvesting.
The MED technology is better known in seawater desalination due to its low electrical energy consumption, low operation cost, high thermal efficiency and compactness. MED is a process that is suitable for use on a range of water types including brackish, seawater and industrial process water requiring a thermal energy source to drive the distillation process. We propose to use the waste heat stream of the geothermal power station to power the MED process. In such a process, the process feed is heated in the first stage by the geothermal waste heat stream, thereby vaporising the feed under reduced pressure. The vapour from the first stage is then condensed in the next successive stage and giving up its latent heat to drive the evaporation of further amounts of process feed. This process is repeated across each effect (stage) within the plant, increasing the degree of internal energy recovery. Economic operation is commonly considered to be viable in the temperature range between about 65-100°C.