Metallurgy Notes for Class 12, IIT JEE and NEET

The topic is called General Principles and Processes of Isolation of Elements, or metallurgy. It is a part of the inorganic chemistry syllabus of JEE Main and JEE Advanced, and it connects thermodynamics (Gibbs energy) and electrochemistry (electrolysis) with the chemistry of the metals. Check the latest syllabus of your board and of NEET for the exact topic list. These Metallurgy Notes for Class 12 explain the whole process in the order in which a metal is obtained from its ore. For the other chapters, see eSaral’s Class 12 Chemistry notes.

Metallurgy Notes for Class 12 for IIT JEE and NEET

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India's Best Exam Preparation for Class 12th - Download Now

India's Best Exam Preparation for Class 12th - Download Now

India's Best Exam Preparation for Class 12th - Download Now

India's Best Exam Preparation for Class 12th - Download Now

Basic Terms in Metallurgy Notes for Class 12

All ores are minerals, but not all minerals are ores. For example, both clay and bauxite contain aluminium, but only bauxite is an ore of aluminium.

Important Ores of Metals in Metallurgy Notes for Class 12

Concentration of the Ore in Metallurgy Notes for Class 12

Concentration (benefaction) removes the gangue. The method depends on the ore.

• Froth flotation with a depressant: to separate $\mathrm{ZnS}$ from $\mathrm{PbS}$, NaCN is added as a depressant. It stops $\mathrm{ZnS}$ from forming a froth with the collector, so $\mathrm{PbS}$ floats.
• Leaching of bauxite (Baeyer’s process): the ore is treated with concentrated NaOH, and then the solution is neutralised with $\mathrm{CO_2}$:

$$\mathrm{Al_2O_3 + 2NaOH + 3H_2O \rightarrow 2Na[Al(OH)_4]}$$ $$\mathrm{2Na[Al(OH)_4] + CO_2 \rightarrow Al_2O_3\cdot xH_2O\downarrow + 2NaHCO_3}$$

The hydrated alumina is filtered and heated to give pure $\mathrm{Al_2O_3}$.

• Cyanide process for silver and gold:

$$\mathrm{4Ag + 8CN^- + 2H_2O + O_2 \rightarrow 4[Ag(CN)_2]^- + 4OH^-}$$ $$\mathrm{2[Ag(CN)_2]^- + Zn \rightarrow [Zn(CN)_4]^{2-} + 2Ag}$$

Calcination and Roasting in Metallurgy Notes for Class 12

Reduction of the Oxide to the Metal in Metallurgy Notes for Class 12

Flux and slag: if the ore has an acidic impurity such as $\mathrm{SiO_2}$, a basic flux such as $\mathrm{CaO}$ is added: $\mathrm{CaO + SiO_2 \rightarrow CaSiO_3}$ (slag). If the impurity is basic (for example $\mathrm{FeO}$ or $\mathrm{CaO}$), an acidic flux such as $\mathrm{SiO_2}$ is used.

Thermodynamic Principles and the Ellingham Diagram in Metallurgy Notes for Class 12

A reduction is feasible if the Gibbs energy change is negative:

$$\Delta G^\circ = \Delta H^\circ - T\Delta S^\circ, \qquad \Delta G^\circ = -RT\ln K$$

For a reduction that has $\Delta H^\circ > 0$ and $\Delta S^\circ > 0$, $\Delta G^\circ$ becomes negative above the temperature $T = \dfrac{\Delta H^\circ}{\Delta S^\circ}$. For the thermodynamics, see eSaral’s Thermodynamics short notes.

Ellingham diagram: it is a plot of $\Delta G^\circ$ against the temperature for the formation of oxides, taken per mole of $\mathrm{O_2}$.

