Sorting it out: meeting the screening challenges associated with mixed C&D materials requires automation and a few helping hands.
Screening equipment, from trommels to vibrating screens to finger screens to disc screens, can facilitate further separation and sorting of mixed C&D material by sizing the material as it moves through the system, creating an overs line that is hand picked and an unders line that is separated into its constituent parts using automation in the form of magnets, air classification systems and sink tanks. The diversity of the materials in the stream makes this a challenging, though achievable, process.
"The only consistent thing about mixed C&D material is its inconsistency," Bill Guptail, director of process sales for General Kinematics, Crystal Lake, Ill., says.
While it's not difficult to separate functionally, he says the percentage of material that will constitute the overs and unders lines is difficult to know when setting up a system.
"When you design a system where you split material flows, it is difficult to know what will go on through the unders screen so you can design the downstream processing appropriately," Guptail says.
Dirk van der Wal, managing director for screening equipment manufacturer Waltec, based in the Netherlands, says C&D material doesn't have to be difficult to separate, as long as the correct processing principles are used. "This is explained by the fact that the costs are calculated by weight, and that C&D waste is a relatively heaW waste," he says. "The effects of this sort of process are therefore immediately very positively or negatively noticeable."
Scott Jable of Lubo USA, a screening and recycling equipment manufacturer headquartered in Stamford, Conn., says that mixed C&D material is difficult to separate in light of its complex composition. "You can get carpeting grabbing hold of rebar that is still wrapped up in concrete," he says.
This complexity requires more sophisticated separation systems, Jable says. "It requires a little more automation to do it successfully."
The challenge lies in sorting a large volume of material while keeping labor costs low, yet still producing clean end products. "To accomplish all three of these is difficult and a great measure of success," says Jable.
A look at the end products that can be created from a mixed C&D stream helps to illustrate the diversity of material contained within.
Generally, the products created from mixed C&D material are aggregates--created from the concrete, asphalt, block and brick in the stream--treated and untreated wood, ferrous and nonferrous metals, gypsum, cardboard and fines, which can be used for ADC (alternative daily cover). Of course, the number of products C&D recyclers produce varies based on their local markets. Material from new construction will vary from demolition debris, Guptail says, adding that systems should be versatile enough to handle material strictly from new construction projects as well as material from demolition sites.
The composition of incoming material is an important factor in determining the appropriate screening and sorting system for a company.
"All separation systems start with a correct analysis of the composition of the material to be processed at a desired volume per hour," van der Wal says.
He adds that nearly all systems work on the principle of degrees of separation that involve screens, air classification, float tanks and magnets as well as hand sorting.
"The first thing that has to happen is a pre-sort of material before it goes onto the system," Jable says. "There are always prohibitives that you don't want to put into the system," he says, citing couches and kitchen sinks as two such examples.
Jable likes using vibratory feeders to feed material to the system because they help to spread material out. "A conveyor doesn't do that. You get lumps and clumps," he says.
However, John O'Neill, vice president of sales and marketing for McCloskey International, Peterborough, Ontario, Canada, prefers using trommels to feed material into the system because they can handle irregular shapes and do not clog easily, making them lower in maintenance, he says.
But O'Neill says vibrating screens are preferable in applications where the material is sized anywhere from 3/8 inches to 1/4-inch minus. "You get the right screen cloth on there, and it can size the material a lot faster than a trommel screen could," he says.
Jable then suggests another pre-sort on the mezzanine to remove smaller residuals, such as plastic sheeting and toasters.
At this point, he suggests a large cut to remove 2-foot-minus material so that manual pickers can sort the overs line easily because the volume of material on the conveyor is reduced.
"On the overs line of the large screen, you can pull all commodities and let the residue go off the end as a negative sort or let the aggregate be the negative sort if there is a lot of that on their line," Jable adds.
He stresses the need for flexibility when it comes to screening equipment. "A good system should be modular," Jable says. "You may start off small and have a single line without much automation. A good system will allow you to grow easily, add a second line, air systems and magnets."
Guptail recommends designing a system based on volume, saying that the overs line should be designed to handle most of the system's volume. "The unders line takes 60 percent by weight, but only 30 percent by volume," he says.
