Modified phosphogypsum fertilizer anti-caking agent and preparation method thereof

Abstract

This application provides a modified phosphogypsum fertilizer anti-caking agent and its preparation method. The modified phosphogypsum fertilizer anti-caking agent comprises the following raw materials in parts by weight: 90 parts dehydrated phosphogypsum, 3-7 parts long-chain diamide phosphogypsum modifier, and 3-7 parts fumed silica; wherein the long-chain diamide phosphogypsum modifier is obtained by amidation reaction of C12-C18 monobasic fatty acids with the amino groups at both ends of a straight-chain C2-C6 aliphatic diamine. In preparation, the above raw materials are mixed and then subjected to heat treatment to obtain the modified phosphogypsum fertilizer anti-caking agent. This application uses the long-chain diamide phosphogypsum modifier to hydrophobically modify dehydrated phosphogypsum, and uses fumed silica to improve the powder dispersion state during the modification process, so that the resulting anti-caking agent has both powder isolation function and good hydrophobic properties, which can reduce the degree of fertilizer particle agglomeration under pressure conditions and improve the fertilizer anti-caking effect.

Description

Technical Field

[0001] This application relates to the field of fertilizer anti-caking agents, specifically to a modified phosphogypsum fertilizer anti-caking agent and its preparation method. Background Technology

[0002] Fertilizers are essential substances in agricultural production used to replenish soil nutrients and promote crop growth. For ease of storage, transportation, and mechanized application, fertilizers are typically prepared as granular or powdered products. However, during actual storage and transportation, fertilizers are susceptible to clumping due to environmental humidity, temperature changes, stacking pressure, and their own hygroscopic properties. Clumping not only affects the product's appearance and flowability but also increases the difficulty of crushing, screening, and application. In severe cases, it can lead to uneven fertilization, clogging of fertilization equipment, and consequently, impacting fertilizer utilization efficiency and the stability of agricultural production.

[0003] Fertilizer clumping is typically related to the processes of hygroscopic absorption, dissolution, and recrystallization on the granule surface. When fertilizer granules are exposed to a humid environment, the granule surface readily absorbs moisture and forms a liquid film, causing some soluble salts to dissolve. During subsequent changes in temperature, humidity, or pressure, the dissolved salts recrystallize, forming bridging structures between adjacent fertilizer granules—so-called salt bridges. As the number and strength of salt bridges increase, the fertilizer granules gradually stick together and clump together. This problem is particularly pronounced for urea, nitrate fertilizers, and some compound fertilizers due to their high hygroscopicity.

[0004] To reduce the risk of fertilizer caking, existing technologies typically involve adding anti-caking agents to fertilizers. Common anti-caking agents include inorganic powders, organic hydrophobic agents, and composite anti-caking agents. Inorganic powder anti-caking agents primarily reduce direct contact between fertilizer particles by coating them on the surface or distributing them in the contact areas between particles, thus lowering the probability of salt bridge formation. Organic hydrophobic agents mainly reduce the wettability of the particle surface, decreasing the spreading and accumulation of moisture on the particle surface. Composite anti-caking agents usually combine organic hydrophobicity with inorganic isolation properties.

[0005] Talc is one of the more commonly used anti-caking agents for powdered fertilizers. Its main component is hydrated magnesium silicate, which has a platy structure and certain lubricity, enabling it to form a physical insulating layer on the surface of fertilizer granules, thereby reducing particle adhesion. However, talc mainly comes from natural talc mines and requires processing through mining, crushing, washing, drying, and fine or ultrafine grinding, which limits both raw material availability and processing costs. Furthermore, while talc has a certain insulating effect, its anti-caking effect may decrease under high humidity, long-term storage, or high pressure conditions, making it difficult to fully meet the demand for low-cost, highly stable anti-caking agents during long-term fertilizer storage and transportation.

[0006] Phosphogypsum is a major industrial byproduct generated during the wet-process phosphoric acid production, and its main component is calcium sulfate. With the development of the phosphorus chemical industry, the production and stockpiling of phosphogypsum are substantial, making its resource utilization a crucial issue for the industry. Purified phosphogypsum, after purification, drying, calcination, and grinding, possesses advantages such as wide availability, low cost, and easy particle refinement. From the perspective of resource utilization and reducing the cost of anti-caking agents, it has potential application as a fertilizer anti-caking powder material.

[0007] However, phosphogypsum and talc differ significantly in composition and surface properties. Talc is a layered silicate mineral powder, while phosphogypsum, after dehydration, is primarily a calcium sulfate powder. Calcium sulfate powder has a certain degree of polarity, making it prone to reacting with moisture. If used directly as an anti-caking agent for fertilizers, its surface hydrophobicity and anti-wetting properties are insufficient, and it may still absorb moisture in humid environments, thus hindering the inhibition of liquid film formation and salt bridge formation in the fertilizer particle contact area. Furthermore, the small particle size of dehydrated phosphogypsum ultrafine powder makes it prone to agglomeration during processing and use, leading to decreased uniformity of its distribution on the fertilizer particle surface and thus affecting its anti-caking effect.

