Extraction of nickel from chromitiferous overburden by roasting with a CO-CO2-N-2 gas mixture

Chromitiferous overburden containing nickel in oxide form is reduced with C O-CO2-N-2 gas mixtures. This converts the nickel oxide to metallic nickel a nd restricts the iron oxide to the magnetite state to extract the nickel. R eduction experiments were carried out on 500-g samples in a bench-scale ver tical reactor. The tests were run. to establish the process parameters, inc luding the optimum partial pressure of CO, the reducing gas flow rate, gran ule size and roasting time. The reduced calcine is then subjected to an amm onia-ammoniacal carbonate leach to extract nickel selectively as its amine complex. More than 90% of the nickel could be extracted with a volumetric C O/(CO+CO2) ratio of 0.57 at 750 degreesC and a roasting time of 90 min. The addition of 2.5% (by weight) charcoal to the granules improved nickel extr action by 11% to 19%. Nickel leaching efficiency with ammoniacal solution i s compared to the bromine-methanol method.