NPK mineral fertilisers and method for preparing same

Abstract

The present invention relates to a solid NPK mineral fertiliser containing silicon and sulphur, wherein the potassium and silicon are in the form of potassium silicate, the method for preparing the NPK mineral fertilizer, and the use thereof.

Description

[0001] DESCRIPTION

[0002] TITLE: NPK Mineral Fertilizers and Their Preparation Process

[0003] FIELD OF INVENTION

[0004] The present invention relates to the field of mineral fertilizers and more particularly to solid NPK compound fertilizers containing bioavailable silicon and sulfur, in particular soluble sulfur, their preparation process, as well as their use in agriculture.

[0005] STATE OF THE ART

[0006] Mineral fertilizers are designed to improve agricultural yields. Their use is crucial for ensuring food security for future generations. Today, it is very difficult to achieve good yields in agriculture without fertilizer inputs, whether mineral or organic. Mineral fertilizers in agriculture can be simple or compound. Simple mineral fertilizers contain only one nutrient: nitrogen (N), phosphorus (P), or potassium (K). Binary mineral fertilizers contain two nutrient elements: NK, NP, or PK. Tertiary mineral fertilizers contain all three NPK elements. There is a wide variety of NPK tertiary fertilizers, depending on the NPK ratio (N / P₂O₅ / K₂O content), the nature of the raw materials used, and their production processes.

[0007] The addition of other macronutrients such as sulfur (S), calcium (Ca) and magnesium (Mg) and trace elements such as iron (Fe), boron (B), copper (Cu), zinc (Zn), manganese (Mn) and molybdenum (Mo) is essential to complement the effects of each of these chemical fertilizers.

[0008] Sulfur is classified as a macroelement, like calcium and magnesium, essential for crop productivity and quality. Although considered a secondary element, plants have equal, and sometimes greater, sulfur requirements than phosphorus. It is considered an essential constituent of certain amino acids and is a component of chlorophyll, enzymes, and several vitamins. It is also responsible for strengthening plant defense mechanisms. The preferred form of sulfur assimilation by crops is the sulfate ion (SO4). 2). It is also introduced in the form of thiosulfate (S2O3 2 ) or elemental sulfur (S). These last two forms require oxidation by soil microorganisms to make them assimilable by plants.

[0009] Silicon is the second most abundant element in the periodic table in the Earth's crust. It is listed by the international soil community as the fourth most important plant fertilizing element after nitrogen, phosphorus, and potassium (NPK). It is very present in the soil, but usually as crystalline silica or bound to other elements as low-solubility silicate minerals that are unavailable to plants. Its so-called bioavailable form is orthosilicic acid, Si(OH)₄, which is present in very small amounts in the soil. However, orthosilicic acid solutions are not thermodynamically stable, leading to their accelerated transformation by polymerization into polysilicic acid, which is unavailable to plants.

[0010] Research conducted over several years by agronomists and plant scientists on the effect of bioavailable silicon on the development and life of various crops has highlighted its benefits and advantages for plant health, soil quality, and environmental protection (see Bélanger et al., Phytopathology, 2003, 93(4), 402-412; Britez et al., 2002, New Phytologist, 156, 437-444; Agarie et al., Journal of Experimental Botany, 1996, 47(5), 655-660). Studies have shown that silicon fertilization effectively improves plant resistance to various biotic and abiotic stresses, including diseases, mineral nutrient deficiencies, water deficits, frost, ultraviolet radiation, and salinity.It also has a positive effect on agricultural yields, with lower input costs and improved mineral and commercial quality of the fruit by preserving its water content post-harvest. Silicon therefore acts positively and efficiently at all stages of plant development.

[0011] On land, the results showed that silicon has a positive effect on the arrangement and distribution of mineral elements, thus improving the physicochemical properties of soils. Another advantage of its use is the reduction of mobile heavy metal content in the soil. The results also showed that it has a positive impact on environmental protection. Indeed, silicon is able to replace certain major nutrients and, as such, allows for the rationalization of fertilizer use, water conservation (up to 30% savings), and a reduction in the use of fungicides. In summary, both under normal conditions and with climate change, silicon is considered the most suitable mineral amendment for sustainable agriculture. Its application as a fertilizer is widespread throughout the world.Indeed, there are several formulations of mineral fertilizers containing silicon on the market. They can be classified as liquid silicon fertilizers and solid silicon fertilizers.

