Preparation method of caustic soda

Abstract

The invention belongs to the technical field of caustic soda, and provides a preparation method of caustic soda. The method comprises the following steps: primary brine refining; performing secondary brine refining by adopting three-tower system, operating two towers, and regenerating in one tower for later use; in the electrolysis process, a high-current-density and natural circulation bipolar electrolytic cell ionic membrane method is adopted; a light salt brine dechlorination process; concentrating the alkali liquor; a chlorine gas treatment process; a hydrogen treatment process; and a waste gas treatment process. The raw and auxiliary materials adopted by the method are easy to obtain, convenient and safe to transport, low in raw material price and stable in technical and economic indexes; the caustic soda obtained by the high-current-density natural circulation bipolar electrolytic cell ionic membrane method is high in quality, can meet the quality requirements of chemical fiber, pharmacy and other industries on high-purity caustic soda, and is low in energy consumption and investment. No waste is produced in chlor-alkali production through the ionic membrane method, and the pollution problem of asbestos waste generated in alkali production through a diaphragm method and the like does not exist.

Description

Technical Field

[0001] The present invention relates to the technical field of caustic soda, and particularly to a preparation method of caustic soda. Background Art

[0002] Different bipolar electrolyzers have the following characteristics: ① Chlorine Engineering BiTAC electrolyzer: The unit area is 3.276 m, the operating current density is 5.5 KA / m, the rated current density is 6 KA / m, the electrodes have a baffle and a current equalizing structure, the current and the electrolyte concentration are evenly distributed, and the voltage drop is small. ② Wood-Denora electrolyzer: The unit area is 2.72 m 2 , the operating current density is 5.0 - 6.0 KA / m, the rated current density is 6 KA / m, the unit cell is composed of two half-shells of the cathode and the anode, which is convenient for disassembly and assembly, and the replacement and maintenance time is short. It is suitable for a relatively high electrolyzer operating pressure. The Wood-Denora electrolyzer has been incorporated into Chlorine Engineering. ③ Asahi Kasei NCH electrolyzer: The unit area is 2.7 m 2 , the operating current density is 5.0 - 5.5 KA / m, the rated current density is 6.0 KA / m, the electrolyzer operating pressure is high, and the Asahi Kasei NCH electrolyzer can provide both the electrolyzer and the ion exchange membrane at the same time. ④ North Chemical Machine MBC electrolyzer: The unit area is 2.7 m 2 , the operating current density is 4.5 - 5.0 KA / m, which is close to the characteristics and technical level of the Asahi Kasei electrolyzer, and the quotation is slightly lower. Each electrolyzer technology has its own characteristics and all are mature technologies.

[0003] For the dechlorination technology of brine, there are two methods: air stripping method and vacuum dechlorination method. The chlorine gas produced by the air stripping method has a low purity and can only be treated as waste gas, which reduces the chlorine recovery rate, and the operating conditions are relatively poor, so it has basically been phased out. Since the liquid caustic soda produced by the electrolyzer has a concentration of only 32 wt%, it is not conducive to long-distance transportation, and most of them need to produce 50 wt% liquid caustic soda and 99 wt% solid caustic soda. Therefore, first, the 32 wt% caustic soda solution needs to be concentrated to 50 wt%. For the concentration of caustic soda solution, there are generally single-effect, double-effect, and triple-effect processes. For the double-effect and triple-effect processes, there are also forward flow and reverse flow. Since the single-effect energy consumption is too high, the steam consumption is generally 1.2 - 1.3 t / tNaOH, so it is not adopted. The forward flow process has a lower energy utilization rate than the reverse flow process, so the reverse flow evaporation process is preferably selected. For the double-effect reverse flow rising film evaporation of the same scale, the investment is about 50% of that of the triple-effect reverse flow falling film evaporation, the building area is only 40 - 50% of that of the triple-effect falling film evaporation, the steam consumption is about 800 kg / tNaOH, while the steam consumption of the triple-effect falling film evaporation is about 500 kg / tNaOH.

