A PROCESS FOR MODIFYING PARTICULATE SOLIDS AND PARTICULATE SOLIDS PREPARED THEREBY

Abstract: The invention provides a process for modifying a particulate solid comprising a water soluble inorganic salt, the salt being selected from the group consisting of halides, nitrates, phosphates and sulfates, so as to decrease the rate of dissolution of the salt in aqueous solution, the process comprising contacting the salt with at least reagent which reacts with one or more ions of the salt to form a reaction product which coats the salt, the reaction product containing at least one ion of the water soluble inorganic salt as a component thereof and having a water solubility not more than half that of the original uncoated water soluble inorganic salt.

Description: A PROCESS FOR MODIFYI NG PARTI CULATE SOLI DS AND PARTI CULATE SOLIDS PREPARED THEREBY The present invention relates to water soluble inorganic salts having substantially water insoluble coatings and to processes for the preparation thereof. More particularly the present invention relates to the water soluble fertilizers provided with substantially water insoluble coatings for slow/sustained release thereof. Water soluble salts are of extensive use in aqueous solutions. Especially salts selected from the group consisting of halides, nitrates, phosphates and sulfates are of extensive use in the fertilizer industry. Such salts often contain cations of Li, Na, Mg and Ca and preferably contain cations which also serve as nutrients and micronutrients e.g., K, NH., Fe, Mn, Cu, Co, Mo and Zn. In certain cases, however, slow release of the salt into the aqueous solution is required. An important example is slow release fertilizers. Use of slow release fertilizers instead of direct application of the fertilizer as a soluble salt provides many advantages such as: increased efficiency, reduced number of applications, reduced soil toxicity, reduced nutrient losses and thereby lower contamination of underground water. In the prior art literature there are described two basic approaches for the preparation of slow release salts, i.e., the preparation of. low solubility formulations or the coating of a water soluble salt. SUBSTITUTESHEET Tne first aoproacn involves the preparation of low solubility compounds sucn as: urea formaldehyde (Hasegawa and Onishi, JP63, 134,593, Stajer, Glaser, Vidensky and Vosolsobe (CS 2588147), syngemte (Von Maessennausen, Czikkely and Jung, DE 3,518,369) ana magnesium ammonium phosonate (JP 82 67,089); or water soluble salts granulated, pressed or sintered with other compounds such as gypsum (Koike JP 61,295,294), coal (Fujimori BR 88 03,282), coal ash (Oshikata, JP 60,239,381) and silicates (JP 59 04,397). The main disadvantage of low-solubility compounds or formulations is the high cost of raw materials and/or manufacturing. Coatings of soluble salts comprise polymers (Hansen, L'S 3,223,518, Showa, JP 60,103,093; U.S. 3,223,518), sulfur, wax (Ξibner, DE 3,321,053), pitch (Zou, CM 1,030,745), soybean formulation (NL 34 03,505), salts of fatty acids (Pipko, Manor and Ziv, EP 276,179), sodium silicate (Koππssarov and Panfileva SU 1,353,767), iron phospnate (Eibner DE 3,321,053), magnesium ammonium phosphate (Ninon JP 81,164,090) and gypsum or cement (JP 59,50,086). Current coating material and processes involve many disadvantages such as low resistance to moisture penetration, explosivity with certain fertilizers, high cost raw materials, introducing into the soil of non-degradable materials and difficulties in achievement of well adhering complete coatings. The main disadvantages, however, relate to complicated and expensive coating processes, to reduction of SUBSTITUTESHEET nutrient content, to addition of oalast and to reαuceα freedom in tailoring fertilizer compositions. As is known, even tne preferred prior art processes of coating witn a polymer are complex, multistage o p erations which involve elements from polymer processing such as the use of a solvent .vnich may cause safety ana environmental difficulties and which has to be evaoorateα and recovered, curing, cross-linking and condensation. These operations are expensive, require know-how ana equipment wnicn is strange to the fertilizer industry and in some cases, incompatible with the fertilizer due, e.g., to relatively high temperatures or to formation of explosive compositions with K!