Method for preparing raw material phosphoric acid for monopotassium phosphate by multi-stage synergistic purification wet-process phosphoric acid

Abstract

The invention discloses a method for preparing raw material phosphoric acid for monopotassium phosphate by multi-stage synergistic purification of wet-process phosphoric acid, and relates to the technical field of wet-process phosphoric acid purification. The method specifically comprises the following steps: firstly, adding part of SiO2 into dilute phosphoric acid, and pumping the dilute phosphoric acid into a defluorination tower for stripping and defluorination; adding the rest SiO2, pumping into a flash chamber, and carrying out vacuum concentration and defluorination; pumping into a desulfurization reaction tank, adding phosphoric ore pulp according to a calcium-sulfur ratio, clarifying the slurry in a thickener after reaction, and filtering the upper overflow clear liquid to obtain desulfurized phosphoric acid; adding sodium sulfide, reacting at normal temperature, heating, continuously reacting, and carrying out solid-liquid separation to obtain dehydrophosphoric acid; and enabling the de-heavy phosphoric acid to sequentially pass through three activated carbon fiber filtering devices which are connected in series, so as to obtain the raw material phosphoric acid which can be directly used for producing the monopotassium phosphate after being purified. A gradient purification system is constructed through concentration defluorination, phosphoric ore pulp desulfurization, sodium sulfide heavy component removal and activated carbon fiber decoloration in a multi-stage process cooperation mode, and step-by-step efficient removal of fluorine, sulfur, heavy metal and pigment in wet-process phosphoric acid is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of wet-process phosphoric acid purification, and in particular to a method for multi-stage coordinated purification of raw phosphoric acid for preparing potassium dihydrogen phosphate from wet-process phosphoric acid. Background Art

[0002] The production of potassium dihydrogen phosphate typically uses wet-process phosphoric acid and potassium chloride as raw materials. An extractant reacts with a solvent in a homogeneous phase, and the potassium dihydrogen phosphate is separated through salting out and cooling. As the grade of phosphate ore decreases, the impurity content in wet-process phosphoric acid produced by acid hydrolysis of phosphate ore increases. These impurities eventually enter the potassium dihydrogen phosphate product, resulting in insufficient purity and excessive impurities, making it difficult to meet regulatory requirements. This necessitates multiple additional purification steps, significantly reducing production efficiency and increasing production costs. Therefore, in order to prepare high-quality and stable potassium dihydrogen phosphate, companies strictly control the impurity indicators in their raw material wet-process phosphoric acid. Usually, the wet-process phosphoric acid needs to undergo steps such as defluorination, desulfurization, deweighting, and decolorization. The existing technologies CN1483666A and CN1769163A both use targeted purification to obtain industrial-grade phosphoric acid and food-grade phosphoric acid. However, the raw phosphoric acid used in the production of fertilizer-grade potassium dihydrogen phosphate does not essentially need to meet such high standards as industrial-grade phosphoric acid. Such a treatment method will result in excessively high purification costs. Therefore, there is an urgent need to develop a method for purifying wet-process phosphoric acid for preparing potassium dihydrogen phosphate. Summary of the Invention

[0003] The object of the present invention is to provide a method for multi-stage coordinated purification of phosphoric acid as a raw material for preparing potassium dihydrogen phosphate from wet-process phosphoric acid, thereby solving the problem that there is no purification method for phosphoric acid as a raw material for the production of fertilizer-grade potassium dihydrogen phosphate.

