Method for preparing semi-hydrated gypsum through in-situ crystal transformation

Abstract

The invention discloses a method for preparing semi-hydrated gypsum through in-situ crystal transformation, and belongs to the technical field of solid waste resource utilization. The method comprises the following steps: heating a crystal modifier solution to generate crystal modifier steam, and enabling the crystal modifier steam to fully contact and react with loosely stacked ardealite to realize in-situ crystal transformation so as to obtain semi-hydrated gypsum. According to the present invention, the crystal modifier steam in-situ reaction and the impurity synchronous removal are performed, the complex pretreatment is not required, the efficient conversion of the phosphogypsum is achieved under the mild condition, the obtained product has characteristics of high whiteness, low impurity content, simplified process, low energy consumption and reagent recycling, and the problems of complex process flow, high energy consumption, poor product performance and the like in the prior art are effectively solved.

Description

Technical Field

[0001] The invention belongs to the technical field of solid waste resource utilization, and particularly relates to a method for preparing hemihydrate gypsum by in-situ crystallization. Background Art

[0002] Phosphogypsum is an industrial waste residue discharged from the wet process of phosphoric acid production in the phosphorus chemical industry. In addition to phosphorus and fluorine, it also contains phosphoric acid, silicon, magnesium, iron, aluminum, and organic impurities. Because it is rich in impurities such as phosphorus, fluorine, and alkali metal salts, simply dumping it in landfills would occupy arable land and pollute the environment. Transforming it into hemihydrate gypsum is a promising and cost-effective treatment method. However, existing technologies for this process present numerous challenges.

[0003] First, phosphogypsum has a high impurity content and a wide variety of impurities, with significant differences between regions and different processes. Harmful impurities such as soluble phosphorus, eutectic phosphorus, and soluble fluorine can seriously affect the performance and quality of hemihydrate gypsum, resulting in low product strength and poor stability. Therefore, pretreatment and impurity removal are necessary before crystallization. Common methods such as water washing, neutralization, ball milling, and flotation generally have the disadvantages of being cumbersome, costly, and difficult to completely remove impurities. For example, while water washing can remove soluble impurities, it consumes a large amount of water resources and produces wastewater, increasing the difficulty of subsequent treatment. Flotation requires the addition of flotation reagents, which has potential impacts on the environment.

[0004] Secondly, most of the existing phosphogypsum crystallization technologies are non-in-situ crystallization methods, such as autoclaving, hydrothermal method and normal pressure salt solution method. The autoclaving process is relatively easy to implement, but the production cycle is long, the cost is high, the product quality fluctuates greatly and the strength is relatively low. The hydrothermal method has been industrialized and the product has good crystal form, stable quality and excellent mechanical properties, but it needs to be carried out in a high temperature and high pressure liquid phase system, with high equipment requirements and high energy consumption. The normal pressure salt solution method has mild reaction conditions, but it is still in the laboratory research stage. The concentrated salt solution affects the subsequent dehydration and washing and product purity, and it is difficult to adapt to large-scale industrial applications.

[0005] Furthermore, the crystallization process is complex, requiring precise control of multiple factors such as temperature, pressure, acidity, and reaction time. Any inaccuracy can negatively impact the crystallization effect and product quality. The process also requires the addition of large quantities of crystallization agents, regulators, and other reagents, increasing production costs and potentially posing environmental risks. Some reagents (such as unconventional polycarboxylic acids, macromolecules, or surfactant-based crystallization agents) are expensive and unsuitable for large-scale production. Existing technologies commonly suffer from low hemihydrate gypsum yields and inefficient resource utilization, leading to increased production costs and reduced economic benefits. Furthermore, the product suffers from low whiteness, limiting its adoption in applications requiring high whiteness.

