Selective rare earth element extraction using high-pressure acid leaching of slags arising from the smelting of bauxite residue

During acid leaching of bauxite residue (red mud), the increase in dissolution of rare-earth elements (REEs) is associated with a substantial co-dissolution of iron; this poses problems in the downstream processing (i.e. solvent extraction or ion exchange). Six different slags generated by reductive smelting of the same bauxite residue sample were treated by high-pressure acid leaching (HPAL) with HCl and H2SO4 to selectively extract REEs. Thus, up to 90 wt% of scandium was extracted from the slags using H2SO4 at 150 degrees C, while with HCl the extraction of scandium reached up to 80 wt% at 120 degrees C. The extraction of yttrium, lanthanum and neodymium was above 95 wt% when HCl was used as a reagent, but it was much lower ( < 20 wt%) with H2SO4, presumably due to the formation of a double sulfate (NaLn(SO4)(2).nH(2O)) and/or due to the adsorption on the surface of silicon/aluminium-oxides compounds. In addition, HPAL of bauxite residue slags led to a significant co-dissolution of aluminium ( > 90 wt%, 18 g L-1), while the concentration of the remaining iron ( > 60 wt%) was of 3 g L-1 in the leachate. The co-dissolution of silicon and titanium was lower than 5 wt%.