JP2005000823A - Method for treating geothermal water - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the adsorption efficiency of arsenic in geothermal water by making silica adsorption on iron hydroxide harder. <P>SOLUTION: First, geothermal water at least containing silica and arsenic and taken out from underground is charged into an oxidizing tank 1, and pretreatment of controlling the pH of the geothermal water to ≥9 and further adding an oxidizer thereto is performed. Next, the geothermal water is intermittently fed to a treatment tank 2, ferric ions are added to the treatment tank 2 per feed to precipitate arsenic, and sediment produced by the precipitation is separated from supernatant fluid. Then, when new geothermal water is treated in the treatment tank 2, the separated sediment is charged to the treatment tank 2 and is repeatedly used. Further, the pH of the treatment liquid in the treatment tank 2 is controlled to 3 to 5 on the treatment, and the repetition number of the use of the sediment is made ≥5 times. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for treating geothermal water taken out from underground, and more particularly, to a method for treating geothermal water that can remove arsenic (As) contained in geothermal water with high efficiency.
[0002]
[Prior art]
In recent years, geothermal power plants have attracted attention. In such a power plant, power is generated by driving a turbine for power generation using high-temperature and high-pressure steam coming out of the underground.
At this time, depending on the place where the steam is collected, not only the steam but also high-temperature geothermal water may spout from the basement. Such geothermal water often contains arsenic (As). For example, when this geothermal water is used for heating, arsenic must be removed from the geothermal water. The same applies to hot spring water as geothermal water used for bathing.
[0003]
Conventionally, for example, an iron hydroxide coprecipitation method has been used as a method for treating such geothermal water.
In this iron hydroxide coprecipitation method, ferric ions are added to geothermal water containing arsenic, for example, an aqueous solution of ferric sulfate is added and reacted to form iron hydroxide and coprecipitate arsenic. Is a method of removing the starch produced by sand filtration or the like.
[0004]
However, this conventional iron hydroxide coprecipitation method does not always have sufficient arsenic removal efficiency, and the inventors of the present application have conducted research to further improve the removal efficiency. And when processing new geothermal water, it discovered that it was effective to use the isolate | separated starch repeatedly.
[0005]
However, a technique for repeatedly using starch as a method for treating wastewater containing heavy metals has been described in, for example, Patent Document 1 (Japanese Patent Laid-Open No. 11-314094).
This is because the sulfide-based precipitate produced by adding a sulfiding agent at a pH of 7 or higher to wastewater containing arsenic and other heavy metals and subsequently adjusting the pH to 7.5 or higher remains in the liquid. It is a method of adding ferric ions and separating the produced starch from the liquid. At this time, the wastewater treatment method in which the filtered starch is added simultaneously with or before or after the addition of the next ferric ion. It is.
[0006]
[Patent Document 1]
Japanese Patent Laid-Open No. 11-314094
[Problems to be solved by the invention]
However, in the conventional method of adding starch at the same time as before or after the addition of ferric ion, even if this is applied to geothermal water as it is, the removal efficiency of arsenic is not always sufficient. There was no problem. The reason is that when geothermal water is adjusted to pH 7.5 or more and ferric ions are added, silica is also easily adsorbed to iron hydroxide, so that arsenic adsorption is inhibited accordingly. It is possible. Therefore, the conventional method of adding starch cannot be simply applied.
[0008]
The present invention has been made in view of the above problems, and an object of the present invention is to provide a method for treating geothermal water in which silica is hardly adsorbed on iron hydroxide and arsenic adsorption efficiency in geothermal water is improved. And
[0009]
[Means for Solving the Problems]
In order to achieve such an object, the geothermal water treatment method of the present invention supplies, at least intermittently or continuously, geothermal water containing at least silica and arsenic and taken out from the underground to the treatment tank. In a method for treating geothermal water in which ferric ions are added to coprecipitate arsenic and the starch produced by the coprecipitation is separated from the supernatant,
In the treatment in the treatment tank, the separated starch is used in the treatment tank, and the treatment liquid in the treatment tank is treated at a pH of 3 ≦ pH ≦ 5.
In addition, a flocculant may be used during the precipitation of the starch. Examples of the flocculant include cationic polymer flocculants. Separation of the starch is performed by an appropriate method such as a precipitation method or a magnetic separation method.
