Directional solidification is a type of solidification that occurs within castings. In other words, it is when the liquid metal solidifies from the farthest end of the casting to closest end.
Directional solidification can also be used as a purification process. Most impurities are more soluble in the liquid phase than in the solid phase during solidification. As a result, these impurities will be pushed by the solidification. This causes the finished casting product to have a lower concentration of impurities than the feedstock material.
The last solidified metal, however, will be filled with impurities. Thus, that last piece can be recycled.