A bleaching apparatus and bleaching method for concentrated nitric acid production

Abstract

The application discloses a bleaching device and bleaching method for concentrated nitric acid production and belongs to the technical field of concentrated nitric acid production.The bleaching device comprises a nitric acid concentration tower, a magnesium nitrate heater, a concentrated nitric acid condenser, a gas-liquid separator and a bleaching tower, the nitric acid concentration tower is connected with the concentrated nitric acid condenser and the magnesium nitrate heater respectively, the concentrated nitric acid condenser is connected with the gas-liquid separator, and the gas-liquid separator is connected with the bleaching tower.The application reduces system resistance and improves system negative pressure stability;the empty tower speed of the bleaching tower is increased, one bleaching tower can be used by multiple sub-systems, the process flow is shortened, the investment and maintenance cost are reduced, the nitrate content in the dilute magnesium nitrate solution is simultaneously reduced to below 0.05%, the corrosion rate of the dilute magnesium nitrate heater is reduced, and the service life is prolonged.

Description

Technical Field

[0001] This invention belongs to the field of concentrated nitric acid production technology, specifically relating to a bleaching device and bleaching method for concentrated nitric acid production. Background Technology

[0002] Currently, most concentrated nitric acid production uses the magnesium nitrate process. Dilute nitric acid and concentrated magnesium nitrate are mixed in a weight ratio of 1:3-4 to form a magnesium nitrate-nitric acid-water ternary mixture, which is then added to the upper part of the stripping section of a concentration tower. Steam with a nitric acid content of over 80% from the stripping section enters the rectification section. After continuous rectification, steam with a nitric acid content of over 98% is obtained from the top of the rectification section. This steam passes through a bleaching tower and then enters a concentrated nitric acid condenser to obtain concentrated nitric acid. Approximately two-thirds of the concentrated nitric acid is returned to the rectification section as reflux acid; approximately one-third of the concentrated nitric acid goes to the bleaching tower to remove nitrogen oxides, yielding a qualified finished acid product. Uncondensed gas in the concentrated nitric acid condenser is recovered by the gas-liquid separator to the tail water tank. In the stripping section, the magnesium nitrate solution absorbs water and its concentration drops to 60%-68%. It flows from the bottom of the stripping section into the magnesium nitrate heater, where the dilute magnesium nitrate solution boils. Secondary steam enters the bottom of the stripping section, providing the heat required for the distillation process. After the concentration of the magnesium nitrate solution is initially increased, it flows into the dilute magnesium nitrate tank. The dilute magnesium nitrate enters the evaporator and is concentrated before entering the concentrated magnesium nitrate tank, and then circulated to the high-level concentrated magnesium nitrate tank.

[0003] The above-mentioned production process has the following three main problems: ① The nitrate content of the dilute magnesium nitrate solution in the magnesium nitrate heater is generally above 0.5%, which causes serious corrosion to the heat exchange tubes; ② The magnesium tail water contains more than 2% nitrate, and some even more than 5%, which cannot be fully recovered, resulting in high treatment costs; ③ The bleaching of the finished acid is done by hot bleaching, the negative pressure of the concentration system is unstable, and the process is long, with high consumption and high maintenance costs.

[0004] CN108557786A discloses a production equipment for producing concentrated nitric acid using the magnesium nitrate process. The top of the concentration tower 2 is connected to the gas-liquid separator 3, the top of the gas-liquid separator 3 is connected to the tail ejector 4, the bottom of the gas-liquid separator 3 is connected to both the bleaching tower 5 and the top of the concentration tower 2, and the bottom of the concentration tower 2 is connected to the dilute magnesium heater 6. This invention improves the concentration tower structure, reducing the concentration of magnesium tail water to below 0.1%, thus lowering the nitrate content. However, it still suffers from problems such as a long process flow, high consumption, and high maintenance costs. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a bleaching apparatus and bleaching method for producing concentrated nitric acid.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A bleaching apparatus for producing concentrated nitric acid includes a nitric acid concentration tower, a magnesium nitrate heater, a concentrated nitric acid condenser, a gas-liquid separator, and a bleaching tower;

[0008] The nitric acid concentration tower is connected to a concentrated nitric acid condenser and a magnesium nitrate heater, respectively. The concentrated nitric acid condenser is connected to a gas-liquid separator, and the gas-liquid separator is connected to a bleaching tower.

