WO1991004791A1

WO1991004791A1 - Causing liquid/solid interaction

Info

Publication number: WO1991004791A1
Application number: PCT/GB1990/001514
Authority: WO
Inventors: Jeremy Belmore JOSEPH; Michael Cox; Rodney Charles Squires
Original assignee: Crossflow Microfiltration Limited
Priority date: 1989-10-05
Filing date: 1990-10-04
Publication date: 1991-04-18
Also published as: FI921510A0; EP0491006A1; IE903568A1; AU6545390A; GR900100733A; GR1000737B; JPH05502819A; CA2067380A1; PT95527A; GB8922503D0; PL287197A1; FI921510A

Abstract

In order to remove e.g. nitrate from water with a short contact time, the water is mixed with small particles of ion-exchange resin in a tank (1), to form a suspension of a concentration of 1.25 to 5 % w/w. The particle mesh size is e.g. from 5 $g(m)m to 0.5 mm. The suspension is passed from the tank (1) into a crossflow filtration unit (3) where treated water is filtered off at (4). During the contact time in the tank (1) and in the pipework, the nitrate content of the water is significantly reduced. The ion-exchange resin is returned to the tank (1) and can be recirculated on average about four times. A bleed of the suspension is taken from the tank (1), is regenerated, and is recycled to the tank (1).

Description

CAUSING LIQUID/SOLID INTERACTION

Background of the Invention

The invention relates generally to causing a liquid to interact with a solid. Normally, mass transfer will occur between the liquid and the solid, but this is not necessarily so. The mass transfer can be to or from the liquid. The invention may be applied for instance to leaching, ion-exchange, crystallisation, drying, specific solution, sorption, roasting, solid-particle-catalysed liquid-phase reactions, and liquid-solid reactions, and the application of these processes to chemical reactions in general. During contact, ion or other exchange may take place, or one or more compounds may be absorbed, absorbed or chemisorbed, or released; the compounds will usually be in the form of ions or molecules, but they could be colloidal or other agglomorations, or mixtures or in some other form. Any number of chemical species may be involved, depending on the requirements of the particular process. If the invention is applied to ion exchange, t can be used for the contact step and/or for the regeneration or elution step. Specifically, the invention has been developed in the course of ion exchange procedures for removing nitrate from water, but can be particularly useful in reducing the hardness of water, selectively removing or regaining materials such as pollutants or metals f'rom water, and adsorbing colour.

The normal procedure is to bring the liquid into contact with the solid, separate the liquid from the solid, regenerate the solid, and bring further liquid into contact with the solid. The solid, eg the sorbing/releasing substrate, would usually be insoluble, eg resin, clay, mineral or glass.

To date, the standard procedure for operating a solid/acqueous liquid system was to support the solid in a column and to operate a batch procedure, passing the process liquid, closing off the process liquid, passing regenerating solution, and then passing the process liquid. This procedure was relatively complicated. Furthermore, the solid particle sizes could not be lower than around 0.4 to 0.5 mm, otherwise the surface tension of water prevented sufficiently rapid draining under gravity, and general practice was to use particle sizes of 0.5 mm upwards. The solid was usually in the form of beads (one standard bead size was 1.1 mm), which may have had a coating of the reactive material, but normally were completely made of the reactive material. The natural swelling and contraction of the solid causes cracking and spalling, and the small particles so formed can clog the bed of solid.

As an alternative to this procedure, continuous gravity settling procedures have been proposed. In this case, the solid must be of such a size that the rate of descent (or differential rate in an up-flowing liquid) is reasonably rapid. The general view is that the bead diameters must be at least 0.5 mm, for sufficiently rapid settling. These procedures are complicated because beads may stick in valves or the valving used may crush the beads, leading to unwanted clogging and insufficiently rapid sedimentation. A specific proposal has been made in GB-A-1 070 251, providing a counter current process where the solids are in effect in suspension. There is no discussion of the sizes of the solid particles but, in practice, it is found that a diameter of at least 0.5 mm is required for successful operation. As another instance, in EP-A-0 010 969 fouling and excessive back pressure can occur.

The separation of fine solid particles can be difficult, it being impractical to use filter columns. Thus it has been suggested that the particles can be coagulated or agglomerated, for instance when using magnetic particles.

It is desirable to provide a continuously operating system where the concentrations remain fairly constant and where the amount of solid required is kept as low as possible.

