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Metallurgy is a very useful scientific technique that is used for the economic extraction process of metals from their compounds. Metals can be in both forms natural and combined states. The impurities in the ores are known as matrix or gangue. The metals obtained in large amounts in their free state are Gold, Copper, Platinum, and Silver. To obtain the other form of pure metals, one has to obtain it through general principles of chemistry. Several such metals have a revolutionary impact on mankind for their uses in worldwide industries. To obtain the metals in their pure form, the general principles and processes of Isolation of elements from their ores have the following steps- Concentration of the ore, Isolation of the metal from its ore and Purification of the metal.
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Key Terms: Metallurgy, Metals, Ores, Isolation, Elements, Principles, Thermodynamic, Purification, Extraction, Concentration of Ore
Occurrence of Elements
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A metal that exists in the metallic form, either as an alloy or as a pure metal is a native metal. Some examples of the same are Cobalt, Cadmium, Indium, etc. Aluminium is the first largest element that exists in nature and iron is the second largest.
- An alloy is a mixture of two or more elements but the majority is taken by metals. An alloy makes the metal harder, resistant to corrosion and less brittle. Some examples are Stainless steel, titanium, and high carbon sheets. These are also used in jet engines.
- Minerals are naturally occurring homogenous inorganic solid substances that have a definite chemical composition whereas Ores are naturally occurring homogenous inorganic solid substances through which metal can be extracted economically.
Occurrence of Elements
There are several steps involved in the extraction of pure metal from Ores. The major steps are as follows:
- Concentration of the Ores
- Isolation of the metal from its concentrated ores
- Purification of the metals
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Concentration of Ores
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It is the process of the isolation of the ores from gangue. The process includes an upward stream of water used. The process of removal of unwanted material from the ores is known as the Concentration or Dressing or Benefaction of the Ores.
The different procedures for the concentration of ores are
- Hydraulic Washing
This process is based on the difference between the gravity of the particles of the gangue and the weight of the metal. This process can also be called ‘Gravity Separation’ So due to gravity the heavy particles i.e. the metal, settle down. Here the crushed ores mixed with the upward stream of running water. The lighter particles of the matrix wash away in the running water.
Hydraulic Washing
- Magnetic Separation
This process is based on the principle of the magnetic properties of the non-magnetic gangue. The powdered ore is placed on the roller belt which is the magnetic belt. If the ore particle is capable of being attracted to the magnetic field, this process can be used. Magnetic material gets attracted to the belt & stays & gets separated. Usually used for Iron and Manganese Ores.
Magnetic Separation
- Froth Floatation Process
In this process Separation of Hydrophobic materials from Hydrophilic takes place i.e. gangue is separated from the Sulphide Ores. The suspension of powdered ore is prepared by using water. Then the froth stabilizers, collectors & suspension are added.
The process uses the difference in wetting characteristics of minerals & the gangue. Gangue is wetted by water & minerals are wetted by oil & a mixture is prepared of crushed ore. The light-weighted froth formed to rise. It contains minerals that can be separated.
Froth Floatation Process
- Leaching
This process can be used when an ore is soluble in a solvent. It contains Iron oxide, Silicon Dioxide and Titanium Oxide. The process Leaches out as sodium aluminate leaving the rest of the impurities.
- Leaching of Alumina from Bauxite
Bauxite is an ore of Aluminium. The concentrated solution of the sodium hydroxide is used for concentrating the ore. The carbon dioxide is passed through & hydrated bauxite is obtained. The hydrated alumina is heated & dried to obtain pure bauxite.
Extractions of Crude Metal from Concentrated Ore
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Ore is considered the Crude Metal. It is a solid mass through which a pure metal can be procured. In the procedure, the ore should be converted into a form that is suitable for the reduction. And further goes into reduction.
The following steps are the main steps in the extraction of the Metals:
- Convert Ores to form the oxides
- Reduction of Metal oxides
Convert Ores to Oxides
Oxygen is more electronegative than the other elements. For oxides, it is easier to accept the electron pair & be reduced. Further steps for the same are
- Calcination: The process of heating a substance in the limited supply of oxygen. The unwanted matter leaves the metal oxide. The temperature of the Metal should be below the melting point.