[0004] To solve the above technical problems, the present invention adopts the following technical solution: a multi-stage collaborative purification method for wet-process phosphoric acid to prepare phosphoric acid as a raw material for potassium dihydrogen phosphate, characterized by comprising the following steps: S1. Concentration and defluorination: Weigh SiO2 according to a silicon-fluorine ratio of 0.3 to 0.5, first add part of SiO2 to dilute phosphoric acid, pump the dilute phosphoric acid into the defluorination tower for stripping and defluorination; after defluorination, add the remaining SiO2 to the phosphoric acid and pump it into the flash chamber for vacuum concentration and simultaneous defluorination; S2. Phosphate slurry desulfurization: The concentrated phosphoric acid obtained in step S1 is pumped into a desulfurization reaction tank, and phosphate slurry is added at a calcium-sulfur ratio of 1.1 to 1.3. The reaction is stirred at a temperature of 60 to 80°C, and the pH of the reaction slurry is controlled to be 2.8 to 3.2; the slurry after the reaction is clarified in a thickener, and the overflow is filtered to obtain desulfurized phosphoric acid; S3 sodium sulfide dephosphorylation: sodium sulfide was added to the desulfurized phosphoric acid in step S2 and stirred thoroughly, the reaction was first carried out at room temperature for 30 to 40 minutes, then the temperature was raised to 45 to 50 ° C and the reaction was continued for 20 to 30 minutes, and the dephosphoric acid was obtained after solid-liquid separation; S4. Activated carbon fiber decolorization: The dephosphoric acid obtained in step S3 is sequentially passed through three activated carbon fiber filter devices connected in series to obtain the purified raw phosphoric acid that can be directly used in the production of potassium dihydrogen phosphate. The filter pore size of the three activated carbon fiber filter devices in series decreases successively.

[0005] A further technical solution is that step S1 is specifically as follows: S1-1. Select purity ≥99%, specific surface area ≥200m 2 / g of SiO2, weighed at a silicon-fluorine ratio of 0.3 to 0.5; S1-2. After 60% SiO2 is added to dilute phosphoric acid and mixed evenly, it is pumped into the defluorination tower for steam defluorination. The vacuum degree in the defluorination tower is controlled at 65 to 70 kPa, and the low-pressure steam pressure at a temperature of 135 to 140 ° C is filled. The phosphoric acid defluorination temperature is controlled at 75 to 85 ° C; S1-3. The preliminary defluorinated phosphoric acid obtained at the bottom of the defluorination tower is pumped out and the remaining 40% of SiO2 is added and mixed evenly. After heating, it enters the flash chamber for concentration and defluorination. During the process, the vacuum degree in the flash chamber is controlled at 65-70kPa, and the temperature for concentrated defluorination of phosphoric acid is controlled at 75-85℃.

[0006] A further technical solution is to utilize the residual heat of the concentrated defluorinated phosphoric acid to carry out the reaction in step S2 without additional heating.

[0007] A further technical solution is that the amount of sodium sulfide added in step S3 is calculated according to the following formula: m(Na2S)=k1·[M]·V / (1+e −0.3t ) Where: [M] is the concentration of heavy metals in phosphoric acid, in mg / L; V is the amount of phosphoric acid treated, in m³; t is the reaction time, in h, and k1 = 1.05 to 1.15.

[0008] A further technical solution is that step S4 is specifically as follows: S4-1. Steam condensate is introduced into the activated carbon fiber filtration device for continuous flushing for 2 to 3 minutes to compact each layer of the activated carbon fiber membrane; S4-2. The phosphoric acid is first passed into the first activated carbon fiber filter device, controlling the filter pore size to 0.4 to 0.5um; S4-3. The filtered phosphoric acid is passed into a second activated carbon fiber filter device, controlling the filter pore size to 0.2 to 0.3um, and filtered and decolorized at 0 to 0.6MPa; S4-4. The filtered phosphoric acid is passed into a third activated carbon fiber filter device, controlling the filter pore size to 0.1 to 0.2um to obtain purified phosphoric acid; S4-5. Use pulse steam to flush the activated carbon fiber filter device for 30 to 60 minutes continuously, with a steam pressure of 0.2 to 0.4 MPa and a frequency of 2 Hz.

[0009] A further technical solution is that the raw phosphoric acid which can be directly used for the production of potassium dihydrogen phosphate after purification has a P2O5 content of ≥38%, SO4 2- Content ≤0.5%, F- content ≤0.2%, solid content ≤0.2%, Fe2O3 content ≤0.1%, Al2O3 content ≤0.8%, CaO content ≤0.5%, MgO content ≤1.2%, total arsenic content ≤20mg / kg, total chromium content ≤10mg / kg, total cadmium content ≤0.5mg / kg, total lead content ≤20mg / kg, total mercury content ≤0.1mg / kg.

