JPH09290231A - Treatment of heavy metal-containing ash - Google Patents

Abstract

PROBLEM TO BE SOLVED: To surely prevent the elution of a heavy metal from a treated matter by easily, efficiently and safely treating a heavy metal containing ash without executing a pH adjustment. SOLUTION: Water, chitosan (or a chitosan soln.) and a phosphoric acid-based metal fixing agent are mixed to. the heavy metal containing ash and the mixture is kneaded. In this way, the heavy metal is stably and surely fixed by allowing the heavy metal in the heavy metal containing ash such as lead and cadmium to react with the phosphoric acid-based metal fixing agent to fix as a hardly soluble salt such as lead phosphate and cadmium phosphate, and also forming a hardly soluble complex with the chitosan or the chitosan soln. and the heavy metal to stably and surely fix the heavy metal.

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 or industrial waste is incinerated at an incinerator, fly ash (hereinafter, incinerated ash and fly ash are referred to as heavy metal-containing ash) is generated along with incinerated ash and combustion exhaust gas. Conventionally, a bag filter or an electric dust collector has been installed to capture the fly ash, and the collected fly ash and incinerated ash have been effectively used for landfill and ground construction.

By the way, since the heavy metal-containing ash thus obtained contains heavy metals, when the heavy metal-containing ash is used for landfilling, etc., a treatment for preventing elution of the heavy metal in the heavy metal-containing ash is performed. Need to be applied.

[0004] Conventionally, as a method for preventing heavy metal-containing ash from eluting heavy metals, 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. For this reason, the chemical treatment method is emphasized.

As a chemical treatment method, a specific amount of water-soluble phosphoric acid or a salt thereof is added to fly ash or a mixture of fly ash and incinerated ash in the presence of calcium hydroxide to prevent elution of cadmium and lead. A method (Japanese Patent Publication No. 4-61710) has been proposed.

As a method of treating ash containing an alkali heavy metal, the fly ash is washed in a fly ash dissolving tank, and the alkali content and the soluble heavy metal in the fly ash are transferred into the fly ash dissolving water. Is neutralized by reacting with CO ₂ gas in a gas-liquid reaction tank, and carbonation of heavy metals is carried out (JP-A-1-2843).
No. 82) has been proposed.

A method of kneading a sulfur-based chelating agent with fly ash has also been proposed.

[0008]

Among the above-mentioned conventional methods, in the method of adding water-soluble phosphoric acid or a salt thereof described in Japanese Patent Publication No. 4-61710, when the pH of the treated ash is 8 or less, cadmium is sufficiently contained. There is a problem that it cannot be fixed.

The method for carbonating heavy metals described in JP-A-1-284382 requires a dehydrator and a filter, which is industrially disadvantageous in terms of equipment.

Further, the method of kneading the sulfur-based chelating agent has a problem that hydrogen sulfide gas is generated when an acid is used together during the treatment.

The present invention solves the above-mentioned conventional problems, and
An object of the present invention is to provide a method for easily, efficiently and safely treating heavy metal-containing ash without requiring adjustment to reliably prevent elution of heavy metals from a treated material.

[0012]

The method for treating heavy metal-containing ash according to the present invention comprises adding heavy metal-containing ash to water and chitosan or a chitosan solution and a phosphoric acid-based heavy metal fixing agent, and kneading the heavy metal-containing ash. It is characterized by immobilizing heavy metals in ash.

In the present invention, heavy metal-containing ash is mixed with water and chitosan or a chitosan solution and a phosphoric acid-based heavy metal fixing agent and kneaded to remove heavy metals such as lead and cadmium in the heavy metal-containing ash from phosphoric acid. Reacts with heavy metal fixing agents to immobilize as a poorly soluble salt of lead phosphate, cadmium phosphate, etc., and forms an insoluble complex between chitosan or a chitosan solution and heavy metal to stably and reliably immobilize heavy metal. To do.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.

In the present invention, the heavy metal in the heavy metal-containing ash is fixed by kneading the heavy metal-containing ash with water and chitosan or a chitosan solution and a phosphoric acid-based heavy metal fixing agent.

In the present invention, water containing ash containing heavy metals,
There is no difference in treatment effect due to the difference in the order of addition of chitosan or a chitosan solution and a phosphoric acid-based heavy metal fixing agent, and in the present invention,
After adding water to the heavy metal-containing ash, chitosan and a phosphoric acid-based heavy metal fixing agent may be added, or these may be added simultaneously. Further, the order of addition of the chitosan solution and the phosphate-based heavy metal fixing agent is not limited, and they may be added simultaneously. Further, water and chitosan and / or phosphoric acid-based heavy metal fixing agent, or chitosan solution and phosphoric acid-based heavy metal fixing agent may be mixed in advance and added as a slurry.

When the heavy metal-containing ash is treated by adding water, chitosan powder and a phosphoric acid-based heavy metal fixing agent, the amount of water added to the heavy metal-containing ash is 5 to 50% by weight, particularly 20%.
It is preferably about 40% by weight. The amount of chitosan powder added to the heavy metal-containing ash is 0.5% by weight or more,
It is particularly preferably 1 to 5% by weight.

