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 manipulate a metal oxide or sulphide's chemical form at high temperature, using either carbon (as fuel and reductant) or a controlled gas atmosphere, to move the metal value toward a form that is easier to reduce, separate, dissolve or volatilize downstream.

(a) Carbothermic reduction. Carbothermic reduction uses carbon (coke or coal), rather than another metal or electrolysis, as the reducing agent for a metal oxide, exploiting the fact that carbon's own oxidation product, CO₂, becomes thermodynamically more stable than most metal oxides as temperature rises (the Ellingham line for $\mathrm{C\rightarrow CO_2}$ has an unusually steep negative slope). A representative example, lead smelting from litharge:

In practice the active reductant is very often CO rather than solid C directly, regenerated in situ by the Boudouard reaction $\mathrm{C+CO_2\rightarrow2\,CO}$, so a carbothermic furnace runs on a self-sustaining CO/CO₂ cycle once it is hot — the same mechanism that reduces iron oxide in a blast furnace (Question 5).

(b) Zinc fuming. Zinc fuming recovers the zinc that would otherwise be lost in a lead or copper smelting slag. Coal or coke, together with air, is injected (through submerged tuyeres) into the molten slag bath; the injected carbon reduces the dissolved ZnO in situ:

Because zinc boils at only 907 °C, well below furnace bath temperature, it leaves the melt as vapour rather than staying dissolved. In the freeboard above the bath it is deliberately reoxidised by excess air to a fine ZnO fume, which is carried out with the offgas and collected in a baghouse:

The fume is a saleable zinc-oxide product (or feed to a hydrometallurgical zinc plant), and the discarded slag is left with a much lower, more environmentally acceptable, residual zinc content.

(c) Magnetizing roast. A magnetizing roast is a mild reducing roast applied to a weakly magnetic iron oxide (haematite, Fe₂O₃, or the hydroxide goethite) to convert it to strongly magnetic magnetite, Fe₃O₄, so the ore can afterwards be upgraded on an ordinary low-intensity magnetic separator:

The atmosphere is held only mildly reducing (a controlled CO/CO₂ ratio) — enough to stop at Fe₃O₄ without pushing on to metallic iron or wustite. It is the standard beneficiation route for low-grade, fine-grained hematite/goethite ores (e.g. weathered itabirite) that cannot 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 a "dead" (oxidizing) roast run hotter and with excess air, which drives the reaction on to the oxide and releases the sulphur as SO₂ gas instead:

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.