Table of Contents
As discussed previously, the primary types of solid wastes generated by the mining industry are overburden and waste rock from surface mining, waste rock from underground mining, bulk tailings from metal-ore and non-metal mineral beneficiation and milling processes and refuse from coal preparation-plant processes. The methods commonly employed for disposal of these wastes in each of the industry segments (that is, coal mining and metal-ore and non-metal mineral mining) are described below.
Mining Coal Tailings Disposal
As has been pointed out earlier in this section, immense quantities of wastes are produced by coal mining activities. Many of the regulations governing these wastes have only recently been enacted, and the Surface Mining Control and Reclamation Act of 1977 (PL 9587) will be a major factor influencing coal mining waste management in the future. The discussion of coal mining waste disposal practices which follows is organized by waste type, since the practices commonly employed are a function of this factor.
Overburden
Overburden from surface coal mines consist of soils, gravels, shales, coaly shales, and other unconsolidated material and, occasionally, some bedrock which overlies the coal seam. Surface mining is conducted by two basic methods: contour strip mining and area mining. Contour strip mines are used in hilly or rolling country, such as occurs in the Appalachian coal mining region. Overburden excavations follow the contour of the coal outcrop along the hillside, resulting in long sinuous bands of strip-mined land surrounding the hill. Additional cuts are made into the hillside until the ratio of overburden to coal becomes economically unfavorable. Extraction of the coal may continue by auger mining methods. Generally, the undisturbed overburden then collapses into the empty auger holes, and some surface subsidence may occur.
Prior to strict regulations, disposal of overburden during contour mining consisted of leaving the spoil scattered over the natural slope below the cut. The resultant hillside shelf was bordered by a highwall on the uphill side and a steep slope on the downhill side, both of which were subject to erosion and oxidation. The Surface Mining Control and Reclamation Act of 1977 now specifically regulates this type of surface mining. As a result, standard contour strip mining methods are increasingly being modified to facilitate contour regrading to minimize overburden handling, and to contain overburden within the mined areas. The modified box-cut or pit-storage mining method is one such technique. Only the overburden removed from the first cut is disposed of downs lope. Overburden from the subsequent cut is deposited into the original cut, and this backfilling process is continued in a stepwise manner. Runoff diversion structures are often constructed ups lope of the mining area to minimize erosion.
Box-cut mining employing two cuts is another modified method designed to facilitate backfilling. The terrace created by this technique is frequently sloped toward the highwall; however, this condition is ameliorated by replacing topsoil and by revegetation.
The second type of strip mining, area mining, occurs extensively in relatively flat-lying lands of the Midwest and West. This method consists of digging successive parallel trenches, followed by backfilling each trench as the next one is dug. Typically, large areas of land are disturbed, and the distance from the first trench to the last is often a mile or more. After mining, this area is covered with ridges of overburden, which now must be reclaimed and returned to pre-disturbance contours as provided for by the Surface Mining Control and Reclamation Act of 1977.
It is interesting to note that, with the enactment of the Surface Mining Control and Reclamation Act of 1977, overburden is now regarded by many to be a resource rather than a waste, due to its use in reclamation. The Act also requires the segregation of topsoil from underlying strata during stripping. During subsequent reclamation, the topsoil is spread over the backfilled overburden. It is anticipated that this practice will minimize the effects of surface mining by providing a substrata for revegetation of mined land.
Waste Rock
Two major methods of underground mining of coal are employed. Room-and-pillar mining is the basic method used in the United States today; however, longwall mining (practiced widely in Europe) is being practiced with increasing frequency in the United States. Room-and-pillar mining may leave as much as 60 percent of the coal in supporting pillars after initial mining. Later, some of these pillars are removed as equipment retreats from the mined area. Long-wall mining permits increased recovery of coal by extracting coal along a single face which is much longer than used in room-and-pillar mining. Artificial roof support is provided, and advancement through the coal seam proceeds at a faster rate.
Much of the waste rock which results from underground mining arises during mine development. Very little waste rock results during actual mining, as nearly all the mined material is sent to either a screening plant (that is, tipple) or a washing plant, where waste is rejected as refuse. Waste rock which does result is typically disposed of in a surface dump site, located near the mine portal. In some instances, waste rock is disposed of underground in inactive areas of the mine. Surface-disposed waste rock is often used for construction purposes by a mining operation. These uses include construction of impoundment dikes, roads, haulways, and as fill, if required. Often, the surface location used as a waste-rock dump site is subsequently the site used for disposal of coal preparation-plant refuse.
