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Author: Subject: castable refractory mixtures
Eclectic
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[*] posted on 15-9-2007 at 05:58


If you want a mix to embed the heating elements, alundum cement works well. AFAIK, it's just alumina with 1-3% phosphoric acid binder. You could use a clay binder if you don't like phosphoric acid, but after firing the acid will be bound up as aluminum phosphate.

I'm thinking of experimenting with milled zircon (Zircopax) with a colloidial alumina binder, maybe with a bit of magnesium formate so as to get a transient corderite phase while firing.

I was recommending Celite as a loose backfill insulation.

For a castable insulating refractory, you can mix in sawdust. It burns out leaving a porous structure.

[Edited on 9-15-2007 by Eclectic]
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[*] posted on 15-9-2007 at 06:06


The pottery suply only had hydrated alumina, not calcined alumina. Can hydrated alumina be used as well or does it have to be calcined first?

I'll probably use the calcium aluminate cement/chamotte mix anyway, its cheaper and cures rapidly at room temperature.




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[*] posted on 15-9-2007 at 06:09


Hydrated alumina is fine if what you want is to mix it with clay to make a higher melting refractory. You can mix calcium aluminate with clay if you want to have something cheaper that will set up at room temp. You could experiment with powdered aluminum, calcium hydroxide, and clay, to see if you can come up with a high temperature version of the foamed concrete building material.

Calcium Aluminate (Ciment Fondu) is good for up to 1600 C?
Higher then any temps you will get with a Nichrome or Kanthal heating element.

[Edited on 9-15-2007 by Eclectic]
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[*] posted on 11-6-2009 at 01:55


How about silicon carbide and Al oxide? Will Nichrome and kanthal do well in that? I've seem a formula for one that contains;

25.4% Al2O3
9.3% SiO2
0.4% FeO3
3.5% CaO
59.8% SiC
0.3% Alkalines

The thing I like about this is it' s thermal conductivity of 8.1W/mK where as the high aluminates are around 2 and below. I'm assuming this a good thing if using it as a mantle.
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[*] posted on 4-7-2009 at 00:40


A few ideas about refractories:

1. Considering the amount of material needed for a furnace, you should try to minimize the use of things that you can't get in 50 lb. sacks at a reasonable price.

2. If you can improvise the facilities, pre-fired bricks are much easier to work with than castable or rammed mixes. Due to their surface area, they can be dried much more evenly and more quickly. Thoroughly fired bricks are more structurally sound than fired-in-place mixes, and more importantly, most of the shrinkage is done with, so unpleasant cracking is avoided.

3. For insulating refractory, you want some organic filler to burn out. Although perlite will work, its low melting temperature will severely limit the service temperature of the refractory. Sawdust is probably cheapest, if you can find a source (I have not yet succeeded.), but if you don't mind a headache you can shred the polystyrene foam blocks used for packaging. The shavings weigh almost nothing, so they easily become airborne, and static effects cause them to stick to everything. I suggest shredding with a motorized wire brush and collecting the results in a freshly emptied shop-vac.

4. Unless you have calcium aluminate cement or something of that sort, the best thing for your fired refractory to turn into is 3:2 mullite, 3(Al2O3)2(SiO2). Here's an alumina-silica phase diagram from Tim's site. Straight Al2O3 has a higher melting point, but it tends to lose more strength as it approaches its melting point. It is best to err on the side of alumina, however, because free silica goes through phase changes with changes in temperature, especially if it soaks for a long time, and the associated volume changes fuck things up.

While it is possible to make mullite by firing finely milled alumina and silica in the appropriate ratio, Al2O3 is very expensive. It is best to start with one of the high pressure stable aluminosilicate minerals (kyanite, sillimanite, and andalusite) and add alumina to make up the difference. The aluminosilicate minerals are all unstable at atmospheric pressure, so they decompose to mullite and silica at high temperature. Because kyanite is the highest pressure form, it decomposes at the lowest temperature (around 1350 C ish). It also undergoes a volume expansion, but finer mesh kyanite changes its dimensions less because the expansion instead goes to reducing porosity. Here's a PDF put out by Virginia Kyanite that explains more:

www.kyanite.com/assets/Virginia%20Kyanite%20and%20Mullite.pd...

If, like me, your local pottery supply shop doesn't stock kyanite, you can use a cheap kaolin clay such as Edgar Plastic Kaolin, but more alumina will be necessary in that case.

5. I have recently discovered that the recycled cellulose fibers sold as building insulation carbonize but do not burn in the flame of a propane torch, and show incredible insulation performance. The side of the wad of fiber directly heated by the torch was carbonized, while the other side remained unchanged. Considering the very low cost of this insulation, it may work well as a sacrificial loose fill around a firebrick inner wall. If the loose fill space is well sealed by a casing and perhaps high temperature silicone sealant, the fiber should combust just enough to maintain a neutral/reducing atmosphere in the space. It probably wouldn't work well as organic filler for firebrick, however, as what I've read suggests that it is treated with boric acid to provide fire resistance, and this would probably flux the refractory.

