<p>Traditional copper smelting slag cleaning processes for matte recovery are limited by the use of non-renewable and high-cost fossil reductants and fluxes. Fly ash with tremendous yield and low cost is one of industrial solid waste, and mainly composed of carbon, silicon, and calcium oxides. In this study, fly ash instead of fossil reducing agent and conventional flux was used to clean copper smelting slag. The pyrolysis characteristics of fly ash and the effects of fly ash addition on copper slag properties and copper loss were investigated. At 1300&#xa0;°C, 60&#xa0;min settling time and 15 wt.% fly ash addition, the copper content in the slag decreased to 0.51 wt.% and the Fe₃O₄ content decreased to 2 wt.%. The reaction mechanism of fly ash on copper slag cleaning was elucidated, which revealed that the alkaline oxide cations Ca²⁺ and Mg²⁺ in fly ash are conducive to the decomposition of Si–O bond in the slag to form free O²⁺, thereby simplifying the silicate network structure in the slag and reducing the slag viscosity. This study obtained an excellent recovery efficiency for matte and reduces the cleaning cost by approximately 31%, which opened up a new approach for treating waste with waste and realized the purposes of energy saving and cost reduction, and has wide application prospects.</p> Graphical Abstract <p></p>