Chlor-alkali chemical reverse osmosis concentrated water purification system and method

Abstract

The application provides a chlor-alkali chemical reverse osmosis concentrated water purification system and method, the purification system comprises: a filter tank, which is internally provided with acid oxidation modification, alkali modification or metal modification activated carbon for adsorbing organic matters in the concentrated water; an oxidation reaction tank connected with the filter tank, which is internally provided with sodium hypochlorite solution for oxidizing ammonia nitrogen in the concentrated water and decomposing organic matters; a reduction reaction tank connected with the oxidation reaction tank, which is internally provided with sodium thiosulfate solution for removing residual chlorine in the concentrated water; a precipitation reaction tank connected with the reduction reaction tank, which is internally provided with barium chloride solution for removing sulfate ions in the concentrated water; and a cation exchange column connected with the precipitation reaction tank, which is internally provided with strong acid cation exchange resin or chelating resin for adsorbing impurity cations in the concentrated water. The chlor-alkali chemical reverse osmosis concentrated water purification system and method can deeply remove organic matters, total nitrogen, sulfate ions and multivalent cations, and has low operation cost and no secondary pollution.

Description

Technical Field

[0001] This application relates to the field of wastewater treatment technology, and in particular to a purification system and method for reverse osmosis concentrate from chlor-alkali chemical plants. Background Technology

[0002] The chlor-alkali chemical industry generates large quantities of wastewater with high salinity, high hardness, and containing organic and nitrogenous pollutants during the production of products such as caustic soda and polyvinyl chloride. Reverse osmosis (RO) membrane technology is used to concentrate and reuse this wastewater. However, the RO process produces concentrated wastewater comprising 20% ​​to 30% of the raw water volume. Not only is sodium chloride highly concentrated (typically reaching 60 g / L to 100 g / L), but organic matter (characterized by TOC, total organic carbon) and total nitrogen (mainly ammonia nitrogen, containing small amounts of organic nitrogen) are also simultaneously enriched to high levels (TOC can reach 30 ppm to 100 ppm, and total nitrogen can reach 20 ppm to 60 ppm). Direct discharge of this concentrated wastewater would cause a severe environmental burden; further evaporation and crystallization of the concentrated wastewater would be extremely energy-intensive, and the crystallized salt would be difficult to utilize due to impurities.

[0003] Conventional technologies for treating reverse osmosis concentrate include advanced oxidation processes, wet oxidation or supercritical water oxidation, biological denitrification, and breakpoint chlorination. Advanced oxidation processes (such as Fenton oxidation and ozone catalytic oxidation) require highly acidic conditions, consume large amounts of reagents, generate secondary pollution such as iron sludge, and have limited efficiency in removing organic matter from high-salt substrates, resulting in TOC concentrations exceeding 10 ppm after treatment. Wet oxidation processes require operation at high temperatures and pressures of 100°C to 180°C, leading to high equipment costs, high energy consumption, and high safety risks. Biological denitrification methods, such as denitrification filters, suffer from severely inhibited microbial activity in high-salt, high-chloride environments, requiring the addition of salt-tolerant bacteria and carbon sources, resulting in poor operational stability. While breakpoint chlorination using sodium hypochlorite oxidation can remove nitrogen and carbon, it requires highly acidic conditions (H+). + The process, conducted at concentrations >0.5 mol / L, resulted in severe equipment corrosion, and the high TOC content in the wastewater interfered with sodium hypochlorite oxidation. Furthermore, it failed to address the removal of sulfate and impurity cations. Therefore, a more effective reverse osmosis concentrate purification method is urgently needed. Summary of the Invention

[0004] In view of this, the purpose of this application is to propose a purification system and method for reverse osmosis concentrate in chlor-alkali chemical industry to solve or partially solve the above-mentioned technical problems.

[0005] Based on the above objectives, the first aspect of this application provides a purification system for reverse osmosis concentrate in chlor-alkali chemical industry, comprising: a filter tank containing acid-oxidized, alkali-modified, or metal-modified activated carbon for adsorbing organic matter in the concentrate; an oxidation reaction tank connected to the filter tank containing a sodium hypochlorite solution for oxidizing ammonia nitrogen in the concentrate and decomposing organic matter; a reduction reaction tank connected to the oxidation reaction tank containing a sodium thiosulfate solution for removing residual chlorine from the concentrate; a precipitation reaction tank connected to the reduction reaction tank containing a barium chloride solution for removing sulfate ions from the concentrate; and a cation exchange column connected to the precipitation reaction tank containing a strongly acidic cation exchange resin or chelating resin for adsorbing impurity cations in the concentrate.

[0006] Further, the activated carbon includes nitric acid-modified activated carbon, sulfuric acid-modified activated carbon, sodium hydroxide-modified activated carbon, potassium hydroxide-modified activated carbon, iron-supported modified activated carbon, copper-supported modified activated carbon, manganese-supported modified activated carbon, and nitric acid-iron-supported modified activated carbon; the specific surface area of ​​the activated carbon is 800 m² / g to 1200 m² / g, the height of the activated carbon is 1.5 m to 2.5 m, and the filtration rate of the filter pool is 5 m / h to 15 m / h.

