How to Separate Aluminum Metal From Its Ore

Aluminum is one of the most extensively used metals in most industries.

The natural ore that is mined and from which 99 percent of the world’s supply of aluminum metals is extracted is hydrated aluminum oxide or bauxite. Bauxite is impure aluminum oxide that contains silicon dioxide, iron oxides and titanium dioxide, among others. The extraction is a chemical purification process of the aluminum oxide called the Bayer process, with subsequent electrolysis called the Hall-Heroult process to extract the aluminum metal from its purified oxide form.

Things You'll Need

• bauxite ore
• ore crushing machine
• cryolite mixture
• High temperature facility
• filtering facility
• electrolysis facility
• high current electricity source
• furnace

Instructions

1. Mix the mechanically crushed bauxite with hot concentrated caustic soda or sodium hydroxide (NaOH) in a grinding mill to produce a liquid solution or slurry with fine ore particles.

2. Bring the solution to high temperatures, typically ranging from 140 to 240 degrees C with pressures of up to 35 atmospheres. The solution will then separate the aluminum oxides and silicon but not the other oxide impurities, which do not react with NaOH.

3. Filter the solution to remove insoluble impurities like iron oxides. After the treatment, the solution becomes a mixture of aluminum oxides and silicon oxides.

4. Introduce carbon dioxide by bubbling the gas through the solution. The gas forms a relatively weak carbonic acid solution that neutralizes the sodium hydroxide in the first step. This process precipitates the aluminum oxide, while leaving the silicates in the solution.

5. Filter the solution to separate the aluminum oxide (also called alumina) from the silicates. Once done, you have a purified but hydrated aluminum hydroxide.

6. Heat the solution to evaporate the water content at temperatures ranging from 1100 to 1200 C. This creates purified aluminum oxide

7. Introduce molten cryolite into the purified aluminum oxide. Cryolite is a mixture of sodium fluoride and aluminum fluoride, and heated to about 980 C to melt the solids. The cryolite mixture melts at a much lower temperature than aluminum oxide would. After the process, you now have molten aluminum oxide ready for electrolysis.

8. Electrolyze the molten aluminum oxide using electricity at a low 4 to 5 volts with a high current of 50,000-150,000 amps. This is referred to as the Hall-Herault process, which produces a 99 percent aluminum purity. Electricity used to extract aluminum is high, where extracting a pound of the metal requires 6 to 8 kilowatt-hours of energy. The process results in aluminum ions being deposited as aluminum metal at the cathode (the bottom and sides of the electrolysis cell), while the oxygen ions goes to the anode. As the anode gets oxidized, continuous replacement of the anode is required.

9. Collect the molten aluminum by using a crucible to siphon the metal off the cathode. The collected molten metal is then cast into ingots or molds.

References

• Chemguide: Aluminum
• Elmhurst: Conversion of Bauxite Ore to Aluminum Metal