阅读说明：本技术 废水的处理方法 (Method for treating waste water ) 是由 原口大辅 林浩志 于 2018-12-27 设计创作，主要内容包括：该废液的处理方法具有：铝溶解工序,在酸性废液中溶解铝,以分离成第1处理水和重金属还原沉淀物；石膏回收工序,在pH4以下的酸碱度下,在所述第1处理水中添加钙化合物,以分离成第2处理水和石膏；重金属共沉淀工序,在所述第2处理水中添加三价铁化合物,以分离成第3处理水和重金属共沉淀物；铝及氟去除工序,在所述第3处理水中添加碱,以分离成第4处理水和含有铝及氟的沉淀物；及中和工序,在所述第4处理水中添加碱,以分离成碱中和处理水和重金属氢氧化物的中和沉淀物。(The method for treating waste liquid comprises: an aluminum dissolution step of dissolving aluminum in the acidic waste liquid to separate into 1 st treated water and a heavy metal reduction precipitate; a gypsum recovery step of adding a calcium compound to the 1 st treated water at a pH of 4 or less to separate the treated water into 2 nd treated water and gypsum; a heavy metal coprecipitation step of adding a ferric iron compound to the 2 nd treated water to separate the 2 rd treated water and a heavy metal coprecipitate; an aluminum and fluorine removal step of adding an alkali to the 3 rd treated water to separate the 3 rd treated water into 4 th treated water and a precipitate containing aluminum and fluorine; and a neutralization step of adding an alkali to the 4 th treated water to separate the treated water into alkali-neutralized water and a neutralized precipitate of a heavy metal hydroxide.)

1. A method for treating a waste liquid, which comprises recovering gypsum having a low fluorine content from an acidic waste liquid containing fluorine and heavy metals and removing the heavy metals, characterized by comprising:

an aluminum dissolution step of dissolving aluminum in the acidic waste liquid to convert fluorine in the solution into fluoroaluminate ions and stably dissolve the fluorine, and to generate a heavy metal reduction precipitate, thereby separating the heavy metal reduction precipitate into 1 st treated water and the heavy metal reduction precipitate;

a gypsum recovery step of adding a calcium compound to the 1 st treated water at a pH of 4 or less to form gypsum, after removing the heavy metal reduction precipitate, and separating the gypsum into 2 nd treated water and the gypsum;

a heavy metal coprecipitation step of adding a trivalent iron compound to the 2 nd treated water after removing the gypsum to generate a precipitate of ferric hydroxide and coprecipitating heavy metals in the solution to the precipitate, thereby separating into 3 rd treated water and a heavy metal coprecipitate;

an aluminum and fluorine removal step of adding an alkali to the 3 rd treated water after removing the heavy metal coprecipitate, adjusting the pH to 5.5 to 9.5 to suppress the amount of the precipitate, and simultaneously generating a precipitate containing aluminum and fluorine, thereby separating the precipitate into 4 th treated water and the precipitate containing aluminum and fluorine; and

and a neutralization step of removing the precipitate containing aluminum and fluorine, and then adding an alkali to the 4 th treated water to adjust the pH to 9.5 to 11.8 to generate a neutralized precipitate of a heavy metal hydroxide, thereby separating the neutralized precipitate into alkali-neutralized treated water and the neutralized precipitate of the heavy metal hydroxide.

2. The method for treating waste liquid according to claim 1, wherein,

in the aluminum and fluorine removal step, the pH of the 3 rd treated water is adjusted to pH5.5 to 7.0 to suppress the amount of precipitates and to suppress the precipitation of arsenic and zinc, thereby precipitating fluorine and aluminum.

3. The method of treating waste liquid according to claim 1 or 2,

the acidic waste liquid containing fluorine and heavy metals is waste water of a non-ferrous metal smelting plant.

Technical Field

The present invention relates to a method for treating waste water, which comprises recovering gypsum having a small fluorine content from acidic waste water (for example, waste water from a nonferrous metal smelting plant) containing heavy metals such as copper, arsenic and zinc in addition to sulfuric acid, fluorine and chlorine, and further, sufficiently removing the heavy metals from a residual liquid at low cost.

The present application claims priority based on patent application No. 2017-250886, which was filed in japan on 12/27/2017, the contents of which are incorporated herein by reference.