[0006] For example, CN119638232A discloses a method for regulating the crystallization of phosphogypsum, which mainly involves mixing phosphogypsum with sulfuric acid and stirring the mixture to react, and then controlling the sulfuric acid concentration and reaction time to achieve the transformation of dihydrate gypsum into α-hemihydrate gypsum and anhydrous gypsum. However, this method has obvious shortcomings: on the one hand, the phosphogypsum needs to be pretreated to remove impurities, otherwise the impurities will affect the quality of the product; on the other hand, its crystallization process is relatively complicated, and the reaction conditions need to be strictly controlled, and the yield is not high. At the same time, the whiteness of the hemihydrate gypsum obtained is low, which limits its scope of application. CN119977373A discloses a method for preparing anhydrous gypsum by ultrasonically coordinating the crystallization of phosphogypsum. Although the whiteness is improved to a certain extent by ultrasonic technology, this method requires the use of ammonium persulfate as an additive, and has strict requirements on the ultrasonic environment and heating conditions. The process is complicated, which increases the production cost and operation difficulty, and is not conducive to large-scale industrial application.

[0007] In summary, existing technologies for converting phosphogypsum into hemihydrate gypsum suffer from significant deficiencies in pretreatment and impurity removal, the crystallization process, and product quality (strength, yield, and whiteness). Research and development of more efficient, environmentally friendly, and economical innovative processes and technologies are urgently needed to promote the efficient utilization and sustainable development of phosphogypsum resources. Summary of the Invention

[0008] The technical problem to be solved by the present invention is that the existing method for preparing hemihydrate gypsum by crystallization of phosphogypsum has the problems of complex process flow and low whiteness and yield of the product.

[0009] In order to achieve the above application purpose, the technical solution adopted in this application is as follows.

[0010] The present invention provides a method for preparing hemihydrate gypsum by in-situ crystallization: a crystallization agent solution is heated to vaporize it into crystallization agent steam, and a sufficient amount of the crystallization agent steam is allowed to fully contact and react with phosphogypsum to perform crystallization and obtain hemihydrate gypsum.

[0011] The mass concentration of the above-mentioned crystal conversion agent solution is 1%~20%.

[0012] The crystal-changing agent solution is at least one of a citric acid solution, a sodium lauryl sulfate solution, and a malic acid solution.

[0013] The above-mentioned method for preparing hemihydrate gypsum by in-situ crystallization is implemented in a reaction system, which includes an in-situ crystallization system and a steam generation system; the in-situ crystallization system includes a reaction chamber, a porous gas pipe, a turning device, a gas collecting device and a condensing device; wherein the gas collecting device is arranged above the reaction chamber, and the gas collecting device is connected to the condensing device through a pipeline; the steam generation system is connected to the in-situ crystallization system through a steam pipeline.

[0014] The bottom of the reaction chamber is treated with anti-seepage treatment, and porous gas pipes are evenly laid in strips at the bottom of the reaction chamber; the pore diameter of the porous gas pipes is 5-50 mm.

[0015] The above-mentioned phosphogypsum is loosely stacked in the reaction chamber of the in-situ crystallization system in the form of strips with a stacking thickness of 20 to 50 cm.

[0016] The above-mentioned crystal-transforming agent solution is heated in a steam generating system and vaporized into crystal-transforming agent steam, and the heating temperature is controlled to be 60°C~150°C.

[0017] The above-mentioned crystal-transforming agent vapor is transported to the porous gas pipe through the steam pipeline, and the transport rate is controlled to be 3~8m / s.

[0018] The above-mentioned crystallization agent vapor enters the reaction chamber through a porous gas pipe, fully contacts with the phosphogypsum to carry out crystallization, and the crystallization time is controlled to be 6 to 12 hours. During the crystallization process, the phosphogypsum pile is loosened every 30 to 180 minutes using a turning device.

[0019] The volatilized gas and crystal-transforming agent vapor during the above-mentioned crystal transformation process are collected by a gas collecting device and then condensed and recovered by a condensing device.

[0020] The method for preparing hemihydrate gypsum by in-situ crystallization provided by the present invention has the following beneficial effects compared with the prior art.

