Mineral Exploration Definitions

Mineral is a homogeneous inorganic substance that occurs naturally, usually in crystalline form with a definite chemical composition.

The common rock-forming minerals (RFM) are quartz, orthoclase feldspar (KAlSi3O8), plagioclase feldspar (CaNaAlSi3O8), albite, mica group such as muscovite and biotite.

The common ore-forming minerals (OFM) are hematite (Fe2O3), cassiterite (SnO2), chalcopyrite (CuFeS2), sphalerite (ZnS), galena (PbS), baryte, gypsum (CaSO4), apatite, etc.

Rock is an assemblage of mineral(s) formed under natural process of igneous, sedimentary and metamorphic origin. The common rocks are basalt, granite, quartzite, sandstone, limestone, marble and mica-schist.

Rock and Minerals

Ore This concept has undergone radical changes over the years. The Institution of Mining and Metallurgy, UK, currently defines “Ore as a solid naturally occurring mineral aggregate of economic interest from which one or more valuable constituents may be recovered treatment.

Ore

Ore deposit is a natural concentration of one or more minerals within the host rock. It has a definite shape on economic criteria with finite quantity (tonnes) and average quality (grade). The shape varies according to the complex nature of the deposit such as layered, disseminated, veins, folded and deformed. It may be exposed to the surface or hidden below stony barren hills, agricultural soil, sand, river and forest.

Ore Minerals maybe classed as  primary (Hypogene) or econdary (supergene).

Hypogene minerals were deposited during the original period of rock formation or mineralization.

Supergene minerals were formed during a later period of mineralization, usually associated with weathering and other near-surface proccesss, leading to precipitation of the secondary minerals from descending solutions “meteoric waters”.

Ore Deposits and Minerals

Protoreis an altered rock mass or primary mineral deposit having uneconomic concentration of minerals. It may be further enriched by natural processes to form ore. These are low-grade residual deposits formed by weathering, oxidation, leachingand similaralteration. The protore can turnintoan economicdepositwith advance technologyand/or increaseof price. It can be exploited for kaolin, iron and nickel due to sufficient enrichment of the respective metals.

Protore

Prime commodity” is the principal ore mineral recovered from the mines.

Associated commodities” are the associated minerals recovered as by-products along with the main mineral.

Trace elements” In general all ore deposits contain number of valuable “trace elements” that can be recovered during processing of ore.

"The prime commodity of a zinc lead- copper-silver mine is zinc, and the associated commodities are lead and copper. The expected value-added trace elements are cadmium, silver, cobalt and gold. The value of all prime commodity, by-products and trace elements are considered collectively for valuation of the ore/mine."

Prime Commodity, Associated Commodity and Trace Element

Ganague minerals is the associated minerals or rocks, having no significant or least commercial value, are called “gangue” minerals. Pure chalcopyrite having 34.5% Cu metal in copper deposit and sphalerite with 67% Zn metal in zinc deposit are hosted by quartzite/mica-schist and dolomite respectively. The constituent minerals of quartzite, mica-schist and dolomite are called the gangue minerals. A list of common gangue minerals are Quartz - Barite- Calcite - Clay minerals All types - Dolomite - Feldspar All types - Garnet All types - Gypsum - Mica All types - Pyrite – Pyrrhotite.Quartz - Barite- Calcite - Clay minerals All types - Dolomite - Feldspar All types - Garnet All types - Gypsum - Mica All types - Pyrite – Pyrrhotite.

     

The raw oreis milled before the separation of the ore minerals from the gangue by various beneficiation processes. The concentrate is fed to the smelter and refinery to produce 99.99% metal. 

Tailing is the rejects of the process plant which are composed of the gangue minerals. Tailings are used as support system by backfilling of void space in the underground mines. Alternatively, it is stored in a tailing pond and is treated as waste. High-value metals can be recovered by leaching from tailing in future. Tailing of Kolar gold mine, India, historically stored at tailing dam, is being considered to recover gold by leaching without any mining cost.

Ganague Minerals and Tailing

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Twin Galaxy Stones

11 Black Opal 

Twin Galaxy Stones - A stunning pair of nearly identical Lightning Ridge Black Opals. These rare stone are both more valuable than diamonds.

Source Here

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Gas Control (Underground Coal Mining)

Some mines do not have a gas problem of any type, the quantity of gas in the coal being so low that only small percentages (or even zero) are ever detected in the air. In such mines, other than continuously or regularly checking gas levels to ensure conditions have not changed, no special processes are required.
Note that this comment refers to naturally occurring gas coming from the seam or surrounding strata. The ventilation system still has to deal with introduced gases such as diesel exhaust, shotfiring fumes and blackdamp (an atmosphere deficient in oxygen).
There are two ways of controlling gas levels in the atmosphere within a mine:

• Provide sufficient air such that gas concentrations in the air are always below levels which would create safety issues (usually including a generous factor of safety).
  
• Capture the gas and contain it until removed to a location where it is not a danger to the mine (usually in pipe ranges or in boreholes).
  

Operationally, the first option is simplest as it does not involve any additional mining procedure. Practically and financially there are limits however, as an increase in the quantity of diluting air requires an increased fan performance and/or additional roadways to reduce resistance. Both of these come at a cost. Additional roadways may also require additional mining units to maintain continuity of production (with longwalls particularly) and this may then require additional ventilation of itself, compounding the problem.
There are 4 ways of capturing/ containing gas to keep it out of airways:

• Drilling boreholes into the coal seam (possibly also in adjacent seams) prior to mining and connecting these to a pipe range to remove any gas to a place where it can be dealt with, most commonly the surface. Usually a vacuum is applied to the pipe range to enhance gas capture. This process is known as "pre-drainage".
  
• Drilling boreholes in roof or floor strata to capture gas released from the strata and flowing through it towards mine airways. Such holes would normally be attached to a pipe range as for pre-drainage. This process is known as "post drainage" as it captures gas released by the mining process or "cross measure drainage" as the boreholes are drilled across the strata beds.
  
• Removing gas from live goaf areas, either using pipes installed through goaf seals and attached to a pipe range as above or by using boreholes drilled from surface down to strata just above the goaf. Vacuum may be applied to such boreholes to assist in gas removal. This process is known as "goaf drainage". The use of boreholes to surface may be limited by surface land use. It should also be noted that even though reference is made to goaf "seals", it is not possible to seal a live goaf as the face area must remain open until mining is completed and the equipment removed.
  
• Sealing off worked out areas of the mine, including standing goaf areas, to contain gas still being made within those workings or goaf areas. Whilst there is always likely to be some leakage, high quality seals may limit contamination of the mine ventilation. Flooding of worked out areas is another effective way of controlling gas liberation, however it may introduce other risks of inrush to future mine workings.
  

Note that these comments relate to controlling gas levels in the airways. Gas drainage for the purpose of eliminating outbursts may involve the same pre-drainage process as above but the target for gas removal is only the area in which development roadways are to be driven. It may also be the case that sufficient gas is pre-drained to avoid outbursts but the remaining gas can still cause problems if released into the atmosphere.

