CN1075045C - A process for manufacturing compound fertilizers - Google Patents

Abstract

A process for the preparation of N and P and/or K containing granular fertilizer products having better properties, wherein (a) contacting a finely divided N and/or P and/or K containing fertilizer with water produces a wet mixture, and (b) granulating the wet mixture into fertilizer granules, and optionally, (c) further processing said fertilizer granules to obtain said granulated fertilizer product. Stage (a) comprises: (i) dividing the finely sized fertilizer material into at least two parts, the first part constituting from about 10% to about 5% by weight of the whole material, (ii) contacting the first part with water to obtain a % weight water slurry, (iii) the slurry is contacted with the remainder of the raw material to obtain a wet mix with a water content of about 1% to 5% by weight.

Description

How to make compound fertilizer

The present invention relates to a method for preparing a granular fertilizer product containing N and P and/or K, wherein:

(α) contacting a finely divided fertilizer material containing N and/or P and/or K with water to produce a wet mixture, and

(b) granulating the wet mixture into a granular fertilizer, and optionally,

(c) further processing the above fertilizer granules to obtain said granular fertilizer products.

In nitrates containing NPK (N=nitrogen, P=phosphorous, K=potassium) several chemical reactions take place. These chemical reactions should be done during the manufacturing process to avoid further chemical reactions. These continuing post-reactions cause many undesirable phenomena such as reduced crushing strength, surface cracking, increased caking tendency, pulverization of the salt surface leading to dust generation during handling, and low wear.

The reaction between ammonium nitrate and potassium chloride results in the formation of potassium nitrate and ammonium chloride.

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In principle the reaction between ammonium nitrate and potassium chloride could be suppressed with non-reactive potassium chloride, but this approach is not attractive given the low availability of non-reactive potassium chloride. Slurries with relatively low water content can also be used to quench the reaction. However, in practice such stringent conditions make this method a very sensitive one.

Another common way to accomplish the reaction between ammonium nitrate and commercial potassium chloride is to use excess ammonium nitrate or excess potassium chloride to allow complete conversion of either potassium chloride or ammonium nitrate. Complete conversion can also be achieved, for example, by premixing ammonium nitrate and potassium chloride in a separate mixer before the granulator to increase the reaction time of the initial components. This type of method cannot be used in the case of a molar excess of potassium chloride because the mixture is too viscous to granulate.

Complete conversion can be achieved using finely divided KCl or using preheated KCl. However, these methods require high energy consumption and thus entail correspondingly high production costs.

A complete conversion of ammonium sulfate is usually required to prevent continued post-reaction. Formation of double salts such as 2NH ₄ NO ₃ ^* (NH ₄ ) ₂ SO ₄ , 3NH4NO ₃ ^* -(NH ₄ ) ₂ SO ₄ leads to particle agglomeration and degradation. The completion of the reaction can be promoted by pulverizing the ammonium sulfate. Another possibility is to work with anhydrous slurries. Adsorbed moisture accelerates the completeness of the reaction during handling and storage. The double salt thus formed has greater solubility in water, and mobile ions are easy to salt out from the particles. This leads to a high caking tendency.

In the case of using ammonium sulfate and potassium chloride, potassium sulfate will be produced according to the following equation.

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The generated potassium sulfate contains almost no unreacted, and generates a series of solid solutions (NH ₄ , K) ₂ SO ₄ . In the case of finely divided ammonium sulfate and potassium chloride, the reaction is completed during storage. When both components are larger crystals, the reaction proceeds at the surface of the crystals but if there is not enough water to accelerate the reaction, the reaction will not proceed further.

Ammonium double salt also forms a series of solid solutions 2(NH ₄ ,K)NO ₃ ^* (NH ₄ ,K) ₂ SO ₄ . This series of solid solutions is easier to dissolve in water than the original components. Due to the higher solubility, there are mobile ions to accelerate further reactions. When such compounds are present in compound fertilizers, they cause serious quality problems related to high caking.

The reaction between ammonium sulfate and ammonium nitrate or potassium nitrate can be handled in a similar manner as above.

