Dechlorination method for wet-process phosphoric acid process

Abstract

The invention discloses a dechlorination method of a wet-process phosphoric acid process, and belongs to the technical field of dechlorination of wet-process phosphoric acid. The method comprises the following steps: adding ground phosphorite into a reaction tank A, carrying out decomposition reaction on the phosphorite and high-temperature slurry to obtain slurry a, pumping the slurry a into a reaction tank B, adding concentrated sulfuric acid and a sodium persulfate solution, activating sodium persulfate in the inherent high temperature, strong acid and Fe2 environment, carrying out oxidation dechlorination reaction, and carrying out solid-liquid separation to obtain low-chlorine phosphoric acid and phosphogypsum; finally, the reaction tank B is connected with a tail gas washing system, water is used for circulating washing, then alkali liquor is used for countercurrent washing, and the pollution discharge standard is met. The oxidation dechlorination process is integrated in the wet-process phosphoric acid production process for the first time, chloride ions are efficiently oxidized into chlorine to be removed by utilizing the strong oxidizing property of sodium persulfate, the dechlorination rate can reach 95% or above, the chlorine content in phosphoric acid is remarkably reduced, equipment corrosion is relieved, the product quality is improved, and the method is simple in process, low in energy consumption, low in cost, green, environmentally friendly and suitable for industrial production. The method is suitable for industrial application.

Description

Technical Field

[0001] This invention relates to the field of wet-process phosphoric acid production technology, specifically to a method for simultaneously removing chloride ions introduced into the raw materials during the wet-process phosphoric acid production process. Background Technology

[0002] As a core raw material in the global phosphate chemical industry, the quality of wet-process phosphoric acid directly impacts the production process and product quality. With the continuous development of phosphate rock resources, the utilization rate of phosphate rock has increased significantly. Chlorine is mainly present in the form of soluble salts such as NaCl, KCl, or fluorochloroapatite. For example, the chloride content of some phosphate rocks in the Middle East, North Africa, and Yunnan-Guizhou region of China is as high as 0.3-2.0%, far exceeding the tolerance limit of 0.05-0.1% for traditional wet-process phosphoric acid. The gradual depletion of high-quality, low-chlorine phosphate rock globally has led to the normalization of phosphate rock utilization, and the resulting chloride ion problem is increasingly becoming a key factor restricting the industry's development.

[0003] The use of phosphate rock led to a significant increase in chloride ion content in wet-process phosphoric acid, resulting in a series of process and product problems. At the process level, Cl... - High-temperature acidic media can trigger pitting corrosion and stress corrosion cracking in equipment, reducing the lifespan of critical equipment such as stainless steel reactors and evaporators by more than 50%, and drastically increasing maintenance costs. Meanwhile, Cl... - It also damages the crystal structure of calcium sulfate, leading to a 20-30% decrease in filtration rate and a reduction in P2O5 recovery rate; during the concentration stage, it easily causes chloride scaling, increasing system energy consumption by more than 30%. At the product level, industrial-grade phosphoric acid (HG / T4069-2008) requires Cl... - ≤100ppm, food grade (GB1886.15-2015) requires Cl - ≤10ppm, while the Cl in phosphoric acid produced directly from phosphate rock is... - The content can reach 1000-5000ppm, which seriously restricts the high-end application of the product.

[0004] Currently, chloride ion removal in wet-process phosphoric acid mainly relies on dechlorination technology at the finished phosphoric acid stage. However, efficient and applicable dechlorination methods are lacking in the wet-process phosphoric acid production process, failing to effectively mitigate chloride ion corrosion of production equipment. Existing dechlorination methods, such as water washing, suffer from significant P2O5 loss, difficult wastewater treatment, and limited dechlorination efficiency; chemical precipitation faces challenges such as high cost, introduction of new impurities, and difficulty in handling precipitates; solvent extraction limits its industrial application due to high investment costs, complex operation, and difficulty in utilizing residual acid. Therefore, given the increasing prevalence of phosphate rock production, there is an urgent need for multidisciplinary research to develop new, green, environmentally friendly, low-energy-consumption, low-cost, high-efficiency, and easy-to-operate dechlorination processes to provide technological support for the high-quality development of the phosphoric acid chemical industry. Summary of the Invention

