GB2173120A - Centrifugal contacting apparatus for solvent extraction - Google Patents

Abstract

A single-stage centrifugal contacting unit for solvent extraction comprises a central rotor shaft 6 formed as a single integral piece together with the annular separation cup 7 of the contacting unit within which incoming liquid phases, mixed in chamber 9 for good transfer of components, arrive via duct 14 and are re-separated by centrifugal force into two phases separately expelled, by rotor-mounted blades defining impellers 17, 18, through liquid outlets, 29, 30. Shaft 6 has moreover an internal drain channel 27, 28 to the exterior, connected with the bottom of said cup 7. Drive means for a plurality of units connected in countercurrent liquid flow can be constituted by a support for the contacting units surrounding a single motor with a vertical shaft driving, by a gear wheel, a plurality of gear wheels mounted one on each motor shaft. <IMAGE>

Description

SPECIFICATION Centrifugal contacting apparatus for solvent extraction The present invention relates to centrifugal contacting units and to apparatus including such units, suitable for solvent extraction.

Such equipment can be used for recovering from liquid materials, compounds or chemical elements.

In particular it is suitable for treating industrial liquids for the recovery of materials such as precious metals. In a particular application such apparatus, embodied as an array comprising a number of centrifugal contacting units, is useful for studies and researches on a pilot scale. Separation and recovery by solvent extraction of actinide elements from solutions of irradiated nuclear fuel, or of uranium from thorium by means of selective solvents can also be effected.

Centrifugal contacting units are already utilised both in the nuclear and non-nuclear industries.

They are used in research and development reprocessing or irradiated nuclear fuels, but may also be advantageously used outside of the nuclear context since they consume less energy, and are ecologically more acceptable, than other processes.

In reprocessing of nuclear waste, centrifugal contacting units have the advantage over other known extraction contacting units (of pulsed column type or of the mixer-settler type) of being able to handle large quantities of liquids with an extremely short contact time between phases, whereby they are suitable for the reprocessing of high burn-up nuclear fuels in a manner which limits the radiolysis of the solvent.

Centrifugal contacting units may also be utilised for the treatment of industrial liquids to recover precious metals or to eliminate harmful elements.

Solutions to be treated should be clarified; this may be carried out with centrifugal clarifiers, decanters, etc., and is important with pulsed column or mixer settlers contacting units, in order to avoid the clogging of the apparatus.

Known types of centrifugal contacting apparatus have certain disadvantages. For example, one known type of centrifugal contacting unit, has in a single machine body, several extraction stages arranged in a vertical sequence. Mixers and separators are located on a single rotary axis, the whole assembly being powered by a single motor. Clogging in any of the stages unavoidably causes the shut-down of the whole system.

Moreover, a known kind of single-stage contacting unit has a mixer arranged underneath. The liquid rises owing to the centrifugal forces, and then exits. Such con#tacting unit does not have pumping systems for the transport of liquids and this adversely affects the efficiency of the apparatus.

Systems are also known comprising a plurality of contacting units arranged in sequence, in a horizontal straight line, and powered by a common transmission belt.

In other known single-stage contacting units, the liquids exit by gravity, and therefore still without the help of pumping systems with disadvantages as before. In yet another known type of singlestage centrifugal contacting unit, which is provided with pumping means, the body of each rotor is powered by a motor of its own; a multistage system cannot readily be based on this since a number of motors corresponding to the number of stages is required. This entails problems of structure and bulkiness of construction, and problems of maintaining uniform constant rotational speed for all units.

In the known contacting units the rotor has as assembly of a separation cup and rotary shaft in several parts joined together. This gives balance problems; fine machining accuracy is needed, and is expensive without even then providing any certainty of a perfect alignment of the parts.

Another problem in known centrifugal contacting units is that when the contacting unit is stopped at the end of an operative cycle the liquid phases present in the separation cup exit laterally. They traverse the lower body of the unit through a venting hole in the fixed part of the unit. Corrosive agents present in the liquid phases may therefore reach the bearings of the rotor, which constitute the most vulnerable part of the unit, and damage them.

Transport of the liquid phases from one contacting unit to another (when these are assembled together) is conventionally effected by one of two working arrangements, a horizontal or a "staircase" arrangement. In the first case the two phases are caused to travel by centrifugal expulsion units.

In the second one phase (usually the aqueous one) is pumped by a centrifugal expulsion unit while the other (usually the organic one) is transferred by gravity. The horizontal arrangement is the most suitable for processing operations where space is limited and the dimensions are standardised (for instance glove-boxes) but needs an actuating member for the array of contacting units which is compact and of limited dimensions. The present invention relates to an operating arrangement of the horizontal type.

