JP4041977B2 - Method and apparatus for processing solution containing selenium - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a processing method and a processing apparatus for a solution containing selenium, and is obtained, for example, by washing water with dust such as dust (for example, dust obtained by collecting exhaust gas discharged from a cement kiln, incineration fly ash, or the like). The present invention relates to a processing method and a processing apparatus for efficiently removing selenium contained in a solution.
[0002]
[Prior art]
In recent years, a technique for using waste such as dust discharged from a garbage incineration facility as a cement raw material has been put into practical use. At that time, in order to remove heavy metals and chlorine contained in the waste, the waste is washed with water.
However, selenium eluted from the waste is contained in the wastewater obtained by washing the waste with water. Selenium is an element whose drainage standard value is set to 0.1 mg / liter or less, and it is necessary to remove the wastewater so that the content is below the drainage standard value before discharging it into the external environment. There is. Therefore, various methods for treating waste water containing selenium have been developed.
[0003]
For example, ferrous compound and ferric compound are added to waste water containing selenium to adjust the liquidity to alkaline, and tetravalent selenium and hexavalent together with the produced ferrous hydroxide and ferric hydroxide. A wastewater treatment method for precipitating selenium is known (see Patent Document 1).
In this wastewater treatment method, after adding the ferrous compound and ferric compound to the wastewater, the pH of the wastewater is once adjusted to 3 or less, and then the pH is adjusted to 8-12 by adding an alkali source. is doing.
[0004]
[Patent Document 1]
JP-A-2002-126758 (Claim 1, Claim 3 and Paragraph 0017 of page 5, column 5)
[0005]
[Problems to be solved by the invention]
In the technique described in the above publication, a relatively large amount of a ferrous compound is added to generate a precipitate containing selenium. Specifically, in the examples in the above publication, the addition amount of the ferrous compound is determined so that the divalent iron ion is 5,000 or more with respect to selenium 1 by weight ratio.
Therefore, the present invention reduces the amount of ferrous compound (compound for supplying divalent iron ions) necessary for removing selenium in the solution to be treated, and also reduces the amount of addition. Regardless of the selenium concentration in the solution after treatment, the treatment method and treatment apparatus for the solution containing selenium, which can reduce the selenium concentration below the drainage standard value (specifically, the residual selenium concentration is 0.1 mg / liter or less). The purpose is to provide.
[0006]
[Means for Solving the Problems]
As a result of intensive studies to solve the above-mentioned problems, the present inventor added an acid such as hydrochloric acid to a solution containing selenium such as a filtrate after washing with dust so that the pH is adjusted to an acidic region. In addition, after adding the ferrous compound, the solution is circulated in one direction in the flow path, and an alkali agent such as calcium hydroxide is added at a predetermined point in the flow path. If the pH on the flow side is adjusted to the alkaline region, the generation of a solid component consisting of selenium and ferrous hydroxide is promoted, and as a result, the amount of ferrous compound added to remove the same amount of selenium The present invention was completed.
[0007]
That is, in the method for treating a solution containing selenium of the present invention (Claim 1), an acid such as hydrochloric acid is added to the solution containing (A) selenium as necessary to adjust the pH to an acidic region (for example, pH 4 or lower). And a ferrous compound addition step of adding a ferrous compound such as ferrous chloride to obtain a solution containing selenium and divalent iron ions, and (B) the selenium and divalent While a solution containing iron ions is circulated in one direction in the flow path using a fluid flow means such as a pipe, an alkaline agent such as calcium hydroxide is used at a predetermined point in the flow in the flow path. And adjusting the pH of the downstream side (downstream side) of the point to an alkaline region (for example, pH 8 to 12) to obtain a solution containing selenium and ferrous hydroxide, C) Filtration or the like of the above solution containing selenium and ferrous hydroxide Thus by solid-liquid separation, and the solid fraction containing selenium and ferrous hydroxide, characterized in that it comprises a solid-liquid separation to obtain a liquid fraction which selenium is removed.
