HOT PRESSING OF PARTICULATE MATERIALS

Description

[0001] The present invention relates to the hot pressing of particulate materials and containers for use in such processes.

[0002] It is desirable to form dense block of material from particulate starting materials and processes have been proposed for use in the safe disposal of radioactive waste wherein hot pressing of the particulate material in a container occurs. The hot pressing may be in a uniaxial pressing process or in an isostatic process. Furthermore in the ceramics field parts of machines can be machined from blocks of ceramic material produced in a hot pressing process from particulate starting materials.

[0003] AU-B-72825/81 discloses a canister for use in processing radioactive waste material. The canister has a thin cylindrical wall with a constant cross-section. The canister is filled with radioactive waste in granule form, and then sealed before being heated and axially compressed to sinter the waste.

[0004] Particularly for use in connection with the disposal of radioactive waste in a synthetic rock matrix, it has been proposed, e.g. in EP-A-0115311, to pour the starting materials into a generally cylindrical container having a bellows-like or convoluted side wall before the bellows is closed and subjected to the hot pressing stage. The convoluted side wall has a serpentine shape. Especially, as in EP-A-0115311, when a screen is used to prevent the particulate starting material entering the convoluted region of the side wall, upon compression, the container mainly achieves reduction in volume by axial compression and portions of the wall fold and form a series of radially outwardly extending flange-like formations.

DISCLOSURE OF THE INVENTION

[0005] The present invention is directed towards the provision of containers for hot pressing of particulate materials in which a more advantageous final shape is achieved.

[0006] The pre-characterising part of claim 1 is based on EP-A-0115311, and the distinguishing features of the present invention are set out in the characterising part of claim 1.

[0007] According to the present invention, there is provided a metal thin-walled container having a first end closed by a first end wall and a second end adapted to be closed by a lid after filling with particulate material, a side wall having outer portions and a reduced diameter portion intermediate the first and second ends and a pair of radially inwardly directed portions extending from the outer portions the reduced diameter portion, the side wall being such that when the container has been filled, closed and subjected hot isostatic pressing, the container undergoes significant axial compression, the radially inwardly directed portions closely approach or contact one another and the particulate material is compressed characterised in that each outer portion is outwardly curved to provide a shallow convex structure and the outer portions provide most of the axial extent of the side wall, whereby the volume of the compressed particulate material closely approaches the volume of an imaginary cylindrical envelope in which the compressed container can be accommodated.

[0008] Although the container may have two or more axially spaced reduced diameter portions, an important embodiment of the invention is one in which a single reduced diameter portion is provided.

[0009] Preferably, the side wall has a smoothly curved profile provided at the axial ends thereof. This feature is especially beneficial when the invention is applied with hot isostatic pressing as radially inward compression takes place in this zone. However, embodiments of the invention can also utilise hot uniaxial pressing and for this purpose, preferably, a restraining ring is provided around the reduced diameter portion and extending between the adjacent radially inwardly extending portions of the side wall.

[0010] The relative dimensions of the portions of the container may be varied according to the scale of the embodiment used and the materials adopted. Generally it has been found that especially beneficial embodiments of the invention are ones where the radially inwardly directed portions extend radially a relatively large distance compared with the spacing therebetween.

[0011] Preferably the radially inwardly extending portions extend radially to an extent of about 10% to 25% of the diameter of the container. Also preferably the spacing between adjacent radially inwardly extending portions is of the order of 5% of the diameter of the container.

[0012] Further preferably the radial dimension of the radially inwardly directed portions is in the range of 10% to 20% of the diameter of the container and the spacings between the radially inwardly directed portions is about 5% of the diameter of the container.

[0013] Also preferably the spacing between adjacent radially inwardly directed portions of the side wall is about 10% of the axial dimension of the generally cylindrical portions of the side wall leading to the radially inwardly directed portions. Preferably the side wall of the container has a smooth change of shape with a transition portion extending substantially in a plane transverse to the axis of the container between the reduced diameter portion and the adjacent portion of the container.

