Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
N 4 - 7 CANADA
4~7~4
S ILICEOUS BONDED REFRACTOR~
Background of the Invention
Bonded silicon carbide bodies have been known and
used for many years. A number of different materials ;~
have been used to bond the grains or particles of silicon
carbide together to form the desired shape, such as clays
and glass-forming mixtures of various composition to form
conventional ceramic and vitreous bonded shapes, pitch
and other tarry matter to form coke-residue bonded bodies,
and more recently silicon and silicon alloys fired under
proper conditions so as to react with constituents of the
ambient atmosphere to form refractory nitride and/or
carbide bonds. The bonded silicon carbide bodies obtained
with these various prior art bonding compositiona and
methods have been satisfactorily used for many purposes,
especially in the refractory field. However, regardless
of the type of product heretofore provided, each specific
one has had its own particular disadvantages and limita~
tions of use. For example, the coke-residue bonded
articles have been unduly susceptible to oxidation at
elevated temperatures, and the vitreous-bonded and clay-
bonded articles have shown a tendency at higher tempera-
tures to soften and lose their strength with loss of
desirable load-bearing ability. ~ -
A refractory body should be able to withstand high
temperatures without oxidation; it should also have the
ability to resist sudden changes in temperatures without
cracking or warping. It should have the necessary mechan-
ical strength to permit its use in required refractory
construction~ Many refractory compositions have been
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developed in an effort to fulfill these requirements. In
many cases, however, the resulting refractory, while
superior in some respects, is deficient in others. Because
of this there is a continual need for improved refractory
bodies of new compositions which will meet those demands
of a special nature which require a combination of proper-
ties not to be found in present refractories. The present
invention provides refractory bodies or shapes having dis-
tinctive compositions which are made by practical methods.
When properly processed the bodies are characterized by
having high density, strength stability at high tempera-
tures, excellent oxidation resistance under severe con-
ditions, and stability in the presence of corrosive gases.
Summary of the Invention
The present invention provides refractory bodies
comprising silicon carbide with small amounts o finely
divided silicon metal and fumed amorphous silica added
as a bonding material. A small amount of barium sulfate
or a similar material is included as a sealing agent.
The mixture may be shaped into articles by any of the
well-known methods of formation, such as mechanical com-
paction or pressing and then fired in an oxidizing
atmosphere at a temperature and period of time sufficient
to convert the silicon metal to a siliceous bond, giving
high density and strength to the resulting refractory
structure.
Description of the Preferred Embodiment
The refractory bodies of this invention are formed
from mixtures which may comprise from about ~0 to about
95 percent of silicon carbide refractory aggregate or
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grain, with the remainder comprislng about 1/2 to about 2
percent of finely divided amorphous silica, from about
2-1/2 to about 3-1/2 percent of a sealing agent such as
barium, sulfate, small amounts of clay plasticizers,
refractory fines, temporary binders and finely divided
silicon metal. In this disclosure and the following claims,
all percentages are by weight unless specifically designa-
- ted otherwise. Oxides of metals such as barium, sodium,
magnesium, calcium, aluminum and iron, may also be present
as impurities in concentrations of less than about one ~ `
percent. ~ ;
The finely divided amorphous silica is a recovered
fume by-product and has a composition of about 91 percent i
silica with about seven percent alumina, the remainder
comprising trace amounts of metals such as boron, mag-
nesium, iron, calcium, and sodium. The average particle
size of this material is about 0.05 microns. The silica
acts as one of the bonding components in the refractory
bodies of the invention. Finely divided barium sulfate
is used as a sealing agent to prevent excessive oxidation
of the refractory grains during the firing process. Other
sealing agents which may be used are alumina silicates
such as feldspar, or magnesium silicates such as talc.
The silicon metal aids in the attainment of high
density in these refractory bodies through the process of
oxidation during firing and subsequent reaction with other
oxides to form a refractory siliceous bonded bond in situ.
This bond is particularly tenacious if the oxidation of
the silicon takes place after it becomes welded to and
before it melts and diffuses into the refractory particles.
The silicon metal is ground to at least 200 mesh but may
be supplied as 6Q0 mesh and finer. The content of silicon
metal added to the refractory mix may vary from about 1/2
to about 3 percent. Water may be added to plasticize the
mixture which may then be shaped into the desired form,
using any of the well-known methods of formation, such
as mechani~al compaction or pressing. The formed article
i5 then dried and fired. During firing it is necessary
to expose the refractory to an oxidizing atmosphere at
temperatures of 2200F and above to develop complete
bonding. During this firing about 1 to 6 percent of
silica is formed in the article by the oxidation of the
silicon bonding metal. The final silica content of the
reractory body after firing may then range from about
1/2 to about 8 percent.
