China Nonferrous Metallurgy

To address the problems of low grade of crude cobalt hydroxide and severe magnesium co-precipitation in the conventional magnesium oxide （MgO）-based wet cobalt precipitation process， an ammonium salt-MgO synergistic cobalt precipitation process was proposed in this study. By introducing ammonium salts such as ammonium sulfate， ammonium chloride， and ammonium bicarbonate， the cobalt precipitation efficiency was improved and impurity entrainment was suppressed. Using the iron-removal liquor from the hydrometallurgical treatment of copper-cobalt ores in the Democratic Republic of the Congo as the raw material， the effects of different ammonium salt types， ammonium sulfate concentration （0. 1-0. 3 mol/L）， and slurry-conditioning time （0-3 h） on cobalt precipitation rate， product grade， and residual magnesium ion content were systematically investigated. In addition， the synergistic mechanism of the ammonium salt-MgO system for cobalt precipitation was explored. The results showed that the ammonium sulfate-MgO synergistic system exhibited the best performance. Under the conditions of 0. 2 mol/L ammonium sulfate concentration，5% addition，0. 5 h slurry-conditioning time，25℃， and a stirring speed of 300 r/min， the cobalt grade of the product reached 52. 45%， while the magnesium ion content decreased to 1. 23%， representing a 78. 4% reduction compared with the conventional process， and meeting the first-grade standard （Co≥30%， Mg≤3%）. The role of ammonium sulfate in the MgO-based cobalt precipitation system can be attributed to two aspects. On the one hand， it suppresses excessive local alkalinity through dynamic pH buffering， thereby reducing the formation of magnesium hydroxide. On the other hand， it accelerates MgO hydration through interfacial activation， promotes the selective precipitation of cobalt ions， and inhibits magnesium co-precipitation. This process resolves the key problem of magnesium entrainment in the traditional MgO cobalt precipitation process， and provides an innovative solution for the efficient recovery of low-grade cobalt resources and the sustainable development of new energy materials， with significant economic and environmental benefits.