Eva Gallagher on sun 21 nov 04
Maybe someone knows the answers to this as I have not been able to find the
answer. Lots of castable recipes call for kyanite. However I have lots of
mullite (calcined kynaite) - about 60 mesh - would that work? Presumably
during firing the kyanite when it changes into mullite, it "meshes" with
other ingredients and as the mullite is already "changed" it may not "mesh"
as well and the resultant casting is not as strong.
Does anybody know?
Regards,
Eva Gallagher
Deep River, Ontario
Logan Oplinger on mon 22 nov 04
On Sun, 21 Nov 2004 13:35:40 -0500, Eva Gallagher wrote:
>Maybe someone knows the answers to this as I have not been able to find the
>answer. Lots of castable recipes call for kyanite. However I have lots of
>mullite (calcined kynaite) - about 60 mesh - would that work? Presumably
>during firing the kyanite when it changes into mullite, it "meshes" with
>other ingredients and as the mullite is already "changed" it may not "mesh"
>as well and the resultant casting is not as strong.
>Does anybody know?
>
>Regards,
>
>Eva Gallagher
>Deep River, Ontario
Hello Eva,
A possible answer to your question.
A quick search on the internet for "Kyanite Calcined" yeilded the following
result at:
http://www.kyanite.com/refractory.html
http://www.kyanite.com/kyanite.html
...."Raw kyanite is used to assist in the firing (or fire in) of most
refractories because its large and irreversible thermal expansion helps to
counteract the normal shrinkage inherent in most other refractory raw
materials. In addition, its conversion to mullite imparts a high level of
refractoriness, thermal shock resistance, and hot strength to products made
from it.".....
Since the mullite you have will not undergo the same irreversible thermal
expansion as kyanite, if used in the castable, it cannot counteract the
shrinkage of the castable refractory as it is fired to completion or
maturity.
Similar, more detailed information is at:
http://www.digitalfire.ab.ca/cermat/material/951.html
I hope this helps.
Logan Oplinger
Another Tropical Island
Logan Oplinger on tue 23 nov 04
On Tue, 23 Nov 2004 13:54:44 +1030, Ivor and Olive Lewis
wrote:
>Dear Logan,
>Since Mullite is prepared by heating Kyanite it might be that the
>"Irreversible changes in expansion" have already taken place and that
>Mullite will be neutral in this respect on heating.
>Your quotation seems to infer that Mullite does not expand as it is
>heated, that its coeff of expansion is Zero. Is this true?
>Best regards,
>Ivor.
Hello Ivor,
The quotation, I took directly from the web site. My comments may not have
been as clearly stated as they should have been. I did not mean to say
that mullite does not expand on heating. Adding mullite to the castable
refractory mix will not have the same effect as adding kyanite. Mullite
will expand and contract on heating and cooling. In addition to thermal
expansion, the kyanite will undergo an irreversible change as its crystal
structure converts to that of mullite. This change is an increase in
volume which counteracts the decrease in volume of the other materials in
the castable as it matures during "firing in".
Logan
Ivor and Olive Lewis on tue 23 nov 04
Dear Eva Gallagher,
Looking at the molecular formulae for these two compound, Mullite
would seem to have a greater degree of refractoriness.
Only a professional petrologist would be able to make comments about
the structural changes that take place as a result of firing a mixture
with Kyanite as an ingredient as compared to one which substituted
Mullite in the refractory mixture.
Best regards.
Ivor Lewis.
Redhill,
S. Australia.
Ivor and Olive Lewis on tue 23 nov 04
Dear Logan,
Since Mullite is prepared by heating Kyanite it might be that the
"Irreversible changes in expansion" have already taken place and that
Mullite will be neutral in this respect on heating.
Your quotation seems to infer that Mullite does not expand as it is
heated, that its coeff of expansion is Zero. Is this true?
