Metal detectors are totally different than simple magnets. They are not going to trap metals but will detect metal presence in a product stream and possibly reject it making them very useful metal separators for process industries handling bulk solids. This page is essentially focused on gravity fall - free fall metal detectors.

1. Technology of metal detectors

How is the detection performed ?

When metal is passing through the metal detector, it will change the magnetic field that is applied by the detection coil of the metal detector. This disruption will allow the detection of the presence of metal.

Figure 1 : Gravity fall metal separator working principle

2. Rejection

How are metal detectors removing foreign bodies ?

Metal detectors can be equipped with a rejection. It takes most often the shape of a flap that is opened as soon as the metal detector spots the presence of metal in the product flow. It is the principle used by Safeline metal detectors for example.

Each time a diversion occurs, some product is lost, it is important not to have false detection otherwise a lot of losses will be generated.

The product passing through the metal detector is susceptible to create a disturbance of the magnetic field which can lead to false positive and the rejection of good product. It is therefore required to teach the metal detector what is the "spectra" of the normal product so that it can distinguish in between normal product and contaminant.

The metal detector performance must be checked on a regular basis by using test balls that are introduced on top of the metal detector. The test is repeated several times and the detector must each time reject the test ball to be considered fit for production.

3. Choosing the right metal detector

What are the parameters guiding the choice of metal separators ?

The following parameters must be taken into account when chosing a metal detector : the product throughput to handle, the location of the metal detector and the sensitivity targeted

Table 1 : Metal Detector design criteria

4. Metal detectors manufacturers

It is very important to select properly a metal detector to have a good and reliable metal detector and thus to select reputable suppliers of metal detection systems. The following companies can supply metal detectors and can be contacted to investigate their equipment :

• Sesotec metal detector
• Safeline metal detector
• Loma metal detector

(Note that PowderProcess.net has no contact with those companies)

Other related sections

Sifter / Vibrating sieves

Magnets