Mechanisms Underpinning the Mobilization of Iron and Magnesium Cations from Victorian Brown Coal Fly Ash

Abstract

This paper reported the leaching of Yallourn brown coal fly ash uniquely rich in magnesioferrite (MgFe2O4) using hydrochloric acid, by varying acid concentration, liquid-to-solid (L/S) ratio, temperature, and time, to understand the mechanisms underpinning the extraction of iron and magnesium. Results indicate that Yallourn fly ash is composed of a densely packed, crystalline ash matrix, mainly in the form of MgFe2O4 and maghemite (γ-Fe2O3). Thermal cleaving at 100 °C and above liberates embedded octahedrally coordinated Fe3+, a controlling step for the leaching process. Increments from 100 to 200 °C did little to enhance extraction of Fe and Mg in single-stage leaching. Except at 70 °C, the thermal decomposition for the elution of both metals was complete within 1 min, while increasing the L/S ratio and acid concentration had negligible influence. The application of two-stage leaching gave protons a larger driving force to access embedded unreacted species. Subsequently, the extraction yields of both Fe and Mg went beyond 80%. The reductive leaching of Fe3+ with the assistance of a small amount of marcasite (FeS2) is the crucial reaction improving the breakage of framework at 100 °C and above. Consequently, the concentration of eluted Fe2+ accounted for 60% and up to 90% in 100 and 200 °C leachates, respectively.