PTA wastewater sodium bromide efficient recovery process

Abstract

The application discloses a PTA wastewater sodium bromide high-efficiency recovery process, and belongs to the technical field of terephthalic acid production wastewater treatment and bromine resource recycling, and comprises the following steps: pretreatment softening: providing a group of terephthalic acid production process generated bromine-containing wastewater, introducing the bromine-containing wastewater into a chelating resin unit at a volume space velocity, removing calcium and magnesium ions in the bromine-containing wastewater, obtaining softened water, strengthening a desalting step through re-filtration, effectively washing the intercepted bromine ions into the product water side by using the physical elution effect of dialysis water, overcoming the Donnan effect under high concentration, and converting the waste liquid from a complex carbonate, bromide and sulfate ternary system into a simple sulfate and bromide binary system through an acidification and decarburization step, and eliminating the interference of carbonate on a membrane system.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment and bromine resource recovery in the production of purified terephthalic acid, specifically a high-efficiency sodium bromide recovery process for PTA wastewater. Background Technology

[0002] In the production process of purified terephthalic acid, the treatment of bromine-containing waste gas usually adopts regenerative thermal oxidation or regenerative catalytic combustion incineration technology. After the tail gas produced by incineration is absorbed by alkaline solution, it will form a process wastewater with high bromide, high alkalinity and high salinity. This type of wastewater not only contains high concentrations of bromide ions, but also enriches a large amount of bicarbonate, carbonate and calcium and magnesium hardness ions.

[0003] Currently, the treatment of such high-salinity wastewater mainly focuses on achieving discharge standards or simple physicochemical degradation. When attempting to recover sodium bromide from it, existing technologies generally face the following technical challenges: the complex ionic composition of the wastewater poses a significant challenge to efficient salt separation; since carbonates, sulfates, and bromides coexist in the system, carbonate ions in a high pH environment can easily alter the charge distribution characteristics of the separation membrane surface during purification using membrane separation technology; this change in charge environment will enhance the non-specific retention of monovalent anions by the membrane surface, leading to a significant decline in salt separation efficiency.

[0004] Inorganic scaling risks severely restrict the operational stability of the system. Although there are softening pretreatment methods in the existing technology, under high recovery rate conditions, the trace hardness ions and high concentrations of carbonates remaining in the wastewater are very likely to form microcrystalline scale such as calcium carbonate inside the pores of the separation membrane or on the heat exchange surface of the evaporator. This scaling phenomenon not only causes a significant and irreversible decline in membrane flux in a short period of time, but also increases the operating pressure drop of the system and may even cause physical damage to the membrane elements.

[0005] Furthermore, in the final product purification stage, due to the lack of effective stripping of complex multi-component salt systems, traditional direct evaporation processes often result in the co-precipitation of various inorganic salts. This leads to the production of sodium bromide solid containing a large amount of sulfate and carbonate impurities, with product purity far below the standards for industrial recycling. It can only be treated as a low-value mixed salt and cannot achieve closed-loop high-value utilization of bromine resources. Therefore, how to achieve high recovery rate and high purity extraction of bromine from PTA wastewater while ensuring long-term stable operation of the system is a pressing problem to be solved in this field.

[0006] The information disclosed in the background section above is only intended to enhance the understanding of the background of this disclosure, and therefore may include information that does not constitute prior art known to those skilled in the art. Summary of the Invention

[0007] The purpose of this invention is to provide a high-efficiency sodium bromide recovery process for PTA wastewater to solve the problems mentioned in the background art.

[0008] The technical solution of the present invention includes the following steps:

[0009] Step S1, Pretreatment Softening: Provide a stream of bromine-containing wastewater generated during the production of purified terephthalic acid, and treat the bromine-containing wastewater with... The volume hourly space velocity is introduced into the chelating resin unit to remove calcium and magnesium ions from the bromine-containing wastewater, thereby obtaining softened water.

[0010] Step S2, Carbon Conversion: The softened water is acidified and decarbonized under normal temperature and pressure by adding sulfuric acid to adjust the pH value to [value missing]. The bicarbonate and carbonate ions are converted into sulfate and free carbon dioxide, and the generated carbon dioxide is removed by blowing air to obtain a decarbonated solution.

[0011] Step S3, Enhanced Salt Separation by Re-filtration: The decarbonated solution is fed into a filter with a molecular weight cutoff of [missing value]. nanofiltration systems, in Membrane concentration and separation are performed under operating pressure. Dialysis water is added to the nanofiltration concentrate in the nanofiltration system for refiltration and washing to obtain nanofiltration permeate enriched with sodium bromide and nanofiltration concentrate enriched with sodium sulfate, respectively.

[0012] Step S4, Evaporation and Purification: The nanofiltration permeate is sequentially subjected to reverse osmosis concentration treatment and evaporation crystallization treatment to obtain high-purity sodium bromide solid.

