JP2015123393A - Method and system for extracting and removing heavy metals in soil - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a system to control the elution amount and the content of heavy metals to reference values or less, by removing the heavy metals from soil produced in tunnel construction.SOLUTION: A removal system of heavy metals contained in soil, includes: soil washing equipment for eluting the heavy metals contained in the soil into water solution by mixing the soil containing the heavy metals produced in tunnel excavation and the water solution of washing agent; solid-liquid separation equipment separating slurry containing the soil into the soil and the water solution; and heavy metal collection equipment collecting the heavy metals from the water solution. Target heavy metals are As, Cd, Cr, Hg, Cu, Se, Pb, F, B and these salts and these ions.

Description

  The present invention relates to a system and a removal method for removing heavy metals from soil containing heavy metals. In particular, the present invention relates to a system for treating soil generated during tunnel construction in situ, removing heavy metals contained in the soil, and obtaining soil of a level that can be reused or discarded.

  Surrounded by the sea on all sides, Japan's soil, which has many volcanoes and hot springs, is rich in naturally-occurring heavy metals such as arsenic, cadmium, lead, boron, fluorine, and chromium. Therefore, heavy metal exceeding the standard value may be contained in the residual soil generated by closed tunnel construction. In the soil countermeasures law (old law) enforced on February 15, 2003, soils where it is judged that the possibility of specific harmful substances exceeding specified standards being exclusively derived from nature are not applicable. It was. However, with the enforcement of the revised Soil Contamination Countermeasures Law on April 1, 1990, it became subject to regulation whether it was artificial or naturally derived. Therefore, when the residual soil generated by excavation of the tunnel exceeds the standard value stipulated by the law, it cannot be backfilled or discarded. In order to reclaim or discard such soil, it is necessary to treat so that the amount of elution of heavy metals from the soil and the content value are below the standard value. The generated residual soil is temporarily stored, transferred to another location, and disposed of after removing heavy metals such as arsenic. However, heavy metals are stored in the storage and transfer process. There is a risk of leakage or elution.

  Patent Document 1 proposes a method in which insolubilization treatment of heavy metal-contaminated soil is integrated with a process of excavation work by a sealed shield machine. In patent document 1, the injection port for inject | pouring a heavy metal insolubilizing material is provided in the cutter front surface or side part of a sealed shield machine, and a heavy metal insolubilizing material is directly inject | poured into the soil generated by excavation. As a result, the contaminated soil is treated on the spot without being exposed to the ground, so that the untreated soil is not exposed to the air, and there is no risk of scattering or runoff / elution due to wind and rain. Patent Document 1 does not mention what kind of substance is used as the heavy metal insolubilizing material.

  Patent Document 2 proposes an adsorption / insolubilizing agent for treating naturally-derived arsenic contained in tunnel shears. In Patent Document 2, a powdered form obtained by kneading natural zeolite having a high cation exchange capacity with a liquid heavy metal adsorbent produced by adding a neutralizer containing an iron salt and an aluminum salt to a calcium / magnesium compound produced in an iron factory. Or a granular heavy metal adsorption treatment agent has been proposed.

JP 2006-316599 A JP2011-136311A

  An object of this invention is to provide the system for removing heavy metals from the soil which generate | occur | produces in tunnel construction, and making the elution amount and content of heavy metals below a reference value. In particular, the present invention provides a method for extracting and removing arsenic-containing soil generated by tunnel excavation on the spot, thereby obtaining soil in which both the arsenic elution amount and the content are below the reference value.

