WO1997018880A1

WO1997018880A1 - Extraction method and apparatus

Info

Publication number: WO1997018880A1
Application number: PCT/GB1996/002813
Authority: WO
Inventors: Christopher John Broan
Original assignee: British Nuclear Fuels Plc
Priority date: 1995-11-17
Filing date: 1996-11-15
Publication date: 1997-05-29
Also published as: GB9523542D0; ZA969599B; AU7581896A

Abstract

A method and apparatus (10) are described for the extraction of a solute from an aqueous feed solution into an aqueous stripping solution, the method comprising the steps of providing at least one stream of each of said feed solution (42) and said stripping solution (46) passing through a continuous phase of a non-polar carrier liquid (28); said carrier liquid (28) having therein a chemical having an affinity for ions of at least one species in said solute in said feed solution; each of said at least one stream of feed (42) and stripping (46) solutions being under the influence of a first and a second high voltage AC electrostatic field. The first and second high voltage AC electrostatic fields are chosen to be at a frequency above conventional mains frequency of 60 Hz such that a specific desired size range of droplets are stabilised by being at their natural frequency and in resonance with the applied field. A desired size range may lie between about 250 and 400 mum and are sufficiently small at the minimum end to prevent passage across baffle means (26) and not too large such that the efficiency of mass transfer between the aqueous (42, 46) and non-aqueous phases (28) is significantly impaired.

Description

EXTRACTION METHOD AND APPARATUS

The present invention relates to improvements to the so- called "electrostatic pseudo liquid membrane" (ESPLIM) method of separation of metal ions from aqueous solutions.

Chinese patent application number CN 86101730A describes a separation technique which enables the purification of aqueous solutions and concentration of solutes in aqueous solutions .

The technique includes the steps of passing droplets of an aqueous feed solution which it is desired to purify and/or from which it is desired to extract metal ions for example, under the influence of gravity, through a first region of a non-polar carrier liquid in which is dissolved a chemical having high affinity for the metal ion or ions to be removed whilst simultaneously subjecting the droplets to a high voltage electrostatic field so as to break up the droplets into a multiplicity of much smaller droplets in order to increase their surface area to volume ratio. The metal ions are complexed by the dissolved chemical into the carrier liquid and are driven, principally by the concentration gradient so formed, to a second region in the non-polar carrier liquid through which is passing under the influence of gravity a stream of droplets of an aqueous "stripping" solution which has a chemically higher affinity for the metal ion than the complexing chemical in the carrier liquid. The stripping solution droplets are also simultaneously subjected to a high voltage electrostatic field so as to break them up into a multiplicity of much smaller droplets and so also to increase their surface area to volume ratio. The metal ions are thus concentrated into the stripping solution and the aqueous feed solution is largely purified of the metal ions. As the very small droplets of the purified feed solution and the stripping solution,- the former now having a lower concentration of the metal ions and the latter now having a high concentration of the required metal ions; pass out of the high voltage electrostatic field, they coalesce and fall under gravity into mutually separated first and second collecting vessels, respectively, and from which they can be removed.

The first and second regions of the carrier liquid are separated by a barrier or baffle which is intended to allow substantially uninterrupted flow and passage of the carrier liquid to and from the first and second regions but, is also intended to impede or prevent the passage of the aqueous feed solution from the first region into the second region and, the passage of the stripping solution from the second region to the first region.

Whilst the ESPLIM technique has produced very good results in terms of high concentration factors of up to 300 between the feed solution and the stripping solution, after reaction, the efficiency of the process could be improved by reducing the leakage of the raw aqueous feed solution to the second region and leakage of the stripping solution with the extracted metal ions to the first region through the barrier. Leakage of raw feed solution to the second region is not merely a question of dilution of the more highly concentrated metal ions in the stripping solution but may also involve the contamination of the concentrated stripping solution if, for example, only one metal ion from two or more is being selectively extracted from the feed solution.

Similarly, leakage of the concentrated stripping solution from the second region to the first region and into the "purified" aqueous feed solution or raffinate will also cause contamination thereof and decrease the efficiency of the process.

Where the streams of feed and stripping solutions are broken up into droplets of less than about 1mm in diameter, a further problem arises in that it is difficult to avoid the formation of an emulsion between the aqueous solutions and the non-polar carrier liquid. The formation of an emulsion significantly increases the leakage rate between the first and second regions of the reaction vessel since the small aqueous droplets are more easily entrained by an emulsion.