[0021] (1) The present invention innovatively uses crystallization agent vapor to directly react with loosely stacked phosphogypsum at atmospheric or near atmospheric pressure, achieving efficient in-situ crystallization. High-temperature crystallization vapor not only effectively promotes the transformation of calcium sulfate dihydrate into calcium sulfate hemihydrate, but also removes some soluble impurities, significantly reducing the reliance on complex pretreatment processes. Through the selective adsorption of crystallization agent vapor molecules on different crystal surfaces, combined with crystal anisotropy and high temperature conditions, the morphology of calcium sulfate hemihydrate crystals can be effectively controlled, thereby obtaining a product with better performance.

[0022] (2) The process of the present invention is greatly simplified and easy to operate and control. The main control parameters are only steam temperature, delivery rate, reaction time and turning frequency, which avoids the precise control of multiple parameters such as temperature, pressure, acidity, etc. in the prior art. At the same time, the condensation recovery system can collect and recycle the volatilized crystallization agent vapor and reaction gas, significantly improving the utilization rate of the crystallization agent, reducing reagent consumption and production costs, and minimizing the risk of environmental pollution.

[0023] (3) In terms of product quality and resource utilization efficiency, the present invention achieves a phosphogypsum conversion rate of more than 90%. The content of hemihydrate gypsum in the obtained hemihydrate gypsum product is greater than 70%, the whiteness is higher than 80%, and the content of harmful impurities fluorine (F) and phosphorus (P) is less than 0.05 mg / Kg, which effectively solves the problems of low strength, poor stability, low whiteness and residual impurities affecting the application of existing products.

[0024] (4) The method of the present invention has low energy consumption, relatively simple equipment requirements, and is easy to scale up. The crystallization is carried out at a mild temperature of 60°C to 150°C, which is much lower than the high-pressure hydrothermal method, and the energy consumption is significantly reduced. The strip-type stacking combined with the turning device and porous gas path design ensures uniform and sufficient contact between steam and materials, making it suitable for large-scale continuous production. The present invention provides a practical, economical and environmentally friendly technical solution to completely solve the environmental pressure brought about by the large-scale storage of phosphogypsum, realize its efficient resource utilization and sustainable development of the industry. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 is a schematic diagram of the reaction system; In the figure: 1 is the steam generation system, 2 is the in-situ crystallization system, 3 is the steam pipeline; 2-1 is the reaction chamber, 2-2 is the porous gas pipe, 2-3 is the turning device, 2-4 is the gas collection device, and 2-5 is the condensation device; Figure 2 SEM images of the phosphogypsum before treatment and the gypsum product obtained in Example 1; In the figure: a) is phosphogypsum before treatment, b) is gypsum product; Figure 3 SEM images of the phosphogypsum before treatment and the gypsum product obtained in Example 2; In the figure: a) is phosphogypsum before treatment, c) is gypsum product; Figure 4 SEM images of the phosphogypsum before treatment and the gypsum product obtained in Example 3; In the figure: a) is phosphogypsum before treatment, d) is gypsum product. DETAILED DESCRIPTION

[0026] In order to make the technical problems, technical solutions and beneficial effects to be solved in the present application clearer, the present application will be further described in detail below in conjunction with the embodiments. It will be understood by those skilled in the art that the following examples are only used to illustrate the present invention and should not be considered as limiting the scope of the present invention. Unless otherwise defined, all scientific and technological terms used herein have the same meanings understood by those of ordinary skill in the art.

[0027] The invention discloses a reaction system for implementing a method for preparing hemihydrate gypsum through in-situ crystallization.

[0028] In one embodiment of the present invention, the reaction system is as follows Figure 1 As shown, it includes an in-situ crystallization system 2 and a steam generating system 1; the in-situ crystallization system 2 includes a reaction chamber 2-1, a porous gas pipe 2-2, a turning device 2-3, a gas collecting device 24 and a condensing device 2-5; wherein, the in-situ crystallization system 2 is in a sealed state, the bottom of the reaction chamber 2-1 is treated with an anti-seepage treatment, and the porous gas pipe 2-2 is evenly laid in strips on the bottom of the reaction chamber 2-1; the gas collecting device 2-4 is arranged above the reaction chamber 2-1, and the gas collecting device 2-4 is connected to the condensing device 2-5 through a pipeline; the steam generating system 1 is connected to the in-situ crystallization system 2 through a steam pipe 3; in addition, valves are also provided at the pipe outlets of the steam pipe 3 and the condensing device 2-5.

