Capturing Ferrous Contaminants
Characteristics such as stability, extreme heat resistance, chemical inertness and suitability for use in mass-produced products have placed ceramics among the most versatile of materials for today's low- and high-tech markets. However, as the range of applications for ceramic products has expanded, product purity has become an increasingly important consideration, both for cosmetic and functional reasons. Metal scrap and fines can be deposited in the raw materials used in ceramic production at virtually any stage of transport, storage and processing. Normal equipment operation produces minute metal scraps worn away by abrasion, and equipment vibration can sometimes shake loose entire machine parts into the material flow. All of these contaminants have the potential to damage your production equipment and adversely affect the appearance and marketability of your products.
Fortunately, with the help of magnetic metal separation equipment, the most common of these metal contaminants-ferrous debris-can be efficiently and economically removed from the dry and liquid components used in ceramic manufacturing.
Magnetic Metal SeparatorsToday's magnetic metal separation equipment has evolved into a highly effective means of capturing iron objects, ferrous fines and even weakly magnetic debris. Because these separators remove ferrous material from the product stream by magnetic attraction, they have the advantage of eliminating ferrous contaminants without sacrificing good material.
Some manufacturers use electronic metal detectors to detect and remove both ferrous and nonferrous industrial metals. Electronic detectors can sense the presence of contaminants such as stainless steel, aluminum, copper, brass, bronze, tin, lead and other conductive metals, as well as the much more common tramp iron. Systems can be set up to actuate a valve or other device to automatically reject contaminated material, and operators can monitor electronic detectors from a remote location through a personal computer to track rejection events, compare contamination levels of different vendors' products, or evaluate machine wear. However, metal detection is a higher-cost solution than magnets, and most ceramic applications don't require the "all-metal" detection-and-rejection approach.
Magnetic separation equipment suitable for ceramic and other dry solids applications comes in many forms. Selecting the right unit(s) to meet your needs requires evaluating a number of factors; your choice depends on what you produce, how you produce it, how you transport materials in your plant and how your plant is laid out.
Magnet SelectionProduct densities, flow rates, operating temperatures, and the type and ferrous content of contaminants you wish to remove will determine the type of magnet you should select for your separation equipment. Separators equipped with economical ceramic magnets are well suited to removing most tramp iron, including larger debris and objects. Rare earth magnets have the extra magnetic strength needed to capture fines, embedded metal particles and marginally magnetic contaminants, and they can also improve separation performance when product densities and flow rates are high. Alnico magnets (made of aluminum, nickel and cobalt) are typically the best choice for applications exposed to operating temperatures above 400∞F because they retain about 85% of their room-temperature magnetic properties up to 1000∞F.
Types of Magnetic SeparatorsMost magnetic metal separators are built around either cartridge or plate magnets. While some "self-cleaning" options for these separators may need electric or pneumatic connections to function, the permanent magnets themselves require no external power. In general, the choice between these two basic types depends largely on the characteristics of the materials you want to clean and the material handling system in which the separator will be used.
If the dry bulk solids you process flow easily and are non-abrasive, and if your separation equipment will be installed to handle vertical gravity flow, your natural choice will probably be a cartridge-based separator, such as a grate or drawer magnet. If your products exhibit poor flow characteristics, or if your separator will be installed at an angle or in a pneumatic conveying line, you will need a plate-based separator, such as a plate housing magnet or some type of in-line magnet. Highly abrasive products can wear through the tubular steel bodies of cartridge magnets at a surprisingly fast rate, so plate magnet separators are also a good choice if you are working with coarse or extremely abrasive materials.
Permanent Magnetic Cartridges
Permanent magnetic cartridges are an efficient and economical way to pull ferrous debris from the material stream. They resist demagnetization, last essentially forever, and require no electrical connections or cables. They can be purchased individually and are available loaded with permanent ceramic magnets, with alnico magnets for high-temperature applications, or with high-energy permanent rare earth magnets that are especially well suited for capturing and removing fine metal particles and weakly magnetic materials.
