Method for producing neutral recycled soil using low-concentration carbon dioxide

Abstract

To provide a neutral regenerated soil production method with low concentration of carbon dioxide gas which can produce neutral regenerated soil having a pH of 8.6 or less in large quantities in a short time without concentrating the low concentration of carbon dioxide gas having a concentration of 10% or less.SOLUTION: A neutral regenerated soil production method with low concentration of carbon dioxide gas includes: a modification solidification step of adding a PS (paper sludge) ash-based modifier to construction sludge or construction generated soil and mixing the resultant to produce modification solidification-treated soil; a dissociation granulation step of dissociating and granulating the modification solidification-treated soil to produce dissociation granulation-modified soil; a classification step of classifying the dissociation granulation-modified soil using a sieve to produce classified soils; a laying step of laying the classified soils in a layered form in a pit; and a neutralization step of supplying a low concentration of carbon dioxide gas having a concentration of 10% or less to a bottom of the pit to neutralize the classified soils.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for producing neutral recycled soil, which modifies alkaline and soft construction sludge and efficiently fixes low-concentration carbon dioxide gas as calcium carbonate in the modified soil to produce neutral recycled soil.

Background Art

[0002] Construction sludge discharged at construction sites often contains cement and lime components and usually exhibits alkalinity with a pH of 11 or more. After such construction sludge is brought into an intermediate treatment plant, quicklime or cement-based solidifying agents are added for modification treatment, and then it is reused as recycled soil or landfilled at a final disposal site. When reused as recycled soil, it often exhibits strong alkalinity with a pH exceeding 11, and thus there is a problem that its utilization applications are limited.

[0003] To address such problems, the inventors have already obtained a patent related to "Recycled Soil Production System and Production Method" (see Patent Document 1) and a patent related to "Neutral Recycled Soil Production System and Production Method by Carbon Dioxide Fixation Technology" (see Patent Document 2).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] The present invention proposes a solution to the problems related to CO2 concentration that have not been solved in the prior patent applications. That is, in the above two patented inventions, a method for producing neutral regenerated soil was devised by contacting highly pure (concentration 95% or more) carbon dioxide gas with alkaline construction soil. Therefore, there was a problem that it was difficult to neutralize efficiently and at a practical level with relatively low-concentration (concentration 10% or less) CO2 discharged from factory chimneys and the like.

[0006] An object of the present invention is to provide a method for producing neutral regenerated soil using low-concentration carbon dioxide gas, which can produce a large amount of neutral regenerated soil with a pH of 8.6 or less in a short time without concentrating low-concentration carbon dioxide gas with a concentration of 10% or less.

Means for Solving the Problems

[0007] That is, the present invention includes the following matters. According to the method for producing neutral regenerated soil using low-concentration carbon dioxide gas of the present invention, a modification and solidification step of adding a PS (paper sludge) ash-based modifier to construction sludge or construction-generated soil and mixing them to produce a modified and solidified treatment soil, a loosening and granulation step of loosening the modified and solidified treatment soil and granulating it to produce a loosened and granulated modified soil, a classification step of sieving and classifying the loosened and granulated modified soil to produce a classified soil, a laying step of laying the classified soil in layers in a pit, a neutralization step of supplying low-concentration carbon dioxide gas with a concentration of 10% or less to the bottom of the pit to neutralize the classified soil, and is characterized by including the above.

[0008] By going through the above steps, it is possible to produce a large amount of neutral regenerated soil with a pH of 8.6 or less in a short time without concentrating low-concentration carbon dioxide gas with a concentration of 10% or less.

[0009] Also, according to the method for producing neutral regenerated soil using low-concentration carbon dioxide gas of the present invention, the laying step is characterized in that the classified soil is laid in the pit from the bottom in descending order of particle size.

[0010] As a result, the voids between the classified soils become larger than those in the case where the particle sizes are uniform for each layer and classification by particle size is not performed (the dry density of the classified soil becomes smaller), and the air permeability increases, so that the neutralization time can be shortened.

[0011] Further, according to the method for producing neutralized recycled soil using low-concentration carbon dioxide of the present invention, the classified soil laid on the uppermost layer in the laying step has a particle size of less than 10 mm, and the classified soil laid on the lowermost layer has a particle size of 40 mm or more and less than 75 mm.

