Method for producing phosphoric acid

Abstract

To provide a phosphoric acid production method which can produce high-purity phosphoric acid from a waste aqueous phosphoric acid solution containing the Si element.SOLUTION: A phosphoric acid production method includes the steps of: (A) mixing a waste aqueous phosphoric acid solution (X) containing the Si element with hydrogen fluoride to obtain a liquid mixture (Y); (B) crystallizing the liquid mixture (Y) to obtain a liquid mixture (Z) containing a phosphoric acid crystal; and (C) separating phosphoric acid as a solid from the liquid mixture (Z).SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a method for producing phosphoric acid.

Background Art

[0002] Phosphoric acid is produced by oxidizing gaseous phosphorus obtained by reducing phosphate-containing phosphate ore as a raw material in an electric furnace or the like to P 2 O 5 and dissolving it in water, or by a so-called dry method or a wet method of decomposing phosphate ore with sulfuric acid. However, since the demand for phosphate ore has increased and it has become difficult to obtain, the development of a method for producing phosphoric acid using raw materials other than phosphate ore is required.

[0003] Phosphoric acid is widely used in semiconductor manufacturing processes (especially etching processes) and liquid crystal manufacturing processes. In particular, phosphoric acid for semiconductor manufacturing is called electronic industry grade (EL grade), and a quality with a lower amount of impurities such as Si element and higher purity than industrial grade is required. The phosphoric acid used in semiconductor manufacturing processes and liquid crystal manufacturing processes has an increased Si element content after repeated use and eventually becomes unsuitable for use, and is discarded as a waste phosphoric acid aqueous solution.

[0004] Conventionally, attempts have been made to use waste phosphoric acid aqueous solutions from semiconductor factories as raw materials instead of phosphate ore, and to regenerate the phosphoric acid contained in the waste phosphoric acid aqueous solutions to produce phosphoric acid. For example, Patent Document 1 describes "a method for recovering phosphoric acid from a metal-containing mixed acid aqueous solution containing a metal, phosphoric acid, and at least one acid other than phosphoric acid, (1) distilling the mixed acid aqueous solution to distill off acids other than phosphoric acid and water, and (2) melt-crystallizing the residual liquid containing phosphoric acid and water under conditions where phosphoric acid crystallizes, and separating phosphoric acid as a solid and the metal as an aqueous solution, respectively. A method for recovering phosphoric acid from a metal-containing mixed acid aqueous solution." Patent Document 1 describes that according to this method, metals can be separated from metal-containing mixed acid aqueous solutions such as metal-containing acid waste liquids.

Prior Art Documents

Patent Document

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, the inventor has found that the phosphoric acid obtained by the method described in Patent Document 1 does not sufficiently reduce the Si element contained in the etching waste liquid. That is, the phosphoric acid produced by the method described in Patent Document 1 was of a quality that could not be used in semiconductor manufacturing. Therefore, an object of the present invention is to provide a method for producing phosphoric acid capable of producing high-purity phosphoric acid from a waste phosphoric acid aqueous solution containing Si element.

[0007] Based on the background art as described above, an object of the present invention is to provide a method for producing phosphoric acid capable of producing high-purity phosphoric acid from a waste phosphoric acid aqueous solution containing Si element.

Means for Solving the Problems

[0008] The inventors of the present invention have intensively studied to solve the above problems. As a result, they have found that the above problems can be solved by having the following configuration, and have completed the present invention. The present invention relates to, for example, the following [1] to [4]. [1] Step (A) of mixing a waste phosphoric acid aqueous solution (X) containing Si element and hydrogen fluoride to obtain a mixed solution (Y), Step (B) of crystallizing the mixed solution (Y) to obtain a mixed solution (Z) containing phosphoric acid crystals, and Step (C) of separating phosphoric acid as a solid from the mixed solution (Z) A method for producing phosphoric acid, comprising: [2] The method for producing phosphoric acid according to [1], wherein the content of Si element in the waste phosphoric acid aqueous solution (X) is 2 ppm or more. 〔3〕The method for producing phosphoric acid according to 〔1〕 or 〔2〕, wherein the amount of hydrogen fluoride is such that the molar ratio of F element to Si element (F / Si) in the mixed solution (Y) is 5 to 500. 〔4〕The method for producing phosphoric acid according to any one of 〔1〕 to 〔3〕, further including a step (D) of subjecting the liquid obtained after performing the step (C) to steam distillation.

