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Application of Eh-pH Diagrams on Acid Leaching Systems for the Recovery of REEs from Bastnaesite, Monazite and Xenotime

: Bastnaesite, monazite and xenotime are rare earth minerals (REMs) that are typical sources for rare earth elements (REEs). To advance the understanding of their leaching and precipitation behavior in different hydrometallurgical processes, Eh-pH diagrams were constructed and modiﬁed using the HSC 9.9 software. The aqueous stability of rare earth elements in H 2 O and acid leaching systems, i.e., the REE-Ligands-H 2 O systems, were depicted and studied based on the Eh-pH diagrams. This study considers the most relevant lixiviants, their resulting equilibrium states and the importance in the hydrometallurgical recovery of rare earth elements (REMs). A literature review was performed summarizing relevant Eh-pH diagrams and associated thermodynamic data. Shifting stability regions for REEs were discovered with additions of acid ligands and a narrow stability region for soluble REE-(SO 4 /Cl/NO 3 ) complexes under highly acidic conditions. As such, the recovery of REEs can be enhanced by adjusting pH and Eh values. In addition, the Eh-pH diagrams of the major contaminants (i.e., Fe, Ca and Al) in leaching systems were studied. The resulting Eh-pH diagrams provide possible insights into potential passivation on the particle surfaces due to the formation of an insoluble product layer.


Introduction
Rare earth elements (REEs) are indispensable constituents in many industrial applications including fuel cells, mobile phones, permanent magnets, lamp phosphors, rechargeable batteries and catalysts [1,2]. With the increasing demand and concerns over restricted or constricting supply, increased importance has been assigned to the improvement and recovery of REEs from REE-bearing minerals. The principal mineral sources for REEs are monazite, bastnaesite and xenotime [3]. In addition to the primary mineral sources, it has also become increasingly crucial to recycle REEs from end-of-life products, such as permanent magnets, fluorescent lamps, batteries and catalysts, which contain a fair amount of REEs [4][5][6][7]. REE primary and secondary sources are often treated with physical separation processes followed by hydrometallurgical methods. In broad terms, froth flotation is used to produce a REE mineral concentrate followed by leaching, solvent extraction and/or selective precipitation to extract the REEs from their mineral matrix [3,8].
The behavior of REEs during leaching processes is typically evaluated using Eh-pH diagrams. Eh-pH diagrams, also known as Pourbaix diagrams, have played a significant role in understanding the aqueous stability of species that are thermodynamically stable within certain regions of redox potential and pH. The application of Eh-pH diagrams is especially useful when multiple elements, which may interact with each other, co-exist in a solution. In addition, the study of Eh-pH diagrams can help improve the feasibility of the recovery/recycling of REEs by presenting the stable REE species in aqueous solutions regarding the critical processing conditions (i.e., pH, Eh and acid ligands). The Eh-pH diagrams of rare earth metals were initially reported by Pourbaix in 1966 [9], which Agarwal et al. (2018) [15] studied monazite and bastnaesite systems with a focus on the yttrium (Y) species in sulfate, nitrate and chloride solutions. The results showed that free Y 3+ ions are dominant in all considered systems below pH 5 while YSO 4 + and YCl 2+ co-exist with Y 3+ in the matrix of sulfuric and hydrochloric acids, respectively. Eh-pH diagrams were also compared with speciation diagrams, which depicted the distribution of the Y species in solution as a function of pH at a constant Eh value. The findings from the speciation and Eh-pH diagrams were consistent. Although Agarwal's work revealed the aqueous speciation of yttrium after acid leaching, further speciation investigations are needed to describe the leaching and precipitation processes.
The objective of this study was to determine, from the literature, the appropriate thermodynamic values required to construct Eh-pH diagrams for systems not previously studied and, by so doing, promote a deeper understanding of the dissolution and precipitation processes that are critical to the extraction of REEs from RE minerals. It is anticipated that this work will provide insights into the processing of a wide variety of primary and secondary REE sources. As an example of secondary sources, other researchers found that coal and coal byproducts could be a promising alternative source for REEs [16][17][18][19]. RE phosphate minerals (apatite, monazite and xenotime) are discovered as most commonly presented rare earth mineral in coal sources [20][21][22]. There is also evidence to support the presence of ion-exchangeable REEs in coal byproducts [23,24]. To investigate the thermodynamic aqueous equilibrium of several pertinent systems, literature was consulted for the appropriate elements combined with the associated lixiviant. As shown in Table 1, there have been significant contributions made in the domains of interest. This work will seek to explore the equilibrium data needed to study the effects on leaching and precipitation based on the data from previous leaching tests (as shown in Table 2).

