claudia schaerer on wed 26 aug 98
Hello!
I am a Canadian potter working in Ghana with a local NGO
(non-governmental organization) known as GRATIS (Ghana Regional
Appropriate Technology Transfer Industrial Service). GRATIS has several
ITTUs (Intermediate Technology Transfer Units) established throughout
Ghana. One of the ITTUs is implementing a graphite crucible project
that is having problems with the composition of various bodies. The
60-70 kg capacity crucibles perform moderately well when melting
aluminum or bronze, but when melting cast iron, multiple cracks occur.
Let me explain the process as best as I can:
Bauxite = 60.7%, alumina, ball clay and kaolin are all available locally
in Ghana. Only the graphite is imported from U.K. (is actually mined in
Madagascar). The bauxite is calcined, then all ingredients are mixed in
an old bakery mixer, before the crucible is formed in a Jolly press,
which is basically a spinning mold with the arm going down. When dry,
the crucible is glazed (green) on the outside and sometimes on the
inside.
The crucibles are fired in a pottery kiln at 1250 - 1300C. The
crucibles are fired in old 500 kg crucibles used as saggers in the kiln.
The crucibles are carefully stacked inside these large crucibles,
stuffed with graphite all around, and the top sealed with clay so no
oxidation will take place during the firing. The firing can last
anywhere between 20-30 hours. Many tests have been done. Aluminum can
be melted 2-3 times with the same crucible and then the crucible cracks
horizontally and vertically. When melting cast iron, the cast iron
cannot even be brought to the melting point before the crucible begins
to crack. The cracking crucibles are 60 and 70 kg capacity. The 5 kg
crucibles do not crack.
Some of the furnaces being used are tilting. Those that do not tilt -
the crucible is removed with tongs. The furnaces are quite basic - a
fan and burner blows the flame in a circular motion inside a round
refractory kiln, with a cover to keep the crucible heated evenly.
Furnace oil or diesel fuel is used to operate the furnace.
The plan is to produce 25, 60, 70 and 100 kg crucibles.
Formula #1 below was developed by someone previously, but did not
perform well:
MATERIAL MASS (%)
Ball clay 38%
Bauxite Grog -30 +60 21%
Bauxite Grog -60 to dust 7%
Graphite medium 17%
Graphite fine 17%
Formula #2 below was developed by the Ceramic Dept. of the University of
Kumasi, Ghana, but performed even more poorly than Formula #1:
MATERIAL MASS (%)
Ball clay 45%
Bauxite Grog +30 7.5%
Bauxite Grog -30 +60 7.5%
Graphite medium 28%
Graphite fine 12%
Any suggestions on how to tackle this problem would be most
appreciated!!
* What about the idea of firing the crucibles in graphite or without
graphite?
* Should the crucibles be fired hotter, cooler, or under a reducing
atmosphere?
* Is glazing needed inside and/or outside, or should the crucibles be
self-glazing?
* Could be crucibles be pounded into the mold rather than using the
Jolly process? Could a hydraulic press be used to press the crucible
into the mold?
My feeling is that the formulas are too low in alumina or too high in
silica. The graphite crucible does seem to be the crucible of choice
since it transfers heat so well.
Please send any suggestions to: FernandDaigle@hotmail.com
Thanks in advance!
Fernand Daigle
c/o Bolga ITTU, Box 298
Bolgatanga
Ghana, West Africa
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Gavin Stairs on thu 27 aug 98
At 09:36 AM 8/26/98 EDT, you wrote:
....
>My feeling is that the formulas are too low in alumina or too high in
>silica.
What a lovely problem! Thanks for sending it to the list.
I suspect that several variables must be right to make the large crucibles.
My assumption is that the result is to be a classic sinter, with the
maximum graphite content, and the minimum silica content, because of the
problem of silica phase transitions. The question then becomes, what is
the ideal proportional mix of large and small graphite powders, for the
optimal packing density, and what is the proper binder. The firing and
forming questions may then become more clear.
The best reasonable binder is probably a mullite, or enhanced alumina
kaolin. If this is the case, then you might achieve it by a mixture of
kaolin and bauxite. This would have to be milled quite fine, and fired
very high, perhaps above 1300C. It would also require pressure packing of
the mould, possibly by pounding in a damp state.
