Ferric silicate precipitates relevant to geothermal systems: Delineation of their complex formation

Abstract

The interest in the utilization of geothermal energy has increased exponentially in the past few decades, and researchers internationally are currently focusing on improving harvesting methods and promoting it due to its numerous benefits compared to traditional energy sources. Corrosion and scaling are two of the significant problems in modern geothermal industry that occur during the harvesting of geothermal energy. Scaling occurs due to the variety of anions and cations that the majority of geothermal reservoir waters contain. High levels of dissolved Fe³⁺ and silicate ions cause the formation of the elusive “iron silicate”, the latter term usually referring to ferric silicate. Its identity, as it is formed in geothermal waters, differs from its geological counterparts. Usually, deposits that contain Fe and Si are referred to as “iron silicate”, revealing very little about its true identity. This research is focused on revealing the true nature of the so-called “ferric silicate”, performing a series of synthesis experiments under various conditions that take into account iron and silicate concentrations (at the supersaturation regime), solution pH, temperature and different sources of iron. Although solutions containing 150 ppm silicate (no ferric) are stable and do not undergo any polycondensation at pH = 7.0, the presence of ferric induces variable silicate loss from solution. Its severity is enhanced as the ferric concentration increases. Ferric loss from solution in the presence of silicate ions is also severe, regardless of the silicate concentration. Both silicate and ferric loss from solution are pH-dependent processes, with a maximum observed around pH = 7.0. This loss is accompanied by precipitate formation, which is corroborated by turbidity measurements. The solubility of ferric silicate (under the conditions studied) increases with temperature. There is a roughly 4-fold solubility increase when the temperature is increased from 25 °C up to 220 °C. However, enhanced ferric incorporation into the final precipitate was observed at higher temperatures. The ferric silicate precipitates were characterized by several techniques, such as powder XRD, ATR-IR, SEM, and EDS, and showed that ferric silicate is an amorphous precipitate with variable Fe:Si atom ratios and a random Fe distribution throughout the solid. These results can serve as a roadmap for ferric silicate precipitation, taking into account the water chemistry of a specific scale-forming brine.