Hall-Heroult process, energy requirements

The Hall-Heroult process is a prodigious consumer of electrical energy. The energy required to produce 1 ton of aluminum from ore is more than twice that required to produce 1 ton of copper and ten times that for 1 ton of steel. More than 75% of this energy is consumed in the reduction of alumina to aluminum metal. The reasons for this high energy consumption have been presented in Table 6.18. The theoretical energy requirement for... 

The Alcoa chlorine process uses about 30% as much electrical energy as the Hall-Heroult process. A1C13 melts at a much lower temperature than the Al203/Na3[AlF6] mixture, so less energy is required to heat the electrolysis container. The product, chlorine gas, is recycled in the Alcoa chlorine process, which keeps the cost down. Also the electrodes do not have to be replaced, as they do in the Hall-Heroult process. However, the main disadvantage is that the Alcoa chlorine process is more dangerous to workers since chlorine is a toxic gas. 

The Hall-Heroult process uses huge amounts of electric energy. For this reason, aluminum often is produced in plants built close to large hydroelectric power stations, where electric energy is abundant and less expensive. The vast amount of electricity needed to produce aluminum from ore is the prime reason that it is especially important to recycle aluminum. Recycled aluminum already has undergone electrolysis, so the only energy required to make it usable again is the heat used to melt it in a furnace. 

The use of large amounts of electrical energy in electrolysis makes production of aluminum from ores an expensive metallurgy. Methods for recycling used A1 use less than 10% of the energy required to make new metal from bauxite by the Hall-Heroult process. 

The energy required to produce aluminum by the Hall-Heroult process is considerable—about 14,000 kilowatt-hours per ton of aluminum. (A watt equals one joule per second, so a kilowatt-hour equals 3.6 milhon joules.) To put this in perspective, note that a typical household might use about 1000 kilowatt-hours per month. Therefore, the energy used to produce one ton of aluminum is sufficient to supply electricity to about 14 homes for one month. Recycling of aluminum would save much of this energy. Only about 7% of the energy required to produce aluminum from aluminum oxide is needed to obtain aluminum metal from used cans and scrap. 

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