Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
333
REFRACTORY MATERI AL
Field of Invention
This invention relates to the use of a relatively
stable acidic aqueous colloidal zirconia sol as the bonding
medium for specific refractories.
Back~round and Prior _
A procedure that is well-known has been used in the
past for making cexamic shapes, namely mixing a binder and a
gelling agent with a refractory and allowing the mix to
chemically set or gel to form a bond and then firing the
body. Typically many shapes have been made using sodium
silicate r potassium silicate, colloidal silica, and
hydrolyzed ethyl silicate as bonds. However, to obtain the
greatest refractoriness of a body, a bond leaving a residue
of a more refractory o~ide is preferable. For example,
alumina and zirconia produce high temperature bonds for
reractories.
U.S. 4,025,350 shows the use of an aqueous solution of
a zirconium salt with a gelling inducing agent and a gelling
delaying agent and a refractory powder to form a refractory
article. This composition requires additional gelling
agents for control thereby increasing costs and control
problems. Also the by-products of the ~elat.ion of the
zirconium salt would need to be eliminated from the
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refractory during firing. There is also an added cost of
the zirconium salt versus the oxide.
U.S. 4,201,594 describes ~he binding of refractory
materials using zirconium salts and incorporating gelling
agents and gel delaying agents. For the same reasons these
compositions are less than desirable.
U.S. 2,984,576 describes an unfired mixture of a
refractory material bonded with a zirconia or hafnia sol in
which the percent of solids in the dispersed phase is at
least 30%. This patent does not describe the specific
refractories useful with the present stable acidic zirconia
sol but on}y as a bond for a variety of refractories.
U.S. 3,758,316 describes the process for producing a
refractory from a refractory powder and a binder precursor
which would include colloidal zirconia, but also requires
the addition of a gelling agent.
Brief Summary of Invention
The basic principle of the present invention is to
make a refractory mix comprisiny a refractory material and a
stable acidic zirconia sol having a fine particle size and
acidic pH. The refractory is composed of an active portion
and, if desired, a relatively inert portionO
Detailed Description of Invention
One would expect that highly refractory materials would
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be relatively inert to the zirconia sol. Howe~er, i~ has
been found that a number of refractories are not totally
inert to the sol and actually react with the sol to cause
yelation of the sol. Very rapid gels or slow ~els can be
produced depending upon the particular type of active
refractory, its particle size distribution, and its
percentage in the refractory mix. Some examples of active
re~ractories which will cause gelation with the zirconia sol
are alkali and alkaline earth metal aluminates, silicates,
zirconates, stannates, titanates, zirconium sllicates and
oxides. Specific examples include calcined maynesium oxide,
electrically ~used magnes.ium oxide, calcium oxide,
electrically ~used calcium oxide, mono calcium aluminate,
calcium aluminate cements, fused cordierite, high alkali
glasses, magnesium aluminate, magnesium aluminum silicate,
magnesium zirconate, magnesium silicate, magnesium zirconium
silicate, magnesium ferrite, magnesium titanate, magnesium
stannate, calcium zirconate, calcium silicate, calclum
zirconium silicate, calcium titanate, calcium stannate,
barium zirconate, barium aluminum silicate, barium
aluminate, barium zirconium silicate, barium stannate,
barium titanate, barium silicate, strontium zirconate,
strontium stannate, strontium zirconium silicate, strontium
silicate, strontium aluminum silicate, strontium titanate,
electrically fused calcium oxide stabilized zirconia,
electrically ~used magnesium oxide stabilized zirconia, iron
chromite, Zeolex 23, wollastonite, bentonite, strontium
aluminate, forsterite, calcium aluminum silicate, ~luorspar,
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fluorbarite, lithlum zirconate, li.thium aluminate, lithium
silicate, lithium aluminum silicate, lithium titanate,
lithium zirconium silicate, and other refractory materials
which are reactive with the zirconia solO Some relatively
non-reacting refractory materials are monoclinic zirconia,
hafnia, alumina, bauxite, mullite, sillimanite, zircon,
ceria, thoria, silioon nitride, silica and other minerals
which do not contain any large amounts in their structure of
the alkaline and alkaline earth metallic oxides or
impurities present that may react with the sol.