Background

Wastewater from non-ferrous metal smelters contains a large amount of heavy metals such as copper, arsenic and zinc in addition to sulfuric acid, fluorine and chlorine, and when the wastewater is discharged to the outside of the system, it is necessary to sufficiently remove these heavy metals in order to meet drainage regulations. In addition, since the wastewater is generally strongly acidic wastewater containing sulfate ions, calcium compounds are often added to the wastewater to be neutralized, but it is expected that gypsum produced by the neutralization treatment is recovered and reused.

As a method for treating waste water from non-ferrous metal smelteries and the like, the following methods are known.

(a) A method for treating waste acid, comprising: a primary sulfidation step of mixing a sulfidizing agent with a waste acid produced by copper smelting to sulfide a heavy metal, and separating the mixture into a primary slurry containing the obtained sulfide precipitate and a primary clarified liquid; a gypsum production step of mixing a neutralizing agent with the primary clarified liquid to convert sulfuric acid into gypsum, and performing solid-liquid separation to obtain a final gypsum solution; and a secondary vulcanization step of mixing a vulcanizing agent with the final gypsum solution to vulcanize the heavy metals and separating the mixture into a secondary slurry containing the obtained vulcanized precipitate and a secondary clarified liquid, wherein the secondary slurry separated into the secondary clarified liquid in the secondary vulcanization step is returned to the primary vulcanization step and mixed with the waste acid (patent document 1).

(b) A method for producing waste acid gypsum, wherein a Ca-containing alkaline agent is added to waste acid obtained from waste gas generated in non-ferrous metal metallurgy to neutralize the waste acid, and fluorine contained in gypsum generated in the neutralization treatment is washed with water or sulfuric acid (patent document 2).

(c) A method for treating waste sulfuric acid, which comprises adding aluminum to fluorine-containing waste sulfuric acid in an amount of 0.5 times or more the amount of fluorine contained in the waste sulfuric acid, and then neutralizing the resultant mixture with an alkaline agent to a pH of 5.6 or less (patent document 3).

(d) A method for treating wastewater, which comprises adding an aluminum salt to wastewater containing at least one of fluorine, selenium and a compound thereof to form flocs, then carrying out precipitation separation, adding a liquid chelating agent to the separated supernatant to cause a reaction, and adding an aluminum salt to the reaction solution to cause solid matter to coagulate, and then carrying out solid-liquid separation (patent document 4).

Patent document 1: japanese patent No. 6206287

Patent document 2: japanese patent laid-open publication No. 2017-105651

Patent document 3: japanese examined patent publication No. 59-34644

Patent document 4: japanese patent laid-open No. 9-192675

In the treatment method of patent document 1, the heavy metals dissolved by the sulfidizing agent are precipitated and removed as sulfides, but when the sulfidizing agent is added to a strongly acidic solution like the waste water of the non-ferrous metal smeltery, harmful hydrogen sulfide gas is generated, which is dangerous and has a problem in terms of safety. Further, the sulfiding agent of the volatilized hydrogen sulfide component does not contribute to the removal of the heavy metal precipitate, and therefore the reaction efficiency is low. Further, although gypsum is produced in the primary vulcanization step in this treatment method, fluorine in the wastewater cannot be removed in the vulcanization treatment, and thus a large amount of fluorine is mixed into gypsum.

The treatment method of patent document 2 is a method of washing gypsum mixed with fluorine with water or sulfuric acid, but in this example, 50mL of washing liquid is required for 10g of gypsum, and the washing liquid is discharged as a large amount of waste water. This increase in wastewater is both environmentally and economically disadvantageous. Further, since gypsum having a large amount of fluorine is washed, fluorine in gypsum is not sufficiently reduced in the case of washing omission, stirring, insufficient washing, or the like. In order to stabilize the washing, the amount of the solution to be added to the gypsum may be increased to lower the solid content concentration, but this still increases the amount of the washing liquid or the waste water, and further treatment of the waste water containing the fluorine-containing washing water needs to be performed again.

In the treatment method of patent document 3, aluminum is added to fluorine-containing waste sulfuric acid to keep fluorine in solution, and a calcium compound is added thereto to produce gypsum, and then solid-liquid separation is performed, but a large amount of aluminum, fluorine, and heavy metals are dissolved in the filtrate from which gypsum is separated, and the treatment of aluminum, fluorine, and heavy metals in the solution becomes a problem.

