What Happens When You Burn Steel Wool?

[Mick West]

I've repeated the oldest version of this type of thing. Burning steel wool. This time I tried to catch more "sparked" spheres than "wired" spheres. To do this I suspend the wool (0000 grade) above a magnet under some paper. Sparks fell and the sphere were caught.
View attachment 32010

Revisiting this again, I used a similar setup, but this time made a little tray with white paper, then collected the spheres with a white sticky label

I noticed more small spheres. Previous I'd been heaping the collected spheres onto a glass slide, and all those round tiny spheres must have just rolled off, doh!

I also tweeted with a chemistry professor, who said the spheres were probably mostly iron, as iron oxide would be ejected as a powder.


Source: https://twitter.com/Professor_Hal/status/1043585975350386688

 

[Mick West]

I also took some video. I found if I position a jewler's loupe in front of my iPhone, it makes a quite powerful macro lens. Then I could use the 240fps 1080p video mode to get quite good close-up video.

Source: https://www.youtube.com/watch?v=_cdMVC2XAw0

 

[Mick West]

The explain what's going on with the steel wool I think we have to figure out how much energy is released when a certain amount of iron is burnt. How many joules for burning 1 mg?

There seems to be a few confounding factors, but it seems to calculate the heat produced in combustion you look up the "heats of combustion" for each thing in the combustion equation, and then see what the difference is. We have:

4 Fe (solid) + 3 O2 (gas) ==> 2 Fe2O3

Hard to find numbers as I'm a bit chemistry challenged. Here's some random things I found for now:


http://adsabs.harvard.edu/full/1950PA.....58..458R

The heat of combustion of iron is 1,700 cal/gm., or roughly twice that of TNT.

https://archive.org/stream/ost-chem...lPhysicalChemistry#page/n245/search/WATER-EQU

Those two things seem close, 1,700 cal = 7113 joules per gram, vs 6700 joules per gram in the second. Let's use 7000 J/g

To raise the temperature of 1 gram of iron by one degree C requires 0.45 Joules. So to melt it, we need to raise it by 1600 (rounding up a bit), so 1600*0.45 =720 joules.

So, to melt a gram of iron you need to burn 720/7000, or approximately 10% of that gram.

Reality will be more complex, but I think that's the right ballpark.

 

[Mick West]

Newer text, higher number, but same ballpark
http://www.mdpi.org/ijms/papers/i8070688.pdf

c∆UFe (heat of combustion of iron wire)=9330.32±2.93 joule/g

 

[Oystein]

Initially, in a hypothetical closed system containing only 1 gram of iron and just the right amount of oxygen to burn 1 gram of iron, the heat of combustion goes into the product - iron oxide - which has the combined mass of 2 Fe + 3 O. You need to use the heat capacity of iron oxide to determine how hot that products gets (theoretical, lossless maximum). Does it get hotter than 1500 °C? Then it might melt some additional Fe. Does the product itself stay below 1500 °C? Then there is no way burning iron gets to melt some adjacent iron.

Of course you have losses.

Radiation is one, and difficult to get a grip on (highly dependent on effective surface:volume ratio, which in turn is dependent on particle size and shape.

Another important loss is to the air that passes through: Roughly speaking, for every part of O that passes the burning iron, there are 4 part N that also pass the burning iron. You heat that N to very nearly the same temperature as the products - or rather, iron oxigen and passing N share the heat of combustion.

Fe has an atomic mass of 56, O of 16. So for 2x56 parts of Fe, you need 3x16 parts O to form iron(III)oxide. In other words, per 1 g of Fe, you need 0.43 g of O to form 1.43 g of iron oxide. Along the way, 0.37 g of N are also sucked into the action. Apply the heat capacities of Fe2O3 and N2, multiply them with their respective masses and with the same temperature T as the variable to solve for, put the heat of combustion of 1 g of Fe on the other side, and you know the maximum temperature.


Further, do not forget the heat required to change the phase of iron (and possibly iron oxide) from solid to liquid.

 

[Mick West]

Of course you have losses.

So would you lean towards the blobs being mostly molten iron oxide? That would imply a near-complete combustion of the iron.

 

[Mauro]

Further, do not forget the heat required to change the phase of iron (and possibly iron oxide) from solid to liquid.

This is true, heat of fusion of iron is 13.81 kJ/mol (wikipedia), or ~ 13.81kJ/56 =~ 246.6J/g which must be added to the 720 J/g calculated by Mick to rise the temperature of the iron to ~1600 °C (melting point = 1538 °C, wikipedia).

Calculating how much heat goes into the unreacted iron, how much in the reaction products, how much is lost to convection and irradiation etc. looks quite difficult to me, but with 7000 J/g available from combustion and ~970 J/g needed to melt the iron, energy does not seem a problem.

The microspheres look very much to be (mostly) Fe just from having being in a liquid phase (or they would not be spheres) and by the shiny appearance. There do exist iron oxides which are shiny and have a liquid phase (magnetite Fe3O4, hematite alpha-Fe2O3..) but I doubt they can be produced by burning iron in air (surely not magnetite).

