Question 6 of 7: Zinc Production (20 marks)

Question 6 — 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.

(a) Pyrometallurgical flow sheet.

(b) Description of the pyrometallurgical route. Sphalerite (ZnS) concentrate is first roasted in air, oxidizing it to zinc oxide calcine and evolving $\text{SO}_2$ that is captured and converted to sulphuric acid on-site: $2\text{ZnS}+3\text{O}_2\rightarrow2\text{ZnO}+2\text{SO}_2$. The powdery calcine is agglomerated by sintering (partial fusion on a moving grate, burning residual sulphur and gluing fine particles into a strong, permeable sinter) so it will not choke the reduction furnace. The sinter is then charged, with coke, into a reduction unit — historically a horizontal or vertical retort, in modern practice more often an Imperial Smelting Furnace (ISF, a sealed blast furnace that co-produces lead) — where carbothermic reduction occurs above zinc's boiling point (907 °C):

$$\text{ZnO}+\text{C}\rightarrow\text{Zn}(g)+\text{CO}$$

Because zinc is a vapour at furnace temperature and reoxidizes almost instantly on contact with the CO₂-bearing furnace gas as it cools, the hot Zn/CO gas stream is quenched in seconds by spraying it through a bath of molten lead in a splash condenser; zinc dissolves briefly in the lead spray, is chilled below the point where the reverse reaction can proceed, and then separates from the lead on standing as a lighter, largely immiscible liquid layer (a Pb–Zn liquation), giving crude zinc at roughly 98.5 % purity. This crude metal is upgraded to Special High Grade (99.99 %) by fractional (New Jersey) redistillation if a higher purity is needed for galvanizing or die-casting alloys.

(c) Hydrometallurgical flow sheet.

(d) Description of the hydrometallurgical route. As in the pyro route, the sulphide concentrate is first roasted to a porous ZnO calcine (again capturing $\text{SO}_2$ as acid feedstock). The calcine is leached in dilute sulphuric acid — in practice the spent, acidified electrolyte returning from the electrowinning step, closing the acid loop — dissolving zinc as sulphate: $\text{ZnO}+\text{H}_2\text{SO}_4\rightarrow\text{ZnSO}_4+\text{H}_2\text{O}$. Iron and other insoluble gangue (often precipitated deliberately as jarosite or goethite to control iron without excessive zinc loss) and unreacted lead/silver residues are filtered off as leach residue. The resulting zinc sulphate solution still carries small amounts of copper, cadmium and cobalt, which are more electropositive than zinc and would otherwise deposit preferentially at the cathode, ruin current efficiency, and (particularly cobalt) cause dangerous redissolution of already-plated zinc; these are removed by cementation, stirring in zinc dust so the more noble metals displace onto the dust and are filtered out as a cement cake ($\text{Cu}^{2+}+\text{Zn}\rightarrow\text{Cu}+\text{Zn}^{2+}$, and similarly for Cd, Co). The purified $\text{ZnSO}_4$ solution is then electrolyzed between aluminum cathodes and lead–silver anodes: zinc deposits as high-purity metal (typically 99.99 %, "Special High Grade" directly off the cathode, no redistillation needed) while oxygen evolves at the anode and regenerates sulphuric acid, which is recycled to the leaching stage. Zinc is periodically stripped from the cathodes, melted and cast — a continuous roast–leach–electrowin (RLE) cycle that today produces most of the world's primary zinc, ahead of the pyrometallurgical route, because it yields directly marketable purity without a separate refining step.