The Invention

The invention provides a method of causing a liquid to interact with a solid, comprising forming a suspension of particles of the solid in the liquid and moving the solid particles and the liquid in the same direction while the liquid interacts with the solid particles, then removing the liquid from the suspension, preferably by crossflow filtration, and recycling the solid particles. The invention also provides plant for carrying out the method of the invention.

The invention, particularly when using a combination of the cocurrent movement of the liquid and discrete particles of the solid and crossflow filtration (or a related barrier filtration method), can take advantage of the enhanced kinetics of e. g. microresins without the drawbacks of excessive pressure drop and danger of fouling or clogging which is expe'rienced with column arrangements, or the attrition and related mechanical blocking and losses commonly associated with continuous systems.

The method of the invention is simple and efficient. Plant sizes can be reduced and equipment costs lowered. The method can be continuous or nearly continuous.

The invention can be used for a contacting stage and for a regeneration stage, or used only in a regeneration stage or only in a contacting stage.

Normal valving can be incorporated if desired because splitting or comminution of the particles is no longer of great significance as the operating particle size can be very small. Thus the invention allows use of fine material, whether it is originally fine or breaks down to fine material as a result of attrition in the process.

Solid Particle Size

The particles can have any form which is suitable, e. g. fibres or platelets or the more traditional spheres and spheroids. For spherical or spheroidal particles, the sizes can be considered as sieve pass sizes (giving the nominal diameter), and in general the sizes herein are sieve or mesh pass sizes.

Constraints on the maximum size of the particles are related to the economic and operating conditions of individual processes by a number of different parameters, such as volume, surface area, minimum or maximum presented area, maximum presented area, minimum dimension and maximum dimension. Though a standard size of about 1 mm (or specifically 1.1 mm) can be used, particularly a large plants, the particles are preferably small compared to those used in standard procedures, so that the effective specific surface is larger and the exchange/sorption/solution/reaction kinetics are more rapid. Relative reduction in size can increase the apparent number of sites available for action but, more importantly, can enhance the reaction kinetics. There is better random contact, particularly at low concentrations. The particle size can be optimised to suit the particular process requirements, but solids can be used as supplied, e. g. with no preliminary sieving to remove fines. In general, the particle size is preferably smaller than that selected for use, for instance, in packed columns or filter beds. The particle size is preferably less than about 0.5, 0.4 or 0.3 mm, or less than 0.25 or 0.2 mm. Generally, although the particles need not be formed by a comminution technique, the particle size can be that of a powdered solid. Quite often, the upper limit on particle size is that imposed by reaction kinetics and also pumping and the energy needed to maintain large particles in suspension. There is not usually any advantage in using large particles, but there is no reason why they should not be used. Nonetheless, an advantage of the invention is that one can use particles of a size smaller than normal.

There is no theoretical lower limit to the particle size that can be used, though in practical terms particles of less than about 0.2 or 0.1 microns will not be used. The size of 0.1 microns is determined by the cut off size of a microfilter, but this size could be reduced further if an ultrafilter were used. A minimum practical size of 0.4 or 0.5 microns is more realistic. However, particles of less than 50 micron tend to form a cream in water and be difficult to dewater.

To give a specific example, for the ion-exchange resin used in the Examples below there is no real advantage seen in having particle sizes less than 100 micron. Particle size ranges may be 50 to 120 microns or 100 to 200 microns, or for activated carbon 100 to 600 microns. Ratio of Solid to Liquid

The concentration or w/w ratio of solid to liquid can be chosen as appropriate. For instance, some adsorbent solids have relatively low capacity and need to be used in high concentration. Some suspensions or slurries can remain fluid at up to 60% solids whilst others are solid at 2%; the concentration must be such as to permit suspension and pumping. Within the need for the suspension to remain pumpable, the properties, and actual solids content, of the suspension will vary depending on the application. For instance, the proportion of solids may be as low as 0.2% or even lower, or the proportion may be over 20% or over 30%. In a plant, there can be different concentrations in different parts, for instance in the treatment stage and in the regeneration stage. In general, the concentration of the solid in the liquid can be relatively low during the cocurrent movement and interaction, e. g. below about 10% w/w, preferably from about 5% down to about 1%.

