Google for "spontaneous combustion of aluminum"....you'll find a variety of information that may fit your situation.

Orenda

Thanks Orenda1168

I did that and got the "maybe"

Kind of hoping for a more definitive answer... or "maybe" is all I will get....

Insufficient information. Quantity of aluminum powder, powder size, age of the powder (aluminum self-passivates, with an oxide film slowly thickening in air), how well it was mixed with a) oxidizer & b) solid pieces acting as heat sinks, ambient temperature, etc. all may influence. If an aqueous cutting fluid was used in machining, probably the aluminum chip surfaces were pre-oxidized by combination of cutting heat & water, so less reactive.

Can only say maybe. Possibly, very sharp thin edges catalyzed the oxidation of oil, picked up some heat & then ignited.  But, note that in the thermite reaction, a magnesium ribbon is used to ignite the reaction (between aluminum powder & iron oxide).

Generally, aluminum powder is considered hazardous for particle diameters < 420 microns (0.0168").  See
Recommendations for storage and handling of aluminum powders and paste, 4th Edn., The Aluminum Association (2006). Free.
http://www.aluminum.org/Content/NavigationMenu/The_Industry/Powder_and_Paste/TR2_2006.pdf

National Fire Protection Association (NFPA) standards:
NFPA 651, Standard for the Machining and Finishing of Aluminum and the Production and Handling of Aluminum Powders [651 seems to have been withdrawn & incorporated into 484 below]
NFPA 484, Standard for Combustible Metals, Metal Powders, and Metal Dusts. $42.50 non-member price.
http://www.nfpa.org/aboutthecodes/AboutTheCodes.asp?DocNum=484

Kenvlach -

Thanks for your help...  Since this was a "trash" or "scrap" type fire that burned up - no one will ever know exactly what was in the container.  

So "MAYBE" is probably as good an answer as there is.  

What now needs to be done - is to establish guidelines to prevent any repeat.

Thansk again.

Since most machinists use a feed rate around .001" per tooth so their tools, and the job, will last longer, most of the chips in the bucket were probably well under the 420 micron threshold in at least one dimension.

You could:

** Use metal waste containers for chips (this may be a legal requirement in your area anyway).

** Encourage your machinists to use higher feed rates.  At ~.005" per tooth, you start getting frangible thick curled chips that fly away from the tool and don't wrap around it in "birds nests", at least in 20xx aluminum.  And you get your parts sooner.

Mike Halloran
Pembroke Pines, FL, USA

If you believe the work done by DMIC in '66, then no way.  Ignition temp for Al fines in air is over 1,200F.
There is a greater chance that the oil and solvents along with some rags was the ignition source.  Many light oils and solvents have ignition temps plenty low for this.

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From the Aluminium Federation (UK):
'does aluminium burn?'.
The answer is, of course, "No".  
http://www.alfed.org.uk/templates/alfed/content.asp?PageId=111

Interesting paper, especially if you've heard (the common misconception) that the British destroyer Sheffield sunk by Argentina in 1982 was due to a burning aluminum superstructure: The Ministry of Defence says it was steel.

Anyway, solid-fueled rockets commonly use fine aluminum powder or flake for fuel, but the Al particles must be small enough that the ignition heat isn't carried away from the surface.  As EdStainless says, ignition is pretty difficult.

I've even intentionally oxidized molten aluminum alloys up to 1400 ^oC. Due to the protective oxide film that forms on both solid & liquid Al, alloying with Mg or Zn is necessary for a reasonable reaction rate [but nowhere near 'burning'].

Thank you everyone for your input...

At the least we have made a few "house keeping" changes to insure that this remote possibility cannot happen.