Coffee shell as a green reductant for iron recovery from pyrite cinder: synergy of biomass-driven reduction and acid leaching

Abstract

The inefficient recovery of iron from pyrite cinder (PC), a hazardous iron-rich byproduct of sulfuric acid production, remains a critical challenge due to chemical inertness of hematite and risks of toxic H₂S generation. This study proposes a sustainable strategy integrating biomass reductive roasting with oxalic acid-assisted leaching to achieve high-efficiency iron recovery. Using coffee shell (CS) as a renewable reductant, PC was converted into reduced pyrite cinder (RPC) at 700 ℃ for 3 h (mass ratio PC:CS = 1:1), where hematite (Fe₂O₃) was stepwise reduced to reactive Fe₃O₄, FeO, and Fe(0), while simultaneously removing 35.78 % sulfur. The resultant RPC exhibited a porous structure with 21.4-fold increased surface area versus PC, facilitating rapid iron dissolution. Coupled with oxalic acid leaching (n(H₂C₂O₄)/n(Fe) = 1:65), over 97 % iron extraction was achieved within 20 min under mild conditions (60 °C, 20 % H₂SO₄, L/S = 5:1 mL·g⁻¹), outperforming untreated PC (<27 %) and yielding leachates with Fe(Ⅱ) concentrations up to 2.14 mol·L⁻¹. Thermodynamic and kinetic analyses revealed that the process shifted from interfacial reaction control (PC) to diffusion-dominated mechanisms (RPC), driven by enhanced reducibility and microstructural modification. High-purity ferrous oxalate dihydrate was recovered at 85.6 % efficiency from RPC leachate via direct precipitation, avoiding the energy-intensive photoreduction required for Fe(Ⅲ)-rich PC leachate. This strategy achieved an overall iron recovery rate of 83.7 % from raw PC to battery-grade ferrous oxalate dihydrate, synchronizing hazardous tailings utilization, critical metal recovery, and agricultural waste upcycling while offering a sustainable blueprint for circular resource economies.