De-arsenic process for 105% polyphosphoric acid production by thermal method

Abstract

This invention discloses an arsenic removal process for producing 105% polyphosphoric acid using the thermal method, belonging to the field of phosphoric acid production technology. The invention includes the following steps: (1) Preparation of the arsenic removal agent solution: Polyvinylpyrrolidone, water, and phosphorus pentasulfide are mixed to obtain an arsenic removal agent solution; (2) Primary arsenic removal: The arsenic removal agent solution is added to 107% polyphosphoric acid for arsenic removal reaction; (3) Preparation of arsenic removal agent microspheres: Sodium hydroxide, water, metakaolin, and rice bran are mixed to obtain a suspension, which is then dripped into white oil, filtered, washed, dried, calcined, and sieved to obtain arsenic removal agent microspheres; (4) Secondary arsenic removal using the arsenic removal agent microspheres; (5) Filtration. The arsenic removal process of this invention can reduce the arsenic content in 105% polyphosphoric acid produced by the thermal method to below 0.5 PPM, meeting the needs of special industries such as lithium hexafluorophosphate electrolyte, pharmaceutical synthesis, electronics, and food in domestic and international markets.

Description

Technical Field

[0001] This invention relates to the field of phosphoric acid production technology, specifically to an arsenic removal process for producing 105% polyphosphoric acid using the thermal phosphoric acid method. Background Technology

[0002] Arsenic belongs to Group Nitrogen in the periodic table. In 105% polyphosphoric acid, elemental arsenic is the most difficult to remove due to its high density, high viscosity, and long, dense chains. There is no consensus on the form of arsenic in thermal phosphoric acid production, but elemental arsenic, as a primary poison, is highly toxic, with its compounds, arsenic trisulfide and arsenic pentasulfide, being even more toxic. The arsenic content significantly impacts the quality of downstream products; higher content leads to lower quality. Currently, domestic fine phosphate chemical companies using the thermal process to produce 105% polyphosphoric acid according to industry standards maintain an arsenic content between 50 PPM and 100 PPM. However, 105% polyphosphoric acid is mainly used in lithium hexafluorophosphate electrolytes, pharmaceutical synthesis, electronics, food, and other specialized industries, where the requirements for arsenic content are even stricter, generally requiring it to be below 0.5 PPM.

[0003] Currently, arsenic removal methods for polyphosphoric acid mainly include the yellow phosphorus low-arsenic method and the arsenic removal agent method. In the yellow phosphorus low-arsenic method, the arsenic content of 105% polyphosphoric acid produced by the thermal process primarily comes from the raw material yellow phosphorus. In my country, major yellow phosphorus producers are concentrated in Yunnan, Guizhou, and Sichuan provinces. The phosphate rock sources in these provinces differ, resulting in varying arsenic contents and thus different arsenic contents in the produced yellow phosphorus. According to national standard yellow phosphorus quality analysis reports, the arsenic content of yellow phosphorus in Yunnan ranges from 130 PPM to 200 PPM, in Guizhou from 310 PPM to 450 PPM, and in Sichuan from 250 PPM to 380 PPM. Based on actual production experience, with a recovery rate of 2.944 for 105% polyphosphoric acid, the arsenic content of 105% polyphosphoric acid produced from yellow phosphorus in most production areas does not meet the food-grade low-arsenic standard for 105% polyphosphoric acid. If arsenic removal is achieved using the low-arsenic method with raw material yellow phosphorus, the arsenic content of the phosphate rock used to produce yellow phosphorus must be less than 0.001%. However, due to limitations in national phosphate rock resources, no domestic yellow phosphorus manufacturer has currently reduced the arsenic content of its yellow phosphorus through technological upgrades or other production methods. In the arsenic removal agent method, arsenic removal agents mainly include sulfides. However, because polyphosphoric acid has characteristics such as high density, high viscosity, and long, dense chains, directly adding sulfides as arsenic removal agents not only results in incomplete arsenic removal but also presents significant challenges in the subsequent filtration process. The filtration time is excessively long, and the finished product is not clear and transparent enough, affecting product quality. Summary of the Invention

