GB2116159A - A process for manufacturing granular compound fertilizers - Google Patents

Abstract

In the process, phosphoric acid, sulfuric acid or nitric acid or a mixture thereof is reacted with ammonia gas in a tubular reactor (1), then in a mixing tank (2); the reaction solution is concentrated in tank (2) by evaporation of water to obtain a high temperature slurry having a liquid content of 40-90% by volume, and the high temperature slurry is then injected by spray means (14) into a granulation zone of a spouted bed granulation apparatus (15) or fluidized granulation apparatus to cause the thus-sprayed slurry to stick to priming particles fed separately into the apparatus which float in the granulation zone, thereby forming enlarged granules. The granules contain two or more of the agronomically effective elements N, P and K in proportions adjustable- if desired-by solid, liquid or aqueous fertilizers or diluents which are added to the slurry before granulation. <IMAGE>

Description

SPECIFICATION A process for manufacturing granular compound fertilizers This invention relates to a process for manufacturing granular compound fertilizers.

It has been known to prepare a solution of an ammonium phosphate, ammonium sulfate or a mixture thereof, which serves as a raw material for granular compound fertilizers, from ammonia and phosphoric acid or sulfuric acid or a mixture of these acids while making use of a tubular reactor. Reference may be made, for example, to a report written by B.R. Parker, M.N. Norton and D.G. Salladay (reported at FAI-IFDC Seminar 1977 in New Delhi, India). Compared with carrying out the reaction in a neutralizing reaction tank equipped with an agitator, the tubular reactor approach has several merits. For instance, the reactor has a simple structure, is easy to maintain and the reaction heat may be effectively utilized for the evaporation of water and the granulation and drying operations.

The aforesaid solution is then fed to a drum-shaped granulator or blunger, mixed with a large quantity of recycled solid particles as well as, optionally, another solid fertilizer substance and/or solid diluent into granules, thereby providing a granular substance having a water content of 2-5% by weight. This granular substance is thereafter dried with hot air in a rotary drier and fed to a classifier for classification, thereby obtaining a final product, i.e., a compound fertilizer of a desired particle size range. Excessively large particles are recycled to the granulator after pulverization and excessively small particles are recycled to the granulator as they are.

The above process generally requires that the quantity of solid particles recycled to the granulation apparatus should be 2 to 8 times that of the final product drawn out of the system.

The object is to maintain the granulator and rotary drier under good operation conditions with respect to the granulatability of each compound fertilizer or the water or energy balance in the granulation system, although the above figures vary depending on the components contained in the compound fertilizer. Thus, it is generally required to recycle to the recycling line even a part of particles of the desired particle size which are obtained in the classifier.

An object of this invention is to provide a process for efficiently manufacturing a granular compound fertilizer which contains at least two agronomically effective components selected from N (nitrogen), P (phosphor) and K (potassium).

According to the present invention, there is provided a process for manufacturing a granular compound fertilizer containing at least two agronomically effective components selected from N, P and K, which comprises reacting phosphoric acid, sulfuric acid or nitric acid or a mixture thereof and ammonia in a tubular reactor to form a solution containing ammonium hydrogenphosphate, ammonium dihydrogenphosphate, ammonium sulfate, ammonium nitrate or a mixture thereof; introducing the solution into a mixing tank and either evaporating the water contained therein to obtain a high temperature slurry having a liquid content of 40-90% by volume or evaporating the water contained therein and, at the same time, mixing a solid fertilizer substance or solid diluent having particle sizes of 50-1,000 ,um or a molten liquid or aqueous solution of a fertilizer substance with the solution to obtain a high temperature slurry having a liquid content of 40-90% by volume; spraying the thus-obtained high temperature slurry into the spacing of a granulation zone of a spouted bed granulation apparatus or fluidized bed granulation apparatus, thereby causing the thus-sprayed slurry to stick to priming particles floating in the spacing and thus forming enlarged granules; and drying and/or cooling the thusenlarged granules.

The invention will now be described in more detail by way of example only with reference to the accompanying drawing, which is a flow sheet illustrating one embodiment of the invention.

In a process according to this invention, ammonia and phosphoric acid, nitric acid, sulfuric acid or a mixture thereof are reacted in a tubular reactor. The preferred acid to be reacted with ammonia is phosphoric acid, or a mixture of phosphoric acid and nitric acid or sulfuric acid.

