What is meant by Vapour phase refining? Write any one example of the process which illustrates this technique, giving the chemical equations involved.

OR

Write and explain the reactions involved in the extraction of gold.

In the vapour phase refining, the metal to be refined is converted into its volatile form which is collected and decomposed to give pure metal. This two technique is successful under two conditions:

(i) The metal should form a volatile compound with the given reagent

(ii) The volatile compound should be easily decomposable so recovery is easy.

An example of vapour phase refining is the Mond Process for refining Nickel. Here, nickel is heated in a stream of carbon monoxide to form a volatile complex named as nickel tetracarbonyl. This complex is decomposed at a higher temperature to obtain pure metal. The reactions are given as:

OR

Gold is a noble metal which is extracted from ores by leaching with reagents. Extraction of gold is carried out by leaching the metal with CN^-, which can be a dilute solution of NaCN or KCN, in the presence of air with O₂. This is an oxidation reaction given by Au → Au⁺. The metal is later recovered by displacement reaction where zinc acts the reducing agent.