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.