Process method and system for preparing alkali from ammonia circulation sodium sulfate waste

Abstract

The invention discloses a process method and system for preparing alkali from sodium sulfate waste through ammonia circulation, belongs to a sodium sulfate waste treatment technology in the field of chemical environmental protection, and aims to solve the problems of single treatment route, poor economic benefit, low resource utilization rate and the like in the prior art. A multi-system collaborative process route comprising a sodium sulfate waste pretreatment and decalcification system, a sodium bicarbonate reaction system, an ammonia recovery reaction system, a carbonization synthesis system and a hydrogen chloride expelling system is adopted, and corresponding units are flexibly started according to the characteristics of raw materials, so that integrated treatment of sodium chloride and sodium sulfate is realized; by constructing a closed cycle of ammonia recovery and carbonization synthesis, the consumption and cost of external raw material ammonia are remarkably reduced, and through targeted decalcification and dechlorination steps, harmful impurities in the system are effectively removed, long-term stable operation and product purity of the system are guaranteed, and the method is suitable for industrial production. Therefore, an efficient, economical and reliable industrial path is provided for resourceful treatment of the high-salinity wastewater.

Description

Technical Field

[0001] This invention relates to a process and system for producing alkali from sodium sulfate waste using ammonia recycling. Specifically, it relates to an integrated process and system that combines the alkali production process of high-salt sodium sulfate waste containing sodium chloride with a carbonization synthesis system, while simultaneously achieving efficient recycling of ammonia media. This technology belongs to the field of sodium sulfate waste treatment in the chemical and environmental protection sector. Background Technology

[0002] In many industrial sectors, including chemical engineering, metallurgy, coking, lithium battery manufacturing, and sodium-based desulfurization, the treatment of high-salinity wastewater is a common challenge. This type of wastewater has a complex composition, often containing multiple salts such as sodium chloride and sodium sulfate. Direct discharge or storage poses a serious threat to the environment. Therefore, resource-based treatment of high-salinity wastewater is not only necessary to meet increasingly stringent environmental protection requirements but also an inevitable trend towards achieving a circular economy and recovering valuable resources. For example, the sodium sulfate component (such as byproduct Glauber's salt, Na₂SO₄·10H₂O) can undergo a metathesis reaction with ammonium carbonate to convert it into ammonium sulfate (nitrogen fertilizer) and sodium carbonate (soda ash), thus achieving a high-value-added transformation from "waste" to "product."

[0003] Currently, there has been considerable exploration of resource utilization technologies for single salts. For example, Chinese patent CN110304641B reports a process for producing alkali from sodium sulfate, specifically involving the crystallization reaction of a pre-reaction slurry composed of solid sodium sulfate, solid ammonium bicarbonate, and frozen salt in a carbonization reactor to produce solid sodium bicarbonate, frozen salt, carbon dioxide, and ammonia. The carbon dioxide and ammonia are then returned to the carbonization reactor to continue reacting with the reaction slurry. This process can increase the conversion rate of ammonium bicarbonate to over 98%. Another example is Chinese patent CN11432947U, which reports an equipment and method for producing ammonium bicarbonate and soda ash using sodium sulfate and CO2. Specifically, it utilizes high-concentration CO2 from industrial solid waste sodium sulfate / sodium sulfate, coking, metallurgy, power plants, lime kilns, and other industrial furnaces, as well as chemical by-product ammonia water, as raw materials to co-produce ammonium bicarbonate, sodium carbonate, and ammonium sulfate.

[0004] However, these existing technological approaches primarily focus on addressing the single issue of sodium sulfate waste. When faced with more complex high-salinity wastewater systems where sodium chloride and sodium sulfate coexist, the following limitations become apparent: (1) Lack of comprehensive treatment solutions: Existing processes are mostly "single-line operations" and lack integrated technical solutions that can simultaneously and synergistically treat multiple salts such as sodium chloride and sodium sulfate, making it difficult to realize the resource utilization of the entire high-salt wastewater system. (2) Poor economic efficiency of resource-based products: For example, the technology of producing chlor-alkali using waste sodium chloride is not economically viable because the raw material sodium chloride itself is cheap and chlor-alkali enterprises have low acceptance of waste salt raw materials. At the same time, products such as sodium carbonate produced from waste salt are subject to access restrictions on raw material sources in product standards. (3) Engineering and cost bottlenecks: The core unit operations such as salt separation and evaporation crystallization of high-salinity wastewater are costly (often exceeding 50 yuan / m³). 3 Furthermore, the process is complex, and although experience has been accumulated in engineering, challenges remain in technology integration, equipment optimization, and operational stability, which has affected large-scale promotion. (4) Poor overall economic benefits: Except for a few routes (such as the production of soda ash from low-concentration miscellaneous salts), the overall economic benefits of high-salt wastewater resource utilization treatment are still weak, which cannot effectively stimulate enterprises' willingness to invest and implement.

[0005] In summary, existing technologies mostly focus on the treatment of single salts, lacking comprehensive and synergistic treatment solutions for high-salinity wastewater such as sodium chloride and sodium sulfate. Furthermore, existing systems demonstrate poor economic feasibility, hindering industrial-scale adoption. Therefore, there is an urgent need to construct a novel multi-system synergistic process flow that can efficiently treat high-salinity wastewater while effectively controlling industrial costs and maximizing resource utilization, thereby promoting the practical application of this technology in industrial settings. Summary of the Invention

[0006] The purpose of this invention is to provide a process and system for producing alkali from sodium sulfate waste using an ammonia-cycle recycling method. This method employs a multi-system synergistic process route comprising a sodium sulfate waste pretreatment and decalcification system, a sodium bicarbonate reaction system, an ammonia recovery reaction system, a carbonation synthesis system, and a hydrogen chloride removal system. By flexibly activating corresponding units according to the characteristics of the raw materials, integrated treatment of sodium chloride and sodium sulfate is achieved. Through the construction of a closed-loop cycle for ammonia recovery and carbonation synthesis, the consumption and cost of external raw material ammonia are significantly reduced. Furthermore, targeted decalcification and dechlorination steps effectively remove harmful impurities from the system, ensuring long-term stable operation and product purity. This provides an efficient, economical, and reliable industrial path for the resource-based treatment of high-salinity wastewater.

[0007] A process system for producing alkali from sodium sulfate waste using ammonia recycling includes a sodium sulfate waste pretreatment and decalcification system, a sodium bicarbonate reaction system, an ammonia recovery reaction system, a carbonation synthesis system, and a hydrogen chloride removal system. The sodium sulfate waste pretreatment and decalcification system is only activated when the sodium chloride mass concentration in the sodium sulfate waste is ≥5%. It is used to dissolve and clarify the sodium sulfate waste. The clarified liquid is sent to a thickener for solid-liquid separation after evaporation and crystallization. The dilute phase of the thickener is returned to the evaporation and crystallization process. The concentrated phase is separated by centrifugation. The obtained solid reacts with the sodium chloride mother liquor from the subsequent process for decalcification. The obtained liquid phase is returned to the evaporation and crystallization process. The sodium bicarbonate reaction system is used to perform solid-liquid separation on a slurry formed by mixing sodium sulfate waste, dilute sodium sulfate solution, ammonium bicarbonate and primary frozen salt. The obtained solid phase is used to prepare sodium bicarbonate product, and the obtained liquid phase is sent to the subsequent reaction process. The primary frozen salt is obtained by primary frozen crystallization. The ammonia recovery reaction system is used to separate the solid and liquid phases of the slurry generated by the reaction of calcium oxide with the liquid from the CO2 stripping tower. The resulting solid phase is used to prepare calcium sulfate products, and the resulting liquid phase is treated by the deammoniation tower to generate the dilute sodium sulfate solution required by the sodium bicarbonate reaction system. The carbonization synthesis system is activated to react concentrated ammonia with CO2 to generate ammonium bicarbonate required for the sodium bicarbonate reaction system. The hydrogen chloride removal system is activated only when the sodium chloride mass concentration in the sodium sulfate waste is ≥5%. It is used to treat the liquid phase from the sodium bicarbonate reaction system and react it with concentrated sulfuric acid, while heating to remove hydrogen chloride gas. Calcium carbonate powder is then added to adjust the pH of the solution. In the sodium sulfate waste pretreatment and decalcification system, the solvent used to dissolve the sodium sulfate waste is the secondary condensate generated in the evaporation and crystallization process, and the operating temperature of the evaporation and crystallization process is controlled above 80°C.

