JPH08299933A - Treating agent for refuse-incineration dust and treatment method - Google Patents

Abstract

PURPOSE: To surely stabilize the heavy metal in dust at a low cost and to eliminate secondary pollution by composing a treating agent of an alkali metal silicate as the main component, adding the agent to a refuse-incineration dust along with water and kneading the mixture to treat the dust. CONSTITUTION: When the elution of the heavy metal in the dust generated as refuse is incinerated in a waste treatment plant, etc., a treating agent consisting essentially of an alkali metal silicate is composed, and the silicate can be allowed to exist as water glass. The treating agent is added to the refuse-incineration ash along with water, and the mixture is kneaded to treat the dust. Further, the treating agent is added by 1-7wt.% as SiO2 , based on the dust. Although the silicate is not specially restricted, sodium orthosilicate, sodium metasilicate, etc., are exemplified.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a treating agent and a treating method for preventing elution of heavy metals in soot dust generated by incineration of garbage in a cleaning plant or the like.

[0002]

2. Description of the Related Art In an incinerator such as a cleaning plant, about 10% of the ash (main ash) generated by the combustion of dust is transferred to the exhaust gas as soot (fly ash ... fly ash). It is collected by an electric dust collector to prevent its release into the atmosphere. However, the collected dust is
Since it contains harmful heavy metals such as lead, cadmium, mercury, chromium, and arsenic, it goes without saying that the heavy metals are eluted by rainwater, etc. May cause serious environmental pollution.

Under these circumstances, due to the 1992 revision of the law, soot and dust was designated as specially controlled general waste, and the standard value was set in the elution test based on the Environmental Agency Notification No. 13 (hereinafter simply referred to as elution test). It is obliged to dispose of the excess dust after detoxifying it by chelating heavy metals, extracting acid, solidifying with cement, etc. Furthermore, since April 1995, the elution standard for lead, arsenic, etc. has been strengthened, and for example, lead is regulated to 0.3 mg / l or less.

By the way, in a cleaning plant, since harmful gases such as HCl and SOx are generated in the combustion gas of dust, as a means for removing them, there is a method of blowing calcium hydroxide into the flue gas in a powder form or a collection method. A method of applying a wet treatment to waste gas after dust is adopted. Therefore, in the case of soot dust collected through the former blowing of calcium hydroxide, the pH of the eluate in the elution test becomes very high and often exceeds pH 12. On the other hand, the soot dust eluate that does not undergo the blowing of calcium hydroxide generally has a pH of 9
It is about -10.

However, it has been confirmed from the results of the elution test of soot dust so far that the elution of the designated heavy metal hardly occurs in the soot dust that has not been blown with calcium hydroxide. On the other hand, in soot dust that has been blown with calcium hydroxide, that is, in soot dust with a very high eluent pH, chromium,
The elution amount of cadmium, arsenic, and mercury does not exceed the respective reference values, but the elution amount of lead, which is an amphoteric metal, tends to be very high. In addition, zinc, which is not included in the designated heavy metals but is not harmless, has a large elution amount at high pH, which causes a problem in environmental hygiene. Therefore, the pH of the eluate
High dust content requires some treatment to prevent the elution of lead and zinc. It should be noted that lead and zinc can be prevented from being eluted by neutralizing the aqueous dispersion of soot and dust to adjust the pH to around 10, but the soot and dust collected in a dry state is purposely dispersed in water and dried after the treatment. This is not practical as a waste treatment method because the process operation becomes complicated and the burden of equipment cost is large.

Conventionally, as means for fixing heavy metals in soot and dust, a method of adding a sulfidizing agent such as sodium sulfide or sodium hydrosulfide to water and soot to form sulfides, and sodium dimethyldithiocarbamate with water and soot Was used to chelate and immobilize heavy metals, but the former method generated highly poisonous hydrogen sulfide in the acidic range, and the latter method specified it as a deleterious substance by decomposing sodium dimethyldithiocarbamate. Since it produces thiuram, which is extremely strict with respect to wastewater standards, there was a risk of secondary pollution. Therefore, at present, instead of sodium dimethyldithiocarbamate, sodium diethyldithiocarbamate or a chelating agent such as a polymer having a dithiocarbamate group obtained by adding carbon disulfide to polyethyleneimine is used, or a heavy metal is phosphoric acid. A method using an orthophosphoric acid preparation that is immobilized as a salt is being used.

