Preparation method of 5N-grade ammonium perrhenate

Abstract

The invention belongs to the technical field of scattered metal salt purification, and particularly relates to a preparation method of 5N-grade ammonium perrhenate. In the prior art, two methods are generally adopted when 5N-grade ammonium perrhenate is prepared: 1, an ammonium perrhenate crude product is adsorbed and separated by ion exchange resin to obtain 5N-grade ammonium perrhenate; and 2, heating and decomposing the ammonium perrhenate crude product to form rhenium heptoxide, separating and purifying the rhenium heptoxide to obtain 5N-grade rhenium heptoxide, and absorbing the 5N-grade rhenium heptoxide by ammonia water to obtain 5N-grade ammonium perrhenate. Wherein the method 1 is wider in application, but the preparation process has the following problems: 1, the purification process is complicated and the efficiency is low; 2, the loss of ammonium perrhenate is large, and the purification rate is low; the invention discloses a preparation method of 5N-grade ammonium perrhenate. The 5N-grade ammonium perrhenate is prepared by separating and purifying an ammonium perrhenate crude product by adopting macroporous adsorption resin. The preparation method provided by the invention has the characteristics of simple process and high purification rate.

Description

Technical Field

[0001] The present invention belongs to the technical field of rare metal salt purification, and specifically relates to a method for preparing 5N grade ammonium perrhenate. Background Art

[0002] There are usually two methods for preparing 5N grade ammonium perrhenate in the prior art: 1. The crude ammonium perrhenate is adsorbed and separated by an ion exchange resin to obtain 5N grade ammonium perrhenate; 2. The crude ammonium perrhenate is heated and decomposed to form rhenium heptoxide, the rhenium heptoxide is separated and purified to obtain 5N grade rhenium heptoxide, and the 5N grade rhenium heptoxide is absorbed by ammonia water to obtain 5N grade ammonium perrhenate. Among them, Method 1 is more widely used, but there are the following problems in its preparation process: 1. The purification process is complex and the efficiency is low; 2. The loss of ammonium perrhenate is large and the purification rate is low.

[0003] In 2023, "CN 117509739A" disclosed a preparation process of 5N grade high-purity ammonium perrhenate. The process includes the following steps: (1) Preparation of a rhenium-containing stock solution: The purchased crude ammonium perrhenate with a purity of more than 90% is stirred and dissolved in pure water, and after precisely filtering off suspended impurities, a clear and transparent rhenium-containing extraction stock solution is obtained; (2) Preparation of a pure ammonium perrhenate solution: The rhenium-containing extraction stock solution undergoes an extraction reaction with a reverse-flowing rhenium extraction organic phase to obtain a rhenium-rich organic phase and a raffinate respectively; the rhenium-rich organic phase and the purified organic phase after being fully washed with pure water are subjected to back extraction to obtain a back extract, and this back extract is the pure ammonium perrhenate solution; (3) Defatting of the pure rhenium back extract: The pure ammonium perrhenate solution is adsorbed and defatted by a defatting resin to obtain a defatted high-purity rhenium solution; (4) Concentration, crystallization and drying: The defatted high-purity rhenium solution is subjected to vacuum evaporation concentration, crystallization, liquid-solid separation and then drying to obtain the final product, 5N grade ammonium perrhenate, and the total rhenium recovery rate can reach 97.13%. It can be seen that this preparation method is very complex.

[0004] In 2024, "CN 119038611A" disclosed a method for purifying ammonium perrhenate. It includes: primary dissolution and crystallization, secondary dissolution and crystallization, resin preheating, ion exchange, resin rhenium washing, resin regeneration, resin acid washing, and ammonium perrhenate neutralization and suction filtration processes. Among them, in the secondary dissolution and crystallization step, hydrogen peroxide with a mass of 1.2 - 1.8% of the ammonium perrhenate crystals is added for oxidation. By controlling the addition amount of hydrogen peroxide, not only the chromaticity of the prepared ammonium perrhenate is good, but also the purity of the ammonium perrhenate after purification can reach more than 99.995%, and the purification effect is good. The purification process of the ammonium perrhenate purification method described in this invention is very complex and the purity of the ammonium perrhenate after purification is only 99.995% and does not reach the 5N level.

