Production of sintered ndfeb magnets from scrap alloy powders

313 Pages·2017·8.12 MB·English

by  Enrique Herraiz Lalana

PRODUCTION OF SINTERED NDFEB
MAGNETS FROM SCRAP ALLOY
POWDERS
by
ENRIQUE HERRAIZ LALANA
A thesis submitted to the University of Birmingham for
the degree of DOCTOR OF PHILOSOPHY
School of Metallurgy and Materials
College of Engineering and Physical Sciences
University of Birmingham
November 2016
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Abstract
Rare earth metals are at the highest supply risk of all metals used for clean-energy technologies in part
due to supply restrictions from the main suppliers in China. Rare earth elements are essential to
produce the highest energy product rare earth permanent magnets which are used in advanced green
applications such as the generators in wind turbines and the motors in electric vehicles. This supply
‘crisis’ has been addressed by many authors by, for example, reducing or substituting the rare earths
for less critical elements, or by finding new primary resources. The focus of this thesis, however, is on
the recycling of rare earth magnets.
Previous work has shown that hydrogen is an effective process gas which can be used to break down
the NdFeB magnets into a friable powder containing Nd Fe BH and NdH . It has also been shown
2 14 x 2.7
that it is possible to re-sinter this material into new magnets. However, the secondary material has
higher oxygen content than the primary material, which reduces the quantity of Nd for liquid phase
sintering.
The aim of this work was to maximise the magnetic performance, by investigating the (1) effect of
particle size of the hydrogenated NdFeB powder, (2) addition of neodymium hydride to aid liquid
phase sintering and hence magnetic performance; and (3) separation techniques to remove the oxidised
Nd prior to liquid phase sintering. Uncoated end-of-life sintered NdFeB magnets from voice coil
motors were exposed to hydrogen at room temperature to break them down into a friable hydrogenated
NdFeB powder. The hydrogenated powder was then milled, sieved, aligned, pressed and sintered into
recycled magnets. The resulting magnets were then tested on a permeameter (at room temperature,
75 °C and 150 °C) to measure the magnetic properties. Scanning electron microscopy was used to
assess the microstructures of the final magnets. Mechanical properties such as Vickers hardness and
fracture toughness were also studied.

