AU749213B2 - A coating process for fertilizers - Google Patents

Description

Regulation 3.2

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

ORIGINAL

Name of Applicant: Hi-Fert Pty Ltd Actual Inventors: Karl Heinrich Walter Roslyn Jane Baird Address for Service: MADDERNS, 1st Floor, 64 Hindmarsh Square, Adelaide, South Australia, Australia Invention title: "A COATING PROCESS FOR FERTILIZERS" Details of Associated Provisional Application No: PP 2105 dated 2nd March 1998 The following statement is a full description of this invention, including the best method of performing it known to us.

o*° Naeo plcn: iFr t t Acua netos KalHirihWie Ro*y Jan Bar Adrs fo Sevc:C-MDEN,1tFlo,6 idas*qae Adlie*ot usrla utai Inetontte OTNGPOES O ETIIES Deal ofAscae rvsoa plctonN:P 15dtd2 d Mrh19 Th folwn sttmn isafl ecito fti netoicuigtebs meho of pefrigi konts I YI- The following invention relates to a method of coating a readily-available, finelyground elemental sulphur onto the surfaces of granular fertilizers, but especially onto granular monoammonium phosphate.

BACKGROUND TO THE INVENTION Until about 40 years or so ago, the mineral plant nutrient sulphur received, despite its importance in the metabolic functions of plants, very little attention from agronomists or manufacturers of fertilizers. This was mainly due to the fact that: the symptoms of sulphur deficiencies resemble those of nitrogen deficiencies and were often explained as nitrogen deficiencies, low analysis fertilizers often contained relatively large quantities of thids nutrient, prior to the 1960's, sulphur deficiencies would only occur in localized areas throughout the world, the high :degree of air pollution would supply enormous quantities of atmospheric sulphur to soils, and many of the pesticides employed contained sulphur (ie lime sulphur, Bordeaux mixtures etc)- Deficiency of the mineral nutrient sulphur has a very noticeable retarding effect upon the growth of the plant. As mentioned before, the symptoms of sulphur deficiencies are very similar to those encountered in nitrogen-deficient plants, meaning that the plants are uniformly chiorotic, exhibiting a spindly, thin-stemmed and stunted growth.

In addition to these direct deficiency effects of sulphur, the work performed by L B Fenn and Associates at the Texas Agricultural Experiment Station, at El Paso, shows that the addition of incremental sulphur to very calcareous soils increases the waterextractable and desorbable Ca, Mg and P content of those soils [L B Fenn, R M Taylor and C A Pety Jr, (1987) J Plant Nutrition, 10. 26-22811. While Fenn's work, and especially Flocker's work, appears to be somewhat absurd, since it shows that plants grown on non-calcareous soils absorb more calcium than those grown on highly calcareous soils [W J Flocker and W H Fuller, (1956), "Availability of calcium in calcareous soils", Soil Sci Soc Amer Proc, 20 38-391], in regard to these findings, it behooves us to remember that calcium carbonate is only sparingly soluble and that the solubility products for the various calcium carbonates are as follows: Ksp CaCO3 2.8x10- 9 Ksp Aragonite 6.OxlO- 9 and Ksp Calcite 4.5xl10 9 It is thought that this sub-optimal availability of Ca in these type of soils is due to the fact that plant roots do not produce sufficient CO 2 through respiration, to produce adequate quantities of Ca ions. In addition, Fenn was able to demonstrate that the presence of Ca, Mg and K ions in the soil not only stimulates the uptake of N by plants, but also considerably reduces the loss of nitrogen in calcareous soils.

In view of these findings, it is not surprising that a number of workers in the field of fertilizer technology have concentrated their efforts into the incorporation of elemental sulphur into granular high analysis fertilizers. With regard to these ::::technologies, the following two types of coating processes are disclosed in various :earlier patents: Coating by Means of Spraying Molten Sulphur onto the Surface of Granular Fertilizers, especially Urea.