[0010] Compared with the prior art, the present invention has the following beneficial effects: 1. Provide a concentrated defluorination, phosphate slurry desulfurization, sodium sulfide deweighting, activated carbon fiber decolorization, multi-stage process collaborative construction of a gradient purification system to achieve efficient step-by-step removal of fluorine, sulfur, heavy metals and pigments in wet-process phosphoric acid.

[0011] 2. Control the total amount of defluorinating agent during the defluorination stage, add the defluorinating agent SiO2 in batches before entering the defluorination tower and flash chamber, and strictly control the defluorination temperature and vacuum degree to further enhance the defluorination effect, so that the fluorine recovery rate reaches more than 92.4%, and the obtained fluorosilicic acid concentration reaches 12-16%, which can be directly used in the subsequent fluoride salt production process.

[0012] 3. Utilizing waste heat from the concentrated defluorination stage, the desulfurization temperature is maintained, achieving energy savings. Simultaneously, by controlling the calcium-sulfur ratio and adjusting the phosphate slurry dosage, a primary desulfurization rate of over 93% is achieved. During the desulfurization process, the slurry pH is controlled at 2.8-3.2, resulting in the formation of short, columnar gypsum (aspect ratio ≤ 2.0), which significantly increases its settling velocity and accelerates solid-liquid separation.

[0013] 4. Use mathematical formula to accurately calculate the amount of sodium sulfide, and react at room temperature to make Hg²⁺, Pb²⁺, As 3 ⁺Preferential precipitation, then heating to remove Cd²⁺ and Cr²⁺, without introducing new impurities, making the heavy metal directional removal rate ≥90%.

[0014] 5. By controlling the filtration pore size through three gradient layers, the system achieves a primary decolorization rate of phosphoric acid exceeding 87.5%. Simultaneously, pulsed steam flushing is used to regenerate the activated carbon fibers, reducing the membrane flux decay rate to less than 5% per 100 hours, significantly extending its service life. Compared to existing technologies using activated carbon for decolorization, this technology produces no activated carbon waste residue and reduces regeneration costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 It is a process flow chart of the present invention.

[0016] Figure 2 is the calculation result of sodium sulfide in Example 1.

[0017] Figure 3 Schematic diagram of the process for decolorization of activated carbon fiber.

[0018] Figure 4 This is a test report of the decolorized paste phosphoric acid in the examples. DETAILED DESCRIPTION

