Question 4 of 7: Light Metals Production (20 marks)

Question 4 — Light Metals 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) The silicothermic (Pidgeon) process. The process reduces calcined dolomite with ferrosilicon under vacuum, and produces 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 first calcined at 1100–1200 °C in a rotary or shaft kiln to drive off both carbonates and leave a mixed oxide known as dolime:

The dolime is ground, blended with ferrosilicon containing about 75–80 % Si (with a little fluorspar as catalyst), and briquetted for density and gas permeability. Briquettes are loaded into horizontal nickel–chromium alloy retorts, roughly 300 mm diameter and 3 m long, evacuated to 10–100 Pa and externally heated to 1150–1200 °C. The reduction is:

Two features deserve emphasis. First, magnesium is produced as a vapour well above its 1090 °C boiling point at atmospheric pressure, so it travels continuously to a water-cooled condenser and deposits as solid "crown" crystals of 99.95–99.98 % purity. Second, the reaction is not thermodynamically favourable at atmospheric pressure at 1200 °C: it is driven forward only because vacuum keeps the magnesium partial pressure low, continuously removing product — Le Châtelier's principle used as a process design tool, and the single most important point an answer should make. Lime's role is chemical, not incidental: binding silica as dicalcium silicate lowers SiO₂ activity and makes the reduction much easier than with pure magnesia.

A cycle takes 8–12 hours; the retort is cooled, the crown removed, and the spent Ca₂SiO₄ briquette cake discharged. Crowns are remelted under a protective flux and cast into ingot. The route's virtues are simplicity, low capital cost, tolerance of small scale and exceptional purity; its faults are heavy manual handling, retort life limited by creep, and an energy intensity of roughly 10–11 kg coal-equivalent per kilogram of magnesium.

(b) The Hall–Héroult process. Alumina melts at 2054 °C and, being an ionic solid, cannot be reduced by carbon to metal in any practical furnace. Charles Hall and Paul Héroult independently solved this in 1886 with the same insight: dissolve the alumina in molten cryolite, Na₃AlF₆, and electrolyse the solution at a temperature the plant can sustain.

The electrolyte. The bath is cryolite carrying 2–6 % dissolved alumina, with aluminium fluoride and calcium fluoride added to depress the liquidus, operating at 950–970 °C. Cryolite dissolves alumina readily, conducts well, is less dense than aluminium so the metal settles beneath it, and is not itself decomposed at the cell voltage.

The cell. A steel casing lined with carbon forms the cathode connection; the working cathode surface is the pad of molten aluminium on the cell floor. Prebaked carbon anodes, made from petroleum coke and pitch, are suspended into the bath from above. A frozen crust of bath and alumina insulates the cell at the sides, and fresh alumina is fed through it by point feeders.

The electrochemistry. Aluminium deposits at the metal pad, $\mathrm{Al^{3+}+3\,e^-\rightarrow Al}$, and oxygen discharged at the anode immediately attacks the carbon. The overall cell reaction is:

The carbon anode is a reactant: about 0.4–0.5 kg of carbon is consumed per kilogram of aluminium, and anode replacement on a fixed cycle is a major part of potroom labour; consuming the carbon also substantially lowers the required cell voltage compared with liberating free oxygen.

Operation. Modern cells run at 150–400 kA and about 4.2 V, of which only 1.2 V is the thermodynamic decomposition potential — the remainder covers anode overvoltage and ohmic drop, and that resistive loss keeps the bath molten. Current efficiency is 92–96 %. Energy consumption is 13–15 kWh per kilogram, which is why smelters site at cheap hydroelectric power — the Canadian industry concentrates in Québec and at Kitimat, British Columbia. Metal is siphoned from the pad every one to two days at 99.7–99.9 % purity, and cells are connected in series in potlines of 100–300 units.