# Conversion of Sodium-Rare Earth Double Sulfate Salts Prepared from Spent NiMH Batteries: Comparison of Precipitation Pathways as Oxalate, Hydroxide, and Carbonate

Clément Laskar, Boris Guzhov, Antoine Barnabé, Morgane Josse, Béatrice Biscans, Laurent Cassayre

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## To cite this version:

Clément Laskar, Boris Guzhov, Antoine Barnabé, Morgane Josse, Béatrice Biscans, et al., Conversion of Sodium-Rare Earth Double Sulfate Salts Prepared from Spent NiMH Batteries: Comparison of Precipitation Pathways as Oxalate, Hydroxide, and Carbonate. Journal of Sustainable Metallurgy, 2025, 10.1007/s40831-025-01173-5. http://dx.doi.org/10.1007/s40831-025-01160-9

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# Conversion of Sodium-Rare Earth Double Sulfate Salts Prepared from Spent NiMH Batteries: Comparison of Precipitation Pathways as Oxalate, Hydroxide, and Carbonate

C. Laskar<sup>1,2</sup> · B. Guzhov<sup>1</sup> · A. Barnabé<sup>3</sup> · M. Josse<sup>1</sup> · B. Biscans<sup>1</sup> · L. Cassayre<sup>1</sup>

Received: 16 January 2025 / Accepted: 11 June 2025

© The Author(s) 2025

## Abstract

Valorizing solid leach residues rich in critical metals, as the rare earth elements (REEs), while reducing reactant and water consumption, is a major challenge in hydrometallurgical processing. In the case of spent nickel-metal hydride (NiMH) battery processing, the most direct way to recover light REEs (La, Ce, Nd, Pr) from the pregnant leach solution (PLS) is by selective precipitation of REE sodium double sulfate salts (REE-DSS), NaREE(SO₄)₂·H₂O. These salts then have to be post-treated to remove the elements Na and S. In primary REEs production processes, the main option is to convert REE-DSS to hydroxides. However, for NiMH batteries, the presence in the PLS of other metals, particularly nickel (~46 g/L), and potassium, raises the question of alternative routes for REE-DSS conversion to avoid the co-precipitation of these elements. In this work, a thermodynamic and experimental study was carried out on industrially prepared NiMH leachates to compare the conversion of REE-DSS into hydroxides, carbonates, or oxalates, focusing for the precipitation reactions on the yields, kinetics, selectivity, and water consumption. While the high yields and fast kinetics are similar for all three routes, it appears that precipitation as REE oxalate (REE₂(C₂O₄)₃·10H₂O) avoids co-precipitation of nickel, thus reducing by a factor of 5 the amount of water required during the initial washing of the REE-DSS to remove the impurities. However, REE oxalate salts are limited in further aqueous processing compared to hydroxide and carbonate salts because of their poor solubility, while they are suitable to thermal decomposition ways.

## Graphical Abstract

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**Keywords** NiMH battery recycling · Circular hydrometallurgy · Rare-earth elements · Selective precipitation · Thermodynamic modeling of rare earth
Introduction

The nickel-metal hydride (NiMH) batteries were introduced in the 1990's as a replacement of Ni-Cd batteries and constituted the majority of the batteries used in electric vehicles in 2000 [1]. After a peak around 2012-2013 (more than half of electric vehicles used NiMH batteries), NiMH usage dropped due to replacement by lithium-ion batteries [2]. Accordingly, important amounts of spent NiMH batteries are discarded and represent hazardous wastes that need to be correctly disposed. Besides, spent NiMH batteries contain specific metal elements that represent valuable secondary resources [3, 4], whose large-scale recovery could account for significant resources for new battery manufacturing [5]. This interest is mainly due to their high-Ni content (40-50 wt% of the black mass powder prepared from spent NiMH), but the anodes of these batteries, made of REENi5 mischmetal, also contain significant amounts of light rare earth elements (REEs; La, Ce, Nd, Pr; 10-15 wt%) which could be recovered and reused [6, 7]. Today, the most valuable light REEs are Nd and Pr, because they are key for Nd-Fe-B permanent magnets manufacturing [8, 9], while a mixture of REEs with a typical molar ratio as used in NiMH batteries is of less economic importance, especially since lithium batteries are now widely used as a replacement for NiMH batteries. However, the high stress on Li demand could lead to a mix between Li-ion batteries and NiMH batteries in the future [10], and it remains important to find ways to recover the strategic metals contained in our wastes, even if the economic return is not favorable at a given time.

In the frame of hydrometallurgical processes, several methods can be used for the recovery of REEs from acid leachates (HCl or H₂SO₄ media). Solvent extraction is an option [11-14], offering excellent purity and yields, but it involves complex and costly multi-step processes. Furthermore, even if treatments based on solvent extraction methods and aiming at producing separate feeds of individual REE have been developed for the recycling of permanent magnets or lamp phosphors [15], not many have been reported for the processing of NiMH batteries [6]. Alternatively, an efficient and robust way consists in grouped selective precipitation by formation of sodium REE double sulfate salts (REE-DSS), according to the overall reaction:

$$REE_{(aq)}^{3+} + 2 SO_{4}^{2-} + Na_{(aq)}^{+} + H_{2}O \rightarrow NaREE(SO_{4})_{2} \cdot H_{2}O_{(s)} \quad (1)$$

This approach is already used for other acidic leachates of REEs-bearing minerals (e.g., bastnasite[16], monazite[17], bauxite[18], eudialyte [19]). It is applicable in a wide concentration range compared to other separation methods adapted to lower concentration of metals, such as membrane separation or ion exchange [20]. However, the presence of

nickel only in trace amounts (i.e., <100 ppm Ni) in REEs-bearing minerals does not result in the same selectivity issues during downstream processing as observed with BM NiMH, where nickel is present at very high concentrations (> 40 wt% Ni). The La/Ce/Nd/Pr ratios also remain constant, irrespective of the material, because, within REEs, the elements La, Ce, Nd, Pr, and Sm form a group of sodium double sulfate salts with very low solubility (<1 mmol/L) in water [21]. The addition of 2-3 times of stoichiometric requirement of sodium sulfate therefore leads to almost complete precipitation of these elements [22, 23], producing an easily filterable and storable solid [24]. In the early 2020's, Porvali et al. [25, 26] and Zielinski et al. [27] proved very efficient (> 95%) REEs recovery yield from NiMH battery leachates using sodium sulfate. Takano et al. [28] implemented this reaction in a 300 liters reactor and reported excellent La and Ce recovery yields at 80 °C. This method is thus particularly simple and robust to selectively recover REEs dissolved from spent NiMH leachates at industrial scale.

