JP4516780B2 - Heavy metal fixing material, cement-based solidifying material, manufacturing method of heavy metal fixing material, manufacturing method of ground improvement material, and processing method of soil to be processed - Google Patents

Description

The present invention is a heavy metal immobilizing material having excellent insolubilizing effects of heavy metals contained in the cement and contaminated soil, etc., a method of manufacturing the same, a method for manufacturing soil improvement material obtained by utilizing the heavy metal immobilizing material, processing method processing target soil And a cement-based solidifying material .

  Dealing with soil contaminated with heavy metals is one of the major problems of modern society, and as one of the permanent countermeasures, containment of contaminated soil with cement or solidifying material is generally applied. However, this method is easily affected by soil properties (particle size, water content, organic matter content), and as a result, immobilization of heavy metals is limited. Further, depending on the soil quality, there has been a problem that hexavalent chromium derived from cement is eluted exceeding the soil environmental standards.

  As a countermeasure against elution of hexavalent chromium from the solidified soil with a cement-based solidifying material, addition of various reducing agents is known (for example, Patent Document 1). Examples of the reducing agent include ferrous sulfate, sulfite, thiosulfate, sulfides such as sulfur, calcium sulfide, and iron sulfide, slag containing these sulfides, hydrides such as hydrogen sulfide, Na, K, Mg Metal powders such as Fe, Zn are used. These reducing agents are useful as a countermeasure for elution of hexavalent chromium, but it is generally known that the strength of the solidified soil is lowered when the amount added is large.

  Among these, when ferrous sulfate, for example, is added as a reducing agent, although there is a certain immediate effect, hexavalent chromium is gradually eluted during the process of hydration of the cement. As described above, ferrous sulfate as a reducing agent has a problem in terms of sustainability of the reducing action over the medium and long term. In addition, for example, volcanic ash cohesive soil that is relatively easy to elute hexavalent chromium in cement is treated as soil, and sulfide is included as a reducing agent in order to suppress elution of hexavalent chromium from this treated soil. When slag is added to a mixture of volcanic ash clay and cement, 30% or more of slag must be added. When a large amount of sulfide-containing slag is added in this way, the strength of the hardened solidified soil is significantly reduced. Therefore, it is not suitable for ground improvement materials.

  In addition, sulfite, thiosulfate, sulfur, iron sulfide, Mg, Fe, and Zn have insufficient effects of immobilizing heavy metals, and hydrogen sulfide, Na, and K can be handled from the standpoint of toxicity and stability. There was a problem that it was difficult.

  On the other hand, calcium sulfide is less likely to cause the above-described problems, has a particularly large effect of insolubilizing heavy metals, and exhibits sufficient performance for suppressing heavy metal elution when added in a small amount. Therefore, it can be said that calcium sulfide is useful for immobilizing heavy metals without reducing the strength of the solidified soil.

As such a method for producing calcium sulfide, a method for producing calcium sulfide by adding coal or carbon monoxide to phosphate gypsum and reducing it (Non-Patent Document 1) is known. In this method, a reducing agent is added to phosphate gypsum so as to be equal to or more than the number of equivalents necessary for reduction of gypsum (number of reducing equivalents of reducing agent / number of oxidation equivalents of gypsum ≧ 1), and heat treatment is performed, and the purity is 30 mass % Or more of calcium sulfide is obtained.
JP 2003-146728 A U.S. Department of The Interior, Report of incubation No. 9323 14p "Recovery of sulfur from phosphophyssum: Conversion of calcium sulfate to calcium sulfide"

  However, the calcium sulfide obtained by the method of Non-Patent Document 1 described above needs to be pulverized to an average particle size of 2 to 20 μm to increase the specific surface area of the calcium sulfide particles in order to exhibit sufficient heavy metal immobilization ability. There is. For this reason, it takes time to grind calcium sulfide, which not only increases the energy cost, but also causes problems such as poor handling due to fine powder.

  Furthermore, calcium sulfide generates hydrogen sulfide by the action of moisture and carbon dioxide in the air during storage and transportation, and further mixing with cement, solidified material, etc., and its odor becomes a problem. In particular, when the relative humidity was about 80% due to rain or the like, hydrogen sulfide was generated so much that it was very difficult to handle.

  Accordingly, the present invention provides a heavy metal immobilization material that is excellent in immobilization of heavy metals and strength development of solidified soil, and has reduced generation of hydrogen sulfide even under high humidity, its production method, and its heavy metal immobilization material An object of the present invention is to provide a method for producing a ground improvement material and a method for treating a soil to be treated using slag.

As a result of intensive studies, the present inventors have used the above-described problems when heavy metal immobilization materials containing not only calcium sulfide but also calcium sulfate are contained in a specific ratio. The present inventors have found that this can be solved and have completed the present invention. That is, in order to solve the above-mentioned problem, the heavy metal immobilization material according to the present invention includes gypsum and a reducing agent such that the ratio of the number of reducing equivalents of the reducing agent to the number of equivalents of gypsum is 0.3 to 0.9. Is obtained by heat-treating a raw material containing 4 to 19% by mass of calcium sulfide and 57 to 95% by mass of calcium sulfate.

