Separation anxiety: setting up efficient conveying and magnetic separation lines for C&D takes careful planning but minimal maintenance.
Magnets not only divert valuable scrap metals from incoming streams of material, they also aid in the creation of clean wood and aggregate products. But in order to achieve an acceptable level of metallic recovery and clean aggregate products, the manner in which material is presented to the magnet is important, and conveyors play an integral part in the process.
While conveyors may seem like they merit little consideration, the sheer variety could start one's head spinning: Flat idler belt, apron, troughed, vibratory feeder and slider bed are just a few. And because they serve to unite the entire processing system, it makes sense to took into the merits of each conveyor type.
How do C&D recyclers go about determining the system that will offer them the most efficient separation of a variety of materials? Well, they start at the end.
LAST THINGS FIRST
"In determining magnetic and separation needs, a customer must first have the knowledge of the market for a particular geographic area," Brian Sinram of Sherbrooke O.E.M. Ltd. of Sherbrooke, Quebec, says. "Keeping what end product he or she has in mind, the customer will then determine the potential volume of recycling he or she can achieve."
John O'Neill, vice president of sales and marketing for McCloskey Brothers Manufacturing, headquartered in Peterborough, Ontario, says lines are typically set up so that overs (or large pieces) are screened out early in the process. A primary screen would then sort material in the 10- or 12-inch-minus range. The resulting material to the material entering a manual picking station.
Secondary screens can help to remove the fines fraction, O'Neill says, leaving behind relatively clean material for hand sorters to pick through. The rest of the material will be conveyed on to the magnet, which will divert the ferrous portion of the stream. In a few cases, eddy current magnets may also be employed to recover copper, brass and aluminum scrap,
"Our distinction regarding C&D systems has to do with the fractioning of the material in two lines, which are 10-inch minus and overs," Sinram says.
Sherbrooke designs systems using its welded Z-pan apron conveyor, which meters material on to a General Kinematics finger screen. Sinram says this method helps to improve sorting efficiency on the overs line.
After the material has been divided into two fractions, Sinram says that optimal sorting and metal removal can be achieved by elevating the overs to the sorting platform, while the unders are transferred to the magnet. "From there, the unders are elevated to a second screening where it is possible for multiple sizes to be automatically separated," he says.
Determining the size and type of magnet to employ in a C&D sorting line hinges on a number of factors, such as total processing capacity; belt width, angle and speed; and the depth of the burden.
LAWS OF ATTRACTION
Perhaps the most basic question that C&D recyclers have to consider when selecting a magnet is whether they want a permanent magnet or an electro magnet.
Permanent magnets are non-electric magnets and may be the better choice for C&D recyclers with portable or mobile setups because they do not require electricity to operate.
Electro magnets, however, may be the better choice for companies with stationary plants, as they often offer stronger magnetism and more functionality than permanent magnets, Dennis Ciccotelli of Steinert US, St. Petersburg, Fla., says.
He says the types of magnets available to C&D recyclers range from drum magnets, which are capable of lifting large, heavy pieces of ferrous metal from a moving stream of material, to magnetic head or tail pulleys, which can draw material from within a pile and retain it while other materials fall away, to overband or crossbelt magnets, which are suspended above the conveying line.
"In some applications, two magnets are installed for optimal sorting, which is referred to as an automated system," Sinram says. "As a general rule, the drum magnet is used in scrap yards. Even though it requires a slightly higher investment, it is well worth it since the drum magnet is more efficient and maintenance free."
Sinram adds, "Currently, most of our customers are converting from their existing overhead magnets to the drum magnet, which has resulted in higher levels of efficiency and a much faster payback time. After all, the C&D business follows the scrap yard business on a large scale."
Ciccotelli says that rubber belt conveyors and vibratory feeders are used most often to present material to a magnet. "They can be in a variety of positions. Usually, it can be a vibratory feeder fit high, or what we call near 12 o'clock, on the magnet," he says. "Or you can start the vibratory feeder low at around 6 o'clock, what they call underfed."
He says underfed drums work better on material of medium size, while overfed drums work better for larger and longer material.
"If we were using rubber belts," Ciccotelli says, "they would be underfed mostly."
Steinert manufactures agitating drum magnets, which means they have several poles that are opposite one another, which causes the material to flip, releasing trapped dirt, Ciccotelli says. "This gets you cleaner scrap and better ferrous recovery," he adds.
