JPH10249316A - Treatment of heavy metal-containing ash - Google Patents

Abstract

PROBLEM TO BE SOLVED: To surely prevent an elution of a heavy metal from a heavy metal- containing ash by treating easily and inexpensively by adding a small amount of a fixing agent irrespective of a kind and shape of the heavy metal-containing ash or without using a pH adjusting agent. SOLUTION: The heavy metal is fixed by adding water, a chelating agent and starch to the heavy metal-containing ash and kneading the mixture. The heavy metal is fixed as a compd. hardly soluble in water by adding the water, the chelating agent, the starch and a phosphoric acid based heavy metal fixing agent to the heavy metal-containing ash and kneading the mixture. The starch has a helical structure using six glucose groups as a unit, and a fixing effect of the heavy metal is far more stabilized by adsorbing directly or incorporating the formed hardly soluble compd. or an unreacted heavy metal in the helical structure.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating heavy metal-containing ash, and more particularly, to a method for easily and efficiently converting heavy metal-containing ash.
In addition, the present invention relates to a method for safely treating and reliably preventing heavy metals from being eluted from a treated product.

[0002]

2. Description of the Related Art When municipal waste and industrial waste are incinerated in an incineration plant, fly ash is generated along with incineration ash and combustion exhaust gas. Fly ash in the combustion exhaust gas is collected by a bag filter or an electric dust collector. Further, when incinerated ash and fly ash are melted in a melting furnace, molten fly ash is generated. On the other hand, ash called electric furnace dust is generated from a metal refining electric furnace. Since these incineration ash, fly ash, molten fly ash and electric furnace dust all contain heavy metals (hereinafter referred to as “heavy metal-containing ash”), these heavy metal-containing ash can be converted from ash to heavy metal. It is used for landfill, ground works, etc. after performing immobilization treatment to insolubilize heavy metals contained so as not to elute and contaminate the environment.

[0003] Of these heavy metal-containing ash, incineration ash is not yet legally required to be treated. However, in recent years, from the viewpoint of environmental protection, the number of landfill facilities that independently apply landfill standards has been increasing, and a treatment for preventing elution of heavy metals is required.

On the other hand, fly ash contains a lot of heavy metals such as lead and cadmium.

Meanwhile, the combustion waste gas, typically, a method of spraying hydrogen chloride, SO _x, and contain acid gases such as NO _x, in order to remove these acidic gases, hydrated lime powder before dust collection (Dry treatment) and a method of washing with an aqueous alkali solution after dust collection (wet treatment) are used. In the case of the former dry treatment, the fly ash produced becomes strongly alkaline because excessively sprayed strong alkaline slaked lime is mixed. For this reason, fly ash generated in an incinerator in which dry waste gas treatment is performed is called “alkali fly ash”. Alkali fly ash is eluted water generated when it comes into contact with water
H becomes high, and lead which is an amphoteric metal is easily eluted at a high concentration. On the other hand, when the latter wet waste gas treatment is performed, the fly ash generated is collected under conditions where the slaked lime is not sprayed. It is called "active fly ash." Neutral fly ash is usually weakly acidic, but may be slightly alkaline. This is because in an incinerator for municipal garbage and industrial waste, lime may be generated during incineration depending on the type of garbage and industrial waste, and the generated lime accompanies fly ash. Therefore,
The eluate produced when neutral fly ash comes into contact with water exhibits various properties from weakly acidic to weakly alkaline.

[0006] Electric furnace dust generated in a metal refining electric furnace may become weakly alkaline due to the accompanying lime added for refining. It is.

Hitherto, as a method for preventing the heavy metal-containing ash from eluting the heavy metal, a melting method, a cement solidification method, an extraction method using an acid or other solvent, a chemical treatment method, and the like have been proposed. Many chemical treatment methods are used in terms of treatment cost and the like.

As a chemical treatment method, as one of the methods for preventing the elution of heavy metals from fly ash, a chelating agent such as a dithiocarbamic acid is added to alkali fly ash to fix heavy metals as a water-insoluble substance. There is a way.

In addition, fly ash or a mixture of fly ash and incinerated ash
A method of preventing the elution of lead and cadmium by adding a phosphate-based heavy metal fixing agent in the presence of a calcium hydroxide source (Japanese Patent Publication No. 4-61710) has also been proposed.

As a method for detoxifying fly ash from an incinerator for municipal garbage, α is disclosed in JP-A-7-136615.
A method of adding sodium silicate and / or a hydrogen phosphate compound, a method of adding allophane having a SiO ₂ / Al ₂ O ₃ molecular ratio of 2 or more, and the like. ing.

[0011]

Among the above conventional methods, a method of adding a chelating agent such as a dithiocarbamic acid is
When applied to alkaline fly ash with a high content of heavy metals such as lead,
The amount of the chelating agent required for fixing the heavy metal increases, and the processing cost increases. Further, after the treated fly ash is landfilled, if it is affected by acid rain or the like, there is a problem that the chelate is decomposed and heavy metals may be eluted.

