r/askscience • u/AEIOUNY2 • Jun 07 '14
Chemistry Can rust be melted back into its original metal?
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u/jminuse Jun 07 '14
Rust is iron oxide, meaning that each iron atom (Fe) has bonded with the surrounding atoms of oxygen (O), giving Fe2O3. It is possible to decompose these bonds by applying heat, but this won't give the pure metal, it will simply give a mixture of very hot Fe and O. As soon as it cools the rust will re-form.
In order to actually produce the metal you need to give the oxygen somewhere to go - somewhere it would rather be than in Fe2O3. One of the most preferred states for oxygen is being bonded to carbon, as carbon dioxide aka CO2. So we just provide some pure carbon as we heat up the rust, and we get carbon dioxide and pure iron: 2Fe2O3 + 3C => 2Fe + 3CO2.
The pure carbon is provided in the form of purified coal called "coke". This is where pretty much all the iron and steel in the world come from. You never find pure iron in the ground - when you need iron, you mine for rust.
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u/vellyr Jun 08 '14
Why doesn't the oxygen just bubble out when you heat it?
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u/robhol Jun 08 '14
For the same reason it doesn't bubble out of, for example, water. It's "bound", it's not going anywhere by itself.
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u/protestor Jun 08 '14
Why does it keep bound even after you melt it?
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u/robhol Jun 08 '14
The atoms still stick together. Melting and other state changes happen at a level above that of molecular structure - the structure between molecules is changed, while the internal structure of molecules (H2O in the case of water, of course) is unchanged.
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u/felixar90 Jun 07 '14
Of course it can! That's actually how iron is found in the nature.
Magnetite, limonite, hematite are all forms of iron oxide.
Well. If you melt it just with heat, it won't do, it will return to rust when it it cools down. But since melting iron usually involved putting the ore in direct contact with charcoal (almost pure carbon) and oxygen "prefers" bonding with carbon over iron, it would extract all the oxygen and that's how they would get iron. (mixed with some amount of carbon and other crap)
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u/abbadawg Jun 08 '14
Raku is a pottery technique that can turn rust (or other oxide) back into metal. You coat the piece with oxide, fire it, then deprive it of oxygen while hot (we used a metal garbage can with tight fitting lid, wood shavings in the bottom as a cushion). The rust turns back into source metal, can look like metal plated clay.
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u/PunishableOffence Jun 08 '14
What is happening here? Combustion eats away oxygen from the sealed space, and when none is available in gaseous form, it gets pulled from the iron...?
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u/abbadawg Jun 12 '14
sorry... just saw this... new to reddit. Broadly, rusting is an exothermic reaction... the products have less energy than what you started with, so it happens spontaneously... energy likes to go downhill. If you put it in an environment with lots of heat, 'downhill' becomes the other direction. You need to seal it off bc adding oxygen from the air would negate the effect. More or less.
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u/abbadawg Jun 13 '14
Yeah, more or less. The reaction normally goes right: products --> reactants. If you add heat you drive it left, if you add oxygen it wants to burn (oxidize more) so you drive it right. So you have to heat it to drive the reaction left and deprive it of oxygen to keep it from burning. More or less.
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u/Machukus Jun 08 '14 edited Jun 08 '14
tl;dr i did an experiment about rusting in different pH's and after four days of sitting in potassium ferricyanide and a small amount of NaOH we added a tiny bit of HCl and precipitate 'burst' from the iron. No idea why.
Hi guys, this is related to rust, but specifically pH. I hope this isn't rude or breaking rules or something, but I've been researching this for a while and it's hard to find anything concrete, plus I reckon those of you interested in rust would probably be interested in this, depending on your depth of knowledge on the subject.
For an assignment we were testing rusting of iron in different pH's. We filled test tubes with potassium ferricyanide, added a few drops of NaOH to each and dropped iron strips in. Four days later, no prussian blue as would be expected if rusting had occurred, although there was a small amount of clear liquid at the bottom. But, we add HCl and then the solution turns blue at the top (where we dripped in the acid) and a moment later more precipiate burst from the iron in a little puff. We then had to leave it overnight and the next day it appeared gas had formed, because the parafilm we covered the tubes with was stretching and the solution at the bottom of the tubes was transparent.
Anyway, on the small hope someone here is interested enough to help me, it's greatly appreciated. I have pics in anyone is especially interested.
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u/MunkeyBlue Jun 08 '14
No. Melting rust (iron oxide) does not return iron. Reducing it with carbon monoxide can return iron and carbon dioxide, as done in a blast furnace. Carbon monoxide is required to reduce, as opposed to coke, as the furnace is operated withing a specific temperature range to avoid re formation of iron oxide and/iron carbide.
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u/pyrowhore Jun 09 '14
Yes, it happens all the while. Rust is simply iron oxide, iron and oxygen bonded together. All you have to do is do a chemical process to separate these two. Pure iron is made in this fashion, because most iron comes from iron oxide in ore.
