Method for purifying sylvite from biomass ash

Abstract

A method for purifying sylvite from biomass ash comprises the following steps: 1) acid leaching: mixing the biomass ash with an acid solution, stirring, and carrying out solid-liquid separation to obtain potassium-containing acid leaching liquid and leached ash; 2) impurity removal: adding alkali into the potassium-containing pickle liquor to adjust the pH value to 11-13, adding sodium carbonate, stirring, and carrying out solid-liquid separation to obtain a potassium-containing solution and a metal precipitate; 3) concentration: concentrating the potassium-containing solution; 4) evaporative crystallization: performing evaporative crystallization on the concentrated potassium-containing solution to obtain potassium salt and a residual sodium-containing solution; 5) drying: drying the potassium salt to obtain a potassium salt product; and bipolar membrane electrodialysis: introducing the residual sodium-containing solution into a bipolar membrane electrodialysis device for bipolar membrane electrodialysis treatment, recycling the obtained acid solution to the step 1), and recycling the obtained alkali to the step 2). According to the method, high-purity sylvite can be obtained, the product value is high, the process is simple, the cost is low, and cyclic utilization of water resources is realized.

Description

Technical Field

[0001] The present invention relates to the field of resource recycling, and in particular to a method for extracting and purifying potassium salt from biomass fly ash. Background Art

[0002] As a new type of renewable and important energy source, biomass has natural "zero carbon" characteristics and unique "negative carbon" potential, providing strong support for achieving the "dual carbon" goal. Therefore, biomass power generation has received strong support from the country and has become a relatively important means to achieve carbon neutrality. Biomass ash will be produced after biomass combustion for power generation. Studies have shown that more than 10 million tons of biomass are directly burned for power generation in my country each year. According to this method of use, the average ash yield of biomass is about 6%-7%, and nearly 7 million tons of biomass ash can be produced each year. In recent years, with the transformation of my country's energy strategy, this number has continued to increase. Similar to other wastes, more than 90% of biomass ash is currently landfilled. This traditional treatment method occupies land and is also a waste of resources. How to dispose of biomass ash at low cost and harmlessly has become an important problem that needs to be solved urgently.

[0003] Biomass ash contains abundant plant potassium resources. In view of the current situation that my country's mineral potassium resources are scarce and difficult to develop, if potassium salt can be extracted from biomass ash reasonably, effectively and economically, it will not only solve the solid waste problem of power plants, but also alleviate the current shortage of potassium resources. There have been relevant reports on the extraction of potassium from biomass at home and abroad. The water washing method or leaching method is mainly used to extract potassium elements, and many research results have been achieved. However, there are currently the following problems in the purification of potassium salt from biomass ash: 1) The leaching solution contains a large amount of impurities, such as calcium, magnesium, aluminum, heavy metals, etc., and the fertilizer prepared without impurities has the disadvantages of low fertilizer efficiency and low product value; 2) The cost of using water to extract potassium elements is low, but the potassium extraction rate is not high, which is not suitable for biomass ash with a low proportion of water-soluble potassium; 3) The cost of adding reagents in the impurity removal section of the process of preparing high-purity potassium salt from biomass ash is high, and the secondary wastewater generated is difficult to treat. Therefore, a simple, efficient and economical extraction and purification technology of high-purity potassium salt from biomass ash is urgently needed. Summary of the invention

[0004] The following is an overview of the subject matter described in detail herein. This overview is not intended to limit the scope of protection of this application.

[0005] In view of the problems existing in the prior art, the purpose of this application is to provide a method for extracting and purifying high-purity potassium salt from biomass ash, using acid leaching to extract potassium from biomass ash, and obtaining high-purity potassium salt after the supernatant is removed, concentrated, evaporated, crystallized, and dried. The remaining liquid enters bipolar membrane electrodialysis to regenerate acid and alkali for reuse, and the leached ash residue after impurities removal can be used to prepare high-quality building materials or adsorbents. The method can obtain high-purity potassium salt with high product value, simple process, low cost, and realize water resource recycling.

