Device for treating waste sodium slag through improved melting method

Abstract

The utility model discloses a device for treating waste sodium slag by an improved melting method, which comprises a melting kettle, wherein a first bottom discharge hole of the melting kettle is connected with a first material inlet of a first refining separation kettle; a first upper discharge hole of the melting kettle is connected with a second material inlet of the second refining separation kettle; a second upper discharge hole of the first refining separation kettle is connected with a second material inlet of the second refining separation kettle; a third upper discharge port of the second refining separation kettle is connected with a feed port of a second vacuum filter, a solid discharge port of the second vacuum filter is connected with a product tank, and a liquid discharge port of the second vacuum filter is connected with a second liquid recovery tank. The device for treating the waste sodium slag is simple in process, low in cost and high in separation efficiency, the purified metal sodium is high in purity and can be directly applied to synthesis of downstream products, the adopted solvent can be recycled, and no three wastes are generated.

Description

Technical Field

[0001] The utility model belongs to the field of chemical raw material preparation and recovery, relates to waste sodium slag, and specifically relates to a device for treating waste sodium slag by an improved melting method. Background Technique

[0002] In the process of producing metallic sodium by molten electrolysis of sodium chloride, in order to reduce the melting point of the electrolyte and improve the electrolysis characteristics, calcium chloride and barium chloride need to be added to the electrolyte to form a ternary electrolyte system. After separating metallic sodium from the electrolytic cell, a certain amount of electrolytic slag will remain, commonly known as sodium slag. The production amount of sodium slag is about 3% of the production amount of metallic sodium. Sodium slag contains 70 - 75% metallic sodium, about 15% metallic calcium, and other components are mainly sodium oxide, calcium oxide, etc. Although sodium slag contains more than 70% metallic sodium, due to the presence of a large amount of impurities such as sodium oxide and calcium oxide, it cannot meet the production requirements. Sodium slag is inexpensive but belongs to hazardous waste, and it is difficult to handle. Long-term storage will also bring relatively large potential safety hazards. However, its potential economic benefits are relatively large. Extracting metallic sodium from sodium slag can not only improve economic benefits but also recycle and reuse hazardous waste to eliminate safety hazards and environmental pollution. Developing a recovery technology for sodium slag is of great significance. Currently, the common industrial production methods for treating metallic sodium slag are as follows:

[0003] 1. Extrusion method:

[0004] CN2846437Y and CN2846436Y propose to use a physical method to melt the metallic sodium in the sodium waste residue into a liquid state, while the remaining waste residue still remains in a solid state, and separate the metallic sodium in the metallic sodium waste residue by filtration and extrusion.

[0005] 2. Vacuum distillation method:

[0006] CN104294054A and CN102634671A propose to separate by using the different boiling points of sodium and impurities through vacuum distillation.

[0007] 3. Sub-boiling distillation with thermal radiation synergistic distillation:

[0008] CN103757435A and CN203700465U propose to heat by radiation to heat the liquid surface of the liquefied metallic sodium, so that molecular evaporation can be achieved in a non-boiling state of the liquid surface, thereby avoiding the violent convection caused by boiling and enabling the evaporation and condensation of liquid droplets containing impurities, thus improving the distillation efficiency.

[0009] 4. Vacuum distillation:

[0010] CN103667708A, CN111363922A, and CN212560390U utilize the differences in boiling points and vapor pressures between elements to separate the main metal and various impurity components under certain temperature and vacuum conditions.

[0011] 5. Molten salt heating method:

[0012] CN107574318A melts the molten salt into a molten salt liquid; heats the sodium slag to above the melting point of sodium to form a solid-liquid mixture, and then adds the solid-liquid mixture to the molten salt liquid to obtain a pure molten sodium metal liquid on the upper layer of the molten salt liquid.

[0013] 6. Molten electrolysis method:

[0014] Patents CN108048872B, CN203700555U, CN108624913A and "Research on Purifying Metallic Sodium with Na·β—Al_2O_3 Diaphragm" introduce the use of electrolytic purification.

[0015] 7. Direct heating filtration method:

[0016] The methods adopted by patents CN206188863U, CN109136579B, and CN208378965U are to remove impurities by filtration in the molten state of sodium.

[0017] 8. Heating solvent filtration method:

[0018] The method introduced in Liu Zongming's "Collection of Chemical Experiment Operation Experiences" (Higher Education Press, 1989) is to separate sodium in the molten state from other impurities based on the difference in their states in the solvent.

