*2.1. Materials and Analytical Procedures*

For the preparation of all solutions used in the present work, de-ionized water and ACS-grade reagents were used. Ion-adsorption clay ores of African origin (courtesy of Tantalus Rare Earths AG) were tested in this study. X-ray Fluorescence (XRF, Brucker AXS S2 Ranger, Brucker, Madison, WI, USA) was employed to determine the overall bulk chemical composition of the solids, whereas the REE content was determined by aqua regia digestion (HCl:HNO3 3:1 *v*/*v*) at 220 ◦C for 1 h (Ethos EZ microwave system, Milestone, Sorisole, Italy) followed by inductively coupled pslasma optical emission spectrometry on the filtered solution diluted with 5% HNO3 (ICP-OES, Agilent 720 series, Agilent Technologies, Santa Clara, CA, USA). The composition of all liquid phases following leaching was analyzed by ICP-OES (Agilent 720 series).

#### *2.2. Batch Leaching Tests*

The baseline experiments involved 50 g clays, 0.125 mol/L (NH4)2SO4 (i.e., 0.25 mol/L NH4 <sup>+</sup> exchange ions) as the main lixiviant, ambient conditions, liquid to solid (L:S) ratio of 2:1 (*v*/*w*), moderate stirring to ensure slurry suspension (300–500 rpm), and 30 min total time, following experimental procedures developed earlier (Moldoveanu and Papangelakis 2012; 2013). At the beginning of the experiment, each of the chelating agents listed in Table 1 were added to the slurry in 1:1 and 2:1 stoichiometric excess, respectively, with respect to the total content of adsorbed cations (i.e., REEs and impurities). Previous work determined the desorption kinetics to be very fast (<15 min) and independent of leaching conditions

such as temperature, pH, and agitation, which influence only terminal extraction levels (Moldoveanu and Papangelakis, 2013). At the end of each experiment, the pregnant leach solution (PLS) was separated by vacuum filtration. The solid residue was washed twice with deionized water of adjusted pH 5 (L:S = 2:1), dried in the oven at 50 ◦C (overnight), weighed, and stored, while the mother liquor and wash water collected after filtration were diluted with 5% (*v*/*v*) HNO3 and analyzed for REE content.

The following formula was employed to quantify the total REE (TREE) extraction (as %):

$$\% \text{ TREE extracted} = \frac{\text{TREE}\_{\text{in, clay}} - \text{TREE}\_{\text{ng,final}}}{\text{TREE}\_{\text{in,clay}}} \times 100$$

where: TREEin,clay = mass of TREE contained in the initial amount of leached ionic clays (mg) and TREEaq,final = mass of TREE contained in the final leaching solution + wash solution (mg), (determined as concentration, units of mg/L, and converted to mass by multiplying with the volume of respective solution).

#### **3. Results and Discussion**

*3.1. Ore Composition*

The bulk chemical composition of the ion-adsorption ore presented in Table 2 is characteristic of the typical weathered ores containing mixed alumino-silicates, mainly kaolinite/halloysite (Al2(Si2O5)(OH)4), quartz (SiO2) and mica (KAl3Si3O10(OH)2), consistent with the overall compositions described in literature ([16,59,60]); the total REE (TREE) content, was determined to be 0.13% (*w*/*w*).

**Table 2.** Overall chemical composition of clays (XRF, major elements only, >0.1 wt%).


The individual and relative REE content, respectively, as shown in Table 3, indicates that the ore contains about 78.5% LREE and 21.5% medium and heavy REE.

**Table 3.** Individual REE content and relative REE distribution in ore (ICP-OES).


*3.2. Batch Ion-Exchange Leaching Tests with Ammonium Sulfate and Chelating Agents*

The ion-exchange leaching performance with 50% less ammonium sulfate (i.e., 0.25 mol/L NH4 +) in the presence of the chelating reagents will be compared to the baseline case of 0.5 mol/L NH4 <sup>+</sup> alone, as well as with the TREE extraction levels achieved with EDTA (method adapted from [61,62]).

