A low-alkalinity composite phosphorus-fluorine curing agent for phosphogypsum and its preparation method

Abstract

This application provides a low-alkalinity composite phosphorus-fluorine curing agent for phosphogypsum and its preparation method. The composite phosphorus-fluorine curing agent comprises the following raw materials in parts by weight: 100 parts by weight of granulated blast furnace slag powder, 30-60 parts by weight of modified metakaolin, 10-20 parts by weight of silica fume, 2-10 parts by weight of inorganic coagulant, 0.1-1 parts by weight of chelating and dispersing agent, 0.1-0.3 parts by weight of dodecylbenzenesulfonic acid, 0.15-0.3 parts by weight of sodium citrate, and 0.05-0.15 parts by weight of hydroxypropyl methylcellulose ether. The modified metakaolin is obtained by coating amorphous aluminum hydroxide onto the surface of metakaolin using a urea hydrolysis precipitation method. By synergistically introducing surface-functionalized modified metakaolin and a process control agent network into the composite curing agent system, the rapid formation of an early dense microstructure can be simultaneously promoted while ensuring good workability of the slurry. This results in improved early strength, long-term stability, erosion resistance, and control of harmful ion leaching risk in the obtained cured body.

Description

Technical Field

[0001] This application relates to the field of phosphogypsum resource utilization technology, specifically to a low-alkalinity composite phosphorus-fluorine curing agent for phosphogypsum and its preparation method. Background Technology

[0002] Phosphogypsum is a major industrial byproduct generated during the wet-process phosphoric acid production. Approximately 4-5 tons of phosphogypsum are produced for every ton of phosphoric acid produced. Phosphogypsum contains soluble phosphates (0.5%-3%), fluorides (0.1%-1.5%), and trace heavy metals. If directly stockpiled or simply treated, it can easily leach into soil and groundwater through rainwater, posing a significant environmental hazard. Furthermore, the low utilization rate of phosphogypsum (less than 40%) has become a core bottleneck restricting the green transformation of the phosphoric acid chemical industry.

[0003] Existing phosphogypsum curing technologies mostly use alkaline materials such as cement and calcium hydroxide as curing agents. Although they can achieve a certain curing effect through physical encapsulation and chemical bonding, they have significant drawbacks: First, the alkalinity of the system is too high (pH value often exceeds 12), which easily triggers alkali-aggregate reaction, leading to cracking of the cured body in the later stage, while also inhibiting plant growth and failing to meet the needs of resource utilization; Second, the curing efficiency of phosphorus and fluorine is limited, with the curing rate of soluble phosphorus and fluorine mostly below 90%, and there is still a risk of leaching after long-term use; Third, the cost is high, with large fluctuations in the price of main materials such as cement, and additional admixtures such as retarders and early strength additives are required, further increasing the processing cost.

[0004] To address the aforementioned issues, existing technologies have developed low-alkalinity curing systems based on sulfoaluminate cement and magnesium-based cementitious materials. However, these systems still suffer from limitations such as limited functionality, poor adaptability to high-impurity phosphogypsum, and complex preparation processes. For instance, while sulfoaluminate cement-based curing agents can control the pH value between 8 and 9, their curing of fluorides relies on a single calcium source, making them prone to curing saturation in high-fluoride phosphogypsum. Magnesium-based systems, on the other hand, suffer from low early-stage strength and demanding hydrothermal reaction conditions, hindering their large-scale application.

[0005] Therefore, developing a composite curing agent that combines low alkalinity, high phosphorus and fluorine curing rate, early strength, and low cost has become an urgent need for current phosphogypsum treatment technology. Summary of the Invention

[0006] This application provides a low-alkalinity composite phosphorus-fluorine curing agent for phosphogypsum and its preparation method. The aim is to provide a solution that can improve the early strength of the cured body while ensuring good workability of the slurry, and efficiently capture and chemically fix phosphorus and fluorine ions in the growing three-dimensional network skeleton, thereby improving the excellent mechanical properties and environmental safety of the phosphogypsum cured body.

[0007] In a first aspect, this application provides a low-alkalinity composite phosphorus-fluorine curing agent for phosphogypsum, comprising the following raw materials in parts by weight:

[0008] 100 parts by weight of granulated blast furnace slag powder, 30-60 parts by weight of modified metakaolin, 10-20 parts by weight of silica fume, 2-10 parts by weight of inorganic coagulant, 0.1-1 parts by weight of chelating and dispersing agent, 0.1-0.3 parts by weight of dodecylbenzene sulfonic acid, 0.15-0.3 parts by weight of sodium citrate, and 0.05-0.15 parts by weight of hydroxypropyl methylcellulose ether;

[0009] The modified metakaolin was obtained by coating amorphous aluminum hydroxide onto the surface of metakaolin using a urea hydrolysis precipitation method.

[0010] According to this application, by introducing modified metakaolinite, in conjunction with an active matrix and additives, a synergistic curing system is constructed in situ at the molecular-particle multi-scale interface in the complex chemical environment of phosphogypsum treatment. This system comprises an active nucleation platform, a network weaving mechanism, and a process control network. This system optimizes reaction kinetics under low alkalinity conditions, promotes the formation of early dense structures, and achieves efficient capture and stable storage of harmful ions such as phosphorus and fluorine, thereby solving the problems of low early strength and poor stability of cured bodies inherent in traditional curing methods.

