Elsevier

Journal of Membrane Science

Volume 545, 1 January 2018, Pages 259-265
Journal of Membrane Science

Separation of lanthanum(III), gadolinium(III) and ytterbium(III) from sulfuric acid solutions by using a polymer inclusion membrane

https://doi.org/10.1016/j.memsci.2017.09.085Get rights and content

Highlights

  • A polymer inclusion membrane for selective extraction of lanthanides was developed.

  • Its composition was 45% di-(2-ethylhexyl) phosphoric acid (D2EHPA) and 55% PVC.

  • Yb(III), Ga(III) and La(III) represented heavy, middle and light lanthanides.

  • Complete extraction, stripping and separation of these ions was achieved.

  • Yb(III): (D2EHPA)2 in the extracted complex is 1: 3 ((D2EHPA)2 – D2EHPA dimer).

Abstract

This study demonstrates for the first time the possibility of selective separation of heavy, middle and light lanthanide ions, represented by Yb(III), Gd(III) and La(III), using a polymer inclusion membrane (PIM) composed of 45 wt% di-(2-ethylhexyl) phosphoric acid (D2EHPA) and 55 wt% poly(vinyl chloride) (PVC). Complete and selective extraction of these three lanthanide ions was achieved by varying the pH of the sulfuric acid feed solution (i.e. Yb(III) at pH 0.25, Gd(III) at pH 1.25 and La(III) at pH 2.25). The extracted ions were back-extracted completely into solutions containing sulfuric acid at concentrations of 7.0, 1.0 and 0.3 mol L−1, respectively. A comprehensive study of the Yb(III) extraction system revealed that the Yb(III): (D2EHPA)2 ratio in the extracted complex was 1:3, where (D2EHPA)2 refers to the D2EHPA dimer. The corresponding thermodynamic extraction constant was determined to be 7.45 × 104. Assuming the same stoichiometry for the La(III) and Gd(III) complexes, the thermodynamic extraction constants of these two lanthanides were determined on the basis of their extraction isotherms as equal to 0.776 and 81.2, respectively. The initial flux values for the three lanthanide ions studied ranged from 6.98 to 3.85 × 107 mol m−2 s−1 in the extraction experiments and from 12.4 to 5.76 × 107 mol m−2 s−1 in the back-extraction experiments.

Introduction

Lanthanides are employed in a variety of applications such as in doping materials to tune their optical, electronic and magnetic properties and as components of polishing powders and industrial catalysts [1], [2], [3]. The similar physiochemical properties of lanthanides, naturally found together in ores such as basanite, monazite and xenotime, make their separation and purification a challenge [4], [5].

The hydrometallurgical manufacturing of lanthanides is often based on the use of sulfuric acid for their leaching from ore concentrates [2] or dissolution of discarded components containing lanthanides (e.g. permanent magnets) [1]. Industrially the separation of lanthanides from their sulfuric acid solutions is conducted using solvent extraction (SX) where multiple extraction steps are necessary to obtain the purity required for the corresponding applications [2], [4], [6]. Acidic extractants are commonly used for the selective extraction of the lanthanide ions in hydrometallurgical processes, making use of the small differences between the formation constants of the corresponding extractant-lanthanide complexes [2], [4]. The SX mechanism of lanthanide ions involving acidic extractants such as carboxylic or organophosphorus extractants is represented by Eq. (1) [4], [6], [7], [8].Ln(III)aq+3HLorgLnL3org+3H+aq

One such extractant, di-(2-ethylhexyl)phosphoric acid (D2EHPA), a monoprotic acid, is widely used in SX to selectively extract metal cations. In the case of lanthanide cations, the pH of the feed solution should be adjusted appropriately in accordance with the Hard and Soft Acids and Bases Theory so that the harder heavy lanthanides are extracted at lower pH values than the softer lighter lanthanides [2], [4], [5].

D2EHPA has already been used to selectively extract all the lanthanide cations through commercial extraction processes such as TALSPEK used in the nuclear fuel industry [4], [6]. However, there are serious environmental and health issues associated with the use of commercial SX systems, mainly due to the large volumes of volatile, flammable and toxic diluents required [9].

Polymer inclusion membranes (PIMs) are a relatively new type of liquid membranes commonly made of poly(vinyl chloride) (PVC) or cellulose triacetate (CTA) as the base-polymer and an extractant which is often referred to as carrier [9]. Separation involving PIMs is an attractive green alternative to traditional SX because the use of large volumes of diluents is eliminated and much smaller quantities of extractants are required [9]. In a PIM separation system the extraction and back-extraction processes take place simultaneously on either side of the membrane thus simplifying the overall separation process and allowing it to be conducted in a continuous flow-through fashion [9]. PIMs are more stable than supported liquid membranes and having a relatively long lifetime they can be reused a number of times although some decrease in the membrane permeability after several successive applications is often observed. The usual reason for this unwanted effect is the leaching of the membrane liquid phase because of the finite water solubility of its constituents [10]. Various approaches to improve PIM stability and permeability which involve the use of other base-polymers (e.g. poly(vinylidene fluoride-co-hexafluoropropene) co-polymer (PVDF-HFP) [11]) or adding crosslinking polymers to the membrane composition [12] have produced promising results.

