CA2052109A1

CA2052109A1 - Calcium aluminate refractory for aluminum metal contact applications

Info

Publication number: CA2052109A1
Application number: CA 2052109
Authority: CA
Inventors: Thomas R. Kleeb; Jack L. Cunningham
Original assignee: Individual
Current assignee: Dresser Industries Inc
Priority date: 1990-10-01
Filing date: 1991-09-24
Publication date: 1992-04-02
Also published as: FR2667310A1; AU8457891A; BR9104209A

Abstract

CALCIUM ALUMINATE REFRACTORY
FOR ALUMINUM METAL CONTACT APPLICATIONS

ABSTRACT OF THE INVENTION
A castable refractory mix especially useful as monolithic linings and shapes for the containment of molten aluminum. In accordance with an exemplary embodiment, the mix contains as major components about 50 to 95 wt.% fused calcium aluminate aggregate and 5 to 50 wt.% calcium aluminate cement binder.

Description

~ ~3iJ.
CALCIUM ALUMINATE REFRACTORY
FOR ALUMINUM METAL CONTACT APPLICATIONS

CROSS~REFERENCE TO RELATED APPLI ATIONS
This application is a continuation in part of co-pending U.S. patent application Serial No. 07/S91,lol, filed October 1, 1990, and having the same title and inventors as the present application.

BACKGROUND OF T~E__NVENTION
This invention rel~tes to monolithis refractories and, more particularly, it concerns an improved lining for aluminum furnaces, especially reverberatory furnaces, holding furnaces, crucibles, ladles, troughs, filter boxes, induction furnaces, and the like.
Commonly, pressed brick and monoliths are used to construct the lining of conventional aluminu~ furnaces.
For reasons of economics and ease of installation, monoliths are preferred especially in certain areas of the furnace, for example, such as in the hearth, lower sidewall, belly band, upper sidewalls, and roof of an aluminum reverberatory furnace. The monoliths are formed on site or as preforme~d shapes by various forming methods, such as cast:ing, gunning, ramming, trowelling, or the like.
Typical monolithic lining materials for aluminum melting or processing furnaces are aluminum or~hopho~phate bonded plasti~s and ramming mixes, castables containing alumino-silicate ~ggregates, ca~tables containing aluminum-resistant addi~ives (such as frit~ and boron-containing co~pounds), casta~les containing fluoride salts, or ~astables with chrome-containing aggregates. While Pach of th~se li~ing materials offers a reasonable servioe life, each type of mat~rial has di~tinct disadvantages. For example, the phosphate bo~ded monolith~ have relatively poor shelf life and therefore must be used soon a~ter manufacture. These monoliths al80 must be heated to 2 ~ 3 about 930F to fully develop an insoluble phosphate bond. A further disadvantage of phosphate bonded materials is the eventual breakdown o~ the phosphate bond and incorporation of elemental phosphorous into the melt as an undesirable contaminate~
Castables containing alumino-silicate aggregates can have a poor sarvice life because the molten melt can react with the silica portion of the aggregate and reduce the silicate to silicon. This reaction contaminates the metal with undesirable levels of silicon, causes refractory loss, and contributes to corundum buildup on the refractory.
Much research activity in recent years has involved addition of ~inc borosilicate, barite, and other barium and boron compounds to refractories used to line aluminum furnaces. These additives have shown the ability to slow down the rate of molten aluminum penetration, thereby, minimizing the reaction with the refractory. one flaw with this approach is that these additives sometimes cause the mix to have a short shelf life. Solu~le boron species and soluble silicates in the frits can react with the binder and produce a retarded set. Another flaw w:ith frits is that they tend to flux the refractory a1: high temperatures.
One problem associated with monoliths containing flu~rid~ salts is that if ~hermal surges occur in the furnac~s, wh~ch can often ~ccur in gas fired furnaces, th~ h~gh temporatures cause the fluoride salts to vaporiz2 thu~ rendering the re~ractory prone to molten aluminum p~netration and reaction. Chrome ore containing castables can be, under cer~ain operating condition~, altered to hexavalent chromium or hexavalen chromium compound~ which may be toxic and very expensive to dispose of.
In li~ht o2 the ~or~yoing, there is a need for an improved monolithic refractor~ for aluminum furnaces and which ha~ a stable shel~ life, contains a minimal level of silica, does not contaminate molten aluminum with undesirable elements, and does not contain chro~ium compounds.