[0007] Furthermore, the molar ratio of sodium hypochlorite in the sodium hypochlorite solution to total nitrogen in the concentrated water is 15:1 to 25:1, the pH value of the liquid in the oxidation reaction tank is 7 to 8, the reaction temperature of the oxidation reaction tank is 20°C to 35°C, and the reaction time is 30 min to 90 min.

[0008] Furthermore, the molar ratio of sodium thiosulfate in the sodium thiosulfate solution to the total residual chlorine in the concentrated water is 1.1:1 to 1.5:1, and the reaction time in the reduction reaction tank is 5 min to 15 min.

[0009] Furthermore, the molar ratio of barium chloride in the barium chloride solution to sulfate ions in the concentrated water is 1:1 to 1.05:1.

[0010] Furthermore, the strongly acidic cation exchange resin includes a 001×7 type resin, the chelating resin includes an aminophosphonic acid type resin, the flow rate of the cation exchange column is 10 BV / h to 20 BV / h, and the exchange capacity is 1.0 eq / L to 1.5 eq / L.

[0011] A second aspect of this application provides a method for purifying concentrated water from reverse osmosis in chlor-alkali chemical plants, using the purification system for concentrated water from reverse osmosis in chlor-alkali chemical plants as described in the first aspect above. The purification method includes: adsorbing organic matter in the concentrated water through the filter tank; oxidizing ammonia nitrogen in the concentrated water and decomposing organic matter through the oxidation reaction tank; removing residual chlorine from the concentrated water through the reduction reaction tank; removing sulfate ions from the concentrated water through the precipitation reaction tank; and adsorbing impurity cations in the concentrated water through the cation exchange column to obtain purified water.

[0012] Further, the modification method of the activated carbon includes: immersing granular activated carbon in a nitric acid solution or sulfuric acid solution with a mass fraction of 10% to 30%, stirring in a constant temperature water bath at 60°C to 80°C for 2 to 4 hours, then washing with deionized water until neutral, and drying at 105°C to constant weight to obtain the first activated carbon; immersing the first activated carbon in a transition metal salt solution of 0.1 mol / L to 0.5 mol / L for 12 to 24 hours, filtering, drying at 105°C, and then calcining at 300°C to 500°C under nitrogen protection for 2 to 4 hours to obtain the modified activated carbon.

[0013] Furthermore, the concentration of sodium chloride in the purified water is 60 g / L to 100 g / L, the total nitrogen concentration is less than 1 ppm, the total organic carbon concentration is less than 7 ppm, the residual chlorine concentration is less than 0.1 ppm, the sulfate ion concentration is less than 10 ppm, and the total hardness is less than 1 ppm.

[0014] Furthermore, the purification method further includes: transporting the purified water to the salting unit of the ion-exchange membrane caustic soda system.

[0015] As can be seen from the above description, the purification system and method for reverse osmosis concentrate in chlor-alkali chemical industry provided in this application include: a filter tank containing acid-oxidized, alkali-modified, or metal-modified activated carbon for adsorbing organic matter in the concentrate; an oxidation reaction tank connected to the filter tank containing sodium hypochlorite solution for oxidizing ammonia nitrogen in the concentrate and decomposing organic matter; a reduction reaction tank connected to the oxidation reaction tank containing sodium thiosulfate solution for removing residual chlorine in the concentrate; a precipitation reaction tank connected to the reduction reaction tank containing barium chloride solution for removing sulfate ions in the concentrate; and a cation exchange column connected to the precipitation reaction tank containing strong acid cation exchange resin or chelating resin for adsorbing impurity cations in the concentrate. Testing showed that the purified water from this system had a sodium chloride concentration of 60 g / L to 100 g / L, a total nitrogen concentration of less than 1 ppm, a total organic carbon concentration of less than 7 ppm, a residual chlorine concentration of less than 0.1 ppm, a sulfate ion concentration of less than 10 ppm, and a total hardness of less than 1 ppm. This system effectively addresses the problems of existing technologies, such as harsh treatment conditions, high residual concentrations of total nitrogen and total organic carbon, significant decrease in activated carbon adsorption efficiency under high salinity, ineffective removal of sulfate and residual chlorine, and high treatment costs. This purification system and method for chlor-alkali chemical reverse osmosis concentrate is simple and convenient, can operate under mild conditions, and can deeply remove organic matter, total nitrogen, sulfate ions, and polyvalent cations. It also boasts low operating costs and no secondary pollution. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in this application or related technologies, the drawings used in the description of the embodiments or related technologies will be briefly introduced below. Obviously, the drawings described below are only embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 This is a schematic diagram of the purification system for reverse osmosis concentrate in chlor-alkali chemical industry, as described in this application. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this disclosure clearer, the following detailed description is provided in conjunction with specific embodiments.

[0019] It should be noted that, unless otherwise defined, the technical terms used in the following embodiments have the same meaning as commonly understood by those skilled in the art. Unless otherwise specified, the experimental reagents used in the following embodiments are conventional biochemical reagents; and the experimental methods described are conventional methods.

