Aggregates Manager Digital Magazine
Issue link: http://read.dmtmag.com/i/214806
OPERATIONSExperience ILLUSTRATED Voices of Dana Boyd "W ▼ hen we look at our conveyor program, we break it down into three areas — safety, quality, and productivity," says Dana Boyd, vice president of operations for Cloverdale, Ind.-based North American Limestone Corp. (NALC). "And each of those areas can be broken down further." On the product quality side, fugitive dust adds a coating on clean product stockpiles, creating a problem with fines, Boyd says. In some applications, oversize material may bounce or spill onto conveyors that lead to stockpiles of smaller sized product, which creates gradation issues. Similarly, material buildup with carryback of fines will eventually fall off and affect gradation of coarser product. When looking at productivity, dust from conveyors is abrasive to conveyor bearings, motors, and gearbox breathers. Dust also will build up between troughing idlers and on the fins of fans and motors, restricting the cooling ability for the latter and causing them to overheat — ultimately leading to downtime. "Spillage represents product that we want to sell. It should be in the stockpile. Fugitive dust also represents wasted product for our fine grind operation," Boyd notes. "You can actually generate several tons of fugitive fines a day without even knowing it. If it is allowed to settle around the plant, it builds up. It can build around the base of your conveyors, trapping moisture and increasing oxidation, which can eventually lead to support failure." For each issue a producer encounters, Boyd says solutions can be put into place. For instance, fugitive dust around the site is typically suppressed with water. It also is controlled through use of chutes, hoods, and/or rubber skirting at transfer points. Venturis and baghouses help to collect fugitive fines without the use of water. The best answer for reducing carryback is the belt cleaner, but Boyd says placement is critical. "Scrapers are usually installed at the head pulley area, but for clean stone applications, you don't want those scrapings falling into your stockpiles," he says. "The excessive fines falling off the belt will create pockets of those fines in your coarse product stockpiles." For NALC's coarse stone products, the facility has installed rubber disc return rollers because fine material does not stick to them. Belt cleaners then scrape the fines into receiving boxes on the tail section of conveyors at transfer points, keeping fines off the ground and out of coarse or clean product stockpiles. The fines can be "recycled" into NALC's fine grind operation. "It's always a tweaking process, chasing bottlenecks," Boyd says. "What works today might not work tomorrow, and there is up front cost involved with these solutions. But because operations are working leaner, anything we can do to minimize labor is part of our cost benefit analysis." Larry Goldbeck C ▼ leaning up after conveyors is a hazardous, costly endeavor, says Larry Goldbeck, manager of conveyor technology for Martin Engineering of Neponset, Ill. For that reason, it pays to put measures into place that reduce the issues producers face with conveyor systems. "I walk by hundreds of conveyors that have no belt cleaners," Goldbeck says. "Producers are always wanting to increase production, but if they keep the material on the belt in the first place, production would increase," he continues. "Carryback without belt cleaners amounts to about 1 to 3 percent of production. Sounds minimal, but day in and day out, it can be substantial." Goldbeck says he follows seven principles for controlling fugitive material on conveyor belts: Belt cleaners work wonders to reduce carryback; "but just one never does it completely," says Goldbeck. A primary cleaner will catch about 85 percent of carryback. A secondary cleaner will catch an additional 12 to 13 percent. A tertiary cleaner will catch 1 to 1.5 percent more. Install the cleaners back to back. "But don't just walk away, then," adds Goldbeck. "You have to maintain them." Make sure the belt is rated for the correct pounds per inch of width (PIW), and that the trough angle and aspect ratio are correct so the belt will track. Impact idlers and slider beds support the belt at the load zone to protect it and keep material from bouncing and spilling. These rest against the belt in the load zone and help keep material from spilling. This protects rubber dust seals and helps to contain material. Chutes, hoods, and rubbers will keep the material on the belt where it enters the load zone. "If your transfer points are totally open, how are you going to keep material on the belt?" Goldbeck asks. Chutes and hoods work here in the same manner. Goldbeck says the above principles are containment measures. Collection (through vacuum methods such as venturis and bag houses) and suppression (via water spray — with or without chemical additives) round out the methods to control fugitive material on their conveyor systems. He says if these principles are properly applied, producers will get the best possible performance and safety from their conveyors. "Your conveyors will never be 100-percent clean. There will be carryback. But would you rather be shoveling daily or just once a month?" Goldbeck says. AGGREGATES MANAGER