General Principles of Metallurgy
From rock to refined metal — the universal sequence of concentration, reduction and refining, and the thermodynamics, captured in the Ellingham diagram, that decides which reducing agent will work
- The difference between a mineral and an ore, and the meaning of gangue, flux and slag.
- How ores are concentrated — hydraulic washing, magnetic separation, froth flotation, leaching.
- Calcination vs roasting, and the routes for reducing an oxide to its metal.
- How the Ellingham diagram decides which reducing agent succeeds.
- The major refining methods, including electrolytic and zone refining.
- The extraction of iron (blast furnace) and aluminium (Hall–Héroult).
Minerals, Ores & the Steps
Metals occur in nature combined as minerals — naturally occurring compounds. A mineral from which a metal can be extracted profitably and conveniently is an ore. Every ore comes mixed with earthy impurities called gangue (or matrix). The whole craft of metallurgy is the sequence that turns ore into pure metal.
Two helper terms recur throughout: a flux is added to combine with the gangue, and the fusible product they form is the slag, which floats off and is removed.
Concentration of the Ore
The first task is to remove the gangue. The method chosen exploits a physical or chemical difference between the ore and its impurities — density, magnetism, surface wetting, or solubility.
| Method | Based on | Used for |
|---|---|---|
| Hydraulic washing | density difference | heavy oxide ores (haematite) |
| Magnetic separation | magnetic property | \(\ce{Fe3O4}\), chromite, wolframite |
| Froth flotation | surface wetting | sulphide ores (galena, zinc blende) |
| Leaching | chemical solubility | bauxite, gold & silver ores |
Conversion to the Oxide
Most reductions work best on an oxide, so carbonate and sulphide ores are first turned into oxides. The two heating treatments — calcination and roasting — differ in whether air is present.
Calcination drives off water and \(\ce{CO2}\) from hydrated or carbonate ores in limited air; roasting oxidises sulphide ores in a current of air. Both end at the metal oxide, ready for reduction.
Reduction to the Metal
The oxide must now lose its oxygen. Which reducing route is used depends on the metal's reactivity — cheap carbon for moderately reactive metals, electrolysis for the most reactive.
| Route | Reaction / principle | For metals like |
|---|---|---|
| Carbon (smelting) | \(\ce{ZnO + C ->[\Delta] Zn + CO}\) | Zn, Fe, Sn, Pb |
| Thermite (aluminothermic) | \(\ce{Cr2O3 + 2Al -> 2Cr + Al2O3}\) | Cr, Mn (where C fails) |
| Self-reduction | \(\ce{2Cu2O + Cu2S -> 6Cu + SO2}\) | Cu, Pb, Hg sulphides |
| Hydrometallurgy | \(\ce{2[Au(CN)2]- + Zn -> [Zn(CN)4]^2- + 2Au}\) | Au, Ag |
| Electrolytic | melt electrolysis | Na, Mg, Al, Ca (most reactive) |
The Ellingham Diagram
Whether a reduction will actually happen is a question of thermodynamics. The Ellingham diagram plots the standard free energy of oxide formation, \(\Delta G^\circ\), against temperature for many metals. The more negative a metal's line, the more stable its oxide — and a metal whose line lies below another's can reduce that other oxide.
Most metal-oxide lines slope upward (gas is consumed, \(\Delta S^\circ<0\)). But forming \(\ce{CO}\) from carbon produces gas (\(\Delta S^\circ>0\)), so its line slopes downward and eventually drops below the metal lines — which is exactly why carbon becomes an excellent, cheap reductant at high temperature.
Refining the Metal
The crude metal still carries impurities. The refining method is matched to the metal's properties — its boiling point, melting point, or the ease of forming a volatile compound.
| Method | Principle | Used for |
|---|---|---|
| Distillation | low boiling point | Zn, Hg |
| Liquation | low melting point | Sn |
| Electrolytic refining | impure anode → pure cathode | Cu, Ag, Au, Al, Zn |
| Zone refining | fractional crystallisation | Si, Ge (semiconductors) |
| Vapour phase | volatile compound formed & decomposed | Ni (Mond), Ti/Zr (van Arkel) |
Extraction of Iron
Iron is won from haematite (\(\ce{Fe2O3}\)) in a towering blast furnace, charged with ore, coke and limestone. Hot air blasted in burns the coke; the carbon monoxide it makes is the true reducing agent.
