In this article we will discuss about the methods and procedures used to control water pollution from coal and metal industry.
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The water pollution problems of coal mining have received great attention. Most of these water pollution problems are not completely unique to the coal industry. The mining of metals such as iron, lead, zinc and copper sometimes has acid drainage problems similar to those of coal.
Gold mining, phosphate mining, sand and gravel washing and other mineral dressing operations have the problem of suspended solids in their effluents. These suspended solids differ from coal’s problem principally in the colour of their effluent.
Wherever thermal energy is required in large quantities, coal is one fuel which is always considered and most frequently used to produce this energy. Coal is the most valuable mineral mined, and coal mining is the largest segment of the mining industry. As bituminous coal is the principal type mined, this article will be devoted primarily to water pollution control from the mining and processing of bituminous coal.
Coal Mines:
The water discharged from bituminous coal mines cannot be readily characterised. It is improper to call this water an industrial waste as it has not been used in the mining process. In fact, it is an unwanted and costly intruder to the mining operation and in an operating mine it must necessarily be handled and disposed of so that mining can continue.
Mine drainage also issues from many abandoned mines, exceeding in quantity and pollutional effect the drainage from active mines. Because water pollution from acid mine drainage is the largest pollution problem of the coal mining industry, it is proper to consider it here even though it is not an industrial waste in the usual sense.
The drainage from bituminous coal mines today may vary from highly acid waters to water of drinking quality, and good quality drainages are known even in normally acid regions. Acid mine drainage must be understood rather than characterised and usually prevented rather than treated.
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To understand acid mine drainage, a working knowledge of the geology, hydrology, chemistry and bacteriology involved is necessary. The acid constituents of almost all acid mine drainages originate from oxidative destruction of iron disulphide.
Iron disulphide in the crystalline form of pyrite is frequently found in the thin layers or partings between layers of coal in a coal bed. Additionally, pyrite may be found in the coal substance itself, both in the form of small nodules dispersed throughout the coal and in large masses or lens- shaped concretions occurring at random in certain coal seams. Pyrite is often found in layers of slate, sandstone and other rock overlying the coal.
The pyrite in and near coal seams may appear as bright, sparkling, yellow crystals characteristic of fool’s gold or as a gray-black, hard, pyritic mass. Early investigators, on the basis of appearance and oxidation rate, hypothesised that the iron sulphides in coal consisted of pyrite, marcasite and pyrrhotite.
However, pyrrhotite is iron monosulphide, FeS and chemical analyses quickly demonstrated that the mineral was a disulphide compound. The pyrite crystal is isometric while marcasite is orthorhombic. X-ray diffraction patterns of these two crystals are easily distinguished and studies of the iron disulphide in several coal seams have readily identified the presence of pyrite, whereas marcasite has not been found.
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It is concluded that the iron disulphide mineral associated with and near coal seams is pyrite rather than marcasite. The high rate of oxidation of this material is presumed to result from the small particle size and the intimate association with carbonaceous materials.
Pyrite reacts with dry atmospheric oxygen to form ferrous sulphate and sulphur dioxide. In the presence of moisture the reaction proceeds more rapidly and ferrous sulphate and sulphuric acid are formed. When the pyritic material is exposed to atmospheric oxygen under wet conditions, bacterial action also may take place.
The bacterial action may somewhat accelerate the rate of oxidation of pyrite; however, the degree of acceleration is a subject of controversy among the various researchers who have investigated it. The acidity of mine drainage is a function of the degree of contact between flowing water and oxidised acid-producing materials and the amount of alkaline material which the water may contact and dissolve either before or after its contact with acid-producing materials.
The pH of mine drainage cannot be correlated with the acidity of this drainage and hence is not a suitable yardstick for determining the effect which a given mine drainage might have when discharged into a particular stream.
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The lack of correlation between pH and acidity is often confusing and has led many people, both mine operators and water pollution control officials, to erroneous conclusions about the quality of mine drainage water.
The lack of correlation between pH and acidity, however, can be understood if we remember that mine drainage is a mixture of unstable chemical compounds that are continuously changing under ambient conditions.
Some of these unstable compounds include unreacted alkali materials, ferrous sulphate, ferric sulphate, a variety of ferric hydroxide sulphate complexes and sulphuric acid that may be occluded and mechanically held in the precipitates.
