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(11) | EP 0 211 533 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | A pressure pad for hot pressing bellows |
| (57) A laminated pressure pad (8) comprising a lamination of a first layer (1), a second
layer in the form of a a central refractory block (3,4,5) which may also be layered
and a third steel layer (2). The first layer (1) is resistant to high temperatures
and pressures which occur in a high temperature press and has a coefficient of thermal
conductivity which is greater than that of each other layer. The laminate structure
is secured together by a series of bolts (6) and nuts (7). |
[0001] The present invention is related to a pressure pad for a press, in particular, a press operable at high pressure and temperature. Also, the invention is related especially, but not exclusively, to such a pressure pad for use in a process whereby radioactive nuclear waste material is immobilised in a synthetic rock structure formed at high temperature and pressure in a compressible, bellows-type canister. An intimate mixture of radioactive nuclear waste material and synthetic rock forming material is placed in the canister which is then closed before the hot pressing step.
[0002] The present applicants have developed a process and apparatus for carrying out such hot pressing operations using an electrical induction furnace within which the bellows-type canister is received. At least one hydraulic ram is provided for applying and maintaining high axial pressures on the canister for extended periods of time during which high temperature is maintained. One example of such an apparatus and process is described in the applicant's co-pending European patent application numbers 81303221.6 and 833049745( publn.nos.44,692 4 102,246 resp.)
[0003] Refractory pressure pads are required for abutment with the ends of the canister and these pads must absorb high compression loading at high temperature. Typical operating temperatures are in the range 1150 C to 1200°C and pressures in the range 14 to 21 MPa. For economic reasons it is important to maximize the through-put of the apparatus and it is necessary to bring the canister and its contents up to the necessary working temperature before pressures are applied for the formation of the synthetic rock. Furthermore it is important that the apparatus can function for lengthy periods with minimum maintenance requirements and the apparatus should be of simple yet reliable design to facilitate where necessary periodic replacement of worn parts in a high activity cell.
[0004] It is important to design the process to take account of thermal conditions and in particular the thermal effects of introducing into the press a bellows-type canister at a temperature substantially lower than the final working temperature.
[0005] The present invention is directed towards providing an arrangement characterised by a new pressure pad arrangement which provides improvements, modifications and alternatives to previously known arrangements.
[0006] Accordingly, in a first aspect of the present invention provides a laminated pressure pad (8) resistant to high temperatures and pressures present in a high temperature press, the pad comprising a first layer (1) arranged to engage an article (10) to be compressed and at least one other layer (2,3,4,5), and characterised by the coefficient of thermal conductivity of the first layer (1) being greater than that of the or each other layer.
[0007] Preferably, the first layer is of a high thermally- conductive material, (such as a metallic material), whereas the or at least one of the other layers is of a low thermally- conductive material, (for instance, a ceramic or refractory material).
[0009] Preferably, a second layer of a material having a coefficient of thermal conductivity which is greater than that of the or each other layer is provided on the side of the pad remote from the first layer. In a preferred embodiment, the said other layer of the pad is a single block of refractory material (such as that manufactured by Kaiser Aluminium and Chemical Corporation under the trade name Kriform CCM). However, in the alternative two or three layers of refractory material could be used.
[0010] In a preferred embodiment, the said first layer is a grade of steel selected to resist the high temperatures and pressures which are applied, said other layer is a refractory material and a further outer layer of a suitably selected grade of steel is provided, the structure being fixed together by bolts passing through the structure. Conveniently the bolts are of the same material as said first layer.
[0011] In a second aspect of the present invention there is provided a method of hot pressure a metal canister (10) within an induction furnace with laminated pressure pads as hereinbefore described, characterised in that it includes the steps of placing a metal canister (10) filled with a particulate supply material on a lower pressure pad (8'), raising the canister into the induction furnace (12) until the top of the canister, (10) abuts with a lower surface (1') of an upper pressure pad (8") heating the canister contents and providing a substantially uniform heating of all surfaces of the metal canister (10) as a result of heat radiated by a susceptor plate (9) lining the inside of the induction furnace (12) and heat radiated from the thermally conductive layers (1',1") of each pressure pad (8',8") during pressing of the canister (10).
