Reduction crystallization of Ni, Cu, Fe and Co from a mixed metal effluent

Removal and recovery of heavy metals from effluent are major concerns due to diminishing fresh water resources, depletion of exploitable ores and human and environmental health concerns. The objective of this work was to efficiently recover heavy metals from effluent in their elemental form as metallic powder by reduction crystallization. This method recovers metals in a pure form and enables them to be directly used. Experiments were conducted using mixed metal solutions of Ni, Cu, Co, and Fe in a 20 L Perspex batch reactor using hydrazine as a reducing agent and nickel powder as seeding material. Ni, Cu, Co and Fe were effectively reduced to their elemental states with removal efficiencies of over 99% for Ni and Co and about 98% for Cu and Fe. Residual concentrations obtained for Ni, Co and Fe were below 0.05 mg/L and below 1.20 mg/L for Cu. Based on the evolution of the particle size distribution (PSD) and its derived moments the dominant particulate processes identified were aggregation, growth and breakage with the possibility of nucleation in the presence of Fe. However, particle size enlargement was largely due to aggregation.