By Steve Fiscor, Editor-in-Chief
More than 20 new prep plants have been constructed in the U.S. during the last five years and a number of older plants have recently completed upgrades that were long overdue. A noticeable trend among both new construction and upgrades is the increased use of large scale Banana screens. While plant operators admire the performance of these screening systems, high vibration frequency, capacities and velocities place a lot of stress on the screen deck and screen media.
Historically, coal operators have had mixed emotions about screen media. Prep plant purists prefer profile wire, while others see advantages in modular synthetic media. Similar to any new technology, when the concept of urethane screen panels was introduced in the 1970s, some coal operators had a positive experience while others did not. Simply put, synthetic media did not have enough openings to allow the heavy media and water to drain fast enough therefore steel would do the job better. During the last 40 years, the technology has improved to the point where urethane screen media has a greater level of acceptance now and it’s growing.
“As coal companies consolidate, we are seeing a change of attitude about steel vs. urethane,” said Ron Bennett, coal industry sales manager at Polydeck Screen. “That trend will continue. There are some people who still think urethane doesn’t work, but we have been very successful applying it in the coalfields. We are seeing people get more comfortable with it over the last three to five years.”
The largest manufacturer of synthetic screen media is Polydeck, located in Spartanburg, S.C. The company’s primary product lines include modular polyurethane and rubber screen panels, and the frame systems to support the media.
The differences between steel and urethane should be obvious, but they are not. One would think steel is harder and therefore would last longer. That may be a common assumption, explained Bennett, but more often he hears plant operators say they tried urethane a number of years ago and it just didn’t work. Aside from brute strength and tradition, other differences between steel and urethane are more subjective, such as effective wear life and ease of handling during maintenance.
Because the Polydeck system uses 1- x 1-ft or 1- x 2-ft screen panels, one of the biggest advantages for modular screening media is the ability to solve a wear problem in one area of the deck. For instance, Banana screens are hard on screening media. “Many of these new plants are using more water and creating higher velocities than was used in the past,” Bennett said. “We have created different panel styles that improve wear in these high velocity areas.” Maintenance personnel can replace just the worn panel instead of an entire section of steel media. “That modular approach also gives us the advantage on Banana screens of alternating rows of media to fine tune the overall performance,” said Dennis Zeiger, marketing manager, Polydeck. “You can put a row of panels with dams to slow material down or change the slot widths. You can adjust the material flow over the deck to achieve production objectives.”
From a maintenance standpoint, modular screening media is much simpler, easier and safer to handle than steel decking, Bennett said. “It takes less people and time to change out the decking and, from a safety standpoint, instead of handling a 4- x 10-ft piece of punch plate, the maintenance crews are handling 1- x 2-ft urethane panels, there is no comparison,” Bennett said. One man can crawl into a vibrating screen and change out a couple of panels compared to a maintenance crew using a crane to remove a whole section of steel.
Handling Tougher Conditions
Ask any coal miner and they will quickly lament that all of the good coal is gone. No one knows this better than the screening professionals like Polydeck that serve the industry. To get at lower quality seams, mines are having to work through clay partings and that clay has an adverse impact on a screen’s performance. Prep plants wash coal and that means water will be added and removed from the process, and the clay becomes a sticky nuisance plugging apertures and blinding screens. Heavy clay build-up is a major problem for many prep plants