Development and Screening of Resins to Recover Rare Earth Elements (REE) and Scandium from Different Sources

Originally presented at Extraction 2018, August 26-29, 2018. Please visit Springer for more information about the conference or how to purchase the paper or proceedings.
  
Abstract​
Natural and technogenic sources of rare earth elements (REE) and other rare and scattered metals are remarkable for their chemical diversity. The pregnant solutions going to processing for the valuable metals have very complex compositions. Both targeted elements and competing impurities are present in the solutions in different chemical forms. Thus processing of such materials demands application of different technological tools addressing numerous specific tasks on the way to the final product. Screening work was undertaken to identify the best resins among developmental and commercial products addressing typical cases in the hydrometallurgy of scandium and REE. The cases outline typical sources of scandium and REE, such as streams of TiO₂ production, extracted phosphoric acid, barren uranium solutions and others. Specially designed ion exchange resins can be effective tools.
Keywords: REE, Scandium, Thorium, Sorption, Ion Exchange, Separation, Recovery, Sorption, Desorption, Selectivity
 
Introduction
Modern technologies consume many advanced materials based on elements, among others, such as rare earth elements (REE). Yttrium and scandium, thanks to their chemical similarity to REE, are often considered in science and technology as belonging to the REE group. Though the REE market size is relatively small, these elements are critically important for some high-tech applications (1).
Complex compounds of oxide, phosphate, and carbonate or fluoride nature represent most of the REE minerals. Common constituents in such minerals are titanium, iron, thorium, uranium and other metals (2).
Thorium is a common undesired impurity in produced REE concentrates because of its radioactivity. The same is true of uranium (3). The issue of thorium and REE separation becomes especially important in the processing of monazite minerals, which are rich in both thorium and REE (4), (5).
The chemistry of polyvalent cations in aqueous solutions is very complex (6). The trivalent lanthanides are hard acids by the Pearson definition and as such, they complex with the hard bases like F^-, CO₃^2-, SO₄^2-, PO₄^3- and OH^- (7). All of the latter, except fluoride, can be characterized as “oxygen” ligands. Presumably some common impurities, i.e. thorium, ferric iron, titanium, zirconium behave similarly. It makes the task of their separate extraction complicated.
A variety of technical tasks related to extraction of REE and auxiliary operations are considered in many reviews (8).
 
Methods
The last four resins in the below table are developmental products that are not available yet on a commercial scale.

Artificial solutions simulating real ones were used in the work. Not all lanthanides were included in the experiments. Typically, 1 to 3 elements were taken to represent light, medium and heavy rare earths in different experiments.

Scandium from Streams of TiO₂ Production by Sulfate Route
One of the earliest works on ion exchange separation of scandium used strong acid cation exchange resin (9). Resins with different functionality have been examined for scandium separation from REE (10). Later, it was shown that phosphorus-containing resins are more effective for sorption of scandium (11), (12), (13).

Sulfate production of TiO₂ pigment is comprised of several stages. When the acid leach solution is diluted, the titanic acid precipitates as a result of hydrolysis. At this step, the hydrolysis sulfuric acid is obtained. This stream contains 200‑250 g/l sulfuric acid, 15-17 mg/l scandium, around 3-5 g/l titanium, 30-40 g/l of total iron, as well as thorium, zirconium, aluminum and other impurities. The hydrolysis acid may be diluted further before disposal. Definitely, such streams are of interest as potential feeds for scandium recovery. Two solutions modeling the real hydrolysis sulfuric acid were prepared (see below).

Sorption was investigated in column with flowrate of 0.5 BV/hour. Due to technical limitations, the sorption runs were stopped overnight and resumed next morning. In total 20 BV of solution was passed through the column.

In the solution #1 the resins GS176, S957, S958 and S956 and in the solution #2 the GS206 and GS218 were tested.

When 20 bed volumes of the feed had passed through the resin beds, the resins were collected and analyzed for metals. Data of the metal loadings on the tested resins are presented in the table below.

Desorption of scandium is most effective from GS218 resin. In addition, due to higher scandium loading at sorption, the average concentration of scandium in the desorbate is also higher than for other resins. Desorption from S958 is also effective but the grade of the average desorbate is lower. It is interesting to notice that the GS176 desorption profile, which is also effective, is shifted right by about 1 BV. It is as if this resin requires more carbonate for scandium desorption and reflecting a chemical difference of this resin.

Scandium is successfully desorbed from resins GS176, S958 and GS218. Thorium is well desorbed from all resins. Titanium poses the biggest problem at desorption. This metal can be stripped from the resins by fluoride reagents.  However, this is not a desirable method because of the high corrosivity of fluoride ion in acidic media.