In the treatment method of patent document 4, aluminum salt is added to the wastewater to adjust the pH to 6 to 8, thereby precipitating aluminum hydroxide and causing gypsum and calcium fluoride (CaF) as SS components (Suspended Solids) in the wastewater₂) The aluminum hydroxide flocs are taken in and a part of fluoride ions are adsorbed to the aluminum hydroxide to be removed, but the precipitate is a mixture of gypsum and a fluorine-containing precipitate, and thus it is difficult to effectively utilize the precipitate as a resource. In addition, copper or arsenic contained in the waste liquid cannot be sufficiently removed.

Disclosure of Invention

The present invention solves the above problems of the conventional treatment methods and provides a treatment method having an excellent effect of removing fluorine and heavy metals contained in a waste liquid.

The present invention is a method for treating waste liquid, which solves the above problems with the following configurations.

[1] A method for treating a waste liquid, which comprises recovering gypsum having a low fluorine content from an acidic waste liquid containing fluorine and heavy metals and removing the heavy metals, characterized by comprising: an aluminum dissolution step of dissolving aluminum in the acidic waste liquid to convert fluorine in the solution into fluoroaluminate ions and stably dissolve the fluorine, and to generate a heavy metal reduction precipitate, thereby separating the heavy metal reduction precipitate into 1 st treated water and the heavy metal reduction precipitate; a gypsum recovery step of adding a calcium compound to the 1 st treated water at a pH of 4 or less to form gypsum, after removing the heavy metal reduction precipitate, and separating the gypsum into 2 nd treated water and the gypsum; a heavy metal coprecipitation step of adding a trivalent iron compound to the 2 nd treated water after removing the gypsum to generate a precipitate of ferric hydroxide, and adsorbing heavy metals in the solution to the precipitate to perform coprecipitation, thereby separating into 3 rd treated water and a heavy metal coprecipitate; an aluminum and fluorine removal step of adding an alkali to the 3 rd treated water after removing the heavy metal coprecipitate, adjusting the pH to 5.5 to 9.5 to suppress the amount of the precipitate, and generating a precipitate containing aluminum and fluorine to separate the precipitate into 4 th treated water and the precipitate containing aluminum and fluorine; and a neutralization step of removing the precipitate containing aluminum and fluorine, and then adding an alkali to the 4 th treated water to adjust the pH to 9.5 to 11.8 to generate a neutralized precipitate of a heavy metal hydroxide, thereby separating the neutralized precipitate into alkali-neutralized treated water and the neutralized precipitate of the heavy metal hydroxide.

The method of treating a waste liquid according to the above [1], wherein in the aluminum and fluorine removal step, the pH of the 3 rd treated water is adjusted to pH5.5 to 7.0 to suppress the amount of precipitates and to suppress the precipitation of arsenic and zinc, thereby precipitating fluorine and aluminum.

[ 3] the method for treating a waste liquid according to the above [1] or [2], wherein the acidic waste liquid containing fluorine and heavy metals is a waste water of a nonferrous metal smelting plant.

According to the method for treating a waste liquid of the present invention, a waste liquid treatment method capable of more effectively removing fluorine and heavy metals contained in a waste liquid can be provided.

Drawings

FIG. 1 is a process diagram schematically showing a method for treating waste liquid according to the present invention.

FIG. 2 is a graph showing changes in fluorine concentration and arsenic concentration in example 2.

FIG. 3 is a graph showing changes in the amount of precipitate formed and the aluminum concentration in example 3.

FIG. 4 is a graph showing changes in the fluorine concentration, arsenic concentration and zinc concentration in example 4.

Detailed Description

Hereinafter, an example of the embodiment of the present invention will be described in detail with reference to the drawings, but the present invention should not be construed as being limited to the embodiment.

