Title of Invention	A PROCESS FOR THE INCORPORATION OF PLANT NUTRIENTS AS COMPONENTS OF A SEMI- PERMEABLE COATING OF A GRANULAR,PHOSPHATIC OR NON-PHOSPHATIC FERTILIZER
Abstract	The present invention is directed to a method for producing a coated granular fertilizer by producing in situ on the surface of the granules a coating comprising one or more ammonium magnesium phosphate and/or potassium magnesium phosphate compounds. Additional plant nutrients, such as elemental sulphur, may be incorporated in the coating. The invention also provides a coating system for the encapsulation of granular water soluble phosphatic or non-phosphatic fertilizers with a semi-permeable coating comprising one or more ammonium magnesium phosphate and/or potassium magnesium phosphate compounds.

Title of Invention

A PROCESS FOR THE INCORPORATION OF PLANT NUTRIENTS AS COMPONENTS OF A SEMI- PERMEABLE COATING OF A GRANULAR,PHOSPHATIC OR NON-PHOSPHATIC FERTILIZER

Abstract

The present invention is directed to a method for producing a coated granular fertilizer by producing in situ on the surface of the granules a coating comprising one or more ammonium magnesium phosphate and/or potassium magnesium phosphate compounds. Additional plant nutrients, such as elemental sulphur, may be incorporated in the coating. The invention also provides a coating system for the encapsulation of granular water soluble phosphatic or non-phosphatic fertilizers with a semi-permeable coating comprising one or more ammonium magnesium phosphate and/or potassium magnesium phosphate compounds.

