- THE MAGAZINE
Magnet MaterialsRecently, increases in the strength of permanent magnets have been extraordinary, and the advent of rare earth permanent magnets has spurred the design of high-intensity permanent magnetic circuits. Certain permanent magnetic circuits designed with rare earth magnets now generate a magnetic force many times greater than the force generated by conventional ferrite or even rare earth magnets currently available.
Rare Earth MagnetsOriginally thought to be rare, the metallic elements with an atomic number between 57 and 71 are classified as "rare earth." Because samarium cobalt (number 62 on the periodic table) was the first material used to make this new generation of magnets, they were called "rare earth."
The newest rare earth magnets consist of neodymium-iron-boron (neodymium is number 60 on the periodic table). The first production neodymium-iron-boron tube-type magnet circuit that came to market developed a surface field of approximately 4,800 gauss. Strength levels have been increasing over the past ten years and are now producing surface fields in excess of 10,000 gauss, compared to the original rare earth or the 1,000 and 2,000 gauss range for conventional magnet circuits.
Applications for rare earth magnets range from removing small ferrous particles from liquid streams to dry, free flowing products. And while magnet manufacturers make many different strengths of rare earth magnet material, the rare earth magnets coupled with the other magnet circuit components (stainless and mild steel) in separation equipment are many times stronger than geometrically similar ceramic magnets.
Protecting Fine China and Other CeramicsMagnetic traps are typically arrays of easily removed, finger-like magnetic tubes and are commonly used for removing ferrous contaminants from liquid or slurried products in pipelines or troughs. Traditional applications involve food products such as soups or chocolate, but these high-powered separators are ideal for the ceramic industry as well, because they exert such a powerful attracting force on small ferrous contamination.
Rare earth traps installed in slurry lines remove fine particulate that would discolor the finish of fine china after firing. Small bits of metal react during the firing process and discolor the finish of the china. Super-strength rare earth magnetic traps collect fine metal contamination and reduce the number of defects resulting from product contamination.
Traps also remove small bits of metal that may contaminate the surface texture of the finished item or material, causing the item to fail during firing. Even a small piece of contamination within an item could literally explode during the firing process-increasing the number of defects, damaging other products or harming processing equipment.
Larger pieces of tramp metal can be detected and removed with rare earth circuits prior to the grinding process. Typically, this is done where raw materials are processed for further use. Undetected tramp metal could otherwise damage the processing equipment, resulting in unnecessary downtime and repair expense.
Rare earth traps are also used as quality control check devices to monitor the performance of electromagnetic filters. The rare earth trap is typically installed after the filter to ensure it is removing all the fine ferrous contamination in the slurry. If any ferrous material is found on the magnet, the filter installation is reviewed for contamination build-up or other operational problems.
The rare earth traps collect tramp metal and hold it until an operator removes the element from the assembly and physically removes the accumulated tramp metal. Periodic maintenance is the most critical factor for rare earth trap magnets' performance. Careful review of new installations will help the user establish the appropriate cleaning frequency to enhance performance of the equipment and reduce product defects.
Detecting ContaminantsElectronic metal detectors are frequently used in tandem with magnetic separators to provide added assurance that ferrous and non-ferrous contaminants are removed from products and process streams. Digital circuitry in the detectors provides high sensitivity and allows monitoring on systems moving from 1 fpm to 100 mph (30 cm/min to 45 m/sec).
Detectors typically include variable delay reject triggering, tachometer devices that allow for variable product speeds and computer memory storage of multiple detectors for precise rejections.