ACID MING

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Prediction, quantification and implementation of mitigation measures are the key enhancement of environmental standard.

Acid mine drainage (AMD) in general refers to the outflow of acidic water from metal mines or coal mines (including

abandoned mines). AMD is one of the most perpetual pollution problems which occur world-wide in the mining areas. It

refers to the distinctive type of wastewater that originates from the weathering and leaching of sulphide minerals present in

coal and metal ore bodies.

In fact, AMD from abandoned coal mines affects the quality of both groundwater and surface water. Drainage results from

various mining methods performed in the watershed. These methods include underground mining, strip mining, and auger

mining. The mining process exposes iron. AMD Formation AMD forms as a result of the dissolution of sulphides, mainly

pyrite (FeS2) and pyrrhotite (FeS) under oxidizing conditions in air and water. This oxidation releases H+ ions and lowers

the surrounding pH to acidic levels. Acidic drainage will subsequently leach additional metal ions from the adjacent rocks

and deposit them.

AMD is a problem because the vast majority of natural life is designed to live and survive at, or near, pH 7 (neutral). The

drainage acidifies the local watercourses and so either kills or limits the growth of the river ecology.

Mining operations lacking sufficient neutralizing carbonate minerals are at greatest risk of environmental degradation and

usually require engineering intervention to minimize the problem. Although prevention of AMD is the most desirable option, a

cost-effective prevention method is not yet available. The most effective method of control is to minimize penetration of air

and water through the waste pile using a cover, either wet (water) or dry (soil), which is placed over the waste pile.

Early diagnosis

espite their high cost, these covers cannot always completely stop the oxidation process and generation of AMD. Application

of more than one option might be required. Early diagnosis of the problem, identification of appropriate prevention/control

measures and implementation of these methods to the site would reduce the potential risk of AMD generation, AMD

prevention/control measures broadly include use of covers, control of the source, migration of AMD and treatment of

sulphide (pyrite) and unremoved coal contained in the sandstone overburden to air and water.

These oxidizing conditions result in an increase of acidity, which subsequently decreases the pH and increases the

concentrations of dissolved metals. These consequences lead to an overall degradation of water quality and the inability to

support aquatic life.

Mineral production is an important component of the economy for many countries, and in some cases it can be the major

source of international revenue. However, mining and mineral production operations that are not well managed can

contaminate groundwater and surface water in the form of AMD, and can adversely affect the health of nearby communities

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that rely on this source for drinking-water or agriculture. Extractive industries include mining of mineral deposits (principally

metal-bearing ores and coal deposits), oil and natural gas production and quarrying for building and road-making materials.

Water sources

Poorly operated or abandoned mine sites are often significant sources of water contamination; contaminants of particular

health concern from these sources include heavy metals and mineral-processing chemicals, such as cyanide. Water

pumped from abandoned mine shafts and open-cut pits is often used for water supply and is generally safe and reliable.

However, this water sources may sometimes be contaminated by mineral processing chemicals, acid mine drainage (AMD)

and waste disposal. These risks must be considered and assessed to determine whether such water sources are safe to be

used for drinking water supply.

Effects of Mine Drainage

Mine drainage is a complex of elements that interact to cause a variety of effects on aquatic life that are difficult to separate

into individual components. Toxicity is dependent on discharge volume, pH, total acidity, and concentration of dissolved

metals. pH is the most critical component, since the lower the pH, the more severe the potential effects of mine drainage on

aquatic life.

The overall effect of mine drainage is also dependent on the flow (dilution rate), pH, and alkalinity or buffering capacity of the

receiving stream. The higher the concentration of bicarbonate and carbonate ions in the receiving stream, the higher the

buffering capacity and the greater the protection of aquatic life from adverse effects of acid mine drainage. Alkaline mine

drainage with low concentrations of metals may have little discernible effect on receiving streams.

Acid mine drainage with elevated metals concentrations discharging into headwater streams or lightly buffered streams can

have a devastating effect on the aquatic life. Secondary effects such as increased carbon dioxide tensions, oxygen

reduction by the oxidation of metals, increased osmotic pressure from high concentrations of mineral salts and synergistic

effects of metal ions also contribute to toxicity. In addition to chemical effects of mine drainage, physical effects such as

increased turbidity from soil erosion, accumulation of coal fines and smothering of the stream substrate from precipitated

metal compounds may also occur.

Effects on the Biodiversity

Benthic (bottom-dwelling) macro invertebrates are often used as indicators of water quality because of their limited mobility,

relatively long residence times, and varying degrees of sensitivity to pollutants. Unaffected streams generally have a variety

of species with representatives of all insect orders. Like many other potential pollutants, mine drainage can cause a

reduction in the diversity and total numbers, or abundance, of macro invertebrates and changes in community structure.

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Moderate pollution eliminates the more sensitive species. Severely degraded conditions are characterized by dominance of

certain taxonomic representatives of pollution-tolerant organisms, such as earthworms (Tubificidae), midge larvae

(Chironomidae), alderfly larvae (Sialis), fishfly larvae (Nigronia), cranefly larvae (Tipula), caddisfly larvae (Ptilostomis), and

non-benthic insects like predaceous diving beetles (Dytiscidae) and water boatmen (Corixidae).

