Production method of large-particle ammonium sulfate

Abstract

The embodiment of the invention provides a production method of large-particle ammonium sulfate. The production method comprises the following steps: 1) grading an ammonium sulfate raw material in a grading unit to obtain a first ammonium sulfate crystal of which the particle size is greater than or equal to a first threshold value d and a second ammonium sulfate crystal of which the particle size is smaller than the first threshold value d; (2) the first ammonium sulfate crystal enters a crystallization unit along with the ammonium sulfate mother liquor to serve as a seed crystal for crystallization; the second ammonium sulfate crystals enter a crystal elimination unit along with the ammonium sulfate mother liquor for crystal elimination; 3) in the crystal elimination unit, heating the ammonium sulfate mother liquor with the second ammonium sulfate crystal to dissolve into clear liquid, and conveying the clear liquid to a crystallization unit; 4) feeding crystal mush containing large-particle ammonium sulfate generated in the crystallization unit into a separation unit, concentrating, grading, centrifuging and drying to obtain a large-particle ammonium sulfate product; and the ammonium sulfate mother liquor with middle crystals concentrated and graded by the separation unit overflows from the upper end to the crystallization unit. According to the method, the large-particle ammonium sulfate is prepared by adopting forced grading and directly taking the medium crystals as the seed crystals, so that the efficiency of preparing the large-particle ammonium sulfate is greatly improved.

Description

Technical Field

[0001] The present application belongs to the field of chemical technology, and specifically relates to a method for producing large-particle ammonium sulfate that is compatible with a caprolactam ammonium sulfate neutralization reaction crystallization device. Background Art

[0002] Caprolactam is an important raw material for chemical fibers and engineering plastics. Currently, the main industrial process for caprolactam production is the cyclohexanone-hydroxylamine route, based on the cyclohexanone oxime Beckmann rearrangement. The liquid-phase Beckmann rearrangement, catalyzed by oleum, produces a rearrangement liquid containing oleum and caprolactam. To neutralize the oleum in the rearrangement liquid, two treatment methods are available: one is to neutralize the rearrangement liquid with ammonia, refine the upper amide oil, extract the ammonium sulfate mother liquor with benzene, and then evaporate and crystallize it to produce solid ammonium sulfate. The other is an integrated neutralization reaction crystallization process, where the rearrangement liquid and ammonia are added via a nozzle to a neutralization crystallizer, where the heat of the neutralization reaction evaporates the water to produce solid ammonium sulfate. The neutralization reaction crystallization process combines neutralization and evaporation in a single reactor, offering numerous advantages. For example, it fully utilizes the heat of the neutralization reaction to evaporate water, resulting in efficient energy utilization; it eliminates the need for extraction and stripping of the ammonium sulfate mother liquor, reducing benzene consumption; and it shortens the process flow, requiring less equipment investment, space, and operating costs. Due to its good economic efficiency, this process is widely used in newly built caprolactam plants.

[0003] However, the neutralization reaction and crystallization are carried out in the same crystallizer. The presence of amide oil and impurities affects the growth of ammonium sulfate crystals, resulting in the average particle size of ammonium sulfate solid particles obtained by this process being around 0.8 mm, and the proportion of large ammonium sulfate particles larger than 2 mm is less than 5%.

[0004] Large-particle ammonium sulfate (particle size ≥ 2mm) offers numerous physical advantages over traditional powdered or small-particle ammonium sulfate, making it highly favored in agricultural and industrial applications. For example, it offers excellent strength, resisting pulverization and dust generation during loading and unloading; it exhibits good fluidity, resisting agglomeration during transportation and storage; it can be mechanized for even spraying during fertilization, with minimal wind dispersion and loss; it exhibits no adhesion to crop leaves and stems, preventing leaf burn; and it also provides a slow-release effect, improving fertilizer utilization. Due to these advantages, large-particle ammonium sulfate is more expensive than powdered or small-particle ammonium sulfate.

[0005] To improve the economic efficiency of their products, various companies are developing crystallization-based large-particle ammonium sulfate plants in conjunction with their caprolactam plants. The first method, which treats the fuming sulfuric acid in the rearrangement solution, separates the neutralization reaction from the evaporation crystallization process. This ammonium sulfate process eliminates the impact of amide oil and impurities on crystal growth at the source, and allows for controllable crystallization conditions, enabling the production of large-particle ammonium sulfate. However, this process still suffers from long process steps, high investment costs, and inefficient energy utilization, and the proportion of large-particle ammonium sulfate produced generally remains below 50%.

[0006] The Chinese patent, published with publication number CN217887958U, redesigns the neutralization reaction and crystallization process. The ammonium sulfate neutralization reaction is performed separately in a reactor, recovering steam generated by the reaction heat. Ammonium sulfate crystallization occurs in a flash separator. This separation of the two reactions significantly reduces the cooling water required for ammonium sulfate crystallization, while also providing enhanced operability, stability, and favorable growth of ammonium sulfate crystals. However, the ammonium sulfate mother liquor is not further treated to remove amide oil and impurities, which still affects the crystallization process. The proportion of large ammonium sulfate particles after implementation has not been disclosed. Sinopec Refining and Chemical Engineering (Group) Co., Ltd. utilizes a crystallization unit combining a single-effect DTB neutralization evaporator and a second-effect OSLO evaporation crystallizer. This separates the neutralization evaporation and evaporative crystallization processes, fully utilizing the heat of dissolution of gaseous ammonia and the neutralization heat of the sulfuric acid and ammonia neutralization reaction. The unit also utilizes the secondary steam from the neutralization crystallizer as a heat source, eliminating the need for external steam. This reduces circulating water consumption by 25% and increases the proportion of ammonium sulfate particles larger than 2 mm to 30%-40%. However, it still suffers from the high investment required for multiple crystallizers and a low proportion of large particles.

