Pilot-scale optimization of atmospheric acid leaching for enhanced nickel and cobalt recovery from laterite ore

Nickel and cobalt are critical metals predominantly extracted from laterites. However, deposits with high iron content (>15%) and SiO2/CaO + MgO ratios (∼1.8) limit the applicability of conventional pyrometallurgical routes. In this scenario, hydrometallurgical processes, particularly leaching technologies, have emerged as promising alternatives for treating low-grade and complex ores. This study evaluated the technical potential of atmospheric sulfuric acid leaching as an alternative to high-pressure methods, using a chlorite- and lizardite-rich lateritic ore in pilot-scale experiments. Sodium sulfite, which serves as a reducing agent, was introduced to induce the in situ formation of SO2, thereby reducing Co³+ to soluble Co2+ and enhancing the overall dissolution kinetics of nickel and cobalt. Using factorial experimental design and central composite design, we identified residence time and leached liquor recirculation as critical variables, increasing the recovery of nickel by 33.7% (8 h with recirculation vs. 2 h without recirculation). Under optimized conditions, the recoveries of nickel and cobalt were 94.13% and ∼100%, respectively, with a corresponding reagent consumption of 770 kg of H2SO4 per ton of ore and 8.21 g/L of Na2SO3. The reproducibility was confirmed at the pilot scale, with experimental deviations lower than 5%. These results demonstrated the operational robustness of the process and highlighted its potential for the valorization of marginal deposits under industrially relevant conditions.