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.

(a) Pidgeon (silicothermic) process. Calcined dolomite (a 1:1 mix of MgO and CaO, from roasting $\text{CaMg}(\text{CO}_3)_2$) is ground, briquetted with powdered ferrosilicon (75 % Si) reductant, and loaded into horizontal steel retorts. Each retort is externally heated to about 1150–1200 °C while a vacuum pump holds the internal pressure at roughly 0.1 mmHg, and the overall reduction is

$$2(\text{MgO}\cdot\text{CaO}) + \text{Si} \rightarrow 2\text{Mg}(g) + \text{Ca}_2\text{SiO}_4$$

The reaction is strongly endothermic and has a positive standard free energy at atmospheric pressure at these temperatures; it is only driven forward because the vacuum keeps the magnesium partial pressure far below its equilibrium value, continuously pulling the equilibrium toward the vapour side (Le Chatelier). Magnesium leaves the hot zone as vapour and condenses as crystalline "crown" in a water-cooled steel condenser fitted to the cool end of the retort, from which it is later removed, melted under a protective flux and cast into ingot. Because each retort is charged, reacted for several hours and discharged as a discrete batch, the Pidgeon process is labour- and energy-intensive per tonne, but its low capital cost has made it the dominant world magnesium production route (overwhelmingly sited in China).

(b) Magnatherm process. The Magnatherm process is a semi-continuous, electrically heated variant of silicothermic reduction that replaces the Pidgeon retort with a large, refractory-lined, submerged-electrode furnace operating under partial vacuum. Calcined dolomite, ferrosilicon and a small addition of alumina (or bauxite) are charged continuously; the alumina lowers the melting point of the calcium-silicate by-product enough that it forms a fluid slag (a calcium aluminosilicate) rather than the solid dicalcium silicate of the Pidgeon process, which can then be tapped continuously from the furnace rather than dug out as a solid batch residue. Resistance heating through the slag itself supplies the reaction heat far more efficiently than external retort heating, and magnesium vapour is drawn off continuously to a condenser. The result is substantially higher throughput per unit and lower labour intensity than the batch Pidgeon retort, at the cost of higher capital investment and electrical energy consumption; it was used industrially (e.g. by Alcan/Norsk Hydro derivatives) as an intermediate-scale alternative between Pidgeon retorts and full electrolytic reduction.

(c) Electrolytic process. The electrolytic route (the Dow-type or IG Farben-type process) starts from anhydrous magnesium chloride, produced either by dehydrating $\text{MgCl}_2\cdot6\text{H}_2\text{O}$ recovered from seawater or brine, or by chlorinating magnesium oxide from calcined dolomite or magnesite. The $\text{MgCl}_2$ is dissolved in a molten chloride electrolyte (typically $\text{MgCl}_2$–$\text{NaCl}$–$\text{CaCl}_2$) held around 680–750 °C in a steel cell fitted with a diaphragm that keeps the anode and cathode compartments separate. Electrolysis decomposes the salt:

$$\text{MgCl}_2(l) \rightarrow \text{Mg}(l) + \text{Cl}_2(g)$$

Molten magnesium, being less dense than the melt, rises to the cathode compartment surface where it is periodically skimmed off, while chlorine gas evolves at the graphite anode and is captured and recycled back to the chlorination step that produces the feed $\text{MgCl}_2$ — making the process self-sufficient in chlorine. Compared with the Pidgeon and Magnatherm thermal routes, electrolysis needs a much larger capital investment (cell halls, rectifiers, chlorine handling) but is more energy-efficient at large scale and produces a higher and more consistent purity metal, which is why it dominates production outside China (e.g. historically Dow Chemical in the US, and current North American/European producers).