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SUMMARY C)F THE INVENTION
In accordance with the present invention, the afore~sntioned disadvantages have been substantially overcome by a fused calcium aluminate aggr~gate which is bonded with a conventional calcium aluminate cement.
Such a combination of aggregate and cement is commonly re~erred to in the industry by the term "refractory concrete". The fuced aggregate of the present invention i8 preferably a low iron dodecacalciu~
heptaluminate (12CaO-7Al203) compound which is commonly used in the steel industry as a desulfurization agent.
Another type of ~used calcium aluminate aggregate useful in the monolith of the present invention is a higher iron material which predominately consists of the phase monocalcium aluminate (CaO Al203). The preferred binder for the monolith of tha pres~nt invention is any commercially available calcium aluminate cement which has an alumina content of 40 to 80 wt.% and is sized -100 mesh.
In addition to the calcium aluminat~ aggregate and binder, in accordance with the present invention, other refractory materials, such as f ine amorphous silica, cru~hed firebrick, calcined or crude fireclay, calcined bauxite, or calcined alumina may be employed up to about 25 wt. % as fillers. Small additions of other material~ which enhance the resistance of the monolith to mol en alu~inum may be added to tha mix.
Accordingly, a principal object of the present invention i~ to provide a calcium aluminate bonded, calcium aluminate based castable having high strength, good abrasion re~istance, and which does not r0act with molten aluminum. Another and more speci~ic object of the invention is the provision of a castable re~ractory mix which includes about 50 to 95 wt.% fused calcium alu~in~te aggregate and 5 to 50 wt.~ calcium aluminate cement binder. Other objects and further scope of applicability of the present invention will become apparent from the detailed description to follow.

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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Table I outlines the chemical and physical properties of three fused calcium aluminate aggregates used in the following examples. Aggregates A and B
were low iron materials (1% Fe203) from two different commercial suppliers. Both aggregates essentially consisted o~ the phases dodecacalcium heptaluminate (12CaO~7A1203) and tricalcium aluminate (3CaO A1203~.
Commercial aggregate C contained high levels of iron oxide (18% Fe203) and consisted of the phases monocalcium aluminate (CaO Al203) and lesser amou~ts of dodecacalcium heptaluminate (12CaO Al203), and dicalciu~n ferrite (2CaO-Fe203). All aggregates were crushed and graded into appropriate fractions ~or preparation of castables.
Aggregates A, B, and C were evaluated in conventional castable systems which contain 30 wt.%
calcium aluminate cement as the binder (Table II, see mixes 1, 2, and 3). In mixes 4 and 5 aggregates ~ (low iron) and D (high iron), respectively, were ev~luated in low cement castables. Mixes 4 and 5 also contained a plus-addition of a conventi.onal phosphate wetting agent. Sufficient water was added to each mix to achieve a cas~able consistenc:y. Table III shows tha screen analysis of mixes 1 to 5. Table IV lists the physical proper~ies of the mixes and results of tests ~peci~ic to the al~minum industry. The mixes were compared to a standard superduty fireclay castable made with the same t~pe and amount of cement used in mixes 1, 2, and 3.
In term~ o~ physical properties, mixes containing the hiqh iron aggregate tended to have the best set of properties - the highest density, highest strength, and the lowest abrasion loss. This was th~ case in both the high cement and the low cement systems. In a molten aluminum test, all the mixes show~d no metal penetration and either slight or no metal adherence.

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This indicated no or minimal reaction occurred between the refractorY and molten aluminum. Shapes made from mixes 3, 4, and 5 were anal~zed to determine the degree of metallic contamination of the aluminum bath which could occur as a result of reaction between the rePractory and the molten aluminum. As shown in Table IV, the amount of contamination was well below the acceptable upper limits.

