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Drepanon

First smelt failure post-mortem analysis

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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:

2uO3WUe

This one becomes somewhat magnetic after roasting:

XGFwpLk

  • 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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Hi there!

Thank you for your kind words. As it turns out, I have news about the project: basically, we're idiots.

Using the experience we gathered during our previous attempt, we set up a new bloomery. We're still using a combination of precast blocks to build the furnace, except this time we're going with clay chimney liners (rated to a very high temperature) instead of cinder blocks. We're still lining the entire thing with clay for a nice, thick clay wall. The furnace was way better than the first one, and this time we were able to monitor the temperatures at 3 different heights very closely (more to follow).

We used the same fan as we did the first time, but we now have some doubts it is still able to maintain a high enough air flow when the counter pressure builds up in the furnace. What are you guys using?

Anyway, we made a very simple mistake, but still gathered some interesting results. Last time, we used a bowl to measure our dose of ore (one small saucepan being approximatively 500 g of ore). So without thinking too much about it, we re-used the same pan.

As it turns out, since I have been so much more selective about the quality of the ore (aiming for ore that had about 90 % magnetic content, as suggested by Lee earlier), the bulk density of my ore has dramatically changed. Our saucepan was now holding 2.5 times the mass of ore it did on our first attempt! This is great news for the ore quality, since it does mean that we're charging much more iron oxides into the furnace. However, it also means that we charged the ore too fast by a factor of 2.5 in the furnace!

We were aiming for a 4:1 fuel/ore ratio to begin with, then increasing up to a 1:1 ratio. Because of our mistake, we actually began with a 1.6:1 ratio (which works well enough), and went as high as a 0.4:1 ratio (which is terrible). So basically, after a brilliant start, the furnace died out being saturated with ore.

This is quite evident on the temperatures that we recorded:

Tnhni6J.jpg

Please note that T1 denotes the temperature closest to the bottom of the furnace, T3 denotes the temperature closest to the top and T2 is somewhere in between. We're more interested in the variation of temperatures than in the actual values, which may vary based on the depth of the thermocouple.

The first thing we can see is that based on the evolution of the T3 temperature, it took about 1.5-2 hours to pre-heat the furnace (which is not much since we used pre-cast blocks). We waited for this temperature to stabilize to begin the reduction.

Then, we see that there is a ~40 minutes delay between a variation of the ore charging rate and the temperature. This is due to the ore "hang time" in the furnace, and it correlates almost exactly with the fuel consumption rate we observed (by measuring the speed at which the top surface of the fuel/ore mix goes down in the furnace).

So about 40 minutes after we introduced ore (at an actual ore/fuel ratio of 1:1.6), we observed a gradual increase of the bottom temperature, which was expected (yay!). This increase was even more noticeable when we increased the ratio up to almost 1:1. But then we went way to far and everything went off, as you can see on the graph.

So what next?

Well, we do believe that we have some appropriate ore and thankfully we still have more than enough to try again. This time, obviously, we'll be more cautious about the ore weight. I expect us to build a new furnace next month, so stay tuned!

Edited by Drepanon

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Great recap, and nice graph. Thanks for taking the time to get us this data! Don't doubt that everyone's taking notes. :)

-Jeff

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Your getting there....
 Your ore looks a lot like some of mine. Limonite.. from central NC

 Like your graph... Do you have a sight viewer at the tuyere? That is the best way of seeing any temp. problems right away.

 Are you on the Face book smelters page ?  https://www.facebook.com/groups/361798240526981/?ref=bookmarks

 keep trying, it looks like your getting close... 

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15 hours ago, Mark Green said:

Do you have a sight viewer at the tuyere? That is the best way of seeing any temp. problems right away.

We do! That's actually how we noticed the furnace was dying off, we initially thought the temperatures droping were a sign of a thermocouple failure (which was kind of expected as we used K type thermocouples up to 1200 °C). We did see the furnace going darker and darker.

Nice Facebook group, I'm joining :)

Thank you for your kind words.

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So we kept going, and here's the summary of our latest attempt/failure (well, kinda).

  • Ore

We used the same ore as before. We definitely know it has been used in bloomeries for centuries (more here) and based on our previous experiments, we feel it's appropriate. We did try to make it as iron-rich as possible by "filtering" the roasted ore with a powerful magnet.

xkBnyrO.jpg?1

When crushing it to pieces, it has such a tendency to crumble into powder that we decided to roll with it and pulverize everything. We simply put some water in it before feeding it into the bloomery in order to avoid the blast from dispersing it everywhere.

71HLwDB.jpg?1

In total, we prepared about 20 kg of ore, aiming for Lee's 0.35 g/cm²/min rule of thumb in a 20x20 cm furnace. This time, we made sure to measure the ore mass each time, though!

  • Air

We identified the air blower as a weak point in our previous attemps. Our previous attempts used a blower that didn't have the required pressure capacity. We changed it for a squirrel cage blower that is able to push up to 375 l/min at a 22 mmH2O (a couple of milibars) pressure. We used water manometers in order to verify both the static pressure and the volumetric flow during the attempt.

