JP2015110203A - Soil decontamination method and soil decontamination system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for efficiently decontaminating a heavy metal-containing excavated soil at low cost.SOLUTION: The soil decontamination method includes: an iron powder addition step of adding iron powder 3 for adsorbing heavy metals to a heavy metal-containing excavated soil 2 generated in excavating construction; and an iron powder separation step of supplying the excavated soil 2 which contains iron powder 38 with heavy metals adsorbed in the iron powder addition step to a specific gravity in water separator 20 for gravity concentration in water and separating the iron powder 38 with adsorbed heavy metals from the excavated soil 2 with the specific gravity in water separator 20.

Description

  The present invention relates to a contaminated soil purification method and a contaminated soil purification system, and more specifically to a technique that enables excavated soil containing heavy metals to be purified efficiently and at low cost.

  When heavy metal is included in the target ground for excavation work, excavated soil containing heavy metal is generated. In that case, iron powder, which is a heavy metal adsorbent, is added to the excavated soil and stirred to adsorb the heavy metal in the excavated soil to the iron powder, and this iron powder is recovered by a magnetic separator to purify the excavated soil. There are things to plan. As a purification technique for such contaminated soil, for example, a first step of adding a chemical that promotes the movement of water, iron powder, and heavy metal to the soil contaminated with heavy metal, and stirring the heavy metal in the soil on the iron powder. And the soil purification method (refer patent document 1) etc. which consist of the 2nd process which isolate | separates the iron powder which carry | supported the heavy metal in the 1st process from soil with a magnetic separator next, etc. are proposed.

JP 2000-51835 A

  However, the magnetic separator described above is expensive and has a limited processing capacity, and it is necessary to install, install, and operate multiple units in order to apply it to the processing of excavated soil generated in large quantities during excavation work. Arise. For this reason, the cost of introducing and operating the magnetic separator further increases, and it becomes necessary to secure a corresponding installation space in a limited area of the construction site, which is not practical.

  Accordingly, an object of the present invention is to provide a technique that enables the excavated soil containing heavy metals to be purified efficiently and at low cost.

  The method for purifying contaminated soil of the present invention that solves the above problems includes an iron powder adding step of adding iron powder for heavy metal adsorption to excavated soil containing heavy metal generated by excavation work, and the heavy metal is adsorbed by the iron powder adding step. Supplying the excavated soil containing the heavy metal adsorbed iron powder to an underwater specific gravity sorter for performing specific gravity sorting in water, and separating the heavy metal adsorbed iron powder from the excavated soil by the underwater specific gravity sorter; and , Including.

  According to this, the reliable iron powder fractionation processing based on the difference in specific gravity between the iron powder after heavy metal adsorption (heavy metal adsorption iron powder) and excavated soil becomes possible. Moreover, such a process can be performed at a lower cost compared to the case of using an expensive magnetic separator. In addition, the underwater specific gravity sorter has a higher processing capacity than the magnetic sorter, and even when a large amount of excavated soil is generated, there is no need to introduce a large number of units during the treatment. Therefore, by adopting such an underwater specific gravity sorter, it is possible to efficiently purify contaminated soil without occupying a limited space at the excavation site. Therefore, excavated soil containing heavy metals can be purified efficiently and at low cost.

  Further, in the above-described contaminated soil purification method, it is preferable that the method further includes a specific gravity adjustment step of adjusting the specific gravity so that the specific gravity of the mud including the excavated soil in the specific gravity sorting water tank of the underwater specific gravity sorter is a predetermined specific gravity. Is preferred.

  According to this, in order to accurately select the specific gravity in the water, it is possible to make the property of the muddy water low viscosity of a predetermined degree, and to easily separate the iron powder (heavy metal adsorbed iron powder) and the excavated soil in the water. As a result, the processing accuracy for selecting the specific gravity in water can be improved. For example, if the excavated soil contains a large amount of clay, the fluid viscosity of the mud treated by the underwater specific gravity sorter can be increased, and a good iron powder separation process is possible.

  Further, in the above-described contaminated soil purification method, after excavating soil containing the heavy metal adsorbing iron powder is supplied to a centrifuge and performing the process of separating the heavy metal adsorbing iron powder from the excavated soil, the iron powder separation In the process, the heavy metal adsorbed iron powder mixed with excavated soil separated from the excavated soil by the separation process in the separation process is supplied to an underwater specific gravity sorter, and the heavy metal adsorbed iron powder is separated from the excavated soil. You may do it.

  According to this, the centrifuge performs faster iron powder fractionation processing, and the separation obtained by this centrifuge (heavy metal adsorbed iron powder mixed with some earth and sand) is accurate by the underwater specific gravity sorter. It is possible to perform a good iron powder fractionation process, and it is possible to perform an iron powder fractionation process with excellent processing efficiency and accuracy.

