A method for preparing potassium sulfate from chlor-alkali industry electrolysis salt mud

Abstract

This invention relates to the field of potassium sulfate technology and discloses a method for preparing potassium sulfate from electrolytic salt mud in the chlor-alkali industry, comprising the following steps: S1 The salt mud is leached and subjected to solid-liquid separation to obtain a potassium-rich leachate and leaching residue; S2 The potassium-rich leachate is acidified and extracted to obtain a loaded organic phase and a raffinate phase; S3 The raffinate phase is separated to obtain potassium sulfate I and filtrate I, and filtrate I is evaporated and crystallized to obtain potassium sulfate II and filtrate II; filtrate II is acidified and reused in the extraction process of S2; S4 The extracted phase is back-extracted to obtain a sodium chloride solution and a deloaded organic phase, and the deloaded organic phase is reused in the extraction process of S2. This invention reduces the environmental pollution and resource waste caused by the indiscriminate dumping of electrolytic salt mud in the chlor-alkali industry, alleviates my country's dependence on imported potash fertilizer, does not introduce other impurities that affect the quality of potassium sulfate, has a short reaction time, and produces a high-quality product.

Description

Technical Field

[0001] This invention relates to the field of potassium sulfate technology, and more specifically, to a method for preparing potassium sulfate from electrolytic salt mud in the chlor-alkali industry. Background Technology

[0002] China is a major agricultural country, and potash fertilizer is an indispensable fertilizer in agricultural production. Common potash fertilizers include potassium chloride and potassium sulfate. However, with the continuous development of agriculture in my country, the planting volume of chlorine-sensitive crops such as sugar and fruit crops, tea, and tobacco is constantly increasing. China's demand for chlorine-free potash fertilizer is growing, and the requirement for "low chlorine" potash fertilizer is becoming increasingly stringent. Potassium sulfate has always been considered a representative of chlorine-free potash fertilizer. Therefore, research on the preparation of low-chlorine potassium sulfate is of great significance.

[0003] Currently, the methods for preparing potassium sulfate can be mainly classified according to the reaction mechanism as follows: pyrolysis (Mannheim process), metathesis, assemblage, and extraction. If classified according to the raw materials used, they can be mainly classified as follows: potassium sulfate production from potassium salt ore, potassium sulfate production from high-salt brine / seawater, and potassium sulfate production from industrial waste / wastewater.

[0004] The Mannheim process is a classic method for preparing potassium sulfate. The first step involves reacting potassium chloride and concentrated sulfuric acid at low temperature to produce potassium bisulfate and hydrogen chloride. The second step involves reacting potassium bisulfate and potassium chloride at high temperature (typically 500-600℃) to produce potassium sulfate, with hydrochloric acid as a byproduct. The advantage of this method is its mature technology, but its disadvantage is that the second step requires high-temperature reaction, and the concentrated sulfuric acid at high temperatures causes severe corrosion to the equipment, typically requiring replacement annually.

[0005] The metathesis method utilizes the reaction of sulfate and potassium chloride to produce potassium sulfate. This method requires low investment and simple equipment, but the product quality is low. Chinese patent CN 107857282 A provides a method for producing potassium sulfate using the metathesis method. This process first reacts sodium sulfate and potassium chloride at room temperature to produce potassium sodium sulfate, and then reacts potassium sodium sulfate with potassium chloride to produce potassium sulfate. This process requires the addition of pure potassium sulfate crystals to induce crystallization to produce potassium sulfate, and the purity of the obtained potassium sulfate is slightly low.

[0006] The associative process involves the association of an organic associating agent with sulfate ions, followed by the replacement of the associated sulfate ions with chloride ions. The replaced sulfate ions then combine with potassium ions in the aqueous phase to form potassium sulfate. This method offers mild reaction conditions, but the process is complex, requires precise operation, and suffers from unstable production. The Shenyang Chemical Comprehensive Utilization Research Institute applied for a patent related to the associative process for potassium sulfate production. This patent was purchased by Yantai Zhongxing Chemical Co., Ltd. in Shandong Province in 1993. After three years of design work, the plant finally started production normally, but it was forced to shut down after only a few months due to various reasons.

