Some metals like gold
and silver are extracted from their concentrated ores by leaching. They are
dissolved in suitable reagents like acids or bases leaving behind insoluble
impurities. The metal is recovered from the solution by crystallization. For
Example, Silver ore is leached with dilute solution of sodium cyanide.
Silver dissolves forming a complex, Sodium Argento Cyanide. The solution
is further treated with scrap zinc to get the precipitate of silver.
Similarly, native
gold is leached with potassium cyanide solution and it is recovered from the
solution by addition of scrap zinc.
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