Recycling of SmCo magnets by removal of iron via oxidative leaching

Samarium (Sm), one of the rare earth elements (REEs), and cobalt (Co) are used in the production of SmCo permanent magnets, which have excellent temperature stability, corrosion resistance and oxidation resistance. Of the two types of SmCo magnets, one contains a high amount of iron, approximately 15.2 percent. This paper focuses on the recycling of iron (Fe)-bearing SmCo. In this study, an oxidative leaching process with nitric acid was developed to eliminate iron through in situ hydrolysis and to dissolve REEs and Co. The influence of experimental conditions on the formation of an amorphous iron compound through the hydrolysis of Fe3+ in a nitric acid environment was thoroughly examined based on a Taguchi orthogonal array. The optimal parameters for oxidative leaching were determined to be an acid concentration of 3 mol/L, a solid-to-liquid ratio of 1/10, and a process temperature of 60 °C. Background In 2023, the EU updated its Critical Raw Materials (CRM) list, identifying key elements for the years ahead [1]. With REEs and Co included in the CRM list, the recovery of these elements from SmCo magnets is becoming increasingly important to meet demand [2-5]. In this study, we propose a novel method for recycling Fe-bearing SmCo magnets. An oxidative leaching process is developed for the first time, in which iron is removed from the leach liquor through in situ hydrolysis in a nitric acid medium, resulting in the elimination of iron during the leaching stage and the dissolution of cobalt and REEs into the solution. We explain the underlying mechanism of oxidative leaching through chemical equations. Furthermore, we determine the optimal oxidative leaching parameters using statistical analysis combined with Taguchi plots. Methods Taguchi orthogonal array design was used for the oxidative leaching experiments. The philosophy of employing the Taguchi design method in this study lies in minimizing the iron dissolution and maximizing the extractions of Sm and Co during oxidative leaching by identifying the optimal leaching parameters. The oxidative leaching parameters, along with their levels are detailed in Table 1. Fig. 1 (a) SEM micrograph, (b) chemical composition and (c) EDX spectrum of SmCo magnets. Fig. 2 Taguchi plots for (a) Sm, (b) Co and (c) Fe. Fig. 3 (a) Elemental mapping of the leach residue, (b) EDX spectrum and (c) chemical composition of the selected areas in the SEM micrograph. Results and discussion Figure 1 illustrates the results obtained from the energy- dispersive X-ray spectroscopy (EDX) analysis of the SmCo magnet powders. This figure includes a scanning electron microscopy (SEM) micrograph, a tabulated representation of the chemical composition of the magnet powders, and the EDX spectrum. These data provide valuable insights into the elemental composition and physical structure of the magnet powders, which are crucial for developing an effective recycling process. The impacts of nitric acid concentration, solid-to-liquid ratio and temperature on leaching efficiency were evaluated. For the creation of the Taguchi plots, the signal-tonoise approach was chosen. For Sm and Co, the “larger is better” approach was selected. Conversely, for Fe, the “smaller is better” approach was chosen as the objective is to minimize the extraction of Fe while achieving high extractions of Sm and Co. This is consistent with the expectation that oxidative leaching should result in high Sm and Co extractions and low Fe extraction. Figure 2 illustrates the Taguchi plots for Sm, Co and Fe. A new approach is introduced to address the removal of iron from Fe-bearing SmCo magnets during oxidative leaching. This innovative idea involves selectively leaching Co and Sm while avoiding the leaching of iron. To achieve this, nitric acid is used to create highly oxidative conditions specifically targeted at iron, resulting in the hydrolysis of iron to form an amorphous Fe compound. This method appears to be a promising way to separate and retain valuable elements (Sm and Co) while effectively isolating and converting iron into a more manageable form. It showcases a strategic approach to enhance the efficiency of recycling processes for SmCo magnets. Elemental mapping of the leach residue was conducted using SEM equipped with EDX analysis. Figure 3 includes (a) the elemental mapping of the leach residue, (b) the EDX spectrum and (c) the chemical composition of selected areas in the SEM micrograph. As illustrated in Fig. 3, the leach residue contains Fe, oxygen (O), zirconium (Zr) and copper (Cu). A small amount of aluminum (Al) was observed, due to the use of an Al stub in the analysis. After the oxidative leaching, Sm and Co were not observed in the leach residue. Conclusion This paper thoroughly investigates the behavior of iron during the oxidative leaching of Fe-bearing SmCo magnets. The research demonstrates that iron ions hydrolyze to form an amorphous iron compound during oxidative leaching, allowing for the successful separation of Sm and Co from Fe in a single step. Within the investigated experimental conditions, the range of nitric acid concentration affects the extraction of Sm and Co, but it does not significantly influence Fe. Statistical results reveal that the solid-to-liquid ratio plays a significant role in Fe hydrolysis. Under the specified conditions of acid concentration of 3 mol/L, temperature of 60 °C, and solid-to-liquid ratio of 1/10, Sm and Co extractions reached approximately 99 percent. Furthermore, Fe was effectively eliminated from the system through Fe hydrolysis during oxidative leaching under these conditions. In summary, this proposed oxidative leaching method for eliminating Fe from Fe-bearing SmCo magnets and achieving high Sm and Co extractions in a single step is promising. The findings from this research provide valuable insights into the Fe hydrolysis mechanism, laying the foundation for a selective leaching process for Sm and Co. Selected references EU Commission (2023) Critical Raw Materials: ensuring secure and sustainable supply chains for EU’s green and digital future. EC: Brussels, Belgium. 16 March 2023 Alonso E, Sherman AM, Wallington TJ, Everson MP, Field FR, Roth R., Kirchain RE (2012) Evaluating rare earth element availability: A case with revolutionary demand from clean technologies. Environ Sci Technol 46(6):3406–3414 Mancheri NA (2012) Chinese monopoly in rare earth elements: supply–demand and industrial applications. China Report 48(4): 449–468 Habib K, Wenzel H (2014) Exploring rare earths supply constraints for the emerging clean energy technologies and the role of recycling. J Clean Prod 84:348–359 Harper EM, Kavlak G, Graedel TE (2012) Tracking the metal

Recycling of SmCo magnets by removal of iron via oxidative leaching

Authored by:
Elif Emil-Kaya, Bernd Friedrich, Merve Papakci

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