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Author: Subject: castable refractory mixtures
rift valley
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[*] posted on 11-12-2004 at 07:51


I made my first mold today. It came out better then expected but I don't think the Al was hot enough. Do you guys know of any inexpensive ways I can make a thermometer that could withstand molten Al. It would make the whole process much easier if I knew the them. of the melt. I have been taking a bunch of pictures of the whole process, I can post them if anyone else is interested.
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[*] posted on 14-12-2004 at 11:01


Check out this video from popular science.

You can click on the link on the page to play the video, an aluminum I-beam gets ate up badly over the course of an hour, the link is about how aluminum amalgamates with mercury to form aluminum oxide which flakes off, just as we discussed earlier in the thread.




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Rosco Bodine
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[*] posted on 14-12-2004 at 11:26


Probably so as not to greatly publicize the truth of what is an obscure and highly destructive knowledge which would not be wanted to be better known , the authors fudged on the technical details in their misinformation about "mercury paste" . The active agent is not any such fictitious "mercury paste" at all . The amalgamating agent is in fact a liquid which consists of corrosive sublimate ( they don't call it that for nothing ) which is mercuric chloride , dissolved in hydrochloric acid . The liquid is effective as an aerosol , a stream from a "water pistol" or syringe , or dripped or painted onto the aluminum object to be destroyed . The reaction zone migrates into fresh unreacted material , and the white oxide comes off rapidly like a white "snake" similar to a pyrotechnic snake . Five cents worth of the corrosive "weapon" can very quickly destroy millions of dollars worth of aluminum whatever , and do it silently ,
covertly , reducing the target to a large pile of fluffy white powder .
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[*] posted on 14-12-2004 at 14:41


Interesting. Just how quickly does it work? It's something to keep my mind nervously alert on my next trans-Atlantic airline flight.

I've seen HgCl2 applied as a methanol solution make aluminum do the flaking into powdery oxide trick, but the effect didn't seem truly catalytic. The reaction stopped after a time, while there was still plenty of (beverage can) aluminum left. I supposed that the mercury was being carried away mingled with the aluminum oxide.




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Rosco Bodine
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[*] posted on 14-12-2004 at 15:02


The air reaction is definitely a visible rate , and to my eye looks more rapid than the rate of attack of concentrated NaOH upon unamalgamated Al . Would have to take a fairly heavy piece of aluminum barstock and time the reaction to see exactly what the actual rate is and it is likely dependant upon temperature and humidity . IIRC the white powder snake would come off the surface of the Al at a length of the ashlike material of maybe one inch per minute , so it isn't all that slow . It is definitely fast enough to consume an aircraft in a time measured in hours not days .
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[*] posted on 14-12-2004 at 15:51


I think the time lapse on that video at that site is 1 hour. As long as the inital concentration of mercury is low this would be a good way to make the aluminum oxide as the starting material for refractories, although I doubt it, does a similar thing happen for magnesium? I've never heard about it though.

[Edited on 12/14/2004 by BromicAcid]




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Rosco Bodine
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[*] posted on 14-12-2004 at 16:15


Never tried magnesium but it would probably be even more vigorous based upon the activity , however the solubility rate of the metal in the amalgam is the limiting factor . Also any impurities in the alloy could cause complication , so the more pure the metal the better . The mercury layer should be fairly heavy for best results , a good fluid layer of liquid mercury in excess of the amount required to maintain the ongoing solution of the metal , so that the amalgam maintains a freely fluid consistency and doesn't form a sludge with the metal being dissolved and transported through the layer . The mercury layer is actually in fluid motion as it completes a loop from the metal being dissolved upwards to the air where it is dumped as oxide , and then returning to the surface of the metal to pick up more , sort of like a conveyor belt for metal ions .
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[*] posted on 15-12-2004 at 20:19


I need to make a lid for my furnace, I am going to use a terra cotta flower pot base and I will fill it with some fireplace refractory crack repair cement mixed with mostly perlite, just enough cement to hold it together. I saw this on another website for a refractory mix but it didn't look too sturdy, for a lid it should be fine though. I'll let you know how it works out
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[*] posted on 15-12-2004 at 21:39


