EP0628970B1 - Decanter centrifuge for the treatment of nuclear products - Google Patents

Description

The present invention relates to a centrifugal decanter with continuous evacuation of the solid for the treatment of nuclear products.

Machines of this type are known which generally include a mechanical zone which is separated from the active zone by means of a support structure in which a removable biological protection slab is provided. The machine is generally a pendulum decanter with cycles which comprises a tank integral with the carrying structure and inside which is driven in rotation a decanter bowl. In the usual way, the machine comprises a loading tank and a tubing which projects a jet of liquid under pressure for the extraction of the decanted solid.

The tank is provided with an evacuation of the clarified liquid, a retractable supply of the suspension to be decanted and a draining of the wet solid after decantation.

Cycle operation of the machine requires periodic stops in the flow rate, which involves storing the substance to be decanted. The decanted solid is removed by means of a jet of pressurized liquid which transforms it into a mud which risks blocking the evacuation duct. In addition, the decanted solid comprises, due to its evacuation in wet form, a quantity of liquid which decreases the quality of decantation of the substance.

The machine bowl is provided with a large diameter in order to be able to place the various elements such as the loading tank and the tubing. The large size of the bowl requires a low speed of rotation due to mechanical stress, which implies poor decantation of the substance. Finally, the larger the diameter of the bowl, the greater the volume of substances to be treated, which is contrary to the treatment of nuclear products.

Also known from French Patent No. 2,039,546 belonging to the applicant a centrifugal decanter which comprises a fixed tank inside which rotates a suspended cylindrical bowl. The cylindrical bowl is supported by a drive shaft which is supported by fixed bearings. These are held on a base so that the bowl and the shaft are raised relative to the fixed tank at the same time time that a removable slab allowing the closure of a passage opening provided in a support structure.

In addition, the centrifugal decanter does not operate continuously, since it is mandatory to considerably slow down its speed in order to remove the cake (solid material) contained in the bowl.

In addition, the centrifugal decanter does not include a device to be uncoupled without risk of contamination, that is to say so that there is no communication between the active area and the mechanical area.

It is these drawbacks that the present invention intends to remedy more particularly.

The purpose of the continuous decanter according to the present invention is to reduce the criticality threshold for the same mass of treated product modifying its geometric shape while improving its quality.

To this end, the continuous decanter according to the invention comprises in the mechanical zone an assembly which allows the rotational drive of a decanter bowl and an Archimedes screw arranged in the active zone and sealed means consisting of a bore drilled in the removable slab of the supporting structure, at least one of the shoulders of which has an O-ring, while the decanter bowl is driven in rotation by a shaft passing through the removable slab by a bore to be connected by via a disc with a sleeve guided axially in a frame to cooperate with a reduction gear so that there is play between the shoulder and the disc in the operating position to allow the mechanical assembly to be uncoupled from the bowl and of the screw to be able to intervene on them without risk of contamination.

In addition, the continuous decanter has no bearing or bearing for axially guiding the decanter bowl and the Archimedes screw in the active area.

The appended drawing, given by way of example, will allow a better understanding of the invention, the characteristics which it presents and the advantages which it is capable of providing:

Fig. 1 is a longitudinal section illustrating the continuous decanter according to the present invention.

Fig. 2 is an enlarged section showing the device for coupling the bowl and the Archimedes' screw with the mechanical assembly.

Fig. 3 and 4 are views illustrating the various steps to be carried out to completely or partially remove the mechanical assembly.

Fig. 5 and 6 are sections representing the different stages, after total removal of the mechanical assembly, of the extraction of the bowl and of the Archimedes screw from the active area in a sealed container or "castle".

There is shown in fig. 1 and 2 a centrifugal decanter 1 with evacuation continues from the solid treaty. The decanter 1 comprises a mechanical zone M which is separated from an active zone A by means of a support structure 2 in which a removable slab 2 a of biological protection is provided. The slab 2 a is pierced with a first bore 2 b which communicates co-axially with a second bore 2 c of smaller diameter passing through the rest of the said slab to form a passage between the two zones A and B.

The first bore 2 b is separated from the second 2 c by two successive shoulders 2 d , 2 e , the reason of which will be seen more clearly below.

In the zone M is placed a metal frame 1 a which rests on the supporting structure 2. The frame 1 a supports in the known manner a motor 1 b which drives in rotation by means of one or more belts a reduction gear 1 tsp .