\LN0, containing fertilizers. Current processes of coating witn inorganic materials are also tedious, require several stages and may require binders. Thus, coating with MgNH.PO. comprises spraying of H,P0- solution followed by dusting with MgO ana SiO-, and tnen spraying NH.OH solution or ammomation with gaseous ammonia. With current coating materials ana processes, consiaerable amounts of coating are required (up to 50" of fertilizer's weight). Nutrient content thus decreases and expensive raw-materials are consumed, balast such as polymers, sodium silicate and calcium stearate, making uo a substantial component of tne coateα materials. Cost cf raw matc- a s ana of coating processes, toxicity ana coating builcup in the soil thus limit tne use of coateα fertilizers. SUBSTITUTESHEET With this state of the art in mind, it is an object c: the present invention to provide a process for modifying a particulate solid comprising a water soluble inorganic salt, said salt being selected from the group consisting of halides, nitrates, phosphates and sulfates, so as to decrease the rate of dissolution of such salts in water. It is another object of the present invention to provide a new and simpler method of coating which is compatible with the fertilizer industry, enables the utilization of low-cost raw materials, decreases the reduction in nutrient content and addition of balast and enables the preparation of tailor made fertilizers. In accordance with the present invention it has now been found that the surface of the water soluble, inorganic salt particles can be converted through reaction with suitable reagents into a water insoluble coating thereby decreasing the dissolution of the water soluble salt. It was surprisingly found that such coating can be formed even when the reagent is applied in aqueous solution. Thus, the present invention provides a process for modifying a particulate solid comprising a water soluble inorganic salt, said salt being selected from the group consisting of halides, nitrates, phosphates and sulfates, so as to decrease the rate of dissolution of said salt in aqueous solution, said process comprising contacting said salt with at least one reagent which reacts with one or more ions of SUBSTITUTESHEET said salt to form a reaction product which coats said salt, said reaction product containing at least one ion of said water soluble inorganic salt as a component thereof and having a water solubility not more than half that of the original uncoated organic salt. The term particulate solids as used herein concerns particulate solids that range from soluble salts only to compositions that contain at least one water soluble salt as one constituent amongst others that may be water soluble or water insoluble. Thus, particulate table salt consisting of 100% sodium chloride is included in the present invention as is a particulate dry soup containing about 10% sodium chloride. For further clarification of the term "particulate solid" for the purposes of the present invention, several fertilizer materials are listed below. These were chosen since controlling the rate of dissolution of particulate fertilizer is a major, though by no means exclusive, object of this invention. In this connection it is appropriate to mention that the term "rate of dissolution" is commonly used loosely with respect to the interaction of particulate commercial materials with aqueous solutions; the implication being that as the truly water soluble constituents dissolve, the particle disintegrates. This clearly applies to particulate fertilizer that frequently contains insoluble constitutents. Thus examples of particulate solids which can be modified according to the above process include: SUBSTITUTESHEET β a) muriate of potash, which is over 95% KC1 and the remainder of which is mainly NaCl, both of which are fully water soluble: b) granulated "triple super phosphate" which is made fr om phosphate rock and wet process acid and contains soluble monocalci urn phospnate as the main constituent and may contain some 20% of insoluble solids originating in the rock (silica, iron oxide...) and in clay used as a binder for the granulation process; c) granulated NPK made of ammonium phosphate, muriate of potash arid phosphate rock, which may contain over 50% of the water insoluble rock. As stated hereinbefore, the water soluble salts present in the particulate solids to which this invention refers are halides, sul¬ fates, nitrates and phosphates. Naturally, when anions of these salts report to an aqueous solution an equivalent amount of cations do so. As explained hereinafter said reagent can react with either said anion or its companion cation. In a preferred embodiment of the present invention there is provided a process for modifying a particulate solid comprising a water soluble inorganic salt, so as to decrease the rate of dissolution of said salt in aqueous solution, said salt being selected from the group consisting of halides, nitrates, phosphates and sulfates and optionally including cations of K, MH Λ , re, Mn, Cu, Co, Mo and Zn, said process comprising contacting said salt with at least SUBSTITUTESHEET one reagent whic. reacts with one or more ions of said salt to form a reaction product which coats said salt, saiα reaction product containing at least one of said ions as a component thereof and having a water solubility not more tnan half that of the original uncoated organic salt. In an especially preferrec emoodiment of the present invention there is provided a process for modifying a particulate solid comprising a water soluble inorganic