General Principles and Processes of Isolation of Elements

Welcome to HSLC Guru. This chapter covers the principles, methods, and operations used to extract metals from their ores. You will learn about ores and minerals, the steps of metallurgy, the role of thermodynamic and electrochemical principles, and the industrial extraction of important metals such as aluminium, copper, iron, and zinc, all aligned with the ASSEB Class 12 Chemistry syllabus.

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Summary

Occurrence of Metals: Metals occur in nature both in free (native) form and in combined form as compounds. The naturally occurring substances obtained by mining are called minerals. A mineral from which a metal can be extracted profitably and conveniently is called an ore. The non-metallic impurities (sand, clay, rocky material) associated with an ore are termed gangue or matrix. Common types of ores include oxide ores (haematite, bauxite, zincite), sulphide ores (zinc blende, copper glance, galena), carbonate ores (calamine, malachite), and halide ores (cryolite, rock salt). Important principal ores include: Iron — haematite (Fe2O3), magnetite (Fe3O4), siderite (FeCO3); Aluminium — bauxite (Al2O3.2H2O), cryolite (Na3AlF6); Copper — copper pyrites (CuFeS2), copper glance (Cu2S), malachite [CuCO3.Cu(OH)2]; Zinc — zinc blende (ZnS), calamine (ZnCO3), zincite (ZnO).

Steps of Metallurgy: Metallurgy is the science and technology of extracting metals from their ores and refining them. The major steps are: (a) Crushing and grinding of the ore. (b) Concentration of the ore (dressing or benefication) — removal of unwanted gangue. Common methods are hydraulic washing or gravity separation (for heavy oxide ores), magnetic separation (for magnetic impurities or magnetic ores like magnetite), froth flotation (for sulphide ores using pine oil and air), and leaching (chemical separation, e.g. bauxite by NaOH in Bayer’s process, or noble metals like gold and silver by NaCN). (c) Conversion of the concentrated ore to its oxide by calcination (heating in absence or limited supply of air, used for carbonate and hydrated ores) or roasting (heating in excess air, used for sulphide ores). (d) Reduction of the oxide to metal using carbon (smelting), self-reduction (auto-reduction, as in Cu and Hg), hydrogen, more electropositive metal (thermite reduction), or electrolytic reduction for highly reactive metals like Na, K, Mg, Al. (e) Refining or purification of the crude metal.

Refining methods include: Distillation (for low-boiling metals like Hg and Zn), liquation (for low-melting metals like Sn and Pb), poling (for blister copper), electrolytic refining (for Cu, Ag, Au, Al, Zn — impure metal as anode and pure metal sheet as cathode in suitable electrolyte), zone refining (for ultra-pure semiconductors like Ge, Si, Ga), and vapour-phase refining through Mond’s process for Ni (using CO to form Ni(CO)4) and Van Arkel method for Zr and Ti (using I2 vapour). Thermodynamic principles are illustrated through the Ellingham diagram, a plot of standard Gibbs free energy change (ΔG°) of formation of metal oxides versus temperature. A reduction is feasible only if the overall ΔG° is negative; a metal can reduce the oxide of any other metal whose ΔG° line lies above its own at that temperature. The diagram also explains why C is a good reducing agent at high temperature and why Al cannot be reduced by carbon below ~2000°C economically.

Electrochemical principles apply to electrolytic reduction. The minimum voltage required corresponds to ΔG° = -nFE°, so a more reactive metal can displace a less reactive metal from the solution of its salt. Metallurgy of important metals: Aluminium is extracted from bauxite by Bayer’s process (leaching with NaOH) followed by Hall-Héroult electrolysis of Al2O3 dissolved in molten cryolite (Na3AlF6) with fluorspar at about 950°C using carbon electrodes. Copper is extracted from copper pyrites by concentration (froth flotation), roasting, smelting (forming matte Cu2S + FeS), Bessemerisation in a Bessemer converter giving blister copper, and finally electrolytic refining. Iron is extracted from haematite in a blast furnace using coke and limestone; carbon monoxide reduces iron oxide and slag (CaSiO3) is removed; the product is pig iron, which is converted to wrought iron and steel. Zinc is extracted from zinc blende by froth flotation, roasting to ZnO, and reduction with coke followed by distillation. Metals so obtained are widely used: Al in transmission lines, packaging, alloys; Cu in electrical wiring; Fe in construction and machinery; Zn for galvanizing and in batteries.

