Mineral sulphide (MS)-lime (CaO) ion exchange reactions (MS+CaO=MO+CaS) and the effect of CaO/C mole ratio during carbothermic reduction (MS+CaO+C=M+CaS+CO(g)) were investigated for complex froth flotation mineral sulphide concentrates. Phases in the partially and fully reacted samples were characterised by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The primary phases during mineral sulphide-lime ion exchange reactions are Fe3O4, CaSO4 Cu2S, and CaS. A complex liquid phase of Ca2CuFeO3S forms during mineral sulphide-lime exchange reactions above 1173 K. The formation mechanisms of Ca2CuFeO3S liquid phase are determined by characterising the partially reacted samples. The reduction rate and extent of mineral sulphides in the presence of CaO and C increase with the increase in CaO/C ratio. The metallic phases are surrounded by the CaS rich phase at CaO/C>1, but the metallic phases and CaS are found as separate phases at CaO/C1 and the reacted samples are excessively sintered.