Method for strengthening phosphoric acid melt crystallization process through ultrasound

Abstract

The invention belongs to the field of electronic chemical purification, and particularly relates to a method for strengthening a phosphoric acid melt crystallization process through ultrasound and residual liquid circulation, which comprises the following steps: adding a high-concentration phosphoric acid raw material into a static melt crystallizer, cooling and graining, and enhancing the purity by adopting ultrasound in a sweating stage, high-concentration uncrystallized raffinate is cooled by an external circulation coil heat exchanger and then flows back to a crystallizer, crystallization continues to be carried out, the yield is increased, and the Fe content can be reduced to 20 ppb from 73 through crystallization purification. The concentration of phosphoric acid adopted by the method is greater than or equal to 85%, the problems of poor impurity removal effect, low yield and long consumed time of a conventional static crystallization method are solved, the uncrystallized residual liquid is fully recycled, the energy consumption is reduced, and the method has high applicability.

Description

Technical Field

[0001] The invention belongs to the field of electronic chemical purification, and in particular relates to a method for ultrasonically strengthening a phosphoric acid melting crystallization process. Background Art

[0002] Phosphoric acid is an important chemical raw material, widely used in fertilizers, food, pharmaceuticals and other fields. Among them, electronic-grade phosphoric acid with ultra-high purity can be used for etching and cleaning chips. The metal content in phosphoric acid determines the quality of electronic-grade phosphoric acid.

[0003] Electronic grade phosphoric acid is prepared by purification of wet process phosphoric acid or thermal process phosphoric acid. The current purification methods include ion exchange, extraction, precipitation, membrane separation and crystallization. Among them, the melt crystallization method has the advantages of low energy consumption and no introduction of other impurities, and is therefore given special attention.

[0004] Chinese patent CN 117049501 A discloses a method for preparing electronic-grade phosphoric acid by ultrasonic-assisted melt crystallization. Electronic-grade phosphoric acid is prepared by ultrasonically enhanced two-stage suspension melt crystallization process, and its quality can reach SEMI G3 grade. However, this method is time-consuming, the loss of phosphoric acid in the two crystallization washing processes is large, and the poor heat transfer effect of suspension crystallization results in a low yield.

[0005] Patent CN201810283633.6 discloses a method for preparing electronic-grade phosphoric acid by liquid membrane crystallization. This method can prepare electronic-grade phosphoric acid with low energy consumption, but it has the disadvantages of low production efficiency and difficulty in achieving scaled-up production.

[0006] Patent CN202410366863.4 discloses an improved static melt crystallization device, which can realize forced convection between the material and the cold and hot medium during the crystallization process, improve the heat exchange efficiency, thereby improving the crystallization purity and shortening the crystallization time. However, the liquid accumulation tank below the device is easy to cause crystals to enter the pump body, reducing the service life of the liner pump. At the same time, there is a lack of insulation device. When the material concentration is too high, it is easy to crystallize in the circulation pipeline and cause blockage. Static melt crystallization of phosphoric acid has problems such as insufficient mass transfer driving force and uneven distribution of impurities. The heat exchange area of ​​the crystallizer limits the upper limit of the yield of a single crystallization. In the later stage of crystallization, the crystals adhere to the heat exchange surface, and the residual liquid cannot contact the heat exchange surface to cool down and crystallize. This part of the residual liquid is often discharged. The recycling of the residual liquid is generally through secondary crystallization, which prolongs the crystallization time and reduces production efficiency. Summary of the invention

[0007] In order to solve the above problems, the present invention aims to provide a method for strengthening the phosphoric acid melt crystallization process, which uses ultrasound to strengthen the crystal nucleation and sweating stages to increase the driving force of mass transfer. The high-concentration residual liquid is cooled in an external heat exchanger and circulated back to the crystallizer for continued crystallization, thereby increasing the yield of a single crystallization, shortening the production time, improving the production efficiency, and improving the purification yield and quality of electronic-grade phosphoric acid.

[0008] The technical solution of the present invention is as follows: A method for enhancing the phosphoric acid melt crystallization process by ultrasound comprises the following steps: Step 1) feeding, phosphoric acid raw material is added into a static melt crystallizer; Step 2) Cooling crystallization: program temperature control to lower the temperature of the circulating water in the crystallizer, turn on ultrasound, add seed crystals when the acid temperature is lower than the crystallization point, and continue to cool the circulating water to the end of crystallization; Step 3) Drain residual liquid: open the bottom valve of the crystallizer to drain the residual liquid; Step 4) Residual liquid cooling and reflux: The high-concentration residual liquid is discharged into the shell side of the coil heat exchanger, cooled to below the corresponding crystallization point, and refluxed to the crystallizer to continue crystallization.

