Concentrated seawater co-production process based on chlorine cycle and membrane-salt pan cooperation

Abstract

The invention discloses a concentrated seawater co-production process based on chlorine cycle and membrane-salt pan synergy, which comprises the following steps: extracting bromine from seawater desalinated concentrated seawater, neutralizing to obtain bromine-extracted concentrated seawater, and concentrating one part of bromine-extracted concentrated seawater by electrodialysis ED to obtain liquid salt; the residual concentrated seawater and light salt brine generated by electrodialysis ED concentration enter a salt pan to be sunned to prepare salt, and the obtained industrial salt and liquid salt prepared by electrodialysis are jointly used as electrolysis caustic soda raw materials; liquid salt obtained through electrodialysis and industrial salt obtained through salt pan tedding serve as raw materials to be prepared into saturated salt water, caustic soda is produced through an ionic membrane electrolysis method, chlorine as an electrolysis byproduct is mainly used for PVC production, the other part is used for concentrated seawater bromine extraction, bromine, chlorine and sodium chemical resources are fully utilized in the concentrated seawater utilization process, and therefore a seawater utilization industrial chain is formed.

Description

Technical Field

[0001] This invention relates to the field of seawater utilization technology, and in particular to a concentrated seawater co-production process based on chlorine cycle and membrane-salt field synergy. Background Technology

[0002] The seawater desalination industry is developing rapidly, but the direct discharge of its byproduct, concentrated seawater, not only wastes valuable chemical resources such as bromine, chlorine, and sodium, but may also lead to abnormally high salinity in nearshore waters and damage to the ecological environment. Existing technologies for utilizing concentrated seawater suffer from drawbacks such as single-resource recovery, fragmented industrial chains, and high energy consumption. For example, residual liquid after bromine extraction is not fully utilized, salt production from salt pans is inefficient and heavily dependent on natural conditions, and the chlorine byproduct of electrolysis is not recovered in a closed-loop system, resulting in insufficient comprehensive resource utilization and hindering the green and sustainable development of the seawater desalination industry. Therefore, there is an urgent need to develop a comprehensive concentrated seawater utilization process that can achieve multi-resource synergistic recovery and a closed-loop cycle of energy and materials to solve the dual challenges of resource waste and environmental pressure. Summary of the Invention

[0003] In view of this, the present invention provides a concentrated seawater co-production process based on chlorine cycle and membrane-salt field synergy.

[0004] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: A process for co-producing concentrated seawater based on chlorine cycle and membrane-salt field synergy includes: Step (1): Desalination of seawater, extraction of bromine from concentrated seawater, electrodialysis of concentrated seawater after bromine extraction coupled with salt field sun-drying. The desalinated seawater is used to extract bromine. After neutralization, the extracted concentrated seawater is obtained. A portion of the extracted concentrated seawater is concentrated by electrodialysis (ED) to obtain liquid salt. The remaining concentrated seawater and the brine produced by electrodialysis (ED) are sent to salt fields to produce salt. The industrial salt obtained is used together with the liquid salt produced by electrodialysis as raw material for electrolytic caustic soda. Step (2): Electrolysis to produce caustic soda and chlorine to produce PVC Using liquid salt obtained by electrodialysis and industrial salt obtained by sun-drying salt fields as raw materials, saturated brine is produced. Caustic soda is produced by ion-exchange membrane electrolysis. The chlorine gas produced by electrolysis is mainly used for PVC production, and another part is used for bromine extraction from concentrated seawater. This makes full use of bromine, chlorine and sodium chemical resources in the process of utilizing concentrated seawater, thus forming a seawater utilization industrial chain.

[0005] Preferably, an air-blowing-acid absorption process is adopted: chlorine gas, a byproduct of electrolysis, is introduced into the concentrated seawater from the seawater desalination process to oxidize bromide ions into elemental bromine; then air is introduced to blow out the elemental bromine, which is then absorbed by a dilute sulfuric acid solution to form a bromine-rich liquid; finally, high-purity bromine is obtained through distillation purification, and the residual liquid after absorption is returned to the neutralization process.

[0006] Preferably, the process involves electrodialysis (ED) concentration: after bromine extraction, the neutralized solution is pumped into an electrodialysis device, where sodium chloride and other salts are enriched under the action of an electric field to obtain liquid salt with a mass fraction of 20%-25%. The resulting brine, with a salt mass fraction of <3%, is transported to the salt fields. The remaining concentrated seawater after bromine extraction is mixed with the ED brine and introduced into graded salt fields. Water is removed through natural evaporation, and impurities such as Glauber's salt are precipitated sequentially to obtain industrial salt with a purity ≥95%. The mother liquor from the salt fields is returned to the seawater desalination system for recycling.

[0007] Preferably, the ion-exchange membrane electrolysis method is used: liquid salt is mixed and dissolved with industrial salt, and impurities such as calcium and magnesium ions are removed by refining to prepare saturated brine; the refined saturated brine is passed into an ion-exchange membrane electrolysis cell, where an electrolysis reaction occurs under the action of direct current, producing chlorine gas at the anode and caustic soda solution and hydrogen gas at the cathode; the caustic soda solution is evaporated, concentrated, and separated into solid and liquid components to obtain a 32% concentration liquid caustic soda or a 99% solid caustic soda product.

