Notes: Longwall Mining Also see Chap 20.1, SME Mining Engineering Handbook Terminology: - Panel width (262m average in 1999, typical range: 200-360m center to center) - Panel length (2470m average in 1999, typical range: 900 – 5280 m) - Head entry (usually 3 entry system) (total width ~ 30-110 m) - Tail entry (usually 3 entry system) - Recovery room - Setup room - Barrier pillar - By the set up rooms, to protect the bleeder entries, solid block, 60-150m, (can be the bleeder system too) - By the recovery rooms, to protect the mains (90-150m, solid block)
Next panel gates being driven (this defines the next panel) Mined out (current panel)
Intake Return Setup room, where longwall starts
Head gate Head entries Bleeders Recovery Room (where LW stops) Barrier pillar
LONGWALL PANEL
Barrier pillar
Panel Width Panel Length
Tail entries (next to old panel) Tail gate
MINED OUT LONGWALL PANEL
Applicability: Depth 60-820m. Works best at greater depths as the roof will collapse easily. Seam thickness 1.1 – 4.0 m (multiple lifts possible for greater thickness) Deposit should be large to justify high costs Seam dip preferably less than 150 Roof must cave. Massive sandstone or limestone can cause problems.
Panel Width: Effect of increasing the width - reduced number of panels - reduced development costs - increase in recovery rate - increase in production. However, after a certain maximum width (~300m), gains negligible. Operational difficulties such as difficulty in maintaining a straight face line and hence, support line and conveyor line is common. If the face is not straight, roof falls can occur ahead of the shearer. Direction of advance
Caving/roof fall line Potential problems
This is because roof falls/caving typically occur in a straight line. If one side of the face is ahead of the other (or as happens most of the time, ahead of the middle), the caving process starts, hurting the face conditions where it lags behind. -
reduced number of face moves. However, each move is longer due to increased equipment to be moved.
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Longwall face moves are very intricate and time consuming. Each face contains tens of heavy supports, armored face conveyor (AFC), shearer, stage loader, power center and the regular conveyors. All these have to be disassembled and moved to the new face. It is challenging not only due to the logistics and scheduling problems, but also due to the fact that handling large and heavy equipment is not easy to handle and move in confined spaces.
higher capital investment higher power and structural requirements at the face for unstable roofs and unidirectional cutting, longer exposure times for roof between the toeline of supports and the face
Number of Panel entries: - minimum of three entries should be planned for the head gate and tail gate, even though, a lot of mines operate with two entries only (per gate). When the three entries are being developed for each gate, it allows for easy movement of man, material and air (one entry for man movement and intake air, one entry – neutral - for material, and one for return air). Once the gates are developed, man, material and intake air use the main gate or head gate, while the return is via the tail entry. Size of chain pillars: - One can use yield pillar design (for deep mines) or other pillar design strategies we discussed before. Use ALPS, the NIOSH program. In most mines, this may be done based on past practices.
Supports: - face support consists of self advancing supports such as 2-leg or 4-leg chock-shields. Each units’ support capacity ranges from 300 to 1200 tons. Units are about 1.3-1.6m wide. - head entry support is a combination of hydraulic props, roof bolts, or 3-set beams Stress: - front abutment stress is felt about 160 m ahead of the face. However, increase in stress is negligible. The stress starts increasing at a distance of 50-60m from the face. The peak front abutment (1.5-5 times the cover load) is felt 1-5 m from the face. The face itself is destressed. - The highest stress is at the ends of the face as this is where the side abutment meets the front abutment stress. In the figure below, “goaf” refers to gob.
From http://www.uow.edu.au/eng/longwall/abutment.htm
Shearer dimension: - 36-42 in web - drum: 75-80% of face height - 13-82 fpm
Production: - Coal cut by the shearer gets loaded onto the AFC, which takes it to the crusher. The crusher outputs the coal into a stage loader which dumps it into the main conveyor. - nomograph gives cycle production - unidirectional or bi-directional cutting
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In unidirectional cutting, the shear cuts when it travels from the tail gate to the head gate. In a double ended shearer, the leading drum cuts the coal while the trailing drum is either free or cuts the floor. On the way back to the tail entry, only cleaning of loose coal is done. In bi-directional cutting, the coal is cut in both directions.
Manpower (about 10-13: 1 supervisor, 6-10 miners, 2 mechanics, 1 electrician)
Equipment details (from a mining machinery company) as in Year 2000 (240-300m wide face): - Shearer, 1400 hp, $2.1 M - AFC, 2400 hp (1200 head gate, 1200 tail gate), $2.9-3.0 M - Shields, $100-130K each, 146 for 240 m face, totaling $17-20 M - Crusher, 350-400 hp, $190K - Stageloader, 350-400 hp, $500K - Face voltage is typically 2300/4160 Total cost of equipment for a face, 240-300 m in width, is approximately $25 M Total power requirement approximately 4500 hp (not including that required for supports).
More Details from a Coal Company The shearer costs about $1.75 million (this is a Joy 4L-S). The cutting motors are about 600 hp each and the tram motors are, I think, about 70 hp. A 1000-foot AFC + crusher + stage loader costs about $5 million. (Joy quotes these as a unit.) The AFC is capable of handling 3000 tph and has a total of about 2700 hp (three drives, 900 hp each). The crusher will have another 200-400 hp (depends on the space available), as will the stage loader. 1250-ton shields, 1.75-meters wide, will cost about $110,000 each. (A 1000-foot face will take 176 shields.) Seven of the shields must be larger than the others (they must push the stage loader assembly and the drives at the head gate and tail gate), and will cost about $135,000 each. Don't forget to buy a few spares. Other panel equipment and their power consumption Hydraulic pumps for the shields: Probably about $600,000 (??). These are four units of 200-300 hp (??-sorry, I forgot to ask about these), only two of which operate most of the time. Lighting -- about $100,000; Monorail unit --- $???; The "mule train" components (emulsion car, water pressure pump, electrical transformer and electrical control units) -- about $500,000 to $1million. Total power consumption for the face The power center for the face is usually sized for 5000 KVA. The shearer will cut at a steady 3000tph, so you can get some idea of the power draw (pretty close to the 1200 hp capacity). The AFC will also draw (I believe) about 2000 hp on a steady basis when the pan is full. For a longwall mine with three continuous miner units for development, I would use a 20,000 KVA main transformer at the mine. Quote for the entire face is about $33 million. This includes the spare equipment, but not the section belt, the electrical controls or any of the retrieval equipment to move the face. Ordinarily, a spare of everything except for the mule train, the hydraulic pumps and shields is purchased. Then, when the face is moved, only these items need to be moved. The rest of the items are sent out for rebuild after they are taken off the face, and the rebuilt items are then placed in the subsequent face. Thus, there is an AFC, stageloader, shearer etc. for panels 1,3,5,7, etc. and a second set for panels 2,4,6,8, etc. This keeps move time down to 4-5 days versus the 20-30 days if everything must be disassembled, rebuilt and reassembled. (And, the difference in capital cost is only the time-value of money for the items involved.) Here is a summary of the Joy equipment: Original Face
Next Face+Spare Units
Shearer: 1,750,000 AFC, Stageloader, Crusher: 5,000,000 Shields: 19,540,000
1,750,000 5,000,000 820,000
Totals:
7,570,000
26,290,000
Joy Longwall System
www.joy.com
