Alkali recovery method and system

Abstract

The invention discloses a method and a system for recovering alkali. The method comprises the following steps: providing or receiving alkali ash; obtaining an alkali-ash characteristic ratio; a dissolution temperature is selected according to the alkali-ash characteristic ratio, at the dissolution temperature, the absolute value of the subtraction of the saturation solubility ratio of potassium sulfate and sodium sulfate in water and the alkali-ash characteristic ratio is defined as the ratio difference, and the ratio difference is larger than or equal to 0.1 and smaller than or equal to 20; mixing the alkali ash and water to achieve dissolution equilibrium at the dissolution temperature, and forming a mixed solution containing an aqueous solution in which potassium sulfate and sodium sulfate are dissolved and undissolved potassium sulfate and / or sodium sulfate; filtering the mixed solution to obtain a filtrate and a filter cake; after the filtrate is pretreated, bipolar membrane electrodialysis treatment is carried out, and separated acid aqueous solution, alkali aqueous solution and saline solution are obtained. The method is suitable for the alkali ash under various combustion conditions in the industry, realizes high reuse rate of sodium sulfate, full recycling of all components of the alkali ash, low energy consumption and zero waste liquid discharge, and has both environmental protection property and economical efficiency.

Description

Technical Field

[0001] This invention relates to the field of alkali recovery technology, and more particularly to a method and system for alkali recovery, especially a method and system for alkali recovery used in industrial combustion processes or pulp and paper processes. Background Technology

[0002] In general industrial combustion processes, such as coal-fired power generation, biomass combustion, and waste incineration, alkali ash is produced. Especially in the alkali recovery process of pulp and paper manufacturing, the accumulation of alkali ash leads to the enrichment of chlorine and potassium elements within the system, resulting in equipment corrosion, scaling, and decreased efficiency. Currently, the mainstream alkali recovery systems for removing chlorine and potassium are still largely used in most factories (especially older systems) using the less efficient traditional discharge method (chlorine and potassium removal efficiency of only 20%-40%). This is mainly because this method requires the least investment for modification, and although the operating cost is higher, many factories consider it a "fixed cost." Processes such as evaporation and crystallization, on the other hand, require huge initial investments, complex systems, high operational technical requirements, and precise control of the mother liquor discharge point; otherwise, even if built, they are difficult to effectively remove chlorine and potassium.

[0003] Currently, most commercially successful cases in the pulp industry employ total dissolution evaporation crystallization, and all are imported from abroad. For example, Veolia's Enhanced Chloride Removal Process (ECRP) is used at Chongqing Lee & Man Paper. This project crystallizes sodium sulfate crystals in the first stage and a mixed salt mainly composed of potassium chloride and sodium chloride in the second stage. This technology has high steam energy consumption and high equipment investment costs. Furthermore, it does not consider the simultaneous saturation of sodium sulfate and potassium sulfate in soda ash, making it impossible to crystallize high-purity sodium sulfate crystals in the first stage. Andritz's ARC method (Ash Re-Crystallization, one-stage evaporation crystallization) utilizes the difference in solubility between potassium and sodium salts. After dissolving the soda ash, it evaporates and concentrates to recover sodium sulfate, which is then reused in the boiler. The mother liquor is partially recycled, and excess mother liquor is discharged from the system for treatment. This method can achieve a potassium ion removal rate of approximately 70% and a chloride ion removal rate of approximately 85%, but the sodium ion loss rate is approximately 25%. Andritz's Ash Leaching process utilizes the difference in solubility between potassium and sodium salts. An appropriate amount of cleaning condensate is added to dissolve chloride and potassium ions, while solid sodium sulfate is reused and the filtrate is discharged. However, this process also fails to account for the co-saturation of sodium sulfate and potassium sulfate, making it ineffective for potassium removal. All of the above-mentioned current processes struggle to effectively remove potassium from high-potassium ash, and generally suffer from high investment costs, high energy consumption, environmental pollution due to wastewater discharge, and resource waste.

[0004] Patent CN120285605A discloses a process and system for recovering sodium sulfate, sodium carbonate, and potassium sulfate (sodium sulfate) from soda ash and removing chloride ions. This process sequentially dissolves and thermally crystallizes the soda ash filtrate to produce sodium sulfate, sodium carbonate, and a first mother liquor. The first mother liquor is then cooled and crystallized to obtain potassium sulfate and a second mother liquor. The second mother liquor is diluted and then enters a sodium filtration unit to separate carbonate, sulfate, and chloride ions. The sodium filtrate is returned to the dissolution unit for recycling. However, practical applications have the following problems: when the soda ash is potassium-rich, resulting in a high potassium content in the soda ash solution (i.e., reaching saturation), sodium sulfate and potassium sulfate crystallize simultaneously during the first thermal crystallization step, making subsequent steps impossible. Furthermore, this scheme requires evaporation crystallization equipment, cooling crystallization equipment, and a sodium filtration unit, making the process complex, requiring high equipment investment, and difficult to commercialize; the discharge of concentrated chloride water will cause environmental pollution.

[0005] Patent CN113998712A discloses a method for removing chloride ions and recovering sodium sulfate and sodium carbonate from alkaline ashing filtrate. This method first uses electrodialysis to separate monovalent and divalent anions, allowing potassium chloride and sodium chloride to enter the concentrated water side. Subsequently, the desalinated water side undergoes evaporation crystallization, cooling crystallization, and adsorption treatments, claiming a sulfate recovery rate of over 90%. However, its practical application suffers from the following problems: the electrodialysis membrane has no selectivity for sodium sulfate and potassium sulfate (both remain on the desalinated water side). If both reach saturation simultaneously, sodium sulfate and potassium sulfate will crystallize out concurrently. While the crystals are recycled, potassium sulfate also returns to the system, making effective separation of potassium and sodium impossible. Furthermore, this scheme involves multiple processes such as electrodialysis, evaporation crystallization, cooling crystallization, and adsorption, making operation complex and difficult to commercialize in actual production.

[0006] Patent CN110877900B discloses a method and system for the resource recovery of mixed salts. The method involves dissolving the mixed salts in 5-20 times their volume of water, then using bipolar membrane electrodialysis to obtain mixed acid, mixed alkali, and dilute brine. The mixed acid is sent to a sodium filtration system to separate chloride and sulfate ions. The mixed alkali is concentrated into concentrated alkali, and the resulting desalinated water is further concentrated via membrane filtration or evaporation before being dried in a spray dryer. This scheme does not consider sodium-potassium separation or sodium sulfate reuse, and therefore is not suitable for treating soda ash solutions. Taking a fully dissolved soda ash solution as an example, in bipolar membrane electrodialysis, both potassium and sodium are monovalent positive ions, which will not selectively remain in the brine chamber or migrate to the alkali chamber, making effective separation of potassium and sodium impossible. Therefore, it cannot meet the potassium removal requirements of an alkali recovery system. Furthermore, this technology uses a fully dissolved process, requiring the addition of a large amount of water (5-20 times the volume), resulting in a huge volume of solution requiring electrodialysis treatment, leading to very high investment, energy consumption, and operating costs.

