Kinetics of Magnesium Extraction from Activated Serpentine by Carbonic Acid

Abstract

Ex situ mineral carbonation heavily depends on the ability of cations (e.g., Mg2+) to form stable carbonates. The abundant serpentine minerals contain sufficient magnesium to fill this role. Serpentine partially dehydroxylated by thermal pre-treatment, which has a nearly amorphous state (meta-serpentine), has been shown to enhance overall carbonation yield. However, the dissolution kinetics of meta-serpentine under the carbonic acid system remains largely unresolved within the literature. In this thesis, the kinetics of magnesium extraction from thermally activated serpentine under the CO2-H2O system was investigated for the purpose of identifying the underlying rate-limiting mechanisms. The extraction rates of magnesium and silica from thermally activated serpentine were measured over a wide range of conditions, covering different reaction temperatures (303–473 K), pressures (10–160 bar), mass loading factors (0.03–1 min), and particle sizes (20–180 μm) using a continuous fluidised bed reactor under the saturated CO2-H2O system. Extraction of silica almost always followed the equilibrium solubility of amorphous silica. Magnesium is believed to be released by the protons associated with the MgO-CO2-H2O equilibria. The kinetic implications of all the measured magnesium extraction rates were evaluated using a far-from-equilibrium pH dependence mechanism. At undersaturated conditions, the remarkable unifying rate behaviour across various experiments implied that the extraction of magnesium is more-or-less proportional to the bulk equilibrium concentration of protons. In addition, the associated term for activation energy with the rate law appeared negligible. At saturated conditions magnesium extraction was inhibited, which was attributed to precipitation of secondary phases in the porous substrate.