Method and system for preparing acid and base by bipolar membrane electrodialysis treatment of high-salinity waste liquid

Abstract

The application relates to the technical field of high-salinity waste liquid treatment, and discloses a method and system for preparing acid and alkali by treating high-salinity waste liquid through bipolar membrane electrodialysis. The method comprises the following steps: collecting water quality characteristic parameters to calculate an initial current density, starting electrodialysis; performing quadratic fitting on a membrane impedance sequence in a sliding window to extract an acceleration, and giving a warning and reducing a flow when the acceleration exceeds a threshold value; after the flow is reduced, reverse power supply is used to strip a scale layer through ion reverse migration, so that online scale removal is realized; and a health index is calculated based on a cumulative reverse power supply duration and a membrane impedance drift rate, and operation parameters are dynamically adjusted. The application solves the technical problems that membrane surface scaling during the process of treating high-salinity waste liquid through bipolar membrane electrodialysis cannot be timely warned and online cleaned.

Description

Technical Field

[0001] This application relates to the field of high-salt waste liquid treatment technology, and in particular to a method and system for preparing acids and bases from high-salt waste liquid by bipolar membrane electrodialysis. Background Technology

[0002] Bipolar membrane electrodialysis technology, which decomposes salt solutions into corresponding acids and bases under the drive of an electric field, has broad application prospects in the resource utilization of high-salt wastewater. In the process of preparing acids and bases from high-salt wastewater, traditional bipolar membrane electrodialysis systems apply a DC electric field to cause anions and cations to migrate to their respective electrodes. Simultaneously, the hydrolysis layer of the bipolar membrane dissociates water molecules into hydrogen ions and hydroxide ions, thereby achieving the conversion of salt into acids and bases. Compared with traditional chemical neutralization methods, this technology has advantages such as low energy consumption, high product purity, and no secondary pollution, and has already been applied in fields such as saline wastewater treatment, organic acid preparation, and industrial wastewater resource utilization.

[0003] However, existing bipolar membrane electrodialysis technology faces membrane fouling and scaling problems during long-term continuous operation, hindering its industrial-scale promotion. When polyvalent cations such as calcium and magnesium ions in high-salt wastewater migrate towards the cathode under the influence of an electric field, they form an insoluble scale layer on the cation exchange membrane surface with anions such as carbonate and sulfate. This leads to a continuous increase in membrane impedance, a decrease in current efficiency, and an increase in energy consumption. Traditional solutions mainly include shutdown for chemical cleaning and physical flushing. However, shutdown cleaning causes production interruptions and capacity losses, the use of chemical cleaning agents increases operating costs and may damage membrane materials, and physical flushing has limited effectiveness in removing dense scale layers. More importantly, existing technologies lack a real-time early warning mechanism for the scaling state on the membrane surface. Remedial measures are often only taken after membrane impedance has significantly increased and system performance has severely deteriorated. At this point, the scale layer has solidified and is difficult to remove, significantly increasing the difficulty of cleaning and the risk of membrane damage. Summary of the Invention

[0004] This application provides a method and system for preparing acids and bases by treating high-salt waste liquid using bipolar membrane electrodialysis. This method solves the technical problem of difficulty in real-time early warning and online cleaning of membrane surface scaling during the treatment of high-salt waste liquid using bipolar membrane electrodialysis, thereby improving the continuous operation stability of the system and the service life of the membrane.

[0005] In a first aspect, this application provides a method for preparing acids and bases from high-salt waste liquid using bipolar membrane electrodialysis, the method comprising:

[0006] Step S1: Collect water quality characteristic parameters of the pretreated high-salt waste liquid, calculate the initial current density and start bipolar membrane electrodialysis;

[0007] Step S2: Within the sliding time window, perform quadratic polynomial fitting on the continuously acquired real-time membrane impedance value sequence, extract the quadratic term coefficients of the fitting equation as the membrane impedance acceleration, and generate an early warning signal and reduce the current density when the membrane impedance acceleration exceeds a preset threshold.

[0008] Step S3: After the current density is reduced for a preset time, the current is reduced to zero. A reverse current of a preset ratio of the current density after the current reduction is applied and maintained for a preset time. The electrostatic repulsion force generated by the reverse migration of ions is used to peel off the scale layer on the surface of the ion exchange membrane. The membrane impedance value before and after the reverse current is applied is measured to calculate the impedance reduction rate as the cleaning effect. After the reverse current is applied, the current is restored to the value before the current reduction. A cycle of current reduction operation and reverse current cleaning is established to realize online descaling in the continuous acid and alkali preparation process.

[0009] Step S4: Calculate the membrane health index based on the cumulative reverse energizing time and the membrane impedance baseline drift rate, and adjust the operating parameters of the cycle according to the membrane health index.

[0010] Secondly, this application provides a method system for preparing acids and bases from high-salt waste liquid using bipolar membrane electrodialysis, the method system comprising:

[0011] The data acquisition module is used to collect water quality characteristic parameters of the pretreated high-salt waste liquid, calculate the initial current density, and start bipolar membrane electrodialysis.

[0012] The generation module is used to perform quadratic polynomial fitting on the continuously acquired real-time value sequence of membrane impedance within a sliding time window, extract the coefficients of the quadratic term of the fitting equation as the membrane impedance acceleration, and generate an early warning signal and reduce the current density when the membrane impedance acceleration exceeds a preset threshold.

