Morphology of vanadium slags were investigated by scanning electron microscopy (SEM). The mineralogical phases were characterized by energy disperse X-ray spectrometry (EDS) and powder X-ray diffraction (XRD). The obtained results show that spinels and silicates are the major phases in the vanadium slag, and V is concentrated in Fe x V 3-x O 4 and Mg x (V, Ti) 3-x O 4 . Both the spinel grain size and volume fraction in the slag with higher V 2 O 3 content are much larger than that with lower V 2 O 3 content. (Fe, Mn) 2 SiO 4 and (Fe, Mn)SiO 3 have a higher proportion in the slag with lower V 2 O 3 and higher SiO 2 content. The relation among cooling conditions and grain size and volume fraction of spinels is also discussed. It is found that low cooling rate and long holding time benefit spinel crystal growth, especially for the interval of 1200-1250 °C. Both mean diameter and volume fraction of spinels could achieve or exceed the industry vanadium slag when holding more than 45 min.

Synthetic vanadium slags were melted at 1673 K and then cooled to a predetermined temperature at different cooling rates. The mineralogical phases and crystallisation behaviour were investigated based on optical microscopy, SEM, EDS and XRD analyses. The results show that spinels and silicates were the major phases in the complex vanadium slag samples, and V is enriched in the spinel phases, i.e. Fe x V 32x O 4 and Mg x (V,Ti) 32x O 4 . The apparent diameter of spinel crystal grains decreases with increasing cooling rate, e.g. from 14 to 5 mm at 1523 K. Crystal grain size increases significantly with increasing holding time. It is also found that the grain sizes of spinels at higher temperatures, 1573 and 1623 K, are considerably smaller than at lower temperatures, 1473 and 1523 K. At lower temperature, large blocked spinel crystals between grey silicate phases are formed, whereas there only a few smaller blocked spinels when performed at higher temperature. Better control of the process cooling parameters will benefit vanadium slag quality.

This paper investigated the growth of spinel crystals in vanadium slag under different heat treatment conditions. The growth mechanism was clarified by SEM + EDS and statistical analyses based on the CSD theory. The results showed that all CSDs follow lognormal distributions. Long holding time and low holding temperature below 1300°C benefit the growth of spinel crystals. We proposed that the growth mechanisms of the spinel crystals consist of two regimes: in the first regime, nucleation control with a decaying rate within the temperature range of 1400 to 1250°C; in the second regime, surface control when the temperature is below 1250°C. The mean crystal diameter changes from 16.6 to 26.2 μm with an increase in holding time at 1250°C. The relationship between the mean diameter of the spinel crystals and the holding time in the present study can be best described by the equation D = 16.32t 0.11 . This journal is

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