JP2012081387A - Washing method of cyanide-contaminated soil - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce disposal cost by reducing the generation amount of concentrated contaminated soil when washing cyanide-contaminated soil.SOLUTION: The contaminated soil is formed into slurry; a polymer coagulant is added to the slurry and mixed and agitated to form flock by selectively coagulating only the earth particles of the sizes of 5-10 μm or larger in the slurry, and fine particles of less than 5-10 μm are dispersed in the slurry without causing coagulation. The cyanide is extracted in a dissolved state from the precipitated and separated flock by the alkali extraction, then cleaned soil consisting of the earth particles of 5-10 μm or larger is obtained by neutralizing, stabilizing and dewatering. The extracted water containing the cyanide in the dissolved state and supernatant of the slurry containing the fine particles of less than 5-10 μm are further subjected to the chemical clarification and dewatering to obtain the concentrated contaminated earth containing cyanide.

Description

  The present invention relates to a technique for cleaning contaminated soil, and more particularly to a cleaning method for low-concentration cyan contaminated soil.

The site of a city gas production plant is often contaminated with cyanide compounds, benzene, and heavy metals emitted from the coal gas production process, and purification is required.
As a purification technique for cyan contaminated soil, for example, a cleaning technique as shown in Patent Document 1 or Patent Document 2 is known.

  With these soil cleaning technologies, environmental pollutants (heavy metals, mineral oil, cyanide, etc.) in contaminated soil are more contaminated with soil organic matter (humus), clay, silt, etc. than coarse particles such as sand and gravel. Since a large amount of fine particles are adsorbed and retained, it is advantageous for the purification to separate and remove coarse particles and fine particles for efficient treatment.

For example, in the soil washing method disclosed in Patent Document 1, a fine particle component adsorbing a pollutant is separated by a hydrocyclone using a contaminated soil as a slurry, and then a suspension containing the fine particle component. The coagulant is added and stirred to coagulate and settle, and the coagulated sludge is dehydrated and disposed as concentrated contaminated soil (dehydrated cake).
In this case, the hydrocyclone is a device that is convenient for classifying the soil slurry and has a high processing capacity. However, the hydrocyclone having a classification point smaller than 20 to 30 μm has a small processing capacity because of its small size, and thus a large amount of soil slurry. Since it requires a large number of cyclones and pumps to treat the wastewater, it is not practical, so in the field of soil washing, it is the limit that the classification point is about 20-30 μm. A hydrocyclone having a diameter of 63 to 125 μm is generally used.

  In addition, the method for purifying contaminated soil disclosed in Patent Document 2 is for soil contaminated with hazardous substances such as heavy metals and cyan, adding an extractant, kneading with a mixer, leaching with a leachate, The harmful substances are removed by liquid separation. In this case as well, the particles are classified at around 150 μm using a drum washer, vibrating sieve, hydrocyclone, spiral classifier, etc., and the particles having a particle size of 150 μm or more are purified. I am doing so.

Japanese Patent Laid-Open No. 2006-11697 JP 2000-157964 A

In the case of cleaning treatment after classifying contaminated soil as described above, it is necessary to dispose of all classified fine particles as concentrated contaminated soil (dehydrated cake) even if it is contaminated soil with a low concentration. Therefore, there is a request to reduce the classification point as much as possible in order to reduce the amount of disposal soil and disposal costs.
In other words, it is normal for soils in various parts of Japan to contain fine particles of 63 μm or less in a dry specific gravity ratio of about 25 to 40%. In regions containing a large amount of clay and silt, fine particles are contained in the entire soil. Since it may indicate 50-60%, when the classification point is set to about 63-125 μm to 150 μm in the conventional cleaning method, it is necessary to treat and dispose of most of the contaminated soil substantially. Therefore, a large cost is required for that.

  Therefore, if the classification point is reduced to, for example, about 5 to 10 μm and the fine particles thus classified are processed and disposed of, the amount of soil to be disposed and the disposal cost can be greatly reduced. As described above, it is difficult to make the classification point sufficiently smaller than 63 to 125 μm without lowering the processing efficiency with the conventional classification means including the conventional hydrocyclone. In fact, the development of effective and appropriate means is desired.

