An innovative slag cleaning and gypsum wastes treatment technique was presented.

The results verified the feasibility and reliability of the innovative technique.

CaSO₄ can efficiently and preferentially sulfurize Cu₂O and CoO into sulphide.

92.04% Cu and 95.62% Co lost in initial slag can be selectively recovered.

Ca-rich and Fe-poor slag property was more beneficial to minimize Cu and Co losses.