JP7541449B2 - Fine aggregate manufacturing method and contaminated soil treatment system - Google Patents

Description

The present invention relates to a method for producing fine aggregate and a system for treating contaminated soil.

The smaller the soil particle size, the greater the amount of harmful substances such as radioactive materials and heavy metals that are adsorbed per unit weight. Therefore, by wet classifying small-grained silt and clay, the soil is separated into gravel and sand with reduced contamination concentrations and silt and clay with high contamination concentrations, thereby reducing the volume of contaminated soil (Patent Document 1).
In this way, contaminated soil is classified for the purpose of purification, and the purified gravel sand after classification can be used as fine aggregate for concrete.
In other words, the classification and washing equipment for contaminated soil also serves as a production facility for fine aggregate.

JP 2015-221421 A

Fine aggregate for concrete is required to have sufficient strength, and specific strength standards are set out in the Standard Specifications for Concrete. Therefore, if the purified sand does not meet the above standards, it is necessary to remove only the weak sand from the purified sand. However, there has been no method to remove only the weak sand from the purified sand, and it has been difficult to stably produce fine aggregate that meets the standards.
The present invention has been made in consideration of the above circumstances, and an object of the present invention is to provide a method for producing fine aggregate that can stably produce fine aggregate having sufficient strength to be used as fine aggregate for concrete while reducing the volume of contaminated soil, and a system for treating contaminated soil.

In order to achieve the above object, the present invention employs the following configuration.
[1] Water is added to the contaminated soil and it is crushed using a crusher.
After crushing, the coarse and fine particles are removed from the contaminated soil to obtain a slurry of gravel and sand.
The obtained gravel sand is crushed to have either an oven-dry density of less than 2.5 g/ ^cm3 or a water absorption rate of more than 3.5% by mass.
The method for producing fine aggregate comprises recovering the crushed gravel sand from which the crushed gravel sand has been removed as fine aggregate.
[2] A method for producing fine aggregate as described in [1], in which organic matter is separated from the crushed gravel and sand by sedimentation due to differences in specific gravity using an upflow separator, and then the crushed gravel and sand is removed.
[3] A method for producing fine aggregate according to [1] or [2], in which the gravel sand after the crushing is classified multiple times to obtain multiple fractions, the fraction having the smallest particle size range is removed as the crushed gravel sand, and the other fractions are recovered as fine aggregate.
[4] The method for producing fine aggregate according to [2] or [3], further comprising separating organic matter from the gravel sand fraction having the smallest particle size range among the fractions recovered as the fine aggregate.
[5] a crusher for crushing the hydrated contaminated soil;
a wet sieve for separating coarse particles from the contaminated soil after crushing by the crusher;
a pre-crushing classifier for removing fine particles from the contaminated soil after crushing by the crusher;
A crusher adjusted to crush gravel having a bone dry density of less than 2.5 g/ ^cm3 or a water absorption rate of more than 3.5 mass% contained in the slurry-like gravel obtained after processing with the wet sieve machine and the pre-crushing classifier;
A contaminated soil treatment system comprising: a post-crushing classifier that classifies the gravel and sand crushed by the crusher from the gravel and sand after treatment by the crusher.
[6] The contaminated soil treatment system described in [5], wherein the crusher is one or more selected from a two-phase flow sand washing method (Hakurijet), an attrition scrubber, a ball mill, and a rod mill.
[7] A contaminated soil treatment system as described in [5] or [6], further comprising an organic matter separator for separating organic matter from the gravel sand after the crushing.

The fine aggregate manufacturing method and contaminated soil treatment system of the present invention can stably produce fine aggregate that has sufficient strength for use as fine aggregate in concrete while reducing the volume of contaminated soil.

1 is a schematic diagram of a contaminated soil treatment system according to one embodiment of the present invention.

A contaminated soil treatment system according to one embodiment of the present invention will be described with reference to FIG.
The processing system of this embodiment comprises, in this order, a crusher 11, a wet sieve machine 12, a first classifier 21 (pre-crushing classifier), a crusher 13, an organic matter separator 14, a second classifier 22, a third classifier 23, and a fourth classifier 24 (post-crushing classifier).
In this embodiment, a coagulation sedimentation device 31 and a pressure filtering device 32 are further provided.

