GB2171026A - Liquid-liquid extraction - Google Patents

Abstract

A method of effecting liquid- liquid contact between an aqueous liquid medium and an organic hydrophobic liquid medium capable of undergoing mass transfer with the aqueous medium and apparatus suitable for use therewith are described. The extraction is effected in a mixer-settler (1) in which an a.c. electric field is applied to at least one of the upper and lower interfaces (13, 14) thereby to assist in controlling the position of that interface within the chamber. This results in improvement in the volumetric throughput and the single mass transfer efficiency and a reduction of aqueous and organic entrainment. <IMAGE>

Description

SPECIFICATION Liquid-liquid extraction This invention relates to liquid-liquid extraction.

There are many industrial processes in which liquid-liquid extraction is practised. Such processes involve intimately contacting an aqueous phase with an organic phase that is substantially immiscible with the aqueous phase so that a component present in one phase can undergo mass transfer from that phase to another. Such mass transfer can involve a variety of mechanisms at the molecular level. For example, water or alkaline solutions can be used to remove acidic components or contaminants from organic solutions.

Another type of mass transfer, which is widely used in hydrometallurgical processes, involves extraction from a dilute aqueous solution of metal ions contained therein using a suitable chelating agent dissolved in an organic solvent, for example an aromatic hydoxy-oxime dissolved in kerosene for copper extraction.

In such mass transfer processes it is usual to mix the two phases so as to form a dispersion of one phase in the other. After a suitable residence time in the mixing zone the dispersion is then allowed to settle out under gravity and the disengaged phases are then separated before further processing takes place. For best results the two phases should be mixed in an approximately 1:1 ratio by volume since this maximises the inter-phase interfacial area in the dispersion and hence maximises the rate of mass transfer, besides improving the phase separation and the entrainment performance.The forms of apparatus used for such mass transfer processes are often termed "mixer-settlers" Conventionally mixer-settlers include separate mixing and settling chambers, the mixing chamber usually being relatively compact and containing an impeller to produce the desired dispersion; often such mixing chambers are cuboid or in the form of a vertical cylinder.

The settling chamber is usually relatively shallow and has a much larger horizontal area of cross section than the mixing chamber so as to facilitate disengagement of the phases. Particularly when using flammable solvents, such as kerosene, there is a considerable fire hazard. In addition, in cases where dilute solutions are being treated, it may be necessary to recycle large volumes of one of the phases in order to get the desired throughput of phases; for example, process considerations may dictate that the phases are fed to the mixer settler at a feed rate ratio of 50:1 by volume, so that, in order to operate the mixing chamber at a feed rate ratio of the phases of about 1:1, it is necessary to recycle 49 volumes of one of the phases for every one volume of the other phase that is fed to the mixer-settler.Further details of such mixer-settlers can be obtained, for example, from a paper entitled "Design of Large Scale Mixer Settlers" by J.B. Lott, G.C.I. Warwick, and J.B.Scuffham, SME Transactions, 252 (March 1972). A design of mixer-settler in which a central mixing tank is arranged to feed radially a number of separate segment-shaped settling tanks arranged in circular fashion around the mixing chamber is described in CB-A-1507665; in this design coalescer pads of "KnitMesh DC" are provided in each settling tank.

Also of interest are US-A-3206288, US-A-3844723, US-A-3984331, GB-A-2024645, FR-A-2276853 and AU-A-503940.

Other designs of apparatus for carrying out liquid-liquid contact have involved passage of the liquid phases in countercurrent through vertical or inclined drums, often divided into several compartments by transverse partitions and fitted with stirrers. Examples of such designs are to be found in US-A-2029691, US-A-22 18080, US-A-2667407, US-A-2735755, US-A-2850362, US-A-3032403, US-A-3062627, GB-A-210774, GB-A-791025, GB-A-1091554 and GB-A-1314124. In at least the majority of these designs it is difficult to control the phase ratio in the dispersion, particularly in the event of a change in flow rate, i.e. an interruption or a surge, of one or both of the liquids being treated.

A more recent design of mixer-settler has been proposed in which a single vessel is used to effect both the mixing and settling steps of the mass transfer process. This has been termed a combined mixer-settler (or CMS, for short). Its design utilises an impeller to form the dispersion in a mixing zone in the vessel, above and below which mixing zone there are provided baffles to permit formation of layers of the lighter phase and of the heavier phase respectively. The disengaged lighter phase is allowed to overflow a first weir whilst the disengaged heavier phase is recovered via a riser to overflow a second weir.

