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Author: Subject: castable refractory mixtures
Rosco Bodine
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[*] posted on 5-1-2005 at 20:07


Quote:
Originally posted by Cyrus
I've been looking at the ACPH binders, and the preparation seems simple enough, but isn't boiling phosphoric acid bad for glassware? :(

If glass can't be used, what can?


That high of a temperature is not required for making the ACPH , all you are doing is forming an aqueous solution of the complex by adding phosphoric acid to a solution of the aluminum salt . Glass or even a stainless steel cooking pot should do fine . Think of it as making a pot of mildly corrosive ACPH "salt solution" . I'm not really sure it will even be viscous when freshly made , at the dilution it is to be used . I have an impression of the ACPH solution as sort of being the "special binder water"
you use to wet down your aggregate mix or "mud" . If the other components are
chosen correctly , it should cold set like mortar . But it will also be a sort of "greenware" from which the water will have to be first slowly dried out by evaporation at normal temperature , then slowly baked out to further dry , and brought up to firing temperature to vitrify and complete the hardening .
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[*] posted on 19-1-2005 at 12:38


Don't know why I haven't posted this link before :

Castable refractories for a Soda kiln

Must be a pretty tough hot-face mix to stand up to salt glazing!
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[*] posted on 10-3-2005 at 12:10


Today I made my very first refractory mix:

-33% portland cement
-65% Perlite (I think it is perlite, it's white, doesn't melt in blue flame and it's very light)
-2% 400 mesh Al powder
(measurements are ballpark ranges, I may have used quite a bit more portland cement now that I think about it ...)


It's curing now ... hoping for beginners luck :cool:
It foamed up pretty good, like chocomousse :P air bubbles in it no bigger then 1mm diameter.

EDIT: I'm confident that it was a very bad conductor for heat, but it was as weak as chalk, even after 30 hours of curing.
It broke in several pieces as I tried to remove it from it's form :(

For this relalatively low temperature application (RBF heater), I think it's best to stick to a normal concrete mix with SiO2.
Would 50/50 portland cement to sand do?



[Edited on 12-3-2005 by BrAiNFeVeR]




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[*] posted on 18-4-2005 at 18:48


Ok, this is kind of a late reply, but if by RBF you are referring to round bottomed flask, than any real refractory is overkill.

Pyrex (this is what you are using, correct?) melts/softens/oozes/because there's no real melting point at around 800 deg. C. That's barely even glowing red (at least my pyrex). Concrete contains the flux CaO, which doesn't really even become active until much higher temperatures, maybe 1200 or 1300 deg. C. That's why axehandle's concrete based furnaces don't melt. So, pretty much anything you use will work fine unless you are planning to melt the flask.

I would suggest a mixture of sodium silicate and perlite. Extremely insulating, cheap, hard at low temps, easy, and it won't melt at your low temperatures. Now, you may have some spiffy setup where the heating wire itself gets really hot (as in bright yellow glowing hot) but even then I think you may be ok.

Ok, I've "finished" my study of kaolin (china clay type stuff) based ceramics, using talc, graphite, alumina, silica, and grog as additives. (and some other stuff) I've got a paper and graphs mostly written up. They should be coming soon. It's kind of abstract, but very interesting. Oh, BromicAcid, that will be part of the paper for you.

Now I should get slip casting some crucibles.

I'm also building a new furnace with diatomaceous earth, perlite, and aluminum chlorophosphate (I'm pretty sure I'll use that binder).




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[*] posted on 20-4-2005 at 06:28


The concrete is holding up pretty good, besides the fact that it cracked.
After 3 days curing I placed the resistor wire and during the first firing I noticed the crack, but it is still very strong.

I think it just needed some flexibility ...

To prevent it from developping a second crack and thus fall apart, I wrapped it in glaswool isolation with some metal wire strapped around it to keep the glaswool in place and give the entire contraption something to hold it together.
Even the concrete against the red hot wire seems to hold up good.

Heat transfer efficiëncy is about 80% so that's pretty good too :-)
It takes some fidling with the concrete , the wire and some electronics, but I'm very happy with the results.




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[*] posted on 20-4-2005 at 10:00


I haven't had time to pursue my experiments with the ACPH binder
and the various refractory mixtures in which it would likely be a good performer .