However, agitating drum magnets are not advisable for heavy material, Ciccotelli says. "We tend to want to hold on to those items and just bring them around so that the lose direct around them falls off."
Harold Bolstad of Dings Magnetics, Milwaukee, Wis., says locating the magnet at the head pulley offers the best recovery. "The best way to present material to the magnet would be an inline application where the magnet is suspended at the head pulley, going the same direction as the material is flowing. As the material comes off the end of the belt, it is free falling. You have material that has a lot of air gaps and openings so the magnet can easily reach into the burden at that point when it is suspended in the air and pull metal through it up to the magnet," he says. Because there is less material to prevent the magnet from reaching into that burden, you get better separation, Bolstad says.
Crossbelt magnets sit at a right angle to the direction of the flow prior to the head pulley, Bolstad says. He says it is more difficult for a magnet to penetrate the depth of the burden in this position.
"If the magnet has to be a foot away, then we need a stronger magnet than if we could be 6 inches away," Ciccotelli says. "Size is a function of distance. Magnetism is a property that decreases with distance and also with heat."
Protecting the magnet from excessive heat created by friction in the case of permanent magnets or from electricity in the case of electro magnets is a key consideration. Therefore, makers of electro magents insulate or anodize their electric coils to protect the core of the magnet. "Some manufacturers fill their case voids with liquids so that they help to cool it off," Ciccotelli says. "We use an air-cooled case because it is friendlier out there in the field because there is less of a problem with leaks," he adds.
In terms of burden depth, Ciccotelli says that most recyclers have problems with burdens deeper than 12 inches. Instead, he suggests that a depth in the range of 3 to 6 inches is best. "If it is a foot deep, a strong magnet will pick up the metal, but it will also trap a lot of dirt."
A fraction of C&D recyclers also employ eddy current separators on their lines to separate nonferrous metals like copper, brass and aluminum. These separators are generally located at the tail end of the processing system, Bolstad says.
Ciccotelli says that nonferrous recovery tends to work best with a smaller particle size. "Eddy current separators will work with a fraction under 6 inches." Eddy currents are not able to efficiently handle large pieces of brass or copper such as pipe joints because they are so heavy, he adds.
Sinram says most C&D recyclers do not include eddy current separators in their system configuration because the amount of material would not justify the extra investment in terms of increase in efficiency. "Furthermore, if we look at the fact that the burden depth on the conveyor is normally about 10 inches, it can render the eddy current separation inefficient," he adds.
After deciding on the proper magnet for their needs, C&D recyclers will be pleased to learn that little maintenance is involved in keeping the magnet operating at peak performance.
MAINTAINING ONE'S INVESTMENT
Sherbrooke's Sinram suggests weekly cleaning and inspections of conveying lines and magnets to his customers. He also advises them to respect the manufacturer's guidelines and to keep essential spare parts inventoried in order to avoid costly or prolonged downtime.
Bolstad recommends examining magnets for broken or damaged parts on a regular basis so that the core is not compromised. He also suggests setting up a lubrication schedule for the bearings.
"If your electrical coming in is constant and the rectifier supplies the appropriate incoming voltage to the magnet, there is not a lot of maintenance to be done," he says of Ding's electro magnets.
"With an electro magnet, you need to change the oil every two years or so or after so many hours of operation," Bolstad adds. "Because a permanent magnet requires no cooling oil, you are just lubricating bearings."
In addition to regular maintenance, Ciccotelli says C&D recyclers can consider optional accessories, such as wear covers, limiting rings and armored belts, to help increase the life of magnets by confining scrap metal and protecting against impact and abrasion.
"We have found that wear covers and limiting rings on a magnet help protect the magnet from getting any core damage," he says. "That is very important because the core is the heart of the drum, and you don't want it to be infiltrated with magnetic material that can get in through a worn-out shell or holes in the shell."
Ciccotelli says that magnetic dirt that enters the magnet through holes in the shell could eventually cover the core and cancel out the magnet. "That is how most operators lose a magnet."
He adds, "A wear cover is a way to prolong the life and prevent any damage to the maximum part of the investment. It adds 10 percent to 15 percent to the cost of the magnet, but makes the asset virtually perpetually renewable just by changing the wear part."
The author is associate editor of Construction & Demolition Recycle and can bee-mailed at email@example.com.
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|Title Annotation:||Operations Focus|
|Publication:||Construction & Demolition Recycling|
|Date:||Jan 1, 2005|
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