[0019] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0020] Example 1: (1) Concentrated defluorination: Take dilute phosphoric acid with a P2O5 content of 28.3% (too low a concentration will increase steam consumption, while too high a concentration may increase the viscosity of phosphoric acid and affect mass transfer efficiency) as follows After metering, 0.72 kg / t of active SiO2 (99% purity, specific surface area 220 m² / g, total addition amount 1.12 kg / t phosphoric acid, SiO2 dosage here calculated based on a silicon-fluorine ratio of 0.33) is added to the defluorination tower for defluorination. The amount of defluorination circulating acid in the defluorination tower is three times the feed amount to ensure sufficient gas-liquid contact. Steam pressure is 0.3 MPa, temperature is 160°C, and the phosphoric acid temperature is raised to the defluorination window through direct steam heating. The gas velocity in the tower is maintained at 0.5 m / s to ensure that fluoride escapes with the steam. The tower bottom temperature is controlled at 110°C, the tower top temperature is controlled at 90°C, and the operation is carried out under slight negative pressure. The material residence time is 20 minutes, and defluorinated phosphoric acid is obtained. Its indicators are as follows: The remaining 0.4 kg / t of active SiO2 is added to the defluorinated phosphoric acid exiting the defluorination tower and fed into a graphite tube heat exchanger. Mixed with the heated circulating acid, it is then introduced into the flash chamber. The phosphoric acid temperature is controlled at 78°C, with a w (P2O5) concentration of 43%. The evaporation chamber vacuum is 67 kPa, the low-pressure steam pressure is 0.40 MPa, and the temperature is 138°C. A pressure regulating valve is used to stabilize the steam pressure. Adjusting the dilute phosphoric acid feed rate and steam flow control concentration promotes the conversion of HF and H2SiF6 into SiF4 for discharge. The final fluorine recovery rate reaches 92.41%, and the fluosilicic acid solution concentration is 16.5% (GB / T 4348.1 titration method). The concentrated phosphoric acid specifications are as follows: (2) Desulfurization of phosphate slurry: Concentrated phosphoric acid is pumped into the desulfurization reaction tank and added to the phosphate slurry at a calcium-sulfur molar ratio of 1.25. The reaction was stirred at 70°C for 1 hour at a stirring rate of 400 r / min. The pH of the reaction slurry was maintained between 2.8 and 3.0, producing short columnar gypsum (aspect ratio of 1.8). After the reaction, the material was pumped into a thickener for clarification. The bottom residue acid was transported to other phosphoric acid units for reuse, and the supernatant overflow was filtered to remove sediment. Testing showed a desulfurization rate of 93.06% for this batch of phosphoric acid, meeting process requirements. The desulfurized phosphoric acid specifications are as follows: (3) Sodium sulfide step-by-step deweighting: Take the desulfurized wet-process phosphoric acid and filter it through a plate and frame filter press to remove suspended impurities. The amount of sodium sulfide is calculated according to the formula based on the heavy metal content of the phosphate: m(Na2S)=k1·[M]·V / (1+e −0.3t ) Where: [M] is the heavy metal concentration in phosphoric acid, in mg / L; V is the phosphoric acid treatment volume, in m³; t is the reaction time, in h, and k1=1.12. Based on the analysis of phosphoric acid composition, 0.793 kg of sodium sulfide is required per ton of phosphoric acid. Sodium sulfide is added slowly, over a period of 10-30 minutes. Stepwise degassing is performed, first reacting at room temperature for 40 minutes to remove Hg²⁺, Pb²⁺, and As. 3 ⁺Precipitation is prioritized, and then the temperature is raised to 50°C for 20 minutes to remove Cd²⁺ and Cr²⁺. Synchronous mechanical stirring promotes mixing, and the stirring speed is controlled at 400r / min to fully precipitate the sulfide ions and heavy metals. After the reaction is completed, a centrifuge is used for solid-liquid separation. After testing, the weight removal rate after the step precipitation is 90.46%, which meets the process requirements. The indicators of phosphoric acid after weight removal are as follows: (4) Activated carbon fiber decolorization: First, steam condensate is introduced to activate the membrane for 2 minutes. The operating pressure of the laminated membrane is 0.2 MPa. The membrane layer is compacted and the fallen fibers are discharged. After the phosphoric acid is pretreated with unreacted phosphate slag and suspended solids by the first activated carbon fiber filter with a pore size of 0.5 μm, it is sent to the second activated carbon fiber filter with an activated carbon fiber membrane with a pore size of 0.2 μm. After the liquid fills the equipment, the exhaust valve is closed and the discharge valve is slowly opened to 1 / 3 of the opening. The phosphoric acid is filtered and decolorized at a pressure of 0.3 MPa to remove pigments and other organic impurities in the phosphoric acid to obtain purified phosphoric acid. The water-insoluble matter is then filtered through the third activated carbon fiber filter with a pore size of 0.1 μm to obtain decolorized phosphoric acid. The decolorization rate is 87.63%.

[0021] When the discharge effect is not good, stop feeding and flush the membrane with low-pressure steam for 45 minutes to regenerate. The outlet valve is fully open, the steam pressure is 0.2~0.4MPa, the frequency is 2Hz, and when the steam condensate becomes dark in color, wash it with water to achieve membrane regeneration.

[0022] The product indicators obtained by using the decolorized phosphoric acid in the production of potassium dihydrogen phosphate are as follows: Appearance: white crystalline powder, free of mechanical impurities; potassium dihydrogen phosphate mass fraction is 99.01%, water-soluble phosphorus pentoxide content is 51.63%, potassium oxide content is 33.94%, moisture content is 0.1%, chloride content is 0.37%, water-insoluble matter content is 0.18%, pH value is 4.71, total cadmium content is not detected, total mercury content is 0.079 mg / kg, total arsenic content is 2.94 mg / kg, total lead content is not detected, and total chromium content is 8.9 mg / kg.

