A process for upgrading sodium nitrite

Abstract

This invention provides a method for preparing sodium nitrite, belonging to the field of chemical technology. The method includes the following steps: S1, guiding the slurry from the bottom of the thickener of a crystallizer to a primary vertical centrifuge for centrifugation and dehydration, obtaining sodium nitrite solid crystals and sodium nitrite crystallization mother liquor; S2, introducing the sodium nitrite solid crystals into a washing tank, adding soft water and saturated sodium nitrite reflux liquid, and mixing thoroughly under the action of a stirrer; the washed sodium nitrite solid crystals are then transferred to a secondary vertical centrifuge for centrifugation and dehydration, yielding high-quality sodium nitrite and sodium nitrite filtrate. Most of the saturated sodium nitrite overflowing from the secondary centrifuge returns to the washing tank to adjust the solid content entering the secondary vertical centrifuge; a small portion of the overflow overflows to a filtrate tank equipped with a stirrer, and the filtrate is pumped into the original mother liquor pipeline via a discharge pump. This invention's method for preparing sodium nitrite yields high-quality sodium nitrite with a high yield.

Description

Technical Field

[0001] This invention relates to the preparation of sodium nitrite, and belongs to the field of chemical technology. Background Technology

[0002] Sodium nitrite (NaNO2) is an inorganic salt formed by the combination of nitrite ions and sodium ions. It is a white or pale yellow orthorhombic crystal or powder with a relative density of 2.168 (0℃), a melting point of 271℃, and decomposes at 320℃. Sodium nitrite has a high content of chloride and sodium nitrate, and its moisture content is also too high, failing to meet the requirements for superior grade. Existing horizontal reciprocating centrifuges cannot meet the requirements for superior grade, and the washing device of horizontal reciprocating centrifuges is not very effective in removing chloride. Furthermore, after centrifugation, it needs to be further dried using a disc dryer to meet the required moisture content. This results in increased steam energy consumption, increased labor costs, unstable product quality, low efficiency, and a lack of competitive advantage in the market.

[0003] Sodium nitrite products contain chloride ions and sodium nitrate, both of which are highly soluble in water. These chloride ions are carried into the product by the water it carries. The principle is as follows: A primary centrifuge is used to dry the slurry from the thickener. The dried crystals will contain a small amount of water and dissolved sodium nitrate and chloride ions. The crystals fall into a washing tank, where soft water is added to wash and dilute the small amounts of chloride ions and sodium nitrate carried by the crystals. The washed slurry is then sent to a secondary centrifuge for further drying. This process reduces the sodium nitrate and chloride ion content in the product, thus increasing the purity of the sodium nitrite.

[0004] During the dilution process, a small portion of sodium nitrite crystals dissolves in the water, thus improving product quality while reducing yield. The larger the amount of softened water added, the higher the dilution factor for impurities, resulting in lower chloride and sodium nitrate content in the final product. However, the more sodium nitrite dissolved in the water, the lower the yield of premium-grade products. Currently, the sodium nitrite dried by horizontal reciprocating centrifuges has a high moisture content and must be further dried using a disc dryer to meet the required moisture content. Furthermore, the chloride in the sodium nitrite product cannot be washed away by the rinsing water device of the horizontal centrifuge, resulting in a high chloride content that does not meet the premium-grade standards. It can only be sold as first-grade or qualified products, leading to lower selling prices and profits.

[0005] Sodium nitrite production methods can be broadly classified into two categories: one is the reduction method, which involves reducing sodium nitrate with lead, calcium, or hydrogen as a reducing agent under certain temperature conditions to produce sodium nitrite. The problem with this method is that the process is complex and the cost is high. The second category is the main production method at home and abroad, which involves using alkaline solution to absorb nitrogen oxide gas and co-produce sodium nitrate and sodium nitrite: (1) Direct method: In the direct method, ammonia and air are oxidized to NO by a platinum catalyst in an oxidation furnace. After the reaction heat is recovered by a waste heat boiler, the waste heat boiler contains high concentrations of nitrogen oxide and nitrogen dioxide gas. Then, the concentrated nitrogen oxide and nitrogen dioxide gas from the oxidation furnace is absorbed by an alkaline solution. The absorption rate is 98.5% to 99%. After absorption, the NaNO2:NaNO3 ratio in the neutralized liquid is greater than 20:1, and the nitrogen oxide concentration in the tail gas is less than 0.2%. (2) Tail gas absorption method: This can be divided into three types: nitric acid tail gas absorption method, nitric acid tail gas concentration method, and gas mixing method. (3) Double decomposition method: The double decomposition method uses nitric acid to directly act on alkalis to produce sodium nitrate, but this method is limited by resources and raw material sources and is not suitable for large-scale development.