 

[Mick West]

Some of the spheres here are sufficiently round and shiny as to be able to resolve a rough image of my window with a sweatshirt over a tripod in front of it.

On the left is an actual reflective sphere from a similar position, showing the window, which is also visible in the large center microspheres. Here much of the reflection is the microscope itself.

Somewhat Escheresque

 

[Oystein]

So would you lean towards the blobs being mostly molten iron oxide? That would imply a near-complete combustion of the iron.

No, I have no opinion either way. Just trying to explain that the heat of combustion goes into the products (inlcuding the O that goes into the oxide), and also into the nitrogen that accompanies the O in air, and that nothing can get hotter than the products, including any adjacent iron that does not get oxidized. These losses limit the temperature that such iron could have been exposed to, and also limit the amount that could get molten IF there was enough heat to exceed the melting point + do the phase transition.

I wouldn't know how to determine from theoretical considerations how much of the iron in any iron particle or thread actually burns.

 

[Oystein]

... There do exist iron oxides which are shiny and have a liquid phase (magnetite Fe3O4, hematite alpha-Fe2O3..) but I doubt they can be produced by burning iron in air (surely not magnetite).

But my McCrone Particle Atlas from 1973 or 1974 does show iron-rich microspheres, which it describes as "magnetite", appearing as typical particles in ashes from coal plants and from a municipal waste incinerator - thus from combustion on air (albeit under conditions engineered to be pretty efficient)

 

[Mick West]

I wouldn't know how to determine from theoretical considerations how much of the iron in any iron particle or thread actually burns.

I'd think it would be quite complicated, as you have to consider the large scale geometry and small scale molecular structure, as well as the composition and temperature of the surrounding gasses over time.

 

[Mauro]

But my McCrone Particle Atlas from 1973 or 1974 does show iron-rich microspheres, which it describes as "magnetite", appearing as typical particles in ashes from coal plants and from a municipal waste incinerator - thus from combustion on air (albeit under conditions engineered to be pretty efficient)

Burning Fe in air is quite a messy reaction and I'd expect many (and even all) kinds of 'iron oxides' will be produced. It's surprisingly hard to find good data about this process but from the scant references I found it should be 'mostly Fe2O3'. Some magnetite will be produced too, but magnetite requires reducing conditions [one way to prepare magnetite is to reduce 'rust' Fe2O3 in an hydrogen atmosphere: 3Fe2O3 + H2 → 2Fe3O4 +H2O (wikipedia)], which is not surprising because the Fe in Fe2O3 (hematite, rust) is in a higher oxidation state than in Fe3O4 (magnetite). The remaining main iron oxide is FeO, which is even more reduced than magnetite.

So, let me correct myself.. the microspheres in Mick's picture look (from their shape and shiny appearance) to have a high Fe content, and they are probably a mixture of all the possible iron oxides in all possible oxidation states (but mostly Fe2O3, at the highest Fe(III) oxidation state) all the way down to, probably, Fe(0) (unreacted Fe). The composition will depend a lot on the exact combustion conditions: starting material (there will also be other Fe compounds from impurities etc., and impurities will probably influence the overall chemistry of the reactions), oxygen availability (not only in the bulk, but also in the micro-environment where each sphere formed), temperature etc.

So there's really no hope to

... to determine from theoretical considerations how much of the iron in any iron particle or thread actually burns.

There is also no hope to determine from theoretical considerations which the final composition of the spherules will be, except that they will contain Fe and oxygen in variable proportions. But in any case the fly ash data from coal powerplants and incinerators, and Mick's experiment, are enough to say that iron-rich microspheres are a common occurrence in combustion processes which do not involve thermite at all.


Uhh I was forgetting the disclaimer: I'm no chemist, I only have the basic chemist background of an engineer. Apologies for any idiocy I may have written, corrections are welcomed.

 

[Mick West]

I'm no chemist, I only have the basic chemist background of an engineer.

Lots more than I have! I was always much better at physics than chemistry.

Professor Hal, from Twitter, is an actual chemistry professor, and said:

The balls are molten Fe. Fe wants to be with Fe. Not air. Best way to do that is to form sphere. Like drops of fat on soup. There will be some oxide but most ejected as dust.

I'm not sure how much thought he put into it, but he stuck by it in later communication. The also reminded me of this 1987 paper by Karim and Mehta from the Journal of Fire Sciences why engages in some speculation as to what is happening. This was posted on the old Microspheres thread years ago.
https://www.metabunk.org/attachments/journal-journal-of-fire-sciences-1987-272-pdf.6119/

First some interesting observation on the autoignition temperature - thinner at lower temperatures obviously, not a huge difference, but every little helps. Not temp in K, so the range is actually 377°C to 417°C - very low when the melting point of iron is 1538°C

They discuss the products:

