A Proposed Mining System for Thin Seam Mining

Introduction

This chapter outlines the concept of a system for the underground mining of thin seams. Its development has taken into account the experience of the past, as discussed in previous chapters, the more recent developments in mining machine guidance and system integration, and the industrial and commercial environment within which modern mining takes place in the United States. (Holman, McPherson, and Loomis, 1999b).

Overview of the Mining Layout

This new mining layout was designed taking into account ventilation, development cost, haulage, equipment maneuverability, materials transportation, and percentage of coal recovered. In this layout large blocks of coal are developed, similar to longwall panels. (Fig 7.1) The nominal dimensions of these blocks are 1000 feet wide by 5000 feet in length. In this design the longer the panel, the higher the recovery that is attained.

The layout of the complete section encompasses a block with two entries on each side. Three longitudinal entries bisect the block into two panels, with the middle entry used for the section belt conveyor. This conveyor entry is shared between the two panels. Three parallel entries run along the shorter ends of the panel blocks. The first entry on the outby side of the panel is used for transportation of supplies. All of these entries serve as main intakes for the ventilation system. The three entries on the inby end of the panel serve as ventilation main returns. Each of the long parallel entries that run the length of the panel, connecting main intakes to main returns, are equipped with regulators to control the air flow distribution through the panel. (Holman, McPherson, and Loomis, 1999b).

The production stalls are cut parallel to each other in a herring bone pattern. These are 500-foot long production cuts that are roughly 13 feet in width, consistent with the capacity of the continuous miner employed, with support pillars between each stall. The width of the support pillars is dependent on the competency of the country rock, coal strength, depth of overburden, and strength of backfill, if used. An average value for a pillar width at 1000-feet of overburden with moderately intact rock, strong coal, and a 30 inch mining height would be 11 feet wide with no backfill, and 5.9 feet wide with a 1000- psi strength backfill. (Donovan, 1998).

The herring bone pattern was selected for length of cut and maneuverability of equipment. Production cuts that angle at 45 degrees from the adjacent entries are accessible by continuous haulage units of the type discussed in Chapter 6. This allows the haulage units to remain in one continuous string, instead of having to be disassembled and put back together each time a production cut is completed.

Haulage in this system is accomplished using a series of chain conveyors. The continuous miner fragments the coal from the solid at the face of the stall and loads it onto the receiving unit of the stall conveyor train. This in turn is connected and loads on to the secondary conveyor train, which is located in the adjoining access entry. The coal travels down the secondary conveyor, which turns into a 45-degree angled, open crosscut to the section belt conveyor in the center entry. This belt conveyor loads on to the main conveyor at the outby end of the panel.

At the outby end of block is a pumping station. A backfill slurry is pumped through a pipe range to fill into the mined out stalls. By sealing these mined-out stalls, leakage pathways are inhibited, requiring the air to travel in the pathways that have been designated as the ventilation network. (Holman, McPherson, and Loomis, 1999b).

Figure 7.1 Mining Layout

Equipment

For mining of this kind, many pieces of equipment will be needed for the functions of development, roof support, mining, haulage, and back filling. Strata conditions will determine the type of equipment needed for developing the main entries where personnel will be working, but otherwise the equipment for this type of mining is essentially the same.

For purposes of development, continuous miners can be used to cut coal and rock in soft strata. For harder country rock a road header might need to be employed. In these open entries, roof support will be achieved through the use of roof bolts. Roof support in the stalls will not be provided. For the purpose of roof bolting in the entries, a twin boomed roofbolter is recommended (Fig 7.2). A scoop for material transport and general utility should be maintained on each section.

The mining in the stalls may be carried out by a Fairchild type of auger continuous miner that cuts from side to side. (Fig 6.3) This miner off loads onto a stall conveyor train such as the Archveyor or the system produced by Long-Airdox Inc. (Fig 7.3) This haulage unit is composed of repeated components and stretches to a length of 520 feet. The secondary haulage system in the adjoining entry is also envisioned to be a version of the Long Airdox Full Dimension unit with elongated bridge sections. (Holman, McPherson, and Loomis, 1999b).

Figure 7.2 Roof Bolter (Tamrock Inc., 1995) reproduced with permission

Figure 7.3 Conveyor Placement

Figure 7.4 Detail of chute transfer point from the stall conveyor train to the secondary conveyor

Figure 7.5 Detail of Transfer from secondary conveyor on to section belt conveyor

Coal Transportation

The coal is transported from the continuous miner within the stall to the mains by means of three conveyor systems, the stall conveyor train, the secondary conveyor unit in the adjoining entry, and the section belt conveyor in the center entry.

