Method for preparing special fertilizer for high-added-value crops by in-situ low-temperature conversion process

Abstract

The invention provides a method for preparing a special fertilizer for crops with high additional value by an in-situ low-temperature conversion process, which comprises the following steps: mixing potassium chloride and concentrated sulfuric acid, carrying out microwave two-stage conversion, carrying out three-stage ammoniation synchronous desorption crystallization, carrying out vacuum filtration separation, and producing potassium sulfate by a rotary flash evaporation technology. Meanwhile, high-quality hydrochloric acid and sodium chloride are prepared through four-stage similar-concentration countercurrent absorption reaction tail gas, crystallized mother liquor is treated through an MVR-centrifugal double-effect evaporation technology to prepare ammonium chloride potassium, efficient recycling of all separated mother liquor is achieved, no three wastes are discharged in the whole production process, and the production cost is low. The treatment or separation system of hydrogen chloride tail gas and crystal liquid of different types of devices can be met, and the tail gas and waste liquid treatment problem in the potassium sulfate production process is further effectively solved.

Description

Technical Field

[0001] The present invention relates to a method for preparing high - value - added special fertilizers for crops by an in - situ low - temperature conversion process, belonging to the technical fields of inorganic salt industry and chemical fertilizer industry. Background Technique

[0002] Potassium sulfate, as an efficient chlorine - free potassium fertilizer, has dual value in agricultural production. It can not only supplement the potassium element required by crops but also provide sulfur nutrients. Restricted by the insufficient reserves of potassium salt deposits in China, there is a significant gap between the current production capacity and the actual demand, and the annual import dependence is relatively high. Currently, there are mainly four technical routes for industrial production: one is the extraction process based on salt lake brine, the second is the decomposition technology of potassium salt ore, the third is the preparation by the double - decomposition reaction of potassium chloride and sulfate, and the fourth is the synthesis method of directly converting potassium chloride and sulfuric acid.

[0003] Chinese Patent Document 2016100449151 discloses a method for producing potassium sulfate with by - product potassium ammonium chloride by low - temperature conversion. This method uses concentrated sulfuric acid for low - temperature two - stage conversion, three - stage ammoniation with synchronous desorption and crystallization, vacuum filtration separation, and rotary drying technology to produce potassium sulfate. At the same time, high - quality hydrochloric acid is prepared by four - stage counter - current absorption of reaction tail gas with similar concentrations, and the mother liquor after crystallization is treated by a parallel - flow triple - effect evaporation process to produce by - product potassium ammonium chloride. However, the above method still has disadvantages and problems such as low removal rate of impurity ions and high energy consumption. Summary of the Invention

[0004] Aiming at the deficiencies of the existing technology, the present invention provides a method for preparing high - value - added special fertilizers for crops by an in - situ low - temperature conversion process.

[0005] The technical scheme adopted by the present invention is as follows:

[0006] A method for preparing high - value - added special fertilizers for crops by an in - situ low - temperature conversion process, the steps are as follows:

[0007] (1) Potassium chloride and concentrated sulfuric acid are fully stirred and mixed evenly. The obtained mixture is subjected to two - stage microwave conversion in sequence. The temperature of the first - stage conversion is 100 - 110 °C, the reaction time is 0.3 - 0.7 h, the temperature of the second - stage conversion is 120 - 140 °C, and the reaction time is 0.8 - 1.2 h to generate potassium bisulfate slurry and hydrogen chloride gas;

[0008] (2) The potassium bisulfate slurry generated in step (1) is added to water for dissolution, mixed with potassium chloride, and then a mixed gas containing gaseous ammonia is introduced into it for three - stage ammoniation reaction. After the three - stage ammoniation reaction, crystallization, vacuum filtration, and rotary flash evaporation are carried out to obtain potassium sulfate solid and a filtrate containing potassium ammonium chloride;

[0009] (3) The filtrate in step (2) is evaporated to a water content of 50 - 60 wt% through an MVR - centrifugal two - effect evaporation system, and then spray - dried after hydrocyclone separation, thus obtaining the by - product potassium ammonium chloride;

[0010] (4) The hydrogen chloride gas generated in step (1) is absorbed by four - stage counter - current absorption with similar concentrations to produce hydrochloric acid;

[0011] (5) Potassium sulfate obtained in step (2) is added with phosphoric acid, urea and a filler, and after mixing, a special fertilizer for high - value - added crops is obtained.

