Magnetic Separation: How To Select The Best Metal Detector?

Source: Eriez
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Eriez "How To Choose & Use Industrial Metal Detectors" Guide

By: Bill Dudenhoefer

While foreign object detection holds a certain degree of importance in virtually every industry imaginable, end products for the pharmaceutical market must meet the most rigid purity standards. Because tablets and capsules are manufactured for human consumption, even the minutest particles must be completely extracted. Before anything can be removed, it must first be detected. And, because hygiene and quality criterion for the pharmaceutical is regularly upgraded, pharmaceutical manufacturers must proactively search for newer, better solutions to detect foreign objects.

When used properly, magnetic separation equipment can virtually eliminate ferrous metal contamination from dry and liquid products. Magnetic separators are available in a wide variety of designs to meet manufacturers' specific needs. For maximum extraction and results, the right equipment should be chosen for the particular job. Selecting the proper magnetic separator requires an understanding of magnetic properties, the process application and environmental elements that exist in each specific installation. Equipment should be selected based on the materials being processed and the many application considerations.

Magnet Materials and Styles

Magnet material generally refers to the magnetic pressings or castings used to develop the magnetic field within the separator. This material may be cut and arranged in a linear fashion using other materials to create a magnetic circuit. Depending on the desired outcome, circuits may be designed to cast a shallow-wide magnetic field, a deep-narrow field or anywhere in between. This is why it is often difficult to compare competing products made of similar material. Once the circuit is assembled, it is usually encased within stainless steel to protect the magnet material from wear associated with product flow, damage or from contaminating the feed material.

Different types of magnet material such as Alnico, Ceramic and Rare Earth have their own unique advantages and disadvantages. Depending on various factors related to a specific application, any of these materials could be appropriate for pharmaceutical applications. It is essential to work with a qualified magnetic supplier to determine the ideal magnet material needed.

Application Considerations

Temperature: Permanent magnetic materials lose strength when exposed to elevated temperatures. Some losses are reversible—when the temperature is returned to normal, the magnetic strength returns. Permanent magnets heated beyond certain temperatures (which depend on the specific material) may also suffer irreversible loss that cannot be recovered by cooling. When specifying a magnet application, it is important to keep in mind the ambient as well as any clean-in-place (CIP) temperatures to ensure a proper magnet design suitable for long-term separation performance.

Flow Rate: Magnetic separators perform best when the contamination is presented to the surface of the separator. It is best to select a magnetic separator that provides for a thin burden depth over or under the magnet to ensure the magnet will have the best opportunity to capture the ferrous contamination.

Flow Characteristics: Many products exhibit different flow characteristics when damp or moist. Large chunks may plug an opening or gap in the separator. So the product must be able to flow freely through the selected magnetic separator.

Process Issues: How the material will be presented to the separator is crucial. If the material is not metered, then the equipment must be capable of handling a surge flow. If the system can't be stopped for cleaning, a self-cleaning magnet is required. Also be aware of any ferrous material in the area that may create a hazard for magnet handling. Two last considerations are the amount of contamination to be removed and the level of product purity that is required.

Type of Material Being Processed: Assessing the material being processed is a vital step in selecting the proper magnetic separator. Product generally falls into three different categories: dry, moist or liquid. Within each of these groups, a wide range of product variation exists.

Separation Equipment

Plate Magnets: These are used in the bottom of an inclined chute or suspended above a thin burden of material on a belt conveyor or stainless steel vibratory feeder to remove occasional pieces of ferrous contamination. Rare Earth models do a superior job on very fine or weakly magnetic contamination.

In a typical chute installation, the magnetic material adheres to the magnet face while the product slides across the face of the magnet. The magnetic field attracts and holds ferrous material until the plate is removed for cleaning. The magnet is usually hinged and swung away from the chute and cleaned manually. Plate Magnets are simple and economical to install and there is an extensive range of designs available. Plate magnet designs include Round Pipe Separators (RPS), Hump Magnets and Deep Reach Separators.

Grate Magnets: These consist of one-inch diameter (25mm) magnetic tubes in a grid formation designed to allow feed material to cascade through the grate, spreading magnetic protection through the cross-sectioned area of a pipe, chute or hopper. Ceramic Magnet models are effective in removing relatively large ferrous contamination, while Rare Earth models do a much better job on fine or weakly magnetic material.

The simplest of grate magnet designs incorporate a single layer for use in a hopper, forcing raw materials to pass through the grates as material feeds from the hopper. Multiple row units improve separation effectiveness. Some applications will utilize a single row of magnets and incorporate them into a grate housing assembly of the user's design. Some magnet manufacturers will also fabricate housings that include one or multiple rows of magnetic grates, depending on the level of protection required for the application. These units are common for vertical chutes of free flowing materials. Self-cleaning units are also available, enabling a completely automated cleaning process.

Pulley and Drum Magnet Designs: Magnetic Pulleys replace standard conveyor head pulleys and effectively convert the conveyer into a self-cleaning magnetic separator. As the conveyed material passes the head pulley and discharges in its natural trajectory, the magnets "scalp" large tramp metal from the burden, then discharge it as the belt pulls away from the backside of the pulley.

Drum Magnets are self-cleaning magnet assemblies that continuously remove tramp metal from a product flow. The magnet assembly is enclosed within a stainless steel drum and fixed in position. The drum rotates around the magnet, conveying material through the magnetic field.

Liquid Line Trap Magnets: Magnetic Traps employ powerful tube magnets that are specifically designed for liquid product flows. The traps are available in many designs, but basically include an inlet port to match the existing pipeline, a magnetic element to collect the tramp metal contamination, then a transition back to the existing pipe. Ceramic Magnet models are effective in removing chunky metals, but Rare Earth models do a much better job on fine or weakly magnet contamination such as rust or scale.

Metal Detectors

Modern metal detectors reliably operate for long periods of time, often in adverse environments, with little attention or maintenance. They can be configured for minimal interference with established process flow. The power required to operate both the detector and the reject device (if any) is minimal, even if a special conveyor is needed to pass the product through the metal detector. In cases where a process cannot be stopped to deal with detected metal contamination, state-of-the-art metal detectors can be configured to reject a contaminated product automatically, even if the possible rejection point is some distance from the detector. The detector can also keep detailed records of detected metal—including the date, time and estimated size—so that the suspicious product can be isolated.

One new, high-tech, innovative metal detector for the pharmaceutical industry is specifically designed to improve process purity through detection of minute metal contaminants during the production of any capsule or tablet-based product. This unit's advanced electronic design simplifies setup, ensures reliable operation, provides instantaneous recovery from phase adjustments and requires minimal operator training. The five-minute QuickStart feature allows users to pass sample product, test and begin production in less than five minutes. By fixing the downslope of the unit's polished stainless steel head at 30 degrees, lengthy timing adjustments of downstream reject equipment are eliminated. For added product purity, an easy-clean chute reduces cross-contamination during product changes.

Recent advances in magnetic separation technology have resulted in a variety of separators specifically developed for the treatment of fine, high-purity materials. The technology to produce the highest quality, purest and safest products for the pharmaceutical industry is improving every day. With so many choices, it is imperative that pharmaceutical manufacturers thoroughly understand their options and needs. The first step in properly selecting magnetic separation equipment is finding a reliable, progressive magnet manufacturer that not only offers the most up-to-date products and services, but also provides close customer support during selection as well as ongoing maintenance assistance.

Click Here To Download:
Eriez "How To Choose & Use Industrial Metal Detectors" Guide

Bill Dudenhoefer is a technical representative at Eriez Magnetics (Erie, Pa.).