Method for preparing anhydrous calcium sulfate by ammonium salt cooperating with phosphogypsum fluidization low-temperature crystal transformation

Abstract

The application provides a method for preparing anhydrous calcium sulfate by ammonium salt synergistic phosphogypsum fluidization low-temperature crystal transformation, which comprises the following steps: mixing phosphogypsum raw materials and ammonium salt, and then passing in 50-400 mL / min of mixed gas of nitrogen and oxygen to obtain fluidized materials, and then performing fluidized roasting, under the mixed gas of nitrogen and oxygen, heating to 200-350 DEG C, constant temperature roasting, stopping the passing in of oxygen, increasing the flow rate of nitrogen to be consistent with the flow rate of the mixed gas of nitrogen and oxygen, cooling with the furnace under nitrogen, and obtaining fluidized roasting products; increasing the flow rate of nitrogen to 500-800 mL / min, and then performing high-speed airflow sorting, and then the silicon dioxide is left in the fluidized bed layer, the anhydrous calcium sulfate is taken out by the nitrogen, and is collected in a cyclone classifier to obtain anhydrous calcium sulfate. The application can effectively solve the problems of high energy consumption at high temperature, low heat and mass transfer efficiency in the traditional calcination process, and the problems of difficult purification of impurity elements, low product purity and the like in the wet crystal transformation process.

Description

Technical Field

[0001] This invention belongs to the field of resource utilization technology of phosphogypsum, specifically relating to a method for preparing anhydrous calcium sulfate by low-temperature fluidized bed crystallization of phosphogypsum in conjunction with ammonium salts. Background Technology

[0002] Phosphoric acid is an important raw material for the production of phosphate fertilizers. The wet process for producing phosphoric acid generates a large amount of phosphogypsum. On average, producing 1 ton of phosphoric acid produces approximately 4.5 to 5.5 tons of phosphogypsum. Its main component is calcium sulfate dihydrate (CaSO4·2H2O), and it contains inorganic impurities such as silicon, phosphorus, fluorine, and iron, as well as organic impurities such as humic substances and flotation reagents. Its complex state makes it difficult to utilize directly. According to incomplete statistics, my country's annual phosphogypsum production is approximately 75 million tons, with a comprehensive utilization rate of only 45%, and the accumulated stockpile exceeds 830 million tons. The stockpiling of phosphogypsum not only causes serious harm to the ecological environment but also restricts the development of phosphate chemical enterprises. Therefore, it is urgent to achieve large-scale resource utilization of bulk solid waste phosphogypsum, which is of vital practical significance for maintaining the sustainable development of my country's agriculture.

[0003] Phosphogypsum has a wide range of applications, including as a cement retarder, agricultural soil conditioner, and in the production of building gypsum board. With my country's increasing emphasis on environmental protection and resource utilization, the large-scale, resource-based, and high-value utilization of phosphogypsum has attracted widespread attention. Currently, the preparation of anhydrous calcium sulfate from phosphogypsum is an important direction for achieving its high-value utilization, with the main preparation processes being pyrometallurgical calcination and wet crystallization. While the traditional pyrometallurgical calcination process is technically mature, it generally suffers from drawbacks such as high energy consumption at high temperatures, large footprint, and low heat and mass transfer efficiency due to the use of rotary kilns and fluidized bed furnaces. Furthermore, although the wet crystallization process offers milder reaction conditions and avoids the high energy consumption associated with high-temperature processes, it also faces challenges such as difficulty in purifying impurities in the reaction system and controlling product purity. In summary, these problems severely restrict the large-scale resource utilization of phosphogypsum towards high-quality anhydrous calcium sulfate. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a method for preparing anhydrous calcium sulfate by fluidized bed conversion of ammonium salt and phosphogypsum in response to the shortcomings of the prior art. This method can effectively solve the drawbacks of the traditional pyrometallurgical calcination process, such as high high-temperature energy consumption, low heat and mass transfer efficiency, and the difficulty in purifying impurity elements and low product purity in the wet conversion process.

