Recycling of neodymium-iron-boride magnet waste by selective sulfation roasting

Abstract

Neodymium–iron-boride (NdFeB) magnets are the strongest commercially available, and as such are used in technologies where high performance is needed, such as electric vehicles, computer hard drives, and high efficiency wind turbines. The manufacture of these magnets consumes a large amount of rare earths, and as supply concerns exists for these materials, recycling of magnet scrap is seen as a possible domestic source. A difficulty in processing is that iron makes up the majority of the magnet, and the subject of this research is the development of a selective recycling method for the contained rare earths. Two pathways for selective sulfation roasting were investigated. The principle for both of these is to perform a roasting process which results in the formation of rare earth sulfate and an iron oxide by taking advantage of the differing thermal stabilities of the phases. A water leach can then effectively separate the soluble rare earth sulfates from the insoluble iron oxides. The first method investigated involved first pugging the magnet material with sulfuric acid, followed by a drying and roasting stage. It was found that on water leaching almost all of the rare earths dissolved, with almost all of the iron being left in the residue. A scale up effort was undertaken with a bench scale semi-continuous rotary kiln, which proved successful. An economic study on this process was performed. The second method investigated was the sulfation of magnet material by roasting in an air-sulfur dioxide atmosphere. A thermal gravimetric analysis experimental setup was constructed and iron oxide and neodymium oxide pellets reacted under various conditions. The mass gain curves were compared against a constructed grain model. It was found that the majority of the neodymium could be extracted utilizing a water leach. Very little iron was found to dissolve at any of the conditions investigated. Scoping studies involving the gas phase sulfation of magnet powder were also performed. It was found that magnet material was resistant to forming rare earth sulfates, but the addition of alkali earth sulfate promoter helped greatly, and the majority of rare earths could be extracted.