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4 COAL CLEANING, STORAGE, AND TRANSPORT Information is sparse concerninq the evaluation of potential trace- element problems encountered in the cleaninq, storaqe, or transport of coal from the mininq and processinq point to the ultimate consumer. The potential contribution of undesirable concentrations of trace elements to the environment as a result of the storaqe or transport techniques currently used for coal in the United States has been questioned (Swanson et al., 1974). The purpose of this section is to examine the trace elements that are known to occur in coal and their potential impacts durinq coal cleaninq, storaqe, or transport. COAL CLEANING Coal cleaninq is conventionally based on the principle that the particles of crushed coal differ from each other in the relative proportions of included mineral matter and orqanic combustible material. Most coal cleaninq occurs at or near the mininq area, often just before the coal is transported. Because mineral-rich particles have hiqher densities than orqanic-rich particles, they can be separated by processes that use a liquid medium with a specific qravity that will optimize the separation of coal from mineral matter. A detailed description of an intens~ve coal-cleaninq process is qiven by Ford et al. (1976). However, coal cleaninq does not eliminate any elements, it simply separates the mineral-associated ones from the orqanic fraction of the coal. The trace-element content of the cleaninq water is tremendously enriched, which qreatly maqnifies the runoff problems beyond those encountered in mininq and storaqe. However, none of the referenced studies contain data suitable for use in this report. Althouqh chanqed in concentration and form, the disposal of mineral constituents is still necessary. The cleaninq operation miqht dissolve some elements, which could result in a water-disposal problem. The averaqe percent removal of arsenic, lead, manqanese, mercury, and selenium durinq coal cleaninq as calculated from data in Ford et al. (1976) ranqed from 28 percent for selenium to 41 percent for arsenic (Table 13). The Southern California Edison Company Mojave Generatinq Station (at Bullhead City, Nevada) has a unique coal-delivery system (Environmental 57
58 TABLE 13 Percentage of Trace Elements Removed by Coal Cleaninga,b Percenta2e Removed Element Average Range Arsenic 41 11-67 Lead 32 8-63 Manganese 37 9-76 Mercury 30 3-68 Selenium 28 2-61 asource: Ford et al. (1976). bcoals used were bituminous from Kentucky, West Virginia, Pennsylvania, Oklahoma, Kansas, and Alabama. Science and Technology, 1976). Coal is transported from the Black Mesa field (Arizona) as a 1:1 coal-water slurry in an 18-in. pipeline over a distance of 275 mi (440 km). On arrival, coal is centrifuqed at 1000 G, and 75 percent of the water is removed before the coal is introduced into the boilers. Such a system could combine transportation with a deqree of cleaninq and separation of undesired constituents. However, such a separation miqht result in dissolution of undesired constituents and increased water-disposal problems. Samples of the coal slurry water obtained at the pipeline terminus were found to be hiqhly acidic, with an averaqe pH of 3.3 for six samples. The acidic condition was attributed to the presence of pyritic materials in the coal. Slurry water was also elevated in total soluble salt content, with an electrical conductivity of 3.1 mmho/cm, correspondinq to approximately 2000 ppm of total dissolved solids. Calcium, sodium, and maqnesium were the dominant cationic species contributinq to the salinity of the slurry water (A. L. Paqe, University of California, Riverside, personal communication, September 1978). Coal preparation plant wastes and reduction of sulfur and trace elements have been studied by the National Research Council (NRC) Committee on Accessory Elements' Coal Panel (1979). Because of this, further discussion on coal preparation will be limited. However, the slurries or residues remaininq from coal-cleaninq processes should be site-specifically analyzed and monitored to control the concentration of trace elements oriqinatinq from this process. STORAGE AND TRANSPORT Accordinq to Gleason (1976), dust has been one of the major problems encountered with the stockpilinq, loadinq, and unloadinq of western coals. Control was implemented at the Montana facilities throuqh the use of paved roads. Waterinq trucks were equipped to spray unpaved haul routes and storaqe piles. In some instances, a surfactant was added to the water, and about once a week a crustinq aqent was applied to the