[0011] Specifically, granulated blast furnace slag powder and silica fume serve as the main active matrix, and their core role is to provide the active calcium source (Ca) necessary for constructing the cementitious network under alkaline conditions. 2+ ) and silicon source (SiO4) 4- The modified metakaolin with amorphous aluminum hydroxide coating provides basic strength support for the final solidified body. This amorphous coating layer, with its high chemical activity and abundant interfacial hydroxyl groups, acts as a pre-defined "active nucleation platform," lowering the nucleation barrier of aluminosilicate hydration products, guiding and accelerating the directional growth of the gel network on its surface, and optimizing the formation efficiency and density of the early structure. Inorganic coagulants provide multivalent metal ions (such as Fe). 3+ Al 3+ (etc.), its function is similar to that of crosslinking agents and precipitants. On the one hand, it can promote the rapid crosslinking of the gel network, and on the other hand, it can form insoluble precipitates with harmful ions.

[0012] When the aforementioned highly active matrix and coagulant directly interact in the high-phosphorus, high-fluoride, and high-impurity ion environment introduced by phosphogypsum, the inorganic coagulant promotes rapid network formation, while harmful ions in the phosphogypsum (such as phosphate and fluoride ions) compete for and consume the reaction; and the rapid reaction leads to flash coagulation of the slurry, resulting in loss of fluidity. Forming a uniform and dense microstructure requires a certain reaction time. This application introduces a functional additive system containing chelating dispersants and small-molecule organic acids / surfactants as a process control network. Through chelation, it preferentially complexes polyvalent metal ions in the solution, achieving "controllable slow release" of ions, avoiding flash coagulation of the slurry due to instantaneously high ion concentrations, ensuring necessary operating time and slurry fluidity. The additive molecules form a "dispersion coat" with strong charge and steric hindrance on the particle surface. Through electrostatic repulsion and steric hindrance synergy, it achieves stable dispersion of solid particles, improving slurry uniformity. The additive anchored at the interface constitutes a dynamic "interfacial ion buffer," whose chelating groups can preferentially complex Ca2+ released by the inorganic coagulant. 2+ Fe 3+ Al 3+ The presence of polyvalent ions enables the controlled, slow-release and directional transport of ions to the active interface of the modified metakaolin, rather than their disordered precipitation with phosphorus and fluoride ions in solution. In this system, sodium citrate further fine-tunes the interfacial ionic environment, while cellulose ether provides water retention and viscosity stabilization, ensuring the reaction continues. Coagulant ions, along with phosphorus and fluoride ions, are simultaneously captured and transported to the active interface. Guided by the modified metakaolin, they react with silicate ions provided by the main active matrix and are orderly integrated into the in-situ grown three-dimensional aluminosilicate network framework, achieving controllable progression from "controlled transport" to "directional reaction" and then to "stable solidification."

[0013] Through the orderly synergy of the aforementioned multi-component and multi-mechanism components, a composite system with high early strength, dense microstructure, and strong chemical stability was ultimately formed in the solidified body. This system can not only effectively imprison harmful ions through physical encapsulation, but also firmly solidify phosphorus and fluoride ions within the network by forming stable chemical bonds, thereby reducing their environmental migration risk. This allows the resulting composite solidifying agent to exhibit excellent performance and environmental safety when treating complex solid wastes such as phosphogypsum.

[0014] In some embodiments, the inorganic coagulant includes polyferric sulfate, polyaluminum sulfate, and calcium aluminate powder.

[0015] In some of the above embodiments, the amorphous aluminum hydroxide layer on the surface of the modified metakaolin is rich in hydroxyl groups and possesses high surface energy and chemical activity. When the inorganic coagulant hydrolyzes, it releases Fe... 3+ Al 3+When polyvalent cations are present, these cations preferentially and rapidly adsorb and accumulate at the active interface, increasing the local concentration and utilization efficiency of ions at the interface and avoiding disordered diffusion and consumption of ions in the solution. Fe enriched at the active interface 3+ Al 3+ And the continuous supply of Ca from calcium aluminate 2+ It can react with active silicate ions (SiO4) dissolved from slag and silica fume. 4- The reaction occurs directly on the surface of modified metakaolin, which promotes the epitaxial growth of the aluminosilicate gel network with the modified particles as the core. This not only accelerates the formation speed of the early network, but also makes the generated gel structure grow more orderly and dense with the active interface as the anchor point.

[0016] In some embodiments, the chelating and dispersing aids include PESA (polyepoxysuccinic acid) and PBTCA (2-phosphonobutane-1,2,4-tricarboxylic acid).

[0017] In some of the above embodiments, the abundant hydroxyl groups in the amorphous aluminum hydroxide layer on the surface of the modified metakaolinite strongly adsorb onto the carboxylic acid and phosphonic acid groups on the PESA / PBTCA molecular chains, achieving "pre-anchoring" of the dispersant on the surface of the active particles. This provides the particles with a "dispersion coat" possessing strong electrostatic and steric hindrance, preventing their aggregation in complex slurries and improving distribution uniformity. Furthermore, the pre-anchored dispersant can form a dynamic "ion buffer" at the particle interface, chelating and releasing the Ca2+ released by the inorganic coagulant through a chelation-slow-release effect. 2+ Fe 3+ Al 3+ The directional and controllable transport of polyvalent ions to the highly active surface of modified metakaolinite optimizes the nucleation and growth efficiency of aluminosilicate gel networks at the interface.