[0012] Thus, liquid soluble silicon fertilizers generally contain aqueous solutions of chelated alkali silicates. CN109384524A discloses a process for manufacturing liquid silicon fertilizer based on potassium silicate complexed with EDTA salts and boric acid, and stabilized with urea.

[0013] CN105399506A discloses a liquid silicon fertilizer formulation based on potassium silicate complexed with organic acids such as citric acid and glyconic acid. The formulation also includes phosphorus and trace elements such as boron and zinc.

[0014] CN111517849A discloses a process for preparing a silicon-based composite liquid fertilizer based on potassium silicate, sodium silicate, or potassium-sodium silicate, having a pH of 12-13 regulated with sodium hydroxide or potassium hydroxide. The disclosed liquid fertilizer is enriched with boron and molybdenum and stabilized with urea.

[0015] None of the liquid fertilizers mentioned above are NPK fertilizers. Furthermore, they use alkaline silicate solutions with a high pH (above 10), requiring significant dilution with water before use. Consequently, these solutions are not economically viable.

[0016] Furthermore, the problem with liquid fertilizers based on orthosilicic acid is their thermodynamic stability. Over time, these liquid solutions lead to the production of polysilicic acid through polymerization, and consequently, the silicon becomes unavailable to plants. Moreover, some of the fertilizers mentioned previously use sodium silicate, which has a negative effect on soil quality. Therefore, it is not advisable to use such fertilizers in the long term.

[0017] A solid fertilizer with bioavailable silicon is therefore more advantageous, compared to a liquid fertilizer, with regard to transport, handling, storage and ease of application.

[0018] WO2012 / 016394 discloses a process for synthesizing a solid fertilizer based on silicon, calcium, magnesium, and sulfur from metallurgical industry waste. It involves treating a mixture of steel slag, blast furnace slag, iron smelting furnace slag, and fly ash with an acid mixture comprising sulfuric acid, nitric acid, phosphoric acid, and hydrochloric acid, followed by granulation of the finished product. However, fertilization with silicate-containing slags presents certain drawbacks, such as the low solubility of the slags and the high presence of impurities.

[0019] W02018 / 169411 discloses a process for stabilizing monosilicic acid with humic substances after solubilizing silica with alkalis. This same fertilizer, containing bioavailable silicon, was subsequently used to formulate solid organo-mineral NPK fertilizers (W02019 / 098854). These organo-mineral fertilizers are mechanically mixed with alkali silicates complexed with humic substances in ratios ranging from 90:10 to 10:90, which can lead to considerable dilution of the major fertilizing elements (NPK). Another drawback of both processes is that drying the product can convert the monosilicic acid chelates back into silica. Consequently, the very high moisture content of the finished product can increase transportation costs.Besides the very high cost of humic substances, the disclosed processes require a residence time for incubation of 3 days followed by 2 days of open-air drying, which makes them difficult to extrapolate to industry and not economically feasible.

[0020] WO2019 / 098853 discloses a solid NPK fertilizer product enriched with silicon in the form of amorphous silicon dioxide. The physical mixing ratio of the mineral NPK fertilizer to the amorphous silicon dioxide is 20:80 to 80:20 and 10:90 to 90:10. However, the dissolution kinetics of amorphous silica are very slow, and its mechanical mixing in the form of very fine particles (nanometer or micrometer) with solid NPK fertilizers results in a heterogeneous and inconsistent final product. The mixing ratios used allow for significant dilution of the major fertilizing elements, and consequently, the amount of NPK nutrients in the fertilizer will, in most cases, be insufficient to meet fertilization requirements, thus leading to very low crop yields.

[0021] Currently, the two main sources of potassium used in the manufacture of NPK tertiary mineral fertilizers are potassium chloride (KCl) and potassium sulfate (K₂SO₄). However, the use of these two products presents several drawbacks and leads to a negative environmental impact. Their high solubility poses a risk of soil leaching, which can lead to groundwater contamination by chlorides, sulfates, and metals. Furthermore, chloride ions combined with these products degrade soil quality by increasing salinity, which is one of the most limiting factors for crop productivity and quality.