Advantages of the Invention

[0009] According to the present invention, high-purity phosphoric acid can be produced from a waste phosphoric acid aqueous solution containing Si element.

Brief Description of the Drawings

[0010]

Figure 1

Embodiments for Carrying Out the Invention

[0011] Next, the present invention will be specifically described. The description of "A to B" regarding a numerical range means A or more and B or less unless otherwise specified. Also, % means mass %.

[0012] <Method for Producing Phosphoric Acid> One aspect of the present invention is a method for producing phosphoric acid, including a step (A) of mixing a waste phosphoric acid aqueous solution (X) containing Si element and hydrogen fluoride to obtain a mixed solution (Y), a step (B) of crystallizing the mixed solution (Y) to obtain a mixed solution (Z) containing phosphoric acid crystals, and a step (C) of separating phosphoric acid as a solid from the mixed solution (Z). This method for producing phosphoric acid can be rephrased as a method for producing high-purity phosphoric acid, a method for purifying phosphoric acid, or a method for recovering phosphoric acid.

[0013] <Step (A)> Step (A) is a step of mixing a waste phosphoric acid aqueous solution containing Si element (also referred to as waste phosphoric acid aqueous solution (X)) and hydrogen fluoride to obtain a mixed solution (also referred to as mixed solution (Y)).

[0014] In the case where the waste phosphoric acid aqueous solution (X) contains phosphoric acid and Si element, their content ratios are not limited as long as they are contained. Examples of the waste phosphoric acid aqueous solution (X) include waste liquids containing phosphoric acid generated in semiconductor manufacturing processes or liquid crystal manufacturing processes, and waste liquids containing phosphoric acid generated in etching processes for semiconductor manufacturing or liquid crystal manufacturing are preferred.

[0015] Phosphoric acid means orthophosphoric acid (H 3 PO 4 ). Phosphoric acid is dissolved in water in the waste phosphoric acid aqueous solution (X). The phosphoric acid contained in the waste phosphoric acid aqueous solution (X) may be condensed phosphoric acid. When the phosphoric acid contained in the waste phosphoric acid aqueous solution (X) is condensed phosphoric acid, the content ratio of phosphoric acid in the waste phosphoric acid aqueous solution (X) is calculated by converting the condensed phosphoric acid into orthophosphoric acid (H 3 PO 4 ). The content ratio of phosphoric acid in the waste phosphoric acid aqueous solution (X) is preferably 75 to 95% by mass, more preferably 80 to 90% by mass. When the content ratio of phosphoric acid in the waste phosphoric acid aqueous solution (X) is not within the above range, it is preferably concentrated or diluted by a known method such as vacuum concentration or atmospheric pressure concentration so as to be within the above range before use. When the content ratio of phosphoric acid in the waste phosphoric acid aqueous solution (X) is within the above range, the effects of the present invention are more likely to be exerted.

[0016] The chemical form and crystal system of the Si element are not limited. For example, it may be contained in the waste phosphoric acid aqueous solution (X) as a single crystal or polycrystal of Si, or as a Si-containing compound such as SiO 2 , Si 3 N 4 , H 2 SiF 6 , Si(OH) 4 . The Si element is preferably contained in the waste phosphoric acid aqueous solution (X) as SiO 2 . The chemical form and crystal system of the Si element may be one kind or two or more kinds.

