JP7261703B2 - Method for treating heavy metal-containing ash - Google Patents

Description

The present invention is a treatment method for immobilizing harmful heavy metals such as lead, cadmium, mercury, arsenic, selenium, and hexavalent chromium contained in heavy metal-containing ash, such as incineration ash and fly ash discharged from garbage incineration plants. It is about.

From the viewpoint of prolonging the landfill life of final disposal sites, the melting treatment of bottom ash and fly ash from waste incinerators, and the direct gasification and melting of wastes are being carried out. Since ash melting furnaces and gasification melting furnaces use high-temperature refractory materials with a high chromium content, excess elution of hexavalent chromium, in particular, is a concern in the treatment of heavy metals in fly ash discharged from treatment facilities.

Fly ash generally contains harmful heavy metals such as Hg, Pb, Cd, hexavalent Cr, As, and Se. 1) Chemical treatment, 2) Melt solidification, 3) Cement solidification, and 4) Extraction with acid or other solvent are obligatory methods for treating these harmful heavy metals. Among these treatment methods, the chemical treatment method is often adopted because it can be performed with relatively simple equipment and at low cost. As the chemical, an organic chelating agent is used, or an inorganic chemical such as phosphoric acid or sulfide is used. Among these methods, the method using an inorganic chemical requires caution in handling due to acid corrosion and odor. Therefore, in recent years, the method using a chelating agent having a dithiocarbamic acid group has become mainstream.

A chelating agent can form a strong complex with cationic heavy metals (heavy metals that are stable as cations in an aqueous solution) such as Hg, Pb, and Cd to immobilize them. However, heavy metals such as hexavalent Cr, As, and Se, which are stable as anions such as chromic acid, arsenic acid (or arsenous acid), and selenic acid (or selenous acid), cannot be fixed in the eluate.

As a heavy metal treatment agent for safely treating such heavy metal contaminants, a chelating agent to which a reducing agent is added (hereinafter also referred to as a mixed agent) disclosed in Patent Document 1 is commercially available. It is also used for heavy metal processing of gasified molten fly ash.

At this time, as the amount of reducing agent necessary for immobilization of hexavalent chromium (hexavalent Cr, Cr ⁶⁺ ), the addition amount equivalent to hexavalent chromium is insufficient, and immobilization cannot actually be achieved with this amount. For this reason, conventionally, the required addition amount is determined by experiments, and usually more reducing agent than necessary is added in consideration of composition fluctuations.

Hexavalent chromium is immobilized by reducing hexavalent chromium to water-insoluble trivalent chromium with a reducing agent. Theoretically, in a heavy metal elution test (hereinafter referred to as elution test) of a product treated with a mixed agent containing a reducing agent and a chelating agent, the form of chromium when reaching an equilibrium state depends on the pH of the eluate and the standard It is determined by the oxidation-reduction potential (hereinafter also referred to as Eh) at the hydrogen electrode. When the pH of the eluate is 12, the stable region of trivalent chromium is entered when Eh is -30 mV or less, and the elution of hexavalent chromium is suppressed.

Eh is usually obtained by adding the potential difference between the reference electrode and the standard hydrogen electrode to the potential (hereinafter also referred to as ORP) measured by an ORP meter using a silver chloride electrode or calomel electrode as a reference electrode. Then, if it is determined that the form of chromium is in the region of trivalent chromium from the temperature, pH, and Eh of the eluate, it means that hexavalent chromium has become trivalent chromium, and hexavalent chromium Immobilization will take place.

Patent No. 5493788 specification

An object of the present invention is to simultaneously treat cationic heavy metals and anionic heavy metals contained in ash, and to treat anionic heavy metals without adding a reducing agent more than necessary. and

In order to achieve the above object, the present invention has the technical features of the following items (1) to ( 5 ).

