Microwave Vacuum Carbothermic Reduction and Sulphidation of a Low Grade Nickeliferous Silicate Laterite Ore

Abstract

The international resources of nickel sulphides are quickly diminishing. In order to satisfy forthcoming nickel demands, the feasible mining of nickel laterite deposits is imperative. Nickel laterites cannot be easily treated since the nickel is finely disseminated throughout the ore. Therefore, very expensive leaching and smelting processes are required to process nickel laterite ore. The incentive for the present research was to develop a new carbothermic reduction process for nickel laterite ore that would produce a higher grade of nickel than current industrial techniques. Microwave Vacuum Reduction Processing (MVRP) of a nickeliferous silicate laterite ore, followed by magnetic separation was performed. The variables investigated included: processing time, microwave power, system pressure, use of argon as an inert gas, charcoal addition, pyrite addition, sample mass, dewatering of the sample and magnetic field intensity. The optimum conditions were determined to be a processing time of 5 minutes, microwave power of 1100 W, pressure of 11 kPa, 6% charcoal addition, 30 g sample mass and magnetic separation using a WHIMS at 1A. These conditions produced a high grade magnetic concentrate which contained 21.0% nickel with a corresponding nickel recovery of 69.6%. The use of a vacuum atmosphere reduced the partial pressure of oxygen, increased the rate of reaction of the sample, and the lowered the reaction temperature of the process. When sulphur was added to a sample in the form of pyrite, less microwave energy was used, and a higher maximum temperature was reached than a sample without pyrite. The use of an argon atmosphere resulted in high nickel grades of 7.16 to 9.24%, with moderate to high nickel recovery values of 37.64 to 88.77%. Regarding the tests performed in air, a processing time greater than 10 minutes was found to be detrimental to the nickel recovery due to oxidation of the sample. The presence of magnetite, Fe3O4, indicated that the reduced sample was oxidized during microwave processing (overheating from a long processing time) or once the sample was removed from the applicator (air exposure). The nickel was recovered as ferronickel, primarily kamacite, α•(Fe,Ni) or taenite, γ•(Fe,Ni) in higher grade concentrates.