JP2020165683A - Decontaminated mud treatment method - Google Patents

Abstract

To provide a decontaminated mud treatment method which allows for more quick separation of a solid content and a water content of mud to thereby allow for reduction of the water content of the mud.SOLUTION: The decontaminated mud treatment method according to the present invention includes a dehydration promotion step of adding a dehydration accelerator including anhydrous gypsum to muddy water including decontaminated mud, and solid-liquid separation step of separating solid content and water content of the muddy water, where the anhydrous gypsum has an average particle diameter D50 in the range of 5-100 μm, and a ratio (I(020)/I(102)) of a peak intensity (I(020)) of the (020) plane to a peak intensity (I(102)) of the (102) plane, among all the peak intensities obtained by X-ray diffraction, of 5 or greater.SELECTED DRAWING: None

Description

The present invention relates to a method for treating decontaminated mud.

If there is a water area such as a dam or lake in the radioactively contaminated area, decontamination work of sediment in that water area will be carried out.
The removed soil dredged for the decontamination work of the bottom of the lake (hereinafter referred to as "decontamination mud") is a muddy substance containing a large amount of water, so it is difficult to store it as it is. .. Regarding the treatment of decontamination mud, there is a method of coagulating, then storing the coagulation decontamination mud in a storage bag, and further storing it in a concrete container (see, for example, Patent Document 1).
As a coagulant used for coagulating decontaminated mud, there is a coagulant containing gypsum powder (see, for example, Patent Documents 2, 3 and 4).

Japanese Unexamined Patent Publication No. 2016-17772 JP-A-2010-172883 JP-A-2010-172882 JP-A-2003-94100

By reducing the water content of the mud, transportation and storage become easier. Therefore, it is desired to rapidly separate solid content such as mud and water contained in mud water to achieve solid-liquid separation at an early stage. However, since it takes time for the coagulants described in Patent Documents 1 to 3 to coagulate the subject, it has been difficult to perform rapid solid-liquid separation.
An object of the present invention is to provide a method for treating decontaminated mud that can more quickly separate solid content and water to reduce the water content of the mud.

The method for treating decontaminated mud according to the present invention includes a dehydration promoting step of adding a dehydration promoting material containing anhydrous gypsum to muddy water containing decontaminated mud, and solid-liquid separation for separating solid content and water in the muddy water. A method for treating decontaminated gypsum, which includes a step, the anhydrous gypsum has an average particle diameter D50 of 5 to 100 μm, and among the peak intensities obtained by X-ray diffraction, the peak intensity (I) of the (020) plane. The ratio (I ₍₀₂₀₎ / I ₍₁₀₂₎ ) of ₍₀₂₀₎ ) to the peak intensity (I ₍₁₀₂₎ ) of the surface ₍₁₀₂ ) is 5 or more.

According to the present invention, there is provided a method for treating decontaminated mud that can more quickly separate solids and water to reduce the water content of the mud.

It is a scanning electron microscope (SEM) photograph of the surface of natural anhydrous gypsum. It is an SEM photograph of the surface of industrial anhydrous gypsum.

Hereinafter, embodiments of the present invention will be described in detail.
The method for treating decontaminated mud of the present invention includes a dehydration promoting step of adding a dehydration promoting material containing anhydrous gypsum to muddy water containing decontaminated mud, and a solid-liquid separation step of separating solid content and water in the muddy water. And include. Each step will be described below.

[Dehydration promotion process]
First, a predetermined dehydration accelerator is added to muddy water containing decontaminated mud. Dehydration of the decontaminated mud is promoted by adding a predetermined dehydration promoting material. The dehydration accelerator used in the present invention can be obtained by mixing components such as a polymer with anhydrous gypsum. Examples of the anhydrous gypsum include natural anhydrous gypsum (produced in Thailand). Its physical properties are shown below in comparison with industrial anhydrous gypsum (manufactured by Asahi Glass Co., Ltd.).