Figure 1 shows the comparative TREE extraction levels obtained for addition of chelating agents in 1:1 and 2:1 stoichiometric ratios with respect to the total content of adsorbed cations (i.e., REE and impurities), following the procedure described in Section 2.2. It can be observed that 1:1 addition of chelating agents to 0.25 mol/L NH4 <sup>+</sup> (as sulfate) resulted in 6–10% increased extraction when compared to lixiviant alone, except for glycine and asparagine, which showed no/very little improvement. This finding is in accordance with the values of logβ shown in Table 1 and the fact that the stability of a chelate increases with the number of functional groups on the ligand ([49,50]). Moreover, EDDS, NTA and aspartic acid reached levels of extraction close to that achieved by 0.5 mol/L NH4 +, denoting sustained high TREE recovery with 50% less lixiviant. The use of NTA-Na3 showed only marginal improvement when compared to the acidic form (NTA), but it may prove a better environmental choice due to lower final acidity levels. EDTA led to the highest TREE extraction, but this reagent was employed only as a measure of maximum

extraction achievable and is not being considered as an option due to the reasons explained in Section 1.

**Figure 1.** Influence of chelating agent addition on the TREE extraction with ammonium sulfate (ambient conditions, 30 min, L/S = 2/1).

Figure 1 indicates that the 2:1 stoichiometric excess did not lead to appreciable improvement in TREE extraction; this finding is in conformity with data reported that lanthanides form ML and ML2 complexes with ligands having four or fewer coordinating sites but only ML complexes with higher dentate ligands ([49,53]). As higher excess of chelating agent does not seem necessary/useful, the 1:1 ratio brings the additional benefit of minimum environmental impact.

In terms of individual REE behavior, a certain selectivity was noticed towards Y and the light REEs (La to Gd, with the exception of Pr and Ce), as these elements exhibited up to 30% higher extraction than the heavy REEs. This trend was explained by the higher charge density associated with the HREEs, which leads to stronger adsorption on clays. Individual REE extraction levels with 0.25 mol/L NH4 <sup>+</sup> and 1:1 chelating agents are shown in Table S1 of the Supplementary Materials.

### *3.3. Process Implications—Seawater as Lixiviant*

The immediate implication of the chelation-assisted ion-exchange leaching process pertains to the possibility of using seawater for leaching solution preparation, instead of fresh water and ammonium sulfate. The average natural seawater composition, in terms of major elements, is given in Table 4, and shows Na and Mg chlorides and sulfates as the main components; although the authors showed in previous studies that Na and Mg are less efficient than NH4 <sup>+</sup> as exchange cations for REEs ([13,15]), it is hypothesized that the presence of ligands has the potential to improve seawater's performance and increase the REE recovery. This would reduce the overall hydrometallurgical plant freshwater consumption, eliminate the ammonia pollution, lessen recycling requirements, and open up the possibility of returning the final purified streams to the sea without risk of contaminating the soil.


**Table 4.** Average natural seawater composition—major elements [63].

The authors prepared synthetic seawater (SSW) of similar composition and performed comparative leaching experiments of ion-adsorption clays with SSW only (corresponding roughly to 0.45 mol/L exchange cations as NaCl), and SSW + selected chelating agents. These cases were compared to the TREE extraction levels achieved when using the very efficient ammonium sulfate lixiviant (both 0.5 mol/L and 0.25 mol/L NH4 +, respectively); the results are depicted in Figure 2.

**Figure 2.** Influence of chelating agent addition on the TREE extraction with simulated seawater (ambient conditions, 30 min, L/S = 2/1, 1:1 chelating agents).

It can be observed that 1:1 addition of chelating agents to SSW resulted in noticeably increased extraction when compared to leaching with SSW alone (again, with the exception of glycine and asparagine) from ~5% for aspartic acid to ~20% for EDDS and NTA-Na3 (notwithstanding 30% for EDTA), reaching levels close to ones achieved with 0.25 mol/L NH4 +. Individual REE extraction levels with SSW and 1:1 chelating agents are shown in Table S2 of the Supplementary Materials.

#### *3.4. Behaviour of Aluminum*

Previous studies conducted by our group [15], as well as other researchers ([7,64,65]) revealed that the main impurity associated with the ion-adsorption ores is Al, which follows identical desorption kinetics to REEs during leaching with (NH4)2SO4, with no selectivity window. The potentially high aluminum concentration in the leachate has a negative impact on the whole downstream REE recovery process, as it leads to excessive consumption of the precipitation reagent (generally oxalic acid, [66]).

The authors evaluated the Al concentration in the leachate (as mg/L) produced during ion-extraction leaching of REEs with ammonium sulfate and SSW alone, and with 1:1 ratio chelating agents under the conditions selected; the results are summarized in Figure 3. The case for 2:1 excess is not shown here as it held no added benefit for REE extraction, as shown in Figure 1, but it extracted slightly more Al. Aluminum extraction levels with 0.25 mol/L NH4 <sup>+</sup> and 2:1 chelating agents is shown in Figure S1 of the Supplementary Materials.