Most acid mine drainages contain large amounts of sulphate, calcium, magnesium and iron and may also contain aluminium, manganese and other heavy metals. The actual amount of each component and the ratio between components vary over extremely wide ranges, depending upon the peculiarities of each specific location.
An average acid mine discharge would contain perhaps 500-1000 mg/l acidity as CaCO3, 100-300 mg/l iron, approximately 2000 mg/l sulphate and total dissolved solids of approximately 3000 mg/l. The pH might vary from about 6.0 down to 2.8. Acid mine drainages rarely contain a high concentration of chloride and almost never contain oxidisable organic materials.
Water discharged from the preparation of coal is not greatly dissimilar to the water discharged from sand and gravel washing or other mineral preparation operations. The principal water pollutant in these waste-waters is suspended solids.
Additionally, the water may contain an increased but not usually troublesome amount of dissolved inorganic solids such as calcium or magnesium sulphates and iron. Waste-water from coal preparation plants is sometimes acidic in character, but usually it is deliberately kept alkaline to minimise corrosion of the processing equipment.
This water has a negligible content of oxidisable organic materials. Historically, chlorides were at one time dissolved in the water of certain preparation processes to give the solution a high specific gravity and to assist in the separation of extraneous mineral matter from coal. These processes have largely disappeared and chlorides are rarely a problem in waste-waters from coal preparation plants.
Waste Disposal and Pollution Prevention:
The process through which acid mine drainage is formed and discharged to cause water pollution may be illustrated as a 6-link chain. Figure 18.1 shows this chain and the part each link plays in the formation and discharge of acid mine drainage. Any treatment or abatement procedure intended to lessen the pollutional effect from acid mine drainage must necessarily break this chain at one or more of its links.
Transport Water:
The acid oxidation products of pyrite must be dissolved and transported to the streams before they can cause water pollution. If these products are not dissolved and carried away by flowing water, they remain in the mine and cause no pollution.
Control of water flow, in both underground and surface mining operations, is probably the most effective method available to minimise the amount of acid discharged from a mine. The principle, as stated for application to mining situations, is that the contact between water and acid-producing materials shall be minimised.
In surface mining, the contact between water and acid-producing materials can be minimised by diverting the flow of surface runoff and flowing stream waters around and away from the mining operation.
This may take the form of high wall diversion ditches, drains and conduits to carry flowing water through or around the mining operation and re-channelling or diverting of streams away from the mine. Contact between water which does gain entry to the mine and acid-forming materials can be minimised by removing this water from the mine as quickly as possible after it accumulates.
In underground mining, contact between flowing water and acid-producing materials can be minimised by sealing off the surface of the earth above the mine to close cracks, fissures, sink holes and other openings when they can be detected, by picking up water as close as possible to its points of entry in the mine and by conducting it through and out of the mine either in closed conduits or in ditches or sewers that prevent further contact of the water with acid-producing materials.
Acid mine drainage as it discharges from a mine is a complex solution of ferrous and ferric iron salts, calcium, magnesium, sometimes manganese and other sulphate salts. The flow of mine drainage may vary from only a few gallons per hour to millions of gallons per day, depending on the specific drainage point being considered.
In some of the drier areas of the country, where annual evaporation equals or exceeds annual rainfall, it is possible to impound low flows of acid drainage and hold them permanently in a pond so they do not flow into any stream. Particularly in surface mines this may be accomplished by proper handling of the overburden. No cases are known where this has proved practicable for underground mining.
Where extremely low flows of acid drainage are encountered, they can sometimes be neutralised through the use of lime or related alkalies. Because of the costs and other factors involved, neutralisation is usually uneconomical and completely impractical.
In those situations where it can be used, it is a difficult operation, especially on continuous flows.
Special equipment is required, which will compel intimate contact between the individual particles of alkali and the acid water. As acid mine water containing dissolved iron is neutralised, the iron precipitates and tends to coat individual particles of alkali, rendering unavailable and useless, the remaining alkaline material in the particle.
Under some conditions it may be practicable to neutralise small standing pools of acid mine drainage before these pools are discharged to the streams. Such neutralisation can be accomplished by the proper application of lime to the acid water. However, neutralisation again is difficult because of the coating effect of the precipitating iron hydroxide.
