Extracting aluminium by electrolysis sim

What the sim shows

Aluminium oxide (alumina) is purified from bauxite. On its own it melts only at about 2050 °C, hotter than steel melts, and a pot that hot would lose more than twice the heat. Dissolved in molten cryolite, the mixture is molten at about 950 °C, which saves energy, and the melt conducts well. Once molten, the ions are free to move. Al³⁺ ions go to the carbon lining, the cathode (−): Al³⁺ + 3e⁻ → Al. The molten aluminium (it melts at 660 °C) is slightly denser than the electrolyte (2.3 against 2.1 g/cm³), so it sinks, collects at the bottom as a silver layer and is siphoned off. O²⁻ ions go to the carbon anodes (+): 2O²⁻ → O₂ + 4e⁻. At about 960 °C the oxygen reacts with the carbon, C + O₂ → CO₂, so bubbles of carbon dioxide stream from under the anodes, the anodes burn away and each block must be replaced after about 25 days. Overall: 2Al₂O₃ → 4Al + 3O₂, with 12 electrons transferred. Alumina is 54/102 = 52.9 % aluminium by mass. Time moves only when you press Wait (1 hour or 1 day). Model and limits: the section shows 2 of the cell's 20 blocks, drawn to scale, with each block's height in proportion to the carbon left (1100 kg new); the blocks start at different ages because a working pot changes them one at a time, and the beam lowers them to keep a 45 mm gap. Aluminium made = I × t ÷ 96 500 ÷ 3 × 27 (Faraday's law at 100 % efficiency, about 67 kg an hour at 200 kA); real pots reach about 95 %, so about 64 kg. The line current wanders by up to ±0.6 % from one Wait to the next, so your points scatter slightly. Carbon used, 0.42 kg per kg of aluminium, is a typical figure; the minimum, if all the oxygen made CO₂, is 0.33 kg, and real pots use more because the hot anodes also burn in air. The pool starts empty so you can see it grow; a working pot keeps a layer about 20 cm deep. In a real pot the current itself keeps the electrolyte hot: Heat stands in for start-up. Voltage (about 4.5 V), energy and the heat-loss comparison (heat lost taken as proportional to the temperature above the room) are illustrative. The ion view uses the school model of Al³⁺ and O²⁻ ions; in the real melt they are held in complex ions with fluoride. Faraday calculations are beyond IGCSE core and O-Level, so the mass made is shown as a readout only; Singapore O-Level asks only for the idea that reactive metals such as aluminium are extracted by electrolysis.

• Steel collector bar: takes the current out of the carbon cathode (−)
• Carbon anode blocks (+): height in proportion to the carbon left (1100 kg new)
• Section shows 2 of the pot's 20 anode blocks, to scale

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