[0022] Therefore, there remains a need for a stable NPK fertilizer that combines the three major plant nutrients (nitrogen, phosphorus, and potassium), silicon, and sulfur, both in a soluble and plant-available form that is easily transportable, handleable, storable, applicable, and soil-neutral. FIGURES

[0023] Figure 1: Illustrative diagram of one embodiment of the process of the invention.

[0024] SUMMARY OF THE INVENTION

[0025] Thus, the invention relates to a solid NPK mineral fertilizer comprising nitrogen, phosphorus, potassium, sulfur and silicon, characterized in that the potassium and silicon are, preferably essentially, in the form of potassium silicate.

[0026] Another aspect of the invention relates to the use of the fertilizer according to the invention to improve the growth and / or yield of a plantation.

[0027] Another aspect of the invention relates to a process for preparing an NPK mineral fertilizer according to the invention, comprising the following steps: a) preparing an aqueous potassium silicate solution by mixing active silica and a potassium hydroxide solution, b) preparing a slurry comprising diammonium phosphate (DAP) and ammonium sulfate by mixing ammonia (NH3), sulfuric acid, and a solution S A p comprising phosphoric acid (H3PO4), c) mixing of the aqueous potassium silicate solution obtained in step a) with the slurry obtained in step b) to obtain a slurry comprising the NPK mineral fertilizer, d) drying of the NPK mineral fertilizer.

[0028] Another aspect of the invention also relates to an NPK mineral fertilizer that can be obtained by the process according to the invention.

[0029] Other aspects of the invention are as described below.

[0030] Definitions

[0031] Any range of values ​​designated by the expression "from a to b" means the range of values ​​from a to b (that is, including the strict bounds a and b).

[0032] For the purposes of the present invention, the term "a" or "an" means "one or more" or "at least one".

[0033] The term "approximately" means that the value in question may be 5% lower or higher, in particular 1% higher, than the stated value. The term "NPK mineral fertilizer" means that the fertilizer comprises nitrogen (N), phosphorus (P), and potassium (K) in the form of mineral salts. For the purposes of this invention, this term also includes NPK fertilizers comprising silicon (Si) and / or sulfur (S).

[0034] DETAILED DESCRIPTION OF THE INVENTION

[0035] The inventors developed an NPK fertilizer that met the stated needs. In particular, they made a surprising discovery: using potassium silicate in the fertilizer yields a product with soluble and bioavailable silicon. They also made it possible to utilize one of the byproducts produced during phosphoric acid production: H2SiF6.

[0036] Solid NPK mineral fertilizer

[0037] Thus, a first aspect of the invention relates to a solid NPK mineral fertilizer comprising nitrogen, phosphorus, potassium, sulfur and silicon, characterized in that the potassium and silicon are, preferably essentially, in the form of potassium silicate.

[0038] The fertilizer according to the invention may have the following contents: a nitrogen content ranging from 5% to 25%, preferably from 10% to 20%, preferably from 12.5% ​​to 16.5%, typically about 14% or 15%, and / or a phosphorus content ranging from 10% to 35%, preferably from 12% to 33%, typically from 15% to 30%, and / or a potassium content ranging from 10% to 25%, preferably from 10% to 20%, preferably from 12.5% ​​to 16.5%, typically about 14% or 15%, and / or a silicon content ranging from 1% to 10%, preferably from 2% to 8%, preferably from 3% to 7%, typically about 4% or 4.5%, and / or a sulfur content ranging from 1% to 16%, preferably from 3% to 15%. %, preferably from 4% to 12%, the content of each element being expressed by weight relative to the total weight of the fertilizer.

