Electrodialysis recovery and reuse method of sodium nitrate washing liquid in process for producing boehmite by nitric acid method

Abstract

The invention belongs to the field of water treatment and industrial wastewater recycling, and provides an electrodialysis recovery and reuse method of sodium nitrate washing liquor in a process for producing boehmite by a nitric acid method. According to the method, washing liquid is shunted according to washing levels and enters preceding-stage electrodialysis, a membrane stack is composed of an anion exchange membrane and a cation exchange membrane and is provided with an acid blocking and acid limiting migration unit, pH signals are collected on the dilution side, the pH change rate is calculated, and under the condition that the circulation flow of the dilution side is kept constant, a controller conducts multi-gear adjustment on working current based on the pH change rate; the water produced at the fresh water side of the preceding-stage electrodialysis serves as reuse water and is connected into a low-stage washing water replenishing pipeline and is combined with fresh pure water, and the liquid at the concentration side is further concentrated by a post-stage electrodialysis and terminal evaporation unit to obtain sodium nitrate concentrated liquid; and the reuse water circulation pipeline is provided with a pollution discharge branch and a bypass treatment branch, ultrafiltration or adsorption treatment is carried out on the circulation water, and online chemical cleaning is carried out on each electrodialysis membrane stack, so that resource reuse and pollution load reduction are realized.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of water treatment and industrial wastewater resource recycling, and relates to a method for recovering and recycling sodium nitrate washing liquid in a process for producing boehmite by a nitric acid method. BACKGROUND

[0002] The nitric acid method process is widely used in the production process of inorganic salts and related chemical products. In the reaction, separation and post-treatment links, the material usually needs to be washed in multiple stages to bring out the entrained mother liquor and soluble impurities, thereby generating high-salinity washing liquid / process solution mainly containing sodium nitrate. Such washing liquid belongs to typical salt-containing industrial wastewater / high inorganic salt load wastewater. In addition to containing high concentrations of nitrate and sodium salt, it often also contains residual acid, fine solids, dissolved silicon and colloidal impurities, as well as a small amount of organic matter or metal ions, and the composition fluctuates significantly with the working condition. It is difficult to be simply disposed as ordinary wastewater, and it is also difficult to be directly recycled under the condition of lacking selective separation means, otherwise it is easy to cause impurity accumulation, corrosion intensification and subsequent process operation fluctuation in the system.

[0003] The existing treatment methods include dilution recycling, neutralization and sedimentation, evaporation crystallization, ion exchange, membrane separation and electrodialysis, etc. Among them, evaporation crystallization and traditional neutralization and sedimentation are more inclined to end-of-pipe reduction or salt enrichment, and it is difficult to build an integrated liquid flow distribution and recycling scheme with the upstream washing unit; ion exchange needs frequent regeneration and generates regenerated waste liquid; pressure-driven membranes such as reverse osmosis are limited by osmotic pressure in high-salinity systems and are prone to scaling and plugging. As an electrically driven desalination method, electrodialysis based on ion exchange membranes has process advantages in selective migration of inorganic ions and realization of salt water graded utilization, and is suitable for the treatment of sodium nitrate-containing washing liquid. It can also further realize the closed-loop management of "treatment-recycling-emission reduction", but in actual application, it still faces problems such as membrane fouling caused by silicon and colloids in the feed, acidity drift on the dilution side caused by the migration of acidic components, concentration polarization and operation fluctuation under the condition of approaching the limiting current, lack of current density grading control combined with membrane stack structure configuration and online monitoring signal, and insufficient coordination management of recycled water distribution, circulating blowdown and bypass purification between different washing stages, which makes it difficult to balance membrane stack stable operation and system water quality stability, and easily causes water quality fluctuation and frequent maintenance. SUMMARY

[0004] In view of the deficiencies in the prior art, the purpose of the present application is to provide a method for recovering and recycling sodium nitrate washing liquid in a process for producing boehmite by a nitric acid method. The washing liquid is collected and pretreated in stages and then enters the electrodialysis, the pH on the dilution side is collected and the change rate is calculated, the circulation flow is constant, the controller adjusts the current grading and the limit current according to the change rate; the concentrated liquid is concentrated by subsequent electrodialysis and evaporation to obtain sodium nitrate concentrated liquid, thereby meeting the needs of actual production.

[0005] To achieve the above object, the present application adopts the following technical solutions:

[0006] In a first aspect, the present application provides a method for recovering and recycling sodium nitrate washing liquid in a process for producing boehmite by nitric acid method, comprising the following steps:

[0007] S1, the sodium nitrate washing liquid generated in the process by nitric acid method is divided into primary washing liquid, secondary washing liquid and tertiary washing liquid according to the washing order;

[0008] S2, the primary washing liquid is sequentially subjected to coarse filtration and precision filtration, and is subjected to silicon removal and colloidal protection treatment to obtain an electrodialysis feed;

[0009] S3, the electrodialysis feed is adjusted to a preset temperature window and an acidity window, and then enters a first-stage electrodialysis ED-1, the ED-1 adopts a membrane stack composed of an anion exchange membrane and a cation exchange membrane, and an acid blocking and limiting migration unit is arranged; a pH signal is collected at the dilution side of the ED-1 and a pH change rate signal is calculated, the circulation flow rate of the dilution side of the ED-1 is kept constant, and a controller adjusts the current of the ED-1 in stages based on the pH change rate signal; when the pH signal exceeds a preset range, the controller reduces the current of the ED-1 to a preset current value, and the ED-1 outputs recycled fresh water and concentrated liquid in the ED-1;

[0010] S4, the concentrated liquid in the ED-1 enters a second-stage electrodialysis ED-2 for concentration, and the ED-2 outputs a concentrated liquid before a terminal;

[0011] S5, the concentrated liquid before the terminal enters a terminal concentration unit for evaporation concentration to obtain a concentrated sodium nitrate liquid;

[0012] S6, the recycled fresh water is introduced into the tertiary washing water supply pipeline and the secondary washing water supply pipeline, respectively, and fresh pure water is introduced into the tertiary washing water supply pipeline and the secondary washing water supply pipeline;

[0013] S7, a blowdown branch and a bypass treatment branch are arranged on the recycled fresh water circulation pipeline, respectively;

[0014] S8, the membrane stack of the ED-1 and the ED-2 is subjected to online chemical cleaning CIP.

