Today, most plants that utilize magnetic separation equipment require periodic testing to ensure that their separation process is operating at acceptable levels. Magnetic strength can deteriorate due to excessive heat, shock, and other factors. A primary tool used in such testing is the pull test kit. A pull test kit is a simple device used to verify that magnetic strength of your equipment meets the required standard. Records are typically maintained to observe any changes over time. The testing is fairly straightforward and can easily indicate problems if done properly. The more difficult question, however, is what to do if a magnetic separator requires attention. This is a much more complex issue because magnetic separation technology has changed greatly over the last 30 years. Magnetic material has been developed that creates far more energy, and there are higher grades of material that can resist heat degradation. These new magnets are much stronger magnetically, physically efficient (require less space), and are much more application and product specific.
During the 1970s, ceramic magnets were the standard for magnetic separation. Standard ceramic magnets are lightweight, rated at 1500-2500 gauss, and can deliver approximately one to three pounds of pull using a half-in. steel ball in a standard pull test kit. Alnico and Samarium Cobalt magnets were developed for high-temperature applications but strength limitations and higher prices limit their use in today’s magnetic separators. In 1988, when the first NdFeB or “rare earth” elements (i.e., neodymium, iron, and boron) were developed for magnets, pull ratings of nearly five pounds and 4000-6000 gauss were possible. Thanks to the advances in magnetic technology, especially since 2005, these rare earth or neodymium magnets can now deliver more than 10,000 gauss and 14 lb of magnetic pull with the same half-in. steel ball. Additionally, new methods of harnessing magnetic strength have been developed that result in lower magnetic loss, secondary coatings can improve wear resistance, new products have made magnetic separation possible in more problematic installations, and various methods of automated magnet cleaning allow virtually hands-off, 24/7 operation of separation equipment.
Although magnetic separators have changed considerably over time, the basic function of magnetic separation remains essentially the same. Effective magnetic separation of tramp metal from a product stream is a function of many product variables including density, viscosity, temperature, particle size, abrasiveness, percentage of tramp to total product volume that is to be separated, and moisture content. Application-based criteria are of equal importance and include considerations such as whether the process lines are gravity fed or pressurized, process velocity, and burden depth. Maximum performance of any magnetic separation process is achieved by making sure that the product passes as close to the magnetic surface as possible and at the lowest velocity possible. In low-volume applications this is relatively easy but higher volume processes, especially with products that are prone to bridging, can pose more of a challenge, especially where separation equipment size is limited by plant layout.
So what does this mean to users of magnetic separation equipment? Well, when you do have a problem with a magnetic separator it may be a simple case of replacing it with a new, identical unit, reviewing the entire system to identify new possibilities, or in the case of a new process line, looking at the options available before you finalize design plans. Choosing the right separation equipment can be a challenging task and a thorough review of your particular application with your magnet supplier is always a good idea before investing in a new magnet. Your supplier’s representative will discuss all aspects of your application with you and make recommendations based on product and process characteristics and will work with you to find the most cost-effective solutions to fit within the available footprint. Don’t “go it alone”; your magnet suppliers are industry experts that specialize in magnetic separation. They will be more than happy to help you find the “right” equipment to protect your product integrity and downstream equipment.
Marvin Angleton is manager, technical services and quality, Bunting Magnetics Co., Newton, KS, and has more than 27 years of manufacturing experience. He directs the Technical Services Department that includes the design engineering, inside sales, estimating, and research & development teams.