[0013] The measurement module is used to reduce the current to zero after the current density is reduced for a preset time, apply a reverse current at a preset ratio of the current density after the current reduction and continue for a preset time, use the electrostatic repulsion force generated by the reverse migration of ions to peel off the scale layer on the surface of the ion exchange membrane, measure the membrane impedance value before and after the reverse current is applied to calculate the impedance decrease rate as the cleaning effect, and restore the current to the value before the current reduction after the reverse current is applied, thus establishing a cycle of current reduction operation and reverse current cleaning to achieve online descaling in the continuous acid and alkali preparation process.

[0014] The calculation module is used to calculate the membrane health index based on the cumulative reverse energizing time and the membrane impedance baseline drift rate, and adjust the operating parameters of the cycle according to the membrane health index.

[0015] In the technical solution provided in this application, by performing a quadratic polynomial fitting on the continuously acquired real-time membrane impedance value sequence within a sliding time window and extracting the quadratic coefficient of the fitting equation as the membrane impedance acceleration, an early warning of the fouling trend on the membrane surface is achieved. Compared with the traditional hysteresis response method that only monitors voltage mutations, this application can trigger an early warning signal and actively reduce the current density before the fouling enters the rapid accumulation stage by monitoring the second derivative physical quantity of impedance growth acceleration. Thus, intervention measures are taken before the fouling layer solidifies, avoiding the fouling from accumulating to a degree that is difficult to remove. The technical solution involves reducing the current density for a preset time, then reducing the current to zero and applying a reverse current at a preset ratio of the reduced current density. This utilizes the electrostatic repulsion generated by the reverse migration of ions to peel off the scale layer on the surface of the ion exchange membrane. Simultaneously, hydrogen ions in the acid chamber migrate in reverse to the membrane surface and dissolve the scale layer, such as calcium carbonate. This achieves online descaling while operating, avoiding the production capacity loss caused by the need for shutdown cleaning in traditional technologies. The impedance reduction rate is calculated by measuring the membrane impedance values ​​before and after reverse energization as a quantitative indicator of the cleaning effect, providing data support for subsequent cleaning parameter optimization. Establishing a cycle of reduced current operation and reverse energization cleaning allows the system to periodically maintain the membrane surface while continuously producing acids and alkalis, maintaining production continuity.

[0016] The technical solution based on calculating the membrane health index using cumulative reverse current duration and membrane impedance baseline drift rate quantitatively integrates two attenuation mechanisms: electrochemical stress accumulation caused by frequent polarity reversals and irreversible blockage of membrane pores. This establishes a dynamic assessment system for membrane health. When the membrane health index falls below a health threshold, the system automatically extends the current-reducing runtime and lowers the reverse current intensity during the cycle. This achieves a dynamic balance between maximizing production capacity and protecting the membrane within its remaining lifespan, avoiding the dilemma of excessive cleaning accelerating membrane aging or insufficient cleaning leading to performance degradation. In particular, the introduction of the membrane impedance acceleration algorithm transforms traditional passive response cleaning into proactive predictive intervention. This allows the system to anticipate risks and adjust operating strategies based on scaling kinetics, demonstrating the core contribution of the algorithm model in the specific application scenario of membrane fouling early warning and control. Compared to traditional methods that rely on experience to set fixed cleaning cycles, the algorithm-driven strategy of this application can adapt to the dynamic changes in scaling rates under different water quality conditions and operating stages, significantly improving the system's intelligence level and adaptability to complex operating conditions. Attached Figure Description

[0017] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a schematic diagram of an embodiment of the method for preparing acids and bases from high-salt wastewater by bipolar membrane electrodialysis in this application.

[0019] Figure 2 This is a schematic diagram illustrating the impedance drop rate variation under different reverse energizing parameters in the embodiments of this application;

[0020] Figure 3 This is a schematic diagram comparing the energy consumption and production rate of different operating strategies in the embodiments of this application. Detailed Implementation

[0021] This application provides a method and system for preparing acids and bases from high-salt wastewater using bipolar membrane electrodialysis. The terms "first," "second," "third," "fourth," etc. (if present) in the specification, claims, and accompanying drawings are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments described herein can be implemented in a sequence other than that illustrated or described herein. Furthermore, the terms "comprising" or "having" and any variations thereof are intended to cover a non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0022] For ease of understanding, the specific process of the embodiments of this application is described below. Please refer to [link / reference]. Figure 1 One embodiment of the method for preparing acids and bases by bipolar membrane electrodialysis treatment of high-salt waste liquid in this application includes:

[0023] Step S1: Collect water quality characteristic parameters of the pretreated high-salt waste liquid, calculate the initial current density and start bipolar membrane electrodialysis;

[0024] Specifically, water quality parameters such as total dissolved solids content, calcium and magnesium ion concentration, chemical oxygen demand, and conductivity of the pretreated high-salt waste liquid are collected in real time using an online ion chromatograph. These parameters are organized into a water quality characteristic data matrix as the basis for subsequent calculations. The scaling risk coefficient is calculated by weighted summation of calcium and magnesium ion concentrations. At the same time, the limiting current density of the waste liquid is measured using a three-electrode electrochemical workstation. This limiting current density reflects the maximum current intensity that ions can withstand for migration without water dissociation side reactions. Finally, based on the target acid and alkali concentrations, preset treatment time, and water quality characteristic parameters, the relationship between voltage, current, and impedance is calculated using Ohm's law, and the relationship between ion migration flux and current is calculated using Faraday's law. The optimal initial current density that can reach the target concentration within the predetermined time without exceeding the limiting current density is derived by solving a system of simultaneous equations.

