A step-by-step modification calcination preparation method of phosphogypsum solid waste

Abstract

The application discloses a kind of phosphogypsum solid waste's step-by-step modification calcination preparation method, belong to solid waste treatment technical field.The method includes: (1) pre-coating treatment: the original state of containing attached water dihydrate phosphogypsum is mixed with composite modifier and is ground, and pre-coated phosphogypsum is obtained;(2) temperature control calcination: the pre-coated phosphogypsum obtained in step (1) is sent into calcining kiln, and dehydration reaction is carried out at 165 DEG C~180 DEG C temperature, and hemihydrate gypsum powder is obtained;(3) hygroscopic aging: the hemihydrate gypsum powder obtained in step (2) is cooled and aged 24h~48h, and the components in powder are used to absorb moisture in air to carry out hydration reaction, and the final product is obtained.The method is free of water washing, short process, and the obtained phosphorus building gypsum standard consistency water consumption is 45%~50%, and 2 hour compressive strength is greater than or equal to 6.5MPa.

Description

Technical Field

[0001] This invention relates to the field of solid waste treatment technology, and in particular to a stepwise modified calcination preparation method for phosphogypsum solid waste. Background Technology

[0002] Phosphogypsum is an industrial waste residue produced during the wet process of phosphoric acid production. Its main chemical composition is calcium sulfate dihydrate (CaSO4·2H2O). Since phosphogypsum inevitably contains impurities such as soluble phosphorus, fluorine compounds and a small amount of organic matter, the building gypsum powder produced by directly calcining it usually has performance defects such as high standard consistency water content, large fluctuation in setting time, and low mechanical strength of hardened body, making it difficult to meet the requirements of superior grade in the national standard GB / T 9776-2008 for building gypsum. Currently, there are two main technical routes for the treatment of phosphogypsum waste in the industry: one is a large-scale water washing process, which removes soluble impurities by washing with a large amount of clean water. Although this can improve the properties of gypsum, it generates a large amount of phosphorus and fluoride-containing wastewater. The other is a process of forced water spraying and subsequent ball milling after calcination. This involves dehydrating dihydrate gypsum into hemihydrate gypsum, and then spraying water to allow the small amount of anhydrous type III gypsum generated to absorb water and transform into hemihydrate gypsum. However, this method suffers from uneven moisture distribution, which can easily lead to localized over-wetting or insufficient rehydration, causing fluctuations in product quality. Furthermore, the subsequent secondary ball milling process further increases costs.

[0003] Therefore, this invention is proposed to solve the above-mentioned technical problems. Summary of the Invention

[0004] The purpose of this invention is to provide a stepwise modified calcination preparation method for phosphogypsum solid waste, comprising the following steps: (1) Pre-coating treatment: The original dihydrate phosphogypsum containing attached water is mixed and ground with a composite modifier to obtain pre-coated phosphogypsum; (2) Temperature-controlled calcination: The pre-coated phosphogypsum obtained in step (1) is fed into a calcining kiln and dehydrated at a temperature of 165℃~180℃ to obtain hemihydrate gypsum powder. (3) Moisture absorption and aging: The hemihydrate gypsum powder obtained in step (2) is cooled and aged for 24h~48h. The components in the powder absorb moisture from the air to carry out a hydration reaction, and the final product is obtained.