[0004] The purpose of this invention is to address the aforementioned problems in the existing technology and provide an arsenic removal process for the thermal phosphoric acid production of 105% polyphosphoric acid. This arsenic removal process can reduce the arsenic content in the thermally produced 105% polyphosphoric acid to below 0.5 PPM. The low-arsenic 105% polyphosphoric acid of this invention can meet the needs of special industries such as lithium hexafluorophosphate electrolyte, pharmaceutical synthesis, electronics, and food in domestic and international markets.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows:

[0006] The arsenic removal process for producing 105% polyphosphoric acid from thermal phosphoric acid includes the following steps:

[0007] (1) Preparation of arsenic removal agent solution: Polyvinylpyrrolidone is added to water and mixed evenly, then phosphorus pentasulfide is added and mixed evenly to obtain arsenic removal agent solution. The mass ratio of polyvinylpyrrolidone, phosphorus pentasulfide and water is 0.01-0.05:2-3:10.

[0008] (2) One-time arsenic removal: 107% polyphosphoric acid is injected into the arsenic removal reaction tower, and then the prepared arsenic removal agent solution is added to the arsenic removal reaction tower to carry out the arsenic removal reaction. The amount of the arsenic removal agent solution added is 3-4L / ton of polyphosphoric acid.

[0009] (3) Preparation of arsenic removal agent microspheres: Sodium hydroxide was dissolved in water to obtain a mixture. Metakaolin and rice bran were added to the mixture and mixed evenly to obtain a suspension. White oil was heated to 80-90℃ and stirred. While stirring, the suspension was dripped into the white oil to obtain a mixture. The temperature of the mixture was controlled at 60-70℃ and allowed to stand for 1-2 hours. The mixture was filtered, and the filter residue was washed with boiling water, dried, calcined, and sieved to obtain arsenic removal agent microspheres.

[0010] (4) Secondary arsenic removal: The temperature of the polyphosphoric acid after the first arsenic removal in step (2) is controlled at 85-95℃, and the arsenic removal agent microspheres prepared in step (3) are added. The mixture is stirred at 100-150 rpm for 20-40 min.

[0011] (5) Filtration: After the polyphosphoric acid after the second arsenic removal in step (4) is filtered by a plate and frame filter press, the filtrate is collected, compressed air is introduced into the filtrate for aeration, oxidation and blowing, and then process water is added to adjust the acid concentration to 105% to obtain 105% polyphosphoric acid after arsenic removal. The elemental arsenic content of the 105% polyphosphoric acid after arsenic removal is <0.5PPM.

[0012] Furthermore, in step (2), the conditions for the arsenic removal reaction are controlled as follows: the reaction temperature is 95-110℃, the reaction time is 1-3h, and the stirring speed is 70-80rpm.

[0013] Furthermore, in step (3), the mass ratio of sodium hydroxide, water, metakaolin, and rice husk is 0.5-3:5-10:0.5-1:0.1-0.5.

[0014] Furthermore, in step (3), the stirring speed is 350-400 rpm; the calcination temperature is 350-450℃, the calcination time is 3-5 h; and the sieve mesh size is 60-80 mesh.

[0015] Furthermore, in step (3), the amount of the suspension added is 0.5-1 times the volume of the white oil.

[0016] Furthermore, in step (3), the white oil is No. 100 white oil.

[0017] Furthermore, in step (4), the amount of arsenic removal agent microspheres added is 0.1-0.3 times the weight of polyphosphoric acid.

[0018] Furthermore, in step (5), hydrogen sulfide gas is continuously released during the aeration process of the filtrate. The released hydrogen sulfide gas is introduced into the exhaust fan pipeline. Under the action of the exhaust fan, the hydrogen sulfide gas is sent into the hydrogen sulfide absorption tower, absorbed by industrial phosphoric acid spray, and discharged into the atmosphere through the exhaust stack after being washed by the alkali washing tower.

[0019] The beneficial effects of this invention are as follows:

[0020] The arsenic removal process of this invention can reduce the arsenic content in 105% polyphosphoric acid produced by thermal method to less than 0.5 PPM. The low-arsenic 105% polyphosphoric acid obtained after arsenic removal can meet the needs of special industries such as lithium hexafluorophosphate electrolyte, pharmaceutical synthesis, electronics, and food in domestic and foreign markets, and can better serve society.

