REE solvent extraction through TODGA

December 17, 2025 Critical Raw Materials  |  Mining  |  REE

The rare earth elements (REEs) can be categorized into two main groups: the light rare earth elements, consisting of La, Ce, Pr, and Nd, and the middle and heavy rare earth elements, which include Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y. Sc is considered a dispersed element, while Pm is not naturally occurring. Due to its unique physical and chemical characteristics, rare earth elements find extensive applications in various industries including agriculture, petrochemicals, metallurgy, machinery, luminescent materials, hydrogen storage materials, environmental protection technologies, medicine, and other sectors.

The increasing demand for rare earth elements has led to the depletion of natural reserves, necessitating their extraction from secondary sources such as permanent magnets, waste fluorescent lamps, coal combustion products, industrial dumps, and more. However, recovering REEs from these sources is challenging due to their complex compositions and the extremely low content of rare earth metals present.

At present, liquid-liquid extraction techniques are extensively applied for the recovery of rare earth elements (REEs) from acid leachates of permanent magnets. These methods offer several advantages, including the ready availability of synthetic organic extractants, high extraction efficiency for REEs, and the capability to selectively extract individual rare earth elements.

TODGA

In recent years, a new extractant called diglycolamides (DGAs) has been developed, specifically designed for environmental protection during the extraction processes. Among several derivatives of diglycolamide extractants, N,N,N¹,N¹-tetraoctyl diglycolamide (TODGA) was found as the most promising candidate due to its high solubility in paraffinic solvent, poor solubility in aqueous medium, good chemical and radiation stability, and significantly high distribution ratio (D) value for trivalent actinides such as Am(III) and Ln(III).

Ansari et al. (2012) [1] summarized the solubility of the extractants in water. It shows that DGA derivatives with shorter alkyl chains, such as propyl and butyl, were found to be soluble in water. This solubility in water makes them suitable for use as masking or sequestering agents for f-block metal ions [2], indicating their potential application in aqueous environments. On the other side, DGA derivatives with longer alkyl chains were practically insoluble in water but freely soluble in paraffinic solvents. This solubility behavior makes them suitable for the extraction of actinides, particularly in organic solvent-based extraction processes. The open-chain ether dicarboxylic acid diamides (diglycolamides) also exhibited very high D values for rare earth ions, such as La(III) and Yb(III) from picrate and nitrate media at a pH 5.

TODGA has also shown lower water solubility compared to other extractants [3-5]. Another advantage of TODGA is its rapid extraction of rare earth ions from nitrate media, even at highly acidic conditions. This eliminates the requirement to use alkali solutions to adjust pH levels, significantly reducing aqueous waste. Research has shown that TODGA facilitates the swift separation of rare earths from nitric acid leaching liquor obtained from phosphate ore.

Solvent extraction performance

The concentration of rare earth metal ions in the organic phase after extraction can be quantified by the distribution coefficient (D), separation (a) and percentage extraction (E), defined as follows:

Where D_A, D_B: Distribution coefficients (or distribution ratios) of metals A and B, respectively, at equilibrium:

Aqueous complexation

In order to deeply understand the relationship between DGA structure and its performance, many studies were mainly aimed at understanding the unique complexation behavior of DGA which leads to their exceptionally high affinity for trivalent actinides and lanthanides as well as the complexation behavior of DGA with REE ions.

Solvent extraction is a specialized process designed for selectively separating and concentrating metal ions from an aqueous solution, utilizing an organic solution as a medium. This procedure typically involves three key stages: extraction, scrubbing, and stripping. During the extraction stage, the aqueous solution containing the target metal (referred to as “M”), often present at low concentrations alongside other dissolved substances (such as metals or impurities), is combined with an organic solvent containing an extractant (designated as “L” or “LH”). In this phase, the metal of interest undergoes a reaction with the extractant to form a chemical complex. This complex exhibits greater solubility in the organic phase compared to the aqueous solution. Consequently, the desired metal is effectively transferred into the organic phase.

The complexation of diglycolamide ligand (L) with Mⁿ⁺ can be written step by step as follows:

Pathak et al. (2009) [6] studied for the complexation of Eu³⁺ with TODGA through time resolved luminescence spectroscopy under varying ligand-to-metal (L/M) ratios in an ethanol – water (3:2 ratio) mixture suggested the formation of 1:1, 1:2, and 1:3 species, Eu(TODGA)³⁺, Eu(TODGA)₂³⁺, and Eu(TODGA)₃³⁺, respectively.

Role of acid nature and aggregation phenomena

Due to TODGA’s amphiphilic characteristics, a comprehensive understanding of coordination chemistry and surfactant chemistry is imperative to explain its distinct affinity for trivalent lanthanides and actinides. In nonpolar solvents, such as n-dodecane, the presence of water and acid catalyzes the formation of a polydisperse mixture comprising TODGA monomers and aggregates, even in the absence of any metal ions.

Similarly, the extraction of metal ions by TODGA (L) can be represented by the following equilibrium reaction:

Where x is the number of TODGA molecules extracted with an acid molecule. The extracted species may be the type HA.xL for nitric acid and for perchloric and hydrochloric acid. The subscripts (aq) and (org) represent the aqueous and the organic phases, respectively, and A^– is the counter-anion associated with the metal ion or acid.

