Basic refractory

Discussion in 'Furnaces and their construction' started by Guyfromalabama, Jan 11, 2021.

  1. I have a large amount of what I believe is magnesium oxide in the form of thermite furnace shards that I picked up from the side of the railroad where it was tossed aside after the rails were welded together. Has anyone ever used this as part of a hotface or to insulate their furnace walls?
  2. DavidF

    DavidF Administrator Staff Member Banner Member

    Aluminum oxide maybe?? Have a picture?
    Don't think anyone has ever tried this before, but I could be mistaken.
  3. It's good stuff: up to 2200 deg C service temperature, resistant to chemicals but has decent thermal conductivity so not that good for an insulator but would probably work as aggregate with ciment fondu based concrete to make a dense refractory.
  4. DavidF

    DavidF Administrator Staff Member Banner Member

    Is it magnesium oxide, or aluminum oxide??
  5. Don't know about the original poster but sintered magnesium oxide is good stuff, they make specialty lab crucibles out of the stuff too. Aluminium oxide melts at 2072 degrees C vs 2800 deg C for magnesium oxide.
    Last edited: Jan 11, 2021
    DavidF likes this.
  6. rocco

    rocco Silver

    My money is on aluminum oxide. I say that because I find it difficult to believe the railway crews would be using a magnesium based thermite as the chances of accidental ignition would be dangerously high compared to an aluminum based thermite.
  7. DavidF

    DavidF Administrator Staff Member Banner Member

    Yes but he might be referring to some type of mold or liner material and not the thermite itself????
    Thermite is fun stuff!! :D
  8. Iron oxide- aluminium thermite peaks out at 45oo deg F or 2500 deg C so that's hot enough to melt aluminium oxide moulds.
  9. IMG_20210112_025429.jpg IMG_20210112_025351.jpg IMG_20210112_025341.jpg IMG_20210112_025518.jpg

    These are the shards I was referring to. When heated with a propane torch and left to cool, whatever binder holding it together is gone and I'm left with what looks like very fine white sand.
    I know that they were used to contain the thermite (aluminum and iron oxide combination) reaction based on what I've read and videos where I've seen the shell broken apart at the end.
    And I also have the iron riser and slabs/sprues leftover. IMG_20210112_025646.jpg IMG_20210112_025730.jpg
    IMG_20210112_025817.jpg IMG_20210112_025817.jpg
    I believe the riser is pure iron and the slabs are pure iron with a little aluminum. When I first found them years ago: I was searching for pig iron around the railroad next to an abandoned iron plant (sloss furnace in birmingham Al) and thought I'd hit the jack pot.

    I'm not sure if these pictures are of any use, however I'll take better ones in a few hours.

    Attached Files:

  10. metallab

    metallab Silver

    It is aluminum oxide Al2O3 as thermite used for rail welding is powdered Al with Fe2O3.
    The slag remains are very heat resistant (up till 2000 C) but are bad insulators.
    You can grind or hammer it to fine gravel and mix it with ordinary cement, but still it is a poor insulator. You can use it as an inner lining of a Kaowool furnace wall or lid.
  11. You are right I believe, about the thermite reaction using aluminium powder and not aluminium oxide powder. My mistake there. But why would they use aluminium oxide in a furnace that is used to melt iron oxide, with the temperatures of the aluminothermic reaction reaching up to 2500°C? I would be just as happy with aluminum oxide tbh, I simply want to be sure.
    IOW what makes you think it's aluminium oxide?
    Thank you!
  12. Here are better pictures.

    The shards of the furnace

    This is a sequence of me heating a piece of one to glowing and the letting it cool to what looks like very fine sand that, if touched, simply falls off.
    IMG_20210112_153438.jpg IMG_20210112_153419.jpg
    IMG_20210112_153455.jpg IMG_20210112_153622.jpg IMG_20210112_153616.jpg

    This is a better picture of the slabs or iron with aluminium contamination
    And the riser of pure iron.
    I have no idea what I'll eventually do with these. In my "this is free" zeal, I picked up sooo much of these.
  13. These are broken pieces of the rail mould. I'm not sure what it's made of either. Its very heat resistant as well. It may just be silica sand colored red.

    Attached Files:

  14. The shard you have looks exactly like resin bound silica sand for sand casting moulds: the glue lasts just long enough for the metal to solidify before the heat causes it to burn away releasing the metal inside.
  15. Petee716

    Petee716 Gold Banner Member

    Ah, finally, a diagnosis for my apparent mental illness! I also suffer from the more common "these are cheap" zeal. Lol.

  16. Ah, and that would be the red or white? Well at the very least I won't be needing to sift silica sand anytime soon.
  17. The white stuff.
  18. garyhlucas

    garyhlucas Silver

    I have done thermite welding of copper ground wires to structural steel and ground rods but the molds were graphite and reusable quite a few times. Fun stuff to do!
  19. metallab

    metallab Silver

    And Mg has a low boiling point (1100 C), much lower than the melting point of iron (1539 C), so it is useless. The boiling point of the reducing metal is the temperature limit of the reaction, hence a thermite reaction is 2500 C, which is the boiling point of Al.
  20. I agree. I thought the refractory might have been made of magnesium oxide or zirconium oxide because I have read that it is used in alkaline refractories used for melting steel. I am still not sure what it is TBH, only that it does not budge under an O/A torch and that it does appear to have high thermal conductivity, which isn't a good thing for purposes that I would have used it for. Perhaps it could be used to make a specific crucible if, at some point, I research more on how it acts when fired like a typical silicon carbide crucible.
    Jimmymmm likes this.

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