A method for co-production of food-grade phosphoric acid and ammonium phosphate salt by wet-process phosphoric acid fractional utilization

Abstract

This invention relates to a method for the graded utilization of wet-process phosphoric acid and the co-production of food-grade phosphoric acid and ammonium phosphate, belonging to the field of phosphoric acid chemical technology. The method first purifies the raw phosphoric acid, with the main process involving multi-stage extraction and back-extraction to obtain food-grade phosphoric acid. For the byproduct raffinate acid, no complex purification is required; instead, a slurry process is used directly to prepare fertilizer. When producing diammonium phosphate, the final neutralization pH is controlled at 8.0-8.5, and the degree of ammoniation is 1.8-2.0. When producing monoammonium phosphate, the raffinate acid is mixed with slag acid and filter solids, and a small amount of raw material acid is added to neutralize to a pH of 4.0-4.5, with a degree of ammoniation of 1.0-1.1. This invention overcomes the problems of high slurry viscosity, equipment blockage, and low product grade caused by the direct fertilizer production of raffinate acid with a simplified process. Furthermore, it coordinates with the disposal of system waste in monoammonium phosphate production, truly achieving high-value utilization and clean production of all components of wet-process phosphoric acid.

Description

Technical Field

[0001] This invention relates to the field of phosphorus chemical technology, and in particular to a method for the graded utilization of wet-process phosphoric acid and the co-production of food-grade phosphoric acid and ammonium phosphate. Specifically, it relates to a graded utilization method for wet-process phosphoric acid, and more particularly to a process that integrates the main process of wet-process phosphoric acid purification with the direct slurry method for preparing ammonium phosphate fertilizer from the by-product raffinate, thereby realizing the high-value utilization of all components of phosphorus resources. Background Technology

[0002] In recent years, with the increasing depletion of high-grade phosphate rock resources globally, the large-scale utilization of medium- and low-grade phosphate rock has become an inevitable choice for the sustainable development of the phosphate chemical industry. Wet-process phosphoric acid produced from medium- and low-grade phosphate rock using the sulfuric acid process has significantly higher impurity content (iron, aluminum, magnesium, calcium, sulfate, fluoride ions, etc.) than that produced from traditional high-quality phosphate rock. This poses a more severe challenge to downstream purification processes and the utilization of byproducts. Driven by the strong concept of a circular economy, how to achieve full utilization of all components per unit mass of phosphate rock resources, especially how to utilize the large amount of residual acid produced as a byproduct during the solvent extraction and purification process of wet-process phosphoric acid, has become a key benchmark for measuring the core competitiveness of phosphate chemical enterprises, and also a prominent bottleneck restricting the industry's green and low-carbon transformation.

[0003] Currently, the capacity of wet-process phosphoric acid purification plants built or under construction both domestically and internationally exceeds 5 million tons per year (based on P2O5). Considering the proportion of residual raffinate produced during the purification process (approximately 35%–45% of the total raw acid), the annual amount of residual raffinate produced as a byproduct is enormous. The P2O5 content in this residual raffinate can still reach as high as 23%–30%, with a phosphorus resource value reaching billions of yuan per year. However, due to its inherent defects of extremely high slurry viscosity and poor processing performance caused by its high impurity content (especially the content of sesquioxides such as magnesium, aluminum, and iron, typically in the range of 3.5%–8.0%), the utilization efficiency of this huge phosphorus resource pool has long been low. The common industry practice of "diluting the residual raffinate with large amounts of fresh raw acid and then barely using it" not only consumes a large amount of high-quality raw acid that could have been used to manufacture high-value-added products, but also results in extremely unstable plant operation and large fluctuations in product quality, failing to fundamentally solve this technical problem. Therefore, developing a novel technical route that allows for the direct, stable, and large-scale utilization of raffinate acid through a slurry process without requiring large-scale dilution of fresh phosphoric acid or the addition of any external chemical additives, and solely through precise control of process parameters, has extremely significant industrial and economic value.

[0004] Wet-process phosphoric acid is the cornerstone of the phosphorus chemical industry, with downstream products covering a wide range of fields such as agricultural fertilizers, industrial phosphates, food additives, and new energy materials. Under the macro-background of sustainable resource utilization, how to efficiently and cleanly utilize wet-process phosphoric acid in its graded processes, especially how to make high-value use of the large amount of residual acid produced as a byproduct during purification, has become a core bottleneck restricting the transformation and upgrading of the phosphorus chemical industry.

[0005] Currently, the mainstream technical route for producing high-value-added products using wet-process phosphoric acid is solvent extraction purification. This method selectively extracts phosphoric acid from wet-process phosphoric acid into the organic phase using an organic solvent. After washing and back-extraction, high-purity refined phosphoric acid is obtained for the production of food-grade and industrial-grade phosphoric acid or phosphates. This route produces high-purity products, but the system is complex. During the extraction process, most of the metallic cations such as iron, aluminum, magnesium, and calcium, as well as anionic impurities such as sulfate and fluoride ions, are enriched in the raffinate aqueous phase, forming a byproduct—raffinate acid—accounting for approximately 35%-45% of the total acid content. The phosphorus pentoxide (P₂O₅) content in the raffinate acid can still be as high as 23% to 30%, indicating significant phosphorus resource value. However, its high impurity content makes its subsequent utilization extremely difficult.

[0006] The difficulty in utilizing raffinate stems from its unique chemical composition and phase structure. In the solvent extraction process of wet-process phosphoric acid, organic extractants (typically a compound extractant consisting of TBP, sulfonated kerosene, and one or more modifiers) exhibit a high selective complexation ability towards phosphoric acid molecules. However, most of the metal cations such as iron, aluminum, magnesium, and calcium, as well as anions such as sulfate and fluoride ions in the raw acid, are repelled into the aqueous phase due to their extremely weak affinity for the organic phase. As multi-stage countercurrent extraction proceeds, the concentration of impurities in the aqueous phase continuously accumulates and concentrates. The final raffinate typically contains 4.0%–8.5% sesquioxides (R₂O₃ = Fe₂O₃ + Al₂O₃), 1.2%–3.5% MgO, and SO₄²⁻. 2- The content ranges from 1.5% to 2.5%. These impurities exist in concentrated phosphoric acid media in various complex ionic, complex, and even colloidal forms.

[0007] When residual acid reacts with ammonia to neutralize, as the pH value gradually increases, multiple complex chemical reactions will occur sequentially and simultaneously in the system: (1) Fe 3+ Al 3+ During neutralization, amorphous hydrous ferric phosphate and aluminum phosphate colloidal precipitates are formed; (2) Mg 2+ In an alkaline environment (especially when pH>7.5), it forms a precipitate of aqueous basic magnesium phosphate complex, which has extremely strong gelling properties; (3) Ca 2+ With SO4 2- , These substances combine to form fine crystals such as CaSO4·2H2O and CaF2. These precipitates exhibit extremely high specific surface area and strong hydration capacity within the conventional operating pH range (typically pH > 8.5), forming a semi-solid system with a gel-like network structure. This causes the slurry viscosity to surge dramatically to the thousands or even tens of thousands of millipascals per second (mPa·s). This not only leads to frequent blockages in pipeline transportation, pumping, atomization, and granulation processes, but also severely deteriorates granulation and drying efficiency, ultimately resulting in poor particle strength, high pulverization rate, and poor appearance. Furthermore, the presence of a large amount of impurities can also cause quality problems such as dilution of total nutrients and a sharp decline in the percentage of water-soluble phosphorus in available phosphorus.

[0008] The traditional uses of residual acid are mainly as follows:

[0009] One method involves blending the residual acid with fresh, low-impurity wet-process phosphoric acid and then producing agricultural-grade diammonium phosphate (DAP) via a tubular reactor or a pre-neutralization-ammoniation-granulation process. This method is essentially a "dilution and impurity reduction" strategy. To ensure stable operation of the production unit and basic product quality, a large amount of fresh raw material acid must be added, and the blending ratio of residual raffinate acid is usually strictly limited. If the proportion of residual raffinate acid increases, during neutralization, high-concentration impurity ions will generate a large amount of amorphous, high-water-content colloidal phosphate double salt precipitate, leading to a sharp increase in slurry viscosity. This causes frequent blockages in pipeline transportation, atomization, and granulation, preventing continuous and stable production. Simultaneously, key indicators such as total nutrient content and the percentage of water-soluble phosphorus in available phosphorus will decline sharply, and in severe cases, the product may even fail to meet quality standards. This simple blending method not only fails to fundamentally solve the problem of residual raffinate acid utilization but also consumes a large amount of high-quality raw material acid that could have produced high-value products, resulting in extremely poor economic efficiency.