  Further, in the above-described contaminated soil purification method, the heavy metal adsorbed iron powder mixed with the excavated soil is supplied to the centrifuge, the heavy metal adsorbed iron powder is separated from the excavated soil, and the excavated soil obtained by the separation is obtained. A series of steps of supplying the mixed heavy metal adsorbing iron powder again to the centrifuge is performed one or more times, and then, in the iron powder separation step, the excavation is separated from the excavated soil by the centrifuge. The iron powder mixed with soil may be supplied to an underwater specific gravity sorter to separate the heavy metal adsorbed iron powder from the excavated soil.

  According to this, in addition to the effect by additionally using the above-described centrifuge, by repeatedly using iron powder, the amount of iron powder used in the iron powder addition process is suppressed, and iron powder is contained and added. It is possible to reduce the scale of various devices that perform the operation and to streamline the contaminated soil purification process.

  In the above-mentioned centrifuge, a solid mixed liquid is poured into a cylindrical container constituting the centrifuge so as to draw a vortex from the circumferential direction. A liquid having a function of colliding with the inner wall of the container for recovery and discharging the liquid from the center of the cylinder can be employed. Since the centrifugal separator having such a function, so-called cyclone, has a simple mechanism, the separation performance is generally inferior to that of a forced swirl centrifugal separator in separating sediment from muddy water. It is said that. However, when using the above-mentioned cyclone for the purpose of separating iron powder whose specific gravity is sufficiently larger than the earth and sand, it is considered that the cyclone of the forced swirl type can be used, and stable processing capacity This is suitable for a situation where a large amount of excavated soil is continuously treated.

  Moreover, in the above-mentioned contaminated soil purification method, the underwater specific gravity sorter may be a general purpose machine for soil washing.

  According to this, it is not necessary to design and order a dedicated underwater specific gravity sorter for each construction site, and it is possible to carry out iron powder separation processing using existing products, and it is possible to reduce equipment introduction costs and labor.

  In addition, the contaminated soil purification system of the present invention includes an iron powder adding device that adds iron powder for heavy metal adsorption to excavated soil containing heavy metals generated by excavation work, and the heavy metal is added by the addition operation of the iron powder adding device. Excavation soil transport device that supplies the excavated soil containing the adsorbed heavy metal adsorbed iron powder to an underwater specific gravity sorter that performs specific gravity sorting in water, an underwater specific gravity sorter that separates the heavy metal adsorbed iron powder from the excavated soil, It is characterized by providing.

  According to the present invention, excavated soil containing heavy metals can be purified efficiently and at low cost.

It is a figure which shows the whole structure containing the contaminated soil purification system of this embodiment. It is a flowchart which shows the example of a procedure of the contaminated soil purification method of this embodiment. It is a figure which shows the structural example 1 of the underwater specific gravity sorter in this embodiment. It is a figure which shows the structural example 2 of the underwater specific gravity sorter | selector in this embodiment. It is a figure which shows the procedure of the iron powder diversion frequency confirmation test in this embodiment. It is a figure which shows the specific test result (table) of the frequency | count of iron powder diversion in this embodiment. It is a figure which shows the specific test result (graph) of the iron powder diversion frequency in this embodiment. It is a figure which shows the verification experiment result (table | surface) of the contaminated soil purification method in this embodiment.

  Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a diagram illustrating an overall configuration including a contaminated soil purification system 1 according to the present embodiment, and FIG. 2 is a flowchart illustrating an exemplary procedure of the contaminated soil purification method according to the present embodiment. In this embodiment, the case where the contaminated soil purification system 1 is applied to excavation work by the muddy water type shield method will be described. Also, it is assumed that the excavated soil generated by the excavation work contains arsenic as a heavy metal.

  In the contaminated soil purification system 1 illustrated in FIG. 1, the shield machine 10 rotates the cutter head to excavate the face, and mixes the sand and excavated soil of the face cut by the cutter head with mud water. Is pumped out of the mine by a mud system 26 comprising an appropriate pump and piping. The muddy water thus pumped is sent to the solid-liquid separator 11. This solid-liquid separator 11 is a muddy water treatment device that separates muddy water and excavated soil, and the above-mentioned muddy water is extracted from excavated soil by using a vibrating sieve or the like (extracted with a vibrating sieve or the like) and muddy water. To separate.

  The construction-generated soil separated by the solid-liquid separator 11 is disposed as general residual soil. On the other hand, the above-described mud water 2 separated by the solid-liquid separator 11 is supplied to the adjustment tank 12 via a predetermined system such as a mud pipe or a mud pump. In the adjustment tank 12, the supplied mud water 2 is temporarily stored, and after component adjustment, the mud water 2 is supplied to the face via the mud feed system 25.

