The present invention is well suited to the classification of crystals and will be described in the context of crystallisation. However, it will be appreciated that the invention is equally suitable for the classification of any particles with time dependent size or density such as in precipitators.

BACKGROUND ART Many chemicals and food products are manufactured and purified by crystallisation. Here it is important to control the product size distribution of the crystals. This presents significant difficulties when the required product size distribution is narrow.

Theoretically, sieves should provide perfect separation given that a particle will only pass through the sieve if it is smaller than the openings. However, the particles must be given a sufficient amount of time on the sieve to ensure that a complete separation is achieved. Furthermore, if the particles readily adhere to each other they become difficult to separate using a sieve. Sieves are also prone to clogging which reduces their efficiency.

Elutriators separate particles according to their settling velocity. Liquid flowing upwards at a specific velocity will carry slower settling particles to the top while permitting faster settling particles to collect at the bottom of the vessel. Unfortunately elutriators are not time efficient, particularly when the separation size is relatively small.

Cyclones are much faster and can provide remarkably high through-puts Unfortunately, they have a relatively high energy consumption and precise size distributions are hard to achieve.

In an attempt to address these deficiencies, US 4,978,509 to Maimoni describes a Lamella Settler Crystalliser. This crystalliser uses a phenomenon first investigated in the 1920s for the separation of blood corpuscles. It has been found that by placing inclined plates or lamella in a fluid flow in which particles are suspended, the sedimentation rate of the particles is accelerated. Particles from the flow would deposit onto the inclined plates at a rate dependent on the velocity of the flow, the length of the plate, the width or spacing between the plates and the plates angle of inclination to the flow path.

Using this principle in the Maimoni crystalliser apparently provides a relatively high product output with a lower energy consumption and lower"shear" (breakage of large crystals) than hydrocyclones. Unfortunately, this type of crystalliser cannot be used to provide a precise or narrow size distribution, as fines can be entrained into the coarse product, and only one product size fraction can be produced.

It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

SUMMARY OF THE INVENTION Accordingly, in a first aspect, the present invention provides a particle classifier for segregating particles of a predetermined size or density range from a suspension of the particles wherein the size and/or density of the suspended particles varies with time, the classifier including: a chamber for receiving a fluid flow in which the particles are suspended;

one or more plates mounted within the chamber at an angle to the flow path for segregating particles of a size or density greater than a predetermined minimum from the suspension ; means to create uniform flow conditions immediately upstream of the inclined plate or plates; and, means to withdraw the segregated particles from the chamber.

Preferably, the means to create uniform flow is adapted to fluidise the suspension. In a further preferred form, an array of the inclined plates are positioned parallel to each other such that the array extends transverse to the flow path.

To fractionate several different particle size categories, two or more of the arrays of parallel inclined plates can be mounted sequentially within the flow path. To deposit progressively smaller particles at each array, the inclined plates in successive arrays are progressively longer and/or more closely spaced than the previous array.

In some embodiments, the particles are crystals growing within the fluid. In other embodiments, the particles are precipitants from the fluid. When used as a crystal classifier, the chamber is generally upright such that the fluid flow is upward from beneath the inclined plates to an overflow collector above the plates, the overflow collector in fluid communication with heat exchange means where in use, the fluid is heated to dissolve ultra-fine crystals not segregated and then recirculated to the chamber.

One particularly preferred embodiment further includes withdrawal means for withdrawing crystals immediately beneath the uppermost array of plates, the withdrawal means being operatively responsive to sensor means adapted to monitor the population of the crystals immediately beneath the uppermost array of plates wherein the rate of withdrawal is controlled such that a substantially constant crystal population within the

chamber is maintained. In this embodiment, the sensor means a pressure transducer attached to the chamber wall immediately beneath the uppermost array of plates and the withdrawn crystals may be dissolved by heating and combined with the recirculated flow.

The crystal classifier may also include means to promote evaporation of the fluid above the uppermost array of plates to enhance growth of ultra-fine crystals into crystals large enough to be segregated. Optionally an agitator may be used to enhance ultra-fine crystal growth.