[0013] Preferably, the bromine-containing wastewater is high-bromine, high-alkalinity, and high-salinity wastewater generated from bromine-containing waste gas produced in PTA production after incineration by RTO or RCO and absorption by alkaline solution;

[0014] The pH value of the bromine-containing wastewater is The concentration of bromide ions was The concentration of bicarbonate is The carbonate concentration is .

[0015] Preferably, in step S3, the refiltration and rinsing operation separates the residual bromide ions in the nanofiltration concentrate to the nanofiltration permeate side, controls the dialysis water replenishment rate to be 0.5-1.5 times the nanofiltration concentrate discharge rate, and controls the mass percentage solid content of the nanofiltration concentrate to be 15%-25%.

[0016] The dialysis water is either pure water or reverse osmosis permeate produced by the reverse osmosis concentration treatment in step S4.

[0017] Preferably, in step S3, after the refiltration and washing operation, the overall bromine recovery rate in the nanofiltration permeate is controlled at over 95%, while the mass fraction of sulfate in the nanofiltration permeate is controlled at below 0.1%.

[0018] Preferably, in step S4, the evaporation crystallization process uses an MVR or MEE evaporation crystallization system.

[0019] Preferably, after step S4, a closed-loop circulation system for the nanofiltration concentrate is further included, specifically comprising:

[0020] Step S5, Freeze-crystallization treatment: The nanofiltration concentrate is introduced into a freezing system, and the freezing temperature is controlled within the range of -5℃ to 5℃ to freeze-crystallize sodium sulfate decahydrate, i.e., Glauber's salt.

[0021] Step S6, Bipolar Membrane Electrodialysis Treatment: The precipitated Glauber's salt is dissolved back to prepare a solution with a mass concentration of 15%-25%, and then introduced into the solution with a current density of... The bipolar membrane electrodialysis system dissociates water to generate regenerated sulfuric acid and regenerated caustic soda.

[0022] Preferably, the regenerated sulfuric acid is refluxed to step S2 to neutralize bicarbonate ions, thereby achieving acid self-sufficiency.

[0023] The regenerated caustic soda is used to absorb the carbon dioxide released during acidification and stripping in step S2, generating sodium carbonate, which is then returned to the PTA main unit as tail gas absorbent, thus achieving self-sufficiency in alkali.

[0024] Preferably, the sodium bromide solid recovered by the process has a purity greater than 98.5%.

[0025] The bromine recovery rate of the entire recovery process system is greater than 96%.

[0026] This invention provides an improved process for the efficient recovery of sodium bromide from PTA wastewater, which has the following improvements and advantages compared with the prior art:

[0027] 1. By strengthening the salt separation step through heavy filtration and utilizing the physical washing effect of dialysis water, the retained bromide ions are effectively washed into the product water side, overcoming the Dornan effect under high concentration. Through the acidification and decarbonation step, the waste liquid is transformed from a complex ternary system of carbonate, bromide and sulfate into a simple binary system of sulfate and bromide, eliminating the interference of carbonate ions on the membrane system.

[0028] 2. This scheme combines freeze crystallization with bipolar membrane electrodialysis to convert waste sodium sulfate into regenerated sulfuric acid and regenerated caustic soda. The regenerated sulfuric acid is recycled to the carbon conversion step to adjust the pH value, achieving acid self-sufficiency. The regenerated caustic soda is used to absorb carbon dioxide generated during acidification and generates sodium carbonate, which is then recycled to the PTA main unit as tail gas absorbent, thus constructing a complete circular ecological chain. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0030] Example 1:

[0031] This invention provides a high-efficiency sodium bromide recovery process for PTA wastewater, comprising the following steps: Step S1, pretreatment softening: providing a stream of bromine-containing wastewater generated during the production of purified terephthalic acid, and softening the bromine-containing wastewater by... The volume hourly space velocity is introduced into the chelating resin unit to remove calcium and magnesium ions from bromine-containing wastewater, resulting in softened water.

[0032] Bromine-containing wastewater is high-bromine, high-alkalinity, and high-salinity wastewater generated from bromine-containing waste gas produced in PTA production after incineration by RTO or RCO and absorption by alkaline solution; the pH value of bromine-containing wastewater is... The concentration of bromide ions was The concentration of bicarbonate is The carbonate concentration is ;

[0033] This embodiment, as the preferred implementation, provides a specific PTA-containing bromine wastewater with a pH of 8 and a bromide ion concentration of [missing information]. The concentration of bicarbonate is The carbonate concentration was 800 mg / L. During the pretreatment softening stage, the bromine-containing wastewater was treated with... The low flow rate introduces the chelating resin unit; the chelating resin is a chelating resin with high selectivity for divalent metal ions, preferably a macroporous chelating resin with iminodiacetic acid or aminophosphonate functional groups that has high complexing selectivity for divalent metal ions.

