Recent developments in the carbonation of serpentinite derived Mg(OH)2 using a pressurized fluidized bed

Abstract

By using a pressurized fluidized bed (PFB) reactor for the carbonation of serpentinite derived Mg(OH)2 we want to utilize, for the purpose of minimizing process energy requirements, the exothermic reaction that leads to the binding of CO2 in a stable and environmentally benign form, MgCO3. Recent results show that most of the carbonation takes place only minutes into the experiment, suggesting that a carbonate layer forms on top of the Mg(OH)2 core and inhibits further carbonation. However, this problem might be eliminated/reduced by increasing the fluidization velocity. On the other hand, the suddenly lowered reactivity might also be the result of MgO formation, which is much less reactive than the initial Mg(OH)2. These and other aspects of gas/solid carbonation using a PFB being investigated include CO2 pressure, temperature, particle size, particle amount (or bed size) and water injection. The influence of temperature and pressure on the carbonation of Mg(OH)2 is evident, but the other factors are less clear. For instance, water has been shown by others to catalyze magnesium carbonation, but our results obtained so far using small amounts (0.1–2% vol- H2O/vol- CO2) of H2O injected into the CO2 stream have not confirmed this. This does not mean that H2O injection could not be beneficial, but implies that its effect has hitherto been clouded by other factors. Most of the experiments have been performed using commercially produced (Dead Sea Periclase Ltd.) Mg(OH)2, but progress in our Mg extraction process for magnesium silicate rock has resulted in sufficient amounts of Mg(OH)2 for use in our PFB reactor as well. Preliminary tests have been promising resulting in around 50% magnesium carbonation conversion in less than 10 minutes for Mg(OH)2 particles of 250–425 μm at 500 °C and 20 bar. In comparison, the Dead Sea Mg(OH)2 particles of same size fraction resulted in only 27% conversion in otherwise similar conditions. This is apparently a result of much lower porosity (0.024 vs. 0.24 cm3/g) and specific surface area (5.4 vs. 46.9 m2/g) for the Dead Sea Periclase sample.