[0015] Preferably, the coarse filtration and the precision filtration are two-stage series filtration, and the ratio of the precision of the coarse filtration to the precision of the precision filtration is (10-50):(0.1-1); the ratio of the treatment volume flow rate of the precision filtration to the filtration effective area is (200-2000) L·h -1 :1m 2The ratio of the recycled fresh water flow introduced into the third washing makeup water line to the recycled fresh water flow introduced into the second washing makeup water line is (1-5):1, and the ratio of the fresh pure water makeup water flow to the recycled fresh water makeup water flow is (0.1-30):100.

[0016] Preferably, the silicon and colloidal protection treatment includes a silicon removal adsorption bed, the adsorption medium of the silicon removal adsorption bed is selected from at least one of alumina particles, zirconia particles, and alumina-zirconia composite oxide particles, the ratio of the volume of the silicon removal adsorption bed to the volume of the primary washing liquid is 1:(20-200), the ratio of the volume flow rate flowing through the adsorption bed to the volume of the adsorption bed is (3-30):1h -1 , and the ratio of the mass of the adsorption medium to the mass of the primary washing liquid is (0.1-2):100.

[0017] Preferably, the online chemical cleaning CIP of step S8 includes an alkali washing-water washing-acid washing-water washing sequence; the ratio of the alkali washing liquid used in the alkali washing to the internal liquid holdup volume of the membrane stack is (2-20):1, and the ratio of the mass of sodium hydroxide in the alkali washing liquid to the mass of the alkali washing liquid is (0.1-1.0):100; the ratio of the acid washing liquid used in the acid washing to the internal liquid holdup volume of the membrane stack is (2-20):1, the ratio of the mass of nitric acid in the acid washing liquid to the mass of the acid washing liquid is (0.05-0.5):100; and the ratio of the water washing liquid used in a single water washing process to the internal liquid holdup volume of the membrane stack is (2-20):1.

[0018] Preferably, the number of acid blocking and acid limiting migration units in the membrane stack of ED-1 is 1:(5-50) of the total number of membrane pairs; nitric acid is added to the electrodialysis feed in step S3, and the mass ratio of the nitric acid to the primary washing liquid is (0.01-0.5):100; the pH of the electrodialysis feed is in the range of 1.5-4.0, and the temperature of the electrodialysis feed is 20-40℃.

[0019] Preferably, the ratio of the dilution circuit volume flow rate to the concentration circuit volume flow rate of ED-1 is (1-3):1; the ratio of the dilution circuit circulation amount to the fresh feed amount of the dilution circuit is (1-10):1; the ratio of the concentration circuit circulation amount to the discharge amount of the concentration circuit is (2-20):1; the ratio of the dilution circuit volume flow rate to the effective membrane area of ED-1 is (100-800)L·h -1 :1m 2 , and the ratio of the concentration circuit volume flow rate to the effective membrane area of ED-1 is (80-600)L·h -1 :1m 2 .

[0020] Preferably, the ratio of the operating current to the limiting current of ED-1 is (0.5-0.9):1; the ratio of the operating current to the limiting current of ED-2 is (0.4-0.85):1; the ratio of the current adjustment range of the controller to the current is 0.05-0.4; and the circulating flow rate on the dilution side of ED-1 is kept constant such that the ratio of the fluctuation range of the circulating flow rate of the ED-1 dilution circuit to the set circulating flow rate is no greater than (0.5-5):100.

[0021] Preferably, the graded adjustment includes multiple current levels, with the number of current levels being 3-8. The ratio of the current setting difference between adjacent current levels to the normal operating current setting value is (1-20):100. The preset range is the difference between the upper and lower pH limits on the ED-1 dilution side, which is 0.2-1.5. When the pH signal exceeds the preset range, the ratio of the current setting of the preset level to the normal operating current setting value is (30-80):100. The pH change rate signal is the ratio of the pH difference between adjacent sampling periods to the sampling period, wherein the sampling period is 2-60s.

[0022] Preferably, the ratio of the volumetric flow rate of the ED-2 concentration loop to the volumetric flow rate of the dilution loop is 1:(0.5-2); the ratio of the circulation rate of the concentration loop to the discharge rate of the concentration loop is (3-30):1; and the ratio of the volumetric flow rate of the concentration loop to the effective membrane area of ​​ED-2 is (150-1200) L·h. -1 1m 2 In step S4, the sodium nitrate mass fraction of the concentrate before the terminal is 18-20%; in step S5, the ratio of the amount of water removed by evaporation to the amount of feed liquid entering the terminal concentration unit is (1-15):100.

[0023] Preferably, in step S7, the ratio of the sewage discharge volume of the sewage branch to the total flow rate of the recycled freshwater in the recycled freshwater circulation pipeline is (0.2-5):100; the ratio of the bypass treatment volume of the bypass treatment branch to the recycled freshwater circulation volume of the recycled freshwater in the recycled freshwater circulation pipeline is (0.5-20):100; the bypass treatment branch is connected to a bypass treatment unit, which is selected from at least one of activated carbon adsorption, resin adsorption, and ultrafiltration; when the bypass treatment unit is an ultrafiltration unit, the ratio of the ultrafiltration permeate volume to the ultrafiltration influent volume is (85-99):100; when the bypass treatment unit is an adsorption bed, the ratio of the bypass treatment volumetric flow rate to the adsorption bed volume is (5-100):1h. -1 .

[0024] Before the pretreated washing solution enters the electrodialysis unit, particulate solids and some aggregates are removed, reducing channel blockage and membrane surface coverage. Dissolved silica exists in the form of silicic acid and its condensation polymers, and is fixed at the solid-phase interface through coordination bonding, surface condensation, and hydrogen bonding on the surface of alumina, zirconium oxide, or their composite oxides. Colloidal protection treatment changes the surface charge and hydration layer state of the particles, regulates the electrostatic interaction between particles, weakens heterogeneous nucleation, bridging aggregation, and deposition of colloids on the membrane surface, and reduces local mass transfer resistance and membrane potential disturbance caused by the capping layer.

