Key factors affecting Ni leaching kinetics from laterite nickel ores in a citric acid–ammonium bisulfate system: shrinking core model and experimental validation

An atmospheric pressure leaching process using a citric acid-ammonium bisulfate mixture was proposed to extract Ni from low-grade laterite nickel ore with transition-type characteristics with high efficiency. The effects of stirring speed, citric acid concentration, ammonium bisulfate concentration, leaching temperature, leaching time, and liquid-to-solid ratio on the leaching behavior of multiple metal elements were systematically investigated low-grade laterite nickel ore with a particle size below 32.90 μm. Under the optimal conditions: stirring speed, 400 rpm; citric acid concentration, 1.5 mol/L; ammonium bisulfate concentration, 0.7 mol/L; leaching temperature, 90 °C; leaching time, 150 min; and liquid-to-solid ratio, 10 mL/g. The leaching efficiencies of Ni, Co, Mn, and Mg reached 94.1 %, 92.1 %, 93.0 %, and 73.2 %, respectively, whereas the leaching efficiency of Fe was 43.8 %, demonstrating favorable metal selectivity and enrichment performance. Kinetic analysis revealed that the leaching of Ni followed the shrinking core model and was jointly controlled by interfacial chemical reaction and diffusion processes, with an apparent activation energy of 38.60 kJ/mol. Particle size analysis showed that the introduction of ammonium bisulfate facilitated the shrinking and disintegration of mineral particles. Furthermore, XRD, FT-IR, and SEM-EDS characterizations confirmed that the mixed-acid system effectively disrupted the crystalline silicate structures and iron-rich phases in the ore, thereby promoting Ni liberation and enhancing its leaching efficiency. This study provides a feasible technical route for the low-temperature, atmospheric-pressure, and environmentally friendly extraction of Ni from low-grade laterite nickel ore, and offers valuable insights for subsequent metal separation and high-value resource utilization.