• The slope of each line is $-\Delta S^\circ$. When a metal forms a solid oxide, a gas ($\mathrm{O_2}$) is used up, so $\Delta S^\circ$ is negative, and the line has a positive slope.
• The line for $\mathrm{2C + O_2 \rightarrow 2CO}$ has a negative slope, because the number of moles of gas increases. The line for $\mathrm{C + O_2 \rightarrow CO_2}$ is almost horizontal.
• The slope of a line increases at the melting point and the boiling point of the metal, because $\Delta S^\circ$ becomes more negative.
• Use: at a given temperature, a metal can reduce the oxide of another metal if its own oxide line lies below the line of that oxide. Carbon (as CO) can reduce an oxide above the temperature at which the $\mathrm{C \rightarrow CO}$ line goes below the line of that oxide. This is why $\mathrm{ZnO}$ needs a high temperature, and why Al can reduce $\mathrm{Cr_2O_3}$.
• Limits: the diagram tells only about the thermodynamic feasibility. It does not give the rate of the reaction, and it is for the standard states.

Extraction of Aluminium (Hall–Héroult Process) in Metallurgy Notes for Class 12

1. Purification of bauxite by Baeyer’s process (shown above) gives pure $\mathrm{Al_2O_3}$.
2. Electrolysis: pure $\mathrm{Al_2O_3}$ is dissolved in molten cryolite ($\mathrm{Na_3AlF_6}$) with some fluorspar ($\mathrm{CaF_2}$). This lowers the melting point of the mixture from about $2000\,^\circ\mathrm{C}$ (pure $\mathrm{Al_2O_3}$) to below about $1000\,^\circ\mathrm{C}$, and it increases the electrical conductivity. The cell has a carbon lining (the cathode) and graphite anodes.

$$\text{Cathode: } \mathrm{Al^{3+} + 3e^- \rightarrow Al}$$ $$\text{Anode: } \mathrm{C + O^{2-} \rightarrow CO + 2e^-}, \qquad \mathrm{C + 2O^{2-} \rightarrow CO_2 + 4e^-}$$ $$\text{Overall: } \mathrm{2Al_2O_3 + 3C \rightarrow 4Al + 3CO_2}$$

The anodes burn away, so they are replaced from time to time. The liquid aluminium collects at the bottom of the cell.

Extraction of Copper from Copper Pyrites in Metallurgy Notes for Class 12

1. Concentration of $\mathrm{CuFeS_2}$ by froth flotation.
2. Roasting removes $\mathrm{S}$ and $\mathrm{As}$ as oxides, and converts part of the ore to $\mathrm{Cu_2S}$ and $\mathrm{FeS}$ (and part of $\mathrm{FeS}$ to $\mathrm{FeO}$).
3. Smelting with sand ($\mathrm{SiO_2}$) in a reverberatory furnace removes iron as slag, and gives matte ($\mathrm{Cu_2S}$ with a little $\mathrm{FeS}$): $\ \mathrm{FeO + SiO_2 \rightarrow FeSiO_3}$.
4. Bessemer converter: air is blown through the molten matte with some silica.

$$\mathrm{2FeS + 3O_2 \rightarrow 2FeO + 2SO_2}, \qquad \mathrm{FeO + SiO_2 \rightarrow FeSiO_3}$$ $$\mathrm{2Cu_2S + 3O_2 \rightarrow 2Cu_2O + 2SO_2}, \qquad \mathrm{2Cu_2O + Cu_2S \rightarrow 6Cu + SO_2}$$

The copper is obtained as blister copper (the $\mathrm{SO_2}$ that escapes forms blisters on the surface). It is refined by electrolytic refining.

Extraction of Iron (Blast Furnace) in Metallurgy Notes for Class 12

The charge of roasted ore (mostly $\mathrm{Fe_2O_3}$), coke and limestone is fed from the top, and hot air is blown from the bottom.

$$\mathrm{C + O_2 \rightarrow CO_2}, \qquad \mathrm{CO_2 + C \rightarrow 2CO}$$ $$\mathrm{Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2}$$ $$\mathrm{CaCO_3 \rightarrow CaO + CO_2}, \qquad \mathrm{CaO + SiO_2 \rightarrow CaSiO_3\ (slag)}$$

The iron that is obtained is pig iron (about 4% carbon). Cast iron has about 3% carbon, and wrought iron (the purest form) has about 0.1% to 0.25% carbon.