The method for treating waste liquid according to the present embodiment is a method for recovering gypsum having a low fluorine content from an acidic waste liquid containing fluorine and heavy metals and removing the heavy metals, the method comprising: an aluminum dissolution step of dissolving aluminum in the acidic waste liquid to convert fluorine in the solution into fluoroaluminate ions and stably dissolve the fluorine, and to generate a heavy metal reduction precipitate, thereby separating the heavy metal reduction precipitate into 1 st treated water and the heavy metal reduction precipitate; a gypsum recovery step of adding a calcium compound to the 1 st treated water at a pH of 4 or less to form gypsum, after removing the heavy metal reduction precipitate, and separating the gypsum into 2 nd treated water and the gypsum; a heavy metal coprecipitation step of adding a trivalent iron compound to the 2 nd treated water after removing the gypsum to generate a precipitate of ferric hydroxide and coprecipitating heavy metals in the solution to the precipitate, thereby separating into 3 rd treated water and a heavy metal coprecipitate; an aluminum and fluorine removal step of adding an alkali to the 3 rd treated water after removing the heavy metal coprecipitate, adjusting the pH to 5.5 to 9.5 to suppress the amount of the precipitate, and generating a precipitate containing aluminum and fluorine to separate the precipitate into 4 th treated water and the precipitate containing aluminum and fluorine; and a neutralization step of removing the precipitate containing aluminum and fluorine, and then adding an alkali to the 4 th treated water to adjust the pH to 9.5 to 11.8 to generate a neutralized precipitate of a heavy metal hydroxide, thereby separating the neutralized precipitate into alkali-neutralized treated water and the neutralized precipitate of the heavy metal hydroxide.

Fig. 1 is a process diagram showing an outline of a method for treating waste liquid according to the present embodiment.

The acidic waste liquid containing fluorine and heavy metals, which is a treatment target in the present embodiment, is, for example, waste water generated in a step of smelting a sulfide mineral of a nonferrous metal such as copper smelting. Generally, the waste water from non-ferrous metal smelteries is strongly acidic waste water with a pH of 0.8 to 2.0, which contains heavy metals such as copper, arsenic and zinc, and also contains sulfuric acid and fluorine.

[ aluminum dissolution procedure ]

The method for treating waste liquid of the present embodiment includes the following aluminum addition step: dissolving aluminum in an acidic waste liquid containing fluorine and heavy metals to convert fluorine in the solution into fluoroaluminate ions to be stably dissolved, and generating a heavy metal reduction precipitate, thereby performing solid-liquid separation into 1 st treated water and the heavy metal reduction precipitate.

By dissolving aluminum, aluminum ion (Al)³⁺) Is supplied to the solution, and forms the aluminum ions and fluorine ions (F) in the solution as shown in the following formulas (1) to (3)^-) Complexed fluoroaluminate ion (AlF)²⁺、AlF₂ ⁺、AlF₃ ⁰) Therefore, fluorine is stably dissolved in the solution.

Al³⁺(aq)+F^-(aq)→AlF²⁺(aq) (1)

AlF²⁺(aq)+F^-(aq)→AlF₂ ⁺(aq) (2)

AlF₂ ⁺(aq)+F^-(aq)→AlF₃ ⁰(aq) (3)

Since fluorine ions in the solution and aluminum ions form fluoroaluminate ions and are stably dissolved, calcium fluoride (CaF) is suppressed even when a calcium compound is added in the next step₂) Can prevent calcium fluoride from being mixed into the gypsum to be produced, and can obtain gypsum with a small amount of fluorine.

In addition to this, the present invention is,when aluminum is dissolved, a reduction reaction occurs as shown in the following formulas (4) and (5). For example, copper ions (Cu) in solution because the ionization tendency of aluminum is greater than that of copper²⁺) Reduced and precipitated by dissolution of aluminum, or reduced copper and arsenate ions (AsO)₃ ^3-) The reaction produces a copper arsenide precipitate. In addition, since the ionization tendency of zinc is close to that of aluminum, most of zinc is dissolved in the solution and remains.

2Al(s)+3Cu²⁺(aq)→2Al³⁺(aq)+3Cu(s) (4)

Al(s)+3Cu(s)+AsO₃ ^3-(aq)+6H⁺→Al³⁺(aq)+Cu₃As(s)+6H₂O (5)

The amount of aluminum dissolved is preferably in a range of 0.4 to 0.8 Al/F in terms of a molar ratio with respect to the amount of fluorine in the solution. When the amount of aluminum dissolved is 0.3 mol per Al/F, complex ionization of fluorine is insufficient, and therefore, when gypsum is produced, the amount of fluorine in gypsum increases, and when the amount of Al/F is 0.4 mol or more, the amount of fluorine in gypsum can be greatly reduced.

