NZ250554A - Preparation of coated granular fertiliser in which fertiliser granules are mixed with dilute mineral acid, sulphur and/or other nutrient and acid neutralising agent - Google Patents

Description

New Zealand Paient Spedficaiion for Paient Number £50554 •• 250554 ♦ Priori ; . . .ZjLu.V^-tSZ-.

Coin^iieie Specification Filed: ?sV.\'V.^» Class: !?>.<? Publication Date: P.O. Journal, No: 2 7JUN TO. ..(s^ NEW ZEALAND PATENTS ACT, 1953 NO * *i'i / f r ■ * No.: Date: COMPLETE SPECIFICATION IMPROVEMENTS IN AND RELATING TO THE COATING OF GRANULAR FERTILIZERS ^ l/WT JOHN DONNELLY JOHNSTON, a British citizen, of 5 Perrott Street, Elizabeth Park, South Australia 5113, Australia hereby declare the invention for which I /jfter-pia.y that a patent may be granted to me^jitf^and the method by which it is to be performed, to be particularly described in and by the following statement: - - 1 - /* ' t • v 2 5 J AM 1994 •• I 250554- IMPROVEMENTS IN AND RELATING TO THE COATING OF GRANULAR FERTILIZERS This invention relates to a method of coating granular fertilizer materials with mineral nutrients, such 5 as sulphur and/or trace elements.

As is well known, plants depend on mineral nutrients for their growth and development. About thirteen elements derived from the soil are indispensable for all plant growth. They are called plant nutrients. Plants use six 10 of the elements in relatively large amounts; namely, nitrogen, phosphorus, potassium, sulphur, calcium and magnesium. These are called "major" nutrients and are constituents of many plant components such as proteins, nucleic acids and chlorophyll and are essential for 15 healthy plant growth. The other nutrients are required in small or trace quantities and are referred to as "micronutrients" or "trace elements" and have a variety of functions in plant metabolism. The "micronutrients" include copper, zinc, manganese, molybdenum, boron, iron, 20 cobalt and chlorine. Fertilizers provide plant nutrients.

An important part of fertilizer manufacture is particulation or granulation i.e. preparing the finished product as particles, preferably well-rounded, that can be stored and transported without damage to the product and 25 which can be spread evenly in the field. Whilst granulation has considerably improved the physical quality of fertilizers, this has led to many difficulties in regard to the incorporation of trace elements and sulphur into these granular products. This difficulty was largely 30 overcome by incorporation of the sulphur and the trace elements into single superphosphate fertilizer during manufacture. However, there is a continuing trend towards the production of concentrated fertilizers such as triple superphosphate, di-ammonium phosphate, mono-ammonium 35 phosphate, and urea in large manufacturing facilities closely associated with phosphate ore bodies and natural gas producers, and transporting these finished fertilizers around the world for use in widely varying agricultural systems and soil types. Manufacture at such facilities 40 has meant that the inclusion of trace elements and sulphur //* o%. - 2 - ;fv O C ! A M iQO/1 r •• 250554 into these products at the point of manufacture is not commercially viable.

In consequence of this, alternative means are continually being sought to incorporate sulphur and trace 5 elements into these concentrated fertilizers, mainly in the form of a "coat" around the surface of the particle.

While the coating of particles has been normal practice for many years in, for example, the pharmaceuticals and food industries, the fertilizer 10 industry has employed coating techniques mainly for the suppression of dustiness during handling and for the prevention of caking during storage.

T.P. Highett (Commercial Fertilizers - Conference 108(1) 23 1964) of the Tennessee Valley Authority, 15 Alabama, USA, proposes the use of oils and waxes as binders to cause micronutrients to adhere to the surface of fertilizer granules. A disadvantage of this proposal is that the oils and wax are slowly absorbed into pores of the fertilizer, thus reducing adhesion of the 20 micronutrients. Philen et al in US patents 3,423,199; 3,520,651 and 3,523,019 disclose the use of aqueous ammonium nitrate as a binder for the coating of granular ammonium nitrate with zinc, iron and manganese. Cicco in U.S. patent 3,560,192 proposes the use of aqueous zinc 25 chloride to bind micronutrients in powder form onto the surface of fertilizer granules.

