Anhydrous phosphorus gypsum and its preparation method

Abstract

This invention discloses anhydrous phosphogypsum and its preparation method, belonging to the field of gypsum building materials technology. The preparation method provided by this invention includes the following steps: mixing phosphogypsum, quicklime, and water evenly, aging to obtain pretreated phosphogypsum; mixing the pretreated phosphogypsum with graphite powder and calcining at 500-550℃ to obtain the final product. The preparation method of anhydrous phosphogypsum provided by this invention can obtain high-whiteness, anhydrous phosphogypsum without harmful impurities at a low calcination temperature of 500-550℃, with a whiteness as high as 0.7 or higher. This not only significantly reduces production energy consumption but also effectively improves the basic reactivity of the product. This invention uses a wet-mixing lime neutralization + aging method to pretreat the phosphogypsum, which can effectively remove soluble fluoride phosphorus. The removal process is simple, energy consumption is very low, drying is unnecessary, preparation cost is low, and the preparation process does not generate new waste.

Description

Technical Field

[0001] This invention relates to the field of gypsum building materials technology, and in particular to a phosphorus-anhydrous gypsum and its preparation method. Background Technology

[0002] The information disclosed in the background of the invention is only intended to enhance understanding of the overall background of the invention and should not necessarily be regarded as an admission or any form of suggestion that the information constitutes the prior art already known to a person skilled in the art.

[0003] Phosphogypsum is a major solid waste product from the hydrometallurgical process of producing phosphate compound fertilizer from phosphoric acid. Phosphoric acid is also a basic raw material for lithium iron phosphate. The adjustment of the energy structure and the rapid development of new energy sources will inevitably increase the demand for phosphoric acid, leading to a rapid increase in phosphogypsum production. Currently, the global annual emission of phosphogypsum exceeds 300 million tons, while the effective utilization rate is only about 20%. Phosphogypsum is a high-grade gypsum resource produced as a byproduct of industrial phosphoric acid smelting, but harmful impurities (mainly including organic matter, phosphorus, and fluorine) limit its utilization in building materials. Currently, the main applications of phosphogypsum include low-temperature calcination to prepare building gypsum, autoclaving or atmospheric pressure methods to prepare high-strength gypsum, and high-temperature calcination to prepare anhydrous gypsum. Because harmful impurities can affect production speed and reduce product performance indicators, certain pretreatment measures are necessary to remove them.

[0004] Currently, the main measures to address the adverse effects of impurities in phosphogypsum on product performance include: physical methods (water washing, flotation), chemical methods (acid leaching, lime neutralization), and heat treatment (high-temperature calcination). Among these, the lime neutralization + high-temperature calcination process is relatively simple and can simultaneously address the performance degradation issues caused by organic matter and soluble fluorine and phosphorus, thus attracting considerable attention. High-temperature calcination of phosphogypsum to prepare anhydrous gypsum is considered a simple and feasible treatment method.

[0005] Currently, the calcination temperature for preparing anhydrous phosphogypsum using the high-temperature calcination method is usually between 800 and 1000°C. Below this temperature, the soluble fluorine and phosphorus in phosphogypsum are difficult to convert into insoluble compounds. However, the current high-temperature calcination method for preparing anhydrous phosphogypsum has many shortcomings, mainly including: (1) high energy consumption; (2) low whiteness of the product. This is because when the temperature reaches about 500°C, the pyrite in phosphogypsum will be converted into iron oxide, causing the color of the anhydrous gypsum to change from white to red. The higher the temperature and the longer the holding time, the darker the color; (3) low reactivity of the product. Anhydrous anhydrous gypsum can be generated at 300°C. Further increasing the temperature will increase the density of the anhydrous gypsum crystals, thereby reducing the reactivity, significantly prolonging the setting time, and reducing the early strength.

[0006] Lowering the preparation temperature of anhydrous phosphogypsum is beneficial for improving the product's reactivity and whiteness, and saving production energy. However, a key challenge in lowering the calcination temperature is removing the influence of harmful impurities. Washing phosphogypsum with water can effectively remove organic matter and soluble fluoride-phosphorus compounds. For example, patent CN106977123A discloses a citric acid-water washing process for removing fluoride impurities from phosphogypsum. This method involves adding citric acid solution to the phosphogypsum for washing, followed by further washing, and finally, further impurity removal and drying. However, this method uses lime to neutralize water for multiple washes of the phosphogypsum sample, resulting in large water consumption and generating a large amount of wastewater containing ionic impurities. This creates new pollution while utilizing wastewater, failing to meet green environmental protection requirements. Furthermore, this method requires a dedicated drying process, increasing technical complexity and preparation costs, making it difficult to implement in actual production.