In the prior art, it is known that an AC dispersion field of mains frequency, i.e. 50 to 60 Hz, is used to break up the feed and stripping solution streams. It has been found that aqueous droplets dispersed in a non-conducting organic phase which is exposed to electric fields at frequencies below the natural vibrational frequencies of the droplets, are caused to break up into smaller droplets at high field intensities m a manner which may generate an emulsion

In view of the fact that the ratio of surface area to volume of the droplets is inversely proportional to the average droplet size, it is desirable for the droplet size to be as small as possible consistent with not producing an emulsion, so as to maximise the surface area to volume ratio in the interests of increasing efficiency of the ESPLIM method.

It is an obiect of the present invention to decrease the leakage between the two regions in the carrier liquid and so to improve the efficiency of the separation method.

It is a further object of the present invention to control the droplet size to that which optimises the surface area to volume ratio consistent with minimising leakage.

According to a first aspect of the present invention, there s provided a method for the extraction of a solute from an aqueous feed solution into an aqueous stripping solution, the method comprising the steps of providing at least one stream of each of sa d feed solution and sa d stripping solution passing through a continuous phase of a non-polar carrier liquid; said carrier liquid having therein a chemical having an affinity for ions of at least one species m said solute m said feed solution, each of said at least one streams of feed and stripping solutions being under the influence of a first and a second high voltage AC electrostatic field, respectively, for at least a part of their passage time through said carrier liquid so as to break up said streams into a multiplicity of droplets of each of said solutions; providing baffle means between said at least one streams of each of the feed and stripping solutions, the baffle means being to minimise transfer of feed solution towards said stripping solution stream and transfer of said stripping solution towards said feed solution stream,- said baffle means also being positioned between the separate high voltage electrostatic fields to which the feed and stripping solution streams are subjected; and, providing mutually separated receiving means to collect the streams of said feed and said stripping solutions after they pass out of said high voltage electrostatic field; said method being characterised in that said first and second electrostatic fields are selected at a frequency which corresponds to the natural vibrational frequency of at least some of a desired size range of the aqueous droplets in the carrier liquid.

The ideal droplet size distribution for the ESPLIM method is for all the drops to be the same size . Such a size is just large enough to permit the baffle to prevent droplets from diffusing across it thus causing leakage. Conversely, droplets larger than this minimum size will have a poorer surface area to volume ratio and, hence, mass transfer between the aqueous solutions and the carrier will be less efficien .

The experimental results for a 50Hz field show a bimodal distribution of droplet sizes as shown in Figure l. As the electrostatic intensity of the field is increased, the relative proportion of droplets m the two maxima changes but, the position of the maximum m each of the two distributions does not change significantly. Such a bimodal distribution as shown m Figure 1 is effectively the worst possible droplet size distribution for promoting efficient operation of the ESPLIM method. A high proportion of the larger droplets may have a poor surface area to volume ratio thus significantly decreasing the extraction efficiency for such droplets whilst the small droplets will be very susceptible to leakage by passing through the baffle between the feed and stripping zones.

The frequency of the high voltage AC electrostatic field is desirably greater than mams frequency (50-60Hz) and at such a frequency that the aqueous feed and stripping solution streams are broken up into smaller droplets than about 0.5mm without the formation of an emulsion.

Earlier work has shown that drops of a first liquid phase suspended in a second immiscible liquid phase have a certain natural frequency of vibration. The application of a periodic force to such droplets, such as by means of an

AC electric field or an ultrasonic transducer, causes the droplets to oscillate at the frequency of the applied force. When this force is m resonance with the natural frequency of the droplets, the amplitude of the vibrations of the droplet will be large. However, when the applied force is not in resonance w th the natural frequency, the droplet is more prone to break up into a number of smaller droplets. It has been proposed that the natural frequency of a droplet depends principally on the droplet size and upon the densities and viscosities of each of the two liquid phases. In the ESPLIM system and method, as mentioned above, the droplets of the dispersed aqueous phase are typically in the range from about 50μm to 600μm in diameter. By analogously applying the correlation referred to above for calculating the natural frequency of droplets, the natural frequency of even the largest of these will be substantially above the 50-60HZ frequency of mains power supplies. Tt sho l be noted that in the case of AC power supplies, there are two points of maximum impulse per cycle thus, the "effective" frequency is twice the nominal frequency of the AC supply. For the system water/10% di- (2-ethyl-hexyl) phosphoric acid (D2EHPA) in Isopar M (trade name) , a kerosene, a natural frequency of 220Hz for a 60Oμm drop is suggested which corresponds to an AC field frequency of 110Hz. The results of drop size measurements for the application of an electrostatic field at 50Hz show no droplets as large as 600μm thus, indicating that any original droplets of this size have been broken down into much smaller droplets, many of sufficiently small size to promote leakage across the baffle.

