Method and Apparatus for Rapidly Solidifying and Cooling Melted Products Based on Metal Oxides by Continuous Casting

The invention concerns a method and apparatus for rapidly solidifying and cooling molten metal oxides and metal oxide-base compositions by continuous casting.

In many industrial processes an attempt is made to obtain rapid solidification and cooling of melted products based on metal oxides, for reasons either of safety or of convenience: for example, when exhausted drosses or slags from electro-metallurgical operations have to be taken rapidly to the tip without the risk of transporting them in the liquid or partially solidified state, or for structural reasons, when rapid cooling is necessary to give the product certain physical properties. For example, abrasives based on corundum and on corundum and zirconia are known to have mechanical properties which improve commensurately with the rapidity with which the molten product, manufactured by fusion in an electric furnace, is cooled. This fact has been pointed out particularly in French Pat. No. 1 332 975 in the name of CARBORUNDUM Company, which recommends casting in a graphite mold, in French Pat. No. 2 127 231 in the name of PECHINEY directed to casting the melted product onto steel balls, in French Pat. No. 2 133 595 in the name of TREIBACHER CHEMISCHE WERKE AG directed to casting onto iron spheres, and in French Pat. No. 2 166 082 in the name of NORTON directed to casting between solid metal plates 1.6 to 12.7 mm apart.

For obvious reasons it would be desirable for the rapid cooling to take place continuously. In the case of drosses and slags, as a matter of fact, cooling in ingot molds, which is the usual practice, is very slow and necessitates a large number of molds, which take up space in the workshop. In the case of abrasives with fine crystallization, casting between closely spaced metal plates or onto steel balls leads to very low productivity.

A certain number of processes for cooling rapidly by continuous casting have previously been described.

French Pat. No. 1 319 102 to NORTON describes the casting of electro-melted alumina, in a thin layer, onto a hollow cylinder which is cooled by sprinkling water onto its internal surface.

U.S. Pat. No. 3,993,119, and French Pat. No. 2 290 266 to NORTON, describe an arrangement for casting between stationary, thick metal plates, through their base onto an endless conveyor. The conveyor passes over rollers which causes the plates to move apart and the blocks of cooled abrasive to be separated and drop into a receiving means.

French Pat. No. 2 422 462 to NORTON describes another arrangement for continuous casting between cooled metal belts which are kept in contact with the solidifying abrasive by a series of guiding rollers.

These various pieces of apparatus are mechanically complex and ill adapted to cheap, continuous mass production of electro-melted abrasives with fine crystallization.

The present invention is based on the use of at least two cooled, horizontal cylinders mounted for rotation about parallel horizontal axes as a means for cooling electro-melted products.

The method of rapidly solidifying and cooling melted products based on metal oxides, by continuous casting, according to the invention, comprises: feeding the melted product into the convergent space formed by two horizontal cylinders, turning in opposite directions at a substantially equal peripheral speed, the distance between their axes being equal to the sum of the radius of each cylinder plus at least about one millimeter, cooling the walls of the cylinders, controlling the rotary speed of the cylinders so that the melted product is not completely solidified until it passes beyond an imaginary plane passing through the axes of rotation of the cylinders, and retrieving the solidified product from the divergent space formed between the cylinders.

The apparatus for carrying out the method in a preferred embodiment essentially comprises a controllable feed for the molten product, two metal cylinders mounted for rotation with their axes parallel and substantially horizontal, adapted to be rotated in opposite directions at a substantially equal peripheral speed, the distance between their axes being equal to the sum of the radius of each cylinder plus at least about one millimeter, at least one cooling means and means for setting the cylinders into snychronous rotation, in opposite directions and at a controllable speed, and a means for retrieving the solidified product.

FIG. 1 is a vertical section through an apparatus for carrying out the method of the invention.

FIG. 2 shows an apparatus similar to that of FIG. 1 with a modified feed for the melted product.

FIG. 3 is a schematic showing of details of the solidification method of the present invention.

FIGS. 4, 5 and 6 show means for sealing the ends of a trough formed between two horizontal cylinders.

FIGS. 7a, 7b, 7c and 7d show four alternate arrangements of two cylinders in accordance with the present invention.

The melted product to be solidified may come directly from a melting furnace 1 or from a storage means, such as a ladle, only the lower part of which has been shown.

At the lower part of the furnace 1 there is a casting orifice 2, which can be sealed by any known means and which is preferably made of graphite or possible refractory material. The molten product 3 flows through the orifice 2 by actuating a sealing plug 4.

A means for rapid, continuous cooling comprises two cylinders 5 and 6, the respective longitudinal axes 7 and 8 of which are parallel and substantially horizontal. An imaginary plane 9 passing through the axes 7 and 8 is horizontal in the case shown, but this arrangement is not indispensable, as can be seen from FIGS. 7a, 7b, 7c and 7d. The cylinders are arranged with a space of at least about one millimeter between them at the closest point, that is to say, in the plane 9 of their axes so as to form a molten product receiving trough therebetween. This distance may be increased, according to the type of product being cast, e.g. up to 10 to 20 millimeters, although the invention is not restricted to this particular value.

