Method for comprehensively utilizing phosphogypsum and byproduct ammonium gypsum

Abstract

The invention relates to a method for comprehensively utilizing ardealite and producing ammonium gypsum as a byproduct, and belongs to the technical field of industrial solid waste resource utilization. The method for comprehensively utilizing the ardealite and producing the ammonium gypsum as the byproduct comprises the following steps: A, pretreating the ardealite, namely aging the ardealite in air for 36-48 hours, and adding CaO which accounts for 0.5-0.7 wt% of the ardealite to neutralize the ardealite until the pH is 6.5-7.0; b, synthetic reaction: feeding the phosphogypsum, the crude ammonium sulfate and the water according to the molar ratio of 1: 1.5-2.0: 10, and stirring and reacting for 60-120 minutes at 150-300rpm to generate slurry containing ammonium gypsum; and C, water-flying separation: carrying out water-flying separation on the slurry containing the ammonium gypsum, respectively filtering an upper layer solution and a lower layer solution after water-flying separation, filtering the upper layer, drying to obtain the product ammonium gypsum, filtering the lower layer to obtain a solid phase which is insoluble impurities, and filtering to obtain a liquid which is high-temperature ammonium-rich mother liquor. The method is low in cost and environmentally friendly.

Description

Technical Field

[0001] The invention relates to a method for comprehensively utilizing by-product ammonium gypsum from phosphogypsum, and belongs to the technical field of industrial solid waste resource utilization. Background Art

[0002] Phosphogypsum is a major byproduct of wet-process phosphoric acid production, with global emissions of approximately 200-300 million tons annually. With the development of the phosphate fertilizer industry, phosphogypsum production has increased annually. Its primary component is calcium sulfate dihydrate (CaSO4·2H2O), but it contains impurities such as free acid, heavy metals (such as cadmium and lead), radioactive elements (uranium and thorium), and silicates. This makes its storage polluting and difficult to recycle. Most of it remains stored outdoors. The harmless treatment and high-value utilization of phosphogypsum can achieve a win-win situation for both environmental protection and economic benefits.

[0003] Current research on phosphogypsum focuses on removing impurities from it to ensure it meets standards for use in building materials, or modifying its surface to enhance its physical or chemical properties for various applications. Phosphogypsum reacts with other substances: Phosphogypsum reacts with potassium chloride to produce potassium sulfate and calcium chloride. It also reacts with carbon dioxide and ammonia to produce calcium carbonate and ammonium sulfate. This chemical reaction not only effectively sequesters atmospheric carbon dioxide but also produces valuable byproducts: ammonium sulfate and calcium carbonate. Both products have significant economic and industrial applications due to their high added value. However, the reaction is slow, and the resulting calcium carbonate is encapsulated or deposited onto the phosphogypsum, slowing the reaction.

[0004] CN108796612A discloses a method for producing calcium sulfate whiskers and ammonium sulfate by cyclically decomposing phosphogypsum. Nitric acid or sulfuric acid is used to decompose phosphogypsum and produce whiskers, but the method has high energy consumption and complicated waste acid treatment.

[0005] CN110049949A discloses a method for releasing impurities from calcium-based minerals. The method comprises reacting a calcium-based mineral containing impurities with an aqueous solution of one or more ionic salts at a temperature of approximately 85°C or higher, wherein at least one of the calcium-based mineral and the one or more ionic salts comprises sulfate, and at least one of the calcium-based mineral and the one or more ionic salts comprises an ammonium salt, and the concentration of the solution is approximately 25% by mass or higher, thereby forming complex salt crystals and releasing impurities. However, this method suffers from high reaction temperatures (≥85°C) and low circulation efficiency due to accumulation of impurities in the mother liquor.

[0006] CN116621212A discloses a method for preparing high-purity and high-white calcium sulfate from phosphogypsum, which adopts gravity separation, such as separation of impurities by surfactant, but has poor separation effect on fine particles and the whiteness of the product is insufficient. Summary of the Invention

[0007] The purpose of the present invention is to provide a new method for comprehensively utilizing phosphogypsum and producing by-product ammonium gypsum.

[0008] To achieve the purpose of the present invention, the method for comprehensively utilizing by-product ammonium gypsum from phosphogypsum of the present invention comprises:

[0009] A. Phosphogypsum pretreatment: aging the phosphogypsum in air for 36-48 hours, and adding 0.5-0.7 wt% of CaO to the phosphogypsum to neutralize it to a pH of 6.5-7.0;

[0010] B. Synthesis reaction: adding phosphogypsum: (NH4)2SO4 in crude ammonium sulfate: water at a molar ratio of 1:1.5-2.0:10, stirring at 150-300 rpm for 60-120 minutes to generate a slurry containing ammonium gypsum;

[0011] C. Water separation: The slurry containing ammonium gypsum is subjected to water separation. The upper and lower layer solutions after water separation are filtered separately. The upper layer is filtered and dried to obtain the product ammonium gypsum. The solid phase after filtering the lower layer is insoluble impurities, and the liquid left after filtration is the high-temperature ammonium-rich mother liquor;

[0012] Both steps B and C were carried out at a constant temperature of 75°C.

