JP2023176819A - Decanter type centrifugal separator, classification point setting method, and classification processing method - Google Patents

Abstract

To provide a method for setting a classification point of a decanter type centrifugal separator capable of reducing solid particle content having a particle size of less than 75 μm to be finally disposed using neither additive nor a filtering device.SOLUTION: A decanter type centrifugal separator 10 acquires a fine particle fraction mixing ratio and a coarse particle fraction recovery ratio from a particle size distribution of a fine particle fraction and a coarse particle fraction obtained by separating particles in a raw liquid 130 with a set particle size being a reference. A classification point of the decanter type centrifugal separator is set to a particle size of less than 75 μm by adjusting at least a height of a weir among rotation speed of an outer body bowl, differential rotation speed, throughput of the raw liquid of the decanter type centrifugal separator per unit time, and the height of the weir so as to further decrease the fine particle fraction mixing ratio and further increase the coarse particle fraction recovery ratio.SELECTED DRAWING: Figure 1

Description

There is an application for exception to loss of novelty.

The present invention relates to a centrifugal separation technique for classifying solid particles contained in wastewater and the like.

Patent Document 1 discloses a waste liquid treatment method in which an organic flocculant is added when waste liquid is separated into solid and liquid using a decanter type centrifugal separator, thereby flocculating suspended substances contained in separated water.

In addition, it is generally known that the smaller the size of soil particles, the higher the concentration of contaminants; for example, it is known that radioactive cesium is mainly adsorbed and fixed on fine clay particles. Based on this knowledge, several methods have been proposed to decontaminate soil containing radioactive materials.

For example, Patent Document 2 discloses a classification cleaning treatment method based on the knowledge that radioactive cesium is adsorbed and accumulated in relatively large amounts on soil particles smaller than 75 μm. According to Patent Document 2, by adding a swelling inhibitor to contaminated soil at the start of classification and cleaning of contaminated soil containing a polymeric water-absorbing resin, the amount of cesium that is relatively adsorbed to soil particles less than 75 μm is reduced. can be done.

Furthermore, according to the method for cleaning radioactively contaminated soil disclosed in Patent Document 3, sludge is separated into liquid and slurry with a particle size of 0.075 mm (75 μm) to 5 mm using a cyclone separator, and the slurry is subjected to decanter type centrifugation. The water is dehydrated by the device, and the water and the liquid from the cyclone separator are filtered by the filtration device. This separates fine soil with a particle size of less than 0.075 mm that contains high concentrations of radioactive cesium.

JP 2017-047358 Publication JP2020-163265A JP 2016-045113 Publication

However, in the above-mentioned Patent Documents 1 and 2, it is necessary to add an organic flocculant, a swelling inhibitor, etc. in order to separate the fine particles, which increases and complicates the processing steps. Further, in Patent Document 3, a filtration device is required to separate soil particles having a particle size of less than 0.075 mm, which results in an increase in the number of installations and a complicated configuration.

Furthermore, in Patent Documents 2 and 3, soil particles with a particle size of less than 75 μm are considered to contain highly concentrated radioactive cesium and are disposed of as final disposal, so they are not used as recycled soil, and therefore it is difficult to reduce the volume of the final disposal soil. Unachievable.

Therefore, the purpose of the present invention is to provide a decanter-type centrifugal separator capable of reducing the volume of solid particles with a particle size of less than 75 μm for final disposal without using additives or filtration devices, a method for setting the classification point thereof, and a method for setting the classification point. The purpose is to provide a processing method.

According to one aspect of the present invention, an outer bowl includes an inner wall that is rotatably supported around a rotation axis and extends in the direction of the rotation axis, and a side wall that crosses the rotation axis; an inner barrel screw rotatably supported around a shaft; a drive mechanism that rotates the outer barrel bowl and the inner barrel screw at different rotational speeds; and an adjustment mechanism that adjusts the distance between the inner wall of the outer bowl and the plurality of liquid discharge ports as a weir height, and a A classification point setting method for a decanter-type centrifugal separator that separates a stock solution containing a predetermined particle size, the method comprising: rotating the outer bowl and the inner screw to separate the particles in the stock solution into a preset particle size; Separate into fine particles and coarse particles as a standard, and from the particle size distribution of the separated fine particles and coarse particles, calculate the mixing ratio of fine particles in the coarse particles and the coarse particles in the stock solution. of the outer body bowl so as to further reduce the fine particle mixing rate and increase the coarse particle recovery rate. By adjusting at least the height of the weir among the rotational speed, the differential rotational speed between the outer bowl and the inner screw, and the height of the weir, the classification point of the decanter type centrifugal separator can be set to a particle size of less than 75 μm. It is characterized by being set to .

By adjusting at least the rotational speed of the outer bowl, the differential rotational speed between the outer bowl and the inner screw, and the height of the weir, the classification point can be set to a particle size of less than 75 μm, and the addition It is possible to reduce the volume of fine particles with a particle size of less than 75 μm for final disposal without using agents or filtration equipment.

According to one aspect of the present invention, it is preferable that the rotational speed of the outer bowl is set to a centrifugal force that causes a difference in sedimentation speed between coarse particles and fine particles of the stock solution. With this setting, it is possible to avoid a situation where the fine particles settle together with the coarse particles due to strong centrifugal force when the rotational speed of the outer body bowl is high, making it impossible to separate the coarse particles and the fine particles.

According to one aspect of the present invention, an outer bowl includes an inner wall that is rotatably supported around a rotation axis and extends in the direction of the rotation axis, and a side wall that crosses the rotation axis; It has an inner barrel screw rotatably supported around a shaft, and a drive mechanism that rotates the outer barrel bowl and the inner barrel screw at different rotational speeds, and sets a stock solution containing particles with different particle sizes. The decanter-type centrifugal separator separates particles according to their particle size, and the outer bowl has a plurality of liquid discharge ports formed at equal intervals on the side wall at positions equidistant from the rotation axis, The distance between the inner wall of the body bowl and the plurality of liquid discharge ports is the height of a weir, and the side wall of the outer body bowl has an adjustment mechanism for adjusting the height of the weir, and the outer body bowl and the inner body bowl have an adjustment mechanism for adjusting the height of the weir. The present invention is characterized in that the height of the weir is adjusted by the adjustment mechanism while the rotational speed of the barrel screw is set to a predetermined value, and the classification point is set to a particle size of less than 75 μm.

By adjusting the height of the weir, the classification point can be set to a particle size of less than 75 μm, and the volume of fine particles less than 75 μm for final disposal can be reduced without using additives or filtration equipment. It can be achieved.

The adjustment mechanism includes a plurality of through holes provided in the side wall of the outer body bowl at positions corresponding to the plurality of liquid ejection ports, respectively, and a plurality of through holes provided in positions corresponding to the plurality of liquid ejection ports, respectively, at positions offset from a center point. It is preferable to include a plurality of circular plates, and a fixture for removably fixing each of the circular plates to the side wall in a desired direction so that the liquid discharge port and the through hole overlap. The height of the weir can be easily changed by simply changing the orientation of each circular plate.

The outer barrel bowl and the inner barrel screw are rotated to separate the particles in the stock solution into fine particles and coarse particles based on a set particle size, and the separated fine particles and coarse particles are separated. From the particle size distribution of At least one of the rotational speed of the outer bowl, the differential rotational speed between the outer bowl and the inner screw, and the height of the weir is configured to further reduce the coarse particle mixing rate and increase the coarse particle collection rate. By adjusting the height of the weir, the classification point can be set to a desired particle size of less than 75 μm. Therefore, it is possible to efficiently concentrate heavy metals and radioactive cesium without using additives or filtration equipment, and it is possible to recycle coarse particles with a particle size larger than the classification point.

According to one aspect of the present invention, the classification point of the decanter centrifugal separator is set to a particle size of less than 75 μm using the classification point setting method described above, and the classified coarse particles are washed and classified multiple times. It is characterized by carrying out. This allows heavy metals and cesium to be concentrated in the fine particles below the classification point, making it possible to newly increase the recycling of the fraction from the classification point to 75 μm.

According to the present invention, it is possible to reduce the volume of solid particles having a particle size of less than 75 μm and to be finally disposed of without using additives or filtration equipment. For example, radioactive cesium is adsorbed in large quantities in fine clay particles, so among particles with a particle size of less than 75 μm, we separate the fine particles, which are less than the particle size of the classification point, and the coarse particles, which are larger than the particle size of the classification point. By using the remaining soil particles as recycled materials, it is possible to reduce the volume of soil particles for final disposal.