These acids and their proportions can be selected in accordance with the agronomically effective components contained in each intended compound fertilizer and the concentrations thereof.

Where phosphoric acid or a mixed acid of phosphoric acid and another acid is used, the resulting ammonium phosphate has maximum solubility when its molar ratio of ammonia to phosphoric acid is 1.4 to 1. If the molar ratio exceeds 1 .4 to 1, the solubility of the ammonium phosphates drops abruptly and solid ammonium phosphates deposit, resulting in clogging of the tubular reactor. Accordingly, it is desirable to maintain the molar ratio of ammonia to phosphoric acid in a tubular reactor within 1.3-1.6 to 1, and more particularly, within 1.3-1.5 to 1.

Ammonia, which is reacted further with phosphoric acid, is added in a subsequent step as will be described later in this specification.

A tubular reactor is similar to a short pipe structurally, and therefore it has a small volume. It is thus unsuitable to employ such a tubular reactor for adding and mixing a solid, molten liquid or aqueous solution to and with a solution of ammonium phosphates, ammonium sulfate or ammonium nitrate resulting from a reaction between ammonia and the above-described acid.

Therefore, a reaction mixture formed in the tubular reactor is then introduced into a mixing tank, where it is reacted further with ammonia in order to increase the molar ratio of ammonia to phosphoric acid further. The water is caused to evaporate from the reaction mixture owing to its sensible heat and reaction heat given off in the mixing tank.It is also possible to heat the mixing tank e.g. with steam and to evaporate the water if insufficient heat is available for the evaporation of the water from the reation mixture. Furthermore, it is also possible to add while stirring various kinds of fertilizer substances in the form of a solid, molten liquid or aqueous solution to the reaction mixture in order to adjust the proportion of each component in the intended compound fertilizer and its concentration in the mixing tank. It may be feasible to add a solid diluent together with such fertilizer substances or as their substituent. The particle sizes of these solids should range from 50-1,000 ym.

The liquid content of the thus-formed slurry ranges from 40 to 90% by volume, and preferably, 40 to 60% by volume. If the liquid content does not reach 40% by volume, it is difficult to achieve smooth transportation of the slurry from the mixing tank to the granulation apparatus by means of a pump. On the other hand, if the water content of the slurry exceeds 30% by volume, it is necessary to have more water evaporate in the granulation apparatus, thereby leading to an increased energy demand. Such a high water content is accompanied by another drawback in that the water content of the granular substance to be obtained will be high and a separate drier will be required.

Although fine solid particles are contained in the thus-obtained high temperature slurry, these solid particles have particle sizes within the above range and are uniformly dispersed in the slurry, thereby making it possible to feed the slurry to a spouted bed or fluidized bed granulation apparatus, which will be described below, by means of a pump. Owing to the nature of the slurry, spray nozzles of the granulation apparatus will be protected from clogging. It is preferred to conduct the agitation of the reaction mixture in the mixing tank at a speed of 1 50-500 r.p.m. It is generally preferred to maintain the slurry at 80-130"C in the mixing tank, although the temperature can vary depending on the composition and the like of the intended compound fertilizer.The residence time of the slurry in the mixing tank is preferably 20 minutes or less, and particularly, 16:) minutes or less. A residence time exceeding 20 minutes is not preferred because it tends to induce a quality alteration to the compound fertilizer due to interactions among the raw material fertilizer substances.

The high temperature slurry prepared in the mixing tank is then sprayed into a granulation spacing of a spouted bed or fluidized bed granulation apparatus by a pump through spray nozzles and is allowed to stick to priming particles floating in the granulation spacing. As such priming particles, may be employed excessively large granules (after pulverization) and excessively small granules, both of which granules are discharged from the granulation apparatus and separated from granules of the desired particle size range by a classifier. Any known spouted bed granulation apparatus or fluidized bed granulation apparatus may be employed in the present invention.