[0007] Therefore, how to provide a simple preparation method for phosphorus-containing anhydrous gypsum with high whiteness and free of harmful impurities that does not generate waste is an urgent problem to be solved. Summary of the Invention

[0008] In view of this, the present invention provides anhydrous phosphorus gypsum and its preparation method, which solves the problem that the existing preparation process of anhydrous phosphorus gypsum cannot simultaneously meet the requirements of no waste generation and high whiteness of the product without harmful impurities.

[0009] In a first aspect, the present invention provides a method for preparing anhydrous phosphorus gypsum, comprising the following steps:

[0010] Phosphogypsum, quicklime, and water are mixed evenly and aged to obtain pretreated phosphogypsum.

[0011] The pretreated phosphogypsum and graphite powder are mixed and then calcined at 500-550℃ to obtain the final product.

[0012] Preferably, the mass ratio of the phosphogypsum, quicklime and water is 100:(1-3):(15-20).

[0013] Preferably, the aging time is 40 to 60 hours.

[0014] Preferably, the mass ratio of the pretreated phosphogypsum to graphite powder is 100:(0.5-1).

[0015] Preferably, the calcination time is 0.5 to 3 hours.

[0016] Preferably, the calcination process is carried out in an air atmosphere.

[0017] Preferably, the heating rate to 500-550°C is 5-15°C / min.

[0018] Preferably, the particle size of the phosphogypsum is 100-200 mesh, and the particle size of the quicklime is 100-200 mesh.

[0019] Preferably, the particle size of the graphite powder is 1–20 μm.

[0020] Secondly, the present invention provides anhydrous phosphate gypsum prepared by the above preparation method.

[0021] Compared with the prior art, the present invention has achieved the following beneficial effects:

[0022] (1) The method for preparing anhydrous gypsum provided by the present invention can obtain anhydrous gypsum with high whiteness and no harmful impurities at a low calcination temperature of 500-550℃. The whiteness can be as high as 0.7 or more, which not only greatly reduces production energy consumption, but also effectively improves the basic reactivity of the product.

[0023] (2) The present invention uses a wet lime neutralization + aging method to pretreat phosphogypsum, which can effectively remove soluble fluorine phosphorus. The removal process is simple, the energy consumption is very low, no drying treatment is required, the preparation cost is low, and the preparation process does not generate new pollution waste. Detailed Implementation

[0024] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0025] This invention provides a method for preparing anhydrous phosphorus gypsum, comprising the following steps:

[0026] Phosphogypsum, quicklime, and water are mixed evenly and aged to obtain pretreated phosphogypsum.

[0027] The pretreated phosphogypsum and graphite powder are mixed and then calcined at 500–550°C to obtain the final product.

[0028] In existing phosphogypsum pretreatment processes, the reaction rate of fluorine, phosphorus, and calcium in the solid phase environment is slow in the dry lime neutralization method, requiring higher reaction temperatures to accelerate the reaction (e.g., CN106006643A); the water washing method generates new waste (e.g., CN106977123A); and the lime neutralization method for preparing slurry increases the preparation and drying processes and costs (e.g., CN112871457A). This invention addresses this by using a wet lime neutralization + aging phosphogypsum pretreatment method. This method adjusts the pH value of the phosphogypsum, accelerating the combination of soluble phosphorus and fluorine at low temperatures to form insoluble substances. These insoluble substances do not participate in the hydration reaction of the gypsum and remain in the anhydrous gypsum without adversely affecting product performance. This solves the problem of the adverse effects of soluble fluorine and phosphorus ions on product performance without generating waste.

[0029] Current methods for improving the whiteness of phosphogypsum products generally involve flotation. However, flotation is a complex process that generates new waste and has high production costs. Another existing technology involves adding ammonium chloride to calcined phosphogypsum to whiten the product. However, chloride ions in gypsum are considered harmful ions and their content is strictly controlled. National standards for building gypsum require chloride content to be below 0.2%. Furthermore, ammonium chloride releases ammonia during calcination, necessitating additional flue gas treatment equipment and increasing production costs.

