JP7145637B2 - Method for treating selenium-containing soil and rock - Google Patents

Description

The present invention relates to a method for treating selenium-containing soil/rock.

Naturally occurring selenium exceeding the elution amount standard (0.01 mg/L) of the Soil Contamination Countermeasures Law may be detected from rocks such as mudstone and shale excavated during construction of mountain tunnels. When the standard is exceeded, the total content of selenium contained in soil and rocks is usually about 0.5 to 1 mg/kg, and the amount of selenium eluted by rainwater is about 0.01 to 0.05 mg/L. is.

Soil and rocks containing selenium above the standard cannot be disposed of as construction soil and must be disposed of as contaminated soil. As a conventional disposal method, it is landfilled in a designated disposal site, but due to the tightness of the disposal site, there is concern that it will be difficult to continue the same disposal in the future (Non-Patent Document 1) , 2).

Edited by Masashi Kamon and Takeshi Katsumi, Public Works Research Institute/Public Works Research Center/Technical Review Committee for Ground Contamination, Handbook for Dealing with Soil Containing Naturally Occurring Heavy Metals Generated in Construction Work, ``Introduction to Publication'' and ``Supervisor's Message'', Taisei Publishing Publishing, 2015 Sachi Kitaoka, Investigation and countermeasures for heavy metals derived from nature, Global Environment, Vol.15, No.1, p.23-30, 2010.

As a result of extensive studies by the present inventors, the amount of selenium eluted from naturally occurring soils and rocks exceeds the standard value as described above, but it is a level that can be sufficiently reduced by appropriate chemical treatment. it was thought.

The present invention provides a method for treating selenium-containing soil and rock, which can reduce the selenium content in soil and rock.

[1] An extraction step of contacting soil or rock particles containing selenium with an aqueous solution containing hypochlorite to obtain an extract containing selenium, wherein hypochlorite relative to the total mass of the aqueous solution A method for treating selenium-containing soil and rocks, wherein the content of is 0.1 to 1.0% by mass.
[2] The method for treating selenium-containing soil/rock according to [1], wherein the pH of the extract is more than pH 4 and less than or equal to pH 7.
[3] The method for treating selenium-containing soil or rocks according to [1] or [2], comprising a crushing step of crushing soil or rocks containing selenium to obtain the particles.

According to the method for treating selenium-containing soil and rocks of the present invention, it is possible to obtain purified soil in which the selenium content contained in the selenium-containing soil and rocks generated by excavation during construction is reduced. Purified soil can be disposed of in the same manner as conventional construction soil.
Furthermore, according to the method for treating selenium-containing soil and rocks of the present invention, it is not necessary to increase the concentration of hypochlorite in the aqueous solution, and rather more selenium can be extracted than when the concentration is excessively high. can. As a result, consumption of hypochlorite can be reduced when processing a large amount of soil and rocks.

1 is a schematic diagram of an example of a treatment system applicable to treatment of selenium-containing soil/rock; FIG. 4 is a graph showing the relationship between the concentration of hypochlorite in the aqueous solution for extraction used in the test and the selenium extraction rate.

《Method for treating selenium-containing soil and rocks》
A first aspect of the present invention has an extraction step of contacting soil or rock particles containing selenium with an aqueous solution containing hypochlorite to obtain an extract containing selenium, and A method for treating selenium-containing soil and rocks, characterized in that the content of hypochlorite is 0.1 to 1.0% by mass. The treatment method can be applied to selenium-bearing soils or selenium-bearing rocks.

The soil includes coarse-grained or clay-like inorganic substances produced by weathering of rocks, organic substances derived from organisms, and the like, and is generally called soil.
The rocks are minerals or mixtures thereof, and may contain metal components, natural glass components, and the like. In this specification, soil and rock may be collectively referred to as soil/rock.