[0017] The content rate of Si element in the waste phosphoric acid aqueous solution (X) is preferably 2 ppm or more, more preferably 5 ppm or more, still more preferably 10 ppm or more. The upper limit of the content rate of Si element in the waste phosphoric acid aqueous solution (X) is not limited, but is preferably 1000 ppm or less, more preferably 200 ppm or less, still more preferably 100 ppm or less. In the prior art, the higher the content rate of Si element, the more difficult it is to remove the Si element from the waste phosphoric acid aqueous solution. However, when the content rate of Si element in the waste phosphoric acid aqueous solution (X) is within the above range, the effects of the present invention are more likely to be exerted.

[0018] The waste phosphoric acid aqueous solution (X) may contain an acid other than phosphoric acid, a metal element, and / or a metalloid element other than Si element. Examples of the acid other than phosphoric acid include acetic acid, nitric acid, hydrochloric acid, sulfuric acid, hydrofluoric acid, etc. Examples of the metal element include metal elements to be etched (e.g., Cu, Al, Fe, Mo, Cr, Ag, etc.). Examples of the metalloid element other than Si element include metalloid elements used in semiconductors (e.g., B, As, Ge, Sb, Te, Se, Po, At). These may be one kind or two or more kinds.

[0019] The content rates of the acid other than phosphoric acid, the metal element, and the metalloid element other than Si element in the waste phosphoric acid aqueous solution (X) are in a range that does not inhibit the effects of the present invention. For example, the content rate of the acid other than phosphoric acid in the waste phosphoric acid aqueous solution (X) is, for example, 10% by mass or less, and also, for example, 1% by mass or more. The content rate of the metal element in the waste phosphoric acid aqueous solution (X) is, for example, 1% by mass or less, and also, for example, 0.01% by mass or more. The content rate of the metalloid element other than Si element in the waste phosphoric acid aqueous solution (X) is, for example, 1% by mass or less, and also, for example, 0.01% by mass or more.

[0020] The temperature of the waste phosphoric acid aqueous solution (X) is not particularly limited. For example, it is 0°C or higher, preferably 10°C or higher, and also, for example, 40°C or lower, preferably 30°C or lower.

[0021] Hydrogen fluoride may be gaseous or liquid hydrogen fluoride at normal temperature, or it may be an aqueous solution of hydrogen fluoride (hydrofluoric acid, hydrofluoric acid). As hydrogen fluoride, hydrofluoric acid containing about 46 to 53% by mass of hydrogen fluoride is preferred.

[0022] The method of mixing the waste phosphoric acid aqueous solution (X) and hydrogen fluoride to obtain the mixed solution (Y) is not particularly limited, and known methods such as stirring and mixing, pump circulation mixing, and pipeline mixing can be used. The mixed solution (Y) is preferably stirred for 1 to 60 minutes, more preferably 10 to 20 minutes, after mixing the waste phosphoric acid aqueous solution (X) and hydrogen fluoride.

[0023] The quantitative ratio of the waste phosphoric acid aqueous solution (X) and hydrogen fluoride to be mixed is not particularly limited, and the waste phosphoric acid aqueous solution (X) and hydrogen fluoride may be mixed at any quantitative ratio. The amount of hydrogen fluoride to be mixed is preferably such that the molar ratio of F element to Si element (F / Si) in the mixed solution (Y) is 5 to 500, more preferably 10 to 100, and even more preferably 15 to 50. When F / Si in the mixed solution (Y) is within the above range, phosphoric acid with low contents of Si element and F element can be obtained. In particular, when F / Si in the mixed solution (Y) is equal to or higher than the above lower limit value, the Si element contained in the phosphoric acid (α) described later tends to be less. Also, when F / Si in the mixed solution (Y) is equal to or lower than the above upper limit value, the F element contained in the phosphoric acid (α) described later tends to be less. In addition, when the waste phosphoric acid aqueous solution (X) contains hydrogen fluoride, the number of moles of F element in F / Si is the total value of the F element of hydrogen fluoride contained in the waste phosphoric acid aqueous solution (X) and the F element of hydrogen fluoride added to obtain the mixed solution (Y).