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Development Face Ventilation

To maintain a ventilation circuit it is necessary to have an intake and a return airway to the innermost point to be ventilated. As roadways are mined, the last section has only a single roadway so another arrangement has to be made.
At one time this was done by erecting a temporary "wall" along one side of the roadway by hanging a flexible, airtight material known as brattice on timber props and extending the brattice across the last completed cut-through so air was forced to flow up to the face.
This system works quite well and is still in use in some mines where ventilation is not too arduous (e.g. bord and pillar mining). The brattice wall tends to be leaky and it is difficult to get good flow at the face for distances over 80m. The brattice also restricts the working width or the width of the return path (or both).
The adoption of longer pillar centres (>100m) in longwall mines coupled with higher in situ gas regimes experienced in deeper mines has necessitated that mines utilize a more effective ventilation system comprising electrically powered auxiliary fans attached to ventilation ducting to course air through the working areas. Auxiliary fans add another complication and another piece of machinery, but give more positive flow and more flexibility in sequencing.
A common misconception among operators is that face ventilation can be improved by increasing the airflow to a panel. This is true for brattice ventilation, but for fan ventilation the face flow depends entirely on what the auxiliary fan will pull. Extra air to the panel will not improve face ventilation quantity.
It is essential to have more airflow in the panel than the auxiliary fan is capable of pulling otherwise recirculation will occur and if there is any gas make the concentration of gas in the air will gradually increase. It is usual to maintain more flow than the open circuit capacity of the fan, typically 30% more than the open circuit capacity added as a safety factor.
Because an auxiliary fan has a pressure rise through it, it is possible for the ventilation pressure at the start of the return to be slightly higher than the intake and recirculation through a stopping or door is possible. This can be avoided by erecting a baffle behind the auxiliary fan outlet to cause a pressure drop at that point.

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VENTILATION Secondary or Panel Airways

Secondary or Panel Airways

There is a tendency for development panels to be mined for longer distances, in order to maximize the length of longwall panels, without increasing the number of headings. At the same time production rates from longwall equipment have increased. The result, combined with increasing face lengths, is ever more arduous ventilation requirements and high ventilation pressures as noted previously.
Airways around longwall face ends are also frequently obstructed by equipment, windrows of coal, brattice wings and secondary support all of which hinders air flow.
Flows and pressures at regulators can be very high and provision must be made in some cases to allow the safe passage of personnel through these devices (e.g. an enclosed steel tunnel with air lock to one side of the roadway).

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VENTILATION Main Airways

As mines become larger and production rates increase the quantity of air flowing in the main intakes becomes large and ventilation effectiveness and cost is usually the main factor deciding the number of main airways required. For other purposes it would often be the case that only two intakes, one each for coal haulage and personnel/materials transport, and one return are necessary. Additional headings are to reduce the resistance to air flow and /or to reduce air velocities. For ventilation, the more airways the better, but after a point the incremental benefit is small; for development costs, the fewer the better – the best compromise is required.
Air velocity also has to be considered where air flows are large relative to:

• Safety and comfort of personnel working in windy locations or walking against high flows
  
• Raising of dust and its ability to settle, mostly in conveyor and transport roads
  

The requirement for high air flows and, increasingly, high pressures dictates a high standard of ventilation control devices. Where access doors are required for personnel or machinery at least double doors forming air locks are required with pneumatically or hydraulically operated doors becoming more common.
Minimising leakage through stoppings, doors, etc is important, but mostly from an efficiency point of view. Removing leakage actually removes parallel air paths and thereby increases overall mine resistance and the result may be that minimal additional air is generated at the face areas within the mine.
Main airways should be mined as straight as possible and kept as free of obstructions as possible, a factor often overlooked when siting major items of equipment. At least the presence of such equipment should be taken into account when ventilation planning is carried out.
A further issue to be considered in mine design and ventilation planning is the siting of large motors (e.g. maingate conveyor drives) and other heat generating sources (including high powered diesel equipment) particularly in mines in Central Queensland that experience both a high geothermal gradient and high ambient temperatures (and humidity levels) through summer months. The application of air cooling systems is being increasingly favoured in such environments.

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VENTILATION Main Fans

Note that the following is written using fan in the singular for the main ventilation. In fact where a large fan duty is required it is common to use 2 or 3 fans in parallel to provide the duty. The reasons for this are:

• Fan duties are becoming very large and the physical size of fans that could meet such duties singularly become impractical
  
• With multiple fans partial ventilation can be maintained in the event that one is stopped, a feature which is considered very beneficial.
  

The main fans which ventilate large mines are major power consumers, having large motors which run virtually continuously, usually only being stopped for maintenance purposes. Great care is therefore required in design and specification in order to avoid unnecessary expenditure. The fan duty required often varies greatly over the life of a mine as the workings become more extensive and it is usually best to make provision for varying the fan performance. This can be done by installing inlet vanes, which restrict the fan inlet size, or by varying the fan speed. Speed can be varied by changing motors, by use of gears or by use of variable drive systems (VVVF drives are being increasingly used).
Air quantity flowing at the main fan is large and unless care is taken with the design of airways and ducting considerable power can be wasted in overcoming the resistance and shock losses which result. Airways should be as straight and smooth as possible and sudden changes of direction should be avoided as far as practical. At the same time there is a requirement that provision be made to protect main fans from damage in the event of an explosion. This is done by mounting the fans away from the direct line of the main airway, which usually involves a 90^o deflection (or close to it), and providing a device in line with the airway that is designed to fail under pressure and release the explosion pressure (e.g.doors or a weak section of duct).
Airways should be kept as clear as possible, though it is common for main return shafts to involve winders with the associated shaft fittings.
High air flows in shafts can cause problems:

• Because of the effect of turbulence on stability of cages, counterweights or ropes
  
• If shafts are wet high velocities can result in water being held in suspension causing unstable flow (air velocities between 8 and 12 m/s should be avoided; above this range water will be carried up the shaft and will have to be dealt with at the fan site and part of the fan power cost will actually be for pumping water).
  

As fan duties become more onerous, the ability to provide ventilation using only fans on the surface becomes increasingly difficult, especially for mines with coal liable to spontaneous combustion. The use of "booster fans" sited underground and acting in series with the surface fan will become more common. While such strategies may be preferred they raise difficulties in relation to interlocking surface and underground fans, avoiding recirculation and separately ventilating underground fan motors to ensure gas-laden air does not pass over the motor(s).

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Common Operational Problems

Gate Roads Misaligned

If the main and tailgates are driven off line far enough it is possible the longwall block is too wide or the space between the gate road pillar ribs is too narrow for the face equipment to fit. The only solution if this occurs is to strip coal off the rib(s) in question to allow the wall to fit and install additional support to cater for the extra width. It is best for such alignment problems to be identified early, preferably while the development machinery is still at that location so it can be used to correct them. On occasions a relatively small misalignment (which the face equipment should be able to handle) becomes a problem if the face creep is not controlled when the face reaches that point. Road widening can then only be carried out by jack-picking, shotfiring or removal of maingate equipment to allow machine access.