In contrast to most plant nutrients, which are mainly held back by physicochemnical process in the root zone of soils, nitrogen is extremely mobile and is easily leached out of the root zone. The aim of these coating processes is to reduce the losses of nutrients by the spraying of molten sulphur, molten wax or bitumen onto the fertilizer granules. The sulphur or other molten substances solidify to form a nearly impervious shell on the surface of the granules, restricting the release of nutrients from the granule. Typical examples of these coating processes were disclosed by P S Fleming, US Patent No 3576613, 27/ 04/1971; J L Smith, US Patent No 4032319, 28/ 06/1977; W R Ali, US Patent No 4081264, 28/ 03/ 1978 etc. The congealed sulphur, having a relatively low surface area, will only be slowly oxidised by the thio-bacilli in the soil and will only become available over a period of several years.

The Establishment of a Tenaciously Adhering Coating containing Readily Available Sulphur in a Finely Divided State.

The aim of these coating processes is to generate, on the surface of the granules, a matrix containing finely-divided sulphur, which will be readily oxidised by the thiobacilli in the soil and thus be readily available to the plant. The following patent specifications disclose typical examples of this technology: In Australian Patent No 601099, K H Walter teaches the coating of granular fertilizers using elemental sulphur, with the aid of ammonium sulphate as a coating additive.

This method has been found to work very well with single, double and triple superphosphate. The resulting coating on these types of fertilizers adheres very strongly and has a high abrasion resistance. However, ammonium phosphates do not lend themselves very well to this method of coating.

ooooo Lefroy et al in International Patent Specification No PCT/AU91/00459 suggest the use of water-soluble adhesives, such as PVA, sodium lignosulphonate or calcium lignosulphonate, for the production of coated fertilizer granules. Unfortunately, the coating established by this method requires an additional drying step for the establishment of a strongly adhering coating. This is not only expensive, but poses possible problems with dust explosions.

In Australian Patent Application No 52601/93 (669403), J D Johnston claims a process for producing a coated granular fertilizer product by mixing fertilizer granules with a dilute mineral acid, a desired quantity of a component including sulphur and/or at least one supplemental micronutrient, and an acid neutralising agent, thereby causing liberation of heat energy which removes water as water vapour and at the same time causing precipitation of at least one reaction product of the acid and neutralising agent which binds the component to the surface of the granules in the form of an adherent coat. Unfortunately, the heat of reaction generated is far too low to evaporate the quantity of water introduced into the system by using dilute mineral acids, meaning that an additional drying step would have to be incorporated, and -B n furthermore the resulting product would, as a result of using a non-nutrient neutralising agent for the neutralisation of the acid, have a very low nutrient content.

In Australian Patent Specification No 45576/96, Hi-Fert Pty Ltd discloses the use of MgO and (NH 4 2 HP0 4 for the sulphur-coating of urea. The coat formed by this operation does not adhere very tenaciously to the urea granule.

Sulphur coated onto the surface of di-ammonium phosphate (DAP) with MgO as a coating additive, as disclosed in Australian Patent Application No 77314/98, forms an abrasion-resistant, tenaciously adhering coat. This is due to the fact that the MgO will immediately react with water, forming the rather reactive Mg(OH) 2, which in turn will react with the DAP, forming in situ the heavily hydrated ammonium struvite. The matrix of struvite [(NH4)MgPO4.6H 20] will encapsulate the granules, and will embed the finely divided sulphur.

Unfortunately, neither ammonium nor potassium struvite can be formed in situ on the surface of mono-ammonium phosphate (MAP), which normally has a pH of about 4.5. Since both of these struvites will only form if the pH is above 7.5, we looked for other compounds which might be able to form, in situ, a strongly adhering matrix into which sulphur could be embedded. In our search for a better adhering sulphur coating, we investigated fully other complex magnesium ammonium phosphates, such as hannayite [Mg3(NH4)2(HPO4) 4 .8H 2 0] and schertelite, and found that these two compounds were useless as far as coating of MAP was concerned.