[0018] In some embodiments, the modified metakaolin is prepared by the following steps:

[0019] Metakaolin was dispersed in an aqueous solution containing aluminum salt and urea. Heating caused the urea to hydrolyze and generate hydroxide ions, which reacted with aluminum ions on the surface of the metakaolin to form a precipitate reaction, resulting in metakaolin coated with amorphous aluminum hydroxide, which was then used as modified metakaolin.

[0020] In some of the above embodiments, amorphous aluminum hydroxide is deposited on the surface of metakaolin using a urea hydrolysis precipitation method. The resulting amorphous structure, due to its high reactivity, large specific surface area, and abundant surface hydroxyl groups, alters the interfacial properties of metakaolin. This active interface not only serves as a highly efficient aluminum source and rapidly participates in the reaction, but also acts as a molecular anchor point, strongly adsorbing with chelating dispersing agents (PESA / PBTCA) to achieve stable particle dispersion. Simultaneously, it acts as an "ion trap," preferentially enriching multivalent ions released by the inorganic coagulant, guiding the directional and dense growth of the aluminosilicate gel network on its surface.

[0021] In some embodiments, the modified metakaolin is prepared by the following steps:

[0022] 100 parts by mass of metakaolin were dispersed in an aqueous solution containing 4-10 parts by mass of aluminum salt and 8-20 parts by mass of urea. The mixed suspension was heated to 80-95°C and kept at this temperature for 2-6 hours with stirring. After the reaction was completed, the mixture was cooled, filtered, washed, and freeze-dried to obtain metakaolin with amorphous aluminum hydroxide coating on the surface, which was used as modified metakaolin.

[0023] In some of the above embodiments, the reaction conditions and dosage ratios in the preparation process of modified metakaolin are specifically described. Under these conditions, metakaolin with amorphous aluminum hydroxide coated on the surface can be obtained. Through the above reaction conditions, metakaolin forms a strong chemical bond with the metakaolin substrate through in-situ precipitation, rather than physical mixing, ensuring the stability of the "core-shell" structure. The amorphous structure and nanoscale particles provide a physicochemical basis for anchoring and chelating dispersion aid molecules and efficiently adsorbing (enriching) coagulant polyvalent metal ions.

[0024] In some embodiments, the inorganic coagulant comprises polyferric sulfate, polyaluminum sulfate, and calcium aluminate powder in a mass ratio of 1:(0.5~2):(2~4).

[0025] In some of the above embodiments, the inventors discovered that when the mass ratio of polyferric sulfate to polyaluminum sulfate is controlled within the range of 1:0.5~2, both rapid precipitation of phosphate ions and efficient fixation of fluoride ions can be ensured, while flash coagulation is avoided through the slow-release regulation of the chelating dispersant. When the amount of calcium aluminate is 2~4 times that of polyferric sulfate, sufficient Ca can be provided. 2+ This process generates stable minerals such as hydroxyapatite while maintaining a low-alkalinity environment in the system.

[0026] In some embodiments, the mass ratio of the polyepoxysuccinic acid (PESA) to 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTCA) is (1~3):1.

[0027] In some of the above embodiments, the inventors discovered that when the mass ratio of polyepoxysuccinic acid (PESA) to 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTCA) in the chelating dispersing agent is controlled within the range of (1~3):1, the system can achieve better dispersion stability and reaction controllability. This may be because, at this ratio, PBTCA forms a "buffer tank" by effectively chelating the polyvalent ions released by the inorganic coagulant through the phosphonic acid group. However, these ions need to be directionally transported to the active interface on the modified metakaolin surface through the dispersion channels established by PESA and the polymer brush interface to participate in network construction, reducing disordered precipitation with phosphorus and fluoride ions in the solution. The dispersion layer established by PESA is easily shielded by the charge of free polyvalent ions and becomes ineffective under high ionic strength environments, while PBTCA can maintain stability by clearing free ions. When the ratio exceeds the range of (1~3):1, excessive PESA leads to insufficient ion removal, resulting in instability of the dispersion layer and ineffective ion precipitation; excessive PBTCA leads to obstructed transport channels, preventing the ions from being directionally enriched.

[0028] In some embodiments, the granulated blast furnace slag powder has a specific surface area ≥480 m². 2 / kg, 28d activity index ≥98%, CaO content 38~45%.

[0029] In some of the above embodiments, when the granulated blast furnace slag powder used meets the requirement of a specific surface area ≥ 480 m², 2 When the curing agent is formulated with a synergistic specification of / kg, 28d activity index ≥98%, and CaO content 38~45%, it exhibits better early strength development and long-term stability. Its high specific surface area ensures rapid particle dissolution, providing CaO to the system under low alkalinity conditions. 2+ With active silicate ions, this is the kinetic basis for maintaining the rapid growth of the gel network; the extremely high activity index means that its gelation potential is almost fully released, which guarantees the final strength.

[0030] In some embodiments, the metakaolin has a loss on ignition ≤ 4.5% and an Al2O3 content ≥ 40%. Based on the above embodiments, under the above specifications of loss on ignition and Al2O3 content, metakaolin itself is a highly active aluminum-silicon source, which can contribute more active aluminum and silicon components to the system; more importantly, it provides an ideal reaction substrate for subsequent surface coating of amorphous aluminum hydroxide, ensuring that the modification process is efficient and uniform.

