Method for purifying manganese chloride

Abstract

The invention belongs to the field of manganese chloride purification, and particularly relates to a method for purifying manganese chloride. The purification method comprises the following steps: drying manganese chloride, heating under a vacuum condition, pre-distilling the dried manganese chloride, and then heating for main distillation. According to the purification method disclosed by the invention, no chemical reaction occurs, no auxiliary additive is added, no three wastes are generated, green production is realized, and the purification process flow is greatly simplified. Purified manganese chloride is high in purity, and a raw material source is provided for a manganese chloride electrolysis system to prepare high-purity metal manganese.

Description

Technical Field

[0001] The invention belongs to the field of manganese chloride purification, and particularly relates to a method for purifying manganese chloride. Background Art

[0002] With the rapid development of ultra-large-scale integrated circuits (VLSI), ultra-high-purity copper wiring (purity ≥6N) in interconnect technology has become difficult to meet the requirements of chips below the 14nm process node. Adding 0.25wt% to 1wt% manganese to copper can effectively reduce the diffusion of Cu atoms into the substrate, preventing electromigration from adversely affecting chip performance. Therefore, the preparation of high-purity manganese (purity ≥5N) is particularly important.

[0003] Conventional manganese sulfate electrolysis has high sulfur and selenium contents in manganese metal, making it difficult to achieve high purity. Manganese chloride electrolysis has become a new development direction for producing high-purity manganese.

[0004] Currently, purification technologies for manganese chloride are concentrated in the wet process. Solvent extraction is often used to extract manganese chloride aqueous solutions, but due to the varying extraction effects of different ions, chemical precipitation and membrane separation techniques are often combined. Deep purification of manganese chloride electrolytes, using manganese chloride and ammonium chloride aqueous solutions as raw materials, requires multiple precision filtrations and multi-stage ion adsorption. The wet process is complex, producing waste gas, wastewater, and waste residue during the purification process, and subsequent treatment of these "three wastes" remains problematic.

[0005] Patent application number 202410759534.6 discloses a method for obtaining high-purity manganese chloride through synergistic extraction. The method comprises the following steps: S1. Extracting and purifying a crude manganese solution containing impurities with a C272 extractant, a P507 extractant, and light white oil or sulfonated kerosene in a certain proportion using a synergistic extractant formed in several interconnected extractors; S2. Saponifying the synergistic extractant with sodium hydroxide or ammonia to obtain an organic phase containing manganese and a raffinate; S3. Stripping the organic phase containing manganese with a stripping agent of 10%-40% hydrochloric acid in an extractor. The collected heavy phase is concentrated, crystallized, and dried to obtain a high-purity manganese chloride solution. This patent application utilizes a solution extraction method, which is complex and involves the disposal of "three wastes."

[0006] Patent application number 202410760144.0 discloses a method for preparing high-purity manganese tetraoxide using manganese chloride as a raw material through precipitation oxidation, followed by steam calcination and purification. This method involves mixing manganese chloride with a surfactant and a catalyst, stirring the mixture with liquid caustic soda, and then introducing air for oxidation to produce the primary product, manganese tetraoxide. Subsequently, high-temperature calcination, while maintaining a constant flow of a mixture of steam and air, yields the purified, high-purity manganese tetraoxide. This method produces high-purity manganese tetraoxide with low impurity content and good dispersibility. The preparation process utilizes auxiliary materials such as surfactants and catalysts, and high-temperature calcination consumes a lot of energy. Summary of the Invention

[0007] Based on the above technical background, the main purpose of the present invention is to provide a method for purifying manganese chloride to overcome the shortcomings of the prior art.

[0008] To achieve the aforementioned object of the invention, the technical solutions adopted by the present invention include:

[0009] The present invention provides a method for purifying manganese chloride, which comprises:

[0010] The manganese chloride is dried, and the temperature is raised under vacuum conditions to perform pre-distillation on the dried manganese chloride, and then the temperature is raised to perform main distillation.

[0011] Preferably, the drying conditions are: drying at 80-90° C. for 90-150 min.

[0012] More preferably, the drying condition is: drying at 85° C. for 120 min.

[0013] The pre-distillation conditions are: the pre-distillation vacuum degree is 10 -3 ~10 -2 Pa, heat to 450-550°C at a heating rate of 5-12°C / min, and keep at this temperature for 20-45 min.

