Sorting Equipment Focus--On Screen

EASY AS ONE, TWO, THREE. Jeff Leech of New Bedford Waste Services, New Bedford, Mass., says that his current plant is set up to make large volumes of alternative daily cover (ADC) with a lot of fines. "The plant is not set up for high yields of wood for boiler fuel or mulch. For that, we would have two grinders—one for the wood only and one for all the other material to be ground up for either ADC or easier transport to disposal."

To make its ADC, New Bedford uses two Action slotted screens before the picking line. One takes out the 6-inch minus, the second removes the 2-inch minus. "The goal is to give the pickers a manageable spread of material in order to recover the maximum amount of product possible," says Leech.

He says that he has run most every screen type for C&D, including a trommel, star and finger. Leech likes the slotted screen because of its simplicity and durability. "It has almost no down time. When it does go down, it is usually a $7 V belt that takes 15 minutes to replace."

A variety of screens is what Downtown Diversion and its sister company Looney Bins operate in their two Los Angeles area plants. "At our small operation we have a ReTech trommel," says Mike Hammer. That unit has worked well for the company in processing C&D, he says, but is very limited in the number of tons it can do per hour. "The starscreens at our other operation are capable of doing a whole lot more."

That plant has three Lubo starscreens, virtually one right after the other. The first one scalps off 8-inch minus, the second takes the 2-inch, and by the time the third one gets the 1/2-inch minus, the material has also been through two picking stations.

Hammer likes the quality of product he gets from the screens, but cautions that when using a star as the primary screen, one must do presorting to remove long, stringy materials that can wrap around the stars. "Otherwise, you are constantly cleaning," he says.

A brief presort should be done, according to Ray Kvedaras of Cooper Tank Transfer in Brooklyn, N.Y. He advocates using a grapple to pull out heavy metal and garbage.

Hammer agrees, adding that for presorting to be economically viable, "you have to be careful and do it efficiently, as the margins are thin in this industry."

However, at Cooper Tank Transfer, Kvedaras uses a General Kinematics fingerscreen as the first screen because of the question of wrapping. His company operates on a little more than an acre and processes 1,500 tons per day, so there is no room for pre-sorting.

The operation mostly makes ADC. For its second screen Cooper Tank Transfer has the Bezner "bucket" screen, a simple unit from Europe that is rarely seen here. Kvedaras says he got the screen as part of an old sorting system the company picked up some time ago. However, he says he would consider having a starscreen as the second part of the processing line because that type of screen can really provide a much finer cut. "Also, you don’t have to allow room for an inclined unit." Kvedaras says, which is very important when working in confined quarters.

He thinks that the product being made isn’t as important as the density of the material coming into the plant. "Most screen manufacturers gauge production with tonnage. This is the most deceiving myth to people who don’t understand densities," Kvedaras says. "Mixed C&D waste could range from 400 pounds per yard to 800 pounds per yard. So ask the manufacturer what density their production rate is based on and get the rate in yards per hour. For C&D you should get the heaviest screen you can get."

He also believes that C&D recyclers need to screen more than once to make good products. "Two is good, three is best," he says.

Three mechanized screening stations are part of Waterway Materials’ two-line recycling plant in Chesapeake, Va.—star, air separation and float tank, in that order. Jimmy Sissons says that the screens are part of a total design system. Even before the material hits the starscreen, the C&D material has been through a presort where the excavator operators are pulling out oversized steel and concrete, long wire, couches, mattresses and the like before the remainder is fed into a pre-grinder to be brought down to a 2-foot-minus. After the pre-grinder another manual presort removes textiles and plastics. Then a Machinex starscreen removes the 1-1/2-inch minus material that is stacked for use as ADC. "The starscreens are good at getting out fines from wet material because all that bouncing and jiggling will loosen up the material," Sisson says.

The air separation and float tank are used in tandem to remove heavy products, mostly aggregates that can be crushed by the company’s crushing plant. Wood for hog fuel and a mulch filler should be left, Sissons says.

What is an even more important screening technique, Sisson says, is to control the inflow of material into the plant. "When we started looking at systems we heard that you never know what you are going to get in the C&D waste stream, and to a degree that is true," he says. "But you can control that change with a relatively narrow bandwidth that will allow you to run the plant at an optimum rate."

ALL WET. One piece of equipment that many of the operators interviewed disagreed on was the float tank, a device that separates the heavies from the light materials using water. Hammer of Downtown Diversion considered putting one in when installing the larger recycling plant, but hesitated because of concerns about regulators and stormwater. However, his company is looking at the newest float tank designs and may still install one.