Full Text	This invention relates to (a) a process which allows the addition of micronutrients and secondary nutrients to granular fertilizers and (b) a process to reduce the dissolution of nutrients from water-soluble granular fertilizers, and more particularly to coating-systems which depend upon hydrated ammonium magnesium phosphates and their isomorphs with potassium for the establishment of tenaciously adhering coatings either (a) to carry micro and/or secondary nutrients, or (b) to reduce the rate of release of water-soluble nutrients. In brief, it relates to the production, by coating processes, (a) of micro- and/or secondary nutrient-carrying granular primary nutrient-carrying fertilizers (the primary nutrient being nitrogen, phosphorus or potassium), or (b) of slow release granular fertilizers carrying very water-soluble compounds of nitrogen, potassium or phosphorus. In common terminology, the term micro-nutrients refers to such elements as Cu, Mn, B, Zn, Co, Fe, Se and Mo, and the term secondary nutrients refers to such elements as S, Mg, Ca and Na. From a purely physiological point of view, it is very difficult to justify the classification of plant nutrients into primary, secondary and micronutrients, this classification being solely based upon the concentration of the nutrient in the plant tissue. Thus, this classification is not based on the value of the nutrient in regard to the physiological function it has to fulfil in the plant, which is always very specific and cannot be performed if the nutrient is absent or not present at the required concentration. The term slow release fertilizer refers mainly to water-soluble nitrogen or potassium fertilizers, in which the rate of dissolution of water-soluble primary nutrient(s) from the fertilizer granule into the soil has been significantly reduced with the object of reducing losses through leaching of water-soluble nutrient(s) from the granule to the subsoil, thereby providing a more permanent source of nutrient(s) which is/are readily available to the growing plant. Over the last three decades or so, a number of patents have taught the incorporation of micronutrients as well as secondary nutrients into granular fertilizers by coating - 1A- operations. The following patents describe nutrient addition by coating technologies: H.H.Nau, US Patent 3353949,21 Nov 1967, "Incorporation of Secondary and Micronutrients in Granular Fertilizers", teaches the incorporation of powdered micronutrients, which were previously ground to pass a 400 mesh Tyler test sieve (aperture of 38 urn), by mixing said powdered material into granular NPK fertilizer. This mixture is sprayed, while in the mixer, with a solution such as 50%w/w NH4NO3 or 50%w/w of blackstrap molasses, to stick the powder to the granule surfaces. We think that the use of NH4NO3 solution, which is a strongly oxidising agent, in conjunction with elemental S is from a safety point of view not a practical proposition, while the use of such a solution with Mn++ ions, which are readily available to plants, would convert the Mn++ ions into agronomically unavailable Mn3+ ions. Otis D. Philen, Julius Silverberg and Melvin M Norton, US Patent 3423199,21 Jan 1969, "Adding Micronutrients to Granular Fertilizers", teach the incorporation by mixing of granular fertilizers with pulverized micronutrient compounds in the presence of a concentrated aqueous solution that interacts to bond the micronutrient firmly to the surface of the granules. The interaction may be chemical or physical or both. In their examples, granular NH4NO3, which had previously been mixed with 10%w/w ZnO and 2.5%w/w kieselguhr (diatomaceous earth) in a rotary mixer, was then sprayed for 1 minute while in the mixer with a 70%w/w NH4NO3 solution. The resulting product consisted of NH4NO3 coated with a complex double salt, Zn5NH4(NO3)2(OH)9.3H2O, which held the unreacted portion of the ZnO in a tenaciously bonded coating. In another example, they teach the incorporation into granular triplesuperphosphate (TSP) of a 13%w/w mixture, consisting of powdered oxides of Zn, Mn, and Fe, in a mixer by spraying this mixture with a 2.8%w/w ammonium polyphosphate solution. While the polyphosphate approach appears to work, the unavailability of polyphosphate materials outside USA prevents the use of this method in most countries. As far as NH4NO3 as a carrier for micronutrients is concerned, the hazards associated with such a powerful oxidising agent make it very 2 unattractive for this application. The decomposition of NH4NO3 is promoted by traces of Cl, free acid and a number of bivalent metals which will, at temperatures as low as 170°C, reach violent dimensions, Otis D. Philen, Julius Silverberg and Melvin M, Norton, US Patent 3520651,14 July 1970, claim in this patent the double salt 3 Zn{OH)2.NH4NO3 as a new compound, which is formed in situ on the surface of granular fertilizers produced by the method described in their US Patent 3423199. Otis D, Philen, Julius Silverberg and Melvin M, Norton, US Patent 3523019,4 Aug 1970, claim in this patent the formation of complex tri-zinc diammonium dipyrophosphate Zn3(NH4)2(P2O7).H2O, or complex zinc ammonium hydroxy nitrates, when fertilizer granules are coated according to their previously described patents. Since most nitrates are relatively hygroscopic, the resulting products are, in the main, very unstable and hygroscopic. G.D. Cicco, US Patent 3560192,2 Feb 1971, suggests in his patent that a free-flowing granular fertilizer can be produced from 42.2%w/w of (NH4)2SO4 and 57.8 %w/w TSP by spraying the said mixture in a rotating mixer with an aqueous solution of 66%w/w of ZnCl2, Sufficient powdered anhydrous ZnSO4 is then added to bring the granules to apparent dryness, the product then being dusted with 3%w/w of ZnO for the production of a free-flowing dry product with good adhesion of the zinc. While this method at first produces a relatively dry coated product, the calcium in the TSP will react with the Cl- in the ZnCl2, forming very hygroscopic calcium chloride. J.W. Church, US Patent 386712418 Feb 1975, "Granular Water-Soluble Micronutrient-Containing Fertilizer", employs adducts formed during the reaction in situ between urea and such water-soluble salts as FeSO4.7H2O, MgSO4.7H2O and CuSO4.5H2O for the formation of tenaciously adhering coatings on ammonium nitrate. Since international regulations governing the transport of ammonium nitrate-containing fertilizers and ammonium nitrate mixtures generally restrict the 3 content of inflammable substances to 0.2 to 0.4%, and urea mixtures with NH4NO3 are used in the mining industry as safety explosives in lieu of dynamite, this method is not a very practical proposition from a safety point of view. Karl H. Walter and Denis Wetherley, AU 554749,15 March 1985, "Production of Granular Trace Element Fertilizers", teach the incorporation of micronutrients into phosphatic fertilizers by treating said phosphatic fertilizers with a mineral acid while the granular fertilizer material is tumbled in a rotary tumbling or coating device in the presence of a particulate micronutrient material, which is bonded to the surfaces of the fertilizer granules while said surfaces are wetted by the said acid. This patent discloses that such micronutrients as Cu, Co, Mn and Zn can be incorporated into the granular fertilizer. Unfortunately, this method can only be employed if the acid employed for the creation of a tacky layer on the surface of the fertilizer granules does not increase the free acidity content of the phophatic fertilizer. This means it can only be used on ammonium phosphate fertilizers. This is due to the fact that any additional acid added to ammonium phosphate fertilizers does not increase the free acid content of these products, but forms either NH4H2PO4 or (NH4)H5(PO4)2 according to: 2(NH4)2HPO4 + H2SO4 ? 2NH4H2PO4 + (NH4)2SO4 or 4NH4H2PO4 + H2SO4 ? 2NH4H5(PO4)2 + (NH4)2SO4 Furthermore, it does not allow the incorporation of elemental sulphur into phosphatic fertilizers. This is due to the fact that elemental sulphur reacts with sulphuric acid forming sulphurous acid, which decomposes into sulphur dioxide and water according to: Another setback of this method lies in the fact that it cannot be used for the incorporation of additional micro or secondary nutrients into triple or double 4 superphosphate without causing serious problems of air pollution by releasing such highly pungent and toxic substances as SiF4 and H2F2. Karl H. Walter, US Patent 5152821,27 Jan 1989, "Addition of Supplemental Macro & Micro Nutrients to Granular Phosphatic Fertilizers", showed that it is possible to employ minute quantities of water in lieu of concentrated H2SO4 by (a) placing the phosphatic base fertilizers in a tumbling bed of particulates in a rotary mixer cum coating device and (b) creating on the surface of the phosphatic base fertilizer a tacky film by spraying said tumbling bed of granules with minute quantities of water before adding the sulphur and (NH4)2SO4, or secondary or micro nutrients. While this technology has proven itself in the marketplace, it will not allow the incorporation of additional nutrients into non-phosphatic fertilizers. Furthermore, the coating produced on the surface of ammonium phosphates will not withstand too well the relatively rough handling often encountered in machinery in the factory and on the farm. Roderick D.B. Lefroy and Graeme J. Blair, PCT/AU91/00459, 5 Oct 1990, "Fertilizer Coating Process", teaches the establishment of a layer of water-soluble adhesives on the surface of a moving bed of granules before adding a first portion of an additive carrying the additional nutrient to coat said granule. This spraying/additive operation is sequentially repeated until the desired level of additional nutrient has been added. The binders disclosed are polyvinyl alcohol and sodium lignosulphonate. In addition, it is stated in the embodiment of this specification that the coating of the granules comes about by physical bonding between the granule, adhesive and additives, and not by chemical bonding. This seems highly unlikely, as it means that during this method of coating the ions in the nutrients added to the granule would disobey the laws of basic chemistry and not react with ions in the base fertilizer. Since the binders used in this patent consist of an aqueous solution of the organic adhesive, the removal of water introduced during this coating operation presents a major problem in large scale operation and has so far prevented its use on an industrial scale. In addition, it is extremely difficult to remove surplus free water from most coated products, since most of the complex reaction products are heavily 5 hydrated and lose their water of crystallisation at a relatively low temperature, meaning that these coated products could be classified as being heat-sensitive and thus require expensive drying equipment designed for low temperature operation. John D. Johnston, AU 669403, 21 Dec 1993, discloses in this patent specification that the problems created by strong acid during coating operations with regard to air pollution could be resolved by the use of diluted acid and subsequent neutralisation of the acid introduced during this method of coating with the help of a neutralising agent. Unfortunately, the use of diluted acid introduces relatively large quantities of free water, which have to be removed from the coating system if a dry, strongly-adhering coat is to be produced. It is erroneously claimed that the heat of reaction produced during the neutralisation of the free acid would remove water as water vapour. Since this is not the case, the problems with this method are similar to those in Lefroy's patent, meaning that it does not resolve the problems associated with free water in the finished coated product. There can be little doubt that the art disclosed in these patent specifications does not allow effective coating of such base fertilizers as granular muriate of potash, sulphate of potash, sulphate of ammonia, or mono- or di-ammonium phosphate, either with elemental sulphur or other additional secondary nutrients or with micronutrients. We have now found that a struvite, or even better a hydrated ammonium magnesium phosphate or potassium magnesium phosphate-based, coating system provides an alternative and effective method for the coating of such products. Furthermore, it provides a method for reducing the release of nutrients from water-soluble fertilizers. Since plant nutrients can only be utilized in the root zone of a soil, which is more or less the cultivated part of the soil, it is of the utmost importance that nutrients in artificial fertilizers are not removed from this zone. 6 Loss of nutrients from this zone can occur as a result of erosion of the topsoil by wind or water or by lixiviation with water into the subsoil. From the point of view purely of nutrient leaching, the loss of P or K nutrients can be neglected as long as the clay content of the soil is > 5.0%w/w and the soil has a normal Fe, Al, Ca and Mg content and until the capacity of the soil to retain these two nutrients is completely satisfied. This means that about 90% of soils on a worldwide basis will not pose any problems in regard to leaching of P or K. The same does not apply to nitrogen, which is easily transported down the soil profile. The extent to which these three nutrients are carried down the soil profile depends largely on the climate, the soil type and the amount of water-soluble nutrients present in the soil solution. Generally speaking, nutrient losses by leaching are worst on freely-draining soils in high rainfall areas having a low CEC (cation exchange capacity) and, of the three major plant nutrients, phosphate is always leached at the lowest rate. The same applies to potassium ions, which are very tightly bound to many minerals, especially to clay minerals of the 2:1 type (meaning that the crystal lattice is made up of 2 silica sheets and 1 alumina sheet) such as illite, vermiculites and weathered mica. In contrast to P and K, which are mainly retained in the root zone by physico-chemical processes, the leaching characteristics of nitrogen in the soil depend largely upon the type of nitrogen (NO3-, NH4+) as well as the texture of the soil. In contrast to NO3-, which is extremely mobile in the soil, NH4+ ions are easily adsorbed by soil particles and very strongly adsorbed by such negatively-charged clay minerals as illites, vermiculites and montmorillonites. Thus, it is not surprising that the amount of NO3 nitrogen leached from soils is considerably greater than that in an ammonium form. There can be little doubt that leaching losses are considerably increased with an increase in nitrogen application rates, meaning that the plant roots cannot utilize the N applied at high application rates. 7 For this reason, farmers have tried to overcome these nutrient leaching losses by employing either less soluble fertilizers or using split applications of these fertilizers. As far as phosphorus is concerned, this can easily be achieved by the application of finely ground reactive phosphate rock, partly acidulated phosphate rock, basic slag, silicophosphates etc. With regard to nitrogen, there are, besides blood-and-bone, meatmeal and cyanamide, no other low solubility fertilizers, and with respect of K, there exists not a single slowly available source of K. Thus, there can be little doubt that there exists a great need for slow release nitrogen and potassium fertilizers. Over the last three decades or so, a number of slow release fertilizers have been developed, mainly with regard to N, by either (a) the production of such nitrogenous fertilizers as urea-formaldehyde or isobutylidene di-urea (IBDU) or (b) the encapsulation of very soluble N fertilizers in a coating consisting of such hydrophobic materials as elemental sulphur, wax, bitumen or polymers. The prior art for these slow or controlled release fertilizers is described in the following patents: Nitrogenous Fertilizers and NFK Fertilizers: R. L. Stansbury, C.S. Lynch and Kamil Sor, US 3276 857,4 Oct 1966 (US Reissue 27238,23 Nov 1971), "Slow Release Fertilizer Pellets", teach the mixing of solid (NH4)2SO4 with a molten binder such as wax or asphalt, before mixing it with either powdered gypsum or limestone to give a friable particulate mixture. This mixture is then extruded and coated with molten wax. Stansbury et al suggest that the temperature of the binder should be well above its softening point in order to ensure that the individual (NH4)2SO4 particles are substantially coated. The high power consumption for the extrusion step in this method, as well as the low value of sulphate of ammonia fertilizers, have prevented the use of this method on a large industrial scale. 8 O. Detmer et al, US 3365288,23 Jan 1968, "Slow Release Coating for Fertilizers", teach the production of a slow release fertilizer by the coating of a very soluble fertilizer with a drying oily copolymer obtained by the polymerization of a mixture of butadiene and a-methylstyrene containing 50-90% butadiene. The coating is applied to the fertilizer particles heated to about 110-120°C in a rotating drum, preferably in the presence of such drying accelerators as peroxides or azo compounds. While this method has been used, it is too expensive to be used on fertilizers for large scale application. R.C. Fox, US 3372019,5 Mar 1968, "Slow-Release Coated Fertilizer", teaches the production of a slow release fertilizer by the coating of the fertilizer granules with a mixture consisting of 25-95wt% of wax and 5-75% of a resinous material, either (a) a copolymer of an unsaturated ester containing a total of 3 to 7 C atoms with olefins having 2 to 4 C atoms or (b) polymers and copolymers having from 2 to 4 C atoms. The resinous material should have a molecular weight > 10 000. The resulting product consists of olefinic ethylacrylate. H.Hecht and H. Schwandt, US 3708276,2 Jan 1973, "Controlled Release Fertilizer", spray granular fertilizers with an aqueous emulsion of polyvinyl chloride-polyvinylidene chloride copolymer and Zn dust. This method is far too expensive to be used on bulk fertilizers. O.H. Mueller, US 4023955,17 May 1977, "Controlled Release Fertilizer", prepares a slow release fertilizer by (a) coating the fertilizer granules with a cement partially hydrated with 2 to 30wt% of water and (b) applying a thin second coating of a semipermeable elastomer consisting of styrene-butadiene acrylic ester and alkyd resins. A final third coating of partially hydrated cement, which can contain micronutrients, is applied. L.S. Wittenbrook and EL. Scheiderer, US 4082533,4 April 1976, "Coated Controlled Release Fertilizer and its Manufacturing Process", teaches the coating of a water-soluble fertilizer in a particulate form, such as prilled urea, by firstly applying 9 masonry cement or a mixture of masonry cement and bentonite, before applying a second coating consisting of 5 to 25wt% of a thermoplastic hydrocarbon resin and wax. The fertilizer granules are slightly wetted before the cement is dusted onto the surface, and then heated before the second coating of a copolymer of ethylene and vinyl acetate, a thermoplastic hydrocarbon resin and paraffin wax is applied. P.S. Fleming, US 3576613, 27 April 1971, "Coating Fertilizer with Sulphur to Control Dissolution", discloses that spraying molten sulphur on fertilizer granules will produce a semipermeable shell of solidified sulphur having a low impact strength. Unless the surface is completely covered with S, the substrate will dissolve quickly in H2O. In view of its hydrophobic nature, molten sulphur does not readily wet the surface of the fertilizer granules, meaning that a relatively heavy coating is required to obtain a slow release effect in the coated product. Fleming claims that dusting the fertilizer granules first with 0.2-3.0wt% of a powdered material, such as carbon black; Zn borate, carbonate, chromate or sulphide; Mg hydroxide or silicate; or another