[0014] Preferably, the vacuum degree of the pre-distillation is 5×10 -3 Pa. The temperature was raised to 500°C at a rate of 10°C / min and kept at this temperature for 30 min.

[0015] The main distillation conditions are: the main distillation vacuum degree is 10 -3 ~10 -2 Pa, heat to 600-700°C at a heating rate of 3-5°C / min, and keep at this temperature for 30-180 min.

[0016] Preferably, the main distillation conditions are: the main distillation vacuum degree is 5×10 -3Pa, and then heated to 650 °C at a heating rate of 4 °C / min and kept at this temperature for 90 min.

[0017] According to a preferred embodiment of the present invention, the pre-distillation and main distillation are preferably carried out in a vacuum distillation furnace.

[0018] The vacuum distillation furnace includes a distillation cover, a crucible, a primary condensation tower, a secondary condensation tower, an upper heating zone and a lower heating zone. Figure 1 shown.

[0019] A primary condensation tower is installed above the distillation hood, and a secondary condensation tower is installed above the primary condensation tower. The crucible is located inside the distillation hood and is used to store dried manganese chloride. The lower heating zone is installed outside the distillation hood, and the upper heating zone is located outside the primary condensation tower.

[0020] The upper and lower heating zones are used to heat the manganese chloride in the crucible, while the condensation tower is used for condensation. The primary condensation tower is equipped with a temperature control device to ensure that low-boiling point impurities are deposited in the secondary condensation tower and manganese chloride is deposited in the primary condensation tower, achieving zoned condensation.

[0021] Preferably, the crucible is made of boron nitride or alumina, the primary condensation tower is made of molybdenum, and the secondary condensation tower is made of 316 stainless steel. Each condensation tower has 5 to 10 layers of condensation trays, preferably 8 layers. The primary and secondary condensation towers are connected by bolts and sealed with rubber gaskets. The entire condenser is installed from the top of the condensation tower.

[0022] Evacuate the vacuum distillation furnace until the vacuum degree reaches 900-1100Pa, then increase the pumping speed; when the vacuum degree reaches 8-12Pa, start the diffusion pump for preheating; after preheating for about 0.5-2h, when the vacuum degree is maintained at 8-12Pa, close the roughing valve, open the main valve, and perform high vacuum operation to reach the target vacuum degree 10 -3 ~10 -2 Pa.

[0023] Preferably, the vacuum distillation furnace is evacuated until the vacuum degree reaches 1000Pa, and the pumping speed is increased; when the vacuum degree reaches 10Pa, the diffusion pump is turned on for preheating; after preheating for about 1h, when the vacuum degree is maintained at 10Pa, the roughing valve is closed, the main valve is opened, and a high vacuum operation is performed to reach the target vacuum degree of 5×10 -3 Pa.

[0024] The dried manganese chloride powder is spread all over the bottom of the crucible. The thickness of the manganese chloride powder layer is controlled to be 5 to 30 mm. The surface of the layer is made corrugated with rake teeth to increase the evaporation area.

[0025] Generally, at a given temperature, the evaporation rate of manganese chloride increases with increasing vacuum. Increasing the vacuum reduces residual gas molecules in the system, increases their mean free path, and prevents collisions and kinetic energy loss during migration, thereby increasing the distillation rate. This also reduces the incorporation of impurities like oxygen from the air, significantly improving product purity.

[0026] If the heating rate is too low, the manganese chloride will evaporate and be lost during the heating process; if the heating rate is too high, the components of the equipment will be damaged and the temperature inside the manganese chloride raw material will be uneven, thereby affecting the distillation effect.

[0027] Precise temperature control significantly impacts purification results. During the pre-distillation stage, temperatures that are too low can make it difficult to volatilize and separate low-boiling-point impurities, thus affecting subsequent purification. High temperatures can cause the manganese chloride to volatilize, reducing product yield. During the main distillation stage, low temperatures can reduce the distillation rate of manganese chloride, while high temperatures can cause high-boiling-point impurities to volatilize, affecting product purity.

[0028] The present invention adopts high vacuum, precise temperature control, and staged temperature increase to improve distillation efficiency and achieve high-purity preparation of manganese chloride.

[0029] The pre-distillation conditions are: at a vacuum degree of 10 -3 ~10 -2 Under the condition of Pa, the upper and lower heating zones are heated simultaneously at a rate of 5-12°C / min to 450-550°C, and the distillation valve is opened. The lower heating zone is kept at 450-550°C for 20-45 minutes for pre-distillation, while the upper heating zone is also maintained at 450-550°C. During this stage, low-boiling-point impurities evaporate and condense in the secondary condensation tray.