[0006] To achieve the above object, the present application provides a method for purifying potassium salt from biomass ash, comprising the following steps: 1) Acid leaching: Mix the biomass ash with the acid solution, stir, separate the solid and liquid, and obtain the potassium-containing acid leaching solution and the ash residue after leaching; 2) Impurity removal: add alkali to the potassium-containing acid leaching solution to adjust the pH to 11-13, add sodium carbonate and stir, separate the solid and liquid, and obtain a potassium-containing solution and a metal precipitate; 3) Concentration: Concentrate the potassium-containing solution; 4) Evaporation and crystallization: evaporate and crystallize the concentrated potassium-containing solution to obtain potassium salt and the remaining sodium-containing solution; 5) Drying: drying the potassium salt to obtain a potassium salt product; and Bipolar membrane electrodialysis: The remaining sodium-containing solution is passed into a bipolar membrane electrodialysis device for bipolar membrane electrodialysis treatment, and the obtained acid solution is reused in step 1), and the obtained alkali is reused in step 2).

[0007] In an exemplary embodiment, in step 1), the acid solution includes but is not limited to any one or a combination of at least two of hydrochloric acid, nitric acid and sulfuric acid.

[0008] In an exemplary embodiment, the acid solution is sulfuric acid, wherein sulfate ions can precipitate with impurity calcium ions, thereby reducing the concentration of impurities in the potassium-containing acid leaching solution and reducing the difficulty of subsequent treatment.

[0009] In an exemplary embodiment, in step 1), the equivalent molar ratio of hydrogen ions in the acid solution to potassium ions in the biomass ash is 1:1-8:1; for example, 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, etc. or any two of them as range values ​​between endpoint values, but are not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0010] In an exemplary embodiment, in step 1), the ash-water ratio of the biomass ash and the acid solution is 1:2-1:10; for example, 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, 1:10, etc. or any two of them as range values ​​between the endpoint values, but not limited to the listed values, other unlisted values ​​within the numerical range are also applicable. The "ash-water ratio" here refers to the ratio of the mass (g) of the biomass ash to the volume (ml) of the acid solution.

[0011] In an exemplary embodiment, in step 1), the stirring time is 5 min-2 h; for example, 5 min, 20 min, 30 min, 1 h, 1.5 h, 2 h, etc. or any two of them as the range value between the endpoint values, but are not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0012] In an exemplary embodiment, in step 1), the stirring speed is 200 r / min-500 r / min; for example, 200 r / min, 300 r / min, 400 r / min, 500 r / min, etc. or any two of them as the range value between the endpoint values, but are not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0013] In an exemplary embodiment, in step 1), the temperature at which biomass ash and acid solution are mixed is 20°C-80°C; for example, 20°C, 25°C, 40°C, 60°C, 80°C, etc. or any two of them as range values ​​between endpoint values, but are not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0014] In an exemplary embodiment, in step 1), in order to further reduce the amount of reagent added and the amount of purification treatment and reduce costs, step 1) also includes multiple acid leaching.

[0015] In an exemplary embodiment, the multiple acid leaching steps include: adding the acid solution and the biomass ash to the potassium-containing acid leaching solution obtained after the previous solid-liquid separation, mixing, stirring, and solid-liquid separation; repeating multiple times until the supernatant after the last acid leaching is used in step 2). In order to reduce the amount of acid added, reduce the cost of the reagent, and reduce the difficulty of processing in subsequent steps, the acid solution and the biomass ash are added to the potassium-containing acid leaching solution each time to replace part of the acid solution.

[0016] In an exemplary embodiment, the number of acid leaching is 2-4 times.

[0017] In an exemplary embodiment, during each acid leaching process, the ash-to-water ratio of the biomass ash added and the acid solution is 1:2-1:10; the stirring time is 5 min-2 h, and the stirring speed is 200 r / min-500 r / min; the temperature of the biomass ash and the acid solution is 20°C-80°C.

[0018] In an exemplary embodiment, in step 2), the base is an alkaline aqueous solution or a solid that is alkaline when dissolved in water; wherein the alkaline aqueous solution includes but is not limited to aqueous solutions of sodium hydroxide, potassium hydroxide, calcium hydroxide, etc., and the solid that is alkaline when dissolved in water includes but is not limited to solids such as calcium oxide, sodium hydroxide, potassium hydroxide, etc.

