Note: Descriptions are shown in the official language in which they were submitted.
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.
2~5~3~
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7 2~43~
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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2~3~
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.
12 ~5~31~
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~.
