CA2054314A1 - Cement-free silicon carbide monoliths - Google Patents

Abstract

CEMENT-FREE SILICON CARBIDE MONOLITHS

ABSTRACT OF THE INVENTION
A lime-free silicon carbide based refractory containing about 87% silicon carbide sized -10 mesh, about 5% alumina sized -325 mesh, about 3% silica sub-micron size, and about 5% alkali-phosphate modified alumino-silicate binder. This material finds particular applicability in the construction of cast shapes and troughs for non-ferrous metal production and as a refractory lining for boiler tubes.

Description

2~3~
CEME~T-FREE SILICON CARBIDE MONOLITHS

BACKGROUN~ OF THE INVENTION
This invention relates to silicon carbide based monoliths and, more particularly, it concerns a lime-free silicon carbide refractory which can be used in various non-ferrous applications, such as aluminum production, ferroalloy production, tin production, copper production, and also as a refractory covering for boiler tubes.
It is recognized that the highest wear and maintenance area in an alu~inum reverberatory ~urnace i3 the belly band area. Thi~ area comprise~ the fluctuating zone between molten metal and air. The belly band zone is subjected to corundum (Al203) buildup (due to oxidation of the metal), reactions between the refractory and the molten metal, reactions between the refractory and cover fluxe~ on the molten metal, mechanlcal scrapp$ng to remove the tough corundum buildup, and high combustion temp~ratur0s ~inc~ a gas flame i~ directed onto the surface of the molten bath.
As the belly band area become~ worn or eroded, the upper sidewall of the furnace becomes undercut and hence become~ unstable. This necQssitates bringing the furnace off line and repairing the belly band. Current refractory materials u-~ed to line the belly band are phosphate bonded high alumina bricX and monollths as well as zircon brick. Although these materials perform reasonably well, localized wear at the belly band i8 a problem. Hence, there is an ongoing need for a more durable and stablQ belly band refractory.
Ferroalloy~, such as 75% Si/25%Fe are typically formed ln submerged arc furnace~. In such a ~urnaco, the temperaturQ of the molten ferroalloy is very high, typically near 3200-3300-F. The molten alloys are tapped from the furnace and poured through troughs into receiving ingots. The troughs are typically made from cement bonded 60% SiC/alumina castables. Tha castables ar~ typically cast lnto rectangular shape~ about 37 x 2~3~
2 _ 17-5/8 x 6". Carbon paste is then rammed over the rectangular shapes to form a trough-shaped configuration. These carbon troughs protect the blocks from the effects of the initial heat. Even though this carbon trough lining material wor~s reasonably well, a more durable and stable refractory is desired.
In waste-to-energy incinerators, boiler tubes filled with cooling water ar~ used to transfer heat from the confineR of the incinerator. Boiler tubes are used to construct the walls of the combustion cha~ber in the incinerator. Because the boiler tubes cannot withstand direct contact with the high temperatures in the interior o~ the incinerator, they are lined with a high conductivity refractory. Historically, silicon lS carbide-based rePractories hav~ been used for this purpose. These re~ractorie~ have been bonded with silicates, phosphates, or calci~ aluminate cements.
While sur~ace liPe has been adequate, a longer refractory life of the boiler tubQ lining is desired.

2~ 4 SUMM~BX_OF ~ INVENTION
In accordance with the present invention, the aforementioned shortcomings and limitaticns of conventional refractorie~ for the described applications have been substantially overcome by development of a lime-free silicon carbide composition.
In accordance with a preferred embodiment, the invention encompasse~ either a mix or a cast shape contai~ing about 87% silicon carbide graded -10 mesh, about 5% fine alumina, about 3% ~ine ~ilica, and about 5~ alkali phosphatQ modified alumino-silicate. This composition may also contain additions of com~on aluminum penetration inhibitors, such as about 5% of a baria-based compound and about 3~ of a boro ilicate compound. These addition ar~ added at the expen~ of the fine silicon carbid~ addition.
Accordingly, a principal ob~ect of the present invention is to provide a cement-free silicon carbide based refractory adapted for u e in non-ferrou~
application~. Another and more ~pecific ob~ect of the invention is the provi~ion of such a refractory which i8 usQful as a refractory coverin~ ~or boilsr tubes.
Other ob~ects and further 8COp~ 0~ applicability of the present invention will become apparent fro~ the detailed description to follow.