[0008] Therefore, although phosphogypsum has low cost and resource utilization value, how to improve the surface wettability of dehydrated phosphogypsum powder, reduce its tendency to absorb moisture and agglomerate in fertilizer systems, and enable it to stably play the role of powder isolation and anti-caking are still technical problems that need to be solved in its application as a fertilizer anti-caking agent. Summary of the Invention

[0009] This application provides a modified phosphogypsum fertilizer anti-caking agent and its preparation method, aiming to solve the problem that the anti-caking effect of dehydrated phosphogypsum is poor when it is used directly as a fertilizer anti-caking agent. By modifying the dehydrated phosphogypsum with hydrophobicity, its hydrophobic properties are improved, thereby reducing the degree of agglomeration of fertilizer particles under pressure.

[0010] In a first aspect, this application provides a modified phosphogypsum fertilizer anti-caking agent, comprising the following raw materials in parts by weight: 90 parts dehydrated phosphogypsum, 3-7 parts long-chain diamide phosphogypsum modifier, and 3-7 parts fumed silica; wherein the long-chain diamide phosphogypsum modifier is obtained by amidation reaction of C12-C18 monobasic fatty acids with the amino groups at both ends of a straight-chain C2-C6 aliphatic diamine.

[0011] According to this application, by modifying dehydrated phosphogypsum with a long-chain diamide phosphogypsum modifier and combining it with fumed silica to improve the powder dispersion state during the modification process, the hydrophobic properties of the resulting anti-caking agent can be improved, so that when it is used as a fertilizer anti-caking agent, it can reduce the degree of fertilizer agglomeration under pressure conditions, thereby improving the fertilizer anti-caking effect.

[0012] Specifically, dehydrated phosphogypsum primarily serves as the inorganic powder matrix in anti-caking agents. After dehydration, it reduces its free water and water of crystallization content and acts as a powder separator between fertilizer particles. However, the main component of dehydrated phosphogypsum is calcium sulfate, which still retains a certain degree of polarity. If used directly as an anti-caking agent, it is easily wetted by water, making it difficult to effectively reduce moisture spread and salt bridge formation in the contact area with fertilizer particles, thus limiting its anti-caking effect.

[0013] This application employs a long-chain diamide phosphogypsum modifier, obtained by amidation reaction of C12-C18 monobasic fatty acids with the amino groups at both ends of a straight-chain C2-C6 aliphatic diamines, to modify dehydrated phosphogypsum. This long-chain diamide phosphogypsum modifier molecule contains long-chain hydrocarbon groups and a diamide structure. The long-chain hydrocarbon groups reduce the wettability of the dehydrated phosphogypsum powder surface, giving the powder surface better hydrophobic properties. The diamide structure facilitates the distribution of the modifier on the surface of the dehydrated phosphogypsum during thermal mixing, allowing the long-chain hydrocarbon groups to act on the phosphogypsum powder surface along with the modifier, thereby transforming the dehydrated phosphogypsum from a powder easily wetted by water into a modified powder with better hydrophobicity.

[0014] In this application, fumed silica is mainly used to improve the powder dispersion state during the modification process. Fumed silica has a small particle size and a high specific surface area, which allows it to be distributed between dehydrated phosphogypsum particles, reducing the tendency of powder agglomeration during the modification process and enabling the long-chain bisamide phosphogypsum modifier to act more uniformly on the surface of dehydrated phosphogypsum.

[0015] Therefore, dehydrated phosphogypsum provides the basis for powder isolation, long-chain diamide phosphogypsum modifier improves the hydrophobicity of the dehydrated phosphogypsum surface, and fumed silica improves the dispersion and anti-agglomeration effects during the modification process. The combination of these three components gives the resulting modified phosphogypsum fertilizer anti-caking agent both powder isolation and good hydrophobic properties. After being added to fertilizer, this anti-caking agent can distribute on the surface of fertilizer particles and in the contact areas between particles, reducing direct contact between fertilizer particles and decreasing the wetting of water and the formation of salt bridges in the particle contact areas, thereby improving the anti-caking effect of the fertilizer under pressure conditions.

[0016] In some embodiments, the C12-C18 monocarboxylic acid includes a first C12-C18 monocarboxylic acid and a second C12-C18 monocarboxylic acid, and the first C12-C18 monocarboxylic acid and the second C12-C18 monocarboxylic acid are different, and the molar ratio of the first C12-C18 monocarboxylic acid and the second C12-C18 monocarboxylic acid is 1:0.8-1.2.

[0017] In some of the above embodiments, the long-chain bisamide phosphogypsum modifier is obtained by reacting two different C12-C18 monobasic fatty acids with a straight-chain C2-C6 aliphatic diamine, allowing the introduction of aliphatic segments with different structures into the resulting long-chain bisamide phosphogypsum modifier system. Compared to long-chain bisamide modifiers formed from only a single monobasic fatty acid, using two different monobasic fatty acids to obtain a long-chain bisamide phosphogypsum modifier system can regulate the softening, spreading, and chain segment stacking state of the modifier during heating and mixing, reducing the possibility of local enrichment or uneven coverage caused by the regular arrangement of a single long-chain structure.

