Laboratory investigation of silica removal from geothermal brines to control silica scaling and produce usable silicates

Abstract

Laboratory and field jar test experiments show that desilication of geothermal brines may be achieved by treating the brine with various metal cations at elevated pH to precipitate metal silicates. Below ∼90 °C and at elevated pH, metal treatment of synthetic and field brines precipitated only amorphous or poorly crystalline metal-rich silicates of little commercial value. Treatment of brine with alkaline-earth metals below ∼90 °C, except Mg, tends to co-precipitate metal carbonates. Laboratory reactions conducted at ∼130 °C demonstrated that certain metal ions may react with silica in brine to precipitate crystalline compounds of commercial value. For example, kerolite clay was precipitated upon treating synthetic and field brines with Mg at 130 °C, while under similar conditions, sodalite and Zeolite P were precipitated upon treatment with Al. Treatment of brines with transition and heavy metals only precipitated amorphous silicates at ∼130 °C, except in the case of addition of a combination of Mg and Fe, which precipitated a smectite clay. Desilication of brine and precipitation of minerals may add value to geothermal power projects by simultaneously controlling scale deposition and producing salable products. By decreasing dissolved silica concentrations in brines, undersaturation of amorphous silica is achieved. Controlled precipitation of silica from brine in crystallizers, tanks or ponds can mitigate silica scaling potential in brine-handling equipment.