Cochise College           Student Papers in Geology

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Roger Weller, geology instructor

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.