[0008] Preferably, the acetylene polymerization process is adopted: using chlorine gas, a byproduct of electrolysis, and acetylene as raw materials, an addition reaction is carried out under the action of mercuric chloride catalyst to generate vinyl chloride monomer (VCM); after VCM is purified to remove impurities, a suspension polymerization reaction is carried out under the action of an initiator to generate polyvinyl chloride (PVC) resin; after centrifugation, dehydration, drying, and sieving, a PVC product with uniform particle size is obtained, and the tail gas generated during the polymerization process is treated to meet emission standards.

[0009] The present invention achieves the following technical effects compared to the prior art: (1) High resource utilization rate: It realizes the synergistic recovery of multiple resources such as bromine, chlorine and sodium in concentrated seawater, with a bromine recovery rate of ≥85% and a sodium chloride utilization rate of ≥90%. The electrolysis by-product chlorine is used in a closed loop for bromine extraction and PVC production, with no waste. (2) Energy consumption and cost optimization: The combined salt production of electrodialysis and salt field evaporation improves efficiency by more than 40% compared with the traditional single salt field evaporation, and reduces energy consumption per ton of salt by 30%; chlorine recycling reduces the cost of purchased oxidants, resulting in significant economic benefits; (3) Environmental protection and sustainability: The concentrated seawater is utilized as a resource throughout the entire process, with no direct discharge of wastewater. The mother liquor from the salt field is recycled to avoid marine pollution. The closed-loop design of the industrial chain reduces carbon emissions and is in line with the concept of green development. (4) Strong process stability: The mature ion membrane electrolysis and acetylene method for PVC preparation are adopted, combined with precise control by electrodialysis, resulting in stable product quality (bromine purity ≥99.5%, caustic soda purity ≥99%, and uniform PVC particle size) and minimal restriction by natural conditions. Attached Figure Description

[0010] Figure 1 This is a flowchart illustrating the comprehensive utilization of concentrated seawater according to the present invention. Figure 2This is a flowchart of the bromine extraction process from concentrated seawater according to the present invention; Figure 3 This is a flowchart of the electrodialysis coupled with salt field sun-drying process of the present invention; Figure 4 This is a flowchart of the process for producing caustic soda by electrolysis of concentrated seawater according to the present invention. Figure 5 This is a flowchart of the PVC preparation process of the present invention. Detailed Implementation

[0011] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0012] Example 1: This invention discloses a concentrated seawater co-production process based on chlorine cycle and membrane-salt field synergy, wherein the total installed capacity is shown in Table 1; Table 1:

[0013] Based on domestic investment estimates for similar large-scale products, the total investment for this project is approximately 12.8 billion yuan. This includes: a 0.66 million tons / year concentrated seawater bromine extraction project (116 million yuan); a 470,000 tons / year (100% NaCl) electrodialysis concentration project for concentrated brine (696 million yuan); and a 1 million tons / year ion-exchange membrane caustic soda and 1.25 million tons / year PVC co-production project (12 billion yuan). The chlor-alkali production from the beach requires 970,000 tons / year of industrial salt, necessitating approximately 100 square kilometers of land.

[0014] Based on the industry average consumption index for related products, when the electricity price is 0.4 yuan / kWh, the cost per ton of bromine is 9292 yuan / ton, and the total annual cost is 61.32 million yuan; based on the industry average, the total annual cost of caustic soda and PVC is estimated to be about 9 billion yuan.

[0015] The project's annual output value is approximately 12.21 billion yuan. Based on a bromine price of 45,000 yuan / ton, the annual output value is 297 million yuan; based on a 32% caustic soda price of 1,300 yuan / ton, the annual output value is 4.03 billion yuan; and based on a PVC price of 6,300 yuan / ton, the PVC output value is 7.88 billion yuan.

[0016] The project's annual profit and tax revenue totals 3.54 billion yuan.

[0017] Example 2: This embodiment addresses the supporting needs of small and medium-sized seawater desalination projects, optimizing process scale and parameters to achieve a balance between efficient resource utilization and cost control, as detailed below: The total investment for the project is approximately RMB 3.86 billion, including RMB 38 million for a 0.2 million / year concentrated seawater bromine extraction project, RMB 222 million for a 150,000 / year (100% NaCl) electrodialysis concentration project, and RMB 3.6 billion for a 300,000-ton ion-exchange membrane caustic soda co-production project with a 375,000-ton PVC production capacity. The project also requires 291,000 tons / year of industrial salt for beach salt production, covering an area of ​​approximately 30 square kilometers.

[0018] Cost accounting: When the electricity price is 0.42 yuan / kWh, the cost per ton of bromine is 9,180 yuan / ton, with a total annual cost of 18.36 million yuan; the total annual cost of caustic soda and PVC is approximately 2.73 billion yuan.