  In view of the above circumstances, in the present invention, when cleaning soil contaminated with cyan, the contaminated soil to be treated is made into a slurry, and a polymer flocculant is added to the slurry and mixed and stirred, so that 5 to 10 μm in the slurry. Only the above earth particles are selectively agglomerated to form a floc, and fine particles of less than 5 to 10 μm are dispersed in the slurry without agglomerating, and the floc is settled and separated from the slurry, and an alkali is removed from the floc. After extracting cyan as a dissolved state by extraction, the floc is neutralized and stabilized and dehydrated to obtain purified soil composed of soil particles of 5 to 10 μm or more, and contains cyan as a dissolved state extracted from the floc The extracted water and the supernatant water of the slurry containing fine particles of less than 5 to 10 μm are further coagulated and precipitated to give a coagulated slur. It is characterized in that concentrated soil containing cyan is obtained by dehydrating di.

In the present invention, efficient separation of fine particles of less than 5 to 10 μm, which is difficult to achieve with a physical method such as hydrocyclone, is separated by a coagulation precipitation method that is a chemical method. By deliberately performing an incomplete coagulation sedimentation process by omitting a part of the coagulation sedimentation method, only 5-10 μm or more of soil particles are selectively separated as flocs so as to be finer than 5-10 μm. This makes it possible to classify particles.
In the present invention, cyan is extracted as a dissolved state from the soil particles after classification of the fine particles as described above by alkali extraction, and the extracted water and the supernatant water containing fine particles of less than 5 to 10 μm are further added. By aggregating and precipitating and dehydrating, the concentrated contaminated soil to be disposed of is concentrated to a high concentration of cyanide, and the soil particles of 5-10 μm or more after cyan extraction are neutralized and stabilized. Treated and reclaimed soil is obtained.
Therefore, according to the present invention, a large amount of soil particles having a size of 5 to 10 μm which must be disposed of in the conventional general cleaning method can be recovered as reusable purified soil. The amount of soil generated can be greatly reduced, and the cost required for disposal can be greatly reduced.

It is a flowchart which shows embodiment of the washing | cleaning method of this invention. It is a figure which shows the principle of an incomplete coagulation precipitation method and an alkali extraction method. It is a figure which shows the specific classification example by this invention. It is explanatory drawing about the incomplete agglomeration precipitation method. It is explanatory drawing about the normal coagulation precipitation method. It is a figure which shows the characteristic of the cyan pollution soil which this invention makes object.

  Prior to describing the embodiment of the present invention, the characteristics of the cyan contaminated soil in the former site of the city gas production factory targeted by the present invention will be described.

FIG. 6 shows the particle size distribution and load curve (relationship between soil particle size and pollutant content) of cyan contaminated soil at the site of the city gas manufacturing plant.
From the load curve data of the total cyan content shown in FIG. 6 (b), the total cyan content of the whole soil is 14 mg / kg, whereas the total cyan content of fine particles of 38 μm or less is 100 mg / kg. It can be seen that it is extremely high, that is, cyan is unevenly distributed on the fine particle side.

The main cyanide compound in the soil contaminated with cyanide at the site of the city gas manufacturing plant is iron cyanide complex (such as iron ferrocyanide) that is considered to be almost harmless, not highly toxic free cyanide. The solubility of this iron cyanide complex is greatly influenced by the pH and redox potential, and in the acidic to neutral pH range and in the moderate aerobic to anaerobic state, the solubility of the iron cyanide complex is quite small (less than 1 mg / L). In the alkaline and highly aerobic state, their solubility is relatively high.
Therefore, it is considered that this kind of cyan-contaminated soil is scrubbed (rubbed) in an alkaline solution and subjected to alkali extraction, thereby effectively extracting cyan and purifying fine particles.

The present invention is based on the above knowledge about cyan-contaminated soil, and a cyan-contaminated soil is obtained by a special coagulation precipitation method (hereinafter referred to as “incomplete coagulation precipitation method”) and an “alkali extraction method” described below. The main objective is to wash
The “incomplete flocculation precipitation method” in the present invention enables efficient separation of fine particles of less than 5 to 10 μm, which is difficult to achieve with a physical method such as hydrocyclone, by the flocculation precipitation method which is a chemical method. However, a part of the ordinary coagulation precipitation method is omitted, and an incomplete coagulation precipitation process is intentionally performed.

