AN APPARATUS AND A PROCESS FOR CONTINUOUS PARTICLE COATING

Abstract: A process for continuously producing coated particles and in particular, particles in which the coating is uniform, continuous and of minimal thickness is described, together with an apparatus for conducting the coating process in which a plurality of coating cells connected in series are provided wherein the particles are pneumatically conveyed during the coating application and in which controlled recirculation of particles occurs within each cell and a controlled spray is applied to the particles in order to produce a substantially uniform distribution of coating.

Description: AN APPARATUS AND A PROCESS FOR CONTINUOUS PARTICLE COATING Field of the Invention This invention relates to the coating of parti- culate materials. More particularly, the invention con¬ cerns a process and related apparatus for coating parti- culate materials as, for example, a particulate water soluble fertilizer or pharmaceutical material giving it a prolonged and controlled release rate. Background of the Invention Coatings are commonly applied to particulate mate- rials as, for example, may be required to control the rate of release of an active ingredient in the material over an extended period. One such application for such coatings is slow-release fertilizers in which the release rate of the fertilizer is controlled in order to extend the period of time over which the active ingredients, i.e. the nutrients, are delivered. Some commercial fertilizers, of which urea is a ty¬ pical example, are water soluble and dissolve rapidly when in contact with water. Consequently, when fertili- zers of this type are employed for agricultural or horti¬ cultural purposes most of the nutrients are rapidly dis¬ solved when the fertilizers are placed in contact with moisture present in the soil. It is well known that the rate of release of nutrients from the affected fertili- zers can be extended, and even controlled, by enveloping the fertilizer particles in a coating suitable for the purpose. These fertilizers are referred to as "slow re¬ lease fertilizers" or "controlled release fertilizers" and are used extensively on lawns, gardens and on horti- cultural and agricultural crops. Coating of particulate materials is also useful in applications other than fer¬ tilizers, such as for example in the pharmaceutical indu- RECTIFIED SHEET stry for obtaining slow release of orally administered medicaments. In the present specification, although reference is made throughout to the coating of fertilizers, it should be understood that the same technique can be used for the coating of other active ingredients. When particles of fertilizer are coated for the above-stated purpose, it is desirable in order to provide a consistent controlled release of nutrients, that the thickness of the coating be substantially uniform. Also, because the coating material dilutes the fertilizer thereby decreasing the amount of plant nutrients per unit weight of coated product, it is necessary to keep the layer or layers of coating materials applied to the par- tides as thin as possible. The requirements for the pro¬ duction of the coated fertilizer product not only in¬ crease the production cost of the concerned fertilizer product but also add to the costs involved with transpor¬ tation, storage and application of the fertilizer mate- rial. Attempts have been made to improve the cost effect¬ iveness of coating particulate materials. U.S. Patent No. 3,241,520 granted March 22, 1966 to D.E. Wurster, et al. , for example, describes an apparatus in which a plurality of layers of the same or different coating materials are applied to particles during sequential flow through a plurality of coating and subsidence zones occupying a series of cells. According to the teaching of this pa¬ tent, the particles to be coated are entrained in an air stream conducted through a diffused spray of the dis¬ solved coating material in the coating zone of a cell. After coating, the particles are deposited in the subsi¬ dence zone of the cell to avait continued movement into the succeeding cell in the series. The process described in this patent suffers the disadvantage that there can be no particle recirculation through the spraying zones in the respective cells. Although the patent describes em- RECTIFIED SHEET bodiments of the invention that contemplate particles re- circulation such teachings are limited to batch-type ope¬ ration of the unit. An alternative manner of multi-cell production of coated particles is described in U.S. Patent No 5,211,985 granted May 18, 1993 to A.R. Shirley, Jr. et al. and as¬ signed to ICI Canada, Inc. This patent discloses an appa¬ ratus and process for continuously producing polymer- coated particles in a plurality of series-connected fluidized beds, in each of which the particles are con¬ ducted essentially randomly through a coating material spray. The production system disclosed in this patent is deficient in that it does not permit a controlled appli¬ cation of coating material to the substrate particles. Accordingly, an accurate control of the release rate of the particulate material cannot be obtained. Consequently, while both of these patents illu¬ strate methods and apparatus for coating particles, nei¬ ther of the patents teaches or suggests methods and appa- ratus suitable for the cost effective manufacture of a coated product in which the coating is essentially uni¬ form in thickness, thereby leading to a coated product with a controlled release of the enclosed ingredients. It is to the amelioration of this problem, therefore, to which the present invention is directed. Summary of the Invention Accordingly, the present invention provides an im¬ proved method for obtaining a uniformly coated product having substantially complete surface coverage by the ap¬ plication of several thin layers of coating materials, rather than a single thick coating. A first aspect of the present invention relates to an apparatus for applying coatings to discrete particles, comprising means for defining a substantially closed chamber; a plurality of partitions disposed on longitudi¬ nal spacing throughout