From ferronickel slag to value-added refractory materials: A microwave sintering strategy

The present study proposes a novel strategy for preparation of refractory materials from potentially hazardous ferronickel slag by microwave sintering of the slag with addition of sintered magnesia in which a series of chemical reactions were involved. This strategy was developed based on examination of the phase transformations and microstructural changes of the slag during microwave sintering through X-ray diffraction (XRD) analysis and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) analysis, which determined the properties of refractory materials derived from the slag. It was shown that under microwave irradiation there existed rapid transformation of the olivine phase in the slag to high-melting point phases, including forsterite and spinels (e.g., magnesium iron chromate spinel, magnesium chromate spinel, and magnesium iron aluminate spinel). As a result, a high-quality refractory material with refractoriness of 1730 °C, bulk density of 2.80 g/cm³, apparent porosity of 1.6%, and compressive strength of 206.62 MPa was obtained by microwave sintering of the slag at 1350 °C for only 20 min with addition of 25 wt % sintered magnesia. Because the microwave sintering strategy not only elevated the refractoriness by 70 °C, but also reduced the heating duration required by the conventional approach by 6 times, it demonstrated apparent technological superiority and wide application prospect in preparing superior-quality refractory materials from ferronickel slag and relevant industrial waste, which contributed to conservation of resources and energy as well as environmental protection.