In our search for better coating systems, we have found that CaO, Ca(OH)2 and even CaSO4 are excellent coating additives, not only for the sulphur coating of MAP, but also for such granular fertilizers as di-ammonium phosphate (DAP), single superphosphate (SSP), double superphosphate (DSP), triple superphosphate (TSP), compacted Sulphate of Ammonia (SoA), Sulphate of Potash (SoP), as well as Langbeinite (K-Mag), which is a mineral [K2SO4.2MgSO 4 found in many potash deposits of the world.

ra-~4r Th7s~~--~r~w SUMMARY OF THE INVENTION Accordingly, the present invention relates to a system or method for the production of coated granular fertilizers having elemental sulphur incorporated in the coating, said coating system or method being characterised by the use of calcium oxide (CaO), calcium hydroxide (Ca(OH)2) or calcium sulphate (CaSO 4 as a coating additive. It should be noted that references to calcium sulphate, throughout the present specification and claims, are (where appropriate) inclusive of hydrates, such as the hemihydrate (Plaster of Paris) and the dihydrate.

The present method provides a method for coating a base fertilizer, being a granular phosphate-containing or sulphate-containing fertilizer, with a coating incorporating elemental sulphur. The coating method comprises: placing granules of the base fertilizer in a mixing-cum-coating device, adding water or an aqueous solution comprising soluble plant nutrient(s), and adding a mixture of sulphur and a coating additive selected from calcium oxide, calcium hydroxide and calcium sulphate. The terms "granular" and "granules", as used throughout the specification and claims, include within their scope prilled products.

The base fertilizer may be, for example, mono-ammonium phosphate (MAP), diammonium phosphate (DAP), single superphosphate (SSP), double superphosphate (DSP), triple superphosphate (TSP), Sulphate of Ammonia (SoA), Sulphate of Potash (SoP) or Langbeinite (K-Mag), and is preferably porous.

DETAILED DESCRIPTION OF THE INVENTION The addition of CaO, Ca(OH)2 or CaSO4 to such sulphate-containing fertilizers as SoA, SoP and K-Mag will form, in situ on the surface of the fertilizer granules, CaSO4.2 H 2 0 and some complex double salts according to: Ca(OH) 2 2H20 K2S04 CaSO 4 .2H 2 0 2KOH [1] Ca(OH) 2 2K 2 SO4 H 2 0 CaK 2

(SO

4 2 .H20 2KOH [2] 2Ca(OH) 2 4(NH 4 2

SO

4 2Ca(NH 4 2

(SO

4 2.H20 4NH 3 2H 2 0 [3] In the case of non-phosphatic fertilizers, the reactions between the fertilizer and the additive, which might be CaO, Ca(OH)2, or even CaSO 4 (eg Plaster of Paris: CaSO4 1/2H 2 0) as a source of Ca, are rather more complex than indicated in equations and We expect that the following compounds can be formed during the coating of these sulphate-containing fertilizers: GYPSUM: CaSO 4 .2H 2 0 is widely distributed in nature and is commonly present in evaporites.

SYNGENITE: CaK 2 (S0 4 2

.H

2 0 is found in nature in evaporites, especially in potash •deposits, and is formed in mixed fertilizers. It forms an isomorphous series with koktaites.

KOKTAITE: Ca(NH4) 2

(SO

4 2

.H

2 0 is normally found in gypsum and anhydrite deposits in evaporites.

POLYHALITE: Ca2K2Mg(SO4) 4 .2H 2 0 is generally found in nature in potassium salt deposits.

We have found that CaO, Ca(OH)2 and CaSO4 are excellent coating additives for the coating of sulphur onto granular MAP. In contrast to Mg, the doubly charged Ca ion does not react with ammonium phosphates or alkali phosphates to form highly hydrated mixed phosphatic salts, which are stable at ambient temperatures. These double salts of alkali phosphates with doubly charged metal ions have been known for over one hundred years [H Rose, (1849) Annal Physik, 77, 292].

