IE862024L - Granulating devide and plant. - Google Patents

Abstract

1. Granulation apparatus (1) having a granulation stage (B) comprising at least part of the length of a drum (2, 101) mounted so as to rotate about a roughly horizontal axis, means for admitting (7, 103, 102) a particulate substrate for producing granules issuing into the interior of the drum and in the vicinity of one end thereof, means (20, 107) for entraining the particulate substrate and granules over part of the travel of the drum and means (17b, 102) for extracting granules to the exterior of the drum issuing in the vicinity of the opposite end to that at which said admission means issue, characterized in that is at least one fluidized bed device (3, 3a, 3b, 108) located within the drum, in such a way that at least part of the substrate and granules entrained by the drum drops onto the or each fluidized bed device and then drops again into the drum, means being provided for discharging to the outside of the drum the fluid issuing from said or each fluidized bed device. [EP0216653A2]

Description

2 The present invention relates to granulating devices for the production of granules especially from a Sprayable product based on a substance which is, for example, crystalIizable, and soluble or insoluble in an aqueous medium, this substance being molten, mixed or dissolved.

In known technology, there are at present two entirely different basic processes for manufacturing granules from a molten substance, these processes being employed, in particular, in the field of fertilizers and of sulphur.

In a first granulation process known as prilling, the molten substance is sprayed at the top of a tower and the droplets are cooled as they fall in the tower. T.his process makes it possible to obtain granules of excellent quality but their particle size range is limited to between 2 and 3 mm. These sizes do not always meet the present requirements of the users who wish to have access to granules which are larger in size. In addition, a prilling tower constitues a costly unit, the installation of which can present problems.

The second process makes use of a rotary drum of a mixer or a pan in which molten salt is sprayed onto small granules which, as a result of coating and cooling, yield granules which are larger in size. These small granules are, in fact, produced partly by the granulator itself after the granules which it produces have been screened, and the relationship of the quantity of granules which are reintroduced into the drum to the quantity * of granules which leave is called the recycle ratio, these quantities being expressed in weight.

In this granulation process, as in all the other known processes, cooling means have to be provided to remove the heat of crystallization of the molten salt.

A means of removing this heat and permitting the cooling and consequently the hardening of the granules consists in greatly increasing the recycle ratio in order to increase thereby the quantity of cold material entering the drum. Under these conditions, the recycle ratio can attain values from 2:1 to 5:1, which is a disadvantage in a production unit insofar as it results in a considerable oversizing of the plants on the one hand and, on the other hand, in the consumption of large quantities of energy for processing the material which circulates in a closed loop.

It therefore becomes necessary to absorb excess heat of granulation by using a cold source external to the granulating device.

Thus, US Patent A-4,213,924 describes a granulating device with a rotary drum in which a molten salt is sprayed onto a curtain of granules falling inside the drum and in which cooling of the granules is produced by injecting water into the drum, this water being vaporized as it absorbs the heat which is to be removed.

However, this process is difficult to operate insofar as the cooling by water evaporation demands strict and continuous monitoring of the operating conditions. These conditions, which are determined essentially by the temperature and the relative humidity of air, must be such that the molten salt is not capable of hydration. Such hydration conditions can vary very widely depending on the nature of the salt employed, with the result that the working conditions of the rotary drum are strictly determined by the nature of the salt, which consequently allows no flexibility in use. In addition, the water entering the drum has to be expelled by means of a stream of air, and this causes problems during the removal of dust from this moist air. Furthermore, this granulating device requires spraying in the form of very fine droplets and consequently the use of nozzles with very fine orifices, with a particularly high spraying pressure, of the order of 50 bars, to produce very fine mists. Pressure which is as high as this is very difficult to apply to molten nitrates which could be liable to detonate under these conditions.

To overcome these disadvantages, the invention aims at providing a granulating device of the rotary drum type, which permits efficient cooling of the granules 4 without the introduction of water and with a recycle ratio which is as low as possible, while providing granules of a h igh quali ty.