At the commencement of a new stall, the nearly 500 ft of the articulated series of chain units that comprise the stall conveyor train will lie alongside the secondary conveyor unit. As the continuous miner advances into the stall, the stall conveyor unit will follow, and retract as the miner is withdrawn at the completion of a 500 ft cut. The out bye end of the stall conveyor train will comprise a curved-jib or chute type of unit attached to a slide rail running the length of the secondary conveyor and loading on to that conveyor (Figure 7.3).

The secondary conveyor will be a little more than 1000ft long but will comprise a series of alternating intermediate mobile bridge carriers and elongated forms of piggyback bridge conveyors (Figures 7.4 and 7.5). This flexible arrangement allows the outby sections to turn into a 45 degree angled crosscut and to load on to the belt conveyor in the center entry. Individual units of the secondary haulage system may be disconnected and stored inby temporarily as the stalls are completed and the panel is mined in retreat. In addition to progressively reducing the active length of the secondary conveyor, this may also be necessary to provide the space required to initiate each new stall.

The initial 1000 ft length of secondary conveyor is suggested to allow the panel to retreat for 500 ft before the complete secondary conveyor has to be moved back, under its own power, to the next angled cross-cut leading to the center entry. Hence, in this arrangement, those crosscuts would be 500 ft apart. Shortening its length to an intermediate value between 500 and 100 feet could reduce the capital cost of the secondary conveyor. This would necessitate more frequent moves of that conveyor, and angled crosscuts that were closer together. In order to facilitate movements of both the stall conveyor train and the secondary conveyor, it is important that excessive spillage is not allowed to accumulate. (Holman, McPherson, and Loomis, 1999b).

Guidance System for this Layout

The guidance system that will control the continuous miner will utilize a 120° scanning laser array, angular transducers, a micro processor with signal transmission capabilities, a Natural Gamma Radiation CID unit, and a radar based rib thickness monitoring unit.

The Natural Gamma Radiation CID unit will be used to inform the miner when it begins cutting out of seam material. A signal will be sent to the microprocessor, which will, in turn, signal the miner to lower its cutting height. The radar based rib thickness monitor will come into use starting with the second production stall. It will send a radar signal perpendicular to the stall to measure the distance between the miner and the previous stall. The unit will send its results to the microprocessor who will adjust the miner heading to maintain alignment, and proper support pillar width.

The scanning laser array will be able to read bar codes on machine reflectors up to 50 meters away. The bar codes will allow the processor to identify the machines and better keep track of machine location and process location data. These data will be used to maneuver the haulage conveyor around the 45° turn into the production stall.

The scanning laser array will be mounted on a mobile frame unit. (see Figure 5.1). This frame unit will have three legs. Two of these legs will be located long the rib, framing in the active production cut, while the third will be located in the middle of the access entry way. The scanning laser array will be mounted on the central leg. The miner and haulage train will pass underneath the frame unit. The legs will be equipped with hydraulic cylinders, and computerized leveling for the laser array. The legs will also have wheels on their bases to allow for easy movement the frame from one cut to the next.

Figure 7.6: Mobile Guidance Frame Unit

The two legs that frame in the production cut are located in positions that are known to the microprocessor. The computer also knows that the production cuts are to be made 45° to the access entry. The computer can adjust the miner to ensure that it is cutting a 45° cut. The computer knows the locations of the corners of the ribs, the angle at which the cut is made, and the length that the miner has progressed. This allows the computer to mathematically model the ribs of the cut, and check for collisions with the equipment. (Figure 5.2).

Figure 7.7: Mobile Guidance Frame Unit in use

The angular transducers will be installed at all of the pivot points of the haulage train. The scanning laser array will provide the identity of at least one unit in the train at all times. The position of the entire production train can be calculated in reference to this unit and its surroundings. This is possible because the dimensions of all of the units are known quantities, the angles between the units are reported by the angular transducers, and the position of at least one units in reference to its surroundings will be provided by the laser array.

There will be an operator interface box where an operator can stop the sequence at any time and manually guide the system. Safety cut off lines to stop the system will be located on all pieces of machinery and on the transmission box.

Series of events during automated mining

First the continuous miner and the haulage train are backed up away from the next production cut. The miners position the framework at the location of the next cut. The secondary conveyor train must be partially disassembled to have enough room to face up the continuous miner. The position of the production train will be computed by the microprocessor utilizing the data obtained by the laser array, the angular transducers, and the know dimensions of the units of the train. 

