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Metallurgy is the process of extracting pure metal from its ore. Naturally, most of the metals are found in their combined state. So it becomes essential to extract the pure form of metal for various other domestic and commercial purposes.
Minerals from which metals can be easily extracted are called Ores. The ore that is mined from the earth usually contains earthy impurities such as dirt, sand and rocks. These impurities are called Gangue. Once the gangue is removed, the ore is subjected to a further purification process to remove the impurities.
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Key Terms: Metallurgy, Matallurgical Process, Ores, dirt, sand, rocks, Pulverization, Gangue, metal
Metallurgical Process
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The various steps involved in Metallurgy are as follows:
Crushing/Grinding
The crushing of the ore into a fine powder in grinders is the first step in any metallurgical process. This is known as Pulverization.
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| General Properties of Non Metals | Physical Properties of Metals and Non-metals | Thermal Conductivity |
| Redox reaction | Specific Heat Capacity | Calorimetry |
Concentration of Ore
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The purification or concentration of ore is done by various methods such as:
- Hydraulic washing
- Froth floatation
- Magnetic Separation
- Leeching
(i) Hydraulic Washing
It is based on the principles of gravity separation where density differences between the ore and the impurities separate the ore from the impurities. In this process, the powdered ore is washed by a stream of running water. The lighter gangue particles are washed away and the heavier ores are left behind. This process is used in the concentration of heavy metals such as Tin (Sn), Lead (Pb) and Iron (Fe).
Separation of concentration of ore
(ii) Froth Floatation
The powdered ore is suspended in water along with a small amount of collectors (such as pine oil) to enhance the non-wettability of ore particles. When air is blown into the mixture, the froth containing the lighter ore particles forms a layer on the surface. The gangue particles sink to the bottom. The froth is then removed and dried to recover the concentrated ore. This process is suitable for the concentration of sulphide ores.
(iii) Magnetic Separation
This method is used when there is a difference in the magnetic properties between the ore and the gangue particles. The powdered ore is placed on a moving conveyor belt that has electromagnetic rollers underneath. If the gangue particles are magnetic, they tend to get attached to the belt while the ore particles fall and get collected below. The ore of tin, Tinstone, is separated from Wolfram, a gangue particle by this method.
(iv) Leeching
It is a chemical method of ore concentration. This method is used when the ore is soluble in a suitable solvent while the impurities remain undissolved. Pure Alumina is extracted from bauxite ore by this method. The following are the process involved:
- Treatment with Sodium Hydroxide dissolves the alumina to form Sodium aluminate
Al2O3 (Impure) + 2NaOH \(\rightarrow\) 2NaAlO2 + H2O
- On hydrolysis, Aluminium hydroxide is precipitated
NaAlO2 + 2H2O \(\rightarrow\) Al(OH) + NaOH
- The precipitate on the subsequent process of filtration, washing, drying and heating gives pure Alumina (Al2O3)
2Al(OH) \(\rightarrow\) Al2O3 (Pure) + 3H2O
Conversion of Ore Into Oxide of Metals
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Since obtaining metals directly from the carbonates and sulphides is a tedious process, these concentrated ores are converted to their metal oxides from which the metals can be easily extracted by reduction. Depending upon the ore and the reactivity of the metals, the conversion to metal oxides can be done either by the process of Calcination or Roasting.
(i) Calcination
The Calcination process is used to convert carbonate ores into metal oxides. Here, the concentrated ores are heated at high temperatures in the absence of air. During Calcination, all the volatile matter escapes from the ore thus leaving behind the metal oxide. Some of the examples are:
- Calcination of Zinc Carbonate
ZnCO3(s) \(\rightarrow\) ZnO(s) + CO2(g)
- Calcination of Hydrated Ferric Oxide
Fe2O3.xH2O(s) \(\rightarrow\) Fe2O3 (s) + xH2O(g)
- Calcination of Dolomite ore
CaCO3.MgCO3(s) \(\rightarrow\) CaO(s) + MgO(s ) + 2CO2(g)
(ii) Roasting
The Sulphide ores are converted into metal oxides by the process of roasting. The concentrated ore is heated in a reverberatory furnace with a steady supply of air at a temperature below the melting point of the metal. Some of the examples are:
- Roasting of Zinc Blende ore
2ZnS + 3O2 \(\rightarrow\) 2ZnO + 2SO2
- Roasting of Galena ore
2PbS + 3O2 \(\rightarrow\) 2PbO + 2SO2
- Roasting of Copper Sulphide
2Cu2S + 3O2 \(\rightarrow\) 2Cu2O + 2SO
Note: The Chloride ores are not affected by Calcination / Roasting
Reduction
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The metal oxide thus obtained is subjected to reduction to extract the metals. The reducing agents used in this process depend upon the reactivity of the metal. Highly reactive metals such as Na and K undergo electrolytic reduction processes whereas less reactive metals such as Cu, Fe and Sn undergo chemical reduction with CO (Coke), Al, H2 as reducing agents.
Reduction using Carbon: When reduction takes place using carbon in the form of coke as a reducing agent, it is called Smelting.