[0012] In this way, by laying the classified soil with a coarser particle size on the lower layer, the neutralization efficiency can be improved by utilizing the neutralization characteristic that the coarser the classified soil, the longer the time required for neutralization, and ensuring a longer contact time of CO2 in the lower layer. Further, by laying the modified soil with a finer particle size on the uppermost part, the sealing degree of the upper part of the soil layer is increased, and it can function as a lid for preventing the dissipation of CO2 from the surface layer.

[0013] Further, according to the method for producing neutralized recycled soil using low-concentration carbon dioxide of the present invention, the neutralization step is when the volume of the classified soil laid in the pit in the laying step is assumed to be V, an initial supply step of supplying the low-concentration carbon dioxide at a volume flow rate of 5V to 20V for 0.1 to 1 hour from the start of supply, a later supply step of supplying the low-concentration carbon dioxide at a volume flow rate of 8V to 40V after the low-concentration carbon dioxide fills the pit, a middle supply step of supplying the low-concentration carbon dioxide at a volume flow rate of 80V to 320V between the initial supply step and the later supply step is included, and the total supply time of the low-concentration carbon dioxide is 4 to 24 hours.

[0014] Thus, by adjusting the supply amount of low-concentration carbon dioxide gas and the contact time with the classified soil, the neutralization of the classified soil can be carried out efficiently in a short period of time.

[0015] Further, according to the method for producing neutral recycled soil using low-concentration carbon dioxide gas of the present invention, it is characterized by including a scooping step of scooping out the classified soil having a particle size of less than 20 mm after the neutralization step.

[0016] Thereby, the removed classified soil is transferred to the product stockyard and used as neutral recycled soil as it is.

[0017] Further, according to the method for producing neutral recycled soil using low-concentration carbon dioxide gas of the present invention, the classified soil having a particle size of 20 mm or more that has undergone multiple cycles of neutralization treatment is mixed with the neutral recycled soil obtained by the method for producing neutral recycled soil using the low-concentration carbon dioxide gas.

[0018] Thereby, the construction sludge received in the pit can be utilized without remainder.

[0019] Further, according to the method for producing neutral recycled soil using low-concentration carbon dioxide gas of the present invention, the modified solidified treated soil is characterized by exhibiting strong alkalinity with a pH of 11 or more.

[0020] That is, according to the present invention, it is possible to neutralize such strongly alkaline construction sludge.

Advantages of the Invention

[0021] According to the invention of the present invention, it is possible to provide a method for producing neutral recycled soil using low-concentration carbon dioxide gas, which can produce a large amount of neutral recycled soil with a pH of 8.6 or less in a short time without concentrating low-concentration carbon dioxide gas with a concentration of 10% or less.

Brief Description of the Drawings

[0022]

Figure 1

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Mode for Carrying Out the Invention

[0023] Hereinafter, a method for producing neutralized recycled soil using low-concentration carbon dioxide according to an embodiment of the present invention will be described with reference to the drawings. First, construction sludge discharged at a construction site is transported to an intermediate treatment facility. Here, the construction sludge transported to the intermediate treatment facility is an industrial waste exhibiting strong alkalinity. Further, the cone index (qc) of this construction sludge is less than 200 kN / m 2 and less.

[0024] (Step S1: Modification and solidification step) Here, the alkaline construction sludge brought into the intermediate treatment facility will eventually solidify if left as it is. However, due to space constraints in the intermediate treatment facility, it cannot be left (cured) for a long time as it is. Therefore, it is common to add a solidifying agent exhibiting strong alkalinity for modification and perform treatment so that it can be handled quickly. In this case, there are cases where the pH becomes 12 or more due to the additional addition of the solidifying agent.

[0025] In the present invention, such prior art is directly applied to modify and solidify construction sludge. That is, in a modification and solidification treatment area (not shown) of an intermediate treatment facility, an alkaline improver is added to the construction sludge for modification and solidification. As the alkaline improver, a PS (paper sludge) ash-based water-absorbing modifier is used.

