Thermodynamic interaction and iron passivation mechanism of copper–cobalt sulfide concentrates during pressure leaching

Copper–cobalt sulfide flotation concentrate received from Comika Mining Kamoya project in Congo contains around 24.2% copper, 7.1% cobalt, 22.9% sulfur, 13% iron, and a small amount of other metals. Under the condition of temperature 190 °C, oxygen partial pressure 6 atm, leaching time 2 h, liquid-to-solid ratio 5:1, and initial acidity 0 g/L, metal recovery rates of over 98% can be achieved for both copper and cobalt. Copper that exists in chalcocite, bornite, chalcopyrite, and carrollite was almost completely leached, while most of copper in limonite remains in the residue. Cobalt that exists in carrollite can be completely leached, while the unleached cobalt remains in the pyrite and limonite phases. During pressure leaching, sulfur was oxidized into sulfate. Meanwhile, iron undergoes a dissolution reaction and generates high-valent iron ions, which can be partially oxidized into metal sulfides and partially precipitated under low acidity conditions. The hematite encapsulation is the main reason for the incomplete leaching of copper and cobalt.