WO2012101398A1

WO2012101398A1 - Magnet recovery

Info

Publication number: WO2012101398A1
Application number: PCT/GB2012/000063
Authority: WO
Inventors: Ivor Rex Harris; John Speight; Allan Walton; Andrew Williams
Original assignee: The University Of Birmingham
Priority date: 2011-01-24
Filing date: 2012-01-24
Publication date: 2012-08-02
Also published as: GB201101201D0; GB2487656A; GB201201215D0; WO2012101398A8

Abstract

A method for removing one or more selected rare earth magnet(s) from an assembly comprising a plurality of rare earth magnets comprises exposing at least the selected rare earth magnet(s) 1o hydrogen gas in situ, to effect hydrogen decrepitation of said magnet(s) whereby a rare earth particulate material is produced, and separating the rare earth particulate material from the rest of the assembly. The method may be carried out by portable apparatus 20 which comprises a reaction cell 22 for attaching to a surface of the magnet 18 and/or assembly 10 to enclose a surface of the selected rare earth magnet 18, a fixing means for releasably securing the reaction cell 22 to the surface of the magnet 18 and/or assembly 10, and a hydrogen gas control system 24, 26 connectable to the reaction cell 22, which controls the supply of hydrogen gas to the reaction cell 22. The method and apparatus find application in the recovery of rare earth magnets from large assemblies such as rotors and turbines.

Description

Magnet recovery

The present invention relates to a process and an apparatus for recovering magnets. In particular, the invention relates to a process and an apparatus for recovering rare earth magnets from assemblies.

Rare earth magnets, in particular permanent magnets of the NdFeB type (neodymium iron boron magnets) and SmCo (samarium cobalt), are known for their high magnetic flux combined with high coercivity compared with conventional magnets. SmCo magnets are available in two different compositions, namely SmCo₅ and Sm₂(Co, Fe, Cu, Zr)₁₇. The transition metal content of the latter composition is rich in cobalt but also contains other metals such as iron and copper, and is commonly referred to as Sm₂Coi₇. Rare earth magnets have found application in a wide range of electronic goods and particularly in "green" technologies such as wind turbine generators and electric motors in electric and hybrid vehicles.

The supply of materials for rare earth magnets, particularly the supply of neodymium (Nd) and dysprosium (Dy), is limited and demand is expected to exceed supply. This will cause material prices to increase and this could limit the use of NdFeB magnets and the development of green technologies unless alternative magnets or sources of supply can be found. Dy is used as an additive to NdFeB for high coercivity magnets used in motor applications. The limited supply of Nd, Sm and Dy is a major concern to many developed economies and some countries are now classifying these as strategic materials.

NdFeB magnets are the most common of the rare earth magnets and are manufactured in two forms; fully dense magnets produced by a sintering process and bonded magnets, a cheaper form with a lower performance where magnetic particles of NdFeB are bonded into a net-shape structure with resins.

The strongly magnetic nature of rare earth magnets means that once they are assembled into a structure they are difficult to remove if, for instance, they are damaged or if they are assembled with the wrong polarity.

Currently, disassembly is carried out by mechanical means using tools such as chisels and hammers to break up and remove the magnet. This is a labour intensive process that can result in damage to surrounding high value components. The magnet shards can be attracted to other magnets in the assembly and therefore are difficult to capture and remove. Typically, the whole assembly is jet-washed to remove any magnetic fragments and then re-machined. There is a significant cost in terms of machining, time in removing the magnets and the loss of magnet material.

An additional complication in some applications of rare earth magnets is that a number of magnets are assembled together into a component. In some applications, the magnets are themselves large and are located in large structures, such as wind turbines, that are difficult to disassemble in order to access the magnets and the components into which they are incorporated.

There is thus a strong need for a cost-effective, efficient and sustainable method of removing rare earth magnets from assembled components.