[0005] In summary, the prior art generally has problems of complex preparation processes and low purification rates when preparing 5N grade ammonium perrhenate. Summary of the Invention

[0006] The technical problem to be solved by the present invention:

[0007] The problems existing in the preparation of 5N ammonium perrhenate in the prior art, such as complex purification process, large loss and low purification rate.

[0008] The technical solution adopted by the present invention:

[0009] The crude ammonium perrhenate is separated by adsorption to obtain 5N ammonium perrhenate; specifically, the crude ammonium perrhenate is separated by adsorption to obtain 5N ammonium perrhenate, which is characterized in that the preparation method is carried out according to the following procedures:

[0010] (1) Pack the adsorption resin into the adsorption column;

[0011] (2) The crude ammonium perrhenate is subjected to solid loading and elution, and the high-purity ammonium perrhenate solution is collected;

[0012] (3) The high-purity ammonium perrhenate solution is distilled and dried to obtain 5N ammonium perrhenate;

[0013] Among them, the passing rate of the crude ammonium perrhenate through a 200-mesh sieve = 100%.

[0014] The above is the basic technical solution of the present invention. The differences between the present invention and the prior art are as follows: 1. The present invention uses an adsorption resin to purify the crude ammonium perrhenate and separate the impurity ions therein; 2. The present invention uses solid loading, elution, and collection of a high-purity ammonium perrhenate solution. The present invention utilizes the property that the solubility of ammonium perrhenate in water changes greatly with temperature, and uses an adsorption resin for purification and removal of impurity ions. Through solid loading, a selective dissolution process of the crude ammonium perrhenate is added on the basis of the existing adsorption resin separation, which promotes the effect of adsorption separation. The basic principle of the present invention is as follows: First, the crude ammonium perrhenate is subjected to solid loading and dissolved by adding deionized water. In this process, based on ammonium perrhenate, salts with a solubility greater than that of ammonium perrhenate will dissolve preferentially, and salts with a solubility less than that of ammonium perrhenate will dissolve last. When this process proceeds, a distribution curve of the concentrations of three types of salts entering the adsorption resin over time is formed, that is, the salt solution with a solubility greater than that of ammonium perrhenate enters the adsorption resin earliest, and the salt solution with a solubility less than that of ammonium perrhenate enters the adsorption resin last, which is very beneficial for improving the separation effect of the adsorption resin; Then, during the process of the loading solution flowing from top to bottom in the adsorption column, the salt solution with a solubility greater than that of ammonium perrhenate is less likely to enter the internal pores of the adsorption resin and precipitate crystals compared to the ammonium perrhenate solution, and can leave the adsorption column faster. The salt solution with a solubility less than that of ammonium perrhenate is more likely to enter the internal pores of the adsorption resin and precipitate crystals compared to the ammonium perrhenate solution, and is more difficult to leave the adsorption column. The overall result is that the salt with a solubility greater than that of ammonium perrhenate leaves the adsorption column earliest, and the salt with a solubility less than that of ammonium perrhenate leaves the adsorption column last, realizing the purification and separation process of ammonium perrhenate from other salts. To promote the effect of the above-mentioned solid loading on adsorption separation, the present invention needs to reduce the particle size of the crude ammonium perrhenate through technical means such as crushing and grinding. The smaller the particle size of the crude ammonium perrhenate, the larger the specific surface area, and the more uniform the process of dissolving the crude ammonium perrhenate with deionized water, which is more conducive to distinguishing ammonium perrhenate from other impurities using the solubility difference. Conversely, it is the opposite. The inventor determined through comparative experiments that when using crude ammonium perrhenate with a 200-mesh sieve passing rate of 100% for solid loading, the effect of the entire separation and purification process is the best, and further reducing the particle size of the crude ammonium perrhenate has no significant effect on improving the adsorption separation effect.

[0015] The above is the basic technical solution of the present invention. To facilitate those skilled in the art to understand and implement the technical solution of the present invention and achieve the predetermined effect. Preferably, the mass ratio of the adsorption resin in step (1) to the crude ammonium perrhenate in step (2) is 25-35:1; the adsorption resin in step (1) is a medium-polarity macroporous adsorption resin. In view of the property of ammonium perrhenate precipitating crystals in water, the present invention needs to use a medium-polarity macroporous adsorption resin as the adsorbent, which has balanced adsorption strength and adsorption capacity, and will not have problems such as large adsorption capacity, high adsorption strength, difficult elution, or small adsorption capacity, low adsorption strength, and easy elution.