[0010] Secondly, residual raffinate acid is mixed with other low-grade phosphoric acid materials such as slag acid and sedimentation tank sludge for direct use in fertilizer production. For example, existing technologies have attempted to neutralize residual raffinate acid by mixing it with wet calcium sulfate slag. However, this method further increases the total amount of impurities and solids in the system, leading to a further deterioration in the rheological properties of the neutralized slurry. This further reduces the total nutrients and available phosphorus content of the product, resulting in a lower product grade and weak market competitiveness.

[0011] Among them, sludge acid is solid-containing sludge acid produced during the wet phosphoric acid sedimentation process. Its P2O5 content is typically in the range of 18% to 42%, and the solid content (mainly calcium sulfate, fluorosilicate, and unreacted phosphate rock powder, etc.) is generally between 15% and 35%, with high levels of impurities such as iron, aluminum, and magnesium. Filter solids are solid filter cakes produced during the wet phosphoric acid desulfurization, defluorination, or filtration processes. The main components are phosphogypsum (CaSO4·2H2O), sodium fluorosilicate (Na2SiF6), unreacted phosphate rock powder, and metal phosphate precipitates, with a P2O5 content generally in the range of 5% to 18%. These low-grade phosphorus-containing solid wastes also contain considerable phosphorus resource value, but in traditional processes, they are considered a "burden" that must be disposed of externally, incurring not only high transportation and storage costs but also facing increasingly stringent environmental regulations.

[0012] Thirdly, some attempts are made to purify residual acid through complex stepwise precipitation using external precipitants (such as sodium salts and fluorides) before reuse. While this method improves product quality to some extent, it inevitably introduces new impurities such as sodium ions, increasing reagent costs, extending the process, and generating large amounts of difficult-to-treat mixed waste liquid or residue. Overall, its economic and environmental benefits are not ideal, greatly limiting its industrial application.

[0013] Furthermore, when dealing with large-scale raffinate streams, these methods of adding precipitants make solid-liquid separation of the precipitate extremely difficult, resulting in high moisture content in the filter cake and significant phosphorus loss, which further reduces the economic performance of the process.

[0014] In summary, existing technologies for utilizing residual raffinate acid are limited to treating it as a low-value waste and passively digesting it. This approach either sacrifices a large amount of high-quality raw material acid for dilution or introduces new impurities and processing steps. A technical route has not yet been found that uses residual raffinate acid as the main raw material and can directly, stably, and economically produce qualified or even superior grade ammonium phosphate fertilizers on a large scale without relying on a large amount of fresh raw material acid.

[0015] Therefore, the phosphate chemical industry urgently needs to develop a brand-new process that can fundamentally change the disposal mode of residual raffinate, transforming it from a "burden" into a "resource." Under simple, stable, and economical industrial conditions, it can achieve large-scale production of high-quality ammonium phosphate products using residual raffinate as a direct raw material with little or no admixture of raw material acid, while simultaneously realizing the high-value utilization of the main process purified acid, truly achieving the full-component graded high-value utilization of wet-process phosphoric acid. Summary of the Invention

[0016] To address the shortcomings of existing technologies, the purpose of this invention is to provide a method for the graded utilization of wet-process phosphoric acid to co-produce food-grade phosphoric acid and ammonium phosphate. The core concept of this method is to classify and configure the products of the main process of wet-process phosphoric acid extraction and purification—high-purity back-extraction acid is used to produce high-value-added products (such as food-grade phosphoric acid), while the residual raffinate is not simply treated or diluted in large quantities, but is directly produced using an optimized slurry ammoniation process through synergistic compounding with low-grade materials such as slag acid and filter solids within the system, thereby stably and economically producing qualified monoammonium phosphate or diammonium phosphate products. This maximizes the value of phosphorus resources and achieves zero waste discharge in the entire process system.

[0017] The above-mentioned objective of this invention is achieved through the following technical solutions:

[0018] This invention provides a method for wet-process phosphoric acid fractionation and utilization to co-produce food-grade phosphoric acid and ammonium phosphate, comprising the following steps:

[0019] Step 1: After the raw material wet-process phosphoric acid is subjected to desulfurization, defluorination, filtration, arsenic removal and heavy metal removal in sequence, it is concentrated once to obtain concentrated acid with a phosphorus pentoxide mass concentration of 50% to 55%.

[0020] Step 2: The concentrated acid is subjected to multi-stage extraction to obtain an organic phase loaded with phosphoric acid and residual acid; the organic phase loaded with phosphoric acid is washed, finely desulfurized and defluorinated, back-extracted and finely filtered to obtain back-extracted acid; the back-extracted acid is subjected to secondary concentration, decolorization, gas stripping defluorination and bleaching to obtain food-grade phosphoric acid.

[0021] Step 3: The residual acid obtained in Step 2 is processed according to the following sub-steps to prepare the ammonium phosphate product:

[0022] Step 3.1 Preparation of diammonium phosphate: Take the raffinate, or a mixture of the raffinate and raw material wet-process phosphoric acid, and neutralize it with ammonia at room temperature. Control the pH value at the end of the reaction to be 8.0-8.5, and the NH3:H3PO4 molar ratio to be 1.8-2.0. Use the slurry process and dry it to obtain the diammonium phosphate product.

[0023] Step 3.2 Preparation of monoammonium phosphate: Take the raffinate, mix it with slag acid and / or filter solid, and then neutralize it with ammonia. Control the pH value at the end of the reaction to be 4.0-4.5, and the molar ratio of NH3:H3PO4 to be 1.0-1.1. Use the slurry process, and obtain the monoammonium phosphate product after drying.

[0024] In a preferred embodiment of the present invention, the extractant in step 2 is a composite extractant composed of two or more reagents selected from tributyl phosphate, sulfonated kerosene, isopropyl ether, isooctanol, n-butanol, isoamyl alcohol, cyclohexanol, and n-octanol, wherein the volume ratio of tributyl phosphate is 0.6 to 0.8; the extraction ratio is 4 to 6:1; the extraction temperature is 50°C; the number of extraction stages is 4 to 6 stages of countercurrent extraction; and the extraction equipment is one or more of the following: a mixing and clarifying tank, a vibrating sieve plate tower, a pulse tower, and a rotary disc tower.

[0025] In a preferred embodiment of the present invention, in step 1, the desulfurization treatment involves adding a stoichiometric amount of phosphate rock slurry or calcium carbonate to the raw material wet-process phosphoric acid, and stirring the reaction at 55°C to 75°C for 1 to 2 hours to precipitate calcium sulfate ions; the defluorination treatment involves adding a stoichiometric amount of sodium carbonate or sodium hydroxide to precipitate sodium fluorosilicate ions; and the arsenic removal and heavy metal removal treatment involves adding sulfide precipitants such as sodium sulfide or phosphorus pentasulfide, and stirring the reaction at 40°C to 70°C for 60 to 90 minutes.

[0026] According to one embodiment of the present invention, in step 3.1, when a mixture of the residual raffinate and the raw material wet-process phosphoric acid is used, the mass percentage of the residual raffinate in the mixture is greater than or equal to 50%.

[0027] According to one embodiment of the present invention, in step 3.2, the sludge acid is solid sludge acid produced during the wet phosphoric acid sedimentation process, and the filter cake is a solid filter cake produced during the wet phosphoric acid desulfurization, defluorination or filtration process; the mixing mass ratio of the residual acid, sludge acid and filter cake is 100:(5-30):(2-10).

[0028] More preferably, in step 3.2, the mixing mass ratio of the residual acid, slag acid, and filter solids is 100:(8-18):(3-7). Within this ratio range, the amount of slag acid and filter solids added is sufficient to provide enough heterogeneous nucleation sites to improve the rheological properties of the slurry, without causing difficulties in slurry transportation due to excessive solid content, and the total nutrient content of the final product can be stably maintained above 56%.

[0029] According to one embodiment of the present invention, in step 3.1, the neutralization reaction is carried out in a pre-neutralization tank or a tubular reactor; in step 3.2, the neutralization reaction is carried out in a pre-neutralization tank with stirring.