[0030] Preferably, the pre-distillation conditions are: at a vacuum degree of 5×10 -3 Under the conditions of 1.5 Pa, the upper and lower heating zones are simultaneously heated to 600-700°C at a rate of 3-5°C / min, and the distillation valve is opened. The lower heating zone is maintained at 600-700°C for 20-45 minutes for pre-distillation, while the upper heating zone is also maintained at 500°C. During this stage, low-boiling-point impurities evaporate and condense in the condensation tray of the secondary condensation tower.

[0031] The main distillation conditions are: at a vacuum degree of 10 -3 ~10 -2Under the condition of Pa, the lower heating zone is heated at a rate of 3-5°C / min to the main distillation temperature of 600-700°C, and the temperature is kept at this temperature for 30-180 minutes for main distillation. Then, the lower heating zone is stopped from heating, and the upper heating zone is cooled at a rate of 8-12°C / min to 300-450°C and kept at this temperature. After the lower heating zone stops heating and cools to 300-450°C, the upper heating zone is stopped from heating and keeping at this temperature. After the upper and lower heating zones are cooled to about 80-120°C, the distillation valve, roughing valve, fore valve and mechanical pump are closed in sequence.

[0032] Preferably, the main distillation conditions are: at a vacuum degree of 5×10 -3 Under the conditions of Pa, the lower heating zone was heated at a rate of 4°C / min to the main distillation temperature of 650°C and held at this temperature for 90 minutes for main distillation. Heating in the lower heating zone was then stopped, and the upper heating zone was cooled at a rate of 10°C / min to 400°C and held there. After the lower heating zone was stopped and the temperature was lowered to 400°C, heating and holding in the upper heating zone was stopped. After the upper and lower heating zones were cooled to approximately 100°C, the distillation valve, roughing valve, foreline valve, and mechanical pump were sequentially closed.

[0033] The present invention achieves purification of manganese chloride under high vacuum conditions by precisely controlling the distillation temperature. The purified manganese chloride is high in purity, and no other auxiliary materials are added during the purification process, and no three wastes are generated, requiring no post-processing.

[0034] The beneficial effects of the present invention are:

[0035] (1) The method for purifying manganese chloride of the present invention is a physical separation method, which does not involve chemical reaction and does not require the addition of other auxiliary materials. No waste water, waste gas, or waste residue is generated, thus avoiding the "three wastes" problem and the complicated post-treatment process, solving environmental protection problems, realizing green production, and greatly simplifying the purification process.

[0036] (2) The purity of manganese chloride obtained by the purification method of the present invention can reach above 5N, which provides a reliable source of raw materials for the preparation of high-purity metallic manganese in the manganese chloride electrolysis system and has good application prospects in the field of manganese chloride electrolysis system.

[0037] (3) The purification method of the present invention does not use extractants, acids, bases, etc., which reduces the cost of raw material preparation and shortens the preparation process.

[0038] (4) The raw material adaptability of the present invention is good. Since the melting point of manganese chloride is quite different from that of impurities, the content and distribution of impurities have little effect on the purity of the product. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 Shows a schematic structural diagram of a vacuum distillation furnace;

[0040] Figure 2 Shows the temperature control curve of the distillation stage;

[0041] Figure 3 The SEM photograph of the manganese chloride prepared in Example 2 is shown.

[0042] Explanation of Figure Numbers

[0043] 1- Distillation hood;

[0044] 2- Crucible;

[0045] 3-First-stage condensation tower;

[0046] 4- Secondary condensation tower;

[0047] 5-upper zone heating zone;

[0048] 6-Lower heating zone. DETAILED DESCRIPTION

[0049] The present invention will be described in detail below, and the characteristics and advantages of the present invention will become clearer and more distinct with the following description.

[0050] ICP-MS analysis of commercially available manganese chloride raw materials revealed the following main impurity elements:

[0051] Table 1

[0052]

[0053] Example

[0054] The present invention is further described below through specific examples. These examples are only intended to illustrate the present invention and are not intended to limit the scope of the present invention.

[0055] Example 1

[0056] A vacuum distillation furnace, comprising a distillation hood 1, a crucible 2, a primary condensation tower 3, a secondary condensation tower 4, an upper heating zone 5 and a lower heating zone 6, as shown in FIG. Figure 1 shown.