[0007] All three patents mentioned above employ a complete dissolution process, handling large volumes, and do not consider reusing the recovered sodium sulfate in the alkali recovery system to reduce the external addition of sodium sulfate. Furthermore, none of them address the issue of ineffective separation of sodium sulfate and potassium sulfate in solutions under potassium-rich soda ash conditions. Moreover, the crystallization process suffers from high equipment investment, high energy consumption, mother liquor discharge, and high operating costs.

[0008] In summary, developing an energy-efficient and high-performance process for removing chlorine and potassium from alkali ash can effectively remove chlorine and potassium while maximizing the reuse of sodium sulfate in the alkali recovery system and achieving comprehensive resource utilization of all components of alkali ash. This has significant industrial application value and practical significance. Summary of the Invention

[0009] The purpose of this invention is to provide a method and system for alkali recovery. This method can effectively remove potassium ions and achieve efficient separation of chlorine and potassium in alkali ash, while providing an extremely high sodium sulfate recovery rate. Furthermore, all components can be recycled and reused, reducing the amount of sodium sulfate required to replenish the alkali recovery system, and there is almost no waste solution discharge. This invention is applicable to alkali ash under various combustion conditions in industry, achieving a high sodium sulfate recovery rate, full resource utilization of all components in the alkali ash, low energy consumption, and zero waste liquid discharge, thus combining environmental friendliness and economy.

[0010] To achieve the above objectives, the present invention provides a method for alkali recovery, comprising the following steps: S1: providing or receiving alkali ash, wherein the alkali ash contains potassium sulfate and sodium sulfate; S2: measuring the weight ratio of potassium sulfate to sodium sulfate in the alkali ash and defining it as the characteristic ratio of the alkali ash; S3: selecting a dissolution temperature based on the characteristic ratio of the alkali ash, wherein at the dissolution temperature, the absolute value of the difference between the ratio of the saturated solubility of potassium sulfate and sodium sulfate in water and the characteristic ratio of the alkali ash is defined as the ratio difference, wherein the ratio difference is greater than or equal to 0.1 and less than or equal to 20; S4: mixing the alkali ash and water to obtain a mixed solution, and dissolving the solution at the specified temperature. S5: Filter the mixture to obtain a filtrate and a filter cake; wherein the filtrate contains an aqueous solution of dissolved potassium sulfate and sodium sulfate, and the filter cake contains undissolved potassium sulfate and / or sodium sulfate; S6: Pretreat the filtrate to remove at least some of the divalent metal ions, to obtain a pretreated filtrate; S7: Perform bipolar membrane electrodialysis on the pretreated filtrate to obtain separated acidic aqueous solution, alkaline aqueous solution, and salt aqueous solution.

[0011] In some embodiments, step S1 includes the following steps: S1-1: heating the dilute black liquor to obtain concentrated black liquor; S1-2: burning the concentrated black liquor to obtain melt and flue gas; and S1-3: collecting solid dust from the flue gas to obtain alkali ash.

[0012] In some embodiments, step S1-1 includes heating the dilute black liquor using a multi-effect evaporator to obtain clean condensate.

[0013] In some embodiments, after step S5, a step S5-1 is also included: determining which of the weight percentages of potassium sulfate and sodium sulfate in the filter cake is higher.

[0014] In some embodiments, after step S5-1, the following steps are further included: when the weight percentage of sodium sulfate in the filter cake in step S5-1 is greater than the weight percentage of potassium sulfate, a sodium-rich filter cake is obtained; the sodium-rich filter cake is burned together with the concentrated black liquor in step S1-2 to convert the sodium sulfate in the filter cake into sodium sulfide; when the weight percentage of potassium sulfate in the filter cake in step S5-1 is greater than the weight percentage of sodium sulfate, a potassium-rich filter cake is obtained; the potassium-rich filter cake is dried and pulverized to obtain potassium fertilizer.

[0015] In some embodiments, after step S7, the following step is also included: heating the brine solution from step S7 together with the dilute black liquor from step S1-1 to obtain a concentrated black liquor.

[0016] In some implementations, at the dissolution temperature, the ratio difference is greater than or equal to 0.2 and less than or equal to 9.

[0017] In some embodiments, the dissolution temperature is higher than or equal to 0°C and lower than or equal to 60°C.

[0018] In some embodiments, the dissolution temperature is higher than or equal to 10°C and lower than or equal to 55°C.

[0019] In some embodiments, in step S4, the weight percentage of soda ash in the mixture is 30% to 65%.

[0020] In some embodiments, in step S4, the weight percentage of soda ash in the mixture is 40% to 55%.

[0021] In some embodiments, in step S7, the acidic aqueous solution contains hydrochloric acid and sulfuric acid, and the volume molar concentration of hydrochloric acid is greater than that of sulfuric acid; the alkaline aqueous solution contains sodium hydroxide and potassium hydroxide; and the salt aqueous solution contains sodium sulfate, potassium sulfate, and sodium chloride.

[0022] In some embodiments, step S7 includes the following steps: controlling multiple independent parameters of the bipolar membrane electrodialysis treatment, including setting the hydrogen ion concentration in the acidic aqueous solution to 1 mol / L to 2 mol / L, the hydroxide ion concentration in the alkaline aqueous solution to 1.5 mol / L to 3.0 mol / L, and the sulfate ion concentration in the salt aqueous solution to 150 g / L to 250 g / L.

[0023] In some embodiments, after step S7, an acid treatment process is further included, in which an acidic aqueous solution is used in the pulping process, and an alkali treatment process is included, in which an alkaline aqueous solution is used in the pulping process.

[0024] In some embodiments, step S6 includes pretreatment comprising resin adsorption, precision filtration, activated carbon adsorption, or a combination thereof.

[0025] In some embodiments, step S7, the bipolar membrane electrodialysis treatment includes using a bipolar membrane electrodialysis device, which includes multiple repeating units composed of a bipolar membrane, a cation exchange membrane, and an anion exchange membrane.

[0026] In some embodiments, the anion exchange membrane comprises an acid- and alkali-resistant anion exchange membrane or a monovalent selective anion exchange membrane.

[0027] In some embodiments, in step S4, the water may include cleaning condensate, clean water, or a combination thereof.

[0028] To achieve the above objectives, the present invention provides an alkali recovery system comprising: a dissolving tank for receiving and mixing alkali ash and water to form a mixed solution; the dissolving tank having a temperature control device capable of controlling the mixed solution at a pre-selected dissolving temperature to achieve substantial dissolution equilibrium; wherein the dissolving temperature is selected based on a ratio difference, wherein the ratio difference is greater than or equal to 0.1 and less than or equal to 20; the ratio difference is the absolute value of the difference between the ratio of the saturated solubility of potassium sulfate and sodium sulfate in water and the weight ratio of potassium sulfate and sodium sulfate in the alkali ash at the dissolving temperature; a filter connected to the dissolving tank for filtering the mixed solution to separate filtrate and filter cake; a pretreatment unit connected to the filter for pretreating the filtrate to remove at least a portion of the divalent metal ions in the filtrate to obtain a pretreated filtrate; and a bipolar membrane electrodialysis device connected to the pretreatment unit for performing bipolar membrane electrodialysis treatment on the pretreated filtrate to obtain separated acid-water solution, alkali-water solution, and salt-water solution.