[0030] More preferably, in step 3.1, when a tubular reactor is used for the neutralization reaction, the residence time of the material in the tubular reactor is controlled to be 5-20 seconds, and the outlet temperature of the slurry after the reaction is 80℃-110℃. Using a tubular reactor can significantly shorten the reaction cycle, reduce the equipment volume, reduce ammonia escape loss, and improve ammonia utilization rate to over 98%.

[0031] According to one embodiment of the present invention, in step 3, flexible production of diammonium phosphate and monoammonium phosphate products can be achieved on the same set of equipment by switching between executing step 3.1 or step 3.2, depending on market demand.

[0032] More specifically, the flexible production process is as follows: when switching from diammonium phosphate (DAP) to monoammonium phosphate (MAP), the ammonia feed path is stopped, the amount of DAP masterbatch in the system is reduced to the minimum value under normal production, the liquid level in the pre-neutralization reactor is reduced to 20% to 40% of the full level, and then the raw material is switched from residual raffinate acid or a mixture of residual raffinate acid and fresh raw material acid to a mixture of residual raffinate acid and slag acid and / or filter solids. The ammonia flow rate is adjusted to control the pH value at the neutralization endpoint from 8.0-8.5 to 4.0-4.5. The entire switching process can be completed within 2 to 6 hours without system evacuation and complete cleaning.

[0033] According to one embodiment of the present invention, in step 2, the residual acid is an acidic solution containing 23% to 30% P2O5 by mass and rich in iron, aluminum, and magnesium impurities, which is discharged from the aqueous phase during the multi-stage extraction process.

[0034] The present invention also provides a monoammonium phosphate product, which is prepared by step 3.2 of a wet-process phosphoric acid fractionation method for co-producing food-grade phosphoric acid and ammonium phosphate salt according to the above embodiments.

[0035] The present invention also provides a diammonium phosphate product, which is prepared by step 3.1 of a wet-process phosphoric acid fractionation method for co-producing food-grade phosphoric acid and ammonium phosphate salts according to the above embodiments.

[0036] The present invention also provides a method for utilizing wet-process phosphoric acid with no waste residue discharge, comprising all steps of the method described in any of the above embodiments, wherein, in step 3.2, the sludge acid and / or filter solids are all consumed as raw materials.

[0037] The present invention also provides a method for modifying a wet-process phosphoric acid purification device, which connects the by-product extraction acid pipeline of the device with the sludge acid conveying pipeline and the filter solid discharge device, adds a mixing tank, and connects it with the original slurry-process monoammonium phosphate or diammonium phosphate production system, so as to realize a method for graded utilization of wet-process phosphoric acid and co-production of food-grade phosphoric acid and ammonium phosphate salts as described in the above embodiments.

[0038] In summary, compared with the prior art, the present invention has at least one of the following beneficial technical effects:

[0039] It truly realizes the full-component graded high-value utilization of wet-process phosphoric acid: the high-purity phosphoric acid produced by the main process can be developed into food-grade and industrial-grade products, while the branch process converts all by-product raffinate and waste residue into fertilizer products with market competitiveness, completely changing the passive situation of traditional factories where "main products make money and by-products consume money".

[0040] The process is extremely simple and stable, solving the engineering challenge of large-scale utilization of raffinate acid: abandoning the traditional complex and inefficient "dilution and impurity reduction" or "addition of precipitant" routes, it directly converts raffinate acid into a slurry that can be stably transported and processed within the framework of the slurry process by precisely controlling the pH value and ammonia degree at the neutralization endpoint. This fundamentally alleviates the problems of pipeline blockage and equipment scaling, and provides a guarantee for the long-term stable operation of the unit.

[0041] Zero discharge of waste residue within the system has been achieved: In the route for preparing monoammonium phosphate, the residual raffinate acid is creatively co-treated with low-grade solid waste such as slag acid and filter solids, and they are converted into fertilizer products together, eliminating the environmental pressure and disposal costs of waste residue discharge, and achieving a double harvest of environmental and economic benefits.

[0042] The product solutions are flexible and highly adaptable to the market: the process can be flexibly switched between monoammonium phosphate and diammonium phosphate, which can quickly respond to market changes and greatly improve the company's operational flexibility.

[0043] Low investment, low cost, and easy to industrialize: The "slurry method" neutralization and granulation process upon which this invention is based is a very mature and widely used technology in the phosphorus chemical industry. The core improvement of this invention lies in the optimization of process parameters and the reconstruction of material routes. It does not require the addition of a large number of expensive special equipment; existing equipment can be implemented with slight modifications. The return on investment is extremely high, making it highly valuable for large-scale industrial promotion.

[0044] This invention innovatively constructs a synergistic production model of "high-purity main product + zero-waste branch line". In traditional wet-process phosphoric acid purification, there is a lack of systematic connection between the main process and the auxiliary process. The residual acid and solid waste are disposed of independently, resulting in high disposal costs and significant phosphorus loss. This invention directly connects the residual acid pipeline from extraction and purification with the slag acid pipeline of the settling tank and the filter solids discharge device, establishing a mixing and proportioning unit, and seamlessly connecting it with the subsequent slurry fertilizer production system, forming a closed material flow cycle. In this cycle, the high-purity phosphoric acid produced by the purification main process can be developed into high-value-added products such as food-grade, pharmaceutical-grade, and electronic-grade phosphoric acid and phosphates, while the branch line process converts all residual acid and solid waste into competitive fertilizer products, completely changing the passive operating situation of traditional factories where "main products make money, and by-products consume money".

[0045] This invention replaces the traditional material impurity reduction route with process parameter control, eliminating the need for dilution and external additives. It creatively proposes a precise operating window of pH 8.0-8.5 and NH3:H3PO4 1.8-2.0 for the diammonium phosphate route. Within this window, while the dominant stable phase of diammonium phosphate ((NH4)2HPO4) is formed, impurities such as iron, aluminum, and magnesium precipitate as particulates with relatively good crystallinity, low specific surface area, and weak hydration capacity, thus controlling the slurry viscosity at the source. This technical approach differs fundamentally from the traditional "dilution for impurity reduction" or "external surfactant addition" routes—traditional routes reduce the apparent concentration of impurities through external intervention or forcibly reduce slurry viscosity, while the route of this invention refines the chemical thermodynamic reaction conditions, adhering to the principle of "impurities precipitating as needed," causing them to precipitate in a phase form with minimal impact on the slurry's rheological properties during ammoniation. This approach demonstrates significant cutting-edge nature and innovation.

[0046] Using "system waste residue" as a nucleating agent, this invention achieves "waste treatment with waste" and synergistic efficiency enhancement in the monoammonium phosphate (MAP) route. This invention creatively introduces slag acid and filter solids (traditionally considered solid waste) into the raffinate ammoniation system. The main components of the slag acid and filter solids are calcium sulfate (CaSO4·2H2O), fluorosilicates, and unreacted phosphate rock powder. Under the weakly acidic environment of pH 4.0-4.5 in step 3.2 of this invention, these fine solid particles can serve as heterogeneous nucleation sites, inducing the orderly attachment and growth of iron and aluminum phosphate precipitates on the particle surface, thereby avoiding the formation of a network gel structure caused by the random generation of a large amount of amorphous colloidal precipitates in the liquid phase. Furthermore, the phosphoric acid-containing medium entrained in the slag acid can react with ammonia to convert into MAP under this pH environment, and its phosphorus component is effectively recovered and incorporated into the product. The residual phosphorus in the unreacted phosphate rock powder in the filter solids can also be further released during the ammoniation process. Therefore, this scheme not only does not increase the solid waste burden of the system, but also achieves synergistic effects among various materials through the "nucleation induction effect" and the "residual phosphorus activation effect", ultimately achieving the goal of zero solid waste emissions for the entire system.

[0047] A flexible switching mechanism for the equipment was established to enable rapid, on-demand capacity conversion. By simultaneously setting up two parallel technical routes, steps 3.1 and 3.2, in the residual acid utilization stage, and establishing corresponding parameter switching operating procedures, the company can flexibly choose between diammonium phosphate (high ammonium phosphate route) and monoammonium phosphate (high utilization capacity route) based on factors such as market fertilizer price fluctuations, changes in upstream residual acid production rates, and inventory pressure of slag acid filter solids, without adding large-scale dedicated equipment or conducting comprehensive line shutdowns for maintenance. This mechanism gives the production equipment extremely high resilience against market risks, which is one of the core competitive advantages that traditional single-product structure purification equipment lacks.

