Zone Refining
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	Zone refining is a material purification process that exploits the thermodynamics of phase diagrams to achieve lower impurity content than can be obtained by chemistry. In the top diagram, a material of composition Ca (red bar) starts to freeze at the temperature Ta, and the first solid (green bar) has a lower  impurity concentration, Cb, than the liquid, Ca.   A distribution coefficient, k = Cb/Ca describes this: the larger k the greater the purification. As T decreases, the solid composition tracks the solidus line and the liquid composition the liquidus line. This process continues until the isotherm intersects the solidus line at the original mean composition. The concentration profile along a rod of the material melted in a local zone that is passed along the bar is shown as n = 1 in the bottom diagram. Increasing the number of molten zones passed through the material sweeps the impurities to one end, much of the bar being purified by several orders of magnitude. The final solid line is for the passage of 20 zones through the material.