To increase production of concrete mixes containing coal fired power generation plant ash products, Charah Inc. opened a new manufacturing plant in Emporia, VA, which also serves as a warehouse and distribution center for other concrete industry products. Profit margins are slim in this high volume business, so management 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 process.
Turning waste into profitable products
Charah is a pioneer in utilizing the byproducts of coal combustion - which are typically dumped into landfills - to produce aggregates for concrete mixtures and concrete products. According to President and CEO Charles Price, for every 10 tons of coal burned, a power plant generates about 1 ton of ash. The fine, powdery residue is called fly ash. Bottom ash is primarily the larger more course particles that range from gravel-sized nuggets down to grains of sand.
Said Price, 'we strive to find new ways to utilize coal combustion products to benefit both the utility industry and the environment.' Recently, the company was a recipient of the Coal Combustion Products Partnership (C2P2) Innovation Award from the U.S. Environmental Protection Agency. The award honors companies that have demonstrated outstanding achievements in the constructive use of coal combustion products (CCPs).
Charah developed better methods of processing bottom ash and sorting it by size to ensure consistent batch-to-batch, lot-to-lot properties, and substitutes it for a portion of the sand or other material that is mixed with cement to produce the Project Mix concrete mix and other additional concrete products that it markets to contractors and DIYers.
The company also sells its premium coal-ash-derived aggregate in pure form to large-scale manufacturers of lightweight concrete blocks and precast concrete products. Called PriceLite , it meets ASTM standards for clay lumps and friable particles, organic impurities, unit weight, stain test and loss on ignition.
Charah's Nathan Boone, vice president of operations, says, 'Lighter concrete blocks generally command a premium price. Using the PriceLite mixture drops the weight of concrete blocks from about 32 pounds (14.5 kg) to 25 or 26 pounds (11.3 or 11.8 kg) depending on the producer, making them easier for masons to handle.'
Mixing 6,800 lbs (3087 kg) in three minutes
According to Boone, the profitability of the operation hinged in part on the ability of the mixing equipment to meet quality, speed and cost requirements. The right mixer would produce 100 percent uniform blends rapidly while preventing product degradation and saving energy. After investigating vertical shaft mixers, the company specified a rotary batch mixer from Munson Machinery, Utica, NY.
Producing bottom ash for the retail concrete market begins when it is collected by Charah at the power plant site. The aggregate is screened on vibrating wet screens at the power plant to eliminate oversize particles and excessive fine particles. Once the material is qualified to be marketed as PriceLite , the aggregate is hauled by contract haulers to the packaging facility in Virginia where it is then dried and cooled on fluid bed drying equipment before being placed in 120-ton storage silos until the 90 cu-ft (2.6 cu-m) Munson 700-TH-90-AR rotary drum mixer is ready for a cycle.
The rotary mixer consists of a horizontal, rotating drum with a stationary inlet at one end and a stationary outlet with a discharge gate at the other. A self-adjusting face seal at the inlet allows dust-free operation. Internal baffles (mixing flights) and lifters create a four-way mixing action as the drum rotates on two heavy-duty trunnion rings.
The entire packaging facility is automated. When the control panel initiates 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 located on the opposite side of the mixer, after which, both weigh hoppers discharge into the inlet of the rotary batch mixer.
'We're loading about 6,800 lbs. (3084 kg) of material per batch into the mixer,' said Boone, 'below its rated capacity of 8,000 lbs. (3629 kg).
Although the application is a batch process, the mixer's drum rotates continuously, with the internal mixing flights tumbling, folding, cutting and turning the material in a multi-directional manner throughout the filling, mixing and discharging phases, achieving 100 percent batch uniformity in less than three minutes, and preventing the separation of ingredients of varying bulk densities and particle sizes and shapes.
'The blender's ability to gently tumble the materials yet mix them homogeneously is vital to the mix,' added Boone. 'We need homogeneity for uniform product in the bags and to ensure the product properly performs for customers.
'But we also need coarse material in a mix,' continued Boone. 'Rock and stone can be up to one-half inch in size, (12.7 mm), which achieves the necessary end product strength not always possible with finer sized particles. Additionally, some types of bottom ash can be friable, which means it can be easily crushed. The vertical shaft mixers we investigated had big plows, also mixing in a horizontal direction, but with a force that might crush the ash material. A rotary drum mixer seemed to offer both the thorough and gentle mixing action we needed.'
After three minutes of mixing time, the discharge gate opens and the internal flights elevate the material and direct it through the gate as the drum rotates, fully discharging the batch with no residual into a 100 cu-ft (12.7 cu-m), 9,000 lb (4082 kg) capacity surge bin located below the mixer.
The surge bin holds blended material until a sensor on the bagging equipment signals a door on the bin bottom to open, discharging material onto a flexible belt conveyor that leads to a bagging machine with a capacity of 10 to 12 bags/minute. Charah is the only company in the country to package cement mixtures in plastic two-handled bags rather than paper bags. Plastic virtually eliminates common problems of paper packaging, such as dust and breakage. 'We believe that plastic packaging is the way of the future,' adds Boone. 'As bagging technology improves we expect our bagging rate to increase.'
Within seconds of the previous batch's discharge from the mixer, a subsequent batch of aggregate and powdered ingredients, which were being weigh batched during the mixing cycle, are released into the rotary batch mixer. 'We don't need to shut off between batches,' said Boone. 'The mixed batches exit the drum so cleanly, there is no cleanup or prepping of the mixer required between batches, so there are no delays between batches. This enables us to run about ten batches an hour.'
Since wash down is unnecessary, the mixer never needs to dry. 'This feature was also critical to us,' said Boone, 'because the last thing we want in is moisture, which will start hydrating the cement. Other mixers usually mention washout doors and cleanout doors, which I do not routinely need. I have a dry product and want to avoid the possibility of moisture entering equipment, so I liked the fact that the mixer moves all material out at discharge.'
Tumbling action prevents degradation, saves energy
Unlike stationary blenders whose agitators are forced through the material throughout relatively long cycle times, Rotary Batch Mixers create a gentle tumbling action over short cycle times, with two outcomes: product degradation is virtually eliminated, and power consumption is greatly reduced. Charah's mixer requires only a 15 hp (11.2 kW) motor to mix 6,800 lbs. (3084 kg) batches, less than one-third of the power required with stationary blenders of equivalent capacity. 'Because our profitability numbers were going to be tight,' said Boone, 'we found the electrical savings of a 15 horsepower (11.2 kW) motor versus two 30 horsepower (22.4 kW) motors found on other blenders pretty significant.'
The rock and stone comprising concrete mixes are abrasive materials that will gouge, dent and scratch surfaces. To reduce wear all product contact surfaces are constructed of abrasion resistant AR235 steel. Depending on the material mixed and the cycle times, flights and baffles can last from 3 to 5 years before replacement. The flights and baffles are bolted to brackets welded to the drum wall, allowing rapid replacement.
After operating the Munson rotary batch mixer for one year at the Emporia facility, Charah has ordered a second one for its new Midwest facility, which is currently under construction.