[0034] The chelating resin unit utilizes the specific complexing ability of its functional groups for divalent cations to deeply remove calcium and magnesium ions from wastewater. The low-flow-rate operation mode aims to extend the contact time of the resin bed, ensuring that hardness ions are removed to the ppb level, thereby preventing inorganic salt scaling in the subsequent membrane system under high recovery rate conditions. Online monitoring shows that... At a flow rate, the hardness of the effluent is stably controlled at [value missing]. Although the processing efficiency is relatively low, it provides the highest safety margin. Based on this, this embodiment further compares key indicators at different flow rates: if the flow rate is increased to... Although the pressure drop only increased slightly However, the hardness of the effluent fluctuated and increased to ;

[0035] If upgraded to The hardness of the effluent quickly exceeded the nanofiltration influent threshold. Data shows that in the pursuit of the ultimate removal rate, < Under the working conditions, It is a necessary condition to ensure that the resin exchange kinetics are fully carried out, and it is reasonable to sacrifice some flux in exchange for the highest removal depth.

[0036] Step S2, Carbon Conversion: The softened water is acidified and decarbonized under normal temperature and pressure by adding sulfuric acid to adjust the pH value. The bicarbonate and carbonate ions are converted into sulfate and free carbon dioxide, and the generated carbon dioxide is removed by blowing air to obtain a decarbonated solution.

[0037] In this embodiment, 98% concentrated sulfuric acid is precisely added to the softened water to adjust the pH to 3. The strong acidic environment disrupts the ionization equilibrium of carbonates, completely converting bicarbonate and carbonate ions into sulfate and free carbon dioxide, which are then removed by blower stripping. The gas-liquid ratio of the blower stripping is controlled at (20-50):1 to ensure a carbon removal efficiency greater than 99%. Specifically, the amount of sulfuric acid added is approximately 3.8 kg / m³ of wastewater, which is finely adjusted in real time according to the alkalinity of the influent to control the pH at 3. This ensures that the carbonate system is completely destroyed, transforming the wastewater system from a complex carbonate / bromine / sulfate ternary system into a simple sulfate / bromine binary system. This eliminates the Dornan effect interference of carbonates on subsequent nanofiltration salt separation, creating thermodynamically favorable conditions for high-purity separation.

[0038] Step S3, Enhanced Salt Separation by Re-filtration: The decarbonated solution is fed into a filter with a molecular weight cutoff of [missing value]. nanofiltration systems, in Membrane concentration and separation are performed under operating pressure. Dialysis water is added to the nanofiltration concentrate in the nanofiltration system for refiltration and washing to obtain nanofiltration permeate enriched with sodium bromide and nanofiltration concentrate enriched with sodium sulfate, respectively.

[0039] In step S3, the refiltration and washing operation separates the residual bromide ions in the nanofiltration concentrate to the nanofiltration permeate side, controls the dialysis water replenishment rate to be 0.5-1.5 times the nanofiltration concentrate discharge rate, and controls the mass percentage solid content of the nanofiltration concentrate to be 15%-25%; the dialysis water is pure water or reverse osmosis permeate produced by the reverse osmosis concentration treatment in step S4;

[0040] In this embodiment, a molecular weight cutoff of [molecular weight] was selected. nanofiltration membranes, in It operates under operating pressure; during the refiltration and washing operation, pure water is used for dialysis, and the dialysis water replenishment rate is strictly controlled to be 0.5 times the nanofiltration concentrate discharge rate, while the solid content of the nanofiltration concentrate is controlled at 15%; by adding dialysis water at a lower rate and controlling the solid content at a lower rate, the concentration polarization layer on the membrane surface can be destroyed by the concentration gradient with lower energy consumption, so that monovalent bromide ions, with a smaller radius, can more easily pass through the membrane pores into the permeate side, while divalent sulfate ions, with a larger radius and more negative charge, are efficiently retained;

[0041] After repeated filtration and washing, the overall bromine recovery rate in the nanofiltration permeate is controlled at over 95%, while the sulfate mass fraction in the nanofiltration permeate is controlled at below 0.1%.

[0042] Testing showed that the overall bromine recovery rate in the nanofiltration permeate in this embodiment reached 96.2%, and the sulfate mass fraction was only 0.08%, achieving efficient separation of bromine and sulfur.

[0043] Step S4, Evaporation and Purification: The nanofiltration permeate is successively subjected to reverse osmosis concentration treatment and evaporation crystallization treatment to obtain high-purity sodium bromide solid;

[0044] In step S4, the evaporation crystallization process uses an MVR or MEE evaporation crystallization system;

[0045] The sodium bromide solid obtained through the process has a purity greater than 98.5%; the bromine recovery rate of the entire recovery process system is greater than 96%.

[0046] In this embodiment, the evaporation crystallization process specifically adopts MVR, a mechanical vapor recompression evaporation crystallization system. Since the carbonate has been completely removed and the sulfate has been controlled at an extremely low level in the front-end steps S2 and S3, the evaporation process exhibits the characteristics of pure single-salt crystallization kinetics, avoiding the phenomenon of mixed salt co-precipitation, and finally obtaining high-purity sodium bromide solid with a purity of 98.8%.

