Process for the production of combined fertilizers

Abstract

A process for making a combined fertilizer comprising a first nitrogen-based fertilizer, such as urea or ammonium nitrate, and one or more further components chosen among: nitrogen-based fertilizers, being different from the first nitrogen-based fertilizer and nutrients, wherein the combined fertilizer is made by a process of granulation in a fluid bed, the fluid bed being preferably in a vortex condition.

Claims

1. A process for making a combined fertilizer comprising: i) a first nitrogen-based fertilizer; ii) one or more second nitrogen-based fertilizer(s) different from said first nitrogen-based fertilizer, and/or one or more nutrient(s), collectively called one or more further components, wherein said combined fertilizer is made by a process of granulation in a fluid bed comprising the steps of: providing a first liquid feed to a first region of a granulation environment, providing a second liquid feed to a second region of said environment, wherein solid seeds acting as starting points for the granulation process are fed to said first region of the granulation environment, and said second region is downstream said first region; and wherein in the first region of the granulation environment, the first liquid feed solidifies on the solid seeds forming a binder layer around the solid seeds, and the second liquid feed has the form of a micronized slurry of said one or more further components in the first nitrogen-based fertilizer, spraying the second liquid feed over the granules previously formed in the first region; wherein the micronized slurry contains solid crystals of said one or more further components, and is sprayed in the form of droplets having an average size at least 5 times the average size of said crystals; and wherein the seeds have an average size of 500-1000 microns, the binder layer has a thickness of 200 to 400 microns, and said crystals have a size not greater than 100 microns.

2. The process according to claim 1, wherein said solid seeds and said first and second liquid feed include said first nitrogen-based fertilizer and said one or more further components.

3. The process according to claim 1, further comprising: dissolving said one or more further components in a liquid flow of the first nitrogen-based fertilizer, obtaining a liquid melt; splitting said liquid melt into a first portion and a second portion; using said first portion of liquid melt to provide said first liquid feed of the granulation environment, and using said second portion of liquid melt to provide said second liquid feed as a micronized slurry.

4. The process according to claim 3 wherein: said step of dissolution is carried out with a ratio between said one or more further components and the first fertilizer which is below the eutectic point and the temperature is controlled so that no solid phase is present in the obtained liquid melt; a further amount of said one or more further components is added to said second portion of liquid melt, in order to form said micronized slurry.

5. The process according to claim 3, wherein said second portion of the liquid melt is larger than the first portion.

6. The process according to claim 5, wherein said first portion is 5% to 30% of the total flow.

7. The process according to claim 5, wherein said second portion is 70% to 95% of the total flow.

8. The process according to claim 1, wherein the solid seeds are generated with any of the following techniques: crushing a portion of the granules delivered by said process of fluid-bed granulation; or drying or pastillation of a liquid.

9. The process according to claim 1, wherein the fluid bed of granulation has a vortex condition, where at least one vortex with a horizontal axis is established in the fluidized bed.

10. The process according to claim 9, wherein the fluid bed has a double-vortex condition including two substantially parallel and counter-rotating vortex with a horizontal axis.

11. The process according to claim 1, wherein: at least one of said first and second nitrogen-based fertilizers contain urea or ammonium nitrate; said nutrients comprise any of: sulphur, potassium, phosphorous, calcium and composite thereof.

12. The process according to claim 11, wherein said nutrients further comprise one or more microelements.

13. The process according to claim 12, wherein the one or more microelements comprise zinc, copper, manganese, chlorine, and/or molybdenum.

14. The process according to claim 1, wherein a liquid melt obtained by dissolving said one or more further components in a liquid flow of said first nitrogen-based fertilizer has a temperature lower than the temperature of the first liquid nitrogen-based fertilizer, reducing the temperature-sensitive formation of not desired by-products.

15. The process according to claim 1, wherein an additive is added to a liquid or a solid feed or injected inside the granulation environment to form a protective layer of the granule, said protective layer having a thickness of 50 to 300 microns.

16. The process according to claim 15, wherein a hydrophobic additive is added to provide protection from humidity.

17. The process according to claim 15, wherein said protective layer has the thickness of 100 to 200 micron.

18. The process according to claim 15, wherein said additive comprises one or more of: blending of carbonates, sulphate or phosphate salts, metal oxides.

19. The process according to claim 18, wherein said additive is combined with an organic matter comprising a wax formulation, an oil based solution, or a cellulose based suspension.

Description

DESCRIPTION OF FIGURES

(1) FIG. 1 is a scheme of the process for making UAS (Urea Ammonium Sulphate) according to an embodiment the invention.

(2) FIGS. 2 and 3 are schematic cross sections of the granulator of FIG. 1.

(3) FIG. 4 is a section of a granule obtainable with the process of FIG. 1.

(4) FIG. 5 is a more detailed scheme of another embodiment of the invention.

(5) FIG. 6 is an experimental diagram of the UAS melting temperatures.

