Plate magnets are simple, economical to install and effectively remove occasional pieces of tramp metal. These units function best when positioned in the bottom of an inclined chute or suspended above a thin burden of material on a belt conveyor or stainless steel vibratory feeder.
In a typical chute installation, ferrous material adheres to the magnet face while the product slides across the face. The contaminants remain on the plate until removed by cleaning. The magnet usually is hinged and swung away from the chute and cleaned manually.
Chemical processing operations that use odd-shaped, round, oval or round-cornered hoppers benefit from installation of grate magnets. These typically have 1-in. (25-mm) diameter magnetic tubes in a grid formation designed to allow feed material to cascade through the grate, effectively spreading magnetic protection over the pipe, chute or hopper cross-section. This is the best type of separator to use with small free-flowing granular or pelletized material.
Grate separators come in designs to suit almost any application. The simplest use a single layer of magnetic tubes in a hopper; materials must pass through the grates before they leave the hopper. Grates in housings feature one or more layers of tubes, depending on the level of protection required. These units frequently are installed in vertical chutes of free-flowing materials.
Drawer-type grates are designed specifically for use in vertically mounted closed chutes and ducts. They come complete with handle and drawer front. These models are easily inspected and cleaned by sliding the magnetic drawer from an opening made inside the chute.
Rotating grate magnets remove both large pieces of unwanted iron as well as minute ferrous particles from material flows that tend to clog and bridge when passed through conventional flat-grate magnets. The dual-action unit rotates a number of powerful magnetic tubes through the material. The magnets attract and hold the unwanted iron while the rotation prevents material from packing and plugging the processing line. Such units provide excellent separating efficiency on many finely ground cohesive materials, including gypsum, barium carbonate, Fuller's earth and lime.
Easy-to-clean grate versions feature push/pull operation that strips accumulated tramp metal without the need to physically handle the magnet. Completely automated self-cleaning units also are available.
When a grate magnet can't fit within the confines of a processing operation, a series of tube magnets just as effectively can remove ferrous contamination from materials ranging from dry powders and granules to liquid products in tanks. Tube magnets also can serve as inspection tools or for fabricating grates for a unique requirement.
Typically, tube magnets come in 1-in. (25-mm) diameter, with available lengths from 4 in. (100 mm) up. Material temperature and the type of ferrous contaminant determine the appropriate magnet. Ceramic and alnico tubes can handle removal of tramp metal (nuts, bolts, washers, etc.). Fine, weakly magnetic contaminants require RE tubes.
One of the most popular and effective magnetic separators is a ferrous trap. It often is used in-line to remove tramp metal or fine ferrous contaminants from liquids or slurries.
The traps are designed to protect flow lines, prevent abrasive wear, reduce incidents of jammed pumps and ensure final product is free of iron contamination. They can serve in lines ranging from under 2 in. (50.8 mm) to 18 in. (457 mm) or more. They handle materials with temperatures up to 400°F (204°C), with special models available for temperatures to 850°F (454°C). Some magnetic traps designed for pipelines 2 in. and smaller can withstand pressures up to 150 psi.
There also are magnetic traps that remove rust, scale or screen wire from difficult-to-flow or chunky products such as chemical slurries. Their special body design works well where finger-style traps would plug because of large product size or where delicate products would break if forced against a baffle.
A good starting point in selecting magnetic separation equipment is a complete process survey and appraisal of plant operations by an experienced professional. By working with an expert, you can find out if magnets currently installed are providing optimum protection, learn what magnets can and can't do, and discover the advantages of the newer RE technologies.
BILL DUDENHOEFER is manager, separation products, for Eriez, Erie, Pa. Email him at firstname.lastname@example.org.