[Crushing process]
The size of soil and rock discharged from a construction site usually forms lumps of several centimeters to several tens of centimeters. These lumps are reduced to fine particles in the crushing process. For example, by using a known crusher such as a jaw crusher or a rod mill, the long diameter (particle diameter) of the particles is preferably 10 mm or less, more preferably 5 mm or less, and even more preferably 2 mm or less. .
When finely pulverized to the above particle size or less, the extraction efficiency of selenium by hypochlorous acid can be enhanced, and the amount of selenium remaining inside the mass can be reduced. The lower limit of the particle size is not particularly limited, but if it is too small, handling and recovery become difficult, so it is preferably 10 μm or more, for example. Generally, when soil or rock is crushed by a known crusher, it is relatively easy to control the upper limit, but it is difficult to make the particle size constant. Therefore, the crushed material obtained by the crushing process is a mixture of various particle sizes. In this specification, the term "crushing" is synonymous with "crushing".

[Extraction process]
As a method of contacting the particles contained in the crushed material obtained in the crushing step with an aqueous solution containing hypochlorite (hereinafter sometimes referred to as hypochlorite aqueous solution), for example, the crushed material a method of dropping or spraying the hypochlorite aqueous solution into the hypochlorite aqueous solution;
When an aqueous hypochlorite solution contacts the particles, selenium is extracted from the particles into the aqueous hypochlorite solution. The selenium to be extracted may be selenium alone (metallic selenium) or a compound containing selenium.

Examples of hypochlorites include salts of hypochlorous acid and alkali metals such as sodium hypochlorite, potassium hypochlorite and lithium hypochlorite, and hypochlorite salts such as calcium hypochlorite. Salts of chloric acid and alkaline earth metals are mentioned. Among them, a salt with an alkali metal is preferable, and sodium hypochlorite is more preferable, since it is easy to extract selenium at a predetermined concentration. The type of hypochlorite contained in the hypochlorite aqueous solution may be one or two or more. When two or more types of hypochlorite are included, their total concentration is the predetermined concentration described later.

The concentration of hypochlorite relative to the total mass of the hypochlorite aqueous solution is 0.1 to 1.0% by mass from the viewpoint of increasing the selenium extraction efficiency and reducing the amount of hypochlorite used. , preferably 0.2 to 0.8% by mass, more preferably 0.3 to 0.7% by mass.
If it is less than 0.1% by mass, selenium cannot be sufficiently extracted, and if it exceeds 1.0% by mass, the extraction efficiency of selenium will decrease, and if it exceeds 1.0% by mass, the concentration will be further increased. Raising it will have the opposite effect.

The hypochlorite aqueous solution in contact with the particles becomes an extract containing selenium. The pH of the hypochlorite aqueous solution in the contact state is preferably more than pH4 and pH7 or less, more preferably more than pH4 and pH6 or less, and particularly preferably more than pH4 and pH5 or less. Within the preferred pH range, the selenium extraction efficiency can be further enhanced. Hypochlorous acid changes to chlorine when the pH becomes 4 or less, and its oxidizing power is reduced.

The pH of the hypochlorite aqueous solution can vary depending on the ingredients contained in the particles. The above preferred pH range is preferably the pH after contacting the particles and after varying according to the composition of the particles. Specifically, it is preferable that the pH of the extract at the end of the extraction step is within the above preferred pH range.

The pH of the hypochlorite aqueous solution may be adjusted in advance before the contact, or may be adjusted during the contact, assuming the pH after contact with the particles.
For example, when the pH of the purified water brought into contact with the particles becomes alkaline due to the components of the particles, the pH of the hypochlorite aqueous solution is adjusted in advance to be about 1 to 2 more acidic than the above preferred pH range. It is preferable to keep According to this method, after the contact, the pH of the hypochlorite aqueous solution is changed to the alkaline side by the above components, and can be stabilized in the above-mentioned suitable pH range. The pH of the hypochlorite aqueous solution is measured by a conventional method in accordance with JIS Z 8802:2011.
As a method of adjusting the pH of the hypochlorite aqueous solution to the acidic side, there is a method of dropping an acid. Examples of the acid to be added dropwise include inorganic acids such as hydrochloric acid and organic acids such as acetic acid.

