Technical Field

The present invention relates to vibratory calciners. Calcination is a process in which particulate material is subjected to elevated temperature to remove volatiles and to achieve a reactive powder product. By way of example only, the present invention will illustrated by application to a particularly demanding application in the calcination field, namely the calcinat of synthetic rock precursor loaded with nuclear waste material which is to be immobilised in the final syntheti rock. Particularly for handling high level radioactive waste, the most stringent protective measures and safety precautions must be taken. As will be apparent to those skilled in the art, most importantly the equipment must h an inherent design which permits it to operate with at mo simple maintenance to be conducted only with manipulators and a very long working life is needed. Furthermore, a relatively compact structure is desirable in view of the very high cost of building an active cell which is a safe containment structure for handling radioactive materials.

Summary of the Invention

The present invention, at least in one embodiment, can be especially useful in providing major advances and advantages for disposal of high level radioactive waste i synthetic rock but the invention is not limited to these applications and the apparatus and methods described here can advantageously be applied to analogous manufacturing methods and equipment design.

The present invention broadly provides an apparatu for calcining particulate material, the apparatus compris

(a) a calcining structure defining a path between first and second vertically spaced locations, (b) means for introducing the particulate material at the first location, (c) means for discharging calcined particulate material at the second location, (d) heating means for establishing a calcining temperature along at least part of the path for calcining the particulate material, (e) vibrator means for vibrating the calcining structure to cause advance of the particulate material along the path, (f) the calcining structure being closed for providing a controlled environment along the path and (g) the calcining structure providing ramp means inclined at a shallow angle to the horizontal and extending along the path for supporting the particulate material and being generally serpentine such that the path length is relatively large compared with the distance between the first and second locations in the vertical direction and composed with the dimensions of the calcining structure in a horizontal direction.

As will be described in more detail below, the present invention can be embodied in various forms of whic two important forms are:

(i) an apparatus in which the ramp means comprises series of separate ramp elements partially overlapping one another and providing a zig-zag path down which the particulate material is adapted to be moved under operatio of the vibrator means; and

(ii) an arrangement in which the ramp means is a generally spiral tubular processing duct and in this case the vibrator means can be a means to move the particulate material upwardly through the duct.

In the first specific embodiment, the calcining structure in horizontal cross-section can be generally rectangular with the ramp elements rigidly connected to th side walls, each ramp element extending from one side wall and terminating just short of the opposite side wall.

The vibrator means preferably comprises a primary vibrator connected to a central base portion of the calcining structure and adjacent a lower discharge duct, in addition an intermittent pulse vibrator is advantageo added to ensure reliable control of the process and stea advance of the particulate material.

The apparatus preferably includes at opposite end inlet and outlet connections to a supply of gas for providing a controlled atmosphere within the calcining structure. For example for the purpose of calcining synthetic rock particulate material incorporating high l radioactive waste, a counter-current flow of a mixture o hydrogen and nitrogen is desirably utilised, the mixture typically comprising 3.5% hydrogen. Preferably, the vibrator means comprises a vibrat unit mounted to vibrate a base portion of the calcining structure. The vibration mode typically has a frequency about fifteen hundred cycles per minute with an amplitud about three millimetres with a lift and twist motion. In the second specific embodiment of the inventio continuous duct may be provided for example of spiral fo In this specific aspect, there is provided an apparatus calcining particulate material, the apparatus comprising support structure in the form of a generally spiral tubu processing duct, means for mounting the duct with the ax around which the duct extends being a vertical axis, an inlet at one end of the duct adapted to be connected to means for supplying the particulate material and an outl at the opposite end of the duct for discharging the calc material, vibrator means for vibrating the duct in a mod cause an advancement of the particulate material from th inlet to the outlet and heating means for establishing a calcining temperature in the tubular duct for calcining particulate material. In the second embodiment of the invention, the

tubular duct may be wound helically with the inlet located at the bottom end and the outlet extending from the upper end of the helical shape, the tubular duct being closed and having means for connection to atmosphere control means whereby the calcination can take place under a controlled atmosphere.

The form of the tubular duct can be varied to suit the application but it has been found especially beneficial to provide a generally rectangular cross-sectional shape. Advantageously the tubular duct has contiguous turn which abut and support one another and the bottom face of the tubular duct is supported by resting on a multiplicity of spaced support elements which are rigidly connected to the base plate. In the interests of providing an efficient, compact and practical device, the apparatus preferably includes a discharge duct extending from the outlet of the tubular du to a location below tubular duct for discharging the particulate material into a collection hopper. Furthermor preferably the apparatus includes a supply hopper arranged at a higher elevation than the inlet to the tubular duct a having a feed device to supply the particulate material progressively to the tubular duct. Preferably a screw fee device is provided and flexible tubular couplings connect the screw feed device to the inlet to the tubular duct and connect the outlet of the tubular duct to the discharge du which leads to the collection hopper.