[0005] The core mechanism of this invention is as follows: During the mixing process, the 15%–25% water contained in the undisturbed dihydrate phosphogypsum forms a thin water film on the particle surface. When the alkaline neutralizing agent (such as quicklime) comes into contact with the water, it reacts with free acids, soluble phosphorus, and other impurities on the phosphogypsum particle surface, generating water-insoluble calcium salts. The aluminum ions generated after the surface coating agent (such as aluminum sulfate) dissolves also combine with impurities such as fluorine to form insoluble compounds. These newly formed solids adhere to the particle surface, fixing the soluble impurities. Under the mechanical action of mixing and grinding, the modifier components further bind to the phosphogypsum surface, and the film-forming aid (such as sodium lignosulfonate) plays a dispersing and wetting role, helping the generated insoluble substances to spread more evenly on the phosphogypsum surface. Ultimately, a coating layer mainly composed of inorganic precipitates and organic components forms on the surface of the phosphogypsum. During the calcination stage, this coating layer on the surface of the phosphogypsum increases the resistance to the outward discharge of internal crystal water, making the dehydration process more gradual. This reduces the generation of type III anhydrous gypsum due to local overheating or excessively rapid dehydration, resulting in a final product dominated by hemihydrate gypsum. This is beneficial for reducing the amount of water required for standard consistency and improving hardening strength. During the aging stage, the hygroscopic adjusting agent (such as anhydrous sodium sulfate) added to the powder beforehand absorbs moisture from the air, creating a locally humid environment around the particles. This promotes the conversion of any small amount of type III anhydrous gypsum that may be generated during calcination into hemihydrate gypsum. On the other hand, it also provides moisture for the continued reaction of residual alkaline substances (such as Ca(OH)2) and residual soluble impurities (such as P2O5). This hygroscopic adjusting process proceeds gradually inside the powder, effectively eliminating the unstable phase of type III anhydrous gypsum, thereby stabilizing the mechanical properties and setting time of the product.

[0006] Preferably, the adhering water content of the undisturbed dihydrate phosphogypsum in step (1) is 15% to 25%. Adhering water is the necessary medium for the chemical reaction between quicklime and aluminum sulfate and impurities on the surface of phosphogypsum. If the water content is less than 15%, it cannot provide enough reaction medium, resulting in incomplete neutralization and coating of impurities; if the water content is more than 25%, the material will be too wet, which will lead to low grinding efficiency and clumping, and it will also be impossible to form a uniform coating layer.

[0007] Preferably, the composite modifier in step (1) is composed of the following raw materials in parts by weight: 50-80 parts of alkaline neutralizer, 10-25 parts of surface coating agent, 5-15 parts of film-forming aid, and 5-10 parts of moisture-absorbing and restoring agent; and the amount of the composite modifier added is 1.4%-3.0% of the dry basis mass of the original dihydrate phosphogypsum.

[0008] Preferably, the alkaline neutralizing agent is one or more of quicklime or carbide slag; the surface coating agent is aluminum sulfate. Specifically, quicklime is preferred in this invention.

[0009] Preferably, the film-forming aid is sodium lignosulfonate; and the moisture-absorbing and restoring agent is anhydrous sodium sulfate.

[0010] Preferably, the mixing and grinding time in step (1) is 15 to 30 minutes.

[0011] Preferably, the heat preservation time of the calcining kiln in step (2) is 1~2 hours.

[0012] Preferably, the mixing and grinding in step (1) is carried out in a ball mill, and the mass ratio of grinding media to material is (2~5):1. Specifically, the grinding media is one or more of steel balls, ceramic balls, or zirconia balls, and steel balls are preferred in this invention.

[0013] Preferably, the aging process in step (3) is carried out in an environment with a relative humidity of 50% to 80%. Controlling the humidity range of the aging environment is beneficial for the stable absorption of moisture by the moisture-absorbing and restoring agent, anhydrous sodium sulfate, and ensures the uniformity of hydration restoration.

[0014] Preferably, the final product is phosphogypsum, with a standard consistency water content of 45%~50% and a 2-hour compressive strength ≥6.5MPa. Phosphogypsum is a building gypsum made from phosphogypsum, and its main component is β-hemihydrate calcium sulfate.

