Text 25255, 67 rader
Skriven 2008-12-22 11:44:48 av Robert Bashe (2:2448/44)
  Kommentar till text 25232 av Michiel van der Vlist (2:280/5555)
Ärende: Dust explosions
=======================
Michiel van der Vlist wrote to Robert Bashe on Sunday December 21 2008 at
14:57:

MV> Sodium will definitely burn in dry air. I have had it happen to myself
MV> in a moment of lapse when handling the stuff.

It will corrode (the reaction is 2Na + 2 HOH -> 2 NaOH + H2), but will not
"burn", i.e. oxidize spontaneously in really _dry_ air (80% N2, 20% O2). I've
also worked with the alkali metals. Possibly your hands were moist, as they
usually are, and you touched the metal surface by accident.

MV> But it will not ignite spontaneously. Contrary to what happens when it
MV> comes in contact with water...

Actually, it doesn't ignite even then. It reacts with the water to afford
sodium hydroxide and hydrogen, which then combines with the oxygen in the air
and ignites (really explodes, with popping noises) because of the exothermic
reaction of sodium with water. Makes a nice firework if the police aren't
watching (naturally illegal). I heard that students from MIT had dumped pieces
of sodium and potassium into the Charles River from the bridge over to Bosten
from Cambridge (right next to MIT) several times during my four years there.
Never saw the fireworks, though.

RB>> it reacts exothermically with the humidity in the air (or with water
RB>> as such) and the reaction produces hydrogen, which often ignites when
RB>> it mixes with the oxygen in the air.

MV> Yep, that is why there are kept under petroleum.

"Petroleum" in this case meaning mineral oil, not the volatile stuff.

RB>> The same applies to the other alkali metals (potassium, lithium), all
RB>> of which are so soft that you'd have a problem making a finely
RB>> divided dispersion of them.

MV> Sodium can easely be cut with a knife. Don't know about the others,
MV> never handled them in metallic form. If they are indeed like sodium, I
MV> think it will be impossible to make a dust cloud of them.

I've worked with lithium (to make lithium organics) and still have a pound or
so of metallic potassium in the cellar I collected one time in a lab. All the
alkali metals are soft, and unless you had the proper equipment to do it under
a non-reactive atmosphere (nitrogen), I doubt that you'd be able to produce
much of a dust. The most I've ever seen is a kind of sodium "grit" (used to
prepare organosodium compounds) and I have no idea how it was produced.

RB>> I've heard of magnesium fires, but not magnesium explosions,

MV> Fires yes. Its inflammability has made magnesium impopular in
MV> industry. I would not want to machine it on a lathe.

As far as I know, it's only used in alloys when machining is involved. r
possibly it's machined under inert gas (I think in this case argon, since I
believe magnesium also reacts with nitrogen).

RB>> so I don't know if it's possible to mill the metal fine enough to
RB>> permit an explosion in air.

MV> Milling magnesium would be very risky in itself. Fire hazard...

I wouldn't care to try. The ribbon I had was flammable enough to burn if you
held a match to it for a few seconds.

Cheers, Bob

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