Zone Refiner Theory

During zone refining or zone melting short molten zones move slowly through an elongated sample of crystalline material.   A molten zone traversing a solid sample has two liquid-solid interfaces; a melting interface and a freezing interface. At the melting interface the sample is merely melted and mixed with the contents of the molten zone. At the freezing interface the crystals tend to be higher in purity than the liquid phase.

Extraordinarily high purification of chemicals can be obtained by repeatedly passing the molten zone or zones through the sample. The impurities are concentrated at the end of the sample, generally in the direction of the movement of the molten zone.

Mathematically this can be expressed as a constant, K, known as the distribution coefficient.  If the concentration of impurity in the solid (crystalline) phase is Cs and the concentration of impurity in the liquid phase is Cl then K is Cs/Cl.  If K is less than 1 (the usual case), the impurities will follow the movement of the liquid zone; if K is greater than 1, the impurities will travel in the opposing direction.

Free Software

Design Scientific has developed a short computer program that allows the rapid calculation of Zone Refiner operation.  This program can be downloaded free by filling out the form below.  In the example displayed below, a zone rate of 10 cm/hour is selected and the purification profile of an impurity having a distribution coefficient (K) of 0.5 at the 1% level is simulated. Twenty curves are displayed indicating four hours of operation or the passage of 20 molten zones. Each line represents one passage of a molten zone through the sample; the migration of the impurities to one end of the sample is readily apparent. This Zone Refiner Simulator Program helps one to understand the power of this technique.

How the program operates

The Zone Refiner Simulator Program divides the 10 cm sample length into 1000 increments. As each increment freezes at the face of the moving frozen zone the impurity will usually have a lower concentration in the crystalline phase than in the liquid phase. This ratio is known as the distribution coefficient (K). The program adds the amount of impurity frozen out of the solid increment to the total concentration of the impurity in the entire molten zone (the molten zone is 100 increments or 1 cm long). This assumes that stirring in the molten zone is fast relative to the zone movement. As each molten zone passes through the sample the cumulative effect of the melting/freezing process can be easily studied. When the molten zone moves to the end of the sample container, the computer program adds the impurity concentration to the smaller molten zone. This causes the concentration of the impurity to appear to have a discontinuity at a distance of 1 cm from the end.

Examination of the concentration curves shows why starting with reasonably pure material is important if high purity is desired. As the impurities accumulate at one end of the sample they tend to limit the purification level possible for at least the last half of the sample.