[0047] Following step S4, a closed-loop circulation system for the nanofiltration concentrate is also included, specifically: Step S5, freeze crystallization treatment: the nanofiltration concentrate is introduced into a freezing system, and the freezing temperature is controlled within the range of -5℃ to 5℃ to precipitate sodium sulfate decahydrate, i.e., Glauber's salt; Step S6, bipolar membrane electrodialysis treatment: the precipitated Glauber's salt is back-dissolved to prepare a solution with a mass concentration of 15%-25%, and then introduced into a current density of... The bipolar membrane electrodialysis system adopts a three-chamber membrane stack structure, which consists of bipolar membranes, anion exchange membranes and cation exchange membranes arranged alternately to form acid chambers, alkali chambers and salt chambers, respectively, so that water dissociates to generate regenerated sulfuric acid and regenerated caustic soda.

[0048] The regenerated sulfuric acid is returned to step S2 to neutralize bicarbonate ions, achieving self-sufficiency in acid; the regenerated caustic soda is used to absorb the carbon dioxide stripped out by acidification in step S2 to generate sodium carbonate, which is then returned to the PTA main unit as tail gas absorbent, achieving self-sufficiency in alkali.

[0049] In this embodiment, the freezing temperature is controlled at -5℃ in step S5. The low temperature environment significantly reduces the solubility of sodium sulfate, promoting the efficient precipitation of Glauber's salt. In step S6, the precipitated Glauber's salt is dissolved back into a solution with a mass concentration of 15%, and then introduced into a current density of... The bipolar membrane electrodialysis system utilizes the water dissociation characteristics of the bipolar membrane interface layer to generate acids and bases in situ. The generated regenerated sulfuric acid is directly recycled for acidification and decarbonization in step S2, and the generated regenerated caustic soda is used to absorb carbon dioxide and is returned to the PTA main unit, thus constructing a closed-loop ecological chain of waste salt resource utilization and acid and base self-sufficiency.

[0050] Example 2:

[0051] A high-efficiency sodium bromide recovery process for PTA wastewater includes the following steps: Step S1, pretreatment softening, the pH value of the bromine-containing wastewater is 8-9, and the bromide ion concentration is... ;

[0052] In this embodiment, the pH value of the bromine-containing wastewater treated was 8.5, and the bromide ion concentration was [missing value]. The concentration of bicarbonate is The carbonate concentration is , bromine-containing wastewater The flow rate is introduced into the chelating resin unit. The medium flow rate balances the relationship between resin adsorption kinetics and bed pressure drop while ensuring treatment efficiency, and takes into account the utilization rate of resin adsorption capacity. This is to quantify the impact of flow rate on system performance.

[0053] In this embodiment, parameter sensitivity tests were conducted under the same influent water quality: when the flow rate was set to 5 BV / h, the bed pressure drop was... The resin adsorption capacity utilization rate was 78%, but a mass transfer dead zone occurred due to the excessively slow flow rate; when the flow rate was set to... At that time, the bed pressure drop rose sharply to Furthermore, insufficient contact time led to a reduction in resin adsorption capacity utilization to 65%, and premature hardness penetration in the effluent. In contrast, The bed pressure drop at the flow rate stabilizes at The resin adsorption capacity utilization rate reached a peak of 88%, and the hardness of the effluent was stable and met the standard, thus establishing this flow rate as the hydraulically optimal solution under this water quality condition.

[0054] Step S2, carbon conversion, adjust pH to 3-4;

[0055] In this embodiment, sulfuric acid is added to the softened water to adjust the pH value to 3.5. The pH value of 3.5 is set as the balance point between acid consumption and conversion efficiency, which can ensure the extent of decarbonization reaction, reduce the amount of alkali required for subsequent neutralization, and reduce the risk of corrosion of equipment materials by strong acid environment.

[0056] Step S3: Re-filtration to enhance salt separation;

[0057] In step S3, the refiltration and rinsing operation controls the dialysis water replenishment rate to be 0.5-1.5 times the nanofiltration concentrate discharge rate;

[0058] In this embodiment, the decarbonated solution is fed into a solution with a molecular weight cutoff of [missing value]. nanofiltration systems, in Separation under operating pressure; innovatively using reverse osmosis permeate produced by step S4 reverse osmosis concentration treatment, realizing water resource recycling within the system; controlling the dialysis water replenishment rate to be 1.0 times the nanofiltration concentrate discharge rate, and controlling the solid content of the nanofiltration concentrate to be 20%; the 1.0 times replenishment rate combined with the 20% solid content creates a stable dialysis gradient on the membrane surface; by enhancing solute diffusion, the retained bromide ions are effectively washed into the permeate side, overcoming the Dornan repulsion effect under high concentration;

[0059] Step S4: Evaporation and purification;

[0060] In step S4, the evaporation crystallization process uses an MVR or MEE evaporation crystallization system;

[0061] In this embodiment, the evaporation crystallization process adopts MEE, a multi-effect evaporation crystallization system, which utilizes multi-stage heat energy reuse to reduce steam consumption.

