A permanent magnet is a magnet that 'retains its magnetic properties in the absence of an inducing field of current' (Oxford Dictionary). Permanent magnets can retain their magnetic properties for a very long time if cared for properly. A lifetime of a permanent magnet can be hundreds of years, losing only 1 per cent of its magnetisation strength every 100 years. However, temperature changes and other factors outside the design specifications can shorten the lifespan. In most cases, however, the lifetime of permanent magnets vastly exceeds the lifetime of the products they are nested in, such as electric vehicles or wind turbines, which in principle makes them potentially suitable for re-use or refurbishment.
Characteristics affecting circularity
While permanent magnets are in principle highly durable and fit for a circular economy, it is more complex in practice. Permanent magnets come in various types and some of the most powerful ones contain rare earth elements (REEs), which is the focus here. Common REE permanent magnets include sintered neodymium-iron-boron (NdFeB) magnets and samarium-cobalt (SaCo) magnets of different subtypes.
To complicate things further, permanent magnets come in numerous grades, reflecting their strength and heat resistance. Once the magnet alloy has been formed into a permanent magnet it often needs to be coated to protect it from corrosion. This is typically the case with NdFeB magnets, as they contain iron (Fe), which is highly corrodable.
There are more than 20 types of permanent magnet coatings used for NdFeB magnets, such as:
- chrome
- epoxy
- Everlube
- gold
- nickel
- nickel-copper-nickel
- nickel-copper-nickel and parylene
- nickel-copper-nickel and PTFE
- nickel-copper-nickel and rubber
- parylene
- phosphate passivation
- polytetrafluoroethylene (PTFE)
- rubber
- tin
- tin and parylene
- titanium
- titanium nitride
- zinc
- zinc chromate
- zinc and rubber
Permanent magnets are also designed with different orientations of the magnetism, to fit the design of the motor or generator. This adds another layer of diversity to the permanent magnet stock found in the economy.
The different permanent magnet characteristics affect the re-usability and recyclability of the magnets. The key message is that, to enable the re-use and recycling of permanent magnets, downstream actors engaged in revalorisation of used permanent magnets need to have access to accurate information about what is contained in the magnets recovered from products, their characteristics and technical properties.