[0005] The purpose of this invention is to provide a dechlorination method for wet-process phosphoric acid production. This method involves introducing highly efficient chloride ions into the process before phosphate rock crystallization. Based on the strong oxidizing properties of sodium persulfate, an oxidation method is used to oxidize chloride ions into chlorine gas during the wet-process phosphoric acid preparation process, achieving the targeted conversion of chloride ions into stable and easily separable chlorine gas for removal before the wet-process phosphoric acid comes into contact with the equipment. This addresses the problems of equipment corrosion, increased energy consumption, high costs, complex operation, difficult wastewater and precipitate treatment, and low dechlorination efficiency caused by phosphate rock in the wet-process phosphoric acid production process.

[0006] To solve the above technical problems, the present invention adopts the following technical solution: A method for dechlorination in a wet-process phosphoric acid production process, comprising the following steps: (1) Phosphate rock decomposition: The ground phosphate rock is added to reaction tank A and decomposed with high temperature slurry to obtain slurry a. The temperature of the decomposition reaction is 70-150℃ and the reaction time is 1-5h. (2) Oxidation and dechlorination reaction: Slurry a is pumped into reaction tank B, concentrated sulfuric acid and Na2S2O8 solution are added, and oxidation and dechlorination reaction is carried out under high temperature conditions. After solid-liquid separation, low-chlorine phosphoric acid and phosphogypsum are obtained. The temperature of the oxidation and dechlorination reaction is 70-150℃ and the reaction time is 1-5h. (3) Tail gas purification: The reaction tank B is connected to the tail gas scrubbing system. First, water is used for circulating scrubbing. After the concentration of fluorosilicic acid is detected to be in the range of 10-15%, the scrubbing liquid is discharged, and then 0.1-2m 3 The exhaust gas is washed twice with alkaline solution in a countercurrent manner to meet emission standards.

[0007] In step (1), the mass ratio of the aforementioned phosphate rock to the high-temperature slurry is 1:8-15, the aforementioned high-temperature slurry is a mixture of phosphoric acid solution and gypsum, wherein the mass ratio of gypsum to phosphoric acid solution is 1:2.3-4, and the concentration of the aforementioned phosphoric acid solution is 25-45%.

[0008] In step (1), the aforementioned slurry a is a mixture of phosphoric acid and calcium sulfate slurry, the temperature of the aforementioned decomposition reaction is 90-110℃, the reaction time is 3h, and the aforementioned reaction tank A is a dissolving tank.

[0009] In step (2), the aforementioned concentrated sulfuric acid is industrial sulfuric acid with a mass concentration of 98%. The amount added is calculated based on the amount of concentrated sulfuric acid used in phosphate rock. The mass ratio of phosphate rock to concentrated sulfuric acid is 1.0-1.5:1.

[0010] In step (2), the mass concentration of the aforementioned Na2S2O8 solution is 10-50%, and the amount added is such that the mass ratio of Na2S2O8 to chloride ions is n(Na2S2O8):n(Cl⁻)=0.5-7:1.

[0011] In step (2), specifically, the mass concentration of the aforementioned Na2S2O8 solution is 30%, and the amount added is such that the mass ratio of Na2S2O8 to chloride ions is n(Na2S2O8):n(Cl⁻)=5:1.

[0012] In step (2), the high temperature of the aforementioned oxidation and dechlorination reaction is 90-110℃, the reaction time is 3h, and the aforementioned reaction tank B is a crystallization tank.

[0013] In step (3), the aforementioned washing system consists of three stages of washing towers. The first stage uses water for circulating washing of the tail gas to remove most of the HF and some of the SiF4. The second and third stages are countercurrent washing to completely remove Cl2, residual SiF4, and HF. The aforementioned second-stage countercurrent washing uses 1m 3 The alkaline solution is circulated and washed once per hour, for 1-5 hours. The aforementioned three-stage countercurrent washing involves circulating alkaline solution and washing once per hour, for 1-5 hours.

[0014] In step (3), the aforementioned exhaust gas is a mixture of chlorine, silicon tetrafluoride, hydrogen fluoride and sulfur dioxide.

[0015] In step (3), the aforementioned alkaline solution is one or more of sodium carbonate, sodium bicarbonate, or sodium hydroxide.