[0008]
In the method of the present invention, an alkaline agent such as calcium hydroxide is added at a predetermined point in the flow path while allowing the acidic solution after the addition of the ferrous compound to flow in one direction in the flow path. Therefore, ferrous hydroxide produced at the point where the alkaline agent is added and on the downstream side of the solution moves in one direction together with selenium in the solution, and forms a coprecipitate with selenium. Become. At this time, the formation of the coprecipitate proceeds efficiently under the condition that the existing precipitate does not exist and selenium and ferrous hydroxide flow together. Therefore, even if the addition amount of the ferrous compound is reduced as compared with the conventional amount, the concentration of selenium in the solution after the treatment can be reduced to a wastewater standard value or less (residual selenium concentration is 0.1 mg / liter or less). .
[0009]
In the method for treating a solution containing selenium according to the present invention, the solution containing selenium and ferrous hydroxide is stored between the step (B) and the step (C), and the selenium and water in the solution are stored. A precipitation growth step of precipitating ferrous oxide as a solid content can be included (claim 2).
If comprised in this way, the solid part (coprecipitate) which consists of selenium and ferrous hydroxide can be produced | generated more reliably.
[0010]
The apparatus for treating a solution containing selenium according to the present invention (Claim 3) is (a) a pretreatment tank for storing a solution containing selenium, and a first treatment such as ferrous chloride is provided in the pretreatment tank. A pretreatment tank having a ferrous compound supply port for supplying an iron compound and an acid supply port for supplying an acid such as hydrochloric acid into the pretreatment tank to adjust the pH of the solution to an acidic region When the solution, a flow path for flowing in one direction, a predetermined point in the middle of the flow in the flow path, of calcium hydroxide containing the selenium discharged from (b) above pretreatment reservoir Supplying an alkaline agent as described above, adjusting the pH of the solution containing selenium on the downstream side of the point to an alkaline region, and supplying an alkaline agent to produce a solid component composed of selenium and ferrous hydroxide A flow path (for example, a pipeline) provided with a mouth, and (c) from the flow path It is characterized in that it comprises a solid-liquid separation means as filtration devices and the like for solid-liquid separation of a solution containing solids comprising the said selenium ferrous hydroxide issued.
The apparatus for treating a solution containing selenium according to the present invention stores the solution containing selenium between the flow path (b) and the solid-liquid separation means (c) and precipitates a solid content in the solution. A sedimentation tank for the purpose (claim 4).
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail.
First, the processing method of the solution containing selenium of the present invention will be described.
[Ferrous compound addition process]
In this step, an acid is added to the solution containing selenium as necessary, and the ferrous compound is added to the solution in a state where the pH of the solution is adjusted to an acidic region, and selenium and divalent iron ions are added. Is a step of obtaining a solution containing
Examples of the solution containing selenium include alkaline filtrates obtained by washing selenium-containing substances such as dust and incineration ash with water.
Here, as soot, for example, soot discharged from a cement kiln and related equipment (for example, dust obtained by collecting exhaust gas discharged from a cement kiln), a melting facility for garbage and incineration ash, etc. Incinerated fly ash discharged from waste, incinerators for garbage and sewage sludge, etc.
Examples of the incineration ash include incineration ash discharged from the furnace bottom of the incineration facility.
[0012]
The washing with water can be performed by a treatment method such as putting selenium-containing material such as dust and water into the mixing tank and stirring for a predetermined time.
The alkaline filtrate can be obtained, for example, by solid-liquid separation of a slurry comprising a selenium-containing substance such as dust and water and filtration. In the present specification, the term “filtrate” is used to include not only the liquid content separated by filtration but also the liquid content obtained by any solid-liquid separation method (for example, centrifugation, etc.). It is done.
The pH of the alkaline filtrate may be as high as about 12 to 13 in the case of a filtrate obtained by rinsing soot such as dust obtained by collecting exhaust gas discharged from a cement kiln. .
