JP2006143976A - Method and apparatus for reduction of leaching out of heavy metals in improved matter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of realizing easily and economically a method of solidifying the ground firmly and preventing leaching out of heavy metals from the solidified improved matter as thoroughly as possible. <P>SOLUTION: The method comprises a mixing process of mixing a cement type ground-improving material with a liquid hardening agent which combines with heavy metals to produce only slightly soluble compounds and exerts alkalinity enhancing the action of precipitation and fixing within improved ground and a mixing treatment process of injecting the cement type improving material added and mixed with the liquid hardening agent into the ground while stirring with a stirring blade to construct improved matter. The liquid hardening agent is a 70 wt.% or higher solution: it contains a sodium salt as the main ingredient, is alkaline and has a content of chloride ion of 1-10 wt.% and a content of sodium ion of 1-10 wt.%. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention is a mixed processing method in which a ground improvement body is constructed by stirring the ground with a stirring blade while injecting a cement-based improvement material into the upper layer, middle layer, deep layer, etc. of the ground. The present invention relates to a method and apparatus for reducing elution of heavy metals in an improved body using a liquid curing agent for reducing the amount of elution.

When building structures and buildings such as roads and revetments on soft silty clay ground and soft sandy ground, ensuring stability, preventing settlement, preventing side movement, liquid For the purpose of preventing erosion and the like, a mixed processing method is performed in order to solidify the upper layer, middle layer, deep layer, etc. of the ground with a cement-based improving material to ensure a solid ground.
As shown in FIG. 2, a solid improvement body 26 is obtained by stirring and solidifying the cement-based improvement material while being injected into the ground 31. The improved body 26 is improved to various shapes such as a pile type, a wall type, a lattice type, and a block type. As the cement-based improving material used in this construction method, ordinary portland cement and blast furnace cement B type as well as cement-based solidifying material are used. If necessary, an AE water reducing agent delayed type admixture may be used to maintain the fluidity of the cement slurry.
The elution of heavy metals such as hexavalent chromium from the improved body 26 is a hydrate in which the heavy metals of hexavalent chromium eluted from the cement by the hydration reaction are simultaneously generated in the process of mixing and solidifying the soil and the cement-based improving material. This occurs especially when it cannot be fixed sufficiently, and there is a case where heavy metals of hexavalent chromium exceeding the soil environment standard may be eluted when improving volcanic ash clay, which significantly inhibits hydrate formation.

  Moreover, the improved body improved by the mixed treatment method has a problem of leaching characteristics when the improved body water generated after the setting and hardening of cement comes into contact with the surrounding soil. It is desirable that the rate of leaching of the improved body or the improved body water should be as small as possible, so that heavy metals encapsulated in the improved body due to water ingress should not appear in the water. . For this purpose, the strength of the improved body has a decisive significance, and the strength of the improved body, in particular its pressure strength and surface strength, also does not fall below a minimum to avoid heavy metals being released into the environment as a result. It is necessary to do so.

Next, regarding the long-term soundness of the solidified improved body 26, as shown in FIG. 2, the strength of the improved body 26 decreases as the material age (hydration reaction) elapses after construction at the boundary of the improved body. Has been confirmed.
For example, in the case of the pile type shown in FIG. 2, it is desirable for the soundness that the diameter D of the improved body 26 does not deteriorate for a long time, but depending on the soil conditions of the ground 31 around the improved body, Ca outflow 32 occurs, and an improved body peripheral deterioration portion 33 is generated. The thickness d of the improved body peripheral deterioration portion 33 gradually increases, and accordingly, the strength of the improved body peripheral deterioration portion 33 is extremely increased. There will be low-strength soil containing cement. Heavy metals such as hexavalent chromium which have been insolubilized in the improved body peripheral deterioration portion 33 constructed as a part of the improved body 26 as well as a reduction in strength of the improved body peripheral deterioration portion 33 of a part of the improved body 26 May be eluted.

