A permanent magnet (PM) is a magnet that "retains its magnetic properties in the absence of an inducing field of current" (Oxford Dictionary). PMs can retain their magnetic properties for a very long time if cared for properly. A lifetime of a PM can be hundreds of years, losing only 1% of its magnetization 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 PMs vastly exceed the lifetime of the products they are nested in, such as EVs or wind turbines, hence in principle making them potentially suitable for reuse or refurbishment.
Characteristics affecting circularity
While PMs are in principle highly durable and fit for a circular economy, this is more complex in practice. PMs come in various types, some of the most powerful ones containing rare earth elements (REE) which is the focus here. Common REE PMs include sintered neodymium iron boron magnets (NdFeB) and samarium cobalt magnets (SaCo) of different sub-types.
To complicate things further, PMs come in numerous grades, reflecting their strength and heat resistance. Once the magnet alloy has been formed into a PM 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 PM coatings used for NdFeB magnets:
- Nickel-copper-nickel (Ni-Cu-Ni)
- Rubber
- Nickel (Ni)
- Epoxy
- Zinc (Zn)
- Gold (Au)
- Tin (Sn)
- Titanium (Ti)
- Titanium nitride (TiN)
- Parylene
- Everlube
- Chrome
- Polytetrafluoroethylene (PTFE)
- Nickel-copper-nickel and rubber
- Zinc and Rubber
- Nickel-copper-nickel and parylene
- Nickel-copper-nickel and PTFE
- Tin and parylene
- Zinc chromate
- Phosphate passivation
PMs are also designed with different orientations of the magnetism, to fit the design of the motor or generator it is intended for. This adds another layer of diversity to the PM stock found in the economy.
The different PM characteristics affect the reusability and recyclability of the magnets. Hence, the key message is that to enable the reuse and recycling of PMs, downstream actors engaged in revalorization of used PMs need to have access to accurate information about what is contained in the magnets recovered from products, their characteristics and technical properties.