[0019] In an exemplary embodiment, in step 2), the equivalent molar ratio of the carbonate ion of sodium carbonate to the multivalent metal ions in the potassium-containing acid leaching solution is 1-1.2; for example, 1, 1.05, 1.1, 1.15, 1.2, etc. or any two of them as range values ​​between the endpoint values, but are not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0020] In an exemplary embodiment, in step 2), the stirring time is 5 min-20 min; for example, 5 min, 10 min, 20 min, etc. or any two of them as the range value between the endpoint values, but are not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0021] In an exemplary embodiment, in step 2), the stirring speed is 200 r / min-500 r / min.

[0022] In an exemplary embodiment, in step 2), adding alkali to the potassium-containing acid leaching solution to adjust the pH to 11-13 includes: directly adding alkali at one time to adjust the pH to 11-13 or adding alkali in steps to adjust the pH to 11-13; preferably, adding alkali in steps can greatly reduce the amount of reagent added, avoiding the precipitation and redissolution of aluminum, chromium and arsenic ions in the process of directly adjusting the pH to 11-13, resulting in waste of alkali.

[0023] In an exemplary embodiment, the stepwise addition of alkali to adjust the pH to 11-13 comprises a) adding alkali to the potassium-containing acid leaching solution obtained in step 1) to adjust the pH; b) solid-liquid separation to obtain supernatant and metal precipitate; c) adding alkali to the obtained supernatant again to adjust the pH; d) Repeat steps b) and c), add alkali to the supernatant obtained finally to adjust the pH to 11-13, add sodium carbonate, stir, separate the solid and liquid, and the obtained potassium-containing solution is used in step 3).

[0024] In an exemplary embodiment, the step-by-step alkali addition method is a three-step alkali addition process, wherein in step a), the pH of the potassium-containing acid leaching solution is adjusted to 7-9, and this step can remove more than 99% of aluminum and iron impurity ions and most of the heavy metal ions in the potassium-containing acid leaching solution after solid-liquid separation; in step c), the pH of the supernatant is adjusted to 10-11, and this step can remove most of the magnesium ions after solid-liquid separation; in step d), the pH of the supernatant is adjusted to 11-13, and this step can remove more than 99.8% of the impurity ions in the potassium-containing acid leaching solution except sodium ions after solid-liquid separation.

[0025] In an exemplary embodiment, in step d), the pH of the supernatant is adjusted to 12-13.

[0026] In an exemplary embodiment, in order to reduce the amount of alkali added, reduce the cost of reagents, and reduce the difficulty of processing in subsequent steps, a mixture of biomass ash and alkali (e.g., cheap calcium oxide / calcium hydroxide) can be used in step a) to replace part of the alkali to reduce the introduction of other impurity ions caused by the addition of alkali.

[0027] In an exemplary embodiment, in step 3), the concentration method is evaporator concentration or membrane concentration.

[0028] In an exemplary embodiment, in step 3), the concentration multiple is 2-4 times.

[0029] In an exemplary embodiment, in step 4), the evaporation crystallization is selected from one or more of single-effect evaporation crystallization, multiple-effect evaporation crystallization, and mechanical vapor recompression evaporation crystallization.

[0030] In an exemplary embodiment, in step 5), the bipolar membrane electrodialysis device is a conventional "acid-salt-alkali" three-chamber membrane stack structure, wherein the alkali chamber, salt chamber and acid chamber are arranged in sequence, the alkali chamber and the salt chamber are separated by a cation exchange membrane, the salt chamber and the acid chamber are separated by an anion exchange membrane, and the acid chamber and the alkali chamber are separated by a bipolar membrane. The salt chamber is initially charged with the remaining sodium-containing solution of step (4), the acid chamber is initially charged with water or dilute acid, and the alkali chamber is charged with water or dilute alkali. Under the action of the electric field, the water molecules in the bipolar membrane dissociate to generate hydroxide ions and hydrogen ions, the hydrogen ions combine with the anions in the acid chamber to obtain an acid solution, and the hydroxide ions combine with the cations in the alkali chamber to obtain an alkali solution. The acid solution obtained in the acid chamber can be reused in step (1), and the alkali obtained in the alkali chamber can be reused in step (2).

