Volatilisation of trace elements during reduction of iron ore by hydrogen: statistical analysis, kinetic study and environmental assessment

Reduction of iron ore directly by H₂ has been developed as an alternative technology for iron production, which can alleviate pollutant emissions by eliminating the needs of cokemaking and sintering processes. However, the behaviour of trace elements (especially the volatilisation of heavy metal(loid)s) in iron ore during reduction by H₂ is still unclear and the relevant research is scarce in literature. This study aimed to provide an insight into the volitilisation behaviour of trace elements and the environmental assessment during reduction of Australian iron ore with H₂ from room temperature to 1000 °C. The trace element concentrations in the iron ore at different reduction tempatures were analysed along with changes in chemical structure and surface morphology using X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy and scanning electron microscopy. Results showed that the extent of As, S, Li, Sn and Pb volatilisation varied with temperature with nil, 22%, 82%, 82% and 72% respectively remaining in the sample at 1000 °C. Simutaneously, stepwise reduction of iron ore (goethite → wustite) with temperature was confirmed. Pearson's method of correlation was employed to determine the correlations between pairs of key indexes (LOI 1000, mass loss%, O loss% and loss ratios of elements) of goethite reduction. Results indicated that S loss exhibited a high correlation (r = 0.977) with Pb loss, implying the simultaneous volatilisation of Pb and S as well as possible presence of Pb and S-containing compounds, such as PbS in the sample. Li and Sn showed the coincidental simultaneous volatilisation as indicated by the highest correlation (r = 1.000 with p ≤ 0.01). The kinetic analysis indicated that the volatilisation of above elements best fitted diffusion models (D1 or D3) with activation energy (E) ranging within 11.65–26.75 kJ/mol. Risk assessment analysis demonstrated a much higher risk score of iron ore reduciton at 1000 °C (value of 94.758) than 200 and 500 °C (value of 1–3) due to obvious volatilisation of As and Pb at high temperature. Life cycle impact assessment (LCIA) data confirmed four valid impact categories with human toxicity being the most significnat with the major contribution ascribed to As element. Generally, the hazardous elements released from the ore reduction process can be eliminated before their discharge to the environment.