High-pressure acid leaching of laterite ores: Effect of acid and solid content on Ni and Co yield under non-isothermal and isothermal conditions

The increasing demand for nickel (Ni) in electric vehicle batteries necessitates efficient extraction methods from laterite ores. High-Pressure Acid Leaching (HPAL) is a prominent hydrometallurgical technique known for its rapid kinetics and high (>90 %) metal dissolution. This study examines the influence of acid-to-ore (A/O) ratios (0.25, 0.35, and 0.45) and slurry solid contents (22, 26, and 30 wt.%) on Ni yield and selectivity under both non-isothermal (100 °C, 1.2 bar; 150 °C, 4.7 bar; 200 °C, 18.9 bar; 265 °C, 50.5 bar) and isothermal (265 °C) conditions. Under non-isothermal conditions, Ni leaching efficiency improved with temperature, achieving a maximum yield of 94.67 ± 0.02 wt.% at 265 °C, compared to 50.86 ± 0.02 wt.% at 200 °C, using an A/O ratio of 0.45 and 26 wt.% solids over 60 min. Isothermal experiments at 265 °C revealed that higher A/O ratios enhanced Ni yield, reaching 94.45 ± 0.01 wt.% at 60 min with 26 wt.% solids and an A/O of 0.45. However, this increased yield was accompanied by higher dissolution of Fe (1.07 ± 0.12 wt.%) and Al (38.84 ± 0.07 wt.%), reducing selectivity to 0.73 ± 0.01. Conversely, an A/O ratio of 0.25 at 26 wt.% solids achieved a higher selectivity of 3.83 ± 0.05 but with a lower Ni yield of 79.14 ± 0.03 wt.%. Lower slurry solid content further improved Ni leaching; at 22 wt.% solids and an A/O of 0.45, Ni yield reached 95.87 ± 0.01 wt.% with a selectivity of 0.89 ± 0.02 at 60 min. Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD) analyses of leached residues indicated that high temperatures facilitate the transformation of primary minerals like goethite and gibbsite into hematite and hydronium alunite. This transformation is more pronounced at higher A/O ratios, affecting the mineralogical composition of the post-leach solid residue. • Over 90 wt.% Ni was extracted in 30 min using optimal acid and solid content. • Selectivity depends on the balance between acid concentration and solid load. • High Ni/Co selectivity and increased Fe/Al precipitation occurred at high temperatures. • XRD and SEM-EDS analyses showed a phase transformation that supports metal yield. • Ni yield at 265 °C, 60 min, A/O 0.45: 94.67 wt.% (non-isothermal) vs 94.45 wt.% (isothermal).