( 1 ) Chemical treatment of ash containing heavy metals stable as cations in aqueous solutions and hexavalent chromium stable as anions in aqueous solutions,
using a mixed drug containing a reducing agent and a chelating agent,
The amount of the mixed chemical to be added is determined so that the oxidation-reduction potential after stabilization of the eluate of the chemical-treated ash treated with the mixed chemical becomes an oxidation-reduction potential capable of reducing hexavalent chromium to trivalent chromium. A method for treating heavy metal-containing ash, comprising:
For the ash collected in advance, an elution test was performed by changing the amount of the mixed chemical added. Determine the correlation between the amount of the mixed chemical to be added to obtain a stable potential for reduction to valent chromium and the oxidation-reduction potential of the eluate after the dissolution test,
A method for treating heavy metal-containing ash, characterized in that, based on the above correlation, whether or not hexavalent chromium elution is prevented is judged from the oxidation-reduction potential of the eluate after the elution test of chemically treated ash actually treated.

In order to quickly determine whether or not hexavalent chromium elution is prevented, the oxidation-reduction potential of the eluate after the elution test is preferably measured within as short a time as possible, not more than one day after the elution test is completed. More preferably early, such as within 6 hours, and even more preferably immediately after, such as within 1 hour. However, as will be described later, the redox potential of the eluate changes over time until it stabilizes 7 to 10 days after the end of the dissolution test. It is important not to change.

( 2 ) When the oxidation-reduction potential of the eluate after the elution test of the chemical-treated ash is completed is +15% or less of the oxidation-reduction potential of the eluate after the elution test, which is obtained based on the correlation. The method for treating heavy metal-containing ash according to ( 1 ) above, wherein it is determined that the elution prevention of hexavalent chromium is appropriate.

( 3 ) In the chemical treatment of ash containing heavy metals that are stable as cations in aqueous solutions and hexavalent chromium that is stable as anions in aqueous solutions,
using a mixed drug containing a reducing agent and a chelating agent,
The amount of the mixed chemical to be added is determined so that the oxidation-reduction potential after stabilization of the eluate of the chemical-treated ash treated with the mixed chemical becomes an oxidation-reduction potential capable of reducing hexavalent chromium to trivalent chromium. A method for treating heavy metal-containing ash, comprising:
Measurement results of the copper content (Cu (mass%)) and lead content (Pb (mass%)) in the ash collected in advance, and the amount of lead when the amount of the mixed chemical added to the ash was changed. Coefficients k1 and k2 in the following empirical formula (a) regarding the amount of the mixed chemical required to prevent the elution of lead from the added amount of the mixed chemical that can reduce the elution amount to 0.3 mg / L or less seek,
Using the same type of ash as the ash, the added amount (F) of the mixed chemical required to prevent lead elution calculated from the empirical formula (a), and the mixed chemical and humidified water were added to the ash and kneaded. In the case of performing an elution test on the above, the amount of the mixed chemical added that can make the elution amount of lead less than the elution standard value when the added amount of the mixed chemical is changed (B), and the amount of hexavalent chromium Addition amount (D) of the same mixed drug that can make the elution amount less than the elution standard value, and addition amount of the same mixed drug required to make the oxidation-reduction potential at the standard hydrogen electrode after stabilization 0 mV or less (E) and,
For the ash to be treated, the added amount of the mixed chemical necessary for preventing lead elution obtained from the empirical formula (a) is given by {(B/F) average value}×{(D/B) average value}× A method for treating heavy metal-containing ash, characterized in that the value obtained by multiplying {the average value of (E/D)} is used as the amount of the mixed chemical to be added.
Addition amount of mixed chemicals required to prevent lead elution (% by mass)
= k1 × Cu (% by mass) + k2 × Pb (% by mass) (a)

( 4 ) Regarding the ash to be treated, the added amount of the mixed chemical required to prevent lead elution, which is obtained from the empirical formula (a) in the method for treating heavy metal-containing ash in ( 3 ) above, and the heavy metal in ( 3 ) above A method for treating heavy metal-containing ash, characterized by adding a larger amount of the mixed chemical than the amount of the same mixed chemical to be added required in the method for treating the contained ash.

( 5 ) The mixed drug is characterized by using either a chelating drug containing a reducing agent or a drug obtained by mixing a chelating drug containing no reducing agent with a reducing agent having compatibility with the chelating drug. The method for treating heavy metal-containing ash according to any one of the above (1) to ( 4 ).