[0009] Preferably, the gas-liquid separator is connected to the nitric acid concentration tower.

[0010] Preferably, the temperature at the top of the bleaching tower is 90-110℃, and the negative pressure is -3 to -5KPa.

[0011] Preferably, the bleaching tower is provided with an air inlet and a bleaching gas recovery outlet.

[0012] More preferably, the gas-liquid separator is connected to the nitric acid concentration tower.

[0013] A bleaching method for producing concentrated nitric acid using the aforementioned bleaching apparatus includes the following steps:

[0014] (1) Nitric acid concentration: Dilute nitric acid solution and concentrated magnesium nitrate solution are mixed and fed into a nitric acid concentration tower. After distillation and rectification, concentrated nitric acid vapor is obtained.

[0015] (2) Cooling: The concentrated nitric acid vapor obtained in step (1) is cooled to obtain liquid concentrated nitric acid;

[0016] (3) Separation: The liquid concentrated nitric acid obtained in step (2) is separated into gas and liquid to obtain concentrated nitric acid;

[0017] (4) Air bleaching: The concentrated nitric acid obtained in step (3) is bleached with air to remove nitrogen oxides and obtain the finished concentrated nitric acid.

[0018] Preferably, the mass ratio of the dilute nitric acid solution and the concentrated magnesium nitrate solution in step (1) is 1:3-4.

[0019] More preferably, the dilute nitric acid solution has a mass fraction of 60-70%, and the concentrated magnesium nitrate solution has a mass fraction of 70-80%.

[0020] Preferably, the nitric acid content in the concentrated nitric acid vapor in step (1) is 98% or higher.

[0021] Preferably, the dilute magnesium nitrate solution produced in step (1) is heated and denitrated, and the resulting secondary steam is refluxed to the distillation step as a heat source, and the resulting dilute magnesium nitrate enters the dilute magnesium nitrate storage tank.

[0022] Preferably, the tail gas generated by the gas-liquid separation in step (3) is recovered by the tail injector, and the acidic water enters the magnesium tail water circulation tank for recycling.

[0023] Preferably, the concentrated nitric acid portion in step (3) is used as reflux acid and refluxed to the distillation step.

[0024] More preferably, the amount of reflux acid used is 60-70% of the mass of concentrated nitric acid.

[0025] Preferably, the conditions for air purging in step (4) are: negative pressure -3 to -5 kPa, air temperature 5-40°C, and air moisture content not higher than 0.1%; more preferably, the air moisture content is 0.04%.

[0026] Preferably, the temperature of the finished concentrated nitric acid is 90-110℃.

[0027] Preferably, the bleaching gas generated by the air bleaching in step (4) is recovered by an air jet, and the acidic water enters the magnesium tail water circulation tank.

[0028] The beneficial effects of this invention are:

[0029] (1) The concentrated nitric acid bleaching of the present invention no longer uses concentrated nitric acid vapor, but instead uses air-cooled bleaching, optimizing process parameters. The temperature at the top of the bleaching tower is controlled at 90-110℃, and the generated concentrated nitric acid can be directly fed into the finished acid storage tank. Furthermore, since the concentrated nitric acid bleaching no longer uses concentrated nitric acid vapor, the bleaching tower is no longer connected in series with the concentrated acid system. The concentrated nitric acid vapor directly enters the concentrated nitric acid condenser to condense the concentrated nitric acid vapor into a liquid state, reducing system resistance and improving system negative pressure stability. At the same time, the empty tower velocity of the bleaching tower is increased, allowing multiple subsystems to share one bleaching tower, shortening the process flow and reducing investment and maintenance costs.

[0030] (2) During the air cooling process of the present invention, the negative pressure is controlled at -3 to -5 kPa, the boiling point of the solution is reduced, which can reduce the content of N2O4 produced by the acid thermal decomposition of the finished product and improve the quality of the concentrated nitric acid of the finished product.

