Hint: In metallurgy, refining refers to the purification of metals. During refining, no chemical change occurs. The final material is chemically similar to the original material. impurities are more soluble in molten state than in solid state.
Complete answer: Zone refining is used to obtain highly pure elements. Silicon, germanium and indium are some examples of the metals that are purified by zone refining. These ultra pure elements are used in the preparation of semiconductors. Zone refining technique is based on the principle that impurities are more soluble in molten state, as compared to the solid state. Take a rod of impure elements. Place a heater below the rod. Move the heater slowly from one end of the rod to the other end. When heat is supplied with the help of the heater to a particular region of the rod, this particular region melts. When the heater is slowly moved from one end of the rod to the other end of the rod, the molten region of the element also moves slowly, along-with the heater, from one end of the rod to the other end. Now impurities being more soluble in molten state, than in solid state, the impurities move along-with the molten region, leaving behind the pure element in the solid state. This process is continued till the heater reaches the other end of the rod. Then, the direction heater is reversed and slow movement of the heater is continued. This process can be repeated several times till desired purity of element is obtained. By using this method, the impurities are concentrated in one end of the rod and then this rod is cut and discarded. The remaining portion of the rod contains highly pure elements.
Note: When germanium is refined by zone refining, only 7-10 ppm impurities remain.Thus, zone refining can be used to obtain ultra-pure elements. One of the advantages of zone refining is that highly pure metal is obtained. One of the disadvantages of zone refining is that it is a slow process.