[0024] <Step (B)> Step (B) is a step of crystallizing the mixed solution (Y) to obtain a mixed solution (Z) containing crystals of phosphoric acid. In other words, it is a step of precipitating phosphoric acid as crystals in the mixed solution (Y). The mixed solution (Z) is a crystal slurry (suspension) containing crystals of phosphoric acid and a liquid.

[0025] Since crystallization in step (B) can efficiently obtain highly pure phosphoric acid, it is usually carried out by a method (cooling method) of lowering the solubility of phosphoric acid by cooling to precipitate crystals.

[0026] In the case of the cooling method, in step (B), the mixed liquid (Y) is cooled from room temperature until the temperature at which phosphoric acid crystallizes is reached, and that temperature is maintained. The cooling method is not particularly limited, and it may be cooled by a known method. For example, it can be cooled by bringing the container containing the mixed liquid (Y) into contact with a refrigerant maintained at 0°C to 15°C.

[0027] The temperature reached by cooling is not particularly limited as long as it is the temperature at which phosphoric acid crystallizes. From the viewpoints of yield and purification efficiency, the temperature reached by cooling is preferably 0°C to 20°C, more preferably 5°C to 15°C. Note that the temperature reached by cooling is the liquid temperature of the mixed liquid (Y).

[0028] The cooling rate is not particularly limited. The holding time at the temperature at which phosphoric acid crystallizes is not particularly limited as long as it is sufficient time for phosphoric acid to crystallize. The holding time at the temperature at which phosphoric acid crystallizes can be appropriately adjusted according to temperature conditions and the scale of the apparatus, and is, for example, 60 minutes to 600 minutes.

[0029] In step (B), seed crystals may be added to the mixed liquid (Y) or the mixed liquid (Z). Adding seed crystals can promote the crystallization of phosphoric acid. The seed crystals may be phosphoric acid crystals, and the size, shape, purity, etc. are not limited. The addition amount of the seed crystals is not particularly limited, but since the influence of the purity of the seed crystals on the purity of the obtained phosphoric acid is small, it is preferably 0.0001% by mass to 1.0% by mass, more preferably 0.001% by mass to 0.1% by mass, still more preferably 0.01% by mass to 0.1% by mass, based on the mixed liquid (Y) or the mixed liquid (Z). When adding seed crystals in the cooling method, it is preferable to add the seed crystals to the mixed solution (Y) or the mixed solution (Z) after the temperature of the mixed solution (Y) or the mixed solution (Z) has reached a temperature suitable for the crystallization of phosphoric acid or lower. The addition temperature of the seed crystals is preferably 15°C or lower, more preferably 10°C or lower.

[0030] <Step (C)> Step (C) is a step of separating phosphoric acid from the mixed solution (Z) as a solid. By step (C), phosphoric acid crystals can be obtained as a solid from the crystal slurry (suspension) containing phosphoric acid crystals and liquid. The solid obtained by step (C) is also referred to as phosphoric acid (α). The separation method is not particularly limited as long as phosphoric acid can be separated from the mixed solution (Z) as a solid, and known methods such as centrifugation and filtration can be used.

[0031] After performing step (C), a liquid (hereinafter also referred to as liquid (α)) is obtained. Liquid (α) is the liquid obtained after separating phosphoric acid from the mixed solution (Z) as a solid, and is the liquid component of the mixed solution (Z), that is, the liquid component of the mixed solution (crystal slurry) containing phosphoric acid crystals.

[0032] Steps (B) and (C) may be repeated. That is, in step (B), instead of the mixed solution (Y), molten phosphoric acid (α) or an aqueous solution of phosphoric acid (α) may be crystallized to obtain a mixed solution (Z) containing phosphoric acid crystals. From the perspective of good handling, in step (B), it is preferable to crystallize an aqueous solution of phosphoric acid (α) instead of the mixed solution (Y) to obtain a mixed solution (Z) containing phosphoric acid crystals. The content of phosphoric acid in the aqueous solution of phosphoric acid (α) is preferably 75 to 95% by mass, more preferably 80 to 90% by mass. The phosphoric acid obtained by repeating steps (B) and (C) is hereinafter also referred to as phosphoric acid (β). Since phosphoric acid (β) is obtained by recrystallizing phosphoric acid, the amount of F element derived from hydrogen fluoride tends to be reduced. The number of repetitions of steps (B) and (C) is not particularly limited.