[0026] Note that, as described in Example 1 below, as a result of conducting tests, it was found that the neutralization time of construction sludge can be significantly shortened by adding a PS ash-based modifier. Therefore, in the present invention, it is an essential condition to use a PS ash-based modifier as the alkaline improver.

[0027] Note that the addition amount of the PS ash-based modifier depends on the water content state of the construction sludge, but usually it is in the range of 5 to 30% by mass ratio with respect to the solid component in the construction sludge, and it is more preferable if it is 10 to 20%. It has been confirmed in laboratory experiments that no remudding occurs due to the generation of water after the neutralization reaction with CO2 in the construction sludge with 20% or more of the PS ash-based modifier added. Also, the pH value of the construction sludge (hereinafter referred to as modified and solidified treated soil) after adding the PS ash-based modifier is not limited.

[0028] In such a modification and solidification process, by adding an alkaline solidifying agent to the construction sludge, the soft construction sludge is surely modified and solidified, and semi-solid modified and solidified treated soil is generated.

[0029] (Step S2: Loosening and granulation process) The semi-solid lump-shaped modified and solidified treated soil is in the process of hydration reaction, and if left as it is, it will solidify into a lump. Therefore, the semi-solid modified and solidified treated soil is crushed and granulated using a loosening and granulation technique (for example, refer to Japanese Patent Laid-Open No. 2015-127050).

[0030] Here, the determination of the suitability of loosening and granulation is performed by the cone index test method (JIS A 1228). Usually, the cone index of the modified and solidified treated soil is 100 kN / m 2If it is above, loosening granulation is possible. However, in order to produce more granulated construction sludge (hereinafter referred to as loosening granulation modified soil) by loosening granulation, the cone index of the modified solidified soil should be 400 kN / m 2 or more and less than 1800 kN / m 2 is preferable. The machine (loosening granulation means) used for the loosening granulation treatment may be an excavator, but a crusher such as a soil improvement machine (plant) having a crushing function is preferable.

[0031] Since the loosening granulation modified soil obtained in such a loosening granulation process is granulated, the surface area of the particles increases, making it easy to come into contact with air and carbon dioxide gas, and it is adjusted to an appropriate particle size.

[0032] Specifically, since the PS ash-based modifier is added to the modified solidified soil, granulation is promoted, and most of the particle sizes of the loosening granulation modified soil are 40 mm or less. Here, when the loosening granulation modified soil is generated in an actual plant, as an approximate value, the loosening granulation modified soil with a particle size of 40 mm or more and less than 75 mm is 10%, the loosening granulation modified soil with a particle size of 40 - 20 mm is about 10%, the loosening granulation modified soil with a particle size of 20 - 10 mm is about 40%, and the loosening granulation modified soil with a particle size of 10 mm or less is about 40%.

[0033] In addition, the loosening granulation modified soil is in a state where it is easy to dry due to the increased surface area. At the same time, it is also in a state where it is easy to reduce the pH by reacting with carbon dioxide in the air.

[0034] (Step S3: Classification process) Next, the loosened granulated modified soil is sieved for classification. Specifically, sieves with apertures of 40 mm, 20 mm, and 10 mm are prepared, and the loosened granulated modified soil is classified into four types of classified soils with different particle sizes: classified soil with a particle size of 40 mm or more and less than 75 mm (hereinafter referred to as 40 mm oversize material), classified soil with a particle size of less than 40 mm and 20 mm or more (hereinafter referred to as 40 - 20 mm material), classified soil with a particle size of less than 20 mm and 10 mm or more (hereinafter referred to as 20 - 10 mm material), and classified soil with a particle size of less than 10 mm (hereinafter referred to as 10 mm undersize material). A vibrating sieve is preferably used as the sieving machine.

[0035] (Step S4: Laying process) FIG. 1 is a schematic view showing a pit into which the classified soil according to the embodiment is introduced. As shown in FIG. 1(a), the pit 2 is a rectangular space formed on the ground. At the bottom of the pit 2, an inflow pipe 4 for supplying carbon dioxide gas, formed of a heat-resistant pipe or the like, is buried in advance. A blower 8 for supplying carbon dioxide gas to the inflow pipe 4 is connected to the end of the inflow pipe 4 above the ground.

