Development of a continuous silica treatment strategy for metal extraction processes in operating geothermal plants

The extraction of rare metals like lithium (Li) from geothermal fluids is a promising alternative to conventional mining. Membrane distillation (MD) could support energy-efficient fluid treatment enabling further freshwater production. For the operation of geothermal plants and MD uncontrolled precipitation of silica (Si) represents a major hurdle. Herein, we demonstrate the transfer of a Si treatment from lab to field demonstrator scale, tested under conditions of an operating geothermal power plant. For the treatment, lime precipitation was chosen showing good Si reduction rates using artificial fluids. The high alkaline conditions of this process (pH > 10) in combination with the high salinities of the geothermal brines (TDS > 100 g/L) are transferred into real geothermal environment with a newly developed numerical design calculation. The resulting demonstrator consists of three major process steps - 1) Si-reduction, 2) liquid/solid separation, and 3) post-concentration using MD. The Si treatment efficiently reduced 98 % of Si in <5 min reaction time, without influencing the lithium concentration negatively. The MD resulted in Li concentrations of ~500 mg/L while producing fresh water. Beyond the approval of the concept, neuralgic points for improvement were identified expanding fundamental knowledge about the material use of geothermal fluids.