Method and apparatus for separating ions and solids from a fluid Field of the invention The present invention relates to a method and apparatus for the removal of ions from a fluid. The invention has particular application in the removal of salts from liquids.

Background Various techniques are available for the removal of dissolved ions from a liquid. These include distillation, reverse osmosis, electrodialysis, freezing and ion exchange.

Distillation is a process in which the liquid is evaporated, leaving the salt behind. The high energy cost of distillation can be reduced by using a vacuum to lower the boiling point or by using multiple stages.

Reverse osmosis is a technique in which pressure, generated by a pump, forces the liquid through a semipermeable membrane against the osmotic pressure to produce pure liquid, leaving behind a more concentrated salt solution on the other side of the membrane.

Electrodialysis involves the use of ion-permeable membranes to filter out negatively and positively charges ions.

In the freezing technique, the salt containing liquid is frozen with the result that pure crystals of the liquid are formed and the dissolved salt is left in concentrated pockets of higher salinity.

In ion exchange, selected ions are stripped from fluids. The normal method for regeneration of the media is to introduce a counter-ion solution that effectively has a higher potential than the media. Hence, target ions are expelled with the counter-ion solution leaving behind a clean media for adsorption of the target ions in the next cycle. There are a number of disadvantages with this process: The media is not usually highly charged even if it could be, because the stripping agent would not be able to overcome the higher potential.

This low charge implies that the charge carrying capacity of the media is not as high as its maximum capability.

The size of the unit (and therefore the capital cost) is inversely proportional to the potential of the media. A high potential results in a relatively smaller device and hence lower capital cost.

* Chemicat feedstocks as used in conventional processes are an extra contaminant in the discharge. Further, if it is a manual system, it leads to labour and operational. costs, and if automatic it results in servicing, maintenance and capital costs. e Most of the stripping agents are extremely corrosive, necessitating the ion exchange system be fabricated from costly, exotic materials.

'Because of the low capacity of the ion exchange systems it has not been an economically viable method for desalination of seawater. It is usually used for brackish or mildly contaminated water where the salt load is significantly less than seawater 'Conventional ion exchange media contains material bulk that does not add to the ion exchange capacity resulting in increased size and cost for a given output.

Disclosure of this Invention We have found that it is possible to use particulate charged material to remove or strip ions from a liquid.

Accordingly, in a first aspect, the present invention provides a method of separating designated ions from a fluid containing the ions, the method comprising contacting the fluid with charged particulate material such that at least a portion of the designated ions are electrostaticaily bound to the charged particulate material, to produce a fluid effluent having a reduced concentration of the designated ions.

By designated ions we mean any selected ions that may be present in the liquid, for example, those ions of a salt dissolved in the liquid. The designated ions may be sodium ions and chloride ions, although it will be clear that the present invention has application to other ions.

The fluid may be a solution comprised primarily of sodium chloride and water. The fluid may be seawater.

Preferably, the fluid is contacted with the charged particulate material by passing fluid through the charged particulate material.

The particulate material may be any permanently charged or pofarised material carrying a charge or capable of inducing a charge in a non charged particle. The charge may be negative or positive and is selected on the basis of the ions to be removed from the fluid. For example, sodium chloride

dissolved in water has a net negative charge, in which case the charged particulate material used will have a positive charge.

Preferably the charged particulate material is an electret material.

Electrets are known in the art. Further description of electrets can be found in the publication"Electrets and Related Charge Storage Phenomena", Baxt, L. and Martin, P. (1968), by the Electrochemical Society, New York.

The electret material may be any material that it capable of retaining a charge. Examples of common electret material include, but are not limited to, polyvinylidenedifluoride (PVDF), polytetrafluoroethylene (PTFE) polyethylene terephthalate (PET), polyethylene (PE) and polypropylene (PP).

The term"particulate"as used herein in relation to the charged particulate material is intended to include particles of any shape including particles in the shape of a sphere (eg beads), flake, rod or fibre.

The charged particulate material may be formed by methods known in the art. For example, in the case of electret beads formed from a polymer, the selected polymer material is melted in a container pressurised with an inert gas. The temperature and pressure of the container are closely controlled then, at the appropriate time, when the material is in an optimal state, a stream of the melted material is ejected from a nozzle and discharged as atomise droplets into a container. On discharge, the droplets, which form quickly into solid spheres of a predetermined dimension have a high voltage source connected to the discharging nozzle, which cause the beads upon cooling to retain a permanent monopolar electrostatic charge. The amount and polarity of the charge depends on the voltage and the properties of the polymer material and the use to which the beads are to be put. For example, when the method of the first aspect is used to strip the salt from seawater, the charge put on the charged particulate material is a positive polarity. Fibre, sheet and rod-like electrets are processed similarly, but use an extruder to produce the material to be charged, which is then passed over high voltage combs whilst still in molten or semi molten form. Upon cooling, some 20 percent of the charge is lost and thereafter the charge remains stable.

Preferably, the particles of the charged particulate material have a size in the range of less that about 5mm, more preferably in the range 1 to 100lu.

