Method for preparing high-purity anhydrous II-type gypsum by autoclaving industrial byproduct gypsum pressed bricks

Abstract

A method for preparing high-purity anhydrous II-type gypsum through brick pressing and steam pressing of industrial byproduct gypsum comprises the steps that S1, raw material treatment is conducted, specifically, a phosphogypsum raw material, an alkaline material and a composite crystal modifier are mixed to obtain a mixture, the mixture is aged and then subjected to brick pressing, and a mixture block is prepared; s2, performing steam curing and crystal transformation: stacking the mixed material blocks on a steam curing trolley, feeding the mixture blocks into a steam curing kettle, and performing steam curing at the temperature of lower than 195 DEG C to obtain gypsum bricks; s3, homogenizing and drying: naturally homogenizing and drying the gypsum bricks in a homogenizing silo, and crushing and grinding when attached water is reduced to 3%, so as to obtain the high-purity anhydrous II-type gypsum. The II-type anhydrous gypsum is prepared by adopting a method of steam curing, natural homogenization drying and crushing and grinding, and the steam curing temperature is controlled within 195 DEG C, so that the treated product is ensured to reach a safe and stable state, and the pH value of the treated product is stabilized at 6-9. Anhydrous II gypsum is prepared from a large amount of consumed phosphogypsum, direct resource utilization is achieved, the process is simple, energy consumption is low, and the national green and energy-saving requirements are met.

Description

Technical Field

[0001] This invention belongs to the field of industrial solid waste resource utilization technology, and specifically relates to a method for preparing high-purity anhydrous type II gypsum by steam pressing of industrial by-product gypsum bricks. Background Technology

[0002] Type II anhydrous gypsum, also known as insoluble hard gypsum, is a stable anhydrous calcium sulfate phase formed by the dehydration of dihydrate gypsum (CaSO4·2H2O), hemihydrate gypsum (CaSO4·0.5H2O), or Type III anhydrous gypsum after high-temperature calcination. Compared to other gypsum phases (such as Type III anhydrous gypsum or hemihydrate gypsum), Type II anhydrous gypsum has the characteristics of high chemical inertness, good water resistance, and excellent mechanical strength, making it of significant application value in many industrial fields.

[0003] Currently, the mainstream processes for converting phosphogypsum into type II anhydrous gypsum can be divided into two categories: 1. Acid treatment method, typically represented by the scheme described in patent CN115849427A. This method uses a mixed system of high-concentration phosphoric acid (30%-40% P2O5) and hydrofluoric acid to treat phosphogypsum, supplemented by sulfuric acid solution for reaction at a specific temperature. Its advantage lies in its ability to effectively control the product grain size and reduce silicon content, but it has significant environmental drawbacks—the waste acid generated after the reaction is difficult to treat and easily causes secondary pollution.

[0004] 2. High-temperature calcination method, taking patent CN113307522A as an example, this technology removes water of crystallization through high-temperature calcination at 700-800℃. Combined with a crystal form control unit to precisely regulate temperature and residence time, it can obtain a high-purity product with an AⅡ crystal form content of ≥98%. Although this method can simultaneously degrade organic impurities, it requires maintaining high-temperature operating conditions, resulting in high energy consumption and significantly increasing production costs.

[0005] Patent CN 120364962A proposes a method for preparing type II anhydrous gypsum using a steam curing process. The method involves mixing phosphogypsum raw material with type II anhydrous gypsum seed crystals and then steam curing the mixture at a temperature below 195°C to obtain an intermediate powder. The intermediate powder is then dried at a temperature below the steam curing temperature to obtain type II anhydrous gypsum. Compared to acid treatment and high-temperature calcination methods, this preparation method features low energy consumption, environmental optimization, and high conversion rate. However, this method directly uses the powder in the steam curing reactor, resulting in a long reaction time, making large-scale industrial production difficult. Furthermore, it fails to treat phosphorus and fluorine impurities in the product, affecting its subsequent applications. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a method for preparing high-purity anhydrous type II gypsum by steam curing phosphogypsum bricks, thereby solving the problems of long reaction times and the presence of phosphorus and fluorine impurities in the product in the prior art. This method achieves deep impurity removal, directional crystal transformation, and high performance enhancement of the phosphogypsum product.

