North American Mining (NAM) is the material extraction contractor drawing on limestone that remains far from exhausted, even though the quarry has operated since 1929. The geological situation, however, has made extraction a more difficult task, as the remaining limestone strata are interlayered with shale. Due to this, quarrying with conventional methods such as drilling and blasting would have led to contamination of the material. The Ash Grove Cement Company therefore decided to use a Wirtgen Surface Miner from NAM for the job. As the 280 SM(i) enables selective extraction of the primary resource, it can guarantee outstanding material purity and is the ideal choice for projects in areas with sensitive infrastructures.
The 280 SM(i) cuts, crushes, and loads the extracted material in one single pass. This means that processes such as pre-crushing, or conventional extraction processes such as drilling and blasting, are no longer necessary. This not only enables enormous savings in terms of costs, but also the extraction of materials in areas that are otherwise hard to access or prohibit the use of drilling and blasting due to the difficulty of obtaining appropriate permits.
The use of Surface Miners opens up entirely new opportunities for the extraction of limestone in the quarry at Louisville. The extraction of the material takes place in a selective, layer-by-layer process: The valuable limestone resource is cleanly separated from waste materials such as shale before crushing with extremely high output and efficiency. Considering a compressive strength of 110 MPa (16,000 PSI) for the limestone here, the 280 SMi achieves a mean cutting rate of approximately 400 t/h – with 87.5% of the material produced in the process being crushed into pieces smaller than the specified maximum size of 7.6 cm. In the waste rock (shale and overburden), the machine achieves a mean cutting rate of approximately 650 t/h. The cut and crushed material is loaded directly into 70-ton dump trucks driving alongside the Surface Miner. The process is fast and continuous, with a fresh truck being filled with crushed limestone every four minutes – around the clock, five to six days a week.
Due to the high compressive strength (extreme hardness) of limestone, the cutting tools used here are subjected to enormous stresses. The 280 SM(i) in use in Louisville is ideally prepared to take on this challenge, and achieves maximum cutting rates with minimal pick wear.
"The machine does a great job. I'm really impressed with the round-shank picks and the quick-change toolholder. They make it so easy to change worn picks that we can get back to work again after stopping for only a couple of minutes," says David Ashby, machine operator from North American Mining, who has more to say about the machine: 'One of the advantages of the movable counterweight and the 90° slewing angle of the discharge conveyor is that they let you work very close to the high quarry walls. Although the machine is very big, its simply fantastic manoeuvrability makes it very useful for working in tighter spots.
Apart from its efficiency, the machine also impresses with a range of environmentally friendly aspects: The number of extraction processes is reduced from four to one, while lower exhaust, noise and dust emissions not only make everyday work on the site much easier, but also offer new opportunities for quarrying rock close to populated areas. 'We can now extract limestone from deposits we could hardly have reached before', explains Nic Haubruge, Business Development Manager at North American Mining. "In many cases, the use of drilling and blasting is simply no longer a viable option, however, with this machine we can develop new deposits with minimal dust emissions, minimal noise emissions, and no vibrations."
