Undesired mineral precipitation in technical plants is a widespread problem - especially in geothermal power plants. Due to the high salt concentrations in the geothermal fluids and the cooling and pressure relief during energy production, the supersaturation of the fluids on certain minerals such as silicates, carbonates and sulphates is likely. Especially the precipitation of silicates due to supersaturation is problematic, since amorphous silicate phases are characterized by a rapid growth rate. Mineral precipitations in the heat exchanger and other plant components reduce the efficiency of a geothermal power plant and cause downtimes for maintenance and cleaning work.
The BMBF-funded BrineMine project is investigating the use of geothermal fluids for drinking water production and raw material extraction by using membrane processes. The focus is on reverse osmosis and membrane distillation, which is used to concentrate the solids dissolved in water and separate the drinking water. To prevent scaling, a pre-treatment of the fluids by selective removal of potential scaling agents is necessary. The focus of this work is the removal of dissolved silica from a model solution whose composition was modelled on the chemical composition of a thermal spring in Chile.
In laboratory experiments three different methods to reduce the silica concentration were tested: The addition of silica crystal nuclei, of calcium compounds and of caustic soda. Lithium as a potential raw material to be extracted serves as an indicator for the element-specific mode of action of the precipitation methods. The efficiency of the different methods was determined by time series experiments, in which the changes in concentration after addition of the precipitant were observed. The silica germination method proved to be ineffective. By adding calcium oxide/hydroxide or caustic soda, a reduction of the silica concentration by at least 80% could be achieved. The reduction is achieved by the formation of calcium silicate hydrate phases (C-S-H phases). The ideal formation conditions are a pH value of 10.5 and a molar calcium/silicon ratio of 2 to 1. The methods act element specifically. The calcium oxide or hydroxide method reduces the silicon dioxide concentration in solution by more than 95% and is therefore a very efficient precipitation method to drastically reduce silicon from geothermal fluids.