[0031] (3) In the air cooling process of the present invention, by controlling the air temperature at 5-40℃ and the water content at no more than 0.1%, the nitrate content in the dilute magnesium nitrate solution is significantly reduced to less than 0.05%, which reduces the corrosion rate of the dilute magnesium nitrate heater and extends its service life. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the bleaching apparatus for producing concentrated nitric acid according to the present invention;

[0033] In the diagram, 1 is the nitric acid concentration tower, 2 is the magnesium nitrate heater, 3 is the nitric acid condenser, 4 is the gas-liquid separator, and 5 is the bleaching tower. Detailed Implementation

[0034] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0035] Before further describing specific embodiments of the present invention, it should be understood that the scope of protection of the present invention is not limited to the specific embodiments described below; it should also be understood that the terminology used in the embodiments of the present invention is for describing specific embodiments and not for limiting the scope of protection of the present invention.

[0036] A bleaching apparatus for producing concentrated nitric acid, such as Figure 1 The aforementioned components include a nitric acid concentration tower 1, a magnesium nitrate heater 2, a concentrated nitric acid condenser 3, a gas-liquid separator 4, and a bleaching tower 5.

[0037] The nitric acid concentration tower 1 is connected to the concentrated nitric acid condenser 3 and the magnesium nitrate heater 2, respectively. The concentrated nitric acid condenser 3 is connected to the gas-liquid separator 4, and the gas-liquid separator 4 is connected to the bleaching tower 5.

[0038] Preferably, the temperature at the top of the bleaching tower 5 is 90-110℃, and the negative pressure is -3 to -5KPa.

[0039] Preferably, the bleaching tower 4 is provided with an air inlet and a bleaching gas recovery port.

[0040] In some preferred embodiments, such as Figure 1 The concentrated nitric acid condenser is a water-cooled condenser.

[0041] In some preferred embodiments, the gas-liquid separator 4 is connected to the nitric acid concentration tower 1.

[0042] The present invention also provides a bleaching method for producing concentrated nitric acid using the aforementioned bleaching apparatus, comprising the following steps:

[0043] (1) Nitric acid concentration: Dilute nitric acid solution and concentrated magnesium nitrate solution are mixed and fed into nitric acid concentration tower 1. After distillation and rectification, concentrated nitric acid vapor is obtained.

[0044] (2) Cooling: The concentrated nitric acid vapor obtained in step (1) is cooled by concentrated nitric acid condenser 3 to obtain liquid concentrated nitric acid;

[0045] (3) Separation: The liquid concentrated nitric acid obtained in step (2) is separated into concentrated nitric acid by gas-liquid separator 4;

[0046] (4) Air bleaching: The concentrated nitric acid obtained in step (3) is bleached in the bleaching tower 5 to remove nitrogen oxides and obtain the finished concentrated nitric acid.

[0047] Preferably, in step (1), the mass ratio of the dilute nitric acid solution to the concentrated magnesium nitrate solution is 1:3-4, the mass fraction of the dilute nitric acid solution is 60-70%, and the mass fraction of the concentrated magnesium nitrate solution is 70-80%.

[0048] Preferably, the nitric acid content in the concentrated nitric acid vapor in step (1) is 98% or higher.

[0049] Preferably, the conditions for air purging in step (4) are: negative pressure -3 to -5 kPa, air temperature 5-40°C, and air moisture content not higher than 0.1%; more preferably, the air moisture content is 0.04%.

[0050] Preferably, the temperature of the finished concentrated nitric acid in step (4) is 90-110℃.

[0051] In some preferred embodiments, the air is dehumidified by freezing using a fan, and the pretreated air enters the bleaching tower.

[0052] In some preferred embodiments, the dilute magnesium nitrate solution produced in step (1) is heated and denitrated, and the resulting secondary steam is refluxed to the distillation step as a heat source, and the resulting dilute magnesium nitrate enters the dilute magnesium nitrate storage tank.

[0053] In some preferred embodiments, the tail gas generated by the gas-liquid separation in step (3) is recovered by the tail injector, and the acidic water enters the magnesium tail water circulation tank for recycling.

[0054] In some preferred embodiments, the concentrated nitric acid in step (3) is used as reflux acid and refluxed to the distillation step. Specifically, the amount of reflux acid is 60-70% of the mass of concentrated nitric acid.

