A process for preparing ammonium sulfate, sodium bicarbonate, and / or sodium carbonate from chemical salt nitrate mixtures as raw materials.

Abstract

This invention relates to a process for preparing ammonium sulfate and sodium bicarbonate and / or sodium carbonate from a mixture of chemical salts and nitrates. The process includes the following steps: (1) washing the mixed salts to purify Na₂SO₄; (2) a metathesis reaction to generate NaHCO₃; (3) washing to obtain sodium bicarbonate, optionally calcining to generate Na₂CO₃; (4) deammoniation; (5) primary evaporation to recover sodium sulfate; and (6) secondary evaporation to prepare ammonium sulfate. Using the method of this invention, sodium bicarbonate and / or sodium carbonate and ammonium sulfate products with comparable or even higher purity can be prepared with significantly lower energy consumption than existing technologies such as cold or hot methods. Furthermore, compared to the case where the mixed salts are directly subjected to a metathesis reaction without washing, the prepared sodium bicarbonate and / or sodium carbonate and ammonium sulfate products have significantly higher purity.

Description

Technical Field

[0001] This invention relates to the field of inorganic chemical engineering, specifically to a process for preparing ammonium sulfate, sodium bicarbonate, and / or sodium carbonate from chemical salts (nitrate mixtures) as raw materials. Background Technology

[0002] With the rapid development of modern coal chemical, pharmaceutical, pesticide, printing and dyeing, and new energy materials (such as lithium batteries) industries in my country, large amounts of high-salinity wastewater and industrial waste salt are generated during industrial production. Among these, mixed salts with sodium sulfate (Na2SO4) and sodium chloride (NaCl) as the main components are particularly common. These waste salts typically contain impurities such as organic matter and heavy metals, posing significant environmental hazards and incurring high disposal costs. Traditional landfill or open-air storage methods not only occupy substantial land resources but also pose serious environmental pollution risks, becoming a bottleneck restricting the sustainable development of related industries. Therefore, developing economical, efficient, and clean technologies for the resource utilization of industrial waste salt, transforming it into high-value-added chemical products and achieving "turning waste into treasure," has significant environmental and economic value.

[0003] Using mixed salts (mainly sodium sulfate and sodium chloride) as raw materials to prepare high-value-added chemical products is an important way to realize the resource utilization of industrial waste salts. Among them, the process route of producing sodium bicarbonate and co-producing ammonium sulfate by reacting sodium sulfate and ammonium bicarbonate in a metathesis reaction has received widespread attention in recent years due to its strong adaptability of raw materials and high market value of products. The core of this technical route is: first, to convert sodium sulfate into sodium bicarbonate through a metathesis reaction, and then to treat the reaction mother liquor to separate and recover the ammonium sulfate product.

[0004] The double displacement reaction between sodium sulfate (Na2SO4, commonly known as Glauber's salt) and ammonium bicarbonate (NH4HCO3) is a liquid-solid double displacement precipitation reaction, and its core chemical equation is as follows:

[0005]

[0006] Under normal to moderate temperature conditions (typically 20-50℃), ammonium bicarbonate dissolves and ionizes to release NH4. ⁺ and HCO3 ⁻ , with Na in the solution ⁺ and SO4 ²⁻Ion exchange occurs. Because sodium bicarbonate (NaHCO3) has a much lower solubility in water than sodium sulfate, ammonium sulfate, and ammonium bicarbonate, NaHCO3 preferentially precipitates out as crystals, thus separating it from the reaction system. This reaction has the advantages of wide availability of raw materials and high-value products: sodium sulfate can be obtained from natural sodium sulfate, chemical by-product sodium sulfate, coal chemical concentrated brine by-product salt, battery recycling waste salt, etc.; ammonium bicarbonate is a bulk industrial product; sodium bicarbonate (baking soda) or sodium carbonate (soda ash) obtained through further calcination is a basic chemical raw material, and ammonium sulfate is an important nitrogen fertilizer.

[0007] After the metathesis reaction, sodium bicarbonate solid (commonly known as "heavy alkali") is obtained through solid-liquid separation, and the remaining liquid phase is called heavy alkali mother liquor. The main components of this mother liquor include: (NH4)2SO4 (the target product), unreacted Na2SO4 (which needs to be recycled), dissolved NH4HCO3 (which needs to be recovered or removed), and water. The essence of co-producing ammonium sulfate is to efficiently separate the ammonium sulfate product from the above complex mother liquor system, while simultaneously returning the unreacted sodium sulfate to the reaction section for recycling. This separation process is crucial to the economic feasibility of the sodium sulfate-to-alkali production process.

[0008] Based on the different operating temperatures of the mother liquor separation process, existing technologies can be divided into two technical routes: "cold separation" and "hot separation".

[0009] Cold separation technology utilizes the difference in solubility of components at low temperatures (typically -5°C to 10°C) to separate salts. At low temperatures, the solubility of Na₂SO₄·10H₂O (sodium sulfate decahydrate) and NH₄HCO₃ decreases significantly, making them prone to crystallization and precipitation, while the solubility of (NH₄)₂SO₄ remains relatively high at low temperatures and remains in the mother liquor. A typical process is as follows: the metathesis mother liquor is cooled to -5°C to 10°C, precipitating a mixed crystal containing Na₂SO₄·10H₂O and NH₄HCO₃ (commonly known as "cold precipitation salt"), which is then filtered and returned to the metathesis reaction section; the mother liquor after cold precipitation mainly contains (NH₄)₂SO₄, which is then concentrated by evaporation, crystallized, and dried to obtain ammonium sulfate product. The main disadvantages of cold separation are: the need for low-temperature refrigeration equipment (such as refrigeration units), resulting in high energy consumption and large equipment investment; and the tendency for fine crystals to form during the cold precipitation process, making filtration difficult.

[0010] The thermal separation process utilizes the characteristic that the solubility of (NH4)2SO4 increases significantly at high temperatures, while the solubility of Na2SO4 remains essentially unchanged or even decreases slightly to achieve salt separation. A typical process is as follows: the mother liquor from the metathesis process is heated to approximately 78°C, and then concentrated by evaporation, causing Na2SO4 to crystallize out (because its solubility does not increase with temperature). After filtration and separation, the crystals are returned to the reaction section. The hot mother liquor after Na2SO4 separation (mainly containing (NH4)2SO4 and a small amount of Na2SO4) is cooled to room temperature (e.g., 25°C), at which point a double salt crystal, Na2SO4·(NH4)2SO4·4H2O, precipitates. The mother liquor after double salt separation is then concentrated by evaporation to obtain ammonium sulfate. The separated double salt is returned to the previous process for further processing.

[0011] For industrial waste salts, especially mixed salts containing sodium chloride and nitrates, the existing technology for preparing sodium carbonate / sodium bicarbonate and co-producing ammonium sulfate using the metathesis method still faces the following key bottlenecks:

[0012] First, achieving both high product yield and purity is difficult. NaCl and Na₂SO₄ often coexist as mixed salts in chemical waste. When the sodium chloride content in the raw material is high, the metathesis reaction and subsequent mother liquor separation process face a series of severe technical challenges. The presence of sodium chloride causes a salting-out effect on the dissolution and reaction of sodium sulfate, leading to incomplete sodium sulfate conversion. The sodium bicarbonate product often contains impurities such as sodium sulfate or ammonium sulfate, resulting in decreased product purity. Simultaneously, accumulated chloride ions in the mother liquor inhibit the forward shift of the metathesis reaction equilibrium, reducing the single-pass conversion rate of sodium ions. To improve yield, some processes require the addition of salting-out agents (such as diethylene glycol in CN116375055B and alcohols in CN120664563A), increasing reagent costs and the burden of subsequent separation. In closed-loop processes, sodium chloride does not participate in the metathesis reaction and accumulates continuously with the mother liquor circulation. The accumulated Cl... - This will further exacerbate the salting-out effect, affecting the concentration factor of the subsequent evaporation and crystallization process, causing Na2SO4 to precipitate prematurely and affecting the quality of (NH4)2SO4.

