Copper Mining  Elmhurst College
Ore Crushing Water Reclamation/Tailings Copper Smelter  Chemistry Department
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Flotation Concentration
Asarco Mining Operations in Arizona
Mission Mine, Tucson
Text edited by Rob Vugteveen, Director, Asarco Mineral Discovery Center

 

Ore Concentration by Froth Flotation

Remember that only 0.67% of the ore is copper. The copper minerals and waste rock are separated at the mill using froth flotation. The copper ore slurry from the grinding mills is mixed with milk of lime (simply water and ground-up limestone) to give a basic pH, pine oil (yes, it comes from trees -- a by-product of paper mills) to make bubbles, an alcohol to strengthen the bubbles, and a collector chemical called potassium amyl xanthate (or the potassium salt of an alkyl dithiocarbonate).
 
The xanthates are added to the slurry in relatively small quantities. Xanthate is a long hydrocarbon (5 carbons) chain molecule. One end of the chain (the ionic dithiocarbonate) is polar and sticks to sulfide minerals while the other end is nonpolar, containing the hydrocarbon chain is hydrophobic -- it hates being in the water and is attracted to the nonpolar hydrocarbon pine oil molecules.
 
Raising the pH causes the polar end to ionize more and to preferentially stick to chalcopyrite (CuFeS2) and leave the pyrite (FeS2) alone. Air is blown into the tanks and agitated like a giant blender, producing a foamy froth. The chalcopyrite grains become coated with xanthate molecules with their hydrophobic ends waving around trying desperately to get out of the water.
 
They attach themselves to the oily air bubbles which become coated with chalcopyrite grains as they rise to the surface and flow over the edge of the tank. In this manner through a series of steps the copper ore is concentrated to an eventual value of over 28% copper. Waste rock particles do not adhere to the bubbles and drop to the bottom of the tank. The waste material that comes out of the bottom of the tanks at the tail end of this process is called "tailings." It is nothing more than ground-up rock with the copper minerals removed.

    Middlings Thickener:

    The graphic on the left is a "middlings thickener". The bubbles that flow over the edge of the first set of flotation tanks (inside the mill building) end up in this thickener. By then the bubbles have broken and the slurry is poured into the center of this round tank. The solid material settles to the gently sloping bottom and is pushed toward the center by a systems of rakes that slowly revolve around the tank.

    The thickened slurry is pumped back into the mill for further processing. The clarified water flows under the small dam, that you can see just inside the perimeter of this tank, flows over the side, and is pumped back to the mill for reuse.

    That stuff floating on top is "almost money" - just chalcopyrite that hasn't sunk in the tan yet. A water spray on the opposite side of that radial walkway helps sink it.

 

Air Bubble:

Text edited by Rob Vugteveen, Director, Asarco Mineral Discovery Center
The graphic on the left shows an air bubble surrounded by grains of chalcopyrite that has been coated with xanthate. The pine oil acts as a frother only, providing the air bubbles that the xanthate sticks to.