SPECIAL REPORT: MATERIALS HANDLING/POWDER PROCESSING: Eliminating 'Separation Anxiety'

September 1, 2010
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Magnetic separators and metal detectors remove unwanted contaminants and play an important role in ensuring quality control.



A variety of unwanted items, such as metal, wood, glass or poisonous/deleterious substances, can find their way into the ceramic manufacturing process. Some contaminants occur as a result of processing, while others are simply inherent to the product itself.

Before raw materials enter processing facilities, they may already be laden with contaminants. Industrial sands used in the glass industry and calcium carbonate used in construction are mined, and the equipment used to harvest these materials comprises numerous metal components. As these components wear and break, rogue metal shards often fall into raw materials. In addition, human error can also result in tools being dropped into the raw materials.

Once in the manufacturing facility, raw materials are put through a number of processes before becoming a final product. Process systems can malfunction and allow foreign material to be introduced into product anywhere along the line. For example, a bearing on a crusher or grinder might fail, which could cause the equipment to spew metal shavings into the material. Metal chips can also accidentally drop onto the production line when equipment is cleaned, adjusted or repaired.

Although the source of tramp metals must be found and eliminated, the most important job of production personnel is to implement safeguards that capture and remove contaminants before they pollute final products or damage other equipment. Various types of equipment are used to capture tramp metals, including magnetic separators, metal detectors and mechanical separators.

Since the need to ensure that tramp metals do not contaminate final products is so critical, these three types of equipment are used in tandem; the use of one does not eliminate the use of the others. Equipment is not placed randomly along the processing line. Instead, each piece of equipment has a specific function and plays an important role in quality control. Production personnel are advised to consult with equipment manufacturers to ensure that the appropriate equipment is selected and placed at the proper point in the production line.

A drawer magnet.

Magnetic Separators

Magnetic separators are installed in any product line where ferrous contaminants must be removed to ensure and protect product purity, prevent damage to machinery, and eliminate hazards associated with tramp metals. As the name implies, magnetic separators use high-strength magnets to attract, capture and retain ferrous metal.

Magnetic separators are classified as type A, B or C, in accordance with the type of magnetic circuit used. Processing lines are generally designated into three main areas (applications): primary, secondary and finishing. The size of the tramp metal to be removed determines whether a type A, B or C circuit is used. Type A is recommended for small fragments, while types B and C are recommended for larger tramp metals.

Once the circuit type is determined, the application is examined and the corresponding family of products-which can range from plate magnets and magnetic chutes to grate magnets and drawer magnets-is recommended. Depending on the specific application, the magnetic separator may also require custom features, special finishes or coatings.

With a gravity flow metal detector, product flows through the metal detector and either into the next processing station or a reject bin.

Metal Detectors

In most cases, the bulk of tramp metals can be captured by magnetic separators. Although the product is fairly clean when it gets to the secondary production processes, minute particles of tramp metals may still exist and could make their way into the final product. The use of a metal detector can mitigate this risk.

A metal detector identifies contaminants that have a conductive quality (electrical conductivity or electrical signal) differing from that of the product being processed. A metal detector “reads” the “signature” electronic signal of conductivity of a product as it flows through a process. Each product is catalogued as a specific “recipe” of components. If the metal detector senses something that falls outside of its programmed recipe, an alarm is triggered. This tells the operator that a foreign object is contained in that product. The contaminated product can then be removed and the source of the contamination can be investigated.

As the product travels into the aperture of the metal detector, it flows through a three-coil balance system consisting of two receiving coils and a sensing coil. The sensitivity of the metal detector, or its ability to pick up contaminants, varies in relation to where the product sits within the aperture. The least sensitive area is in the center of the aperture, whereas the greatest degree of sensitivity occurs near the walls. Because a relationship exists between the size of the aperture and the size of the product, specific metal detectors are indicated for different final products.

Metal detectors are not “one-size-fits-all” solutions. Bulk conveyor metal detectors are placed underneath a conveyor belt to sense contaminants in a product after it passes through magnetic separators and before it moves into grinders and crushers. In a gravity flow metal detector, unpackaged products-such as ceramic powders-are conveyed in gravity flows through metal detectors. A pipeline metal detector allows pumped products, such as ceramic slurries, to flow through the metal detector. Rejected product is separated from the main processing line via a diverter valve. Finally, a tunnel conveyor metal detector involves running packaged products through a tunnel conveyor along a belt.

Two fundamental factors must be considered when evaluating metal detectors: sensitivity and stability. The equipment must be sensitive enough to find tramp metals, and it must be stable and reliable enough to find them consistently.

The relationship between these factors is even more important. As sensitivity goes up, stability goes down. If the sensitivity level is set too high, “false trips” occur and can result in costly product line shutdowns. If the sensitivity level is set too low, the metal detector will not find the contaminants. Metal detectors must balance the need to retain proper sensitivity in order to meet stringent quality demands while continuing to meet production goals.

Raw material flows through a grate magnet, and metal contaminants adhere to the magnetic material housed in the tubes.

Finer Points

Metal detectors can find ferrous, nonferrous and stainless-steel contaminants, while magnetic separators find only ferrous or stainless-steel contaminants. To achieve maximum sensitivity, products must be oriented in the same direction when entering a metal detector. A magnetic separator will work efficiently and effectively without regard to product orientation. In addition, magnets cannot “false trip.” Magnets are not affected by “noise” interference, fluctuation in current, or vibrations; all of these factors could affect the performance of a metal detector.

A metal detector is placed toward the end of the processing sequence. If the metal detector finds contaminants in a packaged product, that product cannot be used and is considered to be lost. Should multiple packages be found with contamination, the financial loss could be tremendous. Magnets, however, remove tramp metal from a product. The clean product moves through the magnetic separator and travels on to the next process. No product is lost.

Magnets are available in either self-clean or manual-clean models. Metal detectors can only function by automatically rejecting product that is detected to have metal contamination, which is either manually inspected or discarded. Magnets are tested with a pull test kit, while metal detectors are tested with test sticks. Both are considered to be dependable means of performance evaluation. When used and maintained properly, magnetic separators and metal detectors are extremely reliable. c

For additional information regarding metal detectors or magnetic separators, contact Magnetic Products, Inc. at 683 Town Center Dr., Highland, MI 48357; (248) 887-5600; fax (248) 887-6100; or visit www.mpimagnet.com.

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