IszTwIu.jpg?1

  • Furnace

Sadly, we did not have enough kaolin clay at hand to do the furnace entirely out of clay. We decided to do the bottom part out of refractory bricks, jointed and lined with Lee Sauder's clay recipe. The tuyère was made out of a ceramic kiln support (that performed absolutely perfect).

DhysYuk.jpg?1

4CVnrUX.jpg?1

umsSZsB.jpg?1

Everything was then coated with a custom clay with a charcoal-based charge. This method has been described by a French smelter (Moretti), we heavily relied on his suggestions.

dSc5LAL.jpg?1

bwmOBtE.jpg?1

The upper half of the furnace was constructed with clay, double-wall chimney liners. This is certainly not as appropriate as building the entire thing out of refractory bricks (or even 100 % kaolin clay), but we tried to keep things cost-effective. As things turn out, this may be one of the reasons we failed in the end. As before, porous cement blocks were used to insulate the furnace (and keep the temperature bearable around it).

FiBKm2T.jpg?1

FpFsRuz.jpg?1

Air was fed into the furnace through a Y attachment with an acrylic window (with a 3D-printed support :) ).

img]

  • Results

After a few hours of preheating, we started feeding ore into the furnace. We aimed for a 13 mmH2O air pressure, as suggested by Moretti. The starting point was decided both by the temperatures getting more or less stable, and the appearance of a beautiful blue flame on the top of the furnace.

HtHFHCg.jpg?1

The results were very similar to our previous attempt. From the very begining, we seemed to have troubles keeping the temperature high enough: through the viewport, we could see dark matter falling in front of the tuyère instead of a jam-like substance. About 8 kg of ore were fed inside the furnace before we weren't able to feed charcoal and ore inside the furnace anymore because of the formation of a solid slag.

DLrGYmv.jpg?1

As before, we had 3 thermocouples along the furnace to monitor the temperatures. And as before, we saw the following trends:

  1. a more or less constant temperature at the top of the furnace (T1);
  2. a slow, regular increase of the middle temperature (T2);
  3. a sharp increase of the lower temperature (T3), seemingly indicating the onset of the reduction reaction, followed by the temperature collapsing.

As we were breaking the furnace down, we discovered a large mass of solid, spongeous slag around the tuyere, filling the furnace and explaining why we weren't able to feed anything anymore. This mass adhered to the furnace walls and filled the whole width of the duct, it wasn't limited to the immediate vincinity of the tuyere.

HtHFHCg.jpg?1

However, upon close inspection of the slag, we discovered that large numbers of metallic iron "pebbles" were visible at the surface.

vvASAQc.jpg?1

Some whiteish pieces of slag were covered of small (<1 mm diameter) iron "dots".

Conclusions

  1. In a way, we did make some iron (just not in any useful quantity) :p
  2. We believe that the reduction reaction was able to start, but was not sustainable, probably because of a too low temperature.
  3. One factor for the temperature being too low could be the air flow that was too important, thus cooling down the slag too fast.
  4. Lee Sauder's clay recipe held fantastically well. Next time, our furnace could be made entirely out of kaolin clay, or in refractory bricks joined and lined with kaolin clay (same principle as with the bottom third of this furnace, but all the way in).
  5. The ceramic kiln support tuyère was a great solution that could be used for numerous runs before needing a replacement.
  6. The fact that we used clay chimney liners could have led to some of the heat getting out of the furnace (either through the liner or between the two walls).
  7. Our ore is most certainly approriate and will be used again.

What are your thoughts on this?

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Interesting!  You may have had too much air, or, conversely, not enough.  Not enough will allow the base of the charge to cool.  I would suggest this is what happened, and give it a little more air.  OR, it may be that it was enough air, but that the slag blocked the tuyere, causing everything to cool off.  We keep an iron rod handy to poke through the tuyere when we see slag in front.  Too much air and you'll get more and more liquid slag, and also cast iron rather than a bloom.

I have found that insulating the top half of the furnace is not necessary, and may not be desirable, except for operator comfort.  I have seen Mark Green have great success with the furnace walls around 10mm thick at the top, 20mm in the middle, and 150mm at the bottom.  I suspect this allows heat to radiate, thus keeping the charge at the top from getting too hot too fast and turning into cast iron.  Speaking of which, that is what those little prills of iron in your slag look like.  

I need to do some more experimenting myself, that's about all i can observe on this run of yours.  Oh, and I like the tuyere!  I may have to steal that idea...

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Hi guys!

Long time no see :)

We gave the whole bloomery thing another go this summer, with mixed results. Here is the debriefing:

 

  • Ore

The ore was the same as earlier, that is, limonite that we "filtered" based on its magnetic properties after roasting. This time, however, we were concerned about the powdery nature of the ore, and we decided to aggregate it with water and a small amount of flour "glue". We basically made an ore cake :)

 

20190331_143650s.jpg

 

I think that was OK, and it did indeed help avoiding ore getting blasted away from the furnace.

