© The Minerals, Metals & Materials Society 2018
Boyd R. Davis, Michael S. Moats, Shijie Wang, Dean Gregurek, Joël Kapusta, Thomas P. Battle, Mark E. Schlesinger, Gerardo Raul Alvear Flores, Evgueni Jak, Graeme Goodall, Michael L. Free, Edouard Asselin, Alexandre Chagnes, David Dreisinger, Matthew Jeffrey, Jaeheon Lee, Graeme Miller, Jochen Petersen, Virginia S. T. Ciminelli, Qian Xu, Ronald Molnar, Jeff Adams, Wenying Liu, Niels Verbaan, John Goode, Ian M. London, Gisele Azimi, Alex Forstner, Ronel Kappes and Tarun Bhambhani (eds.)Extraction 2018The Minerals, Metals & Materials Serieshttps://doi.org/10.1007/978-3-319-95022-8_236

Separation and Purification of Rare-Earth Elements Based on Electrophoretic Migration (PART II)

P. Hajiani1  
(1)
INNORD/GéoMégA, 75 Boulevard de Mortagne, Boucherville, QLD, J4B 6J4, Canada
 
 
P. Hajiani
Email: phajaini@geomega.ca

Abstract

Rare earth elements (REEs) are critical materials in many leading-edge technology products. However, REE separation outside China has remained a challenge in addressing environmental concerns of current production. The author has worked on a technique employing the emphasised variability in the electrophoretic mobility (μi) of REE for the purpose of REE separation . In continuation of the results presented in IMPC 2016 [1], this contribution summarizes the progress achieved in 2016 and 2017. The major goal was to increase the REEs concentration by a factor of 1000 and to attenuate the consequent drawbacks, specifically joule heating effect. Amphiprotic hydroxylic solvent was selected to replace water, which has a major impact on the complexation mechanism and buffer requirement. Non-aqueous media result in a significant drop in the specific molar conductivity of the electrolyte , whilst μi reduces several times only. Moreover, a quasi-steady state electrophoretic separation in conjunction with temperature gradient focusing is adapted to improve scalability.

Keywords

Rare earth elementsSeparationElectrophoresisTemperature gradient focusing

Introduction

The current dominant REE separation technology, solvent extraction , is more than 50 years old and can be assumed as a huge chemical plant (solvent intensive). The objective of this project is to develop a sustainable alternative REE separation process by exploiting variances in other properties of REE rather than that employed in solvent extraction .

Previously, the author demonstrated continuous separation of rare earth ions based on electrophoretic migration across a traverse laminar flow of a complexing agent , so-called free flow electrophoresis (FFE) [1]. Although FFE separation of REEs has been successfully exercised in the laboratory, scaling the technology to operate in a high throughput refinery plant has been considered to be the major technical challenge. There are several key factors which would affect the scale-up adversely. For instance, FFE devices and the corresponding separation mechanism depend on the development of laminar flow, of background electrolyte (BGE), between two plates spaced several millimeters apart. Although other dimensions of the separation cell have no theoretical limit, the plate spacing is considered a major limitation to the design of high throughput separation system. Uniform flow distribution in between plenty of stacked separation units would be a potential issue in scaling up the design.

The other limitation of conventional electrophoresis separation is the maximum analyte concentrate (REE ions) in the separation medium. In preliminary results, the total REE concentration never exceeds several tenths of a milligram per liter [1]. Increasing the analyte concentration requires more complexing agent per liquid volume and leads to high electrolyte conductivity and intensifies Joule-heating. Consequently, the energy efficiency of the method will drop, and heavy cooling is required on the plates along the bed. Besides, excessive Joule-heating creates hot spots and promotes bubble formation within the bed which disrupts laminar flow regime. Furthermore, Joule-heating changes physical properties (i.e. fluid viscosity , equilibrium constants and electrophoretic mobilities) and may promote pure-zone broadening, which adversely affects the FFE separation . Therefore, FFE suffers from serious limitations in applications requiring more than several tenths of a grams of analyte per liter.

Following a rigorous literature review and lab work, a combination of multiple resolutions was conceived to address abovementioned challenges to improve REE electrophoretic separation . The approach required a specific separator design, which may not match what is required for FFE. Major modifications could be classified in three groups: (1) Separation medium: Non-aqueous (nAq) medium was selected instead of aqueous. Hence, molar specific conductivity of the concentrated REE solution was significantly improved. Also, REE complexation does not need any buffer or pH control within the chamber. (2) Focusing: A temperature gradient focusing (TGF) is adapted to focus REEs after separation and reduce zone broadening in the chamber. (3) Apparatus: Separation does not occur in a laminar flow regime anymore; thus, separation chamber dimensions are not limited by low Reynolds number, as is the case for FFE.