[0023] Example 2: (1) Concentrated defluorination: Take dilute phosphoric acid with a P2O5 content of 22.8% (too low a concentration will increase steam consumption, while too high a concentration may increase the viscosity of phosphoric acid and affect mass transfer efficiency). The indicators are as follows After metering, 0.72kg / t of active SiO2 (99% purity, 220m² / g specific surface area, 1.2kg / t phosphoric acid added in total, the SiO2 dosage here is calculated based on a silicon-fluorine ratio of 0.4) is added to the defluorination tower for defluorination. The amount of defluorination circulating acid in the defluorination tower is 4 times the feed amount to ensure sufficient gas-liquid contact. The steam pressure is 0.4MPa and the temperature is 150°C. The phosphoric acid temperature is raised to the defluorination window by direct steam heating, and the gas velocity in the tower is maintained at 0.8 m / s to ensure that fluoride escapes with the steam. The tower bottom temperature is controlled at 120°C, the tower top temperature is controlled at 100°C, and the operation is carried out under slightly negative pressure. The material residence time is 25 minutes. Defluorinated phosphoric acid is obtained, and its indicators are as follows: The remaining 0.48 kg / t of active SiO2 is added to the defluorinated phosphoric acid exiting the defluorination tower and fed into a graphite tube heat exchanger. Mixed with the heated circulating acid, it is then introduced into the flash chamber. The phosphoric acid temperature is controlled at 78°C, with a w (P2O5) content of 43%. The evaporation chamber vacuum is 67 kPa, the low-pressure steam pressure is 0.40 MPa, and the temperature is 138°C. A pressure regulating valve is used to stabilize the steam pressure. Adjustments to the dilute phosphoric acid feed rate and steam flow control the concentration, promoting the conversion of HF and H2SiF6 into SiF4 for discharge. The final fluorine recovery rate reaches 90.78%, and the fluosilicic acid solution concentration is 16.1% (GB / T 4348.1 titration method). The concentrated phosphoric acid specifications are as follows: (2) Desulfurization of phosphate slurry: Concentrated phosphoric acid is pumped into the desulfurization reaction tank and added to the phosphate slurry at a calcium-sulfur molar ratio of 1.25. The reaction was stirred at 70°C for 1 hour at a stirring rate of 400 r / min. The pH of the reaction slurry was maintained between 2.8 and 3.0, producing short columnar gypsum (aspect ratio of 1.8). After the reaction, the material was pumped into a thickener for clarification. The bottom residue acid was transported to other phosphoric acid units for reuse, and the supernatant overflow was filtered to remove sediment. Testing showed that the desulfurization rate of this batch of phosphoric acid reached 92.01%, meeting process requirements. The desulfurized phosphoric acid specifications are as follows: (3) Sodium sulfide step-by-step deweighting: Take the desulfurized wet-process phosphoric acid and filter it through a plate and frame filter press to remove suspended impurities. The amount of sodium sulfide is calculated according to the formula based on the heavy metal content of the phosphate: m(Na2S)=k1·[M]·V / (1+e −0.3t ) Where: [M] is the concentration of heavy metals in phosphoric acid, in mg / L; V is the phosphoric acid treatment volume, in m³; t is the reaction time, in h, k1=1.12. According to the analysis of phosphoric acid composition, 0.475 kg of sodium sulfide is required per ton of phosphoric acid. Sodium sulfide is added slowly, with the addition time controlled within 10-30 minutes, and then the weight is removed in steps. First, react at room temperature for 40 minutes to remove Hg²⁺, Pb²⁺, and As. 3 ⁺Precipitation is prioritized, and then the temperature is raised to 50°C for 20 minutes to remove Cd²⁺ and Cr²⁺. Synchronous mechanical stirring promotes mixing, and the stirring speed is controlled at 400r / min to fully precipitate the sulfide ions and heavy metals. After the reaction is completed, a centrifuge is used for solid-liquid separation. After testing, the weight removal rate after the step precipitation is 90.11%, which meets the process requirements. The indicators of phosphoric acid after weight removal are as follows: (4) Activated carbon fiber decolorization: First, steam condensate is introduced to activate the membrane for 2 minutes. The operating pressure of the laminated membrane is 0.2 MPa. The membrane layer is compacted and the fallen fibers are discharged. After the phosphoric acid is pretreated with unreacted phosphate slag and suspended solids by the first activated carbon fiber filter with a pore size of 0.5 μm, it is sent to the second activated carbon fiber filter with an activated carbon fiber membrane with a pore size of 0.2 μm. After the liquid fills the equipment, the exhaust valve is closed and the discharge valve is slowly opened to 1 / 3 of the opening. The phosphoric acid is filtered and decolorized at a pressure of 0.3 MPa to remove pigments and other organic impurities in the phosphoric acid to obtain purified phosphoric acid. The water-insoluble matter is then filtered through the third activated carbon fiber filter with a pore size of 0.1 μm to obtain decolorized phosphoric acid. The decolorization rate is 86.75%.