[0007] To achieve this objective, the present invention adopts the following technical solution:

[0008] A method for preparing high-purity anhydrous type II gypsum by steam pressing of industrial by-product gypsum bricks includes the following steps:

[0009] S1. Raw material processing: Phosphogypsum raw material is mixed with alkaline materials and composite crystallizing agent to obtain a mixture. After aging, the mixture is pressed into bricks to obtain mixed material blocks.

[0010] S2, Steam curing crystallization: The mixed material blocks are stacked on a steam curing trolley and then sent into a steam curing kettle for steam curing at a temperature below 195°C to obtain gypsum bricks;

[0011] S3. Homogenization and drying: The gypsum bricks are placed in a homogenization chamber for natural homogenization and drying. When the attached water content is reduced to 3%, they are crushed and ground into powder to obtain high-purity anhydrous type II gypsum.

[0012] Preferably, the weight ratio of phosphogypsum, alkaline material, and composite crystallizing agent in S1 is as follows:

[0013] 98-100 parts of phosphogypsum

[0014] 0-2 parts of alkaline material

[0015] 0.4-0.7 parts of composite crystal conversion agent.

[0016] Preferably, the alkaline material is lime or carbide slag.

[0017] Preferably, the composite crystallization agent comprises the following components in parts by weight:

[0018] Sodium citrate 0.001-0.002 parts

[0019] Sodium sulfate 0.01-0.02 parts

[0020] Modified nano-TiO2 0.05-0.1 parts

[0021] 0.03-0.07 parts of chitosan quaternary ammonium salt

[0022] Ionic liquid [BMIM]Cl 0.2-0.5 parts

[0023] 0.01-0.03 parts of cerium nitrate.

[0024] Preferably, the modified nano-TiO2 is prepared by:

[0025] Nano-TiO2 was dispersed in an ethanol-water solution, KH-550 was added, and the mixture was ultrasonically dispersed at 50°C for 1 hour. After centrifugation and washing, it was dried at 100°C to ensure that the hydroxyl coverage of the nano-TiO2 surface was ≥90%. The volume ratio of ethanol to water was 3:1, and KH-550 was added at 3% of the mass of TiO2.

[0026] Preferably, the preparation method of the chitosan quaternary ammonium salt is as follows:

[0027] Chitosan and 3-chloro-2-hydroxypropyltrimethylammonium chloride were dissolved in isopropanol at a mass ratio of 1:2, stirred in a water bath at 60°C for 6 hours, and the degree of quaternization was controlled by infrared spectroscopy to be ≥85%. After vacuum drying, the mixture was pulverized to a particle size ≤50μm, wherein the degree of deacetylation of the chitosan was ≥90%.

[0028] Preferably, the preparation method of the composite crystallization agent is as follows:

[0029] Based on the above weight proportions, the ionic liquid [BMIM]Cl, cerium nitrate, sodium citrate and sodium sulfate are ground to 150 mesh and mixed with modified nano TiO2 and chitosan quaternary ammonium salt. The mixture is stirred at 200-300 rpm for 30-60 min at 50°C to form a uniform powder.

[0030] Preferably, during the pressing of bricks in S1, the attached water content of the material is controlled at 8%-10%, and the bricks are pressed and formed under a pressure of 10-20MPa, with a brick density of 2.0-2.4g / cm³ and a porosity of ≤15%.

[0031] Preferably, the steam curing crystallization in S2 is specifically carried out as follows: the steam curing cart is sent into the steam curing kettle, the temperature inside the steam curing kettle is raised to 160-195℃ using saturated steam, the pressure is 0.5-1.3MPa, and the kettle is opened after reacting for 2-4 hours.

