Next Article in Journal
Hot Deformation Behavior of a Co-Spray-Formed Al-Si/7075 Bimetallic Gradient Alloy
Next Article in Special Issue
Influence of Dysprosium Compounds on the Extraction Behavior of Dy from Nd-Dy-Fe-B Magnet Using Liquid Magnesium
Previous Article in Journal
The Effect of Solidification Rate on the Microstructure and Mechanical Properties of Pure Magnesium
Previous Article in Special Issue
Viscosity and Structural Investigation of High-Concentration Al2O3 and MgO Slag System for FeO Reduction in Electric Arc Furnace Processing
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Metals
Volume 11
Issue 8
10.3390/met11081265
metals-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Chelation-Assisted Ion-Exchange Leaching of Rare Earths from Clay Minerals

by
Georgiana Moldoveanu
and
Vladimiros Papangelakis
*
Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, ON M5S 3E5, Canada
*
Author to whom correspondence should be addressed.
Metals 2021, 11(8), 1265; https://0-doi-org.brum.beds.ac.uk/10.3390/met11081265
Submission received: 29 June 2021 / Revised: 28 July 2021 / Accepted: 6 August 2021 / Published: 11 August 2021
(This article belongs to the Special Issue Recovery and Recycling of Valuable Metals)
Browse Figures
Versions Notes

Abstract

:
The effect of biodegradable chelating agents on the recovery of rare earth elements (REE) from clay minerals via ion-exchange leaching was investigated, with the aim of proposing a cost-effective, enhanced procedure that is environmentally benign and allows high REE recovery while reducing/eliminating ammonium sulfate usage. 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 also explored, in order to offer a process alternative for situations with restricted access to fresh water (either due to remote location or to lower the operating costs). Screening criteria for the selection of chelating agents were established and experiments were conducted to assess the efficiency of selected reagents in terms of REE recovery. The results were compared to extraction levels obtained during conventional ion-exchange leaching procedures with ammonium sulfate and simulated sea water only. It was found that stoichiometric addition of N,N′-ethylenediaminedisuccinic acid (EDDS) and nitrilotriacetic acid-trisodium form (NTA-Na3) resulted in 10–20% increased REE extraction when compared to lixiviant only, while achieving moderate Al co-desorption and maintaining neutral pH values in the final solution.

1. Introduction

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 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 ([7,13,14,15,16]). 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 co-precipitation/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 [33,34,35]. 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 ([37,38,39,40,41]). 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 predominantly 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:
  • Extraction strength: capable of forming stable, strong complexes with the target metals
  • Selectivity towards target metals (i.e., low impurity co-extraction)
  • Low toxicity and high biodegradability
  • Cost-effective
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:
(a)
Poly carboxylic acids (PCA) and amino-poly carboxylic acids recently under study in Europe as alternative chelating agents for heavy metals removal from wastewaters and contaminated soils [34]: citric acid, nitrilotriacetic acid (NTA), aspartic acid, N,N′-ethylenediaminedisuccinic acid (EDDS) and ethylenediaminetetraacetic acid (EDTA)
(b)
Natural amino acids investigated for hydrometallurgical applications due to their complexing action towards transitional metals ([28,29,30]): glycine and asparagine.
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.
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].

2. Materials and Methods

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+ 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 %):
%   TREE   extracted = TREE in ,   clay −   TREE aq , final TREE in , clay   ×   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).
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.

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+ 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+ (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 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+ 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+ 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.
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.
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+ and 2:1 chelating agents is shown in Figure S1 of the Supplementary Materials.
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 Figure 2 and Figure 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+ and OH− 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.
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+ (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+ 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://0-www-mdpi-com.brum.beds.ac.uk/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.