Rare Earth Minerals in Leaching Systems
Bastnaesite, monazite and xenotime account for the majority of the REE production from a variety of recognized RE ores [8,25]. For the construction of Eh-pH diagrams, these three minerals are considered as the likely dominant RE sources and thus were the focus in the construction of the corresponding Eh-pH diagrams for the RE minerals-ligands-H 2 O systems during acid leaching. The major species studied in the leaching of bastnaesite (chemical formula: (Ce, La)(CO 3 )F) are Ce, La and F. Monazite (chemical formula: (Ce, La, Nd, Th)PO 4 ) is a RE phosphate mineral with a relatively high HREE composition compared to bastnaesite [8]. The Eh-pH diagrams for monazite leaching focused on the (Ce/La/Nd/Th)-PO 4 -Ligands-H 2 O systems. Xenotime (chemical formula: Y(PO 4 )), unlike bastnaesite and monazite, is particularly enriched in large quantities of yttrium and other heavy rare earth elements. The development of the Eh-pH diagrams for xenotime leaching systems consist mostly of yttrium and acid ligands in H 2 O and an objective of the study.
Concentrate samples of the three RE minerals (bastnaesite, monazite and xenotime) were obtained from the Mineralogical Research Company (San Jose, CA, USA) and used in leaching experiments to provide data for comparison with the Eh-pH diagrams. The monazite sample, which originated from Eureka Farm 99 (Stiplemans mine operation), Namibia, contained about 10% total REEs, including 5.7% Ce, 3.2% La, and 1.0% Nd, while the other REEs accounted for less than 0.1%. As such, the chemical formula representing the mineral type is most likely (Ce 0.6 La 0.3 Nd 0.1 )PO 4. The bastnaesite sample ((Ce 0.5 La 0.25 Nd 0.25 )CO 3 F) was mined from Oregon No. 2 pegmatite from an operation located in Jefferson County, CO. The sample contained 25% total REEs (TREEs), which included 12.5% Ce, 4.3% La, 4.3% Nd and significantly smaller quantities of the other light REEs. The xenotime sample was a product of a mine located in Norway and contained 5.5% yttrium, which accounted for over 63% of the TREEs. Thorium was the second highest, representing 2.5% by weight of the total.

Leaching Procedures
The mineral samples were crushed and ground in a shatter box to a top size of 15 microns. Leaching experiments were conducted using a triple neck round bottom flask with a total reflux condenser. Temperature was maintained using a water bath at 25 • C. Agitation was provided using a magnetic stirrer at 530 rpm. Leaching experiments were carried out using 1 M H 2 SO 4 , 1 M HCl and 1 M HNO 3 aqueous solutions with 10 g/L solids concentration. Slurry samples were collected at the following times after test initiation: 5, 15, 30, 60, 90 and 120 min. The solids and liquid samples were immediately separated within one minute of collection using a micro filter. Elemental recovery was calculated using the following expression: where c L is the elemental concentration in the leachate, V L is the leachate volume, c f is the concentration in feed, and m f is the mass of the feed solids.

Construction of Eh-pH Diagrams Using the HSC Software
To simulate the leaching conditions in H 2 SO 4 , HNO 3 and HCl systems, the concentrations of the elements (i.e., S/Cl/N) that form the corresponding acid ligands (i.e., SO 4 /Cl/NO 3 ) were input as 1 M when constructing the Eh-pH diagrams. The Eh-pH diagrams were generated using HSC 9.9 under 25 • C and 1 bar. The Eh value is presented as relative to the Standard Hydrogen Electrode (SHE) potential. Due to higher temperatures decreasing the solubilities of RE phosphates, the temperature was held constant at 25 • C when constructing the Eh-pH diagrams and comparing different chemical systems. The molarities of REEs were calculated based on the concentrations from the chemical assay.