[0025] Step S2: Within the sliding time window, perform quadratic polynomial fitting on the continuously acquired real-time membrane impedance value sequence, extract the quadratic term coefficients of the fitting equation as the membrane impedance acceleration, and generate an early warning signal and reduce the current density when the membrane impedance acceleration exceeds the preset threshold.

[0026] Specifically, the passivation risk of the membrane is predicted in advance by using the second time derivative of the membrane impedance, i.e., acceleration. Traditional methods only monitor voltage changes, which are lagging signals. However, this invention calculates the real-time value of the membrane impedance every 30 seconds by measuring the ratio of voltage to current. Six impedance data points are continuously collected within a sliding window of 180 seconds. A quadratic polynomial fitting is performed on these six data points to obtain a fitting equation. The physical meaning of the coefficient of the quadratic term in this equation is the acceleration of impedance growth. When the impedance acceleration continuously exceeds a preset threshold, it indicates that the fouling on the membrane surface has entered a rapid accumulation stage. At this time, an early warning signal is generated, and a risk factor is calculated based on the ratio of acceleration to threshold. The larger the risk factor, the higher the passivation risk, and the greater the corresponding decrease in current density. By reducing the current in advance, the ion migration rate is slowed down, thereby delaying the fouling process.

[0027] Step S3: After reducing the current density for a preset time, reduce the current to zero, apply a reverse current at a preset ratio of the reduced current density for a preset time, and use the electrostatic repulsion force generated by the reverse migration of ions to peel off the scale layer on the surface of the ion exchange membrane. Measure the membrane impedance value before and after reverse current application to calculate the impedance decrease rate as the cleaning effect. After the reverse current application ends, restore the current to the value before the current reduction, and establish a cycle of current reduction operation and reverse current cleaning to achieve online descaling in the continuous acid and alkali preparation process.

[0028] Specifically, after a preset duration of current reduction operation, the current is first steadily reduced to zero in a ramp manner to eliminate electrode polarization. Then, the current is applied in reverse, causing sodium ions that were originally migrating from the salt chamber to the alkali chamber to migrate in the reverse direction. This reversal of ion flow direction generates an outward electrostatic repulsion force on the calcium or magnesium salt scale layer that has been deposited on the surface of the cation exchange membrane. At the same time, hydrogen ions in the acid chamber migrate in reverse to the membrane surface and react with the scale layer such as calcium carbonate to dissolve. The cleaning effect is quantitatively evaluated by measuring the membrane impedance value before and after reverse current application and calculating the impedance decrease rate. After cleaning, the current is restored to the value before current reduction in a ramp manner to continue normal acid and alkali preparation. By establishing a fixed cycle of current reduction operation and reverse cleaning, cleaning is achieved while running, avoiding the production capacity loss caused by the need to shut down the machine in traditional technology.

[0029] Step S4: Calculate the membrane health index based on the cumulative reverse energizing time and the membrane impedance baseline drift rate, and adjust the operating parameters of the cycle period according to the membrane health index.

[0030] Specifically, a quantified assessment system for membrane health is established to address the problem of shortened membrane lifespan caused by frequent polarity reversals. An electrochemical stress accumulation index is calculated by accumulating the current intensity and duration of each reverse current flow. This index reflects the electrochemical shock load borne by the membrane. Simultaneously, the membrane impedance baseline value, i.e., the lowest impedance value after each cleaning, is monitored. The drift rate is calculated by comparing the current baseline value with the initial baseline value. A higher drift rate indicates irreversible blockage or degradation of the membrane's internal pores. The electrochemical stress index and impedance drift rate are weighted and summed according to preset weighting coefficients to obtain a membrane health index. When the health index is below a threshold, the membrane is determined to be in a sub-healthy state. The current reduction cycle duration is automatically extended, and the reverse current intensity is reduced to minimize damage to the membrane, achieving a dynamic balance between production capacity and membrane lifespan.

[0031] In one specific embodiment, step S1 includes:

[0032] The total dissolved solids content, calcium ion concentration, magnesium ion concentration, chemical oxygen demand, and conductivity of the pretreated high-salt wastewater were collected by online ion chromatography to construct a water quality characteristic data matrix.

[0033] Based on the calcium and magnesium ion concentrations in the water quality characteristic data matrix, the scaling risk coefficient is calculated according to the scaling risk coefficient calculation formula.

[0034] The limiting current density of the pretreated high-salt waste liquid was determined using a three-electrode electrochemical workstation.

[0035] Based on the water quality characteristic data matrix and the limiting current density, combined with the target acid and alkali concentration and the preset treatment time, the initial current density is calculated in reverse by solving a system of equations using Ohm's law and Faraday's law.

