How many Coats of Satanite to High Temperature Wool

Discussion in 'Furnaces and their construction' started by Melterskelter, Jan 8, 2019.

  1. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    The reason the Satanite over wool holds up well may very well be that the wool is compliant enough to tolerate the heat cyclic movement. If so, rigidizing the wool under the Satanite may be unhelpful unless in doing so, it also more closely matches the thermal expansion of the two materials. This is what I want to put to the test on a small scale. I've used high alumina mortar to coat a lot of different refractories. There are two issues; the initial shrinkage of the mortar when curing (which is worse the more it's thinned), and then the heat induced cyclic movement. On IFB, it does well but will eventually delaminate, curl and flake. There are different mortars that more closely match the expansion rate of IFB. Over time this also fractures mortared IFB-IFB joints.

    I think it really depends on whether the reason the Satanite separates from the lid is just gravity or because the wool is breaking down and its physical properties at the Satanite interface.

    Still, I think it's encouraging the vertical Satanite surfaces are doing better. What does the surface where your burner impinges look like?

    Sounds like Ironsides has quite a few iron melts on a Satanite painted hotface/wool furnace....maybe he'll weigh in with how his has held up.

    Not many (high) insulating materials fare well with metal contact. IFB does the same thing, just not quite as fast. I did eye that bubble alumina when I bought some Satanite.

    That'll do, and only modest addition of mass.

    Best,
    Kelly
     
  2. Interesting discussion. Of course I have a few comments.:rolleyes:

    It takes a thermal gradient to protect the wool. That is, if you exceed the melting temperature of the wool inside the furnace, you have to have heat flow through the wall to get the temperature down. Think of it this way, if the interior is 3,000F and the shell is 200F (after everything has become stable) with only 2" wool insulation (no brick or castable), then the hot face of the wool is at 3,000F, the 1" depth is at 1,400F and the outside is 200F. So the gradient will be 140F per 0.1". 0.1" deep the temperature will be 2,860F (3,000-140) and at 0.2" deep the temperature will be 2,720F, then at 0.3" 2,580F, Bingo! Under 2,600F. These numbers are not exactly true, there are lots of surface effects but for the most part they don't cause major temperature differences.

    That is all after the temperature gradient has stabilized. That may never take place in your heating cycle, it may not take place in an hour and a half. You might find if you try to maintain the same temperature inside for four hours you will have a total refractory collapse. Maybe not, but maybe. That is one advantage of a high density refractory shell: it takes longer to come up to temperature than the overall heating cycle takes, essentially protecting everything through a long warm up cycle. My brick furnace is a low mass furnace compared to poured refractory, but higher mass than wool. However the wool will fail faster at melting temperature than the brick will, the brick binder starts to melt but the packed aggregate still has some residual strength. That is what has happened when you remove what looks like a good brick and it is brittle and crumbles. There is a big advantage in furnace life to heat and cool, heat and cool. It protects the refractory. In an industrial furnace running full tilt 24 hours per day with a shutdown every couple of years the refractory system must be much more robust than in a short batch furnace. Cooling of the furnace walls to remove the heat becomes important.

    Satanite has essentially no insulating value, and the only insulating value of the wool is the entrained air. I can visualize fingers of Satanite stabilizing melted wool close to the surface. It is also reasonable to expect the arch to melt first with progressively lower temperatures as you go down the inside of the furnace. There may be melted wool under the Satanite on the sidewall but it is not causing an issue because the Satanite is maintaining a firm shell.

    If you go to a brick arch it's definitely a good place for 3,000F brick, Satanite to stabilize the hot face, and ITC-100 to reflect the radiant energy so you melt metal before refractory. But long hold times at above the melting temperature of any refractory will, umm, melt it.

    After you put castable on the floor are you adding a sacrificial layer of wool? It's cheap and doesn't have to last.
     
    Melterskelter likes this.
  3. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    If the failure mechanism really is breakdown of the wool, it would definitely be beneficial if there was some intermediate insulating layer between the Satanite and wool that was higher refractory than wool. It might not need to be much. The Satanite isn't necessarily at furnace temperature if it's conducting heat to what lies behind it but probably is pretty close on the surface. I don't think it's fair to say it has no insulating value, it's just not as good of an insulator as the other materials. If you made a crucbile out of dense refractory and mortar, I think yo'd convinced it has some insulating value. In others words it's not a good conductor.