[0012] Preferably, the molar ratio of potassium chloride to concentrated sulfuric acid in step (1) is 1:(0.9 - 1.3), and the concentration of concentrated sulfuric acid is 98 wt%.

[0013] Preferably, in step (1), the microwave power of the first - stage conversion is 3.5 - 4.5 kW, the reaction time is 0.5 h, the microwave power of the second - stage conversion is 1.5 kW, and the reaction time is 1 h.

[0014] Preferably, the molar ratio of potassium bisulfate slurry to water in step (2) is 1:1.5.

[0015] Preferably, the molar ratio of the addition amount of potassium chloride to potassium bisulfate slurry in step (2) is 1:1.2.

[0016] Preferably, in step (2), the tertiary ammoniation reaction is specifically as follows: A mixed gas of ammonia gas with a pressure of 0.08 MPa and an ammonia volume concentration of 60% and compressed air is introduced, the ammoniation reaction temperature is controlled at 40 °C, and the reaction is carried out for 1 h to obtain a primary ammoniation reaction solution; For the obtained primary ammoniation reaction solution, a mixed gas of ammonia gas with a pressure of 0.08 MPa and an ammonia volume concentration of 60% and compressed air is introduced, the ammoniation reaction temperature is controlled at 50 °C, and the reaction is carried out for 1 h to obtain a secondary ammoniation reaction solution; For the obtained secondary ammoniation reaction solution, a mixed gas of ammonia gas with a pressure of 0.08 MPa and an ammonia volume concentration of 60% and compressed air is introduced, the ammoniation reaction temperature is controlled at 60 °C, and the reaction is carried out for 1 h to obtain a tertiary ammoniation reaction solution.

[0017] Preferably, in step (2), a dynamic crystallizer is used, the crystal - growing temperature is 35 °C, the crystal - growing time is 1 h; the vacuum degree of vacuum filtration is 0.08 MPa; the temperature of rotary flash evaporation is 35 °C, and the flash - evaporation time is 1 h.

[0018] Preferably, the rotation speed of the dynamic crystallizer in step (2) is 370 - 400 rpm.

[0019] Further, the rotation speed of the dynamic crystallizer in step (2) is 380 rpm.

[0020] Preferably, in step (3), in the MVR - centrifugal two - effect evaporation system, one - effect evaporation system is MVR evaporation, with a temperature of 80 °C, a vacuum degree of 0.18 MPa, and an evaporation time of 1 h; the two - effect evaporation system is a centrifugal evaporation system, with a centrifugal force of 2,000 g, a temperature of 60 °C, and an evaporation time of 1.5 h.

[0021] Preferably, in step (3), the inlet temperature of spray drying is 180 °C.

[0022] Preferably, in step (4), the four - stage similar - concentration counter - current absorption is that hydrogen chloride gas enters the first - stage graphite absorber and then enters the first - stage absorption tank, and desalted water is used as the absorbent for absorption; the gas obtained after the first - stage absorption enters the second - stage graphite absorber and then enters the second - stage absorption tank, and desalted water is used as the absorbent for absorption. The liquid obtained after the second - stage absorption flows counter - currently to the first - stage absorption tank for absorption; the gas obtained after the second - stage absorption enters the third - stage absorption tower, and desalted water is used as the absorbent for absorption. The liquid obtained after the third - stage absorption flows counter - currently to the second - stage absorption tank for absorption; the gas obtained after the third - stage absorption enters the fourth - stage absorption tower, and desalted water is used as the absorbent for absorption. The liquid obtained after the fourth - stage absorption flows counter - currently to the third - stage absorption tower for absorption, and the obtained gas is introduced into the alkaline absorbent solution.

[0023] Preferably, in step (4), the alkaline absorbent solution is 5% NaOH.

[0024] Preferably, in step (4), the system pressure is 0.18 - 0.21 MPa.

[0025] Preferably, in step (5), the mass ratios of potassium sulfate, phosphoric acid, urea, and filler are: potassium sulfate slurry 25 - 40%, phosphoric acid 20 - 25%, urea 30 - 35%, and filler 5 - 25%.

