Selective mining as an alternative to drilling and blasting

Online Editor

A high-performance surface miner proves to be a dependable and safe means for extracting primary resources swiftly and effectively in a single operation.

In a quarry in Louisville, Nebraska, a Wirtgen Surface Miner 280 SM(i) is being used for the layer-by-layer and environmentally friendly extraction of limestone from previously undeveloped reserves.

Limestone for the production of cement in the Ash Grove Cement Company plant, one of the largest cement producers in the USA and Canada, has been quarried here in Louisville since 1929. The extraction of the material in the quarry has been contracted out to North American Mining (NAM).

Although the quarry in Louisville has been in operation for almost 100 years, the valuable limestone reserves are far from exhausted. The geological situation has, however, made extraction a more difficult task: 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. Ash Grove therefore decided to use a Wirtgen Surface Miner from NAM for the job. As the 280 SM(i) enables selective extraction of resources, it guarantees high material purity and is ideal for use in areas with sensitive infrastructures.

Limestone – shale – waste rock – clean separation

The 280 SM(i) cuts, crushes and loads the extracted material in a single pass: there is no need for pre-crushing or conventional extraction methods such as drilling and blasting. This not only reduces the otherwise enormous costs, but also enables mining in areas that are difficult to access or sensitive areas where it’s hard to get permits for drilling and blasting.

The use of the Surface Miner in the quarry in Louisville opens new opportunities for the extraction of limestone. The extraction takes place selectively, layer-by-layer: the valuable primary resource, limestone, is cleanly separated from waste rock such as shale before crushing takes place – with exceptional power and efficiency.

Considering a compressive strength of 110MPa (16,000psi) for the limestone in the quarry, the 280 SM(i) achieves a mean cutting rate of approximately 400t/h – with 87.5% of the material extracted being crushed into pieces smaller than the targeted maximum size of 7.6cm. In the waste rock (shale and overburden), the machine achieves a mean cutting rate of approximately 650t/h. The cut and crushed material is loaded directly into ready and waiting 70 tonnes dump trucks. The process is fast and continuous: a fresh truck is filled with crushed limestone every four minutes – around the clock and on five to six days a week.

Minimal wear & high manoeuvrability

Due to the high compressive strength (extreme hardness) of the limestone, the cutting tools used 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 really appreciate the round-shank picks and the toolholder. They make it so easy to replace worn picks and get back to work again after only a couple of minutes,” reports David Ashby, machine operator at North American Mining. “One of the advantages of the sliding counterweight and the 90-degree slewing angle of the discharge conveyor is that they let you work right up to the high sides of the quarry. Although the machine is very big, it’s still very useful for working in tighter spots. The machine’s manoeuvrability is simply fantastic.”

Eco-friendly tech

Apart from its efficiency, the machine comes with a range of environmentally friendly aspects: the reduction of extraction process steps from 4 to 1, lower exhaust emissions, less noise and less dust not only make the everyday work on 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,” says Nic Haubruge, business development manager at North American Mining. “In many cases, we simply can’t use drilling and blasting anymore. And this machine has now enabled us to develop new deposits with minimal dust emissions, minimal noise emission and no vibrations.”