Aspects of the present invention are as follows.
1. A soil cleaning device for mixing heavy metal-containing soil generated by tunnel excavation and an aqueous cleaning agent solution, and eluting the heavy metals contained in the soil into the aqueous solution;
A solid-liquid separator that separates the slurry containing the soil into a soil and an aqueous solution;
A heavy metal recovery device for recovering heavy metals from an aqueous solution;
A system for removing heavy metals contained in soil, including
2. The system of claim 1, wherein the heavy metals are selected from the group consisting of arsenic, cadmium, chromium, mercury, copper, selenium, lead, fluorine, boron and their salts, and their ions and combinations thereof.
3. 3. The system according to 1 or 2 above, wherein the cleaning solution contains water, an acidic substance, a metal chloride, and a fluoride.
4). 4. The system according to 3 above, wherein the acidic substance is selected from the group consisting of hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, oxalic acid, citric acid, tartaric acid, ascorbic acid, lactic acid, sulfamic acid, hydrofluoric acid, and mixtures thereof.
5. 4. The system of claim 3, wherein the metal chloride is selected from the group consisting of lithium chloride, sodium chloride, potassium chloride, cesium chloride, magnesium chloride, calcium chloride, strontium chloride, barium chloride, ammonium chloride and mixtures thereof.
6). The system of claim 3, wherein the fluoride is selected from the group consisting of sodium fluoride, potassium fluoride, ammonium fluoride, ammonium hydrogen fluoride, magnesium fluoride, calcium fluoride, and mixtures thereof.
7). The system according to any one of 3 to 6, wherein the aqueous cleaning solution further contains a dispersant.
8). The system of claim 7, wherein the dispersant is selected from the group consisting of polyethylene glycol, hydroxyethyl cellulose, hydroxypropyl cellulose, dodecyltrimethylammonium chloride, sodium dodecyl sulfate, octaethylene glycol monododecyl ether, and mixtures thereof.

  Hereinafter, the present invention will be described in detail.

  The present invention mixes a heavy metal-containing soil generated by tunnel excavation and a cleaning agent aqueous solution, and elutes a heavy metal contained in the soil into the aqueous solution, and a slurry containing the soil, The system for removing heavy metals contained in the soil includes a solid-liquid separation device that separates the aqueous solution into an aqueous solution and a heavy metal recovery device that recovers heavy metals from the aqueous solution.

  Tunnel excavation is a road tunnel for passing railways, vehicles, people, etc., for power generation by methods including general tunnel excavation methods such as shield method, TBM (tunnel boring machine) method, mountain method, etc. It means excavating soil to construct a conduit tunnel. The soil generated by tunnel excavation is so-called residual soil generated by the above construction method. The system according to the present invention is for treating a heavy metal contained in the remaining soil prior to disposal. Here, “heavy metal” is not “heavy metal” in the original chemical meaning, but refers to all harmful substances specified by the soil environmental standards except organic compounds. That is, heavy metals in the present invention mean arsenic, cadmium, chromium, mercury, copper, selenium, lead, fluorine, and boron. Since many of these heavy metals exist in the form of salts and ions in the soil, the salts or ions of these heavy metals are also included in the “heavy metals” of the present invention. That is, in the present specification, “arsenic” also means “arsenic salt” and “arsenic ion”. Heavy metal salts refer to, for example, sulfide salts, carbonate salts, sulfate salts, and hydroxide salts. These salts are dissolved in water in the soil and exist in various ion forms.

  The system of the present invention includes a soil cleaning device for mixing heavy metal-containing soil generated by tunnel excavation and a cleaning agent aqueous solution, stirring in a slurry state, and eluting the heavy metals contained in the soil into the aqueous solution. . The soil cleaning apparatus can include a soil cleaning tank for bringing the soil into contact with the cleaning agent aqueous solution, a stirrer for stirring the soil slurry, and a cleaning agent tank for adding the cleaning agent.

  As the cleaning agent, those having a function of expelling heavy metals contained in the soil from the soil and eluting them into the aqueous cleaning solution are suitable. Many heavy metals are strongly fixed to the soil and often cannot be washed simply by contact with water. The cleaning agent has the action of first dispersing the soil uniformly in water to increase the contact area with the cleaning agent, and then having the action of peeling heavy metals (and their ions) from the dispersed soil, and It is required to have the effect of expelling the heavy metals (and their ions) that have been peeled off from the soil. Therefore, it is desirable that the cleaning agent used in the present invention is in the form of an aqueous solution containing an acidic substance, metal chloride, fluoride and optionally a dispersing agent.

  Here, the acidic substance serves to weaken the negative charge existing in the soil and neutralize it. The negative charge existing in the soil can be weakened by the acidic solvent, and ions of heavy metals (mainly in the form of cations) that are strongly fixed to the negative charge can be easily removed. Any acidic substance may be used as long as it exhibits acidity in the state of an aqueous solution. Examples of suitable acidic substances having an action of weakening negative charges existing in soil include hydrochloric acid, sulfuric acid, Examples thereof include nitric acid and phosphoric acid, and other examples include organic acids such as oxalic acid, citric acid, tartaric acid, ascorbic acid, lactic acid, and sulfamic acid. A mixture of two or more of these acidic substances can also be used.