Leaching of REE-Bearing Minerals
As the main component in monazite and bastnaesite minerals, Ce has the ability of changing its valence between +3 and +4. The recovery of Ce is thereby more complicated than that of other REEs in leaching solutions. Efforts were made to leach Ce from pure RE minerals, and the results of Ce recoveries were discussed in this study for monazite and bastnaesite leaching. The results of applying different lixiviants on recovering Ce from monazite are shown in Figure 1. The recoveries of Ce in 1 M H 2 SO 4 , 1 M HCl and 1 M HNO 3 were only 0.5, 0.7 and 0.7%, respectively. Based on the Eh-pH diagrams produced for the REE-PO 4 -SO 4 -H 2 O system by Kim et al. [12], a surface reaction involving the hydrated insoluble forms of Nd and La may create a product layer that prevents the additional leaching of Ce in the H 2 SO 4 leaching system. This hypothesis could possibly explain the low Ce recovery-less than 0.4% when using H 2 SO 4 as a lixiviant ( Figure 1). To further investigate the existing species in solution, the speciation diagram for Ce 3+ in the Ce-SO 4 -H 2 O system was plotted using data obtained from Visual MINTEQ 3.1 (KTH, Stockholm, Sweden), as shown in Figure 2. Due to the lack of roasting or strong oxidative conditions, the formation of Ce 4+ in the aqueous system is not anticipated. For that reason, only Ce 3+ species (i.e., Ce(SO 4 ) 2 − , Ce 3+ , CeOH 2+ and CeSO 4 + ) are considered in Figure 2. The dominant Ce species that exist in the given system are cerium sulfate complexes, i.e., Ce(SO 4   The leaching results of Ce from bastnaesite are shown in Figure 3. Improved recoveries are observed in 1 M HNO 3 and 1 M H 2 SO 4 , whereas the recovery in 1 M HCl leaching is comparatively lower. According to research performed by Shuai et al. [13], Ce 4+ and F − mainly exist as [CeF 2 ] 2+ in sulfuric acid. The cumulative stability constant of [CeF 2 ] 2+ is higher than the solubility product constant value of REEF 3 . The stronger coordination between Ce 4+ and F − result in the formation of soluble complexes in solution. However, as reported by Shuai et al., Ce 4+ is reduced to Ce 3+ during acid leaching and the latter forms insoluble CeF 3 with free F − ions [13], which explains the low recovery of Ce in our HCl leaching tests when compared to other lixiviants as presented in Figure 3.

Eh-pH Diagrams of Rare Earth Mineral Leachates
To address the systems that have not been previously studied (Table 1), the Eh-pH diagrams of bastnaesite leaching in a H 2 SO 4 system, monazite in HCl and HNO 3 systems and xenotime in H 2 SO 4 , HCl and HNO 3 systems were investigated. The results and discussions are provided in the following sections.