[0032] A composite crystallization agent induces the conversion of phosphogypsum raw materials into high-purity type II anhydrous gypsum under steam curing conditions. During the crystallization process, the alkaline material reacts with the soluble phosphorus / fluorine released from the dihydrate gypsum, simultaneously solidifying impurities. By enhancing the thermal conductivity of the material through compaction, the reaction time is shortened, ultimately converting phosphogypsum into type II anhydrous gypsum. The pH value of the phosphogypsum is 2-4, while the pH value of the high-purity type II anhydrous gypsum product is 6-9, with a significant reduction in the content of water-soluble phosphorus and fluorine impurities.

[0033] Compared with existing inventions, the present invention has the following advantages:

[0034] This invention employs a steam curing, natural homogenization and drying followed by crushing and grinding method to prepare Type II anhydrous gypsum. The steam curing temperature is controlled below 195℃ to ensure that the treated product reaches a safe and stable state, with a pH value stable between 6 and 9. This method eliminates the need for drying equipment and can be used to produce anhydrous Type II gypsum from large quantities of phosphogypsum, enabling direct resource utilization. Furthermore, the process is simple, energy-efficient, and meets national green energy-saving requirements.

[0035] Modified nano-TiO2: anatase type (5-10nm), surface hydroxyl modification, photothermal synergistic degradation of organic matter, fixation of silicon impurities to form perovskite solid solution;

[0036] Chitosan quaternary ammonium salt (CTS-QA): Quaternization degree ≥85%, chelates Fe³+ / Al³+, directionally adsorbs gypsum dihydrate crystal faces, and induces short columnar growth of gypsum dihydrate.

[0037] Ionic liquid [BMIM]Cl: The eutectic solvent reduces the activation energy for crystal transformation, Cl⁻ inhibits crystal growth, and reduces porosity;

[0038] Cerium nitrate: Rare earth ions substitute Ca²⁺, lattice distortion induces equiaxed crystals. Attached Figure Description

[0039] Figure 1 The image is a 5000x scanning electron microscope image of anhydrous gypsum II prepared in Example 1.

[0040] Figure 2 The image is a 15,000x scanning electron microscope image of anhydrous gypsum II prepared in Example 1.

[0041] Figure 3 The image is a 20,000x scanning electron microscope image of anhydrous gypsum II prepared in Example 1.

[0042] Figure 4 The image is a 30,000x scanning electron microscope image of anhydrous gypsum II prepared in Example 1.

[0043] Figure 5 The image is a 50,000x scanning electron microscope image of anhydrous gypsum II prepared in Example 1.

[0044] Figure 6 The image shows the XRD pattern of anhydrous gypsum II prepared in Example 1, indicating that the obtained sample is anhydrous gypsum II. Detailed Implementation

[0045] The embodiments of the present invention will be described in detail below. The embodiments described below are implemented based on the technical solution of the present invention, and detailed implementation methods and specific operation processes are given. However, the protection scope of the present invention is not limited to the embodiments described below.

[0046] In the embodiments, the composite crystallization agent comprises the following components in parts by weight:

[0047] Sodium citrate 0.001-0.002 parts

[0048] Sodium sulfate 0.01-0.02 parts

[0049] Modified nano-TiO2 0.05-0.1 parts

[0050] 0.03-0.07 parts of chitosan quaternary ammonium salt

[0051] Ionic liquid [BMIM]Cl 0.2-0.5 parts

[0052] 0.01-0.03 parts of cerium nitrate.

[0053] The preparation method of modified nano-TiO2 is as follows: nano-TiO2 is dispersed in an ethanol-water solution, KH-550 is added, ultrasonically dispersed at 50℃ for 1h, centrifuged and washed, and dried at 100℃ to make the hydroxyl coverage of nano-TiO2 surface ≥90%, the volume ratio of ethanol to water is 3:1, and KH-550 is added at 3% of the mass of TiO2.

[0054] The preparation method of chitosan quaternary ammonium salt is as follows: chitosan and 3-chloro-2-hydroxypropyltrimethylammonium chloride are dissolved in isopropanol at a mass ratio of 1:2, stirred in a water bath at 60°C for 6 hours, and the degree of quaternization is controlled by infrared spectroscopy to be ≥85%. After vacuum drying, the mixture is pulverized to a particle size ≤50μm, wherein the degree of deacetylation of the chitosan is ≥90%.

