Integrated recycling of basic oxygen furnace slag by applying an innovative microwave pretreatment process

To move towards a circular economy, which is a European directive, it is necessary to increase the rate of internal recycling of basic oxygen furnace (BOF) slag. However, the use of BOF slag as an iron-bearing material is hampered by the slag's high phosphorous content. It is necessary to separate the iron-containing phases from the undesired phosphorus-containing phases. In this study, an innovative pretreatment process was developed consisting of microwave irradiation of BOF slag to improve the subsequent grindability and magnetic separation steps. The BOF slag was irradiated using a multimode microwave oven in batch operation with a maximum output power intensity of 4 kW and a frequency of 2.45 GHz. Afterwards, the microwave-pretreated BOF slag was comminuted to a particle size in the range of 56 to 100 μm. Finally, magnetic separation was applied to recover the iron-bearing materials. The results indicated that microwave irradiation generated intergranular fractures, which led to the improved liberation and grindability of the BOF slag. They suggested that a larger particle size and higher power intensity are beneficial for intergranular fracture formation. This should be considered in larger-scale testing to further increase treatment efficiency. Furthermore, it was observed that microwave irradiation under an ambient air atmosphere led to the additional oxidation of wüstite in the BOF slag. This resulted in increased magnetic susceptibility, enhancing the separation efficiency in comparison to the untreated reference samples.