The suspended ferric hydroxide carried with acid mine drainage can sometimes be reduced by permitting the water to stand in a lagoon for many hours or days before releasing it to the stream. Additionally, lagooning of acid mine drainages that contain large amounts of ferrous sulphate may permit the sulphate to oxidise and deposit some of its iron content.
When these procedures are applicable and where space and property availability permit, they are considered and used. The precipitated and settled iron hydroxide progressively fills lagoons of this type and the sludge must be removed or a new pond constructed after prolonged use.
Stream pollution by coal mine drainage occurs only after the natural alkalinity of the receiving water has been used up and the concentration of undesirable elements has reached such a point that it renders the receiving waters unsuitable for their proposed downstream uses.
The ultimate downstream effect of mine drainage frequently can be minimised by discharging these waters as uniformly as possible from the mine. This may be accomplished through an elaborate system of pumping controls to hold the water level in the mine at a particular point and to discharge water from the mine in direct relationship to rate of inflow.
Additionally, surface impoundments can be built and controlled so that water pumped intermittently from a mine is released as uniformly as practicable to the receiving streams. Under these conditions the effect on the receiving stream is relatively uniform and the downstream effects are minimised.
Control of the discharge of black solids from a coal cleaning operation is quite similar to the control of suspended solids from any other mineral preparation operation. The black solid material is finely divided clay, black shale and other minerals, along with coal.
Normally, the first step in clarification of coal washery waste-waters is to concentrate the solid material through the use of hydraulic cyclones or thickeners. From these operations the clarified water is returned to the coal washing circuit while the high-solids water is further concentrated.
The underflow from the final clarification stage, usually a thickener, may be passed through filtration equipment such as a drum type continuous filter or more normally, may be pumped to a settling pond where the solids are permitted to settle.
The clarified water from the settling pond may be either returned to the preparation circuit or discharged into a receiving stream. As new preparation plants are designed and built, more and more effort is being expended to make these plants closed-circuit units so there is normally no process water discharge into the streams.
Most coal preparation plants have suspended solids removal equipment installed and operating; however, there are frequent complaints from water pollution control officials that this equipment does not perform satisfactorily.
Probably the most common problem other than normal maintenance is overloading of the clarification system so it cannot function properly. This condition could be overcome by designing and installing larger equipment and clarification systems are often unwisely designed at absolute minimum safety factors.
A more immediate and economic solution usually can be attained by properly balancing the water circuits in the coal washing operations so the minimum amount of water necessary for proper coal preparation is used and discharged to the clarification system.
Proper and constant attention to this detail may forestall the requirement of installing larger clarification equipment and may at the same time yield better water pollution control.
There have been specific instances where the fine coals recovered from washery water clarification have had economic value; in a few cases their value has been great enough to defray a considerable portion of the expense of water clarification.
Normally, however, the solids recovered from water clarification operations are of no economic value and must be disposed of by permanent impoundment, by disposal with other solid refuse where it will be permanently stored, or by disposal in specially constructed pits, so it does not again become waterborne and pollute the streams.
Metal Mining:
This article covers industrial waste-waters resulting from the removal of ores from the ground and from their subsequent treatment to produce an ore concentrate. Emphasis in this article is on those metallic ores that are mined in substantial amount in various parts of the world.
These include iron, copper, zinc, lead, molybdenum and uranium. Of less significance are metal mining operations involving aluminium, tungsten, vanadium, gold, manganese, magnesium, mercury and lithium. Vanadium concentrate is chiefly obtained as a by-product from uranium ore milling operations and may eventually be recovered in substantial quantity from wastes of the phosphorus industry.
The metal mining industry is not unique with respect to control of industrial waste-waters. There is similarity between the mining procedures and subsequent ore upgrading steps in coal mining, metal mining and nonmetal mining.
The metal mining industry produces substantial mine drainage, as does the coal mining industry and contends with a paucity of water in arid areas, as do many nonmetal mining industries. Most ore processing steps, require water.
Many of them produce waste-waters. Although some metal mining operations bring in water to alleviate dust, it is usual that water is drained or pumped from the mine or pit in order to avoid flooding. Often this results in an acid discharge from the operation.
Water is, of course, necessary in ore washing operations, in wet grinding, jigging, tabling, aqueous classification and chemical processing. It is axiomatic that water is also used for power generation, cooling and for sanitary purposes.