[0016] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention is the first to successfully integrate oxidative dechlorination into the wet-process phosphoric acid production process, achieving highly efficient removal of chloride ions without altering the existing main equipment. The optimal solvent ratio (n(Na2S2O8):n(Cl)) is achieved. - With a ratio of 5:1, the dechlorination rate can reach over 95%, reducing the chloride ion content in the acid solution from over 1324 ppm to 136 ppm, effectively solving the application problems of phosphate rock.

[0017] 2. Utilizing the high temperature, strong acid, and Fe in the oxidation-removal chlorine reaction itself. 2 The sodium persulfate is activated by the environment without the need for additional catalysts. By utilizing the strong oxidizing properties of sodium persulfate, chloride ions are oxidized into chlorine gas, breaking through the technical bottleneck that traditional methods struggle to overcome in this process.

[0018] 3. By removing chloride ions at the source, equipment corrosion can be fundamentally alleviated. Experiments have shown that this method can significantly reduce the corrosion rate of stainless steel in the system from severe corrosion >1.50 mm / year to 0.46 mm / year, and improve the corrosion level from "unsuitable" to "good", thereby significantly extending equipment life and reducing maintenance costs.

[0019] 4. The solvent sodium persulfate used in this invention is inexpensive, and the introduced sulfate ions have no negative impact on the existing phosphoric acid production system; the chlorine gas generated in the reaction can be introduced into the existing tail gas scrubbing system and treated by circulating absorption with a 1% alkaline solution, without any risk of secondary pollution, thus combining economic benefits and environmental friendliness.

[0020] 5. The technical method of the present invention can be directly embedded into existing wet-process phosphoric acid production equipment without large-scale modification of existing equipment. It requires less investment, is simple to operate, and has broad prospects for industrial application. Attached Figure Description

[0021] Figure 1 Flowchart of the dechlorination preparation method for wet-process phosphoric acid. Detailed Implementation

[0022] The present invention will be specifically described below through embodiments. It should be noted that the following embodiments are only used to further illustrate the present invention and should not be construed as limiting the scope of protection of the present invention.

[0023] 1. Reaction principle The chemical essence of this invention lies in the innovative use of the inherent high temperature, strong acid, and Fe²⁺ environment of wet-process phosphoric acid to activate sodium persulfate without the need for additional catalysts. By utilizing the strong oxidizing properties of sodium persulfate, chloride ions are oxidized to chlorine gas, which is then released. The entire process involves the following key chemical reactions: (1) The decomposition reaction formula of phosphate rock is: Ca5F(PO4)3+10H3PO4→3H3PO4+5Ca(H2PO4)3+HF (2) The reaction formula for the formation of phosphogypsum in the oxidative dechlorination reaction is: 5Ca(H2PO4)3+5H2SO4→10H3PO4+5CaSO4·nH2O (3) Chemical reaction formula for oxidation and dechlorination: 1) Overall reaction: S2O8²⁻ + 2Cl⁻ → 2SO4²⁻ + Cl2 2) Stepwise reaction mechanism: a) Activation: Persulfate (S₂O₈²⁻) is activated by heat and Fe²⁺ to generate sulfate free radicals (SO₄⁻): The reaction formula is: b. Oxidation: Sulfate radicals oxidize Cl⁻ to generate chlorine radicals (Cl): The reaction formula is: SO4 − +Cl − →SO4 2− +Cl c. Combination: Chlorine free radicals combine to form chlorine gas. The reaction equation is: 2 Cl → Cl2 2. Specific Implementation Examples The present invention will be specifically described below through embodiments. It should be noted that the following embodiments are only used to further illustrate the present invention and should not be construed as limiting the scope of protection of the present invention.

