Note: Descriptions are shown in the official language in which they were submitted.
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MOLDABLE REFRACTORY COMPOSITION AND -
PROCESS FOR PREPARING THE SAME
BACKGROUND OF THE INVENTION
a) Field of the invention
The present invention relates to a moldable refractory composition
for use to prepare refractory pieces that can handle molten metals, especially
molten aluminum.
More particularly, the invention relates to a refractory composition
which, after casting, can be unmolded within a very short period of time that
can be
as small as 10 minutes.
b) Brief description of the prior art
There are numerous refractory products known in the art, which are
resistant to molten metals, like molten aluminum, and are therefore useful for
handling the same. Amongst these refractory products, one can mention those
prepared from compositions containing one or more heat resistant inorganic
materials, like asbestos fibers, in admixture with a colloidal silica that is
used as a
primary binder.
As an example of such refractory products and compositions for
preparing the same, reference can be made to U.S. patent No. 3,224,927 of 1965
to BROWN et al, which discloses a composition for preparing heat resistant
products that can be formed and/or molded into rods, tubes or blocks. This
composition comprises inorganic fibers, preferably asbestos fibers, bonded
with
from 2 to 20% by weight of colloidal silica and from 0.20 to 1.5% by weight of
a
positively charged starch, the above percentages being based on the weight of
inorganic fibers in the composition. The above mentioned, positively charged
starch
is used inter alia as a secondary binder to impart strength to the cast
products until
a silica bonding can be developed by heating.
As another example of refractory products of the above mentioned
type and compositions for preparing the same, reference can also be made to
U.S.
patent Nos. 4,873,209 of 1989 and 5,033,721 of 1991, both to GNYRA, which
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disclose a castable composition for preparing light weight refractory products
that
can be used in metal foundries especially as cementing material or as surface
finishing or "face-off' material to form a protective coating layer to protect
an
underling article. The composition disclosed in these patents comprises a
mixture
of ceramic fibers and wollastonite powder and an aqueous colloidal silica
having a
solid content of 5 to 25% by weight. It can also comprise milled and/or
screened
vermiculite to adjust the density and strength of the final product. In use,
the
composition disclosed in the above patents is efficient. However, it has the
drawback of taking about 24 hours to dry and harden at room temperature,
thereby
making the casting process very time consuming.
It is already known in the art that one may control and actually
accelerate the setting of ceramic coating compositions containing colloidal
silica as
a binder, by adding thereto a phosphorus compound like phosphoric acid and its
salts, especially trivalent metal phosphates. In this connection, reference
can be
made to U.S. patent Nos. 2,995,453 of 1961 to NOBLE et al; 3,041,205 of 1962
to
ILER and 3,650,783 of 1972 to YATES. However, the compositions disclosed in
these patents, are essentially useful as refractory binders, coatings or
adhesives,
and not for the preparation of cast pieces.
On the other hand, U.S. patent No. 5,362,690 of 1994 to DELVAUX
et al discloses and claims a castable refractory composition for use in the
prosecution of cast refractory pieces, having a phosphate salt as a primary
binder.
The composition comprises 1 part by weight of a phosphoric acid solution
having
a concentration ranging from 40 to 85% and its primary acidic function
partially
neutralized by reaction with vermiculite. It also comprises from 0.1 to 2.5
parts by
weight of a mineral, preferably wollastonite, that is refractory and of
fibrous structure
and may react with one part of the acidic functions of the phosphoric acid
that are
still free or not neutralized yet. It further comprises from 0.1 to 2.5 parts
by weight
of another mineral such as serpentine, Mg0 or synthetic olivine, containing at
least
one cation that may react with another part of the acidic functions of the
phosphoric
acid that are still free or not saturated yet and from 0.28 to 1.45 parts by
weight of
water. When the composition disclosed in this patent is processed, it passes
from
a granular state to a plastic state, and then to a liquid state, without need
for any
external heating. At that time, it may be cast to produce refractory pieces
having
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good thermal insulation characteristics, and in particular, a very good
behaviour with
respect to molten aluminium.
SUMMARY OF THE INVENTION
It has now been discovered, and this is the basis of the present
invention, that one may control in a very efficient and reliable manner the
mixing
and molding times of a moldable refractory composition containing wollastonite
as
a heat-resistant inorganic material, and an aqueous solution of colloidal
silica as a
primary binder, if one adds to this composition an aqueous solution of
phosphoric
acid wherein the phosphoric acid has its primary acidic functions partially
neutralized with vermiculite.