[0007] Most domestic caprolactam plants utilize the second integrated ammonium sulfate production process, which involves neutralization reaction and crystallization. To produce large-particle ammonium sulfate, a large-particle ammonium sulfate crystallization unit must be designed in conjunction with the caprolactam ammonium sulfate plant. The ammonium sulfate crystals or slurry produced by the neutralization and crystallization process are dissolved and then recrystallized to produce large-particle ammonium sulfate. In order to increase the proportion of large-particle ammonium sulfate, numerous bottlenecks must be addressed, such as crystal size classification, fine crystal elimination, supersaturation control, and rational energy utilization. Summary of the Invention

[0008] In view of the above-mentioned problems existing in the prior art, the purpose of the embodiments of the present application is to provide a production method that is compatible with the existing caprolactam ammonium sulfate neutralization reaction crystallization device (hereinafter referred to as the caprolactam ammonium sulfate device) and can improve the yield of large-particle ammonium sulfate.

[0009] The technical solution adopted in the embodiment of the present application is a method for producing large-particle ammonium sulfate, comprising the following steps:

[0010] 1) The ammonium sulfate raw material uses the ammonium sulfate mother liquor of the caprolactam ammonium sulfate device as a dispersion medium and a conveying medium, and is forcedly classified in a classification unit to classify first ammonium sulfate crystals with a particle size greater than or equal to a first threshold value d and second ammonium sulfate crystals with a particle size less than the first threshold value d;

[0011] 2) the first ammonium sulfate crystals classified in step 1) enter the crystallization unit along with the ammonium sulfate mother liquor as seed crystals for crystallization; the second ammonium sulfate crystals classified enter the decrystallization unit along with the ammonium sulfate mother liquor for decrystallization;

[0012] 3) In the crystallization unit, the ammonium sulfate mother liquor containing the second ammonium sulfate crystals is heated and dissolved into a clear liquid, and then transported to the crystallization unit for cooling or reduced pressure evaporation to provide supersaturation; in the crystallization unit, the seed crystal consumes the supersaturation generated during the cooling or evaporation process and grows;

[0013] 4) The crystal slurry containing large-particle ammonium sulfate generated in the crystallization unit enters the separation unit for concentration and classification. The concentrated crystal slurry is further centrifuged and dried to obtain a large-particle ammonium sulfate product; when the separation unit is concentrated and classified, the ammonium sulfate mother liquor with medium crystals overflows from the upper end to the crystallization unit for further growth.

[0014] In an optional embodiment, the saturated ammonium sulfate mother liquor formed after the temperature is increased and the crystallization kettle of the crystallization unit is crystallized is continuously transported to the classification crystallizer of the crystallization unit via a feed pump, and the supersaturation generated by the cooling or reduced pressure evaporation process is consumed in the classification crystallizer and grows;

[0015] By controlling the speed of the feed pump or the vacuum degree during the evaporation crystallization process in the classification crystallizer, the supersaturation of the classification crystallizer is controlled, and the density of the slurry in the classification crystallizer is maintained at 1.300 kg / m 3 ~1.400kg / m 3 ;

[0016] The particle size monitoring unit is used to monitor the proportion of ammonium sulfate crystals <0.5 mm in the classification crystallizer, so that the secondary nucleation of the classification crystallizer is maintained at a low level.

[0017] In an optional embodiment, the fine crystals generated by secondary nucleation in the grading crystallizer of the crystallization unit overflow from the overflow port at the top of the grading crystallizer to the mother liquor tank; the particle size of the overflowing fine crystals is controlled by adjusting the height of the overflow port.

[0018] In an optional embodiment, the mother liquor carrying the second ammonium sulfate crystals in step 3) is heated by the outer circulation pipeline of the crystallization kettle of the crystallization unit through the crystallization heat exchanger and crystallized to form a saturated mother liquor, and the saturated mother liquor is transported by the feed pump to the classification crystallizer and evaporated under reduced temperature or reduced pressure to provide supersaturation for the crystallization process;

[0019] The heat source of the decrystallization heat exchanger is provided by the negative pressure steam at the top of the neutralization reaction crystallizer of the caprolactam ammonium sulfate device after being compressed by the steam recompression system.

[0020] In an optional embodiment, the grading crystallizer of the crystallization unit adopts a crystallizer with particle size classification including a guide tube, a baffle and an agitator; the baffle divides the interior of the grading crystallizer into a dynamic zone and a static zone. Under the action of the agitator and the guide tube, the ammonium sulfate mother liquor forms an internal circulation in the dynamic zone, and ammonium sulfate particles with a certain particle size are fully suspended and consume supersaturation to grow; in the static zone formed by the baffle and the wall of the multi-stage crystallizer, large particles of ammonium sulfate are settled; a small amount of fine crystals generated by secondary nucleation are suspended in the upper part of the grading crystallizer, and overflow from the overflow port at the upper part of the grading crystallizer to the mother liquor tank.