Face Misaligned

This can be a case of the face not being straight or of too great a difference between main gate and tail gate positions (face not at right angles to the gate roads). In extreme cases this can result in the same effect as misalignment of gate roads as described in the cutting process section and the face equipment will be too short to reach both edges of the block. Too great a curve along the face can result in there being insufficient flexibility between the pans and they lock together when being pushed over. Straightening curves on the face will mean some sections of the face do not advance as rapidly as others and it is possible for roof control to be problematic at such places.
In all cases it is best to identify the misalignment before serious problems are caused and carry out straightening cuts or fly cuts as appropriate. If the stage of the face being too short has been reached the ribs will have to be stripped at one or both ends of the face to allow correction cuts to be made. If pans are locked, it is usually possible to free them with some "jiggling" of pans and chocks. Loss of roof control will have to be rectified by means described below.

Loss of Control of Roof Along Face

This can result from:

• Face misalignment
  
• Inadequate support capacity
  
• Inadequate hydraulic pressure
  
• Failure to properly set supports (i.e. low initial setting pressures)
  
• Excessive delays in face advance
  
• Localised area of weak roof
  

The loss of roof control can vary from broken roof over the supports, through minor loss of roof material ahead of the supports, up to major cavities ahead of the supports. More often than not, this is a progressive deterioration and the sooner action is taken to correct the issue, the better. Measures that can be taken or issues to be faced include:

• Correction of face misalignment which has been dealt with above
  
• Inadequate support capacity - an issue which cannot be corrected and a new set of supports is called for. A decision would have to be taken to persevere with bad roof conditions for the rest of the block or to stop and attempt to recover the equipment. Inadequate set pressures can lead to roof failures which replicate those due to inadequate support capacity, hence routine monitoring of set pressures is required to prevent such problems and/or eliminate this cause.
  
• Inadequate hydraulic pressure may result from a problem with pumps, but is more often a result of poor support maintenance and an accumulation of leaks (internal and external) preventing correct operation. The ability of chocks to operate up to the design yield load should be checked regularly.
  
• Whilst excessive delays may have already occurred for a variety of factors, it is essential that they be evaluated to identify root causes and develop counter-measures to eliminate potential re-occurrences and/or plans for more effective introduction of control measures in the event of a re-occurrence.
  
• Localised areas of weak roof cannot be avoided, but some roof reinforcement may be called for (local bolting, grout injection, etc).
  

Once a degree of control is lost, the steps in recovery are usually:

• Avoid all unnecessary delays and try and maintain continuous production (24 hour, 7 day operations if necessary) to keep the face advancing as steadily as possible.
  
• When moving supports try and ensure they are advanced while still in contact with the roof (pressure relieved but not lowered) to retain broken roof material in place as much as possible
  
• Advance supports as close as possible behind the cutting machine and keep the unsupported roof area to a minimum
  
• Place timber in cavities above the supports if possible to maintain as uniform a roof loading as possible
  
• If it is considered the area of broken/fallen roof is unlikely to be caught, then stop the face and carry our secondary support. This again may vary from localized bolting to grouting, including strata injection ahead of the face.
  
• If a major cavity has formed it may be necessary to stop for a longer period and fill the cavity with some type of grout or light-weight cement to prevent further loss of strata and to give an artificial roof for supports to act against.
  
• In extreme cases where supports have become ironbound (with or without a cavity ahead of them), it will be necessary to jack-pick or shotfire the roof above to release them.

Loss of Roof Control in the Gate Roads

This would normally be a result of inadequate roadway support design, a localized area of weak roof or high stress or excessive delays in face advance. Again the result can vary from nuisance broken roof up to major falls or the roof lowering onto the longwall maingate equipment. In the tailgate it is frequently possible to continue operating the face even past a fall as only the tailgate drive extends into the gate road (possibly supports as well if the face has crept towards the tailgate). A shearer can clean-up in front of this drive to allow it to move forward, provided the fall has stabilized.
The maingate however is full of equipment and is a major work area for personnel, so any roof deterioration has to be made safe and cleaned-up in order to continue operations.
If roof problems become apparent in the tailgate it is often quite easy to install additional support to prevent a major failure; often passive support such as timber chocks or cans (possibly cuttable cans which have a resin or fibre-glass tube instead of steel) are quickest and easiest. If the roof closes to the extent that chocks and/or the tailgate drive cannot lower enough to pass through it may be possible to cut the roof with the shearer to create the height. If the roof has failed totally and a fall has occurred, it may be possible to continue without securing the fall, as noted above.
Whatever options are considered for the tailgate, the points to be borne in mind are:

• It must be possible to continue operating without exposing personnel to unsafe roof; is it possible to continue without access to the area in question?
  
• Is ventilation still adequate or is the tailgate airway too restricted?
  
• Are there alternative egress considerations which require the tailgate to be maintained for safe travel?
  
• Is there roof in place for the longwall supports to bear against?
  

For maingate roof problems, the considerations are much the same, but in this case:

• Personnel access will always be required so the roof must be made safe and there are always chocks and equipment in the roadway; by-passing it is not an option
  
• Any fall would have been on the conveyor or maingate equipment, so would have to be recovered. Any cavity will need to have some arrangement made to provide a "roof" for the longwall supports to bear against, which can be done by filling with lightweight concrete, erecting a steel frame, placing timber over the chocks when they reach the cavity, etc
  
• Lowering roof usually affects equipment ahead of the face (in particular the crusher) so using the shearer to gain height is not an option; the roof height will have to be re-established by either brushing, jack picking or shotfiring.
  

If roof stability problems occur in roadways adjacent to main or tailgates, those roadways are likely to be required in the future. Recovery of poor roof areas or falls will most likely be necessary, but may not be for the current longwall. In such cases recovery work may be scheduled later providing there is no further deterioration.

AFC tipped forward

It has been noted that the shearer attitude depends on the AFC. If the AFC is tipped towards the face the shearer drum will tend to cut deeper into the floor and this will cause the AFC to tip even more when it is pushed forward. If this is allowed to continue it can reach a point where it becomes very difficult to recover.
This situation can result from:

• Poor control of the floor horizon by operators e.g.cutting too deep
  
• Soft floor, allowing the toe of the pans to dig in
  
• Floor heave under the rear of the pans
  

To correct the problem, the attitude of the AFC has to be corrected, usually by lifting the pans and packing underneath with some material (generally timber) and/or by jack-picking and removing floor material from under the pans. The support canopies can be used as lifting devices to lift pans but care must be exercised to ensure the correct attachments and appropriate slings or chains are used to do this safely. It can be a long, slow process.
In some cases, there are two attachment points for the push cylinders at different levels on the pans which can assist in directing the push up or down to some extent, as required.
It is equally possible, though less common, that the AFC will tilt up at the face and the longwall will tend to climb into the roof. Provided this is not a result of poor or faulty design, this is usually quite easy to correct by cutting below the pan level.

