Method for inhibiting reduction of acid concentration of steam stripping defluorination tower in wet-process phosphoric acid purification process

Abstract

The invention discloses a method for inhibiting reduction of the acid concentration of a steam stripping defluorination tower in a wet-process phosphoric acid purification process, which comprises the following steps: 1) after heavy removal and coarse decoloration of phosphoric acid with the concentration of 79% obtained by concentration of concentration equipment, introducing the phosphoric acid into a membrane filter for filtration, and caching the phosphoric acid in a clear liquid aeration tank; 2) monitoring whether the temperature of the phosphoric acid flowing out of the heat exchanger reaches 100-120 DEG C in real time, and if the temperature of the phosphoric acid is lower than 100 DEG C, increasing the flow rate of steam entering the heat exchanger to 1.5-2.2 m < 3 > / h; and 3) defluorinating phosphoric acid in the steam stripping defluorination tower, feeding the defluorinated phosphoric acid into a premixer through a pipeline, mixing the defluorinated phosphoric acid with hydrogen peroxide, and sequentially feeding the mixture into a first decolorizing tower and a second decolorizing tower which are connected in series through a pipeline. After the method is used, phosphoric acid with the mass concentration of 75-77% and the chromaticity of 10-20 black units can be directly obtained without increasing the concentration of phosphoric acid in concentration equipment.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of phosphoric acid production, and particularly relates to a method for inhibiting decrease of acid concentration in a stripping defluorination tower in a wet-process phosphoric acid purification process. BACKGROUND

[0002] In existing phosphoric acid production, in order to obtain phosphoric acid with an acid concentration of more than 75%, the filtered phosphoric acid needs to be concentrated, and the existing process has the following problems: 1. After defluorination of the phosphoric acid through the stripping defluorination tower, the phosphoric acid concentration decreases by 4-6% compared with the feed concentration, and sometimes the acid concentration decreases by up to 8%. The existing treatment can only be to re-enter the concentration device and then be subjected to stripping defluorination to increase the acid concentration, but this operation increases the production cost and reduces the production efficiency, and especially in large-scale industrial production, the energy waste problem is serious. In order to reduce the energy waste and ensure the product acid concentration, the phosphoric acid concentration of the two-effect concentration outflow entering the stripping process needs to be increased to 83% to offset the decrease in the acid concentration. However, this feed concentration exceeds the design bearing range of the concentration device, which increases the corrosion rate of the production device by 40%, shortens the service life from 5 years to 3 years, and increases the annual maintenance cost by 800,000 yuan.

[0003] 2. In the existing production method, the concentrated phosphoric acid needs to react with hydrogen peroxide to remove organic impurities, and the existing decolorization process has low hydrogen peroxide oxidation removal efficiency, and the TOC (total organic carbon content in the phosphoric acid) after the decolorization treatment is 65 ppm. In order to ensure the TOC removal rate, the amount of hydrogen peroxide needs to be increased, and the production cost increases by 12%. With the growth of market demand for electronic-grade and food-grade refined phosphoric acid, the existing process cannot directly obtain phosphoric acid with a colority of 20 Hazen units and a mass concentration of more than 75%.

[0004] The information disclosed in the background section is only intended to increase the understanding of the general background of the application and should not be considered as admission or in any form as recognition that this information constitutes prior art with respect to the application. SUMMARY

[0005] The present application provides a method for inhibiting decrease of acid concentration in a stripping defluorination tower in a wet-process phosphoric acid purification process, which can effectively control the decrease of the phosphoric acid concentration after defluorination, achieve an effect of controlling the decrease of the acid concentration within 2%, reduce the TOC value of the phosphoric acid after decolorization to 40-45 ppm, reduce the amount of hydrogen peroxide by 30% under the condition of the same production amount, effectively reduce the production cost, and obtain phosphoric acid with a mass concentration of 75-77% and a colority of 10-20 Hazen units after decolorization.