59 inactive parts of the work yard or to the sides of coal storage piles to control dust and inhibit rainwater percolation. Ehrlich (1976) also reported that the handling and storage of some western coals (mainly lignite and subbituminous coal) present a problem in that the coals are friable and may spontaneously ignite. Prevention of spontaneous combustion in these coals requires that they be either compacted, intermittently agitated, or stirred during storage in stockpiles or during rail transport and that their moisture content be carefully monitored. The Committee on Environment and Public Planning (Swanson et al., 1974) of the Geological Society of America has noted the need to control coal dust during handling and storage to prevent dust explosion and spontaneous combustion hazards. They also recognized that trace metals may be leached or redistributed from coal storage areas, resulting in localized elevated levels of trace-metal concentrations. Coal and other fugitive dust emissions have been studied by Amick et al. (1974). They suggest that a total emission inventory must be used to ascertain impacts, to provide baseline data, and to predict effectiveness of long-range control strategies in meeting future air- quality needs. According to Ross (1977), elevated trace-element concentrations have been found associated with soils surrounding coal storage or other areas susceptible to blowing dust near railroad transportation areas. In this literature review concerning trace-element entry into the soil environment, such factors as aerial deposition, pH, fallout during rain, and weathering are cited as topics of concern. More definitive trace-element research has been performed on the contaminants that originate when coal is processed (Wewerka et al., 1976a). Although the emphasis was on coal refuse dumps, the results might have application to coal stored on ground facilities or to blowing materials from rail or haul roads. Another rather extensive review on the chemistry and behavior of trace elements in coal preparation wastes was performed for the U.S. Environmental Protection Agency and the U.S. Energy Research and Development Administration (now the Department of Energy) (Wewerka et al., 1976b). It was assumed that most of the knowledge concerning trace elements and minerals in raw coal could be logically applied directly to coal wastes. It was also stated that sufficient data exist to show that harmful or toxic quantities of elements (e.g., manganese, cobalt, nickel, and zinc) might be released to the environment in the coal preparation processes. Reported data and literature suggest that those trace elements that might be a problem in transportation could be largely controlled or removed by the coal washing or preparation process. The major environmental contribution noted has been leachates through or around coal storage piles exposed to weathering. Rainwater percolating through coal piles may cause local soil contamination depending on the physics and geochemistry of the affected soil. Any water draining from coal storage areas would have to be treated in the same way as drainage water from surface mining operations. Coal dust seems to be the major pathway for the transport of trace elements blown from open trucks, railroad gondolas, or barges onto
60 surrounding soil or water adjacent to the haul routes. The limited literature on this subject considers it as a physical problem that could be easily controlled by covering the transport carriers, dampening with water, or coating with a chemical film to control removal by the wind or by rain percolation. Pipeline coal slurry transport was not a concern, as the worst problem encountered might be an accidental spill or leakage that would cause no more than a short-term water-pollution situation (by slurry contamination) rather than a long-term cumulative transport problem for trace elements. Because there is little literature, data, or information concerning trace elements in transport or storage, it appears likely that these procedures make no significant contribution to the distribution of toxic trace elements in the environment. Coal is a stable, rather safely transported fuel that lacks many of the dangers or problems of leakage, toxicity, explosiveness, flammability, or similar problems encountered with other fuels, such as oil and natural gas. It is doubtful that the storage or transport methods currently in use in the United States will present any severe hazards, because most existing difficulties could be rather easily controlled by feasible and economical methods of wetting, covering, or otherwise limiting potential water-leachate or blowing-dust problems. HEALTH EFFECTS Occupational Health Occupational health impacts from trace metals mobilized during coal transport are unknown, but worker exposure might occur through inhalation of coal dust during loading and unloading of the coal. Estimates of coal dust lost during transport range up to roughly 1 percent of the total tonnage (Council on Environmental Quality, 1973). Losses are from fugitive emissions during loading and unloading and windblown coal dust during transit. Losses vary with type of coal, method of loading and unloading, condition of rail cars, moisture and content of the fines, speed of the train, and wind speed. These are uncontrolled emission estimates, however. Several methods are available to reduce fugitive dust emissions. These include negative pressure hoods over loading operations, wind guards and covers for gondola cars, and chemically sealing or dust-proofing the surface of each load (Szabo, 1978). However, the extent to which use of such methods reduces total emissions is unknown. Thermal drying of coal can produce large amounts of dust, and workers at mechanical coal-cleaning plants with thermal drying equipment could be affected. Most thermal drying of coal is done in the Appalachian region, although only 13 percent of coal mechanically cleaned nationwide in 1975 was thermally dried. West Virginia (with 51 percent of the nation's thermal drying plants) accounted for 45 percent of the total, and Virginia (with 11 percent of these plants) accounted for 11 percent (U.S. Bureau of Mines, 1977, p. 438).