[0033] Phosphoric acid (α) has a significantly reduced Si element content compared to the waste phosphoric acid aqueous solution (X) through steps (A) to (C). Phosphoric acid (α) preferably has an Si content of 2 ppm or less, more preferably 1 ppm or less, and even more preferably less than 1 ppm. Since phosphoric acid (α) has a very low Si element content, it can be used as an electronic industry grade (EL grade) phosphoric acid for semiconductor manufacturing and the like.

[0034] Although not bound by theory, the mechanism by which the Si element content is significantly reduced in phosphoric acid (α) is speculated as follows. For example, when the waste phosphoric acid aqueous solution (X) contains a large amount of silicon dioxide (SiO 2 ), by mixing the waste phosphoric acid aqueous solution (X) with hydrogen fluoride, the fluorine of hydrogen fluoride and silicon dioxide form hexafluorosilicate ions ([SiF 6 ( 2- )) and the silicon dioxide particles dissolve and ionize, making them more likely to remain in the aqueous solution. Therefore, it is speculated that the Si element is hardly contained in the phosphoric acid (α) after crystallization and remains in the liquid (α).

[0035] <Other steps> The method for producing phosphoric acid of the present invention may include any of the following steps (D), (E), and / or (F) in addition to steps (A), (B), and (C).

[0036] (Step (D)) Step (D) is a step of dissolving the solid obtained in step (C) in water to form an aqueous solution and filtering the aqueous solution. The filtration method is not particularly limited, and known methods can be used. The solid obtained in step (C), that is, the phosphoric acid crystals, contain solid impurities (particles, e.g., SiO 2, although metal fine particles may remain, step (D) makes it easier to remove such impurities. Therefore, the method for producing phosphoric acid of the present invention preferably includes step (A), step (B), step (C), and step (D). According to this production method, it is easy to obtain high-purity phosphoric acid with a reduced Si element content rate compared to phosphoric acid (α).

[0037] (Step (E)) Step (E) is a step of subjecting the liquid obtained after performing step (C) to steam distillation, in other words, a step of steam-distilling liquid (α). The method of steam distillation is not particularly limited, and known methods can be used. Steam distillation may be performed, for example, by blowing steam into liquid (α) for a predetermined time and then decomposing the liquid components, or by so-called stripping in which liquid (α) is sprayed and steam is blown from below it.

[0038] The time for performing steam distillation is not particularly limited, but the time for blowing steam is preferably 1 hour or more, more preferably 4 hours or more. The longer the time for blowing steam, the easier it is to further reduce the content rate of F element and / or Si element in phosphoric acid (γ) described later. From the viewpoint of reducing production costs, the time for blowing steam can be, for example, 8 hours or less.

[0039] Although the content rate in liquid (α) is lower than that in the waste phosphoric acid aqueous solution (X), since it contains phosphoric acid, by steam-distilling liquid (α), it is possible to obtain phosphoric acid with a significantly reduced Si element content rate compared to the waste phosphoric acid aqueous solution (X). Therefore, the method for producing phosphoric acid according to the present invention preferably includes step (A), step (B), step (C), and step (E). The phosphoric acid produced by this production method is hereinafter also referred to as phosphoric acid (γ). Although phosphoric acid (γ) has a higher Si element content than phosphoric acid (α), since the Si element content is sufficiently reduced, it can be used as industrial-grade phosphoric acid. Further, according to this production method, since phosphoric acid (α) and phosphoric acid (γ) can be recovered from both the solid and the liquid after crystallization, most of the phosphoric acid contained in the waste phosphoric acid aqueous solution (X) can be reproduced as high-grade phosphoric acid.