Bastnaesite Leaching
Bastnaesite (chemical formula: (Ce, La)(CO 3 )F) is a major REE-bearing mineral [26]. The primary species in bastnaesite leachate are Ce, La and F, with the corresponding acid ligands in the lixiviants. In most hydrometallurgical processes, bastnaesite is typically roasted under conditions that eliminate carbonate. Therefore, when constructing the Eh-pH diagrams for bastnaesite leaching, CO 3 2− is excluded from the system due to the removal of carbonates or the lack of carbonate compounds.
The concentration of the element S composing the acid ligand (i.e., SO 4 2− ) was 1 M for the construction of the Eh-pH diagrams of bastnaesite leaching in a H 2 SO 4 system. The molarities of REEs were calculated based on a chemical assay. The Eh-pH diagrams were generated using HSC 9.9 under 25 • C and 1 bar as presented in  As previously shown in Figure 3, the recovery of Ce in bastnaesite reaches 90, 76 and 94% after 2 h of leaching using 1 M H 2 SO 4 , 1 M HCl, and 1 M HNO 3 , respectively. Due to the effect of Ce 4+ and Ce 3+ on the system, several Eh-pH diagrams are proposed (refer to the bastnaesite summary in Table 1). In a system where bastnaesite is roasted and oxidized, it may be assumed that Ce mainly exists as a tetravalent species. As shown in Figure 5, Ce 4+ forms a hydrated Ce(SO 4 ) 2 salt, which dominates the water stability region. Anecdotal evidence of the lack of solubility of Ce 4+ compounds is corroborated by processes that roast bastnaesite to selectively leach other REEs. It was found in literature that ionic Ce (Ce 3+ and Ce 4+ ) has a strong ability to coordinate with F − and forms a Ce-F complex in the presence of sulfuric acid [14]. The possible existence of a Ce-F complex is indicated by the light blue stability region of CeF 3 below the lower water stability line in the Eh-pH diagram ( Figure 5). However, the stability region of CeF 3 was overlapped by Ce(SO 4 ) 2 ·5H 2 O within the water stability region due to its lower stability compared to Ce(SO 4 ) 2 ·5H 2 O. It seems that, when Ce-F and Ce-SO 4 compounds co-exist in the system, Ce 4+ tends to complex with SO 4 2− in the form of Ce(SO 4 ) 2 with excess SO 4 2− ions.
Although Figure 5 suggests a solid form as a Ce-SO 4 complex at equilibrium in the presence of Ce 4+ , the leaching recovery shown in Figure 3 indicates that Ce is soluble at low pH values of around 0 in a 1 M H 2 SO 4 solution. Due to the variance between the experimental results in Figure 3 showing excellent solubility of cerium from unroasted bastnaesite and the theoretical Eh-pH diagram proposed in Figure 5 indicating an insoluble form of cerium, the Eh-pH diagram was modified to exclude Ce(SO 4 ) 2 ·5H 2 O while holding all other parameters constant. The reason to discount the Ce 4+ species (i.e., Ce(SO 4 ) 2 ·5H 2 O) is that, due to the lack of pre-roasting/pre-oxidizing or the addition of a strong oxidizer, it is unlikely that Ce 4+ will form across the water stability region, even under highly acidic and oxidative conditions. The modified Eh-pH diagram of the Ce-F-SO 4 -H 2 O system is shown in Figure 6. Without the formation of Ce(SO 4 ) 2 ·5H 2 O, the stability region of Ce 3+ occurs within a pH range of −2 to 2. The change of Ce species in the modified Eh-pH diagram shows that Ce could exist stably in its ionic Ce 3+ form at pH values lower than 2 in a 1 M H 2 SO 4 leaching system in the absence of Ce 4+ . This comparison is significant in that the oxidization state of Ce appears to be the single most important factor in the propensity to leach with the associated impact of the oxidation rate of Ce in aqueous systems and the Eh of the solution. In summary, the un-oxidized bastnaesite-sulfate system is better described by the Eh-pH diagram of Figure 6 rather than the more exact thermodynamic predictions presented in Figure 5 given the timescale of the leach test that provided the recovery values presented in Figure 3.
Lanthanum, as shown in Figure 7, also has the ability of complexing with F − and forms insoluble LaF 3 . However, the La-SO 4 compound is soluble in the water stability region within the pH range of 1.5 to 8.5. This finding suggests promising La leachability in sulfuric acid systems for bastnaesite leaching.