[0031] In some embodiments, the silica fume has a SiO2 content ≥94% and a specific surface area ≥22000 m². 2 / kg. Based on the above embodiments, under the above SiO2 content and specific surface area specifications, the silica fume has extremely high purity and is almost entirely composed of active amorphous silica, which can ensure that it can reliably participate in the formation of aluminosilicate networks as a highly efficient silicon source; while the high specific surface area endows it with nanoscale particle characteristics and a good reaction interface, enabling it to dissolve rapidly and provide active silicate ions. Its surface can also serve as an additional nucleation substrate, working synergistically with the active interface of modified metakaolin to promote the uniform precipitation and growth of hydration products, thereby optimizing the rheology of the slurry and improving its strength.

[0032] Secondly, this application provides a method for preparing a low-alkalinity composite phosphorus-fluorine curing agent for phosphogypsum, comprising:

[0033] Provide raw materials for the composite phosphorus-fluorine curing agent according to the first aspect;

[0034] The raw materials are mixed to obtain a low-alkalinity composite phosphorus-fluorine curing agent for phosphogypsum.

[0035] During the powder compounding process, the active interface of the modified metakaolin can preferentially adsorb with chelating and dispersing agent molecules, achieving "pre-anchoring" and "pre-dispersion" of functional components. Thus, when mixed with phosphogypsum and water in the subsequent process, it can instantly and orderly activate a multi-synergistic mechanism of "active nucleation-process control-network weaving" to obtain a composite curing agent with excellent workability, high early strength and efficient phosphorus and fluorine stabilization capabilities.

[0036] In some embodiments, the method includes:

[0037] The modified metakaolin, the chelating and dispersing agent, the dodecylbenzene sulfonic acid and the sodium citrate are first mixed and then mixed in a high-efficiency shear mixer at room temperature for 10-30 minutes to obtain a pre-dispersed functional composite powder.

[0038] The pre-dispersed functional composite powder is mixed with the ultrafine granulated blast furnace slag powder, the silica fume, the inorganic coagulant and the hydroxypropyl methylcellulose ether in a second mixing process, and mixed at 20~30℃ for 5~15 minutes to obtain phosphogypsum low-alkalinity composite phosphorus-fluorine curing agent dry powder.

[0039] In some of the above embodiments, the control of the duration and shear intensity of the first mixing stage allows the carboxylic acid groups and phosphonic acid groups on the molecular chain of the chelating dispersant (PESA / PBTCA), as well as the polar groups of the dodecylbenzenesulfonic acid and sodium citrate, to adsorb onto the abundant interfacial hydroxyl groups of the amorphous aluminum hydroxide coating layer on the modified metakaolin surface through hydrogen bonding, electrostatics, and coordination. This achieves pre-anchoring and pre-dispersion of the functional additive molecules on the surface of the active particles. This step lays the foundation for the functional pre-assembly centered on the modified particles in subsequent mixing, constructing a microscopic process control network from the source. In the subsequent second mixing stage, at a relatively low temperature (20~30℃) and a relatively short time (5~15 minutes), the pre-assembled functional composite powder is uniformly compounded with a large amount of main active matrix and coagulant on a macroscopic scale. These mild mixing conditions aim to prevent the desorption of anchored additive molecules due to excessive mechanical energy input or frictional heat, or to prevent premature pre-hydration reactions of trace components (such as coagulants), thereby ensuring the stability of the composite dry powder during storage. Through the above-described stepwise, controlled dry mixing process, the functional components in the final composite curing agent dry powder are orderly distributed and pre-assembled at the particle scale. When this dry powder comes into contact with a complex aqueous system containing phosphogypsum, it can instantly and synergistically activate multiple mechanisms, including active interface-guided nucleation, controlled slow-release ion transport, and ultra-dispersion stabilization of the reaction environment, thereby simultaneously ensuring good workability of the slurry, high early strength of the cured body, and efficient stabilization of phosphorus and fluoride ions.

[0040] Compared with the prior art, the beneficial effects of this application are at least as follows:

[0041] This application introduces a surface-functionalized metakaolin and a process-regulating agent network synergistically into a composite curing agent system, thereby constructing a triple synergistic curing system of "active nucleation platform - network weaving - process regulation" in situ during the curing process. This system enables directional transport of reactants, interface-guided nucleation, and controlled network growth at the molecular-particle multi-scale, effectively resolving the contradiction between the competitive consumption of reactive ions and the need for rapid solidification in low-alkalinity, high-impurity environments. Simultaneously, through chelation-slow-release mechanisms and ultra-dispersion stabilization, it simultaneously promotes the rapid formation of early-stage dense microstructures while ensuring good workability of the slurry, and efficiently captures and chemically fixes harmful ions such as phosphorus and fluorine within the growing three-dimensional network framework. Therefore, the resulting cured body exhibits improved early strength, long-term stability, erosion resistance, and control over the risk of harmful ion leaching. Detailed Implementation

[0042] 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.

[0043] 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.

[0044] 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.

[0045] In this specification, unless otherwise specified, "parts" refers to "parts by weight".

[0046] 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.

[0047] Ultrafine blast furnace slag powder, with a specific surface area of ​​500 m² 2 / kg, 28d activity index 98.6%, CaO content 41%.