salt, said process comprising contacting particles of said particulate solid with at least one reagent which reacts with one or more ions of said salt to form a reaction product which coats said particles. The invention also provides a particulate solid comprising a water soluble inorganic salt, said salt being selected from the group consisting of halioes, nitrates, phosphates ana sulfates, and saiα salt having a substantially water insoluble coating, said coating having been formed by contacting said salt with at least one reagent which reacts with one or more ions of said water soluble salt to form a reaction product which coats said salt, said reaction product containing at least one ion of said water soluble organic salt as a component thereof and having a water solubility not more than half that of the original uncoated organic salt. In especially preferred embodiments the invention provides a particulate solid comprising a water soluole inorganic salt, saiα salt SUBSTITUTESHEET being selected from the group consisting of halides, nitrates, phosphates and sulfates, and optionally including cations of K, NH., Fe, Mn, Cu, Co, Mo and Zn, said salt having a substantially water insoluble coating, said coating having been formed by contacting said salt with at least one reagent which reacts with one or more of said ions to form a reaction product which coats said salt, said reaction product containing at least one of said ions as a component thereof and having a water solubility not more than half that of the original uncoated organic salt. The invention also provides a particulate solid according to the present invention comprising a water soluble inorganic salt wherein particles of said particulate solid are provided with a substantially water insoluble coating, said coating on said particles having been formed by contacting said salt with at least one reagent which reacts with one or more of said ions to form a reaction product which coats said particles, said reaction product containing at least one of said ions as a component thereof and having a water solubility not more than half that of the original uncoated organic salt. In an especially preferred embodiment of the present invention said reaction product has a water solubility less than 10% of that of the original uncoated water soluble inorganic salt. SUBSTITUTESHEET In a preferred embodiment, the reagent is applied in a combination with at least one of the group comprising surfactants, viscosity, modifiers, water-repellants and binders, to improve mechanical strength, adhesivity to the particle and to further reduce solubility in water. In another preferred embodiment, additional operations are effected for further improvement of the solid particulate properties. SUBSTITUTESHEET One example is treatment by a surface active agent (which is arice to the reagent or applied at a subsequent stage). Such agent might be selected, for example, from those used for anticaking of crystals or for flotation (e.g. amines). The polar side of the surface active agent (surfactant) interacts with the reaction product in the particulate cover, while the hydrophobic end decreases interaction with water and thereby affects solubility. This hydrophobic end also provides for further coating of the particle by hydrophobic solvents as hydrocarbons. Other treatments may include thermal treatment for affecting the coating's characteristics (e.g., converting gypsum dihydrate to hemi-hydrate), a second treatment by the same reagent or treatment by a second reagent (e.g., treatment by a base to convert CaHPO, in the coating to the less soluble Ca,(P0.) ? ), etc. In EPA publication No. 276179 there is described and claimed a method for the manufacture cf physically prepared slow-release fertilizers, wherein the fertilizer particles are coated by at least one layer of a substantially water insoluble metal salt of an organic acid containing from 6 to 30 cabon atoms, particularly between 12 and 20 carbon atoms, or of water insoluble metal salts of a mixture of two or more such organic acids,, the metal salt(s) coating being formed in-sit' from the organic salt(s) and metal oxide or carbonate. SUBSTITUTESHEET As will be realized, nowever, tne aDove method suffers from tne disadvantages that it requires the introduction ana use cf bctn an organic acid or salt and a metal oxide or caroonate to interreact with each otner to form a coating around the fertilizer particles as opposed to tne present invention in which water soluble fertilizer particles tnemselves actively interact with the reagent and ions of said water soluble fertilizer form an integral component of the formed water insoluble coating. Furthermore, tne present invention assures that the reaction to form the water insoluble coating takes place on the surface of the water soluble salt, since in the present invention this salt functions as one of the reactants, which is not the case with said prior art process. This further assures that