Iron phosphate production method

Abstract

Provided is an iron phosphate production method with which iron phosphate can be easily obtained. In a precipitation formation step (S1), an alkali is added to a phosphorus-containing liquid to obtain a precipitate. In a dissolution step (S2), the precipitate is dissolved with an acid to obtain a phosphate solution. In a re-precipitation formation step (S3), an alkali is added to the phosphate solution to obtain a precipitate again. In the re-precipitation formation step (S3), the pH of the phosphate solution is adjusted to be less than 4.0. In the re-precipitation formation step (S3), the pH of the phosphate solution is preferably adjusted such that the purity of iron phosphate in the obtained precipitate is at least 90 mass%. It is preferable to repeat the dissolution step (S2) and the re-precipitation formation step (S3).

Description

Iron phosphate manufacturing method

[0001] The present invention relates to a method for producing iron phosphate.

[0002] While phosphorus is used in a wide range of fields, including agriculture, food, medicine, and industry, it is a resource that is unevenly distributed around the world, with producing countries limited to China, the United States, Morocco, etc. The phosphorus resources distributed in Japan are phosphorus products such as yellow phosphorus and crude phosphoric acid, as well as phosphate rock, which is the raw material for these products, and the country is entirely dependent on imports.

[0003] The phosphorus concentration of molten pig iron tapped from a blast furnace is about 0.1 mass %. The phosphorus concentration in steelmaking slag produced by subjecting this molten pig iron to conventional treatments (dephosphorization treatment, decarburization refining) is about 0.1 mass %. ２ O ５ The phosphorus content in steelmaking slag is low, at most about 5% by mass, calculated as a percentage of the total phosphorus content. For this reason, steelmaking slag has been used as a civil engineering material such as a roadbed material, and the phosphorus in steelmaking slag has not been recovered. However, in recent years, the price of phosphorus resources has risen sharply due to the depletion of phosphate rock and the monopolization of phosphate rock by China, the United States, and other countries. Therefore, the phosphorus in steelmaking slag generated during steel smelting is being reconsidered as a valuable phosphorus resource.

[0004] Patent Document 1 discloses a method for producing (recovering) a phosphorus compound using dephosphorized slag, which is a type of steelmaking slag, as a starting material.

[0005] Japanese Patent Application Laid-Open No. 2022-150640

[0006] As a phosphorus compound to be produced (recovered) from a starting material such as steelmaking slag, for example, iron phosphate is desired. However, the method described in Patent Document 1 involves many processes such as cation exchange, and therefore requires a long time and is complicated to obtain the target phosphorus compound. In addition, in order to produce iron phosphate using the method described in Patent Document 1, it is essentially necessary to add an additional raw material containing iron.

[0007] The present invention has been made in view of the above points, and an object of the present invention is to provide a method for producing iron phosphate that allows iron phosphate to be obtained simply and easily.

[0008] As a result of extensive research, the present inventors have found that the above object can be achieved by adopting the following configuration, and have completed the present invention. That is, the present invention provides the following [1] to [8]. [1] A method for producing iron phosphate, comprising: a precipitation step of adding an alkali to a phosphorus-containing liquid to obtain a precipitate; a dissolution step of dissolving the precipitate with an acid to obtain a phosphoric acid liquid; and a reprecipitation step of adding an alkali to the phosphoric acid liquid to obtain another precipitate, wherein the pH of the phosphoric acid liquid is adjusted to less than 4.0 in the reprecipitation step. [2] A method for producing iron phosphate according to [1] above, wherein the pH of the phosphoric acid liquid is adjusted in the reprecipitation step so that the purity of iron phosphate in the obtained precipitate is 90% by mass or more. [3] A method for producing iron phosphate according to [1] above, wherein the dissolution step and the reprecipitation step are repeated. [4] A method for producing iron phosphate according to [3] above, wherein the dissolution step and the reprecipitation step are repeated until the purity of iron phosphate in the precipitate obtained in the reprecipitation step reaches a threshold value or higher. [5] The method for producing iron phosphate according to [3] above, wherein the dissolving step and the re-precipitation step are repeated depending on the phosphorus and iron contents in the phosphorus-containing liquid. [6] The method for producing iron phosphate according to any of [1] to [5] above, further comprising a phosphorus-containing liquid preparation step before the precipitation step, in which components contained in a phosphorus-containing starting material are leached in acid to obtain the phosphorus-containing liquid. [7] The method for producing iron phosphate according to [6] above, wherein the starting material is at least one phosphorus-containing compound selected from the group consisting of steelmaking slag, sewage sludge, and phosphorus-containing waste liquid. [8] The method for producing iron phosphate according to any of [1] to [7] above, wherein the phosphorus-containing liquid contains at least the elements phosphorus and iron.

