Mining Methods and Environmental Issues in Developing Nations

• Introduction
  • Mineral Deposits vs. Ores
    
    • A mineral deposit is a naturally-occurring concentration of a particular mineral - examples?
      
    • An ore is a mineral deposit that can be economically developed
      
    • Mineral Deposits vs. Ores
      
    • One judge of ore quality is the Concentration Factor
      
      • CF = C_{host rock}/C_{average crust}
      • Other judges of ore quality include location, economic variables, political variables, extraction technology, and environmental variables.
  • World Distribution of Mineral Resources
    
    • Generally uneven - determined by geologic history and tectonic setting
      
    • High metal concentrations along active or extinct plate boundaries (American Cordillera)
      
    • A relatively small population in the industrialized countries now consume the vast majority of the world’s mineral resources, BUT the resources come from ALL nations.
      
  • Formation of Mineral Deposits
    
    • Igneous
      
      • gravitational settling of early, dense minerals on floor of magma chamber
        
      • e.g. Bushveld Intrusion, South Africa
        
      • Layers of chromite and platinum in the Bushveld Intrusion, South Africa
        
    • Metamorphic
      
      • country rocks around an igneous intrusion are changed by heat and chemical reactions with hydrothermal fluids
        
      • contact aureole
        
    • Hydrothermal
      
      • minerals precipitate from hydrothermal solutions above intrusions, leaving veins or disseminated deposits
        
      • Porphyry Copper Deposits, such as Bingham Canyon, Utah
        
    • Sedimentary
      
      • Preferential settling of denser minerals from flowing water
        
      • Placer deposits: glacial deposit containing particles of valuable mineral
        
        • gold placer deposits of California
          
        • placer diamond deposits of South Africa
          
      • Gold Placer Deposits
        
    • Weathering
      
      • removes soluble components of rock, leaving behind concentrated ore
        
      • Bauxite aluminum ore in the tropics
        
    • Evaporation
      
      • leaves a precipitated salt layer
        
      • Middle-East Seas
        
      • desert lakes world-wide.
        
    • Secondary enrichment
      
      • primary deposit is further concentrated by groundwater dissolution followed by re-precipitation
        
      • Copper deposits of Arizona.
        
• Mining Methods
  
  • Surface Mining
    
    • Responsible for 2/3 of world annual mineral production
      
    • Open-pits, strip mines, quarries
      
      • Employs enormous equipment
        
      • Reclamation is expensive and sometimes fails
        
  • Underground Mining
    
    • Cheaper reclamation, but overall more expensive
      
    • Less productive than surface mining
      
    • Generally more dangerous to miners
      
    • Potentially less wasteful than surface mining
      
    • Depth limited to about 4 km by high pressures
      
  • 4-stage evolution of a Producing Mine
    
    • Exploration
      
    • Evaluation
      
    • Development
      
    • Production
    • The first 3 may continue after production starts
      
  • Day to Day Activities at a Working Mine
    
    • Expose the ore
      
    • Transport and stockpile the ore
      
    • Dress the ore
      
      • Includes crushing and concentrating the ore before extracting the element of interest
        
      • Crushing is done in progressively fine stages
        
    • Concentration methods include
      
      • Flotation Separation
        
      • Gravity Separation using water or heavy liquids
        
      • Magnetic Separation
        
    • Extractive Metallurgy
      
      • Pyrometallurgy (smelting)
        
        • Melt concentrate in furnace
          
        • Separate metal from slag by distillation or immiscibility
          
        • Byproducts include gas, vaporized metals, and dust
          
      • Hydrometallurgy
        
        • Dissolve or leach metal from ore or concentrate
          
        • Solvents include sulfuric acid, ammonium, mercury, and sodium cyanide solutions (NaCN dissolves gold)
          
        • High-grade ores are vat-leached; low-grade ores are heap-leached
          
        • Gold in sulfides is roasted first, creating SO2.
          
      • Electrometallurgy
        
      • Electric current is used to deposit metal on cathodes
        
      • Often used to purify metal produced by pyrometallurgy
        
• Environmental Issues
  
  • Waste Disposal
    
    • Amount of waste (tailings) depends on ore grade and extraction technology; can be >99 %.
      
    • Finely ground waste rock (tailings) may contain sulfides, heavy metals, and cyanide residue
      
    • Underground mines use waste as stope fill
      
    • Surface mines convert into slurry and pipe to tailings pond or dump in the ocean
      
  • Problems with Tailings Dams in Developing Countries
    
    • Leaks and failures are common
      
    • 1988 – China, dam overtopped
      
      • 700,000 m3 molybdenum waste released
        
      • 20 deaths
        
    • 2000 – China, gas explosion
      
      • 160 deaths
        
  • Improved engineering of tailings dams in developed countries
    
    • Drainage blankets to reduce internal pore pressures
      
    • Upstream construction to increase dam strength
      
    • Impermeable retention dams
      
    • Acid Mine Drainage
      
    • Sulfides, a common waste product, react when exposed to the atmosphere to form sulfuric acid

LINKS

• Minerals and Ores
  • University of Witwatersand, Johannesburg, School of Geosciences (http://www.wits.ac.za/geosciences/)
  • Department of Geological Sciences, California State University Long Beach: contact metamorphism (http://seis.natsci.csulb.edu/bperry/metarock/CONTACT.htm)
  • Bingham Canyon Copper: Finding Chalcopyrite at "The Richest Hole on Earth" by Steve Voynick, Rock&Gem Contributing Editor (http://www.rockhounds.com/rockgem/articles/bingham_canyon.html)
  • Placers alluviaux (http://pujol.chez.tiscali.fr/placer/placers_alluviaux.html) (in French)
  • ALLUVIAL EXPLORATION & MINING (http://www.kanada.net/alluvial/mining.html)
• Mining Methods
  • Prize Mining Corp. (formerly Muskox Minerals Corp.)(http://www.muskoxminerals.com/s/Home.asp)
  • Geotimes: South Africa’s Geological Gifts by Mike G.C. Wilson (http://www.geotimes.org/dec03/feature_SAfrica.html)
  • Chamber of Mines, South Africa (http://www.bullion.org.za/welcome.htm)
  • Ore Crushing, Asarco Mining Operations in Arizona, Mission Mine, Tucson. Text edited by Rob Vugteveen, Director, Asarco Mineral Discovery Center (http://www.elmhurst.edu/~chm/vchembook/331orecrushing.html)
  • Metso Minerals (http://www.metsominerals.com/)
  • Mining @ Martha (http://www.marthamine.co.nz/ore_process.html)
  • Mining Methods (http://pangea.stanford.edu/~kurt/kurt-mining-methods.html)
  • USGS: Cut and Fill Stoping (http://imcg.wr.usgs.gov/usbmak/anat12.html)

Introduction | Mining Methods | Environmental Issues | Links | top