On the other hand, as a countermeasure against soil contamination, the mixed treatment method is applied when excavation and removal cannot be performed for relatively deep contamination, and cement-based improvement materials are injected into the ground contaminated with toxic substances such as heavy metals. By position agitation, it is contained so as not to elute from the ground. Solidifying soil contaminated with heavy metals does not reduce the content of heavy metals, but elution is basically prevented by solidifying with this construction method.
However, with the progress of the deterioration of the improved body accompanying the elution of the Ca amount from the improved body to the surrounding unimproved ground part, harmful substances contained in the improved body are eluted.

The solidification method as a countermeasure against soil contamination consists of injecting a slurry-like cement-based improving material or, if necessary, an improving material such as cement having a low pH into the contaminated soil, stirring and mixing the improving material and the contaminated soil, It is a method of containing contaminated soil in the improved body using reaction. As the cement improving material, blast furnace cement type B, ordinary Portland cement, or cement-based solidifying material is used. When it is necessary to suppress the increase in pH as much as possible, a cement suitable for the purpose is used.
JP2002-274907 JP 2002-265253 A JP 2001-342461 A JP 2001-316158 A JP 2001-311077 A

Therefore, the present inventors have made various studies in order to reduce elution of hexavalent chromium eluted from the solidified improved body and heavy metals due to deterioration of the improved body. It was found that by adding a liquid curing agent and mixing it with soil, elution of hexavalent chromium and other heavy metals from the solidified improved product can be suppressed.
The first problem to be solved by the present invention is to provide a method and an apparatus for solidifying the ground firmly and preventing elution of heavy metals from the solidified improved body as much as possible.
The second problem to be solved by the present invention is to provide a method and apparatus for solving the first problem in a simple and economical manner.

The present invention is a method for reducing heavy metal elution in a mixed processing method in which a stirring blade is propelled while injecting a slurry-like cement-based improving material into the ground to be improved, and the ground is stirred and mixed to form a stable improved body. A method and apparatus for reducing elution of heavy metals in an improved body by suppressing the elution of hexavalent chromium and other heavy metals from the solidified improved body by adding a liquid curing agent to the cement-based improved material .
The liquid curing agent contains sodium salt as a main component in a solution of 70% by weight or more, exhibits alkalinity, has a chlorine ion content of 1 to 10% by weight, and a sodium ion content of 1 The content of cobalt is 0.01 to 0.1% by weight.

  According to the present invention, the cement-based improved material is added with a liquid curing agent exhibiting alkalinity to enhance the action of binding to heavy metals to form a poorly soluble compound and precipitating and fixing in the improved body. In addition to solidifying the ground firmly, elution of heavy metals from the solidified improved body can be prevented as much as possible. Therefore, the mixed treated soil, which is an improved body, can provide a stable soil for a long time without deterioration.

  According to the present invention, a sodium salt is contained as a main component in a solution of 70% by weight or more, exhibits alkalinity, has a chlorine ion content of 1 to 10% by weight, and has a sodium ion content of 1 10% by weight and the content of cobalt is 0.01 to 0.1% by weight. The liquid curing agent exhibiting alkalinity is used, so that the method of the present invention can be realized easily and economically. In particular, it is effective for a large amount and a wide range of processing, and the total cost can be reduced.

The method and apparatus for reducing elution of heavy metals in the improved body according to the present invention have the following configuration.
(1) The cement-based improving material generates calcium hydroxide (Ca (OH) ₂ ) in the course of its hydration reaction, and becomes alkaline. The hydroxyl group (OH ⁻ ) dissociated from (Ca (OH) ₂ ) generates a hardly-soluble hydroxide precipitate with heavy metals, and precipitates and fixes in the improved body.
In the method and apparatus of the present invention, by adding a specific curing agent to be described later, the precipitation / fixing action in the improved body is effectively performed. This curing agent is preferably liquid.
For example, hexavalent chromium has high solubility in water, and the magnitude of this solubility depends on the pH of the groundwater, the oxidation-reduction potential, and the like. Hexavalent chromium is reduced in a reducing atmosphere to become trivalent chromium, so that hexavalent chromium tends not to elute. Since trivalent chromium is chemically stable, its toxicity is significantly reduced.
The specific curing agent may be a powder, and in this case, it is mixed with water in a cementitious improvement material.
In the following examples, a liquid curing agent is used.