To this end, the subject of the invention is a 5 granulating device containing a granulation stage which comprises at least a part of the length of a drum mounted to move in rotation around a substantially horizontal axis, means for allowing a particulate substrate intended to form granules to enter, opening into the interior of the drum and in the vicinity of one end of the latter, 10 means for entraining the particulate substrate and the granules over a part of the travel of the drum, and means for discharging the granules outside the drum, opening into the vicinity of the end away from the end into which the According to other characteristics: Spraying means are provided inside the drum for spraying a product intended to coat the substrate 25 particles and the granules being formed.

The, or each, fIuidized-bed device is fixed.

The, or each, fluidized-bed device can be oriented around an axis which is substantially parallel to the 30 lengthwise axis of the drum.

The granulating device comprises a fluidized-bed device sloping upwards in the direction/opposite to the rotation direction of the drum.

The fluidized-bed device is arranged so that the 35 granules fall in the vicinity of the top end of this fluidized-bed device.

A part of the inner periphery of the drum, close to the end of the latter into which the granule discharging : means open, is generally smooth, to form a stage for shaping and smoothing the granules.

The invention will be understood better from the reading of the following description of various embodiments, given solely by way of example and made with reference to the attached drawings, in which: Fig. 1 is a perspective view, with cutaway, of an embodiment of the granulating device of the invention; Fig. 2 is a view in cross-section, on a larger scale, along the line 2-2 of Fig. 1; Fig. 3 is a view similar to that of Fig. 2, illustrating an alternative embodiment of the device of Fig. 1; Fig. 4 is a flow sheet o.f a granulating plant using the granulating device of the invention; and Figs 5 & 6 are diagrammatic views of three further embodiments of the device of the invention.

The granulating plant illustrated diagrammatica 11y in Fig. 4 is intended to produce granules, from any sprayable product based on a crystal Iizable substance, su-fh as, for example, nitrates, especially ammonium nitrate, sulphur, potassium hydroxide, sodium hydroxide or urea, or mixtures of such or other substances with water, particularly an aqueous slurry of ammonium phosphate or a mixture produced by nitric digestion of phosphates.

This plant comprises a granulating device 1 of the type having a rotary drum 2 arranged substantially horizontally, inside which there is a fluidized-bed device 3, connected to an air blower 4 outside the drum, this fluidized-bed device being fixed and thus not being integral in rotation with the drum. A bar for spraying 5, hot, a product to be granulated extends inside the drum and into the lower part of the latter under the fluidized-bed device 3, this bar being connected, outside the drum, to a homogenizing tank 6, intended to feed the product to be sprayed to this bar 5.

In addition, the granulating device is connected upstream to a spout 7 for feeding small granules of a diameter, for example, of between 800 microns and 3 mm, these granules being intended to serve as a substrate for 6 the formation of larger granules by coating with the cry-stallizable substance sprayed with the product. This spout 7 opens into the interior of the drum in the vicinity of a first end and, at the opposite end, granule outlet means are provided, these means cooperating with a 5 discharge conveyor 8 which carries the granules to the downstream section of the plant.

The conveyor 8 is adapted to discharge the granules onto a first screening device 9 which separates the 10 granules of a marke-table size, for example from 3 to 5 mm, from other granules of a larger or smaller size. Granules of marketable size are conveyed to a fluidized-bed device 10 intended to complete the cooling of the granules before the latter are coated with a layer of a 15 substance such as amines, carbonates, talc and the like, in a coater 11, this coating layer being intended to pre" vent agglomeration of the granules when they are stored.

The granules rejected as oversize at the first screening device 9 are ground (in 12) and: 20 a) are poured into the homogenizing tank 6, where they are converted, by melting, into a sprayable product. This tank is additionally fed with fresh product to be sprayed, via a channel 12a; b) are recycled directly to the drum 2. 25 The granules rejected as undersize at the first screening device 9 are reprocessed at a second screening device 13 where the undersize granules, of a size below 800 microns to 1 mm, for example, are poured into the homogenizing tank 6 to be converted into a sprayable pro-30 duct therein. The granules which do not pass through the second screening device 13, that is to say of a size between approximately 800 microns and 3 mm, are conveyed to the drum 2 by means of a channel 7.

