JP4894139B2 - Method and apparatus for treating phosphoric acid-containing liquid - Google Patents

Description

The present invention relates to a method and an apparatus for treating a phosphoric acid-containing liquid , in which a phosphoric acid-containing liquid is reacted with an iron salt to form poorly soluble iron phosphate to be separated and removed.

  Some wastewater discharged from IC chip manufacturing processes, liquid crystal device manufacturing processes, painting ground treatment processes, etc. contains phosphoric acid, and biological wastewater such as sewage and human waste such as organic wastewater. Contains phosphoric acid. Such a phosphoric acid-containing liquid cannot be discharged into the environment as it is, and even when water is recovered, phosphoric acid becomes an impurity, so that it is required to remove or recover after removing phosphoric acid. Furthermore, since phosphoric acid is a valuable resource, it is useful to recover it.

Conventionally, slaked lime is generally used as a method for removing phosphoric acid in a phosphoric acid-containing liquid, and hardly soluble Ca ₅ (PO ₄ ) ₃ OH is generated and separated. In this method, since the slaked lime used is inexpensive and the treatment pH is 8 to 11, when heavy metals coexist, there is an advantage that they can be treated simultaneously as hydroxides. However, since organic substances cannot be removed by this method, when organic substances coexist, a separate treatment for removing organic substances is required.

  On the other hand, the treatment method using an iron salt is neutral at pH 6 to 7, so that organic substances such as COD components and coloring components can be removed together with phosphoric acid. For this reason, when these organic substances coexist, the process using an iron salt may be performed. This method is a method in which an iron salt is added to a phosphoric acid-containing liquid and reacted with phosphoric acid. Phosphoric acid and iron salt are reacted at pH 6 to 7, and the poorly soluble salt of phosphoric acid produced is treated from the liquid to be treated. Separate and remove. In this method, organic substances can be removed together with phosphoric acid, but there is a drawback that the sludge produced is bulky and has poor dewaterability.

In general, a neutralization method called HDS method (High Density Solids) is performed as a method for reducing the water content of a dehydrated cake. This method is used to produce a sparingly soluble Ca salt such as Ca ₅ (PO ₄ ) ₃ OH, CaF ₂ , or a sparingly soluble heavy metal water such as Fe (OH) ₃ , Zn (OH) ₂ , Al (OH) _3. In the case of generating oxides, the separated sludge is returned and mixed with alkali, and this alkali mixed sludge is supplied to the reaction tank and reacted with the wastewater to neutralize it. Can be generated. In this method, sludge is used as a crystal precipitation nucleus. Crystals grow by precipitation of a hardly soluble salt, and sludge rich in dewaterability is produced.

  In Patent Document 1, in a method for treating phosphoric acid-containing wastewater in which phosphoric acid is solid-liquid separated as a sparingly soluble calcium salt, a part of the generated sludge is returned and mixed with a calcium compound, and then mixed with raw water, to be treated. A method for treating a phosphate-containing solution that precipitates phosphoric acid in water as a calcium salt is shown. According to this method, a sludge having a high concentration of sludge to be produced, a sludge generation amount being reduced, a sludge dehydration rate being high, and a sludge having a good dewaterability can be obtained.

However, since the method of Patent Document 1 is a method in which phosphoric acid is reacted with a calcium salt at a high pH, this is the case when phosphoric acid and an iron salt are reacted on the weak alkali side to produce poorly soluble iron phosphate. Even if the HDS method is applied as it is, the intended effect cannot be obtained. That is, when phosphoric acid is reacted with an iron salt, the residual phosphoric acid concentration can be lowered at pH 6-7, but even if the HDS method is applied at this pH, sludge having good dewaterability cannot be obtained. Therefore, even if the HDS method employed in Patent Document 1 is applied as it is to react phosphoric acid with an iron salt, sludge having good dewaterability cannot be obtained.
Japanese Patent Laid-Open No. 9-1154

  An object of the present invention is to produce a treatment liquid having a low concentration of phosphoric acid and organic matter in a process of reacting a phosphoric acid-containing liquid with an iron salt to produce and separate and remove poorly soluble iron phosphate. A treatment method and apparatus for a phosphate-containing liquid that has a high sludge concentration, good dewaterability, a fast sludge dewatering rate, a dehydrated sludge with a low cake moisture content, and a reduced sludge generation amount. Is to provide.