[0021] In an optional embodiment, in step 4), the slurry containing large-particle ammonium sulfate coming out of the bottom of the classification crystallizer of the crystallization unit first enters the second particle size classifier of the separation unit for concentration and classification. The concentrated slurry is discharged from the bottom of the second particle size classifier into a centrifuge for centrifugal separation, the filtrate goes to the mother liquor tank, and the solid goes to the dryer. After drying, a large-particle ammonium sulfate product is obtained; the slurry with medium crystals with a particle size of 1.0 mm to 2.0 mm classified by the second particle size classifier overflows into the classification crystallizer of the crystallization unit for continued growth. The second particle size classifier can be selected from one of a thickener and a slurry kettle with particle size classification.

[0022] In an optional embodiment, in step 1), the first threshold d=0.5 mm to 1.2 mm;

[0023] The mass of the first ammonium sulfate crystals in the ammonium sulfate raw material of step 1) accounts for 15% to 30% of the total mass of the ammonium sulfate raw material; and / or

[0024] The solid-liquid mass ratio of the ammonium sulfate raw material to the ammonium sulfate mother liquor in the step 1) is 5% to 15%; and / or

[0025] In the step 1), the solid-liquid mass ratio of the slurry sent to the crystallization unit after classification by the classification unit is 15% to 25%; and / or

[0026] In the step 1), the solid-liquid mass ratio of the slurry sent to the decrystallization unit after classification by the classification unit is 5% to 12%.

[0027] In an optional embodiment, the crystallization temperature of the classification crystallizer of the crystallization unit in step 2) is 20° C. to 50° C.; and / or

[0028] The crystallization temperature of the crystallization kettle of the crystallization unit in step 3) is 60° C. to 90° C.

[0029] In an optional embodiment, the particle size of the fine crystals overflowing from the upper part of the classification crystallizer to the mother liquor tank is less than 0.8 mm; and / or

[0030] The solid-liquid mass ratio of the slurry continuously discharged from the bottom of the grading crystallizer to the separation unit is 40% to 60%.

[0031] In an optional embodiment, the mass of the medium crystals overflowing from the upper end of the second particle size classifier of the separation unit into the classification crystallizer accounts for 6% to 12% of the mass of the crystals in the slurry at the bottom of the classification crystallizer; and / or

[0032] The solid-liquid mass ratio of the crystal slurry discharged from the lower end of the second particle size classifier is 80% to 100%.

[0033] Compared with the prior art, the embodiments of the present application have the following advantages:

[0034] 1. In view of the current situation that the ammonium sulfate prepared by the caprolactam neutralization reaction crystallization device has a certain size, a grading unit is used to forcibly grade the ammonium sulfate raw material from the source. Medium crystals with a certain particle size enter the crystallization unit as seeds, and fine crystals enter the decrystallization unit to be completely dissolved into a clear liquid and then transported to the crystallization unit for cooling to provide supersaturation for crystal growth. Compared with the conventional dissolution-recrystallization process, this decrystallization-cooling (or evaporation) coupled crystallization process saves the energy consumption of completely dissolving the crystals, and uses the medium crystals directly as seeds to prepare large-particle ammonium sulfate, greatly improving the efficiency of preparing large-particle ammonium sulfate.

[0035] 2. While the evaporation crystallization process requires controlling supersaturation through temperature, vacuum, and evaporation rate, the supersaturation in the cooling crystallization process is controlled solely by the feed rate of the decrystallization kettle pump, making it more convenient and reliable. Therefore, the decrystallization-cooling coupled crystallization process is the preferred method. However, from an energy consumption perspective, the decrystallization-evaporation coupled crystallization process can reduce the amount of mother liquor recycled, thereby reducing energy consumption.

[0036] 3. The particle size monitoring unit monitors the proportion of fine crystals <0.5mm in the crystallization unit, maintains the secondary nucleation of the crystallization unit at a low level, and utilizes the grading function of the grading crystallizer in the crystallization unit. An overflow port is set at its upper end to allow the secondary nucleated fine crystals to overflow into the mother liquor tank, which is then sent from the mother liquor tank to the grading unit for grading and then to the decrystallization unit for decrystallization, so as to eliminate the influence of secondary nucleation in the crystallization process to the greatest extent.

[0037] 4. The negative pressure steam from the top of the crystallizer of the neutralization reaction of the caprolactam ammonium sulfate unit is compressed by the steam recompression system (MVR) to provide heat energy for the crystallization unit, reducing steam consumption.

[0038] 5. The second particle size classifier of the separation unit is used to perform secondary classification on the slurry coming from the crystallization unit. The medium crystals in the slurry overflow to the crystallization unit to continue growing. The concentrated slurry is centrifuged and dried to obtain large-particle ammonium sulfate product. Large-particle ammonium sulfate with a particle size of more than 2 mm accounts for more than 90%, and the market demand can be met without the need for screening steps.

[0039] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention.

