The objectives for the MMG are:
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To develop a flexible unit to process scrap rare earth magnets.
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To optimise the processing conditions of the pilot plant so as to produce powders with the characteristics required by magnet manufacturers.
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To characterise the powders made in the pilot plant.
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To supply powders made from recycled rare earth magnets to consortium partners for magnet manufacture.
In parallel, an EPSRC-funded PhD student will be looking to develop the appropriate processing condition to take the powder produced by the pilot plant and generate usable permanent magnets.
The project is partially funded by the DTI and by the following partners:
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PowdermatriX (Project Managers)
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Secure IT Recycling Ltd
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Less Common Metals Ltd
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Magnet Applications Ltd
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Precision Magnetics Ltd
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Birmag Ltd
Detailed Background
A possible large scale, future application of the Hydrogen Decrepitation (HD) process could be in the recycling of SmCo5, Sm2(Co,FeCu,Zr)17 and NdFeB-type magnets. The HD-process converts the magnets to a powder due to the expansion of the material on hydrogen absorption.
Recycling of NdFeB magnets by this means has been proposed by Rivoirard et al [1] and by Zakotnik et al [2] and these workers reported very encouraging results on the production of anisotropic powder from degassed HD powder obtained from sintered NdFeB magnets. Such an application will be of growing importance with the expected growth in electric vehicles and with increasing emphasis on energy conservation.
This is likely to demand greater degrees of energy efficiency and materials recycling, particularly in the case of automotive applications, where "end of life" rules are already being implemented. Such measures can be justified in terms of environmental impact (metal contamination of land-fill sites), energy saving (CO2 reductions) and resource depletion (particularly for the rarer elements such as Dy).
In the case of the HD powders based on SmCo5 and Nd2Fe14B, it would be necessary to handle the powder in an inert atmosphere because of the known reactivity of these powders. A major advantage of using the HD-process would be the ability to avoid a further build-up of the oxygen content in the powder during recycling.
Such a build-up would inhibit subsequent processing options for the powder. The presence of hydrogen in the powder reduces the intrinsic coercivity and hence the material is not directly usable as a permanent magnet, e.g. by resin bonding. Possible routes to recycle the powder into permanent magnets are:
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Vacuum degassing to remove the hydrogen followed by coating, alignment and bonding.
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Vacuum degassing, pre-aligning and hot pressing into fully dense magnets.
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Hydrogenation, disproportionation, desorption, recombination (HDDR) processing to give high coercivity powder for bonding or hot pressing.
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Milling, alignment / pressing and vacuum sintering into fully dense magnets.
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The powder could be blended with fresh powder and processed in one of the ways above.
Previous studies have also looked to process the powder by surface treatment [3] or to remove carbon and free carbides at grain boundaries [4] or oxygen removal by using calcium vapour [5]. Some recycling processes simply look to recover the rare earth elements for subsequent reuse [see for example reference 6].
The work at Birmingham concentrates on two methods of recycling NdFeB sintered magnets, both of which start with the use of the HD process to generate powder from scrap magnets.
The powder is either further processed into coercive powder by degassing of the hydrogen or the powder is reused to make fully dense sintered magnets capable of being used in applications requiring lower costs and/or somewhat reduced magnetic properties.
[1] S. Rivoirard, J.G. Noudem, P. de Rango, D.Fruchart, S. Liesert and J.L. Soubeyroux, Proc. 16th Int. Workshop on Rare-Earth Magnets and Their Applications (Sendai Japan) (2000) p 347.
[2] M. Zakotnik., A.J. Williams and I.R. Harris, 18th workshop on High Performance magnets and their Applications (Grenoble France) vol. 1, (2004) p 267.
[3] T. Horikawa, M. Itoh, S. Suzuki and K. Machida, Journal of Magnetism and Magnetic Materials 271 (2004) p 369.
[4] R.O. Suzuki A. Saguchi, W. Takahashi, T. Yagura and K. Ono, Mat. Trans. 42, no 12, (2001), p 2492.
[5] A. Saguchi, K. Asabe, T. Fukuda, W. Takahashi and R.O. Suzuki, Journal of Alloys and Compounds 408-412 (2006) p 1377.
[6] O. Takeda, T.H. Okabe and Y. Umetsu, Journal of Alloys and Compounds 408-412 (2006) p 387.