Grates are the next step up from individual cartridges. Grates consist of multiple cartridges welded into a round or rectangular stainless steel framework, or tray, so that they can easily be placed in dry or liquid flow lines or hoppers, either with or without mounting hardware. For maximum effectiveness, choose cartridges and grates that combine an adequate reachout with a strong holding force at the surface. Various baffle configurations offer slightly higher performance by increasing the exposure to the magnetic field.
Multi-tray drawer magnets are designed to handle a wide range of separation tasks in gravity conveying systems. Permanent magnetic cartridges arranged in grate-like trays provide the separation action. Drawer magnets can be configured with two, three or more of these trays arranged so that the magnetic cartridges are staggered to increase contact with the product flow. Material moves in a zigzag pattern as it falls through one cartridge tray to the next. Each additional tray increases cartridge-product contact and debris capture.
In most standard configurations, ferrous debris is removed from the cartridges by sliding the trays out of the housing and wiping them off by hand. Self-cleaning drawer magnets are designed so that the magnetic cartridges and the tubes that cover them extend outside the housing. As the tube assemblies are pulled out of the housing, grommets wipe the tubes clean all the way to their nonmagnetic ends. This ensures that each cartridge is fully wiped and prevents captured tramp iron from reentering the housings. Both manual and pneumatic self-cleaning models are available. Pneumatic units can be installed in hard-to-reach locations because they operate at the touch of a remote switch or by automated control packages.
Plate magnets come in a variety of styles and magnetic strengths to cover a wide range of dry solids applications in the ceramic industry. Simple mounting hardware makes installation in rectangular spouting easy. In such applications, plate magnets typically open very much like a hinged hatch for manual cleaning. Permanent plate magnets resist demagnetization, last essentially forever, and require no electrical connections or cables. Most models are available with either ceramic or rare earth magnets. (Alnico magnets would be too large and heavy for use in most plate configurations.)
New high-energy magnets can be used to achieve superior ferrous metal separation in chutes, spouting and even suspended applications. Such magnets use specially designed neodymium loads that can often provide three to five times the gauss and holding force at the magnet surface, while maintaining the same field depth and powerful reachout characteristics that make plate magnets so useful. This combination of strong reachout and holding forces can help minimize fines in powders and colorants, and can capture and hold tramp iron and ferrous fines even in dense, high-flow-rate applications. Neodymium loads can also be 40-60% lighter in weight, making them easier to handle.
Plate Housing Magnets
Plate housing magnets resist bridging and choking and remove tramp iron and ferrous fines from more flow-resistant materials. The housings are usually made of stainless steel and mount easily to enclosed spouting or directly on production equipment. A baffle is usually installed at the top of the housing to help break up clumps and direct product flow over plate magnets on either side of the housing. However, just as plate magnets provide somewhat less protection against ferrous fines than stacked grate magnets, plate housing magnets are likely to be less efficient at capturing contaminants than multi-tray drawer magnets.
One of the advantages of installing plate magnets in housings is having the choice of self-cleaning options. Most plate housing magnets come in both manual and pneumatic models. Manual self-cleaning models have an interior drawer that carries captured debris out past the plate magnets for safe, convenient disposal as the drawer is pulled from the housing. Pneumatic self-cleaning models can be installed without regard to accessibility. They operate at the touch of a remote switch or an automated control. In most standard configurations, the individual plate magnets are hinged to the housing like doors and swing out for easy cleaning.
In-line magnets are engineered to remove both ferrous fines and larger pieces of tramp iron from many types of dry particulates as they travel through pneumatic or gravity-flow conveying lines. Some in-line magnets have a tapered, exposed-pole magnetic cartridge with a "nose cone." These units are designed to direct the flow of material around the cartridge, which is suspended in the center of the housing for optimum contact with the material.
Other pneumatic and gravity in-line magnets feature a design that allows unobstructed product flow over the enclosed plate magnet. In horizontal installations, the magnet is usually located at the bottom of the housing, while in vertical installations it is located on the side. These units typically have tapered transitions that guide material directly over the face of the plate magnet. The plate magnets are hinged to swing away from the housing for easy external cleaning to avoid accidental spills of captured ferrous debris back into the product stream.
Many center-flow, pneumatic and gravity in-line magnets are available with either permanent ceramic magnets or high-energy permanent rare earth magnets. In-line separators typically come in line sizes ranging from 2 to 12 in. and larger, and should be equipped with gaskets made of an appropriate material (usually silicone) to eliminate leakage.