This position calculation will be constantly updated anytime one of the variables changes. While constantly checking for intersection between the safety buffer around the units and the objects in their environment (ribs, other equipment, etc.), the microprocessor guides the production train through its production cycle utilizing the data from the CID unit, and rib monitor. Once the miner has mined a few feet into the face. The secondary haulage train is reassembled.

Mining progresses until the production stall is mined out. The system automatically shuts down, and awaits the mine personnel to give it the command to back the haulage train and miner out of the cut. The order is then repeated for the next cut. When the system has mined down 500 feet of panel length, the secondary haulage train must be advanced. The secondary haulage conveyor is then moved down 500 feet to the next open crosscut.

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Integrated Mining and Haulage Systems

Introduction

The most widely used method of transporting coal away from a continuous mining machine at the present time is the shuttle car (Section 2.3). There are typically two or more shuttle cars operating in a room and pillar operation. This introduces problems of traffic control, cable handling and delays. Each shuttle car requires an operator for guidance and control. The intermittent nature of the operation inherent in separating the mining and haulage units, and the necessary involvement of personnel renders this system unsuitable for autonomous mining in a thin seam environment. It becomes necessary to combine the mining and coal clearance processes into an integrated system. Manufacturers of mining equipment have already developed such systems. This chapter outlines three examples that show promise for thin seam mining.

The “Archveyor” Automated Mining and Continuous Haulage Unit. 

This system was introduced in Chapter 3 as one of the techniques of highwall mining in surface operations. The Archveyor is a long flexible chain conveyor that was developed by Arch Technology, a subsidiary of Arch Coal, Inc. (Figs 6.1, 6.2) It has been used in conjunction with a continuous miner to mine in both highwall and underground environments. Arch’s coal mining complex in Wyoming has operated an Archveyor high wall system since 1992 in the Hanna Basin. (Walker, 1997) These automated machines require only two employees to mine coal from highwall type environments. Since the operators are not located near the dangerous high wall area or the coal mining face, the Archveyor has been operated, since its introduction, for the last six years without a lost time accident. (Walker, 1997).

The mining sequence for this type of setup is relatively simple. The continuous miner begins making a cut and off loads onto the Archveyor. At this time the Archveyor is in conveyor mode, with its hydraulic jacks lifting it off the ground allowing the conveyor to turn. As the miner advances to the point where its boom is almost out of range of the Archveyor, the miner begins to cut upward in the seam allowing coal to collect at the foot of the face until the Archveyor is advanced forward. While this is occurring, the Archveyor moves its load farther down its length towards the tail end to free up a length of belt. Then the hydraulic jacks are retracted and the conveyor rests on its return flights. By running the conveyor in the opposite direction, the Archveyor is propelled forward. The jacks are extended again, and the cycle recommences. This process is repeated for the entire length of the heading, and is utilized for every production cut. (Walker, 1997).

This mining cycle presents problems for thin-seam applications. In thin seams there is no room for an upward cut by a milling head of a continuous miner. This eliminates the possibility of using the foot of the face for storage of fragmented coal while the conveyor repositions itself. In any automated thin-seam mining method, difficulties also arise in finding a way for a milling head continuous miner to pass coal back from the head to the miner’s conveyor. For this reason an auger head continuous miner that swings side to side is more likely to be successful. (Figure 6.3).

This system also has to deal with the extra space needed to raise and lower the conveyor on the hydraulic jacks. Sufficient headroom may not be available in an underground thin-seam coal operation. New advances on the Archveyor may remedy some of these problems. The newly designed Archveyor will be able to mine coal seams as thin as 28 inches (Stickel, 1998). Automation of these machines has been achieved by combining programmable logic controls with inclinometers and ring-laser gyroscopes.

Guidance reliability has been established and proven with success in multiple pass mining. User friendly controls minimize the need for specially trained technical personnel. (Walker, 1997).

In thin-seam underground mining, the infrastructure of development openings is where personnel are required to work or travel. These entries must, therefore, provide greater headroom than the height of the seam itself. The method of excavating development sections for thin-seam underground mining depends primarily on the type of overlying and underlying rock. If the roof is composed of soft shale or similar rock, then a continuous miner may be adequate for the job. If the roof is composed of a harder type of rock, such as sandstone, a road-header would probably be a better choice.

In highwall mining no roof support is employed. Future underground thin-seam mining will require that only the development sections, where personnel work, will be supported. This is possible because of the coal pillars that are left in between each cut, and the small width of each cut. This ensures good natural roof in the heading. (Donovan, 1998).