[0024] When the discharge effect is not good, stop feeding and flush the membrane with low-pressure steam for 45 minutes to regenerate. The outlet valve is fully open, the steam pressure is 0.2~0.4MPa, the frequency is 2Hz, and when the steam condensate becomes dark in color, wash it with water to achieve membrane regeneration.

[0025] The product indicators obtained by using the decolorized phosphoric acid in the production of potassium dihydrogen phosphate are as follows: Appearance: white powder, free of mechanical impurities; mass fraction of potassium dihydrogen phosphate is 99.15%, water-soluble phosphorus pentoxide content is 51.7%, potassium oxide content is 34.08%, moisture content is 0.12%, chloride content is 0.18%, water-insoluble matter content is 0.15%, pH value is 4.47, total cadmium content is 0.001 mg / kg, total mercury content is 0.0032 mg / kg, total arsenic content is 2.15 mg / kg, total lead content is not detected, and total chromium content is 3.12 mg / kg.

[0026] Example 3: (1) Concentrated defluorination: Take dilute phosphoric acid with a P2O5 content of 25.2% (too low a concentration will increase steam consumption, while too high a concentration may increase the viscosity of phosphoric acid and affect mass transfer efficiency). The indicators are as follows After metering, 1.03 kg / t of active SiO2 (99% purity, 220 m² / g specific surface area, total addition amount 1.73 kg / t phosphoric acid, SiO2 dosage calculated here based on a silicon-fluorine ratio of 0.5) is added to the defluorination tower for defluorination. The defluorination circulating acid in the defluorination tower is 5 times the feed rate to ensure sufficient gas-liquid contact. Steam pressure is 0.5 MPa, temperature is 160°C, and the phosphoric acid temperature is raised to the defluorination window through direct steam heating. The gas velocity in the tower is maintained at 1.0 m / s to ensure fluoride escapes with the steam. The tower bottom temperature is controlled at 130°C, the tower top temperature is controlled at 110°C, and the operation is carried out under slight negative pressure, with a material residence time of 30 minutes. Defluorinated phosphoric acid is obtained, and its specifications are as follows: The remaining 0.7 kg / t of active SiO2 is added to the defluorinated phosphoric acid exiting the defluorination tower and fed into a graphite tube heat exchanger. Mixed with the heated circulating acid, it is then introduced into the flash chamber. The phosphoric acid temperature is controlled at 78°C, with a w (P2O5) concentration of 43%. The evaporation chamber vacuum is 67 kPa, the low-pressure steam pressure is 0.40 MPa, and the temperature is 138°C. A pressure regulating valve is used to stabilize the steam pressure. Adjustments to the dilute phosphoric acid feed rate and steam flow control the concentration, promoting the conversion of HF and H2SiF6 into SiF4 for discharge. The final fluorine recovery rate reaches 91.36%, and the fluosilicic acid solution concentration is 16.3% (GB / T 4348.1 titration method). The concentrated phosphoric acid specifications are as follows: (2) Desulfurization of phosphate slurry: Concentrated phosphoric acid is pumped into the desulfurization reaction tank and added to the phosphate slurry at a calcium-sulfur molar ratio of 1.25.

