Mining Methods and Environmental Issues in Developing Nations

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 = Chost rock/Caverage crust Other judges of ore quality include location, economic variables, political variables, extraction technology, and environmental variables.

EARTH: An Introduction to Physical Geology 7th ed / Tarbuck & Lutgens fig 21.2 The annual per capita consumption of nonmetallic and metallic mineral resources for the United States is nearly 10,000 kilograms (11 tons)! About 94 percent of the materials used are nonmetallic. (After U.S. Bureau of Mines)

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 Left: Sample of Merensky Reef showing from top to bottom: feldspathic pyroxenite, top chromite stringer, pegmatoidal feldspathic pyroxenite with sulphides, basal chromite stringer, mottled anorthosite (sample courtesy of Mr Kevin Boerst). http://www.wits.ac.za/geosciences/

Formation of Mineral Deposits Metamorphic country rocks around an igneous intrusion are changed by heat and chemical reactions with hydrothermal fluids contact aureole http://seis.natsci.csulb.edu/bperry/metarock/CONTACT.htm

Formation of Mineral Deposits Hydrothermal minerals precipitate from hydrothermal solutions above intrusions, leaving veins or disseminated deposits Porphyry Copper Deposits, such as Bingham Canyon, Utah http://www.britannica.com/eb/topic?eu=62441&type=17

Porphyry Copper Deposits Bingham Canyon Copper http://www.rockhounds.com/rockgem/articles/bingham_canyon.html

Formation of Mineral Deposits 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 http://pujol.chez.tiscali.fr/placer/placers_alluviaux.html

Gold Placer Deposits http://www.kanada.net/alluvial/mining.html An overview of an example of a gold mining operation supplied by airplane.

Gold Placer Deposits http://www.muskoxminerals.com/s/Home.asp

Formation of Mineral Deposits Weathering removes soluble components of rock, leaving behind concentrated ore Bauxite aluminum ore in the tropics

Formation of Mineral Deposits Evaporation leaves a precipitated salt layer Middle-East Seas desert lakes world-wide. Saltworks in San Francisco Bay

Formation of Mineral Deposits Secondary enrichment primary deposit is further concentrated by groundwater dissolution followed by re-precipitation Copper deposits of Arizona.

Coal mine, Mpumalanga Province, South Africa 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 http://www.geotimes.org/dec03/feature_SAfrica.html Coal mine, Mpumalanga Province, South Africa

Mining Methods 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

Underground Mining http://www.bullion.org.za/welcome.htm

Underground Mining http://www.bullion.org.za/welcome.htm

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 http://www.bullion.org.za/welcome.htm

Day to Day Activities at a Working Mine Dress the ore Includes crushing and concentrating the ore before extracting the element of interest Crushing is done in progressively fine stages http://www.elmhurst.edu/~chm/vchembook/331orecrushing.html

Day to Day Activities at a Working Mine Concentration methods include Flotation Separation Gravity Separation using water or heavy liquids Magnetic Separation http://www.metsominerals.com/inetMinerals/mmhome.nsf/FR?ReadForm&ATL=/inetMinerals/mmcontent2.nsf/WebWID/WTB-040518-2256B-DDBE5

Day to Day Activities at a Working Mine Extractive Metallurgy Pyrometallurgy (smelting) Melt concentrate in furnace Separate metal from slag by distillation or immiscibility Byproducts include gas, vaporized metals, and dust

Day to Day Activities at a Working Mine Extractive Metallurgy 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.

Day to Day Activities at a Working Mine Extractive Metallurgy 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

Environmental Issues 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

Environmental Issues 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

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http://www3. interscience. wiley http://www3.interscience.wiley.com:8100/legacy/college/skinner/0471152285/lecture_notes/ch21.pdf http://www.marthamine.co.nz/ore_process.html http://pangea.stanford.edu/~kurt/kurt-mining-methods.html http://imcg.wr.usgs.gov/usbmak/anat12.html#anatmineh84