such powdered material, will decrease the contact angle between the granule and the molten sulphur and thus allow the establishment of a satisfactory coating with much less S. He found that 5-10%w/w of sulphur on urea produced excellent results in agronomic testwork in the field. J.L. Smith, C.R. Crowley and R.B. Humberger, US 4032319, 28 June 1977, "Sulphur Coated Composite Fertilizer", teach the establishment of a non-uniform sulphur coating containing such modifiers as water-swellable clays on granular ammonium, as well as calcium phosphates, using a fluid bed or other suitable coating devices. The process is performed in an inert atmosphere. The finished coated product contains 10-40wt% sulphur. Potassium Fertilizers: Masao Hamammoto, Seiichi Kamo and Kiyoshi Nakayama, US 3698885,17 Oct 1972, "Slow Release Potassium Phosphate Fertilizer", teach the production of a slow release fertilizer by the calcination at 700 to 1100°C for 20 to 30 minutes of a mixture consisting of caustic potash and H3PO4 having a P2O5:CaO:K2O molar ratio of 10 1.0 :1.7-3.7 :1.2-2.0. The phosphate rock to phosphoric molar ratio expressed as P2O5 was 0.2-1.6 :1.0. An addition of 0.05-2.5wt% of B2O3 as borax or H3BO4 and 0.05-2.5 moles of MgO/mole of P2O5 increased the citrate solubility of the product, and reduced the calcination temperature as well as the time of calcination.MgO was supplied either as the oxide or in the form of such magnesium-containing minerals as serpentine, fosterite etc. After calcination, the product was sprayed at 100°C with an inorganic acid or its ammonium salt to neutralise the material without increasing the water-solubility of the K component. It is claimed that the main constituent of this slow release fertilizer is CaKPO4 which has at 30°C a solubility of 0.1-0.2g K2O/100g H2O. G.B. Blouin, US3877415,15 April 1975, "Apparatus for Applying Coatings to Solid Particles", describes an apparatus which provides a homogeneous, dense mass of sized particles in random motion so that highly uniform coatings of the same or of different solids can be applied to each particle by conventional spray-coating with the liquified coating material(s). The apparatus consists of a rotary drum containing lifting flights. A novel deflector pan is fixed in space inside the upper section of the drum, and deflects particles falling from the lifting flights to the side of the drum, where they form a narrow, dense falling cascade or curtain. The coating material is sprayed onto the cascading particles, preferably as they free-fall after leaving the lower edge of the pan. If desired, some or most of the coating material may be directed onto the top edge of the moving bed, including the juncture of the cascade therewith. In examples of coating granular urea with molten sulphur, the use of this cascading or falling bed apparatus resulted in a more uniform and effective coating of S and increased the capacity by over 100% over equipment not fitted with a cascade apparatus. While some of the methods disclosed in these specifications are utilized on an industrial scale, there can be little doubt that the methods proposed in most of these patent specifications will increase the cost of the finished product to such an extent that these slow release fertilizers can only be used in agricultural activities which 11 give a high return. For most agricultural activities, however, these coated fertilizers are far too expensive. There can be little doubt that there exists a great need for (a) a more reliable coating method for the addition of finely ground and agronomically available elemental S to mono-ammonium or di-ammonium phosphate, as well as to non-phosphatic fertilizers, and (b) a less expensive coating system for the production of less expensive slow release fertilizers. In addition, it is somewhat illogical that most prior art coating systems rely on the establishment of semi-permeable coatings, which are from an agronomic point of view largely useless. It is the object of the present invention to provide methods for the coating of such finished granular fertilizers as Di-ammonium phosphate (DAP), Mono-ammonium phosphate (MAP), Muriate of potash (MOP), Sulphate of potash (SOP), Sulphate of ammonia (SOA) and Potassium magnesium sulphate (K-Mag) for the purpose of (a) reducing the rate of dissolution of the said fertilizer or (b) adding such supplemental nutrients as finely ground elemental sulphur or, in nonphosphatic fertilizers, such supplemental nutrients as Copper (Cu), Zinc (Zn), Manganese (Mn), Cobalt (Co), Boron (B), Molybdenum (Mo), etc, these methods overcoming some of the problems experienced with prior methods or processes. The present invention relates to a method for producing a coated granular fertilizer by producing in situ on the surface of the granules a coating comprising ammonium magnesium phosphate, potassium magnesium phosphate, a hydrate of ammonium magnesium phosphate, a hydrate of potassium magnesium phosphate, or a mixture thereof. Additional plant nutrients, such as elemental sulphur or supplemental plant nutrients (Cu, Zn, Mn, Co, B, Mo, etc:, in ionic form), may be incorporated in the coating. Preferably, the added plant nutrients are selected from elemental sulphur, copper oxide, copper sulphate, basic copper sulphate, copper carbonate, basic copper carbonate, zinc oxide, zinc sulphate, basic zinc sulphate, zinc carbonate, basic zinc 12 carbonate, sodium molybdate, calcium molybdate, manganous oxide, manganous sulphate, basic manganous sulphate, manganous carbonate, basic manganous carbonate, orthoboric acid {H3BO3), metaboric acid (H2BO2), tetraboric acid (H2B2O7), calcium tetraborate, cobalt sulphate, cobalt carbonate and cobalt hydroxide. According to one aspect of the invention, a process for (a) the incorporation of plant nutrients as components of a semipermeable coating of a granular, phosphatic or non-phosphatic fertilizer, and (b) the reduction of the rate of dissolution of water-soluble nutrients from said fertilizer, comprises forming in situ, on the surface of the granules of the fertilizer, a coating comprising one or more ammonium magnesium phosphate and/or potassium magnesium phosphate compounds. According to a second aspect of the invention, a coating system for the encapsulation of granular, water-soluble, phosphatic or non-phosphatic fertilizers with a semipermeable coating is provided and, according to a third aspect of the invention, a coating system for the incorporation of plant nutrients as components of a semipermeable coating of a granular, phosphatic or non-phosphatic fertilizer is provided. In each case, the coating comprises one or more ammonium magnesium phosphate and/or potassium magnesium phosphate compounds. The ammonium magnesium phosphate and/or potassium magnesium phosphate compounds are, for example, ammonium magnesium phosphate, potassium magnesium phosphate or hydrates of either of these compounds. In particular, the compounds may be ammonium magnesium phosphate hexahydrate (struvite), ammonium magnesium phosphate monohydrate, potassium magnesium phosphate hexahydrate and/or potassium magnesium phosphate monohydrate. In one embodiment, the present invention provides a method for the encapsulation of very water-soluble nitrogenous, phosphatic and potassic fertilizers with a coating consisting of such plant nutrients as Mg, P and N, by producing in situ on the surface of individual granules a coating consisting of struvite (ammonium magnesium 13 phosphate hexahydrate), the monohydrate of ammonium magnesium phosphate, or similar ammonium magnesium or potassium magnesium phosphates. Struvite is a mineral, which has been well-known for a long time, and is widely distributed in nature. It is found in guano, which is a substance found in great abundance on some coastlines or islands frequented by seabirds and originates from their excrements, in dung deposits in caves, as well as in urinary calculi, such as bladder, kidney and gall bladder stones. The monohydrate of ammonium magnesium phosphate is a slowly available and non-burning source of nitrogen and phosphorus which was in the past used largely as a speciality fertilizer for fruit crops and ornamentals. According to W.L. Lindsay, A.W. Frazier and H.F. Stephenson, (1962) Soil Sci. Am. Proc. 26,446-452, it is a fertilizer so:il reaction product, and can be somewhat troublesome in trickle irrigation if ammonium phosphates are dissolved in magnesium-containing alkaline waters. Ammonium magnesium phosphate hexa-hydrate, or struvite, has the following formula: NH4MgPO4.6H2O. At 80°C, it loses 5 moles of water to form the monohydrate, having the following composition: NH4MgPO4.H2O, and according to Bridger dehydrates at 200°C to the anhydrous salt [G.L. Bridger, M.L. Salotsky and R.W. Staroska, (1962) J.Agr.Food Chem. 10,181-188]. According to W.L. Lindsay et al, (1962) Soil Sci. Am. Proc. 26, 446-452, and A.W. Frazier, J.P. Smith and J.R. Lehr, (1966) J.Agr. Food Chem. 14, 522-529, ammonium magnesium phosphates form, with potassium magnesium phosphates, an isomorphous series of compounds. The hexahydrate of the potassium salt will, at 60°C, lose 5 moles of water to form the mono-hydrate of potassium magnesium phosphate. The anhydrous salt is formed between 100° and 150°C. 14 These orthophosphates of ammonium and potassium with magnesium are excellent slow release fertilizers for P,N or K, We have found in our work that it is possible to form in situ, on the surface of such primary nutrient-carrying fertilizers as Muriate of potash (MOP), Sulphate of Potash (SOP), Di-ammonium phosphate (DAP) or Mono-ammonium phosphate (MAP), a semi-permeable coating consisting either of ammonium or potassium magnesium phosphate or a mixture thereof according to the following reactions: FORMATION OF AMMONIUM MAGNESIUM PHOSPHATES: From MAP and MgO, at temperatures below 50°C: NH4H2PO4 + MgO + 5H2O ? NH4MgPO4.6H2O (Stravite) [1] at temperatures above 80°C; NH4H2PO4 + MgO ? NH4MgPO4.lH2O [2] From DAP and MgO, at temperatures below 50°C: (NH4)2HPO4 + MgO + 5H2O ? NH4MgPO4H2O + NH3 [3] at temperatures above 80°C: (NH4)2HPO4 +MgO ? NH4MgPO4.H2O + NH3 [4] FORMATION OF POTASSIUM MAGNESIUM PHOSPHATES IN THE PRESENCE OF A SALT OF POTASSIUM AND MgO AND AN AMMONIUM PHOSPHATE From MAP, MgO, and K2SO4 or KCl, 15 at temperatures below 50°C: 2NH4H2PO4 + 2 MgO + K2SO4 + 10H2O ? 2[KMgPO4.6H2O] + (NH4)2SO4 [5] at temperatures above 60°C: 2NH4H2PO4 + 2 MgO + K2SO4? 2[KMgPO4.H2O] + (NH4)2SO4 [6] From DAP, MgO, and K2SO4 or KCl, at temperatures below 50°C: 2(NH4)2HPO4 + 2 MgO + 2 KCl + 10H2O ? 2[KMgPO4.6H2O] + 2NH3+2NH4Cl [7] at temperatures above 60°C: 2(NH4)2HPO4 + 2 MgO + 2 KCl? 2[KMgPO4.6H2O] + 2NH4CI+2NH3 [8] As far as the reactions with DAP are concerned, we have observed that the greatest part of the ammonia liberated will either be absorbed or react with either the ammonium phosphate or impurities in the fertilizer. While the coating reactions at an elevated temperature give an excellent coating system, the coating is preferably performed at ambient temperatures. This preference is due to the fact that the low temperature reactions will yield a potassium or ammonium magnesium phosphate having a considerably higher state of hydration and thus will produce a much drier coated product. In addition, coating systems established at elevated temperatures will not only contain their complex magnesium phosphates in a lower state of hydration than those established at ambient temperatures, but will lose a great deal of their original strength during their transition to a higher state of hydration at ambient temperatures. This is most likely due to the fact that a change in the state of hydration will, most of the time, lead to a change in the specific volume of the hydrated compounds. Since ammonium magnesium phosphate forms an isomorphous series with potassium magnesium phosphates, it is most likely that a mixture of these two isomorphous salts, rather than a pure potassium magnesium salt, will be obtained in reaction [5], [6], [7] and [8]. 16 Our work has shown that these reactions not only allow the production of a tenaciously adhering semi-permeable coating, which will considerably reduce the dissolution of water-soluble nutrients on such diverse fertilizers as MAP, DAP, MOP, SOP and urea, but also allow these fertilizers to be coated with finely ground sulphur and other supplementary nutrients. With regard to such divalent metallic supplementary nutrients as copper, zinc, manganese and cobalt, these Me2+ ions are likely to react with such singly charged metallic-type Me+ ions as K+, Na+ and NH4+ to form the respective Me+Me2+PO4 double salts. The state of hydration of these isomorphous salts is more or less similar to that of the ammonium magnesium phosphate. The coating operation (a) for the establishment of a slow release coating system, or (b) for the incorporation of an additional nutrient, comprises introducing the fertilizer into a granulating device at a feedrate such as to produce a bed of granules within the granulating device, which is rotated at a speed which will ensure mixing with coating components to form an adherent coating on the fertilizer granules, and adding coating components being water and plant nutrients. Preferably, the feedrate is such as to form a deep bed of granules within the granulating device, and the rotation speed ensures that the granules tumble and cascade in such a manner that the bed of granules exhibits excellent mixing and compaction characteristics, so as to result in the formation of a dense and tenaciously adhering coating. At least part of the coating water should be added to the granules, to form a tacky surface, before addition of any dry coating components. The total amount of coating water used can vary between 1.0 and 7.0 wt%. If the granular fertilizer is a phosphatic fertilizer, magnesium oxide or magnesium hydroxide may be added, and will react with the surface of the fertilizer granules to form a coating comprising an ammonium magnesium phosphate compound. 17 If the granular fertilizer is a porous non-phosphatic fertilizer, a phosphatic fertilizer (eg DAP or a mixture of DAP and MAP, in a fine state of division), together with magnesium oxide or magnesium hydroxide, may be added to form a coating comprising an ammonium magnesium phosphate or a potassium magnesium phosphate compound. In either case, it is preferable to utilise 0.25 to 5.0 wt% of magnesium oxide or magnesium hydroxide. If a phosphatic fertilizer is to be added during the coating operation (for example, because the base fertilizer granules are non-phosphatic), the phosphatic fertilizer can be added in dry form, as an aqueous suspension or in solution. In particular embodiments, (a) the phosphatic fertilizer is DAP or a mixture of DAP and MAP, and is added as a dry powder; (b) the phosphatic fertilizer is DAP or a mixture of DAP and MAP, the whole or part of which is added in an aqueous suspension; or (c) the phosphatic fertilizer is DAP or comprises DAP, the whole or part of which is added in solution. The coating operation of the present invention is best performed in equipment normally employed in agglomerative granulation processes. The most commonly employed agglomerative granulation devices are the rolling drum granulator, and the inclined pan or disk granulator. While there are many other granulating devices, such as the "Sackett Star Granulator", the "Constant Density Falling Curtain Granulator", the "Multiple Cone Drum Granulator" or the "SAI-R Internal Recycle Granulator" (SAI =Scottish Agricultural Industries), which could be used for these coating processes, we consider these devices to be mere modifications to the conventional drum granulator. Many modifications involve changes to the internal surfaces of the drum to improve the tumbling and cascading actions, as well as the mixing action in the bed of granules. While these improvements might slightly improve the degree of mixing and agitation, or in other words the mechanically-forced contact within the bed of granules to be coated, they 18 often increase the amount of troublesome build-up of wet material on the internal surfaces of the drum. Despite the fact that in previous work, we obtained excellent results with disk or pan granulators having a disk diameter to disk rim height ratio less than 2, we still prefer a drum granulator equipped with annular rings to any other type of coating or granulating device. Our dislike of pan granulating devices is due largely to the fact that the effects of such important basic design parameters as disk diameter, rim-height, angle of inclination of disk, and rotational speed of disk upon the residence time, as well as the quality of the material to be coated, are not understood at all and make any scale-up of disk devices a doubtful undertaking. After nearly 40 years of research into all possible coating as well as granulating devices, we are still convinced that a cylindrical drum granulator meeting the following design specifications is well-suited for the present coating operations: DRUM DESIGN PARAMETERS: Drum length = 3000mm Drum length to drum radius ratio = 2.4:1 Annular ring height to radius ratio = >0.3:l Angle of decline of drum = Nominal fractional bed volume as % of total drum volume = >30% Coating time = >15 minutes Rotational speed of drum = >40% (all measurements are in metres) Ncs is the critical speed, which is defined as the speed of rotation at which the centrifugal force acting upon a particle, at a distance of "r" from the centre of the drum, equals the gravitational force on the said granule when it is in the zenith of rotation. This means that: g = r?2; whereby 19 g is the acceleration due to gravity (= 9.81 m sec-2); 2r is the diameter of the drum in meters; and ? is the angular velocity in radians per second. Thus, the critical speed Ncs is as follows: This equation will only apply to dry materials having a smooth surface in a clean drum. For filthy drums with wet and sticky materials, the critical speed will be somewhat lower than 50% of that for dry materials. We have found that a coating drum meeting these design specifications will produce a tumbling bed of granules, which has excellent mixing characteristics and, because of its depth, will continuously subject the granules in the said tumbling bed to a more severe pounding than in a shallow bed, thereby ensuring that the coating formed on the surface of the granules is not only densified but pushed into the pores of the granules. The densifying mechanical forces in the tumbling bed 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. While it is possible to establish a coat in a shallow bed, which subjects the individual particles to a much lower degree of compaction, the coating water requirements for the establishment of a coating in a shallow bed will be considerably higher than those in a deep bed. The stronger adherence of the coatings applied in a deep bed is most likely due to the fact that the individual particles in the coating established in a deep bed system are densified to such an extent that they are held together by the much stronger forces of cohesion, and not merely by forces of adhesion which are produced by filling the voids between the individual particles of the coating with water, as is the case in a shallow bed. The chemical reactions disclosed in the present specification are far more complex than written in the various equations, which might for this reason not represent a true picture of the reactions occurring during these coating operations. This is 20 clearly demonstrated by the fact that magnesium oxide is likely to react first with water, forming the very reactive magnesium hydroxide, before it reacts with the MAP or DAP. It has to be stressed, however, that MgO is rarely found as such as a natural mineral, but mostly occurs in metamorphic limestones and dolomite, in ejects of volcanos, and in serpentine rock. Magnesium oxide or magnesia of commerce is industrially produced by the burning of magnesite (MgCO3), or from seawater or brine by precipitation with calcium hydroxide as brucite Mg{OH)2, which is then burned to form MgO. If magnesite or brucite is burned at low temperatures, a very reactive caustic or "light burned magnesia" will be produced. At higher burning temperatures, a "hard burned sintered magnesia" will be formed, which cannot be employed in the coating processes disclosed in the present patent specification. The specifications for the "light burned caustic magnesia" are as follows: "Light Burned Magnesia" (EMAG 75) MgO = 95-97wt% CaO = 2.5-3.5wt% SiO2 = 0.8-1,0wt% L.o.I = 1.5-3.5wt% Spec, Surface Area = 50 m2/g Reactivity = 80 -100 sec. This highly reactive "light burned magnesia" can be employed for the formation of ammonium or potassium magnesium phosphate based coating systems by the following methods. 