Process combination for the production of phosphate granules from phosphate-containing raw materials

Abstract

The invention relates in one aspect to a process for producing a coated phosphate-containing fertilizer comprising the following steps: a) mixing a phosphate-containing starting material with at least one mineral acid, wherein the phosphate-containing starting material comprises at least one ash from the combustion of sewage sludge and / or animal excrement; b) incubating the mixture obtained in step a); c) granulating the mixture of acid and phosphate-containing starting material incubated in b); d) completely or partially drying the granules obtained in step c); e) coating the completely or partially dried granules from step d) with a liquid phosphate-containing suspension, wherein the process is characterized in that the mineral acid in step a) is present in a 0.2-fold to 1.5-fold stoichiometric ratio, preferably in a 1-fold to 1.5-fold stoichiometric ratio.is added to the phosphorus contained in the phosphate-containing starting material.

Description

DESCRIPTION

[0001] The invention relates to a process for producing a coated phosphate-containing fertilizer from phosphate-containing starting materials, such as ash from the incineration of sewage sludge and / or animal excrement. Background and state of the art

[0002] Fertilization improves the supply of nutrients to plants and thus promotes their growth. Consequently, the quality of the products and thus crop yields improve. In agriculture, forestry, and horticulture, fertilizers are used to supplement the nutrient supply. The reason for this is that the nutrients necessary for ideal plant growth are often not present in the soil in optimal form. Thus, nutrients are often not provided by nature in sufficient quantities and, moreover, are removed from the soil in significant quantities by harvested crops (Schubert 2011).

[0003] Fertilizers generally consist of macronutrients and micronutrients. Macronutrients include nitrogen, potassium, and phosphorus. Different processes are used to prepare nutrients for improved plant nutrition. Depending on their origin and production, fertilizers are classified as commercial, natural, mineral, or secondary fertilizers (Fink 1992). The latter is a collective term for predominantly organic waste from various products (meat-bone meal, compost, sewage sludge, or sewage sludge ash). A differentiation based on nutrient elements is also possible. Single-nutrient fertilizers contain a single nutrient (nitrogen, phosphate, or potassium). The combination of several nutrients in one product is called a complex fertilizer. Common commercial examples are phosphate-potassium fertilizers (PK) or nitrogen-phosphate fertilizers (NP).

[0004] Phosphorus is a mineral found in phosphates, for example. It is one of the most important nutrients in the Earth's ecosystem. The majority of global phosphorus or phosphate consumption, approximately 80%, is used for fertilizers and animal feed. Intensive agriculture is not possible without appropriate phosphate fertilization. In 2010, approximately 40 million tons were used as fertilizer. Phosphate as a fertilizer cannot be substituted by other substances. Global reserves are declining rapidly. Phosphate ores are mined industrially from deposits (e.g., in North Africa) and are significantly contaminated with heavy metals such as cadmium and uranium.

[0005] Germany has no natural phosphate deposits and therefore has to import enormous quantities of mineral-bound phosphates every year. In the 1995 / 1996 financial year, approximately 450,000 tons of phosphate were used in German agriculture, compared to 300,000 tons in 2014 / 2015 and 200,000 tons in 2018 / 2019. The consumption trend strongly reflects the price development for this fertilizer, but also the stricter legal requirements for the use of fertilizers. A large portion of the phosphate demand is currently also covered by organic residues and is not included in the aforementioned quantity. Sewage sludge is a form of organic waste. However, its application is classified as ecologically critical. For this reason, the Sewage Sludge Ordinance was amended in 2017 and came into force in October 2017: For municipalities over 50.For areas with a population of 1,000, the recovery of phosphorus from wastewater treatment plants and thus the recycling of phosphorus is mandatory. The goal of this measure is to gradually close the phosphorus material cycle and thus reduce Germany's dependence on imports. After a transitional period of 10 years, it will no longer be possible to apply sewage sludge directly to agricultural land as farmyard fertilizer.

[0006] Limited global resources and stricter legal frameworks for the use of secondary fertilizers are the prerequisite for tapping existing raw material potential through environmentally friendly recycling and developing marketable products using innovative process technologies.

[0007] According to the Federal Statistical Office, the total amount of municipal sewage sludge in Germany in 2015 was approximately 1.8 million Mg (equivalent to 1.8 million tons) of dry matter (DM) (Destatis 2017). Sewage sludge disposal in the Federal Republic of Germany is carried out through material recycling in agriculture and landscaping, as well as thermal disposal, primarily in mono- or co-incineration plants. Thermal treatment of sewage sludge represents the preferred and most important disposal method, accounting for approximately 64% (2015).

[0008] Current thermal sewage sludge disposal processes can be divided into mono-incineration, co-incineration, and special thermal processes. Co-incineration encompasses utilization in cement plants, coal-fired power plants, or waste incineration plants. In addition to the already established technologies for the thermal disposal of sewage sludge, research has also been conducted for several years into alternative utilization options that are only rarely used in Germany. These include, for example, low-temperature conversion, hydrothermal carbonization (HTC), and pyrolysis. In 2010, 1,003,749 Mg TS of sewage sludge was thermally utilized in mono- and co-incineration plants. Of this, approximately 43% went to mono-incineration plants, approximately 43% was co-incinerated in power plants, 9% served as refuse-derived fuel in cement plants, and a further 4% was incinerated (Lehrmann 2013).Assuming a constant percentage distribution of sewage sludge disposal quantities in co-incineration and mono-incineration plants, 498,642 Mg TS of the municipal sewage sludge generated in Germany was disposed of in existing mono-incineration and co-incineration plants in 2015 (Destatis 2017, Lehrmann 2013).

[0009] The material recycling of sewage sludge in agriculture is declining. On the one hand, public acceptance of this disposal method is declining. On the other hand, the Sewage Sludge Ordinance (AbfKlärV) completely prohibits the direct material use of sewage sludge for fertilization purposes on a large portion of soils, particularly in areas used for growing vegetables and fruit, permanent grassland, and in certain water protection areas. The reasons for this are the size of wastewater treatment plants, the high phosphorus content, and the tightening of limit values ​​for heavy metals and organic pollutants. To prevent a possible long-term concentration of pollutants in the soil, in the future only sewage sludge that complies with the requirements of the Sewage Sludge Ordinance and fertilizer law from sewage treatment plants with a capacity of less than 50,000 PE will be permitted for use in agriculture.Larger wastewater treatment plants must provide for phosphorus recovery if their sewage sludge contains more than 20 g phosphorus per kg dry matter of sewage sludge. This can be achieved directly from the sewage sludge with a recovery rate of 50% or down to a residual concentration below 20 g phosphorus per kg dry matter of sewage sludge. Based on the phosphorus recovery rates currently achieved in corresponding pilot plants, only a few processes for phosphorus recovery from sewage sludge enable a sufficient reduction in phosphate levels for subsequent co-incineration of the depleted sewage sludge (Montag et al. 2014). Therefore, the current trend is towards the further expansion of mono-incineration capacities and the optimization of phosphate recovery processes from sewage sludge or wastewater with regard to their recovery potential. Alternatively, phosphorus can also be recovered from the sewage sludge ash from mono-incineration.The statutory minimum recovery rate here is 80%. Operators must submit corresponding technology-neutral concepts for their wastewater treatment plants by the end of 2023. Subsequent implementation must be completed by 2032 for wastewater treatment plants with 50,000 PE or more, and by 2029 for those with 100,000 PE or more (source: UBA).