Coal Preparation-Plant Refuse
Coal preparation occurs in three stages: 1 crushing and sizing, or basic cleaning; (2) hydraulic separation, or standard cleaning; and (3) dense (heavy)-medium separation, or complete cleaning. Flotation is an additional process that is increasingly being used during third-stage cleaning to increase recovery and cleaning of fine coal.
Plants which perform first-stage cleaning only (that is, screening) to size the coal are called tipples. Refuse at tipples results from hand cobbing of impurities from raw coal prior to screening. Water is not used in the cleaning process at tipples.
Water is used in second- and third-stage cleaning, and plant-employing these stages of cleaning are, therefore, referred to as washing plants. Second-stage cleaning (that is, hydraulic separation) is accomplished largely by jigs and tables to achieve gravity separation of coal from impurities. The impurities may or may not be dewatered prior to disposal as refuse.
Heavy-medium separation, or third-stage processing, is generally reserved for metallurgical coal. Sized coal is cleaned in the third stage by hydraulic transport of coal through a vessel into which magentite or sand is added to effect the desired fluid density. A fluid density intermediate between the densities of coal and non-coal material effects a stratification of material according to specific gravity. Non-coal material, or reject, separated in this manner is generally dewatered and disposed of as refuse.
A third-stage process which is gaining increasing use is flotation. This process allows increased recovery and cleaning of fine coal which previously has generally been discarded with refuse. The reject from flotation is typically thickened and hydraulically transported to an impoundment area.
Refuse from coal cleaning can generally be classified as either coarse or fine. Separation between the two is usually accepted as the number 4 sieve. At washing plants using second- or third-stage cleaning, coarse refuse typically constitutes 70 to 80 percent by weight of the total refuse produced. The disposal methods for coarse and fine refuse generally differ, since fine refuse is most often disposed of in slurry form, while coarse refuse is not.
Washing-plant refuse fines typically are hydraulically transported to ponds for sedimentation of solids and clarification of the water for reuse or discharge. Pond seepage-prevention measures are not extensively employed, and pond lining practices are not known in this industry. At operations where slurry ponds have large drainage areas above pond elevation, surface runoff diversion is often practiced to reduce the influx of water from this source.
A major advantage of fine-refuse disposal by submersion in a settling pond is that oxidation of pyritic material, with the resultant production of acid, is effectively reduced due to the much lower diffusion of oxygen in water than in air.
Past practice for coal washing-plant slurries generated at some strip-mining operations in the Midwest was disposal in successive rows of old spoil banks or, more commonly, in the last furrow of such mines. The principal environmental concern at such operations was seepage through the semipermeable spoil banks and subsequent contamination of ground water. For this reason, the placing of slurry ponds or other type of water impoundment on mine spoil is considered to be a poor practice and is now generally discouraged.
An alternative to pond disposal of washing-plant slurries is dewatering of the slurry at the preparation plant and subsequent disposal of the fine refuse with coarse refuse. This practice is gaining increasing use, as a greater number of washing plants now incorporate closed process-water loops. Slurry dewatering generally involves mechanical thickening and filtering.
A disposal alternative at underground mining operations is hydraulic backfilling of washing-plant refuse. This practice was reportedly first employed in 1864 at an anthracite mine in Pennsylvania. This practice has not gained widespread usage, however, due to unfavorable economics at currently operating mines. However, it has been concluded that the economics of underground disposal could improve in future mines specifically designed for this purpose.
The primary method of disposal of coarse preparation-plant refuse is land disposal in piles. In the past, environmental concerns were given little consideration, and the waste disposal site may have been adjacent to the preparation plant, over the nearest hillside, or in a stream bed. However, present-day state and federal laws governing refuse disposal generally prescribe more rigorous methods for disposal-site selection and waste-pile construction.
Several types of piles, such as valley-fill, side-hill, and waste-heap, are now commonly employed. Valley-fill piles are frequently located above a slurry pond or sedimentation-pond system. For pollution control, the valley is filled starting at the head and proceeding downslope. Refuse is disposed of in benches. Each bench is sloped to drain away from the next working bench, and finished benches are covered with soil as required for reclamation procedures. Ideally, the pile is constructed to a height such that drainage from the finished surface can be readily directed into the ponds, thereby eliminating the need to construct and maintain ditches along the side of the valley.
Side-hill refuse disposal is similar to the valley-fill method, except that a permanent ditch is constructed immediately above the upper limit of the pile. Both uphill natural drainage and surface drainage from the finished portion of the pile are routed to the ditch and drained away from the pile.