[0007] The extraction method involves first reacting sulfuric acid or ammonium sulfate with potassium chloride, then extracting the hydrochloric acid or ammonium chloride from the solution using an extractant. Finally, the mother liquor is concentrated and crystallized, or the mother liquor is directly crystallized to obtain potassium sulfate. Chinese patent CN 101746788 A provides a method for preparing potassium sulfate using extraction. This process first involves mixing potassium chloride and sulfuric acid, then adding an extractant composed of trioctylamine, n-butanol, isoamyl alcohol, sec-octyl alcohol, and kerosene. After extraction, the mixture is allowed to stand, separated into layers, and the potassium sulfate crystallized from the raffinate is dried to obtain the product. However, this method uses a large amount of organic solvents, and the chloride ion content of the potassium sulfate product is only <1%.

[0008] The method for producing potassium sulfate from potash ore involves a series of physical and chemical changes to obtain potassium sulfate from potash-containing ore. Generally, the potash ore is coarsely crushed, finely crushed, and screened, then subjected to a high-temperature reaction, followed by concentration and crystallization to obtain potassium sulfate. This method requires high temperatures and is energy-intensive; moreover, the purity of the obtained potassium sulfate is generally low. Chinese patent CN108349743 A provides a method for producing potassium sulfate from potash ore. This process involves contacting the potash ore with water to obtain mining brine, then contacting the mining brine with magnesium chloride to obtain a mixture of potassium-containing magnesium sulfate. This mixture is concentrated, and then the potassium-containing magnesium sulfate is contacted with magnesium sulfate and potassium sulfate to obtain potassium magnesium sulfate. Finally, water is used to remove magnesium sulfate from the potassium magnesium sulfate to obtain potassium sulfate. However, this method results in a low potassium recovery rate from the potash ore.

[0009] The common method for producing potassium sulfate from high-salinity brine / seawater involves spreading and concentrating the brine / seawater in salt fields, followed by evaporation crystallization or chemical precipitation. However, this method requires large-scale salt field construction, has a long production cycle, high energy consumption, and low efficiency. Another method uses adsorbents to directly adsorb potassium ions from the brine / seawater and then uses desorption to extract potassium and prepare potassium sulfate. However, this method is limited by the adsorption capacity and exchange capacity of the adsorbent. Chinese patent CN 115707656 A provides a method for extracting potassium sulfate from sun-dried seawater. This process involves spreading the sun-dried seawater, further concentrating it, and forcibly evaporating it to obtain a potassium-containing mixture. Sodium chloride is removed from the potassium-containing mixture using reverse flotation. The resulting low-sodium potassium-containing mixture undergoes a first-stage decomposition conversion at 10-40℃ and a second-stage conversion at 50-70℃ to obtain potassium sulfate. However, this method is time-consuming, and the quality of the obtained potassium sulfate product is relatively low, with a K2O content of only around 51%. SDIC Xinjiang Lop Nur Potash Co., Ltd. also produces potassium sulfate using the brine from Lop Nur's salt lake. The process involves evaporating and converting the Lop Nur brine to obtain potassium magnesium alum, followed by flotation and separation. The potassium magnesium alum is then converted into soft potassium magnesium alum, which is then converted into potassium sulfate by slightly brackish water. However, the quality of the potassium sulfate product obtained by this process is relatively low, with a chloride ion content of 1.06%.