[0013] Phosphogypsum itself is in powder form, and the particle size is not very uniform, but it does not affect the effect.

[0014] In a specific embodiment, the B synthesis reaction is fed according to a molar ratio of phosphogypsum: crude ammonium sulfate: water of 1:1.5-1.8:10, and the reaction is stirred at 250-300 rpm for 90-120 minutes; the purity of the crude ammonium sulfate is preferably 67-73%.

[0015] In a specific embodiment, the purity of the ammonium gypsum in step C is above 98.4%, preferably above 98.9%.

[0016] In a specific embodiment, the method further includes: D mother liquor circulation: evaporating and concentrating the high-temperature ammonium-rich mother liquor, cooling and crystallizing, and solid-liquid separation to obtain a low-temperature ammonium-rich mother liquor and refined ammonium sulfate, and the low-temperature ammonium-rich mother liquor is returned to step B as the raw material for the next cycle.

[0017] In one embodiment, the temperature of the cooling crystallization and the temperature of the low-temperature ammonium-rich mother liquor are both 50°C.

[0018] In a specific embodiment, the cooling crystallization is a gradient crystallization with a cooling rate of 4 to 7°C / min.

[0019] In a specific embodiment, the method further includes the preparation of refined calcium sulfate: the ammonium gypsum is eluted 3 to 4 times at a solid-liquid ratio of 1:0.5 to 1, each time for 30 to 40 seconds, to obtain an eluent and calcium sulfate dihydrate with a purity of ≥99.8%.

[0020] In a specific embodiment, the method further comprises the preparation of refined calcium sulfate: the ammonium gypsum is eluted three times at a solid-liquid ratio of 1:0.5, each time for 30 seconds; preferably, the water temperature of the elution is room temperature.

[0021] In one embodiment, the method includes returning the eluent to step B as a raw material for the next cycle.

[0022] In one embodiment, the low-temperature ammonium-rich mother liquor and the eluent are returned to step B as raw materials for the next cycle for a number of cycles of 6 or less, preferably 5 to 6 times;

[0023] The purity of the ammonium gypsum obtained after the cycle is above 95.7%.

[0024] Beneficial effects:

[0025] The method of the present invention can produce high-purity ammonium gypsum, refined ammonium sulfate and refined calcium sulfate by using phosphogypsum with high impurity content and crude ammonium sulfate, which is not only low-cost but also environmentally friendly.

[0026] The method for comprehensively utilizing by-product ammonium gypsum of phosphogypsum of the present invention has low energy consumption for synthesizing ammonium gypsum at 75°C. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 A process flow chart of the present invention.

[0028] Figure 2 Phase diagram of the ternary system ammonium sulfate-calcium sulfate-water phase equilibrium experiment at 50°C and 75°C according to an embodiment of the present invention.

[0029] Figure 3 for Figure 2 A partial enlarged view of .

[0030] Figure 4 for Figure 3 XRD patterns of the four points E1, E2, F1, and F2.

[0031] Figure 5 This is the XRD pattern of the ammonium gypsum in Table 2, where 1#-12# are the ammonium gypsum samples numbered 1-12 in Table 2.

[0032] Figure 6 This is an electron microscope image of the ammonium gypsum prepared in Example 1.

[0033] Figure 7 This is the infrared image of the ammonium gypsum prepared in Example 1. DETAILED DESCRIPTION

[0034] To achieve the purpose of the present invention, the method for comprehensively utilizing by-product ammonium gypsum from phosphogypsum of the present invention comprises:

[0035] A. Phosphogypsum pretreatment: aging the phosphogypsum in air for 36-48 hours, and adding 0.5-0.7 wt% of CaO to the phosphogypsum to neutralize it to a pH of 6.5-7.0;

[0036] B. Synthesis reaction: adding phosphogypsum: (NH4)2SO4 in crude ammonium sulfate: water at a molar ratio of 1:1.5-2.0:10, stirring at 150-300 rpm for 60-120 minutes to generate a slurry containing ammonium gypsum;

[0037] C. Water separation: The slurry containing ammonium gypsum is subjected to water separation. The upper and lower layer solutions after water separation are filtered separately. The upper layer is filtered and dried to obtain the product ammonium gypsum. The solid phase after filtering the lower layer is insoluble impurities, and the liquid left after filtration is the high-temperature ammonium-rich mother liquor;

[0038] Both steps B and C were carried out at a constant temperature of 75°C.