[0009] According to the present invention, a method for producing iron phosphate that allows iron phosphate to be obtained simply and easily can be provided.

[0010] 1 is a flowchart showing the flow of a method for producing iron phosphate according to the present embodiment; 2 is a graph showing the relationship between the pH of a phosphoric acid solution and the transfer rate of each element from the phosphoric acid solution to a precipitate; and 3 is a graph showing the relationship between the pH of a phosphoric acid solution and the transfer rate of each element from the phosphoric acid solution to a precipitate.

[0011] [Method for Producing Iron Phosphate] The method for producing iron phosphate of this embodiment will be described with reference to Figures 1 to 3. Figure 1 is a flowchart showing the flow of the method for producing iron phosphate of this embodiment. The method for producing iron phosphate of this embodiment is, in outline, a method for recovering a phosphorus compound containing iron phosphate (a calcined product obtained by calcining a precipitate described below) from a phosphorus-containing liquid.

[0012] As shown in Fig. 1, the method for producing iron phosphate of this embodiment includes a precipitation step (S1), a dissolution step (S2), and a re-precipitation step (S3) in this order. Also, as shown in Fig. 1, the method for producing iron phosphate of this embodiment may include a phosphorus-containing liquid preparation step (S0) before the precipitation step (S1). Each step in the method for producing iron phosphate of this embodiment will be described below.

[0013] <Phosphorus-containing liquid preparation step (S0)> In the phosphorus-containing liquid preparation step (S0), a phosphorus-containing liquid is prepared. For example, components contained in the starting material are leached in acid to obtain an acidic phosphorus-containing liquid. In this case, more specifically, the starting material is added to the acid, stirred, and then filtered. This yields a residue (I) and a leachate (A). The obtained leachate (A) is the phosphorus-containing liquid.

[0014] The acid used in the phosphorus-containing solution preparation step (S0) is not particularly limited, but may be at least one selected from the group consisting of nitric acid, hydrochloric acid, sulfuric acid, phosphoric acid, and citric acid, with nitric acid or hydrochloric acid being preferred, and nitric acid being more preferred. The concentration of the acid is not particularly limited, but is preferably 0.1 to 1.5 M, and more preferably 0.3 to 0.8 M.

[0015] The starting material is a phosphorus-containing compound (including a mixture of phosphorus-containing compounds with other compounds), such as CaO, P ２ O ５ , Al２ O ３ and Fe ｔ However, considering that iron phosphate is ultimately recovered, the starting material contains at least P ２ O ５ and Fe ｔ It is preferable that the starting material (phosphorus-containing compound) contains O. Specific examples of such starting materials (phosphorus-containing compounds) include steelmaking slag such as dephosphorization slag and decarburization slag; sewage sludge; phosphoric acid-containing waste liquid; etc. When the starting material is, for example, steelmaking slag, its particle size is not particularly limited. When the starting material is steelmaking slag, the pH of the acid used in the phosphorus-containing liquid preparation step (S0) is preferably 1.0 or less, more preferably 0.5 or less. The ratio of the starting material to the acid is not particularly limited as long as the pH of the acid used satisfies the above range. However, as the ratio of the starting material increases, the amount of Si eluted increases, and the resulting phosphorus-containing liquid may gel. Therefore, it is preferable to set this ratio by carefully considering the conditions of each step described below. When the starting material is an acidic solution, it is not necessary to add the starting material to the acid, and the starting material may be used directly as the phosphorus-containing liquid.