Contact Stage

Any form of turbulent or other suspension can be used in which the solid particles and the liquid are mixed intimately and randomly - for instance, the intimate mixing can be achieved eg by having a tank with a propeller in it or providing a fluidised bed. The suspension need not be a stable suspension, ie. the solid particles may sink or rise if there is no flow or no agitation, the technique being a type of fluidisation technique. In general terms, the particles will be discrete, ie. not aggregated.

The solid and liquid may be put together prior to introduction to a chamber where the intimate and random mixing occurs. However, in one procedure, after forming the suspension, the suspension is passed along a long supply line with a substantial residence time, to the filter in which the separation is effected; the supply line can be say 1 to 2.5 m long. In general, contact times are preferably greater than about 1 or 2 minutes and less than about 20 minutes, say about 4 or 5 minutes. The time is chosen as appropriate to the particle sizes, the degree of agitation, and the reaction kinetics.

Separation Stage

The separation or liquid removal stage can be carried out in any suitable way. It is not necessary that the separation stage should occur in a chamber different from that in which the contact stage is carried out. However, the mixture is preferably passed to a filter, which can be any suitable filter but is preferably a crossflow filter, which may be used in a dead-end mode. The advantage of using a dead-end mode is that a much higher flux can sometimes be obtained, possibly twenty times as great. The filter can for instance be as described in GB-A-2 185 906 or US 4 765 906. The separation stage can involve the steps of building up a membrane, building up a cake of the contacting material, possibly regenerating the contacting material and washing it while on the filter, breaking up the cake, eg with rollers as described in GB-A-2 185 906 or US-A-4 765 906, opening up the closed end if it is a dead-end filter and flushing out; alternatively, if there is a bag filter, turning the filter inside out with an inserted member such as a piston. A flexible filter is preferred; nevertheless, the filter need not be flexible - a flat sheet filter could be used, and the cake subsequently scraped off the filter. In some cases, the contact stage can occur on or in the filter itself, if appropriate to the particular process; in such a case, the filter can be pre-coated, and then coated with the solid which is then immobilised on the filter in some way; regeneration can then take place on the filter. The use of a filter reduces attrition of the contact material, and also reduces the effect of such attrition in that liquid can be removed even with sub-micron particles present. The use of a crossflow filter enables a constant concentration of the solid in the liquid to be maintained, at least for long periods between filter cleaning operations.

Preferably, sufficient liquid is left after the separation for the solid particles to remain in the form of a suspension, say as a slurry, _. which facilitates recycling.

At the end of the separation phase, the liquid may pass into a further contact stage with the same batch of solid (but regenerated) or a different batch of solid.

Regeneration Stage

The solid may be regenerated, and this may be done at the separation site, or elsewhere. Typical regeneration will be done by immersion in a suitable fluid, eg immersion of ion exchange resin in brine, but any suitable process can be used, such as caustic or acid washing, leaching, gas elution, heating, exposure to light or other electromagnetic radiation, passage of electric current or physical shock. Further washing or flushing may be required after regeneration, to return the solid to a suitable condition for re-use or disposal.

For regeneration, a procedure can be used which is inventive per se. The procedure is a method of contacting a liquid with a solid, comprising forming a suspension of the solid in a liquid, passing the suspension to a filter so that the filter holds the solid back, and then passing a liquid through the filter in the opposite direction to remove the solid from the filter and form a suspension. The filter is preferably in the form of a bag filter so that it has a large retention capacity. The method of operation keeps the filter clean, without requiring any special cleaning step as such. For regeneration, the solid can be suspended in a weak regenerating agent, and then removed from the filter with a strong regenerating agent; for washing, the solid can be suspended in a first washing liquid and then removed from the filter with a second washing liquid. Normally, both procedures will be adopted consecutively, in different filters.

Regeneration is preferably carried out by feeding the solid particles from the means for removing liquid from the suspension back to a chamber in which said suspension is maintained, and drawing suspension from said chamber for regeneration; in other words, the solid particles are preferably recycled without regeneration, a separate regeneration loop being maintained. An alternative is to provide a bleed-off for removing a proportion of the solid particles as they are recycled to the suspension chamber.

Return Stage

The last stage is the return stage, when the regenerated or unregenerated solid may be returned to the start of the procedure for reintroduction at a controlled or measured rate, if the solid does not remain in situ throughout.