[0020] The following describes specific embodiments in conjunction with... Figure 1The technical solution of this application will be described in detail below.

[0021] In some embodiments of this application, a purification system for reverse osmosis concentrate from chlor-alkali chemical plants is provided, such as... Figure 1 As shown, it includes: a filter tank containing acid-oxidized, alkali-modified, or metal-modified activated carbon for adsorbing organic matter in concentrated water; an oxidation reaction tank connected to the filter tank containing sodium hypochlorite solution for oxidizing ammonia nitrogen in the concentrated water and decomposing organic matter; a reduction reaction tank connected to the oxidation reaction tank containing sodium thiosulfate solution for removing residual chlorine from the concentrated water; a precipitation reaction tank connected to the reduction reaction tank containing barium chloride solution for removing sulfate ions from the concentrated water; and a cation exchange column connected to the precipitation reaction tank containing a strongly acidic cation exchange resin or chelating resin for adsorbing impurity cations in the concentrated water.

[0022] In response to the characteristics of high salt concentration (NaCl concentration of 60 g / L to 100 g / L) and high ionic strength of RO concentrate in chlor-alkali chemical industry, the adsorption capacity of conventional activated carbon for organic matter decreases significantly (by 30% to 50%) under high salt conditions. This is because salt ions compete for adsorption sites and block the pores.

[0023] To address this issue, this embodiment employs modified activated carbon, including nitric acid-modified activated carbon, sulfuric acid-modified activated carbon, sodium hydroxide-modified activated carbon, potassium hydroxide-modified activated carbon, iron-supported modified activated carbon, copper-supported modified activated carbon, manganese-supported modified activated carbon, and nitric acid-iron-supported modified activated carbon. The modified activated carbon has a specific surface area of ​​800 m² / g to 1200 m² / g, with significantly enhanced surface polarity. In chlor-alkali RO concentrate, the dynamic adsorption breakthrough time of the modified activated carbon is 1.5 to 2.5 times longer than that of unmodified carbon, and the TOC removal rate remains stable at 80% to 90%.

[0024] The height of activated carbon in the filter tank is 1.5m to 2.5m, for example, 1.5m, 2.0m, 2.5m, etc., with no specific limit. The filtration rate of the filter tank is 5m / h to 15m / h, for example, 5m / h, 10m / h, 15m / h, etc., with no specific limit. This step is used for efficient adsorption and removal of large molecular organic matter and colloids. The TOC removal rate of the effluent is no less than 80% compared to the influent (unmodified activated carbon is usually only 50% to 60%), which can significantly reduce the load on subsequent sodium hypochlorite oxidation and avoid interference of organic matter with the breakpoint chlorination reaction.

[0025] The effluent after activated carbon filtration enters the oxidation reaction tank. Sodium hypochlorite (NaClO) solution is added to the oxidation reaction tank, with the dosage based on available chlorine and a molar ratio of 15:1 to 25:1 to total nitrogen in the water, preferably 20:1 to 22.5:1. Under acidic conditions (pH 4 to 7), the oxidizing power of sodium hypochlorite increases as the pH decreases (at this point, the active chlorine is mainly HOCl), but at the cost of significantly reduced stability (HOCl decomposes, releasing chlorine gas), resulting in poor safety. Under alkaline conditions (pH 7 to 12), as the pH increases, sodium hypochlorite mainly reacts as OCl. - It exists in a form with relatively weak oxidizing power, but its solution stability is good and it does not easily decompose spontaneously. By controlling the pH value of the oxidation reaction tank to 7 to 8 (which can be adjusted with dilute hydrochloric acid or dilute sodium hydroxide), the reaction between HOCl and OCl can be made more stable. - It can coexist, balancing oxidizing and stability (low chemical consumption, good safety), and has the best processing capacity.

[0026] The oxidation reaction tank operates at a temperature of 20℃ to 35℃ for 30 to 90 minutes. An online ORP (oxidation-reduction potential) monitor can be installed during the reaction process to control the NaClO dosage in real time based on the ORP value (controlled within a range of 600mV to 800mV). Under these conditions, sodium hypochlorite oxidizes ammonia nitrogen into nitrogen gas and simultaneously oxidizes and decomposes some recalcitrant organic matter into carbon dioxide and water. The total nitrogen concentration in the effluent is below 1 ppm (ammonia nitrogen concentration below 0.4 ppm), and the TOC concentration is below 7 ppm.

[0027] After the oxidation reaction, residual chlorine remains in the effluent, which can severely damage the ion exchange resin and the ion-exchange membrane in the caustic soda system. Sodium thiosulfate (Na₂S₂O₃) solution can be added to the effluent, with the dosage calculated based on a Na₂S₂O₃:residual chlorine molar ratio of 1.1:1 to 1.5:1. The reaction time is 5 to 15 minutes to completely reduce the residual chlorine in the water to chloride ions, ensuring that the residual chlorine concentration in the effluent is below 0.1 ppm. This step, together with subsequent barium chloride precipitation and cation exchange, helps protect the ion-exchange membrane.