Lefray et al in PCT/AU91/00459 disclose the use of water-soluble adhesives such as polyvinyl alcohol and sodium lignosulphonate to adhere elemental sulphur and 30 trace elements to fertilizer granules. The granules may be urea, mono-ammonium phosphate, di-ammonium phosphate and superphosphate.

In Australian patent 554749, use of concentrated mineral acids, for example 98% w/w sulphuric acid or 62% 35 w/w phosphoric acid, is proposed to create a reactive layer on the surface of phosphate-containing fertilizer granules, particularly ammonium phosphates. Upon the addition of micronutrients to this layer, various reactions occur and bind the micronutrients to the surface 40 of the granules by processes of recrystalisation of. j?-\ t N •; • v /" "-\ !; V •" \ 5717L - 3 - 2 5 JAN 1994 • 250554 various reaction products. This method cannot be used for the incorporation of elemental sulphur into the coat, since elemental sulphur reduces concentrated sulphuric acid to sulphurous acid and sulphur dioxide.

In addition, as pointed out in Australian patent 601099, the method of Australian patent 554749 also gives rise to air pollution by the liberation of pungent and toxic silicon tetrafluoride and hydrogen fluoride, if triple superphosphate or double superphosphate is used as 10 the base fertilizer.

Australian patent 601099 describes a method for the incorporation, into base fertilizers selected from the group consisting of the orthophosphates of calcium, ammonium and potassium, of elemental sulphur and trace 15 elements. The method relies upon the formation of a wet and tacky layer upon the surface of the base granular phosphatic fertilizer, using water only or a solution of a water-soluble salt selected from the sulphates of ammonium or potassium. A variety of chemical reactions between 20 this layer and the water soluble constituents to be affixed to the granules then is utilized, with these reactions producing less water soluble and more hydrated salts which act as binders. Whilst this method yields acceptable coated granules, it is limited in application 25 to the orthophosphates of calcium, ammonium and potassium.

The present invention seeks to provide an improved or alternative method for the coating of finished granular base fertilizers with at least one supplemental nutrient. The base fertilizer may be at least one of triple 30 superphosphate, double superphosphate, single superphosphate, di-ammonium phosphate, mono-ammonium phosphate, ammonium sulphate, urea, potassium sulphate, potassium nitrate and potassium chloride. The supplemental nutrient may be at least one of as sulphur, 35 copper, zinc, manganese, cobalt, molybdenum, boron, iron and phosphates and sulphates of calcium. At least in a preferred form, the method of the invention overcomes or at least reduces one or more of the difficulties and limitations encountered with prior methods and processes 40 for the coating of granules. / < 5717L— - 4 - 2 5 jA?::994- <jCU :■,% i ^ |i' pa gum IJ U 5 5 / In one form, this invention seeks to provide a method for the production of stable, nutritionally well-balanced granular materials from at least one base fertilizer selected from the group consisting of the orthophosphates of calcium, ammonium and potassium, the sulphates of ammonium and potassium, and the nitrate and chloride of 5 potassium and urea, by the addition of coating material comprising at least one supplementary micronutrient and elemental sulphur.

The method of the invention comprises treating granules of base fertilizer material with a dilute mineral acid selected from the group consisting of sulphuric acid and phosphoric acid and mixtures thereof. The base fertilizer granules are mixed with 10 a quantity of the dilute mineral acid sufficient to ensure a uniform wetting of the surface of the granules. A desired quantity of coating material is mixed with the acid wetted granules. Whilst maintaining the components in an intimately mixed condition, an acid neutralising agent selected from the group comprising metal oxides, hydroxides and carbonates then is added in such manner and proportion as to cause an exothermic 15 reaction between the neutralising agent and the acid, and cause the generation of heat sufficient to drive-off water by evaporation. The loss of water by this means results in the rapid precipitation of at least one chemical compound which holds the coating material in a tightly adhering coating on the surface of the base fertilizer granules.

