JP5693992B2 - Method for recovering dissolved iron from wastewater containing various metal ions - Google Patents

Description

  The present invention relates to treatment of wastewater containing multi-metal ions mainly containing dissolved iron discharged from steelworks and steel slag treatment plants, and more particularly, separation of dissolved iron mainly composed of ferrous iron in wastewater. Can be carried out more efficiently and economically than the conventional method, and it is possible to suppress the mixing of metal ions other than iron into the precipitate, which is obtained in a state of reduced moisture content. The present invention relates to a method for recovering dissolved iron from wastewater containing various kinds of metal ions that can effectively use recovered iron.

  Conventionally, when iron is treated in large quantities, such as steelworks and steel slag treatment plants, a large amount of cooling water, surface treatment water, and other wastewater is generated. In addition to iron, various metal ions such as nickel, aluminum, manganese, chromium, zinc, cadmium, tin, and copper are contained. For this reason, when discharging or reusing wastewater, it is required to sufficiently remove these dissolved metals. As a method for removing dissolved metal, generally, neutralizing with an alkali agent to maintain the pH of the wastewater in a neutral to weakly alkaline state, and stirring (aeration treatment) while blowing a large amount of air In this method, the dissolved metal is oxidized with oxygen in the air, and the generated metal hydroxide is precipitated in the form of fine particles and removed by precipitation. However, since the precipitate (sludge) deposited by this method is mixed with metal hydroxides such as iron, zinc and nickel, when this metal hydroxide is reused as a metal resource, There was a problem that oxide could not be used effectively as an iron source because it became an obstacle.

  On the other hand, a method is also known in which the pH of waste water is maintained on the acidic side for treatment. In the acidic pH range, ferrous iron cannot be oxidized by air. In this case, it is oxidized by an oxidizing agent such as hydrogen peroxide or iron-oxidizing bacteria. According to this method, iron can be separated from zinc and nickel and precipitated, but the sludge obtained by this method has poor dewaterability and high water content, so that subsequent sludge treatment becomes difficult. Occurs.

  On the other hand, as a method of reducing the moisture content of the dehydrated cake, after adding an alkali agent to sludge settled and separated in a settling tank, the mixture is returned to the waste water and mixed, and the pH is maintained in a neutral to weakly alkaline state. In this process, dissolved iron in the waste water is oxidized with oxygen in the air to precipitate FeO (OH) particles. According to this method, the metal hydroxide particles can be enlarged to some extent, the dewaterability of the sludge can be improved, and the water content of the dewatered cake can be reduced (see Patent Document 1). As described above, in the method of treating dissolved iron by precipitating FeO (OH) particles, the pH is maintained in a neutral to weakly alkaline state. In existing technology, FeO (OH ) Was not crystallized and processed.

  In any of the above methods, the rate of oxidation of dissolved metal and metal hydroxide [for example, FeO (OH) particles] in the aeration tank (reaction tank) depends on the concentration of dissolved oxygen from the aerated air. The oxidation rate depends on the amount of dissolved oxygen supplied from the aerated air to the dissolved iron and the like flowing into the reaction tank. At this time, if the amount of aerated air is insufficient for various reasons, unreacted dissolved iron and the like are sent to the next settling tank while being dissolved, so in the aggregation / precipitation treatment step (separation step) There is a problem that the metal remains in the treated water without the dissolved metal being sufficiently removed.

  In contrast, the applicant has so far added most of the oxidation of dissolved iron and FeO (OH) in the reaction vessel by oxidation with hydrogen peroxide by adding hydrogen peroxide as an oxidizing agent to the reaction vessel. It is proposed to promote precipitation of FeO (OH) crystal particles by supplementing oxidation with hydrogen peroxide by oxidation with aerated air (see Patent Document 2).

