JP4664786B2 - Insolubilization of heavy metal contaminated soil - Google Patents

Description

  The present invention is to insolubilize contaminated soil containing heavy metals, etc. generated in various fields, that is, contaminated soil containing mercury, arsenic, lead, selenium, chromium, cadmium, boron or fluorine, or incinerated ash. It is related with the method of detoxifying by suppressing elution to the environment.

  Numerous technologies have been developed and put to practical use in order to detoxify contaminated soil containing heavy metals caused by leakage or disposal from on-site equipment and facilities that handle various chemicals and agricultural chemicals. Among them, as a chemical treatment, an insolubilization treatment of heavy metals or the like by removing heavy metals or the like by heating or washing or adding a chemical agent such as a chelating agent is widely adopted.

  As a heat treatment method, Patent Document 1 below describes a method of volatilizing and removing arsenic by reducing arsenic-contaminated soil, and Patent Document 2 mixes chlorine-containing waste with heavy metal-contaminated soil and heats heavy metal content by high-temperature heating. Each of the methods for volatilizing chloride is disclosed. However, these methods cannot remove cadmium and chromium by volatilization, and the application target is limited.

  Patent Document 3 shows a sulfation roasting method in which sulfuric acid is added to a heavy metal-contaminated soil and roasted to gasify some heavy metals, and others are sulfated, but a large amount of corrosive sulfuric acid is used. In addition, it is uneconomical in terms of processing and equipment costs due to the need for low oxygen concentrations.

  On the other hand, as a method for removing or insolubilizing heavy metals by adding chemicals, Patent Document 4 discloses a method in which heavy metals are dissolved with a chelating agent and then desorbed from the chelating agent with an ion exchange resin to regenerate it. . This method is effective for suppressing the elution of lead, but the effect of insolubilization of mercury, cadmium, etc. is unclear, and the ion exchange resin is expensive, which is uneconomical in terms of processing costs.

  Patent Document 5 discloses a method of insolubilizing heavy metals with ferric sulfate and a flocculant and neutralizing them with an acid or an alkali. However, since a large amount of water is required, the facility becomes large-scale. There is.

  In addition, Patent Document 6 discloses a method for insolubilizing mercury or cadmium with sodium sulfide, but it is difficult to confirm that the aqueous solution of the sulfiding agent and the soil are in contact with each other sufficiently. There is a risk that it will remain and elute, so a large amount of sulfiding agent must be used in excess.

  Unlike these methods, there are methods such as backfilling contaminated soil and segregating with water-proof materials, but it is well known that there are still doubts about reliability and the risk of elution of heavy metals. is there.

Patent Document 7 is a method of the inventors' invention made for the purpose of improving the above problems, by forming crystalline calcium silicate in soil containing lead, chromium, cadmium and the like, This is a method of fixing these heavy metals to suppress elution. However, according to further research, when applying to contaminated soil containing high concentrations of mercury, the elution suppression effect is still insufficient, so that mercury is reduced to the environmental standard value of 0.0005 mg / L or less. It turned out to be difficult to reduce reliably.
JP 2001-104932 A JP-A-8-182983 JP 2003-326246 A Japanese Patent Laid-Open No. 2003-159583 JP 2002-233858 A Japanese Patent Laid-Open No. 7-24441 JP 2002-320952 A

  In the present invention, mercury, arsenic, lead, selenium, chromium, cadmium, boron, fluorine, etc. contained in the contaminated soil, especially arsenic and mercury are insolubilized to suppress desolation to the environment, Providing means to ensure the insolubilization rate and ensure the elution amount is below the environmental standard value, to prevent the risk of elution by remaining untreated, and to improve the economics of treatment and equipment And

(1) Sodium sulfide or sodium hydrosulfide as an insolubilizing agent in soil contaminated with heavy metals, etc. is mixed 0.01 to 1 times the chemical equivalent of heavy metals, etc. contained in the contaminated soil, and the heavy metals are sulfided. Then, calcium compound or both compounds of calcium compound and silica compound are added to the mixed soil after conversion to the sulfide in an amount of 3 to 30% by weight in terms of calcium oxide, and then these mixtures are added in an autoclave. A method for insolubilizing heavy metals and other contaminated soil, characterized by being converted to calcium silicate crystals by hydrothermal treatment at a temperature of 130 to 300 ° C .;
(2) The method for insolubilizing contaminated soil such as heavy metals as described in (1) above, wherein the contaminated soil before mixing with the insolubilizing agent is slurried in advance.
(3) The method for insolubilizing contaminated soil such as heavy metals described in (1) or (2) above, wherein an alkaline agent is added to the contaminated soil before mixing the insolubilizing agent to adjust the pH value to alkaline.