Monazite Leaching
The leaching of monazite using 1 M H 2 SO 4 , 1 M HCl and 1 M HNO 3 resulted in Ce recovery values of 0.5, 0.7 and 0.7%, respectively (Figure 1). The results indicate the poor leachability of monazite under the designed leaching conditions. To further investigate the leaching reaction of Ce in monazite, Eh-pH diagrams of monazite solubility in HCl and HNO 3 systems were developed as shown in Figures 8-15, respectively.
In the HCl leaching system, Ce is leached into the solution in its trivalent state and subsequently forms a soluble complex with Cl − in the solution as indicated by the gray region of Ce 3+ in Figure 8 [13]. A similar result occurs in the HNO 3 leaching system, wherein Ce mainly exists in its trivalent state at pH values lower than 3.5, either as a free ion or complexed with NO 3 − . From pH 3.5 to 11, the main species of Ce are phosphate and oxide compounds, which are insoluble forms. The precipitation of Ce hydroxide occurs at pH values higher than 11.
According to the Eh-pH diagrams, the leachability of Ce is comparatively higher under low pH conditions (pH < 3.5). Under these conditions, Ce is in an ionic trivalent form across the water stability region. However, experimental data indicates that the leaching recovery of Ce is less than 1% in 1 M H 2 SO 4 , 1 M HCl and 1 M HNO 3 solutions (Figure 1). This result is likely due to the occurrence of Ce in the feed material in a crystalline state, which requires more energy for Ce to be leached from the associated minerals [27]. As previously reported, a possible reason for low monazite leaching is the difficulty to chemically decompose the mineral structure of monazite and dissolve the REEs into solution [28][29][30]. As shown in Figure 9 through Figures 11, 13 and 14, La and Nd are not soluble except under very acidic conditions. As such, only surface amounts of Ce may be solubilized and leaching diminishes due to the impenetrability (Nd and La insoluble compounds) into the mineral particle. Furthermore, any leaching results in the formation of insoluble complexes in the leaching solution, such as CePO 4 , LaPO 4 and NdPO 4, which accumulates on the mineral surfaces, thereby preventing a further reaction between the minerals and leaching agents, and consequently resulting in low recovery values. Therefore, more aggressive leaching conditions, such as a higher temperature or stronger acidity, may favor the leaching process.
The Eh-pH diagrams of Th shown in Figures 11 and 15 indicate that, in both the HCl and HNO 3 systems, Th has a wider stability region from pH −2 to 8 due to its ionic forms, i.e., ThH 3 PO 4 4+ and Th(HPO 4 ) 3 2− . This result indicates that Th will likely stay in the solution after being leached rather than precipitating as a solid.
Unlike other rare earth elements, which are only stable in their trivalent state (REE 3+ ), Ce commonly exists in trivalent (Ce 3+ ) and tetravalent states (Ce 4+ ) in solution. Tetravalent Ce (Ce 4+ ) is more stable than its trivalent state (Ce 3+ ). Insoluble complexes formed by Ce 4+ and phosphate (i.e., CePO 4 ) show a higher stability than their sulphate or hydroxide forms [31]. The Eh-pH diagrams of Ce species in the Cl − and NO 3 − systems indicate that Ce cannot exist in solutions as its ionic tetravalent form (Ce 4+ ). Instead, it either exists as Ce 3+ or forms complexes as CeClO 4 2+ and CeNO 3 2+ in the presence of Cl − and NO 3 − , respectively, as indicated by Figures 8 and 12. In other words, if Ce is pre-oxidized to Ce 4+ , it will not be easily leached into solution and the separation between Ce 4+ and other REE 3+ species can therefore be achieved.

Xenotime Leaching
The Eh-pH diagrams for xenotime leaching in the H 2 SO 4 , HCl and HNO 3 systems are shown in Figures 16-18. The gray region in all three charts represent the ionic region of Y 3+ (in the Y-PO 4 -SO 4 and Y-PO 4 -NO 3 systems) and YCl 2+ (in the Y-PO 4 -Cl system), respectively. The wide regions of yttrium phosphate overlapping with the water stability region (within the two dash lines) indicate that the leaching of xenotime in H 2 SO 4 , HCl and HNO 3 was limited by the formation of a phosphate precipitant. The recovery of Y in xenotime was 17, 8 and 11% using 1 M H 2 SO 4 , 1 M HCl and 1 M HNO 3 , respectively. In sulfate systems, the formation of ionic species YSO 4 + at pH values from 0 to 0.5 indicates higher leachability of xenotime in sulfuric acid. This correlates well with our experimental data where the highest recovery occurs when using sulfuric acid is used as the lixiviant (Figure 4).