[0012] In order that the invention may be more fully understood, an embodiment thereof will now be described by way of example and with reference to the accompanying drawings in which:
Figure 1 is a side elevation, in partial section, of a pressure pad for use in a high temperature and pressure sintering press;
Figure 2 is a side elevation, also in partial section, of a high temperature and pressure sintering press incorporating upper and lower pressure pads as shown in Figure 1, with the press in a preheating mode;
Figure 3 is a side elevation, again in partial section, of the sintering press shown in Figure 2 but in a loading mode;
Figure 4 is a side elevation, once again in partial section, of the sintering press shown in Figures 2 and 3 but in an initial sintering mode; and
Figure 5 is a side elevation, yet again in partial section, of the sintering press shown in Figures 2 to 4 but in a final sintering mode.
[0013] Referring first to Figure 1, a laminated pressure pad 8 is illustrated and comprises a lamination of a first layer 1, a central refractory block 3, 4, 5 and a third steel layer 2. The drawing also illustrates in dotted lines an alternative arrangement in which the central refractory block can be replaced by three layers of refractory material. If it is desired the characteristics of the respective layers could differ from layer to layer within such a laminated refractory block.
[0014] The first layer 1 is formed from a steel such as grade MA 956 manufactured by INCOMAP LIMITED which is resistant to the high temperature and pressures which occur in the process and is more thermally conductive than refractory materials. The refractory block 3 is preferably of a moulded refractory material manufactured by Kaiser Aluminium and Chemical Corporation Limited under the trade mark "Kriform CCM". The outer layer 2 is also preferably of steel such as grade 321 stainless steel. The laminate structure is secured together by a series of five spaced bolts 6 (only one of which is shown in the drawing) each bolt being secured by a nut 7. The bolts 6 and nuts 7 are all of grade MA 956 steel.
[0015] Reference will now be made to Figures 2 to 5 which
illustrate a high temperature pressing arrangement for forming synthetic rock incorporating radioactive nuclear waste. The pressure pad laminate 8 shown in Figure 1 is used in the arrangement of Figures 2 to 5 to provide a pressure pad 8' which fits on the head of a water cooled ram 15 and also, in an inverted configuration, for a fixed upper pressure pad 8". The pad 8" is fixed to a press frame 14.
[0016] Figure 2 illustrates the apparatus in a preheating mode in which the ram 15 has been partly raised to insert most of the lower pressure pad 8' into the heating zone of an induction furnace comprising heating coils 13 imbedded in an annulus of refractory material 12 with a metallic susceptor sleeve 9 located within the coils 13. The induction coil 13 causes the generation of heat in the susceptor sleeve 9. The cylindrical zone within the sleeve is heated to a temperature of about 1200 thereby heating at least the upper region of the lower pressure pad 8' and the lower region of the upper pressure pad 8" to this temperature.
[0017] Figure 3 illustrates loading of a bellows-type canister 10 onto the top of the pressure pad 81. -The canister contains an intimate mixture of a radioactive nuclear waste material and synthetic rock forming material which, after hot pressing, comprises three titanate materials, namely Hollandite BaAl2Ti6O16, Zirconolite CaZrTi 207 and Perovskite CaTiO3. plus rutile titanium oxide Ti02 and a small amount of metal alloy. It will be appreciated that withdrawal of the pressure pad 8' from within the susceptor sleeve causes thermal shock on the pressure pad due to radiation of heat therefrom to the surrounding environment and furthermore when the canister 10 is placed on the pressure pad it absorbs heat from the first metal layer 1'.
[0018] The loaded lower pressure pad 8' is then raised into an initial heating mode as shown in Fig. 4. The upper surface of the canister 10 contacts the lower surface of the metallic lamination 1 of the upper pad 8". By virtue of the radiant heat from the inductively heated susceptor sleeve 9, the temperature of the canister 10 and its contents is raised to a minimum of l1500C. As shown in Figure 4 heat from the susceptor sleeve 9 flows to the pads 8, 8', particularly through their respective layers 1, 1', as indicated by arrows B and also heat flows by conduction into the canister 10 from these layers as indicated by arrows C.