[0039] Phosphogypsum itself is in powder form, and the particle size is not very uniform, but it does not affect the effect.

[0040] In a specific embodiment, the B synthesis reaction is fed according to a molar ratio of phosphogypsum: (NH4)2SO4 in crude ammonium sulfate: water of 1:1.5-1.8:10, and the reaction is stirred at 250-300 rpm for 90-120 minutes; the purity of the crude ammonium sulfate is preferably 67-73%.

[0041] In a specific embodiment, the purity of the ammonium gypsum in step C is above 98.4%, preferably above 98.9%.

[0042] In a specific embodiment, the method further includes: D mother liquor circulation: evaporating and concentrating the high-temperature ammonium-rich mother liquor, cooling and crystallizing, and solid-liquid separation to obtain a low-temperature ammonium-rich mother liquor and refined ammonium sulfate, and the low-temperature ammonium-rich mother liquor is returned to step B as the raw material for the next cycle.

[0043] In one embodiment, the temperature of the cooling crystallization and the temperature of the low-temperature ammonium-rich mother liquor are both 50°C.

[0044] In a specific embodiment, the cooling crystallization is a gradient crystallization with a cooling rate of 4 to 7°C / min.

[0045] In a specific embodiment, the method further includes the preparation of refined calcium sulfate: the ammonium gypsum is eluted 3 to 4 times at a solid-liquid ratio of 1:0.5 to 1, each time for 30 to 40 seconds, to obtain an eluent and calcium sulfate dihydrate with a purity of ≥99.8%.

[0046] In a specific embodiment, the method further comprises the preparation of refined calcium sulfate: the ammonium gypsum is eluted three times at a solid-liquid ratio of 1:0.5, each time for 30 seconds; preferably, the water temperature of the elution is room temperature.

[0047] In one embodiment, the method includes returning the eluent to step B as a raw material for the next cycle.

[0048] In one embodiment, the low-temperature ammonium-rich mother liquor and the eluent are returned to step B as raw materials for the next cycle for a number of cycles of 6 or less, preferably 5 to 6 times;

[0049] The purity of the ammonium gypsum obtained after the cycle is above 95.7%.

[0050] The specific embodiments of the present invention are further described below in conjunction with examples, but the present invention is not limited to the scope of the examples.

[0051] Example 1

[0052] Operation process

[0053] like Figure 1 As shown, the phosphogypsum powder was aged in air for 48 hours, the pH was neutralized by adding 0.6wt% CaO to the phosphogypsum to resolve the residual acidity, and then dried. In the primary synthesis cycle, the reaction system was established with a H2O: (NH4) 2SO4 molar ratio of 1:0.15 (mass ratio 3:2). In the subsequent low-temperature ammonium-rich mother liquor cycle, no water was added, and this parameter was eliminated in the subsequent cycle. The synthesis conditions are: reaction temperature of 75°C, phosphogypsum: crude (NH4) 2SO4 molar ratio of 1:1.5 (172:153 mass ratio), reaction time of 90 minutes under mechanical stirring at 250rpm, see the phase diagram for details. Figure 2 and Figure 3 ,Depend on Figure 2 It can be found that at 75°C, the crystalline phase region of double salt ammonium gypsum is larger than that of ammonium gypsum at 50°C. The size of the phase region due to temperature changes, to a certain extent, reflects the changes in the stability range of the solid phase. The larger the ammonium gypsum phase region, the more stable the ammonium gypsum solid phase can be in a wider range of compositions. This also means that the salt easily exists in a crystalline state under various conditions such as water-salt ratios. Its crystalline phase is relatively stable and not easily lost due to small changes in conditions. Figure 3 The key data points are detailed in Table 1:

[0054] Table 1. Phase diagram key point data

[0055]

[0056] From the phase diagram, we can see that at 75°C, not only is the solubility of gypsum greater, but the crystalline phase region of the generated ammonium gypsum is also larger.

[0057] Phase separation yields an upper stratum containing ammonium gypsum, which is then subjected to water leaching to obtain the product. The two phases are then vacuum filtered through a Buchner vacuum funnel apparatus to yield: (1) ammonium gypsum in the upper filtrate, and (2) insoluble impurities in the lower residue. The resulting mother liquor constitutes a high-temperature ammonium-rich mother liquor (T = 75°C).