Scandium in Uranium In-Situ Leaching Liquors
Scandium and REE are often found in uranium minerals and leached by sulfuric acid together with uranium. A significant amount of natural uranium is recovered by in situ leaching (ISL) technology. Uranium is extracted from the pregnant solution as uranyl sulfate anionic complexes while all elements in cationic form pass in to the sorption filtrate solution. Scandium, thorium and REE remain as cations under these conditions and these metals can be recovered downstream. Table 4 represents a typical analysis of an ISL filtrate solution after uranium recovery.

Here we can consider La as presenting LREE, Y and Yb for HREE.

Sorption experiments were carried out in batch mode at resin to solution volumetric ratio of 1:400 for 24 hours with continuous agitation. Then metals were analyzed in resin and solution. Two development resins, S958 and GS176, were compared with a commercial aminophosphonic resin AMP. Distribution parameters were calculated not only for individual elements but also for groups of them representing the sum of REE, impurities Me as sum of Th and Fe, and, finally, sum of REE plus Th and Fe considered as impurities with respect to Sc.  Accordingly, distribution coefficients Di were calculated (see table below).

It can be seen that the scandium capacity of GS176 resin is expected to be the highest of all three resins, while at the same time, loading of REE is negligible.

For estimation of selectivity the separation parameters for scandium against other elements and their groups were also calculated (see table below).

All three resins are selective for Sc against other metals in the solution. However, the most important finding is the very high selectivity of GS176 resin for scandium against REE, especially against LREE. We can write the order of selectivity for scandium against REE this way: GS176≫S958≫AMP. The selectivity row against all metals except scandium will be the following: GS176≫S958>AMP.  The high scandium selectivity of the GS176 resin will reduce the costs of downstream processing of the scandium desorbate. In addition, the GS176 resin can be recommended for selective separation of scandium from REE concentrate solutions in weak acid sulfate matrices.

REE in Wet Process Phosphoric Acid Production
REE are often present in significant amounts in the minerals used for production of phosphate fertilizers. Phosphoric acid is extracted from phosphate minerals by strong mineral acids, most often by sulfuric acid. The material flow splits in two, an acidic solution and a solid waste consisting mainly of so-called phosphogypsum, e.g. calcium sulfate contaminated with other materials. REE are distributed between these two phases. These streams used to attract great attention from researchers looking to recover REE (14), (15), (16), (17), (18). Most of the studies have dealt with processing of the phosphogypsum. In our work, we selected the liquid phase as the object of our investigation.

The table below presents the modeling acid solution. The array of elements represents LREE, MREE and HREE.

Recovery of REE from phosphoric acid is a very challenging task.  Sorption was carried out in batch at a resin to solution volumetric ratio of 2:200 for 24 hours with continuous stirring. Then metals were analyzed in both phases. Calculated distribution parameters are presented in the table below.

It can be seen that the strong acid cation exchange resin PPC160 demonstrates the highest loading of LREE and MREE among the resins tested, as well as the best selectivity against iron. At the same time this resin uptakes significant amount of calcium to compare with other resins. S958 and AMP resins prefer HREE, but the AMP resin also loads too much iron. PPC160 has the best selectivity for REE due its strong affinity for LREE and MREE. However if the focus is on recovery of HREE then S958 has the best characteristics in terms of selectivity.

Scandium, with REE and Thorium
Monazite is an important source for both thorium and REE, including scandium. Monazite may be processed by various routes, producing different solid materials and aqueous streams. One such stream is obtained after leaching of the solid residues with nitric acid. Table 9 represents the model composition of such a solution.

Sorption was carried out in batch at a resin to solution volumetric ratio of 1:100 for 24 hours with continuous stirring. Metals were analyzed in both equilibrated phases. Table 10 shows distribution parameters for the metals of interest and impurities.

Conclusions
Recovery, separation and purification of REE and scandium from multicomponent solutions are difficult practical tasks. The chemistry of the systems technologists have to deal with are very complicated. Chemical behavior of the metals of interest, as well as of numerous impurities depends on their relative concentrations, anion background, and acidity. It is impossible to make the best choice of ion exchange resin for a particular case without significant experimental work.

The present work is an attempt to outline several technological tools, which can be useful in selected cases. These cases may be used as patterns of general character and applied to similar tasks. It is recommended to consider each actual case as a system comprising the metal of interest, the specific solution matrix and a resin. Changes to any of these three components will most probably require a re-examination of earlier conclusions arrived at from testwork, for example regarding parameters like the resin selected to achieve the desired separation, sequence of specific technological operations.

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