[0013] Secondly, the process is complex and energy consumption remains high. To separate ammonium sulfate after the metathesis reaction and unreacted sodium sulfate, most existing processes (such as CN120793966A) involve a "low-temperature freezing-high-temperature evaporation" heating and cooling operation. The low-temperature freezing step typically requires cooling the material to -10°C to 25°C to precipitate the sodium sulfate-ammonium sulfate double salt, followed by heating to evaporate and crystallize to obtain ammonium sulfate. This alternating heating and cooling process not only increases equipment investment and operating cycle time but also results in significant energy waste.

[0014] Furthermore, while existing publicly disclosed patents, such as CN201410548937.2 (a process for the co-production of soda ash and ammonium sulfate using a thermal cycling method with sodium sulfate solution or carrier), employ a thermal cycling process, it still requires multiple steps, including preheating at high temperature to remove ammonia, high-temperature evaporation to obtain sodium sulfate, and low-temperature evaporation to obtain ammonium sulfate, making the process relatively complex. CN119528179B (a method for the resource-based preparation of soda ash or sodium bicarbonate from coal chemical concentrated brine) involves multiple processes, including induced crystallization, acidolysis, thermally activated oxidation, metathesis, salting out, and evaporation, resulting in a long processing flow. CN120518097B (a method and apparatus system for preparing sodium carbonate and / or sodium bicarbonate from sodium-based wastewater to co-produce ammonium sulfate) uses gradient crystallization separation based on phase equilibrium principles, but it does not effectively address the limitation of chloride ion accumulation on the evaporation concentration ratio. CN120793966A (a method for preparing sodium carbonate from sodium sulfate waste salt to co-produce ammonium sulfate) employs primary and secondary cold precipitation and requires the addition of an acidifier, still relying on low-temperature freezing operations.

[0015] In summary, existing technologies for processing mixed salts containing sodium chloride and nitrates suffer from problems such as low conversion rates due to chloride ion accumulation, poor product purity, limited evaporation and concentration ratios, and high energy consumption due to alternating hot and cold processes. There is an urgent need to develop a high-efficiency, low-consumption, and pollution-free salt resource utilization process. Summary of the Invention

[0016] This invention is made in view of the aforementioned problems existing in the prior art. The purpose of this invention is to provide an efficient, low-consumption, and pollution-free salt resource utilization process that can co-produce high-purity sodium bicarbonate (and / or sodium carbonate) and ammonium sulfate products with low energy consumption.

[0017] Therefore, the present invention provides a process for preparing ammonium sulfate, sodium bicarbonate and / or sodium carbonate from chemical salt nitrate mixture as raw materials, which includes the following steps:

[0018] (1) The salt-nitrate mixture is mixed and dissolved with water at a temperature of 20-50°C to obtain a solid-liquid mixture, wherein the salt-nitrate mixture includes sodium sulfate and sodium chloride, and the mass ratio of sodium sulfate to sodium chloride is 1:1 to 100:1.

[0019] The water includes pure water, recirculated water, or a combination of pure water and recirculated water;

[0020] The resulting solid-liquid mixture was subjected to solid-liquid separation to obtain a first solid and a first liquid as crude sodium sulfate products.

[0021] Optionally, the first liquid is optionally desalinated and then recycled to step (1) as recirculated water;

[0022] When the water used to mix and dissolve salt and nitrate mixtures consists only of pure water, the mass ratio of salt and nitrate mixtures to pure water is 1:2-4.

[0023] When the water used to mix the dissolved salt and nitrate mixture consists only of recirculated water, the recirculated water is not saturated with sodium sulfate and sodium chloride, and the mass ratio of the salt and nitrate mixture to the recirculated water is 1:2-4;

[0024] When the water used to mix dissolved salts and nitrates includes both pure water and recirculated water, the mass ratio of the salt-nitrate mixture to pure water satisfies the following relationship:

[0025]

[0026] Where m 水 Indicates the mass of pure water added; m 混盐 Indicates the mass of mixed salt added; p NaCl This indicates the mass percentage of NaCl in the mixed salt, where a = 3.0-4.2.

[0027] The mass ratio of the mixed salt and nitrate salt to the recirculated water is 1:2-4;

[0028] (2) The first solid is reacted with ammonium bicarbonate and / or with ammonia and carbon dioxide in water to undergo a double displacement reaction. The reaction products are then separated into solid and liquid phases to obtain a second solid and a second liquid.

[0029] (3) The second solid was washed with water to obtain sodium bicarbonate product and wash water.

[0030] The sodium bicarbonate product is optionally dried and optionally calcined to obtain sodium carbonate product and carbon dioxide gas.

[0031] Optionally, carbon dioxide gas can be recycled to step (2).

[0032] Optionally, the wash water can be recycled to step (1) as recirculated water;

[0033] (4) The second liquid is deaminated to obtain ammonia and deaminated mother liquor, and the ammonia is recycled to step (2).

[0034] (5) The mother liquor for deammoniation is evaporated once, so that the amount of water evaporated once is 20%-30% of the amount of water in the mixture to be evaporated once. The product of the first evaporation is separated into solid and liquid components to obtain a third solid and a third liquid as sodium sulfate products.

[0035] The third solid is recycled to step (2) and mixed with the first solid;

[0036] (6) The third liquid is evaporated a second time, such that the amount of water evaporated in the second evaporation is 30%-40% of the amount of water in the third liquid. The product of the second evaporation is then separated into solid and liquid components to obtain a fourth solid and a fourth liquid, which are ammonium sulfate products.

[0037] The fourth liquid is recycled to step (5) and mixed with the deammoniation mother liquor as the mixture to be evaporated once in step (5).

[0038] Using the method of this invention, sodium bicarbonate and / or sodium carbonate and ammonium sulfate products with comparable or even higher purity can be prepared with significantly lower energy consumption than existing technologies such as cold or hot methods. Furthermore, compared to the case where mixed salt washing is not performed (i.e., the mixed salts are directly subjected to the metathesis reaction step), the prepared sodium bicarbonate and / or sodium carbonate and ammonium sulfate products have significantly higher purity. Attached Figure Description

[0039] To more clearly illustrate the technical solution of the present invention, the embodiments will be described below in conjunction with the accompanying drawings. It should be understood that these drawings are only for the purpose of facilitating a better understanding of the present invention by those skilled in the art, and are not intended to limit the scope of the present invention.

[0040] Figure 1 A flowchart illustrating a process according to an embodiment of the present invention is shown schematically. Detailed Implementation

[0041] To make the inventive objectives, technical solutions, and beneficial technical effects of this application clearer, this application will be described in detail below. It should be noted that the various aspects, features, embodiments, and advantages described in this application can be compatible and / or combined together.

[0042] Unless otherwise specified, the technical terms used in this specification have the same meaning as commonly understood by those skilled in the art.

[0043] Unless otherwise specified, the temperature in this application is room temperature (25°C), the atmosphere is air, and the pressure is atmospheric pressure.

[0044] In this application, unless otherwise specified, the expression "%" used when referring to component content or concentration means "mass %".

[0045] Those skilled in the art will understand that, unless otherwise specified, in this application, a number containing n significant digits after the decimal point actually also includes the result of rounding a number containing more significant digits after the decimal point to n significant digits. For example, 1.0 actually covers all numbers in the range from greater than or equal to 0.95 to less than 1.05; 0.10 actually covers all numbers in the range from greater than or equal to 0.095 to less than 0.105, and so on.

[0046] In this application, unless otherwise specified, even if the term “about” is not used to modify numerical values, the numerical value shall be understood to be modified by “about”; the term “about” includes a deviation of ±5% from the stated numerical value, that is, for the numerical value a, whether or not it is modified by “about”, it shall be understood to represent a range of a ±5%a, that is, 0.95a to 1.05a.

[0047] This invention relates to a process for preparing ammonium sulfate, sodium bicarbonate, and / or sodium carbonate from chemical salts and nitrates as raw materials.

[0048] The process includes the following steps:

[0049] (1) The salt-nitrate mixture is mixed and dissolved with water at a temperature of 20-50°C to obtain a solid-liquid mixture, wherein the salt-nitrate mixture includes sodium sulfate and sodium chloride, and the mass ratio of sodium sulfate to sodium chloride is 1:1 to 100:1.

