Market knowledge: the key to selecting the correct screening system lies in end market demand.
"It depends so much on the material characterization that you have and the size cut you are trying to make," Bill Guptail of General Kinematics, Crystal Lake, Ill., says. General Kinematics produces vibratory process equipment.
"I really believe that before you can think about equipment on a C&D facility, you'd better know what your end markets are," Guptail says.
Bruno Lagace, manager of application engineering for Erin Systems, Portland, Maine, also stresses the importance of the material's characteristics. "First of all, the recycler needs to know exactly what he is going to screen." He says recyclers should consider the size and density of the raw product, maximum particle length, the presence of wrappable materials and whether the material is comprised of new construction debris or demolition debris.
"The recyclers who are looking to achieve a high percentage of recycling will want the screen to maximize screening of fines, but will also want the screen to do a segregation of the material in order to present a steady flow to the pickers," Lagace says.
Scott Jable of Lubo USA, Stamford, Conn., the exclusive U.S. and Canadian distributor of Lubo Screening & Recycling Systems manufactured in The Netherlands, also says starting with end products is key in designing a mixed C&D screening system. In addition to determining the marketable end products for your area, Jable also asserts that the material's incoming condition is an issue. "Is it very much a mixed C&D product? Are there only two products that you are looking to separate? The inbound stream plays a part in that as well," he says.
Steve Cohen, president of The Screen Machine, Reynoldsburg, Ohio, says that after the product mix has been determined, concrete and asphalt recyclers should consider how they will feed the machine. "Are they going to feed it with an excavator, a front-end loader or another conveyor, because that will determine the size of the hopper," he says. The Screen Machine manufacturers portable aggregates screening and crushing equipment.
"The next decision that a customer has to make is whether or not he wants the machine to bc on wheels, on tracks or stationary," Cohen says. "With a track machine, you can literally screen and move continuously," he adds.
C&D recyclers should also consider the need for primary sizing when designing screening systems for their operations.
"The primary screen is often the most difficult thing to get correct," Jable says. "The problem with most primary screening techniques is that they don't allow for big enough screening size.
"For instance," Jable continues, "if your screen can only go up to an 8- or 12-inch screening size ... that means that everything else 8 or 12 inches and larger generally goes to a sorting line. That's a lot of material."
Jable instead suggests using a screen to remove pieces measuring fewer than 18 or 24 inches, reducing the amount of material on the manual picking line. "If your screening size it too small on your primary screen, your pickers are overwhelmed, and you don't get high throughput or high-quality end products," he says.
"I feel that the way to handle C&D is with a primary sizing unit," Guptail says. "Primary sizing is important for no other reason than to make the fines material more easily recovered in automated equipment."
Lagace also notes the importance of removing fines. "You do not want 30 percent to 40 percent of the material on that belt to be fines," he says. "On top of that, if you screen with a fingerscreener, you will have natural segregation of material and your picker will be able to be more effective since the flow of material will be of the same thickness all along the picking line."
Manual screening using picking lines, therefore, remains an important part of the process. "I believe that fully automatic sorting of C&D would be very hard to achieve since the C&D by nature is non-homogeneous and is really tough to sort," Lagace says.
"It's getting more automated all the time," Jable says. "However, there is still no product in the world that is economical that can tell you the difference between a piece of painted wood and a piece of non-palmed wood. Only the human eye can do that economically."
Jable continues, "The bottom line with automation is everything has to be justified. Automation for automation's sake is not necessarily the way to go. With a very large eddy current, for example, if it doesn't pay for itself by reducing labor costs, then it doesn't make any sense to put that in the system."
Guptail recommends a finger screen for primary sizing. "A finger-style screen usually has a big, heavy deck ... and they are able to handle the heavy loadings, which would Ire different from a wire mesh deck on a vibratory screen," he says. "I don't believe vibratory equipment with wire mesh decks are very often used on the big product, because there is too much splinter wood and wire, and things like that will get jammed."
While Guptail says finger screens are most often used for primary sizing, he says a variety of screens are used downstream. Screening equipment generally consists of" the following options: trommel, fingerscreen, star screen and vibratory screens.
"A trommel-type screen is used sometimes to make a secondary removal of a nominal 2-inch product. After that you'll see another star screen-type of a device to do the 3/8-inch cut," he says. The minus 3/8-inch material is then used for fill material.
However, Jable points out that trommels have "a definite limitation on throughput."
Some sources say that trommel screens ate decreasing in popularity with C&D recyclers in light of their large size and the increasing trend toward mobile equipment.
"A trommel is a lift and drop kind of device," Guptail says. "It is probably the preferable method if you ate recovering materials from a landfill, because you are trying to break apart something that is all caked together," he says. "If the material is just coming in off the truck and it hasn't had a chance to cake together, you tend to see more finger screens being used."