[0062] Step S5: Freeze-crystallization treatment; Step S6: Bipolar membrane electrodialysis treatment;

[0063] In step S5, the freezing temperature is controlled at 0℃. At this temperature, the Glauber's salt crystals are larger and have a uniform particle size distribution, making them easier to separate by centrifugation later. In step S6, the Glauber's salt is prepared into a 20% (w / w) solution. Bipolar membrane electrodialysis was performed at a suitable current density. The appropriate current density and solution concentration ensured the stability of the membrane stack voltage of the bipolar membrane system, achieving an optimal balance between energy consumption and acid / alkali production efficiency. This embodiment focuses on the comprehensive balance of process parameters, demonstrating excellent stability and economy.

[0064] Example 3:

[0065] A high-efficiency sodium bromide recovery process for PTA wastewater with a pH value of 8-9;

[0066] This embodiment is designed for high-load operating conditions, treating bromine-containing wastewater with a pH of 9 and a bromide ion concentration of [missing value]. The concentration of bicarbonate is The carbonate concentration is , bromine-containing wastewater The high flow rate introduced into the chelating resin unit is suitable for high-throughput treatment requirements in high-concentration wastewater treatment, preventing excessive accumulation of pollutants in the resin bed. At the specified flow rate, monitoring showed that the water hardness remained below 50 ppb, slightly higher than under low flow rate conditions, but still meeting the nanofiltration feed water requirements, and the treatment efficiency was improved by 3 times. For high-flux requirements, this embodiment compared the limiting flow rate data: At the flow rate, the bed pressure drop is controlled at It is within the safe range of resin mechanical strength;

[0067] If the flow rate is further increased to If the pressure drop exceeds the recommended range, the bed pressure drop will exceed [a certain value]. This leads to an exponential increase in resin breakage rate, and the hardness of the effluent cannot be stably controlled. The following; conversely, if adopted Flow rate and equipment processing capacity will become bottlenecks, therefore... It is to ensure the safety of physical pressure drop. And the maximum production flow rate under the premise of meeting basic softening requirements;

[0068] Step S2, carbon conversion, adjust pH to 3-4;

[0069] In this embodiment, the pH value is adjusted to 4 by adding 50% dilute sulfuric acid to prevent local over-acidity. The addition amount is about 21 kg / m³ of wastewater. The pH value is controlled at 4, which is the upper limit of acidification. This setting aims to save acid consumption to the maximum extent. At the same time, by utilizing the buffering capacity of the high concentration of carbonate ions, the conversion of carbonate to sulfate can still be maintained. This is suitable for scenarios where cost control is extremely sensitive.

[0070] Step S3: Re-filtration to enhance salt separation;

[0071] In step S3, a second filtration and washing operation is performed;

[0072] This embodiment uses a molecular weight cutoff of [missing information]. nanofiltration systems, in Separation is carried out under high operating pressure. Pure water is used for dialysis. The dialysis water replenishment rate is controlled at 1.5 times the nanofiltration concentrate discharge rate, and the solid content of the nanofiltration concentrate is controlled at 25%. The high operating pressure and 1.5 times dialysis water replenishment rate are to cope with the high osmotic pressure and severe concentration polarization caused by the high concentration matrix. Through strong physical washing and high shear force, bromide ions are forced to pass through the membrane pores to ensure the separation effect under the target of high recovery rate.

[0073] Step S4: Evaporation and purification; Step S5: Freeze-crystallization treatment; Step S6: Bipolar membrane electrodialysis treatment.

[0074] In step S5, the freezing temperature is controlled at 5°C. At a relatively high solid content of 25%, a freezing temperature of 5°C is sufficient to achieve good supersaturation, thereby precipitating sodium sulfate and effectively reducing freezing energy consumption. In step S6, the solution is prepared to a high concentration of 25%. Operating at high current density, high current density and high concentration solution are beneficial to improving the output efficiency per unit membrane area and adapting to the needs of large-scale, high-throughput processing. This embodiment demonstrates the process robustness under extreme conditions of high concentration and high throughput.

[0075] Example 4:

[0076] A high-efficiency sodium bromide recovery process for PTA wastewater with a pH value of 8-9;

[0077] In this embodiment, the pH value of the bromine-containing wastewater treated was 8.2, and the bromide ion concentration was [missing value]. The concentration of bicarbonate is The carbonate concentration is The pretreatment softening flow rate is set to .

[0078] Step S2, carbon conversion, adjust pH to ;

[0079] Adjusting the pH value to 3.2, the acidic environment of pH 3.2 can quickly and thoroughly complete the decarbonization reaction, reduce the residence time in the reaction vessel, and improve the reaction kinetic rate;

[0080] Step S3: Re-filtration to enhance salt separation;

[0081] In step S3, a second filtration and washing operation is performed;

[0082] This embodiment uses a molecular weight cutoff of [missing information]. nanofiltration systems, in Operating under pressure. The dialysis water is the reverse osmosis permeate produced in step S4; the dialysis water replenishment rate is controlled at 0.8 times the nanofiltration concentrate discharge rate, and the solid content of the nanofiltration concentrate is controlled at 18%; the 0.8 times washing rate combined with the 18% solid content is a refined optimization of water and energy consumption under the premise of ensuring bromine recovery rate, which can effectively control sulfate leakage and prevent divalent ion penetration due to over-dialysis;

[0083] Step S4: Evaporation and purification; Step S5: Freeze-crystallization treatment; Step S6: Bipolar membrane electrodialysis treatment.