References

  1. Balaram, V. Rare earth elements: A review of applications, occurrence, exploration, analysis, recycling, and environmental impact. Geosci. Front. 2019, 10, 1285–1303. [Google Scholar] [CrossRef]
  2. Zhou, B.; Li, Z.; Chen, C. Global Potential of Rare Earth Resources and Rare Earth Demand from Clean Technologies. Minerals 2017, 7, 203. [Google Scholar] [CrossRef] [Green Version]
  3. Adamas Intelligence. Rare Earth Elements: Market Issues and Outlook. 2019. Available online: www.adamasintel.com/wp-content/uploads/2019/07/Adamas-Intelligence-Rare-Earths-Market-Issues-and-Outlook-Q2-2019.pdf (accessed on 8 August 2021).
  4. Kanazawa, Y.; Kamitani, M. Rare Earth Minerals and Resources in the World. J. Alloys Compd. 2006, 408, 1339–1343. [Google Scholar] [CrossRef]
  5. Gupta, C.K.; Krishnamurthy, N. Extractive metallurgy of rare earths. Int. Mater. Rev. 1999, 37, 197–248. [Google Scholar] [CrossRef]
  6. Murakami, H.; Ishihara, S. REE Mineralization of Weathered Crust and Clay Sediment on Granitic Rocks in the Sanyo Belt, SW Japan and the Southern Jiangxi Province, China. Resour. Geol. 2008, 58, 373–401. [Google Scholar] [CrossRef]
  7. Chi, R.; Tian, J. Weathered Crust Elution-deposited Rare Earth Ores; Nova Science Publishers: New York, NY, USA, 2008. [Google Scholar]
  8. Yang, J.J.; Lin, A.; Li, X.L.; Wu, Y.; Zhou, W.; Chen, Z. China’s Ion-adsorption Rare Earth Resources, Mining Consequences and Preservation. Environ. Dev. 2013, 8, 131–136. [Google Scholar] [CrossRef]
  9. Ganguli, R.; Cook, D.R. Rare Earths: A review of the Landscape. MRS Energy Sustain. A Rev. J. 2018, 5. [Google Scholar] [CrossRef] [Green Version]
  10. Rocha, A.; Schissel, D.; Sprecher, A.; de Tarso, P.; Goode, J. Process Development for the Serra Verde Weathered Crust Elution-deposited Rare Earth Deposit in Brazil. In Proceedings of the 52nd Conference of Metallurgists (COM 2013), Metallurgical Society of the Canadian Institute of Mining, Metallurgy and Petroleum (MetSoc-CIM), Montreal, QC, Canada, 27–31 October 2013. [Google Scholar]
  11. Sanematsu, K.; Kon, Y.; Imai, A.; Watanabe, K.; Watanabe, Y. Geochemical and Mineralogical Characteristics of Ion-adsorption Type REE Mineralization in Phuket, Thailand. Miner. Depos. 2013, 48, 437–451. [Google Scholar] [CrossRef]
  12. Tantalus Rare Earths, AG. Technical Report and Updated Resource Estimate. 2014. Available online: https://intel.rscmme.com/report/Tantalus_Rare_Earths_AG_Tantalus_17-12-2014 (accessed on 8 August 2021).
  13. Moldoveanu, G.; Papangelakis, V. Recovery of rare earth elements adsorbed on clay minerals: I. Desorption mechanism. Hydrometallurgy 2012, 117–118, 71–78. [Google Scholar] [CrossRef]
  14. Moldoveanu, G.; Papangelakis, V. Recovery of Rare Earth Elements Adsorbed on Clay Minerals: II. Leaching with Ammonium Sulfate. Hydrometallurgy 2013, 131–132, 158–166. [Google Scholar] [CrossRef]
  15. Moldoveanu, G.; Papangelakis, V. The Influence of Both Cation and Anion Type on Leaching Efficiency of Lanthanides from Ion-adsorption Ores. IMPC 2016: XXVIII International Mineral Processing Congress Proceedings. In Proceedings of the Metallurgical Society of the Canadian Institute of Mining, Metallurgy and Petroleum (MetSoc-CIM), Montreal, QC, Canada, 11–15 September 2016; ISBN 978-1-926872-29-2. [Google Scholar]
  16. Chi, R.; Tian, J.; Luo, X.; Xu, Z.; He, Z. Basic Research on the Weathered Crust Elution-deposited Rare Earth Ores. In Proceedings of the 52nd Conference of Metallurgists (COM 2013), Metallurgical Society of the Canadian Institute of Mining, Metallurgy and Petroleum (MetSoc-CIM), Montreal, QC, Canada, 27–31 October 2013; pp. 189–199. [Google Scholar]
