Flow of acidic water from the coal or metal mines is referred to as acid mine drainage. This phenomenon is also known as acid rock drainage. It is not necessary that only mining sites exhibit this phenomenon. Transportation corridors and construction sites are also the ones where it takes place. In some places, this phenomenon takes place naturally. Chemical weathering of rocks is the cause of this problem.
The process of sub-surface mining involves digging at great depths. This digging process often goes below the depth of the water table. It requires continuous removal of water to prevent flooding. Once the mining activity at a particular site is abandoned, and if the water is not drained out on a regular basis; the metal sulfides in the mine exposed to the accumulated water and air results in formation of acidic pools.
The condition of mine drainage affects the chemical elements in the soil through changes in pH and temperature levels to a great extent. The level of pH not only reaches zero, but also, at times, can be negative. Temperatures as high as 47°C can be reached. Negative pH results from evaporation of water from acidic pools, making the effluent more acidic by increasing the concentration of hydrogen ions.
Soils with high acidic content become inhospitable for plant growth. Mine drainage, which forms small streams, also affect the aquatic plants and animals adversely, even to the extent of wiping out the entire population of fish in the water body. If such a discharge is released into streams and rivers over which bridges have been constructed, the corrosive properties of the acidic water can even cause structural damage. This phenomenon also renders water useless for drinking purposes.
Treatment usually involves neutralizing the acidic content of the drainage by the introduction of agents that cause precipitation of the metal sulfides. The following measures are used in treating the problem of acid mine drainage effectively.
Lime Neutralization: It is the most commonly used method for treatment of mine drainage. In this treatment, lime slurry is added to a tank that contains mine drainage. The pH level is increased up to 9 with this treatment. In addition to the lime slurry, recycled sludge is also present in the tank. The sludge helps in precipitation of toxic metals that have turned insoluble at the pH of 9. The slurry obtained from this treatment is sent to the sludge settling vessel. Metal-precipitates from this slurry, being heavier, settle down, while clean water stays above and is removed. The metals which settle down can then be recycled and reused.
Metal Sulfide Precipitation: Free sulfides like NaHS, H2S, etc., are used to precipitate base metals in acidic solutions. After this reaction, the resulting solution is treated for the separation of solids and liquids. The effluent which is obtained after separation of solids and liquids is treated in order to reduce the sulfate. Use of biogenic sulfide as an alternative method to precipitate the metals is also used.
Carbonate Neutralization: This process is carried out by adding limestone chips to the site of acid mine drainage. Calcareous materials other than limestone are also used for neutralization of acid.
Hence, we can conclude that acid mine drainage has harmful effects on environment, and these acidic streams should be treated with the techniques given above.
*pH is a measure of acidity or basicity of an aqueous solution. Solutions which have pH less than 7 are acidic in nature, and solutions which have pH levels greater than 7 are basic in nature. pH of pure water is close to 7, and a strong acid such as hydrochloric acid (HCl) has a pH level close to 2.