FIG. 1 is a schematic cross-sectional configuration diagram of a decanter-type centrifugal separator according to an embodiment of the present invention. FIG. 2 is a schematic diagram showing an example of the distribution of coarse particles and fine particles in the decanter centrifugal separator according to this embodiment. FIG. 3 is a perspective view of an orifice plate illustrating a method of adjusting the height of the weir in the decanter centrifugal separator according to this embodiment. FIG. 4 is a schematic diagram showing an example of the distribution of coarse particles and fine particles when the centrifugal force in the decanter centrifugal separator is large (A) and small (B). FIG. 5 is a schematic diagram showing the height of the weir and an example of the distribution of coarse particles and fine particles for explaining the classification principle of the decanter centrifugal separator according to this embodiment. FIG. 6 is a flowchart showing an example of a procedure for determining optimal conditions for centrifugation in the decanter centrifugal separator according to this embodiment. FIG. 7 is a schematic diagram of a soil treatment system including a decanter-type centrifugal separator according to an embodiment of the present invention. FIG. 8 is a schematic diagram showing the mass balance of the total amount of non-radioactive cesium before and after classification in this example. FIG. 9 is a flowchart showing an example of the test soil preparation procedure used in this example. FIG. 10 is a graph in which the coarse particle recovery rate and the fine particle mixing rate in this example were measured under different conditions. FIG. 11 is a flowchart showing an example of the procedure of this test using the decanter type centrifugal separator according to this example.

<Overview of embodiment>
The decanter type centrifugal separator according to an embodiment of the present invention sets the classification point to a particle size finer than the usual 75 μm by adjusting the weir height, main body rotation speed, and differential speed rotation speed, which will be described later. Therefore, without using additives or filtration equipment, just by setting the decanter type centrifugal separator, fine particles with a particle size less than the particle size of the set classification point and coarse particles with a particle size larger than the particle size of the classification point can be separated. be able to.

Here, the height of the weir refers to the distance from the inner wall surface of the outer bowl of the decanter type centrifugal separator to the liquid discharge port. The main body rotation speed refers to the rotation speed of the outer bowl. The differential rotational speed refers to the difference in rotational speed between the outer barrel bowl and the inner barrel screw. The classification point can be set to a particle size of less than 75 μm by adjusting the weir height, body rotation speed, and differential speed rotation speed.

Conventionally, fractions smaller than 75 μm were ultimately disposed of, but the decanter-type centrifugal separator according to the present embodiment can set the particle size smaller than 75 μm as the classification point. For this purpose, by repeating washing and classification multiple times, it becomes possible to recycle the solid particles in the particle size fraction of 75 μm at the classification point.

Hereinafter, the structure and operation of the decanter-type centrifugal separator according to this embodiment, and the method for setting classification points will be described in detail. Note that the cross-sectional shape of the outer barrel bowl, the shape of the inner barrel screw, etc. shown in FIGS. 1 to 3 are illustrated in a simplified manner for the purpose of explanation.

1. Decanter-type centrifugal separator As schematically shown in FIGS. 1 and 2, the decanter-type centrifuge 10 according to the present embodiment includes an outer bowl 101 and an inner screw 102 that are rotatable about a rotating shaft 11 as a common center. has. The outer bowl 101 is rotatable about a rotation shaft 11 by a pulley 103 rotated by a motor (not shown). A rotating shaft 101R of the outer bowl 101 is rotatably supported by a pair of bearing parts 104. The inner barrel screw 102 is rotatably supported within the outer barrel bowl 101 by a support member (not shown). One end of the rotating shaft 102R of the inner barrel screw 102 is connected to a differential speed generator 105, and rotates at a different speed from that of the outer barrel bowl 101. The differential speed generator 105 may be a differential speed motor or a gearbox that changes the speed of the rotation of the outer bowl 101.

The rotating shaft 102R of the inner screw 102 is provided with a feed pipe 106 to which the undiluted sludge solution 130 is supplied, and the centrifugal force caused by the rotation of the inner screw 102 supplies the undiluted solution 130 to the inner wall surface of the outer bowl 101. A plurality of liquid discharge ports 110 are provided on a side wall 111 of the outer bowl 101 perpendicular to the rotation axis 101R at regular intervals, that is, at regular angles on the circumference around the rotation axis 11. Therefore, as illustrated in FIG. 2, the plurality of liquid discharge ports 110 are arranged at a constant distance from the inner wall of the outer body bowl 101, and the distance from the inner wall to the liquid discharge ports 110 is the weir height 111w.

As shown in FIG. 2, when the weir height 111w is high, the water depth h1 of the inner wall of the rotating outer bowl 101 becomes deep, and when the weir height 111w is low, the water depth h1 becomes shallow. The separated liquid is discharged to the outside from the liquid discharge port 110 over a weir with a height of 111w.

Note that the outer barrel bowl 101 and the inner barrel screw 102 are driven by a drive mechanism consisting of a motor and a differential speed generating section 105. That is, the motor that rotates the outer barrel bowl 101 is driven by the drive section 140, and the inner barrel screw 102 is rotationally driven by the differential speed generating section 105. The main body rotation speed of the outer bowl 101 and the differential rotation speed between the outer bowl 101 and the inner screw 102 are set by the rotation speed setting section 141.

When the decanter-type centrifugal separator 10 rotates at high speed, the centrifugal force pushes the stock solution 130 against the inner wall of the outer bowl 101, promoting the settling of solids, and solids with high specific gravity (coarse particles) quickly accumulate on the inner wall. For example, in FIG. 2, when the outer bowl 101 is rotated at high speed for a predetermined period of time, most of the coarse particles indicated by the black circles are deposited in the area surrounded by the inner wall of the outer bowl 101 and the weir of height 111w. , most of the fine particles with low specific gravity indicated by white circles remain suspended in the liquid at the water depth h1. In other words, the rotational speed and rotation duration of the outer bowl 101 are set so that the coarse particles are sufficiently deposited and the fine particles are still in the process of being deposited and in a floating state.

When the inner barrel screw 102 rotates at a predetermined differential speed with respect to the outer barrel bowl 101 in a state where coarse particles have sufficiently accumulated, the solids accumulated on the inner wall of the outer barrel bowl 101 are removed by the conveyor action of the inner barrel screw 102 as shown in FIG. The solids are transferred to the right, dehydrated, discharged from the solid discharge port 112, and discharged from the solid discharge section 121. Further, the fine particles and liquid having a small specific gravity are discharged as a separated liquid from the liquid discharge port 110 and discharged from the liquid discharge part 122.

According to this embodiment, the classification point of the fine particles discharged from the liquid discharge port 110 can be adjusted by the height 111w of the weir provided on the side wall 111. In this embodiment, an adjustment mechanism that can adjust the height 111w of the weir is provided, and this adjustment mechanism can adjust the height 111w of the weir by changing the fixed position of the orifice plate having the liquid discharge port 110. The orifice plate will be explained below with reference to FIG.

As illustrated in FIG. 3, through holes 111a larger than the liquid ejection ports 110 are provided in the side wall 111 at positions corresponding to each of the plurality of liquid ejection ports 110, and an orifice plate 110a is provided to cover each of the through holes 111a. is fixed. The orifice plate 110a is made of a circular plate, and a liquid discharge port 110 is formed at a position off the center of the circular plate. Further, a plurality of fixing through holes are formed at equal intervals around the circular plate, and serial numbers (hereinafter referred to as orifice numbers) are attached to at least half the circumference of the plurality of fixing through holes. The orifice plate 110a is fixed onto the through hole 111a of the side wall 111 of the outer body bowl 101 with a plurality of bolts 110b. A mark 111b is provided on the side wall 111, and the mark 111b is used as a mark for matching the arrangement directions (orifice numbers) of the orifice plates 110a fixed to the plurality of through holes 111a. Each mark 111b is arranged at the same position in each through hole 111a with reference to the radial direction of the outer bowl 101. For example, it is provided on a line extending in the radial direction of the outer bowl 101 from the rotating shaft 11, and at a position passing through each through hole 111a in the diametrical direction. The weir height 111w of the orifice plate 110a can be determined by fixing the same orifice number to each through hole 111a with the same orifice number matching the mark 111b. That is, by fixing the same orifice number on all orifice plates 110a to each mark 111b, the orientation of the orifice plate 110a is determined, and the distance between the off-center liquid discharge port 110 and the inner wall of the outer body bowl 101 is determined. In other words, it becomes possible to adjust the height 111w of the weir. Hereinafter, the direction in which the orifice plate 110a is fixed will be expressed as an orifice number, and the orifice number will represent the weir height 111w. Further, in the following examples, it is assumed that the height 111w of the weir decreases as the orifice number increases.

