Occurrence and principles of extraction of silver
Atomic mass : 108 Valency : 1
Atomic number : 47 Symbol : Ag
Position in the periodic table : Period Number -5, Group Number -11.
Silver occurs both in the native as well as in the combined state.
The important ores of silver are
i) Argentite or silverglance, Ag2S
ii) Horn silver or chlorargyrite, AgCl
iii) Pyrargyrite or Ruby silver, Ag2S . Sb2 S3
The chief ore of silver is Argentite. The silver content in these ores is very small(about 1%). Silver is obtained to a small extent from the gold ores in the kolar fields in Karnataka and in the Anantapur mines.
Extraction of silver from the Argentite ore
Silver is extracted from the argentite ore by the Mac-Arthur and Forrest's cyanide process. The various steps involved in this process are as follows.
The crushed ore is concentrated by froth-floatation process.
2. Treatment of the ore with NaCN
The concentrated ore is treated with 0.4-0.6% solution of sodium cyanide
for several hours. The mixture is continuously agitated by a current of air, so that Ag present in the ore is converted into soluble sodium argento complex.
Ag2S + 4NaCN -- > < - 2Na [Ag(CN)2] + Na2S
Sodium argento cyanide (soluble)
3. Precipitation of silver
The solution containing sodium argento cyanide is filtered to remove insoluble
impurities and filtrate is treated with zinc dust, silver gets precipitated.
2Na [Ag(CN)2] + Zn ® Na2[Zn(CN)4] + 2Ag¯
4. Electrolytic refining
The impure silver is further purified by electrolytic refining. The impure silver is made the anode while a thin sheet of pure silver act as the cathode. The electrolyte is silvernitrate acidified with 1% nitric acid. On passing electricity pure silver gets deposited at the cathode.
5. Extraction of silver from silver coins
Silver coins are Ag-Cu alloys. Ag can be obtained from such an allloy by dissolving the alloy in concentrated HNO3, a solution containing mixture of AgNO3 and Cu(NO3)2 is obtained. The solution is boiled to expel excess of nitric acid, then the solution is treated with con.HCl, silver is precipitated as AgCl. AgCl is separated and converted to silver by fusing with excess Na2CO3.
4AgCl + 2Na2CO3 ® 4Ag¯ + 4NaCl + 2CO2 + O2
The silver thus obtained, is purified by fusion with borax and followed by electrolytic purification.
i) It is a white lustrous metal.
ii) It melts at 1233 o C and boils at 2485 o C.
iii) Molten silver absorbs about twenty times its volume of oxygen which it again expels on cooling. Globules of molten silver are thrown off. This is called "spitting of silver". This can be prevented by covering the molten metal with a layer of charcoal.
i) With dilute HNO3
Dilute HNO3 reacts with silver liberating nitric oxide.
3Ag + 4HNO3 ® 3AgNO3 + NO + 2H2O
ii) With con.HNO3 or con.H2SO4
Hot con.HNO3 or con.H2SO4 react with silver liberating NO2 and SO2 respectively.
Ag + 2 HNO3 ®AgNO3 + NO2 + H2O
2Ag + 2H2SO4 ® Ag2SO4 + SO2 + 2H2O
iii) Action with chlorine
Chlorine attacks silver, forming silver chloride.
2Ag + Cl2 ® 2AgCl
iv) Action of alkalies
Silver is not attacked by alkalies.
Uses of silver
i) Silver salts are used in silvering of mirrors
ii) AgBr is used in photography
iii) Silver salts are used in silver plating.
iv) Silver is used in making electrodes.
v) Silver leaf is used in medicine, while silver amalgam is employed in dental