Characterisation and treatment of Australian salt cakes by aqueous leaching

Salt cakes are complex waste products derived from the melting of aluminium drosses to recover some of the metallic aluminium present. This paper reports the results of detailed characterisation studies on two different Australian salt cakes and proposes a flowsheet that could be used to render the salt cakes less toxic, reduce waste volumes for disposal, and, importantly, recover valuable products from them.Characterisation data showed that the salt cakes contained at least 12 major metals present in at least 19 identifiable phases. Typical total aluminium levels were 37% with the aluminium present in at least 13 phases. Potentially valuable components identified included aluminium metal, salt (NaCl and KCl), and residual aluminium units.Coarse aluminium metal was recovered in a high grade product by grinding and screening. Various aqueous treatments were then investigated to extract soluble salts and decompose aluminium nitrides and carbides present. Variables considered included leach feed size (−2 mm or finely ground), leach time (0–4 h), temperature (25 °C or 60 °C), and lixiviant type (water or 16% w/v NaOH). Typically more than 90% of the chlorides were extracted within 1 h. Full decomposition of nitrides present, however, was only achieved with hot alkaline leaching of finely ground salt cake. Over 40% of the residual alumina present in the leach residues was readily extracted using Bayer digestion conditions.Based on these results, and practical considerations, an integrated flowsheet for the treatment of salt cakes is proposed. This involves crushing and screening the salt cake to recover coarse aluminium metal, followed by wet grinding and screening to recover fine metallic aluminium. Screen undersize material is then leached in cold water and the pulp filtered and washed. The solution is evaporated in solar ponds to recover the dissolved salts for recycling, while the leach residues can be treated in a Bayer digestion plant to recover residual aluminium units.