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Drepanon

First smelt failure post-mortem analysis

13 posts in this topic

Hi all!

I recently got interested in iron smelting and decided to give it a try with a couple of friends. We just got back from our bloomery. Although it was not a complete failure, we were unable to obtain a workable iron bloom. There are a couple of things that we do not understand and we would like to get your advice on them.

  • Furnace construction and protocol

               Ore

Our ore was iron-bearing sandstone that I found near my home:

Z2hpEMx.jpg

We roasted it until bright red. It became quite magnetic after that so I am confident it carried at least some amount of iron oxides. We broke it in small pieces and dust and used all of it. The overall quantity was quite small because of the preliminary nature of the experiment (about 10 lbs).

               Furnace

Our furnace was inspired by this example (Coated Tyle furnace by Skip Williams and Lee Sauder), i.e. made from 10" cinder blocks stacked about 35" high. We dug out underneath the cinder blocks and lined the bottom of the furnace with clay bricks. The total height from bottom to top was about 40". We used a steel pipe about 1" in cross-section to build our tuyère and planted it about 6" from the bottom at a 20 degree downward angle. We measured the air flow at about 5 liters/min, which we thought was at the low end. We added 3 thermocouple ports at various heights to monitor the temperature (but only ended up using one because of cable melting issues). The whole thing was thermally insulated with foam concrete.

               Charcoal

We decided to calibrate our charcoal so that we only used 1" pieces. Otherwise it was classical barbecue charcoal.

  • Results

               Charcoal burn rate and ore charge

We were surprised by the high charcoal burn rate as we naively expected it to be way lower. In the end we had to charge about 1 lb every 10 minutes in order to keep the level steady. We were focused on using a 1:1 ore-to-charcoal so we burned through our small ore quantity in about an hour and a half. We then continued adding charcoal so that ore could travel all the way down the furnace based on the burn rate (about 1 hour to get from the top to the bottom).

               Temperature

We encountered a lot of issues with our thermocouple. We know that the core of the furnace was about 1100-1200 °C (2000-2200 °F) when we added our first ore charge. After that, we tried to play with the air flow to adjust the charcoal burn rate (that we thought was way too high) but since we were a bit anxious we went back to our max flow for the remainder of the experiment. We know that temperatures were about 1200 °C (2200 °F) when ore was present at the bottom of the furnace.

               Slag formation

We had planned for a slag tapping door at the bottom of the furnace. When we opened it (some time after the final ore charge), nothing went out. We observed a lot of dark slag forming through our viewport, and periodically ran a steel rod through the tuyère to remove some of it the best we could.

               Final product

The final product is depicted here:

YL55cfG.jpg

It was a dome-shaped structure located in the center of the furnace, underneath the tuyère (as was expected). However, when putting it on the anvil, we discovered it was mostly moss-like slag. It certainly contained iron oxides as it was observed to be magnetic when cool. Some places had traces of the original redish ore. We found no indication that a reduction reaction of any kind had taken place. We theorize that we only got slag with dissolved iron oxides.

A lot of material from the foundation bricks was vitrified and had mixed with the slag. We think that the bricks were not rated to temperatures that high and might have polluted the furnace, preventing the bloom from forming, but we have no proof of this.

  • Questions

               Do you have any indications about what went wrong inside the furnace?

               We believe that either:

                    - The bricks were not adapted for building the bottom of the furnace. We are planning on using Mr. Sauder's furnace clay recipe (that we used for the furnace door here, which went well) next time. Our bricks might have polluted the furnace.

                    - Our ore charge rate was very bad. We should have begun with 1:4 ore-to-charcoal ratio and increased progressively to 1:1.

               Do these hypotheses make sense? Do you have any other recommendation?

More generally, we would be very interested in your opinion about this experiment.
 
Many thanks!
 

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You may want to find out more about the quality of your ore first. The final output of the furnace depends on how much iron goes in.

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7 hours ago, Jesus Hernandez said:

You may want to find out more about the quality of your ore first. The final output of the furnace depends on how much iron goes in.