The heating means can be of any suitable form but preferably is arranged as a furnace structure extending around the vertical axis of the support structure; for example the furnace heating elements can be arranged in a cylindrical form and an insulation structure is provided above and below the support structure.

For many applications temperature and corrosion resistant metal alloys are used with advantage for the

calcining structure. For example, the calcining of synthetic rock components can be conducted to a temperatu of about 850°C and at these temperatures alloys such as Sandvik 253MA or Inconel 601 materials can be used. However, other embodiments of the invention may be applicable to even higher temperature calcination and for that purpose the duct is to be fabricated from materials resistant to higher temperatures. For example, very high temperature metal alloys or ceramic materials could be considered where calcining temperatures of the order of 1500°C are required.

Many calcining operations require a controlled atmosphere and embodiments of the present invention can readily provide for highly reliable and effective control the atmosphere. Advantageously a counter current gas flo can be provided by providing means for connecting a select gas supply to the outlet from the tubular duct and a gas discharge system connected at the region of the inlet to t tubular duct whereby counter current gas flow may be provided.

This latter option is especially important for calcining synthetic rock incorporating high level nuclear waste as a reducing gas atmosphere must be provided during the calcining step. A most beneficial arrangement is one which a mixture of hydrogen and nitrogen as a reducing gas is supplied into the collection hopper and this flows in counter current through the processing duct.

For the second specific embodiment, preferably two off-gas pipes are connected to an inlet pipe which leads into the inlet to the tubular spiral processing duct and arranged such that a first pipe removes water vapour resulting from the decomposition that takes place in the calcining step and oxides of nitrogen are discharged into second pipe.

In order to control the atmosphere reliably, a further inlet for nitrogen gas is provided between the connection point to the device for supplying the particulat material and the inlet end of the spiral tubular duct. For example nitrogen gas is supplied to pressurize this portion of the system.

According to a second aspect of the invention there is a provided a method of calcining a particulate material comprising using apparatus as described above in any one of the embodiments indicated. The method is especially advantageous when applied to the difficult problem of calcining synthetic rock precursor which incorporates high level radioactive waste. For this embodiment a controlled atmosphere must be provided with an off-gas processing system to deal with radioactive volatile components release in a small quantity during the calcination and in particula it is necessary to deal with cesium, ruthenium and molybdenum.

It has surprisingly been found that a most beneficia range of characteristics can be obtained particularly with tests in relation to synthetic rock precursor components. The advantages which may be achieved include the following at least with a preferred embodiment of the invention:

(a) a relatively compact apparatus can be built at a economic cost and only a relatively small amount of expensive active cell volume is required to accommodate th apparatus;

(b) the apparatus is intrinsically simple with few parts liable to require maintenance and a long working lif measured in many years should be achievable;

(c) a very high throughput rate is achievable while simultaneously producing a very desirable particle structu in the resultant material; the particulate material advanc at a highly uniform rate along the duct and a residence ti as short as about 2 minutes can be achieved while providin full calcination reliably and achieving an excellent

particle structure. In particular a granulated type structure which does not cake or have fine dust particles an unacceptable quantity and which can readily be poured subsequent processing steps is achievable. This is most important as the calcined product, when incorporating radioactive waste, is intended to be poured into containe which are then subjected to hot pressing operations to fo the final synthetic rock and reliable handling of the calcine product is vital; (d) the vibration along the duct has been found to keep the particulate material in continuous agitation and can ensure excellent contact between the gas atmosphere a the powder;

(e) the granular structure of the resultant calcin powder has been found to have good pourability, low dust generation, good packing density and is easy to handle;

(f) because the system provides progressive progre of the particulate material through the duct, reliably al particles have sufficient residence time in the calcinati zone to achieve full de-nitration in contrast to results achievable in continuous fluidised bed calciners where so powder is likely to have a very short residence time;

(g) most importantly, a controlled atmosphere can provided in embodiments of the invention without using an mechanical sliding seals such as would be required in a rotary calciner and this aids minimum maintenance requirements; and

(h) the equipment can be designed such that when components of the apparatus wear out replacement is quite feasible in an active cell.

