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I've made aluminum powder in a ball mill a few times, but my last batch began to smoke and caught on fire soon after I poured it from the tumbler. Nothing unusual was afoot. It was poured into a paper plate. The ball mill aggregate had steel bearings and a few rounded rocks, same as I had used before.

The only difference was I didn't place any charcoal along with the aluminum during the milling? So why would there be spontaneous combustion?

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  • $\begingroup$ And the aluminum was from foil, same roll I had used before. $\endgroup$ – Jeffrey T Smith Feb 12 '17 at 13:34
  • $\begingroup$ I suspect that when you use carbon, it is effectively diluting the freshly activated aluminum so that it cannot reach autoignition temerature. $\endgroup$ – airhuff Feb 12 '17 at 21:40
  • $\begingroup$ I read the charcoal coats the powder to prevent the reaction, but wasn't sure exactly why the Al combusts with air contact. I imagine it is reacting with oxygen or moisture. $\endgroup$ – Jeffrey T Smith Feb 13 '17 at 0:16
  • $\begingroup$ It would depend on temperature, but I suspect that with carbon the reactions $$\ce{Al2O3 + 3C -> 2Al + 3CO,\\ Al2O3 + 3CO -> 2Al + 3CO2,\\ 4Al + 3C -> Al4C3}$$ will also have an effect ($\ce{C ->[O2] CO + CO2}$ as well). (reposted comment due to silly mistake) $\endgroup$ – Linear Christmas Feb 13 '17 at 0:23
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It rapidly oxidized. If the ball mill is sealed for long periods of time the oxygen inside the ball mill is consumed coverting aluminum to aluminum oxide. When the lid is opened the aluminum is rapidly oxidized with a exothermic reaction. If the temperature gets hot enough it can spontaneously ignite.

The reason for the carbon is it coats the aluminum particles to prevent them from oxidizing.

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