[0021] This invention employs a two-step arsenic removal method. In the first step, an arsenic removal agent solution is prepared using phosphorus pentasulfide and polyvinylpyrrolidone to form a liquid arsenic removal agent. The phosphorus pentasulfide liquid arsenic removal agent decomposes the arsenic in the orthophosphoric acid of polyphosphoric acid upon heating, allowing hydrogen sulfide to react more effectively with the arsenic and precipitate it. Polyvinylpyrrolidone acts as an auxiliary agent in the arsenic removal process. In the second step, arsenic removal agent microspheres are prepared and then used to further treat the polyphosphoric acid after the first arsenic removal, achieving thorough arsenic removal and a final arsenic content below 0.5 PPM. Simultaneously, the use of arsenic removal agent microspheres in the initial treatment acts as a filter aid in subsequent filtration, reducing the filtration difficulty, shortening the filtration time, and resulting in a product with low color, clear transparency, and effectively improved product quality. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments.

[0023] Example 1

[0024] The arsenic removal process for producing 105% polyphosphoric acid from thermal phosphoric acid includes the following steps:

[0025] (1) Preparation of arsenic removal agent solution: Polyvinylpyrrolidone with a molecular weight of 10,000 is added to water and mixed evenly, and then phosphorus pentasulfide is added. After mixing evenly, an arsenic removal agent solution is obtained. The mass ratio of polyvinylpyrrolidone, phosphorus pentasulfide and water is 0.02:2.5:10.

[0026] (2) Primary arsenic removal: After the phosphorus combustion process increases the concentration of industrial phosphoric acid to 107% polyphosphoric acid, the 107% polyphosphoric acid is pumped into the arsenic removal reaction tower through a circulating acid pump. Then, the prepared arsenic removal agent solution is added to the arsenic removal reaction tower through a metering pump to carry out the arsenic removal reaction. The amount of the arsenic removal agent solution added is 3.5L / ton of polyphosphoric acid. The conditions for the arsenic removal reaction are controlled as follows: the reaction temperature is 105℃, the reaction time is 2h, and the stirring speed is 75rpm.

[0027] (3) Preparation of arsenic removal agent microspheres: Sodium hydroxide was dissolved in water to obtain a mixture. Metakaolin and 100-mesh rice bran were added to the mixture and mixed evenly to obtain a suspension. The mass ratio of sodium hydroxide, water, metakaolin, and rice bran was 1:8:0.7:0.3. No. 100 white oil was heated to 85°C and stirred at 400 rpm. While stirring, the suspension was dripped into the No. 100 white oil. The volume of the suspension added was 0.6 times the volume of the white oil to obtain a mixture. The temperature of the mixture was controlled at 65°C and allowed to stand for 1.5 h. The mixture was filtered, and the filter residue was washed twice with boiling water, dried, calcined at 400°C for 4 h, and passed through an 80-mesh sieve to obtain arsenic removal agent microspheres.

[0028] (4) Secondary arsenic removal: The temperature of the polyphosphoric acid after the first arsenic removal in step (2) is controlled at 90°C, and the arsenic removal agent microspheres prepared in step (3) are added. The weight of the added arsenic removal agent microspheres is 0.2 times that of the polyphosphoric acid. The mixture is stirred at 120 rpm for 30 min.