The present invention is the following method and apparatus for treating a phosphate-containing liquid.
(1) a first reaction step of reacting a phosphoric acid-containing liquid with iron salt mixed sludge at pH 3 to 5,
A second reaction step in which the reaction solution in the first reaction step is adjusted to pH 6-7 and further reacted;
A solid-liquid separation step for solid-liquid separation of the reaction liquid in the second reaction step;
A sludge return process for returning a part of the separated sludge separated in the solid-liquid separation process;
A mixing process in which iron salt is mixed with return sludge to make iron salt mixed sludge;
The iron salt mixed sludge saw including a mixing sludge supplying step of supplying the first reaction step,
The amount of the return sludge mixed with the iron salt is a method for treating a phosphoric acid-containing liquid in which the amount of iron salt liberated in the iron salt mixed sludge is 200 to 1500 mg / l (as Fe) .
(2) The method according to (1) above, wherein the amount of the iron salt to be reacted with phosphoric acid is 1 to 3 equivalents with respect to phosphoric acid in the liquid to be treated.
(3) The method according to any one of (1) to (2) above, which comprises an aggregating step between the second reaction step and the solid-liquid separation step.
(4) a first reaction tank for reacting the phosphoric acid-containing liquid with iron salt mixed sludge at pH 3 to 5,
A second reaction tank in which the pH of the reaction liquid in the first reaction tank is adjusted to 6 to 7 and further reacted;
A pH adjusting agent injection path for injecting a pH adjusting agent into each of the first reaction tank and the second reaction tank;
A solid-liquid separation device for solid-liquid separation of the reaction liquid in the second reaction tank;
A sludge return path for returning a part of the separated sludge separated by the solid-liquid separator;
A mixing device that mixes iron salt with return sludge to make iron salt mixed sludge,
An apparatus for treating a phosphoric acid-containing liquid, comprising: a mixed sludge supply passage for supplying iron salt mixed sludge to the first reaction tank.
(5) The apparatus according to (4) above, wherein a coagulation tank is provided between the second reaction tank and the solid-liquid separator.

  The phosphoric acid-containing liquid to be treated in the present invention is a liquid containing ortho or pyrophosphoric acid, and waste water, sewage, and human waste discharged from an IC chip manufacturing process, a liquid crystal device manufacturing process, a coating ground treatment process, and the like. And biologically treated water from other organic wastewater. Here, phosphoric acid is a concept including a salt, and includes one that dissociates a phosphate ion that reacts with an iron salt. The concentration of phosphoric acid contained in the liquid to be treated is not limited, but a liquid of about 20 to 1000 mg / l is suitable for the treatment, and in the case of a high concentration, it may be diluted and reacted.

  In the present invention, as the first reaction step, the phosphoric acid-containing liquid is reacted with the iron salt mixed sludge at pH 3 to 5 in the first reaction tank. The iron salt mixed sludge is a mixture of return sludge and iron salt obtained by returning a part of the separated sludge obtained in the subsequent mixing step. The iron salt to be mixed with the returned sludge is preferably a ferric salt such as ferric chloride, ferric sulfate, or polyiron. A ferrous salt such as ferrous chloride or ferrous sulfate may be used, but in this case, it is necessary to add it and oxidize it by aeration with air. The amount of iron salt reacted with phosphoric acid is the amount necessary to react with phosphoric acid in the liquid to be treated to produce an insoluble salt (ferric phosphate). On the other hand, it can be 1 to 3 equivalent times, preferably 1.5 to 2 equivalent times. When the iron salt is present in the return sludge or the liquid to be treated, the shortage can be added. The iron salt to be mixed with the separated sludge can be added as a 5 to 38% by weight aqueous solution.

The return sludge may be one obtained by returning the separated sludge separated in the subsequent solid-liquid separation step as it is, or may be one that is concentrated and returned. The amount of return sludge to be mixed with the iron salt, Ri Ryodea required for use as nuclei for precipitation, the iron salt which is liberated iron salt mixed sludge 200~1500mg / l (as Fe), preferably 300 to It shall be the amount corresponding to 600 mg / l (as Fe). The return amount of the separated sludge for making such a return sludge amount varies depending on the phosphoric acid concentration of the amount of liquid to be treated, but generally 20 to 70 times by weight with respect to SS generated from the inflow treatment liquid, Preferably it can be 30-50 weight times (as dry matter). If it is less than 20 times by weight, unadsorbed iron increases on the sludge surface, and gel-like sludge may be generated. Moreover, when it exceeds 70 weight times, all iron salts will adsorb | suck to sludge, cohesion property will worsen, and there exists a possibility that treated water may become muddy. For this reason, it is preferable to determine the return amount of the separated sludge within the above range so that an appropriate amount of iron salt is adsorbed on the sludge surface and there is an appropriate amount of unadsorbed iron salt having a coagulation action. When the iron salt is insufficient, the iron salt may be added in the second reaction step.

