Ion exchange resin beads are commonly used for selective removal of dissolved mineral constituents from water. Certain of these dissolved mineral constituents, whether in the form of cations or anions, are particularly undesirable. For example, calcium and magnesium ions increase the "hardness"of the water and sulphate ions contribute to corrosion and scaling problems in industrial applications. These ions are commonly found in water extracted from mining operations, making disposal of this water problematical.

Ion exchange resin beads are used selectively to absorb cations or anions onto the resin beads and in so doing become progressively loaded with contaminant ions. Regeneration of these resin beads therefore becomes necessary in order to remove the contaminant ions from the resin.

Conventionally regeneration is effected by bringing the resin into contact with a regenerating liquor. This regenerating liquor may be an aqueous solution of Hydrochloric acid or sodium Hydroxide, depending on the type of resin being treated. A typical regeneration reaction involving the use of sulphuric acid is set out below: R-Ca + H2SO4 (aq)-R-H2 + CaS04 where: R-Ca represents a cation resin loaded with calcium ions; H2SO4 (aq) represents an aqueous solution of sulphuric acid which is used as the regenerating agent; R-H, represents the regenerated resin; and CaS04 represents calcium sulphate or CaS04., H, O The calcium sulphate has a low solubility in water and as a result precipitates from the solution in the form of small hydrated calcium sulphate particles.

These particles tend to precipitate onto the surface of the resin thereby rendering the resin at least partially ineffective for further cation removal.

An example of the regeneration reaction for an anionic resin is as follows: R-SO4 + Ca (OH) 2 (aq) (OH) 2 + CaS04 where: R-SO4 represents an anionic resin loaded with sulphate ions; Ca (OH) 2 (aq) represents an aqueous solution of calcium hydroxide or lime which is used as the regenerating liquor; R- (OH) 2 represents the regenerated resin; and CaS04 represents calcium sulphate or gypsum.

The problem of the precipitation of the calcium sulphate onto the resin beads has been counteracted by the inclusion of small seeding particles of insoluble regeneration product into the solution before the regeneration procedure is commenced. It has been found that by the introduction of the seeding particles, the calcium sulphate preferentially precipitates onto the seeding particles rather than onto the resin. The difficulty then follows in separating the seeding particles from the resin beads.

This problem has been addressed in US 5,269,936 which discloses a method of treating ion exchange resin which includes the steps of introducing a liquor for treating the resin in substantially vertical upflow into a treatment zone in order to produce a fluidized bed comprising the resin and seeding particles interspersed with each other, and separating the resin from the seeding particles through entrainment of the resin on a screen by the liquor being withdrawn from the fluidized bed in a substantially horizontal flow direction.

This is achieved by pumping regenerating liquid into the bottom of a reaction vessel, which is provided with a vertically oriented cylindrical screen and siphoning the liquor and seeding particles through the screen, out through the bottom of the vessel.

A problem with the method of US 5,269,936 is that the regeneration and separation takes place in the same vessel which lends to scaling of the screen with gypsum from a super saturated solution, necessitating frequent cleaning of the screen surface. The cleaning of the screen surface interferes with the efficient operation of the process.

SUMMARY OF THE INVENTION According to a first aspect of the invention there is provided a process for regenerating ion exchange resin including contaminant ions, the method comprising the steps of : introducing a regenerating agent, seeding particles and the ion exchange resin into a reaction zone; reacting the regenerating agent with the contaminant ions of the ion exchange resin to form an insoluble regeneration product; allowing the insoluble regeneration product to precipitate onto the seeding particles; transferring the resin beads and the seeding particles from the reaction zone to a separating zone; and separating the ion exchange resin from the seeding particles in the separating zone by entraining the seeding particles in a liquid passing through a screen having apertures sized to retain the resin and to allow the seeding particles to pass through the screen.

Typically, the separating zone includes an impeller which is rotated to draw the liquid and entrained solid particles through the screen.

Preferably, the liquid is passed upwardly through the separating zone to form a fluidised bed comprising the resin beads and the seeding particles.

The screen is typically arranged in the form of an open topped cylindrical screen extending vertically into the fluidised bed.

Typically the insoluble regeneration product is calcium sulphate.

According to the second aspect of the invention there is provided a process for regenerating ion exchange resin beads including contaminant ions, the method comprising the steps of : passing a regenerating agent upwardly through a fluidised bed comprising seeding particles and the resin beads; allowing the regenerating agent to react with the contaminant ions of the ion exchange resin to form an insoluble regeneration product which precipitates, at least in part, onto the seeding particles; locating within the fluidised bed a screen having apertures sized to retain the resin beads and to allow the seeding particles to pass through the screen; and rotating an impeller to draw seeding particles entrained within the regenerating agent through the screen and to pump the entrained seeding particles away from the fluidised bed.