  According to this heavy metal immobilization material, it is excellent in immobilization of heavy metals, sustainability of medium- and long-term reduction action, and strength development of solidified soil, and generation of hydrogen sulfide is reduced even under high humidity. . In addition, when the content of calcium sulfide is less than 4% by mass, the effect of immobilizing heavy metal becomes insufficient when a heavy metal immobilizing material is added to cement or the like and the effect of immobilizing heavy metal is reduced. When the amount of calcium is more than 19% by mass, hydrogen sulfide is generated in a high humidity environment, which is not preferable. Moreover, when calcium sulfate is less than 57 mass%, when it is added to cement or the like to obtain a solidified material, the amount of ettringite generated after the solidification treatment becomes insufficient, and the strength of the solidified soil is lowered, which is not preferable. In addition, when the amount of calcium sulfate is less than 57% by mass, the amount of cement clinker in the solidified material decreases even if the amount of heavy metal immobilizing material is increased and added to cement, etc., so that sufficient strength development is achieved. Cannot be obtained. When the amount of calcium sulfate is more than 95% by mass, the calcium sulfide content is relatively reduced, and sufficient heavy metal immobilization ability cannot be obtained, which is not preferable. In addition, Cd, Cr, Hg, Se, Pb, As etc. are mentioned as a heavy metal fixed by this heavy metal fixing material.

  Here, the heavy metal immobilization material is hermetically accommodated in the container so that the ratio of container volume (mL) / heavy metal immobilization material weight (g) = 10, and is maintained at 20 ° C. at an initial relative humidity of 80%. It is preferable that the hydrogen sulfide concentration in the container after standing for 24 hours is 2.0 ppm or less.

  With such a heavy metal fixing material, even if the relative humidity becomes as high as about 80% due to rain or the like, the odor does not become a problem, and therefore, handling becomes easy.

  Further, the method for producing a heavy metal immobilization material according to the present invention includes a raw material containing gypsum and a reducing agent so that the ratio of the number of reducing equivalents of the reducing agent to the number of oxidizing equivalents of gypsum is 0.3 to 0.9. A heavy metal immobilization material is produced by heat treatment in a reducing atmosphere.

  According to this method for producing a heavy metal immobilization material, gypsum is reduced, and a heavy metal immobilization material containing 4 to 19% by mass of calcium sulfide and 57 to 95% by mass of calcium sulfate can be produced.

  In this method for producing a heavy metal immobilizing material, it is preferable to heat-treat the raw material at 600 ° C. to 1100 ° C. When the heat treatment temperature is lower than 600 ° C, the reduction reaction of gypsum tends not to proceed sufficiently as compared with the case of 600 ° C or higher. In addition, when the heat treatment temperature is higher than 1100 ° C., a part of gypsum is decomposed and sulfur oxides are generated as compared with the case of 1100 ° C. or lower, so that it is necessary to take environmental measures for exhaust gas.

  Moreover, it is preferable that a reducing agent contains a carbon containing material. Here, the carbon-containing material refers to a substance containing carbon, and includes an organic compound containing a carbon atom as a part of the structure in addition to simple carbon. By using a carbon-containing material as a reducing agent, reduction of gypsum can be promoted.

  Moreover, the manufacturing method of the ground improvement material which concerns on this invention hardens the mixture of the cement-type solidification material containing the above-mentioned heavy metal fixing material and cement, and a process target soil.

  According to this manufacturing method, a ground improvement material having a sufficient volume can be obtained by curing a bulky mixture of soil to be treated and cement-based solidifying material. In addition, since the heavy metal fixing material has an excellent heavy metal fixing function, the heavy metal contained in the cement is sufficiently fixed by the heavy metal fixing material. Therefore, a ground improvement material can be obtained. Here, as the soil to be treated, for example, volcanic ash cohesive soil generated with construction work can be effectively used.

  In this ground improvement material manufacturing method, the cement-based solidified material preferably contains 0.5% by mass or more of calcium sulfide and 5.5% by mass or more of sulfur trioxide. Thereby, for example, even when volcanic ash clay is used as the soil to be treated, heavy metals can be more sufficiently fixed, and sufficient strength can be obtained even in a hardened ground improvement material.

  Moreover, the processing method of the process target soil which concerns on this invention is characterized by mixing and hardening the cement-type solidification material containing the above-mentioned heavy metal fixing material and cement to the process target soil containing a heavy metal.

  According to this treatment method, the soil to be treated is solidified by cement, and the elution of heavy metals in the soil to be treated is sufficiently suppressed by the excellent heavy metal immobilization function of the heavy metal immobilizing material described above.

  In this method of treating soil to be treated, the cement-based solidified material preferably contains 0.5% by mass or more of calcium sulfide. Thereby, the heavy metal contained in the soil to be treated can be more sufficiently fixed.

  The heavy metal immobilization material according to the present invention is excellent in immobilization of heavy metals, sustainability of medium- and long-term reduction action, and strength development of solidified soil, and reduces generation of hydrogen sulfide even under high humidity. Is done.

  Moreover, according to the manufacturing method of the heavy metal fixing material which concerns on this invention, the above heavy metal fixing materials can be manufactured.

  Moreover, according to the manufacturing method of the ground improvement material which concerns on this invention, the ground improvement material by which elution of heavy metal was fully suppressed can be manufactured by utilizing the above heavy metal fixed materials.

  Furthermore, according to the processing method of the process target soil which concerns on this invention, elution of the heavy metal contained in a process target soil can fully be suppressed by utilizing the above heavy metal fixed materials.

  Hereinafter, preferred embodiments of the heavy metal immobilization material, the heavy metal immobilization material production method, the ground improvement material production method, and the treatment target soil treatment method according to the present invention will be described.