  On the other hand, the contaminated soil purification system 1 of the present embodiment for purifying contaminated soil includes an arsenic adsorption tank 14, a centrifuge 15, an iron powder pretreatment tank 18, an iron powder stock tank 19, an underwater specific gravity sorter 20, and an iron powder tank. 21 and a transfer system for mud and solution between them. Further, with the progress of excavation by the shield machine 10 described above, the muddy water 2 is stored in the surplus tank 13, and the muddy water 2 that exceeds a certain reference amount is stored in the arsenic adsorption tank 14 in the contaminated soil purification system 1. Shall be supplied.

  Here, in the contaminated soil purification method of this embodiment illustrated in the flow of FIG. 2, the muddy water 2 supplied from the surplus tank 13 is stored in the arsenic adsorption tank 14, and is taken out from the iron powder tank 21. A predetermined amount of iron powder 3 is charged and stirred (s101). It is assumed that a plurality of arsenic adsorption tanks 14 are arranged as shown in FIG. 1, and the same number of iron powder stock tanks 19 are arranged correspondingly. FIG. 1 illustrates a configuration in which three arsenic adsorption tanks 14A to 14C and iron powder stock tanks 19A to 19C are arranged. A plurality of iron powder stock tanks 19 may be arranged for one arsenic adsorption tank 14. The arsenic adsorption tank 14A and the iron powder stock tank 19A, the arsenic adsorption tank 14B and the iron powder stock tank 19B, and the arsenic adsorption tank 14C and the iron powder stock tank 19C are connected by predetermined piping systems 40 to 42, respectively. Moreover, each arsenic adsorption tank 14A-14C is connected with the excess tank 13 by the predetermined piping systems 43-45.

  In such a configuration, when performing the above-described step s101 (iron powder addition process), first, the arsenic adsorption tank 14A is filled with the muddy water 2 supplied from the excess tank 13, and a predetermined amount of the iron powder 3 is mixed with water. The iron powder mixed solution is put into the arsenic adsorption tank 14A, and stirring work is performed. After that, if the muddy water 2 is supplied from the surplus tank 13, the iron powder 3 is charged into the next arsenic adsorption tank 14B, the muddy water 2 supplied from the surplus tank 13 is filled with the arsenic adsorption tank 14B, and the agitation work I do. Further, if the muddy water 2 is supplied from the surplus tank 13, the iron powder 3 is put into the next arsenic adsorption tank 14C, the muddy water 2 supplied from the surplus tank 13 is filled with the arsenic adsorption tank 14C, and stirring work is performed. Do. As described above, according to the supply of the muddy water 2 from the surplus tank 13, the operations such as the introduction of the iron powder 3 and the stirring with the muddy water 2 are performed in the order of the arsenic adsorption tank 14A → the arsenic adsorption tank 14B → the arsenic adsorption tank 14C. To do.

  The amount of iron powder 3 put into the arsenic adsorption tank 14 increases as the specific gravity of the mud 2 stored in the arsenic adsorption tank 14 increases, that is, as the amount of excavated soil increases.

  It is known that iron powder has a property of efficiently adsorbing and immobilizing heavy metals such as arsenic, selenium, hexavalent chromium, cadmium, lead and cyanide. Therefore, if this iron powder is added to the mud water 2 including the excavated soil and stirred sufficiently, arsenic adhering to the excavated soil particles in the mud water 2 is adsorbed on the surface of the iron powder and is fixed to the iron powder 3. It will be. Hereinafter, the arsenic-adsorbed iron powder 3 is referred to as arsenic-adsorbed iron powder 38 (heavy metal-adsorbed iron powder).

  Subsequently, in the arsenic adsorption tank 14 described above, the stirring operation of the mud water 2 and the iron powder 3 is continued for a predetermined time, and after the mud water 2 and the iron powder 3 are sufficiently mixed, the iron powder-added mud water 4 which is this mixed liquid is used. Is supplied to the centrifugal separator 15 to separate the arsenic-adsorbed iron powder 38 from the iron powder-added mud water 4 (s102). The arsenic adsorption iron powder 38 obtained by this separation may contain earth and sand that could not be separated from the arsenic adsorption iron powder 38. Since it is re-introduced, the contaminated earth and sand are returned to the muddy water, so there is no problem. Further, the iron powder-added muddy water 4 obtained in each of the arsenic adsorption tanks 14A to 14C is centrifuged at different timings between the arsenic adsorption tanks 14A to 14C in order to prevent the arsenic adsorption iron powder 38 having different diversion frequency from being mixed. Supply to the separator 15.

  A cyclone can be employed as the centrifuge 15 described above. The cyclone throws a solid mixed liquid into the cylindrical container so as to draw a vortex from the circumferential direction, so that the arsenic adsorbed iron powder 38 (and excavated soil) having a high specific gravity is applied to the inner wall of the cylindrical container by centrifugal action. The liquid (in this case, muddy water) is recovered by colliding, and has a function of discharging from the center of the cylinder.