Thus, it has been discovered in the method of the present invention that the optimum frequency to apply to the aqueous feed and stripping streams is that frequency of the applied electrostatic field where the desired optimum droplet size range of the aqueous solutions is stabilised. We have found that the larger the droplet size, the lower the natural frequency required to stabilise the droplet. However, even at the largest droplet sizes, the natural frequency is nevertheless greatly in excess of ordinary mains frequencies of 50-60Hz.

Stabilisation of a droplet of a particular size will be controlled purely by the balance of forces between the electrostatic field and the surface tension of the drople . Where the force applied, which is proportional to the field, exceeds the surface tension, which increases as the droplet size decreases, the droplet is broken up by the applied force. In contrast to this, when the field is in resonance with droplets of a given size then, as stated above, droplets of a similar range of sizes will be less prone to breaking up in the field than droplets with a substantially different frequency. However, if the force applied by the electrostatic field is sufficiently great, then even droplets with a natural frequency in resonance with the applied field may be further dispersed. At mains frequencies, the size of a droplet which would theoretically be stabilised is physically too large having regard to surface tension effects and the destabilising effect of the applied field intensity.

Thus, the optimum frequency range for efficient operation of the ESPLIM method and apparatus is that frequency such that the drop size which is stabilised is the smallest which is excluded by the baffle means. A higher frequency may lead to the stabilisation of droplets which are smaller and thus able to pass through the baffle means and so increase leakage. A lower frequency will stabilise droplets with a poorer than optimum surface area to volume ratio. In particular, a frequency which stabilises drops of a larger size than the average drop size formed at a frequency of 50-60Hz, where no droplets are in resonance, will be counter productive and lead to reduction in extraction efficiency.

It has been found that a frequency which will stabilise the greatest proportion of droplets within the size range of about 250 to about 400μm is desirable. Ideally, conditions which maximise t-he proportion of droplets lying about- a mean droplet size of about 325μm is desirable.

Stability of a particular desired droplet size is not only promoted by the frequency per se but also by the field intensity. Increasing field intensity increases the force which is applied to each droplet and which has to be resisted by droplet surface tension. Furthermore, increasing intensity also tends towards droplet instability and the formation of emulsions which, as mentioned above, is highly undesirable from the point of view of preventing leakage. Therefore, intensity should be maintained at a level consistent with promoting and maintaining stability of a particular desired droplet size.

Examples given above are for the system of water containing 10% D2EHPA in Isopar M kerosene. Since the natural frequencies of droplets are dependent inter alia upon not only the droplet size but also the density and viscosity of each of the two phases, it will be clear to the person skilled in the art that considerable variation in specific perameters relating to frequency and intensity may occur with different liquid systems.

It has also been found that the rate of mass transfer between the aqueous phase and the carrier liquid is also significantly improved when the field intensity corresponds to the natural frequency of the stabilised droplets which are in resonance with the applied field. Thus, the method and apparatus of the present invention provides a synergistic effect in that for any given droplet size which is in resonance with the applied field, the rate of mass transfer from the drop in the case of the feed solution and to the drop in the case of the stripping solution is significan ly improved over prior art methods where the droplets are not in resonance with the applied field.

Furthermore, it is also pointed out that a minimum droplet size of about 250μm which is consistent with preventing leakage across the baffle may also change in the future and that this particular minimum drop size is consistent with baffle design and technology currently available. In future, baffle means may be developed whereby significantly lower droplet sizes may effectively be prevented from being transferred across the baffle and hence, higher frequencies stabilising smaller droplet sizes, without leakage may become practicable. In such circumstances, of course, rates of extraction from smaller droplets will be higher leading to higher efficiencies in the ESPLIM method. Thus, improvements in technology may occur which may permit the use of higher frequencies and correspondingly smaller drop sizes as baffle design and technology improves.

The streams of the feed and/or stripping solutions may be constituted by continuous streams or by streams of droplets which are themselves disintegrated into much smaller droplets by the action of the relatively high frequency electrostatic fields which are applied thereto.

Frequencies applied to the feed and stripping solution streams may be the same or different depending upon the particular size range which it is desired to stabilise.