According to the present invention, the success or failure of immobilization of hexavalent chromium is determined by estimating the Eh after stabilization from the Eh of the eluate after the elution test of chemical-treated ash treated with a mixed chemical of a reducing agent and a chelating agent. This makes it possible to make a quick judgment, quickly carry out chemical-treated ash, and grasp and control an appropriate mixed chemical amount. In addition, by combining conventional techniques for determining the amount of chelating agent to be added to prevent elution of cationic heavy metals, it becomes possible to prevent elution of both cationic heavy metals and anionic heavy metals.

FIG. 4 is a diagram showing the relationship between the amount of mixed chemicals added, the amount of Pb and Cr ⁶⁺ eluted, and the Eh of the eluate. FIG. 10 is another diagram showing the relationship between the amount of mixed chemicals added, the amount of Pb and Cr ⁶⁺ eluted, and the Eh of the eluate. FIG. 4 is a diagram showing the relationship between the amount (E) of the mixed drug added required to make the Eh after stabilization equal to or less than 0 mV and the Eh immediately after the dissolution test is completed. Necessary amount of mixed chemicals to be added obtained from the heavy metal concentration obtained by an automatic measuring device for heavy metal concentration in fly ash, and necessary to reduce the Pb elution amount to 0.3 mg / L or less, which was obtained from the mixed chemical addition test. FIG. 10 is a diagram showing the correlation with a mixed chemical addition amount.

The present invention was perfected using experimental techniques. The method will be described below based on the same method.

The experiments and tests described below were performed with multiple fly ash. Table 1 shows the heavy metal content and heavy metal elution amount for the eight types of fly ash used. The elution amount of mercury was below the elution standard value (0.005 mg/L or less) for all eight types of fly ash used. In Table 1, "T-Cr" means the total chromium concentration including hexavalent chromium.

[Examination example 1]
Using fly ash 1 and fly ash 2 described in Table 1, a mixed agent (manufactured by Tosoh Corporation, product number: TS-400, type of chelating agent: piperazine carbodithioate potassium salt system) was added to 100 parts by mass of fly ash. Agent, type of reducing agent contained: (not disclosed by the manufacturer), content of reducing agent: 2 to 5% by weight) 0 to 12 parts by weight, humidifying water 28 to 40 parts by weight (mixed 40 parts by mass of chemicals and humidified water) were added and kneaded well, and an elution test was performed over 6 hours according to Notification No. 13 of the Japanese Environment Agency. Temperature, pH, ORP and heavy metal concentrations (Pb, Cr ⁶⁺ ) were measured for the eluate filtered through a glass cloth filter with a pore size of 1 μm. The ORP was measured from immediately after the end of the dissolution test until 10 days later. The term "end of the dissolution test" means the point of time when the dissolution test is objectively judged to be completed. The data measured within 1 hour after the end of the dissolution test was defined as the data "immediately after the end of the dissolution test". Table 2 shows the results.

1 and 2 show the relationship between the amount of mixed chemicals added, the amount of Pb and Cr ⁶⁺ eluted, and the Eh of the eluate obtained using the data in Table 2. FIG. 1 relates to fly ash 1 and FIG. 2 relates to fly ash 2. FIG. As described above, Eh is a reference electrode and a standard known in advance in the ORP measurement results from publicly known document 1 (MS Today, Vol. 10, No. 4, p. 14 (April 2001)). It was obtained by adding the potential difference with the hydrogen electrode.

As a result, when the pH of the eluate is 12.2 to 12.5, it was found that the known document 2 (Nobuyuki Tsuneoka et al.: Effect of adsorption/reduction action of soil on hexavalent chromium eluted from cement-improved soil, Japan Society of Civil Engineers Papers, No.764/III-67, 133-145, 2004.6) in the theoretical Eh region (-30 to -50 mV), the elution amount of hexavalent chromium is 0.05 mg, which is the lower limit of determination. It was confirmed that it can be reduced to about /L. In addition, it was confirmed that Eh of about 0 mV is sufficient in order to make the elution amount of hexavalent chromium equal to or less than the elution standard value of 1.5 mg/L.