As described below, the height 111w of the weir is set so that the classification point is a particle size of less than 75 μm, in this case a particle size of 20 μm. When the stock solution 130 is supplied through the feed pipe 106 in this state, it is separated and dehydrated. Coarse particles exceeding 20 μm are discharged from the solid discharge section 121 as dehydrated solids, and fine particles of 20 μm or less are discharged as a separated liquid from the liquid discharge section 122. Each is discharged continuously.

2. Adjustment of the classification point The general method of using the decanter type centrifugal separator 10 is to increase the rotational speed (rotation speed of the main body) of the outer bowl 101, and fine-grain particles based on the set particle size under high centrifugal force. Separate the fine particles from the coarse particles. However, when classifying particles of about 20 μm as in this embodiment, if the centrifugal force is too large, most of the fine particles of less than 20 μm will settle together with the coarse particles, as shown in FIG. 4(A). Put it away. For this reason, differences in sedimentation speed due to differences in specific gravity are less likely to occur, and even fine particles are discharged to the solid discharge side. As shown in FIG. 4(B), by reducing the centrifugal force to a certain extent, it is possible to create a difference in the sedimentation speed of fine particles and coarse particles.

As can be seen from the verification data described below, the tendency is that the higher the centrifugal force is, the smaller the particle size discharged to the separated water side is, and the lower the centrifugal force is, the larger the particle size is. However, although it is possible to classify soil by adjusting the centrifugal force alone, it is not possible to reduce the proportion of fine particles mixed into the classified soil.

As illustrated in Figure 5, by rotating with a low centrifugal force, a difference in sedimentation rate is created between coarse particles exceeding 20 μm and fine particles below 20 μm, and in this state, the classification point is 20 μm. It is necessary to adjust the height 111w of the weir so that

As a specific example, it is assumed that the distribution of coarse particles and the distribution of fine particles intersect (almost separated) in the area near the weir height 111w=h2 due to the difference in sedimentation speed. do. At this time, if the weir height 111w is higher than h2 (h3>h2), most of the coarse particles will be discharged to the solid side, resulting in a high coarse particle recovery rate, but many fine particles will also be discharged to the solid side. As a result, the fine particle contamination rate also increases. Conversely, when the weir height 111w is lower than h2 (h2>h4), all fine particles are discharged to the separated liquid side and not to the solid side, so the fine particle mixing rate is low, but many coarse particles are Since the coarse particles are also discharged to the separated liquid side and the coarse particles discharged to the solid side are reduced, the coarse particle recovery rate also becomes low.

For example, let us assume that a low centrifugal force of 200 G or less causes a difference in sedimentation speed between the above-mentioned coarse particles and fine particles. In this state, depending on the weir height 111w, when the coarse particle recovery rate is high, the fine particle mixing rate also becomes high, and when the coarse particle recovery rate is low, the fine particle mixing rate also becomes low. It is desirable to have the highest coarse particle recovery rate and lowest fine particle mixing rate, so the main body rotation speed, differential speed rotation speed, and rotation speed per unit time that optimize both the coarse particle recovery rate and the fine particle mixing rate are desirable. What is necessary is to search for a combination of processing amount and weir height 111w as an optimal solution.

As mentioned above, the main body rotation speed is not high as shown in Fig. 4 (A) where most of the fine particles less than 20 μm settle together with the coarse particles, but the coarse particles and fine particles shown in Fig. 5 are Set a low main body rotation speed (for example, a low centrifugal force of 200 G or less) and a differential speed rotation speed so that the distribution is consistent with the particle content. In this distribution state, it is important to set the weir height 111w in accordance with the water depth h2 near the boundary between 20 μm or more and less than 20 μm. As a concrete image, the height of the weir is set to 111w so that fine particles less than 20 μm are discharged to the separated water side before they settle, and the contamination of fine particles less than 20 μm is minimized. The differential speed rotation speed is set so that coarse particles are moved to the solid discharge port 112.

Furthermore, the amount of mud (undiluted solution 130) processed per unit time also affects the fine particle mixing rate. The throughput affects the retention time of the muddy water inside the decanter of the decanter-type centrifugal separator 10. If the amount of treatment is small, the residence time becomes longer, so the time for fine particles to settle becomes longer, and the fine particle contamination rate increases. Conversely, when the amount of treatment is large, the residence time is shortened, so the time for fine particles to settle is shortened, and the fine particle mixing rate becomes low. Note that the main body rotation speed, differential speed rotation speed, and weir height 111w may change depending on the total amount of mud to be treated.

3. Determination of optimal conditions As illustrated in Figure 6, test soil is first prepared (step 201), followed by an orifice (height of the weir) test and its analysis and evaluation (step 202), a main body rotation speed test and its analysis. - Evaluation (step 203), differential speed rotational speed test and its analysis/evaluation (step 204), throughput test and its analysis/evaluation (step 205) are executed in order. From these test results, optimal conditions for the decanter centrifugal separator 10 are determined using the fine particle mixing rate, coarse particle recovery rate, and non-radioactive cesium reduction rate as indicators (step 206). According to this embodiment, steps 202 to 206 are repeatedly executed. That is, the initial conditions are determined in the first execution (steps 202 to 206), and the conditions are finely adjusted by sequentially repeating this process, and the optimal conditions are finally determined. In this way, if the main body rotational speed, differential speed rotational speed, processing amount per unit time, and weir height 111w are different, the coarse particle collection rate, the fine particle mixing rate, the reduction rate of non-radioactive cesium, and the How the capacity rate changes is evaluated, and based on the evaluation results, optimal setting values for the main body rotation speed, differential speed rotation speed, and weir height 111w of the decanter centrifugal separator 10 are determined.

The test to determine these optimal conditions is called a preliminary test, and the test conducted using these optimal conditions is called a main test. The processing system used in the test will be described below with reference to FIG. Although the present embodiment exemplifies the classification of soil, the present invention is not limited thereto, and can also be applied to the classification of heavy machinery and rare metals in sludge contained in factory wastewater and the like.

4. Treatment System As illustrated in FIG. 7, test soil and fresh water (described later) are added to a test soil receiving tank 301, and the mud is transferred to a desilting tank 302 using a submersible transfer pump (not shown). This transported mud is referred to as sample a. After stirring in the desilting tank 302, the slurry is transferred to a cyclone vibrating sieve 303 by a submersible transfer pump (not shown). The cyclone vibrating sieve machine 303 separates the muddy water supplied by the submersible pump from the muddy water receiving tank 305 and the muddy water from the desilting tank 302, transfers the liquid to the muddy water receiving/cleaning tank 306, and transfers the solids to a flexible container bag. (Flexible Container) 304. The classified solid contained in this flexible container 304 is sand with a size of 75 μm or more, and will hereinafter be referred to as sample e (classified soil A).

The muddy water receiving/cleaning tank 306 has a submersible pump and a submersible agitator for transferring muddy water to the muddy water receiving tank 305, and a plurality of muddy water receiving/cleaning tanks having the same structure are connected in parallel. The muddy water receiving/cleaning tank 306 receives the liquid from the cyclone vibrating sieve machine 303 and the muddy water from the muddy water receiving tank 305, and exchanges the muddy water between it and the muddy water receiving tank 305 using fresh water from a fresh water storage tank 310, which will be described later. Repeat cleaning as you go. The muddy water before cleaning at this time is designated as sample b, the eluted portion of the muddy water after cleaning is designated as sample c, and the muddy water after cleaning is designated as sample d. Note that the muddy water after washing is obtained by desilting classified soil B, which has been classified by centrifugation to be described later, with clean water and stirring it again. Sample c is the amount eluted into water, and sample d is the amount adsorbed to the soil.

In the decanter-type centrifugal separator 10 according to the present embodiment, muddy water after cleaning is supplied as a stock solution 130 from a muddy water receiving/washing tank 306 through a transfer pump P and a flow meter FM. After separation and dehydration, the decanter-type centrifugal separator 10 discharges coarse particles exceeding 20 μm as dehydrated solids to the solid receiving tank 307 . As mentioned above, this dehydrated solid is sand and silt with a size of 21 to 75 μm, and will hereinafter be referred to as sample f (classified soil B). The separated fine particles (silt and clay) of 20 μm or less are discharged to the classified mud water receiving tank 308. The separated water discharged into the classified mud water receiving tank 308 is referred to as sample g.