The hepta-hydrate of calcium ammonium phosphate [CaNH 4

PO

4 .7H 2 0] is only stable at very low temperatures and will decompose at ambient temperatures into calcium phosphates and ammonia [A W Frazier, J R Lehr and J P Smith, (1964) J Agr Food Chem, 12, 198-201].

After considering all aspects, we have come to the conclusion that the following compounds will most likely be formed during coating of MAP with sulphur, employing CaO or Ca(OH)2 as a coating additive. The type of compound which will be formed will be a function of the pH, the temperature and the concentration of the liquid phase.

MONOCALCIUM AND DICALCIUM PHOSPHATE: 2(NH 4

)H

2 P0 4 +CaO+2H 2 0 Ca(H2PO4) 2. H 2 0 2NH 4 0H [4] (monocalcium) a.

NH

4

H

2

PO

4 Ca(OH)2 H 2 0 CaHPO 4 .2H 2 0 NH 4 OH (dicalcium) HYDROXYAPATITE: Caio(P0 4 6

(OH)

2 Despite the fact that the pH prevailing on the surface of an MAP granule would generally be considered as too low to form hydroxyapatite, we believe that the high concentration of calcium hydroxide ions on the surface could lead to the formation of this microcrystalline compound. We base this belief upon the fact that, during this coating operation, the resulting coat dries off considerably faster than in any other coating operation. We think that this is due to this precipitated phosphate having such a small particle size that it starts to behave like a colloid, meaning that it can sorb enormous quantities of water, without getting sticky. It is this phenomenon which is responsible for the widespread use of precipitated calcium phosphate as an inhibiting agent against caking of hygroscopic and sticky materials. This compound, which is (from a thermodynamic point of view) one of the most stable calcium phosphates, will be formed as follows: 6 NH4H 2 P0 4 10 Ca(OH)2 Cal0(P04)6(OH) 2 6NH3 18 H 2 0 [6] CALCIUM AMMONIUM PHOSPHATE: CaNH 4

PO

4

.H

2 0 This compound is found in ammoniated fertilizers and is a reaction product of ammonium phosphates with calcareous minerals.

NH

4

H

2

PO

4 Ca(OH)2 CaNH 4

.PO

4

.H

2 0 H 2 0 [7] DICALCIUM MONOAMMONIUM HEPTAHYDROGEN PHOSPHATE: Ca2NH 4

H

7 (PO4)4.2H 2 0 This acidic salt is formed as follows: 4 [NH 4

H

2

PO

4 2CaO Ca2NH4H7(PO)4.2H20 +3NH3 [8] MONOCALCIUM DIAMMONIUM MONOHYDROGEN PHOSPHATE: Ca(NH 4 )2(HPO 4 )2.H20 This compound is stable in air and is found in mixed fertilizers, as well as in soils.

NH

4

H

2

PO

4 Ca(OH)2 Ca(NH 4 )2(HPO 4 2

.H

2 0 H 2 0 [9] When considering these reactions involving phosphates, it must be remembered that all phosphatic high analysis fertilizers are manufactured from commercial phosphoric acid, which (depending upon the type of phosphate rock which was used in its manufacture) will contain certain impurities (notably, sulphates, silicofluorides, fluorides, Al, Fe, Mg and organic matter). Since many of these impurities will react with the coating additive the reactions taking place are much more complex than indicated in equations to We have found in our work that it is possible to use this method of coating, which employs CaO, Ca(OH) 2 or CaSO4 for the establishment of a strongly and tenaciously adhering coating, to hold in place up to 20%w/w of agronomically available elemental sulphur in a fine state of division.

9 This coating method will, however, work best if the granular fertilizer is porous, and was not manufactured by the tumbling of large crystals.