loss of reagent due to precipi¬ tation away from the surface of the salt does not take place. Although the process cf the present invention can be applied to all kincs cf ater soluble salts, tne following description concentrates on the treatment of water soluble fertilizers in orαer to modify these into fertilizers with slow release properties. Thus, as in said European application the fertilizer to be coated according to the present invention, may be selected from the well known water scluole fertilizers such as: potassium nitrate, ootassiu- sulfate, ammonium nitrate. monopotassium pnosohate, ammonium ohosonates, mixtures of N. P.K. fertilizers, etc. SUBSTITUTESHEET The main nutrients providec tnrougn fertilizers are N, ? ano K. Micro-nutrients consist of Fe, Mn, Cu, Co, Mo, In, etc. N is proviαed as ammonium salts (e.g., (NH „S0 Λ ), NO.*,- salts (e.g., Ca(N0-.) 2 , NaNC,) and urea. is provided in its soluble salts such as KC1 ana KoSO. and P as pnosonates. Of higher value are fertilizers m whicn both the amon and the cations comprise a nutrient, e.g. KNO, ( H 4 ) 2 HP0 4 , H 2 P0 and NH_, 0 3 . Reagents forming water insoluble salts with the anion or the cation of the water soluble salts are suitable for tne process. Thus, e.g., on reacting O . particles witn rLSiFg, the surface cf tne particle is converted according to the reaction: K-.SO4 ÷* rUSiF, > K 2 SiF 6 + H 2 S0 4 . The modified particle thus comprises K^SO. coated with K^S F^- with substantially no decrease of nutrient content. However, even on reaction with the otner ion, nutrient replacement is low and the replaced nutrient can oe recovered as in the reaction: 2 S0 4 + Ca(N0 3 ) 2 > CaS0 4 ■«■ KN0 3 . In another preferred embodiment the reagent can be chosen to also add micro-nutrients to tne fertilizer. Thus, e.g, ZnCHN.^ζSO. ) 2 or cobalt ammonium nitrate are suitable insoluble coatings of ammonium salts. SUBSTITUTESHEET Thus, preferred embodiments of the present invention will include the nutrients N, P or K as a component of the reaction product and especially preferred embodiments of the present invention will also incorporate one or more micronutrients selected from Fe, Mn, Cu, Co, Mo and Zn in said coating. Reagents for the process are those forming water insoluble salts with at least one of the ions comprising the fertilizer. A person versed in the art can choose reagents according to solubility data and according to crystalographic know-how. According to the present invention, it will now be realized that, e.g., K salts can be coated with K 2 Ca(S0 4 ) 2 , K 3 Co(N0 2 )g and K 2 SlF g ; NH 4 + salts form insoluble coatings such as H 4 CaP0 4 , NH 4 CoP0 4 - H 4 gP0 4 and H 4 MnP0 4 ; soluble calcium phosphates or Ca(NC ) can be coated with insoluble salts such as CaCO^, CaF-, Ca laurate, Ca oxalate, Ca 3 (P0 4 ) 2 and Ca tartarate; water soluble phosphate can be coated with water insoluble phosphates such as FeP0 4 and water soluble sulfates with water- msoluble sulfates such as CaS0 4 and BaS0 4 . In carrying out the process of the present invention, reagents may be applied in vapor phase (NH 3 , SiF α ), in non-aqueous liquid, as a dispersion, emulsion or even in aqueous solution. The reagent may also be sprayed over the solid particles. It has thus been found that on introducing particles comprising a water-soluble salt into gently mixed, relatively concentrated solution SUBSTITUTESHEET of the reagent at about ambient temperature, a water insoluble coating is immediately formed on the surface of the salt particles which prevents considerable losses of the soluble salt through dissolution in the aqueous solution. The rate of release of the water soluble salt is dependent on coating thickness, which can be affected by parameters of reagent application. Thus in using the reagent in an aqueous solution, concentration of the aqueous solution, contact time and temperature. pH, reagent nature and additives to the solution, affect coating thickness and thereby the rate of release of the soluble salt. The rate of release is also affected by solubility of the coating wherein less soluble coatings provides for slower release. Unlike in other processes of coating water soluble particles, elevated temperatures are not required. The process is thus suitable for coating particles of unstable compounds such as HN 4 N0-,. Particles coated in the process of this invention can be further coated by other coating materials such as wax. SUBSTITUTESHEET Modified particulate solids obtained according to the invention, having a low rate of dissolution in aqueous solutions, provide excellent starting material for additional treatments for further improvement of their properties, i.e., improvement of mechanical strength, further reduction of rate of dissolution, etc. Such additional treatment may, for example, use components such as surfactants, binders and polymeric materials dissolved or dispersed in aqueous solution. Such option is, in fact, impractical with unmodified