[0036] Next, as shown in FIG. 1(b), the classified soil is laid in the pit 2 in a loosely packed state in the order of 40 mm oversize material 10, 40 - 20 mm material 12, 20 - 10 mm material 14, and 10 mm undersize material 16.

[0037] In this way, by classifying the loosened granulated modified soil and laying it in the pit 2 in a loosely packed state from the bottom in descending order of particle size, the air permeability of carbon dioxide gas in the classified soil can be increased. That is, since the particle sizes of the classified soil are uniform for each layer, the voids between the classified soils become larger (the dry density of the classified soil becomes smaller) than when the particle sizes are not classified, and the air permeability increases, leading to a shortening of the neutralization time.

[0038] Also, it is a generally known fact that the larger the particle size of the classified soil, the better its air permeability (sand has better air permeability than clay, and gravel has better air permeability than sand). Therefore, if a material with a coarse particle size is laid in the lower layer and a material with a fine particle size is laid in the upper layer, a soil layer space with better air permeability can be created in the lower layer. As a result, forced injection of CO2 from the inflow pipe laid in the lower layer (see Japanese Patent No. 6975757) becomes unnecessary. In addition, by laying a classified soil with a coarse particle size in the lower layer, the neutralization characteristic that the coarser the classified soil, the longer the time required for neutralization is utilized, and the neutralization efficiency can be improved by ensuring a longer contact time of CO2 in the lower layer. Furthermore, by laying a modified soil with a fine particle size on the topmost part, the sealing degree of the upper part of the soil layer is increased, and it can function as a lid to prevent the dissipation of CO2 from the surface layer.

[0039] (Step S5: Neutralization process) In this state, the blower 8 is driven, and as shown in Fig. 1(c), low-concentration carbon dioxide gas with a concentration of 10% or less (of course, carbon dioxide gas with a concentration of 10% or more may also be used.) is supplied to the inflow pipe 4.

[0040] Here, assuming that the volume of the classified soil laid in the pit 2 in the above-mentioned laying process (step S4) is V, in the initial stage (0.1 to 1 hour after the start of supply), the pressure of the blower 8 is throttled so that the low-concentration carbon dioxide gas is gently supplied at a volume flow rate of 5V to 20V (initial supply process), and then the pressure of the blower 8 is increased to gradually increase the supply amount of the low-concentration carbon dioxide gas and supply it at a volume flow rate of 80V to 320V (mid-term supply process). When the low-concentration carbon dioxide gas fills the pit 2, the pressure of the blower 8 is throttled again so that the low-concentration carbon dioxide gas is supplied at a volume flow rate of 10V to 40V (late-stage supply process). Regarding the filling of the low-concentration carbon dioxide gas in the pit 2, it may be detected by a detection device, and an instruction to throttle the pressure of the blower 8 may be automatically given.

[0041] After the low-concentration carbon dioxide gas fills the pit 2 and a predetermined time has elapsed, turn off the blower 8 to stop the supply of the low-concentration carbon dioxide gas. Note that the time from the start of the supply to the stop is about 4 to 24 hours. Therefore, the time required for the neutralization of the classified soil is also approximately 4 to 24 hours.

[0042] In this way, by adjusting the supply amount of the low-concentration carbon dioxide gas and the contact time with the classified soil, the neutralization of the classified soil can be carried out efficiently in a short period of time.

[0043] Also, by supplying the low-concentration carbon dioxide gas, which is heavier than air, from the inflow pipe 4 buried at the bottom of the pit 2, the low-concentration carbon dioxide gas is gradually deposited in the pit 2, and the low-concentration carbon dioxide evenly spreads in the pit 2. Therefore, when the low-concentration carbon dioxide gas is supplied to the upper layer, the neutralization is completed simultaneously from the lower layer to the upper layer.

[0044] (Step S6: Scooping process) Next, stop the supply of the low-concentration carbon dioxide gas, and scoop out and remove the 10 mm underlayer material 16 and the 20 - 10 mm material 14 from the pit 2. The removed classified soil is transferred to the product stockyard and used as neutral recycled soil as it is.

