Metal detectors are used in food and pharmaceutical industries to detect metal contamination in products or packets. The metal contamination may occur in food and pharmaceutical products due the presence of metallic pieces or components (viz., fasteners, pins, buds, eroded or corroded metallic parts etc.) in ferrous, non-ferrous, or stainless-steel materials. Detection of such metal contamination is important and in this detection process, high accuracy and reliability are desirable. Thus, a metal detector plays an important role in ensuring product safety. In pharmaceutical industry there is a possibility that the products get contaminated by metallic parts either magnetic or non-magnetic. This is due to metal-object contamination (ingredient contamination, mixing-process errors and machinery failures) and due to the use of various process equipment viz. reactors, agitated and storage vessels, classifiers, dryers, pumps, valves, pipes etc. in the synthesis, processing and storage and most of these equipment, pipes, valves and pumps are made up of metals.

In many pharmaceutical processing, unsaturated organic compounds that contain double and or triple bonds need to be hydrogenated. Platinum black, palladium black, copper metal, copper oxide, nickel oxide, aluminium and other materials have subsequently been developed as hydrogenation catalysts. Abrasive materials can wear mill parts and screens. In these operations, there a probability always exists the products get contaminated with various metallic components. The final food and pharmaceutical products must free from metal contamination as per GMP standards. Metal detectors are mostly used for US State Department of Agriculture (USDA), Hazard Analysis Critical Control Point (HACCP) or US Food and Drug Administration (USFDA) Certifications in India and other countries.

Metal detectors with advanced microprocessor-based technology are highly advantageous for checking coarse-grained food stuffs such as popcorn, cornflakes and soup noodles.

These are different types of metal detectors based on certain operating principle. Some commonly used metals detectors are discussed below: –

Electromagnetic induction measurement:- The operation of these metal detectors is based on the principles of electromagnetic induction. Metal detectors contain one or more inductor coils that are used to interact with metallic elements on the food or pharmaceutical products. Metallic contaminant in the product creates high frequency magnetic field within the detector coil, which in turn activates a reject flap by means of a solenoid. The detector is designed in such a way that it automatically removes the metal components form tablets without production interruption. The metallic contaminant is reliably rejected with very little loss of material due to the extremely fast and short activation of the reject flap of the detector. This type of metal detectors is mostly used to detect the presences of metal contamination in continuous tablets manufacturing process. This type of metal detector can be easily cleaned and are compatible easily for all tablet press. It has high sensitivity to all metals including the most difficult non-magnetic stainless steel. Its performance is maintained stable throughout the life.

Multi-frequency transmission and receiving: – This type of metal detector is working on the principle of multi-frequency transmission and receiving. Multi-coil arrangements are used for the transmission and receiving of multi-frequency in metal detection. The multi-frequency created by the multi-coil arrangement improves the signal viewed by the receiver. Compared with a single transmitter with two receivers, multi-coil arrangements can improve detection performance (as measured by the diameter of the metal sphere size that can be detected) of the instrument by up to 20 per cent.

Radio frequency signals: – Radio frequency metal detectors contain a transmitter antenna that produces radio frequency signals ranging from 300 kHz to 1 MHz. Two receiver antennas sit on each side of the transmitter at equal distance from the transmitter. When the system is balanced and there is no magnetic or conductive inside the aperture of the metal detector, the difference of the two signals is zero, signifying that no metal is present. When metal is present and traveling through the detector, a detectable imbalance is created. The accuracy of such metal detectors depends on the following three factors. The closeness of the metal that is being detected to the antennas or coils (i.e., the aperture or opening size). The signals have two components:

  • one is magnetic (X), and
  • one is conductive (R).

These components enable the system to detect metal foreign objects that are mainly conductive and have a small amount of magnetism such as in 316-alloy stainless steel. As a result, detection of conductive metal objects relies on a different signal analysis compared with a ferrous metal containing iron. Most metals are both magnetic and conductive which change with the size of the metal. An off-line laboratory instrument such as an X-ray fluorescence spectrometer is necessary to determine full metal composition. In some applications, the ability of the system to ignore the signals that can be caused by the uncontaminated product passing through the metal detector is crucial.

Very low frequency measurement: – Metal detectors based on the operating principle of Very Low Frequency (VLF) measurement principle are the most commonly used. The metal detectors contain two sets of coils, namely, transmitter coil and receiver coil. Electricity is passed through the transmitter coil to create a magnetic field. This constantly pushes the electricity into the ground and pulls it back up. The magnetic field so generated interacts with any metallic or conductive object that comes in its way. The receiver coil passes the electric current whenever the metal detector passes over a conductive object. This amplifies and sends the frequency of the current to the control box. Metal detectors, using VLF technology, detect metals and determine the difference between different types of metals and the depth at which they are located.


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