Due to the presence of KOH in NiMH electrolytes, K also co-precipitates in the solid product [25, 29], and it was further shown that the REE-DSS form a solid solution where the respective REE molar ratio is similar to that of the initial leach solution, resulting in typical chemical formula such as (Na₀.₉K₀.₁)(La₀.₆₅Ce₀.₂₄Nd₀.₀₇Pr₀.₀₄)(SO₄)₂·H₂O [29]. Additionally, the precipitation product contains nickel and other metal elements (Al, Fe, Co, Mn), derived from the residual solution trapped in the filter cake after REE-DSS precipitation. While Na and K are part of the structure of the REE-DSS compound, most of the impurities can be removed by appropriate washing of the filter cake. Depending on the washing protocol (and specifically the amount of water used) and the filtration equipment, typical nickel residual contents are 0.5 to 25 mg of Ni per g of REE-DSS [25, 29].

The REE-DSS obtained must then be purified in order to produce valuable REE compounds such as oxides. Ahn et al. [30] showed that the direct heat treatment (in air, up to 1200 °C) of the double sulfate precipitates led to partial sulfate removal through SOₓ volatilization, but could still not provide acceptable product due to high Na content. In primary REEs production from minerals, the main option consists in converting the REEs sulfate salts into REEs hydroxides by caustic soda (Eq. 2):

$$NaREE(SO_4)_2 \cdot H_2O_{(s)} + 3 NaOH_{(aq)} \rightarrow REE(OH)_{3(s)} + 2 Na_2SO_4_{(aq)} + H_2O \quad (2)$$

Regarding REE separation from NiMH battery feedstocks, Wu et al. [31] prepared hydroxides by alkalinization (Eq. 2), followed by redissolution in HCl solution and crystallization of REECl₃ compounds. The REE hydroxides can also be dissolved using HCl as a feed for solvent extraction,
e.g., PC-88A (purified 2-ethylexyl hydrogen 2-ethylhexyl phosphonate) or D2EHPA (di-(2-ethylhexyl) phosphoric acid) [16, 23]. An alternative to solvent extraction was investigated by Porvali et al. [32]. The first step is also the conversion of the REE-DSS into REEs hydroxides in NaOH media according to (Eq. 2). Then, the method uses Ce oxidation and Ce(OH)₄ low solubility at pH > 2, conversely to other trivalent REE hydroxides, soluble at pH < 6. However, the authors reported that the high sulfur content in the Ce(OH)₄ product obtained is a problem for the subsequent valorization of Ce [32]. Another route is the conversion of REE-DSS into carbonates [30] according to (Eq. 3), with subsequent thermal decomposition at 800 °C leading to the formation of a mixture of REE oxides [33]:

The oxalate route can be more selective for impurities (Ca, Mn, Al, sulfate) compared to the carbonate route but requires more reactant to reach similar efficiency [36, 37].

In this work, we have focused on the conversion of REE-DSS prepared from a single industrial NiMH battery material (Fig. 1), using hydroxide, carbonate or oxalate route, with caustic soda, sodium carbonate or oxalic acid, respectively. This choice is not straightforward, because the REE-DSS compounds obtained from NiMH leachates have different compositions (individual REE contents, the presence of K) and impurities (mainly Ni) than REE-DSS prepared from primary extraction. The three routes finally produce a significant amount of sodium sulfate (Na₂SO₄) waste. Such a waste is a classical waste in hydrometallurgical processes.

$$2 \text{NaREE}(\text{SO}_4)_2 \text{H}_2\text{O}_{(\text{s})} + 3 \text{Na}_2\text{CO}_{3(\text{aq})} \rightarrow \text{REE}_2(\text{CO}_3)_3 \cdot x \text{H}_2\text{O}_{(\text{s})} + 4 \text{Na}_2\text{SO}_{4(\text{aq})} + (2-x) \text{H}_2\text{O} \quad (3)$$

At last, the REE-DSS can be converted into oxalate salts according to (Eq. 4), which can also be calcined to prepare REE oxides [34, 35]:

One option for its valorization is to recover NaOH and H₂SO₄ by salt splitting technology [38], but this remains expensive [39]. We have investigated the selectivity of the

$$2 \text{NaREE}(\text{SO}_4)_2 \cdot \text{H}_2\text{O}_{(\text{s})} + 3 \text{H}_2\text{C}_2\text{O}_{4(\text{aq})} \rightarrow \text{REE}_2(\text{C}_2\text{O}_4)_3 \cdot x \text{H}_2\text{O}_{(\text{s})} + \text{Na}_2\text{SO}_{4(\text{aq})} + 3 \text{H}_2\text{SO}_{4(\text{aq})} + (2-x) \text{H}_2\text{O} \quad (4)$$

conversion toward major (Na, K, S) and minor elements (Ni, Co, Fe, Al) from this leachate, as such a step is specific of

<figure><img src="image_4.png" /><figcaption>**Fig. 1** Process flowsheet for the grouped recovery of REEs compounds from spent NiMH batteries. Three different reactants (hydroxides, carbonates, and oxalates) were investigated in this work for conversion of REEs double sulfate salts. The generated amount of products, reactants, and waste is normalized with 100 g of initial BM NiMH, according to results from this work and [29]</figcaption></figure>
NiMH leachates compared to common leachates from mineral processing [37, 40, 41]. Thus, in order to minimize the consumption of reactants and water, according to circular hydrometallurgy principles [39], the washing step appears as central in this process. Typically, we have shown in a previous work that the volume of washing water required to obtain a low-Ni REE-DSS (1 mg/g REE DSS) is 5 times greater than that required for a high-Ni REE-DSS (15 mg/g REE DS) [29]. The selectivity of the three routes has been compared through both thermodynamic and experimental approaches, focusing on the three main REEs (La, Ce, and Nd) from NiMH batteries and the three main impurities (K, Na, and Ni) contained in the REE-DSS.