[0008] The decalcification reaction involves a double displacement reaction between the solid obtained after evaporation and crystallization and a dilute sodium sulfate solution from the ammonia recovery reaction system. This reaction converts the calcium chloride in the dilute sodium sulfate solution into calcium sulfate precipitate, which is then discharged from the system. The conditions for the decalcification reaction are: reaction temperature 20-80℃, pH value 5-7, stirring speed 50-300 rpm, and the reaction endpoint is defined as the saturation of sodium chloride in the solution. In the sodium bicarbonate reaction system, a mixed slurry is prepared by a metathesis reaction of sodium sulfate waste (after pretreatment and decalcification) or sodium sulfate waste with a sodium chloride mass concentration of <5%, a dilute sodium sulfate solution, ammonium bicarbonate, and primary frozen salt. The metathesis reaction is carried out in a sodium bicarbonate metathesis crystallizer under the following conditions: temperature 20–100°C, pressure 0–0.3 MPa, pH 7–10, stirring speed 50–200 rpm, and reaction time 1–6 hours.

[0009] The mixed slurry was sent to a sodium bicarbonate thickener for solid-liquid separation. The resulting concentrated slurry was then filtered, centrifuged, and dried to obtain sodium bicarbonate product. The resulting clear liquid was processed as follows: I. When the sodium chloride mass concentration in sodium sulfate waste is <5%, the resulting clear liquid is directly sent to the primary freeze crystallizer; II. When the sodium chloride mass concentration in sodium sulfate waste is ≥5%, the resulting clear liquid is first sent to the hydrogen chloride removal system and then enters the primary freeze crystallizer.

[0010] The clear liquid obtained from the primary freeze crystallizer is the primary freeze salt used in the sodium bicarbonate reaction system. The crystals precipitated from the primary freeze crystallizer are successively passed through the primary crystallizer thickener and filter. The resulting mother liquor is sent to the CO2 stripping tower, and the resulting filter cake is returned to the sodium bicarbonate reaction system.

[0011] The mother liquor obtained from the filter is heat-exchanged with the effluent from the CO2 stripping tower before being fed into the CO2 stripping tower. Before the heat exchange, the temperature of the mother liquor is 20-50°C, and the effluent temperature of the CO2 stripping tower is 80-100°C. After the heat exchange, the temperature of the mother liquor rises to 60-90°C, and the effluent temperature of the CO2 stripping tower drops to 50-80°C.

[0012] The gas phase at the top of the CO2 stripping tower is condensed, and the condensate is sent to the ammonia water supply tank of the carbonization synthesis system. The uncondensed gas is sent to the dilute ammonia water absorption system or the first tail gas scrubbing tower for treatment. The resulting dilute ammonia water is used to prepare concentrated ammonia water or converted into ammonium sulfate for reuse. In the ammonia recovery reaction system, the effluent from the calcium oxide and CO2 stripping towers is subjected to an ammonia recovery metathesis reaction to produce a reaction slurry. The reaction is carried out in an ammonia recovery metathesis reaction crystallizer, under the following conditions: temperature 10–60°C, pressure 0–0.3 MPa, and reaction time 1–6 hours.

[0013] The ammonia gas generated by the ammonia recovery metathesis reaction is condensed by a surface condenser. The condensate is sent to the concentrated ammonia water storage tank of the carbonization synthesis system, and the uncondensed gas is sent to the dilute ammonia water preparation system or the second tail gas scrubbing tower. The condensation temperature of the surface condenser is controlled between 0 and 30°C.

[0014] The reaction slurry is sent to an ammonia recovery thickener for solid-liquid separation. The resulting concentrated phase is then subjected to primary filtration, pH adjustment with concentrated sulfuric acid, secondary filtration, and drying to obtain calcium sulfate product. The clear liquid obtained from the ammonia recovery thickener and primary filtration is sent to a deammoniation tower for further processing.

[0015] The overhead vapor from the deammoniation tower is sent to a secondary stripping tower for further processing. The bottom solution of the deammoniation tower is used as a dilute sodium sulfate solution in the sodium bicarbonate reaction system. The operating conditions of the deammoniation tower are as follows: The temperature range is 50–100℃, and the pressure range is 0–0.3 MPa. The gas phase at the top of the secondary stripping tower is condensed, and the condensate is returned to the secondary stripping tower. The uncondensed gas is sent to the concentrated ammonia water circulation absorption system. The bottom solution of the secondary stripping tower is returned to the deammoniation tower. The operating temperature of the condensation is controlled between 0 and 30°C. The concentrated ammonia water circulation absorption system is used to produce concentrated ammonia water and send it to the feeding tank of the carbonization synthesis system.

[0016] The carbonization synthesis system includes a main carbonization tower and a secondary carbonization tower connected in series. Concentrated ammonia water and CO2 gas are fed into the main carbonization tower for reaction. The resulting gas phase is fed into the secondary carbonization tower. After solid-liquid separation, the resulting slurry has a solid phase of ammonium bicarbonate required for the sodium bicarbonate reaction system, and the liquid phase is recycled to the concentrated ammonia water preparation system. The concentrated ammonia water in the carbonization sub-tower comes from the feed tank of the carbonization synthesis system. The concentrated ammonia preparation system and the concentrated ammonia circulation absorption system together constitute the concentrated ammonia production process, which are responsible for the recovery and concentration of ammonia from different sources, and finally produce concentrated ammonia of consistent concentration for use by the carbonization synthesis system.

[0017] The operating conditions of the main carbonization tower are: temperature 0~100℃, pressure 0~1 MPa; the operating conditions of the secondary carbonization tower are: temperature 0~100℃, pressure 0~1 MPa.

[0018] The gas phase at the top of the carbonization sub-tower is sent to the second tail gas scrubbing tower after passing through the cleaning and recovery tower, while the liquid phase at the bottom of the carbonization sub-tower is returned to the tower for recycling reaction. In the hydrogen chloride removal system, the liquid phase in the sodium bicarbonate reaction system is sequentially passed through a CO2 stripping tower and a primary freeze crystallizer. The precipitated ammonium chloride reacts with concentrated sulfuric acid under the following conditions: temperature 60–100°C, pH < 6, and stirring speed 50–300 rpm.

[0019] In the hydrogen chloride removal system, calcium carbonate powder is added to the mother liquor obtained from the primary freeze crystallizer to adjust the pH value of the solution. The adjustment conditions are: temperature 10-30℃, pH value 8-9.

[0020] A process for producing alkali from sodium sulfate waste using ammonia recycling, employing the above-mentioned process system, When the sodium chloride mass concentration in the sodium sulfate waste is ≥5%, the sodium sulfate waste is sequentially processed through a sodium sulfate waste pretreatment and decalcification system, a sodium bicarbonate reaction system, an ammonia recovery reaction system, and a carbonation synthesis system for recycling. The liquid phase of the sodium bicarbonate reaction system is sent to the hydrogen chloride removal system for further processing. When the sodium chloride mass concentration in the sodium sulfate waste is <5%, the sodium sulfate waste is sequentially recycled through a sodium bicarbonate reaction system, an ammonia recovery reaction system, and a carbonization synthesis system.

[0021] This invention is achieved through the following technical solution: Compared with the prior art, the present invention has the following advantages and beneficial effects: (1) This invention realizes the synergistic treatment and resource utilization of high-salinity wastewater. Through multi-system synergistic process design, it can simultaneously treat mixed high-salinity wastewater or solid waste containing sodium chloride and sodium sulfate, eliminating the complex salt separation steps in traditional processes and significantly reducing investment costs and land area. In addition, the system can flexibly activate the corresponding treatment unit according to the sodium chloride content in the raw materials, realizing the adaptive treatment of high-salinity wastewater with different components and broadening the application scope.

[0022] (2) This invention achieves closed-loop recycling of ammonia medium through the coordinated operation of the ammonia recovery reaction system and the carbonization synthesis system, which greatly reduces the consumption cost of external raw material ammonia, and can convert sodium sulfate waste into high-value-added sodium bicarbonate products, while producing calcium sulfate as a by-product, thus achieving high efficiency and economy in the resource utilization of waste.

[0023] (3) By introducing decalcification and dechlorination process units into the system, the present invention can effectively remove harmful impurity ions in the system, prevent equipment scaling and corrosion, and ensure the long-term stable operation of the system. At the same time, through optimized process parameter control and material circulation path, the product quality and purity of the main product sodium bicarbonate and the by-product calcium sulfate are ensured.

[0024] (4) Compared with traditional high-salinity wastewater treatment processes, this invention has a significant advantage in treatment cost, with a substantial reduction in unit treatment cost. While solving environmental problems, it also creates economic benefits through resource utilization, forming a virtuous cycle economy model and providing a feasible technical path for the treatment of high-salinity wastewater in the chemical industry.

