Refining Process Against Impurities

A single type of process is not effective for extracting all types of metals. Different metals have different properties. According to those properties, suitable techniques against impurities are developed and put to use to extract them with maximum purity. Some of the refining processes are explained below:

Liquation 

The basic principle of liquation is the difference in the melting points of impurities and the metal. When the impure metal is allowed to melt, the element with a lower melting point melts first. It is later separated. 

Examples:

• Purification of tin metal: The impure tin metal contains Cu, Fe, W, etc., as impurities. It is placed on the sloping hearth of a reverberatory furnace and gently heated. The temperature of the furnace reaches the melting point of tin first as it has a lower melting point. 

Tin melts earlier than the impurities and hence flows down the inclined hearth. The solid impurities are left behind on the hearth. The pure tin metal is collected in a cast iron vessel in the molten state. The metal obtained in this manner is called pig iron.

• Purification of crude zinc: Crude zinc is melted on the sloping hearth of a reverberatory furnace. Molten zinc flows down, while the solid impurities are left behind on the hearth. 

Fractional Distillation

The basic principle of this process is the difference in the boiling points of the metal and impurities. If the metals are volatile, the impurities in them are non-volatile, and vice-versa. Metals like Zn, Cd and Hg are purified by fractional distillation. 

Zone Refining (Fractional Crystallisation) 

Zone refining is used when metals are required in very high purity. Silicon and germanium used in semiconductors are required in highly pure form. Hence, they are purified by this method. This method is based on the principle that after some time of melting of an impure metal, pure metal recrystallises upon gradual cooling and deposits crystals in pure form. At the same time, the impurity remains in liquid form, and it moves ahead towards the end of the rod.

In fractional crystallisation, the impure metal to be refined is taken in the form of a rod. A circular heater H is fitted around this rod and slowly moved along its length. At the extreme left of the impure germanium rod, the heater melts a narrow zone of the rod. As the heater moves rightwards, the molten metal cools down and crystallizes to give pure metal at region X of the rod. The impurities earlier present in region X of the germanium rod pass on to region Y, which is the adjacent molten zone. 

The heater continues to shift rightwards into newer and newer molten zones. Ultimately, the impurities reach the extreme right end, Z, of the germanium rod. This end of the rod is left with all the impurities and discarded. The rod thus obtained is highly pure germanium metal. In addition to germanium, silicon and gallium are also used as semiconductors. They are also refined by the zone refining method.

Chromatography

Different components of a mixture are differently adsorbed on an adsorbent. The mixture is put in a liquid or gaseous medium, and it is moved through the adsorbent. After that, the adsorbed components are removed by using a suitable solvent. 

The chromatographic method is named based on the physical states of the moving medium and the adsorbent material and also based on the passage of the moving medium. Chromatography is useful for the purification of elements that are present in minute quantities. In such cases, the chemical properties of impurities are not very different from those of the desired element. There are several chromatographic techniques, such as paper chromatography, column chromatography, and gas chromatography.

Electrolytic Refining

Metals such as Cu, Ni and Al are refined electrolytically. Impure metal is used as the anode, and pure metal is used as the cathode. The cell consists of three liquid layers. The bottom layer is impure molten metal; the middle layer is a fused salt layer containing the electrolyte; the top layer is pure metal. At the anode (bottom layer), metal passes with solutions as ions. And at the cathode (top layer), these ions are reduced to form pure metal. In operation, molten metal is added to the bottom of the cell, and pure metal is drawn off the top. 

Vapour Phase Refining

Vapour phase refining is carried out through the Van Arkel-De Boer process. Small amounts of pure metals (Ti, Zr, or Bi) can be produced by this method. This process is based on the fact that iodides are the least stable of all halides. 

The impure element is heated with iodine to produce a volatile iodide. It is decomposed by passing the gas over it. The element is deposited on the filament, and iodine is recycled.

Impure Ti + 2I2  → TiI4 →Ti + 2I2

50-250 degree C Tungsten filament

Conclusion

Metals extracted from ores contain impurities, and they have to be refined using appropriate techniques. The procedure selected is based on the behaviour and chemical properties of impurities and the metal. Some of the refining processes used to remove impurities are liquation, distillation, zone refining, and electrolytic refining.