More particularly, it has been discovered that by adding an aqueous
phosphoric acid solution partially neutralized with vermiculite to a mixture
containing
wollastonite and an aqueous solution of colloidal silica, one may reduce to
less than
2 hours and actually to as low as a few minutes, the molding time required to
cast
refractory pieces.
Thus, a first object of the present invention is to provide a moldable
refractory composition comprising a matrix consisting essentially of:
- 8 to 25% by weight of an aqueous phosphoric acid solution
having a concentration of phosphoric acid ranging from 40 to 85% by weight,
said
phosphoric acid having up to 50% of its primary acidic functions neutralized
by
reaction with vermiculite; and
- 75 to 92% by weight of a mixture containing wollastonite and
an aqueous suspension from 20% to 40% by weight of colloidal silica in a
weight
ratio of the aqueous suspension to wollastonite ranging from 0.5 to 1.2.
Preferably, the aqueous phosphoric acid solution has a concentration
of phosphoric acid equal to about 55% by weight and the primary acidic
functions
of the phosphoric acid contained in the aqueous phosphoric acid solution are
neutralized by reaction of 1 part by weight of vermiculite per 5 parts by
weight of
said aqueous phosphoric acid solution.
Preferably also, the aqueous suspension contains about 40% by
weight of colloidal silica and about 60% by weight of water and the
wollastonite
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consists of a mixture of about 2 parts by weight of wollastonite NYAD 400~ and
about 1 part by weight of wollastonite NYAD G~. In this preferred embodiment,
the
matrix also comprises from 10 to 20% by weight of the aqueous phosphoric acid
solution partially neutralized with vermiculite.
F o r some applications, the composition may further comprisesa
reinforcing material selected from the group consisting of refractory fibers,
glass
fibers and fabrics made of said fibers.
A second object of the invention is to provide cast refractory pieces
obtained by casting into a mold a composition as defined hereinabove and
letting
this composition harden in the mold.
As indicated hereinabove, the hardening time is usually lower than
2 hours and can be as low as a few minutes.
Preferably, after the composition has hardened, the piece is
unmolded and can be subjected to a heat treatment at a temperature equal to or
lower than 400°C.
A third object of the invention is to provide a process for preparing a
moldable refractory composition for the production of cast refractory pieces,
comprising the steps of:
a) neutralizing with vermiculite up to 50% of the primary acidic
functions of phosphoric acid contained in an aqueous solution having a
concentration of phosphoric acid ranging from 40 to 85% by weight;
b) mixing wollastonite with an aqueous suspension containing up
to 40% by weight of colloidal silica in a weight ratio of said aqueous
suspension to
said wollastonite ranging from 0.5 to 1.2; and
c) mixing the partially neutralized phosphoric acid solution
obtained in step (a) with the mixture obtained in step (b) to obtain the
requested
moldable refractory composition, the partially neutralized phosphoric acid
solution
obtained in step (a) being present in a percentage ranging from 8 to 25% by
weight
with respect to the total weight of said partially neutralized phosphoric acid
solution
and the mixture obtained in step (b).
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DETAILED DESCRIPTION OF THE INVENTION
As mentioned hereinabove, the process for the preparation of the
moldable refractory composition according to the invention comprises a
plurality of
5 steps.
The first one of these steps consists in neutralizing up to 50% of the
primary acid function of phosphoric acid with vermiculite. This step is
disclosed in
great detail in the above mentioned U.S. patent No. 5,362,690.
The degree of neutralization of phosphoric acid can be determined
by acido-basic titration carried out with an automatic titration device like
the one
sold under the trademark, EMTROHM using NaOH (0.1 M) as titration agent. The
concentration of the H3P04 solution that is used in accordance with the
present
invention ranges from 40% to 85% by weight and is preferably equal to about
55%.
The standard kinetic of neutralization of phosphoric acid is shown in
Table 1, which gives the results obtained when 1 kg of vermiculite sold by
GRACE
under the trademark ZONOLITE~ 3 is reacted with 5 kg of a 55% of H3P04
solution
sold by ALBRIGHT & WILSON. During the experiment, the container containing the
vermiculite and the phosphoric acid solution was left at ambient temperature,
and
samples were taken after 2, 4, 24, 48 and 96 hours of reaction and subjected
to
chemical analysis. One can see that after 96 hours, 44% of the primary acidic
function were neutralized. One can also see that the secondary and tertiary
acidic
functions of phosphoric acid were not neutralized by vermiculite.