Method

Non-aqueous Media

Non-aqueous (nAq) electrolyte solution has been studied and applied in chemistry [2]. Solvation and coordination chemistry of three valent metals (e.g. REEs) are significantly different when their salt is dissolved in a nAq solvent [3]. It has been shown that the ionization constants of weak acids and bases are different from that in aqueous solution [4]. Both abovementioned phenomena have a major impact on REE complexation and therefore, electrophoretic mobility variation when applied in electrophoretic separation of REEs [5]. Application of nAq solvent in electrophoresis separation has been subjected to many investigations [6, 7]. However, the advantages of such a system is a controversial topic to date [8–10].

The author observed that the advantage of nAq solvent in this application is twofold. First, REE complexation with carboxylic acids occurs readily, since dissociation constant of weak acids are much less in nAq [9] and their complexes are much more stable. Therefore, complex formation does not require a buffer and precise pH control . Second, the electrophoretic mobility of hydronium is much lower in nAq solution, resulting in a lower electric current under the equivalent chamber dimensions, REE concentration and applied voltage [9]. Consequently, much higher REE concentration can be accommodated in a nAq solution than in an aqueous medium subjected to the same voltage in a similar geometry.

Also, the author observed that in a nAq solution loaded with a single REE salt, specific conductivity is an accurate measure of REE electrophoretic mobility. Such a behavior is not observed in aqueous electrolyte conductivity, which is seriously impacted by the hydronium. Subsequently, the author carefully measured the specific molar conductivity of 4 REE -nitrates in different nAq medium which were complexed with several anionic and neutral chelating agents at 25 °C in the absence of any buffer [11]. The author observed that a carboxylic anion dissolved in an amphiprotic hydroxylic solvent stabilizes REE in concentrations up to 30 g/L, while maintaining sufficient differences in elemental mobilities for an effective electrophoretic separation .

Focusing

The electrophoresis separation is based on the migration and thus, distribution of charged analytes according to their effective electrophoretic mobilities (μi). The effective mobility of analytes depends on their effective charge density, which is calculated based on the complexed ligand-net charge and solvated hydraulic diameter. During electrophoresis migration, although the analytes are separated based on μi, pure zones broaden with time (or space if moving) due to dispersion. Dispersion causes adjacent pure zones to overlap and reduce the effectiveness of the separation .

To oppose dispersion, equilibrium gradient separation methods [12] are proposed to sharpen the pure zone boundaries and to limit pure band broadening as a focusing technique. Basically, focusing technique let a charged analyte to migrate in the opposite direction of the flow and get accumulated (focused) at a point of balanced net forces (focal point) (Fig. 1). Many focusing techniques have been incorporated with electrophoresis to boost the effectiveness of analyte fractionation [13]. For instance, Rotofor fractionator (Bio-Rad) uses isoelectric focusing technique (IEF) in a free liquid for protein separation [14]. IEF separates and focuses compounds at their isoelectric point (pI) within a pH gradient, which is created and maintained in a buffer system [15]. However, IEF may not be suitable for REE separation in terms of scalability, since it requires a controlled pH gradient along the separation chamber and requires a strong buffer system. Creation and control of a pH gradient is challenging and expensive, even in a relatively small separator. Another intrinsic limit of having a buffer system for this application has been discussed in Sect. 1.
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Fig. 1

Variation of net force versus axial position of analyte provides formation of a stagnant equilibrium zone for similar compounds

Adapted from Giddings and Dahlgren [12]

Amongst other focusing techniques, TGF suits this application the best [13]. TGF is a method which combines Joule-heating and locally controlled cooling to create a longitudinal temperature gradient along the separation channel. The temperature gradient affects viscosity of the fluid and the degree of dissociation of the electrolytes (i.e. an equilibrium constant of a metal complexation reaction is temperature dependant) [13]. This effect results in a gradient of electrophoretic mobility for each analyte. Promoted by hydronium ion which has much higher mobility compared to complexed metal ions, a longitudinal conductivity gradient along the separation chamber forms which induces an electric field gradient. Therefore, a longitudinal gradient in migration velocity of complexed metal ions is bestowed to the combined effect of electric field gradient and electrophoretic mobility gradient. As illustrated in Fig. 2, higher conductivity attenuates the electrophoretic force on specific REE ion in hot zone compared to the cold zone. By applying a of a creeping convective counter-flow, a stagnant zone is created within the chamber where separated analyte (REE ) are accumulated or focused (Fig. 2).
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Fig. 2

Schematic of a pseudo steady state binary separation of element A•(green) (positive charge) and element B•(yellow) (positive charge) when μ•(green) > μ•(yellow). A creeping counter-flow of solvent provides an opposing convective force field along the separation chamber. Electrophoretic force is attenuated from left to right due to the increasing temperature . Analyte A•(green) migrates and focuses in the hotter zone since it has higher electrophoretic mobility

