The separation of zinc and cadmium by distillation

An important element that must be recovered from zinc is cadmium, which is separated by distillation. The alloys of zinc with cadmium are regular solutions with a heat of mixing of 8300 Xcd fzn J gram-atom and the vapour pressures of the elements close to the boiling point of zinc (1180K) are 

The metals can be separated by simple evaporation until the partial pressure of cadmium equals that of pure zinc, i.e. 

It follows that die separation of cadmium must be carried out in a distillation column, where zinc can be condensed at the lower temperamre of each stage, and cadmium is preferentially evaporated. Because of the fact that cadmium-zinc alloys show a positive departure from Raoult s law, the activity coefficient of cadmium increases in dilute solution as the temperature decreases in the upper levels of the still. The separation is thus more complete as the temperature decreases. 

A distillation column is composed of two types of stages. Those above the inlet of fresh material terminate in a condenser, and are called tire rectifying  

In this equation, all of the terms except y +i and x but including x, are constant. Hence the relationship between and x is linear with a slope of L j(L + D) and a line representing the relationship on a graph of y vs jc must pass tlrrough y = jcd when x — jcd, since tire vapour and the liquid have the same composition in the product. This is called the rectifying operating line in a graphical representation of tire distillation process. 

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