FRACTIONAL CRYSTALLIZATION

Two types of fractional crystallization to consider:

  1. Batch Fractionation - where crystals are removed from the liquid in a batch, i.e., more than one crystal involved.
  2. Pure Fractionation - where each individual crystal, once it is formed is removed from the liquid, such that the liquid composition is constantly changing.

Batch Fractionation

Start with Composition X - 75% An - 25% Di.
  1. From X1 to X2, temperature drops.
  2. At X2, T = 1500°C, An begins to crystallize and is in equilibrium with liquid.
  3. From X2 to X3, temperature continues to fall, An + L, the liquid composition moves down the liquidus, towards the eutectic, to maintain equlibrium with the solid An crystallized in the interval X2 to X3. The proportions of An:L are changing, An increasing and L decreasing, as a result of the An crystallizing from the liquid.
  4. At X3 (T = 1400°C), we have An in equilibrium with liquid, proportions 47% An and 53% L. If we now fractionate, remove, all of the An crystals formed over the 100°C drop in T, from X2 to X3, we are left with liquid, L3. This liquid is referred to as a residual liquid as it the residue left over after An crystals were removed them from system.

    Once the An has been removed we are left with a liquid having a composition of 48% Di and 52% An. After removal of the An this 'new' liquid continues to cool.

    In the interval from L3 to X4, An crystallizes and the liquid composition once again moves down the liquidus surface towards the eutectic.

  5. At X4, we have 19% An and 81% L in equilibrium with each other. If we now fractionate the An, we are left with a 'new' liquid with a composition represented by L4, (42% An and 58% Di).

    This new liquid once again cools, and as it cools it again crystallizes An from L4 to X5. The Liquid composition moves down the liquidus surface towards the eutectic.

  6. At X5, at the temperature of the eutectic, Di begins to crystallize with An. The temperature of the system stays at the temperature of the eutectic until all the liquid is consumed. The final solid is a mixture of An and Di, in the proportions represented by the liquid composition L4 (the composition of the last residual liquid produced as a result of fractionation.

Pure Fractionation

Start with Composition X - 75% An - 25% Di

For pure fractional crystallization, as each grain of An is formed it is removed from the system such that a residual liquid is produced, giving a new starting liquid which produces a new grain of An, which when removed yields a new residual liquid.

The path followed during pure fractionation is a series of steps, with the drop representing the crystallization of An and the step representing the removal of the An. The result is that the liquid composition is constantly changing, in order to maintian equilibrium, moving down the liquidus surface towards the eutectic. Because the An is being constantly removed the overall system is becoming enriched in Di.

The last liquid will have a composition of the eutectic and will yield a solid mixture of An and Di, in the proportions given by the eutectic. The bulk compostion of this solid will be the eutectic composition.