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(1) Ores : Argentite (silver glance)
Ag2S Horn silver (AgCl)
Ruby silver (Pyrargyrite)
3Ag2S.Sb2S3.
(2) Extraction : Cyanide process or Mac Arthus-Forrest cyanide process : This method depends on the fact that silver, its sulphide or chloride, forms soluble complex with alkali cyanides in the silver. This implies that silver compounds will dissolve in solution of alkali cyanides in the presence of blast of air.
4Ag + 8NaCN + 2H2O + O2
?
4Na[Ag(CN)2] + 4NaOH
air
or 4Ag + 8CN– + 2H2O + O2
? 4[Ag(CN)2]– + 4OH–
Ag2S + 4NaCN
? 2Na[Ag(CN)2] + Na2S
AgCl + 2NaCN
? Na[Ag(CN)2] + NaCl
The reaction with the sulphide is reversible and accumulation of Na2S
must be prevented. A free excess of air is continuously passed through the solution which oxidizes
Na2S into sulphate and thiosulphate.
2Na2 + 2O2 + H2O → Na2S2O3 + 2NaOH
Na2S2O3 + 2NaOH + 2O2 → 2Na2SO4 + H2O
2Na[Ag(CN)2] + 4NaOH + Zn → Na2ZnO2 + 4NaCN + 2H2O + 2Ag
(3) Extraction of Ag from argentiferrous lead (PbS + Ag2S)– Parke’s Process : It is based upon the following facts (i) Molten Zn and Pb are immiscible, zinc forms the upper layer (ii) Ag is more soluble in molten Zn (iii) Zn-Ag alloy solidifies earlier than molten Pb (IV) Zn being volatile can be separated from Ag by distillation. Ag is purified by cupellation.
Properties of Silver: Silver is a white lustrous metal, best conductor of heat and electricity. Being soft, it is alloyed. The silver alloy used for making jewellery contain 80% Ag and 20% Cu. The composition of a silver alloy is expressed as its purity i.e. the amount of Ag present in 1000 parts of the alloy Ag does not react with dilute HCl or dil.
H2SO4 and aqua regia but reacts with dil. HNO3
and conc. HNO3
forming NO
and NO2
respectively. Chlorine also reacts with Ag
to form
AgCl.
Hot conc. H2SO4
reacts with Ag
forming SO2
like Cu
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