[0010] The sewage sludge ash produced after the incineration of sewage sludge is a powdered raw material with an average P2O5 content of around 20%, making it a valuable raw material for the production of phosphate fertilizer. However, the phosphate contained in the sewage sludge ash is only minimally available to plants without further treatment and therefore cannot be used directly as a phosphorus fertilizer. The invention relates to improvements compared to existing processes with regard to quality characteristics of the fertilizer to be produced, such as macro- and micronutrient contents and their solubilities, and the incorporation of pesticides and microbiologically active substances. At the same time, improvements result in the strength of the granules during storage, handling, and use, and a minimization of energy requirements and the associated costs.

[0011] In the well-known RecoPhos process (wet granulation process), it has already been proven that plant-available phosphates are produced from sewage sludge ash after digestion with a mineral acid, making this product suitable as a fertilizer.

[0012] The advantage of wet granulation processes is that they are extremely cost-effective, both in terms of the required investment and operating costs.

[0013] The disadvantage of this process is that the sewage sludge ash is not 100% mixed with the mineral acids required for conversion, resulting in acid esters on the one hand and unconverted sewage sludge ash on the other. This creates problems with the transport and spreading equipment in subsequent applications. Due to the materials they are made of, the acid esters are exposed to severe corrosion and thus permanent destruction. The formation of granules is difficult to control with this process, and a high dust content is sometimes to be expected. This leads to deposits on chain drives, gears, and adjustment mechanisms of the transport and spreading equipment. This situation leads to considerable strain on the drive systems, even leading to failure due to broken chains or leaking bearings, thus generating considerable maintenance costs.

[0014] The Seraplant process, which is now used on an industrial scale, is a fluidized bed spray granulation and avoids these disadvantages by producing a suspension of sewage sludge ash, water and mineral acids.

[0015] With a suitable selection of dispersion systems (for fluidized bed granulation), a nearly 100% mixture of sewage sludge ash and mineral acids can be assumed, thus avoiding the acid esters described above – and the associated disadvantages in later applications. The Seraplant process is the only industrially implemented process for phosphorus recovery to date. In this process, sewage sludge ash is reacted with phosphoric acid in an aqueous suspension to form water-soluble phosphate. The resulting suspension is granulated in a fluidized bed. The high degree of chemical disruption results in very good plant availability. Fluidized bed granulation produces clearly defined and stable, dust-free granules, which are therefore ideal for application using fertilizer spreaders or underfoot fertilization.

[0016] The disadvantages of the Seraplant process are its very high energy requirements. This results from the heat required to heat the drying air, which is used to remove approximately 50% water from the suspension, as well as from electrical energy, primarily for the supply and exhaust fans for the drying air and the compressors for the spray air. The investment required for a fluidized bed system is also considered very high. This results in high process costs for the Seraplant process or comparable processes.

[0017] Thus, the expert is faced with the dilemma of either choosing a procedure that meets the cost requirements but not the quality requirements or vice versa.

[0018] Consequently, there is an unmet need in the professional world for new processes for the cost-effective production of high-quality fertilizers based on organic raw materials such as sewage sludge or its ashes. Object of the invention

[0019] The object of the invention is to provide an energetically and application-technically optimized process combination for the production of phosphate granules from sewage sludge ash while simultaneously improving quality and expanding the application spectrum.

[0020] The process according to the invention aims to convert the phosphate contained in sewage sludge ash in such a way that it is available to plants. The resulting granules should be as free-flowing as possible, dust-free, and with a grain size of 1-5 mm, in the range of mineral fertilizers currently available on the market.

[0021] Increasing price increases for the necessary inputs, such as raw materials and energy for production, as well as sharply rising transportation costs, are leading to significant increases in fertilizer prices. Therefore, there is an urgent need to develop new, energy- and thus cost-saving fertilizers that offer comparable fertilizing performance to conventional, state-of-the-art fertilizers. Summary of the invention

[0022] The object of the invention is achieved by the features of the independent claims. Advantageous embodiments of the invention are described in the dependent claims.

[0023] In a preferred embodiment, the invention relates to a process for producing a coated phosphate-containing fertilizer comprising the following steps a) mixing at least one phosphate-containing starting material with at least one mineral acid, wherein the at least one phosphate-containing starting material comprises at least one ash from the incineration of sewage sludge and / or animal excrement, b) incubation of the mixture obtained in step a), c) granulation of the mixture of acid and phosphate-containing starting material incubated in b), d) complete or partial drying of the granules obtained in step c), e) coating the fully or partially dried granules from step d) with a liquid phosphate-containing suspension, the method being characterized in that the at least one mineral acid is added in step a) in a 0.2-fold to 1.5-fold stoichiometric ratio, preferably in a 1-fold to 1.5-fold stoichiometric ratio, to the phosphorus contained in the phosphate-containing starting material.

[0024] In preferred embodiments, water is additionally added in step a) and / or the at least one mineral acid is added as an aqueous solution and / or at least one phosphate-containing starting material is added in step a) as a moist (mixed with water) solid and / or at least one phosphate-containing starting material is mixed and / or wetted with water before or during step a).

[0025] Consequently, in some embodiments, at least one phosphate-containing starting material in step a) is a moist (mixed with water) solid and / or an aqueous suspension. In some embodiments, the at least one mineral acid added in step a) is an aqueous solution.

[0026] Preferably, the mixture obtained in step a) and incubated in step b) is a (slightly) moist, non-aqueous, loose-solid mixture and does not represent a liquid suspension.

[0027] In embodiments, the mineral acid is added in a 0.2 to 1.5 times stoichiometric ratio. In embodiments, the mineral acid is added in a 0.2-fold to 1.5-fold, 0.1-fold to 5-fold, 0.5-fold to 1.5-fold, 1.75-fold to 1.5-fold, 1-fold to 2-fold, 0.1-fold, 0.2-fold, 0.3-fold, 0.4-fold, 0.5-fold, 0.6-fold, 0.7-fold, 0.8-fold, 0.9-fold, 1-fold, 1.1-fold, 1.2-fold, 1.3-fold, 1.4-fold, 1.5-fold, 1.6-fold, 1.7-fold, 1.8-fold, 1.9-fold, 2-fold, 2.1-fold, 2.2-fold, 2.3-fold, 2.4-fold, 2.5 times, 2.6 times, 2.7 times, 2.8 times, 2.9 times, 3 times, 3.5 times, 4 times, 4.5 times, 5 times the stoichiometric ratio. These numerical values ​​may also include intermediate ranges and deviations of + / - 10%, according to the invention.