Extraction in HNO₃ solution

The transfer of acid into the solvent phase and subsequent alterations in the acidity levels of both the aqueous and organic phases play a crucial role in the extraction process. The migration of acid during solvent extraction can ultimately dictate the outcome of the system, making it vital to comprehend acid extraction thoroughly for the success of the process.

As a kind of neutral extractant, TODGA can extract HNO₃ [7-10]. The extraction behavior for trivalent REEs and HNO₃ with TODGA from nitric acid media can be accounted for by the coordination mechanism, and is shown the following equation:

In which K_H⁺ is obtained to be 0.63 (Bell et al. (2012), [11]). The lanthanide extraction constants were calculated to be [8]

The extraction constant illustrates the extractant’s potent capability in extracting Rare Earth Elements, particularly as acidity levels rise, promoting complex formation. As acidity further increases, nitric acid extraction intensifies.

Nitric acid extraction into TODGA and similar diglycolamides is often described by assuming the formation of a 1: 1 adduct, HNO₃∙TODGA [12]. Gujar et al. (2010) and Arisaka et al. (2011) [13, 14] reported that TODGA is assumed to form a 1: 1 HNO₃∙TODGA complex.

Solvent extraction studies from HNO₃ medium suggested that three to four TODGA molecules are complexed with the metal ion in nonpolar diluents like n-dodecane, and only two TODGA molecules in polar diluents such as 1-octanol, 1,2-dichloroethane, and nitrobenzene. Wang et al. (2017) [3] observed that TODGA extracted Eu³⁺ nearly at the mole ratio of 4: 1 through plotting the logD values with log[TODGA]_org.

Effect of diluent nature

Various organic solvents, including aliphatic and aromatic hydrocarbons, ketones, and high alcohols, are employed as media for dissolving the extractant. The effectiveness of these organic solvents is primarily determined by their polarity and affinity for hydrogen bonding. They significantly influence extraction efficiency by facilitating solvation through hydrogen bonding and forming specific interactions with the extractable complexes. These interactions can enhance the hydrophobicity of the extractable complexes, promoting their stability in the organic phase and ultimately leading to improved extraction outcomes.

Sasaki et al. (2007) [5] reported an increase in distribution of lanthanides and actinides by TODGA with polarity of the diluents. These observations support the outer sphere interaction of solvated cation with the solvated anion in the organic phase as shown in equation 8.  It appears that polar diluents such as nitrobenzene, 1-octanol, and 1,2-dichloroethane do not support the formation of TODGA aggregates, unlike nonpolar diluents such as n-hexane and n-dodecane [1]. Furthermore, it is evident that the nature of the extracted species with trivalent Ln/An varies depending on the nature of the counteranion.

n-dodecane

Zhu et al. [15, 16] conducted a comprehensive investigation into the extraction behavior of 75 elements from a nitric acid medium using TODGA dissolved in n-dodecane. The study revealed that under conditions of relatively high acidity, particularly for trivalent and tetravalent ions with ionic radii ranging from 87 to 113 pm and 83 to 94 pm respectively, the D values exceeded 1000, and the extracted chemical forms were determined to be M(TODGA)₃(NO₃)_n or M(TODGA)₄(NO₃)_n (n = 3, or 4). This was especially prominent for Ln³⁺ and An³⁺ ions. The ionic radii of lanthanide Ln(III)), actinide, An(III), and tetravalent actinide (An(IV)) ions fall within this range. Therefore, TODGA exhibits strong coordination with trivalent and tetravalent lanthanide and actinide ions.

There is a phenomenon which occurs during solvent extraction processes where the organic phase splits into two phases depending on the concentration and the complexation of metal – ligand called the third phase. The maximum concentration of metal ions that can be loaded in the organic phase without the third-phase formation is termed the limiting organic concentration (LOC). TODGA systems are susceptible to third phase formation when lanthanide concentrations surpass the LOC. This threshold is influenced not only by metal loading, but also by external conditions such as temperature and the identity of the aqueous-phase anion. In particular, perchlorate ions significantly lower the LOC and promote third phase formation. This effect is attributed to their ability to disrupt the hydrogen-bonding network of water, thereby enhancing the transfer of metal–ligand complexes into the organic phase and destabilizing the phase boundary.

Other diluents

Research also shows compounds with carboxylic acid groups enhanced complexation with heavy lanthanides at high pH, significantly reducing D. At elevated pH, separation between REEs diminished due to flat D-value profiles, indicating reduced selectivity.

OLI MSE for REE solvent extraction

OLI., a global electrolyte chemical technology leader which uses its proprietary Mixed Solvent Electrolyte (MSE) [17-19] thermodynamic model can model this complex behavior because of its capability to accurately reproduce solid-liquid, vapor-liquid, and liquid-liquid equilibria and chemical speciation in various electrolyte-containing mixtures in a range of temperatures.

