DE19639444A1 - Removal of selenium from effluent by adding reducing and neutralising agents - Google Patents

Abstract

A method (M1) for the treatment of effluent contg. selenium by adding a reducing agent and neutralising agent to reduce and remove selenium comprises the addn. of iron (II) sulphate and a neutralising agent to the effluent, heating the soln. to 40-80 deg C in air to reduce selenium and removing the residue. Also claimed are two other similar methods (M2) and (M3).

Description

The invention relates to a method for the treatment of selenium-containing Wastewater, it relates in particular to a process for the treatment of selenium containing waste water; with the help of which it is possible to Efficiency of removal from wastewater and improve it Treatment effectively precipitating resulting iron-containing precipitates use.

Because in the wastewater generated in the treatment of anode sludge If the concentration of selenium is relatively high, the selenium must be eliminated be removed from the wastewater.

As a method for removing selenium from the waste water, for example, JP-B-48-30558 describes a method in which Fe ^{2+ is added to} the selenium-containing waste water and then Cu ^{2+ is added} at a pH of 3 to 5 is used to promote the oxidation and hydrolysis of Fe ²⁺ and the pH is brought into a range of 5 to 6 to precipitate and separate selenium together with the hydroxide of Fe ³⁺ .

Also known is a method for adding one or two water-soluble ions such as Ag⁺, Cu ²⁺ , Fe ³⁺ and the like, or of Fe powder to the selenium-containing waste water, to form selenium in the form of an insoluble selenium compound to tie.

Selenium is usually present in the waste water in the form of SeO₃ ^2- (tetravalent selenium). However, it is known that when an oxidizing agent is present in the waste water, SeO₃ ^{2- is converted} into SeO₄ ^2- (hexavalent selenium). As a result, the wastewater contains SeO₃ ^2- and SeO₄ ^2- .

The above conventional methods are effective for removing SeO₃ ^2- , but it is difficult to remove the SeO₄ ^2- . With the conventional methods, therefore, the removal of SeO₄ ^{2- is} insufficient, so that it is impossible to improve the selenium removal efficiency.

Conventional wastewater treatment also turns iron into given so that a large amount of iron hydroxide is formed. It is therefore urgently needed to reduce the amount of sludge.

The aim of the present invention is therefore to provide a process for the treatment of selenium-containing waste water, by means of which not only Se len in the form of SeO₃ ^2- in the waste water, but also selenium in the form of SeO₄ ^2- can be separated. Another object of the invention is to provide a method for recovering the iron in the precipitates accompanying the wastewater treatment and for effectively using the same.

According to a first aspect, the present invention relates to a method for the treatment of selenium-containing waste water by adding a Re detergent and a neutralizing agent to the waste water to selenium to reduce and remove, which is characterized in that egg sen (II) sulfate and a neutralizing agent to a wastewater output material added and mixed with it and the resulting mixed Solution in air is heated to 40 to 80 ° C to reduce the selenium and remove in the form of a thickener precipitate.  

In a preferred embodiment of the invention, sulfuric acid added to the reduced and removed thickener precipitate and the Undissolved residue is filtered through a filter to separate selenium and remove and recover a filtrate, and then the filtrate reused as a reducing agent for the wastewater raw material.

In a second aspect, the present invention relates to a method for the treatment of selenium-containing waste water by adding a Re detergent and a neutralizing agent to the waste water to the Reduce and remove selenium, which is characterized in that iron (II) sulfate and a neutralizing agent are added to the waste water and mixed with it while the wastewater in the air at a tempera is heated from 40 to 80 ° C, that a sedimentation agent is added, to achieve a thickening that a selenium and iron (II) containing Ein thick precipitation in a pre-treatment tank containing the selenium Waste water is added to determine the selenium concentration in the waste water to discontinue, and then the pretreated solution of the above mentioned supply stage is supplied.

