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IV UNDERGROUND COAL MINING TECHNOLOGY A major portion of the current mining research and development effort is focused on improving underground mining because of the continuing productivity decreases in all coal mining and particularly in underground mining (Appendix B). Underground productivity has declined from about 16 tons per man-day in 1969 to about 9.5 tons per man- day in 1975. As will be discussed in detail below, the sensitivity of prices paid for coal to productivity is such that 1 ton per man-day increase implies almost 7 billion (1975) dollars in savings in the price to be paid for coal over the next 10 years. Moreover, productivity losses result in increased miner exposure and, therefore, increased injuries and fatalities in the mines. Thus, productivity is worthy of considerable national attention. A. COAL MINING METHODS Underground coal mining methods are classified according to the method of ground control (an excellent description of all methods is provided in Samuel M. Cassidy, Ed., Elements of Practical Coal Mining, New York: American Institute of Mining Engineers, 1973) as either room and pillar or longwall (shortwall is considered a variation of longwall in this discussion). In 1974, 97 percent of underground coal was produced using the room-and-pillar method and 3 percent, using the longwall technigue. An increase in the relative production of coal using the longwall method is anticipated. Typical room-and-pillar entries or tunnels are 10 to 2H feet wide; pillars range from 20 to 90 feet on a side and may be square or rectangular. Rooms are developed in concentrated areas or "panels" of a mine. After a panel has been developed, pillars may be extracted; if they are not, the method is termed partial extraction. Room-and-pillar coal mining can be classified further as conventional or continuous depending on the method of extraction at the face. In conventional mining, the mining cycle consists of cutting, drilling, blasting, loading, hauling, and roof bolting, and specially designed machines are utilized for each step. In continuous mining, one machine is used to accomplish the cutting, drilling, 25
blasting, and loading steps in the cycle. In 1974, 63 percent of the coal mined by the room-and-pillar method was extracted continuously and 37 percent, conventionally. The unit operations of the mining cycle with continuous miners consist of the following steps: approxiirately 20 feet of entry is mined; the mining machine is iroved to another entry; the area just mined is roof bolted; and the exposed coal and floor is rock dusted. Only about 20 feet of advance per cycle is possible due to the requirement that the operator not advance beyond the roof support line. In cases where the roof is more self-supporting and state laws permit, this advance may be as much as 100 feet per cycle; however, nearly all room-and-pillar production is at a cycle distance of approximately 20 feet. Cycle sequencing, or cut sequencing, refers to the sequence in which cuts are taken in the section to advance a series of rooms and crosscuts. In longwall mining in the United States, a large block of coal from 300 to 700 feet wide and from 1,500 to 7,500 feet long (a 500-foot-by-4,000-foot block is typical) is outlined with two sets of parallel development entries and then is extracted in a single continuous operation. Once a connection is made between the two sets of entries, hydraulically actuated steel supports are placed along the short dimension of the large block outline. After the supports are in place, a special continuous mining machine (a shearer or plow) is used to extract sequential slices along the short end of the longwall panel block. As the machine moves along, the hydraulically powered roof supports advance to provide protection as close to the face as possible and to allow the roof behind the supports to fall. In general, longwall mining can result in higher extraction ratios than room-and-pillar mining as well as greater productivity in terms of tons per man-shift; however, under current Mining Enforcement and Safety Administration (MESA) regulations, longwall and room-and-pillar mining have approximately equal overall extraction ratios although the longwall recovery ratio is considerably higher when measured on a section basis only. In Europe, the emphasis is on increased extraction ratios. Hydraulic mining of coal by large, high-volume monitors has been practiced in thick, steeply dipping seams in Western Canada, but the system has not presently been used in the United States. It is possible that thick seams too deep for surface mining may be mined by some modification of this method in the future. Such a method might also be coupled with a hydraulic transportation system to move the coal-water slurry all the way to a surface washing plant, thereby eliminating many pieces of equipment, such as shuttle cars, belts, and transfer points, which constitute a hazard to workmen. One major company in the United States 26
