GB2237800A - Fertilizer particle and method of preparation - Google Patents

Abstract

A process for agglomerating particulate fertilizer materials in the presence of a fluid phase comprises carrying out the agglomeration in the presence of a lower hydrate of one or more salts or compounds which are agronomically acceptable and under such conditions that at least part of the lower hydrate is hydrated to a higher hydrate during agglomeration. The lower hydrate is preferably selected from the sulphates of calcium and magnesium, and the silicates of potassium, calcium and magnesium optionally in admixture with ferrous sulphate. The agglomeration may be carried out in a rotating drum. The invention also provides a fertilizer agglomerate comprising particles of one or more fertilizer ingredients e.g. ammonium sulphate, ammonium nitrate or urea dispersed within a matrix of calcium sulphate dihydrate.

Description

TITLE: FERTILIZER PARTICLE AND METHOD OF PREPARATION The present invention relates to a fertilizer particle and to a method for its preparation, notably to a fertilizer granule having a matrix of calcium sulphate and to a method for granulating fertilizer ingredients in the presence of a lower hydrate of a salt, notably calcium sulphate, under such conditions that at least part of the lower hydrate is hydrated to a higher hydrate during the production of the granules.

BACKGROUND TO THE INVENTION: Many fertilizer ingredients are used in the form of particulate materials which are applied in solid form to the land. However, it will usually be desired that the particles have a generally uniform and spherical shape and size so that they can be applied readily and at known rates through conventional fertilizer spreader mechanisms. To this end, many fertilizers are put up in the form of agglomerated particles having a particle size within the range 2 to 4 mms.

A typical method for producing such agglomerates comprises tumbling small particles in the presence of one or more liquid phases, notably water, a solution or melt of one or more of the ingredients and/or steam in a rotating drum, pug mill or other device in which a tumbling bed of the particles is formed.

Whilst this method can be used to form agglomerates or granules having adequate strength for subsequent handling from a wide range of mixtures of fertilizer ingredients, this method often cannot be used where formulations are to be made using highly water soluble materials, notably ammonium sulphate, potassium sulphate or ammonium nitrate. Particles from such materials are made either by compacting smaller particles together or by prilling melts of the materials.

Furthermore, water is usually required to provide the liquid phase which causes the particles to adhere initially to one another and subsequently forms crystal bridges between the particles as they are dried. This water must be removed from the product and this usually requires a separate drying step.

We have now devised a method for granulating fertilizer ingredients which enables granules to be produced from readily water soluble fertilizer materials which have adequate strength for subsequent handling. The method of the invention also reduces the amount of water which must be removed from the product to achieve a satisfactorily dry product.

SUMMARY OF THE INVENTION: Accordingly, the present invention provides a method for granulating one or more fertilizer components, which method comprises causing particles of the fertilizer component to agglomerate in the presence of a liquid phase in a tumbling bed of the particles, characterised in that a lower hydrate of calcium sulphate, for example anhydrous calcium sulphate and/or calcium sulphate hemihydrate, is present in the tumbling bed of particles and in that the particles are maintained under such conditions that at least part of the calcium sulphate is hydrated to calcium sulphate dihydrate.

The agglomerated particles contain the calcium sulphate dihydrate or other higher hydrate formed by the hydration of the lower hydrate substantially uniformly dispersed throughout the agglomerate, and we believe that the calcium sulphate dihydrate acts as a matrix to enhance the bonding together of the agglomerate. The invention therefore also provides an agglomerate comprising particles of one or more fertilizer ingredients dispersed within a matrix of calcium sulphate dihydrate.

As indicated below, the lower hydrate for use in the method of the invention and the higher hydrate which forms the matrix in the novel agglomerates of the invention may be selected from the hydrates of a wide range of salts or compounds.

However, for convenience, the invention will be described hereinafter with reference to the use of calcium sulphate.