  Subsequently, the muddy water obtained by separating the arsenic adsorption iron powder 38 from the iron powder-added muddy water 4 by the centrifugal separator 15 such as a cyclone, that is, the muddy water 7 after collecting the iron powder is sent to the slurry tank 16, and the iron powder After collection, the fine sediment contained in the mud 7 is settled, and the sediment is subjected to predetermined volume reduction and dehydration with a press 17 to obtain a cake (s103), and healthy industrial waste that does not contain arsenic. (S104) In addition, if it is the conventional muddy-water shield construction method which does not apply the contaminated soil purification system of this invention, the muddy water discharged | emitted from the surplus tank 13 will be directly supplied to this slurry tank 16, and will pass through cake reduction and dehydration. It will be carried out as industrial waste contaminated with arsenic (hereinafter referred to as contaminated industrial waste).

  On the other hand, the separated product 5, which is the arsenic adsorbing iron powder 38 containing some earth and sand, separated and collected by the process of step s 102 described above, is accompanied by the separation operation of the arsenic adsorbing iron powder 38 in the centrifugal separator 15 described above. And continuously sent to the iron powder pretreatment water tank 18 (s105). In this case, every time the processing in the centrifugal separator 15 for the iron powder-added mud water 4 for each arsenic adsorption tank is completed, the separated products 5 derived from the arsenic adsorption tanks 14A to 14C are not mixed with each other. The separated product 5 stored in the iron powder pretreatment water tank 18 until then is supplied to any of the iron powder stock tanks 19A to 19C (s106). Moreover, in the iron powder stock tanks 19A to 19C, the separated product 5 supplied from the iron powder pretreatment water tank 18 is made into a solution by performing appropriate hydration or the like to generate the iron powder mixed solution 6 (s107).

  As a result of the above processing, when the processing in the centrifugal separator 15 for the iron powder-added mud water 4 is completed for each arsenic adsorption tank 14, the corresponding arsenic adsorption tank 14 becomes empty, and the mud water 2 newly supplied from the surplus tank 13. Can be stored. On the other hand, in the iron powder stock tanks 19A to 19C connected to the corresponding arsenic adsorption tanks 14A to 14C through a predetermined system, the iron powder mixed solution 6 for one batch of each arsenic adsorption tank 14 is stored.

  The iron powder mixed liquid 6 in the iron powder stock tanks 19A to 19C is introduced through piping into the arsenic adsorption tanks 14A to 14C in which the next mud water 2 is measured, and a new iron powder addition process s101 is started.

  Thereafter, the above-described steps s101 to s107 are repeatedly executed within a predetermined number of times based on the number of possible iron powder diversions specified in advance (s108: n to s101). In other words, it may be repeatedly executed up to the limit of the number of divertable times, or may be repeatedly executed less than that depending on the number of possible diversions.

  The arsenic adsorption iron powder 38 in the iron powder mixture 6 added to the muddy water 2 in the arsenic adsorption tank 14 has a limit on the amount of heavy metal that can be adsorbed and fixed per fixed amount, and it is repeatedly used up to this limit amount. Can continue to adsorb and immobilize heavy metals. Therefore, if the above-described separated product 5 (arsenic-adsorbed iron powder 38 containing some earth and sand) collected by the centrifugal separator 15 is repeatedly used every time step s102 is executed, the iron powder can be effectively used. Therefore, the amount of iron powder to be used can be reduced and the cost can be reduced.

  On the other hand, even if the iron powder is not diverted (regression loop at s108: n to s101 in the flow of FIG. 2 is not executed), the next step s109 is executed immediately after the execution of step s107. Good.

  In addition, when the arsenic adsorption iron powder 38 is repeatedly used as described above, the iron powder is gradually lost and the amount thereof is reduced, and the amount of the iron powder contained in the iron powder mixture 6 is less than the specified amount. The unused iron powder 3 may be added from the iron powder tank 21 by an insufficient amount.

  In this way, the above-mentioned steps s101 to s107 are repeated according to the number of times of iron powder diversion, and the above-mentioned step s102 in the final time of the number of diversions, that is, arsenic adsorption from the iron powder-added mud water 4 in the centrifugal separator 15 When the separation process of the iron powder 38 is reached (s108: y), the above-described separated product 5 generated in the centrifuge 15 in this final round is accommodated in a predetermined iron powder stock tank 19, and the iron powder stock tank 19 After being made into the iron powder mixed solution 6 along with appropriate water addition in the water, it is supplied to the muddy water specific gravity adjusting tank 22 and further hydrolyzed, and adjusted to an appropriate specific gravity suitable for an underwater specific gravity sorting process in the underwater specific gravity sorter 20. (S109). An example of this suitable specific gravity will be described later based on the results of a demonstration experiment. Further, the muddy water specific gravity adjusting tank 22 is, for example, a tank having a predetermined capacity for receiving the iron powder mixture 6 from the iron powder stock tank 19, a water supply device for supplying water into the tank, and a stirrer for the stored matter in the tank. A device having an appropriate device configuration such as a configuration including the above can be adopted as necessary.