[0028] The surprising and advantageous effect of the stoichiometric ratio of the at least one mineral acid to the phosphorus contained in the phosphate-containing starting material (0.2-fold to 1.5-fold stoichiometric ratio, preferably 1-fold to 1.5-fold) is a specific solubility rate (dissolution rate) of the phosphate contained in the resulting fertilizer in water.

[0029] In one embodiment of the process according to the invention, the mixing in step a) and / or the incubation in step b) and / or the granulation in step c) is carried out by means of a mixer, granulation plate and / or granulation drum.

[0030] In embodiments, steps a) and / or b) and / or c) of the process according to the invention are carried out by means of wet granulation or a wet granulation process. In embodiments, wet granulation can be carried out by means of a mixer, granulating disc, and / or a granulating drum.

[0031] A clear advantage of granulation as in step c) or a wet granulation process is that it is extremely cost-effective, both in terms of the required investment volume and in terms of operating costs, such as energy costs.

[0032] The inventors have surprisingly discovered that the properties of a fertilizer produced by granulation as in step c) or by wet granulation can be significantly improved if a coating is subsequently applied to the fertilizer. Therefore, according to the invention, it is preferred to subsequently coat (enclose) the granules produced by steps a) to d), preferably using a fluidized bed, and preferably with a suspension of water, phosphate-containing starting material(s), and mineral acid(s).

[0033] In one embodiment of the process according to the invention, the coating in step e) is therefore applied by means of a fluidized bed. In embodiments, the coating is applied in a fluidized bed device or a fluidized bed coater (fluidized bed granulator). Those skilled in the art are familiar with the means for conducting fluidized bed coating and the fluidized bed process.

[0034] The advantage of applying a coating using a fluidized bed is that it can be assumed that the suspension is almost 100% mixed with phosphate-containing starting material(s) and mineral acid(s), which prevents the formation of acid nests and thus the associated disadvantages (risks / hazards for users and machines due to burns and corrosion) during the subsequent application of the resulting fertilizer granules.

[0035] Despite the increased production costs (high energy costs, cost-intensive equipment), the preferred application of the coating by means of a fluidized bed has the previously described advantages with regard to the more homogeneous composition of the resulting coating material (freedom from acid nests) and the high degree of chemical conversion of the starting material.

[0036] A further advantage of the inventive coating of the fertilizer granules according to the invention, e.g., by means of a fluidized bed, is the prevention of the adhesion of fine particles, such as those found in granules produced by wet granulation. Furthermore, such fine particles, if they adhere to the core material of the fertilizer granules produced by wet granulation, can be further bound by the coating.

[0037] Consequently, in embodiments, the process according to the invention comprises the production of a core of a fertilizer granulate according to steps a) to d), preferably by a wet granulation process comprising steps a) - d), wherein the core is subsequently coated with a phosphate-containing suspension in step e), preferably by fluidized bed coating / by means of a fluidized bed process.

[0038] The advantageous effects of the process according to the invention described above are achieved through the synergistic interaction of the coating and the core of the fertilizer. These two components of the fertilizer according to the invention are preferably produced, mixed, and / or applied using different processes. During the mixing or granulation of the fertilizer core, preferably by means of a wet granulation process and / or mixer, a specific, preferably lower or partially incomplete mixing of the starting material with acid occurs, resulting in a lower / partial or partially incomplete reaction of the phosphate contained in the starting material with the added acid. The term "incomplete" here means less than 100%.This in turn leads to a lower water solubility rate (dissolution rate) of the phosphate in the resulting fertilizer core - compared to the solubility rate of the phosphate in the coating.

[0039] The coating is produced or applied from a liquid phosphate-containing suspension of the starting material, preferably using a fluidized bed process, which results in better mixing of the starting material with the aqueous acid solution, which in turn leads to increased / improved reaction of the phosphate with the acid (compared to the core material). This results in a higher water solubility rate (dissolution rate) of the phosphate contained in the coating. Furthermore, the better mixing and the higher liquid content of the coating suspension (during incubation, production, or application) prevent the formation of acid esters, which can certainly remain during mixing and granulation of the core material.The fertilizer coating therefore preferably has low amounts of free acid and a more uniform mixing of the components and / or an increased conversion of the phosphate, compared to the fertilizer core.

[0040] If the core and the coating are combined to form a fertilizer granulate in the process according to the invention, this combination of process features achieves the synergistic effect of producing a fertilizer granulate with an outer shell (coating) of high strength, homogeneity, and increased phosphate solubility in water, contains no acid esters (which can lead to undesirable burns upon contact with the granules), and causes reduced dust formation. Furthermore, the core, which accounts for the larger proportion of the fertilizer, is produced using a cost-effective and energy-saving mixing process that achieves a lower phosphate solubility in water, so that the core achieves a long-term fertilizing effect after application and the rapid dissolution of the coating.

[0041] In summary, a synergistic effect of the process according to the invention is the production of a cost- and energy-saving fertilizer with a long-term fertilizing effect, which, due to its additional coating, initially also achieves a quickly available (short-term) fertilizing effect, and at the same time enables low dust pollution during application and storage and reduced-risk handling by the user.

[0042] In embodiments, a further advantage of producing the core by means of the mixing and granulation process according to the invention, preferably wet granulation and / or using a mixer, is a partially incomplete mixing of the starting material with the acid, which leads to an increased remaining acid concentration in the core of the fertilizer. This remaining and unreacted acid can be released after the fertilizer granules have been applied to the soil and during the dissolution of the core, so that the phosphate of the fertilizer can be slowly further dissolved in the soil. This subsequent / additional dissolution can further improve the solubility of the phosphate contained in the fertilizer, and a long-term fertilizing effect is achieved.

[0043] In embodiments, due to the presence and unreacted acid in the core of the fertilizer granules according to the invention, a lower stoichiometric mixing ratio of acid and starting material can also be used in the coating, since phosphate from the coating, which was not converted due to this small amount of acid during the production of the coating suspension, can still be converted subsequently after application to the soil by the remaining acid in the core of the fertilizer.

[0044] In some such embodiments, in step a), the mineral acid may be added in a 1.5-fold stoichiometric ratio, while the phosphate-containing suspension used for the coating in step e) contains only mineral acid in a 0.2-fold stoichiometric ratio to the starting material.

[0045] In some embodiments, the coating in step e) also comprises complete or partial drying of the granules obtained in step c) according to step d), e.g., in a fluidized bed device. Consequently, in some embodiments, step e) can also be combined / combined with step d) or take place simultaneously.

[0046] Thus, in embodiments, step e) comprises: e) coating and thereby completely or partially drying the granules from step c) with a liquid phosphate-containing suspension.

[0047] In embodiments of the process according to the invention, the liquid phosphate-containing suspension in step e) comprises at least one mineral acid and at least one phosphate-containing starting material. In embodiments of the process according to the invention, the liquid phosphate-containing suspension in step e) comprises at least one mineral acid, water, and at least one phosphate-containing starting material.