[0019] Subsequently, pressure is applied by ram 15 to urge pad 1' and the canister 1.0 upwardly. Pressure in the range 14 21 MPa is maintained until the canister is compressed in size to form compressed canister 10' shown in Figure 5. By this high temperature and pressure method, the contents of the canister are transformed into a synthetic rock incorporating radioactive nuclear waste contained within the compressed canister.
[0020] The lower pad 8' is then withdrawn from the furnace so that the compressed canister 10' and its contents can be removed and another filled canister 10 is loaded onto the lower pressure pad and the cycle repeated.
[0021] As shown by the arrows A in Figure 4, the generally cylindrical side surface of the compressible canister 10 is subjected to direct heat radiation by virtue of the susceptor sleeve 9, such a sleeve and its function and purpose being fully described in our co-pending Patent Application entitled "Inductive Heating Apparatus and Process". Heat transfer to the canister 10 is also greatly enhanced by using the inventive pressure pads 8, 8' with improved conduction being provided by the respective metallic laminations 1. 11, as shown by the arrows B and C in Figure 4.
[0022] Use of at least a preferred embodiment of the invention permits the provision of a relatively simple pressure pad design which nevertheless can accommodate the high temperatures and pressures occurring during the pressing operation. Also the pads can accommodate the thermal shocks which occur during the process and in particular when a pressure pad located on a moving ram is withdrawn from the induction furnace for the purpose of loading a new bellows container. Furthermore, use of the present invention facilitates high press throughput due to the heat flow arrangements as described above which facilitate bringing the entire bellows container and its contents and the pressure pad itself up to the required working temperature before pressure is applied.
1. A laminated pressure pad (8) resistant to high pressures and for use in a high
temperature press, the pad characterised in that it comprises a first layer (1) arranged
to engage an article (1) to be compressed, and at least one other layer (2,3,4,5),
the coefficient of thermal conductivity of the first layer (1) being greater than
that of the or each other layer (2,3,4,5).
2. A laminated pressure pad according to claim 1, characterised in that the said other
layer (2,3,4,5) of the pad is a single block of refractory material.
3. A laminated pressure pad according to claim 1, characterised in that the first
layer (1) is of a high thermally-conductive metallic material and the or at least
one of the other layers (2,3,4,5) is of a low thermally-conductive ceramic material.
4. A laminated pressure pad according to any one of claims 1 to 3, characterised in
that the layers (1,2,3,4,5) are secured detachably together.
5. A laminated pressure pad according to claim 1. characterised in that the first
layer (1) is of a steel of a grade which is highly resistant to high temperature and
pressure applied during a hot pressing operation, said at least one other layer (2,3,4,5)
is provided by a second layer (3,4,5) formed from a refractory material, and the pressure
pad (8) further comprising an outer steel layer (2) connected to the second layer
(3,4,5) on its side remote from the first layer (1), said layers being fixed together
by a plurality of bolts (7) passing through the said layers (1,2,3,9,5).
6. A laminated pressure pad according to any one of claims 1 to 5, characterised in
that it is arranged to fit on the top of an hydraulic ram (8), when in use a metal
canister (10) with a bellows-like wall structure arranged to be placed on the pressure
pad prepared for hot pressing by raising the hydraulic ram (15) into an induction
furnace (12) until the top of the metal canister (10) abuts with a lower surface of
another laminated pressure pad (8") of substantially the same structure as the first
mentioned pad.
7. A laminated pressure pad according to claim 6, characterised in that in a hot pressing
of the metal canister (10), heating of the canister contents is provided by heat radiated
from the highly conductive layer (1',1") of each pressure pad (8',8") in addition to that radiated by induction coils (13) of the induction furnace (2).
8. A method of hot pressure a metal canister (10) within an induction furnace with
laminated pressure pads according to any one of the preceding claims, characterised
in that it includes the steps of placing a metal canister (10) filled with a particulate
supply material on a lower pressure pad (8'), raising the canister into the induction
furnace (12) until the top of the canister, (10) abuts with a lower surface (1') of
an upper pressure pad (8") heating the canister contents and providing a substantially
uniform heating of all surfaces of the metal canister (10) as a result of heat radiated
by a susceptor plate (9) lining the inside of the induction furnace (12) and heat
radiated from the thermally conductive layer (1'.1") of each pressure pad (8!,8") during pressing of the canister (10).