The coating operation, for the incorporation of elemental sulphur onto these granular fertilizers, is best performed in equipment normally employed in agglomnerative granulation processes. The fertilizer granules are preferably porous, with the term "porous" meaning that each granule does not consist of one large crystal, but was produced by either a conventional wet agglomerative granulation process or a dry compaction process. These types of processes will always produce granules having C porous surfaces, which are easier to coat than the smooth surfaces of large crystals.

to*:' The most commonly employed agglomerative granulation devices are the rolling drum granulator, and the inclined pan or disk granulator. While there are many 15 other agglomeration devices, such as the "Sackett Star Granulator", the "Falling Curtain Granulator" etc, which could be employed for the coating of fertilizers, these devices may be regarded as mere modifications of the conventional drum granulator.

W~hile it is possible to obtain excellent coating results with a pan granulator possessing a pan diameter to pan rim height ratio less than 2, we consider the drum granulating device, which is equipped with annular rings, to be superior to any other granulation or coating device.

We employed, in our production scale work, a granulating device which met the following design specifications: DRUM DESIGN PARAMETERS: Drum length to drum diameter ratio 2.4/1 Annular ring height to radius ratio >0.3/1 Angle of decline of drum <1.50 Nominal fractional bed volume as of total drum volume Coating time >15 minutes Rotational speed of drum >40% and <60% of N,, Actual length of drum Actual rotational speed of drum 48.6% of Nc or 13.0 rpm Nc,, is the critical speed, which is defined as the speed of rotation at which the centrifugal force acting upon a granule, at a distance of from the centre of the drum, equals the gravitational force on the said granule when it is in the zenith of its rotation.

This means that: g r) 2 •15 whereby g is the acceleration due to gravity (=9.81 msec- 2 2r represents the diameter of the drum in metres; and o is the angular velocity in radians per second. Therefore, the critical speed is as follows: CRITICAL SPEED Nc, 42.317/i2r rpm We have found that a coating drum meeting these design specifications will produce a cascading bed of granules, which has ideal mixing characteristics for coating operations and, because of its depth, will continuously subject the granules in this cascading, as well as tumbling, bed to a considerably greater pounding than in a shallow bed, thereby ensuring that the coating formed on the surface of the granules is not only densified, but also pushed into the pores of the granules. These densifying mechanical forces are of great importance, if the coating water consumption is to be kept to a minimum, and the coating applied to the granule should be smooth and tenaciously adhere to the finished granule. Despite the fact that it is possible to establish a coating in a shallow bed, the quality of the finished coat is much poorer than that established in a deep bed. This is partly due to the fact that, in a shallow 1 bed, the coating water requirements are considerably greater than those in a deep bed, which means that the voids between the individual crystallites formed, during the reactions taking place during the coating operation, will be considerably larger than the voids in a deep cascading bed of granules.

The crystallites in a deep bed drum coating device are densified to such an extent that they are held together by the considerably stronger forces of cohesion, and not by the much weaker forces of adhesion originating from the voids filled with liquid, as is the case in a shallow bed.

EXAMPLES

The invention will now be described in more detail with respect to the following •Examples, which are illustrative but not restrictive of the present invention.

15 RAW MATERIAL SPECIFICATIONS:

FERTILIZERS:

The fertilizers complied with the following requirements: that they were produced by a size-enlargement process, which will produce a porous granular particle having a total volume of voids varying between and that the size spectrum of the fertilizer lies between <6mm and >2mm. (The size spectrum of the granulate is not a very important factor.)

SULPHUR:

Since the main aim of this sulphur coating process is to provide the plant with the nutrient sulphur, and since plants can only take up sulphur in an ionic form, the elemental sulphur has first to be oxidised in the soil and to be converted into its water-soluble form. This conversion from the elemental form into a water-soluble ionic form can occur as a result of auto-oxidation of elemental sulphur in the soil in the presence of oxygen and moisture, or by thio-bacilli in the soil, which utilize the energy liberated during this oxidation in their metabolic processes.

