Note: Descriptions are shown in the official language in which they were submitted.
}30
BACKGROUND OF THE INVENTION
This invention is directed to an apparatus and method
for casting molten refractory and particula~ly to the very rapid
chilling of molten aluminous abrasive such as the alumina-zirconia
abrasives of the type described in the Rowse and Watson U.S. patent
3,891,408. In casting such molten alumina-zirconia abrasives it
is highly desirable that the product be cast in a very thin layér
80 as to achieve the extremely rapid freezing of the abrasive.
This provides extremely small crystal``size, and where an aluminum-
zirconia eutectic is present, provides extremely small spacingbetween the zirconia rods and platelets in the eutectic mixture.
It also provides for a high "tetragonal" zirconia content in the
resultant crystalline structure. It is also believed that a
substantial content of reduction products is helpful in these
alumina-zirconia abrasives. While the exact function of these
reduction products (either sub-oxides or metallic inclusions) is
not fully understood, it is ~elieved that they provide an improved
performance for many types of grinding.
Whatever the function of the reduction products it is
~o desired to preserve them and accordingly it is a principal object
of the present invention to provide a method and apparatus which
will give extremely rapid cooling of very thin sheets of molten
abrasive.
A further object of the invçntion is to provide such
me~hod and apparatus which will protect the cooling abrasive from
contact with an oxidizing enviroment until the temperature thereof
has been reduced to below red heat, at which point the chilled
abrasive can be safely exposed to oxidizing atmospheres such as
the air and water for removing the final sensible heat therefrom.
PRIOR ART
The continuous casting of metallic sheets has long
been known and is commercially practiced for the manufacture of
sheets, rods and the like by mechanisms such as shown in U.S.
patent 3,805,877 to Ward which is typical of the many patents
assigned to the Southwire Company of Carlton, Georgia. In this
representative patent a metallic belt is run slightly spaced
from a cooled drum and the molten metal is poured into the nip
between the drum and the belt to provi~e a continuous casting.
A related development, which is represented by U.S. patent
3,835,917, provides two Caterpillar treads which conjointly form
opposite sides of a casting space into which molten metal is
poured. In the prior art of casting molten abrasives some of
the closest work is shown in patents owned by the Norton Company,
assignee of the present invention. One of these is V.S. patent
3,377,660 to Marshall et al which describes the casting of a
molten a~raæive on the surface of one drum which is then pressed
against another drum, both of these drums being cooled. A
commercially utilized process is that shown in U.S. patent
3,993,119. This patent shows a number of vertically positioned
heavy cold metal plates which are mo~ed in a continuous stack
under a pouring spout where molten abrasive is poured into the
narrow spaces between the plates. The plates are subsequently
separated a short distance away from the pouring position and
the solidified, but still hot, abrasive is discharged therefrom.
While the aboYe patent does provide an excellent product, it is
~ifficult to proYide an extremely thin spacing between the heavy
m~tal plates. As the spacin~ is decreased there are difficulties
in completely filling the mold spaces and the process becomes
less efficient with more of the poured product ending up as a
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.,
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cap on top of the mold. This cap must be discarded because of
porosity, heavy oxidation and the like.
S~MMARY OF THE INVENTION
The present invention eliminates many of the problems
of the prior art by providing a continuous process for casting
extremely thin sheets of oxide material. It is particularly
applicable to casting of aluminous oxides containing partial,
or complete, reduction products where the cast product can be
provided in an extremely thin layer with very high yields of
thinly cast product. It provides very complete protection of
the cast product against contact by air, moisture and the like
until the product has been cooled below a temperature at which
any adverse reaction such as oxidation or pore forming can be
created by reaction with air or moisture. The device is designed
so that extremely high rates of cooling, utilizing a liquid such
as water, may be employed. It also provides that the surfaces
which are to contact the molten abrasives are completely dry at
the time the abrasive is poured between the two belts as they
are brought into face-to-face position with the very thin layer
of freezing abrasive therebetween. By this means a dense,
extremely finely crystalline, material is produced which has
essentially the same oxidation state as that of the molten stream
being cast. It does not contain induced porosity due to oxidation
of the freezing abrasive or by the generation of stream or other
gases resulting from contact with water during the casting
process.