[0016] The phosphorus-containing liquid prepared in the phosphorus-containing liquid preparation step (S0) preferably contains at least phosphorus (P) and iron (Fe). The phosphorus-containing liquid may further contain elements such as calcium (Ca), aluminum (Al), silicon (Si), magnesium (Mg), and manganese (Mn).

[0017] The phosphorus-containing liquid prepared in the phosphorus-containing liquid preparation step (S0) is acidic. The pH of the phosphorus-containing liquid is preferably less than 4.0, more preferably less than 3.0, and even more preferably less than 1.5.

[0018] <Precipitation Step (S1)> In the precipitation step (S1), an alkali is added to an acidic phosphorus-containing liquid to obtain a precipitate. Specifically, for example, an alkali is added to the phosphorus-containing liquid, and then the liquid is filtered. This results in a filtrate (B) and a precipitate (II). The precipitate (II) is the precipitate.

[0019] As the phosphorus-containing liquid, for example, the leachate (A) obtained in the phosphorus-containing liquid preparation step (S0) is used, but a liquid obtained in another step may also be used.

[0020] The alkali used in the precipitation step (S1) is ammonia water (NH ４ The concentration of aqueous ammonia is not particularly limited, but is preferably 20 to 40% by mass, and more preferably 25 to 35% by mass.

[0021] Depending on the intended use of the final phosphorus compound (iron phosphate), the inclusion of cations may be permitted. In such cases, the alkali is not limited to ammonia water, and sodium hydroxide, potassium hydroxide, etc. may also be used.

[0022] In the precipitation step (S1), the pH of the phosphorus-containing liquid after the addition of alkali is not particularly limited as long as it is a pH at which a precipitate is formed. For example, it is preferable that the pH be in the same range as the pH of the phosphoric acid liquid after the addition of alkali in the re-precipitation step (S3) described below (for example, a pH of less than 4.0).

[0023] <Dissolving Step (S2)> In the dissolving step (S2), the precipitate (II) obtained in the precipitation step (S1) is dissolved using an acid. This results in a phosphoric acid solution. Specifically, for example, the precipitate is added to an acid, followed by stirring and then filtering. This results in a filtrate (C) and a residue (III). The filtrate (C) is the phosphoric acid solution.

[0024] The acid used in the dissolving step (S2) may be, for example, at least one selected from the group consisting of nitric acid, hydrochloric acid, sulfuric acid, phosphoric acid, and citric acid, with nitric acid or hydrochloric acid being preferred, and nitric acid being more preferred.

[0025] Considering the reprecipitation step (S3) described below, the pH of the acid used in the dissolution step (S2) is preferably less than 4.0, more preferably less than 3.0, and even more preferably less than 1.5, although this is not limiting when there is a priority over purity, such as when reducing the amount of waste liquid.

[0026] The pH of the phosphoric acid solution obtained in the dissolving step (S2) is also preferably within the above range (for example, less than 4.0).

[0027] The types of elements contained in the phosphoric acid solution obtained in the dissolving step (S2) are similar to the types of elements contained in the phosphorus-containing solution described above.

[0028] <Re-precipitation Step (S3)> In the re-precipitation step (S3), an alkali is added to the phosphoric acid solution to obtain a precipitate again. Specifically, for example, an alkali is added to the phosphoric acid solution, and then the solution is filtered. This yields a filtrate (D) and a precipitate (IV). The precipitate (IV) is the precipitate.

[0029] As the alkali used in the reprecipitation step (S3), the alkalis described as the alkalis used in the precipitation step (S1) are preferably used.

[0030] In this embodiment, the pH of the phosphoric acid solution is adjusted when alkali is added to the phosphoric acid solution in the re-precipitation step (S3). Please refer to Figures 2 and 3. Figures 2 and 3 are both graphs showing the relationship between the pH of the phosphoric acid solution and the transfer rate of each element from the phosphoric acid solution to the precipitate.