Whatever the actual process of conversion, the sulphur to be coated onto the surface of the fertilizer has to be in a fine state of division or, in other words, has to have a large surface area to be readily oxidized.

The sizings of sulphur to be used were as follows: SIZINGS OF SULPHUR: 99.5%w/w <0.25mm 97%w/w <0.15mm 92%w/w <0.075mm 78%w/w 15 HYDRATED LIME: The hydrated lime used in our testwork met the requirements of the Australian Standard No AS 1672.

In our pilot-plant batch coating device, which can coat about 10kg of fertilizer per batch, as well as in our production plant, in which we can coat 10 to 15 tph of fertilizer, we were able to demonstrate that it is very important to add about 70% of the total water before the ground sulphur containing the hydrated lime is added.

Under no circumstances can the sulphur cum additive be added first. In comparison to the fertilizer, the sulphur cum additive has a very large surface area, which means that it will severely segregate in the tumbling bed and because of its larger surface area will take up more water than the fertilizer, and the wetted sulphur, as well as calcium hydroxide particles, will coalesce with wet <2mm particles of the fertilizer and form small granules. The resulting product will consist of poorly coated fertilizer granules and small granules consisting of sulphur additive and fertilizer fines.

The sprays employed for the incorporation of the water should be sufficiently fine to ensure a proper distribution of the coating water over the cascading bed of fertilizer granules, but not so fine that spray drift will occur and that the inside of the coating drum will be wetted.

In the following Examples, all percentages are on a weight basis, and the coating water is expressed as a %w/w of the total solids content.

EXAMPLE 1 In this testwork in a pilot plant, mono-ammonium phosphate (MAP) was coated with varying quantities of coating additives and of sulphur.

The following table summarises the work performed in the pilot plant: COATING TIME 15 minutes.

Test No MAP Ca(OH)2 Sulphur H 2 0 %w/w %w/w %w/w %w/w 1.1 88.3 1.0 10.7 3.2 1.2 87.3 2.0 10.7 4.8 1.3 78.0 2.0 20.0 An evaluation of these coated products showed that the coat established by this method possessed excellent abrasion resistance and was superior to the coat produced by any other coating method.

EXAMPLE 2 In the following testwork, the fertilizer which was coated was Langbeinite (K-Mag).

These tests (and those of Examples 3 and 4) were again performed in the small pilot batch coating device.

Here again, the resulting coating was excellent.

EXAMPLE 3 In the following testwork, the fertilizer which was coated was Sulphate of Ammonium (SoA).

Test No SoA Ca(OH)2 Sulphur H 2 0 %w/w %w/w %w/w %w/w 3.1 88.0 1.0 11.0 4.1 3.2 87.0 2.0 11.0 4.2 3.3 78.0 2.0 20.0 4.6 The coating adhered tenaciously to the surface of the granules.

EXAMPLE 4 In the following testwork, the fertilizer which was coated was Sulphate of Potash (SoP).

Test No SoP Ca(OH)2 Sulphur H 2 0 %w/w %w/w %w/w %w/w 4.1 88.0 1.0 11.0 4.2 87.0 2.0 11.0 4.8 4.3 78.0 2.0 20.0 4.8 I~ The coated product could only be described as being excellent.

EXAMPLE In the following testwork, the fertilizer which was coated was single superphosphate

(SSP).

Granular single superphosphates (SSP) vary in their granulometry and chemistry depending on their origin. Three different sources were used in our testwork, with all products lying within the specifications below, and all producing quality product.