[0048] Metakaolin, loss on ignition 4.2%, Al2O3 content 43%;

[0049] Silica fume, CAS No. 69012-64-2, SiO2 content 96%, specific surface area 24000 m² 2 / kg;

[0050] Hydroxypropyl methylcellulose ether, model: PA39799;

[0051] Calcium aluminate powder, with an Al2O3 content of 58%, a CaO content of 30%, and a soluble Al2O3 content of 55%;

[0052] Polyferric sulfate, CAS No.: 10028-22-5;

[0053] Polyaluminum sulfate, CAS No.: 10043-01-3;

[0054] Polyepoxysuccinic acid, CAS No.: 51274-37-4, number average molecular weight 1000;

[0055] 2-Butanephosphonate-1,2,4-tricarboxylic acid, CAS No.: 37971-36-1;

[0056] Polycarboxylate superplasticizer, PCA-I, water reduction rate 25%.

[0057] Preparation of modified metakaolin:

[0058] 100 parts by mass of metakaolin were dispersed in 300 parts by mass of deionized water, and 6 parts by mass of aluminum chloride hexahydrate and 12 parts by mass of urea were added sequentially. The mixture was stirred continuously at 85°C for 4 hours. During the reaction, the pH was controlled to 6.5 by adding dilute hydrochloric acid dropwise. After the reaction was completed, the mixture was filtered and separated, and thoroughly washed with deionized water to remove soluble ions. The mixture was then freeze-dried to constant weight to obtain metakaolin with a uniformly coated layer of highly active amorphous aluminum hydroxide, which was used as modified metakaolin.

[0059] Example 1

[0060] Preparation of low-alkalinity composite phosphorus-fluorine curing agent for phosphogypsum:

[0061] Take 100 parts by weight of ultrafine blast furnace slag powder, 45 parts by weight of modified metakaolin, 15 parts by weight of silica fume, 5 parts by weight of inorganic coagulant (where the mass ratio of polyferric sulfate, polyaluminum sulfate, and calcium aluminate powder is 1:1:3), 0.5 parts by weight of chelating dispersant (where the mass ratio of PESA to PBTCA is 2:1), 0.2 parts by weight of dodecylbenzenesulfonic acid, 0.2 parts by weight of sodium citrate, and 0.1 parts by weight of hydroxypropyl methylcellulose ether, and mix them separately. First, add the modified metakaolin, chelating dispersant, dodecylbenzenesulfonic acid, and sodium citrate to a shear mixer, and shear mix them at 25°C and 1200 rpm for 20 minutes to obtain functional composite powder.

[0062] Subsequently, the functional composite powder, along with the ultrafine granulated blast furnace slag powder, silica fume, inorganic coagulant, and hydroxypropyl methylcellulose ether, were added to a mixer and mixed at 25°C for 10 minutes to obtain a low-alkalinity composite phosphorus-fluorine curing agent for phosphogypsum.

[0063] Example 2

[0064] Preparation of low-alkalinity composite phosphorus-fluorine curing agent for phosphogypsum:

[0065] Take 100 parts by weight of ultrafine blast furnace slag powder, 45 parts by weight of modified metakaolin, 15 parts by weight of silica fume, 5 parts by weight of inorganic coagulant (where the mass ratio of polyferric sulfate, polyaluminum sulfate, and calcium aluminate powder is 1:1:3), 0.5 parts by weight of chelating dispersant (where the mass ratio of PESA to PBTCA is 4:1), 0.2 parts by weight of dodecylbenzenesulfonic acid, 0.2 parts by weight of sodium citrate, and 0.1 parts by weight of hydroxypropyl methylcellulose ether, and mix them separately. First, add the modified metakaolin, chelating dispersant, dodecylbenzenesulfonic acid, and sodium citrate into a shear mixer, and shear mix them at 25°C and 1200 rpm for 20 minutes to obtain functional composite powder.

[0066] Subsequently, the functional composite powder, along with the ultrafine granulated blast furnace slag powder, silica fume, inorganic coagulant, and hydroxypropyl methylcellulose ether, were added to a mixer and mixed at 25°C for 10 minutes to obtain a low-alkalinity composite phosphorus-fluorine curing agent for phosphogypsum.

[0067] Example 3

[0068] Preparation of low-alkalinity composite phosphorus-fluorine curing agent for phosphogypsum:

[0069] Take 100 parts by weight of ultrafine blast furnace slag powder, 45 parts by weight of modified metakaolin, 15 parts by weight of silica fume, 5 parts by weight of inorganic coagulant (where the mass ratio of polyferric sulfate, polyaluminum sulfate, and calcium aluminate powder is 1:3:3), 0.5 parts by weight of chelating dispersant (where the mass ratio of PESA to PBTCA is 2:1), 0.2 parts by weight of dodecylbenzenesulfonic acid, 0.2 parts by weight of sodium citrate, and 0.1 parts by weight of hydroxypropyl methylcellulose ether, and mix them separately. First, add the modified metakaolin, chelating dispersant, dodecylbenzenesulfonic acid, and sodium citrate to a shear mixer, and shear mix them at 25°C and 1200 rpm for 20 minutes to obtain functional composite powder.

[0070] Subsequently, the functional composite powder, along with the ultrafine granulated blast furnace slag powder, silica fume, inorganic coagulant, and hydroxypropyl methylcellulose ether, were added to a mixer and mixed at 25°C for 10 minutes to obtain a low-alkalinity composite phosphorus-fluorine curing agent for phosphogypsum.

