SPECIALIST EQUIPMENT O
nly a short drive from author Mark Twain’s boyhood home in the small town of Hannibal, Missouri
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 cement production capacity of 1.2 million short tons (≈1.1 million tonnes) of clinker annually. Having used most of the suitable limestone from surface quarries over the many years of cement production at the location, a 350ft (106m) underground mine now provides the primary source of limestone, which makes up 75% of the raw material required to produce the meal in the company’s Type I/II and Type III Portland cement. Yielding 1.6 million short tons (≈1.4 million tonnes) of limestone annually, the extracted material is transported via truck and high-speed conveyors to move it to the surface, where it is held in a 100,000 ton (≈91,000 tonne) outdoor storage pile, then processed through a four- stage preheater/precalciner dry process cement kiln.
Although the cement plant is a 24-hour operation, the mine works on a 10-hour-per-day, four-day- a-week schedule. Fifty-ton trucks dump 600-650t/h (544-589mt/h) of extracted limestone into an underground hopper that feeds an impact crusher, which reduces the rocks to less than 4in (101mm). T e aggregate is then loaded onto Conveyor 0, a 60in (1,524mm) wide belt travelling approximately 700fpm (3.5m/s) for 400ft (122m). T e rock is discharged through a transfer chute onto Conveyor 1, travelling up the 1,300ft (400m) inclined belt to the surface and dropping 20ft (6m) onto the outdoor storage pile.
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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. “T e two main areas we identifi ed as having unacceptable dust emissions were at the underground crusher hopper and outdoors where the material was discharged onto the stockpile,” explains environmental manager, Leonard Rosenkrans. “With the mine having a single exhaust point, we didn’t want the dust from trucks offl oading 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 T e most common industry solution attempts to address particles released from normal operations, such as loading, unloading, wind and disruption of material, by using water for surface suppression. T e goal is to wet the surface of cargo to promote cohesion of particles to prevent them from becoming fugitive airborne emissions. T e original dust control equipment followed that traditional approach, with a goal of wetting the surface of the cargo in order to promote cohesion of particles and prevent them from becoming fugitive airborne emissions. However, Continental Cement discovered issues with this type of solution when applied to its material handling and stockpiling operations. Operators learned that the water alone was not adequately controlling limestone dust.
Integrating the system with the
plant’s PLC helps operators to monitor and adjust chemical outputs
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