1.0) ALKALI MAGNESIUM PHOSPHATES AS CEMENTING AGENTS IN THE ESTABLISHMENT OF SUPPLEMENTARY NUTRIENT CARRYING COATING SYSTEMS. 1.1) ADDITION OF FINELY GROUND SULPHUR. Since the sulphur incorporated into the coating has to be converted into its water-soluble form by either thiobacilli in the soil, or by auto-oxidation of sulphur in the presence of moisture and oxygen, the sulphur to be incorporated, with the aid of a monovalent alkali magnesium phosphate coating as a binding medium, has to have a high surface area in order to be agronomically useful. The sizings of the sulphur 21 used were as follows: Sizings of Sulphur: 1.1.1) To DAP. Our research work has shown that the addition of the caustic magnesium oxide to the ground sulphur during the milling of the sulphur will lead to an excellent distribution of this additive throughout the ground sulphur. Furthermore, we have found that supplying magnesium in the form of a Mg salt admixed either as a solid in the ground sulphur or dissolved in the coating water does not improve the quality of the coating, with regard to its resistance to abrasion or adherence to the base fertilizer. In addition, we have found that replacing 50wt% of the chemically reactive magnesia by the stoichiometrically equivalent quantity of MgSO4.3H2O results in a coating which is slightly inferior to that produced by MgO on its own. The rate of MgO addition was such that the finished coated fertilizer would contain between 1 and 5wt% of struvite (NH4MgPO4.6H2O). An evaluation of this work showed that very little would be gained by forming in situ more than lwt% (expressed as a wt% of the coated product) of struvite. Example 1: Test Series into the Coating of DAP with Sulphur. In this series of tests, the appropriate quantity of MgO was added to the ground sulphur in the sulphur milling plant to yield, on the basis of the finished product, varying struvite contents in the coating. The following table summarises the raw materials employed in this series of tests. 22 TABLE NO. 1 RAW MATERIALS EMPLOYED DURING THE COATING OF DAP WITH 12wt% OF SULPHUR (wt% expressed on basis of dry solids) [Sulmix] = ["S"] = S° + MgO Nominal Struvite Content wt% 1 2 3 4 5 DAP wt% 89.15 88.96 88.76 88.55 88.33 [MgO in "S"] wt% 1.69 3.38 5.09 6.76 8.45 [S°in"S"] wt% 98.31 96.62 94.41 93.24 91.55 ["S"] wt% 10.85 11.04 11.24 11.45 11.67 Coating H2O wt% 1.75 1.80 1.80 1.90 2.20 The wt% in [ ] represents the concentration of S° and MgO in Sulmix [S]. The water-soluble sulphur content of DAP was assumed to be 1.5wt% as S. The residence time in a production plant was in the order of 18 to 19 minutes at a production rate of 10 tonnes/hour. 1.1.2) To such Non-phosphatic Fertilizers as MOP, SOP and K-Mag. Example 2: The Coating of MOP, SOP and K-Mag with 10wt% of Finely Ground Sulphur. While it is considerably simpler to incorporate elemental sulphur by means of coating it onto phosphatic fertilizers, it might under certain local conditions be advantageous to supply sulphur in the form of S-coated non-phosphatic fertilizers. This coating method will, however, only work if the non-phosphatic granular fertilizers are porous, and were not produced by a process which involves the tumbling of large crystals, but instead were produced by either a conventional wet agglomerative granulation process, or a dry compaction process, which results in granules having an internal void space of about 15% - 35%. In the following testwork, the phosphate required for the formation of the struvite was admixed with the sulphur before milling in the form of di- 23 ammonium phosphate, which had a purity of 95wt%. The appropriate quantity of MgO was added during the coating operation. Since the quantities employed for the various non-phosphatic fertilizers were more or less the same, the raw material data presented in the following table apply to MOP, SOP and K-Mag. The DAP employed did not contain any appreciable quantities of S. TABLE NO. II RAW MATERIALS EMPLOYED DURING THE COATING OF MOP, SOP AND K-MAG WITH 10wt% OF SULPHUR (wt% expressed on basis of dry solids) [Sulmix "P" = [SP] = S° + (NH4)2HPO4] Nominal Struvite Content wt% 1 2 3 4 5 MOP, SOP or K-Mag wt% 89.23 88.39 87.44 86.38 85.20 [Sin[Sp]] wt% 94.34 88.70 83.00 77.30 71.70 [DAPin[Sp]] wt% 5.66 11.3 17.00 22.70 28.30 [SP] wt% 10.60 11.27 12.05 12.94 13.95 MgO wt% 0.17 0.34 0.51 0.68 0.85 Coating H2O wt% 1.75 1.80 1.85 1.90 2.10 The coating time employed in these small scale tests was always 15 minutes. It was found that the struvite-based sulphur coats adhered tenaciously to the base fertilizer. 1.2) THE ADDITION OF MICRONUTRIENTS TO NON-PHOSPHATIC FERTILIZERS. While, generally speaking, most micronutrients are very easily incorporated into phosphatic fertilizers, as well as urea, there is no method available as yet which will allow the incorporation of such micro-nutrients as Copper, Cobalt, Boron, Zinc and Molybdenum into MOP, SOP or K-Mag. 24 We have now found that these micronutrients can be coated onto these non-phosphatic base fertilizers by forming struvite in situ on the surface of these granular fertilizers. We have found that struvite is an excellent coating agent for the incorporation of these micronutrients, as long as the following raw materials are used: TABLE NO. III RAW MATERIALS EMPLOYED IN THE MICRONUTRIENT TEST SERIES COMPOUND FORMULA NUTRIENT MINIMUM SIZINGS wt% wt% µm Cupric oxide CuO 75 - 78 Cu 90 150 Zinc oxide ZnO 79-80 Zn 99.9 53 Boric Acids Ortho H3BO3 16 -17.5 B 75 150 Meta HBO2 22-24.5B 75 150 Tetra H2B4O7 25-27 B 75 150 Calcium borate Ca(BO2)2 16-17 B 75 150 Calcium tetraborate CaB4O7 20-22 B 75 150 Sodium molybdate anhydrous Na2MoO4 45 Mo 90 17 Since these coatings rely upon the crystals of struvite to cement them together, we used a struvite content of 2wt% on the basis of the finished product for incorporating the following micronutrients: 25 TABLE NO. IV PRODUCTION PARAMETERS FOR THE INCORPORATION OF MICRONUTRIENTS INTO NON-PHOSPHATIC FERTILIZERS (Nominal Struvite Content = 2wt%) (COATING WATER EXPRESSED AS WT% OF TOTAL SOLIDS) Final Micronutrient Content of Fertilizer wt% 5Cu 5Zn 2B 0.034 Mo MOP, SOP or K-Mag wt% 81.822 82.082 78.842 88.285 MgO wt% 0.338 0.338 0.338 0.338 DAP wt% 11.30 11.30 11.30 11.30 CuO (76.5 wt% Cu) wt% 6.54 - - - ZnO (79.5 wt% Zn) wt% - 6.28 - - Boron (21 wt% B) wt% - - 9.52 - Molybdenum (45 wt% Mo) wt% - - - 0.077 Coating Water wt% 1.9 2.1 1.9 2.0 The coatings produced by this method adhered extremely well to the non-phosphatic fertilizers. 2.0) SLOW RELEASE COATINGS BY THE IN SITU ESTABLISHMENT OF AN ALKALI MAGNESIUM PHOSPHATE COATING ON THE SURFACE OF GRANULAR WATER-SOLUBLE FERTILIZERS. In order to establish the permeability of these alkali magnesium phosphate coatings with regard to nutrients, we coated varying quantities of struvite-type coatings onto phosphatic, as well as non-phosphatic, granular fertilizers. The nominal struvite content was calculated on the basis of the finished coated fertilizer, and represents an indirect measure of the different coating thicknesses. 2.1) SLOW RELEASE COATING OF MAP OR DAP. Since the quantities of raw materials employed in the coatings of MAP or DAP 26 were more or less the same, the raw materials used in this test series have been combined into one table (below). Example 3: Slow Release MAP and DAP. TABLE NO. V SLOW RELEASE AMMONIUM PHOSPHATES HAVING A VARYING STRUVITE CONTENT (Coating water expressed as wt% of total solids) Nominal Struvite Content wt% 1 5 10 15 20 MAP or DAP wt% 99.83 96.16 98.31 97.46 96.62 MgO wt% 0.17 0.84 1.69 2.54 3.38 Coating H2O wt% 1.0 2.0 3.0 3.5 45 The coating granules produced in these tests possessed excellent physical properties, and showed a considerable reduction in their rate of dissolution with regard to P and N, 2.2) NON-PHOSPHATIC SLOW RELEASE FERTILIZERS. Here again the experimental parameters for the slow release coating of MOP, SOP and K-Mag were more or less the same and, for this reason, the data quoted in the following table apply to all three of these fertilizers. 27 TABLE NO, VI RAW MATERIAL REQUIREMENTS FOR SLOW RELEASE MOP, SOP OR K-MAG FERTILIZERS HAVING A VARYING STRUVITE CONTENT, (Source of PO43-, crystalline DAP, 95% pure) (Coating water expressed as wt% of total solids) Nominal Stravite Content wt% 1 5 10 15 20 MOP,SOP or K-Mag wt% 99.26 96.33 92.65 88.97 85.30 DAP (95% pure) wt% 0.57 2.83 5.66 8.49 11.32 MgO wt% 0.17 0.84 1.69 2.54 3.38 Coating H2O wt% 1.0 1.90 2,9 3.5 4.2 In addition, we have found that the evolution of NH3, generated during the reaction of finely divided DAP with MgO or Mg(OH)2, can be considerably reduced, if part of the DAP (as a source of P) is replaced by MAP. The following table shows the composition of the raw materials employed in tests in which 25%, 50% or 75% of the DAP was replaced by MAP as a source of P. TABLE NO. VII RAW MATERIAL REQUIREMENTS FOR SLOW RELEASE MOP, SOP OR K-MAG FERTILIZERS HAVING A 20 wt% STRUVITE CONTENT AT DIFFERENT LEVELS OF DAP REPLACEMENT BY MAP (Source of PO43-: fine MAP and DAP, 95 wt% pure) (Coating water expressed as wt% of total solids) Nominal Replacement wt% 0 25 50 75 100 of DAP by MAP MOP,SOP or K-Mag wt% 85.3 85.66 86.03 86.39 86.76 DAP (95% pure) wt% 11.32 8.49 5.66 2.83 0 MAP (95% pure) wt% 0 2.47 4.03 7.40 9.86 MgO wt% 3.38 338 3.38 3.38 3.38 Coating Water wt% 4.2 4.3 4.1 4.5 4.2 28 The coatings produced on these non-phosphatic fertilizers adhered tenaciously to the various base fertilizers, which showed a considerable reduction in their release of water-soluble nutrients with an increase in the stravite content of the coating. Furthermore, we have found that it is possible to incorporate the DAP or mixtures of DAP with MAP, during this process, either as a boiling saturated aqueous solution of DAP or as a cold suspension of DAP in water. The quantities of water employed in this series of tests were the same as those shown in Tables Nos VI and VII. In addition, we have found that the rate of dissolution of the water-soluble nutrients can be further decreased by spraying 1 wt% to 4 wt% of molten slack wax onto the bed of tumbling coated granules at the outlet end of the coating device. This forms a sealing layer over the coated granules. Slack waxes are paraffinic byproducts obtained during the refining of crude oil, and have melting points between 42°C and 73°C. While microcrystalline waxes are excellent sealers of pores etc, we prefer paraffinic waxes, which have lower melting points and are less viscous than microcrystalline waxes. The granules should remain for at least another 3 to 4 minutes in the bed of rumbling granules, after spraying with the molten wax. After a total residence time of about 15 to 25 minutes, the coatings produced by this method will adhere tenaciously to the granules, without any subsequent drying step, The coated products produced by these different coating processes, which are based upon the in situ formation of ammonium magnesium phosphates and their isomorphs, were dry and the coating was firm and forcefully affixed to the granules of the base fertilizers, thus providing a simple, reliable as well as inexpensive coating method (a) for the incorporation of such supplemental nutrients as Cu, Zn, Mn, Co, B, Mo, as well as finely ground S to phosphatic as well as non-phosphatic fertilizers and (b) for the reduction of the rate of dissolution of the highly water-soluble nutrients from said fertilizers by encapsulating said fertilizers into this coating. 29 It should be appreciated that the coatings based upon struvite and its isomorphs, which are formed in situ on the surface of the fertilizer granules in a deep bed of tumbling particulates, do not consist of matter which is of no value to the plants, but are in themselves very useful slow release fertilizers containing such valuable plant nutrients as P, Mg, N and/or K, and thus do not act as mere diluents, but instead add value to the fertilizer. The successful execution of the processes for the in situ formation of struvite and its isomorphs depends not only upon the correct proportioning of the raw materials disclosed in the present specif ication, but also the correct operation of the deep bed rotary drum coating device, which will ensure that a tenaciously adhering coating can be established for either the addition of supplementary nutrients to a fertilizer or the reduction in its rate of dissolution, without employing any artificial drying process, which is expensive from an investment, as well as an operating cost, point of view. Finally, it must be stressed that a number of alterations and/or additions may be introduced into the operation of the processes disclosed in the present invention without departing from the spirit or ambit of the present invention. 30 -31-We claim 1. A process for (a) the incorporation of plant nutrients as components of a semi- permeable coating of a granular, phosphatic or non-phosphatic fertilizer, and (b) the reduction of the rate of dissolution of water-soluble nutrients from said fertilizer, which process comprises forming in situ, on the surface of the granules of the fertilizer, a coating comprising compounds selected from the group consisting of one or more ammonium magnesium phosphate and potassium magnesium phosphate compounds, said process comprising the steps of introducing said fertilizer into a granulating device so as to produce a bed of granules within said granulating device, said granulating device being rotated at a speed which will result in the granules in said bed tumbling and cascading, with the bed of granules having mixing and compaction characteristics such as to ensure the formation of a densely adhering coating on the granules from added coating components, being water and plant nutrients. 2. A process as claimed in claim 1, wherein the granular fertilizer is a phosphatic fertilizer, and the coating components comprise magnesium oxide or magnesium hydroxide which reacts with the surface of the fertilizer granules to form, in situ, a coating comprising an ammonium magnesium phosphate compound. 3. A process as claimed in claim 1 or 2, wherein the granular fertilizer is a phosphatic fertilizer selected from the group consisting of di-ammonium phosphate (DAP), mono-ammonium phosphate (MAP) and mixtures thereof. 4. A process as claimed in claim 1, wherein the granular fertilizer is a porous non- phosphatic fertilizer, and the coating components comprise a phosphatic fertilizer and either magnesium oxide or magnesium hydroxide, which react to form, in situ, a coating comprising an ammonium magnesium phosphate or a potassium magnesium phosphate compound. 2. -32- 5. A process as claimed in claim 2 or 4, wherein 0.25 to 5.0 wt % of magnesium oxide or magnesium hydroxide is added. 6. A process as claimed in claim 1 or 4, wherein the granular fertilizer is a non- phosphatic fertilizer selected from the group consisting of muriate of potash (MOP), sulphate of potash (SOP), potassium magnesium sulphate (K-Mag) and mixtures thereof. 7. A process as claimed in claim 6, wherein the non-phosphatic fertilizer is MOP, SOP or K-Mag. 8. A process as claimed in claim 6 or 7, wherein the coating components comprise DAP, MAP or a mixture of DAP and MAP. 9. A process as claimed in any one of claims 6 to 8, wherein the coating components comprise DAP or a mixture of DAP and MAP, and are added as a dry powder. 10. A process as claimed in any one of claims 6 to 8, wherein the coating components comprise DAP or a mixture of DAP and MAP, the whole or part of which is added in an aqueous suspension. 11. A process as claimed in any one of claims 6 to 8, wherein the coating components comprise DAP, the whole or part of which is added in solution. 12. A process as claimed in any one of claims 1 to 11, wherein the ammonium magnesium phosphate or potassium magnesium phosphate compound is a hydrate. 5. -33- 13. A process as claimed in claim 12, wherein the ammonium magnesium phosphate or potassium magnesium phosphate compound is ammonium magnesium phosphate hexahydrate (struvite), ammonium magnesium phosphate monohydrate, potassium magnesium phosphate hexahydrate or potassium magnesium phosphate monohydrate. 14. A process as claimed in claim 1, wherein the amount of water which is added is from 1.0 to 1.7 wt%. 15. A process as claimed in claim 1, wherein the rate of dissolution of water-soluble nutrients is further reduced by spraying molten slack wax onto the bed of tumbling coated granules. 16. A process as claimed in claim 15, wherein the molten slack wax is added to the fertilizer granules at the outlet end of the granulating device, thereby forming a sealing layer over the coated granules. 17. A process as claimed in claim 15 or 16, wherein the amount of slack wax which is added is from 1 to 4 wt %. 18. A process as claimed in claim 1, wherein the total residence time of the fertilizer granules in the granulating device is from 15 to 25 minutes. 19. A process as claimed in any one of claims 1 to 18, wherein said coating incorporates one or more plant nutrients selected from the group consisting of sulphur and compounds of copper, zinc, molybdenum, manganese, boron and cobalt. 13. -34- 20. A process as claimed in claim 19, wherein said coating incorporates ground elemental sulphur. 21. A process as claimed in claim 19, wherein said coating incorporates one or more copper compounds selected from the group consisting of copper oxide, copper sulphate, basic copper sulphate, copper carbonate and basic copper carbonate. 22. A process as claimed in claim 19, wherein said coating incorporates one or more zinc compounds selected from the group consisting of zinc oxide, zinc sulphate, basic zinc sulphate, zinc carbonate and basic zinc carbonate. 23. A process as claimed in claim 19, wherein said coating incorporates sodium molybdate, calcium molybdate or a mixture of sodium molybdate and calcium molybdate. 24. A process as claimed in claim 19, wherein said coating incorporates one or more manganese compounds selected from the group consisting of manganous oxide, manganous sulphate, base manganous sulphate, manganous carbonate and basic manganous carbonate. 25. A process as claimed in claim 19, wherein said coating incorporates one or more boron compounds selected from the group consisting of orthoboric acid (H3BO3), metaboric acid (H2BO2), tetraboric acid (H2B2O7) and calcium tetraborate. 26. A process as claimed in claim 19, wherein said coating incorporates one or more cobalt compounds selected from the group consisting of cobalt sulphate, cobalt carbonate and cobalt hydroxide. 26. -35- th Dated this 15th day of July, 1998. The present invention is directed to a method for producing a coated granular fertilizer by producing in situ on the surface of the granules a coating comprising one or more ammonium magnesium phosphate and/or potassium magnesium phosphate compounds. Additional plant nutrients, such as elemental sulphur, may be incorporated in the coating. The invention also provides a coating system for the encapsulation of granular water soluble phosphatic or non-phosphatic fertilizers with a semi-permeable coating comprising one or more ammonium magnesium phosphate and/or potassium magnesium phosphate compounds.

