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Mick Maxen

Roasting ore

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My limited understanding of why we roast ore is....

 

To try and reduce impurities and get rid of the moisture which is evident from the cracking and splintering as its roasting. This in turn makes it easier to reduce the ore into pieces small enough for our smelting purposes.

 

Is this correct and can you add anything else to the question of why we do it ? Also why does it change the rock/ore from non magnetic to magnetic.

 

Mick.

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That is correct, but there's a little more to it. Roasting limonite/goethite/hematite doesn't just dry it, it drives off the chemically bound water that is always present in those ores, kind of like the difference between plain borax (which is a decahydrate) and anhydrous borax. This does two things: It does make it easier to crush because of the spaces left by the water and hydroxides which in turn means more surface area in the ore for the carbon monoxide to get in and react with the oxygen to leave iron, and it also means your charcoal is not wasting energy doing that for you in the stack, a thing that results in much lower smelting efficiency.

 

It makes nonmagnetic ores magnetic because the same process changes the chemical structure of the remaining iron oxide from nonmagnetic Fe2O3 to magnetic Fe3O4. Which raises the question, do you need to roast magnetic ores?

 

The answer is usually yes, but it depends on the ore. Magnetic hematite or maghemite and specular ore still have a little chemically bound water in them, so that helps. Magnetite does not have any bound water, but it will crush up more easily and it also benefits from the microscopic holes produced by roasting. Consider it a sort of preheat cycle.

 

In other words, you certainly can smelt without roasting, but the lower grade the ore the more it will benefit and the greater efficiency and thus greater yield you'll get for a given ore. That processed magnetite you and Owen have is already roasted and crushed, so the hard part up to that point has been done. If you score some sandy, crumbly stuff from the Forest of Dean, roasting will help that quite a bit.

 

How's that?

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Well, I certainly liked the answer Alan. It did raise another question for me though: Does anyone pre-heat their ore right before adding it to the stack? Basically if you can get it a couple hundred degrees or so before adding would that help at all? Also, don't forget that the thermal expansion of the rock (ore) can/will crack it, even without a chemical reaction helping to break things down.

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How's that ? That is a very fine reply that has answered all my questions, Thanks Alan.

 

Getting rid of moisture was the wrong way of putting it but you knew what I meant.

 

We have used the sandy crumbly stuff from the Forest of Dean quite a few years ago, but I think it was just added in with the magnetite as we did not have much of it.

 

The last smelt Owen and I did was with a few members of The Wealdon Iron Research Group. We used locally sourced Siderite from The Weald area that Owen picked up from a river bed. We roasted that as the chemical analysis showed it to be about 34% Fe. We put 50kgs through the furnace of which only 17 kilos was Fe, if the analysis was correct and had a bloom of about 9 kilos. The bloom was entirely iron which proved to be quite tricky to work.

 

Mick.

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Jerrod, I don't think any small scale smelters preheat the ore, but that is sort of what hot blast does for the big boys.

 

Mick, I was hoping you wouldn't mention siderite! I have no idea what roasting does to that other than crack it up. Although one would think iron carbonate would make good steel...

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That is a very informative post on roasting ore Alan. I was not aware it was beneficial with magnetite. I do not trying to hijack this thread Mick but, you mentioned doing a smelt with WIRG. I am curious if you or Owen had any pictures of the ore that was used ? I have looked at the pictures of clay ironstone ore on the WIRG website but its kind of hard to see any details and there's no pics of a fractured piece. I would like to have a visual comparison to some clay ironstone ore I have been roasting, if possible. Thanks !

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An additional point or two to add to Alan's excellent answer'

 

Roasting can also reduce sulfur, especially if left out in the weather for a few months to make sulfuric acid.

 

Also that your goal would not be to roast it all to magnetic. Theoretically that would be a sign of overroasting, since it takes more energy to smelt from magnetite than from hematite.

 

Jerrod, if you want to preheat your ore, just make your furnace 4" taller ;)

 

I don't remember what it does to siderite either. ...

 

Lee

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Very good additions, Lee! I totally forgot about weathering and desulfurizing.

 

And to clarify on the over-roasting thing, you mean after roasting, say, a nice brown limonite/goethite, it should be reddish and just a little bit magnetic rather than black and strongly magnetic? I've only done the roasting once so far, and I figured from reading your notes that about an hour for fist-sized chunks, bringing them to a low red heat is all that is required.

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Is the weathering done before roasting, after, or both? I would think after, but that is just an educated guess.

 

"Just make your furnace 4" taller." That made me chuckle. I guess there really wouldn't be much of a difference; I was thinking exposure to the smelting atmosphere in a pre-heated state may make a difference, but that is happening lower in the furnace so it really is just pre-heating up top.

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Weathering is done before roasting to give the sulfur a chance to bathe in rainwater, which carries off many of the nasties. Think tailings pile. After roasting you want to keep the ore as dry as possible to prevent the re-uptake of water undoing the chemical de-watering roasting accomplishes.

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I was thinking you would want to roast and therefore break up the ore to better expose more surface area to the rainwater. Any re-hydrating would then be taken care of with a second roast or in those extra 4" in the furnace. As a metallurgist I hate sulfur with a fiery passion. Very possible that I am over analyzing the situation.