JP-A-7-214073 JP-A-7-51684

The above-described conventional method for treating wastewater containing multi-metal ions, as described below, has room for improvement, particularly from the point of recovering and reusing dissolved iron. First, in the method of mixing an alkaline agent with the returned sludge, metal hydroxides such as iron, zinc, and nickel are mixed and precipitated, so when this metal hydroxide is reused as a metal resource, other than iron There is a problem that the metal hydroxide cannot be effectively used. Similarly, in the method using hydrogen peroxide as the oxidizing agent, it is necessary to adjust the pH to 6.5 to 8.3, so iron and zinc, nickel, etc. cannot be separated and recovered as an iron source. It is not suitable for reuse. According to the study by the present inventors, in particular, in the case of wastewater mainly composed of ferrous ions among dissolved iron, when other metal ions are contained, the resulting sludge is settled and concentrated. In addition, the dewaterability is lowered, and subsequent sludge treatment is not easy. Further, according to the study by the present inventors, when the FeO (OH) conversion is advanced, the start-up of the processing is slow, and there is a tendency that good processing cannot be performed in the initial stage of the processing. As described above, although a method for treating the wastewater by maintaining the pH on the acidic side is known, since the sludge obtained by this method is sludge mainly composed of Fe (OH) ₃ , the moisture content However, the sedimentation tank area becomes large when attempting to settle by gravity, and the sedimentation tank to be used needs to have a specification having acid resistance. there were.

  Therefore, the object of the present invention is to highly suppress the mixing of metal components other than iron into the generated sludge when treating wastewater containing various metal ions mainly containing dissolved iron mainly composed of ferrous iron. In addition, selective precipitation of iron components is possible, and the water content is low and the dewaterability of sludge is remarkably improved, so that iron can be recovered and reused economically from wastewater. Another object of the present invention is to provide an economical and useful method for recovering dissolved iron from wastewater containing various metal ions that contributes to effective use of resources. Another object of the present invention is to provide a method for recovering dissolved iron from useful multi-metal ion-containing wastewater that can be satisfactorily treated from the start of the treatment.

  The above object is achieved by the present invention described below. That is, the present invention uses a multi-metal ion-containing wastewater mainly composed of ferrous iron as a raw water and selectively oxidizes iron ions from the multi-metal ions in the raw water by continuously oxidizing the raw water. A method for recovering dissolved iron in raw water to be precipitated and separated into a neutralization oxidation tank of a treatment system for treating the raw water, adjusting the pH of the raw water to 3.5 to 6.0, Add δ-FeO (OH) as a catalyst in the sum oxidation tank and oxidize the dissolved iron with an oxidizing agent to produce metal hydroxides mainly composed of iron (III) hydroxide particles. Settling a sludge containing the metal hydroxide in a settling tank disposed downstream of the tank, and providing a return process for returning a portion of the settling sludge to the neutralization oxidation tank, In this step, a part of the sludge separated and precipitated is introduced into the sludge reaction tank, and the sludge reaction tank A method for recovering dissolved iron from wastewater containing various metal ions is characterized by treating raw sludge by adding an alkaline agent to the sludge and treating the raw water continuously while returning the treated sludge to the neutralization oxidation tank. To do.

  The following are mentioned as a preferable form of this invention. In the returning step, the amount of sludge to be returned into the neutralization oxidation tank is set to be 41 times or more of the theoretical metal hydroxide amount generated from the raw water in the neutralization oxidation tank. How to recover dissolved iron. In addition to the treatment system for treating the raw water, a preparation system for δ-FeO (OH) used in the treatment system is provided, and at least when the treatment system is started up, the preparation system for the δ-FeO (OH) Δ-FeO (OH) crystals produced by adding an alkali agent and hydrogen peroxide to an iron-based reagent containing divalent iron and mixing and reacting to produce δ-FeO (OH) crystal particles. A method for recovering dissolved iron in which fine particles are quickly added to a neutralization oxidation tank in the treatment system. A method for recovering dissolved iron, wherein the oxidizing agent is at least one selected from hydrogen peroxide, ozone, and hypochlorous acid.

  According to the present invention, when processing wastewater containing various metal ions mainly containing dissolved iron mainly composed of ferrous iron, iron in which metal components other than iron are highly suppressed in the generated sludge. Economy that contributes to the effective use of resources, because it can selectively precipitate components, and the water content is low and the dewaterability of sludge can be significantly improved, so that iron can be recovered from wastewater and reused. A method for recovering dissolved iron from wastewater containing various metal ions is provided. According to a preferred embodiment of the present invention, there is provided a method for recovering dissolved iron from useful multi-metal ion-containing wastewater that can be satisfactorily treated from the start of the treatment.

It is a schematic diagram of the processing flow of this invention. It is the figure which showed the processing flow of the comparative example 1 typically.