In addition, pretreatment is a process in which selenium is pre-treated is decorated by adding the iron (II) containing thickener precipitate, that arises in the treatment of selenium-containing wastewater and the Neutralizing agent, while the selenium-containing waste water in the pre treatment tank is heated to a temperature of 40 to 80 ° C in air and a sedimentation agent is added to cause thickening filtering to remove and remove the residue, and the resulting filtrate together with a supernatant thickener Liquid is supplied to a treatment tank.

The invention will now be described with reference to the accompanying Drawings explained in more detail. It shows  

Fig. 1 is a graph showing the influence of the atmosphere and the temperature of the waste water raw material and the Fe addition amount on the removal of selenium;

Fig. 2 is a graph showing the influence of the atmosphere and the temperature of the waste water raw material on the settling rate of selenium and iron hydroxide;

Fig. 3 is a diagram tert treatment steps according to the invention erläu; and

Fig. 4 is a diagram that he explains further treatment stages according to the invention.

The method according to the invention for the treatment of selenium-containing Wastewater is described in more detail below.

First, selenium-containing waste water or a SeO₃ ^2- and SeO₄ ^2- containing waste water is heated to a temperature in the range of 40 to 80 ° C in air using a heater. In this case, iron (II) sulfate as a reducing agent is added in an amount corresponding to 0.5 to 3.0 g / l as Fe ²⁺ ion in the waste water raw material, and then a neutralizing agent such as sodium hydroxide, calcium hydroxide or the like is added ., added, whereby the pH of the resulting mixture is adjusted to a range from 8 to 10.

After adjusting the pH, the mixed solution is stirred for 10 to 30 minutes. In this stage, the Fe ²⁺ in the mixed solution is oxidized to Fe ³⁺ and the reduction of SeO₃ ^2- and SeO₄ ^2- is promoted to form selenium, which is adsorbed by Fe (OH) ₂ and fixed to it, and a sedimentation agent is added to carry out a sedimentation treatment so as to precipitate selenium together with Fe (OH) ₂.

It is important that the wastewater at this stage of treatment is treated with a Heater is always heated to 40 to 80 ° C.

Fig. 1 shows the influence of the atmosphere and the temperature of the waste water on the Se removal efficiency, and it is a graph showing the relationship between the addition amounts of Se (mg / l) and Fe (g / l). As can be seen from Fig. 1, the Se removal efficiency in a nitrogen atmosphere is good when the temperature of the waste water raw material becomes lower. However, the difference between the atmospheres becomes smaller at a temperature of 60 ° C. As a result, the Se removal efficiency in the air is improved when the temperature of the sewage raw material is not below 50 ° C. In order to improve the Se removal efficiency, it is therefore necessary to treat the waste water in the air and to keep the temperature of the waste water at 55 to 65 ° C.

Recently, it was also found that the reduction of SeO₄ ^2- , which is hardly reduced in addition to SeO₃ ^2- , is facilitated by bringing the temperature of the waste water to 55 to 80 ° C. The Fe (OH) ⁺ layer formed on Fe (OH) ₂ and the surface of the verdigris particles become thicker by increasing the temperature and the reduction reaction of SeO₄ ^2- is promoted.

Fig. 2 is a graph showing the influence of the atmosphere and the temperature of the waste water raw material on the settling rate of selenium and iron hydroxide when a reducing agent and a neutralizing agent are added to the selenium-containing waste water and stirred to combine the selenium precipitate together with iron hydroxide. As can be seen from Fig. 2, no influence of the atmosphere can be determined, while the settling time is considerably extended when the temperature of the waste water starting material in the air is 60 ° C. That is, if the temperature of the waste water raw material in the air is kept at 60 to 80 ° C, an improvement in the settling rate is remarkable. Preferably, the temperature of the waste water raw material is within the range of 55 to 65 ° C.

In addition, the increase in the settling speed in Fig. 2 means that the amount of fine particles escaping from the thickening agent in the form of a suspensoid can be reduced to decrease the amount of heavy metal discharged in the form of SS.