[0006] The present application provides a method for inhibiting decrease of acid concentration in a stripping defluorination tower in a wet-process phosphoric acid purification process, which comprises the following steps: 1) The phosphoric acid with a concentration of 79% obtained after concentration by the concentration device is filtered by removing heavy substances and rough decolorization, and is stored in a clean acid aeration tank. The phosphoric acid in the clean acid aeration tank is heat-exchanged with steam in a heat exchanger, and then is heated to 100-120℃ before entering a stripping defluorination tower; 2) The temperature of the phosphoric acid flowing out of the heat exchanger is monitored in real time to determine whether it reaches 100-120℃. If the temperature of the phosphoric acid is lower than 100℃, the steam flow rate into the heat exchanger is increased to 1.5-2.2 m 3 / h. If the temperature of the phosphoric acid is higher than 120℃, the steam flow rate into the heat exchanger is reduced to 1.2-1.5 m 3 / h; 3) After defluorination of the phosphoric acid in the stripping defluorination tower, the phosphoric acid is mixed with hydrogen peroxide in a premixer, and then sequentially enters a first decolorization tower and a second decolorization tower in series through pipelines. The mass concentration of the phosphoric acid discharged is 75%-77%, the colority is 10-20 Hazen units, and the TOC value is 40-45 ppm; The heat exchanger used is a 91m2 vertical tube-type graphite heat exchanger, which can resist corrosion by 120℃ phosphoric acid and has a heat transfer coefficient of 300 W / (m2・℃).

[0007] Preferably, the steam inlet of the heat exchanger is connected to a steam main through a steam pipe, and a steam flow meter, a steam thermometer, a pressure gauge, and a temperature automatic control valve are arranged on the steam pipe. A discharge pipe is arranged on the phosphoric acid outlet of the heat exchanger, and a phosphoric acid temperature sensor is arranged on the discharge pipe. The phosphoric acid temperature sensor is connected to the temperature automatic control valve in a control connection mode using Siemens S7-1200 PLC and a fuzzy PID algorithm. The temperature of the phosphoric acid flowing out of the heat exchanger is monitored in real time to determine whether it reaches 100-120℃. If the temperature of the phosphoric acid is lower than 100℃, the opening of the temperature automatic control valve is increased to increase the steam flow rate into the heat exchanger to 1.5-2.2 m 3 / h. If the temperature of the phosphoric acid is higher than 120℃, the opening of the temperature automatic control valve is reduced to reduce the steam flow rate into the heat exchanger to 1.2-1.5 m 3 / h.

[0008] Preferably, the system comprises a branch pipe, a protection valve, and a preheating valve. Protection valves are arranged on the steam pipes on both sides of the temperature automatic control valve. The branch pipe is connected to the steam pipes outside the protection valves at both ends. The preheating valve is arranged on the branch pipe. When preheating of the heat exchanger is needed, the protection valves and the temperature automatic control valve are closed, and the preheating valve is opened.

[0009] Preferably, the system comprises a flow regulating valve and a flow display. The flow regulating valve is arranged on the discharge pipe and is electrically connected to the flow display.

[0010] Preferably, the system comprises an acid pickling condensate tank, a clean condensate tank, an electric conductivity detector, and a pneumatic two-position valve. A drain pipe is arranged on the water outlet of the heat exchanger; the drain pipe is connected to the pickling condensate tank through a pickling branch pipe; the drain pipe is connected to the cleaning condensate tank through a cleaning branch pipe; A pneumatic two-position valve is arranged on the cleaning branch pipe and the pickling branch pipe respectively; each pneumatic two-position valve is electrically connected to the conductivity detector; When the conductivity detector detects that the pH of the discharged condensate is less than 6.5, the pneumatic two-position valve on the pickling branch pipe is opened, and the pneumatic two-position valve on the cleaning branch pipe is closed; When the conductivity detector detects that the pH of the discharged condensate is 7.0-7.5, the pneumatic two-position valve on the cleaning branch pipe is opened, and the pneumatic two-position valve on the pickling branch pipe is closed.

[0011] Preferably, it comprises: a hydrogen peroxide storage tank; the hydrogen peroxide storage tank is connected to the hydrogen peroxide inlet pipeline of the premixer.

[0012] Preferably, the discharge pipe is connected to the feed inlet pipeline of the premixer.

[0013] Preferably, it comprises: a delivery pump and a control valve; the delivery pump and the control valve are arranged on the pipeline connecting the membrane filter and the supernatant aeration tank.

[0014] Preferably, it comprises: a control valve; the control valve is arranged on the pipeline connecting the stripping defluorination tower and the premixer.

