Solution:

• Zone refining is a method of purifying crystals, in which a narrow region of a crystal is melted, and this molten zone is moved along the crystal.
• The molten region melts impure solid at its forward edge and leaves a wake of purer material solidified behind it as it moves through the ingot.
• The impurities concentrate in the melt, and are moved to one end of the ingot.
• Zone refining was invented by John Desmond Bernal and further developed by William Gardner Pfann as a method to prepare high purity materials, mainly semiconductors, for manufacturing transistors.
• Eg: silicon, germanium etc are purified by this process.