Full Text

This invention relates to (a) a process which allows the addition of micronutrients and secondary nutrients to granular fertilizers and (b) a process to reduce the dissolution of nutrients from water-soluble granular fertilizers, and more particularly to coating-systems which depend upon hydrated ammonium magnesium phosphates and their isomorphs with potassium for the establishment of tenaciously adhering coatings either (a) to carry micro and/or secondary nutrients, or (b) to reduce the rate of release of water-soluble nutrients. In brief, it relates to the production, by coating processes, (a) of micro- and/or secondary nutrient-carrying granular primary nutrient-carrying fertilizers (the primary nutrient being nitrogen, phosphorus or potassium), or (b) of slow release granular fertilizers carrying very water-soluble compounds of nitrogen, potassium or phosphorus.
In common terminology, the term micro-nutrients refers to such elements as Cu, Mn,
B, Zn, Co, Fe, Se and Mo, and the term secondary nutrients refers to such elements as S, Mg, Ca and Na. From a purely physiological point of view, it is very difficult to justify the classification of plant nutrients into primary, secondary and micronutrients, this classification being solely based upon the concentration of the nutrient in the plant tissue. Thus, this classification is not based on the value of the nutrient in regard to the physiological function it has to fulfil in the plant, which is always very specific and cannot be performed if the nutrient is absent or not present at the required concentration.
The term slow release fertilizer refers mainly to water-soluble nitrogen or potassium fertilizers, in which the rate of dissolution of water-soluble primary nutrient(s) from the fertilizer granule into the soil has been significantly reduced with the object of reducing losses through leaching of water-soluble nutrient(s) from the granule to the subsoil, thereby providing a more permanent source of nutrient(s) which is/are readily available to the growing plant.
Over the last three decades or so, a number of patents have taught the incorporation of micronutrients as well as secondary nutrients into granular fertilizers by coating
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operations. The following patents describe nutrient addition by coating technologies:
H.H.Nau, US Patent 3353949,21 Nov 1967, "Incorporation of Secondary and Micronutrients in Granular Fertilizers", teaches the incorporation of powdered micronutrients, which were previously ground to pass a 400 mesh Tyler test sieve (aperture of 38 urn), by mixing said powdered material into granular NPK fertilizer. This mixture is sprayed, while in the mixer, with a solution such as 50%w/w NH4NO3 or 50%w/w of blackstrap molasses, to stick the powder to the granule surfaces. We think that the use of NH4NO3 solution, which is a strongly oxidising agent, in conjunction with elemental S is from a safety point of view not a practical proposition, while the use of such a solution with Mn++ ions, which are readily available to plants, would convert the Mn++ ions into agronomically unavailable Mn3+ ions.
Otis D. Philen, Julius Silverberg and Melvin M Norton, US Patent 3423199,21 Jan 1969, "Adding Micronutrients to Granular Fertilizers", teach the incorporation by mixing of granular fertilizers with pulverized micronutrient compounds in the presence of a concentrated aqueous solution that interacts to bond the micronutrient firmly to the surface of the granules. The interaction may be chemical or physical or both. In their examples, granular NH4NO3, which had previously been mixed with 10%w/w ZnO and 2.5%w/w kieselguhr (diatomaceous earth) in a rotary mixer, was then sprayed for 1 minute while in the mixer with a 70%w/w NH4NO3 solution. The resulting product consisted of NH4NO3 coated with a complex double salt, Zn5NH4(NO3)2(OH)9.3H2O, which held the unreacted portion of the ZnO in a tenaciously bonded coating. In another example, they teach the incorporation into granular triplesuperphosphate (TSP) of a 13%w/w mixture, consisting of powdered oxides of Zn, Mn, and Fe, in a mixer by spraying this mixture with a 2.8%w/w ammonium polyphosphate solution. While the polyphosphate approach appears to work, the unavailability of polyphosphate materials outside USA prevents the use of this method in most countries. As far as NH4NO3 as a carrier for micronutrients is concerned, the hazards associated with such a powerful oxidising agent make it very
2

unattractive for this application. The decomposition of NH4NO3 is promoted by traces of Cl, free acid and a number of bivalent metals which will, at temperatures as low as 170°C, reach violent dimensions,
Otis D. Philen, Julius Silverberg and Melvin M, Norton, US Patent 3520651,14 July 1970, claim in this patent the double salt 3 Zn{OH)2.NH4NO3 as a new compound, which is formed in situ on the surface of granular fertilizers produced by the method described in their US Patent 3423199.
Otis D, Philen, Julius Silverberg and Melvin M, Norton, US Patent 3523019,4 Aug 1970, claim in this patent the formation of complex tri-zinc diammonium dipyrophosphate Zn3(NH4)2(P2O7).H2O, or complex zinc ammonium hydroxy nitrates, when fertilizer granules are coated according to their previously described patents. Since most nitrates are relatively hygroscopic, the resulting products are, in the main, very unstable and hygroscopic.
G.D. Cicco, US Patent 3560192,2 Feb 1971, suggests in his patent that a free-flowing
granular fertilizer can be produced from 42.2%w/w of (NH4)2SO4 and 57.8 %w/w
TSP by spraying the said mixture in a rotating mixer with an aqueous solution of
66%w/w of ZnCl2, Sufficient powdered anhydrous ZnSO4 is then added to bring the
granules to apparent dryness, the product then being dusted with 3%w/w of ZnO
for the production of a free-flowing dry product with good adhesion of the zinc.
While this method at first produces a relatively dry coated product, the calcium in
the TSP will react with the Cl- in the ZnCl2, forming very hygroscopic calcium
chloride.
J.W. Church, US Patent 386712418 Feb 1975, "Granular Water-Soluble Micronutrient-Containing Fertilizer", employs adducts formed during the reaction in situ between urea and such water-soluble salts as FeSO4.7H2O, MgSO4.7H2O and CuSO4.5H2O for the formation of tenaciously adhering coatings on ammonium nitrate. Since international regulations governing the transport of ammonium nitrate-containing fertilizers and ammonium nitrate mixtures generally restrict the
3

content of inflammable substances to 0.2 to 0.4%, and urea mixtures with NH4NO3 are used in the mining industry as safety explosives in lieu of dynamite, this method is not a very practical proposition from a safety point of view.
Karl H. Walter and Denis Wetherley, AU 554749,15 March 1985, "Production of Granular Trace Element Fertilizers", teach the incorporation of micronutrients into phosphatic fertilizers by treating said phosphatic fertilizers with a mineral acid while the granular fertilizer material is tumbled in a rotary tumbling or coating device in the presence of a particulate micronutrient material, which is bonded to the surfaces of the fertilizer granules while said surfaces are wetted by the said acid. This patent discloses that such micronutrients as Cu, Co, Mn and Zn can be incorporated into the granular fertilizer. Unfortunately, this method can only be employed if the acid employed for the creation of a tacky layer on the surface of the fertilizer granules does not increase the free acidity content of the phophatic fertilizer. This means it can only be used on ammonium phosphate fertilizers. This is due to the fact that any additional acid added to ammonium phosphate fertilizers does not increase the free acid content of these products, but forms either NH4H2PO4 or (NH4)H5(PO4)2 according to:
2(NH4)2HPO4 + H2SO4 ? 2NH4H2PO4 + (NH4)2SO4 or 4NH4H2PO4 + H2SO4 ? 2NH4H5(PO4)2 + (NH4)2SO4
Furthermore, it does not allow the incorporation of elemental sulphur into phosphatic fertilizers. This is due to the fact that elemental sulphur reacts with sulphuric acid forming sulphurous acid, which decomposes into sulphur dioxide and water according to:

Another setback of this method lies in the fact that it cannot be used for the incorporation of additional micro or secondary nutrients into triple or double
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superphosphate without causing serious problems of air pollution by releasing such highly pungent and toxic substances as SiF4 and H2F2.
Karl H. Walter, US Patent 5152821,27 Jan 1989, "Addition of Supplemental Macro & Micro Nutrients to Granular Phosphatic Fertilizers", showed that it is possible to employ minute quantities of water in lieu of concentrated H2SO4 by (a) placing the phosphatic base fertilizers in a tumbling bed of particulates in a rotary mixer cum coating device and (b) creating on the surface of the phosphatic base fertilizer a tacky film by spraying said tumbling bed of granules with minute quantities of water before adding the sulphur and (NH4)2SO4, or secondary or micro nutrients. While this technology has proven itself in the marketplace, it will not allow the incorporation of additional nutrients into non-phosphatic fertilizers. Furthermore, the coating produced on the surface of ammonium phosphates will not withstand too well the relatively rough handling often encountered in machinery in the factory and on the farm.
Roderick D.B. Lefroy and Graeme J. Blair, PCT/AU91/00459, 5 Oct 1990, "Fertilizer Coating Process", teaches the establishment of a layer of water-soluble adhesives on the surface of a moving bed of granules before adding a first portion of an additive carrying the additional nutrient to coat said granule. This spraying/additive operation is sequentially repeated until the desired level of additional nutrient has been added. The binders disclosed are polyvinyl alcohol and sodium lignosulphonate. In addition, it is stated in the embodiment of this specification that the coating of the granules comes about by physical bonding between the granule, adhesive and additives, and not by chemical bonding. This seems highly unlikely, as it means that during this method of coating the ions in the nutrients added to the granule would disobey the laws of basic chemistry and not react with ions in the base fertilizer. Since the binders used in this patent consist of an aqueous solution of the organic adhesive, the removal of water introduced during this coating operation presents a major problem in large scale operation and has so far prevented its use on an industrial scale. In addition, it is extremely difficult to remove surplus free water from most coated products, since most of the complex reaction products are heavily
5

