Copper smelter slag produced at Mount Isa Mines represents a complex flotation feed due to its unique mineralogical characteristics and the conditions under which it forms. Rapid cooling of molten slag during smelting and converting results in extremely fine copper sulfide grain sizes, with mineralogical analysis indicating that more than half of the copper sulfide mass is below 10 µm and approximately 27 per cent is below 5 µm. Liberation at conventional grind sizes is correspondingly limited, with only ~50 per cent of copper sulfides reporting as liberated in the RHF slag head sample. These mineralogical constraints fundamentally shape the flotation response of slag and underpin the metallurgical behaviour observed across laboratory and plant studies.