According to a first aspect of the present invention there is provided a method for removing one or more selected rare earth magnet(s) from an assembly comprising a plurality of rare earth magnets, comprising the steps of

exposing at least the selected rare earth magnet(s) to hydrogen gas in situ, to effect hydrogen decrepitation of said magnet(s) whereby a rare earth particulate material is produced, and

separating the rare earth particulate material from the rest of the assembly.

The term "assembly" will be understood to mean any mechanical, electric or electronic device, machine or apparatus, or a part thereof comprising multiple components, including, but not limited to, computer hard drives, speakers, dynamos, tools, motors, generators, turbines and the like. The method of the present invention is particularly suited for use with relatively large assemblies such as large motors, generators, wind turbines, MRI scanners and the like.

The phrase "one or more selected rare earth magnet(s)" will be understood to mean some but not all of the magnets present in the assembly. Thus the method is suitable for removing some selected (e.g. damaged or mis-orientated) magnets from an assembly while leaving the remaining magnet(s) in tact.

By 'in situ' it will be understood that the selected rare earth magnet(s) is exposed to hydrogen gas while it is in its normal position within the assembly, without having to be moved or separated from any other components to which it is normally attached. The assembly may be part of a larger structure. The magnet may be exposed to hydrogen gas after the assembly is removed from the larger structure. Alternatively, the magnet may be exposed to hydrogen gas while the assembly still forms part of the larger structure. In both cases, the magnet is considered to be in situ since it is not removed from the assembly prior to decrepitation.

Hydrogen Decrepitation (HD) is a known process for breaking rare earth alloys, such as NdFeB, into powder, as described by McGuiness et al. ("Hydrogen : Its use in the processing of NdFeB-type magnets : Journal of Less Common Metals , Vol 172-174, 1991). In the decrepitation process hydrogen preferentially enters the rare earth rich grain boundaries in the material. The hydrogen then reacts with the NdFeB matrix grains forming an interstitial hydrogen solution with a -5% volume expansion. The differential volume expansion of the crystal structure due to ingress of the hydrogen causes the brittle structure to fracture so that grains break away from the material forming a fine powder. The fracture process is predominantly inter-granular in nature.

In the case of SmCo₅ magnets the reaction with hydrogen is very different to NdFeB. In this case the magnets are essentially single phase and the reaction with hydrogen involves the formation of a SmCoH_x hydride with a well defined plateau pressure. The appreciable volume expansion involves the decrepitation of the brittle intermetallic compound into a fine powder.

In the case of Sm₂Coi7, the hydrogen forms a range of solutions (depending upon the H₂ pressure) and there is no distinct plateau as observed in SmCo₅. The volume expansion again results in decrepitation of the brittle intermetallic.

It is known that rare earth magnets can be recycled using the process of hydrogen decrepitation (HD), but the process has previously only been applied to individual magnets which have already been separated from an assembly. In the present invention, the selected magnet(s) is exposed to hydrogen gas in situ. Thus, the present invention allows a rare earth magnet, or a number of selected rare earth magnets (e.g. magnets which are damaged) to be broken up by hydrogen decrepitation and removed from an assembly without the need to first physically separate the selected magnet(s) from the assembly. This makes removal and replacement of a damaged rare earth magnet quicker, easier and cheaper, and reduces or eliminates the risk of causing damage to the other components or functioning magnets of the assembly. The decrepitated particles produced by the process can be collected for reprocessing to form new magnets and the assembly can be made serviceable by completing any necessary repairs and fitting a replacement magnet.

In an embodiment, the method further comprises an initial step of providing a barrier around magnets which are not selected for removal from the assembly to prevent exposure of said magnets to hydrogen gas and protect them from decrepitation. Magnets which are damaged can therefore be selectively decrepitated by exposure to hydrogen, while functioning magnets that do not need replacing are protected from the hydrogen by the barrier. In a particular embodiment, the whole of the assembly is exposed to hydrogen gas.

