Production method of phosphoric acid with low iron ion content

Abstract

The invention discloses a method for producing phosphoric acid with low iron ion content, which comprises the following steps of: conveying liquid yellow phosphorus to a phosphorus combustion furnace, combusting and reacting with oxygen in air to generate phosphorus pentoxide gas, and using a phosphorus spray gun to assist combustion during the period; combusted phosphorus pentoxide gas is conveyed to a hydration absorption tower under negative pressure and absorbed by low-concentration phosphoric acid circulated by a concentrated acid pump to generate crude phosphoric acid, and the crude phosphoric acid reacts with hydrogen sulfide, is filtered by a semi-finished product filter and then enters a transfer tank; when the specific gravity of phosphoric acid in the hydration absorption tower meets the process requirement, arsenic removal operation is carried out, crude phosphoric acid enters an arsenic removal reactor after being fully mixed with a gas purifying agent in a pipeline mixer, then enters a precipitator, overflows to an arsenic removal filter from the precipitator, and then enters a degassing tower to remove excessive gas purifying agent; degassing, and adjusting the concentration in an adjusting tower to obtain a finished product; according to the method, fluorine lining is carried out on the heat exchanger pipeline by controlling the acid temperature, and finally the iron ion content in the finished product phosphoric acid is successfully controlled to be 200 ppb.

Description

Technical Field

[0001] The invention belongs to the technical field of phosphoric acid impurity removal, and in particular relates to a method for producing phosphoric acid with a low iron ion content. Background Art

[0002] Yellow phosphorus is an important raw material for the manufacture of organophosphorus pesticides. Yellow phosphorus is also used to produce phosphorus compounds such as thermal phosphoric acid, phosphorus trichloride, phosphorus oxychloride, phosphorus pentasulfide, and is used in industries such as oil extraction, printing and dyeing, papermaking, leather making, alloys, and washing. Yellow phosphorus can also be used to manufacture flame retardant materials and raw materials for lithium batteries.

[0003] Yellow phosphorus usually contains impurities, which mainly come from raw materials and production processes. Common impurities include metal impurities (such as arsenic, silicon, iron, sulfur, etc.), organic impurities (such as oil), and mechanical impurities that have not been rinsed and filtered clean. Among them, arsenic is one of the most difficult impurities to remove, and it is also one of the impurities with a higher content in yellow phosphorus. There are two methods for purifying yellow phosphorus: chemical and physical methods. The chemical purification method involves adding specific chemical reagents to react chemically with impurities in yellow phosphorus to generate soluble compounds or precipitates, which are then removed through filtering, washing, and other steps. The physical separation method uses the differences in physical properties between yellow phosphorus and impurities (such as density, melting point, boiling point, etc.) to separate impurities through distillation, crystallization, filtration, and other methods.

[0004] Chinese invention patent CN109607501B discloses a yellow phosphorus purification process, including the following steps: Ⅰ heating and stirring yellow phosphorus and hydrogen peroxide, separating yellow phosphorus, and washing; Ⅱ heating and stirring the yellow phosphorus after washing in step Ⅰ and a dearsenicizing adsorbent, separating yellow phosphorus, and washing; Ⅲ adding the yellow phosphorus after washing in step Ⅱ to an oxidizing enhancer and nitric acid, heating and stirring, separating yellow phosphorus, and washing; Ⅳ heating and stirring the yellow phosphorus after washing in step Ⅲ and a deironifying adsorbent, separating yellow phosphorus, and washing. The effect of this scheme is high arsenic removal rate and iron removal rate, but the actual removal effect is unknown; while the iron ion content of the thermal phosphoric acid production method of the prior art is relatively high, usually reaching more than 500ppb, the main reason is that the main component of the production equipment is iron, and phosphoric acid is a medium-strong acid, which has a certain corrosiveness to iron products, and a large amount of reaction heat is generated in the production process of phosphoric acid, which makes the production system at a higher temperature level, aggravating the corrosion of phosphoric acid to the equipment, making it difficult to control the iron ions in the product below 200ppb.

[0005] Therefore, the present invention provides a method for producing phosphoric acid with a low iron ion content, which is used to solve the technical problems existing in the above-mentioned background technology. Summary of the invention

[0006] In view of the problems raised by the above background technology, the purpose of the present invention is to provide a method for producing phosphoric acid with a low content of iron ions, to optimize the process of removing yellow phosphorus and to reduce the iron ion content in yellow phosphorus.