By adjusting one or both of the two weirs to set the settled interface (that is to say the interface between the phases under flow conditions but with the impeller stopped) at a predetermined level in the mixing zone, the phase ratio in the dispersion can be maintained substantially at the desired ratio, e.g.

about 1:1 by volume, even though the ratio of the feed rates of the two phases to the vessel may be disparate, e.g. the feed rate ratio may be about 10:1 by volume. In this way mixing can be carried out under near ideal conditions and external recycle of one of the phases is not needed if the process requirements dictate a feed rate ratio to the apparatus, e.g. about 10:1 by volume or more, that is different from the 1:1 by volume ratio that is desired in the dispersion. With this design, the apparatus can be run so that an interruption or a surge in supply of one of the liquid phases to the apparatus does not upset the phase ratio in the dispersion in the mixing zone. Further particulars of this design can be found, for example, in GB-A-1601567.A multi-stage development of this design is described in EP-A-0008189. A modified version in which one or both baffles is omitted is disclosed in EP-A-0036283.

It has long been recognised that, although the mixing stage of a conventional mixer-settler can be relatively compact, a relatively large separate settling stage must be provided in order to allow disengagement of the two phases present in the dispersion to occur essentially completely. Hence the horizontal area of the settling tank must be made large enough to allow layers of the disengaged phases to form above and below the dispersion band, that is to say the bank within which phase disengagement is taking place.

This horizontal area is usually the limiting factor in determining the maximum throughput of the apparatus. As the throughput is increased so the dispersion band will tend to increase-in depth until it fills the settling tank and overflows one or both of the weirs and is discharged to the downstream processing steps, usually a most undesirable event. In many designs of mixer-settler involing separate mixing and settling chambers, the mixing chamber is usually deeper than the settling tank. In order to preserve the necessary hydrostatic balance between the two vessels it is necessary to support the bottom of the settler above ground level or to provide a pit for the mixer. Either of these expedients adds greatly to constructional costs.

Although the combined mixer-settler design of GB-A-1601567, EP-A-0008180 and EP-A-0036283 offers the great advantage of reduced cross-sectional area and obviates the need to support a large settling tank with its bottom some distance above ground level or to place the mixer in a pit, as well as permitting wide variations in feed rate ratio of the phases in operation, it would be desirable to improve further the volumetric throughput of such a design while maintaining a high single stage mass transfer efficiency and achieving low entrainment of both the organic phase in the discharged aqueous phase and the aqueous phase in the discharged organic phase.

Amongst proposals for improving the rate of disengagement of the phases of a dispersion, and hence volumetric throughput, has been use of electric fields. For example, EP-A-0051463 suggests applying a unidirectional varying electric field (i.e. a pulsed d.c.

voltage) across a flow path of a dispersion between a mixer and a settler. Other prior proposals which utilise electric fields to assist coalescence of dispersions include DE-C-733842, US-A-2364118, US-A-4226690, GB-A-1046317 and GB-A-1 582040.

Although the use of electric fields in conjunction with two vessel, conventional mixersettlers can increase volumetric throughput, maintaining a high single stage mass transfer capability and minimising aqueous inorganic entrainment levels, the level of organic entrainment in the discharged aqueous phase is greatly increased to levels which render such designs commercially unacceptable.

It is accordingly an object of the present invention to improve the maximum throughput potential of combined mixer-settlers of the type disclosed in the afore-mentioned GB-A-1601567, EP-A-0008189 and EP-A-0036283, while at the same time achieving high single stage mass transfer efficiencies and low entrainment of both aqueous phase in the discharged organic phase stream and organic phase in the discharged aqueous phase stream.

The present invention is based upon the surprising discovery that both the volumetric throughput and the single mass transfer efficiency of combined mixer settlers of the aforementioned type can be simultaneously improved by applying an a.c. electric field to one or both of the interfaces between the dispersion and the separate organic and aqueous phases. These improvements can be achieved in spite of the fact that turbulent conditions, normally considered not conductive to high phase separation rates, prevail within the dispersion in the mixing zone and at its interfaces with the disengaged media, while the increase in the mean drop size of the dispersion as a result of the application of the electric field would be expected to reduce, not increase, mass transfer rates as a result of the reduction in interfacial area.Further, while the expected reduction in aqueous entrainment in the discharged organic phase is obtained as a result of applying an a.c. electric field to the upper interface, the organic entrainment in the discharged aqueous phase is markedly reduced compared with the levels obtained at the same volumetric separation rate in a two vessel, conventional, mixer settler whose phase separation rate has also been enhanced by the application of an a.c. electric field. This performance aspect of the combined mixer settler design is all the more surprising when the significantly more turbulent conditions which prevail both within and at the interfaces of the dispersion within a combined mixer settler, and which would be normally considered inappropriate for low entrainment generation, are compared with the relatively quiescent conditions prevailing within, and at the interfaces of, the dispersion either within the settler of a two vessel, conventional mixer settler or in a vessel or duct situated between a mixer and its accompanying settler.

It is also a further surprising aspect of the invention that both the organic entrainment levels in the discharged aqueous phase and the aqueous entrainment levels in the discharged organic phase are further reduced by supplying the combined mixer settler with dispersion produced in an external mixer rather than with discrete organic and aqueous phase feeds. Volumetric throughput of this system remains high while, as expected, the efficiency of the contact stage increases as a result of the addition of an additional mixer.