[0025] During electrodialysis, an applied electric field establishes a potential gradient. Sodium ions migrate along the fixed negatively charged channels of the cation exchange membrane, while nitrate ions migrate along the fixed positively charged channels of the anion exchange membrane. Ions in the dilution chamber are removed and accumulate in the concentration chamber, while water molecules migrate along the electrodialysis and osmosis processes. The decrease in solute concentration on the dilution side creates a boundary layer concentration gradient and induces concentration polarization. Under conditions of local ion depletion, water at the membrane interface dissociates to generate hydrogen ions and hydroxide ions, whose migration and recombination alter the acid-base balance on the dilution side. The transmembrane migration of acidic species also includes the carrier migration and structural diffusion of hydrogen ions in the aqueous channels of the membrane phase, as well as the diffusion and interfacial dissociation rebalancing of molecular nitrate. The acid-blocking and acid-limiting migration units reduce the net transmembrane flux of acidic species by altering diffusion paths and effective migration resistance and enhancing co-ion repulsion. When the dilution side circulation flow rate is constant, the boundary layer thickness and mass transfer coefficient fluctuate around the fixed operating conditions. The pH signal collected online is sampled and differentially analyzed to obtain the rate of change, and the controller adjusts the current in stages accordingly. The current setting changes the migration flux and the interface potential gradient, thereby changing the contribution of concentration polarization and water dissociation. When the pH exceeds the limit, the current is reduced or kept at the preset value to change the acidity evolution trend.

[0026] The concentrate enters the downstream electrodialysis stage for further solute enrichment, followed by evaporation and concentration. The solvent phase change removes the solute, increasing its mass fraction and altering its activity coefficient and ion association state. Sodium nitrate and impurity ions partition between the liquid and, possibly, the solid phases. When the recovered freshwater is returned to the washing circuit, the sludge branch provides a clean removal channel for impurities. Bypass treatment removes organic matter, colloids, and some impurity ions through adsorption, ion exchange, or sieving, reducing their accumulation in the cycle.

[0027] Compared with the prior art, the beneficial effects of the present invention are as follows: The present invention diverts the sodium nitrate washing liquid produced by the nitric acid process according to the washing stage, and preferably sends the high-concentration stage into the segmented electrodialysis system after filtration, desiliconization and colloidal protection pretreatment. This ensures that the feed salinity, acidity and silica content of the feed entering the ion exchange membrane stack are within a suitable operating window, improving the salt separation efficiency and operational stability of the sodium nitrate system by electrodialysis. The front-stage membrane stack, by configuring acid blocking and acid migration limiting units, maintains a constant circulation flow rate on the dilution side while introducing current gradation adjustment and over-limit current control based on pH change rate, thereby controlling the migration of acidic components, concentration polarization and operation under near-limiting current conditions. Fluctuations are suppressed within a controllable range, reducing membrane fouling and stabilizing voltage, current, and current efficiency. Recycled freshwater is distributed back to the low-level washing water supply line in stages. The circulation line is equipped with a sewage branch and a bypass ultrafiltration / adsorption treatment unit to reduce the accumulation of organic matter, colloids, and fine particles that are difficult to be effectively retained by electrodialysis in the system. The terminal evaporation unit only treats the terminal concentrate enriched by electrodialysis to reduce the evaporation load. With the help of online chemical cleaning procedures and pretreatment steps, the continuous operation cycle of the membrane stack can be extended, the frequency of maintenance downtime and operating energy consumption can be reduced, and it is easy to integrate with the existing nitric acid process production unit in a modular manner to achieve the synergistic recovery of sodium nitrate concentrate and recycled water. Detailed Implementation

[0028] The technical solution of the present invention will be described in detail below with reference to specific embodiments. The embodiments described herein are specific implementations of the present invention and are used to illustrate the concept of the present invention; these descriptions are explanatory and exemplary and should not be construed as limiting the implementation of the present invention or the scope of protection of the present invention. In addition to the embodiments described herein, those skilled in the art can also adopt other obvious technical solutions based on the content disclosed in the claims and the specification of this application. These technical solutions include those that make any obvious substitutions and modifications to the embodiments described herein.

[0029] The chemical reagents used in the embodiments and comparative examples of this invention are all commercially available products and have not undergone any further purification treatment.

[0030] Example 1

[0031] This embodiment provides a method for the electrodialysis recovery and reuse of sodium nitrate washing solution in the nitric acid process for producing boehmite. The method specifically includes the following steps:

[0032] S1, the washing solution containing sodium nitrate produced by the nitric acid process is divided into primary washing solution, secondary washing solution and tertiary washing solution according to the washing level;

[0033] S2 involves coarse filtration followed by fine filtration of the primary washing solution. The coarse filter has a pore size of 35 μm, and the fine filter has a pore size of 0.2 μm. The ratio of the processing volumetric flow rate to the effective filtration area for the fine filtration is 300 L·h.-1 1m 2 After filtration, silica removal and colloidal protection treatment are performed. The silica removal adsorption bed medium is alumina-zirconia composite oxide particles. The volume ratio of the silica removal adsorption bed to the primary washing liquid is 1:150, and the volumetric flow rate through the adsorption bed is 5 h⁻¹. -1 The ratio of the mass of the adsorption medium to the mass of the primary washing solution is 2:100.