[0028] Water that has undergone dechlorination still contains a certain concentration of sulfate ions (SO42-). 2-High concentrations of sulfate ions can cause sodium sulfate to deposit on the membrane surface during electrolysis in an ion-exchange membrane caustic soda system, reducing current efficiency. Barium chloride (BaCl2) solution is slowly added to the water at a dosage of 1.0 to 1.05 times (preferably 1.01 to 1.03 times) the molar amount of sulfate ions in the concentrated water. The reaction produces barium sulfate (BaSO4) precipitate, which is removed by separation in a sedimentation tank or through filtration equipment (such as sand filtration or microfiltration). After treatment, the sulfate ion concentration in the water can be reduced to below 10 ppm. The collected barium sulfate precipitate, after washing and drying, can be used as a raw material for barium salt production or as a filler in paints, achieving resource recycling.

[0029] Even after the above treatment, the water still contains trace amounts of calcium (Ca). 2+ ), magnesium (Mg) 2+ ), iron (Fe) 3+ Impurity cations such as hydrogen ions (H+) are removed. The effluent is passed through an exchange column loaded with a strongly acidic cation exchange resin (such as 001×7 type resin) or a chelating resin (such as aminophosphonic acid type resin), with the flow rate controlled at 10 BV / h to 20 BV / h and the working exchange capacity approximately 1.0 eq / L to 1.5 eq / L. The resin adsorbs the aforementioned impurity cations and exchanges them for hydrogen ions (H+). + The total hardness of the effluent (calculated as CaCO3) is less than 1 ppm. After the resin is saturated, it can be regenerated with 5% to 8% hydrochloric acid. The regeneration waste liquid can be neutralized with other alkaline wastewater in the plant area (such as acetylene purification wastewater) and then reused.

[0030] Through a five-stage synergistic purification process, the total nitrogen in the RO concentrate is reduced to below 1 ppm, TOC to below 7 ppm, sulfate to below 10 ppm, total hardness to below 1 ppm, and residual chlorine to below 0.1 ppm. All indicators meet or exceed the standards for ion-exchange membrane caustic soda feed brine (typically requiring TOC < 10 ppm, total nitrogen < 5 ppm, sulfate < 20 ppm, and total hardness < 2 ppm). The purified water obtained after this system meets the stringent requirements of ion-exchange membrane caustic soda treatment, achieving a leap from wastewater to high-end raw material. This truly realizes a closed-loop resource recovery system, transforming traditionally difficult-to-treat high-salt organic wastewater into a high-purity sodium chloride solution that can be directly fed into the ion-exchange membrane caustic soda system. Simultaneously, the byproduct barium sulfate precipitate can be used as an industrial raw material, achieving dual recovery of water resources and salt. Compared to evaporation and crystallization processes, the cost per ton of water treated is reduced by 40% to 60%, and no impurity salt solid waste is generated.

[0031] Furthermore, this purification system employs multi-stage synergy, with each unit playing an irreplaceable role. Through a specific sequence of "activated carbon pre-adsorption - NaClO main oxidation - reduction dechlorination - sulfate removal via precipitation - resin purification," it achieves a synergistic effect between physical adsorption and chemical oxidation (activated carbon removes macromolecular organic matter and free chlorine, reducing the oxidation load and lowering the NaClO dosage by 20% to 30%). Compared to a purification sequence that prioritizes oxidation followed by adsorption, this system not only effectively protects activated carbon from excessive oxidation by residual chlorine but also reduces the dechlorination burden. The reduction dechlorination stage effectively protects subsequent ion exchange resins and ion-exchange membranes, demonstrating the integrity and irreversible sequence of the process chain.

[0032] This embodiment utilizes surface-modified activated carbon, which significantly enhances its adsorption selectivity and capacity in high-concentration NaCl solutions through acid oxidation, alkali treatment, or metal loading. The modified activated carbon improves the TOC removal rate in RO concentrate by 20% to 30% compared to unmodified activated carbon, effectively overcoming the high-salt inhibition effect. It also extends the service life and regeneration cycle of the activated carbon, reducing replacement frequency and operating costs.

[0033] This chlor-alkali chemical reverse osmosis concentrate purification system boasts a high degree of automation and stable operation. The NaClO dosage can be precisely controlled via online ORP monitoring, preventing overdosing that could lead to reagent waste and increased burden on subsequent dechlorination. The activated carbon filter can be designed with two parallel tanks for online regeneration and continuous operation. The process is simple to operate and easily automates. Operating under mild conditions significantly reduces energy consumption and costs. All units operate at ambient temperature, pressure, and near-neutral conditions, eliminating the need for high-temperature, high-pressure, or strongly acidic environments, thus substantially reducing equipment investment and maintenance costs.

[0034] In some embodiments of this application, a method for purifying concentrated water from reverse osmosis in chlor-alkali chemical industry is provided. This method uses the purification system for concentrated water from reverse osmosis in chlor-alkali chemical industry as described in any of the above embodiments. The purification method includes: adsorbing organic matter in the concentrated water through the filter tank; oxidizing ammonia nitrogen in the concentrated water and decomposing organic matter through the oxidation reaction tank; removing residual chlorine from the concentrated water through the reduction reaction tank; removing sulfate ions from the concentrated water through the precipitation reaction tank; and adsorbing impurity cations in the concentrated water through the cation exchange column to obtain purified water.