[0010]
Thereby, when the pH of the treatment liquid in the treatment tank is in the range of 3 ≦ pH ≦ 5, silica is hardly adsorbed, and the arsenic adsorption efficiency can be improved. If the pH is less than 3, there is an inconvenience that the generation of iron hydroxide is insufficient and the arsenic removal efficiency is low. If the pH exceeds 5, the silica is adsorbed, so that the arsenic adsorption efficiency decreases. Come.
In this case, it is more desirable to perform the treatment by setting the pH of the treatment liquid in the treatment tank to 3.5 ≦ pH ≦ 4.5. Silica adsorption is more reliably suppressed.
[0011]
In addition, in order to achieve the above object, the geothermal water treatment method of the present invention intermittently supplies geothermal water containing at least silica and arsenic and extracted from the underground to the treatment tank. In a method for treating geothermal water in which ferric ions are added to coprecipitate the arsenic, and the starch produced by the coprecipitation is separated from the supernatant.
When new geothermal water is treated in the treatment tank, the separated starch is repeatedly used in the treatment tank, and the treatment liquid in the treatment tank has a pH of 3 ≦ pH ≦ 5. In addition, the number of repeated uses of the starch is set to 5 or more.
[0012]
Thereby, when the pH of the treatment liquid in the treatment tank is in the range of 3 ≦ pH ≦ 5, silica is hardly adsorbed, and the arsenic adsorption efficiency can be improved. If the pH is less than 3, there is an inconvenience that the generation of iron hydroxide is insufficient and the arsenic removal efficiency is low. If the pH exceeds 5, the silica is adsorbed, so that the arsenic adsorption efficiency decreases. Come.
Also in this case, it is more desirable to perform the treatment by setting the pH of the treatment liquid in the treatment tank to 3.5 ≦ pH ≦ 4.5. Silica adsorption is more reliably suppressed.
In addition, since the number of repeated uses of starch is 5 times or more, the effect of starch improves the adsorption efficiency of arsenic, and the amount of ferric ions added can be reduced accordingly, reducing the processing cost. It can be greatly reduced. When the number of repetitions is 5 times or more, the starch capacity becomes almost constant, and the processing efficiency is stabilized.
[0013]
Furthermore, before supplying geothermal water to the said processing tank as needed, while setting the pH of this geothermal water to 9 <= pH, it is set as the structure which performs the pretreatment which adds an oxidizing agent.
As the oxidizing agent, for example, sodium hypochlorite or hydrogen peroxide is used. In general, arsenic is also referred to as trivalent arsenic (hereinafter also referred to as “As (III)”) mainly present as arsenous acid in geothermal water and pentavalent arsenic (hereinafter referred to as “As (V)”) mainly present as arsenic acid. ) And dissolved, but it has been found that arsenic is present as As (III) in geothermal water.
[0014]
Here, when the pH value of the geothermal water is increased as pretreatment, As (III) is efficiently oxidized to As (V). As the pH value increases, As (III) is efficiently oxidized to As (V). Further, since an oxidizing agent is added, As (III) in the geothermal water is more reliably changed to As (V), and arsenic in the geothermal water can be easily removed by the iron hydroxide coprecipitation method. .
[0015]
Moreover, before supplying geothermal water to the said processing tank as needed, while setting pH of this geothermal water to 9 <= pH, it is set as the structure which performs the pretreatment by air aeration. As a result, As (III) in the geothermal water becomes As (V) more reliably, and arsenic in the geothermal water can be easily removed by the iron hydroxide coprecipitation method.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the processing method of the geothermal water which concerns on embodiment of this invention is demonstrated. The geothermal water to be treated in this embodiment contains at least silica and arsenic and is taken out from the underground. The geothermal water has, for example, a trivalent arsenic (As (III)) concentration of 3.4 mg / l and a pH of 8.4.
[0017]
In the present embodiment, as shown in FIG. 1, an oxidation tank 1 and a treatment tank 2 are provided, pretreatment is performed in the oxidation tank 1, and then geothermal water in the oxidation tank 1 is supplied to the treatment tank 2. Here, ferric ions are added to co-precipitate arsenic for every supply to the treatment tank 2, and then the supernatant is taken from the treatment tank 2 and the starch produced by the co-precipitation is put into the treatment tank. Try to leave. And when processing new geothermal water with the processing tank 2, the isolate | separated starch is put into a processing tank and it uses repeatedly.
