JPH11300372A - Treatment of selenium-containing liquid - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an extremely practical means as a method for removing selenium from a selenium(VI)-contg. liq. by which the entire selenium is surely removed below the effluent standard value at ordinary temps. at a low cost and in a short time. SOLUTION: An aq. ferrous salt soln. is added to a selenium(VI)-contg. liq. to perform reduction in a non-heated state. In this case, the liq. being treated is regulated to pH 8.5-9.5 in the first stage, to pH 1.5-7.5 in the second stage and to pH 8.5-9.5 in the third stage, the reduced liq. is kept at >=pH 11, air is blown into the liq. to oxidize the ferrous salt, then the liq. is neutralized to pH 5.8-7, and the precipitates are separated from the liq.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a treatment method for removing selenium from a liquid containing hexavalent selenium, and more particularly to a practical method for ensuring that all selenium is below a specified value of a drainage standard by treatment in a normal temperature range. Regarding the processing method.

[0002]

2. Description of the Related Art Wastewater discharged from an absorption tower of a flue gas desulfurization unit of various factories including a factory for manufacturing colored glass products, wastewater used to extinguish coal in a thermal power plant,
Wastewater and the like generated from the process of treating the electrolytic precipitate of metal and the process of manufacturing the selenium rectifier contain a relatively high concentration of toxic selenium. Since the specified value based on the selenium effluent standard is 0.1 mg / L or less, the wastewater must be subjected to advanced treatment to remove selenium to the specified value or less, and selenium is scarce as a resource. It is desirable to collect and reuse. However, although there have been many proposals for removing selenium from this wastewater, a treatment method that is sufficiently satisfactory in terms of removal efficiency and treatment cost has not yet been established.

In general, many of the selenium contained in various waste water coexist in the form of SeO ₃ ^2- (4-valent selenium) and SeO ₄ ^2- (6-valent selenium). For the former tetravalent selenium, ferric salt is dissolved in wastewater and hydrolyzed,
Although it can be easily removed by a method of adsorbing and coprecipitating with the precipitated ferric hydroxide, it is known that hexavalent selenium can hardly be removed by this method. Therefore, in a method of treating selenium-containing wastewater proposed in recent years, it is common to reduce and add hexavalent selenium by adding a reducing agent and then perform adsorption and coprecipitation on iron precipitates.

[0004] For example, in the treatment method of Japanese Patent Application Laid-Open No. 9-150164, in the preliminary treatment in the first stage, the solid-liquid wastewater containing selenium heated to 40 to 80 ° C. (preferably 55 to 65 ° C.) A precipitate containing ferrous hydroxide obtained by separation and a neutralizing agent are added and pre-reduced, and the precipitate is filtered by precipitating, and the filtration residue is separated and removed. In this treatment, the solution temperature is maintained in the above range, ferrous sulfate and a neutralizing agent are added and reduced, and the same sedimentation treatment and filtration are performed. And the filtrate and the supernatant of the sedimentation treatment are discharged. Also, Japanese Patent Application Laid-Open No. 9-299964
In the treatment method of No. 1, the liquid temperature is 50 to 95 ° C. (preferably 8
Ferrous salt was added to the selenium-containing wastewater
Next, after adjusting the pH to 7 to 13 and maintaining the solution for a certain period of time, the pH is set at least two times lower than before while maintaining the liquid temperature, and the solution is maintained for a certain period of time. The selenium thus reduced is removed by solid-liquid separation. I do. Further, JP-A-9-15536
In the treatment method of No. 3, the selenium-containing waste water is brought into contact with a selenium alloy such as ferro-selenium to reduce hexavalent selenium, a ferrous salt is added to the liquid to be treated after the reduction, and then neutralized to remove air. By blowing, selenium co-precipitates with the precipitated iron component and is removed by solid-liquid separation.

