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Got some free sulfur. Went to the site, and they put a pallet with 3 barrels on it. One of the barrels was badly corroded, but looked like I could get it home. I had plastic barrels at home that I could transfer the material to.

About 10 km from home I noticed I was leaving a trail of smoke. Got in back with a shovel, and found that I had spots of burning sulfur. I smothered as best I could, got home, and spent the next two hours unloading and putting out blobs of burning sulfur. Did a number on my truck box lining in a few spots.

The lids continued to smoke -- it was a very slow motion fire, with some form of corrosion that would burn across the surface at around an inch an hour.

The smoke smelled strong of sulfur dioxide.

The bulk of the sulfur was unchanged by all this, and I was able to get it into plastic pails and barrels with no problem.

all the barrels had been stored outside, and in the non-corroded barrel had standing water on top of the sulfur.

  1. What happened in storage? Since sulfur always seems to have an SO2 odour, I assume that either some bacteria makes a living processing S to SO2, or there is a very slow reaction at normal temperatures.

  2. What corrosion product would be flammable?

  3. Why would it start to burn when transported, instead of in the former owner's parking lot?

Sulfur combustion on barrel:

Sulfur combustion on barrel

A different lid:

Different Lid

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    $\begingroup$ Static electricity? Maybe a spark happened when you began to move the barrels around? $\endgroup$ – Curt F. Sep 26 '17 at 20:18
  • $\begingroup$ My guess is tribology. Pieces of the sulfur were banging together as you went down the road which created a spark that started the fire. $\endgroup$ – MaxW Sep 26 '17 at 21:28
  • $\begingroup$ Back in the days of sailing ships with wooden hulls, spontaneous ignition was a huge issue. $\endgroup$ – Ivan Neretin Sep 27 '17 at 7:25
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    $\begingroup$ Why would you need sulfur? I would take a lot of things i get for free, but sulfur is not among them. $\endgroup$ – Karl Sep 27 '17 at 8:31
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    $\begingroup$ Sulfur is used in horticulture to lower soil pH. There is a bacteria that makes it's living turning S into SO3, and so creates dilute sulfuric acid. Normally applied at a rate of 1-2 pounds per square meter to drop soil pH by one pH unit. Used it at heavier applications you can drop soil pH down to about 4. Now the only thing that will grow is moss. Easy way to control weeds at edges of things. $\endgroup$ – Sherwood Botsford Sep 27 '17 at 20:46
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Iron sulfide FeS is pyrophoric: flammable, and while it can start just on exposure to air, the presence of UV light will help. As is usual, one of the key components is the surface to volume ratio. A massive lump is unlikely to catch fire. A crusty scale of FeS, such as forms in tanks used to store sour oil (H2S content) is more likely to combust when air is let into the tank.

Deactivation of Pyrophoric Iron Sulfides

From the descriptions in the abstract above, I suspect that I had one of the other iron sulfides. Whatever I had did not burn fast.

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There is an iron sulfide that will burn in air. Presumably this formed in the sealed barrel then moving tore open areas thinned by corrosion and air entered the barrel and ignited the iron sulfide that then ignited the sulfur , maybe.

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