  17. Tian, J.; Tang, X.; Yin, J.; Chen, J.; Luo, X.; Rao, J. Enhanced Leachability of a Lean Weathered Crust Elution-Deposited Rare-Earth Ore: Effects of Sesbania Gum Filter-Aid Reagent. Metall. Mater. Trans. B 2013, 44B, 1070–1077. [Google Scholar] [CrossRef]
  18. Luo, X.; Ma, P.; Luo, C.; Chen, X.; Feng, B.; Yan, Q. The Effect of LPF on the Leaching Process of a Weathered Crust elution-Deposited Rare Earth Ore. In Proceedings of the 53nd Conference of Metallurgists (COM 2014), Metallurgical Society of the Canadian Institute of Mining, Metallurgy and Petroleum (MetSoc-CIM), Vancouver, BC, Canada, 28 September–1 October 2014. [Google Scholar]
  19. Zhang, Z.; He, Z.; Yu, J.; Xu, Z.; Chi, R. Novel Solution Injection Technology for In-Situ Leaching of Weathered Crust Elution-Deposited Rare Earth Ores. Hydrometallurgy 2016, 164, 248–256. [Google Scholar] [CrossRef]
  20. Xiao, Y.; Feng, Z.; Huang, X.; Huang, L.; Chen, Y.; Wang, L.; Long, Z. Recovery of rare earths from weathered crust elution-deposited rare earth ore without ammonia-nitrogen pollution: I. Leaching with Magnesium Sulfate. Hydrometallurgy 2015, 153, 58–65. [Google Scholar]
  21. Xiao, Y.; Feng, Z.; Huang, X.; Huang, L.; Chen, Y.; Wang, L.; Long, Z. Recovery of rare earth from the ion-adsorption type rare earths ore: II. Compound leaching. Hydrometallurgy 2016, 163, 83–90. [Google Scholar]
  22. Ogwuebu, C.; Chileshe, V. Coordination chemistry in mineral processing. Miner. Process. Extr. Metall. Rev. 2000, 21, 497–525. [Google Scholar] [CrossRef]
  23. La Brooy, S.R.; Linge, H.G.; Walker, G.S. Review of Gold extraction from Ores. Miner. Eng. 1994, 7, 1213–1241. [Google Scholar] [CrossRef]
  24. Xu, B.; Kong, W.; Li, Q.; Yang, Y.; Liu, X. A Review of Thiosulfate Leaching of Gold: Focus on Thiosulfate Consumption and Gold Recovery from Pregnant Solution. Metals 2017, 7, 222. [Google Scholar] [CrossRef]
  25. Li, J.; Miller, J.D. A Review of Gold Leaching in Acid Thiourea Solutions. Miner. Process. Extr. Metall. Rev. 2006, 27, 177–214. [Google Scholar] [CrossRef]
  26. Senanayake, G. Gold leaching by copper(II) in ammoniacal thiosulphate solutions in the presence of additives. Part I: A review of the effect of hard–soft and Lewis acid-base properties and interactions of ions. Hydrometallurgy 2012, 115–116, 1–20. [Google Scholar] [CrossRef] [Green Version]
  27. Tavakoli Mohammadi, M.R. Ammonia leaching in the copper industry: A review. In Proceedings of the XXVI International Mineral Processing Congress—IMPC 2012, New Delhi, India, 24–28 September 2012. [Google Scholar]
  28. Oraby, E.A.; Ecksteen, J.J. The selective leaching of copper from a gold–copper concentrate in glycine solutions. Hydrometallurgy 2014, 150, 14–19. [Google Scholar] [CrossRef]
  29. Oraby, E.A.; Ecksteen, J.J. The leaching of gold, silver and their alloys in alkaline glycine–peroxide solutions and their adsorption on carbon. Hydrometallurgy 2015, 152, 199–203. [Google Scholar] [CrossRef]
  30. Oraby, E.A.; Ecksteen, J.J. The leaching and adsorption of gold using low concentration amino acids and hydrogen peroxide: Effect of catalytic ions, sulphide minerals and amino acid type. Miner. Eng. 2015, 70, 36–42. [Google Scholar]
  31. Eivazihollagh, A.; Svanedal, I.; Edlund, H.; Norgren, M. On chelating surfactants: Molecular perspectives and application prospects. J. Mol. Liq. 2019, 278, 688–705. [Google Scholar] [CrossRef]
  32. Leštan, D.; Luo, C.; Li, X. The use of chelating agents in the remediation of metal-contaminated soils: A review. Environ. Pollut. 2008, 153, 3–13. [Google Scholar] [CrossRef] [Green Version]
