Acid leaching process of an ultramafic mine tailing for indirect CO2 mineralization

The extraction of divalent metals from ultramafic mine tailings for indirect CO₂ mineralization is a promising strategy for large-scale carbon sequestration. This study investigates the acid leaching process of an ultramafic nickel tailing, rich in magnesium silicates, using two different acids: hydrochloric acid and citric acid. The aim was to evaluate the leaching efficiency of key metals, including magnesium, calcium, and iron, under varying operational conditions. These conditions included acid concentration (0.5 to 2 mol/L), solid-to-liquid (S/L) ratio (0.01 to 0.5 g/mL), temperature (25 to 65 °C), and multi-stage leaching. The experimental results show that both acids are effective in leaching the target metals, but with differing efficiencies. HCl, being a strong acid, exhibited higher leaching efficiency, particularly for Mg and Fe, due to its complete dissociation and greater ability to break down the mineral structure. Citric acid, a weaker organic acid, demonstrated more moderate leaching efficiency, especially for Mg, with a relatively lower impact from increased acid concentrations. The efficiency of metal extraction in both systems decreased with higher S/L ratios, likely due to reduced surface area for leaching, and the formation of more viscous slurries that hindered the movement of the leachant. Temperature had a distinct effect on the two acid systems. In the HCl system, increasing temperature resulted in a decrease in leaching efficiency for some elements like manganese, nickel, and sulfur, likely due to precipitation or changes in the solubility of certain metal species. In contrast, in the citric acid system, elevated temperatures improved the leaching efficiency for elements such as Mg, Fe, and Mn, enhancing the complexation and solubility of metals. However, temperature had minimal effect on other elements that do not readily form stable complexes with citric acid. The findings highlight that both HCl and citric acid can be effectively used for the acid leaching step in indirect CO₂ mineralization, with HCl providing higher extraction efficiencies for faster leaching and broader applicability to various metals, while citric acid offers a more environmentally friendly, less corrosive alternative. The results provide valuable insights into optimizing the leaching conditions to improve the overall efficiency of indirect mineral carbonation, which could contribute to more sustainable mining practices and enhanced CO₂ sequestration technologies.