[0036] Specifically, the water quality characteristic data matrix is ​​constructed by arranging the five collected water quality parameters into an input vector according to a fixed dimension. Total dissolved solids (TDS) reflects the overall salt concentration level of the waste liquid; calcium and magnesium ion concentrations are directly related to the scaling tendency on the membrane surface; chemical oxygen demand (COD) characterizes the membrane fouling risk that organic matter content may cause; and conductivity reflects the impact of total ion concentration on current transmission efficiency. These five parameters together form the data basis for subsequent calculations. The scaling risk coefficient is calculated using a weighted sum of calcium and magnesium ion concentrations. The weighting coefficient is determined based on the difference in deposition rates of the two ions on the membrane surface, with calcium ions given a higher weight due to their tendency to form calcium carbonate crystals. The calculated scaling risk coefficient is used to determine whether an additional hardening pretreatment step is needed before bipolar membrane electrodialysis.

[0037] When the limiting current density is measured using a three-electrode electrochemical workstation, the working electrode is the membrane material under test, the reference electrode provides a stable potential reference, and the auxiliary electrode forms a current loop. The voltage is gradually increased using a linear sweep voltammetry method, and the current response is recorded. The current density corresponding to the plateau region where the current no longer increases linearly with the voltage is the limiting value. Exceeding this value will cause a large amount of water molecules to dissociate on the membrane surface, producing hydrogen and oxygen side reactions, reducing current efficiency. In the reverse calculation of the initial current density, Ohm's law relates voltage, current, and the sum of membrane impedance and solution resistance, while Faraday's law relates the amount of charge passing through to the amount of acid or alkali produced. Knowing the target acid or alkali concentration and the treatment time, the required total charge can be calculated. Dividing this by the effective membrane area and time yields the required current density. This calculated value must be lower than the limiting current density to ensure process stability, while simultaneously being higher than the minimum current density threshold required to maintain effective ion migration.

[0038] In one specific embodiment, step S2 includes:

[0039] Voltage and current values ​​during bipolar membrane electrodialysis are collected at preset sampling intervals, and the real-time membrane impedance is calculated based on Ohm's law.

[0040] Multiple real-time values ​​of membrane impedance are obtained within a sliding time window of a preset length to construct a membrane impedance time series.

[0041] The membrane impedance time series was fitted with a quadratic polynomial to obtain the fitting equation.

[0042] The coefficients of the quadratic term in the fitted equation are extracted as the membrane impedance acceleration. The membrane impedance acceleration is compared with a preset threshold. When the membrane impedance acceleration exceeds the preset threshold, an early warning signal is generated. The danger factor is calculated based on the early warning signal. The current density reduction is determined based on the danger factor, and a current reduction operation is performed.

[0043] Specifically, the real-time membrane impedance is calculated by dividing the measured voltage by the current using Ohm's law to obtain the total resistance. This total resistance comprises three parts: the membrane bulk resistance, the resistance of the fouling layer on the membrane surface, and the solution resistance. The membrane bulk resistance is constant in the initial stage of operation, the solution resistance changes slowly with ion concentration, while the fouling layer resistance increases rapidly with increasing fouling thickness, becoming the dominant factor. Therefore, the trend of the total resistance mainly reflects the evolution of the fouling state on the membrane surface. The sliding time window setting ensures that each calculation is based on continuous data points within a recent period. The window continuously moves forward over time to maintain data timeliness. Multiple real-time membrane impedance values ​​collected within the window length are arranged chronologically to form a membrane impedance time series, which completely records the dynamic change trajectory of the membrane impedance within a specific time period.

[0044] The quadratic polynomial fitting uses the least squares method to solve for the three coefficients of the fitting equation. The constant term of the fitting equation represents the impedance reference value at the beginning of the window, the linear coefficient of the first term represents the linear growth rate of the impedance, and the quadratic coefficient represents the rate of change of the growth rate itself, i.e., acceleration. When scaling enters the rapid accumulation stage, the impedance not only increases but also the growth rate accelerates, which is reflected in a significant increase in the quadratic coefficient. The physical meaning of this coefficient is the change in impedance per square unit time. The hazard factor is calculated by the ratio of the membrane impedance acceleration to a preset threshold. The larger the ratio, the more serious the deviation of the current scaling rate from the normal level, corresponding to a greater current reduction intervention. A piecewise linear mapping relationship is established between the current reduction amplitude and the hazard factor. When the hazard factor is in different intervals, it corresponds to a current reduction amplitude of 15% and 25%, respectively. The current reduction operation is carried out by a controllable power supply to smoothly adjust the current according to a preset ramp rate to avoid sudden shocks.

[0045] In one specific embodiment, step S3, which involves reducing the current to zero after operating at reduced current density for a preset time, includes:

[0046] After operating at reduced current density for a preset time, the current is linearly reduced to zero from the reduced value according to a preset ramp rate.

[0047] After the current drops to zero, the system is allowed to remain stationary for a preset stabilization period to eliminate electrode polarization effects.