[0040] When the method for producing phosphoric acid according to the present invention includes step (A), step (B), step (C), and step (E), a step of mixing the liquid obtained after performing step (C) and hydrogen fluoride may be included between step (C) and step (E). By this step, it becomes easier to further reduce the content of the Si element in phosphoric acid (γ). The quantitative ratio of the liquid obtained after performing step (C) and hydrogen fluoride is not particularly limited. As the molar ratio (F / Si) of the F element to the Si element in the liquid after mixing the liquid obtained after performing step (C) and hydrogen fluoride, 6.0 or more is preferable, 6.0 to 100.0 is more preferable, and 8.0 to 50.0 is even more preferable.

[0041] (Step (F)) Step (F) is a step of purifying and / or concentrating the liquid obtained in step (E). The method of purification is not particularly limited, and known methods can be used. Examples of the purification method include a method of adsorbing organic substances on activated carbon. The method of concentration is not particularly limited, and known methods can be used. Examples of the concentration method include atmospheric pressure concentration, reduced pressure concentration, and a method by multiple-effect evaporation. Among these, reduced pressure concentration is preferable. The conditions for reduced pressure concentration are not limited as long as phosphoric acid can be concentrated. For example, it can be performed under reduced pressure to 1 kPa to 100 kPa at a temperature condition of 50°C to 180°C. More specific conditions can be exemplified by 93°C and 9 kPa, for example.

[0042] By step (F), the phosphoric acid content in the liquid obtained in step (E) can be increased. Therefore, the method for producing phosphoric acid of the present invention preferably includes step (A), step (B), step (C), step (E), and step (F). According to this production method, high-purity phosphoric acid with a phosphoric acid content higher than that of phosphoric acid (γ) can be obtained. Similar to phosphoric acid (γ), this phosphoric acid has a higher Si element content than phosphoric acid (α), but the Si element content is sufficiently reduced, so it can be used as industrial-grade phosphoric acid.

Examples

[0043] Next, examples of the present invention will be shown and described in more detail, but the present invention is not limited by these.

[0044] <Example 1> To 2,700 g of an 85% phosphoric acid aqueous solution (Shimonoseki Mitsui Chemicals, Inc., 85% purified phosphoric acid), 4.2 g of activated silica (SiO 2 )(ICN activated silica manufactured by Fujifilm Wako Pure Chemical Corporation, product number: 590-31265) was added and heated and dissolved in a dryer at 180°C. After adding ultrapure water and diluting and adjusting to a phosphoric acid content of about 85%, the insoluble matter was removed with a cartridge filter to obtain a Si-containing phosphoric acid solution with a Si element content of 28 ppm and containing no F element. This was designated as a Si-high-containing phosphoric acid aqueous solution A simulating a waste phosphoric acid aqueous solution. 400.0 g of the Si-high-containing phosphoric acid aqueous solution A was placed in a container, and hydrofluoric acid (HF, hydrofluoric acid) (manufactured by Fujifilm Wako Pure Chemical Corporation, product number: 082-03525) was further added to obtain a mixed solution. The addition amount of hydrofluoric acid was determined by previously calculating the weight such that the molar ratio F / Si of F element to Si element in the mixed solution was about 30. Also, the weight of the actually added hydrofluoric acid was measured, and based on that value, F / Si was calculated. The mixed solution was stirred and mixed for 15 minutes using a stirrer. Next, the mixed solution was cooled in a constant temperature bath at 5°C. When the liquid temperature reached about 7°C, 0.3 g of seed crystals (2 to 4 pieces of about 0.1 g with a length of 2 cm and a width of 0.5 cm) were added, and then it was cooled in the constant temperature bath at 5°C for 4 hours for crystallization to obtain a mixed solution (crystal slurry) α containing phosphoric acid crystals.

