Charles Martin Hall pictured at the right was an American chemist, who discovered an inexpensive method for the isolation of pure aluminium from its compounds. The same electrolytic process was discovered concurrently by the French chemist Paul L.T. Heroult and is therefore known as the Hall-Heroult process. It became the basis for the aluminium industries both in the United States and in Europe.

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Hall was born in Thompson, Ohio, on December 6th 1863. He became interested in chemistry, and more specifically in finding an inexpensive method for producing aluminium.  While an undergraduate at Oberlin College. After his graduation in 1885, Hall set up laboratory at home and began work on the purification of aluminium. He had the idea that if he could find a non-aqueous solvent for aluminium oxide, he could produce metallic aluminium by electrolysis, using carbon electrodes. On Feb. 23, 1886, Hall found that molten cryolite, which is the mineral sodium aluminium fluoride, was the solvent he needed for the process; using the cryolite and aluminium oxide and homemade batteries, he produced his first small globules of aluminium. (Hall's experimental work)

Hall had trouble-finding backers for his process. Eventually he went to Pittsburgh, where a small group formed the Pittsburgh Reduction Company, which grew to be the first largest producer of aluminium in the United States. This company later became the Aluminium Company of America.

Hall spent the rest of his life developing both his process and the aluminium industry. In 1911 he was awarded the Perkin Medal for his work. Hall died a very rich man in Daytona Beach, Fla., on Dec. 27, 1914 aged just 51.

 The method

  1. The bauxite (red-brown solid), which is aluminium oxide mixed with impurities, is extracted from the Earth.
  2. The extracted aluminium oxide is then treated with alkali, to remove the impurities. This results in a white solid called aluminium oxide or alumina.
  3. The aluminium is then transported to huge tanks. The tanks are lined with graphite, this acts as the cathode. Also blocks of graphite hang in the middle of the tank, and acts as anodes.
  4. The aluminium is then dissolved in molten cryolite - this lowers the melting point, which reduces the total costs of the process.
  5. Electricity is passed  through and electrolysis begins. Electrolysis is the decomposition of a compound using electricity.
  6. When dissolved, the aluminium ions and oxide ions in the alumina can move.

This is a picture of what a Hall cell looks like. It clearly shows the conditions necessary for the electrolysis to occur.

 

 At the cathode:

The aluminium ions receive electrons to become atoms again:

This is a reduction reaction since electrons are gained.

 At the anode:

The oxide ions lose electrons to become oxygen molecules, O2:

This is an oxidation reaction since electrons are removed..