Process for the metathesis of sodium sulfate and ammonium bicarbonate to produce sodium bicarbonate and co-produce ammonium sulfate

Abstract

This invention relates to the field of chemical production technology and discloses a process for producing sodium bicarbonate and co-producing ammonium sulfate from sodium sulfate via metathesis. The steps include: evaporating and concentrating a refined sodium sulfate solution to separate anhydrous sodium sulfate; subjecting a portion of the mother liquor to flash cooling to precipitate sodium chloride for early impurity removal; reacting anhydrous sodium sulfate with ammonium bicarbonate via metathesis, and drying the resulting crystals to obtain sodium bicarbonate; acidifying the metathesis mother liquor with sulfuric acid, then sending it to a cooling crystallizer to cool and separate the double salt, and extracting a portion of the mother liquor as waste material; adding ammonium hydrate dissociation promoter to the remaining mother liquor and evaporating and concentrating it to finally separate the ammonium sulfate product. This invention maintains the phase balance of the circulating system by pumping out waste and separating sodium chloride in advance, while using a promoter to inhibit sodium impurity entrainment and co-precipitation of double salt during the ammonium sulfate crystallization process, thereby improving the purity of the product crystals and ensuring long-term continuous operation of the process.

Description

Technical Field

[0001] This invention relates to the field of chemical production technology, specifically to a process for the metathesis of sodium sulfate and ammonium bicarbonate to produce sodium bicarbonate and co-produce ammonium sulfate. Background Technology

[0002] Using sodium sulfate and ammonium bicarbonate as raw materials, a metathesis reaction is used to produce sodium bicarbonate (baking soda) and co-produce ammonium sulfate, which is a common process route for the comprehensive utilization of sodium sulfate resources. This process usually requires multiple recycling of the mother liquor after reaction separation to improve raw material conversion rate and reduce production costs.

[0003] In actual continuous production, small amounts of soluble impurities such as sodium chloride carried in the raw materials accumulate continuously within the system as the mother liquor circulates. When entering the mother liquor evaporation and concentration stage, the accumulated sodium chloride easily co-precipitates with anhydrous sodium sulfate, causing a shift in the system's phase equilibrium and increasing the pressure on the front-end impurity removal process. In the drying process after obtaining the intermediate product sodium bicarbonate, due to the limited thermal stability of this substance, if the actual heating temperature of the material cannot be strictly controlled during conventional drying operations, some sodium bicarbonate can easily decompose into sodium carbonate, resulting in a decrease in the total alkalinity index of the baking soda product.

[0004] Furthermore, the mother liquor from the metathesis reaction has a complex composition, containing unreacted bicarbonates and high concentrations of sodium ions. During subsequent extraction and evaporation crystallization of the co-product ammonium sulfate, the residual carbonates alter the system environment, and the high concentration of sodium ions easily occupies lattice defect sites in ammonium sulfate, causing impurity entrainment, or directly reacts with sulfate ions in the system to form sodium ammonium double salts, which then precipitate together. These phenomena reduce the purity of the fertilizer-grade ammonium sulfate product, increase the difficulty of treating the circulating mother liquor, and restrict the long-term stable operation of the metathesis process.

[0005] Therefore, this invention proposes a process for the metathesis of sodium sulfate and ammonium bicarbonate to produce sodium bicarbonate and co-produce ammonium sulfate, in order to overcome the shortcomings of the prior art. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a process for the metathesis of sodium sulfate and ammonium bicarbonate to produce sodium bicarbonate and co-produce ammonium sulfate. This process solves the problems in existing processes for the metathesis of sodium sulfate to produce sodium bicarbonate and co-produce ammonium sulfate, such as the enrichment of impurities in the circulating mother liquor leading to salt-nitrate co-precipitation, the susceptibility of sodium bicarbonate to heat-induced deterioration during the drying process, and the easy occurrence of sodium impurity entrainment and double salt co-precipitation during the ammonium sulfate crystallization stage. These problems result in limited product purity and difficulty in long-term continuous and stable operation of the system.

[0007] To achieve the above objectives, the present invention provides the following technical solution: a process for the metathesis of sodium sulfate and ammonium bicarbonate to produce sodium bicarbonate and co-produce ammonium sulfate, comprising the following steps: Sodium carbonate and sodium hydroxide are added to a sodium sulfate leaching solution to react, and a purified sodium sulfate solution is obtained after solid-liquid separation. The refined sodium sulfate solution is evaporated and concentrated to separate anhydrous sodium sulfate crystals and centrifuged mother liquor. A portion of the centrifuged mother liquor is extracted and fed into a flash cooling crystallizer. Sodium chloride crystals are precipitated by controlling the cooling system. The remaining mother liquor is recycled to produce nitrate. The obtained anhydrous sodium sulfate crystals and ammonium bicarbonate were fed into a metathesis reactor for reaction, and primary sodium bicarbonate crystals and metathesis mother liquor were separated. After washing the primary sodium bicarbonate crystals, they are sent to an airflow dryer for drying, and the material is discharged to obtain dry sodium bicarbonate product. The resulting metathesis mother liquor and the washing liquid were combined, and sulfuric acid was added dropwise for acidification treatment to convert the residual ammonium bicarbonate into ammonium sulfate. The acidified mother liquor is sent to a cooling crystallizer to cool down. After the precipitated double salt solids are separated, a portion of the volume of the generated cooling crystallization mother liquor is extracted as waste material and led out of the circulation loop. The remaining cooled crystallization mother liquor is fed into an ammonium sulfate evaporator and a hydrated ammonium dissociation promoter is added for evaporation and concentration to obtain fertilizer-grade ammonium sulfate product. The hydrated ammonium dissociation promoter is used to regulate interfacial hydrogen bonding to suppress sodium impurity entrainment and double salt co-precipitation.

[0008] By introducing multi-stage evaporation and cooling crystallization physical separation steps, and combining them with metathesis and acidification processes, the material flow of anhydrous sodium sulfate to sodium bicarbonate and ammonium sulfate conversion has been largely streamlined. In the initial nitrification stage, a portion of the mother liquor is extracted for flash cooling, which helps to separate and remove accumulated sodium chloride impurities in the early stages of the system, thus helping to maintain phase equilibrium throughout the process. Regarding the ammonium sulfate evaporation and crystallization step, the addition of a hydrated ammonium dissociation promoter affects the ion hydration layer structure within the concentration system. This promoter reduces the probability of sodium ions occupying lattice defect sites in ammonium sulfate, thereby inhibiting the crystallization kinetics of sodium ions and sulfate ions forming sodium-ammonium double salts, which is beneficial for improving the crystal purity of fertilizer-grade ammonium sulfate products.

[0009] Preferably, when sodium carbonate and sodium hydroxide are added to the sodium sulfate leaching solution for reaction, the molar ratio of sodium carbonate to calcium ions in the solution is controlled to be 1.05 to 1.20:1, and the molar ratio of sodium hydroxide to magnesium ions in the solution is 2.10 to 2.40:1. When the density of the extracted centrifuged mother liquor is concentrated to 1.28–1.32 g / cm³ 3 The mixture is then introduced into the flash cooling crystallizer, and the temperature of the flash cooling system is controlled at 50-55℃.

[0010] A specific ratio of sodium carbonate to sodium hydroxide can effectively promote the precipitation of calcium and magnesium ions, thereby reducing scaling problems in equipment during subsequent evaporation. For highly concentrated high-chlorine mother liquor, setting the flash cooling temperature range to 50-55 degrees Celsius precisely avoids the intersection region of the solubility curves of sodium chloride and anhydrous sodium sulfate. This operation keeps the system primarily in the sodium chloride saturation region, largely preventing salt-nitrate co-precipitation and improving the purity of the recovered sodium chloride.

[0011] Preferably, when the anhydrous sodium sulfate crystals and ammonium bicarbonate are fed into the metathesis reactor for reaction, the molar ratio of anhydrous sodium sulfate to ammonium bicarbonate is controlled to be 1:2.05 to 2.15. During drying in an airflow dryer, the actual heating temperature of the sodium bicarbonate material is maintained at 50–55°C by adjusting the feed rate and hot air volume.

[0012] A slight excess of ammonium bicarbonate can shift the reversible reaction towards the product, thereby increasing the conversion rate of anhydrous sodium sulfate. In the subsequent drying process, the actual heating temperature of 50 to 55 degrees Celsius provides the latent heat of vaporization required to remove free water from the material surface, and is just below the critical temperature for the thermal decomposition of sodium bicarbonate. This plays a crucial role in preventing the thermal decomposition of sodium bicarbonate into sodium carbonate, contributing to the stability of the product's total alkalinity.

