Extraction of copper from copper-iron and copper-nickel-iron sulfide concentrates by a double roast-leach process

A fundamental investigation into a double roast-leaeh process to extract copper from sulfide concentrates containing iron and nickel has been undertaken. The double roast-leach process involves three steps: dead roast to remove sulfur from concentrates, segregation roast to produce metallic copper, and a selective leach of copper with acidic Cu(II) sulfate/acetonitrile (AN)/H20 or Cu(II) chloride/NaCl/h2O solutions. The first part of this work investigates the application of the double roast-leach process for -the recovery of copper from copper-iron and copper-nickel-iron sulfides under a variety of roast conditions. The extent of sulfur removal from the calcine and the formation of copper ferrite were studied to test their effect on the subsequent leaching and recovery of copper. It was found that sulfur in the calcine tends to hold copper as CuS and decrease the copper recovered by leaching, whi1st the formation of ferrite has less effect on the subsequent segregation and copper leaching. A study of the effect of temperature, carbon addition, chloride addition and time on segregation roasting revealed that with about 10% carbon addition, a high efficiency of copper segregation and leaching was achieved over a relatively wide range of roasting temperatures and leaching conditions. However strict control of the roasting temperature to about 670C and carbon addition was necessary to achieve selective segregation and extraction of copper from nickel and iron-bearing sulfide concentrates. The second part of this work compares and contrasts the dissolution rates and mechanism of copper and nickel metals, NiO, Fe3O4, CuFe204 and NiFe2O4 which are the components of the segregated calcine in both the Cu(II) sulfate/AN/H20 and Cu(II) chloride leach solution systems. Both electrochemical and kinetic studies were carried out on pure synthetic minerals by using rotating disc electrodes under controlled potentials and by leaching the minerals in various leach solutions of different Eh. Both approaches complemented each other and gave good agreement and correlation. It was found that the dissolution rate of copper in both the aqueous chloride and aqueous AN/sulfate leach systems was determined by Cu(II) diffusion. The dissolution rate of nickel in aqueous chloride solutions was also determined by Cu(II) diffusion, but in aqueous AN/ sulfate solutions it was inhibited by the passivation of the nickel surface. However, under practical leach conditions there is sufficient chloride present in the segregated calcine to prevent passivation from occurring. In the absence of nickel passivation the dissolution of nickel was also controlled by Cu(II) diffusion in sulfate media. By contrast, the dissolution of magnetite was found to be affected mainly by the redox potential and proton activity(aH+)of the solution and basically unaffected by stirring. Magnetite reacted most rapidly in the presence of Cu(I) at potentials cathodic to its rest potential, and the reaction rate was directly proportional to aH+. Thus to minimise the dissolution of magnetite it was necessary to leach the calcine at high Eh and high pH. The dissolution rates of hematite and nickel, copper and zinc ferrites were determined in dilute HC1. As found with magnetite, reducing agents such as Cu(I) favoured the acid dissolution of hematite but hindered the dissolution of nickel(II) oxide. This was supported by cyclic voltammogram studies of carbon pastes of and ferrites these materials. Overall, it was established that both acidic Cu(II) sulfate/AN/ H20 and Cu(II) chloride/NaCl/H20 solutions are suitable for selective leaching of copper following double roasting