*1.1. Background*

Rare earth elements (REEs) are a collection of fourteen of the fifteen naturally-occurring lanthanides (excluding promethium), further grouped, depending on the atomic number, into "light" rare earth elements (LREEs)—La, Ce, Pr, and Nd, and "middle & heavy" (HREEs)—Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Yttrium (Y) and scandium (Sc) are also considered "rare earths", as they occur alongside lanthanides in the same ore deposits and have similar properties [1]. Due to their unique physical and chemical properties, REEs became progressively more indispensable to the modern industry, with increasing demand in specific fields such clean energy, aerospace, and sustainable technology sectors. It is estimated [2] that the demand for REEs from clean technologies will reach 51.9 thousand metric tons (kt) rare earth oxide REO in 2030, with Nd and Dy, respectively, comprising 75% and 9% of the demand. Adamas Intelligence [3] forecasted that magnet rare earth oxide demand (Nd, Pr, Dy, and Tb) will increase at a compound annual growth rate of 9.7%, and the value of global magnet rare earth oxide consumption will rise fivefold by 2030, from \$2.98 billion in 2019 to \$15.65 billion at the end of the decade.

REEs occur as accessory minerals in various rocks, but the most commercially significant sources, as reviewed by Kanazawa and Kamitani [4], are fluorocarbonates (bastnaesite), phosphates (monazite and xenotime) and weathered crust elution-deposited rare earth ores (ion-adsorption clays). Carbonate and phosphate sources, despite being high grade, are associated with elevated recovery costs due to mining, beneficiation, and the need of aggressive conditions to dissolve the REEs [5]. Ion-adsorption type deposits are substantially

**Citation:** Moldoveanu, G.; Papangelakis, V. Chelation-Assisted Ion-Exchange Leaching of Rare Earths from Clay Minerals. *Metals* **2021**, *11*, 1265. https://doi.org/ 10.3390/met11081265

Academic Editors: Jean François Blais, Srecko Stopic and Dariush Azizi

Received: 29 June 2021 Accepted: 6 August 2021 Published: 11 August 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

lower grade than other lanthanide sources; however, this disadvantage is largely offset by the easier mining and processing costs, the relatively low content of radioactive elements (Th, U), and high HREE content [6]. The ion-adsorption ores contain 0.03 to 0.3 wt.% REEs, out of which generally 60–80% occur as physically adsorbed species on clays, recoverable by ion-exchange leaching [7]. Despite the low grades, ion-adsorption ores account for ~35% of China's total REE output and ~80% of the world's HREE production [8,9]. While at the present China is the only country to commercially produce REE from ion-adsorption ores, recent geological surveys have led to the discovery and investigation of similar deposits in South America [10], Thailand [11], and Africa [12].

The conventional method of processing the ion-adsorption ores is by ion-exchange leaching using monovalent sulfate or chloride salt solutions at ambient temperature ([7,13–16]). During leaching, the physisorbed REE are substituted on the substrate by the exchange ions and transferred into solution as soluble sulfates or chlorides.

Ammonium sulfate is the established lixiviant for the recovery of lanthanides from ion-adsorption ores by either heap or in-situ leaching, due to its high extraction efficiency and low product contamination [16]. However, recent trends in ion-adsorption ore research are focused on minimizing the usage of ammonium sulfate in an effort to reduce ammonia pollution of surface and ground waters, either by adding certain leaching-enhancing additives to the conventional (NH4)2SO4 lixiviant or by evaluating alternative leaching reagents. Tian et al. [17] investigated small additions of natural organic reagents such as Sesbania gum (plant-derived), Luo et al. [18] assessed humic and fulvic acid additions, while Zhang et al. [19] proposed a novel "targeted solution injection" method for in-situ leaching, that would optimize the use of ammonium sulfate. Regarding alternative lixiviant use, Rocha et al. [10] evaluated NaCl and NH4Cl, whereas Xiao et al. [20,21] assessed the use of an MgSO4-CaSO4 combination; however, the use of non-ammonium-based reagents such as sodium or magnesium salts generally leads to decrease in REE production and poor product purity due to their lower extraction efficiency [15] and possibility of coprecipitation/entrainment during subsequent processing stages. Nevertheless, as the grade of ion-adsorption ores is generally low and the recovery of REEs needs to be maximized in order to economically justify the process, the use of the most efficient extraction lixiviant (i.e., ammonium sulfate) is advisable, but employed in conjunction with operating practices designed to ensure minimum environmental impact.