<Heavy metal immobilization material>
First, an embodiment of the heavy metal immobilization material according to the present invention will be described. The heavy metal immobilizing material contains 4 to 19% by mass of calcium sulfide and 57 to 95% by mass of calcium sulfate. Here, calcium sulfide has an effect of suppressing elution of heavy metals contained in contaminated soil, cement, or cement-based solidified material. Therefore, according to this heavy metal fixing material, when added to cement clinker, cement, solidifying material, etc., and soil is solidified, Cd, Cr, Hg, Se contained in the soil and cement are obtained by the action of calcium sulfide. Heavy metals such as Pb and As are sulfided and insolubilized. Therefore, elution of heavy metals from the solidified soil can be prevented.

  In addition, when calcium sulfide is less than 4% by mass, a sufficient heavy metal fixing function cannot be obtained when it is added to cement or the like as compared with the case of 4% by mass or more to obtain a solidified material. In addition, when the amount of calcium sulfide is less than 4% by mass, increasing the amount of heavy metal immobilizing material added to cement or the like will immobilize heavy metal, but increase the amount of heavy metal immobilizing material added. Therefore, since the ratio of cement clinker in the cement-based solidified material is reduced, sufficient strength development cannot be obtained. When the amount of calcium sulfide in the heavy metal immobilization material is more than 19% by mass, the container volume (ml) / sample (heavy metal immobilization material) weight (g) ratio is 10 at 20 ° C. initial relative humidity 80%. If the heavy metal immobilizing material is accommodated in the container and sealed, and allowed to stand for 24 hours, the hydrogen sulfide concentration in the container becomes higher than 2.0 ppm, which is not preferable.

  In addition, calcium sulfate produces ettringite and develops sufficient strength in the solidified soil. When calcium sulfate is less than 57% by mass, when it is added to cement or the like to form a solidified material, the amount of ettringite generated after the solidification treatment becomes insufficient and the strength of the solidified soil is lowered, which is not preferable. In addition, when the amount of calcium sulfate is less than 57% by mass, the amount of cement clinker in the solidified material decreases even if the amount of heavy metal immobilizing material is increased and added to cement, etc., so that sufficient strength development is achieved. Cannot be obtained. When the amount of calcium sulfate is more than 95% by mass, the calcium sulfide content is relatively reduced, and sufficient heavy metal immobilization ability cannot be obtained, which is not preferable.

  FIG. 1 is a schematic view showing a cross section of heavy metal immobilization material particles. As shown in FIG. 1, the heavy metal immobilization material is mainly composed of calcium sulfide and calcium sulfate, and calcium sulfide adheres to the surface of the massive calcium sulfate. For this reason, the specific surface area of calcium sulfide in this heavy metal immobilization material is larger than the specific surface area of calcium sulfide in a conventional heavy metal immobilization material in which calcium sulfide and calcium sulfate are homogeneously mixed. Therefore, the conventional heavy metal immobilization material does not exhibit a sufficient heavy metal immobilization function unless it is pulverized to an average particle size of 2 to 20 μm, whereas the heavy metal immobilization has calcium sulfide attached to the surface of calcium sulfate. The material can exhibit an excellent heavy metal immobilization function to an extent that the average particle size is pulverized to 300 μm or less, more preferably to an average particle size of 100 μm or less. The average particle size of the heavy metal immobilizing material can be measured with a laser diffraction particle size meter.

  Incidentally, general-purpose cement-based solidifying materials are usually composed of cement clinker and gypsum such as anhydrous gypsum, hemihydrate gypsum, and dihydrate gypsum. If the blending amount of gypsum is small, the amount of ettringite produced after the solidification treatment is insufficient, and sufficient strength development cannot be obtained with viscous soil or organic clay soil. In addition, when the amount of gypsum is too large, not only the amount of cement clinker is relatively reduced, but also the amount of ettringite produced is too large, leading to excessive expansion, resulting in sufficient strength development. I can't. For this reason, as a general-purpose cement-type solidification material, what mix | blended 6-9 mass% of gypsum normally in conversion of sulfur trioxide is used.

<Method for producing heavy metal immobilization material>
Next, an embodiment of a method for producing a heavy metal immobilization material according to the present invention will be described.

  A method for producing a heavy metal immobilization material is obtained by reducing a raw material in which gypsum and a reducing agent are mixed so that the ratio of the reducing equivalent number of the reducing agent to the oxidation equivalent number of gypsum is 0.3 to 0.9 in a reducing atmosphere. In this case, the heat treatment is performed. According to this manufacturing method, gypsum is reduced, and it becomes possible to produce a heavy metal immobilization material containing 4 to 19% by mass of calcium sulfide and 57 to 95% by mass of calcium sulfate.

  The amount of the reducing agent added to the gypsum is 0.3 to 0.9 times the number of equivalents required for the reduction of the gypsum, that is, the equivalent of the gypsum oxidation equivalent. The ratio of the number of reducing equivalents of the reducing agent to the number is 0.3 to 0.9. When using wood or waste plastic as a reducing agent, calculate the reduction equivalents taking into account the reduction by dry distillation gas such as hydrocarbon, hydrogen, carbon monoxide, etc. in addition to carbon produced by carbonization, and the number of oxidation equivalents of gypsum The ratio of the number of reducing equivalents of the reducing agent to the amount is 0.3 to 0.9. Also, if a part of the generated dry distillation gas does not contribute to the reduction of gypsum, such as when using an internal heat rotary kiln or a carbonization furnace that burns dry distillation gas as a heat source, it contributes to the reduction reaction. It is preferable to add a reducing agent so that the ratio of the number of reducing equivalents of the reducing agent to the number of oxidizing equivalents of gypsum is 0.3 to 0.9 after subtracting the dry distillation gas.