[0005] However, although these treatment methods are described as being able to finally remove selenium to 0.1 mg / L or less of the wastewater standard, they are practically impossible means for applying to the treatment of wastewater generated in large quantities. Is hard to say. That is,
In the former two treatment methods, it is necessary to carry out the reaction while maintaining the liquid to be treated at a high temperature, and in order to raise the temperature of a large amount of wastewater, enormous heat energy is consumed, and in addition, as a reducing agent Since the amount of iron salt used is large and a large amount of processing residue is generated, it is difficult to adopt it in terms of both processing cost and environmental protection. In addition, the latter treatment method requires the use of an expensive selenium alloy, which makes it impossible to treat a large amount of wastewater in terms of cost, and requires a long time such as tens of hours for reduction. After all it is unrealistic.

Although many other proposals have been made regarding the treatment of selenium-containing wastewater, they are not practical due to the treatment cost and the like, and a selenium removal efficiency as high as claimed can not be obtained in practice. Due to difficulties, none of them has been established as an established processing method.

The present invention has been made in view of the above circumstances, and as a treatment method for removing selenium from a liquid containing hexavalent selenium, a method for removing all selenium in a short time at low cost by a treatment in a normal temperature range, particularly at a low temperature. It is an object of the present invention to provide an extremely practical means that can be reliably removed to a specified value or less.

[0008]

Means for Solving the Problems In the course of diligent studies to achieve the above object, the present inventors first examined factors that have not established a satisfactory selenium removal method in the prior art at the time of each processing operation. It was considered that the behavior of selenium in water was not sufficiently grasped, and various tests described below were performed in order to know the behavior accurately.

In these tests, non-heating (room temperature 24 ° C., maximum temperature at which the liquid temperature reached 29 ° C.) was carried out throughout the treatment steps, and the reduction treatment step was sealed in order to cut off contact with air. In addition, in order to adjust to the composition of the general selenium-containing wastewater, the liquid to be treated contains sulfate ions, which are contained in the normal wastewater and are also components that hinder the reduction of hexavalent selenium, and inorganic salts such as sulfuric acid. Sodium and sodium chloride were added at a high concentration of 15000 mg / L each. Moreover, those in the ferrous salt as the reducing agent dissolved in hydrochloric acid solution 0.4N ferrous sulfate _{(FeSO 4 · 7H 2 O)} , selenium oxide is tetravalent selenium (SeO ₂₎ water And selenous acid (H ₂ SeO ₃ ) dissolved in water, and sodium hexaselenate (Na ₂ SeO ₄ ) dissolved in water is used for hexavalent selenium, and caustic soda and hydrochloric acid are used for pH adjustment. Was used.

<Test 1> [Component Concentration in Liquid to be Treated] Sodium sulfate (as Na ₂ SO ₄ ): 15000 mg / L Sodium chloride (as NaCl): 15000 mg / L Ferrous sulfate (Fe ²⁾ each p of ⁺ a) as ···· 1500mg / L 4-valent selenium (Se ⁴⁺⁾ ······ 20mg / L [test method] table 1 below describes a liquid to be treated having the above composition
After air was blown into the liquid under non-heating (room temperature 24 ° C.) while adjusting to H, ferrous ions were oxidized by aeration for 20 minutes. Filter with 5C filter paper,
Total selenium (T-Se) and tetravalent selenium (Se ⁴⁺ ) in the filtrate
Was measured. The results are shown in Table 1 below.

[0011]

[Table 1]

As is clear from the above Table 1, although the original liquid does not contain hexavalent selenium,
Hexavalent selenium is generated in a pH range of 10 or less by aeration for one minute. Therefore, this hexavalent selenium is nothing but the one generated by the oxidation of tetravalent selenium, that is, the one generated by the ancestral phenomenon. This is because even if hexavalent selenium in wastewater is reduced to tetravalent selenium by adding ferrous salt,
This means that the reduced tetravalent selenium or the tetravalent selenium present in the original wastewater can return to hexavalent selenium in the oxidation process by the next aeration. It is a fatal problem in removing below. From Table 1, it can be seen that the production of hexavalent selenium increases as the pH decreases, whereas the removal rate of tetravalent selenium by adsorption to the precipitated iron component increases as the pH decreases.