the chamber for dividing the cham- RECTIFIED SHEET ber into a plurality of cells; means forming a substan¬ tially vertically extending conduit having open upper and lower ends spaced vertically from adjacent upper and lower surfaces of the chamber; gas distribution means for supplying operating gas to the bottom of each cell and for directing it in vertical streams to the top thereof, the gas distribution means including devices for direct¬ ing a first gas flow through the interior of the conduit to develop a high velocity gas flow therein and a second flow of gas externally of the conduit at a velocity less than that of the first gas flow; means for supplying coating material in a diffused spray to the interior of the conduit; means for supplying discrete particles to be coated to a cell at one end of the chamber whereby the particles are entrained in the operating gas for con¬ trolled circulation through the cell in an upward direct¬ ion through the interior of the vertically extending con¬ duit in contact with a spray of coating material therein and in a downward direction in a subsidence zone exteri- orly of the conduit; means for discharging coated product from a cell at the other end of the chamber; means for discharging operating gas from an upper region of the chamber; and means forming openings in each of the parti¬ tions for effecting the flow of particles into succeeding cells. Another aspect of the invention relates to a pro¬ cess for applying a coating to discrete particles com¬ prising the steps of supplying particulate material to a substantially closed chamber; conducting the particles through a plurality of cells within the chamber; within each cell, circulating the particles between co-direct¬ ional gas streams defining a high velocity coating zone and a lower velocity subsidence zone; supplying coating material in a diffused spray to the fluid stream in the high velocity zone to contact the particles circulating therethrough; controlling the velocity of the fluid streams in the respective zones to recirculate the par- RECTIFIED SHEET tides therebetween before discharging them to a succeed¬ ing cell; and discharging coated product from the last cell in the chamber. It is thus an object of the invention to provide coated particles having a controlled, preferably narrow distribution of coating thickness in order that the coat¬ ings on the respective particles are substantially uni¬ form. Another object of the invention relates to a coat- ing process and apparatus for the conduct thereof in which particles are coated in such a manner that narrow coating weight distribution between the respective par¬ ticles is provided. Still another object of the present invention re- lates to a coating process and apparatus for the conduct thereof in which the particles are coated in multiple stages. Still another object of the invention is to provide a coating process and apparatus for the conduct thereof in which the residence time of the particles in the coat¬ ing spray in each stage can be controlled so that the coating applied to the particles is substantially even and consistent. Still another object of the invention is to provide a process and apparatus for the conduct thereof for the continuous production of coated particles. These and other objects of the invention are accom¬ plished by providing a multistage apparatus for the con¬ tinuous production of discrete coated particles. The ap- paratus includes a plurality of series-connected coating cells containing coating zones in which discrete sub¬ strate particles to be coated are conducted along co- directionally flowing gas streams of unequal velocity. Within each cell the particles are caused to move up- wardly in the higher velocity gas stream which defines the coating zone of each cell and downwardly under the influence of gravity in the subsidence zone of each cell. RECTIFIED SHEET The upwardly flowing gas stream in the subsidence zone serves to cushion the gravity-induced downward movement of the particles. Spraying means are disposed in the coating zone in the respective cells whereby a coating material, preferably in the form of droplets of a so¬ lution of the coating material in a liquid solvent car¬ rier, is deposited on the respective particles during • their residence time in the coating zones. Moreover, re- circulation of the particles between the coating and sub- sidence zones in the respective cells enables a plurality of coating applications to be made while the particles are in each cell. Means provided to control the gas flows to the re¬ spective zones in each cell enable the thickness and weight distribution of coating material applied to the particles to be accurately regulated. Also, the gas is supplied at elevated temperatures to the respective zones of the cells whereby the solvent carrier of the coating deposited on the particles can be evaporated in order to produce a coating of compact substantially even thickness on the particle surface. Means are also provided for continuously feeding and controlling the feed rate of discrete substrate par¬ ticles to the initial cell in the apparatus and for con- veying the particles through succeeding cells thereof to a point of continuous discharge in the terminal cell in the series. For a better understanding of the invention, its operating advantages and the specific objectives obtained by its use, reference should be made to the accompanying drawings and description which relate to a preferred em¬ bodiment thereof. Brief Description of the Drawings Figure 1 is a schematic elevational representation of a multi-cell apparatus according to the present inven¬ tion; RECTIFIED SHEET Figure 2 is an enlarged fragmentary schematic ele- vational representation of a single cell according to the present invention; Figure 3 is a diagram showing the rate of dissolu- tion of fertilizer particles coated according to the pre¬ sent invention as a function of the quantity of coating applied and the number of coating application; Figure 4 is a schematical view similar to Figure 2 showing the effect produced by tilting the central duct slightly; Figure 5 is a graphical representation of theore¬ tical Particle Residence Time Distributions for 2, 3, 4, 5, 10 and 14 cells arranged according to the invention; Figure 6 is a graphical representation of the Par- tide Residence Time Distribution obtained in an experi¬ mental apparatus employing five cells according to the invention compared with the Theoretical Residence Time Distribution curve for five cells; Figure 7 is a graphical representation of the Par- tide Residence Time Distribution in an apparatus with seven cells, together with the integral of the distribu¬ tion curve representing an age distribution of the par¬ ticles at the exit of the last cell; Figure 8 is a partial perspective view of a commer- cial production embodiment of the present invention; Figure 9 is a sectional plan view of the apparatus shown in Figure 8; and Figure 10 is an enlarged view of part of the duct tilting mechanism employed in the apparatus of Figure 8. Description of Preferred Embodiment Figures 1, 2 and 4 of the drawings show a schematic representation of coating apparatus 10 according to the invention. The apparatus 10 includes a substantially closed chamber 12 defined by a plurality of upstanding side and end walls, 14 and 16 respectively, and a roof 18. The interior of the chamber 12 is divided into a plu- RECT1F1ED SHEET rality of cells 20, here shown as being seven in number, by a plurality of longitudinally spaced, upstanding par¬ titions 22 which extend transversely between the opposed side walls 14 of the apparatus 10, and have their upper ends spaced below the roof 18 to define a plenum space 52. Within the lower portion of each cell 20 is posi¬ tioned, substantially vertically disposed, a duct 24, that is desirably of hollow, rotation symmetrical form but which may be of conical or other shapes. The duct 24 has its lower end 26 vertically spaced from the bottom of the cell 20, defined by a gas distributor plate 28, with the upper end 29 spaced well below the top of the parti¬ tion 22. As shown, the ducts 24 divide each cell 20 into a coating zone 30 defined by the interior region of the duct and a subsidence zone 32 which occupies the space exteriorly of the duct. The cross section of the cell 20 shall preferably be of regular shape, e.g., substantially rectangular, circular, hexagonal, or octogonal, or combi- nations thereof. In a typical example, the diameter of the duct 24 shall preferably be between 100 and 250 mm (approximately 4 to 10 inches) . The height of the duct 20 shall prefer¬ ably be between 400 and 1,000 millimeters (approximately 15 to 40 inches) . The adjustable distance between the lower end of the duct 26 and the gas distributor plate 28 shall preferably be between 5 to 50 mm (about 5/8 to 2 inches) more preferably between 10 and 30 mm. The area of the cross section of the subsidence zone 32 shall prefer- ably be three to six times that of the duct 24. The height of the partition 22 over the upper end of the duct 29 shall preferably be between 1.5 and 3 meters (approxi¬ mately 5 to 10 feet) , enough to prevent particles flying over the partition into adjacent cells. Each of the partitions 22 contains a transversely elongated opening 34 for connecting adjacent cells 20 in series. Each of the openings 34 is provided with a verti- RECTIFIED SHEET cally adjustable closure plate 36 that operates to in¬ crease or decrease the overflow height through the parti¬ tion 22 as well as increase or decrease the size of the opening 34 in order to control the rate of transfer of particles to the succeeding cell. Further, each of the partitions 22 contains an opening 35 disposed adjacent the bottom of each cell 20 for connecting adjacent cells in series. Each of the openings 35 is provided with a closure plate 37 that ef- fects the transfer of particles to succeeding cells when charging and emptying the apparatus. The gas distributor plate 28 is advantageously made common to all of the cells 20 and is a perforated plate that separates the chamber 12 from operating gas supply apparatus, which is indicated generally as 38 in the drawings. The gas distributor plate 28 may be formed as a substantially flat plate containing a plurality of open¬ ings 39 as represented in the drawings hereof. The number of holes per unit of plate area is preferably greater in the coating zone than in the subsidence zone. Alterna¬ tively, the gas distributor plate 28 may be formed of a heavy woven screen material (not shown) . In either event, the size of the perforations or openings in the plate shall be such as to prevent the particles from falling through the plate from the cells 20 to the subadjacent gas supply apparatus 38. Gas supply apparatus 38, as schematically repre¬ sented in the drawings, includes in association with each cell 20, a manifold chamber having a supply pipe 41 for supplying conditioned bed gas in amounts regulated by ap¬ propriate valves (not shown) upwardly into the subsidence zone 32 of the cell. Concentrically placed within the manifold chamber 40 is a conductor 42 containing valving (not shown) for supplying conditioned gas into the coat- ing zone 30 within the interior of the duct 24. The gas employed for system operation is typically air, but may also be other gases such as nitrogen, carbon RECTIFIED SHEET dioxide, or similar gases, or mixtures thereof whose pro¬ perties are substantially inert with respect to the coat¬ ing and particulate material being processed. In the practice of the invention, the operating gas is supplied in regulated amounts to the cells 20 at an elevated tem¬ perature to provide drying. The temperature is selected in dependance of the nature of the liquid carrier and the thermal stability of active ingredient. The coating material, which may be dissolved, emul- sified or dispersed in an organic solvent or mixture of solvents, is supplied to the coating zone 30 through an atomizing nozzle 43 at the terminal end of a supply pipe 44 containing a flow control valve 46. Coating materials suitable for use in the process include sulfur, petroleum waxes, chemical resins, and the like. As shown in the drawings, the nozzle 43 is placed between the gas di¬ stribution plate 28 and the lower end 26 of the tube 24. Alternatively, the