[0048] With a short investment payback period and strong applicability, this invention utilizes the "slurry method" neutralization granulation process, a mature and widely used technology in the phosphate chemical industry, with hundreds of operating units already in operation domestically. The core improvements of this invention lie in the optimization of process parameters (neutralization endpoint pH, degree of ammoniation, and material ratio) and the reconstruction of the material route (extracted acid-residue acid-filter solids mixing pipeline). It eliminates the need for extensive and expensive specialized equipment; existing units can be modified simply by adding mixing tanks and altering pipelines. The cost per ton of product is low, the investment payback period is short, and it has significant value for large-scale industrial application. This ensures that this invention can cover various application scenarios, from large-scale phosphate chemical complexes to medium-sized phosphate fertilizer plants, achieving a balance of economic, social, and environmental benefits. Attached Figure Description

[0049] Figure 1 This is a schematic diagram of a process for wet-process phosphoric acid fractionation and co-production of food-grade phosphoric acid and ammonium phosphate salts according to an embodiment of the present invention. Detailed Implementation

[0050] The technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0051] Reference Figure 1 A wet-process phosphoric acid fractionation method for co-producing food-grade phosphoric acid and ammonium phosphate includes the following steps:

[0052] Step 1: Pretreatment of raw wet-process phosphoric acid: The raw wet-process phosphoric acid is subjected to desulfurization, defluorination, filtration, arsenic removal and heavy metal removal in sequence, and then concentrated once to obtain concentrated acid with a P2O5 mass concentration of 50% to 55%.

[0053] Step 2, Extraction, Purification and Preparation of High-Purity Phosphoric Acid: The concentrated acid is subjected to multi-stage extraction to obtain an organic phase loaded with phosphoric acid and residual acid; the organic phase loaded with phosphoric acid is washed, finely desulfurized and defluorinated, back-extracted and finely filtered to obtain back-extracted acid; the back-extracted acid is subjected to secondary concentration, decolorization, gas stripping defluorination and bleaching to obtain a high-purity phosphoric acid product, such as food-grade phosphoric acid;

[0054] Step 3, Fertilizer utilization of residual raffinate: The residual raffinate obtained in Step 2 is processed according to the following sub-steps to prepare ammonium phosphate products:

[0055] Step 3.1 Preparation of diammonium phosphate: Take the residual raffinate obtained in step 2, or a mixture of residual raffinate and raw material wet-process phosphoric acid, and neutralize it with ammonia at room temperature. Control the pH value at the end of the reaction to be 8.0-8.5, and the molar ratio of NH3:H3PO4 to be 1.8-2.0. Use the slurry process and dry it to obtain the diammonium phosphate product.

[0056] Step 3.2 Preparation of monoammonium phosphate: Take the residual acid obtained in step 2, mix it with the slag acid and / or the filter solid produced by the process filtration, and then neutralize it with ammonia. Control the pH value at the end of the reaction to be 4.0-4.5, the NH3:H3PO4 molar ratio to be 1.0-1.1, and use the slurry process. After drying, the monoammonium phosphate product is obtained.

[0057] Step 3.3: Depending on market demand, you can choose to execute either Step 3.1 or Step 3.2 to flexibly use the residual acid in the production of monoammonium phosphate or diammonium phosphate.

[0058] The technical solution of this invention is based on a deep understanding and systematic reconstruction of the material characteristics of the entire wet-process phosphoric acid production process. The technical mechanism by which this invention achieves the triple goals of "graded utilization, full consumption, and economic feasibility" is explained in detail below.

[0059] (I) A systematic concept for the hierarchical utilization of primary and secondary processes

[0060] The core of this invention lies in the concept of "separation." Step 2, the multi-stage extraction and purification process, is the "purification line" of the entire process. Utilizing the precise selectivity of solvent extraction, it extracts approximately 55%-65% of the high-quality phosphorus components from the raw phosphoric acid in an extremely high-purity form (back-extraction acid). This line aims for ultimate purity, targeting high-value-added markets such as food, pharmaceuticals, and new energy. Simultaneously, approximately 35%-45% of the phosphorus components and the vast majority of associated impurities (iron, aluminum, magnesium, etc.) are "separated" into the residual raffinate. Traditionally, this material is considered "inferior" and requires complex re-purification before utilization. This invention breaks with this conventional understanding, treating it as a special "composite phosphorus resource" with a self-consistent chemical composition, directly transforming it into a fertilizer product with a huge market potential through the optimized ammoniation process in step 3. In this way, the main process is unburdened, branch processes are applied according to the specific materials, and the entire system has no low-value materials flowing out, maximizing the graded value of phosphorus resources.

[0061] (II) Feasibility and operational window for directly preparing diammonium phosphate using residual raffinate in step 3.1

[0062] The key to this step lies in the precise selection of the neutralization endpoint pH (8.0-8.5) and the degree of ammoniation (NH3:H3PO4 molar ratio 1.8-2.0). This operating window was determined by the inventors' team after conducting extensive experiments and systematically studying the chemical reaction thermodynamics and slurry rheological properties of the complex ammoniation system of residual acid.

[0063] In traditional raffinate utilization processes, to achieve high ammonification, the pH is often neutralized to above 8.0, or even 8.5-9.0. However, in raffinate systems with high impurities, excessively high pH can cause impurities such as magnesium and aluminum to form extremely viscous colloidal hydroxides or basic phosphates. This is the main culprit for difficulties in slurry transport, granulator wall adhesion, and scaling in the drying tower. This invention precisely controls the neutralization endpoint within the relatively mild range of 8.0-8.5, and, combined with the characteristics of the slurry process, while ensuring that the main component of diammonium phosphate ((NH4)2HPO4) is a thermodynamically stable phase, it maximally suppresses the tendency of impurities such as magnesium and aluminum to form high-viscosity colloidal precipitates. Experiments have shown that within this window, the impurity precipitates exist in the form of relatively well-crystallized particles, with significantly lower specific surface area and water demand than the colloidal substances formed at higher pH levels, thus allowing the slurry to maintain process-acceptable fluidity.

[0064] To more intuitively illustrate the crucial role of this technical window, the inventors designed a systematic slurry viscosity comparison experiment. Under the premise of keeping all other conditions completely consistent, only the endpoint pH value of the residual acid neutralization reaction (with all residual acid fed and no fresh raw material acid mixed in) was changed, and the apparent viscosity of the slurry at 80℃ was measured. The results are shown in Table 1.

[0065] Table 1. Comparison of slurry viscosity under different neutralization endpoint pH conditions in the total extraction residual acid ammoniation system

[0066]

[0067] As clearly seen from the data in Table 1, when the pH value increases from 7.8 to 8.0, the slurry viscosity only increases from 2250 mPa·s to 2850 mPa·s, a moderate increase. However, when the pH value jumps from 8.5 to 8.8, the slurry viscosity jumps sharply from 3920 mPa·s to 6850 mPa·s, an increase of over 75%. This non-linear viscosity abrupt change strongly suggests a phase transformation of the precipitate in the system—a rapid transformation from a well-crystallized particulate precipitate to a colloidal precipitate with high water content and high specific surface area. This phenomenon precisely confirms the non-obviousness of the technical window of this invention and the repeatability of its technical effects. Furthermore, although the slurry viscosity is lowest at pH=7.2, the degree of ammonification is severely insufficient, and monoammonium phosphate (NH4H2PO4) is the main product. Although the proportion of water-soluble phosphorus in the product is high, the total nitrogen content is unqualified, and it cannot be sold as a qualified diammonium phosphate product. The operating window (pH=8.0-8.5) of this invention cleverly avoids the "critical inflection point zone" where the slurry viscosity soars dramatically, while ensuring the thermodynamic stability of the diammonium phosphate main component, thus achieving the optimal balance between product quality and process feasibility.

[0068] Furthermore, this step explicitly states that the raw material can be "raffinate acid, or a mixture of raffinate acid and raw material wet-process phosphoric acid." This provides the process with great flexibility and economy. When the composition of the raffinate acid is relatively good or the product is positioned as a general first-grade product, the entire raffinate acid can be fed directly to achieve complete digestion of by-products. When there are higher requirements for the water-soluble phosphorus in the product or the impurity content of the raffinate acid is abnormally high, a small amount of fresh raw material acid can be mixed in for appropriate adjustment. However, the essence of this invention is that even if mixed, the proportion is far lower than that of the traditional "dilution and impurity reduction" method. The core of the device operation relies on the optimized neutralization window, rather than the dilution of a large amount of raw material acid, thus resulting in significant economic benefits.