~ t is also possible to use this system as a bond or
various fibers made from aluminosilicates, low alkali
glasses, alumina, zirconia, silica, and various organic
fibers such as cotton, rayon, nylon, other synthetic fibers.
The aqueous zirconia sols used in the examples given in
this specification are acidic in nature ranging in pH from
about 0.3 to 6Ø The particle size of the zirconia
particle is generally small, on the order of 25 millimicrons
and smaller. The sol is stabilized by acids such as nit.ric,
hydrochloric, acidic, etc. The gelling action of the sol
with the "active" refractory is believed to he due to a
reactio~ of the acid with the "active" refractory, producing
a "salt", which rPaction raises the pH thereby lowering the
sol stability. Also, the salt formed possibly catalyzes the
gelling of the sol. This gelling action bonds the refrac-
tory into a strong body.
Several ~actors govern the characteristics of the
refractory body bonded with the zirconia sol. The type
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333
of acid in the sol, the particle size and age o~ the
501, the percentage of zirconia in the sol, the percentage
and type of "active" refractory in the mix, its particle
size distribution, temperature, and mixing conditions.
The listing of potential "active" refractories shows
the presence in many cases of an alkaline or alkaline earth
type oxide present in the structure of the refractory or
that the "active" re~ractory is subject to reaction with an
acid. The presence of such "active" refractories, serving
to react with the sol not only causes gelation but also
might serve as sinteriny aids Eor certain refra~tory
systems. The comparative scratch hardness of bonded
refractory shapes after firing serves as a measure of
sintering action by the "active" refractory.
One procedure for utilizing this invention is to
produce cast refractory shapes by mixing the zirconia
sol with at least one "active" refractory. The balance
of the refractory may include a relatively inert refractory.
In some instances, depending upon the nature of the active
refractory, the total refractory may be of the active type.
In other instances, the "active" refractory may be a very
minor portion of the total refractory in the mix. Particle
size distribution and chemical nature of the active
refractory are two of the major factors in determining the
amount of "active" refractory constituent.
Various refractory shapes can be cast using this inven-
tion to produce practical products, such as metal melting
crucibles, boats, tundishes, pouring ladles, pouring cups,
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333
tubes, rods, slabs, bricks, saggers, kiln ~urniture, kiln
car tops, open hearth door facings, kiln parts, pouri~g
nozzles, furnace liners, and others. Such m:ixtures can also
be used to cast dental and jewelry molds ~or metal casting.
In particular, some of these mixes are especially suitable
for molds Eor casting superalloys, stainless steels,
niobium, tantalum, titanium, and molybdenum. By selection
of a high temperature inert refractory, or low~activity
'lactive" reEractory, such as zirconia, hafnia, ceria,
alum.ina, yttria, lanthana, a Eoundry mold can be
produced having an extremely high PCE value ancl havlnc~
low reactivity to some oE the above-mentioned reactiv~
metals.
If desirable, pressing mixes can be made which will
"set" or "gel" in predetermined times in order that a
refractory shape may be made by pressing and then become set
or gelled.
Thin or thick Eilms may be made ~rom mixes which will
"set" or "gel" in predetermined t.i.mes in order tha-t a
re~ractory shape may be made by pressing and then become set
or gelled.
Thin or thick Eilms may be made from mixes which may be
cast on a belt or ~orm and then becoming gelled or set.
Coatings may be dipped or sprayed on to a form or shape, and
then allowed to gel.
Mixes according to this invention may be Eormed into
shapes by injecting molding. Present ceramic injection
molding techniques usually call Eor various temporary bonds
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for the refractory body to allow for ease of molding.
Examples are costly waxes, resins, plastics, etc. These
organic materials are burned out without leaving a high
temperature bond, and shrinkage occurs during loss of
organic material. The present invention provides a "green"
bond and a fired bond in the refractory body. This
technique can be used to mold various-intricate shapes such
as spindles, nozzles, ceramic cores ~or metal castings,
ceramic turbine blades and vanes, shell mold parts for metal
casting, and various other shapes as desirecl
~ primary application ~or this invention is to make
cast refractory bodies which will set ox gel.l at controlled
times. A proportion of "active" refractory may be adjusted
according to the set time required for the mass. This
percentage varies with the particular "active" refractory.