[0045] 400 g of the crystal slurry α was centrifuged for 2 minutes for separation, and the solid α containing crystals and the liquid α were recovered respectively. A part of the obtained solid α was heated and melted at 40°C, diluted with ultrapure water so that the phosphoric acid content rate became 85% by mass, and then filtered through a cartridge filter to obtain an aqueous phosphoric acid solution α. A part of the obtained solid α was heated and melted at 40°C, and the content rates of phosphoric acid, Si element, and F element were analyzed respectively.

[0046] The content rate of phosphoric acid was analyzed by colorimetry (ammonium vanadomolybdate method: colorimetric determination method using a color developing solution obtained by adding nitric acid and ammonium molybdate to ammonium metavanadate), the content rate of Si element was analyzed by ICP mass spectrometry (Agilent: Agilent 7900 ICP-MS), and the content rate of F element was analyzed by ion electrode method (HORIBA LAQUA fluoride ion electrode).

[0047] <Comparative Example 1> An experiment similar to that of Example 1 was conducted except that hydrofluoric acid was not added to the high-Si-containing aqueous phosphoric acid solution A. <Comparative Example 2> An experiment similar to that of Example 1 was conducted except that sulfuric acid (manufactured by Kanto Chemical Co., Inc., sulfuric acid (1+1) product number: 37928-02) was added to the high-Si-containing aqueous phosphoric acid solution A so that the molar ratio S / Si of S element to Si element in the mixed solution was about 30 instead of hydrofluoric acid. <Comparative Example 3> An experiment similar to that of Example 1 was conducted except that nitric acid (manufactured by Kanto Chemical Co., Inc., nitric acid Ultrapur product number: 28163-1B) was added to the high-Si-containing aqueous phosphoric acid solution A so that the molar ratio N / Si of N element to Si element in the mixed solution was about 30 instead of hydrofluoric acid.

[0048] <Example 2> The Si source added to the 85% phosphoric acid aqueous solution was changed from active silica (SiO 2 ) to silicon nitride (Si 3 N 4 )(manufactured by High Purity Chemical Research Institute Co., Ltd., silicon nitride product number: SII09PB), and the resulting solution was designated as Si-high-content phosphoric acid aqueous solution B. An experiment similar to Example 1 was conducted, except that Si-high-content phosphoric acid aqueous solution B was used instead of Si-high-content phosphoric acid aqueous solution A.

[0049] <Comparative Examples 4 to 6> An experiment similar to Comparative Examples 1 to 3 was conducted, except that Si-high-content phosphoric acid aqueous solution B was used instead of Si-high-content phosphoric acid aqueous solution A, and the resulting examples were designated as Comparative Examples 4 to 6, respectively.

[0050] <Examples 3 to 5, Comparative Examples 7 to 9> The amount of active silica (SiO 2 ) added to the 85% phosphoric acid aqueous solution and the heating and dissolution time were changed to obtain Si-high-content phosphoric acid aqueous solutions C, D, and E with Si contents of 33, 36, and 59 ppm, respectively. An experiment similar to Example 1 was conducted, except that Si-high-content phosphoric acid aqueous solutions C, D, and E were used instead of Si-high-content phosphoric acid aqueous solution A, and the resulting examples were designated as Examples 3 to 5, respectively. An experiment similar to Comparative Example 1 was conducted, except that Si-high-content phosphoric acid aqueous solutions C, D, and E were used instead of Si-high-content phosphoric acid aqueous solution A, and the resulting examples were designated as Comparative Examples 7 to 9, respectively.

[0051] <Examples 6, 7> An experiment similar to Example 1 was conducted, except that the addition amount of hydrofluoric acid (HF) was changed so that F / Si was 10.9 and 22.1, respectively, and the resulting examples were designated as Examples 6 and 7.

