Analysis of bauxite residue components responsible for copper removal and related reaction products

Bauxite residue is a solid waste produced during alumina production process, and the storage of that in China reached 0.6 billion tons with an increase of more than 70 million annually. Bauxite residue can be used to remove heavy metals from water. This study analyzed components of bauxite residue responsible for copper removal, removal process and accompanying reaction products. Calcite (CaCO3), hematite (Fe2O3) and sulfur-Fe are main components contributing to copper removal. Sulfur in bauxite residue works with iron to remove copper. All these components reacted with copper immediately as bauxite residue was added. Reaction time of sulfur-Fe and carbonate was 5 min and 1 h, respectively. And hematite reacted until complete removal of copper (>2 h). Sulfur quickly reacted with coexisting iron to remove copper, producing chalcopyrite (CuFeS2), cubanite (CuFe2S3) and bornite (Cu5FeS4). Carbonate in bauxite residue reacted with copper, producing tenorite (CuO), copper hydroxide (Cu(OH)2), malachite (Cu2(OH)2CO3), carbonate cyanotrichite (Cu4Al2(CO3,SO4)(OH)12·2H2O), chalconatronite (Na2Cu(CO3)2·3H2O), nakauriite (Cu8(SO4)4(CO3)(OH)6·48H2O) and callaghanite (Cu2Mg2(CO3)(OH)6·2H2O). Copper precipitated through reaction with hematite to produce delafossite (CuFeO2). After removal reaction, the existing forms of copper in bauxite residue comprised carbonate-bound (73.6%–85.7%), iron oxide-bound (5.6%–23.8%), organic matter/sulfide-bound (0.5%–9.0%) and residual forms (0.9%–2.0%). In conclusion, removal of copper using bauxite residue features a more complex reaction than adsorption.