    I quite liked my IFB (electric) furnace. It was 2300 IFB. Even though it appeared unchanged, the material in the vicinity of the resistive coils had definitely degraded. The mortared joint had many micro-fractures from frequent and rapid heat cycling and the difference in thermal expansion of the IFB and mortar.

    Dense castable is about 150lb/ft3, insulating castable 75lb/ft3, IFB 37.5lb/ft3 (depending upon grade), insulating fiber board 18lb/ft3, and wools ~8lb/ft3. The insulating values tend to go the opposite direction as density/mass. As mass decreases so does is initial time to temperature. Unfortunately, strength and general durability tend to decrease with decreasing density/mass and higher insulating value.

    The place where I buy my refractories build all kinds of ovens and ladles. The engineer there told me professionally built furnaces have materials that are fit for purpose and from a refractory standpoint hold up quite well. The biggest difference is commercial furnaces don't see the frequent temperature cycles that hobby furnaces do and that's what's very hard on rigid refractory materials.

    That may be a good compromise of (low) mass and durability......always compromises.

    Best,
    Kelly
     
    The Desert Yeti likes this.
  4. The difficult part is finding the thin layer to put between Satanite and wool. I think the thin Satanite slurry may create that.

    True Satanite, and anything else, including steel, actually has "no" insulating value. I did say essentially no, but compared to the wool, or brick, it is so minimal you can ignore it when trying to visualize what is happening to the insulation system. If wool is 140F per inch, then 0.125" layer of wool is 20F. A thick layer of Satanite gains you much less than 20F, so we're talking about 10F or less contribution in a 2,800F temperature change. That is essentially nothing.
    When I read that many months ago I believed then that you had hot coils and reflected radiant energy which melted the binder in your brick, not thermal cycling. That is exactly the condition we see in brick which has been over temperature. In my experience repeated thermal cycling will result in those cracks joining and making macro fractures. Had your brick been thicker I don't think you would have seen the same failure of your system. Thermal gradient would have left enough good brick behind to retain integrity.
    Good concise summary. It can be said that the density is correlated with the air content. Packing wool in a closed space (just like fiberglass hone insulation between studs) lowers it's insulating value. Window glass is not a very insulator, and when it is spun into fiberglass insulation and packed back to a similar density as window glass, it has the same insulating value as window glass).
    The continuous operation of large industrial furnaces is also a big factor. Cycling is hard on refractories, but we still get an advantage that we don't run our furnaces for much more than an hour.

    My brick furnace seems to be holding up well but I won't be surprised to find crumbling brick behind the Satanite now that I'm getting more hours with oil firing. I've had the interior lining out once and the brick seemed OK but I'm sure I'm degrading it.

    I've read of lots of home operators who have failed their furnaces trying to run an extended period of time to get up to temperature. Their furnace worked well for short periods of time but when they tried to melt iron, for instance, and didn't have enough burner, the extended run time caused damage through temperature stabilization (they lost their protective startup gradient). I've also repaired many industrial furnaces which have been run too hot and failed the refractory which was only evident when the steel began to fail. Most furnace operators trade off efficiency of operation at higher temperatures with life of refractory at lower temperatures. Ask the refractory engineer whether their molten iron ladle design requires a short duration, gradient based design to be suitable. Do they design for pour time, or could their ladle refractory stand continuous contact with hot metal? Of course that can't happen as the contents quickly cool, but what do they design for? I suspect the design takes into account the mass of the shell and its slow rise as the contents cool.

    My arch is 2,600F brick and I wish I'd made it of 3,000F brick now that I'm thinking of melting iron. I initially planned on covering it with wool but soon realized I'd rather lose the heat than heat the unsupported brick.
     
  5. Melterskelter

    Melterskelter Gold Banner Member

    This makes perfect sense.

    I may be unwittingly seeing binder breakdown of what looked like intact wool at the Satanite/wool interface. Clearly the wool had not glassified as it still looked white and fibrous. I will look a little more closely to see if that thin layer that separated with the Satanite seems extremely friable consistent with binder breakdown.