[0015] Preferably, it comprises: a control valve; the control valve is arranged on the pipeline connecting the heat exchanger and the premixer.

[0016] The application can produce beneficial effects, including: 1) The method for inhibiting the decrease of acid concentration in the stripping defluorination tower in the wet-process phosphoric acid purification process provided by the application can control the acid concentration within 2% after operation, and the TOC content is reduced to 40-50 ppm after decolorization treatment, the hydrogen peroxide usage is reduced by 30% compared with the existing process, the production cost is effectively controlled, and the phosphoric acid with a colority of 20 Hazen units and a mass concentration of more than 75% can be directly obtained without multiple concentration.

[0017] 2) The method for inhibiting the decrease of acid concentration in the stripping defluorination tower in the wet-process phosphoric acid purification process provided by the application can effectively reduce the steam consumption by 2.2 t / h by controlling the steam flow according to the temperature of the phosphoric acid entering the stripping tower.

[0018] 3) After the application of the method for inhibiting the decrease of acid concentration in the stripping defluorination tower in the wet-process phosphoric acid purification process provided by the application in the Yunnan Sanhuan Zhonghua ammonium dihydrogen phosphate production device, the decrease of acid concentration in the stripping defluorination tower is reduced from 6.2% to 2.5%, the annual cost savings are 664,100 yuan, and the equipment life is extended to 8 years. At present, industrial verification has been completed, and it is suitable for popularization in 50-100 kt refined acid devices in the same industry. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 The schematic diagram of the device for reducing the acid concentration of the stripping defluorination tower in the wet-process phosphoric acid purification process in at least one embodiment of the present application; LEGEND: Membrane filter 13, supernatant aeration tank 116, heat exchanger 2, steam flow meter 1, steam temperature meter 11, pressure gauge 111, phosphoric acid temperature sensor 112, temperature automatic control valve 115, protection valve 114, preheating valve 113, stripping defluorination tower 3, flow regulating valve 312, electric conductivity detector 121, hand control valve 122, pneumatic two-position valve 123, steam backflow interface 126, acid washing condensate tank 124, clean condensate tank 125, first decoloring tower 4, second decoloring tower 41, premixer 5, hydrogen peroxide storage tank 51, delivery pump 14, control valve 141. DETAILED DESCRIPTION

[0020] In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application. The components of the embodiments of the present application described and shown in the drawings herein can be arranged and designed in various different configurations.

[0021] Therefore, the following detailed description of the embodiments of the present application provided in the drawings is not intended to limit the scope of the claimed present application, but only represents selected embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without making creative efforts fall within the scope of protection of the present application.

[0022] The technical means not described in detail in the present application and not used to solve the technical problems of the present application are set according to the common knowledge in the art, and various common knowledge setting modes can be implemented.

[0023] Reference Figure 1The application provides a method for inhibiting the decrease of the acid concentration in the stripping defluorination tower in the wet-process phosphoric acid purification process, which comprises a membrane filter 13, a clear liquid aeration tank 116, a heat exchanger 2, a steam flow meter 1, a steam temperature meter 11, a pressure gauge 111, a phosphoric acid temperature sensor 112, a temperature automatic control valve 115, a protection valve 114, a preheating valve 113, a stripping defluorination tower 3, a flow regulating valve 312, a flow display, an electric conductivity detector 121, a hand control valve 122, a pneumatic two-position valve 123, a steam backflow interface 126, an acid washing condensate tank 124, a cleaning condensate tank 125, a first decoloring tower 4, a second decoloring tower 41, a premixer 5, a hydrogen peroxide storage tank 51, a conveying pump 14 and a control valve 141. The phosphoric acid with a concentration of 79% obtained after concentration by the concentration equipment is treated by the membrane filter 13 after the removal of heavy substances and rough decoloring, so that the content of the impurities such as arsenic and lead can be effectively reduced and stored in the clear acid aeration tank 116 which is communicated with the pipeline of the membrane filter 13; the liquid outlet of the clear acid aeration tank 116 is communicated with the liquid inlet of the heat exchanger 2, and the pipeline communicated is provided with the conveying pump 14 and the control valve 141. The phosphoric acid is preheated in the heat exchanger 2, and the temperature of the phosphoric acid at the outlet of the heat exchanger 2 is 100-120 ℃. The outlet of the heat exchanger 2 is communicated with the inlet of the stripping defluorination tower 3, and the pipeline communicated is provided with the flow regulating valve 312. The flow regulating valve 312 has a flow monitoring function, and the obtained phosphoric acid feeding flow result is displayed through the flow display which is electrically connected with the flow regulating valve 312, so that the operator can obtain the flow, and adjust the opening of the flow regulating valve 312 according to the flow. Whether the temperature of the phosphoric acid entering the stripping defluorination tower 3 reaches 100-120 ℃ is monitored in real time. If the temperature of the phosphoric acid is lower than 100 ℃, the steam flow rate entering the heat exchanger 2 is increased to 1.5-2.2 m 3 / h. If the temperature of the phosphoric acid is higher than 120 ℃, the steam flow rate entering the heat exchanger 2 is reduced to 1.2-1.5 m 3 / h.