(2) The cement-based improving material generates various hydrates by the hydration reaction, and these hydrates have an ability to fix heavy metals by substitution solid solution or surface adsorption.
The hexavalent chromium in the cement-modified soil with a high pH is eluted from the cement-improved soil into the surrounding unmodified ground because the surrounding unmodified ground is generally lower in pH than the cement-modified soil. It is because it will be in the state which is easy to adsorb | suck.
The liquid curing agent used in the present invention does not advance this adsorption action.
That is, the presence of the liquid curing agent reduces the rate at which hexavalent chromium is adsorbed on the surrounding soil.

(3) In the liquid curing agent added to the cement-based improving material, the hydrate produced by the progress of the hydration action with the age of the material fills the voids and densifies the hardened body structure. This structure has the effect of physically containing heavy metals by densification.

  The cement-based improving material has a solidifying and insolubilizing effect that is physically and chemically stable against harmful substances of heavy metals due to the synergistic effect of (1), (2) and (3).

A conventionally known general mixing treatment method, that is, a mixing treatment method in which an improved body is constructed by stirring the ground with a stirring blade while injecting a cement-based improving material into the ground will be described with reference to FIG.
In the mixing treatment method, various machines are used, and the embodiment shown in FIG. 1 is an example.
Reference numeral 10 denotes an agitator. The agitator 10 is supplied with a cement-based improving material from a cement-based improving material hopper 11, and is supplied with water for a hydration reaction from a water supply unit 12 as a diluting solution. 13 is stirred.
Further, as a curing agent used in the method and apparatus of the present invention, a liquid curing agent for detoxifying heavy metals is charged from the liquid curing agent hopper 14 and mixed and stirred.

A grout liquid as a cement-based improving material is obtained by stirring and mixing, and this grout liquid is pumped from the grout pump 15 to the pressure feed pipe 16. The amount of grout liquid in the pressure feeding pipe 16 is measured by the flow meter 17, and the injection pressure to the ground 31 is measured by the pressure gauge 18.
When injecting the grouting liquid, the valve 28 and the valve 30 are closed and the valve 19 is opened to pump the grouting liquid to the agitation shaft 22 of the mixing processor 20 and inject it into the ground 31 from the discharge hole 25 at the tip of the agitation shaft 22. At this time, the excavation blade 24 and the stirring blade 23 are provided at the tip of the stirring shaft 22 rotated by the driving device 20, and the ground 31 is excavated downward by the excavation blade 24 so that the ground is mixed by the stirring blade 23 so that the grout liquid is mixed. 31 is stirred.
After the injection of the grouting liquid, the hydration action proceeds in the ground 31 as the material ages, and a strong improvement body 26 is formed.
The grout liquid from the discharge hole 25 may be injected when the ground 31 is being excavated downward, or may be injected when the stirring shaft 22 is raised. Further, the stirring shaft 22 may be two or more axes.
After the injection is finished, the valve 19 is closed, the valves 28 and 30 are opened, and the compressed air is sent from the compressor 27 via the compressed air supply pipe 35 to the pressure feed pipe 16, so that the cleaning liquid from the cleaning liquid tank 29 is washed with the cleaning liquid supply pipe. The grout liquid sucked through the valve 36 and remaining in the pressure feed pipe 16 from the valve 19 is sprayed to the ground 31 to clean the inside of the pressure feed pipe 16.