Thereafter, a polymer flocculant is put into the classified mud water receiving tank 308, the supernatant liquid is transferred as clean water to the solid receiving tank 307, the filtered water tank 309, and the fresh water storage tank 310, and the sediment is filtered in the filtered water tank 309. The collected water is transferred to the fresh water storage tank 310.

5. Preliminary Test The coarse particle recovery rate, fine particle contamination rate, non-radioactive cesium reduction rate, and volume reduction rate due to differences in main body rotation speed, differential speed rotation speed, and weir height 111W were analyzed for samples a to g above. The optimum conditions for the decanter centrifugal separator 10 are determined based on the evaluation.

The purpose of the preliminary test is to obtain basic data such as the weight of non-radioactive cesium in the soil after classification, the recovery rate of coarse particles, the mixing rate of fine particles, and the volume reduction rate.

<Fine particle mixing rate>
The mixing rate of fine particles less than 20 μm can be determined by performing particle size distribution analysis using laser diffraction scattering on the coarse particles of 20 μm or more that have been classified by the decanter centrifuge 10, and using the following formula. I can do it.
Fine particle mixing rate (%) =
{(Amount of particles less than 20μm after classification)/(Amount of coarse particles classified to 20μm or more)}×100

<Recovery rate of coarse particles with particle size of 20 μm or more>
The recovery rate of coarse particles with a particle size of 20 μm or more is determined from the weight of coarse particles with a particle size of 20 μm or more classified by the cyclone vibrating sieve 303 and the weight of coarse particles with a particle size of 20 μm or more classified by the decanter type centrifugal separator 10. It is determined using the following formula. Since the weight of the coarse particles of 20 μm or more classified by the cyclone vibrating sieve 303 is several m ³ of muddy water, the weight of the particles in the classified muddy water is determined from the throughput, water content, and specific gravity. This is subjected to particle size distribution analysis using a laser diffraction scattering method, and the weight is determined from the amount (%) of coarse particles of 20 μm or more.
Coarse particle recovery rate =
{(Weight of particles of 20 μm or more in classified soil B (kg-dry)/(Weight of particles of 20 μm or more in mud after washing (kg-dry))) × 100

Here, the weight of particles of 20 μm or more in muddy water after washing (kg-dry) = processing amount (m ³ ) x specific gravity of muddy water after washing x (100 - water content (%)),

Weight of particles of 20 μm or more in classified soil B (kg-dry) = Weight of particles of 20 μm or more in mud after washing (kg-dry) - Separated water amount (m ³ ) x Specific gravity of separated water x (100 - Water content (%) )) × Amount of particles larger than 20 μm (%)
It is.

<Non-radioactive cesium total reduction rate>
Figure 8 shows an example of the material balance of the total amount of non-radioactive cesium. The less the fine particles of less than 20 μm are mixed into the coarse particles of 20 μm or more, the lower the weight of non-radioactive cesium is. From this, if the contamination rate of fine particles less than 20 μm in one classification with the decanter centrifugal separator 10 is 30% or less, there will be no adhesion of non-radioactive cesium to coarse particles with a particle size of 20 μm or more. Assuming that, in the example of FIG. 8, the total non-radioactive cesium reduction rate is 91.9%. However, in this test, since the test soil is derived from agricultural land and contains humus such as organic matter, it is assumed that there is a certain amount of coarse particles of 20 μm or more and fine particles of less than 20 μm fixed to the coarse organic matter. Guessed.

In this test, the coarse particles after the first classification are washed and classified twice more, thereby gradually crushing the coarse particles of 20 μm or more and the fine particles of less than 20 μm that are stuck to the coarse particles of organic matter. It can be assumed that the total amount of non-radioactive cesium will be reduced as a result of the classification. However, since non-radioactive cesium is water-soluble and classified soil contains water, the total amount of non-radioactive cesium is evaluated by subtracting the amount of non-radioactive cesium eluted into water corresponding to the moisture content.

Total non-radioactive cesium reduction rate (%) =
{1-(Non-radioactive cesium weight after classification/Non-radioactive cesium weight before classification)}×100

The total non-radioactive cesium reduction rate evaluates how much the weight of non-radioactive cesium (Cs-133) in the classified soil is reduced compared to the weight of non-radioactive cesium in the muddy water before classification. FIG. 8 illustrates the material balance when the fine particle mixing rate is 30% and the coarse particle recovery rate is 90%. In addition, it was assumed that there was no fixation of non-radioactive cesium in coarse particles or contamination of humus such as organic matter. In this case, the total non-radioactive cesium reduction rate is calculated using the following formula.
Total non-radioactive cesium reduction rate (%) = 1-(0.081mg/1mg) x 100 = 91.9

5.1) Test implementation method As shown in Figure 9, according to the test soil preparation procedure, the cesium chloride concentration was mixed with water to make it homogeneous, and the mixture was adjusted to become muddy water with a concentration of 5% ± 1%. . Adjustments were made according to the following procedure for each condition of the preliminary test described below. Based on the same muddy water and muddy water concentration of 10% ± 2%, various samples a to g with different setting conditions were collected to analyze and evaluate the classification effect due to weir height 111W, main body rotation speed, and differential speed rotation speed. went.

<Setting conditions>
The main body rotational speed, differential speed rotational speed, and orifice number indicating the weir height 111w set in the preliminary test are as shown in Table 5-1 below.

(a) For X, adjust the rotation speed that had the highest classification effect among conditions 1 to 11.
(b) Y adjusts the differential speed rotation speed that had a high classification effect under conditions 1 to 11.
(c) For Z, the orifice No. that had the highest classification effect among conditions 1 to 10 was adopted.
(d) For Z1, adjust the orifice number that had the highest classification effect among conditions 1 to 8.

In addition, a summary of the collection locations for samples a to g in the preliminary test and main test is as shown in Table 5-2 below. Here, white circles indicate samples used in the preliminary test, and black circles indicate samples used in the main test.

Samples a to g are test soil, cyclone classified soil (classified soil A), centrifugal classified soil (classified soil B), desilting water, muddy water before washing, muddy water after washing, separated water, and concentrated soil (polymer added to separated water) One sample was taken three times from soil (to which a flocculant was added and dehydrated with a filter cloth) and mixed, and the items shown in the table below were measured and evaluated. The cyclone-classified soil (classified soil A) and centrifugal classification soil (classified soil B) were carried out separately after the start of the classification process, during the process, and after the process.

As for the cleaning method, an ejector was used. Since the performance of the ejector pump is 1.0 m3/min and the lift height is 10 m, the operating time for one wash was controlled according to the amount of muddy water in the test.
One cleaning time (min) = Test slurry volume (m ³ ) / 1.0 (m ³ /min) x 5

5.2) Measurement Results The measurement and analysis results of particle size distribution (laser diffraction scattering method) are shown in Table 5-3 below.

In the above table, the sample specimens of the test soil and classified soil A were extracted from 500 g each of the test soils used and mixed. The analysis conditions for the separated water are such that fine particles tend to aggregate more easily than the soil properties, and if the mass balance is calculated using the value without dispersion, it will be less than the actual weight of classified soil B, so internal dispersion is necessary. The analysis value obtained after 3 minutes was adopted.

The errors in the analysis values of particle size distribution (laser diffraction scattering method) are shown in Table 5-4 below. Here, under the above condition 1, samples were taken every 1.5 m ³ of processing volume to check for errors. Further, here, the test was conducted multiple times under the same conditions, and the value of n indicates the number of times the test was conducted.

The results of the moisture content analysis are shown in Table 5-5 below. Here, samples of the test soil and classified soil A were extracted from 500 g each of the test soil used and mixed.

The results of the ignition loss analysis are shown in Table 5-6 below. Here, samples of the test soil and classified soil A were extracted from 500 g each of the test soil used and mixed.

The results of the specific gravity analysis are shown in Table 5-7 below.

5.3) Analysis of preliminary test results and comprehensive evaluation A preliminary classification test will be conducted using the test soil, and the results of the evaluation of the classification effect will be summarized in a list. Regarding the setting of initial conditions, we started the test based on the conditions for reducing the specific gravity of shield mud water that is performed at general civil engineering sites, but the analysis result showed that the fine particle content of classified soil B was higher than expected. We first verified the classification effect of orifice number under conditions where the main body rotation speed was set low and the centrifugal force was set low. Next, after verifying the scraping speed based on the differential rotation speed, verification was performed using the difference in centrifugal force depending on the main body rotation speed. Finally, we adjusted each condition based on the above verification results and conducted a test assuming the main test.