[0015] The beneficial effects of this invention are: This invention utilizes the 15%~25% attached water of the original dihydrate phosphogypsum as a reaction medium, eliminating the need for traditional large-scale water washing and pre-drying processes. By introducing a composite modifier, a pre-coating layer is formed on the surface of the phosphogypsum particles in a micro-aqueous environment, directly passivating and solidifying free phosphorus and fluorine impurities. During the calcination and post-treatment stages, this pre-coating layer increases the resistance to the outward discharge of internal crystal water, making the dehydration process smoother. Combined with the hygroscopic adjusting agent added to the powder in the aging stage, which adsorbs moisture from the air and forms a localized humid environment around the particles, these two factors promote the conversion of a small amount of type III anhydrous gypsum that may be generated during calcination into hemihydrate gypsum, effectively suppressing the over-burned phase of type III anhydrous gypsum that easily causes quality fluctuations. The final prepared phosphogypsum has a standard consistency water content of 45%~50% and a 2-hour compressive strength ≥6.5MPa, meeting and exceeding the national standard requirements for building gypsum GB / T 9776-2008. Attached Figure Description

[0016] Figure 1 Flowchart of a stepwise modified calcination preparation method for phosphogypsum solid waste; Figure 2 SEM image of uncoated phosphogypsum dihydrate from Comparative Example 1; Figure 3 Here is a SEM image of the pre-coated phosphogypsum from Example 2; Figure 4The graphs show the performance comparison of the products prepared in Examples 1-3 and Comparative Examples 1-3. Detailed Implementation

[0017] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] Example 1 This embodiment provides a stepwise modified calcination preparation method for phosphogypsum solid waste, comprising the following steps: (1) Pre-coating treatment: Take the raw material of dihydrate phosphogypsum and test it. The mass fraction of the adhering water is 15%. Prepare a composite modifier with the following composition by weight: 50 parts quicklime, 10 parts aluminum sulfate, 5 parts sodium lignosulfonate, and 5 parts anhydrous sodium sulfate. The total amount of modifier added is 1.4% based on the dry weight of the raw phosphogypsum. Put the above materials into a ball mill together. The mass ratio of steel balls to materials is 2:1. Mix and grind for 15 minutes to form a pre-coating layer on the surface of the phosphogypsum particles with the modifier, and obtain pre-coated phosphogypsum.

[0019] (2) The obtained pre-coated phosphogypsum is fed into a calcining kiln and the calcination temperature is controlled at 165℃. The temperature is maintained for 2 hours to complete the dehydration reaction and obtain hemihydrate gypsum powder.

[0020] (3) The obtained hemihydrate gypsum powder is naturally cooled to room temperature and placed in an environment with a relative humidity of 50% for 48 hours to age. The powder components absorb moisture from the air to carry out a hydration reaction, and the final product is obtained.

[0021] Example 2 This embodiment provides a stepwise modified calcination preparation method for phosphogypsum solid waste, comprising the following steps: (1) Pre-coating treatment: Take the raw phosphogypsum dihydrate and test it. The water content of the adhering material is 20%. Prepare a composite modifier with the following composition by weight: 65 parts quicklime, 18 parts aluminum sulfate, 10 parts sodium lignosulfonate, and 7 parts anhydrous sodium sulfate. The total amount of modifier added is 2% based on the dry weight of the raw phosphogypsum. Put the above materials into a ball mill with a steel ball to material mass ratio of 4:1 and grind for 22 minutes to form a pre-coating layer on the surface of the phosphogypsum particles, thus obtaining pre-coated phosphogypsum. The SEM image of the pre-coated phosphogypsum of this invention is shown below. Figure 3 As shown (2) The obtained pre-coated phosphogypsum is fed into a calcining kiln and the calcination temperature is controlled at 172°C. The temperature is maintained at this temperature for 1.5 hours to complete the dehydration reaction and obtain hemihydrate gypsum powder.

[0022] (3) The obtained hemihydrate gypsum powder was naturally cooled to room temperature and then placed in an environment with a relative humidity of 65% for 36 hours to undergo a hydration reaction by absorbing moisture from the air, thus obtaining the final product. The preparation method is shown in the process flow diagram. Figure 1 As shown.

[0023] Example 3 This embodiment provides a stepwise modified calcination preparation method for phosphogypsum solid waste, comprising the following steps: (1) Pre-coating treatment: Take the raw material of dihydrate phosphogypsum and test it. The mass fraction of the adhering water is 25%. Prepare a composite modifier with the following composition by weight: 80 parts quicklime, 25 parts aluminum sulfate, 15 parts sodium lignosulfonate, and 10 parts anhydrous sodium sulfate. The total amount of modifier added is 3% based on the dry weight of the raw phosphogypsum. Put the above materials into a ball mill together. The mass ratio of steel balls to materials is 5:1. Mix and grind for 30 minutes to form a pre-coating layer on the surface of the phosphogypsum particles with the modifier, and obtain pre-coated phosphogypsum.