When the product produced in this type of casting
apparatus is an alumina-zirconia abrasive and the casting rate
and belt speed is maintained such that the cast layer is about
l/16" thick the rod spacing in the alumina-zirconia eutectic is
2~
on the order of 1000-1800 angstroms and the percent tetragonal
zirconia is on the order of 70% or higher. While the exact
nature of the oxidation state of the product is not known, the
gain on ignition of the product can be as high as .66% which
indicates the presence of considerable reduction products (e.g.
sub-oxides of zirconia or alumina or zirconium and metallic
aluminum) in the cast product. The product typically is extremely
dense, showing almost complete freedom from any porous product
of the type encountered when such an abrasive is cast onto a
plate with one surface exposed to the air. It is also free of
the "cap" which can be characteristic of a lot of product poured
onto a mold of the type shown in 3,993,119 when there is inade-
quate filling of the mold and much of the product freezes at the
top of the mold rather than in the space between the plates.
According to a broad aspect, the invention relates
to:
Apparatus for rapidly chilling molten oxide to
form thin sheets of finely crystalline solid oxide, a pair or
metall$c belts, guide means for advancing the belts into face-
to-f~ce relation, means for applying a cooling medium to the
ba~k ~urfaces of the belts as they move into face-to-face re-
lation, means for pouring a molten oxide into the nip between.
the approaching faoes of the belts, and means for advancing said
belts fr~m the pouring position to a discharge position where
the belts are separated and the solidified oxide sheets are
dischargeæ therefrom; said belts being so arranged that a force
tends to push the belts into face-to-face contact between the
pouring position and the discharge position.
43~
~ According to another broad aspect, the invention
relates to:
Proce-s for rapidly chilling molten oxide to form
thin ~heets of finely crystslline solid oxide, comprising the
steps of advancing a pair of metallic belts into face-to-face
position, applying a cooling liquid to the back surfaces of the
belts as they move into face-to-face po~ition, pouring a lten
oxide into the nip between the approaching faces of the belts,
And ~dvancing said belts from the pouring position to a discharge
position where the beltg are separated and the solidifi~d oxide
sheets are discharged therefrom~ and arranging the cooling
belts so that a force tends to push the belts into face-to-face
contact between the pouring position and the discharge position.
DETAILED DESCRIPTION OF T~E INVENTION
Reference should be had to Figures 1, 2 and 3 which
illustrate one preferred embodimentof the invention. These
figures are schematic, diagramatic, representations of one
embodiment, Fig. 1 being a side view of the cas~ing apparatus,
Fig. 2 being an end view (with portions removed) taken from the
right side of ~ig. 1, and ~ig. 3 being a section taken along
the line 3-3 of Fig. 1.
Referring now to Fig. 1 there are shown two thin
metallic belts, a lower belt 10 and an upper belt 12. AS seen
from Figs. 2 and 3 the upper belt 12 is considerably wider than
the lower belt 10 so that its edges extend a substantial distance
beyond the edge of the lower belt 10 when the two belts are held
in a sandwiched arrangement and with their back surfaces bein~
sprayed by copious quantities of cooling water. The lower belt
10 passes over rolls 14, 15 and 16 and the upper belt passes
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over roll 18 and disc roll 20. As seen better in Fig. 2 disc
roll 20 is formed of a number of narrow discs 20a which are
axially aligned but spaced apart on a common shaft 23. Disc
` roll 20 thus acts to support the metal belt 12 as if it were
passing over a solid drum but it maintains the back surface of
the belt free for application of copious quantitites of cooling
liquid. A plurality of lower spray nozzles 22 and upper spray
nozzles 24 are provided for spraying the back surfaces of the
lower and upper belts, respectively, with water to maintain
these surfaces below red heat, while the belts sandwich the
solidifying and cooling layer of abrasive 30 therebetween. A
furnace for melting the abrasive is schematically shown at 26
with a pouring spout 28 from which a stream of molten abrasive
30 is poured. This molten abrasive 30 is poured into the nip
11 between the belts 10 and 12 as they are brought together.