The reason why iron (II) sulfate is added in an amount corresponding to 0.5 to 3.0 g / l as Fe ²⁺ ion is based on the fact according to the invention that when the amount of Fe ^{2 +} Ion is low, Se removal is poor, while as the addition amount of Fe ²⁺ ion increases, the cost of chemicals becomes high and the amount of residue increases.

The reason why the pH of the mixed solution by adding the neutralizing agent is set in a range from 8.0 to 10.0, is also due to the fact that when the pH we is less than 8 or more than 10, the Se removal efficiency is poor becomes. The pH is preferably within the range of 8.5 to 9.5.

Then the mixed solution is applied after the above treatment Sedimentation agent, for example a flocculant with a high Molecular weight or the like added to the flocculation, concentration and discontinuation of the selenium formed in the mixed solution Promote substance while the supernatant is separated and is discarded.

In addition, after concentration, the thickener precipitate is added with sulfuric acid to adjust the pH to about 5, and then the resulting solution is filtered to separate and remove selenium and to recover the filtrate. A suitable amount of iron (II) sulfate can be added to this filtrate to adjust the Fe ²⁺ concentration to be added to the waste water raw material for recycling, if necessary
The treatment according to the invention is specifically described below with reference to FIG. 3.

A waste water 1 containing selenium is fed to a heating and mixing tank 2 . A waste water raw material 3 is heated in the heating and mixing tank 2 to 55 to 65 ° C and kept at this temperature, while ferrous sulfate 4 is added as a reducing agent. Then the waste water starting material 3 is transferred to a selenium removal reaction tank 5 in which a neutralizing agent 6 , e.g. B. NaOH or the like. Added and stirred for about 30 minutes to adjust the pH to 8.5 to 9.5, whereby the selenium is reduced.

Thereafter, the mixed solution in the selenium removal reaction tank 5 is transferred to a tank 7 for the addition of the sedimentation agent, and a sedimentation agent 8 is added, then it is transferred to a thickener 9 . Through the thickening treatment in the thickener 9 , selenium and Fe (OH) ₂ are precipitated together and separated as a thickener precipitate 12 . On the other hand, a supernatant liquid 10 is discharged into a liquid storage tank 11 . In addition, it is preferable that the heating of the waste water raw material is carried out by using an electric heater or by blowing steam or the like. As the selenium removal reaction tank 5 , if necessary, several tanks may be used.

Then the thickener precipitate 12 is transferred to a recovery tank 13 in which sulfuric acid 14 is added and filtered through a filter 15 , whereby selenium is retained in a residue and removed while the Fe ²⁺ solution is recovered. That is, the resultant filtrate 16 is kept for a period of time in a storage tank 17 in which ferrous sulfate is added to the filtrate stored in this tank if necessary to adjust the Fe ²⁺ concentration. After adjustment, the iron (II) sulfate solution is reused as iron (II) sulfate 4 , which is added to the heating and mixing tank 2 .

The invention also relates to a method in which the thickener precipitate is reused for a pretreatment in which it is added to the selenium-containing waste water in order to reduce the selenium concentration in the waste water. That is, the thickener precipitate 12 in the thickener 9 is a coprecipitate of selenium and iron (Fe (OH) ₂), so that selenium and iron (II) are present in the thickener precipitate 12 simultaneously. Therefore, when the thickener precipitate 12 is supplied to the selenium-containing waste water, it can act as a reducing agent. Since SeO₃ ^2- and SeO₄ ^2- in the selenium-containing wastewater are reduced by the iron (II) (Fe ²⁺ ) in the thickener precipitate with the formation of selenium, the selenium is removed as indicated in the following reaction equation:

H₂SeO₄ + 9Fe (OH) ₂ → Se + 3Fe₃O₄ + 10H₂O

Thereafter, the supernatant thickener liquid formed in this pre-treatment step (B) is introduced into a main treatment tank (selenium removal reaction tank) 5 in a waste water treatment step (A) containing selenium (treatment for reduction and removal of selenium). This pretreatment B pre-reduces the selenium concentration in the wastewater, so that the treatment in the subsequent wastewater treatment stage A becomes more effective and the amount of iron (II) sulfate used is preferably reduced.