As described above, the present invention treats contaminated soil with heavy metals and the like, converts these harmful heavy metals and the like into insoluble sulfides with an insolubilizing agent consisting of sodium sulfide or sodium hydrosulfide, and then converts the calcium compound to Administration, further hydrothermal treatment to convert these to calcium silicate crystals such as tobermorite, thereby sealing the insoluble sulfide, so that heavy metals can be reliably suppressed below the environmental standard value, and treatment, The equipment is very economical.

  FIG. 1 shows a flow of basic means for carrying out the present invention. The target contaminated soil to be treated is soil containing mercury, arsenic, lead, selenium, chromium, cadmium, boron, fluorine, etc., especially soil that contains mercury and arsenic or cadmium. In the present invention, insolubilization treatment of heavy metals and the like can be similarly applied to incineration ash and the like containing these heavy metals and the like.

First, an insolubilizing agent such as heavy metal is mixed in the contaminated soil, and the heavy metal or the like is changed to a poorly soluble compound with low solubility. As the insolubilizing agent, sodium sulfide or sodium hydrosulfide is preferably used, but hydrogen sulfide can also be used. Sulfides such as heavy metals generated in this step are, for example, cadmium sulfide: 5.0 ^* 10 ^ (− 27), lead sulfide: 3.0 ^* 10 ^ (− 28), or mercury sulfide: 1 ^* 10 ^ (− As shown in 53), it is hardly soluble in water, and basically an insoluble treatment is performed here.

  The amount of the insolubilizing agent added may be 0.01 to 1 time equivalent, preferably 0.1 to 1 time equivalent to the chemical equivalent of heavy metal in the contaminated soil. Most of the heavy metals contained in contaminated soil and incinerated ash have a variety of chemical structures and forms that are unclear, and do not necessarily require a stoichiometric insolubilizer calculated from the content alone. In some cases. Therefore, the minimum amount of insolubilizing agent required to reduce the elution amount of heavy metals or the like below the environmental standard value may be used in an amount corresponding to the above lower limit value. On the other hand, if the addition amount of the insolubilizing agent exceeds the above upper limit and is too large, hydrogen sulfide may be generated during hydrothermal treatment in the subsequent step. In addition, what is necessary is just to implement a simple small-scale experiment beforehand in order to determine the addition amount of an insolubilizing agent in the said range.

Next, in the present invention, the calcium compound or both of the calcium compound and the silica compound are mixed into the contaminated soil after the insolubilization treatment, and further hydrothermal treatment is performed. By this reaction, calcium silicate crystals such as tobermorite are produced, and these encapsulate sulfides such as poorly soluble heavy metals formed in the previous step, thereby ensuring an elution suppressing effect. As the calcium compound, quick lime, slaked lime, cement, gypsum, or the like is used, and as the silica compound, quartz, stone powder, glass powder, water glass, or the like is used, and cement, coal ash, incinerated ash, or the like is common as both compound sources. And when the silica content in the contaminated soil to be treated is insufficient, stone powder or coal ash may be used.

The compounding amount of the calcium compound or both of the calcium compound and the silica compound is 3 to 30% by weight in terms of the amount of calcium oxide in the mixed soil, thereby suppressing the bulk of the treated soil finally obtained. Can do.

Hydrothermal treatment of contaminated soil containing a calcium compound or both a calcium compound and a silica compound is carried out in an autoclave at a temperature of 130 to 300 ° C., preferably 150 to 200 ° C., and the reaction time (curing time) is 1 to 1. 24 hours, preferably 2 to 8 hours is sufficient, and the processing pressure is preferably saturated water vapor pressure. By performing hydrothermal treatment under these conditions, a particularly large amount of tobermorite can be produced.

