Sustainable recovery of high-purity nickel from real electroplating sludge <i>via</i> optimized leaching–precipitation process

This study presents an integrated process combining inorganic acid leaching and metal precipitation to recover nickel from real electroplating sludge, offering a sustainable approach suitable for direct application in production facilities in Vietnam. The leaching step was optimized using 5% H₂SO₄ at a solid/liquid (S/L) ratio of 1 : 10, a temperature of 55 °C, and a duration of 40 min, achieving a Ni leaching efficiency of 99.70% while also enabling the effective dissolution of Fe and other coexisting metals, thus demonstrating the high overall leaching efficiency of the process. The resulting leachate was subsequently treated with NaOH to precipitate nickel, with key parameters including pH (2-11), and initial [Ni]₀ concentration (500-4000 mg L^-1) systematically examined. Optimal conditions were identified at pH 9, and an initial [Ni]₀ concentration of 2000 mg L^-1, yielding a recovery efficiency greater than 98%. XRD analysis confirmed that the precipitate consisted predominantly of crystalline NiOOH and Ni(OH)₂ with negligible impurity phases. SEM images revealed uniform polyhedral crystals, and EDX spectra verified nickel as the dominant element with minimal contaminants. Collectively, these findings demonstrate that the proposed process delivers both outstanding metal recovery and minimal secondary pollution, providing an economical, environmentally benign, and practical solution for small-scale industrial treatment of electroplating sludge.