[0025] (5) The process flow design of this invention is reasonable, the system units are closely connected, the operating parameters are clear, and it is easy to realize industrial scale-up and promotion. It is particularly suitable for the treatment of high-salt wastewater generated by industries such as sodium-based desulfurization, coking, smelting, and lithium battery manufacturing, and has good market application prospects.

[0026] In summary, this invention not only effectively solves the technical challenges of treating high-salinity wastewater, but also achieves significant results in terms of economic and environmental benefits, providing the chemical industry with an efficient, economical, and reliable technical route for wastewater treatment and resource utilization. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the process flow of the present invention (I).

[0028] Figure 2 This is a schematic diagram of the process flow of the present invention (II). Detailed Implementation

[0029] The invention's objective, technical solution, and beneficial effects will be further explained in detail below.

[0030] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the claimed invention. Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0031] This invention proposes an integrated process and system capable of synergistically treating high-salt sodium sulfate waste containing sodium chloride. It organically combines sodium sulfate alkali production with a carbonization synthesis system, achieving efficient recycling and resource utilization of the ammonia medium. This technology effectively solves the problems of existing treatment routes being singular, economically inefficient, and having low resource utilization rates, providing an efficient, economical, and highly resource-efficient feasible path for high-salt wastewater treatment in the chemical and environmental protection fields. The technological innovation of this invention lies primarily in its ability to directly treat high-salinity wastewater or mixed salt solid waste, eliminating the complex salt separation steps of traditional processes. This results in a system with advantages such as lower investment, smaller footprint, and simplified process, achieving the same alkali production or treatment scale, significantly reducing the production cost per unit product and demonstrating promising prospects for large-scale industrial application. Furthermore, while addressing the environmental issues of high-salinity wastewater and mixed salt solid waste, this invention also achieves a balance between environmental and economic benefits by constructing a closed-loop system for resource recycling. It possesses significant industrial application value, particularly in the comprehensive treatment and resource utilization of high-salinity wastewater and its derivative mixed salt solid waste in the chemical and environmental protection industries, providing effective technical support for promoting the green and sustainable development of these industries.

[0032] The sodium sulfate high-salt waste containing sodium chloride that is treated in this invention refers to mixed salt waste formed by mixing a certain proportion of sodium chloride into sodium sulfate waste. For example, sodium sulfate waste generated from sodium-based desulfurization can be mixed with sodium chloride wastewater generated from industries such as lithium battery industry and steel smelting (e.g., chlor-alkali industry, seawater desalination or other chlorine-containing processes) in a certain proportion to produce high-salt wastewater with a sodium sulfate content of 10-20% and a sodium chloride content of 20-30%, and then treated using the process system of this invention for resource recovery.

[0033] The process flow of this invention is shown below. Figure 1 and Figure 2 As shown, the system includes a sodium sulfate waste pretreatment and decalcification system, a sodium bicarbonate reaction system, an ammonia recovery reaction system, a carbonization synthesis system, and a hydrogen chloride removal system. Different treatment systems and processes are selected based on the sodium chloride content (≥5% or <5%) in the sodium sulfate waste (mixed salt waste), as detailed below: Process Flow 1: When the sodium chloride mass concentration in sodium sulfate waste is ≥5%, the sodium sulfate waste is sequentially processed through a sodium sulfate waste pretreatment and decalcification system, a sodium bicarbonate reaction system, an ammonia recovery reaction system, and a carbonation synthesis system for recycling. The liquid phase from the sodium bicarbonate reaction system is sent to the hydrogen chloride removal system for further processing. See [link to relevant documentation]. Figure 1 .

[0034] Specifically, 1) In the sodium sulfate waste pretreatment and decalcification system, the sodium sulfate waste is dissolved and clarified. The clarified liquid is then sent to a thickener for solid-liquid separation after evaporation and crystallization (≥80℃). The dilute phase of the thickener is returned to the evaporation and crystallization process, and the concentrated phase is separated by centrifugation. The resulting solid reacts with the sodium chloride mother liquor from the subsequent process for decalcification, and the resulting liquid phase is returned to the evaporation and crystallization process.

[0035] Optionally, in this invention, the solvent used to dissolve sodium sulfate waste is secondary condensate generated during the evaporation and crystallization process; the operating temperature of the evaporation and crystallization process is controlled above 80°C; the decalcification reaction is a double displacement reaction, using the solid obtained after evaporation and crystallization with a dilute sodium sulfate solution from the ammonia recovery reaction system to convert calcium chloride in the solution into calcium sulfate precipitate for discharge from the system. The conditions for the decalcification reaction are: reaction temperature 20–80°C, pH value 5–7, stirring speed 50–300 rpm, and the reaction endpoint is defined as the sodium chloride in the solution reaching saturation.

[0036] 2) In the sodium bicarbonate reaction system, the liquid phase after decalcification (sodium chloride reaches saturation), dilute sodium sulfate solution, ammonium bicarbonate and primary frozen salt are subjected to a double decomposition reaction to obtain a mixed slurry. The mixed slurry is subjected to solid-liquid separation. The resulting concentrated slurry is filtered, centrifuged and dried to obtain sodium bicarbonate product. The resulting clear liquid is first sent to the hydrogen chloride expulsion system and then enters the primary frozen crystallizer.

[0037] Optionally, in this invention, the metathesis reaction is carried out in a sodium bicarbonate metathesis crystallizer, under the following conditions: temperature 20–100°C, pressure 0–0.3 MPa, pH 7–10, stirring speed 50–200 rpm, reaction time 1–6 hours, and the reaction equation is as follows: Na2SO4+ 2NH4HCO3→2NaHCO3↓+ (NH4)2SO4 The mixed slurry contains sodium sulfate and sodium chloride. The mixed slurry is sent to a sodium bicarbonate thickener for solid-liquid separation. Optionally, the operating conditions of the sodium bicarbonate thickener are: temperature 20-60℃, solid content 10-40%.

[0038] After solid-liquid separation in a sodium bicarbonate thickener, the resulting slurry can be used for the following processes in the preparation of sodium bicarbonate products: filtration (e.g., vacuum belt filter, candle filter, or plate and frame filter press, with filtration accuracy controlled within the range of 5–20 μm); centrifugation (e.g., horizontal scraper discharge centrifuge, piston pusher centrifuge, or screw discharge filter centrifuge, with centrifugal factor controlled within the range of 500–1500 and operating temperature controlled within the range of 20–60℃); and drying (e.g., vibrating fluidized bed dryer, spray dryer, or flash dryer, with drying temperature controlled within the range of 80–150℃).

[0039] After solid-liquid separation in the sodium bicarbonate thickener, the resulting clear liquid contains sodium sulfate, ammonium sulfate, ammonium chloride, and ammonium bicarbonate. This clear liquid is first sent to the hydrogen chloride removal system and then enters the primary freeze crystallizer. The crystals precipitated in the primary freeze crystallizer are mainly ammonium sulfate and ammonium chloride. After passing through the primary crystallizer thickener and filter (5-20 μm, optional equipment as above), the resulting mother liquor is sent to the CO2 stripping tower. The resulting filter cake mainly consists of sodium sulfate, sodium chloride, and ammonium bicarbonate, which is returned to the sodium bicarbonate double decomposition reaction crystallizer for recycling of sodium sulfate and sodium chloride. The clear liquid obtained from the primary freeze crystallization is the primary freeze salt, mainly composed of sodium sulfate, sodium chloride, and ammonium bicarbonate. It is also returned to the sodium bicarbonate double decomposition reaction crystallizer for recycling to improve sodium yield.

[0040] For CO2 stripping towers, the mixture of mother liquor obtained from the filter is first heat-exchanged with the liquid effluent from the CO2 stripping tower before being fed into the CO2 stripping tower. Before the heat exchange, the temperature of the mother liquor is 20-50℃, and the liquid effluent temperature of the CO2 stripping tower is 80-100℃. After the heat exchange, the temperature of the mother liquor rises to 60-90℃, and the liquid effluent temperature of the CO2 stripping tower drops to 50-80℃.

[0041] The CO2 stripping tower is used to decompose ammonium bicarbonate in the mother liquor. The reaction equation is as follows: NH4HCO3→ NH3↑ + CO2↑ + H2O The effluent from the CO2 stripping tower (containing sodium sulfate, sodium chloride, ammonium sulfate, and ammonium chloride) is sent to the ammonia recovery reaction system. The gas phase at the top of the CO2 stripping tower (containing ammonia and CO2) is condensed, and the condensate is sent to the ammonia water supply tank of the carbonization synthesis system. The uncondensed gas is sent to the dilute ammonia water absorption system or the first tail gas scrubbing tower for treatment. The resulting dilute ammonia water is used to prepare concentrated ammonia water or converted into ammonium sulfate for reuse.