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Question Answers (1 Mark)

Q1. What is an ore?

Answer: A mineral from which a metal can be extracted profitably and conveniently is called an ore.

Q2. Name the principal ore of aluminium.

Answer: Bauxite, with the formula Al2O3.2H2O.

Q3. Define gangue.

Answer: The unwanted earthy and rocky impurities present along with the ore are called gangue or matrix.

Q4. Which method is used to concentrate sulphide ores?

Answer: Froth flotation method is used to concentrate sulphide ores.

Q5. What is the chemical formula of cryolite?

Answer: Cryolite is Na3AlF6.

Q6. Why is roasting carried out for sulphide ores?

Answer: Roasting in excess air converts sulphide ores into oxides which are easier to reduce.

Q7. Name a metal which is purified by zone refining.

Answer: Germanium (also Silicon, Gallium and Indium) is purified by zone refining.

Q8. What is matte in copper extraction?

Answer: Matte is a mixture of Cu2S and FeS obtained during smelting of roasted copper ore.

Q9. Which method is used to refine nickel?

Answer: Mond’s process (vapour-phase refining using CO).

Q10. Name the reducing agent used in the blast furnace for iron.

Answer: Carbon monoxide (CO) is the main reducing agent in the blast furnace.

Q11. Give the formula and name of one carbonate ore of zinc.

Answer: Calamine, ZnCO3, is a carbonate ore of zinc.

Q12. Why is hydraulic washing used for oxide ores?

Answer: Because oxide ores are heavier than the gangue and can be separated based on differences in specific gravity by a stream of running water.

Q13. What is thermite reaction? Give one example.

Answer: A reaction in which a metal oxide is reduced by powdered Al with the liberation of huge amount of heat is called a thermite reaction. Example: Fe2O3 + 2Al → 2Fe + Al2O3 + heat.

Q14. Name the depressant used in froth flotation to separate ZnS from PbS.

Answer: Sodium cyanide (NaCN) is used as a depressant; it forms a complex with ZnS and prevents it from floating, while PbS is floated.

Question Answers (2-3 Marks)

Q0. Why do most metals occur in nature in combined state rather than free state?

Answer: Most metals are reactive and have high tendency to lose electrons. They readily combine with elements like O, S, Cl, and CO3 to form stable compounds. Hence they are mostly found in combined form as oxide, sulphide, carbonate, or halide ores. Only very less reactive (noble) metals like Au, Ag and Pt are found in the native (free) state.

Q1. Differentiate between calcination and roasting.

Answer: Calcination is heating of carbonate or hydrated ore in the absence or limited supply of air to drive off volatile matter and water; e.g., ZnCO3 → ZnO + CO2. Roasting is heating of sulphide ore in excess of air to convert it to its oxide and to remove sulphur as SO2; e.g., 2ZnS + 3O2 → 2ZnO + 2SO2. Calcination is for carbonates/hydroxides; roasting is for sulphides.

Q2. Explain the principle of froth flotation method.

Answer: Froth flotation works on the principle that sulphide ore particles are preferentially wetted by oil (pine oil) while gangue particles are wetted by water. When air is blown through a mixture of finely powdered ore, water, and pine oil, the ore particles attach to the air bubbles and rise to the surface as a froth, which is skimmed off. The gangue settles at the bottom. Depressants like NaCN can be added to selectively prevent flotation of one sulphide over another.

Q3. Why is the reduction of metal oxide easier if the metal formed is in liquid state at the temperature of reduction?