In the nitric acid phosphate process rock phosphate is converted to a form of phosphorus which is useful to plants. This phenomenon occurs when nitric acid reacts with phosphate rock, at which point phosphorus is liberated as phosphoric acid. The resulting mixture of acids is neutralized to the desired extent with ammonium in a subsequent step. The calcium nitrate formed in these processes can be removed by crystallizing it as calcium nitrate tetrahydrate, but this has the disadvantage of an expensive crystallization treatment step and a large amount of by-products. Calcium can also be precipitated with acids (eg phosphoric acid) in a number of ammonia reactors, the disadvantages of this method being, for example, the need for many reactors and the need to store the ammonium of the acid, which implies high investment and complicated operation.

In the method of making compound fertilizers by steam granulation, solid raw materials are crushed and mixed together, followed by a granulation stage where steam or water is added. In these methods, granulation is sensitive to granulation conditions such as moisture content, temperature, etc., and the physical properties of the final product are poor. In the manufacture of nitrate-containing products, the continuous reaction of ammonium nitrate and potassium chloride for a long time results in strong crystalline bridges between the particles, indicating their strong tendency to agglomerate.

During granulation in the granulator, ammonia or ammoniated solution is reacted with sulfuric or phosphoric acid in the TVA ammonium granulator to produce a slurry. The disadvantage of this method is the need to supply liquid raw materials such as acid and ammonia.

Mechanical blends or bulk blends allow production in various proportions relative to their initial component contents. However, bulk blending is only a practical proposition as long as the fertilizer raw materials used in the preparation of bulk blended fertilizers are well granulated and the particle size distributions are not only close but very similar. Granules of urea, ammonium sulphate, potassium chloride, ammonium phosphate, diammonium phosphate having very narrow size ranges are difficult to obtain practically with conventional granulation equipment such as rotating drums or disks or cylindrical blenders. These factors affect the physical properties of the blended fertilizer, especially when the blending components are granular urea, granular ammonium phosphate, dense potassium chloride and coarse crystalline ammonium sulfate, different particle size distributions also cause non-uniform distribution in the field model.

In the compaction method, different raw materials are fed through a compactor and the raw material particles are compacted by pressure. The product properties are not good due to the high abrasion which leads to the formation of dust during product handling.

It has now been found that compound fertilizers having simplified process schemes and good physical properties can be obtained through improved manufacturing methods. The present invention relates to a process for the preparation of granular fertilizer products containing N and P and/or K, wherein

(a) contacting a finely divided fertilizer material containing N and/or P and/or K with water to produce a wet mixture, and

(b) granulating the wet mixture into fertilizer granules, and optionally

(c) further processing the above-mentioned fertilizer granules to obtain said granular fertilizer products,

Its essential feature lies in the contacting step:

(a) includes the following substeps:

(i) dividing the finely sized fertilizer material into at least two parts, the first part constituting from about 10% to about 50% by weight of the whole finely sized fertilizer material,

(ii) contacting a first portion of said finely sized fertilizer material with water and/or steam to obtain a slurry having a water content of 5% to 30% by weight; and

(iii) The slurry is contacted with the remainder of the finely divided fertilizer material to obtain a wet mixture having a water content of about 1% to 5% by weight.

Therefore, it can be seen that the above-mentioned disadvantages of the prior art methods can be avoided by a new method for preparing granular fertilizers containing N and P and/or P and/or K, wherein at least the finely sized fertilizer raw material is divided into Two parts, wherein the first part constitutes from about 10% to about 50% by weight of the whole finely sized fertilizer material, said first part being contacted with water to obtain 5% to 30% by weight of water. A water slurry which is contacted with the remainder of the finely divided fertilizer material to obtain a wet mixture with a water content of about 1% to 5% by weight.

The wet mixture is then granulated into fertilizer granules, and optionally the fertilizer granules are further processed to obtain the granulated fertilizer product.

Solid commercially available fertilizer materials can be used in the present invention. The fertilizer raw material is finely sized and preferably comminuted. Nitrate-containing NK and/or NP and/or NPK fertilizers are advantageous as the first part of the finely sized fertilizer raw material according to the invention. The first part, that is, the part that is in contact with water to obtain the water slurry can also be non-nitrate NP and/or NPK fertilizer containing ammonia. According to a preferred embodiment of the present invention, the NK or NP or NPK fertilizer comprises ammonium sulphate, potassium chloride, monoammonium phosphate and diammonium phosphate.