[0035] This purification method achieves extremely high treatment depth, meeting the stringent requirements of ion-exchange membrane caustic soda treatment. Through five-stage synergistic purification, it reduces total nitrogen in RO concentrate to below 1 ppm, TOC to below 7 ppm, sulfate to below 10 ppm, total hardness to below 1 ppm, and residual chlorine to below 0.1 ppm. All indicators meet or exceed the standards for ion-exchange membrane caustic soda feed brine (typically requiring TOC < 10 ppm, total nitrogen < 5 ppm, sulfate < 20 ppm, and total hardness < 2 ppm), achieving a leap from wastewater to high-end feedstock. It solves the technical problems of existing technologies, such as stringent treatment conditions, high residual concentrations of total nitrogen and total organic carbon, significant decrease in activated carbon adsorption efficiency under high salinity, ineffective removal of sulfate and residual chlorine, effluent not meeting the feed requirements of ion-exchange membrane caustic soda systems, and high treatment costs. This purification method for chlor-alkali chemical reverse osmosis concentrate is simple and convenient, can operate under mild conditions, can deeply remove organic matter, total nitrogen, sulfate ions, and polyvalent cations, and has low operating costs and no secondary pollution.

[0036] In some embodiments, the method for modifying activated carbon is selected from any one or a combination of the following:

[0037] Acid oxidation modification: Granular activated carbon (iodine value ≥800 mg / g) was impregnated in a 10% to 30% (w / w) nitric acid or sulfuric acid solution, stirred in a constant temperature water bath at 60℃ to 80℃ for 2 to 4 hours, then washed with deionized water until neutral, and dried at 105℃ to constant weight to obtain acid-oxidized activated carbon. Acid oxidation can introduce a large number of oxygen-containing functional groups (such as carboxyl groups, phenolic hydroxyl groups, and lactone groups) onto the surface of activated carbon, enhancing its hydrogen bonding and ion exchange adsorption capacity for polar organic compounds. Experiments show that the equilibrium adsorption capacity of activated carbon modified with nitric acid for TOC in RO concentrate is 35% to 50% higher than that of unmodified activated carbon under a high salt background (NaCl concentration 80 g / L).

[0038] Alkali modification: Activated carbon is immersed in a 5% to 15% sodium hydroxide or potassium hydroxide solution at room temperature for 6 to 12 hours. After washing until neutral, it is dried to obtain alkali-modified activated carbon. Alkali modification can remove ash from the surface of activated carbon, increase the specific surface area and pore volume, and introduce alkaline oxygen-containing functional groups to enhance the adsorption of weakly acidic organic matter.

[0039] Metal-supported modification: Activated carbon is impregnated in a 0.1 mol / L to 0.5 mol / L transition metal salt solution (such as ferric nitrate solution, copper nitrate solution, or manganese sulfate solution) for 12 to 24 hours. After filtration, it is dried at 105°C and then calcined at 300°C to 500°C for 2 to 4 hours under nitrogen protection to obtain metal-modified activated carbon. The supported metal oxides can serve as additional adsorption sites, adsorbing nitrogen-containing organic matter and polar organic matter through coordination. Iron or manganese oxides are preferred, as they can also catalyze the decomposition of some organic matter, forming a synergistic "adsorption-degradation" effect.

[0040] The modified activated carbon preparation process used in this embodiment is mature, and the reagents used (nitric acid, iron salt, etc.) are all commonly used industrial chemicals. Its adsorption capacity is increased by 35% to 50%, its service life is extended by 1.5 to 2.5 times, and it can reduce the subsequent consumption of sodium hypochlorite by 15% to 25%. The overall operating cost is reduced by 10% to 15% compared with the scheme using ordinary activated carbon.

[0041] In some embodiments, activated carbon can be jointly modified by first undergoing acid oxidation followed by metal loading, or by undergoing acid oxidation after alkali modification, achieving a synergistic effect of multiple mechanisms. Specifically, granular activated carbon is impregnated in a 10% to 30% (w / w) nitric acid or sulfuric acid solution, stirred in a constant-temperature water bath at 60°C to 80°C for 2 to 4 hours, then washed with deionized water until neutral, and dried at 105°C to constant weight to obtain first activated carbon; the first activated carbon is then impregnated in a 0.1 mol / L to 0.5 mol / L transition metal salt solution for 12 to 24 hours, filtered, dried at 105°C, and then calcined at 300°C to 500°C for 2 to 4 hours under nitrogen protection to obtain modified activated carbon. Testing has shown that it exhibits the best purification effect.

[0042] In some embodiments, the concentration of sodium chloride in the purified water is 60 g / L to 100 g / L, the total nitrogen concentration is less than 1 ppm, the total organic carbon concentration is less than 7 ppm, the residual chlorine concentration is less than 0.1 ppm, the sulfate ion concentration is less than 10 ppm, and the total hardness is less than 1 ppm.