According to a second aspect of the present invention, there is provided an apparatus for the extraction of a solute from an aqueous feed solution into an aqueous stripping solution, the apparatus comprising vessel means for containing a continuous non-polar carrier liquid, the carrier liquid having therein a chemical having an affinity for ions of at least one species in said solute in said feed solution; means for providing at least one stream of each of said feed and stripping solutions through said carrier liquid in said vessel means,- electrode means for applying a first and a second high voltage AC electrostatic field to each of said feed and stripping solution streams, respectively so as to cause said streams to break up into a multiplicity of small droplets,- baffle means positioned between the electrode means for establishing said first and second high voltage electrostatic fields, said baffle means allowing the movement of said carrier liquid but minimising transfer across the baffle of feed and stripping solutions,-

π mutually separate receiving means for collecting said feed and stripping solutions after they have passed through said first and second high voltage electrostatic fields, respectively; the apparatus being characterised by the means for generating said first and second high voltage AC electrostatic fields generating said fields at at least a frequency above 60Hz and at a natural vibrational frequency of at least some of a desired size range of droplets of said aqueous feed and stripping solutions.

As stated above, the natural vibrational frequencies of the aqueous droplets are partially dependent upon their size but also upon the density and viscosity of the two immiscible phases. Therefore, there may be an upper frequency level consistent with a minimum droplet size below which it would not be worthwhile producing owing to the unavoidable formation of an emulsion at very small droplet sizes.

The generating means will desirably have a frequency or range of frequencies above the usual mains supply frequencies of 50 to 60Hz so as to be able to deal with different immiscible liquid systems.

In order that the present invention may be more fully understood, an example will now be described by way of illustration only with reference to the accompanying drawings of which: Figure 1 shows a graph of droplet size range distribution of a known ESPLIM system subjected to a frequency of 50- 60Hz at various intensities;

Figure 2 shows a schematic arrangement of apparatus showing the basic operation and method of the prior art ESPLIM method and is further used to describe the method and apparatus of the present invention,- and

Figure 3 which shows a graph cf observed droplet size distribution compared with simulated droplet size distributions under resonant conditions.

Referring now to Figure 2 which shows a schematic cross section through an apparatus 10 for carrying out the ESPLIM method of separation according to the prior art. The apparatus 10 comprises a reaction tank or vessel 12 which is divided at its upper portion by a wall 14 into an extraction cell 16 and a stripping cell 18. At the lower end of the tank 12 there is a wall 20 which divides the tank into two receiving vessels or settling tanks 22, 24 for the purified feed solution or raffinate and, for the concentrated extractant in the stripping solution, respectively. Situated between the upper wall 14 and the lower wall 20 is a baffle 26 which allows an organic carrier liquid 28, in this case kerosene, to move freely throughout the vessel 12. Electrodes 30, 32 are situated in the extraction cell side 16 and between which a first high voltage AC electrostatic field may be applied. Electrodes 34, 36 are situated in the stripping cell side 18 and between which a second high voltage AC electrostatic field may be applied. In each of the cells 16, 18 at least one of the electrodes is insulated with, for example, a coating of polytetrafluoroethylene (PTFE) to prevent short circuiting within each cell. A controllable high tension supply, indicated as attached by the dashed lines to an altemating high voltage supply 33, 37, respectively, is provided for the electrodes so as to establish a desired potential therebetween. A supply source such as a conduit 40 is provided above the extraction cell 16 to supply a stream of feed solution 42 which is to be purified, into the carrier liquid 28. The conduit 40 has connected thereto pump means (not shown) and a reservoir tank (not shown) to provide a continuous controlled supply of aqueous feed solution at a controlled rate. Another supply source such as a second conduit 44 is provided above the stripping cell 18 to supply a stream 46 of aqueous stripping solution into the carrier liquid 28. The conduit 44 also has connected thereto pump means (not shown) and a reservoir tank (not shown) to provide a continuous controlled supply of stripping solution at a controlled rate. Each of the settling tanks 22, 24 have conduits 50, 52 to enable the raffinate 54 and the concentrate 56 to be drawn off as the level in each tank rises or as required. The raffinate and concentrate are pumped to collection vessels (not shown) for disposal or further processing as required.

