Shantanu Kumar

111MN0394

Design and performance of Highwall Mining

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 3-

6 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:

Highwall Equipment:

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 Electrical cables for cutter

o Hydraulic lines for shear

o Cooling water lines for cutter motors

o Methane sensor cable

o Control cable

o Hoses protected by steel plates and links.

o Hose chain approximately 330 meters.

o 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 Stacking Pushbeams saves space

o Narrow bench or trench

o Free travelling.

o Quickly to relocate mine to mine.

o Less capital is required than underground mining technologies.

o Short delivery times