- Roasting: In this process, the ore is heated in the furnace in the normal supply of air. The temperature of the metal should be within the melting point.
Reduction of Metal Oxides
Metal oxides are heated in the presence of a suitable reducing agent that will give a pure metal. The reducing agent used is usually Carbon. Oxygen combined with Carbon gives a pure form of metal.
Refining
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The various methods still do not confirm the refined purity of the metal. So to remove the remaining impurity after the extraction of the metals they undergo the following techniques for the refining process.
- Distillation: It is a simple technique in which the metals are heated beyond the melting point & the impure parts of the metal are evaporated through which pure metal is obtained.
- Liquation: This technique is useful for metals with low melting metals such as tin. The process makes the metal pure as during the process the high melting metal do not move & the low melting metal move to the sloping hearth.
- Zone Refining: In this refining technique, at the end of the impure metal rod, a mobile heater is fixated. Through this, the pure metal gets crystallized & the impurities pass to the molten zone. Silicon is one of the metals to be refined through this method.
- Chromatographic Methods: In this technique, an absorbent is used as a column & through its absorbing properties can absorb different components. The components can be absorbed in different levels of the column. Another solvent is used as the eluent to use these components & be purified.
- Electrolytic Refining: The principle of electrolysis is used in this technique. The pure metal is used as a cathode & an impure metal is used as an anode. Refining of Zinc and Copper is done through this method.
- Vapour Phase Refining: This technique requires two steps. Firstly, the metal is converted into its volatile compound & then further it is decomposed to give pure metal.
Electrolytic Refining
Uses of Iron, Zinc, Copper & Aluminium
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Some of the uses of some important metals are:
Uses of Iron
- Iron can be used as a Catalyst in many reactions.
- It is used for the process of Metallurgy.
- It is used for the purposes like manufacturing toys, utensils, and steel.
- Wrought Iron is used in the making of wires chains, etc.
Uses of Zinc
- Zinc is used in multivitamin tablets.
- It is used for automobiles.
- It is used as a component of electrochemical cells & batteries.
- It is used as a reducing agent in dyestuffs & paints.
- It is used for galvanizing the Iron.
Uses of Copper
- It is used in alloys.
- It is used in electrical bus bars.
- Copper is also used for treating ailments.
- It is used in circuit boards.
- It has antimicrobial properties.
- It is also used in the construction of monuments, buildings, etc.
Uses of Aluminium
- It is used in electrical supplies. Al is used for the distance power lines.
- It is used in the making of bodies of mobile phones, laptops, etc.
- It is used in the making of decorative items, chairs, picture frames, etc.
Thermodynamics Principles of Metallurgy
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The study of thermodynamics is a link between thermal energies. It is the transfer of energy that occurs during physical & chemical transformation. Thermodynamics is considered an important metallurgical process. The thermodynamics can be applied to the Process of Ore extraction. The most crucial notion of thermodynamics is Gibbs Free Energy. ΔG° if negative, then the reaction will be spontaneous. ΔH° represents the change of enthalpy. ΔS° is the change of temperature in entropy.
ΔG = ΔH - TΔS
The equation that connects the Equilibrium constant & the Gibbs Free Energy is
ΔGo = RTlnKeq
The constant of Equilibrium is Keq.
For the ΔG to become negative, the entropy change should be positive & temperature to be increased. The TΔS value should be exceeding the enthalpy reaction of change. If ΔG becomes positive, that is because it is coupled with the reaction that has a large negative ΔG. The Ellingham diagram is used as the tool in the extraction of the metal in Metallurgy.
Things to Remember
- Two principles of Chemistry, namely Extraction and Isolation are used to obtain the pure form of the metal.
- Metals present at the top of the reactive series are highly reactive metals.