I did find it mentioned in one of the ACPH related patents that it performed the best of any of the known binders which were tested for their bindering properties on particles of fullers earth type refractory materials . That statement lends support to my idea of using the calcined fullers earth porous absorbent type materials which are used as artificial soil for aquatic plants , and for maintenance of athletic fields . Some smaller percentage of the
lower fired " kitty litter " type of material which disintegrates when saturated and helps to fill the small spaces between the larger particles of the calcined material supposedly strengthens the bond of the entire mass
which is bindered with ACPH .

Also gleaned from the patents regarding portland mixtures , the white portlands that are used for tile grout and other finish work are better performers in refractories where portland is to be a
part of the mixture , than are the dark gray colored portlands .

[Edited on 21-4-2005 by Rosco Bodine]
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[*] posted on 20-4-2005 at 15:27


I've found diatomaceous (fuller's) earth at the $1 store as kitty litter. It doesn't seem to fall apart at high temperatures. Is that what you were referring to as the kitty litter material which fell apart at high temps?

I found some phosphoric acid also at the dollar store... its only about 1.3 M H3PO4 from what I can tell though.




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[*] posted on 20-4-2005 at 16:48


DE and fullers earth are different materials . DE filter aid for pool filters is
a white high silica porous material like microscopic snowflakes made of quartz .

Fullers earth is used for kitty litter of the clay type , but this is simply dried clay and
will get pasty when saturated with water .

The calcined material has been high fired and it retains its structure when wet , and
it is refractory , to what extent I'm not sure , but it should be a good candidate
in a refractory aggregate since it is porous and has some strength of its own ,
where some other porous materials like perlite are not really structural and could weaken the composite if used in too great a proportion , especially if never fired hot enough to actually melt the perlite to leave glass lined voids where once was the perlite .
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[*] posted on 15-5-2006 at 20:11


Roscoe did you ever settle on a satisfactory castable refractory?

Axehandle used Portland cement as a binder in his castable refractory, and he estimates that his furnace reached about 1300C. But as Organikum has stated Portland cement is not supposed to hold up at high temperatures. Can anyone explain axehandle's success with Portland cement?

I have done some rough calculations on the power demand of a simple tube furnace, neglecting heat loss at the ends. I used this equation, taken from a heat transfer text:

P = 2(pi)kL(Ti-To)/(ln Ro/Ri)

with k = thermal conductivity of the insulation
pi = 3.14
L = length of furnace
ln = natural log
Ri = inside radius of furnace insulation
Ro = outside radius of furnace insulation

This formula is interesting to play with. What struck me was that the conductivity of the insulation is a bigger factor than the thickness of the insulation.

The power calculation results confirmed to me how important it is to get that conductivity as low as possible.

[Edited on 16-5-2006 by Magpie]




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[*] posted on 15-5-2006 at 21:18


Where's Axe's post saying so?

It's certainly possible if you don't use much, or use a lot of alumina or other refractory oxide.

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[*] posted on 15-5-2006 at 21:33


Tim,

See:

http://www.sciencemadness.org/talk/viewthread.php?tid=2171#p...

4th post down.




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[*] posted on 21-7-2007 at 12:34


I am going to need some castable (or rammable) refractory as well in the near or less near future (electric furnace! I have 100m 1mm Kanthal A-1 wire, good for 1400°C!).
I have been told that perlite is not refractory enough to be used for fire-facing refractories since it is essentially a sort of glass and melts below 1000°C.

I am thinking about using vermiculite (the stuff that is often used for packaging dangerous liquid chemicals) as the porous constituent of the refractory since Ullmann says it withstands up to 1200°C which would be adequate for my temperature requirements. The higher price and lower availability of vermiculite compared to perlite does not matter since I'm going to build one or two furnaces and not a series.

What do you think about that? Can vermiculite be used as a superior replacement for perlite?
My refractory mix would then consist of vermiculite, portland cement or fire cement and maybe bentonite (cat litter), and water.




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[*] posted on 21-7-2007 at 13:12


Vermiculite is about as bad as perlite. It just stains more (due to Fe(II) content, which is a flux on par with Ca).

Don'tm even bother with PCC!

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[*] posted on 21-7-2007 at 13:27


What do you mean by PCC?

What does the Fe content of the vermiculite have to do with its refractory properties? References suggest vermiculite is more heat resistant than perlite. Obviously the Fe does not act as a flux or its amount is not sufficient for this if the vermiculite is so heat resistant.
I have even read that vermiculite is used as a constituent of insulating refractory bricks.