hydrated and lose their water of crystallisation at a relatively low temperature, meaning that these coated products could be classified as being heat-sensitive and thus require expensive drying equipment designed for low temperature operation.
John D. Johnston, AU 669403, 21 Dec 1993, discloses in this patent specification that the problems created by strong acid during coating operations with regard to air pollution could be resolved by the use of diluted acid and subsequent neutralisation of the acid introduced during this method of coating with the help of a neutralising agent. Unfortunately, the use of diluted acid introduces relatively large quantities of free water, which have to be removed from the coating system if a dry, strongly-adhering coat is to be produced. It is erroneously claimed that the heat of reaction produced during the neutralisation of the free acid would remove water as water vapour. Since this is not the case, the problems with this method are similar to those in Lefroy's patent, meaning that it does not resolve the problems associated with free water in the finished coated product.
There can be little doubt that the art disclosed in these patent specifications does not allow effective coating of such base fertilizers as granular muriate of potash, sulphate of potash, sulphate of ammonia, or mono- or di-ammonium phosphate, either with elemental sulphur or other additional secondary nutrients or with micronutrients.
We have now found that a struvite, or even better a hydrated ammonium magnesium phosphate or potassium magnesium phosphate-based, coating system provides an alternative and effective method for the coating of such products. Furthermore, it provides a method for reducing the release of nutrients from water-soluble fertilizers.
Since plant nutrients can only be utilized in the root zone of a soil, which is more or less the cultivated part of the soil, it is of the utmost importance that nutrients in artificial fertilizers are not removed from this zone.
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Loss of nutrients from this zone can occur as a result of erosion of the topsoil by wind or water or by lixiviation with water into the subsoil.
From the point of view purely of nutrient leaching, the loss of P or K nutrients can be neglected as long as the clay content of the soil is > 5.0%w/w and the soil has a normal Fe, Al, Ca and Mg content and until the capacity of the soil to retain these two nutrients is completely satisfied. This means that about 90% of soils on a worldwide basis will not pose any problems in regard to leaching of P or K. The same does not apply to nitrogen, which is easily transported down the soil profile. The extent to which these three nutrients are carried down the soil profile depends largely on the climate, the soil type and the amount of water-soluble nutrients present in the soil solution. Generally speaking, nutrient losses by leaching are worst on freely-draining soils in high rainfall areas having a low CEC (cation exchange capacity) and, of the three major plant nutrients, phosphate is always leached at the lowest rate.
The same applies to potassium ions, which are very tightly bound to many minerals, especially to clay minerals of the 2:1 type (meaning that the crystal lattice is made up of 2 silica sheets and 1 alumina sheet) such as illite, vermiculites and weathered mica. In contrast to P and K, which are mainly retained in the root zone by physico-chemical processes, the leaching characteristics of nitrogen in the soil depend largely upon the type of nitrogen (NO3-, NH4+) as well as the texture of the soil.
In contrast to NO3-, which is extremely mobile in the soil, NH4+ ions are easily adsorbed by soil particles and very strongly adsorbed by such negatively-charged clay minerals as illites, vermiculites and montmorillonites. Thus, it is not surprising that the amount of NO3 nitrogen leached from soils is considerably greater than that in an ammonium form.
There can be little doubt that leaching losses are considerably increased with an increase in nitrogen application rates, meaning that the plant roots cannot utilize the N applied at high application rates.
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For this reason, farmers have tried to overcome these nutrient leaching losses by employing either less soluble fertilizers or using split applications of these fertilizers.
As far as phosphorus is concerned, this can easily be achieved by the application of finely ground reactive phosphate rock, partly acidulated phosphate rock, basic slag, silicophosphates etc.
With regard to nitrogen, there are, besides blood-and-bone, meatmeal and cyanamide, no other low solubility fertilizers, and with respect of K, there exists not a single slowly available source of K. Thus, there can be little doubt that there exists a great need for slow release nitrogen and potassium fertilizers.
Over the last three decades or so, a number of slow release fertilizers have been developed, mainly with regard to N, by either (a) the production of such nitrogenous
fertilizers as urea-formaldehyde or isobutylidene di-urea (IBDU) or
(b) the encapsulation of very soluble N fertilizers in a coating consisting of such
hydrophobic materials as elemental sulphur, wax, bitumen or polymers.
The prior art for these slow or controlled release fertilizers is described in the following patents:
Nitrogenous Fertilizers and NFK Fertilizers:
R. L. Stansbury, C.S. Lynch and Kamil Sor, US 3276 857,4 Oct 1966 (US Reissue 27238,23 Nov 1971), "Slow Release Fertilizer Pellets", teach the mixing of solid (NH4)2SO4 with a molten binder such as wax or asphalt, before mixing it with either powdered gypsum or limestone to give a friable particulate mixture. This mixture is then extruded and coated with molten wax. Stansbury et al suggest that the temperature of the binder should be well above its softening point in order to ensure that the individual (NH4)2SO4 particles are substantially coated. The high power consumption for the extrusion step in this method, as well as the low value of sulphate of ammonia fertilizers, have prevented the use of this method on a large industrial scale.
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O. Detmer et al, US 3365288,23 Jan 1968, "Slow Release Coating for Fertilizers", teach the production of a slow release fertilizer by the coating of a very soluble fertilizer with a drying oily copolymer obtained by the polymerization of a mixture of butadiene and a-methylstyrene containing 50-90% butadiene. The coating is applied to the fertilizer particles heated to about 110-120°C in a rotating drum, preferably in the presence of such drying accelerators as peroxides or azo compounds. While this method has been used, it is too expensive to be used on fertilizers for large scale application.
R.C. Fox, US 3372019,5 Mar 1968, "Slow-Release Coated Fertilizer", teaches the production of a slow release fertilizer by the coating of the fertilizer granules with a mixture consisting of 25-95wt% of wax and 5-75% of a resinous material, either (a) a copolymer of an unsaturated ester containing a total of 3 to 7 C atoms with olefins
having 2 to 4 C atoms or (b) polymers and copolymers having from 2 to 4 C atoms. The resinous material should have a molecular weight > 10 000. The resulting product consists of olefinic ethylacrylate.
H.Hecht and H. Schwandt, US 3708276,2 Jan 1973, "Controlled Release Fertilizer", spray granular fertilizers with an aqueous emulsion of polyvinyl chloride-polyvinylidene chloride copolymer and Zn dust. This method is far too expensive to be used on bulk fertilizers.
O.H. Mueller, US 4023955,17 May 1977, "Controlled Release Fertilizer", prepares a slow release fertilizer by (a) coating the fertilizer granules with a cement partially hydrated with 2 to 30wt% of water and (b) applying a thin second coating of a semipermeable elastomer consisting of styrene-butadiene acrylic ester and alkyd resins. A final third coating of partially hydrated cement, which can contain micronutrients, is applied.
L.S. Wittenbrook and EL. Scheiderer, US 4082533,4 April 1976, "Coated Controlled Release Fertilizer and its Manufacturing Process", teaches the coating of a water-soluble fertilizer in a particulate form, such as prilled urea, by firstly applying
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masonry cement or a mixture of masonry cement and bentonite, before applying a second coating consisting of 5 to 25wt% of a thermoplastic hydrocarbon resin and wax. The fertilizer granules are slightly wetted before the cement is dusted onto the surface, and then heated before the second coating of a copolymer of ethylene and vinyl acetate, a thermoplastic hydrocarbon resin and paraffin wax is applied.
P.S. Fleming, US 3576613, 27 April 1971, "Coating Fertilizer with Sulphur to Control Dissolution", discloses that spraying molten sulphur on fertilizer granules will produce a semipermeable shell of solidified sulphur having a low impact strength. Unless the surface is completely covered with S, the substrate will dissolve quickly in H2O. In view of its hydrophobic nature, molten sulphur does not readily wet the surface of the fertilizer granules, meaning that a relatively heavy coating is required to obtain a slow release effect in the coated product. Fleming claims that dusting the fertilizer granules first with 0.2-3.0wt% of a powdered material, such as carbon black; Zn borate, carbonate, chromate or sulphide; Mg hydroxide or silicate; or another such powdered material, will decrease the contact angle between the granule and the molten sulphur and thus allow the establishment of a satisfactory coating with much less S. He found that 5-10%w/w of sulphur on urea produced excellent results in agronomic testwork in the field.
J.L. Smith, C.R. Crowley and R.B. Humberger, US 4032319, 28 June 1977, "Sulphur Coated Composite Fertilizer", teach the establishment of a non-uniform sulphur coating containing such modifiers as water-swellable clays on granular ammonium, as well as calcium phosphates, using a fluid bed or other suitable coating devices. The process is performed in an inert atmosphere. The finished coated product contains 10-40wt% sulphur.
Potassium Fertilizers:
Masao Hamammoto, Seiichi Kamo and Kiyoshi Nakayama, US 3698885,17 Oct 1972, "Slow Release Potassium Phosphate Fertilizer", teach the production of a slow release fertilizer by the calcination at 700 to 1100°C for 20 to 30 minutes of a mixture consisting of caustic potash and H3PO4 having a P2O5:CaO:K2O molar ratio of
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1.0 :1.7-3.7 :1.2-2.0. The phosphate rock to phosphoric molar ratio expressed as P2O5 was 0.2-1.6 :1.0. An addition of 0.05-2.5wt% of B2O3 as borax or H3BO4 and 0.05-2.5 moles of MgO/mole of P2O5 increased the citrate solubility of the product, and reduced the calcination temperature as well as the time of calcination.MgO was supplied either as the oxide or in the form of such magnesium-containing minerals as serpentine, fosterite etc. After calcination, the product was sprayed at 100°C with an inorganic acid or its ammonium salt to neutralise the material without increasing the water-solubility of the K component. It is claimed that the main constituent of this slow release fertilizer is CaKPO4 which has at 30°C a solubility of 0.1-0.2g K2O/100g H2O.
G.B. Blouin, US3877415,15 April 1975, "Apparatus for Applying Coatings to Solid Particles", describes an apparatus which provides a homogeneous, dense mass of sized particles in random motion so that highly uniform coatings of the same or of different solids can be applied to each particle by conventional spray-coating with the liquified coating material(s). The apparatus consists of a rotary drum containing lifting flights. A novel deflector pan is fixed in space inside the upper section of the drum, and deflects particles falling from the lifting flights to the side of the drum, where they form a narrow, dense falling cascade or curtain. The coating material is sprayed onto the cascading particles, preferably as they free-fall after leaving the lower edge of the pan. If desired, some or most of the coating material may be directed onto the top edge of the moving bed, including the juncture of the cascade therewith. In examples of coating granular urea with molten sulphur, the use of this cascading or falling bed apparatus resulted in a more uniform and effective coating of S and increased the capacity by over 100% over equipment not fitted with a cascade apparatus.
While some of the methods disclosed in these specifications are utilized on an industrial scale, there can be little doubt that the methods proposed in most of these patent specifications will increase the cost of the finished product to such an extent that these slow release fertilizers can only be used in agricultural activities which
11

give a high return. For most agricultural activities, however, these coated fertilizers are far too expensive.
There can be little doubt that there exists a great need for (a) a more reliable coating method for the addition of finely ground and agronomically available elemental S to mono-ammonium or di-ammonium phosphate, as well as to non-phosphatic fertilizers, and (b) a less expensive coating system for the production of less expensive slow release fertilizers.
In addition, it is somewhat illogical that most prior art coating systems rely on the establishment of semi-permeable coatings, which are from an agronomic point of view largely useless.
It is the object of the present invention to provide methods for the coating of such finished granular fertilizers as Di-ammonium phosphate (DAP), Mono-ammonium phosphate (MAP), Muriate of potash (MOP), Sulphate of potash (SOP), Sulphate of ammonia (SOA) and Potassium magnesium sulphate (K-Mag) for the purpose of (a) reducing the rate of dissolution of the said fertilizer or (b) adding such supplemental nutrients as finely ground elemental sulphur or, in nonphosphatic fertilizers, such supplemental nutrients as Copper (Cu), Zinc (Zn), Manganese (Mn), Cobalt (Co), Boron (B), Molybdenum (Mo), etc, these methods overcoming some of the problems experienced with prior methods or processes.
The present invention relates to a method for producing a coated granular fertilizer by producing in situ on the surface of the granules a coating comprising ammonium magnesium phosphate, potassium magnesium phosphate, a hydrate of ammonium magnesium phosphate, a hydrate of potassium magnesium phosphate, or a mixture thereof. Additional plant nutrients, such as elemental sulphur or supplemental plant nutrients (Cu, Zn, Mn, Co, B, Mo, etc:, in ionic form), may be incorporated in the coating. Preferably, the added plant nutrients are selected from elemental sulphur, copper oxide, copper sulphate, basic copper sulphate, copper carbonate, basic copper carbonate, zinc oxide, zinc sulphate, basic zinc sulphate, zinc carbonate, basic zinc
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carbonate, sodium molybdate, calcium molybdate, manganous oxide, manganous sulphate, basic manganous sulphate, manganous carbonate, basic manganous carbonate, orthoboric acid {H3BO3), metaboric acid (H2BO2), tetraboric acid (H2B2O7), calcium tetraborate, cobalt sulphate, cobalt carbonate and cobalt hydroxide.
According to one aspect of the invention, a process for (a) the incorporation of plant nutrients as components of a semipermeable coating of a granular, phosphatic or non-phosphatic fertilizer, and (b) the reduction of the rate of dissolution of water-soluble nutrients from said fertilizer, comprises forming in situ, on the surface of the granules of the fertilizer, a coating comprising one or more ammonium magnesium phosphate and/or potassium magnesium phosphate compounds.
According to a second aspect of the invention, a coating system for the encapsulation of granular, water-soluble, phosphatic or non-phosphatic fertilizers with a semipermeable coating is provided and, according to a third aspect of the invention, a coating system for the incorporation of plant nutrients as components of a semipermeable coating of a granular, phosphatic or non-phosphatic fertilizer is provided. In each case, the coating comprises one or more ammonium magnesium phosphate and/or potassium magnesium phosphate compounds.
The ammonium magnesium phosphate and/or potassium magnesium phosphate compounds are, for example, ammonium magnesium phosphate, potassium magnesium phosphate or hydrates of either of these compounds. In particular, the compounds may be ammonium magnesium phosphate hexahydrate (struvite), ammonium magnesium phosphate monohydrate, potassium magnesium phosphate hexahydrate and/or potassium magnesium phosphate monohydrate.
In one embodiment, the present invention provides a method for the encapsulation of very water-soluble nitrogenous, phosphatic and potassic fertilizers with a coating consisting of such plant nutrients as Mg, P and N, by producing in situ on the surface of individual granules a coating consisting of struvite (ammonium magnesium
13