[0018] Specifically, the fatty acid segments formed by different monobasic fatty acids differ in hydrophobicity, chain flexibility, and stacking ability. Using two different C12-C18 monobasic fatty acids in a molar ratio of 1:0.8-1.2 allows the long-chain diamide phosphogypsum modifier to possess both good hydrophobic properties and good thermal spreadability, which is beneficial for forming a more uniform hydrophobic modified layer on the surface of dehydrated phosphogypsum powder. Therefore, in the above embodiments, by combining the first and second C12-C18 monobasic fatty acids, the uniformity of modification on the surface of dehydrated phosphogypsum by the long-chain diamide phosphogypsum modifier can be improved while maintaining the hydrophobic effect of the long-chain hydrocarbon groups, resulting in a modified phosphogypsum anti-caking agent with better hydrophobic properties and anti-caking effect.

[0019] It should be noted that when using two different C12-C18 monobasic fatty acids to prepare a long-chain diamide phosphogypsum modifier, the resulting modifier can be a mixed system composed of different long-chain diamide components, which at least contains fatty chain segments introduced by the two C12-C18 monobasic fatty acids; thus, it can exert a better hydrophobic modification effect.

[0020] In some embodiments, the first C12-C18 monocarboxylic acid is a C12-C18 branched monocarboxylic acid, and the second C12-C18 monocarboxylic acid is a C12-C18 straight-chain monocarboxylic acid.

[0021] In some of the above embodiments, by having both branched-chain and straight-chain monobasic fatty acids participate in the amidation reaction, the long-chain diamide phosphogypsum modifier system can simultaneously possess both branched-chain and straight-chain aliphatic segments. The straight-chain aliphatic segments have better hydrophobicity and segment stacking ability, which is beneficial for forming low-surface-energy hydrophobic regions on the surface of dehydrated phosphogypsum powder; the branched-chain aliphatic segments have relatively lower regularity, which can reduce the tendency of long-chain diamide molecules to over-crystallize and locally accumulate during heating and mixing, thereby improving the uniformity of the modifier's spreading on the surface of dehydrated phosphogypsum powder.

[0022] Compared to long-chain diamides formed solely from straight-chain monobasic fatty acids, the long-chain diamide phosphogypsum modifier system prepared by combining branched-chain monobasic fatty acids and straight-chain monobasic fatty acids is more advantageous in balancing the density and uniformity of the hydrophobic layer. In the above embodiments, the branched-chain monobasic fatty acid segments are mainly used to improve the thermal spreadability and surface uniformity of the long-chain diamide phosphogypsum modifier, while the straight-chain monobasic fatty acid segments are mainly used to provide a stronger hydrophobic stacking effect. The combination of the two is beneficial for forming a more uniform and denser hydrophobic modified layer on the surface of dehydrated phosphogypsum, thereby improving the hydrophobic properties and anti-caking effect of the modified phosphogypsum fertilizer anti-caking agent.

[0023] In some embodiments, the first C12-C18 monobasic fatty acid is isostearic acid, and the second C12-C18 monobasic fatty acid is stearic acid.

[0024] In some of the above embodiments, isostearic acid and stearic acid are both C18 monobasic fatty acids with relatively long hydrophobic hydrocarbon groups. Compared to monobasic fatty acids with shorter carbon chains, the long-chain diamide phosphogypsum modifier formed by C18 monobasic fatty acids has a stronger hydrophobic segment effect, which is more conducive to reducing the wettability of the dehydrated phosphogypsum powder surface, thereby improving the hydrophobic performance of the modified phosphogypsum fertilizer anti-caking agent. Meanwhile, isostearic acid is a branched monobasic fatty acid, while stearic acid is a straight-chain monobasic fatty acid. Their combination can maintain the hydrophobic effect of the C18 long chain while also taking into account the spreadability of the modifier during thermal mixing and the accumulation effect of the hydrophobic segments, which is beneficial for the long-chain diamide phosphogypsum modifier to act more uniformly on the surface of dehydrated phosphogypsum. Therefore, using isostearic acid and stearic acid as monobasic fatty acid components is beneficial to further improve the hydrophobic performance and anti-caking effect of the modified phosphogypsum fertilizer anti-caking agent.

[0025] In some embodiments, the dehydrated phosphogypsum is obtained by purifying phosphogypsum and then drying it at 90-110°C, calcining it at 700-800°C to dehydrate it, followed by grinding and grading.