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[*] posted on 21-7-2007 at 14:17


You've never dealt with iron if you don't think it's a flux!

I tried vermiculite with fireclay and it got nice and gooey up in the yellow to yellow-white range (around 1200C).

"Insulating" and "refractory" are two words that mean different things in different contexts. Plain old fiberglass wool is an excellent insulator, and refractory enough to insulate a cooking oven. Sure as hell isn't suitable for a kiln though.

PCC = Portland Cement Concrete.

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[*] posted on 21-7-2007 at 18:47


GC I also have some plans for building a tube furnace. I was going to do that this summer but I am way behind schedule.

My maximum temperature was to be 1350C. What is your maximum design temperature?

I have most of the parameters worked out but have not yet purchased any materials. I had also selected 1mm Kanthal A-1 for a heating element.

For a castable refractory I had identified 3 possibilities that are available to me:

Harbison-Walker Mizzou, 1650C
Harbison-Walker Kast-O-Lite, 1428C
Thermal Ceramics Kaolite Light Weight, 1372C

Thermal conductivity for these are, respectively, 1.04, 0.41, and 0.22 W/m-K.

There should be similar products available in your part of the world. They run about $40/25 kg.

From the reading I have done it is important that any refractory touching the heating element be low in iron (<1%).

I can provide more design "rules of thumb" if you are interested.

[Edited on by Magpie]




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[*] posted on 21-7-2007 at 19:02


Not quite on topic of refractory mixtures but for a small tube furnace you should consider buying a ceramic tube. It's easy to insulate it with glasswool and you're nearly done. Just arrange the heating elements and set up the controllers.
Of course it is a bit expensive solution but IMO it makes the construction and future maintenance a lot easier. :)

Also, garage chemist as you live in Germany you can get ceramic tubes with ease from Haldenwanger.
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[*] posted on 22-7-2007 at 01:15


I already have built a super-low-cost tube furnace for up to 1100°C (it uses NiCr 80/20 as the wire). I have not mixed or purchased any castables for it, I simply built it out of "Ytong", which is a form of highly porous brick that is used to build entire houses out of due to its good insulating properties. It is sold in large blocks very cheap and can be cut easily with a woodsaw.

It is not meant to be made hot, but it does not get soft at all even at 1100°C and insulates reasonably well.
The downside is that it shrinks at such a heat and develops cracks. The tube furnace broke in two after a few firings. I put strong wire around it to hold the pieces together. It holds up fine like that now.
Although I still dont have the quartz tube for it! I cant use the tube furnace for anything at the moment. Its going to take at least two weeks from now for the quartz tube to arrive.

Here is the link to the pictures of my tube furnace:
http://www.versuchschemie.de/topic,9256,0,-Bau+eines+Rohrofe...

As you can see, the fire-facing side has been covered with a layer of fire mortar sold at the home store for repairing ovens. It was difficult to apply, and I certainly wouldnt do it again if I had to rebuild the tube furnace. The Ytong had to be drenched in water in order to not suck the water right out of the fresh mortar, and all that water had to be baked out again before the first firing which took a whole day at medium power setting.

The heating spirals at the ceiling of the furnace are held in place by a lot of hooks which are made of the same wire and glued into the Ytong with waterglass. (Dont use waterglass with Kanthal wire, it will ruin it! Kanthal must not contact any materials containing SiO2 or silicates when hot, the manufacturer warns against that. Nichrome seems to hold up fine to waterglass and SiO2 though.).
As seen in the last picture, the spirals have sagged considerably in some places which led me to use even more wire hooks for the ceiling spirals. They dont sag any more now.


IPN, I plan on making a new tube furnace for at least 1200°C using Kanthal wire wound around a ceramic carrier tube. Unfortunately, Alsint tubes are extremely expensive (over 100€). I am hoping to get one cheaper via ebay.
Haldenwanger also doesnt seem to deal with individuals.

The wire around the Alsint tube will be embedded in a mass made of MgO and Al2O3 (as Brauer recommends it) and thermally insulated by loosely poured Vermiculite in a box. So no castable required for this one either.
Read up in Brauer, they have an excellent section on building all sorts of laboratory furnaces!

