Acid Mine Drainage and Gold Cyanidation

Question 1

Oxidation of Pyrite

Answer 1

Fe goes from +2 to +3
Oxygen is the most common oxidizing agent for environments that are in contact with the atmosphere
4 moles of pyrite reacts with oxygen and water to release 16 moles of H+
This reaction occurs naturally during pyrite weathering at or near the surface of the Earth, accelerated by mining or quarrying
Ferric hydroxide or yellow iron oxide = insoluble yellow-orange precipitate produced

Question 2

Pyrite can occur in shales that may be

Answer 2

interbedded with coal seams, especially those that formed in marine coastal swamps

Question 3

Strip mining of these coal seams commonly causes

Answer 3

acidification of local surface waters because of the oxidation of pyrite. The supply of oxygen and water is usually unlimited

Question 4

Microbial Influences

Answer 4

• Bacterial microbe Thiobacillus ferrooxidans catalyze the oxidation of pyrite
• This microbe is colourless, rod-shaped, aerobic, and airborne bacteria. Convert insoluble metals to soluble states.
• Also is a highly acidophilic bacterium that obtains its energy through the oxidation of iron

Question 5

Fluids in living organisms and seawater have strong

Answer 5

buffing potential. In contrast, rainwater does not

Question 6

Where are acid soils commonly found?

Answer 6

High rainfall areas, rainforests, jungles and forests

Question 7

Where are alkaline soils commonly found?

Answer 7

Low rainfall areas, deserts

Question 8

What effect does rainfall have on soil pH?

Answer 8

Rain (H2O) naturally combines with CO2 in atmosphere to form a weak acid (H2CO3). Clean rainwater has a pH of 5.6-5.7.

Question 9

The natural pH range for most plants is _____, between ____

Answer 9

Acidic, 5.5-7.0

Question 10

True or false: sulfide minerals like pyrite that are underwater will not weather significantly?

Answer 10

True b/c concentration of dissolved oxygen is 25k x lower than atmosphere

Question 11

What is the geological name for a rusty exposure of rock?

Answer 11

Gossans

Question 12

What are some treatments for acid mine drainage?

Answer 12

Adding buffer materials
Limiting exposed rock area
Compacting acid-generating rock
Covering rock with sealing layers

Question 13

Current approaches to AMD and one common aspect: they delay or prevent oxidation. All have limitations

Answer 13

• Physical barriers (wet or dry cover): slowed sulfide oxidation, only short-term effective. Wet cover suitable at specific sites where complete inundation is established, but requires high maintenance costs. Dry cover - plastic liners are expensive and rarely used for large volumes of waste
• Bactericides: suppress oxidation, only effective on fresh tailings and short-lived, not a permanent solution. Application may be toxic to aquatic organisms.
• Chemicals barriers on sulfide surfaces (applying organic and/or inorganic coatings): effective in preventing AMD. Among inorganic silica is most promising, stable, acid-resistant, and long lasting.

Question 14

Health Impacts of Acid Mine Drainage

Answer 14

• Acid will leach from the rock as long as the source rock is exposed to air and water and until sulphides are eached out -> process can lead to 100s or 1000s of years
• Acid is carried off the mine site by rainwater or surface drainage and deposited into nearby streams, rivers, lakes, and groundwater
• Acid mine drainage is responsible for physical, chemical, and biological degradation of stream habitat. It jeopardizes fish and the animals that feed on them.

Question 15

What are some physical properties of gold?

Answer 15

Malleable, soft, pounded thin, drawn out to make a thin wire, made into shapes, isn’t affected by water or oxygen = no rust/tarnish

Question 16

Gold is usually bonded to other metals such as

Answer 16

Silver

Question 17

What role does cyanide play in gold processing?

Answer 17

CN helps extract the gold from other metals, and leach it into the surrounding water (Causes chemical rxn allowing gold to dissolve in pulp)

Question 18

What temperature is the smelter?

Answer 18

1600ºC

Question 19

What are impurities in the smelting process called?

Answer 19

Slag

Question 20

Gold Cyanidation

Answer 20

• Used in 90% of Au production
• Controversial due to very toxic nature of cyanide
• Spill can have a devastating effect on rivers, sometimes killing everything for several miles downstream

Question 21

Baia Mare Cyanide Spill (2000)

Answer 21

• Was a leak of cyanide near Baia Mare, Romania, into the Somes River by gold mining company Aurul, a joint-venture of the Australian company Esmeralda Exploration and the Romanian government.
• Polluted waters eventually reached Tisza and then the Danube, killing large numbers of fish in Hungary and Romania.
• Spill has been called the worst environmental disaster in Europe since the Chernobyl disaster

Question 22

Health Effects of Cyanide

Answer 22

Toxicity of cyanide is expressed as concentration that is lethal to 50% of the exposed pop (LC50)
1) Inhalation: LC50 for gaseous hydrogen cyanide is 100-300ppm. Inhalation of cyanide in this range results in death within 10-60mins
2) Ingestion: LC50 for ingestion is 1-3mg/kg of body weight
3) Absorption through the eyes and skin: LC50 is 100mg/kg of body weight
- Cyanide interferes with cell’s ability to use O2, causing cellular suffocation -> depression of CNS and respiratory arrest

Question 23

Cyanide is cheap, effective, and biodegradable (degrades in sunlight), its high toxicity led to it being

Answer 23

banned for Au extraction in Montana and Wisconsin, the Czech Republic, and Hungary. Currently protests in Romania calling for a ban on the use of cyanide in mining

Question 24

Response to Baia Mare Spill

Answer 24

• EU requires that “the concentration of CN is reduced to the lowest possible lvl using best available techniques”, and all mines started after May 1, 2008 cannot discharge waste containing over 10ppm CN.
• Companies must also put in place financial guarantees to ensure clean-up after the mine is finished
• Mining industry has come up with a voluntary “CN Code” that aims to reduce environmental impacts with third-party audits of a company’s cyanide management