[0057] A primary condensation tower 3 is mounted on top of the distillation hood 1, and a secondary condensation tower 4 is mounted on top of the primary condensation tower 3. The crucible 2 is located inside the distillation hood 1 and is used to store dried manganese chloride. The lower heating zone 6 is mounted outside the distillation hood 1, and the upper heating zone 5 is disposed outside the primary condensation tower 3.

[0058] Crucible 2 is made of boron nitride, primary condensation tower 3 is made of molybdenum, and secondary condensation tower 4 is made of 316 stainless steel. Both primary condensation tower 3 and secondary condensation tower 4 are equipped with eight condensation trays. The primary and secondary condensation towers 3 and 4 are connected by bolts and sealed with rubber gaskets. The entire condenser is installed from the top of the condensation tower.

[0059] Example 2

[0060] Commercially available manganese chloride powder (the main impurity elements contained are shown in Table 1) was placed in a vacuum drying oven and dried at 85°C for 120 min. Finally, it was taken out and vacuum-sealed in a sealed bag for later use.

[0061] Take some dried manganese chloride powder and place it in the crucible 2 of the vacuum distillation furnace described in Example 1. Figure 1 As shown, the manganese chloride powder layer is spread across the entire bottom of crucible 2, with a thickness of 5-30 mm. Use rake teeth to create a corrugated surface to increase the evaporation area. Crucible 2, made of boron nitride or aluminum oxide, is placed in distillation hood 1. The condensation area is divided into a primary condensation tower 3 and a secondary condensation tower 4. The entire condenser is installed from the top. After tightening the furnace lid, vacuum is applied.

[0062] First, start the mechanical pump and slowly open the roughing valve and the fore valve. When the vacuum reaches 1000Pa, increase the pumping speed. When the vacuum reaches 10Pa, start the diffusion pump for preheating. After preheating for about 1h, when the vacuum is maintained at 10Pa, close the roughing valve, open the main valve, and perform high vacuum operation to reach the target vacuum of 5×10 -3 Pa, start heating and conduct pre-distillation.

[0063] The upper heating zone 5 and the lower heating zone 6 are heated to 500℃ at the same time at a rate of 10℃ / min, and the distillation valve is opened; the temperature of the lower heating zone 6 is kept at 500℃ for 30 minutes for pre-distillation, and the temperature of the upper heating zone 5 is maintained at 500℃. During this stage, low-boiling point impurities evaporate and condense in the secondary condensation tray. The heating program is as follows: Figure 2 shown.

[0064] The lower heating zone 6 is heated to the main distillation temperature of 650℃ at a rate of 4℃ / min, kept at this temperature for 60 minutes for main distillation, and then the heating is stopped. The upper heating zone 5 is cooled to 400℃ at a rate of 10℃ / min and kept at this temperature until the lower heating zone 6 stops heating and the temperature drops to 400℃. Then the upper heating zone 5 stops heating. After the upper heating zone 5 and the lower heating zone 6 are cooled to about 100℃, the distillation valve, roughing valve, fore valve and mechanical pump are closed in turn. The temperature control curve is as follows: Figure 2 shown.

[0065] Open the vent valve, open the furnace lid, remove the condensate tray, collect the condensed manganese chloride in the primary condensate tray area, and vacuum seal it to prevent oxidation. Remove the distillation hood and crucible.

[0066] After the experiment, the condenser and distillation chamber were disassembled. The disassembled condenser plate, distillation cover, crucible, and other components were soaked in deionized water for 2 hours, then ultrasonically cleaned for 30 minutes. Finally, they were rinsed again with anhydrous ethanol and allowed to dry. The cleaned and dried components were placed in a vacuum drying oven at 80°C for 60 minutes. After cooling to room temperature, the components were removed and sealed in a sealed bag for later use.

[0067] Example 3

[0068] Commercially available manganese chloride powder (the main impurity elements contained are shown in Table 1) was placed in a vacuum drying oven and dried at 80° C. for 150 min. Finally, it was taken out and vacuum-sealed in a sealed bag for later use.

[0069] Take some dried manganese chloride powder and place it in the crucible 2 of the vacuum distillation furnace described in Example 1. Figure 1 As shown, the manganese chloride powder layer is spread across the entire bottom of crucible 2, with a thickness of 5-30 mm. Use rake teeth to create a corrugated surface to increase the evaporation area. Crucible 2, made of boron nitride or aluminum oxide, is placed in distillation hood 1. The condensation area is divided into a primary condensation tower 3 and a secondary condensation tower 4. The entire condenser is installed from the top. After tightening the furnace lid, vacuum is applied.

