Choose the Right Magnetic Separator

May 25, 2011
Correct selection and installation location will improve removal of weak and fine contaminants

Using magnets to remove ferrous contaminants is well established in chemical processing. However, effective performance requires a basic understanding both of the various types of magnetic separators available today and the physical nature of the product being purified.

Incoming liquid or solid ingredients can pick up contaminants from a variety of sources, including ship holds, truck beds and rail cars. Contamination also may originate within a plant from material handling, grinding, crushing or general abrasion. For example, dry material that free-falls through a carbon steel or stainless steel chute can get contaminated simply by scraping against the chute.

Grate Magnet
Figure 1. Grid of magnetic tubes can go in pipes, chutes and hoppers.

If not caught quickly, the resulting fine ferrous metal contamination can cause plenty of problems. These include costly breakdown of downstream equipment, lost profits from equipment downtime, potential violation of Hazard Analysis and Critical Control Point (HACCP) standards and reduced purity that leads to product that must be scrapped or sold at less than full value.

These problems can be reduced  --  if not virtually eliminated  --  by using magnetic separation equipment. Such separators remove ferrous materials such as nails, scale, bolts, welding rod and other contaminants from dry or liquid processing streams. Units now are available with magnets made from alloys of rare earth (RE) elements, which are substantially stronger than traditional alnico or ceramic materials.

Choosing the best magnetic separator for an application ultimately depends upon several critical factors, including temperature, flow rate, flow characteristics and process issues.

Temperature. Magnetic materials lose strength when exposed to elevated temperatures. They recover some strength when the temperature returns to normal. Permanent magnets heated beyond certain temperatures may suffer irreversible loss in strength. It's important to consider not just ambient, but any Clean in Place (CIP) temperatures to ensure the magnet chosen is suitable for long-term separation performance.

Rotating Grate
Figure 2. Rotation prevents plugging and packing of fine cohesive materials.

RE magnetic separators  --  although more expensive  --  capture fine contaminants at higher temperatures more effectively than units with conventional magnets. Standard RE magnets suit temperatures up to approximately 150°F; specially designed RE models can operate in temperatures as high as 400°F or more, depending upon the application.

Flow rate. Magnetic separators perform optimally when the contamination is presented to the surface of the separator. A unit that provides for a thin burden depth over or under the magnet will capture ferrous contamination most effectively.

Flow characteristics. Many products exhibit different flow characteristics when damp or moist. Are there large chunks that may plug an opening or gap in the separator? Will the product flow freely through the selected equipment? For example, finely ground aluminum powder with any significant moisture content won't flow between the tubes in a grate magnet assembly even if the tubes are nearly 1 in. (25 mm) apart.

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.

Magnetic Hump
Figure 3. Unit suits less-than-free-flowing materials that will cascade down a chute.

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.

As I've already pointed out, magnetic separators come in a variety of forms. Here's a brief rundown of the most common types.

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.

U Trap
Figure 4. In-line separator can handle liquid or slurry product flows.

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 [email protected].