[0016] Further preferably, the medium-polarity macroporous adsorption resin is one of HPD-450, NKA-9, D301, AB-8, and XAD-7. The inventor determined through comparative experiments that using the medium-polarity macroporous adsorption resins of HPD-450, NKA-9, D301, AB-8, and XAD-7 has the best effect. The reason is that the pore sizes of the medium-polarity macroporous adsorption resins of HPD-450, NKA-9, D301, AB-8, and XAD-7 are more conducive to the balance of the adsorption and desorption processes of ammonium perrhenate crystallization.

[0017] Preferably, the content of the crude ammonium perrhenate ≥ 99.0%. The present invention is applicable to the preparation of 5N-grade ammonium perrhenate using crude ammonium perrhenate with a content ≥ 99.0% as the raw material. Among them, 99.0% is also the common content of commercially available industrial-grade ammonium perrhenate. Using crude ammonium perrhenate with a content ≥ 99.0% as the raw material to prepare 5N-grade ammonium perrhenate can reduce the raw material cost for preparing 5N-grade ammonium perrhenate.

[0018] Preferably, the adsorption column in step (1) is provided with an external heat exchange surface; the external heat exchange surface is selected from one of a jacket and a semi-circular tube. The inner surface of the jacket or the semi-circular tube and the outer surface of the adsorption column form a heat exchange medium chamber, and the heat exchange medium chamber has a heat exchange medium inlet and a heat exchange medium outlet. When the heat exchange medium enters the heat exchange medium chamber from the heat exchange medium inlet and then leaves the heat exchange medium chamber from the heat exchange medium outlet, the heat exchange medium transfers heat or cold to the system inside the adsorption column through the outer wall of the adsorption column to achieve the purpose of heat or cold transfer, thereby maintaining the temperature of the system inside the adsorption column. To improve the heat exchange effect, a spiral baffle can also be provided inside the jacket to increase the flow rate of the heat exchange medium in the heat exchange medium chamber, increase the convective heat transfer coefficient on the outside, and promote the increase of the total heat transfer coefficient. A thermal insulation layer is provided outside the external heat exchange surface to reduce heat dissipation, which is more conducive to maintaining the temperature inside the adsorption column constant.

[0019] Preferably, the solid sample loading process in step (2) is as follows: ammonium perrhenate crude solid is loaded onto the upper part of the adsorption resin loaded inside the adsorption column in step (1), and deionized water is evenly added to the upper part of the ammonium perrhenate crude solid under the condition of the first preset temperature to perform sample loading; the elution process is: deionized water is evenly added to the upper part of the adsorption resin under the condition of the second preset temperature to perform elution. Different from the prior art ammonium perrhenate solution sample loading process, the present invention adopts ammonium perrhenate solid sample loading. The selective dissolution process of ammonium perrhenate crude product is added during the solid sample loading process, which is a beneficial supplement to the resin adsorption separation process. During the solid sample loading process, the sample loading speed can be controlled by controlling the adsorption column at the first preset temperature. When the first preset temperature is high, the sample loading speed is fast; when the first preset temperature is low, the sample loading speed is slow. In actual operation, when the content of ammonium perrhenate crude product is low, a lower first preset temperature can be adopted, which is easier to increase the content of ammonium perrhenate crude product to the 5N level. For raw materials with an ammonium perrhenate crude product content much higher than 99.0%, a higher first preset temperature can be adopted, which can accelerate the solid sample loading speed and reduce the time of the entire adsorption separation process. After the solid sample loading is completed, the ammonium perrhenate adsorbed on the medium-polarity macroporous adsorption resin is eluted by controlling the adsorption column at the second preset temperature. During the elution process, inorganic salts with a solubility greater than that of ammonium perrhenate leave the adsorption resin first, and inorganic salts with a solubility less than that of ammonium perrhenate leave the adsorption column later. A 5N grade ammonium perrhenate solution is obtained in the middle section of the entire elution process.