The preferred embodiment will hereinafter be described with reference to the accompanying drawing. A raw material feed of phosphoric acid, sulfuric acid, nitric acid or a mixture thereof is charged into a tubular reactor 1 through a line 5 or 6. A part of the whole ammonia required to make the molar ratio of ammonia to phosphoric acid 1.4 to 1, is supplied through a line 7 to carry out a reaction. The reaction mixture from the tubular reactor 1 is introduced into a mixing tank 2 by coupling an outlet nozzle of the tubular reactor 1 directly to the mixing tank 2.The mixing tank 2 is provided with a high-speed agitator 3 which may be rotated at 1 50-500 r.p.m., whereby completing in a short period of time the mixing of the rection mixture with a solid raw material containing agronomically effective components such as N, P and/or K, or an aqueous solution or molten liquid thereof, or 2 solid diluent fed into the mixing tank 2 via a line 8 or 9. On the other hand, the remaining part of the whole ammonia which is excessive to the above-mentioned ratio, is supplied into the mixing tank 2 through a line 10, thereby achieving the desired molar ratio. While carrying out these mixing or reaction operations, the water is evaporated from the reaction mixture by virtue of reaction heat generated in the tubular reactor 1 and/or mixing tank 2.The thus-evaporated water is sent together with 2 small quantity of ammonia gas via a line 1 2 to a gas scrubber 11, where it is washed with water, phosphoric acid, sulfuric acid or a mixture thereof supplied through a line 28. Water, phosphoric acid, sulfuric acid or a mixture thereof which has absorbed ammonia is fed to the tubular reactor 1 through a line 29. The thus-washed gas is released into the atmosphere.

The above-mentioned solid raw material containing the agronomically effective components includes, for example, urea, ammonium sulfate, ammonium nitrate, ammonium phoshates, ammonium chloride, potassium phosphates, calcium superphosphate, calcined phosphate fertilizer, potassium sulfate, potassium chloride, potassium metaphosphate, a urea-aldehyde condensate fertilizer, magnesium silicate, calcium silicate, various trace elements such as Zn, Mn, Mo, Cu, Fe and B, etc. Examples of molten liquids containing agronomically effective components include urea, ammonium nitrate and the like. Examples of raw material solid diluents include gypsum, and bentonite.The aqueous solution containing agronomically effective components includes, for example, an aqueous solution containing water-soluble substances chosen from the above fertilizer substances A predetermined amount of priming particles is supplied through a line 1 7 into a spouted bed granulation apparatus 1 5. An upwardly-directed spouted bed of the priming particles is formed within the granulation apparatus 1 5 by a flow of the heated air supplied through a line 16.At the same time, the high temperature slurry is supplied from the mixing tank 2 through a pump 4 and line 1 3 to the granulation apparatus 1 5 and sprayed upwardly through slurry spray nozzles 14 provided in the spouting bed granulation apparatus 1 5 at a lower location thereof, thereby causing the thus-sprayed liquid droplets to stick to the floating priming particles and to become larger while drying the thus-enlarged granules with the air flow.

The water evaporated from the high temperature slurry and the air flow generated in the granulation apparatus 1 5 and containing fine particles are sent through a line 1 8 to a separator 1 9, where the fine particles are removed, and then released into the atmosphere. The fine particles caught by the separator 1 9 are conveyed to a storage tank 22.

After enlargement in the granulation apparatus 1 5, the resulting granules (having a wide variety of particle sizes from large to small) are fed through a line 20 to a classifier 21, where they are classified into three groups, i.e., those having diameters within the desired particle size range and to be finished to a final product, those having particle sizes larger than the upper limit of the desired particle size range and those having particle sizes smaller than the lower limit of the desired particle size range. Excessively large granules are fed to a pulverizer 25 while excessively small particles are temporarily stored in the storage tank 22. The final product is cooled down in a cooler 23 and sent outside the system through a line 24.The excessively large particles are pulverized by the pulverizer 25 and then recycled to the classifier 21 through a line 26. The excessively small particles, which have been temporarily stored in the storage tank 22, are recycled via line 1 7 to the granulation apparatus 1 5. In order to control the particle size distribution at the outlet of the granulation apparatus 15, the amount of excessively small particles to be recycled is controlled by a feeder 27.