# Preliminary Thermodynamic Analysis

## Calculation's Objectives and Method

A preliminary thermodynamic analysis was performed to evaluate the reaction yields (REEs recovery) and selectivity (vs Na, K, S, and Ni) expected during the preparation of REE hydroxides, carbonates, and oxalates from a solution prepared by dissolving REE-DSS in water. Due to database limitations and the low content of Nd and Pr in the REE-DSS (ca. 10% of the total REEs), the calculations focused on La and Ce as the main REEs. The initial solution used for the calculations consisted of 0.01 mol of (Na₀.₉K₀.₁)(La₀.₇Ce₀.₃) (SO₄)₂, (near its solubility limit – see section 4), 10⁻⁴ mol of NiSO₄, and 1 kg of water at 25 °C (corresponding to 1.0, 0.4 and 6.10⁻³ g/L of La³⁺, Ce³⁺ and Ni²⁺, respectively). The amount of nickel sulfate (15 mg/g REE-DSS) represents the Ni content in the REE-DSS as measured in our previous work after minimal washing [29]. The calculations consisted in adding an excess of 1 M solutions of either NaOH, Na₂CO₃, or H₂C₂O₄ to the initial solution. Equilibria were computed with a calculation step of 5.10⁻⁴ mol of reactant at 25 °C and atmospheric pressure.

# Computational Tool and Database

The calculations were carried out with the PhreeqC software [42], employing the LLNL aqueous model for coefficient activity calculations, which is a variant of the Debye-Hückel model. The main database used for describing solid and aqueous species was llnl.dat (V8R6), which covers sulfate, hydroxide, and carbonate chemistry. As the database did not contain the description of REEs and Ni oxalates, we included parameters for both aqueous species and solid compounds from other sources, mostly based on the recent survey by Zhang et al. [43] (see online supplementary Table S-1). Aqueous species (H₂C₂O₄(aq), HC₂O₄⁻(aq), C₂O₄²⁻(aq), NiC₂O₄(aq), Ni(C₂O₄)₂²⁻(aq), CeC₂O₄⁺(aq), LaC₂O₄⁺(aq)) were imported from the Thermochimie_eDH_v10d database [44]. Equilibrium constants describing solid oxalate compounds were taken from various sources, as detailed in Table 1. All parameters added to llnl.dat are provided in the online supplementary material (Table S-1).

# Calculation Results and Discussion

The main outputs of the equilibrium calculations are shown in Fig. 2. Depending on the reactant, the pH increases (with hydroxide and carbonate) or decreases (with oxalic acid) from an initial value of 5.7 (Fig. 2a). Complete precipitation of REEs (> 99.9%) can be achieved with all 3 reactants (Fig. 2b, c and d), without any co-precipitation of Na, K, or SO₄. A noticeable difference between the three precipitation routes is that much higher REEs/Ni selectivity is expected with the oxalic acid. Indeed, all Ni should co-precipitate as nickel hydroxide when using NaOH or Na₂CO₃, while Ni is expected to precipitate only in the presence of an excess of oxalic acid. Given the equilibrium constants of REEs and Ni solid oxalates and aqueous complexes (see Table S-1), the selectivity of oxalate media is attributed to a large solubility difference rather than complexation effects. Note that the calculation considered the two allotropic forms

<table><caption>**Table 1** Summary of solid species taken into account for thermodynamic modeling</caption><thead><tr><th>Type of compound</th><th>Chemical formula</th><th>Thermodynamic data source</th></tr></thead><tbody><tr><td>Hydroxides</td><td>Ce(OH)<sub>3</sub>, La(OH)<sub>3</sub>, Ni(OH)<sub>2</sub></td><td>llnl.dat<sup>b</sup></td></tr><tr><td>Carbonates</td><td>Ce<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>•8H<sub>2</sub>O, La<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>•8H<sub>2</sub>O, NiCO<sub>3</sub>,<br>Na<sub>2</sub>CO<sub>3</sub>, Na<sub>2</sub>CO<sub>3</sub>•H<sub>2</sub>O, Na<sub>2</sub>CO<sub>3</sub>•7H<sub>2</sub>O, K<sub>2</sub>CO<sub>3</sub>•1.5H<sub>2</sub>O, K<sub>2</sub>CO<sub>3</sub>•1.5H<sub>2</sub>O, KHCO<sub>3</sub>,<br>KNaCO<sub>3</sub>•6H<sub>2</sub>O</td><td>llnl.dat<sup>b</sup></td></tr><tr><td>Oxalates<sup>a</sup></td><td>Ce<sub>2</sub>(C<sub>2</sub>O<sub>4</sub>)<sub>3</sub>•9H<sub>2</sub>O, La<sub>2</sub>(C<sub>2</sub>O<sub>4</sub>)<sub>3</sub>•9H<sub>2</sub>O<br>Ce<sub>2</sub>(C<sub>2</sub>O<sub>4</sub>)<sub>3</sub>, La<sub>2</sub>(C<sub>2</sub>O<sub>4</sub>)<sub>3</sub><br>α-NiC<sub>2</sub>O<sub>4</sub>•2H<sub>2</sub>O, β-NiC<sub>2</sub>O<sub>4</sub>•2H<sub>2</sub>O</td><td>Lange's Handbook [45]<br>Xiong [46]<br>Hummel et al. [47]</td></tr><tr><td>Sulfates</td><td>Na<sub>2</sub>SO<sub>4</sub>•10H<sub>2</sub>O, K<sub>2</sub>SO<sub>4</sub></td><td>llnl.dat<sup>b</sup></td></tr></tbody></table>