Preferably, the fertilizer ingredient(s) being agglomerated are readily water soluble as compared to dicalcium monohydrogen phosphate dihydrate; and the calcium sulphate dihydrate matrix provides at least 10%, notably from 12.5% to 35%, by weight of the total granule.

The invention can be applied to the agglomeration of a wide range of fertilizer ingredients, but is of especial use in the agglomeration of compositions which contain a major proportion of a readily water soluble ingredient, notably compositions in which at least 70% of the nitrogen in the composition is provided by ammonium sulphate, ammonium nitrate or urea. For convenience, the invention will be described hereinafter in terms of the agglomeration of a mixture containing predominantly ammonium sulphate.

As indicated above, the agglomeration can be carried out in a number of ways, but is preferably carried out by tumbling a bed of solid particles in a rotating drum type granulator in the presence of water and/or steam to provide the liquid phase. For convenience, the invention will be described hereinafter in terms of such a granulation processs.

The granulation is carried out by feeding the desired solid particles to the rotating drum and supplying the liquid phase required to cause the particles to agglomerate to the desired extent. As is known in the art, the agglomeration is affected by the amount of the fluid phase and the temperature at which granulation is carried out. We prefer to use a solid feed material which does not present an excessive surface area to be wetted, for example we prefer to use a crystalline ammonium sulphate feed having a particle size in the range 0.5 to 2.0 mms.

The amount of water or steam required to achieve satisfactory granulation will also depend upon the temperature at which granulation is carried out. We prefer to use the minimum amount of added water required and to carry out the granulation at a temperature of from ambient to 90 C, preferably at from 60 to 850C. The optimum amount of free water or steam in the mixture being granulated can readily be determined for any given granulation process by simple trial and error, but will usually lie within the range 3 to 8% by weight. The water is preferably added by spraying water or steam onto the tumbling bed of particles within the granulation drum, by use of aqueous ammonium sulphate solution as part of the feed to the granulation drum and/or by passing steam into the tumbling bed of particles.

In the process of the invention, the particulate feed contains a lower hydrate of calcium sulphate, which is preferably fed as a powder or crushed crystalline solid feed, optionally premixed with part or all of the ammonium sulphate solid feed.

As indicated above, the calcium sulphate dihydrate formed at least in part during the granulation process preferably provides at least 10% by weight of the dried granular product.

Typically, it is preferred to provide from 15 to 25% by weight of the solid feed materials to the granulator in the form of calcium sulphate lower hydrate. However, if desired, calcium sulphate dihydrate may be incorporated in the feed material as a diluent or by virtue of the recycle of fines material, to adjust the fertilizer content of the granular product to the desired value. It will be appreciated that the calcium sulphate provides both calcium and sulphur and this needs to be accounted for when establishing the composition to be fed to the granulator.

The term lower hydrate is used herein to denote a material which is capable of taking up further water by way of water of hydration and which preferably also forms an higher hydrate which precipitates out, for example as interlocking crystals, to form the matrix within which the other fertilizer ingredients are bound. The term thus includes anhydrous calcium sulphate, calcium sulphate hemihydrate and mixtures thereof with themselves or with calcium sulphate dihydrate.

The lower the degree of hydration in the feed materials, the greater the amount of water it will take up on re-hydration, and hence the lower the amount of water which needs to be removed during any drying stage to produce a dry granule.

However, the conditions required to ensure adequate hydration during granulation of anhydrous calcium sulphate may be detrimental to the granulation process, for example they may require excessive temperatures or acidity or require excessive time. It is therefore preferred to use a calcium sulphate feed material which has an average formula CaSO4.nH20, where n has a value of less than 2, notably from 0.5 to 1.0. As indicated above, this is preferably provided by calcium sulphate hemihydrate, which can be of commercially available form and purity.

The calcium sulphate lower hydrate undergoes at least partial hydration during the granulation process to form calcium sulphate dihydrate which precipitates as interlocking crystals. This has the effect of forming the matrix throughout the granule within which the other ingredients are bound to enhance the granule strength. The hydration also has the effect of taking up at least part of the water fed to the granulation process so that less water needs to be removed from the final product to give a stable dry product.

