Principles of Metallurgy
1. What This Chapter Covers
Gold and silver as jewellery, copper and aluminium as conducting wires, iron for utensils — metals are everywhere in daily life. But the chapter opens with the question that matters: do metals exist in nature in the same form in which we use them?
Mostly they do not. Getting from rock to metal is what this chapter is about.
Metallurgy is the process of extraction of metals from their ores.
The book puts it in historical perspective. Human history in terms of materials had a Bronze Age and an Iron Age, named for the metals people began to use — bronze being an alloy of copper and tin. Today more than 75 per cent of the known elements are metals.
The chapter is allotted 7 periods in December — the smallest allotment in the book — and runs from textbook page 237 to page 252.
2. How Metals Occur in Nature
The earth's crust is the major source of metals. Sea water also holds soluble salts such as sodium chloride and magnesium chloride.
Whether a metal is found free or combined depends on one thing — how reactive it is:
- Gold, silver and copper are among the least reactive, and are found in nature in the free state, also called the native state.
- Most other metals are more reactive and are therefore found combined.
Two definitions follow, and the difference between them is economic rather than chemical:
Minerals are the elements or compounds of metals that occur in nature in the earth's crust.
Ores are those minerals from which the metal can be extracted without economical loss.
The book's example makes the distinction concrete. Aluminium is the most common metal in the Earth's crust and occurs in most minerals — but it is not economically feasible to extract it from most of them. The usual ore is bauxite, which contains 50 to 70 per cent aluminium oxide.
That is also the answer to the Think and discuss question: all ores are minerals, but not all minerals are ores, because a mineral only becomes an ore when extraction from it pays.
Table-1: Activity 1, identify the metal in each ore
| Ore | Formula | Metal | Ore | Formula | Metal |
|---|---|---|---|---|---|
| Bauxite | Al₂O₃·2H₂O | Al | Zincite | ZnO | Zn |
| Copper Iron Pyrites | CuFeS₂ | Cu | Rock salt | NaCl | Na |
| Zinc Blende | ZnS | Zn | Cinnabar | HgS | Hg |
| Magnesite | MgCO₃ | Mg | Magnetite | Fe₃O₄ | Fe |
| Epsom salt | MgSO₄·7H₂O | Mg | Galena | PbS | Pb |
| Horn Silver | AgCl | Ag | Gypsum | CaSO₄·2H₂O | Ca |
| Pyrolusite | MnO₂ | Mn | Lime stone | CaCO₃ | Ca |
| Haematite | Fe₂O₃ | Fe | Carnallite | KCl·MgCl₂·6H₂O | Mg |
Table-2 asks you to sort these into oxides, sulphides, chlorides, carbonates and sulphates. Doing it reveals that most ores are oxides and sulphides — which is exactly why group 16 is called the chalcogen family, from chalco meaning ore and genus meaning produce.
The reactivity bands
| Band | Metals | How they are found |
|---|---|---|
| High reactivity | K, Na, Ca, Mg, Al | Never found free in nature |
| Moderate reactivity | Zn, Fe, Pb, Cu | Mainly as oxides, sulphides and carbonates |
| Low reactivity | Hg, Ag, Pt, Au | Found even in the free state |
3. The Three Stages of Extraction
Extraction of a metal from its ore has three stages:
- Concentration or dressing of the ore
- Extraction of the crude metal
- Refining or purification of the metal
4. Stage I: Concentration or Dressing
Ores mined from the earth come contaminated with large amounts of soil and sand. Those impurities have a name:
The impurity present in the ore is called gangue.
Concentration or dressing means getting rid of as much unwanted rocky material as possible. The physical method chosen depends on a difference between the physical properties of the ore and the gangue.
Table-3: the four dressing methods
| Method | Process | The property it exploits |
|---|---|---|
| Hand picking | Ore particles are picked out from the impurities by hand | A visible difference in colour or size |
| Washing | Crushed ore is kept on a sloping surface and washed with a controlled flow of water; the less dense impurities are carried away and the denser ore particles stay behind | Density |
| Froth flotation | The finely powdered ore is put in water in a flotation cell and air is blown through under pressure; the froth carries the ore particles to the surface while impurities settle at the bottom | Sulphide ores are not wetted by water, while the impurities are |
| Magnetic separation | The powdered ore passes on a moving belt between a magnetic and a non-magnetic wheel, and the magnetic material is drawn aside | One of the two is magnetic and the other is not |
5. The Activity Series
Before the second stage the book builds the tool that decides everything that follows.
The activity series is the arrangement of metals in decreasing order of their reactivity.