Eh-pH Diagrams of the Main Contaminants
Commonly associated contaminants in REE systems include iron, calcium and aluminum. The Eh-pH diagrams showed some solubility of REE 3+ /Ce 3+ in low pH regions. However, the actual leach recovery values were lower than anticipated. A possible explanation is the formation of a product or precipitation layer on the surface of the minerals by contaminants. The product layer may block the mineral surface, thereby inhibiting the reaction between the acids and the minerals. Similar phenomena have been reported by other researchers [15,31,32]. To further understand the impact of contaminants on leaching, Eh-pH diagrams (Figures 19-22) of the three major contaminating elements, i.e., Fe, Ca and Al, were generated for the 1 M H 2 SO 4 , 1 M HCl and 1 M HNO 3 systems, respectively. In the three acid leaching systems, Fe shows the ability to form different species of precipitants, which may be the main factor that causes the passivation effect on the mineral surfaces.
The Eh-pH diagram of Fe, Ca and Al in the sulfuric acid leaching system ( Figure 19) shows the presence of goethite (FeO·OH), hydrated rhomboclase (H 3 OFe(SO 4 ) 2 ·3H 2 O) and calcium ferrite (CaO·Fe 2 O 3 ), under corresponding leaching conditions. However, the stability region of H 3 OFe(SO 4 ) 2 ·3H 2 O and Fe(SO 4 ) 3 ·5.03H 2 O overshadowed the stability region of Fe 3+ and FeO·OH across the water stability region (from pH −2 to 12). The stable occurrence of H 3 OFe(SO 4 ) 2 ·3H 2 O and Fe(SO 4 ) 3 ·5.03H 2 O contradicts the fact that Fe 3+ and FeO·OH should be the dominant Fe species in the sulfate system. Moreover, (H 3 OFe(SO 4 ) 2 ·3H 2 O and Fe(SO 4 ) 3 ·5.03H 2 O) may form at time scales beyond those of typical leaching due to slow kinetics. Therefore, the Eh-pH diagram for the Fe-Ca-Al-NO 3 -H 2 O system was modified by omitting H 3 OFe(SO 4 ) 2 ·3H 2 O and Fe(SO 4 ) 3 ·5.03H 2 O, while keeping all other conditions the same. As shown in the modified Eh-pH diagram (Figure 20), the dominant Fe species under pH 1 is either Fe 3+ (at Eh above 0.65 V vs. SHE) or Fe 2+ (at Eh below 0.65 V vs. SHE). As pH and Eh increase, FeO·OH starts to form and dominates the stability region in the sulfate system. The Eh-pH diagram for the Fe-S-H 2 O system at 298 K reported by Bernardez et al. also corroborates our findings [33].
In the modified Eh-pH diagram ( Figure 20), goethite (FeO·OH) is considered the main product that could cause passivation on the mineral surface. The formation of FeO·OH can be described as: For REE leaching, the formation of FeO·OH, Fe 3 O 4 and CaO·Fe 2 O 3 (as shown in Figure 20) can inhibit the leaching process by preventing contact between the mineral surfaces and the leaching agent in solution.
In HCl leaching systems, Fe mainly exists at a +3 state at pH values lower than 6.0 and starts to form a FeO·OH precipitate at pH values above 6.0, as shown in Figure 21. In HNO 3 systems, Fe exists as either Fe 2+ or Fe 3+ below pH 1.0, as shown in Figure 22. Generally, Fe exists in its ionic forms at low pH and low Eh conditions, which indicates that the leaching of RE minerals containing Fe may be more favorable under these conditions.

Thermodynamic Data from the HSC Database and Other Sources
The data used in generating the Eh-pH diagrams are listed in Table 1 (see Appendix A). The various Gibbs free energies are listed because there are different values of the same species reported in other resources due to the difference in crystallinity. The species with multiple values reported are indicated in Table 1. It is noted that the Gibbs free energy for ThH 2 PO 4 2+ varies from −434.174 to −723.550 kcal/mol in the HSC 9.9 and the HSC 5.11 databases, respectively, which causes differences in the corresponding Eh-pH diagrams. In addition, new species that are not included in the HSC 9.9 database but are reported in other publications are also included in Table 1 (see Appendix A). For example, the Gibbs free energy for an insoluble cerium sulphate, Ce 2 (SO 4 ) 3 ·8H 2 O, is excluded in the HSC database but reported in other studies by Dean and Wagman [34,35]. The importance of having all the available thermodynamic data of the requisite species is that the effect of different thermodynamic data and the presence of different species on the Eh-pH diagrams can be determined. Future work will be focused on importing the missing species and updating the thermodynamic data to see how the Eh-pH diagrams change accordingly. The comparison between Eh-pH diagrams resulting from different thermodynamic data will also help to gain a better understanding of how species with different crystallinities behave chemically in solution.

