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).