[0029] In some embodiments, the system further includes, before the dissolving tank: an evaporator, which receives high-temperature steam to treat the dilute black liquor, forming concentrated black liquor and cleaning condensate; a boiler, connected between the evaporator and the dissolving tank, for burning the concentrated black liquor to obtain ash, melt, high-temperature steam and carbon dioxide; and a mixer, connected to the boiler, for mixing the cleaning condensate and dilute white liquor to form green liquor.

[0030] In some embodiments, the system further includes: a storage tank connected to the filter for receiving and processing the filter cake; when the weight percentage of sodium sulfate in the filter cake is greater than the weight percentage of potassium sulfate, a sodium-rich filter cake is obtained, which is then returned to the boiler for combustion together with the concentrated black liquor; or when the weight percentage of potassium sulfate in the filter cake is greater than the weight percentage of sodium sulfate, a potassium-rich filter cake is obtained, which is then dried and pulverized to form potassium fertilizer.

[0031] In some implementations, the high-temperature steam obtained from the boiler can be reused in the evaporator.

[0032] In some embodiments, the brine solution is fed into a dilute black liquor for mixing before entering an evaporator to obtain a concentrated black liquor.

[0033] In some embodiments, acidic aqueous solutions are used in the acid treatment process of the pulping process, and alkaline aqueous solutions are used in the alkaline treatment process of the pulping process.

[0034] In some implementations, at the dissolution temperature, the ratio difference is greater than or equal to 0.2 and less than or equal to 9.

[0035] In some embodiments, the dissolution temperature is higher than or equal to 0°C and lower than or equal to 60°C.

[0036] In some embodiments, the dissolution temperature is higher than or equal to 10°C and lower than or equal to 55°C.

[0037] In some embodiments, the dissolving tank is operated such that the weight percentage of soda ash in the mixture is 30% to 65%.

[0038] In some embodiments, the weight percentage of soda ash in the mixture is 40% to 55%.

[0039] In some embodiments, the bipolar membrane electrodialysis device can control multiple independent parameters, including setting the hydrogen ion concentration in the acidic aqueous solution to 1 mol / L to 2 mol / L, the hydroxide ion concentration in the alkaline aqueous solution to 1.5 mol / L to 3.0 mol / L, and the sulfate ion concentration in the salt aqueous solution to 150 g / L to 250 g / L.

[0040] In some embodiments, the pretreatment unit uses resin adsorption, precision filtration, activated carbon adsorption, or a combination thereof.

[0041] In some embodiments, the bipolar membrane electrodialysis apparatus includes multiple repeating units consisting of a bipolar membrane, a cation exchange membrane, and an anion exchange membrane.

[0042] In some embodiments, the anion exchange membrane comprises an acid- and alkali-resistant anion exchange membrane or a monovalent selective anion exchange membrane.

[0043] In some embodiments, the water includes cleaning condensate, clean water, or a combination thereof.

[0044] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0045] (1) The traditional process of removing potassium chloride from alkaline ash by complete dissolution does not take into account the enrichment of potassium element in alkaline ash, and it is difficult to achieve effective potassium removal. The technical solution of the present invention can be applied to the process of removing potassium chloride from alkaline ash under different working conditions.

[0046] (2) Traditional alkaline ash complete dissolution potassium removal process, whether it is evaporation crystallization, freeze crystallization or conventional membrane treatment, requires a lot of steam or electricity, resulting in high operating costs and low efficiency for enterprises. This invention efficiently collects a portion of potassium or sodium salts as sludge cakes at a certain dissolution temperature, and then treats the filtrate with bipolar membrane electrodialysis. The amount of filtrate to be treated is greatly reduced, which is extremely energy-saving, environmentally friendly and economical, and has high potassium removal efficiency.

[0047] (3) While efficiently removing potassium chloride, this invention also maximizes the recycling of Na2SO4 into the alkali recovery system, reduces the amount of sodium sulfate added to the system, maintains the normal operation of the alkali recovery system, and greatly reduces the operating costs of enterprises.

[0048] (4) This invention is the first to apply bipolar membrane electrodialysis technology to the process of removing potassium chloride from alkaline ash, simultaneously realizing acid and alkali regeneration and salt separation, avoiding the high energy consumption and waste discharge of traditional evaporation crystallization or cooling crystallization, which causes secondary pollution.

[0049] (5) Through process coupling, the present invention converts elements such as potassium, chloride, and sulfate in soda ash into potassium fertilizer or potassium alkali (KOH), acid, and dilute brine, which are then recycled to the pulping or alkali recovery system, thus achieving closed-loop circulation and zero emissions.

[0050] (6) This invention utilizes the cleaning condensate as a dissolving medium, reducing fresh water consumption and improving energy and resource utilization efficiency. Furthermore, the cleaning condensate of this invention has high purity and extremely low hardness (very low Ca and Mg content), significantly reducing the risk of scaling in the dissolution and subsequent pretreatment stages, thus ensuring stable system operation. In addition, alkaline condensate and alkaline ash are similarly miscible, providing a stable chemical environment that does not cause drastic pH fluctuations, which is beneficial for the smooth operation of the dissolution process.

[0051] (7) The present invention has a simple process and low operating cost. It not only efficiently removes potassium chloride impurities from alkali ash, but also realizes the resource recycling of each component, solves the corrosion and scaling problems of the alkali recovery system, and reduces the addition of external chemicals. It has significant environmental and economic benefits and is suitable for large-scale industrial applications. Attached Figure Description

[0052] Figure 1 This is a schematic flowchart of an alkali recovery method in some embodiments of the present invention.

[0053] Figure 2 This is a schematic diagram of an alkali recovery system in some embodiments of the present invention.

[0054] Explanation of key figure labels:

[0055] 100: Methods for alkali recovery;

[0056] S1, S1-1, S1-2, S1-3, S2, S3, S4, S5, S5-1, S5-A, S5-B, S6, S7: Steps;

[0057] 10: Diluted black liquor;

[0058] 12. ST: High-temperature steam;

[0059] 18: Clean the condensate;

[0060] 20: To thicken the black liquor;

[0061] 22: Molten material;

[0062] 30: Alkali ash;

[0063] 40: Mixture;

[0064] 50: Filtrate;

[0065] 52: Filter cake;

[0066] 62: Sodium-rich filter cake;

[0067] 64: Potassium-rich filter cake;

[0068] 66: Green liquid;

[0069] 70: Filtrate after pretreatment;

[0070] 82: Acid aqueous solution;

[0071] 84: Alkaline aqueous solution;

[0072] 86: Salt solution;

[0073] 200: Alkali recovery system;

[0074] CO2: Carbon dioxide;

[0075] CT: Storage tank;

[0076] DS: Dissolving tank;

[0077] ED: Bipolar membrane electrodialysis device;

[0078] FT: Filter;

[0079] MX: Mixer;

[0080] RB: Boiler;

[0081] PR: Preprocessing unit;

[0082] VE: Evaporator;

[0083] W1: Water;

[0084] W2: Thin white liquid. Detailed Implementation

[0085] To make the content of this invention more detailed and complete, the following provides an illustrative description of the embodiments and specific implementation methods of this invention.