DETAILED DESCRIPTION

(6) FIG. 1 is a scheme of an embodiment of the invention where the at least one nitrogen-based fertilizer is urea and the at least one nutrient is ammonium sulphate (AS).

(7) Reference 1 denotes a urea melt having high purity, preferably 95% or more. In some embodiments, for example when the urea melt 1 is delivered by a two-stage evaporation unit, the purity may be greater than 99%, for example 99.7%.

(8) Stream 2 contains solid ammonium sulphate which is dissolved in the urea melt 1 by a mixing device 3. The quantity of AS relative to urea is preferably below the eutectic point and the temperature after mixing is above the solidification temperature of pure urea so that the resulting melt 4 is a pure liquid without a solid phase (clear melt).

(9) The granulation process takes place in a granulator 5. Said granulator 5 in this example has basically a first zone 5A, a second zone 5B and a third zone 5C in this order from an inlet end to an outlet end.

(10) A first portion 6 of said melt 4 is directly fed to the granulator 5, more in detail to the first zone 5A, by suitable means such as sprayers or the like. In this zone, the melt is contacted with solid seeds 12 as will be further explained hereinbelow.

(11) A second portion 7 of the melt 4 is further processed and added with another AS-containing stream 8 in a wet milling section 9. By adding the additional stream 8, the ammonium sulphate exceeds the eutectic point leading to the formation of a slurry 10 containing solid crystals of AS in the liquid phase. The slurry 10 is basically a micronized dispersion of crystals of AS into a liquid containing both urea and AS.

(12) Preferably the crystals of AS in the slurry 10 have a size ranging from 10 to 100 microns, more preferably even smaller for example ranging from 1 to 50 microns. In some embodiments the wet milling section 9 may include multiple wet milling stages in order to reach a required small size of the solids dispersed in the liquid flow.

(13) Said slurry 10 is sprayed in the second zone 5B of the granulator 5, via suitable sprayers 11.

(14) Further inputs of the granulator 5 include solid seeds 12 and fluidization air 13.

(15) The seeds 12 can be, for example, small crystals of AS or small particles of urea and AS. The seeds 12 for example may be crystals of AS taken from the feed 2 or small particles of urea and AS obtained by crushing some of the granules 14 delivered by the granulator 5 or by solidifying a dedicated small portion of the urea melt 4.

(16) The granulator operates as follows. In the first zone 5A, the seeds 12 are contacted with the melt 6 which forms a thin first layer around the seeds. The so obtained granules are contacted with the slurry 10 in the subsequent zone 5B, leading to the progressive formation of larger granules. The zone 5C is a cooling down zone where the structure of the granules is stabilized.

(17) Stream 14 of granules is the end product of the granulator 5. As mentioned above, a portion of said granules 14 may be internally recirculated and crushed to generate the seeds 12, in some embodiments. The crushing of granules may generate some solid matter under the minimum size of the seeds; this solid matter (fines) is used in the mixing device 3 where it is dissolved in the urea 1. Prior to the mixing device 3, the fines can be further reduced in size, e.g. milled, if necessary.

(18) Preferred features and parameters of the process of FIG. 1 are the following.

(19) The urea melt 1 has a temperature around 130-140 C. depending on the concentration; ammonium sulphate in stream 2 corresponds to an amount of 7% to 9% (mass) of the urea, which is below the eutectic point of about 10%. Hence the melt 4 is a pure liquid being the temperature after mixing around 125-135 C. The AS-containing stream 2 is preferably at ambient temperature, for example 25 C. The ammonium sulphate being colder than the urea is an advantage since cooling in the mixing device 3 reduces the temperature sensitive formation of undesired by-products such as biuret.

(20) The temperature of the slurry stream 10 is preferably controlled at around 125-135 C., e.g. by pre-heating the solid 8 in a suitable pre-heater. The slurry sprayers 11 are preferably designed to produce droplets having an average size of 100 to 300 microns, thus being significantly greater than the size of the crystals in the slurry.

(21) Accordingly, in the region 5A the 500-1000 microns seeds are covered with a 200-400 microns layer of urea and ammonium sulphate; then in the region 5B the particles are sprayed with the 100-300 microns droplets of hot slurry until a desired size of granules (typically 2 to 4 mm) is reached. In the region 5C, the granules are cooled to around 70 C. The granules leaving the granulator 5 are further cooled to 40-50 C. before storage.

(22) FIGS. 2 and 3 are exemplary cross sections of the granulator 5. In both cases, a whirling motion is established by a suitable arrangement of the sprayers 11. FIG. 2 relates to a single-vortex embodiment and FIG. 3 to a double-vortex embodiment.

(23) The vortex or double-vortex arrangement creates an upper wetting zone, where the granules are contacted with liquid or slurry from the sprayers 11 (which also provides the momentum to maintain the rotational state of the vortex) and a lower zone of solidification of the liquid layer deposited on the granules.

(24) FIG. 4 discloses the structure of granules obtainable with the above process, showing a core 16 (corresponding to seeds 12), an inner layer 17 around the core 16, formed in the region 5A (binder layer) and a layer 18 formed in the region 5B and made of solid slurry. In some embodiments a further outer layer including an additive (e.g. hydrophobic additive) is also obtained.

