Extraction of Crude Metal from Concentrated Ore

Concentration removes the gangue, but the concentrated ore is still a compound, not a metal. Before it can be reduced to the free metal, it usually has to be converted into a chemical form that reduction can actually work on — and that form is almost always the oxide, because oxides are comparatively easy to reduce.

So isolating a metal from its concentrated ore breaks into two major stages:

1. Conversion of the ore to an oxide
2. Reduction of that oxide to the metal

(a) Conversion to oxide

Calcination. Calcination is simply strong heating, usually in limited or no air, that drives off volatile matter (water of crystallisation, $CO_2$ from carbonates, etc.), leaving the oxide behind:

Roasting. Roasting is heating the ore in a regular, controlled supply of air, at a temperature kept below the metal's melting point — the goal is to convert sulphides to oxides without melting the metal itself. For sulphide ores:

Copper sulphide ores are typically roasted in a reverberatory furnace (Fig. 6.3) — a shallow, boat-shaped hearth where the charge is heated by radiant heat from the roof rather than direct contact with the fuel. As the cutaway shows, the charge is fed in through hoppers at either end, supported over a magnesite-lined hearth, with air and oil supplying the heat and silica added into the mix. If the ore itself contains iron, silica is deliberately added before heating, so the iron oxide can be removed as a separate liquid layer — iron oxide "slags off" as iron silicate, floating clear of the copper:

The copper itself survives roasting as copper matte, a molten mixture of $Cu_2S$ and $FeS$ rather than pure metal — the fuller extraction sequence for copper is picked up again in Section 6.4.1. The $SO_2$ gas released during roasting isn't simply vented — it's captured and put to use in manufacturing $H_2SO_4$, which is a nice example of a metallurgical by-product feeding straight into another industrial process.

(b) Reduction of oxide to metal …