National Academies Press: OpenBook

Coal Mining (1978)

Chapter: SURFACE MINING

« Previous: UNDERGROUND COAL MINING TECHNOLOGY
Suggested Citation:"SURFACE MINING." National Research Council. 1978. Coal Mining. Washington, DC: The National Academies Press. doi: 10.17226/18766.
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Suggested Citation:"SURFACE MINING." National Research Council. 1978. Coal Mining. Washington, DC: The National Academies Press. doi: 10.17226/18766.
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Suggested Citation:"SURFACE MINING." National Research Council. 1978. Coal Mining. Washington, DC: The National Academies Press. doi: 10.17226/18766.
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Suggested Citation:"SURFACE MINING." National Research Council. 1978. Coal Mining. Washington, DC: The National Academies Press. doi: 10.17226/18766.
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Suggested Citation:"SURFACE MINING." National Research Council. 1978. Coal Mining. Washington, DC: The National Academies Press. doi: 10.17226/18766.
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Suggested Citation:"SURFACE MINING." National Research Council. 1978. Coal Mining. Washington, DC: The National Academies Press. doi: 10.17226/18766.
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Suggested Citation:"SURFACE MINING." National Research Council. 1978. Coal Mining. Washington, DC: The National Academies Press. doi: 10.17226/18766.
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Suggested Citation:"SURFACE MINING." National Research Council. 1978. Coal Mining. Washington, DC: The National Academies Press. doi: 10.17226/18766.
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Suggested Citation:"SURFACE MINING." National Research Council. 1978. Coal Mining. Washington, DC: The National Academies Press. doi: 10.17226/18766.
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Suggested Citation:"SURFACE MINING." National Research Council. 1978. Coal Mining. Washington, DC: The National Academies Press. doi: 10.17226/18766.
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Suggested Citation:"SURFACE MINING." National Research Council. 1978. Coal Mining. Washington, DC: The National Academies Press. doi: 10.17226/18766.
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Suggested Citation:"SURFACE MINING." National Research Council. 1978. Coal Mining. Washington, DC: The National Academies Press. doi: 10.17226/18766.
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V SURFACE MINING Surface-mining methods, which generated less than 5 percent of production in 1930, accounted for more than 50 percent of production in 1975. Surface mining is classified as contour, area, open pit, and auger. Contour mining is used in hilly or mountainous areas (primarily in the East) where the coal is recovered from a narrow contour strip, 50 to 100 feet wide, that follows the coal seam around the irregular topogrphy. (Coal remaining after the final strip is too deep for economical recovery by surface mining and must be mined using auger or underground methods). In area mining, the coal is still recovered in narrow strips, 90 to 200 feet wide, but the surface is more nearly level or dipping with the coal, and wide sections of coal may be removed without encountering excessive overburden. Open-pit mining involves the removal of overburden in a larger, more rectangular pit (e.g., 1,000 feet wide by 2,000 feet long) than in area mining and the systematic moving or deepening of the pit to reirove thick coal seams. In all three of the above cases, the overburden is backfilled into the mined-out area. In auger mining, coal is extracted by drilling large-diameter holes, usually on the highwall face of the last cut in a strip or contour mine. This method has only limited application and therefore is not reviewed in detail in this report. The sequence of steps in contour, area, and open-pit mining is similar: topsoil removal, overburden drilling and blasting, overburden removal, coal drilling and blasting, coal loading and hauling, overburden backfilling and grading, topsoil placement, and revegetation. Auxiliary operations include coal processing, water supplies, environmental protection, and land restoration. Productivity and capital requirements are affected by mining method, seam thickness, overburden depths, and reclamation required. A. OVERBURDEN REMOVAL The overburden generally is loosened by blasting with an explosive (e.g., ammonium nitrate) placed in holes 6 to 15 35