  FIG. 2 illustrates the method of the present invention in which the contaminated soil is slurried by hydration prior to sulfidation. In this method, the moisture content of various contaminated soils is indeterminate, so the slurry can be mixed in advance to reduce the viscosity of the contaminated soil. There are benefits that can be carried out. Therefore, the amount of water may be determined appropriately experimentally and does not need to be specified.

FIG. 3 shows the method of the present invention in which an alkaline agent such as caustic soda is added after water addition to adjust the pH of the contaminated soil slurry to alkaline. This method is intended to efficiently advance the sulfiding action during the sulfidation process in the next step under alkalinity and improve the sulfiding effect. Under acidic conditions, sulfide ions (S ²⁻ ) become hydrogen sulfide. Liberates and impairs the sulfurization effect. The addition amount of the alkaline agent may be appropriately determined experimentally and does not need to be specified.
(Example) The raw soil equivalent to the contaminated soil commonly used in the present invention and the comparative example has arsenic and mercury elution amounts exceeding the environmental standards. That is, the amount of elution of heavy metals and the like is the following value based on the result of analysis in accordance with the attached table of Environmental Agency Notification No. 46 “Environmental Standards Concerning Soil Contamination”.

As 0.045mg / L
Hg 0.76mg / L
The mercury content of the raw soil was 3900 mg / kg on a dry soil basis.

Table 1 shows the implementation conditions and results of the four examples of the present invention. Both according to the process of FIG. 2, first add water to the raw soil material from by adjusting the water content of 30%, were mixed with sodium sulfide insolubilizing agent, mixing quicklime 25 weight percent calcium of compound Then, hydrothermal treatment was performed at 200 ° C. for 5 hours under saturated steam pressure in an autoclave. Sodium sulfide was administered in four stages within a range of 0.001 to 0.05% with respect to the dry weight of the raw soil.

  Table 2 shows the implementation conditions and results of four comparative examples. Comparative Example 1 is an example in which cement is mixed and cured at room temperature, 2 is an example in which sulfurizing agent and cement are mixed and cured at room temperature, 3 is an example in which quick lime is mixed and hydrothermally treated, and 4 is an example in which a chelating agent and quicklime are mixed and hydrothermally treated.

  Comparing the two tables, the arsenic elution amount is 0.001 mg / L or less, and no substantial difference can be seen, but the mercury elution amount is significantly reduced in the present invention compared to the comparative example. It turns out that an effect is very remarkable. Needless to say, this is due to a synergistic effect of insolubilization of heavy metals in contaminated soil by sulfidation and their sealing with tobermorite crystals. That is, in the method of the comparative example, the elution amount of mercury cannot be suppressed below the environmental standard, but in the present invention, it can be surely reduced below that. However, the result of Example 4 is slightly inferior to the results of 1 to 3 in that the effect of insolubilizing arsenic and mercury is slightly inferior. In addition, since the mercury content of the raw soil was 3900 mg / kg (0.39%) on a dry soil basis, the required amount of sodium sulfide having a stoichiometric value equal to the mercury content was equal to the dry soil weight. However, although all of Examples 1 to 3 are considerably smaller than the equivalent amount of the same stoichiometric value, mercury elution can be suppressed. This clearly shows that the sealing effect by the tobermorite crystal contributes.

2 shows a basic flowchart of the method of the present invention. Fig. 3 shows an evolutionary flowchart of the method according to the invention. 2 shows a further development flowchart of the method of the invention.

Claims (3)

Sodium sulfide or sodium hydrosulfide as an insolubilizing agent was mixed in the soil contaminated with heavy metals, etc., in an amount 0.01 to 1 times the chemical equivalent of heavy metals contained in the contaminated soil, and the heavy metals were converted to sulfides. After that, the calcium compound or both the calcium compound and the silica compound are added to the mixed soil after conversion to the sulfide in an amount of 3 to 30% by weight in terms of calcium oxide, and then the mixture is added in an autoclave at 130 to 300%. A method for insolubilizing soils contaminated with heavy metals, which is converted to calcium silicate crystals by hydrothermal treatment at a temperature of ℃.   The method for insolubilizing contaminated soil such as heavy metals according to claim 1, wherein the contaminated soil before mixing with the insolubilizing agent is slurried in advance.   3. The method for insolubilizing contaminated soil such as heavy metals according to claim 1 or 2, wherein an alkaline agent is added to the contaminated soil before mixing the insolubilizer to adjust the pH value to alkaline.

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