As the two belts leave the disc roll 20, with the cooling layer
of abrasive therebetween, they are held in their sandwiched re-
` lationship by two sets of rollers 3~ which are mounted in frame
38, at least one of these frames being pressed towards the other
by means such as springs schematically indicated at 40 in Fig. 3.In the preferred embodiment the rollèrs 36 are commercial conveyor
rollers which are fairly widely spaced apart so as not to inter-
fere, in any appreciable extent, with the spraying of the water
; on the outer surfaces of the sandwiched metal belts. Additional
tensioning means may be provided as desired to maintain the
belts in adequate contact with the drive rolls.
The molten abrasive which contacts the cold, dry metal
;; sheets freezes almost instantaneously at the surface of the sheets.
The spacing between the two sheets, as they are brought togethPr,
is controlled by the rate at which the abrasive is poured there-
between and the rate at which the sheets are moving. When the
sheets are moving fast and the abrasive flow is slow the thick-
ness of the freezing abrasive layer can be maintained extremely
thin, on the order of a few hundreths of an inch. When the
speed of the belts is slower, and there is the same rate of flow
of abrasives, the thickness of the freezing layer can be made
greater, on the order of 1/16 inch and even as thick as 3/16ths
of an inch.
As the abrasive passes between the two belts and is
held there while it freezes and cools, the sensible heat is
rapidly removed so that, as the belt opens up and the abrasive
passes from the surface thereof as the bottom belt goes over
the drum 16 the abrasive chips 30 are at black heat. At this
point they will not be subject to deleterious oxidation and
they can be either quenched in water or just allowed to cool
to room temperature in air.
Several additional means are preferably provided for
assuring that the two facing surfaces of the belts are dry.
This includes a pair of squeegees 36 which serve to remove most
of the water which is on the belt surfaces. Preferably, addi-
tional flame drying is accomplished as shown schematically at
50 and 52 where a plurality of flames are directed against an
; extended area of the back surfaces of the belts so that the
belts are heated above the boiling point of water and thus no
liquid can remain on these surfaces as they are brought together
in the nip 11 to receive the molten abrasive. Any water on the
surface in the nip 11 would, of course, be instantaneously
- vaporized causing porosity in the freezing abrasive and also
reaction with sub-oxides and metallic constituents in the molten
abrasive mixture. The squeegees and heating means are also
~12'~
preferably supplemented by air blasts from nozzles 54 and 56
which assist in removing bulk water from the surfaces to be
dried. A shield 60 is preferably provided to keep the cooling
water away from the lower belt 10 as it travels on its return
path.
If desired those faces of the belts which are to be
brought into face-to-face contact for receiving the molten
abrasive layer therebetween can be coated with a mold coating
such as a layer of carbon black, for example, to improve the flow
of the molten abrasive across the surface of the belt and thereby
- improve the thermal contact between the molten abrasive and the
metal belts. Such a carbon black coating can be provided by the
use of an oxygen-deficient acetylene torch.
In another embodiment of the invention, schematically
indicated in Fig. 4, the two belts are not provided as continuous
belts but are provided as long strips of metal which can be fed
from two separate coils and subsequently wound up on take-up
reels positioned beyond the discharge roll 16. In this case the
- apparatus is essentially the same as shown in Figs. 1, 2 and 3
(although most portions of the apparatus have been eliminated
from the drawing) with the exception that a supply reel is pro-
vided at 41 for feeding a long length of metal belt 12 to the
' disc drum 20, through the apparatus, over the return roll 18 and
thence to take-up reel 42. Similarly the bottom belt 10 can be
fed from a supply 44 thereof, over the roll 14, over discharge
roll 16 and on to another take-up roll 46. With this embodiment
of the invention it is only necessary that the discontinuous
strips be long enough to provide for feeding of sufficient foot-
~` age of metal belts to the apparatus so as to take care of a
complete pour from the electric furnace 26. This system has the
advantage that absolute prevention of moisture entering the nip
is assured. With this arrangement the continuous belts can be
used just once or they can be used many times depending upon the
preferred economics of the casting operation.
While one specific form of the invention has been
described above (with one additional modification of belt feed)
it is apparent that numerous other modifications of the invention
will be obvious to one skilled in the art and such modifications
are intended to be included within the scope of the claims
appended hereto. For example instead of using a cooling medium
such as water, the preferred embodiment, another cooling medium
such as solid carbon dioxide powder or low temperature gas (e.g.
Nitrogen at -195C) can be sprayed against the back surface of
the belt.