  Next, the solution whose specific gravity is adjusted as described above is supplied to the underwater specific gravity sorter 20 (s110). In the underwater specific gravity sorter 20, the arsenic adsorbed iron powder 38 is selected and separated from the above solution having an appropriate specific gravity in the muddy water specific gravity adjusting tank 22 by an underwater specific gravity sorting process (s111).

  FIG. 3 is a diagram showing a configuration example 1 of the underwater specific gravity sorter 20 in the present embodiment. The underwater specific gravity sorter 20 in this case includes a pair of water tanks 50A and 50B for storing the solution to be treated, that is, the iron powder mixed liquid 6, and each time one water tank is filled with the iron powder mixed liquid 6, The supply destination of the iron powder mixture 6 from the iron powder stock tank 19 is switched to the other water tank. In the example of FIG. 3, the water tank 50A currently accepts the supply of the iron powder mixture 6 from the iron powder stock tank 19 described above, and the muddy water 2 (excavated soil) and the arsenic adsorbed iron that form this iron powder mixture 6 Both separation processes due to the difference in specific gravity of the powder 38 are in progress. On the other hand, the water tank 50B has already been filled with the iron powder mixture 6 and a predetermined time has elapsed, and as a result of the progress of the specific gravity selection in water, the arsenic adsorbed iron powder 38 having a large specific gravity is deposited on the bottom, and the supernatant above it. In this state, iron powder-free muddy water 39 having a specific gravity smaller than that of iron powder is stored.

  In the state illustrated in FIG. 3, as described above, the iron powder mixed liquid 6 is stored in the water tank 50A and the specific gravity selection process proceeds, while the iron powder-free muddy water 39 that is the supernatant in the water tank 50B, that is, the sludge is removed. Send to slurry tank 16. In the slurry tank 16, the fine sediment contained in the sludge sent from the water tank 50B is settled, and the sediment is subjected to predetermined volume reduction and dehydration by the press 17 to obtain a cake (s112). It is carried out as a healthy industrial waste that does not contain (s113). On the other hand, the arsenic adsorbed iron powder 38 in the water tank 50B separated and collected by the process in step s111 is taken out from the predetermined discharge port 55 and carried out as contaminated industrial waste (s114), and the process ends. .

  In addition to the configuration illustrated in FIG. 3, the underwater specific gravity sorter 20 may employ a general-purpose machine for washing earth and sand as illustrated in FIG. 4. FIG. 4 is a diagram showing a configuration example 2 of the underwater specific gravity sorter 20 in the present embodiment. As shown in FIG. 4, the underwater specific gravity sorter 20 shown here includes a water tank 50 having a triangular cross-section and a bottom surface gently rising and inclined toward the water surface, and mixing of iron powder from the iron powder stock tank 19 into the water tank 50. The iron powder mixture supply means 51 for guiding the liquid 6 is installed on the bottom surface of the water tank 50, one end of which protrudes above the water surface of the water tank 50, and above the water surface of the water tank 50 among the transport surfaces of the belt conveyor 52. Arsenic adsorbing iron powder collecting means 53 such as a scraper that scrapes off the arsenic adsorbing iron powder 38 adhering to the conveying surface at the protruding portion, and the arsenic adsorbing iron powder collecting means 53 is provided on the wall surface in the vicinity of the water surface of the water tank 50. An overflow port 54 is provided.

  In the water tank 50 supplied with the iron powder mixed liquid 6 from the iron powder mixed liquid supply means 51, the arsenic adsorbed iron powder 38 having a large specific gravity in the iron powder mixed liquid 6 settles on the belt conveyor conveyance surface at the bottom of the water tank. The belt conveyor 52 operates continuously, and the arsenic adsorbed iron powder 38 that has settled and adhered to the conveyance surface is conveyed toward the water surface of the water tank 50. Also, the arsenic adsorbed iron powder 38 that has adhered to the conveyor surface of the belt conveyor and conveyed to the surface of the water tank 50 is scraped and collected by an arsenic adsorbed iron powder collecting means 53 such as a scraper. The arsenic-adsorbed iron powder 38 collected here becomes a processing target in the above-described step s114 and is carried out as contaminated industrial waste. On the other hand, in the water tank 50, the iron powder-free muddy water 39 that is the supernatant and fills the water tank, that is, sludge, is discharged from the wall surface near the water surface of the water tank 50 through the overflow port 54 to the outside of the water tank, and sent to the slurry tank 16. After volume reduction by press 17, dehydration and cake formation (s112), it is carried out as healthy industrial waste (s113).