[0010] The production of potassium sulfate from industrial waste / wastewater mainly relies on the composition and characteristics of the waste / wastewater, employing appropriate preparation methods. Chinese Patent CN 114455611 A discloses a method for producing potassium sulfate from potassium-containing solid waste generated during the calcination of bauxite to produce calcium aluminate. This process involves first subjecting the potassium-containing solid waste to multi-stage countercurrent leaching with water, followed by solid-liquid separation. The resulting liquid is rich in potassium ions and sulfate ions. This liquid is then evaporated, cooled, and crystallized to obtain potassium sulfate. However, this method is time-consuming, requiring 5-8 stages of continuous countercurrent leaching, with each stage lasting 1-2 hours. Chinese patent CN 105645439 A provides a method for producing potassium sulfate from high-salinity industrial wastewater. The process involves first using agents such as lime, sodium carbonate, sodium hydroxide, and PFS to remove most of the impurities from the high-salinity industrial wastewater, then further removing impurities using an ultrafiltration membrane, followed by concentration using a reverse osmosis membrane, preliminary salt separation using a nanofiltration membrane, and then further concentration. After producing sodium sulfate using a thermal or freeze-drying method, potassium sulfate is then prepared using a metathesis method. However, this method has a long process flow and high energy consumption in the preparation of sodium sulfate. Summary of the Invention

[0011] The technical problem solved by this invention:

[0012] To address the shortcomings of existing technologies, this paper provides a method for extracting potassium sulfate from electrolytic salt mud in the chlor-alkali industry. This method reduces environmental pollution and resource waste caused by the indiscriminate dumping of electrolytic salt mud in the chlor-alkali industry, alleviates the problem of my country's reliance on imported potash fertilizer, does not introduce other impurities that affect the quality of potassium sulfate, has a short reaction time, and produces high-quality products.

[0013] The technical solution adopted in this invention is as follows:

[0014] To address the aforementioned technical problems, the present invention aims to provide a method for extracting potassium sulfate from electrolytic salt sludge in the chlor-alkali industry. Based on this, the specific implementation scheme is as follows:

[0015] A method for preparing potassium sulfate from electrolytic salt mud in the chlor-alkali industry includes the following steps:

[0016] S1 salt mud is leached and separated into solid and liquid components to obtain potassium-rich leachate and leaching residue. The leaching is then used to leach potassium and chloride ions from the electrolytic salt mud in the chlor-alkali industry.

[0017] Specifically, the leaching agent used includes water, and the mass ratio of salt mud to leaching agent is 1:(1.4~1.8); the process parameters of the leaching process are: temperature of 20~40℃, time of 10~25min; stirring is carried out during the leaching process, and the stirring speed is 200-500r / min.

[0018] S2 potassium-rich leachate was acidified and extracted to obtain an organic-supported phase (extract phase) and a raffinate phase.

[0019] Specifically, concentrated sulfuric acid (18.4 mol / L) is used for acidification. The potassium-rich leachate contains chloride ions, and the molar ratio of concentrated sulfuric acid to chloride ions added to the potassium-rich leachate is (0.9–1.1):1. The components of the extractant used for extraction crystallization, by volume percentage, include 60–70% trioctylamine, 25–35% n-octanol, and 0–25% sulfonated kerosene; the extraction ratio O / A = (1.8–2.2):1; the process parameters for extraction crystallization are: temperature 30–40℃ and time 5–10 min. Acidification and extraction crystallization are used to remove chloride ions from the potassium-rich leachate while simultaneously obtaining a portion of potassium sulfate.

[0020] The S3 raffinate phase is separated to obtain potassium sulfate I and filtrate I. Filtrate I is evaporated and crystallized to obtain potassium sulfate II and filtrate II. Filtrate II is acidified and reused in the S2 extraction process.

[0021] Specifically, potassium sulfate I is obtained by rinsing and drying; in this technology, a small amount of water is used for rinsing and replacement followed by drying.