[0013]
In the present invention, an acid is added to the solution containing selenium as necessary. For example, when an alkaline filtrate obtained by washing the above dust or the like with water as a solution containing selenium is used, the pH of the filtrate is high, so an acid is added to adjust the pH of the solution to an acidic region. It is necessary to.
Examples of the acid include hydrochloric acid, sulfuric acid, nitric acid and the like.
The pH of the solution containing selenium is preferably adjusted to 4.5 or less, more preferably 4.0 or less, and particularly preferably 3.5 or less by adding an acid. By adjusting the pH to 4.5 or less, divalent iron is easily dissolved as ions in the liquid, and the amount of ferrous compound added to remove substantially all of selenium is further increased. Can be reduced.
[0014]
Examples of the ferrous compound include ferrous salts such as ferrous chloride, ferrous sulfate, and ferrous nitrate. In the solution containing selenium, divalent iron ions for producing ferrous hydroxide that can adsorb and coprecipitate selenium in the alkaline agent addition step (B) by adding the ferrous compound. Can be supplied to.
The amount of ferrous compound added varies depending on the type of solution to be treated, but in the case of a filtrate having a selenium concentration of about 1 mg / liter obtained after washing with dust, divalent iron ions / selenium. The weight ratio is preferably 2,000 to 4,000.
When the addition amount does not reach the lower limit of the above numerical range, selenium may not be sufficiently removed. When the addition amount exceeds the upper limit of the above numerical range, the object of the present invention for reducing the addition amount of the ferrous compound cannot be sufficiently achieved.
[0015]
Here, the relationship between the supply of divalent iron ions and the removal of selenium will be described as follows.
Selenium is mainly composed of tetravalent selenium (specifically, selenite ion represented by SeO ₃ ^2- ) and hexavalent selenium (specifically, selenate ion represented by SeO ₄ ^2-). ) Present in the solution to be treated.
When the ferrous compound is added to the solution containing selenium, the divalent iron ions are sufficiently dissolved in the liquid under the condition that the pH is adjusted to the acidic region. Since divalent iron ions have a reducing action, hexavalent selenium in the liquid is reduced to tetravalent selenium.
Therefore, selenium in the solution exists as tetravalent selenium, not hexavalent selenium, which is difficult to remove, and is easily adsorbed and coprecipitated by the solid content (precipitate) of ferrous hydroxide. Can be removed.
[0016]
In the present invention, for example, an acid waste solution containing a ferrous compound can be used as a supply medium for the ferrous compound. Here, the acid waste liquid refers to a waste liquid containing acid generated in various processing steps in the factory.
Specific examples of the acid waste liquid containing the ferrous compound include, for example, pickling waste liquid. Here, the pickling waste liquid is a contaminated acid cleaning liquid (for example, hydrochloric acid waste liquid) discharged from a pickling process in a steel-related factory (for example, an iron manufacturing factory, a wire manufacturing factory, a special steel factory, etc.). Say. The concentration of divalent iron ions in the pickling waste liquid is, for example, about 50 to 100 g / liter.
[0017]
Pickling means removing oxides or other compounds from the base metal by chemical or electrochemical action. Specifically, the scale or rust layer formed on the surface of the base metal is removed. In order to do so, it refers to an operation of immersing and cleaning in an acid solution such as hydrochloric acid for a relatively long time.
By using an acid waste liquid (for example, pickling waste liquid) containing a ferrous compound as the supply medium for the ferrous compound, the amount of industrial chemicals such as hydrochloric acid and sulfuric acid is reduced, and the processing cost of the present invention is reduced. (Chemical costs) can be reduced.
[0018]
In the method of the present invention, trivalent iron ions can be added together with divalent iron ions. Since trivalent iron ions are easily precipitated even at a lower pH than that of divalent iron ions, the divalent iron ions serve as a core when the divalent iron ions are precipitated, and quickly precipitate the divalent iron ions. be able to. However, when only trivalent iron ions are present without divalent iron ions, only tetravalent selenium precipitates, and hexavalent selenium remains in the solution. Therefore, in the present invention, the presence of divalent iron ions is essential.
