A case of ferrous sulfate addition enhancing chalcopyrite leaching

It is generally accepted that ferric ions as an oxidant are effective for leaching chalcopyrite but ferrous ions contribute to the leaching only as a source of ferric ions. However, this paper found that several chalcopyrite samples were more effectively leached in ferrous sulfate solution than in ferric sulfate solution. Chalcopyrite samples from four different sources were leached in 0.1 mol dm^-3 sulfuric acid solution containing 0.1 mol dm^-3 ferrous sulfate or ferric sulfate at 303 K in air for 168 h. Three samples were more effectively leached in the ferrous sulfate solution than in the ferric sulfate solution. Especially, with the sample from the Akenobe mines, Hyogo, Japan, the amount of copper extracted with ferrous sulfate was about five times larger than that with ferric sulfate. By using the Akenobe sample, leaching experiments and oxygen consumption measurements were carried out under various conditions. The amount of extracted copper increased markedly with increasing ferrous sulfate addition and decreasing pH. During the leaching experiments, most of the soluble iron was present in the ferrous form. By adding ferrous sulfate, proton consumption increased. The mole ratio of elemental sulfur to extracted copper was about 2. When the leach solution was purged with nitrogen, the amount of copper extracted was negligible even with ferrous sulfate. By adding ferrous sulfate, dissolved oxygen consumption on the sample surface increased. From these results, it was concluded that ferrous ions enhance the following reaction for the Akenobe sample: CuFeS₂ + O₂ + 4H⁺ = Cu²⁺ + Fe²⁺ + 2S⁰ + 2H₂O. The importance of this effect in the bacterial leaching of chalcopyrite is discussed.