Metal menace

By and |  February 28, 2014

Suspended magnets and industrial metal detectors play a vital role in eliminating tramp metal and protecting downstream equipment.

Removing damaging tramp metal from aggregate processing is one of the most important procedures to not only enhance product purity but also protect expensive operating equipment. While there are multiple ways to search for and remove both ferrous and nonferrous tramp metals from the product stream, one of the most efficient methods is the use of industrial metal detectors.

Today’s metal detectors are designed to fit within a range of belt widths and can be field-adjusted to fit most conveyor configurations. Manufacturers also offer a variety of options that help personnel quickly and efficiently locate and remove the tramp metal once the offending material passes through the detector.

How does tramp metal enter the production stream? Any place where heavy machinery is involved is a potential source for wayward metal. Examples are loading equipment where pieces of bucket wear liners or cheek plates break off and fall into the product stream. Front end loader bucket teeth, rock bolts and even tools can find their way into the aggregate before entering transfer points, chutes and crushers.

Magnets and metal detectors work together
Metal detectors act as the “last line of defense” before the aggregate enters a critical stage of operation within the processing plant. Prior to this stage, however, most mining and quarry operations use some type of suspended magnet positioned above the primary conveyor belt to remove any large pieces of magnetic tramp metal embedded in the aggregate.

These “workhorse” magnets remove large amounts of ferrous materials conveyed in heavy burden depths on almost any type of conveyor or chute. They are ideal for separation applications on fine or coarse materials, large or small tonnages. The suspended magnet is usually positioned right before a transfer point so the belt (and subsequent downstream equipment) does not suffer damage caused by larger tramp metal.

The metal detector is then placed after the suspended magnet and before any screens, cone or jaw crushers, transfer points or other processing equipment. Should any ferrous, nonferrous or stainless steel metal pass through, the metal detector detects it and signals the conveyor belt to stop, even when the material is conveyed on steel corded belts.

This is particularly important if the suspended magnet cannot remove metal that is embedded in the rock or piece of concrete. The metal detector has the capability to locate tramp metal trapped in deep burden depths and signal operators before the metal can damage other equipment.

Most metal detectors can be set for higher detection sensitivity, ensuring that even the smallest problematic tramp metal can be detected; thus providing better protection for crushers, screeners and conveyor belts.  This ability to locate metal particulates even on high-speed belts saves operators time and money because of less equipment downtime. Metal detectors can also be used effectively on highly conductive products such as gold, copper and iron ore, giving operators far more flexibility and protection.

Once installed, these metal detection systems operate reliably for long periods of time, frequently in adverse environments, with little attention or maintenance. The power required to operate both the metal detector and reject device is minimal; a standard 115V or 230V line is sufficient.

Choosing the right metal detector for the job
Because every metal contamination problem is unique in some way, the appropriate equipment for detection is not always apparent, particularly to plant operators and maintenance personnel who may not be aware of all the possible options when choosing a metal detector. Understanding the differences between metal detection systems and product options is particularly helpful before purchasing and installing such a system in your operation.

Industrial metal detectors are engineered for different applications. For example, the Eriez Model 1230 metal detector is primarily installed on conveyor belts carrying coal, minerals, aggregates and other bulk materials. It can be set up to ignore conductive or magnetic ores such as magnetite and pyrite, even if they are carried by high-tension, steel-corded belts. Eriez’ Model 1220 metal detector is less sensitive and is used mostly to detect large pieces of ferrous and nonferrous metals like bucket teeth.  The size of the tramp metal that must be found and removed is also a consideration. Cone crushers with 12-in. openings can easily let a 1-in. piece of metal pass through without damage. Further down the line, as crusher and screen openings get smaller, a metal detector becomes more important to locate the smaller pieces of metal to prevent equipment damage.

Most metal detectors are available in any belt width and can be field adjusted to fit most conveyor configurations.

Take note
Should any ferrous, nonferrous or stainless steel metal pass through, the metal detector detects it and signals the conveyor belt to stop, even when the material is conveyed on steel corded belts.

John Klinge is product manager, metal detection for Eriez. He can be reached at 814-835-6000 or by email at

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