The two cylinders 5 and 6 are set in synchronous rotation in opposite directions, i.e., counter-rotated, in the directions of the arrows by a drive means including a dynamic means of any known type which will enable the speed of rotation to be varied. One of the cylinders may be mounted on resilient bearings.

The two cylinders 5 and 6 thus define between them a convergent space 10, or trough into which the molten product to be cooled is introduced, and a divergent space 11 in which the solidified product is retrieved.

The two cylinders 5 and 6 are made of a material, such as of metal, which is a good conductor of heat, and they must have good mechanical strength. It is possible to fabricate the cylinders from steel, pure copper or preferably copper alloy, which has its hardness increased by known alloying constituents such as iron, cadmium, chromium or again copper steel or copper covered with a band of steel. The cylinders also may be solid, in which case cooling can be effected only by spraying or sprinkling them externally with water and avoiding any contact between the water and the molten product or they may, prefereably, be hollow and have internal cooling means and optional external cooling means.

The internal part of each cylinder comprises a chamber 12, 13 with water or other suitable cooling fluid circulating through it. The fluid is introduced and discharged through a plurality of holes 14, so as to provide effective and uniform cooling for the walls 15.

The two ends of the convergent space 10 into which the product to be solidified is fed have to be sealed. This can be done by providing an end wall member 16, see FIG. 4, of larger diameter at both ends of each of the cylinders, the member 16 coming into sealing contact with the enlarged diameter of the opposite cylinder. However, this arrangement definitively fixes the spacing between the cylinders, which may not be acceptable for all modes of operation. A different sealing method, see FIG. 5, comprises fitting the opposed ends of each cylinder with a disc 17 which is large enough in diameter to provide a rotary frictional seal relative to the opposite cylinder end.

FIG. 6 schematically illustrates another means, comprising two stationary sealing members 18, preferably made of graphite or a compact or fibrous refractory material, for sealing the ends of the molten material receiving trough formed between the cylinders 5 and 6.

FIGS. 7a-7d show various exemplary arrangements of the cylinders, e.g., 7a with the plane of the axes horizontal, 7b with the plane of the axes slightly inclined, 7c inclined at about 45.degree. and 7d substantially vertical. With respect to 7a and 7b the molten product may simply be cast into the convergent space or trough between the cylinders. With respect to FIGS. 7c and 7d it is necessary to feed it in through a nozzle 22 which sealingly engages the opposed cylinders.

In the various cases the cylinders are preferably theoretically of the same diameter. Since they rotate synchronously they will have an equal peripheral speed. If this speed were different for each cylinder there would be friction between the less rapid cylinder and the solid product, with excessive wear. It would also be possible for the two cylinders to be of unequal diameters and for the speed of rotation to be controlled individually so as to obtain an equal peripheral speed, although this arrangement does not appear to have any special advantages.

The diameter of the cylinders and their length may be chosen between relatively broad limits, and the invention is not restricted to these. The diameter may range from one to a few tens of centimeters to one meter and more, and the length may be up to and even over one meter. The upper limits will often be fixed by the available facilities for construction and machining the surface of a cylinder. Generally speaking, however, cylinders of large diameter will be more suitable for precise control of the height of the molten product 20 in the convergent space 10.

In FIG. 1 the liquid product is fed into space 10 simply by letting it flow by gravity. The rate of flow is controlled so as to keep the liquid product 20 at a substantially constant height in the convergent space 10. Two lateral gates 21 limit the danger of having too large a surface of contact between the wall of the cylinders and the liquid product 20, in the case of the liquid level rising excessively. This would result in premature solidification of the product, which would make the apparatus seize up.

In cases where the product has to be solidified in the absence of a reactive atmosphere, e.g., away from air, as with electro-molten abrasives based on corundum or corundum zirconia, circulation of inert or reducing gas may be provided in the convergent space 10. This is symbolized by the two arrows above the gates 21.

Another means of casting the melted product so as to prevent it from coming into contact with the atmosphere comprises using a nozzle 22, see FIGS. 2 and 7a which molten liquid-sealingly bears on the walls of the cylinder with a gentle, tight friction. This nozzle 22 is preferably made of graphite, and in FIG. 2 the upper part 23 may be made of a flexible or elastic material.

As will be appreciated from FIG. 3 the apparatus operates as follows: The common speed of the cylinders and the flow rate of the cooling water are controlled so that there is still a liquid film of molten product 20, 24, i.e., the common plane of the axes of rotation of the cylinders. This film must be thick enough to allow the already solidified "walls" of the solidified product 25 to be deformed, and not too thick to prevent the liquid from "leaking" or flowing downwardly. The film of molten product extends a vertical distance h below the plane 24 of the axes of the cylinders 5 and 6, and this distance h must not be less than about one centimeter. When the apparatus starts up, and particularly if there is a large gap between the rollers, it may be advantageous to seal the space between the rollers with a means such as asbestos braiding (not shown), which will be forced out of the nip of the cylinders as soon as they start turning.