[0005] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a method for preparing anhydrous calcium sulfate by fluidized bed transformation of ammonium salt and phosphogypsum at low temperature, the method being as follows:

[0006] S1. After mixing phosphogypsum raw material and ammonium salt, place it inside a fluidized bed. Introduce a mixed gas of nitrogen and oxygen at a flow rate of 50 mL / min to 400 mL / min through the air inlet at the bottom of the fluidized bed to obtain fluidized material.

[0007] S2. The fluidized material obtained in S1 is subjected to fluidized roasting to obtain a fluidized roasting product.

[0008] The fluidized bed roasting process is as follows: under a mixed gas atmosphere of nitrogen and oxygen with a flow rate of 50 mL / min to 400 mL / min, the temperature is increased from room temperature to 200℃ to 350℃ at a heating rate of 5℃ / min to 15℃ / min, and roasted at a constant temperature for 10 min to 40 min. Then, the oxygen supply is stopped, the flow rate of nitrogen is increased to match the flow rate of the mixed gas of nitrogen and oxygen, and the furnace is cooled to 25℃ to 50℃ under a nitrogen atmosphere.

[0009] This step enables fluidized low-temperature crystallization and simultaneous impurity removal. By introducing ammonium salts during fluidized calcination, this invention achieves "low-temperature crystallization and simultaneous purification and impurity removal" of anhydrous calcium sulfate. The acidic medium and gases (NH3, H2SO4) released by the thermal decomposition of ammonium salts at lower temperatures can disrupt the lattice stability of calcium sulfate dihydrate (CaSO4·2H2O), reducing the crystal transformation activation energy, thereby achieving low-temperature crystallization of phosphogypsum. Simultaneously, the thermal decomposition products of ammonium salts react with impurities such as F, P, Si, and organic matter in phosphogypsum to generate volatile products such as HF, P2O5, and SiF4, thus achieving simultaneous impurity removal under low-temperature conditions. This effectively improves the product performance (purity and whiteness) and yield of anhydrous calcium sulfate, while shortening the impurity removal process.

[0010] S3. Increase the flow rate of nitrogen to 500 mL / min ~ 800 mL / min, and perform high-speed gas flow separation on the fluidized calcination product obtained in S2. Silica remains inside the fluidized bed, while anhydrous calcium sulfate is carried out by the nitrogen and collected in a cyclone classifier to obtain anhydrous calcium sulfate, thereby achieving further purification of the anhydrous calcium sulfate generated by crystallization.

[0011] Preferably, the ammonium salt in S1 is one or more of ammonium sulfate, ammonium bisulfate, and ammonium persulfate.

[0012] Preferably, the mass ratio of the phosphogypsum raw material and the ammonium salt in S1 is 1: (3.50% to 10.00%).

[0013] Preferably, the volume fraction of oxygen in the nitrogen and oxygen mixture in S1 and S2 is 5% to 50%; the introduction time of the nitrogen and oxygen mixture in S1 is 2 min to 15 min.

[0014] Compared with the prior art, the present invention has the following advantages:

[0015] 1. Compared with traditional pyrometallurgical calcination furnaces (rotary kilns, fluidized bed furnaces), this invention employs a fluidized bed calcination method, which enables full contact between the fluid material and the gas, resulting in a uniform temperature field distribution and avoiding problems such as particle agglomeration and localized overburning. Therefore, the method proposed in this invention can effectively shorten the crystallization time of phosphogypsum, reduce high-temperature energy consumption, and greatly improve the reaction rate.

[0016] 2. By introducing ammonium salts during the fluidized bed roasting process, this invention achieves "low-temperature crystallization and simultaneous purification" of anhydrous calcium sulfate. The acidic medium and gases (NH3, H2SO4) released by the thermal decomposition of ammonium salts at lower temperatures can disrupt the lattice stability of calcium sulfate dihydrate (CaSO4·2H2O), reducing the activation energy of crystal transformation, thereby achieving low-temperature crystallization of phosphogypsum. Simultaneously, the thermal decomposition products of ammonium salts react with impurities such as F, P, Si, and organic matter in phosphogypsum to generate volatile products such as HF, P2O5, and SiF4, thus achieving simultaneous impurity removal under low-temperature conditions. This not only effectively improves the product performance (purity and whiteness) and yield of anhydrous calcium sulfate but also shortens the impurity removal process.

