In Situ Spectroscopy as a Tool to Decipher Monomer and Polymer Deposition of Silica Scale

Abstract

Silica scale deposition remains a challenging constraint to geothermal energy utilization. This is because silica deposition proceeds via several interconnected pathways, each with a complex dependence on variables in both the fluid composition, the nature of the solid surfaces and fluid flow conditions. In the solution phase silica polymerization produces nanometre sized colloidal SiO2 particles via many reactions and intermediates. Silica deposition on a metal surface can proceed via solution phase monomeric silicate (H4SiO4) and/or colloidal silica (SiO2(am)) binding to metal oxide layers that are present on metal surfaces. The progression of silica deposition depends on the interactions between the various reaction pathways and the chemical, electrostatic and fluid flow forces involved. In this paper we demonstrate the use of in situ Attenuated Total Reflectance Infrared (ATRIR) spectroscopy to differentiate between the various pathways in real time and as a function of the fluid and surface properties. In this technique an iron oxide that is representative of the oxide formed on a metal surface is deposited onto an infrared (IR) transparent crystal which is placed in a flow cell within an IR spectrophotometer. IR spectra can be collected while H4SiO4 and SiO2(am) bearing solutions are flowed through the cell and the Si-O stretching region of the spectra (700-1300 cm-1) can yield both quantitative and structural information about the silicate species formed on the oxide surface. In this way the surface chemistry of H4SiO4/SiO2(am) can be studied in situ under the desired conditions of pH, Si concentration, ionic strength, and (with a heated flow cell) temperature.