[0045] Regarding the 40 - 20 mm material 12 with a low neutralization rate remaining in the lower layer, it is repeatedly used in the neutralization treatment (Step S4: Laying process, Step S5: Neutralization process) of subsequent cycles and continues to be neutralized. The finally neutralized 40 - 20 mm material 12 is crushed to less than 20 mm by a crusher and then mixed with the 10 mm underlayer material 16 and the 20 - 10 mm material 14 scooped out in Step S6: Scooping process and used as neutral recycled soil.

[0046] Note that the 40 mm overlayer material 10 located in the bottom layer has a small generation amount, so it is continuously used as a ventilation layer as it is. By continuously using it for a certain period, the pH drops below 8.6. Finally, it is crushed to less than 20 mm by a crusher and used as neutral recycled soil.

[0047] Therefore, it is not necessary to separately purchase single crushed stones as in the prior art (see Japanese Patent No. 6975757), and it is possible to recycle classified soil with a large particle size. According to the invention according to this embodiment, neutralized soil with a pH of 8.6 or less can be mass-produced in a short time within 24 hours without concentrating low-concentration carbon dioxide gas with a cone index of 800 kN / m 2 or more and a concentration of 10% or less, and a method for producing neutralized soil using low-concentration carbon dioxide gas can be provided.

[0048] In addition, by gently supplying low-concentration carbon dioxide gas without pressurizing it, the CO2 fixation efficiency can be increased and the dissipation to the outside of Pit 2 can be minimized. Furthermore, the construction sludge received in Pit 2 can be used without any remainder.

[0049] In the invention of the prior application (see Japanese Patent No. 6975757), by supplying high-concentration carbon dioxide gas with a purity close to 100% from the bottom of the pit together with high-pressure air, it was possible to neutralize the regenerated soil up to a maximum soil layer thickness of about 3 m (about 150 m in one pit 3 ). However, in practice, the practical level was up to a soil layer thickness of 1.5 m. On the other hand, in the invention according to this embodiment, although the concentration of carbon dioxide gas is less than one-tenth of the conventional level, it is possible to neutralize the construction sludge with a soil layer thickness of 3 m or more.

[0050] The following effects can be expected as compared with the conventional method. (1) It is a low-cost technology. The conventional technology proposed a neutralization method that purchases and uses industrial carbon dioxide gas with increased purity by concentrating carbon dioxide gas. In contrast, in the present invention, low-concentration carbon dioxide gas with a low concentration can be directly used for neutralization. For example, the carbon dioxide gas concentration in the exhaust gas discharged from an oil refining plant is said to be about 10%. If the CO2 in such exhaust gas can be directly used without concentration, it will lead to a significant cost reduction. Furthermore, since no equipment or power for injecting CO2 is required, neutralization at a low cost can be realized as compared with the conventional technology.

[0051] (2) It is a technology that contributes to carbon neutrality. Carbon dioxide emitted from factories is listed as one of the causes of global warming as a greenhouse gas. By using this carbon dioxide for the neutralization of recycled soil, CO2 is fixed in the soil as carbonate, which becomes one of the carbon recycling technologies. Furthermore, by efficiently and massively fixing low-concentration carbon dioxide in classified soil, it leads to carbon neutrality (in some cases, carbon negativity), and is strongly linked to social contributions towards the realization of a decarbonized society.

[0052] (3) The recycling rate of construction sludge increases. By reforming alkaline construction sludge, whose current usage is limited, into high-quality neutral recycled soil, it can be used not only as a wide range of ground materials but also as a vegetation base material. In addition, recycled soil with a particle size over 20mm, which has not been used and has been discarded so far, can also be used as neutral recycled soil, so an improvement in the recycling rate of construction sludge can be expected.

[0053] (4) It does not require special machinery, so it has high versatility. Compared with the prior patent (Patent No. 6975757), the additional machinery is only a classifier (such as a vibrating sieve) for classification and a crusher for crushing granulated coarse-grained reformed soil. Therefore, by applying general-purpose machinery, it is possible to manufacture neutral recycled soil.

[0054] (5) The recycled soil is neutral and of high quality The recycled soil shows neutrality with a pH of 5.8 - 8.6, and the cone index ensures 800 kN / m 2 or more. Furthermore, it can be finished into high-quality recycled soil that does not easily remudify.

