Rock Products

JAN 2018

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32 • ROCK products • January 2018 www.rockproducts.com I t's no secret that limestone is the dustiest substance in the cement production process, largely because it's the pri- mary ingredient. Particulate matter is tightly regulated at the state and federal level, both in and out of the workplace. Managers at Continental Cement Co., in Hannibal, Mo., clearly understand the potential impact of fugitive dust emissions on workers and the local community, and they take preemptive steps to create the safest operational envi- ronment possible. "We opened the underground limestone mine in 2013 and brought it to full commission in 2015," said Leonard Rosen- krans, environmental manager at Continental Cement. "Dust generation is unique to each operation, and it's only when the mine is fully operational that adjustments can be made, so once we identified problem areas, we immediately sought solutions." An Efficient Process Only a short drive from author Mark Twain's boyhood home in the small town of Hannibal on the banks of the Mississippi River, Continental Cement has been in operation since 1903. Over more than a century, the company has continued to improve and increase operations to reach a current production capacity of 1.2 million short tons of clinker annually. Although the cement plant is a 24-hour operation, the under- ground mine works on a schedule of 10 hours per day, four days a week. Fifty-ton trucks dump 600 to 650 tph (544 to 589 mt/h) of extracted limestone into an underground hopper that feeds an impact crusher, which reduces the rocks to 4-in. (101 mm) minus. The aggregate is then loaded onto Conveyor 0, a 60-in. (1524- mm) wide belt traveling approximately 700 fpm (3.5 m/s) for 400 ft. (122 m). The rock is discharged through a transfer chute onto Conveyor 1, travelling up the 1,300-ft. (400-m) inclined belt to the surface and dropping 20 ft. (6 m) onto the outdoor storage pile. With plans to potentially add another kiln in the near future, demanding even more production from mine operations, the need for dust control is critical to the operation. "The two main areas we identified as having unacceptable dust emissions were at the underground crusher hopper and outdoors where the material was discharged onto the stock- pile," Rosenkrans explained. "With the mine having a single exhaust point, we didn't want the dust from trucks offloading into the hopper to travel through the mine shaft, lowering visibility and reducing air quality. Also, with a high drop at Conveyor 1 when the storage pile is low, we didn't want dust to travel long distances on windy days. Unfortunately, our previous dust control equipment wasn't doing the job." Solutions That Unveil Further Obstacles Small particles are released from normal operations, such as loading/unloading, transferring or crushing material. At 200 microns or less in diameter (200 micrometers), a particle can remain airborne and ride ambient air currents for long distances. At particle sizes less than 50 microns (roughly half the diameter of a human hair) they become non-visible to the naked eye and can penetrate the body's natural defenses (cilia, mucus, etc.), entering deep into the lungs and poten- tially causing long-term health issues. The most common industry solution attempts to address particles released from normal operations, such as load- ing, unloading, wind and disruption of material, is by using water for surface suppression. The goal is to wet the surface of cargo to promote cohesion of particles to prevent them from becoming fugitive airborne emissions. Controlling Dust Underground Dust Control Limestone Mine's Dust Strategy Treats the Flow, Not the Dust. By Mark S. Kuhar Spraying the material after discharge allows the chem- ical additive to better penetrate the cargo stream.

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