The resulting refractory mix can be then mixed with a
suitable amount of the zirconia sol to a heavy pouring
consistency and poured or cast into a mold form and al]owed
to set. Particle size distribution oE the refractory ~ix
may be varied according to the desired results, strength,
settling within the mold, and gel times. It is usually
advantageous to allow adequate time for satisfactory mixing
of the refractory before casting into a mold. This depends
upon the size of the mold and the equipment used to handle
the mix. If a small volume hand mix is used, mixing can
usually be carried out in a very short period of time such
as one to two minutes and then the mix can be adjusted to
gel or set very rapidly. I prefer a relatively fast gel
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time of 5 to 30 minutes for relatively fast production of
shapes. It may be desirable to remove bubbles from the mix
and to incorporate suitable wetting and defoaming agents to
make a relatively bubble-ree or void-free mass. Time may
be needed to completely wet in the mass and to deair before
casting can be made. Ideally, gelation should occur ~s soon
as practical after pouring.
To illustrate this invention, the data in Table 1 shows
the percentage of active refractory that might be mixed with
an inert reractory, such as tabular alumina, tc~ produce
specific set or gel t.imes. The refractory is mixed Witll the
zirconia sol containing 20% ZrO2 and having a pH of 0.6.
The alumina portion was composed of 50% 325 mesh and finer
tabular alumina and 50% 60 mesh and finer tabular alumina as
supplied by Alcoa. The active refractory percentage is
calculated on the basis of the total amount of refractory
used for the final mix.
The samples indicated in Table 1 all had good green
strength and when fired separately to 1200E', 1800F,
2000F, and 2500F had excellent fired strengths.
Another series of similar experiments to those in Table
1 were carried out according to Table 2 in which the tabular
alumina refractory base was 25% 325 mesh and finer and 75%
60 mesh and finer. This Table shows the gel times for the
various mixes using the active refractory. These were mixed
with the same 7irconia sol as was used in Table 1. After
gelling these samples had excellent green strength and after
firing to the same temperature condi.tions had excellent
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fired strength. In all cases, the stxength at 2500F was
greater than that fired to temperatures below 2500F.
Some unique characteristics were noted about the
compositions described in Table ~. ~ series of test
specimens approximatel~ thick, 1" wide and 2.375"
long were prepared in a mold using the same compositions as
prepared in Table 1. They were allowed to set after
gelation for 30 minutes and then removed from the mold.
After removing from the mold, the specimen was set out in
the air to air dry overnight and then oven dried for 4 hours
at 120C to remove all the water Erom the shape and then
placed into a dessicator ~or cooling. It was then removed
and immediately measured. It was noted that all specimens
showed some shrinkage Erom the mold dimension on the order
of about one-half to one percent. After the specimens were
dried, they were then heated to a temperature of 1200F and
maintained at that temperature for 2 hours and then allowed
to cool to room temperature and remeasured. After measur-
ing, the specimens were then reheated to 1800F and held Eor
2 hours at temperature, cooled, and then remeasured. This
same heating was carried out separately at 2000F and
2500F, after which time measurements were made on the
specimens. It was noted that on many specimens some very
small to fairly sizeable permanent expansion occurred after
cooling. The data in Table 2 shows the permanent expansion
obtained on a number of the specimens cast. The negative
value indicates shrinkage. The remainder of the figures
indicate permanent expansion.
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It can be observed from this Table that some substan-
tial expansions occur on certain specimens. These expan-
sions are not necessarily related to the proportion of
active refractory but are definitely attributed to the
presence of the active refractory. Each composition pro-
bably acts in a different manner and produces different
reaction products which govern the amount of expansion
obtainable. This may be a means for minimizing shrinkage
during firing of refractory bodies utilizing this zirconia
sol bonded system. Normally when considerable sintering
occurs on firing a refractory to a high temperature,
considerable shrinkage occurs with the sintering. It should
be noted that several compositions in the tabulation show
relatively low shrinkage even when ~ired at 2500F. Table 3
shows a similar series of measurements made on specimens
using the tabular alumina refractory containing 2S% 325 mesh
and finer and 75% 60 mesh and finer ~article sizes with the
corresponding "active" refractory.