[0050] The water includes pure water, recirculated water, or a combination of pure water and recirculated water;

[0051] The resulting solid-liquid mixture was subjected to solid-liquid separation to obtain a first solid and a first liquid as crude sodium sulfate products.

[0052] Optionally, the first liquid is optionally desalinated and then recycled to step (1) as recirculated water;

[0053] When the water used to mix and dissolve salt and nitrate mixtures consists only of pure water, the mass ratio of salt and nitrate mixtures to pure water is 1:2-4.

[0054] When the water used to mix the dissolved salt and nitrate mixture consists only of recirculated water, the recirculated water is not saturated with sodium sulfate and sodium chloride, and the mass ratio of the salt and nitrate mixture to the recirculated water is 1:2-4;

[0055] When the water used to mix dissolved salts and nitrates includes both pure water and recirculated water, the mass ratio of the salt-nitrate mixture to pure water satisfies the following relationship:

[0056]

[0057] Where m 水 Indicates the mass of pure water added; m 混盐 Indicates the mass of mixed salt added; p NaCl This indicates the mass percentage of NaCl in the mixed salt, where a = 3.0-4.2.

[0058] The mass ratio of the mixed salt and nitrate salt to the recirculated water is 1:2-4;

[0059] (2) The first solid is reacted with ammonium bicarbonate and / or with ammonia and carbon dioxide in water to undergo a double displacement reaction. The reaction products are then separated into solid and liquid phases to obtain a second solid and a second liquid.

[0060] (3) The second solid was washed with water to obtain sodium bicarbonate product and wash water.

[0061] The sodium bicarbonate product is optionally dried and optionally calcined to obtain sodium carbonate product and carbon dioxide gas.

[0062] Optionally, carbon dioxide gas can be recycled to step (2).

[0063] Optionally, the wash water can be recycled to step (1) as recirculated water;

[0064] (4) The second liquid is deaminated to obtain ammonia and deaminated mother liquor, and the ammonia is recycled to step (2).

[0065] (5) The mother liquor for deammoniation is evaporated once, so that the amount of water evaporated once is 20%-30% of the amount of water in the mixture to be evaporated once. The product of the first evaporation is separated into solid and liquid components to obtain a third solid and a third liquid as sodium sulfate products.

[0066] The third solid is recycled to step (2) and mixed with the first solid;

[0067] (6) The third liquid is evaporated a second time, such that the amount of water evaporated in the second evaporation is 30%-40% of the amount of water in the third liquid. The product of the second evaporation is then separated into solid and liquid components to obtain a fourth solid and a fourth liquid, which are ammonium sulfate products.

[0068] The fourth liquid is recycled to step (5) and mixed with the deammoniation mother liquor as a mixture to be evaporated once.

[0069] Figure 1 A flowchart illustrating a process according to an embodiment of the present invention is shown schematically. Reference will be made below. Figure 1 The present invention will now be described in detail.

[0070] Step (1): Wash with mixed salt to purify Na2SO4

[0071] In step (1),

[0072] The salt-nitrate mixture is dissolved in water at a temperature of 20-50°C to obtain a solid-liquid mixture, wherein the salt-nitrate mixture comprises sodium sulfate and sodium chloride, and the mass ratio of sodium sulfate to sodium chloride is 1:1 to 100:1.

[0073] The water includes pure water, recirculated water, or a combination of pure water and recirculated water;

[0074] The resulting solid-liquid mixture was subjected to solid-liquid separation to obtain a first solid and a first liquid as crude sodium sulfate products.

[0075] Optionally, the first liquid is optionally desalinated and then recycled to step (1) as recirculated water;

[0076] When the water used to mix and dissolve salt and nitrate mixtures consists only of pure water, the mass ratio of salt and nitrate mixtures to pure water is 1:2-4.

[0077] When the water used to mix the dissolved salt and nitrate mixture consists only of recirculated water, the recirculated water is not saturated with sodium sulfate and sodium chloride, and the mass ratio of the salt and nitrate mixture to the recirculated water is 1:2-4;

[0078] When the water used to mix dissolved salts and nitrates includes both pure water and recirculated water, the mass ratio of the salt-nitrate mixture to pure water satisfies the following relationship:

[0079]

[0080] Where m 水 Indicates the mass of pure water added; m 混盐 Indicates the mass of mixed salt added; p NaCl This indicates the mass percentage of NaCl in the mixed salt, where a = 3.0-4.2.

[0081] The mass ratio of the mixed salt and nitrate to the recirculated water is 1:2-4.

[0082] Mixed salts refer to industrial waste salts or high-salinity wastewater containing mainly sodium sulfate (Glauber's salt) and sodium chloride (salt). They typically originate from industries such as coal chemical, pharmaceutical, pesticide, printing and dyeing, and new energy battery materials (e.g., lithium carbonate and lithium hydroxide production).

[0083] In an embodiment, the salt-nitrate mixture comprises sodium sulfate and sodium chloride, for example, consisting essentially of sodium sulfate and sodium chloride, or consisting of sodium sulfate and sodium chloride.

[0084] In the embodiments, the mass ratio of sodium sulfate to sodium chloride in the salt-nitrate mixture is 1:1 to 100:1, for example, 2:1 to 50:1, for example, 4:1 to 30:1. For example, relative to 1 part by mass of sodium chloride, the amount of sodium sulfate can be 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42. 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100 parts by weight, or within the range defined by any two of them.

[0085] In some embodiments, sodium sulfate may constitute 50-99% by mass relative to the total mass of sodium sulfate and sodium chloride, for example, 60-98%, 70-97%, or 80-96% by mass, for example, 50.0, 50.5, 51.0, 51.5, 52.0, 52.5, 53.0, 53.5, 54.0, 54.5, 55.0, 55.5, 56.0, 56.5, 57.0, 57.5. 58.0, 58.5, 59.0, 59.5, 60.0, 60.5, 61.0, 61.5, 62.0, 62.5, 63.0, 63.5, 64.0, 64.5, 65.0, 65.5, 66.0, 66.5, 67.0, 67.5, 68.0, 68.5, 69.0, 69.5, 70.0, 70.5, 71.0, 71.5, 72.0 72.5, 73.0, 73.5, 74.0, 74.5, 75.0, 75.5, 76.0, 76.5, 77.0, 77.5, 78.0, 78.5, 79.0, 79.5, 80.0, 80.5, 81.0, 81.5, 82.0, 82.5, 83.0, 83.5, 84.0, 84.5, 85.0, 85.5, 86.0, 86.5 87.0, 87.5, 88.0, 88.5, 89.0, 89.5, 90.0, 90.5, 91.0, 91.5, 92.0, 92.5, 93.0, 93.5, 94.0, 94.5, 95.0, 95.5, 96.0, 96.5, 97.0, 97.5, 98.0, 98.5, 99.0% quality, or within the range defined by any two of them.

[0086] In some embodiments, the combination of sodium sulfate and sodium chloride may constitute 90-100% by mass, for example 95-100%, for example 98-100% by mass, for example 90.0%, 90.5%, 91.0%, 91.5%, 92.0%, 92.5%, 93.0%, 93.5%, 94.0%, 94.5%, 95.0%, 95.5%, 96.0%, 96.5%, 97.0%, 97.5%, 98.0%, 98.5%, 99.0%, 99.5%, 100.0% by mass, or within the range defined by any two thereof, based on the dry weight of the inorganic matter (i.e., the total mass of the substances excluding water and organic matter).

[0087] In some embodiments, the salt-nitrate mixture may be anhydrous or contain a small amount of water, such as 0-10% by mass, for example 0, 0.1, 0.2, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, 9.5, 10.0% by mass, or within the range defined by any two of these.

[0088] In some embodiments, the salt-nitrate mixture may be free of organic matter or contain a small amount of organic matter, such as 0-5% by mass of organic matter, for example 0, 0.1, 0.2, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0% by mass, or within the range defined by any two of these.

[0089] The water used for the mixing and dissolving can be pure water (freshly supplied water, i.e., makeup water), and / or various process waters generated in subsequent steps and recycled, also referred to in this application as recycled liquid or recycled water.

