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Question Video: Identifying the Metal Fluoride That Is Used as an Additive to the Alumina-Cryolite Mixture in Aluminum Extraction Chemistry

In addition to cryolite, what other metal fluoride is included in the molten mixture used to dissolve alumina?


Video Transcript

In addition to cryolite, what other metal fluoride is included in the molten mixture used to dissolve alumina? (A) BaF2, (B) CaF2, (C) LiF, (D) NaF, (E) MgF2.

First of all, bauxite is the name of impure aluminum ore mined from the earth. During the Bayer process, bauxite is converted into pure alumina, or Al2O3. Alumina is then converted to aluminum metal through the Hall–Héroult process. The melting point of alumina is over 2000 degrees Celsius. Alumina is therefore mixed with molten cryolite, or Na3AlF6, which lowers the melting point of alumina. At around 900 degrees Celsius, alumina dissolves in the cryolite and forms a molten electrolyte. An electrolyte is a substance with freely moving ions that can conduct electricity.

The alumina–cryolite mixture can then be broken down through the process of electrolysis in an electrolytic cell like the one shown. An electrical current is passed through the electrolyte solution using positively charged anodes and the negatively charged cathode. At the cathode, each Al3+ ion gains three electrons. This reduction process forms liquid aluminum metal that sinks to the bottom and can be collected. Oxide ions lose electrons at the anodes. This oxidation process produces oxygen gas, which bubbles off. Unfortunately, the process of electrolysis to form aluminum metal is very expensive. And it takes a lot of energy to run high electrical current through many electrolytic cells at the same time.

In addition, it takes a lot of energy to keep the electrolyte mixture hot enough to be molten. Certain additives can be added to the alumina–cryolite mixture to lower the melting point and increase the electrical conductivity, which in turn lowers the cost and amount of power needed for the reaction. One of these additives is calcium fluoride, or CaF2, which is also known as fluorspar.

When looking at the answer choices, we see that calcium fluoride is one of the options. In conclusion, the metal fluoride additive that is included in the molten mixture used to dissolve alumina is CaF2, or answer choice (B).

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