CA1112430A

CA1112430A - Abrasive casting apparatus and process

Info

Publication number: CA1112430A
Application number: CA317,721A
Authority: CA
Inventors: John J. Scott
Original assignee: Norton Co
Current assignee: Saint Gobain Abrasives Inc
Priority date: 1977-12-19
Filing date: 1978-12-11
Publication date: 1981-11-17
Also published as: NO153757B; GB2010736A; FR2422462B1; DE2854680C2; FR2422462A1; JPS5499781A; NO153757C; GB2010736B; BR7808298A; JPS5814816B2; NO784213L; DE2854680A1

Abstract

ABSTRACT OF THE DISCLOSURE
A continuous process for casting a molten oxide such as an aluminous oxide abrasive is provided wherein two thin metal belts are brought into face-to-face relation and the molten oxide is poured into the nip between the belts as they come into face-to-face relation. The freezing abrasive maintains the belts slightly paced apart on the order of some small fraction of an inch (e.g. 1/16) and the back surface of the belt is flooded with copious quantities of cooling fluid (e.g. water) to maintain the belts below red heat. The belts are held in closely spaced re-lation for a sufficient time to permit the abrasive to be cooled below red heat. The belts are then separated and the abrasive is discharged from between the belts. At this point the abrasive is no longer affected by contaminants in the air, this aspect of the invention being particularly important in those embodiments of the invention where the molten abrasive (e.g. alumina zirconia abrasive) contains reduction products such as sub-oxides or ele-mental metal particles. In a preferred embodiment of the invention the top belt can be considerably wider than the bottom belt so that there is no water maintained on an upwardly facing surface which is subsequently to contact the molten abrasive.

Description

}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 *trademar~

., ~ 'ri.' -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 -5a-Z~3~
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.

Claims

The embodiment of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Apparatus for rapidly chilling molten oxide to form thin sheets of finely crystalline solid oxide, a pair or metallic belts, guide means for advancing the belts into face-to-face relation, means for applying a cooling medium to the back surfaces of the belts as they move into face-to-face re-lation, means for pouring a molten oxide into the nip between the approaching faces of the belts, and means for advancing said belts from the pouring position to a discharge position where the belts are separated and the solidified oxide sheets are discharged 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.

2. Apparatus for rapidly chilling molten oxide to form thin sheets of finely crystalline solid oxide, a pair of metallic belts, guide means for advancing the belts into face-to-face relation, means for applying a cooling medium to the back surfaces of the belts as they move into face-to-face re-lation, means for pouring a molten oxide into the nip between the approaching faces of the belts, and means for advancing said belts from the pouring position to a discharge position where the belts are separated and the solidified oxide sheets are discharged therefrom, the amount of cooling medium being sufficient to maintain the back surfaces of the belts below red heat; 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.

3. Apparatus for rapidly chilling molten oxide to form thin sheets of finely crystalline solid oxide, a pair of metallic belts, guide means for advancing the belts into face-to-face relation, means for applying a cooling fluid to the back surfaces of the belts as they move into face-to-face relation, means for pouring a molten oxide into the nip between the approaching faces of the belts, and means for advancing said belts from the pouring position to a discharge position where the belts are separated and the solidified oxide sheets are discharged therefrom, the upper belt being sufficiently wider than the lower belt so that the upper belt extends beyond both edges of the lower belt at all times while cooling fluid is being applied to the back surfaces of the face-to-face belts;
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.

4. Apparatus for rapidly chilling molten oxide to form thin sheets of finely crystalline solid oxide, a pair of metallic belts, guide means for advancing the belts into face-to-face relation, means for applying a cooling liquid to the back surfaces of the belts as they move into face-to-face rela-tion, means for pouring a molten oxide into the nip between the approaching faces of the belts, and means for advancing said belts from the pouring position to a discharge position where the belts are separated and the solidified oxide sheets are discharged therefrom, the speed of the belts and the distance between the point where the belts come into contact and where they separate being so related that the oxide is cooled to below red heat before being discharged from between the belts;
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.

5. Apparatus for rapidly chilling molten oxide to form sheets of finely crystalline solid oxide, a pair or metallic belts, guide means for advancing the belts into face-to-face relation, means for applying a cooling liquid to the back surfaces of the belts as they move into face-to-face relation, means for pouring molten oxide into the nip between the approaching faces of the belts, and means for advancing said belts from the pouring position to a discharge position where the belts are separated and the solidified oxide sheets are discharged therefrom, the guide means including a plurality of axially spaced discs around which the upper belt passes; 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.

6. The apparatus of Claim 5 wherein there are spray nozzles positioned between the discs for spraying the back surface of the upper belt as it approaches the molten oxide pour point.

7. Apparatus for rapidly chilling molten oxide to form thin sheets of finely crystalline solid oxide, a pair or metallic belts, guide means for advancing the belts into face-to-face relation, means for applying a cooling liquid to the back surfaces of the belts as they move into face-to-face relation, means for pouring a molten oxide into the nip between the approaching faces of the belts, and means for advancing said belts from the pouring position to a discharge position where the belts are separated and the solidified oxide sheets are discharged therefrom, the speed of the belts and the dis-tance between the point where the belts come into contact and where they separate being so related that the oxide is cooled to below red heat before being discharged from between the belts and means for heating the surfaces of the belts before they are returned to the pouring position to vaporize any cooling fluid on the surfaces to be brought into face-to-face relation;
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.

8. The apparatus fo claim 7 wherein said surface heating means includes a flame directed against said belt.

9. The apparatus of claim 4 wherein said belts are fed from separate coils thereof and means are provided for rewinding each belt into a coil after it has passed the dis-charge position.

10. Process for rapidly chilling molten oxide to form thin sheets of finely crystalline 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 position, pouring a molten oxide into the nip between the approaching faces of the belts, and advancing said belts from the pouring position to a discharge position where the belts are separated and the solidified 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.

11. Process for rapidly chilling molten oxide to form thin sheets of finely crystalline 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 position, pouring a molten oxide into the nip between the approaching faces of the belts, and advancing said belts from the pouring position to a discharge position where the belts are separated and the solidified oxide sheets are discharged therefrom, the speed of the belts and the distance between the point where the belt comes into face-to-face position and where they separate being so related that the oxide is cooled to below red heat before being discharged from between the belts; 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.