<Test 2> [Component Concentration in Liquid to be Treated] Sodium sulfate (as Na ₂ SO ₄ ): 15000 mg / L Sodium chloride (as NaCl): 15000 mg / L Ferric chloride (Fe ^{3 +} as) ···· 1500mg / L 4-valent selenium (as Se ⁴⁺⁾ ······ 21mg / L [test method] of the following table 2 according to the liquid to be treated with the above composition in the same manner as in test 1 After aeration for 20 minutes while adjusting to each pH, no. The solution was filtered through a 5C filter paper, and the concentrations of total selenium (T-Se) and tetravalent selenium (Se ⁴⁺ ) in the filtrate were measured in the same manner as described above. The results are shown in Table 2 below.

[0014]

[Table 2]

As is clear from Table 2, even if the liquid to be treated is aerated in the presence of a ferric salt, hexavalent selenium is hardly produced. Although the difference between the results of Test 2 and Test 1 is theoretically difficult to elucidate, the ancestral phenomena in which tetravalent selenium is oxidized to generate hexavalent selenium during the aeration treatment process is based on ferric (Fe) ³⁺ ) alone, suggesting that it is caused by the coexistence of ferrous iron (Fe ²⁺ ).

Thus, the results of Tests 1 and 2 provide an important index for setting the conditions of each treatment step in selenium removal through the reduction-aeration-adsorption step using ferrous salt as a reducing agent. That is, in order to finally make the total selenium concentration equal to or less than the regulation value of the wastewater standard, first, the hexavalent selenium concentration in the liquid to be treated is reduced to 0.1 mg / L or less in the reduction treatment, and then the aeration treatment is performed as described above. In order to avoid the ancestral phenomenon, the ferrous iron is oxidized by setting the pH to 11 or more, and in the subsequent adsorption treatment, the pH is lowered to efficiently adsorb tetravalent selenium to the precipitated iron component. In the next test, the liquid to be treated containing hexavalent selenium is treated under the condition setting based on the above index.

<Test 3> [Component Concentration in Liquid to be Treated] Sodium sulfate (as Na ₂ SO ₄ ): 15000 mg / L Sodium chloride (as NaCl): 15000 mg / L Ferrous sulfate (Fe ^{2 +} as) as ···· 1500mg / L 6-valent selenium (Se ⁶⁺⁾ ······ 20mg / L [test method] the liquid to be treated having the above composition (stock solution pH 1.5) unheated under (room temperature (At 24 ° C.) to reduce the hexavalent selenium by adjusting the pH to 8.5 to 9.5 while maintaining treatment 50 for 50 minutes and treatment II for 80 minutes to reduce the hexavalent selenium. By blowing air, aeration is performed for 20 minutes to oxidize ferrous ions, and then the pH is increased to 6.5.
Selenium was adsorbed on the precipitated iron component (mainly ferric hydroxide), and then filtered through No. 5C filter paper, and all selenium (T-Se) and tetravalent selenium in the filtrate were adjusted. The concentration of (Se ⁴⁺ ) was measured. The results are shown in the following Table 3 together with the pH and the holding time of each treatment step.

[0018]

[Table 3]

As is evident from the results in Table 3, even if the reduction step is as long as 80 minutes, the amount of hexavalent selenium is 0.41 mg / L.
And it remains in a considerable proportion. This implies that hexavalent selenium cannot be sufficiently reduced even in a long period of time in the reduction treatment in the normal temperature range, and it is basically difficult to reduce the final total selenium concentration below the regulation value of the wastewater standard. Represents.

The following summarizes the behavior of selenium in water based on the results of Tests 1 to 3 and technical information conventionally known. Tetravalent selenium is well adsorbed by precipitated iron components such as ferric hydroxide, but hexavalent selenium is hardly adsorbed. Tetravalent selenium tends to be easily oxidized to hexavalent selenium when brought into contact with air in the presence of divalent iron, but does not show this tendency in the presence of trivalent iron alone. The lower the pH, the higher the adsorption rate of tetravalent selenium to the precipitated iron component, while the lower the pH, the lower the production of hexavalent selenium due to the oxidation described above. Although metal selenium is stable to dilute acids and alkalis, in most treatment methods, reduction of hexavalent selenium is limited to tetravalent selenium, and is rarely reduced to metal selenium. The reduction of hexavalent selenium with ferrous salts is incomplete at room temperature by simply increasing the time.

