Is iron sulfide flammable?

This article will answer the following question: “Is iron sulfide flammable?”. We will discuss if iron sulfide (pyrite) is considered fire hazardous and other important characteristics of the compound.

Is iron sulfide flammable?

Yes. Iron sulfide is considered very flammable, even pyrophoric. The material can suffer thermal decomposition, liberating metallic iron in a very flammable form. Iron sulfide is already an important cause of fire and explosion accidents in some facilities.

What is iron sulfide?

Iron sulfides are materials composed of iron and sulfur.

There are many kinds of iron sulfides, but the most popular is probably Pyrite (FeS2), also known as “fool’s gold” due to its visual resemblance. 

Although, a few pyrite deposits can have some actual quantities of gold mixed with the mineral.

Iron sulfide can be considered a source of metallic iron, a flammable compound.

The name “pyrite” comes from the greek “pyritēs lithos”, meaning “stone or mineral which strikes fire”.

During the 16th and 17th centuries, pyrite was used in firearms to provide ignition for bullets, especially for wheel-lock pistols. The friction mechanism creates a spark capable of igniting gunpowder.

It may look obvious that iron sulfide is flammable, but that’s not so simple. Several chemical reactions take place to allow iron sulfide to generate heat, and this heat doesn’t look like fire at all.

What other iron sulfides are there?

Iron sulfide is any material that has at least one iron and one sulfur on each molecule. In this article, we will only discuss FeS2 and FeS.

Other types:

  • Iron(II) sulfide, FeS
  • Greigite, Fe3S4 (cubic)
  • Pyrrhotite, Fe1−xS (where x = 0 to 0.2) 
  • Troilite, FeS, the endmember of pyrrhotite
  • Mackinawite, Fe1+xS (where x = 0 to 0.1) 
  • Marcasite, orthorhombic FeS2
  • Pyrite, cubic FeS2 (fool’s gold)
  • Arsenopyrite, FeAsS, or Fe(As-S), Fe(III) mixed arseno-sulfide
  • Iron(III) sulfide, Fe2S3
  • Iron-sulfur clusters, includes both synthetic and biological
  • Iron–sulfur protein

Can pyrite burn?

We already said that pyrite can generate sparks, and was even used as a source of ignition for gunpowder so, of course, it has some relation with fire. But can it burn?

To explain the relationship between iron sulfide, heat and fire we must first show how fire works and how it takes place.

How does fire works?

Fire is the result of a chemical reaction. When a material gets in contact with oxygen (in the air), suffers oxidation, and generates heat, we can say that the material had burned.

But not all forms of heat are fires, and not all burnings have flames.

When wood burns in a fireplace, the fire sustains itself as long as it has oxygen, heat, and fuel available. These three are the ingredients for every fire.

Fuels are always organic stuff. Every time we burn something we are actually using the molecules of something that once lived. We are using their carbon and hydrogens to produce energy.

It’s easy to picture this when we think about fossil fuels. Crude oil originated from ancient marine forms of life, which stayed millions of years under high pressure. Natural gas is formed similarly.

Coal, although, came from ancient forests which stayed buried for a long time. When humans produce charcoal from wood we speed up this process.

A fire is a constant generation of energy in the form of heat. This energy came from the chemical bondings of the fuel molecule. Ignition sources only make the reaction starts right away.

Pyrite, although, is not an organic compound. It’s simply made of iron and sulfur. Fire simply can’t happen on it, but it can still unleash a lot of heat and serve as an ignition for other fuels to start a fire.

How do flames appear?

Flames of a usual fire, such as the one in the fire’s triangle picture above, are not only the result of the energy being unleashed.

When wood (or any other solid fuel) burns, its flames usually present themselves in yellow or red-like colors, and have a certain height. Flames are where the combustion reaction is taking place.

In the flames, there are airborne particles of the fuel. They’re heated by the combustion, acquiring an incandescent form. This is why the fire has those colors.

There are different temperatures along the flames, which explains the different colors, and they’re intimately related to the incomplete combustion of the fuel.

Complete combustion generates carbon dioxide and water, only. These compounds have no color on their own.

No flames would be seen from the burning (decomposition) of iron sulfide.

How does pyrite decompose?

Knowing how fire works now we can discuss what happens when pyrite burns. What happens with pyrite is much more oxidation than combustion, but since it generates heat, is considered a burning.

FeS2 can decompose itself in low temperatures. The byproducts generated depend if the compound is heated in the presence or absence of oxygen.

If there’s no oxygen (air), the product will not burn readily, but it will decompose until generating metallic iron, which is quite flammable.

In the absence of oxygen, the following transformations will happen after heating iron disulfide in a dried place:
pyrite→pyrrhotite→troilite→iron. 

It will also generate sulfur compounds.

Check the reference Ferro et al (1989) for further information.

How does pyrite burns?

Pyrite decomposes thermally if heated, which can be a big fire hazard. Even explosions can happen. When pyrite burns, first it generates metallic iron, and then iron burns to produce heat.

Saying that iron disulfide (FeS2) is flammable is half true because it can’t burn on its own. 