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Longwall Top Coal Caving

Longwall top coal caving (LTCC) is a special type of longwall mining applicable to very thick seams (greater than 4.5m) where good quality coal is being left because "conventional" longwall equipment has not yet been designed to operate successfully beyond around 5m mining height. It enables an increased recovery for only an incremental additional cost.
The method originated in Europe but has been developed in China in more recent years and is used there quite extensively and successfully.
The lower section of the seam is cut by a conventional longwall set-up except that the longwall supports have a longer rear canopy extending past the base into the goaf. The extended canopies have a sliding door fitted into them.
An additional AFC is attached to the rear of the chocks and runs directly below the canopy openings.
As the face moves forward, the coal left above the section cut by the machine falls onto the extended canopies, providing the goaf is caving normally. The sliding doors in the canopies are sequentially opened, and the coal falls through onto the rear mounted AFC. The maingate stage loader is extended beyond the face conveyor to enable the rear mounted AFC to discharge coal directly onto it and carry coal to the maingate conveyor system.

Diagramatic view of principle of longwall top coal caving

The sliding canopy doors are opened and closed in a controlled manner to ensure the conveyor is loaded efficiently and to prevent stone being taken out when all the coal has been recovered from a particular section.
Not all the coal would be recovered, but recoveries of 75-80% of the full seam in the block are achievable. The system also allows the initial mining height to be reduced to a preferred working height without losing coal.

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Stress/Strata Control

Along the Longwall Face

Along the face, the roof control function is a matter of providing sufficient resistance to control the dead load of the mass of strata which is breaking, or has broken, away from the bulk of the overlying strata. The chocks are not resisting the whole vertical stress field, most of which is redistributed to the solid coal ahead of the face (the "front abutment load") and to re-compacted material in the goaf behind the face (the "rear abutment load"). Provided the chock load capacity is sufficient to control the movement of this dead load so that the roof in front of (and preferably above) the chocks remains intact, face conditions will remain good.
Horizontal stress is not really a major factor in roof control along the face. There will be zero stress in the direction perpendicular to the face once a goaf has formed and stress on the long axis of the face will not create problems. Horizontal stress in the floor however can give rise to floor heave under the pans which can become a major issue. The angle of the pans controls the attitude of the shearer and if the rear of the pans is lifted it often becomes necessary to straighten them before continuing, a difficult and time consuming operation.
Because reliance is being placed on controlling roof lowering which is expected to occur, major problems may arise if a face is stopped for any length of time. It is possible that a stable condition may be attained where movement ceases. It is equally possible that movement will continue to the point where the roof starts to fail ahead of the face and even to the point where the chock legs are fully closed and cannot yield further, a state known as being "iron bound".
Ideally a longwall face should operate as near to continuously as possible and at a steady rate in order to maintain good face conditions. It has often been the case that a relatively minor problem which has caused the face to stand for a period leads to a deterioration in conditions which then compounds into a major event.
Some degree of movement not only ahead of the chocks but slightly ahead of the face can be useful. This movement and resulting load on the uncut coal can cause crushing and fracturing leading to a much reduced power demand on the shearer, sometimes to the extent that it becomes a loading machine rather than a cutting machine. While this effect can be beneficial it should not be a design aim as it occurs near the point where roof control is lost and this would not be an ideal operating state. Even if roof control is maintained initially, too much load on the face coal will lead to slabbing, creating a wider expanse of unsupported roof with roof control problems possibly ensuing, which is exacerbated in high seams.

 

In the Gateroads and Associated Roadways

The gate roads are frequently major causes of concern with regard to strata control around a longwall panel. As well as having to have sufficient support to remain stable during development and for the period before the longwall passes, they have to withstand the redistributed stresses and abutment loads which arise ahead of, behind and to the sides of the advancing face
Around the face ends there is also a redistribution of horizontal stresses. It is often the case that one end of the face is protected by an adjacent goaf (in a "sress shadow" area) and horizontal stresses are carried in higher strata horizons away from the immediate roof. If the current longwall is extended beyond the old goaf, the stress shadow effect is lost and a stress concentration is likely instead. There will be a stress concentration at the other end of the face and such stress concentrations can be up to twice the normal stress levels and can cause compressive failure of the roof strata. It is preferable for the face to be oriented so that such concentration is applied to the minor principal stress rather than the major, but this is not always practical, especially where stress fields change orientation.
During development, roof strata support design is based on high residual horizontal stress levels. These dissipate after the longwall has retreated, often resulting in roof failures due to the lack of confining stress.
Additional support to that required during development is often needed and is usually provided by means of extra long bolts of some kind (or even trusses in weak strata), sometimes in the floor as well as the roof.
In the tailgate it is often practical to install passive support before it is affected by the face, usually in the form of "cans" (large diameter, thin walled steel tubes filled with lightweight concrete) or timber chocks. It is preferable for these to be set to one side of the roadway and still allow vehicle access to the tailgate drive area. If this is not possible, then it is best to leave the installation until just before the effects of the face become apparent.
Because of the conveyor and equipment in the maingate and associated roadway(s) passive support is not usually an option and fairly heavy secondary bolting is often carried out. It is best if this can be done well in advance of the approach of the longwall, ideally between the completion of development and before the longwall start, if there is sufficient time available.
It is ideal if an assessment of strata conditions can be made during development and the extent of secondary support required in different sections planned in detail rather than a blanket approach to the full length of the roadways. More dense support, which may include full mesh coverage of ribs is often applied adjacent to longwall start and finish positions, particularly the latter which has to remain stable under the abutment load for an extended period during longwall recovery.
As the longwall retreats, passive support is often installed in cut-throughs as the face passes when access is no longer required. This is to prevent the goaf running down the cut through and affecting the intersection in the remaining roadway, if it is required to remain open.
Another important issue affecting the stability of the gate roads is the design of chain pillars. These have to be such that they remain intact during the life of two longwalls (assuming longwall retreat and re-use of the second roadway at the maingate end as a tailgate for the second longwall).
LVA - Longwall Visual Analysis – commercial website for company dealing in face & strata data analysis