The Eh of the eluate gradually decreased and stabilized after 7 days. Further, the amount of the mixed chemical added necessary for preventing the elution of Cr ⁶⁺ was slightly larger than the amount of the mixed chemical added necessary for preventing the elution of Pb. Furthermore, the amount of the mixed drug added that makes the Eh after stabilization 0 mV or less is greater than the amount of the mixed drug added required to prevent the elution of Cr ⁶⁺ , and the amount of the mixed drug added prevents the elution of Cr ⁶⁺ from 0.2 mg/L. It was possible to suppress the following.

Hereinafter, the "addition amount" may be simply referred to as "amount". Table 3 shows the amount of the mixed drug required to prevent the elution of Pb and Cr ⁶⁺ , the amount of the mixed drug that Eh is 0 mV or less after 10 days from the elution test, and the elution with this mixed drug amount, which was obtained from Figures 1 and 2. Eh and the like immediately after the end of the test are shown.

The "Cu, Pb content and reaction equivalent mixed chemical amount (A)" in Table 3 is determined by the following formula 1. In Table 3, the "mixed chemical amount (C) corresponding to the reducing agent amount corresponding to the Cr ⁶⁺ content and the reaction equivalent" is obtained by the following equation 2. The chelating agent preferentially reacts with metals Au<Pt<Ag<Hg<Cu,<(H)<Pb<Sn . , the content and the reaction equivalent amount (A) of the mixed chemical agent is a measure of the amount of the mixed chemical agent necessary for preventing Pb elution.

Mixed drug amount (A) [% by mass] = (Cu content [g/kg]/Cu atomic weight + Pb content [g/kg]/Pb atomic weight) x molar mass of chelate/(concentration of chelate/100) x (100/1000) (Formula 1)
Here, the molar mass of the chelate contained in the above Tosoh product number: TS-400, that is, the potassium salt of piperazine carbodithioate is 314.6 [g/mol], and the concentration of the chelate in TS-400 is 35. [% by mass].

Amount of mixed drug (C) [% by mass]=(Cr ^{6 +} content [mg/kg]/Cr atomic weight)/1000×110×1.5/(3.5/100)×(100/1000)... (Formula 2)
Here, the concentration of the reducing agent contained in the mixed chemical is 3.5 [mass %]. Also, 1.5 is the reaction equivalent ratio of the reducing agent required to reduce Cr ⁶⁺ to Cr ³⁺ .

As shown in Table 3, the mixed chemical amount (C) corresponding to the amount of reducing agent for the Cr ⁶⁺ content and the reaction equivalent is 0.29% by mass for fly ash 1 and 0.52% by mass for fly ash 2. be done. However, in order to keep the Cr ⁶⁺ elution amount below the elution standard value of 1.5 mg/L when the pH of the eluate is in the range of 12.2 to 12.5, Eh after 10 days must be 0 mV or below. be. The amount of mixed chemical (E) required for that purpose is 7.89% by mass for fly ash 1 and 9.41% by mass for fly ash 2. In other words, there is a difference of about 18 to 27 times between the Cr ⁶⁺ content and the equivalent mixed drug amount (C). From the results shown in Table 3, it can be seen that the amount of mixed chemicals corresponding to the required amount of reducing agent cannot be calculated by simple equivalent calculation. The amount of the mixed drug required to reduce Cr ⁶⁺ in the eluate to Cr ³⁺ must be the amount that makes Eh 0 mV or less after 10 days as described above, and the amount of the mixed drug equivalent to this amount must be added. As shown in Table 2, it was found that the Cr ⁶⁺ elution amount was 1.5 mg/L or less.

[Examination example 2]
Using fly ash 3 to fly ash 8, 100 parts by mass of fly ash, 0 to 10 parts by mass of a mixed agent (Tosoh product number: TS-400), and 30 to 40 parts by mass of humidifying water (mixed 40 parts by mass of chemicals and humidified water) were added and kneaded well, and an elution test was performed according to Notification No. 13 of the Japanese Environment Agency. Temperature, pH, ORP and heavy metal concentrations (Pb, Cr ⁶⁺ ) were measured for the eluate filtered through a glass cloth filter with a pore size of 1 μm. The ORP was measured from immediately after the end of the dissolution test until 10 days later. Table 4 shows the results.

