The effects of temperature, slag chemistry and oxygen partial pressure on the behaviour of chromium oxide in melter slags

[摘要] ENGLISH ABSTRACT: This thesis details results obtained in an experimental study conducted to determine theeffects of operating temperature, oxygen partial pressure, bulk chromium oxide contentand bulk FeOx/MgO ratio on the solubility of chromium oxide in melter type slags in theplatinum industry.Two PGM-containing layers in the Bushveld Complex in South Africa, the Merensky andUG2 reefs, are currently being mined for the extraction of base metals and platinumgroup metals (PGM). While the Merensky reef is a pyroxenitic layer, the UG2 reef is aplatiniferous chromitite seam. Due to a gradual depletion in Merensky ore reserves,platinum producers have been moving towards the processing of more UG2 concentrates,which are higher in chromium oxide content. The technical difficulties associated withthe smelting of concentrates with high chromium oxide contents is a matter of concern.The formation of chromite spinels in melts increases liquidus temperatures andviscosities and subsequently hampers tapping of slags and mattes from furnaces. Bottombuild-up from the smelting of high chromium oxide containing concentrates could reduceeffective furnace volume.From the literature reviewed it was found that very few published investigations coveredmelt compositions and oxygen partial pressures similar to those encountered in theplatinum industry. Relevant studies were found to deal with significantly lower bulkchromium oxide and iron oxide contents. It became clear that a need exists forinformation on the behaviour of chromium oxide and its effects on phase chemistry andstability in melter slags.It was decided to study the phase equilibria through drop-quench experiments using sixsynthetic slags with bulk FeOx/MgO ratios between 0.6 and 1.9 and bulk chromium oxidecontents between 1.2 and 7 wt%. Temperatures investigated were 1400, 1500 and1600°C. The oxygen partial pressure was varied between 6.8x10-10 atm at 1400°C to8.3x10-5 atm at 1600°C. Experiments were conducted in a sealed vertical tube furnaceand the required oxygen partial pressure in the furnace tube was maintained bycontrolling the flow rates of purified CO and CO2 gas mixtures through the tube.Reaction products were quenched after a reaction time of between 20 and 24 hours,depending on temperature, and the phase compositions were analysed by microprobe.The experimental study revealed that chromium oxide partitions very strongly into thespinel phase relative to the liquid phase, especially at lower temperatures, and higheroxygen partial pressures and bulk chromium oxide contents. The solubility of chromiumoxide in the liquid phase was found to increase with increasing temperature anddecreasing oxygen partial pressure.An increase in bulk chromium oxide contents of 1 wt%, under otherwise constantconditions, resulted in an increase in slag liquidus temperature of approximately 100°Cover the range of temperatures investigated. At 1500°C and bulk chromium oxidecontents of 3.7 and 6.4 wt% a reduction in oxygen partial pressure from 1.1x10-5 to1.1x10-7 atm resulted in increases in soluble chromium oxide of 0.9 and 2.0 wt%,respectively. A further decrease in oxygen partial pressure to 6.7x10-9 atm resulted inincreases in soluble chromium oxide of 2.8 and 4.7 wt%, respectively.Experimental results were compared to values predicted by the multi-phase equilibrium(MPE) model developed by CSIRO, and found to agree well. Slag basicity was not variedexperimentally and therefore the model was used to predict its effect on the solubility ofchromium oxide in the liquid phase and the stability of crystalline phases. At constanttemperature, an increase in basicity resulted in a decrease in the solubility of chromiumoxide in the liquid phase as well as stabilisation of the spinel phase.It was concluded that practicable combinations of one or more of four main factors,namely increased operating temperature and decreased bulk chromium oxide content,slag basicity and oxygen partial pressure, should be applied and evaluated in a plantenvironment to optimise furnace operation. The MPE model would be a valuable tool inpredicting the outcomes of such investigations.

[发布机构] Stellenbosch University