Choose the Right Magnetic Separator

Correct selection and installation location will improve removal of weak and fine contaminants

By Bill Dudenhoefer, Eriez

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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.

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.

KEY FACTORS FOR MAGNETIC SEPARATORS
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.

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?

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