[0024] (2) The obtained pre-coated phosphogypsum is fed into a calcining kiln and the calcination temperature is controlled at 180°C. The temperature is maintained for 1 hour to complete the dehydration reaction and obtain hemihydrate gypsum powder.

[0025] (3) The obtained hemihydrate gypsum powder is naturally cooled to room temperature and placed in an environment with a relative humidity of 80% for 24 hours to age. The powder components absorb moisture from the air to carry out a hydration reaction, and the final product is obtained.

[0026] Comparative Example 1 This comparative example simulates the traditional direct calcination process without coating or aging. The SEM image of the uncoated undisturbed dihydrate phosphogypsum of this invention is shown below. Figure 2 As shown.

[0027] Untreated phosphogypsum dihydrate with an adsorbed water content of 20% was directly fed into a calcining kiln without any modifiers. The calcination temperature was controlled at 172℃ and held for 1.5 hours. After exiting the kiln, it was not aged but directly cooled before performance testing.

[0028] Comparative Example 2 The preparation process of this comparative example is exactly the same as that of Example 2, except that aluminum sulfate is not added to the composite modifier. The formula is adjusted to: 65 parts quicklime, 10 parts sodium lignosulfonate, and 7 parts anhydrous sodium sulfate (the total amount of modifier added is still 2.0%). Other components and process parameters remain unchanged.

[0029] Comparative Example 3 The preparation process of this comparative example is exactly the same as that of Example 2, except that anhydrous sodium sulfate is not added to the composite modifier. The formula is adjusted to: 65 parts quicklime, 18 parts aluminum sulfate, and 10 parts sodium lignosulfonate (the total amount of modifier added is still 2.0%). Other components and process parameters remain unchanged.

[0030] Actual test The final products prepared in Examples 1-3 and Comparative Examples 1-3 were tested according to the national standard GB / T 9776-2008 "Building Gypsum". The standard consistency water requirement was determined using a consistency meter, the setting time using a Vicat apparatus, and the mechanical strength using a flexural and compressive strength testing machine. Simultaneously, 50g of sample was weighed, soaked in distilled water for 24 hours, and then dried at 40℃±4℃ until constant weight (constant weight is defined as the difference between two weighings with a 1-hour drying time interval not exceeding 0.05g). The sample was then crushed, passed through a 0.2mm sieve, and the water of crystallization content was determined according to Chapter 8 of GB / T 5484-2000. The hemihydrate calcium sulfate content was obtained by multiplying the measured water of crystallization content by 4.0278. The data and effects of Examples 1-3 and Comparative Examples 1-3 are shown in [the table below]. Figure 4 As shown in Table 1, the data is as follows.