[0018]
First, the pretreatment in the first oxidation tank 1 will be described. An alkalizing agent is added to the geothermal water to be treated. Sodium hydroxide is preferred as the alkalinizing agent. Thereby, the pH of the geothermal water which was originally pH 8.4 is set to 9.0 or more, for example, pH 10. By this treatment, trivalent arsenic (As (III)) is oxidized to pentavalent arsenic (As (V)).
In this state, an oxidizing agent is further added. As the oxidizing agent, for example, sodium hypochlorite or hydrogen peroxide water is suitable. This ensures that trivalent arsenic (As (III)) is oxidized to pentavalent arsenic (As (V)). Moreover, it is good also as a structure which performs the pretreatment by air aeration.
[0019]
Next, processing in the processing tank 2 will be described. First, as shown in FIG. 1A, the first process will be described. The geothermal water in the oxidation tank 1 is transferred to the treatment tank 2, and the pH of the treatment liquid is 3 ≦ pH ≦ 5, preferably 3.5 ≦ pH ≦ 4.5. In this pH adjustment, for example, sulfuric acid is used. If the pH is too low, sodium hydroxide is used as the alkalizing agent.
[0020]
In this state, ferric ions are added to coprecipitate arsenic. For example, a ferric sulfate solution is added. Under the condition that the concentration of trivalent arsenic (As (III)) is 3.4 mg / l, it is effective that the amount of ferric ion added is 70 mg / l to 120 mg / l each time. If it is less than 70 mg / l, the processing efficiency is inferior. On the other hand, if it exceeds 120 mg / l, the amount of Fe increases, so that the cost increases and the profitability is impaired. Desirably, the amount of ferric ion added is 80 ± 5 mg / l each time. Thereby, a coprecipitation process is performed. In this case, since the pH of the treatment liquid in the treatment tank 2 is 3 ≦ pH ≦ 5, preferably 3.5 ≦ pH ≦ 4.5, the silica in the treatment liquid is hardly adsorbed, and the adsorption efficiency of arsenic is increased accordingly. Can be improved.
In this case, a flocculant may be added. Examples of the flocculant include cationic polymer flocculants.
Thereafter, the starch is precipitated by a precipitation method, the supernatant is taken from the treatment tank 2, and the starch produced by the coprecipitation is left in the treatment tank. Alternatively, at this time, another appropriate method such as a magnetic separation method may be used.
[0021]
Next, as shown in FIG.1 (b), the process after the 2nd time in the processing tank 2 is demonstrated. Similarly to the above, the treated liquid in the oxidation tank 1 is transferred to the treatment tank 2 where the starch is left. In the same manner as described above, the pH of the treatment liquid is 3 ≦ pH ≦ 5, preferably 3.5 ≦ pH ≦ 4.5. In this pH adjustment, for example, sulfuric acid is used. If the pH becomes too low, sodium hydroxide, for example, is used as an alkalizing agent.
[0022]
In this state, ferric ions are added to coprecipitate arsenic. For example, a ferric sulfate solution is added. Under the condition that the concentration of trivalent arsenic (As (III)) is 3.4 mg / l, the amount of ferric ion added is 70 mg / l to 120 mg / l each time, preferably as described above. 80 ± 5 mg / l. Thereby, a coprecipitation process is performed. In this case, since the pH of the treatment liquid in the treatment tank 2 is 3 ≦ pH ≦ 5, preferably 3.5 ≦ pH ≦ 4.5, the silica in the treatment liquid is hardly adsorbed, and the adsorption efficiency of arsenic is increased accordingly. Can be improved.
[0023]
Further, since the starch is repeatedly used, the arsenic adsorption efficiency is improved by the effect of the starch, and the amount of ferric ions to be added can be reduced accordingly, and the processing cost can be greatly reduced. When the number of repetitions is 5 times or more, the starch capacity becomes almost constant, and the processing efficiency is stabilized.
In this way, geothermal water is treated repeatedly.
[0024]
In FIG. 2, the processing method of the geothermal water which concerns on another embodiment is shown. This is provided with an oxidation tank 1, a treatment tank 2, and a precipitation tank 3, and pre-treats in the oxidation tank 1, and then continuously transfers the geothermal water of the oxidation tank 1 to the treatment tank 2, Here, ferric ions are added continuously or intermittently to co-precipitate arsenic, and the supernatant is taken out from the treatment tank 2 and transferred to the precipitation tank 3 to further precipitate starch, and the supernatant and starch are separated. In this case, another appropriate method such as a magnetic separation method may be used. Then, the starch is returned to the treatment tank 2. Therefore, the starch generated in the processing tank 2 and the starch recovered in the precipitation tank 3 are accommodated in the processing tank 2. When starch increases and accumulates in the processing tank 2, it is collected from the lower end of the processing tank 2 in a timely manner. The treatment in the oxidation tank 1 and the treatment in the treatment tank 2 are performed in the same manner as described above. Unlike the above, since the processing can be performed continuously, the processing efficiency is improved.