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Installation/Relocation

The initial installation of a set of longwall equipment and subsequent relocations, which involve recovery, transport and installation operations, are major logistical operations and require detailed planning in advance and close supervision during the exercise. The planning should start prior to development as cut-throughs need to be located at particular spacings around longwall start and finish points to assist recovery and installation operations. It can also be advantageous to mine additional access points (often referred to as "chutes") along a face start line and at the recovery or finish line, where these are practical. The extra access points can assist in reducing the installation and recovery time.
Some mines adopt the practice of pre-driving a longwall recovery roadway and mining the longwall face into this roadway to finish. This procedure has the advantage of eliminating the need for the "bolt-up cycle" (a very slow process) while approaching the finish point to secure the roof where the supports are to be removed. This would already be done in the pre-driven roadway. The big risk with this procedure is a massive failure of the narrowing pillar of coal as the face approaches the pre-driven roadway; such a failure would make the face recovery very difficult and possibly hazardous. Only a few mines are sufficiently confident of the stability of this narrowing pillar to accept the risk.
Most mines hire additional labour and equipment for the installation, recovery and relocation work as it is difficult to justify the cost of retaining these resources for use during a period of around four weeks (depending on the size of the wall and distance to be travelled), maybe once a year (depending on longwall block lengths and production rates). Further, much of the equipment is specialized (eg special vehicles to transport supports and the shearer) and would otherwise remain idle for much of the time.
The opportunity is usually taken during most longwall changeouts (i.e. relocations) to bring key components of the longwall (e.g. shearer, AFC drives, possibly supports) to the surface for overhaul, repair or replacement. Again, much of the overhaul and repair work is outsourced to OEM's and external workshops.
The need for non-mine resources makes forecasting and scheduling important as such resources are frequently in heavy demand and have to be reserved well in advance. Workshop space and resources for equipment overhaul and repair must also be available to ensure the equipment can be turned around and sent back without delaying recommencement of the longwall.
In most cases, equipment has to be recovered and re-installed in a particular order which places additional constraints on the process.
Some mines are fortunate enough to be able to carry some extra items of longwall gear and are able to install this on the new face in advance of the completion of the previous wall. This takes a lot of the pressure off the relocation process, but the extent to which extra gear can be purchased depends on a cost/benefit analysis for that particular mine. The ability to share equipment between a number of mines in a group can also be a great advantage in this regard, but this would limit the degree to which equipment can be tailored to the needs of individual mines.
Because of the number of items of equipment, much of it heavy and bulky, it is important that transport routes underground are planned in detail and these routes are cleaned and prepared in advance of the move. It is ideal if one-way routes can be arranged to avoid having to shunt large equipment to enable other vehicles or equipment to pass.
Storage areas must be allocated to place equipment which may be recovered early then re-installed late in the process and to place equipment when installation proceeds slower than the recovery process. It may be necessary to prepare storage areas on the surface as well, particularly if major overhaul or repair is required for supports.
The whole process has to be integrated with other mine operations which in most cases need to continue with minimal disruption.
Apart from the planning, the change process generally starts when the face is 10-15m from the finish line. At around this point it is necessary to start installing support in the roof at the face so that stable roof can be maintained as longwall supports are removed. This will usually involve installation of roof bolts, possibly with steel roof straps or mesh, or a strong, flexible, plastic mesh. In lower height seams it is also common to cut extra height on the face over the final 10-15m to facilitate support and recovery operations.
At the final two shears extension pieces are attached to the chock/AFC connections to allow the AFC to be pushed over while leaving the chocks back. This is necessary to gain sufficient width ahead of the chocks to allow chocks to be withdrawn across the face line.
Once production is completed, removal of gate road equipment can commence while the coal face is supported, often with rib bolts and mesh (which may be a continuation of the plastic roof mesh used). Sprayed grout has also been used to support the coal face.
An aspect of relocation which does not always receive the attention it should is ventilation. If, as is normal, recovery and re-installation operations occur at the same time, then effectively the mine has an additional panel to ventilate during this period. The ventilation required at the old face to deal with gas may reduce as the gas make will reduce when production ceases, however the use of a number of high powered diesel equipment may require just as much or even more ventilation . Often it is not possible to set-up the long term ventilation circuit properly because access needs may prevent construction of control devices - time and resources may therefore be required to complete the ventilation changes after completion of the relocation and before production can recommence.
The process of recovery, transport and re-installation will vary from mine to mine or even within mines depending on mining conditions, distances equipment has to be transported, the amount of equipment to be sent to surface, resources available and at times personal preference.
The Longwall 20 to Longwall 21 Changeover Recovery Manual is an example of a plan used for one relocation at one mine which is reasonably typical. Other mines may have particular problems, a different transport system or other factors giving rise to variations, but in general the processes will be similar. The additional height available in thicker seams makes some aspects easier, but the equipment to be moved is typically heavier and bulkier.

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Ventilation/Gas Control

The ability to provide adequate ventilation to a longwall panel can be a major factor in the success of an installation, especially in gassy conditions. Most gassy mines now use some degree of gas pre-drainage of the seam being worked which greatly reduces the gas to be dealt with during longwall extraction. However in many cases much of the gas make from a longwall panel comes from sources in the roof and/or floor and requires some form of post drainage of strata or goaf drainage, or otherwise must be handled by the mine ventilation network.
The tendency for longwall panels to become longer and wider and for machinery to become larger increases panel resistance making the ventilation task more challenging, particularly when development constraints from time or cost considerations lead to the need to minimize the amount of development which is acceptable. Generally, the thinner the seam, the more difficult the challenge becomes.
The provision of high ventilating pressures by the use of large surface fans and/or underground booster fans is becoming more frequent, but this entails high capital and operating costs and is not always an option in seams liable to spontaneous combustion. The fact that all longwalls have some airflow through goaves means that particular attention is required to methods and standards where spontaneous combustion is a risk. All longwalls, especially those which do not extract the full seam, leave some coal in the goaf as well as chain pillars each side, and this remaining coal in a poorly ventilated goaf can form ideal situations for spontaneous combustion to occur.
There are 3 basic ways of ventilating a longwall panel:

• "U Ventilation" where intake air comes in the maingate and returns in the tailgate; there is no connection to any airways behind the face line. If a longwall advancing face is ever used, this is the only means of ventilation available.
  
  

  U Ventilation

  
  
  
• "R Ventilation" in which the main ventilation circuit is as for U ventilation, but there is another connection (a "bleed") maintained to return airways behind the face.
  

  R Ventilation

• "Z or Y Ventilation" where both main and tailgates carry intake air and all the return air is carried to main returns behind the face. With this system a variable regulator on the maingate intake is often used to adjust the balance between face flow and maingate intake flow.
  
  

  Z or Y Ventilation

There are other possible variations, especially if more roadways are available at each end of the face, and there can be special cases where layouts are not quite normal, however the majority of faces use one of the above arrangements.
For U and R ventilation, the air flow across the face itself is from main to tail (in the opposite direction to the coal flow and sometimes referred to as "antitropal" ventilation). For Z ventilation the flow is from tail to main (or "homotropal"). If Uni-di cutting is used the air flow direction will govern the direction of cutting.
With regard to controlling gas concentrations in the airways (and face temperatures if these are a problem), the greater the air flow the better in general. However on a longwall face where there is invariably a lot of dust and fine coal, air velocities of more than 3-4 metres/second become very uncomfortable and this may limit the quantity of air which can reasonably be supplied. This is one reason that U ventilation may be unsuccessful.
With R ventilation, the gas make from the face and goaf is split in 2 directions, some diluted by the face air and some by air which passes the face and becomes a bleed to other main returns. With the Z system all the gas passes to the returns behind the face, but extra intake air is added to the flow from the face adjacent to the maingate.
There can be two problems with returning face air through the tailgate end of the face:

• There will be a tendency for air to sweep the goaf edge behind the supports and bring gas over the tailgate end of the face and the tailgate drive. To avoid gas problems in this area it is often necessary to erect a brattice wing from the goaf edge past the tail gate drive and allow air from the goaf to mix and be diluted further outbye. This entails erecting and maintaining brattice in often poor working conditions.
  