On the other hand, in the method of adding a phosphoric acid-based heavy metal fixing agent described in Japanese Patent Publication No. 4-61710, the fixing agent is used for the alkali fly ash containing a large amount of unreacted slaked lime. This leads to the problem that the pH of the eluate does not decrease and lead elution cannot be sufficiently prevented.

Further, a method of adding a pregelatinized starch and / or dextrin, a method of adding a sodium silicate and / or a hydrogen phosphate compound described in JP-A-7-136615, and a method of adding SiO ₂ / Al ₂ O ₃ In the method of adding allophane having a molecular ratio of 2 or more, in order to treat fly ash containing a large amount of heavy metals, it is necessary to add a large amount of the fly ash. There is a problem that heavy metals cannot be fixed below the landfill reference value.

The present invention solves the above-mentioned conventional problems, and makes it possible to produce heavy metal-containing ash irrespective of its type and properties.
An object of the present invention is to provide a method for easily and inexpensively treating by adding a small amount of a fixing agent without using a pH adjuster, thereby reliably preventing elution of heavy metals from heavy metal-containing ash.

[0015]

The method for treating heavy metal-containing ash according to the present invention comprises mixing heavy metal-containing ash with water, a chelating agent and starch, or a phosphoric acid-based heavy metal fixing agent and kneading the mixture. It is characterized in that heavy metals in the contained ash are immobilized.

In the present invention, a chelating agent or a chelating agent and a phosphate-based heavy metal fixing agent are reacted with heavy metals in a heavy metal-containing ash to be fixed as a poorly water-soluble compound. Starch has a helical structure with six glucose groups as a unit, and the resulting hardly soluble compound or, in some cases, unreacted heavy metal is directly adsorbed or immobilized by incorporating it into the helical structure. The effect is further stabilized.

[0017]

Embodiments of the present invention will be described below in detail.

The chelating agent used in the present invention includes:
A water-soluble compound having a hydrophilic chelate-forming group in the base, which selectively reacts with a heavy metal ion to form a strong chelate bond and form a heavy metal complex that is hardly soluble in water is preferable. Examples of such a chelating agent include various dithiocarbamic acids and / or their water-soluble salts. Examples of dithiocarbamic acid include dimethyldithiocarbamic acid, diethyldithiocarbamic acid, dibutyldithiocarbamic acid, and biperazinedithiocarbamic acid. Further, as these water-soluble salts, alkali metal salts such as sodium salt and potassium salt,
Examples thereof include aliphatic primary amine salts, aliphatic secondary amine salts, and polyamine salts. These chelating agents may be used alone or in combination of two or more.

There are no particular restrictions on the base plant of the starch, such as starch by mauzu, sweet potato starch, wheat starch,
Corn starch, tapioca starch and the like. In addition, processed starch, for example, so-called soluble starch, improved solubility in cold water by acting an acid on starch, cationized starch, pregelatinized starch, phosphoric acid starch, carboxymethylated starch And dextrin, which is obtained by partially hydrolyzing starch by the action of heat, acid, alkali or the like, such as roasted dextrin or enzyme-modified dextrin. These starches are 1
The seeds may be used alone or in combination of two or more.

Further, as the phosphoric acid-based heavy metal fixing agent,
Phosphoric acid or a water-soluble salt thereof is used. Among them, phosphoric acid includes orthophosphoric acid, hypophosphorous acid, metaphosphorous acid, pyrophosphorous acid, orthophosphorous acid, hypophosphoric acid, metaphosphoric acid, pyrophosphoric acid Acid, triphosphoric acid, condensed phosphoric acid, etc., and the water-soluble salt thereof is a sodium salt having a Na / PO ₄ molar ratio of 1.0 to 4.0 or a K / PO ₄ molar ratio of 1.0 to 4.0. 4.0 potassium salt. These phosphate-based heavy metal fixing agents may be used alone or in combination of two or more.

The method of the present invention is carried out by adding water and the above-mentioned chelating agent and starch, or further a phosphoric acid-based heavy metal fixing agent to a heavy metal-containing ash and kneading the mixture.

The amount of water added to the heavy metal-containing ash is 10 to
The content is preferably set to 60% by weight, but there is no great difference in the effect of the water content after the treatment, and the amount of water added can be arbitrarily set from the viewpoint of workability and the like.

The order of addition of the water, chelating agent, starch, and / or phosphate-based heavy metal fixing agent is not particularly limited, and they may be added in any order or may be added simultaneously. In addition, water may be mixed with one or more of a chelating agent, starch, and a phosphate-based heavy metal fixing agent in advance and added in the form of a solution or slurry.