In a continuous procedure, there may be a slow bleed-off of the -solid particles for discharge or regeneration, which is replaced by a slow feed of solid particles whilst the major portion of the solid particles is re-circulated. The solid particles can be recycled on average at least about twice before regeneration or discharge, e. g. about three or four times. Large recirculation improves the utilisation of the solid by using its full capacity, enables higher concentrations of solid to be used, and makes the quality of the product more consistent. Specific Uses of the Invention

The liquid is not necessarily water, but it will be so in many cases. The invention can be particularly useful when removing unwanted materials from water which are in low concentration, for instance reducing concentrations of nitrate, boron, strontium or caesium. An advantage of the invention is that low pressure differentials can be used for the filtering, say less than about 350, 250 or 200 kPa and down to about 150, 100 or 50 kPa, allowing large volumes to be treated, say more than

3 about 10 or 20 m /hour; this can be particularly useful in the water industry.

Other uses envisaged include:

using or regenerating powdered activated carbon employed for purifying liquids, e. g. to remove pesticides or other trace organic pollutants such as trihalomethanes;

using powdered activated carbon to remove emulsions such as paint from water, or to remove organics such as oil and butane gas from a water/oil/gas emulsion - normally the carbon would not be regenerated. using or regenerating magnetite or other regenerable, or non-regenerable, coagulating agents;

using biologically and other naturally derived absorbents in treating process fluids such as radioactive wastes (here special filters such as woven carbon or glass fibre, or stainless steel or phosphor bronze, may be used);

continuous ion exchange, in general - normally the ion exchanged would be regenerated.

Preferred Embodiments

The invention will be further described, by way of example, with reference to the accompanying drawings, in which: -

Figures 1 to 4 are schematic diagrams of three different plants for carrying out the method of the invention;

Figures 5a_ and 5b is a diagram of a pilot plant; and

Figure 6 is a view, partly in vertical section, of a sock filter in the plant of Figures 5a_ and 5b. Throughout, the same references are used for components carrying out similar functions.

Figure 1

The contacting solid and the process liquid are mixed in a tank 1 and are pumped out through a large diameter, long flexible hose 2 wound around a drum, giving substantial contact time, the hose 2 acting as a contact chamber. The length of the hose.2 may be 1.75 m, giving an approximate residence time of one minute at a flow rate of 20 1/m. Adequate mixing is required so that sufficient contact occurs, and if desired, a stirrer can be included in the mixing tank 1, though the pumping action will normally give good contact in the hose 2; both in the stirred mixing tank 1 and in the hose 2, the solid and the liquid are moving in cocurrent. The substantial residence time in the hose 2 enables the tank 1 to be smaller and/or the residence time in the tank 1 to be shorter. The mixture then passes to a crossflow filtration unit 3 from which the clean process fluid i.s withdrawn at 4.

At least two methods of regeneration are possible. In a first method, the regenerant (regenerating agent) is fed from a storage tank (not shown) onto the solid on the filter so that the solid is regenerated on the filter. After regeneration, the solid is removed from the filter, to pass along a recycle line 5 back to the mixing tank 1. In a second method, the solid is removed from the filter and passed to a contact tank, the solid then being regenerated, filtered, washed and returned to the mixing tank 1. The method used depends upon the ease of regeneration, though basically the first method is faster and easier than the second.

An alternative procedure can be used, particularly when it is cheaper to discharge the solid (which may be the case with e.g. powdered activated carbon), though the procedure can be used when the solid is later regenerated. Without regeneration on the filter 3, the solid is recirculated along the line 5 as a concentrated slurry or suspension. In this way, the solid can be re-circulated for instance a mean 20 times before being regenerated or discharged. The recirculation can occur during the normal crossflow filtration, or a cycle of removing the solid from the filter can be incorporated so that the solid so removed is immediately recirculated.

Standard components are shown in a conventional manner, including level gauges, one-way valves, stop valves and pressure gauges. There is a flow meter 6 and an electronic control 7 for a main pump 8. The filter 3 can be as disclosed in US 4 765 906.

Figure 2

A dead-ended crossflow filtration unit 11 is used, ie. a crossflow filtration unit with a closed outlet value. A contact chamber is not shown, but may be included if desired. However, sufficient contact may occur in the tank 1 and in the pipework. A recycle line 12 is shown, upstream of the filter 6. The regeneration can be as above.