A hopper 14 is also provided for introducing fil-35 lers into the homogenizing tank 6 by means of a belt conveyor 15 and a metering device. The purpose of these fillers may be, for example, to reduce the percentage of nitrogen in the granules in the case where the salt processed is a nitrate.

At the end of the drum 2 away from that at which the fLuidized-bed device 3 is fed with air, the air Leaving this device is discharged into a washing tower 16 in which a dilute solution of the crystalIizable substance employed in this device is circulated. This solution is subsequently reconcentrated in a unit provided for this purpose before be.ing poured into a homogen izing tank or is recycled directly to the latter.

The granulating device 1 will now be described in greater detail with reference to Figs. 1 and 2. At each of its ends, the drum 2, arranged substantially horizontally, comprises an annular rim 17a, 17b, directed radially inwards to partly close each of its faces. It is supported on its outer periphery by conventional means, which are not shown, and is driven in the direction of arrow R, for example by a crown gear 18 meshing with a pinion 19 connected to a motor, which is not shown. The motor is preferably associated with a speed controller enabling the speed of rotation of the drum to be regulated at an optimum value which is between 35 and approximately 45% of the critical speed, that is to say the speed above which the centrifugal force causes the granules to adhere to the drum wall.

To entrain the granules and to lift them over a part of the travel of the drum 2, the latter is fitted with suitably oriented, lengthwise lifters 20.

The fine granule feed spout 7 opens into the interior of the drum 2 in the vicinity of a first end of the latter, passing through the opening formed by the corresponding annular rim 17a.

At the opposite end, a hopper 21 is provided under the drum to receive the granules which overflow from the latter above the corresponding annular rim 17b, this hopper 21 subsequently discharging the granules onto the conveyor 8.

A duct 22 for feeding air into the fIuidized-bed device 3, connected to the blower 4, and a feed duct 23 connecting the tank 6 to the spraying bar 5 which is fitted with nozzles 24, pass through the end opening of the 8 drum in the vicinity of the hopper 21. The duct 23 is preferably of the jacketed type to enable the product to be kept up to temperature by steam circulating under pressure.

The fIuidized-bed device 3 extends lengthwise in side the drum and substantially in the centre of the latter so that its grid 2'5 is preferably sloping slightly upwards in the direction of rotation R of the drum. The slope angle can be varied and may be, for example, of the 10 order of 5° to the horizontal, so that the granules lifted by the lifters 20 fall back in the"vicinity of the top end of the fluidization grid 25, and then, after staying above the grid 25 for a short time, fall back into the drum. To enable the slope of this fluidized-bed 15 device to be regulated, the latter is advantageously mounted capable of oscillating about an axis which is substantially parallel to the lengthwise axis of the drum.

In the case where the device described above operates with a molten salt, especially a nitrate, the 20 granules 20 introduced via the spout 7 into the rotating drum are subjected to a number of cycles of coating and cooling with air to a temperature close to ambient temperature, before leaving the drum, this cycle being capable of being analysed as follows.

The nozzles 24 apply a coating film of molten salt to the granules falling as a curtain from the device 3. These granules are then entrained by the lifters 20 over the grid 25 onto which they fall to be cooled efficiently, while the salt film deposited by spraying crystallizes, before falling back to the bottom of the 30 drum and restarting a new cycle of granule growth due to coating and crystallization. The granules gradually move in the drum in the direction of the outlet end from which they overflow above the annular rim 17a and fall into the hopper 21.

By virtue of this device, the particle size range of the granules produced may be changed at will by modifying one or more of the following parameters: The slope of the fluidization grid of the fluidized-bed device, to modify the residence time of the 9 granules on this grid; The flow rate of the cooling air through the fluidized-bed grid, to obtain accurate control of the granule bed temperature; The recycle ratio and the rate of spraying of the crystal Iizable substance, to modify the available quantity of substance per granule, that is to say the thickness of each successive coating layer; and The slope of the lifters 20 and the speed of 10 rotation of the drum, to modify the granule mixing regime and the frequency with which the granules are coated and cooled.