[0007] In order to achieve the above technical objectives, the technical solution adopted by the present invention is as follows:

[0008] A method for producing phosphoric acid with a low iron ion content, the method comprises the following specific steps:

[0009] S1: transporting liquid yellow phosphorus to a phosphorus burning furnace to react with oxygen in the air to generate phosphorus pentoxide gas, during which a phosphorus spray gun is used to assist combustion;

[0010] S2: The phosphorus pentoxide gas after combustion is transported to the hydration absorption tower by negative pressure, and is absorbed by the low-concentration phosphoric acid circulated by the concentrated acid pump to generate crude phosphoric acid. The crude phosphoric acid reacts with hydrogen sulfide and is filtered through the semi-finished product filter before entering the transfer tank;

[0011] S3: When the specific gravity of phosphoric acid in the hydration absorption tower meets the process requirements, the arsenic removal operation is carried out. The crude phosphoric acid is fully mixed with the gas purifier in the pipeline mixer and enters the arsenic removal reactor, then enters the precipitator, and then overflows from the precipitator to the arsenic removal filter, and then enters the degassing tower to remove excess gas purifier. After degassing, it enters the adjustment tower to adjust the concentration to obtain the finished product.

[0012] It is further defined that the air pressure of the phosphorus spray gun in S1 is stabilized between 400 kPa and 500 kPa.

[0013] It is further defined that S2 specifically requires controlling the wall temperature of the hydration absorber to be lower than 40°C, the inlet temperature of the first-stage venturi to be lower than 70°C, the inlet temperature of the circulating cooling water to be lower than 38°C, the bottom acid temperature of the hydration absorber to be controlled between 65 and 80°C, and the acid specific gravity of the hydration absorber to be controlled between 1.60 and 1.71.

[0014] It is further defined that the pressure of the gas purifier in S3 is between 40 and 60 kPa, and the degassing pressure of the degassing tower is between 2 kPa and 5 kPa.

[0015] It is further defined that a compressed air bag is used in S1 and the air is drained once every hour to ensure that there is no condensed water and other iron filings impurities in the air.

[0016] It is further defined that process water is added to the hydration absorption tower in S2 to carry out hydration reaction, and the process water uses demineralized water that has been treated with anion and cation exchange.

[0017] It is further defined that the conductivity of the desalted water is less than 30 us / cm, the sodium ion is less than 100 ppb, and the iron ion is less than 30 ppb.

[0018] It is further defined that the liquid level of the tower is adjusted to be less than 80% in S3,

[0019] It is further defined that the low-concentration phosphoric acid circulated by the concentrated acid pump in S3 is also a circulating acid, and when the iron ion content of the circulating acid is less than 150 ppb, the circulating acid and the gas purifier are fully mixed in the pipeline mixer and then enter the arsenic removal reactor;

[0020] The product after arsenic removal in S3 is a semi-finished product. When the circulating acid iron ion is less than 150 ppb, the semi-finished product after arsenic removal does not pass through the degassing tower, but directly enters the adjustment tower from the transfer tank for primary degassing, and then is transported to the adjustment tower for secondary degassing.

[0021] It is further defined that the semi-finished product is a colorless, transparent, viscous liquid, and the parameters of the semi-finished product include a hydrated absorption tower acid specific gravity of 1.690 to 1.71, sulfate ≤10ppm, arsenic ≤0.05ppm, iron ion ≤200ppb, easily oxidizable material ≤50ppm and deionized water conductivity ≤10us / cm.

[0022] Beneficial effects of the present invention:

[0023] The invention provides a method for producing phosphoric acid with a low content of iron ions. The method improves yellow phosphorus impurity removal treatment and post-treatment process control. First, liquid yellow phosphorus is transported from a yellow phosphorus transfer tank to a phosphorus burning furnace through a yellow phosphorus delivery pump for combustion, reacts with low-concentration phosphoric acid in a hydration absorption tower to generate crude phosphoric acid, and then is transported to a hydration absorption tower. The crude phosphoric acid reacts with hydrogen sulfide and is filtered through a semi-finished product filter, and then enters a transfer tank. When the specific gravity of the phosphoric acid in the hydration absorption tower meets the process requirements, an arsenic removal operation is performed. The crude phosphoric acid is fully mixed with a gas purifier in a pipeline mixer and enters an arsenic removal reactor, then enters a precipitator, and then overflows from the precipitator to an arsenic removal filter, then enters a degassing tower to remove excess gas purifier, and after degassing, enters an adjustment tower to adjust the concentration to obtain a finished product.