[0013] Preferably, when adding sulfuric acid for acidification, concentrated sulfuric acid with a mass fraction of 98% is added, the reaction temperature is controlled at 40-55°C, and the addition is stopped when the pH value of the system drops to 3.5-4.5. The mixed tail gas containing ammonia and carbon dioxide that escapes during the metathesis reaction and acidification treatment is sent to a dilute sulfuric acid absorption tower for absorption at 15-30°C. The resulting ammonium sulfate absorbent is returned to the acidified mother liquor or the ammonium sulfate evaporation and crystallization section.

[0014] By adjusting the pH of the system to a specific acidic range through the dropwise addition of concentrated sulfuric acid, the residual ammonium bicarbonate in the mother liquor can be further converted into ammonium sulfate, effectively reducing the adverse effects of carbonate impurities on subsequent crystallization processes. Furthermore, the tail gas escaping during the reaction is absorbed by dilute sulfuric acid at room temperature, and the ammonia in the gas phase is converted into ammonium sulfate and circulated back into the system along with the absorbent liquid, reducing nitrogen loss during the production process.

[0015] Preferably, the temperature in the cooling crystallizer is reduced to 25–30°C; The volume of waste material extracted for discharge accounts for 3% to 8% of the total volume of the cooling crystallization mother liquor; The amount of the ammonium hydrate dissociation promoter added is 0.05% to 0.20% of the total mass of the crystallization mother liquor.

[0016] After the mother liquor precipitates double salts in the cooling crystallizer, a portion of the liquid is drawn off as waste material, effectively providing an outlet for soluble impurities in the process cycle. This approach helps mitigate the increase in liquid phase viscosity and the deviation of the material phase diagram caused by multiple circulations of the mother liquor, which is beneficial for maintaining the continuous operation of the entire production system.

[0017] Preferably, the ammonium hydrate dissociation promoter is composed of a physical blend of the following components in the indicated mass percentages: Polymethacrylic acid-sodium propylene sulfonate copolymer solids 65%–75%; Sodium dodecylbenzenesulfonate solids: 25%–35%.

[0018] Preferably, the polymethacrylic acid-sodium propylene sulfonate copolymer is polymerized from methacrylic acid and sodium propylene sulfonate, with a polymerization molar ratio of 1:1.20 to 1.50, and the number average molecular weight of the polymethacrylic acid-sodium propylene sulfonate copolymer is 3200 to 4800.

[0019] Sodium dodecylbenzenesulfonate typically imparts the desired surface activity to the system, reducing solution tension. In polymethacrylic acid-sodium propylene sulfonate copolymers, the carboxyl and sulfonic acid groups distributed along the chain segments undergo a certain degree of ionization in concentrated ammonium sulfate solution. Considering the aforementioned limitations on the polymerization molar ratio and number-average molecular weight, the copolymer molecules tend to maintain a more extended conformation in the liquid phase. Through the combined effects of steric hindrance and electrostatic repulsion, the polymer chain can act on the hydration layer of ammonium and sulfate ions, reducing the strength of the hydrogen bond network on the ammonium ion surface. This micro-tuning enhances the dehydration rate of ammonium ions at the solid-liquid interface, which not only promotes the regular growth of ammonium sulfate crystals but also significantly inhibits the generation of structural defects and the abnormal intercalation of surrounding sodium impurities.

[0020] Preferably, the ammonium hydrate dissociation promoter is prepared according to the following steps: The methacrylic acid and sodium propylene sulfonate were dissolved in deionized water to prepare a monomer aqueous solution. Ammonium persulfate was dissolved in deionized water to prepare an initiator aqueous solution; the monomer aqueous solution and the initiator aqueous solution were simultaneously and uniformly added dropwise using a double dropwise addition method to carry out the polymerization reaction, and the reaction was maintained at a temperature for curing after the addition was completed. After cooling, the pH of the system was adjusted with alkaline solution, and then dried to obtain a solid polymethacrylic acid-sodium propylene sulfonate copolymer. The solid polymethacrylic acid-sodium propylene sulfonate copolymer and the solid sodium dodecylbenzene sulfonate are physically blended at room temperature according to the target ratio and stirred evenly to obtain the final product.

[0021] Preferably, during the polymerization reaction using the double-drop method, the stirring speed is controlled at 250 rpm, and the dropping time is 2.0 h to 2.5 h.

[0022] Preferably, the polymerization reaction is carried out at a temperature of 75–85°C; The temperature for heat preservation and curing is 85℃, and the curing time is 2 hours; After cooling, the alkaline solution used for adjustment is a 20% sodium hydroxide aqueous solution, which is used to adjust the pH of the system to 6.5–7.5.

[0023] The double-drop addition method used in the polymerization reaction helps maintain a relatively stable monomer-to-radical concentration ratio within the reaction system. With carefully controlled temperature, stirring conditions, and reaction time parameters, the risk of side reactions such as chain transfer or explosive polymerization is reduced, resulting in a more concentrated molecular weight distribution in the final product. The subsequent curing process primarily consumes unreacted monomers, while adjusting the pH with an alkaline solution promotes the conversion of acidic groups on the polymer chains into stable sodium salt forms. After this neutralization treatment, the polymer solids maintain good solubility and dispersion even when added to a high-salt, strong-electrolyte ammonium sulfate mother liquor.

[0024] This invention provides a process for the metathesis of sodium sulfate and ammonium bicarbonate to produce sodium bicarbonate and co-produce ammonium sulfate. It has the following beneficial effects: 1. This invention utilizes a portion of the mother liquor from the nitrate production process for flash cooling and crystallization to pre-separate sodium chloride. In the subsequent cooling and crystallization stage, a portion of the mother liquor is extracted and used as waste material for the discharge system. This multi-stage collaborative waste discharge technique continuously removes accumulated sodium chloride and other soluble impurities from the process cycle, mitigating the increase in viscosity and phase diagram drift of the circulating mother liquor. This maintains the phase balance of the entire production system and ensures long-term continuous and stable operation of the metathesis process.

[0025] 2. In the sodium bicarbonate drying stage, this invention controls the actual heating temperature of the material below the thermal decomposition critical point by adjusting the feed and hot air parameters, thus preventing the deterioration of sodium bicarbonate and stabilizing the total alkalinity of the main product. Simultaneously, the separated metathesis mother liquor is acidified with concentrated sulfuric acid to completely convert unreacted bicarbonate into ammonium sulfate, and dilute sulfuric acid is used to absorb the mixed tail gas escaping from the reaction. This combined approach not only eliminates the interference of carbonates on subsequent crystallization and separation processes but also enables the recovery and reuse of escaping nitrogen, reducing material loss.

[0026] 3. In the ammonium sulfate evaporation and crystallization process of this invention, a hydrated ammonium dissociation promoter formed by the physical blending of polymethacrylic acid-sodium propylene sulfonate copolymer and sodium dodecylbenzene sulfonate is added. This promoter, relying on its relaxed conformation and surface activity in the concentrated mother liquor, weakens the hydrogen bond network on the surface of ammonium ions through steric hindrance and electrostatic interactions. This micro-regulation increases the dehydration rate of ammonium ions at the solid-liquid interface, blocks the path for surrounding sodium ions to intercalate into the lattice defect sites of ammonium sulfate, inhibits impurity entrainment and co-precipitation of sodium-ammonium double salts, and improves the crystal purity of the co-produced fertilizer-grade ammonium sulfate. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the process flow of the present invention; Figure 2 This is a comparison of ion chromatograms of the test solution containing the precipitated solid product of the present invention. Figure 3 The following is a comparison chart of the physicochemical properties of the by-product ammonium sulfate of the present invention, wherein (a) is a distribution chart of total nitrogen mass fraction of each test sample, and (b) is a distribution chart of sodium content in crystals of each test sample. Figure 4 This is a comparison chart of the changing trends of chloride ion concentration in the mother liquor during the multi-cycle process of this invention; Figure 5 This is a monitoring graph showing the change of free ammonia concentration at the exhaust port over time according to the present invention. Detailed Implementation

[0028] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] The main raw materials and reagents used in the following examples and comparative examples have the following sources and specifications. Reagents not specifically mentioned are all commercially available analytical grade or higher grade products.

[0030] The sodium sulfate leaching solution is taken from the saline waste liquid by-product of industrial gypsum production equipment. Its main solute components and mass concentration range are: sodium sulfate 150-180 g / L, sodium chloride 15-25 g / L, calcium ions 50-80 mg / L, magnesium ions 20-40 mg / L, water as solvent, and the initial pH value of the solution is 6.5-7.5.

[0031] Methacrylic acid (CAS No. 79-41-4), sodium propylene sulfonate (CAS No. 2495-39-8), ammonium persulfate (CAS No. 7727-54-0), and sodium dodecylbenzene sulfonate (CAS No. 25155-30-0) are all commercially available products.