In another aspect, the present invention provides a method of segregating particles of a predetermined size or density distribution from a suspension of the particles wherein the size and/or density of the suspended particles varies with time, the method including: positioning one or more plates in a flow of the suspension such that the plate or plates are inclined to the flow path in order to segregate particles from the flow; and, adapting the flow to provide uniform flow characteristics immediately upstream of the plate or plates such that the particles segregated are within the predetermined size or density distribution.

By ensuring uniform flow conditions are maintained immediately upstream of each array of plates, the size distribution of the particles deposited at each array can be closely controlled. Withdrawing the deposited crystals from the chamber so that no more crystal growth occurs can provide a crystal product with a narrow size distribution. Providing a number of separate sets of plates in the flow path allows the crystals to be progressively classified or fractionated into a number of size ranges.

BRIEF DESCRIPTION OF THE DRAWINGS A preferred embodiment will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a schematic representation of a particle classifier according to the present invention; Figure 2 is a schematic representation of the trajectory of a particle over an incline plate of the present invention; Figure 3 is a schematic representation of another form of the particle classifier according to the present invention; Figure 4 is a schematic representation of an alternative embodiment with a varying cross-section fluidization chamber; and Figure 5 is a schematic representation of an embodiment where chamber shape follows the lamellae.

BEST MODE Referring to the figures, the particle classifier has a chamber 1 through which flows a suspension of particles that vary in size or density with time. This particular embodiment will be described with reference to a crystalliser for segregating crystals growing in a super saturated solution.

Inclined plates 2 mounted within the chamber 1 at an angle to the flow segregate crystals bigger than a predetermined minimum size. As best seen in figure 2 each pair of inclined plates will capture particles coarser than a critical particle 16 which is the smallest particle that will be deposited on that particular plate. Particles finer than the critical particle will report to the next stage above the plates in the chamber 1 whereas particles larger than the critical particle will deposit onto the plate and report to the

section of the chamber below the plate. If the crystal continues to grow, then it automatically settles through to the section of the chamber having the appropriately sized plates.

Fluidiser 3 ensures that the flow condition at the openings to each array of plates is uniform which in turn ensures that the critical particle size for each plate in a particular array is constant. It will be appreciated that a variable flow profile across an array of plates decreases the control of the crystal size captured. Therefore, by maintaining the uniform flow conditions throughout the entire chamber through fluidisation of the flow, a narrow size distribution of crystal product can be withdrawn from the chamber at sections 4,5 and 6.

Referring to figures 1 and 3, each array of inclined plates is progressively longer and the distance between each plate shorter in order to separate at finer sizes. The fluid, known as the"mother liquor", is pumped up through the base of the vessel fluidising any crystals present in the lower vertical section. Finer crystals and ultra-fines, generated by collisions between larger crystals, pass up through the first array of plates 7 and into the second section 5 of the chamber 1. A further classification occurs as the crystals are fluidised through the array of plates 8. Only the very finest crystals (the ultra-fines) pass through the final array 9 and out of the chamber 1 through the over flow 10.

Referring to figure 3, the liquor from the over flow 10 may be passed through a heat exchanger 11 to dissolve any ultra-fine crystals. Although these ultra-fines are important in a continuous operation, it is preferable that they do not pass up through the chamber 1 with the mother liquor. With their high surface area, they would tend to grow at the expense of the larger crystals and they can affect the size range present between

each set of plates. Therefore, once the crystals have been redissolved into the liquor, they are returned to the fluidiser 3 at the base of the chamber 1.

The crystalliser operates in a super saturated state. Super saturation is achieved in the usual ways, either by cooling the crystalliser to a given temperature or by controlling evaporation with a vacuum 12 at the top of the chamber 1.

The crystalliser may be operated in either a"batch"or"continuous"mode. In the continuous mode fresh feed fluid may enter the system at 13. The feed fluid is generally in the form of a liquid and therefore does not disturb the crystal classification process.

The feed fluid simply supplies the material needed for the growth of the existing population of crystals.

The feed entry temperature is adjusted by a water jacket 14 provided with hot water flow at a controlled rate and/or temperature to achieve the desired feed temperature input at fluidiser 3.