Conclusions
In this work, a literature review was performed relating to the Eh-pH diagrams of three RE minerals, i.e., bastnaesite, monazite and xenotime, in aqueous systems. Eh-pH diagrams of RE minerals in hydrometallurgical systems that were not previously reported in the literature were developed in this study. Furthermore, leaching tests using RE minerals in the considered aqueous systems, i.e., acid leaching using H 2 SO 4 , HCl and HNO 3 , were performed to provide experimental data for comparison with theoretical findings from the corresponding Eh-pH diagrams.
Using 1 M inorganic acid concentrations at 25 • C with 10 g/L solid concentration, the Ce recovery from bastnaesite leaching showed over 90% recovery in the 1 M sulfuric acid and 1 M nitric acid leaching systems, whereas 1 M hydrochloric acid provided about 75% leaching recovery. The Eh-pH diagrams of REEs associated with bastnaesite leaching in the Ce-F-SO 4 -H 2 O system indicate that Ce has a strong affinity for SO 4 2− and forms a Ce-SO 4 complex, under the excess SO 4 2− . It was interpreted from the modified Eh-pH diagram and the speciation plot that, in bastnaesite leaching using 1 M H 2 SO 4 , Ce mainly existed in ionic (Ce 3+ ) form, which resulted in a high leaching recovery of Ce. However, it might form Ce(SO 4 ) 2 pentahydrate under strongly oxidative conditions. Under the same leaching conditions, Ce in monazite showed leaching recoveries lower than 1%, regardless of the acid type. The Eh-pH diagrams of monazite indicate that Ce can stably exist as Ce 3+ in both HCl and HNO 3 systems. However, the leaching recovery of Ce in these two solutions was lower than 1%. A possible reason for the low Ce recovery in monazite may be the passivation effect by other insoluble species formed during leaching, such as NdPO 4 and LaPO 4 . In addition, the strong crystallinity of monazite minerals indicates that more intensive leaching conditions, such as higher temperature and stronger acidity, may be required to chemically decompose the monazite structure and promote the Ce recovery.
Yttrium leaching recovery from xenotime showed that sulfuric acid is a preferential leaching reagent with a relatively higher Y recovery (over 15%) compared to nitric and hydrochloric systems. The Eh-pH diagrams of xenotime in leaching solutions indicate that Y has some solubility in the H 2 SO 4 solution under oxidative and highly acidic conditions, which results in the highest recovery among the three lixiviants tested.
According to the Eh-pH diagrams of contaminating elements in the three leaching systems, Fe is likely to form precipitants under specific conditions and block the surface of minerals, thereby inhibiting the recovery of other REEs during leaching. It is suggested that the leaching recovery can be enhanced in the stability regions where Fe and other contaminating elements exist in ionic forms.
The compilation of thermodynamic data (Gibbs free energy) of possible RE mineral species reported in the literature presented the hypothesis that such differences were the result of variations in the crystallinity of REE mineral forms. Funding: This research was funded by the Department of Energy of United States (DOE-FE0029900), project titled "Recovery of Rare Earth Elements from Coal Byproducts: Characterization and Laboratory-Scale Separation Tests". This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process or service by trade name, trademark, manufacturer or otherwise does not necessarily constitute or imply its endorsement, recommendation or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

Informed Consent Statement: Not applicable.
Data Availability Statement: Data sharing not applicable.

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

Appendix A
The thermodynamic data of the involved species was compiled from the HSC database and supplemented from other available resources, as shown in Table 1.