View Full Version : Aluminum grenade

01-22-2003, 02:12 AM
Found an article in a back issue of MODELTEC magazine (Feb '96)which describes the dangers of mixing aluminum dust, moisture and a sealed container. According to the article aluminum dust will suck up the oxygen in water vapor or moisture to form oxides which leaves a bunch of hydrogen behind. Particles of 0.017" and smaller is considered dust. Any ideas on how to collect aluminum swarf for casting parts at a later date without the risk of fire or explosion? Any bad experiences with aluminum dust at all?


01-22-2003, 11:54 AM
I've been just collecting alum. swarf from milling and band sawing and saving it in an open coffee can. Planning to dump it in the next time I fire the furnace up and have a melt going. I just wanted to try it and see what happens. Don't have a lot of high hopes for getting any useful aluminum from it, since I've found aluminum cans to be largely a waste of effort.
I wasn't aware of any combustion hazard. (Are you meaning a 'spontaneous' combustion, eg. hazard potential with oily rags?) Of course aluminum (oxide, I think) is the main ingredient in thermite, which yields an extremely intense flash fire which is used for some welding applications. But as I understand it, that requires the presence of iron oxide along with the aluminum.

01-22-2003, 12:18 PM
In one of the shops I worked in about 10 years ago, we had 2 beltsanders with seperate dust collectors. One was marked for aluminum only and the other for titanium only. Some new guy didnt pay attension to the sighns and sanded titanium on the aluminum sander, and the dust collecter blew up, along with a 8x12ft piece of siding from the building. No one got hurt but it sure rocked the surrounding hangers. In the accident report it was concluded that aluminum dust in fine particals and sparks didnt mix.

01-22-2003, 01:38 PM
Anything that is "oxidized" is already burnt, so aluminum oxide is safe, actually it is what some grinding wheels are made of.

Iron oxide is also safe, and is also an abrasive.

Safe is relative, though.

Thermite as I understand it is raw aluminum dust and iron oxide. The reaction reduces the iron by thermally breaking up the iron oxide, and letting the aluminum grab the oxygen, as aluminum is more reactive.

That is how most any ore reduction is done, you produce a reducing zone in a process reactor, typically by providing a supply of a reactive material to grab the oxygen, and input energy (heat) to pull off the oxygen from the oxide.

The burning (oxidation) of the aluminum releases more heat than is required to get the same amount of oxygen loose from the iron, so it is exothermic and you get mighty hot molten iron. You have to have heat to start it, though, hence the magnesium starter typically used.

Thus aluminum dust is dangerous and may ignite. Aluminum will burn in the block or fabricated form also, but usually won't unless as part of a larger fire. It just takes too much energy to heat the block, and the heat is carried away faster than it is created given the limited reaction area (surface area of material). Dust has an awful lot of surface area per pound, compared to a lump, so it CAN burn quite well.

As far as pulling oxygen out of water,, dunno. Maybe. Maybe not. Water does corrode some alloys badly, but I don't know if it actually supplies the oxygen.

[This message has been edited by Oso (edited 01-22-2003).]

01-22-2003, 08:00 PM
Way I read(past tense read) it, Thermite gets its oxy from the iron. OSO has given a good run down on the way it works.

So far as removing oxygen from air- I was warned many times at ship yard that compartments on ship that had been sealed were dangerous because the iron rusts (oxidizes) and depletes oxygen from air. I would guess same thing would apply to Aluminum, especially the dust.

The Fire codes have extentsive instructions on aluminum powder and how to handle. It is considered potentialy explosive. Most metals, if powdered are same.

I told this last time (last summer?) we discussed this topic- I once saw a man ignite aluminum propeller grinding with a match. I have not been able to do it myself, but I got my powder from aluminum other than propellers. I have one ofthe old Audels welding books that give proportion of aluminum and iron oxide. Best I remember there are two iron oxides, one makes thermit, one does not. ANd I have now told morethan I really know about this, but if any one wants more info, be specific and I will try to find the book again.


