Acid Mine Drainage Causes, Consequences and Remediation Dr. David M. Hunter EVOCRA.COM.AU 29 August 2019
Introduction My Details Dr David M. Hunter Res Resea earch Engineer eer Evocra P: 0423-209-917 E: david.hunter@evocra.com.au
Why care about AMD? Global Water Crisis Total Volume of Water on Earth 1383 km 1,386 x 10 15 m 3 Or 1.4 Sextillion Litres Slide References USGS, 2016.
Why care about AMD? Global Water Crisis 97.5% Salt Water 2.5% Fresh Water 1371 km 1,351 x 10 15 m 3 35 x 10 15 m 3 406 km Slide References USGS, 2016.
Why care about AMD? Global Water Crisis Total Volume of Water 68.6% Unaccessible 30.4% Liquid Fresh Water on Earth 243 x 10 14 m 3 106 x 10 14 m 3 1,386 x 10 15 m 3 273 km 360 km Slide References USGS, 2016.
Why care about AMD? Global Water Crisis 0.003% Renewable Fresh Water 428 x 10 11 m 3 43 km 0.8% Liquid Fresh Water 0.00004% Australian Renewable 106 x 10 14 m 3 Fresh Water Approx. 1000x 492 x 10 9 m 3 Sydney Harbour 10 km Slide References UN Water, 2018. USGS, 2016. World Bank, 2019.
Why care about AMD? Global Water Crisis *Data from 2014. Australia Kenya Global Total Renewable Fresh Water Total Renewable Fresh Water Total Renewable Fresh Water 42,800,000 Billion Litres 492,000 Billion Litres 21,000 Billion Litres Renewable Fresh Water Per Renewable Fresh Water Per Renewable Fresh Water Per Capita Capita Capita 5,930,000 Litres 21,000,000 Litres 440,000 Litres Water Stress Water Stress Water Stress 13% 5% 14% Slide References Knoema, 2019. UN Water, 2018. World Bank, 2019.
Why care about AMD? Global Water Crisis *Data from 2014. Australia Total Renewable Fresh Water 492,000 Billion Litres Renewable Fresh Water Per Capita 21,000,000 Litres Water Stress 5% Slide References Knoema, 2019. UN Water, 2018. World Bank, 2019.
Why care about AMD? Global Water Crisis Jakarta, Indonesia - 25 cm per year Houston, TX, USA - 5 cm per year Beijing, China - 10 cm per year Mexico City, Mexico - 90 cm per year Slide References Asmelash, 2019. Cheema, 2019. Mellino, 2016.
Why care about AMD? Environmental Effects ACID MINE DRAINAGE Biological Physical Chemical Ecological • Behavioural • Substrate Modification • Increased acidity • Habitat modification • Respiratory • Increase in stream • Reduced pH • Food-chain velocity • Reproduction bioaccumulation • Destruction of • Turbidity • Osmoregulation • Loss of food source or bicarbonate buffering • Sedimentation system • Acute and chronic prey • Increase in soluble metal • Adsorption of metals • Elimination of sensitive toxicity concentrations • Death of sensitive species onto sediment • Increase in particular • Decrease in light • Food chain species metals • Migration or avoidance modification penetration *Adapted from Gray, 1997. Slide References Gray, 1997.
Why care about AMD? Mining Economics Water is vital to mining operations for: • Transport of materials (slurries/suspensions) • Mineral processing operations – gravity separation/flotation/screening etc. • Dust suppression • And many others. Mining companies invest heavily in water infrastructure. Maximising water recovery and re-use. Prevention and mitigation is better (and cheaper) than cure. In 1997, Harries estimated the average cost of managing AMD in Australia to be $60m per year. Slide References Australian Bureau of Statistics, 2019. Harries, 1997. Ossa-Moreno, 2018.
What is Acid Mine Drainage (AMD) ? Defining AMD Acid Rock Drainage (ARD), Acid Metalliferous Drainage (also AMD), Neutral Mine Drainage (NMD) and Saline Drainage (SD) are also common terms. All of these generally occur due to the oxidation of sulfide minerals via exposure to oxygen and water. Type pH Dissolved Metals/Sulfur Acid Drainage <6.5 Generally High Neutral Drainage >6.5 Low Saline Drainage >6.5 High Acid drainage can be either natural (erosion/weathering) or anthropogenic (human activity) in nature. Acid drainage are generally referred to as acid mine drainage when cause by human activities, mining in particular. Slide References Australian Government, 2016. International Network for Acid Prevention, 2019. Jacobs & Testa, 2014.
What is Acid Mine Drainage (AMD)? Basic Chemistry Pyrite, pyrrhotite, chalcopyrite are common minerals that are known to be “acid generating” minerals. FeS 2 + 7/2O 2 + H 2 O → Fe 2+ + 2SO 42- + 4H + Fe 2+ + 1/4O 2 + H + → Fe 3+ + 1/2H 2 O FeS 2 + 14Fe 3+ + 8H 2 O → 15Fe 2+ + 2SO 42- + 16H + Fe 3+ + 3H 2 O ⇌ Fe(OH) 3 + 3H + Rate limiting factor is primarily the availability of oxygen. AMD generation is often catalysed by acidophilic bacteria that oxidise metals and sulfur. Slide References Australian Government, 2016. International Network for Acid Prevention, 2019. Jacobs & Testa, 2014.