[0004] For chlorine treatment, chlorine is mostly washed with chlorine water and cooled, and then the moisture in the chlorine is absorbed with concentrated sulfuric acid as the medium. Its process technologies include: packed tower + foam tower, multi-stage packed tower, and packed tower + bubble cap tower, etc. The drying effect of the packed tower + foam tower is poor, and the water content of chlorine after drying is generally 300 - 500 ppm; the multi-stage packed tower has a large floor area and a large investment; the water content of chlorine after drying in the packed tower + bubble cap tower can be reduced to 50 ppm, or even below 30 ppm. It has fewer equipment and a small floor area, and is suitable for realizing automatic control. For chlorine transportation, there are chlorine pumps with concentrated sulfuric acid as the liquid ring and screw turbine machines (sets). The chlorine pump has a large consumption of sulfuric acid, a large floor area, a small exhaust volume, a large number of equipment units, and a large power consumption, and is suitable for the chlorine transportation of small-scale production devices; the screw turbine machine (set) has a large exhaust volume, a low power consumption, a small number of equipment units, a small floor area, and a high degree of automation, but the one-time investment is slightly larger.

[0005] The hydrogen produced by electrolysis has a high temperature, a large water content, and contains a small amount of NaOH. Therefore, it is usually washed and cooled to lower the temperature of hydrogen, reduce the water content, and remove the small amount of NaOH, and then sent out after compression and cooling. Hydrogen washing is divided into three methods: direct washing with production water, washing with alkaline circulating water, and closed-loop cooling circulation washing of the water in the washing tower by itself. The direct washing method with production water is prone to scaling and has a large water consumption; the method of washing with alkaline circulating water requires the construction of a separate alkaline circulating water pool; the closed-loop cooling circulation washing method of the water in the washing tower by itself removes heat through a heat exchanger to form a closed loop. There are two forms of hydrogen transportation: Roots blowers and water ring pump compressor units. The output pressure of the Roots blower is relatively low.

[0006] During the start-up, shutdown, and accident states of electrolysis, the chlorine-containing waste gas and accident chlorine need to be absorbed by alkali solution to prevent chlorine leakage accidents and ensure that the waste gas meets the discharge standards. For waste gas treatment, there are process technologies such as tank absorption, single-tower absorption, and two-tower absorption. Among them, the tank absorption has a poor effect, cannot operate continuously, and has poor safety; the quality of the tail gas in the single-tower absorption is unstable; while the two-tower series operation can overcome the above disadvantages, and it should be the first choice for large-scale devices.

[0007] Therefore, it is of great significance to obtain a preparation method of caustic soda with easily available raw and auxiliary materials, convenient transportation, safety, and low raw material prices. Summary of the Invention

[0008] The purpose of the present invention is to provide a preparation method of caustic soda in order to overcome the deficiencies of the prior art.

[0009] In order to achieve the above-mentioned invention purpose, the present invention provides the following technical solutions:

[0010] The present invention provides a preparation method of caustic soda, which comprises the following steps:

[0011] 1) Primary brine refining;

[0012] 2) The refined secondary brine adopts a three-column system, with two columns operating and one column in regeneration standby;

[0013] 3) In the electrolysis process, an ion-exchange membrane method with a high current density and a natural circulation bipolar electrolytic cell is adopted;

[0014] 4) In the process of dechlorination of dilute brine, a vacuum dechlorination method is adopted, with a liquid-ring vacuum pump as the vacuum source;

[0015] 5) In the caustic soda concentration process, a three-effect countercurrent falling-film evaporation process is adopted;

[0016] 6) In the chlorine treatment process, a packed tower + bubble-cap tower is used for drying, and a screw-type turbine unit is used for transportation;

[0017] In the hydrogen treatment process, a scrubbing tower uses the self-closed-loop circulation of water for scrubbing, and a large water-ring pump compressor unit is used for transportation;

[0018] 7) In the waste gas treatment process, a process of absorbing waste gas by circulating alkali liquor in series in two towers is adopted, and the waste gas is chlorine.

[0019] Preferably, in step 1), the saturated crude brine from the salt dissolving tank flows into the baffle tank, where sodium carbonate, caustic soda, and sodium hypochlorite are added for a refining reaction; the crude brine after the refining reaction is filtered in the intermediate tank to obtain a clear liquid; sodium sulfite is added to the clear liquid to remove chlorine, obtaining the primary refined brine.

[0020] Preferably, in step 2), the primary refined brine is pressurized and sequentially flows to the brine storage tank and the ion exchange resin tower, where multivalent cations are removed in the ion exchange resin tower to obtain the secondary refined brine; the gauge pressure for pressurization is 0.35 - 0.45 MPa; the multivalent cations include calcium ions and magnesium ions, and the content of multivalent cations in the secondary refined brine is ≤ 20 ppb.

