Editor’s Note: This article is based on our free webcast “Tramp Metal detection as a crucial tool for preventing loss in cement production” delivered by Cesare Riva, expert on cross-belt analyzers, mineral products, and bulk weighing and monitoring solutions.

What is Tramp Metal?

“Tramp metal” refers to any metallic scrap that accidentally finds its way onto conveyors and into bulk material during the cement manufacturing process. Tramp metal can originate from a variety of different sources, consisting of bucket teeth, manganese steel mantles, bore crowns, bar scrap, chains, and various types of tools.

In the cement industry, tramp metal is the arch enemy of the crusher. In fact, it presents a significant risk of blocking and damaging crushers, shredders, belt-conveyors and other processing equipment. As a result, a tramp metal detection is essential in order to ensure continuity of operation and protecting the safety of workers.

Tramp Metal Detection

Tramp metal detection is carried out by metal detectors installed directly on convey belt lines. Tramp metal detection is the only reliable and cost-effective method of protecting expensive crushers, shredders, conveyor belts and in-line weighing, sampling and monitoring equipment. By detecting the presence of metal objects that conventional magnets may miss, tramp metal detection provides reliable protection to your processing equipment. Investing in a tramp metal detection system is an essential part of protecting cement production operations from costly production delays and safety risks.

During tramp metal detection, solid material passes across a metal detector. When tramp metal is detected, it can either be removed manually or automatically. With manual removal, tramp metal detection triggers an alarm relay and stop the belt – this allows a safe manual removal of the tramp metal to prevent it from entering the downstream process. In automatic removal, tramp metal detection activates a bypass chute to automatically reject the tramp without stopping the belt.

Tramp Metal Detection Technologies

There are two main technologies for tramp metal detection: inductance systems and eddy current systems. The most effective for tramp metal detection in cement mining and other bulk industries is the eddy current system.

Inductance-Type Tramp Metal Detection

Inductance-type tramp metal detection system, make use of an oscillating magnetic field produced by a wire coil. When a conductive or magnetic piece of tramp metal passes over the coil, the oscillations change. This change is detected by the electronics and an alarm is tripped.

There are some advantages to induction-type tramp metal detection: in particular, they tend to be more sensitive than eddy current-type systems. However, there are also some limitations. First, in order to be detected, the tramp metal must be more conductive than the material it’s buried in. In addition, Changes in the belt loading, or the presence of water, can cause false positives. Crucially, non-conductive ore, such as manganese, can’t be detected using this type of tramp metal detection system.

Eddy Current-Type Tramp Metal Detection

In eddy current-type tramp metal detection system, a pulsed current is applied to a coil, which induces a primary magnetic field around it. When tramp material moves through the field, the primary field induces eddy currents in the tramp metal. The eddy currents induce their own secondary magnetic fields around these pieces of tramp metal.

These secondary magnetic fields generated by metallic materials on the conveyor can be differentiated by observing the rate at which they decay once the primary field is switched off. This decay time is a signature of the material, be it ore or tramp metal. Electronics in the detector can discern the difference in the signal between valuable material and tramp by activating its measuring window only after the magnetic field from the material has decayed, but before the magnetic field from the tramp metal has ended.

One of the primary advantages of this type of tramp metal detection is that it can be used to detect ferrous and non-ferrous metals alike, such as manganese steel or copper. Eddy current tramp metal detection systems also completely ignore water, and changes in belt loading have minimal effects. The disadvantage, however, is limited range: the distance between the tramp and the coil is relevant and maybe be a disadvantage in certain applications.

Choosing the Most Effective Tramp Metal Detection System

While there is no one-size-fits-all solution for tramp detectors, there are several basic features you should look for.²

A tramp metal detection system should be designed to accommodate belt conveyors, moving cement or other bulk materials.
Any effective tramp metal detection system must be insensitive to the material transported, with high magnetic permeability and electrical conductivity to prevent high false alarm rate.
For applications in the cement industry, a tramp metal detection system should have the capacity to quickly and accurately detect tramp when buried in wet, conductive materials.
The system should allow easy installation: no need to cut the belt or use special tools.
A robust tramp metal detection system should be resistant to moisture and impact in order to be fit for challenging applications. Offset swing arms can protect the coils and reduce breakage, while an under-the-belt single coil assembly eliminates risk of damage from high burden depths and interference from overhead metal objects. Look for a fiberglass support structure that is resistant to moisture, and impact- and moisture-resistant PVC coils.
Temperature control is important for cold-weather applications.
An operator-friendly interface with easy-to-read indicators ensures that the equipment will be used properly.

When choosing a tramp detection system, it’s also advisable to look for the following technical features:

The controller should enable adjustment of sensitivity of the device to output signals, and should ideally allow the user to adjust the measurement window in the decay time to prevent false positives or passed tramp metal.’
A user-adjustable operating frequency enables elimination of interference and false alarms from motors, radios and other external sources of radiofrequency noise.
A reliable tramp metal detection system should be able to minimize cross-talk and have inputs for speed pickup.

On top of the above technical features, look out for these mechanical features:

The most important feature is configuration of the coils: either dual coil (tunnel) or under-belt. Dual coil systems are suitable for most applications as they are not sensitive to tramp metal location. Under-belt coil systems are required for applications with very low burden depths, or in cases where very large bulk pieces are present that would damage the upper coil in a tunnel system.
Frame assembly and coil bonding should reduce moisture problems – for example, by using a potting compound.
The overall system should have resistance to mechanical shocks.
A dual coil system should have a cantilevered swing arm, enabling the upper coil to allow swing away in case of high tonnage applications when extra burden may occur.

These optional accessories can simplify tramp metal detection and help manage your plant more effectively:

Clip detectors to avoid false trips due to the passage of repair clips or conveyor belt junctions
High pile detectors to monitor the correct position of the swing arm after the passage of extra burden
Spray or flag drop markers to quickly identify the location of tramp metal in the material when the belt is stopping
Beacon lights to inform operators when the belt is stopping

The Benefits of Tramp Metal Detection Systems

A reliable and high-quality tramp metal detection system can make the difference between a safe, productive site and thousands of dollars in damaged equipment – or even worse.

Tramp metal detection systems are a vital part of preventing machine breakdown and minimizing repairs. Detecting tramp metal as soon as it enters the conveying system helps to minimize downtime and maintenance costs. Tramp metal detection provides an economical and reliable means to protect crushers, conveyors and other processing equipment; assuring continuous and uninterrupted operation at your cement plant.

A reliable tramp metal detection system not only protects your equipment, but your workers as well. Tramp metal detection systems are vital for worker safety, preventing unnecessary risks from equipment damaged by tramp metal. Tramp metal entering a crusher can result in dangerous high-energy ejections, which pose a significant safety risk to workers and can lead to serious injury or death.

Finally, tramp metal detection minimizes waste and ensures a more productive site. Detecting tramp metal that originates from an upstream process serves as a warning system, allowing maintenance to be carried out on damaged equipment before more serious failures can occur.

Tramp metal detection systems are crucial tools in cement production; enabling cement manufacturing operations to run smoothly, reliably and safety. With the right tramp metal detection technology in place, operators and owners can rest assured that if and when scrap metal gets into bulk materials, they won’t have to worry about expensive equipment being damaged and turned into scrap itself.

References and Further Reading