[0048] In some embodiments, the at least one mineral acid is sulfuric acid. In other embodiments, hydrochloric acid, nitric acid, phosphoric acid, other acids, or mixtures thereof can be used alternatively and / or in addition to sulfuric acid. The acids or acid mixtures used in each case depend, for example, on the desired properties of the final fertilizer and its desired chemical composition.

[0049] In embodiments, a value in percent (%) specifies a value in weight percent (% w / w).

[0050] In one embodiment of the process according to the invention, the liquid phosphate-containing suspension used for coating in step e) has a dry matter content of between 40 and 60 weight percent (% w / w). In embodiments of the process according to the invention, the liquid phosphate-containing suspension used for coating in step e) has a dry matter content of ≤ 65, ≤ 60, ≤ 55, ≤ 50, ≤ 40, ≤ 30, ≤ 20 weight percent (% w / w). In preferred embodiments, the dry matter content in the coating suspension applied in step e) is substantially lower (e.g., by at least 10%, at least 20% w / w) than that of the core material according to step a).

[0051] In embodiments of the process according to the invention, the liquid phosphate-containing suspension used for coating in step e) has a moisture content between 40 and 60 percent by weight (% w / w). In one embodiment of the process according to the invention, the liquid phosphate-containing suspension used for coating in step e) has a moisture content of between 30 and 70, between 20 and 90, between 40 and 80, between 10 and 95, between 50 and 80, or of ≥ 10, ≥ 20, ≥ 30, ≥ 40, ≥ 45, ≥ 50, ≥ 55, ≥ 60, ≥ 65, ≥ 70, ≥ 75, ≥ 80, ≥ 85, ≥ 90, ≥ 95, preferably of at least 40 percent by weight (% w / w).

[0052] Consequently, it is preferred that the coating suspension be prepared and / or incubated with a high liquid content so that its components, such as sewage sludge ash(s), acid, and water, are both better mixed (resulting in a sprayable suspension) and can react more effectively chemically. This preferably leads to a high degree of homogeneity (mixing) of the resulting suspension and to better conversion of the phosphates contained in the sewage sludge ash.

[0053] In some embodiments, the coating process in step e) is controlled such that (only) a coating of the fertilizer granules from step d) takes place and no granulation, since otherwise the target grain size of the final granules may be exceeded. Furthermore, in some embodiments, it may also be preferable to ensure that the wet granules (core material) are adequately protected by the coating, e.g., from abrasion or other damage.

[0054] In the process according to the invention, therefore, a mechanically strong coating (protective layer) made of a mixture of, for example, sewage sludge ash and acid is preferably applied. In this case, mechanically strong preferably means, for example, increased resistance to abrasion of the coating, which can be achieved, for example, by increased compressive strength of the coating. The increased strength of the coating can, as explained herein, be achieved, inter alia, by increased homogeneity (better mixing of the components) of the coating suspension and / or an increased degree of conversion of the starting materials. These factors are achieved, inter alia, by the increased liquid content of the coating suspension (e.g. compared to the core material) and preferably also by application by means of a fluidized bed.The application of several thin coating layers (as occurs in fluidized bed coating) may also be preferred in embodiments, whereby the stability and hardness of the coating is additionally increased.

[0055] The fertilizer according to the invention has the advantage, due to its coating, which is also referred to as “coating,” that the abrasion of the fertilizer during transport, storage and application, and thus also the dust content in the fertilizer, is significantly reduced.

[0056] In one embodiment of the process according to the invention, the liquid phosphate-containing suspension applied as a coating in step e) is obtained by incubating at least one phosphate-containing starting material with at least one mineral acid. In embodiments, to produce a liquid phosphate-containing suspension, the mineral acid is added to the phosphorus contained in the phosphate-containing starting material in a 0.2-fold to 1.5-fold stoichiometric ratio, preferably in a 0.2-fold to 0.5-fold stoichiometric ratio.

[0057] The surprising effect of the 0.2-fold to 1.5-fold stoichiometric ratio, preferably of a 0.2-fold to 0.5-fold stoichiometric ratio of the at least one mineral acid to the phosphorus contained in the phosphate-containing starting material is a specific solubility, preferably water solubility, of the phosphate in the coating of the resulting fertilizer.

[0058] In embodiments, this preferred specific stoichiometric ratio of acid to phosphate in the suspension used for coating (step e), in conjunction with the specific stoichiometric ratio of acid to phosphorus contained in the phosphate-containing starting material of the granules produced in steps a) to d) ("granule core"), results in the phosphate in the coating having a higher water solubility rate (speed of dissolution in water) than the phosphate in the "granule core." This has the advantageous and surprising effect that, in these embodiments, a fertilizer granule is obtained which comprises a coating with high (phosphate) water solubility, which causes faster dissolution of the coating and thus a rapid release of phosphate into the soil.In these embodiments, however, the core of the granules comprises a phosphate with a lower (slower) water solubility rate, so that the phosphate is released from the core of the granules more slowly and over a longer period of time. This results in a slow-release fertilizer granule with a controllable depot effect.

[0059] Therefore, in one embodiment of the process according to the invention, the phosphate, preferably P2O5, in the coating applied in step e) has an increased solubility rate in water (faster dissolution rate) than the phosphate, preferably P2O5, of the granules obtained in step d), and preferably a water solubility of >10%. In embodiments, the phosphate of the coating has a water solubility of >15%, >20%, or >25%.

[0060] In one embodiment of the process according to the invention, the phosphate, preferably P2O5, contained in the granules produced in steps a)-d) has a lower solubility rate in water (slower dissolution rate) than the phosphate, preferably P2O5, in the coating applied in step e), and preferably a water solubility of <10%. In embodiments, the phosphate, preferably P2O5, contained in the granules produced in steps a)-d) has a water solubility of <15%, <20%, <25%, or <30%.

[0061] According to the invention, a different solubility rate (a different speed of dissolution of the phosphate contained in water) can be achieved by using different manufacturing, mixing, or application methods for the fertilizer core and the coating. Such a different solubility rate is not necessarily equivalent to a significantly different chemical (absolute) water solubility of the phosphate in the respective fertilizer / fertilizer granule components. Consequently, in embodiments, the phosphate in the coating of the fertilizer / fertilizer granule according to the invention can have a similar chemical (absolute) solubility in water as the phosphate contained in the core of the fertilizer / fertilizer granule, although the speed / rate of dissolution in water of the respective phosphate (of the two fertilizer granule components) may differ due to the different manufacturing or application methods.Application methods of the respective fertilizer granule components differ. In other words, in embodiments, the respective absolute (chemical) water solubilities of the phosphates in the coating and the core differ little, hardly, or not at all, while their actual water solubility rate (i.e., the rate of complete dissolution in water) differs. In embodiments, however, the water solubility of the core and the coating also differs significantly or greatly from each other, e.g., by at least 5-10%, by at least 10-20%, by at least 5-25%, by at least 10%, by at least 1%, 2%, 2.5%, 3%, 4%, 5%, 7.5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, or more.