On the frame 1 a and below the reducer 1 c is welded along a vertical axis a tube 1 d in which is provided a bearing 1 e . Inside the bearing 1 e are arranged ball or roller bearings for the axial guidance of a hollow sleeve 1 f , one of the ends of which cooperates by means of an intermediate piece 1 i with the reducer 1 c for its rotational drive. The sleeve 1 f has a flange 1 g which is placed co-axially inside the bore 2 b of the biological slab 2 a . Inside the bore 2b, the flange 1g is fixed by screws on an intermediate disc 3 which is secured to an axle 4 has through the bore 2 c. The axis 4a is secured to a bowl decanter 4 provided in the active region A for driving it in rotation. Inside the bowl 4 is arranged in the known manner an Archimedes screw 5 secured to a shaft 5 a which crosses the axis 4 a of the bowl and which is guided axially inside the sleeve 1 f to come cooperate with the reducer 1 c .

The underside of the intermediate disc 3 is provided just above the shoulder 2 d of the bore 2 b determining therewith an operating clearance X. In addition, the shoulder 2 d has an O-ring 2 f whose outside diameter is slightly smaller than that of the intermediate disc 3.

In the active area A and around the bowl 4 is provided a tank 6 which is fixed to the underside of the support structure 2 so that it surrounds the removable slab 2 a . The tank 6 has in its upper part a discharge 6 a of the clarified liquid. In addition, in the lower part of the tank 6 is provided a fixed supply 6 b for the introduction of the substance to be decanted. The tank 6 has below the feed 6 b a chute-shaped opening 6 c for the extraction of the decanted solid.

A 1- hour double-sided joint is placed on the one hand around the periphery the bore 2b and on the other hand on the outside face of the bearing 1e to constitute a sealed separation between the active zone A and the mechanical zone M during the operation of the decanter 1.

Note that the bowl 4 and the screw 5 are not axially supported in their lower parts and conical by bearings or bearings to facilitate on the one hand their removal and on the other hand the intervention on the mechanical assembly when a breakdown occurs, as will be seen better below.

The continuous decanter 1 described above operates on the same principle as all the other continuous decanters.

The motor 1b , via the reducer 1c , drives the settling bowl 4 and the Archimedes screw 5 at different speeds.

The radioactive substance to be treated is fed into the settling bowl 4 is carried out by the fixed tube 6 b so that said substance arrives in the middle of the thread of the screw 5. The speed difference between the bowl 4 and the screw 5 makes it possible to direct the solid decanted into the lower or conical part of the decanter for its extraction through the orifice 6 c of the tank 6. On the other hand, the clarified liquid performs the opposite path to be evacuated by overflow by means of the tubing 6 a .

An essential feature of the continuous decanter 1 described above is that when a failure occurs in the mechanical assembly located in zone M, an operator can intervene without risk of being contaminated.

Indeed, the removal of the entire mechanical assembly, that is to say for example from the frame 1 a , from the motor 1 b to the reducer 1 c or even the bearing 1 e and the sleeve 1 f , can be carried out without risk of transfer of radioactive particles between the two zones.

In fig. 3, the first step has been shown that the operator must perform before any intervention in order to make zones A and M completely sealed from each other. For this, the operator raises the static seal for 1 hour so as to slide it on the outside wall level 1 e . Then, he removes the screws which hold the flange 1 g of the sleeve 1 f on the intermediate disc 3 secured to the axis 4 a of the bowl 4. When the uncoupling is carried out, it is noted that the underside of the disc 3 comes resting on the shoulder 2 d of the bore 2 b to crush the seal 2 f to constitute a perfect seal between the two zones. It can be seen that the clearance X provided between the underside of the disc 3 and the shoulder 2 d is zero, given that the bowl 4 is lowered vertically inside the tank 6 without risking bearing on the periphery of the upper tube 6 a . Finally, the operator dismantles the reducer 1 c so that the axis 5 a of the screw 5 is released. It can be seen that when the axis 5 a is axially free, the screw 5 descends vertically to come to bear via its thread on the conical part of the bowl 4.

When these different steps are carried out, the operator can then freely disassemble the entire mechanical assembly such as the bearing 1 e and the sleeve 1 f in order to replace the different bearings if necessary, as shown in fig. 4. Similarly, the operator can change the reducer 1 c and the motor 1 b if the case arises.