In the above mixed processing method, a liquid curing agent for detoxifying heavy metals is introduced into the agitator 10 from the liquid curing agent hopper 14, and from the cement-based improvement material and water supply unit 12 from the improvement material hopper 11. Stir and mix with water. The addition amount of the liquid curing agent is set within a range of 0.3 to 15% by weight (C × wt%) with respect to the cement-based improvement material according to the property of the soil to be improved in the mixed processing method. The amount of the liquid curing agent added to the cement-based improving material is appropriately determined in consideration of the effect of addition. For example, if the soil to be improved shows properties with a high content of heavy metals, increase the ratio of the liquid hardener added to the cement-based improvement material, and if it shows properties with a low content, Reduce the proportion of hardener added. In addition, as described above, the strength of the improved body is extremely important for reducing the elution of heavy metals, but in order to obtain this strength, generally, according to the properties of the ground per 1 m ^{3 of the} soil to be improved, Moreover, the addition amount of a cement-type improvement material shall be 70-300 kg by the target intensity | strength. Therefore, the liquid hardening agent is added when the addition amount of the cement-based improvement material is small, and when it is large, it is decreased. It is necessary to adjust so that a hardening | curing agent may be added, and, thereby, elution of heavy metal can be reduced as much as possible.
An example of the best formulation of this liquid curing agent is as follows.
A sodium salt is contained as a main component in a solution of 70% by weight or more and exhibits high alkalinity (specifically, pH = 10.4).
The content of chloride ions is 1 to 10% by weight (specifically, 4.6% by weight on average).
The content of sodium ions is 1 to 10% by weight (specifically, an average of 4.8% by weight).
The cobalt content is 0.01 to 0.1% by weight (specifically, an average of 0.06% by weight).
The content of calcium ions is 0.1% by weight or less.
The evaporation residue (measured at 10.5 ° C. for 24 hours) is 10 to 20% by weight (specifically, 16.7% on average).
Density 1.149 g / cm ³ .
Viscosity 4.4 m · Pas.

The reason why the above composition is adopted is as follows.
(1) The cement-based improving material generates calcium hydroxide (Ca (OH) ₂ ) in the course of its hydration reaction, and becomes alkaline. The hydroxyl group (OH ⁻ ) dissociated from (Ca (OH) ₂ ) generates a hardly-soluble hydroxide precipitate with heavy metals, and precipitates and fixes in the improved body.
The liquid curing agent used in the method and apparatus of the present invention makes this precipitation / fixing action more effective.
For example, hexavalent chromium has high solubility in water, and the magnitude of this solubility depends on the pH of the groundwater, the oxidation-reduction potential, and the like. Hexavalent chromium is reduced in a reducing atmosphere to become trivalent chromium, so that hexavalent chromium tends not to elute. Since trivalent chromium is chemically stable, its toxicity is significantly reduced.
When oxalic acid was added to the oxidizing atmosphere with a liquid curing agent to convert to a reducing atmosphere, the hexavalent chromium elution concentration decreased to a fraction of that before the conversion.
Hexavalent chromium is reduced to chemically stable trivalent chromium by a reducing substance, so that hexavalent chromium is less likely to be generated when reducing earth and improving material are mixed.
The hydration reaction at the initial age of the cement mineral (C ₃ A) that produces a monosulfate phase having a high hexavalent chromium fixing ability is very remarkable. Since fixation of valent chromium acts simultaneously, elution of hexavalent chrome is further suppressed.

(2) The cement-based improving material generates various hydrates by the hydration reaction, and these hydrates have an ability to fix heavy metals by substitution solid solution or surface adsorption.
The hexavalent chromium in the cement-modified soil with a high pH is eluted from the cement-improved soil into the surrounding unmodified ground because the surrounding unmodified ground is generally lower in pH than the cement-modified soil. It is because it will be in the state which is easy to adsorb | suck.
The liquid curing agent used in the method and apparatus of the present invention does not advance this adsorption action.
That is, due to the presence of the liquid curing agent, the rate at which hexavalent chromium is adsorbed from the improved body 26 to the surrounding unmodified ground is reduced.
Iron oxide is very commonly present on the surface of the soil particles, and in the surrounding unmodified ground where the content of iron oxide is low, the adsorption of hexavalent chromium increases due to the decrease in pH due to the presence of the additive.
In this way, hexavalent chromium does not elute to the surrounding unmodified ground by being strongly adsorbed by the improved body.