Figure 3

The plant of Figure 3 is similar to that of Figure 1. A vessel 13 is shown for the collection of treated water withdrawn at 4, and a resin feed tank 14 is shown for feeding the mixing tank 1. There is a line 15 for feeding untreated water. However, a further crossflow filter 16 is shown in a second recycle line 17 which also includes equipment 18 for regenerating the resin and washing it. The filter 16, which can be used in the crossflow or dead-end mode, removes further fluid from the resin before it is passed to the equipment 18. The bulk of the resin can be recycled through the line 5, sufficient being recycled through the line 17 to keep a suitable regeneration level.

Figure 4

The contacting stage of the plant of Figure 4 is similar to that described in relation to the alternative procedure of Figure 1. A process liquid is fed at 21 and a resin suspension or slurry is fed at 22 into a feed tank 1. The suspension in the feed tank 1 is drawn off at 23 by means of a feed pump 8 and pass through a crossflow filter 3. In the filter 3, the liquid is removed and a suspension or slurry of the solid with reduced liquid content is returned at 5 to the feed tank 1 whilst the removed liquid exits at 4.

The regeneration procedure is countercurrent regeneration in a number of cocurrent steps. There is a bleed 24 which is pumped by a variable-stroke displacement pump 25 into a thickening device 26 of any suitable type, illustrated as a rotary vacuum filter. The pump 25 controls the solid concentration and hence the solid detention time in the system. The filtrate is withdrawn at 27 and is returned to the feed tank 1. The thickened solid passes into a tank 28 containing weak regenerant, where it forms a suspension in the regenerant. The suspension is pumped by a pump 29 through a reversible sock filter 30 and the filtrate is returned to the tank 28, the solid being held back in the filter 30. After a sufficient number of cycles, valves 31, 32 are closed and valves 33, 34 are opened. There is a further tank 35 containing strong regenerant, which is then pumped by a pump 36 in the reverse direction through the filter 30, backwashing the filter 30 and picking up the solid retained in the filter 30 and forming it into a suspension which is carried to a head tank 37 (if required). The suspension of solid in strong regenerant is drawn off the head tank 37 at 38 and passed to a second thickening device 39 of any suitable type, illustrated as a rotary vacuum filter. The filtrate from the device 39 is drawn off by a pump 40 and is returned to the strong regenerant tank 35, optionally by way of the head tank 41.

For rinsing, the thickened resin from the thickening device 39 is passed to a rinse tank 42 to which rinsing liquid is added; here it is mixed with a rinsing liquid to form a suspension, and the suspension is pumped by a pump 43 through a second sock filter 44 and returned to the rinse tank 42. After a suitable number of cycles, valves 45, 46 are closed and valves 47, 48 are opened. There is a resin solution tank 49 to which make-up liquid is added, and this liquid is pumped by a pump 50 in the reverse direction through the sock filter 44 to carry away the solid held back in the sock filter 44 and form a suspension in the tank 49. From the tank 49, a positive displacement pump 51 pumps the regenerated solid in suspension back to the feed tank 1. Surplus rinse liquid is removed from the tank 42 and spent regenerant is removed from the tank 28.

The spent solution is run off continuously and in a small stream, and concentrations around the system are substantially constant. Thus there is no requirement for large balancing tanks and blending.

Figures 5a and 5b

Figures 5a_ and 5b should be joined along their respective right and left margins.

The pilot plant of Figures 5a and 5b is based on the plant of Figure 4.

In the contact stage (Figure 5a_), a number of filters 3 are used, divided into three banks each serviced by its own pump 8 - the banks of filters 3 can be operated independently or together to achieve the desired capacity. The filter 3 are curtain modules as disclosed in Figures 11, 12, 17, 18a and 18b of US 4 765 906, each with a back-pressure ball valve 60. The filters 3 drain into a tray 61 which in turn has flap valved outlets so that it can drain at 4 into a treated water tank 62, or can drain at 63 into the feed tank 1 if the quality is unacceptable. There is a cleaning device for cleaning the filters 3, generally as described in US 4 765 906; there is an arrangement for drawing treated water from the treated water tank 62 and pumping it with a pump 64 through a hose reel 65 to moveable spray nozzles 66 for cleaning the filters 3, the spray nozzles 66 being carried by a transporting device 67. The feed tank 1 is equipped with a jet mix pump 68 to ensure that the solid and liquid form a good suspension. There^"is a helical meter 69 in the feed line 21. The treated water tank 62 contains a V-notch weir 70 forming a separate compartment 71 from which treated water is pumped by a pump 72 (controlled by a level switch 73) to a service

3 water tank 74 of say 2 m capacity.