[0009] Step 5) Sweating: Turn on the ultrasonic device, raise the temperature of the circulating water to the sweating temperature, keep warm, and discharge sweat; Step 6) Crystal melting: The circulating water is heated to the crystal melting temperature, kept warm, and the material circulation pump is started to melt the crystals through hot acid to discharge the finished phosphoric acid.

[0010] Furthermore, in step 1), the phosphoric acid concentration of the raw material is greater than or equal to 85%, and the total metal content is less than 200 ppb. A static melt crystallizer with a plate heat exchanger is used, and the material is 316L stainless steel sprayed with fluoroplastic or lined with PTFE.

[0011] Furthermore, in step 2), the seeding temperature is 2-3° C. lower than the corresponding crystallization temperature, the cooling rate is 1-5° C. / h, and the final temperature is 10-20° C. Ultrasound is turned on for 2 minutes every 10 minutes, and the frequency is 30-50 kHz.

[0012] Furthermore, in step 4), the phosphoric acid residual liquid with a concentration greater than or equal to 83% is discharged into the coil heat exchanger, first quickly heated to 40°C to eliminate the seed crystals, and then cooled to 2-3°C below the corresponding crystallization point, and the reflux time is 0.5-1h. If the concentration is lower than 83%, no reflux is performed.

[0013] Furthermore, in step 4), the pipeline transportation of supercooled phosphoric acid should be kept cool and continuous throughout the entire process, and the material should not be stationary in the pipeline to avoid crystallization.

[0014] Furthermore, in step 5), ultrasound is turned on for 2 minutes every 10 minutes, the frequency is 30-50 kHz, the sweating temperature is 28-32°C, and the insulation time is 1-2 hours.

[0015] Furthermore, in step 6), the crystallizer circulating water is heated to 35-40°C and kept warm for 2-4 hours, the acid temperature in the finished product tank is maintained at 35-40°C, and the circulation flow rate is 60-90L / h.

[0016] The advantages of the present invention are: The present invention utilizes an ultrasonic method to strengthen the melt crystallization process. The method reduces the supercooling required for nucleation in the crystal nucleation stage, and the crystallization rate is easy to control. Ultrasound strengthens the migration of metal impurities in the sweating stage, improves the separation effect of crystallization, and thus improves the purity of the finished acid. The uncrystallized residual liquid is cooled and refluxed, and the time can be shortened from 21 hours to less than 13 hours compared with two batches of crystallization, thereby improving the yield and reducing the subsequent waste acid treatment cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 The process flow chart of this method is shown in FIG.

[0018] Figure 2 Schematic diagram of the device of this method. DETAILED DESCRIPTION

[0019] The following will be combined with the specific embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. The embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work belong to the protection scope of the present invention.

[0020] Example 1 (1) Add 3000 ml of 85% phosphoric acid solution into a static crystallizer with a plate heat exchanger; (2) Place a PTFE-lined ultrasonic generator in the reactor, and cool the circulating water from 30°C at a rate of 3°C / h. When the acid temperature in the reactor drops to 18°C, turn on the ultrasound for 3 min every 10 min at a frequency of 30 kHz, and add 4 g of seed crystals. After the crystal nucleus growth is observed through the window and phosphoric acid begins to crystallize, turn off the ultrasonic generator and cool the circulating water to 10°C at a rate of 3°C / h. (3) Open the discharge port for 1 h to discharge the uncrystallized residual liquid; (4) The residual liquid concentration measured by density method was 83.20%. The residual liquid was discharged into the coil heat exchanger and rapidly heated to 40°C and then rapidly cooled to 12°C. It was refluxed to the crystallizer for further crystallization within 1 hour. (5) The circulating water was heated to 28°C at a rate of 6°C / h, and ultrasound was turned on for 3 min every 10 min at a frequency of 30 kHz for 2 h; (6) The circulating water is heated to 40°C and kept warm for 3 hours. The material circulation pump is turned on and the crystals are completely melted. The valve is opened to discharge the material to obtain the finished phosphoric acid.