When the pH of the hypochlorite aqueous solution is adjusted to 1 to 4, chlorine gas is generated. Chlorine gas should be properly vented to avoid inhalation by animals and humans.

Examples of components of the particles that make the purified water alkaline include sodium, calcium, and potassium. The soil/rock may contain these components.

The form in which the soil/rock contains selenium may be a form in which free selenium or a compound containing selenium is adsorbed to the soil/rock, or a form in which selenium or a compound containing selenium is contained in minerals constituting the soil/rock. It may be in the form of a solid solution, or may be in the form of being contained as an element constituting minerals. Here, whether selenium is dissolved in the minerals that make up the soil and rocks should be confirmed by an elemental analysis method using EDS (energy dispersive X-ray spectrometer) or XPS (X-ray photoelectron spectroscopy). can be done.
Minerals containing selenium as a constituent element include, for example, sarknite (FeSe ₂ ), curta ore (CuSe ₂ ), trogtalite (CoSe ₂ ), and penroseite {(Ni, Co, Cu) Se ₂ }.

The soil/rock may contain a sulfur component. When this sulfur component comes into contact with hypochlorous acid, sulfuric acid is produced, and the sulfuric acid elutes selenium.
Examples of soils and rocks containing sulfur include sulfur (S), pyrite (FeS ₂ ), pyrrhotite (Fe _1-x S _x ; x=0.09 to 0.17), molybdenite ( MoS ₂ ), chalcopyrite (CuFeS ₂ ), galena (PbS), howellite (MnS ₂ ), base ore (NiS ₂ ), catielite (CoS ₂ ), lauraite (RuS ₂ ), errichmannite ( OsS ₂ ), birmannite {(Cu, Ni, Co, Fe) S ₂ }, fukuchiko (Cu ₃ FeS ₈ ), cinnabar (HgS) and other sulfide minerals.

The temperature of the hypochlorite aqueous solution when the particles are brought into contact with the hypochlorite aqueous solution is, for example, about 10 to 60.degree.
If the state in which the hypochlorite aqueous solution and the particles are brought into contact is a mixture in which the particles are contained in the hypochlorite aqueous solution, the mixture is stirred to further increase the extraction efficiency of selenium. can be done.

The selenium extracted from the soil/rock with the hypochlorite aqueous solution can take the form of selenium oxoacid. Here, the selenium oxoacid ion includes selenate ion (SeO ₄ ²⁻ ), hydrogen selenate ion (HSeO ₄ ⁻ ), selenite ion (SeO ₃ ²⁻ ), hydrogen selenite ion (HSeO ₃ ^- ).
Hypochlorous acid can oxidize the oxoacid of selenium from tetravalent to hexavalent. The oxoacid of tetravalent selenium tends to adsorb relatively strongly as an inner sphere complex to hydroxyl groups on the surface of soil/rock particles. On the other hand, the oxoacid of hexavalent selenium is relatively weak and readily adsorbs to hydroxyl groups on the surface of soil/rock particles as an outer sphere complex. Therefore, oxidizing the oxoacid of selenium from tetravalent to hexavalent with hypochlorous acid is advantageous in extracting the oxoacid of selenium into the hypochlorite aqueous solution.

The smaller the particle size, the shorter the time for contacting the hypochlorite aqueous solution with the particles to extract selenium. For example, if the particle size is 10 mm or less, the time is preferably about 1 hour to 24 hours, and if the particle size is 2 mm or less, the time is preferably about 10 minutes to 2 hours.
The standard for completion of extraction is, for example, that the total selenium content in the soil/rock after extraction treatment is reduced to about 0 to 50% of that before treatment, and that the selenium elution amount reaches the elution amount standard value (0.01 mg / L ) are as follows.

