15. Acid mine drainage and gold Cynanidation Flashcards
Charge-balanced Equation for Oxidation of Pyrite
4FeS2 + 15O2 + 14H2O → 4Fe(OH)3 + 8SO42- + 16H+
What valence change does pyrite have in ferrite hydroxide?
+2 to +3
Is Fe getting oxidized or reduced
Iron is getting oxidized, because valence number increases
What is the most common oxidizing agent in environment in the atmosphere?
Oxygen
How many moles of H is released?
16 moles
When does oxidation of pyrite occur?
During pyrite weathering at or near the surface of the earth
What can pyrite weathering be sped up by
Accelerated by mining and quarrying
What is the colour of Ferric hydroxide
Yellow orange
Insoluble
Precipitate
Where else can pyrite be found
In shales
Interceded with coal seams
Especially in marine coastal swamps
What does strip mining of pyrite cause
Acidification of local surface waters due to oxidation of pyrite by exposure to oxygen and water
What bacteria helps oxidation of pyrite to ferric oxide and hydrogen ions
Thiobacillus ferroxidans
What are some characteristics of Thiobacillus ferroxidans?
Colourless
Rod shaped
Aerobic
Airborne
What does Thiobacillus ferroxidans do
Convert insoluble metals to soluble state
Is Thiobacillus ferroxidans acidic or alkalinic?
Highly acidophilic (1.5-2.0)
Obtains energy through oxidation of iron
What are some coal and metal inferiors sulfide deposited around the world
Coal
Copper
Copper-lead-zinc
Lead-zinc
Gold
Uranium
Silver
Tin
Nickel
Does a small change in ph scale represent small or large change
Large change since it is logarithmic
Which fluids have strong, buffering capacity
Fluids in living organisms and seawater
Which fluids have weak buffering capacity
Rain water
Where are acid soils commonly found?
Rainforests and jungles
Where are alkaline soil commonly found
Deserts
What effect does rainfall have on soil pH?
Rainwater naturally combines with carbon dioxide in the atmosphere to form a weak acid (h2co3)
What is the pH level of clean rain water?
5.6-5.7
What is the natural pH range for most plants? Is it acidic or alkaline?
Acidic 5.5-7
sulphide minerals like pyrite that are underwater will not Weather significantly
True, this is because they are not exposed to oxygen as much as if it was above water
What is the geological name for a rusty exposure of rock?
Gossan
What are some treatments for acid mine drainage
- Add a buffer material like limestone.
- Limit the area of exposed rock.
- Compacting acid generating rock.
- Covering the rock with sealing layers
What are some long-term treatments for acid mine drainage
- Bacteria inhibition
- Physical by water cover or dry cover
3.desulfurization - Chemical barrier passivation either by inorganic or organic coating
- Electro chemical cover.
What is water cover and dry cover
Water cover: flooding under water, storage of spoils
Dry cover: using soil, organic materials, synthetic materials, and vegetation
What is an organic and inorganic coating?
Inorganic: silica, phosphate, alkaline compound
Organic: lipids ,fatty acid, oxalic acid
What are dry covers used for?
To minimize the production of acidic effluents from sulphide bearing waste, the bottom becomes impermeable but groundwater can exit down
What is the current approach to AMD?
Delay or prevent oxidation
Limitations: physical barrier
What dry barriers have short term effectiveness.
Wet: specific to where inundation is established, high maintenance cost
Dry: plastic liners are expensive and rarely used for large volumes of waste to clean the room
Limitations: bactericides
Suppress oxidation, but are only effective on fresh tailings and short-lived
Not permanent solution
Toxic to aquatic organisms
Limitations: chemical barriers
Good at preventing AMD
What is the most promising stable, acid, resistant, and long, lasting coating in organic coatings
Silica
How does acid reach lakes?
-Acid will leach from rock as long as the source rock is exposed to air and water until sulphides are leached out
- process that lasts hundreds and thousands of years
- Acid is carried off mine site by rainwater and surface drainage, deposited into nearby streams and rivers
Acid mine drainage hazards
Physical, chemical and biological degradation of stream habitats
Affects fishes, and those who eat it
What are some physical properties of gold
Malleable
Soft
Pounded thin
drawn out to make a thin wire
doesn’t rust or tarnish
Gold is usually bonded with which metal
Silver
What role does cyanide play in gold processing?
Helps extract gold from other metals, and leach to surrounding water
What temperature is the smelter
1600 degrees celsius
What are impurities in the smelting process called
Slag
How often is gold cyanidation used
90% of gold production
What is the chemical formula of gold dissolution
4 Au(s) + 8 NaCN(aq) + O2(g) + 2H2O(l)→ 4 NaAu(CN)2 + 4 NaOH(aq)
Remediation of the CN formula
CN− + [O] → OCN−
OCN− + 2H2O → HCO−3 + NH3
Where did Baia Mare cyanide spill occur
Romania into Somes River
When did Baia Mare cyanide spill occur
2000
Where did spill reach and what happened
Reached Tisza and Danube
Killing large numbers of fish in Hungary and Romania
Worst disaster since chernobyl
how did spill occur
Bad weather and bad structure of dams
which company is responsible for cyanide spill
Aurul ( Australian )
how is the toxicity of cyanide expressed in concentration?
lethal to 50% of exposeed population LC50
ways to get cyanide ( 3 categories )
- Inhalation
- Ingestion
- Absorption through eyes and skin
Inhalation how much
Gaseous hydrogen cyanide
100-300 parts per million
death in 10-60 minutes
Ingestion how much
1-3 mg per kilogram of body weight
absorbtion through eyes and skin how much
100 mg per kilo of body weight
how does cyanide affect body
- interferes with cell’s ability to use oxygen
- cellular suffocation
- CNS depression and respiratory arrest
what has mining industry come up with to combat cyanide exposure
voluntary cyanide codes for management
EU needs concentration of cyanide to be at what?
cannot discharge more than 10ppm
financial guarantees for clean up as well
