h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

The comment on zinc extraction is not totally accurate. It can be made by oxidation of zinc sulphides to oxides followed by reduction to metal with carbon. Approximately 10% of the world's zinc is made this way. The vast majority is made by the roast leach electrowin RLE process. The zinc sulphide is oxidized to zinc oxide (calcine) in roasters. The sulphur is recovered as a sulphuric acid by-product.

The calcine is leached in dilute sulphuric acid to make an impure zinc sulphate solution. The solution is purified by raising the pH to about 5. This precipitates the hydrolyseable impurities, in particular iron, arsenic, antimony, and germanium. The solution is further purified to remove copper, cadmium, cobalt and nickel by cementation using zinc dust.

Zinc is then recovered as a sheet by electrowinning (electrolysis), which is a good trick as zinc's redox potential is a about 0.7 V the wrong side of hydrogen. Dilute sulphuric acid is produced at the anode which is recycled to the calcine leaching step. Finally zinc is melted, alloyed and cast for shipment and sale.

The zinc industry faces many challenges; recently it has been price, but inherently the disposal of many of its by-products: iron residues, arsenic and cadmium are problematic.