inches in diameter. The amount of explosive varies but generally is about 0.5 pound per cubic yard. Equipment used to remove the overburden depends on the type of minino, stripping ratio (thickness of overburden compared to the coal seam thickness), thickness of overburden, rate of production, and number of seams being mined. For contour and area mining, overburden depths up to a maximum of 150 feet are normal for draglines and stripping shovels. Open-pit mining at greater depths is being contemplated for the thick seams in the Nest. Contour mining uses large bulldozers, front-end loaders, or relatively small truck and shovel combinations. New environmental requirements limit downslope spoil quantities and favor returning the land to approximately the original contour. Large stripping shovels and walking draglines are the key tools in some contour mining and in most area mining. Walking draglines have buckets that range from 10 to 220 cubic yards and have boom lengths from 175 to 400 feet. In the past decade, draglines have become the most frequently used type of equipment. Their capital cost is lower and their operating costs are somewhat higher, but they can handle greater overburden depths than large stripping shovels. The spoil transport distance of both machines is limited by machine size and boom length. Where the upper layer of overburden is particularly soft and blasting is unnecessary and where topsoil recovery is a key consideration, bucket wheel excavators can be used in conjunction with stripping shovels or draglines for removing the top layers and placing them across the pit on top of previously deposited spoil piles. The bucket wheel excavator has been primarily used for extending the economic depth of existing stripping shovels. The handling of topsoil in surface mining has become especially important in reclamation practices, separate steps may be required to remove, stockpile, and replace the topsoil after mining is completed. Topsoil replacement is generally done with scrappers while the recontouring of spoil piles is accomplished with large bulldozers that often are fitted with specialized blades. Ratios of overburden to coal vary widely from mine to mine and region to region and may range from 1 to 40 cubic yards per ton of coal. The higher stripping ratios occur in the East and the Midwest and the very low ratios, in the thick seam western coal deposits of Montana, Wyoming, and North Dakota. 36

B. COAL LOADING, HAULING, AND HANDLING Surface mining coal recovery varies from about 80 percent in the thin eastern and midwestern seams to tetter than 95 percent in the thick western seams. After the overburden is removed, some coal seams may be mined directly with shovels, but in most cases, holes from 3 to 6 inches in diameter are drilled through the coal and shot with explosives. Thin seams of coal may be ripped with heavy rippers on large bulldozers to loosen it for loading. The coal is loaded with shovels and front-end loaders varying from 5 to 40 cubic yards in capacity. Improved design has allowed front-end loaders to be more competitive with the loading shovel in recent years. If improved wheel excavators can be developed in the future, they could well substitute at least in part for the loading shovel. Coal hauling from the larger surface mines generally is accomplished using 80- to 180-ton bottom-dump trucks while 10- to 30-ton end-dump trucks are used in smaller mines and mines with steep grades (e.g., contour mines in the East). Haulage distances range from less than 1 mile to as much as 10 miles. Overland cable and belt conveyor systems and mine railroads are in operation between fixed points and some automated rail haulage is used. It is possible in the future that suitable in-pit crushing facilities will be developed along with readily moveable and flexible conveyors capable of delivering the coal directly from the loading face to secondary crushing and other processing facilities. Unit-train coal shipments are a major part of present and planned coal transport systems. Adequate storage and rapid load-out therefore have become essential parts of the processing facility. Open storage with stackers and bridge- type bucket recovery systems are used for mine-mouth power plants. Silos with gravity feed are popular for loading directly into rail cars. Trough storage with conveyor trippers for stockpiling or radial stackers are used in conjunction with bottom-of-the-pile vibrating or rotary plow reclaimers for unit-train load-out systems. Some over-the- track conical pile storage with direct loading into rail cars is used. Load-out rates for such facilities vary from 3,000 to 5,000 tons per hour. Dust problems now tend to dictate the use of covered conveyors, transfer points, and storage facilities, and the new Environmental Protection Agency coal processing plant regulations will accelerate this trend. C. ENVIRONMENTAL ASPECTS Water requirements are usually a minor problem for surface mining operations although the mine may be built in conjunction with a high water consuming facility such as a 37