[0090] Those skilled in the art will understand that when the process has just begun and no subsequent steps have been performed, the water is pure water (also known as makeup water); when subsequent process steps are performed and process water (recirculated water) such as the first liquid is generated, the water may include recirculated water (e.g., the first liquid), or pure water and recirculated water (e.g., the first liquid).

[0091] In some implementations (e.g., during the initial start-up phase, when process water, such as the first liquid, has not yet been generated before subsequent steps are performed), the water used to mix and dissolve the salt-nitrate mixture comprises only pure water. When the water used to mix and dissolve the salt-nitrate mixture comprises only pure water, the mass ratio of the salt-nitrate mixture to pure water is 1:2-4, for example 1:2.0-4.0, 1:2.5-3.5, 1:2.8-3.2, 1:2.9-3.1, or 1:3.0. For example, relative to 1 part by mass of the salt-nitrate mixture, the mass of water may be 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4.0, or within the range defined by any two of these.

[0092] In some implementations (e.g., during the initial start-up phase, when subsequent steps have been performed to produce process water, such as a first liquid, but the process water, such as the first liquid, is not yet saturated with sodium sulfate and sodium chloride), the water used to mix the dissolved salts and nitrates consists only of recirculated water. When the water used to mix the dissolved salt-nitrate mixture consists only of recirculated water, the recirculated water is not saturated with sodium sulfate and sodium chloride, and the mass ratio of the salt-nitrate mixture to the recirculated water is 1:2-4, for example 1:2.0-4.0, 1:2.5-3.5, 1:2.8-3.2, 1:2.9-3.1, or 1:3.0. For example, relative to 1 part by mass of the salt-nitrate mixture, the mass of water can be 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4.0, or within the range defined by any two of these.

[0093] In some implementations (e.g., during steady-state operation, i.e., when subsequent steps have been performed to produce process water, such as a first liquid, and the process water, such as the first liquid, is saturated with sodium sulfate and sodium chloride), the water used for mixing the dissolved salt-nitrate mixture comprises pure water and recirculated water. When the water used for mixing the dissolved salt-nitrate mixture comprises pure water and recirculated water, the mass ratio of the salt-nitrate mixture to pure water satisfies the following relationship:

[0094]

[0095] Where m 水 Indicates the mass of pure water added; m 混盐 Indicates the mass of mixed salt added; p NaClThe mass percentage of NaCl in the mixed salt is expressed as a = 3.0-4.2, for example 3.0, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4.0, 4.1, 4.2, or within the range defined by any two of these, preferably a = 3.2-3.8, preferably 3.4-3.6, for example 3.5, and

[0096] The mass ratio of the salt-nitrate mixture to the recirculated water is 1:2-4, for example, 1:2.0-4.0, 1:2.5-3.5, 1:2.8-3.2, 1:2.9-3.1, or 1:3.0. For example, relative to 1 part by mass of the salt-nitrate mixture, the mass of water can be 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4.0, or within the range defined by any two of these ratios.

[0097] By controlling the amount of pure water used in the process, the NaCl in the mixed salt can be basically / fully dissolved, while the sodium sulfate is dissolved less and mainly / mostly in solid form.

[0098] In some embodiments, when the water used to mix the dissolved salts comprises pure water and recirculated water, the recirculated water is already saturated with sodium sulfate and sodium chloride.

[0099] Those skilled in the art will understand that as the first liquid is produced and recycled, sodium sulfate and sodium chloride will accumulate therein, increasing in concentration and gradually approaching saturation; when the first liquid is recycled, for example, 20-25 times, the sodium sulfate and sodium chloride therein become saturated.

[0100] The mixing and dissolving temperature is not particularly limited and may be, for example, 20-50°C, such as 25, 30, 35, 40, 45, 50°C, or within the range defined by any two thereof, such as 30-40°C, such as 35°C. The mixing and dissolving may be carried out under stirring, the stirring time of which is not particularly limited and, taking into account the process efficiency of fully dissolving sodium chloride, may be, for example, 5-60 minutes, such as 5, 10, 15, 20, 25, 30, 40, 50, 55, 60 minutes, or within the range defined by any two thereof.

[0101] In some embodiments, the mixing and dissolution is carried out at 35°C with stirring, wherein stirring is continued for 30-40 minutes.

[0102] The presence of NaCl in the mixed salt is the root cause of low conversion rate, chloride ion accumulation, and difficulty in evaporation and concentration in subsequent metathesis reactions. In step (1), the mixed salt is washed by dissolving it in water to separate NaCl from the mixed salt, thereby purifying Na2SO4 and obtaining high-purity Na2SO4 (crystallized in the form of Glauber's salt), reducing the chloride ion load entering the subsequent recycling system from the source. At the same time, this step can also remove some organic matter in the mixed salt by using the dissolution-crystallization process to ensure the quality of the final product. In the embodiment, the obtained solid-liquid mixture is then subjected to solid-liquid separation to obtain a first solid as crude sodium sulfate product and a first liquid ( Figure 1 The “waste liquid” in “waste liquid desalination” shown in the diagram.

[0103] The solid-liquid separation can be carried out by any suitable method known in the art, such as centrifugation or filtration. The solid-liquid separation can be carried out, for example, while hot, and the first solid obtained is crude sodium sulfate product (high-purity Na2SO4), which is directly used in the next metathesis reaction.

[0104] Then, optionally, the first liquid is subjected to salt separation treatment (i.e., Figure 1 The “waste liquid desalination” shown in the figure is recycled to step (1) as recirculated water (e.g., as all or part of the recirculated water).

[0105] In some embodiments, the first liquid is recycled to step (1) as recirculated water (e.g., as all or part of the recirculated water), for example after desalination treatment (i.e., Figure 1 The waste liquid is desalinated as shown in the figure, or recycled to step (1) as recirculated water (e.g., as all or part of the recirculated water) without desalination treatment.

[0106] Water (e.g., the first liquid) can be reused, and after multiple uses, the content of Na2SO4 and NaCl in it reaches the co-saturation point.

[0107] The salt separation process can be carried out by any method known in the art (e.g., thermal salt separation, freeze crystallization salt separation, and nanofiltration membrane salt separation). The salt separation process can remove at least a portion of sodium sulfate, at least a portion of sodium chloride, or at least a portion of sodium sulfate and at least a portion of sodium chloride from the first liquid.

[0108] The core of thermal salt separation is temperature control, utilizing the significant solubility difference between the two salts at different temperature ranges: at high temperatures, Na₂SO₄ has a relatively low solubility and will preferentially precipitate; while the solubility of NaCl changes very little with temperature and remains stable in the mother liquor. For example, at approximately 110-120℃, the co-saturation concentration of Na₂SO₄ is only 4.4%, far lower than the 25.9% of NaCl.

[0109] Thermal salt separation can be carried out by high-temperature evaporation (heating a saturated solution to obtain anhydrous sodium sulfate crystals). Thus, anhydrous sodium sulfate crystals can be removed from the first liquid, and the mother liquor after sodium sulfate separation can be recycled to step (1) as recirculated water (e.g., as all or part of the recirculated water).

[0110] Freeze-crystallization and salt separation take advantage of the steep low-temperature solubility curve of Na2SO4. When the temperature decreases, it will precipitate out in large quantities in the form of sodium sulfate decahydrate (Na2SO4·10H2O, commonly known as Glauber's salt), while the solubility of NaCl remains basically unchanged.

[0111] Salt separation by cryogenic crystallization can be achieved by freezing the solution at low temperatures (e.g., cooling the solution to about -5°C, at which point the co-saturation concentration of Na2SO4 in the solution can be reduced to about 0.71%, and sodium sulfate will preferentially crystallize out). Thus, sodium sulfate can be removed from the first liquid, and then the mother liquor after sodium sulfate separation can be recycled to step (1) as recirculated water (e.g., as all or part of the recirculated water).

[0112] Nanofiltration membrane separation is a precision separation technology that is becoming increasingly widely used, and it is particularly suitable for processing complex salt-nitrate mixed solutions. It utilizes the selective permeability of nanofiltration membranes to ions of different valence states: monovalent Na⁺ and Cl⁻ can easily pass through the membrane (entering the product water side), while divalent SO₄²⁻ is effectively retained by the membrane (remaining on the concentrate side).