<footnote><sup>a</sup>Solid compounds KOH, NaOH, Na₂C₂O₄, and K₂C₂O₄ were not considered in the calculation due to their high solubility in water (> 30 g/L; [45])</footnote>

<footnote><sup>b</sup>Solubility constants from llnl database are detailed in Table S-2</footnote>
<figure><img src="image_5.png" /><figcaption>**Fig. 2** **a** Calculated precipitation yields of REEs (REEs = 0.7La + 0.3Ce) and pH evolution according to the precipitation agent. Precipitation yields (REEs, Ni) as a function of the amount of reactant **b** NaOH, **c** Na₂CO₃, and **d** H₂C₂O₄, with a molar</figcaption></figure>

ratio Ni/(La + Ce) = 0.01. Sodium and sulfate are not foreseen to precipitate, according to **e** comparison of REEs precipitation yield as a function of the reactions stoichiometry

of Ni oxalates (α- and β-NiC₂O₄·2H₂O), revealing that the α form is more stable (Fig. 2d). Another difference is that stoichiometric amounts of NaOH or Na₂CO₃ lead to complete REEs precipitation, while an excess (ratio > 1.3) of oxalic acid is required (Fig. 2e). This is due to the stability of REEC₂O₄⁺(aq) species that consume some oxalic acid.

The poor selectivity of basic reactants (hydroxides and carbonates) for metal ions such as Fe²⁺, Mn²⁺, or Zn²⁺

is well known, as these ions co-precipitate with REE hydroxides in the same pH range of 6–8 [23]. The superior selectivity of oxalic acid was for instance evidenced by Silvia et al. [37] in a solution containing 3.8 g/L of REEs and 0.92 g/L of Mn²⁺. For low-concentration nickel solutions, such as encountered in processing of REE-DSS from NiMH leachates, our calculations show that oxalic acid is also expected to be more selective than NaOH or Na₂CO₃.
<table><caption>**Table 2** Elemental compositions of the industrial pregnant leach solution, the washed REE-DSS, (Na<sub>0.9</sub>K<sub>0.1</sub>La<sub>0.65</sub>Ce<sub>0.24</sub>Nd<sub>0.07</sub>Pr<sub>0.04</sub>(SO<sub>4</sub>)<sub>2</sub>·2H<sub>2</sub>O), and the dissolved REE-DSS solution</caption><thead><tr><th>Element<sup>a</sup></th><th>Industrial PLS<sup>b</sup><br>(g/L)</th><th>Washed REE-DSS<sup>b</sup><br>(mg/g of solid)</th><th>Dissolved<br>REE-DSS<br>solution<br>(mg/L)</th></tr></thead><tbody><tr><td>Ni</td><td>45.83±1.22</td><td>1.2</td><td>5±1/51±1<sup>c</sup></td></tr><tr><td>La</td><td>8.64±0.25</td><td>310.3</td><td>996±5</td></tr><tr><td>Ce</td><td>3.10±0.09</td><td>113.3</td><td>382±1</td></tr><tr><td>Nd</td><td>1.04±0.03</td><td>35.7</td><td>122±1</td></tr><tr><td>Na</td><td>nd<sup>d</sup></td><td>69.0</td><td>241±2</td></tr><tr><td>K</td><td>0.78±0.05</td><td>0.87</td><td>51±2</td></tr></tbody></table>

Nevertheless, the amount of oxalic acid should be carefully controlled to avoid the precipitation of nickel oxalate, which was reported in direct leaching of NiMH anodes with oxalic acid [48]. Furthermore, since the calculations are based on a non-consolidated thermodynamic database, the results should be interpreted with caution and require experimental comparison.

# Experimental Materials and Methods

## Chemicals and Materials

Industrial REE-DSS were obtained according to the protocol described in Zielinski et al. [27], by the addition of Na₂SO₄ solution into a pregnant leaching solution (PLS) of an industrial black mass of NiMH battery powder from SNAM Groupe. After precipitation, the REE-DSS was washed with 2.5 L of water (for a dried mass of 45 g of solid) at room temperature, to remove the residual elements from the PLS, yielding to a Ni concentration of 1 mg/g of solid, compared to 24 mg/g for the non-washed solid. Elemental analysis of the PLS solution and the washed REE-DSS is reported in Table 2 (data from [29]).

These REE-DSS were used to prepare a 5 L leachate, by dissolving an excess of solid in water over 3 days at 25 °C under stirring, followed by Büchner filtration. For the precipitation experiments, 600 mL samples were taken from the filtrated mother liquor. In some cases, 5.0 mL of a 0.10 mol/L nickel sulfate solution (NiSO₄·6H₂O, ACS Reagent, ≥ 98%, Sigma-Aldrich) was added to model the leachate that would be obtained from a non-washed REE-DSS. The composition of the leachates is provided in Table 2. Other metal

impurities (Fe, Al) were not considered because their concentrations were below 0.2 mg/g of REE-DSS.

The precipitation reactants are oxalic acid (≥ 99.0%, anhydrous, Sigma Aldrich), sodium carbonate (BioXtra, ≥ 99.0%, Sigma-Aldrich), and sodium hydroxide (ACS reagent, anhydrous, ≥ 99.0%, Sigma-Aldrich). These powders were dissolved in deionized water to achieve concentrations of 1 M for oxalic acid and sodium carbonate solutions and 2 M for the NaOH solution. All chemicals were used as received, without any further purification.

# Experimental Reactor and Protocol

## Experimental Device

The reactor was a double jacket 1 L reactor, thermostated with a cryostat (AC200, Thermofisher), equipped with a four-blade Teflon stirrer and a torque meter (set at 230 rpm), and filled with 600 mL of the dissolved REE-DSS solution. A platinum temperature probe (Pt100 Duplex Probe, TC Direct) and a combined pH electrode (LL-Unitrode SC, Metrohm) were immersed in the solution to measure the temperature and pH, respectively, throughout the reaction. The pH acquisition was carried out by the Tiamo software (at 0.1 Hz).

