TECHNICAL FIELD

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

It is desirable to form dense blocks 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.

Particularly for use in connection with the disposal of radioactive waste in a synthetic rock matrix, it has been proposed 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 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 QF THE INVENTION

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.

According to the present invention, there is provided a generally cylindrical, 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 providing a reduced diameter portion intermediate the first and second ends, the side wall having generally cylindrical portions and radially inwardly directed portions extending therefrom to the reduced diameter portion, the radially inwardly directed portions being adjacent to one another, the side wall being such that, when the container has been filled, closed and subjected to hot isostatic pressing, the container undergoes significant axial compression of the container occurs and the radially inwardly directed portions closely approach or contact one another 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.

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.

Preferably, the side wall of the container between an end thereof and the reduced diameter portion is generally barrel-shaped and a smoothly curved profile is 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.

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.

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.

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.

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.

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.

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.

The invention also extends, in another aspect, to a method • of hot isostatic pressing of a particulate material in utilising a container in any one of the forms described above.

Use of the present invention offers surprising and major advantages, at least when preferred embodiments are used. 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 several kilometres into the earth's crust are being contemplated.

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

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.

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

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

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.

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.

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. 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.

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.

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.

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

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.

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.

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.

Although the invention has been described with reference to particular examples, it should be appreciated that it may be embodied in many other forms. For instance the number of reduced diameter portions may be varied to suit the process and the material contained within the container.