[0027] The reaction was stirred at 70°C for 1 hour at a stirring rate of 400 rpm. The pH of the reaction slurry was maintained between 2.8 and 3.0, producing short columnar gypsum (aspect ratio of 1.8). After the reaction, the material was pumped into a thickener for clarification. The bottom residue acid was transported to other phosphoric acid units for reuse, and the supernatant overflow was filtered to remove sediment. Testing showed a desulfurization rate of 92.72% for this batch of phosphoric acid, meeting process requirements. The desulfurized phosphoric acid specifications are as follows: (3) Sodium sulfide step-by-step deweighting: Take the desulfurized wet-process phosphoric acid and filter it through a plate and frame filter press to remove suspended impurities. The amount of sodium sulfide is calculated according to the formula based on the heavy metal content of the phosphate: m(Na2S)=k1·[M]·V / (1+e −0.3t ) Where: [M] is the heavy metal concentration in phosphoric acid, in mg / L; V is the phosphoric acid treatment volume, in m³; t is the reaction time, in h, and k1=1.12. Based on the analysis of phosphoric acid composition, it is calculated that 0.490 kg of sodium sulfide is required per ton of phosphoric acid. Sodium sulfide is added slowly, with the addition time controlled within 10-30 minutes. Stepwise degravimetry is performed, first reacting at room temperature for 40 minutes to remove Hg²⁺, Pb²⁺, and As. 3 ⁺Precipitation is prioritized, and then the temperature is raised to 50°C for 20 minutes to remove Cd²⁺ and Cr²⁺. Synchronous mechanical stirring promotes mixing, and the stirring speed is controlled at 400r / min to fully precipitate the sulfide ions and heavy metals. After the reaction is completed, a centrifuge is used for solid-liquid separation. After testing, the weight removal rate after step precipitation is 90.24%, which meets the process requirements. The indicators of phosphoric acid after weight removal are as follows: (4) Activated carbon fiber decolorization: First, steam condensate is introduced to activate the membrane for 2 minutes. The operating pressure of the laminated membrane is 0.2 MPa. The membrane layer is compacted and the fallen fibers are discharged. After the phosphoric acid is pretreated with unreacted phosphate slag and suspended solids by the first activated carbon fiber filter device with a pore size of 0.5 μm, it is sent to the second activated carbon fiber filter device with an activated carbon fiber membrane with a pore size of 0.2 μm. After the liquid fills the equipment, the exhaust valve is closed and the discharge valve is slowly opened to 1 / 3 of the opening. The phosphoric acid is filtered and decolorized at a pressure of 0.3 MPa to remove pigments and other organic impurities in the phosphoric acid to obtain purified phosphoric acid. The water-insoluble matter is then filtered through the third activated carbon fiber filter device with a pore size of 0.1 μm to obtain decolorized phosphoric acid. The decolorization rate is 87.16%.

[0028] When the discharge effect is not good, stop feeding and flush the membrane with low-pressure steam for 45 minutes to regenerate. The outlet valve is fully open, the steam pressure is 0.2~0.4MPa, the frequency is 2Hz, and when the steam condensate becomes dark in color, wash it with water to achieve membrane regeneration.

[0029] The product indicators obtained by using the decolorized phosphoric acid in the production of potassium dihydrogen phosphate are as follows: Appearance: white powder, free of mechanical impurities; mass fraction of potassium dihydrogen phosphate is 98.38%, water-soluble phosphorus pentoxide content is 51.33%, potassium oxide content is 33.94%, moisture content is 0.2%, chloride content is 0.51%, water-insoluble matter content is 0.11%, pH value is 4.67, total cadmium content is not detected, total mercury content is not detected, total arsenic content is 1.85 mg / kg, total lead content is 0.0005 mg / kg, and total chromium content is 4.12 mg / kg.

[0030] Although the invention has been described herein with reference to a number of illustrative embodiments thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the scope of this disclosure.

Claims

1. A multi-stage collaborative purification method for preparing phosphoric acid as a raw material for potassium dihydrogen phosphate from wet-process phosphoric acid, characterized in that The steps include: S1. Concentration and defluorination: Weigh SiO2 according to a silicon-fluorine ratio of 0.3 to 0.5, first add part of SiO2 to dilute phosphoric acid, pump the dilute phosphoric acid into the defluorination tower for stripping and defluorination; after defluorination, add the remaining SiO2 to the phosphoric acid and pump it into the flash chamber for vacuum concentration and simultaneous defluorination; S2. Phosphate slurry desulfurization: The concentrated phosphoric acid obtained in step S1 is pumped into a desulfurization reaction tank, and phosphate slurry is added at a calcium-sulfur ratio of 1.1 to 1.

