Jan. 15, 2016
ScienceEnvironment

Efficient Recycling of Rare Earth Permanent Magnets

Applications and Present Situation

  • Melt spinning: The material is melted using an induction coil and thereafter cast onto a rotating copper wheel.  © Fraunhofer Project Group IWKSMelt spinning: The material is melted using an induction coil and thereafter cast onto a rotating copper wheel. © Fraunhofer Project Group IWKS
  • Melt spinning: The material is melted using an induction coil and thereafter cast onto a rotating copper wheel.  © Fraunhofer Project Group IWKS
  • Figure 1: Chemical composition of  a Nd-Fe-B magnet from the scrap (Data in mass-%). © Fraunhofer Project Group IWKS
  • Figure 2: Melt spun flakes and scanning electron microscope images of the flake microstructure with grain sizes on the nanoscale. © Fraunhofer Project Group IWKS

Authors: Oliver Diehl1, Eva Brouwer1, Alexander Buckow1, Roland Gauß1, Oliver Gutfleisch1,2

Many advanced technologies are based on the use of functional materials. Due to their chemical composition and microstructure these materials offer certain macroscopic properties useful for their specific applications. Modern vehicles contain a large number of electronic micromotors, electric actuators and powertrains are widely used also in the mechanical engineering industry. Audio devices and hard disks have become an integral part of the private as well as of the business life.
For the future more and more energy will be produced via wind power and electric mobility will replace present mobility technologies, which are based on fossil fuels [1].

Applications and Present Situation
In order to ensure an operation as fault-free and efficient as possible these technologies operate with strong permanent magnets. Magnets containing rare earths on the base of neodymium-iron-boron offer by far the highest energy density. A partial substitution of neodymium by dysprosium enables the use of these permanent magnets even at higher temperatures, which is especially important for the use in electric motors. Due to a strongly increasing demand in many fields of applications the need for these materials has risen continuously over the past decade.

Shortage
The supply of neodymium and dysprosium worldwide as well as rare earth elements in general is rated as critical. Rare Earths occur as oxides in nature. Additionally the mined materials are always a mixture of several rare earth oxides. Therefore, in the laborious primary production the rare earth elements have to be extracted, concentrated, separated, reduced and finally transferred into alloys. A significant proportion of the elements gained this way is used for the production of permanent magnets.
As in the future a rising demand is expected, there will be no way of bypassing a recycling and partlial substitution of these elements on the long run. Beneath this requirement the utilization of the critical elements located in scrap magnets offers some more benefits.

Compared with the energy-intensive, the immediate environment negatively impacting primary mining, magnets from recycled material have the potential to be cheaper and to leave a smaller ecological footprint. Recycling would furthermore reduce the dependence of companies from global economic and geopolitical developments and offers a chance to establish an important value chain on a local level.

Recycling Approaches
Regarding a recycling of permanent magnets it is important to know, that there is not a single kind of Nd-Fe-B alloy. The ternary alloy serves as a base alloy which is variously modified for specific applications in order to optimize properties like temperature stability, corrosion behavior and magnetic characteristics.
Figure 1 shows exemplarily the chemical composition of a scrap magnet. Apart from neodymium, iron and boron other rare earth elements, dysprosium and praseodymium, have been added as well as other elements to optimize this magnet for a specific electric motor application.
At present no procedure is established on an industrial-scale allowing the recycling of rare earth permanent magnets from end-of-life products and therefore the rare earth elements dissipate in metal scrap [2] [3].
In principle a recycling of scrap magnets is conceivable at three levels: Direct Reuse, elemental and alloy recycling [2] [4].

Direct Reuse
Both for economic and ecological aspects this approach appears to be the best solution. But regarding the fact that features like alloy composition and shape have changed over time and are always specified for certain applications, it becomes clear that a direct reuse will only be possible for individual cases, as far as also no irreversible damages, especially corrosion damage, are present [4] [5].

Elemental Recycling
This includes all approaches aiming on the extraction of single elements or compounds. Research concentrates on pyro- and hydrometallurgical processes, partly used in primary production but also very energy-intensive, as well as gas phase reactions and bioleaching [2] [4].

Alloy Recycling
At this level the scrap magnet is recycled as an alloy. As the economically significant rare earth metals neodymium, dysprosium and praseodymium are mainly used for the production of permanent magnetic alloys, it is obvious to establish a recycling process for the whole alloy. At this level several short-loop-recycling procedures are pursued, allowing the production of a starting material for new permanent magnets from scrap magnets in only a few steps [2]. One of these processes is the method introduced here, the melt spinning technology.