[0034] S3, after adding nitric acid to the electrodialysis feed and adjusting the pH to 3.4 and temperature to 22℃, the feed enters the first stage of electrodialysis, ED-1. The ED-1 membrane stack uses anion exchange membrane Neosepta AMX and cation exchange membrane Neosepta CMX to form a membrane pair. The ratio of the number of acid-blocking and acid-limiting migration units to the total number of membrane pairs is 1:35. The pH signal is collected on the ED-1 dilution side, and the pH change rate signal is calculated differentially according to a sampling period of 30 seconds. The circulation flow rate on the ED-1 dilution side is kept constant, and the ratio of the circulation flow rate fluctuation to the set circulation flow rate is no greater than 4:100. The controller adjusts the ED-1 current in stages based on the pH change rate. There are 6 current stages, and the ratio of the current setting difference between adjacent stages to the normal operating current setting value is 1:100. The ratio of the ED-1 operating current to the limiting current is 0.78:1. E The difference between the upper and lower limits of the pH preset range on the D-1 dilution side is 1.0. When the pH exceeds the limit, the current is adjusted down to the preset current. The ratio of the preset current to the normal operating current setting is 65:100. The ratio of the controller's current adjustment amplitude to the current is 0.25. The ratio of the volumetric flow rate of the dilution circuit to the volumetric flow rate of the concentration circuit in ED-1 is 2.5:1. The ratio of the dilution circuit circulation rate to the fresh feed rate of the dilution circuit is 10:1. The ratio of the concentration circuit circulation rate to the concentration circuit discharge rate is 2:1. The ratio of the dilution circuit volumetric flow rate to the effective membrane area is 550 L·h. -1 1m 2 The ratio of the volumetric flow rate of the concentration loop to the effective membrane area is 80 L·h. -1 1m 2 ; ED-1 output recovers and reuses fresh water and ED-1 concentrate;

[0035] S4, the ED-1 concentrate is introduced into the second stage of electrodialysis, ED-2. The ratio of the operating current to the limiting current of ED-2 is 0.40:1, the ratio of the volumetric flow rate of the ED-2 concentration circuit to the volumetric flow rate of the dilution circuit is 1:0.5, the ratio of the circulation rate of the concentration circuit to the discharge rate of the concentration circuit is 30:1, and the ratio of the volumetric flow rate of the concentration circuit to the effective membrane area is 900 L·h. -1 1m 2 The sodium nitrate mass fraction of the concentrated solution before the output terminal is controlled at 18.8%;

[0036] S5, the concentrated liquid before the terminal enters the forced circulation evaporation and concentration unit, and the ratio of the amount of water removed by evaporation to the amount of feed liquid entering the terminal concentration unit is 15:100, to obtain sodium nitrate concentrate.

[0037] S6, the recycled fresh water is introduced into the tertiary wash water supply line and the secondary wash water supply line respectively. The ratio of the recycled fresh water flow rate introduced into the tertiary wash water supply line to the recycled fresh water flow rate introduced into the secondary wash water supply line is 4:1. Fresh pure water is also introduced. The ratio of the fresh pure water supply flow rate to the recycled fresh water supply flow rate is 5:100.

[0038] S7. A sewage discharge branch and a bypass treatment branch are set up in the freshwater recycling pipeline. The ratio of the sewage discharge volume of the sewage discharge branch to the total flow rate of the freshwater recycling pipeline is 3:100. The ratio of the bypass treatment volume in the bypass treatment branch to the flow rate of the freshwater recycling pipeline is 0.5:100. The bypass treatment unit is an ultrafiltration unit. The ratio of the ultrafiltration permeate volume to the ultrafiltration influent volume is 97:100.

[0039] S8. Perform online chemical cleaning (CIP) on ED-1 and ED-2 membrane stacks, following the sequence of alkaline washing-water washing-acid washing-water washing. The volume ratio of the alkaline washing solution used to the liquid holding capacity inside the membrane stack is 10:1, and the mass ratio of sodium hydroxide in the alkaline washing solution to the mass of the alkaline washing solution is 0.5:100. The volume ratio of the acid washing solution used to the liquid holding capacity inside the membrane stack is 10:1, and the mass ratio of nitric acid in the acid washing solution to the mass of the acid washing solution is 0.5:100. During the water washing process, the volume ratio of the water washing solution used per cycle to the liquid holding capacity inside the membrane stack is 10:1.

[0040] Example 2

[0041] This embodiment provides a method for the electrodialysis recovery and reuse of sodium nitrate washing solution in the nitric acid process for producing boehmite. The method specifically includes the following steps:

[0042] S1, the grading method is the same as in Example 1;

[0043] S2 involves a two-stage cascade filtration process for the primary washing solution. The coarse filter has a pore size of 10 μm, and the fine filter has a pore size of 1.0 μm. The ratio of the processing volumetric flow rate to the effective filtration area for the fine filter is 1900 L·h. -1 1m 2 The silica removal adsorption bed medium is alumina particles. The volume ratio of the silica removal adsorption bed to the primary washing liquid is 1:30, and the volumetric flow rate through the adsorption bed is 30 h⁻¹. -1 The ratio of the mass of the adsorption medium to the mass of the primary washing solution is 0.2:100, and colloidal protection treatment is completed.

[0044] S3, add nitric acid to the electrodialysis feed, with a nitric acid to primary wash solution mass ratio of 0.5:100. Adjust the pH to 1.5 and temperature to 40℃ before entering ED-1. The ED-1 membrane stack uses Fumasep FAS and FKS series membranes to form membrane pairs, with the ratio of acid-blocking and acid-limiting migration units to the total number of membrane pairs being 1:5. The pH signal is collected on the ED-1 dilution side, and the pH change rate is calculated differentially according to a sampling period of 60 seconds. The circulation flow rate on the dilution side is kept constant, and the fluctuation amplitude of the circulation flow rate is no greater than 0.5:100 compared to the set circulation flow rate. The number of current levels for current adjustment is 3, the ratio of the current setting difference between adjacent levels to the normal operating current setting value is 20:100, and the ratio of the ED-1 operating current to the limiting current is 0.50:1. The difference between the upper and lower limits of the preset pH range is 0.2. When the pH exceeds the limit, the current is adjusted down to the preset current. The ratio of the preset current to the normal operating current setting is 80:100. The ratio of the controller's current adjustment range to the current is 0.05. The ratio of the ED-1 dilution circuit volumetric flow rate to the concentration circuit volumetric flow rate is 1:1. The ratio of the dilution circuit circulation rate to the dilution circuit fresh feed rate is 1:1. The ratio of the concentration circuit circulation rate to the concentration circuit discharge rate is 20:1. The ratio of the dilution circuit volumetric flow rate to the effective membrane area is 120 L·h. -1 1m 2 The ratio of the volumetric flow rate of the concentration loop to the effective membrane area is 600 L·h. -1 1m 2 ED-1 outputs recycled freshwater and concentrate;

[0045] S4, ED-1 concentrate enters ED-2, the ratio of ED-2 operating current to limiting current is 0.85:1, the ratio of ED-2 concentration circuit volumetric flow rate to dilution circuit volumetric flow rate is 1:2, the ratio of concentration circuit circulation rate to concentration circuit discharge rate is 3:1, and the ratio of concentration circuit volumetric flow rate to effective membrane area is 150 L·h. -1 1m 2 The mass fraction of sodium nitrate in the final solution was controlled at 20.0%.