  Metal chloride means alkali metal chloride or alkaline earth metal chloride. The metal cation in the metal chloride functions to expel the cation into the cleaning solution by replacing the heavy metal cation that has been removed from the soil. Any metal chloride may be used as long as it is a metal chloride, but as a metal chloride in which the metal cation portion is relatively easily replaced by a heavy metal cation, lithium chloride, sodium chloride, potassium chloride, cesium chloride, magnesium chloride , Calcium chloride, strontium chloride, barium chloride, ammonium chloride and the like, and a mixture of two or more thereof is preferably used.

  Fluoride means alkali metal fluoride or alkaline earth metal fluoride. Since the fluoride ion in the metal fluoride has an action of destroying the structure of the soil, it can be suitably used. Fluoride ion reacts with silicon contained in the soil and breaks the silicon-oxygen bond, so that the structure of the soil collapses due to this action, and the space containing heavy metal cations expands, and heavy metal cations Can be easily driven out. Any metal salt may be used as long as it is a metal fluoride, but in consideration of stability during use, for example, sodium fluoride, potassium fluoride, ammonium fluoride, ammonium hydrogen fluoride, magnesium fluoride, Calcium fluoride can be mentioned, and it is also possible to use a mixture of two or more of these.

  The dispersing agent that may be optionally contained in the present invention has a function of uniformly dispersing soil and sand in the soil and facilitating diffusion in water. That is, the dispersant is used to bring the soil into good contact with the cleaning component of the cleaning agent of the present invention. The dispersant is preferably a compound having both a lipophilic part adsorbed on the soil and a hydrophilic part having an affinity for water. As the dispersant used in the present invention, polyethylene glycol, hydroxyethyl cellulose, hydroxypropyl cellulose, dodecyltrimethylammonium chloride, sodium dodecyl sulfate, and octaethylene glycol monododecyl ether are suitable, and two or more of these dispersants are mixed and used. You can also.

  In addition to the cleaning agents used in the present invention, additives that are usually used as cleaning agents can be appropriately added according to the situation at the site of use. For example, an antioxidant, a stabilizer, a foaming agent and the like can be used.

  The system of the present invention has a solid-liquid separation device that separates the slurry after eluting heavy metals contained in the soil into the aqueous cleaning solution solution using the cleaning agent into the soil and the aqueous solution. The solid-liquid separation device is a device for separating the cleaning agent aqueous solution in which heavy metals are dissolved and the soil that does not contain heavy metals, and a soil storage tank that mainly stores the solid-liquid separator and the separated soil It consists of. As the solid-liquid separator, any apparatus can be used as long as it can dehydrate the soil, but it is preferable to use a centrifuge, a filter press, or the like. Soil that no longer contains heavy metals should be temporarily stored in a soil storage tank, treated appropriately, and then discarded or landfilled.

  The system of the present invention recovers heavy metals for recovering heavy metals from an aqueous cleaning solution obtained after separating an aqueous cleaning agent solution in which heavy metals are dissolved from a soil not containing heavy metals using a solid-liquid separator. Have the device. The heavy metal recovery device is mainly composed of a recovery tank that performs processing for recovering heavy metals from the cleaning agent aqueous solution generated from the solid-liquid separation device, and a waste water storage tank for storing the water after such processing. The Here, the process of recovering heavy metals from the cleaning agent aqueous solution means introducing a substance capable of adsorbing heavy metals. Substances that adsorb heavy metals include, for example, inorganic compounds (iron, calcium, etc.), various adsorbents (active carbon, zeolite, chelate resin, etc.), viscosity minerals (bentonite, talc, clay, etc.), organic polymer compounds (anionic) Resin, such as polyacrylic acid soda, acrylic acid-acrylic acid ester copolymer, sodium acrylate-acrylamide strong polymer, or starch-acrylic acid-sodium acrylate copolymer; nonionic resin such as polyacrylamide, alkyl Cellulose or polyethylene oxide), solidifying agents (cement, lime, etc.) or solidifying retarders (citric acid, etc.). These substances are added to a cleaning agent aqueous solution in which heavy metals are dissolved, and are appropriately stirred to adsorb the heavy metals onto the adsorbent, thereby recovering the heavy metals. The treated wash water, which no longer contains heavy metals, is temporarily stored in a wastewater storage tank and disposed of after appropriate treatment.

