Metals and Non-Metals - Part XI

Refining of metals
Free metals obtained by various reduction processes may have several impurities which may be other metals. For example while obtaining Na, K is a common metal that may be separated along with Na. Thus K is an impurity here. These impurities have to be removed by processes known as refining. Refining is nothing but purification of metals. There are various methods of refining; some of which we will discuss below.

i) Liquation method : In liquation method, metals with low melting points are refined. Metals like Sn, Pb, Bi have low melting points compared to the impurities in the metal. A block of impure metal is placed on a sloping furnace. The temperature of the furnace is maintained a bit above the melting point of the metal to be refined. The metal melts and flows off the slope, it is then collected and cooled.

ii) Distillation method : Zn, Cd and Hg form vapours easily. They can be heated and distilled out from their impurities. Impure metal block is heated at a temperature so that the metal atoms start to evaporate. The temperature is then held constant. The vapours are condensed and separated out in a container called receiver. Non-volatile impurities are left behind in the distillation chamber.

iii) Oxidation method : Sometimes impurities are able to get oxidized more easily than the metal itself. In this case oxidative method is used. For example if impurities are S, C, Si or P, they can get oxidized more easily than the metal itself. For example in case of pig iron Fe, these non-metals are present as impurities. When air is passed over hot molten pig iron, these non-metals get oxidized to CO₂, SO₂, P₂O₅ and can be removed easily.

iv) Electrolytic refining : In this method electrolysis is used to refine metals. Metals like Cu, Zn, Sn, Pb, Ag, Au are refined by electrolysis method. In an electrolytic cell, a block of impure metal is made into the anode, a thin strip of pure metal is made into a cathode, and an electrolyte is made out of a suitable metal-salt of the metal to be refined. When an electric current is passed through the cell, ions from the anode enter the electrolyte. The same number of metal ions from the electrolyte gets deposited on the cathode. This is a preferential deposition. Impurities remain in the electrolyte. Some of the impurities may be deposited below the anode. As an example we will study electrolytic refining of copper,

Electrolytic refining of copper : In an electrolytic tank, acidified copper sulphate (CuSO₄ + dil H₂SO₄) solution forms the electrolyte. A block of impure copper is made into an anode by connecting the positive terminal of a power supply (battery). A thin strip of highly pure copper metal is the cathode of the cell. The negative terminal of the power supply is connected to it.  

A small electric current is passed through the cell.  Atoms from the anode enter the electrolyte. The copper from the anode gets converted into copper sulphide. An equal number of copper atoms from the solution get deposited on the cathode. This is to keep the concentration of the solution constant. Impurities from the anode block either remain in solution or collect below the anode, as they are unable to displace copper form the sulphate solution. The impurities remain insoluble in the electrolyte and they are called anode mud.

Copper sulphate solution contains ions of Cu⁺⁺ and SO₄^--. The following reactions take place at the anode and cathode when an electric current is passed.  

Pure copper is scraped or removed from the cathode. Anode becomes thinner as the electrolysis process proceeds. Some important metals like gold and silver are present in the anode mud. These can be recovered separately.

High purity metals
Besides the processes for refining metals discussed above, sometimes, some more processes have to be employed to obtain really high purity metals. Highly pure metals, with impurities less  0.1%  are needed in semiconductor industries, atomic energy application, space applications, etc. We will discuss here two methods, namely, Van Arkel method and zone refining method used for getting ultra pure Ti and Ge respectively.

1) Van Arkel method : In this process metals are converted into other metal-compounds like metal-iodides. Metal-iodides are decomposed easily and highly pure metals are then obtained. For example in a pure block of Ti, Fe can be an impurity. This block is heated in iodine at a temperature of 250°C. Ti converts into TiI₄ but the impurities do not. TiI₄ is passed over hot tungsten filament so that TiI₄ is decomposed and ultra pure Ti is obtained. The reaction is shown below.  

2) Zone refining : This method applies the fact when a metal crystallizes on cooling, impurities are automatically expelled as they do not form part of the crystal. Ge metal is refined by zone refining. Impure germanium is made into a rod. A heater in a circular geometry heats the rod from all direction. The heater can be moved over the length of the rod. First a piece of ultra pure Ge (seed) is attached to the rod on the left hand side. The heater is placed over it. The Ge seed and the impure Ge rod melts. The heater is moved to the right slowly. The molten seed and the Ge from the rod re-crystallize as the temperature drops. The ultra pure Ge crystal grows in size. The impurities from the Ge rod are expelled from the re-crystallized Ge. As the heater is moved to the right, the ultra-pure Ge moves in the left direction and the impurities move in the opposite direction. Other metals refined by this process are Si and Ga.

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