[0026] In some of the above embodiments, the purified phosphogypsum is first dried at 90-110°C to remove free water and reduce the moisture content of the raw material. Subsequently, it is calcined at 700-800°C to further remove the water of crystallization, transforming it into dehydrated phosphogypsum. This drying and calcination dehydration process reduces the adverse effects of moisture introduced into the phosphogypsum powder on its anti-caking effect. After grinding and grading, the calcined and dehydrated phosphogypsum yields dehydrated phosphogypsum powder with a finer particle size and more suitable particle size distribution. This provides better contact and dispersion when mixed with long-chain diamide phosphogypsum modifiers and fumed silica, and also facilitates its distribution as an anti-caking agent between fertilizer particles.

[0027] In some embodiments, the fineness requirement of the dehydrated phosphogypsum is that the residue on a 600-mesh sieve is less than 10%.

[0028] In some embodiments, the linear C2-C6 aliphatic diamine is 1,3-propanediamine.

[0029] In some of the above embodiments, the 1,3-propanediamine molecule contains amino groups at both ends, enabling it to undergo amidation reactions with C12-C18 monobasic fatty acids, thereby forming a long-chain diamide phosphogypsum modifier. Compared to aliphatic diamines with shorter or longer carbon chains, the trimethylene linkage structure formed by 1,3-propanediamine has a moderate length, which is beneficial for the resulting long-chain diamide molecule to possess both a certain degree of chain segment flexibility and diamide structural stability, thereby improving the hydrophobic properties and anti-caking effect of the modified phosphogypsum fertilizer anti-caking agent.

[0030] In some embodiments, the specific surface area of ​​the fumed silica is 80~250 m². 2 / g.

[0031] In some of the above embodiments, fumed silica has a high specific surface area, which can play a role in dispersing and anti-agglomeration during the mixing and modification process of dehydrated phosphogypsum and long-chain diamide phosphogypsum modifier, and is conducive to the more uniform distribution of long-chain diamide phosphogypsum modifier on the surface of dehydrated phosphogypsum.

[0032] In some embodiments, the long-chain diamide phosphogypsum modifier is prepared by the following method: C12~C18 monobasic fatty acids are mixed with straight-chain C2~C6 aliphatic diamines and reacted at 100℃~110℃ for 2h~4h under a nitrogen atmosphere to obtain a long-chain diamide phosphogypsum modifier. The molar ratio of C12-C18 monobasic fatty acids to straight-chain C2-C6 aliphatic diamines is 2:(1.01-1.1).

[0033] In some of the above embodiments, by subjecting C12-C18 monobasic fatty acids to an amidation reaction with straight-chain C2-C6 aliphatic diamines under a nitrogen atmosphere, the influence of side reactions such as oxidation can be reduced, allowing the monobasic fatty acids to react more fully with the amino groups at both ends of the diamine to form a long-chain diamide structure. Controlling the reaction temperature to 100℃-110℃ and the reaction time to 2h-4h facilitates a stable reaction and yields a long-chain diamide phosphogypsum modifier suitable for surface modification of phosphogypsum. Simultaneously, a slight excess of the straight-chain C2-C6 aliphatic diamine relative to the C12-C18 monobasic fatty acid enhances the reactivity of the monobasic fatty acid. Furthermore, the relatively low boiling point of the straight-chain C2-C6 aliphatic diamine allows for subsequent removal of the excess portion through vacuum removal or drying, thereby reducing the adverse effects of excess diamine on the performance of the resulting modifier.

[0034] Secondly, this application provides a method for preparing a modified phosphogypsum fertilizer anti-caking agent, comprising: Provide the raw materials included in the modified phosphogypsum fertilizer anti-caking agent according to any embodiment of the first aspect; The raw materials are mixed and then heated to obtain the product.

[0035] According to this application, by mixing dehydrated phosphogypsum, long-chain diamide phosphogypsum modifier and fumed silica and then heating the mixture, the long-chain diamide phosphogypsum modifier can be softened under heating conditions and distributed on the surface of dehydrated phosphogypsum. At the same time, fumed silica can be used to reduce the agglomeration tendency of the powder during the mixing process, thereby obtaining a modified phosphogypsum fertilizer anti-caking agent with good hydrophobic properties and anti-caking effect.

[0036] In some embodiments, the heat treatment conditions include: treatment at 105~115°C for 0.5~5 hours.

[0037] In some of the above embodiments, controlling the heat treatment temperature at 105~115℃ is beneficial for the long-chain diamide phosphogypsum modifier to soften and distribute more evenly on the surface of dehydrated phosphogypsum; controlling the treatment time at 0.5~5h is beneficial for the raw materials to be fully mixed and for the surface modification to be completed.

[0038] Compared with the prior art, the beneficial effects of this application are at least as follows: This application uses dehydrated phosphogypsum as the inorganic powder matrix of the anti-caking agent and modifies it hydrophobically with a long-chain diamide phosphogypsum modifier. Simultaneously, fumed silica is used to improve the powder dispersion state during the modification process, resulting in a modified phosphogypsum fertilizer anti-caking agent that combines powder isolation and good hydrophobic properties. Therefore, this application can overcome the problem of dehydrated phosphogypsum being easily wetted by water and having insufficient anti-caking effect when used directly as a fertilizer anti-caking agent. It enables the anti-caking agent to reduce the degree of fertilizer particle agglomeration under pressure after being added to fertilizer, thereby improving the fertilizer anti-caking effect. Attached Figure Description

[0039] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0040] Figure 1 Infrared spectrum of the long-chain diamide phosphogypsum modifier-A in Example 1. Detailed Implementation

[0041] The various embodiments or implementation schemes in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments.