## Experimental Protocols

In each experiment, 10–15 mL of a reactant solution was added in 3–5 min. After 120 min, the suspension was recovered (solution + solid) and settled to remove the majority of the supernatant. The remaining solution was then centrifuged (5 min at 6000 rpm) to discard the remaining supernatant, and the precipitate was washed 2 times, one time with 50 mL of ethanol, and again 1 time with 50 mL of water, and the resulting material was then dried at 50 °C for 14 h.

As evidenced by the equilibrium calculations (Fig. 2e), the amount of reactant required to reach full REEs precipitation may vary. Specifically, we expect that an excess of oxalic acid is necessary because of the formation of stable oxalic complexes in solution with REEs. In the first part of our experimental study, we tested several stoichiometric excess of oxalic acid (between 1.2 and 2.0), at two temperatures (25 and 60 °C), to investigate the precipitation kinetics and yields. According to the results and thermodynamic calculations, the second part of the study consisted in comparing precipitation yields and selectivity at a similar stoichiometric excess (1.4) for the three reactants (NaOH, Na₂CO₃, H₂C₂O₄).

<footnote><sup>a</sup>The focus was on the three main REEs</footnote>

<footnote><sup>b</sup>Data from [29]</footnote>

<footnote><sup>c</sup>Prepared by NiSO<sub>4</sub> addition to simulate non-washed REE-DSS</footnote>

<footnote><sup>d</sup>nd' means not determined</footnote>
## Chemical and Structural Characterizations

During a test run, the precipitation rate was measured by analyzing the dissolved metal concentrations in samples. The suspension was collected using a 0.8 mm tube and filtered through a 0.25 µm syringe filter before elemental analysis. The leachate samples were analyzed in a 3 wt% HNO₃ matrix by inductively coupled plasma-optical emission spectroscopy (ICP-OES; Horiba Ultima Expert) for the quantification of Na, K, La, Ce, Nd, and Ni. The aqueous ICP standards were delivered by SCP Science (Villebon-sur-Yvette, France).

The composition of the final solid was also investigated by ICP-OES, after fully mineralizing it at high temperature in aqua regia. The uncertainty in ICP-OES concentration measurements was <2% for all elements and samples. For concentrations below the detection limit, we used the detection limit values as maximum values for yield calculations: Ni (0.01 ppm), La (0.2 ppm), Ce (0.06 ppm), Nd (0.04 ppm), Na (0.02 ppm), K (0.04 ppm).

The precipitation yield $Y_{i(t)}^P(\%)$ for the species at the time $t$ of the experiment was calculated from the analysis of the liquid samples:

$$Y_{i(t)}^P(\%) = 100 \left( 1 - \frac{c_{m,i}(t) \cdot V_{tot}(t)}{c_{m,0} \cdot V_0} \right), \quad (5)$$

where $c_{m,i}(t)$ is the concentration of species i in the leachate sample (g/L) at the time $t$, $c_{m,0}$ is the concentration of species i in the initial leachate sample (g/L), $V_{tot}(t)$ is the volume (L) of the solution at the time $t$, and $V_0$ is the initial volume (L) of the solution. The overall REEs precipitation yield was calculated by considering the three REEs according to

$$Y_{REE(t)}^P(\%) = 100 \left( 1 - \frac{n_t^{La} + n_t^{Ce} + n_t^{Nd}}{n_0^{La} + n_0^{Ce} + n_0^{Nd}} \right), \quad (6)$$

where $n_t^X$ represent the number of moles of the element X in the solution at a given time $t$, and $n_0^X$ the initial number of moles of the element X.

The structural characterization of the synthesized solid phases was performed by powder X-ray diffraction analysis (XRD; BRUKER D8 Endeavor), with the ICCD-PDF and Crystallography Open Database (COD) database. The thermogravimetric analysis (TGA; TGA/DSC1 Mettler Toledo) was performed under air (10 mL/min) from 25 to 800 °C, at a heating rate of 5 °C/min.

For particle observation, a scanning electron microscope-field emission gun (SEM-FEG, JEOL JSM 7100F) equipped with an energy dispersive X-ray analyzer (EDX; Oxford ASDD X-Max detector) was used. The SEM-FEG operated at an accelerating voltage of 10 kV and a working distance of 10 mm. The EDX measurements to determine

the Ni concentration were below the detection limit for all samples, because the main band energies of La and Ni are close (M-La 0.833 keV vs Lα-Ni 0.855 keV).

## Results

### Solubility of REE-DSS in Water

The solubility of REE-DSS in water was determined to be 4.0 g/L at 25 °C. This result is consistent with the solubility of NaLa(SO₄)₂.H₂O (3.6 g/L [21]) and KLa(SO₄)₂.H₂O (4.8 g/L [49]), given that the solid contains both Na and K. Based on the critical review from Cassayre et al. [6], REE-DSS are less soluble than individual REE sulfate salts. The leachate composition is provided in Table 2. The pH of the dissolved REE-DSS solution is 5.7 ± 0.2, which is consistent with the value obtained from equilibrium calculation (5.7). This low initial solubility of REE-DSS could be increased by using an acidic starting solution as the solubility of REE-DSS is three times greater at pH 0.5 than at pH 6 at 25 °C [21], particularly for oxalate precipitation, as oxalate precipitation occurs at acidic pH (Fig. 2a).

### Influence of Process Parameters on Oxalate Precipitation Yield

First, we focused on the oxalate route to determine the required stoichiometric excess to reach a complete precipitation of REEs. We have investigated the REE precipitation reaction depending both on the excess of the acid reactant and the temperature over a 30 min reaction period. The data (Table 3 and Fig. 3) show that an oxalic acid excess of 1.2 does not achieve complete REE precipitation at either 25 or 60 °C, with yields about 92–93%, which is consistent with equilibrium calculations at 25 °C (Fig. 2). With an excess of 2.0, precipitation yields exceed 98% at both temperature, with a reaction completed after 15 min.

