From compost to topsoil and mulch to concrete, trommel screens are fast becoming a popular way to process high-quality compost, mulch and topsoil. However, a trommel screen is not limited to these markets. Owners are finding new ways to utilize these versatile sizing machines, and along the way, discovering new markets for the once considered off-size product.
For the novice, a trommel screen is basically a sizing machine. Material (compost, topsoil, wood waste, mulch) is placed into a hopper using a wheeled loader. The material in the hopper is then fed through a rotating circular drum that features a screen. The screen is designed to let material pass through that meets a desired size indicated by the size of the screen openings. Material that flows through the screen is the desired final product.
According to Tim O’Hara of Wildcat Manufacturing, trommel screens offer a great deal of versatility, no matter the product.
“We have customers using a trommel screen to process construction and demolition [C&D] material, concrete and wood waste,” he said. “In addition, a trommel screen has the ability to process wet material, giving business owners a big advantage in the spring, allowing them to start processing material weeks earlier than if they used a deck screener.”
Another advantage of a trommel screen over a deck screener are the brush systems that continuously clean the screens helping to keep openings clear of debris and promote better sizing. Since the trommel screen features a rotating drum and screen, the lifting and dropping action offers more mixing of the material and better separation.
“In addition, trommel screens are mobile,” said O’Hara. “A contractor can easily move the unit from one site to the next, which is easier than moving the raw material.”
New Markets Abound
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 pre-screening arena.
The booming biomass and ethanol industry are two major new markets for trommel screens. These industries use pneumatic feeding systems to transport the wood biomass to the burners. However, 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 air flow in the pneumatic feeding tubes, can act like a sandblaster and deteriorate the burner, which costs millions of dollars.
Another movement under way is pre-screening. Wood waste operations are using trommel screens to pre-screen wood waste prior to the grinding operation by removing non-organic material that could cause damage to the grinder. One overlooked application is the screening of crushed concrete contaminated with topsoil. Two saleable products can be created in just one pass through the trommel screen. However, it is important to note that a grizzly should be placed over the feed hopper, which is basically a grid that removes large material from the raw material before it goes into the drum to be processed. Heavy duty screens also should be used.
Finding the Right Fit
Trommel screens are not high-tech units, but there are a few features that should be considered.
“Look for a unit with a low-profile hopper to provide the loader operator with improved visibility for the loading function,” said O’Hara. “In addition, a low profile hopper will help eliminate the need to build dirt ramps on your screening sites. Also look for a unit with a large capacity hopper that can handle the bucket on your wheeled loader.”
Other features to consider are conveyors that can load directly into trucks and can rotate 180 degrees to provide more versatility in stacking. O’Hara also suggested selecting a manufacturer who has a strong dealer network and parts support. Due to their design, trommel screens are pretty low maintenance and feature few grease points, but do require routine belt and brush adjustment.
When it comes to screens, the options are limitless. Screens can range from .12 to 6 in. (.3 to 15 cm) in grid size and can vary from slotted to fine-gauge wire to stainless steel. Changing from one screen to another takes about 45 minutes per panel or the entire drum can be changed with another already outfitted with the necessary screens. O’Hara added that screens should have a useful life of around 3,000 to 4,000 hours.
Before buying a unit, the trommel screen should be carefully matched to production needs. O’Hara used a simple formula for this match-making process.
“We start by gathering the customer’s total annual volume, then ask how many hours a day they plan to operate the trommel screen and wrap up by selecting a screen size,” said O’Hara. “This information is calculated and we can determine the trommel screen production required on a daily, weekly or monthly basis to meet their annual production goals.”
O’Hara added that moisture content and the type of material also can affect the calculation.
Trommel Creates a New Revenue Stream
Jo Counts is general manager of Carolina Materials Corporation, a recycling and construction and demolition debris facility located in Lexington, S.C.
The company recycles concrete, asphalt and wood waste from land clearing and building projects, which helps to save landfill air space. Customers include residential and commercial contractors in a two county area. The company receives and stockpiles the debris and reduces the material using screeners and grinders.
“Most of our customers are contractors and the products we produce from our recycling efforts generally are material they can use back on their projects, such as aggregates, landscape mulch and topsoil,” said Counts.
One of the main byproducts of Carolina Materials Corporation is topsoil. The company will process and sell more than 10,000 tons (9,072 t) of topsoil annually. To attain this volume, great care is taken to capture as much value as possible from every load of material that arrives at the facility.
Before any of the material to be recycled is processed, it first passes through a prescreener. The objective is to remove any dirt that may be included in the debris. When processing wood waste, pieces smaller than 12 in. (30.5 cm) in length and 3 in. (7.6 cm) in diameter are removed during the prescreen process. 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 then further processed using a trommel screen to remove any pieces that are over .5 in. (1.3 cm) in size.
“We use a Wildcat 516 trommel screen with either a 5/8-inch or 1/2-inch screen to process the topsoil,” said 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. The overs are introduced back into the stockpile and this helps our facility achieve zero waste from the wood grinding operation.”
In terms of marketing the end-product there’s not much to it.
“Our marketing has really been word of mouth,” said Counts. “Many of our customers are the contractors who are delivering debris to our facility. In addition, we have a number of landscapers, municipalities and homeowners who routinely purchase topsoil from us.”
The trommel screen is an important component to the success of Carolina Materials Corporation.
“The trommel screen allows our company to make use of the waste product from the wood grinding operation that would otherwise have become landfill debris,” said Counts. “It also produces an additional revenue stream for Carolina Materials Corporation from material that normally is discarded.”
The market for trommel screens is expanding every year as they are becoming a more integral part of an operation’s equipment fleet.
“As the biomass, compost and colored mulch markets expand, we really see an increased need for trommel screens,” said O’Hara. “When a customer can screen and color mulch in one pass it can result in significant savings in handling costs. We also see potential applications in the plastic recycling market.”