Table 5 shows the amount of mixed chemicals required to prevent elution of Pb and Cr ⁶⁺ for fly ash 1 to 8, obtained by creating figures similar to FIGS. 1 and 2 of Study Example 1, from the elution test. The amount of the mixed drug at which Eh becomes 0 mV or less after 10 days, and the Eh immediately after the end of the dissolution test with this amount of mixed drug are shown. The Eh immediately after the dissolution test in the mixed drug amount (E) required to reduce the Eh to 0 mV or less after 10 days was 78.35 mV (-131.65 mV in ORP) on average in Study Examples 1 and 2. That is, if the Eh of the chemical-treated ash treated with the mixed chemical immediately after the elution test is completed is about 78.35 mV, the Eh after stabilization (after 7 to 10 days) is about 0 mV, and Pb and Cr ⁶⁺ are eluted. The amount will be suppressed below the elution standard value.

In order to judge the success or failure of immobilization of Cr ⁶⁺ from the Eh of the eluate immediately after the elution test of the fly ash treated with the mixed chemical, it is necessary to predict the Eh 10 days after the elution test. .

FIG. 3 shows the correlation between the amount of mixed drug (E) required to make the Eh after stabilization shown in Table 5 0 mV or less and the Eh immediately after the dissolution test. An approximation derived from the data plotted in FIG. 3 is y=−4.3114x+110.59
More, for example, the Eh of the eluate immediately after the end of the elution test of the treated product obtained by adding and kneading 5% by mass of a mixed drug (manufactured by Tosoh Corporation: TS-400) to fly ash is 89.0 mV (at 25 ° C. If the ORP is lower than −117 mV), the elution of Cr ⁶⁺ can be suppressed below the elution standard value. Of course, the elution of Pb is also suppressed below the elution standard value. However, considering variations in data, as shown in the figure, it is considered appropriate to control the potential within +15% or less, preferably within a range of about ±15% of the potential obtained by the approximation formula.

In the above description, the best results were obtained by using Eh immediately after the dissolution test was completed, but the present invention is not limited to this. Instead of the Eh immediately after the dissolution test is completed, the Eh at any point before the eluate is stabilized can be used in the same manner. For example, the present invention does not preclude the use of Eh within 3 days after the end of the dissolution test, or Eh within 1 day after the end of the dissolution test.

In addition, in the above, the dissolution test was conducted with the dissolution time set to 6 hours in accordance with the provisions of Notification No. 13 of the Japanese Environment Agency, but the dissolution time is not limited to 6 hours. Judging objectively, the elution test can be completed in a shorter time if the elution of heavy metals is sufficient. It is important not to change in the middle.

[Examination example 3]
Using fly ash 3, 100 parts by mass of the fly ash, 0 to 10 parts by mass of a chelating agent that does not contain a reducing agent (product number: TS-300 manufactured by Tosoh), and compatible with the chelating agent. 0.35 parts by mass of dipotassium sulfide as a reducing agent is added to 10 parts by mass of the chelating agent, and 30 to 40 parts by mass of humidifying water (40 parts by mass of the chelating agent + humidifying water) are added and well kneaded to obtain a Japanese environment. A dissolution test was performed according to Government Notification No. 13. Temperature, pH, ORP and heavy metal concentrations (Pb, Cr ⁶⁺ ) were measured for the eluate filtered through a glass cloth filter with a pore size of 1 μm. The ORP was measured from immediately after the end of the dissolution test until 10 days later. Using the results, graphs similar to those in Figs. 1 and 2 of Investigation Example 1 were created, and the amount of the mixed drug required to prevent the elution of Pb and Cr ⁶⁺ , and the mixed drug whose Eh was 0 mV or less after 10 days from the elution test. Eh immediately after completion of the dissolution test with this mixed drug amount was determined. Table 6 shows the results.

The amount of the mixed drug required to prevent the elution of Pb and Cr ⁶⁺ , the amount of the mixed drug at which Eh becomes 0 mV or less after 10 days from the elution test, and the Eh immediately after the elution test with this mixed drug amount, etc. Similar test results were obtained with Ash 3. By selectively using a reducing agent that does not impair the stability of the piperazine carbodithioate potassium salt-based chelating agent, the same Pb and an effect of preventing elution of Cr ⁶⁺ was obtained. The reducing agent used in Investigation Example 3 was dipotassium sulfide, which was combined with the potassium salt of TS-300.

