By Steve Fiscor, Editor-in-chief

Screening machines serve several functions in the modern prep plant. First and foremost is size separation, but they also dewater and remove impurities. The U.S. coal industry varies greatly by region and so do screening needs. Not only is the hardness of the coal a concern, but other issues such as rock, clay and acidic water affect a screen’s operation.

The size of the machines continues to grow as well. While some prep plant operators have adopted wider screens, others are reluctant to make the leap. Many of the new U.S. plants are being built as a single-line process and wider screens are probably the best way to increase throughput in that type of system. Aside from capacity, the larger machines also reduce the screening footprint inside the plant—one large screen instead of multiple smaller screens.

Mines in China and Australia are routinely now using 14-ft wide screens. Assuming the U.S. follows suit, many vendors believe 14-ft screens could becomes a reality in the U.S. soon. Right now, the largest screen in a U.S. prep plant is 12-ft wide.

Except for the deck, most screening machines today are pretty much maintenance free. As far as woven wire vs. urethane panels, all of the vendors agree that there are practical applications for both types of decking. Woven wire offers better open area. From a maintenance standpoint and cost of operation, however, it’s difficult for woven wire to compete with the modular urethane panels. Many of the companies that make screen machines also have a division or sister company that makes and sells screening media.

Tabor Offers New Screening Machine, Media
Tabor Machine Co. recently announced the availability of its widest screening machine yet. “During the last several years, we have seen more use of wider screens,” said John Casey, sales and applications engineer, Tabor Machine. “During the 1980s, the mainstay for screening machines was the 6- x 16-ft single deck horizontal screen. The push now is to 14-ft wide, or even 16-ft wide screens.”

The trend toward the bigger machine is continuing albeit a slower pace than what’s happening abroad. “Only one of our customers is using 12-ft screens,” Casey said. “We have customers that refuse to buy a 10-ft screen. They would rather run two circuits with multiple screens. If one circuit goes down, they can still run the plant at half capacity.”

The company introduced a new line of 12-ft multi-slope and horizontal screens during September 2010. “We recently received our first order for the new 12-ft screen,” Casey said. “Those machines are scheduled to go into production within the next month or so.” Tabor screening machines are field proven, well built and that engineering design carries through to the 12-ft screen and eventually the 14-ft screen, Casey explained.

From a maintenance standpoint, Tabor stuck with standardized parts for the new line. “Whether it’s a 12-ft Banana or a 6-ft single-deck screen, we are still using the same parts,” Casey said. “The mines won’t have to carry a lot of extra, new parts to service the machines. The maintenance costs per ton processed decreases on the larger units, but components for a 12-ft machine will be more expensive than a 6-ft machine. Labor costs would be less.”

When it comes to screen media, Casey believes there are applications for both woven wire and urethane. “Urethane’s strongest points are wear life and safety with the ease of replacement,” Casey said. “To say one is better than the other would be foolish.”

One of Tabor’s sister companies, Norris Screen, recently developed a new modular screen media: the Phase 3 Pinless (P3P) system.

“With the P3P, Norris has the ability to offer a snap-in woven wire panel or a snap-in perforated plate,” Casey said. “Instead of replacing a 4- x 8-ft piece of woven wire, the maintenance crews handle a 1- x 4-ft piece of woven wire. That has a huge safety advantage especially with the sloping decks.” With the pinless system, Casey explained, the plant technicians no longer have to worry about dropping pins into the underflow or the chute work. They simply snap in the panel and they are off to the races—a quick, seamless transition.

Derrick Pushes the Envelope for Clay Removal
Derrick designs and manufactures high frequency vibrating machines and screen surfaces used to screen a wide variety of materials and its Stack Sizer has redefined the concept of efficient, fine particle wet screening. It offers high capacity with a minimal amount of floor space. Using as many as five decks operating one above the other, it can be fitted with the company’s urethane screen surfaces for fine coal separation.

“With the Stack Sizer, we have been able to demonstrate to prep plant designers and managers how our technology can offer an improvement over traditional processes,” said Paul Brodzik, product applications manager, Derrick Corp. “It can replace sieve bends or cyclones and perform a similar fine (75 to 100 micron) separation for them.”