[0071] Example 4

[0072] Preparation of low-alkalinity composite phosphorus-fluorine curing agent for phosphogypsum:

[0073] Take 100 parts by weight of ultrafine blast furnace slag powder, 45 parts by weight of modified metakaolin, 15 parts by weight of silica fume, 5 parts by weight of inorganic coagulant (where the mass ratio of polyferric sulfate, polyaluminum sulfate, and calcium aluminate powder is 1:1:5), 0.5 parts by weight of chelating dispersant (where the mass ratio of PESA to PBTCA is 2:1), 0.2 parts by weight of dodecylbenzenesulfonic acid, 0.2 parts by weight of sodium citrate, and 0.1 parts by weight of hydroxypropyl methylcellulose ether, and mix them separately. First, add the modified metakaolin, chelating dispersant, dodecylbenzenesulfonic acid, and sodium citrate to a shear mixer, and shear mix them at 25°C and 1200 rpm for 20 minutes to obtain functional composite powder.

[0074] Subsequently, the functional composite powder, along with the ultrafine granulated blast furnace slag powder, silica fume, inorganic coagulant, and hydroxypropyl methylcellulose ether, were added to a mixer and mixed at 25°C for 10 minutes to obtain a low-alkalinity composite phosphorus-fluorine curing agent for phosphogypsum.

[0075] Example 5

[0076] Preparation of low-alkalinity composite phosphorus-fluorine curing agent for phosphogypsum:

[0077] Take 100 parts by weight of ultrafine blast furnace slag powder, 45 parts by weight of modified metakaolin, 15 parts by weight of silica fume, 5 parts by weight of inorganic coagulant (where the mass ratio of polyferric sulfate, polyaluminum sulfate, and calcium aluminate powder is 1:1:3), 0.5 parts by weight of PCA-I, 0.2 parts by weight of dodecylbenzenesulfonic acid, 0.2 parts by weight of sodium citrate, and 0.1 parts by weight of hydroxypropyl methylcellulose ether, and mix them separately. First, add the modified metakaolin, PCA-I, dodecylbenzenesulfonic acid, and sodium citrate into a shear mixer, and shear mix them at 25°C and 1200 rpm for 20 minutes to obtain functional composite powder.

[0078] Subsequently, the functional composite powder, along with the ultrafine granulated blast furnace slag powder, silica fume, inorganic coagulant, and hydroxypropyl methylcellulose ether, were added to a mixer and mixed at 25°C for 10 minutes to obtain a low-alkalinity composite phosphorus-fluorine curing agent for phosphogypsum.

[0079] Comparative Example 1

[0080] Preparation of low-alkalinity composite phosphorus-fluorine curing agent for phosphogypsum:

[0081] Take 100 parts by weight of ultrafine blast furnace slag powder, 45 parts by weight of metakaolin, 15 parts by weight of silica fume, 5 parts by weight of inorganic coagulant (where the mass ratio of polyferric sulfate, polyaluminum sulfate, and calcium aluminate powder is 1:1:3), 0.5 parts by weight of chelating dispersant (where the mass ratio of PESA to PBTCA is 2:1), 0.2 parts by weight of dodecylbenzenesulfonic acid, 0.2 parts by weight of sodium citrate, and 0.1 parts by weight of hydroxypropyl methylcellulose ether, and mix them separately. First, add the metakaolin, chelating dispersant, dodecylbenzenesulfonic acid, and sodium citrate to a shear mixer, and shear mix them at 25°C and 1200 rpm for 20 minutes to obtain functional composite powder.

[0082] Subsequently, the functional composite powder, along with the ultrafine granulated blast furnace slag powder, silica fume, inorganic coagulant, and hydroxypropyl methylcellulose ether, were added to a mixer and mixed at 25°C for 10 minutes to obtain a low-alkalinity composite phosphorus-fluorine curing agent for phosphogypsum.

[0083] Comparative Example 2

[0084] Preparation of low-alkalinity composite phosphorus-fluorine curing agent for phosphogypsum:

[0085] Take 100 parts by weight of ultrafine blast furnace slag powder, 45 parts by weight of modified metakaolin, 15 parts by weight of silica fume, 5 parts by weight of inorganic coagulant (where the mass ratio of polyferric sulfate, polyaluminum sulfate, and calcium aluminate powder is 1:1:3), 0.5 parts by weight of chelating dispersant (where the mass ratio of PESA to PBTCA is 2:1), 0.2 parts by weight of dodecylbenzenesulfonic acid, and 0.2 parts by weight of sodium citrate, and mix them separately. First, add the modified metakaolin, chelating dispersant, dodecylbenzenesulfonic acid, and sodium citrate into a shear mixer, and shear mix them at 25°C and 1200 rpm for 20 minutes to obtain functional composite powder.

[0086] Subsequently, the functional composite powder, along with the ultrafine granulated blast furnace slag powder, silica fume, and inorganic coagulant, were added to a mixer and mixed at 25°C for 10 minutes to obtain a phosphogypsum low-alkalinity composite phosphorus-fluorine curing agent.