Answer: The entropy of a liquid is higher than that of a solid. When the metal formed is in the liquid state, ΔS becomes more positive for the reduction reaction. As ΔG = ΔH − TΔS, a more positive ΔS at high temperature makes ΔG more negative, which makes the reduction thermodynamically more feasible.

Q4. What is leaching? Give one example.

Answer: Leaching is a chemical method of concentration in which the ore is treated with a suitable reagent that dissolves the metal compound but not the impurities. Example: bauxite (Al2O3.2H2O) is leached with hot concentrated NaOH; alumina dissolves as sodium aluminate while iron oxide and silica remain insoluble. Another example is the cyanide leaching of gold and silver using NaCN solution.

Q5. Explain the role of cryolite in the extraction of aluminium.

Answer: In the Hall-Héroult process, cryolite (Na3AlF6) plays three important roles: (i) it dissolves alumina and acts as a solvent, (ii) it lowers the fusion temperature of alumina from about 2050°C to around 950°C, saving energy, and (iii) it increases the electrical conductivity of the molten electrolyte, making electrolysis efficient.

Q6. What is the role of limestone in the blast furnace?

Answer: Limestone (CaCO3) decomposes to CaO and CO2 in the blast furnace. The CaO produced acts as a basic flux that combines with the acidic silica (SiO2) impurity in the ore to form fusible calcium silicate (slag): CaO + SiO2 → CaSiO3. The slag floats on the molten iron and is tapped off separately.

Q7. Why is froth flotation method used only for sulphide ores?

Answer: Sulphide ore particles are preferentially wetted by oils such as pine oil, while the gangue (oxides, silicates) is wetted by water. The sulphide particles attach to the oil-coated air bubbles, rise to the top, and form a froth. The selectivity of pine oil towards sulphide minerals is the reason this method is restricted to sulphide ores.

Q8. Write the chemical reactions taking place in the Hall-Héroult cell.

Answer: The electrolyte is molten Al2O3 in cryolite. At the cathode (carbon lining): Al3+ + 3e- → Al. At the anode (carbon): C + O2- → CO + 2e-, and C + 2O2- → CO2 + 4e-. Overall: 2Al2O3 + 3C → 4Al + 3CO2. Carbon anodes are gradually consumed and replaced periodically.

Q9. Explain why electrolytic reduction is used for highly reactive metals.

Answer: Highly reactive metals like Na, K, Mg and Al have very stable oxides with strongly negative ΔG° of formation. These oxides cannot be reduced by ordinary reducing agents like C or CO at economical temperatures. Electrolytic reduction provides the required electrical energy to make ΔG = -nFE less than zero for the decomposition of their molten salts, so the metal is liberated at the cathode.

Question Answers (5-7 Marks)

Q1. Describe the extraction of aluminium from bauxite by the Hall-Héroult process.

Answer: Aluminium is mainly extracted from bauxite (Al2O3.2H2O). The extraction takes place in two stages: purification of the ore (Bayer’s process) and electrolytic reduction (Hall-Héroult process).

Concentration (Bayer’s process): Powdered bauxite is leached with hot concentrated NaOH at 473–523 K and 35–36 bar pressure. Alumina dissolves as sodium aluminate while Fe2O3 and SiO2 remain insoluble: Al2O3 + 2NaOH + 3H2O → 2Na[Al(OH)4]. The solution is filtered to remove insoluble residue (red mud).

The filtrate is then neutralised by passing CO2 to precipitate hydrated alumina: 2Na[Al(OH)4] + CO2 → Al2O3.xH2O + 2NaHCO3. The precipitate is filtered, washed and calcined at 1470 K to obtain pure Al2O3.

Electrolysis (Hall-Héroult process): Pure alumina is dissolved in molten cryolite (Na3AlF6) with a small amount of fluorspar (CaF2) at about 950°C. Electrolysis is carried out in a steel tank lined with carbon (cathode) using carbon anodes. Reactions: at cathode Al3+ + 3e- → Al; at anode C + O2- → CO + 2e- and C + 2O2- → CO2 + 4e-. Molten Al collects at the bottom and is tapped off. The carbon anodes are gradually consumed and must be replaced.