In some embodiments, the step of providing a barrier comprises applying a coating to the magnets which are not selected for removal from the assembly (i.e. the non- selected magnets). The coating may be any suitable substance which can be easily applied to the magnets and which provides a barrier to hydrogen gas. Obtaining a barrier completely impenetrable to hydrogen diffusion is difficult but any coating is suitable that slows down the rate of diffusion of hydrogen sufficiently such that the hydrogen does not reach the non-selected magnet material during the duration of the decrepitation process. The coating may be chosen so that it can be easily removed from the magnets. The coating may be a liquid which dries to form a hard coating, for example a nitrocellulose polymer dissolved in a solvent or a masking coating used in the semiconductor industry (e.g. photoresist masking materials). The coating may be applied to the magnets by any convenient method, for example by spraying or by brushing the coating on to the surface of the magnet. Alternatively, the barrier may constitute a tape, e.g. masking tape, which is applied to the non-selected magnets.

The method may comprise a further step of removing the barrier from the non- selected magnets after exposure of the assembly to the hydrogen gas (i.e. after the decrepitation process is completed). When a coating forms the barrier, the coating may be removed from the non-selected magnets by using a solvent.

In an alternative embodiment, only the selected rare earth magnets (i.e. the magnets which are to be removed from the assembly) are exposed to hydrogen gas, such that the decrepitation process is localised. In a particular embodiment, a reaction cell is attached to one or more surfaces of the selected magnet(s) or to the adjacent surfaces of the assembly either side of the selected magnet(s), such that only the selected magnet(s) is/are exposed to the hydrogen gas. The hydrogen gas may be applied to the magnet surface as a jet or as a static gas. Using this method a small volume of hydrogen can be applied selectively to the magnet(s) which is/are to be removed, without affecting the other magnets of the assembly. The reaction cell may remain attached to the magnet(s) or assembly until the decrepitation process has been completed. Therefore, if one or more magnets of an assembly are damaged and/or suffer a decrease in power, it would be possible to remove these magnets in situ without having to disassemble or move the whole assembly. This method is particularly useful for replacing magnets in large assemblies, such as wind turbines. A single reaction cell may be used to apply hydrogen gas to a surface of multiple magnets in an assembly. Alternatively, multiple magnets may be exposed to hydrogen gas simultaneously by using multiple reaction cells.

In an embodiment, the rare earth magnet is NdFeB. In an alternative embodiment, the rare earth magnet is SmCo. The SmCo magnet may be SmCo₅ or Sm₂Coi₇.

In an embodiment, the method further comprises a step of penetrating the surface of the selected magnet(s) prior to decrepitation. Penetration of the surface of the magnet(s) initiates decrepitation and helps to speed up the decrepitation process. The surface may be penetrated by any suitable means. The magnet(s) may be scratched or scored physically, for example using a scalpel or other sharp implement, or it may be penetrated using a low power laser. The surface may be penetrated to a depth of at least 5 microns or at least 10 microns. The surface of the magnet may be penetrated in a number of places to provide multiple sites for the decrepitation reaction to start. Some commercial magnets are coated to protect against oxidation, for example by a nickel coating. Therefore, this step may be particularly applicable where the magnet is coated (for example, by a nickel coating), or where an oxide layer has formed on the surface of the magnet. In this case, the surface may be penetrated to a depth sufficient to expose the rare earth magnet material underneath the coating or oxide layer. Disrupting the coating or the surface oxide layer facilitates access of the hydrogen gas to the magnet beneath.

The selected rare earth magnet(s) to be removed from the assembly may be exposed to pure hydrogen gas, or it may be exposed to a mixture of hydrogen with one or more inert gases, for example nitrogen or argon. In an embodiment, the magnet(s) is exposed to a non-explosive atmosphere comprising hydrogen. In a particular embodiment, the magnet(s) is exposed to a mixture of gases comprising no more than 10% hydrogen, no more than 5% hydrogen, no more than 3% hydrogen, no more than 1 % hydrogen, no more than 0.5% hydrogen or no more than 0.5% hydrogen. The use of a non-explosive gas mixture simplifies the processing equipment and makes handling of the gas safer, This might be particularly important if the selected magnet is in an assembly which still forms part of a larger structure, where it would be hazardous to use a potentially explosive mixture.

