Productivity is paramount for bulk material handlers, yet meeting customer demand for product purity entails considerable challenges. Regulations from the U.S. Food and Drug Administration (FDA) and U.S. Department of Agriculture (USDA), including the Hazard Analysis and Critical Control Point (HACCP) program, are especially stringent on any type of metal contamination. 

Regulation is one reason attention to production-line product purity has never been greater, but not the only one.

Processors invest millions of dollars annually into inspection and detection equipment, proactively reducing the risk a foreign object or contaminant embeds into a bulk product, and in the worst cases, making its way through distribution channels to consumers.

Best-practice manufacturing operations include detailed inspection systems to help limit product recalls and liability claims.

Yet with increased automation comes a need to ensure contaminants — such as fine ferrous and non-ferrous metal particles — never enter the processing stream. Foreign objects ingested undetected can damage downstream equipment and contaminate raw materials.

The most sensitive metal detectors identify ferrous and nonferrous metals down to sub-millimeter sizes, to prevent fine-metal contamination in dry-bulk processing.

However, we believe, and our business supports the notion, that processors are discovering that matching a magnetic separator with a highly sensitive metal detector is the best defense against contamination.

Why install both?

Well, a magnetic separator is never 100 percent effective. The same is true for a metal detector. But in combination the two give as close to a perfect product yield as possible. Removing ferrous contamination first, with a magnet, means the metal detector will “reject” less often, reducing waste product.

Roots of challenge

Metal contamination comes from a variety of sources. Incoming materials may contain fine metal objects from transportation vessels, whether truck bed, rail car, barge or ship’s hold. Contamination can come from a loading station, silo, refinery or within the plant itself, due to material processing, grinding, crushing or general abrasion. 

Fine metals enter a production stream 1) with the raw materials; 2) when humans lose track of coins, pens or tools; or 3) due to equipment component wear or failure. Detecting broken machinery pieces before major damage occurs saves money.

By removing contamination early in the production stage, the right magnetic separator alongside the metal detector forestalls damage to grinders, ovens, screeners and other dry material processing and handling equipment.

Several type magnetic separators and metal detectors work well together in dry-bulk processing.

A “double-team” approach is most often seen in eliminating metal from whole bulk foods — such as almonds, coffee and cocoa beans, raisins and the like — as well as more granular bulk items like sugar, flour, crushed chili peppers and various spices.

A magnet removes ferrous contamination while the metal detector gets what the magnet misses, as well as nonferrous metals, i.e. aluminum, copper, brass and stainless steel.

A bevy of applications

Metal Gate Magnet

Grate magnets remove small and fine tramp iron from dry, free-flowing products.

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 protects things like 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 contaminants, such as work-hardened stainless steel, from contaminating the product mix.

Grate magnets are made of one-inch diameter (25mm) 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.

A “Rota-Grate,” removes metal from light, fluffy material that tends to clog and bridge when passed through small openings. It rotates powerful magnetic tubes through the material, the action of which attracts and holds the unwanted metal, while rotary action stops the material from packing or plugging the processing line.

Metal Plate Magnet

Plate magnets also remove ferrous material from a dry product flow.

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 it. 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 very 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 range from 1.18 inch (30 mm) to 9.84 inch (250 mm).

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 as bulk material is transported on a conveyor belt and before final packaging (heading for wash down or after roasting, for example). 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.

Final checks

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 the customer finds contamination. 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 marketing manager – sanitary, with Eriez.