Chromium is an important industrial metal that is produced from chromite ore. This ore is a spinel structured solid solution of (Mg,Fe)O•(Al,Cr,Fe)2O3. The problem faced by industrial smelters is that the recovery of chromium metal is low at 85-90% (Hayes (2004)). Through analysis it was found that two reduction zones were present in the smelting of the ore in a Submerged Arc Furnace (SAF): a gas/solid and a liquid slag reduction zone. The reducibility of the ore in both these zones needs to be improved for the recovery of chromium metal to be increased.
Through analysis of the reduction process it was found that a decrease in the effective grain size of the chromite mineral improved its reducibility (Soykan et al (1991)). Examination of the crystal structure of the chromite mineral found that a crystal structure transformation of the ore occurred through high temperature oxidation. This changed the spinel M3O4 chromite mineral structure to a (Al,Cr,Fe)2O3 - M2O3 structure; which resulted in a decreased the effective grain size of the mineral. This formed the hypothesis:
The reducibility of chromite ore can be improved by high temperature pre-oxidation to a M2O3
To prove this hypothesis phase equilibria, using powdered oxides, and reduction experiments, using Samancor FM pellets, were undertaken. Industrial ore samples from Zimbabwe and India were also characterised to test their amenability to the process of high temperature pre-oxidation. A risk assessment (JSA) was undertaken on each step in the procedure to identified and minimise the risk to personnel and equipment.
There was no information available on the five component MgO-FeO-Fe2O3-Al2O3- Cr2O3 system in the literature. The pseudo-ternary (MgO+FeO)-Fe2O3-(Al2O3+Cr2O3) diagram was proposed for the representation of the system at set Cr/Al ratios. The data generated from the equilibrium experiments only demonstrated the trends associated with the phase equilibria of the system as the samples did not reach equilibrium. Oxidated industrial chromite ore samples from Zhao and Hayes (2004) were then analysed and it was found that these samples had reached equilibrium after only a short equilibration time. It was proposed that:
Equilibrium in the system could be achieved from synthetic spinel mixtures.
The reduction work that was conducted focused on investigating the reduction of preoxidised Samancor Chrome FM pellets. Pre-oxidation was carried out at 1200oC for one hour in air. Reduction of the pre-oxidised pellets was carried out in a CO gas stream and simulated the conditions in the SAF. The experiments raised the temperature of the pellet from 100oC to 1300oC at two different heating rates of 120oC/hr and 600oC/hr. Samples were then quenched and analysed using scaning electron microscopy (SEM). It was found that:
The M2O3 phase had formed from the pre-oxidation process and the effective grain size had been reduced after reduction at both heating rates.
This meant that an improvement in the reducibility and dissolution rate of ore in the SAF was achievable.
Industrial ore samples from both Zimbabwe and India were analysed using SEM and electron probe microscopy (EPMA). SEM work showed that the grain size of these ore samples was comparable to that of the Samancor pellets used in the reduction work. The chromite compositions of the ore samples were defined by EPMA and it was found that they all contained high concentrations of FeO. This meant that:
These ores were amenable to the pre-oxidation process providing an effective decrease in the chromite grain sizes.
It was recommended that further work be undertaken on the following:
• Phase equilibria of the MgO-FeO-Cr2O3-Fe2O3-Al2O3 system by oxidation of synthetic spinels
• Plant trials in SAF using pre-oxidised chromite ores.