[0014] A further advantageous feature which preferably also is utilised is the provision at one or preferably both of the ends of a configuration to permit axial displacement of the transverse end wall during the process. Preferably this is achieved by the side wall curving smoothly inwardly at the end of the container to terminate in an axially directed skirt directed away from the body of the container, the skirt having a diameter similar to the reduced diameter portion of the container and a flanged end wall or lid being utilised within the skirt, its flange also being outwardly directed and welded to the free end of the skirt, whereby the annular region at the end of the container around the outside of the skirt is displaced axially during the hot compression.

[0015] Prior to hot isostatic pressing it is necessary to evacuate the filled container and for this purpose, preferably the end wall having the lid is provided with an evacuation tube which is sealed e.g. by crimping when a vacuum is established.

[0016] Hot isostatic pressing of a particulate material can be performed by utilising a container in any one of the forms described above.

[0017] Use of the present invention offers surprising and major advantages. For example where the production of a synthetic rock matrix incorporating radioactive waste is desired, the previous proposals have resulted in compressed bellows-like containers in which of the order of 40% or more of the volume has not contained the densified synthetic rock and accordingly the costs of providing storage are proportionately higher due to wasted space. This is a very substantial penalty when storage in bores extending severai kilometres into the earth's crust are being contemplated.

[0018] A further requirement is that the compressed container reliably achieves a predictable shape in the process so as to facilitate subsequent handling and storage.

[0019] Although a significant amount of effort has gone into developing hot isostatic processes, some of which use containers with bellows-like or convoluted side walls having a serpentine shape, major problems and indeed serious damage to components of the isostatic press occur when a fault such as a pin hole occurs in the container. It is very difficult to monitor the compression of a container in a hot isostatic press. Under very high gas pressures which must be used, if there is a pin hole in the container a high pressure approaching that of the surrounding gas occurs in the container thereby preventing compression and densification of the contents. A serious problem arises when the pressure is removed for the purpose of retrieving the container from the press; if the pressure is reduced rapidly, and this is usually desirable for economy of operation, the gas pressure inside the container becomes much higher than the surrounding gas and a dramatic axial elongation of the prior art type bellows containers occurs with resultant damage to parts of the press against which impact occurs. However, with the container of the present invention only a limited re-expansion is possible.

[0020] In the case of production of blocks of ceramic material from particulate precursors, it would be desirable to produce disc-like or cylindrical blocks which can then later be machined into parts. However, if the pressing process took place in containers with bellows-like wall structures as described in the prior art, a costly exercise of machining away containers would occur. Use of the present invention, however, permits a closely predictable disc-like shape to be achieved with relatively little loss of material and only a small amount of machining is needed to transform the compressed material into a disc-like ceramic block. Such a disc-like ceramic block would be achieved by sawing through the final container at the region of the or each reduced diameter portion and machining off the thin metal sheath.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] By way of example only, the present invention will be further illustrated with reference to the accompanying drawings, of which:-

Fig. 1 schematically illustrates a filled container embodying the present invention and before compression;

Fig. 2 is a schematic representation of the container of Fig. 1 after compression, but neither Fig. 1 nor 2 are drawn to scale or exactly to scale with one another; and Fig. 3 is a schematic cross-section through a second embodiment of container.

BEST MODES FOR CARRYING OUT THE INVENTION

[0022] Referring first to Fig. 1, the container 10 has a generally cylindrical side wall 11, a base wall 12 and a top cap or lid 13 from which an evacuation tube 14 extends. The side wall 11 has two barrel-shaped portions 15 and 16 which have a shallow convex structure in sectional view and a reduced diameter intermediate portion 17 connected to the major barrel-shaped portions by transition wall portions 18 which extend approximately at right angles to the axis of the container.

