State of the art

Inside a Hard Disk Drive (HD), the magnet is only a small volumetric fraction compared to the whole component. For this reason, the most efficient recycling is by removing the magnet from the chassis, thus avoiding the complications caused by grinding the entire HD to fraction the materials only afterwards.

Various techniques are proposed to recover the magnetic material and reintroduce it into the new production chain:

Hydrogen decrepitation-HD and Hydrogenation disproportionation desorption and recombination HDDR, techniques that use hydrogen, are mainly chosen as a solution when the material is oxidised.
Rapid solidification (melt-spinning)
Chemical processes that recover not the alloy but the individual elements that make it up; this solution is characterised by high costs and the production of environmentally harmful waste.

All these solutions involve the use of large amounts of energy, the use of hazardous and difficult-to-manage chemicals (such as H2), a high environmental impact (such as many of the techniques for chemically extracting Nd) or generally very high process times or relatively low yields.

In general, once the device containing the magnet is found, three possibilities open up for its reuse:

Type 1: Reuse
Type 2: Recycle the magnet by grinding it to obtain magnetic alloy powder again.
Type 3: Process the material to separate the raw materials and reuse them for new applications. This option represents indirect recycling, referred to as downgrading, which involves a more significant energy expenditure; an example is chemical dissolution.

The entire activity within the project is configured as an action of true recycling of the material as a NdFeB alloy since it is difficult to implement both the reuse of components with special shapes (type 1 recovery), as well as poor workability as a result of the high hardness, and the separation of the individual elements, under conditions of economic and environmental sustainability, (type 3 recovery). The research applied in this project proposes a substantial advancement of the second recovery method (type 2), selected as the most sustainable of the three.

The limitations of the current type 2 processes are the energy expenditure and the still uncompetitive cost due mainly to the chemical reduction processes to remove the oxide component formed during grinding.

What are the production methods for permanent magnets?

Sintering. The magnets inside HDs are made almost exclusively from sintered materials, i.e. they are compacted and then treated at a high temperature in a controlled atmosphere to densify and acquire suitable mechanical and magnetic properties. In this treatment, the material is not brought to a melting point but to a temperature sufficient to allow atomic diffusion within the material to eliminate the voids between the powders and subsequently, through cooling, impart substantial changes to the alloy's microstructure. At the same time, the best specific magnetic and mechanical properties are obtained.

Plastomagnets. New technologies that do not involve sintering are also used to produce magnets from powders. They are suitable for the production of components that are less powerful from a magnetic point of view but equally interesting from an industrial and technological point of view, such as plastomagnets consisting of a thermoplastic or thermosetting matrix and Neodymium-Iron-Boron (NdFeB) powder. The latter is booming and derives its considerable industrialisation potential from plastic injection or compression moulding. Their main advantages are the possibility of being manufactured in complex shapes and their better magnetic properties than traditional ceramic magnets. Furthermore, NdFeB plastic magnets allow the production of components 'on demand', i.e. customised in almost all parameters: they allow the magnetic properties to vary over a wide range thanks to the amount of polymeric binder and other possible additives between the magnetic metal particles. Furthermore, they are easier to process than sintered materials. Plastomagnets also open the way to further technological possibilities of composite materials: hybrid magnets consist partly of permanent magnetic materials and partly of magnetically soft materials. The possibility of implementing this technology in the currently booming world of 3D printing (additive manufacturing) should also not be overlooked. Precisely given the importance of 3D printing techniques in the generation of complex-shaped components, which can maximise the magnetic characteristics of the material in terms of design optimisation, one activity of the research project will be to determine the applicability of the plastomagnetic mixtures produced to the additive manufacturing sector.

ITALIAN VERSION