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Project

Recovery of rare earths from bauxite residue leachates by functionalised sorbents

Bauxite residue, better known as red mud, is a waste product of the alumina industry, but with many unexploited values. The estimated inventory of bauxite residue in operating and closed alumina refineries reaches the value of several billion of tonnes. Stockpiling of huge amounts of residue can create environmental problems. Bauxite residue comprises iron, aluminium, titanium, sodium and even more interestingly, valuable rare-earth elements (REEs). Although a lot of research has been done on bauxite residue valorisation, to date there are no large-scale applications of bauxite residue yet. The REEs, and scandium in particular, are generally more enriched in residues originating from karst bauxites. Interestingly, bauxites found in the southern region of Europe belong to the group of karst bauxites. Europe currently has no operating REE mine, although these elements are getting an increasing role as materials for the transition to cleaner energy and the production of high-tech devices.

In this PhD thesis sorbents were synthesised and investigated for the recovery of REEs from bauxite residue leachates. The leachates can be prepared by direct leaching of bauxite residue with mineral acids, or as a processing step after recovery of other valuable metals. The leachates typically comprise low concentrations of REEs, whereas base elements like iron or aluminium are concentrated. Liquid-solid extraction is a suitable method for recovery of elements from dilute streams. This process requires sorbents selective for elements of interest. Supported ionic liquid phases (SILPs) and crystalline zirconium-phosphate (α-ZrP) were synthesised, characterised and examined for the REEs recovery and separation from other element present in bauxite residue leachates. Moreover, hybrid materials prepared by grafting of short n-alkyl chains (ethyl, n-propyl and n-butyl) to titanium(IV) phosphate functionalised mesoporous MCM-41 silica were tested for mutual separation of REEs.

Two routes for SILPs preparation were investigated: physical impregnation of ionic liquids on a solid support or chemical immobilisation by covalent bonding of the ionic liquid anion to the solid support. The SILP betainium sulfonyl(trifluoromethanesulfonylimide) poly(styrene-co-divinylbenzene), [Hbet-STFSI-PS-DVB], prepared by covalent bonding of ionic liquid to the resin resulted in a sorbent suitable for scandium recovery. The SILP was tested batchwise to reveal its basic performance for scandium recovery from synthetic acidic solutions, such as selectivity, kinetics, influence of pH and sorption capacity. [Hbet-STFSI-PS-DVB] was packed in a column and detailed studies on REEs separation from base elements were performed under column operating conditions. [Hbet-STFSI-PS-DVB] performance for separation of REEs and the base elements in bauxite residue was evaluated with real acidic bauxite residue leachates. The leachates were obtained by direct leaching of bauxite residue by acids and by high-pressure leaching of a slag generated after smelting reduction of bauxite residue for iron recovery. In both cases, a proof-of-principle for the REEs recovery and separation from base elements by the [Hbet-STFSI-PS-DVB] was confirmed. The performance of [Hbet-STFSI-PS-DVB] was further evaluated in a simplified study with a simulated H2SO4 leachate. From this leachate, iron was selectively precipitated and the remaining solution was subsequently tested for the REEs recovery by the [Hbet-STFSI-PS-DVB]. After precipitating iron, scandium uptake from the sulfate media was boosted. Moreover, the purity of scandium obtained after column chromatography was superior to the purity obtained after ScPO4 precipitation.

Furthermore, the selectivity for scandium over iron of several metal(IV) phosphates was investigated from acidic solutions. It was found that α-ZrP exhibits a strong affinity towards scandium, superior to other tested metal(IV) phosphates. Therefore scandium uptake by α-ZrP was investigated in depth. After evaluating batchwise its performance for scandium recovery, α-ZrP was packed in a column. Scandium separation by the α-ZrP column was examined with a leachate obtained by direct leaching of bauxite residue with HCl. A prominent selectivity for scandium over the vast majority of other elements present in bauxite residue was observed.

Due to the small differences between the different REE, they are often recovered from leachates by liquid-liquid or liquid-solid extraction as a group of elements. Still, for certain applications their separation into individual elements is required. The grafted MCM-41 materials were examined for separation of mixtures of REEs. The separation between scandium and lanthanum was found to be remarkable, whereas neodymium and dysprosium separation was comparable to the separation obtained in liquid-liquid extractions with typical extractants such as tributyl phosphate (TBP).

Date:16 Mar 2015 →  2 Jul 2018
Keywords:Scandium, Lanthanides, Adsorption, Critical metals, Rare earths
Disciplines:Analytical chemistry, Pharmaceutical analysis and quality assurance, Inorganic chemistry, Organic chemistry
Project type:PhD project