nozzle 43 may be positioned within the tube 24 adjacent its lower end 26, or even flush with the gas distributor plate 28, as desired, provided the spray from the nozzle 43 is directed upwardly into the coating zone 30 within the interior of the tube 24. In the operation of the invention, particulate ma¬ terials in the form of granules, particles, prills, or the like, as a substrate or core are processed on a con¬ tinuous basis through a series of coating cells 20, in each of which a specified portion of the total coating material is applied while the particles are caused to circulate at a controlled rate through the coating zone 30 in each cell. Although the described embodiment of the invention contains seven cells 20, it will be understood that a larger or smaller number of cells can be employed without departing from the spirit of the invention which contemplates, in its broadest aspect, the provision of a plurality of series-connected cells in each of which the coating operation, as hereinafter described, is con¬ ducted. By providing a plurality of cells connected in RECTIFIED SHEET series, the probability of particles being insufficiently or excessively coated is substantially reduced as com¬ pared with an apparatus having only a single cell. As the number of cells in an apparatus of the described type in- creases, the probability of a particle by-passing the coating spray in all cells is reduced. Similarly, the probability of a particle being excessively sprayed is also reduced. These effects are graphically illustrated in Figure 5 where the theoretical particle residence time distributions are shown for two, three, four, five, ten and fourteen cells arranged according to the invention. It will be obvious that e.g. two cells only cannot fulfill any high demands for uniformity of the coatings. For many applications, five to ten cells arranged accord- ing to the invention will yield an adequate quality coat¬ ing. For very compact coatings of substantially even thickness, it may be necessary to apply twenty to thirty cells arranged according to the invention. Accordingly, the described invention contemplates the apparatus 10 being provided with two or more cells 20. In a seven cell apparatus, as described, the proba¬ bility of a particle not being sufficiently coated in one cell, or remaining for an extended period in an excessive number of cells, is adequately small for many practical applications. This is graphically illustrated in Figure 7. It will appear that only 9% of the particles will exit the apparatus before having been retained for 50% of the average residence time. Only 8% of the particles will be retained for more than 150% of the average residence time. It is obvious that the apparatus must be con¬ structed in such a way that residence time distributions close to the theoretical ones can be obtained. An experiment was carried out on an apparatus with five cells according to the invention: Steady state operation was established feeding 150 kg/h of white urea particles into the apparatus. At a time called zero, 5 kg of black colored urea particles RECTIFIED SHEET were fed into the apparatus while maintaining all operat¬ ing parameters constant. Samples of the exit particles were taken evey sixth minute for 2-1/2 hours. The average residence time was about one hour. The concentration of black particles in the samples was determined by counting and the residence time distribution was calculated. This curve is shown in Figure 6, and for comparison, the theo¬ retical curve is also shown. It is obvious from these il¬ lustrations that the apparatus according to the invention performs exceptionally close to the theoretically pos¬ sible limit. The apparatus 10 according to the invention ope- rates in the following way: Conditioned operating gas is supplied in controlled amounts through the gas distribu- tor plate 28 from the manifold chamber 40 and from the gas conductor 42. Operating gas to the cell 20 is regu¬ lated in such a manner that the velocity of the gas ad¬ mitted to the coating zone 30 exceeds that supplied to the subsidence zone 32. Simultaneously therewith, coating material in a solvent carrier is introduced at a con¬ trolled, uniform rate into the coating zone 30 as a fine¬ ly atomized spray through the nozzle 43. When there is more than one duct 24 in a cell, there is a corresponding number of coating zones 30 in each cell. While in the principle embodiment of the invention one duct 24 is sub¬ stantially centrally disposed in each cell, it is con¬ ceivable to employ more than one duct in each cell with a corresponding number of coating zones 30. Operating gas flows to the respective zones 30 and 32 are such that particles introduced into the gas streams are carried in the high velocity gas stream through the coating zone within the duct 24 and pass through the spray of coating material from the coating spray nozzle 43 whereby drop¬ lets of coating material impinge on the surface of the particles during their residence time within the coating zone. Upon emerging from the top end of the coating zone 30, the particles are discharged into the more quiescent, RECTIFIED SHEET lower velocity region thereabove and fall down into the subsidence zone 32 wherein the effects of gravity and the upwardly directed lower velocity stream of operating gas therein cooperate to effect a cushioned migration of the particles in a plug flow manner downwardly through this zone. The evaporation of the solvent part of the coating starts in the upper end of the coating zone 30, and con¬ tinues in the lower velocity region. A fraction of the particles corresponding to the feed rate to the apparatus 10 will migrate through opening 34 into the succeeding cell. Within the subsidence zone 32, the evaporation of the solvent is completed, leaving a compact layer of the coating material on each particle. Adjacent to the bottom of each cell 20, the particles re-enter the higher velo¬ city gas flow to the coating zone 30, whereby the par¬ ticles are recirculated through the coating spray to acquire another layer of coating material. The operating gas admitted to the coating and sub- sidence zones 30 and 32 flows upwardly