[0048] Specifically, the linear reduction of current to zero at a preset ramp rate after current reduction avoids the instantaneous impact of sudden current changes on the membrane and electrodes. The ramp rate setting needs to balance the current reduction speed with system stability. Too fast a rate will cause the double layer structure on the electrode surface to not have time to adjust, resulting in overpotential spikes. Too slow a rate will prolong the non-production period and reduce overall efficiency. Linear current reduction allows the charge distribution on the electrode surface to gradually return to equilibrium. The settling time after the current drops to zero is used to eliminate the electrode polarization effect accumulated during operation. The polarization effect manifests as a concentration polarization layer and an electrochemical polarization layer formed on the electrode surface. The former forms diffusion resistance due to the ion concentration gradient near the electrode, while the latter forms an activation barrier due to the lag in electrode reaction kinetics. During the settling period, the concentration polarization layer recovers a uniform concentration distribution through natural diffusion, and the intermediate products in the electrochemical polarization layer complete the reaction or decomposition, ensuring that the electrode is in a clean and controllable initial state when reverse current is applied subsequently, avoiding residual polarization interference with the establishment of reverse current and the rapid reversal of ion migration direction.

[0049] In one specific embodiment, step S3, which involves applying a reverse current at a preset ratio to the reduced current density for a preset duration, includes:

[0050] The current is applied in reverse at a preset ramp rate to the preset ratio of the current density after the current drop to the reverse current value.

[0051] Maintain the reverse current value for a preset reverse energizing duration;

[0052] After the preset reverse energizing time has ended, the reverse current is reduced to zero according to the preset ramp rate.

[0053] Specifically, the reverse current application process also employs ramp rate control to avoid membrane stress concentration caused by sudden changes in current direction. The reverse current value is set to a preset ratio of the current density after current reduction to ensure that the reverse driving force is sufficient to peel off the scale layer without being too strong and damaging the membrane structure. This ratio is usually set to 80% to balance cleaning effect and membrane safety. After the reverse current is established, the ion migration direction is completely reversed. Sodium ions that originally entered the alkali chamber from the salt chamber through the cation exchange membrane are now migrating back from the alkali chamber to the salt chamber, while hydrogen ions in the acid chamber pass through the cation exchange membrane in the reverse direction to reach the membrane surface. The hydrogen ions undergo an acidolysis reaction with the deposited calcium carbonate or calcium sulfate scale layer, converting the insoluble salt into soluble ions that re-enter the solution. At the same time, the electrostatic repulsion force generated by the reverse electric field at the bottom of the scale layer further promotes the removal of the scale layer from the membrane surface.

[0054] The preset reverse current duration is based on the balance between scale dissolution kinetics and ion migration flux. If the duration is too short, the scale will only partially dissolve and the cleaning will be incomplete. If the duration is too long, it will increase ineffective energy consumption, and the accumulation of reverse-migrating ions may form new deposits on the other side of the membrane. The optimal duration was determined through experimental calibration to maximize the impedance reduction rate of a single cleaning cycle. After the reverse current is applied, the reverse current is reduced to zero again using a ramp method. This process also needs to eliminate the polarization effect generated by the reverse operation to prepare for the subsequent restoration of the forward current. After the reverse current is reduced to zero, the charge distribution and ion concentration gradient on the electrode surface tend to reach equilibrium during a short rest period. At this time, the membrane system is in a clean and neutral state without polarization interference, and it can be safely and quickly switched back to the forward current mode to continue the preparation of acids and bases.

[0055] In one specific embodiment, step S3 involves measuring the membrane impedance before and after reverse current application and calculating the impedance drop rate as the cleaning effect. After reverse current application, the current is restored to the value before current reduction, establishing a cycle of current reduction operation and reverse current cleaning to achieve online descaling in the continuous acid and alkali preparation process, including:

[0056] Before applying a reverse current at a preset ratio of the current density after the current reduction is reversed, the membrane impedance value is measured by electrochemical impedance spectroscopy as the membrane impedance value before cleaning.

[0057] After the reverse current was reduced to zero, the membrane impedance value was measured by electrochemical impedance spectroscopy and used as the membrane impedance value after cleaning.

[0058] The impedance decrease rate is calculated based on the membrane impedance values ​​before and after cleaning.

[0059] After the reverse current drops to zero, the current is restored to the value before the current reduction at a preset ramp rate;

[0060] The operation of repeatedly reducing current density for a preset time, reducing the current to zero, applying reverse current, and restoring the current to the value before the current reduction is performed forms a cycle.

[0061] Specifically, the process of measuring membrane impedance using electrochemical impedance spectroscopy involves applying a small-amplitude AC perturbation signal to both sides of the membrane. The scanning frequency range covers different relaxation processes from high to low frequencies. The impedance response in the high-frequency region reflects the resistance of the membrane bulk and the solution, the mid-frequency region reflects the charge transfer process, and the low-frequency region reflects the diffusion mass transfer resistance. The real part of the membrane impedance is extracted from the real intercept in the high-frequency region of the Nyquist plot as the membrane impedance value before cleaning. This value comprehensively reflects the superposition effect of the membrane bulk resistance and the fouling resistance. The membrane impedance value after cleaning is also measured immediately after the reverse current returns to zero using electrochemical impedance spectroscopy. At this time, the fouling has been partially or completely removed. The measured impedance value mainly consists of the membrane bulk resistance and the residual fouling resistance. The impedance decrease rate is calculated by dividing the difference between the impedance values ​​before and after cleaning by the impedance value before cleaning and then multiplying by 100%. This indicator directly quantifies the effectiveness of a single pulse cleaning in removing fouling; a higher impedance decrease rate indicates a more significant cleaning effect.