Extraction management methods include, for example, reducing the total selenium content of soil and rocks after extraction treatment to about 0 to 50% of that before treatment, or reducing the selenium elution amount to the elution amount standard value (0.01 mg/ L) Set a purification target such as below. Next, an extraction experiment is performed in advance using a portion of the soil/rock to be treated, and extraction conditions that satisfy the purification target are determined. As extraction conditions, the concentration and pH of hypochlorite in the hypochlorite aqueous solution, the liquid-solid ratio of the hypochlorite aqueous solution and the soil / rock to be treated, the hypochlorite aqueous solution and treatment The contact time of the soil and rocks to be the target of is mentioned. After that, a larger extraction process is performed under the conditions determined by the extraction experiment. In a large-scale extraction process, it is preferable to analyze the soil/rock after the extraction process at a frequency of one sample per 100 m ³ and conduct a sampling inspection to confirm that the purification target is met.

[Separation process]
After the extraction, the liquid extract is separated from the liquid extract (soil/rock dispersion liquid) containing the residue of the soil/rock. As the separation method, depending on the size of the particles contained in the residue, for example, a method of stepwise separation from large particles to small particles can be used.
Specifically, for example, in the first step, the soil particles (residue) are filtered through a sieve with a mesh size of about 2 mm to remove soil particles (residues) with a particle diameter of about 2 mm, and in the second step, soil particles with a diameter of up to about 75 μm are removed using a hydrocyclone. Except for , there is a method of adding a flocculating agent to the extract in the third step to generate flocs containing the remaining fine soil particles, flocculating and precipitating them to remove them.

Since the particles removed in each of the above steps are clean particles from which selenium has been removed, after dehydration by returning the pH to a neutral range, the particles can be treated as construction soil with less environmental impact. The treatment method is not particularly limited, and for example, a solidification material having a cone index of 200 kN/m ² or more can be obtained by a known solidification treatment performed on construction soil.

The crushing process, the extraction process, and the separation process described above can be performed by, for example, the processing system shown in FIG.

《Treatment system for selenium-containing soil and rocks》
The processing system of FIG. 1 is used for processing for extracting selenium from selenium-containing soil/rock, and has the following configuration. That is, a crusher 1 that makes fine particles (crushes) the soil/rock R1 containing naturally occurring selenium, and the crushed soil/rock particles are brought into contact with an aqueous solution of hypochlorite (extraction agent in the figure), An extraction tank 2 for obtaining an extract containing selenium, a sieve 3 for separating soil/rock residue from the extract, a cyclone 4 for separating soil/rock particles from the extract that has passed through the sieve 3, and a cyclone 4. A turbid water treatment device 5 that separates fine particles remaining in the passed extract by flocculating with a coagulant to obtain an extract as a supernatant liquid from which soil and rocks are removed, and precipitates collected by the turbid water treatment device 5. a dewatering device 7 such as a filter press for dewatering the sediment stored in the mud storage tank 6; , the sieve 3, the sand dehydration sieve 8, and the dehydration device 7 collect (aggregate) the residues, particles and precipitates derived from the soil and rocks, mix them and adjust the pH and moisture content of the mixture, and a mixer 9 for final purified soil R2.

