CA1044704A - Siliceous bonded refractory - Google Patents

Abstract

SILICEOUS BONDED REFRACTORY

Abstract of the Disclosure The strength and density of fired articles made from silicon carbide are improved by the addition of about 1/2 to 3 percent of finely divided silicon metal. Optimum strength is achieved when the articles contain a small amount of silica in addition to the silicon metal. When the refrac-tory mixtures are fired at 2500 to 2700°F in an oxidizing atmosphere, the silicon reacts with oxygen and other constitu-ents of the mix to form a strong refractory siliceous bonding network.

Description

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 7(~4 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 .. . ............... . .
... . . . . ..
.. . . , ~ .

~;0~7al~L
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:

:.
. .

~4 ~
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 . ~ . . . ... . . .

~L47~
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 ~6--- . . . ~
.. . ., . ~ , .; , " . .
: , . . .
., . ~ . .

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.

Claims (4)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A bonded refractory body comprising from about 90 to about 95 percent silicon carbide, from about 1-1/2 to about 8 percent silica, from about 2-1/2 to about 3-1/2 percent sealing agent, and less than about 1 percent of oxidic impurities, said silica including from about 1 to about 6 percent of in situ oxidized silicon metal.

2. A refractory body according to claim 1 in which said sealing agent comprises a material selected from the group consisting of barium sulfate Feldspar and talc.

3. A method for making a bonded refractory body, comprising the steps of:
(a) forming a mixture comprising from about 90 to about 95 percent silicon carbide, from about 1/2 to about 2 percent silica, from about 2-1/2 to about 3-1/2 percent sealing agent, from about 1/2 to about 3 percent silicon metal, and less than about 1 percent of oxidic impurities, (b) shaping said mixture into a compacted body, and (c) heating said compacted body under an oxi-dizing atmosphere at a temperature of at least 2200°F to form the bonded refractory body.

4. A method according to claim 3 in which said heat-ing is at a temperature ranging from about 2500°F to about 2700°F for a time period ranging from about 6 to about hours.