Q2. Explain the extraction of iron from haematite in a blast furnace with reactions.

Answer: Iron is mainly extracted from haematite (Fe2O3). The concentrated ore is mixed with coke and limestone and fed at the top of a blast furnace, while a hot blast of air is blown in from the bottom. The furnace has zones at different temperatures.

At the bottom zone (1500–2000 K): C + O2 → CO2; CO2 + C → 2CO. The CO produced is the main reducing agent.

Middle zone (900–1500 K): reduction of iron oxides — 3Fe2O3 + CO → 2Fe3O4 + CO2; Fe3O4 + 4CO → 3Fe + 4CO2.

Upper zone (500–900 K): Fe2O3 + 3CO → 2Fe + 3CO2. Limestone decomposes: CaCO3 → CaO + CO2. CaO acts as flux: CaO + SiO2 → CaSiO3 (slag).

Molten iron and slag collect at the bottom; slag floats on iron and is tapped separately. The product is pig iron (~4% C). It is later converted to wrought iron (lowest C) and steel (controlled C).

Q3. Discuss the Ellingham diagram and its applications in metallurgy.

Answer: An Ellingham diagram is a plot of standard Gibbs free energy of formation (ΔG°f) of metal oxides per mole of oxygen consumed versus temperature.

Features: Most lines slope upwards because the formation of metal oxides consumes gas (entropy decreases, so −TΔS is positive and ΔG° becomes less negative as T rises). The carbon line (C → CO) slopes downwards because the entropy increases (a mole of solid gives a mole of gas), so at high temperature carbon becomes a powerful reducing agent.

Interpretation: The line lower on the diagram represents a more stable oxide. A metal whose ΔG° line lies below another can reduce the oxide of the second metal at that temperature.

Applications: (a) Predicting the choice of reducing agent at a given temperature. (b) Determining the minimum temperature at which a metal oxide can be reduced by C. (c) Explaining why Al and Mg cannot be reduced economically by C at moderate temperatures and require electrolysis. (d) Justifying thermite reactions, e.g. 2Al + Cr2O3 → Al2O3 + 2Cr.

Limitations: It gives no information about reaction rate (kinetics) and only considers thermodynamic feasibility. The interpretation also assumes equilibrium between the reactants and products.

Q4. Describe the various methods of refining of metals with examples.

Answer: Several methods are used to purify a crude metal depending on the nature of the metal and impurities:

(a) Distillation: Used for volatile metals such as Zn and Hg; impure metal is heated and pure metal distills off, leaving non-volatile impurities behind.

(b) Liquation: Used for low melting metals like Sn and Pb. The metal is melted on a sloping hearth so the liquid metal flows down leaving the higher melting impurities.

(c) Poling: Molten impure copper containing Cu2O is stirred with green wooden poles; the hydrocarbons released reduce Cu2O back to Cu.

(d) Electrolytic refining: Impure metal is made the anode and a thin sheet of pure metal the cathode in a solution of metal salt. Pure metal deposits on cathode while soluble impurities remain in solution and insoluble impurities settle as anode mud (containing Ag, Au). Example: refining of Cu using CuSO4 + dilute H2SO4.

(e) Zone refining: Based on the principle that impurities are more soluble in molten state than in solid state. A movable heater passes through a rod of impure metal; a molten zone moves with the heater and pushes impurities to one end which is later cut off. Used for Si, Ge, Ga, B.

(f) Vapour-phase refining: Mond’s process for Ni — Ni + 4CO → Ni(CO)4 (volatile) which on heating to 460 K gives back pure Ni; Van Arkel method for Zr and Ti — the impure metal reacts with I2 to form a volatile iodide which decomposes on a hot tungsten filament to give pure metal.

(g) Chromatographic methods: Used for very small amounts of substances and based on differential adsorption on a stationary phase.

Q5. Describe the extraction of zinc from zinc blende, mentioning the role of each step.

