Refining plays a crucial role in metallurgy. Any metal which has been extracted from its ore is usually impure in nature. This impure metal which is extracted is called crude metal. Refining is a method of removing impurities in order to obtain metals of high purity. The impurities are removed from crude metal by various methods based on the properties of the metal and the properties of impurities. Some of the methods involved in the purification of crude metal are:
- Zone refining
- Vapour phase refining
- Chromatographic methods
Distillation: This method is used for the purification of metals which possess a low boiling point such as mercury and zinc. In this process the impure metal is heated above its boiling point so that it can form vapours. The impurities do not vaporize and hence they are separated. The vapours of the pure metal are then condensed leaving the impurities behind.
Liquation: In this method, melting point of the metals are taken into consideration. Metals with low melting points are purified using this process. The melting point of the impurities is higher than the metal. The metals are converted into liquid state by supplying heat at a temperature slightly above their melting point. Pure metal melts and flows down from the furnace leaving the impurities behind.
Electrolysis: In this process the impure metal and a strip of pure metal is used. The crude metal is made to act as an anode and pure metal is made to act as a cathode. They are dipped in an electrolytic bath which contains the soluble salt of the same metal. As electricity is passed through the solution, the less basic metal moves towards the anode mud leaving the more basic metal in the solution. For example copper is purified using this method.
Zone refining: Metals such as germanium, silicon, gallium, indium and boron are made free from impurities using this method. In this process the impure metal is attached to a circular mobile heater at one end. As the heater is moved, the pure metal crystallizes out and the impurities pass on to the adjacent part of the metal. This way the impurities get accumulated at the other end of the rod which is cut in order to obtain the pure metal.
Vapour phase refining: In this type of purification, the metal should form a volatile compound in the presence of a reagent and it should easily decompose in order to recover the metal. The metal is transformed into its volatile compound. This volatile compound then undergoes decomposition in order to give pure metal. For example: nickel is purified in this manner.
Chromatographic method: In this method the crude mixture is put into liquid or gaseous medium. This medium is moved through an adsorbent. Different components of the mixture are adsorbed at different levels of the column. These components of the mixture are removed by using suitable solvents.
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