[0024] The outlet of the stripping defluorination tower 3 is communicated with the inlet of the premixer 5, the pipeline communicated is provided with the control valve 141, and the hydrogen peroxide inlet of the premixer 5 is communicated with the pipeline of the hydrogen peroxide storage tank 51. The phosphoric acid and the hydrogen peroxide are premixed in the premixer 5 and then decolored. The outlet of the premixer 5 is communicated with the pipeline of the first decoloring tower 4, and the outlet of the first decoloring tower 4 is communicated with the inlet of the second decoloring tower 41. Thus, the phosphoric acid after defluorination treatment is continuously subjected to decoloring treatment. The pipeline communicated between the second decoloring tower 41 and the first decoloring tower 4 is provided with the premixer 5 which is communicated with the pipeline of the hydrogen peroxide storage tank 51.

[0025] The steam inlet of the heat exchanger 2 is communicated with the steam main pipe for passing steam into the heat exchange layer of the heat exchanger 2, and the condensed water outlet of the heat exchanger 2 is respectively communicated with the pickling condensed water tank 124 and the cleaning condensed water tank 125 through pipes for recovering the water formed after the steam is condensed. The steam flow meter 1, the steam temperature meter 11, the pressure gauge 111 and the temperature automatic control valve 115 are arranged on the pipe communicated with the steam main pipe of the steam inlet of the heat exchanger 2. The discharge pipe is arranged on the phosphoric acid outlet of the heat exchanger 2, and the phosphoric acid temperature sensor 112 is arranged on the discharge pipe. The phosphoric acid temperature sensor 112 is electrically connected with the temperature automatic control valve 115, which is used for controlling the steam flow into the heat exchanger 2 according to the phosphoric acid temperature of the phosphoric acid outlet of the heat exchanger 2. When the phosphoric acid outlet temperature is lower than 100℃, the opening of the temperature automatic control valve 115 is increased to realize the effective temperature increase of the phosphoric acid. When the phosphoric acid outlet temperature is higher than 120℃, the opening of the temperature automatic control valve 115 is reduced to realize the effective temperature decrease of the phosphoric acid, so as to realize the accurate regulation and control of the phosphoric acid temperature. The method can realize the reliable control of the phosphoric acid temperature within ±1℃.

[0026] In an embodiment, the steam backflow interface 126 is arranged on the drain pipe. The steam in the upper part of the drain pipe is backflowed to the steam main pipe through the pipe, so as to avoid the steam directly rushing into each tank.

[0027] The drain pipe is arranged on the condensed water outlet of the heat exchanger 2, and the electric conductivity detector 121 and the hand control valve 122 are arranged on the drain pipe. The end of the drain pipe is respectively communicated with the acid water branch pipe and the cleaning water branch pipe. The other end of the acid water branch pipe is communicated with the pickling condensed water tank 124, and the other end of the cleaning water branch pipe is communicated with the cleaning condensed water tank 125. The pneumatic two-position valve 123 is arranged on the acid water branch pipe and the cleaning water branch pipe respectively, and the pneumatic two-position valve 123 on the acid water branch pipe and the cleaning water branch pipe is electrically connected with the electric conductivity detector 121, so as to realize the purpose of opening the pneumatic two-position valve 123 on the acid water branch pipe and the cleaning water branch pipe respectively according to the pH value of the condensed water discharged from the drain pipe, so as to realize the classified storage of the condensed water according to the change of the pH value of the condensed water, which is beneficial to the subsequent use.

