FACE AREA HAULAGE
FACE AREA HAULAGE:-
With virtually all main coal haulage systems within the seam being by conveyors which start from a more or less fixed point, some arrangement is needed to bridge the gap from the coal cutting machine to that point.
For a longwall face the gap is bridged by the AFC and BSL, both of which are dealt with under longwall equipment and will not be discussed here.
For other mining systems, a more flexible arrangement is required as coal has to be transported from other roadways back to a fixed point, with frequent changes of the mining location. By far the most common method of transport at present is the use of "shuttle cars".
Typical shuttle car viewed from discharge end.
Trailing cable connects to closest corner, driver located on opposite side
Shuttle cars are electric powered, rubber tyred vehicles - essentially a large trough with a scraper conveyor running along the floor, a driver's position mounted to one side and a jib section at one end of the conveyor which can be raised for discharging the load. They are four wheel steered, four wheel driven vehicles with power supplied by a trailing cable reeled onto and off a cable reeler mounted opposite the driver's position.
It is normal for two cars to be used in one mining section with one car being loaded by the continuous miner at the face while the second car is discharging its load at the conveyor. Whilst being loaded, the scraper conveyor in the shuttle car is run in short bursts to drag the load towards the opposite end of the car until the car is full. That car then heads out to the conveyor discharge point, passing the other car at a shunting position on the way.
On reaching the conveyor, the unloading jib section is raised to enable coal to be discharged into the "breaker feeder" or directly onto the conveyor (depending on the mine's relevant equipment and practices) and the scraper conveyor is run until the car is empty. This car will then return to the mining machine or to the passing shunt to await the other car.
Shuttle cars have proved to provide a robust, flexible and generally reliable haulage system and are still very popular. They do suffer from a number of disadvantages:
- Trailing cables are prone to damage as a result of being run over, squashed against the ribs or other equipment or jammed and pulled apart and can be a safety hazard as well as a common source of downtime.
- Trailing cables essentially restrict the number of cars servicing a miner to two, without a risk of tying the cables in knots.
- The haulage is intermittent (unless a car is used as a temporary bunker behind the continuous miner).
- The distance between the mining face and the conveyor discharge point is limited by the length of the trailing cable, unless other special arrangements are adopted.
Other equipment has been developed to try and overcome some of these problems:
- Battery powered shuttle cars were developed to avoid trailing cables and provide more flexibility; some success and certainly more flexible, but batteries are bulky and heavy and require time to charge up and cool before use – this entails a charging station and battery changing equipment. All of this can be just as troublesome and hazardous as using cables, so is only beneficial if the extra flexibility is required.
- Diesel powered shuttle cars were similarly developed to avoid the use of trailing cables and provide more flexibility; certainly more flexible than either cable or battery powered cars and haulers and are unrestricted in respect to distance between loading and discharge points. Their biggest drawback is the production of diesel fumes and heat in the face area which can create unpleasant conditions even with good ventilation.
- Another development has been the use of "ram cars" or "haulers" in place of shuttle cars: these are articulated vehicles with one end carrying the power system and the driver position, the other end consisting of a large tray or bin with a push plate to eject the load. The push plate is operated by a hydraulic cylinder. Ram cars or haulers are operated similarly to shuttle cars except the cars have to be turned between the loading and discharge locations, loading and unloading being at the same end of the vehicle. Ram cars/haulers are usually diesel or battery powered.Neither of these systems have received wide acceptance to date.
- Flexible conveyor trains; various types have been produced including both floor mounted and roof mounted continuous conveyor systems with both systems offering some degree of operational flexibility. The discharge end of the flexible conveyor runs above the end section of the main conveyor to enable the flexible conveyor to discharge onto the panel conveyor as the flexible conveyor follows the continuous miner through the development sequence, The face end of the flexible conveyor is attached to the rear of the continuous miner or is self propelled and kept at that position. With such an arrangement the flexible conveyor is always in place between the miner and the main conveyor, providing continuous haulage. The main disadvantage to date is the high capital cost of such systems and the high maintenance costs associated with the flexible belting. The flexible conveyor trains, at this stage, cannot handle right angle bends and necessitate formation of diamond shaped pillars, which may in some circumstances be prone to crushing on pillar ends and/or result in larger intersections than otherwise preferred. Some mines also consider that the angled cut-throughs that result do not suit most other aspects of mining. Both roof and floor mounted flexible conveyor systems were trialed in Australian mines during the late 1980's with limited success. However there is now a resurgence of interest in these systems as mines seek to improve gate road development rates,
- Bridge conveyor systems: again various types have been produced including both chain type conveyors and conventional conveyor belt systems. Bridge sections are typically short (6m on conveyor bridges and 16m on chain type bridge systems) and are self propelled (the conveyor bridge systems are now fitted with PLC controls which enable the entire system to be operated by one person whereas the chain type require an operator on each bridge section). Again angled cut-throughs are preferred (70o) to facilitate transition between adjacent roadways. Depending upon seam (and hence mining) height the discharge end of these systems can either run over or beside the main conveyor to enable the bridge conveyor to discharge on the panel conveyor as the bridge conveyor follows the continuous miner through the development sequence. Both bridge systems provide a continuous haulage system similar to the flexible conveyor train systems while both have only recently been introduced into Australian mines (although a chain type bridge system was operated in one Australian mine during the 1990's with limited success)
With all the car or hauler arrangements, some fcaility is needed to load efficiently at the panel conveyor. At least a "boot end" is required, a trough arrangement with a top opening slightly narrower than the conveyor belt. The return roller for the conveyor is at the end of the boot nearest the face, so the belt runs under the opening in the top of the boot to carry the coal away. The coal can be loaded on top of the boot, the pile reducing at the rate that the belt can remove the coal. Care is therefore required to ensure that loading is not too fast otherwise spillage will occur. The belt will be fully loaded whilst there is coal left in the boot.
A better means of transferring the coal from car to belt is to use a "breaker feeder". This is a large hopper fitted with a scraper conveyor in the floor leading to a narrow outlet, approximately the width of the conveyor belt, and often containing a coal breaker at the outlet. The use of a feeder enables the car to discharge at maximum rate while the feeder controls the feed rate onto the belt to the desired rate. This is particularly important as high shuttle car discharge rates have been proven to result in higher development rates, whilst controlled feed out rates from the feeder will prevent overloading of the conveyor and the ensuing spillage. While the breaker on the feeder may not be as effective as a crusher in reducing product size, they are generally effective in breaking large lumps of coal or stone into more manageable sizes and improving conveyor loading.
Flexible conveyor trains and bridge conveyor systems feed directly onto the panel conveyor and obviate the need for a breaker feeder. Recent trials with both systems do however highlight the need to introduce a capability to size any large lumps that may be produced if high productivity levels are to be sustained. This can be done either by fitting an in-line breaker into the throat of the continuous miner discharge boom or onto the first bridge section behind the continuous miner.
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