[0024] Example 1 Dechlorination methods in wet-process phosphoric acid production: (1) Phosphate rock decomposition: 10 kg of ground phosphate rock is added to reaction tank A and decomposed with 100 kg of high temperature slurry to obtain slurry a. The decomposition reaction temperature is 100℃ and the reaction time is 3 h. (2) Oxidation and dechlorination reaction: Slurry a is pumped into reaction tank B, 6.8 kg of 98% concentrated sulfuric acid and 30% Na2S2O8 solution are added, and the oxidation and dechlorination reaction is carried out under high temperature conditions. After solid-liquid separation, low-chlorine phosphoric acid and phosphogypsum are obtained. The temperature of the oxidation and dechlorination reaction is 100℃ and the reaction time is 3h. The amount of Na2S2O8 solution added is based on the mass ratio of Na2S2O8 to chloride ions n(Na2S2O8):n(Cl⁻)=5:1. (3) Tail gas purification: Reactor B is connected to the tail gas scrubbing system. The scrubbing system consists of three-stage scrubbing towers. The first stage uses water for circulating scrubbing. After detecting that the concentration of fluorosilicic acid is in the range of 10-15%, the scrubbing liquid is discharged. The second stage uses a countercurrent scrubbing tower with a 1m... 3 The exhaust gas is washed once with countercurrent alkali solution for 3 hours. The third-stage countercurrent alkali solution is used for washing once with circulating alkali solution for 3 hours, and the exhaust gas meets the emission standards.

[0025] Example 2 Dechlorination methods in wet-process phosphoric acid production: (1) Phosphate rock decomposition: 15 kg of ground phosphate rock was added to reaction tank A and decomposed with 80 kg of high temperature slurry to obtain slurry a. The decomposition reaction temperature was 150℃ and the reaction time was 1 h. (2) Oxidation and dechlorination reaction: Slurry a is pumped into reaction tank B, 10 kg of 98% concentrated sulfuric acid and 10% Na2S2O8 solution are added, and the oxidation and dechlorination reaction is carried out under high temperature conditions. After solid-liquid separation, low-chlorine phosphoric acid and phosphogypsum are obtained. The temperature of the oxidation and dechlorination reaction is 150℃ and the reaction time is 1h. The amount of Na2S2O8 solution added is based on the mass ratio of Na2S2O8 to chloride ions n(Na2S2O8):n(Cl⁻)=0.5:1; (3) Tail gas purification: Reactor B is connected to the tail gas scrubbing system. The scrubbing system consists of three-stage scrubbing towers. The first stage uses water for circulating scrubbing. After detecting that the fluorosilicic acid concentration is in the range of 10-15%, the scrubbing liquid is discharged. The second stage countercurrent scrubbing uses a 2m... 3The exhaust gas is washed once with countercurrent alkali solution for 1 hour. The third-stage countercurrent alkali solution is used for washing once with circulating alkali solution for 1 hour, and the exhaust gas meets the emission standards.

[0026] Example 3 Dechlorination methods in wet-process phosphoric acid production: (1) Phosphate rock decomposition: 12 kg of ground phosphate rock was added to reaction tank A and decomposed with 110 kg of high temperature slurry to obtain slurry a. The decomposition reaction temperature was 110℃ and the reaction time was 2 h. (2) Oxidation and dechlorination reaction: Slurry a is pumped into reaction tank B, 9 kg of 98% concentrated sulfuric acid and 20% Na2S2O8 solution are added, and the oxidation and dechlorination reaction is carried out under high temperature conditions. After solid-liquid separation, low-chlorine phosphoric acid and phosphogypsum are obtained. The temperature of the oxidation and dechlorination reaction is 110℃ and the reaction time is 2h. The amount of Na2S2O8 solution added is based on the mass ratio of Na2S2O8 to chloride ions n(Na2S2O8):n(Cl⁻)=2:1. (3) Tail gas purification: The reaction tank B is connected to the tail gas scrubbing system. The scrubbing system consists of three-stage scrubbing towers. The first stage uses water for circulating scrubbing. After detecting that the fluorosilicic acid concentration is in the range of 10-15%, the scrubbing liquid is discharged. The second stage uses a countercurrent scrubbing tower with a 1.5m... 3 The exhaust gas is washed once with countercurrent alkali solution for 2 hours. The third-stage countercurrent alkali solution is used for washing once with circulating alkali solution for 2 hours, and the exhaust gas meets the emission standards.