According to one aspect of the present invention there is provided a method of effecting liquid-liquid contact between an aqueous liquid medium and an organic hydrophobic liquid medium capable of undergoing mass transfer with the aqueous medium, comprising: providing a chamber containing a body of each of the aqueous and organic liquid media;; agitating the liquid media within a mixing zone in the chamber so as to maintain therein a dispersion band which contains a dispersion of droplets of the aqueous medium dispersed within the organic medium, the droplets of the aqueous medium being of a size such that upon standing under gravity the dispersion will substantially completely disengage into two separate liquid layers, the volume ratio of the media in the dispersion band corresponding substantially to a selected value, and the mixing zone being disposed within the chamber so that there are formed above and below the dispersion band respectively an upper layer of lighter medium and a lower layer of heavier medium; continuously supplying at least one of the aqueous and organic liquid media to the mixing zone at a respective preselected feed rate;; allowing disengaged lighter medium to pass upwardly across an upper interface between the dispersion band and the upper layer; allowing disengaged heavier medium to pass downwardly across a lower interface between the dispersion band and the lower layer; applying an a.c. electric field to at least one of the upper and lower interfaces, thereby to assist in controlling the position of that interface within the chamber; and recovering from at least one of the upper and lower layers an amount of the corresponding medium at a rate substantially equal to the rate of supply of that medium to the mixing zone, thereby to maintain the volume ratio of the media in the dispersion band substantially at the selected value.

Preferably both of the aqueous and organic liquid media are continuously supplied to the mixing zone, each at a respective preselected feed rate, and both lighter medium and heavier medium are continuously recovered from the upper and lower layers respectively, each at a rate substantially equal to the rate of supply of that medium to the mixing zone. The two media can be supplied to the mixing zone at any desired ratio, for example from about 1:1000 to about 1000:1 by volume. Often the ratio of the feed rates of the liquid media to the mixing zone will lie, for example, in the range of from about 100:1 to about 1:100 by volume. For many applications this ratio will lie in the range of from about 20:1 to about 1:20, e.g. about 10:1 to about 1:10.

The media can be supplied through separate supply lines to the mixing zone. Alternatively they can be pre-mixed in one or more external mixers and supplied to the mixing zone already in dispersion form. Such external mixers can comprise, for example, static mixers or stirred tanks.

On the other hand it is alternatively possible to operate so that one only of the aqueous and organic media is continuously supplied to the mixing chamber at a preselected feed rate and that medium only is continuously recovered from the respective layer at a rate substantially equal to its rate of supply to the mixing zone.

Whichever mode of operation is adopted it is preferred that the volume ratio of the media in the dispersion lies in the range of from about 5:1 to about 1:5. Most preferably it is in the range of from about 2:1 to about 1:2, e.g. about 1:1.

Agitation of the media within the mixing zone is conveniently effected by means of one or more impellers. Such agitation should be sufficient to produce within the mixing zone a dispersion band that it deeper than is usual for a conventional, two vessel, mixer-settler.

Typically the depth of the dispersion band in the process of the present invention is at least about 50 cm, and is more usually at least about 100 cm up to about 200 cm or more.

The or each medium (or a pre-mixed dispersion) can be supplied at any convenient point to the mixing zone, provided that this enables a dispersion of the desired type to be maintained in the mixing zone. Preferably the mixing zone is of uniform cross section and is substantially symmetrical about a vertical axis.

Preferably also an impeller is mounted in the mixing zone for rotation about that vertical axis. Thus, although the or each medium (or a pre-mixed dispersion) can be supplied to a peripheral part of the mixing zone, it is preferred to supply the same to a central part of the mixing zone, even more preferably just under the zone swept by the impeller. By supplying the medium or media to a point just below the zone swept by the impeller, conveniently through a draught tube, the direction of movement of coalescing droplets through the relevant interface is effectively verticaly and there is little or no overall horizontal movement of the coalesced medium with respect to the interface. To minimise the risk of entrainment in the coalesced medium the depth of each of the upper and lower layers is pre ferably at least about 50 cm, and even more preferably at least about 100 cm.In this way any horizontal movement in the respective layer can be confined to a region relatively remote from the upper or lower interface, as the case may be. The mixing zone can be, for example, of circular or of square horizontal section or of regular pentagonal, hexagonal or higher polygonal (e.g. octagonal) horizontal cross section.

Conveniently the step of recovering medium from the respective one of the upper and lower layers includes allowing the medium to overflow a respective weir. In the case of the heavier medium a riser is provided to allow heavier medium to flow to the weir.