There is a cheap material that is used for landscaping on baseball fields and golf courses , to surface running tracks , maybe tennis courts , ect. to keep rain from making the ground muddy and slippery . It is a fullers earth material sort of like kitty litter , but it has been high fired in a furnace until it vitrifies partially , and forms a porous ceramic gravel like material . It is called Quick Dry , made by a company called Profile , and sells for about six dollars for a fifty pound bag .
A variety of the material is sold as soil for aquatic plants , because it won't disintegrate in water like ordinary unfired clay . This grog like material may have usefulness as an aggregate in refractory materials . It is principally magnesium aluminum silicate granules which are 75% empty space , about half open pores and half closed pores , a hard gritty material of density 2.5 . I haven't been able to find anything about it being used as a refractory component by googling it , but it would seem like a good candidate refractory since it is 74% silica and 11% alumina , along with other high melting point materials in the remainder . And it is cheaper than perlite or vermiculite , plus it has structural and dimensional stability , in that it does not swell from moisture . There are several mesh sizes available , but the Quick Dry is the finest mesh . I will probably try this material as at least part of the aggregate and insulating material in experiments with refractory mixtures . My idea here is that it should give strength and good thermal properties cheaply , as well as providing porosity for escaping water vapor when the "green" material is baked out and brought up to vitrification temperature .
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[*] posted on 16-12-2004 at 18:09


It has fluxes in it. (MgO)

Fluxes aren't good in refractories.

The furnace will probably melt at a lower temperature than you'd like.

(Yes, I'm back.);)




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Rosco Bodine
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[*] posted on 16-12-2004 at 18:59


It may have fluxes , but I have no information to suggest it does . The labeling says it is 100% blended clays which have been calcined at high temperature . Anyway MgO is not a flux . MgO is a very high refractory used for embedding the nichrome element inside metal sheaths for hotplates , and also used to line high melting alloy electric arc furnaces .

Maybe you are thinking of PbO or some of the alkali oxides or fluorides .

BTW , I spotted something interesting on the shelf in the pet section , a kitty litter that is pure silica gel , snow white material having refractory properties , melting point 1700 C . It is sort of coarse
about like aquarium gravel . http://www.ultrapearls.com
I have seen much finer mesh silica gel sold in bulk in hobby shops for use in drying flowers for dried flower arrangements . A mixture of these silica gels maybe with perlite and perhaps DE along with a compatable binder would make a good refractory .

[Edited on 17-12-2004 by Rosco Bodine]
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[*] posted on 17-12-2004 at 17:40


Here's a quote from

http://www.digitalfire.ab.ca/cermat/oxide/mgo.html

~
MgO (Magnesium Oxide, Magnesia)
Family: Flux
Weight: 40.300
Expansion: 0.026
Fusion: 2800C


(Sources: Talc, Dolomite, Magnesium Carbonate)

-Together with SrO, BaO and CaO it is one of the Alkaline Earth group of oxides. It has a cubic crystal structure.

-Like CaO, MgO is refractory at lower temperatures, so much so that it can be used to increase opacity, to perform as a matting agent (i.e. magnesium carbonate), and act as a check to glaze fluidity in a manner similar to alumina (e.g. to prevent devitrification or the tendency to produce crystalline surfaces). When mixed with CaO, it is not as refractory.

-It can act as a catalyst in low temperature bodies assisting the conversion of quartz to higher expansion cristobalite (which reduces crazing).

-In high temperature glazes it acts as a flux (beginning action about 1170C) producing viscous melts of high surface tension and opaque and matte glazes. Like CaO, its melting action drastically accelerates at high temperatures.

-The surface tension of MgO-containing melts is less of a problem in reduction.

-Zircon and Magnesia melt at 2800C making them the highest melting oxides. Remarkably, MgO readily forms eutectics with other oxides to melt at surprisingly low temperatures.

-It is valuable for its lower expansion and crazing resistance. When introduced into a glaze it should preferentially replace calcia, baria, and zinc before the alkalis to maintain surface character. Adding too much will generally move the surface texture toward matte or dry.

-MgO is a light oxide and generally is a poor choice for glazes to host bright colors. However, it does work well in earthtone and pastel glazes, especially in high temperature reduction firing. Likewise, it may be harmful to some under-glaze colors.

-Does not volatilize.