[0040] This application describes an overview of various implementations or examples of the technology, and is not a comprehensive disclosure of the full scope or all of the features of the disclosed technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In the drawings, which are not necessarily drawn to scale, like reference numerals may describe similar components in different views. The drawings illustrate various embodiments generally by way of example and not limitation, and together with the description and claims, serve to illustrate the claimed embodiments. Where appropriate, like reference numerals are used throughout the drawings to refer to the same or similar parts.

[0042] Figure 1 This is a flow chart of the production method of an embodiment of the present application.

[0043] Figure 2 This is a flow chart of canceling the forced classification of the classification unit and the secondary classification of the separation unit based on the embodiment of the present application. DETAILED DESCRIPTION

[0044] In order to make the purpose, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions of the embodiments of the present application will be clearly and completely described below in conjunction with the drawings of the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the described embodiments of the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0045] Unless otherwise defined, the technical or scientific terms used in this application should have the usual meanings understood by people with ordinary skills in the field to which this application belongs. The "first", "second" and similar words used in this application do not indicate any order, quantity or importance, but are only used to distinguish different components. "Include" or "comprise" and similar words mean that the elements or objects appearing before the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects. "Connect" or "connected" and similar words are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "Up", "down", "left", "right" and the like are only used to indicate relative position relationships. When the absolute position of the object being described changes, the relative position relationship may also change accordingly.

[0046] In order to keep the following description of the embodiments of the present application clear and concise, detailed descriptions of known functions and known components are omitted in this application.

[0047] An embodiment of the present application provides a method for producing large-particle ammonium sulfate. The production method is used in conjunction with a caprolactam ammonium sulfate neutralization reaction crystallization device (referred to as a caprolactam ammonium sulfate device). The ammonium sulfate powder of a certain size prepared by the caprolactam ammonium sulfate device is used as a raw material, i.e., an ammonium sulfate raw material, to produce an ammonium sulfate product in which large-particle ammonium sulfate accounts for more than 90%.

[0048] It should be noted that the large-particle ammonium sulfate involved in this application refers to granular ammonium sulfate with a particle size ≥2mm; the percentages involved in this application are all mass percentages.

[0049] The production method of the embodiment of the present application comprises the following steps:

[0050] 1) The ammonium sulfate raw material from the caprolactam ammonium sulfate plant is forcedly classified in a classification unit using the ammonium sulfate mother liquor (ammonium sulfate mother liquor can also be called circulating mother liquor) of the caprolactam ammonium sulfate plant as a dispersion medium and a conveying medium, i.e., the ammonium sulfate raw material is forcedly classified from the source; first ammonium sulfate crystals with a certain particle size greater than or equal to a first threshold value d and second ammonium sulfate crystals with fine particle size less than the first threshold value d are classified;

[0051] 2) the first ammonium sulfate crystals with a certain particle size classified in step 1) enter the crystallization unit with the circulating mother liquor as seed crystals for crystallization; the fine crystals (second ammonium sulfate crystals) classified enter the decrystallization unit with the circulating mother liquor for decrystallization;

[0052] 3) In the crystallization unit, the circulating mother liquor containing the second ammonium sulfate crystals is heated and dissolved into a clear liquid, and then transported to the crystallization unit for cooling or reduced pressure evaporation to provide supersaturation; in the crystallization unit, the seed crystals consume the supersaturation generated during the cooling or evaporation process and grow;

[0053] 4) The crystal slurry containing large-particle ammonium sulfate generated in the crystallization unit enters the separation unit for concentration and classification. The concentrated crystal slurry is further centrifuged and dried to obtain a large-particle ammonium sulfate product; when the separation unit is concentrated and classified, the circulating mother liquor with medium crystals overflows from the upper end to the crystallization unit for further growth.

[0054] The production method of the present application is stable in operation, the supersaturation is easy to control, the preparation efficiency of large-particle ammonium sulfate is high, and the yield of large-particle ammonium sulfate with a diameter of ≥2.0 mm can reach more than 90%.

[0055] In some embodiments, the saturated ammonium sulfate mother liquor formed after the temperature rises and crystallization in the crystallization kettle of the crystallization unit is continuously transported to the classification crystallizer of the crystallization unit via a feed pump, and the supersaturation generated by the cooling or reduced pressure evaporation process is consumed in the classification crystallizer and grows. The supersaturation of the classification crystallizer can be controlled by the feed speed of the feed pump or the vacuum degree of evaporation and crystallization in the classification crystallizer, and the density of the slurry in the classification crystallizer is controlled to be maintained at 1.300 kg / m 3 ~1.400kg / m 3 To prevent supersaturation from exceeding the metastable supersaturation during the crystallization process; and to maintain secondary nucleation in the classifying crystallizer at a low level by monitoring the proportion of ammonium sulfate particles <0.5 mm in the classifying crystallizer using a particle size monitoring unit, so as to timely avoid the explosive occurrence of secondary nucleation. The proportion of ammonium sulfate crystals <0.5 mm in the classifying crystallizer can be controlled to <0.8%, for example, <0.5%.