In the case where a slurry is used, for example an aqueous slurry of ammonium phosphate which is sprayed 15 hot, the operation of the granulating device is similar to that described above, apart from the fact that the air injected through the fIuidized-bed device is preheated to a temperature such that it dries the granules by causing the crystallization of the ammonium phosphate sprayed 20 onto these granules and that it vaporizes virtually all the water present in the slurry which is sprayed.

An example of operating conditions for the operation of the granulating device using ammonium nitrate is given below.

Concentration of the crysta 11izable substance in the product sprayed (a) 97 to 99.5% Temperature of the sprayed product 170 to 190°C Recycle ratio (b) 0.8:1 to 1:1 Recycle temperature 60 to 70°C Granulator outlet temperature 95 to 105°C Speed of rotation of the granulator 35 to 45% of Vc* Cooling air flow 1 to 3 l/kg of sprayed product *Vc = critical speed of rotation of the drum (a) = the impurity being chiefly water 35 (b) = recycled granule fraction relative to the leaving product fraction.

An arrangement of this kind makes it possible to obtain granules of very high quality by virtue of the homogeneous and efficient cooling thereof, which is 1 0 carried out Layer by Layer as they are being coated with the sprayed substance. In each growth cycle of the granule, the quantity of material to be cooled on the Latter is restricted to a thin film which is cooled very effi-5 ciently by the stream of air leaving the fluidized-bed device.

Furthermore, .the use of a fluidized-bed device allows the sprayed substance to be dried efficiently, thus making the presence of a dryer downstream of the 10 granulating device no longer necessary. This major advantage in the case of a molten salt which usually contains from 1 to 2Z of water, is found to be remarkable when use is made of a slurry in which the water is virtually all vaporized by the hot air leaving the fluidized-15 bod device.

Granulation efficiency no longer requires, in contrast to the prior art, spraying in the form of very fine droplets and, consequently, product pressure upstream of the nozzles is of the order of a few bars. 20 This results in a saving in the plant and running costs.

Furthermore, owing to the efficiency of the cooling means chosen, the throughput of air supplied to the « fluidized-bed device is relatively Low compared to the traditional methods, and this enables the granulating de-25 vice to be reduced in overall size. The efficient cooling also makes it possible to use air at ambient temperature when a molten crystalIizable substance is used. An advantage of this kind avoids the need for special conditioning of air for the cooling thereof, making the pro-30 cess more economical.

The low value of the recycle ratio enables the operating costs of the device of the invention to be decreased by reducing to the minimum the energy employed for processing the material circulating in a closed cir-35 cuit in the plant. In addition, the granulating device provides a high degree of safety and a great operating flexibility, since it employs neither water nor steam and permits the particle size characteristics to be chosen by modifying one or more of the abovementioned parameters. il The granules obtained are very tough, particularly because of their layered structure, and are substantially spherical in shape and have a substantial ly smooth surface. These granules can therefore be stored in sacks at temperatures of the order of AS to 50°C without requiring the addition of hardening or coating adjuvants, and this allows normal air to be used for cooling them, even in hot climates.

According to yet another advantage, the granules have very low porosity and, in this respect, meet the safety recommendations in the case of nitrates.

Many other alternative forms may.be envisaged, of course, without departing from the scope of the invention Thus, according to a first alternative embodiment illustrated in Fig. 3, the granulating device is equipped with two fluidized-bed devices 3 arranged one above the other. The upper device 3a slopes downwards in the direction of rotation R of the drum and the lower device 3b slopes at substantially the same angle in the opposite direction. Moreover, in order to ensure that the granules which fall on the upper fluidized-bed device may have a sufficiently long residence time, account being taken of the slope of this device, the latter is preferably equipped with a baffle 26. This baffle is arranged above the corresponding fluidization grid and in the vicinity of the upper end of the Latter, extending substantially parallel to the axis of the drum and slightly sloping relative to the vertical in the direction of rotation R of this drum, so that the granules lifted by the lifters 20 fall onto this baffle 26, on which they will slide, to fall back in the vicinity of the upper end of the grid of the nearby fluidized-bed device 3a, thqs ensuring that these granules spend a sufficiently long residence time in the upper fluidized bed. The lower fluidized-bed device is arranged so that the granules falling from the upper fluidized-bed device fall in the vicinity of its high end thus ensuring that the granules spend a maximum residence time on the corresponding fluidization grid before they fall as a curtain to the bottom of the drum to be coated 12 therein with the sprayed product, as described earlier. When the use of more than two fluidized-bed devices is envisaged, these will be arranged so that two neighbouring fluidized-bed devices will slope in mutually opposite 5 directions, so that the granules cascade successively over the various devices provided.

