Question 6 of 7: Aluminum Production (20 marks)

Question 6 — Aluminum 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 Bayer process. The Bayer process extracts pure alumina from bauxite ore by exploiting the amphoteric solubility of aluminium hydroxide in hot caustic soda. Crushed bauxite is digested in concentrated NaOH solution under pressure at 140–250 °C (the temperature set by whether the bauxite is gibbsitic or the harder-to-dissolve boehmitic/diasporic form), dissolving the aluminium-bearing mineral as soluble sodium aluminate while the iron and titanium oxides remain insoluble: $$\mathrm{Al_2O_3\!\cdot\!3H_2O(s) + 2\,NaOH(aq) \longrightarrow 2\,NaAlO_2(aq) + 4\,H_2O(l)}$$ The insoluble residue is settled and filtered off as red mud (part b). The clarified, aluminate-rich liquor is then cooled and seeded with fine gibbsite crystals, which drives the reaction backward and precipitates aluminium hydroxide: $$\mathrm{2\,NaAlO_2 + 4\,H_2O \longrightarrow Al_2O_3\!\cdot\!3H_2O(s) + 2\,NaOH(aq)}$$ regenerating the caustic liquor for recycle to digestion. Finally the precipitated hydrate is calcined at 950–1100 °C to drive off its water of crystallisation and yield anhydrous, smelting-grade alumina: $$\mathrm{2\,Al(OH)_3 \longrightarrow Al_2O_3 + 3\,H_2O}$$ Because the same NaOH liquor is used, precipitated, and regenerated in a closed loop, the Bayer process consumes no reagent beyond makeup soda for what is lost with the mud, which is what makes it economical at multi-million-tonne scale.

(b) Red mud. Red mud is the insoluble residue rejected during Bayer digestion — principally iron oxide (the source of its colour), titanium oxide, silica (largely as an insoluble sodium–aluminium–silicate "desilication product" that also carries away some alumina and soda), and undissolved gangue from the original bauxite. It leaves the process as a highly alkaline (pH ≈ 13) slurry, and its disposal — today mostly by dry stacking with liquor recovery, rather than the older wet lagoon practice — is one of the largest environmental and land-use liabilities of primary aluminium production, at roughly one to two tonnes of red mud per tonne of alumina produced depending on ore grade.

(c) The Hall–Héroult process. The Hall–Héroult process produces metallic aluminium by electrolytic reduction of alumina dissolved in a molten cryolite (Na₃AlF₆) bath, operated at 950–980 °C in a carbon-lined steel cell that serves as the cathode, with one or more consumable carbon anodes suspended in the bath. Cryolite is essential because it dissolves alumina and provides an ionically conducting molten medium at a temperature roughly 1000 °C below pure alumina's own melting point of about 2050 °C. Passing a large DC current through the cell reduces dissolved Al³⁺ at the cathode to liquid aluminium, which is denser than the bath and collects as a pool at the cell bottom, from which it is periodically siphoned (tapped). At the anode, the liberated oxygen reacts with the carbon anode itself rather than evolving as free O₂, giving the overall cell reaction $$\mathrm{2\,Al_2O_3 + 3\,C \longrightarrow 4\,Al + 3\,CO_2}$$ Consuming the anode carbon this way lowers the theoretical cell voltage substantially compared with evolving oxygen gas directly, and the anodes must therefore be replaced continuously as they are consumed. Typical modern cells run at about 4–4.5 V and consume roughly 13–15 MWh of electricity per tonne of aluminium produced, which is why smelters are sited near large, low-cost power sources such as Canadian hydroelectric capacity (e.g. Kitimat, Québec).

(d) Anode effect. The anode effect is a sudden, sharp rise in cell voltage — from the normal 4–4.5 V to as high as 20–40 V or more — that occurs when the dissolved alumina concentration in the bath immediately beneath an anode falls too low. Once alumina is locally depleted, the carbon anode begins reacting directly with fluoride ions from the cryolite bath instead of with oxide ions, forming a thin, poorly wetting gas film (predominantly the perfluorocarbons CF₄ and C₂F₆, with some CO₂) that coats the anode face and drastically increases the electrical resistance between anode and bath. Historically the phenomenon was a major source of the aluminium industry's perfluorocarbon greenhouse-gas emissions, since CF₄ has an atmospheric lifetime of many thousands of years; it is controlled today through tight, automated point-feeder control of alumina addition that avoids the low-alumina condition that triggers it in the first place.