The crusher 11 is a device for crushing soil to which water has been added. The crusher 11 may be any device capable of crushing and loosening lumpy soil in a hydrated state, and a conventionally known crusher such as a secondary paddle mixer or a rod mill may be used.
In a twin-shaft paddle mixer, the shear force generated by the rotation of the paddles acts directly on the soil particles, so it can reliably disintegrate clay lumps without excessively grinding the gravel.

The wet sifter 12 is not particularly limited as long as it is a device that can separate foreign matter from the crushed soil and separate coarse particles such as gravel and coarse sand, but examples of the device include a vibration sifter and an ultrasonic sifter. The wet sifter 12 has a mesh surface inside, and the grain size range of the coarse particles to be left on the mesh surface can be determined by the mesh size of the mesh surface.
In this specification, the particle size is the length of one side of the smallest square mesh of a standard sieve through which a particle can pass.
By using the wet sifter 12, the soil flowing down the mesh surface can be vibrated while washing water is sprayed onto it, thereby separating the soil according to grain size and washing it.

It is preferable to use classifiers that can perform cleaning as well as classification as the first classifier 21, the second classifier 22, the third classifier 23, and the fourth classifier 24. There are dry classifiers and wet classifiers, but it is preferable to use a wet classifier because it can perform cleaning as well as classification.

Preferred classifiers include high mesh separators and wet cyclones. High mesh separators are sedimentation separators that perform separation by using the difference in sedimentation velocity caused by the difference in specific gravity and surface area of the target objects. Even with the same specific gravity, particles with a small specific surface area and large particle size will sink, while particles with a large specific surface area and small particle size will not sink easily. These can be used as classifiers by taking advantage of this property.
A wet cyclone is a classifier that creates a vortex flow and classifies materials using centrifugal force.
As the first classifier 21, a wet vibrating screen may be used instead of a classifier such as a high mesh separator or a wet cyclone.

The first classifier 21, the second classifier 22, the third classifier 23, and the fourth classifier 24 each have a different range of particle sizes to classify, and are adjusted so that relatively large particles are separated on the upstream side, and smaller particles are separated on the downstream side.
For example, in the case of a high mesh separator, the particle size range to be classified can be changed by adjusting the input slurry flow rate to change the upward flow velocity. Also, for example, when part of the slurry that has overflowed the overflow weir is returned to the high mesh separator, the return flow rate can be adjusted to change the upward flow velocity even if the input slurry flow rate is constant, thereby changing the particle size range to be classified.

The crusher 13 is a device that crushes fine aggregate (2 mm to 5 mm, or 2 mm to 10 mm) by shearing, grinding, crushing, and collision.
The crusher 13 may be any type capable of controlling its strength, and examples of the crusher 13 that can be used include a two-phase sand washing type (Hakurijet (registered trademark) manufactured by Maeda Corporation), a grinding type (attrition scrubber), a crushing type (ball mill/rod mill), a trommel with a crushing, washing and classifying mechanism (Hurricane Series (registered trademark) such as Hurricane/Sand Hurricane manufactured by Shinroku Seiki Co., Ltd., Grind Washer manufactured by Kouki Co., Ltd.), a vertical three-shaft multi-crusher (Big Bang manufactured by Fujiki Co., Ltd.), etc. The crusher 13 may be a combination of one type or two or more types of crushers.
The two-phase flow sand washing method (Hakurijet) uses a blowing device to blow slurry-like gravel and sand together with air onto a wall surface or a collision plate, applying an impact force to the gravel and sand. The position of the wall surface or collision plate onto which the gravel and sand are blown may be below or above the water surface.
An attrition scrubber is a device that uses rotating blades to collide sand particles underwater, causing them to rub against each other.

