Sciencemadness Discussion Board

Stupid question about commercial aluminum isolation

metalresearcher - 12-10-2025 at 02:28

Since 140 years aluminum is isolated using the Hall-Héroult process electrolyzing Al2O3 dissolved in cryolite at 950 C. It costs lots of energy (but usually reneable), but issues lots of CO2 because of the carbonaceous anodes of which no alternative is found yet.
But why don't they use halides ? E.g. AlBr3 with a melting point as low as 95 C (anhydrous). AlCl3 sublimes and AlF3 melts above 1200 C. There should be a reason that this is not used.
Is brominating Al2O3 that complicated ? Although purifying Al2O3 from bauxite is already a tedious process, but commercially viable.

DraconicAcid - 12-10-2025 at 07:28

Al2O3, I believe, is not soluble in the liquid halides. Converting bauxite to the anhydrous halides would be even more energy intensive than the current process.

Also, if you were to electrolyze anhydrous aluminum chloride, what oxidation product would you expect- chlorine gas? Difficult to work with, and even more energy-intensive than the generation of carbon dioxide.

[Edited on 12-10-2025 by DraconicAcid]

davidfetter - 12-10-2025 at 12:11

Quote: Originally posted by metalresearcher  
Since 140 years aluminum is isolated using the Hall-Héroult process electrolyzing Al2O3 dissolved in cryolite at 950 C. It costs lots of energy (but usually reneable), but issues lots of CO2 because of the carbonaceous anodes of which no alternative is found yet.
But why don't they use halides ? E.g. AlBr3 with a melting point as low as 95 C (anhydrous). AlCl3 sublimes and AlF3 melts above 1200 C. There should be a reason that this is not used.
Is brominating Al2O3 that complicated ? Although purifying Al2O3 from bauxite is already a tedious process, but commercially viable.


Turning Al2O3 into literally anything else is a gigantic hill to climb. Everything obvious has been tried, and probably more exhaustively than you can easily picture. This is because alumin(i)um metal is so incredibly useful in so many different arenas. As to CO2, if you're looking to minimize its net output in the industrial system, start with the largest ones like concrete and steel and militaries.

Radiums Lab - 12-10-2025 at 15:18

If you elecrolyse halides, you produce carbon halides which are worse than CO2.

bnull - 13-10-2025 at 02:57

The only aluminum halide stable in the presence of moisture is the fluoride. The others absorb water from air and decompose, releasing acid fumes and gaining an oxygen atom or more per molecule. Given the affinity between oxygen and aluminum, it turns out to be a more expensive way of making aluminum oxide from, well, aluminum oxide. Aluminum fluoride has nothing of that.