The rate of hydration is affected by temperature, the water content and composition of the mixture being granulated. If desired an accelerator, for example an acid such as phosphoric and/or sulphuric acid or seed crystals of calcium sulphate dihydrate, can be incorporated into the mixture being granulated to achieve an adequate rate of hydration under the granulation conditions used. However, it will usually be desired that at least part of the take up of the free water by hydration of the lower hydrate occurs after the product has left the granulation vessel, for example in a subsequent maturing stage and/or in a drier.

In place of part or all of the calcium sulphate lower hydrate described above, lower hydrates of other materials which are acceptable in fertilizer compositions can be used. Such hydrates preferably also form a higher hydrate which will precipitate out to form the matrix in the granule. In order to achieve the precipitation from the liquid phase of the mixture being granulated, we prefer to use materials which yield a higher hydrate having a solubity of less than 4 gs per litre at 250C. Examples of other materials include magnesium sulphate and silicates of potassium, calcium or magnesium.However, it is within the scope of the present invention to use a mixture of materials, one of which provides a lower hydrate to take up part of the free water to reduce the load on the drier, the other of which undergoes hydration during granulation to provide the matrix forming materials-, for example a mixture of ferrous sulphate with calcium sulphate hemihydrate.

The invention therefore provides in its broadest aspect a process for agglomerating particulate fertilizer materials in the presence of a fluid phase, which method comprises carrying out the agglomeration in the presence of a lower hydrate of one or more salts or compounds which are agronomically acceptable and under such conditions that at least part of the lower hydrate is hydrated to a higher hydrate during granulation by the take up of water from the fluid phase.

Preferably, the lower hydrate also hydrates to a higher hydrate which is less soluble than the initial hydrate so as to form a matrix of the higher hydrate substantially uniformly dispersed throughout the agglomerated product.

As indicated above, the granulation is carried out under such conditions that at least part of the hydration occurs during the granulation process. In theory, it would be possible to ensure that the amount of water fed to the granulation process and the conditions under which granulation took place were selected so that substantially all the free water fed to the process was taken up by the hydration of the lower hydrate.

However, this is usually impractical and may require excessive residence time of the material within the granulation drum.

It is therefore preferred to dry the material issuing from the granulation drum to provide a dry, storage stable material using conventional drying techniques. If desired, the material from the granulation drum and/or from the drier can be cooled and/or stood to permit hydration of the calcium sulphate to go to substantial completion before any subsequent treatment of the granular product.

The product is a granule comprising the crystals or other particles of the fertilizer material in a matrix of calcium sulphate dihydrate substantially uniformly dispersed throughout the granule. If desired, other materials may be present in the granule, for example pesticides or other agronomically acceptable materials, or the granule can be coated, for example with an oil, amine or a clay dust to improve its handling and/or storage properties.

DESCRIPTION OF THE DRAWING: The invention will now be described with respect to a preferred embodiment thereof and with respect to the accompanying drawing which is a block diagram of the granulation process of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT: A 15:22 N:S feed material comprising the following is fed to the inlet of a rotating drum granulator 1: Ammonium Sulphate: 71.43 parts Calcium sulphate hemihydrate: 15.00 parts Calcium sulphate dihydrate: 8.34 parts The ingredients can be fed separately or can be mixed in a premixer 2. Steam (8 parts) is fed to the tumbling bed of particles via a sparger pipe and spray outlets 3 located above the bed within the drum 1. The drum 1 can carry internal flights 4 or other devices which aid the tumbling of the material within the drum. The temperature of the material in the drum 1 is maintained at between 60 and 800C by the use of steam and recycled material.

The material leaving the granulator drum 1 is passed directly to a rotary drum type drier 5 which is heated using conventional methods to from 80 to 1000C.

