8. Metallurgy – Complete Chapter Notes
Physical & Chemical Properties Reactivity Series Ionic Compounds Concentration of Ores Extraction of Metals Aluminium (Bayer + Electrolysis) Corrosion & Prevention Alloys & Uses
Earth formed ~4.5 billion years ago. Continuous geological processes created various ores, liquids and gases. Matter around us exists as elements/compounds. A classic way to study many together is to classify them: metals, nonmetals (and metalloids). You learned basics earlier; here we go deeper and connect with metallurgy—the science & technology of extracting, refining and protecting metals.
Physical Properties
Metals
- State: Mostly solids; Hg and Ga are liquids at room temperature.
- Lustre: Shiny (tarnish on exposure to air/moisture/reactive gases).
- Ductile & malleable; drawn into wires/ beaten into sheets (basis of foils, wires).
- Conductivity: Good conductors of heat & electricity (diamond is an exception for electricity—see below).
- Hardness: Generally hard; Li, Na, K are soft (cut with knife).
- Melting/boiling points: Usually high; tungsten has highest m.p. (~\(3422^\circ\)C). Alkali metals & Hg, Ga are lower.
- Sonorous: Produce ringing sound when struck.
Nonmetals
- State: Solid or gas; Br\(_2\) is a liquid exception.
- Lustre: Generally dull; iodine crystals are shiny.
- Hardness: Not hard; exception diamond (hardest natural substance).
- Melting/boiling points: Generally low (varies by allotrope).
- Conductivity: Poor conductors; graphite (C allotrope) conducts electricity well.
Heat vs Electricity: Good heat conductors are usually good electrical conductors; diamond is a notable exception (excellent heat conductor, electrical insulator).
Chemical Properties of Metals
Metals are electropositive—they lose electrons easily to form cations.
a) Reaction with Oxygen
Alkali metals react vigorously; some metal oxides dissolve in water to give alkalies (e.g., NaOH, KOH).
b) Reaction with Water
Sodium/potassium: rapid & exothermic. Calcium: slower; floats due to H\(_2\) bubbles.
c) Reaction with Dilute Acids
Relative reactivity (from observations): \(\mathrm{Mg} \gt \mathrm{Al} \gt \mathrm{Zn} \gt \mathrm{Fe}\).
d) Reaction with Nitric Acid
Aqua Regia: Freshly mixed \( \mathrm{HCl}:\mathrm{HNO_3} = 3:1 \) dissolves noble metals (Au, Pt).
e) Displacement (with salts of other metals)
Reactivity Series of Metals
The experimentally determined order (high → low):
- Highly reactive: K, Na, Li, Ca, Mg (react with water/oxygen strongly)
- Moderately reactive: Al, Zn, Fe, Sn, Pb (react with acids/oxygen)
- Less reactive / noble: Cu, Hg, Ag, Au (do not react easily; often found native)
Chemical Properties of Nonmetals
- Acidic/neutral oxides on combustion:
\[ \mathrm{C} + \mathrm{O_2} \rightarrow \mathrm{CO_2}\ (\text{acidic}),\quad 2\mathrm{C} + \mathrm{O_2} \rightarrow 2\mathrm{CO}\ (\text{neutral}) \] \[ \mathrm{S} + \mathrm{O_2} \rightarrow \mathrm{SO_2}\ (\text{acidic}) \]
- With water: Generally no reaction; halogens dissolve/react.
\[ \mathrm{Cl_2} + \mathrm{H_2O} \rightarrow \mathrm{HOCl} + \mathrm{HCl} \]
- With dilute acids: Generally no reaction; exception—chlorine oxidizes HBr:
\[ \mathrm{Cl_2} + 2\mathrm{HBr} \rightarrow 2\mathrm{HCl} + \mathrm{Br_2} \]
- With hydrogen: Form hydrides under suitable conditions:
\[ \mathrm{S} + \mathrm{H_2} \rightarrow \mathrm{H_2S},\quad \mathrm{N_2} + 3\mathrm{H_2} \rightarrow 2\mathrm{NH_3} \]
Driving force: attaining nearest noble gas configuration (octet) — metals lose e\(^-\), nonmetals gain e\(^-\).
Ionic Compounds
Definition: Compounds of oppositely charged ions (cations \(+\) anions) held by electrostatic forces (“ionic bond”). Electrically neutral overall.
Key Ideas
- Crystalline solids (regular ion arrangement). Shape depends on ion sizes and charges.
- Strong attractions → hard, brittle, with high m.p./b.p.
- Soluble in water; insoluble in kerosene/petrol (no new ion–solvent attractions there).
- Electrical conduction: no in solid (ions fixed); yes when molten or in aqueous solution (mobile ions) → electrolytes.