[0006] In existing technologies, sodium nitrite is generally produced using a tail gas absorption process. This involves absorbing the tail gas from the oxidizing furnace gas with water to generate nitric acid, then absorbing it with a soda ash solution to obtain a neutralized liquid (a mixture of sodium nitrite and sodium nitrate). Sodium nitrite is then produced through processes such as evaporation, crystallization, and separation. The mother liquor of sodium nitrite is then converted, evaporated, crystallized, and separated to produce sodium nitrate.

[0007] Compared to first-grade sodium nitrite, superior-grade sodium nitrite has a significant price and market advantage. Previous authorized invention patents disclosed preparation methods all aimed to produce first-grade sodium nitrite. For example, CN1076172A – Direct Method for Sodium Nitrite Production – specifies a common tail gas absorption process, but it cannot produce superior-grade sodium nitrite. Similarly, CN102491376A – A Method and Apparatus for Preparing Sodium Nitrite Using Mother Liquor and CN104326494A – A Method for Producing Sodium Nitrate under Medium Pressure – mention the use of an OSLO crystallizer to produce sodium nitrite. This traditional crystallizer is prone to scaling, making continuous production difficult and producing high-quality products challenging.

[0008] In the production process of sodium nitrite, a small amount of sodium chloride and sodium nitrate inevitably precipitates during crystallization. These sodium chloride and sodium nitrate are trapped around the sodium nitrite crystals or inside large particles, and are not easily removed even after centrifugation and washing. This results in a high content of sodium chloride and sodium nitrate in the product, failing to meet the requirements for superior grade sodium nitrite. CN106744748B, a method for controlling the production of high-quality sodium nitrite, discloses a method for preparing superior grade sodium nitrite through cooling crystallization in a three-stage vacuum cooling crystallizer. However, this method adds water during each crystallization process. The addition of water not only dissolves and removes sodium chloride from the sodium nitrite but also dissolves some sodium nitrite, leading to a decrease in the yield of superior grade sodium nitrite.

[0009] Furthermore, the sodium nitrite mother liquor obtained from the tail gas absorption method is currently used to produce sodium nitrate through processes such as conversion, evaporation, crystallization, and separation. In addition to nitrate ions, the sodium nitrite mother liquor also contains chloride ions, resulting in low purity sodium nitrate obtained through direct crystallization. CN110255586A, a method for precisely controlling the separation of sodium nitrite and sodium chloride to produce molten salt-grade sodium nitrate, discloses a method that sequentially obtains molten salt-grade sodium nitrate through processes such as magnesium chloride removal, sodium chloride removal, and recrystallization. However, this method is cumbersome and has low production efficiency. Summary of the Invention

[0010] To address the above problems, this invention provides a method for preparing sodium nitrite, solving the technical problem that the purity of sodium nitrite products is too low to meet the superior grade, or even if the purity meets the superior grade, the yield is low.

[0011] To achieve the above objectives, the technical solution of the present invention for preparing sodium nitrite includes the following steps:

[0012] S1, a neutralized liquid is prepared by the tail gas absorption process. The neutralized liquid is evaporated and concentrated, then transferred to a crystallizer for cooling and crystallization. The slurry at the bottom of the thickener of the crystallizer is then guided to a first-stage vertical centrifugal dehydrator for centrifugal dehydration to obtain sodium nitrite solid crystals and sodium nitrite crystallization mother liquor.

[0013] S2, the sodium nitrite solid crystals from step S1 are introduced into a washing tank, soft water and saturated sodium nitrite reflux solution are added and mixed thoroughly under the action of a stirrer, and the washed sodium nitrite solid crystals are transferred to a two-stage vertical centrifuge for centrifugation and dehydration to obtain superior sodium nitrite and sodium nitrite filtrate.

[0014] Furthermore, it also includes the following steps:

[0015] S3, the sodium nitrite crystallization mother liquor obtained in step S1 is successively converted, evaporated and crystallized to obtain sodium nitrate solid crystals;

[0016] S4, the sodium nitrate crystals from step S3 are added to a predetermined amount of a mixed solution of saturated sodium nitrate solution and soft water for purification. After stirring for a predetermined time, centrifugation is performed to obtain molten salt grade sodium nitrate and sodium nitrate filtrate.

[0017] Furthermore, the sodium nitrite filtrate obtained in step S2 is returned to the sodium nitrite solid crystallization refining section to replace the saturated sodium nitrite solution for reuse.

[0018] Furthermore, the sodium nitrite filtrate obtained in step S2 is reused until the chloride ion concentration in the filtrate is above 10 g / L. Then, sodium nitrite solid crystals are prepared by a cooling crystallization step. The obtained sodium nitrite solid crystals are then purified to obtain superior sodium nitrite.

