Dissolution of Heat Activated Lizardite for Direct Aqueous Carbonation at Elevated Pressures and at Low Temperatures

Abstract

Sequestration of CO₂ through direct aqueous carbonation is an evolving technology which is being exploited to form stable magnesium carbonates, and thus storing CO₂ permanently. In this study, dissolution experiments of heat activated lizardite were performed at high pressures (100 bar) and low temperatures (40 áµ’C) to explore the effect of the enhanced solubility of CO₂ under these conditions, and how this influences the extent of Mg extraction under these conditions. An important aspect of the investigation was to examine the effect of particle size and solid ratio on the dissolution rate of Mg and Si, in the absence of sodium bicarbonate.

Aliquots of the dissolving lizardite slurry were periodically sampled during the dissolution process, to provide insight into the dissolution mechanism. The dissolution kinetics disclosed two distinct dissolution regimes, with a rapid initial rate of Mg extraction, resulting the fraction of Mg extracted ranging from 30 to 67 % during the first 20 minutes of the dissolution process, following which the dissolution rate decreases dramatically. The fraction of Mg extracted increased with decreasing particle size, with a maximum Mg of 70% observed with a sub-20 micron size fraction.

The results showed that, for a given mass of lizardite, the extent of dissolution of fine particles is higher than larger size particles, assumed to due to the higher total surface area of the smaller size particles. The dissolution of all particle size fractions is initially relatively rapid, due to high extent of CO₂ dissolution, which leads to a lower pH, while during the latter stages of lizardite dissolution, CO₂ dissolution becomes rate limiting .

The rapid decrease in rate of Mg extraction may be a result of the formation of a silica-rich layer around a core of un-leached particle, which presents a diffusion barrier and inhibits the rate of further dissolution. An alternative explanation may be that the dissolved Si in the supernatant solution reaches a concentration close to or in excess of the equilibrium Si, which may then lead to silicon precipitation (polymerisation or gelation). Another alternative explanation of the observed rapid reduction in the rate of Mg dissolution could be the result of the reaction of Mg and Si to form new magnesium silicate phase on the surface of the particle undergoing dissolution, which could inhibit the net rate of dissolution.