(3) In the liquid curing agent added to the cement-based improving material, the hydrate produced by the progress of the hydration action with the age of the material fills the voids and densifies the hardened body structure. This structure has the effect of physically containing heavy metals by densification.

  The cement-based improving material has a solidifying / insolubilizing effect that is physically and chemically stable against harmful substances such as heavy metals due to the synergistic effects of (1), (2), and (3).

The effect of the said liquid hardening | curing agent by the said mixing | blending is demonstrated in detail.
The main components of the sodium salt contained in the liquid curing agent are sodium carbonate and sodium chloride. Sodium carbonate is added mainly for the purpose of preventing the deterioration of mechanical strength due to neutralization by promoting the alkalinity of the solidified improved body when solidifying by adding a liquid hardener to the cement-based improving material. Sodium chloride is a component added for the purpose of promoting cement hydration together with sodium carbonate when a liquid curing agent is added to a cement-based improving material and solidified. Therefore, it is possible to reliably prevent the heavy metal such as hexavalent chromium and the hormone-inhibiting chemical substance from being eluted and diffused from the improved body.

  The reason why the content of chlorine ions in the liquid curing agent is in the range of 1 to 10% by weight is that when less than 1% by weight, the effect of adding chlorine ions supplied by sodium chloride or the like is not exhibited in the improved body, and 10% by weight. This is because if it exceeds 50%, chlorine ions become excessive and the effect of addition to the improved product reaches its peak. Therefore, the amount of chlorine ions added to the cement-based improving material is appropriately determined in consideration of the effect of addition. For example, when the addition ratio of the liquid curing agent to the cement-based improvement material is large, the addition amount of chlorine ions is set to the lower limit side of the above range, and when the addition ratio of the liquid curing agent is small, addition of chlorine ions It is desirable to set the amount on the upper limit side of the range.

  The content of sodium ions in the liquid curing agent is in the range of 1 to 10% by weight. If the content is less than 1% by weight, the effect of adding sodium ions supplied by sodium chloride or sodium carbonate is not exhibited in the improved product, This is because when it exceeds 10% by weight, sodium ions become excessive, and the effect of addition to the improved product reaches its peak. Therefore, the amount of sodium ion added to the cement-based improving material is appropriately determined in consideration of the effect of addition. For example, when the addition ratio of the liquid curing agent to the cement-based improvement material is large, the addition amount of sodium ions is set to the lower limit side of the above range, and when the addition ratio of the liquid curing agent is small, addition of sodium ions It is desirable to set the amount on the upper limit side of the range.

  For liquid hardeners, when this liquid hardener is added to a cement-based improving material to improve it, it promotes the alkalinity of the improved body in the same way as sodium carbonate to prevent the mechanical strength from decreasing due to neutralization. Ammonium salts added for the main purpose are included. The content of this ammonium salt is preferably 2% by weight or less. The ammonium salt is preferably ammonium chloride.

  The content of the evaporation residue in the liquid curing agent is in the range of 10 to 20% by weight. If the content is less than 10% by weight, the total content can be ensured in which the effect of adding the chlorine ions and sodium ions can be expected. In addition, when the amount exceeds 20% by weight, not only the effect of adding the chlorine ions and sodium ions reaches a peak, but also the content of the liquid curing agent cannot be sufficiently ensured.

  The reason why the content of cobalt in the liquid curing agent is 0.01 to 0.1% by weight is to increase the mechanical strength of the improved body by containing a cobalt compound that forms a complex, This is because the substance can be retained and elution can be prevented.

  The reason why the content of calcium ions in the liquid curing agent is 0.1% by weight or less is that when calcium ions are included, the amount of calcium carbonate produced by the reaction with sodium carbonate as the sodium salt is ignored. This is because the calcium carbonate adversely affects the properties of the improved body, and as a result, the elution and diffusion of the harmful substances cannot be effectively prevented.

