US2968551A

US2968551A - Method of sintering compacts

Info

Publication number: US2968551A
Application number: US761798A
Authority: US
Inventors: Edward D North; James A Rode; Gervaise W Tompkin
Original assignee: Mallinckrodt Chemical Works
Current assignee: Mallinckrodt Chemical Works
Priority date: 1958-09-18
Filing date: 1958-09-18
Publication date: 1961-01-17
Anticipated expiration: 1978-01-17
Also published as: DE1189832B; GB897689A; FR1235187A; NL243509A; BE582803A

Description

Jan. 17, 1961 E. D. NORTH ETAL METHOD OF SINTERING COMPACTS Filed Sept. 18, 1958 FIGI.

FIGZ.

United States Patent METHOD or SINTERING COMPACTS Edward D. North, Waterloo, Ill., and James A. Rode, Crestwood, and Gervaise W. Tompkin, Crystal Lake, Mo., assrgnors to Mallinckrodt Chemical Works, St. Louis, Mo., a corporation of Missouri Filed Sept. 18, 1958, Ser. No. 761,798

4 Claims. (Cl. 75-223) The present invention relates to powder metallurgy and more particularly to the production of sintered fuels for use in nuclear reactors.

Briefly, the method of the present invention relates to the formation of sintered compacts of metallic oxides and metals by continuous means. The method employs unsintered compacts of at least one powdered metal substance and sinters these without sacrifice of density of product and without substantial fracture of the compacts or agglomeration thereof. The method is particularly useful for sintering uranium dioxide compacts which are to be used in fuel elements for nuclear reactors.

Among the several objects of this invention may be noted the provision of improved methods for sintering compacts of powdered metals or metallic oxides; the provision of methods of the character described which provide for simultaneously removing heat-fugitive binders, lubricants, orother temporary additives; the provision of methods of the character described which can be carried out with, simple equipment and materials; the provision of methods of the character described which are economical with respect to power consumption; and the provision of methods of the character described which are adaptable either to continuous or semi-continuous operation. Other objects and features will be in part apparent and in part pointed out hereinafter.

The invention accordingly comprises the methods hereinafter described, the scope of the invention being indicated in the following claims.

In the accompanying drawing, in which one form of apparatus useful in carrying out the invention is shown,

Fig. 1 is an elevation partly in section; and,

Fig. 2 is a perspective of a compact.

Corresponding reference characters indicate corresponding parts throughout the drawing.

It is known to form cold compacts of powdered uranium dioxide, usually cylindrical in form, by compressing the powder in suitable dies, and then sintering the compacts at elevated temperatures to obtain dense forms of uranium which are useful in fuel elements for nuclear reactors.

It has also been customary to prepare the uranium dioxide compacts described above with the aid of a small amount, in the order of 0.1 to about 6%, of a heat-fugitive binder such as, for example, paraffin. The uranium dioxide may be enriched or depleted with respect to U or it may contain the naturally occurring proportions of isotopes. While the invention will be more fully described in terms of preparing such products, its usefulness is not so limited. The purpose of the binder is twofold. First, it increases the density of the compacts by reducing interparticle friction and friction between the particles and the walls of the die in which the compacts are formed. Second, it increases the green, i.e., unsintered, strength of the compacts, thus allowing them to be handled with less chance of their becoming broken or deformed before sintering. The binder is then driven off during the subsequent sintering operation. In the case of uranium 2,968,551 Patented Jan. 17, 1961 dioxide, it is important for the binder to be one which is completely removed from the compacts without leaving any carbonaceous residue which might form uranium carbide during the sintering operation. The compacts are next heated to a temperature suflicient to drive off the binder, which in the case of paraffin is in the neighborhood of 400-500" C. Thereafter, the temperature is increased to 1500 C. or above to complete the sintering of the compacts.

The operations of dewaxing and sintering the compacts are often carried out in separate furnaces, the compacts being transferred from one to the other to avoid the time lost in cooling and reheating the furnace when only one furnace is used. To guard against contamination and also to facilitate handling the green compacts, they are usually supported in molybdenum boats during the de waxing and firing operations.

The conventional methods for making such sintered compacts are both laborious and expensive. The necessity for twice loading and unloading furnaces for each batch of compacts is obviously time-consuming, and the use of molybdenum containers is expensive and their capacity is limited. Also, conventional continuous furnaces are usually not practical because of their high cost.

From the foregoing discussion it is apparent that there is need for a simpler and more efficient method of sintering uranium dioxide compacts. Such a method should be readily adaptable to either large or small scale manufacture, and it should be capable of either continuous or semi-continuous operation using simple and readily available equipment and materials.

