Question

Name the most widely used method for the refining of impure metals obtained by various reduction processes. Describe this method with the help of a labelled diagram by taking the example of any metal.

Answer 1

The most important and most widely used method for refining impure metals is electrolytic refining.

Electrolytic refining means refining by electrolysis. Many metals like copper, zinc, tin, lead, chromium, nickel, silver and gold are refined electrolytically.

For the refining of an impure metal by electrolysis:

1. A thick block of the impure metal is made the anode (It is connected to the positive terminal of the battery).
2. A thin strip of the pure metal is made the cathode (It is connected to the negative terminal of the battery).
3. A water soluble salt (of the metal to be refined) is taken as electrolyte.

The apparatus used for the electrolytic refining of copper is shown in the figure. The device comprises an electrolytic tank filled with an acidified copper sulphate solution, which is acidified with dilute sulphuric acid. A substantial block of impure copper serves as the anode, connected to the positive terminal of the battery, while a slender strip of pure copper functions as the cathode, linked to the negative terminal of the battery.

Copper sulphate solution (CuSO₄ solution) comprises copper ions (Cu²⁺) and sulphate ions \[\ce{(SO^{2-}_4)}\]. When an electric current is passed through a copper sulphate solution, the following reactions occur at the two electrodes:

i. The positively charged copper ions, Cu²⁺, from the copper sulphate solution go to the negative electrode (cathode) and, by taking electrons from the cathode, get reduced to copper atoms.

At cathode: \[\ce{\underset{(From electrolyte)}{\underset{Copper ion}{Cu^{2+}}} + \underset{(From cathode)}{\underset{Electrons}{2e-}} -> \underset{(Deposits on cathode)}{\underset{Copper atom}{Cu}}}\]

These copper atoms get deposited on the cathode, giving pure copper metal.

ii. Copper atoms of the impure anode lose two electrons each to the anode and form copper ions, Cu²⁺, which go into the electrolyte solution (this requires less energy than the discharge of \[\ce{SO^{2-}_4}\] ions).

At anode: \[\ce{\underset{(From impure anode)}{\underset{Copper atom}{Cu}} - \underset{(Given to anode)}{\underset{Electrons}{2e-}} -> \underset{(Goes into electrolyte)}{\underset{Copper ion}{Cu^{2+}}}}\]