Why do they use graphite electrodes in the Hall–Héroult process?

Question

In the Hall–Héroult process of extracting aluminium from aluminium oxide, the electrolyte is a liquid mixture of cryolite $(\ce{NaAlF6})$ and aluminium oxide $(\ce{Al2O3}).$ The cathodes and the anodes are made of graphite.

During the electrolysis process, aluminium is deposited at the cathode and oxygen is liberated at the anode. Some of this oxygen reacts with the carbon in the graphite to form carbon-dioxide, thus slowly burning away the anodes. Thus, the anodes have to replaced periodically.

But if this is the case, why do they use graphite anodes at all? Why don't they use anodes of some other, less reactive material? I'm pretty sure there are many out there that would fit the bill.

Answer 1

According to this source, graphite is used as anode material because carbon is easier to oxidize than $\ce{O^2-}$. The anodic oxidation of carbon

$$\ce{C + 2O^2- \rightarrow CO2 + 4e-}$$

proceeds more efficiently than the oxidation of $\ce{O^2-}$ at an inert anode

$$\ce{2O^2- \rightarrow O2 + 4e-}$$

$\ce{C}$ is a much better reducing agent than $\ce{O^2-}$ whose oxidation state is not changed in the reaction with graphite. As a consequence, anodic current density and the deposition rate of aluminium at the cathode will increase.

Answer 2

The basic reason for using graphite as an anode is in Hall–Héroult process in the electrolytic reduction of alumina to aluminum metal is because graphite being an allotrope of carbon and an inert electrode reacts with oxygen to give out carbon dioxide which thus prevents the liberation of oxygen as a final product at the anode.

Had there been any other metal oxygen would have been liberated which would have readily reacted with aluminum (as aluminum has a high affinity for oxygen in comparison to other metals, for example, iron—even if iron reacted with oxygen the aluminum will easily displace it) to form aluminum oxide and the whole process will prove to be useless.

Graphite is also a good conductor of electricity.

Answer 3

It is simple. Graphite anode is used because when carbon react with oxygen liberated at anode it forms $\ce{CO2}$ gas which is a exothermic reaction and maintain the temperature of process and not allow molten aluminum to solidify. Because the density of aluminum in solid form is less dense than cryolite and hence it can't be able to extract from the bottom.

Answer 4

No , carbon dosent reduce alluminium in hall heroult process , it is simply electrolytic reduction and no it is not used to prevent contact of oxygen with alluminium as alluminium is already denser than the solution and sinks to bottom as in this diagram

Actually hall heroult process is done by taking molten mixture of cryolite and fluorspar(95% of the mixture) and about 5% alumina , this is done to reduce melting point of alumina from about 2200 K to around 1000 K , at which they electrolyzed it , the carbon anode is used as fuel to provide energy to keep contents in molten state , you may have heard the fact that about 0.5 kg of carbon is burnt for every 1kg of alluminium produced as it acts as fuel .

Answer 5

Carbon is a better oxidizing agent than oxygen which produces more heat to retain steady temperature