According to the present invention, the green, uranium dioxide compacts are dispersed in a body of roughly spherical, chemically pure, free-flowing granules of a more refractory material such as fused alumina or zirconia, the granules being of such a size that they effectively surround each compact and prevent it from coming in contact with other compacts or with the walls of the furnace. This mixture is then allowed to gravitate slowly through a vertical, substantially tubular furnace filled with the mixture, preferably in a protective atmosphere such as that provided, in the case of uranium dioxide, by a countercurrent stream of hydrogen or steam. The

upper portion of the furnace is provided with heating means so that the compacts are gradually heated to their sintering temperature as they move through the furnace. This temperature is insufficient to sinter the refractory granules. During this period the binder, if one is present, is driven off and carried away with the stream of gas passing through the furnace. The lower end of the furnace, which is unheated, provides a zone wherein the compacts are gradually cooled by the stream of gas to a temperature at which they can be conveniently removed and handled.

It is important for the granular refractory material to be pure with respect to certain impurities, especially iron oxide and silicon dioxide, which may act as fluxes and cause the granules to become fused together or to the compacts themselves at the temperatures employed. The refractory granules should also be free-fiowing and of sufficiently uniform particle size to minimize packing. The particle size is not critical, provided that it is small enough so that the granules effectively separate the individual uranium dioxide compacts from each other and from the wall of the furnace, but not so small as to cause excessive interference with the passage of gas through the furnace. For the purposes of the present invention, a particle size corresponding to about 8-12 mesh has been found generally satisfactory.

The furnace consists essentially of a tube of a refractory material such as, for example, fused zirconium oxide. It should of course be free from any volatile impurities which might contaminate the uranium dioxide.

'It is important to establish a temperature gradient in the mixture between the upper end and the zone of maximum temperature so that the compacts are gradually heated to the sintering temperature. It will be apparent that the movement of the mixture of compacts and granules through the furnace will itself establish such a gradient and no other precautions are usually necessary. However, more heat may be supplied to the furnace at or near the sintering zone than at the upper end of the furnace, if necessary, to establish optimum firing conditions.

The dimensions of the furnace will depend upon its intended capacity and the output of the heating element used. These in turn will depend upon the maximum sintering temperature to be attained and the length of time .the compacts are to be kept at or near this temperature.

It will be clear that these are variable factors, and that the design of a suitable furnace to meet any particular set of conditions is well within the skill of those trained in the art.

The upper portion of the furnace is conveniently heated by means of an electrical resistance element, either placed externally around the furnace or actually embedded in the walls of the furnace itself, an arrangement which is feasible with non-conductive furnace materials. The lower unheated end of the furnace serves both as a cooling zone for the sintered compacts and as a preheater for the stream of protective gas passing through the furnace.

The rate at which the mixture of compacts and alumina pass through the furnace is determined by the rate at which the fired mixture is removed from the lower end of the furnace. This may be accomplished in a simple manner by attaching a length of flexible tubing to the lower end of the furnace. The end of the tube is then closed with a suitable clamp, or even with the fingers, and opened periodically to permit removing a portion of the fire mixture. Alternatively, the lower end may be a constriction of such a size that the mixture is allowed to escape from the furnace at a rate corresponding to the desired rate of passage of the mixture through the furnace. Using the previously described length of flexible tubing, the mixture can be withdrawn from the furnace in the following manner without substantial escape of hydrogen or without interrupting its flow through the furnace. With the lower end closed, the flexible tubing is closed off at a second point a suitable distance from the end, thus isolating the portion of the mixture to be removed. The lower end of the tubing is now opened and the isolated portion is emptied into a suitable receptacle. When the lower end of the flexible tubing is again closed and the upper constriction opened, the contents of the furnace move a corresponding distance downwards. As sintered material is removed from the lower end of the furnace fresh quantities of green mixture are added at the top.

Instead of the simple arrangement of flexible tubing or a constriction at the bottom of the tubing as described above for controlling the passage of the mixture through the furnace, more elaborate mechanical valves are available which may be used for this purpose if desired.

The process of the present invention is useful for sintering compacts at temperatures up to the sintering temperature of the refractory granules. This upper temperature varies depending upon the chemical nature and physical form of the granules which are used. Aluminum oxide (Alundum) bubbles, for example, begin to sinter at temperatures near 1650 C. While zirconia bubbles and sand begin sintering at somewhat lower temperatures, up to about 1600 C. The temperatures given above are illustrative only, and are subject to some variation depending upon the source of the granules and the criteria used to determine the sintering temperature.