[0056] In some embodiments, the grading crystallizer of the crystallization unit has a grading function, which uses the grading function to grade the large-particle ammonium sulfate crystals (fine crystals) generated by secondary nucleation at the top of the grading crystallizer and the large-particle ammonium sulfate crystals at the bottom. The small-particle ammonium sulfate crystals of the upper secondary nucleation overflow from the overflow port at the top of the grading crystallizer to the mother liquor tank to eliminate the influence of secondary nucleation during the crystallization process to the greatest extent. The large-particle ammonium sulfate crystals go to the separation unit with the slurry. The size of the overflowing fine crystals is controlled by adjusting the height of the overflow port. When the position of the overflow port is raised, the size of the overflowing fine crystals decreases, and when the position of the overflow port is lowered, the size of the overflowing fine crystals increases.

[0057] In some embodiments, the circulating mother liquor carrying the second ammonium sulfate crystals is heated and decrystallized to form a saturated mother liquor by passing through the decrystallization heat exchanger through the external circulation pipeline of the decrystallization reactor of the decrystallization unit. The negative pressure (low pressure) steam from the top of the neutralization reaction crystallizer of the caprolactam ammonium sulfate plant is compressed by the steam recompression system (MVR) to provide heat energy for the decrystallization unit. In other words, the heat source of the decrystallization heat exchanger is provided by the negative pressure steam from the top of the neutralization reaction crystallizer of the caprolactam ammonium sulfate plant, further reducing steam consumption.

[0058] The crystallization unit's grading crystallizer is a particle size grading crystallizer comprising a guide tube, baffles, and a stirrer. Crystallization in the grading crystallizer is carried out in the following manner:

[0059] The baffle divides the interior of the grading crystallizer into a dynamic zone and a static zone. Under the action of the agitator and the guide tube, the ammonium sulfate mother liquor forms an internal circulation in the dynamic zone, and the ammonium sulfate particles with a certain particle size are fully suspended and consume the supersaturation to grow; in the static zone formed by the baffle and the wall of the grading crystallizer, large particles of ammonium sulfate are settled; a small amount of fine crystals generated by secondary nucleation are suspended in the upper part of the grading crystallizer and overflow from the overflow port at the upper part of the grading crystallizer to the mother liquor tank; among them, the evaporation crystallization process controls the water evaporation rate and thus the supersaturation by controlling the vacuum degree at the top of the grading crystallizer.

[0060] In some embodiments, in step 4), the slurry containing large-particle ammonium sulfate coming out of the bottom of the classification crystallizer of the crystallization unit first enters the second particle size classifier of the separation unit for concentration and classification. The concentrated slurry is discharged from the bottom of the second particle size classifier and enters the centrifuge for centrifugal separation. The separated filtrate goes to the mother liquor tank and forms a circulating mother liquor with the slurry overflowing from the top of the classification crystallizer to enter the classification unit as a dispersion medium and conveying medium for the ammonium sulfate raw material. The separated solid goes to the dryer and is dried to obtain a large-particle ammonium sulfate product; the slurry with medium crystals with a particle size of 1.0 mm to 2.0 mm classified by the second particle size classifier overflows into the classification crystallizer of the crystallization unit for continued growth.

[0061] In a specific implementation method, the ammonium sulfate feedstock for the classification unit is derived from the thickener discharge, centrifuge discharge, or post-drying bed discharge of a caprolactam ammonium sulfate unit. The ammonium sulfate feedstock is preferably derived from the thickener discharge to reduce the impact of amide oil and maximize energy savings in the caprolactam ammonium sulfate unit.

[0062] Preferably, the classification unit includes a first particle size classifier. The first particle size classifier is selected from a vibrating screen or a cyclone. When the ammonium sulfate raw material is a centrifuge discharge or a drying bed discharge, a vibrating screen is preferably used, and when the ammonium sulfate raw material is a thickener discharge, a cyclone is preferably used. The classification crystallizer of the crystallization unit can be selected from a classification crystallization device with particle size classification having a guide tube, a baffle, and an agitator. That is, the classification crystallizer is a DTB crystallizer (Draft Tube Baffled Crystallizer).

[0063] Furthermore, mother liquor containing high levels of impurities in the mother liquor tank is regularly discharged to the caprolactam ammonium sulfate unit to prevent the accumulation of amide oil and impurities carried over from the ammonium sulfate raw material input, which could affect crystal growth. Fresh, low-temperature, saturated mother liquor is regularly replenished to the mother liquor tank to ensure a sufficient circulation of mother liquor throughout the production unit.

[0064] In some embodiments, the first threshold d in step 1) should be less than 2 mm. Specifically, the first threshold d can be 0.5 mm to 1.2 mm. The particle size of the first ammonium sulfate crystals is greater than or equal to 0.5 mm to 1.2 mm, and the particle size of the second ammonium sulfate crystals is less than 0.5 mm to 1.2 mm.

[0065] The mass of the first ammonium sulfate crystals in the ammonium sulfate raw material in step 1) accounts for 15% to 30% of the total mass of the ammonium sulfate raw material; the solid-liquid mass ratio of the ammonium sulfate raw material to the circulating mother liquor in the classification unit in step 1) is 5% to 15%.

[0066] Furthermore, in step 1), the solid-liquid mass ratio of the slurry sent to the crystallization unit after classification by the classification unit is 15% to 25%; and the solid-liquid mass ratio of the slurry sent to the decrystallization unit after classification by the classification unit in step 1) is 5% to 12%.