[0058] The electron microscope image of ammonium gypsum is shown in Figure 6 , infrared images are available at Figure 7 , in order to better understand Figure 7 Spectra, peaks with greater absorbance are marked to confirm the presence of characteristic groups, e.g.

[0059] Sulfate (SO4) functional group: at 1080-1115 and 610-654 cm -1 The bands observed at correspond to v(SO4) tetrahedral groups.

[0060] Ammonium functional group: The wave number of wavelength 1418 corresponds to NH4 vibration.

[0061] H2O molecules, hydroxide functional groups, and hydrogen bonds: at 740 cm -1 The band at 3230 cm is attributed to the torsional vibration of water molecules. -1 There is a broad band at , which corresponds to the OH stretching mode of structural water.

[0062] Example 2

[0063] For other details, see Example 1. Some reaction conditions and experimental results are detailed in Table 2:

[0064] Table 2. Analysis of conversion rate and product purity under different conditions of phosphogypsum synthesis

[0065]

[0066] The feed ratio in Table 2 is the molar ratio of phosphogypsum to crude (NH4)2SO4. As can be seen from Table 2, at a feed ratio of phosphogypsum to ammonium sulfate of 1:1.0, the phosphogypsum reaction is incomplete, the conversion rate is low, and the product purity is not high. When the feed ratio is increased to 1:1.3, there is still excess calcium sulfate that has not reacted completely. At a feed ratio of 1:1.5, good conversion rate and purity are achieved. However, when the feed ratio is increased again to 1:1.8 and 1:2.0, due to the presence of a certain amount of ammonium sulfate in the mother liquor, some ammonium sulfate cannot be dissolved in the solution and does not participate in the synthesis. In addition, as the ammonium sulfate concentration increases, the viscosity of the liquid increases sharply, reducing the filtration efficiency of the subsequent filtration process. The optimal feed ratio for the reaction is 1:1.5.

[0067] As shown in Table 2, the reaction time and reaction conversion rate are positively correlated. With the increase of time, the conversion rate and product purity continue to increase. When the time reaches 90 minutes, it stops increasing. The optimal reaction time is 90 minutes. Ammonium gypsum is a flocculent substance floating on the upper layer of the liquid. The water-jet centrifugation method is used to separate ammonium gypsum. Different stirring rates lead to different separation conditions. If the stirring rate is too low, the ammonium gypsum cannot be fully separated from the water, resulting in a low conversion rate. If the stirring rate is too high, the insoluble impurities in the bottom layer will be separated along with the ammonium gypsum, resulting in a decrease in the purity of the ammonium gypsum. As shown in Table 2, a stirring rate of 250 r / min achieves the best separation efficiency and product purity.

[0068] Example 3

[0069] The 99.9% pure ammonium gypsum obtained in Example 1 was eluted to prepare high-purity CaSO4 crystals (99.99%) and elution liquid. The elution conditions and experimental results are detailed in Table 3:

[0070] Table 3. Purity analysis of calcium sulfate dihydrate

[0071]

[0072] As shown in Table 3, when the eluent solid-liquid ratio is 1:0.3, ammonium sulfate can not be fully dissolved in water, and gypsum purity is lower in the product, and productive rate is higher than 100%. When the eluent solid-liquid ratio is 1:0.5, purity and productive rate are relatively suitable, and when the eluent solid-liquid ratio is 1:0.8 and 1:1, the eluent is too much and the water flow is too large, causing gypsum to have a high purity but a slightly low productive rate, and using too much water. The number of rinses also has a certain impact on gypsum productive rate and gypsum purity, and the number of rinses is too few to dissolve all ammonium gypsums, and the number of rinses is too many, and gypsum productive rate decreases. The rinsing time is too few, and the ammonium gypsum dissolving is insufficient, and the rinsing time is too much, and efficiency is not high.

[0073] Example 4

[0074] High temperature mother liquor data point w((NH4)2SO4)=40.834%, w(CaSO4)=0.420%

[0075] The high-temperature ammonium-rich mother liquor obtained in Example 1 was evaporated and concentrated and crystallized at a gradient temperature of 5°C / min to obtain high-purity (NH4)2SO4 crystals (>99%) and low-temperature ammonia concentrate (T=50°C). Figure 1 As shown, the low-temperature ammonium solution and the eluent of Example 3 are recycled as raw materials for the subsequent ammonium gypsum synthesis cycle to establish a closed-loop process. The effect of the number of cycles on the purity and yield of the product is detailed in Table 4:

[0076] Table 4 Effect of cycle number on product purity and yield

[0077]

[0078] As can be seen from Table 4, as the number of cycles increases, the impurities of phosphogypsum and ammonium sulfate in the solution gradually increase, resulting in a gradual decrease in the purity of ammonium gypsum and ammonium sulfate. The gypsum conversion rate and purity are not greatly affected, and gradually stabilize after 5 and 6 cycles. The ammonium gypsum conversion rate is 98.5% and the purity is 95.7%, the gypsum conversion rate is 99.8% and the gypsum purity is 99.5%, the ammonium sulfate conversion rate is 97.7% and the ammonium sulfate purity is 95.4%.

