Question 3 of 7: Pyrometallurgical Processes (20 marks)

Question 3 — Pyrometallurgical Processes (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

All five processes are high-temperature gas/solid treatments that change a mineral's chemical form ahead of smelting or leaching; what distinguishes them is the atmosphere — oxidizing, mildly reducing, or chloride-bearing — and, for the four roasts, how far the atmosphere is allowed to push the reaction.

(a) Calcination. Calcination is the thermal decomposition of a carbonate or hydrate mineral by heating in air or an inert atmosphere, driving off $\mathrm{CO_2}$ or combined water without oxidizing the metal itself. A representative example, limestone flux calcination:

Calcination differs from roasting precisely because no sulphide-to-oxide chemistry occurs and the metal's oxidation state is essentially unchanged — it is a decomposition, not a redox reaction. It is the standard first step for a carbonate ore (e.g. calcining limestone for a blast-furnace flux) and also underlies the final stage of the Bayer alumina process, $\mathrm{2\,Al(OH)_3\rightarrow Al_2O_3+3\,H_2O}$ (Question 5).

(b) Oxidizing roast. An oxidizing (or "dead") roast is run hot and with generous excess air to drive a metal sulphide's oxidation all the way to the oxide, releasing the sulphur as $\mathrm{SO_2}$ gas rather than retaining it in the solid:

The off-gas $\mathrm{SO_2}$ is normally captured and converted to sulphuric acid, so a dead roast is chosen whenever the downstream process wants the oxide calcine directly — e.g. as feed to a leaching plant or a reduction smelter — and does not need the sulphur retained as a sulphate.

(c) Magnetizing roast. A magnetizing roast is a mild reducing roast applied to a weakly magnetic iron oxide (haematite, $\mathrm{Fe_2O_3}$, or the hydroxide goethite) to convert it to strongly magnetic magnetite, $\mathrm{Fe_3O_4}$, so the ore can afterwards be upgraded on an ordinary low-intensity magnetic separator:

The atmosphere is held only mildly reducing — a controlled $\mathrm{CO/CO_2}$ ratio — just enough to stop at $\mathrm{Fe_3O_4}$ without pushing on to metallic iron or wustite. It is the standard beneficiation route for low-grade, fine-grained hematite/goethite ores that cannot otherwise be upgraded by magnetic separation as mined.

(d) Sulfating roast. A sulfating roast is a partial-oxidation roast run at a comparatively low temperature (roughly 600–700 °C) and with limited excess air, deliberately converting a metal sulphide to its water-soluble sulphate rather than all the way to the oxide:

This is in direct contrast to the oxidizing (dead) roast of part (b), which is run hotter and with more excess air and pushes the same sulphide on to the oxide with the sulphur lost as gas. A sulfating roast is chosen when the next step is aqueous leaching and a soluble sulphate salt is wanted directly out of the roaster — nickel and cobalt sulphide concentrates are commonly sulfate-roasted for exactly this reason, avoiding a separate acid-leach dissolution step.

(e) Chloridizing roast. A chloridizing roast adds a chloride source, typically common salt, to the roast charge so that the metal value is converted to a volatile or water-soluble chloride rather than an oxide or sulphate. For a base-metal sulphide:

The liberated chloride either volatilizes directly out of the roaster (useful for a metal with a genuinely volatile chloride, letting it be condensed and separated from the rest of the charge) or stays behind as a water-soluble salt that a subsequent water leach recovers selectively. Chloridizing roasting has classically been used to recover silver and gold from complex, refractory ores where neither direct cyanidation nor smelting alone is efficient.