Alternatively, the base fertilizer granules are mixed with the desired quantity of coating 20 material and necessary quantity of neutralising agent and dilute mineral acid is then added in such manner and proportion as to cause the exothermic reaction previously described.

According to the invention, there is provided a process for producing a coated granular fertilizer product wherein granules of fertilizer are mixed with a dilute mineral 25 acid, a desired quantity of a component including sulphur and/or at least one supplemental micronutrient, and an acid neutralising agent, thereby causing liberaJjjSff^^-^ of heat energy which removes water as water vapour and at the same time capShg v ■ 25055 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.

The amount and concentration of the dilute mineral acid and the amount of neutralising agent required to achieve a tightly adhering coating on the surface of the base fertilizer granules is influenced by a number of actors. These factors include, by not limited to: (i) the need to prevent or at least substantially minimize the chemical breakdown of the base fertilizer or — 7 components of the coating material which could lead to the evolution of noxious gases or loss of nutrient in the form of a gas from the base fertilizer; (ii) the amount of liquid required to optimally disperse the coating materials around the surface of the base fertilizer granules; (iii) the residence time in the mixing apparatus -longer residence times can reduce the amount of liquid required to disperse the coating materials around the surface of the base fertilizer; (iv) the amount of coating material to be applied to the base fertilizer; and (v) the acid neutralising power of the neutralising agent.

Where sulphuric acid is used, it is to be at a concentration of not more than about 60% w/w and preferably not more than about 50% w/w when triple superphosphate, double superphosphate, single superphosphate or mixtures thereof, is the base fertilizer; not more than about 80% w/w and preferably not more than about 70% w/w when di-ammonium and mono-ammonium phosphates, or mixtures thereof, is the base fertilizer; not more than about 80% w/w and preferably not more than about 70% w/w when urea is the base fertilizer; not more than about 80% w/w and preferably not more than about 70% w/w when ammonium sulphate is the base fertilizer; not more than about 30% w/w and preferably not more than about 20% w/w when potassium chloride is the base fertilizer; not more than about 80% w/w and preferably not more than about 70% w/w when potassium sulphate is the base fertilizer; not more than about 30% w/w and preferably not more than about 20% w/w when potassium nitrate is the base fertilizer.

Where phosphoric acid is used, it is to be at a concentration of preferably not more than about 75% w/w and more preferably not more than about 60% w/w and^-mpafe* preferably not more than about 50% w/w when triple superphosphate, double superphosphate, single superphosphate, di-ammonium and mono-ammonium phosphates, urea, ammonium sulphate, potassium chloride, potassium sulphate and potassium nitrate, or mixtures thereof is the base fertilizer.

An insufficient quantity of acid can result in poor dispersion of the coating material around the surface of the granules of base fertilizer which upon neutralisation with the neutralising agent can lead to an uneven or patchy coating. The amount of acid is preferably in excess of that required to evenly disperse the coating material. However, the excess most preferably is limited to optimise the balance between the cost of excess acid and the cost of neutralising agent, allowing for the benefit of neutralising agent being able to provide a desirable constituent to be incorporated in the coating at a required level.

The neutralising agent preferably is added at a level sufficient to neutralise the acid. However, an insufficiency of neutralising agent is not detrimental to the coating provided that the amount of residual acid is acceptable. Similarly, an excess level of neutralising agent can be incorporated into the coating provided that the excess of neutralising agent is not too great such as to remain as fines which are not incorporated into the coating.

By careful control of the amount and concentration of the dilute mineral acids, and the amount of neutralising agent, coated granular fertilizers can be produced where the degree of acidity or alkalinity of the coating can be adjusted to improve the efficacy of the fertilizer in particular soil conditions.

