Cochise College Student Papers in Geology
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Roger Weller, geology instructor
wellerr@cochise.edu
Aluminum
by Alaric Evans
Physical Geology
Fall 2007
Refining Aluminum
Aluminum is a word that many people have trouble pronouncing, it is also a metal
that is now used in almost everything now days. It is used for building because
it is strong and light weight, and used for mirrors because it is very
reflective, it reflects UV light better than traditional silver mirrors.
Although aluminum is very cheap and very common now, it used to be incredibly rare until a process, the Bayer Process, was developed for refining bauxite into alumina; this process is named after its creator, Karl Bayer, who developed it in 1888. After that process the alumina is further refined into 99.7% aluminum using the Hall-Heroult process.
To make alumina first you must get your bauxite and crush it into a uniform
size. The crushed pieces are then put into a grinder along with caustic soda.
The caustic soda breaks down the aluminum containing chemicals and the non
aluminum containing rock fragments sink to the bottom. The dissolved portion of
this contains mostly gibbsite, bohmite, and diaspore, and is called slurry.
The slurry is pumped into a digester, a pressurized container headed to 300
degrees Fahrenheit. More caustic soda is added and swirled into this mixture.
The heat and pressure allow more of the aluminum to be dissolved and the iron,
silica, and titanium containing particles to be filtered out.
Then they must filter out the undesirable parts. This is done primarily by
putting it into a large tank and just waiting for the heavier iron, silica, and
titanium to sink to the bottom. It is further refined by filtering.
After they have created a pure gibbsite, bohmite, and diaspore mixture, they put
it into a large tank and allow the alumina to crystallize often old alumina
crystals and put into the mixture for the new alumina crystals to grow on; when
a crystal forms it sinks to the bottom and is carried away for the next stage.
Next they must remove the chemically combined water from the alumina. This is
called calcination. This involves heating up the alumina hydrate up to 2000
degrees. This causes the water to evaporate off leavening aluminum calcite,
alumina. They remove this from the caustic soda and are left with a fine white
powder.
This is where the Hall-Heroult process takes over. Compared to the
Bayer-process, the Hall-Heroult is quite simple. Alumina is placed into a large
tank lined with graphite. Then DC current is sent into the tank. They use 5.24
volts, but the amperage can exceed 100,000 amperes. The alumina is turned into
pure aluminum and carbon dioxide more alumina is pumped into the top to keep
replenishing the cycle while the carbon dioxide is siphoned off. If the power
goes out while doing this the aluminum solidifies inside the tank and it costs
1.6 billion dollars to make a new one.
After this you now have pure aluminum ready to be turned into soda cans.
References:
http://periodic.lanl.gov/elements/13.html
http://www.d.umn.edu/~pmorton/geol2350/2007/powerpoints/chemistry.html
http://www.aughinish.com/bayer.htm
http://www.rocksandminerals.com/aluminum/process.htm
http://www.madehow.com/Volume-5/Aluminum.html