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The purest forms of metals such as germanium and silicon are required for the process of making semiconductors. But it is hard to get the purest form of these metals. In 1951, William G. Pfann developed a refining method to get the purest form of metal, which is later known as zone refining. The zone refining process is also referred as the zone melting, floating-zone process, or travelling melting zone process.
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Keyterms: Metals, Germanium, Silicon, semiconductors, zone refining, zone melting, floating-zone process, travelling melting zone process, crystals, melting process, crystal
Zone Refining
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Zone refining is a modern technique to get the purest form of crystals from the impure elements specifically for the metals by using melting and crystallization processes. In this method, a narrow region of the crystal is first heated so that all the impurities get melted and forms a layer of the molten zone which moves along the areas of the crystal.
Zone refining is normally referred to as the method of purifying a crystal where there is a thin region of the crystal that undergoes a melting process. The molten zone formed is now moved across the crystal.
The impurities present in the metal are melted at the forward edge in the molten zone process and moved through the block of metal. The solidified pure element is left as residues behind in the process.
As they move through the block of metal, the impurities in the metal are concentrated in the melt and are transported to one end of the metal block.
Zone Refining
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Principle of Zone Refining
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The principle of Zone refining is based on the fact that impurity has more solubility in the molten state. Due to this, when a molten metal crystallizes during the cooling process, impurities are automatically excluded as they do not form any part of the pure crystal.
In the process of zone refining, impurities are removed by moving the molten zone through a moving heater, and the formed re-crystallized pure metal is left behind in the form of solid metal. In this gradual process, the zone should be moving as slowly as possible to get a highly purified form of metal.
Zone Refining Process
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In the zone refining process, a portable radiator is fixed toward one side of the impure metal pole which we need to purify. Now, fix the impure metal rod with a circular mobile heater which is fixed in a section loaded up with inert gas. This circular heater warms the pole radially and produces a region where the temperature is consistently raised to the melting point in a plane perpendicular to the axis of the rod. As the heater moves along the pole, the liquid zone also moves down the rod.
Now, heat the radiator very slowly. The impurities and pure metal both get liquefied, however the atoms of pure metal recrystallize while liquid impurities move with the heater or liquid zone. In this manner, after the cycle impurities get collected toward one side while solid pure metal remains at another end. This process is repeated many times to get pure metal.
Applications of Zone Refining Process
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- The zone refining process is normally used to get the purest form of elements. (Specifically the metals such as Germanium and Silicon)
- It is a commonly very effective process in the removal of impurities from semiconductors such as Ge (Germanium), Ga (Gallium), and Si (Silicon).
- Float zone refining or the Zone refining process is used in making the solar cells.
- The process is also used to make a single layer of crystal.
- The process is used for finding the organic and inorganic compounds as well.
- It is highly useful in preparing organic chemical standards for HPLC, absorbance, or fluorescence spectroscopy.
- The process is very useful for all those analytical methods that require the highest purity for standardizing or calibrating instrumentation.
- It is used for the process of crystallization to concentrate an enzyme, peptide, antibiotic, or any other thermally unstable substance.
- The process is used in the concentration of the heat-labile biological materials in preparation of the aqueous solution.
Limitations of Zone Refining Process
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- Zone refining is a very costly process.
- Its applications are restricted to research facility reagents and important synthetics.
- Zone refining process is not carried out effectively when strong-fluid equilibrium isn't good for the impurities.
- It is required to combine zone refining with other techniques to get a high form of purity.
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Things to Remember
- The process to get the purest form of crystals from the impure elements is known as Zone Refining.
- In this process, melting and crystallization processes are used.
- Zone refining is useful to get metals of high purity.
- It is also known as Zone melting, travelling melting zone and zone floating process.
- It is a very effective process to remove impurities from semiconductors such as Germanium (Ge), Gallium (Ga), and Silicon (Si).
- It is a very expensive process and sometimes requires other methods to remove impurities.
Sample Questions
Ques: What is the basic principle of zone refining of metals? (Comptt. All India 2012, 1 Mark)
Ans: Zone Refining is based on the principle that the impurities are more soluble in the molten state than in the solid state of the metal.
Zone Refining
Ques: Describe the principle controlling each of the following processes :>
(i) Zone refining of metals
(ii) Electrolytic refining of metals (All India 2011, 2 Marks)
Ans: (i) Zone refining of metals: This method is based on the principle that impurities are more soluble in the molten state of the metal than in the solid state. In zone refining, a circular mobile heater is fixed at one end of a rod of impure metal. As the heater moves, the molten zone also moves with the heater. Due to this, the pure metal crystallizes and the impurities pass to the adjacent molten zone. To get impurities at one end of the rod, this process is repeated several times.