[0029] In one embodiment of the present invention, the steam pipe 3 and the gas collecting devices 2 - 4 are made of corrosion-resistant materials.

[0030] In one embodiment of the present invention, the diameter of the pores of the porous air pipe 2-2 is 5-50 mm.

[0031] In one embodiment of the present invention, the condensing device 2-5 can be a condensed water condenser, a liquid nitrogen condenser or a dry ice condenser.

[0032] The present invention provides examples 1 to 3 of treating phosphogypsum by using a method for preparing hemihydrate gypsum by in-situ crystallization. Figure 1 The specific experimental process is as follows:

[0033] Example 1: Preparation of hemihydrate gypsum by in-situ crystallization of phosphogypsum, the specific steps are as follows: (1) Phosphogypsum is loosely stacked in the reaction chamber of the in-situ crystallization system in a strip-like manner with a stacking thickness of 20 cm; (2) In the steam generation system, a malic acid crystal-transforming agent solution with a mass concentration of 5% is heated to 100°C to form crystal-transforming agent vapor, which is then transported to the porous gas pipe through the steam pipeline at a speed of 3 m / s; (3) The crystallization agent vapor enters the reaction chamber through the porous gas path and fully contacts with the phosphogypsum to perform crystallization. The phosphogypsum pile is loosened every 30 minutes using a turning device. After 6 hours of reaction, the crystallization is completed.

[0034] SEM images of the gypsum product obtained from the phosphogypsum before treatment and Example 1 ( Figure 2 ) As can be seen, the product produced in Example 1 is dense, complete, and coarse crystals, namely, calcium sulfate hemihydrate crystals. Testing shows that the hemihydrate gypsum produced in Example 1 has a crystal conversion rate of 95%, an α-gypsum content of 78%, a whiteness of 94%, and F and P contents of less than 0.03 mg / kg.

[0035] Example 2: Preparation of hemihydrate gypsum by in-situ crystallization of phosphogypsum, the specific steps are as follows: (1) Phosphogypsum is loosely stacked in the reaction chamber of the in-situ crystallization system in a strip-like manner with a stacking thickness of 50 cm; (2) In the steam generation system, a sodium dodecyl sulfate crystal-transforming agent solution with a mass concentration of 10% is heated to 150°C to form crystal-transforming agent steam, which is transported to the porous gas pipe through the steam pipeline at a speed of 8 m / s; (3) The crystallization agent vapor enters the reaction chamber through the porous gas path and fully contacts with the phosphogypsum to perform crystallization. The phosphogypsum pile is loosened every 40 minutes using a turning device. After 8 hours of reaction, the crystallization is completed.

[0036] SEM images of the gypsum product obtained from the phosphogypsum before treatment and Example 2 ( Figure 3 ) As can be seen, the product produced in Example 2 is a round, elongated crystal, namely, calcium sulfate hemihydrate crystals. Testing shows that the hemihydrate gypsum produced in Example 2 has a crystal conversion rate of 98%, a β-gypsum content of 86%, a whiteness of 90%, and F and P contents of less than 0.04 mg / kg.

[0037] Example 3: Preparation of hemihydrate gypsum by in-situ crystallization of phosphogypsum, the specific steps are as follows: (1) Phosphogypsum is loosely stacked in the reaction chamber of the in-situ crystallization system in a strip-like manner with a stacking thickness of 40 cm; (2) In the steam generation system, a citric acid crystal-transforming agent solution with a mass concentration of 20% is heated to 80°C to form crystal-transforming agent steam, which is transported to the porous gas pipe through the steam pipe at a speed of 5 m / s; (3) The crystallization agent vapor enters the reaction chamber through the porous gas path and fully contacts with the phosphogypsum to perform crystallization. The phosphogypsum pile is loosened every 60 minutes using a turning device. After 12 hours of reaction, the crystallization is completed.