  In addition to solidification and insolubilization with cement-based improvement materials, it is possible to further reduce the impact on the surrounding environment by using a soil-covering / laying-in construction method and a water-impervious construction method that utilize the adsorption properties and water-insulating properties of clay minerals. .

It is explanatory drawing of the method of reducing the elution of the heavy metal in the improvement body in the mixing treatment method by this invention. It is a conceptual diagram of the method of reducing the elution of the heavy metal in the improved body in the mixing treatment method by this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Agitator, 11 ... Cement system improvement material hopper, 12 ... Water supply part, 13 ... Agitator stirrer, 14 ... Liquid hardening agent hopper, 15 ... Grout pump, 16 ... Pumping pipe, 17 ... Flow meter, 18 ... Pressure gauge , 19 ... valve, 20 ... mixing processor, 21 ... drive device, 22 ... stirring shaft, 23 ... stirring blade, 24 ... excavation blade, 25 ... discharge hole, 26 ... improved body, 27 ... compressor, 28 ... valve, DESCRIPTION OF SYMBOLS 29 ... Cleaning liquid tank, 30 ... Valve, 31 ... Ground, 32 ... Ca elution, 33 ... Improved body periphery deterioration part, 34 ... Support ground, 35 ... Compressed air supply pipe, 36 ... Cleaning liquid supply pipe.

Claims (8)

  A liquid curing agent that is added to a cement-based ground improvement material and combined with heavy metals to form a sparingly soluble compound and to enhance the action of precipitation and fixation in the ground improvement body. It is characterized by comprising a mixing step and a mixing treatment step of constructing an improved body by stirring with a stirring blade while injecting the cement-based improving material additionally mixed with the liquid curing agent into soft ground or loose sandy ground A method for reducing elution of heavy metals in an improved body.   The curing agent is in a liquid form, and contains a sodium salt as a main component in a solution of 70% by weight or more, exhibits alkalinity, has a chlorine ion content of 1 to 10% by weight, and a sodium ion content. The method for reducing elution of heavy metals in the improved body according to claim 1, wherein the content of cobalt is 1 to 10 wt% and the content of cobalt is 0.01 to 0.1 wt%.   The hardening agent is liquid, and the hydrate produced by the hydration reaction of the cement-based improvement material enhances the ability to fix heavy metals by surface adsorption so that the pH of the improvement body becomes higher than the surrounding soil. The method for reducing elution of heavy metals in the improved product according to claim 1 or 2, wherein the contents of chlorine ions and sodium ions are adjusted.   The improvement according to claim 1, 2 or 3, characterized in that the improved body of the mixed processing method using a cement-based improving material to which a liquid hardener is added utilizes the adsorption characteristics and water shielding characteristics of clay minerals in addition to solidification and insolubilization. A method for reducing elution of heavy metals in the body.   The addition amount of the liquid curing agent exhibiting alkalinity is set within a range of 0.3 to 15% by weight (C × wt%) with respect to the cement-based improvement material according to the property of the soil to be improved in the mixed processing method. The method for reducing elution of heavy metals in the improved product according to claim 1, 2, 3 or 4.   A cement-based ground improvement material is combined with a mixed slurry by adding an alkaline curing agent to strengthen the action of forming a poorly soluble compound by combining with heavy metals and precipitating and fixing in the ground improvement body. In an improved body comprising a mixing tank, a pumping pipe for pumping the mixed slurry to the injection ground, and an excavating stirrer for stirring while injecting the mixed slurry supplied from the pumping pipe to the ground A device that reduces the elution of heavy metals.   The apparatus for reducing elution of heavy metals in an improved body according to claim 6, wherein a compressed air supply pipe for supplying compressed air is connected to a pressure feeding pipe for feeding the mixed slurry to the injection ground. The apparatus for reducing elution of heavy metals in an improved body according to claim 6 or 7, wherein a cleaning liquid supply pipe for supplying a cleaning liquid to the pressure feeding pipe is connected to a pressure feeding pipe for pumping the mixed slurry to the injection ground.

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