through the cells 20 and exits into the plenum space 52 for discharge from outlet 54 through conduit 56 whence the gas, with en¬ trained solvent vapors, is conducted to appropriate pro¬ cessing apparatus whereby the gas and solvent components of the mixture can be separated in a well known manner and returned to the apparatus 10 for reuse. Hence, a pre¬ ferred embodiment of the apparatus according to the in¬ vention includes means for exhausting spent gas from the apparatus and for recirculating it as operating gas through the apparatus. According to the present invention particulate ma¬ terial to be processed is charged to the apparatus 10 via the particle inlet conduit 48 which communicates with the first cell 20 in the series. Following processing in each cell 20 in the aforementioned manner, the particles are transferred in series flow to the succeeding cells through the openings 34 in the respective partitions un- RECTIFIED SHEET til the particles emerge from the terminal cell via out¬ let conduit 50 as final product having a surface coating of the desired thickness. The transfer from one cell to the next is normally accomplished by gravity flow. It is obvious that this will require a level difference between the cells. For constructional reasons, this is inconve¬ nient when many cells are required. With reference to Fi¬ gure 4, it has been found that by tilting the coating duct 24 a few degrees toward the opening 34 to the suc- ceeding cell in the main flow of the particles direction through the chamber 14, the average level height of the body of particles in the subsidence zone 32 can be kept substantially uniform for all cells. .Angular deflections of the duct axis up to five degrees, preferably between two and three degrees, from the vertical are acceptable for this purpose. The average height of the level of the body of particles is then controlled solely by the set¬ ting of the outlet conduit 50. No detectible adverse ef¬ fects have been found when using this embodiment. The transfer from one cell to the next can also be assisted by other means not shown, e.g., by blow pipes where compressed gas jets push the particles forward. Control of the application of coating material to the substrate particle in each cell 20 can be accurately maintained because, as the weight of particulate mate¬ rials passing through each cell on a time basis, is con¬ trollable by control of the rate of charge of particulate via inlet conduit 48 and, since the supply of coating ma¬ terial to the coating zones 30 is maintained constant, the wieght of coating material applied to the particu¬ lates passing through each cell is determinable. Also, as the total number of passes the particles make through the respective coating zones 30 is a function of the size of the opening at the lower end 26 of the duct (or ducts) 24 and the amount of operating gas suplied to the ducts, the number of layers of coating material applied to each par¬ ticle can be determined. RECTIFIED SHEET The primary controlling factor in the operation of the apparatus is the available bottom opening of the tube 24 and the amount of high velocity operating gas supplied to the coating zone 30 therein. Thus, for example, in a seven cell unit corresponding to that illustrated in Fi¬ gure 1, if operating gas is supplied to the coating zone 30 of the duct 24 at a rate to create four recirculated passes of particulate material within each cell 20, an average particle will, upon discharge from the outlet conduit 50 contain twenty-eight layers of coating mate¬ rial. Similarly, the supply of operating gas to the cells 20 to create eight passes will produce a final product containing fifty-six layers of coating materials, if suf¬ ficient high velocity operating gas is supplied to the interior of the duct 24 to obtain twelve recirculations of the average particle while in each cell 20, the ave¬ rage particle will receive eighty-four layers of coating material. In a preferred embodiment the coating cells are performed and function as described in international pa¬ tent application PCT/DK95/00040 incorporated herein by reference. In said embodiment (which is not shown on the drawing) an annular aperture exists around the nozzle 43, through which aperture most of the gas entering the coat- ing zone 30 is injected, while none or only a minor gas flow penetrates the gas distributing means 28 in the area immediately surrounding the annular aperture. Below the gas distributing means 28 gas guiding walls are provided together with means for imparting a swirling movement to an accelerating flow of gas upwards through the annular aperture around the nozzle which atomizes the coating liquid. By this last mentioned embodiment of the apparatus according to the invention the coating in each cell be- comes very uniform and consequently the result of the to¬ tal continuous coating operation through a series of cells results in an extremely uniform coating. RECTIFIED SHEET In practice, the process is operated to obtain a coated product that is most cost effective. This is a product that meets a market need and is produced at mini¬ mum cost. In the instant process, one can optimize the process to produce a coated product having a desired de¬ gree of release control by utilizing the minimum amount of coating material that need be used. This can be illus¬ trated by reference to the following specific examples of procedures for producing coated urea particles at the rate of 500 kg per hour, which particles have a bulk density of about 0.8 kg per liter and a particle size distribution of from about 1.8 mm to about 3 mm. In the described examples, the stream of operating gas delivered to the subsidence zone 32 in the respective cells 20 has flow rates controlled to between about 0.6 meters per se¬ cond and about 0.9 meters per second and that delivered to the coating zone 30 is delivered at flow rates of between about 8 meters per second and 15 meters per se¬ cond. EXAMPLES Description of the Apparatus Employed Description of the apparatus used in the examples Number of cells = 7 Diameter of duct 24 203.2 mm (8 inches) Height of duct 24 812.8 mm (32 inches) Distance over gas distributor plate 28 variable from 19-38 mm (3/4 - 1-1/2 in¬ ches) Forward tilting angle of duct 24 2 degrees Cell cross-sectional shape equilateral octagon Sidelength of octagon 190.4 mm (7-1/2 in- RECΠFIED SHEET ches ) Cross - sectional area of duct 24 = 0 . 