The waste-heap disposal configuration is largely dependent upon the acid-producing status of the waste; however, design of all waste heaps is cellular in nature. The object is to build and reclaim individual cells to minimize exposed surfaces and to reduce the water-pollution potential. Acid-producing waste heaps are generally more cellular, with much greater usage of poorly permeable material, such as clay, to isolate cells from water.
Refuse placement techniques are practiced to prevent particle size and material segregation during disposal. Coupled with prompt compaction of waste by effective compaction techniques, proper material placement minimizes exposure of waste to oxidizing forces and ensures that waste piles are resistant to ground-water infiltration long after active dumping is ceased. Waste blending is sometimes practiced to increase the impermeability of disposed waste.
Proper siting of refuse piles, construction of subdrains to maintain embankment stability and to reduce water in the pile’s foundation, diversion of natural runoff, and provisions for sedimentation of silt in runoff are examples of management techniques now being used in association with refuse disposal.
Subdrains are commonly required in the rolling countryside of the Appalachian region but are seldom needed in the less rugged topography of the Midwest. Proper construction of the subdrain system is critical. Where overlying refuse is acid-producing, the permeable portion of the system requires isolation from the refuse-pile leachate to prevent contamination of the ground water.
Diversion of runoff away from refuse piles is a commonly practiced method of reducing the overall pollutant loading to ground and surface water.
Siltation basins are often located downslope from refuse piles to minimize discharge of silt-laden runoff. The basins frequently serve other purposes, make-up water ponds , recirulating ponds, receiving basins for slurry-pond waste, or emergency slurry ponds. Siltation basins constructed at the toe of refuse piles are also used to collect refuse leachate and runoff for sedimentation, and for subsequent treatment when necessary.
Methods of sealing the surface of refuse piles to minimize the oxidation of pyritic material, and subsequent formation of acid drainage, have been investigated on an experimental basis. Sealants which have been investigated include sodium silicate, sodium silicate plus aluminate, and carbonate bonding. The long-term success of these sealants and the costs of their full-scale use are still being evaluated, and sealants are not used in the industry at present.
A number of projects have been conducted to investigate possible uses of coal refuse as a base material for parking-lot construction, as a fill material during highway construction, and as a Port land-cement ingredient. In some instances, refuse has been used for these or similar applications. A widespread demand for refuse in such applications has not developed, however, probably due to the marginal physical properties of refuse for such uses and to the acid-forming potential of some refuse.
A summary of waste disposal practices and the extent to which these practices are presently employed in the coal mining and coal preparation industry is presented in table 3-21.
Tailings Disposal of Mining Metal Ores & Industrial Minerals
Mining and ore-processing methods are similar in the metal-ore and non-metal mineral industries, as are waste disposal techniques. The wastes of interest are waste rock, overburden and ore-processing wastes (or tailings).
Federal environmental legislation which has had the greatest impact on the manner in which metal ore and non-metal mineral milling wastes (that is, tailings) are disposed has been the 1972 Water Pollution Control Act (amended by the Clean Water Act of 1977). Similarly, regulations promulgated under the authority of the Uranium Mill Tailings Radiation Control Act of 1978, as amended, will have a major impact are controlled and managed.
Overburden
Overburden is generated by surface mining activities. These include quarrying, open-pit, open-cut, opencast, stripping, placering, and dredging operations.
Overburden may be composed of soil, sand, clay, gravel, boulders, rock, and combinations thereof, as these materials are removed to expose the ore. Where consolidated rock lies over the ore, drilling and blasting are usually required to loosen the material before removal. In other cases, overburden is simply scraped away without blasting.
Present overburden stripping activities entail segregation of soil and subsoil from overburden where required by law. The soil material is either stockpiled or employed directly for reclamation projects. The major volume of non-soil overburden, however, is handled differently throughout the industry. If overburden material is suitable for construction, it may be used for building roads and tailing dams, especially in regions of high relief. If the overburden is wasted, it may be backfilled into mine excavations, deposited in dumps, or placed in storage piles for future disposal.
States with surface-mining reclamation laws usually require backfilling, recontouring, and reclamation of pits. However, adequate space is not always available for backfilling. Overburden swelling, mining technique used, and the age of the mine are factors which determine availability of space for backfilling.
Overburden originating from a single open pit mine may be segregated depending upon chemical or physical characteristics. The different fractions of overburden may be disposed of by methods appropriate to those characteristics. For example, at several surface uranium mines in Wyoming, two general types of overburden are generated; one type consists of strata removed during the stripping process and the other consists of overburden removed from the pit floor near the ore. The second type of material is likely to contain radioactive values much higher than those of the first type of overburden. Althouh most overburden is eventually backfilled into mined-out areas, the wastes containing significant quantities of radioactive materials are disposed in a more secure manner. These wastes are typically backfilled into the deepest part of the pit soon after they are mined to minimize their potential environmental impact.