  • Charcoal

The fuel was basic charcoal, but we did take some time to "calibrate" it into chunks of consistent size (around 3 cm/1").

  • Air

After some measurements, we discovered that our previous arrangement was creating huge pressure drops. For instance, we used a vacuum cleaner creased hose to get air into the furnace. Moreover, we used an "off-the-shelf" ceramic nozzle with a ~ 20 mm inner diameter. Both those things were causing us to lose an unacceptable amount of our available pressure.

We cleaned a lot of things here, and we verified that we were able to a maximum static pressure of 29 mmH2O, and a maximum air flow of around 1200 l/min. We tried to aim for a 250 l/min air flow.

  • Furnace

We used exactly the same construction principle as last time: refractory bricks lined with Lee Sauder's kaolin furnace clay. No complaints here, it held pretty damn well. We did not use any kind of additional insulation (porous cement or otherwise).

  • Results

The smelt was mostly a failure again, but this time we were able to salvage some very interesting information within its remains.

During our previous attempt, we discovered that a large mass of slag was created well above the nozzle, likely blocking the advance of ore/charcoal towards the bottom of the shaft. This time, we got more or less exactly the same thing. Here's a schematic view of what happened:

 

Post-mortem.png

Here's a photo of the whiteish "crust" that I'm talking about with its correct orientation:

 

IMG_4206-1s.jpg

 

The white stuff was mostly on the top, while reddish slag was hanging underneath it and filled the remainder of the shaft. As last time, we did not get any kind of really "liquid" slag gathering at the bottom of the furnace, only this sluggish mass, as can be seen in this photo of the open furnace door:

 

MVI_4186.MP4_snapshot_00.46_[2019.11.12_21.33.59]-1 copie.jpg

 

Inside the white mass, we discovered a few small "beads" about 5 mm in diameter. Apparently, they began to form while descending through the slag, but were stopped somewhere along the way. You can see it at the bottom of the "channel" it created:

 

IMG_4211-1 copie.jpg

 

One of these beads was way bigger than the other ones. As far as I'm concerned, that's the closest we ever were to a bloom :)

 

MVI_4204.MP4_snapshot_00.04_[2019.08.25_17.30.56]-1 copie.jpg

 

The metallic nature of the pebbles was confirmed by a quick trip to the belt grinder:

 

MVI_4200.MP4_snapshot_00.14_[2019.11.12_21.36.46]-1 copie.jpg

 

This time, they were also big enough that we were able to perform a few analyses...including microstructure and spectrometry analyses!

 

The first particle we tried is showing a lot of lamellar pearlite:

 

Particle1.JPG

 

This one I have absolutely zero idea what it is:

 

Particle2.JPG

 

The third one is again showing some lamellar pearlite:

 

Particle3.JPG

 

And here's a spectrometry analysis of 3 different areas within the same particle:

 

EDS.JPG

 

In the 3 particles we tested, we found a large amount of phosphorus. This was expected as I knew this specific ore had an historical reputation for being phosphorus-rich.

 

What do you guys think? :)

 

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What is your burn rate?  That is, how often do you charge your furnace, and how large is the charge?  A good rate to aim for in a furnace of that size is 1kg ore and 1kg charcoal every six to eight minutes to maintain a full shaft.  If the furnace is going slower than that you will freeze it in the furnace, faster and you may end up with cast iron. We don't measure air volume so much as rate of burn in the furnace.

 

The light-colored stuff looks an awful lot like the clay you are using is melting into the charge.  White slag can happen, but it's fairly unusual.

 

That is a LOT of phosphorus!  I am not sure how to lower it, but anything over 5% is going to be very difficult to forge without it cracking.

 

One final speculation:  I wonder if the smaller top of the furnace is the cause of the problem?  Most of use use a straight shaft around 30cm diameter from top to bottom.  a bottle-shaped cross section was indeed used to produce cast iron, but not, so far as I know, bloom iron.

 

 I hate to introduce too many variables at one time, but if I were to do this I would suggest the following changes:

 

1. parallel-sided shaft inside, 30cm bore.

2. raise the tuyere up to 20-25cm above the base of the shaft.

3. lower the tuyere angle to around 15 - 17 degrees.

 

The nose of the tuyere may work better with a more gradual taper, say from 35mm to 25mm over the last 15-20cm.  This will be more important once you start getting blooms.  A sudden constriction will result in a sharper blast that may tend to erode the top of the bloom.  The bloom needs that ~20cm of space under the tuyere to sit in its bath of slag that protects it from oxidation and allows the droplets of iron to coalesce.  If the blast is acting like an oxygen lance on the top of the bloom you will get a bowl-shaped bloom.  This is common enough in antiquity, but we can avoid it to get bigger blooms.  

 

Keep going, you'll get there!  B)

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