While the invention has been described as applied to the dewaxing and sintering of uranium dioxide fuel compacts, the method is equally applicable to the sintering or heat treatment of other solid compositions and substances, such as the sintered metal and metal oxide objects commonly fabricated by powder metallurgy. For example, the method is applicable to the sintering of oxide mixtures which are even more refractory than uranium dioxide, the upper limit being determined by the sintering temperature of the refractory granules themselves and/ or the limitations of the furnace. Instead of an atmosphere of hydrogen or steam, the furnace may be operated with an atmosphere of some other gas, such as, for example, nitrogen, oxygen, argon, carbon dioxide, or air, depending upon the nature of the material being sintered. The green compacts may be prepared by compressing them in a suitable die, or they maybe prepared by slip casting or by extrusion. Moreover, other binders may be used either in addition to or in place of paraffin. For example, other waxes, stearic acid, cellulose derivatives such as carboxymethylcellulose, and heat-fugitive polymeric materials such as polyvinyl alcohol and polyethylene glycol are other materials used for this purpose, while esters of organic acids are frequently used as lubricants for extruded shapes.

From the foregoing description it will also be clear that the process of the present invention can be operated either continuously or semi-continuously using simple equipment. Even without the aid of any mechanical or automatic equipment little handling of the compacts is required since the green mixture is easily prepared and the sintered compacts are readily separated from the fired mixture by a screening operation, whereby the alumina is recovered for use in sintering additional compacts. In addition, a suitable furnace is easily constructed from standard materials and apparatus, no specially manufactured forms or devices being necessary.

Many other variations of the present invention will be apparent to those skilled in the art.

The following examples illustrate the invention.

Example 1 A furnace 1 suitable for use with the method of the present invention was constructed from a 36 in. length of fused zirconium oxide combustion tube 3 having an internal diameter of 1% in. Tube 3 was supported in a vertical position and the upper one-half of its length was heated by surrounding it with a heavy duty resistance heater 5 having a silicon carbide element 7. To control the rate at which the contents of the furnace gravitate through tube 3, a length of rubber tubing 9 was attached to the lower end of combustion tube 3 and closed with a series of pinch clamps 11. An inlet 13 for the introduction of hydrogen was also provided at the lower end of combustion tube 3 above clamps 11 closing the rubber tubing.

The furnace 1 was next filled with fused alumina bubbles 15, i.e., hollow spheres 8-12 mesh in size, and heated to an operating temperature of 1520 C. Hydrogen was introduced into gas inlet 13 at a rate sufficient to exclude all air from the furnace, particularly from the zone at the upper end of the combustion tube where the pellets are heated to the sintering temperature. Bubbles 15 were withdrawn from the bottom of the furnace by manipulating pinch clamps 11 in the manner previously described and recycled to the top of the furnace to establish that they would flow satisfactorily.

When the furnace had reached its operating temperature, green uranium dioxide compacts 17, which had previously been dewaxed, were dropped on top of alumina bubbles 15 at 15-minute intervals along with additional bubbles. These particular compacts were cylindrical in form and had a length of 2 cm. and a diameter of 1 cm. At the same time, alumina bubbles 15 were withdrawn from the lower end of the furnace at such a rate that compacts 17 were drawn down into the firing zone of the furnace at the rate of about 16 inches per hour. Under these conditions, the residence time of the compacts in the firing zone was approximately one hour. The density of the compacts fired in this manner was 10.0-10.2 g./cc.

There was no significant attrition of the compacts during the firing process, nor was there any plugging of the combustion tube during operation. It was also evident that much less power was required to attain a given firing temperature by this method than if the compacts had been fired 'm an open tube.

Example 2 Green uranium dioxide compacts 17 similar to those described in Example 1 but containing 0.5% parafiin and 1% polyvinyl alcohol were also fired in the same furnace. In this case the maximum temperature of the firing zone was 1500 C. and the compacts were passed through the furnace at the rate of about 8 inches per hour. Sintered compacts having a density of 9.0-9.5 g./cc. were produced in this manner.

If the rate at which the compacts passed through the furnace was increased to 16 inches per hour, there was appreciable cracking of the fired compacts, probably because of excessively rapid heat-up with the generation of high internal pressures in the compacts.

Example 3 Example 1 was repeated except that the refractory granular material 15, which serves as a support for the uranium dioxide compacts during the sintering operation, was composed of fused zirconium dioxide rather than fused aluminum oxide. The granules were solid and of a size comparable to those employed in Example 1. The results were essentially comparable to those obtained when the granular refractory material was fused aluminum oxide.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above meth ods without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

We claim:

1. The method which comprises adding freeflowing granules of refractory material having dispersed therein unsintered compacts of at least one powdered metal substance selected from the group consisting of metallic oxides and metals, which compacts sinter substantially below the sintering temperature of the refractory granules, to the top of a vertical, substantially tubular furnace filled with the mixture, only the upper portion of the furnace being heated; removing the mixture of compacts and granules from the lower end of the furnace at a controlled rate so that the mixture slowly gravitates through the furnace and is thereby gradually heated to a temperature sufiicient to sinter the compacts but insufficient to sinter the refractory granules and then gradually cooled; and thereafter separating the sintered compacts from the refractory granules.

