Question

Explain, what happens if bauxite containing iron and silica as impurities is directly subjected to the process of electrolytic reduction without prior purification.

Answer 1

Reduction with respect to Hydrogen$\left({\mathrm{H}}_2\right)$

Electrolytic reduction of Bauxite without prior purification

• Impurities of Iron oxide$\left({\mathrm{Fe}}_2{\mathrm{O}}_3\right)$ and Silica$\left({\mathrm{SiO}}_2\right)$ are mainly present in Bauxite$\left({\mathrm{Al}}_2{\mathrm{O}}_3.2{\mathrm{H}}_2\mathrm{O}\right)$.
• In the electrolytic reduction process ${\mathrm{Al}}^{3+}$Bauxite reduces at cathode by gaining$3{\mathrm{e}}^{-}$.
• If impurity of Iron oxide is present in Bauxite then ${\mathrm{Fe}}^{3+}$ after gaining $3{\mathrm{e}}^{-}$ will deposit at cathode prior to${\mathrm{Al}}^{3+}$.
• Deposition of Iron$\left(\mathrm{Fe}\right)$ on cathode occurs due to the less electropositive nature of Iron than Aluminum.
• The value of standard reduction potential for Aluminum is more negative $\left(-0.57\mathrm{V}\right)$ than that of Iron$\left(-0.44\mathrm{V}\right)$.

Therefore, the deposition of Iron$\left(\mathrm{Fe}\right)$ at the cathode occurs if the electrolytic reduction of Bauxite takes place prior to purification.