Combined multi-step precipitation and supported ionic liquid phase chromatography for the recovery of rare earths from leach solutions of bauxite residues

Rare-earth elements (REEs) were efficiently recovered by a supported ionic liquid phase (SILP) after iron(III) precipitation from simulated bauxite residue (BR) leachates in sulfuric acid media, since leaching of BR with sulfuric acid is more feasible than leaching with other mineral acids. The SILP betainium sulfonyl(tri-fluoromethanesulfonylimide) poly(styrene-co-divinylbenzene) [Hbet-STESI-PS-DVB] exhibits a high selectivity for the rare-earth elements (REEs) over other elements in acidic BR leachates. However, scandium(III) uptake by the [Hbet-STFSI-PS-DVB] from sulfuric acid leachates is difficult due to strong electrostatic interactions of small scandium(III) ions with sulfate anions. Sulfuric acid leachates generally contain high concentrations of base metal ions like iron(III) and this reduces the efficiency of the [Hbet-STFSI-PS-DVB] for uptake of REEs. Therefore, a precipitation step for iron(III) removal with aqueous ammonia solution was introduced, as a simple and economically viable pretreatment step of BR leachate, prior to the REEs recovery by the SILP. Iron(III) precipitation from sulfuric acid BR leachates increased the efficiency of purification by column chromatography. Additionally, scandium(III) phosphate precipitation, after iron(III) removal, was performed in order to compare and assess the optimum route for scandium(III) purification between the two common processes: selective precipitation and elution chromatography. After scandium(III) phosphate precipitation, the recovery and subsequent purification of the remaining REEs on the [Hbet-STESI-PS-DVB] column were also examined, and resulted in a higher purity of these REEs.