[0062] The process of restoring the current to its pre-current-reduction value allows the bipolar membrane electrodialysis system to return to normal acid and alkali production. The recovery process also employs ramp rate control to ensure a smooth transition. After the current rises to the target value, the concentrations in the acid and alkali chambers continue to increase at the rate determined by Faraday's law, while the salt concentration continues to decrease. The entire system re-enters a stable ion migration and water dissociation coupling process. The cyclical execution connects four operational units—current reduction, current zeroing, reverse cleaning, and current restoration—into a repeatable standard process. The duration of each cycle is determined by the preset durations of current reduction, reverse energization, and current switching ramp time. Through multiple cyclic executions, the membrane surface is periodically cleaned and maintained while continuously producing acids and alkalis, preventing scale buildup to the passivation threshold and forcing system shutdown. The key to this cyclical mechanism is controlling the cleaning time to within 20% of the total operating time, ensuring that the effective time ratio for acid and alkali production remains at a high level.

[0063] Figure 2 This is a schematic diagram illustrating the impedance drop rate variation under different reverse energizing parameters in an embodiment of this application. For example... Figure 2 As shown, the horizontal axis represents the reverse current ratio (as a percentage of the current density after current reduction), and the vertical axis represents the impedance drop rate. The figure illustrates the variation of the impedance drop rate with the increase of the reverse current ratio under three different reverse current durations (5 minutes, 10 minutes, and 15 minutes). Experimental results show that when the reverse current ratio increases from 40% to 80%, the impedance drop rate exhibits a significant upward trend, but the growth slows down after further increasing to 100%. At a reverse current ratio of 80% and a current duration of 10 minutes, the impedance drop rate reaches 86%, achieving the optimal balance between cleaning effect and membrane protection.

[0064] In one specific embodiment, step S4 includes:

[0065] Count the cumulative number of reverse power-on attempts and the duration of each reverse power-on attempt, and calculate the total cumulative reverse power-on duration;

[0066] The cumulative electrochemical stress index is calculated based on the total cumulative reverse current duration, the absolute value of the reverse current, the membrane's rated forward current, and the membrane's cumulative rated operating time.

[0067] Collect the current membrane impedance baseline value, and calculate the membrane impedance baseline drift rate based on the current membrane impedance baseline value and the initial membrane impedance value;

[0068] The membrane health index is calculated based on the electrochemical stress accumulation index and the membrane impedance baseline drift rate, according to the membrane health index calculation formula.

[0069] The membrane health index is compared with the health threshold. When the membrane health index is lower than the health threshold, the duration of the current reduction operation in the cycle is extended and the reverse current intensity is reduced.

[0070] Specifically, the cumulative reverse current duration is obtained by summing the reverse current duration during each pulse cleaning. This duration reflects the total exposure time of the membrane under the reverse electric field throughout the entire operating cycle. The calculation of the electrochemical stress accumulation index involves multiplying the cumulative reverse current duration by the absolute value of each reverse current to obtain the total reverse charge, and then normalizing it by dividing by the product of the membrane's rated forward current and the membrane's cumulative rated operating time. The normalized value represents the proportion of the current accumulated electrochemical stress relative to the total stress that the membrane can withstand within its design life. Although the reverse current has a short duration, the fatigue damage it causes to the membrane has a cumulative effect. Each polarity reversal will cause stress concentration and microstructure rearrangement in the ion exchange groups and polymer backbone inside the membrane. Long-term accumulation leads to a gradual decline in the membrane's mechanical strength and selective permeability.

[0071] The membrane impedance baseline value is defined as the lowest membrane impedance value measured after each pulse cleaning. This value eliminates the contribution of reversible fouling and only reflects the resistance of the membrane body and irreversible fouling layer. The initial membrane impedance value is the baseline value measured at the beginning of operation of a brand new membrane as a reference benchmark for its health status. The current membrane impedance baseline value gradually increases over time due to irreversible blockage in the membrane channels and aging of the membrane material. The membrane impedance baseline drift rate is calculated by dividing the difference between the current baseline value and the initial baseline value by the initial baseline value and then multiplying by 100%. The larger the drift rate, the more severe the degradation of the intrinsic performance of the membrane. The membrane health index calculation formula calculates the electrochemical stress accumulation index and the membrane impedance baseline drift rate by weighting them according to preset weighting coefficients and then subtracting them from 100. The weighting coefficients reflect the relative contribution of the two decay mechanisms to the membrane lifetime. The higher the calculated health index value, the closer the membrane is to a healthy state. When the health index is lower than the health threshold, the membrane is determined to have entered the sub-healthy range. At this time, the system automatically adjusts the operation strategy to extend the current reduction operation time and reduce the cleaning frequency per unit time. At the same time, it reduces the reverse current intensity to reduce the stress impact of a single cleaning. By sacrificing some cleaning efficiency in exchange for extending the membrane lifetime, the system maximizes the production capacity within the remaining lifetime.

[0072] Figure 3 This is a schematic diagram comparing energy consumption and production rate under different operating strategies in the embodiments of this application. Figure 3 As shown, the horizontal axis represents five different operating strategies (A to E), the left vertical axis represents unit energy consumption (kWh / kg), and the right vertical axis represents production rate (kg / h). Diagonally filled bars represent unit energy consumption, and horizontally filled bars represent production rate. From strategy A to strategy E, unit energy consumption decreases from 125 kWh / kg to 102 kWh / kg, while the production rate increases from 82 kg / h to 95 kg / h. This result verifies that by optimizing the cycle operating parameters and dynamically adjusting the membrane health index, it is possible to improve the system's acid production efficiency while reducing energy consumption.

