Extraction of silver.
The extraction of silver is carried out by various techniques which usually depend upon the type of ore. The following processes are usually used:
- Cyanide Process.
Mostly silver is now extracted by this procedure. The principle of the procedure is to get Ag+ into solution as dicyanoargentate ion, [Ag(CN2] ¯ in presence of CN ¯ ions. Silver metal and its compounds are easily soluble in alkali cyanide solutions in the presence of air. The metallic silver is deposited from cyanide solution in the presence of reducing agents such as zinc or aluminum (hydroquinone may also be used as investigated by authors).
The finely divide ore, usually silver sulfide, is agitated with about 1% NaCN solution for about 48 hours in a current of air. Silver gradually goes into solution as sodium dicyanoargentate, Na [Ag(CN)2].
If the ore contains AgCl, the reaction is:
The silver is usually deposited from Na [Ag(CN)2] by treatment with Zn.
The precipitated silver is filtered, washed and then melted with KNO3 to oxidize any zinc.
The process of extraction of silver may be shown schematically as:
- Amalgamation Process.
The ore (Ag2S) is ground to a paste with water and treated with cupric chloride (or NaCl and (CuSO4) when Ag2S is converted into AgCl.
The products are mixed with metallic mercury when silver is deposited.
The silver thus obtained may form am amalgam in presence of excess mercury. Excess mercury may be removed by distillation in retorts.
- Extraction from Commercial Lead.
Small quantities of silver present in lead or lead ores can be economically extracted by Parkes’ process. This process is based upon the partition Law. According to that, molten lead dissolves about 1 % Zn and molten zinc dissolves about 1% pb. But Ag is completely soluble in molten zinc. Thus, by adding molten zinc to the fused lead-silver alloy, silver goes in molten zinc forming Zn – Ag alloy which solidifies and floats on the surface of molten lead. Silver is obtained pure by distilling zinc. This process is also called the desilverization of lead.
The small quantity of lead which may remain in silver is removed by Cupellation process. The mixture is placed in a cupellation furnace ( Figure 1.1) and a blast of hot air passed over it. Lead is converted PbO. PbO is blown off leaving behind Silver.