[0032] Preparation Example 1: This preparation example provides a method for preparing a hydrated ammonium dissociation promoter, comprising the following steps: Add 100 parts of deionized water to a three-necked flask equipped with a condenser, mechanical stirrer, and thermometer, and heat to 80°C. Mix methacrylic acid and sodium propylene sulfonate at a molar ratio of 1:1.35, with a total monomer mass of 30 parts, and dissolve in 70 parts of deionized water to prepare a monomer aqueous solution. Weigh 0.45 parts of ammonium persulfate and dissolve it in 15 parts of deionized water to prepare an initiator aqueous solution. Under constant temperature of 80℃ and stirring speed of 250 rpm, the monomer aqueous solution and the initiator aqueous solution are simultaneously and uniformly added to the flask using a double drop method over a period of 2.5 hours. After the addition was complete, the temperature was raised to 85℃ and kept warm for 2 hours. After cooling to room temperature, the pH of the system was adjusted to 7.0 with a 20% sodium hydroxide aqueous solution to obtain an aqueous solution of polymethacrylic acid-sodium propylene sulfonate copolymer. The number average molecular weight was determined to be 4100 by gel permeation chromatography. The aqueous solution was dried to obtain a solid polymethacrylic acid-sodium propylene sulfonate copolymer. The solid polymethacrylic acid-sodium propylene sulfonate copolymer and the solid sodium dodecylbenzene sulfonate were physically mixed at room temperature at a mass ratio of 70:30 and stirred evenly to obtain the ammonium hydrate dissociation promoter.

[0033] Preparation Example 2: This preparation example provides a method for preparing a hydrated ammonium dissociation promoter, comprising the following steps: Add 100 parts of deionized water to a three-necked flask equipped with a condenser, mechanical stirrer, and thermometer, and heat to 85°C. Mix methacrylic acid and sodium propylene sulfonate at a molar ratio of 1:1.20, with a total monomer mass of 30 parts, and dissolve in 70 parts of deionized water to prepare a monomer aqueous solution. Weigh 0.54 parts of ammonium persulfate and dissolve it in 15 parts of deionized water to prepare an initiator aqueous solution. Under constant temperature of 85℃ and stirring speed of 250 rpm, the monomer aqueous solution and the initiator aqueous solution are simultaneously and uniformly added to the flask using a double drop method over a period of 2.0 h. After the addition was complete, the temperature was raised to 85℃ and kept warm for 2 hours. After cooling to room temperature, the pH of the system was adjusted to 6.5 with a 20% sodium hydroxide aqueous solution to obtain an aqueous solution of polymethacrylic acid-sodium propylene sulfonate copolymer. The number average molecular weight was determined to be 3200 by gel permeation chromatography. The aqueous solution was dried to obtain a solid polymethacrylic acid-sodium propylene sulfonate copolymer. The solid polymethacrylic acid-sodium propylene sulfonate copolymer and the solid sodium dodecylbenzene sulfonate were physically blended at room temperature in a mass ratio of 75:25 and stirred evenly to obtain the ammonium hydrate dissociation accelerator.

[0034] Preparation Example 3: This preparation example provides a method for preparing a hydrated ammonium dissociation promoter, comprising the following steps: Add 100 parts of deionized water to a three-necked flask equipped with a condenser, mechanical stirrer, and thermometer, and heat to 75°C. Mix methacrylic acid and sodium propylene sulfonate at a molar ratio of 1:1.50, with a total monomer mass of 30 parts, and dissolve in 70 parts of deionized water to prepare a monomer aqueous solution. Weigh 0.36 parts of ammonium persulfate and dissolve it in 15 parts of deionized water to prepare an initiator aqueous solution. Under constant temperature of 75℃ and stirring speed of 250 rpm, the monomer aqueous solution and the initiator aqueous solution are simultaneously and uniformly added to the flask over a period of 2.5 hours using a double drop method. After the addition was complete, the temperature was raised to 85℃ and kept warm for 2 hours. After cooling to room temperature, the pH of the system was adjusted to 7.5 with a 20% sodium hydroxide aqueous solution to obtain an aqueous solution of polymethacrylic acid-sodium propylene sulfonate copolymer. The number average molecular weight was determined to be 4800 by gel permeation chromatography. The aqueous solution was dried to obtain a solid polymethacrylic acid-sodium propylene sulfonate copolymer. The solid polymethacrylic acid-sodium propylene sulfonate copolymer and the solid sodium dodecylbenzene sulfonate were physically blended at room temperature in a mass ratio of 65:35 and stirred evenly to obtain the ammonium hydrate dissociation accelerator.

[0035] Reference Appendix Figure 1 Example 1: This embodiment provides a process for the metathesis of sodium sulfate and ammonium bicarbonate to produce sodium bicarbonate and co-produce ammonium sulfate, including the following steps: The sodium sulfate leaching solution was pumped into the reactor, with the initial solution temperature controlled at 50℃. Stirring was started, and 15% (w / w) sodium carbonate and sodium hydroxide aqueous solutions were added. The molar ratio of sodium carbonate to calcium ions in the solution was controlled at 1.12:1, and the molar ratio of sodium hydroxide to magnesium ions in the solution was controlled at 2.24:1. The reaction residence time was 45 min. After the reaction, the system was subjected to solid-liquid separation by pressure filtration. The separated slurry was returned to the gypsum workshop, and the filtrate was a purified sodium sulfate solution. The refined sodium sulfate solution, preheated to 85°C by a heat exchanger, is fed into an MVR (Mechanical Vapor Reduction) evaporation unit for concentration at an operating temperature of 105°C. Anhydrous sodium sulfate crystals precipitate within the system. Centrifugation produces wet nitrate, and the centrifugal mother liquor is divided into two parts: one part is returned to the MVR evaporation unit for recycling, and the other part is used as a high-chlorine concentration mother liquor, concentrated to a density of 1.30 g / cm³. 3 The liquid is then drawn into a flash cooling crystallizer, where the temperature is controlled at 52°C. Sodium chloride crystals precipitate from the system, and industrial-grade sodium chloride is produced by centrifugation. The remaining mother liquor is returned to the MVR evaporator to continue producing nitrate. A portion of the process base liquid was introduced into the metathesis reactor. Based on the actual content of anhydrous sodium sulfate in the wet nitrate, the molar ratio of sodium sulfate to ammonium bicarbonate was controlled at 1:2.10. The wet nitrate and solid ammonium bicarbonate prepared above were continuously fed into the metathesis reactor. The forced external circulation heat exchanger was turned on to maintain the reaction temperature at 35°C, the stirring speed was controlled at 120 rpm, and the reaction residence time was controlled at 3.0 h. After the reaction, the slurry was separated to obtain primary sodium bicarbonate crystals and metathesis mother liquor. Primary sodium bicarbonate crystals undergo a two-stage countercurrent washing process. The first stage uses the washing liquid generated from the second stage, while the second stage uses a saturated cold sodium bicarbonate solution at 25°C to minimize the dissolution loss of sodium bicarbonate crystals. If necessary, a small amount of pure steam condensate is added to the second stage washing liquid. The washed, wet sodium bicarbonate is then fed into an airflow dryer for drying. The hot air inlet temperature is set at 130°C, and the actual heating temperature of the sodium bicarbonate material during drying is maintained at 52°C by adjusting the feed rate and hot air volume. The dryer discharges the product to obtain dry sodium bicarbonate. The mother liquor from the metathesis process and the primary washing liquid from the washing process are combined and fed into an acidification reactor lined with polytetrafluoroethylene. Concentrated sulfuric acid with a mass fraction of 98% is slowly added dropwise under stirring, with the reaction temperature controlled at 45°C. The addition is stopped when the pH of the system drops to 4.0. The residual ammonium bicarbonate in the mother liquor is converted into ammonium sulfate. The mixed tail gas containing ammonia and carbon dioxide escaping from this and the preceding process is collected by a closed induced draft device and sent to a secondary dilute sulfuric acid absorption tower for absorption at 25°C. The ammonia in the gas phase is absorbed by the dilute sulfuric acid and converted into ammonium sulfate absorbent. The resulting ammonium sulfate absorbent is returned to the acidified ammonium sulfate-rich mother liquor or recycled in the ammonium sulfate evaporation and crystallization section. Carbon dioxide is discharged after condensation and demisting or collected and treated separately. The mother liquor from the subsequent evaporation and crystallization section is mixed with the acidified ammonium sulfate-rich mother liquor mentioned above and sent to a cooling crystallizer to be cooled to 28°C. The precipitated double salt solid is then centrifuged and returned to the recombinant decomposition reaction process. The cooling mother liquor generated by centrifugation is divided into two streams. 5% of the total volume is extracted as waste material and led out of the circulation loop for downgrading treatment. The remaining cooling crystallization mother liquor is sent to the evaporation and crystallization section. The remaining cooling crystallization mother liquor was fed into an ammonium sulfate evaporator and 0.12% of the total mass of the mother liquor was added as the ammonium hydrate dissociation promoter prepared in Example 1. Evaporation and concentration were carried out at 90°C and a vacuum of -0.06 MPa, resulting in the precipitation of ammonium sulfate crystals. The lower crystal slurry was centrifuged and dried to obtain fertilizer-grade ammonium sulfate product, while the supernatant mother liquor was returned to the cooling crystallization section for recycling.