[0042] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with an embodiment or example is included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0043] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0044] In this specification, unless otherwise specified, "parts" refers to "parts by weight".

[0045] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0046] Preparation of dehydrated phosphogypsum: Industrial purified phosphogypsum containing free water is dried at a low temperature of 100±10℃ for 2 hours to remove free water, and then calcined at a high temperature of 700-800℃ for 2 hours to remove water of crystallization. After the calcined purified phosphogypsum is gradually cooled to room temperature, it is fed into a dry powder ball mill. The material after ultrafine grinding is classified by an air classifier to control the fineness of the product, and finally dehydrated phosphogypsum is obtained. Fineness requirement of dehydrated phosphogypsum: less than 10% residue on a 600-mesh sieve.

[0047] Fumed silica, hydrophobic, with a specific surface area of ​​approximately 150 m². 2 / g.

[0048] Preparation Example 1 Preparation of long-chain diamide phosphogypsum modifier: C12-C18 monobasic fatty acids and 1,3-propanediamine were mixed at a molar ratio of 2:1.05 and placed in a reaction vessel. Nitrogen gas was introduced, and the mixture was heated to 100-110°C. The mixture was stirred while maintaining the temperature for 4 hours to obtain a crude product. The crude product was then dried in a vacuum drying oven at 105°C and a vacuum of 0.04-0.05 MPa for 5-6 hours to obtain long-chain diamide phosphogypsum modifier-A. The C12-C18 monobasic fatty acid is lauric acid.

[0049] Its infrared spectrum is as follows Figure 1 As shown, 3290 cm -1 The N-H stretching vibration of secondary amide (-CONH-) is present at 2817 cm⁻¹. -1 and 2766 cm -1 It is the C-H stretching vibration of a tertiary amine; 1655 cm⁻¹ -1 and 1556 cm -1 These are the C=O stretching vibration and N-H asymmetric stretching vibration of secondary amide (-CONH-), respectively; 1465 cm -1 It is the C-H bending vibration of methylene (-(CH2)-); 722cm -1 It is a long-chain alkane with the -(CH2) group. n - Planar rocking vibration. The structure represented by these peaks is basically consistent with the structure of the target product.

[0050] Preparation Example 2 Preparation of long-chain diamide phosphogypsum modifier: The preparation method is largely the same as in Example 1, except that the C12-C18 monobasic fatty acid is myristic acid, resulting in long-chain diamide phosphogypsum modifier-B.

[0051] Preparation Example 3 Preparation of long-chain diamide phosphogypsum modifier: The preparation method is largely the same as in Example 1, except that the C12-C18 monobasic fatty acid is palmitic acid, resulting in a long-chain bisamide phosphogypsum modifier -C.

[0052] Preparation Example 4 Preparation of long-chain diamide phosphogypsum modifier: The preparation method is largely the same as in Example 1, except that the C12-C18 monobasic fatty acid is stearic acid, resulting in a long-chain bisamide phosphogypsum modifier-D.

[0053] Preparation Example 5 Preparation of long-chain diamide phosphogypsum modifier: The preparation method is largely the same as in Example 1, except that the C12-C18 monobasic fatty acid is isostearic acid, resulting in a long-chain bisamide phosphogypsum modifier -E.

[0054] Preparation Example 6 Preparation of long-chain diamide phosphogypsum modifier: The preparation method is largely the same as in Example 1, except that the C12-C18 monobasic fatty acids are a mixture of palmitic acid and stearic acid in a molar ratio of 1:1, resulting in a long-chain bisamide phosphogypsum modifier-F.

[0055] Preparation Example 7 Preparation of long-chain diamide phosphogypsum modifier: The preparation method is largely the same as in Example 1, except that the C12-C18 monobasic fatty acids are a mixture of isostearic acid and stearic acid in a molar ratio of 1:1, resulting in long-chain bisamide phosphogypsum modifier-G.

[0056] Preparation Example 8 Preparation of long-chain diamide phosphogypsum modifier: The preparation method is largely the same as in Example 1, except that the C12-C18 monobasic fatty acids are a mixture of isostearic acid and stearic acid in a molar ratio of 1:0.5, resulting in a long-chain bisamide phosphogypsum modifier-H.

[0057] Preparation Example 9 Preparation of long-chain diamide phosphogypsum modifier: The preparation method is largely the same as in Example 1, except that the C12-C18 monobasic fatty acids are a mixture of isostearic acid and stearic acid in a molar ratio of 1:1.5, resulting in long-chain bisamide phosphogypsum modifier-I.

