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- Flotation Concentration
Asarco Mining Operations in Arizona
- Mission Mine, Tucson
- Text edited by Rob Vugteveen, Director, Asarco
Mineral Discovery Center
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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).
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- 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.
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- 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.
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- 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.
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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.
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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.
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