Acid Mine Drainage And Its Control Measures

Submitted By:

Sachchidanand Singh

109MN0504

Email Id: sachchi1991@gmail.com

Introduction: What is Acid Mine Drainage (AMD)?

• Highly acidic water with elevated levels of dissolved metals.

• Drainage from surface or deep coal or metal mines and coal refuse piles.

• An important environmental issue in many areas where mining has taken place.

Sources of Acid Mine Drainage

• Mining of gold, silver, copper, iron, zinc, lead (or combined metals), and coal

– During exploration, operation, and closure of mine, from the mine’s:

• dewatering system

• tailings disposal

• waste heaps

Process of Acid Mine Drainage

• Geochemical and microbial reactions during weathering of sulfide minerals (pyrite) in coal, refuse, or mine overburden– Oxidation of sulfide minerals in the presence of

air, water, and bacteria– Formation of sulfuric acid and increase in

acidity– Solubilization of metals due to low pH

Effects of Acid Mine Drainage

• Water resources– Increased acidity– Depleted oxygen– Increased weathering of minerals release of

heavy metals/toxic elements into stream

– Precipitation of Fe(OH)3 bright orange color of water and rocks

Effects of AMD (cont’d)

• Biological resources– Low pH and oxygen content water

unsuitable for aquatic life

– Precipitation of Fe(OH)3

• Increased turbidity and decreased photosynthesis

• Clogging of interstitial pore space in coars aquatic substrate habitat

Effects of AMD (cont’d)

• Biological resources– Elimination of aquatic plants change in

channel hydraulics– Stress on other biota associated with aquatic

habitats

• Human resources– Corrosion of pipes, pumps, bridges, etc.– Degradation of drinking water supplies– Harm to fisheries

Chemistry of Acid Mine Drainage

Reaction 12FeS2 + 7O2 + 2H2O 4Fe 2+ + 4SO4 + 4H+

• weathering of pyrite in the presence of oxygen and water to produce iron(II), sulfate, and hydrogen ions

Reaction 24Fe2+ + 7O2 + 2H2O 4Fe3+ + 2H2O• oxidation of Fe(II) to Fe(III)

• rate determining step

Chemistry of AMD (cont’d)

Reaction 32Fe3+ + 12H2O 4Fe(OH)3 + 12H+

• hydrolysis of Fe(III)

• precipitation of iron(III) hydroxide if pH > 3.5

Reaction 4FeS2 + 14Fe3+ + 8H2O 15Fe2+ + 2SO4

2- + 16H+

• oxidation of additional pyrite (from steps 1 and 2) by Fe(III) -- here iron is the oxidizing agent, not oxygen

• cyclic and self-propagating step

Chemistry of AMD (cont’d)

Overall Reaction4FeS2 + 15O2 + 14H2O 4Fe(OH)3 + 8H2SO4

Treatment

• Chemical Treatment

1. Calcium Oxide

2. Anhydrous Ammonia

• Passive Treatment

1. Wetlands

2. Open Limestone channel

Calcium Oxide

• Its Alkaline in nature

• Addition of calcium oxide increase the PH level of AMD.

• Cause many of the metals present in solution to precipitate as hydroxides and carbonates.

• It has the lowest material cost and is the safest and easiest to handle.

Anhydrous Ammonia• In the gaseous state, ammonia is extremely

soluble and reacts rapidly.

• It behaves as a strong base and can easily raise the pH of receiving water.

• Injection of ammonia into AMD is one of the quickest ways to raise water pH.

• It should be injected into flowing water at the entrance of the pond to ensure good mixing because ammonia is lighter than water.

Wetlands• Wetlands have several functions that aid in

the removal of metals in drainage

• It acts on filtering mechanism of the dense plant root system which catches any of the suspended solid and flocculated particles as they pass through the wetland.

• There are two types of wetland which are used: 1. Aerobic wetlands 2. Anaerobic wetlands

• Aerobic wetlands are shallow (1- to 3-foot deep) ponds; they filled with soil or organic matter. They facilitate natural oxidation of the metals and precipitate iron, manganese, and other metals.

• Anaerobic wetlands are shallow ponds filled with organic matter, such as compost, and underlain by limestone gravel. It used to neutralize acidity and reduce metals to the sulfide form.

Open Limestone Channel

• Its an open, free-flowing channel lined with coarse limestone.

• These systems oxidize and precipitate metals and add alkalinity to the water.

The best method to treatThe best method to treatAMD is preventionAMD is prevention

This can be done by using proper reclamation methods, which prevents air and/or water from reaching the pyritic materials.

References

1. Acid mine drainage remediation options: a review D. Barrie Johnson*, Kevin B. Hallberg,School of Biological Sciences, University of Wales, Bangor LL57 2UW, UK

2. The Science of Treating Acid Mine Drainage and Smelter Effluents By Bernard Aubé, P. Eng., M.A.Sc. EnvirAubé 361 Aumais, Ste-Anne-de-Bellevue Québec, Canada, H9X 4A9

3. en.wikipedia.org/wiki/Acid_mine_drainage

4. www.wvu.edu/~agexten/landrec/water.htm

THANK YOU