[0021] Preferably, in step 3), the secondary refined brine sequentially flows into the brine elevated tank and the anode chamber of the electrolytic cell, and an aqueous sodium hydroxide solution is added to the cathode chamber for electrolysis; during the electrolysis process, chlorine is generated in the anode chamber and hydrogen is generated in the cathode chamber;

[0022] The chlorine and hydrogen are respectively sent to the chlorine treatment process and the hydrogen treatment process; the dilute brine after electrolysis is discharged from the anode chamber and sent to the dilute brine dechlorination process for dechlorination.

[0023] Preferably, in step 4), the dilute brine after electrolysis removes free chlorine in the vacuum dechlorination tower, and the dechlorinated dilute brine is adjusted to a pH value with alkali and then sodium sulfite solution is added for chemical dechlorination; the vacuum degree of the vacuum dechlorination tower is -0.07 - -0.065 MPa; the pH value is adjusted to 8 - 10.

[0024] Preferably, in step 5), the liquid caustic soda produced in the electrolysis process is evaporated with steam by a triple-effect countercurrent falling film evaporation process to remove part of the water, and liquid caustic soda with a mass concentration of 50% is obtained.

[0025] Preferably, in step 6), the wet chlorine gas produced in the electrolysis process is successively washed and cooled, and then sent to a packed tower through a water mist separator to contact countercurrently with concentrated sulfuric acid for the first dehydration; then sent to a bubble cap tower to contact countercurrently with concentrated sulfuric acid for the second dehydration and then subjected to acid mist trapping.

[0026] The wet hydrogen gas produced in the electrolysis process is successively washed, pressurized, cooled, and water mist trapped; the gauge pressure of the pressurization is 0.09 - 0.11 MPa.

[0027] The beneficial effects of the present invention include the following points:

[0028] 1) The raw and auxiliary materials used in the method of the present invention are easily available, convenient for transportation, safe, with low raw material prices and stable technical and economic indicators.

[0029] 2) The caustic soda obtained by the ion-exchange membrane method with a high current density and a natural circulation bipolar electrolytic cell in the present invention has high quality, can meet the quality requirements of high-purity caustic soda in industries such as chemical fiber and pharmaceutical industries, and has low energy consumption and less investment. The ion-exchange membrane method for chlor-alkali production saves 15 - 25% in cost compared with the diaphragm method for caustic soda production. There is no waste in the ion-exchange membrane method for chlor-alkali production in the present invention, and there are no pollution problems such as asbestos waste in the diaphragm method for caustic soda production; the chlorine gas produced has high purity, usually the chlorine gas purity > 99%, and it is a high-quality raw material for organic chlorine production; the hydrogen gas has high purity, usually the hydrogen gas purity > 99%, and it can be used in occasions requiring high-purity hydrogen. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a process flow chart for preparing caustic soda according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0031] The present invention provides a method for preparing caustic soda, which comprises the following steps:

[0032] 1) Primary brine purification;

[0033] 2) Secondary brine purification, adopting a three-column system, with two columns in operation and one column in regeneration standby;

[0034] 3) Electrolysis process, adopting the ion-exchange membrane method with a high current density and a natural circulation bipolar electrolytic cell;

[0035] 4) Dilute brine dechlorination process, adopting the vacuum dechlorination method with a liquid ring vacuum pump as the vacuum source;

[0036] 5) Alkali liquor concentration process, adopting a triple-effect countercurrent falling film evaporation process;

[0037] 6) Chlorine treatment process, using a packed tower + bubble cap tower for drying and a screw turbine unit for conveying;

[0038] Hydrogen treatment process, using a washing tower with closed-loop self-circulation of water for washing and a large water ring pump compressor unit for conveying;

[0039] 7) Exhaust gas treatment process, using a two-tower series alkali solution circulation absorption process for exhaust gas, and the exhaust gas is chlorine.

[0040] In step 1) of the present invention, the saturated crude brine from the salt dissolving tank flows into the baffle tank, where sodium carbonate, caustic soda, and sodium hypochlorite are added for a refining reaction; the crude brine after the refining reaction is filtered in the intermediate tank to obtain a clear liquid; sodium sulfite is added to the clear liquid to remove chlorine, obtaining the primary refined brine.

[0041] In the present invention, the addition amount of sodium carbonate is determined according to the calcium ion content in the raw salt and the excess alkali of 100 - 300 mg / L in the saturated brine; the amount of caustic soda is determined according to the magnesium ion content in the raw salt and 300 - 500 mg / L of the excess sodium carbonate; 30 mg of sodium hypochlorite is added to every 1 L of saturated brine.

