( iii ) Cupellation
This method is used when impure metal contains impurities of other metals which form volatile oxides .
e . g ., traces of lead ore removed from silver ( as volatile PbO ) by this process .
Chemical Methods
Poling This method is used when the impure metal contains impurities of Its own oxide , e . g ., CU2O in blister copper and SnO2 in impure Sn . The molten impure metal is stirred with green wood poles . At this high temperature . wood liberates gases such as CH4 which reduces any oxides present in the metal .
Electro-refining
In this method , impure metal forms the anode while the cathode is a rod or sheet of pure metal . The electrolytic solution consists of a soluble salt of the metal .
On passing electricity , the pure metal gets deposited on the cathode while the insoluble impurities settle down below the anode as anode mud or anode sludge . Metals like Cu , Ag , Au , Cr , Zn , Ni , etc are purified by this method .