<Comparison between natural anhydrous gypsum and industrial anhydrous gypsum>
(Average particle size D50)
The average particle size D50 obtained from the wet particle size distribution for each anhydrous gypsum is summarized in Table 1 below. Water was used as the dispersion medium, and after pretreatment by ultrasonic dispersion, it was determined by a particle size distribution measuring device (manufactured by Microtrack Bell Co., Ltd.).

It was confirmed that the average particle size D50 of the natural anhydrous gypsum was significantly larger than that of the industrial anhydrous gypsum. A large average particle size D50 is advantageous in that the dispersibility in muddy water is improved.
If the average particle size D50 of natural anhydrous gypsum is as small as less than 5 μm, it may not disperse well in muddy water and the function of the dehydrating agent may be impaired. On the other hand, when the average particle size D50 of natural anhydrous gypsum is larger than 100 μm, the particle size deviates from other components, which is not preferable in that segregation may occur. Therefore, in the present invention, natural anhydrous gypsum having an average particle diameter D50 of 5 to 100 μm is used. The average particle size D50 of the natural anhydrous gypsum is preferably 10 to 40 μm, more preferably 15 to 30 μm.

(Dry particle size)
Table 2 below summarizes the dry particle size of each anhydrous gypsum. The dry particle size was measured using a sieve having a mesh size of 150 μm and a sieve having a mesh size of 90 μm.

Natural anhydrous gypsum has been shown to have a lower proportion of particles on a 150 μm sieve than industrial anhydrous gypsum. This result indicates that the particle size of natural anhydrous gypsum is more uniform than that of industrial anhydrous gypsum. It can be seen from the comparison of the SEM photographs of FIGS. 1 and 2 that the natural anhydrous gypsum has a larger particle size than the industrial anhydrous gypsum.

Table 3 below summarizes the contents of the components contained in natural anhydrous gypsum and industrial anhydrous gypsum. The content of each component was determined by ICP analysis.

The content of CaO may be considered equivalent as long as it is within the range of 30-50%, the content of SO ₃ can be considered equivalent as long as it is within the range of 40% to 60% .. Therefore, regarding the components contained, natural anhydrous gypsum is not clearly different from industrial anhydrous gypsum. The rest of each anhydrous gypsum is impure.

(Measurement of crystallites)
Crystallets of natural anhydrous gypsum (made in Thailand) and industrial anhydrous gypsum (manufactured by Asahi Glass Co., Ltd.) were measured using a fully automatic multipurpose X-ray commentator (manufactured by D8 ADVANCE Bruker AGC Inc.). The measurement conditions were as follows.
・ Tube voltage, tube current: 40kV, 40mA
・ Scanning range: 20 to 50 ° ・ Step width: 0.02 ° ・ Measurement time: 1S / step ・ Divergence slit, light receiving slit: 0.3 °
The crystallite was calculated from the peak of the (020) plane by Scheller's equation. The crystallites are calculated automatically by the device.
D = Kλ / (Bcosθ)
(D: crystallite size, K: Scheller constant (0.9), λ: X-ray wavelength B: peak half-ground width, θ: Bragg angle)
The results obtained are summarized in Table 4 below.

It was confirmed that the peak strength and crystallites of the natural anhydrous gypsum were larger than those of the industrial anhydrous gypsum. Therefore, it can be seen that the natural anhydrous gypsum has a higher orientation than the industrial anhydrous gypsum.
Since natural anhydrous gypsum has higher orientation than industrial anhydrous gypsum, it is presumed that it has a fibrous shape grown in the length direction. Table 5 below shows the strength ratio between the (020) plane and the (102) plane.

Based on the results shown in Table, the peak intensity ratio of the natural anhydrous gypsum, of the peak intensity obtained by X-ray diffraction, and (020) plane peak intensity (I _(020)), the (102) plane The ratio (I ₍₀₂₀₎ / I ₍₁₀₂₎ ) to the peak intensity (I ₍₁₀₂₎ ) was specified to be 5 or more. The ratio (I ₍₀₂₀₎ / I ₍₁₀₂₎ ) is preferably 50 or less, more preferably 10 to 40, and most preferably 15 to 30.
If the ratio (I ₍₀₂₀₎ / I ₍₁₀₂₎ ) is too small, the orientation is not high and the fibrous shape grown in the length direction is not exhibited, which is considered to adversely affect the dispersibility in muddy water. .. It was found that the dispersibility in muddy water was improved by specifying the ratio (I ₍₀₂₀₎ / I ₍₁₀₂₎ ) to 5 or more. Considering the range of easily measurable intensity ratios, the upper limit of the ratio (I ₍₀₂₀₎ / I ₍₁₀₂₎ ) can be about 50.