The pressure (or partial pressure where a mixture of gases is used) of hydrogen must be sufficient to break up the magnet structure and turn it into a particulate material. Where a low partial pressure of hydrogen is used, the gas mixture may be non explosive. This has a significant advantage as the gas can be used with less stringent health and safety regulations. However, if the pressure (or partial pressure) of hydrogen is too low the reaction kinetics will be too slow for the process to be commercially viable. If the pressure is too high the reaction vessel will have to be engineered to withstand the higher mechanical stresses, which will increase the complexity and cost and result in smaller reaction chambers. In a series of embodiments the pressure, or partial pressure where a mixture of gases is used, of hydrogen is from 0.01 mbar to 100 bar, from 0.1 bar to 70 bar, from 0.1 bar to 50 bar, from 0.5 bar to 20 bar or from 1 bar to 10 bar. In a particular embodiment, the decrepitation process is carried out at approximately atmospheric pressure.

In some embodiments wherein the method comprises the step of providing a barrier around non-selected magnets to prevent exposure of said magnets to hydrogen gas, the pressure (or partial pressure) of hydrogen is from 0.01 mbar to 100 bar, from 0.1 bar to 70 bar, from 0.1 bar to 50 bar, from 0.5 bar to 20 bar or from 1 bar to 10 bar. In other embodiments, the pressure (or partial pressure) of hydrogen is no more than 50 bar, no more than 20 bar or no more than 10 bar. In a particular embodiment, the assembly is exposed to pure hydrogen gas.

In some other embodiments wherein only the selected rare earth magnets are exposed to hydrogen gas, for example by applying a reaction cell to the selected magnets, the pressure of the gas (either hydrogen or a mixture of gases) is no more than 30 bar, no more than 20 bar or no more than 10 bar. In some cases wherein a reaction cell is attached to the surface of a selected magnet, the gas will leak out from the cell as the magnet breaks up due to the decrepitation process. It may therefore be preferred that the gas is a non-explosive mixture, for example, 5% hydrogen in 95% argon may be used. The gas may also be supplied at a pressure of at least atmospheric pressure, in order to create a positive flow of gas through the cell. For example, the inlet pressure of the gas may be approximately 1.5 bar such that the gas flows out of the cell at approximately atmospheric pressure.

The atmosphere may be static or it may be flowing. If a static atmosphere is used, the level of hydrogen gas may need to be topped-up to maintain the pressure required and replace the gas which is consumed by the decrepitation process. This may be achieved, for example, by connecting the reaction chamber to a tank containing a metal hydride. Alternatively, the gas can be replenished manually by operator intervention at appropriate intervals during the decrepitation reaction (e.g. by opening a valve to allow more gas to flow into the reaction chamber). In an embodiment where the atmosphere is flowing, the flow rate of the gas may be from 5 ml per minute to 50 I per minute.

In a series of embodiments, the decrepitation process (i.e. the exposure of the selected magnets to the hydrogen gas) is carried out at a temperature of from -30°C to 600°C, from -0°C to 400°C, from 5° to 200°, from 10° to 100°C or from 20° to 50°C. In a particular embodiment, the decrepitation process is carried out at approximately room temperature. If the temperature is too high, formation of the hydride will not occur so the magnet(s) will not turn into a particulate material. It is also possible that at high temperatures other parts of the assembly could melt and hinder the recovery of the rare earth particulate material. If the temperature is too low the reaction kinetics will be too slow. Processing at ambient temperature simplifies the processing apparatus as there is no requirement to provide heating facilities. The temperature at which the decrepitation reaction is carried out may be controlled by varying the temperature of the gas stream.