[0031] Compared with the prior art, this application has the following technical effects: (1) This application is applicable to the purification of potassium salts from a variety of biomass ashes, including biomass ashes with high water-soluble potassium content and / or high acid-soluble potassium content. Compared with water, the potassium extraction rate is greatly improved by using acid as the leaching liquid to extract biomass ashes with high acid-soluble potassium content. After the acid leaching liquid is removed from impurities, concentrated, evaporated, crystallized, and dried, the purity of the potassium salt product obtained can reach 97%, meeting the standard of superior agricultural potassium sulfate.

[0032] (2) This application adopts step-by-step alkali adjustment and multiple acid leaching technology. When treating the same amount of biomass ash, the amount of reagent added can be reduced by 45%, and the amount of water treated by evaporation crystallization and bipolar membrane electrodialysis can be reduced by 45%, thereby significantly reducing the purification cost.

[0033] (3) This application utilizes bipolar membrane electrodialysis to achieve water resource recycling without generating secondary wastewater. The leached ash after impurities removal can be used to prepare high-quality building materials / adsorbents, etc.

[0034] Other features and advantages of the present application will be described in the following description, and partly become apparent from the description, or be understood by implementing the present application. Other advantages of the present application can be realized and obtained by the schemes described in the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The accompanying drawings are used to provide an understanding of the technical solution of the present application and constitute a part of the specification. Together with the embodiments of the present application, they are used to explain the technical solution of the present application and do not constitute a limitation on the technical solution of the present application.

[0036] Figure 1 This is a flow chart of a method for purifying potassium salt from biomass ash described in the present application. DETAILED DESCRIPTION

[0037] In order to make the purpose, technical solution and advantages of the present application more clear, the embodiments of the present application will be described in detail below. It should be noted that the embodiments and features in the embodiments of the present application can be combined with each other arbitrarily without conflict.

[0038] The present invention is further described in detail below in conjunction with specific examples, but the examples should not be construed as limiting the present invention.

[0039] In the embodiment of the present application, biomass ash produced by a biomass power plant is used as raw material, and high-purity potassium salt can be obtained through acid leaching, impurity removal, concentration, evaporation crystallization, bipolar membrane electrodialysis, and drying. At the same time, the alkali addition technology and multiple leaching technology are used to greatly reduce the process cost and the difficulty of subsequent treatment. Bipolar membrane electrodialysis is used to regenerate acid and alkali to achieve water resource recycling.

[0040] Unless otherwise specified, the raw materials in this application are all conventional products on the market.

[0041] Example 1 1) 100 g of biomass ash was mixed with hydrochloric acid solution at an ash-water ratio (g / ml) of 1:6, the molar ratio of hydrogen ions in the hydrochloric acid solution to potassium ions in the biomass ash was controlled to be 8:1, the hydrogen ion concentration in the hydrochloric acid solution was 1.75 mol / L, and the mixture was stirred at 300 r / min at 40°C for 30 min, and solid-liquid separation was performed to obtain an acid leaching solution with a potassium ion concentration of 6.8 g / L and ash residue after leaching; 2) adding 27.3 g of sodium hydroxide solid to the potassium-containing acid leaching solution obtained in step 1), adjusting the pH to 11.9, adding 32.0 g of sodium carbonate at an equivalent molar ratio of carbonate to polyvalent metal ions in the potassium-containing acid leaching solution of 1.05, stirring at 300 r / min for 10 min, separating the solid and liquid, and achieving an impurity removal rate of 99.5% in the supernatant except for sodium ions; 3) The supernatant obtained in step 2) was evaporated 3 times using an evaporator; 4) The supernatant obtained in step 3) was evaporated and crystallized to obtain 5.2 g of potassium chloride with a purity of 90.7%; 5) The potassium chloride solid obtained in step 4) is dried at 105° C. for 2 h to obtain 4.9 g of potassium chloride product with a purity of 94.6%; at the same time, the remaining sodium-containing solution obtained in step 4) is passed into the salt chamber of a bipolar membrane electrodialysis device for bipolar membrane electrodialysis treatment, and 0.45 L of a 2 mol / L hydrochloric acid solution is obtained in the acid chamber, and 0.45 L of a 2 mol / L alkaline solution (containing sodium hydroxide and potassium hydroxide) is obtained in the alkali chamber, which can be reused in step 1) and step 2, respectively. The remaining desalted solution in the salt chamber can be returned to step 1) for diluting the acid or alkali.

