Method for purifying phosphoric acid by crystallization method

Abstract

The invention discloses a method for purifying phosphoric acid by a crystallization process, which belongs to the field of chemical engineering and specifically comprises the following steps: (1) heating and concentrating the raw material phosphoric acid: heating and concentrating the raw material wet-process phosphoric acid, and forming metaphosphate precipitate by metal ions in the phosphoric acid; (2) diluting and filtering concentrated phosphoric acid: diluting the concentrated phosphoric acid with water, and filtering and separating metaphosphate precipitate to obtain diluted acid; and (3) crystallization of phosphoric acid: crystallizing the diluted acid to obtain a target product, namely purified crystal phosphoric acid. The wet-process phosphoric acid is heated and concentrated, metal ions in the phosphoric acid form pyrophosphate precipitates, the concentrated phosphoric acid is diluted with water, the precipitated metaphosphate is filtered and separated, the viscosity of the obtained diluted acid is obviously reduced, the diluted acid is suitable for being purified by a crystallization method, and iron, aluminum and magnesium in the obtained purified crystal phosphoric acid are greatly reduced. The phosphoric acid product purified by the method has the advantages of good quality, high value, no secondary pollution and good environmental friendliness.

Description

Technical Field

[0001] The present invention relates to a method for purifying phosphoric acid by crystallization, specifically a method for purifying wet-process phosphoric acid raw materials into phosphoric acid crystals, and belongs to the technical field of chemical production. Background Art

[0002] Phosphoric acid is an important intermediate chemical product, and its uses can be generally summarized into two aspects, namely fertilizer production and industrial phosphate production. Wet-process phosphoric acid is a method for producing phosphoric acid by decomposing phosphate rock with sulfuric acid, and this method accounts for a high proportion in phosphoric acid production. The phosphoric acid produced by the wet-process phosphoric acid process has a high content of impurities, so it is mainly used for fertilizer production. In other industrial fields except fertilizers, it is difficult to widely apply the wet-process phosphoric acid process, and the thermal-process phosphoric acid process is usually used in other industrial fields. Compared with wet-process phosphoric acid, thermal-process phosphoric acid is a production method with high energy consumption and high pollution. If high-quality phosphates need to be produced from wet-process phosphoric acid to meet the requirements of food-grade, pharmaceutical-grade, and electronic-grade products, the wet-process phosphoric acid needs to be purified to produce high-purity phosphoric acid.

[0003] In the phase diagram of the H3PO4-H2O system, near the concentration of pure phosphoric acid (with a mass concentration of P2O5 of 72.4%), there is a crystallization region of anhydrous phosphoric acid. The anhydrous phosphoric acid crystals are transparent monoclinic crystals with a relative density of 1.88 g / cm 3 , and the crystallization point temperature is 42.4°C. Therefore, theoretically, it should be possible to purify phosphoric acid without cooling by means of traditional crystallization methods. Due to the presence of impurities in wet-process phosphoric acid, the viscosity of the system increases, affecting crystal growth, and thus making the separation of crystals from the mother liquor very difficult. It is not feasible to purify phosphoric acid solely by crystallization. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for purifying phosphoric acid by crystallization, so as to reduce the viscosity of the H3PO4-H2O system, promote the growth of phosphoric acid crystals, make the separation of crystals from the mother liquor easy, and realize the production of purifying phosphoric acid by crystallization.

[0005] The present invention adopts the following technical solution: A method for purifying phosphoric acid by crystallization, which comprises the following steps: (1) Heating and concentrating process of raw material phosphoric acid: Using the phosphoric acid product of wet-process phosphoric acid as the raw material, heating and concentrating the raw material from the battery limit in a concentrating device under normal pressure. The gaseous substances generated during heating and concentration are sent to a condenser for condensation, and the generated condensate is sent to the wet-process phosphoric acid device for reuse. The liquid-phase metal ions in the concentrating device form metaphosphate precipitates, which together with the concentrated phosphoric acid go to the dilution and filtration process of concentrated phosphoric acid; (2) Dilution and filtration process of concentrated phosphoric acid: The concentrated phosphoric acid from the heating and concentrating process of raw material phosphoric acid overflows into the dilution pool of the dilution and filtration process. At the same time, the dilution water required for dilution is continuously added to the dilution pool to form a diluted acid containing metaphosphate precipitates in the dilution pool. The metaphosphate precipitates in the dilution pool are filtered and separated, and the filtered metaphosphate is sent to the battery limit for preparing slow-release fertilizers. The filtered diluted acid after filtration goes to the crystallization process of phosphoric acid; (3) Crystallization process of diluted acid after recycling: The diluted acid after filtration is sent to a crystallization device for crystallization of the diluted acid after recycling. After crystallization, the target product, i.e., purified crystalline phosphoric acid, is obtained.