[0024] Since the source of iron ions is mainly the corrosion of production equipment, and the corrosion of phosphoric acid on equipment depends on the material of the equipment and the temperature of use, this solution successfully controlled the iron ion content in the finished phosphoric acid to below 200 ppb by controlling the acid temperature, thereby improving product quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The present invention can be further illustrated by the non-limiting examples given in the accompanying drawings;

[0026] Figure 1 The present invention is a process production flow chart of an embodiment of a method for producing phosphoric acid with a low iron ion content. DETAILED DESCRIPTION

[0027] In order to enable those skilled in the art to better understand the present invention, the technical solution of the present invention is further described below in conjunction with the accompanying drawings and embodiments. The technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in the field without creative work are within the scope of protection of the present invention.

[0028] It should be noted that all directional indications in the embodiments of the present invention (such as up, down, left, right, front, back, etc.) are only used to explain the relative position relationship, movement status, etc. between the components under a certain specific posture (as shown in the accompanying drawings). If the specific posture changes, the directional indication will also change accordingly.

[0029] In addition, the descriptions of "first", "second", etc. in the present invention are only used for descriptive purposes and cannot be understood as indicating or implying their relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features. In addition, the technical solutions between the various embodiments can be combined with each other, but it must be based on the ability of ordinary technicians in the field to implement. When the combination of technical solutions is contradictory or cannot be implemented, it should be deemed that such a combination of technical solutions does not exist and is not within the scope of protection required by the present invention. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0030] Example 1: Figure 1 As shown, a method for producing phosphoric acid with a low iron ion content of the present invention, the specific steps of the method are as follows:

[0031] S1: transporting liquid yellow phosphorus to a phosphorus burning furnace (transport efficiency of 5.5 tons / hour) to react with oxygen in the air to generate phosphorus pentoxide gas, during which a phosphorus spray gun is used to assist combustion;

[0032] S2: The phosphorus pentoxide gas after combustion is transported to the hydration absorption tower by negative pressure, and is absorbed by the low-concentration phosphoric acid circulated by the concentrated acid pump to generate crude phosphoric acid. The crude phosphoric acid reacts with hydrogen sulfide and is filtered through the semi-finished product filter before entering the transfer tank;

[0033] S3: When the specific gravity of phosphoric acid in the hydration absorption tower meets the process requirements, the arsenic removal operation is carried out. The crude phosphoric acid is fully mixed with the gas purifier in the pipeline mixer and enters the arsenic removal reactor, then enters the precipitator, and then overflows from the precipitator to the arsenic removal filter, and then enters the degassing tower to remove excess gas purifier. After degassing, it enters the adjustment tower to adjust the concentration to obtain the finished product.

[0034] In the application of this embodiment, the air pressure of the phosphorus spray gun in S1 is stabilized between 400 kPa and 500 kPa.

[0035] In the application of this embodiment, S2 specifically requires controlling the wall temperature of the hydration absorber to be lower than 40°C, the inlet temperature of the first-stage venturi to be lower than 70°C, the inlet temperature of the circulating cooling water to be lower than 38°C, the bottom acid temperature of the hydration absorber to be controlled between 75 and 80°C, and the acid specific gravity of the hydration absorber to be controlled between 1.60 and 1.71.

[0036] In the application of this embodiment, the pressure of the gas purifier in S3 is between 40 and 50 kPa, and the degassing pressure of the degassing tower is between 2 kPa and 5 kPa.

[0037] In the application of this embodiment, a compressed air bag is used in S1 and the air is drained once every hour to ensure that there is no condensed water and other iron filings impurities in the air.

[0038] In the application of this embodiment, process water is added to the hydration absorption tower in S2 to carry out hydration reaction, and the process water uses demineralized water that has been treated with anion and cation exchange.

[0039] In the application of this embodiment, the conductivity of the desalted water is less than 30 us / cm, the sodium ion is less than 100 ppb, and the iron ion is less than 30 ppb.

[0040] In the application of this embodiment, the liquid level of the tower is adjusted to be less than 80% in S3.

[0041] In the application of this embodiment, the low-concentration phosphoric acid circulated by the concentrated acid pump in S3 is also a circulating acid. When the iron ion content of the circulating acid is less than 150 ppb, the circulating acid and the gas purifier are fully mixed in the pipeline mixer and then enter the arsenic removal reactor;

[0042] The product after arsenic removal in S3 is a semi-finished product. When the circulating acid iron ion is less than 150 ppb, the semi-finished product after arsenic removal does not pass through the degassing tower, but directly enters the adjustment tower from the transfer tank for primary degassing, and then is transported to the adjustment tower for secondary degassing.

