Dissolution and Condensation Kinetics of Silica in Alkaline Solution

Abstract

SummaryChemical loss through long-term rock reactions is a major impediment for practical application of alkaline waterflooding. This paper addresses the viability of soluble silicates reducing hydroxide consumption by siliceous rock reactions. A convenient differential rate method is used to obtain new experimental kinetic data for the dissolution/condensation rates of quartz at 23 and 70°C [73 and 158°F] and amorphous silica at 23 °C [73 °F] over the pH range from 11 to 12 and in 1 M NaCl. The role of added silicates is studied from zero solution concentration to considerably above the solubility limit.Silica dissolution rates are observed to slow as the solubility limit is reached and become negative (i.e., net precipitation occurs) above the solubility limit. The kinetic rates decrease with increasing solution silicate in a parabolic fashion. Most important, the solubility limit of quartz (amorphous silica) controls the deposition rate of quartz (amorphous silica) over the range of pH values and temperatures investigated. That is, crystalline quartz grows directly on solid quartz particles, rather than deposition of amorphous silica with subsequent alteration to the crystalline habit. This means that relatively small amounts of soluble silicates, approaching the solubility limit of quartz, can protect against hydroxide consumption by quartzitic rock reactions.A molecular dissolution/condensation reaction scheme is proposed, based on equilibrium adsorption of unionized silicic acid. With this reaction path, a mathematical model is developed that agrees quite well with the measured kinetic rate data.