If the kaolin-bauxite system were not practical, some improved properties
might be obtained by replacing some of the kaolin with more bauxite and
plastic clay, but only enough to improve the plasticity and green strength.
For fluxing to reduce the firing temperature it might be possible to add
alkali from ash or as soda ash or salt in order to introduce a fugitive
flux. This would not be intended to remain in the matrix at high temperature.
I would do the following: I would do two sets of investigations
separately, and then together.
I would first try to make a high strength mullite bar by combining the
kaolin, bauxite, plastic (ball) clay and minor fluxes. These I would
approach by line blends made up into test bars for rupture strength.
Then I would try to combine the graphite powders in different proportions
to see what proportion gave the densest compact. This could be done with
damp graphite tamped into a small beaker, levelled carefully and weighed,
or the same with a small addition of plastic clay as a lubricant.
I would then combine the best result of these two tests, and then do a line
blend to see what proportions gave the best fired strength in bar samples,
and ultimately in crucibles.
If you need help in establishing some starting points, or anything else,
for that matter, please feel free to e-mail me off list. I hope this is of
some use to you.
Gavin
Gavin Stairs
Stairs Small Systems (S3)
921 College St., # 1-A
Toronto, Ontario, Canada M6H 1A1
(416)530-0419 stairs@stairs.on.ca
D. Kim Lindaberry on thu 27 aug 98
claudia schaerer wrote:
>
> ----------------------------Original message----------------------------
> When melting cast iron, the cast iron
> cannot even be brought to the melting point before the crucible begins
> to crack. The cracking crucibles are 60 and 70 kg capacity. The 5 kg
> crucibles do not crack.
> * Is glazing needed inside and/or outside, or should the crucibles be
> self-glazing?
I can't really answer your questions about crucible formulas but I have
some observations. I don't believe that any crucible I've ever used was
glazed. I might be wrong, but I don't think glazing is what you want to
do to a crucible. Considering that bronze has a melting temp of approx.
2200 F and iron is syrupy at 2400 F and is best poured around 2900 F.
Either of these temp will strip the glazes right out of a crucible. As
for a glase on the outside of a crucible, well even if it didn't run
right off of the sides at theses temps, whenever you touched the
crucible with tongs it would stick to it. Nix the glazing of the
crucibles!
As for pouring iron from a crucible. While it may be possible, it isn't
very pratical. Iron is a rough and corrosive material. It'll destroy any
crucible after on a few pourings. If your goal is to make iron casting
you need to investigate how to build and operate a cupola. A cupola
(iron furnace) melts iron in a chamber using basically coke and forced
air. The book, "Living Materials, A Sculptor's Handbook," by Oliver
Andrews has a basic diagram of how to build a cupola (pronounced >
coop-o-la)
Good luck qith your project
Kim
--
D. Kim Lindaberry
to visit my web site go to: http://www.johnco.cc.ks.us/~klinda
to send e-mail to me use: mailto:klinda@johnco.cc.ks.us
D. Kim Lindaberry on sat 29 aug 98
DS man wrote:
>
> a glaze on the outside of the crucible is vital when the crucible is made of
> graphite. graphite will oxidize rapidly at temps > 1800F and evaporate. when
> the graphite is gone, the metals will wet the crucible and erode it quickly.
> however, most metals do not wet graphite and it has excellent corrosion
> resistance.
DS,
So are you reccommending that a graphite crucible be used for frequent
iron pouring? If the outside of a graphite crucible needs to be glazed
to prevent oxidtion of the graphite, what exactly prevents the oxidation
of the interior of the crucible before the iron reaches a point where it
actually melts? It seems to me that this surface is just as vunerable to
oxidation as the exterior. I don't know. I'm just asking and trying to
better understand what you are saying. Do you cast iron using a graphite
crucible yourself on a regular basis? I've only ever cast iron using a
cupola and I'd be very interested in an easier way of casting it.
I've been under the impression to melt iron in a crucible you would
either trash the crucible after only a few casting, or you would need
specialized equipment that would give you a vacuum or controled
atmosphere, or an electric induction furnace. Are cupolas passe?
Kim
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