The OLI Software Platform V12.5 includes a comprehensive library of chemical species and delivers reliable, science-based thermodynamic property predictions—enabling robust, data-driven simulations across a range of industrial and environmental applications. Its flexible architecture is designed to simulate complex interactive chemistries, making it ideal for both fundamental research and process design.

This framework has already been successfully applied to simulate phase behavior and caloric properties of binary and multicomponent aqueous systems containing rare earth chlorides, sulfates, and nitrates across the lanthanide series. Furthermore, the MSE model has been used to study REE complexation with chelating agents such as TODTA and nitrates, and to support the design of solvent extraction processes for rare earth separation.

OLI MSE simulation results for REE solvent extraction

In Fig. 1, the OLI MSE model very accurately captures the solubility trends of Nd(NO₃)₃ across a wide temperature range. It correctly predicts:

• The solubility limits for various hydrates.
• Phase boundaries between different hydrated forms (e.g., hexahydrate, tetra and monohydrate).
• Ice formation at lower temperatures.

This validates the robustness of the MSE model in reproducing solid-liquid equilibria in single-salt aqueous systems — a critical foundation for more complex multi-phase systems.

Figure 1: Nd(NO₃)₃ – H₂O binary system

Figure 2: Nd(NO₃)₃ – HNO₃ – H₂O ternary system

Fig. 2 extends the Nd nitrate system by introducing nitric acid, simulating the combined effects on speciation and solubility. Accurate modeling of this ternary system is essential for simulating extraction equilibria where both metal nitrates and strong acids coexist.

Figure 3: TODGA – H₂O – n-Dodecane

Fig. 3 shows water solubility in organic phase as a function of TODGA concentration in dodecane. This indicates that diluents control water transport into the organic phase, which is crucial for third-phase risk and extractant hydration effects.

Figures 4 and 5 illustrate how water solubility in the organic phase responds to varying concentrations of TODGA and HNO₃. These trends highlight two key behaviors:

• Figure 4: At a fixed nitric acid concentration, water solubility increases with increasing TODGA concentration. This is attributed to TODGA’s ability to form hydrogen bonds and solvate water molecules, facilitating their transfer into the organic phase.

Figure 5: When TODGA concentration is held constant, increasing aqueous HNO₃ concentration similarly leads to higher water solubility in the organic phase. This is primarily due to co-extraction of HNO₃, which pulls water along via hydration shells.

Figure 4: TODGA – H₂O – n-Dodecane – HNO₃ (Water Solubility)

Figure 5: TODGA – H₂O – n-Dodecane – HNO₃ (Water Solubility)

Figure 6: TODGA – H₂O – n-Dodecane – HNO₃ (HNO₃ Solubility)

Fig. 6 shows how much HNO₃ is extracted into the organic phase. Model and data agreement validate the role of nitric acid in modulating phase behavior and acid transport into the organic phase.

Figure 7: TODGA – H₂O – n-Dodecane – HNO₃ – Nd(NO₃)₃

Fig. 7 is a full solvent extraction system including metal nitrate and examines the limiting organic concentration (LOC) in the context of neodymium nitrate extraction using TODGA in an organic phase. The LOC is reached when the system can no longer maintain a single phase. The MSE model predicts the composition up to and beyond the LOC.

Conclusion

The OLI Mixed Solvent Electrolyte (MSE) thermodynamic framework has been successfully extended to model rare earth element (REE) behavior during solvent extraction with TODGA. This includes robust simulations of metal-ligand complexation, phase equilibria, and third-phase formation under varying acidities and temperatures.

By integrating a wide range of experimental data, including distribution ratios (D values), limiting organic concentrations (LOCs), and solubility curves, the framework enables a multi-property refinement of thermodynamic parameters. The MSE model captures the impact of variables such as nitric acid concentration, TODGA loading, aqueous phase composition, and co-extracted species (e.g., water, HNO₃), all of which govern the separation efficiency and operational constraints of REE solvent extraction systems.

This modeling capability allows for the evaluation of different extraction schemes and the optimization of process conditions to avoid third-phase formation, improve phase stability, and maximize selectivity among lanthanides.

OLI Tools for REE Solvent Extraction with TODGA

OLI provides engineers and chemists with powerful tools for modeling REE chemistry in organic and aqueous phases, including extraction systems based on TODGA. The core of this capability is built into the MSE model, which enables:

• Accurate simulation of liquid–liquid extraction equilibria
• Modeling of REE–ligand complexation with TODGA and other chelators
• Prediction of LOC behavior and third-phase boundaries
• Phase composition tracking across multicomponent systems

Available OLI Software Tools:

• OLI Studio V12.5: Ideal for point-speciation and phase equilibrium calculations
• OLI Flowsheet: ESP V12.5: Supports full process simulation and flowsheet integration of complex extraction circuits

These tools allow process designers to predict system behavior with high fidelity, saving time and reducing reliance on trial-and-error lab work. OLI’s platform is uniquely positioned to simulate REE–TODGA chemistry across a broad range of process conditions including low to high acidity, temperature variations, and variable ligand concentrations.

Contact OLI at https:/www.olisystems.com/contact for more information or to schedule a meeting with an OLI expert.

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