mine-mouth power plant. Water produced in the pit or held in catchment dams in the area where the coal is being mined often is barely adequate for mine use and, in some cases, wells are necessary to provide adequate supplies. The water requirements for road and other dust suppression uses of a 10-million-ton-per-year surface mine, the largest such mines now in operation in the United States, vary from 100,000 to possibly 600,000 gallons per day (or about 60 million gallons or 180 acre-feet per year). The water requirements for irrigation, reclamation, and coal washing would be in addition to this, and off-site water requirements for employee families and services add about 30 acre-feet per year. In other words, such a mine and its employees consume, on and off site, about 210 acre-feet of water per year. Some irrigation may be required, particularly in the West, to ensure the rehabilitation of proper cover after surface mining. Supplemental irrigation usually would be required only in the first year of revegetation, but in semi-arid regions, a small amount of irrigation may be required in the second year. By using native plant species, a self-sustaining cover should be developed that will not require extended irrigation. Table 5 indicates the approximates quantities of irrigation water that will be required under certain seam thickness and rainfall conditions for a large western surface mine. When coal cleaning is required, water is recycled to the maximum extent possible. The only water normally consumed is that which leaves the mine with the coal (either wet coal or the extraneous water driven off by drying) and that which is lost in waste disposal. The water shipped (extraneous as opposed to the inherent moisture in the coal) with the coal should rarely exceed 5 percent by weight or about 500,000 tons of water per year for a 10-million-ton-per-year nrire. The amount of water lost in waste disposal is more difficult to assess because of the variability of evaporation and seepage losses in the waste ponds. Given reasonable control, however, the water lost should be about equal to the water lost in the coal shipped. Assuming this to be the case, a 10-million-ton-per-year wash plant would consume about 1 million tons of water (approximately 250 million gallons or about 800 acre-feet per year). Where water is relatively abundant, excess water discharges from a mine can cause a serious problems that must be recognized, and proper care should be taken to keep water discharge quality within acceptable limits. Acid mine water problems may be encountered in the East but are infrequent in the West. Saline water and soils are problems unique to western coal reclamation. 38

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A recent concern in surface mining has been the disturbance of the hydrological balance in the vicinity of the mine. The dewatering of mines can and does reduce aquifer flows in the mine area at least temporarily and, in some instances, may reduce water levels in an aquifer for some distance from the mine. Serious attention is being given to minimizing hydrological disturbances from a long- term standpoint in terms of the quality and quantity of surface and underground waters. Surface mining does create some local air pollution, primarily in terms of airborn dust. The dust can and is being controlled by covering stationary dust sources and installing suitable dust control facilities, by using water and other dust suppressants in mine areas, by carefully planning and controlling blasting, and by providing for rapid revegetation of exposed surfaces. Until recently, surface mine land reclamation has been a problem. Past operations left massive piles of spoil consisting of unconsolidated surface and rock removed in mining, and forests and other vegetation were destroyed. Such lands have been termed "orphaned lands," and suitable ways and means to reclaim them are being sought. Some, but not all, of the more responsible mining companies began reclamation along with their surface mining activities. In the 1950s and 1960s, individual states enacted regulations to improve control of surface mining, and more land per year now is being reclaimed than is disturbed. Thirty-nine states, including all of the major coal- producing states, have laws to reduce the impact of surface mining. Although there are considerable variations among the basic effects of these laws, all how seem to be achieving an acceptable level of environmental protection for ongoing mining operations. These laws generally contain permit, bonding, and reclamation performance standards and impose penalties for noneonformanee. The need for additional legislative action has largely been preempted by considerably improved federal regulation and state cooperation to mitigate the environmental consequences of surface coal mining. New federal legislation was enacted in 1977. While the issue of regulating surface mining practices on a national basis continues to be debated, the 1977 federal rules are considered an improvement over some of those that existed in the past. They cover the extraction of minerals under permit, lease, or contract. In keeping with the nation's goals of responsibly developing domestic energy resources, including federal coal lands, these regulations should ensure that adequate measures are taken to avoid, minimize, or correct any damage to the land and environment. Furthermore, the Coal Leasing Bill passed over

