Microstructure Evolution and Phase Transformation of Iron Produced from the Sustainable Carbothermic Reduction Process of High-Pressure Acid Leaching Residue

This paper describes the microstructure evolution and phase transformation of residue from the lateritic nickel ore high-pressure acid leaching (HPAL) during carbothermic reduction using palm kernel shell (PKS) charcoal as a reductant. The study consisted of thermodynamic calculations combined with analysis of reduction experiments. The thermodynamic assessment predicted that the main phases formed during the reduction process were γ-Fe metal, liquid metal, slag, and spinel, with abundance of reducing gas (CO and H₂) at the temperature range of 750-1500 °C. The experimental study shows that the resulting product upon cooling was sponge iron or direct reduced iron when HPAL residue-PKS charcoal composites were reacted up to 1400 °C for 45 min. The sponge iron had an average apparent density of 5.8 ± 0.1 g/cm³ and a 90.8 ± 0.4% metallization degree. The microstructure analysis revealed that as the reduction time was increased, small iron nuggets began forming on the surface of the reduced product. By addition of Na₂CO₃, the separation of iron nuggets from slag appeared to be improved, hence enhancing the overall reduction process. Furthermore, iron nuggets’ highest apparent density and metallization degree were obtained at 7 ± 0.1 g/cm³ and 98 ± 0.5%, respectively, when adding Na₂CO₃ of 6 wt %. The phase and microstructure analyses also revealed that the iron nuggets comprised coarse pearlite, eutectic cementite, ledeburite, and sulfides. Thus, this study offers alternative sustainable process conditions for simultaneously handling the HPAL residue using PKS waste to produce metallic iron.