Why Self-Reduction Works — Thermodynamic Explanation

In the Bessemer converter at high temperature, two competing equilibria exist:

1. $Cu_2S + 3/2 O_2 \rightarrow Cu_2O + SO_2$ $\Delta G$° < 0 (spontaneous)
2. $Cu_2O + 1/2 Cu_2S \rightarrow 3/2 Cu + 1/2 SO_2$ $\Delta G$° < 0 (spontaneous)

Combined: $Cu_2S + O_2 \rightarrow 2Cu + SO_2$

Why Cu is Unique Here

On the Ellingham diagram, the Cu2O line sits high (Cu2O is not thermodynamically stable), meaning:

• Cu2O is readily reduced
• Cu2S can provide sufficient reducing power to reduce Cu2O at this temperature
• The reaction is thermodynamically and kinetically favorable

No external reducing agent (C, CO, H2) is needed — this is historically significant: copper was one of the first metals extracted by humans precisely because nature provides this self-reduction route.

Comparison with Other Self-Reduction Metals

Only a few metals with relatively unstable oxides and stable sulphides can undergo self-reduction:

• Cu → primary example
• Pb (partially): PbS + PbO → Pb + SO2 (lead softening process — analogous)

Most metals (Fe, Zn, Al) cannot self-reduce and require external reducing agents.