2.1: Precious metal recovery from bauxite residue

EPA Funded Research Project at the University of Limerick

Precious metal Recovery from Bauxite Residue-Current state of knowledge:

  • Bauxite residues contain high value metals in small volume fractions including rare earths. Depending of the type of deposit, gibbsite, boehmite or diaspora is the principal aluminium component. Bauxite contains only between 30–50% alumina, the rest being silica, various iron oxides, titanium dioxide, but also calcium, sodium and small amounts of zinc, gallium, nickel, vanadium, zirconium, niobium, thorium, uranium and rare earths (Ochsenkühn-Petropoulou et al., 1995). The actual composition of the bauxite residue depends on the type of bauxite, the mining location and the process parameters of the Bayer process. The pH of the bauxite residue before neutralisation process is about 12 and is reduced below 11.5 with the mud farming process. Iron is a main constituent of bauxite residue , and it can make up to 60% of the mass of the bauxite residue its red colour is attributed to the oxidised iron. The average concentration of rare earths in bauxite residue from Greece is reported as 1040 ppm (Ochsenkühn-Petropoulou et al., 1994).

  • A number of techniques have been reported for the recovery of precious metal from the residue including hydro- and pyro- metallurgy. Binnemans et al., (2013) reported hydrometallurgy involves selectively leaching the minor metals from the residue, leaving behind the major components such as iron oxides. The rare earths can be selectively dissolved by digesting bauxite residue with a dilute acid solution made by dissolving SO2 in water, leaving most of the iron undissolved. After leaching, the precious metal can be recovered from the leachate by selective precipitation as the oxalate, or by solvent extraction.

  • Research to date has focused upon the recovery techniques of individual elements from the residue such as scandium and yttrium. For example Ochsenkühn-Petropoulou et al., (2002) reported a comparative study of leaching with different acids has shown that 0.5 M HNO3 is the best leachant to recover 80% of the scandium present and 96% of the yttrium, but the leaching procedure was less efficient for the light lanthanides (30 to 50% recovery).

  • In addition, pyrometallurgical treatments are used to recover iron from the residue and to subsequently concentrate the precious metal in an oxide slag. Then, the hydrometallurgical steps can be preformed on higher concentration slag/residue (post the pyrometallurgical step).

  • The Al-Source project will focus on Irish bauxite residue and investigating a holistic design and optimisation of an integrated metallurgical system for maximum metal recovery the residue.