[0052] <Example 8> 164 g of solid α obtained in the same manner as in Example 1 was heated to 40 °C to be melted and then cooled in a constant temperature bath at 5 °C. When the liquid temperature reached about 7 °C, 0.3 g of seed crystals (2 to 4 pieces of about 0.1 g with a length of 2 cm and a width of 0.5 cm) were added, and cooling and crystallization were carried out in a constant temperature bath at 5 °C to recrystallize phosphoric acid, thereby obtaining a mixed liquid (crystal slurry) β containing crystals of phosphoric acid. 164 g of crystal slurry β was centrifuged for 2 minutes for separation, and solid β and liquid β were recovered respectively. In the same manner as in Example 1, the content rates of phosphoric acid, Si element, and F element in solid β were analyzed.

[0053] For Examples 1 to 8 and Comparative Examples 1 to 9, the experimental conditions and the content rates of phosphoric acid, Si element, and F element in solid α or solid β are shown in Table 1. In Table 1, the yield of phosphoric acid is a value obtained by dividing the amount of phosphoric acid in the obtained solid α or solid β by the amount of phosphoric acid before the crystallization step.

[0054]

Table 1

[0055] From Table 1, the solid α or solid β obtained in Examples 1 to 8 contained phosphoric acid at a higher content rate compared to the Si-high-containing phosphoric acid aqueous solution, and the content rate of the Si element was significantly reduced. In Examples 1 to 8, high-purity phosphoric acid was obtained.

[0056] <Example 9> After putting about 50 g of liquid α obtained in the same manner as in Example 1 into a container, its weight was precisely measured. Then, as shown in FIG. 1, pure water was heated with a mantle heater, and the water vapor generated by boiling was blown into the liquid α in the container for stripping. The heating temperature, the amount of water vapor blown in, and the blowing time were as described in Table 2. Then, the remaining liquid γ in the container was recovered, and the content rates of phosphoric acid, Si element, and F element were analyzed.

[0057] <Example 10> Except for changing the stripping conditions as described in Table 2, stripping of liquid α was performed in the same manner as in Example 9, and each content rate was measured in the same manner as in Example 9.

[0058] <Example 11> Hydrofluoric acid was added to the liquid α obtained in Example 1 to change the content rate of F element to the amount described in Table 2. Except for changing the stripping conditions as described in Table 2, stripping of liquid α was performed in the same manner as in Example 9, and each content rate was measured in the same manner as in Example 9.

[0059] Regarding Examples 9 to 11, the weight of liquid α in the container before stripping, the content rates of phosphoric acid, Si, F, F / Si, the weight of liquid γ in the container after stripping, the content rates of phosphoric acid, Si element, and F element, and the stripping conditions are shown in Table 2.

[0060]

Table 2

[0061] From Table 2, the liquid γ obtained in Examples 9 to 11 contained phosphoric acid at a higher content rate compared to liquid α, and the content rates of Si element and F element were significantly reduced. In Examples 9 to 11, high-purity phosphoric acid was obtained.

Explanation of Symbols

[0062] 1 Pure water 2 Mantle heater 3 Oil bath 4 Liquid (α)

Claims

1. Step (A) of mixing a waste phosphoric acid aqueous solution (X) containing Si element and hydrogen fluoride to obtain a mixed solution (Y), Step (B) of crystallizing the mixed solution (Y) to obtain a mixed solution (Z) containing crystals of phosphoric acid, and Step (C) of separating phosphoric acid as a solid from the mixed solution (Z) are included, the content of Si element in the waste phosphoric acid aqueous solution (X) is 59 ppm or less, A method for producing phosphoric acid, wherein the content of Si element in the solid obtained by step (C) is 2 ppm or less.

2. The method for producing phosphoric acid according to claim 1, wherein the content of Si element in the waste phosphoric acid aqueous solution (X) is 2 ppm or more.

3. The method for producing phosphoric acid according to claim 1, wherein the amount of the hydrogen fluoride is such that the molar ratio (F / Si) of F element to Si element in the mixed solution (Y) is 5 to 500.

4. The method for producing phosphoric acid according to any one of claims 1 to 3, further including step (D) of steam-distilling the liquid obtained after performing step (C).

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