Best regards,
Ivor.
steve baker on tue 23 nov 04
Most any combination of aluminous/siliceous materials will form mullite at some high temperature. This is readily seen in the phase diagram where the mullite field is huge. The key is the refractoriness and other physical properties of the mullite formed, because the chemistry of the material is obviously not uniform throughout the field. Kyanite, when heated, forms a "breed of mullite" that will retain high physical strength at high temperature, the property known as high hot load strength. Mulcoa types of grogs are simply fired kaolin that are converted to mullite, except that the kaolins they start with are more like bauxitic kaolins with high alumina contents, and when they are burned and converted they shrink - they don't expand. If you don't want the expansion of the kyanite, then you can use the calcined form. As far as the expansion rates of fired materials, nothing with any mass comes close to zero. (Low mass space shuttle tiles may be the lowest expanding materials
we are somewhat familiar with.) The low expansion materials we are most familiar with are kiln furniture materials like cordierite, mullite, and silicon carbide. I'm not certain about silicon carbide, but I know cordierite has a lower expansion coefficient than mullite. Problem is that cordierite melts at cone 14 and softens as you approach that. In fact, it will not form in it's initial firing much before cone 13 1/2. It has such a short firing range to form cordierite that nearly all cordierite kiln furniture is actually a combination with mullite, never pure cordierite. It is too difficult to manufacture and fire consistently. If the piece does not see cone 13 1/2, cordierite will probably not form. If it sees cone 14, it may well melt to a blob.
Usually, in a castable, raw kyanite is employed to negate the shrinkage from clays and drying. Exchanging only the kyanite with mullite would result in a cast piece that would be smaller after firing at high temp. The general characteristics of the two would probably be very similar.
Ivor and Olive Lewis wrote:
Dear Logan,
Since Mullite is prepared by heating Kyanite it might be that the
"Irreversible changes in expansion" have already taken place and that
Mullite will be neutral in this respect on heating.
Your quotation seems to infer that Mullite does not expand as it is
heated, that its coeff of expansion is Zero. Is this true?
Best regards,
Ivor.
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Eva Gallagher on tue 23 nov 04
Thanks Logan, Ivor and Steve. I guess the consensus is to use it if it does
not matter that it will shrink - so I will use my mullite for making
prefired slabs for the floor and then buy kyanite for the rest. I think that
I originally bought the mullite to make flameware - but have a hard time
remembering as it was over 25 years ago.
Eva Gallagher,
Deep River, Ontario
----- Original Message -----
From: "steve baker"
To:
Sent: Tuesday, November 23, 2004 8:29 AM
Subject: Re: Mullite vs kyanite in castable
> Most any combination of aluminous/siliceous materials will form mullite at
some high temperature. This is readily seen in the phase diagram where the
mullite field is huge. The key is the refractoriness and other physical
properties of the mullite formed, because the chemistry of the material is
obviously not uniform throughout the field. Kyanite, when heated, forms a
"breed of mullite" that will retain high physical strength at high
temperature, the property known as high hot load strength. Mulcoa types of
grogs are simply fired kaolin that are converted to mullite, except that the
kaolins they start with are more like bauxitic kaolins with high alumina
contents, and when they are burned and converted they shrink - they don't
expand. If you don't want the expansion of the kyanite, then you can use
the calcined form. As far as the expansion rates of fired materials,
nothing with any mass comes close to zero. (Low mass space shuttle tiles
may be the lowest expanding materials
> we are somewhat familiar with.) The low expansion materials we are most
familiar with are kiln furniture materials like cordierite, mullite, and
silicon carbide. I'm not certain about silicon carbide, but I know
cordierite has a lower expansion coefficient than mullite. Problem is that
cordierite melts at cone 14 and softens as you approach that. In fact, it
will not form in it's initial firing much before cone 13 1/2. It has such a
short firing range to form cordierite that nearly all cordierite kiln
furniture is actually a combination with mullite, never pure cordierite. It
is too difficult to manufacture and fire consistently. If the piece does
not see cone 13 1/2, cordierite will probably not form. If it sees cone 14,
it may well melt to a blob.
>
> Usually, in a castable, raw kyanite is employed to negate the shrinkage
from clays and drying. Exchanging only the kyanite with mullite would
result in a cast piece that would be smaller after firing at high temp. The
general characteristics of the two would probably be very similar.
>
> Ivor and Olive Lewis wrote:
> Dear Logan,
> Since Mullite is prepared by heating Kyanite it might be that the
> "Irreversible changes in expansion" have already taken place and that
> Mullite will be neutral in this respect on heating.
> Your quotation seems to infer that Mullite does not expand as it is
> heated, that its coeff of expansion is Zero. Is this true?
> Best regards,
> Ivor.