  Hereinafter, the present invention will be described in more detail with reference to preferred embodiments. FIG. 1 shows a schematic processing flow. The method for recovering dissolved iron in the wastewater containing multi-metal ions according to the present invention comprises adding δ-FeO (OH) as a catalyst to a neutralization oxidation tank of a treatment system for treating raw water and introducing the raw water into the neutralization oxidation tank The pH of the steel was adjusted to 3.5 to 6.0, and the dissolved iron in the waste water was oxidized with an oxidizing agent such as hydrogen peroxide, ozone and hypochlorous acid to mainly contain iron (III) hydroxide particles. It is characterized by producing a metal hydroxide. Further, in the sedimentation tank disposed on the downstream side of the neutralization oxidation tank for performing the above treatment, the sludge containing the metal hydroxide is precipitated and separated, and a part of the precipitated sludge is separated from the neutralization oxidation tank. In this step, a part of the sludge precipitated and separated is introduced into the sludge reaction tank, and the sludge is treated by adding an alkaline agent to the sludge reaction tank. The raw water is continuously treated while returning to the neutralization oxidation tank.

As a result of intensive studies to solve the problems of the prior art described above, the present inventors selectively processed only iron from various metals in the wastewater if the treatment is performed in a state satisfying all the following requirements. Further, the inventors found that the precipitate separated can be sufficiently reused as an iron source, and the present invention has been achieved.
(1) The treatment in the neutralization oxidation tank of the treatment system for treating the raw water is performed in a state where the pH of the raw water introduced into the tank is maintained at 3.5 to 6.0.
(2) Basically, oxidation is performed with an oxidizing agent such as hydrogen peroxide without aeration in the tank. Furthermore, at that time, δ-FeO (OH) is added as a catalyst for treatment, thereby generating a metal hydroxide mainly composed of iron (III) hydroxide particles.
(3) Furthermore, the sludge containing the metal hydroxide is precipitated and separated in a sedimentation tank disposed on the downstream side of the neutralization oxidation tank, and a part of the precipitated sludge is returned to the neutralization oxidation tank. While processing raw water continuously. When returning, a part of the sludge that has been separated by precipitation is introduced into a sludge reaction tank, an alkali agent is added to the tank to treat the sludge, and the treated sludge is returned to the neutralization oxidation tank.

Hereinafter, the steps (1) to (3) will be described.
<Step (1)>
First, the raw water targeted in the present invention is a waste water mainly containing dissolved iron and containing various metal ions such as nickel, aluminum, manganese, chromium, zinc, cadmium, tin, copper, This corresponds to the drainage of cooling water, washing water after pickling, surface treatment water, etc. used when processing a large amount of iron as in steelworks and steel slag treatment plants. These are often acidic at pH 2-3. In the present invention, it is necessary to treat the raw water in the neutralization oxidation tank while adjusting the pH of the raw water to 3.5 to 6.0. For this reason, an oxidizing agent is added while adjusting the pH by adding an alkali agent or an oxidizing agent into the neutralizing oxidation tank. Before the neutralization oxidation tank, a pH adjustment tank that adjusts the pH of the raw water may be provided, but in the present invention, the conventional method treated the pH as 7.5 to 9.0, Since it can process in the acidic side state adjusted to pH 3.5-6.0, Preferably it is about 4.0, it is more advantageous than the processing method of the waste_water | drain mainly containing the conventional dissolved iron. That is, it is possible to easily adjust the pH in the neutralization oxidation tank without providing a separate pH adjusting tank, and the amount of the drug for adjusting the pH can be smaller than before. In the method of the present invention, since the raw water is treated at a pH of 3.5 to 6.0, metals other than iron remain in the water without being precipitated. As a result, dissolved iron can be selectively recovered from the wastewater containing various metal ions. After dissolved iron, which accounts for most of the metals contained in the raw water, is precipitated, other metals remain dissolved in the treated water, but the amount is very small. It can be processed more easily than when it is done. The treated water can be reused by performing advanced treatment such as precipitation and filtration treatment, activated carbon adsorption treatment and the like, if necessary.

<Step (2)>
In the present invention, raw water whose pH is adjusted to 3.5 to 6.0 is oxidized with an oxidizing agent in a neutralization oxidation tank. In the present invention, the oxidation treatment is basically performed with an oxidizing agent without aeration in the tank. As the oxidizing agent, hydrogen peroxide, ozone, hypochlorous acid, or the like can be used. Among them, hydrogen peroxide is particularly preferable because it is economical and does not have a concern about secondary contamination. The amount added when using hydrogen peroxide water depends on the nature of the raw water, but is, for example, in the range of not less than 6 times the theoretical amount required for oxidation calculated from the iron content in the raw water, running cost and treatment If efficiency is considered, it should just add in the range of about 2-3 times.