[0046] S5, the concentrated liquid before the terminal enters the evaporation and concentration unit. The ratio of the amount of water removed by evaporation to the amount of feed liquid entering the terminal concentration unit is 1:100, resulting in concentrated sodium nitrate liquid.

[0047] S6. When recycling fresh water, the ratio of the flow rate of recycled fresh water introduced into the three-stage washing water supply pipeline to the flow rate of recycled fresh water introduced into the two-stage washing water supply pipeline is 1:1. Fresh pure water is introduced, and the ratio of the flow rate of fresh pure water to the flow rate of recycled fresh water is 30:100.

[0048] S7, the ratio of sewage discharge to the total flow rate of recycled freshwater in the recycling pipeline is 0.2:100, and the ratio of bypass treatment capacity to recycled freshwater in the bypass treatment pipeline is 20:100. The bypass treatment branch is equipped with an ultrafiltration unit connected in series with a resin adsorption bed; the ratio of ultrafiltration permeate to ultrafiltration feed water is 85:100, and the ratio of bypass treatment volumetric flow rate to resin adsorption bed volume is 60 h⁻¹. -1 ;

[0049] S8, CIP is performed according to the following steps: alkaline wash-water wash-acid wash-water wash. The volume ratio of the alkaline wash solution used in the alkaline wash to the liquid holding capacity inside the membrane stack is 3:1, and the mass ratio of sodium hydroxide in the alkaline wash solution to the mass of the alkaline wash solution is 0.2:100. The volume ratio of the acid wash solution used in the acid wash to the liquid holding capacity inside the membrane stack is 3:1, and the mass ratio of nitric acid in the acid wash solution to the mass of the acid wash solution is 0.1:100. The volume ratio of the water wash solution used in a single water wash to the liquid holding capacity inside the membrane stack is 3:1.

[0050] Example 3

[0051] This embodiment provides a method for the electrodialysis recovery and reuse of sodium nitrate washing solution in the nitric acid process for producing boehmite. The method specifically includes the following steps:

[0052] S1, the grading method is the same as in Example 1;

[0053] S2, with a coarse filter pore size of 50 μm and a precision filter pore size of 0.1 μm, has a precision filter volumetric flow rate to effective filter area ratio of 1400 L·h. -1 1m 2 The silica removal adsorption bed medium is zirconium oxide particles. The volume ratio of the silica removal adsorption bed to the primary washing liquid is 1:200, and the volumetric flow rate through the adsorption bed is 12 h⁻¹. -1 The ratio of the mass of the adsorption medium to the mass of the primary washing solution is 0.6:100, and colloidal protection treatment is completed.

[0054] S3, add nitric acid to the feed, with a nitric acid to primary wash solution mass ratio of 0.1:100. Adjust the pH to 2.6 and the temperature to 34℃ before entering ED-1. ED-1 uses a Neosepta AMX / CMX membrane pair and incorporates acid-blocking and acid-limiting migration units. The ratio of the number of acid-blocking and acid-limiting migration units to the total number of membrane pairs is 1:50. pH is collected on the dilution side, and the pH change rate is calculated differentially at 10-second sampling intervals. The circulation flow rate on the dilution side remains constant, with the fluctuation amplitude of the circulation flow rate not exceeding 2:100 compared to the set circulation flow rate. The number of current levels for current adjustment is 8, the ratio of the current setting difference between adjacent levels to the normal operating current setting is 8:100, and the ratio of the ED-1 operating current to the limiting current is 0.90:1. pH The difference between the upper and lower limits of the preset range is 1.5. When the pH exceeds the limit, the current is adjusted down to the preset current. The ratio of the preset current setting to the normal operating current setting is 30:100. The ratio of the controller's current adjustment range to the current setting is 0.15. The ratio of the ED-1 dilution loop volumetric flow rate to the concentration loop volumetric flow rate is 3:1. The ratio of the dilution loop circulation rate to the dilution loop fresh feed rate is 8:1. The ratio of the concentration loop circulation rate to the concentration loop discharge rate is 8:1. The ratio of the dilution loop volumetric flow rate to the effective membrane area is 800 L·h. -1 1m 2 The ratio of the volumetric flow rate of the concentration loop to the effective membrane area is 250 L·h. -1 1m 2 ;

[0055] S4, ED-1 concentrate enters ED-2, the ratio of ED-2 operating current to limiting current is 0.60:1, the ratio of ED-2 concentration circuit volumetric flow rate to dilution circuit volumetric flow rate is 1:1.5, the ratio of concentration circuit circulation rate to concentration circuit discharge rate is 20:1, and the ratio of concentration circuit volumetric flow rate to effective membrane area is 1200 L·h. -1 1m 2 The mass fraction of sodium nitrate in the final solution was controlled at 19.4%.

[0056] S5, the ratio of water removed by evaporation to feed liquid entering the terminal concentration unit is 6:100, to obtain concentrated sodium nitrate solution;

[0057] S6, the ratio of the recycled fresh water flow rate introduced into the three-stage washing and replenishment pipeline to the recycled fresh water flow rate introduced into the two-stage washing and replenishment pipeline is 5:1, and the ratio of the fresh pure water replenishment flow rate to the recycled fresh water replenishment flow rate is 0.5:100.