[0062] In embodiments, acid can be added in a 1-fold stoichiometric ratio both to the starting material of the mixture produced in step a) and to the phosphate-containing solution according to step e). Even in such embodiments, a different water solubility rate (dissolution rate) of the finally obtained phosphate in the coating and the core can be achieved. This can be achieved solely by the different process steps, mixing methods, and / or the amount of liquid added in steps a)-d) and step e) in the process according to the invention, e.g., by wet granulation of a moist solid (for fertilizer core) in steps a)-d) and fluidized-bed coating using a suspension (for coating) in step e).

[0063] In the coating manufacturing process, it is incubated as a suspension with an increased liquid content, so that on the one hand the acid can react more efficiently with the starting material during incubation with more free water (than when mixing a moist solid) and at the same time the residue of acid esters is reduced, since this liquid suspension can be mixed better and more evenly during incubation with acid (than a moist solid).

[0064] According to the invention, the water solubility of a substance describes the percentage to which the substance can be dissolved in water. The solubility of a substance indicates the extent to which it dissolves in a solvent or is homogeneously distributed in the solvent as an atom, ion, or molecule and thus dissolves.

[0065] In other words, the surprising fertilizing effect and abrasion resistance of the fertilizer granules according to the invention is ensured by the inventive combination of process steps a) to d) with the coating of the resulting granules in step e), wherein the preferably different solubilities of the phosphate contained in the core and in the coating of the fertilizer are achieved by the preferably different stoichiometric mixing ratios of acid to phosphate in the respective underlying suspensions / mixtures of the starting materials.

[0066] In embodiments, the mineral acid is added in water- and sodium citrate-soluble forms according to the desired conversion reaction and conversion rate of the non-water-soluble phosphorus compounds in the phosphate-containing starting material.

[0067] In embodiments, the mineral acid is added in the process according to the invention in step a) and / or e) in a 0.2-fold to 1.5-fold stoichiometric ratio. In embodiments, the mineral acid is added in a 0.2-fold to 1.5-fold, 0.1-fold to 0.4-fold, 0.2-fold to 0.3-fold, 0.15-fold to 0.35-fold, 0.1-fold to 5-fold, 0.5-fold to 1.5-fold, 1.75-fold to 1.5-fold, 1-fold to 2-fold, 0.1-fold, 0.15-fold, 0.2-fold, 0.25-fold, 0.3-fold, 0.35-fold, 0.4-fold, 0.5-fold, 0.6-fold, 0.7-fold, 0.8-fold, 0.9-fold, 1-fold, 1.1-fold, 1.2-fold, 1.3-fold, 1.4-fold, 1.5-fold, 1.6-fold, 1.7-fold, 1.8-fold, 1.9-fold, 2-fold, 2.1-fold, 2.2-fold, 2.3-fold, 2.4-fold, 2.5-fold, 2.6-fold, 2.7-fold, 2.8-fold, 2.9-fold, 3-fold, 3.5-fold, 4-fold, 4.5-fold, 5-fold stoichiometric ratio. These numerical specifications may also include intermediate ranges and deviations of + / - 10%, according to the invention.

[0068] In some embodiments, the mineral acid is added in a 1 to 1.5-fold stoichiometric ratio. This ratio can enable better digestion under certain conditions.

[0069] In some embodiments, the mineral acid is used in a 0.2 to 1.5-fold stoichiometric ratio relative to the phosphorus content. In some embodiments, the mineral acid is used in a 0.2 to 0.3-fold stoichiometric ratio relative to the phosphorus content, in order to keep the proportion of free acid in the coating (surface of the finished fertilizer granules) as low as possible.

[0070] In one embodiment of the process according to the invention, the liquid phosphate-containing suspension comprises the incubation product of step a).

[0071] In one embodiment of the process according to the invention, the liquid phosphate-containing suspension applied in step e) is previously incubated with the mineral acid for a period of between 30 minutes and 24 hours.

[0072] In embodiments of the invention, prior to the application of the liquid phosphate-containing suspension used for coating in step e), preferably comprising at least one phosphate-containing starting material, at least one mineral acid, and water, the components in the suspension are incubated. In embodiments, the incubation takes place in a mixer or an incubation vessel (such as a stirred vessel). In embodiments, the incubation takes place in a fluidized bed device.

[0073] In one embodiment of the process according to the invention, the coating under point e) is applied by means of a fluidized bed. In embodiments, the entire granulate, i.e., the coating and optionally also the core material, is dried during and / or after the coating is applied by means of a fluidized bed.

[0074] In embodiments, the coating in step e) is applied in several steps and / or layers, e.g., in at least two layers. In embodiments, the most recently applied coating and / or the entire fertilizer granulate is dried after each coating step. In embodiments, in step e), several layers of the coating can be applied to the core material one after the other by means of a fluidized bed device or a fluidized bed process, wherein the drying of the newly applied layer and / or the entire fertilizer granulate can take place simultaneously and / or subsequently. Preferably, the application of one or more layers of coating by means of a fluidized bed enables the simultaneous drying of the respective coating during or immediately after application of the coating.An advantage of applying multiple coatings using a fluidized bed is increased strength of the coating and thus of the entire coated fertilizer granulate.

[0075] In one embodiment of the process according to the invention, the coated granules obtained in step e) comprise a grain size between 1-5 mm, wherein the coating applied in step e) has a thickness between 5-90 weight percent (% w / w) of the granules obtained from step e), and / or preferably has a thickness between 5 and 25 weight percent (% w / w) of the granules obtained from step e).

[0076] Preferably, the process according to the invention produces granules whose surface is designed, e.g. by the coating, in such a way that little dust formation and little abrasion occur during agricultural application of the fertilizer.

[0077] In one embodiment of the process according to the invention, the coating applied in step e) therefore has a higher compressive strength than the granulate obtained in step d), preferably a compressive strength of ≥ 20N. In embodiments, the granulate obtained in step d) accordingly has a compressive strength of < 20 N. In other embodiments, the coating applied in step e) has a higher compressive strength than the granulate obtained in step d), preferably a compressive strength of 2:1 N, ≥ 5 N, ≥ 10 N, ≥ 15 N, ≥ 20 N, ≥ 25 N, ≥ 30 N or ≥ 35 N. In embodiments, the granulate obtained in step d) accordingly has a compressive strength of ≤ 1 N, ≤ 5 N, ≤ 10 N, ≤ 15 N, ≤ 20 N, ≤ 25 N, ≤ 30 N or ≤ 35 N. The preceding ranges for “less than or equal to” (≤) or greater than or equal to (≥) also alternatively include the ranges “less than” (<) or “greater as" (>).

[0078] In one embodiment of the process according to the invention, the fertilizer produced in step e) has a moisture content of between 1 and 20 percent by weight (% w / w). In one embodiment of the process according to the invention, the fertilizer produced in step e) has a moisture content of between 0.1 and 20, between 0.5 and 20, between 0.5 and 10, between 0.1 and 10, between 1 and 10, between 1 and 15, between 1 and 30, or of <0.1, <0.5, <1, <2, <3, <4, <5, <6, <7, <8, <9, <10, <12, <15, <20, <25, or <30 percent by weight (% w / w).