From the above results, it can be seen that the natural anhydrous gypsum has a significantly larger average particle size D50 than the industrial anhydrous gypsum, and also has high crystallinity and high orientation. Natural anhydrous gypsum is naturally produced over a long period of time. On the other hand, in the case of industrial anhydrous gypsum, for example, phosphoric gypsum is produced in a short period of time when phosphoric acid is produced by reacting phosphorus ore with sulfuric acid. Due to the difference in properties based on the difference in crystal formation rate between natural anhydrous gypsum and industrial anhydrous gypsum, and due to these physical properties, the dehydration accelerator containing natural anhydrous gypsum has a dispersibility in water. It is superior to industrial anhydrous gypsum, and it is presumed that it acts effectively as a nucleus when polluted suspended matter aggregates.

<Polymer>
The polymer imparts the effect of bloating of flocs due to aggregation, but the effect is not significantly improved even if it is excessively blended. If 10 to 15 parts by mass of the polymer is used with respect to 100 parts by mass of natural anhydrous gypsum, the desired effect can be sufficiently obtained.

As the polymer, either a synthetic polymer or a natural polymer may be used. As the synthetic polymer, it is preferable to use a combination of a low anion low molecular weight polymer having a weight average molecular weight (Mw) of about 6 million to 12 million and a strong anion high molecular polymer having a Mw of about 13 million to 16 million. The low molecular weight polymer and the high molecular weight polymer can be used in any ratio. For example, the mass ratio (low molecular weight polymer: high molecular weight polymer) can be about 80:20 to 40:60.

As the synthetic polymer, conventionally known ones can be used. For example, a carboxylate-based polymer such as sodium acrylate, a copolymer of a carboxylate and acrylamide, a copolymer of a sulfonate and acrylamide, a copolymer of an alkylaminoacrylate salt and acrylamide, and an alkyl. Examples thereof include a copolymer of an aminomethacrylate salt and acrylamide. Of these, polycarboxylic acid salt-based substances, that is, polymers of carboxylates and copolymers of carboxylates and acrylamide are preferable.

Examples of the polycarboxylic acid in the copolymer of the carboxylate and acrylamide include acrylic acid, methacrylic acid, itaconic acid, maleic acid, and carboxymethyl cellulose.
Examples of the sulfonic acid in the copolymer of the sulfonate and acrylamide include acrylamide 2-methylpropane sulfonic acid, vinyl sulfonic acid, and styrene sulfonic acid.

Examples of the alkylaminoacrylate salt in the copolymer of the alkylaminoacrylate salt and acrylamide include dimethylaminoethyl acrylate, dimethylaminopropylacrylamide, acryloyloxyethyltrimethylammonium chloride, acryloylaminopropyltrimethylammonium chloride, and acryloyl2-hydroxypropyllide. And so on.

Examples of the alkylaminomethacrylate in the copolymer of alkylaminomethacrylate and acrylamide include dimethylaminoethyl methacrylate, dimethylaminopropylmethacrylamide, metaacryloyloxyethyltrimethylammonium chloride, metaacryloylaminopropyltrimethylammonium chloride, and metaacryloyl. 2-Hydroxypropyllide and the like can be mentioned. The composition of the synthetic polymer can be appropriately changed as long as the effect is not deviated.