Although the metal mining industry requires much water, it also produces much water by virtue of its operations penetrating the water table. Moreover, the relative geographic isolation of the industry and the geologic circumstances which seem to concentrate its effort in water-scarce areas have combined to render the industry sensitive to its water problems.
Because of such factors, the industry is knowledgeable with respect to waste-water control, water recovery and water reuse and has been able to maintain its water resources ample for growth. Simultaneously, pollutional aspects of its waterborne wastes have been thoughtfully controlled.
The principal characterisation of metal mining waste-waters is their settleable solids. In various forms, as mud and slimes washed from the ore during processing, as gangue from wet gravity separation operations or froth flotation systems and as undissolved residues from chemical leaching procedures, these suspended solids represent the industry’s major waterborne waste.
This is not surprising when it is borne in mind that most domestic ores contain from 1 per cent to 10 per cent of the desired concentrate, hence the bulk of the ore fed to the mill must be discarded. Another waste, acid drainage from both active and abandoned metal mines, may, as a tonnage flow, dwarf the production of waterborne waste solids.
However, such mine flows are relatively dilute and their dissolved mineral content, converted to potential solids, is minor compared with the waterborne undissolved solids in the waste-water of the industry.
Another source of wastes is the wide variety of reagents used in froth flotation processes. Most of these reagents appear in the effluent from the mill. Some, such as slime depressors, adhere to the waterborne gangue; others remain entrained or dissolved in the process waters.
Finally, the metal mining industry utilises some, though relatively few, leaching operations, usually employing soda ash, caustic soda or acids, chiefly sulphuric. These reagents, or water-soluble products thereof, further add to the waste-water streams of the industry.
The pollutional potential of waste-waters from the metal mining industry is relatively low. There have been and are exceptions to this, such as waterborne wastes from cyanide leaching circuits or waterborne radio-activity from uranium milling operations.
However, cyanide leaching circuits are so few and so isolated as to preclude their further consideration. Water-borne radio-active waste discharged present some problems in uranium milling districts until prompt and decisive action on the part of all concerned corrected the situation.
Except for the relatively negligible input of sanitary wastes (where sanitary sewers enter mine and mill waste streams), wastes from the metal mining industry have no significant biochemical oxygen demand. Moreover, these wastes, on the average, have practically no toxicity.
This is because the processing steps in the industry consist essentially of water washing, gravity separations in a water medium, flotation using small dosages of nontoxic reagents and leaching involving either sodium alkalies or sulphuric acid; and because the potentially toxic metal ions are efficiently scavenged from wastes as part of the milling operation.
The principal control of waste effluents by the metal mining industry is usually accomplished by a settling pond or ponds. Few operating mills may be found without their nearby tailings ponds, settling ponds, impounding dams, lagooning areas or similar devices.
Sometimes these structures take advantage of the local topography, but more often they must be constructed by damming, excavating, diking or a combination of the three. In most metal mining localities, substantial land area is available and the use of much land area for waste disposal is fortunately both technically and economically feasible.
The common tailings pond serves many purposes. It is a primary settler for the gangue, a clarifier for the water, a treatment tank for pH adjustment or chemical precipitation as desired, a water storage area, a surge tank for controlling discharge into public waters and an investment storage because, in many instances, the mill tailings contain secondary values which can be reclaimed at a future time.
It is of particular significance that the reservoir of water in the pond (the pond being necessary per se to settle out solids) is, by virtue of its relatively low burden of dissolved process chemicals, at least a tempting and usually a vital source of water for reuse.
Depending upon the ratio of atmospheric evaporation to precipitation and the ground and effluent characteristics affecting seepage, many tailings ponds never overflow their confines and some may go nearly dry during certain periods of the year.
This is especially true in the arid and semi-arid areas of the country. On the other hand, this situation may be reversed in normal and heavy rainfall areas. In either situation, tailings ponds at least retain substantially all, if not all, of the waste solids from the metal mining operation.
In addition to tailings pond techniques, the industry utilises many innovations for industrial wastewater control primarily dictated by local circumstances. If the effluent to the tailings pond is alkaline, acid mine drainage may also be run into the tailings pond to effect a neutralisation and a better grade of water for reuse.