[0069] (III) Regarding the technical mechanism of preparing monoammonium phosphate by mixing residual raffinate acid, slag acid, and filter solids in step 3.2

[0070] This step is a crucial closed loop in achieving "zero waste residue" emissions within the system, a key aspect of this invention. During the wet-process phosphoric acid production, the settling tank generates "sludge acid" (containing a significant amount of insoluble silicates, calcium sulfate, and metal phosphate sludge), while the filtration section intermittently produces "filter solids" (mainly a mixture of phosphogypsum, fluorosilicates, and unreacted phosphate rock powder). These are also low-grade phosphorus-containing solid wastes. This invention creatively proposes mixing the residual raffinate acid with the sludge acid and filter solids before ammoniation to produce monoammonium phosphate (MAP).

[0071] First, the calcium sulfate (CaSO4·2H2O), fluorosilicates, and other components in the slag acid and filter solids can act as physical nucleation sites or auxiliary seed crystals, improving the aggregation state of the precipitate during ammoniation and thus enhancing the sedimentation and filtration performance of the slurry.

[0072] Secondly, the production pH window for monoammonium phosphate (MAP) (4.0-4.5) is lower than that for diammonium phosphate (DAP). Within this more acidic range, the precipitation behavior of iron and aluminum ions in phosphate differs from that in the alkaline range, resulting in a relatively lower degree of hydration. By mixing the raffinate with other acidic waste residues and neutralizing the pH to this range using ammonia, the phosphoric acid in the raffinate and the citrate-soluble phosphorus in the residue can be converted into MAP, while simultaneously fixing impurities in the solid product. When used as fertilizer, this product provides nitrogen and phosphorus nutrients, and the trace elements such as iron, aluminum, calcium, magnesium, and silicon it contains also supplement the nutrition of certain soils and crops.

[0073] Third, and most importantly, this mixed utilization method ensures that the entire wet-process phosphoric acid grading system produces almost no phosphorus-containing waste residue discharged externally. The residue acid, filter solids, and other materials are all fully utilized as raw materials, transformed into commercially valuable fertilizer products, truly achieving clean production and a circular economy.

[0074] In the synergistic ammoniation system of raffinate acid-residue acid-filter solids, the aforementioned "nucleation site-induced crystallization" effect is not a simple physical mixing, but involves a complex synergistic mechanism of interfacial chemistry and crystallization kinetics. Specifically:

[0075] (1) Heterogeneous nucleation effect: The calcium sulfate (CaSO4·2H2O) microcrystals, fluorosilicate particles, and unreacted phosphate rock powder contained in the slag acid and filter solids provide an interfacial energy barrier much lower than that required for homogeneous nucleation. During ammoniation, when the pH of the system gradually increases from the initial strong acidity to 4.0~4.5, Fe 3+ Al 3+ First, heterogeneous nucleation and precipitation occur on the surface of existing solid particles (solid-liquid interface) rather than in the liquid phase. This "surface anchoring-directional precipitation" mode effectively avoids the phenomenon of a large number of amorphous colloidal precipitates randomly forming and cross-linking into a three-dimensional gel network in the liquid phase.

[0076] (2) Synergistic viscosity reduction effect: Since the iron phosphate / aluminum precipitate is preferentially anchored on the surface of solid particles such as calcium sulfate in the slag acid and filter solid, the concentration of free amorphous colloidal particles in the liquid phase is significantly reduced, and the system cannot form a gel-like network. As a result, the macroscopic viscosity of the slurry is maintained within the pumpable range. The inventors' verification experiments have confirmed that the apparent viscosity of the control sample without the addition of slag acid and filter solid at the same pH=4.5 endpoint is 2680 mPa·s at 80°C; while after adding slag acid accounting for 20% of the mass of residual acid and 5% of filter solid, the slurry viscosity under the same conditions is only 1520 mPa·s, a reduction of 43.3%, which is extremely significant.

[0077] (3) Residual phosphorus activation and co-conversion effect: The phosphoric acid-containing medium entrained in the slag acid and the residual phosphorus of unreacted phosphate rock powder in the filter solid can be gradually neutralized by ammonia and converted into the effective component of monoammonium phosphate within the weakly acidic-weakly alkaline conversion range of the ammoniation reaction. The solubility of citrate-soluble phosphorus in the slag acid increases with increasing temperature and reacts with ammonia to form MAP. This effect transforms the slag acid and filter solid from pure "waste" into "secondary phosphorus resources", further improving the phosphorus yield of the entire system.

[0078] (4) Nutrient Synergistic Effect: In the final monoammonium phosphate product, the impurities such as iron, aluminum, and magnesium from the residual acid, as well as elements such as calcium and silicon from the residue acid and filter solids, exist in citrate-soluble or slow-release forms. This not only does not negatively affect the quality of the fertilizer, but also provides additional auxiliary nutrition for certain nutrient-deficient soils and crops that require micronutrients (such as fruit trees, vegetables, and legumes). The appropriate amount of calcium (Ca) in the product can improve acidic soil, magnesium (Mg) is the central atom of chlorophyll, and silicon (Si) can enhance the mechanical strength of crop stems and lodging resistance. This multidimensional fertilizer configuration of "binary nutrients (N+P2O5) + auxiliary micronutrients" is the additional value brought by this synergistic system.

[0079] (iv) Regarding the product solution flexibility in step 3.3

[0080] This invention, through the parallel setup of steps 3.1 and 3.2, endows the production equipment with extremely high flexibility. Enterprises can quickly switch between monoammonium phosphate (MAP) and diammonium phosphate (DAP) products based on factors such as market fertilizer prices, raw material acid inventory, and the rate of residual acid production. For example, during the peak season for phosphate fertilizer, multiple DAP production lines can be operated; during the off-season or when slag acid and filter solids require centralized processing, the system can switch to a MAP production line. This flexible "one-to-two" production mode is a significant advantage that traditional single-product purification equipment lacks.

[0081] In summary, the method provided by this invention, from a systems engineering perspective, rationally classifies and allocates materials throughout the entire wet-process phosphoric acid purification process. Instead of employing complex secondary chemical purification methods to forcibly purify residual acid, it "goes with the flow," utilizing its inherent composition to directly prepare fertilizer using the simplest and most mature slurry process. Through internal waste co-processing, it achieves clean production throughout the entire process. This solution significantly reduces the cost of utilizing residual acid while ensuring the continuous and stable operation of the equipment and the basic quality of the product, demonstrating outstanding industrial practicality and significant economic benefits.

[0082] It should be noted that, unless otherwise specified, the raw materials and reagents used in each embodiment are all commercially available conventional products or industrial by-products. Unless otherwise specified, the detection methods and standards involved, such as product nutrient content and impurity content, are all in accordance with the national standard GB 10205-2009 Monoammonium Phosphate and Diammonium Phosphate. In conjunction with the technical solution of this invention, the inventors also conducted the following systematic experimental verifications, focusing on the following aspects: a process using raffinate acid as the sole feed for diammonium phosphate; a process using raffinate acid mixed with fresh acid for diammonium phosphate; a process using raffinate acid + slag acid + filtration solids synergistic monoammonium phosphate process; and verification of the product's flexible switching effect.

[0083] Example 1: (Process implementation and product verification of diammonium phosphate preparation using residual acid as the main feedstock)

[0084] A method for wet-process phosphoric acid fractionation and co-production of food-grade phosphoric acid and diammonium phosphate, the specific process route is as follows: Figure 1 As shown.

[0085] Step 1: Pretreatment of raw material wet-process phosphoric acid

[0086] The crude wet-process phosphoric acid produced by a certain phosphate chemical enterprise has the following main indicators: P2O5 26.5%, SO42-22-3 ... 2- 2.2%, F - It contains 1.9% of heavy metals such as arsenic, lead, and chromium, as well as suspended solids.

[0087] First, a measured amount of phosphate rock slurry (approximately 28% P2O5) is added to crude phosphoric acid to initiate a desulfurization reaction. The mixture is stirred at 65°C for 2 hours, allowing sulfate ions to combine with calcium in the slurry to form calcium sulfate dihydrate. Then, an appropriate amount of sodium carbonate is added to defluorinate, producing sodium fluorosilicate. After the reaction is complete, the solution is filtered through a plate and frame filter press to obtain a clear desulfurized and defluorinated liquid.

[0088] Subsequently, a measured amount of sodium sulfide solution was added to the clarified liquid, and the mixture was stirred at 55°C for 45 minutes to precipitate arsenic and heavy metal ions as sulfides. After another precise filtration, preliminarily purified phosphoric acid was obtained.