The a.c. electric field may be applied to both of the upper and lower interfaces, or to one only of these two interfaces. Preferably its peak field strength is in the range of from about 0.03 volts/cm up to about 8000 volt/cm. Usually the peak field strength will be in the range of from about 0.1 volts/cm up to about 2000 volts/cm. Such field strengths can be produced at applied voltages of from about 100v to about 40,000v. The voltage referred to is the peak-to-peak voltage. The frequency may range, for example, from about 0.5 Hz up to about 200 Hz but is conveniently that of the local mains power supply, e.g. 50 Hz or 60 Hz. Although the wave form of the a.c.

field is not critical, it is preferred to use sinusoidal wave forms.

Any suitable means can be used to generate the a.c. electric field. In one arrangement the electric field is generated with the aid of an electrode to which an a.c. voltage is applied and which is preferably immersed in an organic continuous medium within the vessel.

The layer of coalesced aqueous medium can in this case effectively form an earth electrode. Alternatively un uninsulated earth electrode may be provided in conjunction with the or each charged electrode. Conveniently the or each earth electrode has the configuration of a grid.

Although it is also possible to apply an electric field to the relevant adjacent interface by means of an insulated electrode immersed in the aqueous continuous medium, this is generally less effective than if the electrode is immersed in the dispersion or in the organic continuous medium.

If the electrode to be charged is to be immersed in the aqueous continuous medium then it should be insulated. If it is to be immersed in the dispersion adjacent the interface with the aqueous continuous medium then it is desirably insulated; if it is not insulated, then a control device can be provided in this case to detect the position of the interface between the dispersion and the aqueous continuous phase and to connect the a.c. voltage to the electrode only when it is immersed in the dispersion. In this way the risk of shorting when the electrode is immersed in the aqueous phase is avoided. When the electrode to be charged is immersed in the organic continuous medium or in the dispersion adjacent the interface between the dispersion and the organic continuous medium, the electrode can either be uninsulated or insulated.If the electrode is not insulated, then a current limiting device is generally connected in series with the electrode. As the type and thickness of the insulant has an effect on the electric field, any insulant used should, in general, be of minimum thickness consistent with providing effective insulation. As suitable insulating materials there can be mentioned polymethylmethacrylate, polyvinylchloride, and polytetrafluoroethylene.

The or each electrode to be charged is positioned in the vicinity of one of the interfaces.

In the case of the upper interface it can be positioned in the upper layer of coalesced lighter medium preferably just above the upper interface, at the upper interface, or in the dispersion band, preferably just below the upper interface. In the case of the lower interface, the electrode to be charged can be positioned in the dispersion band, preferably just above the lower interface, at the lower interface, or just below the lower interface.

In such arrangements the electric field extends vertically. Alternatively the electric field can extend radially, e.g. by providing that one of the electrodes, i.e. the charged electrode or the earth electrode, extends in annular fashion around the other electrode.

The electrode or electrodes to be charged can be of any convenient shape, e.g. a vertical or inclined plate, a single wire, a plurality of wires or a grid. The electrode to be charged and/or the earth electrode can alterntively be formed as a baffle. The electrode to be charged can equally be arranged in annular fashion with respect to the earth so as to produce a radial electric field. In this case the earth electrode can be the central electrode and the charged electrode the outer electrode, or vice versa. Whatever configuration of electrode is adopted it should preferably occupy as small a proportion as possible of the horizontal internal section of the chamber so as not to restrict unduly the flow of liquid(s) through the apparatus.

The invention further provides apparatus for effecting liquid-liquid contact between an aqueous liquid medium and an organic hydrophobic liquid medium capable of undergoing mass transfer with the aqueous medium, comprising: a chamber for holding a body of each of the aqueous and organic liquid media;; agitator means within the chamber for agitating the liquid media in a mixing zone so as to maintain therein a dispersion band which contains a dispersion of droplets of the aqueous medium dispersed within the organic me dium, the droplets of the aqueous medium being of a size such that upon standing under gravity the dispersion will substantially completely -disengage into two separate liquid layers, the mixing zone being disposed within the chamber so that an upper layer of disengaged lighter medium may form above the dispersion band and separated therefrom by an upper interface and so that a layer of disengaged lower medium may form below the dispersion band and separated therefrom by a lower interface; supply means for continuously supplying at least one of the aqueous and organic liquid media to the mixing zone;; electric field means for applying an a.c. electric field to at least one of the upper and lower interfaces, thereby to assist in controlling the position of that interface within the chamber; and recovery means for recovering from the chamber from at least one of the upper and lower layers an amunt of the corresponding medium at a rate substantially equal to the rate of supply of that medium to the mixing zone, thereby to maintain the volume ratio of the media in the dispersion band substantially at a selected value.

In order that the invention may be clearly understood and readily carried into effect some preferred embodiment thereof will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, wherein: Figures 1 and 2 are respectively a vertical section and a plan view of one form of liquidliquid extraction plant embodying the invention; Figures 3 to 11 are vertical sections through other embodiments of the invention; and Figure 12 is a vertical section through a modified mixer-settler of conventional design.

Throughout the drawings like reference numerals are used to indicate like parts.

