Treatment of salt cakes by aqueous leaching and Bayer-type digestion

Abstract

Salt cakes are toxic waste products which are produced when aluminium drosses are remelted under a salt cover to recover some of the metallic aluminium present. They are expensive to dispose of in waste dumps because they contain many toxic compounds and many water-soluble compounds. However, they also contain many relatively valuable compounds, some of which may be worth recovering. This paper shows that 90% of the Cl, 55% of the Na, and 45% of the K can be extracted by aqueous leaching of −2 mm salt cake for 1 h at 25 °C. Treatment of the aqueous leach residue by Bayer-type digestion at 100 °C and 145 °C for 15 min was also investigated. It was found that elpasolite (K 2NaAlF 6) and metallic aluminium were fully extracted from fine ground material at both temperatures, that all aluminium nitrides were destroyed, and that all the resultant aluminium oxides and hydroxides were dissolved. Some other compounds were also dissolved. The total Al 2O 3 extraction was about 42%; this was comparable with that from typical bauxites. Corundum (Al 2O 3) and diaoyudaoite (NaAl 11O 17) were insoluble in the solution and increased in proportion in the final residue. Sodalite (Na 6[Al 6Si 6O 24] · 2NaCl) had precipitated in all the residues.