Examples of natural polymers include seed polysaccharides such as guar gum, roasted guar gum and roasted bin gum, resin polysaccharides such as arabinogalactan gum and arabiya gum, seaweed polysaccharides such as alginic acid and chitosan, and fruit polysaccharides such as pectin and silium gum. , Rhizome polysaccharides such as starch and konjak, fiber polysaccharides such as cellulose, microbial zansan gum, zancoat, zanflow, curdran, succinoglucan and the like, animal gelatin, casein, albumin, shruck and the like. In addition, starch derivatives, guar gum derivatives, roasted guar gum derivatives obtained by subjecting starch, guar gum, roasted guar gum, cellulose, alginic acid, etc. to treatments such as oxidation, methylation, carboxymethylation, hydroxyethylation, phosphorylation, and cationization. , Cellulose derivative, alginic acid derivative and the like may be used. The composition of the natural polymer can be appropriately changed without departing from the effect.

<Other ingredients>
The dehydration accelerator used in the present invention may contain aluminum sulfate, soda ash, powdered polyaluminum chloride (PAC), diatomaceous earth (P1200) and the like. It is preferable to use these components in the following ratios with respect to 100 parts by mass of natural anhydrous gypsum.
Aluminum Sulfate: 15-30 parts by mass
Alginic acid: 1-10 parts by mass
Soda ash: 10 to 20 parts by mass
Powder PAC: 0.5 to 10 parts by mass
Diatomaceous earth: 5 to 20 parts by mass

<Dehydration accelerator>
The dehydration accelerating material can be produced by blending a predetermined amount of a polymer and components such as aluminum sulfate with natural anhydrous gypsum and mixing them using a powder mixer or the like.
The desired effect can be sufficiently obtained by adding 0.01 to 0.1 parts by mass of the dehydration accelerator to 100 parts by mass of muddy water containing decontaminated mud. A dehydration accelerator is housed in a predetermined container together with muddy water, and is stirred at 100 to 400 rpm using, for example, a stirring blade or the like. About 20 seconds to 2 minutes after mixing, flocs are generated and settle. Dehydration promotion of decontaminated mud can be determined based on the settling time of flocs and the mass of flocs.

[Solid-liquid separation process]
In the muddy water in which the dehydration of the decontaminated mud is promoted, the solid content and the water are separated by using a water permeable member. As the water permeable member, for example, a sieve can be used. The mesh size of the sieve can be appropriately selected, but is preferably in the range of 0.25 mm to 1.8 mm, and more preferably in the range of 0.4 mm to 1.2 mm. When muddy water is supplied on a sieve having a predetermined opening, the solid content and the water content are separated, and a floc with reduced water content is obtained on the sieve.

Solid-liquid separation can also be performed by accommodating muddy water in a bag body made of at least a part of a water-permeable member. Examples of the water-permeable member in this case include a mesh material having a mesh size of 0.25 to 1.8 mm, preferably a mesh material having a mesh size of 0.4 mm to 1.2 mm. The mesh material preferably constitutes the bottom of the bag body, and more preferably the entire bag body is a mesh material.
Moisture can be discharged by accommodating muddy water in a bag and suspending it from above, for example, to obtain dehydrated mud. Since the mud is already contained in the bag, the work such as transportation and storage after dehydration becomes easy.
When the bag containing muddy water is pressurized, the discharge of water is accelerated, so that solid-liquid separation can be performed more quickly. Further, even when the bag containing muddy water is rotated to perform the centrifugation treatment, the discharge of water can be accelerated.

Specific examples of the present invention will be shown below, but these are not limited to the present invention.
<Selection of plaster to be used>
First, the dehydration performance of natural anhydrous gypsum and industrial anhydrous gypsum was confirmed. Specifically, natural anhydrous gypsum and industrial anhydrous gypsum were added to the bottom sediment of the pond, and a floc formation test and a sedimentation test were conducted.

(Preparation of sample)
475 g of water was added to 25 g of reservoir bottom sediment to prepare 500 g of muddy water. A sample was prepared by adding 0.3 g of natural anhydrous gypsum (produced in Thailand) to the obtained muddy water.