Conventionally, two-phase flow sand washing methods (Hakurijet) and attrition scrubbers have been used to remove fine particles adhering to the surface of sand particles, but in the treatment system of this embodiment, they are used as crushers to crush gravel sand, which has low strength.
The crushing strength of the two-phase flow sand washing method (Hakurijet) can be controlled by adjusting the air flow rate of the compressor used to blow the gravel sand.
The crushing strength of an attrition scrubber can be controlled by the solid-liquid ratio, rotation speed, and mixing time. The higher the solid-liquid ratio, the higher the strength. Since the tank volume is fixed, the mixing time is usually controlled by the amount of material fed per unit time.
The crushing strength of a ball mill can be controlled by the solid-liquid ratio, the size and weight of the balls, the rotation speed, etc. The crushing strength of a rod mill can be controlled by the solid-liquid ratio, the diameter and weight of the internal metal rods, the rotation speed, etc.
The crushing strength of the Hurricane Series and Grind Washer can be controlled by the clearance length between the tips of the teeth and blades attached to the outer rotating body and the inner rotating body, the solid-liquid ratio, the rotation speed, etc.
The crushing strength of the Big Bang can be controlled by the solid-liquid ratio, rotation speed, etc.

The organic matter separator 14 may be an upflow type separator.
Upflow separators are devices that separate organic matter mixed with sand by utilizing the difference in settling velocity caused by the difference in specific gravity. Examples of upflow separators include upflow columns, water separators, and high mesh separators.
When using a high mesh separator, the difference in settling speed between sand and organic matter is large due to the difference in specific gravity, so the flow rate (flow velocity) is set lower than when used as a sand classifier.

The coagulating sedimentation device 31 is a device that causes fine particles suspended in the suspension water to be coagulated and settled, and separates them from the water.
The pressure filter 32 is a device that dehydrates the fine particles that have been coagulated and precipitated, and can be, for example, a known pressure filter (belt press or filter press) equipped with a filter made of filter cloth or the like and a press.

According to the contaminated soil treatment system of the present invention, a treatment method for reducing the volume of the contaminated soil A0 can be carried out. Also, a method for producing fine aggregate that can be used as fine aggregate for concrete can be carried out.
Hereinafter, the treatment method using the contaminated soil treatment system of this embodiment, that is, the method for treating contaminated soil and the method for producing fine aggregate, will be described with reference to FIG.

The contaminated soil A0 to be treated by the treatment system of this embodiment is soil contaminated with harmful substances such as radioactive materials such as cesium and heavy metals. However, in the case of soil contaminated with radioactive materials, it is necessary to select and use soil with a contamination concentration equal to or lower than the decontamination rate as the contaminated soil A0 for producing fine aggregate that can be used as fine aggregate for concrete. The decontamination rate is a value calculated by the following formula.
Decontamination rate=1−radioactivity concentration after purification/radioactivity concentration before purification. Contaminated soil A0 is generated during decontamination work or the like, and is carried by a transport vehicle or the like to a treatment site where the treatment system of this embodiment is located.

In the treatment system of this embodiment, first, an appropriate amount of water calculated according to the weight, grain size, radioactivity concentration, etc. of the contaminated soil A0 is added to the contaminated soil A0. The amount of water added is, for example, 3 to 10 times the weight of the contaminated soil A0. The more water added, the easier it is to improve the cleaning effect. The less water added, the easier it is to make the treatment system compact.
Cations may be added to the contaminated soil A0 together with water. Addition of cations reduces the adhesion of fine particles to the gravel in the contaminated soil A0, making it easier to peel the fine particles off from the gravel.

The cation may be a divalent cation or a monovalent cation, but is preferably a divalent cation, since the divalent cation can neutralize the modifier added to prevent clogging of sieves in intermediate storage facilities and the like, facilitating the separation of fine particles.
Examples of divalent cations include alkaline earth metal ions such as magnesium ion, calcium ion, etc. Examples of monovalent cations include alkali metal ions such as potassium ion, sodium ion, etc.
The cations are introduced in the form of salts, such as magnesium chloride, calcium chloride, etc., into the water added to the contaminated soil A0.

The contaminated soil A0 to which water and, if necessary, additives such as cations have been added, is crushed in a crusher 11, and a first slurry A1 is produced in which the crushed contaminated soil A0 is mixed with water.
Next, the first slurry A1 is passed through a wet sieve 12 to separate the coarse particle fraction B into a second slurry A2.