[0042] 3) In the ammonia recovery reaction system, calcium oxide and the liquid from the CO2 stripping tower are reacted with ammonia recovery metathesis to obtain a reaction slurry. The reaction slurry is then subjected to solid-liquid separation. The obtained solid phase is used to prepare calcium sulfate products, and the obtained liquid phase is treated by a deammoniation tower to generate the dilute sodium sulfate solution required by the sodium bicarbonate reaction system.

[0043] Optionally, the above reaction is carried out in an ammonia recovery metathesis reaction crystallizer (calcium-alkali reactor) under the following conditions: temperature 10–60°C, pressure 0–0.3 MPa, and reaction time 1–6 hours. The reaction equation is as follows: (NH4)2SO4+ Ca(OH)2→CaSO4↓+ 2NH3↑+2H2O Na2SO4+ Ca(OH)2→CaSO4↓+2NaOH 2NH4Cl +Ca(OH)2→CaCl2+2NH3↑+2H2O The ammonia gas produced by the ammonia recovery metathesis reaction is condensed by a surface condenser. The condensate (ammonia water) is sent to the concentrated ammonia water storage tank of the carbonization synthesis system, while the uncondensed gases (ammonia gas and water vapor) are sent to the dilute ammonia water preparation system or the second tail gas scrubbing tower. The condensation temperature of the surface condenser is controlled between 0 and 30°C.

[0044] The reaction slurry obtained from the ammonia recovery metathesis reaction contains sodium sulfate, ammonium sulfate, ammonium chloride, and sodium chloride. This slurry is sent to an ammonia recovery thickener for solid-liquid separation. The resulting concentrated phase (mainly containing calcium sulfate) is sequentially subjected to primary filtration, pH adjustment with concentrated sulfuric acid, secondary filtration, and drying to obtain calcium sulfate product. The clarified liquid obtained from the ammonia recovery thickener and primary filtration (mainly containing sodium hydroxide, sodium sulfate, sodium chloride, and calcium chloride) is sent to a deammoniation tower (ammonia stripping tower) for further processing. Optional equipment for primary and secondary filtration includes vacuum belt filters, candle filters, and plate and frame filter presses. The primary filtration accuracy is optionally 30 μm, and the secondary filtration accuracy is optionally 5 μm. When adjusting the pH with concentrated sulfuric acid, a sulfuric acid concentration of 95–98% is selected, and the final pH is adjusted to 5–8. Optional equipment for the drying process includes vibrating fluidized beds, spray dryers, and flash dryers. The drying temperature is optionally controlled within the range of 100–200℃.

[0045] The ammonia stripping tower is used to remove ammonia from the mother liquor to achieve ammonia recycling. During the ammonia stripping tower process, the gas phase at the top of the ammonia stripping tower (mainly containing ammonia) is sent to the secondary stripping tower for further processing. The solution at the bottom of the ammonia stripping tower is a dilute sodium sulfate solution, which is returned to the sodium bicarbonate metathesis reaction crystallizer for recycling. The operating conditions of the ammonia stripping tower are: temperature 50-100℃, pressure 0-0.3 MPa.

[0046] Furthermore, in the secondary stripping tower, the gas phase at the top of the secondary stripping tower is condensed. The condensation operation temperature is controlled within the range of 0 to 30°C. The condensate (concentrated ammonia water) is then returned to the secondary stripping tower, while the uncondensed gas (ammonia) is sent to the concentrated ammonia water circulation absorption system for the production of concentrated ammonia water and then sent to the feed tank of the carbonization synthesis system. The bottom solution of the secondary stripping tower is returned to the deammoniation tower for recycling.

[0047] In this invention, the carbonization synthesis system feed tank and the aforementioned concentrated ammonia water storage tank are ammonia water storage tanks of different concentrations; wherein, the concentrated ammonia water storage tank is used to receive the high-concentration ammonia water recovered in the system, and the carbonization synthesis system feed tank receives concentrated ammonia water that has been configured to meet the reaction requirements. Together, they provide raw materials for the carbonization synthesis system.

[0048] 4) In the carbonization synthesis system, concentrated ammonia water is reacted with CO2 to generate ammonium bicarbonate, which is then returned to the sodium bicarbonate metathesis reaction crystallizer as a raw material. The carbonization main tower and carbonization auxiliary tower are arranged in series. Concentrated ammonia water and CO2 gas are first sent to the carbonization main tower for reaction. The gas phase obtained from the reaction is then sent to the carbonization auxiliary tower. After solid-liquid separation, the solid phase of the slurry obtained from the reaction is ammonium bicarbonate required for the sodium bicarbonate metathesis reaction, and the liquid phase is sent to the concentrated ammonia water preparation system for circulation.

[0049] The operating conditions of the main carbonization tower are: temperature 0-100℃, pressure 0-1 MPa, CO2 inlet flow rate 0.5-3.0 m / s, concentrated ammonia water flow rate 0.01-0.1 m / s, concentrated ammonia water mass concentration 5-30%, the resulting reaction gas phase contains ammonia and CO2, and the resulting slurry contains ammonium bicarbonate; the operating conditions of the secondary carbonization tower are: temperature 0-100℃, pressure 0-1 MPa.

[0050] Furthermore, in the above process, the concentrated ammonia water in the carbonization sub-tower comes from the feed tank of the carbonization synthesis system; the concentrated ammonia water preparation system and the concentrated ammonia water circulation absorption system together constitute the concentrated ammonia water production process, which are responsible for the recovery and concentration of ammonia water from different sources, and finally produce concentrated ammonia water of consistent concentration for use by the carbonization synthesis system; the gas phase at the top of the carbonization sub-tower is sent to the second tail gas scrubbing tower after passing through the cleaning and recovery tower, and the liquid phase at the bottom of the carbonization sub-tower is returned to the tower for circulation reaction. 5) In the hydrogen chloride removal system (hydrogen chloride removal reactor), the clear liquid obtained from the solid-liquid separation of the sodium bicarbonate thickener in the sodium bicarbonate reaction system (mainly containing sodium sulfate, ammonium sulfate, ammonium chloride, and ammonium bicarbonate) is treated and reacted with concentrated sulfuric acid. At the same time, the hydrogen chloride gas is removed by heating. Then, calcium carbonate powder is added to adjust the pH value of the solution.

[0051] In actual operation, the obtained clear liquid is first stripped of ammonium bicarbonate in a CO2 stripping tower, and then ammonium chloride is precipitated by cooling and crystallization. The obtained ammonium chloride is used for the hydrogen chloride removal reaction. The water content of the ammonium chloride used for the reaction is controlled to be ≤30%. The reaction conditions are: pH<6, concentrated sulfuric acid mass concentration 92-100%, heating temperature 60-100℃, and stirring speed 50-300 rpm.

[0052] Calcium carbonate powder (e.g., industrial grade, purity ≥95%) is added to adjust the pH of the cooling crystallization mother liquor and the mother liquor after the hydrogen chloride is expelled from the reaction. Preferably, the pH is adjusted to 8-9 at a temperature of 10-30°C. After pH adjustment, the mixture is filtered using a vacuum belt filter. The resulting solid is calcium sulfate precipitate, and the liquid phase mainly contains sodium sulfate and ammonium sulfate.

[0053] Process Flow 2: When the sodium chloride mass concentration in the sodium sulfate waste is <5%, the sodium sulfate waste is sequentially recycled through a sodium bicarbonate reaction system, an ammonia recovery reaction system, and a carbonization synthesis system. (See [link to relevant documentation]). Figure 2 .

[0054] 1) In the sodium bicarbonate reaction system, ammonium sulfate waste raw material, dilute sodium sulfate solution, ammonium bicarbonate and primary frozen salt are subjected to a double decomposition reaction to obtain a mixed slurry. The mixed slurry is subjected to solid-liquid separation. The resulting concentrated slurry is filtered, centrifuged and dried to obtain sodium bicarbonate product. The resulting clear liquid is directly sent to the primary frozen crystallizer.

[0055] Because the sodium chloride concentration in the ammonium sulfate waste raw material varies (<5%), the resulting clear liquid mainly contains sodium sulfate, sodium bicarbonate, and ammonium sulfate. The crystals precipitated by the primary freeze crystallizer are mainly ammonium sulfate. After passing through the primary crystallizer thickener and filter (5-20μm, optional equipment as in process flow one), the resulting mother liquor is sent to the CO2 stripping tower. The resulting filter cake mainly consists of sodium sulfate and ammonium bicarbonate, which is returned to the sodium bicarbonate double decomposition reaction crystallizer for sodium sulfate recycling. The clear liquid obtained from the primary freeze crystallization is the primary freeze salt, mainly composed of sodium sulfate and ammonium bicarbonate, which is also returned to the sodium bicarbonate double decomposition reaction crystallizer for recycling, thereby improving the sodium yield.

