( 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.