Aggregates Manager Digital Magazine
Issue link: http://read.dmtmag.com/i/756888
AGGREGATES MANAGER Voices of Experience Tim Cassibo Dean Fogal Jarrod Rice B efore Rankin Construction, par- ent company for Port Colborne Quarries (PCQ) in Ontario, Cana- da, purchased the aggregate operation in 2007, PCQ had been a dry crushing and screening facility. But specification demands for asphalt sand — both inter- nally for Rankin's asphalt plants, and externally for other customers — led the company to add a wash plant. "When we put in the wash plant, we knew that two or three years down the road, we'd want a system that could help us recover material that was going into our settling ponds," says Tim Cassibo, operations manager for PCQ. When it became apparent that the quarry was having to clean its settling pond twice a year, Rankin decided it was time to put a means in place to capture material before it could reach the settling pond. "We were dedicating workers and equipment and time to cleaning the pond, which took all of them away from production. It was a cost liability for us," Cassibo says. PCQ installed a HI-G Dewatering Machine, which combines small hydro- cyclones surrounding a radial manifold with a high-G dewatering screen. With the fines recovery system in place, ultra-fines down to the 400/500 mesh level are captured from the wash- ing plant's hydrocyclone effluent, and dewatered to a dry and manageable 80 percent solids by weight — saleable for such products as aglime and dense flowable fill. The ROI for PCQ's fines recovery system was realized in less than two years. "We're now effectively recovering 75 percent of our washing operation's waste material, and we're converting it into a profitable product that we can sell. In addition, we have been able to extend the life of our settling pond by at least four or five times," Cassibo says. "We've gone from cleaning it twice a year to now cleaning it every three or four years." "S everal years ago, we were experiencing problems with our fines recovery process because the belt presses we used for dewatering the ultra-fines required a lot of repairs and personnel time," says Dean Fogal, superintendent for Valley Quarries' Mt. Cydonia plants in Pennsylvania. "We also were devot- ing time and equipment to cleaning our settling ponds." The solution was the installation of a combination of hydrocyclones paired with dewatering screens, which capture approximately 70 percent of solids from the plants' discharge slurry and recover material down to 400 mesh. At the Mt. Cydonia #2 Plant, the remaining 30 percent of the ultra-fine solids (-400 mesh) are sent in a slurry to a radial thickener device, which con- solidates them for dewatering. Valley replaced its belt press with a high-ca- pacity centrifuge to dewater the thick- ener underflow (about 30 percent solids by weight). The centrifuge dewaters the ultra-fines into a stackable, convey- able form that is 72 percent solids by weight. The water from the centrifuge is returned to the thickener feed as a recirculating load. Fogal says the settling pond has been eliminated, because 100 percent of its solids are dewatered, and all of the pro- cess water is recovered. "Our processing plant uses about 2,800 gallons of water per minute — 168,000 gallons per hour," he notes. "Aside from about 2,500 gallons per hour that goes out with the product, the rest of our water circulates back to the plant." He explains that the plant's sand product typically is stacked at 22 percent moisture. The material is allowed to drain off and is sold at 6 to 8 percent moisture. Water that drains from the product is channeled back to the system. "The entire process is much more en- vironmentally friendly and has resulted in cost savings. Plus, we're now able to sell our solid ultra-fines for such prod- ucts as cattle bedding," Fogal says. A ccording to Jarrod Rice, ag- gregates manager for Derrick Equipment Co., mechanical equipment uses the same gravitational principal for separating ultra-fines as a settling pond — requiring a signifi- cantly smaller footprint and making the process more efficient. These machines include hydrocyclones, dewatering screens, and centrifuges. A hydrocyclone, he says, works in the same way as a tornado. As water spins within, heavier solid material is pushed to the outside, where gravity causes it to fall out of the bottom. At the same time, smaller material is pushed upwards in the center of the vortex and flows out of the top as a slurry. "Larger hydrocyclones handle greater volumes, but make a coarser material cut," Rice explains. "Because we want to capture finer material, we make up volume us- ing a series of smaller hydrocyclones." For purposes of capturing material, the heavier (coarser) material in a hy- drocyclone falls to a high-G dewatering screen, from which it can be stacked at up to 80 percent solids by weight. The lighter (finer) material slurry flows to either a thickener system or to the settling pond. "At that point, even if your ultra-fine material flows to a settling pond, you're way ahead in pond maintenance re- quirements," Rice says. "If you process it more, chemical flocculants can be used to cause the solids to consolidate and fall. The underflow then might report to a type of press — or it can be sent to a centrifuge." A centrifuge accepts material continu- ously, rather than in batches. It has an outer bowl that spins at a high rate. The thickener underflow enters as a slurry. Centrifugal energy sends fine solids to the wall and clear water to the center. A conveyor augers the solids out of the discharge end at up to 75 percent solids by weight, from which they can be conveyed and stacked. Water from the centrifuge is sent back to the plant.