[0042] In the present invention, the filtration sequence is membrane filtration feed pump rough filtration, membrane filtration. The rough filtration intercepts particles with a particle size of more than 1.0 mm. The clear liquid coming out of the membrane filtration device is added with sodium sulfite and then sent to the secondary brine refining, and is sent out of the boundary area by the primary brine pump; the concentrated liquid enters the salt sludge tank and is sent to a plate and frame filter press by a slurry pump for filtration. The clear liquid is recycled for salt dissolving, and the salt sludge is sent out of the boundary area.

[0043] In the present invention, in order to maintain a relatively high filtration capacity and a relatively low filtration pressure of the filtration membrane, physical backwashing and chemical cleaning are carried out regularly; for physical backwashing, the refined brine after filtration is pressurized by compressed air to perform an instantaneous reverse flushing on the membrane surface to remove the dirt attached to the membrane surface, and the filtration capacity is restored; the membrane is chemically cleaned with 10 - 15 wt% hydrochloric acid to thoroughly clean the membrane surface and completely restore the filtration flux; the concentration of hydrochloric acid is preferably 11 - 14 wt%, and more preferably 12 - 13 wt%.

[0044] In step 2) of the present invention, the primary refined brine is pressurized and sequentially flows to the brine storage tank and the ion exchange resin tower, and multivalent cations are removed in the ion exchange resin tower to obtain the secondary refined brine; the gauge pressure of the pressurization is preferably 0.35 - 0.45 MPa, and more preferably 0.4 MPa; the multivalent cations are trace multivalent cations, and the multivalent cations include calcium ions and magnesium ions. The content of multivalent cations in the secondary refined brine is preferably ≤20 ppb.

[0045] In the present invention, after the resin tower operates for 48 h, the resin adsorbs Ca 2+ 、Mg 2+The ability to remove polyvalent cations such as etc. decreases, and off-line regeneration is necessary. The regenerant is hydrochloric acid and caustic soda solution. After regeneration, the acidic and alkaline wastewaters are neutralized to pH = 7 - 9 and pumped to the primary brine for salt dissolution.

[0046] In step 3) of the present invention, the secondary refined brine sequentially flows into the brine elevated tank and the anode chamber of the electrolytic cell, and an aqueous sodium hydroxide solution is added to the cathode chamber for electrolysis; during the electrolysis process, chlorine gas is generated in the anode chamber and hydrogen gas is generated in the cathode chamber;

[0047] The chlorine gas and hydrogen gas are respectively sent to the chlorine gas treatment process and the hydrogen gas treatment process; the electrolyzed dilute brine is discharged from the anode chamber and sent to the dilute brine dechlorination process for dechlorination.

[0048] In the present invention, the anode is a titanium coating with an effective area of 2.7 m 2 ; the cathode is a nickel coating with an effective area of 2.7 m 2 ; the effective area of the ion exchange membrane is 2.7 m 2 ; the current density is 1 - 6 KA / m 2 . After electrolysis, there are remaining sodium ions in the solution. At this time, the catholyte in the cathode chamber contains OH - generated by electrolysis, and Na + passes through the ion exchange membrane from the anode chamber into the cathode chamber and combines with OH - generated by electrolysis in the cathode chamber to form a NaOH solution; the original catholyte (aqueous sodium hydroxide solution with a mass concentration of 30.5%) gradually increases in NaOH concentration to 32 wt% during electrolysis, flows to the alkali receiving tank, a part of which is mixed with pure water and then returned to the cathode chamber of the electrolytic cell, another part is cooled and stored and sold as a 32 wt% alkali solution product, and the rest is sent to evaporation and concentration to be concentrated into 50 wt% caustic soda for external sale.

[0049] In step 4) of the present invention, the electrolyzed dilute brine removes free chlorine in the vacuum dechlorination tower. After dechlorination, the dilute brine is adjusted to the pH value and then sodium sulfite solution is added for chemical dechlorination; the vacuum degree of the vacuum dechlorination tower is preferably -0.07 - -0.065 MPa, more preferably -0.069 - -0.068 MPa, and even more preferably -0.067 MPa; the pH value is preferably adjusted to 8 - 10, more preferably adjusted to 9.