    I can see where the ITC would tend to slow the conduction of heat through the wall and thus might help retard inevitable deterioration of the wall and lid. I have not yet applied it as I did not want to waste it on what I feared (partially proven) would be an unstable surface. Mabe I will apply it to part of the lid when I rebuild it to see how much protective effect it has.

    Interesting idea to use sacrificial wool. I am not sure how much actual barrier to iron it might afford as it seems so easily melted/dissolved by iron droplets. Have you found it to make clean out of wayward iron easier. On the plus side it would serve as a very low mass and highly insulating first surface that flame/heat impinges. I would at least rigidize it, though,as I want to minimize the amount of friable wool fibers spewing out the chimney

    Whether the side walls may be still standing despite possible melting of the wool is an interesting question. I suspect the wool is still in contact with the Satanite there as tapping on the surface to listen for a hollow sound fails to detect areas that sound hollow.

    Also, I am wondering what materials the industrial foundries are using that even significantly more refractory as I seem to find few materials available rated to greater than 3000. I'd be interested to know what they can get their hands on.

    Denis
     
  6. Melterskelter

    Melterskelter Gold Banner Member

    Yes, that material looks very interesting. I see it can be had on eBay. Does anyone have any experience with it?

    Edit: I did write the seller this morning regarding its use for my furnace and got a very prompt and articulate response--not always the norm on eBay
    Here is my message "
    "I am interested in using this material in a diesel fired furnace in which I melt cast iron. How well might it be expected to withstand the temperatures encountered? If coated with a high-temperature mortar might its durability improve?

    My current furnace uses 2600 degree ceramic wool covered with a 1/8" coat of Satanite. Unfortunately, it degrades after a few hours with such exposure.

    Denis"

    Here is the response
    "Dennis (Sic):

    The Bubble Alumina may be used as low density aggregate to cast refractory furnace shapes for funace as you have described using high Temperature mortar with which to bond it., I can also be used as loose insulation fill for fiber optic glass production furnaces and other glass furnaces as well. It can be used as a loose bulk, insulation material in complex or occluded cavities, for leveling materials on hearths, or as intermediate insulation in furnaces operating to temperatures as high as 1800 °C. (3272F). You should be able to expect a long life in your application using this material."


    My conclusion is that it might work if used as a filler for Satanite. Engineering the support for the chimney end and for the rim end of the alumina/Satanite blend would need some thought. I would expect some cracking. Making intentional radial joints in the lid liner might be a good idea. I am concluding that Mifco makes their liners segmentally is due to the propensity for uninterrupted rings of castable to crack.
     
    Last edited: Jan 21, 2019
  7. The highest temperature refractory I've used is the Greencast 94, which is advertised as 3,400F maximum recommended temperature. It is dense and not a good insulator but is very durable. It is often use as burner tile (tuyere lining).

    3,000F brick is nice but needs a hot face.
     
  8. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Haven't used it but have seen references to it in Kiln making books as an insulating refractory filler. Those references list max service temperature at 2700F but noted it varied depending upon quality.
    • The bubble alumina on eBay is (seller advancer 53) is just bulk material without binder. It says it is ~38lb/ft3 bulk density. That's between the density of K23 and K26 IFB. Looks like he wants $10/lb....yikes!
    • The bubble alumina at hightemptools.com says it has a density of 86lbs/ft3 but is already mixed with mortar to a trowelable consistency. The latter density is in the range of some of the lighter insulating castable refractories. They want $6/lb, but half of what you are buying is binder/mortar.
    I suspect it would have similar insulating properties to the materials of similar density, albeit at potentially higher refractory. The thing I noticed on the bulk material on eBay was the bubble size between 2mm & 5mm. Wow, hard to imagine a thin brushable material with that size filler.

    Best,
    Kelly
     
  9. Melterskelter

    Melterskelter Gold Banner Member

    Pricey but you only neeed a couple pounds to trowel a lid liner.

    Not planning to use it as a brush-on, but rather to be troweled.

    I am not going there at this time unless someone pops up who has used it and thinks it was great. For right now, I just brushed on a single coat of Satanite to the areas of my lid where the Satanite previously fell off. Happily it causes no issues if it falls into the crucible mis-melt as it floats right up with the dross.