[0043] In the application of this embodiment, the semi-finished product is a colorless, transparent, viscous liquid, and the parameters of the semi-finished product include hydrated absorption tower acid specific gravity of 1.690-1.71, sulfate ≤10ppm, arsenic ≤0.05ppm, iron ion ≤200ppb, easily oxidizable material ≤50ppm and deionized water conductivity ≤10us / cm.

[0044] Embodiment 2:

[0045] The liquid yellow phosphorus is transported to the phosphorus burning furnace to react with oxygen in the air to generate phosphorus pentoxide gas, during which a phosphorus spray gun is used to assist the combustion, and the air pressure of the phosphorus spray gun is stabilized between 400 kPa and 500 kPa;

[0046] The phosphorus pentoxide gas after combustion is transported to the hydration absorption tower by negative pressure, and is absorbed by the low-concentration phosphoric acid circulated by the concentrated acid pump to generate crude phosphoric acid. The crude phosphoric acid reacts with hydrogen sulfide and is filtered through a semi-finished product filter before entering the transfer tank.

[0047] Specifically, the wall temperature of the hydration absorber is lower than 40°C, the inlet temperature of the primary venturi is lower than 70°C, the inlet temperature of the circulating cooling water is lower than 38°C, the bottom acid temperature of the hydration absorber is controlled between 75 and 80°C, and the specific gravity of the acid in the hydration absorber is controlled between 1.60 and 1.71;

[0048] When the specific gravity of phosphoric acid in the hydration absorption tower meets the process requirements, the arsenic removal operation is carried out. The crude phosphoric acid is fully mixed with the gas purifier in the pipeline mixer and enters the arsenic removal reactor, then enters the precipitator, and then overflows from the precipitator to the arsenic removal filter, and then enters the degassing tower to remove excess gas purifier. After degassing, it enters the adjustment tower to adjust the concentration to obtain the finished product;

[0049] Specifically, the pressure of the gas purifier is between 50 and 60 kPa, and the degassing pressure of the degassing tower is between 2 kPa and 5 kPa.

[0050] Embodiment three:

[0051] The liquid yellow phosphorus is transported to the phosphorus burning furnace to react with oxygen in the air to generate phosphorus pentoxide gas, during which a phosphorus spray gun is used to assist the combustion, and the air pressure of the phosphorus spray gun is stabilized between 400 kPa and 500 kPa;

[0052] The phosphorus pentoxide gas after combustion is transported to the hydration absorption tower by negative pressure, and is absorbed by the low-concentration phosphoric acid circulated by the concentrated acid pump to generate crude phosphoric acid. The crude phosphoric acid reacts with hydrogen sulfide and is filtered through a semi-finished product filter before entering the transfer tank.

[0053] Specifically, the wall temperature of the hydration absorber is lower than 40°C, the inlet temperature of the primary venturi is lower than 70°C, the inlet temperature of the circulating cooling water is lower than 38°C, the bottom acid temperature of the hydration absorber is controlled between 70 and 75°C, and the specific gravity of the acid in the hydration absorber is controlled between 1.60 and 1.71;

[0054] When the specific gravity of phosphoric acid in the hydration absorption tower meets the process requirements, the arsenic removal operation is carried out. The crude phosphoric acid is fully mixed with the gas purifier in the pipeline mixer and enters the arsenic removal reactor, then enters the precipitator, and then overflows from the precipitator to the arsenic removal filter, and then enters the degassing tower to remove excess gas purifier. After degassing, it enters the adjustment tower to adjust the concentration to obtain the finished product;

[0055] Specifically, the pressure of the gas purifier is between 50 and 60 kPa, and the degassing pressure of the degassing tower is between 2 kPa and 5 kPa.

[0056] Embodiment 4:

[0057] The liquid yellow phosphorus is transported to the phosphorus burning furnace to react with oxygen in the air to generate phosphorus pentoxide gas, during which a phosphorus spray gun is used to assist the combustion, and the air pressure of the phosphorus spray gun is stabilized between 400 kPa and 500 kPa;

[0058] The phosphorus pentoxide gas after combustion is transported to the hydration absorption tower by negative pressure, and is absorbed by the low-concentration phosphoric acid circulated by the concentrated acid pump to generate crude phosphoric acid. The crude phosphoric acid reacts with hydrogen sulfide and is filtered through a semi-finished product filter before entering the transfer tank.

