Applications

The Ellingham-diagram reasoning of Section 6.4 isn't just theory — it explains, quantitatively, why iron, copper, and zinc are each extracted the way they are.

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(a) Extraction of iron

Concentrated iron oxide ore ($Fe_2O_3$, $Fe_3O_4$) is first calcined/roasted to drive off water, decompose carbonates, and oxidise any sulphides. It's then mixed with limestone and coke and charged into a Blast furnace (Fig. 6.5) from the top.

How the furnace runs. A blast of hot air is blown in from the bottom, where burning coke pushes the temperature up to around 2200 K — this combustion supplies most of the heat the whole process needs. The hot $CO$ and heat rise up through the furnace; as they do, they meet the descending charge and reduce the iron oxides in stages, at progressively lower temperatures the higher up the furnace they go.

Lower zone, 500–800 K — $Fe_2O_3$ is reduced in steps, first to $Fe_3O_4$ and then on to $FeO$:

Limestone decomposes here too, and the resulting $CaO$ removes the silicate impurity as a molten slag that separates cleanly from the iron.

Higher zone, 900–1500 K:

The thermodynamic heart of it is reaction 6.27:

This is exactly the coupling logic of Section 6.4: it's the sum of $FeO$ reduction and carbon oxidation —

On the Ellingham diagram, the Fe→FeO line rises while the C→CO line falls, and they cross at about 1073 K. Above that crossing, $\Delta_rG^\circ(C,CO)$ sits below $\Delta_rG^\circ(FeO,Fe)$, so coke can reduce FeO and get oxidised to CO in return — which is exactly the regime the 900–1500 K zone operates in. As a concrete check: at around 1673 K, $\Delta_rG^\circ$ for $2FeO \rightarrow 2Fe + O_2$ is roughly $+341\ kJ\,mol^{-1}$ and for $2C + O_2 \rightarrow 2CO$ is roughly $-447\ kJ\,mol^{-1}$; summing the two gives about $-53\ kJ\,mol^{-1}$ for reaction 6.27 overall — negative, so the reduction is feasible. The reduction of $Fe_3O_4$ and $Fe_2O_3$ by CO at somewhat lower temperatures follows the same logic, just at the lower-temperature crossing points of their respective curves with the CO/CO₂ line.

What comes out of the furnace. The product, pig iron, still carries about 4% carbon plus smaller amounts of S, P, Si, and Mn. Melting pig iron with scrap iron and coke under a hot air blast gives cast iron — slightly lower in carbon (~3%) but extremely hard and brittle.

Purer still: wrought iron. The purest commercial form of iron, wrought (malleable) iron, is made from cast iron by oxidising away its impurities in a reverberatory furnace lined with haematite. The haematite itself oxidises the carbon:

Limestone is added as a flux, and sulphur, silicon, and phosphorus pass into the slag; the metal is then freed of slag by passing it through rollers.

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(b) Extraction of copper from cuprous oxide

On the Ellingham diagram, the $Cu_2O$ line sits almost at the top, meaning cuprous oxide is thermodynamically easy to reduce — the C/CO and C/CO₂ lines lie well below it, especially past 500–600 K, so heating with coke alone reduces it readily.

In practice most copper ores are sulphides, sometimes with iron mixed in, so they're first roasted/smelted to oxide:

and the oxide is then reduced with coke:

The industrial route runs through a reverberatory furnace with silica mixed in, where iron oxide slags off as iron silicate, leaving copper matte — a molten $Cu_2S$/$FeS$ mixture:

$FeO + SiO_2 \rightarrow FeSiO_3 \qquad(6.34)$ …