[0079] In one embodiment of the process according to the invention, the mixture produced in step a) has a moisture content of between 10 and 30 percent by weight (% w / w). In one embodiment of the process according to the invention, the mixture produced in step a) has a moisture content of between 10 and 50, between 1 and 30, between 5 and 30, between 1 and 20, between 5 and 20, between 1 and 10, between 10 and 20, or of <10, <20, <30, <40, <50, <60 percent by weight (% w / w). In embodiments of the process according to the invention, the mixture produced in step a) comprises a dry matter content of between 70 and 90 percent by weight (% w / w). In embodiments of the process according to the invention, the mixture produced in step a) comprises a dry matter content of ≥ 50, ≥ 55, ≥ 60, ≥ 65, ≥ 70, ≥ 75, ≥ 80, ≥ 85, ≥ 90, ≥ 95 or of more than 90 percent by weight (% w / w).

[0080] For the purposes of the invention, the moisture content refers to the water content of a substance or mixture, i.e. the percentage of water present therein, which is given in percent by weight (% w / w).

[0081] In one embodiment of the process according to the invention, the at least one mineral acid in step a) has a total water content of <80 percent by weight (% w / w).

[0082] A total water content is understood as the total proportion of water contained in a substance or mixture in percent by weight (% w / w).

[0083] In one embodiment of the process according to the invention, an acid of the at least one mineral acid in step a) has a pK s Value of <4. In one embodiment of the process according to the invention, at least one acid of the at least one mineral acid in step a) has a pK s Value of <4.

[0084] In preferred embodiments, a combination of the fluidized-bed coating process (application of a coating using a fluidized bed) with granulation using a mixer and granulating disc enables a specific adjustment of the desired properties of the fertilizer granules. This combination of process features allows the properties of the fertilizer to be individually adapted to the respective requirements regarding its composition, solubility, or other relevant properties.

[0085] In one embodiment, granules are produced and dried in a first step using the wet granulation process, which then have approximately 90-95% of the target grain size. Without an additional coating, the granules produced in this way exhibit the disadvantages listed above, but have the advantage of being very cost-effective.

[0086] The inventors have surprisingly discovered that these disadvantages can be overcome by subsequently applying a coating to the fertilizer. Therefore, according to the invention, it is preferable to subsequently coat (envelop) the granules produced by wet granulation, preferably using a fluidized bed and preferably with the above-described suspension of phosphate-containing starting material(s), water, and mineral acid(s), in order to overcome the previously described disadvantages of wet granulation.

[0087] In some embodiments, it may be preferable to control the process so that (only) coating takes place and no granulation, since otherwise the target grain size may be exceeded. Furthermore, in some embodiments, it may also be preferable to ensure that the "wet granulate" dried in whole or in part in step d) is adequately protected, e.g., against abrasion or other damage, by the coating applied in step e).

[0088] In one embodiment, between 80-90 weight percent (% w / w) of the mass of the fertilizer granules obtained according to the invention are produced by wet granulation in steps a)-d), and 10-20 weight percent (% w / w) of the fertilizer granules are applied as a coating in step e). In other words, in embodiments, the fertilizer granules produced according to the invention comprise or consist of 80-90 weight percent (% w / w) of a material granulated by wet granulation and 10-20 weight percent (% w / w) of a coating.

[0089] Consequently, in these embodiments, the process according to the invention comprises applying a coating in step e) which constitutes 10-20 weight percent (% w / w) of the total weight of the fertilizer granules obtained from step e).

[0090] Furthermore, in these embodiments of the process according to the invention, the grain size during the (wet) granulation in step c) is preferably adjusted such that the grain size obtained comprises 80-90% of the desired grain size of the fertilizer granules obtained from step e).

[0091] In one embodiment, between 80-90% of the mass of the fertilizer granules obtained according to the invention are produced by wet granulation in step c), and 10-20% by a fluidized bed process in step e).

[0092] More preferably, the process according to the invention can both reduce the manufacturing costs and improve the properties of the fertilizer.

[0093] In embodiments, these advantages can be achieved by producing a fertilizer according to the invention which is composed of 80-90 weight percent (% w / w) of a material produced by (inexpensive) wet granulation and 10-20 weight percent (% w / w) of a material produced or applied by (expensive) fluidized bed coating.

[0094] One advantage of granules produced in embodiments using a mixer (alternatively or additionally, granulation on a granulating disc is also according to the invention) is that they are very inexpensive to produce. This results from the lower investment costs for the plant technology and the lower operating costs, which arise primarily from the significantly lower energy requirements of these systems. The low energy costs result from the process itself and the significantly lower proportion of water to be evaporated. While water contents of approximately 40 to 60% are typical for fluidized beds, water contents between 10 and 30% are typical for mixer granulation (and disc granulation). However, the invention is not limited to these percentages; deviations from these percentages in the range of 1-50% are also according to the invention.

[0095] In embodiments of the invention, the at least one phosphate-containing starting material comprises or consists of one or more ashes from the incineration of sewage sludge and / or animal excrement. Sewage sludge ashes are typically obtained by the thermal incineration of sewage sludge, with the sewage sludge (as a form of organic waste) being fed, for example, to thermal disposal through mono- or co-incineration plants. The sewage sludge ash resulting from the incineration of sewage sludge is a powdered starting material that, during mono-incineration, has an average P2O5 content in the range of approximately 20% and thus represents a valuable raw material for the production of phosphate fertilizers.

[0096] In some embodiments, these one or more sewage sludge ash components can be supplemented with digestate or other additives. Digestate is typically understood to be solid or liquid residues obtained from the fermentation of biomass. In some applications, the addition of digestate or other substances can positively influence the effectiveness and (fertilizer) properties of the granules.

[0097] In embodiments of the invention, the mixing in step a) and / or the incubation in step b) of the mixture of at least one phosphate-containing starting material with at least one mineral acid takes place in a mixer. In embodiments of the invention, the mixing in step a) of the mixture of at least one phosphate-containing starting material with at least one mineral acid takes place in an incubation container. In embodiments of the invention, the mixing in step a) and / or the incubation in step b) of the mixture of at least one phosphate-containing starting material with at least one mineral acid takes place in a mixer and / or an incubation container.

[0098] In embodiments of the invention, the mixture obtained from step b) of at least one phosphate-containing starting material with at least one mineral acid is granulated in step c) in a mixer. In embodiments, the mixture obtained from step b) of at least one phosphate-containing starting material with at least one mineral acid is granulated in step c) on a granulating disc. The use of a granulating disc can be particularly advantageous in embodiments for the production of granules with a diameter of more than 2 mm.

[0099] To achieve sufficient mechanical stability for subsequent coating and support, e.g., in a fluidized bed, the moist granules produced in the mixer or on the granulating plate in step c) are pre-dried in a drying apparatus in step d). If the mechanical strength of the core permits, this can also be done in the fluidized bed itself.

