Lowering melting temperatures of high zinc slag derived from co-smelting of primary ores and secondary resources: Regulation method and mechanisms

The synergistic and enhanced bath smelting of primary ores with secondary resources is a key strategy to address the paradox of resource scarcity and recycling of secondary resources in zinc metallurgy. However, due to the high melting temperature of high zinc slags (Zn > 25 %), bath smelting below 1250 °C has not yet been practically implemented. This study proposes a new approach for reducing the melting temperature of high zinc slags by tailoring slag composition and co-smelting secondary resources to promote synergistic melting. The melting properties and regulation mechanisms of high zinc slags were systematically investigated through FactSage thermodynamic calculations, in-situ observation using an ash fusion temperatures auto detecting system, and quenching experiments. The results reveal that CaO/SiO₂ and FeO/SiO₂ mass ratio are the primary factors influencing the melting temperature. Excessively high or low CaO/SiO₂ and FeO/SiO₂ promote the formation of high melting point phases such as zincite, spinel, willemite and melilite, thereby increasing the melting temperature. Components from secondary resources, such as B₂O₃, Na₂O and Cu₂O, can significantly reduce the content of high melting point phase in the melting process, and reduce the melting temperature, while MgO, Cr₂O₃, etc., have the opposite effect. The melting temperature of co-smelting high zinc slag with 15 % sludge and 4 % borax was reduced over 125 °C. In industrial pilot-scale trial exceeding 20 tons, the flow temperature of high zinc slag containing 27.08 % zinc was 1118 °C. This study establishes technical foundations for low-energy and clean co-smelting of secondary resources with primary ores.