[0042] Comparative Example 1 100 g of biomass ash was mixed with water at an ash-to-water ratio (g / ml) of 1:6, stirred at 300 r / min for 30 min, and the solid-liquid separation was performed. The potassium ion concentration in the supernatant was 0.9 g / L, which was 97% lower than that in Example 1.

[0043] Example 2 1) 100 g of biomass ash was mixed with sulfuric acid solution at an ash-water ratio (g / ml) of 1:3, the molar ratio of hydrogen ions in the sulfuric acid solution to potassium ions in the biomass ash was controlled to be 6:1, the hydrogen ion concentration in the sulfuric acid solution was 2.6 mol / L, and the mixture was stirred at 500 r / min for 20 min at 25°C. The solid-liquid separation was performed to obtain an acid leaching solution with a potassium ion concentration of 12.4 g / L and ash residue after leaching; 2) Alkali adjustment in steps: add 10.5 g of calcium hydroxide solid to the potassium-containing acid leaching solution obtained in step 1), adjust the pH to 8.5, separate the solid and liquid, and the removal rate of magnesium, aluminum and iron impurity ions in the supernatant reaches 99.5%, and the removal rate of heavy metal ions reaches 99.0%; in the second step, add 3.1 g of sodium hydroxide solid, adjust the pH to 13.1, add 1.0 g of sodium carbonate according to the equivalent molar ratio of carbonate to multivalent metal ions in the potassium-containing acid leaching solution of 1.2, stir at 300 r / min for 5 min, separate the solid and liquid, and the impurity removal rate of the supernatant except sodium ions reaches 99.7%; 3) The supernatant obtained in step 2) was evaporated 3 times using an evaporator; 4) The supernatant obtained in step 3) was evaporated and crystallized to obtain 7.6 g potassium sulfate with a purity of 93.2%; 5) The potassium sulfate solid obtained in step 4) is dried at 105° C. for 2 h to obtain 7.3 g of potassium sulfate product with a purity of 96.2%. Meanwhile, the remaining sodium-containing solution obtained in step 4) is passed into the salt chamber of a bipolar membrane electrodialysis device for bipolar membrane electrodialysis treatment, and 0.05 L of a 1 mol / L sulfuric acid solution is obtained in the acid chamber, and 0.05 L of a 2 mol / L alkaline solution (containing sodium hydroxide and potassium hydroxide) is obtained in the alkaline chamber. The remaining desalted solution in the salt chamber can be returned to step 1) for diluting the acid or alkali.

[0044] By using sulfuric acid as the acid solution, the concentration of impurity calcium ions in the obtained potassium-containing acid leaching solution is only 0.6 g / L, which is 88%-96% lower than that using other strong acids. In addition, the two-step alkali adjustment technology used in this embodiment reduces the amount of hydroxide ions added by 25% and the amount of sodium carbonate added by 77% compared with direct alkali addition. In addition, the concentration of impurity sodium ions obtained in step (2) is only 6.7 g / L, which reduces the difficulty of subsequent treatment.

