The role of CaO in the extraction of Ni and Mo from carbonaceous shale by calcification roasting, sulphation roasting and water leaching was studied. Experimental results showed that 97.23% Mo and 93.16% Ni were extracted from the Ni-Mo ore by mixing 100 g the raw Ni-Mo ore with 36 g CaO and grinding to less than 106 mu m, then oxidation roasting at 700 degrees C for 2 h, sulphation roasting with 70 ml 98% sulphuric acid at 240 degrees C for 2 h and, water leaching at 98 degrees C for 2 h with a liquid/solid ratio of 2:1. But only 60.67% Ni was extracted for the oxidation roasting without adding CaO. The addition of CaO in the ore is both propitious to environment protection and nickel extraction. XRD analysis showed that the nickel in the calcification roasted ore was in the form of NiO, while the nickel in the oxidation roasted ore was in the form of NiFeO(4). It was found that the NiFeO(4) formed in the oxidation roasted Ni-Mo ore was included by chemically inert iron silicates, which results in the NiFeO(4) irresolvable in sulphuric acid. Experiments confirmed that CaO can decompose not only the NiFe(2)O(4) but also the iron silicates at the roasting temperature. (C) 2011 Elsevier B.V. All rights reserved.