[0067] Preferably, the crystallization temperature of the classification crystallizer of the crystallization unit in step 2) is 20°C to 50°C; and the crystallization temperature of the crystallization kettle of the crystallization unit in step 3) is 60°C to 90°C.

[0068] Preferably, the particle size of the fine crystals overflowing from the upper part of the classification crystallizer to the mother liquor tank is less than 0.8 mm; the solid-liquid mass ratio of the slurry continuously discharged from the bottom of the classification crystallizer to the separation unit is 40% to 60%.

[0069] Preferably, the mass of the medium crystals overflowing from the upper end of the second particle size classifier of the separation unit to the classification crystallizer accounts for 6% to 12% of the mass of the crystals in the slurry at the bottom of the classification crystallizer; the solid-liquid mass ratio of the slurry discharged from the lower end of the second particle size classifier is 80% to 100%.

[0070] Preferably, in the prepared large-particle ammonium sulfate product, the mass proportion of ammonium sulfate particles with a particle size ≥2.0 mm reaches more than 90%.

[0071] The production method of the present application is described in detail below with reference to specific embodiments:

[0072] Example

[0073] like Figure 1As shown, the saturated mother liquor C (temperature is 30°C) of the caprolactam ammonium sulfate device is introduced into the mother liquor tank 7 and transported to the first particle size classifier 1 of the classification unit through the pipeline k at a preset flow rate as the classification suspension (i.e., the circulating mother liquor mentioned above). At the same time, the ammonium sulfate raw material (raw material particle size distribution see Table 1) from the caprolactam ammonium sulfate plant enters the first particle size classifier 1 at a constant speed for forced particle size classification. Crystals of a certain size (particle size > 1 mm) flow with the suspension through pipeline d into the classification crystallizer 2 of the crystallization unit (solid-to-liquid ratio of approximately 20%) as seed crystals. Fine crystals (particle size < 1 mm) flow with the suspension through pipeline e (solid-to-liquid ratio of approximately 10%) into the decrystallization heat exchanger 9 of the decrystallization unit. Negative pressure steam B originating from the top of the neutralization reaction crystallizer of the caprolactam ammonium sulfate plant is compressed by the vapor recompression system (MVR) 8, providing heat energy for the decrystallization heat exchanger 9, completely dissolving the fine crystals into a clear liquid. The liquid then enters the decrystallization kettle 3 (temperature within the decrystallization kettle 80°C) and enters the classification crystallizer 2 via pipeline f at a preset flow rate for cooling and crystallization, obtaining a crystalline suspension. The steam after heat exchange forms condensate, which enters the condensate storage tank 10 and is discharged. The content of the crystalline phase in the classification crystallizer 2 is about 35%, which corresponds to a growth rate of the crystalline phase in the classification crystallizer 2 of about 38 kg / (m 3 h). The slurry in the classification crystallizer 2 was sampled and analyzed. The solid phase particle size distribution is shown in Table 1. The growth rate of the ammonium sulfate crystals was calculated to be 4.5×10-5 mm / s.

[0074] The suspension from the classification crystallizer 2 enters the second particle size classifier 4 of the separation unit via pipeline g. In this second particle size classifier 4, the slurry undergoes secondary classification. Approximately 10% of the solid phase located in the upper portion of the second particle size classifier 4 overflows into the classification crystallizer 2 to continue growing. This solid phase mainly contains medium crystals (particle size of 1.0mm to 2.0mm and a very small amount of fine crystals). 90% of the solid phase is concentrated in the second particle size classifier 4, with a solid phase content of approximately 50%. It then enters the centrifuge 5 to obtain crystals (liquid holdup 2%) and crystallized mother liquor. The crystallized mother liquor enters the mother liquor tank via pipeline j to form a circulating mother liquor with the mother liquor overflowing from the classification crystallizer 2. To prevent the enrichment of amide oil entrained in the ammonium sulfate raw material A from affecting crystal growth, 5% of the total circulating mother liquor in the mother liquor tank is discharged as stream h to the caprolactam ammonium sulfate unit, and fresh mother liquor is provided by the mother liquor supply stream C. After the crystals are dried, the ammonium sulfate product obtained (particle size distribution is shown in Table 1) contains large particles of ammonium sulfate ≥2 mm accounting for more than 90%.

[0075] A particle size monitoring unit is provided in the grading crystallizer 2. The particle size detection unit consists of a particle size monitoring sensor and a particle size monitoring analyzer to monitor the proportion of fine crystals <0.5 mm. The feed amount per unit time to the grading crystallizer 2 is controlled by the feed pump speed of the decrystallization unit, so as to control the supersaturation of the grading crystallizer 2 and control the proportion of fine crystals <0.5 mm to below 0.5%. The grading function of the grading crystallizer 2 is utilized, and an overflow port is provided at its upper end to allow the secondary nucleated fine crystals to overflow into the mother liquor tank 7. The fine crystals are sent from the mother liquor tank 7 to the grading unit for grading, and then enter the decrystallization unit for decrystallization, so as to eliminate the influence of secondary nucleation in the crystallization process to the greatest extent.