Hi,

Thank you for your answer. How do you suggest I should do this?

I've looked into Lee Sauder's "iron dumpling" method, but even with the forge at full blast, all I can do is heat toe inside contents of the dumpling to a nice red hot. No trace of any iron bead inside. The contents went from a 50/50 redish/black mix (roasted ore/charcoal) to an even black mix. The charcoal is still there and I guess the iron ore transformed into magnetite hence the darker color. I wonder what kind of temperature I should aim at to get the iron bead...

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Are there any historical iron mines in the area where you collected the ore? If so old records may show analysis results.

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Yes, my region has a rich (no pun intended) history of iron extraction during the late Middle Ages. I found the ore at the exact spot that is documented in archives. I had no luck finding analysis results, though. I'm trying to get in touch with a geological lab which could help me on this particular matter.

However, the fact that the ore is both quite magnetic and able to get a very nice red color after roasting leads me to think that iron is indeed present. What do you think?

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I do not have the experience some here do, but I can say just because it is magnetic does not necessarily mean it will yield enough iron for a successful smelt.  If your roasted ore only yields 30% iron there is a good possibility it will all be lost to slag.  If your slag is magnetic there is your iron and your answer.

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Antoine- I was just sitting down to reply to your email, and saw that you asked here as well- so I'll just answer here where it might help someone else too.
I've got quite a few suggestions:

Ore: As Jesus suggested, unless you've got decent ore you're going nowhere regardless of anything else. The bloomery process requires ore that is over 50% iron (70% iron oxide) That is the iron content of fayalite, and you only get the amount of iron above that as bloom.
I'm not sure why you weren't able to get the dumpling crucible hot, unless you just got impatient. If you can get the outside white hot and keep it there, it's just a matter of time before the inside reaches that temperature. It usually takes me about 20 minutes to get heat to the inside, and I try to hold it at that heat for 15 or 20 minutes after that.
You can also get an idea of how much iron vs sand is in your ore by grinding a given amount of ore, and dissolving out the iron oxide with muriatic acid, and measuring how much sand remains. This will take several days and repeated changes of the acid, until just the clean quartz remains.

Furnace:The cinder block is a poor choice- you should at least line it with clay- but I bet it will fall to pieces once you get this furnace as hot as it should be. You want clay-based materials, not cement based materials.
The iron pipe as a tuyere will rapidly burn up. You can cover it with good refractory clay, or make an all clay tuyere, or you can make a copper tuyere :
https://www.youtube.com/watch?v=zjE6WFSWglc
 

Air rate/ burn time: I'm guessing your 5 liter/min is a typo- that's off by a factor of about 200 from what you want. The 1:1 ore to charcoal rate is fine, but 1 lb in 10 minutes is way too slow. In a 10" round furnace, I would consume 4 lbs ore and 4 lbs charcoal every 10 minutes. The 10" square would be even faster. A good consumption target is .35 grams of charcoal per minute per square centimeter of furnace cross section.

Total input: Even if everything was right, you'd get nothing with only 10 lbs of ore. That's not even enough to get the furnace hot and start building a slag bowl. You need to run at least 60 or 70 lbs of ore through a 10" round furnace to be effective.

Good luck, hope that helps-

Lee

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I have nearly zero experience here, but isn't the ore to charcoal ratio supposed to be more like 4 charcoal and 1 ore? Please correct if I'm wrong.

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Remember the ratio is by weight rather than volume.  

