The research work presented in this paper focused on the recovery of cobalt from mixed Co-Mn hydroxide precipitates (obtained from sulphate leach liquors of nickel oxide ore), using ammonia-ammonium carbonate leaching. The characterization of the initial mixed hydroxide precipitates, as well as the corresponding leached residue was carried out by X-ray Diffraction, TG-DTA and Scanning Electron Microscopy. Cobalt and manganese precipitation was based on the statistical design and analysis of experiments, in order to determine the main effects and interactions of the precipitation factors, which were the equilibrium pH and the temperature. Co and Mn were precipitated as hydroxides at pH = 10.5 and T = 25 degrees C. using 5 M NaOH as a neutralizing agent, by 99.9% and 99.5%, respectively. The main mineralogical phases were, Mn(3)O(4) (Hausmannite), gamma-Mn(3)O(4) and CoMn(2)O(4), while Co(OH)(2) and Mn(OH)(2) (Pyrochroite) were also present as minor constituents. Cobalt and manganese separation was based on selective cobalt recovery by ammonia-ammonium carbonate leaching of the produced mixed hydroxide precipitate. The factors studied were the ammonia-ammonium carbonate concentration and the solid to liquid ratio. The cobalt recovery efficiency reached 93%. Mn(3)O(4) (Hausmannite) was the main mineralogical phase of the leached residue, while MnCO(3) (Rhodochrosite) and Mn(2)O(3) were also present. Small quantities of cobalt were also observed in the residue as CoMn(2)O(4). (C) 2010 Elsevier Ltd. All rights reserved.