Optimization and Comparative Analysis of Neutralization Agents for Iron, Aluminum, and Chromium Removal in HPAL Pregnant Leach Solution

High-pressure acid leaching (HPAL) pregnant leach solution (PLS) derived from laterite ore contains valuable metals, such as nickel and cobalt, alongside impurities, including iron, aluminum, and chromium. Efficient impurity removal with minimal losses of valuable metals is essential for effective PLS purification. This study investigates the effect of CaCO₃ particle size (− 400 mesh) on precipitation performance, revealing an 11% improvement in aluminum removal efficiency compared to coarser particles. A two-stage precipitation process was found to be optimal. In the first stage (80°C, pH 3.0, 2 h), 96% Fe, 92% Al, and 97% Cr were removed, with minimal co-precipitation of Ni (1.04%) and Co (2.05%). The second stage (70°C, pH 4.5, 1.5 h) achieved 99.9% removal of Fe, Al, and Cr, though with slightly higher Ni loss (3.95%). Morphological analysis revealed that CaCO₃ produced crystalline residues, facilitating improved solid-liquid separation compared to the amorphous residues formed using NaOH and MgO. The thermogravimetric analysis confirmed superior moisture control in CaCO₃-derived residues, showing only 2.88% mass loss at 115°C, significantly lower than residues from MgO and NaOH. These results provide practical guidance for impurity removal in HPAL operations and highlight the effectiveness of CaCO₃ as a neutralizing agent.