[0055] In the invention of this embodiment, the case of neutralizing construction sludge to produce neutral recycled soil is exemplified. However, the present invention can also be applied when using construction-generated soil that is secondarily generated during construction work instead of construction sludge.

[0056] In the present invention, as a result of conducting a neutralization test in which low-concentration carbon dioxide of 10% or less is reacted to achieve neutralization of recycled soil by low-concentration CO2, the following new findings were obtained (details are shown in the examples). 1) When the addition amount of the PS ash-based modifier is increased, the neutralization time is drastically shortened. 2) The smaller the dry density (the larger the gap) of the modified soil, the shorter the neutralization time. 3) The finer the particle size of the modified soil, the shorter the neutralization time. 4) Even when the CO2 concentration is 10% or less, if the conditions of 1) to 3) above are met, neutralization can be achieved within 24 hours (as short as several hours) of contact time. 5) The smaller the CO2 flow rate, the longer the neutralization time, but the higher the CO2 immobilization rate. The above findings are the basis for the present invention to require the steps of steps S1 to 6 described above.

Examples

[0057] Next, examples of experiments conducted using the invention according to the embodiment will be described. [Example 1] Neutralization promoting effect of PS ash-based modifier Initial water content ratio w = 1.0w L of blue clay (liquid limit w L = 40.7%, plastic limit w P = 23.7%), blast furnace cement type B was added at a dry mass ratio of 3% and 6% to prepare simulated sludge. For the sample with 3% cement added, a PS ash-based modifier was further added in an amount of 0 to 20% based on the dry mass of the blue clay to prepare each sample. The prepared samples were sealed and cured for 5 days, and for the sample without the PS ash-based modifier added, soil particles of 9.5 mm or more were removed using a sieve.

[0058] Each of the prepared samples was gently packed into a plastic cup with a diameter of 82 mm to a layer thickness of 30 mm to serve as a specimen. For each sample, 10 specimens were prepared, placed in an incubator as shown in Figure 2, and cured under conditions with different CO2 concentrations to carry out neutralization. In this test, specimens were taken out one by one from the incubator at arbitrary elapsed times, and pH measurements were taken based on the standards of the Japanese Geotechnical Society. In this experiment, the time point when the pH dropped below 8.6 was regarded as the completion of neutralization, and the neutralization completion time t N was defined as such.

[0059] Figure 3 shows the relationship between the neutralization completion time t N and the CO2 concentration for each blended sample prepared. In any blending condition, as the CO2 concentration decreases, t N tends to become longer. However, in the cases where 10% or 20% of the PS ash-based modifier was added, the neutralization completion time was significantly shorter compared to the case without addition. Also, in the range where the CO2 concentration is 2 - 10%, the neutralization completion time is within 24 hours for all cases where the PS ash-based modifier was added. From the above, it was found that adding the PS ash-based modifier is extremely effective in neutralization by low-concentration CO2 gas.

[0060] [Example 2] Influence of density w = 1.0w L to the blue clay, blast furnace cement type B and the PS ash-based modifier were added at dry mass ratios of 3% and 20% respectively to prepare an alkaline simulated sludge. Specimens (dry density ρ d = 0.74, 0.93, 1.16 g / cm 3 ) were prepared by varying the packing method to fill a cup with a diameter of 82 mm up to h = 30 mm, and were cured and neutralized in an incubator with a CO2 concentration of 10%. Approximately 10 specimens were prepared for each density, taken out sequentially during neutralization, and pH measurements were taken. The results are shown in Figure 4. For the case with a smaller dry density ρ d , the time for neutralization was shorter. It was confirmed in this test that the greater the air permeability by increasing the gap, the higher the efficiency of neutralization by CO2.

[0061] [Example 3] Influence of particle size For Kasaoka clay with an initial water content ratio w of approximately 30, 40, and 50% (relatively sticky clay, liquid limit w L = 60.4%, plastic limit w P = 26.0%), blast furnace cement type B and PS ash-based modifiers were each added at a dry mass ratio of 3% and 10% to prepare alkaline sludge. The sample was loosened and sieved, and the samples passing through each sieve were used for incubation curing at a CO2 concentration of 10% in an incubator to be neutralized. The results are shown in Fig. 5. It was confirmed that the smaller the particle size of the modified soil, the shorter the time t N required for neutralization completion.