>
>
____________________________________________________________________________
__
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
>
>
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>
____________________________________________________________________________
__
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>
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Ivor and Olive Lewis on wed 24 nov 04
Dear Steve Baker,
Good to have another soul in the group who is familiar with the
Al2O3-SiO2 phase diagram.
As I read it, if you can get the right molecular mixture of raw
corundum and quartz you will form only Mullite on cooling from the
melt, or if you can sinter for long enough above the Tamman
temperature of the highest melting point substance. If you have any
other molten (or sintered) mixture of these two compounds the result,
on cooling will be mixture of Mullite and one of the Silica phases on
the silica rich end of the scale or Mullite and Corundum on the
alumina side of the scale. Fortunately the Eutectic which forms on the
high silica rich side is high enough not to cause problems in our
field but in metal smelting it could be a nuisance.
A good potential refractory mixture will be baked to sinter, without
the intervention of a liquid phase. It will be compounded to eliminate
or minimise all compounds that might cause fusion. If it is initially
heated to beyond its intended service temperature it should remain
stable over a long period of time when used ot fire pottery.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
steve baker on wed 24 nov 04
Ivor,
I am not real well versed in this stuff, but you enticed me to get out an old phase diagram book and you are right on with your observations. The biggest problem with this phase diagram is that it is cost prohibitive to use pure alumina in most refractories. Therefore, you are always dealing with impurities that can really mess things up.
Steve
Ivor and Olive Lewis wrote:
Dear Steve Baker,
Good to have another soul in the group who is familiar with the
Al2O3-SiO2 phase diagram.
As I read it, if you can get the right molecular mixture of raw
corundum and quartz you will form only Mullite on cooling from the
melt, or if you can sinter for long enough above the Tamman
temperature of the highest melting point substance. If you have any
other molten (or sintered) mixture of these two compounds the result,
on cooling will be mixture of Mullite and one of the Silica phases on
the silica rich end of the scale or Mullite and Corundum on the
alumina side of the scale. Fortunately the Eutectic which forms on the
high silica rich side is high enough not to cause problems in our
field but in metal smelting it could be a nuisance.
A good potential refractory mixture will be baked to sinter, without
the intervention of a liquid phase. It will be compounded to eliminate
or minimise all compounds that might cause fusion. If it is initially
heated to beyond its intended service temperature it should remain
stable over a long period of time when used ot fire pottery.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
______________________________________________________________________________
Send postings to clayart@lsv.ceramics.org
You may look at the archives for the list or change your subscription
settings from http://www.ceramics.org/clayart/
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Ivor and Olive Lewis on thu 25 nov 04
Dear Steve,
The problem with the Al2O3-SiO2 style refractories is the limiting
service temperature imposed by the Eutectic between Cristobalite and
Mullite, ~1650=BA C
If we use natural minerals to concoct our recipes for refractories
then the best starting place is a high quality kaolin. This can be
used as a plasticising and binding agent for Sillimanite, Kyanite or
Mullite. The material resulting from firing such a mixture will not be
prone to some of the problems associated with Fireclay types of
refractory since it avoids introducing free silica which presents two
problems. The first is the potential for making low temperature
melting phases with residual Alkali metal salts or minerals from the
kaolin. The second is avoidance of phase changes associated with
silica compounds which impose mechanical stresses during heating and
cooling. If these are persistent and large in magnitude then the
structure can collapse.
At Redhill we have a wonderful natural refractory, a silica cemented
quartzite. You could fire it up to 1400=BA C and keep it there for years
with no deterioration. But let it cool once and the stuff
disintegrates.
Best regards,
Ivor.
----- Original Message -----
From: "steve baker"
To:
Sent: Thursday, 25 November 2004 4:07
Subject: Re: Mullite vs kyanite in castable
> Ivor,
>
> I am not real well versed in this stuff, but you enticed me to get
out an old phase diagram book and you are right on with your
observations. The biggest problem with this phase diagram is that it
is cost prohibitive to use pure alumina in most refractories.
Therefore, you are always dealing with impurities that can really mess
things up.
>
> Steve
>
> Ivor and Olive Lewis wrote:
> Dear Steve Baker,
> Good to have another soul in the group who is familiar with the
> Al2O3-SiO2 phase diagram.