Specifically, as shown in example 1, when Al/F is 0.3 mol, the amount of fluorine in the gypsum is 0.3 mass% or more. On the other hand, if Al/F is 0.4 mol, the amount of fluorine in the gypsum can be made 0.2 mass% or less.

The ph of the acidic waste liquid from the aluminum dissolution step is preferably 4.0 or less. As shown in example 1, pH4.1 or more is not preferable because arsenic in the solution is adsorbed to gypsum and the arsenic content increases rapidly.

In this way, aluminum is dissolved in the acidic waste liquid to cause a reduction reaction, whereby copper or arsenic in the solution is precipitated as a reduction precipitate, and is removed by solid-liquid separation. When the oxidation-reduction potential is +400mV (vs. SHE) or less, the reduction reaction proceeds well.

In addition, when suspended particles containing fine heavy metals are present in the acidic waste liquid, these suspended particles can be caused to enter the reduced precipitate and can be coagulated and separated. By removing the heavy metal at the initial stage of the treatment step, the heavy metal can be prevented from being mixed into the post-treatment precipitate, and the amount of the neutralizing agent to be added in the post-treatment can be reduced as compared with the conventional process. The main components of the reduced precipitate are copper and copper arsenide, and therefore can be recovered and used as a raw material for copper smelting.

[ Gypsum recovering Process ]

As shown in the following formula (6), the filtrate (first treated water) obtained by subjecting the reduced precipitate formed by dissolving aluminum to solid-liquid separation is added with a calcium compound to form gypsum, and the gypsum is recovered by solid-liquid separation into second treated water and gypsum. Sulfate ions in the solution are removed by the formation of gypsum. As the calcium compound, calcium carbonate, calcium hydroxide, calcium oxide, or limes containing these as a main component can be used.

H₂SO₄(aq)+CaCO₃(s)+H₂O→CaSO₄·2H₂O(s)+CO₂(g) (6)

Since the fluoride ion contained in the filtrate forms a complex ion with the aluminum ion and is stably dissolved, even when calcium carbonate or the like is added, calcium fluoride (CaF) is hardly produced₂) Since fluorine can be prevented from being mixed into gypsum, gypsum with a low fluorine content can be obtained. Further, even if suspended particles containing fine heavy metals are present in the wastewater, since the suspended particles are aggregated and separated in the previous step, gypsum having a low heavy metal content can be recovered. Gypsum is preferably produced at a pH of 4.0 or less. If the pH exceeds 4.0, the heavy metals precipitate as hydroxides or co-precipitate in gypsum, which is not preferable.

[ heavy Metal coprecipitation Process ]

A trivalent iron compound is added to the residue (treated water 2) from which gypsum is separated to form a precipitate of ferric hydroxide, and heavy metals in the solution are adsorbed to the precipitate to perform coprecipitation.

In the residual liquid from which gypsum was separated, a large amount of fluorine remained together with the dissolved aluminum. Although it is possible to generate a precipitate containing aluminum or fluorine by adding an alkali such as calcium hydroxide to the residual liquid and adjusting the ph to a ph of 5.0 or more, and then performing solid-liquid separation, if a large amount of heavy metals such as arsenic or molybdenum are contained in the solution, these heavy metals are excessively mixed into the precipitate containing aluminum or fluorine, so that the quality of the recovered fluorine is greatly reduced, and there is a problem that it is difficult to separate and recover these heavy metals.

Therefore, the treatment method of the present embodiment has the following heavy metal coprecipitation step: heavy metals such as arsenic are selectively removed from the solution prior to the removal of aluminum and fluorine. When a trivalent iron compound such as ferric chloride is added to the residual liquid (treated water 2) from which gypsum has been separated to precipitate ferric hydroxide, heavy metals such as arsenic and molybdenum in the solution are coprecipitated in the ferric hydroxide precipitate, and therefore, the heavy metals such as arsenic and molybdenum can be removed by solid-liquid separation into treated water 3 and the heavy metal coprecipitate. In addition, since arsenic or iron is concentrated particularly at a high concentration in the heavy metal coprecipitate, it can be used as a raw material of an arsenic compound or an iron compound.

When zinc, cadmium, nickel, or the like is dissolved in the residual liquid after the gypsum separation, a part of the residual liquid is also adsorbed to the iron hydroxide precipitate, and thus is removed together with the heavy metal coprecipitate.

