Types of Ore Massive: base metal Cu, Ni, Zn Disseminated: gold, Diamonds Snow Lake 0.16 oz Au / tonne INCO Thompson Ni ore Ekati, NWT, Diamonds Bissett Au
Bedded Ore bodies Potash, Saskatchewan Syncrude, Oil sands, Fort McMurray Black Thunder Coal Mine Wyoming, USA
Extraction process depend on shape, position and value of ore body Open Pit Strip Mining Underground Mine Dissolution: potash, uranium, In situ mobilization: oil shale Heap Leach What are the major environmental issues related to each form of extraction?
Open Pit Bingham Canyon Cu Mine Ekati Diamond Mine Note: The amount of waste rock to be removed to provide reasonable gradient for trucks. Only efficient for large ore bodies close to the surface Can be useful for dumping mine waste at closure
Strip Mining Very efficient for high level flat lying ore bodies. Overburden and top soil can be replaced minimizing environmental damage
Dissolution: potash, uranium In situ mobilization: sulfur, oil shale NaCl or KCl dissolved in water Acid in situ leach of metallic ore Hot water or steam used to mobilize sulfur or oil
Crushing Ore bearing and non ore-bearing rock will be separated as soon as possible What happens to non-ore-bearing rock? Ore-bearing rock will be crushed to the size necessary to liberate the required mineral Heap leach requires only very coarse crushing Much finer crushing for gold and PGE as they are enclosed within other mineral grains Initial crushing underground In mill, crushing in autogenous circuit with feedback of large particles Final crushing in rod or ball mills
Beneficiation (Extraction of metal from rock/mineral) Physical techniques –Gravity Gold from quartz Diamonds Tantalum –Flotation Sulphides from silicates Cu and Ni sulphides NaCl from KCl Chemical Techniques –CN leaching –Roaster oxidizes sulphide to SO 2 Smelting Electrochemical Refining of pure metal Consider the waste generated at each stage
Gravity Separation Whiffle table Separator Fine grained waste goes to tailings pond. What does it contain?
Flotation Cells Ore becomes attached to air bubbles, float, and are collected. Gangue sinks to the bottom of the flotation cells and piped to tailings pond Chemicals added include: Frothers: pine oils & alcohols promote the formation and stability of bubbles. Collectors promote adherence of air bubbles to the mineral. Conditioners: make the surface of the mineral particle either more or less susceptible to concentration. Activators e.g. copper sulfate, lead nitrate, lead acetate Depressants e.g. sodium cyanide, zinc sulfate
Chemical Separation Cyanide leaching for gold –at pH 11 –gold retrieved by Merrill Crowe (precipitation on zinc dust) or Carbon in Pulp (CIP) process Roaster oxidizes sulphide to SO 2 –Releases metal for refining and SO 2 to the atmosphere unless scrubbers are in place
Smelting Separates remaining silicates from metals Slag (molten silicates) pored off Metal pores into bars or sent for final electochemical refining