i
When primary magnets made from cast alloys are hydrogen decrepitated, they are milled to decrease
the particle size to single crystals. However, in a sintered magnet after hydrogen decrepitation it is
unclear if the polycrystalline particles which are already aligned can rotate under an applied field, thus
giving good magnetic performance. Therefore, different milling conditions and sieve sizes were tested
to investigate this. The powder was then re-sintered as described above, resulting in magnets with an
increase in density, remanence and maximum energy product at the expense of coercivity, with
decreasing powder particle size. In all cases, the remanence and coercivity was substantially lower
than for the starting magnet. The decrease in magnetic properties has previously been shown to be due
to the fact that the grain boundary phase oxidises during primary production. Therefore it is necessary
to blend some alloying elements into the hydrogenated powder to replace this oxidized phase.
In the second part of the study, neodymium hydride was blended systematically into the hydrogenated
powder from 1-5 at%. This resulted in recovery of the coercivity of the recycled magnets with an
associated decrease in remanence and maximum energy product. It was found that at 3 at%
neodymium hydride, the coercivity of the recycled magnets was equal to that of the starting material;
and that it was surpassed when larger quantities of neodymium hydride were added. The thermal
stability of remanence and coercivity were also enhanced with such additions. The best magnetic
properties were obtained at additions of 5 at% neodymium hydride, reaching a coercivity of 1400 kA
m-1, remanence of 975 mT, maximum energy product of 179 kJ m-3; and thermal stability of coercivity
and remanence better than in the starting material.
In the third part of the study, an investigation was carried out in order to remove the oxygen from the
recycled magnets by separating the oxygen-rich grain boundary phase from the hydrogenated matrix
phase. To do so, two different techniques were used, namely wet low-intensity magnetic separation
and hydrocyclone separation. The separated powder was assessed by X-ray diffraction, X-ray
fluorescence, inductively coupled plasma and scanning electron microscopy.
ii
Hydrocyclone separation was shown to be a much more effective method than wet low-intensity
magnetic separation in the separation of the oxygen-rich grain boundary phase particles. In addition,
the hard magnetic phase particles were shown to be intact after exposure to water and air at 80 °C.
This may open the door to a new family of water-based recycling processes for scrap NdFeB magnets,
but further work is required.
The resultant powder was blended with neodymium tri-hydride (0, 5, 6 and 7 at%), aligned, pressed
and sintered into recycled magnets. A high remanence of near 1 T was achieved, underlining that hard
magnetic phase particles do not oxidise or corrode substantially during hydrocyclone separation and
drying.
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A Elena y a mi familia
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Acknowledgements
I would like to thank Dr Allan Walton for his supervision, encouragement and advice along the project
especially at the beginning, when I was lost and he came to point the direction to follow. Your help
has been key to the development of this project and me as a researcher and as individual. Hope you
finally learnt Herraiz is my first family name instead of a middle name. Thanks also to Professor
Harris for sharing his knowledge and providing discussions of great interests, although you got wrong
my name. No worries, Enriquo was only one letter away from my real name.
Special thanks to Dr Malik Degri for his advice and support when I started and I did not know much
neither about magnets nor English. These months were of great help and I will always be grateful to
you.
I also would like to thank Dr Richard Sheridan for his unconditional help and support at any time in
every aspect of this project. I would not have been able to get to this of the project without your help.
Once again, thank you very much.
Thanks also go to all members of Magnetic Materials Group for embracing me since the very
beginning and make me feel part of this family. Thanks Vicky Mann, Olli Brooks, Matthew Farr, Alex
Campbell, Muhammad Awais, Jonathan Meakin and Lydia Pickering for your help, discussions and
support along these three years. Thanks to Andy Bradshaw for his technical advice, assistance and
help during this three years and especially when building and fixing the sintering furnace. I would also
like to thanks Wei Zhou for his support and feeding the entire group with such strange Chinese
delicatessen. Thanks to Christian Jonsson for your help, discussions and general friendship.
I would like to thank all EREAN members: ESRs, ERs and supervisors. Special thanks to Iuliana
Poenaru for her help on XRD, Rita Schulze for master lessons on GaBi and everyone that has
contributed to reach this point in my career.