[0023] At each of the axial ends of the container, the side wall 11 has a radially inwardly directed shoulder 19 leading to an axially extending skirt 20 which is outwardly directed and which is secured by a weld 21 to a corresponding skirt 22 of the base wall 12 and lid 13 as the case may be.

[0024] Before the cap 13 is installed, the container is filled with particulate material 23 such as ceramic powder or a mixture of radioactive waste and synthetic rock precursor. As shown in Fig. 1 the evacuation tube 14 leads to a ceramic fibre filter 24 retained within the cap 13 by a perforated screen 25.

[0025] This feature is very important where egress of solid powder material with the gas upon evacuation is to be avoided at all costs such as in the case of treating radioactive material.

[0026] After compression, the tube 14 is crimped or sealed at 26 as shown in Fig. 2. As shown in Fig. 2 after hot isostatic pressing a substantial reduction in axial length occurs and there is also reduction in diameter. The transitional wall portions 18 closely approach or even touch one another and the base wall 12 and end cap 13 are axially displaced inwardly as deformation of the shoulder 19 occurs.

[0027] When the invention is applied to the formation of ceramic materials it will be apparent from Fig. 2 that two ceramic discs can be produced without complicated and expensive machining operations and with little loss of ceramic material.

[0028] For some applications the container can be made of mild steel although for other applications more expensive and higher performing alloys may be needed such as selected grades of stainless steel.

[0029] Referring now to Fig. 3 an alternative embodiment is described which is especially suitable for hot uniaxial pressing. The same reference numerals have been used in Fig. 3 to refer to elements corresponding to those of Figs. 1 and 2.

[0030] Container 10 has a generally cylindrical side wall 11, a base wall 12 and a top 27 which includes a reinforced opening 28. Side wall 11 has four barrell-shaped portions 29, 30, 31 and 32 separated by reduced diameter intermediate portions 17. Portions 17 are connected to the barrell-shaped portions by transitional wall portions 18 which extend approximately at right angles to the axis of the container.

[0031] Metal rings 33 are located around the container in the recesses defined by reduced diameter portions 17 and radially extending portions 18. These rings are useful when the container is to undergo hot uniaxial pressing, during which the restraining rings 33 prevent deflections occurring in the bellows and the resulting convolutions which are a cause of wasted space.

Claims

1. A metal thin-walled container having a first end closed by a first end wall (12) and a second end adapted to be closed by a lid (13) after filling with particulate material (23), a side wall (11) having outer portions (15,16) and a reduced diameter portion (17) intermediate the first and second ends and a pair of radially inwardly directed portions (18) extending from the outer portions to the reduced diameter portion (17), the side wall (11) being such that when the container has been filled, closed and subjected hot isostatic pressing, the container (10) undergoes significant axial compression, the radially inwardly directed portions (18) closely approach or contact one another and the particulate material is compressed characterised in that each outer portion (15,16) is outwardly curved to provide a shallow convex structure and the outer portions provide most of the axial extent of the side wall (11), whereby the volume of the compressed particulate material closely approaches the volume of an imaginary cylindrical envelope in which the compressed container can be accommodated.

2. A container as claimed in claim 1, wherein the radially inwardly directed portions (18) extend radially to an extent of about 10% to 25% of the diameter of the container (10).

3. A container as claimed in claim 1 or 2 and wherein the spacing between adjacent radially inwardly directed portions (18) is of the order of 5% of the diameter of the container (10).

4. A container as claimed in any preceding claim and wherein the spacing between adjacent radially inwardly directed portions (18) of the side wall (11) is about 10% of the axial dimension of the outer portions (15,16).

5. A container as claimed in any preceding claim including a single reduced diameter portion (17) midway between the first and second ends.

6. A container as in any one of claims 1 to 4, wherein the side wall has at least one additional reduced diameter portion (17) and an additional outer portion with a pair of radially inwardly directed portions (18) extending from the outer portions to the additional reduced diameter portion (17).