[0036] Example 1:

[0037] This embodiment provides a process for the metathesis of sodium sulfate and ammonium bicarbonate to produce sodium bicarbonate and co-produce ammonium sulfate, including the following steps: The sodium sulfate leaching solution was pumped into the reactor, with the initial solution temperature controlled at 40℃. Stirring was started, and 15% (w / w) sodium carbonate and sodium hydroxide aqueous solutions were added. The molar ratio of sodium carbonate to calcium ions in the solution was controlled at 1.05:1, and the molar ratio of sodium hydroxide to magnesium ions in the solution was controlled at 2.10:1. The reaction residence time was 30 min. After the reaction, the system was subjected to solid-liquid separation by pressure filtration. The separated slurry was returned to the gypsum workshop, and the filtrate was a purified sodium sulfate solution. The refined sodium sulfate solution, preheated to 80°C by a heat exchanger, is fed into an MVR (Medium-Voltage Reduction) evaporation unit for concentration at an operating temperature of 101°C. Anhydrous sodium sulfate crystals precipitate within the system. Centrifugation produces wet nitrate, and the centrifugal mother liquor is divided into two parts: one part is returned to the MVR evaporation unit for recycling, and the other part is used as a high-chlorine concentration mother liquor, concentrated to a density of 1.28 g / cm³. 3 The liquid is then drawn into a flash cooling crystallizer, where the temperature is controlled at 50°C. Sodium chloride crystals precipitate from the system, and industrial-grade sodium chloride is produced by centrifugation. The remaining mother liquor is returned to the MVR evaporator to continue producing nitrate. A portion of the process base liquid was introduced into the metathesis reactor. Based on the actual content of anhydrous sodium sulfate in the wet nitrate, the molar ratio of sodium sulfate to ammonium bicarbonate was controlled at 1:2.05. The wet nitrate and solid ammonium bicarbonate prepared above were continuously fed into the metathesis reactor. The forced external circulation heat exchanger was turned on to maintain the reaction temperature at 30°C, the stirring speed was controlled at 100 rpm, and the reaction residence time was controlled at 2.0 h. After the reaction, the slurry was separated to obtain primary sodium bicarbonate crystals and metathesis mother liquor. Primary sodium bicarbonate crystals undergo a two-stage countercurrent washing process. The first stage wash uses the washing liquid generated from the second stage wash, while the second stage wash uses a saturated cold sodium bicarbonate solution at 20°C to reduce the dissolution loss of sodium bicarbonate crystals. If necessary, a small amount of pure steam condensate is used to supplement the washing liquid in the second stage. The washed wet sodium bicarbonate is then sent to an airflow dryer for drying. The hot air inlet temperature is set to 120°C, and the actual heating temperature of the sodium bicarbonate material during the drying process is maintained at 50°C by adjusting the feed rate and hot air volume. The dryer discharges the product to obtain dry sodium bicarbonate. The mother liquor from the metathesis process and the primary washing liquid from the washing process are combined and fed into an acidification reactor lined with polytetrafluoroethylene (PTFE). Concentrated sulfuric acid with a mass fraction of 98% is slowly added dropwise under stirring, with the reaction temperature controlled at 40°C. The addition is stopped when the pH of the system drops to 4.5. The residual ammonium bicarbonate in the mother liquor is converted into ammonium sulfate. The mixed tail gas containing ammonia and carbon dioxide escaping from this and the preceding process is collected by a closed induced draft device and sent to a secondary dilute sulfuric acid absorption tower for absorption at 15°C. The ammonia in the gas phase is absorbed by the dilute sulfuric acid and converted into ammonium sulfate absorbent. The resulting ammonium sulfate absorbent is returned to the acidified ammonium sulfate-rich mother liquor or recycled in the ammonium sulfate evaporation and crystallization section. Carbon dioxide is discharged after condensation and demisting or collected and treated separately. The mother liquor from the subsequent evaporation and crystallization section is mixed with the acidified ammonium sulfate-rich mother liquor mentioned above and sent to a cooling crystallizer to be cooled to 30°C. The precipitated double salt solid is then centrifuged and returned to the recombinant decomposition reaction process. The cooling mother liquor generated by centrifugation is divided into two streams. 3% of the total volume is extracted as waste material and led out of the circulation loop for downgrading treatment. The remaining cooling crystallization mother liquor is sent to the evaporation and crystallization section. The remaining cooling crystallization mother liquor was fed into an ammonium sulfate evaporator and 0.05% of the total mass of the mother liquor was added as the ammonium hydrate dissociation promoter prepared in Preparation Example 2. Evaporation and concentration were carried out at 85°C and a vacuum of -0.05 MPa, resulting in the precipitation of ammonium sulfate crystals. The lower crystal slurry was centrifuged and dried to obtain fertilizer-grade ammonium sulfate product, while the supernatant mother liquor was returned to the cooling crystallization section for recycling.

[0038] Example 2:

[0039] This embodiment provides a process for the metathesis of sodium sulfate and ammonium bicarbonate to produce sodium bicarbonate and co-produce ammonium sulfate, including the following steps: The sodium sulfate leaching solution was pumped into the reactor, with the initial solution temperature controlled at 60℃. Stirring was started, and 15% (w / w) sodium carbonate and sodium hydroxide aqueous solutions were added. The molar ratio of sodium carbonate to calcium ions in the solution was controlled at 1.20:1, and the molar ratio of sodium hydroxide to magnesium ions in the solution was controlled at 2.40:1. The reaction residence time was 60 min. After the reaction, the system was subjected to solid-liquid separation by pressure filtration. The separated slurry was returned to the gypsum workshop, and the filtrate was a purified sodium sulfate solution. The refined sodium sulfate solution, preheated to 90°C by a heat exchanger, is fed into an MVR (Medium-Voltage Reduction) evaporation unit for concentration at an operating temperature of 108°C. Anhydrous sodium sulfate crystals precipitate within the system. Centrifugation produces wet nitrate. The centrifugal mother liquor is divided into two parts: one part is returned to the MVR evaporation unit for recycling, and the other part is used as a high-chlorine concentration mother liquor, concentrated to a density of 1.32 g / cm³. 3The liquid is then drawn into a flash cooling crystallizer, where the temperature is controlled at 55°C. Sodium chloride crystals precipitate from the system, and industrial-grade sodium chloride is produced by centrifugation. The remaining mother liquor is returned to the MVR evaporator to continue producing nitrate. A portion of the process base liquid was introduced into the metathesis reactor. Based on the actual content of anhydrous sodium sulfate in the wet nitrate, the molar ratio of sodium sulfate to ammonium bicarbonate was controlled at 1:2.15. The wet nitrate and solid ammonium bicarbonate prepared above were continuously fed into the metathesis reactor. The forced external circulation heat exchanger was turned on to maintain the reaction temperature at 40°C, the stirring speed was controlled at 150 rpm, and the reaction residence time was controlled at 4.0 h. After the reaction, the slurry was separated to obtain primary sodium bicarbonate crystals and metathesis mother liquor. Primary sodium bicarbonate crystals undergo a two-stage countercurrent washing process. The first stage uses the washing liquid generated from the second stage, while the second stage uses a saturated cold sodium bicarbonate solution at 35°C to minimize the dissolution loss of sodium bicarbonate crystals. If necessary, a small amount of pure steam condensate is added to the second stage washing liquid. The washed, wet sodium bicarbonate is then fed into an airflow dryer for drying. The hot air inlet temperature is set at 140°C, and the actual heating temperature of the sodium bicarbonate material during drying is maintained at 55°C by adjusting the feed rate and hot air volume. The dryer discharges the product to obtain dry sodium bicarbonate. The mother liquor from the metathesis process and the primary washing liquid from the washing process are combined and fed into an acidification reactor lined with polytetrafluoroethylene. Concentrated sulfuric acid with a mass fraction of 98% is slowly added dropwise under stirring, with the reaction temperature controlled at 55°C. The addition is stopped when the pH of the system drops to 3.5. The residual ammonium bicarbonate in the mother liquor is converted into ammonium sulfate. The mixed tail gas containing ammonia and carbon dioxide escaping from this and the preceding process is collected by a closed induced draft device and sent to a secondary dilute sulfuric acid absorption tower for absorption at 30°C. The ammonia in the gas phase is absorbed by the dilute sulfuric acid and converted into ammonium sulfate absorbent. The resulting ammonium sulfate absorbent is returned to the acidified ammonium sulfate-rich mother liquor or recycled in the ammonium sulfate evaporation and crystallization section. Carbon dioxide is discharged after condensation and demisting or collected and treated separately. The mother liquor from the subsequent evaporation and crystallization section is mixed with the acidified ammonium sulfate-rich mother liquor mentioned above and sent to a cooling crystallizer to be cooled to 25°C. The precipitated double salt solid is then centrifuged and returned to the recombinant decomposition reaction process. The cooling mother liquor generated by centrifugation is divided into two streams. 8% of the total volume is extracted as waste material and led out of the circulation loop for downgrading treatment. The remaining cooling crystallization mother liquor is sent to the evaporation and crystallization section. The remaining cooling crystallization mother liquor was fed into an ammonium sulfate evaporator and 0.20% of the total mass of the mother liquor was added as the ammonium hydrate dissociation promoter prepared in Preparation Example 3. Evaporation and concentration were carried out at 95°C and a vacuum of -0.08 MPa, resulting in the precipitation of ammonium sulfate crystals. The lower crystal slurry was centrifuged and dried to obtain fertilizer-grade ammonium sulfate product, while the supernatant mother liquor was returned to the cooling crystallization section for recycling.