[0086] This article uses the pulp and paper industry as an example and employs several common terms, such as "black liquor," "green liquor," "white liquor," and "cleaning condensate," which are technical terms in the field to which this invention pertains. Those skilled in the art will understand their meanings without ambiguity. In this article, "pulping black liquor" refers to the black liquor produced during the pulp and paper process. Furthermore, it should be understood that the term "and / or," as used herein, encompasses any combination and all combinations of one or more of the related listed items.

[0087] Figure 1 This is a schematic flowchart of an alkali recovery method in some embodiments of the present invention. The alkali recovery method 100 includes steps S1, S2, S3, S4, S5, S6 and S7. Figure 2 This is a schematic diagram of an alkali recovery system in some embodiments of the present invention.

[0088] Please refer to the following at the same time Figure 1 and Figure 2 The alkali recovery method 100 proceeds to step S1, providing or receiving alkali ash 30, which contains at least potassium sulfate and sodium sulfate. In some embodiments, the alkali ash 30 contains at least potassium sulfate, sodium sulfate, sodium chloride, and potassium chloride, but potassium sulfate and sodium sulfate are the main components.

[0089] In some embodiments, the alkali ash 30 is obtained from the flue gas produced by burning black liquor using dust collection technology. In some embodiments, step S1 includes steps S1-1, S1-2, and S1-3 as described below.

[0090] In step S1-1, the dilute black liquor 10 is heated to obtain the concentrated black liquor 20. In some embodiments, such as Figure 2As shown, the dilute black liquor 10 produced during the pulping process is transported through pipelines into the evaporator VE for heating. After passing through the evaporator VE, the dilute black liquor 10 forms concentrated black liquor 20. In some embodiments, the evaporator VE is, for example, a multi-effect evaporator, which may contain multiple heating units connected in series. After the dilute black liquor 10 is heated by the multiple heating units connected in series using high-temperature steam 12, clean condensate 18 is formed.

[0091] In steps S1-2, the thickened black liquor 20 is burned to obtain melt 22 and flue gas. In some embodiments, such as Figure 2 As shown, thickened black liquor 20 is fed to boiler RB for combustion. The lignin in thickened black liquor 20 becomes the fuel, and the sodium sulfate and sodium sulfite in thickened black liquor 20 react to form sodium sulfide. After combustion, thickened black liquor 20 produces a molten product 22 containing sodium sulfide and sodium carbonate, as well as flue gas containing solid particulate matter (or fly ash), carbon dioxide (displayed as CO2 in the diagram), and high-temperature steam ST. In some embodiments, such as... Figure 2 As shown, the high-temperature steam ST produced by the combustion of the concentrated black liquor 20 in the boiler RB can also be reused in the evaporator VE in step S1-1.

[0092] In steps S1-3, solid dust particles in the flue gas are collected to obtain alkali ash 30. In some embodiments, such as Figure 2 As shown, boiler RB is equipped with an electrostatic precipitator to collect solid dust from the flue gas to obtain alkali ash 30. Alkali ash 30 contains relatively large amounts of sodium sulfate and potassium sulfate, and relatively small amounts of sodium chloride, potassium chloride, and sodium carbonate.

[0093] Please refer to the following at the same time Figure 1 and Figure 2 In step S2 of the alkali recovery method 100, the weight ratio of potassium sulfate to sodium sulfate in alkali ash 30 (potassium sulfate / sodium sulfate) is analyzed and measured using any existing technology and defined as the "alkali ash characteristic ratio".

[0094] In the alkali recovery method 100, step S3 involves selecting a dissolution temperature based on the characteristic ratio of the alkali ash. The absolute value of the difference between the saturated solubility ratio of potassium sulfate and sodium sulfate in water (potassium sulfate / sodium sulfate) and the characteristic ratio of the alkali ash is defined as the "ratio difference." At the selected dissolution temperature, this ratio difference is greater than or equal to 0.1 but less than or equal to 20, for example, 0.1, 0.2, 0.3, 0.4, 0.5, 1, 2, 3, 4, 5, 6, 8, 10, 12, 14, 16, 18, 19, or 20. In this text, the dissolution temperature refers to a suitable dissolution temperature selected based on the difference in saturated solubility of potassium sulfate and sodium sulfate at different temperatures, according to the actual potassium and sodium content in the alkali ash, so that one of the salts in the alkali ash can theoretically be completely dissolved in water. The saturated solubility is defined as the weight of solute that can be dissolved in 100 grams of water, expressed in g / 100g water. Specifically, Table 1 below summarizes the saturated solubility data of potassium sulfate and sodium sulfate in water (unit: g / 100g water). For example, at 40℃, the saturated solubility of potassium sulfate is 14.8g / 100g water, and the saturated solubility of sodium sulfate is 48.8g / 100g water. The ratio of the saturated solubility of potassium sulfate and sodium sulfate in water is 0.30. However, when the measured characteristic ratio of alkali and ash is 0.54, and the dissolution temperature is selected as 40℃, the difference in ratio is 0.24 (i.e., |(0.30-0.54)| = 0.24).

[0095] Table 1:

[0096]

[0097] In some embodiments, at the selected dissolution temperature, the ratio difference is greater than or equal to 0.2 and less than or equal to 9. In some embodiments, the selected dissolution temperature is higher than or equal to 0°C and lower than or equal to 60°C, for example, 0°C, 1°C, 10°C, 15°C, 20°C, 25°C, 30°C, 35°C, 40°C, 45°C, 50°C, 55°C, 59°C, or 60°C. In some specific embodiments, the dissolution temperature is higher than or equal to 10°C and lower than or equal to 55°C.

[0098] Please refer to the following at the same time: Figure 1 and Figure 2In the alkali recovery method 100, step S4 involves mixing alkali ash 30 with water W1 to obtain a mixture 40, which reaches substantial dissolution equilibrium at the stated dissolution temperature. The mixture 40 contains an aqueous solution of dissolved potassium sulfate and sodium sulfate, as well as undissolved potassium sulfate and / or sodium sulfate. In other words, the amount of alkali ash 30 is sufficient to supersaturate at least one of potassium sulfate and sodium sulfate, resulting in precipitation. Specifically, if the selected dissolution temperature allows all potassium sulfate to dissolve, the resulting precipitate contains no or only trace amounts of potassium salt; conversely, if all sodium sulfate dissolves, the precipitate contains no or only trace amounts of sodium salt.

[0099] In some embodiments, when the weight percentage of alkali ash 30 in the mixed solution is greater than a certain value, such as 65%, the content of potassium sulfate and sodium sulfate in the precipitate will be too close, which is not conducive to the separation effect of the overall process, or will cause sodium sulfate to leave the alkali recovery system 200 in subsequent processes and become unusable, as will be described in more detail below. On the other hand, when the weight percentage of alkali ash 30 in the mixed solution 40 is less than a certain value, such as 30%, the proportion of water W1 in the mixed solution is too high, which will lead to a significant increase in power consumption during subsequent electrodialysis treatment, or will result in a relatively low removal rate of potassium sulfate. Therefore, in some embodiments, the weight percentage of alkali ash 30 in the mixed solution 40 is 30% to 65%, for example, about 30%, 35%, 40%, 45%, 50%, 55%, 60%, or 65%. In some embodiments, the weight percentage of alkali ash 30 in the mixed solution 40 is 40% to 55%.