The following are examples of other refractory mixes
used with the acid stabilized zirconia sol and illustrating
the use of "active" refractories.
EXAMPLE I
Composition:
Electrically fused calcium
oxide stabilized zirconium
oxide - 325 mesh 30 grams
E'used Magnesium Oxide -
325 mesh 1 gram
Tabular alumina 60 mesh and
finer 150 grams
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Tabular alumina - 28 + 48
mesh 120 grams
This refractory composition was mixed wi~h 35 ml acid
stabilized zirconia sol containing 20% ZrO2. I~ was then
poured into a rubber mold. The gel time was determined to
be approximately 5 minutes. After 30 minutes, the sample
was removed from the mold and by means of a diamond saw was
cut into test specimens for modulus of rupture measurements.
Unfired strength of this mix was approximately 57 psi.
Samples were fired to 2500Fr held for two hours and cooled
to room temperature, and modulus of rupture was determined
as 575 psi. A similar firing to 2700F for two hours and
then cooling showed a modulus of rupture of 910 psi. A
firing to 2900F for two hours and cooled showed a modulus
of rupture of 1888 psi.
EXAMPLE II
Composition:
Tabular alumina - 325 mesh240 grams
Electrically fused magnesium
oxide 2 grams
This was mixed with 45 ml of the same zirconia sol as
in Example 1. The gel time on this mix was approximately
4-1/2 minutes. The green modulus of rupture was not deter-
mined but specimens fired to 2000F for two hours and cooled
showed a modulus of rupture of 234 psi. Firing to 2500F
for two hours and cooled showed the modulus of rupture to be
1164 psi. Firing to 2700F for two hours and cooling showed
a modulus of rupture of 2995 psi. A specimen fired to
2900F for two hours showed a modulus of rupture of 5674
psi .
r
~ 3 3 3
EXAMPLES III
Composition:
FF zirconium oxide t calcium
stabilized, 325 mesh 170 grams
- 50 + 100 mesh - 325 mesh 160 grams
- 12 + 35 mesh - 325 mesh 80 grams
This refractory composition was mlxed with 30 ml of the
zirconia sol used in Example I. The gel time was 8 minutes.
The modulus of rupture measurements after firing specimens
to the particular temperatures for two hours and testing
after cooling are as follows:
Modulus of Rupture
pounds ~ _nch
Unfired 278
2000F 479
2500F 1888
2700~F 2019
2900F 2623
Test specimens from Examples I, II, and III were also
measured before fi.ring and after each firing and showed the
following percentage permanent expansion (-~) or shrinkage
( ):
Firing _ Examples
Temperature F III III
2000 +0.08 -0.09 -0.11
2500 +0.2g -0.46 -0.51
2700 ~0.40 -1.60 -0.50
The development of some permanPnt exparlsiorl could be
helpful in eliminati.ng or minimizing settling and drying
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333
shrinkage on some compositions, thereby increasing dimen-
sional accuracy in making shapes.
The following is an example of typical shell mold
system possible by the use of this invention:
Composition:
Electrically fused calcium oxide
stabilized zirconium oxide2000 grams
Zirconia sol containing 20~ ZrO2 500 ~rams
Concentrated hydrochloric acid 17 ml.
Wetting agent - Sterox NJ15 drops
This slurry was prepared to a viscosity of 3~ seconds
as measured by the Zahn #4 cup. Sheets of wax, approxi.-
mately l/8" thick and 2 1/2" wide by 5 l/2" long were dipped
into this slurry and immediately stuccoed while wet with a
- 50 + 100 mesh ~irconia of the same composition as used in
the slurry. After dipping several specimens, the slurry was
diluted with the zirconia sol to a viscosity of 15 seconds
and a further dip was applied after the first dip had dried
overnight. While the second coating was still wet, it was
stuccoed with a relatively coarse zirconia granule of a - 12
+ 35 mesh of the same composition as the material in the
slurry. This was repeated for additional coatings and a
final seal was applied, making a total of 6 stucco layers
and 7 slurry layers. Two dips were applied per day through
the final dip. The dipped specimens were then allowed to
dry for 2 days and the wax was melted out. The specimens
were then cut into strips 1" wide, dried, and then tested
for unfired strength. Six specim~ns were tested giving an
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average modulus of rupture value of 500 psi. Additional
specimens were fired for 2 hours to various temperatures
beginning at 2000F and cooled back to room temperature and
tested. The MOR after firing to 2000F was 220 psi. The
MOR after firing to 2200F and cooling to room temperature
was 300 psi. The MOR increased to 1200 psi after firing to
2500F. This indicated a substantial strength was
obtainable on a shell mold composition utilizing this
invention.