[0055] The preparation method of the composite crystallization agent is as follows: according to the above weight parts, the ionic liquid [BMIM]Cl, cerium nitrate, sodium citrate and sodium sulfate are ground to 150 mesh and mixed with modified nano TiO2 and chitosan quaternary ammonium salt, and stirred at 200-300 rpm for 30-60 min at 50℃ to form a uniform powder.

[0056] <Example 1>

[0057] This embodiment provides a method for preparing high-purity anhydrous type II gypsum by steam curing phosphogypsum bricks. The preparation method includes the following steps:

[0058] Mix 99 parts by weight of phosphogypsum and 1 part by weight of lime, and then add 0.4 parts by weight of composite crystallizing agent to obtain a uniform mixture. Control the water content of the mixture to 8%, press it into shape under 15 MPa pressure, stack it on a steam curing trolley, and then send it into an autoclave for steam curing. Keep it at 190℃ for 4 hours. Quickly remove it from the autoclave, place the gypsum bricks in a homogenization chamber for natural homogenization and drying. When the water content is reduced to 3%, crush and grind it to obtain type II anhydrous gypsum with a crystal water content of 0.4%.

[0059] <Example 2>

[0060] This embodiment provides a method for preparing high-purity anhydrous type II gypsum by steam curing phosphogypsum bricks. The preparation method includes the following steps:

[0061] Mix 99 parts by weight of phosphogypsum and 1 part by weight of lime, and then add 0.7 parts by weight of composite crystallizing agent to obtain a uniform mixture. Control the water content of the mixture to 10%, press it into shape under 15 MPa pressure, stack it on a steam curing trolley, and then send it into an autoclave for steam curing. Keep it at 190℃ for 4 hours. Quickly remove it from the autoclave, place the gypsum bricks in a homogenization chamber for natural homogenization and drying. When the water content is reduced to 3%, crush and grind it to obtain type II anhydrous gypsum with a crystal water content of 0.3%.

[0062] <Example 3>

[0063] This embodiment provides a method for preparing high-purity anhydrous type II gypsum by steam curing phosphogypsum bricks. The preparation method includes the following steps:

[0064] 99.5 and 0.5 parts by weight of phosphogypsum and lime were mixed, and 0.5 parts by weight of composite crystallizing agent were added to obtain a uniform mixture. The water content of the mixture was controlled at 9%. The mixture was pressed into shape under a pressure of 15 MPa, stacked on a steam curing trolley, and then sent to an autoclave for steam curing. The mixture was kept at 190°C for 4 hours. The mixture was quickly removed from the autoclave and placed in a homogenization chamber for natural homogenization and drying. When the water content was reduced to 3%, the mixture was crushed and ground to obtain type II anhydrous gypsum with a crystal water content of 0.66%.

[0065] <Example 4>

[0066] This embodiment provides a method for preparing high-purity anhydrous type II gypsum by steam curing phosphogypsum bricks. The preparation method includes the following steps:

[0067] 98.5 and 1.5 parts by weight of phosphogypsum and lime were mixed, and 0.6 parts by weight of composite crystallizing agent were added to obtain a uniform mixture. The water content of the mixture was controlled at 9%. The mixture was pressed into shape under a pressure of 15 MPa, stacked on a steam curing trolley, and then sent to an autoclave for steam curing. The mixture was kept at 190°C for 4 hours. The mixture was quickly removed from the autoclave and placed in a homogenization chamber for natural homogenization and drying. When the water content was reduced to 3%, the mixture was crushed and ground to obtain type II anhydrous gypsum with a crystal water content of 0.52%.

[0068] <Example 5>

[0069] This embodiment provides a method for preparing high-purity anhydrous type II gypsum by steam curing phosphogypsum bricks. The preparation method includes the following steps:

[0070] Mix phosphogypsum and lime in weight ratios of 98 and 2, and then add 0.6 parts by weight of composite crystallizing agent to obtain a uniform mixture. Control the water content of the mixture at 9%, press it into shape under 15 MPa pressure, stack it on a steam curing trolley, and then send it into an autoclave for steam curing. Keep it at 190℃ for 4 hours. Quickly remove it from the autoclave, place the gypsum bricks in a homogenization chamber for natural homogenization and drying. When the water content is reduced to 3%, crush and grind it to obtain type II anhydrous gypsum with a crystal water content of 0.54%.