[0113] Nanofiltration membrane desalination can be performed via membrane separation (passing the solution through a nanofiltration system to separate sodium sulfate-rich concentrate and sodium chloride-rich permeate) and optionally, subsequent fractional crystallization (subsequent evaporation or freeze-crystallization of the two solutions). The evaporation is performed with reference to the high-temperature evaporation in thermal desalination, and the freeze-crystallization is performed with reference to the low-temperature freezing in freeze-crystallization desalination. Thus, sodium sulfate and / or sodium chloride can be removed from the first liquid, and the mother liquor after the separation of sodium sulfate and / or sodium chloride is then recycled to step (1) as recirculated water (e.g., as all or part of the recirculated water).

[0114] The sodium sulfate obtained from the salt separation can be provided to the metathesis reaction step (2).

[0115] By recycling the first liquid to step (1) as recirculated water after optionally undergoing desalination, wastewater generation and discharge are reduced on the one hand, and the utilization rate of resources (e.g., Na2SO4) is improved on the other hand.

[0116] In some implementations, the first liquid is not desalinated and is directly recycled to step (1) as recirculated water.

[0117] In some embodiments, the recirculated water is a first liquid that has undergone desalination or has not undergone desalination.

[0118] In some implementations, when the first liquid has not yet been generated in the process, the water used to mix and dissolve the salt and nitrate mixture consists only of pure water.

[0119] In some implementations, when a first liquid has already been produced in the process and the first liquid is not yet saturated with sodium sulfate and sodium chloride, the water used to mix the dissolved salts and nitrates consists only of the first liquid (e.g., the first liquid that has not undergone salt separation treatment).

[0120] In some embodiments, when a first liquid has already been produced in the process and the first liquid is saturated with sodium sulfate and sodium chloride, the water used to mix the dissolved salts includes water and the first liquid (e.g., the first liquid without salt separation treatment).

[0121] Compared with the case where the mixed salt is directly subjected to the metathesis reaction without performing the mixed salt washing step (1), the sodium carbonate product and ammonium sulfate product obtained by performing step (1) have significantly higher purity.

[0122] Step (2): Double displacement reaction, producing NaHCO3.

[0123] In step (2), the first solid, i.e., sodium sulfate, is subjected to a metathesis reaction with ammonium bicarbonate and / or with ammonia and carbon dioxide in water. The reaction products are then separated into solid and liquid components to obtain the second solid (i.e., NaHCO3) and the second liquid.

[0124] In some embodiments, Na2SO4 and NH4HCO3 or ammonia + CO2 are placed into a metathesis reactor at a molar ratio of Na:CO2 or NH4HCO3:H2O = 0.8-1.2:1:2-10.

[0125] In some embodiments, Na may be 0.8-1.2 mol relative to 1 mol of CO2 or NH4HCO3, for example 0.80, 0.85, 0.90, 0.95, 1.00, 1.05, 1.10, 1.15, 1.20 mol, or within the range defined by any two thereof, for example 0.90-1.10 mol, for example 0.95-1.05 mol, for example 1.00 mol.

[0126] In some embodiments, water may be 2-10 moles relative to 1 mole of CO2 or NH4HCO3, for example 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, 9.5, 10.0 moles, or within the range defined by any two of these, for example 3.0-8.0 moles, for example 4.0-6.0 moles, for example 4.5-5.5 moles, for example 5.0 moles.

[0127] In some implementations, when ammonia + CO2 is used, the molar ratio of ammonia to CO2 can be 1:0.95-1.05, for example 1:1. For example, relative to 1 mole of ammonia, carbon dioxide can be 0.95, 0.96, 0.97, 0.98, 0.99, 1.00, 1.01, 1.02, 1.03, 1.04, 1.05 moles, or within the range defined by any two of these.

[0128] Since the Cl⁻ concentration in the system is already controlled at a low level, the salting-out effect is significantly weakened, and a high conversion rate can be achieved without adding a salting-out agent.

[0129] In some embodiments, the metathesis reaction may be carried out, for example, at 20-50°C, such as at temperatures of 25, 30, 35, 40, 45, 50°C, or within a range defined by any two of these, such as 30-40°C.

[0130] In some embodiments, the metathesis reaction may be carried out, for example, under stirring. The stirring time is not particularly limited and, taking into account process efficiency, may be, for example, 10-180 minutes, such as 80-120 minutes, such as 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180 minutes, or within the range defined by any two of these.

[0131] In this embodiment, after the reaction is completed, the reaction products are subjected to solid-liquid separation to obtain a second solid (i.e., NaHCO3) and a second liquid (…). Figure 1 (The heavy alkali mother liquor).

[0132] The solid-liquid separation can be performed by any suitable method known in the art, such as centrifugation or filtration.

[0133] In this application, metathesis reaction has the meaning known in the art, that is, a reaction in which two compounds exchange ionic components to generate two new compounds. In this invention, it specifically refers to the reaction in which sodium sulfate (N2SO4) and ammonium bicarbonate (NH4HCO3) undergo ion exchange in aqueous solution to generate sodium bicarbonate (NaHCO3) precipitate and ammonium sulfate ((NH4)2SO4).

[0134] In this application, unless otherwise specified, "heavy alkali" refers to the common name for sodium bicarbonate (NaHCO3) solid obtained through solid-liquid separation after the metathesis reaction. This solid is a wet material containing a small amount of entrained mother liquor, requiring further washing, drying, or calcination to obtain a qualified product. The heavy alkali mother liquor refers to the liquid phase remaining after the sodium bicarbonate solid has been separated from the metathesis reaction. Its main components are ammonium sulfate (the target product), unreacted sodium sulfate, dissolved ammonium bicarbonate, and a small amount of dissolved sodium bicarbonate.

[0135] Step (3): Wash to obtain sodium bicarbonate product, and optionally calcine to generate Na2CO3.

[0136] In step (3), the second solid (i.e., heavy alkali or wet sodium bicarbonate) is washed with water to obtain sodium bicarbonate product and wash water. Optionally, the sodium bicarbonate product is dried and optionally calcined to obtain sodium carbonate product and carbon dioxide gas. Optionally, the carbon dioxide gas is recycled to step (2), and optionally, the wash water is recycled to step (1) as recycle water.

[0137] The washing can be carried out using a small amount of pure water. For example, relative to 1 part by mass of NaHCO3, the amount of pure water used can be, for example, 0.05-1.0 parts by mass, such as 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50, 0.55, 0.60, 0.65, 0.70, 0.75, 0.80, 0.85, 0.90, 0.95, 1.00 parts by mass, or within the range defined by any two of these.

[0138] The washing temperature is not particularly limited and may be, for example, 20-50°C, 30-40°C, 20, 25, 30, 35, 40, 45, 50°C, or within the range defined by any two of these.

[0139] The washing process can be, for example, countercurrent washing.

[0140] The resulting wash water can be recycled to step (1) as recirculated water (e.g., as part of the recirculated water).

[0141] In some embodiments, after washing the second solid with water, the resulting (wet) sodium bicarbonate product is dried. The drying can be carried out by any suitable method known in the art. The drying can be carried out, for example, at a temperature of 20-60°C.

[0142] In some embodiments, after washing the second solid with water (and optionally drying it), the resulting sodium bicarbonate product is calcined to obtain sodium carbonate product and carbon dioxide gas.

[0143] The calcination can be carried out at any suitable temperature known in the art. For example, the calcination can be carried out at temperatures of 150-400°C, such as 200-250°C, such as 150, 200, 250, 300, 350, 400°C, or within the range defined by any two of these.

[0144] The calcination time may be appropriately selected depending on the calcination temperature, for example, 1-5 hours, for example, 1-2 hours, for example, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0 hours, or within the range defined by any two of them.

[0145] Through calcination, NaHCO3 decomposes into Na2CO3, H2O and CO2.

[0146] The resulting Na₂CO₃ is collected and can be sold directly as a raw material. The resulting Na₂CO₃ may have a purity of, for example, 98.0% or higher by mass, or, for example, 98.5% or higher.

[0147] The CO2 obtained from the decomposition is cooled, compressed, and stored in a CO2 gas source for recycling, for example, recycled to step (2).