 

Note to those that like it in metal because it makes it easier to machine: That is because it makes the metal have bad properties! Butter is really easy to machine too! Suck it up, take smaller cuts with better bits.

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Thanks for all the great replies.

 

Brandon, I just had a look on the WIRG website and cannot find any photos. Unfortunately I never took any either but what I shall do is the next time Owen and I are smelting I shall take some photos of the siderite as found, then some of it broken open and some after roasting for a comparison.

 

 

Mick.

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Thanks Mick. This weekend I will try to post some pics of this ore in another post and it will be easier to make a comparison that way.

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There a siderite photos on my Bushfire forge Facebook page, I have lots of them on my computer .but no way to acess them at the moment.

Siderite is an iron carbonate ( grey) with a limonite crust on it.

As it It's heated it goes through at least 4 different states of oxide....pretty complicated . Most of the stuff we were smelting was red or black a lot of hematite,maghemite and magnetite and a couple I can't remember.

The siderite loses around 30% in weight when roasted most of that is Co2 and 2% h2o hense the large amounts of shattering explosions.

So the pre roasted ore at 36% iron by weight turnes into a 50% (or so)by weight ore after roasting( cant remember if those % are iron or iron oxide weights.....)

If you did not roast this stuff it could easily blow your charcoal out of the furnace as it explodes its pretty lively.

 

I think that there would certainly be a much better weathering action on this rock as roasted rather raw, I can't see weathering raw siderite more than just he surface mm or so.

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I have just managed to put some pictures up on my Bushfire forge Facebook page of raw siderite ore, there is also an older post called siderite selfie with ore in situ.

Sorry to send to face book but I am in Norway .....and inter media interactions are not so easy.

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I have just managed to put some pictures up on my Bushfire forge Facebook page of raw siderite ore, there is also an older post called siderite selfie with ore in situ.

Sorry to send to face book but I am in Norway .....and inter media interactions are not so easy.

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Weathering is done before roasting to give the sulfur a chance to bathe in rainwater, which carries off many of the nasties. Think tailings pile. After roasting you want to keep the ore as dry as possible to prevent the re-uptake of water undoing the chemical de-watering roasting accomplishes.

Biringuccio says to roast first, then weather.

I leave roasted limonite out for long periods of time, it gets wet, but it doesn't rehydrate, so no biggie there.

 

Lee

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Very good additions, Lee! I totally forgot about weathering and desulfurizing.

 

And to clarify on the over-roasting thing, you mean after roasting, say, a nice brown limonite/goethite, it should be reddish and just a little bit magnetic rather than black and strongly magnetic? I've only done the roasting once so far, and I figured from reading your notes that about an hour for fist-sized chunks, bringing them to a low red heat is all that is required.

 

In practice, I don't think it matters much, you'll always get a mix of magnetite and hematite. As long as you don't start melting it (which wrecks that cracky gas availability), it's ok. I just wanted to discourage the idea of sorting magnetically, which doesn't make sense unless you've got magnetite to start with, because all your best, most reducible ore won't be strongly magnetic.

 

But it is nice to keep them hot for a couple of hours, to be sure the middle of the chunk gets up to temperature.

 

Lee

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Thanks Owen !

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Simply put, a chemical reaction occurs at roasting temp that turns Fe2O3 into Fe3O4 which is magnetic. But like Lee said, just because it isn't magnetic doesn't mean it won't make iron.

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Thanks Zeb,

 

but why is Fe3O4 magnetic ?

 

Roasting ore so that it goes magnetic is a sure way of proving that you have iron present.

 

Mick.

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That one I don't know Mick. As far as field testing for iron present, I've had luck using a piece of hardwood and an oxy/acetelene torch. I carve a nice pocket for some crushed up ore to rest in the hardwood and then use the torch to reduce it in the wooden "crucible". You'll wind up making a small pill of cast iron, but if there iron in the rock, you'll know. I'm still working with the method, but it seems to work. Adjust the torch to be carburizing as well.

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The field testing idea is quite trick Zeb. I had not thought of that.

The why does ore go magnetic when roasted question, I was asked earlier in the week. A few of us dug up some ore in a woods in East Sussex that is steeped in iron making history. When we had found enough, I set up a fire and roasted the ore for a couple of hours. The biggest surprise for the people watching was that the grey rock had turned to a black/red colour and it had turned magnetic. As this was all being filmed for a tv show, the presenter asked "so Mick, why has it turned magnetic" PFM was my reply, which I don't think will be included in the show when it goes out.

 

Something I had not thought about before but had read, was how porous the ore goes after being roasted. As we were pushed for time we pulled a few pieces out of the fire and cooled them down in a bucket of water. A piece of ore about the size of a small orange hissed and fizzed for quite a long time as it cooled down.

 

Mick

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Doing some quick math with molecular weights will have Fe2O3 at 70% iron by mass and Fe3O4 at 72% iron. Not sure if the extra iron is the deciding factor in it being magnetic... 2% doesn't seem like much, but it could be

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