  In addition, when the ratio of the reduction equivalent number of the reducing agent to the oxidation equivalent number of gypsum is less than 0.3, the calcium sulfide content in the heavy metal immobilization material after the heat treatment tends to be less than 4% by mass. When the ratio of the number of reducing equivalents of the reducing agent to the number of oxidizing equivalents of gypsum exceeds 0.9, the calcium sulfide content tends to exceed 19% by mass or more, which is not preferable.

  The raw material containing gypsum and a reducing agent is preferably heat-treated at 600 to 1100 ° C, more preferably 700 to 1000 ° C after pulverization. When the heat treatment temperature is lower than 600 ° C., the reduction reaction of gypsum tends not to proceed sufficiently as compared with the case of 600 ° C. or higher. When the heat treatment temperature is higher than 1100 ° C., a part of gypsum is decomposed and sulfur oxides are generated, so that it is necessary to take an environmental measure for exhaust gas.

  As the heating furnace for heat-treating the raw material, an internal combustion burner type rotary kiln, an external heating type rotary kiln, a double-cylinder rotary kiln type carbonization furnace, a batch type carbonization furnace, or the like can be adopted, and it can be heated to a predetermined temperature. If it does not specifically limit.

  Here, the raw material is not particularly limited as long as it is mainly composed of gypsum, and is commercially available anhydrous gypsum, hemihydrate gypsum, dihydrate gypsum and gypsum board, cast molding gypsum mold, industrial model gypsum mold, etc. However, it is more preferable to use gypsum waste from the viewpoints of economy and environmental protection. When gypsum board waste is used, for example, about 7% by mass of paper is adhered to the gypsum board. However, since the gypsum waste is heat-treated in a reducing atmosphere, it is not necessary to separate and remove the adhered paper. In this case, the adhered paper effectively functions as a part of the reducing agent added to the gypsum. Moreover, it is preferable to remove the metals contained in the gypsum waste material using a magnetic separator, a sieve or the like after the crushing treatment. The gypsum used as a raw material is used after being pulverized, but when mixed uniformly with a reducing agent, it is preferably pulverized to a particle size of 10 mm or less.

  The reducing agent for gypsum preferably contains a carbon-containing material. The carbon-containing material includes an organic compound containing carbon as a part of the structure in addition to a material containing carbon. Carbon-containing materials used as reducing agents include carbon, coal, coke, charcoal, wood, etc., coal ash containing unburned carbon discharged from coal-fired power plants, and gasified slag discharged from coal gasifiers. And wastes such as pulp sludge, waste plastic, waste wood, and felled trees discharged from paper mills.

  According to the heavy metal immobilizing material produced as described above, the container is hermetically accommodated in the container so that the ratio of container volume (mL) / heavy metal immobilizing material weight (g) = 10, and the initial relative humidity is 80%. When the temperature is maintained at 20 ° C., the hydrogen sulfide concentration in the container after standing for 24 hours can be reduced to 2.0 ppm or less. With such a heavy metal immobilizing material, even if the relative humidity is increased to about 80% due to rain or the like, the odor due to the generation of hydrogen sulfide does not become a problem, so that handling becomes easy.

  Further, according to the above method for producing a heavy metal immobilization material, since gypsum is reduced to calcium sulfide at the contact surface between the gypsum particles as a raw material and the reducing agent, calcium sulfide is partially formed on the surface of the heavy metal immobilization material. It is formed so that it adheres, and the remaining part is composed of calcium sulfate. For this reason, the specific surface area of calcium sulfide becomes larger than the case where calcium sulfide and calcium sulfate are mixed homogeneously. Therefore, an excellent heavy metal immobilization function is exhibited by pulverizing the heavy metal immobilization material to a particle size that passes through a sieve having a 300 μm diameter mesh, more preferably a particle size that passes through a screen having a 100 μm diameter mesh. .

  Further, this heavy metal fixing material is suitable as a raw material for cement-based solidifying material, and is pulverized using a known device such as a ball mill, vertical roller mill, vibration mill, pin mill, cement clinker, cement, solidifying material, etc. It can be used by mixing with or mixed with cement clinker or the like and pulverized with cement clinker or the like.

Here, sulfur dioxide in the exhaust gas discharged during the heat treatment of the raw material can be desulfurized by a known method, and the desulfurization method is not particularly limited. As the desulfurization method, for example, a limestone-gypsum method desulfurization apparatus can be used. In this desulfurization apparatus, an absorption tower is filled with a limestone chiller made of limestone powder and water, and exhaust gas is bubbled through the absorption tower to change sulfur dioxide SO ₂ in the exhaust gas into gypsum CaSO ₄ or calcium sulfite CaSO _3. It is a device that absorbs. In addition, you may use magnesium hydroxide etc. instead of limestone as an absorber. Moreover, you may comprise so that slaked lime may be blown in the flue which discharges exhaust gas, it reacts with the sulfur dioxide in exhaust gas, and it collects and collect | recovers by gypsum with a bag filter etc. With such a desulfurization apparatus, the sulfur dioxide concentration can be reduced to the emission standard of sulfur oxides.

<Manufacturing method of ground improvement material>
Next, an embodiment of a method for producing a ground improvement material according to the present invention will be described.

  The manufacturing method of the ground improvement material in this embodiment mixes and hardens the above-mentioned heavy metal fixing material and cement with respect to soil to be treated such as volcanic ash clay. The cement contains a small amount of heavy metal, but the heavy metal contained in the cement is fixed by the heavy metal fixing material having an excellent heavy metal fixing function. Moreover, a ground improvement material having a sufficient volume can be obtained by using not only sandy soil and viscous soil but also volcanic ash clay.