    I did find the denuded areas to be quite firm as they are superficially glassified as evidenced by the dark coloration. Very light areas had the Satanite fall off late in the burn and so were not glassified or firmed up. So, I first tried a MAPP gas torch on those soft areas to try to "toast" them a bit. The MAPP torch really is not up to the task. But, a rosebud on my O/A torch did the job quickly so that a definite crust could be established. Then I just painted on a single coat of Satanite about the consistency of latex house paint---just a little thicker I suppose.

    Tomorrow should be a melt day. We shall see if the Satanite holds at all or just falls off. If differential thermal expansion between the Satanite and crusty wool does not result in delam of the Satanite, then my intuition tells me the crusty surface/Satanite combo might be a winner. For future reference, toasting the wool with the rosebud would be an easy prep step for the wool if this is successful.

    Furnacetopb urn (2).JPG Furnacetopb urn (1).JPG
     
  10. I hope not, but when you fried the wool you probably made it very brittle and may have lost it's strength.

    Have you looked at ceramic fiber modules? You might make a taller arch and stand the wool on edge. If you make it three inches thick it could be sewn to the shell with stainless steel wire in the cool area of the wool, maybe 1" from the shell and 2" from the hot face. A better solution is to make your own module by pleating it and sewing the part that contacts the shell to the shell. If my brick arch hadn't worked I was going to try that. It still needs hot face protection but the wool is a lot stronger across its width rather than its thickness. Probably your Satanite didn't fail, but the wool it was attached to.

    If you want to use brick, you could make a suspended brick arch if you're uncomfortable with cutting the wedges like I did. You can drill crosswise of the brick a little less than halfway through then drill a big hole from the back intersecting the cross hole. Then you run a SS rod through a SS eyebolt in the big hole, or a loop of SS wire in the big hole, and support the brick from above with the eyebolt or wire. If you're running the furnace at 2,800F and the brick exterior is at 400F the halfway point is at 1,600F (half of 2,400 delta plus 400) which 304SS and 316SS will stand. Then coat the bottom with Satanite and seal the seams.

    Sorry about the hijack.
     
  11. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Denis, OIF jogged my memory with the comment about the modules. I was clicking through some of my stored links to sources of refractory materials and the KT refractories site has some very interesting materials. On the landing page, they mention bulk bubble alumina in .2mm-.5mm diameter. In addition to raw materials they have some very interesting materials in both brick and castable. Their corresponding downloadable documents with material properties is a great technical reference if nothing else. If you pursue some of that fine bubble alumina and there are minimums, let me know. I'd be interested in some and sharing the cost.

    http://ktrefractories.com/index.cfm

    Best,
    Kelly
     
  12. Melterskelter

    Melterskelter Gold Banner Member

    you may well be right, but the remaining wool feels pretty tough---more so than the original unheated wool by far. I'll be melting tomorrow, if all goes well. Proof will be whether the roof ends up in the pudding!

    You are right that it was the wool that pulled away with the Satanite. It is possible the wool was weakened by the heat exposure. In other words, being covered by Satanite it did not get a chance to glassify and yet was degraded somewhat. But, it looks to me like really getting after the wool with heat may actually form a tough crust that might help it adhere to a satanite coat.
    Your brick arch sounds like a very reasonable option. Thank you for pointing modules out. I have not yet looked at modules. I am sure I would be more inclined to "roll my own."
    I have no concerns about cutting the brick to form an arch as you did. You set the gold standard on that one. (On the other hand, those old Romans had similar ideas that seemed to have stood the test of time.) Anyway, that is my fall-back option. For now, I am going to experiment with quicker and dirtier approaches that may or very well may not work out. Thanks for the ideas. I appreciate it.

    Kelly's "roll top" method he is talking about in his low-mass electric furnace also looks like it deserves a close look.

    Denis
     
    Last edited: Jan 22, 2019
  13. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I think I have it a lot easier than you because I'm pretty easily within the service temp range of the materials. You're probably pushing it.

    Seems like it may depend on whether the wool actually is breaking down at the Satanite interface and that's contributing to the failure or whether it's just mechanical failure. If the wool is decompoising, the only benefit the construction method offers is bond to the interface between the rolled layers. If you glued the layers with Satanite I think the potential for that benefit is definitely there, but that will also add mass and create a thin conduction path through the wool every where the Satanite exists........unless it was only applied to the layers near the inner surface and across the entire boundary. Not sure how significant that may be. Still think you'd want to rigidize the wool for structural integrity.