For high-volume recyclers, Guptail suggests using a finger screen to make a primary cut of nominal 8-to-10-inch material, sending the overs material along to a picking line and the material under 10-inches on to the unders line, where a magnet will recover the ferrous material in the stream. At this point, the material on the unders line will go on to a secondary finger screen that will remove 2-inch-minus material, while the larger material will go on to the a de-stoner classifier to recover the aggregate.
General Kinematics and Sherbrooke OEM, Sherbrooke, Quebec, Canada, put in a classification system at a facility in the southern U.S. that Guptail says he considers an ideal system. The system, which can process up to 500 cubic yards per hour, uses two dual-deck vibratory finger screens to receive and size material at 8 inches. Following the first cut, heavy-duty apron conveyors elevate and transfer material to a trommel and picking platform. The minus-8-inch material goes on to an automating sorting line.
For recyclers producing less volume, Guptail suggests removing the 2-inch-minus material using a finger screen and manually picking the larger material.
The overs line of a C&D recycling system uses little automation because of the large pieces involved. The strength of air streams employed on the overs line would slow down the processing rate of the system, and magnets would pull large pieces of metal that could swing out and injure a manual sorter, Jable says. However the unders line generally incorporates a great deal of automation.
Guptail also says automation is rare on the overs line because the variability of the material is too great and the magnets and such involved would need to be quite large and powerful.
Some screening systems incorporate crushers or shredders at the front end to reduce the size of material prior to screening. However, not all recyclers or suppliers agree on the merits of such an approach.
"Pre-reduction can help" in some applications, van der Wal says. "Through pre-reduction we get a more homogeneous size, whereby the separation equipment in the production line can be set up more accurately at an average value. This makes [the separation equipment] much more efficient and will raise the quality of the finished product," he says.
"Systems with a pre-reducer often have less downtime and a much higher capacity," van der Wal adds.
However, pre-reduction can sometimes lead to disadvantages. "If there is a lot of trash in the material flow, then pre-reduction sometimes makes it even more difficult to separate effectively, simply because it mixed together everything that you later want to separate," van der Wal says.
Guptail also cautions against reducing material prior to screening, unless a company is making an ADC product, because of the amount of commingling. "Sometimes a combination system is what people will use," he says. "Unpicked material will be crushed and screened for ADC."
MEASURES OF SUCCESS
While a few yardsticks can measure the success of a screening system, for Jable, nothing is more important than the quality of the end products.
"If you don't have high-quality end products, the markets tend to go away," he says. "You need to run as fast as you can with maximum throughput that still allows you to get saleable end products."
Jable also says C&D recyclers should consider their processing rate and the amount of manual labor involved in arriving at their end products. "If a company can do 10 or 12 tons per hour [but] get dirty or unsellable product, they haven't maximized the return on their investment," he says.
Guptail suggests that the percentage of recovered products a company achieves through its screening and sorting system is a good measure of a system's effectiveness. Van der Wal indicates that the best measure for a system is the purity of the finished product because it determines the price of the yield. "You can make a really great product, but if it costs more than it earns, then that makes no sense. This is the difficulty with C&D recycling," he says.
"C&D recyclers must continue to create high-quality end products to be open to the largest variety of end markets," Jable says. "If recyclers create quality to sell, more people are going to buy."
The author is associate editor of Construction & Demolition Recycling and can be reached at firstname.lastname@example.org.
* Scott Jable of screening equipment manufacturer Lubo USA, based in Stamford, Conn., says that optical sorting of CSD material is becoming a more viable prospect.
In optical sorting, a scanner reads the footprint and composition of the material on a conveyor, employing air jets to eject materials based on their analysis into designated recovery bins.
Such systems are advancing to become more realistic for companies processing a high-volume of materials. Jable says optical sorting is much more affordable and more accurate than it was in the past.
Lubo USA is the North American distributor for TiTech Visionsort systems, which are manufactured in Norway. Jable says TiTech has hundreds of installations sorting recyclables of all kinds with accuracies in the 95 percent range.
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|Title Annotation:||Operations Focus; construction and demolition|
|Publication:||Construction & Demolition Recycling|
|Date:||Mar 1, 2005|
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