[0029] (5) Filtration: The polyphosphoric acid after the second arsenic removal in step (4) is filtered by a plate and frame filter press. The filtrate is collected in a collection tank and then flows into an acid collection tank. Compressed air is introduced into the filtrate for aeration, oxidation, and blowing. Hydrogen sulfide gas is continuously removed from the filtrate in the acid collection tank. The filtrate gradually becomes clear and transparent. Then, process water is added to adjust the acid concentration to 105% to obtain 105% polyphosphoric acid after arsenic removal. It is a colorless, transparent, and viscous liquid. Because the arsenic removal agent solution will reduce the concentration of polyphosphoric acid, the concentration of polyphosphoric acid needs to be increased to 107% in order to achieve the required concentration. The elemental arsenic content of the 105% polyphosphoric acid after arsenic removal is <0.5PPM. It is pumped into a hundred-ton storage tank using a finished product pump. The hydrogen sulfide gas removed from the phosphoric acid in the acid collection tank is introduced into the exhaust fan pipe. Under the action of the fan, the hydrogen sulfide gas is sent into the hydrogen sulfide absorption tower, where it is absorbed by industrial phosphoric acid spraying. After being washed by an alkaline washing tower, it is discharged into the atmosphere through the exhaust stack.

[0030] Example 2

[0031] The arsenic removal process for producing 105% polyphosphoric acid from thermal phosphoric acid includes the following steps:

[0032] (1) Preparation of arsenic removal agent solution: Polyvinylpyrrolidone with a molecular weight of 10,000 is added to water and mixed evenly, and then phosphorus pentasulfide is added. After mixing evenly, an arsenic removal agent solution is obtained. The mass ratio of polyvinylpyrrolidone, phosphorus pentasulfide and water is 0.05:2:10.

[0033] (2) Primary arsenic removal: 107% polyphosphoric acid is pumped into the arsenic removal reaction tower through a circulating acid pump and a flow meter. Then, the prepared arsenic removal agent solution is added to the arsenic removal reaction tower to carry out the arsenic removal reaction. The amount of the arsenic removal agent solution added is 4L / ton of polyphosphoric acid. The conditions for the arsenic removal reaction are controlled as follows: the reaction temperature is 110℃, the reaction time is 1h, and the stirring speed is 70rpm.

[0034] (3) Preparation of arsenic removal agent microspheres: Sodium hydroxide was dissolved in water to obtain a mixture. Metakaolin and 100-mesh rice bran were added to the mixture and mixed evenly to obtain a suspension. The mass ratio of sodium hydroxide, water, metakaolin, and rice bran was 0.5:5:0.5:0.1. No. 100 white oil was heated to 90°C and stirred at 350 rpm. While stirring, the suspension was dripped into the No. 100 white oil. The volume of the suspension added was 0.5 times the volume of the white oil to obtain a mixture. The temperature of the mixture was controlled at 70°C and allowed to stand for 1 hour. The mixture was filtered, and the filter residue was washed twice with boiling water, dried, calcined at 350°C for 5 hours, and passed through a 60-mesh sieve to obtain arsenic removal agent microspheres.

[0035] (4) Secondary arsenic removal: The temperature of the polyphosphoric acid after the first arsenic removal in step (2) is controlled at 95°C, and the arsenic removal agent microspheres prepared in step (3) are added. The weight of the added arsenic removal agent microspheres is 0.3 times that of the polyphosphoric acid. The mixture is stirred at 100 rpm for 40 min.

[0036] (5) Filtration: After the polyphosphoric acid after the second arsenic removal in step (4) is filtered by a plate and frame filter press, the filtrate is collected. Compressed air is introduced into the filtrate for aeration, oxidation and blowing. Then, process water is added to adjust the acid concentration to 105% to obtain 105% polyphosphoric acid after arsenic removal. It is a colorless, transparent and viscous liquid. The elemental arsenic content of the 105% polyphosphoric acid after arsenic removal is <0.5PPM.

[0037] Example 3

[0038] The arsenic removal process for producing 105% polyphosphoric acid from thermal phosphoric acid includes the following steps:

[0039] (1) Preparation of arsenic removal agent solution: Polyvinylpyrrolidone with a molecular weight of 10,000 is added to water and mixed evenly, and then phosphorus pentasulfide is added. After mixing evenly, an arsenic removal agent solution is obtained. The mass ratio of polyvinylpyrrolidone, phosphorus pentasulfide and water is 0.01:3:10.

[0040] (2) Primary arsenic removal: 107% polyphosphoric acid is pumped into the arsenic removal reaction tower through a circulating acid pump and a flow meter. Then, the prepared arsenic removal agent solution is added to the arsenic removal reaction tower to carry out the arsenic removal reaction. The amount of arsenic removal agent solution added is 3.5L / ton of polyphosphoric acid. The conditions for the arsenic removal reaction are controlled as follows: the reaction temperature is 95℃, the reaction time is 3h, and the stirring speed is 80rpm.