> As I read it, if you can get the right molecular mixture of raw
> corundum and quartz you will form only Mullite on cooling from the
> melt, or if you can sinter for long enough above the Tamman
> temperature of the highest melting point substance. If you have any
> other molten (or sintered) mixture of these two compounds the
result,
> on cooling will be mixture of Mullite and one of the Silica phases
on
> the silica rich end of the scale or Mullite and Corundum on the
> alumina side of the scale. Fortunately the Eutectic which forms on
the
> high silica rich side is high enough not to cause problems in our
> field but in metal smelting it could be a nuisance.
> A good potential refractory mixture will be baked to sinter, without
> the intervention of a liquid phase. It will be compounded to
eliminate
> or minimise all compounds that might cause fusion. If it is
initially
> heated to beyond its intended service temperature it should remain
> stable over a long period of time when used ot fire pottery.
> Best regards,
> Ivor Lewis.
> Redhill,
> S. Australia.
>
>
______________________________________________________________________
________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your
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> settings from http://www.ceramics.org/clayart/
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>
>
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Ivor and Olive Lewis on thu 25 nov 04
Dear Logan,
Thanks for your clear exposition of the relationships between Kyanite
and Mullite.
I realise that the information was contained in the quotation,
Best regards,
Ivor.
steve baker on thu 25 nov 04
Ivor,
Coke oven refractories are good examples of what you are referring to. Here you have nearly 100% pure silica products that are maintained at temperatures above for cristobalite inversion for years even decades. I knew of one such operation in southern Ohio that had not cold cooled their ovens for like 45 years!
Steve
Ivor and Olive Lewis wrote:
Dear Steve,
The problem with the Al2O3-SiO2 style refractories is the limiting
service temperature imposed by the Eutectic between Cristobalite and
Mullite, ~1650º C
If we use natural minerals to concoct our recipes for refractories
then the best starting place is a high quality kaolin. This can be
used as a plasticising and binding agent for Sillimanite, Kyanite or
Mullite. The material resulting from firing such a mixture will not be
prone to some of the problems associated with Fireclay types of
refractory since it avoids introducing free silica which presents two
problems. The first is the potential for making low temperature
melting phases with residual Alkali metal salts or minerals from the
kaolin. The second is avoidance of phase changes associated with
silica compounds which impose mechanical stresses during heating and
cooling. If these are persistent and large in magnitude then the
structure can collapse.
At Redhill we have a wonderful natural refractory, a silica cemented
quartzite. You could fire it up to 1400º C and keep it there for years
with no deterioration. But let it cool once and the stuff
disintegrates.
Best regards,
Ivor.
----- Original Message -----
From: "steve baker"
To:
Sent: Thursday, 25 November 2004 4:07
Subject: Re: Mullite vs kyanite in castable
> Ivor,
>
> I am not real well versed in this stuff, but you enticed me to get
out an old phase diagram book and you are right on with your
observations. The biggest problem with this phase diagram is that it
is cost prohibitive to use pure alumina in most refractories.
Therefore, you are always dealing with impurities that can really mess
things up.
>
> Steve
>
> Ivor and Olive Lewis wrote:
> Dear Steve Baker,
> Good to have another soul in the group who is familiar with the
> Al2O3-SiO2 phase diagram.
> As I read it, if you can get the right molecular mixture of raw
> corundum and quartz you will form only Mullite on cooling from the
> melt, or if you can sinter for long enough above the Tamman
> temperature of the highest melting point substance. If you have any
> other molten (or sintered) mixture of these two compounds the
result,
> on cooling will be mixture of Mullite and one of the Silica phases
on
> the silica rich end of the scale or Mullite and Corundum on the
> alumina side of the scale. Fortunately the Eutectic which forms on
the
> high silica rich side is high enough not to cause problems in our
> field but in metal smelting it could be a nuisance.
> A good potential refractory mixture will be baked to sinter, without
> the intervention of a liquid phase. It will be compounded to
eliminate
> or minimise all compounds that might cause fusion. If it is
initially
> heated to beyond its intended service temperature it should remain
> stable over a long period of time when used ot fire pottery.
> Best regards,
> Ivor Lewis.
> Redhill,
> S. Australia.
>
>
______________________________________________________________________
________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your
subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
>
>
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>
______________________________________________________________________
________
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> Moderator of the list is Mel Jacobson who may be reached at
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______________________________________________________________________________
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