  In the present invention, δ-FeO (OH) is added to the neutralization oxidation tank when it is oxidized with an oxidizing agent such as hydrogen peroxide, and this is made to function as a catalyst. In this manner, in the presence of δ-FeO (OH), the pH is adjusted to 3.5 to 6.0, and the treatment with the oxidizing agent is performed selectively without containing other metals. It becomes possible to produce a metal hydroxide mainly composed of iron (III) hydroxide particles.

  As shown in FIG. 1, δ-FeO (OH) is provided with a preparation system for δ-FeO (OH) for use in the treatment system separately from the treatment system for treating raw water. The prepared δ-FeO (OH) is preferably added as a catalyst. In other words, in this δ-FeO (OH) preparation system, δ-FeO (OH) crystal particles are prepared by adding an alkali agent and hydrogen peroxide to an iron-based reagent containing divalent iron and mixing and reacting them. Is generated. When the produced δ-FeO (OH) fine crystal particles are added at least to the neutralization oxidation tank in the treatment system at the time of starting the treatment system, and the oxidation treatment with the oxidizing agent is performed. The catalyst. In the example shown in FIG. 1, a part of the sludge obtained by the oxidation treatment using the oxidizing agent in the neutralization oxidation tank described above is returned to the sludge reaction tank, and the alkali agent is added to the sludge and returned. Sludge is added to the neutralization oxidation tank, and the δ-FeO (OH) crystal particles obtained by the above preparation are appropriately neutralized oxidation tank at least when the treatment system is started up or as necessary. It is the composition which can be added to. In this way, even when the oxidation treatment with the oxidant is started in the neutralization oxidation tank, the δ-FeO (OH) particles are efficiently obtained using the fine δ-FeO (OH) crystal fine particles as nuclei. Growth can be performed to selectively agglomerate and precipitate iron. As a result, selective recovery of dissolved iron in raw water can be performed more efficiently.

  The preparation system of δ-FeO (OH) is a portion surrounded by a dotted line in FIG. In the preparation system of δ-FeO (OH), for example, δ-FeO (OH) crystal particles are obtained by adding an alkali agent and hydrogen peroxide to an iron-based reagent containing divalent iron and mixing and reacting them. Generate. In particular, in order to be able to use the wastewater treatment system promptly at the time of start-up, δ-FeO (OH) using an iron-based reagent as a starting material with or prior to the start of oxidation treatment with an oxidizing agent of raw water. It is better to operate the crystal fine particle preparation system. For example, hydrogen peroxide is used to generate δ-FeO (OH) crystal particles from an iron-based reagent, and the generated crystal particles are quickly added as an oxidation catalyst into the neutralization oxidation tank to adjust the pH condition. The raw water in a specific range is oxidized to form a metal hydroxide so that most of the raw water becomes iron hydroxide. According to the study by the present inventors, the δ-FeO (OH) crystal fine particles exhibit the catalytic function more effectively by this configuration, and the raw water is oxidized from the start of the treatment system. Will be done well. When the δ-FeO (OH) crystal fine particles obtained in this way are present in the neutralization oxidation tank, the FeO (OH) generated by the oxidation is promptly generated immediately before use, and the oxidation catalyst. As a result, the crystal particle diameter of the δ-FeO (OH) added as can be grown. As a result, the sludge obtained in the coagulation tank is excellent in cohesiveness and sedimentation.

  According to the study by the present inventors, by adopting a method for precipitating δ-FeO (OH) crystal particles using an iron-based reagent containing divalent iron as described above, the start-up of the treatment and the subsequent treatment There is a significant difference in efficiency, and good treatment can be stably performed from the start of the treatment, and the recovery efficiency of dissolved iron can be improved.