(a) About the fine particle mixing rate The fine particle mixing rate is determined from the analysis results of the particle size distribution (laser diffraction scattering method) of classified soil B. The mixing rate of fine particles is shown in Table 5-8 below.

When orifice No. = 5, comparing the main body rotation speed of 1,800 rpm (900G) under condition 1 with 1,273 (450G) under condition 2, 1,273 (450G) under condition 2 showed a lower fine particle mixing rate. . When orifice No. = 6, condition 3 of 1,800 rpm (900G) showed a lower fine particle mixing rate. When comparing different orifice numbers at the same main body rotation speed, it was found that the smaller the weir height (111w) was, the lower the fine particle contamination rate was.

Therefore, since it is assumed that the difference in classification points due to the difference in specific gravity will occur more easily if the main body rotation speed is lowered, Condition 5 is determined to be near the lower limit of the standard usage range of the decanter type centrifugal separator 10 (main body rotation speed 848 rpm). The classification effect of the height of the weir of 111w was verified. Up to Condition 7, water would flow out from the solid discharge port if the treatment amount was set to 20 m ³ /h, so 12.5 m ³ /h was set as the upper limit treatment amount that would prevent water from flowing out.

As shown in FIG. 10, the results showed that the fine particle content (401) tended to decrease as the height of the weir became shallower. In addition, from condition 8, when the throughput was changed from 12.5 m ³ /h to 20 m ³ /h, the fine particle mixing rate decreased significantly, indicating that the throughput was also a necessary condition for the classification effect.

Comparing Conditions 8 and 9 using a weir height of 111w at a throughput of 20 m ³ /h, the fine particle contamination rate (401) is higher when the weir height is shallower (401), which is the same as the trend from conditions 5 to 7. has become lower.

The classification effect due to the differential rotation speed was performed at a main body rotation speed of 848 rpm (200 G) and 800 rpm (178 G). At 848 rpm (200G), the faster the differential rotation speed, the lower the contamination rate, but at 800 rpm (178G), the faster the differential rotation speed, the higher the fine particle contamination rate (401). Ta.

The classification effect of the main body rotation speed was verified by setting the scraping speed to 10 rpm, orifice number = 14, and setting the main body rotation speed to 848 rpm (200 G), 800 rpm (178 G), and 684 rpm (130 G). As a result, the lowest fine particle mixing rate (401) was shown under the condition of 800 rpm (178 G).

In addition, when orifice No. = 15 in condition 13, the amount of particles larger than 20 μm in the muddy water after washing was larger than in other test conditions, so particles smaller than 20 μm were not discharged in time, and particles smaller than 20 μm were accumulated inside. It is assumed that it was mixed in.

(b) Recovery rate of coarse particles with a particle size of 20 μm or more The recovery rate of coarse particles is calculated by determining how many particles with a size of 20 μm or more in the washed muddy water were recovered in classified soil B. The coarse particle recovery rate is shown in Table 5-9 below.

As can be seen from FIG. 10, regarding the coarse particle recovery rate (402), the higher the body rotation speed and the higher the weir height 111w (deeper the water), the better the recovery rate, but the fine particle mixing rate ( 401) is also on the rise. Since the coarse particle recovery rate (402) significantly decreased under condition 12 (main body rotation speed 684 rpm), if the main body rotation speed was too low, it became difficult to produce a difference in specific gravity, and coarse particles flowed out to the separated water side. Therefore, it is thought that the coarse particle recovery rate (402) decreased.

When orifice number = 15 in condition 13, the amount of particles larger than 20 μm in the muddy water after washing was about 20% higher than in other test conditions, so the accumulated particles larger than 20 μm could not be discharged in time and flowed out to the separated water side. As a result, it is assumed that the coarse particle recovery rate decreased. Classified soil B tended to have a higher moisture content as the fine particle content increased, and the appearance was clearly different.

(c) Total non-radioactive cesium reduction rate In the preliminary test, the test soil contained 1.0 (mg/kg dry soil) of naturally occurring Cs-133, so the total non-radioactive cesium reduction rate was evaluated based on this. Ta. The evaluation method is to measure the amount of non-radioactive cesium (Cs-133) in the washed mud water, classified soil B, and separated water under condition 11, which has the lowest fine particle contamination rate, and calculate the reduction rate based on the material balance by particle size. was evaluated.

First, the densities of organic matter and soil particles in the mass balance are as shown in Table 5-10 below, and the analysis results of the separated water that is not contaminated with particles larger than 20 μm in preliminary test 1 before and after ignition loss are as follows. I asked for more.

In addition,
Particle weight less than 20 μm after ignition loss = 100 x (1-34.3%) = 65.7
Volume of particles less than 20 μm after ignition loss = 65.7/2.56 = 25.66
Soil particle volume = 100/2.009 = 49.79
x = Volume of inorganic matter before loss on ignition - Volume of organic matter after loss on ignition = 49.79-25.66 = 24.12
y = Weight of organic matter before loss on ignition / Volume of organic matter before loss on ignition = 34.3/24.12 = 1.422
It is.

The material balance for Condition 11 is shown in Table 5-11 below.

The material balance for Condition 11 is shown in the table below.

Table 5-12 below shows the non-radioactive cesium (Cs-133) weight (kg-dry) and total amount conversion by particle size distribution of the washed mud water, classified soil B, and separated water. The amount of non-radioactive cesium (Cs-133) (kg-dry) is the concentration of non-radioactive cesium (Cs-133) in muddy water, classified soil B, and separated water after washing, according to the weight of each particle size by particle size distribution. I found it by substituting the values as follows.

5.4) Determination of processing conditions Based on the preliminary test results, analysis and evaluation described above, and the graph in Figure 10, it was found that fine particles were Condition 11 was adopted in this test because the fraction mixing rate was the lowest at 18.3% and the coarse grain fraction recovery rate was at a high level of 81%.

6. Main test 6.1) Main test and evaluation method (a) Preparation of muddy water for test
Two types of test soil (test soil 1 and test soil 2) were collected and adjusted to muddy water with a concentration of 10% ± 1% according to the test soil preparation procedure described above.

(b) Main test The main test is carried out using test mud according to the procedure shown in FIG. 11. That is, in this test, the washing and classification steps were performed three times based on the optimal conditions (here, conditions 11) obtained in the preliminary test. In addition, tests were conducted twice under the same conditions for each test using two types of test soil, each having 2 tons each.

From the test soil obtained in the main test, cyclone classified soil (classified soil A), centrifugation classified soil (classified soil B), desilting water, muddy water before washing, muddy water after washing, separated water, and concentrated soil, the same as in the preliminary test was prepared. Samples were collected according to the collection method and the items shown in the table below were measured and evaluated.

The conditions for this test are as shown in Table 6-1 below. In addition, the washing water in the second and subsequent washing classifications was adjusted so that the muddy water concentration after washing was 3% ± 1%.

The analysis results of non-radioactive cesium concentration are as follows.

The analysis results of particle size distribution (laser diffraction scattering method) are shown in Table 6-3 below.

The analysis results of moisture content are as follows.

The analysis results of loss on ignition are shown in Table 6-5 below.

The specific gravity analysis results are shown in Table 6-6 below.

The weight analysis (unit: kg) is as shown in Table 6-7 below.

(c) Analysis and evaluation of this test result From the results of this test, the fine particle contamination rate by the decanter centrifugal separator 10, the recovery rate of coarse particles with a particle size of 20 μm or more, the non-radioactive cesium concentration reduction effect, and the volume reduction rate. We analyzed this and evaluated the applicable range of soil characteristics targeted by this technology.

c. 1) Fine particle mixing rate The fine particle mixing rate is determined from the analysis results of the particle size distribution (laser diffraction scattering method) of classified soil B. The results are shown in Table 6-8 below.

The fine particle content of Test Soil 1 was higher than the preliminary test results. The cause is that the number of coarse particles of 20 μm or more has increased from the preliminary test, so the scraping could not catch up and fine particles were mixed in with the remaining coarse particles, and the coarse particles and fine particles were caused by insufficient cleaning. The coarse particles were stuck together, and while the coarse particles were being discharged with a screw, the coarse particles crumbled and the fine particles were peeled off, resulting in a high proportion of fine particles. This is also confirmed by the fact that the coarse particle content of the washed mud water decreased by nearly 20% after classification, and that the fine particle content increased by about 13% when classified soil B in Tests 1 and 2 was internally dispersed. It can be seen that there is an increase in Additionally, in the preliminary tests, the same muddy water after washing was reused repeatedly, so the heat from the pump and submersible agitator caused the water temperature to rise, which may also have been affected by the viscosity of the water.