While the description above sets forth certain
broad ranges for material compositions and processing
conditions, it should be understood that narrower ranges
of material compositions and reaction conditions may
give a refractory product with superior properties. In
a preferred embodiment of the invention, a basic mixture
of silicon carbide was first made up and this was divided
into a number of smaller portions with the addition of
various amounts of additives to each portion. Composi-
tion of the basic mixture was as follows:
:.
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TA~LE I
% Pounds Kg.
Pan Mill "C" Silicon Carbide 6/10 11.~ 90 40.8
Pan Mill "C" Silicon Carbide 10/1817.0 130 59.1
Pan Mill "C" Silicon Carbide 18/3423.5 180 81.8 ;
Pan Mill "C" Silicon Carbide D. C. 3.3 25 11.4 ;
Fines ,
Pan Mill "C" Silicon Carbide 34/7018.3 140 63.7
Pan Mill "C" Silicon Carbide -70 17.6 135 61.3 ~ ;
10Silicon Carbide Colloidal Slip 5.4 41 18.7
Barytes (Barium Sulphate) 3.1 24 10.8
TOTAL 100.0
The fractions following the silicon carbide components
refer to U.S. Standard screen sizes, the fraction numerator
giving the screen size through which the particles passed,
while the denominator gives the screen size on which the
particles were retained. Dry lignone was used as a tempo-
rary binder. This is a lignone-sulfonate residue, a
by-product of paperpulp manufacture. The addition of a
commercial wetting agent is helpful in preparing the mix
for molding. The choice of temporary binder and wetting
agent i9 not critical in the practice of the invention.
For an evaluation of the bonding effectiveness of
silic~n metal and amorphous silica additions, varying
amounts of these were added ~o 25 pound test portions of
the above basic silicon carbide mixture. Each mixture
was then pressed into bricks, about 4 inches in both width
and thickness and about 9 inches in length. The bricks
were then dried and fired under an oxidizing atmosphere in
a kiln at a temperature range of about 2500F to about
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2700F for time periods ranging from about 6 hours to ~-
about 8 hours. After cooling, the bricks were tested
for rupture strength. Results of these tests are shown
in Table II.
TABLE II - MODUhUS OF RUPTURE
Brick Load at (Cold
Density Rupture Modulus)
Sample Si% SiO2% gm/cc lbs. psi
..
M-l2-1/2 - 2.63 22,550 3829
M-21-1/4 - 2.61 27,600 4749
M-3 5/8 - 2.61 25,20~ 4336
M-4 - 1 2.60 24,000 4130
M-5 - 1/2 2.60 25,400 4371
M-6 1 1/2 2.62 27,000 4646
M-7 2 1/2 2.62 29,350 5050
In the above Table, the silicon metal used was a
200 mesh powder. The silica was fumed silica, an
extremely fine powder with an average particle size of
about 0.05 microns. Brick densities were determined
after firing by the Archimedian method. The cold modulus
of rupture was determined on a Tinius Olsen testing
machine having a load capacity of 60,000 lbs. Loads
were determined on a brick span length of 7 inches.
Although good refractory articles have been made
with silicon metal additions up to about 5 percent,
the optimum addition is from 1 to 2 percent. It will
be noted from Table II that the addition of small amounts
of amorphous silica also enhance the strength of the
refractory and the combination of silicon metal and
silica gives maximum strength, a preferred combination
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being that of about 2 percent silicon metal and 1/2 per-
cent silica addition to the refractory mi~ture.
An oxidizing atmosphere during firing is necessary
since the silicon metal o~idizes and then combines with
other oxides present to form glass and a microcrystalline
siliceous bonding network in the resulting refractory
article. The bonding network may comprise less than ~-
about 15 percent of the final fired structure. We have
described the making of molded shapes in which the
article is molded and fired in the exact shape and form
in which it is intended for use, such as in firebrick or
similar refractory shapes. Another way of making and
using the refractory bodies of the invention i~ to mold
raw batches of green material into the briquettes or
other shapes and fire as previously described. After
firing, the bodies may be crushed to granular form and
the granules may be used as a loose filtering media or
as catalyst or catalyst carriers. The granular material
may also be bonded by means of sintered metals, vitreous
or ceramic bonds or other bonding materials to form
refractory articles suitable for many industrial uses.