The neutralising agent can, if required, be added as a single charge, as separate charges or continuously over a period of time. It is preferable to add the agent in two successive charges. The first of these achieves partial neutralisation of the acid, and tends to provide a tacky surface layer on the granules. The second completes neutralisation and also attainment of substantially dry granules. However, if the granules still are moist after completion of addition of the agent, they can be subjected to a drying operation such as by blowing heated air through the granules.

The stages of the process can be achieved in a variety of forms of apparatus commonly used in the fertilizer industry, such as ribbon mixers, single or twin shaft paddle mixers, drum batch mixers, pan granulators and drum granulators. One convenient arrangement is to 10 conduct each stage in a rotary drum granulator in which a cascading or tumbling bed of granules provides a good mixing action as well as imparting considerable mechanical forces to the surface of granules. A single drum granulator can be used, with the granules passing, during 15 rotation of this, from an inlet end to a discharge end. The acid can be sprayed on tumbling granules shortly after the inlet end, such as from spray heads, or incorporated into the bed of granules by a sparging device. The coating material and neutralizing agent can be added in 20 turn, after the acid, in a suitably fine particulate form, to the tumbling granules or, alternatively, the coating material and neutralizing agent can be mixed with the fertilizer granules before addition of the acid.

The neutralising agent most preferably is selected 25 from oxides, hydroxides, carbonates and mixtures thereof of at least one of calcium, magnesium, potassium and sodium. These compounds have the benefit of providing acid salts which are incorporated, with benefit, in the coating material. However, other oxides, hydroxides and 30 carbonates can be used, such as those of manganese or zinc, also to achieve a beneficial or at least a neutral effect.

The quantity of coating material can vary quite significantly in accordance with requirements and also the 35 size spectrum of the fertilizer granules. In large part, the quantity of coating material determines requirements for both acid and neutralising agent. However, notwithstanding this, the quantity of neutralising agent typically ranges from about 0.1 to 25 wt%, such as from 2 40 to 20 wt% and most preferably from 5 to 15 wt%, such as •>'- \ / /' o ^ 5717L - 8 - : 2 about 10 wt%, relative to the weight of fertilizer granules.

Adjustment of the strength of the dilute mineral acid within the limits previously defined and corresponding adjustment of the quantity of neutralising agent affords the means by which the coating process is readily able to be controlled.

The principal chemical reactions which occur between the dilute mineral acid and the neutralising agent involved in this coating process include one of more of the following: WITH DILUTE SULPHURIC ACID (WHERE n = 1 to 9) H2S04+Ca0+nH20 15 H2S04+K20 H2S04+Mg0+nH20 H2S04+Na20+nH20 2H2S04+K20+Mg0+nH2C> 3H2S04+K2O+2MgO 20 2H2S04+Ca0+K20 4H2S04+K20+2Ca0+Mg0 6H2S04+K20+4Ca0+Mg0 H2^°4+^aC03+nH2° H2S04+K2C03 25 H2S04+Na2C03+nH20 H2S04+MgC03+nH20 CaSO^.(n+l)h20 k2so4+h2o MgS04.(n+l)H20 Na2SC>4 . (n+l)H20 K2Mg(S04)2.(n+2)H20 K2Mg(S04)3+3H20 K2Ca(S04)2+2H20 K2Ca2Mg(S04)4+4H20 K2Ca4Mg(S04)6+6H20 CaS04.(n+l)H20+C02 K2so4+H2O+CO2 Na2S04.(n+l)H20+C02 MgS04.(n+l)H20+C02 40 5717L 11 V D 2.