TABT~ X

q a t ~ A 3 C
~ ed ~a 1~ ~ ~D ~ n L ~
O~cc:~ion: ;~p~ r 1 ;-~poli~ 2 i~'.?' ~ ;J 'd r ~ o nC ~ ~ r C~alcal An~ly~
( Calcinod 3asi~ ) Si1iC;æ ~C10~) 3.13~ 98 ~.5~;
Alu~ina ( A1203 ) 43 .1 39 . ~ 35 . 7 Tit&rlla ~102) 2~ 0 1.93 rEon Oxid~ t~-23) ' j~ 13.1 (CaOi g9.~ 51.9 38.0 gn;~lA (.~90) 0.50 1.1~ 0.34 ~o~a (Na~0) o.oa 0.02 0 ~3 poe~sh (R20) 0.13 0.11 0 02 E 1th~a ~11 a ~ 0.01 ~ 0.0L
~oeal A~ alyæ~d 3~ 9~ g~jj (D~y 8asi~) SU1~!UE T~lox~d~ O3) 0.17~ 0.06 X-ray Ol~r~c~lon ~n~ly~1 C~Q ~ Al2~ 3 N~ N~ .~

3 CaO ~ A 120 3 ~ M .~ 4 ~2CaCI-7A1~03 ~ .n ~CaO~ 03 Po~03 ~o ~o .~
2C~O - ~-2~3 N~ NO :n ~ul~ p~ G~a~y . 2.~ 2.8~ ~.21 oe ND ~ nol: d~
no~ c~ eelon~

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TABLE I I

Agqe~gae~ A
-3/~10 .~sh ~10/+29 .~h 1~
-28/~6S ~h 10 -- __ __ _ -6g ~o~sh 4 Ag~g II~Q C
-- l/i! iRCh --- 31~

- 4 mss~h -- 3 9 ~ 2 5 ~M~ (600 ~325 ~ h) Ag~o~a~ B
-1/ 2 ~ h - - -- 2 ~ ~ 2 10 r~a~
-10/~2a ~sh - 2 ~ 6 ~ h ~ 9 7 -- 6 ~ h ~ 3 3~ t 5 ~ ~ - 3 2 ~ 2 7 - -Sub-mlcron Slllca -_ __ __ 3 2 Cal~ n~0 C~
(30~ Alumina) 30 30 39 C~lclu~a Aluoln~to C~or~t (70% ~lumlna) ~ 9 6 ?lu~l Ad~ltlor.t Phosph~e~ ttlA51 ~9~ Q . 2~0 . 2 ~a~ng ~c~ .7~~8.50~12.2~10.9 -lo~ 32~ ~

Scr~n A~aly~i~

.`'1 1 ~: 1 2 3 ~ -Scr~n ~n~lysl~
ld on 3 r~h ~ 16 19 13 ~ 7 5 5 ~ ~
a 1l ~ 3 4 5 ~_ 1~ ~ _~ ~ 7~ '' 4 ~ 6 ~ 5 4 ~ 3 2 d _~ _~ 2 2~0 ~iii~h ~ ~L ~ ~ 2 ~

J l 3 ~ ~ ~ , z, ~,, . , t., C3 ~~ ~ ~3 Q C~ a 3 ~
.~ , e s~

s:
3 ~ , 3j ~ a i~3 ~1 155 5~ 5~ 5 s~

In accordance with an exemplary embodiment of the present invention, a calcium aluminate based castable particularly though not exclusively adapted to aluminum contact applications has a mlx of 50 to 95 wt.% fused calcium aluminate aggregate and 5 to 50 wt. % binder.
A first fused calcium aluminate aggregate useful in the present invention contains less than 3 wt.~, preferab1y less than 1 wt.~, iron oxide and is predominately the phase dodecacalcium heptaluminate (12CaO 7A1203).
Alternatively, a second fused calcium aluminate aggregate useful in the present invention contains more than 10 wt.%, but less than 30 wt.% iron oxide, preferably about 20 wt.% iron oxide, and is predominately of the phase monocalcium aluminate (Ca~-Al203).
Also, in accordance with the exemplary embodiment of the present invention, the binder is pre~erably a oalcium aluminate cement either with or without additions of fine silica, fine fused calcium aluminate, fine calcium aluminate cement or mixtures of these additionsO Additionally, the castable refractory mix of the present invention for certain applications can include a ~iller, such as fine or coarse amorphous or crystalline silica, crushed firebrick, calcined or crude fireclay, calcined bauxite, calcined alumina, or mixtures of these fillers in amounts up to about 25 wt.%.
Shapes may be made from the abov~-described exgmplary castable calcium aluminate based mix of the presen invention by conventional shape forming processes such as casting, ramming, gunning, pressing or the like.
Thus it will be appreciated as a result o the present invention, a highly ef~sctive calcium aluminate 35 based castable is provided by which the principal object and others are completely fulPilled. It is contemplated and would be apparent to those skilled in 2 ~ ~ ~

the art from the foregoing description that variations and/or modifications of the disclosed embodiment may be made without departure from the invention.
Accordingly, it is expressly intended that the foregoing description is illustrative of a preferred embodiment only, not limiting, and that the true spirit and scope of the present invention be determined by reference to the appended claims.