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Gracias a la mujer de mi vida, Elena, por su cariño, amor, comprensión y apoyo incondicional durante
estos ocho años en los que tantas cosas hemos vivido juntos. Sabes que sin ti todo esto no habría sido
posible. Me faltan las palabras para describir lo mucho que te agradezco ser mi infinito y mi todo.
También le quiero agradecer a mis padres (y sus parejas) el apoyo y cariño durante todo este tiempo.
No solo durante el doctorado, si no desde que hace 8 años empecé la Universidad. Todo vuestro
esfuerzo y dedicación durante estos años han sido y serán impagables.
Gracias a mis ‘parents in law’ (suegros suena demasiado mal), por estar siempre ahí y, por supuesto,
por haber traído al mundo a la persona más maravillosa del mundo, Elena. También quiero dar las
gracias a mi abuela, mi cuñada, mis tíos y mis primos, tanto de sangre como políticos; aunque en
realidad os siento a todos como sangre propia. No podía olvidarme de mi sobrino y mi hija, Friss y
Terra, cuyos lametazos y anécdotas han sido un apoyo importante estos años.
En un momento así quiero recordar a aquellos que no están pero sin duda se alegrarían de este
momento. Luis, Satur y Charly; siempre estáis presentes y nunca os olvidaremos.
Tampoco me quiero olvidar de mi alma mater, la Universidad Carlos III de Madrid, y en especial de
Elena Gordo, por introducirme en el maravilloso universo de la investigación.
Enrique Herraiz Lalana
University of Birmingham
November 2016
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Table of Contents
Chapter 1. Introduction ...................................................................... 1
1.1. Introduction to the Project ............................................................................................................ 1
1.2. History of Magnetism................................................................................................................... 2
1.3. History of Permanent Magnets ..................................................................................................... 5
1.3.1. Lodestone .............................................................................................................................. 5
1.3.2. Magnetic Steels ..................................................................................................................... 6
1.3.3. Al-Ni-Fe and Al-Ni-Co ......................................................................................................... 7
1.3.4. Ceramic Magnets: Ferrites .................................................................................................... 8
1.3.5. Rare Earth Permanent Magnets ............................................................................................. 8
1.3.5.1. SmCo Alloy .................................................................................................................. 9
5
1.3.5.2. Sm Co Alloy ................................................................................................................ 9
2 17
1.3.5.3. Nd-Fe-B Alloy................................................................................................................ 9
1.3.6. Other Permanent Magnets ................................................................................................... 10
1.3.6.1. Cu-Ni-Fe and Cu-Ni-Co Alloys ................................................................................... 10
1.3.6.2. Fe-Co-Mo Alloy ........................................................................................................... 10
1.3.6.3. Fe-Co-V Alloy.............................................................................................................. 11
1.3.6.4. Pt-Co Alloy .................................................................................................................. 11
1.3.6.5. Cr-Co-Fe Alloy ............................................................................................................ 11
1.3.6.6. Sm-Fe-N Alloy ............................................................................................................. 11
1.4. Rare Earths ................................................................................................................................. 12
1.4.1. Definition and classification ................................................................................................ 12
1.4.2. Applications ........................................................................................................................ 13
1.4.3. Occurrences ......................................................................................................................... 15
1.4.4. Mining and Production ........................................................................................................ 17
1.4.4.1. Rare earth elements-bearing minerals .......................................................................... 17
1.4.4.2. Deposits ........................................................................................................................ 18
1.4.4.2.1. Carbonatite Associated Deposits ........................................................................... 18
1.4.4.2.2. Alkaline Igneous Rocks Associated Deposits ....................................................... 18
1.4.4.2.3. Ion-Adsorption Clay Deposits ............................................................................... 19
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1.4.4.2.4. Monazite-Xenotime Placer Deposits ..................................................................... 19
1.4.4.3. Historical Development ................................................................................................ 19
1.4.4.4. Global Production ........................................................................................................ 20
1.4.4.5. Reserves ....................................................................................................................... 21
1.4.4.6. Major Mines ................................................................................................................. 22
1.4.4.7. Environmental Concerns .............................................................................................. 22
1.4.5. Criticality ............................................................................................................................. 23
1.4.6. Politics, Policies and Price .................................................................................................. 24
1.5. General Aim ............................................................................................................................... 25
Chapter 2. Principles of magnetism ................................................ 26
2.1. The Origin of Magnetism ........................................................................................................... 26
2.2. Magnetic Behaviour ................................................................................................................... 28
2.2.1. Diamagnetism...................................................................................................................... 28
2.2.2. Paramagnetism .................................................................................................................... 29
2.2.3. Ferromagnetism ................................................................................................................... 29
2.2.4. Antiferromagnetism ............................................................................................................ 32
2.2.5. Ferrimagnetism.................................................................................................................... 32
2.2.6. Summary of Magnetic Behaviours ...................................................................................... 33
2.3. Magnetic Domains ..................................................................................................................... 34
2.4. Units of Magnetism .................................................................................................................... 37
2.5. Magnetic Properties.................................................................................................................... 37
2.5.1. Intrinsic Properties .............................................................................................................. 38
2.5.1.1. Saturation Magnetisation .............................................................................................. 38
2.5.1.2. Curie Temperature ........................................................................................................ 38
2.5.1.3. Anisotropy .................................................................................................................... 38
2.5.1.4. Magnetocrystalline Anisotropy .................................................................................... 38
2.5.1.5. Shape Anisotropy ......................................................................................................... 40
2.5.2. Extrinsic Properties ............................................................................................................. 40
2.5.2.1. Hysteresis Loops .......................................................................................................... 40
2.5.2.2. Coercivity ..................................................................................................................... 44
2.5.2.3. Remanence ................................................................................................................... 44
2.5.2.4. Maximum Energy Product ........................................................................................... 44
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