ERTH308 Lecture 15: Acid Mine Drainage + Gold Cyanidation PDF

ERTH 308: Lecture 15 Acid Mine Drainage + Gold Cyanidation Learning Objectives 1) Explain the chemical reaction for the oxidation of pyrite. 2) Describe the rock types that pyrite is commonly found in. 3) Describe the action of microbes on the oxidation of pyrite. 4) Describe several...

ERTH 308: Lecture 15 Acid Mine Drainage + Gold Cyanidation Learning Objectives 1) Explain the chemical reaction for the oxidation of pyrite. 2) Describe the rock types that pyrite is commonly found in. 3) Describe the action of microbes on the oxidation of pyrite. 4) Describe several methods of preventing acid mine drainage. 5) Explain the role of cyanide in gold extraction and processing. 6) Describe the effects of the cyanide spill at Baia Mare, Romania.  Acidic vs. Alkaline https://www.epa.gov/acidrain/what-acid-rain  Charge-balanced Equation for Oxidation of Pyrite: 4FeS2 + 15O2 + 14H2O → 4Fe(OH)3 + 8SO42- + 16H+ Fe experiences a change in valence # from +2 in pyrite to +3 in ferric hydroxide. The Fe is getting OXIDIZED, because it’s valence # increases. The most common oxidizing agent in environments in contact with the atmosphere is O2.  Oxidation of Pyrite: Importance 4FeS2 + 15O2 + 14H2O → 4Fe(OH)3 + 8SO42- + 16H+ So 4 moles of pyrite reacts with oxygen and water to release 16 moles of H+ into the environment. This reaction occurs naturally during pyrite weathering at or near the surface of the Earth. *Accelerated by mining or quarrying Insoluble yellow-orange precipitate, also known as ferric hydroxide or yellow iron oxide.  4FeS2 + 15O2 + 14H2O → 4Fe(OH)3 + 8SO42- + 16H+ Pyrite can occur in shales that may be interbedded with coal seams, especially those that formed in marine coastal swamps. Image result for acid mine drainage http://protectecuador.org/portfolio/acid-mine-drainage/ Strip mining of such coal seams commonly causes acidification of local surface waters because of the oxidation of pyrite by exposure to O2 and H2O. The supply of O2 and H2O is usually unlimited! https://microbewiki.kenyon.edu/index.php/Acidithiobacillus_ferrooxidans Thiobacillus ferrooxidans. Jim Horan, "Acid Mine Drainage Experiments." *increases the rate of oxidation by 1,000,000x compared to just O2! Thiobacillus ferrooxidans are colourless, rod-shaped, aerobic, airborne bacteria. They convert insoluble metals to their soluble state. Thiobacillus ferrooxidans is a highly acidophilic (pH 1.5 to 2.0) bacterium that obtains its energy through the oxidation of iron.  Well-known coal and metalliferous sulfide deposits around the world that are associated with Acid Mine Drainage (AMD) Sahoo et al. (2013) Current Approaches for Mitigating Acid Mine Drainage YouTube: Acid Rock Drainage: The Facts (9:51) Image result for ph scale 1) The pH scale is logarithmic. This means that a small change in pH represents a _______ change in H+ concentration. 2) Solutions that resist changes to pH are called _________. These act like sponges to absorb excess H+. 3) Fluids in living organisms and seawater have strong ____________ capacity. In contrast, rainwater does not. 4) Where are acid soils commonly found? 5) Where are alkaline soils commonly found? 6) What effect does rainfall have on soil pH? YouTube: Acid Rock Drainage: The Facts (9:51) Image result for ph scale 1) The pH scale is logarithmic. This means that a small change in pH represents a large change in H+ concentration. 2) Solutions that resist changes to pH are called _________. These act like sponges to absorb excess H+. 3) Fluids in living organisms and seawater have strong ____________ capacity. In contrast, rainwater does not. 4) Where are acid soils commonly found? 5) Where are alkaline soils commonly found? 6) What effect does rainfall have on soil pH? YouTube: Acid Rock Drainage: The Facts (9:51) Image result for ph scale 1) The pH scale is logarithmic. This means that a small change in pH represents a large change in H+ concentration. 2) Solutions that resist changes to pH are called buffers. These act like sponges to absorb excess H+. 3) Fluids in living organisms and seawater have strong ____________ capacity. In contrast, rainwater does not. 4) Where are acid soils commonly found? 5) Where are alkaline soils commonly found? 6) What effect does rainfall have on soil pH? YouTube: Acid Rock Drainage: The Facts (9:51) Image result for ph scale 1) The pH scale is logarithmic. This means that a small change in pH represents a large change in H+ concentration. 2) Solutions that resist changes to pH are called buffers. These act like sponges to absorb excess H+. 3) Fluids in living organisms and seawater have strong buffering capacity. In contrast, rainwater does not. 4) Where are acid soils commonly found? 5) Where are alkaline soils commonly found? 6) What effect does rainfall have on soil pH? YouTube: Acid Rock Drainage: The Facts (9:51) Image result for ph scale 1) The pH scale is logarithmic. This means that a small change in pH represents a large change in H+ concentration. 2) Solutions that resist changes to pH are called buffers. These act like sponges to absorb excess H+. 3) Fluids in living organisms and seawater have strong buffering capacity. In contrast, rainwater does not. 4) Where are acid soils commonly found? rainforests, jungles 5) Where are alkaline soils commonly found? 