The flexible design of the Archveyor allows it to mine traverse through undulating seams, and to turn at ninety degrees, even off narrow entries. This is accomplished using drive motors every 7.5 meters to run small sections of the chain conveyor and provides both horizontal and vertical flexibility. This powerful system is currently mining seams that dip at up to 30 percent, while operating at well under maximum available power. If necessary the modular design allows for an increase in power per unit length for steeper pitched seams. (Walker, 1997).

Built-in diagnostic and trouble shooting routines aid the operator in monitoring system performance, ensuring more operational time and facilitating predictive and preventive maintenance. Scheduled maintenance can be programmed and alarmed automatically, and production data can be collected through real time data sampling.

This adaptable system was constructed with the ability to incorporate a number of subsystems including atmospheric monitoring and ventilation. Air ducts can be installed along the length of the conveyor to deliver air directly to the face. (Fig 6.2) (Walker, 1997).

Figure 6.1 Archveyor Section

(Arch Technology Corporation) reproduced with permission

Figure 6.2 Archveyor Underground

(Arch Technology Corporation) reproduced with permission

Figure 6.3 Fairchild Continuous Miner

(Fairchild Incorporated) reproduced with permission

The Long-Airdox Full Dimension Continuous Haulage System

A new form of face haulage has been created and marketed by Long-Airdox. This has evolved from a system that was conceived, initially, in 1958, but has undergone constant updates and improvements since that time. Some of these improvements include the introduction of custom designed dual, extended life conveyor chains and many others. (Long-Airdox)

The system contains three main components that are utilized together to transport the coal. These components include the inby mobile bridge carrier (Fig 6.4), the intermediate mobile bridge carrier (Fig 6.5), and the piggyback bridge conveyor. (Fig 6.6) In a standard set-up as shown on Figure 6.7, the inby mobile bridge carrier is fed by a continuous miner and dumps into the first piggyback bridge conveyor. This conveyor dumps into the intermediate mobile bridge carrier, which off-loads into the second piggyback bridge conveyor. This bridge conveyor empties onto the haulage belt of the section. Currently all of the mobile bridge conveyors require operators. The flexibility of the system allows the configuration to be adapted to specific layouts. (Long-Airdox)

This system has consistently set productivity and reliability records with over 125 installations operating across the country. These systems are used in seams ranging in thickness from 30 inches to 13 feet. With haulage capacities of over 30 tons per minute and tram rates up to 85 feet per minute this system will fill the haulage requirements of the majority of continuous miner sections in operation. (Long-Airdox).

These systems are not yet fully automated but the design lends itself to automation. The manufacturer is looking into automating this already versatile mining tool.

Figure 6.4 Inby Mobile Bridge Conveyor

Figure 6.5 Intermediate Mobile Bridge Conveyor

Figure 6.6 Piggyback Bridge Conveyor

(Long-Airdox) reproduced with permission

Figure 6.7 Assembled Continuous Haulage System

(Long-Airdox) reproduced with permission

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Autonomous Mining Machines

Introduction

Any future system of underground mining of thin seams must be completely automated and operate without the presence of personnel actually on the working face. This is a prerequisite, not only because of the limited height of the faces but also to maintain the costs of mining at a competitive level. It is also an unfortunate fact of geology that coal seams seldom lie in a flat uniform plane of constant thickness. For these reasons, mining equipment designed to cut and remove coal from thin seams must be autonomous; that is, it must have a significant degree of artificial intelligence in order to navigate minor anomalies in geology or obstacles along the coal haulage routes.

There are three types of mine navigation; face, local, and global. Face navigation is the process of positioning the mining equipment within the area of the working face. Local navigation is the guidance of mining equipment in the immediate area, excluding the face, encompassing guidance around corners and obstacle avoidance. Global navigation is utilized when maneuvering mobile equipment between various points in the mine. Local and global tasks are very similarly performed, while face navigation is specific to the equipment and the task being performed. (Anderson, 1989).

The required objectives of any guidance system are to align the equipment at the face, achieve proper guidance during cutting, adequately tram through entries, and turn corners. Other requirements include the ability to generate a path by means of a computer model of the mine, and be able to follow that path. This model must be constantly updated to keep up with the changing configuration of the mine. (Anderson, 1989).