[0022] The evaporation crystallization process is performed at least twice; specifically, the two-stage evaporation crystallization method used in this application is as follows: evaporation crystallization includes evaporation crystallization I and evaporation crystallization II; after evaporation crystallization I, potassium sulfate II-1 and filtrate II-1 are obtained; potassium sulfate II-1 is washed and dried to obtain potassium sulfate product II-1, with the washing water accounting for 18% to 25% of the mass of potassium sulfate II-1; filtrate II-1 is subjected to evaporation crystallization II to obtain potassium sulfate II-2 and filtrate II-2. Potassium sulfate II-2 is washed with saturated potassium sulfate solution and dried to obtain potassium sulfate product II-2. Filtrate II-2 is acidified with sulfuric acid and then reused in the extraction process of S2; the concentration of sulfuric acid is 18.4 mol / L; V 滤液II-2 :V 硫酸 =50:(1.6~2.4).

[0023] The aforementioned evaporation crystallization process employs negative pressure constant temperature evaporation, with a temperature of 40–45°C, a rotation speed of 70–100 r / min, and a pressure of -0.1 MPa.

[0024] The S4 extract phase is back-extracted to obtain sodium chloride and a desupported organic phase (extractant). The desupported organic phase (extractant) is reused in the S2 extraction process.

[0025] Specifically, the back-extraction agent used includes sodium hydroxide, with a concentration of 7-8% (wt%); the back-extraction ratio is O / A = 6:(5-7); the back-extraction temperature is 40-50℃; and the reaction time is 2.5-5 min.

[0026] The technical mechanism and beneficial effects of this invention are as follows:

[0027] This invention first utilizes the high water solubility of KCl, the main component of chlor-alkali industrial electrolytic sludge from Huarong Chemical Co., Ltd., to leach potassium salts from the sludge using water as the leaching agent, obtaining a leachate rich in potassium and chloride ions. Then, leveraging the excellent extraction kinetics of trioctylamine, the good ability of n-octanol to prevent the formation of a third phase during extraction, and the superior dilution properties of sulfonated kerosene, chloride ions in the potassium-rich leachate are removed through extraction crystallization to directly obtain a portion of potassium sulfate product. Subsequently, to prevent excessively high chloride ion content in the potassium sulfate product obtained from the first evaporation crystallization, the raffinate filtrate is subjected to two consecutive evaporation crystallizations. The crystallized products from both evaporations are washed to obtain the remaining potassium sulfate. The obtained potassium sulfate products all have a chloride ion content of <0.1% and a K₂O content of >52%. Furthermore, the extract phase can be back-extracted, allowing the extractant to be recycled in the extraction process with good recycling performance. The secondary evaporation concentrate can also be returned to the extraction process for recycling through acidification. This invention reduces environmental pollution and resource waste caused by the indiscriminate dumping of electrolytic salt sludge in the chlor-alkali industry, alleviates my country's reliance on imported potash fertilizer, does not introduce other impurities that affect the quality of potassium sulfate, has a short reaction time, and produces high-quality products. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the process for preparing potassium sulfate from electrolytic salt mud in the chlor-alkali industry, provided in an embodiment of the present invention. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0030] Raw material description

[0031] The chlor-alkali industrial electrolytic salt mud used in each embodiment of this application is all from Huarong Chemical Co., Ltd., and its composition is shown in Table 1.

[0032] Table 1. Ion content in salt mud

[0033]

[0034] Example 1

[0035] 160g of chlor-alkali industrial electrolytic salt mud was weighed and added to a beaker containing 224g of water while stirring. After the addition was completed, the reaction continued for 10 minutes. The reaction conditions during this stage were: temperature 25℃, solid-liquid ratio 1:1.4, and stirrer speed 300r / min. After the reaction stopped, the mixture was filtered, and the beaker wall and filter cake were washed with a small amount of water to obtain a potassium-rich leachate. The potassium content was determined by the sodium tetraphenylborate gravimetric method, and the potassium leaching rate in the chlor-alkali industrial electrolytic salt mud was found to be 95.72%.