[0056] The CO2 stripping tower is used to decompose ammonium bicarbonate in the mother liquor. The mixture of mother liquor obtained from the filter is first heat-exchanged with the effluent from the CO2 stripping tower before being sent back into the CO2 stripping tower. The effluent from the CO2 stripping tower mainly contains sodium sulfate and ammonium sulfate, which is sent to the ammonia recovery reaction system. The gas phase (containing ammonia and CO2) at the top of the CO2 stripping tower is condensed, and the condensate is sent to the ammonia water supply tank of the carbonization synthesis system. The uncondensed gas is sent to the dilute ammonia water absorption system or the first tail gas scrubbing tower for treatment. The resulting dilute ammonia water is used to prepare concentrated ammonia water or converted into ammonium sulfate for reuse. The specific operation is the same as in process flow one.

[0057] 2) In the ammonia recovery reaction system, calcium oxide and the liquid from the CO2 stripping tower are reacted with ammonia recovery metathesis to obtain a reaction slurry. The reaction slurry is then subjected to solid-liquid separation. The obtained solid phase is used to prepare calcium sulfate products, and the obtained liquid phase is treated by a deammoniation tower to generate the dilute sodium sulfate solution required by the sodium bicarbonate reaction system.

[0058] Because the sodium chloride concentration in the ammonium sulfate waste raw material varies (<5%), the resulting reaction slurry mainly contains sodium sulfate and ammonium sulfate, and its reaction equation is as follows: (NH4)2SO4+ Ca(OH)2→CaSO4↓+ 2NH3↑+2H2O Na2SO4+ Ca(OH)2→CaSO4↓+2NaOH The ammonia gas generated from the ammonia recovery metathesis reaction is treated in the same manner as in process flow one.

[0059] The resulting reaction slurry was subjected to solid-liquid separation. The resulting concentrated phase (mainly containing calcium sulfate) was successively filtered once, pH adjusted with concentrated sulfuric acid, filtered twice, and dried to obtain calcium sulfate product. The ammonia recovery thickener and the clear liquid obtained from the first filtration (mainly containing sodium hydroxide and sodium sulfate) were sent to the deammoniation tower for treatment.

[0060] 4) In the carbonization synthesis system, concentrated ammonia water is reacted with CO2 to generate ammonium bicarbonate, which is then returned to the sodium bicarbonate metathesis reaction crystallizer as a raw material. The carbonization main tower and carbonization auxiliary tower are arranged in series. Concentrated ammonia water and CO2 gas are first sent to the carbonization main tower for reaction. The gas phase obtained from the reaction is then sent to the carbonization auxiliary tower. After solid-liquid separation, the solid phase of the slurry obtained from the reaction is ammonium bicarbonate required for the sodium bicarbonate metathesis reaction, and the liquid phase is sent to the concentrated ammonia water preparation system for circulation.

[0061] The system of this invention demonstrates good process adaptability for high-salt waste with different compositions. In process flow two, because the sodium chloride mass concentration in the raw materials is less than 5%, the system does not use the sodium sulfate waste pretreatment and decalcification system or the hydrogen chloride removal system, and the material composition content in some steps differs from that in process flow one. Based on this, process flow two, while using the main equipment, process route, and core parameters of process flow one, only made adaptive adjustments to the relevant steps to address the composition differences.

[0062] This demonstrates that the system of the present invention can effectively treat complex saline wastewater or solid waste (including mixed systems of sodium chloride and sodium sulfate in different proportions) without relying on the complex and costly salt separation steps in traditional processes, significantly reducing equipment investment and floor space requirements, and providing a more economical and operationally feasible solution for the resource-based treatment of high-salt waste.

[0063] The specific implementation of the present invention will be described below with reference to the embodiments. Of course, the scope of protection of the present invention is not limited to the following embodiments.

[0064] Example 1: This embodiment describes the process of treating sodium sulfate and sodium chloride waste generated from the lithium battery industry using process flow one, configuring it into high-salt wastewater with a sodium sulfate content of 15-20% and a sodium chloride content of 25-30%. In the sodium sulfate waste pretreatment and decalcification system, the evaporation crystallization temperature is controlled at 120℃, the decalcification reaction temperature at 35℃, the pH value at 7, and the stirring rate at 280rpm.

[0065] In the sodium bicarbonate reaction system, the reaction temperature for the metathesis reaction was controlled at 35℃, the pressure at 0MPa, the pH at 8, the stirring rate at 280rpm, and the reaction time at 1 hour. The operating conditions for the sodium bicarbonate thickener were: temperature 35℃ and solid content 25%. In the preparation of the sodium bicarbonate product, a vacuum belt filter was used for filtration with a filtration accuracy of 7μm, followed by centrifugation using a horizontal scraper centrifuge with a centrifugation factor of 800 and an operating temperature of 35℃. Drying was performed in a vibrating fluidized bed at a drying temperature of 120℃. In the CO2 stripping tower, before heat exchange, the mother liquor temperature was 20℃, and the outlet temperature of the CO2 stripping tower was 85℃. After heat exchange, the mother liquor temperature rose to 75℃, and the outlet temperature of the CO2 stripping tower dropped to 60℃.

[0066] In the ammonia recovery reaction system, the reaction temperature of the ammonia recovery metathesis reaction is controlled at 35℃, the pressure at 0.1MPa, and the reaction time at 1.5 hours; the condensation temperature of the surface condenser is 15℃; the primary filtration accuracy is 30μm, and the secondary filtration accuracy is 5μm; when adjusting the pH with concentrated sulfuric acid, sulfuric acid with a mass concentration of 98% is selected, and the final pH is adjusted to 7; the drying process adopts a vibrating fluidized bed, and the drying temperature is 120℃; in the ammonia removal tower, the temperature is controlled at 70℃ and the pressure at 0.1MPa.

[0067] In the carbonization synthesis system, the operating conditions of the main carbonization tower are controlled as follows: temperature 35℃, pressure 0.5MPa, CO2 inlet flow rate 1m / s, concentrated ammonia water flow rate 0.05 m / s, and concentrated ammonia water mass concentration 20%; the operating conditions of the secondary carbonization tower are controlled as follows: temperature 35℃, pressure 0.5MPa.

[0068] In the hydrogen chloride removal system, the water content of the ammonium chloride used for the reaction is controlled to be ≤30%. The reaction conditions are: pH 5.5, concentrated sulfuric acid mass concentration 98%, heating temperature 80℃, and stirring speed 200rpm. After adding calcium carbonate powder, the pH is adjusted to 9 at 25℃.

[0069] Example 2: This embodiment describes the process of treating sodium sulfate and sodium chloride waste generated from the lithium battery industry using process flow one, configuring it into high-salt wastewater with a sodium sulfate content of 15-20% and a sodium chloride content of 25-30%. In the sodium sulfate waste pretreatment and decalcification system, the evaporation crystallization temperature is controlled at 110℃, the decalcification reaction temperature at 30℃, the pH value at 6.5, and the stirring rate at 200 rpm.

[0070] In the sodium bicarbonate reaction system, the reaction temperature for the metathesis reaction was controlled at 30℃, the pressure at 0MPa, the pH at 8.5, the stirring rate at 200rpm, and the reaction time at 1.5 hours. The operating conditions for the sodium bicarbonate thickener were: temperature 30℃ and solid content 20%. In the preparation of the sodium bicarbonate product, a vacuum belt filter was used for filtration with a filtration accuracy of 7μm, followed by centrifugation using a horizontal scraper centrifuge with a centrifugation factor of 800 and an operating temperature of 30℃. Drying was performed in a vibrating fluidized bed at a drying temperature of 110℃. In the CO2 stripping tower, before heat exchange, the mother liquor temperature was 25℃, and the outlet temperature of the CO2 stripping tower was 80℃. After heat exchange, the mother liquor temperature rose to 80℃, and the outlet temperature of the CO2 stripping tower dropped to 55℃.

[0071] In the ammonia recovery reaction system, the reaction temperature of the ammonia recovery metathesis reaction is controlled at 30℃, the pressure at 0.2MPa, and the reaction time at 2 hours; the condensation temperature of the surface condenser is 15℃; the primary filtration accuracy is 30μm, and the secondary filtration accuracy is 5μm; when adjusting the pH with concentrated sulfuric acid, sulfuric acid with a mass concentration of 95% is selected, and the final pH is adjusted to 6.5; the drying process adopts a vibrating fluidized bed, and the drying temperature is 110℃; in the ammonia removal tower, the temperature is controlled at 65℃ and the pressure at 0.2MPa.