[0017] 3. In the further separation process of the roasted materials, this invention increases the fluidizing gas flow rate. Under a high-speed airflow field, it utilizes the significant difference in particle size distribution between anhydrous calcium sulfate and silica to achieve efficient separation of fine-grained anhydrous calcium sulfate and coarse-grained silica. This process not only further improves the silica removal effect but also avoids the adverse effects of silica on product performance in subsequent applications. Through graded impurity removal, the obtained anhydrous calcium sulfate has higher purity, whiteness, and more uniform particle size distribution, realizing the high-value application of phosphogypsum.

[0018] The present invention will be further described in detail below with reference to the embodiments. Detailed Implementation

[0019] Example 1

[0020] The phosphogypsum raw material in this embodiment comes from a certain place in Sichuan Province. The mass fraction of particles with a size ≤74μm is 90%, and the total calcium content is 21.10%, the total sulfur content is 18.52%, the total fluorine content is 0.085%, the total phosphorus content is 0.36%, and the total organic matter content in the sample is 9.80g / kg. The ammonium salt is ammonium bisulfate, with a mass fraction of particles with a size ≤74μm of 90%.

[0021] The fluidized bed roasting equipment in this embodiment was purchased commercially from Hefei Kejing, model 0TF-1200X-S-VT.

[0022] The method for preparing anhydrous calcium sulfate by fluidized bed transformation of ammonium salts and phosphogypsum at low temperature in this embodiment is as follows:

[0023] S1. After mixing the phosphogypsum raw material and ammonium salt (ammonium bisulfate), place it inside the fluidized bed. Introduce a mixed gas of nitrogen and oxygen (oxygen volume fraction of 30%) at a flow rate of 150 mL / min through the air inlet at the bottom of the fluidized bed for 6 minutes. The material is in a good fluidized state, and fluidized material is obtained.

[0024] The mass ratio of the phosphogypsum raw material to the ammonium salt is 1:3.50%;

[0025] S2. The fluidized material obtained in S1 is subjected to fluidized roasting to obtain a fluidized roasting product.

[0026] The fluidized bed roasting process is as follows: In an atmosphere of nitrogen and oxygen mixture (oxygen volume fraction of 30%) with a flow rate of 150 mL / min, the temperature is increased from room temperature to 300°C at a heating rate of 12°C / min, and roasted at a constant temperature for 30 min. Then, the oxygen supply system is turned off, the oxygen supply is stopped, the flow rate of nitrogen is increased to match the flow rate of the nitrogen and oxygen mixture, and the furnace is cooled to 30°C in a nitrogen atmosphere.

[0027] This step enables fluidized low-temperature crystallization and simultaneous impurity removal because ammonium bisulfate, which begins to decompose at a relatively low temperature (around 200°C), releases acidic media and gases (NH3, H2SO4). These gases disrupt the lattice stability of calcium sulfate dihydrate (CaSO4·2H2O), lowering the activation energy for crystal transformation. This allows calcium sulfate dihydrate (CaSO4·2H2O) to lose its molecular water and become anhydrous calcium sulfate (CaSO4), thus achieving low-temperature crystallization of phosphogypsum. Simultaneously, these decomposition products (NH3, H2SO4) react with impurities such as F, P, Si, and organic matter in the phosphogypsum to form volatile products such as HF, P2O5, and SiF4, achieving simultaneous impurity removal. This not only effectively improves the product performance (purity and whiteness) and yield of anhydrous calcium sulfate but also shortens the impurity removal process.

[0028] S3. Increase the flow rate of nitrogen to 800 mL / min and perform high-speed airflow separation on the fluidized calcined product obtained in S2. Coarse-grained silica (above +625 mesh) remains inside the fluidized bed, while fine-grained anhydrous calcium sulfate (below -800 mesh) is carried out by the nitrogen and collected in a cyclone classifier to obtain anhydrous calcium sulfate, thereby achieving further impurity removal of the anhydrous calcium sulfate generated by crystal transformation.