Magnetic liquid traps can be used for contaminant capture just about anywhere liquids, pastes and slurries need to be screened. These simple metal separators are ideal for removing ferrous debris from the liquid lines that carry glazing material and colorants for ceramic tile. Magnetic liquid traps provide continuous, dependable protection against tramp iron contamination and come in housings typically made from 316 stainless steel and sized to fit a wide range of applications. Standard fittings are available for tubes and pipes up to 4 in. in diameter. Standard configurations include cartridge-style, single-plate-style, and dual-plate-style traps. Special models are made for sanitary and high-temperature applications. Self-cleaning traps offer convenience and long-term maintenance savings.
Some traps equipped with the new high-energy neodymium rare earth magnets can capture small ferrous particles and weakly magnetic or work-hardened 300-series stainless steel fragments that might slip past other magnets. Depending on the appropriateness of the application, liquid traps can significantly reduce downtime, maintenance, costly wear and tear on machinery, and wasted tile or other ceramic products.
Cartridge-style liquid traps are intended for use with non-fibrous fluids. The trap design forces liquids through a tightly spaced grid of magnetic cartridges to ensure close contact between the product and the magnetic field. Ferrous tramp is magnetically extracted from the liquid flow by the magnets and held on the cartridge surfaces. Wherever large amounts of tramp are anticipated or when service time must be kept to a minimum, self-cleaning traps, which can be cleaned and returned to service quickly, are recommended. The T-shaped body of the trap provides a sump in which heavy nonferrous contaminants such as small stones and nonferrous metal fragments can collect.
Liquid traps built with single- and dual-plate magnets effectively remove ferrous particles from viscous and fibrous liquids and liquids containing large suspended solids. This type of trap is designed to "divert" ferrous contaminants against a plate magnet and into a deep magnetic field, where they are captured and held. The magnetic field creates strong holding forces at either side of a central deflector, where ferrous contaminants can be trapped and protected from the wiping action of the liquid flow. The deflector creates a 180-degree change in the direction of flow to agitate the product and expose contaminants that would otherwise be blocked by suspended solids. A gap between the deflector and the magnet face allows non-ferrous solids to pass unobstructed.
Manual or Self-Cleaning?Self-cleaning and continuous-cleaning models can make heavy contamination and inconvenient locations much easier to manage. They can save operator time and system downtime, and they also let you locate separation equipment where it can perform at peak efficiency-without regard to accessibility for routine cleaning. These points are normally where product accumulation and velocity are as low as possible to allow for maximum separation.
To determine whether an investment in self- or continuous-cleaning separation equipment is justified, you'll want to estimate the amount of tramp metal you expect to capture and evaluate the accessibility of the separation equipment you plan to install.
Achieving Your Separation ObjectivesThe type of separation units you select will affect the degree of exposure control you will be able to maintain under varying flow conditions. Exposure control refers to the exposure of tramp metal to the separator's magnetic field. Exposure relates to the time the metal particles spend in the magnetic field, their distance from the magnet(s) and the strength of the field.
Drawer magnets with two or more rows of cartridges arrayed on 2-in. centers generally have the best exposure control and separation performance of any type of separator, because tramp metal can never pass more than a 1⁄2 in. from a cartridge, no matter what the flow rate.
Grates, plate housings, and pneumatic and gravity in-line magnets also offer good exposure control. However, plate magnets usually suffer from a substantial drop in collection performance when and if the product burden exceeds the flow rates that the magnet was designed to handle.
Numerous factors must be considered when determining your separation needs. Only a few have been presented here. While it's important to be informed, you should also get advice from an established equipment manufacturer or a reputable sales-and-service organization before making your purchase. That way, you can to be sure you end up with the best product choices and placements of magnetic separators to achieve your objectives.
For more information about magnetic metal separation equipment or metal detectors, contact Bunting Magnetics Co., P.O. Box 468, Newton, KS 67114-0468; (800) 835-2526; fax (316) 283-4975; e-mail firstname.lastname@example.org ; or visit http://www.buntingmagnetics.com .