[0100] In some implementations, such as Figure 2 As shown, soda ash 30 is conveyed to a dissolving tank DS and mixed with water W1. The dissolving tank DS has a temperature control device that can control the mixture 40 at the dissolving temperature selected in step S3. Soda ash 30 and water W1 reach substantial dissolution equilibrium at the selected dissolution temperature, forming the mixture 40. In some embodiments, the condensate 18 obtained from the evaporator VE can also be conveyed to the dissolving tank DS as the water W1 required to dissolve the soda ash 30. In other embodiments, the water W1 can be other clean water, condensate 18, or a combination thereof.

[0101] The cleaning condensate 18, used as water W1 in step S4, provides excellent technical benefits. First, it eliminates the need for clean water or at least reduces its usage. Second, the cleaning condensate 18 is obtained through evaporation and condensation, resulting in extremely low water hardness (very low levels of magnesium and calcium ions), fundamentally eliminating the risk of scaling in the dissolving tank DS and the overall alkali recovery system 200.

[0102] Please refer to the following at the same time: Figure 1 and Figure 2In the alkali recovery method 100, step S5 involves filtering the mixture 40 to obtain a filtrate 50 and a filter cake 52. The filtrate 50 contains an aqueous solution of dissolved potassium sulfate and sodium sulfate, and the filter cake 52 contains undissolved potassium sulfate and / or sodium sulfate. The filtration method can be any applicable prior art, such as plate and frame filtration or centrifugal filtration. In some embodiments, such as... Figure 2 As shown, the mixture 40 is sent to the filter FT for filtration to obtain filtrate 50 and filter cake 52. Filter cake 52 is transported to storage tank CT, and filtrate 50 is used for subsequent processes.

[0103] In some embodiments, the alkali recovery method 100 includes additional steps after step S5. After obtaining filter cake 52 by performing step S5, step S5-1 is performed to determine which has a higher weight percentage, potassium sulfate or sodium sulfate, in filter cake 52, and then either step S5-A or step S5-B is selected.

[0104] In step S5-1, if the weight percentage of sodium sulfate in filter cake 52 is greater than the weight percentage of potassium sulfate, step S5-A is performed. Specifically, step S5-A involves forming a sodium-rich filter cake 62 when the weight percentage of sodium sulfate in filter cake 52 is greater than the weight percentage of potassium sulfate. The sodium-rich filter cake 62 is then burned together with the concentrated black liquor 20 described in step S1-2, thereby converting the sodium sulfate in the sodium-rich filter cake 62 into sodium sulfide. In some embodiments, such as... Figure 2 As shown, the sodium-rich filter cake 62 is returned to the boiler RB and undergoes a combustion reaction together with the thickened black liquor 20 in the boiler RB. The sodium sulfate in the sodium-rich filter cake 62 is reacted to form sodium sulfide and becomes part of the melt 22. The melt 22 produced by the boiler RB is conveyed to the mixer MX and mixed with the dilute white liquor W2 to obtain green liquor 66. The green liquor 66 can then undergo a causticization reaction to form white liquor, which can then be returned to the pulping process for reuse.

[0105] Conversely, in step S5-1, if the weight percentage of potassium sulfate in filter cake 52 is greater than the weight percentage of sodium sulfate, step S5-B is performed. Specifically, step S5-B includes forming a potassium-rich filter cake 64 when the weight percentage of potassium sulfate in filter cake 52 of step S5-B is greater than the weight percentage of sodium sulfate. The potassium-rich filter cake 64 is then dried and pulverized to obtain potassium fertilizer. In some embodiments, such as... Figure 2 As shown, the potassium-rich filter cake 64 is conveyed to other devices for drying and crushing to obtain potassium fertilizer, which can be used to make fertilizer for the forest farm.

[0106] The weight percentage of alkali ash 30 in the mixed liquor 40 affects the potassium content in the sodium-enriched filter cake 62 and the sodium content in the potassium-enriched filter cake 64. In step S5-A, if the potassium content in the sodium-enriched filter cake 62 is too high, potassium ions will return to the alkali recovery system 200, resulting in a relatively low potassium removal rate. On the other hand, if the sodium content in the potassium-enriched filter cake 64 is too high, sodium ions will leave the alkali recovery system 200 along with the potassium fertilizer, thus reducing the overall sodium recovery rate of the alkali recovery system 200. Therefore, as described above regarding step S4, in some embodiments, the weight percentage of alkali ash 30 in the mixed liquor 40 is 30% to 65%.

[0107] In some embodiments, the ratio of soda ash characteristics obtained in step S2 and the ratio of the saturated solubility of potassium sulfate and sodium sulfate at the dissolution temperature selected in step S3 can be used to determine which of the two substances, potassium sulfate and sodium sulfate, has a higher content in filter cake 52. However, in other embodiments, the proportion of one substance can be determined by analyzing the composition of filter cake 52.

[0108] Please refer to the following at the same time Figure 1 and Figure 2 The alkali recovery method 100 pretreats the filtrate 50 obtained in step S5 in step S6 to remove at least some of the divalent metal ions, such as divalent iron ions, to obtain a pretreated filtrate 70. In some embodiments, the pretreatment in step S6 includes using resin adsorption, precision filtration (e.g., using an ultrafiltration membrane), activated carbon adsorption, or a combination thereof in the pretreatment unit PR. In some embodiments, the resin used in the resin adsorption method has functional groups for removing metal ions. In some embodiments, such as... Figure 2 As shown, filtrate 50 is transported to pretreatment unit PR, where most of the divalent metal ions in filtrate 50 are removed, resulting in pretreated filtrate 70.

[0109] Please refer to Figure 1 and Figure 2In the alkali recovery method 100, after step S6, step S7 is performed to treat the pretreated filtrate 70 by bipolar membrane electrodialysis to obtain separated acid-water solution 82, alkali-water solution 84, and salt-water solution 86. In some embodiments, the bipolar membrane electrodialysis treatment includes using a bipolar membrane electrodialysis device ED, which comprises multiple repeating units consisting of a bipolar membrane, a cation exchange membrane, and an anion exchange membrane. In some embodiments, the anion exchange membrane comprises an acid- and alkali-resistant anion exchange membrane or a monovalent selective anion exchange membrane. For example, the core material of the acid- and alkali-resistant anion exchange membrane is a cross-linked polystyrene-polyethylene composite, which can be purchased from Hangzhou Lanran Co., Ltd. (China), Asahi Kasei Corporation (Japan), DuPont (USA), Asahi Kasei Corporation (Japan), or Fujifilm Corporation (Japan). The monovalent selective anion exchange membrane can be purchased from DuPont (USA), Asahi Kasei Corporation (Japan), or Fujifilm Corporation (Japan).

[0110] In some embodiments, the acidic aqueous solution 82 obtained in step S7 contains hydrochloric acid and sulfuric acid, wherein the volume molar concentration of hydrochloric acid is greater than that of sulfuric acid. Furthermore, the alkaline aqueous solution 84 obtained in step S7 contains sodium hydroxide and potassium hydroxide. Moreover, the brine aqueous solution 86 obtained in step S7 contains a relatively large amount of sodium sulfate and small amounts of potassium sulfate and sodium chloride.