[0040] Comparative Example 1: Compared with Example 1, the difference is that in the primary sodium bicarbonate crystal drying stage, the feed rate and hot air volume are not adjusted so that the actual heating temperature of the sodium bicarbonate material during the drying process reaches 120°C, while the rest are the same.

[0041] Comparative Example 2: Compared with Example 1, the difference is that no ammonium hydrate dissociation promoter is added when the remaining cooled crystallization mother liquor is fed into the ammonium sulfate evaporator crystallizer; otherwise, they are the same.

[0042] Comparative Example 3: Compared with Example 1, the difference is that the step of extracting 5% of the total volume as waste material to be discharged from the circulation loop is cancelled, and all the cooling mother liquor generated by centrifugation is directly sent to the evaporation and crystallization section. The rest are the same.

[0043] Comparative Example 4: Compared with Example 1, the difference is that the step of adding concentrated sulfuric acid with a mass fraction of 98% for acidification is omitted, and the mother liquor from metathesis and the primary washing liquid generated from washing are combined and directly sent to the cooling crystallizer. All other aspects are the same.

[0044] Comparative Example 5: The difference compared to Example 1 is that the density of the high-chlorine concentrate mother liquor was concentrated to 1.30 g / cm³. 3 Instead of using a flash cooling crystallizer to control the temperature at 52°C, the conventional cooling process is used to cool the crystallizer down to 15°C for salt precipitation. All other aspects are the same.

[0045] Comparative Example 6: Compared with Example 1, the difference is that when the remaining cooled crystallization mother liquor is fed into the ammonium sulfate evaporator crystallizer, only sodium dodecylbenzene sulfonate of the same mass as the accelerator is added, and polymethacrylic acid-sodium propylene sulfonate copolymer is not added, while the rest are the same.

[0046] Test Example 1: Mechanism verification of the effect of polymer accelerators on the crystallization phase and hydrogen bonding of ammonium sulfate: The solid products obtained by centrifugation in the evaporation crystallization section of Examples 1, Comparative Examples 2 and 6 were collected, vacuum dried to constant weight at 50°C, ground and passed through a 200-mesh sieve, and used as X-ray diffraction test samples. At the same time, crystallization mother liquor samples of each of the above examples and comparative examples were collected under the corresponding addition conditions and before the start of evaporation crystallization, and sealed and stored as infrared spectroscopy test samples. Among them, Example 1 was the crystallization mother liquor after adding ammonium hydrate dissociation promoter, Comparative Example 2 was the blank crystallization mother liquor without adding promoter, and Comparative Example 6 was the crystallization mother liquor after adding an equal mass of sodium dodecylbenzenesulfonate.

[0047] The three prepared solid powder samples were scanned using an X-ray diffractometer (Cu-Kα source, operating voltage 40kV, operating current 40mA). The scanning range θ was set to 10° to 60°, the scanning step size was 0.02°, and the scanning speed was 5° / min. The diffraction intensity of each sample was recorded as a function of the diffraction angle, and the phase assignment and quantitative phase analysis of each diffraction peak were performed using PDF standard cards.

[0048] Infrared absorption spectroscopy was performed on three surviving mother liquor samples using an attenuated total reflectance (ATR) attachment to a Fourier transform infrared spectrometer; the spectral range was set to 4000 cm⁻¹. -1 Up to 400cm -1 The resolution is 4cm. -1 Signal accumulation scan 32 times; quantitative analysis 3000cm -1 Up to 3300cm -1 The characteristic absorption peak position of the ammonium ion NH stretching vibration in the region was determined, and the peak wavenumber was recorded. Each test sample was measured in parallel three times. The data in the table is the average of the three test results.

[0049] The experimental data are shown in Table 1: Table 1: Comparison of crystalline phases and infrared absorption peak positions between Example 1 and Comparative Examples 2 and 6 Example 1 99.12 Not detected, below the detection limit for XRD quantitative analysis 0.88 3185.3 Comparative Example 2 84.45 13.56 1.99 3145.2 Comparative Example 6 86.12 11.85 2.03 3147.6 in conclusion: According to the data in Table 1, the ammonium sulfate phase content in the crystalline product obtained in Example 1 reached 99.12%, and no sodium sulfate-ammonium sulfate double salt phase was detected, indicating that the product has high phase purity. In contrast, in Comparative Example 2 without the addition of ammonium hydrate dissociation promoter, the product contained 13.56% double salt phase, and the content of the ammonium sulfate main phase decreased to 84.45%; in Comparative Example 6 with only the addition of the single-component surfactant sodium dodecylbenzenesulfonate, the product still contained 11.85% double salt phase. The above data indicate that under the test conditions, the addition of ammonium hydrate dissociation promoter is beneficial to reducing the proportion of double salt phase in the crystalline product and increasing the content of ammonium sulfate main phase.

[0050] In a multi-component aqueous salt system containing both sodium sulfate and ammonium sulfate, the crystallization behavior is affected by factors such as ionic composition, temperature, concentration, and the circulation state of the mother liquor. The ammonium hydrate dissociation promoter added in Example 1 is a complex system of polymethyl methacrylate-sodium propylene sulfonate copolymer and sodium dodecylbenzene sulfonate, whose molecular structure contains carboxyl / carboxylate groups and sulfonate groups. Infrared spectroscopy results show that the NH stretching vibration peak of ammonium ions in the crystallization mother liquor of Example 1 is at 3185.3 cm⁻¹. -1 It is higher than the 3145.2cm of Comparative Example 2. -1Compared to Comparative Example 6, which measures 3147.6 cm -1 The change in peak position indicates that the hydrogen bonding environment and hydration state around the ammonium ion may change after the addition of a hydrated ammonium dissociation promoter.

[0051] Based on the phase analysis results, it can be concluded that the aforementioned promoters may influence the ion migration and crystal facet growth behavior during the ammonium sulfate crystal growth process through the interaction between polar groups and ammonium ions and water molecules, as well as the adsorption of polymer chain segments at the crystal growth interface, thereby reducing the possibility of sodium salt or sodium-ammonium complex salt co-precipitating with ammonium sulfate. Comparative Example 6, which only added sodium dodecylbenzenesulfonate, still showed 11.85% sodium sulfate-ammonium sulfate complex salt phase in its product, indicating that the inhibitory effect of a single surfactant on the precipitation of the complex salt phase is relatively limited. In summary, the results in Table 1 support the regulatory role of hydrated ammonium dissociation promoters in the phase composition of ammonium sulfate crystals.

[0052] Test Example 2: Feasibility verification of the salt-nitrate phase diagram separation mechanism by flash cooling at a specific temperature: The solid products obtained in Example 1 in the flash cooling crystallizer and the solid products obtained in Comparative Example 5 in the conventional cooling salt precipitation operation were collected. The obtained samples were dried in a vacuum drying oven at 50°C until the mass was constant, ground and mixed evenly as test samples.

[0053] Accurately weigh 0.105 g of each dried original sample and dissolve it in 100 mL of deionized water to prepare sample solutions. Filter the solutions through a 0.22 μm polyethersulfone microporous membrane to remove suspended particulate matter and collect the filtrate for later use.

[0054] The filtrate was tested using an ion chromatograph with an anion exchange column and a mixed aqueous solution of sodium carbonate and sodium bicarbonate as the eluent. The constant flow rate was set to 1.0 mL / min, and the column temperature was maintained at 30 °C. Qualitative discrimination was performed based on the retention time of each anion. The peak area output by the conductivity detector was recorded, and the mass fractions of chloride and sulfate ions in the sample were calculated using the external standard method. These were then converted into equivalent sodium chloride and anhydrous sodium sulfate contents.

[0055] The experimental data are shown in Table 2: Table 2: Comparison of the mass percentage of anions in the solid-phase products of Example 1 and Comparative Example 5.