In the case of urea containing nitrogen, phosphorus and potassium, it is also preferred that the remainder of the finely sized fertilizer feedstock comprises urea.

In the method according to the invention, the finely sized fertilizer material is divided into at least two parts, the first part constituting about 10% to 50% by weight of the whole finely sized fertilizer material. Said first part of the finely sized fertilizer material is then contacted with water to obtain a water slurry containing 5% to 30% by weight of water. In this step, the finely divided fertilizer raw material is brought into good contact with water at 50 to 150° C., preferably in a separate dissolution vessel equipped with a heater and a stirrer. According to a particular embodiment of the invention, said first portion of finely sized fertilizer material is brought into contact with water in two secondary steps, preferably carried out in separate containers. At this stage, the aforementioned chemical reactions are reduced due to the low reaction potential of the prepared slurry. Most of the ammonium nitrate in the slurry has reacted and no further reaction occurs during storage and handling of the fertilizer.

After the slurry is formed, it is contacted with the remainder of the finely sized fertilizer material to obtain a moist mixture having a water content of about 1% to 5% by weight. This is followed by a granulation step of the moist mixture. The slurry is introduced into a granulator such as a rotating drum, cylindrical blender or similar. The contacting of the aqueous slurry with the remainder of the finely sized fertilizer material and said granulation stage are advantageously carried out in the same granulator. According to a specific embodiment of the present invention, the granulation step is carried out in a granulator at a temperature of 50 to 100° C. to agglomerate the finely sized fertilizer raw material into granules. Slightly crushed raw materials with typical _d50 values ranging from 50 to 800 [mu]m can be used for granulation in the present invention.

The granulation step is followed by further processing to obtain the granulated fertilizer product of the invention. Further processing may include a drying step to reduce the moisture content of the fertilizer granules, preferably to about 0.2% to 0.5% by weight. An additional cooling step may also be included to reduce the temperature of the fertilizer granules to about 30 to 55°C. Preferably, further processing may also include classification steps, such as sieving granular fertilizers. The fractionation step separates commercially suitable product in the size range from 1 to 10 mm, preferably in the size range from 2 to 5 mm. In this case it is particularly preferred to return undersized fertilizer particles to the step of contact between said aqueous slurry and the rest of the finely divided fertilizer raw material. The classifying step may also include returning the oversized fertilizer particles that have been crushed to a small particle size to the contacting step.

The invention is further illustrated in the following examples. These examples are for illustration only and do not limit the present invention.

Example 1: Nitrate-containing NPK fertilizer

225kg/t of NP30-10 and 100kg/t of diammonium phosphate are continuously conveyed to the reactor equipped with a mixer by a belt feeder. The temperature of the reaction vessel was maintained at 120°C and the moisture content was adjusted to about 10%.

The slightly non-viscous liquid mixture is passed through a cylindrical blender granulator at a temperature of about 60-75°C. Other raw materials, such as crushed (to about 750 μm) ammonium sulfate 236kg/t, potassium chloride 255kg/t, crushed monoammonium phosphate 163kg/t and filler 14kg/t are also continuously fed to the granulator.

The resulting granules with an outlet temperature of about 60° C. were dried to a moisture content of about 0.4%, sieved into a fraction smaller than 2 mm, a product fraction of 2-5 mm and a fraction larger than 5 mm, and the latter fraction was crushed. The smaller than 2 mm fraction and the crushed fraction are returned to the granulator as solid recycle material.

The stored product granules (NP ₂ O ₅ -K ₂ O 15-15-15) showed no tendency to agglomerate even after several weeks of storage. The resulting product had a crushing strength of about 35N.

Using NK as a raw material, the above-mentioned product was produced by the following steps. Continuously supply 253kg/t of NK (NK ₂ O 24-13) and 290kg/t of monoammonium phosphate to the mixing vessel equipped with a mixer by using a belt conveyor. The temperature of the reaction vessel was maintained at 74°C and a moisture content of about 9% with the addition of steam.