Prior to explaining the “incomplete coagulation precipitation method” in the present invention, first, an ordinary coagulation precipitation method will be explained with reference to FIG.
The reason why particles in water are stably dispersed without agglomeration is that the particle surface is generally negatively charged and repulsion between the particles occurs, and ions and dissolved molecules present around the particles This is because access is blocked.
Therefore, in order to grow into large particles by aggregating action, the surface charge of the particles is neutralized by the opposite charge to weaken the electrostatic repulsion, and the polymer agglomerates with functional groups (adsorptive groups) that are easy to adsorb It is necessary to cross-link the particles with an agent. Therefore, in the ordinary coagulation precipitation method, as shown in FIG. 5, coagulation precipitation treatment is performed by using an inorganic coagulant (PAC, sulfate band, etc.) and a polymer coagulant in combination. It is necessary to do.
In other words, the inorganic flocculant has a strong charge neutralizing action on the particle surface and at the same time adsorbs to the particles and dissolved matter, so it is suitable for collecting all the small particles, but can form only small flocs with low strength. . Therefore, by adding an anionic or nonionic polymer flocculant having a larger molecular weight and an adsorbing group such as an amide group or a carboxyl group to the small flocs, the small flocs are cross-linked and become strong large flocs. And sedimentation is promoted.

  In this way, in the ordinary coagulation precipitation method, a combination of an inorganic coagulant and a polymer coagulant forms a large floc to obtain an aggregation coagulation action. In this case, all particles including fine particles are dispersed. Since it becomes a large floc and settles to become a precipitated sludge, only fine particles of 5 to 10 μm cannot be selectively separated.

Therefore, in the present invention, as shown in FIG. 4, the addition of the inorganic flocculant is omitted and only the polymer flocculant is added, and the incomplete flocculation and precipitation treatment is intentionally performed as described above.
That is, in the case of agglomerating only with the polymer aggregating agent without adding the inorganic aggregating agent, the particle size that can be agglomerated is naturally limited, and particles having a relatively large diameter of 5 to 6 μm or more are as large as in the past. Forms flocs and settles to form precipitated sludge. However, the cross-linking action of the polymer flocculant is difficult to work on fine particles of less than 5 to 6 μm, and no flocs are formed, or even flocs are formed. These are only fine, and since they have a low sedimentation rate, they can flow out as an overflow while floating.
Therefore, it is possible to separate the sedimented sludge (underflow) and the supernatant water (overflow) that settle down as large flocs, thereby reducing the fine particle content of less than 5 to 6 μm and the fine particle content larger than that. Can be separated. However, since the supernatant water is actually overflowed and collected as an overflow stream, fine particles of about 10 μm are also collected along with the overflow stream flowing in the horizontal direction, so the actual classification point is slightly larger. As shown in the figure, it becomes about 5 to 10 μm.

In order to be able to classify and separate fine particles by incomplete aggregation as described above, the addition amount and pH of the polymer flocculant are adjusted appropriately on the premise that the addition of the inorganic flocculant is omitted. However, the amount of the polymer flocculant added may be smaller than that of the normal coagulation precipitation method, and the pH is adjusted so that the coagulation is performed in a pH range different from the appropriate pH of the normal coagulation precipitation method. Good.
For example, the addition amount of the polymer flocculant may be about 1/2 to 2/3 of the amount that is considered appropriate in a normal coagulation precipitation method. Further, in the usual aggregation precipitation method in the case of using an anionic polymer as the polymer flocculant, the pH is generally 7 to 9, but in the present invention, the pH is preferably 9 to 11. Further, the overflow rate of overflow may be appropriately set so that the classification point is 5 to 10 μm as described above in association with the addition amount of the polymer flocculant and the adjustment of pH.

  The gist of the present invention is to sequentially carry out the above-mentioned “incomplete coagulation precipitation method” and “alkaline extraction method” on cyan contaminated soil, thereby enabling efficient cleaning and purification of cyan contaminated soil. FIG. 1 to FIG. 2 show specific embodiments thereof.