[0028] In an embodiment, the discharge outlet of the heat exchanger 2 is communicated with the feed inlet of the premixer 5 through a pipe, so as to realize the control of the process according to the needs of the treated material. The phosphoric acid material which does not need to be treated by the defluorination process can be directly treated by the decolorization process. The amount of hydrogen peroxide required in the decolorization process is effectively controlled, and the TOC content is reduced to 40-50ppm after the decolorization treatment.

[0029] In an embodiment, the discharge outlet of the stripping defluorination tower 3 is communicated with the feed inlet of the next process equipment through a pipe, so as to adjust the phosphoric acid treatment process according to the requirements, and realize that part of the phosphoric acid does not need to be treated by the defluorination process.

[0030] In a specific embodiment, it comprises: the protection valve 114, the preheating valve 113, the branch pipe; the steam pipe is connected to the steam main pipe through the steam pipe, and the temperature self-control valve 115 is arranged on the steam pipe. The temperature self-control valve 115 is respectively arranged at two ends of the protection valve 114; the two ends of the branch pipe are respectively connected to the steam pipes outside the protection valve 114; and the preheating valve 113 is arranged on the steam pipe. When the heat exchanger 2 needs to be started, the preheating valve 113 can be opened and the protection valve 114 can be closed, the steam is input to preheat the heat exchanger 2 to a preset temperature, and then the phosphoric acid is input, so that the continuous production can be stably carried out.

[0031] In a specific embodiment, the heat exchanger 2 is a 91 square meter vertical tubular graphite heat exchanger, which can resist 120 DEG C phosphoric acid corrosion, the heat transfer coefficient is 300 W / (m2·C), and the temperature rise requirement of 40 DEG C under the flow of 15 m3 / h is met.

[0032] In a specific embodiment, the control between the temperature self-control valve 115 and the phosphoric acid temperature sensor 112 adopts Siemens S7-1200 PLC + fuzzy PID algorithm, the steam flow is dynamically adjusted, the temperature control precision is ±1 DEG C, and the response time is less than 10 seconds.

[0033] Running examples are compared. Example 1 1. The feeding conditions are that the phosphoric acid with a concentration of 79% and a temperature of 76-84 DEG C is treated by the method provided in the application and the device before the transformation in the Sanhuang Chemical Industry, respectively. The phosphoric acid entering the membrane filter 13 is obtained after concentration by a concentration device, the material of the concentration device is 904 stainless steel, and the corrosion rate of the concentration device is calculated according to the concentration conditions of the phosphoric acid treated by the concentration device, that is, the concentration concentration is 79%, the concentration temperature is 86-90 DEG C, and the concentration vacuum degree is 11-12 KPa.

[0034] 2. The main process of the device used in the method provided in the application is that the phosphoric acid flows through the heat exchanger 2, the stripping defluorination tower 3, the premixer 5, the first decoloring tower 4 and the second decoloring tower 41. The structure of the device before the transformation is that the membrane filter 13 and the stripping defluorination tower 3 are connected by a pipeline; the stripping defluorination tower 3 and the premixer 5 are connected by a pipeline, and the premixer 5 and the first decoloring tower 4 are connected.

[0035] The obtained results are shown in the following table: Table 1 The data in table 1 (except for the colority) are average results after multiple sampling, and the colority is a range value obtained by multiple measurements. The steam consumption before the transformation is the steam consumption at the defluorination tower.

[0036] As can be seen from Table 1, the method provided by the present application can effectively control the concentration decrease of phosphoric acid after defluorination and decolorization, and at the same time obtain a large amount of condensed water for other production processes. After using the method provided by the present application, the steam consumption is effectively controlled, the TOC value is obviously improved after decolorization treatment, and in the case of obvious improvement of the TOC value, the hydrogen peroxide consumption is effectively reduced. The colority of the obtained phosphoric acid is stable within the required range, and the concentration of the obtained phosphoric acid meets the requirements, and the problem of product unqualification caused by acid reduction does not occur.