Higher pressures and temperatures are required for the decrepitation of Sm₂Coi₇ magnets compared to NdFeB or SmCo₅ magnets. Thus, in an embodiment wherein the rare earth magnet is Sm₂Coi₇, the decrepitation process is carried out at a relatively high temperature and/or a relatively high pressure. Suitably high temperatures include temperatures of at least 70°C,at least 80°C, at least 90°C and at least 100°C. Suitably high pressures include pressures of at least 7 bar, at least 8 bar, at least 9 bar and at least 10 bar. In an embodiment, the surface of the selected magnet(s) may be heated prior to the decrepitation reaction. The surface may be heated to a temperature of from 50°C to 400°C, from 75°C to 250°C or from 100°C to 200°C. Heating the surface of the magnet(s) helps to speed up the decrepitation process. In an alternative embodiment, the hydrogen gas stream is heated prior to coming into contact with the selected magnet, in order to speed up the start of the decrepitation process. It may be easier to increase the temperature of the gas stream than that of the underlying magnet(s).

The selected magnet(s) is exposed to the hydrogen gas for a period of time which depends on a number of factors including the hydrogen gas pressure, the temperature of the decrepitation process, the surface condition of the magnet(s) and the size of the magnet(s). In a series of embodiments, the selected magnet(s) is exposed to the hydrogen gas for a period of time of from 0 minutes to 2 weeks, from 30 minutes to 1 week, from 1 hour to 64 hours, from 2 to 52 hours or from 3 to 48 hours. In another series of embodiments, the selected magnet(s) is exposed to hydrogen for a period of time of from 2 to 8 hours, from 3 to 6 hours or from 4 to 5 hours.

In an embodiment, the selected rare earth magnet(s) is partially or completely demagnetised during the decrepitation process. The resulting particulate material may exhibit soft magnetic properties. Partial or complete demagnetisation allows the particulate material to be more easily separated from the other components of the assembly. For example, NdFeB magnets are demagnetised during the decrepitation process. In an alternative embodiment, the selected rare earth magnet(s) is not demagnetised by the decrepitation process, and produces permanent magnetic particulate material on decrepitation. For example, SmCo magnets are not demagnetised by the decrepitation process. In this embodiment, the method may comprise an additional step of demagnetising the particulate material. The particulate material may be demagnetised, for example, by heating or by reversing the magnetic field.

The rare earth particulate material resulting from the decrepitation process (also referred to as the 'particulate material' or 'particles') is separated from the remaining assembly components. Separation of the rare earth particulate material may be carried out by mechanical means. For example, the particulate material may be removed from the assembly by scraping (e.g. using a spatula) or brushing, or by using a high pressure gas jet or a vacuum (e.g. using a mini-vacuum cleaner). Separation of the particulate material may be carried out during and/or after the decrepitation process. Alternatively, the decrepitated particulate material may be separated from the rest of the assembly by a magnetic or electromagnetic field (e.g. using an electromagnet). The field may be applied during the decrepitation process so that the particles are collected as they are produced. Using this technique it is possible to remove the particles without causing damage to the other magnets or the underlying assembly. The particulate material can then be removed from the assembly to protect it from oxidation so that it can be more easily recycled into new magnets.

According to a second aspect of the present invention there is provided a portable apparatus for in situ removal of a selected rare earth magnet from an assembly comprising a plurality of rare earth magnets, the apparatus comprising

a reaction cell for attaching to a surface of the magnet and/or assembly to enclose a surface of the selected rare earth magnet,

a fixing means for releasably securing the reaction cell to the surface of the magnet and/or assembly, and

a gas control system, connectable to the reaction cell, which controls the supply of hydrogen gas to the reaction cell.