It is constructed by watching how each metal behaves in five standard reactions. Table-4 sets them out — and note that in the printed book this table is turned on its side across the page:
| Metals | Action of oxygen | Cold water | Steam | Dilute strong acids | Chlorine on heating |
|---|---|---|---|---|---|
| K, Na | Form Na₂O and K₂O in limited O₂, but peroxides in excess O₂ | K to Mg displace H₂ from cold water, with decreasing reactivity — K violently, Mg very slowly | K to Fe displace H₂ with steam, without decreasing reactivity — K very violently, Fe very slowly | K to Pb displace H₂ from dilute strong acids with decreasing reactivity — K explosively, Mg very vigorously, Fe steadily, Pb very slowly | All metals react with chlorine on heating to give their chlorides, with reactivity decreasing down the list |
| Ca, Mg, Al, Zn, Fe | Burn with decreasing vigour to form CaO, MgO, Al₂O₃, ZnO, Fe₂O₃ | From Al to Au, no displacement of H₂ from cold water | KCl, NaCl, CaCl₂, MgCl₂, AlCl₃, ZnCl₂, FeCl₃, PbCl₂, CuCl₂, HgCl₂, AgCl, PtCl₃ and AuCl₃ are formed | ||
| Pb, Cu, Hg | Do not burn, but form only a surface layer of oxide — PbO, CuO, HgO | From Pb to Au, no displacement of H₂ from steam | |||
| Ag, Pt, Au | Do not burn or oxidise even on the surface | From Cu to Au, no displacement of H₂ from dilute strong acids |
One detail in that table is worth pausing on. Reactivity decreases down the series for cold water and for acids — but with steam the book says K to Fe all displace hydrogen without decreasing reactivity. The measure the book gives for the chlorine row is also precise: the ordering is read from the heat evolved when one mole of chlorine reacts with the metal.
6. Stage II: Extracting the Crude Metal
The method used to reduce an ore to the metal depends mainly on the position of the metal in the activity series.
A. Metals at the top — K, Na, Ca, Mg, Al
Simple chemical reduction by heating with carbon or CO is not feasible: the temperature required is too high and too expensive.
Electrolysis of their aqueous solutions does not work either, for a specific reason — water would be discharged at the cathode in preference to the metal ions.
What is left is electrolysis of the fused compound. To extract sodium from NaCl, molten NaCl is electrolysed with a steel cathode and a graphite anode:
At the cathode: 2Na⁺ + 2e⁻ -> 2Na
At the anode: 2Cl⁻ -> Cl₂ + 2e⁻
Keeping the ore molten takes a great deal of electricity, so suitable impurities are added to the ore to lower its melting point.
B. Metals in the middle — Zn, Fe, Sn, Pb, Cu
These occur mainly as sulphides or carbonates, and must be turned into oxides before reduction.
Roasting does that for sulphides — heating strongly in excess air:
2PbS + 3O₂ -> 2PbO + 2SO₂
The oxide is then reduced, by one of four routes:
(i) Reduction with carbon. The oxide is reduced by coke in a closed furnace, giving the metal and carbon monoxide:
PbO + C --(1400 °C)--> Pb + CO
(ii) Reduction with carbon monoxide, in a blast furnace:
Fe₂O₃ + 3CO --(blast furnace)--> 2Fe + 3CO₂
(iii) Auto reduction, or self reduction, used for copper. The sulphide ore is partially roasted to give some oxide:
2Cu₂S + 3O₂ -> 2Cu₂O + 2SO₂
Then the air supply is stopped and the temperature raised, so the remaining sulphide reduces the oxide it just made:
2Cu₂O + Cu₂S -> 6Cu + SO₂
(iv) Reduction by a more reactive metal. Highly reactive metals such as sodium, calcium and aluminium displace metals of lower reactivity from their compounds. These displacement reactions are highly exothermic — so much heat is evolved that the metal produced is molten:
TiCl₄ + 2Mg --(850 °C)--> Ti + 2MgCl₂
TiCl₄ + 4Na --(850 °C)--> Ti + 4NaCl
The thermite reaction is the case you are asked about most:
Fe₂O₃ + 2Al -> Al₂O₃ + 2Fe + heat
Cr₂O₃ + 2Al -> Al₂O₃ + 2Cr + heat
The molten iron it produces is used to join the railings of railway tracks or cracked machine parts.
C. Metals at the bottom — Ag, Hg
Their reactivity is so low that their oxides can be reduced by heat alone, and sometimes by displacement from aqueous solution.
