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Abstract

Sm-Co magnets are the second most widely industrially used REE-permanent magnets
(REE: rare earth elements) after Nd-Fe-B. The magnets are used in a wide range of
applications, such as electronics, aerospace and medical applications. Sm-Co provides the
highest known coercivity and thermal stability values in the market. However the magnets
are mainly composed of REE (Sm) and Co, and over the past 10 years the previous
elements have been identified by the European Commission to be critical elements
because of the risk of their supply resources; as their market is controlled by China and
DRC, beside their very high economic importance. Moreover, REE permanent magnets
alone consume about 20% of the REE produced worldwide. Consequently, the recycling of
Sm-Co magnets has been considered as an urgent solution to help maintain a sustainable
source of those very important metals and magnets. In the work described here, Hydrogen
Decrepitation (HD) has been applied as an effective way for the direct recycling of the two
industrially produced types of sintered Sm Co magnets; SmCo5 and Sm2Co17. Hydrogen of a
pressure of only 1 bar, room temperature for 3 hours was sufficient for the decrepitation
of SmCo5, however more severe conditions were required for the decrepitation of Sm2Co17;
20 bar of hydrogen gas for 2 days. The recycled magnets prepared by using more than 98
wt.% of the recycled decrepitated powder showed to have as good magnetic properties as
the industrially produced ones, by using the conventional sintering route. The magnetic
properties of the recycled SmCo5 were remanence (Br) = 0.95 T, intrinsic coercivity (iHc)
> 1500 kA/m and maximum energy product ((BH)max) of 171.1 kJ/m3 and for Sm2Co17
were Br = 1.09 T, iHc > 1500 kA/m and (BH)max = 218.3 kJ/m3. Spark plasma sintering
(SPS) has been studied as an alternative way for the consolidation of the recycled powder,
where it was shown to be an effective route for producing isotropic SmCo5 magnets with
near full density and coercivity values similar to the conventionally sintered magnets. For
Sm2Co17, the magnetic properties of the SPS-ed samples showed deterioration of iHc to
about 200 kA/m, which was related to the microstructure formation during the sintering
process. Anisotropic polymer bonded magnets were also produced from the recycled
decrepitated powder with a very simple process of mixing the magnetic powder with a
polymer binder followed by aligning, pressing and heating the pressed compact. The
polymer bonded Sm2Co17 magnets showed to have promising properties of Br = 0.58 T, iHc
> 1500 kA/m and (BH)max = 56 kJ/m3. Because of the oxidation of the Sm element in
SmCo5 during the preparation of their polymer bonded magnets, their iHc was as low as
250 kA/m. As most of the industrially produced magnets are coated; usually with Ni-Cu-Ni, for protection from corrosion or chipping, and the presence of the coating residuals
would deteriorate the magnetic properties of the recycled magnets, the possibility of
removing the coating material from the recycled powder has been studied. By applying HD
on the coated magnets, it was possible to sieve out the coating residuals from the
decrepitated powder where 98 wt.% of the coating was removed. The dissolution of the
coating material was also studied by using 2M HNO3 and 1 vol.% Br/organic systems,
where it was shown that the latter system can be a promising route for the industrial
application of the removal of the coating on the magnet’s surface in a very short time of
less than 20 min. To complete the picture, the effect of the presence of the coating
residuals on the properties of Sm Co magnets was studied, where it was shown that up to
a coating content of 2 wt.% can be present in SmCo 5 magnets without decreasing its
magnetic or mechanical properties. For Sm2Co17 magnets, 1 wt.% was shown to be the
maximum allowed concentration to be present without sharp negative effects on the
properties of the magnets. Finally strip casting was also studied as an alternative recycling
route of the Sm-Co magnets to be compared with HD, where very low recovery values of
less than 50 wt.% were achieved.