In an embodiment, the apparatus further comprises a sealing means for forming a gas-tight seal between the reaction cell and the surface of the magnet and/or assembly to which it is attached. The sealing means may be chosen from a wide range of known materials suitable for operating under the temperature, pressure and atmosphere conditions required for the reaction. Suitable materials may include rubber, epoxy resins, soft metal seals and silicon rubber materials that are typically used for the manufacture of gaskets. The sealing means may form part of the reaction cell or it may be applied to the intersection between the reaction cell and the surface of the magnet/assembly, after the reaction cell is attached to said surface.

The seal may be formed with the magnet itself (i.e. when the reaction cell is placed directly onto a selected magnet). In this case, eventually the seal will be broken as the decrepitation reaction spreads through the magnet. The apparatus may further comprise a shut-off system which is activated when the hydrogen gas flow increases dramatically as a result of the seal breaking. This prevents any non-selected magnets being exposed to hydrogen gas, For very large magnets the decrepitation reaction could be performed in this manner by sealing a number of reaction cells onto several areas of the magnet. Alternatively, the seal may be made with a material which holds the magnet in position in the assembly (for example epoxy resin).

In an embodiment, the reaction cell is formed from a transparent material. This allows the progress of the decrepitation reaction to be easily followed from the outside.

The fixing means may comprise a clamp or a strap or any suitable means which secures the reaction cell to the surface and allows it to be easily released after the decrepitation process is complete. Alternatively, the fixing means may secure the reaction cell to the surface magnetically or, if the experiment is carried out below atmospheric pressure, by suction.

In an embodiment, the apparatus further comprises a collection means for collecting the rare earth particulate material produced by the decrepitation reaction. In an embodiment, the collection means comprises a mechanical means for collecting the particles, such as a suction means. In another embodiment, the collection means produces a magnetic or electromagnetic field (i.e. the collection means is magnetic). In a particular embodiment, the collection means comprises a magnetic rod made of a material with a high saturation magnetisation compared to the rare earth magnet material, for example iron. An electromagnet or a permanent magnet may be mounted at an end of the rod to provide the magnetic or electromagnetic field. In a further embodiment, the collection means may comprise an inert chamber into which the particles are collected. This protects the particles from oxidation so that they may be more easily recycled into new magnets.

The gas control system may comprise a tank or a store containing a pressurised gas or a metal hydride, which will automatically replenish the hydrogen as it is depleted from the gas in the reaction cell during the decrepitation process. The gas control system may further comprise one or more valves which control the flow of gas into the reaction cell. The valves may be operated manually or they may be automated. In an embodiment, the gas control system comprises a gas pressure regulator which automatically regulates the pressure of gas inside the reaction cell.

The hydrogen gas may be a commercially pure grade or it may be a mixture of hydrogen with an inert gas such as argon or nitrogen. The gases may be stored as a mixture of the required composition, or each gas may be stored separately and mixed by a suitable gas control means before being supplied to the reaction cell.

In an embodiment, the apparatus further comprises a heater for heating the reaction cell and/or the surface of the magnet during and/or prior to decrepitation. In another embodiment, the apparatus comprises a heater (such as a filament) for heating the hydrogen gas stream before it enters the reaction cell. In a further embodiment, the apparatus comprises a temperature control system to monitor and control the temperature inside the reaction cell when the apparatus is in use, i.e. during the decrepitation process. The temperature may be manually controlled, or it may be controlled by a preset programme.

In another embodiment, the apparatus comprises a vacuum pump for evacuating the reaction cell prior to decrepitation.

Certain aspects of the invention will now be described by way of example with reference to the accompanying figures in which:

Figure 1 is a schematic diagram of an apparatus for removal of a selected magnet by hydrogen decrepitation, according to an embodiment of the present invention;

Figure 2a shows a side view of an assembly comprising rare earth magnets;

Figure 2b shows a front view of the assembly of Figure 2a with a reaction cell placed thereon, prior to decrepitation;

Figure 2c shows a side view of the assembly and reaction cell shown in Figure 2b;

Figure 2d shows the assembly and reaction cell of Figure 2c, during the decrepitation process; and

Figure 2e shows the assembly of Figure 2a after the decrepitation process.