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On 4/6/2017 at 9:15 AM, Lee Sauder said:

Antoine- I was just sitting down to reply to your email, and saw that you asked here as well- so I'll just answer here where it might help someone else too.
I've got quite a few suggestions:

Ore: As Jesus suggested, unless you've got decent ore you're going nowhere regardless of anything else. The bloomery process requires ore that is over 50% iron (70% iron oxide) That is the iron content of fayalite, and you only get the amount of iron above that as bloom.
I'm not sure why you weren't able to get the dumpling crucible hot, unless you just got impatient. If you can get the outside white hot and keep it there, it's just a matter of time before the inside reaches that temperature. It usually takes me about 20 minutes to get heat to the inside, and I try to hold it at that heat for 15 or 20 minutes after that.
You can also get an idea of how much iron vs sand is in your ore by grinding a given amount of ore, and dissolving out the iron oxide with muriatic acid, and measuring how much sand remains. This will take several days and repeated changes of the acid, until just the clean quartz remains.

Furnace:The cinder block is a poor choice- you should at least line it with clay- but I bet it will fall to pieces once you get this furnace as hot as it should be. You want clay-based materials, not cement based materials.
The iron pipe as a tuyere will rapidly burn up. You can cover it with good refractory clay, or make an all clay tuyere, or you can make a copper tuyere :
https://www.youtube.com/watch?v=zjE6WFSWglc
 

Air rate/ burn time: I'm guessing your 5 liter/min is a typo- that's off by a factor of about 200 from what you want. The 1:1 ore to charcoal rate is fine, but 1 lb in 10 minutes is way too slow. In a 10" round furnace, I would consume 4 lbs ore and 4 lbs charcoal every 10 minutes. The 10" square would be even faster. A good consumption target is .35 grams of charcoal per minute per square centimeter of furnace cross section.

Total input: Even if everything was right, you'd get nothing with only 10 lbs of ore. That's not even enough to get the furnace hot and start building a slag bowl. You need to run at least 60 or 70 lbs of ore through a 10" round furnace to be effective.

Good luck, hope that helps-

Lee

I love posts like this. I copy them and paste into a word doc saving it as resource material. 

Thanks Lee!

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Hi Lee,

On 6/4/2017 at 6:15 PM, Lee Sauder said:

Antoine- I was just sitting down to reply to your email, and saw that you asked here as well- so I'll just answer here where it might help someone else too.

I was not sure you would read this on the forum. Thank you for taking some time to answer me, much appreciated. As Joshua stated, this is great reading material. Hats off to you sir!

On 6/4/2017 at 6:15 PM, Lee Sauder said:

You can also get an idea of how much iron vs sand is in your ore by grinding a given amount of ore, and dissolving out the iron oxide with muriatic acid, and measuring how much sand remains. This will take several days and repeated changes of the acid, until just the clean quartz remains.

That's a great and simple idea. I'm going to try it on several samples to get an idea of the iron content. I'm thinking about using a paper filter to get rid of the iron chloride solution at the end (and maybe monitoring the pH to know for sure when I don't need to add any more acid).

I'm also going to try to get a rough estimate by comparing the density of iron-bearing sandstone with regular sandstone (that I can find right over or under the ore layer). The use of this specific ore (that is, on this precise location) is well documented in the middle ages, so I'm rather confident about the iron content.

On 6/4/2017 at 6:15 PM, Lee Sauder said:

Furnace:The cinder block is a poor choice- you should at least line it with clay- but I bet it will fall to pieces once you get this furnace as hot as it should be. You want clay-based materials, not cement based materials.
The iron pipe as a tuyere will rapidly burn up. You can cover it with good refractory clay, or make an all clay tuyere, or you can make a copper tuyere :
https://www.youtube.com/watch?v=zjE6WFSWglc

 

I didn't include that in my original post for the sake of brevity, but the cinder blocks were indeed lined with kaolin clay (as was the tuyère). We probably underestimated the thickness that was required though, we're thinking about doing it with a much thicker clay wall next time. The cinder blocks idea originated from this video, where they seem to have wielded "good enough" results.

On 6/4/2017 at 6:15 PM, Lee Sauder said:

Air rate/ burn time: I'm guessing your 5 liter/min is a typo- that's off by a factor of about 200 from what you want. The 1:1 ore to charcoal rate is fine, but 1 lb in 10 minutes is way too slow. In a 10" round furnace, I would consume 4 lbs ore and 4 lbs charcoal every 10 minutes. The 10" square would be even faster. A good consumption target is .35 grams of charcoal per minute per square centimeter of furnace cross section.