The pH value of the 2 nd treated water in the heavy metal coprecipitation step is preferably within a range of pH3.0 to 4.0. If the pH exceeds 4.0, fluorine begins to precipitate in addition to arsenic and the like, and thus arsenic and the like cannot be selectively precipitated. On the other hand, if the pH is less than 3.0, the removal of heavy metals such as arsenic is insufficient. The pH in the gypsum formation step is preferably 4.0 or less, and when a ferric iron compound is added, the pH inevitably decreases, and when the pH significantly decreases, an alkali such as calcium hydroxide may be added to adjust the pH within a range of 3.0 to 4.0.

[ aluminum and fluorine removal Process ]

Cadmium, zinc, and the like are dissolved in the residual liquid (treated water 3) obtained by solid-liquid separation of the heavy metal coprecipitate, and remain after separation of the dissolved aluminum or the fluorine and heavy metal coprecipitate initially contained in the wastewater. Conventionally, as a method for removing zinc, cadmium, or the like in such a solution, it has been known that a hydroxide precipitate is formed by adding a neutralizing agent such as calcium hydroxide so that the ph value is in an alkaline region of ph9.5 to ph 11.8. However, if added toWhen the pH of the neutralizing agent is adjusted to a range of pH9.5 to pH11.8 in one step, Friedel's salts are formed as shown in the following formulas (7) to (9) in addition to the hydroxide: ca₂Al(OH)₆Cl·2H₂O ], couzel salt [ Kuzel's salt: ca₄Al₂(OH)₁₂Cl(SO₄)·5H₂O ], Ettringite (Ettringite: ca₆Al₂(OH)₁₂(SO₄)₃·26H₂O), and the like, and generates a precipitate containing a large amount of chlorine, hydroxyl groups, and water of hydration in addition to aluminum, resulting in an increase in the amount of sludge generated.

2Ca(OH)₂+Al³⁺+2OH^-+Cl^-+2H₂O→Ca₂Al(OH)₆Cl·2H₂O (7)

4Ca(OH)₂+2Al³⁺+4OH^-+Cl^-+SO₄ ^2-+5H₂O→Ca₄Al₂(OH)₁₂Cl(SO₄)·5H₂O (8)

6Ca(OH)₂+2Al³⁺+Cl^-+3SO₄ ^2-+26H₂O→Ca₆Al₂(OH)₁₂(SO₄)₃·26H₂O (9)

Wastewater treatment sludges are typically disposed of repeatedly in a smelting process or landfilled in a final disposal site. When a large amount of wastewater treatment sludge having a high water content is charged, the amount of fuel used in the smelting step increases, and the landfill treatment becomes insufficient due to the increase in the landfill amount, so that it is necessary to avoid an increase in the amount of sludge produced.

In the treatment method of the present embodiment, in order to suppress the formation of the layered double hydroxide and avoid an increase in the amount of sludge produced, the pH of the 3 rd treated water is adjusted to pH5.5 to 9.5, preferably pH5.5 to 6.5, which is slightly lower than the pH of the 3 rd treated water, so that aluminum is selectively precipitated, instead of adjusting the pH to pH9.5 to pH11.8 in one step. Since it is difficult to form a layered double hydroxide at a pH of 5.5 to 9.5, and almost all aluminum in the solution is precipitated as a hydroxide, the pH is adjusted to the above range to avoid an excessive increase in the amount of sludge formation and to form an aluminum precipitate, and the aluminum can be effectively removed by solid-liquid separation. As the neutralizer, calcium hydroxide, calcium oxide, sodium hydroxide, potassium hydroxide, and the like can be used.

In addition, when a calcium compound is used as a neutralizing agent for fluorine in the 3 rd treated water, calcium fluoride (CaF) is generated by reacting with fluorine in the solution₂) Since a fluorine compound is formed, fluorine can be efficiently removed from the solution while removing aluminum. The calcium fluoride produced has good filterability, and the solid-liquid separation ability can be significantly improved. In addition, although sodium fluoride or potassium fluoride generated when sodium hydroxide or potassium hydroxide is used as a neutralizing agent is easily dissolved, fluorine can be removed from the solution at the same time as aluminum is removed since fluorine ions in the solution are adsorbed to aluminum hydroxide precipitates. The precipitates recovered by solid-liquid separation into the 4 th treated water and the precipitates containing aluminum and fluorine mainly contain aluminum or fluorine, and therefore can be utilized as an aluminum resource or a fluorine resource.

