A Study on the Role of Sulphur in the Thermal Upgrading of Nickeliferous Laterite Ores
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As the global demand for nickel continues to increase and the nickel sulphide ore deposits are depleted, there is considerable interest in the exploitation of the nickeliferous laterite ores. However, current processing methods suffer from an inability to effectively concentrate the nickel content of the ore prior to processing, resulting in high costs and significant waste streams. One of the most promising methods of addressing this challenge is the selective reduction of the laterite ore to form a ferronickel alloy, followed by magnetic separation to recover the alloy, forming a nickel concentrate. The current work investigates several phenomena in the selective reduction process with the aim of identifying the conditions which yield the largest ferronickel particles, while still maintaining a sufficiently high nickel grade in the ferronickel. In particular, the effects of varying coal, sulphur, pyrite, and sodium sulphate additions, and reduction temperatures in the range of 1000 to 1200°C, have been investigated. By maximizing the ferronickel particle size it is expected that the nickel grade in the magnetic concentrate can be improved. The best results were achieved for the reduction of a limonite containing 6% coal and 4% elemental sulphur additions at 1100°C for 1 hour. Under these conditions an average ferronickel particle size of 1.59 μm and nickel grade of 4.96% was achieved, compared to 1.01 μm and 8.18% for the same ore and reduction conditions without the addition of sulphur. In general, increased coal and sulphur additions, and increased temperature, correlated with increased ferronickel particle size and decreased nickel grade in the limonite. In the case of a saprolite, the addition of sulphur appeared to decrease the average ferronickel particle size.