  At this time, it is preferable that the heavy metal immobilization material contains 0.5% by mass or more of calcium sulfide and the amount of sulfur trioxide is 5.5% by mass or more. In this case, the heavy metal is more sufficiently immobilized. The hardened ground improvement material can also obtain sufficient strength.

<Treatment method for soil to be treated>
Next, an embodiment of the processing method for the processing target soil will be described.

  In the treatment method of the treatment target soil in the present embodiment, the above-described heavy metal fixing material and cement are mixed and hardened in the treatment target soil containing heavy metal such as contaminated soil. Since the above-mentioned heavy metal immobilization material is excellent in the heavy metal immobilization function, the heavy metal contained in the contaminated soil to be treated is immobilized, and elution of heavy metal can be sufficiently suppressed. Here, the heavy metal fixing material preferably contains 0.5% by mass or more of calcium sulfide. In this case, the heavy metal can be more sufficiently fixed.

  Hereinafter, the content of the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples.

<Production of ground improvement material>
[Production of raw materials for fixing heavy metals]
A gypsum board (thickness 9.5 mm, dihydrate gypsum amount 93% by weight, paper 7% by weight) was crushed with a jaw crusher to separate the paper, and then a sieve having a 3 mm diameter stitch using an atomizer Was pulverized to a full particle size. In addition, a slag discharged from a coal gasification furnace (hereinafter abbreviated as “coal gasification slag”; carbon 70.8 mass%) is measured with a laser diffraction particle size meter, and a ball mill is used so that the average particle size becomes 120 μm. Used to grind. In addition, as shown in Table 1, the waste gypsum board pulverized product and the coal gasified slag pulverized product were mixed at a mass ratio of 97: 3 to 74:26. Raw materials of A, B, C, D-1 to D-6, E, and F were prepared. For each raw material, the mass ratio and weight of gypsum and coal gasification slag are shown in the “raw material blend” column of Table 1.

In addition, each raw material No. In A to F, the number of reduction equivalents of coal gasification slag (a), the number of oxidation equivalents of gypsum in the raw material (b), and the ratio of reduction equivalents of coal gasification slag to the number of oxidation equivalents of gypsum (a / b) ) Was calculated from the following equations (a), (b), and (c). These values are shown in Table 1.
Reduction equivalent number of coal gasification slag in raw material (a)
= Coal gasification slag raw material blending weight x 0.708 / carbon 1 reduction equivalent weight ...
Number of oxidation equivalents of gypsum in raw material (b) = raw material blending weight of gypsum ÷ weight of gypsum 1 oxidation equivalent (b)
Ratio of reduction equivalent number of coal gasification slag to oxidation equivalent number of gypsum = a / b (C)

[Heat treatment of raw materials]
Subsequently, each said raw material No. About A to F, a raw material 40 g is put into a ship crucible, and nitrogen gas is fed at a gas flow rate of 0.5 L / min in a tubular electric furnace (inner diameter 60 mm × height 1000 mm) through which gas can flow. Heat treatment was carried out at 4 ° C. for 4 hours. Thus, a heavy metal fixing material was obtained. Each raw material No. The heat treatment conditions A to F were as shown in the column of “heat treatment temperature (° C.)” in Table 1.

[Target soil for immobilization]
Volcanic ash clay (natural water content ratio: 103.1%, non-contaminated soil, Chiba Prefecture (Kanto Loam)) was used as the target soil for immobilization treatment for producing ground improvement materials.

[Production of cement-based solidified material]
Each raw material No. Each heat-treated product obtained from A to F was measured with a laser diffraction particle size meter LA-500 manufactured by Horiba, Ltd. and pulverized using a ball mill until the average particle size became 90 μm. Subsequently, the pulverized product was added to ordinary Portland cement (sulfur trioxide content: 2.0% by mass) so that the pulverized product was 10% by mass to prepare a cement-based solidified material. The column of “solidification material ratio” in Table 2 shows the mixing ratio of normal Portland cement and heavy metal fixing material when producing a cement-based solidification material (Nos. 1 to 12). In addition, as a comparative example, what made the solidification material only with the general soft soil cement type solidification material was also produced (No. 13).

[Hardening of soil to be fixed]
The treatment of volcanic ash cohesive soil (non-contaminated soil) with the cement-based solidifying material was performed as follows. That is, the cement-based solidified material is added to volcanic ash clay, stirred and mixed, filled into a mold (diameter 5 cm x height 10 cm), compacted, and sealed and cured in a temperature-controlled room at 20 ° C and relative humidity 80%. Cured. After 7 days, a ground improvement material (specimen) was obtained which was removed from the mold and hardened. As a comparative example, a general soft soil cement-based solidified material (sulfur trioxide content 6.5 mass%) was used, and the same treatment as above was performed to obtain a specimen (No. 13).

Here, the addition amount (kg / m ³ ) of the solidifying material to 1 m ³ of the volcanic ash cohesive soil that is the treatment target soil was 200 kg. Here, “solidifying material” in “addition amount of solidifying material” means “solidifying material” mixed according to the mixing ratio of normal Portland cement and heavy metal fixing material shown in the column of “solidifying material ratio” in Table 2. To do.