[0031] Table 1

[0032] As can be seen from the table, the phosphogypsum prepared using the method of the examples has better performance compared to the comparative example. The performance defects of Comparative Example 1 are mainly due to the lack of treatment of soluble impurities and the generation of a large amount of type III anhydrous gypsum during calcination. The examples, by forming a pre-coating layer on the surface of the phosphogypsum particles, achieved in-situ solidification of impurities and regulated the calcination dehydration process, thereby inhibiting the generation of type III anhydrous gypsum and transforming the main crystalline phase into hemihydrate gypsum. Comparing Example 2 with Comparative Example 2 (lacking aluminum sulfate), it can be seen that the lack of aluminum sulfate will lead to an increase in the standard consistency water requirement, a decrease in strength, and a shortened setting time. The aluminum ions provided by aluminum sulfate react with the fluoride ions in phosphogypsum and the calcium ions provided by the alkaline neutralizing agent to generate insoluble calcium fluoroaluminate salts, which coat the surface of the phosphogypsum. The lack of aluminum sulfate weakens the regulation of the dehydration rate, resulting in an increase in type III anhydrous gypsum and thus deteriorating the performance. Comparative Example 2 and Comparative Example 3 (without anhydrous sodium sulfate) show that the absence of anhydrous sodium sulfate increases the standard consistency water requirement, decreases the strength, and places the setting time in the middle range. This indicates that during the aging stage, it promotes the conversion of trace amounts of type III anhydrous gypsum and the secondary solidification of residual impurities through hygroscopic action; the absence of this component will affect the stability of the final phase composition. The hemihydrate calcium sulfate content measured in Comparative Example 1 was only 52.4%. Due to the lack of thermal buffer protection from the pre-coating layer, the dehydration rate of phosphogypsum during calcination was out of control. This not only produced Type III anhydrous gypsum with extremely high water demand and loose particle structure, but also caused some material to transform into Type II anhydrous gypsum (dead calcined phase) that completely lost its hydration activity due to severe local overheating. The dead calcined phase could not be hydrated during the 24-hour immersion test, resulting in a significant decrease in the final measured effective main crystalline phase content. The hemihydrate content of Comparative Example 2 decreased to 61.6%, which proves that without aluminum sulfate, a dense calcium fluoroaluminate coating layer could not be formed on the surface of the phosphogypsum particles. The weakened physical barrier function of the coating layer led to partial overburning. The purity of Comparative Example 3 decreased to 66.3%, which proves that without the moisture-absorbing back-regulating agent, the trace amount of Type III anhydrous gypsum generated during calcination could not be converted back to hemihydrate gypsum through gas phase capture during the aging stage. The residue in the system affected the final purity.

[0033] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A stepwise modified calcination preparation method for phosphogypsum solid waste, characterized in that, Includes the following steps: (1) Pre-coating treatment: The original dihydrate phosphogypsum containing attached water is mixed and ground with a composite modifier to obtain pre-coated phosphogypsum; (2) Temperature-controlled calcination: The pre-coated phosphogypsum obtained in step (1) is fed into a calcining kiln and dehydrated at a temperature of 165℃~180℃ to obtain hemihydrate gypsum powder. (3) Moisture absorption and aging: The hemihydrate gypsum powder obtained in step (2) is cooled and aged for 24h~48h. The components in the powder absorb moisture from the air to carry out a hydration reaction, and the final product is obtained.

2. The preparation method according to claim 1, characterized in that, The adsorbed water mass fraction of the undisturbed dihydrate phosphogypsum described in step (1) is 15%~25%.

3. The preparation method according to claim 1, characterized in that, The composite modifier in step (1) is composed of the following raw materials in parts by weight: 50-80 parts of alkaline neutralizer, 10-25 parts of surface coating agent, 5-15 parts of film-forming aid, and 5-10 parts of moisture-absorbing and restoring agent; and the amount of the composite modifier added is 1.4%-3.0% of the dry basis mass of the original dihydrate phosphogypsum.

4. The preparation method according to claim 3, characterized in that, The alkaline neutralizing agent is one or more of quicklime or carbide slag; the surface coating agent is aluminum sulfate.

5. The preparation method according to claim 3, characterized in that, The film-forming aid is sodium lignosulfonate; the moisture-absorbing and reversing agent is anhydrous sodium sulfate.

6. The preparation method according to claim 1, characterized in that, The mixing and grinding time in step (1) is 15~30 min.

7. The preparation method according to claim 1, characterized in that, Step (2) The heat preservation time of the calcining kiln is 1~2 hours.

8. The preparation method according to claim 1, characterized in that, The mixing and grinding described in step (1) is carried out in a ball mill, and the mass ratio of grinding media to material is (2~5):

1.

9. The preparation method according to claim 1, characterized in that, The aging process described in step (3) is carried out in an environment with a relative humidity of 50% to 80%.

10. The preparation method according to claim 1, characterized in that, The final product is phosphorus-based building gypsum, with a standard consistency water requirement of 45%~50% and a 2-hour compressive strength ≥6.5MPa.

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