Dedusting is no longer a high-priced headache for the Hopkinsville Quarry in Kentucky. One of more than 60 locations owned by Rogers Group Inc., this site has eliminated the need for specialized equipment such as air classifiers or high frequency screens. The operation separates out its excess minus-200 mesh material, while netting a premium aglime product, with the use of a conventional screen -- a new cost-effective and field-proven solution resulting from meticulous analysis and inventive screening ingenuity.

Nearly 50 percent of the material produced at the Hopkinsville Quarry feeds its onsite asphalt plant. Tight specifications combined with astronomical liquid asphalt costs demand a consistent control over the percentage of minus-200 mesh in the mix. Additionally, as the site is rurally located, the facility produces a saleable aglime product from its ultra-fine material. As such, the operation had always opted for dry classification processes using an air separator, or high frequency and screening equipment, which had presented high capital and operating costs.

However, according to the management and crew, this classification equipment was simply not getting the job done, especially when moisture content was a factor. Consequently, the asphalt plant was often sent scrambling into mix adjustments owing to spikes or dips in 200-mesh gradations. At the same time farm customers were faced with clogged spreaders due to an overly coarse aglime product. Management knew the situation had to change, however, they had no immediate or easy answers.

Enter Polydeck Screen Corp. and its team of screening experts, led by New Business Manager Perry Miller. As an instructor, Miller was participating in a Rogers Group onsite training session when the topic turned to the search for a cost-effective dedusting solution. A 20-year-plus screening industry veteran, Miller shared the possibility of a yet untried method, now much more of a potential success owing to Polydeck innovations in screen media designs, molding technology, raw material formulations, and screen panel attachment systems.

With the necessary calculations completed, and after consultations with screen manufacturer Deister Machine Co. Inc., Polydeck presented the Rogers Group engineering team with a proposal to test a new dedusting solution using conventional screening instead of costly specialty classification equipment.

The analytical approach
“We chose the Hopkinsville location for the test because we knew if it would work there, it would work almost anywhere,” said Joseph Krause, project engineer for Rogers Group, who points to site characteristics such as mining from four different ledges, producing specialty products and blends, and having an inconsistent feed. “Those are the very challenges that made the previous use of an air separator very difficult. You can’t adjust them easily, so if your material changes, there is little you can do. Plus, they can’t handle moisture content above 1.6 percent,” he said.

The new dedusting approach began with the installation of a Deister 6-ft. x 20-ft., triple-deck screen. Polydeck worked with Deister to set the rotation, stroke, angles of declination, and speed to an agreed-upon formula that would give the unit the optimum versatility while allowing ease of adjustments.

Next, the screen media was highlighted with the strategic placement of 1-ft. x 2-ft. specially formulated Flexi-style rubber panels with 1 mm x 12 mm openings. “We know of no other panel on the market with this spring-like, flexible nature and such a small opening,” Miller said, explaining that its flexibility is what keeps the screen from blinding.

The panels, he said, are engineered in a “maxi” screen panel design with a narrow outside ridge, and are mounted on Polydeck’s PipeTop II frame system with unique 0.5-in.-wide attachment rails. “This provides maximum open area, unlike typical media systems with mounting styles that create up to 3 in. of dead space where fines may build up,” he said.

“When you’re working with gradations all the way down to minus-200 mesh, it’s all these equipment features combined that enable us to do more work within a smaller footprint, such as a 6-ft. x 20-ft. screen, and still match the level of efficiency that brings the mix into the required specifications. We are also able to control the bed depths on each deck, which truly allows the panels to work their magic,” he said.

But perhaps the most important key to this new approach is the required analysis of the sampling and data. Miller maintained a detailed spreadsheet that documented feed material to the screen, over the screen and through the screen, while noting moisture content and recording the end specifications of both the asphalt dust and the aglime.

Initially, there were considerable variations in gradations, and, after careful examination, it was learned that the inconsistencies in specs were the result of differences in material characteristics per each ledge. In fact, it was discovered that regarding one particular ledge, material was already in spec before it hit the screen. “Once we learned that, we could make adjustments in panels, volume and depth of bed depending upon the ledge being mined, and maintain the desired specifications,” Miller said.

The big benefits
“The way we’re running now, I can predict gradations each day within just a couple percentage points and that means no complaints from aglime customers or from the asphalt plant,” said Eric Steidl, plant manager of the Hopkinsville location. He says the only real variability is caused by the change in ledges, and, to a much lesser extent, by feed rate or moisture content. “We haven’t had any problems with the screens blinding up as we have so much screening area for what we are sending it, and we have enclosed the belt leading to the screen to mitigate any issues caused by rain,” he said.

Steidl said operating an air separator had been a nightmare. “It was heavily dependant upon moisture content and feed rate. If you fed it too much, it would plug and if fed too little, you were cutting too well. There was no easy way to control the cut as you would have to change the speed or the angle of the vanes, and that was a day-long process at best,” he said.

But what Steidl said he really wants other plant managers to know about the new dedusting solution is that “this screen is not a specialized unit. It’s like any other screen I have. I know how to change the bearings, the oil and the media. I know the timing belt, springs and chutes. When you start talking about classifiers, sand screws and dewatering screens -- those are specialized pieces of equipment that the crew doesn’t know much about. If you can go with a conventional screening solution, your headache is over as you already know the equipment.”

Polydeck Screen Corp., www.polydeckscreen.com

Carol Wasson is a veteran freelance writer for the aggregates and construction equipment industries.