According to a third aspect of the invention there is provided a method of separating regenerated ion exchange resin beads from seeding particles carrying an insoluble regeneration product, the method comprising the steps of : passing a liquid upwardly through a fluidised bed comprising the resin beads and the seeding particles; locating within the fluidised bed a screen having apertures sized to retain the resin beads and to allow the seeding particles to pass through the screen; and rotating an impeller to draw seeding particles entrained within the liquid through the screen and to pump the entrained seeding particles away from the fluidised bed.

An embodiment of the invention is described in detail in the following passages of the specification which refer to the accompanying drawings. The drawings, however, are merely illustrative of how the invention might be put into effect, so that the specific form and arrangement of the features shown is not to be understood as limiting on the invention.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS Figure 1 shows a schematic flow diagram of a regeneration process according to the invention; and Figure 2 shows a diagrammatic representation of a cross section through the separation vessel depicted in Figure 1.

DESCRIPTION OF AN EMBODIMENT The regeneration apparatus depicted in Figure 1 includes a reaction vessel 10 and a separation vessel 12.

The reaction vessel 10 has cylindrical upper section and an inverted conical base. A resin inlet pipe 14 is provided at the top of the reaction vessel 10 through which an ion exchange resin for regeneration is introduced into the vessel 10.

A regenerating agent inlet 16 is provided at the bottom of the inverted conical base of the vessel 10 in order to introduce the regenerating agent, typically a liquid, into the reaction vessel 10.

Once the reaction vessel 10 is loaded with the ion exchange resin beads, the regeneration agent either for a cationic or anionic resin, as is described in the background to this specification, is introduced into the vessel 10 through the inlet 16 at a flow rate so as to form a fluidised bed within the vessel 10.

During this phase of the operation of the regeneration process the insoluble regeneration product, typically calcium sulphate, is formed and preferentially precipitates out onto the seeding particles.

After a suitable residence time within the reaction vessel 10, which is determined according to the reaction kinetics of the regeneration reaction forming the insoluble regeneration product, the seeding particles bearing the insoluble regeneration product and the regenerated ion exchange resin beads are withdrawn from the reaction vessel 10 and introduced into the separation vessel 12 by means of pipe 18.

The separation vessel 12 is best illustrated by reference to Figure 2. The separation vessel 12, like the reaction vessel 10. has an upper cylindrical section 20 and a low inverted conical section 22. The separation vessel 12 includes a screen separator 24 mounted within the separation vessel 12.

The screen separator 24 has a cylindrical screen frame 26 within which is mounted a screen 28 which is depicted in dashed outline. At the upper end of the cylindrical screen frame 26 the screen separator has an impeller 30 mounted on the drive shaft 32 of an electric motor 34. Mounted within the screen 28 is a frusto-conical flow director 36. This type of screen separator is known an is more fully described in South African patent no. 91/1342.

The screen 28 is formed of a fine wire mesh having apertures of a size selected so as to allow the seeding particles bearing the insoluble regeneration product to pass through the screen while preventing the resin beads from passing through the screen.

In a preferred embodiment of the invention screen separator scrapers or sweep arms may be provided around the outer surface of the screen so that on rotation of the arms about the screen the arms agitate and dislodge any solids which form or accumulate on the outer surface of the screen.

After reaction of the regenerating agent with the ion exchange resin beads in reaction vessel 10 the beads and the seeding particles, bearing the insoluble regeneration product, are introduced into the separation vessel 12 through inlet 37. The slurry of regeneration beads and seeding particles within the regenerating liquor is allowed to accumulate within the separating vessel to a level generally corresponding to the level indicated in Figure 2. At this stage the impeller 30 is rotated within the upper end of the separating screen 28 drawing the regenerating agent and the seeding particles through the screen and discharging these particles and liquor into the launder 38. The discharge of the liquor into the launder at the top of the separating vessel is advantageous as it allows for flow of the liquor into a further vessel by gravity an does not require the liquor to be elevated to a further vessel, which can be costly.

An outlet pipe 40 is provided to draw the liquor and particles away from the launder 38. It will be appreciated that in this way the seeding particles and the regenerating liquor are extracted from the slurry within the separation vessel leaving the regenerated ion exchange resin beads within the vessel. During the separation phase additional liquid may be introduced into the separating vessel 22 to aid with the separation process.

On completion of the separation process the regenerated ion exchange resin beads are withdrawn from the separating vessel through the outlet 42.

The provision of the separate separation vessel 12 allows the liquor to de- super saturate in the reaction vessel 10 before the liquor reaches the separation vessel 12. This reduces scaling of calcium sulphate on the separating screen 28 in the separating vessel 12. Accordingly, the screen in the process of the present invention requires cleaning less frequently than the screen in the process disclosed in US 5,269,936.

The apparatus described above allows a variety of adaptions in its configuration and construction, all falling within the scope of the present invention. The described embodiment of the invention should accordingly not be construed as being limiting on the scope of this invention.