[0111] In some embodiments, step S7 includes the following steps: controlling multiple independent parameters of the bipolar membrane electrodialysis treatment, including setting the hydrogen ion concentration in the acidic aqueous solution 82 to 1 mol / L to 2 mol / L, the hydroxide ion concentration in the alkaline aqueous solution 84 to 1.5 mol / L to 3.0 mol / L, and the sulfate ion concentration in the saline aqueous solution 86 to 150 g / L to 250 g / L.

[0112] In some embodiments, after step S7, the alkali recovery method 100 further includes the following step: heating the brine solution 86 obtained in step S7 together with the dilute black liquor 10 obtained in step S1-1 to obtain a concentrated black liquor 20. In some embodiments, such as Figure 2 As shown, the pretreated filtrate 70 is fed into a bipolar membrane electrodialysis unit ED for electrodialysis treatment, yielding an acidic aqueous solution 82, an alkaline aqueous solution 84, and a brine aqueous solution 86. The brine aqueous solution 86 is further fed into the pipeline of the dilute black liquor 10, and together with the dilute black liquor 10, enters the evaporator VE for heating and concentration. However, in other embodiments, the brine aqueous solution 86 can enter the evaporator VE via a separate pipeline. Therefore, the sodium sulfate in the brine aqueous solution 86 can re-enter the alkali recovery system 200 and be converted into sodium sulfide through combustion in the boiler RB.

[0113] In the bipolar membrane electrodialysis process, chloride ions in the pretreated filtrate 70 are converted into hydrochloric acid via an electrochemical reaction to form hydrochloric acid in the acidic aqueous solution 82, which is equivalent to removing chloride ions from the dilute black liquor 10. Similarly, potassium ions in the pretreated filtrate 70 are converted into potassium hydroxide via an electrochemical reaction to form potassium hydroxide in the alkaline aqueous solution 84, which is equivalent to removing potassium ions from the dilute black liquor 10. Furthermore, the resulting brine solution is reintroduced into the alkali recovery system 200, where the sodium sulfate is further reacted by combustion in the boiler RB to generate sodium sulfide, significantly improving the sodium recovery rate. Therefore, according to the embodiments of the present invention, potassium and chloride ions in the alkali recovery system 200 can be effectively removed, and a higher sodium recovery rate is provided.

[0114] Furthermore, based on practical experience, the composition of each batch of soda ash 30 is not exactly the same. According to the embodiments of the present invention, a relatively high sodium recovery rate and a relatively high potassium removal rate can be achieved by selecting different dissolution temperatures.

[0115] In some embodiments, after step S7, the alkali recovery method 100 further includes: using the acidic aqueous solution 82 obtained in step S7 in the acid treatment process of the pulping process, and using the alkali aqueous solution 84 obtained in step S7 in the alkali treatment process of the pulping process.

[0116] The following examples and comparative models further illustrate this point.

[0117] Example 1

[0118] After obtaining soda ash 30, the characteristic ratio (weight ratio of potassium sulfate / sodium sulfate) of soda ash 30 was measured to be 0.54. Based on the characteristic ratio of soda ash, the dissolution temperature was selected as 40℃. At 40℃, the saturated solubility ratio of potassium sulfate and sodium sulfate in water is 0.30. Therefore, the ratio difference is 0.24 (i.e., |(0.30-0.54)|=0.24). 666g of soda ash 30 was mixed with 999g of water W1 to obtain a mixture 40 with a weight percentage of 40%, and stirred at 40℃ until thermal and dissolution equilibrium was reached. Then, the mixture 40 was filtered to obtain filtrate 50 and filter cake 52. The concentrations of potassium sulfate, sodium sulfate, and sodium chloride in filtrate 50 were analyzed, and their weights were calculated. Then, the weights of potassium sulfate, sodium sulfate, and sodium chloride in filter cake 52 were obtained through mass balance calculation. The experimental results are summarized in Table 2 below. The filter cake 52 contains 144.06 g of potassium sulfate, 91.94 g of sodium sulfate, and 4.59 g of sodium chloride, resulting in a potassium-rich filter cake 64. After drying and pulverizing the potassium-rich filter cake 64, potassium fertilizer is obtained.

[0119] Table 2:

[0120]

[0121] After pretreatment of filtrate 50 by resin adsorption, pretreated filtrate 70 is obtained, which is then fed into a bipolar membrane electrodialysis unit ED, with an operating voltage of 26.05V and a current density of 80A / m³. 2 Samples of the obtained acidic aqueous solution 82, alkaline aqueous solution 84, and salt aqueous solution 86 were taken and analyzed at running times of 40 minutes, 50 minutes, 60 minutes, and 70 minutes, respectively, to calculate the sodium chloride removal rate, potassium sulfate removal rate, and sodium sulfate reuse rate. The definitions of the above sodium chloride removal rate, potassium sulfate removal rate, and sodium sulfate reuse rate are as follows:

[0122] Sodium chloride removal rate = (1 - weight of sodium chloride in electrodialysis residue / weight of sodium chloride in soda ash) × 100%;

[0123] Potassium sulfate removal rate = (1 - weight of potassium sulfate in electrodialysis residue / weight of potassium sulfate in soda ash) × 100%;

[0124] Sodium sulfate recovery rate = (weight of sodium sulfate in electrodialysis residue / weight of sodium sulfate in soda ash) × 100%;

[0125] The experimental results are summarized in Table 3 below.

[0126] Table 3:

[0127]

[0128] The results of Example 1 show that the removal rates of sodium chloride and potassium sulfate increased with increasing electrolysis time, but the sodium sulfate recovery rate decreased with increasing electrolysis time. At an electrolysis time of 70 minutes, the sodium chloride removal rate was 89.5%, the potassium sulfate removal rate was 79.8%, but the sodium sulfate recovery rate was 63.8%.

[0129] Finally, the brine solution 86 can be reused and mixed with dilute black liquor 10 before entering the evaporator VE (e.g. Figure 2 As shown), the alkaline solution 84 can be reused in the alkaline treatment process of the pulping process, while the acidic solution can be reused in the acid treatment process of the pulping process.

[0130] Examples 2-5

[0131] As described in Example 1, after obtaining soda ash 30 in Examples 2-5, the characteristic ratios of each soda ash 30 (weight ratio of potassium sulfate / sodium sulfate) were measured. A dissolution temperature was then selected based on these ratios, and the ratio difference was calculated at that temperature. The soda ash 30 was then mixed with water W1 in step S4 and filtered in step S5, with some experimental conditions adjusted for different soda ash 30 compositions. The composition of soda ash 30 in Examples 2-5, the weight percentage of soda ash 30 in the mixed solution 40, the dissolution temperature, and the composition of the filtrate 50 and filter cake 52 are also summarized in Table 2 above.

[0132] The filter cake 52 obtained in Example 2 is a sodium-rich filter cake 62, which can be reused for applications such as... Figure 2 The filter cake 52 obtained in Example 3 is a potassium-rich filter cake 64, which can be used to manufacture potash fertilizer and recycled in forest farms. The filter cake 52 obtained in Examples 4 and 5 is a sodium-rich filter cake 62, which can be recycled for purposes such as... Figure 2 The boiler RB shown.