• Access to any part of the tailgate may be difficult (or impossible) while the face is operating because of dust or gas levels, so that any tailgate work can only be carried out during scheduled longwall outages or downtimes.
  

With an R or Z system, an airway can be maintained along the goaf edge behind the face to the first cut through so that the draw at the maingate end of the face is away from personnel and equipment. Maintaining this airway is important; if it closes too tightly for the R system the air flow balance would be lost resulting in increased face gas levels, and for the Z system face return air would have to pass outbye over the maingate work area. In this case brattice may be required to separate the return air from the equipment and personnel which as well as being inconvenient, may limit the face air flow. The use of an R or Z system may also limit the length of chain pillars which can be used to whatever distance a goaf airway can be maintained.
With the R and Z systems, local control of the ventilation is often carried out using pressure measurements at regulators rather than measuring flows, the pressure values for ideal conditions being determined by experience. At times reference is made to "holding gas in the goaf" by adjusting the pressure across it, so the gas does not contaminate the face area. In fact the process is actually holding the gas front in the goaf away from the face. Because the goaf is open ended any gas make must be removed somewhere and it is prevented from coming onto the face by being dragged towards some other location - it is not actually "held" in the goaf.
If it is not possible to control any gas make successfully by diluting it with ventilation, there are three possibilities, either as alternatives or in conjunction:

• The use of "sewer" airways where higher gas levels than that normally permitted by statutory limits are allowed and personnel access is prevented unless production is stopped and gas levels reduced.
  
• Cross-measure post drainage where boreholes are drilled through the strata above and/or below the seam and connected to a gas drainage range to capture some of the gas before it can reach the mine airways.
  
• Goaf drainage where gas is drawn from the goaf cavity either underground by pipes which are open to the goaf area and connected to a gas drainage range or direct to surface through boreholes drilled to (or close to) the goaf cavity from the surface.
  

For the latter two options, suction may be applied to the gas drainage range or surface boreholes to assist gas capture levels.
It may be possible to use the longwall direction of mining in relation to seam dip to assist in gas control; for a goaf where methane predominates, buoyancy effects of the gas will assist in keeping gas away from the face if it is worked down dip; if carbon dioxide predominates working up-dip will assist. It is more likely for this benefit to be obtained as a bonus as other factors are more likely to determine mining layouts and mining directions.

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Alternative Shortwall Mining Method – Gretley Colliery

Another shortwall mining method was developed in the 1980's as a means of realising the benefits of longwall mining in a shallow seam, partial extraction application where there were considerable subsidence constraints under a residential area. A short (approx 30m) longwall face was employed with a short, single ended shearer, and a "round the corner" AFC which eliminated the use of a stage loader. The face was operated blind off the two entry maingate, with an auxiliary fan being used to ventilate the face. The single drum BJD shearer was able traverse across the full width of the maingate roadway to obviate the need for sumping the shearer drum in, with the face being pushed over before the shearer took a full cut across the face. The shearer was subsequently trammed back to the maingate trimming the floor, and the supports advanced before the next shear was taken.
While the method was successfully employed to lift productivity from the mine and as a partial extraction system, the system has not been applied at any other mines in Australia.

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Punch Longwall Mining

This is a rather specialized type of mining which is only applicable where a seam is exposed at the highwall of an open cut mine which has reached the economic limit for open cut mining. It could also be applied where a seam has an extensive natural outcrop, but there are no instances of this in Australia known to the writer.
Two roadways or sets of roadways are driven into the seam from the highwall (or outcrop) to the maximum practical distance to form gate roads and are then connected by driving the longwall installation roadway.
The longwall is then retreated back towards the surface, leaving only a pillar of coal sufficient to prevent the highwall being destabilized.
The longwall is then recovered and the process repeated, possibly using a roadway remaining from the previous block, if available, as one of the gate roads.

Punch mining into an open cut highwall

In this way a large area of coal can be recovered, beyond the open cut limit, without the need for major investment in infrastructure (shafts, main roadways, etc), the coal produced being delivered into the open cut where other necessary facilities (coal prep plant, surface transport loading facilities, stockpiles, etc) already exist.
The term punch mining is only applied where a series of such longwall blocks is extracted, each in effect being a small underground mine. It is not used to describe the case of a large underground mine being developed by driving the mine access from a highwall or outcrop.
Any method of mining could actually be used to extract coal in this way, but it has been included in the longwall section as the method has been developed for use with longwall equipment.

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Shortwall Underground Coal Mining

This method of mining was developed in the late 1960's to take advantage of the then recent development of suitable hydraulic longwall supports, coupled with the productivity and low capital cost of continuous miners and shuttle cars. In effect it gained some of the advantages of longwall mining without the cost of installing a complete set of longwall equipment
An installation roadway was driven as for a normal longwall, but only supports were installed. A continuous miner was then utilised to cut 3.5m wide open ended lifts off the face, with shuttle cars being used to transport coal along and off the face to the maingate belt in lieu of an AFC.
The face length was therefore limited by the length of shuttle car cables then available, but in practice most shortwall faces were considerably shorter than this (<90m).
Supports were connected to a reference rail which was then utilized to pull the 2 or 3 leg supports forward, in a similar manner to the use of an AFC to advance longwall supports.
Shortwall faces could be installed between two gate roads as for a longwall face, but in some cases were mined to a blind end and ventilated by auxiliary fan (not very suitable for gassy seams as the fans could draw from the goaf).
Shortwalls were used in an endeavour to increase the productivity of continuous miners at relatively low capital cost, sometimes as a transition stage while changing a mine to full longwall. In some cases, because they were somewhat more flexible, shortwalls were used to obtain the benefits of longwalls in mines, or parts of mines, where seam discontinuities or mine geometry made the use of full longwalls impractical
The main disadvantages of shortwalls compared to longwalls are:

• The width of the unsupported roof ahead of the chocks is governed by the width of a continuous miner as opposed to a shearer drum.
  
• Personnel have to work adjacent to the face which presents safety issues unless rib support is installed which would greatly slow production.
  
• The use of shuttle cars is by its nature not continuous and brings in all the disadvantages of trailing cables in the face area.

Shortwalls had only mixed success and there have been no shortwall operations in Australia (or elsewhere to the knowledge of the writer) for many years.