[0026] The rotary flash evaporation of the present invention is carried out according to the existing technology; the rotary flash evaporation can be either at atmospheric pressure or under reduced pressure.

[0027] The beneficial effects and advantages of the present invention are as follows:

[0028] 1. The method of the present invention integrates and innovates the two - stage microwave conversion process. The conversion rate of potassium chloride breaks through more than 96%, reducing energy consumption to a greater extent and ensuring stable product quality. At the same time, the reaction tail gas is dust - removed and then washed with dilute sulfuric acid, which not only avoids the environmental pollution caused by ammonia gas but also recovers ammonia resources, realizing the comprehensive utilization of resources.

[0029] 2. The method of the present invention adopts the three - stage ammoniation synchronous centrifugal resolution crystallization technology, which improves the crystal growth rate and crystal size, and enhances the separation efficiency of potassium sulfate solids.

[0030] 3. The method of the present invention integrates technologies such as multi-effect evaporation, steam hierarchical utilization, vacuum filtration, and mother liquor recycling, and develops a process for producing potassium chloride and ammonium chloride by MVR-centrifugal two-effect evaporation, increasing product varieties and saving energy consumption.

[0031] 4. Based on the three-stage falling film absorption of the present invention, demineralized water is used as the absorbent, and a process equipment for producing high-quality hydrochloric acid by four-stage countercurrent absorption with similar concentrations is further developed. The concentration of the by-product hydrochloric acid reaches 36%; under the action of the alkaline absorbent, the final emission concentration of hydrogen chloride tail gas is ≤20 mg / m 3 , far lower than the national standard requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is a process flow block diagram of the in-situ low-temperature conversion process for preparing high-value-added crop special fertilizers of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0033] The technical solutions of the present invention will be further described below in conjunction with the embodiments and the drawings, but the scope of the present invention is not limited thereto.

[0034] The raw materials used in the embodiments are all commonly available on the market.

[0035] Example 1

[0036] A method for preparing high-value-added crop special fertilizers by an in-situ low-temperature conversion process, the steps are as follows:

[0037] (1) Weigh potassium chloride and concentrated sulfuric acid at a molar ratio of potassium chloride to 98 wt% concentrated sulfuric acid of 1:0.9 respectively, and put them into the reaction tank under continuous stirring and stir to mix evenly. The obtained mixture is subjected to two-stage microwave conversion in sequence. The temperature of the first-stage conversion is 100 °C, the microwave power is 3.5 kW, the reaction time is 0.3 h, the temperature of the second-stage conversion is 120 °C, the microwave power is 1.5 kW, and the reaction time is 0.8 h to generate potassium bisulfate slurry and hydrogen chloride gas.

[0038] (2) Dissolve the potassium bisulfate slurry generated in step (1) in water. Mix the potassium bisulfate slurry and water at a molar ratio of 1:1.5, then mix with potassium chloride. The addition amount of potassium chloride is in a molar ratio of 1:1.2 to the potassium bisulfate slurry. Then, introduce a mixed gas containing gaseous ammonia for a three-stage ammoniation reaction. The specific three-stage ammoniation reaction is as follows: Introduce a mixture of gaseous ammonia with a pressure of 0.08 MPa and an ammonia volume concentration of 60% and compressed air, control the ammoniation reaction temperature at 40 °C, and react for 1 h to obtain a primary ammoniation reaction solution; for the obtained primary ammoniation reaction solution, introduce a mixture of gaseous ammonia with a pressure of 0.08 MPa and an ammonia volume concentration of 60% and compressed air, control the ammoniation reaction temperature at 50 °C, and react for 1 h to obtain a secondary ammoniation reaction solution; for the obtained secondary ammoniation reaction solution, introduce a mixture of gaseous ammonia with a pressure of 0.08 MPa and an ammonia volume concentration of 60% and compressed air, control the ammoniation reaction temperature at 60 °C, and react for 1 h to obtain a tertiary ammoniation reaction solution. After the three-stage ammoniation reaction, through crystal cultivation, vacuum filtration, and rotary flash evaporation, potassium sulfate solid and filtrate are obtained. Crystal cultivation uses a dynamic crystallizer with a rotation speed of 370 rpm, a crystal cultivation temperature of 35 °C, and a crystal cultivation time of 1 h; the vacuum degree for vacuum filtration is 0.08 MPa; the rotary flash evaporation temperature is 35 °C, and the flash evaporation time is 1 h.