is presently actively pursuing development of such a hydraulic transport system. 1. BQQf Control Poof and ground control remains one of the irost important aspects of successful coal mining. Strata above coal seams generally are not self-supporting and therefore must be strengthened with artificial supports. Currently in use as supports are hydraulic props, timbers, cribs, roof bolts, posts and cross bars, window sets, and steel arches with and without lagging. In room-and-pillar irining, the type of support used depends on the strength of the rock being supported as well as the length of time the opening must be maintained, the opening geometry, the depth of the coal seam, and other factors, but roof bolting is most widely used with timber, cribs, and other supports being employed if additional support is necessary. To place a bolt, a hole is drilled in the roof strata and the bolt is anchored in the hole using an expansion shell or is cemented in the hole with a resin. In longwall mining, the roof at the working face is supported by the advancing hydraulic props and the unsupported roof falls as the mining machine and props advance. 2. Hauling Methods Once coal is extracted from the face, it must be transported outside the mine to the preparation plant or some other point of use. In discussing hauling systems, it is convenient to divide them into the operations at the face, in the panel, and main haulage. With room-and-pillar mining, the face haulage system usually is not continuous, and an underground electric truck or shuttle car is employed to traverse the distance between the face and some intermediate point. Bridge conveyors sometimes are used instead of trucks to provide a continuous system. Panel and section haulage generally is accomplished using a conveyor belt. In many cases, panel haulage is combined with main haulage and mine cars are used. Main haulage generally can be accomplished using either a conveyor belt or mine cars. with longwall mining, face haulage is accoirplished using a chain conveyor that runs the full length of the longwall face. Section haulage involves a conveyor belt, and main haulage may utilize either a belt or track. 3. Ventilation and Methane Control Methods Ventilation has become an important factor in the design and operation of underground coal mines as regulation concerning allowable methane and dust levels have become 27
more stringent. There are areas in the coal fields where the quantity of methane liberation currently liirits production from the mine as well as the productivity of men and machinery. All underground mines are ventilated with large fans, generally 300 hp each, that provide large voluires of fresh air to dilute explosive gases or dust. Ventilation methods can be divided into the unit operations of face, panel, and mine ventilation systems. Methane air mixtures can be made nonexplosive by dilution with sufficient air. New regulations promulgated in 1969 have stretched ventilation technology to the limit in many areas in order to comply with regulations concerning allowable methane content. As coal mining moves to deeper and gassier mines, operators increasingly may remove the gas by pipe drainage prior to mining in order to decrease the amount of air needed for ventilation since achieving the air volumes required is becoming both costly (shafts, fans, stoppings, etc.) and technically difficult. Coal mine emissions of methane, although frequently representing huge amounts, occur in a highly diluted form in the discharged ventilation streams, usually substantially less than 1 percent methane. There is no technology available today that could economically concentrate this methane into a form useful as commercial pipeline gas once it has been mixed with air. The gas collected from boreholes prior to mining has been of high Btu value, but there has been some question about the legal ownership of this gas (e.g., in western Pennsylvania, where coal and natural gas are both produced, the mineral rights to the coal have been obtained by the coal operators but not the gas mineral rights). However, if an economical method of recovering the gas can be developed, improved safety will serve as a sufficient incentive and the mineral rights question is not expected to be a serious impediment. Recovering methane from coal seams is an important part of the Energy Research and Development Administration research program. B. ENVIRONMENTAL ASPECTS Underground mining operations do not contribute directly to air pollution. In the past, refuse banks have been a source of pollution in the form of dust and slow combustion of waste coal. Coal preparation and transportation are a second source of dust emissions but the probleir is miniirized by enclosing the coal preparation plant and by using dust suppressants on the exposed surfaces of fine sizes of coal during shipment. 28