For the purpose of this method, face navigation only will be considered. The basic assumption is that personnel will guide equipment through the non-face areas. The two most critical aspects of face navigation are the alignment of the continuous miner and of the machine during cutting. It is also important that the miner have the ability to detect rock-coal interfaces, and to stay within the coal seam Coal-rock Interface Detection (CID) technologies are employed for this purpose. (Anderson, 1989)

Coal-Rock Interface Detection (CID)Technologies

One necessary component of any automated coal mining method is seamfollowing technology. This is usually achieved through detecting the coal-rock interface at the boundary of the seam. This has been abbreviated to CID for coal-rock interface detection. There are numerous types of CID technology that are being employed and tested around the world. These include natural gamma radiation (NGR), vibration, infrared, optical / video sensors, radar, and pick force. Each of these methods has its strengths and weaknesses that determine whether it can be utilized for fully automated mining in the underground mining environment. (Mowrey, 1992).

Natural Gamma Radiation (NGR) CID Technology

The first of these methods, natural gamma radiation, is the only method already being employed in commercial mining applications, with over one hundred and fifty units around the world. This method works on the principle that shale, clay, silt, and mud have higher levels of naturally occurring radioactivity than coal. This is due to their content of minute quantities of radioactive potassium (K-40), uranium, and thorium. The attenuation of the NGR by the coal can be used to measure the thickness of the coal between the sensor and the rock interface. The measured NGR decreases exponentially as a function of coal thickness.

This method has many strong features that make it a viable option for use in automated mining operations. It can measure coal thickness readings from 2 to 50 centimeters. The indication is easily read from a display panel. This compact unit can be mounted on the miner itself, where it will not be in the way, and it is applicable in most seams. The most prevalent applications to date have been on longwall units.

There are a few inherent weaknesses in this system that arise from the distribution of radioactive material in the seam. For example, NGR levels vary from seam to seam, requiring the units to be calibrated for each seam in which they will be used. Anotherminor problem is that the NGR levels can vary throughout a seam, depending on the levels of radioactive constituents that were present at the time of geologic deposition.

Another problem that presents itself, at times, is that of rock partings, layers of rock that sometimes intrude into a continuous coal seam. These can show false seam boundaries to the unit by indicating a coal-rock interface within the seam. (Mowrey, 1991).

Vibration Based CID

As coal or rock is being cut, it induces different patterns of vibration. By interpreting the change in vibrations produced, the sensor can detect when the machine has started cutting boundary rock instead of coal. The three types of vibrations that are studied are machine vibration, in-seam seismic, and acoustic vibrations. The strengths and weaknesses of this method vary depending on which type of vibration is being examined. When studying machine vibration the sensors can be mounted on the machine itself so that the sensors are out of the way and need not be remounted as mining progresses. This method has good potential when adaptive signal discrimination technologies are used to help interpret the vibrations. This system also gives immediate feed back when rock starts to be cut, so mining can proceed up to the roof.

In-seam seismic and acoustic sensors must be attached to the coal itself, requiring the sensors to be remounted as mining progresses. This is inefficient, necessitates personnel to be at the working face to mount the sensors, and contradicts one of the main reasons for having an automated mining method, i.e. to eliminate workers from the production face. Hence the in-seam seismic, or acoustic sensing methods are not practical in thin seams. (Mowrey, 1991).

Infrared CID Technology

Different types of strata release different amount of infrared radiation while being cut. This is primarily a factor of their physical characteristics. Infrared sensing devices can measure the values of infrared radiation emitted from the cutting zone. Changes in the intensity of emission can be attributed to changes in the strata being cut. This informs the computer when the miner is leaving the seam, so that corrections can be made.

This method has distinct advantages that merit its further development. The radiation readings can be taken from a location behind the cutting drum, from a remotely mounted sensor, even when the drum is obscured by dust and water sprays. This method can be used under any type of roof, allows coal to be mined up to the roof, and yields an instantaneous response time. (Mowrey, 1991).

Optical / Video CID

The theory that is applied here is that different types of strata have different reflectivities, meaning that they reflect different amounts of light from a similar source.

This physical attribute can be exploited to discern the difference in two materials using a reflected light source. This technology is not very accurate but is greatly improved by the addition of video cameras and image analysis equipment.

These sensors, like the infrared sensors, can be remotely mounted and, with the appropriate video cameras, can see through moderate dust and water sprays. Heavy dust and water can cause problems. Another benefit of this system is that data obtained from the video systems can also be employed for guidance purposes. (Mowrey, 1992).

Radar Based CID

Radar based CID utilizes a single antenna, which transmits and receives Doppler radar pulses. A network analyzer is utilized to control frequency, and for signal analysis.