[0039] (3) Evaporate the filtrate in step (2) through an MVR - centrifugal two-effect evaporation system until the water content is 50 wt%. The first-effect evaporation system is MVR evaporation, with a temperature of 80 °C, a vacuum degree of 0.18 MPa, and an evaporation time of 1 h; the second-effect evaporation system is a centrifugal evaporation system, with a centrifugal force of 2,000 g, a temperature of 60 °C, and an evaporation time of 1.5 h. Then, after hydrocyclone separation, spray drying is carried out at an inlet temperature of 180 °C to obtain the by-product potassium ammonium chloride.

[0040] (4) Subject the hydrogen chloride gas generated in step (1) to four-stage countercurrent absorption with similar concentrations. The system pressure is 0.18 MPa. The hydrogen chloride gas enters the first-stage absorption tank after being absorbed by the first-stage graphite. Demineralized water is used as the absorbent for absorption; the gas obtained after the first-stage absorption enters the second-stage graphite absorption and then enters the second-stage absorption tank, and demineralized water is used as the absorbent for absorption. The liquid obtained after the second-stage absorption flows countercurrently to the first-stage absorption tank for absorption; the gas obtained after the second-stage absorption enters the third-stage absorption tower, and demineralized water is used as the absorbent for absorption. The liquid obtained after the third-stage absorption flows countercurrently to the second-stage absorption tank for absorption; the gas obtained after the third-stage absorption enters the fourth-stage absorption tower, and demineralized water is used as the absorbent for absorption. The liquid obtained after the fourth-stage absorption flows countercurrently to the third-stage absorption tower for absorption to prepare 36% hydrochloric acid, and the obtained gas is introduced into a 5% NaOH solution.

[0041] (5) Add phosphoric acid, urea, and a filler to the potassium sulfate prepared in step (2). The mass ratio is: potassium sulfate 25%, phosphoric acid 20%, urea 30%, filler 25%. After mixing, a high-value special fertilizer for crops is obtained.

[0042] Example 2

[0043] A method for preparing special fertilizers for high-value crops by an in-situ low-temperature conversion process, the steps are as follows:

[0044] (1) Calculate and weigh potassium chloride and concentrated sulfuric acid with a molar ratio of potassium chloride to 98 wt% concentrated sulfuric acid of 1:1.1 respectively, and put them into a reaction tank under continuous stirring and mix evenly. The obtained mixture is subjected to two-stage microwave conversion in sequence. The temperature of the first-stage conversion is 105 °C, the microwave power is 4 kW, the reaction time is 0.5 h, the temperature of the second-stage conversion is 130 °C, the microwave power is 1.5 kW, and the reaction time is 1 h to generate potassium bisulfate slurry and hydrogen chloride gas.

[0045] (2) Dissolve the potassium bisulfate slurry generated in step (1) in water. After mixing the potassium bisulfate slurry and water in a molar ratio of 1:1.5, mix it with potassium chloride. The addition amount of potassium chloride and the molar ratio of the potassium bisulfate slurry is 1:1.2. Then, introduce a mixed gas containing gaseous ammonia into it for a three-stage ammoniation reaction. The specific three-stage ammoniation reaction is as follows: introduce a mixture of gaseous ammonia with a pressure of 0.08 MPa and an ammonia volume concentration of 60% and compressed air, control the ammoniation reaction temperature at 40 °C, and react for 1 h to obtain a first-stage ammoniation reaction solution; for the obtained first-stage ammoniation reaction solution, introduce a mixture of gaseous ammonia with a pressure of 0.08 MPa and an ammonia volume concentration of 60% and compressed air, control the ammoniation reaction temperature at 50 °C, and react for 1 h to obtain a second-stage ammoniation reaction solution; for the obtained second-stage ammoniation reaction solution, introduce a mixture of gaseous ammonia with a pressure of 0.08 MPa and an ammonia volume concentration of 60% and compressed air, control the ammoniation reaction temperature at 60 °C, and react for 1 h to obtain a third-stage ammoniation reaction solution. After the three-stage ammoniation reaction, through crystal cultivation, vacuum filtration, and rotary flash evaporation, potassium sulfate solid and filtrate are obtained. Crystal cultivation is carried out using a dynamic crystallizer with a rotation speed of 380 rpm, a crystal cultivation temperature of 35 °C, and a crystal cultivation time of 1 h; the vacuum degree of vacuum filtration is 0.08 MPa; the rotary flash evaporation temperature is 35 °C, and the flash evaporation time is 1 h.

