Chapter 1 — Metallurgy Exercise Solutions
Metallurgy answers should be written as a sequence: ore, concentration, conversion to oxide, reduction, and refining.
Key Points
| Step | Purpose |
|---|
| Concentration | Removes gangue |
| Calcination | Heating ore without air |
| Roasting | Heating ore in excess air |
| Reduction | Converts metal oxide to metal |
| Refining | Removes final impurities |
Short Answer Pattern
- Define the process.
- Mention where it is used.
- Write one example or equation.
- Add the principle if asked.
Important Concepts
- Froth flotation is used for sulphide ores.
- Magnetic separation works when ore or impurity is magnetic.
- Ellingham diagram helps predict feasibility of reduction.
- Electrolytic refining gives highly pure metal.
Worked Examples
Example 1 — Roasting reaction
Write the roasting reaction of zinc blende (ZnS).
2ZnS + 3O₂ →(heat) 2ZnO + 2SO₂
Roasting = heating a sulphide ore in excess air to form the oxide.
Example 2 — Calcination reaction
Write the calcination reaction of limestone / calamine (ZnCO₃).
ZnCO₃ →(heat) ZnO + CO₂
Calcination = heating a carbonate/hydroxide ore in limited air to form the oxide.
Example 3 — Role of flux (slag formation)
Show how silica gangue is removed in iron extraction.
Flux: CaCO₃ → CaO + CO₂
Slag: CaO + SiO₂ → CaSiO₃ (calcium silicate, the slag)
Flux combines with gangue (SiO₂) to form an easily removable slag.
Common Mistakes
- Confusing roasting and calcination.
- Forgetting the role of flux.
- Writing extraction steps in wrong order.
- Not balancing chemical equations.
Exam Checklist
- Process names are correct.
- Equations are balanced.
- Diagram-based answers mention axis and trend.
- Final metal and impurity removal step are clear.
How to Use This Page
- Read the Key Concepts table for a quick recap before answering any question.
- Attempt MCQs first to test recall, then move to Fill in the Blanks and True or False.
- Use Answer Briefly for 2-mark questions and Answer in Detail for 5-mark questions.
- Check the Common Mistakes to Avoid section before exams to avoid losing marks.
- Go through the Exam Checklist in the last 10 minutes of revision.
Key Concepts (Quick Recap)
| Term | Definition |
|---|
| Metallurgy | The science and technology of extracting metals from their ores |
| Mineral | A naturally occurring substance from which metal may or may not be profitably extracted |
| Ore | A mineral from which a metal can be extracted profitably |
| Gangue | Impurities (sand, clay, rock) present in the ore |
| Concentration / Dressing | Process of removing gangue from the ore |
| Calcination | Heating the concentrated ore in the absence of air to remove volatile impurities |
| Roasting | Heating the concentrated ore in excess of air to convert sulphides to oxides |
| Flux | A substance added to remove gangue as slag |
| Slag | The fusible product formed when flux reacts with gangue |
| Smelting | Reduction of the ore by a reducing agent at high temperature in a furnace |
| Ellingham diagram | A plot of ΔG vs temperature showing the feasibility of reduction of metal oxides |
| Aluminothermic reduction | Using aluminium as a reducing agent for metal oxides (e.g., chromium oxide) |
| Electrolytic refining | Refining impure metal by electrolysis; pure metal deposits on the cathode |
| Zone refining | Method of refining metals by moving a molten zone along a rod to concentrate impurities at one end |
| Liquation | Refining method for low-melting metals where impure metal is heated on a sloping surface |
I. Choose the Best Answer (MCQs)
1. Which method is used for concentrating sulphide ores?
Answer
(a) Gravity separation (b) Froth flotation (c) Magnetic separation (d) Leaching
Answer(b) Froth flotation — Sulphide ores are preferentially wetted by pine oil and float as froth, while gangue is wetted by water and sinks.
2. In the Ellingham diagram, a line that lies below another at a given temperature indicates:
Answer
(a) The metal is less reactive (b) The oxide is more stable (c) The oxide is less stable (d) The metal cannot be reduced
Answer(b) The oxide is more stable — A more negative ΔG means the oxide formation is more favourable, so the oxide is more stable.