[0148] Step (4): Deamination

[0149] In step (4), the second liquid is deaminated to obtain ammonia and deaminated mother liquor, and the ammonia is recycled to step (2).

[0150] The second liquid (i.e.) Figure 1 The heavy alkali mother liquor contains unreacted NH4HCO3, which will decompose to produce NH3 and CO2 during subsequent evaporation. If it is not removed in advance, it will affect the evaporation and crystallization operation and cause material waste.

[0151] By removing ammonia from the second liquid, the impact on subsequent evaporation and crystallization operations is reduced, and the removed ammonia (NH3) can be recycled (recycled to step (2) for metathesis reaction), thereby reducing material waste and saving raw material costs.

[0152] In one embodiment, the deammoniation can be carried out by heating and stripping.

[0153] The deammoniation / heated stripping can be carried out by introducing steam while heating.

[0154] The deammoniation / heated stripping can be carried out, for example, at temperatures of 60-95°C, such as 60, 65, 70, 75, 80, 85, 90, 95°C, or within the range defined by any two of these.

[0155] The steam may be, for example, low-pressure steam, such as steam pressure of 0.1-0.2 MPa.

[0156] Ammonia-containing vapor is obtained by heating and stripping, which is then condensed and recycled for use in the metathesis reaction in step (5).

[0157] Step (5): Evaporate once to recover sodium sulfate.

[0158] In step (5), the deammoniation mother liquor is evaporated once, such that the amount of water evaporated once is 20%-30% of the water volume of the mixture to be evaporated once. The product of the first evaporation is then subjected to solid-liquid separation to obtain a third solid and a third liquid as sodium sulfate products (i.e., Figure 1 (The mother liquor from the first evaporation).

[0159] The deammoniation mother liquor (optionally mixed with the secondary evaporation mother liquor) mainly contains Na2SO4 and (NH4)2SO4. Na2SO4 is preferentially crystallized out through evaporation and concentration.

[0160] The temperature of the first evaporation is not particularly limited and may be, for example, 75 to 120°C, such as 75, 80, 85, 90, 95, 100, 105, 110, 115, 120°C, or within the range defined by any two of them.

[0161] The primary evaporation can be carried out, for example, under a vacuum of 0 to -85 kPa, such as 0, -5, -10, -15, -20, -25, -30, -35, -40, -45, -50, -55, -60, -65, -70, -75, -80, -85 kPa, or within the range defined by any two of these.

[0162] The evaporation can be carried out such that the amount of water evaporated in the first evaporation is 20%-30% of the amount of water in the mixture to be evaporated, for example 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, or within the range defined by any two of them.

[0163] Those skilled in the art will understand that when the secondary evaporation mother liquor and the deammoniation mother liquor are not mixed, the mixture to be evaporated in the first stage refers to the deammoniation mother liquor; when the secondary evaporation mother liquor and the deammoniation mother liquor are mixed, the mixture to be evaporated in the first stage refers to the mixture of the deammoniation mother liquor and the secondary evaporation mother liquor. Therefore, in the initial stage of the process of the present invention, the mixture to be evaporated in the first stage refers to the deammoniation mother liquor; thereafter (e.g., in the steady-state operation stage of the process of the present invention), the mixture to be evaporated in the first stage refers to the mixture of the deammoniation mother liquor and the secondary evaporation mother liquor.

[0164] The product after the first evaporation (i.e., slurry) is subjected to solid-liquid separation (e.g., solid-liquid separation while hot) to obtain a third solid (i.e., sodium sulfate solid) and a third liquid (mother liquor from the first evaporation).

[0165] The solid-liquid separation can be performed by any suitable method known in the art, such as centrifugation or filtration.

[0166] In this embodiment, the third solid (i.e., sodium sulfate solid) is recycled to step (2) and mixed with the first solid to carry out a metathesis reaction.

[0167] Step (6): Secondary evaporation to prepare ammonium sulfate

[0168] In step (6), the third liquid (i.e., the mother liquor from the first evaporation) is subjected to a second evaporation, such that the amount of water evaporated is 30%-40% of the amount of water in the third liquid. The product is then subjected to solid-liquid separation to obtain a fourth solid (i.e., ammonium sulfate) and a fourth liquid ( Figure 1 The second evaporation mother liquor in the process is recycled to step (5) and mixed with the deammoniation mother liquor as the mixture to be evaporated once in step (5).

[0169] The temperature of the secondary evaporation is not particularly limited and may be, for example, 40-90°C, 50-60°C, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90°C, or within the range defined by any two of these.

[0170] The secondary evaporation can be carried out, for example, at a vacuum level of -80 to -95 kPa, such as -80 to -90 kPa, such as -80, -85, -90, -95 kPa, or within the range defined by any two of these.

[0171] The secondary evaporation can be carried out to such that the amount of water evaporated in the secondary evaporation is 30%-40% of the amount of water in the third liquid, for example, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, or within the range defined by any two of them.

[0172] The product after secondary evaporation (i.e., slurry) is subjected to solid-liquid separation (e.g., solid-liquid separation while hot) to obtain a fourth solid and a fourth liquid (mother liquor from secondary evaporation).

[0173] The solid-liquid separation can be performed by any suitable method known in the art, such as centrifugation or filtration.

[0174] After drying, the fourth solid product is obtained as ammonium sulfate solid.

[0175] In this embodiment, the fourth liquid (i.e., the secondary evaporation mother liquor) is recycled to step (5) and mixed with the deammoniation mother liquor to form the mixture to be evaporated once in step (5). That is, in step (5), the deammoniation mother liquor and the fourth liquid can be mixed and then evaporated once.

[0176] Using the method of this invention, sodium bicarbonate and / or sodium carbonate and ammonium sulfate products with comparable or even higher purity can be prepared with significantly lower energy consumption than existing technologies such as cold or hot methods. Furthermore, compared to the case where the mixed salts are directly subjected to a metathesis reaction step without washing, the prepared sodium bicarbonate and / or sodium carbonate and ammonium sulfate products have significantly higher purity.

[0177] In addition, the present invention has the following advantages:

[0178] First, this invention employs a pretreatment dechlorination technology. Utilizing the solubility difference between Na₂SO₄ and NaCl, by controlling the water washing temperature (20-50℃, e.g., 30-40℃, e.g., 35℃) and quantitative water replenishment, NaCl is selectively dissolved and removed to obtain high-purity Na₂SO₄ (NaCl ≤ 2%). A small amount of NaCl entrained in the raw material will be carried into the wet NaHCO₃ material during solid-liquid separation, and after calcination, it will be converted into chloride impurities in Na₂CO₃. The reduction in NaCl in the raw material directly reduces the amount carried into the wet NaHCO₃ material during subsequent solid-liquid separation, thereby reducing the introduction of chloride impurities into the calcined Na₂CO₃ product and ensuring product purity. Simultaneously, the low-chlorine environment also inhibits the accumulation of chloride ions in the mother liquor, preventing the concentration factor of the secondary evaporation process from being affected by premature NaCl precipitation. Therefore, this invention can simultaneously obtain high-purity Na₂CO₃ and (NH₄)₂SO₄ products without adding any salting-out agent.

[0179] Secondly, this invention employs a closed-loop, all-thermal salt separation process, eliminating the necessary low-temperature freezing step in traditional processes. It replaces the alternating hot and cold process with a two-stage thermal separation method of "primary evaporation → secondary evaporation," eliminating the need for refrigeration equipment and achieving highly efficient separation of sodium sulfate and ammonium sulfate. All mother liquor circulates within the system in a closed loop, with no waste discharge. Furthermore, the ammonia recovered from deammoniation and the CO2 generated from calcination can be reused in the metathesis reaction, further reducing the cost of purchased raw materials. In addition, this invention has broad raw material adaptability, capable of processing various salt-nitrate mixtures with different mass ratios of Na2SO4 and NaCl. It exhibits good adaptability to various salt-nitrate mixtures with different mass ratios of Na2SO4 and NaCl without requiring significant adjustments to process parameters, and the process parameter adjustments are simple.

[0180] In summary, this invention provides a green, clean, economical, and energy-saving technical path for the large-scale, high-value utilization of industrial waste salt.