[0050] In the present invention, after removing free chlorine by the vacuum dechlorination method, the content of free chlorine in the light brine is preferably 18 - 22 mg / L, more preferably 20 mg / L; the chlorine gas separated during the dechlorination process is cooled at 50°C by the dechlorination tower condenser, and after separating the moisture, it is returned to the main chlorine gas pipe by the vacuum pump; the condensed chlorinated water and the chlorinated water from the chlorine treatment are sent back to the dechlorination tower for dechlorination recycling. Since the light brine returns to the primary brine refining process for re-saturation after dechlorination and is recycled in this way, the content of chlorate in the light brine also accumulates and increases continuously. In order to decompose the chlorate generated in the anode chamber of the electrolytic cell, a part of the light brine (1 / 10 of the amount of light brine coming out of the electrolytic cell) is sent to the chlorate decomposition tank and excessive hydrochloric acid (twice the normal reaction amount) is added to decompose the chlorate into sodium chloride and chlorine gas for recovery.

[0051] In the present invention, for the dechlorinated light brine from the ion-exchange membrane electrolysis process, a part enters the membrane method for nitrate removal system, and after removing the sulfate radical therein, it is sent to the water distribution tank; another part of the light brine directly enters the water distribution tank, and after being mixed with the filtrate recovered from the salt sludge pressure filtration, the steam condensate from the evaporation process, the secondary steam condensate, the hydrogen washing liquid from the hydrogen treatment, and the process water from other processes for water mixing, it is sent to the salt dissolving tank for salt dissolution by a pressure pump, and the concentrated nitrate water is discharged to the sewage treatment plant.

[0052] In step 5) of the present invention, the liquid caustic soda produced in the electrolysis process is evaporated to remove part of the water by the triple-effect countercurrent falling-film evaporation process to obtain liquid caustic soda with a mass concentration of 50%.

[0053] In step 6) of the present invention, the wet chlorine gas produced in the electrolysis process is washed and cooled in sequence, and then sent to the packing tower through a water mist separator to contact countercurrently with concentrated sulfuric acid for the first dehydration; then it is sent to the bubble-cap tower to contact countercurrently with concentrated sulfuric acid for the second dehydration and then is subjected to acid mist trapping;

[0054] The wet hydrogen gas produced in the electrolysis process is washed, pressurized, cooled, and water mist trapped in sequence; the gauge pressure for pressurization is preferably 0.09 - 0.11 MPa, more preferably 0.1 MPa.

[0055] In the present invention, the cooling of wet chlorine gas and wet hydrogen gas is preferably carried out by using chilled water, the temperature of the chilled water is preferably 5 - 7°C, more preferably 6°C, the cooling temperature of chlorine gas and hydrogen gas is preferably 12 - 15°C, more preferably 13 - 14°C; the gauge pressure for pressurization; the concentration of concentrated sulfuric acid is preferably 97 - 99 wt%, more preferably 98 wt%.

[0056] In the present invention, in order to keep the hydrogen gas pressure in the electrolytic cell constant, a reflux hydrogen gas pipeline is provided at the distribution table and connected to the inlet of the hydrogen gas compressor, and a regulating valve is used for adjustment; the reflux flow rate is automatically adjusted according to the change in the pressure of the main hydrogen gas pipe coming from the electrolysis process to keep the pressure of the hydrogen gas coming from the electrolysis process stable.

[0057] In the present invention, the hydrogen flow rate is determined according to the caustic soda production. Trace amounts of alkali mist are entrained in the hydrogen, and water washing is used for cooling. The water after washing is alkaline.

[0058] The following are detailed descriptions of the technical solutions provided by the present invention in conjunction with embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0059] Embodiment 1

[0060] The preparation process of caustic soda includes the following steps:

[0061] 1) Primary brine purification: The primary brine membrane filtration process of Hangjin Chemical Industry Company is adopted for primary brine purification. Before the saturated crude brine flowing out from the salt dissolving tank flows into the electrolytic cell, it first flows into the baffle tank. Sodium carbonate, caustic soda, and sodium hypochlorite, the refining agents, are respectively added into the baffle tank. Calcium carbonate crystal precipitates are formed by the reaction of sodium carbonate with calcium ions in the crude brine, and magnesium hydroxide colloid precipitates are formed by the reaction of caustic soda with magnesium ions in the crude brine (the addition amount of sodium carbonate is determined according to the calcium ion content in the raw salt and the excess alkali of 200 mg / L in the saturated brine; the amount of caustic soda is determined according to the magnesium ion content in the raw salt and the excess sodium carbonate of 400 mg / L; 30 mg of sodium hypochlorite is added to each 1 L of saturated brine).