[0030] This invention discovers that the whiteness of calcined phosphogypsum is mainly affected by organic impurities and the valence state of iron, and that the organic matter in phosphogypsum decomposes completely at 500℃. Therefore, this invention improves the whiteness of anhydrite products through two methods: (1) setting the calcination temperature to 500-550℃, at which temperature all organic impurities in phosphogypsum can be removed, significantly improving the product whiteness; (2) adding the reducing agent graphite, whose oxidation temperature point matches the calcination temperature, effectively inhibiting the conversion of ferrous iron to ferric iron at 500-550℃, reducing the coloring effect of ferric iron. The high-whiteness anhydrous anhydrite preparation process of this invention is simple, has low preparation cost, generates no new waste, and is beneficial for actual production.

[0031] In this invention, the mass ratio of phosphogypsum, quicklime, and water is 100:(1-3):(15-20). This invention has found that when the quicklime content is too low, soluble ions cannot be completely removed, which adversely affects the product's coagulation and mechanical properties; when the quicklime content is too high, it affects the purity of the final anhydrous gypsum, leaving residues after calcination and increasing the water requirement of the anhydrous gypsum. The water addition amount in this invention satisfies the requirement of creating a micro-liquid phase environment in the aggregated pores of the powder, which is conducive to the combination of soluble fluoride and phosphorus ions with calcium ions to form insoluble precipitates. If the water addition is too low, a liquid phase cannot be formed, and the lime cannot dissolve to produce soluble calcium ions that combine with soluble fluoride and phosphorus ions; if the water addition is too high, a slurry is formed, requiring drying after aging, and the resulting agglomeration necessitates secondary grinding before calcination, increasing process complexity and production costs.

[0032] This invention does not impose any special limitations on the aging process; any aging operation well known in the art can be used. In this invention, the aging time is 40–60 hours, more preferably 45–50 hours, and most preferably 48 hours.

[0033] In this invention, the mass ratio of the pretreated phosphogypsum to graphite powder is 100:(0.5-1). Insufficient graphite powder content cannot effectively inhibit the conversion of iron sulfide to iron oxide, resulting in a decrease in the whiteness of the product; excessive graphite powder content will adversely affect the whiteness of the anhydrous phosphogypsum product.

[0034] In this invention, the calcination time is 0.5–3 hours, more preferably 1–2 hours. During the calcination process, the phosphogypsum gradually dehydrates to form anhydrous phosphogypsum, while the impurities and organic matter are completely decomposed.

[0035] In this invention, the calcination process is carried out in an air atmosphere. The heating rate to 500–550°C is 5–15°C / min, more preferably 8–12°C / min.

[0036] In this invention, in order to enable soluble fluoride and phosphorus ions to fully react with calcium ions to form an insoluble precipitate, the particle size of the phosphogypsum is 100-200 mesh, more preferably 150 mesh; the particle size of the quicklime is 100-200 mesh, more preferably 150 mesh.

[0037] In this invention, the particle size of the graphite powder is 1-20 μm, more preferably 5-15 μm.

[0038] The present invention also provides anhydrous phosphorus gypsum prepared by the above-described preparation method. The anhydrous phosphorus gypsum prepared by the present invention is free of organic matter and soluble phosphorus and fluorine impurities, and has a high whiteness, reaching 0.7 or higher.

[0039] The technical solution of the present invention will be further described below with reference to specific embodiments. In the following embodiments, the particle size of quicklime powder is 150 mesh, and the average particle size of graphite powder is 10 μm.

[0040] Example 1

[0041] This embodiment provides a method for preparing anhydrous phosphorus gypsum.

[0042] (1) Wet pretreatment: Grind phosphogypsum powder through a 150-mesh sieve, weigh 100g of phosphogypsum, 15g of water and 2g of quicklime powder, stir evenly and age for 48h to obtain pretreated phosphogypsum.

[0043] (2) Calcination process: 100g of pretreated phosphogypsum and 0.8g of graphite powder are mixed and placed in a high-temperature furnace. The temperature is raised to 550℃ at a rate of 10℃ / min and kept at that temperature for 2 hours. Then the mixture is cooled to room temperature and taken out to obtain anhydrous phosphogypsum.

[0044] Example 2

[0045] This embodiment provides a method for preparing anhydrous phosphorus gypsum.

[0046] (1) Wet pretreatment: Grind phosphogypsum powder through a 150-mesh sieve, weigh 100g of phosphogypsum, 18g of water and 2.5g of quicklime powder, stir evenly and age for 48h to obtain pretreated phosphogypsum.

[0047] (2) Calcination process: 100g of pretreated phosphogypsum and 0.8g of graphite powder are mixed and placed in a high-temperature furnace. The temperature is raised to 510℃ at a rate of 10℃ / min and kept at that temperature for 2 hours. Then the mixture is cooled to room temperature and taken out to obtain anhydrous phosphogypsum.