Water pollution occurs in the form of solid matter from coal washing and refuse piles and acid drainage. Use of settling basins to remove solid matter from the coal washing water for maximum reuse together with treatment of discharage water is necessary. Mine tailings from beneficiation frequently represent a solid waste disposal problem, particularly because they may contribute to acid drainage problems. Acid water is much more prevalent in the eastern United States, especially the Appalachian Region,1 because of the extensive mining above drainage that results in exposure of acid-producing materials to precolating groundwater and precipitation runoff. The acid water is formed largely from the oxidation of pyrite associated with coal. Acid water formation can occur from natural causes (i.e., from water drainage over surface coal outcroppings and through underground deposits above drainage); however, the problem becomes much more serious when a large amount of pyritic material is exposed during the mining process. Acid mine water can be controlled to some extent by preventing water drainage through coal in operating mines or abandoned mines. When this is impractical, it is possible to neutralize the acid water and filter or settle out iron and other precipitated materials before the water is returned to streams or lakes; suitable methods of control have been developed for some situations. Surface subsidence can be a serious problem. While backfilling may provide limited control in abandoned mines and in active mines that use longwall or partial recovery mining methods, it has not been demonstrated as an economic possibility for underground mining on a large scale.2 In some situations, a solution to the problem of subsidence may be the use of longwall mining or complete extraction room- and-pillar mining since the surface is allowed to subside permanently in a planned and controlled manner. C. PRODUCTIVITY AND SAFETY Because of a combination of factors including more stringent application of health and safety standards, underground productivity declined from about 16 tons per man-shift in 1969 to 9.5 tons in 1975, and this decrease has increased the capital and operating costs for irining coal. Labor, management, and the government have been conscious of the need to provide safer mining conditions and some improvements have resulted; however, it now is becoming increasingly important to maintain safety and improve technology in a regulatory climate that will help to restore productivity to at least the level formerly achieved. 29
Current deep mine capitalization costs in the East are approximately $40 per annual ton (1975 dollars). This is based partially on productivity levels of 11 tons per man- shift. With 3 percent replacement production required per year and assuming 160 million tons of new annual capacity by 1985, approximately 270 million tons of added underground capacity will be required. Thus, a 1 ton per nan-day change in productivity implies approximately a $1 billion change in capitalization required by 1985. Further, labor costs per man-day are approximately $90, including workmen's compensation and other benefits. A first approximation also indicates that a 1 ton per man-day variance in productivity represents a $1-per-ton labor cost in underground mining. Table H gives capital costs based on a 15 percent return on discounted cash flow after taxes and assumes that investment must be made during the first 5 years to achieve 1985 production goals and that investments may be capitalized over an average life of 20 years. In summary, an increase of 1 ton per man-shift in underground coal mining productivity, if achieved now, would save the nation $6.89 billion between now and 1985 in prices paid for coal in noninflated 1975 dollars. A decrease in productivity costs more than an increase saves. Overall underground productivity has declined more than 6 tons per man-shift since 1969; therefore, improvements in underground productivity are worthy of considerable national attention. The social costs of productivity losses also are real and serious. A 1 ton per man-day change in productivity results in a change of 10,000 men employed in underground mining. This change in personnel exposure underground has important safety implications since exposure is a key element in underground mining safety. USBM and MESA data for 1975 reveal that 115 of 140,000 underground coal miners employed lost their lives (or one out of 1,200). Based on present statistics, a loss of 1 ton per man-day in productivity results in 8 or 9 more fatalities per year. D. RESEARCH AREAS 1. Continuous Mining Considerable time is spent moving underground equipment from place to place in continuous mining. Typical productivity per shift for a continuous miner section is tOO tons while the theoretical rate is 10 tons per minute or 3,600 tons per shift in 6 hours available cutting time. This theoretical rate can never be achieved because of transportation, supply, roof support, ventilation, and other problems. Coal mine operators emphasize the need for "keeping the mining machine at the face and in as nearly continuous 30
