Directed precipitation of anhydrous magnesite for improved performance of mineral carbonation of CO2

Abstract

This paper studies the indirect aqueous carbon sequestration via Mg(OH)2 using directed precipitation technique. This technique produces anhydrous MgCO3 (magnesite), the most desirable carbonated phase for sequestration. The formation of magnesite is significantly affected by its kinetics of precipitation in an aqueous carbonation medium. This study considers directed precipitation strategy to control precipitation of anhydrous magnesite through enhancement of the heterogeneous precipitation. Heterogeneous precipitation is implemented using seeding material that could improve the conversion efficiency of the directed carbonation of Mg(OH)2. A ternary phase diagram is achieved which represents the relative concentration of possible precipitated phases: brucite (Mg(OH)2), magnesite and hydromagnesit (Mg5(CO3)4(OH)2·4H2O). The results reveal the fundamental role of heterogeneous precipitation on the magnesite concentration and conversion percentage of Mg(OH)2 wet carbonation process. Two seeding materials, hydrophobic activated carbon and hydrophilic alumina, were tested and the influence of the surface chemistry of varying seeding sites (hydrophobic vs. hydrophilic seeds) was elaborated. At the carbonation temperature of 100°C and 150°C, a heterogeneous precipitation using hydrophilic alumina results in lower concentrations of anhydrous magnesite in precipitated compounds, even as compared to the seedless solution, owing to the hydrophilic properties of alumina. In contrast, use of activated carbon as heterogeneous nucleation sites in an aqueous medium results in a magnesite concentration of around 60% and the corresponding carbonation conversion of about 72% under the controlled condition of 200°C and 30bar CO2 pressure.