The temperature of the entire chamber 1 can also be controlled by an enveloping water jacket 15.

Under continuous conditions, the crystal population in each section is driven by the population of the finest crystals just below the uppermost array 9. This population of crystals corresponds to a specific solids concentration which can be measured easily using a pressure transducer (not shown) on the vessel wall. Ideally this concentration should remain constant. To achieve this, some suspension could be withdrawn from section 4 of the chamber 1. The withdrawn crystals are redissolved and combined with the fluidisation liquor. The withdrawal rate can be controlled by the pressure sensor in response to the measured crystal concentration to maintain a substantially constant crystal population in the chamber 1.

In either mode, specific products can be obtained by withdrawing the suspension from different sections of the chamber 1. The rate at which the particles are withdrawn can be varied to produce the desired particle size distribution. A coarse product with a narrow size range can be obtained simply by withdrawing suspension from the lowest part of the vessel. Conversely, a fine grade crystal may be withdrawn from an upper portion.

It is worth noting that if the rate of withdrawal of the suspension from the lowest portion is decreased, then the concentration of crystals in that section will increase. In turn the rate of removal of these crystals will be unchanged. This behaviour is not produced in a conventional fluidised bed settler where the suspension concentration is dictated by the fluidisation rate. In contrast, this system provides a broad range of possible concentrations at any given fluidisation rate. Particles are continuously entrained out of the lower section, but return after they settle onto the inclined plates.

This results in a concentration higher than that expected for the fluidisation rate. These high concentrations result in higher rates of production.

The ultra-fine crystals in the chamber provide the initial growth sites for new crystals. These new crystals then progress down to the sections of the crystalliser appropriate for their size where they can be withdrawn as product. However, it is important that the number of ultra-fine crystals produced is controlled and not returned with the fluidisation liquor. Therefore it is desirable to enhance their growth while simultaneously limiting their number.

This may be achieved using several methods. Evaporation from the top of the chamber 1 may contribute to the growth of ultra-fine crystals and thereby the number of crystals settling down through the various sections 4,5 and 6 of the chamber 1. A stirrer

(not shown) may also be located at the top of the chamber 1 to agitate the ultra-fine crystals in order to enhance their growth at this point in the system. Conveniently, the array of plates 9 tends to prevent the agitation from influencing the flow in the lower sections of the chamber 1.

A further possibility is to use the crystalliser with a conventional system such as a stirred vessel. The stirred vessel, with its intense agitation, would be used for generating small crystals. The product would be fed continuously to the top of the chamber 1 where further growth could occur. Small crystals would settle into the appropriate stage and continue to grow in the usual way passing to even lower sections when large enough. Of course any remaining ultra-fines would emerge as usual in the overflow 10 and be redissolved by the heat exchanger 11.

Although the fluidization chamber has been described in the embodiments above as typically square in cross-section and of constant cross-section throughout its height, it is also possible to vary the shape and cross-section of the chamber in order to provide additional control. For example, in Figure 4 there is shown a fluidization chamber 17 in which three arrays of lamellae 18,19 and 20 are positioned in areas of the fluidization chamber 21 having differing cross-sections. By controlling the cross-section in this manner, the fluidization rate through each set of lamellae may be individually controlled even though there is a common fluidization rate supplied at 22 at the bottom of the chamber.

Figure 5 illustrates a further embodiment in which the fluidization chamber 22 incorporating arrays of lamellae 23 and 24 has angled side walls in the regions corresponding with the lamellae. For example the side walls in region 25 are angled to conform with the angle of inclination of the plates in lamellae 23, and similarly the side

walls in region 26 are angled to correspond with the angle of inclination in lamellae 24.

It is preferred that the zones 27 between lamellae remain with substantially vertical side walls. This configuration is particularly advantageous in reducing or eliminating any "dead"areas at either end of the lamellae between the inclined plates and the chamber walls.

The invention has been described herein by way of example only. Ordinary workers in this field would readily recognise many variations and modifications which do not depart from the spirit and scope of the broad inventive concept.