**Figure 3.** Aluminum concentration in the leachate (ambient conditions, 30 min, L/S = 2/1, 1:1 chelating agents).

The comparative chart shows that EDTA, NTA (in both forms), citric and aspartic acids prove very good chelating agents for aluminum, achieving much higher Al concentration in the leachate than in the presence of ammonium sulfate alone (0.25 mol/L NH4 +) or SSW, and therefore offering no selectivity towards REE. The lower Al concentration levels obtained with glycine and asparagine (in both cases) must be due to the inferior chelating power of these compounds, as also indicated in the case of REEs. EDDS appears to suppress Al desorption, which, combined with the good REE extraction levels shown in Figures 2 and 3, make it the recommended choice for the chelation-assisted ion-exchange leaching. NTA-trisodium, although extracting more aluminum than EDDS, performed better than the other chelating agents in this aspect, and could be considered a viable alternative to the more expensive EDDS, especially when used in conjunction with seawater.

#### *3.5. Influence of Solution pH on Rare Earths and Aluminum Extraction*

The final (equilibrium) pH in the PLS is an important factor, as it impacts the metal recovery levels and the environment (both during the in-situ leaching procedure and also upon discharge—especially if seawater is to be employed). Table 5 lists the initial pH of the lixiviant solutions (containing the chelating agent but prior to clay addition) and the final (equilibrium) pH of the filtrate; the pHfinal is a combination of the pHinitial and the buffering effect of the clays (due to the existence of H<sup>+</sup> and OH<sup>−</sup> groups also adsorbed on the surface). It was decided not to adjust the pH for a specific value, as the final/equilibrium values were not considered high enough to initiate the hydrolysis process in the presence of the chelating agents (known to expand the pH solubility window for lanthanide species due to the formation of stable aqueous complexes). Additionally, the operating costs will be further lowered in the absence of an unnecessary pH adjusting step.


**Table 5.** Initial and final (equilibrium) solution pH values.

It can be observed that, with the exception of EDDS and NTA-Na3, the pHfinal was generally below four for both ammonium sulfate and SSW, which could explain the high levels of aluminum in the PLS (as these pH levels are below the Al hydrolysis threshold pH of ~4.5). EDTA, citric acid and NTA led to even more acidic levels of around pH 2; these high levels of final acidity would demand thorough in-situ washing in order to bring the soil towards more neutral pH values as well as pre-treatment prior to discharge (in the case of seawater use).

The use of EDDs and Na-Na3 resulted in more neutral values of pHfinal in both lixiviant systems, which, combined with good TREE recovery and relatively suppressed Al co-extraction, may make these the recommended ligands to be employed, especially with sweater. There is no risk of potential TREE loss at these pH values, due to the fact that chelating agents are known to extend the solubility window beyond the hydrolysis values, which is in the range 6.5–7 for lanthanides (depending on the specific REE). Moreover, the higher pH values obtained in the case of EDDS and NTA-trisodium have a beneficial effect on the dissociation extent of the chelating agent and subsequent availability of the ligand for REE.

#### *3.6. Process Considerations Involving Different Types of Clay Ores*

The major components of the ion-adsorption clays usually employed for research (most of them of Chinese origin) are mainly kaolinite/halloysite, with probable fractions of chlorite and illite, which explains the generally low total rare earth element content (usually 0.03–0.3 wt%) due to low cation exchange capacity (CEC) of these clays. Kaolinite and chlorite have a CEC of 5–15 meq/100 g, illite 25–40 meq/100 g, while the CEC of montmorillonite is 80–120 meq/100 g [67]. Based on these figures, it can be inferred that ion-adsorption ores containing clay fractions with higher CEC, such as montmorillonite, smectite, vermiculite, etc., would have a higher initial overall TREE content.