Here, the amount of calcium sulfide and the amount of sulfur trioxide in the solidified material were calculated by the formulas (d) and (e). The respective calculation results are shown in the columns of “Calcium sulfide amount in solidified material (mass%)” and “Sulfur trioxide amount in solidified material (mass%)” in Table 2. Here, “solidifying material” in these columns means the mixing ratio of ordinary portland cement, general soft soil cement-based solidifying material, and heavy metal fixing material shown in the “solidifying material ratio” column of Table 2. It means mixed solidified material.
Calcium sulfide content in solidified material =
The amount of calcium sulfide in the heavy metal immobilization material x the solidification material mixing ratio of the heavy metal immobilization material ... (d)
Amount of sulfur trioxide in solidified material =
(Sulfur trioxide amount in cement x cement solidification material blending ratio + sulfur trioxide amount in heavy metal immobilization material x solidification material blending ratio in heavy metal immobilization material) (e)

[Evaluation of immobilization of heavy metals]
The amount of heavy metal elution was measured for each board improvement material (specimen) after demolding. The results are shown in the column “Elution amount of heavy metal” in Table 2. In the case where the heavy metal concentration is below the detection limit, “<(detection limit value)” is described.

  According to Ministry of the Environment Notification No. 46 of the Ministry of the Environment of Japan, the soil environment standard value of hexavalent chromium is 0.05 mg / L or less. As shown in Table 1, heavy metal fixing material No. B, C, and D-2 to D-6 are obtained by heat-treating at 600 to 1200 ° C. in which the ratio of the reducing equivalent number of the reducing agent to the oxidizing equivalent number of gypsum is 0.3 to 0.9. It is what was done. These heavy metal fixing materials No. Hexavalent chromium elution amount when cement-based solidified material prepared by adding B, C, D-2 to D-6 to ordinary Portland cement is added to volcanic ash clay is less than the soil environmental standard value. It was low (Examples 1-7). However, heavy metal immobilization material obtained by heat treatment at 500 ° C., In the heat-treated product of D-1 (Reference Table 1), hexavalent chromium was eluted exceeding the soil environment standard value (No. 4: Comparative Example 2). In addition, heavy metal fixing material No. In Examples using B, C, and D-2 to D-6, all contained 0.5% by mass or more of calcium sulfide in the solidified material.

[Evaluation of uniaxial compressive strength]
Each specimen was measured for uniaxial compressive strength in accordance with JIS A 1216 “Soil uniaxial compression test method” and in accordance with Environment Agency Notification No. 46, and the amount of heavy metal elution was measured. The evaluation results are shown in the column of “uniaxial compressive strength (kN / m ² )” in Table 2.

According to this result, heavy metal fixing material No. No. using E and F. The uniaxial compressive strength in the specimens 10 to 12 (Comparative Examples 3, 4, and 5) was 850 kN / m ² or less, which was smaller than the uniaxial compressive strength of 900 kN / m ² or more in Examples 1 to 7. Here, when attention is paid to sulfur trioxide in the solidified material, no. It turned out that the sulfur trioxide in 10-12 solidified materials is 5.5 mass% or less. On the other hand, no. 1 (Comparative Example 1) and No. 1 4 (Comparative Example 2) had a sufficiently high uniaxial compressive strength of 940 kN / m ² , but the amount of calcium sulfide in the solidified material was less than 0.5% by mass, and the elution amount of hexavalent chromium was Soil environmental standards were exceeded.

  Therefore, if the solidified material contains 0.5% by mass or more of calcium sulfide and 5.5% by mass or more of sulfur trioxide, the Cr elution amount is less than the soil environment standard, and the sufficient insolubilizing effect is obtained. It was confirmed that the strength development was also good.

[Evaluation of hydrogen sulfide generation]
Moreover, hydrogen sulfide generation evaluation of the heavy metal fixing material after heat processing was also performed. That is, a plurality of polypropylene wide-mouth bottle containers having a diameter of 60 mm and a height of 110 mm are prepared in a constant temperature and humidity chamber at 20 ° C. and a relative humidity of 80%, and each raw material A to F is heat-treated for each bottle container. 30 g each of the heat-treated product obtained by the above was stored and sealed so that the inside of the bottle container was airtight. And the temperature in a thermo-hygrostat was maintained at 20 degreeC, and left still for 24 hours, Then, the bottle container was taken out and the hydrogen sulfide density | concentration in a bottle container was measured with the detection tube by Gastec Corporation. The ratio of the volume (mL) of the bottle container to the weight (g) of the heat-treated product (heavy metal immobilization material) is about 10. Each raw material No. The hydrogen sulfide concentration in the heat-treated products A to F is shown in the column of “hydrogen sulfide concentration (ppm)” in Table 3.

  As shown in Table 3, the ratio of the number of reducing equivalents of the reducing agent to the number of oxidizing equivalents of gypsum is 0.3 to 0.9 (0.3 ≦ (a / b) ≦ 0.9), and 600 to 1200. Heavy metal immobilization material obtained by heat treatment at ℃, The heat-treated product (heavy metal immobilization material) of B, C, D-2 to D-6 has a generation amount of hydrogen sulfide of 2.0 ppm or less after 24 hours under the conditions of 20 ° C. and a relative humidity of 80%. There were few.