[0058] S7, the ratio of sewage discharge volume in the sewage branch to the total flow rate of recycled freshwater in the recycled freshwater circulation pipeline is 5:100, and the ratio of bypass treatment volume in the bypass treatment branch to the recycled freshwater circulation volume in the recycled freshwater circulation pipeline is 12:100; the bypass treatment unit is set up in parallel with the ultrafiltration unit and the activated carbon adsorption bed, the ratio of ultrafiltration permeate volume to ultrafiltration influent volume is 93:100, and the ratio of bypass treatment volumetric flow rate to activated carbon adsorption bed volume is 10h^-1;

[0059] In S8, the ratio of the volume of alkaline washing solution used in CIP to the liquid holding capacity inside the membrane stack is 20:1, and the mass ratio of sodium hydroxide in the alkaline washing solution to the mass of the alkaline washing solution is 1.0:100. The ratio of the volume of acid washing solution used in acid washing to the liquid holding capacity inside the membrane stack is 20:1, and the mass ratio of nitric acid in the acid washing solution to the mass of the acid washing solution is 0.2:100. The ratio of the volume of water washing solution used in a single water washing process to the liquid holding capacity inside the membrane stack is 20:1.

[0060] Example 4

[0061] This embodiment provides a method for the electrodialysis recovery and reuse of sodium nitrate washing solution in the nitric acid process for producing boehmite. The method specifically includes the following steps:

[0062] S1, the grading method is the same as in Example 1;

[0063] S2, with a coarse filter pore size of 25μm and a precision filter pore size of 0.8μm, and a precision filter volumetric flow rate to effective filter area ratio of 800L·h. -1 1m 2 The silica removal adsorption bed medium is a mixture of alumina particles and zirconium oxide particles. The volume ratio of the silica removal adsorption bed to the primary washing liquid is 1:80, and the volumetric flow rate through the adsorption bed is 20 h⁻¹. -1 The ratio of the mass of the adsorption medium to the mass of the primary washing solution is 1.2:100, and colloidal protection treatment is completed.

[0064] S3, add nitric acid to the feed, with a nitric acid to primary wash solution mass ratio of 0.25:100, adjust pH to 4.0, and set temperature to 28℃ before entering ED-1; ED-1 uses a Neosepta AMX / CMX membrane pair and incorporates acid-blocking and acid-limiting migration units, with the ratio of the number of acid-blocking and acid-limiting migration units to the total number of membrane pairs being 1:20; pH is collected on the dilution side, and the pH change rate is calculated differentially at 2-second sampling intervals. The circulation flow rate on the dilution side remains constant, with the fluctuation amplitude of the circulation flow rate not exceeding 5:100 compared to the set circulation flow rate; the number of current levels for current adjustment is 5, the ratio of the current setting difference between adjacent levels to the normal operating current setting is 12:100, and the ratio of the ED-1 operating current to the limiting current is 0.65:1; pH pre-... The upper and lower limits of the range are set to be 0.6. When the pH exceeds the limit, the current is adjusted down to the preset current. The ratio of the preset current to the normal operating current setting is 50:100. The ratio of the controller's current adjustment range to the current is 0.4. The ratio of the ED-1 dilution loop volumetric flow rate to the concentration loop volumetric flow rate is 1.8:1. The ratio of the dilution loop circulation rate to the dilution loop fresh feed rate is 5:1. The ratio of the concentration loop circulation rate to the concentration loop discharge rate is 15:1. The ratio of the dilution loop volumetric flow rate to the effective membrane area is 300 L·h. -1 1m 2 The ratio of the volumetric flow rate of the concentration loop to the effective membrane area is 450 L·h. -1 1m 2 ;

[0065] S4, ED-1 concentrate enters ED-2, the ratio of ED-2 operating current to limiting current is 0.72:1, the ratio of ED-2 concentration circuit volumetric flow rate to dilution circuit volumetric flow rate is 1:1, the ratio of concentration circuit circulation rate to concentration circuit discharge rate is 10:1, and the ratio of concentration circuit volumetric flow rate to effective membrane area is 500 L·h. -1 1m 2 The mass fraction of sodium nitrate in the final solution was controlled at 18.2%.

[0066] S5, the ratio of water removed by evaporation to feed liquid entering the terminal concentration unit is 10:100, to obtain concentrated sodium nitrate solution;

[0067] S6, the ratio of the recycled fresh water flow rate introduced into the three-stage washing and replenishment pipeline to the recycled fresh water flow rate introduced into the two-stage washing and replenishment pipeline is 2.5:1, and the ratio of the fresh pure water replenishment flow rate to the recycled fresh water replenishment flow rate is 15:100.

[0068] S7, the ratio of sewage discharge volume in the sewage branch to the total flow rate of recycled freshwater in the recycled freshwater circulation pipeline is 1:100, and the ratio of bypass treatment volume in the bypass treatment branch to the recycled freshwater circulation volume in the recycled freshwater circulation pipeline is 5:100; the bypass treatment unit is an ultrafiltration unit, and the ratio of ultrafiltration permeate volume to ultrafiltration influent volume is 99:100.

[0069] In S8, the ratio of the volume of alkaline washing solution used in CIP to the liquid holding capacity inside the membrane stack is 5:1, and the mass ratio of sodium hydroxide in the alkaline washing solution to the mass of the alkaline washing solution is 0.8:100. The ratio of the volume of acid washing solution used in acid washing to the liquid holding capacity inside the membrane stack is 5:1, and the mass ratio of nitric acid in the acid washing solution to the mass of the acid washing solution is 0.35:100. The ratio of the volume of water washing solution used in a single water washing process to the liquid holding capacity inside the membrane stack is 5:1.

[0070] Comparative Example 1

[0071] This comparative example provides an electrodialysis recovery and reuse method for sodium nitrate washing liquid in the nitric acid process for producing boehmite. The difference between this method and Example 1 is that in step S2, the primary washing liquid is only subjected to coarse and fine filtration, without silicon removal and colloidal protection treatment. The filtered liquid is directly used as electrodialysis feed into step S3. Other process parameters and operating conditions are exactly the same as in Example 1.

[0072] Comparative Example 2

[0073] This comparative example provides an electrodialysis method for the recovery and reuse of sodium nitrate washing solution in the nitric acid process for producing boehmite. The difference between this method and Example 1 is that the ED-1 membrane stack in step S3 does not have acid blocking and acid migration limiting units. ED-1 is composed of alternating anion exchange membranes and cation exchange membranes. Other process parameters and operating conditions are exactly the same as in Example 1.