[0019] Furthermore, in step S2, the amount of saturated sodium nitrite solution added is 1.5-2 times the weight of the sodium nitrite solid crystals, and the amount of soft water added is 0.3-0.5 times the molar amount of chloride ions contained in the sodium nitrite solid crystals.

[0020] Furthermore, in step S2, the stirring process under the action of the stirrer involves first heating the solution to 70-75℃ and stirring for 30-60 minutes, then cooling it to 30-35℃ and stirring for 15-30 minutes.

[0021] Furthermore, the sodium nitrate filtrate obtained in step S4 is returned to the sodium nitrate solid crystallization refining section as a substitute for the saturated sodium nitrate solution for reuse.

[0022] Furthermore, the sodium nitrate filtrate obtained in step S4 is reused until the chloride ion concentration in the filtrate is above 10 g / L, and sodium nitrate solid crystals are prepared by crystallization. The obtained sodium nitrate solid crystals are then purified to obtain molten salt grade sodium nitrate.

[0023] Furthermore, in step S4, the amount of saturated sodium nitrate solution added is 1.5-2 times the weight of the sodium nitrate solid crystals, and the amount of soft water added is 0.5-0.8 times the molar amount of chloride ions contained in the sodium nitrate solid crystals.

[0024] Furthermore, in step S4, the solution is first heated to 60-65℃ and stirred for 30-60 minutes, then cooled to 20-25℃ and stirred for 15-30 minutes.

[0025] The beneficial effects of the method for preparing sodium nitrite according to the present invention are as follows:

[0026] This invention discloses a method for preparing sodium nitrite, which yields sodium nitrite of superior quality with a high yield. The method involves first obtaining sodium nitrite crystals through cooling crystallization, and then purifying and refining the crystals by adding a predetermined proportion of saturated sodium nitrite solution and soft water. This improves the purity of sodium nitrite without reducing its content, achieving the requirements for superior quality. Sodium nitrite mother liquor is subjected to conventional conversion, evaporation, and crystallization to obtain sodium nitrate crystals. These crystals are then refined by adding a saturated sodium nitrate solution and an appropriate amount of soft water to obtain molten salt-grade sodium nitrate. The process is simple and highly efficient.

[0027] In this invention, the saturated sodium nitrite solution in the sodium nitrite refining process can be reused multiple times. Finally, sodium nitrite crystals can be obtained through cooling crystallization, and then refined again to obtain high-quality sodium nitrite. The process achieves high sodium nitrite yield and eliminates waste. The centrifuged liquid after sodium nitrite refining can also be combined with the sodium nitrite mother liquor and used to prepare sodium nitrate through conversion, evaporation, and crystallization.

[0028] In the sodium nitrite refining process of this invention, the ratio of saturated sodium nitrite solution to soft water is specifically set. Adding too much soft water will result in a greater loss of sodium nitrite and a reduced yield of sodium nitrite, while adding too little soft water will result in the sodium nitrite purity not meeting the requirements of superior grade.

[0029] In this invention, the saturated sodium nitrate solution in the sodium nitrate refining process can be reused multiple times. Finally, sodium nitrate crystals can be obtained through cooling crystallization, and then refined again to produce molten salt-grade sodium nitrate. The centrifuged liquid after sodium nitrate refining can be used in other existing processes to prepare products such as sodium chloride and magnesium carbonate, or for solution preparation in other steps. Attached Figure Description

[0030] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0031] Figure 1 This is a process flow diagram of the sodium nitrite preparation method of the present invention;

[0032] Figure 2 This is a partial process flow diagram of steps S1 and S2 in the sodium nitrite preparation method of the present invention, wherein M101 is a primary vertical centrifugal dehydrator, X101 is a primary hopper, V101 is a washing tank, R101 is a stirrer, M102A is a secondary vertical centrifugal dehydrator A, M102B is a secondary vertical centrifugal dehydrator B, X102A is a secondary hopper A, and X102B is a secondary hopper B. Detailed Implementation

[0033] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Those skilled in the art can make similar extensions without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0034] like Figure 1 and 2 As shown, a method for preparing sodium nitrite includes the following steps:

[0035] S1, a neutralized liquid is prepared by tail gas absorption process, and the neutralized liquid is evaporated, concentrated and cooled to crystallize to obtain sodium nitrite solid crystals and sodium nitrite crystal mother liquor;

[0036] The tail gas absorption method can refer to the sodium nitrite "direct method" production process disclosed in the prior art CN1076172. The first step of this method is to sequentially oxidize, cool and absorb to obtain a neutralized liquid, and use the neutralized liquid for the subsequent preparation steps of this invention.

[0037] The evaporation and concentration are carried out using existing technologies. In this invention, the sodium nitrite content in the solution is 600-800 g / L.