[0027] Example 4 Dechlorination methods in wet-process phosphoric acid production: (1) Phosphate rock decomposition: 9 kg of ground phosphate rock was added to reaction tank A and decomposed with 130 kg of high temperature slurry to obtain slurry a. The decomposition reaction temperature was 90℃ and the reaction time was 4 h. (2) Oxidation and dechlorination reaction: Slurry a is pumped into reaction tank B, 8 kg of 98% concentrated sulfuric acid and 40% Na2S2O8 solution are added, and the oxidation and dechlorination reaction is carried out under high temperature conditions. After solid-liquid separation, low-chlorine phosphoric acid and phosphogypsum are obtained. The temperature of the oxidation and dechlorination reaction is 90℃ and the reaction time is 4h. The amount of Na2S2O8 solution added is based on the mass ratio of Na2S2O8 to chloride ions n(Na2S2O8):n(Cl⁻)=4:1. (3) Tail gas purification: Reactor B is connected to a tail gas scrubbing system. The scrubbing system consists of three-stage scrubbing towers. The first stage uses water for circulating scrubbing. After detecting that the fluorosilicic acid concentration is in the range of 10-15%, the scrubbing liquid is discharged. The second stage uses a countercurrent scrubbing tower with a 0.5m... 3 The exhaust gas is washed once with countercurrent alkali solution for 4 hours. The third-stage countercurrent alkali solution is used for washing once with circulating alkali solution for 4 hours, and the exhaust gas meets the emission standards.

[0028] Example 5 Dechlorination methods in wet-process phosphoric acid production: (1) Phosphate rock decomposition: 7 kg of ground phosphate rock was added to reaction tank A and decomposed with 150 kg of high temperature slurry to obtain slurry a. The decomposition reaction temperature was 70℃ and the reaction time was 5 h. (2) Oxidation and dechlorination reaction: Slurry a is pumped into reaction tank B, 7 kg of 98% concentrated sulfuric acid and 50% Na2S2O8 solution are added, and the oxidation and dechlorination reaction is carried out under high temperature conditions. After solid-liquid separation, low-chlorine phosphoric acid and phosphogypsum are obtained. The temperature of the oxidation and dechlorination reaction is 70℃ and the reaction time is 5h. The amount of Na2S2O8 solution added is based on the mass ratio of Na2S2O8 to chloride ions n(Na2S2O8):n(Cl⁻)=7:1. (3) Tail gas purification: Reactor B is connected to a tail gas scrubbing system. The scrubbing system consists of three-stage scrubbing towers. The first stage uses water for circulating scrubbing. After detecting that the fluorosilicic acid concentration is within the range of 10-15%, the scrubbing liquid is discharged. The second stage uses a countercurrent scrubbing tower with a 0.1m... 3 The exhaust gas is washed once with countercurrent alkali solution for 5 hours. The three-stage countercurrent alkali solution is used for washing once with circulating alkali solution for 5 hours, and the exhaust gas meets the emission standards.

[0029] 3. Comparative Experiment Comparative Experiment 1 (without chlorine removal) This embodiment provides a dechlorination method for a wet-process phosphoric acid process, the specific steps of which are as follows: (1) Phosphate rock decomposition: 10 kg of ground phosphate rock is added to reaction tank A and decomposed with 100 kg of high temperature slurry to obtain slurry a. The decomposition reaction temperature is 100℃ and the reaction time is 3 h. (2) Solid-liquid separation: The slurry a is pumped into the reaction tank B, 6.8 kg of 98% concentrated sulfuric acid is added and reacted for 3 hours. After solid-liquid separation, low-chlorinated phosphoric acid and phosphogypsum are obtained. (3) Tail gas purification: Reactor B is connected to the tail gas scrubbing system. The scrubbing system consists of three-stage scrubbing towers. The first stage uses water for circulating scrubbing. After detecting that the concentration of fluorosilicic acid is in the range of 10-15%, the scrubbing liquid is discharged. The second stage uses a countercurrent scrubbing tower with a 1m... 3 The exhaust gas is washed once with countercurrent alkali solution for 3 hours. The third-stage countercurrent alkali solution is used for washing once with circulating alkali solution for 3 hours, and the exhaust gas meets the emission standards.

[0030] Effect:The indicators of phosphate rock used as raw material for wet-process phosphoric acid production are shown in Table 1, which shows that the CI content in phosphate rock is 862 ppm; the indicators of phosphoric acid without dechlorination process are shown in Table 2, which shows that the CI content in phosphoric acid is 1324 ppm; the indicators of phosphogypsum without dechlorination process are shown in Table 3, which shows that the CI content in phosphogypsum is 117 ppm; the corrosion of metal materials is shown in Table 4, which shows severe corrosion >1.50 mm / year, and is not applicable.