Claims

1. A castable refractory mix useful in aluminum contact applications comprising 50 to 95 wt.% fused calcium aluminate aggregate and 5 to 50 wt.% binder.

2. The refractory mix of claim 1 in which the fused calcium aluminate aggregate contains less than 3 wt.%
iron oxide and consists predominately of the phase dodecacalcium heptaluminate (12CaO?7Al2O3).

3. The refractory mix of claim 2 in which the fused calcium aluminate aggregate contains less than 1 wt.%
iron oxide.

4. The refractory mix of claim 1 in which the fused calcium aluminate aggregate contains more than 10 wt.%, but less than 30 wt.% iron oxide, and consists predominantly of the phase monocalcium aluminate (CaO?Al2O3).

5. The refractory mix of claim 4 in which the fused calcium aluminate aggregate contains about 20 wt.% iron oxide.

6. The refractory mix of claim 1 in which the binder consists of calcium aluminate cement.

7. The refractory mix of claim 1 in which the binder consists of calcium aluminate cement with additions of fine silica, fine fused calcium aluminate, fine calcium aluminate cement or mixtures thereof.

8. The refractory mix of claim 1 in which the mix contains a filler, such as fine or coarse amorphous or crystalline silica, crushed firebrick, calcined or crude fireclay, calcined bauxite, or calcined alumina or mixtures thereof in amounts up to about 25 wt.%.

9. A shape useful as an aluminum contact refractory made from a mix comprising 50 to 95 wt.% fused calcium aluminate aggregate and 5 to 50 wt.% binder.

10. The shape of claim 9 in which the fused calcium aluminate aggregate contains less than 3 wt.% iron oxide and consists predominately of the phase dodecacalcium heptaluminate (12CaO?7Al2O3).

11. The shape of claim 10 in which the fused calcium aluminate aggregate contains less than 1 wt.% iron oxide.

12. The shape of claim 9 in which the fused calcium aluminate aggregate contains more than 10 wt.%, but less than 30 wt.% iron oxide, and consists predominantly of the phase monocalcium aluminate (CaO?Al2O3).

13. The shape of claim 12 in which the fused calcium aluminate aggregate contains about 20 wt.% iron oxide.

14. The shape of claim 9 in which the binder consists of calcium aluminate cement.

15. The shape of claim 9 in which the binder consists of calcium aluminate cement with additions of fine silica, fine fused calcium aluminate, fine calcium aluminate cement or mixtures thereof.

16. The shape of claim 9 in which the mix contains a filler, such as fine or coarse amorphous or crystalline silica, crushed firebrick, calcined or crude fireclay, calcined bauxite, or calcined alumina or mixtures thereof in amounts up to about 25 wt.%.

17. A lining for aluminum furnaces, especially reverberatory furnaces, holding furnaces, crucibles, ladles, troughs, filter boxes, induction furnaces, and the like, comprising a castable refractory mix having 50 to 95 wt.% fused calcium aluminate aggregate and 5 to 50 wt.% binder.

18. The lining of claim 17 in which the fused calcium aluminate aggregate contains less than 3 wt.% iron oxide and consists predominately of the phase dodecacalcium heptaluminate (12CaO?7Al2O3).

19. The lining of claim 18 in which the fused calcium aluminate aggregate contains less than 1 wt.% iron oxide.

20. The lining of claim 17 in which the fused calcium aluminate aggregate contains more than 10 wt.%, but less than 30 wt.% iron oxide, and consists predominantly of the phase monocalcium aluminate (CaO?Al2O3).

21. The lining of claim 20 in which the fused calcium aluminate aggregate contains about 20 wt.% iron oxide.

22. The lining of claim 17 in which the binder consists of calcium aluminate cement.

23. The lining of claim 17 in which the binder consists of calcium aluminate cement with additions of fine silica, fine fused calcium aluminate, fine calcium aluminate cement or mixtures thereof.

24. The lining of claim 17 in which the mix contains a filler, such as fine or coarse amorphous or crystalline silica, crushed firebrick, calcined or crude fireclay, calcined bauxite, or calcined alumina or mixtures thereof in amounts up to about 25 wt.%.