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TABLE 1 -
KINETIC OF NEUTRALIZATION OF 5 KG of H,P04
SOLUTION 55% WITH 1 KG OF VERMICULITE
TIME NEUTRALIZATION
(hours) (%)
2 20
4 28
24 33
48 37
96 44
This kinetic of reaction is reproducible and therefore can be used to
obtain a starting material that is of constant quality, by proper selection of
the
duration reaction. It is also worth mentioning that the reaction speed can be
increased if wanted, by increasing the reaction temperature.
The second step of the process according to the invention consists
in mixing wollastonite with an aqueous suspension of colloidal silica.
Wollastonite is a natural material comprising acicular crystals of
calcium silicate (CaO.Si OZ). In practice, wollastonite from any industrial
source may
be used and no preliminary treatment needs to be carried out. However, in
accordance with a preferred embodiment of the invention, use will preferably
be
made of the wollastonite NYAD G~ and NYAD 400~ sold by NYCO.
The relative proportions of wollastonite NYAD G~ and wollastonite
NYAD 400~ may vary from 0 to 100 and 100 to 0, respectively. Experiments
carried
out by the Applicant have shown that a high proportion of wollastonite NYAD
400
gives to the refractory pieces a very nice surface aspect. However, unmolding
cracks may appear during drying. By increasing the proportion of wollastonite
NYAD
G~, one may eliminate such cracks. As a matter of fact, it has been found that
a
wollastonite mixture containing two parts by weight of wollastonite NYAD 400~
and
one part by weight of wollastonite NYAD G~ is the most efficient mixture for
obtaining refractory pieces with no reinforcing fiber glass fabric therein,
which are
resistant to handling.
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The aqueous solution of colloidal silica that is used, can contain from
20 to 40% by weight of colloidal silica and from 80 to 60% by weight of water.
In
practice, any commercially-available colloidal silica can be used. Among these
commercial products, particular invention can made to the colloidal silica
sold under
the trademark LUDOX~ by DuPONT DE NEMOURS, and particularly of the
colloidal silica LUDOX HS-40~ which has proved to be particularly efficient.
This
LUDOX HS-400~ is an aqueous colloid of silica particles containing about 40%
by
weight of solid material and 60% by weight of water.
The weight ratio of the aqueous suspension of colloidal silica, and
more preferably of LUDOX HS-40~, to the total amount of wollastonite may vary
from 0.5 to 1.2. If this ratio is lower than 0.50, the resulting composition
will be too
dry to be properly molded. If the same ratio is higher than 1.2, the
composition will
be too liquid to be handled and decantation problems will occur.
The third step of the process according to the invention consists in
adding the aqueous solution of phosphoric acid partially neutralized with
vermiculite
to the obtained mixture of wollastonite and the aqueous suspension of
colloidal
silica. Such an addition must be carried in such a manner that the resulting
composition contains from 8 to 25% by weight of the aqueous solution of
phosphoric acid partially neutralized with vermiculite, the balance consisting
of the
wollastonite and the aqueous suspension of colloidal silica.
As aforesaid, the invention is based on the discovery that such an
addition of a solution of phosphoric acid partially neutralized by vermiculite
to a
conventional mixture of wollastonite and colloidal silica permits to
substantially
reduce the working time during the preparation of the composition and the
molding
time that is required to obtain any refractory piece.
As it has already been explained in the "Brief description of the prior
art" given hereinabove, the existing mixtures of wollastonite and colloidal
silica like
LUDOX must be heated after casting in order to permit the unmolding of the
refractory pieces. Indeed, the pieces made from these existing mixtures and
held
in a mold at ambient temperature remain in a gelatinous state, even after a
period
of time of 24 hours, and are therefore very difficult not to say impossible to
unmold
and handle.
On the other hand, the addition of phosphoric acid or of a phosphoric
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acid salt as such, as it has also been proposed in the prior art, does not
permit to
obtain reproducible results as far as the mixture preparation time and the
unmolding
time are concerned.
Very surprisingly, it has now been found that the addition of a solution
of phosphoric acid partially neutralized with vermiculite permits to obtain in
a very
predictable manner a mixture preparation time and an unmolding time that are
much shorter than so far. This is very difficult to explain. However, it seems
that this
phenomena is related to the reaction temperature. Indeed, it has been found
that
the amount of the aqueous solution of phosphoric acid partially neutralized by
the
vermiculite that must be added to the mixture in order to obtain good result,
must
be sufficient (viz. equal to at least 8% by weight) to create, within the
composition
in the mold, a slightly exothermic reaction.
In order to demonstrate the efficiency of the process according to the
invention, different compositions according to the invention were prepared and
tested. The formulations of the so-tested compositions and the unmolding times
of
the refractory pieces that were made from them are reported in Table 2.
