Chemical control of ferric silicate scaling by phosphonate additives under geothermal stresses

The formation, precipitation, and deposition of the so-called "iron silicate" in geothermal waters have been among the problematic issues in some geothermal installations. Our approach aims to systematically study the influence of phosphonate-based chemical additives on the formation of precipitates in the presence of both silicate and ferric (Fe3+) ions. The selection of this type of antiscalants was based on their extraordinary affinity for metal ions in general, and in particular for ferric cations. For this purpose, various phosphonate additives that possess diverse chemical structures and variable numbers of functional groups have been systematically evaluated. These additives are PBTC (2-phosphonobutane-1,2,4-tricarboxylic acid), HEDP (hydroxyethylidene-1,1diphosphonic acid), AMP (amino-tris(methylenephosphonic acid)), HDTMP (hexamethylenediamine-tetrakis (methylenephosphonic acid)), DTPMP (diethylenetriamine-pentakis(methylene phosphonic acid)) and BHMTPAMP (bis-hexamethylenetriamine-pentakis(methylenephosphonic acid)). Experiments were conducted in solutions containing silicate (150 ppm, expressed as SiO2) and iron (150 ppm, as Fe) at pH 7.0. The phosphonate additives used were found to act as stabilizing agents, most likely by complexing the Fe3+cations, thereby preventing ferric silicate formation. The experimental design included a plethora of physicochemical parameters, such as antiscalant concentration, salinity, and temperature. Based on the generated experimental data, numerous functional insights have been generated, contributing to a more complete and comprehensive understanding of the mechanism of ferric silicate control.