[0089] Finally, the phosphoric acid was fed into a graphite evaporator and concentrated under a vacuum of -0.09 MPa and a temperature of 80°C to obtain concentrated acid with a P2O5 mass concentration of 52%, which was then cooled for later use.

[0090] Step 2: Extraction, purification, and preparation of food-grade phosphoric acid

[0091] The concentrated acid obtained in step 1 was extracted using a mixed solvent of tributyl phosphate (TBP) and kerosene as the extractant in a four-stage countercurrent extraction tank. The ratio (organic phase: aqueous phase) was controlled at 3.5:1, and the temperature was 45°C. After extraction, a phosphoric acid-loaded organic phase and raffinate were obtained. The typical composition of the raffinate was: P₂O₅ 46.0%, Fe 1.85%, Al 1.05%, Mg 0.88%, Ca 0.05%, SO₄²⁻. 2- 1.92%.

[0092] The loaded organic phase enters the washing section and is washed countercurrently with dilute monoammonium phosphate solution. It then enters the fine desulfurization and defluorination reactor, where measured amounts of calcium carbonate and sodium hydroxide are added. The mixture is stirred at 55°C for 40 minutes to thoroughly remove residual sulfate and fluoride ions. After clarification and separation, the organic phase enters the back-extraction section and is back-extracted with deionized water at 70°C to obtain back-extracted acid (approximately 32% P2O5).

[0093] The back-extracted acid was further concentrated in an enamel-lined evaporator to increase the P2O5 concentration to 85%. Then, 0.6% (by weight of acid) of powdered activated carbon was added, and the mixture was stirred at 85°C for 1 hour to decolorize. The activated carbon was then removed by a plate and frame filter press. The decolorized acid was sent to a stripping tower for stripping with clean hot air to remove trace organic residues and further defluorinate. Finally, a trace amount of hydrogen peroxide was added for bleaching to obtain a colorless, transparent, food-grade phosphoric acid product that meets all the standards of GB 1886.15.

[0094] Step 3: Fertilizer utilization of residual raffinate – preparation of diammonium phosphate

[0095] Step 3.1: This batch uses only the residual acid from the total extraction process (a byproduct of Step 2) as raw material, without mixing in fresh raw acid. At room temperature and with stirring, the residual acid is pumped into a pre-neutralization tank, and gaseous ammonia is slowly introduced for neutralization. By adjusting the ammonia flow rate, the final pH value is precisely controlled to be 8.2, and the NH3:H3PO4 molar ratio is 1.9. The heat of the neutralization reaction naturally raises the slurry temperature to approximately 80°C. The resulting slurry has good fluidity and can be directly pumped.

[0096] Step 3.2: Pump the neutralized slurry obtained in Step 3.1 into a spray drying tower or spray granulator, and perform atomization (or spraying), drying, granulation, and sieving according to the standard slurry method for diammonium phosphate. After cooling, the dried material yields granular diammonium phosphate product.

[0097] Testing revealed that the diammonium phosphate product obtained in this embodiment contained: 63.8% total nutrients (N+P2O5), 15.2% total nitrogen (N), 48.6% available phosphorus (P2O5), 82.5% water-soluble phosphorus, and 1.0% moisture. All product indicators met the requirements for first-grade diammonium phosphate produced by the slurry method in GB10205-2009. The entire production process was continuous and stable, with no abnormal phenomena such as nozzle clogging or tower wall scaling observed.

[0098] The inventors further measured real-time data on the change of slurry viscosity with pH during the above neutralization reaction process, as shown in Table 2.

[0099] Table 2. Real-time variation of viscosity of residual acid feed slurry in Example 1 with pH (temperature 80℃)

[0100]

[0101] Table 2 clearly shows that within the pH range of 4.5 to 7.5, the slurry viscosity increases approximately linearly with increasing pH, with a relatively mild increase. When the pH rises from 8.0 to 8.5, the viscosity increase is slightly greater, but still remains within the range of 3000 to 4000 mPa·s. This viscosity level is precisely the operating range within which industrial pre-neutralization tanks and slurry pumps can reliably deliver the slurry. Regarding the change in stirring motor current, from an empty acid solution to the neutralization endpoint at pH 8.2, the current increases from 12.5A to 22.5A, an increase within the safe range of the equipment's rated power.

[0102] Example 2: (Optimization of the blending ratio for preparing diammonium phosphate by mixing residual acid with fresh raw material acid)

[0103] This embodiment aims to verify the effect of adding a small amount of fresh raw material acid on improving process stability and product quality when the content of residual acid impurities is high in step 3.1.

[0104] Take the same residual acid as in Example 1, and separately take fresh raw material wet-process phosphoric acid (P2O5 26.5%, Fe 0.15%, Al 0.22%, Mg 0.09%, SO42-). 2- (1.65%). Residual acid and fresh raw material acid were mixed at different mass ratios, and ammoniation experiments were conducted. The neutralization endpoint was controlled at pH=8.2 and the NH3:H3PO4 molar ratio=1.90. Diammonium phosphate was prepared using a slurry process. The results are shown in Table 3.

[0105] Table 3. Comparison of slurry characteristics and product quality under different raffinate acid blending ratios

[0106]

[0107] As can be seen from Table 3: (1) When the proportion of residual acid is 50%~100%, although the viscosity of the slurry increases with the increase of the proportion of residual acid, the viscosity is always within the range of stable operation (2380~3180 mPa·s) under the operating window of this invention (pH=8.2), and the proportion of water-soluble phosphorus in the product is always above 82%, which are all first-class products and fully meet the national standard requirements. (2) Even under the extreme case of 100% residual acid (B5), the scheme of this invention can operate normally without the addition of any surfactant or other additives, and all indicators of the product (total nutrients 63.8%, water-soluble phosphorus 82.5%) reach and slightly exceed the threshold of first-class products. This fully demonstrates that the core function of this invention lies in the precise control of the operating window, rather than relying on the dilution effect of a large amount of fresh acid.

[0108] Example 3 (Process implementation and product verification of monoammonium phosphate preparation by synergistic digestion of residual acid, slag acid and filter solids)

[0109] A method for wet-process phosphoric acid fractionation and co-production of food-grade phosphoric acid and monoammonium phosphate.

[0110] Steps 1 and 2 are exactly the same as in Example 1.

[0111] Step 3: Fertilizer utilization of residual raffinate – Preparation of monoammonium phosphate

[0112] Step 3.1: Take the residual acid produced in Step 2, and separately take the slag acid discharged from the settling tank (P2O5 40.7%, Fe2O3 3.05%, MgO 2.4%, solid content 28.6%) and the filter solid produced by the rotary filter (mainly containing calcium sulfate, unreacted phosphate rock powder, etc.). Mix the three together at a mass ratio of 100:20:5 as the mixed raw material to be processed.

[0113] Step 3.2: Under ambient temperature and stirring, ammonia gas is introduced into the above mixed raw materials to carry out a neutralization reaction. The final pH value of the reaction is precisely controlled to be 4.3, and the molar ratio of NH3:H3PO4 is 1.05. After the reaction is completed, a mixed slurry is obtained. This slurry contains soluble impurities introduced by residual raffinate acid, slag acid, and insoluble particles introduced by filtration solids, but the overall slurry is still pumpable.

[0114] Step 3.3: The slurry obtained in Step 3.2 is fed into a spray drying tower for atomization drying, or into a spray granulator for granulation drying. The hot air temperature and material residence time are strictly controlled to ensure the moisture content meets the standards. After drying, the material is cooled and sieved to obtain the monoammonium phosphate product.

[0115] Testing revealed that the monoammonium phosphate product prepared in this embodiment contained: 57.2% total nutrients (N+P2O5), 10.8% total nitrogen (N), 46.4% available phosphorus (P2O5), 88.9% water-soluble phosphorus, 1.2% sulfate content, and 1.5% moisture. This product not only meets the requirements for first-class monoammonium phosphate produced by the slurry method in GB 10205-2009, but also converts phosphorus-containing solid wastes such as slag acid and filter solids—which would otherwise require external discharge—into part of the product, achieving zero discharge of wastewater, waste gas, and solid waste within the workshop.

[0116] To further explore the systematic impact of different mixing ratios on the synergistic absorption effect, the following extended experiment was conducted, and the results are summarized in Table 4.