[0062] The crude brine after the completion of the refining reaction flows into the intermediate tank by gravity, and after being intercepted by large particle impurities above 1.0 mm by a coarse filter by a membrane filtration feed pump, it is sent to the membrane filtration device. The clear liquid coming out of the membrane filtration device is added with sodium sulfite to remove chlorine, obtaining primary refined brine, which is sent to the secondary brine purification by a primary brine pump; the concentrated liquid enters the salt sludge tank and is sent to a plate and frame filter press by a slurry pump for filtration. The clear liquid is recycled for salt dissolving, and the salt sludge is sent out of the boundary area.

[0063] 2) Secondary brine purification, adopting a three-tower system, with two towers in operation and one tower in standby for regeneration. The primary refined brine is pressurized by a pump (0.4 MPa gauge pressure) and sent to the brine storage tank of the secondary brine purification process, and then pressurized by a pump and sent into the ion exchange resin tower to remove trace amounts of multivalent cations such as trace calcium ions and trace magnesium ions in the brine, so that the content of multivalent cations is less than 20 ppb. The secondary refined brine coming out of the ion exchange resin tower enters the brine high-level tank, and then flows from the brine high-level tank into the anode of the electrolytic cell in the electrolysis process.

[0064] 3) Electrolysis process, adopting the ion-exchange membrane method of a high current density and natural circulation bipolar electrolytic cell, the North Chemical Machine MBC electrolytic cell. The secondary refined brine sequentially flows into the brine high-level tank and the anode chamber of the electrolytic cell, and an aqueous sodium hydroxide solution is added to the cathode chamber for electrolysis; during the electrolysis process, chlorine gas is generated in the anode chamber and hydrogen gas is generated in the cathode chamber; the chlorine gas generated in the anode chamber of the electrolytic cell and the hydrogen gas generated in the cathode chamber are respectively sent to the chlorine treatment process and the hydrogen treatment process; the electrolyzed dilute brine is discharged from the anode chamber and sent to the dechlorination tower of the dilute brine dechlorination process for dechlorination.

[0065] The anode is a titanium coating with an effective area of 2.7 m 2 ; the cathode is a nickel coating with an effective area of 2.7 m 2 ; the effective area of the ion-exchange membrane is 2.7 m 2 ; the current density is 1 - 6 KA / m 2 . After electrolysis, sodium ions remain in the solution. At this time, the catholyte in the cathode chamber contains OH - and Na + that pass through the ion-exchange membrane from the anode chamber into the cathode chamber and combine with the OH - generated by electrolysis in the cathode chamber to form a NaOH solution; the original catholyte (aqueous sodium hydroxide solution with a mass concentration of 30.5%) gradually increases in NaOH concentration to 32 wt% during electrolysis, flows to the alkali receiving tank, part of it is mixed with pure water and then returned to the cathode chamber of the electrolyzer, and the other part is cooled and stored and sold as a 32 wt% alkali solution product, and the rest is sent for evaporation and concentration to be concentrated into 50 wt% caustic soda for external sale.

[0066] 4) The dechlorination process of the brine uses the vacuum dechlorination method with a liquid ring vacuum pump as the vacuum source. The brine generated by electrolysis enters the dechlorination tower, and the free chlorine dissolved in the brine is removed under a vacuum condition of -0.068 MPa. The dechlorinated brine contains 20 mg / L of free chlorine. After adjusting the pH value to 9 with alkali, a sodium sulfite solution is added to completely remove the free chlorine.

[0067] The dechlorinated brine is sent to the primary brine purification process to be used as salt-making water; the chlorine gas separated during the dechlorination process is cooled by the dechlorination tower condenser at 50 °C, and after separating the moisture, it is returned to the chlorine gas main pipe by the vacuum pump; the condensed chlorinated water and the chlorinated water from chlorine treatment are sent back to the dechlorination tower for dechlorination recycling. Since the brine returns to the primary brine purification process for re-saturation after dechlorination and is recycled in this way, the chlorate content in the brine also accumulates and increases continuously. In order to decompose the chlorate generated in the anode chamber of the electrolyzer, part of the brine (1 / 10 of the brine volume coming out of the electrolyzer) is sent to the chlorate decomposition tank and an excessive amount of hydrochloric acid (twice the normal reaction amount) is added to decompose the chlorate into sodium chloride and chlorine gas for recovery.