[0073] The method for preparing acids and bases from high-salt waste liquid using bipolar membrane electrodialysis in the embodiments of this application has been described above. The method system for preparing acids and bases from high-salt waste liquid using bipolar membrane electrodialysis in the embodiments of this application is described below. One embodiment of the method system for preparing acids and bases from high-salt waste liquid using bipolar membrane electrodialysis in the embodiments of this application includes:

[0074] The data acquisition module is used to collect water quality characteristic parameters of the pretreated high-salt waste liquid, calculate the initial current density, and start bipolar membrane electrodialysis.

[0075] The generation module is used to perform quadratic polynomial fitting on the continuously acquired real-time value sequence of membrane impedance within a sliding time window, extract the coefficients of the quadratic term of the fitting equation as the membrane impedance acceleration, and generate an early warning signal and reduce the current density when the membrane impedance acceleration exceeds a preset threshold.

[0076] The measurement module is used to reduce the current to zero after the current density is reduced for a preset time, apply a reverse current at a preset ratio of the current density after the current reduction and continue for a preset time, use the electrostatic repulsion force generated by the reverse migration of ions to peel off the scale layer on the surface of the ion exchange membrane, measure the membrane impedance value before and after the reverse current is applied to calculate the impedance decrease rate as the cleaning effect, and restore the current to the value before the current reduction after the reverse current is applied, thus establishing a cycle of current reduction operation and reverse current cleaning to achieve online descaling in the continuous acid and alkali preparation process.

[0077] The calculation module is used to calculate the membrane health index based on the cumulative reverse energizing time and the membrane impedance baseline drift rate, and adjust the operating parameters of the cycle according to the membrane health index.

[0078] 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 the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing acids and bases from high-salt wastewater using bipolar membrane electrodialysis, characterized in that, The method includes: Step S1: Collect water quality characteristic parameters of the pretreated high-salt waste liquid, calculate the initial current density and start bipolar membrane electrodialysis; Step S2: Within the sliding time window, perform quadratic polynomial fitting on the continuously acquired real-time membrane impedance value sequence, extract the quadratic term coefficients of the fitting equation as the membrane impedance acceleration, and generate an early warning signal and reduce the current density when the membrane impedance acceleration exceeds a preset threshold. Step S3: After the current density is reduced for a preset time, the current is reduced to zero. A reverse current of a preset ratio of the current density after the current reduction is applied and maintained for a preset time. The electrostatic repulsion force generated by the reverse migration of ions is used to peel off the scale layer on the surface of the ion exchange membrane. The membrane impedance value before and after the reverse current is applied is measured to calculate the impedance reduction rate as the cleaning effect. After the reverse current is applied, the current is restored to the value before the current reduction. A cycle of current reduction operation and reverse current cleaning is established to realize online descaling in the continuous acid and alkali preparation process. Step S4: Calculate the membrane health index based on the cumulative reverse current duration and membrane impedance baseline drift rate. Adjust the operating parameters of the cycle according to the membrane health index, including: counting the cumulative number of reverse current cycles and the duration of each reverse current cycle, and calculating the total cumulative reverse current duration; calculating the electrochemical stress accumulation index based on the total cumulative reverse current duration, the absolute value of the reverse current, the membrane rated forward current, and the membrane cumulative rated operating time; the membrane impedance baseline value is the lowest membrane impedance value measured after each pulse cleaning. This value excludes the contribution of reversible fouling and only reflects the resistance of the membrane body and the irreversible fouling layer. Collect the current membrane impedance baseline value, and calculate the membrane impedance baseline drift rate by dividing the difference between the current membrane impedance baseline value and the initial membrane impedance baseline value by the initial membrane impedance baseline value and then multiplying by 100%; calculate the membrane health index according to the membrane health index calculation formula based on the electrochemical stress accumulation index and the membrane impedance baseline drift rate; compare the membrane health index with the health threshold. When the membrane health index is lower than the health threshold, extend the current reduction operation duration and reduce the reverse current intensity in the cycle.

2. The method for preparing acids and bases by bipolar membrane electrodialysis treatment of high-salt waste liquid according to claim 1, characterized in that, Step S1 includes: The total dissolved solids content, calcium ion concentration, magnesium ion concentration, chemical oxygen demand, and conductivity of the pretreated high-salt wastewater were collected by online ion chromatography to construct a water quality characteristic data matrix. Based on the calcium and magnesium ion concentrations in the water quality characteristic data matrix, the scaling risk coefficient is calculated according to the scaling risk coefficient calculation formula. The limiting current density of the pretreated high-salt waste liquid was determined using a three-electrode electrochemical workstation. Based on the water quality characteristic data matrix and the limiting current density, combined with the target acid and alkali concentration and the preset treatment time, the initial current density is calculated in reverse by solving a system of equations using Ohm's law and Faraday's law.