[0100] In embodiments, in step e), a coating is then applied to the previously produced granules in a fluidized bed system. In embodiments, the coating can consist of the same starting material(s) and acid(s) as the previously produced granules. In embodiments, the coating can also be produced from different or additional starting material(s). The starting material(s) and acids comprise at least one phosphate-containing starting material and at least one mineral acid.

[0101] When using a fluidized bed system for coating in step e), granulation should preferably be prevented, since there is a possibility that a desired target grain size will be exceeded.

[0102] In embodiments, the suspension for the coating in step e) is prepared from an aqueous solution having a dry matter content of between 40 and 60 weight percent (% w / w).

[0103] In embodiments, the suspension for the coating in step e) is used for a reaction time of between 0.5 hours (h) and 24 hours (h). In embodiments, the suspension for the coating is used for a reaction time of between 0.25 hours and 72 hours. In embodiments, the suspension for the coating is used for a reaction time of between 1 hour and 48 hours. In embodiments, the suspension for the coating is used for a reaction time of 0.5 hours (h) to 12 hours. In embodiments, the suspension for the coating is used for a reaction time of approx.10 minutes, 15 minutes, 20 minutes, 30 minutes / 0.5h, 1h, 1.5h, 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 5h, 5.5h, 6h, 6.5h, 7h, 7.5h, 8h, 8.5h, 9h, 9.5h, 10h, 10.5h, 11h, 11.5h, 12h, 16h, 24h, 36h, 48h, 72h or between 0.5h-6h, 0.5h-12h, 0.5h-24h, 1h-6h, 1h-12h, 1h-24h, 1h-36h, 1h-48h, Used 6h-12h, 6h-24h, 6h-36h, 6h-48h, 12h-24h, 12h-36h, 12h-48h, 12-72h.

[0104] In some embodiments, long reaction times are preferable to short batch times for better chemical digestion.

[0105] In embodiments, the coating layer, which preferably represents an aqueous chemical digestion of the suspension used, has high mechanical strength in the final fertilizer granulate. High mechanical strength reduces abrasion and the associated dust generation on the fertilizer application machines. According to the invention, the support during the coating process preferably leads to a solidification of the outer layer of the granulate according to the invention, which was previously preferably produced in the mixer or granulating disc in steps a)-d). Advantageously, potentially adhering dust is bound by the coating process according to the invention and thus poses no problems during further handling of the product and agricultural application.

[0106] "Carrier" in the sense of the invention is the binding of remaining or adhering fine particles to a granulate after wet granulation. These particles can be problematic when granulate is applied mechanically, e.g., to floors, and can lead to increased dust formation and soiling of the machinery. When a coating is sprayed over these fine particles, these dust particles are bound, thus preventing problems during application and / or use of the granulate.

[0107] For the purposes of the invention, a coating is preferably stronger or more stable than the core of a fertilizer granulate, since the components are better mixed in the liquid solution. When the material (moist solid) for the fertilizer core is mixed, however (e.g. in a mixer), the moist solid is mixed rather loosely, so that mixing is not ideal (<100%). Increased strength of the coating is also achieved by applying the coating using a fluidized bed process, since this preferably involves applying several thin layers of the suspension one after the other (in step e)), whereas mixing the moist solid in step a) results in a more "porous" / granular mixture. However, the thin and strong coating according to the invention achieves increased overall strength of the fertilizer granulate.

[0108] The coating according to the invention advantageously prevents the contact of potentially remaining acid (or acid nests) in the fertilizer granulate produced by means of a mixer or granulating disc with the outside world, e.g. the users, storage facilities or application machines.

[0109] Due to the improved chemical breakdown of the coating's starting material, e.g., through incubation and application using a fluidized bed, there is less free acid, and in particular, no acid pockets, in the coating itself, making the granules gentle on the material and less prone to corrosion on agricultural machinery. This represents a significant advantage over other inexpensive fertilizer granules produced using mixers or granulating discs.

[0110] Preferably, the coating dissolves upon contact with water and is available to the plants shortly after fertilizer addition, particularly due to the P2O5 content contained in the coating with its increased solubility in water or the increased water solubility of the coating. In contrast, the core of the granules (granules from the mixer / granulating disc from steps a) to d)) preferably dissolves more slowly upon contact with water. These embodiments therefore represent a long-term fertilizer.

[0111] Preferably, phosphate converted by the acid itself supports the adhesion of the fertilizer, whereby in this case too, an improved conversion of the phosphate in the coating suspension can achieve an increased stability of the coating, and thus of the entire fertilizer.

[0112] In preferred embodiments, the appropriate percentage of the fertilizers produced by the two combined processes according to the invention, which make up the fertilizer according to the invention (core and coating), can achieve advantages such as a price reduction, a hard surface, the avoidance of acid pockets, and dust binding. The ratio of short- and long-term plant-available phosphorus fertilizer can be actively adjusted during the process by varying the percentage of the combined fertilizers according to the invention in the core and coating.

[0113] In the context of the invention, percentages refer to weight percent (% w / w) unless otherwise stated.

[0114] In a further aspect, the invention relates to a fertilizer granulate produced by the method according to the invention. All parameters and features of the embodiments of the method described herein are thus also applicable to embodiments of the fertilizer granulate produced by this method.

[0115] In a further aspect, the invention relates to an apparatus and a system for carrying out the method described herein.

[0116] The present disclosure encompasses methods, as well as apparatus and means for performing these methods. Any feature of the invention disclosed in connection with one aspect of the invention is hereby also disclosed in connection with the other inventive aspects disclosed herein. Accordingly, embodiments and features of the invention described in connection with one embodiment of the method disclosed herein are deemed to be disclosed in connection with any other aspect and embodiment of the disclosure.

[0117] The various aspects of the invention are united by, benefit from, are based on, and / or are associated with the common and unexpected advantageous effects of the present method for producing a phosphate-containing fertilizer granulate having improved fertilizing and handling properties. EXAMPLES

[0118] The invention is further described by the following examples. These are not intended to limit the scope of the invention and claims, but rather represent exemplary embodiments of various aspects of the invention, which are provided to illustrate the described invention. Example 1 Wet granulation

[0119] For wet granulation according to an embodiment of the present invention, dry sewage sludge ash is charged into a suitable mixer (in the described experiment, an Eirich production mixer R08W was used).

[0120] A mixture of water and sulfuric acid is then prepared. The sulfuric acid used in this experiment was of technical quality with a concentration of 70%. Of this, 1 kg of technical acid was used per 2.1 kg of sewage sludge ash. (This corresponds to a 1-fold stoichiometric ratio for the sewage sludge ash used.) The acid was mixed with 100 g of water per 1 kg of acid (70%) used. The mixer was started so that the dry sewage sludge ash was swirled. The acid-water mixture was slowly added. The mixing time was approximately 2.5 minutes. Depending on the desired particle size distribution, it may be sufficient to carry out a build-up granulation only in the mixer. If larger granule diameters are required, these can subsequently be produced on a granulating disc. For this purpose, the granules / mixture were transferred from the mixer to a granulating disc, and water was sprayed onto the granules.The inventors' experiments have shown that the granulation effect increases significantly with the increasing diameter of the granulation plate.