(Contents of the test)
Flock formation test: The prepared sample (muddy water to which natural anhydrous gypsum was added) was stirred at a mixing speed of 200 rpm using a general-purpose stirrer (general-purpose stirrer BL300 manufactured by Shinto Kagaku Co., Ltd.). After 1 minute, muddy water was strained using a sieve (sieving mesh 850 μm) for solid-liquid separation, and the mass of flocs on the sieve was measured.
Further, a floc formation test was carried out in the same manner as described above except that the opening of the sieve was changed, and the mass of flocs on the sieve was measured. The results are shown in Table 6 below. The more flocs on the sieve, the more dehydration was promoted.

For comparison, 0.3 g of industrial anhydrous gypsum (manufactured by Asahi Glass Co., Ltd.) was added to the same muddy water (500 g) as described above, and a floc formation test was carried out in the same manner. For solid-liquid separation, a sieve having an opening of 850 μm was used. The mass of flocs on the sieve is as shown in the table below.

As shown in the above table, the sample using natural anhydrous gypsum has a larger total mass remaining on the sieve and the dry mass obtained by removing water from the total mass as compared with the sample using industrial anhydrous gypsum. This is because the sample to which natural anhydrous gypsum is added forms flocs more preferably than the sample using industrial anhydrous gypsum, and more solid content (material contained in the dry mass) in muddy water is taken into the flocs. It is based on suppressing the passage of solids in muddy water through the sieve. That is, it was shown that natural anhydrous gypsum acts more effectively as a nucleus when polluted suspended matter aggregates and easily forms large flocs than industrial anhydrous gypsum, and therefore has higher dehydration promoting performance against muddy water. ..

Precipitation test: The same sample as the sample prepared in the floc formation test was stirred in the same manner as in the floc formation test, and then placed in a 500 ml graduated cylinder and allowed to stand, and the amount of sedimentation was measured. In this test, raw water (muddy water without gypsum added) was also measured. The measurement results are shown in Table 8 below.

As shown in the above table, the sample using natural anhydrous gypsum settles about 70% in a little over 10 seconds and 80% in 1 minute. On the other hand, in the sample using industrial anhydrous gypsum, it took more than 30 seconds to settle 70% and 5 minutes to settle 80%. Compared with industrial anhydrous gypsum, the sample using natural anhydrous gypsum can aggregate and settle the solid content in muddy water at an early stage, and has high dehydration promoting performance. This is based on the fact that natural anhydrous gypsum has better dispersibility in water than industrial anhydrous gypsum and can form large flocs.

As shown in the above results, natural anhydrous gypsum is superior in dehydration promoting performance to industrial anhydrous gypsum. Therefore, it is expected that the solid content and the water content can be separated more quickly to reduce the water content of the mud by treating with the method of the present invention using the dehydration accelerator containing natural anhydrous gypsum.

In the following, a dehydration-promoting material containing natural anhydrous gypsum or industrial anhydrous gypsum is produced, and the obtained dehydration-promoting material is used to treat decontamination mud.

<Preparation of dehydration accelerator>
A sample of 100 parts by mass of natural anhydrous gypsum mixed with polymer, aluminum sulfate, alginic acid (coarse powder), soda ash, powdered polyaluminum chloride (PAC), and diatomaceous earth (P1200) in parts by mass shown in Table 9 below. 1 to 10 dehydration accelerators were prepared.

For comparison, the dehydration accelerator of Sample 11 was prepared with the same formulation as that of Sample 3 except that the natural anhydrous gypsum was changed to industrial anhydrous gypsum.

<Treatment of muddy water>
As a treatment target, 25 g of decontaminated mud (sediment soil collected from a designated pond in Fukushima Prefecture) and 475 g of tap water were mixed to prepare 500 g of mud. The treatment using Samples 1 to 10 containing natural anhydrous gypsum is referred to as Examples 1 to 10, and the treatment using Sample 11 containing industrial anhydrous gypsum is referred to as Comparative Example.