[0058] Example 1 Preparation of modified phosphogypsum fertilizer anti-caking agent: Dehydrated phosphogypsum was added to a mixer. Under stirring, fumed silica, long-chain diamide phosphogypsum modifier-A, and ultrafine phosphogypsum powder were mixed in a mass ratio of 90:5:5. The mixture was then stirred and continued to mix. The temperature was increased to 105~115℃ and maintained for 1 hour to obtain a modified phosphogypsum fertilizer anti-caking agent.

[0059] Example 2 Preparation of modified phosphogypsum fertilizer anti-caking agent: The process is largely the same as in Example 1, except that long-chain diamide phosphogypsum modifier-B is used instead of long-chain diamide phosphogypsum modifier-A.

[0060] Example 3 Preparation of modified phosphogypsum fertilizer anti-caking agent: The results are largely the same as in Example 1, except that long-chain diamide phosphogypsum modifier-C is used instead of long-chain diamide phosphogypsum modifier-A.

[0061] Example 4 Preparation of modified phosphogypsum fertilizer anti-caking agent: The process is largely the same as in Example 1, except that long-chain bisamide phosphogypsum modifier-D is used instead of long-chain bisamide phosphogypsum modifier-A.

[0062] Example 5 Preparation of modified phosphogypsum fertilizer anti-caking agent: The results are largely the same as in Example 1, except that long-chain diamide phosphogypsum modifier-E is used instead of long-chain diamide phosphogypsum modifier-A.

[0063] Example 6 Preparation of modified phosphogypsum fertilizer anti-caking agent: The process is largely the same as in Example 1, except that long-chain diamide phosphogypsum modifier-F is used instead of long-chain diamide phosphogypsum modifier-A.

[0064] Example 7 Preparation of modified phosphogypsum fertilizer anti-caking agent: The process is largely the same as in Example 1, except that long-chain diamide phosphogypsum modifier-G is used instead of long-chain diamide phosphogypsum modifier-A.

[0065] Example 8 Preparation of modified phosphogypsum fertilizer anti-caking agent: The process is largely the same as in Example 1, except that long-chain diamide phosphogypsum modifier-H is used instead of long-chain diamide phosphogypsum modifier-A.

[0066] Example 9 Preparation of modified phosphogypsum fertilizer anti-caking agent: The process is largely the same as in Example 1, except that long-chain bisamide phosphogypsum modifier-I is used instead of long-chain bisamide phosphogypsum modifier-A.

[0067] Example 10 Preparation of modified phosphogypsum fertilizer anti-caking agent: Similar to Example 1, except that the long-chain bisamide phosphogypsum modifier-A was replaced by a mixture of stearic acid bisamide modifier-D obtained in Preparation Example 4 and isostearic acid bisamide modifier-E obtained in Preparation Example 5 at a mass ratio of 1:1.

[0068] Comparative Example 1 Dehydrated phosphogypsum was used as an anti-caking agent.

[0069] Comparative Example 2 Use 600-mesh talc powder as an anti-caking agent.

[0070] Test section I. Hydrophobicity test: 1. Principle After adding a measured amount of powder to water, stirring, and allowing it to stand, the hydrophobic powder floats / suspends in the upper layer and remains unwetted by water; the hydrophilic powder sinks. The percentage of unwetted powder mass to the total powder mass is calculated through separation, filtration, drying, and weighing; this percentage is the hydrophobicity.

[0071] 2. Reagents and Instruments Reagent: Deionized water Instruments: Electronic balance (accuracy 0.001g), standard sieve (20 mesh), beaker (250mL), vacuum filtration device, drying oven 3. Testing Steps a. Sample pretreatment Take the powder to be tested, dry it in an oven at 105±2℃ to constant weight, cool it to room temperature, and then pass it through a 20-mesh sieve to remove large particles and lumps.

[0072] b. Weighing and mixing Accurately weigh the pretreated powder sample m1 (m1 is approximately 5g, and the record should be accurate to 0.001g) and place it in a 250mL beaker; add 100mL of deionized water and stir at a constant speed with a glass rod for 3min to fully disperse the powder; let it stand at room temperature for 30min to allow the system to separate into layers.

[0073] c. Solid-liquid separation and weighing Carefully pour out the upper layer of floating matter and suspension (including powder that has not been fully wetted by water), avoiding disturbing the lower layer of settling water that wets the powder; transfer the upper layer of material to a Buchner funnel pre-lined with quantitative filter paper (already weighed to constant weight m0), and start vacuum filtration until there is no obvious dripping of filtrate, collecting the unwetted powder on the filter paper. Place the filter paper containing the powder in an oven and dry it at 105±2℃ to constant weight (the difference between two weighings ≤0.002g); remove it and place it in a desiccator to cool to room temperature, and accurately weigh the total mass m2 of the filter paper + unwetted powder.

[0074] 4. Calculation formula Hydrophobicity (%) = (m2-m0) / m1×100%, the results are shown in Table 1. Where: m1——mass of the pretreated powder sample, in g; m2 — Total mass of unwetted powder and filter paper, in grams; m0 — the mass of the filter paper, in grams.

