156.

Describe the principle behind each of the following processes:

(i)Vapour phase refining of a metal.

(ii)Electrolytic refining of a metal.

(iii)Recovery of silver after silver ore was leached with NaCN.

(i) Vapour phase refining
Vapour phase refining is the process of refining metal by converting it into its volatile compound and then, decomposing it to obtain a pure metal. The basic principles involved in this process are:

(a) The metal should form a volatile compound with an available reagent, and

(b) The volatile compound should be easily decomposable so that the metal can be easily recovered.

 Nickel, zirconium, and titanium are refined using this method.

(ii) Electrolytic refining is the process of refining impure metals by using electricity. In this process, impure metal is made the anode and a strip of pure metal is made the cathode. A solution of a soluble salt of the same metal is taken as the electrolyte. When an electric current is passed, metal ions from the electrolyte are deposited at the cathode as a pure metal and the impure metal from the anode dissolves into the electrolyte in the form of ions. The impurities present in the impure metal gets collected below the anode. This is known as anode mud.

(iii) In the process of leaching, the finely divided silver is treated with a dilute solution of sodium cyanide while a current of air is continuously passed. As a result, silver passes into the solution.

forming sodium dicyanoargenate (I) while the impurities remain unaffected which are filtered off.

Ag₂S + 4NaCN --->  2Na [Ag(CN₂)] + Na₂S

                                        Sodium dicyanoargenate (I)