  33. Kociałkowski, W.Z.; Diatta, J.B.; Grzebisz, W. Evaluation of Chelating Agents as Heavy Metals Extractants in Agricultural Soils under Threat of Contamination. Pol. J. Environ. Stud. 1999, 8, 159–184. [Google Scholar]
  34. Kołodyńska, D. Chelating Agents of a New Generation as an Alternative to Conventional Chelators for Heavy Metal Ions Removal from Different Waste Waters. In Expanding Issues in Desalination; Ning, R.Y., Ed.; InTech: London, UK, 2011. [Google Scholar] [CrossRef] [Green Version]
  35. Kołodyńska, D. Application of a new generation of complexing agents in removal of heavy metal ions from different wastes. Environ. Sci. Pollut. Res. 2013, 20, 5939–5949. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  36. Haynes, R.J.; Swift, R.S. An Evaluation of the use of DTPA and EDTA as extractants for micronutrients in moderately acidic soils. Plant Soil 1983, 74, 111–122. [Google Scholar] [CrossRef]
  37. Fukuda, Y.; Nakao, A.; Hayashi, K. Syntheses and specific structures of higher-order mixed chelate lanthanide complexes containing terpyridine, acetylacetonate, and nitrate ligands. J. Chem. Soc. Dalton Trans. 2002, 4, 527–533. [Google Scholar] [CrossRef]
  38. Wheelwright, E.J.; Spedding, F.H. The Use of Chelating Agents in the Separation of the Rare Earth Elements by Ion-Exchange Methods; Ames Laboratory ISC Technical Reports; 1955; Paper 101; Available online: http://lib.dr.iastate.edu/ameslab_iscreports/101 (accessed on 8 August 2021).
  39. Mackey, J.L. A Study of the Rare-Earth Chelate Stability Constants of Some Aminopolyacetic Acids. Retrospective Theses and Dissertations, Iowa State University of Science and Technology, Ames, IA, USA, 1960; p. 2797. Available online: http://lib.dr.iastate.edu/rtd (accessed on 8 August 2021).
  40. Rowlands, D.L.G. Observations on the Separation Factors of the Rare Earths with Various Ligands. J. Inorg. Nucl. Chem. 1967, 29, 809–814. [Google Scholar] [CrossRef]
  41. Karraker, R.H. Stability Constants of Some Rare-Earth-Metal Chelates. Retrospective Theses and Dissertations, Iowa State University of Science and Technology, Ames, IA, USA, 1969; p. 1969. Available online: http://lib.dr.iastate.edu/rtd (accessed on 8 August 2021).
  42. Li, Q.; He, Z.; Zhang, Z.; Zhang, T.; Zhong, C.; Chi, R. Study on the recovery of rare earth from weathered crust elution-deposited rare earth ore by citrate coordination leaching. Chin. Rare Earths 2015, 36, 18–22. [Google Scholar]
  43. Wang, L.; Liao, C.; Yang, Y.; Xu, Y.; Xiao, Y.; Yan, C. Effects of organic acids on the leaching process of ion-adsorption type rare earth ore. J. Rare Earths 2017, 35, 1233–1238. [Google Scholar] [CrossRef]
  44. Zhang, H.; Zhang, Z.; Liu, D.; Chai, X.; Chi, R. Permeability Characteristics of Weathered Crust Elution- Deposited Rare Earth Ore In-Situ Leaching with Ammonium Acetate. Min. Metall. Eng. 2019, 39, 110–114. [Google Scholar]
  45. Chai, X.; Li, G.; Zhang, Z.; Chi, R.; Chen, Z. Leaching Kinetics of Weathered-crust Elution-deposited Rare Earth Ore with Compound Ammonium Carboxylate. Minerals 2020, 10, 516. [Google Scholar] [CrossRef]
  46. Chen, Z.; Zhang, Z.; Chi, R. Leaching Process of Weathered-crust Elution-deposited Rare Earth Ore with Formate Salts. Front. Chem. 2020, 8, 1017. [Google Scholar] [CrossRef] [PubMed]
  47. Cristiani, C.; Iannicelli Zubiani, E.M.; Dotelli, G.; Finocchio, E.; Gallo Stampino, P.; Lichelli, M. Polyamine-Based Organo-Clays for Polluted Water Treatment: Effect of Polyamine Structure and Content. Polymers 2019, 11, 897. [Google Scholar] [CrossRef] [Green Version]
  48. Cristiani, C.; Bellotto, M.; Dotelli, G.; Latorrata, S.; Ramis, G.; Gallo Stampino, P.; Iannicelli Zubiani, E.M.; Finocchio, E. Rare Earths (La, Y, and Nd) Adsorption Behaviour towards Mineral Clays and Organoclays: Monoionic and Trionic Solutions. Minerals 2021, 11, 30. [Google Scholar] [CrossRef]
