The physical and chemical reclamation and recycling of elements from black aluminium furnace residues
Dross is the inevitable waste product generated by the world-wide primary and secondary aluminium industries. Around 3.5 million tonnes of white dross and 0.85 million tonnes of black dross are produced per year. Drosses are classified depending on metal content. White dross contains 40% and black dross contains 10% of aluminium. About half of the white dross is currently landfilled along with almost all of the black dross, and the annual cost to the UK industry of landfilling this waste is about £90 million per year. There is no commercially viable method of recovering the aluminium, which is potentially recyclable from these drosses, and no current conventional process has the custom-built capability to avoid their disposal to landfill. The practice of landfilling aluminium dross has raised problems for both industry and environment, because of potential leaching of heavy metals into groundwater from the landfill. The black dross has been physically and chemically characterised. The results show that the dross contains mostly metallic aluminium, aluminium nitride, alumina, sodium chloride, potassium chloride and silica with almost 50% of the dross being a mixture of sodium chloride and potassium chloride. A counter current leaching process has been developed to recycle the salts back to the secondary furnace. The environmental impact of black dross has been investigated using simulated landfill conditions. Leachability studies were carried out using water, and other leach media to model the following conditions: humic acid-containing systems, acid rain (pH 2.5), and acetic acid solutions (pH 5). Up to 60% of the dross was leached out in all media with the production of the saline solution providing the major impact. Only small amounts of aluminium were leached out under all these condition. No leachability behaviour was detected once the black dross was washed. So landfilling washed black dross can minimise both environmental and economical problems. Black dross residue, after washing with water, is enriched with alumina. Added value chemicals (of >95% purity) including aluminium hydroxide, alumina, aluminium chloride, aluminium nitrate, aluminium sulphate and aluminium carboxylates have been prepared using the washed black dross. Preliminary calculations have shown that the technology developed to recycle black dross to added-value chemicals is economically viable. A methodology has been developed which enables low grade alumnium waste, black dross, to be treated, commercially viable chemicals to be prepared, reagent use to be minimised through recycle and the diversion of millions of tonnes of furnace residue from landfill.