[0019] 9. Cold trap method:

[0020] Zhu Rui proposed in the article "Research on the Preparation of Purifying Metallic Sodium" that the solubility of different substances changes with temperature. By heating the metallic sodium to be purified, when the hot metallic sodium flows through a cold trap filled with a metal mesh, due to the sudden drop in temperature, the solubility of impurities decreases exponentially, so that the impurities in the metallic sodium precipitate and deposit on the metal mesh, achieving the purpose of removing impurities.

[0021] 10. Chemical treatment method:

[0022] Zhu Rui also proposed a method in "Research on the Preparation of Purifying Metallic Sodium", which is to add sodium peroxide or trace amounts of water vapor to convert the main impurities into particulate impurities such as calcium oxide or calcium hydroxide for removal.

[0023] 11. Sedimentation method:

[0024] Zhang Jiangang introduced in the article "Progress and Difficulty Analysis of Purification of Metallic Sodium" that the purification of monomer impurities such as calcium is achieved by gravity sedimentation method using the morphological form of calcium and the density difference of sodium.

[0025] Among the currently adopted methods, the products obtained by extrusion method, sedimentation method, cold trap method, direct heating filtration method, etc. have incomplete impurity removal, and new substances that need to be separated are introduced in chemical treatment method, molten salt heating method, etc., while the equipment costs and energy consumption of methods such as vacuum distillation and molten electrolysis method are relatively high. Summary of the Invention

[0026] Aiming at the deficiencies existing in the prior art, the purpose of the present utility model is to provide a device for treating waste sodium slag by improved melting method to solve the technical problem that the separation effect of waste sodium slag of the device for treating waste sodium slag in the prior art needs to be further improved.

[0027] In order to solve the above technical problems, the present utility model is implemented by adopting the following technical solutions:

[0028] A device for treating waste sodium slag includes a melting kettle, and the first bottom discharge port of the melting kettle is connected to the first material inlet of the first refining and separating kettle.

[0029] The first upper discharge port of the melting kettle is connected to the second material inlet of the second refining and separating kettle; the second upper discharge port of the first refining and separating kettle is connected to the second material inlet of the second refining and separating kettle.

[0030] The third upper discharge port of the second refining and separating kettle is connected to the feed port of the second vacuum filter, the solid discharge port of the second vacuum filter is connected to the product tank, and the liquid discharge port of the second vacuum filter is connected to the second liquid recovery tank.

[0031] The present utility model also has the following technical features:

[0032] The second bottom discharge port of the first refining and separating kettle is connected to the feed port of the first vacuum filter, the solid discharge port of the first vacuum filter is connected to the solid recovery tank, and the liquid discharge port of the first vacuum filter is connected to the first liquid recovery tank.

[0033] The third bottom discharge port of the second refining and separating kettle is connected to the feed port of the first vacuum filter, the solid discharge port of the first vacuum filter is connected to the solid recovery tank, and the liquid discharge port of the first vacuum filter is connected to the first liquid recovery tank.

[0034] The melting kettle is provided with a raw material waste sodium slag inlet connected to a waste sodium slag storage tank, and the melting kettle is also provided with a solvent inlet connected to a liquid storage tank.

[0035] The melting kettle is provided with a first stirring paddle, the first refining and separating kettle is provided with a second stirring paddle, and the second refining and separating kettle is provided with a third stirring paddle.

[0036] The melting kettle is provided with a first temperature display, the first refining and separating kettle is provided with a second temperature display, and the second refining and separating kettle is provided with a third temperature display.

[0037] The melting kettle is provided with a first nitrogen inlet and a first nitrogen outlet; the first refining and separating kettle is provided with a second nitrogen inlet and a second nitrogen outlet; the second refining and separating kettle is provided with a third nitrogen inlet and a third nitrogen outlet.

[0038] Compared with the prior art, the utility model has the following beneficial technical effects:

[0039] (Ⅰ) The device for treating waste sodium residue of the utility model has simple process, low cost, high separation efficiency, high purity of the purified metallic sodium, can be directly applied to the synthesis of downstream products, the solvent used can be recycled and reused, and no three wastes are generated.