The unit offers a big savings in square foot area and operational costs. “The Stack Sizer replaced many of the old multi-feeds that Derrick used to make,” Brodzik said. “It’s pretty much the same floor footprint, but the new machine is a little taller and it has double or triple the capacity. With the 75-micron screen panels, we’re now screening even finer size fractions, grabbing all of that coal down to 200 mesh.”

Plant operators have relied on Derrick high frequency systems to reduce ash by screening out the minus 100 mesh (100 micron) clay fraction. “We started at 150 microns and advanced to 100 microns and now we are operating at the 75-micron level,” Brodzik said. “It’s a clay removal device. It knocks down the ash and improves the calorific value of the coal.”

The product from the Stack Sizers reports to centrifugal dryers. “By removing the clay, some of our customer are telling us that the life span on the centrifugal dryers has almost doubled between rebuilds,” Brodzik said. “That’s an unintended consequence with great benefits.”

The Stack Sizer debuted in 2004 and Derrick currently has 30 installation in U.S. coal. The capacity for the 100-micron machine is 9 tons per hour (tph) per deck, or 45 tph per machine. With 75 microns, the machine processes 4.5 tph per deck, or a total of 20 to 25 tph. A typical prep plant would install two machines or sometime four units.

The development of 75-micron apertures for Derrick’s urethane screen surfaces is a remarkable achievement for fine particle separation. With three to four times more effective open area than conventional urethane screens, Derrick claims its fine mesh urethane screen surfaces will not blind with near-size particles. “Maintenance concerns are minimal,” Brodzik said. “Our panels are lasting up to two years. The plant manager checks the panel tension occasionally. Most plants are getting a year before they change the panels.”

Currently, the engineers at Derrick are looking at replacing cyclone banks with a Stack Sizer. “Generally, the screens are more efficient than cyclones,” Brodzik said. “It’s been working well for us in iron ore. In the grinding circuit, we are replacing cyclones with the screens to reduce energy demand. That’s the question we will ask next: Can the Stack Sizer compete with cyclones in a prep plant?”

Customers are also asking Derrick about fines recovering systems for refuse impoundments and settling ponds. “We have hundreds of fines recovery systems in sand plants around the world,” Brodzik said. “Our system should work the same for coal with a few modifications to the pumps. We are waiting for the startup of the first fines recovery system intended for use on a coal impoundment. The performance of that first system should prove the effectiveness of the technology.”

Metso to Launch a Line of Screens for Coal Applications
At one time, Metso screens—or what was Allis Chalmers, then Svedala—were prevalent in the coalfields. The company’s market share may have been as high as 50% in the late 1980s. While many machines are still operating, competition eroded that figure during the leaner times of the 1990s. Now the company is looking at a strategy to re-enter the coal market with not just screening machines, but its entire product line, which includes a full service division with pumps, flotation, etc.

“We are now trying to reinvigorate the side of the Metso business that serves the coal market,” said Scott Snyder, product manager, vibrating equipment NAM, Metso. “Metso has re-established a screening machine factory for coal applications in Columbia, S.C. We are centralizing production at that facility. We’re in an assembly stage now with a qualified supply chain.”

The company had an existing factory with high capacity overhead cranes, which also served as the distribution center for the U.S. “We have renovated part of the factory to produce screening machines,” Snyder said.

Metso did not abandon the screening business entirely. “The majority of our business in screening today is in the construction/aggregates segment,” Snyder said. “We sell a lot of equipment into that segment. We recently released a whole new line of screens for the construction business.”

Snyder also explained Metso will produce a high-quality, high capacity screening machine designed specifically for coal preparation. “We want to re-establish the brand name in the coal business,” Snyder said. “We will place coal screens in 2011. We are targeting some key projects to prove what we have.”

The company will unveil new technology at Coal Prep 2011. Although he would not give specifics, Snyder said the technology Metso will be imparting into its screening machines will be extremely good at handling wet material that tends to blind screens. “We have developed ways to eliminate blinding problems without changing media, but by changing technology,” Snyder said. “Another exciting area for Metso is screen media and the company has developed a new LS system which will allow it to actively compete in the coal market. We will have all of our technology and more on display at Coal Prep 2011.”