[0055] In some preferred embodiments, the bleaching gas generated by the air bleaching in step (4) is recovered by an air jet, and the acidic water enters the magnesium tail water circulation tank.

[0056] Applying the bleaching device of this invention to the improvement of the 150,000-ton concentrated nitric acid magnesium nitrate process can replace the nine independent bleaching towers in the concentrated nitric acid system with one unit. By using pretreated air to bleach the concentrated nitric acid and controlling the air blowing conditions, nitrogen oxides can be effectively removed. The temperature at the top of the tower is controlled at 90-110°C. The bleached concentrated nitric acid can be directly fed into the finished product storage tank without the need for cooling. This eliminates the need for the original nine independent finished product condensers, shortens the process flow, and greatly reduces investment and maintenance costs.

[0057] Compared to the original bleaching process, the improved process of this invention ensures that the nitrate content in the dilute magnesium in the magnesium nitrate heater can be controlled below 0.05%, and the service life of the magnesium nitrate heater is more than 5 years; the nitrate content in the magnesium tail water is also controlled below 0.05%, and it can be used directly as replenishment water for circulating water without treatment, resulting in significant energy saving and consumption reduction; the concentrated nitric acid product is cold-bleached to ensure product quality, reduce investment, and make production more stable. The improved system has a shorter process flow, fewer leaks, and significantly lower maintenance costs.

Claims

1. A bleaching method for producing concentrated nitric acid using a bleaching apparatus, characterized in that, Includes the following steps: (1) Nitric acid concentration: A dilute nitric acid solution and a concentrated magnesium nitrate solution are mixed and fed into a nitric acid concentration tower. After distillation and rectification, concentrated nitric acid vapor is obtained. The mass ratio of the dilute nitric acid solution to the concentrated magnesium nitrate solution is 1:3-4. The mass fraction of the dilute nitric acid solution is 60-70%, and the mass fraction of the concentrated magnesium nitrate solution is 70-80%. The nitric acid content in the concentrated nitric acid vapor is above 98%. (2) Cooling: The concentrated nitric acid vapor obtained in step (1) is cooled to obtain liquid concentrated nitric acid; (3) Separation: The liquid concentrated nitric acid obtained in step (2) is separated by gas-liquid separation to obtain concentrated nitric acid; (4) Air purging: The concentrated nitric acid obtained in step (3) is purged with air to remove nitrogen oxides and obtain the finished concentrated nitric acid; the conditions for air purging are: negative pressure -3 to -5 kPa, air temperature 5-40℃, and air moisture content not higher than 0.1%; the temperature of the finished concentrated nitric acid is 90-110℃. The bleaching apparatus includes a nitric acid concentration tower, a magnesium nitrate heater, a concentrated nitric acid condenser, a gas-liquid separator, and a bleaching tower. The nitric acid concentration tower is connected to both the concentrated nitric acid condenser and the magnesium nitrate heater; the concentrated nitric acid condenser is connected to the gas-liquid separator; and the gas-liquid separator is connected to the bleaching tower.

2. The bleaching method according to claim 1, characterized in that, The temperature at the top of the bleaching tower is 90-110℃, and the negative pressure is -3 to -5 kPa.

3. The bleaching method according to claim 1, characterized in that, The bleaching tower is equipped with an air inlet and a bleaching gas recovery outlet; the gas-liquid separator is connected to the nitric acid concentration tower.

4. The bleaching method according to claim 1, characterized in that, The dilute magnesium nitrate solution produced in step (1) is heated and denitrated, and the resulting secondary steam is refluxed to the distillation step as a heat source. The resulting dilute magnesium nitrate enters the dilute magnesium nitrate storage tank.

5. The bleaching method according to claim 1, characterized in that, The tail gas generated by the gas-liquid separation in step (3) is recovered by the tail injector, and the acidic water enters the magnesium tail water circulation tank for recycling.

6. The bleaching method according to claim 1, characterized in that, The concentrated nitric acid in step (3) is used as reflux acid and refluxed to the distillation step; the amount of reflux acid is 60-70% of the mass of concentrated nitric acid.

7. The bleaching method according to claim 1, characterized in that, The bleaching gas generated by the air bleaching in step (4) is recovered by an air jet, and the acidic water enters the magnesium tail water circulation tank.

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