Eg: Ferric Oxide (Fe2O3) is smelted in a blast furnace to reduce it to Iron.
Fe2O3 + 3C \(\rightarrow\) 2Fe + 3CO
Reduction using Aluminium: When metal oxides are reduced to metals by using aluminium as reducing agents, it is called the Alumino-thermic process.
Eg: Reduction of Manganese dioxide to Manganese by heating with aluminium
3MnO2 + 4Al \(\rightarrow\) 3Mn + 2Al2O3
Electrolytic Reduction: Highly reactive metals such as Na, Mg, Al, etc, cannot be reduced by smelting as they have a higher affinity for oxygen than carbon. Thus the metals are extracted by the process of Electrolytic reduction. The electrolysis of molten salts always yields the metal at the Cathode.
Eg: The electrolysis of molten NaCl yields sodium metal.
Refining
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The metal extracted from its oxides still consists of impurities that should be removed by the process of refining. The most widely used method is the Electrolytic refining process. The various methods of refining are discussed below.
(i) Electrolytic Refining
In this process, the impure metals act as the anode, a strip of pure metal as cathode and a solution of the salt of the metal as an electrolyte. When electricity is passed, pure metal from the anode dissolves in the electrolyte and gets deposited at the cathode. The soluble impurities dissolve whereas the insoluble impurities sink to the bottom and get collected as anode mud.
(ii) Distillation
This method of refining is suitable for metals with low boiling points such as Zinc and Mercury. The pure metal is obtained as a distillate after evaporation on heating.
(iii) Liquation
This method is useful to separate metals of lower boiling point from that of higher boiling point eg: Lead and Tin. The impure metal is made kept on a sloping hearth that separates low boiling point metals from higher boiling point impurities.
(iv) Zone Refining
Semiconductors like Ge, Si, Ga are purified by this process. It is based on the principle that when molten metal is cooled, pure metallic crystals get deposited on the surface.
(v) Vapour Phasing
In this method, the impure metal is treated with a reagent to form a volatile compound that is collected and then decomposed to recover the pure metal.
(vi) Chromatographic method
When the metal to be extracted and the impurities have similar properties, chromatographic refining becomes a suitable method. It is based on the principle that different elements in a mixture are adsorbed differently.
Things To Remember
- Minerals from which metals are extracted are called ores. All ores are minerals but not all minerals are ores.
- The impurities in the ores are called Gangue.
- The process of grinding the ores to a fine powder is called Pulverisation
- Calcination takes place in the absence of air while Roasting takes place in the presence of air.
- When reduction takes place using coke as a reducing agent, it is called smelting.
- During electrolytic refining, the pure metal is always liberated at the cathode.
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Sample Questions
Ques. Differentiate between a mineral and an ore. (2 marks)
Ans. The naturally occurring chemical substances in the form of which the metals occur in the earth along with impurities are called minerals.
The mineral from which metal can be extracted conveniently and economically is called an ore. Thus, all ores are minerals but all minerals are not ores.
Ques. What type of ores can be concentrated by the magnetic separation method? (2 marks)
Ans. The Magnetic separation method is used when either the ore or the impurities associated with it are magnetic in nature.
Example: Chromite (FeO.Cr2O3) -A an ore of chromium
Magnetite (Fe3O4) \(\rightarrow\) an ore of iron.
Ques. What is the role of depressants in the froth floatation process of dressing of ores? (2 marks)
Ans. Depressants are used to prevent certain types of particles from forming the froth with the bubbles.
Example: In the case of an ore containing ZnS and PbS, the depressant used is NaCN. It selectively prevents ZnS from coming to the froth but allows PbS to come with the froth.
Ques. Differentiate between roasting and calcination. Explain the two with the help of suitable chemical equations. (2 marks)
Ans. Roasting: It is a process in which sulphide ore is heated in the presence of oxygen to convert into oxide.
Roasting of Zinc Blende ore
2ZnS + 3O2 \(\rightarrow\) 2ZnO + 2SO2
Calcination: It is a process in which carbonate ore is heated in the absence of air to form oxide.
Calcination of Zinc Carbonate
ZnCO3(s) \(\rightarrow\) ZnO(s) + CO2(g)
Ques. a) How is zinc extracted from its ore?
b) Name two metals that can be used to reduce metal oxides to metals. (2 marks)
Ans. a) By the process of reduction with carbon, Zinc can be extracted from its ore.
ZnO + C \(\rightarrow\) Zn + CO2
b) Aluminum and Magnesium can be used to reduce the metal oxides to metal because of their higher affinity to oxygen.
Ques. What is the principle of the Electrolytic refining process? (2 marks)
Ans. Here the impure metal is made to act as anode and a strip of the same metal in pure form is used as cathode. When they both are put in a suitable electrolyte containing soluble salt of the same metal, the more basic metal remains in the solution and the less basic ones go to the anode mud.
Example : In refining of Cu,
At anode : (oxidation)
Cu \(\rightarrow\) Cu2+ + 2e–
At cathode : (reduction)
Cu2+ + 2e– \(\rightarrow\) Cu
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