Question 5 of 7: Magnesium Production (20 marks)

Question 5 — Magnesium 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.

All three routes reduce magnesium from an oxide (dolomite-derived) or chloride feed, but they differ sharply in how the reduction is driven and in how continuously they can run — the comparison in Figure 5.1 is worth keeping in mind while reading the three descriptions.

(a) The silicothermic (Pidgeon) process. The process reduces calcined dolomite with ferrosilicon under vacuum, producing magnesium directly as a condensed vapour of very high purity. It is a Canadian invention — Lloyd Pidgeon developed it at the National Research Council in Ottawa in the 1940s, first operated at scale at Haley, Ontario. Dolomite is calcined at 1100–1200 °C to dolime:

Dolime is ground, blended with 75–80 % ferrosilicon and a little fluorspar catalyst, and briquetted. Briquettes are loaded into horizontal Ni–Cr alloy retorts, evacuated to 10–100 Pa, and externally heated to 1150–1200 °C, where the reduction occurs:

Magnesium leaves as vapour (well above its 1090 °C atmospheric boiling point) and condenses downstream as high-purity "crown" crystals (99.95–99.98 %). The reaction is not thermodynamically favourable at 1200 °C at atmospheric pressure; only the continuous removal of Mg vapour by vacuum — Le Châtelier's principle used as process design — drives it forward, which is the central point of any description of the process. Being externally heated through the retort wall, the process is inherently batch and labour-intensive, at roughly 10–11 kg coal-equivalent per kg Mg.

(b) The Magnetherm process. Magnetherm (developed by Péchiney, France) is a silicothermic process using the same overall reduction as Pidgeon — dolime plus ferrosilicon(-aluminium) — but the heat is supplied electrically rather than through a furnace wall. Charge is melted in a submerged-arc electric furnace, and the current passing through the resistive slag itself provides the heat needed to sustain the reduction at around 1550–1600 °C, all under a vacuum of a few kPa maintained above the bath:

Because the electric-arc furnace can be charged and tapped semi-continuously (fresh briquettes added and spent slag tapped while the furnace runs), Magnetherm achieves substantially higher throughput per unit than a batch Pidgeon retort and a somewhat better energy efficiency, since the heat goes directly into the reacting charge rather than through a conductive retort wall. It never displaced Pidgeon on total world tonnage, but it was historically significant as the higher-productivity silicothermic alternative.

(c) The electrolytic process. The electrolytic route (the IG Farben / Dow family of processes) is chemically unrelated to the two silicothermic routes: it reduces anhydrous magnesium chloride directly, rather than an oxide, by molten-salt electrolysis:

The electrolyte is a molten chloride bath (MgCl₂ with NaCl/CaCl₂/KCl additions to lower the melting point and improve conductivity) held around 680–750 °C in a steel cell lined with refractory. Graphite anodes evolve chlorine gas, which is collected and recycled to chlorinate fresh feed (magnesite, dolomite, or brine-derived Mg(OH)₂) back to MgCl₂, closing the chlorine loop; magnesium metal, being less dense than the melt, coalesces and rises to a collection zone at the steel cathode, from which it is periodically siphoned under an inert or reducing cover to prevent it re-oxidising or re-combining with the liberated chlorine. Because current can be passed continuously, the electrolytic route runs as a genuinely continuous process and, worldwide, supplies the majority of primary magnesium production — the thermal (Pidgeon-family) routes remain significant mainly where electricity is expensive relative to coal or ferrosilicon and where smaller, more flexible batch plants suit the local market.