Magnesium removal from hydrometallurgical process solutions

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Nickel laterites represent approximately 70% of known nickel resources, however, extraction processes for the material tend to have slender profit margins relative to the mining sector in general and are thus vulnerable to market fluctuations (for which nickel is notorious). Magnesium present in the ores creates a large issue for acid-based hydrometallurgical treatment options as soluble magnesium greatly reduces the effectiveness of the acid. Magnesium is released into solution during the nickel leach stage and, unlike other metals, is not effectively recovered during neutralisation, producing a final magnesium-laden solution. This issue results in significant waste water disposal as opposed to recycle in some regions that can least afford excessive water consumption such as central Australia. A technology for the atmospheric recovery of magnesium from process liquors has been developed at UBC. Addition of phosphoric acid and ammonia to the final laterite process liquors under mildly alkaline conditions will induce precipitation of struvite (NH4MgPO4.6H2O), a highly sought-after fertilizer. Sulfate is subsequently removed via gypsum (CaSO4.2H2O) precipitation, leaving a relatively pure ammonia solution. Ammonia is subsequently recovered via stripping, producing a purified solution to be recycled to the beginning of the process. Upon completion of precipitation optimisation, testing ammonia stripping, examining the influence of other solution species and performing continuous operation of the technology will be ready for the next step toward commercialisation such as piloting. This project will also serve to test the applicability of the process to other types of plant liquors, expanding the possible market significantly. The ability to recover the magnesium from solution in itself is a very important discovery saving vast quantities of water and improving the efficiency of the plant. Performing this task via the formation of a valuable by-product is a significant additional advantage. Estimates suggest that this process could recover > 150,000 tonnes of magnesium from a single plant saving up to 9 billion litres of water. Therefore this process not only represents a significant financial gain but also an associated environmental achievement.