[0021] The above and above indicate the difficulty of highly removing selenium from wastewater containing hexavalent selenium. In other words, hexavalent selenium is reduced only to tetravalent selenium by the usual reduction treatment with ferrous salt, and tetravalent selenium is an ancestral phenomenon of a term induced in the next aeration process for oxidizing ferrous iron. This makes it easy to return to hexavalent selenium, and this hexavalent selenium remains without being adsorbed by the precipitated iron component due to the properties of the term. On the other hand, the term suggests a condition for avoiding the ancestor return phenomenon of the term and increasing the adsorption and removal rate of tetravalent selenium. Is not sufficiently removed. However, means for heating the liquid to be treated to prevent hexavalent selenium from remaining or using a special reducing agent capable of reducing hexavalent selenium to metallic selenium is, as described above, in actual wastewater treatment. Can not be adopted.

Thus, the present inventors have conducted further intensive studies. As a result, even if the reduction time is lengthened, unreduced hexavalent selenium remains because the reduction reaction does not proceed due to the phase equilibrium of the reaction system. It was determined that this was caused by the change in pH during the reduction treatment, thereby shifting the phase equilibrium of the system, thereby changing the concentrations and existing forms of tetravalent and hexavalent selenium in the system, and reached the idea of increasing the processing efficiency. That is, tetravalent selenium has a tendency that the higher the pH, the lower the adsorption rate by the precipitated iron component, that is, the higher the pH, that is, the higher the concentration in the liquid, the higher the pH, but the pH range of the reduction treatment (about 8.5 to 9.5) is The reaction system is [Se ⁶⁺ ⇔S
e ⁴⁺ {adsorption]. In this phase equilibrium, the concentration of hexavalent selenium is necessarily governed by the concentration of tetravalent selenium, but when the pH is changed to a lower side during the treatment, the amount of tetravalent selenium adsorbed on the precipitated iron component is reduced. When the pH is then raised again to the original range, the above phase equilibrium is surely at a lower concentration than before the pH change. That is, the concentration of hexavalent selenium is reduced.

Thus, based on the above findings, when selenium removal through a reduction-aeration-adsorption process using ferrous salt as a reducing agent is carried out in a normal temperature range, the pH during the reduction treatment was lowered. As a result of extensive experimental research on a treatment method involving a pH change of returning above, all selenium can be reliably removed to below the regulation value of wastewater standards by this method, and it is also superior in terms of treatment cost and treatment efficiency. It has been found that the method has a high suitability for treating a large amount of wastewater, and the first method for treating a selenium-containing liquid according to the present invention has been established. That is, according to this treatment method, the reduction treatment can be carried out efficiently in a short time despite the treatment in the normal temperature range, and the newly discovered tetravalent when the above-mentioned pH is lowered to a lower side. Due to the unique behavior of selenium, sufficient reduction efficiency can be achieved even when the amount of ferrous salt used is reduced, so that the amount of solids generated by final solid-liquid separation is reduced, and selenium is contained in this solids. Since selenium is concentrated to a high concentration, there is an advantage that selenium in the liquid to be treated can be efficiently recovered in a highly concentrated state.

Further, the present inventors have focused on the peculiar behavior of tetravalent selenium when the above-mentioned pH is lowered to a lower side, and further examine whether or not a more efficient processing means utilizing this behavior is possible. As a result, p is the same as in the first processing method.
In the case where the treated material is completely filtered after the reduction treatment accompanied by the change in H, selenium in the liquid to be treated concentrates in the filtration residue, and almost no selenium is present in the filtrate. The inventors have found that the aeration-adsorption treatment step in the method can be omitted, and succeeded in constructing a second treatment method for a selenium-containing liquid according to the present invention.