7. A container as claimed in any preceding claim including a metal ring (33) extending around the container between adjacent inwardly directed portions (18) of the side wall (11), so that a metal ring (33) surrounds the or each reduced diameter portion (17) whereby the container may be subjected to hot uniaxial pressing and the or each ring (33) restrains outward deformation of the container (10).

8. A container as claimed in any preceding claim, and wherein each of the outer portions (15, 16) has a smoothly curved profile at its axial ends, the profile turning radially inwardly (19).

9. A container as claimed in any preceding claim and wherein at each of the first and second ends, the side wall (11) curves smoothly radially inwardly (19) and terminates in an axially directed skirt (20) directed away from the body of the container, the skirt having a diameter approximately the same as the reduced diameter portion and a flanged end wall (12,13) being provided to be accommodated within the skirt and to be welded (21) thereto after filling tne container (10), thereby providing an annular region (19) around the skirt to facilitate some axial displacement of the main part of the end wall (12,13) during hot compression of the container (10).

10. A container as claimed in any preceding claim and wherein an evacuation tube (14) is provided at one end of the container (10) whereby after filling and closure, the container (10) may be evacuated and the tube (14) crimped to form a seal (26) whereby hot isostatic pressing may be effected.

11. A method of forming a dense ceramic material comprising filling a container with particulate ceramic precursor material, the container being as claimed in any one of claims 1 to 6 or 8 to 10, evacuating the container and subjecting it to hot isostatic pressing.

12. A method of forming a dense ceramic material comprising taking a container as claimed in any one of claims 1 to 10 and subjecting it to hot uniaxial pressing.

Ansprüche

1. Dünnwandiger Metallbehälter mit einem ersten durch eine erste Abschlusswand (12) abgeschlossenen Ende und mit einem zweiten Ende, das, nachdem ein Teilchenmaterial (23) eingefüllt worden ist, mit einem Deckel (13) verschliessbar ist, mit einer Seitenwand (11), welche Aussenbereiche (15,16), einen Bereich (17) mit reduziertem Durchmesser, der zwischen dem ersten und zweiten Ende liegt, und ein Paar von radial nach innen gerichtete Bereiche (18) aufweist, wobei sich die nach innen gerichteten Bereiche (18) von den Aussenbereichen (15,16) zu dem reduzierten Bereich (17) erstrecken, wobei die Seitenwand (11) so geformt ist, dass, wenn der Behälter gefüllt, geschlossen und isostatischem Heissverpressen ausgesetzt ist, der Behälter (10) bedeutender axialer Kompression ausgesetzt ist und die radial nach innen gerichteten Bereiche (18) sich gegenseitig annähern oder sich berühren und das Teilchenmaterial zusammengepresst wird,
dadurch gekennzeichnet, dass
jeder Aussenbereich (15,16) in Form einer leichten konvexen Struktur nach aussen gebogen ist und dass die Aussenbereiche den grössten Teil der axialen Ausdehnung der Seitenwand (11) gewährleisten, so dass sich das Volumen des zusammengepressten Teilchenmateriales dem Volumen einer imaginären zylinderförmigen Hülle nähert, in die der zusammengepresste Behälter untergebracht werden kann.

2. Behälter nach Anspruch 1, wobei sich die radial nach innen gerichteten Bereiche (18) bis ungefähr 10% bis 25% des Durchmessers des Behälters (10) radial nach innen erstrecken.

3. Behälter nach einem der Ansprüche 1 oder 2, wobei der Abstand zwischen benachbarten radial nach innen gerichteten Bereichen (18) annähernd 5% des Durchmessers des Behälters (10) beträgt.

4. Behälter nach einem der vorhergehenden Ansprüche, wobei der Abstand zwischen benachbarten radial nach innen gerichteten Bereichen (18) der Seitenwand (11) annähernd 10% der axialen Abmessung der Aussenbereiche (15,16) beträgt.

