Process issues. Consider the overall process. How will the material be presented to the separator? Is the product metered or must you handle a surge flow? Can you stop the system for cleaning or is a self-cleaning magnet necessary? Is access available for cleaning? Could ferrous material in the area create a hazard for magnet handling? How much contamination must be removed? What level of product purity is required?
Another process issue is where to install the magnetic separator. Should it be placed prior to an extruding machine? At the discharge end of a screw conveyor? Beneath a hopper? Before the material drops into a bulk bag?
Assessing the material being processed also is a key step in selecting the proper magnetic separator. Materials generally fall into three different categories: dry, moist or liquid. Within each of these groups a wide range of product variation exists. For example, dry products range from fine chemical powders flowing down a chute to large mined rock moving along a high-speed conveyor. Obviously, these vastly dissimilar materials require different separation equipment.
Dry free-flowing granular-type product. If material is small and free-flowing, a grate magnet provides the best opportunity for the ferrous contamination to contact the magnet directly. Grates (Figure 1) used in vertical product flows are easily cleaned, while plate magnets will work well if the material is cascading down an angled chute. Cleaning requires stopping product flow to remove collected iron from the magnet.
A magnetic hump (Figure 2) or radial field cartridge generally is best for decontaminating pneumatically conveyed free-flowing material. These units also require interruption of product flow for cleaning.
Dry product with some bridging. While grates allow for very efficient removal of fine metallic contamination, they don't work if the material can't cascade between the magnetic tubes. Plate magnets don't restrict the flow of material and won't contribute to bridging (build-up of material) if installed beneath a sloped chute. Magnetic humps suit less-than-free-flowing products, as long as the material will cascade down a sloped chute.
Moist, starchy or lumpy products. These materials, such as powdered lime, present flow problems for grate assemblies because of bridging. They also create difficulties for chutes because of their high angle of repose. Separators designed specifically for use with such products are available -- for instance, the Eriez Deep Reach Separator, which has two powerful magnet circuits surrounding a chute to penetrate difficult product flows, and Eriez Rota-Grates (Figure 3), whose rotating action prevents material from packing and plugging the processing line.
Liquid or slurry products. Such materials require use of a magnetic trap. Many traps are similar to grates in that tube magnets are arranged perpendicular to the flow to trap any ferrous materials passing through. A U-trap (Figure 4) employs a flat plate magnet in a shallow body to minimize damage to the product flowing past. U-traps are ideal for stringy, chunky-flow products that wouldn't flow through a series of magnetic tubes.
PLATES, GRATES, TUBES AND TRAPS
As I've already pointed out, magnetic separators come in a variety of forms. Here's a brief rundown of the most common types.