[0062] [Example 4] Influence of CO2 flow rate For blue clay with w = 1.0w L , blast furnace cement type B and PS ash-based modifiers were each added at a dry mass ratio of 3% and 20% to prepare alkaline sludge. Using this sludge, a 1 L specimen with φ = 100 mm and h = 127 mm (dry density ρ d = 0.75 - 0.77 g / cm 3 ) was permeated with CO2 gas at a concentration of 10% at a flow rate Q of 0.5, 1.0, and 1.5 L / min for neutralization (see Fig. 6). In the test, the CO2 concentration ratio C out / C in measured at the inlet and outlet was used to evaluate the neutralization time and the fixed CO2 amount. As the flow rate Q increases, it can be seen from Fig. 7 that the neutralization time t out / C in required to reach C EON = 1.0 becomes shorter. Also, the CO2 immobilization rate F (%) with respect to the inflowing CO2 was determined, and the result of organizing F with respect to Q is shown in Fig. 8. It was found that the CO2 immobilization rate F decreases as Q increases.

[0063] Therefore, in order to shorten the neutralization time, it is advisable to increase the CO2 flow rate. On the other hand, in order to increase the immobilization rate of CO2 in the soil, it is important to decrease the flow rate. Similar to cooking rice, by adjusting the flow rate, the alkaline soil can be efficiently neutralized by CO2.

[0064] From the above embodiments, it was confirmed that even with strongly alkaline construction sludge, it is possible to surely lower the pH and produce high-quality neutral recycled soil.

Explanation of Signs

[0065] 2 pits 4 inlet pipes 8 blowers 10 40 mm oversize material 12 40 - 20 mm material 14 20 - 10 mm material 16 10 mm undersize material

Claims

1. A modification and solidification step of adding a PS (paper sludge) ash-based modifier to construction sludge or construction-generated soil, mixing them, and producing modified and solidified treated soil; A loosening and granulation step of loosening the modified and solidified treated soil and granulating it to produce loosened and granulated modified soil; A classification step of sieving and classifying the loosened and granulated modified soil to produce classified soil; A laying step of laying the classified soil in layers from the bottom in descending order of particle size at the bottom of a pit where single-grain crushed stone is not laid; A neutralization step of supplying low-concentration carbon dioxide gas with a concentration of 10% or less to the bottom of the pit to neutralize the classified soil; A scooping step of scooping the neutralized classified soil with a particle size of less than 20 mm; A method for producing neutral recycled soil using low-concentration carbon dioxide gas, comprising: The classified soil with a particle size of 20 mm or more that has undergone the neutralization step in a plurality of cycles is crushed to a particle size of less than 20 mm, and then mixed with the neutralized classified soil with a particle size of less than 20 mm in the scooping step and reused as neutral recycled soil. A method for producing neutral recycled soil using low-concentration carbon dioxide gas, characterized in that.

2. The classified soil laid in the uppermost layer in the laying step has a particle size of less than 10 mm, The classified soil laid in the lowermost layer has a particle size of 40 mm or more and less than 75 mm. The method for producing neutral recycled soil using low-concentration carbon dioxide gas according to Claim 1.

3. The neutralization step is as follows: When assuming the volume of the classified soil laid in the pit in the laying step as V, An initial supply step of supplying the low-concentration carbon dioxide gas at a volume flow rate of 5V to 20V for 0.1 to 1 hour from the start of supply; A later supply step of supplying the low-concentration carbon dioxide gas at a volume flow rate of 8V to 40V after the low-concentration carbon dioxide gas fills the pit; A middle supply step of supplying the low-concentration carbon dioxide gas at a volume flow rate of 80V to 320V between the initial supply step and the later supply step; Including, The total supply time of the low-concentration carbon dioxide gas is 4 to 24 hours. The method for producing neutral recycled soil using low-concentration carbon dioxide gas according to Claim 1 or 2.

4. The modified and solidified treated soil exhibits strong alkalinity with a pH of 11 or more. The method for producing neutral recycled soil using low-concentration carbon dioxide gas according to any one of Claims 1 to 3.

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