2~4~1 ~

ETAIL~LLL$CRIP~ION OF THE PREFERRE~_EMBODIMENT
In accordance with an exemplary embodiment, the present invention encompasses either a mix or a cast shape containing about 81~ silicon carbide grad~d -l0 mesh, about 5% fine alumina, about 3% fine silica, and about 5% alkali phosphate modified alu~ino~silicate binder. In accordanco with a preferred embodiment, the binder is a commercially availahle product sold under the tradename Lithopix AS-85.
The present mix or shape may alRo contain addition~ of common aluminum penetration inhibitor~, such as about 5% of a baria-based compound and about 3 of a borosilicate co~pound. The~e additions are added at the expen~e of the fine ~ilicon carbide addition~
Table l outlines a portion of the mix development aimed at finding an improved refractory for trough~
used in ferroalloy applications. Mixe~ l to 4 evaluated the effect of gr~nd (+l0 mesh grain3 versus -l0 mesh grain3) and the effect o~ calcined versus reactive alumina. All mixes worOE madQ with the same amount oP ~ine ~ilica and alkall phosphate alumino-silicate binder (Lithopix AS 85). From this ~tudy, it was concluded that the best strength after reheating was obtained with mix 3. This mix had a -l0 mesh grind and contained reacti~e alumina.
M1XQ~ 5 and 6 contained the preferred -l0 me~h grlnd and reactive alumina but were made wlth two level~ e~ ~dry phosphate powder and inorganic sillcate hardener instead o~ the Lithopix AS 85 binder. Theso two mixes had 3ignificantly lower denqitle~ and strengths. Stren~th was particularly low after reheating to 2500-F. This e~ort indicated that the desired propertie~ of high den~ity and high strQngth were primarily lnfluenced by the Lithopix AS 85 binder instead of the grind or type of alumina addition.

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Cast rectangular shapes of mix #3 were made measuring about 37 x 17-5/8 x 6". These shapes were used to construct the trough used to transport molten ferrosilicon to receiving ingot molds. Field trials indicate the lime-free 87~ SiC composition outperformed the standard cement bonded 60~ SiC/alumina cast shapes.
Since this initial trial was successful, it was decided to cast a complete trough using the lime-free SiC mix.
For economic reasons, the lower half of the trough which is not exposed to the metal stream was made from a typical cement bonded 70~ alumina castable and the upper half was made from the more expensive lime-free 87% SiC mix. The cast trough was then covered with carbon paste to provide protection during the initial heat up. A further advantage o~ this de3ign i~ that the upper portion of the used trough can be easily removed and replaced wlth a new insert. This ability further decreases the cost of maintaining the trough.
Two field trials were held utilizing this new deæign and both trials were deemed successful.
It is contemplated that similar troughs incorporating the lime-fr~e 87% SiC mix can be used in the pouring of molten tin and in other ~hops which pour molten copper. Additional proposed applications for this material include cast tiles for use as refractory covers on boiler tubeg.
Table 2 outlines development o~ the lime-free 5iC
mlx for ul3e in the belly band zone o~ alumlnum furnacQs. Mix A in Table 2 is the standard mix developed for use in ferroalloy troughs. Mixe~ B to H
contain additions of common aluminum penetration inhibitors such as a baria-based compound and a borosllicate ba~ed compound. Thesa additives are well known in the art. The addition of these compounds 3S increased density and the strength of the material especially strength after exposure to 1500~F.
comparison o~ mixes B and C suggest that removal of the 2~31~

finest SiC component had a favorable effect on the water requirement of the mix. In this series, mix G
had the bast sat of physical properties as well as a relatively low raw stock cost.

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Table 3 provides a screen analysis of mixes 1-6 of Table l and mixes A-~ of Table ~.

Table 3 Screen Analy~is Table~ 2 - - -lO Mixes: l & 2 3 to 6 A to G H

% Held on +lO mesh 36 ~ 3 15 + 3 lS + 342 + 3 10/28 n 12 30 30 12 28~65 1~ 10 15 15 8 -65 ~ 42 + 3 40 + 3 40 ~ 33a + 3 Wlth re~erenc~ to Table 4, te~t~ on shapes cast from mix G indicatad thi~ compo~ition has a very good re-Qi~tance to molten aluminum.