[0031] Table 1: Raw Material Phosphate Rock Specifications Table 2: Phosphoric acid index (without dechlorination process) Table 3: Phosphogypsum Indicators (without dechlorination process) Table 4: Corrosion of Metal Materials Comparative Experiment 2 (Inefficient Chlorine Removal) This embodiment provides a dechlorination method for a wet-process phosphoric acid process, the specific steps of which are as follows: (1) Phosphate rock decomposition: 10 kg of ground phosphate rock is added to reaction tank A and decomposed with 100 kg of high temperature slurry to obtain slurry a. The decomposition reaction temperature is 100℃ and the reaction time is 3 h. (2) Oxidation and dechlorination reaction: Slurry a is pumped into reaction tank B, 6.8 kg of 98% concentrated sulfuric acid and 30% Na2S2O8 solution are added, and the oxidation and dechlorination reaction is carried out under high temperature conditions. After solid-liquid separation, low-chlorinated phosphoric acid and phosphogypsum are obtained. The temperature of the oxidation and dechlorination reaction is 100℃ and the reaction time is 3h. The amount of Na2S2O8 solution added is based on the mass ratio of Na2S2O8 to chloride ions n(Na2S2O8):n(Cl⁻)=0.5:1; (3) Tail gas purification: Reactor B is connected to the tail gas scrubbing system. The scrubbing system consists of three-stage scrubbing towers. The first stage uses water for circulating scrubbing. After detecting that the concentration of fluorosilicic acid is in the range of 10-15%, the scrubbing liquid is discharged. The second stage uses a countercurrent scrubbing tower with a 1m... 3 The exhaust gas is washed once with countercurrent alkali solution for 3 hours. The third-stage countercurrent alkali solution is used for washing once with circulating alkali solution for 3 hours, and the exhaust gas meets the emission standards.

[0032] Effect:Table 5 shows the indicators of phosphate rock used as raw material for wet-process phosphoric acid production, which shows that the CI content in the phosphate rock is 862 ppm; Table 6 shows the indicators of phosphoric acid without dechlorination process, which shows that the CI content in the phosphoric acid is 884 ppm; Table 7 shows the indicators of phosphogypsum without dechlorination process, which shows that the CI content in the phosphogypsum is 114 ppm; Table 8 shows the corrosion status of metal materials, which is 0.5-1.50 mm / year.

[0033] Table 5: Raw Material Phosphate Rock Indicators Table 6: Phosphoric acid indicators (phosphoric acid prepared using dechlorination process) Table 7: Phosphogypsum Index (with addition of dechlorination process) Table 8: Corrosion of Metal Materials Comparative Experiment 3 (Medium-efficiency chlorine removal) (1) Phosphate rock decomposition: 10 kg of ground phosphate rock is added to reaction tank A and decomposed with 100 kg of high temperature slurry to obtain slurry a. The decomposition reaction temperature is 100℃ and the reaction time is 3 h. (2) Oxidation and dechlorination reaction: Slurry a is pumped into reaction tank B, 6.8 kg of 98% concentrated sulfuric acid and 30% Na2S2O8 solution are added, and the oxidation and dechlorination reaction is carried out under high temperature conditions. After solid-liquid separation, low-chlorine phosphoric acid and phosphogypsum are obtained. The temperature of the oxidation and dechlorination reaction is 100℃ and the reaction time is 3h. The amount of Na2S2O8 solution added is based on the mass ratio of Na2S2O8 to chloride ions n(Na2S2O8):n(Cl⁻)=3:1. (3) Tail gas purification: Reactor B is connected to the tail gas scrubbing system. The scrubbing system consists of three-stage scrubbing towers. The first stage uses water for circulating scrubbing. After detecting that the concentration of fluorosilicic acid is in the range of 10-15%, the scrubbing liquid is discharged. The second stage uses a countercurrent scrubbing tower with a 1m... 3 The exhaust gas is washed once with countercurrent alkali solution for 3 hours. The third-stage countercurrent alkali solution is used for washing once with circulating alkali solution for 3 hours, and the exhaust gas meets the emission standards.