[0036] Take 230 ml of potassium-rich leachate, add 15 ml of concentrated sulfuric acid, and then slowly add the solution at 30°C. The composition is V. 三辛胺 :V 正辛醇 :V 磺化煤油 414 ml of an extractant with a concentration of 60%:35%:5% was added while stirring. After the addition was complete, the reaction continued for 5 minutes before being stopped. The raffinate phase was observed to contain some solid crystals. After standing and phase separation, the raffinate phase was filtered to obtain 36.75 g of wet potassium sulfate residue. This residue was washed with 7.35 g of water and dried at 105 °C to obtain 24.62 g of potassium sulfate. The potassium content was determined by the sodium tetraphenylborate gravimetric method, and the chlorine content was determined by silver nitrate titration. The K₂O content was found to be 53.48%, and the Cl content was [not specified]. - The content was 0.08%; the chloride ion concentration in the obtained raffinate filtrate was 0.46464 mol / L.

[0037] The extract phase was back-extracted using a 7% (wt%) NaOH solution as the back-extracting agent. Under the back-extraction experimental conditions of O / A = 6:5, reaction temperature 40℃, reaction time 2.5 min, and stirring speed 400 r / min, the back-extraction rate reached 100%. The raffinate was a NaCl solution, and the back-extracted extract phase could be reused in the extraction process.

[0038] 200 ml of the raffinate filtrate was taken and subjected to two separate evaporation crystallization processes. The evaporation was carried out under negative pressure and constant temperature conditions: 40℃, 90 r / min, and -0.1 MPa. Each evaporation concentrated the product by a factor of two. After filtration following the first evaporation crystallization, 8.02 g of wet residue was obtained. This residue was washed with 2 g of water and dried at 105℃ to obtain 7.38 g of potassium sulfate. After filtration following the second evaporation crystallization, the residue was washed with a saturated potassium sulfate solution and dried at 105℃ to obtain 3.71 g of potassium sulfate. The Cl- content in the K2SO4 product from the first evaporation crystallization was... - The contents of water-soluble K2O were 0.0500% and 53.47%, respectively, and the Cl content in the secondary evaporation crystallization product K2SO4 was... - The contents of water-soluble K2O were 0.0713% and 53.62%, respectively.

[0039] Add 1.6 mL of sulfuric acid to 50 mL of the secondary evaporation concentrate, then add this to a beaker containing 200 mL of potassium-rich leachate and 13 mL of concentrated sulfuric acid. Slowly add the solution at 30°C, resulting in a composition of V. 三辛胺 :V 正辛醇 :V 磺化煤油 445 ml of an extractant with a ratio of 60%:35%:5% was added while stirring. After the addition was complete, the reaction continued for 5 minutes, then stopped. The mixture was allowed to stand for phase separation, and the raffinate was filtered. The chloride ion concentration in the raffinate filtrate was 0.46596 mol / L. The extractant phase was back-extracted using a 7% (wt%) NaOH solution. Under the back-extraction experimental conditions of a phase ratio of O / A = 6:5, a reaction temperature of 40℃, a reaction time of 2.5 min, and a stirring speed of 400 r / min, the back-extraction rate reached 100%. The back-extracted raffinate was a NaCl solution. The back-extracted extractant phase can be reused in the extraction process.

[0040] Example 2

[0041] 160g of chlor-alkali industrial electrolytic salt mud was weighed and added to a beaker containing 288g of water while stirring. After the addition was completed, the reaction continued for 10 minutes. The reaction conditions during this stage were: temperature 40℃, solid-liquid ratio 1:1.8, and stirrer speed 300r / min. After the reaction stopped, the mixture was filtered, and the beaker wall and filter cake were washed with a small amount of water to obtain a potassium-rich leachate. The potassium content was determined by the sodium tetraphenylborate gravimetric method, and the potassium leaching rate in the chlor-alkali industrial electrolytic salt mud was found to be 97.18%.