Two techniques used for surface disposal of overburden and waste rock are pile and hillside dumping. Hillside dumping employs a local topographic feature as a dumping point, whereas pile dumping occurs without benefit of hillside slopes. These Jumps may not be contoured and reclaimed, depending upon company policy and state laws.
Waste Rock
Underground mining is conducted through adits or shafts by a variety of methods that include room-and-pillar, block caving, timbered-stope, open-stope, shrinkage-stope, sublevel-stope, and others. Waste removal is proportionately much less in underground than in surface mining, but it still requires surface waste disposal areas.
Historically, the underground mining method most often used has been some form of open stone. Generally, to reach the ore or mineral, a shaft is sunk near the ore body. Horizontal passages are cut from the shaft to the various depths necessary to reach the ore. The ore is then removed, hoisted to the surface, crushed, and concentrated. Waste rock may be retained in the mine and used for backfilling of mined-out stopes or may be crushed underground (primary crushing only) and transported to the surface for disposal.
The method typically employed for the surface disposal of waste rock includes dumping in hillside dumps or surge piles. Rock may subsequently be reclaimed from a surge pile and used for construction of roads, berms, and tailing dams.
Mill Tailings
Ore or mineral milling or concentration can be performed by a number of methods, such as gravity concentration, magnetic separation, electrostatic separation, flotation, and leaching. The object, in each case, is to separate the metal from the less valuable matrix rock, or gangue.
The valuable metal-bearing material produced during milling is called concentrate, while the undesirable waste material is called tailings. Since ores are usually crushed and ground before milling, both concentrate and tailings consist of finely ground particles. In the past, mill tailings have been discharged onto hillsides or directly into lakes or streams. These practices are now prohibited by state and federal laws.
Today, the universal disposal method for tailings is the tailing pond. Tailings are slurried with water and pumped to tailing ponds for settling. Frequently, tailings are cyclone-classified before deposition. Coarse sands are used to build-up the tailing dam, while the slimes are deposited inside the pond. The slimes provide some degree of sealing, which slows the rate of seepage from the pond. Many ponds are trenched at the base of the dam or have adjacent toe ponds to collect seepage. The seepage is either recycled, treated, or discharged .
More elaborate tailing-pond sealing methods include: clay liners, vertical clay sealing layers in tailing dams and synthetic pond liners. In the recent past, pond sealing has sometimes been employed in the uranium-ore segment of the industry but was not extensively employed elsewhere. Regulations promulgated under the authority of the Uranium Mill Tailings Radiation Control Act of 1978 now require that steps be taken at uranium mills to reduce seepage of toxic materials into groundwater to the maximum extent reasonably achievable. Such steps are noted to include the installation of Low permeability bottom liners, recycle of tailing solutions, dewatering of tailings, and other appropriate methods. It has been noted, however, that under these regulations the requirements for currently existing uranium tailing ponds will be determined on a site by site basis as obviouslv it would not be possible to line the bottom of an existing tailings impoundment.
An alternative tailing disposal technique practiced at deep underground mines is backfilling into mines.
Coarse tailing fractions are frequently used for construction purposes and as fill, particularly in areas where conventional construction materials are not readily available.
Uranium mill tailings are generally disposed of in tailing ponds; however, management of these wastes differs from tailings derived from other metal-ore operations. The Uranium Mill Tailing Radiation Control Act of 1978 regulates disposal of these wastes, as uranium tailings present special disposal problems. The presence of radionuclides and subsequent breakdown products (such as radium 22b and radon gas, respectively) creates additional potential for contamination of ground water and air space within the soil column.
Tailings disposed of in tailing ponds are generally kept submerged in water and undergo very little oxidation while in this state. As a result, tailing solids do not become subjected to weathering processes until after abandonment of the pond, which allows the pond to drain. In the past, reclamation of abandoned tailing ponds was seldom practiced by mining companies. At present, however, state laws in most mining states require some form of reclamation once a tailing pond has served its useful life.
A number of studies have been conducted to investigate possible commercial uses of mill tailings. The area most often explored has been potential construction applications. Tailings from uranium-ore processing have been used extensively in some areas of the West to construct buildings, etc. This subsequently has been determined to be highly undesirable, however, due to radiation from radioactive components contained in these tailings.
A summary of the solid waste disposal practices employed in the metal-ore and non-metal mineral mining industries is presented in table 3-22.