In operation, the apparatus 10 functions as follows and using as an example the extraction of cobalt metal ions from the feed solution 42 in which the Co ions are present at a concentration of lOOOppm in a 0.IM aqueous sodium acetate solution, the feed solution being supplied at flow rate of 200 ml/hr into the carrier liquid 28. The stripping solution comprises a 1.0M solution of sulphuric acid which is supplied at a flow rate of 10 ml/hr into the carrier liquid. The diluent kerosene carrier liquid 28, Isopar M (trade name) has dissolved therein 10 volume% of di- (2- ethyl-hexyl)phosphoric acid extractant. An AC electrostatic field of 3 V supplied via a transformer from the mains power supply is applied between the electrodes 30, 32 and 34, 36 to establish the first and second electrostatic fields. Ac the relatively large droplets of the feed solution 42 and stripping solution 46 fall into the extraction cell 16 and stripping cell 18, they are subjected to the electrostatic fields between the electrodes 30, 32 and 34, 36 which have the effect of causing the relatively large droplets to break up into a multiplicity of microdroplets 60, 62 thereby greatly increasing the surface area to volume ratio of the two aqueous phases. In the extraction cell 16, the Co ions are extracted from the aqueous solution droplets due to the affinity of the D2EHPA thus causing the concentration of the Co-complex to rise in the extraction cell in the kerosene phase. Due to the concentration gradient so formed, the Co-complex diffuses through the kerosene through the baffle 26 towards the stripping cell 18 where the Co-complex reacts with the microdroplets 62 of the stripping solution where the Co-complex reacts with the sulphuric acid to free the D2EHPA, the Co ions reacting with the sulphuric acid and being concentrated therein. The D2EHPA then migrates back through the baffles 26 to the extraction cell 16 to establish a continuous chemical process. As the reacted droplets 60, 62 pass through the electrostatic fields under the influence of gravity, they eventually pass out of the field s and begin to coalesce into larger droplets 70, 72 which fall into the receiving tanks 22, 24 as appropriate.

In experiments under the conditions described above, the initial feed solution of a Co concentration of 1000 ppm was purified to a concentration of 10 ppm in the raffinate 54, whilst the concentrate 56 had a concentration of 19,750 ppm of Co ions.

Therefore, it will be seen that the method makes it possible to concentrate metal ions to a level where it is both practicable and economic to extract the concentrated metal ions so as to recover and reuse the metal per se. An example of this may be uranium. It is also clear that the feed solution may be so purified as to make disposal easier and/or less hazardous.

As noted above, Figure 1 shows the droplet size range distribution achieved when operating the ESPLIM method at conventional mains frequencies of 50-60Hz. The size range distribution effectively falls into two separate and distinct ranges of about 25-150um at the lower end and about 250-550μm at the upper end. The relative proportions of droplets within these two size ranges vary depending upon the applied field intensity as may be seen from Figure 1. This particular form of bimodal size range distribution which naturally results from using conventional mains frequency, is possibly the worst form of distribution which could occur from the point of view of process efficiency. Firstly, it contains a relatively high proportion of droplets at a size which are too small to be effectively stopped by the baffle means 26 whilst the second size range of 250-550μm contains a relatively high proportion of droplets which are larger than the optimum size with regard to efficiency of the rate of mass transfer between the aqueous and non-aqueous phases.

According to the apparatus of the present invention, the AC field frequencies applied across the electrodes 30, 32 and 34, 36 from the supplies 33, 37 are at a frequency corresponding to the natural vibrational frequency of the droplets 60 and 62, and above a frequency of about 60H∑. The droplets 42, 60 are within a desired size range of about 250 to about 400μm with a medium droplet size of about 325μm for the aqueous/10% D2EHPA/kerosene system described in detail above. The frequency of the supplies 33, 37 is such as to stabilise at least a substantial proportion of the droplets in this size range. The graph shown in Figure 2 shows three droplet size ranges: AJ- *B' ,- and, " C . Size range 'A' corresponds to the size range up to about 150μm which constitutes the first size range distribution of stabilised which is too small to be prevented from passing across the baffle 26 thus reducing process efficiency due to high leakage. Size range 'C represents a size range of stabilised droplets which are too large to pass across the baffle 26 but are also unnecessarily large with regard to surface area to volume ratio which is less than optimum to allow most efficient mass transfer between aqueous droplet 60, 62 and carrier liquid 28. Size range distribution 'B' corresponds to the optimum distribution in that the smallest droplets of about

250μm are large enough to prevent significant leakage across the baffle 26 whilst the largest droplets of about 400μm diameter still allow efficient mass transfer between the carrier liquid 28 and droplets 60, 62. Furthermore, where the droplets resonate at their natural frequency with the applied field from the sources 33, 37, the rate of mass transfer between the aqueous droplets and the carrier liquid 28 is significantly improved.