water-soluble particulate solids, as it would lead to the dissolution of a major part of the unmodified particulate. The reaction product of low water solubility that coats the soluble salt in the modified particulate solid may, in some cases, perform as a low-cost filler in other coatings formed cocurrently with, or subsequently to, it. The large variety of reagents suitable for modifying the solubility of water soluble particles according to the present invention allows freedom in choosing the reagents and adjusting them SUBSTITUTESHEET to reαuirements such as addition of micronutrients, adjustaoi 1ity to acid/base properties of the soil and low cost. The latter point is of high importance as slow release fertilizers formed by current processes are relatively expensive due to high cost of raw material and of processing. Using the present invention thus allows the use of low cost reagents such as H-SiFg solutions. More specifically, it has now been found that introducing granules of KNO, into gently mixed solution at ambient temperature results in their coating with water insoluble K 7 Si " g, wnich changes the solubility of the coated fertilizer. H-,SiFg is a very low cost reagent. In fact, in the neighborhood of wet process acid (WPA) production it may even have a negative value. Fluoride present in phosphate rock in the form of fluoroapatite results in fluosilicic acid in the phosphoric acid. On concentration of the WPA SiF- evaporates and on scrubbing in water H 2 SiFg reforms according to the reaction: 2SiF. + H,0 > H„Slr- ÷ Si0 . A 2 c D 2 Since the coating is formed on reaction with the soluble salt particle and includes at least one of its ions, good adhesion to the particle is ootained avoiding the need of binders and improving the ability to affect the coating of less spherical particles. Formation of inorganic coating reduces the αanger of flammability ana of formation of explosive materials. SUBSTITUTE SHEET The process of the invention allows using reagents in vapor phase, in non-aqueous liquid, as an emulsion or even in aqueous solution. Melting of reagents is not required. In addition reaction of the reagent with the water soluble particle is spontaneous requiring no energy introduction. The process thus does not require energy input or temperature elevation. In fact in most cases low temperature is favorable, which is advantageous for coating of temperature-sensitive compounds such as HN.N0-,. In another preferred embodiment the reagent is introduced in an organic solvent. Thus, amine solvents such as those defined in Israel Patent 57024 corresponding to U.S. Patent No. 4,291,007, the teachings of which are incorporated herein by reference, bind H-,SiF, to form the fluosilicate of the amine according to the reaction R NH- + H-,SiF, > R NH. HSiF, or (R NH. 7 SiF,. n J-n n 4-n 6 n 4-n . o wherein n = 1 , 2 or 3. Reacting this organic phase with a solid particle containing KNO^ for example, results in a reaction of forming a water insoluble coating on the particle according to the reaction (R NH. )_SiF, + 2KN0, > 2R.NH. NO, + 9 SiF,. n 4-n'2 6 3 4 4-n 3 2 6 Introduction of the reagent in the presence of an organic solvent provides several advantages such as: 1. Reaction may be affected and controlled by the properties of the solvent, e.g., it is known that amines prefer nitric acid over SUBSTITUTESHEET fluosilicic acid thereby Droviding a driving force for tne reaction; 2. the solvent may be useα to extract a reagent from its aαueous solution thereny permitting the use of reagents formed as by-products even if in dilute solutions (e.g., H-,SiFg formed on scrubbing Sir, , in phosphoric acid industry); and 3. the solvent may be chosen so as to form tne required organic coating on the top of the insoluble salt which is formed. The extensive know-how in the field of crystalization, and in benaviour of saturated and cf super-saturated solutions can be utilized for adjusting coating's properties. Small amounts of additives may affect the habit of coating and improve product properties. As both fertilizers and soil are inorganic and as fertilizer release and utilization are dependent on water presence, the process of the invention provides for higher compatibility with fertilizer production and utilization. While the invention will now be described in connection with certain preferred embodiments in the following examples so that aspects thereof may be more fully unαerstood and appreciated, it is not intended to limit the invention to these particular embodiments. On the contrary. it is intended to cover all alternatives, modifications and eαuivalents as may be included within the scooe of the invention as defined by the appended claims. Thus, the following SUBSTITUTESHEET examples wnich include preferred embodiments will serve to illustrate the practice of tms invention, it being understood that tne particulars shown are by way of example and for purposes of illustrative discussicn of preferred embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of formulation procedures as well as of the principles and conceptual aspects of the invention. Comparative Example A Potassium nitrate granules with sizes ranging between about 1 mm and 5 mm were introduced into water in w/w ratio of 1:20 respectively. Strong shaking was applied in a mechanical shaker at ambient temperature. Complete dissolution was observed after about 3 minutes. (No insoluble matter was left in the solution). Example 1 Potassium nitrate granules from the batch used in Comparative Example A were introduced into 36% w/w aαueous solution of fluosilicic acid and gently stirred in a mechanical shaker at ambient temperature for a period of about five hours to effect the coating thereof. No indication of potassium nitrate dissolution was found and it is assumed that any potassium nitrate exiting from the coated granules was immediately reacted with the fluosilicic acid still in solution and adds to the coating. Independent precipitation of I SiFg was not SUBSTITUTESHEET ooserveα thus strengthening tne assumption that K 7 SiFg was formeα solely as a coating on tne granules ana not independent thereof. The treated granules were separated from tne solution, washeα and introduced into water in w/w ratio of 1:20 respectively. Strong shaking was applied in a mechanical shaker at ambient temperature. Potassium nitrate dissolution could be visually followed as the coating formed is partially transparent. About eight hours were required for substantial dissolution of the potassium fluosilicate coated potassium nitrate. After 16 hours only the coating remains in solution. This example shows that coating potassium nitrate with potassium fluosilicate slows its dissolution by a factor of 2 orders of magnitude. Having illustrated the principals of the present invention and the carrying out thereof in the above example it will now be simple for men skilled in tne art to carry out the present invention with other ions simply by consulting any handbook and noting the comparative solubility of soluble and substantially insoluble salts. Thus, e.g., from The Handbook of Chemistry and Physics, Chemical Rubber Co. the following listing of solubilities of a gram of salts per 100 gram H-,0 in cold water can be founα, which listings are presented hereinafter for eacn ion, first with regard to water soluble SUBSTITUTESHEET salts and then with regard to less soluble salts which can serve as the reaction product coating for the first batch of water soluble salts. In the following lists, "i" stands for insoluble. SUBSTITUTE SHEET Sodium salts Cold water Sodium nitrate 92.1 Sodium hydrogen oxalate 1.7 Sodium phosphate 1.5 Sodium fluosilicate 0.65 Lithium salts Lithium nitrate 90 Lithium laurate 0.15 Lithium phosphate 0.04 Lithium fluoride 0.27 Magnesium salts Magnesium nitrate hexahydrate 125 Magnesium phosphate Magnesium fluoride 0.008 Magnesium laurate 0.007 Magnesium oleate 0.024 Magnesium oxalate 0.07 SUBSTITUTESHEET K. salts Potassium sulfate 12 Potassium chloride 34.7 Potassium nitrate 13.3 Potassium dihydrogen phosphate 33 Dipotassium hydrogen phosphate 167 Potassium hydrogen oxalate 2.5 Potassium hydrogen tartarate 0.37 Potassium fluosilicate 0.12 Potassium calcium sulfate (syngenite) 0.25 Calcium salts Calcium dihydrogen phosphate 1.8 Calcium nitrate 121 Calcium sulfate 0.2 Calcium carbonate 0.001 Calcium fluoride 0.002 Calcium laurate 0.004 Calcium oxalate 0.0007 Calcium phosphate 0.002 Calcium tartrate 0.03 SUBSTITUTESHEET 1370 24 Ammonium salts Ammonium sulfate 70.6 Ammonium nitrate 118.3 Ammonium dihydrogen phosphate 22.7 Ammonium hydrogen phosphate 57.5 Ammonium calcium phosphate i Ammonium magnesium phosphate 0.02 Ammonium cobalt phosphate i Ammonium manganese phosphate 0.003 Ammonium zinc sulfate 7 Cobalt ammonium nitrate 1.7 Cobalt ammonium sulfate 1.4 Cobalt ammonium chloride 0.4 SUBSTITUTESHEET Iron Salts SUBSTITUTE SHEET /01370 2 Cobalt Salts Cobalt chloride 38 Cobalt nitrate 134 Cobalt citrate 0.8 Cobalt oxalate i Cobalt phosphate i Zinc Salts Zinc chloride 432 Zinc nitrate 114 Zinc oleate i Zinc oxalate 0.0008 Zinc phosphate i Phosphates Potassium dihydrogen phosphate 33 Ammonium dihydrogen phosphate 22.7 Iron (III) phosphate i Cobalt (II) phosphate i Copper (II) phosphate i Zi c phosphate i SUBSTITUTESHEET It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative examples and that the present invention may be embodied in other specific forms without departing from the essential attributes thereof, and it is therefore desired that the present embodiments and examples be considered in all respects as illustrative and not restrictive, reference being made to the appended claims, rather than to the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. SUBSTITUTESHEET