Means may be provided for preventing accumulation of the product on the inner wall of the drum.

The drum may also include, on its inner periphery 10 in the vicinity of its outlet end, a substantially smooth annular .portion intended to enable the granules to roll over each other and over this substantialIy smooth part of the wall, in order to produce granules of a substantially spherical and uniform shape.

Several spraying bars may be provided in the drum and the means for lifting the granules in the drum can assume any suitable shape other then fins.

According to yet another alternative form, the granulating device of the invention may be advantageous-20 ly combined with a conventional granulation tower in which particles are formed by spraying a molten salt at the top of the tower and by the falling drops solidifying into particles by being cooled in an upward air stream injected at the bottom of the tower. 25 In point of fact, in granulation towers, the mean diameter of the particles obtained does not exceed 2.5 mm whatever the nature of the product, and it would be desirable to produce a substantially greater granulation.

Thus, by introducing the particles originating from the tower into the granulating drum of the invention, by feeding the fluidized bed with atmospheric air and by spraying a solution of an appropriate molten salt onto these particles to make them grow, granules with a mean 35 diameter of between 3 and 5 mm are obtained at the drum outlet.

It should be pointed out that this result is obtained nevertheless without raising the usual recycle ratio of the granulation tower, which is of the order 13 of 5 to 10%.

As a result, such an arrangement makes it possible to increase the production capacity of granulation towers, the granulating drum being arranged between the 5 bottom of such towers and the screening plant which is conventionally provided to sift the freshly-formed granules in order to send the reject fraction towards a melting device connected to the spraying means arranged at the top of the tower.

Reference will now be made to the embodiments of the invention which are illustrated in Figs. 5 and 6.

It is known that particulate products produced by compacting have the disadvantage that the particles obtained in this manner have sharp and brittle ridges on 15 their surfaces, which give rise to dust when they are handled.

This disadvantage also applies to particles produced by compacting finer particles, insofar as such particles obtained by compacting tend to crumble and 20 thus to form dust.

Another aim of the present invention is therefore to overcome theSe disadvantages by providing a device which is capable of converting particles with an irregular and brittle surface into granules which have 25 a substantially smooth surface and which will therefore produce only a little dust due to friction when they are handled.

To this end, the invention has the following additional characteristics: the device comprises two stages A and 8, and means are provided for introducing particles into a first stage, or particle smoothing stage, this first stage including means for moistening the particles and means for setting these particles into relative motion 35 with respect to one another, and means for allowing entry ensuring the passage of the moist particles from the first stage towards the second stage B, this second stage consisting of the granulation stage; the first stage comprises at least a part of the length of a rotary drum, with a substantially horizontal axis, into the interior of which part the means for introducing the particles open in the vicinity of one of its ends, the means for allowing moist particles to enter opening in the vicinity of the 5 end away from this part of the drum, whose inner surface is substantially smooth, and means for spraying a liquid onto the particles being also provided inside the said part of the drum; both stages of the device may be arranged in 10 one and the same drum or in two separate drums.

In the definition given above, and in the remainder of the description, particles are understood to mean the particulate crude substrate originating directly from crushing or compacting, and granules are intended 15 to mean the particles obtained after smoothing and drying of the surface of the smoothed and moist particles entering the second stage.

The device iIlustrated in Fig. 5 comprises a drum 101 with a substantially'horizontal axis and 20 mo.unted so as to rotate around this axis. At each of its ends, this drum comprises an annular shell 102, each defining a rim directed radially towards the inside of the drum around a corresponding central opening.

This drum 101 is divided into two p«arts along 25 its length by an annular shell 103 defining an annular rim directed radially inwards and with a central opening.