It is drawing which represented the system of this invention typically. It is drawing explaining the method for measuring the function of the "soil washing apparatus" and the "solid-liquid separation apparatus" of the system of this invention laboratory. It is drawing for demonstrating the method for measuring the function of the "heavy metal collection | recovery apparatus" of the system of this invention laboratory.

  The system of the present invention removes heavy metals from soil by bringing soil containing heavy metals generated by tunnel excavation into contact with a cleaning agent, and drainable or reusable water. Can be used to obtain For example, when the system of the present invention is used for treatment of soil containing a large amount of naturally derived arsenic, it usually depends on the amount of arsenic contained and the type of cleaning agent used, but the soil containing arsenic usually contains A cleaning agent of 1 to 1000 times, preferably 3 to 500 times, more preferably 5 to 100 times based on the volume is used.

  A soil treatment method using the system of the present invention will be described with reference to FIG. The soil to be treated generated by tunnel excavation is transported to a place away from the excavation site on the ground or in the tunnel by a normal method using a locomotive or the like, and placed in the soil cleaning apparatus of the present invention. Soil is put into a soil washing tank in a soil washing apparatus, and a detergent aqueous solution is added thereto from the detergent tank. Here, the arsenic-containing soil and the cleaning agent are stirred and mixed well, and the arsenic in the soil is eluted into the cleaning solution. Thereby, the cleaning agent aqueous solution becomes an arsenic-containing aqueous solution. Next, the slurry is sent to a solid-liquid separator and separated into an arsenic-containing aqueous solution and soil using a solid-liquid separator. The separated soil is stored in a soil storage tank, and the arsenic-containing aqueous solution is sent to the next heavy metal recovery unit. An arsenic adsorbent is added to a heavy metal collection tank in the heavy metal collection apparatus, and is agitated and brought into contact with the arsenic in the arsenic-containing aqueous solution. The obtained arsenic-free water is temporarily stored in a wastewater storage tank and reused in the soil cleaning apparatus of the system of the present invention, or appropriately treated as necessary to be reused as wastewater or industrial water. .

  Since the processing by the system of the present invention is often performed outdoors, the system of the present invention is designed so that it can be suitably used at a temperature of, for example, −20 ° C. to 40 ° C. In order to enhance the effect of washing by the heavy metal washing apparatus of the system of the present invention, the temperature in the soil washing tank can be particularly increased. For example, the cleaning can be performed at 60 ° C. or higher, preferably 80 ° C. or higher, more preferably 100 ° C. or higher. In addition, the agitation when bringing the heavy metal-containing soil into contact with the detergent aqueous solution can use an industrial stirrer, and a large agitator equipped with one or more stirring blades or the like in order to enhance the effect of washing. Can be vigorously stirred, for example, using Further, by providing a plurality of soil cleaning devices and repeating the cleaning a plurality of times, it is possible to transfer heavy metals to the cleaning agent aqueous solution almost certainly.

  The system of the present invention can be used for on-site treatment of soil containing heavy metals generated by tunnel excavation. In particular, the system of the present invention removes arsenic that tends to elute from soil that contains a lot of natural arsenic generated by tunnel excavation, obtains soil that is below the standard value of arsenic elution, and reclaims or discards it on the spot. It is possible to do. On the other hand, the aqueous solution generated by using the system of the present invention can be reused for soil washing as it is, or can be reused on the spot for drainage or other purposes by performing an appropriate treatment.

  A model experiment was conducted to treat heavy metal-containing soil using the system of the present invention on a laboratory scale. An outline of the model experiment is shown in FIG.

Preparation of cleaning agents The following five cleaning agents were prepared:
(1) Preparation of Magnesium Detergent (pH 1) (referred to as “Mg Detergent (pH 1)”) Water (500 mL) was placed in a poly beaker and cooled in an ice bath. Next, 35% hydrochloric acid (30 g, Junsei Chemical), potassium chloride (30 g, Junsei Chemical) and magnesium chloride (30 g, Junsei Chemical) were slowly added and stirred until completely dissolved. Finally, water was added to bring the total volume to 1L.

(2) Preparation of F-based cleaning agent (pH 1) (referred to as “F-based cleaning agent (pH 1)”) Water (500 mL) was placed in a poly beaker and cooled in an ice bath. Next, 35% hydrochloric acid (30 g, Junsei Kagaku), potassium chloride (30 g, Junsei Kagaku) and potassium fluoride (30 g, Junsei Kagaku) were slowly added and stirred until completely dissolved. Finally, water was added to bring the total volume to 1L.

