Question

What is meant by refining of a metal? 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 metals obtained by different reduction processes are usually found to be with some impurities. The process involved in the removal of these impurities from the reduced metals is called purification or refining of metals. There are several refining methods used to purify metals based on the type of metals.
The most widely used method for the refining of impure metals obtained by various reduction processes is electrolytic refining method. Electrolytic refining is a process of purification by electrolysis.  Several metals like copper, zinc, tin, lead, nickel, chromium, gold and silver are refined by this process.

Let us explain electrolytic refining process by considering an example of copper metal. The electrolytic refining tank consists of anode, cathode and electrolytic solution. In the refining of copper metal, a bulky piece of impure copper is made anode (attached to positive terminal) and a thin layer of pure copper is made cathode (attached to negative terminal). The acidified copper sulphate solution (copper sulphate solution with dilute sulphuric acid) is taken as electrolytic solution or electrolyte.

When electricity is passed through the cell, impure copper from the anode (as CU²⁺ ions) melts and travels to the electrolyte. The pure copper metal (CU²⁺ ions) from the electrolyte moves to the cathode and gets deposited there as pure copper. The amount of copper ions from the anode going to the electrolyte will be equivalent to the amount of copper ions collected at the cathode. This keeps the concentration of the electrolyte constant and proceeds the further reaction. Copper ions, being positively charged, get attracted to the negatively charged cathode and are collected or deposited there. 
The copper sulphate electrolyte contains CU²⁺ and `SO_4^(2-)` ions. The reactions that take place at anode and cathode when electricity passed through the electrolytic cell are:

(i) The impure copper atoms from anode lose two electrons to form copper ions and enter the copper sulphate electrolyte.
Anode: 

(ii) The copper ions from the copper sulphate electrolyte move to cathode (negatively charged) by gaining two electrons from cathode and get reduced to copper atoms.
Cathode:  

Thus impure copper atoms, entering copper sulphate electrolyte from anode, get deposited as pure copper atoms at cathode. As the process continues, cathode (pure metal collected) becomes thicker and anode (impure metal) becomes thinner. The metal obtained would be 99.99 percent pure. This pure copper is separated from the cathode later. The impurities are thrown back under the anode as anode mud.​