[0038] SEM images of the gypsum product obtained from the phosphogypsum before treatment and Example 3 ( Figure 4 ) As can be seen, the product produced in Example 3 is a rod-shaped hexagonal crystal, namely, calcium sulfate hemihydrate crystals. Testing shows that the hemihydrate gypsum produced in Example 3 has a crystal conversion rate of 92%, an α-gypsum content of 72%, a whiteness of 81%, and F and P contents of less than 0.05 mg / kg.

Claims

1. A method for preparing hemihydrate gypsum by in-situ crystallization, characterized in that: The crystal-transforming agent solution is heated to vaporize it into crystal-transforming agent steam, and a sufficient amount of the crystal-transforming agent steam is allowed to fully contact and react with the phosphogypsum to perform crystal transformation and obtain hemihydrate gypsum.

2. The method for preparing hemihydrate gypsum by in-situ crystallization according to claim 1, characterized in that: The mass concentration of the crystal conversion agent solution is 1% to 20%.

3. The method for preparing hemihydrate gypsum by in-situ crystallization according to claim 2, characterized in that: The crystal-changing agent solution is at least one of a citric acid solution, a sodium lauryl sulfate solution, and a malic acid solution.

4. The method for preparing hemihydrate gypsum by in-situ crystallization according to claim 1, characterized in that: The method is implemented in a reaction system, wherein the reaction system comprises an in-situ crystallization system (2) and a steam generation system (1); The in-situ crystallization system (2) comprises a reaction chamber (2-1), a porous gas pipe (2-2), a turning device (2-3), a gas collecting device (2-4) and a condensing device (2-5); wherein the gas collecting device (2-4) is arranged above the reaction chamber (2-1), and the gas collecting device (2-4) and the condensing device (2-5) are connected through a pipeline; The steam generation system (1) is connected to the in-situ crystal transformation system (2) via a steam pipeline (3).

5. The method for preparing hemihydrate gypsum by in-situ crystallization according to claim 4, characterized in that: The bottom of the reaction chamber (2-1) is treated with anti-seepage treatment, and the porous gas pipe (2-2) is evenly laid in strips on the bottom of the reaction chamber; The diameter of the pores of the porous air pipe (2-2) is 5-50 mm.

6. The method for preparing hemihydrate gypsum by in-situ crystallization according to claim 4, characterized in that: The phosphogypsum is loosely stacked in the reaction chamber (2-1) of the in-situ crystallization system in the form of strips with a stacking thickness of 20 to 50 cm.

7. The method for preparing hemihydrate gypsum by in-situ crystallization according to claim 4, characterized in that: The crystal-transforming agent solution is heated in a steam generating system (1) and vaporized into crystal-transforming agent steam, and the heating temperature is controlled to be 60°C to 150°C.

8. The method for preparing hemihydrate gypsum by in-situ crystallization according to claim 4, characterized in that: The crystal-transforming agent steam is transported to the porous gas pipe (2-2) through the steam pipe (3), and the transport rate is controlled to be 3-8 m / s.

9. The method for preparing hemihydrate gypsum by in-situ crystallization according to claim 4, characterized in that: The crystallization agent vapor enters the reaction chamber (2-1) through the porous gas pipe (2-2) and fully contacts with the phosphogypsum to perform crystallization, and the crystallization time is controlled to be 6 to 12 hours; During the crystallization process, the phosphogypsum pile is loosened every 30 to 180 minutes using a turning device (2-3).

10. The method for preparing hemihydrate gypsum by in-situ crystallization according to claim 4, characterized in that: The volatilized gas and crystallization agent vapor during the crystallization process are collected by a gas collecting device (2-4) and then condensed and recovered by a condensing device (2-5).

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