Figure 1 shows an assembly 10 comprising a rotor body 12 around which a plurality of magnets 14 is arranged. The magnets 14 are bonded to the rotor body 12 by an epoxy resin layer 16 applied around each side of each magnet 14 and between the magnets 14 and the rotor body 12. At the top of the rotor body 12, as shown, is a damaged magnet 18 which is to be removed from the assembly 10.

A portable apparatus 20 is positioned for removal of the damaged magnet 18 from the assembly 10. The apparatus 20 comprises a Perspex reaction cell 22 which is connected to a gas supply 24 by a gas line 26. The gas supply 24 provides a non- explosive mixture of 3% hydrogen in argon. The flow of gas from the gas supply 24 to the reaction cell 22 is controlled by gas valves 28, 30, 32. In the embodiment shown, the apparatus 20 further comprises a vacuum pump 34 which is controlled by a valve 36. A collection means in the form of a magnet 38 is provided on the surface of the reaction cell 22 to collect the particles 40 produced by the decrepitation process.

As shown in Figure 1 , to remove the selected damaged magnet 18 from the assembly 10, the reaction cell 22 is positioned over the damaged magnet 18 and is sealed against the surface of the epoxy resin 16 using a suitable sealing means (not shown). The reaction cell 22 is held in position over the damaged magnet 18 by any suitable means, such as a strap or a clamp (not shown). To reduce processing time, the surface of the damaged magnet 18 is scratched prior to attaching the reaction cell 22 to the assembly 10.

The reaction cell 22 is flushed with hydrogen gas from the gas supply 24 through the gas line 26 by opening gas valves 28, 30 and 32. When the reaction cell 22 has been purged, the gas line valve 32 is closed to allow the reaction cell 22 pressure to build to a static pressure of 3 bar. The gas pressure can be maintained by a suitable gas pressure regulator means (not shown) positioned between the gas supply 24 and the gas line valve 28.

As an alternative to purging the reaction cell 22 with hydrogen gas, the reaction cell 22 can be evacuated by the vacuum pump 34 after opening valves 36 and 30, leaving valve 32 closed. Once the reaction cell 22 has been evacuated, valve 36 is closed and the cell is filled with hydrogen gas to a static pressure of 3 bar by opening valve 28, with valve 30 remaining open. As soon as the hydrogen gas is introduced into the reaction cell 22, it reacts with the rare earth magnet 18, causing it to be broken down into small particles 40. The particles 40 are collected throughout the decrepitation process by the magnet 38. As the reaction cell 22 is Perspex the progress of the decrepitation reaction can be followed visually to identify the point at which the decrepitation of the magnet 18 is complete. In alternative embodiments wherein the reaction cell 22 is not clear, the decrepitation reaction may be allowed to progress for a predetermined period of time before the supply of hydrogen gas is stopped. Once this point has been reached, the reaction cell 22 can be isolated from the gas supply 24 by closing gas line valves 28 and 30. The reaction cell 22 is then removed from the surface of the assembly 10 and the decrepitated particles 40 collected by the magnet 38 are removed for disposal or recovery.

After removal of the apparatus 20 and the decrepitated particles 40, the rotor body 12 can be cleaned before a replacement magnet is fitted in the position originally occupied by the damaged magnet 18. The new magnet can be fitted to the assembly 10 using a new layer of epoxy resin between the magnet and the surrounding magnets 14 of the rotor body 12.

Figure 2a shows a rotor assembly 42 containing NdFeB rare earth magnets with faulty magnets 44 to be removed mounted in epoxy resin 46. The magnets 44 were scored across the surface using a scalpel blade to give the surface disruptions 48.

