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Question

Name the important ore of silver. Write all the steps and reactions involved in the Cyanide process for the extraction of silver from its ore.

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Solution

The important ore of silver are Argentite (Ag2S) and Horn Silver (AgCl).
Extraction of Silver from ore by cyanide process: In this process the finely powered ore is concentrated by froth flotation process and then treated with dilute NaCN and a current of air is passed into the solution so that Ag present in the ore is converted into soluble sodium argento cyanide complex.
Ag2S+4NaCN2Na[Ag(CN)2]Sodiumargentocyanidecomplex+Na2S
The current of air passed into the solution oxidises Na2S formed in the above reaction into Na2S2O3,Na2SO4 and S thus enables the above reversible reaction to proceed in forward direction.
2Na2S+2O2+H2ONa2S2O3+2NaOH
4Na2S+5O2+2H2O2Na2SO4+4NaOH+2S
Then the solution containing Na[Ag(CN)2] is treated with Zn dust so that Ag gets precipitated.
2Na[Ag(CN)2]+ZndustNa2[Zn(CN)4]+2Agppt
Precipitated Ag is purified by electrolytic method. The cathode is pure silver, anode is block of impure silver and electrolyte is 6% AgNO3 with 1% HNO3. On passing current, impurities get deposited at the anode and pure silver at the cathode. Zn and Cu remain in solution while Au settles down as anode mud.

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