    Best,
    Kelly
     
  14. I don't think commercial modules are of a shape suitable for home foundries. But if you study their design and see how they support with stainless steel which is kept in a cool area of the module it will give you some ideas to "roll your own".
     
  15. Melterskelter

    Melterskelter Gold Banner Member

    I agree that the modules I saw were too large for my use. The principle of their construction may be applicable, though. In a sideways sort of approach I did hang my wool on studs and laced it into place.

    http://forums.thehomefoundry.org/index.php?threads/tuyere-question.512/page-2#post-11246

    But The method I used did not take advantage of the increased strength (you clued me in on a prior post)along the length and width of the wool. So, if the wool were used on edge rather than on the flat, it might exhibit greater and more durable adhesion of mortar to wool. I have been staring off into space thinking about ways I could sew the wool into place with SS wire.

    The weather favors a melt today and all is in place to do a stress test on my furnace lid. More on that later today.

    Denis
     
  16. Melterskelter

    Melterskelter Gold Banner Member

    Lid stress test completed and all of the newly Satanite-coated “toasted” areas held up fine. That was only an hour and 15 min exposure to high heat BUT it does look good with no evidence of breakdown. I’ll hold off on the happy dance until I’ve run it 5 or 6 times with good results. I really hope it holds as toasting the wool would be the lightest and simplest approach.

    The dense castable 1/2” coating on the floor continues to do well.

    Denis


     
  17. Melterskelter

    Melterskelter Gold Banner Member

    3CE5CD3A-7D73-4621-A45E-59767946461B.jpeg I thought I’d post an image of the “toasted” wool to show what it looks like after I hit it for a few seconds with the O/A rosebud. To the left Satanite I applied as a single coat to toasted wool. The bare area I am working on was 1/8” Satanite previously applied
    virgin wool which flaked off. I am presently thinking a single coat applied 1/16” thick will hold upper better. Seems to be true, but needs to stand the test of many heats.

    Besides forming the obvious beads of alumina there is a coalescing of the underlying wool fibers for perhaps 1/16” that seems to provide a better surface for the Satanite.

    Denis
     
  18. DavidF

    DavidF Administrator Staff Member Banner Member

    Coat the wool in sodium silicate, then vacuum bag it to the form to create the shape. Once cured apply the satinite.. just my thoughts on the subject...:D
     
  19. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Just out of curiosity, do you think there is any merit in just not coating the lid? I know what the supposed operating temperature of the wool is supposed to be, and the recent wool furnace build coat everything, and it's the norm in forges, but at 1/16" of Satanite, I have my doubts whether that could be doing much in the way of insulating or thermally protecting the wool. Yah, you should stabilize the wool because the fibers and yada-yada-yada but there are lots of ways to do that.

    It probably needs some surface treatment since the wool is friable and you have high velocity air and fine debris, but what is that surface condition like after you toast it? Good enough? Or if you just used some colloidal silica to initially stabilize the wool, would the furnace temps toast and naturally condition the surface?

    Just invoking Occum's razor.

    If the walls are happy leave them. If the floor is happy with some dense castable; so be it.

    Best,
    Kelly
     
  20. Melterskelter

    Melterskelter Gold Banner Member

    I think Occum had a good idea that should always be trimming back overly complex ideas.

    But, though Satanite at 1/16th inch must have nearly zero insulating qualities, it is pretty clear looking at my lid that the wool is much much more reactive with the hot gases (there is more than pure heat exposure in play) that are swirling in the furnace. Uncoated the wool rapidly breaks down as demonstrated by the severe deterioration that occurred over the course of only part of a melt in areas where the Satanite flaked off mid melt. In those areas there were some regions of wool that melted to only 2/3 its original thickness and 1/4 thick globs of alumina glass formed. But with a full 1.5 hour melt the toasted and then thin Satanite coated wool was unharmed. And no melting of underlying wool occurred in the untoasted but thick coated wool areas.

    My conclusion is Satanite is very resistant to the “corrosive” hot gaseous furnace atmosphere whereas wool is not. In your electric furnace the protective effect may be less evident. But still Satanite will form a hard crust that will prevent aerosolization of wool fibers.

    Trying to understand best I can what I see...

    Denis
     

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