[0031] For example, if the phosphoric acid solution contains 100 parts by mass of element A, and 80 parts by mass of that amount is transferred to the precipitate, the transfer rate of element A is calculated to be 80% by mass. However, the transfer rates shown in Figures 2 and 3 are normalized and therefore have no unit.

[0032] As can be seen from the graphs in Figures 2 and 3, the element concentrations in the precipitates change depending on the pH of the phosphoric acid solution when the precipitates are formed. For example, the lower the pH, the lower the rate at which Al and Si migrate to the precipitates. Furthermore, the higher the pH, the lower the rate at which P migrates to the precipitates.

[0033] Therefore, in this embodiment, when alkali is added to the phosphoric acid solution to generate a precipitate in the reprecipitation generation step (S3), the pH of the phosphoric acid solution is adjusted to a low value. That is, even after the alkali is added to the phosphoric acid solution, the pH of the phosphoric acid solution is prevented from rising too much (the pH of the phosphoric acid solution is maintained at a low value). This makes it possible to increase the purity of iron phosphate in the precipitate (hereinafter simply referred to as "purity"). Note that purity refers to the content of iron phosphate in the fired product (phosphorus compound) obtained by firing the precipitate.

[0034] Specifically, in the reprecipitation step (S3), the pH of the phosphoric acid solution after the addition of the alkali is less than 4.0, preferably 3.5 or less, more preferably 3.0 or less, and even more preferably 2.5 or less. In this case, it is preferable to adjust the pH of the phosphoric acid solution so that the purity is 90% by mass or more.

[0035] The pH adjustment of the phosphoric acid solution to a purity of 90% by mass or more can be performed, for example, by the following procedure: (1) Add alkali little by little to the phosphoric acid solution produced in the dissolution step (S2). The amount of alkali to be added can be determined by trial and error. Alternatively, equilibrium calculation software such as "PHREEQC" can be used to simulate the addition of alkali and perform an analysis to change the pH, thereby determining the amount of alkali to be added that will result in a pH range in which iron phosphate is produced. (2) Collect the precipitate and measure the purity of the iron phosphate. (3) Based on the purity measurement results, perform one of the following: (3-1) If the purity of the iron phosphate is greater than 95% by mass: This indicates excessive purity. In this case, slightly lowering the purity will increase the yield and be more efficient, so return to (1) and add additional alkali. (3-2) If the purity of the iron phosphate is 90% by mass or more but less than 95% by mass: The reprecipitation production step (S3) is complete, and proceed to the next step. (3-3) If the purity of the iron phosphate is less than 90% by mass: Since the purity will not reach 90% by mass or more even if more alkali is added, the reprecipitation step (S3) is completed. In this case, the production of iron phosphate is terminated due to the purity being unsatisfactory, and measures such as reducing the amount of alkali added (increasing the oxidation-reduction potential) are taken, and the production of iron phosphate is restarted from the beginning. Alternatively, as described in the determination step (S5) described below, the dissolution step (S2) and the reprecipitation step (S3) may be considered as one cycle, and multiple cycles may be repeated to increase the purity.

[0036] The pH of the phosphoric acid solution obtained in the dissolution step (S2) (for convenience, referred to as "phosphoric acid solution (S2)") is, for example, less than 4.0. On the other hand, the pH of the phosphoric acid solution after the addition of alkali in the reprecipitation step (S3) (for convenience, referred to as "phosphoric acid solution (S3)") is also less than 4.0. However, the two do not overlap. In other words, even if the pH of the phosphoric acid solution (S2) and the pH of the phosphoric acid solution (S3) are both less than 4.0, the relationship pH of the phosphoric acid solution (S2) < pH of the phosphoric acid solution (S3) is satisfied.