[0181] Example

[0182] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0183] It should be noted that the process described in this invention involves material recycling. During the initial start-up phase, since there is no recycled material in the system, fresh raw materials or pure water can be used instead of recycled material until the system reaches a stable circulation state (i.e., the flow rate and composition of each material no longer change significantly). In the following embodiments, unless otherwise specified, the operational steps after the process enters a stable circulation phase are described. The specific operations during the initial start-up phase are well known to those skilled in the art and will not be repeated here.

[0184] Example 1

[0185] 1. Salt washing to purify Na2SO4

[0186] The salt-nitrate mixture (Na₂SO₄ 89.3%, NaCl 4.3%, composition as shown in Table 1 below) was treated as follows:

[0187] In the initial start-up phase, the salt-nitrate mixture is mixed with pure water at a mass ratio of 1:3, stirred at 35°C for 30 minutes, and then filtered. Then, the newly added salt-nitrate mixture is mixed with the filtered mother liquor at a mass ratio of 1:3, stirred at 35°C for 30 minutes, and then filtered; this operation is repeated until the filtered mother liquor becomes saturated.

[0188] When the mother liquor becomes saturated, it enters steady-state operation.

[0189] During the steady-state operation phase, the salt-nitrogen mixture, saturated mother liquor, and pure water were mixed at a mass ratio of 1:3:0.15 and stirred at 35°C for 30 minutes. After stirring, solid-liquid separation was performed by filtration to obtain high-purity Na2SO4 solid and saturated mother liquor. The saturated mother liquor was recycled to be mixed with the salt-nitrogen mixture and pure water at a mass ratio of 1:3:0.15.

[0190] Table 1 shows the comparison of Na2SO4 content before and after salt washing (under steady-state operating conditions).

[0191] Table 1

[0192]

[0193] 2. Double displacement reaction, producing NaHCO3.

[0194] The Na2SO4 solid, NH4HCO3, and H2O obtained in step 1 were reacted at 35°C for 80 min in a molar ratio of 0.5:1:5, and then separated into solid and liquid by centrifugation to obtain the heavy alkali mother liquor and NaHCO3 solid.

[0195] 3. Calcination produces Na2CO3

[0196] The NaHCO3 solid obtained in step 2 was washed countercurrently with a small amount of pure water (about 0.2 times the mass of the NaHCO3 solid) at a washing temperature of 30°C. The resulting wet alkali was then calcined at 220°C for 1 hour, and the calcined product was Na2CO3.

[0197] 4. Deamination

[0198] The heavy alkali mother liquor obtained in step 2 is heated to 90°C and stripped by low-pressure steam (0.1 MPa). The ammonia-containing steam is condensed and recovered for metathesis reaction (i.e., recycled to step 2).

[0199] 5. Single evaporation: recovery of sodium sulfate

[0200] Evaporation is carried out in the following manner:

[0201] If the secondary evaporation in step 6 has not yet occurred and secondary evaporation mother liquor has not yet been generated, the deammoniation mother liquor from step 4 is subjected to primary evaporation. The amount of water evaporated is approximately 30% of the water volume of the mixed liquor, the evaporation temperature is 115℃, and the vacuum degree is 0 kPa. The slurry is then filtered while hot for solid-liquid separation. The separated sodium sulfate solid is directly returned to participate in the metathesis reaction.

[0202] When the secondary evaporation in step 6 has already produced a secondary evaporation mother liquor, the deammoniation mother liquor from step 4 and the secondary evaporation mother liquor from step 6 are mixed and then subjected to a first evaporation. The amount of water evaporated is approximately 20% of the water volume of the mixture, the evaporation temperature is 115℃, and the vacuum degree is 0 kPa. The slurry is then filtered while hot to achieve solid-liquid separation. The separated sodium sulfate solid is directly returned to participate in the metathesis reaction.

[0203] 6. Secondary evaporation: Preparation of ammonium sulfate

[0204] The cooling mother liquor from step 6 is evaporated and concentrated, with the evaporation temperature controlled at 60℃ and the vacuum degree at -80kPa. The amount of water evaporated is approximately 30% of the cooling mother liquor. While still hot, the mixture is filtered for solid-liquid separation. The separated solid is wet ammonium sulfate, which is dried with hot air at 80℃ to obtain the ammonium sulfate product. The separated liquid is the secondary evaporation mother liquor, which is returned to step 5 for recycling.

[0205] The prepared Na2CO3 has a purity of 99.1% by mass, which meets the requirements of Class II Grade 1 of GB / T 210-2022 "Industrial Anhydrous Sodium Carbonate".

[0206] The prepared (NH4)2SO4 has a nitrogen content of 19.2% by mass and a sulfur content of 21.0% by mass, which meets the requirements of GB / T 535-2020 "Fertilizer Grade Ammonium Sulfate" Type II.

[0207] Example 2:

[0208] Example 2 was carried out in the same manner as Example 1, except that in step 1, a different salt-nitrate mixture (Na2SO4 77.6%, NaCl 17.1%, composition shown in Table 2 below) was used and the salt-nitrate mixture, saturated mother liquor and pure water were mixed in a mass ratio of 1:3:0.6.

[0209] Table 2 shows the comparison of Na2SO4 content before and after salt washing (under steady-state operating conditions) in step (1).

[0210] Table 2

[0211]

[0212] The prepared Na2CO3 has a purity of 98.9% by mass, which meets the requirements of Class II Grade 1 of GB / T 210-2022 "Industrial Anhydrous Sodium Carbonate".

[0213] The prepared (NH4)2SO4 has a nitrogen content of 19.0% by mass and a sulfur content of 21.0% by mass, which meets the requirements of GB / T 535-2020 "Fertilizer Grade Ammonium Sulfate" Type II.

[0214] The comparison between Example 2 and Example 1 shows that even if the proportion of NaCl in the salt-nitrate mixture used as raw material is too high, high-purity Na2SO4 can still be obtained by washing the salt, with little impact on product quality.

[0215] Comparative Example 1:

[0216] Comparative Example 1 was carried out in the same manner as Example 1, except that step 1, the salt washing, was not performed, and all other conditions were the same.

[0217] As a result, the prepared

[0218] The purity of Na2CO3 is 96%, which meets the requirements for qualified products in GB / T 210-2022 "Industrial Anhydrous Sodium Carbonate".

[0219] Furthermore, the nitrogen content of the prepared (NH4)2SO4 is 13.3% and the sulfur content is 21.5%, which does not meet the requirements of GB / T 535-2020 "Fertilizer Grade Ammonium Sulfate" Type II.

[0220] As can be seen from Comparative Example 1, when the salt washing operation in step 1 is omitted, the purity of both Na2CO3 and (NH4)2SO4 prepared is significantly reduced. This is believed to be because, when the salt washing operation in step 1 is missing, NaCl will enter the subsequent metathesis and calcination processes with the raw materials, and will be carried into the wet NaHCO3 material during solid-liquid separation, transforming into chloride impurities in Na2CO3 after calcination. Moreover, the presence of NaCl in the raw materials will cause Na2SO4 to precipitate prematurely during the secondary evaporation process, leading to its entrainment into the ammonium sulfate product, which seriously affects the purity of (NH4)2SO4. To achieve the same purity as the ammonium sulfate product in Example 1, the evaporation concentration factor must be reduced, but this will significantly reduce the product yield.

[0221] Comparative Example 2:

[0222] Comparative Example 2 was carried out in the same manner as Example 1, except that the steps after deammoniation were performed as follows:

[0223] 5. Single evaporation: recovery of sodium sulfate

[0224] The mother liquor from step 4 was evaporated once at a temperature of 78°C and a vacuum of -80 kPa, with approximately 32% of the mother liquor evaporated. The slurry was then subjected to solid-liquid separation while still hot. The separated sodium sulfate solid was directly returned to participate in the metathesis reaction. The liquid, the mother liquor from the first evaporation, was sent for cooling and crystallization.

[0225] 6. Cooling and crystallization: precipitation of double salt (Na2SO4·(NH4)2SO4·4H2O)

[0226] The mother liquor from the first evaporation is cooled and crystallized at 25°C, and the slurry undergoes solid-liquid separation. The separated Na₂SO₄·(NH₄)₂SO₄·4H₂O and ammonium sulfate solids are sent to the first evaporation. The separated liquid, which is the cooling mother liquor, is sent to the second evaporation.