The slightly viscous slurry was passed to a drum granulator at a temperature of about 68°C. Crushed ammonium sulphate 290 kg/t and potassium chloride 197 kg/t were also continuously fed to this granulator.

The resulting granules are dried to a moisture content of about 0.4% and sieved out fractions smaller than 2 mm, product fractions 2-5 mm, and fractions larger than 5 mm, which are then crushed, fractions smaller than 2 mm and crushed Part is returned to the granulator as solid recycle material.

The stored product granules (NP ₂ O ₅ -K ₂ O 15-15-15) showed no tendency to agglomerate even after 8 weeks of storage. A slight introduction of moisture into the product does not increase the tendency to caking. The crushing strength of the pellets is about 60N.

Example 2: Nitrate-free NPK fertilizer

200 kg/t of diammonium phosphate and 133 kg/t of monoammonium phosphate were continuously supplied to a mixing vessel equipped with a mixer. Raw materials are continuously supplied with a belt conveyor. The temperature in the reaction vessel was maintained at 100°C and the moisture content was adjusted to about 12.5%.

The slightly viscous liquid mixture is passed through a drum granulator at a temperature of about 50-60°C. The rest including crushed monoammonium phosphate 79kg/t, crushed (about 750μm) ammonium sulfate 489kg/t and filler 99kg/t were also continuously fed to the granulator.

The resulting granules with an outlet temperature of about 60° C. were dried to a moisture content of about 0.3% and sieved out fractions smaller than 2 mm, product fractions 2-5 mm and fractions larger than 5 mm, which were then crushed. The smaller than 2 mm fraction and the crushed fraction are returned to the granulator as solid recycle material.

The stored product granules (NP ₂ O ₅ -K ₂ O 16-20-0) showed no tendency to agglomerate even after several weeks of storage. The crushing strength of the obtained pellets was about 30N.

Another nitrate-free species (NP ₂ O ₅ -K ₂ O 16-16-8) was prepared by the following steps. Continuously supply 350kg/t of diammonium phosphate to the mixing container equipped with a mixer with a belt conveyor. And steam is used to keep the temperature at 57°C and the moisture content is adjusted to about 18.5%.

The slightly viscous slurry was passed through a drum granulator at a temperature of about 50°C. Potassium chloride 137 kg/t and slightly crushed ammonium sulfate 494 kg/t are also continuously fed to this granulator as well.

The granules formed are dried to a moisture content of about 0.5 to 0.8% and sieved to form a fraction smaller than 2mm, a product fraction of 2-5mm and a fraction larger than 5mm, the latter being crushed. The smaller than 2mm fraction and the crushed fraction are returned to the granulator as solid recycle material.

The stored product granules (NP ₂ O ₅ -K ₂ O 16-16-8) showed no tendency to agglomerate even after several weeks of storage. The crushing strength is about 45N.

Example 3: NPK fertilizer containing urea

Continuously supply 150kg/t of diammonium phosphate and 258kg/t of monoammonium phosphate to the mixing container equipped with a mixer by belt conveyor. The temperature of the reaction vessel was maintained at 98°C and the moisture content was adjusted to about 15%.

The slightly viscous liquid mixture is passed to a drum granulator at a temperature of 50-60°C. Urea 55kg/t, crushed ammonium sulfate 106kg/t, potassium sulfate 404kg/t and magnesium sulfate 20kg/t are also continuously fed to this granulator.

The resulting granules with an outlet temperature of about 50° C. were dried to a moisture content of about 0.3% and sieved out fractions smaller than 2 mm, product fractions 2-5 mm and fractions larger than 5 mm, the latter being crushed. The smaller than 2 mm fraction and the crushed fraction are returned to the granulator as solid recycle material.

The stored product granules showed no tendency to agglomerate even after several weeks of storage. The resulting granules had a crushing strength of about 38N.

Example 4: NPK fertilizer with NPK-containing nitrate as raw material

Continuously supply 200 kg/t of diammonium phosphate and 275 kg/t of NPK (26-7-4) to a mixing vessel equipped with a mixer using a belt conveyor. The temperature in the reaction vessel was maintained at 125°C and the moisture content was maintained at about 12%.