5. Behälter nach einem der vorhergehenden Ansprüche, umfassend einen einzigen Bereich (17) mit reduziertem Durchmesser.

6. Behälter nach einem der Ansprüche 1 bis 4, wobei die Seitenwand mindestens einen zusätzlichen Bereich (17) mit reduziertem Durchmesser und einen zusätzlichen Aussenbereich mit einem Paar radial nach innen gerichteten Bereichen (18) aufweist, wobei sich diese nach innen gerichteten Bereiche (18) von den Aussenbereichen (15,16) zu dem zusätzlichen reduzierten Bereich (17) erstrecken.

7. Behälter nach einem der vorhergehenden Ansprüche, umfassend einen Metallring (33), der den Behälter zwischen benachbarten radial nach innen gerichteten Bereichen (18) der Seitenwand (11) umgibt, so dass ein Metallring (33) den oder jeden Bereich (17) mit reduziertem Durchmesser umgibt, wodurch der Behälter uniaxialem Heissverpressen ausgesetzt werden kann und der oder jeder Ring (33) äussere Deformationen des Behälters (10) verhindert.

8. Behälter nach einem der vorhergehenden Ansprüche, wobei jede der Aussenbereiche (15,16) an seinem axialen Ende ein sanft gebogenes Profil aufweist, wobei die Profile radial nach innen gerichtet sind (19).

9. Behälter nach einem der vorhergehenden Ansprüche, wobei die Seitenwand (11) an jedem der ersten und zweiten Enden sanft radial nach innen gebogen ist (19) und in einem axial gerichteten Rand (20) endet, der vom Körper des Behälters weggerichtet ist und der annähernd denselben Durchmesser aufweist wie der Bereich mit reduziertem Durchmesser und wobei eine geflanschte Abschlusswand (12,13) vorhanden ist, die innerhalb des Randes (20) angeordnet ist und mit diesem verschweisst (21) wird, nachdem der Behälter (10) gefüllt worden ist, wobei ein ringförmiger Bereich (19) um den Rand vorhanden ist, um während dem Heissverpressen des Behälters (10) axiale Verschiebungen des Hauptteils der Abschlusswand (12,13) zu erleichtern.

10. Behälter nach einem der vorhergehenden Ansprüche, wobei an einem Ende des Behälters (10) ein Entlüftungskanal (14) vorhanden ist, so dass der Behälter (10) nach dem Füllen und Verschliessen evakuierbar ist und der Kanal (14) mittels isostatisches Heissverpressen zu einer Abdichtung verformbar ist.

11. Verfahren zur Herstellung eines dichten keramischen Materiales, wobei ein Behälter nach einem der Ansprüche 1 bis 6 oder 8 bis 10 mit keramischem Teilchenmaterial gefüllt wird und der Behälter evakuiert und isostatischem Heissverpressen ausgesetzt wird.

12. Verfahren zur Herstellung eines dichten keramischen Materiales, wobei ein Behälter nach einem der Ansprüche 1 bis 10 uniaxialem Heissverpressen ausgesetzt wird.

Revendications

1. Conteneur à paroi métallique mince ayant une première extrémité fermée par une première paroi d'extrémité (12) et une seconde extrémité adaptée à être fermée par un couvercle (13) après remplissage par un matériau particulaire (23), une paroi latérale (11) ayant des parties extérieures (15,16) et une partie de diamètre réduit (17) intermédiaire entre les première et seconde extrémités, et une paire de parties (18) orientées radialement vers l'intérieur, s'étendant depuis les parties extérieures vers la partie (17) de diamètre réduit, la paroi latérale (11) étant telle que quand le conteneur a été rempli, obturé et exposé à un pressage à chaud isostatique, le conteneur (10) subit une compression axiale significative, les parties (18) orientées radialement vers l'intérieur se rapprochent de très près ou viennent en contact l'une de l'autre, et le matériau particulaire est comprimé, caractérisé en ce que chaque partie extérieure (15, 16) est incurvée extérieurement pour constituer une structure convexe peu marquée et les parties extérieures constituent le principal de l'étendue axiale de la paroi latérale (11), d'où il résulte que le volume du matériau particulaire comprimé approche de près le volume d'une enveloppe cylindrique imaginaire dans laquelle le conteneur comprimé peut être reçu.

