Rare Earth Extraction by Oxalic Acid – Simplified Case in OLI Flowsheet: ESP

Objective

This article demonstrates the development of a simple rare earth precipitation model using OLI Flowsheet: ESP. The example models the selective precipitation of neodymium from a leachate solution using oxalic acid, based on the laboratory system described in the referenced literature.^1,2 The workflow illustrates how OLI’s electrolyte thermodynamic framework can be used to study hydrometallurgical recovery processes relevant to critical materials such as rare earth elements, lithium, cobalt, and nickel.

Disclaimer: The user interface, calculations, and results displayed in this article are from OLI Flowsheet: ESP Version 12.5. Other software versions may appear different due to continual developments to the software.

Background

The model developed in this article is based on the oxalate precipitation process outlined in Reference 1.¹ In an earlier paper, the authors studied the efficiency of leaching spent NdFeB magnets using hydrochloric acid (HCl) and sulfuric acid (H₂SO4), with nickel remaining in the solid waste stream.² Next, in Reference 1, the researchers evaluated the efficacy of oxalic acid for selectively extracting the rare earth components of the magnets as rare earth oxalate complexes, directly from the leach solution.^1,2 

In this study, the authors prepared a model solution for Fe⁺² and Nd⁺³ using FeCl₂ and NdCl₃, respectively, and the pH for the model solution was modified to approximately 1.0.¹ Next, oxalic acid was added at specified stoichiometric amounts at 20°C.¹ They repeated a similar precipitation process on leachate solutions of real NdFeB magnets (~14 g).^1,2 Analysis of this leachate solution indicated that iron existed in the (+2) oxidation state in the leach solution.^1,2

Process Overview in OLI Flowsheet: ESP

The modeled process represents oxalate precipitation of rare earth elements from a sulfuric-acid leachate of NdFeB magnets.^1,2 The model includes small adaptations from the published experimental setup to provide a clear, lab-scale proof-of-concept.

Note: This model is intended only To demonstrate the chemical reactions between oxalic acid and the leach solution, inefficiencies related to solid/liquid separations or kinetic effects were not considered for this model.

Figure 1. Overview of rare earth precipitation process in OLI Flowsheet: ESP^1,2

Leach Solution Representation

A Water Analysis was used to represent the aqueous leachate after sulfuric-acid leaching (Figure 2).

Figure 2. Water Analysis representation of NdFeB magnet solution after leaching with H₂SO₄ ^1,2

• Concentrations were aligned with those reported in Table S2 of the Supplementary Information of Reference 1 for the sulfuric-acid leachate, based on inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis.¹
• Nd³⁺ concentration was entered as 18 g/L (slightly higher than the experimentally reported 15.8 g/L value) to account for additional trace rare earth content (Praseodymium and Dysprosium).¹
• Boron was entered as B(OH)₃ using a conversion factor of 5.72, consistent with OLI species requirements. For further details, please see Conversions for Water Analysis with Elements B, Si, S, and P.
• The Reconciliation calculation was set to match the experimental model solutions’ pH of 1.0.¹
• Electroneutrality was balanced using the makeup ion of SO₄²⁻, consistent with sulfuric-acid leaching.¹

For this example, we have focused only on neodymium, as this is the element for which oxalic acid complexation has been fully captured by the OLI MSE database (as of Version 12.5).

Mixing Leachate with Oxalic Acid

The leachate stream enters a Separator configured for an Isothermal mixing calculation at 20 °C.

• Oxalic acid (100%) was added initially at 2.2×10⁻³ mol/hr, representing a 1:1 stoichiometric ratio with Nd³⁺.¹
• A Manipulator Block was added to vary the oxalic acid flow for sensitivity calculations.

Sensitivity Analysis – Oxalic Acid Amount

To analyze the effect of increased oxalic acid concentration on extraction efficiency, the authors varied the stoichiometric ratios of oxalic acid relative to the rare earth concentration.¹

In OLI Flowsheet, a Sensitivity Analysis was configured using the Manipulator Block’s Factor, Total Flow parameter to match the experimental studies.¹

• Factor, Total Flow range: 1.0–1.4
• Corresponding molar ratios (oxalic acid : Nd³⁺):
  • 1.0:1
  • 1.2:1
  • 1.4:1

For more information on how to set up a Sensitivity Analysis in OLI Flowsheet: ESP, please reference our Support Center article.

Calculator Block – Neodymium Solid Yield

A Calculator Block was added to report neodymium recovery:

• Neodymium inflow: Total MBG amount of Nd(+3) in the leach solution (g/hr)
• Neodymium solid: MBG amount of Nd(+3) in the calculated solid phase exiting the separator (g/hr)
• Calculated variable = (Neodymium solid) / (Neodymium inflow) * 100%

For more information on how to configure a Calculator Block in OLI Flowsheet: ESP, please refer to our Support Center article.

Results

The model predicts precipitation of neodymium(III) oxalate decahydrate (Nd₂(C₂O₄)₃·10H₂O). Most other components remain in the liquid phase. Cobalt is predicted to partially precipitate as Co(OH)₃.

As in the referenced study, predicted recovery increases with oxalic acid dosage (see Figure 3 and Table 1).¹

Figure 3. Sensitivity plot of oxalic acid flow versus predicted neodymium solid yield^1,2

While OLI’s predictions cannot be directly compared to the experimental data due to the model’s exclusion of other rare earths in the initial leach solution, the predicted trend aligns well with experimental behavior and provides a valid basis for further model development.¹

Table 1. Effect of ratio between oxalic acid and rare earth on neodymium recovery – experimental and OLI results¹

Conclusion

This simplified example demonstrates how OLI Flowsheet: ESP can be used to evaluate rare earth precipitation processes using oxalic acid. The workflow provides a foundational structure for creating more detailed hydrometallurgical models, including multi-component rare earth systems or integrated purification processes.

Related OLI Resources

• OLI’s V12 Database Supports Simulation of Solvent Extraction for Recovery of Critical Materials from Spent Lithium-Ion Batteries (LIBs)
• New Chemistry in OLI v12-5
• Webinar – Rare Earth Elements: Developing efficient extraction and purification processes
• Modeling Rare Earth Solvent Extraction with TODGA: A New Thermodynamic Capability Introduction
• Reimagining waste streams for REE production

References

1. Klemettinen, A.; Adamski, Z.; Chojnacka, I.; Leśniewicz, A.; Rycerz, L. Recovery of Rare Earth Elements from the Leaching Solutions of Spent NdFeB Permanent Magnets by Selective Precipitation of Rare Earth Oxalates. Minerals 2023, 13 (7), 846. DOI: 10.3390/min13070846
2. Klemettinen, A.; Żak, A.; Chojnacka, I.; Matuska, S.; Leśniewicz, A.; Wełna, M.; Adamski, Z.; Klemettinen, L.; Rycerz, L. Leaching of Rare Earth Elements from NdFeB Magnets without Mechanical Pretreatment by Sulfuric (H₂SO₄) and Hydrochloric (HCl) Acids. Minerals 2021, 11 (12), 1374. DOI: 10.3390/min11121374