~

I don't have my oxides confused. :P

In other places the fluxing action is not mentioned at all so I understand the confusion, and yes it's used in bricks. Here's a lil' trick to remember which oxides are glass formers, which are fluxes, and which are neutrals.
R= metal O=oxygen

R2O- flux
RO- flux
R2O3- neutral
RO2 - glass former

Of course there are going to be some exceptions, but in general this works.

That silica gel you mention looks interesting- It's got to be pretty porous, and that's good for insulation.

[Edited on 18-12-2004 by Cyrus]




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Rosco Bodine
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[*] posted on 17-12-2004 at 19:09


I think the special case where MgO would act as a flux would be those specific proportions where the eutectic effect is the cause . A similar thing can occur with aluminum phosphate and alumina if the proportions favor the low melting eutectic , but in proportions which are comfortably away from that eutectic the same materials are refractory . The ratios are everything , the same for silicates .
Anyway at some point in the vitrification there are refractory compositions formed from magnesia and other materials , whether the magnesia is a flux or not , it does not simply melt and solvate the other materials but forms higher melting complexes which get even more solid with rising temperature . Perlite does the same thing and so do several other materials which first begin to flow and then resolidify as they react with other materials .

The factor I have been looking at carefully is the acidic or basic or neutral nature of the materials present and how the reactions proceed in the presence of moisture to produce a cold setting effect , and then what follows during intermediate heating as moisture is driven off , and vitrification occurs . The different reactions in each stage have to proceed in a way they overlap so that the composition doesn't crumble . And the products of the final vitrification reactions need to be unchanged by falling temperature and re-exposure to moisture .

There is some magnesia content in nearly all the common clays . I don't think it will be any problem in a phosphate or combination phosphate and silicate complex bindered refractory . Even DE has some magnesia content and DE is most definitely refractory . What is going on in glazes is similar but I think the difference with magnesia as a refractory is all about the proportions and the nature of the other materials being quite different . Even powdered borosilicate glass which has a lot of fluxing action and low melting point can actually be used as a binder in high temperature refractories ,
within certain proportions . Just because the powdered glass is the first thing to melt doesn't cause the aggregate with which it "alloys" to fall apart in the fire .
Actually I am trying to avoid magnesia in the refractory compositions I am considering , but for reason of pH considerations causing a short working life for the cold setting of wet mixtures containing basic refractories . I am trying to keep to neutral and acidic components to have extended working life for the wet mixture , and to accomplish the overlapping sort of binding schemes I mentioned earlier . Unless magnesia has been "dead burned" and granulated fairly coarse , it is too chemically active when wet and reacts too rapidly with binders so that many of the compositions set up much too quickly . Anyway , even though MgO is listed as a constituent in certain clays , I am reasonably certain that this is the type of mineral analysis expression as is common usage in the fertilizer industry , referencing Magnesium content expressed as the oxide , even though the magnesium is likely present as a silicate complex or some other compound that is anything else other than actual MgO , if you follow what I mean . I am experimenting with this material anyway , and it either works or it doesn't . A good old blow torch test on some of the granulated material side by side with some sand might be revealing . I can check the pH of the wetted material too and see what that reveals .
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[*] posted on 18-12-2004 at 07:21


These are the products that they sell at home depot, that I was talking about. http://www.rutland.com/sfp4/link4.htm#600

The castable refractory goes for 13 bucks a bucket and a 32oz bucket of the furnace cement is only three bucks!

This product is made in nearby Vermont so I dont know if it is a nationwide home depot product of just the ones nearby.

P.S. What do you guys know about kaowool (wool made out of ceramic shards I think?) I have seen pictures of furnaces where there is just folded kaowool and no castable refractory.

[Edited on 18-12-2004 by rift valley]
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[*] posted on 18-12-2004 at 08:26


Yeah that Home Depot stuff is strictly regional availability only . There are other things the same way .

The availabiliy issue is exactly why I am sorting through patents looking for the most useful mixtures of OTC materials which can be combined in "off label use" compositions which will serve the purpose . There are three or four compositions which I believe are workable in that regard , and one of the most interesting involves a phosphate modified portland cement binder . It is tedious reading through the patents and picking out the few that seem most useful . When I complete the reading and making notes , a summary of what I find will be posted here .