  49. Cotton, S. Lanthanide and Actinide Chemistry; John Wiley & Sons Ltd.: Chichester, UK, 2006. [Google Scholar]
  50. Choppin, G.R. Comparison of the solution chemistry of the lanthanides and actinides. J. Less-Common Met. 1983, 93, 232–330. [Google Scholar] [CrossRef]
  51. Kettle, S.F.A. Physical Inorganic Chemistry-a Coordination Chemistry Approach; Springer: Berlin/Heidelberg, Germany, 1996; pp. 238–268. [Google Scholar]
  52. Choppin, G.R. Structure and thermodynamics of lanthanide and actinide complexes in solution. Pure Appl. Chem. 1971, 27, 23–41. [Google Scholar] [CrossRef] [Green Version]
  53. Smith, R.M.; Martell, A.E. Critical Stability Constants, Enthalpies and Entropies for the Formation of Metal Complexes of Aminopolycarboxylic Acids and Carboxylic Acids. Sci. Total Environ. 1987, 64, 125–147. [Google Scholar] [CrossRef]
  54. Jones, P.W.; Williams, D.R. Chemical Speciation Used to Assess [S,S’]-ethylenediaminedisuccinic acid (EDDS) as a readily-biodegradable replacement for EDTA in radiochemical decontamination formulations. Appl. Radiat. Isot. 2001, 54, 587–593. [Google Scholar] [CrossRef]
  55. Tandy, S.; Ammann, A.; Schulin, R.; Nowack, B. Biodegradation and speciation of residual SS-ethylenediaminedisuccinic acid (EDDS) in soil solution left after soil washing. Environ. Pollut. 2006, 142, 191–199. [Google Scholar] [CrossRef]
  56. Moeller, T.; Martin, D.F.; Thompson, L.C.; Ferrus, R.; Feistel, G.R.; Randall, W.J. The Coordination Chemistry of Yttrium and the Rare Earth Metal Ions. Chem. Rev. 1965, 65, 1–50. [Google Scholar] [CrossRef]
  57. Berthon, G. The Stability Constants of Complexes of Amino Acids with Polar Side Chains. Pure Appl. Chem. 1995, 67, 1117–1240. [Google Scholar] [CrossRef]
  58. Kiss, T.; Sovago, I.; Gergely, A. Critical Survey of Stability Constants of Complexes of Glycine. Pure Appl. Chem. 1991, 63, 597–638. [Google Scholar] [CrossRef]
  59. Weaver, C.; Pollard, L. The Chemistry of Clay Minerals, 1st ed.; Elsevier Science Ltd.: Amsterdam, NY, USA, 1973. [Google Scholar]
  60. Peng, S. Geological Characteristics and the Prospecting Criteria of the Granite-Weathering Crust Ion-Adsorption Type REE Deposits in Nanling Area, South China; Materials Science Forum, Trans Tech Publications: Zurich, Switzerland, 1991; pp. 33–42. [Google Scholar]
  61. Ding, Z.; Wang, Q.; Hu, J. Extraction of heavy metals from water-stable soil aggregates using EDTA. Procedia Environ. Sci. 2013, 18, 679–685. [Google Scholar] [CrossRef] [Green Version]
  62. Minkina, T.M.; Mandzhieva, S.S.; Burachevskaya, M.V.; Bauer, T.V.; Sushkova, S.N. Method of determining loosely bound compounds of heavy metals in the soil. MethodsX 2018, 5, 217–226. [Google Scholar] [CrossRef] [PubMed]
  63. Atkinson, M.J.; Bingman, C. Elemental composition of commercial sea salts. J. Aquaric. Aquat. Sci. 1997, VIII, 39–43. [Google Scholar]
  64. He, Z.; Zhang, Z.; Yu, J.; Xu, Z.; Chi, R. Process optimization of rare earth and aluminum leaching from weathered crust elution-deposited rare earth ore with compound ammonium salts. J. Rare Earths 2016, 34, 413–419. [Google Scholar] [CrossRef]
  65. Yu, B.; Hu, Z.; Zhou, F.; Feng, J.; Chi, R. Lanthanum (III) and Yttrium (III) Adsorption on Montmorillonite: The Role of Aluminum Ion in Solution and Minerals. Miner. Process. Extr. Metall. Rev. 2020, 41, 107–116. [Google Scholar] [CrossRef]
  66. Chi, R.; Xu, Z. A solution chemistry approach to the study of rare earth element precipitation by oxalic acid. Metall. Mater. Trans. B 1999, 30B, 189–195. [Google Scholar] [CrossRef]