[0041] (3) Preparation of arsenic removal agent microspheres: Sodium hydroxide was dissolved in water to obtain a mixture. Metakaolin and 100-mesh rice bran were added to the mixture and mixed evenly to obtain a suspension. The mass ratio of sodium hydroxide, water, metakaolin and rice bran was 3:10:1:0.5. No. 100 white oil was heated to 80°C and stirred at 400 rpm. While stirring, the suspension was dripped into No. 100 white oil. The volume of the suspension added was 1 times the volume of the white oil to obtain a mixture. The temperature of the mixture was controlled at 60°C and allowed to stand for 2 hours. The mixture was filtered, and the filter residue was washed twice with boiling water, dried, calcined at 450°C for 3 hours, and passed through an 80-mesh sieve to obtain arsenic removal agent microspheres.

[0042] (4) Secondary arsenic removal: The temperature of the polyphosphoric acid after the first arsenic removal in step (2) is controlled at 85°C, and the arsenic removal agent microspheres prepared in step (3) are added. The weight of the added arsenic removal agent microspheres is 0.1 times that of the polyphosphoric acid. The mixture is stirred at 150 rpm for 20 min.

[0043] (5) Filtration: After the polyphosphoric acid after the second arsenic removal in step (4) is filtered by a plate and frame filter press, the filtrate is collected. Compressed air is introduced into the filtrate for aeration, oxidation and blowing. Then, process water is added to adjust the acid concentration to 105% to obtain 105% polyphosphoric acid after arsenic removal. It is a colorless, transparent and viscous liquid. The elemental arsenic content of the 105% polyphosphoric acid after arsenic removal is <0.5PPM.

[0044] Example 4

[0045] The arsenic removal process for producing 105% polyphosphoric acid from thermal phosphoric acid includes the following steps:

[0046] (1) Preparation of arsenic removal agent solution: Polyvinylpyrrolidone with a molecular weight of 10,000 is added to water and mixed evenly, and then phosphorus pentasulfide is added. After mixing evenly, an arsenic removal agent solution is obtained. The mass ratio of polyvinylpyrrolidone, phosphorus pentasulfide and water is 0.01:3:10.

[0047] (2) Primary arsenic removal: 107% polyphosphoric acid is pumped into the arsenic removal reaction tower through a circulating acid pump and a flow meter. Then, the prepared arsenic removal agent solution is added to the arsenic removal reaction tower to carry out the arsenic removal reaction. The amount of the arsenic removal agent solution added is 3L / ton of polyphosphoric acid. The conditions for the arsenic removal reaction are controlled as follows: the reaction temperature is 95℃, the reaction time is 3h, and the stirring speed is 80rpm.

[0048] (3) Preparation of arsenic removal agent microspheres: Sodium hydroxide was dissolved in water to obtain a mixture. Metakaolin and 100-mesh rice bran were added to the mixture and mixed evenly to obtain a suspension. The mass ratio of sodium hydroxide, water, metakaolin, and rice bran was 3:10:1:0.5. No. 10 white oil was heated to 80°C and stirred at 400 rpm. While stirring, the suspension was dripped into No. 10 white oil. The volume of the suspension added was 0.8 times that of the white oil to obtain a mixture. The temperature of the mixture was controlled at 60°C and allowed to stand for 2 hours. The mixture was filtered, and the filter residue was washed twice with boiling water, dried, calcined at 450°C for 3 hours, and passed through an 80-mesh sieve to obtain arsenic removal agent microspheres.

[0049] (4) Secondary arsenic removal: The temperature of the polyphosphoric acid after the first arsenic removal in step (2) is controlled at 85°C, and the arsenic removal agent microspheres prepared in step (3) are added. The weight of the added arsenic removal agent microspheres is 0.1 times that of the polyphosphoric acid. The mixture is stirred at 150 rpm for 20 min.

