Thermodynamic Phase Conversion Mechanism on Copper–Cobalt Slag Cleaning Process Using Gypsum Wastes as Sulfurizing Agent

A promising copper–cobalt slag cleaning technique has been proposed, involving employing gypsum wastes as a sulfurizing agent to sulfurize copper, cobalt, and iron and enrich a condensed matte from copper–cobalt slag. This work thermodynamically investigated the phase conversion mechanism in the Cu2O–CoO–Fe3O4–FeO–CaSO4–C system, at the same time, calculated the equilibrium of the “matte–slag–gas.” The results show that CaSO4 can be selectively reduced to CaS without self-decomposition to CaO and SO2. Cu2O and CoO lost in slag can be sulfurized by CaS to Cu2S and CoS. FexOy tended to stay in oxides. Whereas, in the “matte–slag–gas” equilibrium system, a matte containing Cu2S, CoS, FeS, Fe, Co, and Cu was obtained. In addition, industrial copper flash smelting slag was used to conduct beach-scale experiments to validate the thermodynamic modeling results and detect the copper and cobalt recovery efficiency. The experimental results reveal that more than 92% copper and 94% cobalt were recovered in matte. The sulfur and calcium in gypsum waste transferred to matte and cleaned slag, respectively. Thus, the values in the slag and gypsum wastes can be recycled simultaneously.