While these tolerant organisms may also be present in unpolluted streams, they dominate in impacted stream sections. pH -

Most organisms have a well defined range of pH tolerance. If the pH falls below the tolerance range, death will occur due to

respiratory or osmo-regulatory failure. Low pH causes a disturbance of the balance of sodium and chloride ions in the blood

of aquatic animals. At low pH, hydrogen ions may be taken into cells and sodium ions expelled. The primary causes of fish

death in acid waters are loss of sodium ions from the blood and loss of oxygen in the tissues.

Metals

Heavy metals can increase the toxicity of mine drainage and also act as metabolic poisons. Iron, aluminium, and

manganese are the most common heavy metals which can compound the adverse effects of mine drainage. Heavy metals

are generally less toxic at circumneutral pH. Trace metals such as zinc, cadmium, and copper, which may also be present in

mine drainage, are toxic at extremely low concentrations and may act synergistically to suppress algal growth and affect fish

and benthos.

Sedimentation

Drainage water from acid mine drainage is initially clear but turns a vivid orange colour as it becomes neutralised because of

the precipitation of iron oxides and hydroxides. This precipitate, often called ochre, is very fine and smothers the river bed

with a very fine silt. Thus, small animals that used to feed on the bottom of the stream or ocean (benthic organisms) can no

longer feed and so are depleted. Because these animals are at the bottom of the aquatic food chain, this has impacts higher

up the food chain into fish.

Mitigating the Problem of Acid Drainage

a. Acid Mine Drainage Remediation at the Source - A number of factors influence the development of acid mine drainage

directly at the source and along the pathway. Besides the neutralization potential based on the chemistry of the minerals, the

long term availability and the amount of reactants during the process of weathering are crucial for the quantity of water

getting access to the reactive phases and for the quality of the outflow. Reduction of the accessibility of the material and

limiting the amount of water and oxygen infiltration will have the strongest influence on AMD generation.

Besides technical remediation efforts, natural processes such as clogging of pores, formation of hardpans or cemented

layers, and generation of capillary barriers on the basis of grain size contrasts, are most effective to withdraw cells of the

heap from further weathering.

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b. AMD treatment - The formation and treatment of acid mine drainage is the biggest environmental problems relating to

mining and processing activities in the worldwide. Various methods are used for the sulphates and heavy metals removal

from acid mine drainage in the world.

There are two approaches to controlling Acid Mine Drainage. The first is to reduce or eliminate the source of the AMD. One

method for source elimination seeks to prevent oxidation by replacing the air within the mine with groundwater. This air-with-

water replacement is brought about by sealing any mine openings with an impermeable grouting material. One such material

under investigation is flue gas desulferization (FGD) material, a by-product from coal-fired power plants. This material is

composed of primarily calcium sulphate (gypsum). Another source elimination strategy is to fill the mine with a solid (e.g.,

FGD or a clay slurry) in order to eliminate the oxidation reaction.

The second primary method for mitigating Acid Mine Drainage involves treating the AMD itself in order to remove the

negative impact to the watershed. Chemical, biological, or physical treatments may be used in AMD abatement. Chemical

treatments primarily seek to neutralize the acid through the addition of an alkali (soda ash) with a subsequent sedimentation

basin in order to retain metal precipitates after the pH adjustment. Biological treatments use constructed wetlands, as one

example, for natural attenuation of biological nutrient additions in order to accelerate indigenous activity. Physical treatment

seeks to alleviate the impact through re-routing of streams to circumvent possible problematic geological formations.

In environments where groundwater has been contaminated by waste water and acid mine drainage, microbial sulphate

reduction can be exploited in subsurface permeable reactive barriers. A permeable reactive barrier is a passive, in-situ

technique: groundwater treatment proceeds within the aquifer and long-term maintenance of the installation is unnecessary.

This method consists of installing an appropriate reactive material into the aquifer, so that contaminated water flows through

the material (see figure below).

The reactive material induces chemical reactions that remove the contaminants from the water or otherwise cause a change

that decreases the toxicity of the contaminated water. For the treatment of water contaminated with acid mine drainage, a

number of studies have shown the effectiveness of this method.

Acid mine drainage is a global problem. Cleanup of abandoned mine sites is difficult because of their remote locations and

because these problems will go on for hundreds of years until the mineralized rock is leached free of sulphides and metals.

Typical cleanup strategies for addressing.

Conclusion

Of the huge amount of money spent on acid mine drainage each year, the major portion is spent on treatment. But treatment

is not the best solution to most acid mine drainage problems.

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Treatment has the disadvantage of being necessary for as long as the acid discharge continues and thus requires

manpower, surface facilities, and sludge disposal areas indefinitely.

Since acid drainage results from the oxidation of pyrite associated with coal and overburden strata, limiting the rate of pyrite

oxidation would reduce the amount of acid formed. T. Ferro oxidants normally catalyze the pyrite oxidation and accelerated

the initial acidification of freshly exposed coal and overburden. Inhibiting bacterial activity through the application of

bactericides, therefore, would limit the rate of acid production and, in combination with proper reclamation, would reduce

substantially the total amount of acid produced.

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