3. The reaction is stirred at a temperature of 60 to 80°C, and the pH of the reaction slurry is controlled to be 2.8 to 3.2; the slurry after the reaction is clarified in a thickener, and the overflow is filtered to obtain desulfurized phosphoric acid; S3 sodium sulfide dephosphorylation: sodium sulfide was added to the desulfurized phosphoric acid in step S2 and stirred thoroughly, the reaction was first carried out at room temperature for 30 to 40 minutes, then the temperature was raised to 45 to 50 ° C and the reaction was continued for 20 to 30 minutes, and the dephosphoric acid was obtained after solid-liquid separation; S4. Activated carbon fiber decolorization: The dephosphoric acid obtained in step S3 is sequentially passed through three activated carbon fiber filter devices connected in series to obtain the purified raw phosphoric acid that can be directly used in the production of potassium dihydrogen phosphate. The filter pore size of the three activated carbon fiber filter devices in series decreases successively.

2. The method according to claim 1, wherein: The step S1 is specifically as follows: S1-1. Select purity ≥99%, specific surface area ≥200m 2 / g of SiO2, weighed at a silicon-fluorine ratio of 0.3 to 0.5; S1-2. After 60% SiO2 is added to dilute phosphoric acid and mixed evenly, it is pumped into the defluorination tower for steam defluorination. The vacuum degree in the defluorination tower is controlled at 65 to 70 kPa, and the low-pressure steam pressure at a temperature of 135 to 140 ° C is filled. The phosphoric acid defluorination temperature is controlled at 75 to 85 ° C; S1-3. The preliminary defluorinated phosphoric acid obtained at the bottom of the defluorination tower is pumped out and the remaining 40% of SiO2 is added and mixed evenly. After heating, it enters the flash chamber for concentration and defluorination. During the process, the vacuum degree in the flash chamber is controlled at 65-70kPa, and the temperature for concentrated defluorination of phosphoric acid is controlled at 75-85℃.

3. The method according to claim 1, wherein: In step S2, the residual heat of the concentrated defluorinated phosphoric acid is used to carry out the reaction without additional heating.

4. The method according to claim 1, wherein: The amount of sodium sulfide added in step S3 is calculated according to the following formula: m(Na2S)=k1·[M]·V / (1+e −0.3t ) Where: [M] is the concentration of heavy metals in phosphoric acid, in mg / L; V is the amount of phosphoric acid treated, in m³; t is the reaction time, in h, and k1 = 1.05 to 1.

15.

5. The method according to claim 1, wherein: The step S4 is specifically as follows: S4-1. Steam condensate is introduced into the activated carbon fiber filtration device for continuous flushing for 2 to 3 minutes to compact each layer of the activated carbon fiber membrane; S4-2. The phosphoric acid is first passed into the first activated carbon fiber filter device, controlling the filter pore size to 0.4 to 0.5um; S4-3. The filtered phosphoric acid is passed into a second activated carbon fiber filter device, controlling the filter pore size to 0.2 to 0.3um, and filtered and decolorized at 0 to 0.6MPa; S4-4. The filtered phosphoric acid is passed into a third activated carbon fiber filter device, controlling the filter pore size to 0.1 to 0.2um to obtain purified phosphoric acid; S4-5. Use pulse steam to flush the activated carbon fiber filter device for 30 to 60 minutes continuously, with a steam pressure of 0.2 to 0.4 MPa and a frequency of 2 Hz.

6. The method according to claim 1, wherein: The raw phosphoric acid which can be directly used for the production of potassium dihydrogen phosphate after purification has a P2O5 content of ≥38%, SO4 2- Content ≤0.5%, F- content ≤0.2%, solid content ≤0.2%, Fe2O3 content ≤0.1%, Al2O3 content ≤0.8%, CaO content ≤0.5%, MgO content ≤1.2%, total arsenic content ≤20mg / kg, total chromium content ≤10mg / kg, total cadmium content ≤0.5mg / kg, total lead content ≤20mg / kg, total mercury content ≤0.1mg / kg.

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