  In the above flow example, prior to the iron powder separation process by the underwater specific gravity sorter 20, a processing form is shown in which the prior iron powder separation process is performed by the cyclone 15 as a centrifuge. In addition to these processing forms, the process (s1) involving the centrifuge such as steps s102 to s106 in the flow of FIG. 2 is not performed, and the muddy water 2 (excavated soil) and the iron powder 3 are collected in the arsenic adsorption tank 14. The agitated iron powder-added mud water 4 may be subjected to subsequent specific processing after being subjected to specific gravity adjustment by appropriate addition of water or the like and then directly into the underwater specific gravity sorter 20. In this case, the submerged specific gravity sorter 20 is stored in the water tank 50 in the above-described step s110, and the solution to be subjected to the submerged specific gravity sorting process is the iron powder-added mud water 4.

  In the above description, the iron powder separation step by the underwater specific gravity sorter 20 is executed once. However, the present invention is not limited to this, and may be executed a plurality of times. That is, when a small amount of earth and sand is mixed in the arsenic adsorbing iron powder 38 separated by the underwater specific gravity sorter 20, the separated product is put into the underwater specific gravity sorter 20 again, and the arsenic adsorbing iron powder is more completely obtained. 38 may be separated from the earth and sand.

  Moreover, the identification method of the frequency | count of possible transfer of the iron powder 3 mentioned above is as follows. FIG. 5 is a diagram showing the procedure of the iron powder diversion frequency confirmation test in the present embodiment. That is, in the test container 30, an arsenic mixed solution 35 in which a predetermined amount of water (eg, 1 liter) and arsenic (eg, 0.5 mg) are mixed is prepared, and 4 g of iron powder 3 is added thereto for 1 hour. Allow to stir. This arsenic mixed solution 35 is assumed to be a case where 2% iron powder is added to the muddy water with a liquid-solid ratio of 5 per soil volume.

  After stirring for 1 hour, only the iron powder adsorbing arsenic, that is, the arsenic adsorbing iron powder 38 is taken out from the arsenic mixed solution 35 by filtration or the like. In the process so far, it is synonymous with the process of separating the iron powder 3 from the iron powder-added mud water 4 in the above-described step s102 or s103 being executed once. Further, the pH, EC, and arsenic concentration of the filtrate obtained by solid-liquid separation treatment such as filtration on the arsenic mixed solution 35 were measured.

  Subsequently, the iron powder 3 taken out in the first treatment described above is added to the newly prepared arsenic mixed solution 35 in the test container 30 and stirred for 1 hour, and the arsenic adsorbed iron powder 38 is mixed with the arsenic mixed solution 38 as described above. The work taken out from 35 is performed. This is synonymous with the second separation process executed in step s102. Also in this case, the pH, EC, and arsenic concentration of the filtrate were measured as described above.

  In this embodiment, such a process was repeated 10 times. In addition, as the arsenic mixed solution 35, two solutions having an arsenic concentration of 0.5 mg / l and 0.05 mg / l were prepared and used as test objects. FIG. 6 is a diagram showing a specific test result table 400 of the number of iron powder diversions in the present embodiment, and FIG. 7 is a diagram showing a specific test result graph 500 of the number of iron powder diversions in the present embodiment. As shown in the table 400 of FIG. 6 and the graph 500 of FIG. 7, when the contaminated water having an arsenic concentration of 0.05 mg / L was repeatedly treated with the same iron powder up to 10 times, the arsenic remaining in the filtrate ( As) is 0.002 mg / L or less at any time, and it is clear that the arsenic concentration can be sufficiently reduced even if the diversion is repeated 10 times. On the other hand, when the treatment with the same iron powder is repeatedly performed on contaminated water having an arsenic concentration of 0.5 mg / L, the residual arsenic concentration is 0.001 mg / L or less in the first treatment, but in the second treatment, The residual arsenic concentration was 0.080 mg / L, indicating that the arsenic concentration could not be reduced sufficiently. Considering the concentration level, one treatment for contaminated water with an arsenic concentration of 0.5 mg / L is estimated to correspond to 10 treatments for contaminated water with an arsenic concentration of 0.05 mg / L. The result of the fact that the arsenic concentration could not be sufficiently reduced in the second treatment with respect to the contaminated water of L was as follows. This indicates that the number is more than the number of times and less than 20 times. Thus, once the possible number of times of diversion is possible for contaminated water with an arsenic concentration of 0.5 mg / L, the above test is again performed on the contaminated water with an arsenic concentration of 0.05 mg / L, and the arsenic concentration can be sufficiently reduced. Determine the limit number of diversions, that is, the number of possible diversions. In addition, if the number of possible diversions of the iron powder specified in this way is 15, for example, the actual number of diversions can be determined as 12 or the like in consideration of a predetermined safety factor.