The coarse part of the organic matter remains even after repeated classification, but the fine part is reduced in the same way as the fine part.

c. 2) Coarse particle recovery rate The coarse particle recovery rate is calculated by determining how many particles of 20 μm or more were recovered in classified soil B in the muddy water after washing. The results are as follows.

The reason why the recovery rate was lower than that in the preliminary test in the first classification of main tests 1 and 2 was the same as in the case of the fine particle mixture rate, and the fine particles were peeled off while the coarse particles were being discharged with the screw. It can be seen from the fact that the number of particles increased and the recovery rate improved from the second classification that it is important to perform classification in a state where the sticking of coarse particles and fine particles is eliminated.

In test soil 2, the clay is more aggregated than in test soil 1, and the coarse particles have decreased by about 10% even after the third classification, so it is thought that the aggregates of coarse particles are less likely to crumble compared to test soil 1. It will be done. Furthermore, since the coarse particle content of the separated water was greater than that of test soil 1, the coarse particle content that could not be discharged was flowing out.

c. 3) Reduction rate of total amount of non-radioactive cesium Non-radioactive cesium (Cs-133) was added to the muddy water after cleaning at a rate of 50 g/1 t-dry of soil, and the reduction rate of total amount of non-radioactive cesium was evaluated. The evaluation method was to measure the non-radioactive cesium (Cs-133) concentration in the muddy water after cleaning, classified soil B, and separated water, and evaluate the reduction rate based on the material balance by particle size.

Table 6-10 below shows the material balance of Test 1.

Table 6-11 below shows the material balance of Test 2.

Table 6-12 below shows the material balance of Test 3.

Table 6-13 below shows the material balance of Test 4.

The following shows the weight (kg-dry) of non-radioactive cesium (Cs-133) by particle size distribution of washed mud water, classified soil B, and separated water. The non-radioactive cesium (Cs-133) weight (kg-dry) is the non-radioactive cesium (Cs-133) concentration in muddy water, classified soil B, and separated water after washing, according to the weight of each particle size by particle size distribution. I found it by substituting the values as follows.

In addition, if the particle size distribution values of fine particle mixing rate and coarse particle recovery rate do not match the analytical value of non-radioactive cesium (Cs-133) concentration, calculations should be made based on the particle size distribution value after internal dispersion. As a result, the internal dispersion value was adopted as the value was close to the analytical value for non-radioactive cesium (Cs-133).

Table 6-14 below shows the total amount of non-radioactive cesium (Cs-133) conversion table for Test 1. The numbers in parentheses () indicate the analytical values.

Table 6-15 below shows the total amount of non-radioactive cesium (Cs-133) conversion table for Test 2. The numbers in parentheses () indicate the analytical values.

Table 6-16 below shows the total amount of non-radioactive cesium (Cs-133) conversion table for Test 3. The numbers in parentheses () indicate the analytical values.

Table 6-17 below shows the total amount of non-radioactive cesium (Cs-133) conversion table for Test 4. The numbers in parentheses () indicate the analytical values.

Table 6-18 below shows the total amount reduction rate of non-radioactive cesium (Cs-133).

Regarding the reduction rate, since the amount of non-radioactive cesium (Cs-133) added was added, it was also adsorbed to coarse particles of 20 μm or more, so the reduction rate after the first classification was low based on the preliminary test results. However, we were able to achieve a reduction rate of over 70%.

In Tests 1 and 2, when the number of classifications was increased, the concentration of non-radioactive cesium in coarse particles of 20 μm or more decreased. From this, as can be seen from the particle size distribution after passing through the decanter centrifugal separator 10, the amount of fine particles contained in the separated water after classification is greater than the amount of fine particles in the muddy water before classification. The non-radioactive cesium concentration in the coarse particles is reduced because the fine particles stuck to the surface of the coarse particles are peeled off due to friction between the particles.

In addition, in Tests 3 and 4, contrary to Tests 1 and 2, the non-radioactive cesium concentration in fine particles less than 20 μm decreased with each classification number. This is because the fine particles of clay, which were contained in large amounts in test soil 2, were broken down by passing through the decanter centrifugal separator 10, resulting in aggregates with less non-radioactive cesium attached. It is presumed that the reduction was due to an increase in the amount of fine particles inside the grains.

Table 6-19 below shows the non-radioactive cesium concentration by particle size.

(d) Identification of issues and consideration of countermeasures In this test, test soil 1 was classified twice and test soil 2 was classified once, thereby achieving a fine particle contamination rate of 30% or less.

Regarding the recovery rate of coarse particles, the recovery rate from the second time onwards is greatly improved between the first and second classification. This shows that the difference in particle size distribution between coarse particles and fine particles in muddy water before classification has a large effect on the recovery rate of coarse particles. In the muddy water before the first classification in Tests 1 and 2, the amount of particles less than 20 μm was 70-72%, while in the muddy water before the second classification it was less than 25%, and this difference was the reason for the difference in recovery rate. It is thought that it is connected to.

Another factor is that the muddy water concentration in the preliminary test was about 5%, but the muddy water concentration in the first test was 8%, and the amount of particles larger than 20 μm increased, so coarse particles could not be discharged in time and the internal It can be inferred that the recovery rate decreased from the preliminary test because the soil that had been stagnant in the water flowed out to the separated water side. In addition, in the second and subsequent classifications, the recovery rate of coarse particles improved when the slurry concentration before classification was 2 to 3%, indicating that the soil discharge rate affected the recovery rate of coarse particles. Conceivable.

In order to improve the recovery rate of coarse particles, increase the rotation speed of the differential speed motor according to the amount of coarse particles and increase the soil scraping speed to create a condition where coarse particles do not stay inside. It can be assumed that the recovery rate of coarse particles will be improved.

Regarding the total amount reduction rate of non-radioactive cesium, since the amount of non-radioactive cesium (Cs-133) added was added, non-radioactive cesium was also adsorbed to coarse particles of 20 μm or more. The average reduction rate was 80.1% for the second classification, 13.8% for the second time, and 12.0% for the third time, which was lower than the 93.9% reduction rate for the first classification in the preliminary test (Table 5-12).

In Tests 3 and 4, the average reduction rate was 90.2% in the first classification, 19.1% in the second, and 11.1% in the third.

In all tests, the lower the fine particle mixture rate, the higher the total non-radioactive cesium reduction rate, which shows that the fine particle mixture rate has a large effect on the total non-radioactive cesium reduction. Furthermore, when classified soil B was subjected to internal dispersion, the amount of particles less than 20 μm increased by about 17% at maximum in classified soil B at the first classification, and an increase of about 5% was observed from the second classification onward. The smaller the amount of particles less than 20 μm after internal dispersion, the lower the inclusion rate of fine particles and the total amount of non-radioactive cesium, so it is possible to reduce the adhesion of fine particles to coarse particles in muddy water before classification. It is thought that this will lead to an efficient reduction of the total amount of non-radioactive cesium.

In the improvement measures described below, the differential speed rotation speed was increased in test soil 2, and the test soil used in main tests 1 and 2 was reused (test soil with less adhesion of fine particles to coarse particles). , conducted the test. In addition, since non-radioactive cesium was also adsorbed in coarse particles of 20 μm or more, the difference in adsorption amount between coarse particles and fine particles was not clear. The test was conducted under the condition that 10 mg/kg-dry of cesium was added.

(e) Implementation of improvement measures Since it was not possible to achieve a recovery rate of 90% or more for coarse particles with a particle size of 20 μm or more, the following improvement measures were implemented.

e. 1) Improvement measure 1 (change of differential speed rotation speed)
Using Test Soil 2, the test was conducted again by changing the differential rotation speed from 500 rpm to 850 rpm, with the same main body rotation speed and weir height 111w. Table 6-20 below shows the material balance for improvement measure 1.

The coarse particle recovery rate and fine particle contamination rate of Improvement Measure 1, which increased the differential rotation speed, are shown in Tables 6-21 and 6-22 below in comparison with Tests 3 and 4.

