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has a fully mobile unit installed at the Pingshu coal mine in China that is processing 9,000 mtph. For Pitchford, however, dependability is more important than size. He offers a primary sizer station in Colombia that handles 2,200 mtph as an example. At this mine, the MMD 625 series primary sizer sits below a hopper that integrates an MMD D4 apron feeder. "For that Colombian coal operation, we not only engineered the primary, but the entire system from the secondary sizers to the surge pile where they load the trains heading to the port," Pitchford said. The system in Colombia has a diamond-shaped hopper. "The angle is about 30° and it allows two trucks to dump at the same time into a 400-ton hopper," Pitchford said. "The apron feeder redirects the RoM coal, which is reduced to 8 in. The con- veyor goes to another tower with two additional secondary siz- ers before the coal in transferred to the reclaim pile. There are no screens in the network." On this particular application, MMD had to meet the chal- lenge of reducing the coal to 98%, passing minus 2 in. on a round screen. "That is a fairly tight tolerance for a rather hard bituminous coal," Pitchford said. "If the sizers are not meeting specifications, we hear about it. At first, we had some difficul- ties, because we usually work with a square screen and a 2-in. square piece of coal will not pass through a 2-in. round hole." But MMD and the mine persevered, and the mine has subse- quently purchased two more systems. They keep their perfor- mance data close to the vest for competitive purposes, Pitchford explained, "but they buy their wear components from us, and we are treated with respect when we are on-site." The MMD sizer works well with a hard bituminous coal and it also works well with a soft lignite. MMD has three very suc- cessful installations in North Dakota lignite mines for about 15 years now. "The selling point with lignite operators is the limit- ed amount of fines generated by the sizing process," Pitchford said. "The lignite from one of our customers is shipped to a gasi- fication plant. The plant screens out anything that's minus 0.25 in. and sends it back to the mine. It doesn't want fines. The mine then blends that with another product stream for another customer." In this particular case, after passing the model 625 primary sizer, the lignite is transferred to a secondary sizer. A total of 1,800 tph is reduced to minus 3 in. "When we installed the first machines — the primary and secondary sizers — we reduced the amount of fines by 6% to 13%, depending on the feed," Pitchford said. "They were saving money on re-handling coal. Two years later, they ordered four more machines." Those units have been operating since 1995 with no serious maintenance issues, Pitchford explained. "We have rebuilt them," he said. "We have never had a shaft break or any serious gearbox problems. We provide the mine with spares. If a service engineer is needed, and they order replacement parts from us; MMD does not charge for the service call." How does a sizer react when it is faced with noncrushable material? The MMD machines have a fluid coupler between the motor and the shafts on the machine. The system is designed to withstand a 5-second stall before it shuts down the motor. The technicians reverse the machine to remove the tramp metal. "Nine times out of 10, the steel will pop up and the coal drains down the sides of the machine," Pitchford said. "A good opera- tor can free the tramp metal in 5 to 7 minutes; an unskilled operator will need about 45 minutes." That same piece steel, Pitchford said, can do serious damage to other types of machines, taking them offline for a day or more. MMD has earned its keep in Colombia and North Dakota and now several PRB operators are considering these systems, but money is tight at the moment. p r i m a r y c r u s h e r s c o n t i n u e d 40 www.coalage.com November 2015 An MMD 625 sizer (blue section at the base of hopper above) handles 2,200 mtph. The TerraSource 5000 series Nitroil adjustment system removes tramp metal.