Ores and Metallurgy

Ores and Metallurgy

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

An ore is a naturally occurring rock or mineral from which a metal can be profitably extracted. The worthless rocky material mixed with the ore is called gangue. Metallurgy covers the full sequence: finding the ore, dressing (concentrating) it, converting it to a metal oxide, reducing that oxide to the free metal, and refining the crude metal.

Key facts WAEC loves to test:

• Calcination = heating carbonate ores in the absence of air (e.g. CaCO₃ → CaO + CO₂).
• Roasting = heating sulphide ores in the presence of air (produces SO₂).
• Flux + gangue → slag; basic flux (CaO) for acidic gangue (SiO₂), acidic flux (SiO₂) for basic gangue.
• Iron is extracted in the blast furnace using haematite (Fe₂O₃), coke as fuel/reductant, and limestone (CaCO₃) as flux.
• Steel is iron alloyed with controlled carbon (≈0.2–2%) plus other metals.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Sources of Metals

Metals occur either native (Au, Ag, Pt in uncombined form) or as combined ores classified by the anion: oxides (haematite Fe₂O₃, magnetite Fe₃O₄, bauxite Al₂O₃·2H₂O), sulphides (galena PbS, zinc blende ZnS), carbonates (calcite CaCO₃, cerrusite PbCO₃) and halides/ chlorides (rock salt NaCl, carnallite KCl·MgCl₂·6H₂O). For a deposit to qualify as an ore, the metal concentration must make mining economically worthwhile.

Concentration (Dressing) Methods

The goal is to remove gangue before chemical treatment:

• Hydraulic washing / levigation – gravity separation for heavy oxide ores.
• Magnetic separation – used when ore is magnetic (magnetite) but gangue is not.
• Froth flotation – preferred for sulphide ores; ore particles cling to air bubbles coated with pine oil, while gangue sinks.
• Leaching – dissolving the ore in a suitable reagent (e.g. bauxite in NaOH to form sodium aluminate).

Conversion to Oxide

• Calcination (no air): for carbonate/hydrated ores; releases CO₂ or water.
• Roasting (excess air): for sulphide ores; converts sulphide → oxide + SO₂ gas. Example: 2ZnS + 3O₂ → 2ZnO + 2SO₂.

Reduction to Metal

Choice of reductant depends on the metal’s position in the activity/reactivity series:

• C, CO, H₂ reduce the oxides of moderately reactive metals (Zn, Fe, Sn, Pb).
• Electrolysis of fused oxides/ores is required for very reactive metals (Na, Mg, Al) because carbon cannot reduce them.
• Thermite reaction uses Al powder to reduce oxides of less reactive metals (Cr₂O₃, MnO₂): 2Al + Fe₂O₃ → 2Fe + Al₂O₃ + heat.

The Blast Furnace (Iron)

Charge from the top: haematite + coke + limestone. Hot air (“blast”) is blown in through tuyères at the base. Reactions proceed in zones:

1. C + O₂ → CO₂
2. CO₂ + C → 2CO (main reductant)
3. Fe₂O₃ + 3CO → 2Fe + 3CO₂
4. CaCO₃ → CaO + CO₂, then CaO + SiO₂ → CaSiO₃ (slag, floats on molten iron).

Refining

Crude metal is purified by distillation (Zn, Hg), electrolytic refining (Cu, Al — impure metal as anode, pure as cathode), or zone refining (semiconductors like Si, Ge).

Exam Patterns

WAEC WASSCE frequently asks: name a method of concentration for a named ore, distinguish calcination from roasting, write blast-furnace equations, or explain slag formation. Memorise the chemical formulae of haematite, bauxite, galena and limestone — they appear almost every year.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Alloys of Iron — Carbon Content Matters

• Pig iron from the blast furnace contains ~4% C plus Si, Mn, P, S; brittle, used for castings.
• Cast iron = remelted pig iron, 2–4% C; brittle but strong in compression.
• Wrought iron = <0.1% C, obtained by puddling; malleable, used for chains, anchors.
• Steel = 0.2–2% C with controlled Mn, Cr, Ni etc.; properties vary sharply with carbon content and heat treatment.

Acidic vs Basic Flux — A Common Trap

Examiners often rephrase the rule. The rule is “flux is the OPPOSITE chemical character of the gangue”:

• If gangue is basic (e.g. FeO, CaO) → add acidic flux SiO₂.
• If gangue is acidic (e.g. SiO₂) → add basic flux CaO from limestone. The product (CaSiO₃ or FeSiO₃) is slag, which is skimmed off because it is fusible and less dense than the molten metal.

Thermite and Aluminothermic Welding

The thermite reaction (2Al + Fe₂O₃ → 2Fe + Al₂O₃, ΔH ≈ −850 kJ mol⁻¹) is so exothermic that the iron produced is molten. WAEC may ask why Al is used despite being more reactive than Fe — because Al has a stronger affinity for oxygen (more negative ΔGf of Al₂O₃) and the reaction is self-sustaining once ignited by a magnesium ribbon.

Refining Nuances

• Electrolytic refining of copper: anode = impure blister copper, cathode = pure copper strip, electrolyte = acidified CuSO₄; impurities below Cu (Ag, Au) fall as anode mud, while Zn, Fe go into solution.
• Zone refining: a molten zone is moved slowly along a rod; impurities stay dissolved in the melt and are swept to one end, yielding ultra-pure crystals for semiconductors.

Connections to Other Topics

Ores link directly to acids, bases and salts (acidic gangue vs basic gangue), electrochemistry (electrolytic extraction and refining), air and combustion (roasting, blast-furnace oxygen), and extraction of specific metals (Al from bauxite via the Bayer–Hall process).

Common Mistakes

• Confusing roasting (sulphide → oxide, air present) with calcination (carbonate → oxide, air absent).
• Writing limestone (CaCO₃) instead of its decomposition product CaO as the actual flux.
• Saying carbon “directly” reduces Fe₂O₃ — in practice CO is the principal gaseous reductant at furnace temperatures.
• Forgetting SO₂ from roasting causes acid rain, a frequent environmental question.

Practice Prompts

1. A student roasted 200 g of ZnS (RFM = 97 g mol⁻¹). Calculate the volume of SO₂ produced at s.t.p., and the mass of ZnO formed.
2. Explain why aluminium is extracted by electrolysis while iron is extracted in the blast furnace using carbon, referring to the reactivity series and the energetics of oxide formation.

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