pyrophoric Fe-sulfides
4 messages in this thread
Help — Doug, Lanny, Jerry, Nathan, Michael-if-not-left-on-vacation — everyone
else:
Most of you know, I'm an x-ray diffractionist. It seems I'm about to be asked
to determine whether a particular scale or deposit may have caused a minor
explosion. The material is allegedly an iron sulfide. (I say, "It ain't
nothing till I calls it.") This is probably not a naturally-occurring
material, but a corrosion product or a precipitate.
I'm posting a variation of this message up in Chemistry, but thought maybe some
of you with mining experience might have come across this in your travels.
Here goes:
I have been told that under certain conditions, "iron sulfide" will
self-ignite or spontaneously ignite. What might these conditions be? Either
environmental or morphological/chemical, specific to the sulfide? It seems the
reaction has to do with the decomposition of the Fe-sulfide: I think oxidation
of the iron, freeing sulfur which is flammable. (My boss said the *sulfur*
oxidizes, but I think he mis-spoke himself, — unless he was speaking of that
last spontaneous step!)
Now, Hurlbut and Kline says that marcasite is less stable than pyrite — would
that imply that a finding of marcasite implies potential "pyrophorism"? What
about the pyrrhotites, greigite, mackinawite, or the few Fe-sulfides with no
mineral name? Detectable free sulfur? Anything else you can think of?
Yikes — this is almost certainly going to come my way tomorrow, so if anyone
has any info on this, the sooner the better (I'll be infinitely grateful!).
Thanks — E.
Good luck Elizabeth!
Pyrite, marcasite and others do readily oxidize (some mine dumps have so many
sulfur fumes one can hardly stand to breathe), but I've only heard rumors that
it is possible for them to actually "burn", sort of like "SHC".
In fact, pyrite and marcasite suffer from what is called "pyrite disease" which
is the natural oxidation and decomposition of them under surface conditions,
including a carefully prepared mineral collection. Perhaps, in a damp
underground environment, where the heat energy given off by the oxidation could
not be lost (rock really is a poor conductor of heat), then the increasing
temperature would lead to an increase in the rate of oxidation and it would
become a self generating process.
I am not much for geochemistry, but marcasite is less stable than pyrite. Also,
the sulfur does oxidize. During the oxidation process, the Fe and S are
separated and the S can oxidize directily to sulfur oxides (the smell on mine
dumps as mentioned above) or combine with whatever is handy, and of course the
iron oxidizes. Evidence: gossans and most deposits where iron sulfides have
been oxidized contain iron oxides but little sulfur, rarely, they contain small
amounts of elemenatal sulfur.
Lanny
Elizabeth:
Re: self-combusting sulfides. Grain-size certainly is a factor, w/
increased surface area available for reaction in finer materials, etc.
I recall anecdotes that Kuroko-type massive sulfides in Japan have problems
w/ "self-igniting" ores on occasion. On a field trip to Homestake's McLaughlin
mine in CA, we observed a "smoking face" on one of their benches one damp
afternoon. The material was fine-grained pyrite & stibnite and actually hot to
the touch. (The "smoke" was mostly steam from evaporating rainwater.)
Sulfide oxidation rxns tend to be strongly exothermic – in certain sulfide
smelters, the feedstock actually doubles as "fuel".
Perhaps Gilchrist's (1989; Permagon Press) "Extraction Metallurgy" can give
you some leads – my shelf of ore geology books is rather silent on pyrophoric
phenomena.
Good luck!
Cheers,
Scott M.
Elizabeth – I work in a plant that manufactures sulfuric acid and elemental
sulfur. It is the second process that brings to bear on your question.
In the Claus process of sub-stoichiometric combustion of H2S (hydrogen
sulfide), 1/3 of the H2S is oxidized to S02 which reacts with the remaining H2S
to form S + H2O. Most of the process piping is carbon steel. In the strongly
reducing conditions, with significant sulfur compounds present, a pyrophoric
iron sulfide does form. The steel becomes sulfided, scales off, and deposits
in the Claus catalyst beds.
Whenever we shut the sulfur plant down for catalyst change, we have to slowly
increase the O2 concentration in the process gas to slightly above 0.5% to
promote controlled oxidation of the FeS. If this weren't done, a significant
fire would begin once the catalyst was exposed to the atmosphere.
Given the multiple valences of both iron and sulfur, I couldn't hazard a guess
as to what the actual formula is for the pyrophoric variety.
Hope this helps!
and thanks for the Moab tips….unfortunately I didn't read them until we
returned (today). sure is a cool place for being so d*mned hot ( 111 on
Saturday )
Michael