Referring first to Figs. 1 and 2 of the drawings a mixer-settler 1 comprises a deep rectangular section vessel in which is mounted an impeller 2 fixed on a vertical shaft 3 which can be rotated about its axis by a suitable motor 4. Impeller 2 is of the double-shrouded type with an "eye" in its lower face. As shown, the impeller 2 has 8 blades. However, other designs of impeller may be used, if desired, for example a pump-mix impeller, a marine impeller or a turbine impeller.

Immediately under impeller 2 is positioned the upper open end of an annular draught tube 5, through which the two liquid phases to be mixed are introduced to the vessel. One of the phases (preferably the minority phase if the phases are not supplied at a 1:1 by volume feed rate ratio) is supplied through the inner tube 6 of the annular draught tube 5 from a corresponding supply pipe 7, whilst the other phase is supplied by way of supply pipe 8 to the annular space 9 between the two draught tubes 5 and 6. Typically the ratio of the rates of supply of the two phases is about 10:1 by volume.

Under the mixing action of impeller 2 the two phases are dispersed one in the other to form a mixing zone 10 of mixed phases, from which coalesce lighter and heavier phases to form bodies 11 and 12 of the coalesced lighter and heavier phases respectively. Reference numeral 13 indicates the somewhat turbulent upper interface between the dispersion present in the mixing band 10 and the coalesced lighter phase 11, whilst reference numeral 14 represents the corresponding lower interface between the turbulent dispersion-filled mixing zone 10 and the coalesced heavier phase 12.

In a typical application the lighter phase is, for example, a kerosene solution of a hydroxyoxime, such as that sold under the trade mark Acorga PT 5050, whilst the heavier phase is a dilute acidic aqueous copper sulphate solution obtained by acid leaching of a copper ore. In this case the lighter phase is the organic phase, whilst the heavier phase is the aqueous phase. However, it will be appreciated that in certain circumstances, such as when an organic solvent with a specific gravity of greater than 1 is used, e.g. chloroform, the organic phase may be the heavier phase and the aqueous phase may be the lighter phase. In the following description it is assumed that the apparatus is being used with a lighter phase that is an organic phase and with an aqueous phase that is the heavier phase.

Coalesced organic (lighter) phase overflows weir 15 and exits the apparatus via outlet 16.

On the other hand coalesced aqueous (heavier) phase is recovered via opening 17 and riser 18 before overflowing weir 19 and exiting the apparatus via outlet 20.

The rates at which the aqueous and organic phases exit the apparatus equal the rates at which they are supplied via tubes 5 and 6.

Under steady state flow conditions, with the impeller 2 stopped, the position of the settled interface 21 is determined by the relative heights of the weirs 15 and 19. If desired, at least one of the weirs 15 and 19, e.g. the weir 19, is provided with a vertical plate, whose upper edge can be adjusted vertically to alter the height of the corresponding weir and hence change the position of the settled interface 21. The position of this interface can be determined by inspection of the corresponding level of the interface 71 in the sight tube 72. Normally it will be desirable to arrange for the settled interface 21 to intersect the zone swept by the impeller 2 or possibly to lie a short distance below it.However, under all circumstances the settled interface 21 lies within the mixing zone 10, i.e. between the upper and lower interfaces 13 and 14 of the dispersion band caused by setting impeller 2 into motion. Because the settled interface 21 intersects the zone swept by impeller 2 the phase ratio in the dispersion will be about 1:1 by volume when, the rates of supply of the two media are not too disparate. If, however, the phases are supplied at very disparate flow rates, e.g. at a flow rate of about 100:1 by volume or more, the interfaces 13 and 14 may not be dispersed symmetrically with respect to the impeller 2. In this case weir 19 should be adjusted until the interface 71 again lies midway between the upper and lower interfaces 13 and 14. In this way the phase ratio in the dispersion can be maintained at about 1:1 by volume.

An electrode 22 which can be insulated or uninsulated is positioned in the disengaged organic phase and is connected by a lead 23 to a high tension source 24 arranged to deliver an a.c. voltage in the range of from about 100 v to about 40,000 v, for example an a.c.

voltage of 1240 V and 50 Hz. The coalesced aqueous phase acts as an earth electrode so that a downward a.c. electric field is applied to upper interface 13. By varying the voltage delivered to the electrode 22 a degree of control can be exercised over the position of upper interface 13. In this way the throughput of organic phase can be significantly increased without risk of the dispersion band expanding upwardly and of dispersion overflowing weir 15. If the a.c. electric field is switched off at such an increased organic phase flow rate then the dispersion band may rapidly fill the upper part of vessel 1 and dispersion may overflow weir 15.

As can be seen from Fig. 2, electrode 22 is in the form of a sinuous single wire which can be insulated or not. Alternatively it could be a spirally coiled wire or a grid or culd be a plurality of insulated or uninsulated wires. At all events the cross section of electrode 22 is small in comparison to the area of cross section of vessel 1.