[0025]
[Experimental example]
Next, an experimental example of the geothermal water treatment method according to the present invention will be described. The geothermal water used in this experimental example has a trivalent arsenic (As (III)) concentration of 3.4 mg / l and a pH of 8.4.
(Experimental example 1)
The relationship between the pH value and the degree of oxidation of arsenic in the pretreatment of geothermal water was measured. In this example, the pH value in the geothermal water is changed, and hydrogen peroxide is added 1 time as much as As (III) in the geothermal water for every change and stirred for 10 minutes, and then the oxidation rate of As (III) is obtained. The effect of pH was examined. The pH was adjusted with sodium hydroxide. The results are shown in FIG.
[0026]
From this result, arsenous acid (As (III)) in geothermal water is oxidized to arsenic acid (As (V)) by hydrogen peroxide water as an oxidizing agent, but the pH in geothermal water is 8.1, which is efficient. It can be seen that, in order to oxidize As (III), it is effective to add hydrogen peroxide after the pH is set to 9 or higher.
[0027]
(Experimental example 2)
The pH value of geothermal water and the degree of arsenic removal were measured when ferric ions were added in the treatment tank 2 to coprecipitate arsenic. In this example, As (III) in geothermal water was oxidized with hydrogen peroxide, Fe (III) was added and adjusted to various pH to produce iron hydroxide, stirred for 10 minutes, filtered, filtered The As concentration of the liquid was measured. The additive solution is a ferric sulfate solution. The results are shown in FIG.
[0028]
From this result, it can be seen that arsenic can be effectively removed by treating the treatment liquid at pH 3 ≦ pH ≦ 5, preferably 3.5 ≦ pH ≦ 4.5.
[0029]
(Experimental example 3)
The pH value of geothermal water and the degree of removal of silica when ferric ions were added in the treatment tank 2 to coprecipitate arsenic were measured. In this example, As (III) in geothermal water was oxidized with hydrogen peroxide, Fe (III) was added and adjusted to various pH to produce iron hydroxide, stirred for 10 minutes, filtered, filtered The Si concentration of the liquid was measured. The additive solution is a ferric sulfate solution. The results are shown in FIG.
[0030]
From this result, it can be seen that when the pH of the treatment liquid exceeds 5, the adsorption amount of silica increases. That is, corresponding to the result of Experimental Example 2 above, when the pH of the treatment liquid is 5 or less, silica is hardly adsorbed and arsenic adsorption efficiency is improved.
[0031]
(Experimental example 4)
Repeated starch experiments were performed. In this experiment, starch was repeatedly used, and Fe (III) was added for each repeated use, and the As concentration of the filtrate with respect to the added amount was measured. The addition amount was set to four modes of 40 mg / l, 60 mg / l, 80 mg / l, and 120 mg / l. The results are shown in FIG.
[0032]
From this result, the processing efficiency increases as the number of times the starch adsorbs As, but the processing efficiency is inferior at 60 mg / l or less, but at 80 mg / l or more, the starch and newly added Fe are added. Due to the synergistic effect with (III), the processing efficiency is greatly increased. If it exceeds 120 mg / l, the processing efficiency increases, but the amount of Fe increases, so that the cost increases and the profitability deteriorates. From this, it can be seen that the amount of Fe (III) required to remove As can be reduced by making the addition amount of Fe (III) appropriate each time. Desirably, the amount of ferric ion added is 80 ± 5 mg / l each time.
[0033]
(Experimental example 5)
An experiment was conducted on the influence of the number of repetitions on the starch volume (after 20 minutes of standing) when the starch was repeatedly used. In this example, (1) 200 ml of geothermal water was added to a beaker, Fe (III) was added to 120 mg / l, adjusted to pH 4.0, transferred to a graduated cylinder, and inverted stirring was performed 10 times. . The volume of the starch that settled after standing for 20 minutes was measured. (2) After the measurement, the supernatant water was discarded and the starch was transferred to a beaker. The operations (1) and (2) were repeated. The results are shown in FIG.