[0034] Effect: The indicators of phosphate rock produced by wet-process phosphoric acid production are shown in Table 9, which shows that the CI content in phosphate rock is 862 ppm; the indicators of phosphoric acid without dechlorination process are shown in Table 10, which shows that the CI content in phosphoric acid is 524 ppm; the indicators of phosphogypsum without dechlorination process are shown in Table 11, which shows that the CI content in phosphogypsum is 124 ppm; the corrosion of metal materials is shown in Table 12, which shows a usable corrosion rate of 0.5-1.50 mm / year.

[0035] Table 9: Raw Phosphate Rock Indicators Table 10: Phosphoric acid indicators (phosphoric acid prepared using dechlorination process) Table 11: Phosphogypsum Indicators (with addition of dechlorination process) Table 12: Corrosion of Metallic Materials Comparative Experiment 4 (High-efficiency chlorine removal) This embodiment provides a dechlorination method for a wet-process phosphoric acid process, see... Figure 1 The specific steps are as follows: (1) Phosphate rock decomposition: 10 kg of ground phosphate rock is added to reaction tank A and decomposed with 100 kg of high temperature slurry to obtain slurry a. The decomposition reaction temperature is 100℃ and the reaction time is 3 h. (2) Oxidation and dechlorination reaction: Slurry a is pumped into reaction tank B, 6.8 kg of 98% concentrated sulfuric acid and 30% Na2S2O8 solution are added, and the oxidation and dechlorination reaction is carried out under high temperature conditions. After solid-liquid separation, low-chlorine phosphoric acid and phosphogypsum are obtained. The temperature of the oxidation and dechlorination reaction is 100℃ and the reaction time is 3h. The amount of Na2S2O8 solution added is based on the mass ratio of Na2S2O8 to chloride ions n(Na2S2O8):n(Cl⁻)=5:1. (3) Tail gas purification: Reactor B is connected to the tail gas scrubbing system. The scrubbing system consists of three-stage scrubbing towers. The first stage uses water for circulating scrubbing. After detecting that the concentration of fluorosilicic acid is in the range of 10-15%, the scrubbing liquid is discharged. The second stage uses a countercurrent scrubbing tower with a 1m... 3 The exhaust gas is washed once with countercurrent alkali solution for 3 hours. The third-stage countercurrent alkali solution is used for washing once with circulating alkali solution for 3 hours, and the exhaust gas meets the emission standards.

[0036] Effect: Table 13 shows the indicators of phosphate rock produced by wet-process phosphoric acid as raw material, which shows that the CI content in phosphate rock is 862 ppm; Table 14 shows the indicators of phosphoric acid without dechlorination process, which shows that the CI content in phosphoric acid is 136 ppm; Table 15 shows the indicators of phosphogypsum without dechlorination process, which shows that the CI content in phosphogypsum is 87 ppm; Table 16 shows the corrosion status of metal materials, which is good: 0.05-0.50 mm / year.

[0037] Table 13: Raw Material Phosphate Rock Indicators Table 14: Phosphoric acid indicators (phosphoric acid prepared using dechlorination process) Table 15: Phosphogypsum Indicators (with addition of dechlorination process) Table 16: Corrosion of Metallic Materials Through verification, compared with Experiment 1, without the use of dechlorination methods, the phosphate chloride content was as high as 1324 ppm, resulting in severe equipment corrosion at a rate >1.50 mm / year. Compared with Experiments 2-4, the situation improved significantly after introducing sodium persulfate as a dechlorination agent: In Experiment 2, when the molar ratio of sodium persulfate to chloride ions was 0.5:1, the phosphate chloride content dropped to 884 ppm, the corrosion rate decreased to 0.5-1.50 mm / year, and the corrosion level improved to "usable". In Experiment 3, when the molar ratio of sodium persulfate to chloride ions was 3:1, the dechlorination effect was enhanced, the phosphate chloride content further decreased to 524 ppm, and the corrosion rate also decreased to 0.5-1.50 mm / year. Ultimately, the best results were achieved in comparative experiment 4, where the molar ratio of sodium persulfate to chloride ions was 5:1. The chloride content in phosphoric acid was significantly reduced to 136 ppm, with a chloride removal rate of approximately 95%. At the same time, the equipment corrosion rate was significantly reduced to 0.05-0.50 mm / year, and the corrosion level was improved to "good". The results fully demonstrate that this method, by oxidizing chloride ions into chlorine gas at the front end of the process, can fundamentally and efficiently solve the problem of excessive chlorine content and significantly alleviate equipment corrosion, providing a reliable technical path for the high-value utilization of phosphate rock.