Limestone removes the silica gangue: \(\ce{CaCO3 -> CaO + CO2}\), then \(\ce{CaO + SiO2 -> CaSiO3}\) (slag). Molten iron and lighter slag are tapped off separately.
Extraction of Aluminium
Aluminium's oxide is too stable for carbon to reduce, so it is extracted by electrolysis. First the bauxite ore is purified to pure alumina (\(\ce{Al2O3}\)) by the Bayer process; then alumina is reduced in the Hall–Héroult cell.
Pure \(\ce{Al2O3}\) is dissolved in molten cryolite (\(\ce{Na3AlF6}\)) with fluorspar, which lowers the melting point from over \(2000\,^\circ\text{C}\) to about \(950\,^\circ\text{C}\) and makes the melt conduct. Graphite electrodes carry the current; the carbon anodes are slowly burnt away and replaced.
Putting It to Work
Problem. Which method best concentrates the sulphide ore zinc blende, and why?
Solution. Sulphides are wetted by oil, not water — flotation suits them:
Problem. Classify each: \(\ce{ZnCO3 -> ZnO + CO2}\) and \(\ce{2ZnS + 3O2 -> 2ZnO + 2SO2}\).
Solution. Carbonate in limited air = calcination; sulphide in excess air = roasting:
Problem. Write the aluminothermic reduction of \(\ce{Cr2O3}\) and state why aluminium is used.
Solution. Al's oxide line lies below \(\ce{Cr2O3}\), so Al reduces it:
Problem. Can carbon reduce \(\ce{ZnO}\) at high temperature? Justify thermodynamically.
Solution. Above the crossover the \(\ce{C->CO}\) line is below the \(\ce{Zn->ZnO}\) line:
Problem. What actually reduces \(\ce{Fe2O3}\) in the blast furnace, and write the reaction.
Solution. Carbon monoxide, made from coke, is the reducing agent:
Problem. Which refining method gives ultrapure germanium for semiconductors, and on what principle?
Solution. Impurities concentrate in the melt and are swept to one end:
Chapter Summary
Mineral vs ore; gangue, flux, slag; the three steps — concentrate, reduce, refine.
Hydraulic washing, magnetic separation, froth flotation (sulphides), leaching.
Calcination (limited air, carbonates) vs roasting (excess air, sulphides).
Carbon, thermite, self-reduction, hydrometallurgy, electrolysis (reactive metals).
Lower line reduces higher; the \(\ce{C->CO}\) line slopes down, undercutting metals at high T.
\(\ce{Fe}\) by \(\ce{CO}\) in the blast furnace; \(\ce{Al}\) by Hall–Héroult electrolysis in cryolite.
Problems
For each item, first decide which step it tests — concentration, reduction, the Ellingham diagram, or refining — then apply the relevant rule. Difficulty rises down the list.
- Distinguish a mineral from an ore, and define gangue, flux and slag.
- Describe froth flotation and explain the role of pine oil and a depressant.
- Differentiate calcination and roasting with one example reaction each.
- Which reduction route suits sodium, and why can carbon not be used?
- Write the thermite reaction for manganese from \(\ce{Mn3O4}\).
- State the principle of the Ellingham diagram and explain why the \(\ce{C->CO}\) line slopes downward.
- Using the diagram, explain why carbon reduces \(\ce{ZnO}\) only above a certain temperature.
- List two refining methods and the property each exploits.
- Explain electrolytic refining and what the anode mud contains.
- Write the three reduction reactions occurring in the blast furnace.
- Why is alumina dissolved in cryolite in the Hall–Héroult process?
- Write the cathode and anode reactions in the electrolysis of alumina.