The operator proceeds in the reverse path to reassemble the mechanical assembly of the continuous decanter 1, the seal between the zone M and the zone A remaining perfect because the disc 3 is in abutment against the shoulder 2 d .

In fig. 5 and 6, the continuous decanter 1 is shown without its frame 1 a , that is to say when the operator has carried out its total removal, as described above. The removal of the frame 1 a allows the operator to intervene on the settling bowl 4 and the Archimedes screw 5 when these are defective. This intervention is carried out in the known manner in a sealed container or "castle", the latter being shown in phantom.

Lifting means, not shown, are fixed on the removable slab 2 a to lift the assembly made up of the bowl and the screw inside the watertight "castle". It is noted that the withdrawal is possible, given that the intermediate disc 3 is supported on the shoulder 2 d of the bore 2 b and that the screw is against the conical part of the bowl.

It is noted that the tank 6 remains integral with the slab 2 as well as the supply 6 b . Note that the removal of the bowl 4 and the screw 5 is carried out without human intervention in the active area A. Before removing the "castle" shown in phantom, a sealed drawer closes the opening of the supporting structure 2 in which is placed the slab 2 a so that no communication can be made between the two zones. The installation of another bowl provided with another screw is carried out under watertight "castle". The operator then proceeds to assemble the mechanical assembly described above until the bowl and the screw are connected to the reducer 1 c .

The continuous decanter 1 makes it possible to extract the solid when dry, which considerably improves the quality of decantation of the substance. In addition, continuous operation increases the efficiency of the machine and avoids storing the substance to be decanted.

It is found that reducing the diameter of the screw 5 compared to conventional machines for an equivalent flow rate makes it possible to considerably increase the speed of rotation of the latter. This high speed of rotation of the continuous centrifugal decanters 1 according to the invention and the extraction of the dry decanted solid considerably improves the quality of settling of the substance.

In addition, the elongated shape of the screw 5 modifies the geometry of the treated product which allows, for the same mass of product to be treated, more radioactive substances without ever reaching the criticality threshold.

Claims (7)

1. A continuous decanter for the processing of nuclear products, of the type comprising an active zone (A) separated from a mechanical zone (M) by a load-bearing structure (2), characterised in that a mechanical assembly (1a, 1b, 1c, 1d, 1e, 1f, and 3) is placed in the mechanical zone (M) to permit a decanter bowl (4) and an Archimedes' screw (5) arranged in the active zone (A) to be driven in rotation about a vertical axis and in that hermetic means consisting of a bore (2b) formed in the removable slab (2a) of the load-bearing structure (2), at least one of the shoulders (2d) of which slab comprises an O-ring (2f), whilst the decanter bowl (4) is driven in rotation by a shaft (4a) passing through the removable slab (2a) through a bore (2c), to be connected by means of a disc (3) to a sleeve (1f) guided axially in a frame (la) to cooperate with a reducer (1c) such that there is a play (X) between the shoulder (2d) and the disc (3) in the operating position to make it possible to uncouple the mechanical assembly from the bowl (4) and the screw (5) in order to be able to intervene therein without risk of contamination.
2. A continuous decanter according to Claim 1, characterised in that a joint (1h) surrounds firstly the bore (2b) and secondly the outer wall of a bearing block (1e) to form a seal between the zone (M) and the zone (A) in the operating position.
3. A decanter according to Claim 1, characterised in that the seal between the zones (A and M) is effected by means of the disc (3) which comes to bear on the shoulder (2d) of the bore (2b) to crush the joint (2f) when the mechanical assembly is removed.
4. A decanter according to Claim 1, characterised in that the decanter bowl (4) and the Archimedes' screw (5) are guided axially solely in the zone (M) by means of rolling bearings and a bearing block (1e).
5. A decanter according to Claim 1, characterised in that upon the dismantling of the mechanical assembly, the Archimedes' screw (5) comes to bear on the conical part of the decanter bowl (4).
6. A continuous decanter according to Claim 1, characterised in that the play (X) is zero when the mechanical assembly is removed.
7. A continuous decanter according to Claim 1, characterised in that the feed means (6b) is fixed to the vessel (6) which is integral with the load-bearing structure (2).

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