If desired, a level detector 41 can be provided, such as a microwave detector of the type disclosed in GB-A-2123237, the output of which is used to control the voltage delivered by the source 24 so as to control the position of the upper interface 13 within predetermined limits. Detector 41 can also be arranged to shut off the supply of organic phase to the vessel 1 if the position of interface 13 should rise to such an unacceptably high level that there is a significantly risk of dispersion overflowing weir 15.

Besides helping to control the position of upper interface 13, the a.c. electric field generated by electrode 22 also has the beneficial effect of reducing the aqueous-in-organic entrainment level, thus demonstrating the effectiveness of the a.c. electric field in assisting phase disengagement.

The apparatus of Figs. 1 and 2 has been described as being of rectangular section; it could alternatively be of any other desired horizontal section, e.g. circular section or regular polygonal (e.g. hexagonal) section.

In Fig. 3 an earth electrode 25 in the form of a horiontal grid is positioned in the dispersion band and is connected to earth by lead 26 in order to supplement charged electrode 22. The annular draught tube 5 of Figs. 1 and 2 is replaced by a T-shaped draught tube 42, the aqueous and organic media each being supplied via a respective one of its arms 43 and 44.

In the embodiments of Figs. 1 and 3 the organic (lighter) phase is the majority phase supplied via inlet pipe 8 or 44. Fig. 4 shows a form of apparatus in accordance with the invention in which the aqueous (heavier) phase is the majority phase. In this case the electrode 22 is replaced by an insulated electrode 27 which is situated in the vicinity of the lower interface 14, for example just above the lower interface 14, and is connected by lead 28 to a suitable a.c. source 29, e.g. a 240V 50 Hz mains supply. Interface detector 41 can be used to detect the position of interface 14 and to control the a.c. source 29 and the control valve (not shown) for controlling the supply of aqueous phase through inlet pipe 8.

Fig. 5 illustrates a similar arrangement to that of Fig. 4 but with an earth electrode 30 below charged electrode 27, also positioned in the vicinity of the lower interface 14, for example just above the lower interface 14, which is earthed by way of lead 31. Again.

the aqueous (heavier) phase is the majority phase. Earth electrode 30 conveniently takes the form of a grid electrode.

In the apparatus of Figs. 6 to 9 the feed rates of the two media are approximately equal. In the mixer-settler of Fig. 6 two charged electrodes 22 and 27 are provided, each connected to a high voltage a.c. power supply 24.

Fig. 7 shows a form of apparatus with two charged electrodes 22 and 27 and one earth electrode 25.

The apparatus of Fig. 8 has two charged electrodes 22 and 27 and a single earth electrode 30. Electrode 27 is in the form of a baffle. A similar baffle 45, which is not connected to an electrode, is provided in the vicinity of the upper interface 13.

In the variant of Fig. 9 there are two charged electrodes 22 and 25 and two earth electrodes 25 and 30. In this case the two phases are premixed in a premixer box 32 to which the phases are fed by way of lines 33 and 34. The dispersion from premixer box 32 in this case flows by way of line 35 to draught tube 36.

The mixer-settlers of Figs. 10 and 11 are intended mainly for use in situations in which the organic (lighter) medium is the majority medium. That of Fig. 10 is generally similar to the apparatus of Fig. 1 except that an annular charged electrode 37 is used, whilst the shaft 3 is earthed by way of line 38. A reverse arrangement is used in the mixer-settler of Fig.

11, a central annular charged electrode 39 being surrounded by an outer annular earth electrode 40.

In the various illustrated forms of apparatus the dispersion is of the aqueous-in-organic type.

When the flow rates of the two media are approximately equal the risk of inversion of the type of dispersion from an aqueous-in-organic dispersion to an organic-in-aqueous dispersion can be reduced by adjusting the position of the settled interface 21 appropriately.

Thus, assuming that the aqueous medium is the heavier phase, it will usually be preferred to arrange that the settled interface 21 lies slightly below the zone swept by the impeller 2. On the other hand, if the organic phase is the heavier phase, it will usually be preferred to adjust the weirs 15 and 19 so that the settled interface 21 lies at a level a little above the zone swept by the impeller 2.

In the above description the forms of apparatus illustrated in Figs. 1 to 3, 10 and 11 are described as being best suited for systems in which the process conditions require use of a high organic:aqueous feed flow ratio. In addition such mixer-settlers can be used under conditions in which the process involves use of a 1:1 organic:aqueous feed flow ratio system with rapid coalescence rate characteristics. Similarly, the forms of apparatus of Figs. 4 and 5 have been described for use in low organic:aqueous feed flow ratio systems; they can equally well be used with a 1:1 organic:aqueous feed flow ratio system with rapid sedimentation rate characteristics.In the case of the devices of Figs. 6 to 9, these were described above as suitable for 1:1 organic:aqueous feed flow ratio systems; such devices can also be used with high organic: aqueous feed flow ratio systems with slow coalescence rate characteristics or with low organic:aqueus feed flow ratio systems with slow drainage/sedimentation rates.