In the method according to the first aspect, when the heavy metal-containing ash is treated with a chelating agent and starch, the chelating agent is preferably a dithiocarbamic acid or a 30 to 40% by weight aqueous solution of an alkali metal salt or an amine salt thereof. A liquid chelating agent is preferable, and its preferable addition amount is 0.1 to 5 as a solid content with respect to 100 parts by weight of the heavy metal-containing ash.
Parts by weight, especially 0.3 to 3 parts by weight. Further, as the starch, pregelatinized starch or dextrin having a molecular weight of 1,000 to 1,000,000 is preferable, and a preferable addition amount thereof is 1 to 25 parts by weight as a solid content with respect to 100 parts by weight of heavy metal-containing ash.
In particular, it is 5 to 20 parts by weight. When dextrin is used as starch, the degree of hydrolysis is 3 to 100 in DE value (= {direct reducing sugar (glucose) / total solid content} × 100).
It is preferably 15%.

In the method of claim 2, when the heavy metal-containing ash is treated by adding a chelating agent, a starch and a phosphate-based heavy metal fixing agent, the chelating agent is particularly dithiocarbamic acid or an alkali metal salt thereof. Amine salts are preferred, and the preferred amount of addition is heavy metal-containing ash 1.
It is 0.1 to 5 parts by weight, particularly 0.3 to 3 parts by weight as a solid content based on 00 parts by weight. The starch is preferably a pregelatinized starch or dextrin having a molecular weight of 1,000 to 1,000,000, and its preferable addition amount is 1 to 25 parts by weight, particularly 5 to 2 parts by weight, as a solid content with respect to 100 parts by weight of heavy metal-containing ash.
0 parts by weight. When dextrin is used as starch, the degree of hydrolysis is preferably 3 to 15% in terms of DE value (= {direct reducing sugar (glucose) / total solid content} × 100).

Further, as the phosphoric acid-based heavy metal fixing agent,
Orthophosphoric acid or its sodium salt or potassium salt is preferable, and its preferable addition amount is 1 to 20 parts by weight, particularly 5 to 15 parts by weight as a solid content with respect to 100 parts by weight of the heavy metal-containing ash. In particular, when a water-soluble salt is used as the phosphoric acid-based heavy metal fixing agent instead of a strongly acidic agent such as orthophosphoric acid, the corrosiveness to equipment such as chemical injection equipment and kneaders is significantly increased. Reduced and industrially advantageous.

According to the present invention, a kneaded product with little elution of heavy metals can be obtained by adding water and a chelating agent and starch, or a phosphoric acid-based heavy metal fixing agent to the heavy metal-containing ash and kneading them.

The method of kneading the heavy metal-containing ash with water, the chelating agent and the starch, or further with the phosphoric acid-based heavy metal fixing agent is such that the heavy metal-containing ash and the chelating agent, the starch and the phosphate-based heavy metal fixing agent are uniformly mixed. Any method may be used. This mixture is usually
Landfill disposal.

[0029]

The present invention will be described more specifically below with reference to examples and comparative examples.

In the following Examples and Comparative Examples,
The component composition of the treated fly ash (raw ash) is as shown in Table 1.

[0031]

[Table 1]

Examples 1 to 4 and Comparative Examples 1 to 6 To 100 parts by weight of raw ash A, starch and / or a liquid chelating agent were added in the amounts shown in Table 2 (however, only water was used in Comparative Example 1). In addition to the parts by weight, the mixture was kneaded with a small kneader.

The starch has a molecular weight of 1600 to 3300 and a DE value of 5 to 10 of corn as the base plant.
% Dextrin was used. In addition, liquid chelates include:
A 36% by weight aqueous solution of commercially available potassium piperazine dithiocarbamate was used.

The treated product was subjected to a heavy metal dissolution test according to the Environment Agency Notification No. 13 test method. The measurement of the elution water was performed on the pH and the lead concentration in the elution water, and the results are shown in Table 2.

[0035]

[Table 2]

As shown in Table 2, even when 4 parts by weight of the liquid chelate was used alone, the effect of fixing heavy metals was insufficient (Comparative Example 4). However, by using 2.5 parts by weight of starch, the amount of liquid chelate added was 2%.
Elution of lead could be suppressed in parts by weight (Example 2).

The processing cost of Example 2 is calculated to be 10,500 yen per ton of fly ash, assuming that the starch is 100 yen / Kg and the liquid chelate is 400 yen / Kg. On the other hand, the processing cost of Comparative Example 4 using the liquid chelate alone was estimated to be 16,0 per ton of fly ash.
It becomes 00 yen.

Examples 5 to 10 and Comparative Examples 7 to 14 Starch and / or liquid chelate were added to 100 parts by weight of raw ash B in the amounts shown in Table 3 (however, only water was used in Comparative Example 7), and water was further added to 30 parts by weight. The treatment was carried out in the same manner as in Example 1 except that the kneading was carried out in addition to parts, and the test was carried out in the same manner. The results are shown in Table 3.