The application of coordination chemistry (i.e., the capacity of certain ligands to form stable complexes with metals) to leaching is a well-known and implemented technique in mining and metallurgy, especially for extraction of PGMs, Ag, Cu, and U from ores [22]. For example, as an alternative to the well-established routes of gold leaching with cyanate [23], thiosulphate [24] or thiourea [25], Senanayake [26] comprehensively describes gold leaching by copper(II) in ammoniacal thiosulphate solutions in the presence of various additives. Similarly, while ammonia-based reagents are the traditional abiotic leaching media for copper [27], Oraby and Ecksteen reported on selective leaching of Cu from a Cu–Ag concentrate in the presence of glycine [28] and leaching of Au in a H2O2–glycine medium [29,30]. These processes are, however, dissolution-based leaching, where the ligands are assisting the main lixiviant (either acid or base), by extending the solubility window of metal species in solution via complex formation and thereby enhancing the extraction efficiency.

The use of chelating agents (mostly aminopolycarboxylic and polycarboxylic acids and their salts) for mobilization and removal of toxic metals from metal-contaminated soils is a widely studied and applied low-cost, efficient, soil remediation technique conducted either in-situ (soil flushing) or ex-situ (heap/column leaching [31,32]; chelating agents are able to desorb (mobilize) metals from soil solid phases by forming strong water-soluble compounds stable over a wide range of pH, which are subsequently removed by enhanced phytoextraction or soil washing techniques [33–35]. Synthetic amino-polycarboxylic acids (amino-PCAs) such as ethylenediaminetetraacetic acid (EDTA) and diethylenetrinitrilopentaacetic acid (DTPA) and their analogues are the most widely used industrial chelating agents, with applications in pulp and paper, cleaning, chemical processing, agriculture, and

water treatment [35,36]. In their comprehensive review, Eivazihollagh et al. [31] describe the application of chelating agents to various fields such as wastewater treatment and soil remediation, mineral flotation, organometallic catalysis, and metal recovery, to name a few, and also present the main routes employed for ligand recovery/reuse. Despite obvious advantages such as low cost and good chelating efficiency across the spectrum, these reagents are toxic and exhibit resistance to conventional biological or physico-chemical water treatment destruction methods and show extended persistence in the environment (i.e., low biodegradability).

Lanthanide recovery from ion-adsorption ores is generally performed either as batch, in-situ, or heap leaching, following similar concepts of toxic metals removal during soil remediation procedures. Although the preferential chelation of lanthanides with various specific ligands is a well-known chemistry fact, the applications were initially limited to laboratory-scale techniques ([37–41]). Various applications of chelating agents to REE extraction, especially from ion-adsorption clays, have been developed lately. Li et al. [42] evaluated ammonium citrate to leach the weathered crust elution-deposited rare earth ores. Wang et al. [43] investigated various carboxylic acids as additives to 0.3 mol/L NH4Cl for the leaching of REEs from ion-adsorption ores, while Zhang et al. [44] explored the leaching of rare earth from ion-adsorption ores by ammonium acetate. More recent studies involving the use of chelating/complexing reagents during ion-exchange leaching of rare earths were conducted by Chai et al. [45], who explored ammonium carboxylate–ammonium citrate mixture as lixiviant, and Chen et al. [46], who evaluated formate salts. Similarly, studies conducted by Cristiani et al. made use of the good complex-forming capacity of polyamines to evaluate the efficiency of functionalized clays as sorbents capable of the uptake/removal of heavy metals from polluted aqueous effluents [47] and lanthanides from leachates of electronic wastes [48].

The aim of the present study was to investigate a cost-effective, enhanced ion-exchange leaching procedure that is environmentally benign and allows high REE recovery while reducing ammonium sulfate usage, by employing biodegradable chelating agents in conjunction with the main lixiviant. Additionally, in an effort to further reduce/eliminate ammonia-based leaching, a processing route employing a lixiviant system consisting of simulated sea water (equivalent to about 0.5 mol/L NaCl) in conjunction with chelating agents was explored. Although ion-exchange leaching with NaCl (usually 1 mol/L, according [16]) generally leads to lower total rare earth (TREE) extraction than during leaching with ammonium sulphate, and application of chloride-based reagents has its own challenges (e.g., higher reagent costs and equipment corrosion risks), the use of a naturally occurring, inexpensive, and readily available lixiviant such as seawater is worth evaluating. It is expected that chelating agents will improve TREE extraction with seawater and offer a cost-effective process alternative for situations where access to fresh water and large quantities of chemical reagents is restricted (either due to remote location or to lower the operating costs).

