Acid Mine Drainage and Gold Cyanidation

Question 1

Explain the chemical reaction for the oxidation of pyrite

Answer 1

4 mols of pyrite react with oxygen and water to release 16 mols of H+

Fe is oxidized (valence increases from +2 to +3)

Question 2

Most common oxidizing agent in environment

Answer 2

O2

Question 3

Oxidation of pyrite occurs naturally when

Answer 3

Pyrite weathers at or near earths surface

Question 4

Oxidation of pyrite is accelerated by

Answer 4

Mining

Question 5

Ferric oxide (yellow-iron oxide)

Answer 5

Yellow-orange precipitate in oxidation of pyrite reaction that smothers organisms living on stream bottom

Question 6

What is the chemical formula for ferric oxide

Answer 6

4Fe(OH)3

Question 7

Pyrite occurs in shales that can be interbedded with

Answer 7

Coal seams

Question 8

Coal seams are common in shales in

Answer 8

Marine coastal swamps

Question 9

What often causes acidification of local surface waters

Answer 9

Strip mining of coal seams - oxidation of pyrite by exposure to O2 and H20 causes release of H+

Question 10

Thiobacillus ferrooxidans

Answer 10

Microbe that catalyzes oxidation of pyrite

Question 11

Thiobacillus ferrooxidans vs O2 oxidation

Answer 11

Thiobacillus ferrooxidans 1,000,000x increased rate

Question 12

Thiobacillus ferrooxidans convert insoluble metals to

Answer 12

Soluble state

Question 13

Rod-shaped

Answer 13

Thiobacillus ferrooxidans

Question 14

Aerobic, airborne bacteria

Answer 14

Thiobacillus ferrooxidans

Question 15

Thiobacillus ferrooxidans pH preference

Answer 15

Highly acidic (pH 1.5-2.0)

Question 16

Thiobacillus ferrooxidans energy obtaining reaction

Answer 16

Oxidation of iron

Question 17

Logarithmic nature of pH scale

Answer 17

Small change in pH = large change in H+ concentration

Question 18

Buffers

Answer 18

Solutions that resist changes in pH by absorbing H+

Question 19

Have strong buffering capacity

Answer 19

Fluids in living organisms and seawater

Question 20

Common place for alkaline soils

Answer 20

Deserts

Question 21

Common place for acidic soils

Answer 21

Rainforests, jungles

Question 22

How does rain affect pH of soil

Answer 22

Combines with Co2 to create weak acid

Question 23

pH of clean rainwater

Answer 23

5.6-5.7

Question 24

Natural pH range of plants

Answer 24

acidic (5.5-7)

Question 25

Sulfide minerals (pyrite) will not weather significantly when

Answer 25

Underwater

Question 26

Gossan

Answer 26

Rusty exposure of rock

Question 27

Acid mine drainage treatments

Answer 27

-add buffer - limit area of exposed rock - compact acid-generating rock - cover rock with sealing layers

Question 28

Buffer material for acid mine drainage

Answer 28

Limestone

Question 29

Categories of long-term treatments for AMD

Answer 29

1. bacterial inhibition 2. physical barrier 3. desulfurization 4. chemical barrier 5. electrochemical cover

Question 30

Purpose of AMD treatment

Answer 30

Delay or prevent oxidation

Question 31

Physical barriers

Answer 31

- Slow sulfide oxidation - short-term

Question 32

Wet vs dry physical barriers

Answer 32

Wet: sites with complete induntation and high maintenance costs Dry: plastic liners expensive and cannot be used for large volumes

Question 33

Bactericides pros and cons

Answer 33

- suppress oxidation - short-lived - may be toxic to aquatic organisms

Question 34

Most promising, stable, acid-resistant, long lasting chemical barrier

Answer 34

Silica

Question 35

Chemical barriers

Answer 35

Act on sulfide surfaces

Question 36

Acid will leach from rock as long as

Answer 36

Rock is exposed to air and water and sulphides left to leach out

Question 37

Responsible for physical, chemical, and biological degradation of stream habitat

Answer 37

AMD

Question 38

Physical properties of gold

Answer 38

- Soft + malleable - Doesn't rust or tarnish

Question 39

Gold usually bonded to

Answer 39

Silver

Question 40

Role of cyanide in gold processing

Answer 40

Helps extract it from other metals and leach into surrounding water

Question 41

Temperature of gold smelter

Answer 41

1600 C

Question 42

Slag

Answer 42

Impurities in smelting process

Question 43

Used in 90% of gold production

Answer 43

Gold cyanidation

Question 44

Why is gold cyanidation controversial

Answer 44

Toxic nature of cyanide

Question 45

Date and location of Baia Mare Cyanide Spill

Answer 45

2000, Romania into Somes River

Question 46

LC50

Answer 46

Lethal dose to 50% of population

Question 47

LC50 for gaseous hydrogen cyanide

Answer 47

100-300 ppm, death within 60min

Question 48

LC50 for cyanide ingestion

Answer 48

1-3mg per kg body weight

Question 49

LC50 for cyanide absorption through eyes and skin

Answer 49

100mg per kg body weight

Question 50

How does cyanide act on the body

Answer 50

Interferes with cell's ability to use oxygen - causing cellular suffocation which leads to the depression of the CNS and respiratory arrest

Question 51

Pros of cyanide

Answer 51

Cheap, effective, and biodegradable in sunlight

Question 52

Cyanide banned for gold extraction

Answer 52

Montana, Wisconsin, Czech Republic, Hungary

Question 53

European Union response to Baia Mare

Answer 53

"the concentration of cyanide must be reduced to the lowest possible level using the best possible techniques" No discharge of waste containing over 10ppm of cyanide

Question 54

Currently protesting for cyanide ban

Answer 54

Romania