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Cutting Machines - Coal Ploughes

Coal Ploughs have had little application in Australia, and their main use has been in Europe, particularly in Germany where they were first developed. Essentially a plough is a large mass of steel, usually of a more or less triangular shape when viewed from the coal face or goaf sides, fitted with large "picks" (more like small agricultural plough blade tips) angled from the steel body towards the coal face. The plough height is the working height in the seam being mined (possibly a bit lower if the coal tops can be guaranteed to fall once the coal below is cut. These "picks" act in a fashion similar to chisels and break a narrow web of coal off the face (of the order of 300-400mm thick). In most cases there are no moving parts on a coal plough.
The plough itself is mounted on the front of the AFC and is pushed into the face by push cylinders mounted in the supports. The plough has an endless chain haulage attached to the rear, and is driven through sprockets on electric drive(s) at the face end(s).
The main advantages of ploughs compared to shearers are:

• Cheap
• Simple (no moving parts on the cutting machine itself)
• Relatively low dust make
• Able to keep exposed roof area very small (but a large number of chock movements would be required to maintain this)

Though only a small web is taken, in the right conditions production rates can be comparable to a shearer as the plough is operated at a relatively fast speed along the face.
Some disadvantages are:

• Cutting height is fixed
• Ability to cut stone is limited
• With increasing cutting height, machine stability becomes more problematic
• Grading can only be done using the AFC angle
• There are safety implications with an exposed chain haulage.

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Cutting Processes

It has been stated that a longwall advances by cutting slices off the block. This is relatively simple for hand worked faces and with a coal plough, but with mechanized longwalls using shearers the means of doing this is not as straightforward as it would at first appear because of the complexity of the equipment. To start with, the cutting machine has to cut into the face after each slice is taken to line itself up to cut the next web. It is incapable of cutting at right angles to the face, so has to be eased in at an angle. This is achieved by "snaking" the AFC on which the shearer travels, so that the cutter drum can cut a wedge shaped section of coal until the full depth of the web is attained.
It is possible to cut a full web in one pass and to do this in either direction, a process known as "Bi-directional or Bi-di cutting". An alternative is to cut in one direction only, known as "Uni-directional or Uni-di cutting", often actually cutting only part of the web height in one direction and the remainder in the reverse direction. There are other more complicated processes involving only taking half the web width in one pass and the remainder on the return.
At first glance Bi-di cutting would appear to be the quickest way to advance and frequently this is the case. However there are advantages with the other processes, relating to simplicity of operation, "steering" the face equipment, effect on the coal haulage system, power requirements on the shearer, location of operators in relation to dust sources, etc which can result in better productivity overall. The best system will often be different for different mines, particularly in different seam thicknesses and possibly even with different personnel.
If the full web is cut in one pass, more power for cutting is required and the shearer will move slower than is possible if only part of the seam is cut. This, and the simpler overall process, may allow Uni-di or other processes to attain similar production levels to Bi-di.
If a shearer cuts coal at a given rate, the effect on the haulage system will be different depending on the direction of cut – when cutting towards the tailgate the coal is carried away from the shearer while the shearer is moving away from the cut location; when cutting to the maingate the shearer is travelling in the same direction as the cut coal and so tends to load fresh coal on top of that already on the AFC. The total coal load for each web cut is the same, but there are higher peak loads on part of the AFC when cutting to the maingate. This variation can be evened-out with methods which do not cut the full web in one pass.
There are two primary sources of dust on a longwall face, the cutting machine and during support advance. With Uni-di operations it is possible to keep operators on the intake side of these sources most of the time, particularly with remote or automatic chock operation. With Bi-di cutting this is not possible at all times.
Another aspect of the cutting process which requires strict attention is the straightness of the face and its angle to the gate roads. To begin with, the face equipment is a fixed overall length apart from a small amount of play between items. If the face contains excessive curvature, particularly in the plane of the seam but also to some extent perpendicular to it, it could happen that the face end(s) will be within the longwall block and short of the gate roads. If curvature of the face is too great it is also possible for the pans to become locked and unable to be advanced.
Face alignment is maintained by checks with a string line across the face. If out of alignment, a "straightening cut" is done whereby the AFC pans are only partially advanced by different amounts up to the string line and only a part web is cut across most of the face.
A face will seldom move in exactly the required direction when advanced. If there is a dip across the face, the chocks and pans will tend to move down dip all the time. If the snake is always in the one direction the face will tend to move towards the face end where the snake begins. If chock side shields only touch one adjacent chock, that chock will tend to be pushed away from the one it is touching. If the face line is not perpendicular to the gate roads, the face will tend to move towards the gate which is lagging. All these factors can act together to tend to move the face towards one gate or the other, this movement being known as "face creep". If it is allowed to go too far the maingate equipment can run into one of the ribs and the tailgate end may either run into the rib away from the face or will not reach the edge of the block.
The usual method of "steering" the face is to intentionally cut it at an angle other than a right angle to the gate roads so one end leads the other and the movement so caused counteracts the unintentional movement which is occurring. The required angle is created by cutting a "fly cut", whereby the AFC is set up to a string line set to the required angle. A wedge shaped web is thus cut. It may be necessary to take more than one fly cut to achieve the desired angle.
Note that if too great an angle is present this can have the same effect as curvature on the face where the equipment may not reach the gate roads.
A further aspect of cutting relates to grading. The need for this may arise from:

• Presence of seam discontinuities (faults)
• Gate roads floors cut below normal face cutting height for whatever reason
• Need to cut extra face height (possibly for a number of reasons, but frequently when approaching the end of a block)

Shearer drums have the ability to cut some distance below the bottom of the AFC as well as above the normal roof height, so any profile can be cut within these limits. Whenever grading is required however it is necessary to keep in mind that changes must be made gradually. Longwalls cannot handle sudden changes well in any direction. Grades along the face must be within the limits of vertical movement between pans. In the direction of face advance, large steps are to be avoided (the AFC may jam up against a large step or chock bases may bridge across steps and have poor floor contact or the AFC toe may dig into the floor, especially if soft, and be difficult to level off again).
The position of the drum cutting the roof level can usually be observed visually, especially if there is a good "marker bed" in the strata (which may be the seam roof) or can be judged relative to the chock canopy position. The floor drum may not be so easy as it cannot be readily seen. The use of a measuring stick is a simple but effective means of checking.
Attempts have been made to control cutting height automatically by sensing some strata level (eg seam roof) and maintaining a constant height relative to this, but these attempts have not been entirely successful
Another method of control involves carrying out a manual shear during which a control computer "learns" the cut profile and will then repeat the profile automatically. The "learning" has to be repeated whenever a changed profile is needed for whatever reason.
When cutting with a shearer, an important aspect to be kept in mind at all times is that the angle of the cutting drums is fixed relative to the shearer body and the latter is governed by the alignment of the AFC. If the AFC is tilted towards or away from the face, then so will be the drums and to a position exaggerated by the distance they extend in front of the AFC. If not controlled carefully it can become very difficult to guide a face back to its desired position within a seam, especially if a face begins to dive into a soft floor.

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Advancing and Retreating Longwalls

Advancing Longwalls

In this method, the face start point is close to the main headings, usually leaving a barrier pillar to protect them. Once the face equipment is installed, extraction commences working away from the main headings towards the block limit. Obviously the main and tailgates do not exist prior to the start of extraction and have to be formed at each end of the face as mining progresses. The gate roads are effectively in the goaf and a false rib has to be installed on one side, usually by constructing a small pillar, sometimes using stone cut from the roof in thin seams or using some type of cementitious material brought into the mine. Such gate roads tend to require a very heavy support system (yielding steel arches have often been used).