3. Calcination is the process of heating ore in:
Answer
(a) Excess of air (b) Absence of air (c) Presence of carbon (d) Presence of chlorine
Answer(b) Absence of air — Calcination is done in limited or no air to decompose carbonates and hydroxides.
4. Which reducing agent is used in the aluminothermic process?
Answer
(a) Carbon (b) Hydrogen (c) Aluminium (d) Carbon monoxide
Answer(c) Aluminium — Aluminium has a high affinity for oxygen and acts as a reducing agent in the thermite reaction.
5. In electrolytic refining of copper, the cathode is made of:
Answer
(a) Impure copper (b) Pure copper (c) Graphite (d) Zinc
Answer(b) Pure copper — Pure copper acts as the cathode where pure copper from the anode gets deposited.
6. The slag formed during iron extraction in a blast furnace is:
Answer
(a) CaO (b) SiO₂ (c) CaSiO₃ (d) FeSiO₃
Answer(c) CaSiO₃ — Limestone decomposes to CaO, which reacts with silica gangue to form calcium silicate slag.
7. Zone refining is based on the principle that:
Answer
(a) Impurities are more soluble in the melt than in the solid (b) Impurities are less soluble in the melt than in the solid (c) Metals have low melting points (d) Impurities are magnetic
Answer(a) Impurities are more soluble in the melt than in the solid — As the molten zone moves, impurities concentrate in the molten region and are swept to one end.
8. In the extraction of aluminium by Hall-Heroult process, the electrolyte used is:
Answer
(a) Pure alumina (b) Alumina dissolved in molten cryolite (c) Bauxite dissolved in water (d) Aluminium chloride solution
Answer(b) Alumina dissolved in molten cryolite — Cryolite lowers the melting point of alumina and increases conductivity.
9. Which of the following is an oxide ore?
Answer
(a) Zinc blende (b) Bauxite (c) Galena (d) Cinnabar
Answer(b) Bauxite — Bauxite is Al₂O₃·2H₂O, an oxide ore. Zinc blende is a sulphide, galena is a sulphide, cinnabar is a sulphide.
10. Magnetic separation is useful when:
Answer
(a) The ore is non-magnetic (b) Either the ore or the impurity is magnetic (c) The ore is a sulphide (d) The ore is very light
Answer(b) Either the ore or the impurity is magnetic — Magnetic separation separates magnetic from non-magnetic substances using a magnetic separator.
11. Vapour phase refining of titanium uses:
Answer
(a) Carbon (b) Iodine (c) Chlorine (d) Hydrogen
Answer(b) Iodine — Titanium reacts with iodine to form volatile TiI₄, which decomposes on a hot filament to deposit pure Ti (van Arkel method).
12. The role of cryolite in aluminium extraction is to:
Answer
(a) Act as a reducing agent (b) Lower the melting point of alumina and increase conductivity (c) Remove silica impurities (d) Act as a flux in the blast furnace
Answer(b) Lower the melting point of alumina and increase conductivity — Cryolite reduces the melting point from ~2072 °C to ~950 °C and improves electrical conductivity.
II. Fill in the Blanks
1. The process of removing gangue from the ore is called ________. Concentration / Dressing
2. In froth flotation, the depressant used to separate PbS from ZnS is ________. Sodium cyanide (NaCN)
3. Calcination is carried out in the ________ of air. Absence
4. The flux used to remove acidic impurities like SiO₂ is ________. Basic flux (e.g., CaO or limestone)
5. In the Ellingham diagram, ΔG is plotted against ________. Temperature
6. The reducing agent used in the aluminothermic reduction of Cr₂O₃ is ________. Aluminium (Al)
7. The anode in electrolytic refining of copper is made of ________. Impure copper
8. In zone refining, impurities move towards the ________ end of the rod. One end (the end where the molten zone finally reaches)
9. Bauxite ore is concentrated by ________ method (chemical). Leaching (Bayer's process)
10. The reaction Fe₂O₃ + 3C → 2Fe + 3CO is an example of ________ reduction. Smelting / Carbon reduction
III. True or False
1. Roasting converts carbonate ores to oxides. — False. Roasting converts sulphide ores to oxides. Calcination converts carbonate ores to oxides.
2. Froth flotation is suitable for concentrating sulphide ores. — True.
3. In the Ellingham diagram, a metal whose line lies below another can reduce the oxide of that metal. — True. A metal with a more negative ΔG can reduce the oxide of a metal with a less negative ΔG.