phosphate hexahydrate), the monohydrate of ammonium magnesium phosphate, or similar ammonium magnesium or potassium magnesium phosphates.
Struvite is a mineral, which has been well-known for a long time, and is widely distributed in nature. It is found in guano, which is a substance found in great abundance on some coastlines or islands frequented by seabirds and originates from their excrements, in dung deposits in caves, as well as in urinary calculi, such as bladder, kidney and gall bladder stones.
The monohydrate of ammonium magnesium phosphate is a slowly available and non-burning source of nitrogen and phosphorus which was in the past used largely as a speciality fertilizer for fruit crops and ornamentals.
According to W.L. Lindsay, A.W. Frazier and H.F. Stephenson, (1962) Soil Sci. Am. Proc. 26,446-452, it is a fertilizer so:il reaction product, and can be somewhat troublesome in trickle irrigation if ammonium phosphates are dissolved in magnesium-containing alkaline waters.
Ammonium magnesium phosphate hexa-hydrate, or struvite, has the following formula: NH4MgPO4.6H2O. At 80°C, it loses 5 moles of water to form the monohydrate, having the following composition: NH4MgPO4.H2O, and according to Bridger dehydrates at 200°C to the anhydrous salt [G.L. Bridger, M.L. Salotsky and R.W. Staroska, (1962) J.Agr.Food Chem. 10,181-188].
According to W.L. Lindsay et al, (1962) Soil Sci. Am. Proc. 26, 446-452, and A.W. Frazier, J.P. Smith and J.R. Lehr, (1966) J.Agr. Food Chem. 14, 522-529, ammonium magnesium phosphates form, with potassium magnesium phosphates, an isomorphous series of compounds. The hexahydrate of the potassium salt will, at 60°C, lose 5 moles of water to form the mono-hydrate of potassium magnesium phosphate. The anhydrous salt is formed between 100° and 150°C.
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These orthophosphates of ammonium and potassium with magnesium are excellent slow release fertilizers for P,N or K,
We have found in our work that it is possible to form in situ, on the surface of such primary nutrient-carrying fertilizers as Muriate of potash (MOP), Sulphate of Potash (SOP), Di-ammonium phosphate (DAP) or Mono-ammonium phosphate (MAP), a semi-permeable coating consisting either of ammonium or potassium magnesium phosphate or a mixture thereof according to the following reactions:
FORMATION OF AMMONIUM MAGNESIUM PHOSPHATES:
From MAP and MgO,
at temperatures below 50°C:
NH4H2PO4 + MgO + 5H2O ? NH4MgPO4.6H2O (Stravite) [1]
at temperatures above 80°C;
NH4H2PO4 + MgO ? NH4MgPO4.lH2O [2]
From DAP and MgO,
at temperatures below 50°C:
(NH4)2HPO4 + MgO + 5H2O ? NH4MgPO4H2O + NH3 [3]
at temperatures above 80°C:
(NH4)2HPO4 +MgO ? NH4MgPO4.H2O + NH3 [4]
FORMATION OF POTASSIUM MAGNESIUM PHOSPHATES IN THE PRESENCE OF A SALT OF POTASSIUM AND MgO AND AN AMMONIUM PHOSPHATE From MAP, MgO, and K2SO4 or KCl,
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at temperatures below 50°C:
2NH4H2PO4 + 2 MgO + K2SO4 + 10H2O ? 2[KMgPO4.6H2O] + (NH4)2SO4 [5]
at temperatures above 60°C:
2NH4H2PO4 + 2 MgO + K2SO4? 2[KMgPO4.H2O] + (NH4)2SO4 [6]
From DAP, MgO, and K2SO4 or KCl, at temperatures below 50°C:
2(NH4)2HPO4 + 2 MgO + 2 KCl + 10H2O ? 2[KMgPO4.6H2O] + 2NH3+2NH4Cl [7]
at temperatures above 60°C:
2(NH4)2HPO4 + 2 MgO + 2 KCl? 2[KMgPO4.6H2O] + 2NH4CI+2NH3 [8]
As far as the reactions with DAP are concerned, we have observed that the greatest part of the ammonia liberated will either be absorbed or react with either the ammonium phosphate or impurities in the fertilizer.
While the coating reactions at an elevated temperature give an excellent coating system, the coating is preferably performed at ambient temperatures. This preference is due to the fact that the low temperature reactions will yield a potassium or ammonium magnesium phosphate having a considerably higher state of hydration and thus will produce a much drier coated product. In addition, coating systems established at elevated temperatures will not only contain their complex magnesium phosphates in a lower state of hydration than those established at ambient temperatures, but will lose a great deal of their original strength during their transition to a higher state of hydration at ambient temperatures. This is most likely due to the fact that a change in the state of hydration will, most of the time, lead to a change in the specific volume of the hydrated compounds.
Since ammonium magnesium phosphate forms an isomorphous series with potassium magnesium phosphates, it is most likely that a mixture of these two isomorphous salts, rather than a pure potassium magnesium salt, will be obtained in reaction [5], [6], [7] and [8].
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Our work has shown that these reactions not only allow the production of a tenaciously adhering semi-permeable coating, which will considerably reduce the dissolution of water-soluble nutrients on such diverse fertilizers as MAP, DAP, MOP, SOP and urea, but also allow these fertilizers to be coated with finely ground sulphur and other supplementary nutrients.
With regard to such divalent metallic supplementary nutrients as copper, zinc, manganese and cobalt, these Me2+ ions are likely to react with such singly charged metallic-type Me+ ions as K+, Na+ and NH4+ to form the respective Me+Me2+PO4 double salts. The state of hydration of these isomorphous salts is more or less similar to that of the ammonium magnesium phosphate.
The coating operation (a) for the establishment of a slow release coating system, or (b) for the incorporation of an additional nutrient, comprises introducing the fertilizer into a granulating device at a feedrate such as to produce a bed of granules within the granulating device, which is rotated at a speed which will ensure mixing with coating components to form an adherent coating on the fertilizer granules, and adding coating components being water and plant nutrients. Preferably, the feedrate is such as to form a deep bed of granules within the granulating device, and the rotation speed ensures that the granules tumble and cascade in such a manner that the bed of granules exhibits excellent mixing and compaction characteristics, so as to result in the formation of a dense and tenaciously adhering coating. At least part of the coating water should be added to the granules, to form a tacky surface, before addition of any dry coating components. The total amount of coating water used can vary between 1.0 and 7.0 wt%.
If the granular fertilizer is a phosphatic fertilizer, magnesium oxide or magnesium hydroxide may be added, and will react with the surface of the fertilizer granules to form a coating comprising an ammonium magnesium phosphate compound.
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If the granular fertilizer is a porous non-phosphatic fertilizer, a phosphatic fertilizer (eg DAP or a mixture of DAP and MAP, in a fine state of division), together with magnesium oxide or magnesium hydroxide, may be added to form a coating comprising an ammonium magnesium phosphate or a potassium magnesium phosphate compound.
In either case, it is preferable to utilise 0.25 to 5.0 wt% of magnesium oxide or magnesium hydroxide.
If a phosphatic fertilizer is to be added during the coating operation (for example, because the base fertilizer granules are non-phosphatic), the phosphatic fertilizer can be added in dry form, as an aqueous suspension or in solution. In particular embodiments, (a) the phosphatic fertilizer is DAP or a mixture of DAP and MAP, and is added as a dry powder; (b) the phosphatic fertilizer is DAP or a mixture of DAP and MAP, the whole or part of which is added in an aqueous suspension; or (c) the phosphatic fertilizer is DAP or comprises DAP, the whole or part of which is added in solution.
The coating operation of the present invention is best performed in equipment normally employed in agglomerative granulation processes.
The most commonly employed agglomerative granulation devices are the rolling drum granulator, and the inclined pan or disk granulator. While there are many other granulating devices, such as the "Sackett Star Granulator", the "Constant Density Falling Curtain Granulator", the "Multiple Cone Drum Granulator" or the "SAI-R Internal Recycle Granulator" (SAI =Scottish Agricultural Industries), which could be used for these coating processes, we consider these devices to be mere modifications to the conventional drum granulator. Many modifications involve changes to the internal surfaces of the drum to improve the tumbling and cascading actions, as well as the mixing action in the bed of granules. While these improvements might slightly improve the degree of mixing and agitation, or in other words the mechanically-forced contact within the bed of granules to be coated, they
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often increase the amount of troublesome build-up of wet material on the internal surfaces of the drum.
Despite the fact that in previous work, we obtained excellent results with disk or pan granulators having a disk diameter to disk rim height ratio less than 2, we still prefer a drum granulator equipped with annular rings to any other type of coating or granulating device. Our dislike of pan granulating devices is due largely to the fact that the effects of such important basic design parameters as disk diameter, rim-height, angle of inclination of disk, and rotational speed of disk upon the residence time, as well as the quality of the material to be coated, are not understood at all and make any scale-up of disk devices a doubtful undertaking.
After nearly 40 years of research into all possible coating as well as granulating devices, we are still convinced that a cylindrical drum granulator meeting the following design specifications is well-suited for the present coating operations: DRUM DESIGN PARAMETERS:
Drum length = 3000mm
Drum length to drum radius ratio = 2.4:1
Annular ring height to radius ratio = >0.3:l
Angle of decline of drum = Nominal fractional bed volume as % of total drum volume = >30%
Coating time = >15 minutes
Rotational speed of drum = >40% (all measurements are in metres)
Ncs is the critical speed, which is defined as the speed of rotation at which the
centrifugal force acting upon a particle, at a distance of "r" from the centre of the
drum, equals the gravitational force on the said granule when it is in the zenith of
rotation.
This means that:
g = r?2; whereby
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g is the acceleration due to gravity (= 9.81 m sec-2); 2r is the diameter of the drum in meters; and ? is the angular velocity in radians per second. Thus, the critical speed Ncs is as follows:

This equation will only apply to dry materials having a smooth surface in a clean drum. For filthy drums with wet and sticky materials, the critical speed will be somewhat lower than 50% of that for dry materials.
We have found that a coating drum meeting these design specifications will produce a tumbling bed of granules, which has excellent mixing characteristics and, because of its depth, will continuously subject the granules in the said tumbling bed to a more severe pounding than in a shallow bed, thereby ensuring that the coating formed on the surface of the granules is not only densified but pushed into the pores of the granules. The densifying mechanical forces in the tumbling bed 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. While it is possible to establish a coat in a shallow bed, which subjects the individual particles to a much lower degree of compaction, the coating water requirements for the establishment of a coating in a shallow bed will be considerably higher than those in a deep bed. The stronger adherence of the coatings applied in a deep bed is most likely due to the fact that the individual particles in the coating established in a deep bed system are densified to such an extent that they are held together by the much stronger forces of cohesion, and not merely by forces of adhesion which are produced by filling the voids between the individual particles of the coating with water, as is the case in a shallow bed.
The chemical reactions disclosed in the present specification are far more complex than written in the various equations, which might for this reason not represent a true picture of the reactions occurring during these coating operations. This is
20

clearly demonstrated by the fact that magnesium oxide is likely to react first with water, forming the very reactive magnesium hydroxide, before it reacts with the MAP or DAP. It has to be stressed, however, that MgO is rarely found as such as a natural mineral, but mostly occurs in metamorphic limestones and dolomite, in ejects of volcanos, and in serpentine rock. Magnesium oxide or magnesia of commerce is industrially produced by the burning of magnesite (MgCO3), or from seawater or brine by precipitation with calcium hydroxide as brucite Mg{OH)2, which is then burned to form MgO. If magnesite or brucite is burned at low temperatures, a very reactive caustic or "light burned magnesia" will be produced. At higher burning temperatures, a "hard burned sintered magnesia" will be formed, which cannot be employed in the coating processes disclosed in the present patent specification. The specifications for the "light burned caustic magnesia" are as follows:
"Light Burned Magnesia"
(EMAG 75)
MgO = 95-97wt% CaO = 2.5-3.5wt%
SiO2 = 0.8-1,0wt% L.o.I = 1.5-3.5wt%
Spec, Surface Area = 50 m2/g Reactivity = 80 -100 sec.
This highly reactive "light burned magnesia" can be employed for the formation of ammonium or potassium magnesium phosphate based coating systems by the following methods.
1.0) ALKALI MAGNESIUM PHOSPHATES AS CEMENTING AGENTS IN THE
ESTABLISHMENT OF SUPPLEMENTARY NUTRIENT CARRYING
COATING SYSTEMS.
1.1) ADDITION OF FINELY GROUND SULPHUR.
Since the sulphur incorporated into the coating has to be converted into its water-soluble form by either thiobacilli in the soil, or by auto-oxidation of sulphur in the presence of moisture and oxygen, the sulphur to be incorporated, with the aid of a monovalent alkali magnesium phosphate coating as a binding medium, has to have a high surface area in order to be agronomically useful. The sizings of the sulphur
21

used were as follows:
Sizings of Sulphur:
1.1.1) To DAP.
Our research work has shown that the addition of the caustic magnesium oxide to the ground sulphur during the milling of the sulphur will lead to an excellent distribution of this additive throughout the ground sulphur. Furthermore, we have found that supplying magnesium in the form of a Mg salt admixed either as a solid in the ground sulphur or dissolved in the coating water does not improve the quality of the coating, with regard to its resistance to abrasion or adherence to the base fertilizer. In addition, we have found that replacing 50wt% of the chemically reactive magnesia by the stoichiometrically equivalent quantity of MgSO4.3H2O results in a coating which is slightly inferior to that produced by MgO on its own.
The rate of MgO addition was such that the finished coated fertilizer would contain between 1 and 5wt% of struvite (NH4MgPO4.6H2O). An evaluation of this work showed that very little would be gained by forming in situ more than lwt% (expressed as a wt% of the coated product) of struvite.
Example 1: Test Series into the Coating of DAP with Sulphur.
In this series of tests, the appropriate quantity of MgO was added to the ground sulphur in the sulphur milling plant to yield, on the basis of the finished product, varying struvite contents in the coating. The following table summarises the raw materials employed in this series of tests.
22

TABLE NO. 1
RAW MATERIALS EMPLOYED DURING THE COATING OF DAP
WITH 12wt% OF SULPHUR
(wt% expressed on basis of dry solids)
[Sulmix] = ["S"] = S° + MgO

Nominal Struvite Content wt%
1
2
3
4
5
DAP wt%
89.15
88.96
88.76
88.55
88.33
[MgO in "S"] wt%
1.69
3.38
5.09
6.76
8.45
[S°in"S"] wt%
98.31
96.62
94.41
93.24
91.55
["S"] wt%
10.85
11.04
11.24
11.45
11.67
Coating H2O wt%
1.75
1.80
1.80
1.90
2.20
The wt% in [ ] represents the concentration of S° and MgO in Sulmix [S]. The water-soluble sulphur content of DAP was assumed to be 1.5wt% as S. The residence time in a production plant was in the order of 18 to 19 minutes at a production rate of 10 tonnes/hour.
1.1.2) To such Non-phosphatic Fertilizers as MOP, SOP and K-Mag.
Example 2: The Coating of MOP, SOP and K-Mag with 10wt% of Finely Ground
Sulphur.
While it is considerably simpler to incorporate elemental sulphur by means of coating it onto phosphatic fertilizers, it might under certain local conditions be advantageous to supply sulphur in the form of S-coated non-phosphatic fertilizers. This coating method will, however, only work if the non-phosphatic granular fertilizers are porous, and were not produced by a process which involves the tumbling of large crystals, but instead were produced by either a conventional wet agglomerative granulation process, or a dry compaction process, which results in granules having an internal void space of about 15% - 35%.
In the following testwork, the phosphate required for the formation of the struvite was admixed with the sulphur before milling in the form of di-
23

ammonium phosphate, which had a purity of 95wt%. The appropriate quantity of MgO was added during the coating operation.
Since the quantities employed for the various non-phosphatic fertilizers were more or less the same, the raw material data presented in the following table apply to MOP, SOP and K-Mag. The DAP employed did not contain any appreciable quantities of S.
TABLE NO. II
RAW MATERIALS EMPLOYED DURING THE COATING OF MOP, SOP AND K-MAG WITH 10wt% OF SULPHUR (wt% expressed on basis of dry solids) [Sulmix "P" = [SP] = S° + (NH4)2HPO4]

Nominal Struvite Content wt%
1
2
3
4
5
MOP, SOP or K-Mag
wt%
89.23
88.39
87.44
86.38
85.20
[Sin[Sp]]
wt%
94.34
88.70
83.00
77.30
71.70
[DAPin[Sp]]
wt%
5.66
11.3
17.00
22.70
28.30
[SP]
wt%
10.60
11.27
12.05
12.94
13.95
MgO
wt%
0.17
0.34
0.51
0.68
0.85
Coating H2O
wt%
1.75
1.80
1.85
1.90
2.10
The coating time employed in these small scale tests was always 15 minutes. It was found that the struvite-based sulphur coats adhered tenaciously to the base fertilizer.
1.2) THE ADDITION OF MICRONUTRIENTS TO NON-PHOSPHATIC FERTILIZERS.
While, generally speaking, most micronutrients are very easily incorporated into phosphatic fertilizers, as well as urea, there is no method available as yet which will allow the incorporation of such micro-nutrients as Copper, Cobalt, Boron, Zinc and Molybdenum into MOP, SOP or K-Mag.
24

We have now found that these micronutrients can be coated onto these non-phosphatic base fertilizers by forming struvite in situ on the surface of these granular fertilizers. We have found that struvite is an excellent coating agent for the incorporation of these micronutrients, as long as the following raw materials are used:
TABLE NO. III RAW MATERIALS EMPLOYED IN THE MICRONUTRIENT TEST SERIES

COMPOUND
FORMULA
NUTRIENT
MINIMUM

SIZINGS

wt%
wt%
µm
Cupric oxide
CuO
75 - 78 Cu
90
150
Zinc oxide
ZnO
79-80 Zn
99.9
53
Boric Acids

Ortho
H3BO3
16 -17.5 B
75
150
Meta
HBO2
22-24.5B
75
150
Tetra
H2B4O7
25-27 B
75
150
Calcium borate
Ca(BO2)2
16-17 B
75
150
Calcium tetraborate
CaB4O7
20-22 B
75
150
Sodium molybdate anhydrous
Na2MoO4
45 Mo
90
17
Since these coatings rely upon the crystals of struvite to cement them together, we used a struvite content of 2wt% on the basis of the finished product for incorporating the following micronutrients:
25

