iandol on sun 18 mar 01
Dear Gavin,
For the most part I cannot disagree with what you have to say.
But the from the information available to me it is impossible to believe =
that the Hydrogen and Oxygen ions in the Kaolinite structure are in the =
form of H2O (H-O-H). We are dealing with Hydroxyl Ions (-O-H). You will =
find diagrams in Lawrence and West, Hamer and Hamer, Kingery et al, =
Dana's Mineralogy, Barsoum and Cardew, to name but a few, to support =
that. They all show Hydroxyl ions coordinated with the aluminium ions. =
Sorry Sir, no free water until there is a chemical reaction.
So the processes which occurs on heating seem to be Dehydroxylation =
(minus 2*-O-H) and Dehydrogenation (minus 2-H) not dehydration. Were I =
to heat Blue Vitriol, which has Water of Crystallisation, the chemical =
process would be Dehydration.=20
If you look at the phase diagrams, Nos.313 and 314, in Levin et al you =
will note that the base line temperature is 1400 deg C. And as you say, =
there are many reactions before a mixture containing kaolin would reach =
that temperature, including the quartz phase changes which would show on =
a diagram with a base line at 0 deg C.=20
Thanks for your response. I am sure, together, we are making a lot more =
people think a little more deeply about the technology and science of =
ceramics.
Best regards, and have a happy Easter
Ivor
Gavin Stairs on mon 19 mar 01
At 01:51 AM 3/18/01, iandol wrote:
>Dear Gavin,
>For the most part I cannot disagree with what you have to say.
>But the from the information available to me it is impossible to believe
>that the Hydrogen and Oxygen ions in the Kaolinite structure are in the
>form of H2O (H-O-H). We are dealing with Hydroxyl Ions (-O-H). You will
>find diagrams in Lawrence and West, Hamer and Hamer, Kingery et al, Dana's
>Mineralogy, Barsoum and Cardew, to name but a few, to support that. They
>all show Hydroxyl ions coordinated with the aluminium ions. Sorry Sir, no
>free water until there is a chemical reaction.
>So the processes which occurs on heating seem to be Dehydroxylation (minus
>2*-O-H) and Dehydrogenation (minus 2-H) not dehydration. Were I to heat
>Blue Vitriol, which has Water of Crystallisation, the chemical process
>would be Dehydration.
>If you look at the phase diagrams, Nos.313 and 314, in Levin et al you
>will note that the base line temperature is 1400 deg C. And as you say,
>there are many reactions before a mixture containing kaolin would reach
>that temperature, including the quartz phase changes which would show on a
>diagram with a base line at 0 deg C.
>Thanks for your response. I am sure, together, we are making a lot more
>people think a little more deeply about the technology and science of ceramics.
Hi Ivor,
Yes, you are right: what we are talking about is not water hydrogen bonded
to the aluminosilicate structure. The kaolinite structure implies a
dissociation of the H:OH bond, and a re-association with the unsatisfied
bonds of the aluminosilicate plates of the layered kaolin structure. And,
to be frank, I have never tried to imagine the process by which a feldspar
crystal reorganizes itself to accept the water. It must have something to
do with opening the lattice to leach out the (Na, K, Ca, etc), and
replacing it with water. The initial reaction with the alkali and water
may be the agency for the initial splitting of the water into H and NaOH
for example. The H could then replace the Na in the lattice, which would
imply a greatly stressed lattice, and an impetus to further leach and
hydrolize. The whole reaction would be exothermic, and the reversal, or
partial reversal in the clay maturation reaction is therefore endothermic,
and happens only at elevated temperature. We know at what temperature this
occurs: it is the calcination temperature of our clay body. Thus we know
it is not well defined, and depends on the alkalis and alkali earths
present, among other factors, but the sign of it is that the clay becomes
insoluble in water. What is going on is the replacement of the water with
the alkali fluxes, and the release of water from the combination of the
volatile H and OH. Water is also be released by the clay directly, as it
contains both H and OH in proportion. The volatiles will readily combine
to form water as they leave their binding sites, and indeed, this reaction,
which is effectively a combustion reaction, may be a prime determinant of
the reaction dynamic, supplying local energy to break the clay bonds.
So I agree with you that the reactions are more complex than a simple
dissociation of whole water molecules. However, the stoichiometry of the
reactions makes clear that water is leaving the clay. What is left behind
is a radically restructured aluminosilicate. Instead of the planar
structure of kaolinite, we have a three dimensional lattice, the structure
of which will depend on the fluxes present, among other factors. It will
be aluminosilicate, but it may involve different proportions of aluminum
and silicon, with the excess atoms being forced out of the lattice to form
a second phase. So there is a lot of ion exchange and spatial movement
going on. Characteristically, such a reaction will happen over a
temperature range, and will take time to go to maturity. The time will be
dependant upon temperature in an exponential way. In most clay bodies
these reactions will take place over a range from maybe cone 08 through 1
or so, at a guess, although some changes will take place all the way up to
complete vitrification, which may be at cone 15 or 20. Most if not all of
the water (i.e., H and OH) will have left by some low cone like 04,
however. I'm quite willing to be corrected about the exact temperature
limits of water reactions in firing clay, but I would not expect to find
any large amount to be released above common bisque points. The reactions
above that point will involve mostly reorganization of the silica-alumina
and flux phases. At vitrification, the structure must be quasi-homogenous,
and at least one phase must be fully liquid.
All the best to you,
Gavin
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