3. The method for preparing acids and bases by bipolar membrane electrodialysis treatment of high-salt waste liquid according to claim 1, characterized in that, Step S2 includes: Voltage and current values ​​during bipolar membrane electrodialysis are collected at preset sampling intervals, and the real-time membrane impedance is calculated based on Ohm's law. Multiple consecutive real-time values ​​of the membrane impedance are acquired within a sliding time window of a preset length to construct a membrane impedance time series; The membrane impedance time series was fitted with a quadratic polynomial to obtain the fitting equation. The coefficients of the quadratic term of the fitted equation are extracted as the membrane impedance acceleration. The membrane impedance acceleration is compared with a preset threshold. When the membrane impedance acceleration exceeds the preset threshold, an early warning signal is generated. A danger factor is calculated based on the early warning signal. The current density reduction is determined based on the danger factor, and a current reduction operation is performed.

4. The method for preparing acids and bases by bipolar membrane electrodialysis treatment of high-salt waste liquid according to claim 1, characterized in that, Step S3, which involves reducing the current to zero after a preset time of reducing the current density, includes: After a preset time of operation to reduce current density, the current is linearly reduced to zero from the reduced value according to a preset ramp rate. After the current drops to zero, the system is allowed to remain stationary for a preset stabilization period to eliminate electrode polarization effects.

5. The method for preparing acids and bases by bipolar membrane electrodialysis treatment of high-salt waste liquid according to claim 4, characterized in that, Step S3, which involves applying a reverse current at a preset ratio to the reduced current density for a preset duration, includes: The current is applied in reverse at a preset ramp rate to the preset ratio of the current density after the current drop to the reverse current value. The reverse current value is maintained for a preset reverse energizing time. After the preset reverse energizing time has ended, the reverse current is reduced to zero at a preset ramp rate.

6. The method for preparing acids and bases by bipolar membrane electrodialysis treatment of high-salt waste liquid according to claim 5, characterized in that, In step S3, the membrane impedance values ​​before and after reverse current application are measured, and the impedance decrease rate is calculated as the cleaning effect. After reverse current application, the current is restored to the value before current reduction, establishing a cycle of current reduction operation and reverse current cleaning to achieve online descaling in the continuous acid and alkali preparation process, including: Before applying a reverse current at a preset ratio of the current density after the current reduction is reversed, the membrane impedance value is measured by electrochemical impedance spectroscopy as the membrane impedance value before cleaning. After the reverse current drops to zero, the membrane impedance value is measured by electrochemical impedance spectroscopy as the membrane impedance value after cleaning. The impedance decrease rate is calculated based on the membrane impedance value before cleaning and the membrane impedance value after cleaning. After the reverse current drops to zero, the current is restored to the value before the current reduction at a preset ramp rate; The operation of repeatedly reducing current density for a preset time, reducing the current to zero, applying reverse current, and restoring the current to the value before the current reduction is performed forms a cycle.

7. A method and system for preparing acids and bases by bipolar membrane electrodialysis treatment of high-salt waste liquid, characterized in that, A method for preparing acids and bases from high-salt wastewater using bipolar membrane electrodialysis as described in any one of claims 1-6, the system comprising: The data acquisition module is used to collect water quality characteristic parameters of the pretreated high-salt waste liquid, calculate the initial current density, and start bipolar membrane electrodialysis. The generation module is used to perform quadratic polynomial fitting on the continuously acquired real-time value sequence of membrane impedance within a sliding time window, extract the coefficients of the quadratic term of the fitting equation as the membrane impedance acceleration, and generate an early warning signal and reduce the current density when the membrane impedance acceleration exceeds a preset threshold. The measurement module is used to reduce the current to zero after the current density is reduced for a preset time, apply a reverse current at a preset ratio of the current density after the current reduction and continue for a preset time, use the electrostatic repulsion force generated by the reverse migration of ions to peel off the scale layer on the surface of the ion exchange membrane, measure the membrane impedance value before and after the reverse current is applied to calculate the impedance decrease rate as the cleaning effect, and restore the current to the value before the current reduction after the reverse current is applied, thus establishing a cycle of current reduction operation and reverse current cleaning to achieve online descaling in the continuous acid and alkali preparation process. The calculation module is used to calculate the membrane health index based on the cumulative reverse energizing time and the membrane impedance baseline drift rate, and adjust the operating parameters of the cycle according to the membrane health index.

8. The system according to claim 7, characterized in that, Collect water quality characteristic parameters of the pretreated high-salt wastewater, calculate the initial current density, and start bipolar membrane electrodialysis, including: The total dissolved solids content, calcium ion concentration, magnesium ion concentration, chemical oxygen demand, and conductivity of the pretreated high-salt wastewater were collected by online ion chromatography to construct a water quality characteristic data matrix. Based on the calcium and magnesium ion concentrations in the water quality characteristic data matrix, the scaling risk coefficient is calculated according to the scaling risk coefficient calculation formula. The limiting current density of the pretreated high-salt waste liquid was determined using a three-electrode electrochemical workstation. Based on the water quality characteristic data matrix and the limiting current density, combined with the target acid and alkali concentration and the preset treatment time, the initial current density is calculated in reverse by solving a system of equations using Ohm's law and Faraday's law.

9. The system according to claim 8, characterized in that, After a preset period of operation with reduced current density, the current is reduced to zero, including: After a preset time of operation to reduce current density, the current is linearly reduced to zero from the reduced value according to a preset ramp rate. After the current drops to zero, the system is allowed to remain stationary for a preset stabilization period to eliminate electrode polarization effects.

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