ABSTRACT
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.
ABSTRACT
Electric Arc Furnace Dust (EAFD) is a byproduct of the steelmaking industry, which is one of the most significant and rapidly generated hazardous waste. This material includes recyclable elements such as zinc (Zn) and iron (Fe), exclusively in the form of zinc-ferrite (ZnFe2O4), zincite (ZnO), magnetite (Fe3O4) with some other minor compounds of Si, Mn, Mg, and Pb. A combination of pyro and hydrometallurgical route, also known as hybrid process, has acquired considerable attention to extract Fe and Zn from electric arc furnace dust (EAFD). In the current study, systematic thermodynamic assessments using thermochemical-software FactSage and targeted experimental investigations were carried out to assess the suitability of the caustic-roasting step of this important hybrid process. Thermodynamic calculation of the effect of different parameters on the conversion of EAFD to a suitable feedstock for the next separation stages showed that the major zinc-ferrite (ZnFe2O4) phase of EAFD can be potentially converted to ZnO and sodium-ferrite (NaFeO2) intermediate phase while reacting with NaOH (>2 mol) at temperatures equal or higher than 100 °C. The suitable condition was assessed to an EAFD to NaOH ratio of 1:3 M fraction and a temperature, T > 318 °C at the normal atmospheric condition. Moreover, the selected experimental investigation was also showing a good agreement with the current thermodynamic assessment.