[0006] Preferably, the crystallization in step (3) includes crystallization more than twice, and a stirrer is provided in the crystallization device.

[0007] Preferably, the crystallization steps are specifically as follows: The diluted acid after recycling from the dilution and filtration process of concentrated phosphoric acid enters a primary crystallizer for crystallization, and is centrifugally separated for crystallization. The crystals generated by the primary crystallizer are melted and then enter a recrystallizer for crystallization. The crystals after centrifugal separation are used as the target product and sent to the battery limit. The mother liquor in the recrystallizer returns to the primary crystallizer.

[0008] Preferably, the crystallization temperature of the primary crystallizer is ≥32 °C and the crystallization time is ≤40 minutes; the crystallization temperature of the recrystallizer is ≥37 °C and the crystallization time is ≤40 minutes. The mass concentration of P2O5 in the target product, i.e., purified crystalline phosphoric acid, is ≥65%.

[0009] Preferably, the crystallization temperature of the primary crystallizer is 32 °C - 42 °C, the crystallization time is 20 minutes - 40 minutes; the crystallization temperature of the recrystallizer is 37 °C - 47 °C, the crystallization time is 20 minutes - 40 minutes. The mass concentration of P2O5 in the target product, i.e., purified crystalline phosphoric acid, is 65% - 75%.

[0010] Preferably, the mother liquor in the primary crystallizer goes to a secondary crystallizer for crystallization. The primary crystallization mother liquor from the primary crystallizer crystallizes in the secondary crystallizer, and secondary crystals are obtained by centrifugal separation. The secondary crystals are melted and then returned to the primary crystallizer.

[0011] Preferably, the mother liquor in the secondary crystallizer goes to the tertiary crystallizer for crystallization. The secondary crystallization mother liquor from the secondary crystallizer crystallizes in the tertiary crystallizer, and after centrifugal separation, tertiary crystals are obtained. The tertiary crystals are melted and then returned to the secondary crystallizer; the tertiary crystallization mother liquor is returned to the concentration device in the raw material phosphoric acid heating and concentration process.

[0012] Preferably, the crystallization temperature in the secondary crystallizer is ≥25 °C and the crystallization time is ≤40 minutes; the crystallization temperature in the tertiary crystallizer is ≥18 °C and the crystallization time is ≤40 minutes; the mass concentration of P2O5 in the tertiary crystallization mother liquor is ≤78%.

[0013] Preferably, the crystallization temperature in the secondary crystallizer is 25 °C - 35 °C and the crystallization time is 20 minutes - 40 minutes; the crystallization temperature in the tertiary crystallizer is 18 °C - 28 °C and the crystallization time is 20 minutes - 40 minutes; the mass concentration of P2O5 in the tertiary crystallization mother liquor is 68% - 78%.

[0014] Preferably, in step (1), the mass concentration of P2O5 in the raw wet-process phosphoric acid is 50% - 70%, the phosphoric acid is heated and concentrated to 400 °C - 460 °C in an atmospheric pressure concentration device, and the condensation temperature of the gas phase containing sulfuric acid and phosphoric acid is 100 °C - 140 °C; in step (2), the mass concentration of P2O5 in the metaphosphate is 70% - 80%, and the mass concentration of P2O5 in the recycled dilution acid is 67% - 77%.

[0015] The beneficial effects of the present invention are as follows: In the present invention, the raw material, wet-process phosphoric acid, is heated and concentrated, and metal ions in the phosphoric acid form metaphosphate precipitates; the concentrated phosphoric acid is diluted with water, and the metaphosphate precipitates are filtered and separated to obtain recycled dilution acid; the recycled dilution acid is subjected to primary crystallization, melting, and recrystallization to obtain the target product, purified crystalline phosphoric acid.

[0016] The specific principle of the present invention is: By heating and concentrating the phosphoric acid produced by decomposing phosphate rock with sulfuric acid (hereinafter referred to as wet-process phosphoric acid), metal ions in the phosphoric acid form pyrophosphate precipitates. The concentrated phosphoric acid is diluted with water and the precipitated metaphosphate is filtered and separated. This salt can be used as a good slow-release fertilizer. The viscosity of the obtained recycled dilution acid decreases significantly, making it suitable for purification by crystallization. Through the above operations, fluorine and sulfate radicals in the recycled dilution acid are basically removed, and the iron, aluminum, and magnesium in the phosphoric acid are greatly reduced; during heating and concentration, the organic matter in the wet-process phosphoric acid will be decomposed and carbonized and filtered out together with the metaphosphate precipitate. Therefore, the recycled dilution acid does not contain organic matter. The impurity content of the target product, purified crystalline phosphoric acid, obtained by crystallizing the recycled dilution acid is greatly reduced, and the iron, aluminum, and magnesium in the purified crystalline phosphoric acid are greatly reduced. The phosphoric acid product purified by the present invention has good quality and high value, and no secondary pollution is generated during the whole process, showing good environmental friendliness. Description of the Drawings