  A preparation system including the mixing tank, the oxidation tank, and the neutralization tank shown in FIG. 1 for generating crystal particles of δ-FeO (OH) used in the present invention will be described. In the mixing tank, an iron-based reagent and an alkaline agent are mixed. As an iron-type reagent used in this case, for example, ferrous chloride, ferrous sulfate and the like can be used. Moreover, sodium hydroxide, slaked lime, calcium carbonate, etc. can be used as the alkaline agent. As a mixing ratio thereof, it is preferable that the iron concentration: alkali agent concentration is 1: 2 to 1:30. Mixing is performed with a stirrer. The mixture is then introduced into an oxidation bath and hydrogen peroxide is added with stirring. The amount of hydrogen peroxide to be added is preferably such that the iron concentration: hydrogen peroxide concentration (as a 35 mass% solution) is 1: 0.6 or more. From this process, δ-FeO (OH) fine crystal particles are generated, and then the reaction product is introduced into a neutralization tank and neutralized by adding hydrochloric acid or sulfuric acid as a neutralizing agent. In the present invention, it is preferable to add δ-FeO (OH) thus generated to the neutralization oxidation tank, for example, at the time of starting up the treatment system.

<Step (3)>
In the present invention, sludge containing a metal hydroxide mainly composed of iron (III) hydroxide particles is precipitated and separated in a sedimentation tank disposed downstream of the neutralization oxidation tank, and one of the sludges separated by precipitation is separated. The part is returned to the neutralization oxidation tank, and at that time, a part of the precipitated sludge is introduced into the sludge reaction tank, an alkali agent is added to the tank to treat the sludge, and the treated sludge is removed. Return to the neutralization oxidation tank. The amount of sludge to be returned is preferably at least 41 times the theoretical amount of metal hydroxide generated from the raw water in the neutralization oxidation tank. That is, as a result of conducting a basic study on the amount of sludge to be returned, it was found that when the amount is less than 41 times, aggregation is not sufficient and the treated water SS concentration is> 20 mg / L, and the quality of the treated water tends to be inferior. . Furthermore, when the returned sludge amount was less than 41 times, the water content of the separated sludge tended to increase. On the other hand, the upper limit of the amount of sludge to be returned is not particularly specified, but if the treatment magnification exceeds 120 times, aggregation is promoted too much, and the treated water SS concentration becomes> 20 mg / L, and the quality of the treated water may be inferior. 120 times or less is recommended. Based on the above examination results, it was determined that a suitable range in which more stable operation is possible is about 80 to 120 times, more preferably about 80 to 100 times.

  According to the study by the present inventors, for example, after the sludge is returned in the amount as described above, in the state where the pH of the raw water in the neutralization oxidation tank is adjusted to 3.5 to 6.0, the peroxidation is performed. By performing oxidation treatment with an oxidizing agent such as hydrogen, continuous treatment of raw water can be made more balanced. That is, if comprised as mentioned above, since only the iron content in many types of metal ions contained in raw water will precipitate selectively, it will become possible to collect | recover dissolved iron. According to the study by the present inventors, when treated by the above method, the precipitate (sludge) has a low water content and a remarkably improved dewaterability.

  In the example shown in FIG. 1, after reacting as described above in a neutralization oxidation tank to produce (precipitate) a metal hydroxide mainly composed of iron (III) hydroxide particles, Incorporated into the slag, agglomeration treatment is performed to improve the iron recovery efficiency. That is, by adding a flocculant into the coagulation tank and mixing and stirring, the dissolved iron can be selectively and efficiently precipitated to form sludge. As the flocculant at this time, conventionally known polymer flocculants and the like can be used. The following is performed in the coagulation tank. That is, in the neutralization oxidation tank, the δ-FeO (OH) fine particles in a state of being folded and floating in the raw water without being folded on the surface of the crystal fine particles of δ-FeO (OH) added as a catalyst, Aggregate growth can be performed using δ-FeO (OH) crystal fine particles having a large particle diameter as nuclei. As a result, the recovery efficiency of dissolved iron can be improved. The precipitated sludge is withdrawn from the bottom of the precipitation tank, and a part thereof is returned to the neutralization oxidation tank through the sludge reaction tank. In addition, surplus sludge that has not been returned to the neutralization oxidation tank is sent to a sludge concentration tank or sludge storage tank (not shown), where sludge treatment such as sludge concentration or sludge dewatering is performed, and finally it is recycled as iron-based material. Used.

The present invention will be described more specifically with reference to examples and comparative examples.
[Example 1]
A verification test was conducted using the test plant shown in FIG. Table 1 shows the quality of raw water used in the test. This raw water is waste water from an iron mill, a steel slag treatment plant, a plating factory, or the like, which is a treatment target in the present invention, and contains iron as a main component and contains at least zinc, nickel, and chromium. The pH of the raw water was about 2.

