Indirect Carbonation of Victorian Brown Coal Fly Ash for CO2 Sequestration: Multiple-Cycle Leaching-Carbonation and Magnesium Leaching Kinetic Modeling

In this paper, a closed-loop multistep process which allows leaching and precipitation of magnesium and calcium as carbonate from Victorian brown coal fly ash has been examined. Victorian brown coal fly ash has a distinctively high concentration of alkaline earth metals and low amounts of aluminum and silica. The main objective here is to clarify the dissolution kinetics of magnesium and calcium in regenerative ammonium chloride and subsequent carbonation of dissolved cations. Instead of a once-through test with fresh ammonium chloride, multiple locked circuits were adopted to assess the leaching capability of regenerated ammonium salt, as well as the accumulation of impurities upon the recycling and reuse of the leaching agent. As has been revealed, upon increasing cycles of ammonium chloride use, the extraction yields of both target cations decreased significantly. Their extraction by ammonium chloride was favored by the presence of the oxide form in the original ash sample, with the extraction of calcium occurring much faster than that of magnesium. Both phenomena were in agreement with the thermodynamic equilibrium prediction on the lowest Gibbs function for the dissolution of oxides, especially calcium oxide in ammonium chloride solution. Carbonation results dropped gradually upon the increase in the cycle number; meanwhile, the size and morphology of precipitates were changed from the first to last cycle. By fitting the observed results with a shrinking core model, it was shown that the extraction of Mg2+ followed a pseudo-second-order reaction with a nonconstant ammonium chloride concentration in the film layer on the surface of a solid particle. The activation energy of 20.7 kJ mol(1) was obtained for the dissolution of magnesium from both Hazelwood fly ash and pure MgO in ammonium chloride solution.