Question 4 of 7: Zinc Production (20 marks)

Question 4 — Zinc Production (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.

Both routes start from the same roasted feed — a zinc sulphide concentrate is oxidised to ZnO calcine, generating an SO₂ stream that is fixed as sulphuric acid at both plants — and then diverge completely in how that ZnO is reduced to metal: the pyrometallurgical route reduces it thermally with carbon and condenses zinc vapour, while the hydrometallurgical route dissolves it in acid and reduces the dissolved Zn²⁺ electrolytically.

(a)/(b) Pyrometallurgical route — the Imperial Smelting Process (ISP).

Zinc concentrate (ZnS) is first roasted/sintered on a downdraught sinter strand, producing a hard, permeable ZnO–PbO sinter and driving off SO₂ for sulphuric-acid manufacture:

Sinter is charged with coke to the Imperial Smelting Furnace (ISF), a blast-furnace-type shaft reactor blown with pre-heated, oxygen-enriched air. Zinc oxide is carbothermically reduced:

while any PbO present is reduced and, being non-volatile, simply melts and is tapped as lead bullion from the furnace hearth — the ISF is deliberately a co-smelting furnace for both metals. The critical, and metallurgically distinctive, step follows immediately: at furnace temperature (>1000 °C) zinc leaves as vapour mixed with CO, and the reverse reaction $\mathrm{Zn(g)+CO_2\rightarrow ZnO(s)+CO(g)}$ is fast on cooling, so the hot gas is quenched in seconds by spraying it through a shower of molten lead droplets at about 550 °C in a lead-splash condenser. Zinc dissolves into the lead spray far faster than it can re-oxidise, "freezing in" the metal. The Zn-saturated lead is then cooled; zinc's very limited solubility in lead makes it separate out on liquation as a crude zinc layer, which is refined to 99.99+ % purity by fractional (three-column) distillation, exploiting the large boiling-point gap between Zn (907 °C) and Pb (1749 °C). The lead, now zinc-depleted, is recirculated to the condenser.

(c)/(d) Hydrometallurgical route — Roast–Leach–Electrowin (RLE).

The concentrate is roasted under fully oxidising conditions (a dead roast, hotter and with more excess air than the sulfating roast of Question 3) to ZnO calcine, again generating SO₂ for the acid plant — identical chemistry to the first step of the pyro route. The calcine is then leached in spent (recycled) sulphuric acid from the electrowinning circuit:

Any zinc ferrite (ZnO·Fe₂O₃) formed during roasting resists this mild leach and reports to an insoluble residue, which is either discarded or, in modern plants, treated by a stronger hot-acid or jarosite/goethite process to recover its zinc. The pregnant ZnSO₄ solution then passes through purification: zinc dust is added to cement out, by displacement, the copper, cadmium, cobalt and nickel impurities that would otherwise poison or co-deposit at the cathode — e.g. $\mathrm{Zn(s)+Cu^{2+}(aq)\rightarrow Zn^{2+}(aq)+Cu(s)}$, the same cementation chemistry verified quantitatively in Question 7. Finally, the purified electrolyte is electrowon between lead–silver anodes and aluminum cathodes:

Zinc plates out on the aluminum cathode (stripped and melted into ingots), oxygen evolves at the anode, and the regenerated sulphuric acid is recycled directly back to leaching — the loop that makes RLE self-sustaining in acid and closes the flow sheet shown above.