The slightly viscous liquid mixture was passed into the granulator of the cylindrical blender at a temperature of about 60-65°C. Crushed ammonium sulphate 275 kg/t, potassium chloride 215 kg/t and filler 40 kg/t were also fed continuously to this granulator as well.

The resulting granules were dried to a moisture content of about 0.3% and sieved out fractions smaller than 2 mm, product fractions 2-5 mm, and fractions larger than 5 mm, which were crushed. The smaller than 2 mm fraction and the crushed fraction are returned to the granulator as solid recycle material.

The stored product particles (NP ₂ O ₅ -K ₂ O 16-11-14) showed no tendency to agglomerate even after several weeks of storage. The resulting pellets had a crushing strength of about 50N.

Claims (18)

1. A process for the preparation of a granular fertilizer product containing N and P and/or K, wherein

(a) Contacting finely divided fertilizer raw material containing N and/or P and/or K with water to produce a wet mixture, and

(b) granulating the wet mixture into fertilizer granules, and optionally

(c) Further processing said fertilizer granules to obtain said granulated fertilizer product,

characterized in that the contacting step (a) comprises the following sub-steps:

dividing the finely divided fertiliser material into at least two fractions, the first fraction constituting from 10% to 50% by weight of the total finely divided fertiliser material,

(ii) contacting a first portion of the finely divided fertiliser feedstock with water and/or steam to obtain a slurry comprising from 5% to 30% by weight water;

(iii) contacting the slurry with the remainder of the finely divided fertiliser material to obtain a wet mixture having a water content of from 1% to 5% by weight.

2. A method according to claim 1, characterised in that the first part of the finely divided fertilizer raw material is a NK and/or NP and/or NPK fertilizer containing nitrates.

3. A method according to claim 1, characterized in that said first portion of finely divided fertilizer material is an ammonia-containing non-nitrate NP and/or NPK fertilizer.

4. A process according to claim 1,2 or 3, characterized in that the remainder of the finely divided NK or NP or NPK fertilizer raw material comprises ammonium sulphate, potassium chloride, monoammonium phosphate and diammonium phosphate.

5. A method according to claim 1,2 or 3, characterized in that the remaining part of the finely divided fertilizer raw material comprises urea.

6. A method according to claim 1,2 or 3, characterised in that the first portion of finely divided fertiliser raw material is contacted with water in sub-step (ii) at a temperature of 50 to 150 ℃.

7. The process according to claim 6, characterized in that the contacting is carried out in a further dissolution vessel equipped with a heater and a stirrer.

8. A method according to claim 1,2 or 3, characterised in that the first portion of finely divided fertiliser material is contacted with water in two stages in sub-step (ii).

9. The method according to claim 8, characterized in that the contacting is carried out in a further vessel.

10. A process accordingto claim 1,2 or 3, characterized in that the granulation step (b) is carried out in a granulator at a temperature of from 50 to 100 ℃ to coagulate finely divided fertilizer material into granules.

11. A process according to claim 1,2 or 3, characterised in that the granulation step (c) and the sub-step (iii) of contacting the slurry with the remainder of the finely divided fertiliser material are carried out in the same granulator.

12. A process according to claim 1,2 or 3 characterised in that step (c) includes a drying step to reduce the moisture content of the granular fertilizer to 0.2 to 0.5% by weight.

13. A process according to claim 1,2 or 3 characterised in that step (c) includes a cooling step to reduce the temperature of the granular fertilizer to between 30 and 55 ℃.

14. A process according to claim 1,2 or 3, characterized in that step (c) comprises a fractionation step, the resulting products being recovered in a size range from 1 to 10 mm.

15. A method according to claim 14, characterized in that said size ranges from 2 to 5 mm.

16. The method of claim 13, wherein the classification step comprises recycling undersized fertilizer particles to said contacting sub-step (iii).

17. A method according to claim 13, characterized in that oversized fertilizer granules that have been crushed to smaller granule sizes are recycled to said secondary step (iii).

18. A method according to claim 14, characterized in that oversized fertilizer granules that have been crushed to smaller granule sizes are recycled to said secondary step (iii).