Despite the advantages of PIM-based separation, there is very little research done on lanthanide extraction with PIMs reported in the literature, especially when one considers the large amounts of SX and supported liquid membrane studies that have been conducted in this area [6], [7], [8], [13], [14]. Li Chen and Ji Chen have used poly(vinylidene fluoride) (PVDF) PIMs containing the ionic liquid [A336][P507] for the extraction of predominantly heavy lanthanides such as Lu(III) [15]. Research has also been done for Ce(III) extraction using N,N,N′,N′-tetraoctyldiglycolamide (TODGA) in CTA-based membranes [16], [17]. Ansari et al. have demonstrated that these membranes are capable of transporting La(III), Eu(III) and Lu(III) from 1 M nitric acid solutions [18]. However neither TODGA nor [A336][P507] are commercially available [15], [16], [18]. Additionally, the use of CTA as the base-polymer may pose some stability issues at high acidities often used in lanthanide extraction [2], [4], [9]. Although the studies mentioned above show effective extraction of lanthanides, no membranes have yet been reported for effective lanthanide separation through selective PIM extraction.

Despite the fact that the commercially available extractant D2EHPA has been used successfully for the SX of lanthanides, to the best of our knowledge no PIMs containing D2EHPA have been used for the same purpose. PVC is a common polymer used in making PIMs and is stable at the acidities required for selective lanthanide extraction using D2EHPA, making it ideal as a base-polymer for such applications. Studies with PVC-based D2EHPA PIMs for the selective extraction of Zn(II), U(VI) and Pb(II) have already been conducted, showing that D2EHPA/PVC is a stable PIM composition not requiring plasticisation [19], [20], [21]. Thus, this combination of extractant (i.e. D2EHPA) and base-polymer (i.e. PVC) was chosen in the present study.

The lanthanides can be split into three groups of light (La-Sm), middle (Sm-Ho) and heavy (Ho-Lu) [5], [6]. Lanthanum, gadolinium and ytterbium were selected as representatives of these three groups because of their industrial significance [1], [22], [23]

This paper describes a study on the applicability of a PVC-based PIM containing D2EHPA as the carrier for the selective extraction and separation of La(III), Gd(III) and Yb(III), from their strong sulfuric acid solutions. The PIM-based separation of these metals is characterized in terms of the stoichiometry and extraction constants of the corresponding metal-D2EHPA complexes, along with extraction and back-extraction initial fluxes.

Section snippets

Reagents

D2EHPA (97%, Sigma Aldrich), Yb(NO3)3·5H2O (99.9%, Sigma Aldrich), Gd(NO3)3·6H2O (99.9%, Sigma Aldrich), Arsenazo III (Sigma Aldrich), La(NO3)3·6H2O (96%, Sigma Aldrich) (BDH), H2SO4 (98%, SCI Labscan), HCl (32%, Ajax Finechem), NaOH (98%, Chem Supply), potassium hydrogen phthalate (99.95%, Chem Supply), high molecular weight PVC (Fluka), tetrahydrofuran (THF) (HPLC grade without stabiliser, VWR), and bromothymol blue (BDH) were used as received. Deionized water (resistivity > 18.2  cm,

Results and discussion

D2EHPA was selected as the extractant in the PIMs studied due to the fact that it is readily available commercially and its ability to extract and separate efficiently lanthanides in solvent extraction and supported liquid membrane systems [2], [4], [6]. Previous research on D2EHPA/PVC-based PIMs has shown that the highest D2EHPA concentration at which the membrane is still mechanically stable and can be used in multiple extraction/back-extraction cycles is 45 wt% [21], [28]. Therefore PIMs of

Conclusions

This is the first study that demonstrates a successful PIM-based selective separation of light, middle and heavy lanthanide ions from sulfuric acid solutions using a PIM composed of 45 wt% D2EHPA and 55 wt% PVC. It has been established that La(III), Ga(III) and Yb(III), as representatives of these three groups of lanthanide ions, can be extracted selectively and completely by manipulating the solution acidity (i.e. Yb(III) at pH 0.25, Gd(III) at pH 1.25 and La(III) at pH 2.25). Complete

References (33)

  • P. Sipos

    Application of the Specific Ion Interaction Theory (SIT) for the ionic products of aqueous electrolyte solutions of very high concentrations

    J. Mol. Liq.

    (2008)
  • A.M. St John et al.

    Determination of the intial flux of polymer inclusion membranes

    Sep. Purif. Technol.

    (2013)
  • A.M. St John et al.

    Transport and seperation of uranium (VI) by a polymer inclusion membrane bsed on di-(2-ethylhexyl) phosphoric acid

    J. Membr. Sci.

    (2012)
  • C. Preinfalk et al.

    The industrial applications of rare earth elements

  • M. Cox et al.

    Solvent Extraction Principles and Practice

    (2004)
  • F.A. Cotton et al.
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