[0039] The various concentrations can be determined using standard measurement methods. Nitrogen content can be determined according to ISO 5314:1981. Phosphorus content is expressed as P₂O₅ equivalent and can be determined according to NF EN 15956:2011 and NF EN 15959:2023. Potassium content is expressed as K₂O equivalent and can be determined according to ISO 7407:1983 followed by analysis using inductively coupled plasma optical emission spectroscopy (ICP-OES). Unless otherwise specified, silicon (Si) content can be determined by mineralization with aqua regia, followed by contact with hydrofluoric acid (HF) and analysis by atomic absorption spectrometry. Sulfur (S) content can be determined gravimetrically according to NF EN 15749:2022.

[0040] The fertilizer according to the invention may have the following contents: a nitrogen content ranging from 10% to 20%, preferably from 12.5% ​​to 16.5%, typically about 14% or 15%, a phosphorus content ranging from 12% to 33%, typically from 15% to 30%, a potassium content ranging from 10% to 20%, preferably from 12.5% ​​to 16.5%, typically about 14% or 15%, a silicon content ranging from 2% to 8%, preferably from 3% to 7%, typically about 4% or 4.5%, and a sulfur content ranging from 3% to 15%, preferably from 4% to 12%, the contents of each element being expressed by weight relative to the total weight of the fertilizer.

[0041] In the fertilizer according to the invention, nitrogen can be introduced in the form of ammonia, phosphorus in the form of phosphoric acid, potassium and silicon in the form of potassium silicate, and sulfur in the form of sulfuric acid. According to a preferred embodiment, potassium and silicon are introduced solely in the form of potassium silicate. The fertilizer according to the invention may comprise one or more of the elements in the following forms: nitrogen in the form of nitrate ions (NO₃⁻). 3 ) and / or ammonium ions (NH4 + ), phosphorus in the form of phosphate ions, hydrogen phosphate ions (HPO4 2 ) and / or dihydrogen phosphate ions (H2PO4), sulfur in the form of sulfate ions (SO4 2 ).

[0042] Advantageously, sulfur is present in fertilizer in a soluble form, particularly as sulfate ions (SO4 2), typically in the form of ammonium sulfate.

[0043] The fertilizer according to the invention is in solid form, it can for example be in the form of powder or granules, preferably with a diameter ranging from 1 to 4 mm.

[0044] The fertilizer according to the invention advantageously has a pH ranging from 7 to 8. Typically, the pH can be determined by dissolving the fertilizer in water and measuring the pH of the resulting solution according to standard NF U44-001. It is understood that the fertilizer may include other nutrients and / or micronutrients commonly used in the field. For example, calcium (Ca) and magnesium (Mg) may be cited as nutrients, and zinc (Zn), copper (Cu), boron (B), molybdenum (Mo), and iron (Fe) may be cited as micronutrients. Furthermore, the fertilizer may include any type of fillers and / or additives commonly used by those skilled in the art that are suitable for fertilizers and plant protection products.

[0045] According to an advantageous embodiment, the fertilizer according to the invention is free from components that increase soil salinity, i.e., increase its electrical conductivity, through the dissolution of salts. Examples of such components include sodium salts and chlorides. Soil salinity can be determined by a soil electrical conductivity measurement method according to ISO 11265. Typically, "free from components that increase soil salinity" means less than 0.5% of these components by weight relative to the total weight of the fertilizer, preferably less than 0.1%.

[0046] Use of fertilizer

[0047] Another aspect of the invention relates to the use of fertilizer as defined above to improve the growth and / or yield of a plantation.

[0048] The present invention also relates to a method of fertilizing a plantation comprising the application to said plantation of the fertilizer as defined above.

[0049] The fertilizer can be used on all types of plantings, such as field crops (cereals, oilseeds and protein crops), vegetable crops, arboriculture, greenhouse crops, hydroponic crops and private gardens.

[0050] Fertilizer can be applied by any suitable method, in particular by spreading the fertilizer on the soil in powder or granule form.

[0051] A skilled person is able to determine the amount of fertilizer needed to apply to a plantation based on various factors such as the nature of the plantation, its area, or the nature of the soil.

