Gamma Guidance Enhances Highwall Mining Coal Recovery
Terex SHM’s introduction of gamma sensors on its highwall miners has opened new opportunities for clean coal recovery. Internationally acclaimed mining writer Simon Walker found out more.
 The significance of the natural gamma radiation signature of waste rock such as shale has long been understood in coal mining, with the phenomenon used widely in underground applications to differentiate between coal and waste.
Constantly in pursuit of inovative technologies that maximize coal recovery, West Virginia-based specialist equipment supplier, Terex SHM is introducing Gamma detection and guidance systems
for highwall mining.
The concept behind using natural gamma radiation to guide the cutterhead on a continuous miner or a longwall shearer relies on the fact that shale naturally emits a gamma signature while coal does not. Thus the closer the cutterhead gets to the boundary between coal and shale, the stronger the natural gamma signal that can be picked up by a sensor mounted on the machine. With appropriate control systems installed, the strength of the signal can then be used to limit the vertical range through which the cutterhead can travel which, in cases where it is essential that a coal skin be left in the roof or floor, makes the operator’s task much simpler and more accurate.
Contour Mining is a specialist contract-mining company that began highwall coal recovery using augers in 1985. Today, the company operates eight auger units as well as its two Terex SHM miners, with operations scattered across southern and central West Virginia. Its second SHM highwall miner is working at United Coals’ Century property, located near Buckhannon in the north-central part of the state. Delivered in April 2006, Terex SHM No. 46 has been producing 40,000 tons per month of direct-shipping coal, relying on its natural gamma-detection system to maintain a clean, low-ash run-of-mine product.
Contour Mining’s contract with Amvest involves mining from two seams: the Powellton and No. 2 Gas, which are separated by between 10 and 50 ft of ‘soapstone’ – weak shale that can present problems in terms of entry roof stability from time to time. The seams themselves range from 36 to 54 in thick, with the Terex SHM machine mining 9 ft 6 in-wide entries to depths of 800-850 ft. As Mr. Bundy explained, the company often finds it necessary to pull the cutterhead back out of entries mined into the Powellton seam, so that the pushbeams that transport cut coal back to surface can be cleaned of fallen soapstone before the entry is completed to its final depth.
Enclosed Pushbeams Ensure Clean Coal Recovery
This system makes very clean coal recovery possible since the cut coal is augured back from the cutterhead through the enclosed pushbeams. This strategy not only minimizes direct dilution, but also helps ensure that the equipment is not pinched in the entry by fallen roof rock.
Mining a single entry takes between 12 and 15 hours, depending on the depth and whether a mid-entry clean-off is needed. Contour’s crews operate on two, 12-hour shifts per day, on a four-day on, four-day off rotation, with a total complement of 16 men. The company achieved a major honor in 2005 with a top-three position in the national Sentinels of Safety awards, its crews here having worked over 33,000 man-days without a lost-time accident.
Contour Mining’s second highwall miner, Terex SHM No. 46, has been working in the Pittsburgh and Redstone seams on United Coals’ Century property since May 2006, and is scheduled to move to a new site, some 30 miles away, once permitting has been completed at the end of the year. The key to mining successfully in the Pittsburgh seam, Mr. Bundy explained, is to keep a thin skin of coal in the roof: if this is not done, the weak shale’s and limestone above is all to ready to come down into the entry.
 This machine was the first Terex SHM highwall miner to be equipped with a natural gamma radiation detector. “At first, we had a unit positioned on the top of the cutterhead module so that we could leave this coal skin in the roof,” Mr. Bundy noted. “We knew that we had to leave coal in the roof so that we could get a clean product,” he added. “However, we found that the top of the seam rolls too much, and that trying to keep the cutterhead in the seam was too difficult, so we added a second gamma sensor to the bottom of the cutterhead as well.
“The seam floor is much more regular than the roof,” he went on, “and by following that as well, we have been able to get a much cleaner product. In fact,” he stated categorically, “Without the gamma sensors, we could not have mined the Pittsburg seam here. It was the only way we could make the project work.”
As at Powellton, Contour Mining is driving 9 ft 6 in-wide entries with 4 ft 6 in pillars, with a 15 ft-wide barrier pillar every five holes. This gives around 65% recovery of the in-situ coal, with entries being mined to depths of 800-900 ft. Virtually all of the company’s initial work has been in the Pittsburg seam, with a small residual resource in the overlying Redstone seam – which has been extensively mined underground in the past – being left until the end of campaign.
On Terex SHM No.46, however, the gamma sensors have taken the place of the current-draw measurement, with the thickness of coal remaining being shown on-screen instead. “Our operators are not using amps any more,” Mr. Bundy noted. “They just rely on the gamma readout for all of the cutterhead position control”.
Turning to the issue of costs, Mr. Bundy said that there have been no unwelcome surprises on either Terex SHM highwall miner. “Our operating costs have been right in line,” he confirmed, “and in fact, costs on No. 46 have been lower than we had anticipated”.
As for the service that Contour Mining receives from Terex SHM, he was equally emphatic: “Everything we need, they take care of.”
Using natural gamma sensors on its machine has meant that Contour Mining has been able to work an otherwise unmineable resource. With experience like that, it seems likely that the technology will soon be indispensable in highwall mining, not just an option.
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