[0046] (3) The filtrate in step (2) is evaporated to a water content of 56 wt% through an MVR - centrifugal two-effect evaporation system. The first-effect evaporation system is MVR evaporation, with a temperature of 80 °C, a vacuum degree of 0.18 MPa, and an evaporation time of 1 h; the second-effect evaporation system is a centrifugal evaporation system, with a centrifugal force of 2,000 g, a temperature of 60 °C, and an evaporation time of 1.5 h. Then, after hydrocyclone separation, it is spray-dried at an inlet temperature of 180 °C to obtain the by-product potassium ammonium chloride.

[0047] (4) The hydrogen chloride gas generated in step (1) is absorbed by countercurrent in four stages with similar concentrations, and the system pressure is 0.20 MPa. After being absorbed by the first-stage graphite absorber, the hydrogen chloride gas enters the first-stage absorption tank and is absorbed with demineralized water as the absorbent; the gas obtained after the first-stage absorption enters the second-stage graphite absorber and then enters the second-stage absorption tank, and is absorbed with demineralized water as the absorbent. The liquid obtained after the second-stage absorption flows countercurrently to the first-stage absorption tank for absorption; the gas obtained after the second-stage absorption enters the third-stage absorption tower and is absorbed with demineralized water as the absorbent. The liquid obtained after the third-stage absorption flows countercurrently to the second-stage absorption tank for absorption; the gas obtained after the third-stage absorption enters the fourth-stage absorption tower and is absorbed with demineralized water as the absorbent. The liquid obtained after the fourth-stage absorption flows countercurrently to the third-stage absorption tower for absorption to produce 36% hydrochloric acid, and the obtained gas is introduced into a 5% NaOH solution.

[0048] (5) Potassium sulfate prepared in step (2) is added with phosphoric acid, urea and a filler, and the mass ratio is: potassium sulfate 40%, phosphoric acid 25%, urea 35%, filler 5%. After mixing, a special fertilizer for high-value-added crops is obtained.

[0049] Example 3

[0050] A method for preparing a special fertilizer for high-value-added crops by an in-situ low-temperature conversion process, the steps are as follows:

[0051] (1) Calculate and weigh potassium chloride and concentrated sulfuric acid with a molar ratio of potassium chloride to 98 wt% concentrated sulfuric acid of 1:1.3 respectively. Under continuous stirring, they are put into a reaction tank and stirred and mixed evenly. The obtained mixture is subjected to two-stage microwave conversion in sequence. The temperature of the first-stage conversion is 110 °C, the microwave power is 4.5 kW, and the reaction time is 0.7 h. The temperature of the second-stage conversion is 140 °C, the microwave power is 1.5 kW, and the reaction time is 1.2 h to generate a potassium bisulfate slurry and hydrogen chloride gas.

[0052] (2) Dissolve the potassium bisulfate slurry generated in step (1) in water. Mix the potassium bisulfate slurry and water at a molar ratio of 1:1.5, then mix with potassium chloride. The addition amount of potassium chloride and the potassium bisulfate slurry is at a molar ratio of 1:1.2. Then introduce a mixed gas containing gaseous ammonia for a three-stage ammoniation reaction. The specific three-stage ammoniation reaction is as follows: Introduce a mixture of gaseous ammonia with a pressure of 0.08 MPa and an ammonia volume concentration of 60% and compressed air, control the ammoniation reaction temperature at 40 °C, and react for 1 h to obtain a primary ammoniation reaction solution; for the obtained primary ammoniation reaction solution, introduce a mixture of gaseous ammonia with a pressure of 0.08 MPa and an ammonia volume concentration of 60% and compressed air, control the ammoniation reaction temperature at 50 °C, and react for 1 h to obtain a secondary ammoniation reaction solution; for the obtained secondary ammoniation reaction solution, introduce a mixture of gaseous ammonia with a pressure of 0.08 MPa and an ammonia volume concentration of 60% and compressed air, control the ammoniation reaction temperature at 60 °C, and react for 1 h to obtain a tertiary ammoniation reaction solution. After the three-stage ammoniation reaction, through crystal cultivation, vacuum filtration, and rotary flash evaporation, potassium sulfate solid and filtrate are obtained. Crystal cultivation uses a dynamic crystallizer with a rotation speed of 400 rpm, a crystal cultivation temperature of 35 °C, and a crystal cultivation time of 1 h; the vacuum degree for vacuum filtration is 0.08 MPa; the rotary flash evaporation temperature is 35 °C, and the flash evaporation time is 1 h.