[0084] In step S5, the freezing temperature is controlled at -2℃. In step S6, the concentration of Glauber's salt remelting is 18%, and the current density is... This embodiment verifies the operational stability of the process in a low-energy-consumption mode through fine-tuning of parameters, making it suitable for application scenarios with strict requirements for energy consumption indicators.

[0085] Example 5:

[0086] A high-efficiency sodium bromide recovery process for PTA wastewater, wherein the pH value of the bromine-containing wastewater is [value missing]. ;

[0087] In this embodiment, the pH value of the bromine-containing wastewater was 8.8, and the bromide ion concentration was [missing value]. The concentration of bicarbonate is The carbonate concentration is The pretreatment softening flow rate is set to Step S2: Carbon conversion, adjust pH to... ;

[0088] Adjust the pH value to 3.8. This pH value ensures the decarbonization effect while retaining a trace amount of buffering capacity, making the system more stable.

[0089] Step S3: Re-filtration to enhance salt separation;

[0090] In step S3, a second filtration and washing operation is performed;

[0091] This embodiment uses a molecular weight cutoff of [missing information]. nanofiltration systems, in Operating under pressure, the dialysis water is pure water, and the dialysis water replenishment rate is controlled at 1.2 times the nanofiltration concentrate discharge rate. The solid content of the nanofiltration concentrate is controlled at 22%. This parameter combination is designed for higher concentration feed water. By enhancing the dialysis intensity by 1.2 times, it overcomes the Donan effect obstacle at a high solid content of 22%, ensuring efficient permeation of bromide ions.

[0092] Step S4: Evaporation and purification; Step S5: Freeze-crystallization treatment; Step S6: Bipolar membrane electrodialysis treatment.

[0093] In step S5, the freezing temperature is controlled at 2℃. In step S6, the concentration of Glauber's salt remelting is 22%, and the current density is... This embodiment demonstrates the adaptability of the process in handling fluctuating water quality. By appropriately increasing the dialysis ratio and current density, it effectively addresses changes in the influent load.

[0094] Comparative Example 1:

[0095] This comparative example provides a PTA wastewater treatment process, the steps of which are basically the same as those in Example 2. The pretreatment step S1 is the same as in Example 2, but in step S2, the softened water is not acidified and decarbonized. Instead, the softened water is directly sent to step S3. The softened water is sent directly to the nanofiltration system in step S3 at its original pH of 8.5 for separation. Since no carbon conversion is performed, there are a large number of bicarbonate and carbonate ions in the wastewater, resulting in a high pH environment on the nanofiltration membrane surface. The retention characteristics of carbonate are significantly different from those of sulfate, and calcium carbonate microcrystals are easily formed in the membrane pores. Although softened, trace amounts still pose a risk, significantly affecting the salt separation effect and membrane flux. This aims to verify the necessity of the acidification and decarbonization step for the stability of the membrane system.

[0096] Comparative Example 2:

[0097] This comparative example provides a PTA wastewater treatment process, the steps of which are basically the same as those in Example 2, except that: in step S3, no re-filtration and washing operation is performed, that is, no dialysis water is added to the nanofiltration concentrate, but conventional single nanofiltration concentration is used, and the concentrate is directly discharged after being concentrated to a solid content of 20%; due to the lack of dilution and elution effect of dialysis water, a large number of bromide ions are retained in the high-concentration sodium sulfate concentrate, and due to the influence of concentration polarization, they cannot enter the product water side, which aims to verify the key role of the re-filtration and washing operation in the bromine recovery rate.

[0098] Comparative Example 3:

[0099] This comparative example provides a PTA wastewater treatment process, the steps of which are basically the same as those in Example 2, except that: in step S3, although a re-filtration and washing operation is performed, the dialysis water replenishment rate is only 0.2 times the discharge rate of the nanofiltration concentrate. The low dialysis water volume is insufficient to break the concentration polarization layer and cannot effectively replace the residual bromide ions from the concentrate. This aims to verify the critical value effect of the dialysis water replenishment rate parameter.

[0100] Comparative Example 4:

[0101] This comparative example provides a PTA wastewater treatment process, the steps of which are basically the same as those in Example 2, except that steps S2 and S3 are omitted, and the wastewater softened in step S1 is directly sent to step S4 for evaporation and crystallization. Since the wastewater contains a large amount of carbonates, bicarbonates and sulfates, mixed salts are generated during the evaporation process, and the product quality is extremely poor due to the lack of membrane separation purification. This aims to verify the core position of membrane separation salt separation process in the entire recycling process.

