1. Special Topic in Mining Engineering-II (MN392)
Assignment-1
Issues of Acid Mine Drainage and Its Remedial Measures
Submitted by
Satyabrata Nayak
113MN0487
2. Acid mine drainage treatment,
East Rand, South Africa
AMD waters in the Rio Tinto, Spain
3. Introduction:
• Acid Mine Drainage to the flow of water out of a mine that has a very high acidic(low pH) after being in contact
with air and metal.
• It results from the oxidation of sulfide minerals inherent in some ore bodies and the surrounding rocks.
• Iron sulfide minerals, especially pyrite (FeS2), chalcopyrite (FeS.CuS) and also pyrrhotine(FeS) contribute the most
to formation of Acid Mine Drainage.
• Oxygen (from air or dissolved oxygen) and water (as vapor or liquid) which contact the sulfide minerals directly
cause chemical oxidation reactions which result in the production of sulfuric acid.
Chemistry of Acid Mine Drainage:
Reaction 1
2FeS2 + 7O2 + 2H2O 4Fe 2+ + 4SO4 + 4H+
• Weathering of pyrite in the presence of oxygen and water to produce iron(II), sulfate, and hydrogen ions
4. Reaction 2
4Fe2+ + 7O2 + 2H2O 4Fe3+ + 2H2O
• Oxidation of Fe(II) to Fe(III)
• Rate determining step
Reaction 3
2Fe3+ + 12H2O 4Fe(OH)3 + 12H+
• Hydrolysis of Fe(III)
• Precipitation of iron(III) hydroxide if pH > 3.5
Reaction 4
FeS2 + 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
6. 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
9. 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
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 coarse aquatic substrate habitat
• 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
10. Acid mine drainage: The toxic legacy of gold
mining in South Africa
Baia Mare Gold Mine, was partly owned by the
Australian company
11. Case Study:
Mining near the Iron Mountain Mine in California
began in the 1860's. In 1963 the mine closed, and in
1983 was designated an EPA Superfund site. Water
passing through the mine site has resulted in periodic
fish kills of migrating salmon since at least the 1940's.
The site requires expensive active maintenance in
order to prevent it from also contaminating the
drinking water of nearby communities. The high
levels of acidity and toxic metals have sterilized large
portions of nearby creeks. As with all acid mine
drainage sites, the rocks will remain capable of
generating sulfuric acid for an unknown number of
years, perhaps throughout the entire future human
habitation of the region.
Iron Mountain Mine in California
12. Remedial measures of acid mine drainage:
Lime neutralization:
• The most commonly used commercial process.
• A high-density sludge (HDS) process.
• A slurry of lime is dispersed into a tank containing acid mine drainage and recycled sludge to increase
water pH to about nine.
• At this pH, most toxic metals become insoluble and precipitate, aided by the presence of recycled sludge.
• Optionally, air may be introduced in this tank to oxidize iron and manganese and assist in their
precipitation.
• The resulting slurry is directed to a sludge-settling vessel, such as a clarifier. In that vessel, clean water
will overflow for release, whereas settled metal precipitates (sludge) will be recycled to the acid mine
drainage treatment tank, with a sludge-wasting side stream.
14. Calcium silicate neutralization:
A calcium silicate feedstock, made from processed steel slag, can also be used to neutralize active acidity
in AMD systems by removing free hydrogen ions from the bulk solution, thereby increasing pH.
As the silicate anion captures H+ ions (raising the pH), it forms monosilicic acid (H4SiO4), a neutral
solute.
Monosilicic acid remains in the bulk solution to play many roles in correcting the adverse effects of
acidic conditions.
In the bulk solution, the silicate anion is very active in neutralizing H+ cations in the soil solution.
While its mode-of-action is quite different from limestone, the ability of calcium silicate to neutralize
acid solutions is equivalent to limestone as evidenced by its CCE value of 90-100% and its relative
neutralizing value of 98%.
15. 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.
16. • 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.
Precipitation of metal sulfides:
Most base metals in acidic solution precipitate in contact with free sulfide, e.g. from H2S or NaHS. Solid-
liquid separation after reaction would produce a base metal-free effluent that can be discharged or further
treated to reduce sulfate and a metal sulfide concentrate with possible economic value.
17. • Case Study: Burleigh Tunnel, part of the Clear Creek/Central City Superfund Site,
Colorado:
• This site is located in Idaho Springs, Colorado in a narrow valley with very harsh cold winters and
limited sunlight year-round. This project was in operation for about 3 years before treatment failed for a
variety of reasons and was decommissioned.
• The water exiting the Tunnel is roughly neutral with a pH of 6.5, with discharge averaging 60 gallons per
minute, elevated concentrations of bicarbonate buffer the mine water, and zinc is the metal of most
concern.
• The pilot system installed is described as two “anaerobic compost wetlands in both upflow and
downflow configurations,” they were not designed to treat the entire flow, but only one-fourth, or 15
gpm - approximately 7.5 gpm in each cell.
• Each wetland was a 0.05-acre filled four feet deep with a mixture of an organic-rich compost (96
percent) and alfalfa hay (4 percent). The cells were installed below grade to reduce freezing and the
earthen side walls were bermed.
18. Other Methods:
Surface water diversion
Soil compaction
Dry covers
Covers with sludge
Sealing with clay
Handling tailings
Application of chemicals
19. The best method to treat AMD is prevention:
This can be done by using proper reclamation methods, which prevents air and/or water from
reaching the pyritic material
This can be done by using proper reclamation methods, which prevents air and/or water from reaching
the pyritic materials.
20. Conclusion:
Acid mine drainage (AMD) greatly influences water quality and has high environmental and ecological
impacts. It is therefore required to solve this worldwide problem at the earliest opportunity. There are
several preventive techniques to avoid the generation of AMD, each of them effective for a different
situation. Among them, dry covers and covers with sludge are the more general ones, applicable to most
situations. Although it would be perfect to prevent the generation of AMD, many times it is not completely
possible, requiring corrective techniques to reduce or remove contamination from water. In this case, "in-
line systems" plants are the most effective solution, both in economic and recovery percentage aspects, in
contrast with highly effective but expensive techniques such as treatment plants by ion exchange of by
reverse osmosis. Using a systems approach, a number of new procedures have been developed to
successfully characterize, manage and rehabilitate AMD-generating mine sites and to protect surface and
ground waters from environmental dam- age. The importance of an interactive protocol with clear
management objectives and procedures is vital to successful rehabilitation of such sites and long-term
protection of the environment.