The present invention sets out to provide centrifugal contacting units and apparatus for solvent extraction which eliminates the drawbacks and inconveniences of the previously known apparatus.

In one aspect of the invention consists in a single-stage centrifugal contacting unit for solvent extraction, of the type comprising: a vertical rotor shaft within a stator housing; inlets for two solvent phases; means mounted on the shaft to mix the two phases and thereby increase transfer of a conveyed component between the phases; a structure surrounding, carried by, and spaced from the shaft to define an annular separation cup in communication with the mixed phases, wherein the phases can re-separate under centrifugal force as two layers of different densities; expulsion means for the separated phases; and outlets for the expelled phases: in which unit the separation cup and rotor shaft are an integral structure, and a longitudinal gravity drainage duct extends within the shaft and communicates with the separation cup.

Preferably, the interior of the stator housing is shaped to define two annular chambers, each communicating with one of the outlets, and in which blades are mounted on the shaft to define two expulsion impellers rotatable one within each annular chamber to expel the separated phase therein.

In other aspect the invention consists in apparatus for solvent extraction comprising at least one unit as described above together with drive means for the rotor shaft.

Such apparatus preferably comprises a plurality of the sa#id units with a common drive means, and being mutually interconnected at their liquid inlets and outlets so that the two liquid phases travel within the plurality of units in countercurrent with one another.

The common drive means preferably comprises a single motor with a vertically extending shaft carrying a gear wheel,the plurality of said units being located jointly to surround the motor shaft, each having its rotor shaft provided with a rotor shaft gear wheel which meshes with the motor shaft gear wheel for common operation.

Constructional and functional advantages derive from apparatus of the kind described above. Making the rotor shaft integral with the separation cup resolves the problem of balance. Moreover, the protection of the support bearings of the rotor is assured by the drain channel, which allows the separation cup to empty through the shaft of the rotor, in such a way so that the bearings cannot be reached by corrosive and harmful agents. Moreover, actuation with a single motor and transmission gears gives a structure having a circular arrangement, very simple and compact, which ensures good operating characteristics.

The invention will be further described with reference to the accompanying drawings of an embodiment thereof by way of-non limitative example. In these drawings: Figure 1 is a somewhat diagrammatic side elevation partly sectioned longitudinally of the upper part of a centrifugal contacting unit according to the present invention; Figure 2 is a somewhat diagrammatic side elevation, partly sectioned longitudinally, of an assembly shaft with a separation cup being part of the rotor of a contacting unit slightly modified with respect to that shown in Figure 1; Figure 3 is a somewhat diagrammatic side elevation on a smaller scale, partly sectioned longitudi- nally of an entire centrifugal contacting unit, with slight modifications with respect to that partially shown in Figure 1; and Figure 4 shows in part-section the actuating device of an array of centrifugal contacting units utilising the features shown in the preceding Figures.

A centrifugal contacting unit 1 comprises two main parts, a fixed stator 2 and a rotor 3 rotatable around its own longitudinal axis within the stator 2. In its upper part the stator 2 has two nipples 4 and 5, for the introduction of the liquids to be treated. The main part of the rotor 3 is an assembly having a rotating central shaft 6 and an annular separation cup 7. The upper end of shaft 6 carries radial blades 8, housed within a mixing chamber 9 itself separated from underlying chamber 10 by means of an annular disc 11 provided with through holes, arranged with radial symmetry. Annular disc 11 is carried by the stator 2. Near the shaft 6 the stator is shaped to provide chamber 10 defining an overflow. Under the wall 12 the stator has a further circular wall 13 whereby an annular channel 14 is defined with, and around the upper part of, the shaft 6.The stator 2 has two other sloping internal walls 15 and 16, arranged around its inner periphery, located one above the other, and arranged to form two annular chambers 17 and 18, respectively.

Welded on the free upper lip of the separation cup 7 is a system of formed walls, in a circular ar rangement Firstly, is provided wall 19. Secondly, wall 20, provided in its lower part with a circular array of through holes 21 and defining- with wall 19, a channel 22. Thirdly wall 23 which extends downwards into the separation cup to define with the vertical portion of said wall 10 an annular channel 24, and to define with the shaft 6 an annular channel 14', which communicates at its upper part with said channel 14.

Walls 19 and 20 each hav#e in their upper parts a number of horizontal through holes and carry at these upper parts blades 25 and 26, housed within said chambers 18 and 17, respectively. Blades 25 and 26 are consequently a part of the rotor.

Along the longitudinal central axis of the lower part of shaft 6 is a channel 27 which is open at the bottom end of the shaft and communicates with the bottom of separation cup 7 by means of channel 28.