[0075] II. Anti-caking performance test Weigh 0.2g of the anti-caking agent prepared in each example and comparative example, and mix thoroughly with 50g of powdered urea or potassium nitrate to obtain the test sample. Place the test sample in a tablet press and maintain a pressure of 30kPa for 10 minutes to simulate the agglomeration process of fertilizer under pressure. Then, remove the compressed sample, place it on a 100-mesh standard sieve for sieving, weigh the mass of the residue on the sieve, and calculate the mass percentage of the residue according to the following formula: The percentage of residue on the sieve (%) = mass of residue on the sieve / total mass of the compressed sample × 100%.

[0076] The smaller the percentage of residue on the sieve, the fewer large particles are formed after compression, the easier it is for the sample to pass through the sieve and disperse, and the better the anti-caking performance. The test results are shown in Table 1.

[0077] In addition, referring to Section 6.4.1 (Method 1) of the chemical industry standard HG / T 5520—2019 "Fertilizer Anti-caking Agents", the anti-caking performance of the powder anti-caking agents prepared in each example and comparative example was tested.

[0078] 1. Basic experimental conditions The experimental substrate was a 15-15-15 granular compound fertilizer with a total nutrient content ≥45% and a particle size of 2-4 mm. The powder anti-caking agent in each example and comparative example was added at 0.3% of the fertilizer mass and thoroughly mixed with the sieved fertilizer sample. Accelerated testing conditions were as follows: the sample was placed in a constant temperature and humidity chamber at 40℃±3℃ and 80%±5% relative humidity for 4 hours, then transferred to a vacuum drying oven and dried to constant weight at 35℃ and a vacuum of 0.02 MPa.

[0079] 2. Test Operation Procedures The compound fertilizer sample was passed through a 4.75mm test sieve to separate qualified granules for later use. 500g of the sieved fertilizer sample was weighed, and the corresponding anti-caking agent was added, mixed thoroughly at a ratio of 0.3% of the fertilizer mass. The mass of the petri dish (m1) was weighed, and 20g of the mixed sample was weighed into the petri dish. The total mass of the petri dish and sample (m2) was then weighed. A stainless steel mesh was placed on the sample surface, and a 100g weight was placed on the mesh. The entire sample was placed in a constant temperature and humidity chamber at 40℃±3℃ and 80%±5% relative humidity for 4 hours. After removal, the sample was transferred to a vacuum drying oven and dried to constant weight at 35℃ and a vacuum of 0.02MPa.

[0080] After the sample cooled, it was inverted 6 cm above the platform, allowing it to fall naturally under gravity. It was then sieved using a 4.75 mm stainless steel sieve, ensuring that no external force was used to break any clumps during the sieving process. The remaining clumps in the petri dish were collected, and the total mass (m³) of the petri dish and clumps was measured. Two replicates were performed for each group of samples, and the arithmetic mean was taken as the result.

[0081] The anti-caking rate is calculated using the following formula: W1 = (m2 - m3) / (m2 - m1) × 100% In the formula, W1 represents the anti-caking rate (%), m1 represents the mass of the petri dish (g), m2 represents the total mass of the petri dish and the mixed sample (g), and m3 represents the total mass of the petri dish and the caking mixed sample (g). A higher anti-caking rate indicates less residual caking after accelerated moisture absorption, pressure treatment, and drying, and better anti-caking performance. The test results are shown in Table 1.

[0082] Table 1

[0083] As shown in Table 1, compared with Comparative Examples 1 and 2, the modified phosphogypsum fertilizer anti-caking agents obtained in Examples 1-10 all exhibited higher hydrophobicity, and showed lower residue ratios in the anti-caking performance tests of urea and potassium nitrate. Furthermore, they all showed higher anti-caking rates in the standard rapid method anti-caking performance test. These results demonstrate that this application, by modifying dehydrated phosphogypsum with a long-chain diamide phosphogypsum modifier and combining it with fumed silica to improve the powder dispersion state during the modification process, can significantly improve the hydrophobic properties and fertilizer anti-caking effect of the obtained anti-caking agent.

[0084] Further comparison of Examples 1-4 shows that as the carbon chain length of the fatty acid increases from C12 to C18, the overall hydrophobicity of the resulting anti-caking agent increases, while the overall proportion of residue on the sieve in the urea and potassium nitrate system decreases, indicating that longer fatty acid chain segments are beneficial for improving the hydrophobic effect on the surface of dehydrated phosphogypsum. Comparison of Examples 4 and 5 shows that although the long-chain diamide modifier formed by using isostearic acid alone can maintain a good hydrophobic modification effect, its anti-caking effect is slightly weaker than that of Example 4, which uses stearic acid alone.