Answer: Zinc is mainly obtained from zinc blende (ZnS) and calamine (ZnCO3). The extraction is carried out in the following steps:

(i) Concentration: The crushed ore is concentrated by froth flotation, which selectively floats sulphide particles and separates them from siliceous gangue.

(ii) Roasting/Calcination: The concentrated ore is roasted in excess of air below the melting point of zinc to convert it to ZnO: 2ZnS + 3O2 → 2ZnO + 2SO2. Carbonate ores like calamine are calcined: ZnCO3 → ZnO + CO2.

(iii) Reduction (Smelting): ZnO is reduced by coke at about 1673 K in fire-clay retorts: ZnO + C → Zn + CO. Zinc vapours so produced are condensed.

(iv) Refining: The crude metal (spelter) is purified by distillation or by electrolytic refining using ZnSO4 solution as electrolyte. The metal obtained is used in galvanizing iron sheets and in the manufacture of dry cells.

Role of each step: Concentration removes gangue; roasting/calcination converts sulphide or carbonate to easily reducible oxide; smelting reduces the oxide to metal; refining removes impurities to yield pure zinc.

Q6. Outline the extraction of copper from copper pyrites.

Answer: The principal ore of copper is copper pyrites (CuFeS2). The extraction involves the following steps:

(i) Crushing and concentration: The ore is crushed in a stamp mill and concentrated by froth flotation, which separates sulphide ore particles from gangue.

(ii) Roasting: The concentrated ore is roasted in a reverberatory furnace in excess air to remove moisture, volatile matter, and most of the sulphur and arsenic: 2CuFeS2 + O2 → Cu2S + 2FeS + SO2.

(iii) Smelting: The roasted ore is mixed with coke and silica in a blast furnace; FeS is converted to FeO which combines with silica to form slag (FeSiO3): FeO + SiO2 → FeSiO3. Cu2S together with some FeS forms a fused mass called matte.

(iv) Bessemerisation: Matte is transferred to a Bessemer converter lined with silica. A blast of hot air is blown through it. FeS is oxidised to FeO which forms slag with silica; Cu2S is partly oxidised to Cu2O which then reacts with the remaining Cu2S in self-reduction: 2Cu2O + Cu2S → 6Cu + SO2. Copper obtained is called blister copper (~98% pure) due to evolution of SO2.

(v) Refining: Blister copper is finally refined by electrolytic refining using CuSO4 + H2SO4 as electrolyte. Impure Cu is the anode and a thin strip of pure Cu is the cathode. Anode mud contains Ag and Au, which are recovered as valuable by-products.

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Multiple Choice Questions (MCQs)

Q1. The principal ore of aluminium is:
(a) Cryolite (b) Bauxite (c) Corundum (d) Felspar
Answer: (b) Bauxite

Q2. Froth flotation process is used for the concentration of:
(a) Oxide ores (b) Sulphide ores (c) Carbonate ores (d) Halide ores
Answer: (b) Sulphide ores

Q3. The chemical formula of cryolite is:
(a) AlF3 (b) Na3AlF6 (c) NaAlF4 (d) Al2O3
Answer: (b) Na3AlF6

Q4. Roasting is generally carried out for:
(a) Sulphide ores (b) Carbonate ores (c) Oxide ores (d) Hydrated ores
Answer: (a) Sulphide ores

Q5. Which of the following is used as flux in the blast furnace?
(a) Silica (b) Coke (c) Limestone (d) Cryolite
Answer: (c) Limestone

Q6. Mond’s process is used for refining of:
(a) Copper (b) Nickel (c) Zinc (d) Aluminium
Answer: (b) Nickel

Q7. Zone refining is based on the principle that:
(a) Impurities are more volatile than metal (b) Impurities are more soluble in molten metal than in solid metal (c) Impurities are more soluble in solid than liquid (d) None
Answer: (b) Impurities are more soluble in molten metal than in solid metal

Q8. The reducing agent in the extraction of iron in blast furnace is:
(a) Coke (b) CO (c) H2 (d) Al
Answer: (b) CO