the President's veto on August 4, 1976, will provide the states with aid in solving the environmental and socioeconomic problems resulting from mining. These new laws, however, may create additional problems for the coal industry and, thus, may retard coal development. D. PPODUCTIVTTY In 1975, the average productivity of U.S. surface mines was 26.69 tons per man-day.» Even with increased equipment sizes, surface mine productivity in the East and Midwest probably will do well to remain constant because of increasing stripping ratios, the exhaustion of the more easily mined surface reserves in the East and Interior Regions, and the advent of stricter safety and environmental regulations. Open-pit coal mining as opposed to strip mining has only recently been practiced to any large extent in the West, and its impact on national coal productivity has yet to be felt. Western surface coal mine productivity will range from 50 to 150 tons per man-day or irore. As shovel, truck, and dragline sizes increase and new techniques are developed, the overall average productivity from western surface mines also can be expected to increase. In recent years, safety and environmental requirements have caused a reduction in surface mining productivity. The decrease in productivity resulting from these activities, however, is and should continue to be relatively less than the decrease in productivity experienced in underground mining where manpower levels and safety concerns have been greater. The rapid expansion of high-productivity coal mines in the West should cause the national average for surface-mined coal to improve materially over the next decade. E. WESTERN COAL CAPITAL COSTS For capital cost projections in the West, it was necessary to establish a basis for apportioning yearly incremental production increases between surface and underground mining. Using historical and projected trends, it was assumed that production from western surface mining contributed approximately 87 percent of the total 1975 western coal production and that this amount would increase by 0.5 percent per year through 1981, after which it would remain at the 90 percent level through 1985 (Tafcle 6). It also was recognized that the capital cost per ton of coal depends on whether the incremental tonnage produced by a mining method results from the opening of a new mine or the expansion of an existing mine. Data compiled by the USBM were used to determine an approximate split, and it was estimated that 90 percent of all surface production and 70 percent of all underground western coal production during the next 10 years would come from newly opened mines.2

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It then was necessary to determine an average capital cost per ton of production for each source, and this was done using information published by the Federal Energy Administration and the U.S. Bureau of Mines.2 A calculated average of economic evaluations for both surface and underground mining costs was used to establish a capital cost consisting of the total investment required to bring a new mine up to full production plus the deferred capital costs required to sustain operation. (It should be noted that these capital costs do not include land acquisition, exploration, working capital, or major rail spur or community development projects.) These estimates (in 1975 dollars) then were used to establish total yearly capital requirements (assuming a predominance of the surface mines being developed in the Northern Great Plains and drift mines for underground development) in dollars per ton of annual production as follows: Capital Requirements Surface Underground Initial $ 7.00 $22.00 Deferred 3.00 12.00 Total $10.00 $3H.OO Because of thicker seams and generally more favorable mining conditions in the West, these capital costs are well below those for an eastern mine. Additional analyses indicated that the capital costs associated with expanding an existing surface operation are only 45 percent as great as those involved in opening a new surface mine or about $4.50 per ton of annual production. Likewise, capital costs of expanding underground mines are only 55 percent of those required for a new mine or approximately $18.50 per ton of annual production. Unfortunately, these values do not differentiate between initial and deferred capital and are meant to be only reasonable averages. Using the above data, it was possible to estimate the total capital requirements (in constant 1975 dollars) for western mine development by year from 1976 through 1985 (Table 7).2 In order to put the total capital requirements in proper perspective, the capital figure for each year has been inflated to reflect the latest inflation projections. An average inflation index trade up of three pertinent mining wholesale price indices and a construction wage index has been used. The indicated western coal capital requirement is $5,424 million in current dollars. Operating costs in western mines will vary in 1976 from about $3 to $12 per ton depending on the type and age of the mine. Costs in recent years have escalated rapidly.

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REFERENCES 1. 0.S. Bureau of Nines, Mineral Industries Survey, Weekly Coal Report, April 22, 1977, (Washington, D.C.: U.S. Bureau of Mines, 1977). 2. Richard L. Gordon, Economic Analysis of Coal Supply; An Assessment of Existing Studies, EPRI Report 335 (Palo Alto, California: Electric Power Research Institute, 1975).

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