~able 4 _olten Alu~inu~ Test Mix: G
Alcoa'3 72 hr. Aluminum Cup Te~t U~ing 7075 Alloy at 1500F:
Aluminum Penetration: None Aluminum Adherence: - Moderate Change Maximum in Metal Allowable Chemi~tcy Increase Silicon (Si) -0.09 +0.50 Iron (Fe) 0.0 +O.lO
Magne~ium (Mg) -0~48 ~~~

ll 20a~314 Mixes and cast shapes without the aluminum penetration inhibitors have found successful application in troughs used to transfer molten ferroalloys. This composition presumably will be effective also in tin and copper applicatlons. The mix or cast shapes with aluminum penetration inhibitors should find application in the belly band zone of aluminum reverberatory furnaces.
It is preferred that the silicon carbide in this invention should have a silicon carbide content of at least 90% preferably 95% SiC or higher and that the density of the grain should be at least 2.5 g/cc, preferably 2.9 g/cc and higher.
Also, in accordance with a preferred embodiment, the fine alumina addition should be essentially -150 mesh, preferably -325 mesh. The alumina can be ground calcined alumina or super ground reactive alumina. The alumina content of this powder should be at least 98%, preferably higher. The ine silica addition should be -325 mesh, preferably submicron. Ideally, this addition should be round or spherical to enhance the flow properties oS the castable. The Lithopix AS 85 additive should be sized -150 mesh and have a typical chemical analysis as shown in Table 5.

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Table 5 Che~ical Analv i~ of Lithopi~c AS 85 S i02 59 O 7 A12325. 2 TiO20.01 Fe203 . 06 CaO0.05 MgQ0 . 0 8 P205~ 8.20 Na200.17 K208.15 Total Analyzed 99. 8~

20~31~
1~ _ One means of achieving longer refractory life of boiler tube refractory linings is to increase the ~ilicon carbide content of the refractory. This has been accomplished in accordance with the current invention by use of an alkali-phosphate modified aluminum silicate binder.
Thus, it will be appreciated that as a result of the present invention, a highly effective lime-free silicon carbide refractory is provided by which the principal object a~d others are completely fulfilled.
It is contemplated and will be apparent to those skilled in the art from the foregoing description that variations and/or modification~ of the di~closed e~bodiment may be made without departure from the invention. Accordingly, it is expres~ly intended that the foregoing description is illustrative of a preferred s~bodiment only, not limiting, and that the true spirit and scope of the present invention be determined by reference to be appended claim~.

Claims (18)

1. A cement-free cast shape essentially consisting of about 10 to 97% silicon carbide, graded -10 mesh and finer; 1 to 10% fine alumina; 1 to 10%
fine silica; and 1 to 10% alkali phosphate modified alumino-silicate powder.

2. The cast shape according to claim 1, containing preferably 70 to 97% SiC.

3. The cast shape according to claim 2, wherein said fine alumina is preferably sized -325 mesh.

4. The cast shape according to claim 3, wherein said fine silica is preferably sized sub-micron.

5. A cast shape according to claim 1 which is used in the ferroalloy industry, such as in the transfer of molten alloy into ingots.

6. A cast shape according to claim 1 which is used in the copper industry for handling molten copper.

7. A cast shape according to claim 1 which is used in the tin industry for handling molten tin.

8. A cast shape according to claim 1 which is used to line boiler tubes.

9. A cast shape according to claim 1 which contains typical levels of aluminum penetration inhibitors and is used in an aluminum application such as in the belly band zone of aluminum reverberatory furnaces.

10. A cement-free mix essentially consisting of about 10 to 97% silicon carbide, graded -10 mesh and finer; 1 to 10% fine alumina, 1 to 10% fine silica, and 1 to 10% alkali phosphate modified alumino-silicate powder.

11. The mix according to claim 10, containing preferably 70 to 97% SiC.

12. The mix according to claim 11, wherein said fine alumina is preferably sized -325 mesh.

13. The mix according to claim 12, wherein said fine silica is preferably sized sub-micron.

14. A mix according to claim 10 which is used in the ferroalloy industry, such as in the transfer of molten alloy into ingots.

15. A mix according to claim 10 which is used in the copper industry for handling molten copper.

16. A mix according to claim 10 which is used in the tin industry for handling molten tin.

17. A mix according to claim 10 which is used to line boiler tubes.

18. A mix according to claim 10 which contains typical levels of aluminum penetration inhibitors and is used in a molten aluminum application such as in the belly band zone of aluminum reverberatory furnaces.