Q9. Blister copper contains copper of approximate purity:
(a) 60% (b) 75% (c) 98% (d) 99.99%
Answer: (c) 98%

Q10. Van Arkel method is used to obtain pure:
(a) Cu and Ag (b) Ti and Zr (c) Na and K (d) Fe and Mn
Answer: (b) Ti and Zr

Q11. Which of the following is a sulphide ore?
(a) Bauxite (b) Haematite (c) Galena (d) Calamine
Answer: (c) Galena (PbS)

Q12. The slag formed in the blast furnace during iron extraction is:
(a) FeSiO3 (b) CaSiO3 (c) Al2SiO5 (d) MgSiO3
Answer: (b) CaSiO3

Q13. The purest form of commercial iron is:
(a) Cast iron (b) Pig iron (c) Wrought iron (d) Steel
Answer: (c) Wrought iron

Fill in the Blanks

Q1. The naturally occurring substance from which a metal can be extracted economically is called an _______.
Answer: ore

Q2. The unwanted earthy and rocky impurities present along with the ore are called _______.
Answer: gangue

Q3. In the Hall-Héroult process, _______ is added to lower the fusion temperature of alumina.
Answer: cryolite

Q4. Heating of an ore in the absence or limited supply of air is called _______.
Answer: calcination

Q5. The plot of ΔG° versus T for the formation of oxides is known as the _______ diagram.
Answer: Ellingham

Q6. Vapour-phase refining of nickel is known as _______ process.
Answer: Mond’s

Q7. The crude metal obtained from a Bessemer converter for copper is called _______ copper.
Answer: blister

True or False

Q1. Bauxite is the chief ore of iron.
Answer: False (it is the chief ore of aluminium).

Q2. Roasting is carried out in excess of air.
Answer: True.

Q3. Carbon cannot reduce Al2O3 economically at moderate temperatures.
Answer: True.

Q4. Mond’s process is used for the refining of copper.
Answer: False (it is used for refining nickel).

Q5. Zone refining is based on the difference in solubility of impurities in solid and molten state.
Answer: True.

Q6. Pig iron contains the lowest percentage of carbon among iron forms.
Answer: False (it contains the highest, around 4%).

Glossary

Term	Meaning
Mineral	Naturally occurring substance containing a metal in combined or free state.
Ore	Mineral from which a metal can be profitably extracted.
Gangue	Unwanted earthy or rocky impurities in an ore.
Flux	A substance added to remove impurities by forming slag.
Slag	Fusible product formed by reaction of flux with gangue.
Calcination	Heating of ore in absence or limited supply of air.
Roasting	Heating of sulphide ore in excess of air.
Smelting	Reduction of an oxide ore by carbon in a furnace.
Leaching	Chemical concentration of ore by selective dissolution.
Froth Flotation	Method of concentration of sulphide ores using oil and air.
Ellingham Diagram	Graph of ΔG° of oxide formation vs temperature.
Hall-Héroult	Electrolytic process for extracting Al from alumina in molten cryolite.
Bayer’s Process	Leaching of bauxite with NaOH for purification of alumina.
Bessemerisation	Conversion of matte to blister copper in a Bessemer converter.
Matte	Mixture of Cu2S and FeS formed during smelting of copper ore.
Blister Copper	Approximately 98% pure copper obtained from Bessemer converter.
Mond’s Process	Vapour-phase refining of nickel using carbon monoxide.
Van Arkel Method	Vapour-phase refining of Ti and Zr using iodine.
Zone Refining	Purification based on differential solubility of impurities in molten state.
Electrolytic Refining	Refining of metal by electrolysis with impure anode and pure cathode.
Liquation	Refining of low-melting metals by allowing them to flow on a sloping hearth.
Distillation	Refining of volatile metals by vaporisation and condensation.
Pig Iron	Crude iron obtained from blast furnace containing about 4% carbon.
Wrought Iron	Purest form of commercial iron with very low carbon content.
Steel	Alloy of iron with controlled amount of carbon (0.2–1.5%).