The intensity of the applied field is such that the physical stability of the droplets created by their surface tension is not impaired leading to further breakdown of the preferred range of droplet sizes.

Claims

l. A method for the extraction of a solute from an aqueous feed solution into an aqueous stripping solution, the method comprising the steps of providing at least one stream of each of said feed solution and said stripping solution passing through a continuous phase of a non-polar carrier liquid; said carrier liquid having therein a chemical having an affinity for ions of at least one species in said solute in said feed solution; each of said at least one streams of feed and stripping solutions being under the influence of a first and a second high voltage AC electrostatic field, respectively, for at least a part of their passage time through said carrier liquid so as to break up said streams into a multiplicity of droplets of each of said solutions; providing baffle means between said at least one st eams of each of the feed and stripping solutions, the baffle means being to minimise transfer of feed solution towards said stripping solution stream and transfer of said stripping solution towards said feed solution stream; said baffle means also being positioned between the separate high voltage electrostatic fields to which the feed and stripping solution streams are subjected; and, providing mutually separated receiving means to collect the streams of said feed and said stripping solutions after they pass out of said high voltage electrostatic field; said method being characterised in that said first and second electrostatic fields are selected at a frequency which corresponds to the natural vibrational frequency of at least some of a desired size range of the aqueous droplets in the carrier liquid.

2. ^* A method according to claim l wherein the frequency of the high voltage AC electrostatic field is greater than mains frequency.

3. A method according to claim 1 or claim 2 wherein the frequency of the electrostatic field is greater than the natural frequency of droplets whose diameter is equal or greater than the average diameter generated when a mains frequency AC field of the same intensity is applied and less than the natural frequency of drops small enough to penetrate the baffle means.

4. A method according to claim l or claim 2 wherein the frequency of the electrostatic field stabilises a minimum droplet size of about 250μm.

5. A method according to claim 1 or claim 2 wherein the frequency of the electrostatic field stabilises a maximum droplet size of about 400μm.

6. A method according to any one preceding claim wherein the intensity of the electrostatic field does not cause the breakdown of droplets which have a natural frequency of about the applied electrostatic field frequency.

7. A method according to any one preceding claim wherein the electrostatic field frequencies applied to the droplets of the feed solution stream and to the droplets of the stripping solution stream are different.

8. An apparatus for the extraction of a solute from an aqueous feed solution into an aqueous stripping solution, the apparatus comprising vessel means for containing a 5 continuous non-polar carrier liquid, the carrier liquid having therein a chemical having an affinity for ions of at least one species in said solute in said feed solution,- means for providing at least one stream of each of said feed and stripping solutions through said carrier liquid in said vessel means,- electrode means for applying a first and a second high voltage AC electrostatic field to each of said feed and stripping solution streams, respectively so as to cause said streams to break up into a multiplicity of small droplets; baffle means positioned between the 5 electrode means for establishing said first and second high voltage electrostatic fields, said baffle means allowing the movement of said carrier liquid but minimising transfer across the baffle of feed and stripping solutions,- mutually separate receiving means for collecting said feed and stripping solutions after they have passed through said first and second high voltage electrostatic fields, respectively; the apparatus being characterised by the means for generating said first and second high voltage AC electrostatic fields generating said fields at at least a frequency above 60H∑ and at a natural vibrational frequency of at least some of a desired size range of droplets of said aqueous feed and stripping solutions.

9. Apparatus according to claim 8 wherein the generating o means has a frequency or range of frequencies above mains supply frequencies of 50 to 60H∑.

10. Apparatus according to claim 8 wherein the frequency generated by the means for generating said first and second high voltage AC electrostatic fields is greater than the natural frequency of droplets whose diameter is equal or greater than the average diameter generated when a mains frequency AC field of the same intensity is applied and less than the natural frequency of droplets small enough to penetrate the baffle means.

11. Apparatus according to claim 8 or claim 9 wherein the generating means has a frequency sufficient to stabilise aqueous droplets having a minimum droplet size of about

250μm.

12. Apparatus according to claim 8 wherein the generating means has a frequency which stabilises a maximum droplet size of about 400μm.

13. Apparatus according any one preceding claim from 8 to 11 wherein the generating means is able to supply different frequencies to said first and said second high voltage electrostatic fields.

14. Apparatus according to any one preceding claim from 8 to 13 wherein the electrostatic field intensity stabilises droplets within the size range from about 250 to about

400μm without causing significant further breakdown of droplets.