[0133] The filtrates obtained in Examples 2-5 were pretreated with resin adsorption and then subjected to bipolar membrane electrodialysis. The important electrodialysis experimental parameters and results are summarized in Table 4 below.

[0134] Table 4:

[0135]

[0136] In Example 2, the sodium sulfate recovery rate was as high as 99.4%, the sodium chloride removal rate was 78.7%, and the potassium sulfate removal rate was 76.4%. The experimental results of Examples 3, 4, and 5 are shown in Table 4. The system 200 for alkali recovery achieved efficient removal of chlorine and potassium, and a very high proportion of sodium sulfate was returned to the alkali recovery system 200.

[0137] Comparative Example 1 - Comparison Patent No. CN 110877900B (A Method and System for Resource Utilization of Miscellaneous Salts)

[0138] A comparative experiment was conducted using the same soda ash as in Example 1. 666 grams of soda ash were dissolved in 3330 grams of water (the soda ash concentration was 17% by weight). The mixture was stirred at 50°C for 30 minutes to obtain 3996 grams of filtrate. The volume of liquid required for electrodialysis was three times that of the embodiment of this invention, meaning the power consumption was at least three times higher. After pretreatment as in Example 1, the filtrate underwent electrodialysis. After running for 40, 50, 60, and 70 minutes, the concentrations of each component in the acid and alkaline solutions were analyzed to obtain the sodium chloride and potassium sulfate removal rates. The concentrations of each component in the brine solution were analyzed, and the brine solution was reused in the alkali recovery system to calculate the sodium sulfate recovery rate. The experimental results are summarized in Table 5 below.

[0139] Table 5:

[0140]

[0141] In Table 5, when electrodialysis ran for 50 minutes, the sodium chloride removal rate was 88.4%, while the potassium sulfate removal rate was only 43.2%, indicating that potassium removal was ineffective. The sodium sulfate reuse rate was 74.5%. The embodiment of the present invention is significantly superior to the technical solution of Comparative Example 1.

[0142] If Comparative Example 1 further improves the potassium sulfate removal rate by extending the electrodialysis time, the sodium sulfate removal rate will also increase as the electrodialysis time is extended, meaning the sodium sulfate reuse rate will decrease, resulting in excessive sodium sulfate loss and increased operating costs.

[0143] Comparative Example 2 - Traditional Total Dissolution Evaporation Crystallization Process

[0144] Using the same soda ash as in Example 1, a conventional total dissolution evaporation crystallization process was employed. 666 grams of soda ash were added to 1998 grams of water for total dissolution (25% by weight), and the mixture was stirred at 50°C for 30 minutes. The filtrate was then evaporated and crystallized at 80°C.

[0145] When half of the water evaporated, the evaporation and crystallization process was stopped, and filtration was immediately performed. Subsequently, the main components of the crystals and the solution were analyzed. The results showed that the crystals contained 7.4 grams of sodium chloride, 158.4 grams of potassium sulfate, and 193.0 grams of sodium sulfate. The potassium and sodium salts crystallized simultaneously and could not be effectively separated. In addition, the waste solution contained 35.1 grams of sodium chloride, 52.7 grams of potassium sulfate, and 197.1 grams of sodium sulfate, which needed to be discharged. Based on the alkali recovery system for crystal recovery and the discharge of waste solution, the sodium chloride removal rate was calculated to be 87.3%, the potassium sulfate removal rate to be 21.8%, and the sodium sulfate reuse rate to be 51.3%. It is evident that the traditional evaporation and crystallization process cannot effectively remove potassium, and the discharge of mother liquor also wastes resources and increases the burden on environmental wastewater treatment.

[0146] The foregoing discloses multiple embodiments (or examples) of the present invention and their related technical effects. Different embodiments or examples may have different or the same technical effects. Therefore, any embodiment (or example) of the present invention or the scope of the patent application need not achieve all the objectives, advantages, or features disclosed in the present invention. Furthermore, the content disclosed above is only a part of the many embodiments of the present invention and is not intended to limit the present invention. Any simple equivalent changes and modifications made to the content of the scope of the patent application of the present invention are within the scope of the patent of the present invention. In addition, the abstract and the title of the invention are only used to assist in patent document retrieval and are not intended to limit the scope of the present invention.

Claims

1. A method for alkali recovery, characterized in that: Includes the following steps: S1: Providing or receiving soda ash, wherein the soda ash contains potassium sulfate and sodium sulfate; S2: The weight ratio of potassium sulfate to sodium sulfate in the soda ash is measured and defined as the soda ash characteristic ratio value; S3: Select a dissolution temperature based on the soda ash characteristic ratio. At the dissolution temperature, the absolute value of the difference between the ratio of the saturated solubility of potassium sulfate and sodium sulfate in water and the soda ash characteristic ratio is defined as the ratio difference. The ratio difference is greater than or equal to 0.1 and less than or equal to 20. S4: Mix the soda ash with water to obtain a mixture, and reach substantial dissolution equilibrium at the dissolution temperature; wherein the mixture is an aqueous solution containing dissolved potassium sulfate and sodium sulfate, as well as undissolved potassium sulfate and / or sodium sulfate; S5: Filter the mixture to obtain a filtrate and a filter cake; wherein the filtrate contains the aqueous solution containing dissolved potassium sulfate and sodium sulfate, and the filter cake contains undissolved potassium sulfate and / or sodium sulfate; S6: Pre-treat the filtrate to remove at least some of the divalent metal ions in the filtrate, and obtain a pre-treated filtrate; S7: The pretreated filtrate is subjected to bipolar membrane electrodialysis to obtain separated acid aqueous solution, alkaline aqueous solution and salt aqueous solution.

2. The alkali recovery method as described in claim 1, characterized in that: Step S1 includes the following steps: S1-1: Heating dilute black liquor to obtain concentrated black liquor; S1-2: Combustion of the concentrated black liquor to obtain melt and flue gas; S1-3: Collect the solid dust in the flue gas to obtain the alkaline ash.

3. The alkali recovery method as described in claim 2, characterized in that: Step S1-1 involves heating the dilute black liquor using a multi-effect evaporator to obtain clean condensate.

4. The alkali recovery method as described in claim 2, characterized in that: After step S5, there is also a step S5-1: determining which of the weight percentages of potassium sulfate and sodium sulfate in the filter cake is higher.

5. The alkali recovery method as described in claim 4, characterized in that: Following step S5-1, the following steps are also included: When the weight percentage of sodium sulfate in the filter cake in step S5-1 is greater than the weight percentage of potassium sulfate, a sodium-rich filter cake is obtained. The sodium-rich filter cake is then burned together with the concentrated black liquor in step S1-2 to convert the sodium sulfate in the filter cake into sodium sulfide. When the weight percentage of potassium sulfate in the filter cake in step S5-1 is greater than the weight percentage of sodium sulfate, a potassium-rich filter cake is obtained. The potassium-rich filter cake is then dried and pulverized to obtain potassium fertilizer.