That is, the specific behavior of the above-mentioned tetravalent selenium means that, in addition to the assumed adsorption by the precipitated iron component,
The reason is that selenite, which is tetravalent selenium, easily bonds with divalent iron ions to form hardly soluble iron selenite.
This iron selenite has the property of dissolving in the alkali side region and the strongly acidic region, but has a sufficiently low solubility in the pH range of 2.5 to 8. Therefore, after performing the sufficient reduction treatment accompanied by the pH change, if the pH is set to the low range and the whole filtration is performed, substantially the entire amount of selenium contained in the liquid to be treated is reduced to the precipitated iron component ( Mainly ferric hydroxide)
Contained in the filtration residue as adsorbed to and iron selenite,
In addition, since excess ferrous iron dissolves and moves to the filtrate side, the amount of the filtration residue is further reduced as compared with the first treatment method, and selenium can be recovered as a higher-concentration state. It will be easier.

The first method for treating a selenium-containing liquid according to the first aspect of the present invention is a method for treating a selenium-containing liquid, which comprises adding a ferrous salt aqueous solution to a hexavalent selenium-containing solution and performing the treatment without heating. The pH of the liquid in the first step is adjusted to 8.5 to 9.5, the second step to 1.5 to 7.5, and the third step to 8.5 to 9.5. After the treatment, air is blown into the liquid while maintaining the pH of the liquid at 11 or more to oxidize ferrous iron, and then the liquid is neutralized to pH 5.8 to 7, and then the precipitate is solid-liquid separated. It is assumed that. The second method for treating a selenium-containing liquid according to claim 2 of the present invention is characterized in that, in the reduction treatment performed by adding an aqueous ferrous salt solution to a solution containing hexavalent selenium and without heating, The pH of the solution was adjusted to be 8.5 to 9.5 in the first step, 1.5 to 7.5 in the second step, and 8.5 to 9.5 in the third step. The method is characterized in that the pH of the liquid is lowered to 2 to 7.5 and the whole filtration is performed.

However, in these first and second treatment methods, the holding time of the first stage in the reduction treatment is set to 5 hours.
Claim 3 in which the holding time of the second stage is set to 1 minute or more and the holding time of the third stage to 5 minutes or more, respectively, can be cited as a preferred embodiment.

[0028]

BEST MODE FOR CARRYING OUT THE INVENTION In the first and second methods for treating a selenium-containing liquid according to the present invention, a reduction treatment is performed by adding an aqueous ferrous salt solution to a liquid containing hexavalent selenium without heating. Adjusts the pH of the solution during the reduction treatment, and in the first step, pH 8.
After setting the pH in the range of 5 to 9.5, the pH is reduced to the range of 1.5 to 7.5 in the second stage, and then returned to the original range of 8.5 to 9.5 in the third stage. By 6
The reduction of the selenium (II) proceeds to the utmost, so that the total selenium in the liquid after the final solid-liquid separation is reduced to 0.1 mg / L or less, which is the regulation value of the wastewater standard.

That is, in the pH range of the first stage of the reduction treatment, a phase equilibrium of [Se ⁶⁺ ⇔Se ⁴⁺ ⇔adsorption] is established in the reaction system, and the concentration of hexavalent selenium is equal to that of tetravalent selenium. However, by lowering the pH in the second step, a significant proportion of the tetravalent selenium is excluded from the reaction by increasing adsorption to the precipitated iron component and producing iron selenite. Therefore, when the pH is raised again to the original region as the third step, the above-mentioned phase equilibrium is established at a lower concentration than before the pH change, so that in this third step, Proceeds until the reduction of hexavalent selenium, which was hindered by the phase equilibrium in the first stage, reaches the low concentration of phase equilibrium governed by the reduced concentration of tetravalent selenium. Therefore, the unreduced hexavalent selenium remaining at the end of the reduction treatment step has a low concentration sufficiently lower than the regulated value of the wastewater standard of 0.1 mg / L.

The pH of the first and third steps is as follows:
It is 6 even if it deviates on either side higher or lower than the range of 8.5 to 9.5.
It is not desirable because the reduction efficiency of selenium (IV) decreases. Although the pH of the second stage is broad from the strongly acidic region to the neutral region as described above, the shift of the phase equilibrium to the lower concentration side does not occur on the alkaline side than 7.5, and the pH of the reduction treatment step is reduced. The amount of unreduced hexavalent selenium remaining at the end of time sharply increases.
For pH adjustment, common alkali components and acid components conventionally used for pH adjustment, such as caustic soda and hydrochloric acid, can be used.