Examples: Melting/Boiling (°C)
| Compound | Type | m.p. | b.p. |
|---|---|---|---|
| NaCl | Ionic | 801 | 1412 |
| KCl | Ionic | 772 | 1390 |
| MgO | Ionic | 2852 | 3600 |
| H\(_2\)O | Non-ionic (molecular) | 0 | 100 |
Metallurgy: Concepts & Flow
Metallurgy = extraction of metals from ores and their purification for use.
Occurrence
- Most metals occur as compounds (oxides, sulphides, carbonates, nitrates).
- Unreactive metals (Ag, Au, Pt) often found native.
- Mineral: naturally occurring compound with impurities; Ore: mineral economically viable for extraction; Gangue: earthy impurities (sand, clay, etc.).
Core Steps
- Concentration of ore (remove gangue).
- Extraction/Reduction to metal (method depends on reactivity).
- Refining (purification, often electrolytic).
Concentration of Ores (Beneficiation)
a) Gravity-based (by density)
- Wilfley table: Vibrating, riffled, inclined table with water flow; lighter gangue washes away; heavier mineral collects between riffles.
- Hydraulic separation: Upward water jet in tapering tank; lighter gangue exits at top; heavier ore settles & is collected below.
b) Magnetic Separation
On a belt over a magnetic roller: magnetic ore parts stick & drop near the magnet, nonmagnetic fall farther. Example: cassiterite (SnO\(_2\)) with FeWO\(_4\).
c) Froth Flotation (sulphide ores)
Exploits hydrophobic (ore) vs hydrophilic (gangue). Pine/eucalyptus oils + air → froth. Sulphide particles attach to froth and float; gangue remains wetted and sinks. Used for ZnS (zinc blende), CuFeS\(_2\) (copper pyrites).
d) Leaching
Selective dissolution of ore (leaves gangue). Bauxite concentrated using aqueous NaOH (Bayer) or Na\(_2\)CO\(_3\) (Hall) → soluble sodium aluminate; later precipitated as Al(OH)\(_3\), calcined to Al\(_2\)O\(_3\).
Extraction of Metals (Reduction)
Choice of method depends on place in the reactivity series.
1) Highly Reactive Metals (top of series)
Na, K, Ca, Mg, Al: obtained by electrolytic reduction of their molten salts/oxides.
Cathode: \(\mathrm{Na^+ + e^- \rightarrow Na(l)}\) (reduction)
Anode: \(\mathrm{2Cl^- \rightarrow Cl_2(g) + 2e^-}\) (oxidation)
2) Case Study: Aluminium (from Bauxite)
Ore: Bauxite \((\mathrm{Al_2O_3}\cdot n\mathrm{H_2O})\); Impurities: SiO\(_2\), Fe\(_2\)O\(_3\), TiO\(_2\)
- Concentration (Bayer/Hall):
\[ \mathrm{Al_2O_3 + 2NaOH + 2H_2O \rightarrow 2NaAlO_2 + 3H_2O} \] \[ \mathrm{NaAlO_2 + 2H_2O \rightarrow Al(OH)_3\downarrow + NaOH} \] \[ \mathrm{2Al(OH)_3 \xrightarrow{1000^\circ C} Al_2O_3 + 3H_2O} \]
- Electrolytic reduction of alumina: Dissolve Al\(_2\)O\(_3\) in molten cryolite (Na\(_3\)AlF\(_6\)) with CaF\(_2\) to lower m.p. to ~\(1000^\circ\)C. Steel tank with graphite lining as cathode; graphite anodes.
Cathode: \(\mathrm{Al^{3+} + 3e^- \rightarrow Al(l)}\) (collects at bottom)
Anode: \(\mathrm{2O^{2-} \rightarrow O_2 + 4e^-}\); \( \mathrm{C + O_2 \rightarrow CO_2} \) (anode consumption)
3) Moderately Reactive Metals
- Usually occur as sulphides or carbonates. Convert to oxides first:
Roasting (sulphide):\ \(\mathrm{2ZnS + 3O_2 \rightarrow 2ZnO + 2SO_2}\)
Calcination (carbonate):\ \(\mathrm{ZnCO_3 \xrightarrow{\Delta} ZnO + CO_2}\) - Reduce oxide with C/CO or more reactive metals:
\[ \mathrm{ZnO + C \rightarrow Zn + CO} \] Aluminothermy:\ \(\mathrm{3MnO_2 + 4Al \rightarrow 3Mn + 2Al_2O_3 + \text{heat}}\)
- Thermit reaction (welding rails):
\(\mathrm{Fe_2O_3 + 2Al \rightarrow 2Fe + Al_2O_3 + \text{heat}}\)
4) Less Reactive Metals (bottom)
Often found native; or extracted by simple roasting. Copper from Cu\(_2\)S:
Refining (Purification) of Metals
Metals obtained are impure. Electrolytic refining is common: impure metal as anode, thin pure sheet as cathode, suitable salt solution as electrolyte. Pure metal deposits on cathode; impurities fall as anode mud.