61 Public Health Transport by Unit Train, Barqe, and Truck Trace metals are emitted durinq coal transport throuqh diesel-fuel combustion and from windblown coal particles. The larqe reqional differences in volumes of coal transport result in order-of-maqnitude or qreater differences in emissions of all metals in reqions east of the Mississippi River compared with reqions west of the Mississippi (see Appendix C). Unit trains transport more tonnaqe nationally than either barqes or trucks (Appendix A), thereby accountinq for the larqest share of the emissions. In Northern Appalachia, barqe and truck emissions combined are qreater than unit-train emissions, but unit-train emissions constitute over 50 percent of emissions from coal transport in the Central Appalachia and Central reqions. In the Northwest and Southwest reqions, trains are the primary mode of coal transport; barqe and truck contributions to air pollution are neqliqible by comparison. Public health impacts of trace metals mobilized durinq coal transport are not known. Emissions from low-level sources will be mobilized over shorter distances, with risk of increased exposure for only relatively small populations. Reqional emissions ranqe from 10 ton/yr to 10-3 ton/yr for various metals (Appendix C). Table 14 indicates that arsenic may be mobilized to air in rouqhly equal amounts by the coal combustion process and by transportation. Arsenic in coal is associated with a mineral phase, in pyrite or possibly arsenopyrite (Swaine, 1977), which has physical and chemical properties that are different from the elemental, oxide, and sulfate forms found in combustion products (Bertine and Goldberq, 1971; Davison et al., 1974). Transport by Slurry Pipelines Populations livinq near slurry pipeline routes could be exposed to pollutants mobilized by accidental slurry release, slurry storaqe containment failure, fuqitive dust, or power-use emissions (Mininq Information Services, 1977). The probabilities of such exposures occurrinq and resultant effects have not been quantified and would be confined to local areas. Coal Cleaninq Trace metals mobilized in air and water from effluents of coal preparation constitute potential public health problems (Wewerka et al., 1976b). The effluents from coal preparation and processinq include rock, mineral impurities, and other noncoal solids (known qenerally as coal refuse) and an aqueous effluent ("blackwater"). Up to 2 tons of blackwater, containinq from 4 to 5 percent of suspended coal particles, can be qenerated per ton of coal washed. Blackwater is usually sent to a tailinqs pond, where the solids are allowed to settle and the clarified water recirculated. Washinq coal with water removes dust and fine orqanic material adherinq to the coal.
r> u TABLE 14 Ratios of Amounts of Trace Elements and Iron Mobilized into Air by Steam-Electric Plant Coal Combustion to Amount Mobilized by Coal Supply Process in 1975, by Metal and Regiona,b Re ion Metal Appalachia Central Northwest Southwest Range of Magnitudes Arsenic 2 1 0.3 0 5 0.1-1 Beryllium 20 40 10 600 10-100 Cadmium 300 3,000 20 7 1-1,000 Chromium 100 300 70 300 10-100 Copper 9 20 9 40 1-10 0\ Iron 10,000,000 40,000,000 300,000 80,000,000 lOO,o02-10,ooo,ooo "' Lead 100 500 20 700 10-100 Mercury 4,000 2,000 60,000 2,000 1,000-10,000 Nickel 500 400 50 200 10-100 Vanadium 70 60 1,000 200 10-1,000 Zinc 60 60 800 300 10-100 asource: Appendix C. bincludes extraction (underground, strip, and auger) and transportation (unit train, barge, truck). Air emissions from preparation were reported as being zero. i
63 Density separation uses media with different densities to separate the heavier particles. Seventy-five percent of the total conunercial production of washed coal used density separation methods, which reduce the concentration of certain trace elements in the washed coal. Aluminum, antimony, arsenic, calcium, cadmium, cobalt, copper, gallium, iron, lead, manganese, mercury, molybdenum, nickel, potassium, selenium, titanium, zinc, and zirconium are susceptible to removal from coal by density.separation methods (Wewerka et al., 1976b). Solid wastes from coal cleaning and processing, an overall average of 23 percent by weight of the mined coal, concentrate the components to constitute the most potent source of AMD in the coal-fuel cycle. All such wastes should be returned to the mining sites for suitable disposal to prevent leaching and other forms of escape into the active environment. The finely divided and well-exposed state of coal refuse enables acid-producing and weathering processes to work more effectively. Thirty percent of AMD from abandoned or neglected mines has been attributed to coal-waste materials. There are 3000 to 5000 abandoned coal-waste piles in the United States. Typically, 1.5 to 2.0 pounds of acid (H2 S04 ) and 0.5 to 0.7 pounds of soluble iron are produced per acre of refuse per day, but rates of more than 300 pounds of acid per acre per day have been reported from highly mineralized areas. As with AMD from mines, water supplies are affected, but the direct impact on public health is not known (NRC Study Conunittee to Assess the Feasibility of Returning Underground Coal Mine Wastes to the Mined-Out Areas, 1975). Ignition of Coal-Waste Piles In the late 1960's, an estimated 300 to 500 coal-waste piles were burning, as a result of intentional ignition or by spontaneous heat generated by reactions within the pile. Air pollution from burning refuse piles is highly concentrated in the vicinity of the sources and can have considerable local impact. Many trace elements are transported from the hot zone by the generated vapors and deposited in cooler surrounding areas. Large amounts of the major pollutants--sulfur oxides, nitrogen oxides, hydrocarbons, carbon monoxide, and particulates--are also emitted from the heaps (Wewerka et al., 1976a). Burning piles are generally located near small conununities. In one survey of 292 burning piles, 45 percent were within less than 1 mile of a community of 200 or more people. Nine percent were in areas populated by 10,000 to more than 100,000 people (McNay, 1971). The public health impact of trace elements in air pollution from burning refuse heaps is unknown. Whether or not the air-quality standards are exceeded by burning coal-waste piles, the same emission regulations apply as for any uncontrolled burning, regardless of whether started by spontaneous combustion or by other circumstances.