Here, the preferable holding time of each step in the reduction treatment is about 5 minutes or more in the first step, about 1 minute or more in the second step, and about 5 minutes or more in the third step. If it is shorter than the above range, the reduction of hexavalent selenium is insufficient. On the other hand, if it is too long, the reduction of hexavalent selenium does not hinder, but the whole process takes time and is inefficient. However, although this reduction process is performed in a normal temperature range, it takes only a few minutes to a few pick-ups throughout the entire process, so it can be said that the processing efficiency is extremely good.

In the first treatment method of the present invention, air is blown into the liquid while maintaining the pH of the liquid at 11 or more after the above-mentioned reduction treatment to oxidize ferrous iron, and then the liquid is treated with a pH of 5.8-7.
After the neutralization, the precipitate is subjected to solid-liquid separation. That is, by blowing air and aerating, ferrous iron is oxidatively hydrolyzed to mainly precipitate ferric hydroxide, and the precipitated iron component mainly composed of ferric hydroxide is added to the liquid after the reduction treatment. The tetravalent selenium dissolved therein is adsorbed and migrates to the solid content side, whereby most of the selenium contained in the liquid to be treated is removed together with the precipitate by the subsequent solid-liquid separation.

Here, the pH of the solution in the above-described aeration treatment
Is set to 11 or more in order to avoid the above-mentioned ancestor return phenomenon in which tetravalent selenium is oxidized by contact with air in the presence of divalent iron and returns to hexavalent selenium. As shown in Table 1, when this pH is lower than 11, an ancestral phenomenon is induced, so that it is extremely difficult to finally reduce the selenium concentration to the regulated value of the wastewater standard. The aeration time is set until almost all of the ferrous iron is oxidized to trivalent iron.

The reason why the solution is neutralized to pH 5.8 to 7 after the aeration is to promote adsorption of tetravalent selenium by the precipitated iron component mainly composed of ferric hydroxide, and this pH is too high. On the other hand, there is a problem that even if the adsorption is insufficient, the adsorption of tetravalent selenium is incomplete even if the adsorption is too low, and the drainage regulation value of 0.1 mg / L is not satisfied. Further, considering the drainage standard of pH, the lower limit is preferably set to pH 5.8.

For the final solid-liquid separation, sedimentation treatment with a thickener or the like, filtration with an appropriate filter medium, a combination of sedimentation treatment and filtration of the precipitate can be employed, and in that case, an appropriate method for promoting solid-liquid separation can be used. Any suitable coagulant or sedimentation agent may be added.
The supernatant and the filtrate of the sedimentation treatment can be discharged as they are because the total selenium concentration is sufficiently lower than the regulated value of the wastewater standard of 0.1 mg / L. On the other hand, selenium is concentrated in the sediment and the filtration residue in the sedimentation treatment. However, as described above, the amount of the ferrous salt used as the reducing agent can be reduced. Therefore, selenium in the liquid to be treated can be recovered in a highly concentrated state, and selenium can be easily reused.

On the other hand, in the second treatment method of the present invention, after the above-mentioned reduction treatment, the pH of the solution is reduced to 2 to 7.5, and the whole filtration is performed.

In the second treatment method, when the pH of the liquid was reduced to 2 to 7.5 after the reduction treatment, the solution was dissolved in the liquid as in the second stage during the reduction treatment. Since selenite, which is tetravalent selenium, combines with divalent iron ions to form hardly soluble iron selenite, the tetravalent selenium is transferred to the solid content side without being adsorbed on the precipitated iron component. Can be done. Then, as shown in the results of Example 2 described below, if the above-described sufficient reduction treatment with a change in pH is performed, almost the entire amount of selenium contained in the liquid to be treated is mainly reduced as iron selenite. It is contained in the filtered residue, and the filtrate contains almost no selenium. Even at this stage, the regulation value of the wastewater standard can be sufficiently cleared.

Further, since the excess ferrous iron dissolved in the liquid after the reduction treatment is directly transferred to the filtrate side after filtration, the amount of the filtration residue is reduced by the above-described aeration to convert the surplus ferrous iron into trivalent iron. The amount is much smaller than that of the first treatment method for precipitation. Therefore, selenium can be recovered in a more concentrated state than in the first treatment method, and reuse becomes easier.