[0102] To verify the criticality of the pH control range (3-4) in the acidification and decarbonization process of this invention and the specific differences in the effects of different pH values ​​within the range, an extended verification experiment was set up. The experiment was divided into 6 groups, and the treatment object was the softened water of Example 1. The only difference was the endpoint pH value adjusted in step S2.

[0103] Experimental group setup:

[0104] Outside the boundary control group A: pH=2.0, overly acidic;

[0105] Control group B outside the boundary: pH=5.0, insufficient acidification;

[0106] Control group C outside the boundary, i.e., Comparative Example 1: pH=8.5, not acidified;

[0107] Example Optimization Group D: pH=3.0;

[0108] Example Optimization Group E: pH=3.5; Example Optimization Group F: pH=4.0;

[0109] Test method: Using the same nanofiltration membrane as in Example 1, in The pressure is located at the above The membrane was continuously operated for 24 hours under the operating pressure within the range, and the membrane flux decay rate was recorded. The inorganic carbon removal rate and product purity on the permeate side were also tested.

[0110] Experimental results and data show that: - Control group C outside the boundary, pH 8.5: flux decay rate was as high as 15.6%, membrane surface showed obvious scaling, and salt separation effect was poor; - Control group B outside the boundary, pH 5.0: flux decay rate was 8.2%, and inorganic carbon removal rate was only 85%; indicating that carbonate conversion was incomplete at pH 5, and residual bicarbonate ions still caused scaling and blockage inside the membrane pores; - Control group A outside the boundary, pH 2.0: flux decay rate was 1.1%, which performed well, but acid consumption was 1.5 times that of the pH 3.0 group, and the sulfate rejection rate in the permeate was slightly reduced from 99.9% to 99.2%, which is speculated to be due to damage to the charged layer on the nanofiltration membrane surface caused by excessive acidity; - Optimized groups D / E / F of the example, pH 3.0, 3.5, 4.0: the flux decay rate of the three groups was controlled between 1.2% and 1.8%, and the inorganic carbon removal rate was >99.5%; among them, the pH 3.5 group had the best overall performance, with a decay rate of 1.2% and the highest purity;

[0111] Conclusion: The pH value determined by this invention, in the range of 3-4, is the optimal process window that balances membrane flux stability, salt separation effect, and economic cost; a pH value higher than 4 will lead to incomplete decarbonization and scaling, while a pH value lower than 3 will result in waste of acid and potential risks to membrane performance.

[0112] Testing standards:

[0113] Bromine ion concentration and bromine recovery rate: determined by ion chromatography (IC) using a Dionex ICS-5000+ instrument, in accordance with standard HJ84-2016; the determination was repeated three times and the average value was taken.

[0114] Sulfate content: determined by ion chromatography, referring to standard HJ84-2016;

[0115] Sodium bromide product purity: determined by potentiometric titration, referring to standard GB / T1266-2006;

[0116] Membrane flux decay rate: The change in per unit time of water production was measured by volumetric method after 24 hours of continuous operation.

[0117] Table 1 Performance test data of Examples 1-5 and Comparative Examples 1-4

[0118] Example 1 96.2 98.8 0.08 1.5% Stable operation Example 2 97.1 99.2 0.05 1.2% Stable operation Example 3 96.8 99.0 0.06 1.8% Stable operation Example 4 96.5 98.9 0.07 1.4% Stable operation Example 5 97.3 99.1 0.05 1.6% Stable operation Comparative Example 1 65.4 92.5 1.85 15.6% Severe membrane fouling and carbonate interference with salt separation Comparative Example 2 72.1 98.6 0.06 2.1% A large amount of bromine is lost in the concentrated water. Comparative Example 3 81.5 98.7 0.07 1.9% Insufficient washing water limits recovery rate Comparative Example 4 >99 (All crystallized) 78.4 12.5 N / A The product is a low-value mixed salt and cannot be used.

[0119] As can be seen from the data analysis in Table 1, the present invention’s Examples 1-5 achieved excellent technical results under different operating parameters by using a combination of acidification decarbonization and heavy filtration to enhance salt separation. Specifically, Examples 2 and 5 showed the best overall performance, with bromine recovery rate exceeding 97% and sodium bromide product purity reaching over 99%, while sulfate impurities were controlled at extremely low levels.

[0120] Trend analysis shows that as the dialysis water replenishment rate increased from 0.2 times in Comparative Example 3 to 1.0 times in Example 2, the bromine recovery rate significantly increased from 81.5% to 97.1%. This indicates that the dialysis washing effect plays a decisive role in overcoming the Donnan effect and eluting bromide ions from the concentrate side to the product water side. However, when the rate continued to increase to 1.5 times in Example 3, the recovery rate increase tended to level off at 96.8%, indicating that 1.0-1.2 times is the most cost-effective operating range.