[0021] Example 2 (1) Add 3000 ml of 88% phosphoric acid solution into a static crystallizer with a plate heat exchanger; (2) Place a PTFE-lined ultrasonic generator in the reactor, and cool the circulating water from 30°C at a rate of 3°C / h. When the acid temperature in the reactor drops to 24°C, start the ultrasound for 3 min every 10 min at a frequency of 30 kHz, and add 4 g of seed crystals. After the crystal nucleus begins to grow and phosphoric acid begins to crystallize, turn off the ultrasonic generator and cool the circulating water to 10°C at a rate of 3°C / h. (3) Open the discharge port for 1 hour to discharge the uncrystallized residual liquid; (4) The density method was used to measure the residual liquid concentration, which was 86.21%. The residual liquid was discharged into the coil heat exchanger and rapidly heated to 40°C and then rapidly cooled to 20°C. After 1 hour, the residual liquid was returned to the crystallizer for further crystallization. (5) The crystallizer circulating water was heated to 28°C at 6°C / h, and ultrasound was turned on for 3 min every 10 min at a frequency of 30 kHz for 2 h; (6) The circulating water is quickly heated to 40°C and kept warm for 3 hours. The material circulation pump is turned on and the valve is opened to discharge the material after the crystals are completely melted to obtain the finished phosphoric acid.

[0022] Comparative Example 1 (1) Add 3000 ml of 85% phosphoric acid solution into a static crystallizer with a plate heat exchanger; (2) The circulating water was cooled at a rate of 3°C / h from 30°C. The acid temperature in the kettle dropped to 18°C ​​and 4 g of seed crystals were added. The growth of crystal nuclei was observed through the window. After phosphoric acid began to crystallize, the circulating water was cooled at a rate of 3°C / h to 10°C. (3) Open the discharge port for 1 h to discharge the uncrystallized residual liquid; (4) The residual liquid concentration measured by density method is 83.60%. The residual liquid is discharged into the coil heat exchanger and rapidly heated to 40°C and then rapidly cooled to 12°C. It is then refluxed to the crystallizer for further crystallization within 1 hour. (5) The circulating water temperature is raised to 28°C at 6°C / h and sweating is performed for 2h; (6) The circulating water is quickly heated to 40°C and kept warm for 3 hours until the crystals are completely melted. The valve is opened to discharge the material to obtain the finished phosphoric acid.

[0023] Comparative Example 2 (1) Add 3000 ml of 85% phosphoric acid solution into a static crystallizer with a plate heat exchanger; (2) Place a PTFE-lined ultrasonic generator in the reactor, and cool the circulating water from 30°C at a rate of 3°C / h. When the acid temperature in the reactor drops to 18°C, start the ultrasound for 3 min every 10 min at a frequency of 30 kHz, and add 4 g of seed crystals. After the crystal nucleus begins to grow and phosphoric acid begins to crystallize, turn off the ultrasonic generator and cool the circulating water to 10°C at a rate of 8°C / h. (3) Open the discharge port for 1 hour to discharge the uncrystallized residual liquid. The residual liquid is discharged into the coil heat exchanger, cooled to below the corresponding crystallization point, and refluxed to the crystallizer for further crystallization; (4) The residual liquid concentration measured by density method is 83.20%. The residual liquid is discharged into the coil heat exchanger and rapidly heated to 40°C and then rapidly cooled to 12°C. It is then refluxed to the crystallizer for further crystallization within 1 hour. (5) The circulating water was heated to 28°C at a rate of 6°C / h, and ultrasound was turned on for 3 min every 10 min at a frequency of 30 kHz for 2 h; (6) The circulating water is quickly heated to 40°C and kept warm for 3 hours. The material circulation pump is turned on and the crystals are completely melted. The valve is opened to discharge the material to obtain the finished phosphoric acid.

[0024] Comparative Example 3 (1) Add 3000 ml of 85% phosphoric acid solution into a static crystallizer with a plate heat exchanger; (2) Place a PTFE-lined ultrasonic generator in the reactor, and cool the circulating water from 30°C at a rate of 3°C / h. When the acid temperature in the reactor drops to 18°C, turn on the ultrasound for 3 min every 10 min at a frequency of 30 kHz, and add 4 g of seed crystals. After the crystal nucleus growth is observed through the window and phosphoric acid begins to crystallize, turn off the ultrasonic generator and cool the circulating water to 10°C at a rate of 3°C / h. (3) Open the discharge port for 1 h to discharge the uncrystallized residual liquid; (4) The residual liquid concentration measured by density method is 83.20%. The residual liquid is discharged into the coil heat exchanger and rapidly heated to 40°C and then rapidly cooled to 12°C. It is then refluxed to the crystallizer for further crystallization within 1 hour. (5) The circulating water temperature is raised to 28°C at 6°C / h and sweating is performed for 2 hours; (6) The circulating water is heated to 40°C and kept warm for 3 hours. The material circulation pump is turned on and the crystals are completely melted. The valve is opened to discharge the material to obtain the finished phosphoric acid.