The signals are attenuated as they pass through coal and bounce off the density interface of the confining rock. The attenuation of these waves can be interpreted to find the distance to that interface.

This system has reliable accuracy and operates well under most roof conditions. Another application of this technology is that it may be used to measure the thickness of the ribs, to ensure straight holes in highwall-auger mining. Some inherent problems with this system are that it does not work well in coals with wave dispersing properties, and it requires the transmitter to be within 10 cm of the coal. (Mowrey, 1995).

Pick Force CID

This CID method measures changes in the force exerted on one or more of the picks on a continuous miner. The energy required to break differing types of rock results in varying forces being applied to any given pick. This phenomenon can be used to determine when the mining machine cuts into a different type of strata.

This system could be conveniently integrated into the mining machine, keeping all of its components compact and protected. This system also gives instantaneous feedback when the miner leaves the seam. No system of this type is currently developed for advanced testing. (Mowrey, 1992).

Continuous Miner Guidance Technologies

There are four main types of continuous miner guidance systems that appear at the forefront of this technology. These systems are Laser Based Miner Guidance, Ultrasonic Continuous Miner Guidance, Modular Azimuth Positioning (MAPS) and Angular Position Sensing Systems (APSS). Ultrasonic sensors, scanning laser arrays, and ring laser-gyroscopes are all employed in these miner guidance systems.

Laser Based Miner Guidance

The laser system is composed of four laser-scanning sensors that scan for two retro-reflective targets, and report their angular coordinates. This is accomplished by panning the laser beams in the horizontal plane and recording the angles of the beams when they encounter the targets on the rear of the continuous miner. This information is used to triangulate the position and heading of the miner with respect to the known position of the laser arrays. The computer that processes this information may be programmed and linked to drive the continuous miner using these data. (Anderson, 1989).

This system has acceptable accuracy, but is limited to a range of 100 feet and a 110° field of view. Problems with uneven floor have also caused problems by moving the targets out of the plane of laser scanning. This problem can be corrected with longer targets or more scanners. Another problem with this system is that it can only be used for face navigation, and the lasers have to be moved and re-installed as mining advances. This installation requires workers at the face, making this method inappropriate for remote mining in thin-seam applications. (Anderson, 1989).

Ultrasonic Continuous Miner guidance

Ultrasonic sensors have been utilized for experimental miner guidance. In one application, ultrasonic ranging sensors were arranged in formation on a 27-inch diameter fiberglass ring, the Denning ring, at 15° intervals. This ring was mounted on top of the continuous mining machine. These sensors send out ultrasonic pulses and interpret the reflected waves. The data give the computer the coordinates of ribs, corners, and obstructions that are necessary for miner guidance. The computer is also set up to drive the miner through mine workings and to mine coal. (Strickland and King, 1993)

This system stands out as one of the most promising of those reviewed. The sensors are inexpensive and have few moving parts, or lenses to clean. Measurements can be taken through dust and smoke, and the system is integrated directly into the continuous miner so that no accompanying workers are needed.

A few problems have been encountered such as differences in the reflective surface characteristics, reducing accuracy. For example, some surfaces absorb the sound energy, instead of reflecting it back. This causes those surfaces to appear to be much farther away. The sensors, though relatively tough, can be damaged requiring their replacement. (Strickland and King, 1993).

Modular Azimuth Positioning System

The Modular Azimuth Positioning System (MAPS) employs a ring laser optical gyroscope in cooperation with a Zero-Velocity Update (ZUPT) system. When the miner stops moving the translation and rotation data are fed into the dynamic reference unit and processed, giving the new location and heading of the miner. (Sammarco, 1993).

This system is also integrated into the miner, so that personnel are not needed at the mining face. The ring laser gyroscope has low power consumption, is fast leveling, and requires little re-calibration. This very expensive piece of equipment has to be started from a known location, facing in a known direction. The gyroscope is also sensitive to vibration. The ZUPT unit requires that the miner often make frequent stops that can last over a minute. These time delays are the major drawback of this system. (Sammarco, 1993).

Angular Position Sensing Miner Guidance

This method utilizes a mobile computer framework developed by the Bureau of Mines. This system is basically the same as the laser based guidance system, except that the lasers are mounted on this framework. They must, therefore, be driven to the location where they need to be set up again, as mining progresses. (Anderson, 1989).

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Past Attempts at Underground Mining of Thin Seams

Mole Miners

A mole miner is a machine that can cut a “blind” narrow face entry while being remotely-operated. This technology is composed of several sub-systems ; haulage for coal removal, ventilation, monitoring, and a control and pushing system which advances the miner into the face. Mole miners have been employed in the USA, UK, and the former USSR with less than outstanding results. This method has become more feasible for thin-seam mining with the development of autonomous mining machines and has the advantage that personnel do not enter the extraction zone.