According to this invention, a granular compound fertilizer containing at least two components selected from N, P and # K can be effecitvely manufactured from a reaction mixture which has been obtained by reacting phosphoric acid, sulfuric acid, nitric acid or a mixture thereof with ammonia in a tubular reactor. Since a slurry prepared from the above-mentioned reaction mixture is subjected to granulation in a spouted bed or fluidized bed, it is possible to lower the load of the drying step in the granulation process. In addition, the evaporation of water from the reaction mixture can be effected in the course of the preparation of the slurry by virtue of the reaction heat between ammonia and the acids.Since the reaction mixture is supplied in the slurry form to the granulation step, it is feasible to add, upon preparation of the slurry, any desired fertilizer substance containing desired agronomically effective components in desired concentrations or a solid diluent to the reaction mixture so as to adjust the composition of the intended final product. Accordingly, the process of this invention can be applied to manufacture grannular compound fertilizers of any composition. Furthermore, the deposition of ammonium monohydrogen phosphate and clogging in the tubular reactor are avoided in the present process, since ammonia is reacted with phosphoric acid in two steps, that is, in the tubular reactor and the mixing tank.

The invention will be described further in the foliowing specific examples.

Example 1: A manufacturing test for a grannular compound fertilizer containing N and P in amounts of 18% by weight and 46% by weight (in terms of P205), respectively, and having diameters in a range of from 1 to 3.5 mm was carried out as follows in accordance with the process illustrated in the accompanying drawing: The test was conducted using a tubular reactor of 50 mm in inner diameter and 2,000 mm in length and a mixing tank with an effective volume of 501 and equipped with an agitator rotatable at 200 r.p.m. The spouted bed granulation apparatus was generally of a cylindrical shape and was formed at a lower part thereof into an inverted truncated conical shape.Its diameter was 1,000 mm at the cylindrical part and its effective volume was 250 1.

Into the tubular reactor 1, were directly charged 373 kg/hour of liquid ammonia at 0 C through the line 7, 800 kg/hour (in terms of P205) of 54 wt. % phosphoric acid at 25"C via the line 5 and 99 kg/hr of 98 wt.% of sulfuric acid at 25"C through the line 6. In addition, the reactor 1 was fed with 2,247 kg/hr (in terms of P205) of 32.4 wt. % of phosphoric acid at 30"C through the line 28. The latter phosphoric acid was first used as an absorbent in the gas scrubber 11 to recover ammonia gas accompanied with steam which had been separated in the mixing tank 2 and then charged into the reactor 1 via the line 29. The reaction mixture was then introduced into the mixing tank 2, where it was reacted with 1 92 kg/hr of liquid ammonia at O"C supplied through the line 10, thereby providing 2,985 kg/hr of slurry at 1 10 C: which contained 115% by weight of water and 46% by weight of solid components and consisting principally of diammonium hydrogenphosphate and ammonium sulfate.The resultant slurry was transported through the slurry pump 4 and line 113 and sprayed through the spray nozzles 14 disposed in the spouted bed granulation apparatus 1 5 which were operated under the following operation conditions: Amount of air blown in the 8,300 Nrn3/hr granulation apparatus Temperature of air blown into 1 70etc the granulation apparatus Temperature in the granulation 70-75"C apparatus Amount of priming particles 2,710-2,760 kg/hr supplied Enlarged granules of a compound fertilizer were obtained at the outlet of the line 20 at a rate of 5,180 5,240 kg/hr. The water content in the enlarged granules was 1.5-2.5% by weight.

The thus-granulated compound fertilizer was classified at the classifier 21 into those having diameters within the desired particle size range for the final product, those having excessively large particle sizes and those having excessively small particle sizes which amounted on average to 48.0%, 5.3% and 46.7%, respectively. The final product was obtained at a rate of 2,460-2,510 kg/hr. The granular final product was cooled in the cooler 23 to 45 C, which is a temperature suitable for storage, and discharged out of the system through the line 24.

