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Acid Mine Drainage

Acid Mine Drainage. Terms. Acid Mine Drainage (AMD) Water that is polluted from contact with mining activity Acid Rock Drainage (ARD) Natural rock drainage that is acidic Both produce acidic waters. Sources of Acid Mine Drainage (AMD). Mine Effluent. Mine Dump. Mill Tailings. pyrite

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Acid Mine Drainage

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  1. Acid Mine Drainage

  2. Terms • Acid Mine Drainage (AMD) • Water that is polluted from contact with mining activity • Acid Rock Drainage (ARD) • Natural rock drainage that is acidic • Both produce acidic waters

  3. Sources of Acid Mine Drainage(AMD) Mine Effluent

  4. Mine Dump Mill Tailings

  5. pyrite water + air low pH + metals AMD Chemistry • Pyrite weathering

  6. Acid Mine Drainage Water - from rain and snowmelt + Oxygen - from the air + Pyrite- from the mine Reaction =Sulfuric Acid

  7. AMD Chemistry Iron oxide 4FeS2 + 14 H2O + 15 O2 → 4Fe(OH)3 + 8 SO42- + 16 H+ Overall acid producing

  8. Fe2+ + 2SO4 +2H+ Reaction 1: FeS2(s) + H2O + 7/2O2 Fe(OH)3(s) + 3H+ Reaction 4: Fe3+ + 3H2O THE CHEMISTRY ofACID MINE DRAINAGE pyrite water sulfate acid Reaction 2:* Fe2+ + 1/4O2 +H+ Fe3+ + 1/2H2O Reaction 3: FeS2(s)+ 8H2O + 14Fe3+ 15Fe2+ + 2SO4 + 16H+ * catalyzed by bacteria

  9. AMD Chemistry • Surface area • more surface area, faster rate • smaller grains, more surface area

  10. Characteristics • Increased acidity = decreased pH • Increased metal concentrations • Increased sulfate • Increased suspended solids All four don’t necessarily occur at the same time

  11. Stream Effects Colored waters: • “Yellow boy” • Iron oxides, basically rusting the stream floor • White • Aluminum • Black • Manganese Determined by shifts in pH

  12. Extent of Problem • Colorado • 20,000+ mines • 1,300 miles of streams • Montana • 20,000+ mines • 1,000 miles of streams • Arizona • 80,000+ mines • 200 miles of streams

  13. Treatment • Active v. Passive • Active • physical addition of alkalinity to raise pH • High cost • effective • Passive • Naturally available energy sources • Little maintaince • Driven by volume

  14. Passive Treatment

  15. Active Treatment • Typical treatment processes (“ODAS”) • -oxidation • -dosing with alkali • -sedimentation

  16. Iron Mountain, California Active Treatment

  17. Shift in Mining Techniques • “Old school” • Abandoned mines • Tailings/waste rock piles • ARD • “New School” • Cyanide heap leach mining

  18. “New School” • Cyanide Heap Leach • Extract gold from low grade ore • Ore crushed, placed in open air leach pads • Cyanide sprayed on top • Leaches gold as migrates through ore • Solution drained, gold recovered • Pretty huh?

  19. Summitville, Colorado

  20. Summitville Mine • Rio Grande Headwaters • Elevation 12,800’ • Snowfall: 7-11 m/ year • Population: 700 • 112 stamping machines • Abandoned in early 1900s • Gold prices fell, diminishing returns, weather issues

  21. Summitville • 1984 • Application for mining permit • 1985 • Large scale open pit gold mine • Cyanide leaching • 1986 • Construction. Problems.

  22. Summitville • 1987-1991: Heap Leach Pad • 73 acres • One pile >190’ • No outlet for water

  23. Summitville • 1987-1991 cont • Permit to discharge excess water. Limits in concentrations • Could not meet limits • Fish kills downstream for 17 miles in Alamosa River

  24. Summitville • 1992 • EPA assumes control, $20,000,000 to ‘fix’ • Heap leach pad near overflow, discharging 3,000 gallons/minute through leaks • 200 million gallons of cyanide laced water • Not last till spring snowmelt

  25. Costs • To date: $185 million • Annually: $1.5 million • Taxpayers foot bill • Mine owner cost: $3 million bond

  26. Conclusions • Acid rock drainage is generated at mines and naturally where sulfide minerals are present and the buffering capacity of the water is exceeded. • AMD degradation can be acute because: • 1) Mines act as collectors of groundwater • 2) Water is in contact with high grade ore minerals • 3) Mine dumps and tailings provide dramatically increased surface areas for the interaction of water, oxygen, and sulfide minerals.

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