[0020] More preferably, by introducing hot water into the inner cavity of the external heat exchange surface of the adsorption column, the first preset temperature is controlled to be 45 - 55 °C. The inventor determined through comparative experiments combined with the content of ammonium perrhenate crude product that controlling the first preset temperature to be 45 - 55 °C can achieve the fastest adsorption separation speed under the condition of meeting the final ammonium perrhenate content reaching the 5N level. When the first preset temperature is selected at the lower limit of 45 °C, the final ammonium perrhenate content is high, but the adsorption separation time is long and the water consumption is large. When the first preset temperature is selected at the upper limit of 55 °C, the situation is just the opposite.

[0021] More preferably, by introducing hot water into the inner cavity of the external heat exchange surface of the adsorption column, the second preset temperature is controlled to be 55 - 65 °C. The influence of the second preset temperature on adsorption separation is similar to that of the first preset temperature on adsorption separation as described above, and will not be elaborated here.

[0022] Preferably, the temperature of the distillation process in the control step (3) is 45-65°C; the pressure is -0.08 to -0.097 MPa. The present invention utilizes the equilibrium process of precipitation and dissolution of ammonium perrhenate in the saturated solution of ammonium perrhenate, and uses medium-polarity macroporous adsorption resin to selectively adsorb ammonium perrhenate to achieve its separation from other inorganic impurities. Since the solubility of ammonium perrhenate in water is medium under the second preset temperature condition, the quantity of the finally obtained 5N-grade ammonium perrhenate solution is large. To increase the speed of the distillation process and reduce the distillation time, the temperature of the distillation process in the control step (3) is 45-65°C; the negative pressure distillation process with a pressure of -0.08 to -0.097 MPa is adopted to increase the heat transfer temperature difference and improve the heat transfer speed. In specific implementation, multi-effect distillation or MVR technology can also be used to reduce the consumption of live steam.

[0023] The present invention discloses a preparation method of 5N-grade ammonium perrhenate, which solves the problems of complex process, large loss and low purification rate in the prior art for preparing 5N-grade ammonium perrhenate. Specific embodiments

[0024] The present invention will be specifically described below by way of examples, but the present invention is not limited to these examples.

[0025] Take crude ammonium perrhenate: 500 g (content: 99.0143%), and pulverize it into fine powder with a 200-mesh sieve passing rate of 100% in a centrifugal pulverizer for standby.

[0026] Example 1

[0027] (1) Load the adsorption column:

[0028] Put an appropriate amount of absorbent cotton into a glass adsorption column with a jacket of ∮40*1200 mm, and use a long glass rod to evenly press the absorbent cotton flat on the sand core near the bottom inside the adsorption column. Add 500 ml of deionized water to the adsorption column, and slowly add 850 g of medium-polarity macroporous adsorption resin of model D301. After the resin is added, turn the bottom plug of the adsorption column to drain a part of the deionized water in the adsorption column so that the liquid level of the deionized water in the adsorption column is 0.5-1 cm higher than the upper end face of the adsorption resin. After the resin is loaded, evenly load 120 g of quartz sand on the resin. After the quartz sand is loaded, evenly load 30.000 g of crude ammonium perrhenate on the upper end face of the quartz sand. After the crude ammonium perrhenate is loaded, evenly load 100 g of quartz sand on the upper end face of the crude ammonium perrhenate.

[0029] (2) Preheat:

[0030] Set the heating temperature of the water bath: 50°C. Use a hot water peristaltic pump to transport the 50°C hot water in the water bath into the lower inlet of the adsorption column jacket, and return the hot water from the upper outlet of the adsorption column jacket to the water bath. Preheat the material inside the adsorption column through the hot water in the jacket. The preheating time is about 30 minutes.

[0031] (3) Loading:

[0032] After preheating, use a deionized water peristaltic pump to transport the 50°C deionized water in the beaker into the adsorption column for loading. Set the peristaltic pump speed: 30 r / min, and the deionized water flow rate is about: 30 ml / min. The deionized water entering the adsorption column is distributed by quartz sand and then enters the ammonium perrhenate crude product layer. The ammonium perrhenate crude product slowly dissolves under the flushing of 50°C hot water. The deionized water dissolving the ammonium perrhenate crude product enters the adsorption column and undergoes continuous processes of precipitation crystallization, adsorption crystallization, and dissolution crystallization, gradually forming a distribution of ammonium perrhenate content from low to high and then from high to low in the vertical direction of the adsorption column. When the ammonium perrhenate crude product in the adsorption column is completely dissolved, the loading is completed.