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Table 1
Type of Active Wt. % Active
Sample Refractory RefractorY Gel Time
1 Calcium Aluminate Cement 5.0 Immed.
2 Calcium Aluminate Cement 1.0 8 min.
3 Calcium Aluminate Cemen~ 2.0 4 min.
4 Calcium Aluminate Cement 0.5 45 min.
Magnesium Zirconate 1.0 6 min.
6 Magnesium Zircona~e 0.5 2 hr. -~
7 Magnesium Zirconium Silicate 1.0 Overnight
8 Magnesium Zirconium Silicate 5.0 55 min.
9 Magnesium Zirconium Silicate 7.5 12 min.
MagnesiumlZirconium Silicate 10.0 10 min.
11 MgO T-139 - 325 Mesh 1.0 90 sec.
12 MgO T-139 - 325 Mesh 0.8 2 min.
13 MgO T-139 - 325 Mesh 0.6 4 min.
14 MgO T-139 - 325 Mesh 0.4 10 min.
Calcium Zirconium Silicate 1.0 Overnight
16 Calcium Zirconium Silicate 5.0 20 min.
17 Calcium Zirconium Silicate 3.0 28 min.
18 Calciurn Zirconium Silicate 7.5 7 min.
19 Calcium Zirconate lo0 Overn:lght
Calcium Zi.rcona~e 5.0 1 hr.
21 Calcium Zirconate 7.5 90 sec.
22 Calcium Zirconate 10.0 Immed.
23 CaO 1.0 Instant
24 CaO 0.1 1 hr. +
CaO 0.25 60 sec.
26 CaO 0.5 Instant
27 Iron Chromite 1.0 Overnight
28 Iron Chromite 5.0 30 sec.
29 Iron Chromite 3.0 Overn.ight
Iron Chromite 4.0 2 hrs.
31 Iron Chromite 5.0 9 min.
32 Iron Chro~lte 6.0 5 min.
33 Zeolex 23 1.0 Overnight
34 Zeole~ 23 5.0 Instant
Zeolex 23 2.0 8 min.
36 Zeolex 23 3.0 Lnstant
37 Winco Cordierite 3- 200 Mesh 1.0 Overnight
38 Winco Cordierite - 200 Mesh 5.0 1 hr. +
39 Winco Cordierite - 200 Mesh 7.0 12-15 min.
Winco Cordierite - 200 Mesh 8.0 8 min.
41 Wollastonite 1.0 7-11 min.
1. Manuactured by CoE~ Minerals, King of Prussia, Pa.
2. Trademark of J.M. Huber Corp., Baltimore, Mdo
3. Manufactured by Winco Minerals, E. Aurora, N.Y.
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333
Table 2
_.
Permanent Expansion in Thousandths ol Inch
at Firing Temperature
el Time 1200F 1800F 2000F 2500F
3 4 min. .010 .003 .001- .005-
2 8 min. .002 .001- .003 .004-
6 min. .006 .004 .003 .006-
7 Overnight .004 .003 .004 .009-
10 min. .008 .001 .001- .037-
11 1 1/2 min. .005 .004 .011 .011-
12 2 min. .008 .003 .002 .016-
13 4 min. .002 .000 .002- .020-
14 10 min. .002 .005 .003 .011-
lG Overnight .0:1.6 .01S .016 .013-
18 7 min. .006 .009 .005 .026-
17 28 min. .005 .006 .004 .022-
19 Overnight .004 .003- .001- .007-
27 Overnight .008 .010 .000 .012-
2 hrs. .002 .005 .009 .002-
31 9 min. .003 .001~ .008 .010-
37 Overnight .002 .004 .002- .013-
39 15 min. .001 .005 .007 .024-
8 min. .004 .004 .021-
41 11 min. .005- .005 .004 .025-
24 60 min. ~ .004 .006 .015-
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