The reduction of oxides and halides by reactive metals

An industrial example of the reduction of stable oxides to form a metallic element as a vapour, is the Pidgeon process for the production of magnesium. 

The reaction of magnesium oxide with silicon produces a very small vapour pressure of magnesium 

It was found that the reaction rate could be significantly enhanced if a fluorspar addition is made to the briquette with the objective of producing a small amount of a liquid phase at the operating temperature. There is a eutectic in the Ca0-Si02-CaF2 system at a temperature of 1398K. These studies show the importance of the role of a vapour species to transport chemical potentials in a solid-solid reaction, and also the desirability of introducing a small quantity of a liquid. 

The production of metals which form very stable oxides by the aluminothermic process, such as manganese, chromium and vanadium is carried out with reactants at room temperature which react to provide enough heat to raise the temperature of the products to high temperatures at which the whole system is liquid. The metal phase which is produced can therefore separate from the liquid slag which is formed. The production of chromium serves as a useful 

The small value of the entropy change reflects the fact that only liquids are involved in this reaction. The heat balance in carrying out this reaction may be calculted, according to Hess s law, by calculating the heat change at room temperature, and subtracting the heat required to raise the products to the final temperature. The data for this reaction are as follows  

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