  67. Meunier, A. Clays; Springer: Berlin/Heidelberg, Germany, 2005. [Google Scholar]
  68. Alshameri, A.; He, H.; Xin, C.; Zhu, J.; Xinghu, W.; Zhu, R.; Wang, H. Understanding the role of natural clay minerals as effective adsorbents and alternative source of rare earth elements: Adsorption operative parameters. Hydrometallurgy 2019, 185, 149–161. [Google Scholar] [CrossRef]
Metals 11 01265 g001 550
Figure 1. Influence of chelating agent addition on the TREE extraction with ammonium sulfate (ambient conditions, 30 min, L/S = 2/1).
Figure 1. Influence of chelating agent addition on the TREE extraction with ammonium sulfate (ambient conditions, 30 min, L/S = 2/1).
Metals 11 01265 g001
Metals 11 01265 g002 550
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).
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).
Metals 11 01265 g002
Metals 11 01265 g003 550
Figure 3. Aluminum concentration in the leachate (ambient conditions, 30 min, L/S = 2/1, 1:1 chelating agents).
Figure 3. Aluminum concentration in the leachate (ambient conditions, 30 min, L/S = 2/1, 1:1 chelating agents).
Metals 11 01265 g003
Table
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).
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).
logβ
Hexadentate
(logK1 = logβ)		Tetradentate
(logK1 = logβ)		Tridentate
(logβ)		Bidentate
(logβ)
M3+EDTA 1EDDS 1NTA 1Citric 2Aspartic 2Asparagine 3Glycine 4
La15.511.810.39.58.37.16.1
Ce1612.410.79.68.87.26.4
Pr16.413.111.09.79.17.66.9
Nd16.613.711.29.89.57.87.1
Sm17.114.511.5 8.0
Eu17.314.811.59.8
Gd17.414.911.59.9 8.2
Tb17.915.011.6
Dy18.315.111.7 8.6
Ho18.615.411.8
Er18.916.112.0
Tm19.316.412.2
Yb19.617.012.3 8.9
Lu19.917.612.4
Al16.313.411.411.7 9.36.4
1 [53]; 2 [56]; 3 [57]; 4 [58].
Table
Table 2. Overall chemical composition of clays (XRF, major elements only, >0.1 wt%).
Table 2. Overall chemical composition of clays (XRF, major elements only, >0.1 wt%).
OxideSiO2Al2O3Fe2O3TiO2K2OMnOZrO2CaO
Composition (wt%)40.332.419.62.52.30.30.20.2
Table
Table 3. Individual REE content and relative REE distribution in ore (ICP-OES).
Table 3. Individual REE content and relative REE distribution in ore (ICP-OES).
REEYLaCePrNdSmEuGdTbDyHoErTmYbLuTREE
ppm2223211686425044195963642497351306
%1724.6134.9193.41.44.60.52.80.31.80.75.60.4100
Table
Table 4. Average natural seawater composition—major elements [63].
Table 4. Average natural seawater composition—major elements [63].
ElementNaMgCaKClS (as SO4−)pH
Concentration (mol/L)0.450.050.010.010.530.047–8
Table
Table 5. Initial and final (equilibrium) solution pH values.
Table 5. Initial and final (equilibrium) solution pH values.
Chelating Agent(NH4)2SO4 (0.25 M NH4+)SSW
pHinitialpHfinalpHinitialpHfinal
No ligand5.74.18.83.8
EDTA2.832.62.1
EDDS8.57.16.46
NTA2.33.321.9
NTA-Na38.56.19.25.5
Citric2.52.61.82
Aspartic3.13.74.13.9
Glycine5.84.36.84.1
Asparagine4.84.16.73.8
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Share and Cite

MDPI and ACS Style

Moldoveanu, G.; Papangelakis, V. Chelation-Assisted Ion-Exchange Leaching of Rare Earths from Clay Minerals. Metals 2021, 11, 1265. https://0-doi-org.brum.beds.ac.uk/10.3390/met11081265

AMA Style

Moldoveanu G, Papangelakis V. Chelation-Assisted Ion-Exchange Leaching of Rare Earths from Clay Minerals. Metals. 2021; 11(8):1265. https://0-doi-org.brum.beds.ac.uk/10.3390/met11081265

Chicago/Turabian Style

Moldoveanu, Georgiana, and Vladimiros Papangelakis. 2021. "Chelation-Assisted Ion-Exchange Leaching of Rare Earths from Clay Minerals" Metals 11, no. 8: 1265. https://0-doi-org.brum.beds.ac.uk/10.3390/met11081265

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop