The key difference between Hall Héroult Process and Hoopes process is that Hall Héroult process forms aluminum metal with 99.5% purity, whereas Hoopes process produces aluminum metal with about 99.99% purity.
Hall Héroult process and Hoopes process are important in producing pure aluminum metal. Both of these processes are electrolytic processes. The purity of aluminum metal produced by each process is different from each other.
What is Hall Héroult Process?
Hall Héroult process is the major industrial route for the smelting of aluminum metal. This process involves the dissolving of aluminum oxide or alumina which is obtained from bauxite mineral (through Bayer process) in molten cryolite, followed by electrolyzing the molten salt bath in a purpose-built cell. Typically, this process takes place at 940-980 Celsius degrees in industrial-scale applications. More importantly, this process produces about 99.5% pure aluminum metal. However, we do not use recycled aluminum in this process because that type of aluminum requires no electrolysis. Hall Héroult process tends to contribute to climate change due to the emission of carbon dioxide during the electrolytic reaction.
This process is important because elemental aluminum cannot be produced by the electrolysis of an aqueous aluminum salt since hydronium ion readily oxides elemental aluminum. Usually, aluminum oxide has a very high melting point; therefore, it needs to be dissolved in cryolite in order to lower the melting point. This makes the electrolysis process easier. This process requires a carbon source, which is often coke.
Since this is an electrolysis process, we need to use a cathode and an anode. Usually, the electrodes are made of purified coke. At the cathode, aluminum ions take electrons, forming aluminum metal. At the anode, oxide ions combine with carbon atoms from coke to form carbon monoxide gas. However, in reality, much more carbon dioxide gas is formed than carbon monoxide gas. In this process, cryolite is used to drop down the melting point of alumina because it can dissolve alumina well. Cryolite is also able to conduct electricity; thus, we can use it as the electrolytic medium. Furthermore, cryolite has a low density compared to aluminum metal, which is a requirement for the electrolysis process.
What is Hoopes Process?
Hoopes process is an industrial process useful for obtaining aluminum metal of very high purity. The process was named after the scientist William Hoopes. The aluminum metal that we can obtain from Hall Héroult process has a purity of about 99%. For most applications, that amount of purity is taken as pure aluminum. But for extremely sensitive purposes, this purity is not enough. Therefore, further purification of aluminum can be carried out by the Hoopes process, which is also an electrolytic process.
Hoopes process uses an electrolytic cell which contains an iron tank with carbon at the bottom. For the anode of this cell, a molten alloy of copper, crude aluminum or silicon can be used. This anode forms the lowermost layer of this electrolytic cell. There is a middle layer which contains a molten mixture of fluorides of sodium, aluminum and barium. The next layer is the uppermost layer which contains molten aluminum. The cathode of the cell is two graphite rods that are dipped in molten aluminum.
During the electrolysis process, aluminum ions from the middle layer of the cell tend to migrate towards the upper layer where these ions get reduced, forming aluminum metal by gaining three electrons from the cathodes. Here, an equal number of aluminum ions form in the lower layer at the same time (at the anode). These aluminum ions then migrate to the middle layer. We can obtain pure aluminum tapped off from the upper layer from time to time. The purity of this aluminum is about 99.99%.
What is the Difference Between Hall Héroult Process and Hoopes Process?
Both Hall Héroult process and Hoopes process are electrolytic processes that produce aluminum metal with high purity. However, the key difference between Hall Héroult Process and Hoopes process is that Hall Héroult process forms aluminum metal with 99.5% purity, whereas Hoopes process produces aluminum metal with about 99.99% purity.
Below infographic lists more differences between Hall Héroult Process and Hoopes process in tabular form.
Summary – Hall Héroult Process vs Hoopes Process
For most applications, the purity of aluminum obtained through Hall Héroult process is considered as pure aluminum. But for extremely sensitive purposes, this purity is not enough. In such instances, we need further purification, which is done by Hoopes process. The key difference between Hall Héroult Process and Hoopes process is that Hall Héroult process forms aluminum metal with 99.5% purity, whereas Hoopes process produces aluminum metal with about 99.99% purity.
1. Hall–Héroult process. (2020, September 21). Retrieved October 15, 2020, Available here.