[Examination example 4]
An automatic measuring device for heavy metal concentrations in ash was installed at the inlet chute of the fly ash kneading device to automatically measure the concentrations of Cu and Pb in the fly ash. As this automatic measuring device, those described in Japanese Patent No. 5361698 and Japanese Patent No. 5657112 were used. Further, for the same fly ash, a mixed chemical addition test was performed to determine the amount of the mixed chemical required to reduce the Pb elution amount from the chemical-treated ash to 0.3 mg/L or less. Then, from the test results of about 20 samples, the values obtained by multiplying the analysis results of Pb and Cu by constant constants k1 and k2, and the amount of mixed chemicals required to reduce the Pb elution amount to 0.3 mg/L or less. The above constants k1 and k2 were determined to match. In the fly ash used in this study example 4, the Cu content (mass%) was multiplied by 4.4 as k1, and the Pb content (mass%) was multiplied by 17.6 as k2. could ask for That is, the following formula 3 was able to be obtained.
Mixed drug amount (% by mass) required to reduce the Pb elution amount to 0.3 mg/L or less
= 4.4 × Cu (mass%) + 17.6 × Pb (mass%) (Equation 3)

The above constants vary depending on the type of mixed chemicals used, the type of fly ash, the preparation method of the sample for fluorescent X-ray measurement with an automatic measuring device, and the like. Incidentally, the means for automatically measuring the concentration of heavy metals in the ash at the inlet chute portion of the fly ash kneading device is described in, for example, JP-A-2005-118733 in addition to those described in the above two patent publications. You can also use tools.

Fig. 4 shows the required mixed chemical amount obtained from the heavy metal concentration obtained by the automatic measuring device of the heavy metal concentration in the fly ash, and the Pb elution amount obtained by the mixed chemical addition test to be 0.3 mg / L or less. shows the correlation with the amount of mixed drug required for From this relationship, it was found that plus 1% by mass should be added to the right side of Equation 3 in order to ensure that the elution amount of Pb is 0.3 mg/L or less. In FIG. 4, the diamond-shaped marks represent the data of the fly ash collected in advance to determine the value of the constant in Equation 3, and the circular marks are the same type of fly ash collected from the same facility as the same fly ash 3 to fly ash. Data for Ash 8 are represented.

For fly ash 3 to 8, the amount of mixed chemicals required to prevent Pb elution, which was obtained by an automatic measuring device for heavy metal concentrations in fly ash, and the amount of Pb and Cr ⁶⁺ elution obtained in Study Example 2 were used as the elution criteria. Table 7 shows the amount of the mixed chemicals required to reduce the Eh to the value or less and the amount of the mixed chemicals that makes the Eh of 0 mV or less after stabilization for the eluate of the chemical-treated ash treated with the mixed chemicals.

In Table 7, the mixed drug amount (E) required to make Eh after stabilization 0 mV or less is the mixed drug amount (D) required to make the Cr ^{6 +} elution amount 1.5 mg / L or less. surpassed. In addition, the mixed chemical amount (F) obtained by the above equation 3 also exceeds the mixed chemical amount (B) required to reduce the Pb elution amount to 0.3 mg/L or less. Therefore, it is possible to obtain the amount of the mixed chemical necessary for immobilizing the target heavy metal. In addition, by performing fly ash treatment with the larger amount of mixed chemicals (E) where Eh after stabilization is 0 mV or less and the amount of mixed chemicals (F) obtained by Equation 3, cationic heavy metals and anions It becomes possible to prevent the elution of both heavy metals from the system.