[0038] Cooling crystallization involves cooling the concentrated solution under vacuum to 30-35°C, letting it stand for 1-2 hours, and then centrifuging to obtain sodium nitrite solid crystals.

[0039] S2, add the sodium nitrite solid crystals from step S1 to a predetermined amount of a mixture of saturated sodium nitrite solution and soft water for purification, stir for a predetermined time, and centrifuge to obtain superior sodium nitrite and sodium nitrite filtrate.

[0040] Preferably, the amount of saturated sodium nitrite solution added is 1.5-2 times the weight of sodium nitrite solid crystals, and the amount of soft water added is 0.3-0.5 times the molar amount of chloride ions contained in the sodium nitrite solid crystals; during the stirring process, the solution is first heated to 70-75℃ and stirred for 30-60 minutes, and then cooled to 30-35℃ and stirred for 15-30 minutes.

[0041] Preferably, the obtained sodium nitrite filtrate is returned to the sodium nitrite solid crystallization refining section to replace the saturated sodium nitrite solution for reuse; when the obtained sodium nitrite filtrate is reused until the chloride ion concentration in the filtrate is above 10 g / L, sodium nitrite solid crystals are prepared by a cooling crystallization step, and the obtained sodium nitrite solid crystals are refined to obtain superior sodium nitrite.

[0042] S3, the sodium nitrite crystallization mother liquor obtained in step S1 is successively converted, evaporated and crystallized to obtain sodium nitrate solid crystals;

[0043] The conversion and evaporation steps can be carried out using existing processes well known to those skilled in the art, and the crystallization is carried out by directly cooling to 20-25°C and allowing it to stand for crystallization.

[0044] S4, the sodium nitrate crystals from step S3 are added to a predetermined amount of a mixed solution of saturated sodium nitrate solution and soft water for purification. After stirring for a predetermined time, centrifugation is performed to obtain molten salt grade sodium nitrate and sodium nitrate filtrate.

[0045] Preferably, the obtained sodium nitrate filtrate is returned to the sodium nitrate solid crystallization refining section as a substitute for the saturated sodium nitrate solution for reuse; when the obtained sodium nitrate filtrate is reused until the chloride ion concentration in the filtrate is above 10 g / L, sodium nitrate solid crystals are prepared by crystallization, and the obtained sodium nitrate solid crystals are used to prepare molten salt grade sodium nitrate through a refining step.

[0046] The amount of saturated sodium nitrate solution added is 1.5-2 times the weight of sodium nitrate solid crystals, and the amount of soft water added is 0.5-0.8 times the molar amount of chloride ions contained in the sodium nitrate solid crystals. During the stirring process, the solution is first heated to 60-65℃ and stirred for 30-60 minutes, and then cooled to 20-25℃ and stirred for 15-30 minutes.

[0047] The centrifuge used in the centrifugation process of this invention is a vertical centrifuge, and its structural features are as follows:

[0048] Material: 304 stainless steel; adopts German technology; rotor speed: 3000 r / min; dynamic balancing is performed on the main drum during equipment manufacturing to ensure stable drum operation. The centrifuge body includes an upper cylinder, lower cylinder, cover, main drive shaft, bearing housing, inner drum, outer drum, flow guide device, filter screen, feeding device, drum fixing device, water spraying device, water spraying tank, bearing lubrication device, transmission device, motor adjusting seat, pulley, shock absorber, connecting bolts, etc. The main shaft bearing housing is precision cast and connected to the lower cylinder using a stop joint. The surface roughness of the upper and lower end faces and the stop joint is 1.6 or higher. The upper and lower bearings are NSK bearings from Japan. The lower part of the bearing housing includes a lower cover, a lower bushing, and a lower spacer ring, while the upper part includes an upper cover, an upper bushing, and an upper spacer ring. The bearing end cover and the bearing are sealed with PTFE and a labyrinth seal. The inner and outer parts and the drum are precision cast and machined. Six sets of flow guiding devices are precision machined and used in combination at different angles. By adjusting the angle of the flow guiding devices, the channel length can be changed, and the residence time of the material in the flow guiding channel can be controlled, which can effectively control the final moisture content of the material.

[0049] During centrifugation, material enters the centrifuge's distributor from the top. The distributor continuously fills the guide channel with material. Under the action of centrifugal force, the material flows downward along the guide channel. During this flow, liquid passes through the screen and enters the filtrate chamber, while solid material is discharged into the lower solid material chamber along the guide channel. Utilizing centrifugal force, the centrifuge simultaneously completes centrifugal filtration and centrifugal unloading. The angle of the guide channel can be adjusted arbitrarily, thereby changing the channel length and controlling the residence time of the material within the guide channel to achieve optimal liquid content in the solid material.