Fig. 8 includes baffles 27 and 45, the former of which is used as an electrode. It is of course equally possible to incorporate an upper and/or a lower baffle, whether or not used as an electrode in either case, into any of the other forms of plant illustrated in Figs.

1 to 7 and 9 to 11. As illustrated, baffles 27 and 45 are formed by vertical plates intersecting at right angles to form a so-called egg box baffle. The skilled reader will appreciate that other designs of baffle can be used, for example inclined plate baffles, baffles in the form of pads of mesh (e.g. "KnitMesh D.C." pads; thje word "KnitMesh" is a trade mark), or baffles formed from vertical plates intersecting to form vertical passages of triangular or hexagonal section or a mixture of vertical passages of differing section, such as alternating square and octagonal sections.

Fig. 9 illustrates a separate mixer box 32; a similar mixer box can also be used in conjunction with the apparatus of any of Figs. 1 to 8, 10 or 11.

Each of Figs. 1 to 11 illustrates a single stage mixer-settler apparatus. It will be readily apparent to the skilled reader that several such mixer-settlers can be used in conjunction with one another or with one or more known forms of mixer-settler with the flow pattern through the combined plant being either cocurrent or counter-current as process considerations may require.

Fig. 12 illustrates a conventional mixer-settler which has been modified by fitting an insulated electrode in the settler. This mixersettler includes a mixer 101 and a settler 102.

Aqueous and organic media are supplied by way of lines 103 and 104 to a draught tube 120, whose outlet is positioned immediately below an impeller 105 driven by a motor 106.

Under the mixing action of impeller 105 mixer 101 fills with dispersion and overflows into settler 102 as indicated by arrow 107 to pass under distributor 108 and to form a dispersion band 109. Coalesced organic (lighter) medium overflows weir 110 into launder 111 and is recovered in line 112. On the other hand coalesced aqueous (heavier) medium passes up riser 113 to launder 114 and overflows weir 115 before exiting in line 116. Reference numeral 117 indicates an insulated electrode, to which an a.c. voltage can be supplied by means of power supply 118.

It is surprisingly found that the organic-inaqueous entrainment is very significantly lower when using the apparatus of Fig. 1 than when using that of Fig. 12. In view of the vigorous mixing conditions prevalent in use of the CMStype apparatus of Fig. 1, it is very unexpected that the organic-in-aqueous entrainment level should be much, much lower than in the apparatus of Fig. 12 in which settling of the dispersion occurs under relatively much more quiescent conditions.

Loss of organic medium by entrainment in aqueous raffinate necessitates supplying replacement organic medium to enable the plant to continue to operate satisfactorily. In addition it will usually mean further treatment of the aqueous raffinate prior to discharge to the environment in order to minimise pollution or prior to further processing in order to prevent interference contamination of downstream processes. These factors can represent a significant operating cost for the plant operator and can have a profound effect on the commercial viability of the plant. Hence the benefits of the invention have a significant impact on the cost of operating a CMS plant.

Claims (29)

1. A method of effecting liquid-liquid con tact between an aqueous liquid medium and an organic hydrophobic liquid medium capable of undergoing mass transfer with the aqueous medium, comprising: providing a chamber containing a body of each of the aqueous and organic liquid media; agitating the liquid media within a mixing zone in the chamber so as to maintain therein a dispersion band which contains a dispersion of droplets of the aqueous medium dispersed within the organic medium, the droplets of aqueous medium being of a size such that upon standing under gravity the dispersion will substantially completely disengage into two separate liquid layers, the volume ratio of the media in the dispersion band corresponding substantially to a selected value, and the mixing zone being disposed within the chamber so that there are formed above and below the dispersion band respectively an upper layer of lighter medium and a lower layer of heavier medium; continuously supplying at least one of the aqueous and organic liquid media to the mixing zone at a respective preselected feed rate; allowing disengaged lighter medium to pass upwardly across an upper interface between the dispersion band and the upper layer; allowing disengaged heavier medium to pass downwardly across a lower interface between the dispersion band and the lower layer; applying an a.c. electric field to at least one of the upper and lower interfaces thereby to assist in controlling the position of that interface within the chamber; and recovering from at least one of the upper and lower layers an amount of the corresponding medium at a rate substantially equal to the rate of supplying of that medium to the mixing zone, thereby to maintain the volume ratio of the media in the dispersion band substantially at the selected value.

2. A method according to claim 1, in which both of the aqueous and organic liquid media are continuously supplied to the mixing zone, each at a respective preselected feed rate, and in which both lighter medium and heavier medium are continuously recovered from the upper and lower layers respectively, each at a rate substantially equal to the rate of supply of that medium to the mixing zone.

3. A method according to claim 2, in which the ratio of the feed rates of the liquid media to the mixing zone lies in the range of from about 1000:1 to about 1:1000 by volume.