[0050] (5) Filtration: After the polyphosphoric acid after the second arsenic removal in step (4) is filtered by a plate and frame filter press, the filtrate is collected. Compressed air is introduced into the filtrate for aeration, oxidation and blowing. Then, process water is added to adjust the acid concentration to 105% to obtain 105% polyphosphoric acid after arsenic removal. It is a colorless, transparent and viscous liquid. The elemental arsenic content of the 105% polyphosphoric acid after arsenic removal is <0.5PPM.

[0051] Comparative Example 1

[0052] The arsenic removal process for producing 105% polyphosphoric acid from thermal phosphoric acid includes the following steps:

[0053] (1) Preparation of arsenic removal agent solution: Polyvinylpyrrolidone with a molecular weight of 10,000 is added to water and mixed evenly, and then phosphorus pentasulfide is added. After mixing evenly, an arsenic removal agent solution is obtained. The mass ratio of polyvinylpyrrolidone, phosphorus pentasulfide and water is 0.02:2.5:10.

[0054] (2) Primary arsenic removal: 107% polyphosphoric acid is pumped into the arsenic removal reaction tower through a circulating acid pump, and the prepared arsenic removal agent solution is added to the arsenic removal reaction tower through a metering pump for arsenic removal reaction. The amount of arsenic removal agent solution added is 5L / ton of polyphosphoric acid. The conditions for the arsenic removal reaction are controlled as follows: reaction temperature is 105℃, reaction time is 2h, and stirring speed is 75rpm.

[0055] (3) Filtration: The polyphosphoric acid after the first arsenic removal in step (2) is filtered through a plate and frame filter press. The filtrate is collected in a collection tank and then flows into an acid collection tank. Compressed air is introduced into the filtrate for aeration, oxidation, and purging. Hydrogen sulfide gas is continuously removed from the filtrate in the acid collection tank. Then, process water is added to adjust the acid concentration to 105% to obtain 105% arsenic-removed polyphosphoric acid. According to the experimental comparison, since no arsenic removal agent microspheres were used in this comparative example, the filtration time was about twice that of Example 1.

[0056] Comparative Example 2

[0057] The arsenic removal process for producing 105% polyphosphoric acid from thermal phosphoric acid includes the following steps:

[0058] (1) Preparation of arsenic removal agent microspheres: Sodium hydroxide was dissolved in water to obtain a mixture. Metakaolin and 100-mesh rice bran were added to the mixture and mixed evenly to obtain a suspension. The mass ratio of sodium hydroxide, water, metakaolin, and rice bran was 1:8:0.7:0.3. No. 100 white oil was heated to 85°C and stirred at 400 rpm. While stirring, the suspension was dripped into the No. 100 white oil. The volume of the suspension added was 0.6 times the volume of the white oil to obtain a mixture. The temperature of the mixture was controlled at 65°C and allowed to stand for 1.5 h. The mixture was filtered, and the filter residue was washed twice with boiling water, dried, calcined at 400°C for 4 h, and passed through an 80-mesh sieve to obtain arsenic removal agent microspheres.

[0059] (2) First-time arsenic removal: The temperature of 107% polyphosphoric acid is controlled at 90°C, and the arsenic removal agent microspheres prepared in step (1) are added. The weight of the added arsenic removal agent microspheres is 0.2 times that of polyphosphoric acid. The mixture is stirred at 120 rpm for 30 min.

[0060] (3) Filtration: The polyphosphoric acid after the first arsenic removal in step (2) is filtered by a plate and frame filter press, and then the acid concentration is adjusted to 105% by adding process water to obtain 105% polyphosphoric acid after arsenic removal.

[0061] Comparative Example 3

[0062] The arsenic removal process for producing 105% polyphosphoric acid from thermal phosphoric acid includes the following steps:

[0063] (1) Preparation of arsenic removal agent solution: Phosphorus pentasulfide is added to water and mixed to obtain arsenic removal agent solution. The mass ratio of phosphorus pentasulfide to water is 2.5:10.