[0029] The anhydrous calcium sulfate obtained after high-speed airflow separation has a calcium sulfate (CaSO4) content of 96.55%, a whiteness of 93.12%, a total fluorine content of 0.009%, a total phosphorus content of 0.031%, and an organic matter content of 0.05 g / kg.

[0030] Example 2

[0031] The phosphogypsum raw material in this embodiment comes from a certain place in Yunnan Province. The mass fraction of particles ≤74μm is 50%, of which the total calcium content is 20.14%, the total sulfur content is 16.48%, the total fluorine content is 0.102%, the total phosphorus content is 0.62%, and the organic matter content is 10.72 g / kg. The ammonium salt is ammonium sulfate, with a mass fraction of particles ≤74μm being 50%.

[0032] The fluidized bed roasting equipment in this embodiment is commercially available, model 0TF-1200X-S-VT.

[0033] The method for preparing anhydrous calcium sulfate by fluidized bed transformation of ammonium salts and phosphogypsum at low temperature in this embodiment is as follows:

[0034] S1. After mixing the phosphogypsum raw material and ammonium salt (ammonium sulfate), place it inside the fluidized bed. Introduce a mixture of nitrogen and oxygen (oxygen volume fraction of 5%) at a flow rate of 400 mL / min through the air inlet at the bottom of the fluidized bed for 2 minutes. The material is in a good fluidized state, and fluidized material is obtained.

[0035] The mass ratio of the phosphogypsum raw material to the ammonium salt is 1:7.5%;

[0036] S2. The fluidized material obtained in S1 is subjected to fluidized roasting to obtain a fluidized roasting product.

[0037] The fluidized bed roasting process is as follows: In an atmosphere of nitrogen and oxygen mixture (oxygen volume fraction of 5%) with a flow rate of 400 mL / min, the temperature is increased from room temperature to 350°C at a heating rate of 10°C / min, and roasted at a constant temperature for 10 min. Then, the oxygen supply system is turned off, the oxygen supply is stopped, the flow rate of nitrogen is increased to match the flow rate of the nitrogen and oxygen mixture, and the furnace is cooled to 50°C in a nitrogen atmosphere.

[0038] S3. Increase the flow rate of nitrogen to 500 mL / min and perform high-speed airflow separation on the fluidized calcined product obtained in S2. Coarse-grained silica (above +625 mesh) remains inside the fluidized bed, while fine-grained anhydrous calcium sulfate (below -1000 mesh) is carried out by the nitrogen and collected in a cyclone classifier to obtain anhydrous calcium sulfate.

[0039] The anhydrous calcium sulfate obtained after high-speed airflow separation has a calcium sulfate (CaSO4) content of 97.85%, a whiteness of 94.58%, a total fluorine content of 0.0085%, a total phosphorus content of 0.045%, and an organic matter content of 0.076 g / kg.

[0040] Example 3

[0041] The phosphogypsum raw material in this embodiment comes from a certain place in Guizhou Province. The mass fraction of particles ≤74μm is 80%, of which the total calcium content is 19.82%, the total sulfur content is 18.79%, the total fluorine content is 0.073%, the total phosphorus content is 0.43%, and the organic matter content is 8.78g / kg. The ammonium salt is ammonium persulfate, with a mass fraction of particles ≤74μm being 90%.

[0042] The fluidized bed roasting equipment in this embodiment is commercially available, model 0TF-1200X-S-VT.

[0043] The method for preparing anhydrous calcium sulfate by fluidized bed transformation of ammonium salts and phosphogypsum at low temperature in this embodiment is as follows:

[0044] S1. After mixing the phosphogypsum raw material and ammonium salt (ammonium persulfate), place it inside the fluidized bed. Introduce a mixture of nitrogen and oxygen (oxygen volume fraction of 50%) at a flow rate of 50 mL / min through the air inlet at the bottom of the fluidized bed for 15 min. The material is in a good fluidized state, and fluidized material is obtained.

[0045] In this embodiment, the ammonium salt may also be one or more of ammonium sulfate, ammonium bisulfate, and ammonium persulfate;

[0046] The mass ratio of the phosphogypsum raw material to the ammonium salt is 1:10.00%;

[0047] S2. The fluidized material obtained in S1 is subjected to fluidized roasting to obtain a fluidized roasting product.