Melt Spinning Technology
The process of melt spinning, also known as rapid quenching, has been used for years to produce different kinds of metal alloys and it is also used for the primary production of magnetic material. In industrial manufacturing more than 90% of permanent magnets are produced via sintering [6]. Alternatively melt spinning and hydrogen based processes are established to create nanocrystalline powder, which can be further processed into hot pressed and hot deformed or polymer bonded magnets. [7]. This production route, established for primary production, can also be used for an alloy recycling of scrap magnets.
During melt spinning the scrap magnets are inductively melted in a crucible, as shown in the teaser. This melt, heated to more than 1200°C, is thereafter cast via a nozzle, located in the bottom of the crucible, onto a copper wheel, which is fast rotating and water cooled. Due to the high thermal conductivity of copper the thermal energy is transferred from the melt into the wheel within milliseconds (typical cooling rates for melt spinning are around 1 million K/s) and the melt drop solidifies on the rotating wheel as a thin flake.
The rapid cooling prevents an atomic arrangement in the otherwise typical crystalline lattice. Instead an amorphous structure, where the atoms are arranged without any long-range order, or a nanocrystalline structure, where atoms organize in grains on the nanoscale (Figure 2), is created. The kind of microstructure created in the flakes whilst solidification can be precisely set via process parameters like rotation speed of the copper wheel or temperature of the melt at spinning.
In addition to an optimized microstructure the process enables a modification of the chemical composition with additives and a reduction of the oxygen content, which is especially interesting for heavily corroded material.
The melt spinner at Fraunhofer Project Group IWKS has a capacity of 500 g, thus enables experiments between lab and industrial scale. A first production of recycled permanent magnets with this device and via the described process route has been demonstrated. Further research concentrates on optimization of the process parameters of melt spinning as well as of the following processes to obtain highest possible outputs and best possible magnetic properties.

Summary and Outlook
Recycling of permanent magnets and the contained critical elements bears great economic advantages and offers an alternative source a strategic alloy. The presented process is oriented to a process route established in the primary production of permanent magnets. It was shown on a lab scale that this route is in principal usable for a recycling of these materials as well. This recycling approach enables the reuse of the entire alloy at a relatively low effort in energy and cost.

Affiliations
1Fraunhofer Project Group for Materials Recycling and Resource Strategies IWKS, Hanau, Germany
2Materials Science, TU Darmstadt, Darmstadt, Germany

Contact
Oliver Diehl

Fraunhofer Project Group IWKS
Fraunhofer Institute for Silicate Research (ISC)
Hanau
oliver.diehl@isc.fraunhofer.de

Literature:

[1]          O. Gutfleisch, M. A. Willard, E. Brück, C. H. Chen, S. G. Sankar und J. P. Liu, Adv. Mater. 23, pp. 821-842, 2011. DOI: 10.1002/adma.201002180

[2]          R. Gauß, O. Diehl, E. Brouwer, A. Buckow, K. Güth und O. Gutfleisch, Chemie Ingenieur Technik, Nr. 87 (11), 2015. DOI: 10.1002/cite.201500061

[3]          M. Buchert, A. Manhart und J. Sutter, 2014.

[4]          K. Binnemanns, P. T. Jones, B. Blanpain, T. Van Gerven, Y. Yang, A. Walton und M. Buchert, J. Cleaner Prod. 51, pp. 1-22, 2013. doi:10.1016/j.jclepro.2012.12.037

[5]          U. Bast, R. Blank, M. Buchert, T. Elwert, F. Finsterwalder, G. Hörnig, T. Klier, S. Langkau, F. Marscheider-Weidemann, J.-O. Müller, C. Thürigen, F. Treffer und T. Walter, MORE_Abschlussbericht.pdf, 2014.

[6]          D. N. Brown, Z. Wu, F. He, D. J. Miller und J. W. Herchenroeder, J. Phys.: Condens. Matter 26 (2014) 064202 (8pp). doi:10.1088/0953-8984/26/6/064202

[7]          O. Gutfleisch, J. Phys. D: Appl. Phys. 33, pp. R157-R172, 2000. doi:10.1088/0022-3727/33/17/201

More on urban mining:
http://www.laboratory-journal.com/search/gitsearch/Urban%20Mining

What is a permanent magnet:
http://www.allaboutcircuits.com/textbook/direct-current/chpt-14/permanen...
 

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