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Magnets and Magnetic Separation Q&A

  •   In the context of coffee and tea blending operations, what are the best magnets to use and where should they be placed? Are there magnets available which will pull on stainless steel tramp?

    Many of our customers will screen their coffee and/or tea through our drawer-in-housing with 1 in. Rare Earth magnetic Nedox tubes; Nedox is a synergistic food-grade application on the tube that assists with flowability and is ideal for abrasive product that could otherwise shorten the life of a bare magnetic tube. It is also ideal for clumpy or leafy product that tends to cake or bridge. In your application the recommendation of rare earth is two-fold: you are blending a consumer-use product so capturing the smallest metallic fines is paramount plus the magnet(s) can be positioned throughout the facility to protect your grinders, blenders, and packaging equipment from damage. It is normal to have magnetic separators: 1) at the front of the plant to clean incoming product and avoid unnecessarily contaminating the facility, 2) within the plant to capture tramp iron that enters the stream through the normal course of processing, and 3) at final packaging or load-out to serve as final inspection.
    The ability to capture stainless steel is dependent on the grade and changes to molecular structure of the steel contaminants (filings from chute or blade wear, sheared bolt, etc.). Many manufacturers of processing equipment use 400 series stainless – whether by choice or as specified by the processing facility – explicitly so that downstream magnetic separators can capture these weakly magnetic contaminants as they enter the product stream.

  •   I’m trying to separate iron particles from an aqueous resin solution but instead of separating they form a gel. What is happening and how can I avoid this phenomena with a chemical modifier?

    Is the iron inherently present in the aqueous resin solution or is the contamination occurring upstream somewhere in a dry state where it could more easily be captured? If it is introduced upstream in a liquid state that’s all right as, generally, liquid line magnets are recommended for this type of an application. The choice of which style (magnetic plate or magnetic tube circuitry) is determined by the material content flowing through the magnet: strained, viscous, fibrous, crushed, etc. You make reference to already attempting to separate the particles so it would be helpful to know what method you are using.
    What you are describing sounds like ferrous or ferric hydroxide, and the chemical modifier phenomena is a question for a chemist. I do hope the balance of the information provided is helpful.

  •   Is it possible when working with particles from 40 to 140 mesh to separate magnetite from metallic (Fe) or partially metallic (Fe) particles with magnets?

    Yes, it is possible; however it is almost impossible in a dry state. Most magnetite separation applications are addressed with wet drum magnetic separators. The fluid helps to dissolves the mechanical bonds allowing magnetic separation to occur.

  •   We need to remove all ferrous from recycled tires. The product comes from furnace (after carbonization) on a conveyor belt. After the belt we have a 2 m hopper chute followed by a 1.5 m vertical pipe. The allgaier screen takes out particles >5mm. Fine fraction leaves via 150 mm diam. outlet in to 1.5 m vertical pipe to packing station. The throughput is 850 kg/h (1/3 is ferrous; mainly wire about 20 to 80 mm long). What magnets would you recommend for an efficient separation?

    Our engineers would suggest as a first step the installation of an overhead magnet after the furnace and above the initial conveyor. If you mounted this magnet in an inline configuration at the head pulley and build a collection chute, the magnetic circuit should assist in shaking some of the imbedded wires free. There will likely be some carry-over of rubber into the collection chute, but this will get the bulk of the wire.
    The second step is to place a magnetic drum separator in a position to work in conjunction with the allgaier screener. If the particles >5mm need to be metal free, place the drum before the screener; if only the particle <5mm need to be metal free, place it after the screener. Post screener is the ideal location as it will create a consistent, metered flow across the face of the drum for optimal metal capture. We would suggest a self-cleaning package on these magnets to maximize your facility’s operational efficiency.

  •   What are a list of items I need to consider when selecting a magnetic separator for my plant?

    The best way to ensure the proper magnet – both in material specification and in its operational features – is to complete an application form that most manufacturers will provide. Generally, this form will include questions on flow rate, moisture content, bulk density, conveyance method, and angle of repose how the material is introduced to the magnet. The answers will allow an engineer to ensure the right circuit, spacing of the chosen circuit to allow the product to flow through, around, over or under the magnet and still achieve optimal tramp iron capture. From a mounting and cleaning perspective, it is important to know what equipment the magnet will be mating up to and a choice on cleaning options since most manufacturers provide a range from manual operator clean to automated, self-cleaning equipment.

  •   I have heard the words gauss, force index and pull strength used in terms of measuring a magnet. Is one of these measurement methods more or less reliable than the others?

    Simply put, gauss is a unit of measurement of a magnetic field. Wikipedia defines it as: one maxwell per square centimeter; it equals 1×10−4 tesla (or 100 μT). Force index measures the rate of change of a given magnetic field so the higher the rate of change the larger the magnetic force; think “bigger is better”. Both of those explanations can be confusing to break down into “What does that mean to me and my application?”
    Enter Pull Strength- this is where the rubber meets the road because at the end of the day what really matters is this: will the magnet I choose hold onto contaminants? Pull strength is a measure that is widely accepted in the marketplace as it results from a series of pull tests using either a sphere or plate test piece on the magnetic circuit that is simply pulled from the circuit until it breaks free. The resultant “pull” is recorded in ounces or pounds of “pull strength” required to break the test piece away. After a series of tests, outliers are eliminated and the balance of pulls are averaged to publish a pull strength- also commonly known as “Holding Value”.

    Gauss and Force Index are really the sizzle to Pull Strength’s steak. Pull strength is “an is”- a fair measurement that can be quantified and easily compared amongst manufacturers so the end-user can make a confident decision in choosing the best product for a given application.

  •   Are there different strengths or types of magnets available, and if so, how do I know which one would be best for my application?

    Typically users will choose between ceramic or rare earth permanent magnetic circuits in the dry particulates marketplace. In general, ceramic is chosen when the main goal is “capital” protection- think large tramp iron like nuts, bolts, knives and screwdrivers which cause damage when introduced to downstream equipment. Conversely, rare earth is optimal for “consumer” protection as its strength and holding capabilities far exceed that of the ceramic magnet, allowing it to capture small and very weakly magnetic fines. In all processing facilities there exists an opportunity for metal shavings, filings, iron powder and items like wire mesh to enter the product stream; rare earth magnets will capture this tramp iron throughout the facility if properly located in the processing stream(s). Specialty materials like Alnico and Samarium Cobalt – both used for high temperature applications- and electromagnetic circuits – for larger tramp iron on belt -conveyed product like you would find in a mine or power plant – are also available, although are used to a much lesser extent in this market segment.