[0056] Example 1 59.61 0.95 98.31 1.40 Comparative Example 5 23.42 39.87 38.62 58.95 in conclusion: According to Table 2 and Figure 2According to the data, in Example 1, the chloride ion content of the precipitated solid phase was 59.61%, the sulfate ion content was 0.95%, and the equivalent sodium chloride purity reached 98.31%, while the sodium sulfate content was 1.40%. In Comparative Example 5, the chloride ion content of the precipitated solid phase was 23.42%, the sulfate ion content reached 39.87%, and the equivalent sodium chloride content was 38.62%, while the sodium sulfate content was 58.95%. The above results show that under the test conditions, the solid phase obtained in Example 1 has a higher proportion of sodium chloride and a lower amount of sodium sulfate entrainment; while the solid phase obtained in Comparative Example 5 has a higher proportion of sodium sulfate, indicating that there is a more obvious salt-nitrate co-precipitation phenomenon in its solid phase composition.

[0057] In a ternary system consisting of sodium chloride, sodium sulfate, and water, the solubility of each salt exhibits specific patterns with varying temperatures and ion concentrations. A continuous increase in chloride ion concentration in the concentrated mother liquor leads to changes in the phase equilibrium and crystallization regions of the mixed system. Therefore, by controlling the cooling method and crystallization temperature, the separation efficiency of sodium chloride and sodium sulfate can be influenced to some extent.

[0058] In Example 1, the system was flash-cooled at 52°C to achieve a moderately supersaturated state in the mother liquor, promoting the preferential precipitation of sodium chloride. During the flash-cooling process, some water evaporates, carrying away heat from the system and enhancing the crystallization driving force of sodium chloride. Simultaneously, within this temperature range, the sulfate component remains primarily in the liquid phase, reducing the co-precipitation of sodium sulfate and sodium chloride. In Comparative Example 5, a conventional cooling method was used, lowering the system to 15°C. At this temperature, the co-precipitation of sodium chloride and sodium sulfate decahydrate was more likely, resulting in a significantly higher sodium sulfate content in the resulting solid phase. Therefore, under the test conditions, flash-cooling at 52°C is beneficial for improving the purity of the sodium chloride solid phase and reducing the amount of sodium sulfate entrained.

[0059] Test Example 3: Evaluation of the purity and thermal decomposition resistance of sodium bicarbonate products: Collect the dry sodium bicarbonate products obtained from the drying apparatus in Examples 1, 2, and 3, as well as the dry product from the corresponding discharge of Comparative Example 1. Store each batch of samples separately in a sealed desiccator to prevent absorption of moisture from the air.

[0060] Thermogravimetric analysis (TGA) was used to perform thermal analysis on each dried product. 15.0 mg of each sample was weighed and placed in an alumina crucible. The temperature was programmed under a nitrogen atmosphere. The test gas flow rate was set to 50 mL / min, the heating rate to 10 °C / min, and the test temperature range to 30 °C to 250 °C. Real-time data on the mass change of the sample with temperature was continuously collected by a recording terminal to obtain the corresponding TGA curves.

[0061] The composition of each sample was determined by acid-base titration. 2.000 g of each dried product was weighed and dissolved in carbon dioxide-free deionized water, then transferred to a 250 mL volumetric flask and diluted to volume. 25.00 mL of the test solution was transferred to an Erlenmeyer flask, two drops of phenolphthalein indicator were added, and titration was performed using a hydrochloric acid standard solution of known concentration. The volume consumed when the solution changed from red to colorless was recorded. Subsequently, methyl orange indicator was added to the same solution, and titration continued using the hydrochloric acid standard solution until the solution changed from yellow to orange. The volume of solution consumed in the second titration was recorded. Based on the volumes of hydrochloric acid standard solution consumed in the two titrations, the mass percentages of sodium carbonate and sodium bicarbonate in the sample were calculated.

[0062] The experimental data are shown in Table 3: Table 3: Comparison of the mass percentage of dry sodium bicarbonate products in Examples 1 to 3 and Comparative Example 1 Example 1 99.18 0.52 0.30 Example 2 99.31 0.44 0.25 Example 3 99.25 0.58 0.17 Comparative Example 1 82.37 16.94 0.69 in conclusion: According to the data in Table 3, the mass fraction of sodium bicarbonate in the dry sodium bicarbonate products obtained in Examples 1, 2, and 3 reached 99.18%, 99.31%, and 99.25%, respectively, with corresponding sodium carbonate impurity contents all below 0.60%. In the test results of Comparative Example 1, the main content of sodium bicarbonate decreased to 82.37%, while the sodium carbonate impurity content increased to 16.94%. These results indicate that under the test conditions, the drying methods of Examples 1 to 3 are beneficial in reducing the degree of decomposition of sodium bicarbonate during the drying process and maintaining a high sodium bicarbonate content in the resulting products.

[0063] Sodium bicarbonate decomposes easily upon heating, primarily through the formation of one molecule of sodium carbonate, one molecule of water, and one molecule of carbon dioxide from two molecules of sodium bicarbonate. In this process, although the hot air inlet temperature is set between 120°C and 140°C during the drying airflow operation, the latent heat of phase change absorbed during the evaporation of water adhering to the crystal surface and some interlayer water offsets some of the heat transferred to the solid phase by the hot air. This maintains the actual heating temperature of the solid material between 50°C and 55°C. Within this temperature range, this temperature control method can, to some extent, reduce the decomposition of sodium bicarbonate caused by excessively high bulk material temperatures, thus helping to maintain product composition stability.

[0064] Comparative Example 1 did not involve temperature control intervention, allowing the material to be heated to 120°C. At this temperature, the risk of thermal decomposition of sodium bicarbonate increased significantly, inducing a phase transition during the drying process. This resulted in a large amount of the initially generated primary sodium bicarbonate decomposing into sodium carbonate, leading to a substantial decrease in the final discharge content. Thermogravimetric analysis (TGA) results showed that the sample in Example 1 underwent complete decomposition and weight loss within the heating range, with a total weight loss rate of approximately 36.5%, close to the theoretical weight loss ratio of pure sodium bicarbonate. In contrast, Comparative Example 1, due to the pre-decomposition reaction during the drying stage, had an increased initial sodium carbonate content, resulting in a lower secondary pyrolysis weight loss rate of approximately 30% in the TGA test. These test results correspond to the trend of sodium carbonate impurity content changes in Table 3, indicating that controlling the actual heating temperature of the sodium bicarbonate material is beneficial for reducing thermal decomposition during the drying stage.

[0065] Test Example 4: Purity and key impurity assessment of by-product ammonium sulfate: Fertilizer-grade ammonium sulfate products obtained from the bottom discharge of the ammonium sulfate evaporator crystallizer and centrifugal drying in Examples 1, 2, and 3, as well as Comparative Examples 2, 4, and 6, were selected as test objects. After sampling, the products were sealed and stored in an anhydrous environment for subsequent testing.

[0066] The total nitrogen mass fraction of each sample was determined using a Kjeldahl nitrogen analyzer. 0.500 g of each sample was accurately weighed and placed in a Kjeldahl digestion tube. A mixed catalyst of potassium sulfate and copper sulfate, along with concentrated sulfuric acid, was added. The solution was digested at high temperature on a heating module until it turned a transparent blue-green color. After cooling, the digestion tube was transferred to a distillation unit, where sodium hydroxide solution was automatically added to release free ammonia, which was then carried by steam into a receiving flask containing boric acid absorption solution and a mixed indicator. Finally, the absorption solution was titrated with a standard hydrochloric acid solution of known concentration, and the total nitrogen mass percentage of each sample was calculated based on the volume of hydrochloric acid consumed.

[0067] The sodium content in each sample was determined using flame atomic absorption spectrometry. 1.000 g of each sample was accurately weighed, dissolved in deionized water, and brought to a final volume of 100 mL. For comparative samples with higher predicted concentrations, necessary dilutions were performed. Using an atomic absorption spectrometer equipped with a sodium hollow cathode lamp, the absorbance of the solution was measured at a wavelength of 589.0 nm. The absolute sodium ion content in each sample crystal was calculated in ppm using the established sodium ion standard curve.

[0068] Quantitative determination of inorganic carbonate residues was performed on each sample. An appropriate amount of sample was weighed and dissolved. The content of carbonate and bicarbonate in the sample solution was tested using a gas chromatography method or the inorganic carbon channel of a total organic carbon analyzer. The results were converted into the mass percentage of carbonate and bicarbonate impurities to evaluate the carbon removal and impurity removal effect of the system. Each test sample was measured in triplicate. The data in the table are the average of the three test results.