[0048] The fluidized bed roasting process is as follows: In a mixed gas atmosphere of nitrogen and oxygen (oxygen volume fraction of 50%) with a flow rate of 50 mL / min, the temperature is increased from room temperature to 200°C at a heating rate of 5°C / min, and roasted at a constant temperature for 40 min. Then, the oxygen supply system is turned off, the oxygen supply is stopped, the flow rate of nitrogen is increased to match the flow rate of the mixed gas of nitrogen and oxygen, and the furnace is cooled to 25°C in a nitrogen atmosphere.

[0049] S3. Increase the flow rate of nitrogen to 500 mL / min and perform high-speed airflow separation on the fluidized calcined product obtained in S2. Coarse-grained silica (above +625 mesh) remains inside the fluidized bed, while fine-grained anhydrous calcium sulfate (below -1200 mesh) is carried out by the nitrogen and collected in a cyclone classifier to obtain anhydrous calcium sulfate.

[0050] The anhydrous calcium sulfate obtained after high-speed airflow separation has a calcium sulfate (CaSO4) content of 98.12%, a whiteness of 93.45%, a total fluorine content of 0.006%, a total phosphorus content of 0.024%, and an organic matter content of 0.061 g / kg.

[0051] Comparative Example 1

[0052] The phosphogypsum raw material and ammonium salt in this embodiment are the same as in Example 1.

[0053] The traditional horizontal tube furnace roasting equipment in this comparative example is model OTF-1200X.

[0054] The method for preparing anhydrous calcium sulfate by fluidized bed conversion of ammonium salts and phosphogypsum in this comparative example is the same as that in Example 1, except that the fluidized bed roasting equipment in Example 1 is replaced with a traditional horizontal tube furnace roasting.

[0055] In this comparative example, the materials were piled up in a quartz magnet, resulting in a less uniform temperature field compared to fluidized bed roasting. This led to uneven heating, lower heat and mass transfer efficiency, and a tendency for some phosphogypsum to fail to complete crystallization. Consequently, the roasted product contained a large amount of hemihydrate gypsum (CaSO4·0.5H2O) in addition to anhydrous calcium sulfate. Furthermore, the temperature field resulted in poor material permeability, hindering the migration of impurities and leading to a higher impurity content and lower removal rate (total fluorine content: 0.046%, total phosphorus content: 0.31%, organic matter content: 8.4 g / kg). Therefore, under the same roasting conditions (300℃, 30 min), the conventional horizontal tube furnace roasting of phosphogypsum in this comparative example was less effective than the fluidized bed roasting in Example 1 in terms of both crystallization and impurity removal.

[0056] Comparative Example 2

[0057] The method for preparing anhydrous calcium sulfate by fluidized bed transformation of phosphogypsum in this comparative example is the same as in Example 1, except that no ammonium salt additive is added.

[0058] In this comparative example, under the same calcination regime (300℃, 30 min), the acidic gases released from the thermal decomposition of ammonium salt additives did not lower the activation energy of phosphogypsum lattice transformation. Therefore, under the above calcination regime, the phosphogypsum did not complete the crystal transformation, and the calcined product did not contain anhydrous calcium sulfate, but still contained a small amount of hemihydrate gypsum (CaSO4·0.5H2O) and a large amount of dihydrate calcium sulfate. Impurities were almost not removed (total fluorine content was 0.080%, total phosphorus content was 0.35%, and organic matter content was 8.75 g / kg). This is because the acidic gases released from the thermal decomposition of ammonium salt additives easily react with impurities (F, P, and Si) to generate volatile substances such as HF, P2O5, and SiF4, thus improving the impurity removal efficiency. Therefore, under the same calcination regime (300℃, 30 min), without the synergistic effect of ammonium salt additives on the fluidized bed low-temperature crystal transformation of phosphogypsum, the crystal transformation effect and impurity removal effect are inferior to those of Example 1.