Steel is known to oxidize into the two oxides: Fe203 and Fe,O, when it undergoes intense combustion in oxygen rich environment. If a steel sample is entirely oxidized to the non-magnetic Fe,O,, the weight gain would be 43%, while the corresponding gain in weight would be 38.2% when the steel is oxidized entirely to form the magnetic Fe304. The presence of carbon in the steel wool was evident from microscopic examination of the steel wool sample (Figure 5). It was observed that the color of the residue was glossy black and magnetic, indicating that the oxide in the residue was mainly Fe,O,. The actual weight gain was found to be from 31% to 36%, corresponding to the sample compactness range of 85 kg/ml to 285 kg/m3. The difference in the weight observed between the actual weight gains and that expected from complete oxidation to the lower oxide Fe304 (38.2%) could be attributed to:
a. Loss of a small amount of the residue into the air stream and through the formation of sparks
b. Incomplete oxidation of the steel wool resulting in the formation of FeO, the degree of which was observed to increase with a decrease in the sample compactness
c. Possible formation of other lighter oxides due to the presence of very small amounts of C, Mn, Si, etc. in the steel wool

The loss of residue into the stream increased with a decrease in the sample compactness since the resistance to flow increases with compactness.

They do not mention the possibility of pure Fe.

The residue was examined:

A morphological examination of the residue was carried out. It can be observed from the scanning electron micrographs (Figures 6 and 7) that the residue consists of oxidized and flaked fibers having widths greater than that of the original steel wool fibers. Also, the fiber ends are fused in the form of hollow spheres and some of the fibers are fused together forming large, hollow, oxide lumps. These lumps have attached to them a large number of hollow oxide spheres. The diameters of these spheres are approximately as large as the width of the original fibers. Figure 8 shows a typical view of the hollow oxide sphere. The crystalline appearance of the sphere surface is typical.

They say they are hollow but don't provide an evidence for this. They then discuss the sphere formation, which is where it gets even more speculative:

It is suggested that the formation of the spheres can be explained by the following mechanism. When the steel wool is exposed to the hot moving stream of air, surface oxidation of the fibers begins. Due to the different thermal properties of the hot outer oxidized layer and the unoxidized core of the fibers, the oxidized layer starts to crack, allowing oxygen to penetrate. This oxygen also reacts with the small amount of carbon already present in the steel, eventually forming carbon dioxide. At the same time, oxygen reacts with the steel. Due to the pressure rise between the two oxide layers caused partly by carbon oxides generation, a hollow oxide sphere is ultimately formed. Moreover, the steel wool fibers contain normally some surface and internal defects. The pressure rise in the internal cavities caused by COZ generation, after the surface oxidation, also contributes to the formation of the hollow oxide spheres. The crystalline sphere surface shows the cracked oxide layer. It has been reported that hollow spheres are common in coal ash and ash from volcanoes [4]. The formation mechanism of these spheres, probably, has some similarity to that for the steel wool residue spheres, as both the materials contain carbon.

The line "Due to the pressure rise between the two oxide layers caused partly by carbon oxides generation, a hollow oxide sphere is ultimately formed." reminds me of "some magic occurs here". The biggest question is how such a hollow oxide sphere flows along the fiber (sometimes multiple fibers), in a way at exactly resembles simple melting.

 

[Mick West]

I wonder if there's a way of determining anything from the magnetic properties of the spheres? Like measuring the force of 1g.

Hmm, if were to place it on a microscale, on a tall non-magnetic pedestal (wooden dowel), then I could position a magnet above it at a fixed distance, and measure the decrease in weight, then compare to steel wire. I'm not sure how much less magnetic iron oxide is though.

 

[Mauro]

This was posted on the old Microspheres thread years ago.
https://www.metabunk.org/attachments/journal-journal-of-fire-sciences-1987-272-pdf.6119/

Interesting article.

So steel is know to oxidize to Fe2O3 ('hematite') and Fe3O4 ('magnetite'), and according to the article what they actually got (on the basis of colour, magnetic properties, and weight gain of the sample) was mostly Fe3O4. I will not argue with them on the Fe3O4 (nor on the FeO which they report a bit later), they are the experts, but given magnetite oxides in air to hematite, while one needs an hydrogen (or some other reducing gas) atmosphere to convert hematite to magnetite, I'd still bet on Fe2O3 as the most abundant oxide produced.

It matters little anyway: I think there are now multiple lines of evidence for the production of 'iron-rich' (actually a variable mixture of iron oxides, with or without pure Fe and/or other Fe compounds/impurities) microspheres from combustions in air which do not involve 'thermite'.


At the end of page 275 of the Karim & Metha article there is the following sentence:

It was further observed that in all these tests, the top portion of the samples was partially oxidized showing that the combustion extin-guished before the entire sample was oxidized.

So not all the steel wool burned. They did not report any melting of the unreacted steel wool to be honest, but I would be pretty surprised if there was no melting at all.

 

[Mick West]

So not all the steel wool burned. They did not report any melting of the unreacted steel wool to be honest, but I would be pretty surprised if there was no melting at all.

I think it would be practically impossible for there to be NO melting, the melting point of low carbon steel (as used in steel wool, and A36 structural steel) is 1410°C. The melting point of Fe2O3 is 1,565°C.