01-22-2003, 10:50 PM
I have never had any problems with aluminum dust,swarf,or otherwise.However aluminum thats been alloyed with magnesium is another story this stuff can burn quite easily.As to the oxygen most of the time there is not enough present to make a difference.Although I have seen 316 stainless cryogenic lines burn in a high pressure oxygen atmosphere.The process involved in flame cutting steel is a chemical reaction the fuel gas is used to heat the steel to the threshold temperature and then the oxygen jet is used to chemicaly "rust away"the the heated metal.And as with any simple reaction the more heat you add the faster the reaction rate.A neat experiment is to start a cut and while cutting turn off the fuel gas supply the cut will continue as long as the reaction is above the heat threshold.As far as my own experience almost any finely divided powder in the correct atmosphere can be deadly.If you think aluminum dust is going to be a problem just mix it with some thing you can separate out later like cat litter.

01-24-2003, 02:53 AM
Dust is explosive, period.

Wheat dust
hammer milled metals
Grinding dust
Paint over spray
powder paints
plastic pigments
Coal dust
dirt & cement flour

All bad mojo. I have experienced the Overspray self -ignition while in bulk. It burns like magnesium. Very difficult to put out.

It would be safer to mix borax witht he aluminum - when it melts it becomes a flux, but I do not know if it is compatable with Aluminum. I use it with gold, silver, Platinum, and raw mineral refining & blow pipe analysis.

[This message has been edited by Thrud (edited 01-24-2003).]

01-24-2003, 11:45 AM
Dust of almost any material suspended in air can create an explosive atmosphere.
The question deals with the storage of fine aluminum swarf and dust in the presence of moisture or water. The oxidation process can result in the production of free hydrogen. However the production will not be rapid enough to generate large amounts of free hydrogen. Storage in a clean container, not contaminated by other materials, with a cover that will allow the container to "breathe" should be safe. Hydrogen, being lighter than air will dissipate rapidly.
There are many sources of combustion in the shop. The worst are solvents and gases used in welding and brazing. The list goes on with oily rags and so forth. The main concern in fire proofing the shop is better directed toward these items.
That being said, large quantities of any potentially hazardous material have no place in the shop, and should be removed to an area where minimal damage will occur if ignition results. This is usually considered to be at least 50 feet from any structure.

[This message has been edited by JCHannum (edited 01-24-2003).]

01-24-2003, 01:28 PM
<<aluminum thats been alloyed with magnesium is another story this stuff can burn quite easily.>>

in early 70's a warehouse full of alumag VW engine blocks (i think were made for '69 engines only) caught fire near San Antonio. When all was over even the concrete foundation had burned through. The sky was bright that night.

02-03-2003, 09:29 PM
Thanks for the replies. Like 'lynnl' I am saving swarf for future forging so will keep it all in coffee cans and keep shop and machines clean, and shop & warm and dry. As usual, common sense seems to work OK.


Uncle Dunc
02-04-2003, 04:07 AM
>> Dust is explosive, period.

>> Dust of almost any material suspended in air can create an explosive atmosphere.

I would amend that to say dust of any material that can be oxidized exothermically _may_ be explosive.

Anything that cannot be oxidized or has already been oxidzed is not explosive. Clay dust is not explosive. Silica dust is not explosive. Iron oxide or aluminum oxide dust are not explosive. Wood ash dust is not explosive. Etc.

I don't know enough chemistry to know if there are substances that absorb hear when they are oxidized, but if there are, their dusts are not likely to be explosive.

[This message has been edited by Uncle Dunc (edited 02-04-2003).]

02-04-2003, 07:04 PM
Unc: I am on shakey ground with this one- and it would be so easy to look up (but I feel lazy)- But iron, I think takes more heat to "Burn" than it gives off. If not, seems you could strat cutting with torch and turn Actlyne off and it would continue to cut till the iron ran out. I know you can "cut " a little with the heat off, but not far.

What I am saying is that "cutting " is exothermic but not enough to raise other iron to kindling temp.

02-04-2003, 10:43 PM
How does rock dust explode then? Or is it contaminated with coal dust?

02-05-2003, 12:41 AM
Don't know - except it is dangerous. Under ideal conditions items not normally considered explosive, are.

Clorine compounds in general are oxidizers and causes enormous problems for firefighters. Most people consider them to be "safe" - the facts say otherwise...