[0053] (3) Evaporate the filtrate in step (2) through an MVR - centrifugal two-effect evaporation system until the water content is 60 wt%. The first-effect evaporation system is MVR evaporation, with a temperature of 80 °C, a vacuum degree of 0.18 MPa, and an evaporation time of 1 h; the second-effect evaporation system is a centrifugal evaporation system, with a centrifugal force of 2,000 g, a temperature of 60 °C, and an evaporation time of 1.5 h. Then, after hydrocyclone separation, spray drying is carried out at an inlet temperature of 180 °C to obtain the by-product potassium ammonium chloride.

[0054] (4) Absorb the hydrogen chloride gas generated in step (1) through four-stage countercurrent absorption with similar concentrations. The system pressure is 0.21 MPa. The hydrogen chloride gas enters the first-stage graphite absorber and then enters the first absorption tank after the first-stage absorption, and demineralized water is used as the absorbent for absorption; the gas obtained after the first-stage absorption enters the second-stage graphite absorber and then enters the second absorption tank after the second-stage absorption, and demineralized water is used as the absorbent for absorption. The liquid obtained after the second-stage absorption flows countercurrently to the first absorption tank for absorption; the gas obtained after the second-stage absorption enters the third-stage absorption tower, and demineralized water is used as the absorbent for absorption. The liquid obtained after the third-stage absorption flows countercurrently to the second absorption tank for absorption; the gas obtained after the third-stage absorption enters the fourth-stage absorption tower, and demineralized water is used as the absorbent for absorption. The liquid obtained after the fourth-stage absorption flows countercurrently to the third-stage absorption tower for absorption to produce 36% hydrochloric acid, and the obtained gas is introduced into a 5% NaOH solution.

[0055] (5) Add phosphoric acid, urea, and a filler to the potassium sulfate prepared in step (2). The mass ratio is: potassium sulfate 30%, phosphoric acid 25%, urea 30%, and filler 15%. After mixing, a high-value special fertilizer for crops is obtained.

[0056] It should be noted that, in this document, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprising", "including" or any other variant thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements not only includes those elements, but also includes other elements not expressly listed, or further includes elements inherent to such process, method, article or device.

[0057] Although embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing high value-added crop fertilizer by in-situ low-temperature conversion process, characterized in that: The following steps are involved: (1) Potassium chloride and concentrated sulfuric acid are stirred and mixed uniformly, and the obtained mixture is subjected to two-stage microwave conversion in sequence, wherein the first-stage conversion temperature is 100-110° C., the reaction time is 0.3-0.7 h, and the second-stage conversion temperature is 120-140° C., the reaction time is 0.8-1.2 h, to generate potassium bisulfate slurry and hydrogen chloride gas; (2) adding the potassium hydrogen sulfate slurry generated by the reaction in step (1) into water to dissolve, mixing with potassium chloride, and then introducing a mixed gas containing gaseous ammonia into the water to carry out a tertiary amination reaction. After the tertiary amination reaction, the slurry is subjected to crystal growing, vacuum filtration, and rotary flash evaporation to obtain potassium sulfate solid and a filtrate containing potassium ammonium chloride; (3) The filtrate in step (2) is evaporated to a water content of 50-60 wt% by an MVR-centrifugal double-effect evaporation system, and then subjected to hydrocyclone separation and spray drying at 180° C. to obtain potassium ammonium chloride as a byproduct; (4) subjecting the hydrogen chloride gas generated in the reaction of step (1) to four-stage similar concentration countercurrent absorption to produce hydrochloric acid; (5) adding the potassium sulfate obtained in step (2) to phosphoric acid, urea and a filler, and mixing them to obtain a high value-added crop fertilizer.