If the pH of the solution to be adjusted after the reduction treatment deviates from the above range of 2 to 7.5 on either side, iron selenite may be eluted. Since selenium moves to the filtrate side, it becomes difficult to clear the regulation value of the drainage standard. After the reduction treatment, the holding time in a state where the pH is adjusted to 2 to 7.5 is 1
It should be at least minutes. Needless to say, heating of the liquid is not required at all during this holding.

In both the first and second treatment methods of the present invention, the selenium-containing liquid to be treated is not particularly limited as long as it contains hexavalent selenium, but it is a component that hinders the reduction of hexavalent selenium. Even when high concentrations of certain sulfate ions and inorganic salts are contained, selenium can be removed to a level below the regulation value of the wastewater standard without any problem.

[0041]

EXAMPLES Example 1 [Component Concentration in Liquid to be Treated] Sodium sulfate (as Na ₂ SO ₄ ): 15000 mg / L Sodium chloride (as NaCl): 15000 mg / L Ferrous sulfate (Fe ^{2 +} as) ···· 1500mg / L 6-valent selenium (Se ⁶⁺ as) ······ 20mg / L the liquid to be treated a composition (for stock solution pH 1.5), non-heating temperature (room temperature 24 ° C.) The pH was adjusted using sodium hydroxide and hydrochloric acid while stirring at, and after performing the reduction treatment through the first to third steps under the pH and time conditions described in Table 4 below, the pH was adjusted to 11.5. By blowing air into the liquid while adjusting the pH, ferrous ions are oxidized to precipitate a trivalent iron component mainly composed of ferric hydroxide. .15 and hold for 15 minutes Selenium is adsorbed to precipitated iron component and, then No.
The solution was filtered through a 5C filter paper, and the concentrations of total selenium (T-Se) and tetravalent selenium (Se ⁴⁺ ) in the filtrate were measured. The results are shown in the following Table 4 together with the retention time and pH of each step.

[0042]

[Table 4]

As is evident from the results in Table 4, in the selenium removal treatment at room temperature through the reduction-aeration-adsorption process using ferrous salt as a reducing agent, the pH of the liquid was lowered during the reduction treatment. If a processing means of shaking once and returning to the original pH is adopted, all the selenium after the final treatment can be sufficiently removed to 0.1 mg / L or less, which is the regulation value of the wastewater standard. Thus, the pH value when the above-mentioned pH is shaken to the lower side (second stage) can adopt a wide range of 1.5 to 7.5 as in the treatments A to F, but the pH in the treatment G (pH 8) can be adopted. Like p
When H exceeds 7.5, it can be seen that unreduced hexavalent selenium is remarkably increased, so that the regulated value of the wastewater standard cannot be cleared. The solubility of ferrous iron determined from the solubility product constant of ferrous hydroxide is 44 mg / L at pH 8.
Since the pH and pH are about 440 mg / L at 7.5, it is inferred that ferrous selenite is sufficiently formed in the presence of at least several hundred mg / L of ferrous iron.

Example 2 [Component Concentration in Liquid to be Treated] Sodium sulfate (as Na ₂ SO ₄ ): 15000 mg / L Sodium chloride (as NaCl): 15000 mg / L Ferrous sulfate (Fe ²⁺⁾ 1500 mg / L Hexavalent selenium (as Se ⁶⁺ ) 50 mg / L For the liquid to be treated (stock solution pH 1.5) having the above composition, without heating (room temperature 24 ° C.) By adjusting the pH with caustic soda and hydrochloric acid while stirring, the first step is to adjust the pH
9.10-9.27 for 10 minutes, pH 2 in the second step
4 minutes at 3.44 to 3.45, pH 9 in the third stage.
After performing a reduction treatment through three steps of holding each in the range of 08 to 9.24 for 13 minutes, the pH was adjusted to 3.39, and the mixture was held for 5 minutes. The mixture was filtered with a 5C filter paper.

