Abstract: During the process of copper smelting in Isa furnace, the scraper was worn badly due to hard lump formed in flue, and the flue gas circulation was blocked, which affected the production efficiency and stability. In this paper, from the perspective of process mineralogy, the occurrence state of copper, iron, sulfur and other main elements from the residual heat boiler flue products of the Isa furnace, as well as the mineral phase composition and structure characteristics of hard lump were examined. And the factors leading to the formation of hard lump in flue were analyzed. The results show that the content of copper ferrite, hematite, magnetite, matte and silicate glass phase in hard lump is obviously high. And these mineral phases often adhere and cement copper sulfate in the form of nuclei to form slags with relatively high hardness, which was related to the strong splashing of slag and matte during smelting. Due to the high temperature and oxygen content of the flue gas flow, the slagging continues to grow accompanied by repeated adhesion and secondary reactions. According to the research results, the slagging problem in the flue can be effectively improved by controlling the flow of the spray gun and the negative pressure in the furnace, appropriately reducing the temperature of the dust and expanding the diameter of the vertical flue.