Shortly after making contact with the cold surface of the cylinders, the molten material becomes partly solidified and is carried downwards by the rotation of the cylinders. A very important feature of the invention resides in the rotary speed of the cylinders being controlled as a function of the cooling power of the apparatus so that, in the zone of minimum distance between the cylinders, i.e., the zone defined by the two generatrices of the cylinders located in the horizontal plane of the axes of rotation, part of the product is still in the liquid state. The liquid thus comprises a film in the central zone of the partly solidified product. This film allows the two solid layers that encase it to come together and bond and pass between the cylinders. This would not be possible if solidification were completed when the material reached the zone of minimum distance between the cylinders, since the solid material obtained is generally speaking hardly malleable or completely unmalleable.

The solidified product is retrieved in the divergent space 11 between the two cylinders downstream of the plane 24. According to its nature the solidified product 25 will spontaneously break into blocks of variable dimensions, or it may remain in slab form. The solidified product 25 may be retrieved by any conventional means (not shown), such as a receiving hopper or conveyor belt, or it may drop directly into a crusher or grinder.

In addition, as shown in FIGS. 1 and 2, two rakes or scrapers 26 which are held against the cylinders by resilient means, e.g., springs, may eliminate any traces of solidified product which are left adhering to the outer wall of the cylinders 5 and 6.

Finally, the apparatus may comprise a means for accelerated cooling of the solidified product by sprinkling or spraying on water, and a means for additional cooling of the cylinders, by sprinkling the outer wall, followed by a drying means to guarantee that the wall will be completely free from any trace of water when it reaches the casting zone.

A first embodiment of the apparatus according to the invention is constructed, comprising two hollow copper cylinders, 126 mm in diameter and 300 mm long, arranged so that their axes are parallel, in a horizontal plane and 130 mm apart.

With the apparatus set to a rotary speed of 4.5 revolutions per minute and with water flowing into each cylinder at 40 liters per minute, approximately 6 kg per minute of corundum zirconia containing 20% of ZrC.sub.2 and 68% of Al.sub.2 O.sub.3 was solidified. On leaving the cylinders, the product spontaneously breaks into polygonal pieces with a 3 to 15 mm edge on a thickness of 4 mm. Some pieces break in the median plane of the solidified product 25, i.e., the shrinkage hole or void. There is thus very good preparation for a subsequent crushing process.

In another embodiment of the invention there are two steel cylinders 350 mm in diameter and 1000 mm long. Their parallel axes, which are contained in the same horizontal plane, are 354 mm apart. They are rotated at a speed of 3.5 revolutions per minute.

The cylinders are fitted at one end with a cheek plate 450 mm in diameter, e.g., see FIG. 5, also made of steel. The cylinders are placed heat to foot alongside, so that the cheeks provide a lateral limit to the free space between the cylinders. Each cylinder is cooled by circulating 200 liters of cold water per minute.

42 kg per minute of corumdum zirconia containing 25% ZrO.sub.2 and 75% of alumina is solidified in thin sheets on this apparatus.

A further embodiment comprises two copper cylinders 1000 mm long and 346 mm in external diameter. These are covered with two sleeves made of hard quenched steel, 2 mm thick and 350 mm in external diameter, which are fitted on hot. The copper cylinders are stiffened internally by copper fins arranged in diametric planes.

The fins also encourage heat exchange between the metal of the cylinder and the cooling water. The cylinders are fitted with steel cheeks 450 mm in diameter, as shown in FIG. 5.

The horizontal, parallel axes of these cylinders are contained in an oblique plane inclined at 25.degree. to the horizontal, as generally illustrated in FIG. 7b, and are spaced 355 mm apart.

This arrangement improves the feed of liquid corundum-zirconia to the machine. With the cylinders rotating at 3 revolutions per minute and cooling water being supplied at 220 liters per minute per cylinder, it has been possible to solidify corundum-zirconia, containing 25% of ZrC.sub.2, continuously at a rate of 48 kg/minute.

The invention applies more particularly to rapid solidification and cooling, by a continuous casting technique, of non-metallic melted products based on metal oxides, with a melting point of up to 2000.degree. C. This is the case with respect to white corundum, i.e., pure electro-melted alumina; black corundums i.e., electro-melted alumina containing a few percent of impurities, such as oxides of iron, titanium and silicon; corundum-zirconia; metallurgic drosses and slags based on lime, silica, magnesia, alumina; various metal oxides with melting points between 1000.degree. and over 2000.degree. C.; and special abrasives based on alumina and aluminium oxycarbides.