[0037] Example 2: The difference from Example 1 is that in order to obtain phosphoric acid meeting the index requirement of 75% or more by using the equipment before modification, the concentration of the phosphoric acid discharged from the concentration equipment is 84%; The concentration of the phosphoric acid discharged from the concentration equipment of the device provided by the present application is 79%. The corrosion rate of the concentration equipment is calculated according to the common knowledge in the art and in combination with the concentration, temperature of the phosphoric acid treated and the material of the concentration equipment. The obtained results are shown in the following table: Table 2 As can be seen from the above table, in order to ensure that the phosphoric acid meeting the requirement of 75% concentration is obtained, the acidity of the phosphoric acid in the concentration equipment is increased before the modification of the equipment, which leads to the increase of the corrosion rate of the concentration equipment, thereby shortening the service life of the concentration equipment. However, the generation of acid reduction cannot be avoided, and at the same time, the indexes after decolorization cannot meet the requirements, so that changing the concentration of the phosphoric acid entering the equipment cannot solve the problems existing in the existing production equipment. However, by using the method provided by the present application, the concentration of the phosphoric acid treated by the concentration equipment can be controlled, thereby prolonging the service life of the equipment to 8 years.

[0038] The method has been stably operated in the 62.5 kt / a refined acid device of Yunnan Sanhuan Zhonghua, and the indexes meet the requirements, realizing the unity of social, environmental and economic benefits, and having strong demonstration effect and popularization value: (1) Capacity increase: the annual capacity is increased from 62.5 kt to 67.5 kt, and the additional production is 0.5 kt. According to the sales price of 6400 yuan / t and the production cost of 5800 yuan / t, the annual income is increased by 3000 yuan; (2) Cost saving: the hydrogen peroxide consumption is reduced by 0.5 kg / t, and the annual saving is 59700 yuan; The equipment maintenance cost is reduced from 8000 yuan to 3000 yuan, and the annual saving is 500000 yuan; The condensed water is recovered by 18000 tons / year, and the value is 104400 yuan; (3) Total economic benefit: 300+66.41=366.41 (ten thousand yuan / year), and the investment recovery period is 96 days.

[0039] (4) Safety guarantee: the corrosion rate of the equipment is reduced by 64%, the safety hidden danger such as acid leakage is reduced, and the production safety is improved.

[0040] (5) Resource utilization: condensate recovery rate is increased to 90%, and 18,000 tons of water is saved annually.

[0041] Although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art can make modifications to the technical solutions recorded in the foregoing embodiments, or make equivalent replacements to part of the technical features, and any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A method for suppressing the decrease in acid concentration in the stripping defluorination tower during a wet phosphoric acid purification process, characterized in that, Includes the following steps: 1) After the phosphoric acid obtained by the concentration equipment is concentrated to a concentration of 79%, it is de-weighted and crudely decolorized and then passed into the membrane filter (13) for filtration. It is then buffered in the clear liquid aeration tank (116) connected to the membrane filter (13) by pipeline. The phosphoric acid in the clear liquid aeration tank (116) enters the heat exchanger (2) for heat exchange and is heated to 100~120℃ before entering the stripping defluorination tower (3). 2) Monitor in real time whether the temperature of phosphoric acid flowing out of heat exchanger (2) reaches 100~120℃. If the temperature of phosphoric acid is lower than 100℃, increase the steam flow rate entering heat exchanger (2) to 1.5~2.2 m. 3 / h, if the phosphoric acid temperature is higher than 120℃, reduce the steam flow rate entering the heat exchanger (2) to 1.2~1.5m. 3 / h; 3) After phosphoric acid is defluorinated in the stripping defluorination tower (3), it enters the premixer (5) through the pipeline and is mixed with hydrogen peroxide. Then it enters the first decolorizing tower (4) and the second decolorizing tower (41) in series through the pipeline. The mass concentration of the discharged phosphoric acid is 75%~77%, the color is 10~20 black units, and the TOC value is 40~45ppm. The heat exchanger used (2) is a 91㎡ vertical shell-and-tube graphite heat exchanger that can withstand phosphoric acid corrosion at 120℃ and has a heat transfer coefficient of 300W / (m²・℃).