Conversely, when the mill effluent is acid and particularly when acid mine drainage is also entering the tailings pond, pulverised limestone or lime is frequently added to the effluent ahead of its discharge to the tailings pond to effect pH control of the clarified effluent in the settling area.
In some instances, acid effluent from copper mills containing dissolved copper, or copper-pregnant acid drainage from the mines, is first passed over scrap iron to precipitate the copper, while simultaneously dissolving iron.
The value of the recovered ‘cement’ copper is considerably greater than the cost of lime subsequently used for pH control and iron precipitation in the settling pond. Where flotation reagents, untreated sanitary sewage or other chemicals that might adversely affect the reuse of tailings pond waters are present, activated carbon may be added to the pond influent for the purpose of adsorbing undesired organic matter into the common precipitate.
The impounding of radio-active wastes has produced a problem as to legal responsibility when operations are abandoned and ownership of the property changes hands. Sometimes tailings pond waters are suitable for livestock watering, irrigation and sanitary facilities.
Possibly no other industrial waste control device produces so much reusable water as the tailings pond and at a price that the industry can afford. Conversely, probably no industry is better situated than the metal mining industry, in terms of geographic and technological considerations, to utilise tailings ponds to their fullest advantage.
Copper and uranium are the most significant of the metal mining industries in which pollution potential could be serious, as a result of dissolved substances, if control measures were not being practiced. Iron, zinc, lead, complex ore operations, molybdenum and other segments of the industry have no pollution problems as far as dissolved wastes are concerned and of course, solid tailings are impounded by practically all industry units.
The iron industry, by the very nature of its operations, adds little to its process water other than hardness. It uses water primarily for washing its ores and avidly reuses water from its settling ponds. In some plants, where a heavy medium of finely divided ferrosilicon is involved, a slight solubility of iron in plant water may result. However, clarified water from the iron mining industry can generally be discharged into public waters without detrimental effect.
In zinc production, the usual processing involves mining, crushing, grinding, flotation, thickening and filtering. This industry produces considerable mine drainage, a typical composition of which is 1450 mg/l Zn, 800 mg/l Fe and pH 2.3.
Water from the settling and impounding of zinc mill process tailings tends to be alkaline, so the combination of mine water and clarified tailings pond overflow results in nothing more than a hard water with a pH in the vicinity of 7.0.
The zinc content of drainage from both active and abandoned mines presents a challenging problem with respect to development of an economic process for zinc recovery, but is not considered to be a pollution problem. Essentially, the foregoing statements on the zinc industry apply also to those industries processing lead-zinc, copper-zinc and complex lead-zinc- silver-gold-copper ores.
Mine drainage from these operations has a pH near 7.0 and clarified mill process water from the tailings ponds generally ranges from neutral to slightly alkaline because of the slightly alkaline nature of the mill concentration systems usually employed.
It is not uncommon to find the waste-water from these operations in demand for irrigation and livestock watering, particularly in arid areas. There are a few abandoned mines in which ageing processes have been undisturbed for a long period and the sulphur bodies present have produced so much sulphuric acid that the drainage water has a pH as low as 2.5 or 3.0.
However, it is general practice not to pump water of this acidity; and when such water does overflow the mine, it is usually diluted to a harmless acid strength by the freshet producing the overflow.
The mining of gold, involving cyanide leaching, could beget a water pollution problem, were it not for the fact that gold operations are so limited and their locales so isolated. In one gold operation, using a cyanidation process, waste-water overflows the settling pond at a pH of approximately 9.0 and is almost immediately lost into the ground of the isolated area. In another plant, mill tailings are impounded in a dry mountain canyon, apparently capable of serving this purpose for some generations ahead.
Summarising the foregoing observations, it may be concluded that the mining industry enjoys relatively high standards of industrial waste-water control. This is largely the result of the inherent technology of the industry and the practical necessity of water reuse, but it is no less a tribute to the awareness of the industry of the value of protecting and respecting natural water resources.
An assessment of the long range aspects of industrial waste-water control in the metal mining industry must be based on the premises that the industry already has the situation well in hand, it is not an expanding industry as a whole, it is essentially wed to its ore reserves and metal mining industry processes (statistics notwithstanding) may well consume less rather than more water even though production of some metal concentrates does increase.