[0072] In the carbonization synthesis system, the operating conditions of the main carbonization tower are controlled as follows: temperature 30℃, pressure 0.6MPa, CO2 inlet flow rate 0.8m / s, concentrated ammonia water flow rate 0.07m / s, and concentrated ammonia water mass concentration 18%; the operating conditions of the secondary carbonization tower are controlled as follows: temperature 30℃, pressure 0.6MPa.

[0073] In the hydrogen chloride removal system, the water content of the ammonium chloride used for the reaction is controlled to be ≤30%. The reaction conditions are: pH 6, concentrated sulfuric acid mass concentration 95%, heating temperature 75℃, and stirring speed 180rpm. After adding calcium carbonate powder, the pH is adjusted to 8.5 at 30℃.

[0074] Example 3: This embodiment describes the process of treating sodium sulfate and sodium chloride waste from the smelting industry using process flow one, configuring it into high-salt wastewater with a sodium sulfate content of 15-22% and a sodium chloride content of 28-33%. In the sodium sulfate waste pretreatment and decalcification system, the evaporation crystallization temperature is controlled at 115℃, the decalcification reaction temperature at 33℃, the pH value at 6.8, and the stirring rate at 250rpm.

[0075] In the sodium bicarbonate reaction system, the reaction temperature for the metathesis reaction was controlled at 25℃, the pressure at 0.1MPa, the pH at 8.5, the stirring rate at 250rpm, and the reaction time at 2 hours. The operating conditions for the sodium bicarbonate thickener were: temperature 33℃ and solid content 22%. In the preparation of the sodium bicarbonate product, a vacuum belt filter was used for filtration with a filtration accuracy of 7μm, followed by centrifugation using a horizontal scraper centrifuge with a centrifugation factor of 800 and an operating temperature of 33℃. Drying was performed in a vibrating fluidized bed at a drying temperature of 105℃. In the CO2 stripping tower, before heat exchange, the mother liquor temperature was 30℃, and the outlet temperature of the CO2 stripping tower was 75℃. After heat exchange, the mother liquor temperature rose to 70℃, and the outlet temperature of the CO2 stripping tower dropped to 40℃.

[0076] In the ammonia recovery reaction system, the reaction temperature of the ammonia recovery metathesis reaction is controlled at 33℃, the pressure at 0.1MPa, and the reaction time at 1.5 hours; the condensation temperature of the surface condenser is 15℃; the primary filtration accuracy is 30μm, and the secondary filtration accuracy is 5μm; when adjusting the pH with concentrated sulfuric acid, sulfuric acid with a mass concentration of 92% is selected, and the final pH is adjusted to 6; the drying process adopts a vibrating fluidized bed, and the drying temperature is 115℃; in the ammonia removal tower, the temperature is controlled at 68℃ and the pressure at 0.1MPa.

[0077] In the carbonization synthesis system, the operating conditions of the main carbonization tower are controlled as follows: temperature 33℃, pressure 0.7MPa, CO2 inlet flow rate 0.7m / s, concentrated ammonia water flow rate 0.06m / s, and concentrated ammonia water mass concentration 22%; the operating conditions of the secondary carbonization tower are controlled as follows: temperature 33℃, pressure 0.7MPa.

[0078] In the hydrogen chloride removal system, the water content of the ammonium chloride used for the reaction is controlled to be ≤30%. The reaction conditions are: pH 3, concentrated sulfuric acid mass concentration 98%, heating temperature 70℃, and stirring speed 170rpm. After adding calcium carbonate powder, the pH is adjusted to 9 at 35℃.

[0079] Example 4: This embodiment describes the process of treating sodium sulfate waste generated from sodium-based desulfurization using process flow two, which prepares the wastewater as a high-salt wastewater with a sodium sulfate content of 25-35% and a sodium chloride content of 0-5%. In the sodium bicarbonate reaction system, the reaction temperature for the metathesis reaction was controlled at 35℃, the pressure at 0MPa, the pH at 8, the stirring rate at 280rpm, and the reaction time at 1 hour. The operating conditions for the sodium bicarbonate thickener were: temperature 35℃ and solid content 25%. In the preparation of the sodium bicarbonate product, a vacuum belt filter was used for filtration with a filtration accuracy of 7μm, followed by centrifugation using a horizontal scraper centrifuge with a centrifugation factor of 800 and an operating temperature of 35℃. Drying was performed in a vibrating fluidized bed at a drying temperature of 120℃. In the CO2 stripping tower, before heat exchange, the mother liquor temperature was 20℃, and the outlet temperature of the CO2 stripping tower was 85℃. After heat exchange, the mother liquor temperature rose to 75℃, and the outlet temperature of the CO2 stripping tower dropped to 60℃.

[0080] In the ammonia recovery reaction system, the reaction temperature of the ammonia recovery metathesis reaction is controlled at 35℃, the pressure at 0.1MPa, and the reaction time at 1.5 hours; the condensation temperature of the surface condenser is 15℃; the primary filtration accuracy is 30μm, and the secondary filtration accuracy is 5μm; when adjusting the pH with concentrated sulfuric acid, sulfuric acid with a mass concentration of 98% is selected, and the final pH is adjusted to 7; the drying process adopts a vibrating fluidized bed, and the drying temperature is 120℃; in the ammonia removal tower, the temperature is controlled at 70℃ and the pressure at 0.1MPa.

[0081] In the carbonization synthesis system, the operating conditions of the main carbonization tower are controlled as follows: temperature 35℃, pressure 0.5MPa, CO2 inlet flow rate 1m / s, concentrated ammonia water flow rate 0.05m / s, and concentrated ammonia water mass concentration 20%; the operating conditions of the secondary carbonization tower are controlled as follows: temperature 35℃, pressure 0.5MPa.

[0082] Example 5: This embodiment describes the process of treating sodium sulfate waste generated from sodium-based desulfurization using process flow two, which prepares the wastewater as a high-salt wastewater with a sodium sulfate content of 25-35% and a sodium chloride content of 0-5%. In the sodium bicarbonate reaction system, the reaction temperature for the metathesis reaction was controlled at 30℃, the pressure at 0MPa, the pH at 8.5, the stirring rate at 200rpm, and the reaction time at 1.5 hours. The operating conditions for the sodium bicarbonate thickener were: temperature 30℃ and solid content 20%. In the preparation of the sodium bicarbonate product, a vacuum belt filter was used for filtration with a filtration accuracy of 7μm, followed by centrifugation using a horizontal scraper centrifuge with a centrifugation factor of 800 and an operating temperature of 30℃. Drying was performed in a vibrating fluidized bed at a drying temperature of 110℃. In the CO2 stripping tower, before heat exchange, the mother liquor temperature was 25℃, and the outlet temperature of the CO2 stripping tower was 80℃. After heat exchange, the mother liquor temperature rose to 80℃, and the outlet temperature of the CO2 stripping tower dropped to 55℃.

[0083] In the ammonia recovery reaction system, the reaction temperature of the ammonia recovery metathesis reaction is controlled at 30℃, the pressure at 0.2MPa, and the reaction time at 2 hours; the condensation temperature of the surface condenser is 15℃; the primary filtration accuracy is 30μm, and the secondary filtration accuracy is 5μm; when adjusting the pH with concentrated sulfuric acid, sulfuric acid with a mass concentration of 95% is selected, and the final pH is adjusted to 6.5; the drying process adopts a vibrating fluidized bed, and the drying temperature is 110℃; in the ammonia removal tower, the temperature is controlled at 65℃ and the pressure at 0.2MPa.

[0084] In the carbonization synthesis system, the operating conditions of the main carbonization tower are controlled as follows: temperature 30℃, pressure 0.6MPa, CO2 inlet flow rate 0.8m / s, concentrated ammonia water flow rate 0.07m / s, and concentrated ammonia water mass concentration 18%; the operating conditions of the secondary carbonization tower are controlled as follows: temperature 30℃, pressure 0.6MPa.

[0085] Example 6: This embodiment describes the process of treating sodium sulfate waste generated from sodium-based desulfurization using process flow two, which prepares the wastewater as a high-salt wastewater with a sodium sulfate content of 25-35% and a sodium chloride content of 0-5%. In the sodium bicarbonate reaction system, the reaction temperature for the metathesis reaction was controlled at 25℃, the pressure at 0.1MPa, the pH at 8.5, the stirring rate at 250rpm, and the reaction time at 2 hours. The operating conditions for the sodium bicarbonate thickener were: temperature 33℃ and solid content 22%. In the preparation of the sodium bicarbonate product, a vacuum belt filter was used for filtration with a filtration accuracy of 7μm, followed by centrifugation using a horizontal scraper centrifuge with a centrifugation factor of 800 and an operating temperature of 33℃. Drying was performed in a vibrating fluidized bed at a drying temperature of 105℃. In the CO2 stripping tower, before heat exchange, the mother liquor temperature was 30℃, and the outlet temperature of the CO2 stripping tower was 75℃. After heat exchange, the mother liquor temperature rose to 70℃, and the outlet temperature of the CO2 stripping tower dropped to 40℃.

