The rare earth magnets so important to current and future technologies are produced in a five step process. It begins with getting the raw materials out of the ground and end with manufacture of magnets designed for some particular use. All five steps are currently concentrated in Asia, particularly in China.
Mining and concentration
Mining rare earths is often carried out either by an open-pit process or by in-situ leaching. Once the ore-bearing material is out of the ground, it is run through a crusher or mill to break it into small particles, and then the actual ore is separated out in a process called concentration. This usually involves gravity separation, froth flotation or magnetic separation.
Getting oxides from ore
The result of separation is ore, the raw, unprocessed rock that contains rare earth metals. The actual metal is present in the form of oxides, which are made up of one or more atoms of rare earth elements chemically bonded to one or more oxygen atoms. The process of separating oxide from ore is a chemical one, and may involve roasting, acid leaching, salt or caustic fusion or high temperature sulphation. A second step in the process is the separation of the rare earth oxides from one another, which requires myriad steps of sequential solvent extraction or ion exchange.
From oxides to metals
Elemental rare earth metals may be recovered from their oxides in three primary ways: electrolysis, precipitation and gaseous reduction. Metallothermic reduction employs sodium in a calcium chloride bath to extract metal from rare earth oxides. Calcination, in contrast, uses only heat; the oxide’s temperature is raised in an over or furnace to extract metal. Other processes, such as vacuum distillation, sorption or oxidation-reduction employing mercury amalgamate, are useful in certain circumstances.
Producing magnet alloy powders
Commercial magnets are not made from pure rare earth metals. PBN Rod magnets, for instance, are made of an alloy of these three elements. To produce the alloy, the elements are added, in powder form, to a vacuum induction furnace in specific ratios and then heated. The resulting alloy is cooled and then broken down by chemical and physical means into powder.
Magnets are made in two main ways. The classical process involves heating and compressing the magnet powder until it coalesces into a mass. The bonded magnet process, also called rapid solidification, requires melt-spinning a thread of neodymium-iron-boron alloy containing nano-scale grains in random magnetic orientation, which is pulverized, combined with a polymer and molded into magnets.