TABLE 2
FORMULATIONS AND UNMOLDING TIMES
NO. WOLLASTONITE LUDOX H~P04 UNMOLDING DENSITY
(#) (%) (%) VERMICULITE TIME
1 50 50 0 24 h. -
2 47.5 47.5 5 16 h. -
3 46 46 8 90 min. 1.27
4 45 45 10 20 min. 1.25
5 44 44 12 15 min. 1.28
6 42 42 16 10 min. 1.09
7 40 40 20 8 min. 1.12
8 49 40 11 20 min. 1.25
9 51 38 11 15 min. 1.35
10 52 36 12 15 min. 1.45
11 56 32 12 15 min. 1.45
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In all these formulations, the aqueous solution of phosphoric acid
partially neutralized with vermiculite that was used as starting material was
prepared by mixing 20 kg of H3P04 55% with 4 kg of vermiculite ZONOLITE~ 3 in
an HOBART mixing apparatus. The obtained mixture was filled in a plastic
container and kept on storage during a period of four days before being used.
The so prepared solution of phosphoric acid partially neutralized with
vermiculite was then mixed with a mixture of wollastonite and with an aqueous
suspension of colloidal silica of trademark LUDOX HS-40O in an HOBART mixing
apparatus having a volume of 30 liters. The resulting mixture weighted about
22
kg and was molded into refractory pieces in the form of bricks having the
following
dimensions: 9.5" x 4.5" x 2.5". In all the tested formulations, the mixture of
wollastonite that was used contained two parts of wollastonite NYAD 400~ and
one
part of wollastonite NYAD G~.
The composition whose formulation is numbered 1 had a LUDOX~
to wollastonite ratio equal to 1 and do not contain phosphoric acid partially
neutralized with vermiculite. After 24 hours in the mold, this composition No.
1 was
still in a gelatinous state and difficult to handle.
Under the same conditions of preparation as above, the composition
whose formulation is numbered 2 to and which contained 5% only of the aqueous
solution of phosphoric acid partially neutralized with vermiculite, had an
unmolding
time of 16 hours.
In full contrast, the composition whose formulations are numbered 3
to 7 and which contained 8, 10, 12, 16 and 20% by weight of the aqueous
solution
of phosphoric acid partially neutralized with vermiculite, respectively, had
aqueous
unmolding times of 90, 20, 15, 10 and 8 minutes, respectively.
The composition whose formulations are numbered 8, 9, 10 and 11,
were different from the previous ones, inasmuch their LUDOX~ to wollastonite
ratios were different from 1 and actually equal to 0.82, 0.75, 0.69 and 0.57,
respectively. These compositions contained 11 and 12% by weight of the aqueous
solution of phosphoric acid partially neutralized with vermiculite,
respectively. As
can be seen, the unmolding times of the refractory pieces made from these
compositions ranged from 20 to 15 minutes.
In view of the above, it is obvious that the composition according to
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the invention can be used for the manufacture of the cast refractory pieces of
any
requested shape, like those that are usually needed in the metallurgy
industry.
These compositions can be reinforced by any suitable reinforcing
material. In practice, use be made of refractory fibers, glass fibers or any
fabric
5 made of such fibers. When use is made of a fabric as reinforcing material,
the
composition of the invention can be applied by spraying or dipping. This
permits to
obtain very thin plates. As a matter of fact, it has been found that the
composition
according to the invention can be applied to a mat made of glass fibers, that
can
subsequently be rolled up to form tubes.
10 The invention and its advantages will be better understood upon
reading the following non restrictive examples.
EXAMPLE 1
kg of an aqueous solution of H3P04 having an acid concentration
15 of 55% were mixed in a HOBART mixing apparatus with 4 kg of vermiculite
ZONOLITE~ 3 sold by GRACE. The mixing time was about 2 min. The obtained
mixture was filled in a plastic container that uses hermetically sealed, and
was
stored for a period of four days before being used.
19.5 kg of a suspension of LUDOX HS-40~ and wollastonite was
20 prepared in a HOBART mixing apparatus having a volume of 30 liters, by
incorporating 7.3 kg of wollastonite NYAD 400~ and 3.4 kg wollastonite NYAD G~
to 8.8 kg of LUDOX HS-40~. The suspension of LUDOX and wollastonite was
mixed for about 2 min. at middle speed within the HOBART mixing apparatus.
2.5 kg of the aqueous solution of phosphoric acid partially neutralized
with vermiculite, were added to the 19.5 kg suspension of LUDOX~ and
wollastonite and the resulting mixture was stirred for 2 min. at middle speed
within
the HOBART mixing apparatus.