[0086] In the ammonia recovery reaction system, the reaction temperature of the ammonia recovery metathesis reaction is controlled at 33℃, the pressure at 0.1MPa, and the reaction time at 1.5 hours; the condensation temperature of the surface condenser is 15℃; the primary filtration accuracy is 30μm, and the secondary filtration accuracy is 5μm; when adjusting the pH with concentrated sulfuric acid, sulfuric acid with a mass concentration of 92% is selected, and the final pH is adjusted to 6; the drying process adopts a vibrating fluidized bed, and the drying temperature is 115℃; in the ammonia removal tower, the temperature is controlled at 68℃ and the pressure at 0.1MPa.

[0087] In the carbonization synthesis system, the operating conditions of the main carbonization tower are controlled as follows: temperature 33℃, pressure 0.7MPa, CO2 inlet flow rate 0.7m / s, concentrated ammonia water flow rate 0.06m / s, and concentrated ammonia water mass concentration 22%; the operating conditions of the secondary carbonization tower are controlled as follows: temperature 33℃, pressure 0.7MPa.

[0088] Comparative Example 1: Based on Example 1, this comparative example eliminates the sodium sulfate waste pretreatment and decalcification system and the hydrogen chloride removal system in its process flow. Instead, the high-salt wastewater (sodium sulfate content 15-20%, sodium chloride content 25-30%) is directly sent to the sodium bicarbonate reaction system for treatment, and then recycled by combining the ammonia recovery reaction system and the carbonization synthesis system.

[0089] Apart from the changes mentioned above, the remaining processes and parameters are the same as in Example 1. Any adaptive optimizations made thereto have no substantial impact on this comparison and will not be elaborated further.

[0090] Comparative Example 2: Based on Example 1, the ammonia recovery reaction system is eliminated, and the specific process is adjusted as follows: the effluent from the CO2 stripping tower is directly returned to the sodium bicarbonate metathesis reaction crystallizer to achieve the recycling of sodium sulfate; simultaneously, the gaseous components of the CO2 stripping tower are converted into dilute ammonia water, which is used to prepare the concentrated ammonia water required for the carbonation synthesis system. Concentrated ammonia water needs to be additionally added to the carbonation synthesis system, and then, through the main and auxiliary carbonation towers, the concentrated ammonia water reacts with CO2 to produce ammonium bicarbonate, which is then returned to the sodium bicarbonate metathesis reaction crystallizer.

[0091] Apart from the changes mentioned above, the remaining processes and parameters are the same as in Example 1. Any adaptive optimizations made thereto have no substantial impact on this comparison and will not be elaborated further.

[0092] Comparative Example 3: Based on Example 1, the carbonization synthesis system was eliminated, and the concentrated ammonia obtained in the system was used as a byproduct instead of preparing ammonium bicarbonate, which was then returned to the sodium bicarbonate metathesis reaction crystallizer.

[0093] Apart from the changes mentioned above, the rest of the process and parameters are the same as in Example 1. Any adaptive optimizations made do not affect the conclusions of this comparison, so they will not be repeated here.

[0094] Comparative Example 4: Based on Example 4, the ammonia recovery reaction system is eliminated, and the specific process is adjusted as follows: the effluent from the CO2 stripping tower is directly returned to the sodium bicarbonate metathesis reaction crystallizer to achieve the recycling of sodium sulfate; simultaneously, the gaseous components of the CO2 stripping tower are converted into dilute ammonia water, which is used to prepare the concentrated ammonia water required for the carbonation synthesis system. Concentrated ammonia water needs to be additionally added to the carbonation synthesis system, and then, through the main and auxiliary carbonation towers, the concentrated ammonia water reacts with CO2 to generate ammonium bicarbonate, which is then returned to the sodium bicarbonate metathesis reaction crystallizer.

[0095] Apart from the changes mentioned above, the rest of the process and parameters are the same as in Example 4. Any adaptive optimizations made thereto do not affect the conclusions of this comparison, so they will not be repeated here.

[0096] Comparative Example 5: Based on Example 4, the carbonization synthesis system was eliminated, and the concentrated ammonia obtained in the system was used as a byproduct instead of preparing ammonium bicarbonate, which was then returned to the sodium bicarbonate metathesis reaction crystallizer.

[0097] Apart from the changes mentioned above, the rest of the process and parameters are the same as in Example 4. Any adaptive optimizations made thereto do not affect the conclusions of this comparison, so they will not be repeated here.

[0098] Computer simulations were performed according to the process flow and parameters of the above embodiments and comparative examples, and the following test data were obtained, as shown in Table 1.

[0099] Table 1

[0100] The data in Table 1 above shows that: (1) In Examples 1 to 6 of the present invention, although the sodium bicarbonate production of the two processes decreased slightly, the consumption of by-products and key materials also decreased slightly, but the overall product conversion rate was still high, the product purity did not change and remained stable at 99.5%, which met the national standard requirements and the system operated stably.

[0101] (2) Comparing Example 1 of the present invention with Comparative Example 1, it can be seen that after the pretreatment and hydrogen chloride removal system was removed in Comparative Example 1, the sodium bicarbonate production decreased from 580 tons to 300 tons (a decrease of about 48%), and the calcium sulfate production decreased from 470 tons to 200 tons (a decrease of about 57%). This shows that the present invention can effectively remove harmful impurities such as calcium and chlorine through the sodium sulfate waste pretreatment and decalcification system and the hydrogen chloride removal system, ensuring the stable operation of the system and high product yield under high chlorine conditions.

[0102] (3) Comparing Example 1 of the present invention with Comparative Examples 2 and 3, it can be seen that although Comparative Examples 2 (without the ammonia recovery reaction system) and Comparative Examples 3 (without the carbonization synthesis system) are comparable to Example 1 in terms of product output, they must purchase concentrated ammonia water or ammonium bicarbonate as raw materials, which leads to a significant increase in raw material costs. This shows that the present invention achieves a closed-loop circulation of ammonia medium through the synergy of the ammonia recovery reaction system and the carbonization synthesis system, which greatly reduces the dependence on external raw materials and costs.

[0103] (4) Comparing Examples 4 to 6 of the present invention with Comparative Examples 4 and 5, it can be seen that under low chlorine conditions, although canceling the ammonia recovery reaction system or the carbonization synthesis system does not affect the product yield, it is still necessary to purchase ammonia sources or ammonium bicarbonate, resulting in increased operating costs. Furthermore, this demonstrates that the system of the present invention has process flexibility, and different process flows or specific units can be switched according to the sodium chloride content (≥5% or <5%) in the raw materials, so as to achieve low-cost and high-efficiency resource utilization.

[0104] In summary, this invention successfully solves the industry problems of "high cost, low efficiency, and low product added value" in high-salinity wastewater treatment by combining multi-system collaboration, closed-loop ammonia circulation, precise control of impurities, and intelligent process flow, and has significant environmental and economic value.

[0105] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications or equivalent changes made to the above embodiments based on the technical essence of the present invention shall fall within the protection scope of the present invention.

Claims

1. A process system for producing alkali from sodium sulfate waste using ammonia recycling, characterized in that, This includes a sodium sulfate waste pretreatment and decalcification system, a sodium bicarbonate reaction system, an ammonia recovery reaction system, a carbonization synthesis system, and a hydrogen chloride removal system. The sodium sulfate waste pretreatment and decalcification system is only activated when the sodium chloride mass concentration in the sodium sulfate waste is ≥5%. It is used to dissolve and clarify the sodium sulfate waste. The clarified liquid is sent to a thickener for solid-liquid separation after evaporation and crystallization. The dilute phase of the thickener is returned to the evaporation and crystallization process. The concentrated phase is separated by centrifugation. The obtained solid reacts with the sodium chloride mother liquor from the subsequent process for decalcification. The obtained liquid phase is returned to the evaporation and crystallization process. The sodium bicarbonate reaction system is used to perform solid-liquid separation on a slurry formed by mixing sodium sulfate waste, dilute sodium sulfate solution, ammonium bicarbonate and primary frozen salt. The obtained solid phase is used to prepare sodium bicarbonate product, and the obtained liquid phase is sent to the subsequent reaction process. The primary frozen salt is obtained by primary frozen crystallization. The ammonia recovery reaction system is used to separate the solid and liquid phases of the slurry generated by the reaction of calcium oxide with the liquid from the CO2 stripping tower. The resulting solid phase is used to prepare calcium sulfate products, and the resulting liquid phase is treated by the deammoniation tower to generate the dilute sodium sulfate solution required by the sodium bicarbonate reaction system. The carbonization synthesis system is used to react concentrated ammonia with CO2 to generate ammonium bicarbonate required by the sodium bicarbonate reaction system. The hydrogen chloride removal system is activated only when the sodium chloride mass concentration in the sodium sulfate waste is ≥5%. It is used to treat the liquid phase from the sodium bicarbonate reaction system and react it with concentrated sulfuric acid, while heating to remove hydrogen chloride gas. Calcium carbonate powder is then added to adjust the pH of the solution.