As described above, improvement measure 1 improved the coarse particle recovery rate by approximately 36% compared to tests 3 and 4. Regarding the particle size distribution of separated water, the proportion of coarse particles of 20 μm or more was 23% to 25%, but with improvement measure 1, this was reduced to about 7%. This also shows that the recovery rate of coarse particles can be improved by setting the differential rotational speed according to the amount of coarse particles in muddy water. Since the fine particle mixing rate is generally close to that of Tests 3 and 4, it can be seen that the coarse particle recovery rate can be adjusted by adjusting the differential rotation speed without affecting the fine particle mixing rate.

e. 2) Improvement measure 2 (soil with less adhesion of fine particles)
The soil used in Tests 1 and 2 was reused, and the classification test was conducted three times again under the same operating conditions of the decanter centrifugal separator 10. The material balance is shown in Table 6-23 and Table 6-24.

The coarse particle recovery rate and fine particle contamination rate for improvement measure 2 are shown in Tables 6-25 and 6-26 below.

In Improvement Measure 2, the soil used in Tests 1 and 2 was reused, and as a result of internal dispersion, the particle size distribution was within 1% error for all samples. For this reason, the coarse particle recovery rate was 60% to 61% in the first classification of Tests 1 and 2, but improved to 77.7% to 79.5% in Improvement Measure 2.

Regarding the fine particle content, it was 28.2% to 33.6% in Tests 1 and 2, but improved to 19.9% to 23.7% in Improvement Measure 2. From this, it can be seen that the smaller the amount of fine particles adhering to the coarse particles, the better the recovery rate of coarse particles and the rate of mixing of fine particles.

e. 3) Improvement measure 3 (soil with increased amount of non-radioactive cesium added)
10mg/kg-dry of non-radioactive cesium was added to the separated water classified in this test 3, stirred for 3 days, and then the classified soil without the addition of non-radioactive cesium was classified under the conditions set in this test 3. A test slurry was prepared by mixing with B. The evaluation results of Improvement Measure 3 using this test mud water are shown in Tables 6-27 to 6-30 below.

In improvement measure 3, since non-radioactive cesium was added to the separated water after classification, it was possible to clarify the differences in the non-radioactive cesium concentrations of fine particles, coarse particles, and organic matter.

Even when the non-radioactive cesium concentration increased, the reduction rate was lower than in Tests 3 and 4, but the tendency for the non-radioactive cesium concentration in fine particles to decrease as the number of classifications increased was the same. .

7. Evaluation of test results As already mentioned, the general method of using the decanter centrifugal separator 10 is to increase the centrifugal force by increasing the rotational speed of the main unit to increase the fine and coarse particles based on the set particle size. Separate from grains. However, when classifying around 20 μm, when the centrifugal force is high, most of the fine particles settle together with the coarse particles, and the fine particles are discharged to the solid discharge side, resulting in a difference in sedimentation speed due to the difference in specific gravity. is unlikely to occur. As a tendency, the higher the centrifugal force, the smaller the particle size discharged to the separated water side, and the lower the centrifugal force, the larger the particle size. Therefore, although it is possible to classify soil by adjusting the centrifugal force alone, it is not possible to adjust the proportion of fine particles mixed into the classified soil. Therefore, it is necessary to create a difference in sedimentation speed with a low centrifugal force and adjust the weir height 111w.

At a low centrifugal force of 200 (G) or less, classification of about 20 μm is possible, but as the height of the weir (111w) becomes shallower, although the fine particle mixing rate decreases, the coarse particle recovery rate also tends to decrease. When setting the classification point at 20 μm, it is important to match the weir height 111w to the water depth near the boundary between 20 μm or more and less than 20 μm. In other words, the main body rotation speed, differential speed rotation speed, throughput per unit time, and weir height 111w can be set so that fine particles less than 20 μm are discharged to the separated water side before settling. is important.

The decanter type centrifugal separator 10 according to the present embodiment has a particle size finer than the normal classification point of 75 μm by setting the main body rotation speed, differential speed rotation speed, throughput per unit time, and weir height 111w. For example, the classification point can be set at a particle size of 20 μm. As a result, cesium is concentrated in fine particles less than 20 μm, making it possible to newly increase the recycling of the 20 μm to 75 μm fraction.

In the decanter type centrifugal separator 10, the differential rotation speed is a mechanism that adjusts the scraping speed of the screw for discharging the solid matter accumulated inside the decanter type centrifuge 10, but it depends on the coarse particle content of muddy water. In addition, increasing the scraping speed improves the recovery rate.

In test soil 2, as a result of increasing the differential rotation speed, the coarse particle recovery rate improved to 80%. In order to further improve the recovery rate, the operating conditions for the first, second, and third times were changed, and in the first classification, the main body rotation speed was set to 848 rpm, the weir height was set to 111W, and the orifice number was set to 13. The performance of coarse particle recovery rate and fine particle mixing rate has been improved by increasing the recovery rate and setting the main body rotation speed to 800 rpm in the second and third classification, and setting the weir height 111W to orifice No. 14. It is expected that this will be possible.

At first, we were concerned that the concentration of non-radioactive cesium would not be reduced even if there was a large amount of organic matter, but since we were able to classify organic matter such as humus, we were able to reduce the concentration of non-radioactive cesium by keeping the fine particle content low. I was able to lower it. In addition, in the second and third classification, classified soil B did not contain fine particles of 5 μm or less, and all of them were discharged to the separated water side. Table 7-1 below shows the particle size distribution of classified soil B below 20 μm.

Further, when classification is performed using the decanter type centrifugal separator 10, the coarse particles are reduced the most during the first classification, and a decrease can be seen even after the second classification. Improvement measure 2 described above results in a small reduction because the soil after three classifications is reused. From this, it is considered that the classification efficiency is improved when the decanter type centrifugal separator 10 is used as a pretreatment before classification. Table 7-2 below shows the reduction rate of coarse particles in Tests 1 to 4 and Improvement Measure 2.

The increased fine particles are thought to be due to the fine particles adhering to the surface of the coarse particles peeling off due to friction, rather than the coarse particles being broken down, as the amount of particles of 5 μm or less in particular has increased. Improvement measure 2 uses test soil that has already been classified three times, so the coarse particles are in a state after the fine particles have been peeled off. As the number of classification increases, the number of fine particles of 10 μm to 20 μm increases slightly, which indicates that the particles are gradually polished. From this, it can be seen that the decanter type centrifugal separator 10 also has a certain degree of crushing and polishing effect. Table 7-3 below shows the increase/decrease rate of fine particles.

The non-radioactive cesium concentration reduction rate is greatly influenced by the non-radioactive cesium concentration of the coarse particles, and the lower the coarse particle concentration ratio, the higher the reduction rate of the non-radioactive cesium concentration. The coarse particle concentration ratio was determined from the non-radioactive cesium concentration by particle size of classified soil B in Tables 6-19 and 6-20 for the first time of classification.
Coarse particle concentration ratio = Non-radioactive cesium concentration in coarse particles / Non-radioactive cesium concentration in fine particles

8. Consideration of recycling An example of material balance is shown in Table 8-1.

Regarding the reuse of removed soil of 20 to 75 μm, the results of the preliminary test and the main test show that if the soil is classified three times, the fine particle content can be reduced to less than 20%. Therefore, if the non-radioactive cesium concentration reduction rate is 40.2%, which is the highest based on the test results, if the radioactive cesium concentration in the mud water after cyclone classification is 13,000Bq/kg-dry or less, 8,000Bq/kg-dry It becomes below. Based on the material balance, 8,499 tons, including 6,955 tons of soil after cyclone classification and 1,544 tons of soil classified for the third time using the decanter centrifuge 10, can be reused.

The scope of application is not limited by soil type. The most effective conditions are soil with a soil particle density of 1 to 2.6 (g/cm ³ , 20°C), and a muddy water concentration of 10% or less. The radioactive cesium concentration in the target removed soil varies depending on the radioactive cesium concentration in coarse particles of 20 μm or more, so the scope of application must be confirmed in the removed soil.

9. Summary The results of this test mentioned above reveal the following.

Regarding the coarse particle recovery rate, the second and subsequent classifications are more than 35% better than the first classification, regardless of the soil quality. The reason for the improvement is that when comparing main tests 1 and 2 with the same operating conditions in test soil 1, preliminary test 11, and improvement measures, the particle size distribution of muddy water after washing was finer in main tests 1 and 2. The particle content is 70% to 71%, 89.18% in preliminary test 11, and 64 to 68% in the improvement measure.However, in preliminary test 11, the test soil was used repeatedly 11 times, and the improvement measure was obtained by using the soil classified three times. Preliminary test 11 had the highest recovery rate for coarse particles, followed by the improvement measure. From this, it is thought that the recovery rate of coarse particles is less influenced by the proportion of fine particles in the muddy water after washing and more influenced by the adhesion of coarse particles and fine particles. Another reason is that the concentration of muddy water after washing was about 9% in main tests 1 and 2, 5% in preliminary test 11, and 2 to 3% after the second classification. It is also possible that the effects of Since the coarse particle recovery rate is improved by increasing the differential speed rotation speed and raising the scraping speed when there is a large amount of coarse particles, we believe that it is necessary to set the differential speed rotation speed high in advance. It will be done.

