(259d) Lanthanide Precipitation from Spent Ni-MH Battery Leachates: Kinetic Study and Phase Identification

Securing access to critical metals required for high-performance technologies, particularly the light rare earth elements (REEs = La, Ce, Nd, Pr), has become a major challenge for import-dependent economies such as the EU.The recycling of spent nickel metal hydride (Ni-MH) batteries from hybrid electric vehicles (HEV) serves as an increasingly-attractive secondary source of REEs thanks to the recent development of hydrometallurgical processes.^[1] In order to recover REEs, precipitation from pregnant leach solutions (PLS) in sulfate media, using Na₂SO₄ is often reported in the literature. However, little consideration is given as to whether and how sodium ions influence the precipitation efficiency and selectivity, and detailed phase characterization of the products is rarely reported. This work focuses on a better understanding of the precipitation phenomenon and mass balances in this complex system, by coupling pilot-scale experiments on industrially-sourced PLS, in-depth precipitated particles identification, and thermodynamic calculations.

The initial PLS used in this study originates from the pilot-scale leaching of industrial samples of spent Ni-MH battery powders in H₂SO₄ media, according to a process described in [1]. The metallic content of the battery waste leachate consists of 71%mol Ni, 10 %mol REEs and 19%mol of other metals like Co, Mn and Fe. In the view of metal recycling, it is of great interest to find a selective precipitation pathway to recover REEs from such a complex solution. For each precipitation experiment, 1.5 L of PLS was added to a double-jacketed batch reactor; solution temperature was regulated using a cryostat and pH was monitored. In a standard experiment, a 2.9 M Na₂SO₄ solution was maintained at 40 °C and added to the PLS at constant flow rate, in simple jet, using a membrane pump. Mechanical agitation was implemented with a 3-blade Teflon marine propeller at 400 rpm. The influence of three operating parameters on precipitation efficiency and kinetics was studied: temperature (25 – 60 °C), Na:REEs molar ratio (1:1 – 4:1, by increasing the total addition time of Na₂SO₄ solution at a constant flow rate) and Na₂SO₄ solution addition (all at once or slowly at 7 g.min^-1 implying an initial semi-batch mode). Suspension volumes were regularly sampled, filtered on 0.45 µm syringe filters, and solutions were analyzed by ICP-OES. Precipitation efficiency was calculated as the proportion of the REEs initially present in solution that have precipitated. After 1 h, the suspension was filtered on a P3 Büchner filter and rinsed with demineralized water at room temperature. The obtained crystals were dried in an oven at 80 °C overnight before in-depth characterization using multiple techniques: SEM-EDX, powder XRD, TGA, microwave digestion in aqua regia and ICP-OES.

Results compiled in Figure 1) a) illustrate the evolution of the proportions of Ni, Co, Mn, Fe, La, Ce, Nd and Pr remaining in solution when precipitation is carried out at 60°C, for a Na:REEs molar ratio of 4:1 and a Na₂SO₄ solution flow rate of 7 g.min^-1. The simultaneous drop of pH when REE concentrations start to decrease (Figure 1) b) demonstrates that pH monitoring serves as an indicator of precipitation initiation and therefore allows determining the induction period, which is of about 3 min in this case. Curves in Figure 1) a) flatten out after approximately 30 min, suggesting that equilibrium has been reached. After 1 h, the precipitation efficiencies of La, Ce, Nd and Pr reach 99.3 %, 100.0 %, 99.7 % and 87.6 %, respectively, corresponding to 70 ppm of La, 0 ppm of Ce, 4 ppm of Nd and 52 ppm of Pr remaining in solution. REE precipitation is highly selective in this configuration since 99 % of Ni, 99 % of Co, 98 % of Mn, and 99 % of Fe remain in solution for further recovery. The molar ratios of REEs in the precipitate during the semi-batch and batch modes are reported in Figure 1) c). These ratios remain relatively constant during the 1 h run, suggesting that the same phases form throughout the whole precipitation period.

Precipitation efficiencies and aqueous REE concentrations are summarized in Table 1 for all experimental conditions. This compilation highlights the influence of operating parameters. For a fixed Na:REEs molar ratio of 4:1, increasing the temperature from 25 to 60 °C greatly increases precipitation efficiency of all REEs and yields faster precipitation kinetics; for example, 74.4 % of La is precipitated at 25 °C while at 60 °C, 99.3 % is precipitated. Similarly, increasing the Na:REEs molar ratio from 1:1 to 4:1 at 60 °C, strongly increases the precipitation efficiencies of all REEs. Furthermore, adding the equivalent volume of Na₂SO₄ solution at once yields the same precipitation efficiencies than when the solution is added at a 7 g.min^-1 flow rate but greatly increases precipitation kinetics; however, rapid injection favors nucleation generating very small particles and consequently, after 1 h, filtration was more difficult (low filtration rates). It is interesting to note that, for a given temperature or for a given Na:REEs molar ratio, precipitation efficiencies of the REEs is always as: Ce > Nd > La > Pr.

