ZONE REFINING OF METALS

Zone refining is a kind of process for refining metals. The metals we see today are far from how they are found in the form of ores on the earth’s crust. The origin of metals in the form of ores contains many impurities in many forms such as oxides of the metal, sulphurs and other impurities.

To achieve the metals that are usable and endure high amounts of stress we must refine or in other words purify these metals. This is also called metal extraction.

The refining of metal is based on different properties of metal i.e each type of refining is based on the specific type of metal property and the impurity in it and uses various devices like circular mobile heater.

Refining of metals is an essential part of  metallurgy. Metallurgy deals with the refining of the impure metal to achieve its purest form.

Types of metal refining

• Distillation 
• Liquation 
• Electrolysis 
• Vapour phase refining 
• Chromatographic methods
• Zone refining

Distillation

Only metals that have a low boiling point can be purified using this method. In distillation the metal is heated above the boiling point of the metal which results in the formation of vapours. These vapours are stored in cooling chambers which are moved into another storage unit , the impurities are left behind as the vapours contain only the pure metal which is condensed. Some examples of metals that are purified using distillation are mercury and zinc.

Liquidation

In this method of metal refining, metals with a low melting point are considered. The metal of these low melting points is used for purification. It is imperative that the impurities must have a higher melting point than the metal needed to be extracted. In this process the metals are melted, converting them into a liquid state by heating the metal to a temperature just above the melting point of the metal , this results in the pure metal melting and flowing out of the furnace therefore the impurities are left behind in the furnace.

Electrolysis

In electrolysis, the impure metal must act as anode, which loses ions during the process and the pure metal acts as the cathode which gains ions during the process. Here the impure metal is used as an anode and an additional strip of the pure metal is used in the cathode. These metals are dipped into an electrical solution which is made of the salt of the same pure metal , the salt should be dissolvable. When electricity is passed through this electrolytic solution the metal with lower basicity is transferred to the anode and the metal with higher basicity stays in the solution.

Metals like aluminium and copper are refined using electrolysis.

The reactions are:

Anode: M → Mn+ + ne–

Cathode: Mn+ + ne– → M

Vapour phase refining

In this process of vapour phase refining, the metal must form a volatile compound when there is a reagent present and the compound so formed must easily decompose for the metal to be recovered. This results in the formation of the metal volatile compound and this compound is the one that undergoes the decomposition to provide the pure metal.

Metals like nickel and titanium are refined using the vapour phase refining.

Chromatographic Method

In the chromatographic method of metal refining , the crude mix or the impure metal ore is locked into a liquid or gaseous medium , thuis medium is made to move through an absorbent. This absorbent will absorb different components at different levels of the column. This happens as different components are removed according to the solvent suitable for the impurities.                                  

Zone Refining

The main principle behind zone refining is that impurities are more soluble in molten form in comparison to the solid state of metal.

Zone refining is a very significant technique of zone-melting. In this technique, an ore is refined by letting the pure metal in the ore pass through several molten zones of the rod across it in a single path. As one zone gets heated, impurities flow into the zone from the adjacent region. Therefore by the time the zone moves towards the end of the given ore body, it contains all the impurities in the ore leaving the rest of the ore containing only pure metal. The implementation of zone refining usually uses machines like circular mobile heaters.

Zone refining was initially defined by an American scientist by the name of W.G. Pfann in the early years of the 1950s for the purification of germanium for the manufacturing of transistors. The level of purification was to date never heard of, i.e., less than one fraction of detectable impurity in 10 billion parts of germanium. This technique was taken up in manufacturing transistors across the globe that are also used in devices like a circular mobile heater.

Examples and Limitations of Zone Refining

Zone Refining is used when there is a requirement for ultra-pure metals. These metals are usually used in laboratories and therefore need to be pure to avoid any impurities from altering a chemical reaction in which the metal is being used. Popular metals like tin, lead and bismuth are refined using zone refining when they are required for laboratory usage.

The only limitation of zone refining is that it is very expensive. Zone refining can achieve a very high purity of metals, but in order to attain this purity the resources required to achieve the given temperature for the refining is very costly. Zone refining also cannot be done on all metals, rather it is done only for valuable chemicals and metals for the laboratory. It is also often combined with other methods to achieve even more purity in the reagent required.

Conclusion

Zone refining is one kind of  process of refining metals , the metals we see today are far from what they actually are when they are found in the form ores on the earth’s crust. The origin of metals are ores, these ores contain many impurities in many forms such as oxides of the metal, sulphurs and other impurities.

Zone refining is very time consuming but it is a process that yields a high purity of metals. Hence it is used only for ores that have many impurities and conventional extraction methods would not suffice.