6) What effect does rainfall have on soil pH? YouTube: Acid Rock Drainage: The Facts (9:51) Image result for ph scale 1) The pH scale is logarithmic. This means that a small change in pH represents a large change in H+ concentration. 2) Solutions that resist changes to pH are called buffers. These act like sponges to absorb excess H+. 3) Fluids in living organisms and seawater have strong buffering capacity. In contrast, rainwater does not. 4) Where are acid soils commonly found? rainforests, jungles 5) Where are alkaline soils commonly found? deserts 6) What effect does rainfall have on soil pH? YouTube: Acid Rock Drainage: The Facts (9:51) Image result for ph scale 1) The pH scale is logarithmic. This means that a small change in pH represents a large change in H+ concentration. 2) Solutions that resist changes to pH are called buffers. These act like sponges to absorb excess H+. 3) Fluids in living organisms and seawater have strong buffering capacity. In contrast, rainwater does not. 4) Where are acid soils commonly found? rainforests, jungles 5) Where are alkaline soils commonly found? deserts 6) What effect does rainfall have on soil pH? rain (H2O) naturally combines with CO2 in the atmosphere to form a weak acid (H2CO3). Clean rainwater has a pH of 5.6-5.7. 7) The natural pH range for most plants is _______, between ________. 8) True or false: sulfide minerals like pyrite that are underwater will not weather significantly. Explain! 9) What is the geological name for a rusty exposure of rock? 10) What are some treatments for acid mine drainage? 7) The natural pH range for most plants is acidic, between 5.5 - 7. 8) True or false: sulfide minerals like pyrite that are underwater will not weather significantly. Explain! 9) What is the geological name for a rusty exposure of rock? 10) What are some treatments for acid mine drainage? 7) The natural pH range for most plants is acidic, between 5.5 - 7. 8) True or false: sulfide minerals like pyrite that are underwater will not weather significantly. Explain! True 9) What is the geological name for a rusty exposure of rock? 10) What are some treatments for acid mine drainage? 7) The natural pH range for most plants is acidic, between 5.5 - 7. 8) True or false: sulfide minerals like pyrite that are underwater will not weather significantly. Explain! True 9) What is the geological name for a rusty exposure of rock? gossan 10) What are some treatments for acid mine drainage? 7) The natural pH range for most plants is acidic, between 5.5 - 7. 8) True or false: sulfide minerals like pyrite that are underwater will not weather significantly. Explain! True 9) What is the geological name for a rusty exposure of rock? gossan 10) 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  Long-term treatment for acid mine drainage Sahoo et al. (2013) Current Approaches for Mitigating Acid Mine Drainage  Diagram of a typical dry cover designed to minimize the production of acidic effluents from sulfide-bearing wastes Sahoo et al. (2013) Current Approaches for Mitigating Acid Mine Drainage Current approaches to AMD have one common aspect: they delay or prevent oxidation. However, all have limitations. Physical barriers such as wet or dry cover have slowed sulfide oxidation in several studies; however, both wet and dry barriers exhibit only short-term effectiveness. Wet cover is suitable at specific sites where complete inundation is established, but this approach requires high maintenance costs. When using dry cover, plastic liners are expensive and rarely used for large volumes of waste. Bactericides can suppress oxidation, but are only effective on fresh tailings and short-lived, and do not serve as a permanent solution to AMD. In addition, application of bactericides may be toxic to aquatic organisms. Chemical barriers on sulfide surfaces (applying organic and/or inorganic coatings) are effective in preventing AMD. Among inorganic coatings, silica is the most promising, stable, acid-resistant and long lasting.  Health Impacts of Acid Mine Drainage Acid will leach from the rock as long as the source rock is exposed to air and water and until the sulphides are leached out – a process that can last hundreds, even thousands of years. The 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 not only fish, but also the animals who feed on them. Image result for fish white background  Gold Extraction and Processing YouTube: The Process of Excavating and Refining Gold (4:49) 1) What are some physical properties of gold? 2) Gold is usually bonded to other metals such as __________. 3) What role does cyanide play in gold processing? 4) What temperature is the smelter? 5) What are impurities in the smelting process called?  Gold Extraction and Processing YouTube: The Process of Excavating and Refining Gold (4:49) 1) What are some physical properties of gold? soft, malleable, pounded thin, drawn out to make a thin wire, made into shapes, doesn’t rust or tarnish 2) Gold is usually bonded to other metals such as __________. 3) What role does cyanide play in gold processing? 4) What temperature is the smelter? 5) What are impurities in the smelting process called?  Gold Extraction and Processing YouTube: The Process of Excavating and Refining Gold (4:49) 1) What are some physical properties of gold? soft, malleable, pounded thin, drawn out to make a thin wire, made into shapes, doesn’t rust or tarnish 2) Gold is usually bonded to other metals such as silver. 