There is no limitation on the particle size of the gravel to be removed as coarse particle fraction B, but since gravel of 5 mm or less can be used as fine aggregate for concrete, it is preferable to separate gravel with a particle size exceeding 5 mm as coarse particle fraction B.
If the contaminated soil A0 is appropriately selected, the coarse particle fraction B can be disposed of as is in accordance with waste disposal standards.
The coarse particle fraction B may be used for purposes other than as concrete aggregate, and may be treated in an organic matter separator.

When gravel with a grain size exceeding 5 mm is separated as coarse grain fraction B, the second slurry A2 from which the coarse grain fraction B has been separated contains gravel with a grain size of 2 to 5 mm, sand with a grain size of 0.075 mm or more and less than 2 mm, and silt and clay with a grain size of less than 0.075 mm.

The second slurry A2 is classified by the first classifier 21 to become a third slurry A3 from which the first fine particle fraction S1 has been removed. Note that since the fine particle fraction is also ultimately removed as the second fine particle fraction S2 described below, it does not matter if some of the fine particle fraction remains in the third slurry A3 at this stage.
Although there is no limitation on the particle size of the fine particles removed as the first fine particle fraction S1, it is preferable to remove silt and clay contained in the second slurry A2 having a particle size of less than 0.075 mm as the first fine particle fraction S1.

The reason why the first fine particle fraction S1 is mostly removed before processing in the crusher 13 is that if the fine particle fraction is left behind and processed in the next crusher 13, the fine particle fraction will act as a cushion and the crushing efficiency will decrease.
The removed first fine particle fraction S1 is considered to have a large amount of adsorbed harmful substances, so it is treated in a coagulation settling device 31 to produce a settled sludge S3, and then treated in a pressure filtration device 32 to produce a concentrated residue S4. The concentrated residue S4 is disposed of in a heat treatment facility or the like where heat treatment or chemical treatment is performed, or in a final disposal site.

When gravel with a grain size exceeding 5 mm is separated as coarse grain fraction B, the third slurry A3 from which the first fine grain fraction S1 has been separated contains gravel with a grain size of 2 to 5 mm and sand with a grain size of 0.075 mm or more and less than 2 mm. In other words, the third slurry A3 is a slurry-like gravel sand from which the coarse grain fraction and the fine grain fraction have been removed. The gravel sand in the third slurry A3 may contain a mixture of high-strength gravel sand and low-strength gravel sand.

Therefore, the low-strength gravel sand in the third slurry A3 is crushed by a crusher 13. In order to use the purified gravel sand as fine aggregate, it is required to have a sufficiently large strength. Specifically, it is required to satisfy both the conditions that the bone dry density is 2.5 g/ ^cm3 or more and the water absorption is 3.5 mass% or less (according to the Standard Specifications for Concrete).
The bone dry density and water absorption are values measured by the methods specified in JIS A 1109.

The crusher 13 of this embodiment is adjusted to crush gravel sand having an oven-dry density of less than 2.5 g/ ^cm3 or a water absorption rate of more than 3.5 mass% (hereinafter referred to as "low-strength gravel sand"). The crusher 13 crushes the low-strength gravel sand to produce crushed material with a grain size reduced to the level of silt or clay.

The higher the crushing strength of the crusher 13, the more reliably the low-strength gravel sand can be crushed, but if the crushing strength is too high, even gravel sand that can be used as fine aggregate will be crushed, and the yield of the obtained fine aggregate will decrease. In addition, the crushed gravel sand must finally be treated as the second fine particle fraction S2 in the coagulation sedimentation device 31 and the pressure filter device 32, and sufficient volume reduction will not be achieved.
Therefore, the crusher 13 of this embodiment is preferably adjusted so as not to crush gravel sand having an oven-dry density of 2.5 g/cm ³ or more and a water absorption rate of 3.5 mass % or less.