The extraction tank 2 is connected to a crushed material supply unit 1 a for transferring the crushed soil/rock particles from the crusher 1 to the extraction tank 2 . In addition, the extraction tank 2 is equipped with a stirring blade for mixing and stirring the received soil/rocks and the hypochlorite aqueous solution introduced into the extraction tank 2, and a scrubbing machine for rubbing the soil/rocks together. is provided.
The sieve 3 is connected to a first liquid extract feeding section 2 a that feeds the liquid extract containing residual soil and rocks after extraction from the extraction tank 2 to the sieve 3 .
The cyclone 4 is connected to a second liquid extract feeding section 3 a that feeds the liquid extract containing particles of soil and rock that has passed through the sieve 3 from the sieve 3 to the cyclone 4 .
The turbid water treatment device 5 is connected to a third extract liquid sending unit 4 a that sends the extract containing fine particles of soil and rock that has passed through the cyclone 4 from the cyclone 4 to the turbid water treatment device 5 . Further, an extract fourth liquid sending unit 5 b for sending an extract as a supernatant liquid obtained in the muddy water treatment device 5 to the outside is connected to the muddy water treatment device 5 .
The sludge storage tank 6 is connected to a first sediment transfer section 5 a that transfers the sediment collected by the turbid water treatment device 5 from the turbid water treatment device 5 to the sludge storage tank 6 .
The dewatering device 7 is connected to a second sediment transfer section 6 a that transfers the sediment temporarily stored in the mud storage tank 6 from the mud storage tank 6 to the dehydration device 7 . Further, a first drainage liquid feeding section 7b for feeding the waste water dehydrated by the dehydrating device 7 to the outside is connected to the dehydrating device 7. As shown in FIG.
The sand dehydration sieve 8 is connected to a first particle transfer section 4 b that transfers the soil/rock particles collected by the cyclone 4 from the cyclone 4 to the sand dehydration sieve 8 . Although not shown in FIG. 1 , the sand dehydration sieve 8 is connected to a second waste water feeding section that feeds the waste water dehydrated by the sand dehydration sieve 8 to the outside.
The mixer 9 includes a residue transfer section 3b that transfers the residue of the soil/rock separated by the sieve 3 to the mixer 9, A second transfer part 8a for transferring particles to the mixer 9 and a third transfer part 7a for transferring fine particles of soil/rock dehydrated by the dehydrator 7 from the dehydrator 7 to the mixer 9 are connected. It is
Each part that performs the above connection is composed of known connection members such as pipes, valves, and pumps.

In the example of FIG. 1, the extracted selenium passes through the muddy water treatment device 5 and is treated in the wastewater treatment process.
As a treatment method other than the above, for example, there is a method of insolubilizing selenium in muddy water before flocculating the soil in the muddy water treatment device 5, and flocculating and recovering the insolubilized selenium together with the soil. In this case, since the concentration of selenium contained in the waste water discharged from the muddy water treatment apparatus 5 is low, the waste water treatment can be simplified. If the concentration of selenium in wastewater falls below environmental standards, there will be no need for special wastewater treatment. A known ion adsorbent can be used as a method for insolubilizing selenium. Soil containing selenium is recovered as a dehydrated cake by a dehydrator 7 and is properly treated without being put into a mixer 9. - 特許庁
In addition, as another treatment method other than the above, for example, before sending the extract from the cyclone 4 to the turbid water treatment device 5, the extract from the cyclone 4 is brought into contact with a known ion adsorbent to A method of sending the extract to the turbid water treatment device 5 after reducing the selenium concentration in the extract may be used. This method can also simplify the treatment of waste water discharged from the turbid water treatment device 5 in the same manner as described above.

The total selenium content in the samples used below was measured by JIS K0102: 2013, "67. Selenium hydrogen compound generation ICP emission spectroscopic analysis method".

[Test Example 1]
Rock K containing naturally occurring selenium was crushed to obtain selenium-containing particles having a particle size of 2 mm or less. The total selenium content of the selenium-containing particles was 0.268 mg/kg.
Six types of hypochlorite with a sodium hypochlorite concentration of 0% by mass, 0.1% by mass, 0.3% by mass, 0.5% by mass, 1.0% by mass, or 3.0% by mass An aqueous acid salt solution (adjusted to pH 5-6 with hydrochloric acid) was prepared.
100 g of the selenium-containing particles were added to each of 1000 g of six types of hypochlorite aqueous solutions (liquid-solid ratio: 10), and 5 ml of a hydrochloric acid solution obtained by mixing hydrochloric acid and water at a volume ratio of 1:1 was added for 30 minutes. After shaking, six extracts were obtained. Each extract was centrifuged, and the resulting supernatant was filtered through a membrane filter with a pore size of 0.45 μm to obtain an extract containing selenium.
The reason why the hydrochloric acid solution is added is to prevent the pH of the hypochlorite aqueous solution from shifting to the alkaline side due to the alkaline components of rocks.
The results of measuring the total selenium content in each extract by the above method are shown in the graph of FIG.

