Recovery of valuable materials from aluminium salt cakes

Abstract

Salt cakes, which are nominally waste products derived from aluminium dross melting furnaces, are complex mixtures of some 20 different compounds made up of many different elements. Normally they are regarded as waste products and they are disposed of in toxic waste dumps. However, it is shown here that some components are readily recoverable as high-grade products for recycling or sale and that the residues thus generated can be non-toxic. Recoverable components include metallic aluminium, salt (halite (NaCl) and sylvite (KCl)), alumina-containing compounds, and possibly hydrogen gas. Metallic aluminium is soft and malleable and is not reduced in size by crushing and grinding, whereas the other components in salt cake are soft and brittle or are readily dissolved in water. Hence the coarse metallic aluminium can be readily recovered by crushing and screening and the finer metallic aluminium can be recovered by fine grinding and screening, froth flotation, or possibly electrostatic separation. Aqueous acid or alkaline leaching has also been proposed to recover aluminium metal from salt cake. The halite and sylvite are easily extracted by leaching ground salt cake with cold water and filtering off the saline solution. This solution can be sent to solar evaporation ponds where the water is evaporated and the dry salt harvested for recycling to dross treatment furnaces or other markets. Some of the water-insoluble or oxide aluminium compounds present are soluble in Bayer-type leach solutions and could possibly be sent to a Bayer-type leach plant for production of high-grade alumina for aluminium production. Alternatively, because the oxide aluminium compounds are inert they could be sent to a non-toxic dump. The possibility of integrated flowsheets to recover more than one product in a single plant is also discussed.