TABLE NO. IV
PRODUCTION PARAMETERS FOR THE INCORPORATION OF MICRONUTRIENTS INTO NON-PHOSPHATIC FERTILIZERS
(Nominal Struvite Content = 2wt%) (COATING WATER EXPRESSED AS WT% OF TOTAL SOLIDS)

Final Micronutrient Content of Fertilizer
wt%
5Cu
5Zn
2B
0.034 Mo
MOP, SOP or K-Mag
wt%
81.822
82.082
78.842
88.285
MgO
wt%
0.338
0.338
0.338
0.338
DAP
wt%
11.30
11.30
11.30
11.30
CuO (76.5 wt% Cu)
wt%
6.54
-
-
-
ZnO (79.5 wt% Zn)
wt%
-
6.28
-
-
Boron (21 wt% B)
wt%
-
-
9.52
-
Molybdenum (45 wt% Mo)
wt%
-
-
-
0.077
Coating Water
wt%
1.9
2.1
1.9
2.0
The coatings produced by this method adhered extremely well to the non-phosphatic fertilizers.
2.0) SLOW RELEASE COATINGS BY THE IN SITU ESTABLISHMENT OF AN
ALKALI MAGNESIUM PHOSPHATE COATING ON THE SURFACE OF
GRANULAR WATER-SOLUBLE FERTILIZERS.
In order to establish the permeability of these alkali magnesium phosphate coatings with regard to nutrients, we coated varying quantities of struvite-type coatings onto phosphatic, as well as non-phosphatic, granular fertilizers. The nominal struvite content was calculated on the basis of the finished coated fertilizer, and represents an indirect measure of the different coating thicknesses.
2.1) SLOW RELEASE COATING OF MAP OR DAP.
Since the quantities of raw materials employed in the coatings of MAP or DAP
26

were more or less the same, the raw materials used in this test series have been combined into one table (below).
Example 3: Slow Release MAP and DAP.
TABLE NO. V
SLOW RELEASE AMMONIUM PHOSPHATES HAVING A
VARYING STRUVITE CONTENT (Coating water expressed as wt% of total solids)

Nominal Struvite Content
wt%
1
5
10
15
20
MAP or DAP
wt%
99.83
96.16
98.31
97.46
96.62
MgO
wt%
0.17
0.84
1.69
2.54
3.38
Coating H2O
wt%
1.0
2.0
3.0
3.5
45
The coating granules produced in these tests possessed excellent physical properties, and showed a considerable reduction in their rate of dissolution with regard to P and N,
2.2) NON-PHOSPHATIC SLOW RELEASE FERTILIZERS.
Here again the experimental parameters for the slow release coating of MOP, SOP and K-Mag were more or less the same and, for this reason, the data quoted in the following table apply to all three of these fertilizers.
27

TABLE NO, VI
RAW MATERIAL REQUIREMENTS FOR SLOW RELEASE MOP, SOP OR K-MAG FERTILIZERS HAVING A VARYING STRUVITE CONTENT,
(Source of PO43-, crystalline DAP, 95% pure) (Coating water expressed as wt% of total solids)

Nominal Stravite Content
wt%
1
5
10
15
20
MOP,SOP or K-Mag
wt%
99.26
96.33
92.65
88.97
85.30
DAP (95% pure)
wt%
0.57
2.83
5.66
8.49
11.32
MgO
wt%
0.17
0.84
1.69
2.54
3.38
Coating H2O
wt%
1.0
1.90
2,9
3.5
4.2
In addition, we have found that the evolution of NH3, generated during the reaction of finely divided DAP with MgO or Mg(OH)2, can be considerably reduced, if part of
the DAP (as a source of P) is replaced by MAP. The following table shows the composition of the raw materials employed in tests in which 25%, 50% or 75% of the DAP was replaced by MAP as a source of P.
TABLE NO. VII
RAW MATERIAL REQUIREMENTS FOR SLOW RELEASE MOP, SOP OR K-MAG FERTILIZERS HAVING A 20 wt% STRUVITE CONTENT AT DIFFERENT LEVELS
OF DAP REPLACEMENT BY MAP
(Source of PO43-: fine MAP and DAP, 95 wt% pure)
(Coating water expressed as wt% of total solids)
Nominal Replacement wt% 0 25 50 75 100
of DAP by MAP

MOP,SOP or K-Mag
wt%
85.3
85.66
86.03
86.39
86.76
DAP (95% pure)
wt%
11.32
8.49
5.66
2.83
0
MAP (95% pure)
wt%
0
2.47
4.03
7.40
9.86
MgO
wt%
3.38
338
3.38
3.38
3.38
Coating Water
wt%
4.2
4.3
4.1
4.5
4.2
28

The coatings produced on these non-phosphatic fertilizers adhered tenaciously to the various base fertilizers, which showed a considerable reduction in their release of water-soluble nutrients with an increase in the stravite content of the coating.
Furthermore, we have found that it is possible to incorporate the DAP or mixtures of DAP with MAP, during this process, either as a boiling saturated aqueous solution of DAP or as a cold suspension of DAP in water. The quantities of water employed in this series of tests were the same as those shown in Tables Nos VI and VII.
In addition, we have found that the rate of dissolution of the water-soluble nutrients can be further decreased by spraying 1 wt% to 4 wt% of molten slack wax onto the bed of tumbling coated granules at the outlet end of the coating device. This forms a sealing layer over the coated granules.
Slack waxes are paraffinic byproducts obtained during the refining of crude oil, and have melting points between 42°C and 73°C. While microcrystalline waxes are excellent sealers of pores etc, we prefer paraffinic waxes, which have lower melting points and are less viscous than microcrystalline waxes.
The granules should remain for at least another 3 to 4 minutes in the bed of rumbling granules, after spraying with the molten wax. After a total residence time of about 15 to 25 minutes, the coatings produced by this method will adhere tenaciously to the granules, without any subsequent drying step,
The coated products produced by these different coating processes, which are based upon the in situ formation of ammonium magnesium phosphates and their isomorphs, were dry and the coating was firm and forcefully affixed to the granules of the base fertilizers, thus providing a simple, reliable as well as inexpensive coating method (a) for the incorporation of such supplemental nutrients as Cu, Zn, Mn, Co, B, Mo, as well as finely ground S to phosphatic as well as non-phosphatic fertilizers and (b) for the reduction of the rate of dissolution of the highly water-soluble nutrients from said fertilizers by encapsulating said fertilizers into this coating.
29

It should be appreciated that the coatings based upon struvite and its isomorphs, which are formed in situ on the surface of the fertilizer granules in a deep bed of tumbling particulates, do not consist of matter which is of no value to the plants, but are in themselves very useful slow release fertilizers containing such valuable plant nutrients as P, Mg, N and/or K, and thus do not act as mere diluents, but instead add value to the fertilizer.
The successful execution of the processes for the in situ formation of struvite and its isomorphs depends not only upon the correct proportioning of the raw materials disclosed in the present specif ication, but also the correct operation of the deep bed rotary drum coating device, which will ensure that a tenaciously adhering coating can be established for either the addition of supplementary nutrients to a fertilizer or the reduction in its rate of dissolution, without employing any artificial drying process, which is expensive from an investment, as well as an operating cost, point of view.
Finally, it must be stressed that a number of alterations and/or additions may be introduced into the operation of the processes disclosed in the present invention without departing from the spirit or ambit of the present invention.
30

-31-We claim
1. A process for (a) the incorporation of plant nutrients as components of a semi-
permeable coating of a granular, phosphatic or non-phosphatic fertilizer, and (b) the

reduction of the rate of dissolution of water-soluble nutrients from said fertilizer, which process comprises forming in situ, on the surface of the granules of the fertilizer, a coating comprising compounds selected from the group consisting of one or more ammonium magnesium phosphate and potassium magnesium phosphate compounds, said process comprising the steps of introducing said fertilizer into a granulating device so as to produce a bed of granules within said granulating device, said granulating device being rotated at a speed which will result in the granules in said bed tumbling and cascading, with the bed of granules having mixing and compaction characteristics such as to ensure the formation of a densely adhering coating on the granules from added coating components, being water and plant nutrients.
2. A process as claimed in claim 1, wherein the granular fertilizer is a phosphatic
fertilizer, and the coating components comprise magnesium oxide or magnesium
hydroxide which reacts with the surface of the fertilizer granules to form, in situ, a
coating comprising an ammonium magnesium phosphate compound.
3. A process as claimed in claim 1 or 2, wherein the granular fertilizer is a phosphatic
fertilizer selected from the group consisting of di-ammonium phosphate (DAP),
mono-ammonium phosphate (MAP) and mixtures thereof.
4. A process as claimed in claim 1, wherein the granular fertilizer is a porous non-
phosphatic fertilizer, and the coating components comprise a phosphatic fertilizer and
either magnesium oxide or magnesium hydroxide, which react to form, in situ, a
coating comprising an ammonium magnesium phosphate or a potassium magnesium
phosphate compound.
2.
-32-
5. A process as claimed in claim 2 or 4, wherein 0.25 to 5.0 wt % of magnesium oxide
or magnesium hydroxide is added.
6. A process as claimed in claim 1 or 4, wherein the granular fertilizer is a non-
phosphatic fertilizer selected from the group consisting of muriate of potash (MOP),
sulphate of potash (SOP), potassium magnesium sulphate (K-Mag) and mixtures
thereof.
7. A process as claimed in claim 6, wherein the non-phosphatic fertilizer is MOP, SOP
or K-Mag.
8. A process as claimed in claim 6 or 7, wherein the coating components comprise
DAP, MAP or a mixture of DAP and MAP.
9. A process as claimed in any one of claims 6 to 8, wherein the coating components
comprise DAP or a mixture of DAP and MAP, and are added as a dry powder.
10. A process as claimed in any one of claims 6 to 8, wherein the coating components
comprise DAP or a mixture of DAP and MAP, the whole or part of which is added in
an aqueous suspension.
11. A process as claimed in any one of claims 6 to 8, wherein the coating components
comprise DAP, the whole or part of which is added in solution.
12. A process as claimed in any one of claims 1 to 11, wherein the ammonium
magnesium phosphate or potassium magnesium phosphate compound is a hydrate.
5.
-33-
13. A process as claimed in claim 12, wherein the ammonium magnesium phosphate or
potassium magnesium phosphate compound is ammonium magnesium phosphate
hexahydrate (struvite), ammonium magnesium phosphate monohydrate, potassium
magnesium phosphate hexahydrate or potassium magnesium phosphate
monohydrate.
14. A process as claimed in claim 1, wherein the amount of water which is added is from
1.0 to 1.7 wt%.
15. A process as claimed in claim 1, wherein the rate of dissolution of water-soluble
nutrients is further reduced by spraying molten slack wax onto the bed of tumbling
coated granules.
16. A process as claimed in claim 15, wherein the molten slack wax is added to the
fertilizer granules at the outlet end of the granulating device, thereby forming a
sealing layer over the coated granules.
17. A process as claimed in claim 15 or 16, wherein the amount of slack wax which is
added is from 1 to 4 wt %.
18. A process as claimed in claim 1, wherein the total residence time of the fertilizer
granules in the granulating device is from 15 to 25 minutes.
19. A process as claimed in any one of claims 1 to 18, wherein said coating incorporates
one or more plant nutrients selected from the group consisting of sulphur and
compounds of copper, zinc, molybdenum, manganese, boron and cobalt.
13.
-34-
20. A process as claimed in claim 19, wherein said coating incorporates ground
elemental sulphur.
21. A process as claimed in claim 19, wherein said coating incorporates one or more
copper compounds selected from the group consisting of copper oxide, copper
sulphate, basic copper sulphate, copper carbonate and basic copper carbonate.
22. A process as claimed in claim 19, wherein said coating incorporates one or more zinc
compounds selected from the group consisting of zinc oxide, zinc sulphate, basic zinc
sulphate, zinc carbonate and basic zinc carbonate.
23. A process as claimed in claim 19, wherein said coating incorporates sodium
molybdate, calcium molybdate or a mixture of sodium molybdate and calcium
molybdate.

24. A process as claimed in claim 19, wherein said coating incorporates one or more
manganese compounds selected from the group consisting of manganous oxide,
manganous sulphate, base manganous sulphate, manganous carbonate and basic
manganous carbonate.
25. A process as claimed in claim 19, wherein said coating incorporates one or more
boron compounds selected from the group consisting of orthoboric acid (H3BO3),
metaboric acid (H2BO2), tetraboric acid (H2B2O7) and calcium tetraborate.
26. A process as claimed in claim 19, wherein said coating incorporates one or more
cobalt compounds selected from the group consisting of cobalt sulphate, cobalt
carbonate and cobalt hydroxide.
26.
-35-

th
Dated this 15th day of July, 1998.

The present invention is directed to a method for producing a coated granular fertilizer by producing in situ on the surface of the granules a coating comprising one or more ammonium magnesium phosphate and/or potassium magnesium phosphate compounds. Additional plant nutrients, such as elemental sulphur, may be incorporated in the coating. The invention also provides a coating system for the encapsulation of granular water soluble phosphatic or non-phosphatic fertilizers with a semi-permeable coating comprising one or more ammonium magnesium phosphate and/or potassium magnesium phosphate compounds.

Documents:

01225-cal-1998-abstract.pdf

01225-cal-1998-claims.pdf

01225-cal-1998-correspondence.pdf

01225-cal-1998-description(complete).pdf

01225-cal-1998-form-1.pdf

01225-cal-1998-form-2.pdf

01225-cal-1998-form-3.pdf

01225-cal-1998-form-5.pdf

01225-cal-1998-priority document.pdf

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Patent Number

194469

Indian Patent Application Number

1225/CAL/1998

PG Journal Number

30/2009

Publication Date

24-Jul-2009

Grant Date

24-Jun-2005

Date of Filing

15-Jul-1998

Name of Patentee

HI-FERT PTY LTD.

Applicant Address

1, RICHMOND ROAD, KESWICK, SOUTH AUSTRALIA.

Inventors:

#	Inventor's Name	Inventor's Address
1	KARL HEINRICH WALTER	18 HOBART STREET, HENLEY BEACH, SOUTH AUSTRALIA
2	ROSLYN JANE BAIRD	18 LOTUS DRIVE, ABERFOYLE PARK, SOUTH AUSTRALIA

PCT International Classification Number

C05B7/00, C05D7/00

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	PO8082	1997-07-18	Australia