Selective Nickel Leaching from Cu-Ni Sulfide Ores Using Deep Eutectic Solvents

Traditional hydrometallurgical lixiviants such as sulfuric and hydrochloric acid pose environmental and safety risks due to their corrosive and toxic nature. To advance greener extraction technologies, this study evaluated deep eutectic solvents (DESs) as potential sustainable alternatives for the selective extraction of nickel (Ni) from polymetallic sulfide ore concentrates. Three choline chloride (ChCl)-based DESs, ethaline, reline, and maline, were compared using a factorial experimental design approach. All of the DESs demonstrated selectivity for Ni, indicating their potential for targeted metal extraction with minimal co-dissolution of copper (Cu) and cobalt (Co). Ethaline achieved the highest Ni recovery of 56.3%, while maline achieved moderate Cu recovery (19.6%). The Co recoveries were low (~16%) across all solvents. Factor screening identified the pulp density as the dominant parameter influencing Ni recovery, with lower solid loading enhancing the leaching efficiency. Under optimized conditions (0.5% w/v, 600 rpm, 25 °C, 1 h), a peak Ni recovery of 59.1% was achieved, consistent with the response surface methodology (RSM) model prediction of 58.1%. The recovery ceilings below industrial benchmarks were attributed primarily to the locked nature of pentlandite (<30% liberation), which likely restricted the reactive surface area accessible to the DES lixiviant. The study highlights the potential of ChCl-based DESs as selective, energy-efficient, and environmentally benign lixiviants for nickel extraction.