The first such miner developed in America was by the Union Carbide Company to mine coal outcrops exposed in the highwalls of strip operations, once stripping was no longer economic. (Clark, 1982) This miner was self-propelled, moving its haulage system behind it. The cutting height was kept to no more than 5 ft. The miner maintained its position in the seam utilizing pick force sensing to detect differences in rock hardness.

The most recent application of this technology in the former USSR has been in steep seams that would not require a separate haulage system to remove coal from the working face. Other attempts in more level seams have been discarded. (Clark, 1982).

In the United Kingdom a machine called the Collins miner was created specifically to mine thin seams. (Fig 4.1) Several versions were designed and tested. Many promising results were produced but due to economic factors, including the growing availability of natural gas as an alternate fuel source, the research was not continued.

The Collins miner was intended to cut a 300 ft long entry that had dimensions of 6 ft 3 in wide, from development sections of dimensions 12 ft wide by 7ft 6 in high. Numerous consecutive parallel entries would be driven to obtain the desired production. The width of the rib pillars between the entries was dependent on the thickness of overburden and rock strength. (Clark, 1982).

The cutting head of the Collins miner was composed of three overlapping augers. These were driven from a water-cooled gearbox by a single 120-hp electric motor. Unpowered cutting blades that squared the three overlapping circles removed the upper and lower cusps between the auger holes. A small flight conveyor moved the cut coal to a belt conveyor behind the machine.

The entire machine was mounted on skid plates that were connected to the main frame, by hinges in the front and by lifting jacks at the rear. The jacks controlled the angle of the cutting head. Side jacks were also included in the design to help facilitate horizontal steering. The launching platform was mounted on rails and carried the miner from hole to hole. The platform included jacks for positioning the miner, and pushing cylinders and a pawl mechanism for driving the miner. (Holman, McPherson, and Loomis, 1999b).

Figure 4.1 Collins Miner Layout

(Clark, Cauldon, and Curth, 1982) reproduced with permission

A Brief Overview of Full-Face Miners

Full-face miners are mining machines that span the entire width of the mining face and are advanced or retreated along the entire width at one time. Due to the fact that these systems utilize so much equipment, there must be personnel on the working face for the purpose of maintenance. The size of the equipment has, to this time,limited the application of full-face miners to seam dimensions of no less than 16 inches.

Examples of full-face miners include the Miniwall (Fig 4.2), the In-Seam Miner (Fig 4.3), and the Yarmak Miner (Fig 4.4). All of these systems remove coal from the face by means of a laterally moving, cutting device. (Holman, McPherson, and Loomis, 1999b).

Figure 4.2 “Mini-wall”

(Clark,Cauldon, and Curth, 1982) reproduced with permission

Figure 4.3 In-Seam Miner

(Clark, Cauldon, and Curth, 1982) reproduced with permission

Figure 4.4 Yarmak Miner

(Clark, Cauldon, and Curth, 1982) reproduced with permission

Scraper Boxes

The scraper box was one of the simplest longwall-type systems. Originally scraper boxes were used simply as haulage units on hand-worked longwalls in moderately thin seams. The scraper box was made up of a box that is open at the top, front, and bottom. A scraper blade was hinged into the rear of the box. When drawn forward the blade took on a closed position preventing the contents from leaving the box.

When the box was drawn backwards the blade would rotate into an open position allowing the box to pass over any objects in its path. To give the system more reach, multiple scrapers were usually employed on the same wall. Each scraper carried its contents to the end of its section of wall, dumping it where the next scraper could pick it up and pass it along to the head gate of the wall.

One of the premier scraper box systems was the Haarman scraper box, which utilized a heavy skid board to press the scraper box against the face. The caused the box to take shallow cuts off the face each time it passed over it. In later systems the skid board was removed and a heavy-duty chain that ran the length of the wall took its place.

In these methods the ends of the walls were kept slightly ahead of the center, in a bow shape, to facilitate the movement of the chain. Since the tension in the chain kept the box against the face cutting coal, this system came to be known as the “chain tension scraper box” (Fig 4.5). By removing the skid board, the job of constantly moving the board was also eliminated, hence greatly improving efficiency.

The only job that remained for personnel was the installation of roof supports. (Clark, 1982).