[0070] First, start the mechanical pump, and slowly open the roughing valve and the front valve, and evacuate until the vacuum reaches 900Pa, then increase the pumping speed; when the vacuum reaches 8Pa, start the diffusion pump for preheating; after preheating for about 2 hours, when the vacuum is maintained at 8Pa, close the roughing valve, open the main valve, and perform high vacuum operation to reach the target vacuum of 10 -3 Pa, start heating and conduct pre-distillation.

[0071] The upper heating zone 5 and lower heating zone 6 were simultaneously heated to 550°C at a rate of 12°C / min, and the distillation valves were opened. Lower heating zone 6 was held at 550°C for 20 minutes for pre-distillation, while the upper heating zone 5 was also maintained at 550°C. During this stage, low-boiling-point impurities evaporated and condensed in the secondary condensation tray.

[0072] The lower heating zone 6 was heated at a rate of 5°C / min to the main distillation temperature of 700°C and held at this temperature for 90 minutes for main distillation. Heating in the lower heating zone was then stopped, and the upper heating zone 5 was cooled at a rate of 12°C / min to 450°C and held there. After the lower heating zone was cooled to 450°C, heating and holding in the upper heating zone was stopped. After the upper and lower heating zones cooled to approximately 120°C, the distillation valve, roughing valve, foreline valve, and mechanical pump were sequentially closed.

[0073] Open the vent valve, open the furnace lid, remove the condensate tray, collect the condensed manganese chloride in the primary condensate tray area, and vacuum seal it to prevent oxidation. Remove the distillation hood and crucible.

[0074] After the experiment, the condenser and distillation chamber were disassembled. The disassembled condenser plate, distillation cover, crucible, and other components were soaked in deionized water for 2 hours, then ultrasonically cleaned for 30 minutes. Finally, they were rinsed again with anhydrous ethanol and allowed to dry. The cleaned and dried components were placed in a vacuum drying oven at 80°C for 60 minutes. After cooling to room temperature, the components were removed and sealed in a sealed bag for later use.

[0075] Example 4

[0076] Commercially available manganese chloride powder (the main impurity elements contained are shown in Table 1) was placed in a vacuum drying oven and dried at 90° C. for 90 min. Finally, it was taken out and vacuum-sealed in a sealed bag for later use.

[0077] Take some dried manganese chloride powder and place it in the crucible 2 of the vacuum distillation furnace described in Example 1. Figure 1 As shown, the manganese chloride powder layer is spread across the entire bottom of crucible 2, with a thickness of 5-30 mm. Use rake teeth to create a corrugated surface to increase the evaporation area. Crucible 2, made of boron nitride or aluminum oxide, is placed in distillation hood 1. The condensation area is divided into a primary condensation tower 3 and a secondary condensation tower 4. The entire condenser is installed from the top. After tightening the furnace lid, vacuum is applied.

[0078] First, start the mechanical pump, and slowly open the roughing valve and the front valve, and evacuate until the vacuum reaches 1100Pa, then increase the pumping speed; when the vacuum reaches 12Pa, start the diffusion pump for preheating; after preheating for about 0.5h, when the vacuum is maintained at 12Pa, close the roughing valve, open the main valve, and perform high vacuum operation to reach the target vacuum of 10 -2 Pa, start heating and conduct pre-distillation.

[0079] The upper heating zone 5 and lower heating zone 6 are simultaneously heated to 450°C at a rate of 5°C / min. The distillation valves are then opened. Lower heating zone 6 is held at 450°C for 45 minutes for pre-distillation, while upper heating zone 5 is also maintained at 450°C. During this stage, low-boiling-point impurities evaporate and condense in the secondary condensation tray.

[0080] The lower heating zone 6 was heated at a rate of 3°C / min to the main distillation temperature of 600°C and held at this temperature for 120 minutes for main distillation. Heating in the lower heating zone was then stopped, and the upper heating zone 5 was cooled to 300°C at a rate of 8°C / min and held there. After the lower heating zone was cooled to 300°C, heating and holding in the upper heating zone were stopped. After the upper and lower heating zones cooled to approximately 80°C, the distillation valve, roughing valve, foreline valve, and mechanical pump were sequentially closed.