In the aluminum and fluorine removal step, the ph is preferably adjusted to a range of 5.5 to 7.0 in order to suppress coprecipitation of arsenic or zinc remaining in the 3 rd treated water and to selectively precipitate aluminum and fluorine. For example, fluorine in the solution reacts with calcium hydroxide to form calcium fluoride precipitates, and the fluorine concentration in the solution rapidly decreases in the range of pH4.0 to pH5.5, and becomes almost zero in the vicinity of pH7. On the other hand, since arsenic or zinc in the solution is adsorbed to the calcium fluoride precipitate at a pH in the range of 4.0 to 7.0, the concentration in the solution gradually decreases, and when the pH exceeds 7.0 and becomes a basic region, a hydroxide or calcium salt starts to be partially formed, so that the decrease ratio of the zinc concentration and the arsenic concentration in the solution gradually increases. Accordingly, in order to suppress the formation of zinc or arsenic precipitates and promote the precipitation of aluminum and fluorine, it is preferable to control the pH to be in the range of 5.5 to 7.0. Since the aluminum and fluorine precipitates generated in this pH range are less contaminated with zinc or arsenic, they can be utilized as a firing raw material for aluminum resources or fluorine resources such as cement.

[ neutralization procedure ]

When zinc, cadmium, nickel, or the like remains in the residual liquid (treated water 4) from which the precipitate containing aluminum and fluorine has been separated, a neutralized precipitate of a heavy metal hydroxide such as zinc hydroxide, cadmium hydroxide, or nickel hydroxide is generated by further adding an alkali to the residual liquid so as to adjust the ph to a range of 9.5 to 11.8, and the neutralized precipitate is separated into alkali-neutralized treated water and a neutralized precipitate of a heavy metal hydroxide and removed. If the pH exceeds 11.8, the zinc hydroxide is not preferable because it is redissolved. By adjusting the pH to a range of 9.5 to 11.8, hydroxides such as zinc, cadmium, and nickel remaining in the residual liquid are generated and precipitated, and thus the neutralized precipitate can be removed by solid-liquid separation.

The gypsum with a small amount of fluorine and heavy metals is recovered through the above-mentioned series of treatment steps, and the amount of fluorine and heavy metals in the wastewater is reduced to meet the drainage regulations, so that the wastewater can be discharged to the outside of the system. Further, since the pH value of the alkali-neutralized treated water in the neutralized precipitate removal step is from pH9.5 to 11.8, the pH of the discharge water reference value is preferably from 5.8 to 8.6 for discharging, and therefore reverse neutralization can be performed by adding an acid. The recovered gypsum or precipitate can be effectively used as a cement raw material.

In the treatment method of the present embodiment, stable dissolution of fluorine and generation of heavy metal precipitates are simultaneously performed by dissolving aluminum in wastewater, and therefore, efficient treatment can be performed, and since gypsum is generated in a state where fluorine is stably dissolved in a solution, gypsum with an extremely small amount of fluorine can be obtained without mixing fluorine into gypsum. Further, a large amount of chemicals such as sulfuric acid is not required to clean gypsum. Therefore, the amount of water discharged can be reduced. In addition, the treatment method of the present embodiment does not use a sulfidizing agent, and therefore does not generate hydrogen sulfide, and is safe in working environment.

In the treatment method of the present embodiment, after the gypsum is recovered, heavy metals such as arsenic and molybdenum are adsorbed to the iron hydroxide precipitate to be separated, and therefore, the effect of removing these heavy metals is high and the burden of the removal operation is small.

In the treatment method of the present embodiment, when removing aluminum, the pH is adjusted to a pH range of slightly lower pH5.5 to 9.5 so as to selectively precipitate aluminum, instead of adjusting the pH to pH9.5 to pH11.8 in one step to generate hydroxide, and thus layered double hydroxide is not generated and the amount of sludge generated is not increased. Thus, the burden of sludge treatment is greatly reduced. Specifically, an increase in the amount of fuel used can be avoided in the smelting process of sludge, and the landfill amount can be suppressed in the landfill process to extend the life of the final disposal site.