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This paper reported the leaching of Yallourn brown coal fly ash uniquely rich in magnesioferrite (MgFe 2 O 4 ) 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 MgFe 2 O 4 and maghemite (γ-Fe 2 O 3 ). Thermal cleaving at 100 °C and above liberates embedded octahedrally coordinated Fe 3+ , 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 Fe 3+ with the assistance of a small amount of marcasite (FeS 2 ) is the crucial reaction improving the breakage of framework at 100 °C and above. Consequently, the concentration of eluted Fe 2+ accounted for 60% and up to 90% in 100 and 200 °C leachates, respectively.

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