Back to front: mixed C&D material may need to go through several screening steps before clean, marketable products are made.
Incoming materials can vary depending on the mixture of customers and the geographic region. (Massachusetts customers do not have to worry about palm tree fronds--South Florida recyclers do.)
And perhaps most critically, suppliers say recyclers must set their systems up depending on which end markets they intend to serve and which end products they are striving to create. The best equipment in the world will not yield a profit if it is producing material that has not been upgraded beyond landfill input.
THE END IN MIND
One of the most popular self-improvement books of the 1990s, Steven Coveys Seven Habits of Highly Effective People, listed as its first principle, "Begin with the End in Mind."
That advice seems to apply to setting up mixed C&D processing systems, according to Tim Griffing of Continental Biomass Industries Inc. (CBI), Newton, N.H. "With design, you have to start at the back end of the system and go after the objective," says Griffing.
A mixed C&D recycler who is serving the boiler fuel market will most likely need a much different system than one who is serving the colored mulch market.
In the wood markets alone, specifications for size and purity can vary depending on the product being made. Mixed C&D recyclers may also be striving to produce marketable old corrugated containers (OCC) and other grades of paper; plastic scrap; a clean gypsum product; metals sorted into ferrous and nonferrous streams; and a "fines" (the fraction of smallest particles) product that may be suitable for either alternative daily cover (ADC) at a landfill or perhaps upgraded to another use, such as for creating berms and other land-shaping goals.
But with wood making up perhaps 40 percent of a recycler's material flow, by Griffing's estimation, considering the wood end products can be a good place to Start.
While boiler fuel and mulch are two traditionally large end markets, Griffing says the production of wood pellets made for residential home heating stoves is a growing market.
Pellets can be shipped loose or bagged, with bags sized to fit the capacity of one-day's worth of stove fuel in a residential unit. Griffing says that to a surprising extent, this market is an export market, with pellets shipped as far away as Europe and Japan.
Although C&D wood has to be sorted to ensure that lead-based paint is not a factor, Griffing says that C&D debris has an advantage over forested wood. "C&D wood comes in dry, which gives it a built-in advantage when you are marketing it as a product to burn in a stove," he remarks.
Wood pellets typically are less than 1/4-inch in size, so a mixed system will need to be designed with plenty of grinding and screening steps in place to serve this market.
CHOICES TO BE MADE
After mixed C&D recyclers have determined the specifications they need to meet to serve the best regional end markets, there are still plenty of choices to be made as a system is designed and put into place.
Among the first questions: To what extent does material need to be shredded or down-sized as it makes its way through the system?
"Incoming material should be broken down to a manageable size," says Griffing. Size reduction of materials accomplishes two things he notes: It allows for uniform feeding onto conveyors and screens, and in the case of scrap wood it can help separate metal fasteners such as nails and hinges from the wood.
Even before material is down-sized, however, the human touch in the form of an alert hydraulic material handler operator and sorters at picking stations can help mixed C&D recyclers set aside high-value items (sheet metal and corrugated boxes) and spare the grinding and screening equipment from meeting up with heavy objects such as anvils or railroad spikes or troublesome plastic sheeting that can wrap around the discs on some types of screens.
"Manufacturers and recyclers have tried to mechanize as much as possible, but you still have to sort," says Griffing. "There are too many heavy objects and too many stray objects like plastic bottles that don't belong in an end product."
Automated systems can be configured in a number of different ways, with vibration, drop-through size, density, magnetic properties and flotation properties all put to use in various ways by competing equipment companies and their recycling plant operator customers.
Suppliers General Kinematics Corp. (GK), Crystal Lake, and Sherbrooke OEM, Sherbrooke, Quebec, Canada, have formed an alliance to offer systems that let recyclers take advantage of their wide variety of screening options.
Vibration is also used at the front end of mixed C&D systems, as with Sherbrooke's vibrating apron screens, which the company often offers as an alternative to trommel screens.
CBI works closely with Action Equipment Co. Inc., Newberg, Ore., and often recommends its Taper-Slot screen to help both classify by both size and density as well as moving along mixed C&D materials at the front end of a sorting system. "It's a design that works extremely well with C&D material," says Griffing.
Trommel screens are still on the scene as well, with CBI introducing a new option that features a screen consisting of 32 easily replaceable panels. Thus, when damage occurs to once small part of the screen only, a solitary worker can quickly replace the one panel and the trommel can be up and running again quickly.
Finger screens are another option. Sherbrooke says its finger screens are built to be durable in the harsh C&D climate, and that they are less subject to being rendered ineffective by material that either clogs up trommels or, in the case of wire and plastic sheet, wraps around the discs or stars in those types of screens.
Disc screens and star screens remain as an option, however, as they effectively separate by size (adjustable by the space of gaps between the discs or stars) in a manner that ideally can occur much more quickly than the process of passing through a trommel unit.
A very different animal--sink-float systems that use significant amounts of water to separate materials--can also be considered by mixed C&D operators.
MIXING AND MATCHING
With so many techniques to choose from, determining which ones to use and in which order again can mean starting with expectations are for the end of the system.
Griffing notes that individual makers of mulch may have good reasons to take different paths. If they are colorizing most of their product, they will probably want to screen out most of the fines, since fines "soak up the colorant" and thus add to the product cost.
Those producing a natural mulch product, however, may wish to keep more of the fines in, since a certain amount of dirt lends heft and the proper "feel" to mulch that consumers expect.
The Downtown Diversion mixed C&D facility (profiled in the March-April 2005 issue of C&DR), accepts a wide variety of material that is entered into a sorting system largely supplied by Lubo USA, Stamford, Conn.
The company is currently producing saleable OCC and metals shipments, with most of the wood currently being made into boiler fuel for power plants.
At that time, Downtown Diversions CEO Mike Hammer reported to C&DR that the system was achieving an 80 percent diversion rate, with only 10 to 15 percent of that total made up of ADC.
Thorough sorting systems are a significant expense for mixed C&D recyclers, but getting it right and matching it to end markets can help operators be glad that they made the investment.
The author is editor of Construction & Demolition Recycling and can be contacted at firstname.lastname@example.org.
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|Title Annotation:||OPERATIONS FOCUS|
|Publication:||Construction & Demolition Recycling|
|Date:||Jan 1, 2007|
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