Removal of metal contaminants from plastics materials is frequently done using high-strength magnets based on the element neodymium, but it is just as certain that many, even most, recyclers and processors have little or no idea what neodymium is. Neodymium, which usually labors in obscurity, makes possible the powerful magnets used to decontaminate streams of plastic by removing metals.
Ignoring the contaminants is not an option. It risks damage to barrels, screws, valves, molds, and runners. If contaminants somehow make their way into the parts, the parts themselves may become scrap.
Neodymium can be used in permanent-magnet form to attract ferrous (iron-based) metals by the use of high-strength magnets. It is also used in electromagnet "eddy-current" techniques to repel non-ferrous metals, such as aluminum, copper and brass.
So, what is this important, virtually indispensable stuff? Neodymium is one of the "rare earth" group of elements on the Periodic Table of Elements, a chart originally developed in the 1860s that organizes all known elements according to their various characteristics. The chart evolves over time with the discovery of new elements, but continues to be a respected reference tool to this day.
The rare earth elements, all 17 of them, including neodymium, are really neither rare nor earth, as ordinarily understood. They can also be called "minerals," but the "earth" title seems to have stuck in common usage. A number of them, in addition to neodymium, are useful or even indispensable in electrical, electronic, and computer applications as well.
In addition to neodymium, the rare earths are, (in order of their atomic number, for what it is worth) scandium, yttrium, lanthanum, cerium, praseodymium, promethium samarium , europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium. They are typically named after a location or after a person who was prominent in their discovery.
Suppliers of "high-strength" magnets typically are using the neodymium magnets in their products, but sometimes do not mention or emphasize neodymium in their literature, perhaps because it sounds exotic, complicated, or expensive. Suppliers include Bunting Magnetics Co., Newton, Kan.; Eriez Magnetics, Erie, Pa.; Industrial Magnetics Inc., Boyne City, Mich.; and S+S Inspection Inc., Bartlett, Ill.
Neodymium magnets are an example of a metal that is used to attract ferrous metals or repel nonferrous metals, depending on the setup.
Typical ferrous contaminants include nails, screws, washers, shavings, granular particles, and a wide variety of miscellaneous ferrous contaminants that somehow make their way into the materials stream.
Non-ferrous contaminants include all of the ferrous bits and pieces, in non-ferrous metal forms, plus common objects such as pennies, paper clips, and bottle caps.
High-strength magnets. Usually combined with the more familiar elements iron (Fe) and boron (B), the neodymium iron boron (NdFeB) combination provides "industrial strength" magnetic force that will pull almost any iron-based (ferrous) contaminants out of a plastics material stream. High-strength neodymium-based magnets are incorporated in metal — removal systems in the form of grates, shelves, plates, discs, rolls, tubes and virtually any other shapes into which metal can be fabricated.
Eddy-current setups. Suppliers of high-strength magnets based on neodymium usually offer neodymium-based electromagnetic eddy current systems as well. Removal of non-ferrous metal contaminants is accomplished by using the same neodymium as in powerfully attractive magnets, but is instead set up to provide a strong repellent force for non-ferrous metals.
Based on personal curiosity about the neodymium magnets, I had a look at a local hardware store recently and was able to buy several examples of the neodymium magnets configured as washers in small packs for under $5 per pack. They make a terrific "crack" sound when brought in proximity and contacting each other, without even having been removed from their blister pack. I used one of my own fingers between the packs to demonstrate that the magnets, small ones at least, were indeed strong, but not dangerous.
The upshot is this: The next time you prevent serious damage to plastics processing equipment by trapping ferrous contaminants with a high-strength magnet or non-ferrous contaminants with an eddy-current setup, remember that you probably owe that success to an obscure entry on the Periodic Table of Elements, namely neodymium.
Merle R. Snyder, senior correspondent