[0071] <Comparative Example 1>

[0072] This comparative example provides a method for preparing anhydrous type II gypsum by pressing phosphogypsum bricks and steam curing. The preparation method is identical to that of Example 1, except that the mixture is not pressed into bricks, it is spread on a steam curing trolley, and the spreading thickness is 10 cm. The type II anhydrous gypsum product prepared in this comparative example has a crystal water content of 1.74%.

[0073] <Comparative Example 2>

[0074] This comparative example provides a method for preparing anhydrous type II gypsum by steam curing phosphogypsum bricks. The preparation method is identical to that of Example 1, except that the raw material is replaced with spherical raw gypsum mixed with carbide slag. The type II anhydrous gypsum product prepared in this comparative example has a water of crystallization of 0.76%.

[0075] <Comparative Example 3>

[0076] This comparative example provides a method for preparing anhydrous type II gypsum by steam curing phosphogypsum bricks. The preparation method is identical to that of Example 1, except that the heat treatment time is 2 hours. The type II anhydrous gypsum product prepared in this comparative example has a crystal water content of 0.46%.

[0077] <Comparative Example 4>

[0078] This comparative example provides a method for preparing anhydrous type II gypsum by steam curing phosphogypsum bricks. The preparation method is identical to that of Example 1, except that the composite crystallizing agent is replaced with a mixture of sodium citrate and sodium sulfate. The type II anhydrous gypsum product prepared in this comparative example has a crystal water content of 0.52%.

[0079] The parameters and product water of crystallization records for each of the above embodiments are plotted in Table 1 below.

[0080] Table 1

[0081]

[0082] To better illustrate the technical effects of the present invention, physicochemical analysis and paste preparation experiments were conducted on the prepared Type II anhydrous gypsum product, and the specific data are shown in Table 2 below.

[0083] Table 2

[0084]

[0085] As shown in the table, the water of crystallization in the reaction products of the examples remained below 1%, and the conversion rate of type II anhydrous gypsum exceeded 90%. Compared with examples 1, 2, 3, 4, and 5, after mixing with lime, the water-soluble phosphorus and fluoride in the type II anhydrous gypsum product decreased significantly, the impurity removal rate reached up to 99.55%, the product performance was significantly improved, ensuring that the product reached a safe and stable state, with the pH value stable between 6 and 9, and can be directly utilized as a resource.

[0086] Comparing Example 1, Comparative Example 1, and Comparative Example 3, under the same reaction conditions, the product with directly spread crystals had significantly higher water content of crystals than the product after pressing. Simultaneously, the setting time of Comparative Example 1 was significantly reduced, indicating that the product still contained some hemihydrate gypsum crystals, indicating incomplete reaction and a low conversion rate. Even after reducing the reaction time to 2 hours after pressing, the water content of the product in Comparative Example 3 was still less than 1%, demonstrating that pressing can enhance the thermal conductivity of the material, shorten the reaction time, and reduce reaction energy consumption, demonstrating advanced technology and more significant industrial application prospects. Comparing Example 1 and Comparative Example 4, the composite crystallization agent, through mechanisms such as media regulation, heterogeneous nucleation, crystal face inhibition, nano-reinforcement, and organic toughening, promoted the formation of more uniform and dense anhydrous gypsum crystals, resulting in a superior structure and fewer defects. The product's compressive and flexural strengths were significantly improved, and its toughness was greatly enhanced, broadening its application areas.

[0087] The above embodiments are only for illustrating the technical solutions and features of the present invention, and are intended to enable those skilled in the art to implement them better. They should not be used to limit the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention are within the scope of protection of the present invention. The parts not described in detail are prior art.

