Repeated recycling of calcium carbide slag for MgO extraction from reject brine

Freshwater scarcity has led to large-scale seawater desalination, generating 141.5 million m³ of brine daily. Among the elements in reject brine, magnesium (Mg) is the most abundant after sodium and chloride, making it a promising resource for MgO production. Conventional Mg extraction methods rely on commercial alkalis such as CaO, which are carbon- and cost-intensive. This study introduces an alternative approach using carbide slag, an industrial waste rich in Ca(OH)₂, as a sustainable alkali source. The alkaline solution derived from carbide slag reached a pH of 12.5 and enabled 99.6 % Mg²⁺ extraction from reject brine. The final product MgO presented 80 % purity. To produce 1 ton MgO, the energy consumption was 2.92 GJ with associated CO₂ emissions of 0.19 ton, which was more than ten times lower than the conventional chemical-based recovery process. Furthermore, the results demonstrated the potential application of the recovered MgO as sustainable construction material, as it can sequester CO₂ and develop mechanical strength, thereby further reducing its carbon footprint. In addition, the carbide slag was recycled repeatedly for multiple extraction cycles. After 2 cycles, MgO yield increased by 1.8 times, reducing slag consumption by 46 %. The alkaline solution from the third cycle maintained a pH of 12.4, suggesting the potential for further repeated cycles. Overall, the results demonstrate the technical and environmental feasibility of the proposed approach.