Silver
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SILVER
Silver is a white, lustrous soft transition element. It is characterised by
- Atomic number 47
- Relative atomic mass 107.87
- Relative density 10.5
- Melting point 961.93OC
- Boiling point 2212OC
- Is highly malleable and ductile.
- A good conductor of heat and electricity.
OCCURRENCE
Silver is obtained in native form as well as in ores as argentite (silver glance) Ag2S, silver copper glance (Ag. Sn)2S, pyragyrite or ruby silver, 3Ag2S.Sb2S3 and horn silver AgCl. It is also present in the lead ore galena.
EXTRACTION
Silver is extracted from its ores by the cyanide process. In this process the silver compounds are dissolved in dilute sodium or potassium cyanide (KCN) solution. The silver is dissolved as KAg(CN)2. This process involves the following reactions
Ag2S + 4NaCN è 2NaAg(CN)2 + Na2S
4Na2S + 5O2 + 2H2O è 2Na2SO4 + 4NaOH + 2S
The metallic silver and the horn silver, which are also present, are also dissolved
4Ag + 8NaCN + 2H2O + O2 è 4NaAg(CN)2 + 4NaOH
AgCl + 2NaCn è NaAg(CN)2 + NaCl
RECOVERY OF SILVER
Silver is precipitated from the above solution by the addition of finely divided zinc powder. Silver is precipitated as a dark amorphous mass, while the zinc reacts and goes into the complex. Silver is now refined by the Electrolytic process. In this method, impure silver bars are made the anode. Pure silver “plates” out on a silver cathode and the impurities settle to the bottom as anode mud.