A first part of the drum extending between an end shell 102 and the intermediate shell 103, defines a 30 first stage A of the device of the invention. The second part of this drum, included between the other end shell 102 and the intermediate shell 103, defines the second stageb of the device.

A spout 104 for introducing nonuniformly-surfaced 35 particles opens into the interior of drum in the vicinity of its end included in the first stage, and a nozzle 105 for spraying a liquid, such as water or an aqueous slurry, is provided inside the first part of the drum 101 . This nozzle 105 is directed downwards to spray 1 5 a mass of particles P present in the bottom of the drum and is connected to means of supplying a spraying liquid by means of piping 106. The inner cylindrical wall of this first part of the drum 101 of the first stage A is 5 substantially smooth so that, as the drum rotates, the particles of the mass P are set in motion relative to each other, as will be made clear in the following text.

The second part of the drum 101 (second stage B) is similar to the granulating drum described above and 10 comprises over its entire inner cylindrical wall lenghwise lifters 107, distributed circumferentially over this cylindrical wall, these lifters projecting radially towards the inside of the drum.

The second stage B also comprises a fixed flui-15 dized-bed device 108 connected to a fan (not shown) outside the drum by means of a duct 109. The grid of the fluidized-bed device 108 extends substaxtially over the entire length of the second stage B directed upwards while also sloping upwards in the direction of rotation of the drum so that 20 the most particles originating from the first stage A and lifted by the lifters 107 fall back onto this fluidized-bed device 108 where they are dried and/or cooled before falling into the drum.

Optionally, a bar 110 for spraying a liquid con-25 taining a substance capable of coating the particles, for example a molten salt, a slurry or a suspension, may be provided under the fluidized-bed device 108 so that the nozzles 111 of this bar point in a suitable direction to apply a spray to the curtain of particles falling from the 30 fluidized-bed device 108. This spraying bar 110 is connected to feeding means provided outside the drum, by means of a dust 112.

The device according to this first embodiment operates as follows : monuniformly-surfaced particles pro-35 duced by crushing or compacting are introduced into the first stage A of the drum 101 through the spout 104 ; 16 the particles introduced in this manner are set in motion relative to each other by the rotation of the drum 101 and are sprayed with a liquid, preferably water or an aqueous product, by means of the nozzle 105 to 5 mo isten these particles at least at the surface. In the course of their relative motions, the particles come into con tact with each other so that their asperities, softened by moistening beforehand, are reduced, dust formation being prevented by the moistening of the 10 environment.

By overflowing above the rim provided by the shell 103, the smooth-surfaced moist particles are trans,-ferred into the second stage B, where they are picked up by the rib lifters 107 which lift these particles and 15 let them fall onto the fluidized-bed device 108, where they are dried and/or cooled before falling as a curtain to the bottom of the drum while being coated, if appropriate, as they fall by the substance sprayed via the nozzles 111. The particles repeatedly pass a number 20 of times through this cycle of being picked up by the rib lifters 107 and cooling and/or drying on the fluidized-bed device 108, before leaving the drum 101 in the form of uniformly-sur-f aced granules by overflowing over the rim provided by the shell 102 attached to the outlet 25 end of the drum.

The liquid sprayed by the nozzle 105 in the first stage A may be merely water to moisten, at least at the surface, the mass P of particles whose surface is being smoothed by the mutual contact of these particles 30 with each other, or an aqueous slurry in which the solid phase is in nature identical with or different from that of the particles, in order to moisten them at least at the surface while coating them with the solid phase to take part in their smoothing by covering the remaining asper-35 ities and bridging the hollows between these asperities.

The coating material sprayed, where appropriate, via the bar 110, may be in nature identical with or different from that of the original particles and may be produced in the form of a slurry or a molten salt. In 1 7 the case where this coating substance in the second stage B is in the form of a molten salt, the air blown through the fluidized-bed device 108 serves not only to dry the moist particles leaving the first stage A but also to 5 cool the coating of the molten salt sprayed by the nozzles 111 of the bar 110.

In the embodiment illustrated in fig. 6, the device comprises two separate drums 10la and 101b, equipped at each of their ends with a shell 102 similar to 10 that described above.