When it decomposes in the presence of air (oxygen), this reaction doesn’t even generate heat. We are talking about the following reaction:

FeS2(solid) → Fe(solid) + 2S(gas) + “cold”

This does not liberate heat, and it actually steals heat from the surroundings. So why is iron sulfide flammable?

You guessed right if you thought that what is flammable is the iron that comes as a byproduct of the pyrite decomposition.

 4Fe(solid) + 3O2(gas) → 2 Fe2O3(solid) + Heat

Of course, if you toss the mineral into a fire there’s no way to control or even know that this cold even existed because the energy liberated is far bigger.

We can say that pyrite is very flammable because it generates a lot of heat upon burning. The iron that comes as a byproduct of the decomposition of pyrite is finely divided, so it can burn easily.

To understand more, check the reference from Hu et al (2006), available below.

It’s important to say that many reactions take place and lots of chemicals can be generated if iron sulfide burns with oxygen. We will discuss that shortly.

What are the pyrite hazards?

Pyrite won’t do much harm if it’s left alone. But the problem is that it can decompose by being exposed to air, so it is always hazardous to be in contact with unless you’re in a controlled environment.

Both iron(IV) sulfide (FeS2, our beloved pyrite) and Iron(II) sulfide (FeS) are flammable and hazardous. Pyrite decomposes itself into FeS during thermal decomposition.

The combustion of pyrite creates several byproducts including iron oxides (hematite and magnetite), sulfates (ferrous sulfate  FeSO4 and ferric sulfate Fe2(SO4)3, SO2, SO3).

Ferrous sulfide can spontaneously self-ignite (it’s pyrophoric), causing lots of damage and even explosion. It is commonly produced in gas production facilities and storage tanks.

Being pyrophoric, the material can ignite more easily and serve as an ignition for other less-flammable substance, so the fire hazard is always present in any facility that, purposely or not, produces iron sulfides.

If you wish to understand more about the hazard, check the reference Liu et al (2019).

Examples of ignition sources

An ignition source is anything that generates flames, heat, or sparks. We note:

  • Open flames;
  • Cutting and welding;
  • Hot surfaces;
  • Heat;
  • Lightning;
  • Smoking;
  • Spontaneous ignition;
  • Friction;
  • Static electricity;
  • Electrical Sparks;
  • Stray Currents;
  • Ovens, furnaces, and any heating equipment;
  • Pyrotechnical materials.

You can read more about ignition sources and pyrophoric materials in the reference from Nolan (2019).

Conclusion

Iron sulfide is a very flammable substance that can readily suffer thermal decomposition and combustion. It is an important fire hazard in any kind of operation that produces it

Frequently Asked Questions (FAQS): Is iron sulfide flammable?

is iron sulfide magnetic?

No. Iron sulfide does not possess magnetic properties. 

Although, if it the compound came directly from a mineral source, it might have magnetic materials such as magnetite naturally added to it and, therefore, the material might be considered magnetic.

Also, if the iron sulfide has suffered oxidation, it might have released metallic iron, which is magnetic.

is iron sulfide soluble?

Iron sulfide is not soluble in water, but it does react with acid solutions. Fe2S3 and FeS2 are not soluble, but FeS does dissolves in alkaline muds.

is iron sulfide a mixture?

Iron sulfide (Pyrite, FeS2) is made of iron and sulfur heated and mixed together, but they form a new compound called iron sulfide, which is a single substance.

Citations

https://en.wikipedia.org/wiki/Iron_sulfide
https://en.wikipedia.org/wiki/Iron(II)_sulfide
https://en.wikipedia.org/wiki/Pyrite
https://en.wikipedia.org/wiki/Wheellock
https://en.wikipedia.org/wiki/Iron(III)_sulfide#
https://pubchem.ncbi.nlm.nih.gov/compound/14788#section=2D-Structure
https://education.nationalgeographic.org/resource/petroleum
https://www.ebi.ac.uk/chebi/searchId.do;jsessionid=36842DA31C3B9737F0EE38640B398FB4?chebiId=CHEBI:86471

http://www.ecolog.com/daily_images/1003890388-1003892086.pdf

Ferro, D., Piacente, V., & Scardala, P. (1989). Decomposition enthalpies of iron sulfides. The Journal of Chemical Thermodynamics, 21(5), 483–494. doi:10.1016/0021-9614(89)90165-1 

Hu, G., Dam-Johansen, K., Wedel, S., & Hansen, J. P. (2006). Decomposition and oxidation of pyrite. Progress in Energy and Combustion Science, 32(3), 295–314. doi:10.1016/j.pecs.2005.11.004 

Liu, Sisi, et al. “Study on Oxidation and Self-ignition of Ferrous Sulfide Based on the Thermo-gravimetry Experiments.” International Journal of Chemical Engineering and Applications 10.6 (2019).

Nolan, D. P. (2019). Control of Ignition Sources. Handbook of Fire and Explosion Protection Engineering Principles for Oil, Gas, Chemical, and Related Facilities, 251–270. doi:10.1016/b978-0-12-816002-2.00014-3 

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