(3) Preparation of fluoride-based cleaning agent (pH 2.5) (referred to as “F-based cleaning agent (pH 2.5)”) Water (500 mL) was placed in a poly beaker and cooled in an ice bath. Next, 35% hydrochloric acid (3 g, Junsei Kagaku), potassium chloride (30 g, Junsei Kagaku) and potassium fluoride (30 g, Junsei Kagaku) were slowly added and stirred until completely dissolved. Finally, water was added to bring the total volume to 1L.

(4) Preparation of fluoride-based cleaning agent (pH 12) (referred to as “F-based cleaning agent (pH 12)”) Water (500 mL) was placed in a poly beaker and cooled in an ice bath. Next, sodium hydroxide (5 g, Pure Chemical), potassium chloride (30 g, Pure Chemical) and potassium fluoride (30 g, Pure Chemical) were slowly added and stirred until completely dissolved. Finally, water was added to bring the total volume to 1L.

(5) Preparation of fluoride detergent (pH 14) (referred to as “F detergent (pH 14)”) Water (500 mL) was placed in a poly beaker and cooled in an ice bath. Next, sodium hydroxide (50 g, Pure Chemical), potassium chloride (30 g, Pure Chemical) and potassium fluoride (30 g, Pure Chemical) were slowly added and stirred until completely dissolved. Finally, water was added to bring the total volume to 1L.

(Example 1) Contact between cleaning agent aqueous solution and arsenic-containing soil An attempt was made to elute arsenic from the arsenic-containing soil into the cleaning solution using the cleaning agent aqueous solution. This experiment is a model experiment at the laboratory level of the washing process in the “soil washing apparatus” and the solid-liquid separation process in the “solid-liquid separation apparatus” in the system of the present invention.

  A separable flask (1 L) is filled with arsenic-containing soil (arsenic elution amount: 0.030 mg / L, arsenic content: 10 mg / kg, 200 g) and an aqueous cleaning solution (300 mL), and the temperature inside the flask is maintained at 25 ° C. And stirred at a rotational speed of 300 rpm for 30 minutes. After completion of stirring, suction filtration was performed to separate the soil and the aqueous washing water solution, and the soil was washed with water. The amount of arsenic elution and arsenic content in the collected soil was measured, and the arsenic concentration in the cleaning aqueous solution was measured.

  In addition, the measurement of the amount of arsenic elution from soil follows the method described in the notification based on the Soil Contamination Countermeasures Law (Ministry of the Environment Notification No. 18, March 6, 2003, which stipulates the measurement method related to the soil leaching investigation). The measurement of arsenic content in soil is conducted according to the method described in the notification based on the Soil Contamination Countermeasures Law (Ministry of the Environment Notification No. 19, March 6, 2003, which stipulates the measurement method for soil content survey) It was. Further, the arsenic concentration in the cleaning aqueous solution was measured according to the method described in “JISK0102, 61.3 Hydride Generation ICP Emission Spectroscopic Analysis” of the Japan Industrial Standards Committee.

  In addition to the five types of cleaning agents prepared in the above experiment, a hydrochloric acid aqueous solution (pH 1.0) and a sodium hydroxide aqueous solution (pH 12) were used as the cleaning agents. The results of the experiment are shown below:

  The standard value of arsenic elution amount stipulated in the Soil Contamination Countermeasures Law is 0.01 mg / L. Therefore, the effect of cleaning the arsenic-containing soil can be judged by whether or not it falls below this reference value. The amount of arsenic elution from the soil after washing with water, aqueous hydrochloric acid solution and aqueous sodium hydroxide solution could not fall below the standard value. On the other hand, when Mg-based cleaning agents and F-based cleaning agents (pH 1 and pH 2.5) were used, the arsenic elution amount from the soil was below the reference value. Note that the arsenic content allowed in the soil stipulated in the Soil Contamination Countermeasures Law is 150 mg / kg or less. Therefore, the arsenic content of the used arsenic-containing soil is originally lower than the standard value. That is, it can be said that the soil used in this experiment is not a soil with a particularly high content of arsenic itself but a soil that cannot be landfilled or discarded because it contains arsenic that is easily eluted. As a result of the above washing experiment, arsenic that was easily eluted was removed, and a soil below the reference value could be obtained.