[0117] Table 4. Comparison of slurry characteristics and product quality under different ratios of residual acid, sludge acid, and filter solids.

[0118]

[0119] As can be seen from the data in Table 4: (1) Compared with the ammonium treatment of pure residual acid (D1, slurry viscosity 2680 mPa·s), the slurry viscosity decreased to varying degrees after the addition of residual acid and filter solids. When the ratio of residual acid:residual acid:filter solids = 100:18:7, the viscosity dropped to 1480 mPa·s, a decrease of up to 45%. This fully demonstrates the positive effect of the solid particles in residual acid and filter solids on the rheological properties of the slurry. (2) Within the preferred ratio range (100:8:3 to 100:30:10), the total nutrient content of the product remained at 56.0%~57.8%, and the proportion of water-soluble phosphorus remained at 85.2%~90.8%, both meeting or exceeding the first-class product indicators. (3) When the ratio exceeds the upper limit of the preferred ratio (e.g., D7=100:45:18), although the slurry viscosity and solid waste disposal rate are acceptable, the total nutrient content of the product drops to 53.8%, which is close to or slightly below the threshold of first-class product. This indicates that adding too much slag acid and filter solids will dilute the effective nutrient content of the product. (4) Considering the three dimensions of slurry fluidity, product quality and solid waste disposal rate, the ratio of residual acid: slag acid: filter solids = 100:(8~30):(3~10) is a relatively good range, while 100:(8~18):(3~7) is an even better range. This can ensure that the product meets the first-class product index and achieve a solid waste disposal rate of more than 90% in the system.

[0120] Example 4 (Verification of residual phosphorus recovery rate in the synergistic disposal process of residual acid-sludge acid-filtration solids)

[0121] This embodiment specifically verifies the contribution of slag acid and filter solids to the system's phosphorus yield during the synergistic ammoniation process. Material balance was performed based on the D5 ratio (residual acid: slag acid: filter solids = 100:18:7) in Example 3 to examine the conversion and recovery of various phosphorus forms before and after ammoniation.

[0122] Table 5. Material balance table of phosphorus element before and after synergistic ammoniation

[0123]

[0124] If we disregard the phosphorus contribution from the slag acid and filter solids, considering only the 460.0 kg P2O5 in the residual acid, the 522.0 kg P2O5 in the product already exceeds the amount of residual acid input. This excess of approximately 62.0 kg P2O5 originates from the citrate-soluble and water-soluble phosphorus in the slag acid and the partially activated residual phosphorus in the filter solids. The activation recovery rate of residual phosphorus in the slag acid and filter solids can be estimated as: (62.0-18.9) / (73.3+7.6) × 100% ≈ 53.3%. This indicates that the synergistic ammoniation process of this invention can indeed effectively recover a considerable proportion of residual phosphorus from the slag acid and filter solids and convert it into effective phosphorus components in the product, rather than simply "landfilling" the waste residue into the product. This is the core value of this synergistic enhancement system.

[0125] Example 5 (Verification of the Flexible Switching Effect of Products Using the Same Device)

[0126] This embodiment illustrates the production flexibility of the present invention. On the same production unit, diammonium phosphate is first produced continuously for 7 days under the conditions of Example 1. When the inventory is sufficient, the feed of residual raffinate is stopped and replaced with a mixed feedstock (residual raffinate + sludge acid + filter solids) prepared according to the ratio in Example 3. The ammonia flow rate is adjusted, the neutralization endpoint pH is controlled from 8.2 to 4.3, and the atomization / granulation parameters are adjusted to MAP mode. This allows for seamless switching to monoammonium phosphate production, which continues for 5 days.

[0127] Table 6. Comparison of key parameters for flexible switching from DAP to MAP for the same device

[0128]

[0129] Materials generated during the transition period can be returned to the mixing tank to participate in the reaction again, with almost no waste materials generated. The switching process does not require line shutdown, and the raw material pretreatment and extraction sections always maintain stable operation.

[0130] Throughout the entire 12-day continuous production cycle, the raw material pretreatment and extraction processes remained stable, with 100% of the residual acid being consumed in real time without any backlog. This demonstrates that the method of this invention can achieve flexible "online" switching, making it highly suitable for handling complex market supply and demand and variable raw material compositions.

[0131] Comparative Example 1 (A failed case of direct neutralization of residual acid in traditional full extraction to produce diammonium phosphate)

[0132] This comparative example aims to demonstrate the consequences of not operating within the specified operating window (step 3.1) of this invention. The same residual acid as in Example 1 was used and neutralized to pH 8.8 (outside the 8.0-8.5 range of this invention) by direct ammonia injection at room temperature. Upon reaching a pH close to 8.5, a sharp increase in slurry viscosity and a rapid increase in stirring current were observed. When the pH reached 8.8, the slurry became pasty, lost its fluidity, and could not be piped to the spray gun. Forced shutdown and reactor cleaning revealed a large amount of gelatinous scale adhering to the reactor walls and agitator blades. Analysis of a small amount of dried slurry showed a total nutrient content of only 60.1% and a water-soluble phosphorus ratio of only 70.5%. This comparative example clearly illustrates that for residual acid systems, precise control of the neutralization endpoint is crucial to the success of the process; exceeding the operating window of this invention will lead to irreversible engineering failure and product degradation.

[0133] Comparative Example 2 (Traditional Dilution and Impurity Reduction Process)

[0134] The same residual acid and fresh wet-process phosphoric acid (P2O5 26.5%) as in Example 1 were used and blended at a mass ratio of residual acid to raw acid of 4:6. After mixing, ammonia was introduced to neutralize the mixture until pH=8.2, and the subsequent process was the same as in Example 1. The resulting diammonium phosphate product had a total nutrient content of 64.1% and a water-soluble phosphorus ratio of 83.9%, with quality comparable to or slightly better than that of Example 1. However, this process consumed 60% of the high-quality fresh raw acid used for neutralization. In contrast, Example 1 of this invention produced a first-class product without consuming any fresh raw acid, simply through process optimization, resulting in a significant improvement in economic value and resource utilization efficiency.

[0135] Comparative Example 3 (Comparative verification of scheme 4 in Comparative Document 4 – External surfactant process)

[0136] To verify the technical advantages of this invention over the traditional "dilution and impurity reduction + external surfactant" route, a comparative verification experiment was conducted with reference to the method scheme of CN101613094A. This scheme requires combining the residual raffinate acid with ordinary raw material phosphoric acid to make the sesquioxide content in the combined acid less than 3%, then adding a surfactant to the combined acid, and neutralizing it with ammonia to produce diammonium phosphate.

[0137] The same raffinate acid as in Example 1 was mixed with fresh wet-process phosphoric acid (P2O5 26.5%) at a mass ratio of raffinate acid:raw acid = 35:65, reducing the R2O3 content in the mixed acid to approximately 2.85% (meeting the <3% requirement). Sodium dodecyl sulfate (SDS) surfactant was added to the mixed acid at 0.3% of the acid mass. Ammonia gas was introduced at room temperature to neutralize to pH 8.2. The comparative results are as follows: Under the conditions of the scheme in Comparative Document 4, the apparent viscosity of the slurry at 80°C decreased to approximately 1750 mPa·s, indicating good continuous operation. The total nutrient content of the product was 64.5%, the water-soluble phosphorus content was 91.2%, and the product quality reached the superior grade level. However, this scheme consumed 65% of the high-quality fresh raw material acid used for neutralization and required continuous addition of surfactant (based on an annual DAP production scale of 100,000 tons, approximately 150-200 tons of SDS are needed annually, with annual chemical costs of approximately 1.5-2 million RMB).

[0138] Table 7. Comparison of economic benefits between the present invention and existing technical solutions (based on an annual production of 100,000 tons of phosphate fertilizer products)

[0139]

[0140] The data in Table 7 strongly demonstrates that although the product grade of this invention is slightly lower than that of the high-blending scheme (Grade 1 vs. Grade 1), it eliminates the consumption of a large amount of high-quality fresh raw material acid, avoids the input of all added chemical agents, and fundamentally eliminates the burden and cost of solid waste disposal. Its overall economic benefits far exceed those of existing technologies. In the fertilizer market, the price difference between Grade 1 and Grade 1 products is typically only 50-100 yuan per ton, while the cost savings in raw material acid and chemicals in this scheme can reach several hundred yuan per ton. After offsetting these savings, the overall yield per ton of product in this scheme is far superior to existing processes.

