April 21, 2011 | General

Screening Insights For Compost And Mulch Producers

BioCycle April 2011, Vol. 52, No. 4, p. 30
Details on trommel screen panels, as well as factors that help determine the appropriate capacity screen for an operation, are discussed.
Greg Ehm

WHEN it comes to screens for compost and mulch, the options are many. Not only are there different categories of screens such as trommel, vibrating deck and star, each type comes in a variety of capacities and screen sizes. This article focuses on trommels.
Any screen unit should be sized to the operation’s production needs. “We start by gathering the customer’s total annual volume, then ask how many hours a day they plan to operate the trommel and wrap up by selecting a screen size,” says Scott Eberts of Vermeer, which markets the Wildcat line of screens. “This information is calculated and we can determine the trommel screen production required on a daily, weekly or monthly basis to meet annual production goals.” Moisture content and the type of material can also affect the calculation.
Changing from one screen to another takes about 45 minutes per panel. Alternatively, the entire drum can be changed out with another already outfitted with the screens needed. Eberts adds that screens should have a useful life of around 3,000 to 4,000 hours, however, it really depends on the product being screened and how much heavy contamination (rock, crushed concrete, etc.) is present in the material.
With any type of screen, there are some features that facilitate both loading and output. “Look for a unit with a low-profile hopper to provide the loader operator with improved visibility for the loading function,” says Eberts. “In addition, a low profile hopper will help eliminate the need to build dirt ramps on the screening sites. Also look for a unit with a large capacity hopper that can handle the bucket on your wheeled loader.” Consider conveyors that can load directly into trucks and rotate 180 degrees to provide more versatility in stacking, he adds.

Screen panels are the heart of any trommel screen and have a significant impact on production rate and end product quality. Panels are made of spring steel, high-abrasion or abrasion-resistant wire. Woven together, the wire forms a panel that is available in a variety of sizes, depending on the size and design of the trommel screen drum. The panels fit on the outside of the trommel screen drum and can be designed with openings ranging from as small as one-eighth inch (.3 cm) up to 6 inches (15.2 cm). The standard screen panel opening ranges from three-eighths to one-and one-quarter inches (.9 to 3.1 cm), according to Eberts.
Screen panel manufacturers should have the ability to run bulk density weights on the material being processed and correlate that to the drum size and the amount of material on the screen panel at any given time. This information helps determine the appropriate wire gauge for the specific application. “This testing allows us to precisely match the wire gauge and screen panel opening to the material you are processing,” he explains. “If you’re not taking this extra step you may not be getting the best performance from your screens.”
Another option to consider is tensioned screen panels, which add tensile strength and allow use of a smaller gauge or diameter wire. A smaller gauge wire opens the screening area, meaning there is more area to screen material, which ultimately enhances production. This is especially true when working with wet material, as there is more area for the material to flow through on a tensioned screen panel.
“Tensioned screen panels can also provide benefits to trommel screen operators,” says Eberts. “A trommel screen featuring 10-gauge (3.4 mm) wire will typically out produce a unit using one-quarter-inch (6.3 cm) wire by up to 25 percent at the end of the day. This is based on the same-size screen opening, but using smaller diameter tensioned wire can make a difference.”
Customers wanting to boost production will usually jump to the conclusion that a bigger machine is needed. But that’s not always the case. Wildcat recently worked with a customer who was screening sawdust to be used in the manufacture of pressed fireplace logs. The material specifications were very stringent and the customer had trouble with the sawdust rolling into balls within the trommel drum.
“We contacted our wire supplier and discussed the challenge,” says Eberts. “They created a clean-through screen panel. Instead of every wire being interwoven, every other wire was straight. This minor change modified the momentum of the material within the drum just enough to eliminate the balling so the material wasn’t constantly rolling at the same pace. The customer could successfully screen the product to meet the stringent requirements.”
Another customer was looking to upgrade its machine to achieve a 30 percent increase in production. Rather than purchasing a larger machine, the customer installed slotted screen panels, boosting production by 35 percent.
All screen panels stretch just like a chain or conveyor belt. They need to be inspected weekly to make sure the panels are not loose and are properly tensioned (if using tensioned screen panels). Standard screen panels typically feature an attachment on each end of the panel. If the wires and attachments are not properly crimped, the panels can pull out at the crimped end and begin spreading, adversely affecting screening consistency.

Traditionally, trommel screens have been used for compost, topsoil and mulch, but new markets are emerging that offer expanded opportunities in the energy, value-added mulch and prescreening arenas. Wood recyclers servicing the biomass and ethanol industries are a case in point. Pneumatic feeding systems are used to transport biomass to the burners. These systems have stringent specifications and require the material to be sized, removing the long wood fibers and fines. The long wood fibers can become lodged in the pneumatic feeding tubes resulting in potential jams. On the flip side, fines like sand and dirt, combined with the rapid airflow in the pneumatic feeding tubes, can act like a sandblaster and deteriorate the burner, which costs millions of dollars.
Another application is prescreening. Wood waste operations are using trommels to prescreen wood waste prior to grinding by removing nonorganic material that could cause damage to the grinder. Another application is screening crushed concrete contaminated with topsoil. However, a grizzly – basically a grid that removes large material from the raw material before it goes into the drum – should be placed over the feed hopper. Heavy-duty screens should be used.
Carolina Materials Corporation (CMC) in Lexington, South Carolina recycles concrete, asphalt and wood waste from land clearing and building projects. Customers include residential and commercial contractors in a two county area. CMC receives and stockpiles the debris and reduces the material using screeners and grinders. “Most of our customers can use the material we produce, such as aggregates, landscape mulch and topsoil, back on their projects,” says Jo Counts, General Manager. “We process and sell over 10,000 tons/year of topsoil. To attain that volume, care is taken to capture as much value as possible from every load that comes in.”
Before any of the material to be recycled is processed, it passes through a prescreener to remove any dirt that may be included in the debris. When processing wood waste, pieces smaller than 12 inches in length and three inches in diameter are removed. This material is mixed with the dirt, stockpiled and turned into high-quality topsoil.
Once the stockpile has had time to decompose, which usually takes 30 to 45 days, it is further processed using a trommel screen to remove any pieces over a half-inch in size. “We use a Wildcat 516 trommel screen with either a 5/8-inch or half-inch screen to process the topsoil,” says Count. “The unit operates one to three times per week, based on volume, and produces anywhere from 90 to 120 tons of material per hour depending on the moisture content of the topsoil. Overs are introduced back into the stockpile, which helps our facility achieve zero waste from the wood grinding operation.”

Greg Ehm is a Technical Writer with Two Rivers Marketing in Des Moines, Iowa.

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