[0085] Comparing Examples 4, 6, and 7, it can be seen that the long-chain diamide modifier formed by different fatty acid blends can further reduce the proportion of residue on the sieve. In particular, Example 7, which is a blend of isostearic acid and stearic acid, shows the highest hydrophobicity and the lowest proportion of residue on the sieve. This indicates that the combination of branched-chain fatty acids and straight-chain fatty acids is beneficial to balance the spreading uniformity of the modifier and the effect of the hydrophobic chain segments, thereby further improving the anti-caking performance of the modified phosphogypsum fertilizer anti-caking agent.

[0086] Comparing Examples 7 to 9, it can be seen that Example 7 has a better anti-caking effect than Examples 8 and 9. The above results show that controlling the molar ratio of isostearic acid and stearic acid to a near equimolar range is more conducive to taking into account both the spreading and regulating effect of branched fatty acid segments and the hydrophobic stacking effect of straight fatty acid segments, thereby obtaining better hydrophobic properties and anti-caking effect.

[0087] Comparing Examples 4, 5, 7, and 10, it is evident that Example 10, which uses separately prepared stearic acid bisamide modifiers and isostearic acid bisamide modifiers for physical compounding, exhibits a significantly weaker anti-caking effect compared to Example 7. This may be because, although the physical compounding method simultaneously introduces bisamide components corresponding to both linear and branched aliphatic segments, the softening, crystallization, and distribution states of the two symmetrical long-chain bisamide components differ. During heating and mixing, local enrichment or uneven coverage may still occur, making it difficult to fully utilize the synergistic effect of the branched and linear aliphatic segments. In Example 7, isostearic acid and stearic acid undergo a amidation reaction with 1,3-propanediamine, resulting in a modifier system containing a certain proportion of long-chain bisamide components with stearic acid and isostearic acid segments grafted at both ends, respectively. Introducing both straight-chain and branched aliphatic segments into the same molecule helps reduce local enrichment or uneven coverage caused by differences in softening, crystallization, and distribution among different symmetrical long-chain diamide components. This allows the modifier system to act more uniformly on the surface of dehydrated phosphogypsum during heating and mixing, thereby improving the hydrophobic properties and anti-caking effect of the modified phosphogypsum fertilizer anti-caking agent.

[0088] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A modified phosphogypsum fertilizer anticaking agent, characterized in that, The raw materials include the following parts by weight: 90 parts dehydrated phosphogypsum, 3-7 parts long-chain diamide phosphogypsum modifier and 3-7 parts fumed silica; The long-chain diamide phosphogypsum modifier is obtained by amidation reaction of C12-C18 monobasic fatty acids with the amino groups at both ends of a straight-chain C2-C6 aliphatic diamines.

2. The modified phosphogypsum fertilizer anti-caking agent according to claim 1, characterized in that, The C12~C18 monocarboxylic acids include a first C12~C18 monocarboxylic acid and a second C12~C18 monocarboxylic acid, and the first C12~C18 monocarboxylic acid and the second C12~C18 monocarboxylic acid are different. The molar ratio of the first C12~C18 monocarboxylic acid and the second C12~C18 monocarboxylic acid is 1:0.8~1.

2.

3. The modified phosphogypsum fertilizer anti-caking agent according to claim 2, characterized in that, The first C12~C18 monocarboxylic acid is a C12~C18 branched monocarboxylic acid, and the second C12~C18 monocarboxylic acid is a C12~C18 straight-chain monocarboxylic acid.

4. The modified phosphogypsum fertilizer anti-caking agent according to claim 3, characterized in that, The first C12-C18 monobasic fatty acid is isostearic acid, and the second C12-C18 monobasic fatty acid is stearic acid.

5. The modified phosphogypsum fertilizer anti-caking agent according to claim 1, characterized in that, The dehydrated phosphogypsum is obtained by purifying phosphogypsum and then drying it at 90-110℃, calcining it at 700-800℃ to dehydrate it, followed by grinding and grading.

6. The modified phosphogypsum fertilizer anti-caking agent according to claim 1, characterized in that, The straight-chain C2-C6 aliphatic diamine is 1,3-propanediamine.

7. The modified phosphogypsum fertilizer anti-caking agent according to claim 1, characterized in that, The specific surface area of the fumed silica is 80 to 250 m 2 / g.

8. The modified phosphogypsum fertilizer anti-caking agent according to any one of claims 1 to 7, characterized in that, The long-chain diamide phosphogypsum modifier was prepared by the following method: C12~C18 monobasic fatty acids are mixed with straight-chain C2~C6 aliphatic diamines and reacted at 100℃~110℃ for 2h~4h under a nitrogen atmosphere to obtain a long-chain diamide phosphogypsum modifier. The molar ratio of C12-C18 monobasic fatty acids to straight-chain C2-C6 aliphatic diamines is 2:(1.01-1.1).

9. A method for preparing a modified phosphogypsum fertilizer anti-caking agent, characterized in that, include: Provide the raw materials included in the modified phosphogypsum fertilizer anti-caking agent according to any one of claims 1 to 8; The raw materials are mixed and then heated to obtain the product.

10. The method according to claim 9, characterized in that, The heat treatment conditions include: treatment at 105~115℃ for 0.5~5 hours.

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