[0068] 5) Alkaline solution concentration process, using a three-effect countercurrent falling film evaporation process. The 32 wt% alkaline solution produced in the electrolysis process is sent to the alkaline solution buffer tank, and then pumped to the top of the III-effect heat exchanger by a feed pump. The alkaline solution flowing down along the tube bundle is heated and concentrated to 36 wt% by the secondary steam (the secondary steam generated by the II-effect evaporator). The 36 wt% alkaline solution is pumped to two parallel preheaters by the III-effect alkaline pump, and after being heated by steam condensate and concentrated alkaline solution respectively here, it is sent to the top of the II-effect evaporator. The alkaline solution flowing down along the tube bundle is further concentrated to 42 wt% after being heated by the secondary steam (the secondary steam generated by the I-effect evaporator). The 42 wt% alkaline solution is pumped to two parallel preheaters by the II-effect alkaline pump, and after being heated by steam condensate and concentrated alkaline solution respectively here, it is sent to the top of the I-effect heat exchanger. The alkaline solution flowing down along the tube bundle is heated by saturated steam (175 °C) and its concentration is increased to 50 wt%. The hot alkaline solution is pumped to the preheater by the I-effect alkaline pump to recover the remaining heat, and then cooled to 45 °C by the finished product alkaline solution cooler and enters the 50 wt% alkaline solution tank. Finally, the 50 wt% alkaline solution is sent to the finished product tank for sale by the finished product pump.

[0069] The saturated medium-pressure steam enters the shell side of the I-effect heat exchanger to heat the alkaline solution. The condensed condensate is sent to the primary brine for salt dissolution after heat exchange with 42 wt% and 36 wt% alkaline solutions; the secondary steam generated by the III-effect heat exchanger enters the surface condenser for condensation. The condensate enters the steam condensate tank, and the non-condensable gas is pumped out by a water ring vacuum pump; the steam condensate obtained after the secondary steam generated by the I-effect heat exchanger heats the II-effect evaporator enters the steam condensate tank, and finally is pumped to the primary brine for salt dissolution by the steam condensate pump; the washing water and flushing water are collected and pumped to the 32 wt% alkaline solution buffer tank of this device.

[0070] 6) Chlorine treatment process, using a packed tower + bubble cap tower for drying and a screw type turbine unit for transportation; hydrogen treatment process, using a scrubbing tower with closed-loop self-circulation washing of water and a large water ring pump compressor unit for transportation. The wet chlorine gas produced in the electrolysis process is first washed in the chlorine gas scrubbing tower, then enters the chlorine gas cooler and is cooled with 6 °C chilled water to cool the chlorine gas temperature to 13 °C. After passing through the water mist separator, it is sent to the packed tower to contact countercurrently with concentrated sulfuric acid (98 wt%) for the first dehydration. The chlorine gas coming out of the packed tower is then sent to the bubble cap tower to contact countercurrently with concentrated sulfuric acid (98 wt%) for further dehydration. After passing through the acid mist eliminator, the water content in the chlorine gas is less than 50 ppm, and then it is compressed and pressurized by the chlorine gas compressor and sent to the chlorine gas users;

[0071] The wet hydrogen gas produced in the electrolysis process is first washed in the hydrogen gas scrubbing tower, then pressurized to 0.1 MPa gauge pressure by the hydrogen gas compressor, cooled with 6 °C chilled water to cool the hydrogen gas temperature to 13 °C, and then sent to the hydrogen gas users after removing a certain amount of water mist through the trap. When starting up, the hydrogen gas with a lower concentration is discharged into the atmosphere through the hydrogen gas flame arrester.

[0072] 7) Exhaust gas (chlorine gas) treatment process, adopting a process of two towers in series with caustic soda solution circulating to absorb exhaust gas. When the device starts up, shuts down, or is in an accident state, the chlorine gas is first absorbed in the absorption tower with the absorption liquid from the circulation tank. The tail gas after the absorption reaction then enters the tail gas tower for further absorption with caustic soda solution to make the discharged tail gas meet the standard (chlorine content is lower than 5mg / Nm 3 ) and is then sent out by a fan and discharged into the atmosphere at a height of 25m. At the same time, sodium hypochlorite is produced in this process.