[0043] The purified water is a high-purity sodium chloride solution, which can be directly fed into the salt treatment unit of the ion-exchange membrane caustic soda system, achieving complete resource recovery and reuse. It transforms traditionally difficult-to-treat high-salt organic wastewater into a high-purity sodium chloride solution that can directly enter the ion-exchange membrane caustic soda system, while simultaneously producing barium sulfate precipitate as a byproduct (which can be used as an industrial raw material), achieving dual recovery of water resources and salt. Compared to evaporation and crystallization processes, the cost per ton of water treated is reduced by 40% to 60%, and no impurity salt solid waste is generated.

[0044] Example 1 Preparation of modified activated carbon (using acid oxidation to modify activated carbon) Coconut shell granular activated carbon (particle size 2-3 mm, iodine value 950 mg / g) was washed with deionized water to remove surface dust and dried at 105℃ for 2 hours. 500 g of the dried activated carbon was weighed and immersed in a 20% nitric acid solution (liquid-to-solid volume ratio 3:1), and stirred in a 70℃ constant temperature water bath for 3 hours. After the reaction, the acid solution was discarded, and the activated carbon was repeatedly washed with deionized water until the pH of the washing solution was ≥5.0. The washed activated carbon was placed in an oven and dried at 105℃ to constant weight to obtain nitric acid-modified activated carbon. BET (specific surface area assay) testing showed that the specific surface area decreased slightly from 1050 m² / g before modification to 960 m² / g, but the surface carboxyl content increased from 0.12 mmol / g to 0.56 mmol / g.

[0045] RO concentrate purification A reverse osmosis (RO) concentrate from wastewater at a chlor-alkali chemical plant was taken. Its TOC concentration was 48.5 ppm, and its total nitrogen (TN) concentration was 31.2 ppm (of which ammonia nitrogen accounted for 95%). Cl... - The concentration is 88 g / L, SO4² - The concentration was 1.3 g / L, the total hardness (calculated as CaCO3) was 92 ppm, the residual chlorine concentration was 0 ppm, and the pH value was 7.6.

[0046] The modified activated carbon was loaded into a filter tank (glass adsorption column, 50 mm inner diameter, 2.0 m carbon layer height), and RO concentrate was passed through at a filtration rate of 10 m / h. Continuous operation was performed, with samples taken every 10 BV for testing. Results showed that the TOC concentration in the effluent stabilized between 8.5 ppm and 9 ppm within the first 20 BV of treatment (removal rate approximately 82%). The mixed effluent from the first 20 BV was then used in subsequent steps, with an average TOC concentration of 8.7 ppm and a total nitrogen concentration of 29.8 ppm. (Additionally, the breakthrough point (effluent TOC > 35 ppm) of this filter tank occurred at approximately 50 BV, while the control group using unmodified activated carbon experienced breakthrough at 25 BV.) The pH of the effluent was adjusted to 7.5, and then a sodium hypochlorite solution with an available chlorine content of 10% was added to the oxidation reaction tank. The molar ratio of available chlorine to total nitrogen was 17:1. The ORP value was controlled at 720 mV ± 20 mV using online ORP monitoring. The reaction was carried out at 25℃ with stirring for 60 min. The total nitrogen concentration in the effluent was measured to have decreased to 0.4 ppm, and the TOC concentration to have decreased to 4.8 ppm.

[0047] The residual chlorine concentration in the oxidized water was measured to be 52 ppm. Then, a 10% sodium thiosulfate solution was added to the reduction reaction tank, calculated based on a Na₂S₂O₃ to residual chlorine molar ratio of 1.2:1. After 10 minutes of reaction, the residual chlorine concentration was measured to be 0.03 ppm.

[0048] Slowly add a 10% barium chloride solution to the sedimentation tank of the above effluent. The dosage is equal to the original concentration of SO4²⁻ in the concentrated water. - 1.02 times the molar number. After stirring for 30 min, allow to stand and precipitate; filter the supernatant. Detect SO4²⁻. - The concentration was 5.2 ppm.

[0049] The filtrate was passed through a strongly acidic cation exchange column (001×7 type resin) at a flow rate of 15 BV / h. The total hardness (calculated as CaCO3) in the water was measured to be 0.6 ppm.

[0050] The final effluent concentration was 0.4 ppm for total nitrogen, 4.8 ppm for TOC, and SO4²⁻. - The concentration was 5.2 ppm, the total hardness was 0.6 ppm, and the residual chlorine concentration was 0.03 ppm. All indicators were better than those of Comparative Example 1, which did not use modified activated carbon, and the sodium hypochlorite consumption per ton of water was reduced by 12%.

[0051] Example 2 Preparation of modified activated carbon (using iron-supported modified activated carbon) Take 500g of the same unmodified activated carbon as in Example 1, and immerse it in a 0.2 mol / L ferric nitrate (Fe(NO3)3·9H2O) solution (liquid-to-solid volume ratio 2:1) for 24 hours with shaking at room temperature. Filter the solution and dry the activated carbon at 105℃ for 4 hours. Then, under nitrogen protection in a tube furnace, heat the carbon to 400℃ at a rate of 5℃ / min and calcine it at this temperature for 3 hours. Allow it to cool naturally to room temperature to obtain iron-loaded modified activated carbon (Fe2O3 loading approximately 3.5wt%).

