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Not really a chemistry question since no reactions happening, but couldn't find a more appropriate stack exchange site.

Considering recycling aluminum cans for scrap metal using induction heating. I've seen people do it with very unsophisticated equipment, but what I don't get is Al2O3 is denser (3.95g/mL) than molten aluminum (2.38 g/mL).

I suppose surface tension can stop any oxide that forms at the surface of the melt from sinking, but how strong is that? Is it resistant to mild stirring (e.g. when you skim off the dross)? Are there other examples of denser materials floating?

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A chunk of $\ce{Al2O3}$ would sink in molten aluminum. However, $\ce{Al2O3}$ usually appears in smaller pieces (down to microns), with irregular surfaces, probably filled with air ($\ce{N2}$) from when it was cool. A mix of metallic aluminum and $\ce{Al2O3}$ could allow aluminum to sink thru the solid particles after melting, allowing the particles to rise above the surface of the liquid. $\ce{Al2O3}$ has a very high melting point.

The mass of solid particles would have some very weak structure, and some buoyancy by trapped air. The surface tension of aluminum is very high (~900 dyne/cm) , so it might very well account for the covering of the metal by a heavier substance and the non-wetting of the oxide.

This is reminiscent of floating a needle on water.

In the electrolytic production of aluminum from $\ce{Al2O3}$:

The electrolyte is a molten bath of cryolite ($\ce{Na3AlF6}$) and dissolved $\ce{Al2O3}$. Cryolite is a good solvent for $\ce{Al2O3}$ with low melting point, satisfactory viscosity, low vapour pressure, and density lower than that of liquid aluminum (2 vs 2.3 g/cm3), which allows natural separation of the product from the salt at the bottom of the cell.

References

  1. https://link.springer.com/article/10.1007%2Fs11663-999-0108-4
  2. https://en.wikipedia.org/wiki/Aluminium_smelting
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  • $\begingroup$ actually, I realized it doesn't matter if the aluminum oxide sinks or not, only that it separates when pouring. Watching some videos it seems the slag mixture is more viscous, while the molten Al can be easily poured out $\endgroup$ – Yale Zhang Jul 7 '19 at 11:01

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