2. The process system according to claim 1, characterized in that: In the sodium sulfate waste pretreatment and decalcification system, the solvent used to dissolve the sodium sulfate waste is the secondary condensate generated in the evaporation and crystallization process, and the operating temperature of the evaporation and crystallization process is controlled above 80°C.

3. The process system according to claim 1, characterized in that: The decalcification reaction involves a double displacement reaction between the solid obtained after evaporation and crystallization and a dilute sodium sulfate solution from the ammonia recovery reaction system. This reaction converts the calcium chloride in the dilute sodium sulfate solution into calcium sulfate precipitate, which is then discharged from the system. The conditions for the decalcification reaction are: reaction temperature 20-80℃, pH value 5-7, stirring speed 50-300 rpm, and the reaction endpoint is defined as the saturation of sodium chloride in the solution.

4. The process system according to claim 1, characterized in that: In the sodium bicarbonate reaction system, a mixed slurry is prepared by a metathesis reaction of sodium sulfate waste (after pretreatment and decalcification) or sodium sulfate waste with a sodium chloride mass concentration of <5%, a dilute sodium sulfate solution, ammonium bicarbonate, and primary frozen salt. The metathesis reaction is carried out in a sodium bicarbonate metathesis crystallizer under the following conditions: temperature 20–100°C, pressure 0–0.3 MPa, pH 7–10, stirring speed 50–200 rpm, and reaction time 1–6 hours.

5. The process system according to claim 4, characterized in that: The mixed slurry was sent to a sodium bicarbonate thickener for solid-liquid separation. The resulting concentrated slurry was then filtered, centrifuged, and dried to obtain sodium bicarbonate product. The resulting clear liquid was processed as follows: I. When the sodium chloride mass concentration in sodium sulfate waste is <5%, the resulting clear liquid is directly sent to the primary freeze crystallizer; II. When the sodium chloride mass concentration in sodium sulfate waste is ≥5%, the resulting clear liquid is first sent to the hydrogen chloride removal system and then enters the primary freeze crystallizer.

6. The process system according to claim 5, characterized in that: The clear liquid obtained from the primary freeze crystallizer is the primary freeze salt used in the sodium bicarbonate reaction system. The crystals precipitated from the primary freeze crystallizer are successively passed through the primary crystallizer thickener and filter. The resulting mother liquor is sent to the CO2 stripping tower, and the resulting filter cake is returned to the sodium bicarbonate reaction system.

7. The process system according to claim 6, characterized in that: The mother liquor obtained from the filter is heat-exchanged with the effluent from the CO2 stripping tower before being fed into the CO2 stripping tower. Before the heat exchange, the temperature of the mother liquor is 20-50°C, and the effluent temperature of the CO2 stripping tower is 80-100°C. After the heat exchange, the temperature of the mother liquor rises to 60-90°C, and the effluent temperature of the CO2 stripping tower drops to 50-80°C.

8. The process system according to claim 6, characterized in that: The gas phase at the top of the CO2 stripping tower is condensed, and the condensate is sent to the ammonia water supply tank of the carbonization synthesis system. The uncondensed gas is sent to the dilute ammonia water absorption system or the first tail gas scrubbing tower for treatment. The resulting dilute ammonia water is used to prepare concentrated ammonia water or converted into ammonium sulfate for reuse.

9. The process system according to claim 1, characterized in that: In the ammonia recovery reaction system, the effluent from the calcium oxide and CO2 stripping towers is subjected to an ammonia recovery metathesis reaction to produce a reaction slurry. The reaction is carried out in an ammonia recovery metathesis reaction crystallizer, under the following conditions: temperature 10–60°C, pressure 0–0.3 MPa, and reaction time 1–6 hours.

10. The process system according to claim 9, characterized in that: The ammonia gas generated by the ammonia recovery metathesis reaction is condensed by a surface condenser. The condensate is sent to the concentrated ammonia water storage tank of the carbonization synthesis system, and the uncondensed gas is sent to the dilute ammonia water preparation system or the second tail gas scrubbing tower. The condensation temperature of the surface condenser is controlled between 0 and 30°C.

11. The process system according to claim 9, characterized in that: The reaction slurry is sent to an ammonia recovery thickener for solid-liquid separation. The resulting concentrated phase is then subjected to primary filtration, pH adjustment with concentrated sulfuric acid, secondary filtration, and drying to obtain calcium sulfate product. The clear liquid obtained from the ammonia recovery thickener and primary filtration is sent to a deammoniation tower for further processing.

12. The process system according to claim 11, characterized in that: The overhead vapor from the deammoniation tower is sent to a secondary stripping tower for further processing. The bottom solution of the deammoniation tower is used as a dilute sodium sulfate solution in the sodium bicarbonate reaction system. The operating conditions of the deammoniation tower are as follows: The temperature range is 50–100℃, and the pressure range is 0–0.3 MPa.

13. The process system according to claim 12, characterized in that: The gas phase at the top of the secondary stripping tower is condensed, and the condensate is returned to the secondary stripping tower. The uncondensed gas is sent to the concentrated ammonia water circulation absorption system. The bottom solution of the secondary stripping tower is returned to the deammoniation tower. The operating temperature of the condensation is controlled between 0 and 30°C.

14. The process system according to claim 13, characterized in that: The concentrated ammonia water circulation absorption system is used to produce concentrated ammonia water and send it to the feeding tank of the carbonization synthesis system.

15. The process system according to claim 14, characterized in that: The carbonization synthesis system includes a main carbonization tower and a secondary carbonization tower connected in series. Concentrated ammonia water and CO2 gas are fed into the main carbonization tower for reaction. The resulting gas phase is fed into the secondary carbonization tower. After solid-liquid separation, the resulting slurry has a solid phase of ammonium bicarbonate required for the sodium bicarbonate reaction system, and the liquid phase is recycled to the concentrated ammonia water preparation system. The concentrated ammonia water in the carbonization sub-tower comes from the feed tank of the carbonization synthesis system. The concentrated ammonia preparation system and the concentrated ammonia circulation absorption system together constitute the concentrated ammonia production process, which are responsible for the recovery and concentration of ammonia from different sources, and finally produce concentrated ammonia of consistent concentration for use by the carbonization synthesis system.

16. The process system according to claim 15, characterized in that: The operating conditions of the main carbonization tower are: temperature 0~100℃, pressure 0~1 MPa; the operating conditions of the secondary carbonization tower are: temperature 0~100℃, pressure 0~1 MPa.

17. The process system according to claim 15, characterized in that: The gas phase at the top of the carbonization sub-tower is sent to the second tail gas scrubbing tower after passing through the cleaning and recovery tower, while the liquid phase at the bottom of the carbonization sub-tower is returned to the tower for recycling reaction.

18. The process system according to claim 1, characterized in that: In the hydrogen chloride removal system, the liquid phase in the sodium bicarbonate reaction system is sequentially passed through a CO2 stripping tower and a primary freeze crystallizer. The precipitated ammonium chloride reacts with concentrated sulfuric acid under the following conditions: temperature 60–100°C, pH < 6, and stirring speed 50–300 rpm.

19. The process system according to claim 18, characterized in that: In the hydrogen chloride removal system, calcium carbonate powder is added to the mother liquor obtained from the primary freeze crystallizer to adjust the pH value of the solution. The adjustment conditions are: temperature 10-30℃, pH value 8-9.

20. A process for producing alkali from sodium sulfate waste using ammonia recycling, characterized in that: Using the process system described in any one of claims 1 to 19, When the sodium chloride mass concentration in the sodium sulfate waste is ≥5%, the sodium sulfate waste is sequentially processed through a sodium sulfate waste pretreatment and decalcification system, a sodium bicarbonate reaction system, an ammonia recovery reaction system, and a carbonation synthesis system for recycling. The liquid phase of the sodium bicarbonate reaction system is sent to the hydrogen chloride removal system for further processing. When the sodium chloride mass concentration in the sodium sulfate waste is <5%, the sodium sulfate waste is sequentially recycled through a sodium bicarbonate reaction system, an ammonia recovery reaction system, and a carbonization synthesis system.

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