When controlling the amount of mixed chemicals to be added in actual processing, if a maximum of 3% by mass is added to the amount of mixed chemicals (F) obtained by Equation 3 shown in Table 7, Pb and Cr The amount of elution of ⁶⁺ never exceeds the elution standard value. Here, the mixed medicine amount after this addition is referred to as "controlled mixed medicine amount (1)". Also, if the mixed drug amount (E) at which Eh becomes 0 mV or less is E>B and E>D,
F × {Average value of (B/F)} × {Average value of (D/B)} × {Average value of (E/D)}
By controlling the amount of the mixed chemical added, as shown in Table 8, the amount of the mixed chemical used can be reduced by about 1.6% by mass (8.435-6.837=1.598). The mixed drug amount managed in this way is referred to as a "controlled mixed drug amount (2)", and is the amount of the mixed drug to be added. Table 8 shows a comparison of the amount of each mixed drug. The control mixed chemical amount (2) corresponds to the mixed chemical amount (E) required to make the Eh after stabilization 0 mV or less. By treating the fly ash with the larger amount (F) or the control mixed chemical amount (2), it becomes possible to prevent the elution of both cationic heavy metals and anionic heavy metals. If the mixed drug amount (E) that makes Eh equal to or less than 0 mV does not satisfy the conditions of E>B and E>D, the controlled mixed drug amount (1) is added instead of the controlled mixed drug amount (2). It is sufficient to set the amount of the mixed drug to be added to the desired value.

The amount of chelating agent added necessary for preventing elution of cationic heavy metals such as Hg, Pb, and Cd can be measured using a known method. However, known methods for measuring the amount of chelating agent added necessary for preventing elution of cationic heavy metals are not effective methods for fixing harmful anionic heavy metals such as Cr ⁶⁺ , As, and Se.

The present invention relates to a method for preventing Cr ⁶⁺ elution, which is concerned about excess elution in molten fly ash such as gasification melting and ash melting. By combining the method of the present invention with a conventional technique for determining the amount of chelating agent added to prevent the elution of cationic heavy metals, it becomes possible to prevent the elution of both cationic heavy metals and anionic heavy metals.

Specifically, as described above, the theoretical principle of preventing the elution of hexavalent chromium is reproduced from the relationship between the temperature, pH, and Eh of the eluate of the chemical-treated ash treated with a mixed chemical of a reducing agent and a chelating agent. was done. However, it was confirmed that Eh changed over time. It takes about 7 to 10 days for this Eh to stabilize. For this reason, if it is not possible to quickly determine whether the hexavalent chromium fixation of the chemical-treated ash treated with the mixed chemical is successful or not at the treatment site, a storage period of about 7 to 10 days is required before the chemical-treated ash is transported. put away. In this regard, according to the present invention, the success or failure of immobilization of hexavalent chromium is determined by estimating the Eh after stabilization from the Eh of the eluate after the elution test of the chemical-treated ash actually treated with the mixed chemical. can be determined immediately. As a result, the chemical-treated ash can be carried out quickly, and the required mixed chemical amount can be properly grasped and controlled.

[Example 1]
The automatic measuring device for the heavy metal concentration in the ash described in the above-mentioned Japanese Patent Nos. 5361698 and 5657112 is installed at the inlet chute portion of the fly ash kneading device, and the Cu in the fly ash to be processed is and Pb concentrations were automatically measured. At the same time, the fly ash was sampled, and the contents of T-Cr and Cr ⁶⁺ were chemically analyzed, and the amount of heavy metal elution before the addition of the mixed chemical was measured by the Japanese Environment Agency Notification No. 13 test. Those results are shown in Table 9.

From the contents of Cu and Pb analyzed by an automatic measuring device for heavy metal concentrations, it was possible to obtain the amount (F) of the mixed chemical to be added necessary for Pb immobilization from Equation 3. For fly ash 9 to 11 in Table 9, the mixed agent (manufactured by Tosoh Corporation, product number: TS-400) of the controlled mixed agent amount (1) obtained by adding the mixed agent amount (F) + 3% by mass and humidified water. In addition, a kneading process was performed. In addition, for fly ash 12 to 14, based on the values shown in Table 8, 0.829 × {(D / B ) is 1.098 × {(E/D) average value} is 1.382”. ) and moistened water were added to perform a kneading treatment. Table 10 shows the amount of the mixed chemicals added and the results of the elution test of the kneaded chemical-treated ash. As shown in Table 10, the elution amounts of Pb and Cr ⁶⁺ in the chemically treated ash that were actually treated were both below the elution standard values (Pb: 0.3 mg/L or less, Cr ⁶⁺ : 1.5 mg/L or less). had cleared. In addition, the Eh immediately after the end of the elution test of the eluate of each chemical-treated ash also satisfies the target control value (that is, the range of Eh immediately after the end of the elution test shown in FIG. 3), and the Eh after stabilization is 0 mV. expected to arrive in the vicinity.