(25) Further details of a preferred embodiment are disclosed in FIG. 5, where items and flow lines corresponding to FIG. 1 are denoted with the same numerals for simplicity.

(26) The urea melt 1 is pumped through a melt pump 20 to a feeding pressure which is preferably in the range 8 to 15 bar. The ammonium-sulphate containing stream 2 is obtained by feeding ammonium sulphate 21 from a hopper 22, possibly mixed with fines 23 recirculated from a seed generation loop 24. The mixed granular flow, containing solid ammonium sulphate 21 and fines 23, can be milled in a dry milling unit 25, if appropriate, to further reduce the particle size and facilitate dissolution.

(27) The ammonium sulphate 21 may be in a crystalline form or in a coarser form. The term of crystalline form is used to denote a mean particle size of around 1 mm. A crystalline form is generally suitable for direct feeding to the mixer 3, i.e. without further reduction in the dry milling unit 25; when ammonium sulphate 21 is available in a coarser form, e.g. with a mean particle size of 2 mm or more, the further milling in the unit 25 is preferred.

(28) As per thermal balance in the mixer 3, the mixing of urea 1 and AS-containing stream 2 results in a temperature drop. This temperature drop is a positive feature when working with urea since it reduces the formation of biuret.

(29) The mixer 3 is preferably a low-residence and high-shear machine to induce enough turbulence allowing easy dissolution. Preferably, the AS crystals are further pulverized during the mixing operation so that the contact area between the solvent and the solute is increased to the benefit of a complete dissolution.

(30) The clear melt 4 is divided into a main stream 6, directed to the granulator 5, and a side stream 7 for the formation of a slurry. Usually the side stream 7 is greater than the main stream 6, i.e. the side stream 7 is 70% to 95% of the stream 4 delivered by the mixer 3.

(31) The further ammonium sulphate 8 to be mixed with the side stream 7 is provided, in this embodiment, by a second hopper 26. The solid matter form said hopper 26 is preheated in a plate heater 27, for example to a temperature of 60 to 120 C., and then is directed to the wet milling section 9.

(32) Said section 9 comprises a first wet miller 28, a slurry pump 29 and a second wet miller 30.

(33) The pre-heated solid ammonium sulphate is contacted with the side stream 7 of clear melt in the first wet miller 28, which is designed to disperse and mill down the ammonium sulphate in the liquid melt and to generate crystals with a first average size, e.g. 100 to 500 microns. The slurry pump 29 is provided to push the slurry through the second wet miller 30 which is typically designed to disperse further the solid crystal into the liquid melt leading to a second average size which is finer than the first average size, e.g. 10 to 100 micron. In a further embodiment the second wet miller 30 can include a twin unit in series to reach an even smaller average size of the crystals, preferably 1 to 50 micron. The second wet miller 30 produces the slurry 10 which is fed to the zone 5B of granulator 5.

(34) The seeds 12 are produced by the loop 24 using granules 31 taken from the output 14 of the granulator 5. Said granules 31 are crushed in a crusher 32 and crushed granules pass through a sieve 33 to select particles within a desired size range (e.g. 500 to 1000 microns) which form the seed stream 12. Particles outside this range are sent to lines 34 and 23. Larger particles (e.g. >1000 microns) are sent back to the crusher via line 34; smaller particles (e.g. <500 microns), also termed fines, are sent via line 23 to the feed of the milling unit 25 (if provided) or to the urea/AS mixer 3.

(35) Only a small portion of granules is internally used for the generation of seeds; the rest of granule output 14 (line 35) is for example cooled down in a suitable cooler 36 to form a granular composite fertilizer 37.

EXAMPLE

(36) FIG. 6 provides an example of the transformations taking place in the process of FIG. 5.

(37) Point A of the diagram of FIG. 6 denotes a urea melt 1 having a temperature of around 140 C. Said urea melt 1 is mixed with an ammonium sulphate-containing stream 2 at ambient temperature, providing a clear melt 4.

(38) Point B denotes said clear melt 4, having a temperature of 128 C. and containing 9% of ammonium sulphate.

(39) A stream 6 separated from said clear melt 4 is injected in the first section 5A of the granulator, where it contacts seeds 12 and wherein the temperature is controlled in the range 95 to 105 C., resulting in a solidification process. Said solidification process is evidenced from point B to point C. Point C denotes seeds 12 which are covered by a thin layer of solid having uniform composition.

(40) Said clear melt 4 containing 9% of ammonium sulphate (point B) is further added with ammonium sulphate so as to achieve a concentration of ammonium sulphate of around 30% and obtain a slurry 10, which is identified by point D.

(41) Said slurry 10 is sprayed in the section 5B of the granulator, where it contacts the granules denoted by point C and wherein the temperature is controlled in the range 95 to 105 C., resulting in the progressive formation of larger granules covered by a solid out-layer and identified with point E.