No Speeding

Evolution is an important concept for recyclers of mixed C&D materials. Years of trial and error are helping recyclers and equipment suppliers discover which new techniques are worth expanding and, in some cases, which older methods may be worth retiring.

The path of processing technology change would probably not be described by most as a smooth, paved, six-lane highway. Rather, it is a winding road crowded with potholes and featuring surprises around many of its curves.
Regional differences in material and differing suggestions from equipment suppliers add to the variables, but some common findings do seem to emerge. Interviews with suppliers and recyclers indicate that industry-wide changes can be slow, but the body of knowledge is growing.

 

Hands and Screens

Mixed C&D material lives up to the definition of the word “mixed” and perhaps even surpasses it. Chunks of concrete, pieces of metal, wood in all shapes and sizes, drywall, plastics tarps and empty soft drink bottles are all likely to be part of the container loads being tipped.

In most cases, human contact with the stream remains important to single out both the most valuable materials and the most problematic materials at the front end of the process.

Eric Winkler of Bulk Handling Systems (BHS), Eugene, Ore., says a “positive pick” at the outset remains important so that materials that can make up loads of metal, old corrugated containers (OCC) and clean wood can be separated early on. The majority of C&D sorting systems put into place include stations with chutes and bunkers designed for this purpose.

System Snapshots

Recyclers have a number of options available as they try to turn a commingled stream of C&D materials into marketable products. At Shamrock Disposal in Blaine, Minn., commingled material is dumped at the front of the company’s 20,000-square-foot facility and weighed before a front-end loader stockpiles the material for an excavator. The excavator feeds a hopper on a Continental Biomass Industries (CBI) Annihilator shredding machine that sizes material for a sorting line. The shredder has a 6-inch forged steel, 20,000-pound rotor with reversible tips and an 8,000-pound hydraulically actuated anvil door. “[The shredder] just makes it into a size that you can work with,” says Shamrock Disposal co-owner Rick Gersdorf. “For example, a 12-foot 2-by-6 is hard to maneuver compared with four 3-foot pieces. We typically like to size material to 12 to 18 inches.” After primary reduction by the Annihilator, the material is conveyed to a sorting line, where 12 to 15 laborers separate large pieces into containers that feed concrete bins located down on the main level. The remaining smaller material runs beneath a magnet, and particles smaller than 1 inch are screened out and used for landfill alternative daily cover (ADC). On the main level, a front-end loader loads trucks that deliver the larger pieces to customers that sell scrap material or manufacture recycled products. More than 1,000 miles east, at New England Recycling in Taunton, Mass., loads are dumped onto the floor at the facility, according to facility manager Paul Correia. He says large pieces are manually sorted. Next the remaining material is conveyed to a 0.5-inch trommel screen and then loaded onto another belt and a sorting crew averaging about 24 workers separates the individual materials, feeding individual hoppers at the top of chutes feeding bins below. Bulky trash, cardboard, aggregates, steel, aluminum and wood are sorted in order. On the Pacific Coast, Recovery1, Tacoma, Wash., relies on sorting equipment that includes a two-stage WSM 72-15 disc screen from West Salem Machinery that provides final material sizing. The company’s Terry Gillis says the screen was built to company specifications for a desired mulch particle size of 3-inch nominal. Oversized particles are conveyed back to the shredder. “A disc screen has a free flow, so air can flow up through it much more easily than I perceive an oscillating screen or vibratory screen can do, and we get really good pickup of material we don’t want in our wood product,” says Gillis. “I don’t ask my guys on the sort line to pick out that small plastic and stuff; I just say let . . . the equipment pull it out.” Recovery1 also operates two non-blinding bivi-Tec screens from Aggregates Equipment Inc.—one for dirt and one for gypsum. An oscillating BM&M wood fiber screen and a trommel screen built by the company itself round out Recovery1’s equipment lineup. —Don Talend

Tim Griffing of Continental Biomass Industries (CBI), Newton, N.H., notes that buyers of most end market materials (such as aggregates, wood, OCC and metal) consider plastic an unwelcome contaminant, so the earlier it can be indentified and separated the better. “For the most part, you’ve got to touch the plastic,” he remarks.

Even with end markets developing for some types of plastics in the C&D stream, removing it as a contaminant remains important in its own right. In addition to hand selecting larger plastic objects, Griffing says suction can be used to remove smaller pieces of the “super light” fraction, though in his experience he says recyclers “will also end up with other things” when suction is used.

Beyond hand sorting, screens and specialized conveyors remain the work horses of automated mixed C&D systems, separating materials by size and by density.

Such technology “is still the mainstay” says Winkler. In the Pacific Coast market with which Winkler is most familiar, he says systems often split initially using screens and mechanical separation to create streams sorted by size, such as creating an 8-inch-minus and an 8-inch-plus split.

The smaller fraction can then be further sorted by both size and density so that aggregates, wood and dirt can be effectively separated from each other. The more a recycler chooses to invest in screens or in emerging separation technologies, the cleaner the streams that can be achieved.

Another option available to C&D recyclers is to shred material early in the process, notes Griffing. He says CBI’s Annihilator model is adjustable, but many recyclers say they find that shredding material to 2-foot-minus provides good sorting conditions.

“Two feet has become a preferable size,” he notes. “The advantages [for pickers] are uniform size, uniform material flow, and there is less handling of the material up front.”

At some C&D recycling facilities, “It can save floor space if you don’t have an excavator on the tipping floor crunching material ahead of time,” adds Griffing.

 

Emerging From The Pack

In several other recycling applications, including at auto shredding plants and at municipal material recovery facilities (MRFs), optical sorting, air jets and other emerging technologies are being added to sorting systems.

The role of such new technologies at mixed C&D recycling plants may not be fully clear in the minds of those sources contacted for this story.

“You have to set the table for optical sorting,” Winkler says of the role of that technology within the C&D sector. “A clean stream created by mechanical separation is needed first,” he says.

Winkler says optical sorting can help eject marketable OCC and PET bottles, as examples. He also says, though, “I’ve seen optical technology applied too soon in the process, and when that happens, it flat-out does not work.”

Air separation to remove lighter materials is another technology that has started to gain some momentum. Griffing says air knives can be suitable for a medium-sized fraction ranging perhaps from 2 to 12 inches.

Winkler offers a similar comment and says, as with optical sorting, “You have to properly present the stream. If I send pieces from 2 inches to 20 inches in size to an air system, larger chunks that should be ‘lights’ then become ‘heavies.’ Size it with screens first, then apply air and then maybe optics,” Winkler says.

Varying combinations of existing and emerging technologies are being deployed, and experiments are likely to continue with the end goal of sending out clean shipments in mind.

Griffing says, “No matter where you go in the United States, wood is 35 to 45 percent of the incoming product, or more. And pellets made from clean wood are a growing market, but that is a tight-spec market.”

Many of the other end markets for recyclable materials pulled from the mixed C&D stream have equally strict requirements, whether they are energy markets or manufactured product makers with tight specifications.

With the profit motive in place, recyclers and equipment suppliers are likely to keep tinkering to advance technology and how it is deployed.