[0025] Comparative Example 4 (1) Add 3000 ml of 85% phosphoric acid solution into a static crystallizer with a plate heat exchanger; (2) Place a PTFE-lined ultrasonic generator in the reactor, and cool the circulating water from 30°C at a rate of 3°C / h. When the acid temperature in the reactor drops to 18°C, turn on the ultrasound for 3 min every 10 min at a frequency of 30 kHz, and add 4 g of seed crystals. After the crystal nucleus growth is observed through the window and phosphoric acid begins to crystallize, turn off the ultrasonic generator and cool the circulating water to 10°C at a rate of 3°C / h. (3) Open the discharge port for 1 h to discharge the uncrystallized residual liquid; (4) The circulating water was heated to 28°C at a rate of 6°C / h, and ultrasound was turned on for 3 min every 10 min at a frequency of 30 kHz for 2 h; (5) The circulating water is heated to 40°C and kept warm for 3 hours. The material circulation pump is turned on and the crystals are completely melted. The valve is opened to discharge the material to obtain the finished phosphoric acid. (6) The residual liquid concentration was determined by density method to be 83.6%. The material was fed into the crystallizer and cooled to 15°C by programmed temperature control. Seed crystals were added and steps (2), (3), (4) and (5) were repeated to obtain a second batch of finished phosphoric acid. The typical metal contents and yields of the electronic grade phosphoric acid prepared in Example 1 and Comparative Example 1 are shown in Table 1.

[0026] Table 1

[0027] It can be seen from Example 1 and Comparative Examples 1-4 that the introduction of ultrasound during the crystallization and sweating process can improve the impurity removal effect of crystallization purification; a too fast cooling rate during the crystallization process will lead to a decrease in the impurity removal effect; while crystallizing, the residual liquid is discharged into a heat exchanger to first heat up to eliminate the crystal seeds and then cool down to below the crystallization temperature, and the method of continuing crystallization in a reflux crystallizer (step 4) significantly shortens the time compared to the secondary crystallization of the residual liquid, and the efficiency is improved.

[0028] The technical solution of the present invention is explained through the above embodiments, but the present invention is not limited to the above embodiments, that is, it does not mean that the present invention must rely on the above specific embodiments to be implemented. Any improvements made by technicians in the relevant field based on the present invention, or equivalent replacement of the materials selected by the present invention, etc., fall within the scope of protection of the patent.

Claims

1. A method for enhancing the melt crystallization process of phosphoric acid by ultrasound, characterized in that: The preparation method comprises the following steps: Step 1) Feeding: Phosphoric acid raw material is added to the crystallizer; Step 2) Cooling crystallization: Control the temperature to lower the temperature of the circulating water in the crystallizer, turn on ultrasound, add seed crystals when the acid temperature is lower than the crystallization point, and continue to cool the circulating water to the end point of crystallization; Step 3) Drain residual liquid: open the bottom valve of the crystallizer to drain the residual liquid; Step 4) Residual liquid cooling and reflux: The residual liquid is discharged into a heat exchanger, cooled to below the corresponding crystallization point, and refluxed to the crystallizer for further crystallization; Step 5) Sweating: Turn on the ultrasonic device, raise the temperature of the circulating water to the sweating temperature, keep warm, and discharge sweat; Step 6) Crystal melting: The circulating water is heated to the crystal melting temperature, kept warm, and the material circulation pump is started to melt the finished hot acid crystals and discharge the finished phosphoric acid.

2. The method according to claim 1, characterized in that: Step 1) The phosphoric acid raw material concentration is greater than or equal to 85%, and the total metal content is less than 200 ppb. A static melt crystallizer with a plate heat exchanger is used, and the material is 316L stainless steel sprayed with fluoroplastic or lined with PTFE.

3. The method according to claim 1, characterized in that: Step 2) The seeding temperature is 2-3°C lower than the corresponding concentration crystallization temperature, the cooling rate is 1-5°C / h, and the end temperature is 10-20°C; ultrasound is turned on for 1.5-2.5 minutes every 5-15 minutes, and the frequency is 30-50 kHz.

4. The method according to claim 1, characterized in that: Step 4) The residual liquid concentration is greater than or equal to 83%, and is discharged into the coil heat exchanger. The temperature is first quickly raised to 40°C to eliminate the seed crystals and then cooled to 2-3°C below the corresponding crystallization point. The reflux time is 0.5-1h. If the concentration is lower than 83%, no reflux is performed.

5. The method according to claim 1, characterized in that: Step 5) Turn on ultrasound for 1.5-2.5 minutes every 5-15 minutes, with a frequency of 30-50 kHz, a sweating temperature of 28-32℃, and a heat preservation time of 1-2 hours.

6. The method according to claim 1, characterized in that: Step 6) The crystallizer circulating water temperature is raised to 35-40°C and kept warm for 2-4 hours. The acid temperature in the finished product tank is maintained at 35-40°C with a circulation flow rate of 60-90L / h.

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