Milling Operations: Getting the most out of magnets

by Jeff Gwirtz
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In flour mills, magnets are placed to remove ferrous tramp metal, including iron, steel, fines, rust or scale from the product stream. The required magnetic strength would be a function of material flow rate, density and installation angle, which impacts bed depth and velocity, and limits the size and cross sectional area of the retained ferrous tramp metal.

The magnetic field must be strong enough to capture the ferrous tramp metal, pull it to the magnet’s surface and hold it in place, preventing it from being “washed” off the surface of the magnet and allowing it into the product stream. For any given magnetic strength measurement (pull test or gauss measurement), the magnet would have different separation or protection capability depending on properties of material, physical orientation and the size of the potential contaminant flowing over the magnets surface.

Selection

Magnets are a prevalent piece of equipment in finished product handling systems and have been a target of food protection improvement efforts in flour mills. Seldom are they given much attention once installed. However, third-party food safety inspections, regulation and customers have been demanding a more thorough explanation for our system design and practices during inspections and approval process.

Millers must be prepared to answer questions using sound science and engineering rather than past practice, which is no longer an adequate response for justification.

It is recommended that design parameters of all magnets be identified as part of the magnet maintenance program. Magnets should be properly installed per manufacturer’s recommendation, ensuring magnet contact prevents magnetization of supporting or surrounding material, which results in reduced magnetic strength where needed.

Internationally, millers have suggested anything between 10,000 gauss to 18,000 gauss should be sufficient for most grain processing applications. However, depending on the foreign materials found in your processes/grains/raw materials and/or to minimize risk, you may decide on the higher strength.

Rare Earth Magnets, also known as Neodymium, NdFeB magnets, or NIB, because they are composed mainly of Neodymium (Nd), Iron (Fe) and Boron (B), may be a good option. They are strong magnets with increasing strength as the grade number increases, for example, from N35, N38 and N42. In addition, these magnet grades have maximum operating temperatures of 176° F (80° C) to 392° F (200° C), but may lose magnetization permanently at 590° F (310° C) to 662° F (350° C).

Validation of Magnets

Milling Operations
Table 1: Magnetic properties conversion table
To view the enlarged image, click here.
 
Normally, Rare Earth Magnets (permanent magnet) don’t lose the strength up to 100 years or until there is physical damage on them. Some suggest it is not necessary to check strength of magnets regularly but rather look for any physical damage such as cracked welds or other damage from physical abuse. The pull test or gauss measurement for a magnet in a food safety program should be used to reject a magnet with a significant loss of magnetic field from the properly sized target. A magnet should be inspected and cleaned at least on a daily basis and checked for strength monthly.

In addition, the audit should confirm maximum instantaneous flow is at or below the initial design parameters and the magnet installation position and angle follows the manufacturer’s recommendation and is properly isolated from other equipment and/or spouting. Magnet installations allowing magnetization of the entire spout or failure to cover the spout width are performance compromised.

Gauss meters measure the number of lines of flux or magnetic field density at the surface of the magnet in gauss (or Tesla). It is an accurate instrument for the measurement of the magnetic field. However, its sensitivity will produce different results with the slightest movement when testing a magnet in the production environment.

When monitoring the performance of a magnetic separator over a period of time, accurate comparisons will be difficult. Moreover, Surface Gauss is a point measurement not considering the magnet’s three-dimensional field and interactions with bodies around them. Proper Gauss meter operation requires training, practice and precise placement based on magnet design and orientation that would be quite difficult in the field. It is an instrument best used in the hands of the magnet manufacturer’s design engineers and perhaps quality assurance team.  

An overview of magnetic properties is provided in Table I. It is important to note that magnetic field intensity Oersted and field density Gauss are numerically the same when measured in air (around the magnet).

The magnetic pull test kit described by many industrial magnet suppliers should be adequate for HACCP and other programs’ auditing purposes, as magnets may be damaged and lose strength over time. It would be of value to identify the relationship between magnetic pull test results and gauss measurements. However, such a comparison is like comparing “apples and oranges” as one magnet vendor indicated. Pull Force Testers are used to test the holding force of a magnet that is in contact with a flat steel plate. Pull forces are measured in pounds (or kilograms).

The pull force of a magnet is determined by the amount of force that would be required to break the magnet free if it was attached to a steel plate. Calculation of the pull force requires the assumption that the steel plate is flat and that the force used to pull the magnet would be applied perpendicular to the surface of the plate. Pull force also may be determined by placing the magnet in between two steel plates.

The third method of determining pull force is finding the amount of force needed to dislodge a magnet from another magnet that is identical. Most pull test kits allow for testing based on the first method identified. Testing also is modified to allow for structural features of the magnet and for distance above the surface of the magnet.

Metal/Foreign Object Detector

Milling Operations
Most metal/foreign object detectors for packaged products may be described as a detection tunnel surrounding a belt or tabletop conveyor. Similar detectors surrounding or including a product spout allow product to pass through the detection field as it moves from one floor to another via gravity. Metal or foreign object detectors identify the presence of foreign objects or non-ferrous in a package or product stream and signal mechanical diversion of suspect package or product from the line.

Validation data should ensure that the equipment can detect metal of the appropriate size at different locations on the belt and at different locations in or around the package. For example, if a 50-pound (22.7-kilogram) sack of flour is to be tested, the system could be validated by testing the standards at the leading edge, the tailing edge, and on top of and under the bag. It is suggested that this be done for each product type. The standards might even be tested by inserting a detector into the bag at different locations.

Multiple tests — a minimum of 10 — should be done at each location. The persons doing the testing must also confirm that the settings remain the same throughout the test. Settings should be recorded throughout the test.

The result should be the determination of the location to place the test wands during calibration checks during normal production. The bag should be sent through the center of the tunnel as that is the least sensitive area in conveyor systems. The test standard for future verification must be placed in the location where the magnetometer receives the weakest signal. Rigorous test protocols like this will provide confidence that the system was set up properly and is doing its job.

Conclusion

Magnets and metal detectors require specific design, installation, inspection and verification as part of product safety and quality programs. It is critical that design criteria be well developed and documented. Proper installation is required to ensure the magnet can function optimally within design parameters.

Validation ensures performance against design and expected performance criteria and confirms installation has not compromised design. Inspection and monitoring ensures performance is maintained over time and identifies potential failures and/or operating parameters, which might compromise product safety and quality.
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