Regarding the ceramic blanket , it is good stuff , particularly in thick layers and even better if coated and sealed into a rigid structure . Prices vary widely . eBay is your best buy from what prices I have seen . The fiber I believe is aluminum silicate . There is another material called rock wool carried by some building suppliers for similar purposes as fiberglass but has twice the fire resistance . Rock Wool might be useful as a fiber reenforcement in refractory or as a backup insulation for the ceramic blanket .
Rock wool can be hard to find though because it has almost been compltely replaced by fiberglass for ordinary uses where the higher temperature rating for rock wool isn't needed .
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[*] posted on 20-12-2004 at 15:44


Rockwool can be found cheaply at ANY online hydroponics store and even some local gardening centers. It's used as an inert wicking medium in soiless gardening.
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[*] posted on 20-12-2004 at 17:54


Quote:

P.S. What do you guys know about kaowool (wool made out of ceramic shards I think?) I have seen pictures of furnaces where there is just folded kaowool and no castable refractory.

Ah yes, Kaowool. I recently aquired a substitute for it called Superwool (http://www.607max.com/newpdfs/ukblanket.pdf). I tried to get Kaowool, but apparently it's phased out here due to safety problems with it (silicosis etc).

It's a blanket, 610mm x 7320mm x 25mm.... very reminiscent of white rockwool. Physical properties are in PDF above. Cost me SEK1000 (~= €100).

I intend to line a large metal bucket with it for a big propane based aluminum melting furnace.
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[*] posted on 20-12-2004 at 18:26


I have heard that ceramic fibre really must be coated, for safety. The fibres can break down through thermal cycling, and can be very nasty if they get into your lungs. Something called 'ITC100' seems to be a common coating. Its also supposed to improve furnace performance by re-radiating heat from the walls.
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[*] posted on 20-12-2004 at 21:45
ITC-100


Yes, ITC-100 seems like a near monopoly on rigidizers, and rigidizers are necessary.
Satanite is another product, but I haven't been able to find much on that except that it contains crystoballite and quartz.

IIRC ITC-100 is made of zircon/zirconia, (colloidal?)silica, alumina, sodium silicate, magnesium silicate, and water, but I have no exact proportion data, just general ranges, (ie 5-15%) and have done a test on this, it's not perfect yet, but moderately ok so far.

Rosco, I like the idea of the silica gel spheres, but won't they tend to absorb a lot of water- I mean that's what they are made for. :D. This may make mixing/casting difficult or it may need to be dried/fired very slowly to let off all of the water. Also, I take it you plan on using calcium aluminate or a phosphate as a binder? Where can you get this stuff?

[Edited on 21-12-2004 by Cyrus]




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[*] posted on 21-12-2004 at 03:41


This is confusing.... the information I have is that rigidizers don't have to be used unless the fiber tends to disintegrate yielding harmful particles. The insulator I bought is supposedly normally used without any rigidizers what so ever. Seems more research is in order.

Cyrus, I found this PDF with Satanite technical data: http://www.hwr.com/ci/datasheetsv1/SATANITE.pdf

EDIT: There is an exact chemical analysis in there that should make it possible to mix something similar after a trip to the pottery supplier...


[Edited on 2004-12-21 by axehandle]
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[*] posted on 21-12-2004 at 16:33


Quote:
Originally posted by Cyrus

Rosco, I like the idea of the silica gel spheres, but won't they tend to absorb a lot of water- I mean that's what they are made for. :D. This may make mixing/casting difficult or it may need to be dried/fired very slowly to let off all of the water. Also, I take it you plan on using calcium aluminate or a phosphate as a binder? Where can you get this stuff?

[Edited on 21-12-2004 by Cyrus]