[0050] This equipment enables fully automated continuous centrifugal separation, boasting a high degree of automation and excellent separation effect. It can also be customized to suit different material conditions. It is widely used in industries such as chemical, inorganic salt, food, pharmaceutical, mineral processing, and municipal wastewater treatment.

[0051] The quality standard for premium-grade sodium nitrite should comply with the requirements of GB2367-2006, namely, a sodium nitrite content of ≥99.0%, a sodium nitrate content of ≤0.8%, and a chloride content of ≤0.10%. The quality standard for molten salt-grade sodium nitrate should be a sodium nitrate content of ≥99.5%, a chloride content of ≤0.05%, and a sodium nitrite content of ≤0.01%. The detection methods for each substance's content shall adopt the corresponding methods in GB2367-2006 and GB / T4553-2016.

[0052] Example 1

[0053] A method for preparing sodium nitrite includes the following steps:

[0054] S1, a neutralized liquid is prepared by the tail gas absorption process. The neutralized liquid is evaporated and concentrated to a sodium nitrite content of 750 g / L, cooled to 30°C and allowed to stand for 1.5 h. After centrifugation, sodium nitrite solid crystals and sodium nitrite crystallization mother liquor are obtained.

[0055] S2, the sodium nitrite solid crystals from step S1 are added to a predetermined amount of a mixed solution of saturated sodium nitrite solution and soft water for purification. The mixture is stirred for a predetermined time and then centrifuged to obtain superior grade sodium nitrite and sodium nitrite filtrate. The amount of saturated sodium nitrite solution added is 1.5 times the weight of the sodium nitrite solid crystals, and the amount of soft water added is 0.3 times the molar amount of chloride ions contained in the sodium nitrite solid crystals. During the stirring process, the solution is first heated to 75℃ and stirred for 30 minutes, then cooled to 30℃ and stirred for 15 minutes. The mass fraction of sodium nitrite in the superior grade sodium nitrite product is measured to be 99.8%, the mass fraction of sodium nitrate is 0.2%, and the mass fraction of sodium chloride is 0.04%. The yield of superior grade sodium nitrite is 96.72% (sodium nitrite yield = mass of sodium nitrite in the final product / mass of sodium nitrite in the concentrated solution after step S1 * 100%).

[0056] S3, the sodium nitrite crystallization mother liquor obtained in step S1 is successively converted, evaporated and crystallized to obtain sodium nitrate solid crystals; wherein, the conversion and evaporation steps can be carried out using existing processes well known to those skilled in the art, and the crystallization is carried out by directly cooling to 25°C and allowing it to stand for crystallization;

[0057] In step S4, the sodium nitrate crystals from step S3 are added to a predetermined amount of a mixed solution of saturated sodium nitrate solution and soft water for purification. The mixture is stirred for a predetermined time and then centrifuged to obtain molten salt grade sodium nitrate and sodium nitrate filtrate. The amount of saturated sodium nitrate solution added is twice the weight of the solid sodium nitrate crystals, and the amount of soft water added is 0.5 times the molar amount of chloride ions contained in the solid sodium nitrate crystals. During the stirring process, the solution is first heated to 65℃ and stirred for 30 minutes, then cooled to 25℃ and stirred for another 30 minutes. The mass fraction of sodium nitrate in the molten salt grade sodium nitrate product was measured to be 99.8%, the mass fraction of sodium nitrite was 0.007%, and the mass fraction of sodium chloride was 0.03%. The yield of molten salt grade sodium nitrate was 94.33% (sodium nitrate yield = mass of sodium nitrate in the final product / mass of sodium nitrate in the solution after evaporation and concentration in step S3 * 100%).

[0058] Example 2

[0059] A method for preparing sodium nitrite includes the following steps:

[0060] S1, a neutralized liquid is prepared by the tail gas absorption process. The neutralized liquid is evaporated and concentrated to a sodium nitrite content of 755 g / L, cooled to 35°C and allowed to stand for 2 hours. After centrifugation, sodium nitrite solid crystals and sodium nitrite crystallization mother liquor are obtained.

[0061] S2, the sodium nitrite solid crystals from step S1 are added to a predetermined amount of a mixed solution of saturated sodium nitrite solution and soft water for purification. The mixture is stirred for a predetermined time and then centrifuged to obtain superior grade sodium nitrite and sodium nitrite filtrate. The amount of saturated sodium nitrite solution (the sodium nitrite filtrate obtained in step S2 of Example 1) added is twice the weight of the sodium nitrite solid crystals, and the amount of soft water added is 0.5 times the molar amount of chloride ions contained in the sodium nitrite solid crystals. During the stirring process, the solution is first heated to 70°C and stirred for 60 minutes, then cooled to 35°C and stirred for 30 minutes. The mass fraction of sodium nitrite in the superior grade sodium nitrite product is measured to be 99.7%, the mass fraction of sodium nitrate is 0.4%, and the mass fraction of sodium chloride is 0.06%. The yield of superior grade sodium nitrite is 96.46% (sodium nitrite yield = mass of sodium nitrite in the final product / mass of sodium nitrite in the concentrated solution after step S1 * 100%).