Here, in order to investigate the relationship between the amount of hydrogen sulfide generated and the heavy metal immobilization material (heat-treated product), the composition of the heat-treated product was examined. That is, with respect to the heavy metal immobilization material obtained by heat-treating each of the raw materials A to F as described above, the amount of sulfur and sulfur trioxide in the form of sulfide is measured according to JIS R 5202 “Chemical analysis method for Portland cement”. did. Furthermore, the mass% of the calcium sulfide amount is calculated from the measured sulfur mass% of the sulfur form by the formula (f), and the mass% of the calcium sulfate amount is calculated from the measured mass% of the sulfur trioxide (g). Calculated from the formula. The results are shown in Table 3.
Calcium sulfide amount = sulfide form sulfur amount × (72.14 / 32.06) (f)
Calcium sulfate amount = sulfur trioxide x (136.14 / 80.06) (G)

  Then, no. It was found that the heat-treated products B, C, D-2 to D-6 had a calcium sulfide content of 4 to 19% by mass. Further, a heavy metal immobilization material in which the ratio of the number of reducing equivalents of the reducing agent to the number of oxidizing equivalents of gypsum exceeds 0.9, The heat-treated products of E and F (Reference Table 1) had a calcium sulfide content of 19% by mass or more, and the hydrogen sulfide generation amount after 20 hours at 20 ° C. and a relative humidity of 80% exceeded 2.0 ppm. Therefore, it was found that the calcium sulfide content in the heavy metal immobilization material is preferably 4 to 19% by mass.

[Sulfur dioxide generation during heat treatment]
Further, the exhaust gas during the heat treatment was collected in an aluminum tedlar bag, and the sulfur dioxide concentration in the exhaust gas was measured with a detector tube manufactured by Gastec Corporation. The results are shown in the column of “Sulfur dioxide concentration in exhaust gas (ppm)” in Table 3. Raw material No. In the heat treatment of D-6, a large amount of sulfur dioxide was generated as compared with other raw materials. In any heat treatment of any raw material, sulfur dioxide was removed by a desulfurization apparatus. In addition, if the heat processing temperature of a raw material shall be 600 to 1100 degreeC, raw material No.1. Sulfur dioxide as much as the amount of sulfur dioxide generated during the heat treatment of D-6 is not generated, and the load on the desulfurization apparatus is reduced or unnecessary. Therefore, the heat treatment temperature of the raw material is more preferably 600 ° C. to 1100 ° C. than 600 ° C. to 1200 ° C.

  By the way, heavy metal immobilization material obtained by heat treatment at 500 ° C., In the heat-treated product of D-1 (Reference Table 1), hexavalent chromium was eluted exceeding the soil environmental standard value (No. 4: Comparative Example 2). The heat-treated product of D-1 had a calcium sulfide content (Reference Table 3) of less than 4% by mass. A heavy metal immobilizing material in which the ratio of the reducing equivalent number of the reducing agent to the oxidizing equivalent number of gypsum is less than 0.3, In the heat-treated product of A (see Table 1), hexavalent chromium was eluted exceeding the soil environment standard value (No. 1: Comparative Example 1), but the calcium sulfide content was less than 4% by mass. In addition, the heat-treated products of the raw materials E and F of Comparative Examples 3, 4, and 5 (Reference Table 2) that did not exhibit sufficient uniaxial compressive strength had a calcium sulfate content of less than 57% (Reference Table 3). ). In addition, the raw material No. 1 of Comparative Example 1 that did not exhibit a sufficient hexavalent chromium fixing function. The heat-treated product of A had a calcium sulfate content exceeding 95% (Reference Table 3).

  From the above, heavy metal fixing material No. It was found that the volcanic ash clay treated with B, C, D-2 to D-6 can be used effectively as a ground improvement material to which heavy metals are fixed.

<Solidification of contaminated soil>
[Heavy metal fixing material]
As the heavy metal immobilization material used in the solidification treatment of the contaminated soil, the heat-treated material D-4 shown in Table 1 was employed.

[Target soil for immobilization]
As the immobilization treatment object, the following three types of contaminated soil containing heavy metals were prepared.
(I) Sandy soil (natural water content ratio: 18.5%) Hexavalent chromium contaminated soil: Hexavalent chromium elution amount according to Environment Agency Notification No. 46: 2.5 mg / L
(II) Cohesive soil (natural water content 35.2%) Hexavalent chromium contaminated soil: Arsenic elution amount according to Environmental Agency Notification No. 46: 4.2 mg / L
(III) Sandy soil (natural water content 19.2%) Lead contaminated soil: Elution amount of lead according to Environment Agency Notification No. 46: 4.7 mg / L

[Production of cement-based solidified material]
The method for producing the cement-based solidified material is the same as that for producing the cement-based solidified material in the production of the ground improvement material. That is, the raw material No. The heat-treated product obtained from D-4 was pulverized using a ball mill until the average particle size became 90 μm, and ordinary Portland cement (sulfur trioxide content 2.0 mass) was used so that the pulverized product became 10 mass%. %) To prepare a cement-based solidified material. The column of “solidification material ratio” in Table 4 shows the mixing ratio of normal Portland cement and heavy metal fixing material when producing cement-based solidification material (No. 14, 16, 18). In addition, as a comparative example, what made the solidification material only with the cement-type solidification material for general soft soils was also produced (No. 15, 17, 19).

[Hardening of soil to be fixed]
Treatment of heavy metal-contaminated soil (chromium-contaminated soil using sandy soil, arsenic-contaminated soil using viscous soil, lead-contaminated soil using sandy soil) with the cement-based solidification material It is the same as the hardening process of the immobilization process target soil in manufacture. That is, the cement-based solidified material is added to the heavy metal contaminated soil, stirred and mixed, filled into a mold (diameter 5 cm × height 10 cm), compacted, and sealed and cured in a temperature-controlled room at 20 ° C. and a relative humidity of 80%. Cured. After 7 days, a specimen was removed from the mold and cured (Reference Table 4: No. 14, 16, 18). In addition, as a comparative example, a general soft soil cement-based solidified material (sulfur trioxide content 6.5 mass%) was used, and the same treatment as above was performed to obtain specimens (Reference Table 4 Nos. 15, 17, and 17). 19).