2. Conteneur selon la revendication 1, dans lequel les parties (18) orientées radialement vers l'intérieur s'étendent radialement sur une étendue d'environ 10 à 25% du diamètre du conteneur (10).

3. Conteneur selon la revendication 1 ou la revendication 2, dans lequel l'écart entre les parties adjacentes (18) orientées radialement vers l'intérieur est de l'ordre de 5% du diamètre du conteneur (10).

4. Conteneur selon l'une des revendications précédentes, dans lequel l'écart entre les parties adjacentes (18) orientées radialement vers l'intérieur de la paroi latérale (11) est environ 10% de la dimension axiale des parties extérieures (15, 16).

5. Conteneur selon l'une des revendications précédentes, comportant une seule partie (17) de diamètre réduit à mi-distance entre la première et la seconde extrémité.

6. Conteneur selon l'une quelconque des revendications 1 à 4, dans lequel la paroi latérale a au moins une partie (17) de diamètre réduit additionnelle et une partie extérieure additionnelle avec une paire de partie (18) orientées radialement vers l'intérieur s'étendant depuis les parties extérieures jusqu'à la partie (17) additionnelle de diamètre réduit.

7. Conteneur selon l'une quelconque des revendications précédentes, comportant un anneau métallique (33) s'étendant autour du conteneur entre les parties (18) adjacentes orientées vers l'intérieur de la paroi latérale (11), de sorte qu'un anneau métallique (33) entoure chaque partie (17) de diamètre réduit, d'où il résulte que le conteneur peut être soumis à un pressage à chaud uniaxial et le, ou chaque, anneau (33) limite la déformation du conteneur (10) vers l'extérieur.

8. Conteneur selon l'une quelconque des revendications précédentes, dans lequel chacune des parties extérieures (15, 16) a un profil légèrement incurvé à ses extrémités axiales, le profil tournant radialement vers l'intérieur (19).

9. Conteneur selon l'une quelconque des revendications précédentes, dans lequel à chacune des première et seconde extrémités, la paroi latérale (11) s'incurve légèrement radialement vers l'intérieur (19) et se termine par une jupe (20) dirigée axialement, orientée en s'écartant du corps du conteneur, la jupe ayant un diamètre approximativement le même que la partie de diamètre réduit et une paroi d'extrémité (12, 13) à bride étant prévue pour être logée dans la jupe et y être soudée (21) après remplissage du conteneur (10) réalisant ainsi une région annulaire (19) autour de la jupe pour faciliter certains déplacements axiaux de la partie principale de la paroi d'extrémité (12, 13) pendant la compression à chaud du conteneur (10).

10. Conteneur selon l'une quelconque des revendications précédentes, dans lequel un tube d'évacuation (14) est prévu à une extrémité du conteneur (10), d'où il résulte qu'après remplissage et obturation, on peut faire le vide dans le conteneur (10) et le tube (14) serti pour former un joint (26), d'où il résulte que l'on peut appliquer une pression isostatique à chaud.

11. Procédé pour former un matériau céramique dense comportant le remplissage d'un conteneur avec un matériau particulaire précurseur de céramique, le conteneur étant comme revendiqué dans l'une quelconque des revendications 1 à 6 ou 8 à 10, faire le vide dans le conteneur et l'exposer à une pression isostatique à chaud.

12. Procédé pour former un matériau céramique dense comportant l'étape de prendre un conteneur comme revendiqué dans l'une quelconque des revendications 7 à 10 et de l'exposer à une compression uniaxiale à chaud.

Drawing