Rare earth recovery

Materials World magazine
,
1 Aug 2011
hard disk drive

Dr Allan Walton, Senior Science Research Fellow and Dr Andy Williams, Head of the Magnetic Materials
Group, both at the University of Birmingham, UK, outline methods to retrieve crucial elements that are
in short supply.

Rare earth elements have been highlighted,
by both the EU and USA, as being at
highest supply risk of all metals for clean
technologies. Of particular concern are neodymium
(Nd) and dysprosium (Dy), which are used to
produce permanent magnets. Neodymiumiron-boron (NdFeB) magnets, which often contain Dy,
are used in many high-tech applications and clean
technologies including consumer electronics, motors
in electric/hybrid vehicles and generators in offshore
wind turbines. China produces more then 95% of
the world’s rare earth materials, and last year it cut
export quotas by 30-40%. This led to Nd and Dy
prices rising from around US$30/kg and US$150/kg
to US$420/kg and US$3,000/kg respectively over
the last 18 months.

There are three possible solutions to this material
shortage, which could be employed in unison –
opening rare earth mines in other countries (rare earth
deposits can be found at many locations), using
alternative devices that do not contain rare earth
elements or recycling the existing stock of material
contained within scrap devices.

Most of today’s rare earth magnets are contained
in small quantities within electronic devices such as
hard disk drives (HDDs) that contain two NdFeB
magnets (a resin bonded magnet in the spindle motor
and a sintered magnet in the voice coil motor):

Although there are only around 20 grammes of
sintered NdFeB in each HDD, 600m HDDs were
manufactured in 2008, accounting for more than
25% of the global sintered NdFeB magnet production
for that year.

In order to recycle rare earth magnets, they first
Rare earth recove
Dr Allan Walton, Senior Science Research Fellow and Dr Andy Williams, Head of the Magnetic Materials
Group, both at the University of Birmingham, UK, outline methods to retrieve crucial elements that are
in short supply.
need to be efficiently extracted from the devices,
which is often difficult due to the design of the
equipment. For example, to manually remove the
magnets from a HDD would require the removal of
12-15 security screws. Even then, the sintered magnet
is glued into the voice coil motor and coated with
nickel. If the HDD is shredded, which is often the case
to remove information from the disk, then due to
the brittle nature of the material it tends to break
into a powder and magnetically stick to any other
ferromagnetic components. If the magnets are
removed prior to shredding, it is still not possible to
simply re-use them in new devices because there is a
wide variety of shape and size, depending on the age
of the device and manufacturer of the HDD.
Therefore, in addition to removing the magnets, a
further challenge is to reprocess the material into
new magnets suitable for modern devices.

Hydrogen separation

Researchers at the University of Birmingham have
developed processes for extracting NdFeB magnets
from electrical devices using hydrogen gas. Hydrogen
can be absorbed by a sintered NdFeB magnet at
room temperature and atmospheric pressure in a
process known as hydrogen decrepitation. During
decrepitation, the Nd-rich grain boundary phase
initially absorbs hydrogen, forming an interstitial
hydride (NdH~3). The matrix grains then also absorb
hydrogen, forming Nd2Fe14BH~3 with a five per cent
volume expansion.

This volume expansion at the surface, compared to
the bulk, causes the surface material to break away
from the magnet as a coarse hydrided powder. Flat polycrystalline particles
are produced. An uncoated
sintered voice coil
magnet (VCM) will typically
break up in around 30
minutes. As the lattice
expansion changes the
interatomic spacing,
there is a modification of
the exchange interaction
between neighbouring
atoms and hence a
change in the magnetic
properties. Thus the
hydrided powder loses its
permanent magnetisation
and crucially, for easier
separation, it is no longer
attracted to other soft
magnetic components in
the scrap.