  The iron salt mixed sludge is a mixture of separated sludge (reaction precipitate) and iron salt in a concentrated state, and the iron salt is adsorbed on the sludge surface. When mixed with phosphoric acid-containing liquid, the sludge surface The iron salt adsorbed on the iron reacts with phosphoric acid, and the iron phosphate precipitate produced by the reaction precipitates with the sludge particles as nuclei, and the sludge particle crystals grow. In normal treatment, phosphoric acid and iron salt are reacted at pH 6-7 in order to reduce the residual phosphoric acid concentration. When the HDS method is employed, sludge produced when reacted at pH 6-7. The dehydration property is not improved so much. As a result of investigations by the present inventors, gel sludge is precipitated in the reaction solution at pH 6 to 7 and crystallization does not occur. Therefore, the dewaterability of the generated sludge is not improved so much, but when reacted at pH 3 to 5, crystals are produced. It has been found that the dewaterability of the generated sludge is improved.

  For this reason, in this invention, a phosphoric acid containing liquid is made to react with an iron salt mixing sludge by pH 3-5 in a 1st reaction process. pH 3-5 is the pH of the reaction solution in the first reaction tank, and when pH 3-5 is obtained when the phosphoric acid-containing solution is mixed with iron salt mixed sludge and reacted, When pH adjustment is necessary, it is preferable to inject a pH adjuster into the first reaction tank, and it is not necessary to adjust pH in a mixing apparatus that forms iron salt mixed sludge. As the pH adjuster, an acid or an alkali is used depending on the pH of the phosphoric acid-containing liquid. As the acid, an acid used as a normal pH adjusting agent such as hydrochloric acid or sulfuric acid, and as the alkali, an alkali used as a normal pH adjusting agent such as sodium hydroxide or calcium hydroxide can be used.

  When the phosphoric acid-containing liquid is reacted with iron salt mixed sludge at pH 3-5 as the first reaction step, the iron phosphate precipitate produced by the reaction of phosphoric acid and iron salt precipitates with sludge particles as the core, and sludge The crystal of the particles grows, and a heavy sludge with good dewaterability can be obtained. However, at pH 3-5, the solubility of iron phosphate is high, and treated water with a low phosphoric acid concentration cannot be obtained even if solid-liquid separation is performed as it is. For this reason, in this invention, as a 2nd reaction process, in the 2nd reaction tank, the reaction liquid of a 1st reaction process is adjusted to pH 6-7, and is made to react further. The pH adjustment is performed by injecting the pH adjusting agent (alkali) used in the first reaction step. At pH 6-7, since the solubility of iron phosphate is small, the reaction between phosphoric acid and iron salt present in the reaction solution proceeds and the precipitation of iron phosphate is promoted. As a result, the phosphoric acid concentration in the reaction solution is lowered, and the organic matter in the reaction solution is embraced and removed in the precipitate of iron phosphate, and the organic matter concentration is also lowered.

  The reaction solution in the second reaction step is separated into a separated solution and separated sludge by solid-liquid separation in the solid-liquid separation step. The solid-liquid separation can be performed by each method using a solid-liquid separation apparatus conventionally used, such as sedimentation separation, filtration separation, and membrane separation. Part of the separated sludge is sent from the sludge return path to the mixing step as return sludge for mixing with iron salt in the sludge return step, and the remainder is discharged as waste sludge. The separation liquid can be discharged as a processing liquid as it is, but it may be recovered by post-processing.

In the mixing step, the iron sludge separated in the solid-liquid separation step is added and mixed in a mixing device to form iron salt mixed sludge. The types and amounts of the separated sludge and iron salt to be mixed here are as described above. As the mixing device, it is preferable to use a mixing tank, but a line mixer may be used, and the iron salt supply path is joined to the sludge return path, and iron salt is supplied and mixed in the return sludge. Also good.
In the mixed sludge supply step, the iron salt mixed sludge obtained in the mixing step is supplied to the first reaction tank of the first reaction step through the mixed sludge supply path.

