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Matching Proper Magnet with Metal Detector Ensures Optimum Product Purity

October 7, 2013
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Metal detector conveyor system with flip
Plate magnet
Grate magnet
Eriex rota-grate
Tube magnet

While productivity is the end goal for any company involved in bulk material handling - particularly in the food sector - there are considerable challenges to meet customers' demands for product purity. Regulations coming from the US Food and Drug Association (FDA) and US Department of Agriculture (USDA) and the respective Hazard Analysis and Critical Control Point (HACCP) program are especially stringent on any type of metal contamination.  
    Processors invest millions of dollars annually into inspection and detection equipment, proactively reducing the risk that any foreign object or contaminant will be embedded into a bulk product, eventually making its way to end-use consumers through supermarkets, grocery stores, restaurants, retailers and other distributors.
    Attention to product purity on the production line has never been greater, creating the need for processors to detect and eliminate foreign objects before the end product reaches the consumer. Best practice manufacturing operations now include detailed inspection systems to help limit product recalls and liability claims.
    But with this increased drive toward automation comes a need for higher levels of product purity and ensuring that contaminants - such as fine ferrous and nonferrous metal particles - do not enter the processing stream. Allowing foreign objects to pass through undetected can result in damage to downstream equipment and contamination to raw materials.
    The most sensitive metal detectors, for example, can detect ferrous and nonferrous metals down to sub millimeter sizes, making them ideal to prevent fine metal contamination in dry bulk processing.
    However, many processors are discovering that matching a specific magnetic separator alongside that highly-sensitive metal detector is the best defense against any metal contamination reaching end-use customers.
    Why install both? A magnetic separator is never 100 percent effective in removing contamination. The same is true for a metal detector. But working in combination gives the operator as close to a perfect product yield as possible. Removing ferrous contamination first with a magnet also means the metal detector will reject less often, cutting down on wasted product.
    Any type of fine metal can enter the production process with the raw material or can get into the product because of wear or failure of processing equipment components. Detecting pieces of broken machinery can help solve problems before major damage occurs. There's also the human factor: inevitably, items such as coins, pens, and processing tools will occasionally fall into the product stream.

Eliminating Contamination Early Stops Downtime
Metal contamination comes from a variety of sources. Incoming products may contain fine metal objects from the transportation vessel used to deliver the product, such as a truck bed, rail car, barge, or ship hold. The contamination may also originate in a loading station, silo, refinery, or within the plant itself because of material processing, grinding, crushing, or general abrasion.  
    By removing contamination early in the production stage, the right magnetic separator alongside a metal detector helps prevent damage to grinders, ovens, screeners, and other vital equipment used in dry material processing and handling.

Matching Magnets with Metal Detectors
There are several types of magnetic separators and metal detectors that work well together in dry bulk processing. This “double team” approach is especially relevant in eliminating metal from whole bulk foods such as almonds, coffee and cocoa beans, raisins, etc., and more granular bulk items like sugar, flour, crushed chili peppers, and various spices. The magnet removes the ferrous contamination while the metal detector focuses on any ferrous missed by the magnet as well as nonferrous metals, i.e. aluminum, copper, brass, stainless steel.
    For free-fall applications, the combination of a Rare Earth (RE) grate or plate magnet and a metal separator achieves the best results. Installing both at the beginning of material flow helps protect vital equipment such as grinding mills.
    Grate magnets remove small and fine tramp iron from dry, free-flowing products. They are designed for steep-sloped hoppers, floor openings, vertical closed chutes, and ducts. They prevent ferrous and small, weakly magnetic contamination, such as work-hardened stainless steel, from contaminating the product mix.
    Grate magnets are made of 1-in.-diam magnetic tubes in a grid formation, allowing feed material to cascade through the grate, effectively spreading magnetic protection through the cross sectioned area of a pipe, chute, or hopper.
    Another separator to speed along processing is a Rota-Grate, which removes metal from light, fluffy material that tends to clog and bridge when passed through small openings. This unit rotates a number of powerful magnetic tubes through the material. The magnetic action attracts and holds the unwanted metal and the rotary action prevents the material from packing and plugging the processing line.
    Plate magnets also remove ferrous material from a dry product flow. In a typical chute installation, the magnetic material adheres to the magnet face while the product slides across the face of the magnet. The magnetic field attracts and holds ferrous material until the plate is removed for cleaning. The magnet is usually hinged and swung away from the chute and cleaned manually. Plate magnets are simple and economical to install, as well as efficient at removing occasional pieces of tramp metal.
    Once the dry material cascades through these magnets, the next stop should be through another metal separator to detect all magnetic and non-magnetic metal contamination. Metal contaminants are rejected through a quick-flap reject unit. Pipe sizes for these uniquely designed detectors range from 1.18 to 9.84 in.
    Metal separators are available in wash down or non-wash down designs, depending upon the hygienic nature of the operation and sanitary conditions.
    A tunnel-style metal detector should also be strategically placed during the stage when bulk material is transported on a conveyor belt and before final packaging (heading for wash down or after roasting, for example). These conveyor system metal detectors are available in various belt widths and aperture heights and can be calibrated for different material sensitivities. As the bulk material makes its way through the detector, any lingering metal is detected via an automated reject such as a flip gate or head pulley reject.
    Dry bulk material can pick up trace metal contamination as it goes through a final hopper and into a super sack or bulk bag. For extra protection, a series of tube magnets coupled with another vertical metal detector/separator is recommended before bagging and sealing. This final step prevents an entire sack or bag from being rejected if contamination is found by the customer. It also protects the processor's brand name and reputation.
    Using the right magnet with the proper metal detection technology can safeguard processing equipment and ensure product purity. This tried-and-true “marriage” keeps dry bulk material under strict surveillance at all times and prevents unnecessary machine downtime and costly maintenance.
    John Klinge is product manager—metal detection, Eriez (Erie, PA). For more information, call 800-345-4946 or visit www.eriez.com.

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