[0059] Specifically, the wall temperature of the hydration absorber is lower than 40°C, the inlet temperature of the primary venturi is lower than 70°C, the inlet temperature of the circulating cooling water is lower than 38°C, the bottom acid temperature of the hydration absorber is controlled between 65 and 70°C, and the specific gravity of the acid in the hydration absorber is controlled between 1.60 and 1.71;

[0060] When the specific gravity of phosphoric acid in the hydration absorption tower meets the process requirements, the arsenic removal operation is carried out. The crude phosphoric acid is fully mixed with the gas purifier in the pipeline mixer and enters the arsenic removal reactor, then enters the precipitator, and then overflows from the precipitator to the arsenic removal filter, and then enters the degassing tower to remove excess gas purifier. After degassing, it enters the adjustment tower to adjust the concentration to obtain the finished product;

[0061] Specifically, the pressure of the gas purifier is between 50 and 60 kPa, and the degassing pressure of the degassing tower is between 2 kPa and 5 kPa.

[0062] Embodiment 5: including two improvements

[0063] Improvement point 1: Line the pipeline with fluorine. Line the pipeline from the acid outlet of the heat exchanger to the arsenic removal horizontal tower with fluorine. Change the pipeline from the acid outlet of the heat exchanger → the arsenic removal horizontal tower from two acid pipes (DN50) to one acid pipe (DN80).

[0064] Improvement point 2: Strictly implement the following production process.

[0065] like Figure 1 The process flow chart and table below show:

[0066] Figure 1 Flowchart description: The production involves phosphorus melting section, oxidation section, purification section, and water production section.

[0067] Liquid yellow phosphorus is transported from the yellow phosphorus transfer tank to the phosphorus combustion reaction tower spray gun through the yellow phosphorus transfer pump for quantitative combustion, atomized with compressed air and burned with the support of secondary air to generate P2O5 gas. The by-product steam enters the steam drum above the phosphorus combustion reaction tower, and the steam is then supplied to the production insulation and power generation equipment through the sub-cylinder. Under the action of the exhaust fan suction force, the P2O5 gas enters the hydration absorption tower, hydrates with the sprayed dilute phosphoric acid in the tower, and is absorbed to generate crude phosphoric acid. The crude phosphoric acid reacts with hydrogen sulfide and is filtered through the semi-finished product filter, then enters the 3# transfer tank, and is then transported to the 7# adjustment tower for degassing. The degassed phosphoric acid is transported to the finished product tower through a precision filter.

[0068] In Example 5, the phosphoric acid workshop began tracking and debugging after resuming production on July 15; during the production of electrical acid in the phosphoric acid workshop of the factory, the staff observed the combustion effect of the phosphorus spray gun in the combustion reaction tower (phosphorus burning furnace) to ensure that the yellow phosphorus atomization effect was good during the production of low-iron electrical acid;

[0069] Secondly, the temperature of the air duct is controlled below 680°C by adjusting the frequency of the yellow phosphorus supply pump to ensure that the combustion temperature is within a controllable range.

[0070] The relevant process data control during the commissioning and production of low-iron electrical acid is as follows:

[0071]

[0072]

[0073] The results of circulating acid iron ion detection are as follows:

[0074]

[0075] Debugging conclusion:

[0076] 1. The expected effect was initially achieved on July 25. The difference between the circulating acid iron ion and the finished product was 19ppb, and the difference before fluorine lining was 150ppb, a decrease of about 130ppb.

[0077] 2. Production resumed on July 15, and the electrical acid index requirements were met on July 18, which is 3 days shorter than before.

[0078] Embodiment six:

[0079] In Example 6, the phosphoric acid workshop started tracking and debugging from September 8; in the state of extremely low inventory of yellow phosphorus, the staff ensured that the various process parameters were within the normal range; the temperature of the air guide pipe was controlled below 680°C by adjusting the frequency of the yellow phosphorus supply pump to ensure that the combustion temperature was within the controllable range;

[0080] The relevant process data control situation in the production process of low iron electrical acid is as follows:

[0081]

[0082]

[0083] The test results are shown in the following table:

[0084]

[0085]

[0086] Debugging conclusion:

[0087] 1. In September, it was verified that the circulating acid could not only produce electrical acid without going through the low-aluminate production process line and when the yellow phosphorus inventory was extremely low, but also the difference between the circulating acid iron ion and the finished product was only 31ppb, compared with 150ppb before fluorine lining, a decrease of about 120ppb, achieving the expected effect.

[0088] 2. Electrical acid can still be produced in a short time.