Principle of advancing longwall

Advancing longwalls were once common in Europe in relatively thin seams where packs were constructed using stone, which had to be cut in order to produce sufficient height for the gate roads, and sometimes using coal fines which were not very marketable at one time.
Usually a pillar of coal referred to as a "chain pillar" would be left between adjacent longwall blocks, wide enough to remain intact when carrying the load between two goaves and protect the gate road. Occasionally two longwalls would be operated simultaneously, one each side of a shared maingate (in this case referred to as a "mother gate").

Retreating Longwalls

In this method, the gate roads are first driven from the main headings to the block limits and then connected with a roadway to install the face equipment. The gate roads may be connected to another set of roadways at that point for ventilation/gas control purposes. Once the face equipment is installed, production commences with the face retreating from the limit back towards the main headings, usually to finish at a position so that a barrier pillar is left to protect the latter headings.

Principle of retreating longwall

Because the gate roads are long, it is normally necessary to drive at least two (sometimes more) at each side of the block. That set of roadways which will be used for access onto the longwall and for coal clearance off the longwall are typically called the "maingate roadways" whilst the other roadway or set of roadways is typically referred to as the "tailgate roadways". The latter are used for primary access on occasions, but this is not generally the case.
As the face retreats, the roadways forming the face ends are destroyed and become part of the goaf. The other roadways will remain open if adequately supported and it is common practice for one of the remaining roads at the maingate end (usually only one in any case) to become the tailgate of the next block. The gate road first working pillars then become the chain pillars between the blocks.

Comparison of Advancing and Retreating Longwalls

The advantages of retreating longwalls compared to advancing are:

• Gate road formation is remote from face operations (less congestion at face ends, less supplies into longwall face area, face not held up waiting for gate road preparation or vice versa, no problems of dust production from gate road workings affecting longwall personnel).
  
• No gate or roadway side packs required, so less supplies overall
  
• Longwall block is surrounded by roadways before the longwall starts so knowledge of strata conditions is much better
  
• Gas drainage of adjacent blocks can be carried out starting during development; with longwall advancing the drilling can only be done behind the face after longwall extraction, allowing less drainage time before the next block commences production
  
• With retreat longwall mining, additional gateroads or bleeder roadways behind the goaf area can be developed for ventilation by the development unit if required. Such additional roadways are much more difficult to mine with an advancing longwall
  
• There are more options for ventilation/gas control using additional roadways at the limit of the block
  
• Advancing longwall gateroads typically require extensive maintenance to maintain the roadway cross-section (roof and floor brushing) during the life of the longwall block, whereas retreat longwall gateroads are allowed to collapse behind the retreating face
  

The only real advantages of advancing longwalls are:

• Production can begin earlier as the mine does not have to wait for the gate roads to be developed before longwall production can commence (provided development rates are adequate this should only apply for the first longwall in a mine)
  
• It provides an opportunity for disposal of stone which has to be excavated into gate side packs (this benefit is probably more than offset by the costs involved in pack construction)
  

As far as the writer is aware, only one advancing longwall has been worked in Australia, a hand worked face at Stockton Borehole Colliery operated from the late 1890's to the mid 1950's.

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LONGWALL MINING FOR UNDERGROUND COAL

Overview

In the method of secondary extraction known as longwall mining a relatively long mining face (typically in the range 100 to 300m but may be longer) is created by driving a roadway at right angles between two roadways that form the sides of the longwall block, with one rib of this new roadway forming the longwall face. Once the longwall face equipment has been installed, coal can be extracted along the full length of the face in slices of a given width (referred to as a "web" of coal). The modern longwall face is supported by hydraulically powered supports and these supports are progressively moved across to support the newly extracted face as slices are taken, allowing the section where the coal had previously been excavated and supported to collapse (becoming a goaf). This process is repeated continuously, web by web, thus completely removing a rectangular block of coal, the length of the block depending on a number of factors (see later notes)

Basic longwall mining principle simplified (retreating longwall in this case)

A coal haulage system is installed across the face, on modern faces an "armoured face conveyor or AFC". The roadways which form the sides of the block are referred to as "gate roads". The roadway in which the main panel conveyor is installed is referred to as the "main gate" (or "maingate"), with the roadway at the opposite end being referred to as the "tail gate" (or "tailgate") roadway.
The benefits of longwall mining compared to other methods of pillar extraction are:

• Permanent supports are only needed in the first workings portion and during installation and recovery operations. Other roof supports (longwall chocks or shields on modern longwalls) are moved and relocated with the face equipment.
  
• Resource recovery is very high - in theory 100% of the block of coal being extracted, though in practice there is always some coal spillage or leakage off the face haulage system lost into the goaf, especially if there is a lot of water on the face
  
• Longwall mining systems are capable of producing significant outputs from a single longwall face – 8 million tones per annum or more.
  
• When operating correctly the coal is mined in a systematic, relatively continuous and repetitive process which is ideal for strata control and for associated mining operations
  
• Labour costs/tonne produced are relatively low
  

Disadvantages are:

• There is a high capital cost for equipment, though probably not as high as first appears when compared to the number of continuous miner units which would be required to produce the same output.
  
• Operations are very concentrated ("all eggs in one basket")
  
• Longwalls are not very flexible and are "unforgiving" - they do not handle seam discontinuities well; gate roads have to be driven to high standards or problems will arise; good face conditions often depend on production being more or less continuous, so problems which cause delays can compound into major events.
  
• Because of the unforgiving nature of longwalls, experienced labour is essential for successful operations.
  

A major decision to be made is the size of longwall blocks. Because modern longwalls involve a large number of pieces of equipment (numbers of a magnitude of several hundred items, with many components weighing up to 30 tonnes or more), the process of recovering the equipment from a completed block, transporting it to a new block and then installing it in the new block (often with much of it being taken out of the mine for overhaul on the way) is a very major operation. Apart from the direct cost involved, production and hence income is zero during this period. Bigger longwall blocks will enable the number of relocations to be minimized, however there are limiting factors to the size of longwall blocks:

• The longer the face the more power is required on the face coal haulage system (see later notes on AFC's). The greater the power, the larger the physical size of the drive units (usually there is a drive unit at both ends of the face). The drive units have to fit into the excavation and allow room for access past them, for ventilation across the face and for some degree of roof to floor closure. Also the greater the power, the larger (and therefore heavier) the chain on the face conveyor – these chains have to be manhandled on the face at times and there are practical limitations as to the size of the chain.
  
• In some longwall installations, the heat created by the high power haulage drives may become a factor.
• Both face width and length may be governed by limitations created by lease boundaries, seam discontinuities or variations, already existing mine development and/or ventilation capacity.
  
• The ability of the mine to develop new longwall blocks so that longwall production continuity is not adversely impacted.
  
• Condition of equipment – changing out some items for overhaul or replacement during the life of a longwall block can be problematic, and is best done during a relocation.

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