[0052] Fertilizer preparation process

[0053] Another aspect of the invention relates to a process for preparing an NPK mineral fertilizer as defined above, comprising the following steps: a) preparing an aqueous potassium silicate solution by mixing active silica and a potassium hydroxide (KOH) solution; b) preparing a slurry comprising diammonium phosphate (DAP) and ammonium sulfate by mixing ammonia (NH3), sulfuric acid, and a solution S A p comprising phosphoric acid (H3PO4), c) mixing of an aqueous potassium silicate solution from step a) with the slurry from step b) to obtain a slurry comprising the NPK mineral fertilizer, d) drying of the NPK mineral fertilizer.

[0054] Steps a) and b) can be consecutive or concurrent.

[0055] Step a)

[0056] Step a) allows the preparation of an aqueous potassium silicate solution by mixing active silica and an aqueous potassium hydroxide solution. It can be carried out in a stirred reactor, and / or at a temperature ranging from 80 °C to 120 °C and / or for a duration greater than or equal to 30 minutes, in particular greater than or equal to 1 hour.

[0057] Silica can be in amorphous or crystalline form, preferably it is in amorphous form.

[0058] Preferably, in step a), the Si:K2O molar ratio goes from 0.8:1.2 to 1.2:0.8, typically about 1:1.

[0059] Potassium hydroxide solution advantageously has a KOH concentration ranging from 3 to 5 mol / L.

[0060] The aqueous solution obtained at the end of step a) can advantageously be a viscous solution. It can have a density ranging from 1.3 kg / L to 1.9 kg / L. This viscosity can be obtained by evaporating the water. It can be used in the remainder of the process without further treatment.

[0061] The active silica used in step a) is advantageously prepared from fluorosilicic acid obtained as a by-product of the production of phosphoric acid from phosphate ore. Preferably, the process according to the invention includes a preliminary step a') of preparation of the active silica, carried out before step a), step a') comprising the following successive substeps: i) mixing of fluorosilicic acid (H2SiF6) with an alkaline solution having a pH ranging from 7.5 to 9 to obtain a silica suspension, ii) filtration of the silica, iii) washing of the silica obtained in step ii).

[0062] The alkaline solution is preferably a solution of NaOH, NH3, or NH4OH, preferably NaOH. Step i) can be carried out with stirring for a duration of 1 to 5 hours. Step i) can be carried out at a temperature of 60 °C to 100 °C, preferably 70 °C to 90 °C, typically around 80 °C.

[0063] Step i) thus generates a silica suspension by neutralizing fluorosilicic acid with the alkaline solution. The fluorosilicic acid can have concentrations ranging from 6% to 34% by weight, preferably from 12% to 24%, typically around 18%, relative to the total weight of the solution. The amount of alkaline solution should then be adjusted to obtain a pH ranging from 7.5 to 9.

[0064] Preferably, fluorosilicic acid is obtained from a process for preparing phosphoric acid from phosphate ore, particularly by wet process. For example, fluorosilicic acid can be obtained by scrubbing the fluorinated gases released during the concentration of phosphoric acid.

[0065] This suspension is then filtered (step ii) by any suitable means, and then, in step iii), the silica is washed. The silica washing is an acid wash, which may also include a preliminary wash with water. For example, a nitric acid solution may be used for the acid wash. Such a nitric acid solution may be concentrated to 1 / 8 by volume.

[0066] The silica thus obtained can optionally be dried before its further use, for example for a period of 1 to 10 hours and at a temperature ranging from 70 °C to 130 °C.

[0067] The silica obtained at the end of step iii) can be in powder form. It is said to be active or reactive, that is, capable of being used in step a) of the process according to the invention. It can have one or more of the following characteristics: a purity by weight greater than 98%, a specific surface area ranging from 18 m² 2 / g at 30 m 2 / g, and / or a uniform micrometric morphology. Preferably, it has a purity by weight greater than 99%, a specific surface area ranging from 22 m² 2 / g at 26 m 2 / g and a uniform micrometric morphology.

[0068] Step b) therefore consists of preparing a slurry comprising diammonium phosphate (DAP) and ammonium sulfate, by mixing ammonia (NH3), sulfuric acid and a solution S A p comprising phosphoric acid (H3PO4).

[0069] Step b) can be carried out under stirring in a suitable reactor. This step can be carried out at a temperature ranging from 60 °C to 100 °C, typically around 80 °C.