Brief Description of the Drawings

By way of an example only, embodiments of the invention will now be described with reference to the accompanying drawings of which:

Figure 1 is a schematic elevation of a first embodiment utilising a continuous spiral ramp;

Figure 2 is a second embodiment using a downwardly directed zigzag ramp structure; and Figure 3 is a schematic plan view of the embodiment of figure 2.

Detailed Description of Preferred Embodiments

Both of the illustrated embodiments are particularly adapted for use in a radiation cell and for calcining synthetic rock components incorporating high level radioactive waste.

Referring first to figure 1, the apparatus comprises a helical tubular vibrating duct 10 disposed about a vertical axis and arranged for conveying and calcining particulate material as it moves upwardly along the interio ramp-like surface of the duct. The duct has an inlet portion 11 lowermost and an outlet portion 12 uppermost and is surrounded by a furnace 13 with insulation material not shown in the drawing for providing thermal efficiency. The desired temperature gradient is established along the duct which defines the processing path in accordance with the material to be calcined.

The helical duct comprises a multiplicity of turns 9 of a substantially cylindrical cross-section metal alloy duct, the turns being in abutment one on top of the other and providing a shallow ramp, and the lowermost turn is supported by a series of spaced support feet 14 which are rigidly interconnected to a base plate 15 which is mounted through a series of circumferentially spaced spring mounts 16 to support the tubular duct on a support frame 17. An insulation layer 18 overlays the base plate and is disposed beneath the tubular duct. The base plate is connected to motorised vibrators 19 which typically vibrates the base plate at a frequency of around fifteen hundred cycles per minute and with an amplitude of around three millimetres.

It has been found this will cause particulate material t vibrated within the duct and to move progressively up th shallow ramp-like surface of the helical duct.

The inlet portion 11 to the duct incorporates a connection 20 to a source of nitrogen gas and has a flex coupling 21 connecting the inlet portion to the discharg from a screw feeder 22 introducing the particulate mater progressively from a supply hopper 23. The screw feeder packs the supply material tightly and sufficiently to effectively provide a gas seal and the nitrogen gas pres at the inlet prevents the escape of volatile products resulting from the calcination process. A controlled atmosphere is provided in counter current flow within th helical duct 10. The gases are discharged through first second discharge tubes 24 and 25 which connect to the in portion at spaced locations and respectively are positio for discharging water vapour and oxides of nitrogen to a off-gas system from which radioactive materials can be recovered. The discharge portion 12 of the helical duct is connected to a discharge pipe 26 which. leads downwardly through the base plate 15 to a flexible coupling 27 to a fixed outlet pipe 28 from which the calcined material is discharged into a collection hopper 29 which is sealed. collection hopper 29 has a gas inlet connection 30 for connection to a reducing gas supply whereby this gas move in counter current relationship through the apparatus. calcining synthetic rock with high level radioactive wast a convenient reducing gas is a mixture comprising 3.5% hydrogen and the balance nitrogen (by volume) .

Referring now to figures 2 and 3, the calciner comprises a rectangular cross-section vertical tubular du 40 surrounded by a furnace 41 with insulation surrounding the furnace and extending to top and bottom insulating en walls 42 and 43. Within the duct 40 are provided a serie of ramp plates 44 arranged in an overlapping zigzag

configuration and rigidly attached respectively to the side walls and one of the end walls of the duct. At its lower end the duct 44 tapers to a tubular discharge duct 45 aroun which an inclined angular wall 46 is provided and through which the duct is mounted by support springs 47. The discharge duct 40 terminates with a flexible coupling 48 which in turn connects to an outlet pipe 49 for the calcine particulate material. Above the flexible coupling 48, a primary vibrator 50 is mounted for providing the desired normal vibrations to the support structure for advancing th particulate material. A second vibrator 51 is also provide and adapted to be operated intermittently in a pulse mode t provide greater vibrations when required.

The upper end of the duct 40 is connected to an inle duct 52 which is coupled through a flexible connector 53 to an inlet pipe 54 leading from a supply of the particulate material.

For the purpose of maintaining the desired reducing gas atmosphere in the calciner, a gas inlet duct 55 is provided at the bottom of the duct 40 and connected to a suitable source of a mixture of hydrogen and nitrogen typically comprises 3.5% hydrogen. In the centre of the upper end of the duct 40 a gas discharge duct 56 is provide and this couples through a flexible connector 57 to an off gas pipe 58. The gas discharge duct 56 and the particulate material inlet pipe 54 are supported at the upper end of th apparatus by a spring structure 59.