[0062] S3, the sodium nitrite crystallization mother liquor obtained in step S1 is successively converted, evaporated and crystallized to obtain sodium nitrate solid crystals; wherein, the conversion and evaporation steps can be carried out using existing processes well known to those skilled in the art, and the crystallization is carried out by directly cooling to 20°C and allowing it to stand for crystallization;

[0063] In step S4, the sodium nitrate crystals from step S3 are added to a predetermined amount of a mixed solution of saturated sodium nitrate solution and soft water for purification. The mixture is stirred for a predetermined time and then centrifuged to obtain molten salt grade sodium nitrate and sodium nitrate filtrate. The amount of saturated sodium nitrate solution added is 1.5 times the weight of the solid sodium nitrate crystals, and the amount of soft water added is 0.6 times the molar amount of chloride ions contained in the solid sodium nitrate crystals. During the stirring process, the solution is first heated to 60℃ and stirred for 60 minutes, then cooled to 20℃ and stirred for 30 minutes. The mass fraction of sodium nitrate in the molten salt grade sodium nitrate product is measured to be 99.7%, the mass fraction of sodium nitrite is 0.008%, and the mass fraction of sodium chloride is 0.04%. The yield of molten salt grade sodium nitrate is 93.68% (sodium nitrate yield = mass of sodium nitrate in the final product / mass of sodium nitrate in the solution after evaporation and concentration in step S3 * 100%).

[0064] Example 3

[0065] A method for preparing sodium nitrite includes the following steps:

[0066] S1, a neutralized liquid is prepared by the tail gas absorption process. The neutralized liquid is evaporated and concentrated to a sodium nitrite content of 760 g / L, cooled to 35°C and allowed to stand for 1.5 h. After centrifugation, sodium nitrite solid crystals and sodium nitrite crystallization mother liquor are obtained.

[0067] S2, the sodium nitrite solid crystals from step S1 are added to a predetermined amount of a mixed solution of saturated sodium nitrite solution and soft water for purification. The mixture is stirred for a predetermined time and then centrifuged to obtain superior grade sodium nitrite and sodium nitrite filtrate. The amount of saturated sodium nitrite solution (the sodium nitrite filtrate obtained in step S2 of Example 2) added is twice the weight of the sodium nitrite solid crystals, and the amount of soft water added is 0.5 times the molar amount of chloride ions contained in the sodium nitrite solid crystals. During the stirring process, the solution is first heated to 75°C and stirred for 60 minutes, then cooled to 35°C and stirred for another 60 minutes. The mass fraction of sodium nitrite in the superior grade sodium nitrite product is measured to be 99.5%, the mass fraction of sodium nitrate is 0.7%, and the mass fraction of sodium chloride is 0.08%. The yield of superior grade sodium nitrite is 95.29% (sodium nitrite yield = mass of sodium nitrite in the final product / mass of sodium nitrite in the concentrated solution after step S1 * 100%).

[0068] S3, the sodium nitrite crystallization mother liquor obtained in step S1 is successively converted, evaporated and crystallized to obtain sodium nitrate solid crystals; wherein, the conversion and evaporation steps can be carried out using existing processes well known to those skilled in the art, and the crystallization is carried out by directly cooling to 20°C and allowing it to stand for crystallization;

[0069] In step S4, the sodium nitrate crystals from step S3 are added to a predetermined amount of a mixed solution of saturated sodium nitrate solution and soft water for purification. The mixture is stirred for a predetermined time and then centrifuged to obtain molten salt-grade sodium nitrate and sodium nitrate filtrate. The amount of saturated sodium nitrate solution (the sodium nitrate filtrate obtained in step S4 of Example 1) added is twice the weight of the solid sodium nitrate crystals, and the amount of soft water added is 0.8 times the molar amount of chloride ions contained in the solid sodium nitrate crystals. During the stirring process, the solution is first heated to 65°C and stirred for 60 minutes, then cooled to 20°C and stirred for 30 minutes. The mass fraction of sodium nitrate in the molten salt-grade sodium nitrate product was measured to be 99.5%, the mass fraction of sodium nitrite was 0.010%, and the mass fraction of sodium chloride was 0.05%. The yield of molten salt-grade sodium nitrate was 91.24% (sodium nitrate yield = mass of sodium nitrate in the final product / mass of sodium nitrate in the solution after evaporation and concentration in step S3 * 100%).