The body of the stator 1 has two outlet openings 29 and 30, communicating with said chambers 17 and 18, respectively, and a venting hole 31.

The rotatable mounting of the shaft 6 of the rotor to the body of the stator 2 is achieved by ball bearings 32 and 32' (see Figure 3).

Sealing is achieved by permanent deformation of adjacent machined edges to give a metal-to-metalseal without the interposition of rubber or pike seals between such parts.

Treatment of a liquid containing a substance that has to be extracted from it by means of a solvent is as follows.

Two solutions are prepared, an aqueous- phase FA containing said substance, and an organic phase FO containing said solvent.

The aqueous and organic phases are fed through the respective manifolds 4 and 5 to the centrifugal contacting unit, previously set in rotation. Said two phases first reach the mixing chamber 9, where they are brought to the state of a very dispersed emulsion by means of the blade impeller 8. This emulsion passes towards the underlying chamber 10 through the holes in the annular disc 11 The chamber 9, because of its geometry, allows the formation of emulsion; it has smaller dimensions than chamber 10, so as to obtain the emulsion-in a short time. Chamber 10, where the two phases remain in the emulsion state, allows the exchange of matter between the two phases.

The emulsion passes over the wall 12, runs through the channels 14 and 14' under gravity, along the surface of the shaft 6, and arrives within the separation cup 7. Within this cup the emulsion is centrifugally decomposed into the two original phases FA and FO, which become stratified (according to their specific gravity) in correspondence with an interphase radius determined by the particular operational parameters of the contacting unit.

For maximum efficiency, it is advisable to arrange the interphase radius in a position such that the volumes of the aqueous and organic phases within the separation cup are equal.

The time elapsing between the mixing of the phases (chamber 9) and the arrival of the resulting emulsion within the separation cup 7 is the actual contact time during which the transfer of matter from one phase to the other e.g. FA to FO, takes place.

Centrifugal force causes the aqueous phase FA to pass through holes 21 and channel 22 and arrive within chamber 18. Similarly, organic phase FO rises along the channel 24 and arrives within the chamber 17. Thereafter, phases FA and FO are expelled from said chambers 18 and 17, towards the exterior of the centrifugal contacting unit, through the apertures 30, 29, respectively, by means of the action of the blade impellers 25, 26, respectively.

At the end of each run, i.e. when the rotor of the rotor of the contacting unit is stopped, the separation cup 7 automatically empties through the drain channel 28, 27, and the contents can be collected in a suitable vessel under the shaft 6.

Simple computation can show that, during the operation of the contacting unit, since the centrifugal force is much higher than the force of gravity, leaking cannot occur through the drain channel 28 as long as the rotor is rotating.

The hole 31 does not discharge liquid under normal conditions on stopping the contacting unit; it vents and discharges to the exterior in those (exceptional) cases when feed is much greater than the capabilities of the apparatus.

A particular kind of actuating device suitable for an array of contacting units will now be described with reference to Figure 4.

In such a device a horizontal circular support 33 is fastened by bolts 34 and spacers 35 fastened onto a body 36, which in turn is anchored to a number of legs 37 which rest on antivibrating blocks 38 (in the drawing only one of the legs 37 has been shown).

Body 36 is provided with a central cylindrical bushing 39 carrying in its lower parts support members for an electric motor 40, the shaft 41 of which is rotatably journalled by means of bearings 42 at the centre of said horizontal support 33. This support 33 has a number of circular holes, equally spaced around a circular path. Within each such hole the lower terminal part of a centrifugal contacting unit 1 is inserted, and locked in position by tightening a screw 43.

Transmission of motion from the motor 40 to the individual contacting units 1 is performed through the meshing of a central gear wheel 44, around the upper end of the shaft 41 of the motor, and a toothed wheel 45, arranged around the lower end of the shaft 6 of each centrifugal contacting unit 1.

Vibrations are greatly damped by blocks 38 and by interposing an elastic coupling between motor 40 and toothed wheel 44. Moreover, the support members of the motor 40 where carried by the bushing 39 also comprise an appropriate elastic material.

The central toothed wheel 44, besides transmitting rotary motion, serves as a flywheel of the device and as a cooling means for the rotating members; this latter function is performed by fastening to it a fan (not shown in Figure 4), which, at the operational speed of the device, produces an air motion to ensure acceptable limits for the operating temperatures.

With simple structural modifications the motor 40 may be mounted in an overlying position rather than an underneath position. Also, a lateral, or other position is possible. The controls and auxiliary parts can be located remote from the rotary members.