[0069] The experimental data are shown in Table 4: Table 4: Comparison of various physicochemical properties of by-product ammonium sulfate Example 1 21.12 114 Not detected Example 2 21.05 136 Not detected Example 3 21.14 92 Not detected Comparative Example 2 18.23 29150 Not detected Comparative Example 4 16.47 1850 9.85 Comparative Example 6 18.51 27340 Not detected in conclusion: According to Table 4 and Figure 3 According to the data, the total nitrogen content of the by-product ammonium sulfate solid products obtained in Examples 1, 2, and 3 were 21.12%, 21.05%, and 21.14%, respectively, all close to the theoretical total nitrogen content of ammonium sulfate; the corresponding sodium entrainment amounts were 114 ppm, 136 ppm, and 92 ppm, respectively, which are at a low level. These results indicate that under the test conditions, the products obtained in Examples 1 to 3 have a high content of effective ammonium sulfate components and a low amount of sodium impurities.

[0070] The sodium content of the products obtained in Comparative Examples 2 and 6 was 29,150 ppm and 27,340 ppm, respectively, and the total nitrogen content was 18.23% and 18.51%, respectively. Compared with Examples 1 to 3, the sodium impurity content in Comparative Examples 2 and 6 was significantly increased, while the total nitrogen content decreased accordingly. This result indicates that, under conditions where no ammonium hydrate dissociation promoter is added or only a single surfactant is added, sodium salt entrainment or co-precipitation of sodium-ammonium double salts is more likely to occur in the system, thus affecting the composition of the by-product ammonium sulfate solid product.

[0071] The total nitrogen content of the product obtained in Comparative Example 4 was 16.47%, and the residual amounts of carbonate and bicarbonate were 9.85%. This result indicates that, without acidification treatment, carbonate / bicarbonate components in the mother liquor are more likely to remain or be carried out with the solid-phase product. It should be noted that the residual amounts of carbonate and bicarbonate are the total residual amounts calculated based on the inorganic carbon content, and are not limited to all being present in the form of sodium salt lattice inclusions; they may also include ammonium salt forms, adsorbed bicarbonate, and inorganic carbon components carried in by the mother liquor.

[0072] In a mixed salt system where sodium sulfate and ammonium sulfate coexist, sodium ions may enter the solid-phase product through interstitial entrainment, surface adsorption, or co-precipitation of double salts. The ammonium hydrate dissociation promoter added in Examples 1 to 3 is formed by compounding polymethyl methacrylate-sodium propylene sulfonate copolymer with sodium dodecylbenzene sulfonate. The carboxyl / carboxylate groups, sulfonate groups, and polymer segments in its molecule may interact with ammonium ions, water molecules, and the crystal growth interface, thereby affecting ion migration and crystal growth behavior during the ammonium sulfate crystallization process. Based on the results in Table 4, it can be concluded that the addition of this promoter helps reduce the possibility of sodium impurities co-precipitating with the ammonium sulfate solid phase. The data from Comparative Example 4 also reflect that acidification treatment has a certain effect on reducing carbonate residues. The mother liquor after metathesis may contain incompletely converted ammonium bicarbonate and dissolved sodium bicarbonate. Without concentrated sulfuric acid acidification, these components may decompose or be carried into the solid product with the mother liquor during heating and concentration in the evaporator crystallizer, increasing the residual amounts of carbonates and bicarbonates in the resulting product. In this example, adjusting the pH value for acidification before entering the evaporation stage helps convert bicarbonate components into relatively stable ammonium sulfate components and reduces the risk of deammoniation and carbonate residues during subsequent evaporation and crystallization processes.

[0073] Test Example 5: Impurity enrichment and stability testing of multi-cycle cyclic systems: A continuous operation simulation platform for the process flow of Example 1 and Comparative Example 3 was built, and continuous feeding was carried out with the material throughput scaled proportionally. A cycle was defined as the complete material flow process of metathesis reaction, primary sodium bicarbonate separation, mother liquor acidification and conversion, cooling crystallization separation of double salt, ammonium sulfate evaporation and crystallization to the return of the crystallization mother liquor to the cooling crystallization section.

[0074] Sampling was performed at the end of the 1st, 5th, 10th and 15th cycles of continuous operation on the simulation platform. For Example 1, liquid samples were extracted from the waste discharge branch interface of the cooling mother liquor or the sampling port before the cooling mother liquor diversion. For Comparative Example 3, liquid samples were extracted from the main delivery pipeline of the cooling mother liquor after the centrifuge. The samples were filtered through a 0.22μm filter membrane and then used for later use.

[0075] The mother liquor samples collected in each cycle were titrated with silver nitrate standard titration solution. The chloride ion concentration in the samples was determined with potassium chromate as an indicator. The mass concentration of chloride ions in the system mother liquor at each stage was recorded and calculated.

[0076] During each sampling cycle, the condition of the heat exchange tube wall of the ammonium sulfate evaporator crystallizer in the system was observed simultaneously, and it was recorded whether there was obvious salt precipitation or scaling. Each test sample was measured in parallel three times, and the data in the table is the average of the three test results.

[0077] The experimental data are shown in Table 5: Table 5: Comparison of chloride ion concentration in mother liquor during multi-cycle processes in Example 1 and Comparative Example 3 Cycle 1 2.15 2.18 5th cycle 5.37 10.64 10th cycle 6.12 26.83 15th cycle 6.09 47.91 in conclusion: According to Table 5 and Figure 4 The data shows that in Example 1, the chloride ion concentration of the mother liquor was 2.15 g / L in the first cycle, 5.37 g / L in the fifth cycle, 6.12 g / L in the tenth cycle, and 6.09 g / L in the fifteenth cycle, indicating that under the condition of setting up a waste discharge branch, the chloride ion concentration in the mother liquor tends to be relatively stable after several cycles of operation. In Comparative Example 3, the chloride ion concentration of the mother liquor increased from 2.18 g / L in the first cycle to 47.91 g / L in the fifteenth cycle, showing a trend of continuous increase with the increase of the cycle period, and scaling and crystallization inhibition were observed in the equipment during operation.

[0078] In the aqueous salt system of sodium sulfate and ammonium bicarbonate metathesis, the raw materials and pretreatment processes may introduce small amounts of chloride ions and other easily soluble impurities. In a mother liquor circulation system without effective drainage measures, the main salts can be separated into the solid phase through crystallization, while easily soluble impurities such as chloride ions are difficult to expel with the target crystals and may therefore gradually accumulate in the liquid phase. As water evaporates and the number of mother liquor circulations increases, the concentration of these impurity ions rises, which may affect the solubility balance and crystal growth environment during the ammonium sulfate crystallization process, and increase the risk of salt accumulation on the heat exchanger walls, mixed salt precipitation, or decreased crystallization efficiency.

[0079] In Example 1, before the cooled crystallization mother liquor was returned to the evaporation section, 5% of its total volume was extracted and led out of the circulation loop for degradation treatment. This operation provided an external channel for impurities in the circulation system, allowing readily soluble impurities such as chloride ions to be partially removed from the system via the waste discharge branch. As can be seen from the data in Table 5, the chloride ion concentration in Example 1 showed relatively small changes after the 10th cycle, indicating that under the test conditions, the waste discharge branch helps reduce the risk of continuous chloride ion accumulation and contributes to maintaining the operational stability of the mother liquor circulation system.

[0080] Test Example 6: Ammonia escape rate monitoring and measurement in exhaust gas: Gas sampling and monitoring points were set at the end of the exhaust channels of the evaporation crystallization apparatus corresponding to Example 1 and Comparative Example 4, respectively.

[0081] After the evaporation and crystallization equipment enters the stable heating and concentration operation stage, the probe of the portable pump ammonia gas detector is connected to the exhaust channel. The data acquisition frequency of the detector is set to record once per minute, and continuous monitoring is performed for 120 minutes to obtain real-time dynamic data of the free ammonia concentration at the exhaust port.

[0082] During the continuous monitoring period, a constant flow air sampler was simultaneously connected to an impact absorption tube containing dilute sulfuric acid absorbent to continuously extract gas from the exhaust channel for 45 minutes at a flow rate of 0.5 L / min for on-site sampling.

[0083] After sampling, the absorbent was transferred to a volumetric flask and brought to volume. A suitable amount of sample solution was added to Nessler's reagent for color development. The absorbance of the solution was measured at a wavelength of 420 nm using a UV-Vis spectrophotometer. The average mass concentration of ammonia emitted from the exhaust port during the monitoring period was calculated using the standard curve. Each test sample was measured in parallel three times. The data in the table are the average of the three test results.

[0084] The experimental data are shown in Table 6: Table 6: Comparison of test data on free ammonia concentration at the exhaust port between Example 1 and Comparative Example 4 Example 1 0.42 - 2.87 1.25 1.31 Comparative Example 4 1456.8 - 3218.4 2341.6 2289.5 in conclusion: According to Table 6 and Figure 5 According to the data, the average ammonia concentration recorded by the exhaust gas detector in Example 1 was 1.25 mg / m³. 3 The absorbance assay result was 1.31 mg / m³. 3 The average ammonia concentration recorded by the detector at the exhaust port of Comparative Example 4 was 2341.6 mg / m³. 3 The absorbance assay result was 2289.5 mg / m³. 3 The results obtained from both test methods showed the same trend, indicating that under the test conditions, the free ammonia concentration in the exhaust gas corresponding to Example 1 was significantly lower than that of Comparative Example 4. Comparative Example 4 exhibited high concentration fluctuations during dynamic monitoring, with the highest value recorded by the detector being 3218.4 mg / m³. 3 .