WITH DILUTE PHOSPHORIC ACID (WHERE n = 1 to 19) H3P04+Ca0+nH20 2H3P04+Ca0 2H3P04+3Ca0 5 H3P04+K20 2H3P04+K20 H3P04+Mg0+nH20 2H3P04+3Mg0+nH20 2H3P04+Na20+nH20 10 H3P04+Na20+nH20 2H3P04+3Na20+nH20 2H3P04+K20+2Mg0+nH2O H3P04+CaC03+nH20 2H3P04+CaC03 15 2H3P04+3CaC03 H3P04+K2C03 2H3P04+3MgC03+nH20 2H3P04+3Na2C03+nH20 CaHP04.(n+l)H20 Ca(H2P04)2.H20 Ca3(P04)2+3H20 k2hpo4+h2o 2khp0.+2ho0 4 2 MgHP04.(n+l)H20 Mg3(P04)2.(n+3)H20 2Na2HP04.(n+l)H20 Na2HP04.(n+l)H20 2Na3P04.(n+3)H20 2KMgP04.(n+3)H20 CaHP04.(n+l)H20+C02 Ca(H2P04)2.H20+C02 Ca3(P04)2+3H20+3C02 k2hpo4+h2o+co2 Mg3(P04)2+(n+3)H20+3C02 2Na3P04.(n+3)H20+3C02 The free water generated by these reactions, as distinct from the water taken up as water of crystallisation or constitution by some of the reaction products, as well as the water contained in the dilute mineral acid, is removed from the system in the form of 25 water vapour by virtue of the heat of reaction caused to occur by neutralisation of the acid with the neutralising agent. This results in the production of a granular fertilizer with a stable coat of supplementary nutrient(s).

An especially preferred acid neutralising agent is 30 cement kiln dust, also known lime kiln dust or bypass dust. Cement kiln dust is a by-product of cement manufacture, and is a finely divided, highly alkaline dry powder. It contains a number of chemical compounds but principally the oxides and carbonates of calcium, 35 potassium and magnesium, in varying proportions. A typical analysis of cement kiln dust is shown in Table 1. 40 .■ 1 oA ON •• > ' 5717L -O r/.

TABLE 1 TYPICAL CHEMICAL ANALYSIS OF CEMENT KILN DUST Calcium, expressed as CaO, (%) = 52 .0 Potassium, expressed as K20, (% ) = 11 .1 Magnesium, expressed as MgO, (% ) = 4 .3 Iron, expressed as Fe203, (%) = 3 .0 Aluminium, expressed as A^O^, (%) = 3 .5 Silicon, expressed as Si02, (%) = 13 .6 Titanium, expressed as Ti02, (% ) = 0 .3 Phosphorus, expressed as p2°5' (%) = 0 .03 It has been found that cement kiln dust is an effective acid neutralising agent in the coating process of the present invention. It preferably is used in the 15 proportion of 0.1 to 25 wt%, preferably 1 wt% to 25 wt% of the total composition of the coated granular product. There is a cost advantage to be gained by the use of this material, since cement manufacturers face considerable difficulty in the disposal of this by-product in an 20 environmentally sound and cost effective manner. Such manufacturers thus resort to stockpiling or sale of relatively small quantities at nominal cost as a soil conditioner.

In order to better understand the coating process of 25 this invention, reference is made to the following Examples. In the Examples the base fertilizers were coated using a mixing cum coating drum operating at an output between 0.5 and 2.0 tonnes per hour with a granule bed volume of about 20% of the total drum volume and a 30 residence or coating time of about 5 minutes. The base fertilizers and supplementary nutrients used in the Examples are denoted by the following abbreviations:- - "r 2 5 JAM"- /. triple superphosphate TSP di-ammonium phosphate DAP mono-ammonium phosphate MAP single superphosphate SSP ammonium sulphate AS potassium sulphate PS potassium nitrate PN potassium chloride PC urea UR copper oxide CO zinc oxide ZO manganous oxide MO copper sulphate pentahydrate CS zinc sulphate heptahydrate ZS manganous sulphate tetrahydrate MS sulphur S reactive phosphate rock RPR calcium sulphate dihydrate CSD.

EXAMPLES 1 TO 19 In the Examples the base fertilizer was coated with supplementary nutrients using sulphuric acid of varying concentrations as the reactant precursor. Table 2 shows the amount of ingredients used in each Example, as a weight percentage of the whole mixture.

EXAMPLES 20 TO 31 In these Examples the same procedure was followed as in Examples 1 to 19 except that dilute phosphoric acid was used as the reactant precursor. Table 3 shows the amount of ingredients used in each Example, as a weight 30 percentage of the whole mixture.

