Development of a combined leaching and ion-exchange system for valorisation of spent potlining waste

This work aims to contribute to addressing the global challenge of recycling and valorising spent potlining; a hazardous solid waste product of the aluminium smelting industry. This has been achieved using a simple two-step chemical leaching treatment of the waste, using dilute lixiviants, namely NaOH, H2O2 and H2SO4, and at ambient temperature. The potlining and resulting leachate were characterised by spectroscopy and microscopy to determine the success of the treatment, as well as the morphology and mineralogy of the solid waste. This confirmed that the potlining samples were a mixture of contaminated graphite and refractory materials, with high variability of composition. A large quantity of fluoride was solublised by the leaching process, as well as numerous metals, some of them toxic. The acidic and caustic leachates were combined and the aluminium and fluoride components were selectively extracted, using a modified ion-exchange resin, in fixed-bed column experiments. The resin performed above expectations, based on previous studies, which used a simulant feed, extracting fluoride efficiently from leachates of significantly different compositions. Finally, the fluoride and aluminium were coeluted from the column, using NaOH as the eluent, creating an enriched aqueous stream, relatively free from contaminants, from which recovery of synthetic cryolite can be attempted. Overall, the study accomplished several steps in the development of a fully-realised spent potlining treatment system.