  It is preferable that an aggregating tank is provided as an aggregating step between the second reaction step and the solid-liquid separation step, and the aggregating treatment is performed by injecting an aggregating agent into the second reaction step reaction solution. The flocculant used here is a flocculant that agglomerates fine precipitates and organic substances precipitated in the liquid in the second reaction step together with the returned sludge containing the grown crystals to form a floc. Although a flocculant is used, other flocculants may be used. As the polymer flocculant, polymer flocculants used for the aggregation of inorganic solids such as sodium polyacrylate and polyacrylamide partial hydrolyzate can be used, but those suitable for the aggregation of iron phosphate As long as it is anionic, nonionic, or cationic. As the aggregating operation, a normal aggregating operation such as injecting an aggregating agent and stirring is employed. In this case, if necessary, it can be adjusted to a pH suitable for the flocculant to be injected.

Each of the first reaction tank and the second reaction tank is provided with a pH adjusting agent injection path for injecting a pH adjusting agent. In flocculation tank can be provided with a pH adjusting agent injection path for injecting a pH adjustment agent. Moreover, when using a mixing tank as these tanks and a mixing apparatus, what is necessary is just the structure and magnitude | size which can perform each reaction etc., and these tanks are equipped with the stirring apparatus and other equipment for stirring. be able to.

  In the treatment of the present invention, when the phosphoric acid-containing liquid is reacted with the iron salt mixed sludge at pH 3-5 as the first reaction step, pH 3-5 is suitable for crystal growth, so the reaction between phosphoric acid and iron salt The precipitate of iron phosphate produced by the above method precipitates with sludge particles as nuclei, and the sludge particle crystals grow, producing heavy and highly dewaterable sludge. In this case, it is estimated that most (about 90% or more) phosphoric acid reacts and precipitates. However, at pH 3-5, the solubility of iron phosphate is large, and treated water with a low phosphoric acid concentration cannot be obtained by solid-liquid separation as it is. Therefore, as the second reaction step, the reaction solution of the first reaction step is adjusted to pH 6-7. When the reaction is further carried out, the reaction between phosphoric acid and iron salt present in the reaction solution proceeds, and the precipitation of iron phosphate is promoted. Here, it is assumed that the remaining (about 10% or less) phosphoric acid reacts and precipitates. As a result, the phosphoric acid concentration in the reaction solution is lowered, and the organic matter in the reaction solution is embraced and removed in the precipitate of iron phosphate, and the organic matter concentration is also lowered.

  For this reason, the reaction solution in the second reaction step is subjected to a coagulation treatment as necessary, and then separated into solid-liquid separation into a separation solution having a low concentration of phosphoric acid and organic matter and a separation sludge having good dehydration properties. it can. As a result, the separation liquid can be discharged as a treatment liquid having a low concentration of phosphoric acid and organic matter. Part of the separated sludge is returned to grow crystals and the remainder is discharged, so that the size of the crystals in the sludge can be made constant. Since the separated sludge to be discharged has a high concentration and good dewaterability, a dewatered sludge having a high dewatering rate and a low moisture content of cake can be obtained, and the amount of sludge generated can be reduced.

  In this case, the phosphoric acid in a to-be-processed liquid can be efficiently removed by making the quantity of the iron salt made to react with phosphoric acid 1-3 equivalent times with respect to the phosphoric acid in a to-be-processed liquid. The amount of return sludge mixed with the iron salt is such that the amount of iron salt liberated in the iron salt mixed sludge is 200 to 1500 mg / l (as Fe), so that the phosphoric acid remaining in the first reaction step is reduced. It can be precipitated in the second reaction step to increase the removal rate of phosphoric acid and organic matter. Further, when an aggregation step is provided between the second reaction step and the solid-liquid separation step, the removal rate can be further increased due to the aggregation effect of the precipitated phosphoric acid and organic matter.