[0081] Open the vent valve, open the furnace lid, remove the condensate tray, collect the condensed manganese chloride in the primary condensate tray area, and vacuum seal it to prevent oxidation. Remove the distillation hood and crucible.

[0082] After the experiment, the condenser and distillation chamber were disassembled. The disassembled condenser plate, distillation cover, crucible, and other components were soaked in deionized water for 2 hours, then ultrasonically cleaned for 30 minutes. Finally, they were rinsed again with anhydrous ethanol and allowed to dry. The cleaned and dried components were placed in a vacuum drying oven at 80°C for 60 minutes. After cooling to room temperature, the components were removed and sealed in a sealed bag for later use.

[0083] Experimental example

[0084] Experimental Example 1 SEM test

[0085] The manganese chloride prepared in Example 2 was subjected to SEM testing, and the test results are as follows: Figure 3 shown.

[0086] from Figure 3 The microscopic morphology of the MnCl2 produced by the present invention is spherical / ellipsoidal, with a particle size ranging from 1 to 200 μm. A surface scan reveals uniform distribution of Mn and Cl, with an atomic ratio of approximately 1:2. This demonstrates that the manganese chloride produced by the method of the present invention has uniform particle size and high purity.

[0087] The present invention has been described in detail above with reference to specific embodiments and exemplary examples. However, these descriptions should not be construed as limiting the present invention. Those skilled in the art will appreciate that various equivalent substitutions, modifications, or improvements may be made to the technical solutions and implementations of the present invention without departing from the spirit and scope of the present invention, all of which fall within the scope of the present invention. The scope of protection of the present invention shall be determined by the appended claims.

Claims

1. A method for purifying manganese chloride, characterized in that: The method comprises: The manganese chloride is dried, and the temperature is raised under vacuum conditions to perform pre-distillation on the dried manganese chloride, and then the temperature is raised to perform main distillation.

2. The method according to claim 1, characterized in that The drying conditions are: drying at 80-90° C. for 90-150 minutes.

3. The method according to claim 1, characterized in that The pre-distillation conditions are: the pre-distillation vacuum degree is 10 -3 ~10 -2 Pa, heat to 450-550°C at a heating rate of 5-12°C / min, and keep at this temperature for 20-45 min.

4. The method according to claim 1, wherein The main distillation conditions are: the main distillation vacuum degree is 10 -3 ~10 -2 Pa, heat to 600-700°C at a heating rate of 3-5°C / min, and keep at this temperature for 30-180 min.

5. The method according to claim 1, wherein The pre-distillation and main distillation are carried out in a vacuum distillation furnace; the vacuum distillation furnace comprises a distillation hood, a crucible, a primary condensation tower, a secondary condensation tower, an upper heating zone and a lower heating zone; A primary condensation tower is installed on the upper part of the distillation hood, and a secondary condensation tower is installed on the upper part of the primary condensation tower. The crucible is located inside the distillation hood, the lower heating zone is installed outside the distillation hood, and the upper heating zone is arranged outside the primary condensation tower.

6. The method according to claim 5, characterized in that Each level of the condensation tower is equipped with 5 to 10 layers of condensation trays; The first-level condensation tower and the second-level condensation tower are connected by bolts and sealed with rubber pads.

7. The method according to claim 5, characterized in that The dried manganese chloride is spread all over the bottom of the crucible, and the thickness of the manganese chloride layer is controlled to be 5 to 30 mm.

8. The method according to claim 5, characterized in that The pre-distillation conditions are: at a vacuum degree of 10 -3 ~10 -2 Pa, the upper heating zone and the lower heating zone are simultaneously heated to 450-550°C at a heating rate of 5-12°C / min; the lower heating zone is kept at 450-550°C for 20-45 minutes for pre-distillation, and the temperature of the upper heating zone is also maintained at 450-550°C.

9. The method according to claim 5, characterized in that The main distillation conditions are: at a vacuum degree of 10 -3 ~10 -2 Pa, the lower heating zone is heated at a rate of 3-5°C / min to a main distillation temperature of 600-700°C, and the main distillation is carried out by keeping the temperature for 30-180 minutes.

10. The method according to claim 9, characterized in that After the main distillation is completed, the lower heating zone stops heating, and the upper heating zone is cooled to 300-450°C at a cooling rate of 8-12°C / min and kept warm until the lower heating zone stops heating and cools to 300-450°C, and then the upper heating zone stops heating and keeps warm.

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