In addition, the amount of solidification material (kg / m ³ ) added to 1 m ³ of soil to be treated is 60 kg for chromium-contaminated soil using sandy soil, 100 kg for arsenic-contaminated soil using viscous soil, and sandy soil. The lead-contaminated soil was 60 kg.

[Evaluation of immobilization of heavy metals]
The amount of elution of heavy metal was measured for each specimen after demolding. The result is shown in the column of “heavy metal elution amount” in Table 4.

  According to Ministry of the Environment Notification No. 46 of the Ministry of the Environment of Japan, the soil environment standard value of hexavalent chromium is 0.05 mg / L or less, and the soil environment standard value of arsenic As is 0.01 mg / L or less, The reference value of lead Pb is also 0.01 mg / L or less. No. in Table 4 14 (Example 8), the heavy metal fixing material No. Using a cement-based solidification material prepared by adding D-4 to ordinary Portland cement, the chromium-contaminated soil is treated, so that the hexavalent chromium contained is immobilized, and the elution of hexavalent chromium is based on soil environmental standards. It was found that it was sufficiently suppressed to below the value. On the other hand, no. From the results shown in Fig. 15 (Comparative Example 7), when chromium-contaminated soil is solidified with a general soft soil cement-based solidifying material without using a heavy metal fixing material, the elution amount of hexavalent chromium is suppressed to the soil environmental standard value. I couldn't.

  In Table 4, No. 16 (Example 9), the heavy metal fixing material No. Arsenic As is immobilized by treating arsenic-contaminated soil using cement-based solidified material prepared by adding D-4 to ordinary Portland cement, and leaching of arsenic is sufficiently suppressed to below the soil environmental standard value. I found out that On the other hand, no. From the results shown in Fig. 17 (Comparative Example 8), when arsenic-contaminated soil is solidified with a general soft soil cement-based solidifying material without using a heavy metal immobilizing material, the amount of arsenic eluted is suppressed to the soil environmental standard value. I couldn't. From this, the heavy metal fixing material No. It was found that D-4 sufficiently functions as a heavy metal immobilization material that suppresses the elution of arsenic As heavy metals from contaminated soil.

  No. 18 (Example 10), the heavy metal fixing material No. By treating the lead-contaminated soil using a cement-based solidified material prepared by adding D-4 to ordinary Portland cement, the contained lead Pb is fixed, and the elution of lead is below the soil environmental standard value. It was found to be sufficiently suppressed. On the other hand, no. From the results shown in 19 (Comparative Example 9), when lead-contaminated soil is solidified with a general soft soil cement-based solidifying material without using a heavy metal fixing material, the amount of lead elution is suppressed to the soil environment standard value. I couldn't. From this, the heavy metal fixing material No. It was found that D-4 sufficiently functions as a heavy metal immobilization material that suppresses the elution of lead Pb.

It is the schematic which shows the cross section of heavy metal fixed material particles.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Heavy metal fixed material, 2 ... Calcium sulfide, 3 ... Calcium sulfate.

Claims (12)

Heavy metal fixation obtained by heat-treating the raw material containing the gypsum and the reducing agent in a reducing atmosphere so that the ratio of the reducing equivalent number of the reducing agent to the oxidation equivalent number of the gypsum is 0.3 to 0.9. Chemical material,
A heavy metal immobilization material comprising 4 to 19% by mass of calcium sulfide and 57 to 95% by mass of calcium sulfate. 2. The heavy metal immobilization material according to claim 1, wherein the calcium sulfide is partially attached to the surface of the massive calcium sulfate. The heavy metal immobilization material according to claim 1 or 2, wherein the average particle size is 300 µm or less. When the container volume (mL) / heavy metal immobilization material weight (g) ratio = 10, the container was hermetically accommodated in the container and maintained at 20 ° C. with an initial relative humidity of 80%. The heavy metal immobilization material according to any one of claims 1 to 3, wherein the hydrogen sulfide concentration in the container is 2.0 ppm or less. A raw material containing said plaster as reducing the ratio of the number of equivalents of reducing agent to oxidizing equivalents of gypsum is 0.3 to 0.9 and the reducing agent is heat treated in a reducing atmosphere, 4 calcium sulfide A method for producing a heavy metal immobilization material, comprising producing a heavy metal immobilization material containing ˜19 mass% and calcium sulfate in an amount of 57 to 95 mass% . The method for producing a heavy metal immobilization material according to claim 5 , wherein the raw material is heat-treated at 600 ° C to 1100 ° C. The method for producing a heavy metal immobilization material according to claim 5 or 6 , wherein the reducing agent contains a carbon-containing material. A method for producing a ground improvement material, comprising: curing a mixture of a cement-based solidifying material containing the heavy metal fixing material according to any one of claims 1 to 4 and cement, and a soil to be treated. 9. The method for producing a ground improvement material according to claim 8 , wherein the cement-based solidifying material contains 0.5% by mass or more of calcium sulfide and 5.5% by mass or more of sulfur trioxide. . A soil to be treated comprising a soil to be treated containing heavy metal and a cement-based solidifying material containing the heavy metal immobilization material according to any one of claims 1 to 4 and cement and cured. Processing method. The method for treating soil to be treated according to claim 10 , wherein the cement-based solidified material contains 0.5% by mass or more of calcium sulfide. The cement-type solidification material containing the heavy metal fixing material as described in any one of Claims 1-4, and cement.

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