4. Magnetic separation can be used for all types of ores. — False. Magnetic separation can only be used when either the ore or the impurity is magnetic.
5. Zone refining produces metals of very high purity. — True. Zone refining produces metals of 99.99% purity.
6. In electrolytic refining, the impure metal is made the cathode. — False. The impure metal is the anode; pure metal deposits on the cathode.
7. Bauxite is a sulphide ore of aluminium. — False. Bauxite is an oxide ore (Al₂O₃·2H₂O).
8. Cryolite lowers the melting point of alumina in the Hall-Heroult process. — True.
IV. Match the Following
| Column A | Column B |
|---|
| 1. Gravity separation | (d) Oxide and carbonate ores |
| 2. Froth flotation | (e) Sulphide ores |
| 3. Calcination | (a) Heating in absence of air |
| 4. Roasting | (b) Heating in excess air |
| 5. Smelting | (c) Reduction by carbon at high temperature |
| 6. Electrolytic refining | (f) Impure metal as anode, pure metal as cathode |
| 7. Zone refining | (g) Semiconductor-grade purification |
| 8. Liquation | (h) Low-melting metal on a sloping hearth |
Answer1-d, 2-e, 3-a, 4-b, 5-c, 6-f, 7-g, 8-h
V. Answer Briefly (2-Mark Questions)
1. Differentiate between calcination and roasting.
Answer
Calcination is the process of heating the concentrated ore in the absence or limited supply of air to remove volatile impurities, moisture, and CO₂ from carbonates. Roasting is the process of heating the concentrated ore in excess air to convert sulphide ores to oxide ores and remove sulphur as SO₂. Calcination is used for carbonate and hydroxide ores, while roasting is used for sulphide ores.
2. What is the role of flux in metallurgy? Give an example.
Answer
A flux is a substance added during smelting to combine with gangue (impurities) and form a fusible product called slag, which can be easily separated from the molten metal. For example, in iron extraction, limestone (CaCO₃) acts as a flux. It decomposes to CaO, which combines with silica gangue to form calcium silicate slag:
CaCO₃ → CaO + CO₂
CaO + SiO₂ → CaSiO₃ (slag)
3. State the principle of froth flotation.
Answer
Froth flotation is based on the principle that sulphide ore particles are preferentially wetted by oil (pine oil) and rise to the surface as froth, while gangue particles are wetted by water and sink to the bottom. The froth is skimmed off to obtain the concentrated ore. Depressants like sodium cyanide are used to selectively prevent one sulphide from floating.
4. What is an Ellingham diagram? What information does it provide?
Answer
An Ellingham diagram is a graph plotting the standard Gibbs free energy change (ΔG°) of formation of metal oxides against temperature. It helps predict the feasibility of reduction of a metal oxide by another metal or carbon. A metal whose oxide line lies lower in the diagram (more negative ΔG) can reduce the oxide of a metal whose line lies above it at that temperature.
5. Why is cryolite added to alumina in the Hall-Heroult process?
Answer
Cryolite (Na₃AlF₆) is added to alumina in the Hall-Heroult process for two reasons: (1) It lowers the melting point of alumina from approximately 2072 °C to about 950 °C, saving energy, and (2) It increases the electrical conductivity of the molten mixture, allowing electrolysis to proceed efficiently.
6. What is the van Arkel method? Which metals are purified by this method?
Answer
The van Arkel method is a vapour phase refining process in which an impure metal is heated with iodine to form a volatile compound, which then decomposes on a hot tungsten filament to deposit pure metal. This method is used for purifying titanium and zirconium to very high purity (semiconductor grade).
Ti (impure) + 2I₂ →(heat) TiI₄ →(hot filament) Ti (pure) + 2I₂
VI. Answer in Detail (5-Mark Questions)
1. Describe the extraction of aluminium from bauxite ore with all steps and equations.
Answer
The extraction of aluminium from bauxite involves three main steps: concentration, conversion to alumina, and electrolytic reduction.
Step 1 — Concentration (Bayer's Process): Bauxite ore (Al₂O₃·2H₂O) is crushed and digested with concentrated NaOH at 473–523 K under pressure. Alumina dissolves as sodium aluminate, leaving impurities (red mud) behind. The solution is cooled, seeded with Al(OH)₃, and aluminium hydroxide precipitates out.