[0070] Sulfuric acid may have an H2SO4 content greater than or equal to 95% by weight, preferably greater than or equal to 98% by weight.

[0071] Phosphoric acid may have a phosphorus content ranging from 46% to 54% by weight, preferably from 47% to 53%, preferably from 48.5% to 51.5%, typically about 50%.

[0072] Solution S Ap may have the following contents: a magnesium (%MgO) content ranging from 0.01% to 7%, preferably from 0.1% to 5%, preferably from 0.7% to 2.5%, and / or an aluminum (%Al2O3) content ranging from 0.01% to 7%, preferably from 0.05% to 5%, preferably from 0.1% to 1%, and / or an iron (%Fe2O3) content ranging from 0.01% to 7%, preferably from 0.05% to 5%, preferably from 0.1% to 1%, and / or a sodium (%Na2O) content ranging from 0.01% to 7%, preferably from 0.05% to 5%, preferably from 0.1% to 1%, and / or a sulfur (%SO4) content ranging from 0.1% to 7%, preferably from 0.5% to 5%, preferably from 1.5% to 3.5%, and / or a potassium (%K2O) content ranging from 0.001% to 7%, preferably from 0.01% to 5%, preferably from 0.05% to 1%, the content of each element being expressed by weight relative to the total weight of the solution S AP .

[0073] According to an advantageous embodiment, solution S APcan be prepared by mixing phosphoric acid, optionally industrial, with phosphogypsum (CaSO4·2H2O). It is understood that phosphogypsum is only slightly soluble in phosphoric acid, and that, consequently, the solution S AP is not necessarily homogeneous. Industrial phosphoric acid is understood to mean unpurified phosphoric acid, that is, acid that may have the phosphorus content described above, in the presence of impurities such as metallic impurities like magnesium, calcium, iron, aluminum, etc. Purified phosphoric acid, that is, acid with higher phosphorus content and free of impurities, is also usable within the scope of the present invention.

[0074] Advantageously, phosphogypsum is obtained through a process of preparing phosphoric acid from phosphate ore. According to this embodiment, the solution S APcan then have a solids content ranging from 0.1% to 10%, preferably from 0.5% to 8%, preferably from 2% to 5%. Solution S APmay have the following characteristics: a phosphorus content ranging from 46% to 54% by weight, preferably from 47% to 53%, preferably from 48.5% to 51.5%, typically about 50%; a magnesium (%MgO) content ranging from 0.01% to 7%, preferably from 0.1% to 5%, preferably from 0.7% to 2.5%; an aluminum (%Al2O3) content ranging from 0.01% to 7%, preferably from 0.05% to 5%, preferably from 0.1% to 1%; an iron (%Fe2O3) content ranging from 0.01% to 7%, preferably from 0.05% to 5%, preferably from 0.1% to 1%; a sodium (%Na2O) content ranging from 0.01% to 7%, preferably from 0.05% to 5%, preferably from 0.1% to 1%, a sulfur (%SO4) content ranging from 0.1% to 7%, preferably from 0.5% to 5%, preferably from 1.5% to 3.5%, a potassium (%K2O) content ranging from 0.001% to 7%, preferably from 0.01% to 5%, preferably from 0.05% to 1%, and a solids content ranging from 0.1% to 10%, preferably from 0.5% to 8%.preferably from 2% to 5%, the content of each element being expressed by weight relative to the total weight of solution S, AP .

[0075] The solids content can be determined by gravimetry after a separation step by centrifugation.

[0076] In step b), sulfuric acid and solution S A p are preferentially added to the reactor, then ammonia is added, preferably in gaseous form. Advantageously, ammonia is added until the phosphoric acid in solution S is neutralized. A p and sulfuric acid. Preferably, the NH3:H3PO4 molar ratio is from 1.8 to 2.2, typically around 2. Step c) therefore consists of mixing an aqueous potassium silicate solution obtained in step a) with the slurry obtained in step b) to obtain a slurry including the NPK mineral fertilizer.

[0077] Step c) can be carried out in the same reactor as step b). This step can be carried out at a temperature ranging from 60 °C to 100 °C, typically around 80 °C. This step is typically carried out with stirring. The stirring time typically ranges from 15 to 30 minutes.