The filtrate after the filtration was adjusted to pH 11.
Blowing air while adjusting to the range of 4 to 11.5, 15
After aeration for a minute, the pH was further adjusted to 6.5 and maintained for 10 minutes. The solution was filtered through a 5C filter paper, and the filtrate was entirely selenium (T-Se) and tetravalent selenium (Se ⁴⁺ ).
Was measured, the total selenium was 0.033 mg /
The results were 0.018 mg / L for L and tetravalent selenium, and 0.015 mg / L for hexavalent selenium.

In addition, the filtrate after the above-mentioned filtration
(T-Se) and tetravalent selenium (Se ⁴⁺Measure the concentration)
As a result, all selenium was 0.09 mg / L and tetravalent selenium was 0.1.
073 mg / L, 0.017 mg / L hexavalent selenium
The result was obtained.

On the other hand, with respect to the filtration residue obtained by the total filtration, washing with (1 + 1) hydrochloric acid and washing with 1% caustic soda were separately performed after washing with water, and vitamin C was added to the solution in an acidic environment. A precipitate formed.
Vitamin C generates tetravalent selenium and red metal selenium in an acidic environment. Therefore, selenium dissolved in a strongly acidic or wide alkaline region in the filtration residue is tetravalent selenium. Is confirmed to be mostly tetravalent selenium.

In the treatment of Example 2, it is assumed that the reduction of selenium is almost tetravalent, and even if it is assumed that 10 mg / L of oxygen is present in the liquid to be treated, Is theoretically estimated to be about 135 mg / L, and a selenium concentration of 50 mg
/ L, most of which have migrated into the filtration residue. If the transfer into the filtration residue is performed only by adsorption with ferric hydroxide, it is generally considered that ferric iron is required to be at least 10 times as large as selenium. Considering that only about 2.7 times exist and the amount of filtration residue generated is small, a considerable proportion of selenium is transferred to the filtration residue by precipitation of iron selenite as described above. It is assumed that

[0049]

According to the first aspect of the present invention, as a method for removing selenium from a liquid to be treated containing hexavalent selenium, reduction-aeration-adsorption is performed in a normal temperature range to perform solid-liquid separation. Efficiently removes all selenium below the specified value of wastewater standards efficiently in a short time, uses less reducing agent, can process large amounts of wastewater at low cost, and concentrates selenium at high concentration A very practical method that can be recovered in a state is provided.

According to the second aspect of the present invention, as a treatment method for removing selenium from the liquid to be treated containing hexavalent selenium, by performing a total filtration after reduction in a normal temperature range, it is more efficient than the above treatment method. All selenium can be reliably removed to below the sewage standard value, processing costs and equipment costs are further reduced, and the amount of filtration residue generated is small, and selenium can be recovered in a more concentrated state that is easy to reuse. A method is provided.

According to the third aspect of the present invention, there is an advantage in the above-described processing method that the reduction treatment can be performed efficiently and reliably.

Claims (3)

[Claims] 1. In a reduction treatment performed by adding an aqueous ferrous salt solution to a solution containing hexavalent selenium without heating, the pH of the solution during the treatment is 8.5 to 9.5 in the first stage. In the second step, the pH is adjusted to 1.5 to 7.5, and in the third step, 8.5 to 9.5. After the reduction treatment, air is introduced into the liquid while maintaining the pH of the liquid at 11 or more. A method for treating a selenium-containing liquid, which comprises blowing a ferrous iron to oxidize the liquid, neutralizing the liquid to a pH of 5.8 to 7, and then separating the precipitate into a solid and a liquid. 2. In a reduction treatment performed by adding an aqueous solution of ferrous salt to a solution containing hexavalent selenium without heating, the pH of the solution during the treatment is 8.5 to 9.5 in the first stage. In the second step, the pH is adjusted to 1.5 to 7.5 and in the third step to 8.5 to 9.5. After the reduction treatment, the pH of the solution is reduced to 2 to 7.5, and the solution is completely filtered. A method for treating a selenium-containing liquid. 3. The holding time of the first step in the reduction treatment is set to 5 minutes or more, the holding time of the second step is set to 1 minute or more, and the holding time of the third step is set to 5 minutes or more.
The method for treating a selenium-containing liquid according to any one of the above.