Finally, we choose to select an excess of oxalic acid of 1.4, based on our experiments and calculations at 25 °C, to reach precipitation yields higher than 95%, while minimizing potential Ni co-precipitation, which can occur with an

<table><caption>**Table 3** Final yields of precipitation experiments with oxalic acid at difference temperature and excess of reactant</caption><thead><tr><td>T (°C)</td><td>Oxalic acid excess</td><td>REE precipitation yields (%)</td></tr></thead><tbody><tr><td>25</td><td>1.2</td><td>93.0±2.2</td></tr><tr><td>25</td><td>2.0</td><td>98.1±1.1</td></tr><tr><td>60</td><td>1.2</td><td>91.8±0.4</td></tr><tr><td>60</td><td>2.0</td><td>99.2±0.6</td></tr></tbody></table>
<figure><img src="image_6.png" /><figcaption>**Fig. 3** Precipitation of REEs during oxalic acid addition in function of time, depending on temperature and excess of reactant</figcaption></figure>

excess greater than 1.4. The same stoichiometric ratio was then applied to NaOH and Na₂CO₃ reactants for comparison, even though no excess of reactant is needed according to our thermodynamic calculations. For example, an excess of 1.1 was used for precipitation as hydroxides in the study by Wu et al. [31], yielding to 94.5% of recovery of REE. A higher stoichiometric ratio of 1.6 was used in the work of Porvali et al. [32], with the aim of oxidizing Ce during their process.

## Comparative REE Precipitation Using Hydroxide, Carbonate, and Oxalate Reactants

The comparative study was conducted to investigate the precipitation yields and the selectivity toward nickel with the precipitation of oxalate, carbonate, and hydroxide, under different conditions outlined in Table 4. In each case, the final measured pH values (pH<sub>f-xp</sub>) are in good accordance with thermodynamic predictions (pH<sub>f-calc</sub>). This shows that the selected thermodynamic model is appropriate and provides accurate understanding of the chemical reactions occurring during the conversion.

## Reaction Products

The carbonate and the oxalate salts are well-crystallized solid, according to the SEM images and diffractograms (Fig. 4). Conversely, the hydroxide compound present both crystallized and amorphous nature, with respective thin and large peaks (Fig. 4). The identified phases by XRD are coherent with thermodynamic predictions (Table 1), with the following main phases obtained La(OH)₃, La₂(CO₃)₃•8H₂O, and La₂(C₂O₄)₃•10H₂O, for hydroxide, carbonate, and oxalate routes, respectively. For the carbonate compound, a

<table><caption>**Table 4** Summary of the experimental conditions for the comparative experiments with the additions of sodium hydroxide (NaOH), sodium carbonate (Na₂CO₃), and oxalic acid (H₂C₂O₄) in the dissolved REEDSS solution (m=0.59 kg, pH₁=5.7±0.2), with a stoichiometric excess of the reactant of 1.4</caption><thead><tr><th>#</th><th>Reactant</th><th>NiSO<sub>4</sub> in initial solution</th><th>pH<sub>f-xp</sub></th><th>pH<sub>f-calc</sub></th></tr></thead><tbody><tr><td>REH1</td><td>NaOH</td><td>No</td><td>11.9</td><td>12.0</td></tr><tr><td>REH2</td><td>NaOH</td><td>Yes</td><td>12.0</td><td></td></tr><tr><td>REC1</td><td>Na₂CO₃</td><td>No</td><td>9.7</td><td>9.8</td></tr><tr><td>REC2</td><td>Na₂CO₃</td><td>Yes</td><td>9.4</td><td></td></tr><tr><td>REO1</td><td>H₂C₂O₄</td><td>No</td><td>1.6</td><td>1.7</td></tr><tr><td>REO2</td><td>H₂C₂O₄</td><td>Yes</td><td>1.6</td><td></td></tr></tbody></table>

The pH<sub>f-xp</sub> and pH<sub>f-calc</sub> refer to the measured and calculated pH values after the addition of reactant

second phase, La₂(CO₃)₂•(OH)₂, is also detected in the diffractogram. The high crystallinity of oxalate and carbonate compounds facilitates the recovery of the solid cake during a filtration process. The hydroxides crystals obtained are far bigger (~100 µm size) than in the work of Porvali et al. (~20 µm size; [32]).

Solid composition analysis performed by ICP-OES gives the mass ratio of La, Ce, and Nd in the final hydroxides, carbonates, and oxalates (Table 5). The comparison of the measured REEs content in the products with the theoretical REEs content considering the major phase determined by XRD (La₂(C₂O₄)₃•10H₂O, La₂(CO₃)₃•8H₂O and La(OH)₃) shows, with the closest results (≤1%), that the oxalate compound has the highest purity and the most homogeneous composition. The hydroxide phase shows a large difference (REEs content is 51–55 wt%, while theoretical value is 73
<figure><img src="image_7.png" /><figcaption>**Table 5** REE content of hydroxide (REH-), carbonate (REC-), and oxalate (REO-) compounds, determined by ICP-OES analysis (Exp.), and comparison with the theoretical concentrations (Th.) from solid compounds formula determined by XRD</figcaption></figure>

<table><thead><tr><th rowspan="2">#</th><th rowspan="2">La (wt%)</th><th rowspan="2">Ce (wt%)</th><th rowspan="2">Nd (wt%)</th><th colspan="3">ΣREE (wt%)</th></tr><tr><th>Exp.</th><th>Th.</th><th>Relative<br>deviation,<br>%</th></tr></thead><tbody><tr><td>REH1</td><td>35.7</td><td>14.5</td><td>4.8</td><td>55.0</td><td>73.2</td><td>25</td></tr><tr><td>REH2</td><td>33.6</td><td>13.7</td><td>4.6</td><td>51.8</td><td></td><td>29</td></tr><tr><td>REC1</td><td>33.8</td><td>12.8</td><td>4.2</td><td>50.8</td><td>46.3</td><td>10</td></tr><tr><td>REC2</td><td>29.7</td><td>12.2</td><td>4.0</td><td>45.8</td><td></td><td>1</td></tr><tr><td>REO1</td><td>25.5</td><td>9.7</td><td>3.2</td><td>38.3</td><td>38.7</td><td>1</td></tr><tr><td>REO2</td><td>25.8</td><td>10.0</td><td>3.2</td><td>39.0</td><td></td><td>1</td></tr></tbody></table>

Theoretical ratio was calculated according to the molar ratio in the initial washed solid [29]; compounds formula are the main phases determined by XRD results: La(OH)₃ for hydroxides (REH), La₂(CO₃)₃·8H₂O for carbonates (REC) and La₂(C₂O₄)₃·10H₂O for oxalates (REO)

wt%), probably due to residual water linked with its amorphous nature, despite the long drying time (14 h at 50 °C). Indeed, the initial cake moisture was 60–70% for hydroxides, compared to 20–25% for carbonates and oxalates.

