Cryolite (Na₃AlF₆) is the molten solvent that lets alumina be electrolysed at about 960 °C instead of its own 2,072 °C melting point, and it is the most dangerous material in the aluminum chain: fluoride salts are acutely toxic, and the fume from a working cell contains hydrogen fluoride, which burns lung and bone and whose skin burns can run deep while the pain lags behind. Natural cryolite is rare, so a village would have to make it, and making it is where the worst hazard lies.
Part of Hall-Héroult Electrolysis
The fluoride flux of the aluminum cell: why it is needed, how it is sourced, and the mechanism of fluoride and hydrogen fluoride poisoning.
Why A Fluoride Bath At All
The carrier gives the problem: alumina melts at 2,072 °C, impractical for electrolysis, but dissolved in molten cryolite the mixture melts at only 960 °C. The Phase 5 Aluminum Production article adds that pure cryolite melts at 1,009 °C and that aluminum fluoride and calcium fluoride are used industrially to lower the melting point further; the figures between the two articles differ in framing, and neither is a measurement I can check.
Sources, In Order Of Realism
- Natural cryolite: rare, with Greenland the historical source (carrier); the Phase 5 article calls the main deposits largely exhausted.
- A synthetic mix of sodium fluoride and aluminum fluoride (carrier). The stoichiometry is 3NaF + AlF₃ → Na₃AlF₆. With recalled molar masses (NaF 41.99, AlF₃ 83.98) the mix is 3 × 41.99 = 126.0 g and 83.98 g, giving 60% NaF and 40% AlF₃ by mass. The corpus has no route to either salt except through hydrofluoric acid.
- Cryolite from hydrofluoric acid and sodium aluminate (carrier): 6HF + NaAlO₂ + 2NaOH → Na₃AlF₆ + 4H₂O. The equation balances. Hydrofluoric acid itself comes from fluorite (calcium fluoride) and sulfuric acid, as I recall (CaF₂ + H₂SO₄ → CaSO₄ + 2HF); the corpus names fluorite as a mineral (Fluoride Sources) and sulfuric acid (Sulfuric Acid), and warns that hydrofluoric acid dissolves glass (Container Selection). It gives no safe village procedure, and this article does not supply one.
Worked Example: A Bath Inventory
For the carrier’s smallest box (0.5 m × 0.3 m floor), take a bath 0.1 m deep (assumed): 0.5 × 0.3 × 0.1 = 0.015 m³ = 15 L. At about 2.1 g/cm³ for molten cryolite (recalled), 15 × 2.1 = 31.5 kg of flux, before any is lost to fume and to the cell lining. Fluoride is consumed continuously, as I recall, so a supply for top-up is needed throughout.
What Fluoride Does To The Body
Hydrogen fluoride (HF). The carrier says HF penetrates skin painlessly and dissolves bone, and even small splashes can be fatal. The mechanism, as I recall it and cannot source from the corpus: HF is a small uncharged molecule that passes through skin, then releases fluoride ions that bind calcium and magnesium in the tissue and blood. That kills the cells where it lands and depletes calcium in the body, which can upset the heart’s rhythm. In weak solutions the pain may not start for hours, by which time the damage is deep. Inhaled HF does the same to the lungs, causing fluid build-up that appears after a delay.
Fluoride salts. Sodium fluoride and cryolite dust are acutely toxic if swallowed or inhaled, for the same calcium-binding reason. No dose is given here; the corpus’s fluoride article is for water fluoridation and must not be read as safe for these quantities.
From the cell. A working bath gives off fluoride-bearing fume that includes HF when it meets moisture, as I recall. The carrier’s advice for HF stands: full-body protection, work in open air, calcium gluconate gel immediately available, and “consider whether the aluminum is worth the risk”.
Common Mistakes
- Treating fluoride salt as a harmless flux.
- Storing HF in glass.
- Feeding damp alumina or flux to the hot bath.
- Assuming a splash that does not hurt is harmless.
Safety
Hydrofluoric acid and fluoride fume
HF burns deep and painlessly at first, binds the body’s calcium, and as a gas destroys lung tissue. Never handle it without full-body protection, a face shield and rubber gear, in open air, with calcium gluconate gel at hand before work begins. Do not breathe cell fume, stand upwind of the bath, and keep a second person present. A single splash can be fatal, and there is no field treatment beyond the gel and getting medical care. Do not attempt this on the strength of this article.
What’s Next
- Learn next: Operating Parameters - running the bath
- Combine with: Why Aluminum Matters - the scrap alternative that avoids fluoride