- The steps involved in the extraction of pure metal from ores are- Concentration of the Ores, Isolation of the metal from its concentrated ores and Purification of the metals.
- The total refining of the metal purification depends on the chemical and physical properties of the particular metal.
- Ellingham diagram shows the relation between the stability of a compound and the temperature.
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Sample Questions
Ques. Which method is used for refining of zirconium? Explain. (3 Marks)
Ans. Zirconium and Titanium are refined by the van Ankle process, Here the crude metal is heated in an evacuated vessel with iodine.
Zr + 2I2 → Zr I4
There is the decomposition of the metal iodide on a tungsten filament, electrically heated to about 1800K and pure metal is deposited on the filament.
Zr I4 → Zr + 2I2
Ques. Differentiate between a mineral and an ore. (3 Marks)
Ans. Minerals are the Naturally occurring chemical substances in the form in which the metals occur on the Earth along with the impurities, whereas,
Ores are the mineral form in which metal can be extracted conveniently & economically.
Therefore, All Ores are minerals but all Minerals cannot be Ores.
Ques. What is the function of SiO2 in the metallurgy of copper? (3 Marks)
Ans. During roasting, copper pyrites are converted into a mixture of FeO and Cu2O. To remove basic FeO, acidic flux silica is added during smelting. Now FeO combines with SiO2 (silica) to form ferrous silicate (FeSiO3) slag which floats over molten matte.
SiO32 (Flux) + FeO (Gangue) → FeSiO3 (Slag)
Ques. Write a short note on Mond’s process. (3 Marks)
Ans. In this process, nickel is heated in a stream of carbon monoxide to give a volatile complex, nickel tetra carbonyl.
330K
Ni + 4CO → [Ni(CO)4]
-350K
The carbonyl is heated to a higher temperature
450-470K
[Ni(CO)4] → Ni + 4CO
Ques. The graphite electrodes in the extraction of ‘Aluminium’ by the Hall-Heroult process need to be frequently changed. Why? (3 Marks)
Ans. During the electrolysis, aluminium is deposited at the cathode & oxygen is liberated at the anode and sometimes it reacts with the carbon in the graphite to form carbon dioxide, so the anode burns away slowly. Hence, the anodes need to be replaced periodically.
Ques. Why is the reduction of a metal oxide easier if the metal formed is in a liquid state at the temperature of reduction? (3 Marks)
Ans. The reduction of metal oxide is easier in the liquid state of the metal, as the entropy is more positive, compared to the solid state, in such a manner that the Gibbs free energy becomes more negative and thus, the reduction process occurs readily.
Ques. Outline the principles behind the refining of metals by the following methods :
(a) Chromatographic method
(b) Zone refining method (3 Marks)
Ans. (a) Chromatographic Method: It is based on the principle that different components of a mixture are differently adsorbed on the adsorbent
(b) Zone Refining of Metals: This method is based on the principle that impurities are more soluble in the molten state of the metal (the melt) than in the solid state. A circular mobile heater is fixed at one end of a rod of impure metal in zone refining. As the heater moves, the molten zone of the mod also moves along with it. As a result, pure metal crystallizes out of the melt and the impurities pass to the adjacent molten zone. This process is repeated several times which leads to the segregation of impurities at one end of the rod. Then the end comprising the impurities is cut off. Silicon, boron, gallium etc. can be purified by this process.
Ques. (a) Write the principle of the method used for the refining of germanium.
(b) Out of PbS and PbCO3 (ores of lead), which one is concentrated by the froth floatation process preferably?
(c) What is the significance of leaching in the extraction of aluminium? (3 Marks)
Ans. (a) For the refining of germanium, zone refining is used which is based on the principle that the impurities are more soluble in the melt then in the solid state of the metal.
(b) PbS, i.e, Lead sulphide, is concentrated by the froth floatation process.
(c) The significance of leaching is to concentrate pure alumina from bauxite ore by digesting it with a hot concentrated solution of NaOH to form sodium meta aluminate leaving behind impurities which are then treated with CO2 and form hydrated alumina through precipitation.
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