Cinnabar, HgS, an ore of mercury, is heated in air. It converts first to the oxide, and further heating reduces that to the metal:
2HgS + 3O₂ -> 2HgO + 2SO₂, then 2HgO -> 2Hg + O₂
Displacement from aqueous solution, for silver:
Ag₂S + 4CN⁻ -> 2[Ag(CN)₂]⁻ + S²⁻
2[Ag(CN)₂]⁻ + Zn -> [Zn(CN)₄]²⁻ + 2Ag
Ag₂S is dissolved in a solution such as KCN to give dicyanoargentate(I) ions, and silver is then precipitated by treating with zinc dust.
7. Stage III: Refining the Crude Metal
The metal obtained by reduction is contaminated with unchanged ore, other metals from the ore, and non-metals from the anions. Blister copper from copper iron pyrites, for instance, still contains copper sulphide, iron and sulphur.
Refining is the process of obtaining the pure metal from the impure metal.
Which method is used depends on the nature of the metal and of its impurities:
| Method | How it works | Used for |
|---|---|---|
| Distillation | The molten metal is distilled and the pure metal collected as the distillate | Low boiling metals such as zinc and mercury carrying high boiling impurities |
| Poling | The molten metal is stirred with logs of green wood; impurities leave as gases or are oxidised into a scum, and the reducing gases from the wood prevent the copper from being oxidised | Blister copper |
| Liquation | A low melting metal is made to flow down a sloping surface, leaving high melting impurities behind | Tin |
| Electrolytic refining | The impure metal is the anode, a strip of the pure metal the cathode, in a bath of a soluble salt of the same metal | Copper, zinc |
In electrolytic refining the reactions are simply:
At the anode: M -> Mⁿ⁺ + ne⁻
At the cathode: Mⁿ⁺ + ne⁻ -> M
For copper, with an electrolyte of acidified copper sulphate solution:
At the anode: Cu -> Cu²⁺ + 2e⁻
At the cathode: Cu²⁺ + 2e⁻ -> Cu
The soluble impurities go into solution, while the insoluble ones collect below the anode as anode mud. That mud is not waste — it contains antimony, selenium, tellurium, silver, gold and platinum, and as the book notes, recovering these metals may meet the cost of refining.
8. Corrosion
Rusting of iron (iron oxide), tarnishing of silver (silver sulphide) and the green coating on copper and bronze (copper carbonate) are all corrosion.
Activity 2: what rusting actually needs
Three test tubes, each with clean iron nails:
| Tube | Contents | Nails exposed to | Result |
|---|---|---|---|
| A | Some water, corked | Air and water | Rust |
| B | Boiled distilled water plus about 1 ml of oil, corked — the oil floats and stops air dissolving | Water only | No rust |
| C | Anhydrous calcium chloride, corked — it absorbs any moisture from the air | Dry air only | No rust |
The conclusion is that both air and water are needed; either one alone does nothing.
The chemistry of rusting
The book calls it complex, but treats it as essentially an electrochemical phenomenon. At one spot on the iron surface, oxidation occurs and that spot behaves as an anode:
2Fe -> 2Fe²⁺ + 4e⁻
The electrons released travel through the metal to another spot, which behaves as a cathode, where oxygen is reduced in the presence of H⁺:
O₂ + 4H⁺ + 4e⁻ -> 2H₂O
Where does that H⁺ come from? The book is careful here: it is believed to come from H₂CO₃ formed when carbon dioxide from the air dissolves in water in moist conditions, and possibly from other acidic oxides dissolving from the atmosphere.
The overall reaction is
2Fe + O₂ + 4H⁺ -> 2Fe²⁺ + 2H₂O
and the Fe²⁺ is then further oxidised by atmospheric oxygen to Fe³⁺, which appears as rust — hydrated ferric oxide, Fe₂O₃·xH₂O.
Prevention of corrosion
Prevention matters because it not only saves money but prevents accidents, such as a bridge collapse or the failure of a key component.
- Keep the surface away from the atmosphere — paint it, or coat it with a chemical such as bisphenol.
- Cover it with another metal (tin, zinc) that is inert or reacts itself to save the object, usually by electroplating.
- Provide a sacrificial electrode of another metal such as magnesium or zinc, which corrodes itself but saves the object.
The Do you know? box adds alloying as a way of improving a metal's properties. Pure iron is very soft and stretches easily when hot, so it is never used pure; a little carbon makes it hard and strong, and nickel with chromium gives stainless steel, which does not rust.
Pure gold is 24 carat and too soft for jewellery, so it is alloyed with silver or copper. In India 22 carat gold is generally used — meaning 22 parts of pure gold to 2 parts of silver or copper.