0324 m 2 ( 0 . 35 sq . ft . ) Cross - sectional area of 2 subsidence zone 32 = 0.1426 m (1.53 sq. ft.) Cross-sectional area ratio of subsidence zone 32 and duct 24 = 4.4 Height of partitions 22 over duct end 29 = 1.82 m (6 ft.) Height of the exit weir 53 in the exit opening 50 in last cell = 572 mm (22-1/2 in¬ ches) Example 1 Procedure 1. Open the closure plate 35 and start gas flows. 2. Open the system with 485 kg of urea prills. 3. Adjust preliminary operating gas flow to the respective zones 30 and 32 in each cell 20 so that the particles will move as a fluid and be distributed into all of the seven cells. 4. Adjust the operating gas flow to the coating zone 30 in each cell 20 to a rate of about 8 meters per second so that the average particle in each cell is caused to be recirculated through the coating zones 30 about four times in 8.57 minutes. Operating gas is admitted to the subsidence zones 32 in the respective cells at a rate of about 0.8 meters per second. 5. Close the closure plates 35. 6. Initiate the flow of prills through the inlet conduit 48 at a controlled rate of 485 kg per hour. RECTIFIED SHEET 7. Adjust the temperature of the operating gas supplied to both of the zones 30 and 32 about 60°C (i.e., that sufficient to evaporate the solvent carrier in the respecctive zones) . 8. Initiate the spray of coating material through the nozzle 43 into all cells at a rate of 2.142 kg per hour dry basis. 9. Determine an equilibrium condition at the end of 2-1/2 times the average residence time at which coated product emerging from the dis¬ charge conduit 50 comprises 97% by weight substrate and 3% by weight coating material deposited in twenty-eight layers on each par¬ ticle. Operating Conditions 1. 485 kg per hour urea prills processed. 15 kg per hour coating material applied dis- solved in 485 kg of toluene. 500 kg per hour product having 35 by weight coating produced. 2. An average particle passes through seven cells in sixty minutes. 60/7 = 8.57 minutes residence time in each cell. 3. High velocity operating gas at a velocity of 8 meters per second to one coating zone 30 suffi¬ cient to cause four recirculations of an ave- rage particle through each coating zone in 8.57 minutes. 4. 15/7 = 2.1428 kg of coating material sprayed per hour into the coating zone of each cell 20. 5. Each cell 20 has a working capacity of 500/7 = 71.43 kg. RECTIFIED SHEET . Example 2 Repeat the conditions of Example 1 using a setting of the bottom opening of duct 24 and a flow of operating gas thereto at a rate of about 9 meters per second to cause an average particle to pass thorugh the coating zone 30 in each cell eight times during the 8.57 minutes residence time. The prill product comprises 97% particu¬ late core material and 3% coating material deposited in fifty-six layers. Example 3 Repeat the conditions of Example 1 using a setting of the bottom opening of duct 24 and a flow of operating gas thereto at a rate of about 10 meters per second to cause an average particle to pass through the coating zone in each cell twelve times during residence in each cell. The prill product comprises 97% particulate core material and 3% coating material deposited in the form of eighty-four layers on each particle. Example 4 Repeat Example 1 using 10 kg per hour of coating material per 484 kg per hour core material and an operat¬ ing gas velocity of 8 meters per second to the coating zone 30 and a gas velocity of 0.6 m/s in the subsidence zone 32. The prill product comprises 97.98% core material and 2.02% coating material deposited in twenty-eight layers on each particle. Example 5 Repeat Example 2 using 10 kg per hour of coating material per 485 kg per hour core material, a gas veloci¬ ty of 9 meters per second and a gas velocity of 0.6 m/s in the subsidence zone. The prill product comprises 97.98% core material and 2.02% coating material deposited in fifty-six layers on each particle. RECTIFIED SHEET Example 6 Repeat Example 3 using 10 kg per hour of coating material per 485 kg per hour core material using a gas velocity of 10 meters per second. The prill product com- prises 97.98% core material and 2.02% coating material deposited in the form of eighty-four layers on each par¬ ticle. Example 7 Repeat Example 3 using 485 kg per hour core mate¬ rial and 20 kg per hour coating material, a gas velocity of 10 meters per second and a gas velocity of 0.9 m/s in the subsidence zone. The prill product comprises 96.04% core material and 3.96% coating material. The coating comprises eighty-four layers on each particle. Example 8 Repeat Example 7 using a setting of the bottom opening of duct 24 and a gas velocity of about 12 meters per second providing a coating of 188 layers on each particle. Figure 3 is a diagram illustrating the rate of dissolution of fertilizer particles coated according to the above examples. The desired 7-day release rate for slow and controlled release fertilizers is generally be¬ low fifty percent. Thus, from the diagram it will be seen that the prodution of coated fertilizers having varied amounts of coating will provide diverse rates of release of nutrients. Figures 8, 9 and 10, wherein similar reference numerals are employed to designate similar elements of the inventive apparatus, illustrate the medial portion of one form of commercial production embodiment of the in- vention, the upper portion defining the plenum space 52 and the lower portion defining the manifold chamber 40 being omitted for sake of clarity. As shown, this e bodi- RECTIFIED SHEET ment of the invention, in order to enhance the production capacity of thee equipment, includes a pair of tandem- connected ducts 24 disposed in each cell 20. Moreover, in order to regulate particle flow thorugh the device, the ducts 24 are adapted for angular displacement of their central axes as well as linear movement transversely of the respective cells. Thus, the coating apparatus, indicated generally as 10', comprises