Characteristics of Ellingham Diagram
1 . All the plots slope upwards since ΔG ° becomes more positive when temperature increases , i . e ., stability of oxides decreases .
2 . A metal will reduce the oxide of other metals which lie above it in Ellingham diagram , i . e ., the metals for which the free energy of formation
( ΔG ° f ) of their oxides is more negative can reduce those metal oxides which has less negative ΔG ° f
3 . The decreasing order of the negative values of ΔG ° f of metal oxides is Ca > Mg ( below 1773 K ) > AI > Ti > Cr > C > Fe > Ni > Hg > Ag
Thus , AI reduces FeO , CrO and NiO in thermite reduction but it will not reduce MgO at temperature below 1773 K .
Mg can reduce A12O3 below 162 K but above 1023 K , Al can reduce MgO .
4 . CO is more effective reducing agent below 1073 K and above 1073 K . coke is more effective reducing agent , e . g ., CO reduces F2O3 below 1073 K but above it , coke reduces Fe2O3 .
Coke reduces ZnO above 1270 K .
Refining or Purification of Crude Metals
Physical Methods
( i ) Liquation This method is used for refining the metals having low melting points ( such as Sn . Pb , Hg , Bi ) than the impurities , The impure metal is placed on the sloping hearth and is gently heated . The metal melts and flows down leaving behind the non-fusible impurrties .
( ii ) Distillation This is useful for low boiling metals such as Zn , Hg . The impure liquid metal is evaporated to obtain the pure metal as distillate .