Kinetics of silica polymerization and deposition from dilute solutions between 5 and 180°C

Abstract

Between 5 and 90°C the maximum rate of silica polymerization in dilute neutral solutions at constant supersaturation, increases only slowly (formal Energy of Activation = 3 kcal/mole). From 90 to 180°C the maximum rate decreases to below the rate at 5°C. The reactions show induction periods, whose lengths increase with temperature and decrease with supersaturation. The maximum reaction rates after the period of induction obey approximate 4th order kinetics. As the reaction temperature increases from 5 to 180°C, the average molecular weight of the polymers formed increases from approximately 10 5 to 10 9. A model is proposed, which is consistent with these kinetics. It assumes a slow bimolecular reaction between monomers; as polymers are formed, active sites on them react rapidly with further monomer. Because fewer, but larger, polymers are formed at higher temperatures, the reaction rate is little affected by temperature. Between 5 and 90°C the total silica concentration remains constant throughout the polymerization reaction. Above 90°C, the total silica concentration progressively decreases with rising temperature, as formation of larger polymers leads to silica deposition. Deposition is accelerated by rocking the reaction vessel. At 180°C, in an Incoloy-lined vessel, the total silica concentration falls to below the quartz solubility; possibly some iron or nickel silicates are formed. At 180°C in a gold reaction vessel, the total silica concentration falls to the quartz solubility; the deposit consists of amorphous silica, cristobalite, and a trace of quartz. It is suggested that at elevated temperatures, especially in iron pipes, aerated geothermal waters could deposit silica scales, even though the silica concentration of the water is near, or even below, the amorphous silica solubility at that temperature.