Al₂O₃·2H₂O + 2NaOH → 2NaAlO₂ + 3H₂O
2NaAlO₂ + 4H₂O → 2NaOH + 2Al(OH)₃
2Al(OH)₃ →(heat) Al₂O₃ + 3H₂O
Step 2 — Hall-Heroult Process (Electrolytic Reduction): Pure alumina is dissolved in molten cryolite (Na₃AlF₆) at about 950 °C in an iron tank lined with carbon (cathode). Carbon electrodes act as the anode. On passing electric current, aluminium is deposited at the cathode and oxygen is liberated at the anode.
At cathode: Al³⁺ + 3e⁻ → Al
At anode: 2O²⁻ → O₂ + 4e⁻
Overall: 2Al₂O₃ → 4Al + 3O₂
The oxygen reacts with the carbon anode to form CO₂, so the anode needs periodic replacement. The aluminium obtained is about 99.5% pure.
2. Explain the extraction of iron from haematite in a blast furnace with all reactions.
Answer
Iron is extracted from haematite (Fe₂O₃) in a blast furnace through the following steps:
Step 1 — Concentration: Haematite is concentrated by gravity separation and magnetic separation.
Step 2 — Calcination / Roasting: The concentrated ore is heated in limited air (calcination) or excess air (roasting) to remove moisture and convert FeCO₃ or FeS₂ to Fe₂O₃.
Step 3 — Smelting in the Blast Furnace: The ore is mixed with coke (reducing agent) and limestone (flux) and fed into the top of the furnace. Hot air is blasted from the bottom.
Zone of combustion (lower): C + O₂ → CO₂; CO₂ + C → 2CO
Zone of reduction (middle): 3Fe₂O₃ + CO → 2Fe₃O₄ + CO₂
Fe₃O₄ + CO → 3FeO + CO₂
FeO + CO → Fe + CO₂
Zone of slag formation: CaCO₃ → CaO + CO₂
CaO + SiO₂ → CaSiO₃ (slag)
Molten iron collects at the bottom (hearth) and slag floats above it. Pig iron obtained contains about 4% carbon and impurities. It is further refined to produce cast iron or wrought iron.
3. Discuss the different methods of refining of metals with examples.
Answer
Refining is the process of removing impurities from crude metal to obtain pure metal. The main methods are:
(a) Distillation: Used for low-boiling metals like zinc and mercury. The impure metal is heated; the pure metal vaporises and condenses separately, leaving behind non-volatile impurities.
(b) Liquation: Used for low-melting metals like tin and lead. The impure metal is placed on a sloping hearth and heated slightly above its melting point. The pure metal flows down the slope, while impurities remain behind.
(c) Electrolytic Refining: The impure metal is the anode, pure metal is the cathode, and a solution of the metal salt is the electrolyte. On passing current, pure metal deposits on the cathode, and impurities either dissolve in the electrolyte or fall as anode mud. Used for copper, gold, silver, zinc.
At anode: M (impure) → Mⁿ⁺ + ne⁻
At cathode: Mⁿ⁺ + ne⁻ → M (pure)
(d) Zone Refining: A circular heater moves along a rod of the impure metal. The molten zone sweeps impurities towards one end, which is cut off. Used for semiconductors (silicon, germanium, gallium).
(e) Vapour Phase Refining (van Arkel method): The impure metal reacts with a halogen to form a volatile compound, which decomposes on a hot filament to give pure metal. Used for titanium, zirconium.
Ti + 2I₂ → TiI₄ →(hot filament) Ti (pure) + 2I₂
4. Explain the Ellingham diagram and its applications in metallurgy.
Answer
The Ellingham diagram is a graphical representation of the standard Gibbs free energy change (ΔG°) for the formation of metal oxides plotted against temperature. Each line represents the reaction: 2xM + O₂ → 2MₓO.
Key Features:
- Most lines slope upward (ΔG becomes less negative with increasing temperature) because entropy decreases in oxide formation (fewer gas molecules).
- For reactions involving gases as products (like C + O₂ → CO₂ or 2C + O₂ → 2CO), the lines may slope downward because entropy increases.
- The point where a line crosses ΔG = 0 indicates the temperature above which the oxide becomes unstable and decomposes.