Most preferably, the particles have a size of about 50cm.

In order to ensure that the charged particles efficiently strip ions from the liquid, the particles preferably form a fluidised bed.

The invention will now be described with particular reference to the stripping or removal of salt from seawater, using charged beads as the electret.

However, it will be clear to the skilled reader that the method of the present invention may be used to strip or remove other salts from other fluids.

As the salt particles of seawater have a net negative charge, the positive electrostatic charge on the charged particles-preferably a charge many times higher than the ionic charge on the salt particle-easily captures the salt particle from the fluid. Without seeking to limit the present invention by theory, it is believed that the process continues with the charged beads building up depositions beyond the rigid Stern's layer, and many times further than the normally diffuse Gouy-Chapman layer expected in a normal electrochemical processes encountered in flocculation. The region between these two layers is the plane of shear and the difference in potential at this plane with respect to the fluid is called the zeta potential, or the electrokinetic potential. The zeta potential for the charged particle can be many times greater than an electrochemical zeta potential, even for the most electro-active molecules.

One option in the method of the invention is to discard the charged particulate material once there has been sufficient salt build-up thereon, leaving a substantially desalinated water.

An altemative and preferred option is to regenerate the particulate charged material so that it may be reused for desalination. We have found that the salt loaded charged particulate material may be regenerated by allowing the particles to approach an electrostatic field of sufficient strength for the salt ions to transfer from the charged beads to the higher electrostatically charged body.

Accordingly, in a second aspect, the present invention provides a method of separating designated ions from a fluid containing the ions, the method comprising: (a) contacting the fluid with charged particulate material for a time sufficient for at least some of the designated ions to be electrostatically bound to the charged particulate material to produce an effluent have a reduced concentration of the designated ions; (b) regenerating the charged particulate material resulting from (a) by exposing the charged particulate material to an electrostatic charge generated by electronic means such that at least some of the designated ions bound to the charged particulate material transfer to the electronic means to thereby

effectively restore the charge on the charged particulate material, making them substantially free of the designated ions ; and optionally reusing at least a portion of the regenerated charged particulate material in step (a).

Preferably step (b) is facilitated by agitating the charged particulate material within the fluid, for example, by inducing turbulent flow so that the charged particulate material has a significantly higher probability of touching the electronic means.

The electronic means may be of any suitable shape and/or configuration.

The electronic means may be one or more insulated electrodes. The electrodes may be insulated by a dielectric material. The electrodes may be one or more hollow tubes, made from a wire mesh or the like strategically located within a reactor vessel containing the charged particulate material. The electronic means are preferably connected to a high voltage source so that strong electrostatic field (s) are generated. Preferably, the generated electrostatic field is greater than that of the charged particulate material. More preferably, the generated electrostatic field is many times the electrostatic field on the charged particulate material. The electrostatic charge generated by any suitable means may be in the range of about 1 to 100 kV. From experience it has been found that a voltage in the order of 70kv is adequate. Preferably the voltage supply provides a pulsed voltage, most preferably a square wave voltage. By using a pulsed voltage, salt electrostatically attached to the electrodes goes through a cycle of release and attachment to the electrodes so that the salt may be flushed through and out of the hollow electrodes by a flushing fluid.

In a third aspect, the present invention provides an apparatus for the removal of designated ions from a fluid, comprising : a vessel having an inlet for introducing the fluid into the vessel ; a plurality of charged particulate matter having an electrostatic field contained within the vessel in fluid communication with the inlet ; and an outlet in fluid communication with the charged beads through which fluid having a reduced concentration of designated ions may be removed, and the beads retained in the vessel.

The charged beads may be that described in more detail above. Most preferably, the charged beads are in the form of electret material carrying a charge.

The charged particulate matter may be contained in the apparatus between containment means that are porous to fluid and ions but will not allow the charged particulate matter to pass therethrough- The inlet and outlet may have valves which allow control of fluid flow therethrough. The outlet may be in fluid communication with a conduit the inlet of which is above the confined charged particulate matter.

In a particularly preferred embodiment of an apparatus in accordance with the invention, the apparatus may include means for regeneration of the charged particulate matter.

Accordingly, in a fourth aspect, the present invention provides an apparatus of the third aspect, further comprising means for regeneration of the charged particulate matter.

Preferably the means for regenerating the charged particulate matter is one or more insulated electrodes which are capable of generating an electrostatic field greater than that of the charged particulate matter. Preferably the electrodes are adapted to act as discharge means for fluid having an increased concentration of the designated ions.

The insulated electrodes may be in the form of tubes that are porous to the fluid and designated ions but are impervious to the charged beads.

Preferably the electrodes are connected to a voltage source that is capable of providing a pulsed or continuous voltage to the electrode (s).

Preferably the voltage source is capable of providing a square wave voltage.

The outlets of the electrodes may be in fluid communication with an outlet from the vessel that is below the contained charged beads so that the fluid now concentrated in the desired ions may be removed from the vessel.