The fourth slurry A4 after being processed by the crusher 13 contains high-strength gravel having a grain size of 2 to 5 mm, high-strength sand having a grain size of 0.075 mm or more and less than 2 mm, and crushed pieces of low-strength gravel and sand. That is, the fourth slurry A4 contains crushed pieces of high-strength gravel and sand and low-strength gravel and sand.
The fourth slurry A4 is subjected to removal of organic matter by an organic matter separator 14 to produce a fifth slurry A5. The organic matter is removed because the inclusion of organic matter is undesirable when the slurry is used as fine aggregate for concrete.
The crushing process sufficiently loosens the adhesion of sand and gravel to organic matter, so that the organic matter can be easily removed by the organic matter separator 14.
When the organic matter content of the fourth slurry A4 is sufficiently small, the treatment by the organic matter separator 14 can be omitted. A sufficiently small organic matter content means that the organic matter content complies with the standard (JIS A 1105) set forth in the Standard Specifications for Concrete.

The fifth slurry A5 contains crushed high-strength gravel sand and crushed low-strength gravel sand, and organic matter has been removed. Therefore, by performing classification only once and removing the crushed low-strength gravel sand from the fifth slurry A5, fine aggregate usable as fine aggregate for concrete can be recovered.
However, in this embodiment, classification is performed multiple times (three times in this embodiment) to recover fine aggregate in multiple fractions (three in this embodiment) having different particle size ranges.
The fine aggregate is recovered in fractions to facilitate conformity with the prescribed grading standards (grading distribution) defined in the Standard Specifications for Concrete.

That is, the fifth slurry A5 is first classified by the second classifier 22 into a relatively coarse first fine aggregate P1 and a sixth slurry A6, and the recovered first fine aggregate P1 is temporarily stored in the first collection area 41.
Next, the sixth slurry A6 is classified in the third classifier 23 into a second fine aggregate P2 in the medium particle size range and a seventh slurry A7, and the recovered second fine aggregate P2 is temporarily stored in the second collection area 42.
Next, the seventh slurry A7 is classified in the fourth classifier 24 into relatively fine third fine aggregate P3 and a second fine particle fraction S2, and the recovered third fine aggregate P3 is temporarily stored in the third collection area 43.

There is no limitation on the grain size ranges of the first fine aggregate P1, the second fine aggregate P2, and the third fine aggregate P3. For example, the first fine aggregate P1 can be gravel sand with a grain size exceeding 1.2 mm, the second fine aggregate P2 can be sand with a grain size exceeding 0.3 mm and not exceeding 1.2 mm, and the third fine aggregate P3 can be sand with a grain size of 0.075 mm or more and 0.3 mm or less.
In this case, the sixth slurry A6 contains crushed high-strength gravel sand and low-strength gravel sand with a grain size of 1.2 mm or less, the seventh slurry A7 contains crushed high-strength gravel sand and low-strength gravel sand with a grain size of 0.3 mm or less, and the second fine particle fraction S2 contains crushed low-strength gravel sand at the silt or clay level with a grain size of less than 0.075 mm.

The first fine aggregate P1, second fine aggregate P2, and third fine aggregate P3, which are temporarily stored in the first collection area 41, second collection area 42, and third collection area 43, respectively, are mixed in an appropriate ratio with adjusted fine aggregate P0 to make adjusted fine aggregate P0 that conforms to the specified particle size standard (particle size distribution) defined in the Standard Specifications for Concrete.

The first fine aggregate P1, the second fine aggregate P2, and the third fine aggregate P3 have already had organic matter removed by the organic matter separator 14, but there is a concern that the organic matter has not been sufficiently removed from the third fine aggregate P3 due to its small particle size. Therefore, the third fine aggregate P3 may be further treated by the organic matter separator.

The second fine particle fraction S2 is treated in a coagulation sedimentation device 31 in the same manner as the first fine particle fraction S1 to produce a precipitated sludge S3, and then treated in a pressure filtration device 32 to produce a concentrated residue S4. The concentrated residue S4 is disposed of in a heat treatment facility or the like where heat treatment or chemical treatment is performed, or in a final disposal site.
In addition, since the second fine particle fraction S2 is presumed to have a low concentration of harmful substances, in order to improve the volume reduction rate, a separate coagulation and sedimentation device and pressurized filtration device may be provided in addition to the coagulation and sedimentation device 31 and pressurized filtration device 32, and the second fine particle fraction S2 may be treated separately from the first fine particle fraction S1 and reused.