With a very few exceptions , ( for example US6284688 ) , nearly all of the refractory mixtures will contain some amount of water to be baked out slowly , before the material can be gradually brought up to vitrification temperature . The more porous the aggregate the more of a problem that can be , although the rate at which the moisture is absorbed can also be slow enough that it is desirable in helping to set the binder . The order in which the aggregate and fillers and binders , liquids ect. are mixed is important to regulate the setup time and minimize the water requirement for some compositions . Some advantage may be taken of first dampening slightly the most porous materials and then coating them with another finer dry ingredient which tends to seal them before that aggregate is waterlogged .
The slower absorbtion will then help the cold setup which occurs later , and reduce the amount of physically held water to be baked out . Silica gel formed in situ can be a portion of the refractory binder material in silicate bindered refractories where water glass is the used in the "cement" portion of the composition .
The combination trisodium phosphate / waterglass binder looks more interesting because of simplicity .
I am more in favor of the phosphate binders because they are cheaper and provide better performance . The phosphate binder that is looking interesting there is the ACPH . The phosphates which go through a sol/gel scenario and chemically change through rising temperature seem most promising .
As to where to get them , I don't know . But I have on hand precursors which may allow me to synthesize the materials . For Aluminum Chloride solution I will probably try the method of US4447351 , and then treat the concentrated solution with 85% phosphoric acid to make the ACPH , or I will make the ACPH directly from aluminum amalgam and the acids .
Some ACPH related patents are GB1435988 , GB1322724 , GB1429555 , US4046581 .

But the most simple of all the refractory binders which I found in all the patents is to modify the high temperature chemistry of portland cements , using phosphates , such as described in GB1449484 . The difficulty to be overcome there is the quick setting nature of such compositions . I want a composition which has a good working life , perhaps a half hour to one hour after mixing . So I am studying the patents and looking to adapt the techniques and mixtures for my purpose .

Regarding the ceramic wool , it definitely should be sealed with some solid refractory coating . Several places I have seen it stated not to use the bare material as a hot face , but rather as an insulation layer , even as an outer covering for a cast liner , but definitely not to use the bare ceramic material to line a forge or kiln . It is dangeous because of particulates , and there is mention that the uncoated material also physically deteriorates badly over time , making its replacement more expensive than coating it to prevent that deterioration . It is also reported that fluxes attack the bare hot material like gasoline attacks styrofoam .
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[*] posted on 21-12-2004 at 17:24
fiberfrax, etc.


Not to disagree, just thought I'd say that I've seen the untreated blanket material used on gas furnaces and as the refractory for the combustion chamber of oil burning furnaces. It holds up OK to a jet of burning #2 fuel oil. It is a little fragile to the touch - even new, though.
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[*] posted on 22-12-2004 at 08:40


The ceramic material will degrade after time and how fast probably depends on the intensity of the conditions , but the danger of respirable particulates carries warnings even for handling the cold , new material . Given that certain hazard is increased for the uncoated hot material , especially in contact with a steady draft in a fuel fired furnace , it would seem prudent not to use the ceramic blankets without some sealant . Some of the blanket materials may even contain a binder component which conveys flexibility and helps hold the fibers together during installation , but after the installed material is brought up to high temperature , that binder degrades and with it the physical integrity of the blanket is degraded in the absence of some additional sealant which coats and rigidizes the structure in its post fired permanent installation . To me ,
coating the ceramic fiber material with a heat setting refractory sealant makes good sense from a safety and durability standpoint , and probably also enhances the refractory properties .

Some additional patents of interest regarding aluminum phosphate binders are US3999999 , US4059669 , US3899342 The last of these three is interesting because it mentions something which I have also seen referenced in some other patents relating to phosphate binders . The addition of even a small amount of citric acid to a phosphate bindered refractory , for example one quarter of one per cent of the dry weight of the refractory composition , can result in doubling the fired strength of the virified composition . The solubility of the phosphate is enhanced by formation of an organic phosphate complex which decomposes at high temperatures and modifes the crystallization at high temperatures of the phosphate binder . My speculation about this is that a fine dispersion of graphite probably results from the thermal decomposition of the small amount of organic phosphate , and this modifes the crystallization of the phosphate binder at high temperature , and may even result in the formation of refractory carbides which also contribute to the strength of the refractory .
update: There is another patent which simplifies further the preparation of phosphate binder systems GB595768 . Earlier in this thread the term sol/gel binder system was mentioned , and there is a good description of the concept in US2522548 which explains how phosphate binders work , and how this is analogous to other binder systems which also work on the sol/gel principle .

[Edited on 23-12-2004 by Rosco Bodine]
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[*] posted on 5-1-2005 at 16:32


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?




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[*] posted on 5-1-2005 at 17:04


Some glass is eaten by hot phosphoric acid, some is not. Pyrex is to some extent (not too much, I think), Simax is said to be inert. How about steel? I know that the acid is used for passivating it, but I’m not sure how well it would actually hold it at high temperatures.
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