a series of cells 20 defined by vertical partitions 22 which extend transversely of the side walls 14. Particle outlet 50 penetrates the other end wall. As shown in Fig. 9, the side walls 14 and partitions 22 are so-formed as to cooperatively define cells 20 that are essentially octagonal in section. Each partition contains an opening (not shown) which is framed on two sides by a pair of mounting brackets 60 for retaining a particle flow control plate 62 that covers the partition opening. Plate 62 is formed with an opening 64 which establishes flow communication between adjacent cells 20 and is mov- able by moving the plate 62 with respect to the brackets 60. Each cell 20 contains a pair of ducts 24 defining coating zones 30. The ducts 24 are connected in tandem by strapping plates 66 and 68 which secure the ducts adja- cent their upper and lower ends, respecticely, and that enable the position of the ducts to be adjusted trans¬ versely of each cell. Means are provided to enable axial tilting of the ducts either toward or away from the re¬ spective partitions 22. Tilting of the duct pairs is ef- fected by the connection of ends 70 of the respective strapping plates 66 and 68 to lead screws 72 disposed externally of opposed side walls 14 and threadedly at¬ tached via lock units 74 to mounting studs 76 having aligned openings 78 for reception of the screw. Each lead screw 72 is adapted to threadedly engage a collar 80 seated against rotation in an elongated opening formed by slots 32 in the plates. Accordingly, tilting of the ducts RECTIFIED SHEET toward or away from the adjacent portion 22 is achieved by adjusting the relative fore and aft positions of the respective upper and lower ends of the connected ducts 24 by rotation of the screws 72 within the collar 80. In practice, angular displacements of the ducts 24 of up to about five degrees is contemplated with adjustment capa¬ bility of between two and three degrees being preferred. Also, it will be appreciated that the cooperation of the collars 80 in the slots 82 of the respective strapping plates 66 and 68 enables the position of the connected ducts 24 to be adjusted in the transverse di¬ rection within each cell. Besides these described elements of particle flow adjustment, another element of particle flow regulation is obtained by the ability to vertically adjust the posi¬ tion of the flow control plate 62 on each partition and, thereby, the location of the opening 64 that establishes communication between adjacent cells 20. Thus, in addi¬ tion to the ability to increase particle flow between adjacent cells 20 and thereby reduce the amount of re- circulation of the particles through the respective coat¬ ing zones 30 by tilting the ducts 24 toward the succeed¬ ing cell, as described before in connection with Fig. 4, an increase in particle flow between adjacent cells can also be obtianing by lowering the position of the opening 64 in the respective partitions 22. Conversely, a de¬ crease in particle flow rate between succeeding cells with a concomitant increase in particle recirculation through the coating zones 30 is accomplished by tilting the ducts 24 away from the parition 22 and/or by moving the flow control plate 62 to raise the opening 64 there¬ in. It will be appreciated that, while tilting adjust¬ ments of the ducts 24 and be effected during periods of apparatus operation, adjustments made to the transverse position of the ducts and to the location of the openings 64 between adjacent cells 22 can be made only when appa- RECTIFIED SHEET ratus operation has been terminated and the interior of the chamber 12 exposed. The described apparatus advantageouly contains viewing windows 84 provided in a side wall 14 of each cell to permit observation of the various stages of the particle-coating process. Also, in the described arrange¬ ment, the closure 35 at the bottom of each cell is pi¬ votally secured to the partition wall and an operating handle 86 is provided exteriorly of each cell whereby the closure can be moved between its open and closed posi¬ tions and the loading and unloading of the apparatus fa¬ cilitated. It will be understood that various changes in the details, materials and arrangements of parts which have been herein described and illustrated in order to explain the nature of the invention may be made by those skilled in the art with the principle and scope of the invention as expressed in the appended claims. For example, al¬ though the description herein is directed to apparatus in which the cell structure is essentially rectangular or octagonal in section, it will be understood that the sectional shapes of the described cells can be other than these, such as for example other polygonal shapes, or circular. Also, while the described apparatus employs series-connected cells which are disposed in longitudi¬ nal, end-to-end alignment, other alignments as, for ex¬ ample, a clustered alignment of the cells with annular or similar disposition may be employed in practice of the invention. For example, the cells may be disposed as pa- rallel rows with opposite flow direction, or they may form a circle in which the cells are separated by radial partitions. Due care should, of course, be taken to en¬ sure easy access to operating handles etc. for each cell. RECTIFIED SHEET