[0076] Comparative Example

[0077] like Figure 2 As shown, the saturated mother liquor C (temperature is 30° C.) of the caprolactam ammonium sulfate device is introduced into the mother liquor tank 7 and transported to the first particle size classifier 1 ' of the classification unit through the pipeline k at a preset flow rate as the transport medium (i.e., the circulating mother liquor mentioned above). At the same time, the ammonium sulfate raw material of the caprolactam ammonium sulfate device (raw material particle size distribution is shown in Table 1) enters the first particle size classifier 1' at a certain speed for premixing. Since the classification function is removed, the ammonium sulfate raw material will not be classified in the first particle size classifier 1', but is only mixed into the circulating mother liquor, and then enters the classification crystallizer 2 through pipeline d. Under the classification action of the classification crystallizer 2, fine crystals (from the raw material itself and generated by secondary nucleation) enter the decrystallization heat exchanger 9 of the decrystallization unit through the external circulation pipeline e at a preset flow rate (solid-liquid ratio of 5%). The negative pressure steam B from the top of the neutralization reaction crystallizer of the caprolactam ammonium sulfate device is compressed by the steam recompression system (MVR) 8 to provide heat energy for the decrystallization heat exchanger 9, so that the fine crystals are completely dissolved into a clear liquid, and then enter the decrystallization kettle 3 (the temperature in the decrystallization kettle is 80°C) and enter the crystallizer 2 through pipeline f for cooling to provide supersaturation.

[0078] The suspension from the classification crystallizer 2 enters the second particle size classifier 4' of the separation unit through pipeline g at a preset flow rate. The second particle size classifier 4' uses a thickener. 95% of the solid phase is concentrated in the thickener, and the solid content reaches about 50%. It enters the centrifuge 5 to obtain crystals (liquid holdup 2%) and mother liquor after crystallization. The mother liquor after crystallization enters the mother liquor tank through pipeline j and forms a circulating mother liquor with the mother liquor overflowing from the thickener. In order to prevent the enrichment of amide oil entrained in the ammonium sulfate raw material from affecting the growth of crystals, 5% of the total circulating mother liquor in the mother liquor tank is discharged from h to the caprolactam ammonium sulfate device, and fresh mother liquor is provided by the mother liquor supply flow C. After the crystals are dried, the ammonium sulfate product is obtained (particle size distribution is shown in Table 1).

[0079] Table 1

[0080]

[0081]

[0082] It can be clearly seen from Table 1 that in the production method of the embodiment of the present application using the forced classification of the grading unit and the secondary classification of the separation unit, the large-particle ammonium sulfate product obtained has a large-particle ammonium sulfate with a particle size of ≥2 mm accounting for more than 90%. However, in the production method of canceling the forced classification of the grading unit and canceling the secondary classification of the separation unit, the large-particle ammonium sulfate product obtained has a large-particle ammonium sulfate with a particle size of ≥2 mm accounting for only 30% to 35%. It can be seen that the production method of the present application greatly improves the yield of large-particle ammonium sulfate.

[0083] The present invention reduces the dissolution energy consumption of recrystallization from the source, utilizes the low-pressure steam of the neutralization reaction crystallization device to provide heat energy for the decrystallization unit, further reduces the steam consumption, and adopts the decrystallization-cooling (or evaporation) coupled crystallization process to more easily realize the control of supersaturation and the regulation of the crystallization process, thereby greatly improving the yield of large-particle ammonium sulfate.

[0084] The specific parameters involved in the embodiments of the present invention can be adjusted according to actual production conditions.

[0085] In another aspect, the present invention provides the use of the above-mentioned large-particle ammonium sulfate in the preparation of compound fertilizer.

[0086] On the other hand, the present invention provides a compound fertilizer comprising the above-mentioned large-particle ammonium sulfate.

[0087] The above description is intended to be illustrative rather than restrictive, and those skilled in the art may make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present disclosure. Furthermore, the above examples (or one or more of them) may be used in combination with each other, and it is contemplated that these embodiments may be combined with each other in various combinations or permutations.

Claims

1. A method for producing large-particle ammonium sulfate, characterized in that: The steps include: 1) The ammonium sulfate raw material uses the ammonium sulfate mother liquor of the caprolactam ammonium sulfate device as a dispersion medium and a conveying medium, and is forcedly classified in a classification unit to classify first ammonium sulfate crystals with a particle size greater than or equal to a first threshold value d and second ammonium sulfate crystals with a particle size less than the first threshold value d; 2) the first ammonium sulfate crystals classified in step 1) enter the crystallization unit along with the ammonium sulfate mother liquor as seed crystals for crystallization; the second ammonium sulfate crystals classified enter the decrystallization unit along with the ammonium sulfate mother liquor for decrystallization; 3) In the crystallization unit, the ammonium sulfate mother liquor with the second ammonium sulfate crystals is heated and dissolved into a clear liquid, and then transported to the crystallization unit to cool or evaporate under reduced pressure to provide supersaturation; In the crystallization unit, the seed crystal consumes the supersaturation generated during cooling or evaporation and grows; 4) The crystal slurry containing large-particle ammonium sulfate generated in the crystallization unit enters the separation unit for concentration and classification. The concentrated crystal slurry is further centrifuged and dried to obtain a large-particle ammonium sulfate product; when the separation unit is concentrated and classified, the ammonium sulfate mother liquor with medium crystals overflows from the upper end to the crystallization unit for further growth.