Regarding the fine particle content, it can be reduced to 30% or less in one classification by lowering the height of the weir using a low centrifugal force of 200 G or less, but the coarse particle recovery rate also decreases. Therefore, in order to increase the recovery rate of coarse particles and lower the mixing rate of fine particles, it is considered necessary to change the operating conditions for the first classification and the operating conditions for the second and subsequent classifications. The coarse particle recovery rate can be improved by performing the first classification under the conditions of Preliminary Test 7, and from the second time onwards under the conditions of Preliminary Test 11.

Regarding the total non-radioactive cesium reduction rate, the reduction rate for the first time was reduced by about 5% by performing three classifications, but after the second classification, the coarse particle recovery rate was about 95%. Therefore, each time the number of classifications is increased, the coarse particle content decreases by about 5%. Therefore, considering the coarse particle recovery rate, it seems better to limit the number of classifications to three times or less. We were able to demonstrate that the total amount of non-radioactive cesium could be reduced by approximately 90% by classifying the classified coarse particles of 20 μm or more twice and keeping the fine particle content low. Compared to the muddy water before the first classification, the total non-radioactive cesium reduction rate was 80.1% black soil and 90.2% red soil during the first classification, 2.0% black soil and 4.0% red soil during the second classification, and 2.0% black soil during the third classification. %, red clay 0.7%. Furthermore, it was characteristic that after the second and subsequent classifications, the fine particles less than 5 μm were 0% in the coarse particles, and that organic matter such as humus could also be classified at 20 μm.

Regarding the coarse particle recovery rate, it was improved to 80% by changing the operating conditions in improvement measure 3, but it is likely that it can be increased to 80% or higher by further adjusting the operating conditions. In this test, the amount of non-radioactive cesium added was 5 mg/kg-dry, which is higher than the concentration of non-radioactive cesium that exists in nature, so non-radioactive cesium was also adsorbed to the coarse particles, resulting in non-radioactive cesium being added to the coarse particles after classification. Although the total amount of radioactive cesium has increased, if the adsorption of non-radioactive cesium to the coarse particles is small as evaluated in condition 13 of the preliminary test, the total amount of non-radioactive cesium can be reduced to 90% by performing classification three times. The degree can be reduced. However, regarding the non-radioactive cesium concentration reduction rate, according to the non-radioactive cesium concentration comparison in Table 7-4, when the non-radioactive cesium concentration ratio of coarse particles is 46.7%, once from muddy water after washing to classified soil B The reduction rate is 40% in this classification. Since the non-radioactive cesium concentration is greatly influenced by the concentration of coarse particles of 20 μm or more, it decreases as the radioactive cesium concentration of coarse particles decreases. Regarding the concentration of non-radioactive cesium in coarse particles, the higher the concentration of non-radioactive cesium in the test soil, the higher the concentration of coarse particles tends to be, and the amount of radioactive cesium contained in the removed soil is extremely small compared to the test soil. From this, it was determined that the non-radioactive cesium concentration ratio of the coarse particles could be 46.7% or less, and the non-radioactive cesium concentration reduction rate could be 40% or more, so it was possible to recycle it. In addition, the decanter-type centrifugal separator has the effect of polishing the coarse particles because each time it is classified, the coarse particles decrease and the fine particles increase, so it can be used as a pretreatment to improve classification efficiency. be able to.

The present invention is generally applicable to decanter-type centrifugal separators that separate wastewater or muddy water into fine particles and coarse particles based on a set particle size.

10 Decanter type centrifugal separator 11 Rotating shaft 101 Outer bowl 101R Rotating shaft 102 Inner screw 102R Rotating shaft 103 Pulley 104 Bearing part 105 Differential speed generating part 106 Feed pipe 110 Liquid discharge port 111 Side wall 111w Weir height 112 Solid discharge Outlet 120 Casing 121 Solid discharge section 122 Liquid discharge section 130 Stock solution 140 Drive section 141 Rotation speed setting section

Claims (7)

an outer bowl that is rotatably supported around a rotation axis and has an inner wall extending in the direction of the rotation axis and a side wall that crosses the rotation axis; an inner barrel screw, a drive mechanism that rotates the outer barrel bowl and the inner barrel screw at different rotational speeds, and a plurality of liquid discharge ports provided at equal intervals on the circumference of the side wall of the outer barrel bowl. and an adjustment mechanism that adjusts the distance between the inner wall of the outer bowl and the plurality of liquid discharge ports as a weir height, and the adjustment mechanism adjusts the distance between the inner wall of the outer body bowl and the plurality of liquid discharge ports as a height of the weir, and the adjustment mechanism adjusts the distance between the inner wall of the outer body bowl and the plurality of liquid discharge ports as a height of the weir, and the adjustment mechanism adjusts the distance between the inner wall of the outer body bowl and the plurality of liquid discharge ports as the height of the weir. A method for setting a classification point for a decanter-type centrifugal separator for separating,
rotating the outer barrel bowl and the inner barrel screw to separate particles in the stock solution into fine particles and coarse particles based on a set particle size;
From the particle size distribution of the separated fine particles and coarse particles, the mixing ratio of fine particles contained in the coarse particles and the coarse particle fraction, which is the ratio of the coarse particles after separation to the coarse particles in the stock solution, are determined. Get the recovery rate and
The rotational speed of the outer bowl, the differential rotational speed between the outer bowl and the inner screw, and the decanter centrifugation are set to further reduce the fine particle mixing rate and increase the coarse particle recovery rate. Setting the classification point of the decanter type centrifugal separator to a particle size of less than 75 μm by adjusting at least the height of the weir among the throughput of the stock solution per unit time of the device and the height of the weir;
A classification point setting method for a decanter type centrifugal separator. The decanter type centrifugal separator according to claim 1, wherein the rotational speed of the outer bowl is set to a centrifugal force that causes a difference in settling speed between coarse particles and fine particles of the stock solution. How to set the classification point of the device. 3. The method for setting a classification point for a decanter centrifugal separator according to claim 2, wherein the centrifugal force is 450G or less. an outer bowl that is rotatably supported around a rotation axis and has an inner wall extending in the direction of the rotation axis and a side wall that crosses the rotation axis; a decanter type that separates a stock solution containing particles with different particle sizes at a set particle size, and a drive mechanism that rotates the outer barrel bowl and the inner barrel screw at different rotational speeds. A centrifugal separator,
The outer bowl has a plurality of liquid discharge ports formed at equal intervals at positions equidistant from the rotation axis of the side wall, and the distance between the inner wall of the outer bowl and the plurality of liquid discharge ports is The height of the cough,
The side wall of the outer body bowl has an adjustment mechanism for adjusting the height of the weir,
The height of the weir is adjusted by the adjustment mechanism while the rotational speeds of the outer bowl and the inner screw are set to predetermined values, respectively, and the classification point is set to a particle size of less than 75 μm. Decanter type centrifugal separator. The adjustment mechanism is
a plurality of through holes provided in the side wall of the outer body bowl at positions corresponding to the plurality of liquid discharge ports, respectively;
a plurality of circular plates in which each of the plurality of liquid ejection ports is formed at a position offset from a center point;
a fixture that removably fixes each of the circular plates to the side wall in a desired direction so that the liquid discharge port and the through hole overlap;
The decanter type centrifugal separator according to claim 4, characterized in that it consists of: The outer barrel bowl and the inner barrel screw are rotated to separate the particles in the stock solution into fine particles and coarse particles based on a set particle size, and the separated fine particles and coarse particles are separated. From the particle size distribution of At least one of the rotational speed of the outer bowl, the differential rotational speed between the outer bowl and the inner screw, and the height of the weir are set so as to further reduce the coarse particle mixing rate and increase the coarse particle collection rate. The decanter type centrifugal separator according to claim 4 or 5, wherein the height of the weir is adjusted. A classification process characterized by setting the classification point of the decanter centrifugal separator by the classification point setting method according to any one of claims 1 to 3, and repeatedly washing and classifying the stock solution a plurality of times. Method.

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