Preferably the apparatus of the fourth aspect is adapted to operate in at least two modes : one mode being a ion removal mode in which designated ions are allowed to attach to the beads matter to produce a fluid effluent having a reduced concentration of the designated ions ; and a second regeneration mode in which ions have been electrostatically attached to the charged particulate matter are transferred to the electrode (s) which are charged so as to produce an electrostatic field greater than that of the charged particle. In the latter mode, designated ions deposited on the electrodes are flushed out through the electrodes via the outlet (s) in fluid communication with electrode (s) when the power is turned off.

Embodiments In order that the present invention be more readily understood, the following non-limiting embodiment is provided.

Referring to Figure 1, desalination apparatus 1 comprises a vessel in one embodiment, being a sphere but not being limited to this shape, having inlet 4 for introduction of seawater or other salt containing liquid into the compartment 17 and from there into volume 80 via porous surface 12. The apparatus has an outlet 4a, with valve 6, which is in fluid communication with outlet of electrodes 16 and 18 via region 82 and a second outlet 8, which communicates with the interior of the vessel via a tube 10 having a valve 13.

Porous layers or surfaces 12 and 14 are located within vessel to contain electret polymer beads therebetween. The pore size of the porous layers is such to allow fluid to pass therethrough but prevent the electret beads from passing through the layers. In the present embodiment, the porous surfaces are formed from needle felt having pore size of approximately 10% of bead diameter of 5%.

Electrodes 16 and 18 are located within the chamber 80 and the outlets 7 thereof extend into region 82 which is in fluid communication with outlet 4 These electrodes also act as discharge tubes in that the outlets thereof are in fluid communication with outlet 4a. These electrodes are connected to a high voltage source (not shown) so that they create a strong electrostatic field.

Preferably the electrodes are run at a voltage in the range of about 1-100kV.

The voltage source provides a pulsed wave (positive/negative), which in the preferred embodiment is a square wave.

A detailed view of the electrode construction is shown in Figure 2. Each electrode 30 is in the form of a hollow cylindrical steel cage 32, which has been insulated by coating the electrode in an insulating dielectric material such as PET, PDVF or similar. Needle felt (34) of pore size of approximately 10% of the bead diameter, e. g. 5Am covers the insulated electrode to prevent the electret beads from passing through into the interior of the electrode.

The apparatus typically contains approximately 9x1 0t3 positively charged electret beads of a size located in region 80 between porous layers 12 and 14 as a fluidised bed.

The apparatus may be operated in two modes-a desalination stage and a bead charge regeneration stage.

In the desalination stage, valve 13 is open and valve 6 is dosed.

Seawater is introduced into the vessel 1 via inlet 4, passes into chamber 17 and passes through porous layer 12. The seawater is adjusted so that there is flow of the water through the body of electret beads located in region 80.

Because the salt has a net negative charge, it is attracted to and electrostatically attached to the positively charged beads. Fresh water, stripped of salt passes through porous layer 14 and into conduit 11 and from there is removed from the vessel. A conductivity measuring device (not shown) is located within the flow of fresh water to measure the conductivity of the water and interpret its salt content. The desalination stage is continued until the conductivity measure indicates that the salt content in the desalinated water reaches a predetermined level that is indicative of the electret beads being sufficiently covered with salt and therefore no longer capable of stripping salt at the same rate from inlet seawater. The apparatus then switches to the bead regeneration stage In the bead regeneration stage, valve 13 is closed and valve 8 is opened so that seawater entering inlet 4 passes into region 80, through the electrodes 16 and 18 and out of the apparatus via outlet 4a. The flow of seawater is adjusted so that there is turbulent flow in region 80 to maximise the probability of the electret beads touching or closely approaching the electrodes 16 and 18.

The electrodes are connected to high voltage source so that they generate an electrostatic field in the range of about 1 to 100 kV, which is greater that the electrostatic field of the electret beads. The salt electrostatically attached to the beads in the desalination step are then transferred to the electrode when the beads touch or approach within the feld of the electrodes thereby regenerating the beads to their original net charge and resulting in a salt build up on the electrodes. The voltage applied to the electrodes is pulsed typically as a square wave. This pulsed voltage allows salt deposited on the electrodes to be flushed out by the incoming seawater to produce a stream containing concentrated salt (brine) which passes from region 80 through the electrodes and thereafter out of the vessel via outlet 4a. A conductivity measurement means may be located in the concentrated salt stream to measure salt concentration and thereby monitor and control timing of the bead regeneration step.

During this whole procedure, the fluid flow continues mono-directionally but suitable valving ensures that the now concentrated salt solution takes a different exit from the device than the previously cleaned water. After a

suitable time this cycle is repeated, viz. purification of the salt water then a period of regeneration of the beads and so on.

It will be appreciated that the apparatus sequentially moves through a number of desalination/regeneration cycles. As least part of the concentrated salt solution produced in the regeneration stage may be returned to a desalination stage. A desalination plant may comprise a plurality of apparatus according to the present invention.

Throughout this specification the word"comprise", or variations such as "comprises"or"comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

Any description of prior art documents herein is not an admission that the documents form part of the common general knowledge of the relevant art in Australia.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.