[0017] Figure 1It is the system flow chart of the method for purifying phosphoric acid by crystallization method in an embodiment of the present invention; Figure 2 It is the performance assessment result table of a specific application example of the present invention. Specific embodiments

[0018] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. Based on the following embodiments, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the scope of protection of this patent.

[0019] The system flow of the method for purifying phosphoric acid by crystallization method in an embodiment of the present invention is as Figure 1 shown. The method for purifying phosphoric acid by crystallization method in this embodiment includes the following steps: (1) Heating and concentrating process of raw material phosphoric acid: In the heating and concentrating process of raw material phosphoric acid, the phosphoric acid product of wet-process phosphoric acid is used as the raw material. The raw material from the battery limit is heated and concentrated in a concentrating device under normal pressure. The gaseous substances generated by heating and concentrating are sent to a condenser for condensation, and the generated condensate is sent to the wet-process phosphoric acid device for reuse. The liquid-phase metal ions in the concentrating device form metaphosphate precipitates, which go to the dilution and filtration process of concentrated phosphoric acid together with the concentrated phosphoric acid. In this step, the mass concentration of P2O5 in the raw material wet-process phosphoric acid is 50% - 70%, and the phosphoric acid is heated and concentrated to 400°C - 460°C in the concentrating device under normal pressure. The condensation temperature of the gas phase containing sulfuric acid and phosphoric acid is 100°C - 140°C.

[0020] (2) Dilution and filtration process of concentrated phosphoric acid: The concentrated phosphoric acid from the heating and concentrating process of raw material phosphoric acid overflows into the dilution pool of the dilution and filtration process. At the same time, the dilution water required for dilution is continuously added to the dilution pool to form a diluted acid containing metaphosphate precipitates in the dilution pool. The metaphosphate precipitates in the dilution pool are filtered and separated. The filtered metaphosphate is sent to the battery limit for preparing slow-release fertilizers, and the filtered diluted acid goes to the crystallization process of phosphoric acid. In this step, the mass concentration of P2O5 in the metaphosphate is 70% - 80%, and the mass concentration of P2O5 in the diluted acid is 67% - 77%.

[0021] (3) Crystallization process of diluted acid: The filtered diluted acid is sent to a crystallization device for crystallization of the diluted acid. After crystallization, the target product, i.e., purified crystalline phosphoric acid, is obtained. The crystallization in this step includes crystallization more than twice, and a stirrer is provided in the crystallization device.

[0022] In the present invention, phosphoric acid produced by decomposing phosphate rock with sulfuric acid (hereinafter referred to as wet-process phosphoric acid) is heated and concentrated to above 400 °C. Metal ions in the phosphoric acid form pyrophosphate precipitates. The concentrated phosphoric acid is diluted with water and filtered to separate the precipitated metaphosphate. This salt can be used as a good slow-release fertilizer. The viscosity of the recycled diluted acid obtained decreases significantly and is suitable for purification by the crystallization method. Through the above operations, fluorine and sulfate radicals in the recycled diluted acid are basically removed. The iron content in the phosphoric acid is reduced to 55.76% of the original, the aluminum content is reduced to 48.51% of the original, and the magnesium content is reduced to 56.49% of the original. When heated and concentrated, the organic matter in the wet-process phosphoric acid will be decomposed and carbonized and filtered out together with the metaphosphate precipitate. Therefore, the recycled diluted acid does not contain organic matter. Therefore, the impurity content of the target product, purified crystal phosphoric acid, obtained from the recycled diluted acid by the crystallization method is significantly reduced.