Claims (5)

In the chemical treatment of ash containing heavy metals that are stable as cations in aqueous solutions and hexavalent chromium that is stable as anions in aqueous solutions,
using a mixed drug containing a reducing agent and a chelating agent,
The amount of the mixed chemical to be added is determined so that the oxidation-reduction potential after stabilization of the eluate of the chemical-treated ash treated with the mixed chemical becomes an oxidation-reduction potential capable of reducing hexavalent chromium to trivalent chromium. A method for treating heavy metal-containing ash, comprising:
For the ash collected in advance, an elution test was performed by changing the amount of the mixed chemical added. Determine the correlation between the amount of the mixed chemical to be added to obtain a stable potential for reduction to valent chromium and the oxidation-reduction potential of the eluate after the dissolution test,
A method for treating heavy metal-containing ash, characterized in that, based on the above correlation, whether or not hexavalent chromium elution is prevented is judged from the oxidation-reduction potential of the eluate after the elution test of chemically treated ash actually treated. When the oxidation-reduction potential of the eluate after the elution test of the chemical-treated ash is completed is +15% or less of the oxidation-reduction potential of the eluate after the elution test is obtained based on the correlation, the hexavalent 2. The method for treating heavy metal-containing ash according to claim 1 , wherein it is judged that the elution prevention of chromium is appropriate. In the chemical treatment of ash containing copper and lead, which are heavy metals stable as cations in aqueous solutions, and hexavalent chromium, which is stable as anions in aqueous solutions,
using a mixed drug containing a reducing agent and a chelating agent,
The amount of the mixed chemical to be added is determined so that the oxidation-reduction potential after stabilization of the eluate of the chemical-treated ash treated with the mixed chemical becomes an oxidation-reduction potential capable of reducing hexavalent chromium to trivalent chromium. A method for treating heavy metal-containing ash, comprising:
Measurement results of the copper content (Cu (mass%)) and lead content (Pb (mass%)) in the ash collected in advance, and the amount of lead when the amount of the mixed chemical added to the ash was changed. Coefficients k1 and k2 in the following empirical formula (a) regarding the amount of the mixed chemical required to prevent the elution of lead from the added amount of the mixed chemical that can reduce the elution amount to 0.3 mg / L or less seek,
Using the same type of ash as the ash, the added amount (F) of the mixed chemical required to prevent lead elution calculated from the empirical formula (a), and the mixed chemical and humidified water were added to the ash and kneaded. In the case of performing an elution test on the above, the amount of the mixed chemical added that can reduce the elution amount of lead below the elution standard value when the added amount of the mixed chemical is changed (B), and the amount of hexavalent chromium Addition amount (D) of the same mixed drug that can make the elution amount less than the elution standard value, and addition amount of the same mixed drug required to make the oxidation-reduction potential at the standard hydrogen electrode after stabilization 0 mV or less (E) and,
For the ash to be treated, the addition amount of the mixed chemical necessary for preventing lead elution obtained from the empirical formula (a) is given by {(B/F) average value}×((D/B) average value}× A method for treating heavy metal-containing ash, characterized in that the value obtained by multiplying {the average value of (E/D)} is used as the amount of the same mixed chemical to be added.
Addition amount of mixed chemicals required to prevent lead elution (% by mass)
= k1 × Cu (% by mass) + k2 × Pb (% by mass) (a) For the ash to be treated, the added amount of the mixed chemical necessary for preventing lead elution, which is obtained from the empirical formula (a) in the method for treating heavy metal-containing ash described in claim 3 , and the heavy metal content described in claim 3 . A method for treating heavy metal-containing ash, characterized by adding a larger amount of the mixed chemical than the amount of the same mixed chemical to be added required in the method for treating ash. The mixed drug is either a chelating drug containing a reducing agent or a drug obtained by mixing a chelating drug containing no reducing agent with a reducing agent having compatibility with the chelating drug. A method for treating heavy metal-containing ash according to any one of items 1 to 4 .

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