Flour was used in WW2 to make air/fuel bombs when other materials were not obtainable or in short supply.

02-05-2003, 02:08 AM

If it CANNOT burn, it won't be explosive either.

Explosive mixtures occur when the an oxidizable material is "carbureted" or mixed with the right proportion of oxygen or other "oxidizer".

An element or compound which can attain a lower energy state by combining with another available material will give up energy as it does so. Combination can be with oxygen, or other related element, or something completely different. Oxygen is the usual one for dust, though, its in the air......

Usually, there is some "activation energy" which must be input to "kick-start" the reaction (like a match, spark, etc). Otherwise, the reaction would already have occurred.

If the energy released is more than sufficient to "activate" the reaction of a like amount of material, it will proceed. The more in excess of the necessary starting energy, the faster the reaction will likely proceed.

Dust is small, and requires little energy to heat to reaction temperature, so the effective required activation energy is minimum. (not being rigorous here). And the surface to volume ratio is large, so the reaction is fast due to the material pretty much all reacting at once.

The heat of the neighboring particles will start a dust particle burning, in a hurry. Hence, explosion.

A big lump of coal, while the material theoretically requires the same activation energy per gram either way, takes more energy input to get it "lit" because there is more mass to heat up. Not much chance of explosion of lumps.

Chris Fazio
02-05-2003, 10:19 AM
An exothermic reaction is one that releases energy, whereas an endothermic reaction absorbs energy. The burning of a piece of wood releases heat and light which is an exothermic reaction. Dissolving certain chemicals in a solvent will cause an endothermic reaction which causes the solution to become cold. In order to oxidize something oxygen must obviously be present. It can be obtained from the air or chemically produced during the reaction. Smokeless powder has a oxidizing agent in it that produces the oxygen to make your gunpowder burn inside the cartridge case. An explosion is nothing more than a very, very rapid "burning". Not all dust mixtures with air are explosive,e.g. aluminum oxide {AL2,O3}dust in air does not explode because it has reached it's final oxidation state and cannot react with any more oxygen. Iron oxide can have different oxidation states,e.g. FeO which can continue to oxidize to Fe2,O3 and Fe3,O4 therefore FeO can create explosive conditions when suspended as dust in the air. Most organic compounds {those that contain carbon} are explosive when they are in the form of dust and suspended in air.If we take a block of pure Al and put a torch to it, eventually we will cause it to burn and it will burn at a rate determined by the amount of oxygen available and the surface area of the burning material. If we take this same block and turn it into dust and suspend the dust in air the surface area has increased tremendously and the oxidation reaction proceeds very rapidly {explodes}.


Uncle Dunc
02-05-2003, 02:45 PM

You're probably right, which suggests that iron powder is not very likely to be explosive. My point was that iron oxide powder is definitely not explosive, because the iron has already done all the oxidizing it's ever going to do.

On a related (just barely) question, do I understand correctly that carbon monoxide is not a fuel because adding another oxygen atom to it to make carbon dioxide is not an exothermic reaction, or not very exothermic?

02-05-2003, 07:26 PM
Unc D: Back in the 1800's the chemist produced and distributed (widely) coal gas, also called water gas, illuminating gas and several other names by heating coal and processing it. As near as I remember, the gases varied in contents but were about 50 percent hydrogen, 30 percent methane, 10 percent carbon monoxide(CO). I think they all burned.

Only reason I remember (or was interested in) the contents was that 100 years ago, most high class homes had, believe it or not, hydrogen gas pumped to the living, dining rooms, and bed rooms. Men at work were panic stricken by the thought that a vaccumm furnace was filled with hydrogen gas (extremely high vacumm never the less)) and would explode!!!!!.

So those old ladies of the 19th century knew better how to handle an explosive gas than we highly educated people of to day do. Yep I know, there were explosions. But natural gas explodes also. I bet when DuPont discovers a gas that will replace welding gas, that we can no longer safely handle Actylene. Sort of like freon 12.

And any fool knows CO in small amounts kills and eath is irrevokable!!! But they did handle it and lived through it- I bet death due to explosions and CO poisoning was comparable to todays road kills, AIDs, Big MAc deaths http://bbs.homeshopmachinist.net//smile.gif


Nuff ranting for a day