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Preliminary Investigation of a Novel Process for Recycling
Samarium-Cobalt Magnets by Selective Oxidation
Mitchell Harvey, Mario Caccia, Courtney Young
Metallurgical and Materials Engineering, Montana Technological University
ABSTRACT: SmCo permanent magnets contain up to 77 wt% cobalt with the balance being predominantly
samarium. Having an effective way to recycle SmCo magnets is important for ensuring a continuous supply of
these critical materials. A selective oxidation roasting process has been investigated for converting the samarium
in the magnets to Sm2O₃ while leaving the cobalt in elemental form. Different methods were examined for
liberating and separating the Sm2O₃ and Co metal in the resulting calcined magnets. Results are compared to
those available in the literature.
Keywords: Samarium, Cobalt, Rare Earth Magnets, Recycle, Roasting, Magnetic Separation
INTRODUCTION
Co and Sm were identified as critical materials in the United
States Geological Survey’s 2022 report (USGS 2022). Sm
and Co are used in numerous consumer and military appli-
cations including high-strength SmCo magnets (USGS
2023 Hedrick 2004). Having an effective way to recycle
these SmCo magnets is important to ensure continued
access to these metals.
By controlling the partial pressure of oxygen (pO2)
during roasting, it is possible to selectively oxidize the Sm
in SmCo magnets creating a composite of Sm2O3 dispersed
in a Co metal matrix (Bartlett et al. 1974). For Sm2Co17
magnets, the composite will be 30 vol.% Sm2O3 and 70
vol% Co after oxidation (Swanson et al., 1996). Pure Sm
and Co have vastly different equilibrium oxygen partial
pressures with their respective oxides, that is, the content of
oxygen in the atmosphere needed to oxidize the metal dif-
fers by more than 20 orders of magnitude. Considering the
oxidation reactions for pure Sm and pure Co:
2 Sm(s) +3 ⁄2 O2(g) ⇌ Sm2O3(s)
Co(s) +1 ⁄2 O2(g) ⇌ CoO(s)
The equilibrium oxygen partial pressure (pO2,eq) between
Sm and Sm2O3 is 2.8×10–48 atm at 827 °C while the
pO2,eq between Co and CoO is 1.3×10–15 atm at the same
temperature (Barin 1995). These values indicate that if
the oxygen partial pressure in the atmosphere is kept at or
below 1.3×10–15 atm, only Sm will oxidize. Sm and Co,
however, are not present as pure metals in Sm-Co magnets,
rather they are found as intermetallics. The oxidation of
the intermetallic compound Sm2Co17 at low oxygen partial
pressures can follow one of the two reactions:
Sm2Co17(s) +3 ⁄2 O2(g) ⇌ Sm2O3(s) +17 Co(s)
Sm2Co17(s) +10 O2(g) ⇌ Sm2O3(s) +17 CoO(s)
Unlike the oxidation of the pure metals, the free energy
of formation of the intermetallic is not zero and will thus
affect the equilibrium oxygen partial pressure for each reac-
tion. Using the thermodynamic model developed by Yuan
et al. and neglecting the contribution of the free energies
of the pure elements in the hexagonal close packed lattice,
the free energy of formation of Sm2Co17 at 827 °C was
calculated to be –11 kJ/mol (Yuan et al. 2011). Therefore,
Preliminary Investigation of a Novel Process for Recycling
Samarium-Cobalt Magnets by Selective Oxidation
Mitchell Harvey, Mario Caccia, Courtney Young
Metallurgical and Materials Engineering, Montana Technological University
ABSTRACT: SmCo permanent magnets contain up to 77 wt% cobalt with the balance being predominantly
samarium. Having an effective way to recycle SmCo magnets is important for ensuring a continuous supply of
these critical materials. A selective oxidation roasting process has been investigated for converting the samarium
in the magnets to Sm2O₃ while leaving the cobalt in elemental form. Different methods were examined for
liberating and separating the Sm2O₃ and Co metal in the resulting calcined magnets. Results are compared to
those available in the literature.
Keywords: Samarium, Cobalt, Rare Earth Magnets, Recycle, Roasting, Magnetic Separation
INTRODUCTION
Co and Sm were identified as critical materials in the United
States Geological Survey’s 2022 report (USGS 2022). Sm
and Co are used in numerous consumer and military appli-
cations including high-strength SmCo magnets (USGS
2023 Hedrick 2004). Having an effective way to recycle
these SmCo magnets is important to ensure continued
access to these metals.
By controlling the partial pressure of oxygen (pO2)
during roasting, it is possible to selectively oxidize the Sm
in SmCo magnets creating a composite of Sm2O3 dispersed
in a Co metal matrix (Bartlett et al. 1974). For Sm2Co17
magnets, the composite will be 30 vol.% Sm2O3 and 70
vol% Co after oxidation (Swanson et al., 1996). Pure Sm
and Co have vastly different equilibrium oxygen partial
pressures with their respective oxides, that is, the content of
oxygen in the atmosphere needed to oxidize the metal dif-
fers by more than 20 orders of magnitude. Considering the
oxidation reactions for pure Sm and pure Co:
2 Sm(s) +3 ⁄2 O2(g) ⇌ Sm2O3(s)
Co(s) +1 ⁄2 O2(g) ⇌ CoO(s)
The equilibrium oxygen partial pressure (pO2,eq) between
Sm and Sm2O3 is 2.8×10–48 atm at 827 °C while the
pO2,eq between Co and CoO is 1.3×10–15 atm at the same
temperature (Barin 1995). These values indicate that if
the oxygen partial pressure in the atmosphere is kept at or
below 1.3×10–15 atm, only Sm will oxidize. Sm and Co,
however, are not present as pure metals in Sm-Co magnets,
rather they are found as intermetallics. The oxidation of
the intermetallic compound Sm2Co17 at low oxygen partial
pressures can follow one of the two reactions:
Sm2Co17(s) +3 ⁄2 O2(g) ⇌ Sm2O3(s) +17 Co(s)
Sm2Co17(s) +10 O2(g) ⇌ Sm2O3(s) +17 CoO(s)
Unlike the oxidation of the pure metals, the free energy
of formation of the intermetallic is not zero and will thus
affect the equilibrium oxygen partial pressure for each reac-
tion. Using the thermodynamic model developed by Yuan
et al. and neglecting the contribution of the free energies
of the pure elements in the hexagonal close packed lattice,
the free energy of formation of Sm2Co17 at 827 °C was
calculated to be –11 kJ/mol (Yuan et al. 2011). Therefore,