(Example 2) Adsorption of arsenic from washing water The washing water obtained as a result of bringing the washing aqueous solution into contact with the arsenic-containing soil and separating the solid and liquid contains arsenic that is easily contained in the soil. ing. An attempt was made to adsorb arsenic from an arsenic-containing cleaning aqueous solution. This experiment is a model experiment on a laboratory scale of the heavy metal recovery process in the “heavy metal recovery apparatus” in the system of the present invention. An outline of the model experiment is shown in FIG.

  A syringe was filled with 25 g of A-type zeolite (Tosoh) as an arsenic adsorbent. Arsenic-containing cleaning aqueous solution (arsenic concentration: 0.11 mg / kg, 50 g) obtained in the above-described cleaning experiment (cleaning experiment using F-based cleaning agent (pH 1)) is placed in a syringe, and the syringe piston is pushed to remove the aqueous solution. It was collected. The concentration of arsenic in the collected aqueous solution was measured using an ICP emission analyzer (ICPS-8100, Shimadzu Corporation).

  When the arsenic-containing cleaning aqueous solution obtained in the cleaning experiment using the F-based cleaning agent (pH 1) was treated with A-type zeolite, the arsenic concentration of the obtained aqueous solution was 0.03 mg / kg.

(Example 3) Reuse of cleaning agent aqueous solution The cleaning experiment of Example 1 was performed using the aqueous solution finally obtained by the above arsenic adsorption experiment (arsenic concentration: 0.03 mg / kg). This experiment confirms whether heavy metals can be reused as a cleaning solution by removing heavy metals from the cleaning solution once used.

  In a separable flask (1 L), the aqueous solution (arsenic concentration: 0.03 mg / kg, 150 mL) obtained in Example 2 and the arsenic-containing soil of Example 1 (arsenic elution amount: 0.030 mg / L, arsenic content) : 10 mg / kg, 50 g), the temperature in the flask was maintained at 25 ° C., and the mixture was stirred at a rotational speed of 300 rpm for 30 minutes. After completion of stirring, suction filtration was performed to separate the soil and the aqueous cleaning solution, and the soil was washed with water. When the arsenic elution amount of the collected soil was measured, it was 0.009 mg / L, which was lower than the elution amount reference value 0.01 mg / L.

  In the system of the present invention, it can be seen that the amount of cleaning agent used can be reduced by repeatedly using the cleaning agent.

Claims (8)

A soil cleaning device for mixing heavy metal-containing soil generated by tunnel excavation and an aqueous cleaning agent solution, and eluting the heavy metals contained in the soil into the aqueous solution;
A solid-liquid separator that separates the slurry containing the soil into a soil and an aqueous solution;
A heavy metal recovery device for recovering heavy metals from an aqueous solution;
A system for removing heavy metals contained in soil, including   The system of claim 1, wherein the heavy metals are selected from the group consisting of arsenic, cadmium, chromium, mercury, copper, selenium, lead, fluorine, boron and salts thereof, and ions thereof, and combinations thereof. .   The system according to claim 1 or 2, wherein the aqueous cleaning solution comprises water, acidic substances, metal chlorides, and fluorides.   4. The system of claim 3, wherein the acidic substance is selected from the group consisting of hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, oxalic acid, citric acid, tartaric acid, ascorbic acid, lactic acid, sulfamic acid, hydrofluoric acid, and mixtures thereof. .   4. The system of claim 3, wherein the metal chloride is selected from the group consisting of lithium chloride, sodium chloride, potassium chloride, cesium chloride, magnesium chloride, calcium chloride, strontium chloride, barium chloride, ammonium chloride and mixtures thereof. .   4. The system of claim 3, wherein the fluoride is selected from the group consisting of sodium fluoride, potassium fluoride, ammonium fluoride, ammonium hydrogen fluoride, magnesium fluoride, calcium fluoride, and mixtures thereof.   The system in any one of Claims 3-6 in which cleaning agent aqueous solution contains a dispersing agent further.   8. The system of claim 7, wherein the dispersant is selected from the group consisting of polyethylene glycol, hydroxyethyl cellulose, hydroxypropyl cellulose, dodecyltrimethylammonium chloride, sodium dodecyl sulfate, octaethylene glycol monododecyl ether, and mixtures thereof.

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