In this embodiment, an example in which classification after crushing is performed three times has been shown, but classification after crushing may be performed once or twice. Also, classification may be performed more than three times, for example, six classifiers may be used to perform classification six times.
When classification is performed six times, it is preferable to divide the fine aggregate into six fractions, for example, 5 mm to 2.5 mm, 2.5 mm to 1.2 mm, 1.2 mm to 0.6 mm, 0.6 mm to 0.3 mm, 0.3 mm to 0.15 mm, and 0.15 mm to 0.075 mm. In this case, since the six fractions are the same as the fractions shown in the Standard Specifications for Concrete, it is easy to adjust the fine aggregate to fall within the particle size distribution standard in the adjustment field 50.

11 Crusher
12 Wet sieve machine 13 Crusher 14 Organic matter separator 21 First classifier 22 Second classifier 23 Third classifier 24 Fourth classifier 31 Coagulation sedimentation device 32 Pressurized filtration device 41 First collection site 42 Second collection site 43 Third collection site 50 Adjustment site A0 Contaminated soil A1 First slurry A2 Second slurry A3 Third slurry A4 Fourth slurry A5 Fifth slurry A6 Sixth slurry A7 Seventh slurry B Coarse particle fraction P0 Adjusted fine aggregate P1 First fine aggregate P2 Second fine aggregate P3 Third fine aggregate S1 First fine particle fraction S2 Second fine particle fraction S3 Settled sludge S4 Concentrated residue

Claims (8)

Water is added to the contaminated soil and it is crushed using a crusher.
After crushing, the coarse and fine particles are removed from the contaminated soil to obtain a slurry of gravel and sand.
The resulting slurry-like gravel sand is subjected to a crushing process with the goal of crushing low-strength gravel sand, which corresponds to either gravel sand having an oven-dry density of less than 2.5 g/ ^cm3 or gravel sand having a water absorption rate of more than 3.5 mass%, to obtain crushed material with a particle size reduced to the level of silt or clay;
A method for producing fine aggregate , comprising removing the crushed low-strength gravel sand produced by the crushing treatment and recovering the gravel sand as fine aggregate. The method for producing fine aggregate according to claim 1, wherein when water is added to the contaminated soil, cations are added together with the water and then the soil is crushed by the crusher. 3. The method for producing fine aggregate according to claim 1, wherein the crushed low-strength gravel is separated from the gravel by sedimentation due to differences in specific gravity using an upflow separator to separate organic matter, and then the crushed low-strength gravel is removed. 3. The method for producing fine aggregate according to claim 1 or 2, wherein the gravel sand after the crushing treatment is classified multiple times to obtain multiple fractions, the fraction having the smallest particle size range is removed as crushed material of the low-strength gravel sand , and the other fractions are recovered as fine aggregate. The method for producing fine aggregate according to claim 4 , further comprising separating organic matter from gravel sand of the smallest particle size range among the fractions recovered as the fine aggregate. A crusher for crushing the hydrated contaminated soil;
a wet sieve for separating coarse particles from the contaminated soil after crushing by the crusher;
a pre-crushing classifier for removing fine particles from the contaminated soil after crushing by the crusher;
A crusher adjusted for the purpose of crushing low-strength gravel sand, which corresponds to either gravel sand having an oven-dry density of less than 2.5 g/ ^cm3 or gravel sand having a water absorption rate of more than 3.5 mass% contained in the slurry- like gravel sand obtained after processing with the wet sieve machine and the pre-crushing classifier, into a crushed product with a particle size reduced to the level of silt or clay ;
A contaminated soil treatment system comprising: a post-crushing classifier that classifies the low-strength gravel crushed by the crusher from the gravel treated by the crusher. 7. The contaminated soil treatment system according to claim 6 , wherein the crusher is one or more selected from the group consisting of a two-phase flow sand washing method (Hakurijet), an attrition scrubber, a ball mill, and a rod mill. 8. The contaminated soil treatment system according to claim 6 or 7 , further comprising an organic matter separator for separating organic matter from the gravel sand after the crushing.

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