The material leaving the drier 5 contains a large proportion of particles in the desired size range of 2 to 4 mms and is passed to a screen 6 where the fine material (typically those particles with a size less than 1.7 mms) is removed and can be recycled to the granulator drum 1. The remaining material, containing mostly particles in the desired size range, but also some particles greater than 4 mms, is passed to a rotary cooler 7 where the temperature is reduced from an ex-drier temperature of about 900C to a temperature of about 400C.

The material from the cooler is passed to a screen 8 where the oversize material is removed. The product then passes to store or to further processing, for example to a coating process. The oversize material is fed to a crusher 9 to reduce its size ' to one suitable for re-cycling to the granulator drum 1.

In the above process the free water content of granules produced using a conventional process would be approximately 8% and this water content must be reduced to about 18 if the product is to be starage stable. However, with the process of the invention the free water content is reduced to 4.27% with the remaining 2.73% held as water of hydration. The amount of water to be removed during the drying stage is thus reduced by 39%, representing a saving in heat energy and capital costs in the design and operation of the drier.

The granules produced by the above process have calcium sulphate dihydrate crystals dispersed uniformly throughout the granules and acting as a matrix for the other components of the granule. The granule has a crushing strength similar to that of a commercially available granular compound fertilizer.

When the hemihydrate was omitted from the mixture fed to the granulator, little or no granular product was obtained.

Claims (13)

CLAIMS:

1. A process for agglomerating particulate fertilizer materials in the presence of a fluid phase, which method comprises carrying out the agglomeration in the presence of a lower hydrate of one or more salts or compounds which are agronomically acceptable and under such conditions that at least part of the lower hydrate is hydrated to a higher hydrate during agglomeration.

2. A method as claimed in claim 1 wherein the lower hydrate hydrates to a higher hydrate which is less soluble than the initial hydrate so as to form a matrix of the higher hydrate substantially uniformly dispersed throughout the agglomerated product.

3. A method as claimed in either of claims 1 or 2 wherein the higher hydrate has a water solubility of less than 4 gs per litre at 250C.

4. A method as claimed in any one of the preceeding claims wherein the lower hydrate is selected from the sulphates of calcium and/or magnesium, the silicates of potassium, calcium and/or magnesium, optionally in admixture ferrous sulphate.

5. A method for granulating one or more fertilizer ingredients, which method comprises causing particles of the fertilizer ingredient to agglomerate in the presence of a liquid phase in a tumbling bed of the particles, characterised in that a lower hydrate of calcuim sulphate is present in the tumbling bed of particles and in that the particles are maintained under such conditions that at least part of the lower hydrate is hydrated to calcium sulphate dihydrate during the agglomeration process.

6. A method as claimed in any one of the preceeding claims wherein the lower hydrate has the formula CaSO4.nH20 wherein n has a vals of from 0.5 to 1.0.

7. A method as claimed in any one of the preceeding claims wherein the lower hydrate is present in from 10 to 35% by weight of the dry weight of the mixture being agglomerated.

8. A method as claimed in any one of the preceeding claims wherein the fertilizer ingredient(s) being agglomerated have a water solubility greater than that of dicalcium monohydrogen phosphate.

9. A method as claimed in any one of the preceeding claims wherein at least 70% of the nitrogen content of the fertilizer ingredients is provided by ammonium sulphate, ammonium nitrate and/or urea.

10. A method as claimed in any one of the preceeding claims wherein the agglomeration is carried out at a temperature of from 60 to 850C and at a free water content of from 3 to 8% by weight.

11. A method for agglomerating particulate fertilizer ingredients substantially as hereinbefore described with respect to and as shown in the accompanying drawing.

12. An agglomerate comprising particles of one or more fertilizer ingredients dispersed within a matrix comprising calcium sulphate dihydrate.

13. An agglomerate as claimed in claim 12 when produced by a method as claimed in any one of claims 1 to 11.