[0037] Depending on the starting material used in the phosphorus-containing solution preparation step (S0), the phosphoric acid solution may contain insufficient Fe or P, making it impossible to obtain the desired purity. In this case, additional material may be added in the reprecipitation generation step (S3). For example, if there is a shortage of Fe, iron nitrate or the like may be added as an additional material.

[0038] <Caloring Step (S4)> The method for producing iron phosphate according to this embodiment preferably further includes a calcination step (S4) after the reprecipitation step (S3). The precipitate obtained in the reprecipitation step (S3) is a precursor of the final product (a phosphorus compound containing iron phosphate). By calcining this precipitate, a phosphorus compound containing iron phosphate is obtained as a calcined product. From the viewpoint of crystallizing the resulting calcined product (phosphorus compound), the calcination temperature is preferably 500°C or higher, more preferably 570°C or higher, and even more preferably 650°C or higher. On the other hand, the calcination temperature is preferably 900°C or lower, more preferably 830°C or lower, and even more preferably 750°C or lower. Note that if the precipitate (precursor) obtained in the reprecipitation step (S3) is used as is without calcination, the calcination step (S4) is not necessary.

[0039] <Determination Step (S5)> The method for producing iron phosphate according to this embodiment may further include a determination step (S5) after the re-precipitation step (S3). In the determination step (S5), it is determined whether the purity of the iron phosphate in the precipitate obtained in the re-precipitation step (S3) is equal to or greater than a threshold value.

[0040] Specifically, for example, the precipitate obtained in the reprecipitation step (S3) is first calcined to obtain a calcined product (phosphorus compound) (see calcination step (S4)). Next, an X-ray diffraction (XRD) pattern of the obtained calcined product (phosphorus compound) is obtained, and the type of phosphorus compound contained in the calcined product is identified. This determines whether or not the calcined product contains iron phosphate. If it is determined that the calcined product contains iron phosphate, the calcined product is then subjected to XRF (X-ray fluorescence) analysis to detect and quantify various elements (including Fe and P). Note that oxygen is usually not detected in XRF analysis. The ratio (unit: mass %) of the total mass of Fe and P to the total mass of all elements detected by XRF analysis is calculated as the content (i.e., purity) of iron phosphate in the calcined product (phosphorus compound).

[0041] Thereafter, it is determined whether the purity is equal to or greater than a threshold value (e.g., 90% by mass). The determination may be made by an operator operating the various measurement devices described above, or by a device such as a personal computer (PC) to which the various devices are connected. If the purity is equal to or greater than the threshold value, the above-described series of steps is terminated. On the other hand, if the purity is less than the threshold value, the above-described dissolution step (S2) and re-precipitation generation step (S3) may be considered as one cycle, and multiple cycles may be repeated until the purity is equal to or greater than the threshold value.

[0042] From the viewpoint of application to applications requiring high purity, such as battery materials, the higher the purity threshold of the iron phosphate, the more preferable. Specifically, 80% by mass or more is preferable, and 85% by mass or more is more preferable. On the other hand, when the purity is higher than a certain level, the effect of increasing the purity by repeating the dissolution step (S2) and the reprecipitation generation step (S3) becomes small, and multiple repetitions may be required to further increase the purity. Furthermore, the yield of iron phosphate decreases with increasing number of repetitions. From these viewpoints, the purity threshold of iron phosphate is preferably 99.8% by mass or less, and more preferably 95% by mass or less. A specific example of the purity threshold of iron phosphate is 90% by mass, but this can be changed as appropriate from the above viewpoints.

[0043] Thus, according to the present embodiment, by calcining the precipitate obtained in the reprecipitation step (S3), a calcined product (phosphorus compound) having a high iron phosphate content can be obtained. In this case, no treatment such as cation exchange is required, and therefore iron phosphate can be easily obtained.

[0044] <Preliminary Determination Step (S6)> The method for producing iron phosphate according to the present embodiment may further include a preliminary determination step (S6) after the phosphorus-containing liquid preparation step (S0) and before the precipitation step (S1). In the preliminary determination step (S6), before the precipitation step (S1), it is determined whether the contents of phosphorus (P) and iron (Fe) in the phosphorus-containing liquid are equal to or greater than threshold values.