The drum 101a is included in the first stage A and the drum 101b is included in the second stage B. The drum 10lb is arranged at a slightly lower lever than the drum 101a, and a spout 116 is arranged at the outlet 15 of this drum 101a and its lower end opens into the interior of the drum 10lb in the vicinity of the inlet end of the latter.

With this arrangement, the particles enter the first stage 1 via the spout 104. The smooth moist parti-20 cles obtained at the outlet of the first stage A overflow into the hopper 116, from which they enter the drum 10lb of the second stage B via the spout 117.

In this embodiment, agitation is produced by means of the rib lifters 107 and the drying by means of the 25 fluidized-bed device 108, it being possible, of course, for the latter to be replaced by any other known means of drying.

The means of transport, in this case taking the form of a hopper and a spout, are indicated by way of 30 example, insofar as the relative arrangement of the drums 101a and 101b may be other than that illustrated in Fig. 6, and, in this case, these means of transport may take any form known in the state of the art, for example a belt or bucket conveyor.

This process may be used to treat any kind of appropriate particulate materials, and especially fertilizers, that is to say a material based on a nitrogen - 18 containing salt and/or a phosphate-containing salt and/or potash.

When a slurry is sprayed in the first stage A, it is obviously advantageous to concentrate this slurry S as much as possible to have the least possible quantity of water to remove in the second stage 8.

In the case of particles of potash slag, that is to say a mixture of slags originating from the Thomas steel plants and potash, containing OX of nitrogen, 10 10 to 14% of phosphorus expressed as P2O5 and 14 to 15% of potassium, expressed as the slurry concentration is approximately 40 to 50%.

In the case of a fertilizer of the superphosphate (NP) type, containing 14% of nitrogen, expressed as N2O/ 36% of phosphorus expressed as P2O5 and 0% of 15 potassium, a slurry concentration of approximately 65% has been attained.

The mass flow rate of the slurry may represent from 10 to approximately 15% of the quantity of partic-20 les entering the first stage.

Without coating by means of spraying in the second stage B, the increase in the mass of the particles between the inlet and the outlet of the device of the invention is quite significant, it being possible for 25 this increase to be from 5 to approximately 10% of the initial mass of particles.

To illustrate the effect of the process of the invention on the change in the particle size and on the abrasion resistance of the granules obtained from these 30 particles, an example is given below of the particle size distribution, together with an indication of the abrasion resistance of the nonuniformly-surfaced particles, called entering particles, and of the granules obtained according to the process of the invention from 35 these entering particles, called leaving particles.

In this example, the particles consist of potash slag containing 0% of nitrogen, 14% of phosphorus expressed as P2O5 and 14Z of potassium expressed as K2O, the entering particles being sprayed in the first stage i3 A with a slurry in which the solid phase is of the sane nature as the particles themselves.

Entering particles: particle size distribution: 29.3% > 4.00 mm 4.00 mm > 22.2% > 3.00 mm 3.00 mm > 26.7% > 2.00 mm 2.00 mm > 16.8% > 1.00 mm 1.00 mm > 2.3% > 500 1j 2.7% < 500 u Abrasion test: 90.8% Leaving granules: particle size distribution: 45.5% > 4.00 mm 4.00 mm > 29.1% > 3.00 mm 3.00 mm > 24.9% > 2.0 mm 15 0.5% > 1.00 mm 0 % < 1.00 mm Abrasion test: 98.1% The abrasion test consists in measuring the resistance of the particles or of the granules after they 20 have passed through a slowly rotating ball mill, this mill comprising a 5-litre pot, in which are arranged 2 kg of hard porcelain balls, 20 mm in diameter.

A kilogram of particles, or granules, screened between 2 and 4 mm beforehand, is introduced into the 25 pot. After 15 minutes' rotation at 40 rev/min, the balls are separated off and the contents of the pot are passed over a coarse-mesh screen. The screened product is rescreened on a 2-mm screen and the oversize fraction on this screen is weighed to within 1 gram. 30 The result of the abrasion test is expressed as a percentage as follows: let P be the weight of the oversize fraction on the 2-mm screen; the percentage abrasion is then equal to P x 100 35 1000 The higher the percentage result of the abrasion test, the greater the mechanical strength of the granules or of the particles, that is to say the less likely they are to form dust.