In Examples 1 to 31, cement kiln dust was the acid neutralising agent. In each case, a tenaceously adhering coating with excellent resistance to abrasion was produced around granules of the base fertilizer. Other acid 35 neutralising agents have been used, e.g. sodium carbonate, calcium carbonate and calcium hydroxide. Each of these other agents proved to be effective in producing such coating on the base fertiizer, although their greater cost compared with cement kiln dust is such that the latter is 40 highly preferred. - ''Vj ^ " v. 5717L - 12 - ' ,,,.^4 2 5 JAN iv.-* • • • • TABLE 2 AMOUNT OF INGREDIENTS USED IN EXAMPLES 1 TO 19 Example Base Supplementary Sulphuric Acid Neutralising No . Ferti lizer Nutrient Cone. Amount Agent Type wt% Type wt% %w/w wt% wt% 1 TSP 72.2 S 11.5 49.7 10.5 5.8 2 TSP 70.1 S 14.0 49.7 10.2 5.6 3 TSP 68.2 S 16.4 49.7 10.0 5.5 4 TSP 76.9 ZO 6.2 49.7 10.8 6.2 TSP 74.6 CS 9.0 49.7 10.4 6.0 AS 5.4 KS 5.4 6 TSP 55.1 CO 2.7 49.7 9.5 5.4 MO 4.0 CSD 12.5 7 TSP 73.2 RPR 10.2 49.7 10.7 5.9 8 DAP 80.9 ZO 3.9 67.5 8.7 6.5 9 DAP 71.3 S 10.2 67.5 10.6 7.9 MAP 68.5 CS 8.2 49.7 9.6 5.5 11 UR 75.0 S 9.0 67.5 7.1 8.8 12 UR 68.3 S 4.1 67.5 7.6 12.8 MO 9.6 13 PC 63.6 S 10.2 19.9 13.5 12.7 CS 4.2 14 AS 69.3 ZS 4.4 80.0 9.7 8.6 MS 3.9 SSP 68.6 S 15.0 49.7 10.1 6.3 16 SSP 71.2 CS 10.0 49.7 9.0 9.8 17 SSP 52.3 RPR 26.2 35.0 15.4 6.2 CS 4.0 V 18 PN 64.0 MO 4.0 19.9 12.3 11.7 err "tN* Z S 4.0 c_ CO 3.0 h. >*£J £;} 19 PS 70.0 MO 4.5 70.0 10.0 8.5 '/J ZO 4.0 > ■ ' - 13 5710Ir TABLE 3 AMOUNT OF INGREDIENTS USED IN EXAMPLES 20 TO 31 N. > NJ OT "/ \\ C_. {. >»' *** • vD • 1 Example Base Supplementary- Phosphoric Acid Neutralising No.

Fertilizer Nutrient Cone.

Amount Aaent Type wt% Type wt% %w/w wt% wt% TSP 69.3 S 14.0 50.1 .8 .9 21 TSP 76.4 ZO 4.0 50.1 .4 .7 CO 3.5 22 TSP 70.4 S .0 50.1 11.2 6.1 CS 4.0 ZS 3.3 23 TSP 68.1 S 4.5 50.1 11.9 6.5 RPR 9.0 24 DAP 75.3 S .0 50.1 .5 .7 CO 3.5 MAP 61.7 CS 7.8 50.1 12.3 6.7 ZS 8.0 MS 3.5 26 UR 68.6 S .0 50.1 .6 .8 27 AS 71.3 CO 3.5 50.1 11.1 6.1 ZO 4.0 MO 4.0 28 PC 72.7 s .8 50.1 .7 .8 29 PN 81.1 CO .1 50.1 8.9 4.9 PS 79.3 ZO 6.2 50.1 9.4 .1 31 SSP 64.5 s 4.1 50.1 12.4 6.8 RPR 12.2 - 14 5718L po U*S tr< O'f In the prior art fertilizer coating processes it is necessary that the supplementary nutrient(s) be in a finely divided form to enable the surface reactions between the base fertilizer and the coating material to proceed. It has been found in the coating process of the present invention that the size distribution of the supplementary nutrient(s) can vary within relatively wide limits without affecting the physical integrity of the coat. For example, the reactive phosphate rock (RPR) used in the Examples is that commercially available from North Carolina, United States of America, which possessed the following size distribution: The coating process of the present invention provides a cost advantage in that the supplementary nutrient(s) need not be subjected to further comminution before incorporation in the coat around the granular base fertilizer. A further advantage of this process in terms of ability to incorporate supplementary nutrients possessing a relatively wide size distribution is that means are provided for the production of fertilizer products which possess slow nutrient release characteristics.