First, each dehydration accelerating material was added to muddy water to promote dehydration, and then solid-liquid separation was performed. The treatment performance was evaluated by comprehensively judging the floc formation rate, the size of the flocs, the pH of the clean water, and the transparency of the clean water. Specifically, the muddy water was contained in a 1 liter glass beaker, and 0.3 g (0.06% by mass) of a dehydration accelerator was added to each. Using a general-purpose stirrer (BL300 manufactured by Shinto Kagaku Co., Ltd.), the dehydration accelerator and muddy water were mixed by stirring at a rotation speed of 200 rpm while visually observing.
When the dispersed state of the dehydration accelerator in the muddy water was visually observed, it was confirmed that the dehydration accelerators of Samples 1 to 10 were more dispersed than the dehydration accelerator of Sample 11.

(Flock formation speed)
The time (t _F ) from the start of mixing to the formation of flocs was measured, and the flocs formation rate was evaluated as follows.
⊚: t _F ≤ 30 sec
◯: 30 sec <t _F ≤ 60 sec
Δ: 60 sec <t _F ≤ 120 sec
X: t _F > 120 sec
Even when the flocs were not formed, the flocs formation speed was set to “x”.
For the flock formation speed, "△", "○", and "◎" are accepted. The shorter the floc formation rate, the better the dehydration promoting performance, which leads to a reduction in processing time.

(Flock size)
The muddy water containing the flocs was separated from the solid content (flocs) and the water content using a circular sieve having an opening of 850 μm, and the size of the flocs remaining on the sieve was measured. The size of the flock refers to the size of the flock formed in the muddy water, and was determined visually and by measurement from the outside of the beaker. If the size of the floc is 5 mm or more and 10 mm or less, it is evaluated as “large”, and if it is 1 mm or more and less than 5 mm, it is evaluated as “small”. The larger the flocs formed, the faster the solid-liquid separation can be performed.

(Evaluation of water content under the sieve)
The pH of the water under the sieve was measured with a tabletop pH meter. The pH is preferably about 5-9, more preferably about 5.8-8.6.
The transparency of the water under the sieve was visually inspected and evaluated according to the following criteria.
◎: No floating matter is seen and the water becomes transparent.
◯: There are some floating substances, but the water becomes almost transparent.
Δ: There is some floating matter, and there is some turbidity in the water.
X: There is suspended matter and the water is turbid. Excellent transparency corresponds to the sedimentation of polluted suspended matter in muddy water, that is, the promotion of dehydration.

The floc formation rate, floc size, pH of clean water, and transparency of clean water are summarized in Table 10 below together with a comprehensive evaluation of treatment performance. The comprehensive evaluation was made by comprehensively judging the floc formation rate and the size of the flocs as the main factors, and the transparency and pH of the clean water as the subordinates.

As shown in Table 10 above, in Examples 1 to 10 using the dehydration accelerator containing natural anhydrous gypsum, the solid content in the mud is higher than that in the comparative example using the dehydration accelerator containing industrial anhydrous gypsum. It was confirmed that water and water could be separated quickly and the dehydration performance was improved.

Claims (5)

A method for treating decontaminated mud, which includes a dehydration promoting step of adding a dehydration promoting material containing anhydrous gypsum to mud containing decontaminated mud, and a solid-liquid separation step of separating solids and water in the mud. hand,
The anhydrous gypsum has an average particle diameter D50 of 5 to 100 μm, and among the peak intensities obtained by X-ray diffraction, the peak intensities of the (020) plane (I ₍₀₂₀₎ ) and the peak intensities of the (102) plane (102) I ₍₁₀₂₎₎ ratio of _{(I (020) / I (} 102)) is the processing method of decontaminating mud which is characterized in that 5 or more. The method for treating decontaminated mud according to claim 1, wherein the ratio (I ₍₀₂₀₎ / I ₍₁₀₂₎ ) is 50 or less. The method for treating decontaminated mud according to claim 1 or 2, wherein the anhydrous gypsum is a natural anhydrous gypsum. The method for treating decontaminated mud according to any one of claims 1 to 3, wherein the solid-liquid separation step is performed using a bag body including at least a part of a water-permeable member. The method for treating contaminated mud according to claim 4, wherein the water-permeable member is a mesh material having a mesh size of 0.25 to 1.8 mm.