[0039]

[Table 3]

As shown in Table 3, when the liquid chelate was used alone, the effect of fixing lead was insufficient even when the liquid chelate was added in an amount of 10 parts by weight (Comparative Example 13). But,
By using 2.5 parts by weight of starch, the elution of lead could be suppressed even when the added amount of the liquid chelate was 4 parts by weight (Example 7).

The processing cost of the seventh embodiment is estimated to be 18,500 yen per ton of fly ash, as estimated in the same manner as in the first embodiment. On the other hand, the processing cost of Comparative Example 13 using the liquid chelate alone is 40,000 yen per ton of fly ash.

From the above results, it was confirmed that the combined use of the liquid chelate and the starch reduced the cost of immobilizing lead in fly ash by the liquid chelate.

Examples 11 to 14, Comparative Examples 15 to 35 100 g of raw ash C was placed in a 1 L polybeaker, and 30 g of water was added.
g, and the fixing agents shown in Tables 4 and 5 were further added in the amounts shown in Tables 4 and 5 (however, no additive was used in Comparative Example 15), and the mixture was sufficiently kneaded with a spatula.

The starch used was dextrin having a molecular weight of corn of 2000-3000 and a DE value of 6-8%, and sodium piperazine dithiocarbamate was used as a chelating agent. As the phosphoric acid-based heavy metal fixing agent, monopotassium dihydrogen phosphate (KH ₂ PO ₄ ), orthophosphoric acid (H ₃ PO ₄ ), or monosodium dihydrogen phosphate (NaH ₂ PO ₄ ) is used. Was.

Regarding the obtained processed material, notice 13 of the Environment Agency
The dissolution test was carried out according to the above-mentioned test, and the measurement results of the pH of the eluate and the concentrations of lead and cadmium are shown in Tables 4 and 5. In addition, the availability method (N
The samples were subjected to a dissolution test according to EN 7341), and the concentrations of lead and cadmium in the eluate were measured and converted to the amounts of lead and cadmium eluted per raw ash weight, and the results are shown in Tables 4 and 5.

[0046]

[Table 4]

[0047]

[Table 5]

The following are clear from Tables 4 and 5.

From the untreated fly ash (Comparative Example 15), a high concentration of lead elutes in the test of the Environment Agency Notification No. 13, and a high concentration of both lead and cadmium elute in the availability test.

The chelating agent alone treatment (Comparative Examples 16 to
In 20), a large amount (9% by weight) must be added in order to prevent the elution of lead in Test No. 13 of the Notification of the Environment Agency, and even in this case, the lead elution concentration in the availability test is high.

In the case where the phosphate-based heavy metal fixing agent was used alone (Comparative Examples 21 to 26), the elution of lead could be prevented in both the test No. 13 and the availability test of the Environment Agency, but a large amount (9.8). % By weight). In combination with also three dextrin and NaH ₂ PO ₄ and activated clay (Comparative Example 27-29). Similarly, in order to prevent the elution of Pb in both tests of Environment Agency Notification No. 13 test and availability test Requires the addition of a large amount (at least 7.5% by weight of NaH ₂ PO _{4 and at} least 30% by weight in total).

On the other hand, in the embodiment of the present invention in which a starch, a chelating agent and a phosphate-based heavy metal fixing agent are used in combination (Examples 11 and 12), the amount of addition is small (Example 12). (Total 11% by weight). In both the test of the Environment Agency No. 13 test and the availability test, elution of lead and cadmium could be prevented.

In Examples 13 and 14, only the starch and the chelating agent were used without the addition of the phosphoric acid-based heavy metal fixing agent, and although the availability test results were poor,
Good results were obtained in the Environment Agency Notification No. 13 test.

[0054]

As described in detail above, according to the method for treating heavy metal-containing ash of the present invention, heavy metal-containing ash can be used irrespective of its type and properties and without using a pH adjuster. With the addition of a small amount of the immobilizing agent, the treatment can be performed easily and inexpensively, and the elution of heavy metals from heavy metal-containing ash can be reliably prevented.

In particular, according to the method of claim 2 in which a phosphoric acid-based heavy metal fixing agent is used in combination, heavy metals can be reliably fixed even under severe elution environment conditions such as an availability test.

Claims (2)

[Claims] 1. A method for treating heavy metal-containing ash, comprising adding water, a chelating agent and starch to the heavy metal-containing ash and kneading the mixture to fix the heavy metal in the heavy metal-containing ash. 2. A heavy metal-containing ash, wherein water, a chelating agent, a starch and a phosphate-based heavy metal fixing agent are added to the heavy metal-containing ash and kneaded to fix the heavy metal in the heavy metal-containing ash. Processing method.