It can be seen, therefore, Looking at the numerical values given above, that the granules obtained by means of the process of the invention from the nonuniformly-surfaced particles have a clearly lower propensity to form dust than the particles from which these granules have been formed.

Claims (16)

1. Granulating device containing a granulation stage which comprises at least a part of the length of a drum mounted to move in rotation around a substantially horizontal axis, means for allowing a particulate substrate intended to form granules to enter, opening into the interior of the drum in the vicinity of one end of the latter, means for entraining the particulate substrate and the granules over a part of the travel of the drum, and means for discharging the granules outside the drum, opening into the vicinity of the end away from the end into which the said means of allowing entry open, there being provided at least one fluidized-bed device arranged inside the drum so that at least some of the substrate and of the granules entrained by the drum fall onto this fluidized bed and then fall back into the drum, and means being provided for removing from the drum the fluid leaving the, or each, fluidized-bed device.

2. Device according to Claim 1, wherein spraying means are provided inside the drum for spraying a product intended to coat the substrate particles and the granules being formed.

3. Device according to Claim 1 or 2, wherein the, or each, fluidized-bed device is fixed.

4. Device according tu 1,2 or 3, wherein the, or each, fluidized-bed device can be oriented around an axis which is substantially parallel to the lengthwise axis of the drum.

5. Oevice according to any one of the preceding claims, which comprises a fluidized-bed device sloping upwards in the direction opposite to the rotation direction of the drum.

6. Device according to Claim 5, wherein the fluidized-bed device slopes so that the substrate particles and the granules fall in the vicinity of its top end.

7. Device according to any one of Claims 1 to 4, 22 which comprises at least two flui dized-bed devices spaced vertically and offset sideways, two adjacent devices sloping in opposite directions so that the granules and the substrate partic-5 i les are caused to fall successively in cascade fashion from one fluidized-bed device to another.

8. Device according to Claim 7, wherein a baffle arranged above the upper fluidized- bed device is provided to cause the granules and the 10 substrate particles to fall in the vicinity of the top endof this fluidized-beddevice.

9. Device according to any one of the preceding claims, wherein an annular section of the inner periphery of the drum, close to the end of the 15 drum into which the granule outlet means open is substantially smooth, so that the granules may roll over each other and over the corresponding section of the drum wall to produce granules having a substantially smooth surface and a substantially spherical shape. 20

10. Device according to any one of the preceding claims, wherein the spraying means are situated in the lower part of the drum under the, or each, fluidized-bed device.

11. Device according to any one of the preceding 25 claims, wherein the, or each, fluidized- bed device is supplied with hot air.

12. Device according to any one of the preceding claims, which comprises two stages and wherein means are provided for introducing 30 particles into a first stage or particlesmoo-thing stage, this first stage including means for moistening the particles and means for setting these particles into relative motion with respect to one another, and means for allowing entry ensuring 35 the passage of the moist particles from the first stage towards the second stage, this second stage consisting of the granulation stage.

13. Device according to Claim 12, wherein the first stage comprises at least a part 23 of the Length of a rotary drum , with a substantially horizontal axis, into the interior of which part the means for introducing the particles open in the vicinity of one of its end means for allowing moist particles to enter opening in the vicinity of the end away from this part of the drum, whose inner surface is substantially smooth, and means for spraying a liquid onto the particles being also provided inside the said part of the drum.

14. Device according to Claim 13, wherein the parts of the length of the drum of the first and second stages are defined by one and the same drum the means for allowing the moist particles to enter comprising a shell arranged transversely inside the drum in an intermediate location in respect of the length of the latter and thus defining two parts in the drum, which are associated, in the case of one with the first stage and, in the case of the other, with the second stage, the granules travelling from one end to the other by overflowing in the region of the central opening of the shell.

15. Device according to Claim 13, wherein each of the first and second stages comprises a separate drum, between which the means for allowing entry of the granules are arranged.

16. A granulating device as claimed in any preceding claim, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings F. R. KELLY & CO., AGENTS FOR THE APPLICANTS