According to the present invention, the phosphoric acid-containing liquid is reacted with iron salt mixed sludge at pH 3-5 in the first reaction step, and further reacted by adjusting the reaction liquid of the first reaction step to pH 6-7 in the second reaction step. In the solid-liquid separation step, the reaction liquid in the second reaction step is subjected to solid-liquid separation, the iron salt is mixed with the sludge separated in the solid-liquid separation step, and the obtained iron salt mixed sludge is used as the first reaction step. The amount of return sludge to be returned and mixed with the iron salt was such that the iron salt liberated in the iron salt mixed sludge was 200-1500 mg / l (as Fe). Organic substances can be efficiently removed together with acid, and a treatment liquid with low phosphoric acid and organic substance concentrations can be obtained, and the concentration of produced sludge is high, the dewaterability is good, the sludge dehydration rate is fast, and the moisture content of the cake is high. Low dewatered sludge is obtained and sludge generation amount It can be reduced.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. 1A and 1B are flowcharts showing a method and apparatus for treating a phosphoric acid-containing liquid according to another embodiment of the present invention. In FIG. 1, 1 is a 1st reaction tank, 2 is a 2nd reaction tank, 3 is a coagulation tank, 4 is a solid-liquid separator, 5 is a mixing apparatus. L1 is a liquid path to be treated, L2 to L4 are transfer paths, L5 is a process liquid path, L6 to L7 are pH adjuster paths, L8 is a mixed sludge supply path, L9 is an iron salt supply path, L11 is a sludge return path, L12 Is a sludge discharge path, and L13 is a flocculant supply path. In FIG. 1A, a sedimentation separation tank is used as the solid-liquid separation device 4, and a mixing tank is used as the mixing device 5. In FIG. 1B, a sedimentation separation tank is used as the solid-liquid separation device 4, but the mixing device 5 is configured to join the iron salt supply path L9 to the sludge return path L11, and at the junction. It is connected to the mixed sludge supply path L8.

  The processing method of the phosphoric acid containing liquid by said processing apparatus introduces a phosphoric acid containing liquid from the to-be-processed liquid path L1 to the 1st reaction tank 1 as a 1st reaction process, and mixes iron salt mixed sludge from the mixed sludge supply path L8. The pH adjusting agent is injected from the pH adjusting agent passage L6 to adjust to pH 3 to 5, and reacted with phosphoric acid and iron salt mixed sludge. The resulting insoluble salt of iron phosphate precipitates with sludge as the nucleus, and crystals grow. Subsequently, as the second reaction step, the reaction solution of the first reaction step is introduced into the second reaction tank 2 from the transfer path L2, and a pH adjuster is injected from the pH adjuster path L7 to adjust to pH 6-7, and further. React. As a result, the remaining phosphoric acid and the iron salt react, and the insoluble salt of iron phosphate that is generated entraps the organic substance and precipitates in the liquid. Subsequently, as a coagulation step, the reaction solution of the second reaction step is introduced into the coagulation tank 3 from the transfer path L3, and the coagulant is injected from the coagulant path L13 to perform an aggregation reaction to form a floc. At this time, if necessary, a pH adjusting agent is injected. Subsequently, in the solid-liquid separation step, the aggregation reaction liquid in the aggregation step is introduced into the solid-liquid separation device 2 from the transfer path L4, and solid-liquid separation is performed to separate the separation liquid and sludge. The separation liquid is discharged from the treatment liquid path L5 as a treatment liquid. Part of the separated sludge is returned to the mixing device 5 from the sludge return path L11, and the remainder is discharged from the sludge discharge path L12. In the mixing device 5, iron salt is supplied from the iron salt supply path L9, iron salt is mixed with the returned sludge, and the obtained iron salt mixed sludge is supplied to the first reaction tank 1 from the mixed sludge supply path L8.

FIG. 2A is a flowchart showing the apparatus and method employed in Comparative Examples 4 to 6, in which the mixing apparatus 5 and the sludge return path L11 in FIG. 1A are omitted, and the first from the iron salt supply path L9. Iron salt is supplied to the reaction tank.
FIG. 2 (B) is a flowchart showing the apparatus and method employed in Comparative Examples 7 to 8, the mixing apparatus 5 in FIG. 1 (A) is omitted, and the sludge returned from the sludge return path L11 to the first reaction tank 1 is shown. The iron salt is supplied from the iron salt supply path L9.
FIG. 2 (C) is a flowchart showing the apparatus and method employed in Comparative Examples 9 to 10, in which the pH adjusting agent is supplied from the pH adjusting agent passage L6 to the mixing apparatus (mixing tank) 5 in FIG. 1 (A). The iron salt is supplied to the first reaction tank 1 from the iron salt supply path L9.

The effects of the present invention will be described below with reference to examples and comparative examples.
The liquid to be treated used in Examples and Comparative Examples was liquid crystal production wastewater, and the water quality was pH 1.7, COD: 10.5 mg / l, Al: 6 mg / l, PO ₄ -P: 254 mg / l. is there.
The experiment was performed using the apparatus shown in FIG. The specifications of the apparatus are as follows: first reaction tank 1 volume: 1,000 ml, second reaction tank 2 volume: 1,000 ml, coagulation tank 3 volume: 500 ml, solid-liquid separation tank 4 (thickener) capacity: 5,000 ml, mixing tank 5 volumes: 200 ml.
The operating conditions were as follows: inflow of liquid to be treated: 3,000 ml / hr, return of sludge: 1,200 ml / hr, pH adjusting agent: NaOH solution, pH meter-linked injection, flocculant: polyacrylamide cliff rock PA-331 ( Kurita Industry Co., Ltd., registered trademark) 2-3 mg / l injection, iron salt: ferric chloride (38%) 737 mg / l as Fe (Fe / P = 1.6 molar ratio).

