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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 How to distinguish?. Characteristics. Increased acidity = decreased pH

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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 • How to distinguish?

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

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

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

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

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

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

  9. 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

  10. 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

  11. Passive Treatment

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

  13. Iron Mountain, California Active Treatment

  14. “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?

  15. 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

  16. Summitville • 1984 • Application for mining permit • 1985 • Large scale open pit gold mine • Cyanide leaching • 1986 • Construction. Problems. • HDPE liner ripped during placement, but not fixed.

  17. Summitville • 1987-1991: Heap Leach Pad • 73 acres • One pile >190’ • No outlet for water, only lost through ET • Snowfall underestimated, ET overestimated Now What?

  18. 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

  19. 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

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

  21. SUMMARY • 4 characteristics of AMD • Pyrite weathering (Fool’s gold) source of acidity in many AMD problems • “Old School” and “New School” mining • Cyanide heap leach technology • Summitville example

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