Acid Mine Drainage

Flow of acidic water from the mines of coal or metals 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 acid mine drainage takes place. In some places, acid rock drainage takes place naturally. Chemical weathering of rocks is the cause of this problem.

What is Acid Mine Drainage?

The process of sub-surface mining involves digging out the earth surface at great depths. This digging process often goes below the depth of water table. It requires continuous removal of water to prevent flooding. Once the mining activity at a particular site is abandoned, the water is not drained out on a regular basis; this process leads to this condition.

Effects of Acid Mine Drainage

The condition of mine drainage affects the chemical elements in 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. A temperature of 47°C can be reached. Negative pH results from evaporation of water from acidic pools.

Yellow Boy is a condition where yellow-orange colored iron hydroxide is precipitated. Formation of iron hydroxide takes place if ions of iron come in contact with a neutralizing agent or freshwater. The pH which has earlier gone below 3, is then raised and iron hydroxide gets precipitated.

Treatment

The following treatments 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 is raised 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. Sludge settling vessel is a container, where slurry obtained from this treatment is sent. Metal precipitates from this slurry settle down while clean water stays above and is removed. The treatment tank is used to recycle the metals which settle down.

Metal Sulfide Precipitation: The free sulfides like NaHS, H2S, etc. are used to precipitate base metals in acidic solutions. After this reaction, 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: The carbonate neutralization 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.

The basic information starting from what is acid mine drainage to the treatment measures to be taken are presented above.