80-100%w/w between 1 and 5 mm sized particles and typically analysed as: Phosphorus (Total) 8.8-9.2%w/w Phosphorus (Available) 8.6-8.9%w/w 15 Sulphur as Sulphate 11-13.2%w/w The addition of CaO or Ca(OH)2 to SSP will form, in situ on the fertilizer granules, monocalcium phosphate (Ca(H 2 P0 4 2

.H

2 0) and/or dicalcium phosphate (CaHPO 4 .2H 2 according to the following formulae: 2H 3 P0 4 Ca(OH)2 Ca(H 2 P0 4 2

.H

2 0 H 2 0 Phosphoric acid Calcium hydroxide Monocalcium phosphate Together with the monocalcium phosphate already present in SSP: Ca(OH)2 H20 Ca(H 2 P0 4 )2.H 2 0 2CaHPO 4 .2H 2 0 [11] Calcium hydroxide Monocalcium phosphate Dicalcium phosphate 3Ca(H 2 P0 4 2 *H20 7Ca(OH)2 Cao(P0 4 6

(OH)

2 15H 2 0 [12] Monocalcium phosphate Calcium hydroxide Hydroxyapatite The sulphur is embedded in this tenaciously adhering coating of monocalcium phosphate and/or dicalcium phosphate and/or hydroxyapatite.

T

The following table summarises the work performed in the pilot plant. The coating time was 15 minutes.

Test No SSP Ca(OH)2 Sulphur H 2 0 %w/w %w/w %w/w %w/w 5.1 89 1.0 10 5.2 79 1.0 20 10.0 5.3 78 2.0 20 12.0 These trials resulted in coatings which had superior fugitive dust levels than for any of the other coating methods.

EXAMPLE 6: Production Scale Run In a production run employing the continuous coating drum described earlier in this specification, MAP was coated under the following conditions: Feed rates:

MAP

Sulphur Water 8.9 t/h 1.1 t/h 230 1/h 2.3%w/w The sulphur employed in this production run contained 10%w/w of hydrated lime.

The product produced in this production run was submitted to extensive tests in the production plant as well as in the field. The results of these tests showed that this coating had a good abrasion resistance.

While the present invention has been described in terms of preferred embodiments in 11 order to facilitate better understanding of the invention, it should be appreciated that various alterations or modifications can be made without departing from the principles of the invention. Therefore, the invention should be understood to include all such alterations or modifications within its scope.

*e *o ~0~_111

Claims (10)

1. A method for producing a fertilizer coated with a coating incorporating elemental sulphur, the base fertilizer being a granular phosphate-containing or sulphate-containing fertilizer, said method comprising: placing granules of said base fertilizer in a mixing-cum-coating device, adding water or an aqueous solution comprising soluble plant nutrient(s), and adding a mixture of sulphur and a coating additive selected from calcium oxide, calcium hydroxide and calcium sulphate.

2. A method according to claim 1, wherein the base fertilizer is mono- ammonium phosphate (MAP), di-ammonium phosphate (DAP), single superphosphate (SSP), double superphosphate (DSP), triple superphosphate (TSP), Sulphate of Ammonia (SoA), Sulphate of Potash (SoP) or Langbeinite (K-Mag).

3. A method according to claim 1 or claim 2, wherein the base fertilizer is porous.

4. A method according to any one of claims i to 3, wherein the coating additive is calcium oxide.

A method according to any one of claims 1 to 3, wherein the coating additive is calcium hydroxide.

6. A method according to any one of claims 1 to 3, wherein the coating additive is calcium sulphate.

7. A method according to any one of claims 1 to 6, wherein the sulphur is in finely divided form.

8. A method according to any one of claims 1 to 7 wherein, in step water is sprayed onto the granules of the base fertilizer.

9. A method according to any one of claims 1 to 8, substantially as described herein and with reference to any one of Examples 1 to 6.

10. A coated granular fertilizer produced by the method of any one of claims 1 to to 9. DATED THIS 2ND DAY OF MARCH 1999 HI-FERT PTY LTD By its Patent Attorneys MADDERNS .*:eo 0* 0 o* S o C. e :-C