[0089] In summary, the present invention successfully controls the iron ion content in the finished phosphoric acid to below 200 ppb by controlling the acid temperature, improving the heat exchanger pipeline, and lining the heat exchanger pipeline with fluorine, thereby improving the product quality.

[0090] The above embodiments are merely illustrative of the principles and effects of the present invention, and are not intended to limit the present invention. Anyone familiar with the technology may modify or change the above embodiments without violating the spirit and scope of the present invention. Therefore, all equivalent modifications or changes made by those with ordinary knowledge in the technical field without departing from the spirit and technical ideas disclosed by the present invention should still be covered by the claims of the present invention.

Claims

1. A method for producing phosphoric acid with a low iron ion content, characterized in that: The specific steps of the method are as follows: S1: transporting liquid yellow phosphorus to a phosphorus burning furnace to react with oxygen in the air to generate phosphorus pentoxide gas, during which a phosphorus spray gun is used to assist combustion; S2: The phosphorus pentoxide gas after combustion is transported to the hydration absorption tower by negative pressure, and is absorbed by the low-concentration phosphoric acid circulated by the concentrated acid pump to generate crude phosphoric acid. The crude phosphoric acid reacts with hydrogen sulfide and is filtered through the semi-finished product filter before entering the transfer tank; S3: When the specific gravity of phosphoric acid in the hydration absorption tower meets the process requirements, the arsenic removal operation is carried out. The crude phosphoric acid is fully mixed with the gas purifier in the pipeline mixer and enters the arsenic removal reactor, then enters the precipitator, and then overflows from the precipitator to the arsenic removal filter, and then enters the degassing tower to remove excess gas purifier. After degassing, it enters the adjustment tower to adjust the concentration to obtain the finished product.

2. A method for producing phosphoric acid with a low iron ion content according to claim 1, characterized in that: The air pressure of the phosphorus spray gun in S1 is stabilized between 400 kPa and 500 kPa.

3. A method for producing phosphoric acid with a low iron ion content according to claim 1, characterized in that: Specifically, S2 requires controlling the wall temperature of the hydration absorber to be lower than 40°C, the inlet temperature of the first-stage venturi to be lower than 70°C, the inlet temperature of the circulating cooling water to be lower than 38°C, the bottom acid temperature of the hydration absorber to be controlled between 65 and 80°C, and the specific gravity of the acid in the hydration absorber to be controlled between 1.60 and 1.

71.

4. The method for producing phosphoric acid with a low iron ion content according to claim 1, characterized in that: The pressure of the gas purifier in S3 is between 40 and 60 kPa, and the degassing pressure of the degassing tower is between 2 kPa and 5 kPa.

5. The method for producing phosphoric acid with a low iron ion content according to claim 1, characterized in that: The compressed air bag is used in S1 and is drained once every hour to ensure that there is no condensed water and other iron filings impurities in the air.

6. The method for producing phosphoric acid with a low iron ion content according to claim 1, characterized in that: Process water is added to the hydration absorption tower in S2 to carry out hydration reaction, and the process water uses demineralized water treated with anion and cation exchange.

7. A method for producing phosphoric acid with a low iron ion content according to claim 3, characterized in that: The conductivity of the desalted water is less than 30 us / cm, the sodium ion is less than 100 ppb, and the iron ion is less than 30 ppb.

8. The method for producing phosphoric acid with a low iron ion content according to claim 1, characterized in that: In the step S3, the tower liquid level is adjusted to be less than 80%.

9. The method for producing phosphoric acid with a low iron ion content according to claim 1, characterized in that: The low-concentration phosphoric acid circulated by the concentrated acid pump in S3 is also a circulating acid. When the iron ion content of the circulating acid is less than 150 ppb, the circulating acid and the gas purifier are fully mixed in the pipeline mixer and then enter the arsenic removal reactor; The product after arsenic removal in S3 is a semi-finished product. When the circulating acid iron ion is less than 150 ppb, the semi-finished product after arsenic removal does not pass through the degassing tower, but directly enters the adjustment tower from the transfer tank for primary degassing, and then is transported to the adjustment tower for secondary degassing.

10. The method for producing phosphoric acid with a low iron ion content according to claim 9, characterized in that: The semi-finished product is a colorless, transparent, viscous liquid. The parameters of the semi-finished product include a hydrated absorber acid specific gravity of 1.690 to 1.71, sulfate ≤10ppm, arsenic ≤0.05ppm, iron ion ≤200ppb, oxidizable material ≤50ppm and deionized water conductivity ≤10us / cm.

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