What is Acid Mine Drainage (AMD)? Effect of Biology & Climate Slide References International Network for Acid Prevention, 2019.
What is Acid Mine Drainage (AMD)? Example Sources OPEN-CUT UNDERGROUND WASTE-ROCK TAILINGS DAMS PITS MINES DUMPS Gold King Mine Berkeley Pit Falun Mine Mount Polley Silverton, CO Butte, MT Falun, Sweden Quesnel Lake, Canada Slide References Atlas Obscura, 2010. Parsons, 2016. Bratty et al., 2017. PitWatch, 2019. Gullufsen, 2018. Schlanger, 2015.
What is Acid Mine Drainage (AMD)? Modern vs. Legacy Modern (or Emerging) Legacy In developed countries: AMD has a long history: • • Better managed and understood Iberian Pyrite Belt, Spain • • Regulations and measures in place to prevent Falun Mine, Sweden • AMD De Re Metallica by Agricola published in 1556 In underdeveloped countries: Extent of Legacy AMD: • • Miners (artisan & companies) turning a blind eye 19,300 km of rivers and streams. • • Health implications for locals 72,000 ha of lakes and reservoirs • Often contaminating primary water sources • Example: Macacalder Mine, Kenya Need to continue to prevent emergence of new AMDs while continuing the effort to remediate legacy AMD sources. Slide References Bratty et al., 2017. Johnson & Hallberg, 2005. Ogola, Mitullah & Omulo, 2002.
What is Acid Mine Drainage (AMD)? Predicting AMD Important to predict and understand the potential for acid generation. This is often done using acid base accounting (ABA). Maximum Potential Acidity (MPA) in kg H 2 SO 4 /t = wt.% S x 30.6 Acid-Neutralising Capacity (ANC) => determined by addition of HCl and back titration with NaOH Net Acid Producing Potential (NAPP) = MPA – ANC ANC/MPA Ratio Slide References Australian Government, 2016.
What is Acid Mine Drainage (AMD)? Characterising AMD Once AMD is already generated it is often characterised by its acidity or acidity load – often referred to as tonnes of acidity (TOA) Total Acidity = Acid (H + ) + Latent Acidity (Acidity from dissolved metals) Total Acidity (mg/L CaCO 3 ) = 50 x (3 x [Total Soluble Fe]/56 + 3 x [Al 3+ ]/27 + 2 x [Mn 2+ ]/55 + 1000x10 -pH ) Acidity Load (tonnes CaCO 3 ) = 10 -9 x Volume (L) x Acidity (mg/L CaCO 3 ) Acidity Load (tonnes CaCO 3 /day) = 10 -9 x 86,400 x Flowrate (L/s) x Acidity (mg/L CaCO 3 ) Slide References Australian Government, 2016.
Case Studies of AMD Los Frailes Mine, Spain • Tailings dam failure at Los Frailes Mine in 1998. • 5.5 billion litres of AMD into the Rio Agrio and Rio Guadiamar. • Est. 1.3 - 1.9 billion litres of tailings spilt. • Spill affected 4634 ha. of land, 2600 ha. of which were covered by tailings fines. Value Tailings Solids Tailings Liquids (%) (mg/L) pH - 2.9 Sulfur 45 1200 Arsenic 0.6 0.2 Copper 0.2 17 Iron 45 80 Lead 1 3.5 Zinc 1 450 Slide References Eriksson & Adamek, 2016. Morin & Hutt, 2004.
Case Studies of AMD Gold King Mine, Colorado, USA • Release of AMD from underground mine. • 11 million litres over 9 hrs released into Animas River. • Estimated 24 to 45 tonnes of dissolved metals delivered to Lake Powell, 550 km away. • Release included aluminium, iron, manganese, lead, cadmium, arsenic and copper at a pH of 2.93. • Animas River returned to pre-spill levels within 15 days. • EPA says spill equivalent to 4 to 7 days of normal drainage rate suggests 1.8 to 2.7 ML per day. • Producing approximately 3.5 ML of metal hydroxide sludge per year. Slide References Schlanger, 2015 Sullivan et al., 2017. Weiser, 2018.
Case Studies of AMD Macalder Mine, Kenya • Mine in the Migori Gold Belt. • Population 1.1 million with 32% living below poverty line. • In 2009, only 10.3% of population were educated to high-school level. • 3 schools located within 500 m of mine. • Primarily artisanal mining – unregulated. • Run-off flows into Macalder Stream then into the Kuja River which flows into Lake Victoria. • Lake Victoria is Second largest freshwater lake in the world. • Study found 13.75 mg/L of lead and 8.04 mg/L of arsenic in Macalder Stream. Slide References Knoema, N.D. Migori County, 2016. Ogola et al., 2002.
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