# Why do they use graphite anodes in the Hall-Heroult process?

In the Hall-Heroult process of extracting aluminium from aluminium oxide, the electrolyte is a liquid mixture of cryolite ($\ce{NaAlF_6}$) and aluminium oxide (of course). 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.

## 3 Answers

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.

• And also given that under most conditions carbon dioxide is a gas it leaves no trace in the obtained material ($\ce{CO2 ^}$). – Martin - マーチン May 16 '14 at 16:37

Its simple. Graphite anode is used because when carbon react with oxygen librated at anode it forms 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 cyrolite and hence it can't be able to extract from the bottom.

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