Reductive leaching of iron and magnesium out of magnesioferrite from Victorian brown coal fly ash

This paper for the first time reports the reductive leaching of an iron-rich brown coal fly ash composed principally of a chemically resilient magnesioferrite (MgFe2O4) matrix. The simultaneous mobilization of Fe and Mg out of magnesiorferrite here aims to produce abundant Mg2+ that can convert into high-purity MgCO3, through a mineral carbonation process for CO2 capture, and abundant Fe2+/Fe3+ that can convert into value-added high-purity Fe-rich compounds such as FeOOH. Sulfurbearing compounds, including Na2S, Na2S2O4, and FeS2, were used as reductants, on the basis of the fact that S is one of the inherent elements in fly ash that has a Fe-reductive capability. Synchrotron-based X-ray absorption near-edge spectroscopy was used to quantitatively determine the speciation of Fe (Fe2+ or Fe3+) and S (SO42− or S2−) in the leachate produced. Leaching with Na2S2O4 and FeS2 was found to produce the most Fe2+ (more than 70% of total eluted Fe) in the leachate at 200 °C. Increasing the leaching temperature is beneficial in increasing the reactivity of FeS2, leading to a greater amount of Fe2+ produced at 200 °C, whereas Na2S2O4 reached its best performance at 100 °C. This is due to a quicker dissolution of Na2S2O4 into the leachate to promote the reduction of inherent Fe3+-bearing ash matrix in the liquid phase, whereas FeS2 mainly remains as a solid, which is less reactive. None of the mechanisms involved affected the total Mg2+ cations eluted. Increasing the molar ratio of S to Fe from 0.125 to 0.5 completely reduced all aqueous Fe3+ present to Fe2+ for both reductants. Concurrent with this was an incremental change in total aqueous Fe amount when Na2S2O4 was used. No significant increase in total Fe eluted was observed when FeS2 was used. The fate of S differs for both cases, with S mostly mobilized in the leachate when Na2S2O4 was used while predominantly being in the solid leaching residue in the case of FeS2. In light of this, the use of FeS2 is more promising on a large scale, although it is less active.