Sep 4 2015
Strong permanent magnets are capable of powering wind turbines and electric motors. Highly powerful magnets are made up of rare earth elements, dysprosium and neodymium. Researchers at Fraunhofer are working on a more economic and rapid method for recycling rare earth elements used in permanent magnets.
The rotational energy of the wind is converted into electricity for powering networks. Nowadays, gearboxes are being replaced by powerful permanent magnets for a higher energy yield and fault-free transmission of electricity. Such strong magnets are being used in cars as well for powering electric actuators, which in turn run lighter and smaller windscreen wipers. Numerous servomotors and actuators are placed in various positions inside cars for specific positioning of things, adjustment of side windows and seats.
Neodymium, boron and iron are the principal components of powerful permanent magnets. In addition to these elements, dysprosium is also found often in permanent magnets. Of these elements, boron and iron are easily available but dysprosium and neodyminum are not readily available. Extraction of such rare earth elements is a cumbersome process and requires a high level of input energy. As a result these elements are comparatively expensive and excessive extraction causes ecological imbalance. China is the leading supplier of rare earth elements with over 90% of the elements scattered there. Almost half of the global reserves is available in China.
Transforming Old into New
Hence, scientists have been working on recycling magnets. Until now, this implies that the rare earth elements are extracted from permanent magnets. The extraction process however is cumbersome and costly. Researchers from the Fraunhofer Project Group for Materials Recycling and Resource Strategies IWKS, located in Alzenau and Hanau are taking a different route to recycling of magnets. The Project Group is part of the Fraunhofer Institute for Silicate Research ISC.
In the words of Oliver Diehl, a scientist in the Project Group - “Instead of trying to regain each individual type of rare earth, we focus on recycling the entire material, meaning the complete magnet – and this in only a few steps”. He added - “This process is much easier and more efficient, because the composition of the material is already almost as it should be.”
The scientists have adopted the melt spinning process, also referred to as “rapid solidification”, for recycling. This method has been used for other alloys and has shown reliable results. The magnet is liquefied in a melting pot and then heated to temperature above 1000oC. The liquefied magnet is then directed through a nozzle onto a spinning copper wheel that is water-cooled. The rotational speed of the copper wheel is between 10 and 35m/s. Heat transfer from the liquid to the copper wheel happens within fractions of a second and solidification of the liquid occurs.
These solidified droplets are referred to as “flakes” by the scientists. The internal structure of these flakes is different from the orderly crystal lattice that would have formed in a normal solidification process. The rapid solidification in this process does not allow crystallization; instead a nanocrystalline or an amorphous structure is formed. In the amorphous structure atoms are arranged irregularly, while in the nanocrystalline structure the atoms align themselves as nanometer-sized grains.
The advantage of the melt spinning process is ability to vary the grain sizes, allowing the alteration of areas within the same crystalline structure to be altered. By varying the grain sizes the characteristics of the permanent magnet can be altered. The flakes are then milled and converted into a powder form for further processing.
“We press it into its final shape”, Diehl says.
Recycling of the First Magnet
The scientists have successfully recycled magnets in the demonstration plant that they had set up.
“The demo system can process up to half a kilogram of molten material and is somewhere between a lab and a large-scale plant”, Diehl goes on to specify.
Customizing the characteristics of the magnets is the next step in the research, which the team is doing by modifying the melt spinning process like altering the speed of the copper wheel, or varying the liquid heating temperature during the rapid solidification process. These two factors have a direct influence on the cooling rate, which in turn influences the crystal structure of the flakes.
In a majority of scenarios, the removal of the permanent magnet from the engines is a challenge. Therefore, scientists are working on developing a hassle-free collection cycle for used engines, and alternative engine designs that enable easy disassembly of magnets. The costs involved in the design are however difficult to predict at present.
“The anticipated financial advantage in recycling the magnets depends not only on the recycling process, but also on the price development for rare earth elements”, Diehl says. “The higher the raw material prices for rare earths, the more it will pay off to recycle already existing materials.”