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operation as possible." To do this, two technological breakthroughs are required: a. A method for supporting the roof at the same rate the continuous miner advances, and b. A method for transporting the coal away from the machine at the cutting rate The two examples utilizing field data presented below illustrate the nature of the bottlenecks involved in bolting and transportation. Time studies reveal that 7-foot roof bolts can be drilled and set in the Pittsburgh No. 8 seam in 2.44 minutes. Assuming a 16-foot room and 4-foot bolt spacing, three bolts must be placed across the entry every 4 feet, (3/4 bolt per foot of mining advance). If one could place bolts sequentially, the maximum advance rate, bolting limited, is 0.55 feet per minute. Assuming a 6-foot-by-16- foot mining entry, there are 3.85 tons per foot, resulting in a maximum bolting-limited tonnage of 2.11 tons per minute. If three bolts could be placed simultaneously, which is not now possible, 6.33 tons per minute becomes the boIting-limited production rate. Since the mining machine is capable of 10 tons per minute, roof bolting or, more generally, roof control is an obvious bottleneck to production. During the cycle, the mining machine discharges coal into a shuttle car that travels back and forth to some discharge point. Travel time for a 6-ton shuttle car in a 6-foot coal seam is 3 to 5 minutes. Haul capacity therefore is 1.2 to 2.0 tons per minute using one shuttle car and 2.5 to iÂ».0 tons per minute using two if there is no interference between shuttle cars (but there is usually some). Use of three shuttle cars generally is impractical due to interference in haul roads and difficulty in laying out switch-out points. Thus, one can see that both roof bolting and face haulage currently limit face productivity to about 4OO tons per 6-hour shift for continuous miner sections. 2. Conventional Mining The above comments also apply to conventional sections except that increased attention also must be given to coordination of men and machinery movement. It is clear that the nature of the conventional section cycle does not permit effective utilization of continuous haulage. Improvement in unit operations would increase productivity. None, however, would have the iirpact that 32
improved roof support and transportation could have on underground productivity by keeping the machine in place. 3. Longwall Mining Lonqwall mining is more productive and inherently safer than room-and-pillar mining but also is more capital- intensive and often has serious dust problems. Nevertheless, new production can be put on-stream through placing longwalls in active mines more economically than by opening up new mines, and, during the short term, longwalls should be encouraged in those active mines where they are applicable. Bottlenecks to improved productivity center around equipment maintenance, ground control, and dust concentration along the longwall face. Poof control problems are chiefly tailgate support and falls in front of the supports. While shield supports have greatly reduced falls ahead of the supports, tailgate support and the design of tailgate entries remain serious problems worthy of further research. Another problem in longwall mining concerns mining through abandoned gas and oil wells. Fruitful areas of research concern developing methods to determine whether flow between the gas well and pressurized gas and sand exists as well as suitable procedures for sealing gas wells. One point often overlooked is that one and one-half to two continuous mining sections must be committed to each longwall section for panel development. This being the case, each longwall installation must be evaluated as a package (i.e., as three sections, two continuous and one longwall). Anything that improves continuous or room-and- pillar productivity has a significant impact on longwall mining productivity. 4. Mine Development It takes four to seven years to place underground production on-stream from the time that a decision has been made to establish a new mining property. Capital is tied up without return for those years; therefore, larger margins are required in later years for equivalent discounted cash flow rates. An additional major item in mine development is shaft sinking, and faster, safer, lower cost methods are desirable. A promising possibility is the drilling of large-diameter (18 feet or more) shafts. 33
5. Thick Underground Coal A great deal of the nation's coal resources are located in the West and are characterized by thick seams under deep cover. Although significant tonnages of this coal may not be needed for 25 years, important long-term research concerns underground mining of this coal. A minimum objective should be 50 percent recovery of the resources in the ground. 6. Primary Research Topics Given the problems and needs identified above, the Panel has concluded that research focusing on the following should be given high priority: a. The development of a method for transporting coal from the mining face at rates approaching the ability to cut and load coal at the face b. The development of a method for supporting the roof at lineal rates approaching the ability to cut and load together with solutions for ground-control problems in longwall mining c. The development of methods for me'thane abatement or dilution d. The development of methods for large-diameter blind hole shaft drilling e. The development of methods for efficient mining and recovery of thick underground coal REFERENCES 1. Appalachian Regional Commission, Acid Mine Drainage in Appalachia, (Washington, D.C.: Appalachian Regional Commission, 1969). 2. National Research Council, Underground Disposal of Coal Mine Wastes, (Washington, D.C.: National Academy of Sciences, 1975).