Indeed it was, the type lies into the unit, we measured around 5-6 m^3/h, not l/min. This equates to around 100 l/min. Still not good enough! We're working on improving this.

Follow-up question: do you think the appropriate air flow correlates well with the furnace dimensions?

On 6/4/2017 at 6:15 PM, Lee Sauder said:

Total input: Even if everything was right, you'd get nothing with only 10 lbs of ore. That's not even enough to get the furnace hot and start building a slag bowl. You need to run at least 60 or 70 lbs of ore through a 10" round furnace to be effective.

Duly noted. I'm planning on mining ore on a larger scale next week. We'll try again in a few weeks.

On 6/4/2017 at 6:15 PM, Lee Sauder said:

Good luck, hope that helps-

It really does. Thank you again.

 

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Alright - we're getting ready for our next try. Before that, I wanted to know a little more about the ore we'll be using.

My ore mostly comes from the cliff side I showed in the first post. I was able to classify it in 3 major categories:

  • Banded sandstone

3sY5x6e

I focused on the dark bands that I thought were more likely to have a higher-than-average iron content. Surprisingly, this dark coloration only exists on the surface of the ore up to a depth of about 5 mm. This appears very well after roasting (which renders it quite magnetic):

LrNAJBq

Beneath that, the ore is mainly made of reddish sandstone, much like the layers above and beyond the ore in the cliffside.

  • Dark homogenous sandstone

This type of rock is very dark and appears to have a much larger grain size. The color is uniform throughout the rock. It does not change colors much after roasting and does not become magnetic:

vhMRIsZ

I took it after I saw the same kind of rocks in a quarry nearby, which is supposed to yield iron-bearnig rocks:

v80Oiy1

  • Iron crust

At the bottom of the cliff, I found large crusts of layered deposits. It really looks like thin layers of rust!

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Some of the crusts have very interesting-looking inclusions:

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This one becomes somewhat magnetic after roasting:

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  • Test protocol

I decided to try to use magnetic roasting on the ore. My reasoning is that the ore is mostly made of (hydrated) iron oxides and mostly silica. Roasting the ore *should* transform Fe2O3 into Fe3O4 which is magnetic, allowing me to remove it from the silica.

I roasted small samples (8-10 g) of each of the ore types using a gaz burner. I then powderized the ore and used a magnet to remove everything in the powder that is magnetic. I repeated the experiment on 3 samples for each ore type to verify the results.

Using a precision scale, I weighted the ore before and after roasting (to get an idea of how much it loses), I weighted the powder and the magnetic and amagnetic contents separately. I also checked that magnetic + amagnetic = original weight, just to check that I didn't lose anything in the process.

  • Results

The results are presented below:

  • Banded sandstone : 30-40 % magnetic content
  • Dark homogenous sandstone : below 1 % magnetic content
  • Iron crust : 80-92 % magnetic content

The error (difference between the original weight and the sum of magnetic and amagnetic contents) was less that 0.5 % of the original weight each time.

I was quite amazed at the amount of magnetic content in the iron crust. I don't know how it translates in terms of iron content, but I believe it is quite promising. The dark homogenous sandstone was quite a surprise as it appears to yield very little iron at all. This was confirmed by a test on the similar stuff I found in the nearby quarry, which yielded almost the same results.

  • What now?

Well, according to everything I read (which was confirmed by Mr. Sauder here), I shouldn't use anything below 50 % iron. I also know that too high of an iron content will inhibit slag formation and prevent the reduction reaction from happening at all.

Let's say that the magnetic content translates well in terms of iron content. I'm thinking about mixing the right proportions of iron crust and banded sandstone to aim at something like 67 % magnetic content.

What do you think?

Edited by Drepanon
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The iron crust is the stuff you're looking for! Don't worry about mixing it, just smelt it.

 

 

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