[0074] Comparative Example 3

[0075] This comparative example provides an electrodialysis recovery and reuse method for sodium nitrate washing solution in the nitric acid process for producing boehmite. The difference between this method and Example 1 is that in step S3, the pH signal is still collected on the ED-1 dilution side and the circulation flow rate on the dilution side remains constant, but the pH change rate signal is not calculated. The controller does not adjust the ED-1 current in stages based on the pH change rate. The ED-1 power supply operates in constant current mode. Other process parameters and operating conditions are exactly the same as in Example 1.

[0076] Dissolved silicon content (as SiO2): The test object is the recycled fresh water obtained from the ED-1 dilution side outlet in step S3. The sampling location is set at the sampling valve of the pipeline before the ED-1 dilution side outlet water enters the step S6 graded reprocessing and before it is mixed with fresh pure water. The detection method is the silicomolybdenum blue spectrophotometric method: After sampling, the sample is immediately filtered through a 0.45μm filter membrane. A certain volume of filtrate is added to acidic ammonium molybdate reagent to generate silicomolybdenum heteropolyacid, and then a reducing agent is added to generate molybdenum blue. The absorbance is measured at the selected wavelength on the spectrophotometer. A calibration curve is plotted using silicon standard solution, and the absorbance is converted into the mass concentration as SiO2.

[0077] Turbidity: The test object is the recycled fresh water obtained from the ED-1 dilution side outlet in step S3. The sampling location is the same as above, and the sample is not filtered. The detection method is the turbidity meter scattering light method: calibrate the zero point and range with formalin turbidity standard solution according to the instrument requirements, slowly pour the sample into the colorimetric bottle, remove air bubbles and wipe the outer wall before reading the turbidity on the instrument; if necessary, dilute high turbidity samples according to the specified multiple and convert the turbidity.

[0078] Free nitric acid: The test object is the recycled fresh water obtained from the ED-1 dilution outlet in step S3, and the sampling location is the same as above; the detection method is the sodium hydroxide standard solution titration method: a certain volume of sample is placed in an Erlenmeyer flask, monitored with a pH meter or with phenolphthalein as an indicator, and titrated with a calibrated NaOH standard solution to the endpoint (the pH endpoint or color endpoint remains stable), and the volume consumed is recorded; the molar concentration of free nitric acid is calculated based on the amount of NaOH used and the sample volume and expressed as HNO3.

[0079] Table 1 shows the test results of the electrodialysis recovery and utilization method of sodium nitrate washing solution in the nitric acid process of Examples 1-4 and Comparative Examples 1-3.

[0080] Table 1. Test results of electrodialysis recovery and utilization method of sodium nitrate washing solution in nitric acid process of Examples 1-4 and Comparative Examples 1-3

[0081]

[0082] As shown in Table 1, compared with Example 1, the dissolved silicon content, turbidity, and free nitric acid content of Comparative Example 1 increased; the dissolved silicon content, turbidity, and free nitric acid content of Comparative Example 2 increased; and the dissolved silicon content, turbidity, and free nitric acid content of Comparative Example 3 increased.

[0083] This is because, in Comparative Example 1, after removing the desilicon and colloidal protection, silicic acid, silica condensates, and colloidal particles are retained with the dilution side effluent. Dissolved silica and turbidity increase simultaneously, and colloids deposit on the membrane surface to form a capping layer, increasing local mass transfer resistance. During operation, the deposited layer peels off and enters the effluent channel, further increasing turbidity. In Comparative Example 2, without acid-blocking and acid migration-limiting units, free nitric acid in the dilution side effluent increases, altering interfacial distribution and local potential distribution, making the pH on the dilution side more prone to shift. In Comparative Example 3, without current-level adjustment based on pH change rate and operating at constant current, acidity drift accumulates on the dilution side, free nitric acid rises, and enhanced local polarization increases the probability of colloid deposition and re-peeling on the membrane surface, resulting in an upward shift in effluent turbidity.

[0084] The above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for electrodialysis recovery and reuse of sodium nitrate washing solution in the nitric acid process for producing boehmite, characterized in that, Includes the following steps: S1, the washing solution containing sodium nitrate produced by the nitric acid process is divided into primary washing solution, secondary washing solution and tertiary washing solution according to the washing level; S2, the primary washing solution is subjected to coarse filtration and fine filtration in sequence, and silicon removal and colloidal protection treatment is performed to obtain electrodialysis feed; S3, after adjusting the electrodialysis feed to the preset temperature and acidity windows, it enters the first electrodialysis stage ED-1. ED-1 uses a membrane stack composed of anion exchange membranes and cation exchange membranes, and is equipped with acid blocking and acid migration limiting units. The pH signal is collected on the dilution side of ED-1 and the pH change rate signal is calculated. The circulation flow rate on the dilution side of ED-1 is kept constant. The controller adjusts the current of ED-1 in stages based on the pH change rate signal. When the pH signal exceeds the preset range, the controller reduces the current of ED-1 to the preset current value, and ED-1 outputs recycled fresh water and concentrate from ED-1. S4, the concentrate in ED-1 is fed into the second stage of electrodialysis ED-2 for concentration, and the concentrated solution is output from the terminal of ED-2. S5, the pre-terminal concentrate is fed into the terminal concentration unit for evaporation and concentration to obtain sodium nitrate concentrate; S6, the recycled fresh water is introduced into the tertiary wash water supply pipeline and the secondary wash water supply pipeline respectively, and fresh pure water is introduced into the tertiary wash water supply pipeline and the secondary wash water supply pipeline; S7, sewage branch and bypass treatment branch are set up on the freshwater recycling pipeline respectively; S8, perform online chemical cleaning (CIP) on the membrane stacks of ED-1 and ED-2.

2. The method for electrodialysis recovery and reuse of sodium nitrate washing solution in the nitric acid process for producing boehmite according to claim 1, characterized in that, The coarse filtration and fine filtration are two stages of filtration in series, and the ratio of coarse filtration accuracy to fine filtration accuracy is (10-50):(0.1-1); the ratio of the treatment volumetric flow rate to the effective filtration area of ​​the fine filtration is (200-2000) L·h. -1 1m 2 The ratio of the flow rate of recycled fresh water introduced into the tertiary washing and replenishment pipeline to the flow rate of recycled fresh water introduced into the secondary washing and replenishment pipeline is (1-5):1, and the ratio of the replenishment flow rate of fresh pure water to the replenishment flow rate of recycled fresh water is (0.1-30):

100.