[0048] Example 3

[0049] This embodiment provides a method for preparing anhydrous phosphorus gypsum.

[0050] (1) Wet pretreatment: Grind phosphogypsum powder through a 150-mesh sieve, weigh 100g of phosphogypsum, 15g of water and 1.5g of quicklime powder, stir evenly and age for 48h to obtain pretreated phosphogypsum.

[0051] (2) Calcination process: 100g of pretreated phosphogypsum and 1g of graphite powder are mixed and placed in a high-temperature furnace. The temperature is raised to 550℃ at a rate of 10℃ / min and kept at that temperature for 2 hours. Then the mixture is cooled to room temperature and taken out to obtain anhydrous phosphogypsum.

[0052] Example 4

[0053] This embodiment provides a method for preparing anhydrous phosphorus gypsum.

[0054] (1) Wet pretreatment: Grind phosphogypsum powder through a 150-mesh sieve, weigh 100g of phosphogypsum, 15g of water and 2g of quicklime powder, stir evenly and age for 45h to obtain pretreated phosphogypsum.

[0055] (2) Calcination process: 100g of pretreated phosphogypsum and 0.5g of graphite powder are mixed and placed in a high-temperature furnace. The temperature is raised to 525℃ at a rate of 8℃ / min and kept at that temperature for 2 hours. Then the mixture is cooled to room temperature and taken out to obtain anhydrous phosphogypsum.

[0056] Comparative Example 1

[0057] The difference between this comparative example and Example 1 is that the quicklime powder content is 0.5g.

[0058] Comparative Example 2

[0059] The difference between this comparative example and Example 1 is that the pretreatment involves dry mixing with lime for neutralization. The specific operation steps are as follows:

[0060] Grind phosphogypsum powder through a 150-mesh sieve, weigh 100g of phosphogypsum and 2g of quicklime powder, stir evenly and age for 48 hours to obtain pretreated phosphogypsum; mix 100g of pretreated phosphogypsum with 0.8g of graphite powder, place in a high-temperature furnace, heat to 550℃ at a rate of 10℃ / min, hold for 2 hours, and then cool to room temperature with the furnace to obtain the final product.

[0061] Comparative Example 3

[0062] The difference between this comparative example and Example 1 is that no graphite powder is added to this comparative example.

[0063] Comparative Example 4

[0064] The difference between this comparative example and Example 1 is that the temperature was increased to 450°C at a rate of 10°C / min.

[0065] Comparative Example 5

[0066] The difference between this comparative example and Example 1 is that this comparative example does not undergo wet pretreatment. The specific steps are as follows:

[0067] Grind phosphogypsum powder through a 150-mesh sieve. Mix 100g of phosphogypsum with 0.8g of graphite powder, place the mixture in a high-temperature furnace, heat it to 550℃ at a rate of 10℃ / min, keep it at that temperature for 2 hours, and then cool it to room temperature with the furnace. The product is then ready.

[0068] Comparative Example 6

[0069] The difference between this comparative example and Example 1 is that this comparative example does not undergo wet pretreatment, does not add graphite powder, and calcines the phosphogypsum at 550°C. The specific steps are as follows:

[0070] The phosphogypsum powder is ground through a 150-mesh sieve, then placed in a high-temperature furnace and heated to 550°C at a rate of 10°C / min. It is then held at that temperature for 2 hours and cooled to room temperature with the furnace before being removed.

[0071] Comparative Example 7

[0072] The difference between this comparative example and Example 1 is that this comparative example does not undergo wet pretreatment, does not add graphite powder, and calcines the phosphogypsum at 850°C. The specific steps are as follows:

[0073] The phosphogypsum powder is ground through a 150-mesh sieve, then placed in a high-temperature furnace and heated to 850°C at a rate of 10°C / min. It is then held at that temperature for 2 hours and cooled to room temperature with the furnace before being removed.

[0074] The performance of the final products of Example 1 and Comparative Examples 1 to 7 was measured, as shown in Table 1. Note: The mechanical property data (3d compressive strength, 28d compressive strength) in Table 1 are experimental values ​​without the addition of an active activator.

[0075] Table 1. Performance measurement data of the final products of Examples 1 and Comparative Examples 1-7

[0076]

[0077]

[0078] As can be seen from Table 1, the phosphorus-anhydrous gypsum of Example 1 has the highest whiteness value, reaching 75%, and also has excellent mechanical properties and a high degree of hydration.