2. The method for preparing special fertilizer for high value-added crops according to claim 1, characterized in that: In the step (1), the molar ratio of potassium chloride to concentrated sulfuric acid is 1:(0.9-1.3), and the concentration of concentrated sulfuric acid is 98wt%; the microwave power of the first-stage conversion is 3.5-4.5kW, and the reaction time is 0.5h; the microwave power of the second-stage conversion is 1.5kW, and the reaction time is 1h.

3. The method for preparing special fertilizer for high value-added crops according to claim 1, characterized in that: In the step (2), the molar ratio of potassium bisulfate slurry to water is 1:1.5, and the molar ratio of potassium chloride added to potassium bisulfate slurry is 1:1.2; a dynamic crystallizer is used, the rotation speed is 370-400 rpm, the crystal growing temperature is 35°C, and the crystal growing time is 1 hour; the vacuum degree of vacuum filtration is 0.08 MPa; the rotary flash temperature is 35°C, and the flash time is 1 hour.

4. The method for preparing special fertilizer for high value-added crops according to claim 3, characterized in that: The rotation speed of the dynamic crystallizer in step (2) is 380 rpm.

5. The method for preparing special fertilizer for high value-added crops according to claim 1, characterized in that: The tertiary amination reaction in step (2) is specifically: A mixed gas of gaseous ammonia and compressed air with a pressure of 0.08 MPa and an ammonia volume concentration of 60% is introduced, the amination reaction temperature is controlled at 40° C., and the reaction is carried out for 1 hour to obtain a primary amination reaction liquid; a mixed gas of gaseous ammonia and compressed air with a pressure of 0.08 MPa and an ammonia volume concentration of 60% is introduced into the obtained primary amination reaction liquid, the ammonia reaction temperature is controlled at 50° C., and the reaction is carried out for 1 hour to obtain a secondary amination reaction liquid; The obtained secondary amination reaction liquid was introduced into a mixture of gaseous ammonia and compressed air with a pressure of 0.08 MPa and an ammonia volume concentration of 60%. The amination reaction temperature was controlled at 60° C. and the reaction was carried out for 1 hour to obtain a tertiary amination reaction liquid.

6. The method for preparing special fertilizer for high value-added crops according to claim 1, characterized in that: In the MVR-centrifugal two-effect evaporation system in step (3), the first-effect evaporation system is MVR evaporation, the temperature is 80°C, the vacuum degree is 0.18MPa, and the evaporation time is 1h; the second-effect evaporation system is a centrifugal evaporation system, the centrifugal force is 2,000g, the temperature is 60°C, and the evaporation time is 1.5h.

7. The method for preparing special fertilizer for high value-added crops according to claim 1, characterized in that: In the step (4), the four-stage similar concentration countercurrent absorption is: The hydrogen chloride gas enters the primary absorption tank after being absorbed by the primary graphite, and is absorbed with desalted water as the absorbent; the gas obtained after the primary absorption enters the secondary absorption tank after being absorbed by the secondary graphite, and is absorbed with desalted water as the absorbent, and the liquid obtained after the secondary absorption flows back to the primary absorption tank for absorption; the gas obtained after the secondary absorption enters the tertiary absorption tower, and is absorbed with desalted water as the absorbent, and the liquid obtained after the tertiary absorption flows back to the secondary absorption tank for absorption; The gas obtained after the tertiary absorption enters the quaternary absorption tower and is absorbed with desalted water as the absorbent. The liquid obtained after the quaternary absorption flows back to the tertiary absorption tower for absorption, and the obtained gas is passed into the alkaline absorption liquid.

8. The method for preparing special fertilizer for high value-added crops according to claim 7, characterized in that: The alkaline absorption liquid is 5% NaOH.

9. The method for preparing special fertilizer for high value-added crops according to claim 7, characterized in that: In the step (4), the pressure of the four-stage similar concentration countercurrent absorption system is 0.18-0.21 MPa.

10. The method for preparing special fertilizer for high value-added crops according to claim 1, characterized in that: In the step (5), the mass proportions of potassium sulfate slurry, phosphoric acid, urea and filler are: potassium sulfate slurry 25-40%, phosphoric acid 20-25%, urea 30-35%, and filler 5-25%.

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