9. The Pyrochemical Processes and the Furnaces
Four processes carry the heat-driven part of metallurgy. The prefix in pyrochemical is pyre, meaning heat.
| Process | What is done | Key point |
|---|---|---|
| Smelting | The ore is mixed with flux and fuel and strongly heated, so strongly that the ore is reduced to the metal and the metal is obtained molten | Impurities react with the flux to form slag, which is removed |
| Roasting | The ore is heated in the presence of oxygen or air, below its melting point | The products stay solid; generally done in a reverberatory furnace |
| Calcination | The ore is heated in the absence of air | The ore decomposes — carbonates become oxides |
| Flux | A substance added to remove the gangue by reacting with it | Acidic gangue takes a basic flux; basic gangue takes an acidic flux |
Roasting and calcination are the pair most often confused, and the difference is simply whether air is present:
2ZnS + 3O₂ -> 2ZnO + 2SO₂ (roasting)
MgCO₃ -> MgO + CO₂ and CaCO₃ -> CaO + CO₂ (calcination)
The flux rule works both ways round:
CaO + SiO₂ -> CaSiO₃ — basic flux removing acidic gangue (silica), giving calcium silicate slag
FeO + SiO₂ -> FeSiO₃ — here FeO is the gangue and SiO₂ is the flux, giving ferrous silicate slag
The blast furnace
Smelting is carried out in a specially built furnace called the blast furnace. For haematite ore, coke is the fuel and limestone the flux. The reactions inside are:
2C + O₂ -> 2CO (fuel)
Fe₂O₃ + 3CO -> 2Fe + 3CO₂ (haematite reduced)
CaCO₃ -> CaO + CO₂ (limestone, the flux, calcined to lime)
CaO + SiO₂ -> CaSiO₃ (lime removing the silica gangue as slag)
The parts of a furnace
Every furnace has three parts:
- Hearth — where the ore is kept for heating.
- Chimney — the outlet through which flue (waste) gases leave.
- Fire box — where the fuel is kept for burning.
What differs is how those parts are arranged:
| Furnace | Arrangement |
|---|---|
| Blast furnace | Fire box and hearth combined in one big chamber holding both ore and fuel |
| Reverberatory furnace | Fire box and hearth separated, but the flames from the burning fuel touch the ore in the hearth and heat it |
| Retort furnace | No direct contact between hearth and fire box — even the flames do not touch the ore |
Key words from the chapter
Minerals, ores, froth flotation, thermite process, distillation, poling, liquation, electrolytic refining, smelting, roasting, calcination, blast furnace, reverberatory furnace.
10. Summary
Metallurgy is the extraction of metals from their ores. Metals come from the earth's crust, and whether one is found free or combined is decided by reactivity — Au, Ag and Cu occur native; K, Na, Ca, Mg and Al never do.
Minerals are naturally occurring compounds of metals; ores are the minerals from which extraction pays. So all ores are minerals, but not all minerals are ores — aluminium is the commonest metal in the crust, yet bauxite alone is worth working, at 50 to 70 per cent aluminium oxide.
Extraction has three stages. Concentration removes the gangue by a physical difference — hand picking by appearance, washing by density, froth flotation by wettability for sulphide ores, and magnetic separation when one component is magnetic.
Extraction of the crude metal follows the activity series.
Metals at the top need electrolysis of the fused compound, because carbon reduction is too hot to be economical and aqueous electrolysis discharges water at the cathode instead. Metals in the middle are first roasted or calcined to the oxide, then reduced with carbon, CO, auto reduction or a more reactive metal.
Metals at the bottom need only heat, as cinnabar does, or displacement from solution, as silver does with zinc dust.
Refining matches the method to the impurity: distillation for zinc and mercury, poling with green wood for blister copper, liquation for tin, and electrolytic refining with the impure metal as anode and pure metal as cathode — where the anode mud of Ag, Au and Pt may pay for the whole process.
Corrosion is oxidation of a metal. Activity 2 shows rusting needs both air and water. It is electrochemical: iron is oxidised at an anodic spot, electrons travel through the metal, and oxygen is reduced at a cathodic spot with H⁺ from dissolved CO₂; Fe²⁺ is then oxidised to hydrated ferric oxide. It is prevented by painting, by coating with another metal, or by a sacrificial electrode that corrodes instead.
Finally the heat processes. Smelting reduces the ore to molten metal with flux and fuel; roasting heats in air; calcination heats without air; and flux removes gangue of the opposite chemical character. The blast furnace combines fire box and hearth, the reverberatory furnace separates them but lets the flames touch the ore, and the retort furnace keeps the flames off it entirely.