[0019]
In the present invention, it is desirable to reduce the amount of dissolved oxygen in the solution in order to maintain the stability of the ferrous compound added to the solution containing selenium. That is, in carrying out the present invention, the selenium removal treatment may be performed in a state where the solution containing selenium is exposed to air, but in a non-oxidizing atmosphere (for example, in nitrogen gas), If a removal process is performed, the addition amount of a ferrous compound can further be reduced.
[0020]
[Alkaline agent addition process]
In this step, the solution containing selenium and divalent iron ions obtained in the ferrous compound addition step described above is continuously circulated in one direction inside a fluid circulation means such as a pipe, In this step, an alkaline agent is added at a predetermined point in the middle, and the pH at the downstream side of the point is adjusted to an alkaline region to obtain a solution containing selenium and ferrous hydroxide.
Examples of the fluid circulation means for circulation of the solution used in this step include a pipe line having a predetermined inner diameter and length.
The inner diameter and length of the pipe line are not particularly limited, and can be variously determined depending on conditions such as the amount of the solution to be treated and the concentration of selenium.
In addition, if a screw is installed in a pipe line, the stirring efficiency of the solution which distribute | circulates the inside of a pipe line can be improved.
In addition, a small tank with a stirring blade having a very short residence time can be provided at the place where the alkali agent is added. However, this small tank is distinguished from a storage tank (ordinary chemical addition tank) having a long residence time, and substantially constitutes a part of the flow path.
[0021]
As the alkaline agent, for example, calcium hydroxide, sodium hydroxide, potassium hydroxide and the like are used. Among these, calcium hydroxide is preferable because it can be used as a cement raw material when it is later recovered as a solid component.
In this step, the pH of the solution containing selenium and divalent iron ions is preferably 8.0 to 12.0, more preferably 8.5 to 10.5, and particularly preferably 8. It is adjusted to 5 to 9.5. By adjusting the pH within this numerical range, the amount of selenium adsorbed on and coprecipitated with ferrous hydroxide increases, and the selenium removal rate can be further improved.
[0022]
[Precipitation growth process]
This step is a step of storing the solution containing selenium and ferrous hydroxide obtained in the above alkaline agent addition step and growing selenium and ferrous hydroxide in this solution as a coprecipitate. .
By providing this step, the coprecipitate can be reliably generated even in the case where the coprecipitate of selenium and ferrous hydroxide is not sufficiently generated in the alkaline agent addition step described above. Can do.
In addition, in the above-mentioned alkaline agent addition process, when the coprecipitate of selenium and ferrous hydroxide can be generated sufficiently, this process can be omitted.
[0023]
[Solid-liquid separation process]
In this step, the solution containing selenium and ferrous hydroxide obtained in the above-described precipitation growth step (or alkali agent addition step) is subjected to solid-liquid separation to obtain a solid component containing selenium and ferrous hydroxide. (Precipitate) and a liquid component from which selenium has been removed.
Examples of the solid-liquid separation method include a filtration method using a vacuum filtration device, a filter press, a screw press, etc., and a centrifugation method using a centrifuge.
[0024]
The solid content obtained by solid-liquid separation usually contains calcium hydroxide, heavy metal, and the like derived from soot dust in addition to ferrous hydroxide and selenium. This solid content can be used as a cement raw material after removing heavy metals and the like.
The liquid obtained by solid-liquid separation is a step for removing heavy metals (for example, coagulation precipitation, liquid flow through a chelate resin, etc.) or a step for recovering chlorine (for example, a crystallization method). Wastewater that does not substantially contain selenium (ie, wastewater whose residual selenium concentration is 0.1 mg / liter or less of the wastewater standard value). can do.
[0025]
Next, a processing apparatus for a solution containing selenium according to the present invention will be described with reference to the drawings. FIG. 1 is a diagram schematically showing a processing apparatus for a solution containing selenium according to the present invention.