[0045] Specifically, in the preliminary determination step (S6), first, the content of each element in the phosphorus-containing liquid is measured using an ICP atomic emission spectrometer. Then, it is determined whether the content (total content) of P and Fe in the phosphorus-containing liquid is equal to or greater than a threshold value (e.g., 15% by mass). The determination may be made by an operator operating the above-mentioned various measurement devices, or by a device such as a personal computer (PC) to which the various devices are connected.

[0046] If the P and Fe contents in the phosphorus-containing liquid are less than the threshold values, the purity of the iron phosphate in the precipitate obtained through the subsequent precipitation step (S1), dissolution step (S2), and reprecipitation step (S3) is expected to be low. Therefore, if the P and Fe contents in the phosphorus-containing liquid are less than the threshold values, the dissolution step (S2) and the reprecipitation step (S3) are considered as one cycle, and this cycle is repeated at least twice. The number of repetitions is set appropriately depending on the P and Fe contents in the phosphorus-containing liquid. Note that if the P and Fe contents in the phosphorus-containing liquid are less than the threshold values, it is not necessary to perform the determination step (S5) after the first reprecipitation step (S3).

[0047] On the other hand, when the P and Fe contents in the phosphorus-containing liquid are equal to or greater than the threshold value (or when the P and Fe contents are less than the threshold value but the cycle of the dissolving step (S2) and the re-precipitation generation step (S3) has been repeated at least twice), it is preferable to carry out the determination step (S5) after the re-precipitation generation step (S3) to determine whether the purity of iron phosphate in the precipitate is equal to or greater than the threshold value.

[0048] The threshold value for the P and Fe content (total content) of the phosphorus-containing liquid is, for example, 15% by mass, but may vary somewhat depending on other components in the phosphorus-containing liquid. Therefore, the production of iron phosphate may be repeated, and the threshold value may be adjusted appropriately during the process based on the P and Fe content of the phosphorus-containing liquid and the determination results of the determination step (S5). That is, when a certain threshold value is set and iron phosphate is repeatedly produced, if the purity is frequently determined to be below the threshold value in the first determination step (S5) of each production, the threshold value may be raised. In this case, it is preferable to increase the threshold value so that the Fe / P ratio approaches 1. Conversely, if the purity is frequently determined to significantly exceed the threshold value in the first determination step (S5) of each production, the threshold value may be lowered.

[0049] The present invention will be specifically described below with reference to examples, but the present invention is not limited to the examples described below.

[0050] Examples 1 to 7: First, in the phosphorus-containing solution preparation step (S0), steelmaking slag as a starting material was added to nitric acid (concentration: 0.5 M) and stirred for 0.4 hours to obtain a phosphorus-containing solution (pH: 0.5). The P and Fe contents in the obtained phosphorus-containing solution were equal to or greater than a threshold value (15% by mass). Next, in the precipitation step (S1), ammonia water (concentration: 28% by mass) was added as an alkali to the phosphorus-containing solution while stirring at 200 rpm. The pH of the phosphorus-containing solution after the alkali addition is shown in Table 1 below. A precipitate was thus obtained. Next, in the dissolution step (S2), the precipitate was added to nitric acid (concentration: 0.5 M) and stirred for 0.5 hours to obtain a phosphoric acid solution. The pH of the obtained phosphoric acid solution is shown in Table 1 below. Next, in the reprecipitation step (S3), ammonia water (concentration: 28% by mass) was added as an alkali to the phosphoric acid solution while stirring at 200 rpm. The pH of the phosphoric acid solution after the addition of the alkali is shown in the following Table 1. As a result, a precipitate was obtained.

