GB2263416A

GB2263416A - Solvent extraction

Info

Publication number: GB2263416A
Application number: GB9301160A
Authority: GB
Inventors: George Marshall Gillies; Paul Stephenson
Original assignee: British Nuclear Fuels PLC
Current assignee: Sellafield Ltd
Priority date: 1992-01-24
Filing date: 1993-01-22
Publication date: 1993-07-28
Anticipated expiration: 2013-01-22
Also published as: FR2686525B1; GB2263416B; FR2686525A1; GB9201501D0; JPH05337304A; GB9301160D0

Abstract

A solvent extraction system comprises a pulsed phase mixer column 12, means 18 for supplying a solvent continuous phase to the column, and means 16 for supplying in a counter-current flow, a dispersed aqueous phase to the column to effect mixing between said phases. There are also provided, means 28 for extracting from the said column mixed aqueous and solvent phases, settler 32 for separating the mixed said phases and to which the mixed said phases are arranged to be delivered by the extracting means, and means 40 for returning separated said solvent phase from the separating means to the column. The mixed said phases are extracted at a rate greater than the rate at which the aqueous phase is supplied to the column by the aqueous phase supply means. <IMAGE>

Description

A Solvent Extraction System This invention relates to a solvent extraction system, and more particularly to a pulsed column solvent extraction system.

One of the problems of pulsed column solvent extraction systems is the control of the interface between immiscible solvent and aqueous phases. This is particularly so when the interface is near the bottom of the column (ie solvent continuous) since it can be affected by the pulsing mechanism.

In GB 2217224 A a system is described in which an air lift system is used to deliver mixed phases formed by the pulsed column to an external separator.

Information about the behaviour of the air lift mechanism (inter alia) is measured to provide a means of controlling or shutting down the air lift system in the event of a malfunction. Relatively complicated electronic signal processing if required to process such information.

According to the present invention, there is, provided a solvent extraction system comprising a pulsed phase mixer column, means for supplying a solvent continuous phase to the column, and means for supplying in counter-current flow dispersed aqueous phase to the column to effect mixing between said phases, means for extracting from the said column mixed aqueous and solvent phases, means for separating the mixed phases and to which the mixed phases are arranged to be delivered by the extracting means, and means for returning separated said solvent phase from the separating means to the column, the extracting means being arranged to extract the mixed said phases at a rate greater than the rate at which the aqueous phase is supplied to the column by the aqueous phase supply means.

The separated said solvent phase may be returned to the solvent phase supply means by the separated solvent phase return means so as to be returned to the column, or may be returned at an alternative position such as an intermediate position of the column.

The invention also provides in another aspect a method of solvent extraction in a pulsed column, using the aforementioned system, the method comprising feeding a dispersed aqueous phase and a continuous solvent phase to the column in counter-current flow, extracting mixed said phases from the column, separating the mixed said phases, and returning the separated said solvent phase to the column, the extraction of the mixed said phases being at a rate greater than the rate of feed of the aqueous phase to the column.

By ensuring that the outflow rate of mixed phases from the pulsed column is greater than the feedrate of phases to the column then, in contrast to the known prior art, no control or process measurements are necessary to control the flow within the system successfully.

Embodiments of the present invention will now be further described, by way of example only, with reference to the single Figure, Figure 1 in the accompanying drawing which shows a diagrammatic representation of a solvent extraction system.

Referring now to Figure 1, a solvent extraction system 10 is shown and comprises a column 12 having a number of conventional axially spaced, perforated dispersion plates 14 disposed along the column 12. An aqueous feed line 16 for dispersed aqueous phase connects at one end to an aqueous supply unit 17 and its other end connects near the top of the column 12.

A feed line 18 for a continuous solvent phase connects near the bottom of the column 12. A pulse unit 20 is joined through a line 21 to the column 12 below the solvent feed line 18, and a solvent discharge line 22 connects above the aqueous feed line 16. A pump 26 is connected by a line 28 to the bottom of the column 12, and is arranged to discharge through a line 30 to an annular chamber 34 in a phase separating vessel 32 which is located above the column 12. An inlet 36 between the base of the annular chamber 34 and a central chamber 38 allows flow between the chambers 34, 38. A solvent return line 40 from the annular chamber 34 descends to the solvent feed line 18. An aqueous discharge line 42 in the central chamber 38 but at a lower height in the vessel 32 than the solvent return line 40, discharges to drain (not shown).

In operation, with an aqueous phase feed through the line 16, and a lighter solvent phase feed through the line 18 to the column 12 so as to be in counter-current flow in the column 12, mixing of the solvent phase and the aqueous phase occurs as they pass through the column 12 and as the pulse unit 20 transmits pulses to the column 12 (eg compressed air).