Alshameri et al. [68] evaluated kaolinite (Kao), montmorillonite (Mt), muscovite (Ms) and illite (Ilt) for their adsorptive/and regeneration behaviors towards La3+ and Yb3+ (as proxies for light and heavy REEs, respectively). They concluded that montmorillonite exhibited the highest adsorption and regeneration efficiencies for both La3+ and Yb3+ and noticed a decrease in the order of Mt > Ms > Ilt > Kao. Less intuitively, however, it was reported that Kao had highest extraction efficiencies for both REEs, in the order of Kao > Ilt > Mt > Ms. This behavior was linked to the structure and surface properties of the clays: while the overall high CEC of "pristine" clays allowed for elevated REE adsorption, the lower desorption from 2:1 clays (such as Mt and Ms) was probably due to the difficulty of the desorbing agent in accessing non-surface adsorption sites. Based on these studies, we can conclude that the consumption of more chelating agent during hypothetical leaching of high CEC clays is not a predictable assumption, despite the elevated initial REE content in clays. Only proper experimental assessment can determine the actual REE extraction and chelating reagent consumption from clays other than the ones employed in the present study.

The model for the lanthanide desorption mechanism from clay materials was proposed and described by the authors in a previous paper [13]. The process is a simple ion-exchange reaction between the rare earths physically adsorbed on clays and exchange cations from solution (such as NH4 +, Na+, Mg2+), and the main driving force is the difference in hydration enthalpy between REEs and the exchange cation (i.e., cations with a more negative hydration enthalpy, such as lanthanides, have more affinity towards the aqueous phase). The chelating reagents are expected to preferentially coordinate the rare earths once in solution; due to their large molecular structure it is not expected that the ligands will adsorb on the clay surface.

#### **4. Conclusions**

The present study investigated the effect of chelating agents on the recovery of rare earth elements from clay minerals via ion-exchange leaching, in order to propose an enhanced procedure that is environmentally benign and allows high REE recovery while reducing or eliminating ammonium sulfate usage.

The authors established screening criteria for the selection of optimal chelating agents, conducted experiments in order to evaluate the efficiency of the selected reagents and compared the results with REE extraction levels obtained during conventional ion-exchange leaching procedures with ammonium sulfate. The main reasons for ligand selection were rapid biodegradability, non-toxicity, and high values of stability constants of formation for complexes.

It was found that 1:1 addition of EDDS, NTA (both the acid and tri-sodium form), aspartic and citric acid to 0.25 M NH4 <sup>+</sup> (as sulfate) resulted in 6–10% increased extraction when compared to lixiviant alone, while 2:1 stoichiometric excess did not lead to appreciable improvement in TREE extraction. Although seawater alone did not perform well as lixiviant, 1:1 addition of chelating agents to SSW resulted in noticeably increased TREE extraction (e.g., 20% for EDDS and NTA-Na3), reaching levels close to the ones achieved with 0.25 mol/L NH4 +. Glycine and asparagine did not enhance TREE recovery in either lixiviant system, due to the inferior chelating power of these compounds.

All chelating agents investigated (again, with the exception of glycine and asparagine) achieved considerably higher Al concentration in the leachate than in the presence of ammonium sulfate or SSW alone, and therefore offered no selectivity towards REE, although EDDS and NTA-Na3 appear to slightly suppress Al desorption.

The main implication of this study is the possibility to use simple seawater with added chelating agents as an extracting agent. From a process perspective, the use of EDDS or NTA-Na3 in conjunction with lower NH4 <sup>+</sup> concentrations and especially seawater appears to be the recommended option, as these systems led to high TREE extraction, moderate Al co-desorption and neutral pH values in the PLS. This has the potential to reduce the overall hydrometallurgical plant freshwater consumption, limit/eliminate the ammonia pollution, and open up the possibility of returning the final purified streams to the sea without risk of contaminating the soil—offering an environmentally benign ion-exchange leaching process.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/met11081265/s1, Figure S1: Comparison of Aluminum extraction at various ratios of lixiviant (ammonium sulphate, Am, containing 0.25 mol/L NH4+ ions) and chelating agents. Table S1: Individual rare earth extraction levels with 0.25 mol/L (NH4)2SO4 (AMS) in the presence of various chelating agents (1:1 ratio). Table S2: Individual rare earth extraction levels with simulated sea water (SSW) containing ~0.48 mol/L NaCl in the presence of various chelating agents (1:1 ratio).

**Author Contributions:** Conceptualization, G.M and V.P.; methodology, G.M.; validation, V.P.; formal analysis, V.P.; investigation, G.M.; data curation, G.M.; resources, G.M.; writing-original draft preparation, G.M.; writing, review and editing, V.P.; visualization, G.M.; supervision, V.P.; project administration, V.P. Both authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not Applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available upon request from the corresponding author.

**Conflicts of Interest:** The authors declare no conflict of interest.