  In this example, δ-FeO (OH) was prepared by the δ-FeO (OH) preparation system shown in FIG. 1 when the wastewater treatment system was started up (at the start of operation of the treatment system). Was used. Specifically, in a mixing tank having a capacity of 1 L, 60 mL of a 31% by mass aqueous solution obtained by dissolving ferrous chloride containing divalent iron (manufactured by Hiroshima Wako Co., Ltd.) in water was added. 940 mL of 4 mass% sodium hydroxide (NaOH) aqueous solution was put in and mixed well. Next, the obtained mixture was led to an oxidation tank, and 8,000 mg / L of 35 mass% hydrogen peroxide was added to the oxidation tank, and oxidation treatment was performed with sufficient stirring. Next, this was led to a neutralization tank, and neutralized so that the pH was around 8 using a hydrochloric acid solution as a neutralizing agent. As a result, a precipitate was generated in the neutralization tank, and this precipitate was confirmed to be fine crystal particles of δ-FeO (OH) of about 1 to 20 μm.

In this example, the fine crystal particles of δ-FeO (OH) prepared at the start of the treatment as described above were put into a neutralization oxidation tank at the start of the treatment for treatment. In that case, it added so that the addition amount of (delta) -FeO (OH) might be set to 10,000 mg / L with respect to raw | natural water, and stopped adding after that and processed.
After adding δ-FeO (OH), the raw water is first introduced into a neutralization oxidation tank having a capacity of 15 L, and a 25% by mass solution of sodium hydroxide as an alkali agent is added to adjust the pH to 3. It was adjusted to 5-4.0, and oxidation treatment with hydrogen peroxide was performed in a neutralization oxidation tank in a state where this value was maintained, and iron ions were crystallized as iron hydroxide. At that time, raw water was introduced into the neutralization oxidation tank at a rate of 1 L / min for treatment. Moreover, in this Example, as shown in FIG. 1, a part of the sludge settled in the settling tank was returned to the neutralization oxidation tank through the sludge reaction tank. At that time, in a sludge reaction tank, it was treated with a 25% by mass solution of sodium hydroxide as an alkali agent to form alkali sludge. The amount of sludge that was circulated back was 100 times the amount (weight) of SS generated from raw water. For example, taking the case of raw water treated in the present embodiment as an example, if the Fe ²⁺ containing 400 mg / L, Fe ²⁺ all if the FeO (OH), 400 × 89 /56 = 636mg / L SS occurs. The circulation return amount at this time is 636 × 100 = 63600 mg / L (≈64 g / L). Therefore, when the return sludge concentration is designed to be 200 g / L, 0.32 L of return sludge may be returned.

  As shown in FIG. 1, the mixed liquid containing the precipitated iron hydroxide that has been processed in the neutralization oxidation tank is then sent to the agglomeration tank for aggregation treatment. In the flocculation tank, the polymer flocculant was added to a concentration of 15 mg / L to perform the flocculation treatment. At that time, KEA-730 (trade name: manufactured by Nippon Steel Environmental Engineering Co., Ltd.) was used as a polymer flocculant. Next, the raw water treated after the treatment in the coagulation tank was introduced into the precipitation tank, and the flocs aggregated in the precipitation tank were settled and separated. The sludge (sludge) separated by precipitation was retained and concentrated in the precipitation tank. In this example, a part of the sludge was treated in the sludge reaction tank as described above, and returned to the reaction tank as an alkaline sludge to be circulated and returned. Excess sludge in the coagulation tank was pulled out to complete the treatment. As described above, the above treatment was continuously performed for 500 hours while introducing raw water as actual waste water at a speed of 1 L / min.

  The treated water obtained by the above treatment was sampled every 0.5 to 24 hours, and the iron ions in the treated water were measured. The concentration was 1.0 mg / L or less, and the dissolved iron was separated. It was confirmed that it was extremely efficient. In particular, it has been found that stable treatment can be realized at a high level from the beginning to the end of the treatment. Moreover, since zinc and nickel ions remained in the treated water to the same extent as the raw water, it was found that iron, zinc and nickel could be separated effectively. Furthermore, when SS in the treated water was measured, it was 3 mg / L or less, and it was confirmed that sedimentation separation was efficiently performed. Moreover, as a result of sampling and investigating the properties of sludge discharged as excess sludge every 1-24 hours, it is superior in dehydration compared to the comparative example, is easy to handle, and has an extremely high Fe content. It was confirmed. Furthermore, it was confirmed that the sedimentation rate was remarkably increased. This point will be described later.

