Note: Descriptions are shown in the official language in which they were submitted.
~ ~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
--2--
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
--3--
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
--5--
cement binder. Other objects and further scope of
applicability of the present invention will become
apparent from the detailed description to follow.
2~J~
6~
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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7--
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~
-9~
TABLE I I
2 3
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~ ~
TA3LE III
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~
-12-
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 ~ ~ ~
-13-
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.