[0042] Take 250 ml of potassium-rich leachate, add 10.0 ml of concentrated sulfuric acid, and slowly add the solution at 40°C. The composition is V. 三辛胺 :V 正辛醇 :V 磺化煤油 414 ml of an extractant with a concentration of 70%:30%:10% was added while stirring. After the addition was complete, the reaction continued for 10 minutes before stopping. The raffinate phase was observed to contain some solid crystals. After standing and phase separation, the raffinate phase was filtered to obtain 4.37 g of wet potassium sulfate residue. This residue was washed with 1 g of water and dried at 105 °C to obtain 2.69 g of potassium sulfate. The potassium content was determined by the sodium tetraphenylborate gravimetric method, and the chlorine content was determined by the silver nitrate titration method. The K₂O content was found to be 53.26%, and the Cl content was [not specified]. - The content was 0.06%; the chloride ion concentration in the obtained raffinate filtrate was 0.34606 mol / L.

[0043] The extract phase was back-extracted using an 8% (wt%) NaOH solution as the back-extracting agent. Under the back-extraction experimental conditions of O / A = 6:7, reaction temperature 50℃, reaction time 5 min, and stirring speed 400 r / min, the back-extraction rate could reach 100%. The raffinate was a NaCl solution, and the back-extracted extract phase could be reused in the extraction process.

[0044] 200 ml of the raffinate filtrate was taken and subjected to two separate evaporation crystallization processes. The evaporation was carried out under negative pressure and constant temperature conditions: 45℃, 100 r / min, and -0.08 MPa. Each evaporation concentrated the product by 1.5 times. After filtration following the first evaporation crystallization, 7.66 g of wet residue was obtained. This residue was washed with 1.5 g of water and dried at 105℃ to obtain 5.23 g of potassium sulfate. After filtration following the second evaporation crystallization, the residue was washed with a saturated potassium sulfate solution and dried at 105℃ to obtain 2.36 g of potassium sulfate. The Cl- in the K₂SO₄ product from the first evaporation crystallization was... - The contents of water-soluble K2O were 0.0532% and 53.12%, respectively, and the Cl content in the secondary evaporation crystallization product K2SO4 was... - The contents of water-soluble K2O were 0.0711% and 53.47%, respectively.

[0045] Add 1.6 mL of sulfuric acid to 50 mL of the secondary evaporation concentrate, then add this to a beaker containing 200 mL of salt leaching solution and 8 mL of concentrated sulfuric acid. Slowly add the solution at 40 °C, resulting in a composition of V. 三辛胺 :V 正辛醇 :V 磺化煤油 450 ml of an extractant with a ratio of 70%:30%:10% was added while stirring. After the addition was complete, the reaction continued for 10 minutes, then stopped. The mixture was allowed to stand for phase separation, and the raffinate was filtered. The chloride ion concentration in the raffinate filtrate was 0.34976 mol / L. The extractant phase was back-extracted using an 8% (wt%) NaOH solution. Under the back-extraction experimental conditions of O / A = 6:7, reaction temperature 50℃, reaction time 5 min, and stirring speed 400 r / min, the back-extraction rate reached 100%. The back-extracted raffinate was a NaCl solution. The back-extracted extractant phase can be reused in the extraction process.

[0046] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing potassium sulfate from electrolytic salt mud in the chlor-alkali industry, characterized in that, The steps include the following: S1 salt mud is leached and separated into solid and liquid components to obtain potassium-rich leachate and leaching residue; S2 potassium-rich leachate is acidified and extracted and crystallized to obtain a loaded organic phase and a raffinate phase; the components of the extractant used for extraction and crystallization, by volume percentage, include 60-70% trioctylamine, 25-35% n-octanol, and 0-25% sulfonated kerosene; The S3 raffinate phase is separated to obtain potassium sulfate I and filtrate I. Filtrate I is evaporated and crystallized to obtain potassium sulfate II and filtrate II. Filtrate II is acidified and reused in the S2 extraction process. The S4 loaded organic phase is back-extracted to obtain a sodium chloride solution and a deloaded organic phase. The deloaded organic phase is reused in the S2 extraction process.