It was further demonstrated that compounds of cobalt selected from the group cobalt sulphate, cobalt oxide, and basic cobalt sulphate, compounds of molybdenum selected from the group ammonium molybdate, sodium molybdate and molybdenum trioxide, compounds of boron selected from the group sodium tetraborate, calcium borate and orthoboric acid, and ferrous sulphate, or mixtures from these groups, could be used in the process without detriment to the integrity of the coat.

Finally it is to be understood that various alterations and/or additions may be introduced into the formulations and arrangements of the present invention without departing from the spirit or ambit of the invention. u • + 0. 5mrn + 0.25mm + 0.15mm + 0.075mm 6.0% 43.9% 9 0.5% 99.5% 0 ^ ^ 4

Claims (16)

WHAT WE CLAIM IS:

1. A process for producing a coated granular fertilizer product, wherein granules of fertilizer are mixed with a dilute mineral acid, a desired quantity of a component 5 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. 10

2. A process according to claim 1, wherein the granular fertilizer is at least one of single superphosphate, double superphosphate, triple superphosphate, mono-ammonium phosphate, di-ammonium phosphate, urea, ammonium sulphate, potassium chloride, potassium nitrate, potassium sulphate and ammonium nitrate.

3. A process according to claim 1 or claim 2, wherein the acid neutralising agent is 15 or includes an oxide, hydroxide and/or carbonate of calcium, potassium and/or magnesium.

4. A process according to any one of claims 1 to 3, wherein the acid neutralising agent is or includes cement kiln dust.

5. A process according to any one of claims 1 to 4, wherein the component is or 20 includes elemental sulphur.

6. A process according to any one of claims 1 to 5, wherein the component is or includes at least one of copper, zinc, manganese, cobalt, molybdenum, boron, iron and calcium.

7. A process according to claim 6, wherein the component is or includes a copper 25 compound selected from copper oxide, copper sulphate, basic copper sulph copper cement and mixtures thereof. 16 - 2 1MB £*""

8 A process according to claim 6 wherein the component is or includes a zinc compound selected from zinc oxide, zinc sulphate, basic zinc sulphate and mixtures thereof.

9. A process according to claim 6, wherein the component is or includes a 5 manganese compound selected from manganese oxide, manganous sulphate and mixtures thereof.

10. A process according to claim 6, wherein the component is or includes a cobalt compound selected from cobalt oxide, cobalt sulphate, basic cobalt sulphate and mixtures thereof. 10

11. A process according to claim 6, wherein the component is or includes calcium sulphate and/or a calcium phosphate.

12. A process according to any one of claims 1 to 11, wherein the dilute mineral acid is a solution of sulphuric acid and/or phosphoric acid.

13. A process according to any one of claims 1 to 12, wherein mixing the granular 15 fertilizer with said dilute mineral acid is effected after mixing the fertilizer with thecomponent and neutralising agent.

14. A process according to any one of claims 1 to 12, wherein the component and neutralizing agent are added to the granular fertilizer after mixing the fertilizer with said dilute mineral acid. 20

15. A process according to any one of claims 1 to 14, substantially as hereinbefore described with reference to any one of the examples.

16. A coated granular fertilizer product produced by the process of any one of Claims 1 to 15. dated this dayofp^p^1 A.J. FAisK & SON 25 AFOR THE APPLICANTS - 17