6. I've run across references suggesting that complex carbon shapes may be converted to silicon carbide by heating to extremely high temperature in a bed of silica (sand or fumed?) and loose carbon. Both SiC and carbon are very effective microwave susceptors, so I think it might be possible to form silicon carbide structures (crucibles, burner tubes, resistive heating elements) in a domestic microwave oven from carbon objects formed from powdered charcoal held together with an organic binder (wheat flour?)
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[*] posted on 17-5-2017 at 15:57
Pourable Alumina


I want to make my own Alumina tube, 600mm long, 40mm OD, 32mm ID with a 4mm wall. I made a polyurethane mold and bought some Rescore Al2O3 from Cotronics in a mad shopping rage.

It is frightfully expensive for me. What are the chances of making my own mix than I can pour or somehow ram down a 4mm gap?

I saw someone who combined Calcined Alumina with Veegum (Kaolin) but how do you make that pourable?

https://digitalfire.com/4sight/material/calcined_alumina_41....
made a kiln shelf from 96.5% calcined alumina and 3.5% Veegum

In this patent, someone combines an aqueous phosphate and silica sol as a binder but it doesn't look pourable.

Refractory binder
EP 0824092 A1

A mixture of 1 volume Clinochem P8 at 50% wt polyammonium phosphate (36% "P205") is mixed with 4 volumes of 40% wt silica sol, the mixture is then mixed with tabular alumina grains and powders such as are readily known to users and manufacturers of ceramics at between 60 ccs per kilogram to 80 ccs per kilo, (depending upon the application) and then placed in a mould. At normal working temperatures, 15° - 21°C, gelation occurs in approximately 30 minutes and can be removed within one hour. No accelerator in the form of a finely divided grain is used in this example; it appears that the silica sol causes the phosphate to gel as well as vice versa.


So what are my chances of making something pourable? How does cotronics do it?


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[*] posted on 18-5-2017 at 03:32


I've found myself needing to make a castable refractory for a large oven, it's only going to a max of 1000C but settling on a mix has been confusing to say the least.
Everyone has a different recipe or a variation on a recipe. And when there is a comment steering someone away from using a certain compound or product there's just as many declaring it's vital in their formula. Sodium silicate seems to be a staple, and yet there's so many that will swear you off it because it ages badly or is incompatible with other chemistries.??

So in the end I decided to just use a formula given to me by a refractory company. 4 parts vermiculite, 1 part ciment fondu, 1 part fireclay.
They say add 2 parts water, but it's not nearly enough I found, I needed at least 3 parts and some more to get it to flow into all the parts of the mold and not trap air. And it set so quickly that I barely had time to empty the wheelbarrow with a shovel before it was unable to trowel smooth. A spray bottle with water helped get the top smoothed over. Even when given a decent amount of water it's still a very plastic mix and heavy to work by hand. A few good bangs with a rubber mallet drove off the larger trapped inclusions of air and made the edges crisp and well formed. Adding a bit more water makes it perfectly pourable, but work quickly.

I used linseed oil as a mold release and anything coated in it repelled the concrete mix perfectly well. I suggest getting a bit on your hands before getting any of the cement on them. The Fondu is hellishly drying on the skin. I'd suggest you use a respirator and coat the paper filter with oil too. Nuthin's gettin through that!

The volume reduced by a little more than half when you add water. The original suggestion of 0.6 it's dry volume was quite a bit off.
After setting for a day the mold was opened and the block has a good hollow thunk sound to it even though it's really quite hard. It's 20l by volume but probably closer to 13 to 15 kg even without the loss of water from drying. It should provide the necessary structure and insulation without the specific heat capacity of a sand filled mix.

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[*] posted on 18-5-2017 at 04:00


There is another forum online dedicated to furnace and foundry construction. I read it sometimes, since I built a metal-melting furnace, they helped me find a brick-and-mortar castable refractory source near me. The topic of refractory compositions, and where to acquire refractory material has been covered in almost mind-numbing detail there. I suggest any member interested in furnace design read these two webpages:
alloyavenue.com

backyardmetalcasting.com
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[*] posted on 18-5-2017 at 05:50


Thanks for the link. I will definitely have a look.

In the mean time, I found this;

Keratab Fine Special
Description; A self flowing self de-airing, fine tabular alumina pourable hydraulic setting ceramic.

Applications; Pipes, thin walled tubes, lead-outs, crucibles funnels, complex shapes, burner quarls.