Claims: WHAT IS CLAIMED IS: 1. A process for modifying a particulate solid comprising a water soluble inorganic salt, said salt being selected from the group consisting of halides, nitrates, phosphates and sulfates, so as to decrease the rate of dissolution of said salt in aqueous solution, said process comprising contacting said salt with at least one reagent which reacts with one or more ions of said salt to form a reaction product which coats said salt, said reaction product containing at least one ion of said water soluble inorganic salt as a component thereof and having a water solubility not more than half that of the original uncoated water soluble inorganic salt. 2. A process for modifying a particulate solid comprising a water soluble inorganic salt, as claimed in claim 1, said salt being selected from the group consisting of halides, nitrates, phosphates and sulfates, and optionally including cations of K, NH 4> Fe, Mn, Cu, Co, Mo and Zn, so as to decrease the rate of dissolution of said salt in aqueous solution, said process comprising contacting said salt with at least one reagent which reacts with one or more ions of said salt to form a reaction product which coats said salt, said reaction product containing at least one of said ions as a component thereof and having a water solubility not more than half that of the original uncoated water soluble inorganic salt. 3. A process according to claim 1 wherein said particulate solid is contacted with an aqueous medium containing said reagent. SUBSTITUTESHEET 4. A process according to claim 1 for modifying a particulate solid comprising a water soluble inorganic salt, said process comprising contacting particles of said particulate solid with at least one reagent which reacts with one or more ions of said salt to form a reaction product which coats said particles. 5. A particulate solid comprising a water soluble inorganic salt, said salt being selected from the group consisting of halides, nitrates, phosphates and sulfates, and said salt having a substan¬ tially water insoluble coating, said coating having been formed by contacting said salt with at least one reagent which reacts with one or more ions of said water soluble salt to form a reaction product which coats said water soluble salt, said reaction product containing at least one ion of said water soluble inorganic salt as a component thereof and having a water solubility not more than half that of the original uncoated water soluble inorganic salt. 6. A particulate solid as claimed in claim 5 comprising a water soluble inorganic salt, said salt being selected from the group consisting of halides, nitrates, phosphates and sulfates, and optionally containing cations of K, H 4 , Fe, Mn, Cu, Co, Mn and Zn, said salt having a substantially water insoluble coating, said coating having been formed by contacting said salt with at least one reagent which reacts with one or more of said ions to form a reaction product which coats said salt, said reaction product containing at least one of said ions as a component thereof and having a water solubility not SUBSTITUTESHEET more than half that of the original uncoated water soluble inorganic salt. 7. A particulate solid according to claim 5 wherein said reaction product has a water solubility less than 10% of that of the original uncoated water soluble inorganic salt. 8. A particulate solid according to claim 5 wherein said reaction product is selected from K 2 Ca(S0 4 ) 2 , 3 Co(N0 2 )g and I SiFg, 9. A particulate solid according to claim 5 wherein said reaction product is selected from calcium sulfate, calcium carbonate, calcium fluoride, calcium laurate, calcium oxalate, calcium phosphate and calcium tartrate. 10. A particulate solid according to claim 5 wherein said reaction product includes nitrogen, phosphorus or potassium as a component thereof. 11. A particulate solid according to claim 5 wherein said reaction product further includes a micronutrient selected from Fe, Mn, Cu, Co, Mo and Zn as a component thereof. 12. A particulate solid according to claim 5 comprising a water soluble inorganic salt, wherein particles of said particulate solid are provided with a substantially water insoluble coating, said SUBSTITUTESHEET coating on said particles having been formed by contacting said salt with at least one reagent which reacts with one or more ions of said waster-soluble salt to form a reaction product which coats said particles, said reaction product containing at least one ion of said water-soluble inorganic salt as a component thereof and having a water solubility not more than half that of the original uncoated water-soluble inorganic salt. 13. A particulate solid according to claim 5, wherein said water-soluble inorganic salt is KN0 3 . 14. A particulate solid according to claim 13, wherein said reaction product is K 2 SiF β . 15. A particulate solid according to claim 1, further comprising a surfactant. SUBSTITUTESHEET

Inventors: EYAL AHARON MEIR, EYAL, AHARON, MEIR
Applicants: EYAL AHARON MEIR, WHALLEY KEVIN, YISSUM RES DEV CO, WHALLEY, KEVIN, YISSUM RESEARCH DEVELOPMENT COMPANY OF THE HEBREW
Application year: 1993
Main classification: C05G3
IPC codes: B01J2, C05G3