In the regeneration stage (Figure 5b), each thickener 26, 39 is associated with a respective suction auxiliary unit 75, 76. The filtrate from the thickener 26 can be passed along a line 27a and pumped by a pump 77 to the feed tank 1, or can pass along a line 27b and be pumped by a pump 78 to the treated water tank 62, according to quality. The filtrate from the thickener 39 can be passed along a line 40a_ and returned to the strong regenerant tank 35 by a pump 79, or can be passed along a line 40b and pumped by a pump 80 to the feed tank 1 or via a line 81 to any of the tanks 28, 35, 42 or 49. A regenerant saturator 82 is shown for feeding regenerant to the strong regenerant tank 35. There is an overflow feed 83 from the strong regenerant tank 35 to the weak regenerant tank 28. The weak regenerant tank 28 is provided with a stirrer 84. There is an overflow line 85 from the weak regenerant tank 28 to a waste tank 86 which overflows into a basin 87 pumped to waste by a pump 88.

In the rinsing stage, the rinse tank 42 has an overflow line 89 feeding to the waste tank 86, and the tank 49 has a safety overflow 90 leading to the waste tank 86. The tank 49 has a stirrer 91 for forming a suspension of the solid.

A service water line 92 is provided for start up.

In any of the Figures, two or more mixing tanks 1 can be used in parallel. In Figures 1 to 4, two or more filters 3 or 11 or 16 can be used in parallel; if dead-end mode is employed for the filtration units, this enables a semi-continuous procedure to be operated by switching from filter 3, 11 or 16 to the other.

Figure 6

Figure 6 illustrates the sock filter 30 or 44. The filter 30, 44 comprises two cylindrical casings 101 bolted together at flanges 102. The flanges 102 sandwich between them suitable gaskets, flanges 103 on two stainless steel conical cages 104 which are within the casings, and the flange of a filter cloth 105. The filter cloth 105 is stitched to form a cone of the same size as the conical cages 104 with a flange on the open end of the cone. The cages 104 prevent the filter cloth 105 over-extending and bursting, ie. they suspend and restrain the filter cloth 105. The filter cloth 105 can be formed of polyester and can be of a single ply of the same specification as described with reference to Figures 17 to 18a of US 4 765 906.

Example 1

On a labratory scale, the plant of Figure 1 was used to reduce nitrate concentration in water from 93 to less than 13 mg-N0₃/l. Using " Duolite Microionex AOH" iόn-exchange resin (as supplied by Rohm & Haas) as a powder having particles in the range of 5 to 90 μm, the contact time was one minute. Although a specific ion-exchange resin is mentioned, any suitable powdered anionic resin may be used. The concentration was preferably greater than

0.05%, 0.1% being effective and about 0.2% being preferred - there seems to be no great advantage in having concentrations greater than 0.2% at this particular initial nitrate concentration.

The filter 3 was as described with reference to Figures 5 to 10 and 17 to 18b of US 4 765 906. US 4 765 906 explains how the held-back solid can be removed from the filter 3. The pressure differential was 150 kPa across the filter 3, a suitable range being 100 to 200 kPa.

Example 2

On a labratory scale, the plant of Figure 3 was used for reducing nitrate concentration in water from 100 to 11 mg-NO,/l. The resin as in Example 1 was fed in as a slurry in water at 0.1% w/w concentration, and was filtered by the filter 3 in crossflow mode with part of the slurry recycling via the line 5 to the mixing tank

1. A bleed stream was separated by valves to flow into the filter 16 which was operated in dead-end mode. The product water thus obtained was combined with that from the filter 3 and collected in the tank 13. Once sufficient resin had been collected in the filter 16, a valve was opened allowing brine solution (regenerant) to wash the resin into the tank 18 for regeneration. The brine solution was followed by a small amount of product water to clean the filter 16 before it was re-introduced into service as a dead-end filter.

Other details were as in Example 1.

Example 3

On a pilot plant scale, the plant of Figure 5 was used for reducing nitrate concentration in ground water from about 60 to 19 mg-NO_/l. The throughput was 11.9

3 m /hour. The ground water contained no detectabl* solids. Nine filters 3 were used, each 8 m long.