The process for the treatment of waste water containing selenium, which consists of the pretreatment (B) and the main treatment (A), is described in more detail below with reference to FIG. 4.

In the pre-treatment step (B) the selenium waste water 1 containing 2 is introduced B in egg nen pretreatment tank while the iron (II) thickener precipitate 12 containing formed in the main treatment stage (A), the pre-treatment tank added 2 B together with a neutralizing agent 6 such as NaOH or the like to adjust the pH to about 9, and then the resulting mixture is stirred with heating for about 30 minutes. Then the mixed solution is transferred to a tank 7 B for the addition of the sedimentation agent and with a sedimentation agent 8 ver, after which it is transferred to a thickener 9 B to carry out the thickening treatment for separation into a liquid phase and into a solid phase.

After completion of the treatment, the supernatant liquid B in the thickener 9 B 5 formed is transferred 10 B in a selenium removal reaction tank as the main treatment tank. On the other hand, the precipitate 12 B formed in the thickener 9 B is filtered through a filter 15 B and the resulting filtrate 16 B is mixed with the above-mentioned supernatant liquid 10 B and introduced into the selenium removal reaction tank 5 while the residue (Se) therefrom is removed to dispose of it.

In the selenium removal tank 5 , the temperature of the liquid is heated to 60 ° C. and held at this value, and iron (II) sulfate 4 is added as a reducing agent. Neutralizer 6 is also added to adjust the pH to about 9 and stirring is continued for about 30 minutes. The resulting mixed solution is transferred to the tank 7 to add the sedimentation agent, and the sedimentation agent 8 is added, and then the solution is subjected to a thickening treatment in the thickener 9 to effect solid-liquid phase separation.

The supernatant fluid 10, which is formed during the thickening treatment is discharged in a liquid storage tank 11, while the A thick precipitate is transferred to B 2 in the pretreatment tank 12th

By repeating the above steps, the selenium concentration reduced to a very low value while reusing precipitation.

The invention is illustrated by the following examples, but without to be limited to that.

example 1

The treatment of waste water containing selenium is carried out according to the method shown in FIG. 3. That is, the selenium-containing waste water is heated in air to a temperature in the range of 15 to 60 ° C to form a waste water raw material, iron (II) sulfate in an amount of 0.5 to 2.0 g / L as Fe ²⁺ ions is added up to the amount of dissolved selenium, and also NaOH is added as a neutralizing agent to adjust the pH of the resulting mixed solution to 9. The mixed solution is then stirred for 30 minutes and mixed with Ku rifloc (trade name) as sedimentation agent, then it is transferred to a thickener in order to precipitate selenium and Fe (OH) ₂ together as a thickener precipitate. On the other hand, the constituents of an excess liquid which is formed in the thickener are determined, and results are obtained as shown in Table 1 together with the analysis results of the constituents in the waste water raw material.

Table I

In addition, the recycling operation is carried out using the thickener precipitate as a reducing agent according to the method shown in FIG. 3. That is, the precipitate obtained in the thickener is mixed with sulfuric acid to adjust the pH to 5, then filtered to separate and remove selenium, and then the resulting filtrate is mixed with an appropriate amount of iron (II) - sulfate added to adjust the Fe ²⁺ concentration to 12.5 g / l. The solution thus provided is added to the waste water raw material in the heating and mixing tank 2 . By repeating the above-mentioned operation, the recovery ratio of iron is determined by recycling, thereby obtaining results as shown in Table II below together with the composition of the liquid before and after the treatment.

Table II

As can be seen from Table II, it is possible according to the invention to recover SeO₄ ^2- , which could hardly be recovered in the conventional process, only by heating the waste water starting materials and adjusting the pH, so that the selenium removal ratio from the waste water containing selenium can be improved. In addition, according to the invention, the precipitate which is formed in the treatment for the removal of selenium is efficiently recycled without being disposed of, so that the amount of waste is reduced and resources can be conserved.

