Acid mine drainage
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Acid Mine Drainage. Mining & the Environment. Mine overburden & waste soils (mine tailings) are waste products generated by the mining industry.

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Acid Mine Drainage

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Acid Mine Drainage


Mining & the Environment

  • Mine overburden & waste soils (mine tailings) are waste products generated by the mining industry.

  • When these tailings are exposed to the atmosphere, precipitation and ground or surface water, they can react with oxygen & water to generate products which affect the pH & heavy metal composition of soils & streams


Mine Tailings


Acid Mine Drainage

  • When mineral deposits containing sulfides are mined, they have the potential to produce acid mine drainage.

    • Coal, copper, gold, silver, zinc, lead & uranium

  • AMD is caused by the physical & chemical weathering of the common mineral pyrite (FeS2)


Pyrite

  • Physical weathering of pyrite is necessary to reduce the grain size of the mineral.

    • Miners often accelerated this process by grinding up ores and dumping the overburden in the mine tailings piles

  • When exposed to water & oxygen, pyrite forms sulfuric acid.


Oxidation of Pyrite

4FeS2(s) + 14O2(g) + 4H2O(l) 4Fe 2+(aq) + 8SO42-(aq) + 8H+

  • The ferrous & hydrogen ions are released into the waters that runoff from mine drainage tunnels or tailings piles.

  • The ferrous ions are oxidized to form ferric ions

    4Fe 2+(aq) + O2(g) + 4H+(aq) 4Fe3+(aq) + 2H2O(l)


Oxidation of Pyrite

  • The ferric ion hydrolyzes win water to form an insoluble yellow-orange precipitate called “yellow boy”.

    4Fe3+(aq) + 12H2O(l) 4Fe(OH)3(s) + 12 H+(aq)


AMD in the High Andes, Peru


AMD in Colorado


“Yellow boy” precipitation smothers aquatic plants and animals


4FeS2(s) + 14O2(g) + 4H2O(l) 4Fe 2+(aq) + 8SO42-(aq) + 8H+

4Fe 2+(aq) + O2(g) + 4H+(aq) 4Fe3+(aq) + 2H2O(l)

4Fe3+(aq) + 12H2O(l) 4Fe(OH)3(s) + 12 H+(aq)

4FeS2(s) + 15O2(g) + 14H2O(l) 4Fe(OH)3(s) + 8SO42-(aq) +16H+

smothers organisms living on the stream bottom


Microbial Influences

  • Abiotic oxidation of pyrite is slow.

  • The bacterial microbe Thiobacillus ferrooxidans catalyzes the oxidation of FeS2 to ferric ions and hydrogen ions


Microbial Influences

  • The pH of AMD can less than 3.

  • Other heavy metal ions (zinc, copper, lead, arsenic and manganese) are also soluble in acidic solution & are mobilized

  • Streams are often devoid of life for miles downstream of an AMD source


T. ferrooxidans

  • Acidophilic

    • capable of surviving at low pH’s

  • Autotrophic

    • obtains its carbon by fixing atmospheric CO2

Viewed by electron microscope

magnified 30,000 times


T. ferrooxidans

  • Obtains its energy by the oxidation of either iron or sulfur

    Fe 2+ + 0.25 O2 + H+ Fe 3+ + 0.5 H2O

    H2S + 2O2 SO4 2- + 2H+

    So + H2O + 1.5 O2 SO4 2- + 2H+

    S2O3 2- + H2O + 2O2 2SO4 2- + 2H+


T. ferrooxidans

  • T. ferrooxidans is generally assumed to be obligately aerobic, but under anaerobic conditions, it can be grown on elemental sulfur using ferric iron as an electron acceptor.

    S + 6Fe3+ + 4H2O H2SO4 + 6Fe 2+ + 6H+

    G=-314 KJ/mol


T. ferrooxidans

  • Important in bioleaching processes where anaerobic conditions exist

  • Can also obtain energy from oxidizing Cu+, Se2+, & from oxidation of Sb, U & Mo compounds

Red-orange color due to

production of Fe(III) as

T. ferrooxidans oxidizes Fe(II)


T. ferrooxidans

  • Experiments show that T. ferrooxidans accelerates extraction of copper from ores


Coal Mining and AMD

Upper Conemaugh River Basin, PA


A Little History

  • Nature bestowed Cambria & Somerset Counties, PA a mixed blessing with an abundance of coal & a topography which made it easy to extract

  • Five minable seams of coal provided the energy needed for the Industrial Revolution which made Johnstown one of the largest iron & steel production centers in the world


A Little History

  • The Cambria Iron Company (Andrew Carnegie’s first still mill) was located in Johnstown

  • It later grew into the largest integrated Steel Mill in the world (stretched 14 mi along the Conemaugh & Little Conemaugh Rivers

  • Steel mills used large amount of coal to make coke (fuel for the clast furnaces)


Types of Coal Mines

  • Drift or Slope Mines

    • driven into valley walls near level of coal

    • drain excess water encountered by gravity flow out the entry

  • Shaft Mines

    • pumps used to remove water

    • boreholes drilled to relieve water pressure


Types of Coal Mines

  • Surface Mines

    • uses draglines which can remove up to a depth of 200 ft in a single pass

    • miners left the overburden rock where it acid and metals into streams to add to the discharges from the abandoned deep mines


Water Flows

  • Underground mines may produce thousand gallon per minute flows

  • Strip mines produce less flow


Mine Drainage Wasteland

  • Iron mound precipitated from water discharging from a 300’ deep borehole.

  • Precipitate (up to 9 ft deep) has killed trees


Open Mine Entry

  • Water discharging from drift mine.

  • Discharges from these types of mines

    • 200-800 gpm

    • pH range 2.7-3.2

    • Metal concentrations:

      • 58mg/l Fe

      • 20.9 mg/l Mn

      • 55.4 mg/l Al


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