Extraction of Zinc in Metallurgy Notes for Class 12

$\mathrm{ZnS}$ is concentrated by froth flotation, and then roasted to $\mathrm{ZnO}$. The oxide is reduced with coke at about $1673\ \mathrm{K}$, and the zinc is obtained as vapour, which is condensed:

$$\mathrm{2ZnS + 3O_2 \rightarrow 2ZnO + 2SO_2}, \qquad \mathrm{ZnO + C \xrightarrow{1673\ K} Zn + CO}$$

Refining of Metals in Metallurgy Notes for Class 12

Mond process:

$$\mathrm{Ni + 4CO \xrightarrow{330-350\ K} Ni(CO)_4}, \qquad \mathrm{Ni(CO)_4 \xrightarrow{450-470\ K} Ni + 4CO}$$

Van Arkel method for zirconium:

$$\mathrm{Zr + 2I_2 \rightarrow ZrI_4}, \qquad \mathrm{ZrI_4 \xrightarrow{\text{hot W filament}} Zr + 2I_2}$$

Electrolytic refining of copper: at the anode, $\mathrm{Cu \rightarrow Cu^{2+} + 2e^-}$, and at the cathode, $\mathrm{Cu^{2+} + 2e^- \rightarrow Cu}$. The electrolyte is acidified $\mathrm{CuSO_4}$, and the less active impurities (Ag, Au, Pt) collect as anode mud. The laws of electrolysis are in eSaral’s Electrochemistry notes.

IIT JEE Tips for Metallurgy Notes for Class 12

• Calcination vs roasting: calcination is in the absence of air (carbonates and hydrated oxides), and roasting is in the presence of air (sulphides).
• Match the method with the ore: sulphides go for froth flotation, and then roasting. Carbonates go for calcination. Oxides can be reduced directly.
• Ellingham diagram: the slope is $-\Delta S^\circ$. The oxide line that lies lower belongs to the better reducing agent.
• Flux: an acidic impurity needs a basic flux, and a basic impurity needs an acidic flux.
• Refining: link the method to the property: volatility (distillation, Mond), low melting point (liquation), the semiconductor grade (zone refining) and a very high purity (Van Arkel).
• Electrolytic questions: use $\mathrm{Q = It}$ and Faraday’s laws, with the number of electrons per atom of the metal.

Key Facts in Metallurgy Notes for Class 12

Solved Examples from Metallurgy Notes for Class 12

Example 1 (calcination or roasting): Which process is used for $\mathrm{ZnCO_3}$, and which for $\mathrm{ZnS}$? Write the reactions.

$\mathrm{ZnCO_3}$ is calcined: $\mathrm{ZnCO_3 \rightarrow ZnO + CO_2}$. $\mathrm{ZnS}$ is roasted: $\mathrm{2ZnS + 3O_2 \rightarrow 2ZnO + 2SO_2}$.

Example 2 (temperature for a reduction): A reduction has $\Delta H^\circ = +200\ \mathrm{kJ\,mol^{-1}}$ and $\Delta S^\circ = +200\ \mathrm{J\,K^{-1}\,mol^{-1}}$. Above what temperature is it spontaneous?

$\Delta G^\circ < 0$ when $T > \dfrac{\Delta H^\circ}{\Delta S^\circ} = \dfrac{200000}{200} = 1000\ \mathrm{K}$.

Example 3 (using data from an Ellingham diagram): Suppose that at 1500 K, $\Delta G^\circ$ for $\mathrm{2C + O_2 \rightarrow 2CO}$ is $-370\ \mathrm{kJ}$ and for $\mathrm{2Zn + O_2 \rightarrow 2ZnO}$ is $-330\ \mathrm{kJ}$ (illustrative values). Can carbon reduce ZnO at 1500 K?

