Two examples from the zinc smelting industry

Let it be supposed, in the first place, that there are three phases present, namely, zinc oxide and carbon as immiscible solids, and a vapour phase consisting of CO, CO2 and zinc vapour. (The possibility of forming liquid zinc as a fourth phase will be discussed later.) The system would therefore be divariant if it were an entirely arbitrary mixture of the five species in question. However, the fact that it is prepared from zinc oxide and carbon implies a stoichiometric restriction on the composition of the vapour phase for every atom of zinc vapour there must be one atom of combined oxygen as CO or CO2. 

The system is therefore univariant, so that at any chosen temperature it has a definite total pressure and vapour phase composition. It may be noted that if the process is operated in a furnace open to the atmosphere it is impossible for any steady evolution of zinc oxide to occur until the condition 

The matter may also be discussed by the method of 6 6. The state of the system may be specified by means of the five variables, Py Pzn jPco jPco,- Between these there are four equations  

We consider now the possibility of the presence of an additional phase, namely, liquid zinc. If the temperature of the reaction system is raised above T the pressure will rise above 1 atm. It is found that the displacement to the right in the chemical reactions causes the partial pressure of the zinc vapour to rise more rapidly, with increasing temperature, than the vapour pressure of liquid zinc. The vapour 

Under such conditions, of course, the total number of atoms of metallic zinc iik the system is still equal to the number of atoms of oxygen which are present as CO and CO2. The following equation is .logo , 0(6.7, , 5,13). 

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