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The study provides a method for improving the quality of the refractory material prepared from ferronickel slag by promoting the spinel formation and growth in the slag which was sintered with sintered magnesia and chromium oxide in a broad sintering temperature range from 1200°C to 1500°C. According to the thermodynamic analysis, except for forsterite due to the addition of sintered magnesia, a number of high‐melting point spinel phases can also be formed in the presence of chromium oxide and this trend becomes more apparent with increasing sintering temperature, along with declined presence of low‐melting point clinopyroxene, mainly enstatite. This expectation was verified by conversion of a part of the original phase of ferronickel slag, olivine, to two main spinel phases, including magnesium aluminate spinel and donathite which was produced by the replacement of nontoxic Cr3+ ions with Fe3+ ions in the octahedral vacancies of magnesium chromate spinel. The formation and growth of these spinel phases were promoted by elevating temperature from 1200°C to 1500°C, which accelerated the transition of initially generated enstatite to a glassy phase, in favor of densification. The formation and growth of spinel during sintering contributed to high refractoriness and compressive strength of the resulting refractory materials [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Preparation of refractory materials from ferronickel slag by sintering over a broad temperature range (1200°C to 1500°C) with addition of sintered magnesia was explored. The thermodynamic calculations indicated that the amounts of newly generated high-melting-point forsterite and spinel phases increase with increasing temperature. The experimental analysis demonstrated that elevating the sintering temperature promoted conversion and crystallization of forsterite and spinel phases from the original phase of the slag, with simultaneous reduction of low-melting-point enstatite. There was also rapid growth of spinel grains from about 0.5 μm to 5 μm, which should be controlled by selecting an appropriate temperature. The results showed that, by sintering the slag at 1400°C for 3 h with addition of 20 wt.% sintered magnesia, a high-quality refractory material with refractoriness of 1680°C, bulk density of 2.93 g/cm3, apparent porosity of 1.81%, and compressive strength of 166.62 MPa was obtained. [ABSTRACT FROM AUTHOR]

The chromium-promoted preparation of forsterite refractory materials from ferronickel slag was investigated by microwave sintering of the slag with the additions of sintered magnesia and 0–10 wt% chromium oxide (Cr 2 O 3). The thermodynamic calculations revealed that the addition of Cr 2 O 3 can promote the formations of spinel and liquid phase and maintain high content of forsterite below 1500 °C. The experimental results showed that there existed a stronger promoting effect of Cr 2 O 3 additive on the properties of refractory materials in the microwave field than that in conventional sintering. It was attributed to the preferential formation and growth of spinel with stronger microwave absorption than other phases (e.g., enstatite), the existence of more forsterite, and the enhanced densification in association with the presence of more liquid phase at the same temperature. By microwave sintering of the mixture of ferronickel slag, 25 wt% sintered magnesia, and 4 wt% Cr 2 O 3 at 1350 °C for 20 min, a superior refractory material with refractoriness of 1801 °C, thermal shock resistance of 6 times, bulk density of 2.97 g/cm3, apparent porosity of 1.4%, and compressive strength of 197 MPa was obtained. Compared with that prepared by conventional sintering at 1350 °C for 2 h, the refractoriness and thermal shock resistance were increased by 175 °C and 100%, respectively. The present study provided a novel method for preparing high-quality refractory materials from ferronickel slag and relevant industrial wastes. [ABSTRACT FROM AUTHOR]