On the other hand, when the chitosan solution and the phosphoric acid-based heavy metal fixing agent are added to the heavy metal-containing ash for treatment, it is not necessary to add water separately, but the addition amount of the chitosan solution is small and kneading is difficult. In this case, it is desirable to further add water so that the amount of water with respect to the heavy metal-containing ash is 5 to 50% by weight, particularly 20 to 40% by weight. Further, in this case, the addition amount of the chitosan aqueous solution is such that the addition ratio of chitosan to the heavy metal-containing ash is 0.5% by weight or more, particularly 1 to
Preferably, it is 5% by weight.

Examples of chitosan used in the present invention include chitosan powder obtained by deacetylating chitin contained in the shells of crabs and shrimps, and the deacetylation rate and molecular weight thereof are not particularly limited. Not something.

A chitosan solution is obtained by dissolving such chitosan powder in an aqueous solution of about 0.5 to 2% by weight of hydrochloric acid, nitric acid or the like so that the concentration of chitosan is 0.1 to 2% by weight. An aqueous chitosan solution can be used.

On the other hand, as the phosphoric acid-based heavy metal fixing agent,
Phosphoric acid or a salt thereof is used, of which, as phosphoric acid, orthophosphoric acid and hypophosphorous acid, metaphosphorous acid, pyrophosphorous acid, orthophosphorous acid, hypophosphoric acid, metaphosphoric acid, pyrophosphoric acid,
Triphosphoric acid, condensed phosphoric acid can be mentioned, and as the phosphate,
Examples thereof include salts of phosphoric acid, primary phosphates, and secondary phosphates.

In the present invention, in particular, orthophosphoric acid (H ₃ P
O ₄ ), monosodium dihydrogen phosphate (NaH ₂ PO
₄ ), disodium monohydrogen phosphate (Na ₂ HPO ₄ ),
Condensed phosphoric acid and the like can be preferably used.

In the present invention, the amount of the phosphoric acid and / or phosphate is preferably 1 to 20% by weight based on the heavy metal-containing ash.

According to the present invention, water and chitosan, or a chitosan solution and a phosphoric acid-based heavy metal fixing agent are added to the heavy metal-containing ash and kneaded to obtain a kneaded product with almost no elution of heavy metals. The kneading method of the heavy metal-containing ash and these additives may be any method as long as the heavy metal-containing ash and the chitosan and the phosphoric acid-based heavy metal fixing agent are uniformly mixed. This mixture is usually landfilled.

[0025]

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

In the following Examples and Comparative Examples, the fly ash treated as raw ash is municipal waste incineration fly ash having the composition shown in Table 1.

[0027]

[Table 1]

The details of the chitosans A, B and C used are as follows.

Chitosan A: A powder obtained by deacetylating chitin contained in the shell of a crab with sodium hydroxide, and having a molecular weight of 300,000 and a deacetylation rate of 87 mol%. Chitosan B: Chitosan A is chemically depolymerized. Powder of chitosan C having a molecular weight of 15,000 and a deacetylation rate of 87 mol%: chitin contained in shrimp shell was deacetylated with sodium hydroxide, and had a molecular weight of 30,000 and a deacetylation rate of 94. Mol% powder Examples 1-8, Comparative Examples 1-10 50 g of raw ash was placed in a 500 ml polybeaker, and water and chitosan, or chitosan solution and phosphoric acid-based heavy metal fixing agent were added at the ratio shown in Table 2. The mixture was thoroughly kneaded with a spatula, and the kneaded product was subjected to an elution test according to the Environmental Agency Notification No. 13 test, and the results are shown in Table 2.

As the chitosan solution, chitosan A
Was dissolved in a 1 wt% hydrochloric acid aqueous solution to prepare a 1 wt% chitosan aqueous solution. Orthophosphoric acid was used as the phosphoric acid-based heavy metal immobilizing agent.

[0031]

[Table 2]

The following is clear from Table 2.

That is, in the case of no treatment (Comparative Example 1), Pb and Cd
Elutes. When water and chitosan or a chitosan solution is added (Comparative Examples 2 to 9), the pH is slightly increased by adding a large amount of chitosan, and the elution of Pb and Cd can be prevented to some extent, but it is not sufficient. Further, when only orthophosphoric acid is added (Comparative Example 10), the elution of Pd can be prevented to some extent, but the elution of Cd cannot be prevented.

On the other hand, in Examples 1 to 8 in which water and chitosan or a chitosan solution and orthophosphoric acid were used in combination,
Relatively low pH of 6.5-6.8 with a small amount of addition
Even under the conditions, the elution amount of Pb and Cd can be significantly reduced.

[0035]

As described above in detail, according to the method for treating heavy metal-containing ash of the present invention, heavy metals such as lead and cadmium can be reliably fixed even if the pH of the eluate of heavy metal-containing ash is low. Because it does not contain sulfur, no harmful gases such as hydrogen sulfide are generated during treatment and landfill. No dehydrator or filter is required. Under such effects, heavy metal-containing ash can be easily and efficiently
Moreover, the treatment can be performed safely, and the elution of the heavy metal from the treated product can be reliably prevented.

Claims (2)

[Claims] 1. A method for treating heavy metal-containing ash, which comprises fixing water and chitosan and a phosphoric acid-based heavy metal fixing agent to the heavy metal-containing ash and kneading the mixture to fix the heavy metal in the heavy metal-containing ash. 2. A method for treating heavy metal-containing ash, which comprises fixing a heavy metal in the heavy metal-containing ash by kneading the heavy metal-containing ash with a chitosan solution and a phosphoric acid-based heavy metal fixing agent.