  First, water is added to the contaminated soil to be treated and the coarse particles exceeding 2 mm are removed in advance by wet fluid. Classification by fine particle fraction (overflow).

  With regard to underflow from hydrocyclone, cleaning sand that satisfies environmental standards can be obtained only by selectively separating fine particles containing cyanide by flotation. Since it contains a concentration of cyanide, it is sent to a subsequent aggregation and precipitation step for further processing.

  As described above, cyan is unevenly distributed in fine particles and organic substances less than 5 to 10 μm in the fine particles less than 63 μm that are overflow from the hydrocyclone. Applying the “aggregation precipitation method”, the particles are further classified, and fine particles of 5-10 μm or more containing low concentration cyan are settled and separated as flocs, and fine particles of less than 5-10 μm containing high concentrations of cyan are added. Collect as clear water.

  Since flocs separated by the above incomplete flocculation precipitation method contain a low concentration of cyanide, neutralization is carried out after extracting cyan by adding NaOH and applying the above-mentioned “alkali extraction method”. -Washed soil (recycled soil) that satisfies environmental standards can be obtained by stabilizing and dewatering.

  On the other hand, the supernatant water containing high-concentration cyan separated by the incomplete flocculation precipitation method, the fine particles from the above flotation, and the extracted water from the above alkali extraction step are further subjected to normal flocculation. A precipitation method (a so-called complete coagulation precipitation method using an inorganic coagulant) is carried out, in which all fine particles of less than 5 to 10 μm and dissolved cyan are separated from the water as coagulated sludge, which is pressed and dehydrated. As a result, a concentrated contaminated soil containing a high concentration of cyanide is obtained.

According to the present invention, a large amount of particles having a size of 5 to 10 μm or more that must be disposed of in the past because they are contained in the concentrated contaminated soil can be recovered as reusable purified soil, and thus concentrated contaminated. The amount of soil generated can be greatly reduced, and the cost required for disposal can be greatly reduced.
According to one experimental example, in the case of the conventional method in which the hydrocyclone was classified at 63 μm, it was necessary to dispose 35% of the entire soil to be treated as concentrated contaminated soil. It is confirmed that the concentrated contaminated soil can be reduced to less than 5-8% when classified by 5 to 10 μm, and the amount of concentrated contaminated soil can be reduced to about 1/4 compared with the conventional method. .

  Specifically, as shown in FIG. 3, when a soil sample of 2 kg or less is 100 kg-dry as shown in FIG. 3, the conventional fine particle fraction (overflow) of less than 63 μm classified by hydrocyclone is 35 kg-dry (total In the present invention, the overflow from the hydrocyclone is further classified by incomplete agglomeration and precipitation, and it is necessary to dispose as concentrated contaminated soil. Less fine particles are reduced to 5 kg-dry (5% of the total), and the amount of concentrated contaminated soil to be finally disposed of is only 8 kg-dry (total of recovered from flotation and alkaline extraction) 8%).

In addition, as shown in FIG. 6, the cyan content is higher as the particle size is smaller, and therefore, by treating fine particles selectively as in the present invention, more efficient treatment can be achieved compared to the conventional method. Is possible.
From the above, the cleaning method of the present invention can efficiently treat relatively low concentration of cyan contaminated soil by organically combining the “incomplete flocculation precipitation method” and the “alkali extraction method”. , Extremely reasonable and effective.

Claims (1)

A method for cleaning soil contaminated with cyanide,
The contaminated soil to be treated is made into a slurry, and by adding a polymer flocculant to the slurry and mixing and stirring, only the soil particles of 5 to 10 μm or more in the slurry are selectively aggregated to form a flock, Disperse the fine particles of less than 5 to 10 μm in the slurry without agglomeration,
After the floc is settled and separated from the slurry and extracted from the floc by alkali extraction in a dissolved state, the floc is neutralized and stabilized, and dehydrated to obtain a purified soil composed of soil particles of 5 to 10 μm or more. With
The extracted water containing cyan as a dissolved state extracted from the floc and the supernatant water of the slurry containing fine particles of less than 5 to 10 μm are further coagulated and precipitated, and the coagulated sludge is dehydrated to remove cyan. A method for cleaning cyan-contaminated soil, characterized by obtaining concentrated contaminated soil.

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