[0064] (2) Primary arsenic removal: After the phosphorus combustion process increases the concentration of industrial phosphoric acid to 107% polyphosphoric acid, the 107% polyphosphoric acid is pumped into the arsenic removal reaction tower through a circulating acid pump. Then, the prepared arsenic removal agent solution is added to the arsenic removal reaction tower through a metering pump to carry out the arsenic removal reaction. The amount of the arsenic removal agent solution added is 3.5L / ton of polyphosphoric acid. The conditions for the arsenic removal reaction are controlled as follows: the reaction temperature is 105℃, the reaction time is 2h, and the stirring speed is 75rpm.

[0065] (3) Preparation of arsenic removal agent microspheres: Sodium hydroxide was dissolved in water to obtain a mixture. Metakaolin and 100-mesh rice bran were added to the mixture and mixed evenly to obtain a suspension. The mass ratio of sodium hydroxide, water, metakaolin, and rice bran was 1:8:0.7:0.3. No. 100 white oil was heated to 85°C and stirred at 400 rpm. While stirring, the suspension was dripped into the No. 100 white oil. The volume of the suspension added was 0.6 times the volume of the white oil to obtain a mixture. The temperature of the mixture was controlled at 65°C and allowed to stand for 1.5 h. The mixture was filtered, and the filter residue was washed twice with boiling water, dried, calcined at 400°C for 4 h, and passed through an 80-mesh sieve to obtain arsenic removal agent microspheres.

[0066] (4) Secondary arsenic removal: The temperature of the polyphosphoric acid after the first arsenic removal in step (2) is controlled at 90°C, and the arsenic removal agent microspheres prepared in step (3) are added. The weight of the added arsenic removal agent microspheres is 0.2 times that of the polyphosphoric acid. The mixture is stirred at 120 rpm for 30 min.

[0067] (5) Filtration: The polyphosphoric acid after the second arsenic removal in step (4) is filtered by a plate and frame filter press. The filtrate is collected in a collection tank and then flows into an acid collection tank. Compressed air is introduced into the filtrate for aeration, oxidation and blowing. Hydrogen sulfide gas is continuously removed from the filtrate in the acid collection tank. The filtrate gradually becomes clear and transparent. Then, process water is added to adjust the acid concentration to 105% to obtain 105% arsenic-removed polyphosphoric acid.

[0068] Table 1 shows a comparison of the arsenic removal experimental data of Examples 1-4 and Comparative Examples 1-3.

[0069] Table 1

[0070] Group Original concentration, % Arsenic value before arsenic removal, PPM Arsenic value after arsenic removal, PPM Concentration after arsenic removal, % Example 1 107.54 88 0.092 105.15 Example 2 107.01 92 0.090 105.08 Example 3 107.63 89 0.095 105.24 Example 4 107.52 88 0.484 105.17 Comparative Example 1 107.77 90 5.178 105.49 Comparative Example 2 107.05 90 66.50 105.04 Comparative Example 3 107.13 87 0.899 105.10

[0071] As shown in Table 1, the arsenic content after arsenic removal in Examples 1-4 is all <0.5 PPM, which meets the requirements of special industries such as lithium hexafluorophosphate electrolyte, pharmaceutical synthesis, electronics, and food. The white oil used in Example 4 is No. 10 white oil, which has a poorer arsenic removal effect compared to No. 100 white oil. Comparative Examples 1 and 2 both used a single arsenic removal process without using arsenic removal agent microspheres or arsenic removal agent solution, resulting in a significant increase in the arsenic content of the final product, which is far from meeting the requirements of special industries such as food. Comparative Example 3 did not use polyvinylpyrrolidone in the arsenic removal agent solution, which also led to the arsenic content of the final product rising to >0.5 PPM, also failing to meet the requirements of special industries such as food.

[0072] The analysis report of 105% polyphosphoric acid after arsenic removal process in Examples 1-4 and Comparative Examples 1-3 is shown in Table 2.

[0073] Table 2

[0074]

[0075] As shown in Table 2, the 105% polyphosphoric acid obtained after arsenic removal using the arsenic removal processes in Examples 1-4 meets all relevant standards. However, the arsenic content of the polyphosphoric acid obtained after arsenic removal using the processes in Comparative Examples 1-3 does not meet the requirements of special industries such as food processing. In summary, the two-step arsenic removal method used in this invention works synergistically to achieve thorough arsenic removal, low color and clear transparency of the finished product, and a final arsenic content below 0.5 PPM.