Example 2

The two-step treatment of waste water containing selenium is carried out according to the method shown in FIG. 4 including the pretreatment. In this process, the waste water containing selenium is heated in air to a temperature in the range of 60 ° C. to form a waste water starting material, which is mixed with iron (II) sulfate and also with NaOH to bring the pH to 9 adjust. The mixed solution is then stirred for 30 minutes and introduced into a thickener in order to precipitate selenium together with Fe (OH) ₂ as a thickener precipitate. The thickener precipitate thus obtained is introduced into a pretreatment tank to reduce the selenium concentration in the selenium-containing waste water, as shown in Fig. 4, and the pretreated selenium-containing water is reintroduced into the main treatment tank. This procedure is repeated. The constituents in the liquid after the treatment and the constituents of the waste water raw material are averaged, and the results shown in Table III are obtained.

As can be seen from Table III, the removal of SeO₄ ^2- , which was hardly possible according to the conventional method, is now possible and the ratio of selenium removal from the waste water containing selenium can also be improved. In addition, the precipitate obtained in the treatment for selenium removal can be effectively recycled without disposal, so that the amount of waste can be reduced and the resources can be conserved.

Table III

Claims (8)

1. A process for the treatment of waste water containing selenium by adding a reducing agent and a neutralizing agent to the waste water to reduce and remove selenium, characterized in that iron (II) sulfate and a neutralizing agent are added to and mixed with a waste water starting material and the resulting mixed solution is heated in air to 40 to 80 ° C to reduce selenium and remove it as a thickener precipitate. 2. Process for the treatment of waste water containing selenium Adding a reducing agent and a neutralizing agent to the Ab water to reduce and remove selenium, characterized in that iron (II) sulfate and a neutralizing agent are added to the waste water and mixed with it while the wastewater in the air is at a tem temperature of 40 to 80 ° C is heated to selenium as a thickener precipitate reduce, and that the thickener precipitate added sulfuric acid the undissolved residue is filtered through a filter to remove selenium separate and remove, and that at the same time a filtrate is recovered and all reducing agents for the wastewater raw material in the circuit to be led. 3. The method according to claim 1 or 2, characterized in that the waste water starting material iron (II) sulfate in an amount within the loading range from 0.5 to 3.0 g / l, calculated as the amount of Fe ²⁺ - Ions, is added. 4. The method according to claim 1 or 2, characterized in that the A neutralizing agent is added to the waste water raw material pH of the starting material to a value in the range of 8 to 10 adjust.   5. Process for the treatment of waste water containing selenium Adding a reducing agent and a neutralizing agent to the Ab water to reduce and remove selenium, characterized in that iron (II) sulfate and a neutralizing agent are added to the waste water and mixed with it while the wastewater in the air is at a tem temperature of 40 to 80 ° C is heated that a sedimentation agent is added to carry out a thickening that the selenium and iron (II) contain a thickener precipitate in a pre-treatment tank for the selenium holding waste water is added to the selenium concentration in the waste reduce water, and that after that the pretreated solution of the above called heating stage is supplied. 6. The method according to claim 5, characterized in that it is pretreatment is a process in which selenium is pre-reduced by adding the thickener precipitate containing iron (II), which Treatment of waste water containing selenium is formed, and of neutra lizing agent while heating the selenium-containing waste water in the pre treatment tank in air at a temperature of 40 to 80 ° C, and at to which a sedimentation agent is added to cause thickening filtering to separate and remove the residue, and the resulting filtrate along with a supernatant thicker liquid is introduced into a treatment tank. 7. The method according to claim 5 or 6, characterized in that the Neutralizing agent for both the pre-treatment tank and the main tank treatment tank is added to the pH of the liquid to be treated speed to a range from 8 to 10. 8. The method according to claim 5 or 6, characterized in that the amount of iron (II) sulfate added to the main treatment tank within the range of 0.5 to 3.0 g / l, calculated as the amount of Fe ²⁺ ions, lies.