Claims

1. A method for preparing high-purity anhydrous type II gypsum by steam pressing of industrial by-product gypsum bricks, characterized in that, Includes the following steps: S1. Raw material processing: Phosphogypsum raw material is mixed with alkaline materials and composite crystallizing agent to obtain a mixture. After aging, the mixture is pressed into bricks to obtain mixed material blocks. S2, Steam curing crystallization: The mixed material blocks are stacked on a steam curing trolley and then sent into a steam curing kettle for steam curing at a temperature below 195°C to obtain gypsum bricks; S3. Homogenization and drying: The gypsum bricks are placed in a homogenization chamber for natural homogenization and drying. When the attached water content is reduced to 3%, they are crushed and ground into powder to obtain high-purity anhydrous type II gypsum.

2. The method for preparing high-purity anhydrous type II gypsum by autoclaving of industrial by-product gypsum bricks according to claim 1, characterized in that, The weight ratio of phosphogypsum, alkaline materials, and composite crystallizing agent in S1 is as follows: 98-100 parts of phosphogypsum 0-2 parts of alkaline material 0.4-0.7 parts of composite crystal conversion agent.

3. The method for preparing high-purity anhydrous type II gypsum by autoclaving of industrial by-product gypsum bricks according to claim 1, characterized in that, The alkaline material is lime or carbide slag.

4. The method for preparing high-purity anhydrous type II gypsum by autoclaving of industrial by-product gypsum bricks according to claim 1, characterized in that, The composite crystallization agent comprises the following components in parts by weight: Sodium citrate 0.001-0.002 parts Sodium sulfate 0.01-0.02 parts Modified nano-TiO2 0.05-0.1 parts 0.03-0.07 parts of chitosan quaternary ammonium salt Ionic liquid [BMIM]Cl 0.2-0.5 parts 0.01-0.03 parts of cerium nitrate.

5. The method for preparing high-purity anhydrous type II gypsum by autoclaving of industrial by-product gypsum bricks according to claim 4, characterized in that, The method for preparing the modified nano-TiO2 is as follows: Nano-TiO2 was dispersed in an ethanol-water solution, KH-550 was added, and the mixture was ultrasonically dispersed at 50°C for 1 hour. After centrifugation and washing, it was dried at 100°C to ensure that the hydroxyl coverage of the nano-TiO2 surface was ≥90%. The volume ratio of ethanol to water was 3:1, and KH-550 was added at 3% of the mass of TiO2.

6. The method for preparing high-purity anhydrous type II gypsum by autoclaving of industrial by-product gypsum bricks according to claim 5, characterized in that, The preparation method of the chitosan quaternary ammonium salt is as follows: Chitosan and 3-chloro-2-hydroxypropyltrimethylammonium chloride were dissolved in isopropanol at a mass ratio of 1:2, stirred in a water bath at 60°C for 6 hours, and the degree of quaternization was controlled by infrared spectroscopy to be ≥85%. After vacuum drying, the mixture was pulverized to a particle size ≤50μm, wherein the degree of deacetylation of the chitosan was ≥90%.

7. The method for preparing high-purity anhydrous type II gypsum by autoclaving of industrial by-product gypsum bricks according to claim 6, characterized in that, The preparation method of the composite crystallization agent is as follows: Based on the above weight proportions, the ionic liquid [BMIM]Cl, cerium nitrate, sodium citrate and sodium sulfate are ground to 150 mesh and mixed with modified nano TiO2 and chitosan quaternary ammonium salt. The mixture is stirred at 200-300 rpm for 30-60 min at 50°C to form a uniform powder.

8. The method for preparing high-purity anhydrous type II gypsum by autoclaving of industrial by-product gypsum bricks according to claim 1, characterized in that, When pressing S1 bricks, the attached water content of the material is controlled at 8%-10%, and the bricks are pressed and formed under a pressure of 10-20MPa. The density of the brick blank is 2.0-2.4g / cm³, and the porosity is ≤15%.

9. The method for preparing high-purity anhydrous type II gypsum by autoclaving of industrial by-product gypsum bricks according to claim 1, characterized in that, The specific process of steam curing crystallization in S2 is as follows: the steam curing cart is sent into the steam curing kettle, and the temperature inside the steam curing kettle is raised to 160-195℃ and the pressure is 0.5-1.3MPa using saturated steam. The kettle is opened after reacting for 2-4 hours.

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