[0121] Comparing Example 2 with Comparative Example 1, it can be seen that without acidification and decarbonization, the membrane flux decline rate is as high as 15.6%, and the product purity is only 92.5%. The mechanism is that under high pH conditions, carbonate ions not only easily cause inorganic scaling, but also change the charge distribution on the membrane surface, increasing the non-specific retention of bromide ions. In contrast, the present invention converts the complex carbonate system into a single sulfate system through step S2, eliminating the negative impact of high alkalinity on nanofiltration membranes.

[0122] Furthermore, Comparative Example 4 confirmed that direct evaporation could not obtain high-purity products, with sulfate content as high as 12.5%, which is an unusable mixed salt. The process of this invention not only achieves efficient and high-purity recovery of bromine resources, but also realizes closed-loop utilization of acid and alkali resources through a bipolar membrane system, which has significant economic and environmental benefits.

[0123] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention.

Claims

1. A high-efficiency process for recovering sodium bromide from PTA wastewater, characterized in that, Includes the following steps: Step S1, Pretreatment Softening: Provide a stream of bromine-containing wastewater generated during the production of purified terephthalic acid, and treat the bromine-containing wastewater with... The volume hourly space velocity is introduced into the chelating resin unit to remove calcium and magnesium ions from the bromine-containing wastewater, thereby obtaining softened water. Step S2, Carbon Conversion: The softened water is acidified and decarbonized under normal temperature and pressure by adding sulfuric acid to adjust the pH value to [value missing]. The bicarbonate and carbonate ions are converted into sulfate and free carbon dioxide, and the generated carbon dioxide is removed by blowing air to obtain a decarbonated solution. Step S3, Enhanced Salt Separation by Re-filtration: The decarbonated solution is fed into a filter with a molecular weight cutoff of [missing value]. nanofiltration systems, in Membrane concentration and separation are performed under operating pressure. Dialysis water is added to the nanofiltration concentrate in the nanofiltration system for refiltration and washing to obtain nanofiltration permeate enriched with sodium bromide and nanofiltration concentrate enriched with sodium sulfate, respectively. Step S4, Evaporation and Purification: The nanofiltration permeate is sequentially subjected to reverse osmosis concentration treatment and evaporation crystallization treatment to obtain high-purity sodium bromide solid.

2. The high-efficiency sodium bromide recovery process for PTA wastewater according to claim 1, characterized in that: The bromine-containing wastewater is a high-bromine, high-alkalinity, and high-salinity wastewater generated from bromine-containing waste gas produced in PTA production after incineration by RTO or RCO and absorption by alkaline solution. The pH value of the bromine-containing wastewater is The concentration of bromide ions was The concentration of bicarbonate is The carbonate concentration is .

3. The high-efficiency sodium bromide recovery process for PTA wastewater according to claim 1, characterized in that: In step S3, the refiltration and rinsing operation separates the residual bromide ions in the nanofiltration concentrate to the nanofiltration permeate side, controls the dialysis water replenishment rate to be 0.5-1.5 times the nanofiltration concentrate discharge rate, and controls the mass percentage solid content of the nanofiltration concentrate to be 15%-25%. The dialysis water is either pure water or reverse osmosis permeate produced by the reverse osmosis concentration treatment in step S4.

4. The high-efficiency sodium bromide recovery process for PTA wastewater according to claim 1, characterized in that: In step S3, after the refiltration and washing operation, the overall bromine recovery rate in the nanofiltration permeate is controlled at over 95%, while the mass fraction of sulfate in the nanofiltration permeate is controlled at below 0.1%.

5. The high-efficiency sodium bromide recovery process for PTA wastewater according to claim 1, characterized in that: In step S4, the evaporation crystallization process uses an MVR or MEE evaporation crystallization system.

6. The high-efficiency sodium bromide recovery process for PTA wastewater according to claim 1, characterized in that: Following step S4, a closed-loop circulation system for the nanofiltration concentrate is also included, specifically comprising: Step S5, Freeze-crystallization treatment: The nanofiltration concentrate is introduced into a freezing system, and the freezing temperature is controlled within the range of -5℃ to 5℃ to freeze-crystallize sodium sulfate decahydrate, i.e., Glauber's salt. Step S6, Bipolar Membrane Electrodialysis Treatment: The precipitated Glauber's salt is dissolved back to prepare a solution with a mass concentration of 15%-25%, and then introduced into the solution with a current density of... The bipolar membrane electrodialysis system dissociates water to generate regenerated sulfuric acid and regenerated caustic soda.

7. The high-efficiency sodium bromide recovery process for PTA wastewater according to claim 6, characterized in that: The regenerated sulfuric acid is refluxed to step S2 to neutralize bicarbonate ions, thereby achieving acid self-sufficiency. The regenerated caustic soda is used to absorb the carbon dioxide released during acidification and stripping in step S2, generating sodium carbonate, which is then returned to the PTA main unit as tail gas absorbent, thus achieving self-sufficiency in alkali.

8. The high-efficiency sodium bromide recovery process for PTA wastewater according to claim 1, characterized in that: The sodium bromide solid recovered by the aforementioned process has a purity greater than 98.5%. The bromine recovery rate of the entire recovery process system is greater than 96%.

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