[0023] The crystallization steps are specifically as follows: a. The recycled diluted acid from the concentrated phosphoric acid dilution and filtration process enters the first-stage crystallizer for crystallization, and the crystals are separated by centrifugation; the crystals produced in the first-stage crystallizer are melted and then enter the recrystallizer for crystallization, and the crystals after centrifugation are taken as the target product out of the battery limits; the mother liquor in the recrystallizer is returned to the first-stage crystallizer. Generally, the crystallization temperature in the first-stage crystallizer is ≥ 32 °C, and the crystallization time is ≤ 40 minutes; the crystallization temperature in the recrystallizer is ≥ 37 °C, and the crystallization time is ≤ 40 minutes. The mass concentration of P2O5 in the target product, purified crystal phosphoric acid, is ≥ 65%. In a preferred embodiment, the crystallization temperature in the first-stage crystallizer is 32 °C - 42 °C, the crystallization time is 20 minutes - 40 minutes; the crystallization temperature in the recrystallizer is 37 °C - 47 °C, the crystallization time is 20 minutes - 40 minutes, and the mass concentration of P2O5 in the target product, purified crystal phosphoric acid, is 65% - 75%.

[0024] b. The mother liquor in the first-stage crystallizer goes to the second-stage crystallizer for crystallization. The first-stage crystallization mother liquor from the first-stage crystallizer crystallizes in the second-stage crystallizer, and the second-stage crystals are separated by centrifugation. The second-stage crystals are melted and then returned to the first-stage crystallizer. Generally, the crystallization temperature in the second-stage crystallizer is ≥ 25 °C, and the crystallization time is ≤ 40 minutes. In a preferred embodiment, the crystallization temperature in the second-stage crystallizer is 25 °C - 35 °C, and the crystallization time is 20 minutes - 40 minutes.

[0025] c. The mother liquor in the second-stage crystallizer goes to the third-stage crystallizer for crystallization. The second-stage crystallization mother liquor from the second-stage crystallizer crystallizes in the third-stage crystallizer, and the third-stage crystals are separated by centrifugation. The third-stage crystals are melted and then returned to the second-stage crystallizer; the third-stage crystallization mother liquor is returned to the concentration device in the raw material phosphoric acid heating and concentration process. Generally, the crystallization temperature in the third-stage crystallizer is ≥ 18 °C, and the crystallization time is ≤ 40 minutes; the mass concentration of P2O5 in the third-stage crystallization mother liquor is ≤ 78%. In a preferred embodiment, the crystallization temperature in the third-stage crystallizer is 18 °C - 28 °C, the crystallization time is 20 minutes - 40 minutes; the mass concentration of P2O5 in the third-stage crystallization mother liquor is 68% - 78%.

[0026] In a specific application example, in the step (1), the raw material - wet-process phosphoric acid has a P2O5 mass concentration of 65%. The phosphoric acid atmospheric pressure concentration device heats and concentrates it to 430 °C, and the condensation temperature of the gas phase (including sulfuric acid and phosphoric acid) is 120 °C. In the step (2), the metaphosphate has a P2O5 mass concentration of 77.3%, and the recycled diluted acid has a P2O5 mass concentration of 73.4%. In the step (3): in process a, the crystallization temperature of the primary crystallizer is 37 °C and the crystallization time is 30 minutes; the crystallization temperature of the recrystallizer is 42.4 °C and the crystallization time is 30 minutes. The target product - purified crystalline phosphoric acid has a P2O5 mass concentration of 72.4%; in process b, the crystallization temperature of the secondary crystallizer is 30 °C and the crystallization time is 30 minutes; in process c, the crystallization temperature of the tertiary crystallizer is 23 °C and the crystallization time is 30 minutes; the P2O5 mass concentration of the tertiary crystallization mother liquor is 74.9%.

[0027] The operation results of the above specific application example are as Figure 2 shown in the table. The data in this table are the performance assessment results of an 88 kg / day crystallization method for purifying phosphoric acid (calculated based on 100% P2O5). It can be seen from the Figure 2 table that the impurity content of the obtained target product - purified crystalline phosphoric acid has been greatly reduced. Through calculation, it can be known that under the same P2O5 content, iron is reduced to 3.33% of the original, aluminum is reduced to 1.8% of the original, and magnesium is reduced to 2.54% of the original. During the purification process, the P2O5 yield reaches more than 88%. The iron content in the target product is reduced to 100 mg / kg of the target product. If it is melt-crystallized once again, the iron content can be reduced to 10 mg / kg of the target product.