Powder XRD analyses of the precipitates yield diffraction peaks that lie between those of pure NaLa(SO₄)₂.H₂O and NaCe(SO₄)₂.H₂O double sulfates. Crystal lattice parameters were calculated and exhibit intermediate values between those of pure double sulfates suggesting that precipitates are solid solutions of lanthanide-alkali double sulfates. Structures are assigned to the hexagonal crystal system^[2], which can be observed on a representative SEM micrograph provided in Figure 2. The chemical composition of the solids was determined by TGA (number of water molecules n) and ICP-OES analysis (elemental content of S, Na, La, Ce, Nd and Pr). The resulting solid compositions are gathered in Table 2 as Na_x(REE)₁(SO4)_y.nH₂O, where x and y are the respective Na and S proportions relative to the total REEs content. The water content (n) is close to 1 for each solid, which proves the formation of monohydrate-type crystals. The mass and electrical (3+x–2y) balances yield that both Na and SO₄ contents are lower than the stoichiometric ratios (x<1 and y<2), and that the overall charge of such compounds is about +0.35 – +0.40. This charge balance inconsistency, which is currently investigated, is most likely due to the incorporation of additional ions in the structure. As indicated in Table 2, REE molar ratios in the mineralized precipitates are relatively equivalent for all experiments. Therefore, it is concluded that the same lanthanide-alkali solid solution forms regardless of the temperature or the Na:REEs molar ratio.

In parallel, equilibrium calculations were performed using the mixed-solvent electrolyte model implemented in the OLI software, which includes an accurate thermodynamic description of rare earth sulfates recently developed by Anderko et al.^[3] Calculations were carried out with the various initial PLS conditions (pH, temperature, and ion concentrations). They foresee the precipitation of the compounds NaLa(SO₄)₂.H₂O, NaCe(SO₄)₂.H₂O, NaNd(SO₄)₂.H₂O and NaPr(SO₄)₂.H₂O in all conditions, which is in good agreement with XRD characterizations. The calculated REEs aqueous concentrations, detailed in Table 1, are also rather close to experimental measurements, which indicates that the precipitation reactions are close to thermodynamic equilibrium after 1 h. More specifically, REE aqueous concentrations strictly meet calculated concentrations at 60 °C and for a 4:1 Na:REEs molar ratio. At 60 °C, calculations show that the solubilities of all lanthanide-alkali double sulfates decrease with increasing Na⁺ concentration, which explains why increasing the Na:REEs molar ratio improves precipitation efficiencies at constant pH. Moreover, for a Na:REEs molar ratio of 4:1, increasing the temperature decreases the double sulfates solubilities (apart from NaCe(SO₄)₂.H₂O which shows the opposite trend). Therefore, based on both experimental results and thermodynamic trends, we conclude that the configuration (60 °C and 4:1 Na:REEs molar ratio) yields the best precipitation efficiencies for La, Ce, Nd and Pr (99.3 %, 100.0 %, 99.7 % and 87.6 %, respectively).

Key takeaways of this study are a straightforward approach to perform a grouped extraction of REEs contained in complex solutions originating from the leaching of industrial samples of spent Ni-MH batteries. Using a concentrated Na₂SO₄ solution, highly selective precipitation was obtained after 1 h at 60 °C and for a Na:REEs molar ratio of 4:1, whereby only 70 ppm La, 0 ppm Ce, 4 ppm Nd and 52 ppm Pr remain in solution and more than 98 % of the major elements do not co-precipitate. Thanks to multi-analytical characterization backed by thermodynamic calculations, the precipitated crystals were determined to be lanthanide-alkali double sulfate solid solutions of overall composition Na_0.79La_0.59Ce_0.20Nd_0.07Pr_0.14(SO₄)_1.72.H₂O. Results suggest that the same lanthanide-alkali solid solution forms regardless of the experimental conditions.

[1] M. Zielinski, L. Cassayre, P. Destrac, N. Coppey, G. Garin, B. Biscans, Leaching mechanisms of industrial powders of spent nickel metal hydride batteries in a pilot-scale reactor, ChemSusChem. 4 (2020) 616–628.

[2] A.C. Blackburn, R.E. Gerkin, Sodium lanthanum(III) sulfate monohydrate, NaLa(SO4)2.H2O, Acta Crystallogr. C. 50 ( Pt 6) (1994) 833–838.

[3] G. Das, M.M. Lencka, A. Eslamimanesh, P. Wang, A. Anderko, R.E. Riman, A. Navrotsky, Rare earth sulfates in aqueous systems : Thermodynamic modeling of binary and multicomponent systems over wide concentration and temperature ranges, J. Chem. Thermodyn. 131 (2019) 49-79.