Claims

1. A method for dechlorination in a wet-process phosphoric acid production process, characterized in that: Follow these steps: (1) Phosphate rock decomposition: The ground phosphate rock is added to reaction tank A and decomposed with high temperature slurry to obtain slurry a. The temperature of the decomposition reaction is 70-150℃ and the reaction time is 1-5h. (2) Oxidation and dechlorination reaction: Slurry a is pumped into reaction tank B, concentrated sulfuric acid and Na2S2O8 solution are added, and oxidation and dechlorination reaction is carried out under high temperature conditions. After solid-liquid separation, low-chlorine phosphoric acid and phosphogypsum are obtained. The temperature of the oxidation and dechlorination reaction is 70-150℃ and the reaction time is 1-5h. (3) Tail gas purification: The reaction tank B is connected to the tail gas scrubbing system. First, water is used for circulating scrubbing. After the concentration of fluorosilicic acid is detected to be in the range of 10-15%, the scrubbing liquid is discharged, and then 0.1-2m 3 The exhaust gas is washed twice with alkaline solution in a countercurrent manner to meet emission standards.

2. The dechlorination method for the wet-process phosphoric acid according to claim 1, characterized in that: In step (1), the mass ratio of phosphate rock to high-temperature slurry is 1:8-15, the high-temperature slurry is a mixture of phosphoric acid solution and gypsum, wherein the mass ratio of gypsum to phosphoric acid solution is 1:2.3-4, and the concentration of phosphoric acid solution is 25-45%.

3. The dechlorination method for the wet-process phosphoric acid according to claim 1, characterized in that: In step (1), the slurry a is a mixture of phosphoric acid and calcium sulfate slurry, the temperature of the decomposition reaction is 90-110℃, the reaction time is 3h, and the reaction tank A is a dissolving tank.

4. The dechlorination method for the wet-process phosphoric acid process according to claim 1, characterized in that: In step (2), the concentrated sulfuric acid is industrial sulfuric acid with a mass concentration of 98%, and the amount added is calculated based on the amount of concentrated sulfuric acid used in phosphate rock. The mass ratio of phosphate rock to concentrated sulfuric acid is 1.0-1.5:

1.

5. The dechlorination method for the wet-process phosphoric acid according to claim 1, characterized in that: In step (2), the mass concentration of the Na2S2O8 solution is 10-50%, and the amount added is such that the mass ratio of Na2S2O8 to chloride ions is n(Na2S2O8):n(Cl⁻) = 0.5-7:

1.

6. The dechlorination method for the wet-process phosphoric acid according to claim 5, characterized in that: In step (2), the mass concentration of the Na2S2O8 solution is 30%, and the amount added is such that the mass ratio of Na2S2O8 to chloride ions is n(Na2S2O8):n(Cl⁻) = 5:

1.

7. The dechlorination method for the wet-process phosphoric acid according to claim 1, characterized in that: In step (2), the high temperature of the oxidation and dechlorination reaction is 90-110℃, the reaction time is 3h, and the reaction tank B is a crystallization tank.

8. The dechlorination method for the wet-process phosphoric acid according to claim 1, characterized in that: In step (3), the washing system consists of three washing towers. The first stage uses water for circulating washing to remove most of the HF and some of the SiF4. The second and third stages are countercurrent washing to completely remove Cl2, residual SiF4 and HF. The secondary countercurrent washing uses 1m 3 The alkaline solution is circulated and washed once per hour, for 1-5 hours. The three-stage countercurrent washing involves circulating the alkaline solution once per hour, for 1-5 hours.

9. The dechlorination method for the wet-process phosphoric acid according to claim 1, characterized in that: In step (3), the exhaust gas is a mixture of chlorine, silicon tetrafluoride, hydrogen fluoride and sulfur dioxide.

10. The dechlorination method for the wet-process phosphoric acid process according to claim 1 or 8, characterized in that: In step (3), the alkaline solution is one or more of sodium carbonate, sodium bicarbonate, or sodium hydroxide.

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