Investigation Into the Thermal Upgrading of Nickeliferous Laterite Ore
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Nickeliferous laterite ores are currently processed using complex energy intensive flowsheets. Limited mineral upgrading can be achieved by low-cost mineral processing as the nickel is not found as a separable mineral phase but finely disseminated throughout the host goethite mineral. Whole ore extraction processes are required which result in intrinsically higher capital and operating costs. Market pressure has provided incentives to develop alternative upgrading techniques that can produce a nickel concentrate and reduce the material input to downstream processing facilities. Thermal upgrading through a selective reduction mechanism to produce a ferronickel concentrate has been studied extensively and has shown promising potential. In this research, a two stage selective reduction of nickeliferous laterite ore was investigated at 600oC and 1000 – 1100oC with varying coal and sulphur additions. Experiments showed that the limonite ore could be selectively reduced using a coal additive to a ferronickel and wustite phase. A combination of XRD and bromine/methanol diagnostic leach tests confirmed the presence of metallic nickel and iron in the calcine. Higher degrees of metallization corresponded with higher sulphur additions and growth zone temperatures. Sulphur was added to improve particle growth through the establishment of a Fe-O-S liquid phase, which was found to improve Ni recovery from 13.8% to 75.8% over the range of 0 – 4 wt% S. Ferronickel particles ranging in size from 20 – 60 microns were shown to be present but highly dispersed throughout the upgraded ore. Particle growth improved with higher growth zone temperatures and longer retention times. Magnetic separation of the calcine showed maximum upgrading of grades to 3 – 4 wt% nickel with recoveries ranging from 83.7 – 93.2%. Partial oxidation of wustite particles to magnetite caused the particles to be magnetic and resulted in recovery of unwanted iron oxides. The presence of iron oxide fines was believed to allow for rapid oxidation of wustite phase and also produce slimes that hindered physical separation of the upgraded ore.