[0040] (Ⅱ) The device for treating waste sodium residue of the utility model uses a melting kettle to melt the alkali metal, two refining and separating kettles to separate and solidify the alkali metal from other impurities, and filters other impurities through a filtering device, and the generated metallic sodium can be directly stored in an inert solvent, significantly improving the separation efficiency and separation effect of solid impurities.

[0041] (Ⅲ) In the device for treating waste sodium residue of the utility model, the melting kettle, the first refining and separating kettle and the second refining and separating kettle can all be kept inclined to a certain degree by controlling the kettle body, so as to ensure the separation effect of the discharge port. Description of the Drawings

[0042] Figure 1 It is a schematic structural diagram of the device for treating waste sodium residue by the improved melting method.

[0043] The meanings of the labels in the figure are as follows: 1 - melting kettle, 2 - first refining and separation kettle, 3 - second refining and separation kettle, 4 - first vacuum filter, 5 - second vacuum filter, 6 - first bottom discharge port, 7 - first material inlet, 8 - first upper discharge port, 9 - second material inlet, 10 - second bottom discharge port, 11 - solid recovery tank, 12 - first liquid recovery tank, 13 - second upper discharge port, 14 - third bottom discharge port, 15 - third upper discharge port, 16 - product tank, 17 - second liquid recovery tank, 18 - sodium residue storage tank, 19 - raw material sodium residue inlet, 20 - liquid storage tank, 21 - solvent inlet, 22 - first stirring paddle, 23 - second stirring paddle, 24 - third stirring paddle, 25 - first temperature display, 26 - second temperature display, 27 - third temperature display, 28 - first nitrogen inlet, 29 - first nitrogen outlet, 30 - second nitrogen inlet, 31 - second nitrogen outlet, 32 - third nitrogen inlet, 33 - third nitrogen outlet, 34 - pipeline, 35 - valve, 36 - feeding pump.

[0044] The following further elaborates on the specific content of the present utility model in conjunction with embodiments. Specific embodiments

[0045] It should be noted that all the equipment and raw materials in the present utility model, unless otherwise specified, are all the equipment and raw materials known in the prior art.

[0046] In the present utility model, the various equipment are mainly connected through pipeline 34. Valves 35 and feeding pumps 36 are provided on each pipeline as required and are opened or closed according to process requirements.

[0047] All the valves 35 and feeding pumps 36 in the present utility model are the commonly used valves and feeding pumps in the prior art.

[0048] In the present utility model, the solvent used in the continuous melting and secondary refining method is a single or composite inert solvent with a density greater than that of metallic sodium, such as refined naphthenic oil or high-density white oil, and a single solvent is preferred. The refining of naphthenic oil refers to naphthenic oil that has removed main impurities such as naphthenic acid and does not react with crude sodium residue.

[0049] In the present utility model, the inert solvent for storing metallic sodium is a single or composite inert solvent with a density less than that of metallic sodium, such as low-density white oil.

[0050] Complying with the above technical solutions, the following gives specific embodiments of the present utility model. It should be noted that the present utility model is not limited to the following specific embodiments, and all equivalent transformations made on the basis of the technical solutions of this application fall within the protection scope of the present utility model.

[0051] Embodiment 1:

[0052] This embodiment provides a device for treating waste sodium slag, including a melting kettle 1, as Figure 1 shown, the first bottom discharge port 6 of the melting kettle 1 is connected to the first material inlet 7 of the first refining and separation kettle 2.

[0053] As Figure 1 shown, the first upper discharge port 8 of the melting kettle 1 is connected to the second material inlet 9 of the second refining and separation kettle 3; the second upper discharge port 13 of the first refining and separation kettle 2 is connected to the second material inlet 9 of the second refining and separation kettle 3.

[0054] As Figure 1 shown, the third upper discharge port 15 of the second refining and separation kettle 3 is connected to the feed port of the second vacuum filter 5, the solid discharge port of the second vacuum filter 5 is connected to the product tank 16, and the liquid discharge port of the second vacuum filter 5 is connected to the second liquid recovery tank 17.

[0055] In this embodiment, the melting kettle 1 can be heated by an oil bath coil or by electricity, and oil bath coil heating is preferred.

[0056] In this embodiment, the vacuum filter is preferably a vacuum suction filter.