I also want to build a conventional, top-loading furnace similar to axehandles and Davsters design, and for that I will need some sort of insulating refractory castable. The fire-facing side will probably be made of chamotte bricks, but those dont insulate and will have to be surrounded by castable.
Though I see that the castable wont be subjected to the full heat of the furnace, and perlite will probably do the job with that design...

[Edited on 22-7-2007 by garage chemist]




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[*] posted on 23-7-2007 at 11:07


Quote:
There should be similar products available in your part of the world. They run about $40/25 kg.


Unfortunately, that does not seem to be the case. The is no seller for such products in germany that sells to individuals.
Comparable products are called "Feuerleichtbeton" and are used exclusively for industrial size furnaces in the steelmaking industry and by furnace builders.
No individual builds their own high-temperature furnaces in germany, and those who do dont have castable refractories at hand and have to use replacement materials like Ytong which are not meant to be heated, or refractory bricks which however are not insulating.
Hence I have to mix my own castable refractory.

I again pose my question: Has anyone successfully used vermiculite as the insulating component of such mixes?
On the site of a manufacturer of refractory materials I have again found evidence that vermiculite is more heat-resistant that perlite and would therefore be a preferred ingredient.




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[*] posted on 14-9-2007 at 17:15


Another question: I seem to be having some language problems concerning the definition of some of the raw materials. Dictionaries say that fireclay=chamotte. But the chamotte that ceramics stores carry is pre-fired, e.g. grog. In germany only the name chamotte is known.
I bought 10kg of fine chamotte (0 - 0,2mm) for use as grog in admixture with Ciment Fondu that I got from france. The Ciment Fondu says on the sack that it has to be mixed with refractory aggregates in order for the cured mortar to be refractory. So I bought the chamotte.

But what is the fireclay that is used in the first bucket furnace here:
http://metal.duncanamps.com/foundry/furnace_bucket.php
Is that pre-fired chamotte as well? Because I have never seen unfired fireclay offered for sale anywhere.




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[*] posted on 14-9-2007 at 17:21


Fireclay is unfired clay with a high vitrifying point (> cone 30). Try looking for raw clays, especially china clay (kaolin).

Chamotte = fired clay = grog.

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[*] posted on 14-9-2007 at 17:40


OK, so the stuff I got is prefired.

Is fireclay supplied as a dry powder or as a moist paste like normal clay?
Do you happen to know the german term for fireclay? Dictionaries translate is as Schamotte, which obviously only means the prefired grog. I need to know the term for the unfired material.




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[*] posted on 14-9-2007 at 19:28


I don't.

Fireclay can be had dry or pugged. Often the pugged material is a blend for specific pottery uses (e.g., stoneware), so do ask your supplier if it's raw.

Other terms that come to mind: "lute" (as used to seal flasks in Ye Olden Days) was sometimes a clay composition I think, and you may also encounter the term "loam" (IIRC, Bessemer used the term), which is a sand and clay type of soil.

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[*] posted on 14-9-2007 at 19:53


Something like a china clay or kaolin would make a very high grade fireclay. Any clay with a fusion temp over 1600 C. You can add alumina to make it even more refractory.

You can get these clays either dry in bags, or wet from a pottery supply.

[Edited on 9-14-2007 by Eclectic]

Vermicullite and Perlite have fairly low melting points. Diatomite or Celite are supposed to be a good refractory insulation as a loose fill. If you added a bit of colloidial silica binder to slurry of diatoms and had a way to cast it while vacuum filtering the water out, you could make bricks and sheets of the stuff.

(Sorry if I'm being redundant 12AX7)

[Edited on 9-14-2007 by Eclectic]
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[*] posted on 15-9-2007 at 05:14


Silica is bad news when using Kanthal wire, look it up in the Heating alloys handbook from the Kanthal website- they say embedding masses containing free SiO2 rapidly destroy the heating wire at high temperatures. Any SiO2 must be bound as silicate or compounded with lots of Al2O3.

So the embedding mass for the heating wire in my planned tube furnace can't have SiO2 in it. Perlite both contains SiO2 and only resists temperatures up to 800°C, so a castable mix containing perlite can only be used for back insulation, the mix directly surrounding the heating wire will consist of equal parts calcium aluminate cement (Ciment Fondu) and ground chamotte.

I will first build a small test furnace anyway, that uses Nichrome wire. Nichrome resists SiO2.




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