[0079] In Comparative Example 1, the amount of quicklime was relatively low. During the wet pretreatment stage, soluble fluoride and phosphorus ions could not be completely combined to form insoluble substances, which had an adverse effect on the condensation performance and mechanical properties of the product.

[0080] In Comparative Example 2, a pretreatment process using dry lime was employed, which did not involve a liquid phase. As a result, the reaction rate of Ca ions and soluble fluorine and phosphorus ions was extremely low, leading to a still high content of soluble fluorine and phosphorus in the product, which was detrimental to the coagulation and mechanical properties.

[0081] In Comparative Example 3, without the addition of graphite powder, the whiteness decreased significantly compared to Example 1. This is because, without a reducing agent, the divalent iron sulfide in the phosphogypsum oxidizes to red trivalent iron at around 500°C, reducing the product's whiteness. In Comparative Example 4, the calcination temperature was lowered to 450°C, and the product's whiteness was noticeably reduced. This is because at lower temperatures, organic matter cannot be completely decomposed and removed, thus reducing the product's whiteness. The resulting product also exhibited reduced mechanical properties, primarily due to the adverse effects of residual organic matter on mechanical properties, and the lower amount of type II anhydrous gypsum generated in the product at lower temperatures.

[0082] Comparative Example 5, which did not undergo wet pretreatment, showed lower agglomeration and mechanical properties. This is because the product contained soluble phosphorus and fluorine impurities, which adversely affected its agglomeration and mechanical properties. Comparative Example 5 exhibited higher whiteness than Comparative Example 6 because the graphite powder in Comparative Example 5 suppressed the coloring effect of ferric iron.

[0083] Comparative Example 7 showed that calcining phosphogypsum at 850℃ caused the pyrite in the phosphogypsum to transform into iron oxide, changing the color of the anhydrite from white to red, resulting in a very low whiteness of only 52%. Simultaneously, the high-temperature treatment at 850℃ increased the density of the anhydrite crystals, thereby reducing reactivity, significantly prolonging the setting time, and decreasing compressive strength.

[0084] The impurity content of the final products of Example 1, Comparative Example 4, Comparative Example 6 and Comparative Example 7 was determined, and the results are shown in Table 2.

[0085] Table 2. Impurity content (%) of anhydrous phosphorus gypsum products

[0086] Examples / Comparative Examples organic matter soluble phosphorus soluble fluoride Example 1 0 0 0 Comparative Example 4 0.12 0 0 Comparative Example 6 0 1.3 0.77 Comparative Example 7 0 0 0

[0087] As shown in Table 2, when wet pretreatment is performed but the calcination temperature is low (Comparative Example 4), organic matter is difficult to completely remove. Combined with Table 1, it can be seen that the presence of organic matter adversely affects the whiteness and mechanical properties of the product. Without wet pretreatment and without adding graphite powder, calcining phosphogypsum only at 550℃ (Comparative Example 6) results in the presence of soluble phosphorus and fluoride in the product. Calcination at 850℃ can completely remove soluble phosphorus and fluoride and organic matter, which is a common impurity removal method for anhydrous phosphorus gypsum in the prior art. However, Example 1 of this invention, through wet pretreatment + calcination at 550℃, can remove both organic matter and soluble phosphorus and fluoride simultaneously at a lower temperature.

[0088] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing anhydrous phosphorus gypsum, characterized in that, Includes the following steps: Phosphogypsum, quicklime, and water are mixed evenly and aged to obtain pretreated phosphogypsum. The pretreated phosphogypsum and graphite powder are mixed and then calcined at 500-550℃ to obtain the final product.

2. The preparation method according to claim 1, characterized in that, The mass ratio of the phosphogypsum, quicklime and water is 100:(1-3):(15-20).

3. The preparation method according to claim 1, characterized in that, The aging time is 40 to 60 hours.

4. The preparation method according to claim 1, characterized in that, The mass ratio of the pretreated phosphogypsum to graphite powder is 100:(0.5-1).

5. The preparation method according to claim 1, characterized in that, The calcination time is 0.5 to 3 hours.

6. The preparation method according to claim 1, characterized in that, The calcination process is carried out in an air atmosphere.

7. The preparation method according to claim 1, characterized in that, The heating rate to 500–550℃ is 5–15℃ / min.

8. The preparation method according to claim 1, characterized in that, The phosphogypsum has a particle size of 100-200 mesh, and the quicklime has a particle size of 100-200 mesh.

9. The preparation method according to claim 1, characterized in that, The particle size of graphite powder is 1–20 μm.

10. Anhydrous phosphate gypsum prepared by the preparation method according to any one of claims 1 to 9.

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