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 DumpMine Dump

Mill TailingsMill Tailings

AMD Chemistry

• Pyrite weatheringpyritewater

+ air

low pH+ metals

Acid Mine DrainageAcid Mine Drainage

WaterWater - from rain and snowmelt

++

OxygenOxygen - from the air

++

PyritePyrite - from the mine

Reaction =Reaction =Sulfuric AcidSulfuric Acid

AMD Chemistry

4FeS2 + 14 H2O + 15 O2 → 4Fe(OH)3 + 8 SO42- + 16 H+

Iron oxide

Overall acid producing

THE CHEMISTRY THE CHEMISTRY ofof

ACID MINE DRAINAGEACID MINE DRAINAGE

THE CHEMISTRY THE CHEMISTRY ofof

ACID MINE DRAINAGEACID MINE DRAINAGE

Reaction 1: FeS2(s) + H2O + 7/2O2 Fe2+ + 2SO4 + 2H2H++

Reaction 2:* Fe2+ + 1/4O2 + HH++Reaction 2:* Fe2+ + 1/4O2 + HH++ Fe3+ + 1/2H2O Fe3+ + 1/2H2O

Reaction 3: FeS2(s)+ 8H2O + 14Fe3+ 15Fe2+ + 2SO4 + 16H16H++

Reaction 4: Fe3+ + 3H2O Fe(OH)3(s) + 3H3H++

pyrite water sulfate acid

* catalyzed by bacteria

AMD Chemistry• Surface area

– more surface area, faster rate– smaller grains, more surface area

Characteristics

• Increased acidity = decreased pH

• Increased metal concentrations

• Increased sulfate

• Increased suspended solids

All four don’t necessarily occur at the same time

Stream Effects

Colored waters:• “Yellow boy”

– Iron oxides, basically rusting the stream floor

• White– Aluminum

• Black– Manganese

Determined by shifts in pH

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

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

Passive Treatment

Active TreatmentTypical treatment processes (“ODAS”)

-oxidation-dosing with alkali-sedimentation

Active Treatment

Iron Mountain, California

Shift in Mining Techniques

• “Old school”– Abandoned mines– Tailings/waste rock piles– ARD

• “New School”– Cyanide heap leach mining

“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?

Summitville, Colorado

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

Summitville

• 1984– Application for mining permit

• 1985– Large scale open pit gold mine– Cyanide leaching

• 1986 – Construction. Problems.

Summitville

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

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

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

Costs

• To date: $185 million– Annually: $1.5 million

• Taxpayers foot bill

• Mine owner cost: $3 million bond

Conclusions

• 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.

• Acid rock drainage is generated at mines and naturally where sulfide minerals are present and the buffering capacity of the water is exceeded.