The composition in a liquid form was cast into a mold having 5
cavities to form a corresponding number of bricks of the following dimensions.
9 "
x 4.5" x 2.5". The bricks were unmolded after 20 min. They were dried for 10
hours
at 110°C and fired for 4 hours at 375°C.
Table 3 gives the mechanical characteristics for the obtained bricks.
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TABLE 3
MECHANICAL PROPERTIES
Density 1.25
Cold Crushing Strength ASTM C-133 (psi) 2135
Modulus of Rupture ASTM C-133 (psi) 840
Table 4 gives the cold crushing strength and the modulus of rupture
of the same bricks after heating at 110°C, 400°C, 700°C
and 850°C, respectively.
TABLE 4
MECHANICAL PROPERTIES AFTER HEATING
HEATING TEMPERATURE (°C)
110 400 700 850
Cold Crushing Strength 1805 2135 1611 1668
Modulus of Rupture 916 840 854 860
The refractory pieces that were so obtained had an excellent
behaviour in the presence of molded aluminum alloys. By way of example,
samples
of the so obtained bricks were dipped in a molten alloy containing 95%
aluminum
and 5% magnesium at 850°C. Suitable amounts of magnesium were added
daily
to maintain a concentration of magnesium to the value of 5% within the molten
alloy.
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After 96 hours, the refractory bricks dipped into the molten alloy had no
visually
detectable cracks or damages.
EXAMPLE 2
A mixture of 19.5 kg of a suspension of LUDOX HS-40~ and
wollastonite was prepared in a same manner as disclosed in example 1. 3.5 kg
of
an aqueous solution of phosphoric acid partially neutralized with vermiculite
were
also prepared in the same manner as disclosed in example 1, and were added to
the mixture of LUDOX HS-40~ and wollastonite. The mixture was mixed for 2 min.
at middle speed in an HOBART mixing apparatus.
The composition was cast in a polyurethane mold having five cavities
shaped and sized to form a corresponding number of spoons for sampling molted
aluminum. These spoons had an individual volume of 0.340 liter. There were
easily
unmolded after 10 min. and could support the drying and subsequent filing
steps
without any mechanical damages.
EXAMPLE 3
5 kg of a suspension of LUDOX HS-40~ and wollastonite was
prepared by adding 2.5 kg of wollastonite NYAD 400~ to 2.5 kg of LUDOX HS-40~
in an HOBART mixing apparatus. The obtained suspension of LUDOX~ and
wollastonite was mixed for 2 minutes at middle speed within the HOBART
apparatus.
0.375 kg of an aqueous solution of phosphoric acid partially
neutralized and prepared as disclosed in example 1 was added to these 5 kg
suspension of LUDOX~ and wollastonite.
Pieces of fiber glass fabric of type E having a surface of 12" x 12"
were impregnated with the suspension of LUDOX~ and wollastonite after the
aqueous solution of phosphoric acid partially neutralized with vermiculite was
added
thereto. Each piece of impregnated fabric had a thickness of about 0.75 mm.
The
pieces of impregnated fabrics were individually dried to obtain thin plates,
or were
stacked one above the other to form a plate of high thickness. The fabric that
was
used was made of glass fiber of type E and was of the type 32L sold by PYROTEK
INC. The plates made of a single sheet and those made of sheets stacked to
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achieve the requested thickness were easy to handle after about 2 hours. -
The obtained products were dried for 10 hours at 110°C and fired
for
hours at 375°C. After firing, the percentage by weight of glass fiber
fabric within
the final product was about 35% by weight. The density of the obtained product
was
5 1.60 and its flexion resistance and flexion elasticity modulus (ASTM D-790-
86) were
respectively equal to 3000 psi and 610 kpsi. Its shock resistance measured
according to ASTM D256-87 was about 45 kg cm/cm2.
EXAMPLE 4
A refractory tube 13" long, 0.5" thick and with an internal diameter of
3" was manufactured by rolling a fabric of glass fiber of type 32L having a
width of
13" and a length of 100" about a shaft having a diameter of 3". Just before
rolling,
the fiber glass fabric was impregnated with a suspension of LUDOX~ and
wollastonite to which was previously added an aqueous solution of phosphoric
acid
partially neutralized with vermiculite as described in example 3.
The tube that was so formed was unmolded after 2 hours. It was then
dried at 110°C for 10 hours and fired for 5 hours at 375°C.
After firing, the amount
of fiber glass fabric within the final product of about 35% by weight and the
density
of final product was about 1.60.
It is obvious that numerous modifications could be made to the
preferred embodiments of the invention disclosed hereinabove without departing
from the scope of the invention as defined in the appended claims.
