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Mole•Master™ provides industry leading silo cleaning services.

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Mole•Master™ provides industry leading silo cleaning services.Lonestar Industries, Greencastle, IN

(Buzzi Unicem USA)

Powder and Bulk Engineering, December 1996

Clearing silo buildup is a piece of cake for cleaning service. When compacted cement tied up storage space, a plant hired a silo cleaning service.

Lonestar Industries of Greencastle, Indiana, a large US cement producer, has made cement since 1919. Besides six cement plants, the company operates several ready mix and aggregate plants throughout the US.

At the Greencastle cement plant Lonestar mines most of the raw materials for Portland cement, Portland lime and masonry cement. The cement is made through a wet process that combines calcium, silica, alumina, and iron oxides with water to form a slurry with 34 percent moisture content. The slurry is burned in kiln at 2,700° to form clinker, which is ground with gypsum to produce the gray powdered cement. The energy intensive wet process uses coal as the primary fuel source and post-consumer and manufacturer waste solvents as the secondary fuel.

The Greencastle plant annually produces 720,000 tons of cement. The cement is packaged in 94 and 70 pound paper bags or shipped in bulk by truck or railcar. Cement awaiting packaging or shipment is stored in 36 outdoor silos. Because cement is vulnerable to compaction, explains assistant plant manager David Puzan, "it's critical to keep your product dry and rotated as you move it and ship it to customers."

Cement compacts in silo bank: capacity down 25 percent

Last winter Lonestar had compaction problems with one of their silo banks. The bank includes eight silos with five storage interstices, for a total of 13 storage spaces. The silos, erected in 1928, were covered by a single roof that had a slow leak. Cement in the eight silos was flowing with increasing difficulty. The silos had been built with flat bottoms, which contributed to the flow problems. The five interstices held a discontinued product line including build-up on the walls, which had to be removed to increase plant storage capacity. Engineers estimated the unusable space was 25 percent of the plant's storage capacity.

"During the winter, construction slows, and that's when we fill up for heavy spring shipments." Puzan says. "All the other available silos were approaching capacity. The silos in this bank had ratholed, and the compacted cement plugged the feeder openings. Our goal was to retrieve lost capacity so we could restock for the busy season, and there was some urgency." The plant decided to re-roof the silo bank, clean out the silos and interstices, and reclaim lost storage before spring. Lonestar wanted to salvage as much dead inventory as possible.


Cement plant considers cleaning options

"We knew silo cleaning wasn't work we could do ourselves," Puzan says. "Years ago it was common practice for plant workers to enter silos and try to clean them. It's very dangerous and can be fatal. I don't approve of our workers entering silos; I won't allow it. We contacted several silo cleaning services for competitive bids on the project. These services are professional; they know what they're doing, and they have the equipment to do it efficiently."

Puzan considered a few silo cleaning companies he knew from prior experience. "I've worked with three different cleaning services over the years at various plants," he says. He picked one with which he had a good prior experience. The selected company gave Lonestar a detailed proposal estimating the time to remove the cement buildup. "Their bid was a little higher than others, but they offered a turnkey cleanout. Some services had lower bids but then had associated costs that were job dependent. The service I chose was up front with their proposal."

Cleaning service reclaims lost storage capacity

Lonestar hired the cleaning service, which sent out a crew of three to clear the 13 storage spaces and feeders. The service makes several variations of a proprietary cleaning device called a mole for cleaning service use and for sale. The device is remotely controlled and monitored from the silo roof, eliminating worker entry. The device's cutting heads are made of various materials to suit each particular job. Construction materials include steel, aluminum, stainless steel, and several durable plastic resins. The device can be powered pneumatically or hydraulically.

At the cement plant the service company used the Arch•Master vertical auger device to drill through bridged cement and create a flow channel to the feeder. Next, their Big•Mole system enlarged the flow channel outward to the silo walls.

The crew worked between January and March. "We weren't sure how long it would take because each silo had different compaction problems." Puzan says. "Compacted cement ranged from 200 to 2,000 tons per silo."

"The crew was very flexible. They'd work with us if we needed to transfer cement. If there was a bottleneck, they would move on to another silo. They gave us constant updates on their progress. They worked long days and worked some nights when needed. Their flexibility helped us to minimize costs."

"The weather was not the best through January and February. First we had torrential rain, then we had record snowfall. Two blizzards hit: we had subzero temperatures and over 50 inches of snow in 2 months. Sometimes the weather was so bad we didn't want the crew working, so we sent them home. But there was very little downtime on the project. They stuck with the job and worked well with our people. These guys became part of our team for 3 months."

The cleaning service cleaned the 13 storage areas and freed up the feeders. Some silos had to be cleaner than others. The discontinued product line had to be completely removed from the interstices because Lonestar didn't want to contaminate the incoming fresh cement. After the bulk of the removal was done, the crew used shovels and then brooms to clean down to the floor. "Those silos were cleaned so well that you could sit down and eat a lunch in there," Puzan says.

The compacted Portland cement was reprocessed and reused, returning valuable inventory. Because of the silos' flat bottoms, the cleaning crew left some of the hardened cement in place to create an angle of repose rather than clear it to the floor. "We get a cone effect to funnel the cement into the feeder," Puzan says. "And we don't have to worry about that cement contaminating our product because we're continuing to store the same type of cement in those silos."

Lonestar regained lost storage in time for their busy season. "The crew opened up 25,000 to 30,000 tons of capacity by March." Puzan says. "It enabled us to keep our cement production rate on target and service our customers." Asked whether Lonestar would use the silo cleaning service again if needed. Puzan replied, "Definitely. That's an easy question."

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