Maximum service temperature; 1800°C


Maximum particle size; 0,5mm


Density; 2,85g/cm³

Strength;
Air dry;          35MPa
Fired 1200°C; 80MPa

Mixing; Add +9% water, mix for 2 minutes and pour. Pot life 12 minutes. Chemical analysis;

Al2O3   97%
CaO     3%
SiO2     0.01%
B2O3    0.008%
Fe2O3   0.01%
Na2O    0.25%

Seeing that I don't know much, I have to ask, Does this need an added binder like Phosporic acid or is Calcium Oxide at 3% the binder? Surely there is something not listed here.
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[*] posted on 16-6-2017 at 07:18


I'm pretty sure the missing item is a dry phosphate salt.

People in this thread have mentioned keeping the "free silica" content low to avoid damage to the element.

I'm still not clear on what "free silica" actually means. I am looking to follow this patent;


www.google.com/patents/US5900382

where the binder consists of a wet or dry phosphate and silica sol or silica fume.


EXAMPLE 1

A mixture of 1 volume 50% wt polyammonium phosphate is mixed with 4 volumes of 40% wt silica sol, the mixture is then mixed with tabular alumina grains and powders such as are readily known to users and manufacturers of ceramics at between 60 mls per kilogram to 80 mls per kilo and then placed in a mold.

At normal working temperatures, 150 degree - 21 degree C, gelation occurs in approximately 30 minutes and can be removed within one hour. No accelerator in the form of a finely divided grain is used in this example; it appears that the silica sol causes the phosphate to gel as well as vice versa.


This doesn't seem like free silica to me. So what kind of ingredients would result in a "free Silica"?
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[*] posted on 22-6-2017 at 07:04


Quote: Originally posted by shaft  
Thanks for the link. I will definitely have a look.

In the mean time, I found this;

Keratab Fine Special
Description; A self flowing self de-airing, fine tabular alumina pourable hydraulic setting ceramic.

Applications; Pipes, thin walled tubes, lead-outs, crucibles funnels, complex shapes, burner quarls.



Maximum service temperature; 1800°C


Maximum particle size; 0,5mm


Density; 2,85g/cm³

Strength;
Air dry;          35MPa
Fired 1200°C; 80MPa

Mixing; Add +9% water, mix for 2 minutes and pour. Pot life 12 minutes. Chemical analysis;

Al2O3   97%
CaO     3%
SiO2     0.01%
B2O3    0.008%
Fe2O3   0.01%
Na2O    0.25%

Seeing that I don't know much, I have to ask, Does this need an added binder like Phosporic acid or is Calcium Oxide at 3% the binder? Surely there is something not listed here.


The lower % ingredients are not actually 'ingredients' at all but rather impurities that have to be tolerated IMO.

High temp. bricks are very expensive because the ingredients have to be very pure and there are not so many produced like the more normal (1350C) ones.

http://www.ebay.co.uk/itm/INSULATION-BRICKS-230-x-114-x-76mm...

Somewhat contradictory to this is the fact that you can purchase very high temperature refractory cement for a reasonably price.

Yob
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[*] posted on 10-7-2017 at 23:16


In my country, Australia, things are hard to get. We don't make things anymore.

I did find a castable mix called Shiraflo 90 SB made by Shinagawa, but it's still only 88% Alumina with a lot of silica rocks. I filtered those out and managed to cast a 600mm long tube with a 4mm wall.

I had to use 10% water to make it flowable enough to seep into the gap. They recommend a max of 6.5% water.

I have been trying to make a castable using Calcined Alumina 300 mesh from the pottery store and various phosphate binders as outlined in these patents;

http://patft.uspto.gov/netacgi/nph-Parser?Sect2=PTO1&Sec...

https://www.google.com/patents/US5900382


Also this where they make Aluminium Phosphate in situ;

https://www.researchgate.net/publication/274717245_High-alum...

Here in one example, they use 9.6% Phosphoric acid solution @48% wt concentration. I couldn't do that with Calcined Alumina, it was still a dry powder. I had to use about 60-70%. All results were failures.

I believe I worked out why it doesn't work. I didn't know that Calcined Alumina had such a massive surface area. So now, I want to use Tabular Alumina.

Where the hell do you find a 25kg bag of that? Not in my country. Not that I can see.


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[*] posted on 11-7-2017 at 15:37


Quote: Originally posted by shaft  
In my country, Australia, things are hard
.....
I believe I worked out why it doesn't work. I didn't know that Calcined Alumina had such a massive surface area. So now, I want to use Tabular Alumina.

Where the hell do you find a 25kg bag of that? Not in my country. Not that I can see.




I would think that calcined anything will have a smaller surface area than the stuff that is not calcined?
Perhaps I am not correct.

I suppose that links to stuf is of no use as you want to actually make your own.
http://www.ebay.co.uk/itm/Castable-Refractory-Cement-High-Al...

Yob


[Edited on 11-7-2017 by yobbo II]
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