[0059] This invention can effectively solve the drawbacks of the traditional pyrometallurgical calcination process, such as high energy consumption at high temperatures, low heat and mass transfer efficiency, and the difficulty in purifying impurity elements and low product purity in the wet crystallization process.

[0060] The fluidized bed calcination method used in this invention allows the introduction of gas to loosen and uniformly distribute phosphogypsum within the fluidized bed. Under good fluidization conditions, the heat and mass transfer efficiency between particles is greatly improved, and the calcination products can be effectively prevented from caking and sticking together. This can also effectively prevent the deterioration of reaction kinetics, thereby increasing the reaction rate.

[0061] This invention incorporates ammonium salts in synergistic fluidized bed calcination of phosphogypsum, which helps to further reduce the temperature for removing water of crystallization and simultaneously facilitates the escape of impurities such as F, P, Si, and organic matter, thereby improving the purity and whiteness of the product and achieving an integrated process of "low-temperature crystallization + simultaneous purification" of phosphogypsum. Finally, with the volatilization and removal of most impurities, the material in the fluidized bed mainly consists of silica and anhydrous calcium sulfate, with a significant difference in particle size distribution: coarse particles are predominantly silica, while fine particles are predominantly anhydrous calcium sulfate. Based on this physical characteristic, by controlling the flow rate of the fluidizing gas inside the fluidized bed, finer calcium sulfate particles can be carried by the high-speed airflow into a cyclone classifier for collection, achieving efficient capture of the fine calcium sulfate powder carried out by the airflow, while coarse silica particles remain in the bed, thus separating the two and obtaining a high-quality calcium sulfate product. In summary, this invention has significant advantages: a simple process flow, low equipment energy consumption, and solves the technical drawbacks of traditional pyrometallurgical calcination and wet crystallization processes, which is conducive to promoting the large-scale industrialization of phosphogypsum resource utilization.

[0062] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any way. Any simple modifications, alterations, and equivalent changes made to the above embodiments based on the inventive essence shall still fall within the protection scope of the present invention.

Claims

1. A method for preparing anhydrous calcium sulfate by fluidized bed transformation of ammonium salt and phosphogypsum at low temperature, characterized in that, The method is as follows: S1. After mixing the phosphogypsum raw material and ammonium salt, the mixture is placed inside a fluidized bed. A mixture of nitrogen and oxygen gas with a flow rate of 50 mL / min to 400 mL / min is introduced from the air inlet at the bottom of the fluidized bed to obtain a fluidized material. The ammonium salt is one or more of ammonium sulfate, ammonium bisulfate, and ammonium persulfate. S2. The fluidized material obtained in S1 is subjected to fluidized roasting to obtain a fluidized roasting product. The fluidized bed roasting process is as follows: under a mixed gas atmosphere of nitrogen and oxygen with a flow rate of 50 mL / min to 400 mL / min, the temperature is increased from room temperature to 200℃ to 350℃ at a heating rate of 5℃ / min to 15℃ / min, and roasted at a constant temperature for 10 min to 40 min. Then, the oxygen supply is stopped, the flow rate of nitrogen is increased to match the flow rate of the mixed gas of nitrogen and oxygen, and the furnace is cooled to 25℃ to 50℃ under a nitrogen atmosphere. S3. Increase the flow rate of nitrogen to 500 mL / min~800 mL / min and perform high-speed airflow separation on the fluidized calcination product obtained in S2. Silica remains inside the fluidized bed, while anhydrous calcium sulfate is carried out by the nitrogen and collected in a cyclone classifier to obtain anhydrous calcium sulfate.

2. The method for preparing anhydrous calcium sulfate by fluidized bed transformation of ammonium salt and phosphogypsum at low temperature according to claim 1, characterized in that, The mass ratio of phosphogypsum raw material to ammonium salt in S1 is 1: (3.50% to 10.00%).

3. The method for preparing anhydrous calcium sulfate by fluidized bed transformation of ammonium salt and phosphogypsum at low temperature according to claim 1, characterized in that, The volume fraction of oxygen in the nitrogen and oxygen mixture described in S1 and S2 is 5% to 50%; the introduction time of the nitrogen and oxygen mixture described in S1 is 2 min to 15 min.