Spent hydrodesulphurization (HDS) catalysts are treated with hydrometallurgical leaching processes such as caustic leaching and acid leaching with roasting as a pre-treatment step. In the alkaline leaching processes such as sodium carbonate roasting followed by water leaching, most of the molybdenum and vanadium are selectively leached and most of nickel is left in the residue after the alkaline leaching of spent HDS catalysts. Therefore, it is important to recover nickel from the alkaline leaching residue. The main form of nickel in the alkaline leaching residue is spinel, therefore, it is difficult to extract nickel from the residue using mineral acids.

In the present study, the effect of grinding conditions on the leaching efficiency of nickel and aluminum from the oxidative roasted alkaline leaching residue was investigated. The dry and wet grinding, grinding speed, grinding time and the amount of roasted product in grinding significantly affected the leaching efficiency of nickel and aluminum. The leaching efficiency of nickel and aluminum from dry ground roasted product was high compared with wet ground roasted product. And also, the leaching efficiency of nickel and aluminum was low in the low amount of the roasted product fed in dry grinding and high grinding speed, because the particles were aggregated again.

In this study, we examined the effect of carbon black on the sulfuric acid leaching of a copper ore containing 1.1% Cu occurring as CuFeS₂ (chalcopyrite). Leaching experiments were performed in Erlenmeyer flasks containing 200 mL of the sulfuric acid medium while agitated with a magnetic stirrer at 50°C. The addition of carbon black enhanced the kinetics of chalcopyrite leaching at an initial pH of 1.2. The extraction yield of Cu was 97% in 98 h in the presence of 0.2 g of carbon black, compared with 18% in the absence of carbon black. The acceleration of chalcopyrite dissolution did not occur without direct contact between the ore and the carbon black, suggesting that a galvanic interaction between the two materials plays a role in the improvement of the leaching rate. In the presence of carbon black, the redox potential fell below 600 mV vs. SHE with a decrease in the ratio of Fe(III) to Fe(II), and the leaching rate of chalcopyrite increased markedly. The enhanced kinetics of chalcopyrite leaching in the copper ore could be attributed to dissolution reactions at a low redox potential and to the galvanic interaction between the chalcopyrite and the carbon black.