[0227] 7. Secondary evaporation: Preparation of ammonium sulfate

[0228] The cooling mother liquor undergoes secondary evaporation at 78℃ and a vacuum of -80 kPa, with approximately 30% of the water evaporated from the deammoniation mother liquor. The slurry is then subjected to solid-liquid separation while still hot. The separated solid is ammonium sulfate. The liquid, the secondary evaporation mother liquor, is sent for cooling and crystallization.

[0229] The prepared (NH4)2SO4 has a nitrogen content of 19.1% by mass and a sulfur content of 22.5% by mass, which meets the requirements of GB / T535-2020 "Fertilizer Grade Ammonium Sulfate" Type II.

[0230] However, the energy consumption of Comparative Example 2 and Example 1 was calculated based on the evaporation rate and temperature. For every 1 kg of ammonium sulfate produced, Example 1 required 12.2 kJ of energy, while Comparative Example 2 required 22.5 kJ, which is significantly higher than that of Example 1.

[0231] The above description is merely an exemplary embodiment of the present invention. It should be noted that those skilled in the art can make improvements to the present invention without departing from the inventive concept, and all such improvements fall within the scope of protection of the present invention.

Claims

1. A process for preparing ammonium sulfate, sodium bicarbonate, and / or sodium carbonate from chemical salt nitrate mixtures as raw materials, comprising the following steps: (1) The salt-nitrate mixture is mixed and dissolved with water at a temperature of 20-50°C to obtain a solid-liquid mixture, wherein the salt-nitrate mixture includes sodium sulfate and sodium chloride, and the mass ratio of sodium sulfate to sodium chloride is 1:1 to 100:

1. The water includes pure water, recirculated water, or a combination of pure water and recirculated water; The resulting solid-liquid mixture was subjected to solid-liquid separation to obtain a first solid and a first liquid as crude sodium sulfate products. Optionally, the first liquid is optionally desalinated and then recycled to step (1) as recirculated water; in, When the water used to mix and dissolve salt and nitrate mixtures consists only of pure water, the mass ratio of salt and nitrate mixtures to pure water is 1:2-4. When the water used to mix the dissolved salt and nitrate mixture consists only of recirculated water, the recirculated water is not saturated with sodium sulfate and sodium chloride, and the mass ratio of the salt and nitrate mixture to the recirculated water is 1:2-4; When the water used to mix dissolved salts and nitrates includes both pure water and recirculated water, the mass ratio of the salt-nitrate mixture to pure water satisfies the following relationship: Where m 水 Indicates the mass of pure water added; m 混盐 Indicates the mass of mixed salt added; p NaCl This indicates the mass percentage of NaCl in the mixed salt, where a = 3.0-4.

2. The mass ratio of the mixed salt and nitrate salt to the recirculated water is 1:2-4; (2) The first solid is reacted with ammonium bicarbonate and / or with ammonia and carbon dioxide in water to undergo a double displacement reaction. The reaction products are then separated into solid and liquid phases to obtain a second solid and a second liquid. (3) The second solid was washed with water to obtain sodium bicarbonate product and wash water. The sodium bicarbonate product is optionally dried and optionally calcined to obtain sodium carbonate product and carbon dioxide gas. Optionally, carbon dioxide gas can be recycled to step (2). Optionally, the wash water can be recycled to step (1) as recirculated water; (4) The second liquid is deaminated to obtain ammonia and deaminated mother liquor, and the ammonia is recycled to step (2). (5) The mother liquor for deammoniation is evaporated once, so that the amount of water evaporated once is 20%-30% of the amount of water in the mixture to be evaporated once. The product of the first evaporation is separated into solid and liquid components to obtain a third solid and a third liquid as sodium sulfate products. The third solid is recycled to step (2) and mixed with the first solid; (6) The third liquid is evaporated a second time, such that the amount of water evaporated in the second evaporation is 30%-40% of the amount of water in the third liquid. The product of the second evaporation is then subjected to solid-liquid separation to obtain a fourth solid and a fourth liquid as ammonium sulfate products. The fourth liquid is recycled to step (5) and mixed with the deammoniation mother liquor as the mixture to be evaporated once in step (5).

2. The process as described in claim 1, wherein in step (1): The mixing and dissolution are carried out at a temperature of 30-40°C, for example, 35°C; The mass ratio of sodium sulfate to sodium chloride in the salt-nitrate mixture is 2:1 to 50:1, for example, 4:1 to 30:

1. The combination of sodium sulfate and sodium chloride in the salt-nitrate mixture accounts for 90-100% of the mass of the salt-nitrate mixture, for example, 95-100% of the mass; When the water used to mix the dissolved salts and nitrates consists only of pure water, the mass ratio of the salts and nitrates to pure water is 1:2.5-3.5, 1:2.8-3.2, 1:2.9-3.1, or 1:3.

0. When the water used for mixing dissolved salt and nitrate mixtures consists only of recirculated water, the mass ratio of the salt and nitrate mixture to the recirculated water is 1:2.5-3.5, 1:2.8-3.2, 1:2.9-3.1, or 1:3.

0. When the water used to mix dissolved salts and nitrates includes pure water and recirculated water, the mass ratio of the salts and nitrates to the recirculated water is 1:2.5-3.5, 1:2.8-3.2, 1:2.9-3.1, or 1:3.

0. When the water used to mix dissolved salts and nitrates includes both pure water and recirculated water, the recirculated water is already saturated with sodium sulfate and sodium chloride. a = 3.2-3.8, preferably 3.4-3.6, for example 3.5; and / or The first liquid is either desalinated and then recycled to step (1) as recirculated water, or it is directly recycled to step (1) as recirculated water without desalination.

3. The process as described in claim 1, wherein in step (2): Relative to 1 mole of carbon dioxide or ammonium bicarbonate, sodium sulfate contains 0.8-1.2 moles of sodium, for example 0.95-1.05 moles, or for example 1.00 moles; For every 1 mole of carbon dioxide or ammonium bicarbonate, there are 2-10 moles of water, for example 3.0-8.0 moles, 4.0-6.0 moles, 4.5-5.5 moles, or 5.0 moles. When using ammonia and carbon dioxide, the molar ratio of ammonia to carbon dioxide is 1:0.95-1.05, for example, 1:1; The metathesis reaction is carried out at 20-50°C, for example, 30-40°C; and / or The metathesis reaction was carried out under stirring.

4. The process as described in claim 1, wherein in step (3): The washing temperature is 20-50℃, for example, 30-40℃; The washing is a counter-current washing; and / or The sodium bicarbonate product was calcined to obtain sodium carbonate product and carbon dioxide gas.

5. The process as described in claim 1, wherein in step (4), The deammoniation is carried out by heated stripping, wherein the heated stripping is carried out by introducing steam while heating; Preferably, the heating stripping is carried out at a temperature of 60-95°C; Preferably, the steam has a steam pressure of 0.1-0.2 MPa.

6. The process of claim 1, wherein in step (5): The temperature for primary evaporation is 75-120℃; and / or The evaporation is carried out under a vacuum of 0 to -85 kPa.

7. The process of claim 1, wherein in step (6): The temperature for secondary evaporation is 40-90℃, for example 50-60℃; and / or The secondary evaporation is carried out under a vacuum of -80 to -95 kPa, for example -80 to -90 kPa.

8. The process of claim 1, wherein in step (1): The recirculated water is either the first liquid that has undergone salt separation treatment or the first liquid that has not undergone salt separation treatment; When the first liquid has not yet been generated in the process, the water used to mix and dissolve the salt and nitrate mixture consists only of pure water; When the process has already produced a first liquid and the first liquid is not yet saturated with sodium sulfate and sodium chloride, the water used to mix and dissolve the salt and nitrate mixture consists only of the first liquid, for example, the first liquid before salt separation treatment; and When the first liquid has been generated in the process and the first liquid is saturated with sodium sulfate and sodium chloride, the water used to mix and dissolve the salt-nitrate mixture includes water and the first liquid, for example, the first liquid that has not undergone salt separation treatment.

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