Apparatus

Separation performed in a rotating cylinder apparatus containing free solution of coordinated REE salts, dissolved in an amphiprotic hydroxylic solvent. Fluid stabilization against convective and gravitational disturbances accompanied with radial homogenization inside the separation chamber is best owed to the axial rotation of the cylinder (1 rpm) and polyester screen dividers as shown in Fig. 3a. The separation chamber is comparted to 20 cells, each holding one fraction of separated REEs. The electrophoresed REEs get separated and focused by temperature gradient while a convective counter-flow provided resorting force. Cooling and temperature gradient is maintained by an internal axial ceramic finger (Fig. 3a). Finally, the fractionated solution could be withdrawn rapidly after separation completed without intermixing, using a vacuum harvesting apparatus (Fig. 3b). This device is a homemade modification of the isoelectric fractionator by Bio-Rad under the trademark “Rotofor” [14].
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Fig. 3

The scheme of modified Rotofor fractionation system. a Rotofor equipped with 60 ml separation chamber, divided to 20 fractionation cells, filled with REE solution uniformly. b Vacuum harvesting system

Experimental

Figure 4 depicts set-up assembly of electrophoretic separator of REEs. Rotofor machine (Fig. 4a/b) are filled with REE salt solution in designated nAq solvent. Electrode assemblies holding cathode and anode electrolyte solution are separated from the main chamber by ion exchange membranes. Electrode assemblies and ion exchange membranes provide electrical contact between the separation chamber and the power supply. DC power supply (Fig. 4c) provides DC voltage up to 5000 V (up to 400 W) on Pt electrodes which are 20 cm apart. The separation and focusing test was performed in 6 h run by applying 2500 V DC (12500 V/m). Coolant circulates by a peristaltic pump (Fig. 4d). Coolant inlet temperature at 5 °C and flow rate at about 0.7 l/h, in tandem with electric field intensity are the key variables to create and stabilize longitudinal temperature gradient within the separation chamber. A syringe pump (Fig. 4f) provides the creeping counter-flow and creates convective force in separation chamber. This flow corresponds to the actual electromigration velocity of the analytes and must be adjusted accordingly during the test. In this experiment, the optimum counter-flow was set to about 10 ml/h. After finishing the separation , the solution is collected from compartments by a vacuum pump (Fig. 4h) connected to the harvesting system. Permeable polyester screens in the chamber reduce intermixing between neighbor sectors during quick withdrawn. All assays are analysed by ICP-OES (Optima 8300, PerkinElmer).
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Fig. 4

Modified ROTOFOR (Bio-Rad) assembly for electrophoretic separation of REEs and temperature gradient focusing , INNORD lab, Boucherville, a, b ROTOFOR, c DC Power Supply, d Peristaltic pump as coolant circulator, e Chiller, f Syringe pump which creates counter-flow, g pH/Temp. meter, h Vacuum pump connected to the harvesting system

Results and Discussion

Synthetic equally apportioned lanthanum, europium and ytterbium concentrate were used for the experiments. Separating three elements enabled validating and comparing the results to previous work performed in FFE system [1]. A volume averaged REE concentration over chamber volume is reported since REE concentration is not uniform (Fig. 2) through chamber. Besides, it affects the size of the chamber required for separation of 1 unit mass of REE . In previous work [1] 1 g/L REE solution was injected to the separation channel, carrying a background electrolyte and was diluted to volume average of 3.5 mg/L.

Figure 5 shows separation results of the REE sample with 62.54 mg/L volume averaged REE concentration, which is 18-fold higher than previous work. Figure 5 implies that pure-zone of La is wider than the others and the focusing technique must be intensified.
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Fig. 5

A single pass separation of a synthetic REE -mix in the modified ROTOFOR prototype

Later, the total volume averaged REE concentration was further augmented up to 30 g/L which is over 8000 times higher than previous work (1). The electrophoretic separation of such a concentrated REE solution is several times slower within the same electric field strength. In return, the foot print per kg of REE and energy consumption per kg of REE is significantly reduced.

Conclusion

Electrophoretic separation and temperature gradient focusing of REEs in a rotating cylinder seems to be an adequate separation technique for scaling. The separation prototype demonstrates a significant reduction in footprint as it is several times smaller in comparison with the preceding version. More importantly, separation testing was conducted in a liquid which contained 18 to 800-fold more REE per unit of volume. In addition, power consumption per unit mass of REE has been lowered significantly compared to previous prototype [1]. Moreover, this prototype costs about one tenth of what had been used for FFE [1]. Therefore, increasing the REE concentration and lowering the costs of the separation units were successfully achieved.

This separator works based upon a multi-physics system which includes electro-migration, heat, mass and momentum transfer, which are all strongly interconnect in a time-dependent physics. Therefore, scaling of such a complicated geometry requires a detailed numerical model.

Acknowledgements

This research was partly supported by Industrial Research Assistance Program (IRAP) through the National Research Council Canada (NRC) under project number of 849874.