Reaction Kinetics and Selectivity

The precipitation kinetics are comparable across the 3 routes, with complete La precipitation after 20 min (Fig. 5a). However, only the oxalate precipitation route is selective
<figure><img src="image_8.png" /><figcaption>**Fig. 5** a Lanthanum and b nickel concentrations in the aqueous phase over the time for the three precipitation routes: hydroxides, carbonates, and oxalates. The lanthanum completely precipitates in less than 20 min. In the case of oxalate precipitation, nickel remains preferentially in the solution, without precipitating</figcaption></figure>

for Ni (Fig. 5b). The final elemental precipitation yields (Table 6) show that the oxalate reactant prevents the precipitation of Ni, Na, and K compared to the other reactants. The oxalate route also does not involve the addition of sodium conversely to NaOH and Na₂CO₃ reactants. Additionally, there is less K precipitation with the oxalate route, especially compared to the carbonate route. Our results are in good agreement with previous results for other leachate origins, where oxalate compounds were shown to be more selective for impurities. For example, in a REE sulfuric acid leachate with the presence of impurities (Ca, Mg, Mn, Al, U), the oxalate compounds were also more pure (99.2% purity) than

the carbonates (94.2% purity) [37]. The precipitation yields to obtain hydroxides were obtained in 20 min, at 25 °C with a lower stoichiometric excess (1.4) than in the experiment of Porvali et al. [32] with optimal conditions at 60 °C, an excess of 1.6, and a time of 2 h. However, they used a much higher solid/liquid ratio in their initial NaOH solutions (25 g/L) compared to ours (4.0 g/L).

## Thermal Behavior of REEs Compounds

The thermogravimetric measurements (Fig. 6) enabled a comparison of the thermal behavior of the three REE compounds. For each compound, a temperature of ca. 700 °C was required to complete decomposition in air, forming oxides which are the valuable products for REEs [50, 51]. The carbonate and oxalate compounds exhibit sharp transitions, with dehydration at 100–120 °C, and decarbonation at 400–450 °C, consistent with structural analyses indicating crystallinity. Therefore, it can be deduced that the energy required to transform the carbonate and oxalate compounds into oxides is likely to be similar. On the other hand, the mass loss for the hydroxide compound is very broad, which is characteristic of compounds in gel form. The main conclusion of TGA measurements is that similar temperatures (about 700 °C) are required to convert the compounds into oxides.

According to the mass losses due to dehydration at 100–120 °C, the number of water molecules was calculated using the formula:

$$ \Delta m_{H_2O}(x) = 100 \cdot \frac{x \cdot M_{H_2O}}{M_d + x \cdot M_{H_2O}}, \quad (7) $$

where Δm<sub>H₂O</sub> is the water loss (%), x is the number of water molecules, M<sub>H₂O</sub> is the molar mass (g/mol) of H₂O, and M<sub>d</sub> is the molar mass (g/mol) of the hydrated solid. The number of water molecules determined was about 1 H₂O for hydroxides, 4 H₂O for carbonates, and 9 H₂O for oxalates. This is consistent with XRD analyses

<table><caption>**Table 6** Elemental precipitation yields after 120 min for the three precipitation routes</caption><thead><tr><th>Precipitation yields, Y<sup>P</sup><sub>i</sub></th><th>La</th><th>Ce</th><th>Nd<sup>a</sup></th><th>Ni<sup>a</sup></th><th>Na</th><th>K</th></tr></thead><tbody><tr><td>REH1</td><td>&gt;99.1<sup>a</sup></td><td>&gt;99.4<sup>a</sup></td><td>&gt;98.8<sup>a</sup></td><td>x</td><td>19.1 ± 0.2<sup>b</sup></td><td>2.3 ± 0.4</td></tr><tr><td>REH2</td><td>&gt;99.1<sup>a</sup></td><td>&gt;99.4<sup>a</sup></td><td>&gt;98.8<sup>a</sup></td><td>&gt;99.1<sup>a</sup></td><td>13.6 ± 0.2<sup>b</sup></td><td>1.3 ± 0.2</td></tr><tr><td>REC1</td><td>&gt;99.1<sup>a</sup></td><td>&gt;99.4<sup>a</sup></td><td>&gt;98.8<sup>a</sup></td><td>x</td><td>19.6 ± 0.3<sup>b</sup></td><td>11 ± 2</td></tr><tr><td>REC2</td><td>&gt;99.1<sup>a</sup></td><td>&gt;99.4<sup>a</sup></td><td>&gt;98.8<sup>a</sup></td><td>&gt;99.1<sup>a</sup></td><td>20.8 ± 0.2<sup>b</sup></td><td>8 ± 2</td></tr><tr><td>REO1</td><td>&gt;99.1<sup>a</sup></td><td>&gt;99.4<sup>a</sup></td><td>&gt;98.8<sup>a</sup></td><td>x</td><td>3.4 ± 0.1</td><td>2.0 ± 0.1</td></tr><tr><td>REO2</td><td>&gt;99.1<sup>a</sup></td><td>&gt;99.4<sup>a</sup></td><td>&gt;98.8<sup>a</sup></td><td>7.0 ± 0.1</td><td>4.0 ± 0.1</td><td>0.9 ± 0.1</td></tr></tbody></table>