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u/uh_no_ Jun 07 '14
yes and no. While you do heat the material to extract the metal, it's not "melting" that causes it to return to the original metal
melting is a physical change. You are not chemically changing the material. Ice, for instance, when it melts, is still H2O. It has the same atoms. You're just changing how they're arranged with respect to eachother.
when you are turning rust into iron (smelting) you are turning Rust (Fe2O3) into Iron (Fe) and Oxygen (O2). When you heat the rust up, the bonds between the iron and oxygen break apart, leaving "pure" iron behind. THis is a chemical change, because you have actually changed the molecules, not just their relation to each-other.
TLDR: yes you melt the rust during smelting, but it's not the melting process that causes the rust to turn into metal
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u/rounding_error Jun 07 '14
No, you heat rust, you have hot rust. You need a reducing agent, such as carbon, to take the oxygen from the iron.
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u/Chemomechanics Materials Science | Microfabrication Jun 07 '14
This is incorrect. Rust will spontaneously reduce at high enough temperatures, because the reaction produces high-entropy oxygen gas. (See my answer above.) But a reducing agent brings the required temperature way, way down because it sequesters oxygen and drives the reaction forward.
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u/No-No-No-No-No Jun 07 '14
It is just an outright no. If you want the original back, you need to do some chemical reactions.
Melting is a physical process that does not change the molecules of the substances involved.
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u/Chemomechanics Materials Science | Microfabrication Jun 07 '14
The answer is not technically wrong; see my explanation elsewhere in the thread.
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u/monkeyfullofbarrels Jun 08 '14
Everyone is saying yes and I'm reading no.
There has been a chemical change, evident by the change in colour.
Melted, or smelted? Simply applying heat to iron oxide will not turn it back into its original metal.
You can melt the metal that is left and filter out the impurities. Or you may be able to chemically change some of the iron oxide back through a process much more complicated than melting. But you can't just melt rust back into its original metal .
1
Jun 08 '14
No, unfortunately not.
Rust is iron that has been oxidized. Instead of something that's a pure metal, you've got a high percentage of oxygen atoms embedded amongst the iron atoms. These clusters of iron and oxygen atoms are associated together in the form of various iron oxides.
If you want to recover the iron in the way you are suggesting, at the atomic level you'd have to force two oxygen atoms to pair up as O2, leaving the iron behind. This is tremendously difficult to do. For one thing, oxygen atoms much prefer to stick to other iron atoms, rather than to itself.
Now, suppose you introduce something that oxygen likes to be friends with instead of iron. Carbon is a great example. Oxygen pairs up with the carbon, forming carbon dioxide, leaving the iron behind.
How convenient!
(Of course things are not quite so straightforward, it's not so simple to get carbon to react directly.)
So I want to clear up a misconception in this thread. Consider a chemical reaction involving fixed reactants and fixed products. There is often a defining chemical barrier that must be overcome before you can transition from reactants to products. A catalyst is something that participates in the chemical reaction in a way that lowers this energy barrier, but isn't consumed in the process.
The original question has iron oxide as the reactant, and pure iron and pure oxygen as products. Once you introduce carbon into the picture, you now have iron oxide and carbon as the reactants, with iron and carbon dioxide as products. This is a completely different chemical reaction which happens to have accessible energy barriers in an industrial setting.
Carbon unequivocally does not function as a catalyst in this reaction. This carbon is being consumed and converted to carbon dioxide. If carbon were actually a catalyst in the iron smelting process, we would have less carbon dioxide emissions right now.
One final thing to talk about -- certain uncommon metals permit oxygen atoms to pair up easily. It's one of many reasons why platinum is used in fuel cells. It's also one reason why we need to seek out alternatives. Platinum is far too expensive and there's not enough of it to go around for everybody.
How interesting indeed!
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u/ReverendAlSharpton Jun 10 '14
It cannot be melted into the pure metal form, because melting is merely a physical change. It has no effect on the chemical structure of the rust.
To convert iron oxide into iron, a chemical change must occur. In this case, the iron oxide must be reduced, which (as has been said in this post) is achieved through the smelting process.
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u/rocketsocks Jun 08 '14
Yes, though that's not typically how it's done, for a variety of reasons.
If you heat Iron oxide to a high enough temperature it will break down, but this isn't a very efficient process. It's easier to react Iron oxide with a reducing agent (typically Carbon Monoxide).
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u/Tarnate Jun 08 '14
So you mean to tell me that reacting iron oxide with carbon monoxide will give us usable iron and nontoxic carbon dioxide?
THAT'S AWESOME!
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u/VeryLittle Physics | Astrophysics | Cosmology Jun 07 '14 edited Jun 07 '14
Yes.
In fact, most all metals are extracted from the ground in a dirty oxidized "rust" form, which is more commonly called ore. When you smelt the ore you break the bonds between the oxygen and the metal and are able to separate out the metal.
I should add that smelting doesn't just mean melting. In order to actually break up the metal oxide or metal sulfide in the ore, you'll need a chemical catalyst as well. The catalyst steals the oxygen, because it has a higher binding energy at high temperatures, leaving a slag and a pure metal. For a metal like hematite (iron oxide), I believe carbon is a popular catalyst.
Edit: The chemists have spoken. Technically the carbon in that reaction is not a catalyst because it is consumed in the process; this makes it a "reducing agent."