2. The method for producing large-particle ammonium sulfate according to claim 1, wherein: The saturated ammonium sulfate mother liquor formed after the temperature is increased and the crystallization kettle of the crystallization unit is crystallized is continuously transported to the classification crystallizer of the crystallization unit through a feed pump, and the supersaturation generated by the cooling or reduced pressure evaporation process is consumed in the classification crystallizer and grows; By controlling the speed of the feed pump or the vacuum degree during the evaporation crystallization process in the classification crystallizer, the supersaturation of the classification crystallizer is controlled, and the density of the slurry in the classification crystallizer is maintained at 1.300 kg / m 3 ~1.400kg / m 3 ; The particle size monitoring unit is used to monitor the proportion of ammonium sulfate crystals <0.5 mm in the classification crystallizer, so that the secondary nucleation of the classification crystallizer is maintained at a low level.

3. The method for producing large-particle ammonium sulfate according to claim 1, wherein: The fine crystals generated by secondary nucleation in the classification crystallizer of the crystallization unit overflow from the overflow port on the upper part of the classification crystallizer to the mother liquid tank; the particle size of the overflowing fine crystals is controlled by adjusting the height of the overflow port.

4. The method for producing large-particle ammonium sulfate according to claim 3, wherein: The mother liquor carrying the second ammonium sulfate crystals in step 3) is heated by the outer circulation pipeline of the crystallization kettle of the crystallization unit through the crystallization heat exchanger and crystallized to form a saturated mother liquor, and the saturated mother liquor is transported to the classification crystallizer by the feed pump and evaporated under reduced temperature or reduced pressure to provide supersaturation for the crystallization process; The heat source of the decrystallization heat exchanger is provided by the negative pressure steam at the top of the neutralization reaction crystallizer of the caprolactam ammonium sulfate device after being compressed by the steam recompression system.

5. The method for producing large-particle ammonium sulfate according to claim 1, wherein: The grading crystallizer of the crystallization unit adopts a crystallizer with particle size classification including a guide tube, a baffle and an agitator; the baffle divides the interior of the grading crystallizer into a dynamic zone and a static zone. Under the action of the agitator and the guide tube, the ammonium sulfate mother liquor forms an internal circulation in the dynamic zone, and ammonium sulfate particles with a certain particle size are fully suspended and consume supersaturation to grow; in the static zone formed by the baffle and the wall of the multi-stage crystallizer, large particles of ammonium sulfate are settled; a small amount of fine crystals generated by secondary nucleation are suspended in the upper part of the grading crystallizer, and overflow from the overflow port at the upper part of the grading crystallizer to the mother liquor tank.

6. The method for producing large-particle ammonium sulfate according to claim 1, wherein: In the step 4), the slurry containing large-particle ammonium sulfate coming out of the bottom of the grading crystallizer of the crystallization unit first enters the second particle size classifier of the separation unit for concentration and classification, and the concentrated slurry is discharged from the bottom of the second particle size classifier into the centrifuge for centrifugal separation, the filtrate goes to the mother liquor tank, and the solid goes to the dryer, and the large-particle ammonium sulfate product is obtained after drying; the slurry with medium crystals with a particle size of 1.0 mm to 2.0 mm classified by the second particle size classifier overflows into the grading crystallizer of the crystallization unit for continued growth.

7. The method for producing large-particle ammonium sulfate according to claim 1, wherein: In step 1), the first threshold value d=0.5mm-1.2mm; The mass of the first ammonium sulfate crystals in the ammonium sulfate raw material of step 1) accounts for 15% to 30% of the total mass of the ammonium sulfate raw material; and / or The solid-liquid mass ratio of the ammonium sulfate raw material to the ammonium sulfate mother liquor in the step 1) is 5% to 15%; and / or In the step 1), the solid-liquid mass ratio of the slurry sent to the crystallization unit after classification by the classification unit is 15% to 25%; and / or In the step 1), the solid-liquid mass ratio of the slurry sent to the decrystallization unit after classification by the classification unit is 5% to 12%.

8. The method for producing large-particle ammonium sulfate according to claim 1, wherein: The crystallization temperature of the classification crystallizer of the crystallization unit in step 2) is 20° C. to 50° C.; and / or The crystallization temperature of the crystallization kettle of the crystallization unit in step 3) is 60° C. to 90° C.

9. The method for producing large-particle ammonium sulfate according to claim 3, wherein: The particle size of the fine crystals overflowing from the upper part of the classification crystallizer to the mother liquor tank is less than 0.8 mm; and / or The solid-liquid mass ratio of the slurry continuously discharged from the bottom of the grading crystallizer to the separation unit is 40% to 60%.

10. The method for producing large-particle ammonium sulfate according to claim 6, wherein: The mass of the medium crystals overflowing from the upper end of the second particle size classifier of the separation unit into the classification crystallizer accounts for 6% to 12% of the mass of the crystals in the slurry at the bottom of the classification crystallizer; and / or The solid-liquid mass ratio of the crystal slurry discharged from the lower end of the second particle size classifier is 80% to 100%.

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