For $\mathrm{2ZnO + 2C \rightarrow 2Zn + 2CO}$, $\Delta G^\circ = -370 - (-330) = -40\ \mathrm{kJ}$, which is negative, so the reduction is feasible.

Example 4 (froth flotation): How can $\mathrm{ZnS}$ and $\mathrm{PbS}$ be separated by froth flotation?

Add NaCN as a depressant. It prevents $\mathrm{ZnS}$ from forming a froth, so $\mathrm{PbS}$ floats, and $\mathrm{ZnS}$ stays in the pulp.

Example 5 (a flux): Which flux is used to remove $\mathrm{SiO_2}$, and which to remove $\mathrm{CaO}$?

For the acidic impurity $\mathrm{SiO_2}$, a basic flux $\mathrm{CaO}$ is used: $\mathrm{CaO + SiO_2 \rightarrow CaSiO_3}$. For the basic impurity $\mathrm{CaO}$, an acidic flux $\mathrm{SiO_2}$ is used.

Example 6 (aluminium by electrolysis): What mass of aluminium is obtained from a current of $10{,}000\ \mathrm{A}$ for 1 hour (100% efficiency)? ($\mathrm{Al} = 27$, $F = 96500\ \mathrm{C\,mol^{-1}}$)

$Q = 10000\times 3600 = 3.6\times10^{7}\ \mathrm{C}$. The moles of Al $= \dfrac{Q}{3F} = \dfrac{3.6\times10^{7}}{3\times 96500} \approx 124.4$ mol, so the mass is $124.4\times 27 \approx 3360\ \mathrm{g}$, which is about $3.36\ \mathrm{kg}$.

Example 7 (anode consumption): How much carbon is used at the anode for each kilogram of aluminium, if the overall reaction is $\mathrm{2Al_2O_3 + 3C \rightarrow 4Al + 3CO_2}$?

4 mol Al uses 3 mol C. So 1 mol Al ($27$ g) uses $0.75$ mol C $= 9$ g C. The mass of carbon per kg of Al is $\dfrac{9}{27} \approx 0.33\ \mathrm{kg}$.

Example 8 (iron from haematite): How much iron can be obtained from 1000 kg of pure $\mathrm{Fe_2O_3}$? ($\mathrm{Fe} = 56$, $\mathrm{O} = 16$)

$\mathrm{Fe_2O_3}$ has a molar mass of $160$, and it contains $112$ of Fe. The mass of iron is $1000\times\dfrac{112}{160} = 700\ \mathrm{kg}$.

Example 9 (the Mond process): How is nickel purified by the Mond process, and why is CO not wasted?

Impure Ni reacts with CO at $330$–$350\ \mathrm{K}$ to give volatile $\mathrm{Ni(CO)_4}$. The vapour is heated to $450$–$470\ \mathrm{K}$, and it decomposes to pure Ni and CO. The CO is released again, so it is recycled.

Example 10 (the cyanide process): Write the reactions for the extraction of native silver by the cyanide process.

$\mathrm{4Ag + 8CN^- + 2H_2O + O_2 \rightarrow 4[Ag(CN)_2]^- + 4OH^-}$, and then $\mathrm{2[Ag(CN)_2]^- + Zn \rightarrow [Zn(CN)_4]^{2-} + 2Ag}$. Zinc is a stronger reducing agent than silver, so it displaces silver from the complex.

Common Mistakes in Metallurgy Notes for Class 12 Problems

How to Study Metallurgy Notes for Class 12

1. Read the chapter in your textbook first, and then use these Metallurgy Notes for Class 12 for revision. See eSaral’s Class 12 Chemistry notes for the other chapters.
2. Learn the steps in the order of extraction: concentration, calcination or roasting, reduction and refining. Then learn the Ellingham diagram.
3. Learn the reactions of aluminium, copper, iron and zinc as a set, with the method and the flux.
4. Practise one problem each on the Ellingham diagram, a flux, a refining method and an electrolysis numerical.
5. Revise with eSaral’s JEE Main Chemistry revision notes, and test yourself with eSaral’s JEE Main chapter-wise PYQs.