[0076] Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications and improvements without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention shall be defined by the claims.

Claims

1. An arsenic removal process for producing 105% polyphosphoric acid using thermal phosphoric acid, characterized in that, Includes the following steps: (1) Preparation of arsenic removal agent solution: Polyvinylpyrrolidone is added to water and mixed evenly, then phosphorus pentasulfide is added and mixed evenly to obtain arsenic removal agent solution. The mass ratio of polyvinylpyrrolidone, phosphorus pentasulfide and water is 0.01-0.05:2-3:

10. (2) One-time arsenic removal: 107% polyphosphoric acid is injected into the arsenic removal reaction tower, and then the prepared arsenic removal agent solution is added to the arsenic removal reaction tower to carry out the arsenic removal reaction. The amount of the arsenic removal agent solution added is 3-4L / ton of polyphosphoric acid. (3) Preparation of arsenic removal agent microspheres: Sodium hydroxide was dissolved in water to obtain a mixture. Metakaolin and rice bran were added to the mixture and mixed evenly to obtain a suspension. White oil was heated to 80-90℃ and stirred. While stirring, the suspension was dripped into the white oil to obtain a mixture. The temperature of the mixture was controlled at 60-70℃ and allowed to stand for 1-2 hours. The mixture was filtered, and the filter residue was washed with boiling water, dried, calcined, and sieved to obtain arsenic removal agent microspheres. (4) Secondary arsenic removal: The temperature of the polyphosphoric acid after the first arsenic removal in step (2) is controlled at 85-95℃, and the arsenic removal agent microspheres prepared in step (3) are added. The mixture is stirred at 100-150 rpm for 20-40 min. (5) Filtration: After the polyphosphoric acid after the second arsenic removal in step (4) is filtered by a plate and frame filter press, the filtrate is collected, compressed air is introduced into the filtrate for aeration, oxidation and blowing, and then process water is added to adjust the acid concentration to 105% to obtain 105% polyphosphoric acid after arsenic removal. The elemental arsenic content of the 105% polyphosphoric acid after arsenic removal is <0.5PPM.

2. The arsenic removal process for producing 105% polyphosphoric acid from thermal phosphoric acid as described in claim 1, characterized in that, In step (2), the conditions for the arsenic removal reaction are controlled as follows: the reaction temperature is 95-110℃, the reaction time is 1-3h, and the stirring speed is 70-80rpm.

3. The arsenic removal process for producing 105% polyphosphoric acid from thermal phosphoric acid as described in claim 1, characterized in that: In step (3), the mass ratio of sodium hydroxide, water, metakaolin, and rice husk is 0.5-3:5-10:0.5-1:0.1-0.

5.

4. The arsenic removal process for producing 105% polyphosphoric acid from thermal phosphoric acid as described in claim 1, characterized in that: In step (3), the stirring speed is 350-400 rpm; the calcination temperature is 350-450℃ and the calcination time is 3-5 h; the sieve mesh size is 60-80 mesh.

5. The arsenic removal process for producing 105% polyphosphoric acid from thermal phosphoric acid as described in claim 1, characterized in that: In step (3), the white oil is No. 100 white oil.

6. The arsenic removal process for producing 105% polyphosphoric acid from thermal phosphoric acid as described in claim 1, characterized in that: In step (4), the amount of arsenic removal agent microspheres added is 0.1-0.3 times the weight of polyphosphoric acid.

7. The arsenic removal process for producing 105% polyphosphoric acid from thermal phosphoric acid as described in claim 1, characterized in that: In step (5), hydrogen sulfide gas is continuously released during the aeration process of the filtrate. The released hydrogen sulfide gas is introduced into the exhaust fan pipeline. Under the action of the exhaust fan, the hydrogen sulfide gas is sent into the hydrogen sulfide absorption tower, absorbed by industrial phosphoric acid spray, and discharged into the atmosphere through the exhaust stack after being washed by the alkali washing tower.

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