A 90 cubic-foot Munson 700-TH-90-AR rotary batch mixer blends dry cement mixes in less than three minutes with 100% uniformity. Gentle mixing preserves the texture of the coarse material, which is necessary to maximize the strength of cured products.
The rotary batch mixer totally evacuates blended material, which is gravity-discharged into this 9000-pound capacity surge bin.
Sometimes it pays to be green when a producer can do it efficiently. That was the challenge managers at Charah Inc. faced when they opened a new plant in Emporia, Va.
Their goal was to increase production of concrete mixes containing coal-fired, power generation, plant ash products. The new production facility would also serve as a warehouse and distribution center for other concrete products.
Profit margins are slim in this high-volume production business. So the management team placed great emphasis on the capital and operational costs of the equipment to be purchased, particularly the dry bulk blending equipment at the heart of the mixing process.
Charah is a pioneer in utilizing the coal combustion byproducts, which are typically dumped into landfills, to produce aggregates for concrete mixtures and concrete products. The producer has focused its attention on bottom ash, primarily the larger, more coarse particles that range from gravel-sized nuggets down to grains of sand.
The producer recently received the Coal Combustion Products Partnership (C2P2) Innovation Award from the U.S. Environmental Protection Agency. The award honors companies that demonstrate outstanding achievements in the constructive use of coal combustion products (CCPs). “We strive to find new ways to utilize coal combustion products to benefit both the utility industry and the environment,” says Charles Price, Charah's president.
Charah developed better methods of processing bottom ash and sorting it by gradation to ensure consistent batching. This allows Charah to substitute the fly ash for a portion of the sand or other material that it mixes with the cement. The producer then sells the concrete mix and other concrete products to contractors and the public.
The company also sells its premium coal ash-derived aggregate in pure form to large manufacturers of lightweight concrete blocks and precast concrete products. This aggregate meets the ASTM standards for clay lumps and friable particles, organic impurities, unit weight, stain test, and loss on ignition.
Along with being more economically friendly, masonry products batched with the fly ash aggregate are worker-friendly and profitable. Nathan Boone, Charah's vice president of operations, says lighter concrete blocks generally command a premium price. “The fly ash mixture reduces the concrete block's weight from about 32 pounds to the 25-pound range, making them easier for masons to handle,” he says.
The plant's profitability hinges, in part, on the mixing equipment. Handling the fly ash can be difficult. Boone's investigation for a better way led him from vertical shaft mixers to a rotary batch mixer.
Producing bottom ash for the bagging operation begins at the power plant. The raw material aggregate is screened on vibrating wet screens to remove oversize particles and excessive fines. The output is then hauled to the producer's packaging facility. It is dried and cooled on fluid bed drying equipment before being placed in 120-ton storage silos.
The entire packaging facility is automated. When the control panel starts the batching process, flexible belt conveyors transport ash, sand, and stone from storage silos to a weigh-batch hopper on one side of the blender. Screw conveyors move fine cement powder and fly ash from silos into a weigh-batch hopper on the opposite side of the mixer. Then both weigh hoppers discharge into the inlet of the rotary batch mixer.