Aggregates Manager Digital Magazine
Issue link: http://read.dmtmag.com/i/945822
26 AGGREGATES MANAGER / March 2018 S ometimes the bottleneck at an aggregates operation isn't in the plant itself, but in the pit, points out Phillip Gosnell, eastern divi- sional production manager at Rogers Group, Inc. If a fleet is undersized or operating inefficiently, the plant may out produce the pit — i.e. the truck and loader could be the bottleneck because enough material can't be taken out of the pit and to the crusher(s). To address this, he suggests an operation "start with the cheapest options first, such as operator training, haul road conditions and design, and blasting practices." Although there are numerous benefits with having properly trained operators, a significant one is a reduced potential for the pit fleet to be the bottleneck. "Traveling too far in the loading pattern will equate to longer cycle times and can cause 'black belt' time at the plant," Gosnell says. "This is an obvious sign that something in the pit is the bottleneck." If the blasted material is too large or small, the plant will not be optimized, resulting in different bottlenecks — and usually not the desired one, Gosnell emphasizes. "With good intentions, managers can attempt to save costs or reduce fines generation by spreading the shot patterns," he says. "The repercussions, however, can be lump-size material entering the primary circuit that causes a dramatic reduction in overall plant throughput." Gosnell also cautions that new bottlenecks can materialize, such as transfer points bridging with oversized material. "The offset in blasting costs is usually defeated by the higher cost to produce per hour given the lower production rates and downtime experienced," he says. "Comparing the actual plant performance with simulation software can provide valuable infor- mation on efficiencies and optimization." Taking belt cuts — including crusher ins and outs — and comparing them to manufacturer suggested results will provide a starting point to understanding what the bottleneck should be in an operation. "This information might point to crusher settings, liner conditions, chamber restrictions, or something else," Gosnell notes. "Typically, conveyors are the easiest fix. Increasing the speed or horsepower on a bottlenecked conveyor has been done countless times." Bottlenecks can also be created over time from poor maintenance practices or management decisions. "Screen media is a prime ex- ample," Gosnell says. "Media wire diameters increase to reduce maintenance time changing screen cloth, but it creates a new bottleneck because either the crusher(s) receiving the recirculating load becomes overloaded with undersized material, or the product quality of the finished material suffers. Changes in wire diameter or media type also can have a big impact on material plugging. "The more material plugs, the less efficient the screen and the more undersized material will recirculate back to the crushers — and the undesirable bottleneck starts," he says. A lthough the goal seems to always be more tons per hour across the screen, the real key to optimum screening is maximizing capacity without losing quality and efficiency, explains Joe Schlabach, vice president of marketing and sales for Deister Machine Co., Inc. "Vibrating screens must be properly selected, designed, and applied, and must be operated under the correct parame- ters," he says. If they aren't properly selected and maintained, screening circuits may become big bottlenecks. "A screen operating at 75 percent efficiency allows 25 percent of the undersize material to be rejected with the oversize material," Schlabach says. "This adversely affects production goals and profits." This goes back to the fundamentals of properly feeding a screen — spreading out the load as much as possible, determining the precise operating parameters, and selecting the proper screening media. Maximum screening efficiency results from proper adjustments in speed, stroke, rotation (or throw) direction, and angle of inclination. "Depth of bed of material on the decks is an important factor in screening efficiency," Schlabach says. "Vibrating screens stratify the ma- terial, the process of the smaller particles working their way through the coarser material." The material bed should not reach a depth that would prevent undersize material from reaching the openings before it is discharged. The rule of thumb is depth of bed in dry screening should not exceed four times the opening size measured at the discharge end of the screen." For example, with a ½-inch screen opening, the depth of bed at the discharge end should not be more than 2 inches. "Overloading screens is a common practice, but it leads to a carryover problem and less-efficient screening," Schlabach says. "In addition to properly feeding the screen, you need to apply the proper stroke and speed to get the maximum benefits out of the screen(s). Generally, for small openings, more speed and less stroke is needed. For larger openings, less speed and more stroke is needed. "The operation's equipment manufacturer(s) or supplier(s) is a resource to help with all of this, especially determining the proper media," he advises. "There are so many types of media — wire screen cloth, modular urethanes and rubbers, bolt-down type media — and several varieties within the various types." Ultimately, it all boils down to the operations people talking to their screening media providers to ensure the application matches the goal of the operation. "There is no one given way to guarantee it will be right," Schlabach says. "There will be a lot of experimentation and trial and error. Experience is beneficial to achieve the desired results. You always need to be observing and will learn over time what you need to do to stay in spec and reduce bottlenecks." Voices of Experience Phillip Gosnell Joe Schlabach