＊ 比較例−４の汚泥発生量に対する減容率として表示した。 Examples 1-5, Comparative Examples 1-10:
Examples 1 to 4 and Comparative Examples 1 to 3 are based on the apparatus and method of FIG. 1 (A), Example 5 is based on the apparatus and method of FIG. 1 (B), and Comparative Examples 4 to 6 are based on FIG. 2 (A). According to the apparatus and method, Comparative Examples 7 to 8 are the apparatus and method of FIG. 2B, and Comparative Examples 9 to 10 are the apparatus and method of FIG. The treatment was performed under the above conditions while changing the pH value of 2.
And the TP of the separation liquid (treatment liquid) of the solid-liquid separation device 4, the sludge concentration of the concentrated sludge after leaving the returned sludge or waste mud for a day, and the filter cloth 60 mesh, pressing pressure 2kg assuming a roll press A dehydration test was conducted at / cm ² and the moisture content of the dehydrated cake was measured. The results are shown in Table 1.

* Displayed as volume reduction rate relative to sludge generation amount of Comparative Example-4.

  As shown in Table 1, the sludge volume reduction effect is generated with the maximum pH 4 in the first reaction tank 1. If the pH is less than 3, precipitation of phosphoric acid and iron salt is difficult to generate. On the other hand, if the pH exceeds 5, salt iron becomes iron hydroxide, and the reactivity with phosphoric acid becomes weak. Therefore, the optimum pH is 3.0 to 5. 0, preferably pH 3.5-4.5.

  Phosphoric acid-containing liquid such as wastewater discharged from IC chip manufacturing process, liquid crystal element manufacturing process, painting ground treatment process, or organic wastewater such as sewage and human waste is reacted with iron salt, phosphoric acid It can be used in a processing method and apparatus for separating and removing as iron.

(A), (B) is a flowchart which shows the processing method and apparatus of the phosphoric acid containing liquid by another embodiment of this invention. (A)-(C) is a flowchart which shows the processing method and apparatus of the phosphoric acid containing liquid of a comparative example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 1st reaction tank 2 2nd reaction tank 3 Coagulation tank 4 Solid-liquid separation apparatus 5 Mixing apparatus

Claims (5)

A first reaction step of reacting the phosphoric acid-containing liquid with iron salt mixed sludge at pH 3 to 5,
A second reaction step in which the reaction solution in the first reaction step is adjusted to pH 6-7 and further reacted;
A solid-liquid separation step for solid-liquid separation of the reaction liquid in the second reaction step;
A sludge return process for returning a part of the separated sludge separated in the solid-liquid separation process;
A mixing process in which iron salt is mixed with return sludge to make iron salt mixed sludge;
The iron salt mixed sludge saw including a mixing sludge supplying step of supplying the first reaction step,
The amount of the return sludge mixed with the iron salt is a method for treating a phosphoric acid-containing liquid in which the amount of iron salt liberated in the iron salt mixed sludge is 200 to 1500 mg / l (as Fe) .   The method according to claim 1, wherein the amount of the iron salt to be reacted with phosphoric acid is 1 to 3 times the amount of phosphoric acid in the liquid to be treated. The method according to any one of claims 1 to 2 comprising the aggregation process between the second reaction step and the solid-liquid separation step. A first reaction tank for reacting a phosphoric acid-containing liquid with iron salt mixed sludge at a pH of 3 to 5,
A second reaction tank in which the pH of the reaction liquid in the first reaction tank is adjusted to 6 to 7 and further reacted;
A pH adjusting agent injection path for injecting a pH adjusting agent into each of the first reaction tank and the second reaction tank;
A solid-liquid separation device for solid-liquid separation of the reaction liquid in the second reaction tank;
A sludge return path for returning a part of the separated sludge separated by the solid-liquid separator;
A mixing device that mixes iron salt with return sludge to make iron salt mixed sludge,
An apparatus for treating a phosphoric acid-containing liquid, comprising: a mixed sludge supply passage for supplying iron salt mixed sludge to the first reaction tank. The apparatus of Claim 4 including a coagulation tank between a 2nd reaction tank and a solid-liquid separator.

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