Corrosion of Metals & Its Prevention
Common Observations
- Iron rusts: \(\mathrm{Fe_2O_3\cdot H_2O}\) reddish layer in moist air.
- Copper patina: green \(\mathrm{CuCO_3}\) layer (with CO\(_2\)/moisture).
- Silver tarnish: black \(\mathrm{Ag_2S}\) (from H\(_2\)S in air).
- Aluminium: thin, protective \(\mathrm{Al_2O_3}\) film forms (self-passivation).
Prevention Methods
- Barrier coatings: Paint, oil, grease, varnish (short-term; scratches allow rust beneath).
- Galvanizing (Zn on Fe/steel): sacrificial Zn corrodes first; protects iron. nails, sheets
- Tinning (Sn coat): “Kalhaee” on Cu/Brass utensils to prevent poisonous verdigris; protects food.
- Anodizing: grow thicker oxide on Al/Cu via electrolysis; uniform, hard, protective \(\mathrm{Al_2O_3}\).
- Electroplating: coat less reactive metal (Ag, Au, Cr, Ni) on reactive ones for protection & appearance.
- Alloying: tailor properties; many alloys resist corrosion.
Alloy Composition Properties/Uses Bronze Cu (~90%) + Sn (~10%) Hard, corrosion-resistant; statues, medals Brass Cu + Zn Machinable; fittings, musical instruments Stainless steel Fe (~74%) + Cr (~18%) + C (~8%) Rust-resistant; cookware, surgical tools Amalgam Metal + Hg Dental (Ag amalgam, earlier), extraction (Au amalgam)
Quick Recap: Must-Know Equations & Ideas
Roasting: \(\mathrm{2MS + 3O_2 \rightarrow 2MO + 2SO_2}\) (sulphide \(\rightarrow\) oxide)
Calcination: \(\mathrm{MCO_3 \xrightarrow{\Delta} MO + CO_2}\) (carbonate \(\rightarrow\) oxide)
Electrolysis (NaCl l): Cathode \(\mathrm{Na^+ + e^- \rightarrow Na}\);\ Anode \(\mathrm{Cl^- \rightarrow \tfrac12 Cl_2 + e^-}\)
Thermit: \(\mathrm{Fe_2O_3 + 2Al \rightarrow 2Fe + Al_2O_3 + heat}\)
Exam tip: Always justify the chosen extraction method by position in the reactivity series and the nature of the ore (sulphide vs carbonate vs oxide).
Self-Check (Think & Answer)
- Why is froth flotation preferred for sulphide ores but not for oxide ores?
- State one reason why ionic compounds conduct electricity in molten/aqueous state but not in solid state.
- Explain why aluminium doesn’t corrode rapidly despite being reactive.
- Write electrode reactions for electrolysis of molten \(\mathrm{MgCl_2}\) and \(\mathrm{CaCl_2}\).
Chapter 8 – Metallurgy: Exercise Solutions
1) Write names.
- Alloy of sodium with mercury: Sodium amalgam.
- Molecular formula of the common ore of aluminium: Bauxite, \( \mathrm{Al_2O_3\cdot nH_2O} \) (hydrated alumina).
- The oxide that reacts with both acid and base to form salt and water: Amphoteric oxide — e.g. Aluminium oxide \( \mathrm{Al_2O_3} \) (also ZnO).
- Device used for grinding an ore: Ball mill.
- The nonmetal having electrical conductivity: Graphite (allotrope of carbon).
- Reagent that dissolves noble metals: Aqua regia (conc. \( \mathrm{HCl}:\mathrm{HNO_3}=3:1 \)).
2) Make pairs of substances and their properties.
| Substance | Correct Property | Reason |
|---|---|---|
| a) Potassium bromide (KBr) | 2) Soluble in water | Ionic salt → hydrates/dissolves forming ions. |
| b) Gold (Au) | 4) High ductility | Can be drawn into very fine wires/sheets. |
| c) Sulphur (S) | 1) Combustible | Burns in air to \( \mathrm{SO_2} \). |
| d) Neon (Ne) | 3) No chemical reaction | Noble gas → inert. |
3) Identify the pairs of metals and their ores.
- Bauxite \(\rightarrow\) Aluminium (ii)
- Cassiterite \(\rightarrow\) Tin (iii)
- Cinnabar \(\rightarrow\) Mercury (i)
4) Explain the terms.
a) Metallurgy: Science & technology of extracting metals from ores and refining them for use.
b) Ores: Minerals that can yield metal economically (contain a high % of the metal compound).
c) Minerals: Naturally occurring compounds of metals found in the earth’s crust, often with impurities.
d) Gangue: Unwanted earthy impurities (sand, clay, rock) associated with ores; removed during concentration.