This system, despite all of its advances, still required the use of personnel on the working face, to install roof supports. This gave the system a minimum seam height of 16 inches. The pulling forces needed to move the boxes on the face required a large winch to generate them. Skid boards prevented easy access to the scraper boxes.

In Germany this system produced five tonnes per man shift. The tension used in the chain varied from 4,000 to 8,000 pounds. This system had many problems that kept it from being a widely used system. The shape of the wall coupled with the use of the chain did not provide a sufficient normal force to generate an adequate rate of advance.

Also the bow shaped wall had extra stress accumulated on the lagging section of the wall. This set up also limited the lengths of the wall and greatly hindered the economic potential of this system. (Clark, 1982). In any system where caving is desired behind the working section, it is important to have a straight break line along which the roof can fail. Curved faces do not provide this straight break line and make it more difficult for the roof to fail. If the roof fails to break in a timely fashion, excessive stresses can accumulate on the supports. Keeping the working section straight, or on-line, maintains responsive, predictable caving and facilitates smooth operation of the system.

Figure 4.5 Chain Tension Scraper

(Clark, Cauldon, and Curth, 1982) reproduced with permission

READ MORE - Past Attempts at Underground Mining of Thin Seams

Highwall Mining

Highwall mining is the practice of mining coal by tunneling into the exposed highwall face of a surface mine and removing the coal. This is accomplished in a number of ways including auguring, addcar systems, and Archveyor (Arch Technology Corporation) systems. Highwall mining allows the recovery of coal that would otherwise be lost, and offers high productivity measured in tonnage per man-day. (Walker, 1997). While highwall mining has been developed to extend the recovery of coal from surface mines, it does, additionally, provide an opportunity of examining how such methods may be adapted for the underground mining of thin seams.

Highwall auger systems are composed of three main components: the cutting head or heads, flights for moving the coal and the motorized drive. (Fig 3.1) The cutting head cuts into the face at right angles to the exposed face, while being pushed by the flights and turned by the drive unit. (Fig 3.2) As the head cuts coal, the flights carry the coal back out to the base of the highwall. When the flight reaches the extent of its length, the flight is uncoupled from the drive and another flight is added to the string. This is repeated until the drill string reaches the desired cutting length. Then a new hole is drilled further down the face, leaving a small pillar to support the overburden. (Clark, 1982).

The main weakness of this method is difficulty in maintaining straight holes over long distances. New radar technologies are being developed to maintain hole and to increase the auger range to over 600 feet. (Mowrey, Ganoe, and Monaghan, 1995).

Any curvature of the highwall also has serious effect on the attainable recovery. Concave highwalls cause auger holes to fan out, while convex highwalls cause auger holes to intersect. Both of these situations result in lower recoveries, and intersecting holes also pose a risk of collapse. The highest possible recovery comes from having a straight highwall face. (Fig 3.3) (McCarter, 1992).

The addcar system (Figure 3.4) was also designed for highwall mining. The current system is not applicable for thin-seam application, because it can only be used in seams of more than 90 cm or 35.4 inches in thickness. This system recovers up to 60% of reserves, using 12.5 m-long individually powered Addcars. The standard system depth is 365m, but the new upgraded ‘Highwall Hog’ has an extended range to 500m. The Addcar system utilizes a continuous miner and Addcars that utilize chain conveyors to remove coal from the entry. The advantages claimed for this system over its closest competitors are: 150% more penetration; 90% more annual production capacity; 82% more installed horsepower; and it can produce the same tonnage in a reduced length of highwall. (Walker, 1997).

The newest innovation in highwall mining comes in the form of the Archveyor. (Fig 3.5) This mining system receives its cutting power from a highly modified Joy 12CM continuous miner capable of cutting a 3.8m-wide from 1.8 to 4.9m thick. The Archveyor itself follows the miner into the heading, transporting the coal out. The Archveyor has drive units every 7.5m, enhancing both vertical and horizontal flexibility. A chain conveyor is used for coal transportation and when lowered and reversed, to move the system forward. When mining is underway, hydraulic jacks lift the Archveyor clear of the ground allowing the conveyor to transport coal. (Walker, 1997).

Figure 3.1 Highwall Auger Unit (Walker, 1997) reproduced with permission

Figure 3.2 Overburden Removal (McCarter, 1992) reproduced with permission

Figure 3.3 Auger Hole Pattern (McCarter, 1992) reproduced with permission

Figure 3.4 Highwall Addcar System (Walker, 1997) reproduced with permission

Figure 3.5 Archveyor System (Arch Technology Corporation) reproduced with permission

READ MORE - Highwall Mining