[0051] The precipitate obtained in the reprecipitation step (S3) was then fired at 700°C to obtain a crystallized fired product. An XRD pattern was obtained for the resulting fired product (phosphorus compound) to identify the type of phosphorus compound contained in the fired product. In the "Produced Phase" column of Table 1 below, "A" was entered when iron phosphate was contained as the phosphorus compound, and "B" was entered when other phosphorus compounds or other phases containing amorphous halo peaks were contained in addition to iron phosphate.

[0052] The obtained fired product (phosphorus compound) was subjected to XRF analysis to determine the iron phosphate content (i.e., purity) in the fired product (phosphorus compound) according to the method described above. The results are shown in Table 1 below.

[0053] Examples 8 to 9 In Examples 8 to 9, the processes up to the precipitation step (S1) were carried out, but the dissolution step (S2) and the reprecipitation step (S3) were not carried out (the corresponding columns in Table 1 below are marked with "-"). The precipitates obtained in the precipitation step (S1) were evaluated in the same manner as in Examples 1 to 7. The results are shown in Table 1 below.

[0054]

[0055] <Summary of Evaluation Results> Please refer to the results shown in Table 1. In Examples 1 to 5, in which the pH of the phosphoric acid solution after the addition of alkali was set to less than 4.0 in the reprecipitation step (S3), a phosphorus compound with a high iron phosphate content (90 mass % or more) was obtained without carrying out cation exchange, addition of additional raw materials, or the like.

[0056] In contrast, in Examples 6 and 7, in which the pH of the phosphoric acid solution after the addition of alkali was not adjusted to less than 4.0 in the reprecipitation step (S3), the content of iron phosphate in the obtained phosphorus compound was lower than in Examples 1 to 5.

[0057] Furthermore, in Examples 6 and 7 in which the dissolving step (S2) and the reprecipitation step (S3) were not carried out, the content of iron phosphate in the obtained phosphorus compound was smaller.

[0058] Among Examples 1 to 5, Example 5 had the lowest iron phosphate content of 91 mass %, and when the dissolving step (S2) and the reprecipitation step (S3) were further repeated, a phosphorus compound with an iron phosphate content of 99 mass % was finally obtained. More specifically, the initial dissolving step (S2) and reprecipitation step (S3) constituted the first cycle, and the iron phosphate content gradually increased to 94 mass % after the second cycle, 97 mass % after the third cycle, and 99 mass % after the fourth cycle.

Claims

1. a precipitation step of adding an alkali to the phosphorus-containing liquid to obtain a precipitate; a dissolving step of dissolving the precipitate in acid to obtain a phosphoric acid solution; and a re-precipitation step of adding an alkali to the phosphoric acid solution to obtain a precipitate again, In the reprecipitation step, the pH of the phosphoric acid solution is adjusted to less than 4.

0.

2. 2. The method for producing iron phosphate according to claim 1, wherein in the re-precipitation step, the pH of the phosphoric acid solution is adjusted so that the purity of iron phosphate in the resulting precipitate is 90% by mass or more.

3. The method for producing iron phosphate according to claim 1 , wherein the dissolving step and the re-precipitation step are repeated.

4. 4. The method for producing iron phosphate according to claim 3, wherein the dissolving step and the re-precipitation step are repeated until the purity of iron phosphate in the precipitate obtained in the re-precipitation step reaches a threshold value or more.

5. The method for producing iron phosphate according to claim 3 , wherein the dissolving step and the re-precipitation step are repeated depending on the phosphorus and iron contents in the phosphorus-containing liquid.

6. Further, a phosphorus-containing liquid preparation step is provided before the precipitation step; The method for producing iron phosphate according to any one of claims 1 to 5, wherein in the phosphorus-containing liquid preparation step, a component contained in a starting material containing phosphorus is leached with an acid to obtain the phosphorus-containing liquid.

7. 7. The method for producing iron phosphate according to claim 6, wherein the starting material is at least one phosphorus-containing compound selected from the group consisting of steelmaking slag, sewage sludge, and phosphoric acid-containing waste liquid.

8. The method for producing iron phosphate according to any one of claims 1 to 5, wherein the phosphorus-containing liquid contains at least phosphorus and iron elements.