2. The method for suppressing the decrease in acid concentration in the stripping defluorination tower during the wet phosphoric acid purification process according to claim 1, characterized in that, The steam inlet of the heat exchanger (2) is connected to the steam main pipe through a steam pipe. A steam flow meter (1), a steam thermometer (11), a pressure gauge (111), and a temperature control valve (115) are installed on the steam pipe. A discharge pipe is installed on the phosphoric acid outlet of the heat exchanger (2), and a phosphoric acid temperature sensor (112) is installed on the discharge pipe; the phosphoric acid temperature sensor (112) is connected to the temperature control valve (115) using Siemens S7-1200 PLC + fuzzy PID algorithm. Monitor in real time whether the temperature of phosphoric acid flowing out of heat exchanger (2) reaches 100~120℃. If the temperature of phosphoric acid is lower than 100℃, increase the opening of the temperature control valve (115) until the steam flow rate entering heat exchanger (2) is 1.5~2.2 m. 3 / h, if the phosphoric acid temperature is higher than 120℃, reduce the opening of the temperature control valve (115) until the steam flow rate entering the heat exchanger (2) is 1.2~1.5m. 3 / h.

3. The method for suppressing the decrease in acid concentration in the stripping defluorination tower during the wet phosphoric acid purification process according to claim 2, characterized in that, include: Branch pipe, protection valve (114), preheating valve (113); protection valve (114) is installed on the steam pipes on both sides of the temperature control valve (115); the two ends of the branch pipe are connected to the steam pipe outside the protection valve (114); a preheating valve (113) is installed on the branch pipe. When it is necessary to preheat the heat exchanger (2), the protection valve (114) and the temperature control valve (115) are closed, and the preheating valve (113) is opened.

4. The method for suppressing the decrease in acid concentration in the stripping defluorination tower during the wet phosphoric acid purification process according to claim 2, characterized in that, include: Flow control valve (312), flow display; The flow regulating valve (312) is installed on the discharge pipe and electrically connected to the flow display.

5. The method for suppressing the decrease in acid concentration in the stripping defluorination tower during the wet phosphoric acid purification process according to claim 1, characterized in that, include: Pickling condensate tank (124), cleaning condensate tank (125), conductivity detector (121), pneumatic two-position valve (123); A drain pipe is installed at the outlet of the heat exchanger (2); the drain pipe is connected to the pickling condensate tank (124) through a pickling branch pipe; the drain pipe is connected to the cleaning condensate tank (125) through a cleaning branch pipe. Pneumatic two-position valves (123) are installed on the cleaning branch pipe and the pickling branch pipe respectively; each pneumatic two-position valve (123) is electrically connected to the conductivity detector (121); When the conductivity detector (121) detects that the pH of the discharged condensate is less than 6.5, it controls the pneumatic two-position valve (123) on the acid washing branch to open and the pneumatic two-position valve (123) on the cleaning branch to close. When the conductivity detector (121) detects that the pH of the discharged condensate is 7.0~7.5, the pneumatic two-position valve (123) on the cleaning branch pipe is opened and the pneumatic two-position valve (123) on the pickling branch pipe is closed.

6. The method for suppressing the decrease in acid concentration in the stripping defluorination tower during the wet phosphoric acid purification process according to claim 1, characterized in that, include: Hydrogen peroxide storage tank (51); the hydrogen peroxide storage tank (51) is connected to the hydrogen peroxide inlet pipeline of the premixer (5).

7. The method for suppressing the decrease in acid concentration in the stripping defluorination tower during the wet phosphoric acid purification process according to claim 2, characterized in that, The discharge pipe is connected to the inlet pipe of the premixer (5).

8. The method for suppressing the decrease in acid concentration in the stripping defluorination tower during the wet phosphoric acid purification process according to claim 1, characterized in that, include: The pump (14) and control valve (141) are spaced apart on the pipeline connecting the membrane filter (13) and the clear liquid aeration tank (116).

9. The method for suppressing the decrease in acid concentration in the stripping defluorination tower during the wet phosphoric acid purification process according to claim 1, characterized in that, include: Control valve (141); The control valve (141) is installed on the pipeline connecting the stripping defluorination tower (3) and the premixer (5).

10. The method for suppressing the decrease in acid concentration in the stripping defluorination tower during the wet phosphoric acid purification process according to claim 1, characterized in that, include: Control valve (141); The control valve (141) is installed on the pipeline connecting the heat exchanger (2) and the premixer (5).

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