(ii) Electrolytic refining of metals : In this process, the impure metal acts as anode and a strip of the same metal in pure form is used as cathode. They both are put in suitable electrolyte containing soluble salt of the same metal. The more basic metal remains in the solution and the less basic solution moves to the anode mud.
Example : In refining of Cu-
At anode: (oxidation)
Cu → Cu2+ + 2e–
At cathode: (reduction)
Cu2+ + 2e– → Cu
Ques: Which methods are usually employed for purifying the following metals :
(i) Nickel (ii) Germanium
Mention the principle behind each one of them. (All India 2012, 2 Marks)
Ans: (i) Nickel: Mond process.
In this method, the metal is converted into its volatile compound and collected in a different place. It is then decomposed to give pure metal.
(ii) Germanium: Zone refining.
Zone refining is based on the principle that the impurities of a metal are more soluble in the molten than in the solid state.
Ques: (a) Give an example of zone refining of metals.
(b) What is the role of cryolite in the metallurgy of aluminium? (Delhi 2013, 2 Marks)
Ans: (a) Zone refining is based on the principle that the impurities in a metal are more soluble in the molten state than in the solid state.
(b) The role of cryolite (Na3AIF6) in the metallurgy of Aluminium is to lower the melting point of the mixture and bring conductivity.
Ques: Outline the principles behind the refining of metals by the following methods: (Delhi 2014, 2 Marks)
(i) Zone refining method
(ii) Chromatographic method
Ans: (i) Zone refining of metals: This method is based on the principle that the impurities in the metal are more soluble in the molten state than in the solid state. In this process a circular heater is fixed at one end of a rod of impure metal.
(ii) Chromatographic method : This method is based on the principle that different components of a mixture are differently adsorbed on the adsorbent.
Ques: Describe the principle involved in each of the following processes of metallurgy: (All India 2010, 3 Marks)
(i) Froth floatation method
(ii) Electrolytic refining of metals
(iii) Zone refining of metals
Ans: (i) Froth floatation method: It is used for removing gangue from sulphide ores. In this powdered ore is mixed with collectors and froth stabilisers which enhance non-wettability of the mineral particles and froth stabilisation respectively. The ore comes with froth and gangue remains in the solution.
Froth floatation method
(ii) Electrolytic refining of metals: The impure metal acts as anode and a strip of the same metal in pure form is used as cathode. When both are dipped in the electrolyte containing soluble salt of the same metal, the more basic metal remains in the solution and the less basic metal moves to the anode mud.
Example : In refining of Cu
At anode : (oxidation)
Cu → Cu2+ + 2e–
Af cathode : (reduction)
Cu2+ + 2e– → Cu
(iii) Zone refining of metals: It is based on the principle that impurities are more soluble in the molten state of the metal than in the solid state. This process is repeated several times which leads to the collection of impurities at one end of the rod. Silicon, boron, gallium etc. can be purified by this process.
Zone refining of metals
Ques: i) Name the method of refining of metals such as Germanium. (Delhi 2016, 3 Marks)
(ii) In the extraction of Al, impure Al2O3 is dissolved in the cone. NaOH to form sodium aluminate and leave impurities behind. What is the name of this process?
(iii) What is the role of coke in the extraction of » iron from its oxides?
Ans: (i) Zone refining is used for refining Germanium.
(ii) Leaching of Alumina or Bayer’s process.
(iii) Coke (CO) acts as a reducing agent in the extraction of iron from its oxides in the zone of reduction in blast furnace at 1123 K.
FeO + CO → Fe + CO2
Ques: (a) What is the principle behind ‘zone refining’ of metals? Name an element which is refined by this method. (Comptt. Delhi 2016, 3 Marks)
(b) Write the name of the metal refined by each of the following processes :
(i) Distillation
(ii) Liquation
Ans: (a) Zone refining of metals: This method is based on the principle that impurities present in a metal are more soluble in the molten state of the metal than in the solid state. The process of zone refining is repeated several times which leads to the collection of impurities at one end of the rod. Silicon, boron, gallium etc. can be purified by this process.
Zone refining of metals
Germanium can be refined by the zone refining method.
(b) (i) Zinc can be refined by distillation.
(ii) Lead can be refined by liquation.
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