A "Purolite" A520E ion-exchange resin was used, as a powder having particles in a range of 50 to 100 μm. The resin was dosed by the pump 51 into the feed tank 1 at such a rate that the resin was 0.2% w/w of the raw water, though the dynamics of the system caused the resin concentration in the feed tank 1 to be significantly higher, namely about 1.25%. At steady state, the total load of resin in the system was about 11 kg. The bleed pump 25 was pumping at about 1000 1/h, amounting to 16% of the feed. The pressure in the filters 3 was about 175 kPa, or possibly up to 200 kPa (higher pressures than this could be used if the raw water contained solid matter which fouled the filters 3). The filters 3 removed about 96% of the water from the suspension flowing into the filters 3. The resin on average recycled about four times to the feed tank before being regenerated.

The thickener 26, 39 were operated to a discharge the solids at 50% w/w concentration. The filter 30 was reverse flow backwashed on each regeneration cycle after three minutes. 10% w/w sodium chloride solution was used as the strong regenerant and 5% w/w sodium chloride solution as the weak regenerant.

The desired nitrate concentration was obtained in the treated water, and chlorides were below 200 mg/1. Water recovery was about 84%.

The average retention time in the feed tank 1, pipework and filters 3 was about 4 or 5 minutes. The filters 3 were as disclosed in Figures 11, 12, 17, 18a and 18b of US 4 765 906. However, as there were no solids present in the raw water, no cleaning cycle was required. Some of the resin settled on the weave of the filter support to form a membrane, but the remainder of the resin did not build up a significant layer.

Example 4

Example 3 was repeated with a different raw water.

Nitrate concentration was reduced from 43.3 to 16.5-18 mg-NO /l.

Example 5

Example 5 is as Example 3, with the following parameters different:

Resin dose: 0.8% w/w of the raw water; Concentration in the feed tank: 5% w/w; Total load of resin: 44 kg; Water recovery: 96%.

The present invention has been described above purely by way of example, and modifications can be made within the spirit of the invention.

Claims

Cl aims

1. A method of causing a liquid to interact with a solid, comprising:

forming a suspension of particles of the solid in the liquid and moving the solid particles and the liquid in the same direction while the liquid interacts with the solid particles;

then removing liquid from the suspension; and

recycling the solid particles.

2. The method of Claim 1, wherein liquid is removed from the suspension by crossflow filtration.

3. The method of Claim 1 or 2, wherein the solid particles are continuously recycled.

4. The method of any of the preceding Claims, wherein after removing liquid from the suspension, sufficient liquid is left for the solid particles to remain in suspension, and the solid particles are recycled.

5. The method of any of the preceding Claims, wherein a proportion of the solid particles is continuously removed either for discharge or for regeneration and recycling.

6. The method of any of the preceding Claims, wherein the solid particles are regenerated by continuously passing them around a regeneration loop.

7. The method of Claim 6, wherein there is a chamber in which suspension is maintained, suspension being drawn from said chamber for regeneration.

8. The method of Claim 6 or 7, wherein a suspension is formed of the solid in a weak regenerating agent, the suspension is passed to a filter so that the filter holds the solid back, and a stronger regenerating agent is passed through the filter in the oppos^'ite direction to remove the solid from the filter and form a suspension of the solid in the stronger regenerating agent.

9. The method of Claim 6 or 7, wherein the solid is regenerated by treatment with a regenerating agent, and is then washed by forming a suspension of the solid in a first wash liquid, passing the suspension to a filter so that the filter holds the solid back, and passing a second wash liquid through the filter in the opposite direction to remove the solid from the filter and form a suspension of the solid in the second wash liquid.

10. The method of any of the preceding Claims, wherein the particle size is about 1 mm or less.

11. The method of any of the preceding Claims, wherein the liquid is water and said solid is in the water in low concentration.

12. The method of Claim 11, wherein said solid comprises nitrate ions.

13. The method of any of the preceding Claims, wherein the solid is ion exchange resin.

14. The method of any of the preceding Claims, wherein, as the solid particles and liquid are moved and interact, the concentration of the solid in the liquid is not greater than about 10%.

15. The method of any of the preceding Claims, wherein the solid particles are recycled on average at least about twice before regeneration or discharge.

16. Plant for causing a liquid to interact with a solid comprising: a chamber comprising means for maintaining therein a suspension of solid particles in a liquid, whereby the solid and the liquid can act on one another;

means for feeding the solid and the liquid to the chamber;

means for removing liquid from the suspension; and

means for recycling the solid particles from the liquid removing means to said chamber.

17. The plant of Claim 16, wherein the means for removing liquid from the suspension is a crossflow filter.