3) What role does cyanide play in gold processing? 4) What temperature is the smelter? 5) What are impurities in the smelting process called?  Gold Extraction and Processing YouTube: The Process of Excavating and Refining Gold (4:49) 1) What are some physical properties of gold? soft, malleable, pounded thin, drawn out to make a thin wire, made into shapes, doesn’t rust or tarnish 2) Gold is usually bonded to other metals such as silver. 3) What role does cyanide play in gold processing? cyanide helps extract the gold from the other metals, and leach it into the surrounding water 4) What temperature is the smelter? 5) What are impurities in the smelting process called?  Gold Extraction and Processing YouTube: The Process of Excavating and Refining Gold (4:49) 1) What are some physical properties of gold? soft, malleable, pounded thin, drawn out to make a thin wire, made into shapes, doesn’t rust or tarnish 2) Gold is usually bonded to other metals such as silver. 3) What role does cyanide play in gold processing? cyanide helps extract the gold from the other metals, and leach it into the surrounding water 4) What temperature is the smelter? 1600 degrees C 5) What are impurities in the smelting process called?  Gold Extraction and Processing YouTube: The Process of Excavating and Refining Gold (4:49) 1) What are some physical properties of gold? soft, malleable, pounded thin, drawn out to make a thin wire, made into shapes, doesn’t rust or tarnish 2) Gold is usually bonded to other metals such as silver. 3) What role does cyanide play in gold processing? cyanide helps extract the gold from the other metals, and leach it into the surrounding water 4) What temperature is the smelter? 1600 degrees C 5) What are impurities in the smelting process called? slag  Gold Cyanidation used in 90% of gold production chemical formula for the dissolution of gold: 4 Au(s) + 8 NaCN(aq) + O2(g) + 2H2O(l)→ 4 Na[Au(CN)2](aq) + 4 NaOH(aq) the gold is then recovered from solution Remediation of the CN: CN− + [O] → OCN− OCN− + 2H2O → HCO−3 + NH3 https://en.wikipedia.org/wiki/Gold_cyanidation  Gold Cyanidation controversial due to the very toxic nature of cyanide Infamous cyanide spills: Year Mine Country Incident 1985-91 Summitville US Leakage from leach pad 1980s-present Ok Tedi Papua New Guinea Unrestrained waste discharge 1995 Omai Guyana Collapse of tailings dam 1998 Kumtor Kyrgyzstan Truck overturned Collapse of containment dam 2000 Baia Mare Romania (see 2000 Baia Mare cyanide spill) Helicopter dropped crate into 2000 Tolukuma Papua New Guinea rainforest Truck leaked 200 liters of 2018 San Dimas Mexico cyanide solution into the Piaxtla River in Durango can have a devastating effect on rivers, sometimes killing everything for several miles downstream.  YouTube: Baia Mare Cyanide Spill (7:48) The 2000 Baia Mare cyanide spill was a leak of cyanide near Baia Mare, Romania, into the Someș River by the gold mining company Aurul, a joint-venture of the Australian company Esmeralda Exploration and the Romanian government. The polluted waters eventually reached the Tisza and then the Danube, killing large numbers of fish in Hungary and Romania. The spill has been called the worst environmental disaster in Europe since the Chernobyl disaster. https://en.wikipedia.org/wiki/2000_ Baia_Mare_cyanide_spill By Photo: Délmagyarország/Karnok Csaba - http://www.delmagyar.hu/szeged_hirek/azonnal_olt_a_cian_a_tiszaban/2415983/, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=42070845 https://cyanidecode.org/cyanide-facts/ Health Effects of Cyanide The toxicity of cyanide is typically expressed as the concentration that is lethal to 50% of the exposed population (LC50). 1) inhalation: LC50 for gaseous hydrogen cyanide is 100-300 parts per million. Inhalation of cyanide in this range results in death within 10-60 minutes. 2) ingestion: LC50 for ingestion is 1-3 milligrams per kilogram of body weight 3) absorption through the eyes and skin: LC50 is 100 milligrams per kilogram of body weight. Cyanide interferes with the cell’s ability to use oxygen, causing cellular suffocation. This leads to depression of the central nervous system and respiratory arrest. Although cyanide is cheap, effective, and biodegradable (it degrades in sunlight), its high toxicity has led to it being banned for gold extraction in the US states of Montana and Wisconsin, the Czech Republic, and Hungary. There are currently protests in Romania calling for a ban on the use of cyanide in mining. Although cyanide is cheap, effective, and biodegradable (it degrades in sunlight), its high toxicity has led to it being banned for gold extraction in the US states of Montana and Wisconsin, the Czech Republic, and Hungary. There are currently protests in Romania calling for a ban on the use of cyanide in mining. In response to the 2000 Baia Mare cyanide spill, the European Union requires that "the concentration of cyanide is reduced to the lowest possible level using best available techniques", and all mines started after 1 May 2008 may not discharge waste containing over 10ppm cyanide. Companies must also put in place financial guarantees to ensure clean-up after the mine has finished. The mining industry has come up with a voluntary "Cyanide Code" that aims to reduce environmental impacts with third party audits of a company's cyanide management. https://www.cyanidecode.org/