[0019] Output value and profits and taxes: The project's annual output value is approximately RMB 3.66 billion, of which bromine (priced at RMB 44,500 / ton) has an annual output value of RMB 89 million, 32% caustic soda (priced at RMB 1,280 / ton) has an annual output value of RMB 1.209 billion, and PVC (priced at RMB 6,250 / ton) has an annual output value of RMB 2.362 billion; the total annual profits and taxes are RMB 930 million.

[0020] This embodiment is applicable to the concentrated seawater treatment of seawater desalination projects with a daily output of 50,000 to 100,000 tons. It has a lower investment threshold and requires less land. By optimizing parameters, it can be adapted to the needs of small and medium-sized production capacity. At the same time, it maintains the closed-loop chlorine cycle and multi-resource recovery characteristics. The energy consumption is reduced by more than 25% compared with traditional processes of the same scale. It can be promoted and applied in small and medium-sized chemical industrial parks along the coast, taking into account both environmental benefits and investment returns.

[0021] The above description is merely a preferred embodiment of the present invention and does not constitute any limitation on the technical scope of the present invention. Therefore, any minor modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. A concentrated seawater co-production process based on chlorine cycle and membrane-salt field synergy, characterized in that, include: Step (1): Desalination of seawater, extraction of bromine from concentrated seawater, electrodialysis of concentrated seawater after bromine extraction coupled with salt field sun-drying. The desalinated seawater is used to extract bromine. After neutralization, the extracted concentrated seawater is obtained. A portion of the extracted concentrated seawater is concentrated by electrodialysis (ED) to obtain liquid salt. The remaining concentrated seawater and the brine produced by electrodialysis (ED) are sent to salt fields to produce salt. The industrial salt obtained is used together with the liquid salt produced by electrodialysis as raw material for electrolytic caustic soda. Step (2): Electrolysis to produce caustic soda and chlorine to produce PVC Using liquid salt obtained by electrodialysis and industrial salt obtained by sun-drying salt fields as raw materials, saturated brine is produced. Caustic soda is produced by ion-exchange membrane electrolysis. The chlorine gas produced by electrolysis is mainly used for PVC production, and another part is used for bromine extraction from concentrated seawater. This makes full use of bromine, chlorine and sodium chemical resources in the process of utilizing concentrated seawater, thus forming a seawater utilization industrial chain.

2. The concentrated seawater co-production process based on chlorine cycle and membrane-salt field synergy according to claim 1, characterized in that, The process employs an air-blowing-acid absorption method: chlorine gas, a byproduct of electrolysis, is introduced into the concentrated seawater from the seawater desalination process to oxidize bromide ions into elemental bromine; then, air is introduced to blow out the elemental bromine, which is then absorbed by a dilute sulfuric acid solution to form a bromine-rich liquid; finally, high-purity bromine is obtained through distillation purification, and the residual liquid after absorption is returned to the neutralization process.

3. The concentrated seawater co-production process based on chlorine cycle and membrane-salt field synergy according to claim 1, characterized in that, Electrodialysis (ED) Concentration: After bromine extraction, the neutralized solution is pumped into an electrodialysis unit, where sodium chloride and other salts are enriched under the action of an electric field, resulting in a liquid salt with a mass fraction of 20%-25%. The resulting brine, with a salt mass fraction of <3%, is transported to the salt fields. Salt Field Drying: The remaining concentrated seawater after bromine extraction is mixed with the ED brine and introduced into the graded salt fields. Water is removed through natural evaporation, and impurities such as Glauber's salt are precipitated in sequence, resulting in industrial salt with a purity of ≥95%. The mother liquor from the salt fields is returned to the seawater desalination system for recycling.

4. The concentrated seawater co-production process based on chlorine cycle and membrane-salt field synergy according to claim 1, characterized in that, The ion-exchange membrane electrolysis method is adopted: liquid salt and industrial salt are mixed and dissolved, and impurities such as calcium and magnesium ions are removed by purification to prepare saturated brine; the purified saturated brine is passed into an ion-exchange membrane electrolytic cell, and an electrolytic reaction occurs under the action of direct current. Chlorine gas is generated at the anode, and caustic soda solution and hydrogen gas are generated at the cathode; the caustic soda solution is evaporated and concentrated, and solid-liquid separation is performed to obtain 32% concentration liquid caustic soda or 99% solid caustic soda product.

5. The concentrated water co-production process based on chlorine cycle and membrane-salt field synergy according to claim 1, characterized in that, The acetylene polymerization process is adopted: chlorine gas, a byproduct of electrolysis, and acetylene are used as raw materials. An addition reaction occurs under the action of mercuric chloride catalyst to generate vinyl chloride monomer (VCM). After VCM is purified to remove impurities, it undergoes suspension polymerization under the action of an initiator to generate polyvinyl chloride (PVC) resin. After centrifugation, dehydration, drying, and sieving, a PVC product with uniform particle size is obtained. The exhaust gas generated during the polymerization process is treated to meet emission standards.

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