Claims

1. A method for purifying phosphoric acid by crystallization method, characterized in that, It includes the following steps: (1) Heating and concentrating process of raw material phosphoric acid: Using the phosphoric acid product of wet-process phosphoric acid as the raw material, heating and concentrating the raw material from the battery limit in a concentrating device under atmospheric pressure. The gaseous substances generated during heating and concentration are sent to a condenser for condensation, and the generated condensate is sent to the wet-process phosphoric acid device for reuse. The liquid-phase metal ions in the concentrating device form metaphosphate precipitates, which are sent to the dilution and filtration process of concentrated phosphoric acid together with the concentrated phosphoric acid; (2) Dilution and filtration process of concentrated phosphoric acid: The concentrated phosphoric acid from the heating and concentrating process of raw material phosphoric acid overflows into the dilution tank of the dilution and filtration process. At the same time, the dilution water required for dilution is continuously added to the dilution tank to form a recycled dilution acid containing metaphosphate precipitates in the dilution tank. The metaphosphate precipitates in the dilution tank are filtered and separated, and the filtered metaphosphate is sent to the battery limit for preparing slow-release fertilizers. The filtered recycled dilution acid goes to the crystallization process of phosphoric acid; (3) Crystallization process of recycled dilution acid: The filtered recycled dilution acid is sent to a crystallization device for crystallization of the recycled dilution acid, and the target product, i.e., purified crystalline phosphoric acid, is obtained after crystallization; 2. The method for purifying phosphoric acid by the crystallization method according to claim 1, characterized in that: The crystallization in step (3) includes crystallization more than twice, and a stirrer is provided in the crystallization device.

3. The method for purifying phosphoric acid by crystallization according to claim 2, characterized in that: The crystallization steps are specifically as follows: The recycled dilution acid from the dilution and filtration process of concentrated phosphoric acid enters the primary crystallizer for crystallization, and the crystals are separated by centrifugation. The crystals generated by the primary crystallizer are melted and then enter the recrystallizer for crystallization. The crystals after centrifugal separation are sent to the battery limit as the target product; The mother liquor in the recrystallizer returns to the primary crystallizer.

4. The method for purifying phosphoric acid by crystallization according to claim 3, characterized in that: The crystallization temperature of the primary crystallizer is ≥32 °C, and the crystallization time is ≤40 minutes; the crystallization temperature of the recrystallizer is ≥37 °C, and the crystallization time is ≤40 minutes. The mass concentration of P2O5 in the target product, i.e., purified crystalline phosphoric acid, is ≥65%; 5. The method for purifying phosphoric acid by the crystallization method according to claim 4, characterized in that: The crystallization temperature of the primary crystallizer is 32 °C - 42 °C, and the crystallization time is 20 minutes - 40 minutes; the crystallization temperature of the recrystallizer is 37 °C - 47 °C, and the crystallization time is 20 minutes - 40 minutes. The mass concentration of P2O5 in the target product, i.e., purified crystalline phosphoric acid, is 65% - 75%; 6. The method for purifying phosphoric acid by crystallization according to claim 3, characterized in that: The mother liquor in the primary crystallizer goes to the secondary crystallizer for crystallization. The primary crystallization mother liquor from the primary crystallizer crystallizes in the secondary crystallizer, and secondary crystals are obtained by centrifugal separation. The secondary crystals are melted and then returned to the primary crystallizer.

7. The method for purifying phosphoric acid by the crystallization method according to claim 6, wherein: The mother liquor in the secondary crystallizer goes to the tertiary crystallizer for crystallization. The secondary crystallization mother liquor from the secondary crystallizer crystallizes in the tertiary crystallizer, and tertiary crystals are obtained by centrifugal separation. The tertiary crystals are melted and then returned to the secondary crystallizer; The tertiary crystallization mother liquor returns to the concentrating device in the heating and concentrating process of raw material phosphoric acid.

8. The method for purifying phosphoric acid by crystallization method according to claim 7, characterized in that: The crystallization temperature of the secondary crystallizer is ≥25 °C, and the crystallization time is ≤40 minutes; the crystallization temperature of the tertiary crystallizer is ≥18 °C, and the crystallization time is ≤40 minutes; the mass concentration of P2O5 in the tertiary crystallization mother liquor is ≤78%; 9. The method for purifying phosphoric acid by the crystallization method according to claim 8, wherein: The crystallization temperature of the secondary crystallizer is 25 °C - 35 °C, and the crystallization time is 20 minutes - 40 minutes; the crystallization temperature of the tertiary crystallizer is 18 °C - 28 °C, and the crystallization time is 20 minutes - 40 minutes; the mass concentration of P2O5 in the tertiary crystallization mother liquor is 68% - 78%; 10. The method for purifying phosphoric acid by crystallization according to any one of claims 1 to 9, characterized in that: In the step (1), the mass concentration of P2O5 in the raw material wet-process phosphoric acid is 50% - 70%, the phosphoric acid is heated and concentrated to 400°C - 460°C in an atmospheric pressure concentration device, and the gas-phase condensation temperature of the sulfuric acid and phosphoric acid-containing gas is 100°C - 140°C; in the step (2), the mass concentration of P2O5 in the metaphosphate is 70% - 80%, and the mass concentration of P2O5 in the recycled diluted acid is 67% - 77%.

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