Abstract: Aiming at the slow cooling copper slag of a smelting plant in China, the effects of insulation and slow cooling system on copper recovery index of copper slag flotation were studied by taking holding time and cooling rate as variables. The effects of holding time and cooling rate on the crystallization performance of high temperature copper slag were analyzed by scanning electron microscopy. It was found that copper concentrate with copper grade of 8.206% and copper recovery of 66.95% can be obtained under the optimum cooling conditions of holding time of 2 h and cooling rate of 2℃/min. The appropriate holding time could make the copper-bearing mineral particles fully aggregate in the melting state to form floating copper phase. It was also found that the slower the cooling rate was, the crystalline grain size of copper-bearing phase in slag was larger and the distribution relationship between copper and other gangue mineral phases was simpler. Therefore, a reasonable slow cooling system is conducive to optimizing the index of copper recovery from slow cooling copper slag by flotation.