[0078] Step d)

[0079] Step d) therefore consists of drying the NPK mineral fertilizer. This drying can be carried out by any method known to those skilled in the art on the slurry obtained at the end of step c) at the appropriate temperature, for example, using a hot air stream. Typically, the NPK mineral fertilizer is dried until the moisture content is less than 1.5%.

[0080] The duration and temperature of step d) can be determined by a person skilled in the art according to any further processing of the fertilizer and its uses.

[0081] For example, NPK mineral fertilizer can be dried to a moisture content of less than 3%, preferably less than 1.5%, by weight.

[0082] End of process

[0083] Following step d), the NPK mineral fertilizer may undergo all kinds of usual treatments for conditioning, preservation and / or subsequent use.

[0084] Typically, the process according to the invention may further include, following step d), a step e) of grinding and / or granulating the NPK mineral fertilizer into powder or granule form.

[0085] Grinding can be carried out in a grinder adapted to the quantity of solid to be ground. This allows for obtaining a product with a controlled particle size; for example, the powder obtained after grinding can have a particle size of less than 0.5 mm.

[0086] The fertilizer can also be formed into granules. The conditions and methods of granulation are known to those skilled in the art. For example, after step e), the NPK mineral fertilizer may be in the form of granules with a diameter ranging from 1 mm to 4 mm. Preferably, step e) is a step of grinding the NPK mineral fertilizer into a powder, preferably with a particle size of less than 0.5 mm, followed by granulation of the powder into granules, the granules preferably having a diameter ranging from 1 mm to 4 mm.

[0087] Figure 1 presents a diagram of one embodiment of the process of the invention.

[0088] Another aspect of the invention also relates to an NPK mineral fertilizer that can be obtained by the preparation process described above.

[0089] The aforementioned features of the present invention, as well as others, will be better understood upon reading the following description of several examples of embodiments of the invention, given by way of illustration and not limitation.

[0090] EXAMPLES

[0091] Materials and methods

[0092] 480 grams of NaOH (98% by weight) are dissolved in two liters of water. The resulting solution is slowly added to 1568 grams of H₂SiF₆ (18% by weight). The mixture is heated to 80 °C while stirring. The pH of the mixture should be between 7.5 and 9. The mixture is stirred for 2 hours, and then the resulting suspension is filtered. The resulting cake is washed 3 to 4 times with hot water and then 3 times with 500 mL of nitric acid (1 / 8 by volume). The product is then dried at 105 °C for 4 hours to obtain 100 g of purified active silica in the form of a very fine white powder, with a purity greater than 99% by weight and a specific surface area of ​​approximately 24 m². 2 / g and a uniform micrometric morphology. The reaction yield is approximately 85%.

[0093] Concentrations of H2SiF6 ranging from 6 to 34% by weight can be used, while varying each time the amount of alkali required for neutralization.

[0094] Example 2:

[0095] 212 grams of KOH (85 wt.) are dissolved in 1 L of water. In a reflux flask, this solution is added to 98 grams of the active silica (99 wt.) prepared in Example 1. The mixture is then heated with appropriate stirring to a temperature between 80 and 120°C until a clear, suspension-free solution is obtained. The heating time is preferably greater than one hour. The condenser is then removed, and heating and stirring are continued until a viscous solution with a density of 1595 g / L is obtained. The mass of K₂SiO₃ in the final product obtained according to this second example is 248 grams.

[0096] Different masses of K2SiO3 can be prepared, under the same conditions described in this example, by changing the required masses of SiO2 and KOH.

[0097] Example 3:

[0098] Production of an NPK fertilizer 15-15-15-4, 5%Si-11%S

[0099] In a stirred reactor maintained at a temperature of 80 °C, the following are added:

[0100] 299 grams of phosphoric acid balanced with the appropriate amount of phosphogypsum (CaSO4·2H2O) to obtain phosphoric acid with a suitable solids content (SC), expressed as CaSO4·2H2O. The table below shows the composition of the phosphoric acid used:

[0101] [Table 1]

[0102] 315 grams of concentrated sulfuric acid having a strength of 98.5% in H2SO4.

