# Aluminum powder reaction with air

I have made aluminum powder in a ball mill a few times, but my last batch began to smoke and caught fire soon after I poured it from the tumbler. Nothing unusual was afoot. It was poured into a paper plate. The ball mill aggregate was steel bearings and a few rounded rocks, same as I had used before. The only thing different was I didn't place any charcoal along with the aluminum during the milling? So why the spontaneous combustion?

• And the aluminum was from foil, same roll I had used before. – Jeffrey T Smith Feb 12 '17 at 13:34
• I suspect that when you use carbon, it is effectively diluting the freshly activated aluminum so that it cannot reach autoignition temerature. – airhuff Feb 12 '17 at 21:40
• 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. – Jeffrey T Smith Feb 13 '17 at 0:16
• 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) – Linear Christmas Feb 13 '17 at 0:23