The chelating agents selected are known for good chelating abilities and biodegradability; while some of them have been evaluated for REE ion-exchange leaching before (e.g., citric acid, EDTA, acetate-based), the others are newly applied. More specifically, the authors established screening criteria for the selection of optimal chelating agents, conducted experiments in order to evaluate the efficiency of selected reagents to maintain high REE extraction in the presence of lower lixiviant concentration or simulated seawater, and compared the results with REE extraction levels obtained during conventional ion-exchange leaching procedures.

#### *1.2. Selection of Chelating Agents*

Lanthanides are hard acids with strong preference for electronegative atoms, consequently they bond very well to hard bases, i.e., ligands containing oxygen ([22,49]). In aqueous solutions, complexation always involves substitution of the metal–oxygen bond from solvation water with another metal–oxygen bond from a ligand and the bonds are pre-

dominantly electrostatic [50]. At a molecular level, Kettle [51] explains the affinity between lanthanides and oxygen-containing ligands via the Ligand Field Theory: the 4f orbitals of REEs are well shielded by the 5d and 6s orbitals and do not participate in bonding, undergoing only minimal crystal field splitting; unlike the case for transitional metals (d-block elements), the interactions of lanthanides with ligands are rather dominated by steric and electrostatic effects. Because of this, lanthanides are considered weak field and have affinity towards weak field ligands (e.g., O-containing chelating agents), forming weak field, high spin complexes.

According to Choppin [52], rare earths have high coordination numbers of 8–12 and are thus capable of forming stronger complexes with organic poly-functional ligands than with inorganic ligands due to the possibility of forming multiple metal–oxygen bonds. Furthermore, Smith and Martell [53] indicated that the stability constant values for polydentate ligands are greater than for monodentate ones (i.e., if a bond to one of the donor atoms is broken, the others will hold) and increase with the number of coordinating groups, explaining thus why EDTA is such an efficient (albeit not selective) chelating agent.

In accordance with the challenges described in Section 1.1, the main factors governing the selection of ligands are delineated as following:


Based on these considerations, the most efficient chelating agents (highest binding power) should therefore contain more than one oxygen-containing functional group (carboxyl and/or hydroxyl). The reagents selected for this study are commonly available polydentate compounds known to exhibit good chelating power for heavy metals, low toxicity and superior biodegradability:


In order to avoid introducing additional impurity cations in the system (and thus possibly contaminate the final rare earth product), the acid form (HL) of the chelating agents was selected for this study; however, as it is reported that the amount of available free ligand increases with increasing pH due to improved dissociation [32,35], the tri-sodium form of NTA (NTA-Na3) was also evaluated for comparison purposes. NTA is reported to undergo fast degradation in natural conditions due to action of various bacteria strains from the Proteobacteria subclass, with a half-life of degradation for 100 μg/L NTA of ~31 h (WHO, 1996), while ~80% of EDDS converts to CO2 in 20 days [54,55]. The citric and L-aspartic acids, as well as the small molecule amino acids selected are naturally occurring compounds with known applications in nutritional supplements and food industries, hence of no toxicity.

The efficacy of a chelating agent is usually rated with the overall stability constant of formation of the ligand–metal complexes (given the symbol β). Smith and Martell [53] and Cotton [49] reported that, despite their high coordination numbers (8–12), lanthanides form monodentate and bidentate complexes with ligands having four or fewer coordinating sites, but only monodentate complexes with higher coordinating ligands.

The logβ values can be used to rank different ligands towards a specific metal (the higher the logβ, the stronger, more stable the complex). Table 1 presents the available data on constants of formation (logβ) for complexes of rare earths with the selected chelating agents, as reviewed by various authors; for the multidentate ligands that form only 1:1 complexes, logK1 = logβ.


**Table 1.** Constants of formation (logβ) for complexes of trivalent rare earth ions and aluminum with the chelating agents employed in the present study (25 ◦C, 1 atm, 0.1 mol/L KNO3 ionic strength).

<sup>1</sup> [53]; <sup>2</sup> [56]; <sup>3</sup> [57]; <sup>4</sup> [58].

Data for aluminum is included due to the fact that Al is the main impurity to interfere with the ion-exchange leaching process. Impurities associated with the ion-adsorption ores are usually Na, K, Mg, Ca, Mn, Zn, Al, and Fe [7]; while most of these cations occur as part of the mineral matrix and do not leach out during the mild REE leaching conditions, a significant amount of Al, due to its trivalent state, is physically adsorbed and liable to be desorbed along with the lanthanides during the process [10].