Then, the verification experiment result of the contaminated soil purification method in this embodiment is demonstrated. FIG. 8 is a diagram showing a verification experiment result (table) of the contaminated soil purification method in the present embodiment. In this demonstration experiment, first of all, in order to create a test mud, the material used in Kanagawa Prefecture was crushed with a hammer, crushed with a jaw crusher, hydrolyzed with a high-speed blanker, separated by a cyclone, and a 75 μm screen. After classification, the mud soil of 75 μm or less was put into a 5 m ³ tank, and water was added while measuring the specific gravity with a mud balance to produce muddy water having a target specific gravity of 1.05.

  Subsequently, the capacity of the muddy water in the tank was calculated from the water depth, and the dry soil weight in the tank was calculated as the soil particle density 2.65. Furthermore, 4% of iron powder was weighed with respect to the dry soil weight calculated here, and this was added and stirred to produce a test mud. In this demonstration experiment, iron powder (53NJ) having a wide particle size distribution was used, but a single particle size can also be used.

  A 1 L sample was collected from the test mud produced here with a graduated cylinder, and the composition was measured in the test room. As shown in the “initial” record in the verification test result 700 in FIG. It became 1.063, the amount of iron powder 3.88 g / l, the amount of dry soil 0.101 kg / l, and the amount of iron powder per dry soil 3.83 (%).

In a test using an underwater specific gravity sorter (a general purpose machine for washing earth and sand), first the water tank in the underwater specific gravity sorter is filled with the above test mud, and then the above test mud is supplied. After 5 minutes have passed and the flow rate has stabilized (0.05 m ³ / min, 0.1 m ³ / min), the overflow from the water tank was sampled twice as 1.2 L of sample mud, and the sample mud collected here The iron powder contained was analyzed. This analysis method is as follows.

  First, 1.0 L of the sample muddy water is measured with a graduated cylinder, and the muddy water specific gravity is calculated from the weight. Next, the soil particle density is set to 2.65, and the dry soil weight in the sample mud is calculated from the above-mentioned mud specific gravity. In addition, as a method of calculating the amount of dry soil in the muddy water from the specific gravity of the sample muddy water, for example, when the specific gravity of the muddy water is 1.2, soil volume + water volume = 1 L, soil weight + water weight = 1.2 kg. The dry soil weight is obtained by solving the simultaneous equations of water volume = water weight, soil weight = soil volume × 2.65 (soil particle density).

  Subsequently, the sample mud is stirred with a bar magnet, and the iron powder adhering to the bar magnet is collected. The iron powder collection by the bar magnet is repeatedly performed about 10 times until the iron powder does not adhere to the magnet. Next, the iron powder sampled from the sample muddy water by the rod-shaped magnet was dried in a drying furnace, and completely dried to measure its weight (iron powder weight).

The characteristics of the overflow muddy water, which is the supernatant in the water tank after the specific gravity selection process, as revealed by these measurements, are shown in the “over muddy water” record in the demonstration result 700 of FIG. When the flow rate is 0.05 m ³ / min, the amount of iron powder in the overflow mud is 355 g / m ³ (9.1% of the total iron powder amount of 3880 g / m ³ contained in the “initial” mud), The recovered iron powder amount (described as “under” in the figure) was 3525 g / m ³ (90.9% of the total iron powder amount 3880 g / m ³ contained in the “initial” mud). On the other hand, if muddy water flow rate 0.1 m ³ / min, (25.6% of the total iron content 3880g / ^{m 3} muddy water comprises the "initial") iron powder weight 995 g / ^{m 3} in the overflow mud, to The amount of iron powder recovered from the muddy water (described as “under” in the figure) was 2885 g / m ³ (74.4% of the total iron powder amount 3880 g / m ³ contained in the “initial” muddy water).

According to the verification test result 700, it can be seen that the iron powder can be separated more efficiently than the muddy water at each of the muddy water flow rates, but particularly when the muddy water flow rate is 0.05 m ³ / min.・ It can be seen that the recovery efficiency is good. Further, the specific gravity of the muddy water suitable for the underwater specific gravity sorting process in the underwater specific gravity sorter 20 is 1.05 considering that the efficiency of separation and recovery of iron powder is good in the verification test result 700 of FIG. It can be seen that the range includes ˜1.063.

In addition, among the characteristics of the overflow muddy water, those related to earth and sand are the overflow muddy water when the muddy water flow rate is 0.05 m ³ / min as shown in the record of “over muddy water” in the verification test result 700 of FIG. The amount of earth and sand collected from the muddy water is 33.0 kg / m ³ (32.7% of the total amount of earth and sand contained in the “initial” muddy water is 101 kg / m ³ ). 68.0 kg / m ³ (67.3% of 101 kg / m ³ of the total amount of sand contained in the “initial” mud). On the other hand, when the muddy water flow rate is 0.1 m ³ / min, the amount of earth and sand in the overflow muddy water is 47.5 kg / m ³ (47.0% of the total amount of earth and sand contained in the “initial” muddy water is 101 kg / m ³ ). The amount of earth and sand recovered from the muddy water (denoted as under in the figure) was 53.5 kg / m ³ (53.0% of the total amount of earth and sand contained in the “initial” muddy water of 101 kg / m ³ ). According to the verification test result 700, each muddy water flow rate resulted in the discharge of the corresponding earth and sand together with the iron powder from the muddy water.

