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Acid Mine Drainage

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Coal Mining and AMD. Upper Conemaugh River ... Shaft Mines. pumps used to remove water. boreholes drilled to relieve water pressure ... Mine Drainage Wasteland ... – PowerPoint PPT presentation

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Title: Acid Mine Drainage


1
Acid Mine Drainage
2
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

3
Mine Tailings
4
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)

5
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.

6
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)

7
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)

8
AMD in the High Andes, Peru
9
AMD in Colorado
10
Yellow boy precipitation smothers aquatic
plants and animals
11
  • 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

12
Microbial Influences
  • Abiotic oxidation of pyrite is slow.
  • The bacterial microbe Thiobacillus ferrooxidans
    catalyzes the oxidation of FeS2 to ferric ions
    and hydrogen ions

13
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

14
T. ferrooxidans
  • Acidophilic
  • capable of surviving at low pHs
  • Autotrophic
  • obtains its carbon by fixing atmospheric CO2

Viewed by electron microscope magnified 30,000
times
15
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

16
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

17
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)
18
T. ferrooxidans
  • Experiments show that T. ferrooxidans accelerates
    extraction of copper from ores

19
Coal Mining and AMD
  • Upper Conemaugh River Basin, PA

20
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21
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

22
A Little History
  • The Cambria Iron Company (Andrew Carnegies 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)

23
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

24
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

25
Water Flows
  • Underground mines may produce thousand gallon per
    minute flows
  • Strip mines produce less flow

26
Mine Drainage Wasteland
  • Iron mound precipitated from water discharging
    from a 300 deep borehole.
  • Precipitate (up to 9 ft deep) has killed trees

27
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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