2. The method for preparing potassium sulfate from electrolytic salt mud in the chlor-alkali industry according to claim 1, characterized in that, S1 includes at least one of features (S1-1) to (S1-3): (S1-1) The leaching agent used includes water, and the mass ratio of salt mud to leaching agent is 1:(1.4~1.8); (S1-2) Leaching process: temperature 20~40℃, time 10~25min; (S1-3) Stirring is used during the leaching process at a speed of 200-500 r / min.

3. The method for preparing potassium sulfate from electrolytic salt mud in the chlor-alkali industry according to claim 1, characterized in that, S2 includes at least one of features (S2-1) to (S2-2): (S2-1) The acid used for acidification is concentrated sulfuric acid with a concentration of 18.4 mol / L; (S2-2) The potassium-rich leachate contains chloride ions, and the molar ratio of the amount of concentrated sulfuric acid added to the molar amount of chloride ions in the potassium-rich leachate is (0.9~1.1):

1.

4. The method for preparing potassium sulfate from electrolytic salt mud in the chlor-alkali industry according to claim 1, characterized in that, S2 includes at least one of features (S2-3) to (S2-4): (S2-3) Extraction ratio O / A = (1.8~2.2):1; (S2-4) Extraction and crystallization process: temperature is 30~40℃, time is 5~10min.

5. The method for preparing potassium sulfate from electrolytic salt mud in the chlor-alkali industry according to claim 1, characterized in that, S3 includes feature (S3-1): (S3-1) Potassium sulfate I is obtained by rinsing and drying.

6. The method for preparing potassium sulfate from electrolytic salt mud in the chlor-alkali industry according to any one of claims 1 to 5, characterized in that, S3 includes at least one of features (S3-2) to (S3-3): (S3-2) The evaporation and crystallization process shall be performed at least twice; (S3-3) Evaporation crystallization includes evaporation crystallization I and evaporation crystallization II; evaporation crystallization I yields potassium sulfate II-1 and filtrate II-1; potassium sulfate II-1 is washed and dried to obtain potassium sulfate product II-1; filtrate II-1 is evaporated and crystallized II to obtain potassium sulfate II-2 and filtrate II-2.

7. The method for preparing potassium sulfate from electrolytic salt mud in the chlor-alkali industry according to claim 6, characterized in that, S3 includes features (S3-4): (S3-4) Evaporation crystallization is carried out under negative pressure and constant temperature evaporation, with a temperature of 40~45 ℃, a rotation speed of 70~100 r / min, and a pressure of -0.1 MPa.

8. The method for preparing potassium sulfate from electrolytic salt mud in the chlor-alkali industry according to claim 6, characterized in that, (S3-3) includes feature (S3-3-1): (S3-3-1) Potassium sulfate II-2 is obtained by washing and drying with saturated potassium sulfate solution.

9. The method for preparing potassium sulfate from electrolytic salt mud in the chlor-alkali industry according to claim 6, characterized in that, (S3-3) includes feature (S3-3-2): (S3-3-2) Filtrate II-2 is acidified with sulfuric acid and then reused in the extraction process of S2; the concentration of sulfuric acid is 18.4 mol / L; V 滤液II-2 :V 硫酸 =50:(1.6~2.4).

10. The method for preparing potassium sulfate from electrolytic salt mud in the chlor-alkali industry according to any one of claims 1 to 5, characterized in that, S4 includes feature (S4-1): (S4-1) The back-extraction agent used includes sodium hydroxide, with a concentration of 7-8 wt%; the back-extraction ratio is O / A = 6:(5-7), the back-extraction temperature is 40-50℃, and the reaction time is 2.5-5 min.

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