[0087] Test section

[0088] The low-alkalinity composite phosphorus-fluoride curing agent dry powder obtained from the various examples and comparative examples was premixed with the phosphogypsum to be treated at a mass ratio of 1:4 in a dry state for 1 minute. Then, deionized water was added at a water-to-solids ratio (curing agent + phosphogypsum) of 0.2, and the mixture was stirred using a cement mortar mixer according to GB / T 17671 "Test Method for Strength of Cement Mortar" to prepare a uniform cured slurry. The mixed slurry was divided into two layers and placed into a 40mm × 40mm × 40mm cubic mold. Each layer was vibrated on a vibrating table for 60 seconds to compact and remove air bubbles. Excess slurry was scraped off and the surface was smoothed. The molded molds were placed in a constant temperature and humidity chamber at 20℃ and 90% relative humidity for 24 hours before demolding.

[0089] Early strength test: After demolding, the specimens were placed in saturated lime water at 20℃ and soaked for 3 and 28 days. The test was conducted according to GB / T 17671. The specimens were placed in the center of the testing machine's pressure plate and subjected to a uniform vertical load of 2.4 kN / s until failure. The maximum failure load (F, in N) was recorded, and the compressive strength (Rc, in MPa; A is the bearing area of ​​the specimen, in mm²) was calculated using the formula Rc = F / A. 2 ).

[0090] Leaching Test: The test blocks cured for 28 days and the untreated phosphogypsum were crushed and ground, and all were passed through a 9.5 mm sieve. The pretreated sample was weighed and leached according to the "Solid Waste Leaching Toxicity Leaching Method - Acetic Acid Buffer Solution Method" (HJ / T300-2007): Acetic acid buffer solution with pH=4.93 was added at a liquid-to-solid ratio of 20:1 (L / kg). The mixed sample was placed in a vortexing device and vortexed at 30 r / min for 18 h at 23℃. After vortexing, the sample was filtered through a 0.45 μm microporous membrane, and the filtrate was collected as the leachate. The total phosphorus concentration was determined using inductively coupled plasma optical emission spectrometry (ICP-OES), and the fluoride ion concentration was determined using a fluoride ion selective electrode method. Solidification rate (%) = (Original phosphogypsum leaching concentration - Solidified body leaching concentration) / Original phosphogypsum leaching concentration.

[0091] The test results are shown in Table 1.

[0092] Table 1

[0093]

[0094] According to the test data in Table 1, the compressive strength and curing rate of each comparative example are lower than those of the embodiment of the present invention, indicating that the curing agent provided by the present invention can improve the early strength and curing rate of phosphogypsum. The possible reason is that in Comparative Example 1, unmodified ordinary metakaolin was used instead of modified metakaolin coated with amorphous aluminum hydroxide. Although their chemical compositions are similar, ordinary metakaolin lacks a highly active amorphous aluminum hydroxide coating layer on its surface. Insufficient hydroxyl density and chemical activity on the particle surface prevent it from serving as an efficient "active nucleation platform" to significantly reduce the nucleation barrier of hydration products. This results in slow and disordered early growth of the aluminosilicate network, leading to low early strength at 3 days. Its surface properties also prevent strong adsorption with chelating and dispersing agents (PESA / PBTCA), weakening the pre-anchoring effect of the "process-controlled network." Coagulant ions cannot be efficiently enriched and directionally transported to the interface reaction, and some ions are competitively consumed by phosphorus and fluoride ions in the solution. This not only reduces curing efficiency but also affects the density of the final network structure, thus lowering its 28-day strength. In Comparative Example 2, hydroxypropyl methylcellulose ether was not added to the formulation. During slurry preparation and early hydration, moisture evaporates or migrates more easily, causing incomplete and uneven local hydration reactions. This makes it difficult to form a continuous and homogeneous microstructure within the slurry and may generate micro-defects. Therefore, the physical environment for early strength formation deteriorated, resulting in a decrease in 3-day strength compared to Example 1. At the same time, its 28-day strength was also affected due to uneven moisture distribution potentially affecting the long-term hydration process.

[0095] The results of Examples 1, 2, and 5 show that the type and proportion of chelating dispersants have a certain impact on the early strength and curing rate of the curing agent. This may be because a specific proportion range is optimal for achieving both "steric hindrance dispersion" and "ion chelation and slow release." At this proportion, sufficient PESA molecules can form a fully extended polymer brush on the surface of modified metakaolin and other particles, ensuring the initial dispersibility and workability of the slurry through a strong steric hindrance effect. Simultaneously, sufficient PBTCA molecules, through their unique phosphonic acid groups, can inhibit the release of Ca from the inorganic coagulant in the system. 2+ Fe 3+ Al 3+The efficient and dynamic chelation of polyvalent ions allows for the controlled release of active ions, which are preferentially transported to the active interface of modified metakaolin to participate in the construction of an ordered network, rather than being competitively consumed by phosphorus and fluoride ions in the solution. In Example 2, while increasing the proportion of PESA may slightly enhance dispersibility, the relative insufficiency of PBTCA directly leads to a decrease in the system's ion buffer capacity and control precision. Some polyvalent ions, failing to be chelated and controlled in time, undergo a certain degree of ineffective precipitation reaction in the early stage of slurry mixing, which not only consumes ions used to construct the early gelling network but may also damage the microscopic homogeneity of the slurry. The macroscopic result is impaired formation efficiency and compactness of the early hydration network, leading to a decrease in early strength after 3 days. In Example 5, the conventional water-reducing agent PCA-I, although possessing excellent dispersing and water-reducing capabilities, lacks strong chelating functional groups for polyvalent metal ions in its molecular structure. Therefore, it cannot control ion reaction kinetics. When the coagulant ions are rapidly released, under the competition of high concentrations of phosphorus and fluoride ions, some ions fail to participate in the construction of the main gel network and instead undergo disordered precipitation. This not only consumes effective reactants, resulting in low early network strength, but also the generated ineffective precipitates disrupt the homogeneity and density of the microstructure, leading to a decrease in the fluorine-phosphorus solidification rate.