The sintered magnet in
the voice coil assembly of
a HDD is plated with nickel (Ni). It has been shown
that by processing at elevated temperature and
pressure (170°C and 7 bar) it is possible to
force hydrogen through the coating, resulting in
decrepitation of the magnet and peeling of the coating
into flake-like particles. After hydrogen processing
of manually extracted voice coil assemblies, it has
also been shown that separation of the hydrided
NdFeB from the iron casing, glues, screws and
most of the Ni flakes (see middle image, right) is possible by
a simple sieving operation. In addition, further
processing techniques have been shown to separate
out the finer Ni flakes, producing powders with a Ni
content of less than 0.03wt%.

The work at Birmingham is ongoing with
techniques being developed to allow hydrogen
processing of VCM assemblies at room temperature,
regardless of coatings. The impact of small quantities
of nickel on magnets made from recycled powder is
also being assessed. Part of this work forms the
basis of an EU Framework 7 proposal.

The next step…

A pilot plant has been built at the University with the
aim of separating sintered NdFeB magnets from
scrap electronics. So far, trials have been performed
on manually separated VCM assemblies and pure
NdFeB magnets. Once the material has been loaded
from the top of the vessel, the lid is closed and the
vessel is evacuated using a rotary vacuum pump and
then backfilled with hydrogen.

In the next stage of development, pre-processed
computer HDDs will be used as the feedstock.
Mechanical stages are being introduced to the
reactor in order to shake the powder out of the hard
drives once hydrided. After the powder is extracted from the hard drives, it falls through a valve at the
bottom of the vessel and is collected in a gas-tight
container. The valve on the top of the collection
vessel can be closed and the powder transferred
under argon to the next stage of the recycling
process. The hydrided powder is not pyrophoric, but
it will oxidise on exposure to air. Therefore, protection
under argon is recommended if the material is being
made directly into new magnets. The pilot plant would be capable of extracting 10-40kg of NdFeB
per run, depending on the form of the pre-processed
scrap hard drives.

The powder extracted from the separation vessel is
still in the form of an alloy of NdFeB – although now
containing hydrogen. There are many possible routes
to reprocess this material that either separate the rare
earth elements from the alloy, such as through
solvent extraction, or reprocess the alloy powders
into new magnets.

The University of Birmingham researchers have
already demonstrated that it is possible to produce
new sintered magnets from recycled NdFeB material
on a small scale with greater than 90% of the properties
compared to the starting material. They have also
demonstrated that it is possible to produce magnetic
powders suitable for processing into resin-bonded
magnets with properties appropriate for commercial
applications. The limitation in reprocessing the
extracted powders directly into magnets is that the
composition of the final product is, to a large degree,
controlled by the composition of the magnets
recovered from the scrap. The range of composition
will depend on the nature of the scrap, and therefore
the final product may have to be sold with a range
of properties.

If the recovered NdFeB powder were to be
considered as a high-grade rare earth ore, then
the rare earth elements could be extracted for use
in any application, including re-melting into a
starting alloy for conventional magnet manufacture.
It would provide a supply of rare earth materials
that is many times more concentrated than any
mined source of rare earth elements (around 30%
by weight of the alloy would be rare earth metals).
Unlike many of the mined sources of rare earths,
the recycled material would not contain radioactive
thorium, and the rare earth content would be limited
to neodymium, dysprosium and praseodymium.
Mined sources contain most of the rare earth
elements that are difficult to separate due to their
similar chemical nature.

The research effort on recycling of rare earth
magnets is still a work in progress, requiring
development in many areas to optimise the conditions.
However, it is evident that hydrogen can be used as a
separation tool to extract NdFeB, and although the
current work focuses on HDDs, the technology would
be applicable to other rare earth containing devices.
Two patent applications have been filed by the
University of Birmingham based on this technology.

Further information

Dr Allan Walton is a Senior Science Research Fellow at the University of Birmingham working on the Energy Theme (funded through
HEFCE UK). Email: a.walton@bham.ac.uk. Tel: +44 (0)121 414 3960.

Dr Andy Williams is Head of the Magnetic Materials Group at the University of Birmingham. Tel: +44 (0)121 414 3959. Email:
a.j.williams@bham.ac.uk. Website: www.magnets.bham.ac.uk The University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK