Fundamental investigations on high temperature interactions of alumina-carbon refractories with molten iron at 1550 °C - implications for refractory recycling

This work investigates high temperature (1550 ºC) interactions of alumina-carbon refractories with liquid iron with specific focus on the chemical interactions occurring both in the interfacial region as well as in the bulk of the refractory. The sessile drop technique was used to determine the interfacial wetting behaviour and the phase transformations during the chemical reactions were determined using SEM, EDS and X-ray diffraction. Alumina-carbon refractories were prepared using two types of carbonaceous materials - synthetic graphite and natural graphite. From the experimental results, it was clearly observed that molten iron had penetrated to varying extents into all refractory substrates. The highest penetration was observed for alumina-carbon refractory substrates containing 10% synthetic graphite, while all alumina-natural graphite substrates showed much lower levels of metal penetration. The drop in the contact angles was in direct correspondence with increasing carbon pickup values for these samples. Natural graphite samples also showed lower carbon pickup compared to synthetic graphite samples of similar carbon content. The differences in metal penetration and interfacial wetting behaviour were found to depend on the carbon pickup and the ash content of the refractory substrates, with the ash helping in the formation of an interfacial layer which limited reactions with the metal, and in filling the pores within the refractory. To determine the chemical interaction of Al2O3-10%C refractory molten iron at 1550 °C, investigations were also carried out in argon atmosphere for time periods ranging between 15 minutes to 3 hours. Video images of the iron droplet started to show minor deviations after 30 minutes of contact followed by intense activity in the form of fine aluminium oxide whiskers emanating from the droplet and on the refractory substrate. The interfacial region also changed significantly over time and the formation of small quantities of iron aluminide intermetallics was recorded after 30 minutes as a reaction product in the interfacial region. This shows that alumina cannot be treated as chemically inert at steelmaking temperatures when both carbon and molten iron are simultaneously present. High temperature interactions of recycled refractory (mixture of used and virgin refractory) substrates with liquid iron were also investigated to determine the role of used refractory constituents which contains degraded alumina, carbon and iron contamination from used refractory. The presence of iron and reactive alumina in the refractory substrates was seen to affect both physical and chemical interactions, contact angles and carbon pick-up. The structural integrity and bonding of the recycled refractory was also compromised, and interfacial cracks were observed along with increases in the interfacial area of contact of metal droplet with refractory. Video images also showed whisker formation on metal droplets during 30 minutes of reaction time. The results from this study have shown that metal contaminated used refractories tend to enhance refractory degradation during recycling and are therefore unsuitable for steelmaking applications