The solution processing apparatus 1 containing selenium stores a pretreatment tank 2, a flow path 3 such as a pipe for flowing the solution from the pretreatment tank 2 in one direction, and a solution from the flow path 3. And a solid-liquid separation means 5 for solid-liquid separation of the solution from the precipitation tank 4.
Among these, the pretreatment tank 2 is for supplying a ferrous compound such as ferrous chloride into the pretreatment tank 2 from the inlet of the solution 6 containing selenium and the ferrous compound storage tank 7. A ferrous compound supply port 8, an acid supply port 10 for supplying an acid such as hydrochloric acid into the pretreatment tank 2 from the acid storage tank 9, and a solution for stirring and mixing the solution in the pretreatment tank 2 Stirring means (for example, a stirring blade) 11 is provided.
In the pretreatment tank 2, the solution 6 containing selenium becomes an acidic solution containing selenium and divalent iron ions.
[0026]
In the flow path 3, the end on the upstream side is formed as the drain port 12 of the pretreatment tank 2, and the end on the downstream side is formed as the solution inlet 14 of the precipitation tank 4.
The solution in the pretreatment tank 2 flows into the flow path 3 from the drain port 12 by the pump 13, and then mixed with the alkaline agent 16 supplied from the alkaline agent supply port 15 provided in the middle of the flow path 3. Furthermore, it flows through the flow path 3 and flows into the precipitation tank 4 from the solution inlet 14. The alkaline agent 16 is supplied from the alkaline agent storage tank 18 into the flow path 3 by the pump 17 at a predetermined flow rate.
On the downstream side of the alkaline agent supply port 15 of the flow path 3, the acidic solution containing selenium and divalent iron ions is mixed with the alkaline agent 16 to form a solid component (co-polymerized with selenium and ferrous hydroxide). It turns into an alkaline solution containing sediment.
[0027]
The solution that has flowed into the precipitation tank 4 from the flow path 3 is stirred and mixed by stirring means 19 such as a stirring blade. Then, in the solution in the precipitation tank 4, in addition to the coprecipitate generated in the previous step, further coprecipitate is generated, and finally a solid component (coprecipitate consisting of selenium and ferrous hydroxide) is formed. Product) and a liquid component from which selenium is substantially removed.
The solution containing the coprecipitate in the precipitation tank 4 is discharged from the drain 21 of the flow path 20 by the pump 22 and guided to the solid-liquid separation means (filter device) 5.
In the solid-liquid separation means 5, the solution containing the coprecipitate is separated into a solid component (coprecipitate composed of selenium and ferrous hydroxide) and a liquid component from which selenium has been removed.
Among these, the solid content is used as a cement raw material or the like after a heavy metal removal treatment or the like. The liquid component is discharged into the external environment after being subjected to a chlorine removal process or the like.
[0028]
【Example】
Experiments were performed as follows using a small processing apparatus for experiments.
[Example 1]
20 kg of dust obtained by collecting the exhaust gas discharged from the cement kiln was washed with water in a mixing tank and then filtered to obtain 50 liters of filtrate containing 1 mg / liter of selenium. The pH of this solution was 12.
This solution is supplied to a pretreatment tank (capacity: 3.4 liters) equipped with a stirring blade at a flow rate of 0.1 liter / minute, and hydrochloric acid and divalent iron ions / selenium are contained in the pretreatment tank. Ferrous chloride in an amount of 3,000 by weight was added to adjust the pH to 3.5. The residence time of the solution in the pretreatment tank was about 30 minutes.
[0029]
This solution is guided by a pump into a T-shaped pipe (inner diameter of the pipe: 8 mm, inner diameter of the alkali agent supply port: 4 mm, length from the alkali agent supply port to the end on the downstream side: 27 cm). While being circulated, a slurry of calcium hydroxide was added from the alkali agent supply port. The calcium hydroxide slurry was supplied while adjusting so that the pH of the downstream solution was 8.5 to 9.0. The time required for the solution to move from the alkaline agent supply port to the end on the wake side was 8 seconds.