[0057] As a preferred solution of this embodiment, as Figure 1 shown, the second bottom discharge port 10 of the first refining and separation kettle 2 is connected to the feed port of the first vacuum filter 4, the solid discharge port of the first vacuum filter 4 is connected to the solid recovery tank 11, and the liquid discharge port of the first vacuum filter 4 is connected to the first liquid recovery tank 12.

[0058] As a preferred solution of this embodiment, as Figure 1 shown, the third bottom discharge port 14 of the second refining and separation kettle 3 is connected to the feed port of the first vacuum filter 4, the solid discharge port of the first vacuum filter 4 is connected to the solid recovery tank 11, and the liquid discharge port of the first vacuum filter 4 is connected to the first liquid recovery tank 12.

[0059] As a preferred solution of this embodiment, as Figure 1 shown, the melting kettle 1 is provided with a raw material sodium slag inlet 19 connected to the sodium slag storage tank 18, and the melting kettle 1 is also provided with a solvent inlet 21 connected to the liquid storage tank 20.

[0060] As a preferred solution of this embodiment, as Figure 1 shown, the melting kettle 1 is provided with a first stirring paddle 22, the first refining and separation kettle 2 is provided with a second stirring paddle 23, and the second refining and separation kettle 3 is provided with a third stirring paddle 24. In this embodiment, the first stirring paddle 22 is preferably a stirring paddle with intense stirring that can drive a relatively large amount of solid particles to suspend, such as a curved blade turbine stirring paddle.

[0061] As a preferred solution of this embodiment, as Figure 1 shown, a first temperature display 25 is provided on the melting kettle 1, a second temperature display 26 is provided on the first refining and separation kettle 2, and a third temperature display 27 is provided on the second refining and separation kettle 3.

[0062] As a preferred solution of this embodiment, as Figure 1 shown, a first nitrogen inlet 28 and a first nitrogen outlet 29 are provided on the melting kettle 1; a second nitrogen inlet 30 and a second nitrogen outlet 31 are provided on the first refining and separation kettle 2; a third nitrogen inlet 32 and a third nitrogen outlet 33 are provided on the second refining and separation kettle 3.

[0063] The working process of the device for treating waste sodium slag given in this embodiment is as follows:

[0064] Step 1, melting of waste sodium slag:

[0065] Keep the system dry, turn on nitrogen protection throughout the process. Nitrogen enters from the first nitrogen inlet 28, the second nitrogen inlet 30, and the third nitrogen inlet 32 respectively, and discharges from the first nitrogen outlet 29, the second nitrogen outlet 31, and the third nitrogen outlet 33. Transfer about 200 kg of treated naphthenic oil from the liquid storage tank 20 to the melting kettle 1 through the solvent inlet 21. Then transfer 50 kg of crude sodium slag from the sodium slag storage tank 18 to the melting kettle 1 through the raw material sodium slag inlet 19. Turn on the heating coil of the reaction kettle for heating until the temperature displayed by the first temperature display 25 is between 160 and 170 °C. At this time, the sodium slag begins to slowly melt locally, and some solids deposit. Turn on the first stirring paddle 22 and maintain a relatively fast stirring speed of 800 rpm for about 30 to 90 minutes, then stop stirring. The sodium slag stratifies. Most of the upper layer is the obtained product, and most of the lower layer is other by-products. Tilt the reaction kettle 45 degrees in the direction of the first upper discharge port 8, keep the first bottom discharge port 6 closed, open the first upper discharge port 8, and transfer the upper part of the material to the second refining and separation kettle 3 through the second material inlet 9. Close the first upper discharge port 8 when the liquid level drops below the first upper discharge port 8 and no more material is discharged. Return the melting kettle 1 to the upright position, replenish the treated naphthenic oil to the original liquid level, continue to tilt the melting kettle 1 by 45 degrees, open the first upper discharge port 8 for discharging, close the first upper discharge port 8 when the liquid level drops for the second time and no more material is discharged, open the first bottom discharge port 6, and discharge the remaining solid material to the first refining and separation kettle 2 through the first material inlet 7.