[0121] The produced granules were then dried in a drying cabinet at 90°C.

[0122] For experimental purposes, a portion of the granules was also packaged in moist plastic bags and dried in a fluidized bed two days later. The granules survived this test unscathed.

[0123] The granules produced in the described experiment (using the granulating disc for larger granules) had a particle size distribution between approximately 1 mm and 5 mm (plus a moderate amount of dust that was not granulated and (mainly) loosely adhered to the granules). Fluidized bed coating

[0124] For the coating in the fluidized bed, a suspension of sewage sludge ash and water is mixed, to which sulfuric acid is then added. In the described experiment, the ratio of sewage sludge to acid was a stoichiometric ratio of 25%, based on the complete conversion of the phosphate contained in the sewage sludge ash. The suspension was prepared with a dry matter content of 45%. The suspension was dispersed using a jet mixer. After the acid was added, the suspension was allowed a reaction time of approximately 5 hours, but was also continued to be dispersed during this time.

[0125] For fluidized-bed coating, the dried granules from the wet granulation process were introduced into the fluidized bed. The previously prepared spray suspension was injected using two-fluid nozzles. The nozzle position was a top-spray position.

[0126] The supply air temperature was approximately 225°C, the exhaust air temperature was set to approximately 70°C.

[0127] The supply air flow was adjusted accordingly to create the desired, highly agitated fluidized bed. The discharge was adjusted accordingly to achieve a differential pressure across the fluidized bed of approximately 30-35 mbar.

[0128] After the process was started up and a continuous process was established, approximately 77.6 kg of spray suspension were injected per hour. This corresponds to a solids content of approximately 34.9 kg / h. Approximately 140 kg of previously produced (pre-dried) granulate from the wet granulation were fed into the fluidized bed per hour. The good grain product discharge was approximately 172 kg per hour. Thus, the finished granulate consists of approximately 80% wet granulation material and approximately 20% fluidized bed coating, based on mass.

[0129] Product discharge occurred continuously through a rotary valve, while new granulate from the wet granulation was continuously introduced into the fluidized bed via a rotary valve to maintain a constant bed mass. The coating test described was run continuously for approximately 24 hours, so the process can be classified as "continuous operation" rather than "batch operation." All parameters such as supply air flow, supply air temperature, exhaust air temperature, spray pressure, filter differential pressure, pressure drop across the fluidized bed, and differential pressure across the sieve plate remained constant after approximately 3 hours for the remainder of the test, thus indicating a stable continuous process.

[0130] The product was sieved to the desired grain size using a tumbler screen and the oversize grain was ground using a mill and returned to the fluidized bed process together with the fines using a rotary valve.

[0131] To prevent sticking during cooling, the product was placed in plastic bags and left open to cool. The bags were only sealed after complete cooling.

[0132] On an industrial scale, a corresponding bulk material heat exchanger can be used in embodiments for product cooling.

[0133] In summary, the present experiment shows that the process according to the invention is suitable, as exemplified by the embodiment used here, for converting the phosphate contained in sewage sludge ash in such a way that it is available to plants. Furthermore, the granules obtained according to the invention are free-flowing, dust-free, and have a favorable grain size of between 1 and 5 mm. The process according to the invention also makes it possible to reduce the rapidly rising costs for the necessary operating resources, such as raw materials and energy, which are incurred for the production and transport of fertilizers. Consequently, the invention provides a new energy-saving and thus cost-saving production process for fertilizers that have at least a comparable fertilizing performance to conventional fertilizers, without generating the high costs previously required. FIGURES

[0134] In the following, the invention will be explained in more detail with reference to figures, without being limited to these. Fig. 1: The figure shows an exemplary process comprising the sequence of method steps and means used to carry out the method steps of an embodiment of the method according to the invention.

Claims

[1] A process for producing a coated phosphate-containing fertilizer comprising the following steps a) mixing at least one phosphate-containing starting material with at least one mineral acid, wherein the at least one phosphate-containing starting material comprises at least one ash from the incineration of sewage sludge and / or animal excrement, b) incubation of the mixture obtained in step a), c) granulation of the mixture of acid and phosphate-containing starting material incubated in b), d) complete or partial drying of the granules obtained in step c), e) coating the completely or partially dried granules from step d) with a liquid phosphate-containing suspension, wherein the process characterized byis that the mineral acid in step a) is added in a 0.2-fold to 1.5-fold stoichiometric ratio, preferably in a 1-fold to 1.5-fold stoichiometric ratio, to the phosphorus contained in the phosphate-containing starting material. [2] Process according to the preceding claim, wherein the incubation in step b) and / or the granulation in step c) is carried out by means of a mixer, granulation plate and / or granulation drum. [3] Process according to one of the preceding claims, wherein the coating in step e) is applied by means of a fluidized bed. [4] Process according to one of the preceding claims, wherein the liquid phosphate-containing suspension in step e) comprises at least one mineral acid and at least one phosphate-containing starting material. [5] A process according to any one of the preceding claims, wherein the liquid phosphate-containing suspension used for coating in step e) has a dry matter content of between 40 and 60 weight percent (% w / w). [6] Process according to one of the preceding claims, wherein the liquid phosphate-containing suspension is obtained by incubating at least one phosphate-containing starting material with at least one mineral acid, wherein the at least one mineral acid is added to the phosphorus contained in the at least one phosphate-containing starting material in a 0.2-fold to 1.5-fold stoichiometric ratio, preferably in a 0.2-fold to 0.5-fold stoichiometric ratio. [7] A process according to any one of the preceding claims, wherein the liquid phosphate-containing suspension comprises the incubation product of step b). [8] A process according to any one of the preceding claims, wherein the liquid phosphate-containing suspension from step e) is previously incubated with the mineral acid for a period of between 30 minutes and 24 hours. [9] A process according to any one of the preceding claims, wherein the coated granules obtained in step e) comprise a grain size of between 1-5 mm, and wherein the coating applied in step e) has a thickness of between 5-90 weight percent (% w / w) of the granules obtained from step e), and / or preferably has a thickness of between 5 and 25 weight percent (% w / w) of the granules obtained from step e). [10] A process according to any one of the preceding claims, wherein the P2O5 in the coating applied in step e) has a higher water solubility rate than the phosphate in the granules obtained in step d). [11] Process according to one of the preceding claims, wherein the coating applied in step e) has a higher compressive strength than the granulate obtained in step d), preferably a compressive strength of >20N. [12] A process according to any one of the preceding claims, wherein the fertilizer produced in step e) has a moisture content of between 0.1 and 20% by weight (% w / w). [13] A process according to any one of the preceding claims, wherein the mixture produced in step a) has a moisture content of between 10 and 30 weight percent (% w / w). [14] Process according to one of the preceding claims, wherein the at least one mineral acid in step a) has a total water content of <80 weight percent (% w / w). [15] Process according to one of the preceding claims, wherein an acid of the at least one mineral acid in step a) has a pK s value of <4. [16] Fertilizer granules obtained by a process according to any one of the preceding claims.

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