[0033] (4) Desorption:

[0034] After loading, raise the hot water temperature in the water bath to 60°C. Use a hot water peristaltic pump to transport the 60°C hot water in the water bath into the lower inlet of the adsorption column jacket, and return the hot water from the upper outlet of the adsorption column jacket to the water bath. Keep the material inside the adsorption column warm through the hot water in the jacket. Use a peristaltic pump to transport the 60°C deionized water in the beaker into the adsorption column for desorption. Set the peristaltic pump speed: 45 r / min, and the deionized water flow rate is about: 50 ml / min. Collect the desorbed solution flowing out from the bottom of the adsorption column into multiple 500 ml volumetric flasks in sequence, and control the volume of the desorbed solution in each volumetric flask to be about: 500 ml. In this example, the total number of volumetric flasks for receiving the desorbed solution is 8, numbered 1 # 、2 # 、3 # 、4 # 、5 # 、6 # 、7 # 、8 # in sequence.

[0035] (5) Classification:

[0036] Detect the ammonium perrhenate content in each volumetric flask respectively. Combine the ammonium perrhenate solutions in the volumetric flasks with an ammonium perrhenate content ≥ 99.999% to obtain a high-content ammonium perrhenate solution. Combine the ammonium perrhenate solutions in the low-numbered volumetric flasks with an ammonium perrhenate content < 99.999% to obtain a front portion solution. Use the ammonium perrhenate solution in the high-numbered volumetric flasks with an ammonium perrhenate content < 99.999% as the rear portion solution.

[0037] (6) Distillation and drying:

[0038] Transfer the ammonium perrhenate solution with high content in batches to a 250 ml three-necked flask equipped with a thermometer, a stirring paddle, and a recovery condenser. The three-necked flask is heated by 65 °C hot water. Control the vacuum degree at -0.09 MPa and the temperature at 55 °C to distill off the water. Distill until it becomes viscous. Transfer the material to a vacuum oven and control the vacuum degree of the oven at 0.095 MPa and the temperature at 65 °C for drying for 8 hours to obtain 5N grade ammonium perrhenate: 25.24 g, content: 99.9994%.

[0039] (7) Concentration and crystallization of the front portion and the rear portion:

[0040] Distill and dry the front portion and the rear portion in the same method as that for the ammonium perrhenate with high content. Finally, obtain the front portion: 2.35 g, content: 97.4521%, and the rear portion: 2.27 g, content: 93.2101%.

[0041] Example 2

[0042] The preparation process and process parameters of Example 2 are basically the same as those of Example 1, except that: the input amount of the resin when loading the adsorption column is reduced to: 750 g. Finally, obtain 5N grade ammonium perrhenate: 23.46 g, content: 99.9991%; the front portion: 3.37 g, content: 98.1941%; the rear portion: 3.03 g, content: 95.1389%.

[0043] Example 3

[0044] The preparation process and process parameters of Example 3 are basically the same as those of Example 1, except that: the input amount of the resin when loading the adsorption column is increased to: 1050 g. Finally, obtain 5N grade ammonium perrhenate: 26.45 g, content: 99.9997%; the front portion: 1.97 g, content: 96.5424%; the rear portion: 1.43 g, content: 91.0187%.

[0045] Example 4

[0046] The preparation process and process parameters of Example 4 are basically the same as those of Example 1, except that: the first preset temperature is reduced to 45 °C. Finally, obtain 5N grade ammonium perrhenate: 26.15 g, content: 99.9996%; the front portion: 2.03 g, content: 96.8742%; the rear portion: 1.68 g, content: 91.2253%.

[0047] Example 5

[0048] The preparation process and process parameters of Example 5 are basically the same as those of Example 1, except that: the first preset temperature is increased to 55°C. Finally, 5N grade ammonium perrhenate is obtained: 24.75 g, content: 99.9994%; front portion: 2.61 g, content: 97.6624%; rear portion: 2.48 g, content: 94.1976%.