[0070] In the above embodiments, the sodium nitrite filtrate obtained in step S2 and the sodium nitrate filtrate obtained in step S4 are tested for chloride ion content. When the standard upper limit is reached, the sodium nitrite filtrate is used to prepare sodium nitrite solid crystals through a cooling crystallization step. The obtained sodium nitrite solid crystals are then purified to obtain superior grade sodium nitrite. Similarly, the sodium nitrate filtrate is used to prepare sodium nitrite solid crystals through a cooling crystallization step. The obtained sodium nitrite solid crystals are then purified to obtain superior grade sodium nitrite.

[0071] Comparative Example 1

[0072] Comparative Example 1 is a comparative experiment of Example 1, the difference being that the amount of soft water added in step S2 is 0.2 times the molar amount of chloride ions contained in the sodium nitrite solid crystals. The other step parameters are the same as in Example 1. The final product sodium nitrite test results are as follows: sodium nitrite mass fraction is 98.2%, sodium nitrate mass fraction is 0.9%, and sodium chloride mass fraction is 0.09%; the yield of sodium nitrite is 95.97% (sodium nitrite yield = mass of sodium nitrite in the final product / mass of sodium nitrite in the concentrated solution after step S1 * 100%).

[0073] Comparative Example 2

[0074] Comparative Example 2 is a comparative experiment with Example 1, the difference being that the amount of soft water added in step S2 is 0.6 times the molar amount of chloride ions contained in the sodium nitrite solid crystals. The other step parameters are the same as in Example 1. The final product sodium nitrite test results are as follows: sodium nitrite mass fraction is 99.2%, sodium nitrate mass fraction is 0.4%, and sodium chloride mass fraction is 0.06%; the yield of sodium nitrite is 92.31% (sodium nitrite yield = mass of sodium nitrite in the final product / mass of sodium nitrite in the concentrated solution after step S1 * 100%).

[0075] Comparative Example 3

[0076] Comparative Example 3 was a comparative experiment with Example 1, the difference being that in step S2, the temperature was not raised during stirring, but the stirring was continued at 30°C for 45 minutes. Other step parameters were the same as in Example 1. The final product sodium nitrite test results were as follows: sodium nitrite mass fraction was 98.7%, sodium nitrate mass fraction was 0.9%, and sodium chloride mass fraction was 0.10%; the yield of sodium nitrite was 93.78% (sodium nitrite yield = mass of sodium nitrite in the final product / mass of sodium nitrite in the concentrated solution after step S1 * 100%).

[0077] Comparative Example 4

[0078] Comparative Example 4 is a comparative experiment of Example 1, the difference being that the amount of saturated sodium nitrite solution added in step S2 is 1 times the weight of the sodium nitrite solid crystals. Other step parameters are the same as in Example 1. The final product sodium nitrite test results are as follows: sodium nitrite mass fraction is 98.2%, sodium nitrate mass fraction is 0.8%, and sodium chloride mass fraction is 0.12%; the yield of sodium nitrite is 95.89% (sodium nitrite yield = mass of sodium nitrite in the final product / mass of sodium nitrite in the concentrated solution after step S1 * 100%).

[0079] Comparative Example 5

[0080] Sodium nitrite was prepared according to the method of Example 1 of patent CN106744748B. The sodium nitrite product obtained contained 99.1% sodium nitrite, 0.53% sodium nitrate, and 0.5% sodium chloride. The yield of sodium nitrite was 84.87% (sodium nitrite yield = mass of sodium nitrite in the final product / mass of sodium nitrite in the high-temperature mother liquor * 100%).

[0081] Comparative Example 6

[0082] Comparative Example 6 is a comparative experiment of Example 1, the difference being that the amount of soft water added in step S4 is 0.4 times the molar amount of chloride ions contained in the sodium nitrate solid crystals. Other step parameters are the same as in Example 1. The final product sodium nitrate test results are as follows: sodium nitrate mass fraction is 98.9%, sodium nitrite mass fraction is 0.011%, and sodium chloride mass fraction is 0.06%; the yield of molten salt grade sodium nitrate is 93.89% (sodium nitrate yield = mass of sodium nitrate in the final product / mass of sodium nitrate in the solution after evaporation and concentration in step S3 * 100%).

[0083] Comparative Example 7

[0084] Comparative Example 7 is a comparative experiment of Example 1, the difference being that the amount of soft water added in step S4 is 0.9 times the molar amount of chloride ions contained in the sodium nitrate solid crystals. Other step parameters are the same as in Example 1. The final product sodium nitrate test results are as follows: sodium nitrate mass fraction is 99.4%, sodium nitrite mass fraction is 0.009%, and sodium chloride mass fraction is 0.05%; the yield of molten salt grade sodium nitrate is 90.11% (sodium nitrate yield = mass of sodium nitrate in the final product / mass of sodium nitrate in the solution after evaporation and concentration in step S3 * 100%).