6. The alkali recovery method as described in claim 2, characterized in that: After step S7, the method further includes the following step: heating the brine solution from step S7 together with the dilute black liquor from step S1-1 to obtain the concentrated black liquor.

7. The method for alkali recovery as described in claim 1, characterized in that: At the melting temperature, the ratio difference is greater than or equal to 0.2 and less than or equal to 9.

8. The method for alkali recovery as described in claim 1 or claim 7, characterized in that: The melting temperature is higher than or equal to 0°C and lower than or equal to 60°C.

9. The method for alkali recovery as described in claim 1 or claim 7, characterized in that: The melting temperature is higher than or equal to 10°C and lower than or equal to 55°C.

10. The method for alkali recovery as described in claim 1, characterized in that: In step S4, the weight percentage of the soda ash in the mixture is 30% to 65%.

11. The method for alkali recovery as described in claim 1, characterized in that: In step S4, the weight percentage of the soda ash in the mixture is 40% to 55%.

12. The method for alkali recovery as described in claim 1, characterized in that: In step S7, the acidic aqueous solution contains hydrochloric acid and sulfuric acid, and the volume molar concentration of the hydrochloric acid is greater than that of the sulfuric acid; the alkaline aqueous solution contains sodium hydroxide and potassium hydroxide; and the salt aqueous solution contains sodium sulfate, potassium sulfate, and sodium chloride.

13. The method for alkali recovery as described in claim 12, characterized in that: Step S7 includes the following steps: controlling multiple independent parameters of the bipolar membrane electrodialysis treatment, including setting the hydrogen ion concentration in the acidic aqueous solution to 1 mol / L to 2 mol / L, the hydroxide ion concentration in the alkaline aqueous solution to 1.5 mol / L to 3.0 mol / L, and the sulfate ion concentration in the salt aqueous solution to 150 g / L to 250 g / L.

14. The method for alkali recovery as described in claim 12 or claim 13, characterized in that: After step S7, the process further includes an acid treatment process in which the acid aqueous solution is used in the pulping process, and an alkali treatment process in which the alkali aqueous solution is used in the pulping process.

15. The method for alkali recovery as described in claim 1, characterized in that: In step S6, the pretreatment includes resin adsorption, precision filtration, activated carbon adsorption, or a combination thereof.

16. The method for alkali recovery as described in claim 1, characterized in that: In step S7, the bipolar membrane electrodialysis treatment includes using a bipolar membrane electrodialysis device, which includes multiple repeating units composed of a bipolar membrane, a cation exchange membrane, and an anion exchange membrane.

17. The method for alkali recovery as described in claim 16, characterized in that: The anion exchange membrane includes acid and alkali resistant anion exchange membranes or monovalent selective anion exchange membranes.

18. The alkali recovery method as described in claim 3, characterized in that: In step S4, the water may include the cleaning condensate, clean water, or a combination thereof.

19. A system for alkali recovery, characterized in that: Include: A dissolving tank is used to receive and mix soda ash and water to form a mixture; the dissolving tank has a temperature control device that can control the mixture at a pre-selected dissolving temperature to achieve substantial dissolution equilibrium; wherein the dissolving temperature is selected based on a ratio difference greater than or equal to 0.1 and less than or equal to 20; the ratio difference is the absolute value of the difference between the ratio of the saturated solubility of potassium sulfate and sodium sulfate in water at the dissolving temperature and the weight ratio of potassium sulfate and sodium sulfate in the soda ash; A filter, connected to the dissolving tank, is used to filter the mixture to separate the filtrate and filter cake. A pretreatment unit, connected to the filter, is used to pretreat the filtrate to remove at least some of the divalent metal ions in the filtrate, so as to obtain a pretreated filtrate. A bipolar membrane electrodialysis device is connected to the pretreatment unit to perform bipolar membrane electrodialysis on the pretreated filtrate to obtain separated acidic aqueous solution, alkaline aqueous solution and salt aqueous solution.

20. The alkali recovery system as described in claim 19, characterized in that: The system further comprises, prior to the melting tank: The evaporator is equipped with high-temperature steam to treat dilute black liquor, forming concentrated black liquor and cleaning condensate. A boiler, connected between the evaporator and the dissolving tank, is used to burn the concentrated black liquor to obtain the soda ash, melt, high-temperature steam and carbon dioxide; And a mixer, connected to the boiler, for mixing the cleaning condensate and dilute white liquid to form green liquid.

21. The alkali recovery system as described in claim 20, characterized in that: The system further includes: a storage tank connected to the filter for receiving and processing the filter cake; when the weight percentage of sodium sulfate in the filter cake is greater than the weight percentage of potassium sulfate, a sodium-rich filter cake is obtained, which is then returned to the boiler for combustion together with the concentrated black liquor; or when the weight percentage of potassium sulfate in the filter cake is greater than the weight percentage of sodium sulfate, a potassium-rich filter cake is obtained, which is then dried and pulverized to form potassium fertilizer.

22. The alkali recovery system as described in claim 20, characterized in that: The high-temperature steam obtained from the boiler is recycled back into the evaporator.

23. The alkali recovery system as described in claim 20, characterized in that: The brine solution is fed into the dilute black liquor for mixing, and then enters the evaporator to obtain a concentrated black liquor.

24. The alkali recovery system as described in claim 19, characterized in that: The acidic aqueous solution is used in the acid treatment process of the pulping process, and the alkaline aqueous solution is used in the alkaline treatment process of the pulping process.

25. The alkali recovery system as described in claim 19, characterized in that: At the melting temperature, the ratio difference is greater than or equal to 0.2 and less than or equal to 9.

26. The alkali recovery system as described in claim 19 or claim 25, characterized in that: The melting temperature is higher than or equal to 0°C and lower than or equal to 60°C.

27. The alkali recovery system as described in claim 19 or claim 25, characterized in that: The melting temperature is higher than or equal to 10°C and lower than or equal to 55°C.

28. The alkali recovery system as described in claim 19, characterized in that: The dissolving tank is operated such that the soda ash in the mixture is 30% to 65% by weight.

29. The alkali recovery system as described in claim 19, characterized in that: The soda ash in the mixture accounts for 40% to 55% by weight.

30. The alkali recovery system as described in claim 19, characterized in that: The bipolar membrane electrodialysis device controls multiple independent parameters, including setting the hydrogen ion concentration in the acidic aqueous solution to 1 mol / L to 2 mol / L, the hydroxide ion concentration in the alkaline aqueous solution to 1.5 mol / L to 3.0 mol / L, and the sulfate ion concentration in the salt aqueous solution to 150 g / L to 250 g / L.

31. The alkali recovery system as described in claim 19, characterized in that: The pretreatment unit uses resin adsorption, precision filtration, activated carbon adsorption, or a combination thereof.

32. The alkali recovery system as described in claim 19, characterized in that: The bipolar membrane electrodialysis device comprises multiple repeating units consisting of a bipolar membrane, a cation exchange membrane, and an anion exchange membrane.

33. The alkali recovery system as described in claim 32, characterized in that: The anion exchange membrane includes acid and alkali resistant anion exchange membranes or monovalent selective anion exchange membranes.

34. The alkali recovery system as described in claim 20, characterized in that: The water includes the cleaning condensate, clean water, or a combination thereof.

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