[0066] Step 2, primary refining treatment:

[0067] In the first refining and separation kettle 2 with a volume of 200 L, the reaction system is kept at a temperature between 150 and 160 °C. After the material discharged from the first bottom discharge port 6 of the melting kettle 1 enters, the second stirring paddle 23 is started and maintained at 500 rpm for about 10 to 30 minutes. Then the stirring is stopped. A very small amount of metallic sodium doped in the lower layer is stirred to the upper layer, and most of the solids still settle in the lower layer. The first refining and separation kettle 2 is tilted 45 degrees towards the second upper discharge port 13, the second bottom discharge port 10 is kept closed, and the second upper discharge port 13 is opened to transfer the upper part of the material to the second refining and separation kettle 3. When the liquid level drops below the second upper discharge port 13 and no more material is discharged, the second upper discharge port 13 is closed. The first refining and separation kettle 2 is returned to the upright position, and the treated naphthenic oil is added to the original liquid level. Then the first refining and separation kettle 2 is tilted 45 degrees again, the second upper discharge port 13 is opened for discharging. When the liquid level drops for the second time and no more material is discharged, the second upper discharge port 13 is closed, the second bottom discharge port 10 is opened, and the remaining material is discharged into the first vacuum filter 4. It is cooled to room temperature for filtration. The filtrate obtained from the filtration enters the first liquid recovery tank 12 for recycling, and the filter residue enters the solid recovery tank 11 for further treatment.

[0068] Step 3, secondary refining treatment:

[0069] In the second refining and separation kettle 3 with a volume of 200 L, the reaction system is kept at a temperature between 150 and 160 °C. After the material discharged from the first upper discharge port 8 of the melting kettle 1 and the second upper discharge port 13 of the first refining and separation kettle 2 enters, the six-curved-blade turbine stirring is started and maintained at 500 rpm for about 10 to 30 minutes. Then the stirring is stopped. A large number of metallic sodium beads are evenly distributed above the liquid surface, and a small part of the residues wrapped in the sodium beads settle in the lower layer. The second refining and separation kettle 3 is tilted 45 degrees towards the third upper discharge port 15, the third bottom discharge port 14 is kept closed, and the third upper discharge port 15 is opened to transfer the upper part of the material to the second vacuum filter 5. When the liquid level drops below the third upper discharge port 15 and no more material is discharged, the third upper discharge port 15 is closed. The second refining and separation kettle 3 is returned to the upright position, and the treated naphthenic oil is added to the original liquid level. Then the second refining and separation kettle 3 is tilted 45 degrees again, and the third upper discharge port 15 is opened to discharge the material into the second vacuum filter 5. When the liquid level drops for the second time and no more material is discharged, the third upper discharge port 15 is closed, the third bottom discharge port 14 is opened, and the remaining material is discharged into the second vacuum filter 5. It is cooled to room temperature for filtration. The filtrate obtained from the filtration enters the first liquid recovery tank 12 for recycling, and the filter residue enters the solid recovery tank 11 for further treatment.

[0070] At room temperature, the mother liquor of metallic sodium beads is vacuum filtered in the second vacuum filter 5. The filtrate obtained from the filtration is fed into the second liquid recovery tank 17 for recycling. The sodium beads are stored in the product tank 16, and there is a protective solvent such as low-density white oil inside the product tank 16 to protect the product. The product in the product tank can be further processed into sodium ingots by using a sodium casting machine.

[0071] The processed metallic sodium provided by the present utility model can be applied to related downstream products of metallic sodium, and has no obvious influence on the content and yield.

[0072] Example 2:

[0073] This example presents the application of metallic sodium in the synthesis of alkali metal alkoxides. Among them, the metallic sodium is the finished metallic sodium obtained by using the device for treating waste sodium slag in Example 1 above.

[0074] In a 1000L reaction kettle equipped with a mechanical stirrer, a thermometer, and a reflux condenser, 160 kg of tert-amyl alcohol, 500 kg of dry toluene, and 27 kg of sodium ingots prepared by this method are added. Under nitrogen protection, the mixture is refluxed at 100 - 110 °C for 10 hours. After the metallic sodium completely disappears, the excessive residual tert-amyl alcohol and toluene mother liquor are distilled off at atmospheric pressure, and then the remaining small amount of reaction medium is distilled off under reduced pressure to obtain a whiteish solid of sodium tert-amylate. Low-temperature nitrogen gas (-20 °C) is blown in through the bottom valve of the reaction kettle to cool it to below 40 °C, and the required pale yellow powdery sodium tert-amylate 123.6 kg is obtained by vacuum discharging. The content is 99.7%, and the free alkali is 0.2%, meeting the product requirements.