<footnote><sup>a</sup>Values were below the limit of detection, yielding to minimum precipitation values taken as equal to the detection limit for the considered element</footnote>

<footnote><sup>b</sup>The amount of Na added from the reactant (NaOH and Na₂CO₃) was excluded from the yield calculations</footnote>
<figure><img src="image_9.png" /><figcaption>**Fig. 6** Thermogravimetric analysis of rare earth hydroxide, carbonate, and oxalate compounds (5 °C/min, air 10 mL/min), with (REH2, REC2, and REO2; continuous lines) and without (REH1, REC1 and REO1; dotted lines) the addition of Ni in the leachate</figcaption></figure>

(Fig. 4), which indicated the precipitation of La(OH)₃, La₂(CO₃)₃·8H₂O + La₂(CO₃)₂·2H₂O, and La₂(C₂O₄)₃·10H₂O compounds. The mass loss due to the dehydration of La(OH)₃ is 15.4%, but a total of around 25% was measured, indicating the presence of residual water probably trapped in a gel structure, as La(OH)₃ has no known hydrated form. Due to the amorphous nature of the hydroxide sample (Fig. 4a), the TGA signal does not allow for a more accurate determination of the product's purity. The intermediate number of 4 H₂O molecules for carbonates is consistent from the presence of the two phases determined by XRD, not just the octahydrated phase. It is worth to note that the presence of Ni in the compounds (at concentration ≤ 2 wt% in any cases) does not significantly influence the thermogravimetric behavior.

## Conclusions

In this work, we report a comparison of different conversion routes for double sulfate salts of rare earth elements (REE-DSS) from the leachate of spent NiMH batteries, using hydroxides, carbonates, and oxalates. This paper focused in particular on reducing water consumption during the prior washing step of REE-DSS to remove nickel, which is the main metal impurity in the REE-DSS.

The main results show that the REE precipitation yields and kinetics are similar for the three routes. However, the selectivity toward Ni differs. The oxalate route eliminates Ni (<0.01 wt%), Na (<0.02 wt%), and K (<0.08 wt%) impurities in the final solid, reducing the amount of water needed to

wash the initial REE-DSS by a factor of 5 for a similar final Ni content. XRD, ICP-OES, and TGA analyses of the solid are in good agreement, especially for the REE oxalate salt, which has a more homogeneous structure as shown by the steep TGA curves observed at 100 and 400 °C, compared to those of carbonates and hydroxides.

The oxalic acid route requires a greater amount of reactant than the carbonate and hydroxide routes. A minimum stoichiometric excess of 1.4 is necessary to achieve comparable yields. In the case of the hydroxide route, only one main waste product (Na₂SO₄) is obtained, which reduces the waste diversity compared to the other routes. Using sodium-containing reactants, while sodium is already present in large quantities in the initial REE-DSS, decreases waste diversity in all cases.

Using oxalic acid also requires a greater financial investment. Precipitation under carbonates is the most cost-effective option in terms of reactant cost and stoichiometric consumption. Indeed, sodium carbonate is a low-cost reactant (~$200/mt), compared to sodium hydroxide (~400 $/mt) and oxalic acid (~700 $/mt) [52]. A comprehensive evaluation is still required to determine the precise costs, particularly with regard to water consumption. For instance, oxalic precipitation starts in acidic conditions, enabling the process to begin directly from an acidic solution of dissolved REE DSS, wherein its solubility is three times greater at pH 0.5 than at pH 6 at 25 °C [21]. With the exception of the washing step, the relatively low REE-DSS solubility primarily determines water consumption. Additionally, the residual solution containing a stoichiometric excess of oxalic acid can be
used in subsequent precipitation processes to decrease global oxalic acid consumption.

According to the TGA curves obtained, a thermal treatment above 700 °C results in the formation of REE oxides from the three compounds (oxalate, carbonate, and hydroxide). However, such a treatment is energy-consuming and emits CO₂ for oxalate and carbonate routes. This must therefore be taken into account in the process balance, as twice as much CO₂ is released by the oxalate route. An alternative would be to plan for the direct capture of this pure CO₂ flux. In terms of individual REE valorisation, the option of finally treating REE products by solvent extraction could be considered, as REE hydroxides and carbonates dissolve easily in HCl [23, 53]. For the valorization of poorly water-soluble REE oxalates, a solvent such as ethylene glycol could be used [53], or other techniques such as oxidative dissolution could be employed [54].

**Supplementary Information** The online version contains supplementary material available at https://doi.org/10.1007/s40831-025-01173-5.

**Acknowledgements** We thank the SNAM Groupe for the supply of the black mass powder prepared from NiMH batteries. The authors acknowledge M.-L. De Solan Bethmale, M. Boyer, A. Moreau, G. Pham, and C. Rey-Rouch for their contribution to the experiments, chemical analysis, and solid characterizations. We are grateful to the anonymous referees for comments and for editorial handling.

**Funding** Open access funding provided by Université de Toulouse. This work was partly funded by the Recyl'Batt project (MITI-CNRS, Metallo-Mix AAP2021).

## Declarations

**Conflict of Interest** On behalf of all authors, the corresponding author states that there is no conflict of interest.

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Authors and Affiliations

C. Laskar<sup>1,2</sup> · B. Guzhov<sup>1</sup> · A. Barnabé<sup>3</sup> · M. Josse<sup>1</sup> · B. Biscans<sup>1</sup> · L. Cassayre<sup>1</sup>

<figure><img src="image_10.png" /></figure>

C. Laskar
clement.laskar@gmail.com

¹ Laboratoire de Génie Chimique, Université de Toulouse, CNRS, INP, UPS, 4 Allée Emile Monso, 31400 Toulouse, France

2 Department of Chemistry, KU Leuven, Celestijnenlaan 200F, P.O. Box 2404, 3001 Louvain, Belgium

3 CIRIMAT, Université de Toulouse, CNRS, INP, UPS, 118 Route de Narbonne, 31062 Toulouse, France

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