Using Magnetic Separation to Remove Contamination

Q. We have fine ferrous metal contamination in our pharmaceutical product, a free-flowing powder, even though we have a magnet. How can we remove the contamination?

A. Ferrous metal contamination can be reduced-if not virtually eliminated-with the use of magnetic separation equipment.  An understanding of the various types of magnetic separators, however, and their interaction with the physical nature of the product being purified is important. Temperature, flow rate, flow characteristics, and process issues are crucial factors for effective removal.

Magnetic materials lose strength when exposed to elevated temperatures, although some strength is recovered when the temperature returns to normal. Permanent magnets heated beyond certain temperatures may also suffer irreversible loss (i.e., a reduction in strength that cannot be recovered by cooling). It is important to note ambient and any clean-in-place (CIP) temperatures when choosing a magnet suitable for long-term separation performance. Rare earth (RE) magnetic separators, although more expensive, capture contaminants better at higher temperatures than conventional magnets. Standard RE magnets can work in temperatures up to 150 °F, while other RE models are designed to operate in temperatures up to 250 °F and even up to 400 °F.

Magnetic separators perform best when the contamination is presented to the surface of the separator. It is ideal to select a magnetic separator that provides for a thin burden depth over or under the magnet to ensure the magnet will have the most opportunity to capture the ferrous contamination. Plate, grate, and trap magnets have different characteristics for removing contaminants from different types of materials.

Many products exhibit different flow characteristics when damp or moist. Are there large chunks that may plug an opening or gap in the separator? Will the product flow freely through the selected magnetic separator? For example, a pharmaceutical powder with any significant moisture content will not flow between the tubes in a grate magnet assembly, even if they are positioned nearly one inch (25 mm) apart from each other.

The process design is also important. Issues include how the material is presented to the separator, whether the material is metered or has surge flow that needs to be handled, and where the magnetic separator is installed (e.g., prior to a filling/packaging machine, at the discharge end of a conveyor, beneath a hopper, or before the material drops into a bulk bag).  

A magnetic survey and appraisal of plant operations by an experienced professional can help to determine if your current magnetic installations are giving optimum protection, to learn what magnets can and cannot do, and to discover the advantages of newer technologies.

—Bill Dudenhoefer is manager of Separation Products at Eriez.

If you have a problem with your equipment or process, an industry expert may have the solution. Please send your question to Jennifer Markarian, editor of Equipment and Processing Report, and we may be able to provide an answer in a future issue. All questions will remain anonymous.