[0103] The reaction mixture is neutralized with gaseous ammonia until an NH3:H3PO4 molar ratio of 2 is reached, indicating complete neutralization of both acids and the formation of an NPS (P2O5-NS) slurry. The reactor is then opened, and the equivalent of 248 g of potassium silicate obtained in Example 2 (expressed on a dry basis) is added. The reactor is closed, and the reaction mixture is stirred. The mixture is then collected as a slurry and allowed to cool. The resulting slurry is then dried at a temperature of 60 °C until a final moisture content (H2O) of less than 1.5% by weight is reached. One kg of NPK 15-15-15-4, 5%Si-11%S fertilizer is thus obtained. The product is then ground in a suitable mill to a particle size of less than 0.5 mm. The resulting powder is then formed into granules.

[0104] Example 4:

[0105] Preparation of an NPK fertilizer 14-28-14-4%Si-5%S The preparation process is identical to that of example 3 using:

[0106] 562 g of phosphoric acid having the composition shown in the table below.

[0107] [Table 2]

[0108] 91 grams of concentrated sulfuric acid having a strength of 98.5% in H2SO4.

[0109] The equivalent of 232 grams of potassium silicate (expressed on a dry basis), prepared according to example 2.

[0110] This process allows you to obtain 1 kg of NPK fertilizer 14-28-14-4%Si-5%S.

Claims

DEMANDS 1. Solid NPK mineral fertilizer comprising nitrogen, phosphorus, potassium, sulfur and silicon, characterized in that the potassium and silicon are in the form of potassium silicate.

2. NPK mineral fertilizer according to claim 1, having: a nitrogen content of 5% to 25%, preferably 10% to 20%, preferably 12.5% ​​to 16.5%, typically about 14% or 15%, and / or a phosphorus content of 10% to 35%, preferably 12% to 33%, typically 15% to 30%, and / or a potassium content of 10% to 25%, preferably 10% to 20%, preferably 12.5% ​​to 16.5%, typically about 14% or 15%, and / or a silicon content of 1% to 10%, preferably 2% to 8%, preferably 3% to 7%, typically about 4% or 4.5%, and / or a sulfur content of 1% to 16%, preferably 3%. at 15%, preferably from 4% to 12%, the content of each element being expressed by weight relative to the total weight of the fertilizer.

3. NPK mineral fertilizer according to claim 1 or 2, wherein the sulfur is in a soluble form, in particular as sulfate ions SO4 2- .

4. NPK mineral fertilizer according to any one of claims 1 to 3, wherein the NPK mineral fertilizer is in the form of a powder or granules, preferably with a diameter of 1 to 4 mm.

5. NPK mineral fertilizer according to any one of claims 1 to 4, wherein the NPK mineral fertilizer is free from components increasing soil salinity such as sodium salts or chlorides.

6. A process for preparing an NPK mineral fertilizer as defined in any one of claims 1 to 5, comprising the following steps: a) preparing an aqueous potassium silicate solution by mixing active silica and a potassium hydroxide solution, b) preparing a slurry comprising diammonium phosphate (DAP) and ammonium sulfate by mixing ammonia (NH3), sulfuric acid, and a solution S A P containing phosphoric acid (H3PO4), c) mixing the aqueous potassium silicate solution obtained in step a) with the slurry obtained in step b) to obtain a slurry comprising the NPK mineral fertilizer, d) drying the NPK mineral fertilizer.

7. A preparation process according to claim 6, further comprising, following step d), a step e) of grinding and / or granulating the NPK mineral fertilizer into powder or granules.

8. A preparation process according to claim 6 or 7, comprising a preliminary step a') of preparing the active silica, carried out before step a), step a') comprising the following successive substeps: i) mixing fluorosilicic acid (H2SiF6) with an alkaline solution having a pH from 7.5 to 9 to obtain a silica suspension, ii) filtering the silica, iii) washing the silica obtained in step ii).

9. A preparation process according to claim 8, wherein the fluorosilicic acid is obtained from a process for preparing phosphoric acid from phosphate ore.

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