[0052] RO concentrate treatment The same water quality and purification method as in Example 1 were used. The difference was that the filtration tank used the aforementioned iron-loaded modified activated carbon column (carbon layer height 2.0 m) with a filtration rate of 10 m / h. The results showed that the effluent TOC remained stable at 7.2 ppm to 8 ppm within 25 BV before treatment (average removal rate of approximately 84%), and the breakthrough point (effluent TOC > 35 ppm) appeared at approximately 55 BV, a further extension compared to acid-modified carbon (50 BV). In addition, the iron-loaded carbon exhibited a stronger adsorption affinity for nitrogen-containing organic matter, and the total nitrogen in the effluent decreased by approximately 15% compared to the influent (from 31.2 ppm to 26.5 ppm), providing favorable conditions for subsequent oxidative denitrification.

[0053] The final effluent concentration of total nitrogen was 0.3 ppm and the concentration of total organic carbon (TOC) was 4.2 ppm.

[0054] Example 3 Preparation of modified activated carbon (combined modification: nitric acid oxidation + iron support) First, the activated carbon was modified by nitric acid oxidation according to the method of Example 1, and then dried and calcined with iron load according to the method of Example 2 to obtain the combined modified activated carbon.

[0055] RO concentrate treatment The same water quality and purification method as in Example 1 were used. The difference was that the filter tank used the aforementioned combined modified activated carbon. The results showed that the TOC of the effluent remained stable at 5.5 ppm to 6 ppm within the first 30 BV (average removal rate of 88%), with a breakthrough point of approximately 65 BV.

[0056] The final effluent concentration of total nitrogen was 0.3 ppm and the concentration of total organic carbon (TOC) was 3.8 ppm.

[0057] Comparative Example 1 The same water quality and purification method as in Example 1 were used. The difference was that ordinary activated carbon was used in the filter tank without any modification. The results showed that the activated carbon broke through at 25 BV (effluent TOC > 35 ppm), and the average TOC of the mixed effluent after the first 20 BV was 18.2 ppm (removal rate of only 62.4%). In addition, the molar ratio of NaClO added to the oxidation reactor needed to be increased to 22:1 before the effluent TOC could barely be reduced to 6.9 ppm (close to the upper limit of 7 ppm), and the total nitrogen concentration was 0.6 ppm.

[0058] Although the final effluent quality barely met the standards, the sodium hypochlorite consumption increased by 25% compared to Example 1, and the activated carbon replacement frequency was 2-3 times that of modified carbon, resulting in a significant increase in overall operating costs. This comparative example demonstrates the necessity of activated carbon modification under high salinity conditions.

[0059] Comparative Example 2 The same water quality and purification method as in Example 1 were used. The difference was that activated carbon pre-adsorption was omitted, and NaClO oxidation was carried out directly; all other conditions were exactly the same as in Example 1. The results showed that the total nitrogen concentration in the effluent from the oxidation reactor was 0.7 ppm, but the TOC concentration was 22.3 ppm.

[0060] Analysis of the cause: Large organic molecules that were not removed by activated carbon adsorption consumed more sodium hypochlorite, leading to incomplete oxidation. Furthermore, the actual NaClO consumption increased by 28% compared to Example 1. This indicates that activated carbon pre-adsorption plays an irreplaceable role in ensuring TOC compliance and reducing oxidant consumption.

[0061] Comparative Example 3 The same water quality and purification method as in Example 1 were used. The difference was that the oxidation and adsorption order was changed; NaClO oxidation was performed first, followed by activated carbon filtration. All other conditions remained the same. The results showed that the total nitrogen concentration in the effluent after activated carbon filtration was 0.6 ppm, and the TOC concentration was 5.9 ppm. However, a large amount of residual chlorine was adsorbed on the surface of the activated carbon, shortening its service life by 30%. This indicates that the "adsorption-then-oxidation" order not only effectively protects the activated carbon from excessive oxidation by residual chlorine but also reduces the dechlorination burden.

[0062] Comparative Example 4 The same water quality and purification method as in Example 1 were used. The difference was that sodium thiosulfate reduction dechlorination was omitted, and the effluent from the oxidation reaction tank directly entered the sedimentation reaction tank. Results showed that the residual chlorine concentration in the effluent from the oxidation reaction tank was 52 ppm, and after passing through the sedimentation reaction tank and cation exchange column, the residual chlorine concentration still remained (approximately 45 ppm). When this water was fed into an ion-exchange membrane caustic soda system, after 48 hours of operation, the ion-exchange membrane showed significant performance degradation (tank voltage increased by 8%, current efficiency decreased by 5%). Disassembly and inspection revealed oxidative corrosion on the membrane surface. This indicates that reduction dechlorination is a necessary step for protecting the ion-exchange membrane and cannot be omitted.

[0063] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of this disclosure is limited to these examples; within the framework of this disclosure, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of different aspects of the embodiments of this disclosure as described above, which are not provided in detail for the sake of brevity.