[0141] Comparative Example 4 (Comparative experiment of a mixture of residual acid, sludge acid, and filtered solids, but with pH exceeding the range of this invention)

[0142] To verify the crucial role of precise pH range (4.0-4.5) control in step 3.2, the following comparative experiment was conducted.

[0143] The raw materials were mixed according to the same ratio of residual acid: residue acid: filter solids = 100:20:5 as in Example 3, but the neutralization endpoint pH values ​​were selected as pH=3.2 (below the lower limit of the scope of the present invention), pH=4.8, pH=5.5 (above the upper limit of the scope of the present invention), and pH=4.3 (preferred value of the present invention) for comparison.

[0144] Table 8. Comparison of product characteristics of the mixed system of raffinate acid + residue acid + filter solids under different neutralization endpoint pH values

[0145]

[0146] The results showed that at pH 3.2, the ammonification level was too low, resulting in an excessively high proportion of acidic ammonium phosphate, primarily H3PO4, in the product. This not only led to a low total nutrient content (only 50.5%) but also produced an acidic product, posing a risk of corrosion to packaging materials during storage and transportation. While production was barely possible at pH 4.8, a small amount of DAP phase was observed, causing the N:P2O5 ratio in the product to deviate slightly from the theoretical MAP value (1:5.0). At pH 5.5, the slurry viscosity increased significantly to 3250 mPa·s, and the recovery rate of residual phosphorus in the slag acid also decreased. This further confirms that the pH range of 4.0-4.5 specified in step 3.2 of this invention is a necessary condition for the qualification of monoammonium phosphate products.

[0147] Example 6 (Process adaptability verification when the source of residual acid varies across multiple batches)

[0148] Considering that the composition of residual acid in actual industrial production may fluctuate due to factors such as the batch of raw phosphate rock and extraction operation conditions, this embodiment specifically verifies the universal adaptability of the method of the present invention to residual acid from different sources.

[0149] Samples of residual acid from three different phosphate chemical companies were collected and tested.

[0150] Table 9. Comparison of the main chemical composition of raffinate from different sources and its adaptability test results under the process conditions of this invention.

[0151]

[0152] The revised data in Table 9 more systematically demonstrates that the wet-process phosphoric acid classification and utilization method provided by this invention exhibits broad and reliable technical applicability to raffinate from different phosphate rock deposits with varying impurity contents and compositions. For extremely high-impurity raffinate (Company C) with a total R2O3 content as high as 7.25%, although the slurry viscosity (4380 mPa·s) when directly producing diammonium phosphate from the raffinate is higher than the ideal operating range, only 10%–15% by mass of fresh phosphoric acid is needed to bring the system back to a stable operating state. This blending ratio is still far lower than the traditional "dilution and impurity reduction" process (which typically requires blending more than 50%). More notably, in the monoammonium phosphate route of step 3.2, thanks to the synergistic viscosity-reducing effect of the slag acid and filter solids, as well as the heterogeneous nucleation-inducing effect, even with the high-impurity raffinate from Company C, the slurry viscosity can still be firmly controlled within the excellent pumpable range of 1720 mPa·s, and the total nutrient content and water-soluble phosphorus ratio of the product both meet the national standards for first-class products. This fully demonstrates the correctness, advancement, and engineering practicality of the core technical concept of this invention: "replacing material dilution and impurity reduction with a precise process window".

[0153] In summary, the method of this invention, through systematic material classification and process reconstruction of the entire wet phosphoric acid purification process, has successfully solved the bottleneck of high-value utilization of raffinate acid that has plagued the industry for many years in an unprecedentedly simple, direct, and economical way. At the same time, it has realized the full-scale consumption and value-added transformation of all low-grade phosphorus-containing materials in the system, providing a green, sustainable, and efficient upgrading path for the phosphoric chemical industry.

[0154] The embodiments described herein are preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Therefore, all equivalent changes made in accordance with the structure, shape, and principle of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for wet-process phosphoric acid fractionation and utilization to co-produce food-grade phosphoric acid and ammonium phosphate, characterized in that, Includes the following steps: Step 1: After the raw material wet-process phosphoric acid is subjected to desulfurization, defluorination, filtration, arsenic removal and heavy metal removal in sequence, it is concentrated once to obtain concentrated acid with a phosphorus pentoxide mass concentration of 50% to 55%. Step 2: The concentrated acid is subjected to multi-stage extraction to obtain an organic phase loaded with phosphoric acid and residual acid; the organic phase loaded with phosphoric acid is washed, finely desulfurized and defluorinated, back-extracted and finely filtered to obtain back-extracted acid; the back-extracted acid is subjected to secondary concentration, decolorization, gas stripping defluorination and bleaching to obtain food-grade phosphoric acid. Step 3: The residual acid obtained in Step 2 is processed according to the following sub-steps to prepare the ammonium phosphate product: Step 3.1 Preparation of diammonium phosphate: Take the raffinate, or a mixture of the raffinate and raw material wet-process phosphoric acid, and neutralize it with ammonia at room temperature. Control the pH value at the end of the reaction to be 8.0-8.5, and the NH3:H3PO4 molar ratio to be 1.8-2.

0. Use the slurry process and dry it to obtain the diammonium phosphate product. Step 3.2 Preparation of monoammonium phosphate: Take the raffinate, mix it with slag acid and / or filter solid, and then neutralize it with ammonia. Control the pH value at the end of the reaction to be 4.0-4.5, and the molar ratio of NH3:H3PO4 to be 1.0-1.

1. Use the slurry process, and obtain the monoammonium phosphate product after drying.

2. The method for wet-process phosphoric acid fractionation and utilization to co-produce food-grade phosphoric acid and ammonium phosphate according to claim 1, characterized in that, In step 3.1, when a mixture of the residual raffinate and the raw material wet-process phosphoric acid is used, the mass percentage of the residual raffinate in the mixture is greater than or equal to 50%.

3. The method for wet-process phosphoric acid fractionation and utilization to co-produce food-grade phosphoric acid and ammonium phosphate according to claim 1, characterized in that, In step 3.2, the sludge acid is solid sludge acid produced during the wet phosphoric acid sedimentation process, and the filter cake is a solid filter cake produced during the wet phosphoric acid desulfurization, defluorination or filtration process; the mixing mass ratio of the residual acid, sludge acid and filter cake is 100:(5-30):(2-10).

4. The method for wet-process phosphoric acid fractionation and utilization to co-produce food-grade phosphoric acid and ammonium phosphate according to claim 1, characterized in that, In step 3.1, the neutralization reaction is carried out in a pre-neutralization tank or a tubular reactor; in step 3.2, the neutralization reaction is carried out in a pre-neutralization tank with stirring.

5. The method for wet-process phosphoric acid fractionation and utilization to co-produce food-grade phosphoric acid and ammonium phosphate according to claim 1, characterized in that, In step 3, based on market demand, flexible production of diammonium phosphate and monoammonium phosphate products can be achieved by switching between steps 3.1 and 3.2 on the same set of equipment.

6. The method for wet-process phosphoric acid fractionation and utilization to co-produce food-grade phosphoric acid and ammonium phosphate according to claim 1, characterized in that, In step 2, the residual acid is an acidic solution containing 23% to 30% P2O5 by mass and rich in iron, aluminum, and magnesium impurities, which is discharged from the aqueous phase during the multi-stage extraction process.

7. A monoammonium phosphate product, characterized in that, The monoammonium phosphate product is prepared by step 3.2 of the wet-process phosphoric acid fractionation method according to any one of claims 1 to 6, which co-produces food-grade phosphoric acid and ammonium phosphate.

8. A diammonium phosphate product, characterized in that, The diammonium phosphate product is prepared by step 3.1 of the wet-process phosphoric acid fractionation method according to any one of claims 1 to 6, which co-produces food-grade phosphoric acid and ammonium phosphate.

9. A method for utilizing all components of wet-process phosphoric acid without waste residue discharge, characterized in that, It includes all the steps of the method according to any one of claims 1 to 6, and in performing step 3.2, the sludge acid and / or filter solids are all consumed as raw materials.

10. A method for modifying a wet-process phosphoric acid purification device, characterized in that, The by-product extraction acid pipeline of the device is connected to the sludge acid conveying pipeline and the filter solid discharge device. A mixing tank is added and connected to the original slurry method monoammonium phosphate or diammonium phosphate production system to realize the wet process phosphoric acid graded utilization and co-production of food-grade phosphoric acid and ammonium phosphate salt as described in any one of claims 1 to 6.

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