[Test Example 2]
A rock IS containing naturally occurring selenium was crushed to obtain selenium-containing particles having a particle size of 2 mm or less. The total selenium content of the selenium-containing particles was 0.160 mg/kg.
An extract containing selenium was obtained in the same manner as in Test Example 1 except that the selenium-containing particles were used, and the total selenium content was measured by the above method. The results are shown in the graph of FIG.

[Test Example 3]
A rock IN containing naturally occurring selenium was crushed to obtain selenium-containing particles having a particle size of 2 mm or less. The total selenium content of the selenium-containing particles was 0.311 mg/kg.
An extract containing selenium was obtained in the same manner as in Test Example 1 except that the selenium-containing particles were used, and the total selenium content was measured by the above method. The results are shown in the graph of FIG.

<Test results>
In the graph of each test example, the horizontal axis indicates sodium hypochlorite concentration. The selenium elution rate on the vertical axis is expressed by (mass of selenium contained in the extract)÷(mass of selenium contained in the rock sample)×100 (%).
As shown in Test Example 1, the selenium elution rate of the sodium hypochlorite aqueous solution having a concentration of 0.3% by mass was 51%, which was higher than the other concentrations. Surprisingly, the selenium elution rate decreased as the sodium hypochlorite concentration increased to 0.5% by mass, 1.0% by mass, and 3.0% by mass.
As shown in Test Example 2, the selenium elution rate of the sodium hypochlorite aqueous solution having a concentration of 0.3% by mass was 90%, and the selenium elution rate was excellent. Surprisingly, the selenium elution rate decreased at higher sodium hypochlorite concentrations of 0.5% by mass and 3.0% by mass. The selenium elution rate at a concentration of 1.0% by mass was 100%.
As shown in Test Example 3, the selenium elution rate of the sodium hypochlorite aqueous solution having a concentration of 0.5% by mass was 80%, which was higher than the other concentrations. Surprisingly, the selenium elution rate decreased as the sodium hypochlorite concentration increased to 1.0% by mass and 3.0% by mass.
From the above results, it was found that extraction (elution) using a hypochlorite aqueous solution with a hypochlorite concentration of 0.1 to 1.0% by mass can increase the extraction efficiency of selenium. it is obvious.

Each configuration and combination thereof in each embodiment described above is an example, and known configurations can be added, omitted, replaced, and changed in other ways without departing from the scope of the present invention.

R1... Soil/rock containing selenium, R2... Purified soil, 1... Crusher, 1a... Crushed matter supply unit, 2... Extraction tank, 2a... Extract liquid first feeding unit, 3... Sieve, 3a... Extract liquid second 2nd liquid sending part 3b... Residue transferring part 4... Cyclone 4a... Third liquid sending part for extract 4b... First particle transferring part 5... Muddy water treatment device 5a... First sedimentation transferring part 5b... Extraction Liquid fourth liquid feeding section 6 Mud storage tank 6a Sedimentation second feeding section 7 Dehydrator 7a Sedimentation third feeding section 7b Drainage first liquid feeding section 8 Sand dewatering sieve 8a ... particle second transfer section, 9 ... mixer

Claims (2)

An extraction step of contacting soil or rock particles containing selenium with an aqueous solution containing hypochlorite to obtain an extract containing selenium,
The content of hypochlorite relative to the total mass of the aqueous solution is 0.1 to 0.7% by mass,
pH of the extract is more than pH 4 and pH 7 or less ,
A method for treating selenium-containing soil and rocks , wherein the pH of the aqueous solution containing hypochlorite is 1 to 2 lower than the pH of the extract . 2. The method for treating selenium-containing soil/rock according to claim 1, further comprising a crushing step of crushing the soil or rock containing selenium to obtain the particles.

Images