Using ultra-high-pressure waterjet cutting in underground mining can reduce stope heights and improve mine safety, as less blasting will be required, says University of the Free State (UFS) affiliated lecturer Johann Grobler.

Currently, a typical cycle at a rock face comprises marking drill holes for blasting to remove the valuable part of the advancing face, while also creating a minimum working height for the drill operators, roof support team and workers clearing the rubble. This minimum height is usually about 800 mm, although the reef may only be 250 mm thick. The marked holes are drilled and charged with explosives, after which the mine is evacuated for synchronised blasting, he explains.

In comparison, waterjet cutting is a continuous operation in which a stream of water containing an abrasive material is used under very high pressure to cut rocks from a remote position. It does not require drilling, evacuation, blasting and roof support behind the face. No synchronisation is required and all sections work independently.

The cutting head is small, compared with the average size of a human being and requires much less headroom. It is more selective and cuts only the valuable section of the face, says cutting systems manufacturer MultiCam RSA owner Stiaan Viljoen.

“This technology would allow for better and safer working conditions and much higher yield of ore would be possible,” he says.

While the labour requirements for this technology are likely to be lower than those of conventional methods, specialised training would also have to take place, resulting in a skilled and well-paid labour force in a safer working environment, UFS associate professor Chris Gauert tells Mining Weekly.

He adds that there would be a shift from semiskilled labour to skilled labour to service the equipment and operate the machines.

Additional benefits of using waterjet cutting in underground mining include significantly reduced noise levels, as a result of replacing the percussion drilling machines with an electric motor and pump that can be acoustically isolated.

The exposure of workers to vibration, elevated temperatures and physically demanding work is reduced, as the work is done by mechanical scrapers and water hoses. The potential for fall-of-ground incidents is also reduced, Grobler says.

Further, productivity is increased and costs reduced, Viljoen adds.

“More ore is mined in the same period by the same number of people, which increases the output for the same input, and some underground activities, such as drilling and blasting, will no longer be needed, further reducing costs.”

Waterjet cutting is also likely to lead to increased output capacity of the processing plant without any additional capital expenditure, as the plant’s capacity will be used only for reef material and not for barren material that dilutes the ore.

Grobler adds that waterjet cutting could also provide access to deeper resources.

“Pressure in the rock and high temperatures are now less of a concern and it should allow for deeper mining without the associated risks and costs.”

Challenges
Existing industrial waterjet systems operate on the surface, where the conditions differ dramatically from those underground, Viljoen says.

“The electronic systems will have to be modified to operate using solid-state switching to prevent possible gas ignition and the units will have to be repackaged and designed to be transportable from above ground to the stope area.

“A significant design effort will be needed to get this technically advanced equipment to operate successfully underground,” he says.

These technologies would have to be developed before a testing programme can be started to evaluate the best process of mining using the waterjet system, Grobler adds.

“There is a need for testwork to be done in actual field conditions to identify areas of the technology that will require modification,” UFS mineral resource throughput management director Johann Claassen emphasises.

“A specific test area that has already been identified is how to follow the reef with the cutting head,” he says, adding that there are technologies that can guide the cutting heads, based on laser or ultraviolet light, which can be considered.

Funding, the opportunity to work underground, support from mine management and the freedom to challenge the current way of working will also be required, Viljoen says.

“Finance should be obtained to develop a prototype and a test programme should be implemented to evaluate further progress. Partnerships would have to be built with the relevant industries to ensure the development of a fit-for-use technology that could safely support mining operations in years to come,” Grobler concludes.