[0064] This disclosure is intended to cover all such substitutions, modifications, and variations that fall within the broad scope of the appended claims. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this disclosure should be included within the scope of protection of this disclosure.

Claims

1. A purification system for concentrated water from reverse osmosis in chlor-alkali chemical industry, characterized in that, include: The filter tank is equipped with activated carbon that has been modified by acid oxidation, alkali, or metal to adsorb organic matter in the concentrated water. An oxidation reaction tank, connected to the filtration tank, is equipped with a sodium hypochlorite solution inside, used to oxidize ammonia nitrogen in the concentrated water and decompose organic matter; A reduction reaction tank, connected to the oxidation reaction tank, is provided with sodium thiosulfate solution to remove residual chlorine from the concentrated water; A precipitation reaction tank, connected to the reduction reaction tank, is provided with barium chloride solution inside to remove sulfate ions from the concentrated water; A cation exchange column, connected to the precipitation reaction tank, is provided with a strong acidic cation exchange resin or chelating resin inside, for adsorbing impurity cations in the concentrated water.

2. The purification system for reverse osmosis concentrate in chlor-alkali chemical industry according to claim 1, characterized in that, The activated carbon includes nitric acid-modified activated carbon, sulfuric acid-modified activated carbon, sodium hydroxide-modified activated carbon, potassium hydroxide-modified activated carbon, iron-supported modified activated carbon, copper-supported modified activated carbon, manganese-supported modified activated carbon, and nitric acid-iron-supported modified activated carbon; the specific surface area of ​​the activated carbon is 800 m² / g to 1200 m² / g, the height of the activated carbon is 1.5 m to 2.5 m, and the filtration rate of the filter pool is 5 m / h to 15 m / h.

3. The purification system for reverse osmosis concentrate in chlor-alkali chemical industry according to claim 1, characterized in that, The molar ratio of sodium hypochlorite in the sodium hypochlorite solution to total nitrogen in the concentrated water is 15:1 to 25:1, the pH value of the liquid in the oxidation reaction tank is 7 to 8, the reaction temperature of the oxidation reaction tank is 20°C to 35°C, and the reaction time is 30 min to 90 min.

4. The purification system for reverse osmosis concentrate in chlor-alkali chemical industry according to claim 1, characterized in that, The molar ratio of sodium thiosulfate in the sodium thiosulfate solution to the total residual chlorine in the concentrated water is 1.1:1 to 1.5:1, and the reaction time in the reduction reaction tank is 5 min to 15 min.

5. The purification system for reverse osmosis concentrate in chlor-alkali chemical industry according to claim 1, characterized in that, The molar ratio of barium chloride in the barium chloride solution to sulfate ions in the concentrated water is 1:1 to 1.05:

1.

6. The purification system for reverse osmosis concentrate in chlor-alkali chemical industry according to claim 1, characterized in that, The strongly acidic cation exchange resin includes 001×7 type resin, the chelating resin includes aminophosphonic acid type resin, the flow rate of the cation exchange column is 10 BV / h to 20 BV / h, and the exchange capacity is 1.0 eq / L to 1.5 eq / L.

7. A method for purifying concentrated wastewater from reverse osmosis in chlor-alkali chemical industries, characterized in that, Using the purification system for chlor-alkali chemical reverse osmosis concentrate as described in any one of claims 1-6, the purification method includes: The filtration tank adsorbs organic matter from the concentrated water. The ammonia nitrogen in the concentrated water is oxidized and organic matter is decomposed through the oxidation reaction tank. The residual chlorine in the concentrated water is removed through the reduction reaction tank; Sulfate ions are removed from the concentrated water through the precipitation reaction tank; The water is purified by adsorbing impurity cations in the concentrated water using the cation exchange column.

8. The method for purifying concentrated wastewater from reverse osmosis in chlor-alkali chemical plants according to claim 7, characterized in that, The method for modifying the activated carbon includes: Granular activated carbon is impregnated in a nitric acid solution or sulfuric acid solution with a mass fraction of 10% to 30%, stirred in a constant temperature water bath at 60°C to 80°C for 2 to 4 hours, then washed with deionized water until neutral, and dried at 105°C to constant weight to obtain the first activated carbon. The first activated carbon was impregnated in a transition metal salt solution of 0.1 mol / L to 0.5 mol / L for 12 h to 24 h, filtered, dried at 105 °C, and then calcined at 300 °C to 500 °C for 2 h to 4 h under nitrogen protection to obtain modified activated carbon.

9. The purification method for reverse osmosis concentrate in chlor-alkali chemical industry according to claim 7, characterized in that, The purified water has a sodium chloride concentration of 60 g / L to 100 g / L, a total nitrogen concentration of less than 1 ppm, a total organic carbon concentration of less than 7 ppm, a residual chlorine concentration of less than 0.1 ppm, a sulfate ion concentration of less than 10 ppm, and a total hardness of less than 1 ppm.

10. The method for purifying concentrated wastewater from reverse osmosis in chlor-alkali chemical plants according to claim 7, characterized in that, Also includes: The purified water is then transported to the salting unit of the ion-exchange membrane caustic soda system.

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