Published on March 2, 2014
Design and performance of Highwall Mining Shantanu Kumar 111MN0394
As surface minable coal is depleted, many operators look to highwall mining to extend reserve life. Recent advances in highwall mining equipment have greatly improved productivity, clean coal recovery, and attainable penetration beneath the highwall. Somewhat lagging have been systematic design approaches for ensuring web pillar and overall highwall stability. INTRODUCTION: Surface coal mines typically face higher stripping ratios as mining progresses. When stripping ratios become uneconomic, recovery of coal beneath the final highwall can be accomplished by other means. If the reserve characteristics are sufficient to justify a large capital investment, the mining company may choose to start an underground mine. In some cases, seam characteristics, market demands, or production scheduling can make certain reserve areas not amenable to either surface stripping or underground mining. In these cases, the methods available to surface coal operators are auger and highwall mining. Auger mining was developed in the mid-1940’s as a means of recovering coal beneath the final highwall. Augering machines are, in essence, large drills that bore circular holes of various diameter (typically 1-2 m, or 36 ft) into the coal seam from the pit floor or bench adjacent to the highwall. Although simple, augering machines have several inherent drawbacks that limit their effectiveness. Guidance is difficult, because the auger string consists of segments or flights and is, therefore, not rigid. Auger holes tend to drift downward and in the direction of rotation. Penetration is also limited by the horsepower of the drive system at the highwall. Penetration lengths of 90-150 m (300-500 ft) are typical. To overcome the shortcomings of augering, several manufacturers have developed highwall miners based on the continuous mining machines that have been used in underground coal mining since the 1970’s. Instead of a circular hole, highwall miners create rectangular openings, typically 3.5 m (11.5 ft wide. Because the head of the mining machine can move vertically, highwall miners can adjust to varying seam height, giving them a distinct advantage over augering machines. Advantages of Highwall mining: 1. Greater depth of penetration with almost constant power (305 m penetration) 2. No coal size degrading with increase mining depth. 3. Variable cutting height using adjustable continuous miner cutting head (0.75 m – 3.05 m), extracting 60% to 80% of available coal. 4. Negotiate dips and rolls in the seam because of non-rigid structure of the push beams. 5. High production rate (80 000– 120 000 ton / month) 6. Mine seam dips of +5 degree to -12 degree. The advantages have been discussed in detail below: Cost: Highwall mining is clearly cheaper, due to the lower establishment and development costs. (Already paid for by open cast operation) No in-seam supportor transport systems are required, the cost of ventilation measures is negligible and the capital cost is a fraction of that of a longwall.
Flexibility: Highwall mining can economically access smaller blocks of coal and is more readily able to avoid geological structures or other impediments to production.The mobility of the highwall mining equipment makes it easy to move around a pit, from pit to pit or frommine to mine. Applicability: Because it operates by remote control, highwall mining equipment can work in thin seams without having to cut stone to make height for people to move around, or it can selectively extract a high-quality segment of a seam. The equipment can operate on steeper dips than miners and shuttle cars, and can mine multiple seams without subsidence. Safety: Operations are inherently safer because they are carried out by remote control, so all personnel remain outside the entries. Personnel are therefore not exposed to underground hazards such as roof falls, gas, dust, irrespirable atmospheres, flooding, vehicle movements in confined spaces, etc. With the use of ventilation or inert gas, highwall mining can continue in gassy seam conditions. Coal Recovery: Although at certain times a longwall block or a pillar extraction unit will recover more coal than a highwall mining panel, the overall recovery is drastically reduced by the large areas of pillars that must be left to protect the underground roadways and the odd shapes that cannot be accessed. Even a modern longwall mine will typically recover 85% or less of the in situ coal reserve, marginally higher to the recovery from highwall mining. Productivity: Highwall mining requires fewer personnel, i.e. with the Bucyrus SHS 12 people total (for 3x shifts) can produce 1.5 million tonne PA, compared to around 90 people (3 shifts) to get the same output from an underground mine. A highwall miner system produces more coal per man than an underground continuous mining unit. When factoring the lower capital and labor costs of highwall mining, the cost per tonne is much less than underground mining. Production: Modern highwall mining equipment operate in seam heights of 0,76 meter to 3,05 meter in one pass and seam dips of up to 12 degrees. Coal production per machine ranges from 0,5 million tons per year for low seams to 1,5 million tons per year for high seams, in a continuous operation. Where to do Highwall mining:
Specific Highwall Mining situations Contour Mining: In a mountainous area, the highwall mining system can follow a coal seam along the side of the hill. Open Cast Mining: Highwall mining is used to mine coal from underneath the final highwall, when the strip limit is reached due to economic reasons or surface conditions. Trench Mining: The unit mines coal from both sides of a purpose-prepared trench; this mining method is used when an open pit is not an option. Augered seam: When to Highwall Mining: When economic strip limits are reached. When strip limits are reached by surface conditions. Extend operational life of mine. Transition from surface mining to underground mining. Highwall Operation:
Cutter Head: Interchangeable, for seams 0.8 to 5.0 meters Width 2.9 to 3.5 metres Automatically following seam contour Push beams: Front loader loads into magazine Every 6 meter new push beam is inserted by PTM Operator checks latching and inserts lock pins Augers connect by striker plates
Reel and Chain: o Power chain for o o o o Electrical cables for cutter o Hydraulic lines for shear o Cooling water lines for cutter motors o Methane sensor cable o Control cable Hoses protected by steel plates and links. Hose chain approximately 330 meters. Automatically unwinds/winds into/from channel on push beam.
Tracks: 4 Hydraulically powered tracks articulate 90 degrees for straight and cross travel Circle mode for accurate heading Each track 1 meter vertical movement for adjusting seam dip and floor contour Turning of each track is achieved automatically Anchoring:
Control System: Most of the highwall mining system’s operation is controlled by a Programmable Logic Controller, which provides reliable performance for greater uptime. A comprehensive diagnostics system, including troubleshooting assistance, streamlines maintenance procedures. Equipment Relocation: For quick relocation over long distances, the highwall mining system can be taken apart in modules. Rapid disassembly and reassembly is facilitated by convenient hydraulic and electrical connectors, and all modules are sized for transport using regular public roads. Depending on local conditions, the system can also be transported between sites and without disassembly by heavy haul trucks. Performance of Highwall Mining: Penetration upto 300 meters Can work on dip of up to 15 degrees Monthly production typically around 100,000 tonnes Operates with a 3 / 4 man crew Up to 70% recovery, which is subject to o Coal compression resistance o Overburden load o Seam height / Pillar stability The strength of HW technology: o No ash dilution o Very high up-time o Only crew of four is required.
o o o o o o Stacking Pushbeams saves space Narrow bench or trench Free travelling. Quickly to relocate mine to mine. Less capital is required than underground mining technologies. Short delivery times
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Contour Highwall Mining, LLC (CHM) is a privately-held company headquartered in Summersville, West Virginia. The company has been highwall mining since 2000.