Mixed aqueous and solvent phases are extracted by the pump 26 through the line 28 and are discharged through the line 30 into the annular chamber 34. Phase separation of the mixed phases occurs in the vessel 32 due to their different densities and their immiscibility. Solvent phase in the annular chamber 34 overflows into the solvent return line 40 and is returned by gravity feed to the solvent phase feed line 18. Aqueous phase below the interface with the solvent phase in the vessel 32 flows through the inlet 36 into the central chamber 38 where it overflows into the aqueous discharge line 42 and is discharged to drain by gravity feed. The solvent phase reaching the top of the column 12 is discharged through the line 22.

The rate at which the pump 26 extracts the mixed aqueous and solvent phases is controlled so that it is greater than the rate of feed of the aqueous phase in the supply line 17.

One of the advantages of the invention is that it does not rely on instrumentation to sense an interface in the column 12. Another advantage is that the solvent extraction system is largely self-regulating provided that the flow of the mixed phases from the column 12 is greater than the aqueous feed to the column 12.

It should be noted that there is no aqueous/solvent phase interface in the column 12 below the aqueous feed 16 in operation of the invention.

Although the invention has been described in relation to a phase separation vessel positioned above the column so as to rely on gravity feed for discharge of the separated solvent phase and aqueous phase from the vessel, means such as a pump may be used to discharge the solvent phase and/or the aqueous phase from the vessel. If desired, the vessel may be located below the top of the column if such pump means are provided.

The extraction of the mixed phases should be from below the level of the lowest dispersion plate. The solvent return line 40 may join the column at an intermediate position of the column 12 for some applications of the invention.

The invention has a particular application in the processing of nuclear material, but should also have uses in non-nuclear industries.

Claims

1. A solvent extraction system comprising a pulsed phase mixer column, means for supplying a solvent continuous phase to the column, and means for supplying in a counter-current flow, a dispersed aqueous phase to the column to effect mixing between said phases, wherein there are provided, means for extracting from the said column mixed aqueous and solvent phases, means for separating the mixed said phases and to which the mixed said phases are arranged to be delivered by the extracting means, and means for returning separated said solvent phase from the separating means to the column, the extracting means being arranged to extract the mixed said phases at a rate greater than the rate at which the aqueous phase is supplied to the column by the aqueous phase supply means.

2. A system as claimed in Claim 1, wherein the separated said solvent phase is arranged to be returned to the solvent phase supply means by the separated solvent phase return means so as to be returned to the column.

3. A system as claimed in Claim 1, wherein the separated said solvent phase is arranged to be returned to the column at an intermediate position thereof.

4. A system as claimed in any one of the preceding Claims, wherein the column has a plurality of axially spaced, perforated dispersion plates disposed along the column, the solvent continuous phase supply means is connected to the column at a location below the lowest dispersion plate, and the extracting means is arranged to extract the mixed said phases from the column below the location of the solvent continuous phase supply means.

5. A system as claimed in any one of the preceding Claims, wherein the separated solvent phase return means includes pump means.

6. A system as claimed in any one of the preceding Claims, wherein the dispersed aqueous phase supply means includes an inlet to the column, an outlet for the solvent continuous phase from the column is provided above the inlet, and the separating means is located at a higher position than the outlet.

7. A system as claimed in Claim 5, wherein the dispersed aqueous phase supply means includes an inlet to the column, an outlet for the solvent continuous phase from the column is provided above the inlet, and the separating means is located at a lower position than the outlet.

8. A system as claimed in any one of the preceding Claims, wherein separated said aqueous phase is arranged to be discharged from the separating means by pump means.

9. A method of solvent extraction in a pulsed column, the method comprising feeding a dispersed aqueous phase and a continuous solvent phase to the column in counter-current flow, extracting mixed said phases from the column, separating the mixed said phases, and returning the separated said solvent phase to the column, the extraction of the mixed said phases being at a rate greater than the rate of feed of the aqueous phase to the column.

10. A method as claimed in Claim 9, wherein the separated said solvent phase is returned to the continuous solvent phase being fed to the column, thereby to be returned to the column.

11. A method as claimed in Claim 9, wherein the separated said solvent phase is returned at an intermediate position of the column.

12. A method as claimed in any one of Claims 9 to 11, wherein the mixed said phases are extracted from the column below the continuous solvent phase feed into the column.

13. A method as claimed in any one of Claims 9 to 12, wherein the mixed said phases are extracted from the bottom of the column, and the rate of extraction of the mixed said phases is such as to inhibit the formation of an aqueous/solvent interface in the column below the dispersed aqueous phase feed into the column.

14. A method as claimed in any one of Claims 9 to 13, wherein pump means assist the returning of the separated said solvent phase to the column.

15. A method of solvent extraction substantially as hereinbefore described with reference to the accompanying drawing.

16. Use of the method as claimed in any one of Claims 9 to 15 for the processing of nuclear material by solvent extraction.

17. A solvent extraction system substantially as hereinbefore described with reference to the accompanying drawing.