3. The method for electrodialysis recovery and reuse of sodium nitrate washing solution in the nitric acid process for producing boehmite according to claim 1, characterized in that, The silicon removal and colloidal protection treatment includes a silicon removal adsorption bed. The adsorption medium of the silicon removal adsorption bed is selected from at least one of alumina particles, zirconium oxide particles, and alumina-zirconia composite oxide particles. The volume ratio of the silicon removal adsorption bed to the volume of the primary washing solution is 1:(20-200), and the volumetric flow rate through the adsorption bed to the volume of the adsorption bed is (3-30):1h. -1 The ratio of the mass of the adsorption medium to the mass of the primary washing solution is (0.1-2):

100.

4. The method for electrodialysis recovery and reuse of sodium nitrate washing solution in the nitric acid process for producing boehmite according to claim 1, characterized in that, The online chemical cleaning CIP in step S8 includes an alkaline wash-water wash-acid wash-water wash sequence; the volume ratio of the alkaline wash solution used in the alkaline wash to the liquid holding capacity inside the membrane stack is (2-20):1, and the mass ratio of sodium hydroxide in the alkaline wash solution to the mass of the alkaline wash solution is (0.1-1.0):100; the volume ratio of the acid wash solution used in the acid wash to the liquid holding capacity inside the membrane stack is (2-20):1, and the mass ratio of nitric acid in the acid wash solution to the mass of the acid wash solution is (0.05-0.5):100; the volume ratio of the water wash solution used in a single water wash to the liquid holding capacity inside the membrane stack is (2-20):

1.

5. The method for electrodialysis recovery and reuse of sodium nitrate washing solution in the nitric acid process for producing boehmite according to claim 1, characterized in that, In the ED-1 membrane stack, the ratio of the number of acid-blocking and acid-limiting migration units to the total number of membrane pairs is 1:(5-50); in step S3, nitric acid is added to the electrodialysis feed, and the mass ratio of nitric acid to the primary washing solution is (0.01-0.5):100; the pH of the electrodialysis feed is in the range of 1.5-4.0, and the temperature of the electrodialysis feed is 20-40℃.

6. The method for electrodialysis recovery and reuse of sodium nitrate washing solution in the nitric acid process for producing boehmite according to claim 1, characterized in that, The ratio of the volumetric flow rate of the dilution loop to that of the concentration loop for ED-1 is (1-3):1; the ratio of the circulation rate of the dilution loop to the fresh feed rate of the dilution loop is (1-10):1; the ratio of the circulation rate of the concentration loop to the discharge rate of the concentration loop is (2-20):1; and the ratio of the volumetric flow rate of the dilution loop to the effective membrane area of ​​ED-1 is (100-800) L·h. -1 1m 2 The ratio of the concentration loop volumetric flow rate to the effective membrane area of ​​ED-1 is (80-600) L·h -1 1m 2 .

7. The method for electrodialysis recovery and reuse of sodium nitrate washing solution in the nitric acid process for producing boehmite according to claim 1, characterized in that, The ratio of the operating current to the limiting current of ED-1 is (0.5-0.9):1; the ratio of the operating current to the limiting current of ED-2 is (0.4-0.85):1; the ratio of the current adjustment range of the controller to the current is 0.05-0.4; the circulating flow rate on the dilution side of ED-1 is kept constant such that the ratio of the fluctuation range of the circulating flow rate of the ED-1 dilution circuit to the set circulating flow rate is no greater than (0.5-5):

100.

8. The method for electrodialysis recovery and reuse of sodium nitrate washing solution in the nitric acid process for producing boehmite according to claim 1, characterized in that, The graded adjustment includes multiple current levels, with 3-8 current levels in total. The ratio of the current setting difference between adjacent current levels to the normal operating current setting is (1-20):

100. The preset range is the difference between the upper and lower pH limits on the ED-1 dilution side, which is 0.2-1.

5. When the pH signal exceeds the preset range, the ratio of the current setting of the preset level to the normal operating current setting is (30-80):

100. The pH change rate signal is the ratio of the pH difference between adjacent sampling periods to the sampling period, where the sampling period is 2-60s.

9. The method for electrodialysis recovery and reuse of sodium nitrate washing solution in the nitric acid process for producing boehmite according to claim 1, characterized in that, The ratio of the volumetric flow rate of the ED-2 concentration loop to the volumetric flow rate of the dilution loop is 1:(0.5-2); the ratio of the circulation rate of the concentration loop to the discharge rate of the concentration loop is (3-30):1; and the ratio of the volumetric flow rate of the concentration loop to the effective membrane area of ​​ED-2 is (150-1200) L·h. -1 1m 2 In step S4, the sodium nitrate mass fraction of the concentrate before the terminal is 18-20%; in step S5, the ratio of the amount of water removed by evaporation to the amount of feed liquid entering the terminal concentration unit is (1-15):

100.

10. The method for electrodialysis recovery and reuse of sodium nitrate washing solution in the nitric acid process for producing boehmite according to claim 1, characterized in that, In step S7, the ratio of the sewage discharge volume of the sewage branch to the total flow rate of the recycled freshwater in the recycled freshwater circulation pipeline is (0.2-5):100; the ratio of the bypass treatment volume of the bypass treatment branch to the recycled freshwater circulation volume of the recycled freshwater in the recycled freshwater circulation pipeline is (0.5-20):100; the bypass treatment branch is connected to a bypass treatment unit, which is selected from at least one of activated carbon adsorption, resin adsorption, and ultrafiltration; when the bypass treatment unit is an ultrafiltration unit, the ratio of ultrafiltration permeate volume to ultrafiltration feed volume is (85-99):100; when the bypass treatment unit is an adsorption bed, the ratio of the bypass treatment volumetric flow rate to the adsorption bed volume is (5-100):1h. -1 .

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