Jable adds that Lubo USA's StatScrecn offers a larger screening size and high capacity. "It also offers a high degree of agitation to the stream, which tends to get the fines off everything and also gets you better separation of material in general," he says. "You can "also adjust the screening size merely by changing the rotation speed of the starts. The faster it goes, the less material falls through."
However, Jable also points out that manual pre-sorting is required with a star screen, otherwise stringy, long material could wrap around the spinning axles.
For separating concrete and asphalt material, vibratory screens are often used "in any type of an aggregate industry," Cohen says.
"Many contractors are moving to stainless steel screens that keep the asphalt from sticking to wires and blinding the screen deck. Stainless steel doesn't cling as much as tempered steel does," Gary Pederson, vice president of sales for Major Wire Industries Ltd., Candiac, Quebec, Canada, says. "Crushed concrete creates a considerable amount of dust and fines, so blinding and plugging can happen here as well, particularly if the material is damp or if water must be used for dust suppression in urban areas," he adds. Major Wire manufactures Flex-Mat self-cleaning screen cloth.
Pederson touts the company's urethane and wire screening cloths, which he says allow for more wire vibration to reduce blinding and plugging. "In addition, they provide more open area on the screen cloth, which "allows more material to pass through the screen and increases the producer's hourly output," he says. "This can significantly decrease the producer's operating costs."
Guptail says vibratory screens and star screens are also used in sizing concrete aggregate. But Jable warns, "The thing to be cautious of with things like vibratory and finger screens is jamming. They are counting on gravity to move the material down. If you get sticky or wet material, sometimes you can clog those machines."
TRENDING TOWARD TOMORROW
While Cohen sees increasing interest in mobile equipment among aggregates processors, the trend is not universal. Lagace finds the need for portability rare in mixed C&D systems.
"I really don't believe that portability is an issue. I think that the thing I see the most is that people are not shredding up front," Guptail says. "Once you shred it and mix it, it's hard to get a clean product. It makes sense if you've got a good outlet for ADC (alternate daily cover) material, but there's only so many landfills that are going to want ADC."
While Guptaii sees a decrease in upfront shredding among his customers, Jable has noticed increasing interest from his customers in grinding material as an alternative to primary screening. "I haven't seen a study to see if it's effective," he says, though he does note that the technique increases the maintenance frequency for the grinder's wear parts.
Cohen expects to see more crushing and screening equipment that is designed to work together in the future. "In the future ultimately you'll see plants that are combination screening plants and crusher plants. The holdback to that right now is weight restrictions," he says, referring to the weight restrictions governing highway transportation of these plants.
Jable also notes the trend toward automation. "With the high cost of labor, everyone is trying to automate as much as they possibly can; but, again, you want to be sure the automation is justifiable," he says.
RELATED ARTICLE: Conveying concerns.
The effectiveness of screening equipment often has to do with the way in which material is presented. This is where conveying equipment comes into play.
"We look at C&D as a heavy-duty application," John Poplawski of Hustler Conveyor, St. Charles, Mo., says. Hustler Conveyor manufactures a full line of conveyors as well as trommels and magnetic separation equipment. "Normally, what we do is either go with a steel-belt conveyor with a 7/16" plate. What we do for baler feeds and applications of a lighter duty is use 1/4-inch."
To make the conveyors more durable, Hustler reinforces the belts every 9 inches at the pitch, Poplawski says. "Basically what it does is add another support member."
The piano-hinged belt Hustler manufactures is solid so dirt and debris will not build up, and it features bigger flights, or slats driven on the outer edges of the conveyor by a drive chain. It is also hinged every 2 inches for durability. The double-beaded apron conveyor offers better containment of materials and easier maintenance, Poptawski says.
Bill Guptail of General Kinematics, Crystal Lake, III., says apron conveyors are often used on the feed system. When working with more finely sized material, how you present it to the screening equipment becomes more important, he says. "An apron will go up a little steeper."
"One of the neat trends of the machines now that are coming out is they all have conveyors that are built on to them," Steve Cohen, president of The Screen Machine, Reynoldsburg, Ohio, says. "No longer do you need to haul the crusher, the screening plant and two or thee conveyors."
On the aggregates side of the industry, Cohen says that in addition to durability, the conveyors need to be set up quickly to reduce an operation's downtime. "If customers want to be able to sit in one place and run for several days, then they are going to need to bring a separate conveyor that is very long--an 80- or 100-foot stacker," he says. "But the whole idea of these machines on tracks is that you can make windrows. They key is instead of getting a long conveyor and stockpiling into a really high single pile, you can move the machine along and make smaller piles."
The author is associate editor of Construction & Demolition Recycling magazine and can be contracted e-mail at firstname.lastname@example.org.
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|Title Annotation:||Screening & Conveying Equipment Report|
|Publication:||Construction & Demolition Recycling|
|Date:||Jan 1, 2004|
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