Effects of coexisting zinc and nickel ions on crystallization behaviour and properties of spinel synthesised in chromium-containing wastewater

Electroplating wastewater contains chromium ions, zinc ions, and nickel ions due to the emergence of a multi-layer plating process and unified treatment of mixed wastewater. The effects of coexisting zinc and nickel ions on crystallization behaviour and properties of spinel synthesised in chromium-containing wastewater by ferrite method were investigated in the study. The results indicated that the quality of the supernatant and the stability of synthetic products were barely affected. However, the magnetic property of the synthetic products was significantly increased when the concentration ratio of zinc and nickel ions was appropriate. With the extension of stirring time, the stability of the synthetic product increased, but the crystallinity gradually decreased. Chromium and zinc ions entered the synthetic products preferentially over nickel ions during ferrite synthesis. In addition, the morphology of the synthetic products was irregular and dense, and the lustre distribution was uniform. The distributions of iron, chromium, zinc, nickel and oxygen were overlapped, owing to a single spinel, and the molecular formula of synthetic products was speculated as FexCryZnzNinO4 (3.53 < x < 3.99, 0.2 < y < 0.36, 0.04 < z < 0.05, 0.03 < n < 0.05). Finally, the transport model of metal ions in the mixed wastewater was established, and the synthesis mechanism was revealed.