[0085] Comparative Example 8

[0086] Comparative Example 8 is a comparative experiment of Example 1, the difference being that the amount of saturated sodium nitrate solution added in step S4 is 1 times the weight of the sodium nitrate solid crystals. Other step parameters are the same as in Example 1. The final product sodium nitrate test results are as follows: sodium nitrate mass fraction is 98.1%, sodium nitrite mass fraction is 0.02%, and sodium chloride mass fraction is 0.09%; the yield of molten salt grade sodium nitrate is 93.28% (sodium nitrate yield = mass of sodium nitrate in the final product / mass of sodium nitrate in the solution after evaporation and concentration in step S3 * 100%).

[0087] Comparative Example 9

[0088] Comparative Example 9 was a comparative experiment with Example 1, the difference being that the stirring process in step S4 was not heated, but directly stirred at 25°C for 60 min. Other step parameters were the same as in Example 1. The final product sodium nitrate test results were as follows: sodium nitrate mass fraction was 98.3%, sodium nitrite mass fraction was 0.01%, and sodium chloride mass fraction was 0.08%; the yield of molten salt grade sodium nitrate was 93.35% (sodium nitrate yield = mass of sodium nitrate in the final product / mass of sodium nitrate in the solution after evaporation and concentration in step S3 * 100%).

[0089] Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A method for preparing sodium nitrite, characterized in that, Includes the following steps: S1, a neutralized liquid is prepared by the tail gas absorption process. The neutralized liquid is evaporated and concentrated, then transferred to a crystallizer for cooling and crystallization. The slurry at the bottom of the thickener of the crystallizer is then guided to a first-stage vertical centrifugal dehydrator for centrifugal dehydration to obtain sodium nitrite solid crystals and sodium nitrite crystallization mother liquor. S2, the sodium nitrite solid crystals from step S1 are introduced into a washing tank, soft water and saturated sodium nitrite reflux liquid are added and mixed thoroughly under the action of a stirrer, and the washed sodium nitrite solid crystals are transferred to a two-stage vertical centrifuge for centrifugation and dehydration to obtain superior sodium nitrite and sodium nitrite filtrate. S3, the sodium nitrite crystallization mother liquor obtained in step S1 is successively converted, evaporated and crystallized to obtain sodium nitrate solid crystals; S4, the sodium nitrate crystals from step S3 are added to a predetermined amount of a mixed solution of saturated sodium nitrate solution and soft water for purification. After stirring for a predetermined time, centrifugation is performed to obtain molten salt grade sodium nitrate and sodium nitrate filtrate. The sodium nitrite filtrate obtained in step S2 is returned to the sodium nitrite solid crystallization refining section to replace the saturated sodium nitrite solution for reuse. In step S2, the amount of saturated sodium nitrite solution added is 1.5-2 times the weight of the sodium nitrite solid crystals, and the amount of soft water added is 0.3-0.5 times the molar amount of chloride ions contained in the sodium nitrite solid crystals. Step S2 involves stirring the solution under the action of a stirrer by first heating it to 70-75℃ and stirring for 30-60 minutes, then cooling it to 30-35℃ and stirring for 15-30 minutes.

2. The method for preparing sodium nitrite according to claim 1, characterized in that, The sodium nitrite filtrate obtained in step S2 is reused until the chloride ion concentration in the filtrate is above 10 g / L. Then, sodium nitrite solid crystals are prepared by a cooling crystallization step. The obtained sodium nitrite solid crystals are then purified to obtain superior sodium nitrite.

3. The method for preparing sodium nitrite according to claim 1, characterized in that, The sodium nitrate filtrate obtained in step S4 is returned to the sodium nitrate solid crystallization refining section as a substitute for the saturated sodium nitrate solution for reuse.

4. The method for preparing sodium nitrite according to claim 3, characterized in that, The sodium nitrate filtrate obtained in step S4 is reused until the chloride ion concentration in the filtrate is above 10 g / L. Sodium nitrate solid crystals are then prepared by crystallization. The obtained sodium nitrate solid crystals are then purified to obtain molten salt grade sodium nitrate.

5. The method for preparing sodium nitrite according to claim 1, characterized in that, In step S4, the amount of saturated sodium nitrate solution added is 1.5-2 times the weight of the sodium nitrate solid crystals, and the amount of soft water added is 0.5-0.8 times the molar amount of chloride ions contained in the sodium nitrate solid crystals.

6. The method for preparing sodium nitrite according to claim 1, characterized in that, In step S4, the solution is first heated to 60-65℃ and stirred for 30-60 minutes, then cooled to 20-25℃ and stirred for 15-30 minutes.

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