As shown in Figures 2b and 2c, a Perspex reaction cell 50 was secured in place on the rotor assembly 42 over the faulty magnets 44. A magnet 52 was positioned in contact with an upper surface of the reaction cell 50 to collect the decrepitated powder particles as they were produced. The reaction cell 50 and the collection magnet 52 were held in place by a webbing strap 54 and a clamp 56. A gas mixture of 3% hydrogen in argon was supplied to the reaction cell 50 through a gas line 58. The reaction cell 50 was purged prior to decrepitation. The decrepitation process was carried out at room temperature, with a static atmosphere of 3 bar pressure.

Figure 2d shows decrepitated particles 60 which accumulated in the reaction cell 50 two hours after the start of the decrepitation process. The reaction was stopped after 2 hours and 10 minutes, and the reaction cell 50 and magnet 52 were removed from the rotor assembly 42.

The rotor assembly 42 after the decrepitation process is shown in Figure 2e. The faulty NdFeB magnets have been removed to leave a clear area 62 where a new magnet can be fitted after further preparation to remove any residual decrepitated particulate material 60 and epoxy resin 46 from the rotor assembly 42.

Claims

1. A method for removing one or more selected rare earth magnet(s) from an assembly comprising a plurality of rare earth magnets, comprising the steps of exposing at least the selected rare earth magnet(s) to hydrogen gas in situ, to effect hydrogen decrepitation of said magnet(s) whereby a rare earth particulate material is produced, and

separating the rare earth particulate material from the rest of the assembly.

2. The method according to claim 1 , further comprising a step of providing a barrier around the rare earth magnets which are not selected for removal from the assembly to prevent exposure of said magnets to hydrogen gas, said barrier provision being prior to said exposure to hydrogen.

3. The method according to claim 2, wherein the method comprises a further step of removing the barrier from the magnets after exposure of at least the selected magnets to the hydrogen gas.

4. The method according to claim 2 or claim 3, wherein the whole assembly is exposed to hydrogen gas.

5. The method according to any one of claims 2 to 4, wherein providing the barrier comprises applying a coating to the rare earth magnets which are not selected for removal from the assembly.

6. The method according to claim 5, wherein the coating is applied as a liquid.

7. The method of claim 6, wherein the coating is a polymer dissolved in a solvent, the barrier being formed up on dissolution of the solvent.

8. The method according to claim 1 , wherein only the selected rare earth magnets are exposed to hydrogen gas.

9. A method according to claim 8, wherein a reaction cell is attached to one or more surfaces of the selected magnet or to adjacent surfaces of the assembly either side of the selected magnet, such that only the selected magnet is exposed to hydrogen gas.

10. The method according to claim 8, wherein exposure of the selected magnet to hydrogen is via a hydrogen jet.

11. The method according to any preceding claim, wherein the selected rare earth magnet is a NdFeB or SmCo magnet.

12. The method according to any preceding claim, further comprising the step of penetrating the surface of the selected magnet(s) prior to decrepitation to disrupt a coating or oxide layer.

13. The method according to claim 2, wherein the surface is penetrated to a depth of at least 5 microns.

14. A portable apparatus for in situ removal by decrepitation of a selected rare earth magnet from an assembly comprising a plurality of rare earth magnets, the apparatus comprising

a hydrogen gas control system, connectable to the reaction cell, which controls the supply of hydrogen gas to the reaction cell.

15. The apparatus according to claim 14, additionally comprising a sealing means for forming a gas-tight seal between the reaction cell and the surface of the magnet and/or assembly to which it is attached.

16. The apparatus according to claim 15, further comprising a shut-off system which is activated if the hydrogen gas flow increases dramatically as a result of the seal breaking.

17. The apparatus according to any one of claims 14 to 16, further comprising a collection means for collecting the rare earth particulate material produced by the decrepitation.

18. The apparatus according to claim 17, wherein said collection means includes a suction means.

19. The apparatus according to claim 17, wherein said collection means includes means for generating a magnetic or electromagnetic field.