[0045] Example 3 1) 1000 g of biomass ash was mixed with sulfuric acid solution at an ash-to-water ratio (g / ml) of 1:4, the molar ratio of hydrogen ions in the sulfuric acid solution to potassium ions in the biomass ash was controlled to be 7:1, the hydrogen ion concentration in the sulfuric acid solution was 2.7 mol / L, and the mixture was stirred at 300 r / min at 80°C for 10 min, and solid-liquid separation was performed to obtain a first acid leaching solution with a potassium ion concentration of 10.4 g / L and ash residue after leaching; 1000 g of new biomass ash and sulfuric acid were added to the first acid leaching solution, wherein the source of sulfuric acid was 0.3 L of concentrated sulfuric acid and 0.05 L of 0.8 mol / L sulfuric acid solution obtained by bipolar membrane electrodialysis, the molar ratio of hydrogen ions in the sulfuric acid solution to potassium ions in the biomass ash was controlled to be 7:1, the ash-to-water ratio (g / ml) of new biomass ash to sulfuric acid was 1:4, the hydrogen ion concentration in the sulfuric acid solution was 2.7 mol / L, and the mixture was stirred at 300 r / min at 80°C for 10 min, and solid-liquid separation was performed to obtain a potassium ion concentration of 19.8 g / L of the second acid leaching solution and the ash after leaching; 2) Alkali adjustment in steps: add 500 g of new biomass ash to the second acid leaching solution obtained in step 1), adjust the pH to 7.1, then add 5.8 g of calcium oxide, adjust the pH to 8.1, separate the solid and liquid, and the total removal rate of impurities magnesium, aluminum and iron ions in the supernatant reaches 60.1%, and the removal rate of heavy metal ions reaches 79.5%; in the second step, add 11.2 g of calcium oxide to the supernatant, adjust the pH to 9.8, separate the solid and liquid, and the total removal rate of impurities magnesium, aluminum and iron ions in the supernatant reaches 71.8%, and the removal rate of heavy metal ions reaches 91.2%; in the third step, add strong alkali to the supernatant, wherein the source of the strong alkali is 30 g of sodium hydroxide solid and 0.05 L of 2 mol / L alkali solution (containing sodium hydroxide and potassium hydroxide) obtained by bipolar membrane electrodialysis, adjust the pH to 12.3, add 5.6 g of sodium carbonate according to the equivalent molar ratio of carbonate to multivalent metal ions in the potassium-containing acid leaching solution of 1.1, 300 Stir at r / min for 5 min, separate the solid and liquid, and the removal rate of impurities except sodium ions in the supernatant reaches 99.8%; 3) The supernatant obtained in step 2) is concentrated by 2 times using a reverse osmosis membrane; 4) The concentrated and partially impurity-free potassium-containing solution obtained in step 3) was evaporated and crystallized to obtain 168.3 g of potassium sulfate with a purity of 94.5%; 5) The potassium sulfate solid obtained in step 4) is dried at 105° C. for 2 h to obtain 163.8 g of potassium sulfate product with a purity of 97.1%, which meets the standard of high-quality agricultural potassium sulfate. Meanwhile, the remaining sodium-containing solution obtained in step 4) is passed into the salt chamber of a bipolar membrane electrodialysis device for bipolar membrane electrodialysis treatment, and 0.05 L of 0.8 mol / L sulfuric acid solution is obtained in the acid chamber, and 0.05 L of 2 mol / L alkaline solution (containing sodium hydroxide and potassium hydroxide) is obtained in the alkaline chamber. The remaining desalted solution in the salt chamber can be returned to step 1) for diluting the acid or alkali.

[0046] This embodiment uses the secondary acid leaching technology. Compared with the single acid leaching, when treating the same amount of biomass ash, the amount of alkali added in the impurity removal process is reduced by 45%, the amount of sodium carbonate added is reduced by 50%, and the amount of water treated in evaporation crystallization and bipolar membrane electrodialysis is reduced by 45%, which greatly reduces the operating cost.

[0047] Although the embodiments of the present application have been shown and described above, it can be understood that the above embodiments are exemplary and cannot be understood as limitations on the present application. Ordinary technicians in this field can change, modify, replace and modify the above embodiments within the scope of the present application.

Claims

1. A method for purifying potassium salt from biomass ash, characterized in that: The following steps are involved: 1) Acid leaching: Mix the biomass ash with the acid solution, stir, separate the solid and liquid, and obtain the potassium-containing acid leaching solution and the ash residue after leaching; 2) removing impurities: adding alkali to the potassium-containing acid leaching solution to adjust the pH to 11-13, adding sodium carbonate, stirring, and separating the solid and liquid to obtain a potassium-containing solution and a metal precipitate; 3) Concentrating: concentrating the potassium-containing solution; 4) Evaporation and crystallization: evaporating and crystallizing the concentrated potassium-containing solution to obtain potassium salt and the remaining sodium-containing solution; 5) Drying: drying the potassium salt to obtain a potassium salt product; and Bipolar membrane electrodialysis: the remaining sodium-containing solution is passed into a bipolar membrane electrodialysis device for bipolar membrane electrodialysis treatment, and the obtained acid solution is reused in step 1), and the obtained alkali is reused in step 2).

