Bauxite residue (red mud) is a byproduct from the smelting of aluminum. Like other waste slags from the metallurgy industry, bauxite residue is traditionally put into hazardous-waste landfills, although it contains a mixture of valuable raw materials in low concentrations. A new process is being developed in a four-year, European project, called HARARE (Hydrogen As the Reducing Agent in the REcovery of metals and minerals from metallurgical waste), which began in June 2021.
Led by Sintef (Trondheim, Norway; www.sintef.no), with nine industry and research partners from four European countries, the four-year HARARE project aims to recover copper and aluminum, as well as iron, cobalt, scandium and other critical metals from copper slag and bauxite residue (diagram), without generating CO2 emissions in doing so.
Conventional aluminum smelting processes generate about twice as much red mud as Al metal, and copper smelting generates about 2.2 metric tons of slag for every kilogram of Cu metal produced. Although metallurgical slags contain mineral and metal residues, these have only low concentrations of the metals and minerals of interest, which makes them difficult to recover economically. For example, slag that contains less than 1% copper will normally be regarded as waste and dumped. Other rare and valuable metals and minerals occur in even smaller concentrations.
Iron oxide is the main component in both the slag from flash smelting of copper and the bauxite residue from alumina production in the Bayer process. In the Harare concept, the iron oxide is reduced with hydrogen (instead of carbon) at high temperatures, thus avoiding the release of CO2. Once the iron is recovered, the copper, alumina and other metals can be recovered by metallurgical treatment. After metals are recovered, the remaining residue could be suitable for building materials.