[0075] Product detection method: (Q-320411ATU 002-2017)

[0076] (1) Determination of total alkali: Weigh about 1 - 2 g of the sample (accurate to 0.0002 g) and place it in a dry 250 ml iodine flask. Add about 50 ml of distilled water (boiled and cooled) to dissolve, shake well, and let it stand for 5 min. Add 3 drops of 1% phenolphthalein indicator, and titrate with 0.5 mol / l hydrochloric acid standard solution until it turns colorless as the end point. The total alkali content of potassium tert-butoxide is expressed as a mass fraction and calculated according to the following formula: X 1 = C*V*0.11013 / m*100%, where: C represents the concentration of the hydrochloric acid standard solution (mol / L); V represents the volume of the hydrochloric acid standard solution consumed (ml); m represents the mass of the sample (g).

[0077] (2) Determination of free alkali:

[0078] Add 4 ml of glacial acetic acid and 20 ml of anhydrous methanol into the reaction flask as the mother liquor, just enough to cover the electrode. Titrate with Karl Fischer reagent to the end point. Then weigh about 1 - 2 g of the sample (accurate to 0.0002 g), quickly add it into the reaction flask, titrate with Karl Fischer reagent to the end point, and record the volume of Karl Fischer reagent consumed. The mass percentage of free base is calculated as follows: X 2 = 2.22T * V / m * 100%. In the formula, T represents the titration degree of Karl Fischer reagent to water (g / ml); V represents the volume of Karl Fischer reagent consumed in titrating the sample (ml); m represents the mass of the sample (g); 2.22 is the molar mass ratio of sodium hydroxide to water. Take the arithmetic mean of the results of two parallel determinations as the determination result, and the difference between the results of two parallel determinations shall not be greater than 0.2%.

Claims

1. A device for treating waste sodium slag, comprising a melting kettle (1), characterized in that: The first bottom discharge port (6) of the melting kettle (1) is connected to the first material inlet (7) of the first refining and separation kettle (2); The first upper discharge port (8) of the melting kettle (1) is connected to the second material inlet (9) of the second refining and separation kettle (3); the second upper discharge port (13) of the first refining and separation kettle (2) is connected to the second material inlet (9) of the second refining and separation kettle (3); The third upper discharge port (15) of the second refining and separation kettle (3) is connected to the feed port of the second vacuum filter (5), the solid discharge port of the second vacuum filter (5) is connected to the product tank (16), and the liquid discharge port of the second vacuum filter (5) is connected to the second liquid recovery tank (17).

2. The device for treating waste sodium slag according to claim 1, characterized in that: The second bottom discharge port (10) of the first refining and separation kettle (2) is connected to the feed port of the first vacuum filter (4), the solid discharge port of the first vacuum filter (4) is connected to the solid recovery tank (11), and the liquid discharge port of the first vacuum filter (4) is connected to the first liquid recovery tank (12).

3. The device for treating waste sodium slag according to claim 1, characterized in that: The third bottom discharge port (14) of the second refining and separation kettle (3) is connected to the feed port of the first vacuum filter (4), the solid discharge port of the first vacuum filter (4) is connected to the solid recovery tank (11), and the liquid discharge port of the first vacuum filter (4) is connected to the first liquid recovery tank (12).

4. The device for treating waste sodium slag according to claim 1, characterized in that: The melting kettle (1) is provided with a raw material sodium slag inlet (19) connected to a sodium slag storage tank (18), and the melting kettle (1) is also provided with a solvent inlet (21) connected to a liquid storage tank (20).

5. The device for treating waste sodium slag according to claim 1, characterized in that: The melting kettle (1) is provided with a first stirring paddle (22), the first refining and separation kettle (2) is provided with a second stirring paddle (23), and the second refining and separation kettle (3) is provided with a third stirring paddle (24).

6. The device for treating waste sodium slag according to claim 1, characterized in that: The melting kettle (1) is provided with a first temperature display (25), the first refining and separation kettle (2) is provided with a second temperature display (26), and the second refining and separation kettle (3) is provided with a third temperature display (27).

7. The device for treating waste sodium slag according to claim 1, characterized in that: The melting kettle (1) is provided with a first nitrogen inlet (28) and a first nitrogen outlet (29); the first refining and separation kettle (2) is provided with a second nitrogen inlet (30) and a second nitrogen outlet (31); and the second refining and separation kettle (3) is provided with a third nitrogen inlet (32) and a third nitrogen outlet (33).

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