[0034]
From this result, it can be seen that, in the starch repetition method, the starch volume initially increases, but the starch volume becomes constant after a certain number of times. If the number of repeated uses of the starch is 5 times or more, the starch volume becomes almost constant and the processing efficiency is stabilized.
[0035]
【The invention's effect】
As described above, according to the method for treating geothermal water of the present invention, geothermal water is intermittently or continuously supplied to the treatment tank, and ferric ions are added to the tub to coprecipitate arsenic. At the same time, the starch produced and separated by coprecipitation is used in the treatment tank, and the pH of the treatment liquid in the treatment tank is 3 ≦ pH ≦ 5, preferably 3.5 ≦ pH ≦ 4.5. Since the treatment is performed, the silica is less likely to be adsorbed due to the pH of the treatment liquid in the treatment tank, and the arsenic adsorption efficiency can be improved accordingly. In addition, if the number of repeated uses of starch is increased to 5 times or more, the adsorption efficiency of arsenic can be improved due to the effect of starch, and the amount of ferric ions to be added can be reduced accordingly, greatly increasing the processing cost. Can be reduced. In addition, when the number of repetitions is 5 times or more, the starch capacity is almost constant, and the processing efficiency is stabilized.
[0036]
Further, before supplying the geothermal water to the treatment tank, the pH of the geothermal water is set to 9 ≦ pH and the pretreatment for adding the oxidizing agent is performed. As (III) can be efficiently oxidized to As (V) and an oxidant is added. As a result, As (III) in the geothermal water becomes As (V) more reliably. Arsenic can be easily removed by iron hydroxide coprecipitation.
[Brief description of the drawings]
FIG. 1 is a diagram showing a method for treating geothermal water according to an embodiment of the present invention.
FIG. 2 is a diagram showing a method for treating geothermal water according to another embodiment of the present invention.
FIG. 3 is a graph showing the relationship between the pH value and the degree of arsenic oxidation in the pretreatment of geothermal water according to an experimental example of the present invention.
FIG. 4 is a graph showing the relationship between the pH value of geothermal water and the degree of arsenic removal when ferric ions are added and co-precipitated in a treatment tank according to an experimental example of the present invention. .
FIG. 5 is a graph showing the relationship between the pH value of geothermal water and the degree of silica removal when ferric ions are added to coprecipitate arsenic in a treatment tank according to an experimental example of the present invention. .
FIG. 6 is a graph showing the relationship between the amount of Fe (III) added for each repeated use and the degree of arsenic removal when the starch is used repeatedly, according to the experimental example of the present invention.
FIG. 7 is a graph showing the relationship between the starch capacity and the number of repetitions when the starch is repeatedly used in the experimental example of the present invention.
[Explanation of symbols]
1 Oxidation tank 2 Treatment tank 3 Settling tank

Claims (6)

Geothermal water containing at least silica and arsenic and taken out from the underground is intermittently or continuously supplied to the treatment tank, and ferric ions are added in the treatment tank to co-precipitate arsenic, and the starch produced by the coprecipitation In the method of treating geothermal water that separates things from the supernatant,
When the treatment is performed in the treatment tank, the separated starch is used in the treatment tank, and the treatment liquid is treated at a pH of 3 ≦ pH ≦ 5. Hot water treatment method. 2. The method for treating geothermal water according to claim 1, wherein the treatment liquid in the treatment tank is treated with a pH of 3.5 ≦ pH ≦ 4.5. Geothermal water containing at least silica and arsenic is extracted from the underground and supplied to the treatment tank intermittently, and ferric ions are added to co-precipitate each time it is supplied to the treatment tank. In a method of treating geothermal water that separates the starch from the supernatant,
When new geothermal water is treated in the treatment tank, the separated starch is repeatedly used in the treatment tank, and the treatment liquid in the treatment tank has a pH of 3 ≦ pH ≦ 5. And the processing method of the geothermal water characterized by making the frequency | count of repeated use which the said starch uses into 5 times or more. 4. The method for treating geothermal water according to claim 3, wherein the treatment liquid in the treatment tank is treated with a pH of 3.5 ≦ pH ≦ 4.5. Before the geothermal water is supplied to the treatment tank, the pH of the geothermal water is set to 9 ≦ pH and a pretreatment for adding an oxidizing agent is performed. 4. The method for treating geothermal water according to 4. Before supplying geothermal water to the treatment tank, the pH of the geothermal water is set to 9 ≦ pH, and pretreatment by air aeration is performed. 5. The method for treating geothermal water according to 5.

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