[0049] Example 6

[0050] The preparation process and process parameters of Example 6 are basically the same as those of Example 1, except that: the second preset temperature is decreased to 55°C. Finally, 5N grade ammonium perrhenate is obtained: 26.66 g, content: 99.9997%; front portion: 1.94 g, content: 96.3125%; rear portion: 1.25 g, content: 88.8112%.

[0051] Example 7

[0052] The preparation process and process parameters of Example 7 are basically the same as those of Example 1, except that: the second preset temperature is increased to 65°C. Finally, 5N grade ammonium perrhenate is obtained: 24.51 g, content: 99.9994%; front portion: 2.83 g, content: 97.8420%; rear portion: 2.51 g, content: 94.0255%.

[0053] Comparative Example 1

[0054] The preparation method and process parameters of Comparative Example 1 are basically the same as those of Example 1, except that: the input amount of resin when loading the adsorption column is reduced to: 600 g. Finally, ammonium perrhenate with a relatively high content is obtained: 20.45 g, content: 99.9924%; front portion: 4.75 g, content: 98.4553%; rear portion: 4.65 g, content: 97.0718%.

[0055] Comparative Example 2

[0056] The preparation method and process parameters of Comparative Example 2 are basically the same as those of Example 1, except that: the first preset temperature is increased to 70°C. Finally, ammonium perrhenate with a relatively high content is obtained: 21.55 g, content: 99.9912%; front portion: 4.16 g, content: 97.9542%; rear portion: 4.15 g, content: 97.0096%.

[0057] Comparative Example 3

[0058] The preparation method and process parameters of Comparative Example 3 are basically the same as those of Example 1, except that: the second preset temperature is increased to 80°C. Finally, ammonium perrhenate with a relatively high content is obtained: 20.46 g, content: 99.9814%; front portion: 5.14 g, content: 98.4442%; rear portion: 4.26 g, content: 97.0128%.

[0059] Finally, it should be noted that the above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or perform equivalent replacements for some of the technical features. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A preparation method of 5N ammonium perrhenate, the preparation method comprising: The crude ammonium perrhenate is separated by adsorption to obtain 5N-grade ammonium perrhenate; it is characterized in that the preparation method is carried out according to the following procedures: (1) Pack the adsorption resin into the adsorption column; (2) The crude ammonium perrhenate is subjected to solid loading and elution, and the high-purity ammonium perrhenate solution is collected; (3) The high-purity ammonium perrhenate solution is distilled and dried to obtain 5N-grade ammonium perrhenate; Among them, the passing rate of the crude ammonium perrhenate through a 200-mesh sieve = 100%.

2. The preparation method according to claim 1, characterized in that, The mass ratio of the adsorption resin in step (1) to the crude ammonium perrhenate in step (2) is 25-35:1; the adsorption resin in step (1) is a medium-polarity macroporous adsorption resin.

3. The preparation method according to claim 2, characterized in that The model of the medium-polarity macroporous adsorption resin is one of HPD-450, NKA-9, D301, AB-8, XAD-7.

4. The preparation method according to claim 1, characterized in that, The content of the crude ammonium perrhenate ≥ 99.0%.

5. The preparation method according to claim 1, characterized in that, The adsorption column in step (1) is provided with an external heat exchange surface; the external heat exchange surface is selected from one of a jacket and a semi-circular tube.

6. The preparation method according to claim 1, wherein, The solid loading process in step (2) is as follows: the solid of the crude ammonium perrhenate is loaded onto the upper part of the adsorption resin loaded inside the adsorption column in step (1), and deionized water is uniformly added to the upper part of the solid of the crude ammonium perrhenate under the control of the adsorption column at a first preset temperature condition for loading; the elution process is as follows: deionized water is uniformly added to the upper part of the adsorption resin under the control of the adsorption column at a second preset temperature condition for elution.

7. The preparation method according to any one of claims 5 or 6, characterized in that, By introducing hot water into the internal cavity of the external heat exchange surface of the adsorption column, the first preset temperature is controlled to be 45-55°C.

8. The preparation method according to any one of claims 5 or 6, characterized in that, By introducing hot water into the internal cavity of the external heat exchange surface of the adsorption column, the second preset temperature is controlled to be 55-65°C.

9. The preparation method according to claim 1, characterized in that, The temperature of the distillation process in step (3) is controlled to be 45-65°C; the pressure is -0.08 to -0.097 MPa.