Example 2: A manufacturing test for a grannular compound fertilizer containing N, P and K in amounts of 1 9% by weight, 19% by weight (in terms of P2O5) and 1 9% by weight (in terms of K2O), respectively, and having diameters in a range of from 1 to 4 mm was carried out as follows in accordance with the same process as that employed in Example 1:: Into the tubular reactor 1 in the same manner as that followed in Example 1 were supplied 259 kg/hr of liquid ammonia at 0 C through the line 7, 1,253 kg/hr (in terms of P205) of 54 wt. % of phosphoric acid at 25'C via the line 5, 65 kg/hr of 98 wt. % of sulfuric acid at 25"C through the line 6, and 583 kg/hr (in terms of P205) of 21.7 wt. % of phosphoric acid at 30"C via the line 28.The resulting reaction mixture was introduced into the mixing tank 2 and combined with 1,267 kg/hr of potassium chloride having particle sizes of 1,000 ,um or smaller and maintained at room temperature and 1,196 kg/hr of a 96 wt. % aqueous solution of urea supplied at 11 oec through the line 9, thereby obtaining a slurry having a temperature of 105'C and containing 6% by weight of water and 30.5% by weight of solid components at a rate of 4,1 61 kg/hr.The thus-obtained slurry was conveyed through the slurry pump 4 and line 3 and sprayed through the spray nozzles 1 4 of the spouted bed granulation apparatus 1 5 operated under the following operation conditions: Amount of air blown into the 8,500 Nm3/hr granulation apparatus Temperature of air blown into 50 C the granulation apparatus Temperature in the granulation 55-60 (:: apparatus Amount of priming particles 4,030-4,270 kg/hr supplied Enlarged granules of a compound fertilizer were obtained at a rate of 7,865-8,345 kg/hr.

These granules had a water content of 1-1.5% by weight.

The thus granulated compound fertilizer was classified by the classifier 21 into those having diameters within the desired particle size range for the final product, those having excessively large particle sizes and those having excessively small particle sizes which amounted on average to 49.6%, 1.0% and 49.4%, respectively. The final product was obtained at a rate of 3,840-3,920 kg/hr. The final product was cooled to 45"C in the cooler 23 and then discharged out of the system.

Claims (11)

1. A process for manufacturing a granular compound fertilizer containing at least two agronomically effective components selected from N, P and K, which comprises reacting phosphoric acid, sulfuric acid or nitric acid or a mixture thereof and ammonia in a tubular reactor to form a solution containing diammonium hydrogenphosphate, ammonium dihydrogen phosphate, ammonium sulfate, ammonium nitrate or a mixture thereof; introducing the solution into a mixing tank and either causing its water to evaporate to obtain a high temperature slurry having a liquid content of 40-90% by volume or causing its water to evaporate and, at the same time, mixing a solid fertilizer substance or solid diluent having particle sizes of 50-1,000 ,um or a molten liquid or aqueous solution of a fertilizer substance with the solution to obtain a high temperature slurry having a liquid content of 40-90% by volume; spraying the thusobtained high temperature slurry into the spacing of a granulation zone of a spouted bed granulation apparartus or fluidized bed granulation apparatus, thereby causing the thus-sprayed slurry to stick to priming particles floating in the spacing and thus forming enlarged granules; and drying and/or cooling the thus-enlarged granules.

2. The process according to claim 1, wherein the temperature of the slurry ranges from 8O'C to 1 30 C.

3. The process according to claim 1 or claim 2, wherein the slurry has a liquid content of 40-60% by volume.

4. The process according to any of claims 1 to 3, wherein the said fertilizer substance introduced is a compound containing at least one of N, P and K, or a trace element.

5. The process according to any of claims 1 to 4, wherein the solid diluent introduced is gypsum or bentonite.

6. The process according to any of claims 1 to 5, wherein the molten liquid of the fertilizer substance introduced is molten urea or ammonium nitrate.

7. The process according to any of claims 1 to 6, wherein the residence time of the slurry in the mixing tank is 20 minutes or less and the slurry is agitated at 150 to 500 r.p.m.

8. The process according to any of claims 1 to 7, wherein the priming particles have the same composition as the granular compund fertilizer.

9. The process according to any preceding claim, wherein phosphoric acid or a mixture of phosphoric acid and sulfuric acid or nitric acid is reacted with ammonia in the tubular reactor in such a way that the molar ratio of ammonia to phosphoric acid in the resulting ammonium phosphate in a reaction mixture ranges from 1.3 to 1.6 to 1, and the reaction mixture is then reacted in the mixing tank so that the molar ratio of ammonia to phosphoric acid in the resultant ammonium phosphate falls within a range of from 1.4 to 2.0 to 1.

10. A process for maufacturing a granular compound fertilizer according to Example 1 or Example 2 herein.

11. A process according to claim 1 and substantially as herein described.

1 2. A granular compund fertilizer made by the process claimed in any of claims 1 to 11.