[0079] Comparative Example 1

[0080] NH4Cl was used to replace crude (NH4)2SO4, and CaSO4-NH4Cl-H2O was studied. The input ratio of water: phosphogypsum: ammonium chloride was 1:1:2. The study found that no ammonium gypsum was produced.

Claims

1. A method for comprehensively utilizing by-product ammonium gypsum from phosphogypsum, characterized in that: The method for comprehensively utilizing by-product ammonium gypsum from phosphogypsum comprises: A. Phosphogypsum pretreatment: fully contact the phosphogypsum with air, age it in air for 36 to 48 hours, and add 0.5 to 0.7 wt% of CaO to neutralize the phosphogypsum to a pH of 6.5 to 7.0; B. Synthesis reaction: adding phosphogypsum: (NH4)2SO4 in crude ammonium sulfate: water at a molar ratio of 1:1.5-2.0:10, stirring at 150-300 rpm for 60-120 minutes to generate a slurry containing ammonium gypsum; C. Water separation: The slurry containing ammonium gypsum is subjected to water separation. The upper and lower layer solutions after water separation are filtered separately. The upper layer is filtered and dried to obtain the product ammonium gypsum. The solid phase after filtering the lower layer is insoluble impurities, and the liquid left after filtration is the high-temperature ammonium-rich mother liquor; Both steps B and C were carried out at a constant temperature of 75°C.

2. The method for comprehensive utilization of by-product ammonium gypsum from phosphogypsum according to claim 1, characterized in that: B. Synthesis reaction: Phosphogypsum: (NH4)2SO4 in crude ammonium sulfate: water in a molar ratio of 1:1.5-1.8:10, and the reaction is stirred at 250-300 rpm for 90-120 minutes; the purity of the crude ammonium sulfate is preferably 67-73%.

3. The method for comprehensive utilization of by-product ammonium gypsum from phosphogypsum according to claim 1, characterized in that: The purity of the ammonium gypsum in step C is above 98.4%, preferably above 98.9%.

4. The method for comprehensive utilization of by-product ammonium gypsum from phosphogypsum according to claim 1, characterized in that: The method further includes: D mother liquor circulation: evaporating and concentrating the high-temperature ammonium-rich mother liquor, cooling and crystallizing, and performing solid-liquid separation to obtain a low-temperature ammonium-rich mother liquor and refined ammonium sulfate, and returning the low-temperature ammonium-rich mother liquor to step B as a raw material for the next circulation.

5. The method for comprehensive utilization of by-product ammonium gypsum from phosphogypsum according to claim 4, characterized in that: The temperature of the cooling crystallization and the temperature of the low-temperature ammonium-rich mother liquor are both 50°C.

6. The method for comprehensive utilization of by-product ammonium gypsum from phosphogypsum according to claim 4, characterized in that: The cooling crystallization is a gradient crystallization with a cooling rate of 4 to 7° C. / min.

7. The method for comprehensive utilization of by-product ammonium gypsum from phosphogypsum according to claim 1, characterized in that: The method further comprises the preparation of refined calcium sulfate: the ammonium gypsum is eluted 3 to 4 times at a solid-liquid ratio of 1:0.5 to 1, each time for 30 to 40 seconds, to obtain an eluent and calcium sulfate dihydrate with a purity of ≥99.8%.

8. The method for comprehensive utilization of by-product ammonium gypsum from phosphogypsum according to claim 7, characterized in that: The method further comprises the preparation of refined calcium sulfate: the ammonium gypsum is eluted three times at a solid-liquid ratio of 1:0.5, each time for 30 seconds; preferably, the water temperature of the elution is room temperature.

9. The method for comprehensive utilization of by-product ammonium gypsum from phosphogypsum according to claim 7, characterized in that: The method includes returning the eluent to step B as a raw material for the next cycle.

10. The method for comprehensive utilization of by-product ammonium gypsum from phosphogypsum according to claims 4 and 9, characterized in that: The low-temperature ammonium-rich mother liquor and the eluent are returned to step B as raw materials for the next cycle for a maximum of 6 times, preferably 5 to 6 times; The purity of the ammonium gypsum obtained after the cycle is above 95.7%.

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