Here, in the demonstration experiment result 700 of FIG. 8, the amount of iron powder and the amount of earth and sand indicated by the “under” record are the measured values (the amount of iron powder 3.88 g / 1 = 3880 g / m ³ , dry soil amount 101 kg / m ³ ), which is a value obtained by subtracting the measured value for “over mud”.

  In this embodiment, the system configuration and the processing method corresponding to the case where the excavated soil contains arsenic as a heavy metal have been described. However, as described above, selenium, hexavalent chromium, cadmium, lead, cyanide other than arsenic. For example, the contaminated soil purification system and the contaminated soil purification method of the present embodiment may be applied. In that case, it is necessary to take into account the sedimentation time of heavy metal adsorbed iron powder in mud water. Various conditions such as the tank depth and size, mud flow rate, etc., the amount of iron powder used, the stirring time, the number of possible diversions, etc. The value of is applied to the properties of the heavy metal as necessary.

  According to this embodiment, excavated soil containing heavy metals can be purified efficiently and at low cost. As mentioned above, although embodiment of this invention was described concretely based on the embodiment, it is not limited to this and can be variously changed in the range which does not deviate from the summary.

1 Contaminated soil purification system 2 Muddy water (excavated soil)
3 Iron powder 4 Iron powder-added mud 5 Separation product 6 Iron powder mixture 7 Mud after recovery of iron powder 10 Shield machine 11 Solid-liquid separator 12 Adjustment tank 13 Surplus tank 14 Arsenic adsorption tank 15 Cyclone 16 Slurry tank 17 Press 18 Iron powder Pretreatment tank 19 Iron powder stock tank 20 Underwater specific gravity sorter 21 Iron powder tank 22 Mud specific gravity adjustment tank 25 Mud feed system 26 Waste mud system 30 Test vessel 35 Arsenic mixed solution 38 Arsenic adsorbed iron powder 39 Iron powder free mud 40-45 Piping system 50 Water tank 51 for selecting specific gravity in water 51 Iron powder mixture supply means 52 Belt conveyor 53 Arsenic adsorption iron powder recovery means 54 Overflow port 55 Arsenic adsorption iron powder discharge port

Claims (6)

An iron powder addition process for adding iron powder for heavy metal adsorption to excavated soil containing heavy metal generated by excavation work;
Excavated soil containing heavy metal adsorbed iron powder adsorbed heavy metal by the iron powder addition step is supplied to an underwater specific gravity sorter that performs specific gravity sorting in water, and the heavy metal adsorbed iron powder is Iron powder separation process to separate from excavated soil;
Contaminated soil purification method characterized by including.   2. The contaminated soil according to claim 1, further comprising a specific gravity adjustment step of adjusting a specific gravity so that a specific gravity of the mud including the excavated soil in the specific gravity sorting water tank of the underwater specific gravity sorter is a predetermined specific gravity. Purification method. After the excavation soil containing the heavy metal adsorbed iron powder is supplied to the centrifuge and the heavy metal adsorbed iron powder is separated from the excavated soil, the centrifugal separator performs the separation in the iron powder separation step. Supplying the heavy metal adsorbed iron powder mixed with excavated soil to an underwater specific gravity sorter separated from the excavated soil in the treatment, and separating the heavy metal adsorbed iron powder from the excavated soil;
The contaminated soil purification method according to claim 1 or 2, characterized in that. The heavy metal adsorbing iron powder mixed with excavated soil is supplied to the centrifuge, the heavy metal adsorbed iron powder is separated from the excavated soil, and the heavy metal adsorbed iron powder mixed with excavated soil obtained by the separation is centrifuged. After performing a series of procedures to be supplied again to the separator one or more times, in the iron powder separation step, the iron powder mixed with excavated soil separated from the excavated soil by the centrifugal separator is screened for specific gravity in water Supplying to the machine, separating the heavy metal adsorbed iron powder from the excavated soil,
The contaminated soil purification method according to claim 3.   The contaminated soil purification method according to any one of claims 1 to 4, wherein the underwater specific gravity sorter is a general-purpose machine for soil washing. An iron powder addition device for adding iron powder for heavy metal adsorption to excavated soil containing heavy metal generated by excavation work;
Excavated soil transport device for supplying excavated soil containing heavy metal adsorbed iron powder adsorbed heavy metal by addition operation in the iron powder adding device to an underwater specific gravity sorter for performing specific gravity sorting in water;
An underwater specific gravity sorter for separating the heavy metal adsorbing iron powder from the excavated soil;
Contaminated soil purification system characterized by comprising.