The solution that has passed through the T-shaped pipe is once grown in a precipitation tank (volume: 3.4 liters, solution residence time: 30 minutes) with a stirring blade, and then filtered with a filter press. The solid component consisting of selenium and ferrous hydroxide was separated from the liquid component (filtrate). The concentration of selenium in the filtrate was 0.1 mg / liter or less.
[0030]
[Comparative Example 1]
While supplying the same solution as the processing target liquid used in Example 1 to a pretreatment tank (capacity: 3.4 liters) with a stirring blade at a flow rate of 0.1 liter / min, The same amount of hydrochloric acid and ferrous chloride as in Example 1 were added to adjust to the same pH as in Example 1. The residence time of the solution in the pretreatment tank was the same as in Example 1.
This solution is continuously sent to a reaction vessel (capacity: 3.4 liters, solution residence time: 30 minutes) with a stirring blade by a pump, and calcium hydroxide slurry is added to the reaction vessel to adjust the pH of the solution in the vessel. It was constantly supplied while adjusting to 8.5 to 9.0.
This solution was filtered with a filter press to obtain a solid component composed of selenium and ferrous hydroxide and a liquid component (filtrate). The concentration of selenium in the filtrate was 0.5 mg / liter.
[0031]
【The invention's effect】
According to the method and apparatus for treating selenium of the present invention, it is possible to efficiently treat a solution containing selenium such as waste water obtained by washing dust so that selenium is almost completely recovered and removed as a solid component. it can.
[Brief description of the drawings]
FIG. 1 is a diagram schematically showing an example of a processing apparatus for a solution containing selenium according to the present invention.
[Explanation of symbols]
1 Treatment apparatus for solution containing selenium 2 Pretreatment tank 3 Channel 4 Precipitation tank 5 Filtration apparatus (solid-liquid separation means)
6 Selenium-containing solution 7 Ferrous compound storage tank 8 Ferrous compound supply port 9 Acid storage tank 10 Acid supply port 11 Stirring blade (stirring means)
DESCRIPTION OF SYMBOLS 12 Drain port 13 Pump 14 Solution inflow port 15 Alkali agent supply port 16 Alkali agent 17 Pump 18 Alkali agent storage tank 19 Stirring blade (stirring means)
20 Channel 21 Drain 22 Pump

Claims (4)

(A) First, an acid is added to a solution containing selenium as necessary, and a ferrous compound is added in a state where the pH is adjusted to an acidic region to obtain a solution containing selenium and divalent iron ions. An iron compound addition step;
(B) While allowing the solution containing selenium and divalent iron ions to flow in one direction in the flow path, an alkali agent is added at a predetermined point in the flow of the flow path; An alkaline agent addition step of adjusting the pH of the flow side to an alkaline region to obtain a solution containing selenium and ferrous hydroxide;
(C) Solid-liquid separation of the solution containing selenium and ferrous hydroxide to obtain a solid component containing selenium and ferrous hydroxide and a liquid component from which selenium has been removed. A method for treating a solution containing selenium, comprising:   Precipitation growth in which a solution containing the selenium and ferrous hydroxide is stored between the step (B) and the step (C), and the selenium and ferrous hydroxide in the solution are precipitated as a solid component. The processing method of the solution containing the selenium of Claim 1 including a process. (A) A pretreatment tank for storing a solution containing selenium, a ferrous compound supply port for supplying a ferrous compound in the pretreatment tank, and an acid in the pretreatment tank A pretreatment tank having an acid supply port for supplying;
(B) A flow path for flowing the solution containing the selenium discharged from the pretreatment tank in one direction, for supplying an alkaline agent to a predetermined point in the flow in the flow path. A flow path provided with an alkaline agent supply port;
(C) A solid-liquid separation unit for solid-liquid separation of the solution containing selenium discharged from the flow path, and a solution processing apparatus containing selenium.   The precipitation tank for storing the solution containing the selenium between the flow path (b) and the solid-liquid separation means (c) and precipitating a solid content in the solution. An apparatus for treating a solution containing selenium.

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