5) Write scientific reasons.
- Lemon/tamarind cleans greenish copper vessels: The green layer is basic copper carbonate (patina). Organic acids (citric/tartaric) dissolve it to soluble copper salts: \( \mathrm{CuCO_3 + 2H^+ \rightarrow Cu^{2+} + CO_2 + H_2O} \).
- Ionic compounds have high melting points: Strong electrostatic attractions in the ionic lattice require large energy to overcome.
- Sodium kept in kerosene: Na reacts violently with air/moisture/water forming \( \mathrm{Na_2O/NaOH} \) + \( \mathrm{H_2} \); kerosene isolates it from air/water.
- Pine oil in froth flotation: Acts as frother (stable froth) and helps render sulphide particles hydrophobic, so they attach to air bubbles and float; gangue stays wetted and sinks.
- Anodes replaced during electrolysis of alumina: Oxygen liberated at carbon anodes forms \( \mathrm{CO_2} \) (consuming anode): \( \mathrm{C + O_2 \rightarrow CO_2} \).
6) Copper coin in silver nitrate solution shines — why? Write equation.
Reason: Copper is more reactive than silver; it displaces Ag from \( \mathrm{AgNO_3} \). Deposited silver gives a glitter.
7) Metal A: 2,8,1; Metal B: 2,8,2 — which is more reactive? Write reactions with dil. HCl.
Answer: A (Group 1, Na-like) is more reactive than B (Group 2, Mg-like).
Alkali metals react very vigorously with acids; alkaline earth metals react readily but less violently.
8) Draw neat labelled diagrams.
a) Magnetic Separation Method
b) Froth Flotation Method
c) Electrolytic Reduction of Alumina (Hall–Héroult Cell)
d) Hydraulic Separation Method
9) Write chemical equations.
- Aluminium came in contact with air:
\[ 4\mathrm{Al} + 3\mathrm{O_2} \rightarrow 2\mathrm{Al_2O_3} \quad(\text{thin protective oxide film}) \]
- Iron filings in aqueous copper sulphate:
\[ \mathrm{Fe} + \mathrm{CuSO_4(aq)} \rightarrow \mathrm{FeSO_4(aq)} + \mathrm{Cu} \]
- Ferric oxide with aluminium (thermit):
\[ \mathrm{Fe_2O_3} + 2\mathrm{Al} \rightarrow 2\mathrm{Fe} + \mathrm{Al_2O_3} + \text{heat} \]
- Electrolysis of alumina (in cryolite):
Cathode: \( \mathrm{Al^{3+} + 3e^- \rightarrow Al(l)} \)
Anode: \( \mathrm{2O^{2-} \rightarrow O_2 + 4e^-} \)
Effective cell reaction (with C anodes): \( \boxed{\,\mathrm{2Al_2O_3 + 3C \rightarrow 4Al + 3CO_2}\,} \) - Zinc oxide in dilute HCl:
\[ \mathrm{ZnO} + 2\mathrm{HCl} \rightarrow \mathrm{ZnCl_2} + \mathrm{H_2O} \]
10) Complete the statements (Extraction of Aluminium).
- Ingredients and gangue in bauxite: Ingredient: \( \mathrm{Al_2O_3\cdot nH_2O} \) (hydrated alumina). Gangue: \( \mathrm{SiO_2} \), \( \mathrm{Fe_2O_3} \), \( \mathrm{TiO_2} \).
- Use of leaching during concentration: To selectively dissolve alumina (amphoteric) as sodium aluminate using aqueous \( \mathrm{NaOH} \) / \( \mathrm{Na_2CO_3} \), leaving insoluble impurities (then precipitate \( \mathrm{Al(OH)_3} \)).
- Chemical reactions (Hall’s / Bayer path to alumina):
Leach (Hall): \( \mathrm{Al_2O_3 + Na_2CO_3 \xrightarrow{aq} 2NaAlO_2} \) (simplified)
Precipitation: \( \mathrm{2NaAlO_2 + 3H_2O + CO_2 \rightarrow 2Al(OH)_3\downarrow + Na_2CO_3} \)
Calcination: \( \mathrm{2Al(OH)_3 \xrightarrow{1000^\circ C} Al_2O_3 + 3H_2O} \) - Heating the ore with concentrated caustic soda (Bayer):
\( \mathrm{Al_2O_3 + 2NaOH + 2H_2O \rightarrow 2NaAl(OH)_4} \) \quad or \quad \( \mathrm{Al_2O_3\cdot 2H_2O + 2NaOH \rightarrow 2NaAlO_2 + 3H_2O} \)
11) Classify: Cu, Zn, Ca, Mg, Fe, Na, Li — into reactivity groups.
- Reactive metals (high): Na, Li, Ca, Mg
- Moderately reactive: Zn, Fe
- Less reactive (noble end): Cu