2. The method according to claim 1, characterized in that In step 1), the acid solution includes but is not limited to any one of hydrochloric acid, nitric acid and sulfuric acid or a combination of at least two thereof; Optionally, the acid solution is sulfuric acid.

3. The method according to claim 1 or 2, characterized in that: In step 1), the equivalent molar ratio of hydrogen ions in the acid solution to potassium ions in the biomass ash is 1:1-8:1; and / or The ash-water ratio of the biomass ash to the acid solution is 1:2-1:10; and / or The stirring time is 5 min-2 h and the stirring speed is 200 r / min-500 r / min; and / or The temperature of the biomass ash and the acid solution when mixed is 20°C-80°C.

4. The method according to claim 1 or 2, characterized in that: Step 1) further comprises multiple acid leaching, wherein the multiple acid leaching steps comprise: adding the acid solution and the biomass ash to the potassium-containing acid leaching solution obtained after the previous solid-liquid separation, mixing, stirring, and solid-liquid separation; repeating multiple times until the supernatant after the last acid leaching is used in step 2); Optionally, the number of multiple acid leaching is 2-4 times; Optionally, during each acid leaching process, the ash-water ratio of the added biomass ash to the acid solution is 1:2-1:10; the stirring time is 5 min-2 h, and the stirring speed is 200 r / min-500 r / min; the temperature at which the biomass ash and the acid solution are mixed is 20°C-80°C.

5. The method according to claim 1 or 2, characterized in that: In step 2), the base is an alkaline aqueous solution or a solid that is alkaline when dissolved in water; Optionally, the alkaline aqueous solution includes, but is not limited to, aqueous solutions of sodium hydroxide, potassium hydroxide, and calcium hydroxide; Optionally, the alkaline solid soluble in water includes but is not limited to calcium oxide, sodium hydroxide, and potassium hydroxide.

6. The method according to claim 1 or 2, characterized in that: In step 2), the equivalent molar ratio of carbonate in the sodium carbonate to the multivalent metal ions in the potassium-containing acid leaching solution is 1-1.2; and / or The stirring time is 5 min-20 min; the stirring speed is 200 r / min-500 r / min.

7. The method according to claim 1 or 2, characterized in that: In step 2), adding alkali to the potassium-containing acid leaching solution to adjust the pH to 11-13 includes: directly adding alkali at one time to adjust the pH to 11-13 or adding alkali in steps to adjust the pH to 11-13.

8. The method according to claim 7, characterized in that The stepwise addition of alkali to adjust the pH to 11-13 comprises the following steps: a) adding alkali to the potassium-containing acid leaching solution to adjust the pH; b) solid-liquid separation to obtain supernatant and metal precipitate; c) adding alkali to the supernatant again to adjust the pH; d) Repeat steps b) and c) until the pH of the supernatant obtained is adjusted to 11-13 by adding alkali, sodium carbonate is added, stirred, and the solid-liquid separation is performed, and the obtained potassium-containing solution is used in step 3).

9. The method according to claim 8, characterized in that The stepwise alkali addition method is a three-step alkali addition process, wherein in step a), the pH of the potassium-containing acid leaching solution is adjusted to 7-9, in step c), the pH of the supernatant is adjusted to 10-11, and in step d), the pH of the supernatant is adjusted to 11-13; Optionally, in step d), the pH of the supernatant is adjusted to 12-13.

10. The method according to claim 1 or 2, characterized in that: In step 3), the concentration method is evaporator concentration or membrane concentration; and / or In step 3), the concentration multiple is 2-4 times; and / or In step 4), the evaporation crystallization is selected from one or more of single-effect evaporation crystallization, multiple-effect evaporation crystallization, and mechanical steam recompression evaporation crystallization.

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