Abstract:The iron in copper smelting slag was enriched with high temperature oxidation modification process. The effects of different kinds of additives on precipitation and growing behavior of magnetite phase were analyzed. The changes of quantity of crystallization, grain size and crystal morphology of magnetite phase under additives were investigated with combination of the analysis of phase diagram, and the mechanisms of action of additives were given a preliminary discussion. The results show that addition of 2%~5% CaO is in favor of magnetite precipitation, and 1%~3% addition of CaF2 can promote precipitation, growing and coarsening of magnetite. The new type of composition additives can obviously promote precipitation and growing of magnetite with the volume fraction of 41.5%~42.4% and 50.2~55.6 μm equiaxed grains. It is beneficial to separation of magnetite from modified slag by mineral processing.