2. The method which comprises adding free-flowing granules of refractory material having dispersed therein unsintered compacts of powdered uranium dioxide, which compacts sinter substantially below the sintering temperature of the refractory granules, to the top of a vertical, substantially tubular furnace filled with the mixture, only the upper portion of the furnace being heated; removing the mixture of compacts and granules from the lower end of the furnace at a controlled rate so that the mixture slowly gravitates through the furnace and is thereby gradually heated to a temperature of at least 1500 C. but insufiicient to sinter the refractory granules and then gradually cooled; and thereafter separating the sintered uranium dioxide compacts from the refractory granules.

3. The method which comprises adding free-flowing granules of refractory material dispersed therein unsintered compacts of powdered uranium dioxide and a heat-fugitive binder, to the top of a vertical, substantially tubular furnace filled with the mixture, only the top of the furnace being heated and the furnace further being provided with means for passing a flow of gas through the furnace countercurrent to the direction of movement of the mixture; removing the mixture of compacts and granules from the lower end of the furnace at a controlled rate so that the mixture slowly gravitates through the furnace and is gradually heated to a maximum temperature of approximately 1500 'C., during which time the heat-fugitive binder is driven off and removed with the outgoing gas, and then gradually cooled; and thereafter separating the sintered uranium dioxide compacts from the refractory granules.

4. The method of sintering compacts of a metal substance which comprises surrounding the compacts with free-flowing granules of a more refractory material, allowing the mixture of compacts and granules to gravitate through a vertical, substantially tubular furnace wherein the compacts are gradually heated to a temperature sufiicient to sinter the compacts, but insufficient to sinter the refractory granules, gradually cooling the mixture and separating the sintered compacts from the refractory granules.

References Cited in the file of this patent UNITED STATES PATENTS UNITED STATES PATENT OFFICE CERTIFICATIGN 0F CORRECTION Patent No; 2,968,551 January 17, 1961 Edward Do North et a1,

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3, line 39, for "fire" read fired column 6, line 2 1 after "material" insert having Signed and sealed this 13th day of June 1961a 3 EA L) Attest:

ERNEST W. SWIDER Attesting Officer DAVID L. LADD Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATION 0F CORRECTION Patent Nod $968,551 January 17, 1961 Edward D, North at aly It h'ereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3, line 39, for "fire" read fired column 6, line 24!: after "material" insert having Signed and sealed this 13th day of June 1961 SEAL) Attest:

DAVID L. LADD ERNEST W. SWIDER Attesting Officer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATION OF CORECTIO'N Patent Nod 2368 551 January 17, 1961 Edward D, North e1; alo

It is h'ereby certified that error appears in the above numbered patenfo requiring correction and 'that the said Letters Patent should read as corrected below.

Column 3, line 39, for "fire" read fired column 6, line 241, after "material" insert, having t.

Signed and sealed this 13th day of June 1961.

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attesting Officer Commissioner of Patents

Claims

1. THE METHOD WHICH COMPRISES ADDING FREE-FLOWING GRANULES OF REFRACTORY MATERIAL HAVING DISPERSED THEREIN UNSINTERED COMPACTS OF AT LEAST ONE POWDERED METAL SUBSTANCE SELECTED FROM THE GROUP CONSISTING OF METALLIC OXIDES AND METALS, WHICH COMPACTS SINTER SUBSTANTIALLY BELOW THE SINTERING TEMPERATURE OF THE REFRACTORY GRANULES, TO THE TOP OF A VERTICAL, SUBSTANTIALLY TUBULAR FURNANCE FILLED WITH THE MIXTURE, ONLY THE UPPER PORTION OF THE FURNACE BEING HEATED, REMOVING THE MIXTURE OF COMPACTS AND GRANULES FROM THE LOWER END OF THE FURNACE AT A CONTRLLED RATE SO THAT THE MIXTURE SLOWLY GRAVITATES THROUGH THE FURNACE AND IS THEREBY GRADUALLY HEATED TO A TEMPERATURE SUFFICIENT TO SINTER THE COMPACTS BUT INSUFFICIENT TO SINTER THE REFRACTORY GRANULES AND THEN GRADUALLY COOLED, AND THEREAFTER SEPARATING THE SINTERED COMPACTS FROM THE REFRACTORY GRANULES.