Claims: C L A I M S 1. An apparatus for applying coatings to discrete particles comprising: means (14, 16, 18) defining a substantially closed chamber (12) ; a plurality of partitions (22) disposed on longi¬ tudinal spacing throughout said chamber dividing said chamber into a plurality of cells (20) ; substantially vertically extending conduit means (24) having open upper and lower ends spaced vertically from adjacent upper and lower surfaces of said chamber; gas distribution means (28) for supplying operating gas to the bottom of each cell and for directing it in vertical streams to the top thereof, said means including devices for directing a first gas flow through the inte¬ rior of a said conduit to develop a high velocity gas flow therein and a second flow of gas externally of said conduit at a velocity less than that of said first gas flow; means (43) for supplying coating material in a dif¬ fused spray to the interior of the said conduit; means (48) for supplying discrete particles to be coated to a cell at one end of said chamber whereby said particles are entrained in operating gas for controlled circulation through said cell in an upward direction through the interior of said vertically extruding conduit in contact with the spray of coating material therein and in a downward direction in a subsidence zone (32) exte¬ riorly of said conduit; means (50) for discharging coated product from a cell at the other end of said chamber; means (56) for discharging operating gas from an upper region of said chamber; and means forming openings (34) in each of said parti- tions (22) for effecting the flow of particles into suc¬ ceeding cells. 2. The apparatus according to claim 1 in which said RECTIFIED SHEET conduit means is defined by a hollow, substantially rota¬ tion symmetrical duct. 3. The apparatus according to claim 2 in which said duct is disposed substantially centrally of the cell. 4. The apparatus according to claim 1 in which said coating material is supplied as droplets of a solution, emulsion or dispersion of the coating material in a sol¬ vent and said operating gas is supplied at a temperature sufficiently elevated to evaporate the solvent. 5. The apparatus according to claim 1 including mov¬ able closures (62) closing said openings, said closures being operable to adjust said openings for controlling the flow of particles between succeeding cells. 6. Apparatus according to claim 1 including means for exhausting spent gas from said apparatus and for recir¬ culating it as operating gas through • the apparatus. 7. The apparatus according to claim 6 including a gas exhaust conduit from said chamber and gas inductor means in said conduit for withdrawing spent operating gas from said chamber. 8. The apparatus according to claim 1 including means for displacing the axis of said conduit angularly rela¬ tive to a vertical position for adjusting the rate of transfer of particle into succeeding cells. 9. The apparatus according to claim 6 including means downstream in the gas flow sense for separating solvent vapors from said operating gas whereby said solvent and said operating gas can be recirculated as process mate¬ rials through said apparatus. 10. The apparatus according to claim 1 in which said particles move downwardly in a plug flow manner in said subsidence zone. 11. The apparatus according to claim 1 including means defining a low gas velocity region beyond the discharge end of said coating zone in each cell whereby particles are transferred to said subsidence zone therein. 12. The apparatus according to claim 1 including means RECTIFIED SHEET for independently controlling the flow of operating gas to said coating and subsidence zones respectively whereby the recirculation of particles through said coating zone can be regulated. 13. The apparatus according to claim 1 including ad¬ justable weir means for regulating the discharge of coated product from said chamber. 14. A process for supplying coatings to discrete par¬ ticles comprising the steps of: supplying particulate materials to a substantially closed chamber; conducting the particles through a plurality of cells within the chamber; within each chamber, circulating the particles be- tween co-directional gas streams defining a high velocity zone and a lower velocity subsidence zone; supplying coating materials in a diffused spray to the gas stream in the high velocity zone to contact the particles circulating therethrough; controlling the velocity of the gas streams in the respective zones to recirculate the particles between the respective zones before discharging them to a succeeding cell; and discharging coated product from the last cell in the chamber. 15. The process according to claim 14 including the step of supplying said coating material dissolved, emul¬ sified or dispersed in a solvent carrier. 16. The process according to claim 14 including the step of heating the gas stream conducted through the cells to a temperature sufficient to evaporate the sol¬ vent leaving the coating material on the particles. 17. The process according to claim 14 including the step of controlling the thickness of coating material applied to the particles by independently controlling the velocities of the gas streams in the respective zones of each cell. RECTIFIED SHEET 18. The process according to claim 17 including the step of conducting the particles sequentially through a plurality of cells. 19. The process according to claim 14 in which urea particles are coated with a polymer which is slowly so¬ luble in water. 20. The process according to claim 14 including the step of angularly varying the disposition of the high velocity zone for controlling the circulation of par- tides in the cell. 21. The apparatus according to claim 1 in which said conduit means includes a plurality of ducts disposed within each cell in substantially spaced relation from the walls thereof. 22. The apparatus according to claim 21 in which said conduit means comprises a pair of tandem-connected, la¬ terally spaced ducts in each cell. 23. The apparatus according to claim 22 including means for adjusting the position of said ducts within said cells. 24. The apparatus according to claim 23 in which said position-adjusting means includes means for jointly dis¬ placing the axis of said ducts angularly relative to the vertical for adjusting the rate of transfer of particles into succeeding cells. 25. The apparatus according to either one of claims 8 or 24 in which said axis-displacing means comprises a pair of vertically spaced straps connecting said ducts adjacent the top and bottom ends thereof, said straps having their ends disposed externally of said chamber, and means for adjusting the relative positions of the respective straps to vary the locations of the upper and lower ends of said ducts and thereby alter the inclina¬ tion of the axis thereof. RECTIFIED SHEET

Inventors: LIBORIUS ERIK, LIBORIUS, ERIK
Applicants: LIBORIUS ERIK, NIRO ATOMIZER AS, LIBORIUS, ERIK, NIRO A/S
Application year: 1995
Main classification: B01J2
IPC codes: B01J2