[0085] Ammonium bicarbonate aqueous solution may decompose under heating conditions, producing ammonia, carbon dioxide, and water. Comparative Example 4 eliminated the mother liquor acidification and conversion step; the mother liquor and washing liquid separated by the metathesis reaction directly entered the subsequent processing stage. When this mother liquor enters the evaporation and crystallization equipment with a higher operating temperature, the possibility of decomposition of the residual ammonium bicarbonate components increases, and the generated ammonia may enter the exhaust channel with the secondary steam. Therefore, the ammonia concentration detected at the exhaust port in Comparative Example 4 was relatively high.

[0086] In Example 1, before the mother liquor enters the heated crystallization operation, it undergoes acidification pretreatment with concentrated sulfuric acid. As concentrated sulfuric acid is added dropwise and the pH value drops to 4.0, carbonate and bicarbonate ions in the solution react with hydrogen ions, and ammonium ions react with sulfate ions to form components of the ammonium sulfate system. This acidification treatment helps reduce the risk of ammonia release from ammonium bicarbonate during subsequent high-temperature concentration. Simultaneously, the ammonia-containing tail gas generated in the metathesis and acidification operation sections is guided to the absorption tower via a closed induced draft device and undergoes gas-liquid mass transfer absorption in a dilute sulfuric acid spray-packed environment, converting ammonia gas into ammonium sulfate absorbent, which is then recycled back to the acidified mother liquor or the ammonium sulfate crystallization system. The combination of the above acidification pretreatment and tail gas absorption measures helps reduce the concentration of free ammonia in the exhaust gas and reduces the risk of nitrogenous substances being discharged with the gas phase.

[0087] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A process for the metathesis of sodium sulfate and ammonium bicarbonate to produce sodium bicarbonate and co-produce ammonium sulfate, characterized in that, Includes the following steps: Sodium carbonate and sodium hydroxide are added to a sodium sulfate leaching solution to react, and a purified sodium sulfate solution is obtained after solid-liquid separation. The refined sodium sulfate solution is evaporated and concentrated to separate anhydrous sodium sulfate crystals and centrifuged mother liquor. A portion of the centrifuged mother liquor is extracted and fed into a flash cooling crystallizer. Sodium chloride crystals are precipitated by controlling the cooling system. The remaining mother liquor is recycled to produce nitrate. The obtained anhydrous sodium sulfate crystals and ammonium bicarbonate were fed into a metathesis reactor for reaction, and primary sodium bicarbonate crystals and metathesis mother liquor were separated. After washing the primary sodium bicarbonate crystals, they are sent to an airflow dryer for drying, and the material is discharged to obtain dry sodium bicarbonate product. The resulting metathesis mother liquor and the washing liquid were combined, and sulfuric acid was added dropwise for acidification treatment to convert the residual ammonium bicarbonate into ammonium sulfate. The acidified mother liquor is sent to a cooling crystallizer to cool down. After the precipitated double salt solids are separated, a portion of the volume of the generated cooling crystallization mother liquor is extracted as waste material and led out of the circulation loop. The remaining cooled crystallization mother liquor is fed into an ammonium sulfate evaporator and a hydrated ammonium dissociation promoter is added for evaporation and concentration to obtain fertilizer-grade ammonium sulfate product. The hydrated ammonium dissociation promoter is used to regulate interfacial hydrogen bonding to suppress sodium impurity entrainment and double salt co-precipitation.

2. The process for producing sodium bicarbonate and co-producing ammonium sulfate by metathesis of sodium sulfate and ammonium bicarbonate according to claim 1, characterized in that, When sodium carbonate and sodium hydroxide are added to the sodium sulfate leaching solution for reaction, the molar ratio of sodium carbonate to calcium ions in the solution is controlled to be 1.05 to 1.20:1, and the molar ratio of sodium hydroxide to magnesium ions in the solution is 2.10 to 2.40:

1. When the density of the extracted centrifuged mother liquor is concentrated to 1.28–1.32 g / cm³ 3 The mixture is then introduced into the flash cooling crystallizer, and the temperature of the flash cooling system is controlled at 50-55℃.

3. The process for producing sodium bicarbonate and co-producing ammonium sulfate by metathesis of sodium sulfate and ammonium bicarbonate according to claim 1, characterized in that, When the anhydrous sodium sulfate crystals and ammonium bicarbonate are fed into the metathesis reactor for reaction, the molar ratio of anhydrous sodium sulfate to ammonium bicarbonate is controlled to be 1:2.05 to 2.

15. During drying in an airflow dryer, the actual heating temperature of the sodium bicarbonate material is maintained at 50–55°C by adjusting the feed rate and hot air volume.

4. The process for producing sodium bicarbonate and co-producing ammonium sulfate by metathesis of sodium sulfate and ammonium bicarbonate according to claim 1, characterized in that, When performing acidification treatment by adding sulfuric acid, concentrated sulfuric acid with a mass fraction of 98% is added dropwise, and the reaction temperature is controlled at 40-55℃ until the pH value of the system drops to 3.5-4.5 and then the addition is stopped. The mixed tail gas containing ammonia and carbon dioxide that escapes during the metathesis reaction and acidification treatment is sent to a dilute sulfuric acid absorption tower for absorption at 15-30°C. The resulting ammonium sulfate absorbent is returned to the acidified mother liquor or the ammonium sulfate evaporation and crystallization section.

5. The process for producing sodium bicarbonate and co-producing ammonium sulfate by metathesis of sodium sulfate and ammonium bicarbonate according to claim 1, characterized in that, The temperature in the cooling crystallizer is reduced to 25-30°C; The volume of waste material extracted for discharge accounts for 3% to 8% of the total volume of the cooling crystallization mother liquor; The amount of the ammonium hydrate dissociation promoter added is 0.05% to 0.20% of the total mass of the crystallization mother liquor.

6. The process for producing sodium bicarbonate and co-producing ammonium sulfate by metathesis of sodium sulfate and ammonium bicarbonate according to claim 1, characterized in that, The ammonium hydrate dissociation promoter is composed of the following components in mass percentage by physical blending: Polymethacrylic acid-sodium propylene sulfonate copolymer solids 65%–75%; Sodium dodecylbenzenesulfonate solids: 25%–35%.

7. The process for producing sodium bicarbonate and co-producing ammonium sulfate by metathesis of sodium sulfate and ammonium bicarbonate according to claim 6, characterized in that, The polymethacrylic acid-sodium propylene sulfonate copolymer is polymerized from methacrylic acid and sodium propylene sulfonate, with a polymerization molar ratio of 1:1.20 to 1.50, and the number average molecular weight of the polymethacrylic acid-sodium propylene sulfonate copolymer is 3200 to 4800.

8. The process for producing sodium bicarbonate and co-producing ammonium sulfate by metathesis of sodium sulfate and ammonium bicarbonate according to claim 6, characterized in that, The ammonium hydrate dissociation promoter is prepared according to the following steps: The methacrylic acid and sodium propylene sulfonate were dissolved in deionized water to prepare a monomer aqueous solution. Ammonium persulfate was dissolved in deionized water to prepare an initiator aqueous solution; the monomer aqueous solution and the initiator aqueous solution were simultaneously and uniformly added dropwise using a double dropwise addition method to carry out the polymerization reaction, and the reaction was maintained at a temperature for aging after the addition was completed. After cooling, the pH of the system was adjusted with alkaline solution, and then dried to obtain a solid polymethacrylic acid-sodium propylene sulfonate copolymer. The solid polymethacrylic acid-sodium propylene sulfonate copolymer and the solid sodium dodecylbenzene sulfonate are physically blended at room temperature according to the target ratio and stirred evenly to obtain the final product.

9. The process for producing sodium bicarbonate and co-producing ammonium sulfate by metathesis of sodium sulfate and ammonium bicarbonate according to claim 8, characterized in that, During the polymerization reaction using the double-drop method, the stirring speed was controlled at 250 rpm, and the dropping time was 2.0 h to 2.5 h.

10. The process for producing sodium bicarbonate and co-producing ammonium sulfate by metathesis of sodium sulfate and ammonium bicarbonate according to claim 9, characterized in that, The polymerization reaction is carried out at a temperature of 75–85°C. The temperature for heat preservation and curing is 85℃, and the curing time is 2 hours; After cooling, the alkaline solution used for adjustment is a 20% sodium hydroxide aqueous solution, which is used to adjust the pH of the system to 6.5–7.5.

Images