Milk of lime precipitation of Mg-rich nickel laterite leachate for reactive MgO preparation and potential internal reuse

Continuous Mg 2+ accumulation during hydrochloric acid leaching of nickel laterite ore causes magnesium loss and increases the burden of saline wastewater treatment. In this study, magnesium was recovered from Mg-rich leachate as reactive MgO by precipitation using milk of lime followed by calcination. Simulated MgCl 2 -rich solutions revealed the concentration-dependent precipitation behavior. At low Mg 2+ concentrations, Mg(OH) 2 formed mainly through direct precipitation and produced dense lamellar precursors. At Mg 2+ concentrations of 30 g/L and above, CaMg 2 Cl 6 ·12 H 2 O was detected as a transient chloride-containing double salt during the early precipitation stage, and honeycomb-like porous Mg(OH) 2 precursors were obtained after washing. Calcination of the precursor produced MgO with improved citric acid activity (CAA). The optimized process was validated using purified Mg-rich leachate, achieving a stoichiometry-based Mg precipitation efficiency of 95.47% and producing MgO with approximately 98 wt% purity. The synthesized MgO showed CAA of 22.52 ± 1.19 s, a specific surface area of 60.75 m 2 /g, and Fe removal performance comparable to commercial MgO, supporting its potential internal reuse as a pH adjustment reagent in hydrochloric acid leaching processes.