[0073] The dechlorinated brine from the ion-exchange membrane electrolysis process, a part of it enters the membrane method denitrification system, after removing the sulfate radical in it, it is sent to the water distribution tank; another part of the brine directly enters the water distribution tank, and is mixed with the filtrate recovered from the salt mud pressure filtration, the steam condensate from the evaporation process, the secondary steam condensate, the hydrogen washing liquid in the hydrogen treatment, and the process water from other processes for water distribution, and then is sent to the salt dissolving tank for salt dissolving through a pressure pump.

[0074] Example 2

[0075] Replace the North Chemical Machine MBC electrolytic cell in Example 1 with the Asahi Kasei NCH electrolytic cell, and other conditions are the same as those in Example 1.

[0076] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art of this technology, without departing from the principle of the present invention, several improvements and refinements can still be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A method for preparing caustic soda, characterized in that, It includes the following steps: 1) Primary brine refining; 2) Secondary brine refining, adopting a three-column system, with two columns in operation and one column in regeneration standby; 3) Electrolysis process, adopting the ion-exchange membrane method with a high current density and a natural circulation bipolar electrolytic cell; 4) Fresh brine dechlorination process, adopting vacuum dechlorination method with a liquid ring vacuum pump as the vacuum source; 5) Caustic soda concentration process, adopting a triple-effect countercurrent falling film evaporation process; 6) Chlorine treatment process, adopting a packed tower + bubble cap tower for drying and a screw turbine unit for transportation; 7) Hydrogen treatment process, adopting a scrubber with closed-loop water circulation for washing and a large water ring pump compressor unit for transportation; 8) Waste gas treatment process, adopting a two-column series alkali liquor circulation absorption process for waste gas, and the waste gas is chlorine.

2. The preparation method of caustic soda according to claim 1, wherein In step 1), the saturated crude brine from the salt dissolving tank flows into the baffle tank, where sodium carbonate, caustic soda, and sodium hypochlorite are added for a refining reaction; the crude brine after the refining reaction is filtered in the intermediate tank to obtain a clear liquid; sodium sulfite is added to the clear liquid to remove chlorine, and primary refined brine is obtained.

3. The preparation method of caustic soda according to claim 1 or 2, characterized in that, In step 2), the primary refined brine is pressurized and sequentially flows to the brine storage tank and the ion exchange resin tower, where multivalent cations are removed in the ion exchange resin tower to obtain secondary refined brine; the gauge pressure for pressurization is 0.35 - 0.45 MPa; the multivalent cations include calcium ions and magnesium ions, and the content of multivalent cations in the secondary refined brine ≤ 20 ppb.

4. The preparation method of caustic soda according to claim 3, wherein In step 3), the secondary refined brine sequentially flows into the brine elevated tank and the anode chamber of the electrolytic cell, and an aqueous sodium hydroxide solution is added to the cathode chamber for electrolysis; during the electrolysis process, chlorine is generated in the anode chamber and hydrogen is generated in the cathode chamber; The chlorine and hydrogen are respectively sent to the chlorine treatment process and the hydrogen treatment process; the electrolyzed fresh brine is discharged from the anode chamber and sent to the fresh brine dechlorination process for dechlorination.

5. The preparation method of caustic soda according to claim 4, characterized in that, In step 4), the electrolyzed fresh brine removes free chlorine in the vacuum dechlorination tower, and the dechlorinated fresh brine is adjusted to a pH value with alkali and then sodium sulfite solution is added for chemical dechlorination; the vacuum degree of the vacuum dechlorination tower is -0.07 - -0.065 MPa; the pH value is adjusted to 8 - 10.

6. The method for preparing caustic soda according to claim 4 or 5, characterized in that, In step 5), the liquid caustic soda produced in the electrolysis process is evaporated to remove part of the water by steam through a triple-effect countercurrent falling film evaporation process to obtain a liquid caustic soda with a mass concentration of 50%.

7. The preparation method of caustic soda according to claim 6, characterized in that, In step 6), the wet chlorine gas produced in the electrolysis process is sequentially washed and cooled, and then sent to a packed tower through a water mist separator to contact countercurrently with concentrated sulfuric acid for the first dehydration; then sent to a bubble cap tower to contact countercurrently with concentrated sulfuric acid for the second dehydration and then through an acid mist trap; The wet hydrogen gas produced in the electrolysis process is sequentially washed, pressurized, cooled, and water mist trapped; the gauge pressure for pressurization is 0.09 - 0.11 MPa.

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