[0096] Based on the results of Examples 1, 3, and 4, it can be seen that the mass ratio of polyferric sulfate, polyaluminum sulfate, and calcium aluminate in the inorganic coagulant has a certain impact on the early strength and curing rate of the curing agent. Example 3 increased the proportion of polyaluminum sulfate, resulting in a slight decrease in 3-day strength, while the 28-day strength remained at a high level. The possible reason for this is that increasing the proportion of Al... 3+ The supply significantly enhances the specific complexation and fixation capacity for fluoride ions, which is beneficial for the formation of a stable fluoroaluminate phase. However, in the early reaction kinetics, Al... 3+ The efficiency of promoting rapid precipitation and cross-linking of gelation networks is slightly lower than that of Fe. 3+ Therefore, this formulation sacrifices a small amount of early absolute strength in exchange for superior fluoride fixation potential. Example 4 increased the proportion of calcium aluminate, resulting in lower early strength at 3 days, but good strength at 28 days. This is mainly because calcium aluminate hydrolysis releases Ca. 2+ The process of creating an alkaline environment is relatively gradual, resulting in a slow rate of increase in ion concentration and pH value in the early stages of the system, and a weak initial kinetics for network formation.

[0097] 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 low-alkalinity composite phosphorus-fluorine curing agent for phosphogypsum, characterized in that, The raw materials include the following parts by weight: 100 parts by weight of granulated blast furnace slag powder, 30-60 parts by weight of modified metakaolin, 10-20 parts by weight of silica fume, 2-10 parts by weight of inorganic coagulant, 0.1-1 parts by weight of chelating and dispersing agent, 0.1-0.3 parts by weight of dodecylbenzene sulfonic acid, 0.15-0.3 parts by weight of sodium citrate, and 0.05-0.15 parts by weight of hydroxypropyl methylcellulose ether; The inorganic coagulant includes polyferric sulfate, polyaluminum sulfate, and calcium aluminate powder, wherein the mass ratio of polyferric sulfate, polyaluminum sulfate, and calcium aluminate powder is 1:(0.5~2):(2~4). The chelating and dispersing agent includes polyepoxysuccinic acid (PESA) and 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTCA), wherein the mass ratio of polyepoxysuccinic acid (PESA) to 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTCA) is (1~3):1; The modified metakaolin was obtained by coating amorphous aluminum hydroxide onto the surface of metakaolin using a urea hydrolysis precipitation method.

2. The composite phosphorus-fluorine curing agent according to claim 1, characterized in that, The modified metakaolin was prepared through the following steps: Metakaolin was dispersed in an aqueous solution containing aluminum salt and urea. Heating caused the urea to hydrolyze and generate hydroxide ions, which reacted with aluminum ions on the surface of the metakaolin to form a precipitate reaction, resulting in metakaolin coated with amorphous aluminum hydroxide, which was then used as modified metakaolin.

3. The composite phosphorus-fluorine curing agent according to claim 1, characterized in that, The modified metakaolin was prepared through the following steps: 100 parts by mass of metakaolin were dispersed in an aqueous solution containing 4-10 parts by mass of aluminum salt and 8-20 parts by mass of urea. The mixed suspension was heated to 80-95°C and kept at this temperature for 2-6 hours with stirring. After the reaction was completed, the mixture was cooled, filtered, washed, and freeze-dried to obtain metakaolin with amorphous aluminum hydroxide coating on the surface, which was used as modified metakaolin.

4. The composite phosphorus-fluorine curing agent according to any one of claims 1 to 3, characterized in that, The raw materials meet at least one of the following conditions: 1) The specific surface area of ​​the granulated blast furnace slag powder is ≥480m². 2 / kg, 28d activity index ≥98%, CaO content 38~45%; 2) The loss on ignition of the metakaolin is ≤4.5%, and the Al2O3 content is ≥40%; 3) The silica fume has an SiO2 content ≥ 94% and a specific surface area ≥ 22000 m². 2 / kg.

5. A method for preparing a low-alkalinity composite phosphorus-fluorine curing agent for phosphogypsum, characterized in that, include: Provide the raw materials for the composite phosphorus-fluorine curing agent according to any one of claims 1 to 4; The raw materials are mixed to obtain a low-alkalinity composite phosphorus-fluorine curing agent for phosphogypsum.

6. The method according to claim 5, characterized in that, include: The modified metakaolin, the chelating dispersant, the dodecylbenzene sulfonic acid, and the sodium citrate are first mixed and then mixed in a high-efficiency shear mixer at room temperature for 10-30 minutes to obtain a pre-dispersed functional composite powder. The pre-dispersed functional composite powder is then mixed with the granulated blast furnace slag powder, the silica fume, the inorganic coagulant, and the hydroxypropyl methylcellulose ether in a second mixture at 20-30°C for 5-15 minutes to obtain a phosphogypsum low-alkalinity composite phosphorus-fluorine curing agent dry powder.

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