4. A method according to claim 2 or claim 3, in which the media are pre-mixed to form a dispersion prior to being supplied to the mixing zone.

5. A method according to claim 1, in which one only of the aqueous and organic media is continuously supplied to the mixing chamber at a preselected feed rate and that medium only is continuously recovered from the respective layer at a rate substantially equal to its rate of supply to the mixing zone.

6. A method according to any one of claims 1 to 5, in which the volume ratio of the media in the dispersion lies in the range of from about 5:1 to about 1:5.

7. A method according to any one of claims 1 to 6, in which the step of recovering medium from the respective one of the upper and lower layers includes allowing the medium to overflow a respective weir.

8. A method according to any one of claims 1 to 7, in which the electric field is applied to both of the upper and lower interfaces.

9. A method according to any one of claims 1 to 7, in which the electric field is applied to the upper interface only.

10. A method according to any one of claims 1 to 7, in which the electric field is supplied to the lower interface only.

11. A method according to any one of claims 1 to 10, in which the peak field strength of the electric field is in the range of from about 0.1 volt/cm up to about 2000 volt/cm.

12. A method according to any one of claims 1 to 11, in which the electric field is generated with the aid of an electrode to which an a.c. voltage is applied and which is immersed in an organic continuous medium within the vessel.

13. A method according to claim 12, in which the electric field is generated with the aid of an additional earth electrode.

14. A method according to claim 12 or claim 13, in which at least one of the electrodes is formed as a baffle.

15. A method of effecting liquid-liquid contact between an aqueous liquid medium and an organic liquid medium conducted substantially as herein described with particular reference to any one of Figs. 1 to 11 of the accompanying drawings.

16. Apparatus for effecting liquid-liquid contact between an aqueous liquid medium and an organic hydrophobic liquid medium capable of undergoing mass transfer with the aqueous medium, comprising: a chamber for holding a body of each of the aqueous and organic liquid media; agitator means within the chamber for agitating the liquid media in a mixing zone so as to maintain therein a dispersion band which contains a dispersion of droplets of the aqueous medium dispersed within the organic me dium, the droplets of aqueous medium being of a size such that upon standing under grav ity the dispersion will substantially completely disengage into two separate liquid layers, the mixing zone being disposed within the cham ber so that an upper layer of disengaged lighter medium may form above the dispersion band and separated therefrom by an upper interface and so that a layer of disengaged lower medium may form below the dispersion band and separated therefrom by a lower interface; supply means for continuously supplying at least one of the aqueous and organic liquid media to the mixing zone; electric field means for applying an a.c. electric field to at least one of the upper and lower interfaces, thereby to assist in controlling the position of that interface within the chamber; and recovery means for recovering from the chamber from at least one of the upper and lower layers an amount of the corresponding medium at a rate substantially equal to the rate of supply of that medium to the mixing zone, thereby to maintain the volume ratio of the media in the dispersion band substantially at a selected value.

17. Apparatus according to claim 16, in which supply means are provided for supply both of the aqueous and organic liquid media to the mixing zone, each at a predetermined feed rate, and in which means are provided for recovering from the chamber both disengaged lighter medium and disengaged heavier medium from the upper and lower layers respectively, each at a rate substantially equal to the rate of supply of that medium to the mixing zone.

18. Apparatus according to claim 17, in which pre-mixing means are provided for premixing the media so as to form a dispersion prior to entry to the mixing zone.

19. Apparatus according to claim 18, in which the pre-mixing means comprises one or more stirred tanks.

20. Apparatus according to any one of claims 16 to 19, in which the agitator means comprises an impeller mounted in the mixing zone for rotation about a substantially vertical, axis.

21. Apparatus according to claim 20, in which the mixing zone is substantially symmetrical about the said vertical axis.

22. Apparatus according to claim 20 or claim 21, in which the medium or media is or are supplied to a part of the mixing zone just beneath the zone swept by the impeller.

23. Apparatus according to any one of claims 16 to 22, in which the or each recovery means comprises a weir over which the respective medium may overflow.

24. Apparatus according to any one of claims 16 to 23, in which the electric field means comprises at least one electrode arranged for immersion in a zone wherein there is an organic continuous medium and voltage generating means connected to the insulated electrode for supplying a varying voltage thereto.

25. Apparatus according to claim 24, in which the electric field means further includes an earth electrode.

26. Apparatus according to claim 24 or claim 25, in which an electrode is disposed within the chamber so as to lie in the vicinity of the upper interface in operation of the apparatus.

27. Apparatus according to any one of claims 24 to 26, in which an electrode is disposed within the chamber so as to lie in the vicinity of and above the lower interface in operation of the apparatus.

28. Apparatus according to any one of claims 24 to 27, in which at least one electrode is formed as a baffle.

29. Apparatus for effecting liquid-liquid contact constructed and arranged substantially as herein described with particular reference to any one of Figs. 1 to 11 of the drawings.