[Comparative Example 1]
In this comparative example, the same raw water as that used in the examples was processed using the experimental apparatus that does not return the sludge shown in FIG. The neutralization oxidation tank was adjusted to pH 4.0. When the metal ions in the treated water obtained by the treatment in this way were measured, it was confirmed that the treated water had the same water quality as in the examples. However, as will be described later, in addition to being difficult to handle because the dewaterability of the sludge obtained by the treatment is inferior compared to the examples, the iron concentration in the dewatered cake is relatively It turned out to be lower.

[Evaluation]
About the characteristic of each sludge settled and separated in the Example and the comparative example, the dehydrating property, the solid substance component, and the sedimentation property were investigated with the following method. As a result, it was confirmed that the sludge settled and separated in the examples was remarkably excellent in dewaterability and the iron content was significantly higher than the sludge settled and separated in the comparative example. This is because the sludge obtained by the process of the present invention is a component mainly composed of iron (III) hydroxide from the cake component, and the moisture content is as low as 42% by mass. Therefore, operations such as kneading with other raw materials are performed. Means that it can be used as an iron source. This means that according to the method of the present invention, the treatment of the waste iron-containing wastewater containing mainly dissolved iron and using the wastewater containing various metal ions such as zinc and nickel as raw water is economically performed. . Moreover, since the sedimentation speed is high, it is possible to reduce the size of sedimentation basins such as thickeners and improve the processing efficiency.

(1) Dewaterability of sludge The sludge discharged from the settling tank in the examples and comparative examples was sampled every 1-24 hours, and the dewaterability thereof was examined as follows. Specifically, the SS concentration of the sludge discharged is adjusted to 100 g / L, and pressure dehydration is performed under the condition of using a filter cloth with a ventilation rate of 15 cm ³ / cm ² / sec at a pressure of 0.3 MPa. The water content of the dehydrated cake was measured. The obtained results are shown in Table 2.

(2) Metal ion content of sludge Measurement results of metal ion content of dewatered cake obtained by dewatering sludge discharged from the sedimentation tank in Examples and Comparative Examples by the above method Are shown in Table 2.

(3) Sediment sedimentation The liquid in the coagulation tank in Examples and Comparative Examples is sampled every 1-24 hours, taken into a 7 L graduated cylinder, and settled for 10 minutes from the start of sedimentation. Table 3 shows the sedimentation rate.

Claims (3)

Raw water containing multi-metal ion-containing wastewater mainly composed of ferrous iron as raw water, and continuously oxidizing the raw water to selectively precipitate iron ions from multi-metal ions in the raw water A method for recovering dissolved iron of
The raw water is introduced into the neutralization oxidation tank of the treatment system, the pH of the raw water is adjusted to 3.5 to 6.0, and δ-FeO (OH) is added to the neutralization oxidation tank as a catalyst. Then, oxidized iron is oxidized with an oxidizing agent to form a metal hydroxide mainly composed of iron (III) hydroxide particles. At that time, the above-mentioned δ-FeO (OH) is added to treat the raw water. In addition to the system, a preparation system of δ-FeO (OH) used in the treatment system is provided, and at least when the treatment system is started up, the preparation system of δ-FeO (OH) contains divalent iron. By adding an alkali agent and hydrogen peroxide to an iron reagent, mixing and reacting to produce δ-FeO (OH) crystal particles, and adding the generated δ-FeO (OH) crystal particles Done
Precipitating and separating the sludge containing the metal hydroxide in a precipitation tank disposed downstream of the neutralization oxidation tank; and
A return step for returning a part of the sludge separated and precipitated to the neutralization oxidation tank is provided. In this step, a part of the sludge separated and precipitated is introduced into the sludge reaction tank, and an alkali agent is added to the sludge reaction tank. A method for recovering dissolved iron from multi-metal ion-containing wastewater, wherein the raw water is continuously treated while adding sludge to treat the sludge and returning the treated sludge to the neutralization oxidation tank.   In the returning step, the amount of sludge to be returned into the neutralization oxidation tank is set to be 41 times or more of the theoretical metal hydroxide amount generated from the raw water in the neutralization oxidation tank. The method for recovering dissolved iron according to claim 1. The method for recovering dissolved iron according to claim 1 or 2 , wherein the oxidizing agent is at least one selected from hydrogen peroxide, ozone, and hypochlorous acid.

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