In a particular embodiment the device according to Figure 4 comprises a support 33 provided with six equally spaced holes for receiving six contacting units 1. With this compact arrangement it is possible to treat very large quantities of liquids.

The array operates at 6000 revolutions per minute with a capability ranging from a few liters/hour up to a maximum of 40 liters/hour. With one liter of aqueous phase and one liter of organic or organic phase a chemical process may be tested without a reciprocal pollution of the phases.

The centrifugal contacting units are numbered clockwise, the first being the one within which the organic phase is introduced, and the sixth being the one where the aqueous phase is introduced.

The first and sixth contacting units need not be provided with the blade expulsion impeller 25 since only single phase feed takes place in each case.

In operation, the array is started and brought up to 6000 revolutions per minute. The aqueous phase is introduced to the sixth contacting unit. When it starts to flow out of the first contacting unit the organic phase is introduced into the first contacting unit. Steady state operation is considered to have started at that moment when the two streams start to flow regularly out of the respective outlets of the array.

The flows of the streams of the two phases consequently take place in countercurrent. The expulsion impeller for the organic phase of a given contacting unit transfers this phase onward to the numerically subsequent contacting unit, while the expulsion impeller for the aqueous phase of the said given contacting unit transfers the aqueous phase back to the numerically preceding contacting unit A number of such arrays I, II, etc may be connected with each other following the order 6, 611 - 1",, etc., the upper numeral representing the array. When the need arises of having a number of arrays in this way, it will be necessary to introduce any lacking impellers when the connection of two arrays is effected.

#Connection conduits between contacting units can be stainless steel flexible tubing. Each array can be provided with a sampling system with switching eiectrovalves with simultaneous control, and with a device for the collection of the phases when the contacting units are brought to rest.

Feeding of the solutions may be effected with centrifugal pumps or by gravity or other liquid feeders. Measurement of the flow rates may be carried out with rotary meters after calibration. During steady state operati-on samples can be taken at regular time intervals into suitable flasks. Before the solutions are exhausted, flow-blocking valves are closed and the contacting units are stopped.

Because of the uniform modular nature of the contact units, each array may operate with six or less, down to one, contacting units. Each contacting unit can be easily removed from support 33 by releasing blocking screw 43 and by decoupling of the connections with adjacent contacting units. If a contacting unit becomes clogged it can therefore be replaced with these simple operations without upsetting the remaining system. This is a clear advantage over systems wherein clogging or one stage entails the shut-down of and interference with the whole system.

It is also possible to introduce a third phase into any of the stages of the array of contacting units.

Moreover, by substituting one or more single-stage -contacting units with other similar contacting units but with a different rotor body, larger ranges of equilibrium specific gravities can be dealt with. The simple assembly of contacting units also allows simple liquid washing and cleaning of the conduits of the whole circuit.

The apparatus shows considerable versatility, and can be used to a system comprising a number of contacting units according to the requirements of the particular chemical treatment.

Claims (7)

1. A single-stage centrifugal contacting unit for solvent extraction, of the type comprising: a vertical rotor shaft within a stator housing; inlets for two solvent phases; means mounted on the shaft to mix the two phases and thereby increase transfer of a conveyed component between the phases; a structure surrounding, carried by, and spaced from the shaft to define an annular separation cup in communication with the mixed phases, wherein the phases can re-separate under centrifugal force as two layers of different densities; expulsion means for-the separated phases; and outlets for the expelled phases: in which unit the separation cup and rotor shaft are an integral structure, and a longitudinal gravity drainage duct extends within the shaft and communicates with the separation cup.

2. A unit as claimed in claim 1 in which the interior of the stator housing is shaped to define two annular chambers, each communicating withone of the outlets, and in which blades are mounted on the shaft to define two expulsion impellers- rotatable one within each annular chamber to expel the separated phase therein.

3. Apparatus for solvent extraction comprising at least one unit as claimed-in Claim 1 or Claim 2 together with drive means for the rotor shaft.

4. Apparatus as claimed in Claim 3, comprising a plurality of the said units with a common drive means, and being mutually interconnected at their liquid inlets and outlets so that the two liquid phases travel within the plurality of units in countercurrent with one another.

5. Apparatus as claimed in Claim 4, wherein the common drive means comprises a single motor with a vertically extending shaft carrying a#gear wheel, and the plurality of said units are located jointly to surround the motor shaft, each having its rotor shaft provided with a rotor shaft gear wheel which meshes with the motor shaft gear wheel for common operation.

6. Apparatus for solvent extraction comprising more than one of the pluralities of units claimed in Claim 4 or 5.

7. A contacting unit for solvent extraction or apparatus comprising at least one such unit, substantially as herein described with reference to, and as illustrated in, the accompanying drawings.