Applications: 1. Predicting feasibility of reduction: A metal can reduce the oxide of another metal if its Ellingham line lies below that metal's line at the given temperature. For example, aluminium can reduce Fe₂O₃ because Al₂O₃ has a more negative ΔG.
2. Choice of reducing agent: The diagram helps select carbon or a more electropositive metal as a reducing agent. Carbon can reduce many oxides at high temperatures because the CO line slopes downward.
3. Thermal stability of oxides: Oxides with more negative ΔG values are more thermally stable. For example, MgO is more stable than HgO.
4. Temperature of decomposition: The intersection of a metal oxide line with ΔG = 0 gives the temperature above which the oxide decomposes. Mercuric oxide (HgO) and silver oxide (Ag₂O) decompose at relatively low temperatures.
5. Describe the concentration of ores by different methods.
Answer
Concentration (or dressing) of ores is the process of removing gangue from the ore. The main methods are:
(a) Gravity Separation (Hydraulic Washing): Based on the difference in densities of ore and gangue. The powdered ore is washed with a stream of water on a wilfley table. Lighter gangue particles are washed away while heavier ore particles settle. Used for oxide ores like haematite and tin stone.
(b) Froth Flotation: Used for sulphide ores. The powdered ore is mixed with water, pine oil (frother), and collectors (e.g., xanthates). Air is blown in; sulphide particles attach to oil-coated air bubbles and rise as froth, which is skimmed off. Depressants like NaCN selectively prevent ZnS from floating when separating PbS from ZnS.
(c) Magnetic Separation: Used when either the ore or the impurity is magnetic. The powdered ore is passed over a conveyor belt near a magnetic roller. Magnetic particles are attracted and form a separate heap. Used for magnetite (Fe₃O₄) and wolframite.
(d) Leaching (Chemical Method): The ore is treated with a suitable reagent that selectively dissolves the metal. Bauxite is leached with NaOH (Bayer's process); silver and gold are leached with NaCN or KCN.
Al₂O₃·2H₂O + 2NaOH → 2NaAlO₂ + 3H₂O (bauxite leaching)
Ag₂S + 4NaCN → 2Na[Ag(CN)₂] + Na₂S (silver leaching)
Common Mistakes to Avoid
- ❌ Writing "roasting" when the question asks about carbonate ores. ✅ Roasting is for sulphide ores; calcination is for carbonate/hydroxide ores.
- ❌ Confusing anode and cathode in electrolytic refining. ✅ Impure metal is the anode; pure metal deposits on the cathode.
- ❌ Forgetting to balance chemical equations in extraction steps. ✅ Always check atom and charge balance in every equation.
- ❌ Saying cryolite is the reducing agent in aluminium extraction. ✅ Cryolite is a flux that lowers melting point and increases conductivity; the actual reduction is by electrolysis.
- ❌ Writing that Ellingham diagram plots enthalpy vs temperature. ✅ It plots Gibbs free energy (ΔG°) vs temperature.
- ❌ Confusing gangue and flux. ✅ Gangue is the impurity in ore; flux is the substance added to convert gangue to slag.
- ❌ Omitting the role of depressant in froth flotation. ✅ NaCN acts as a depressant to prevent ZnS from floating when separating from PbS.
- ❌ Writing zone refining is for all metals. ✅ Zone refining is specifically for metals requiring very high purity like semiconductors (Si, Ge).
Exam Checklist
- [ ] Can you list all concentration methods with the type of ore they suit?
- [ ] Can you write balanced equations for calcination and roasting?
- [ ] Can you explain the role of flux and slag with a chemical equation?
- [ ] Can you describe the blast furnace reactions zone by zone?
- [ ] Can you explain the Hall-Heroult process with cathode and anode reactions?
- [ ] Can you interpret an Ellingham diagram to determine feasibility of reduction?
- [ ] Can you differentiate between all five refining methods with examples?
- [ ] Can you write the Bayer's process equations for bauxite concentration?
- [ ] Can you explain the van Arkel method with the chemical equation?
- [ ] Do you know which ores are oxide, sulphide, and carbonate?
- [ ] Can you explain the principle of froth flotation with the role of depressants?
- [ ] Have you practised 5-mark answers with step-by-step extraction processes?