[0007]

However, any of the chelating agents having a dithiocarbamic acid group is likely to decompose over time, resulting in a decrease in the effect of fixing heavy metals, and at the same time, it has almost no immobilizing function for zinc. Not on
At present, there are some that produce thiuram-based compounds corresponding to harmful components even if they are not regulated, and it is not preferable from the viewpoint of environmental hygiene. Further, the orthophosphoric acid preparation requires a large amount of addition to the soot dust in order to fully exert the immobilizing action of heavy metals, and has a fatal drawback that the treatment cost is higher than that of the above chelating agent. .

In view of the above situation, the present invention has an excellent function of immobilizing heavy metals in soot dust in the treatment of dust incineration soot dust, and can exhibit a high elution preventing effect on both amphoteric metals, lead and zinc, and Provide a treatment agent and a treatment method that can make the product non-degradable and do not cause secondary pollution, and can significantly reduce the treatment cost as compared with the case of using a conventional chelating agent having a dithiocarbamic acid group. The purpose is to do.

[0009]

In order to achieve the above object, the invention of claim 1 relates to a treatment agent for refuse incineration and dust containing an alkali metal silicate as a main component.

According to a second aspect of the present invention, in the waste incineration dust treatment agent according to the first aspect, the alkali metal silicate is present as water glass.

A third aspect of the present invention relates to a method for treating dust incinerated dust, which comprises adding a treatment agent containing an alkali metal silicate as a main component to water and dust, and kneading the dust.

According to a fourth aspect of the present invention, in the method for treating waste incineration dust according to the above-mentioned third aspect, the treatment agent containing an alkali metal silicate as a main component is SiO ₂ and the amount is 1 to 7% by weight based on the waste incineration dust. It adopts a configuration of adding in a ratio.

[0013]

Detailed structure and function of the present invention Since the treating agent of the present invention contains an alkali metal silicate as a main component, it is contained in dust when incinerated dust is added together with water and kneaded. Since heavy metals react with alkali silicate and are converted into water-insoluble silicates, elution of heavy metals can be prevented even if soot dust after treatment gets wet with rainwater or the like. And
Soot dust is a toxic gas component (HCl, SOx
The inclusion of calcium hydroxide added to the flue gas for the removal of (), etc.) makes it possible to elute amphoteric metals such as lead and zinc in trace amounts even when the aqueous dispersion becomes highly alkaline with a pH of 12 or more. Can be suppressed to.

Further, since the heavy metal silicate produced by the above treatment is non-decomposable and has the same form as natural minerals such as rocks, the above-mentioned elution preventing effect becomes permanent, and There is no concern that the efficacy will be reduced due to decomposition as in the case of using a chelating agent having a dithiocarbamic acid group and that secondary pollution by a by-product will be caused. Moreover, since the alkali metal silicate, which is a treating agent component, is very inexpensive, there is an advantage that the cost of soot dust treatment can be significantly reduced as compared with the case where the above-mentioned conventional chelating agent is used.

The alkali metal silicate used in the treatment agent of the present invention is not particularly limited, but sodium salts and potassium salts are preferable because they are easily available. For example, sodium orthosilicate (Na ₄ SiO ₄ ). , Sodium metasilicate (Na ₂ SiO ₃ ) and its various proportions of hydrates, sodium disilicate (Na ₂ Si ₂ O ₅ ), sodium tetrasilicate (Na ₂ Si ₄ O ₉ ), potassium metasilicate (K ₂ SiO ₃ ), mixtures of two or more of these, M ₂
O · nSiO ₂ (M is Na or / and K, n = 2 to 4)
And water glass represented by.

Among these, water glass is particularly inexpensive, has a good kneading property with soot dust, and has the property of absorbing carbon dioxide gas in the air to precipitate gel silicic acid. Since it shows a strong adhesive force, the kneaded material with soot dust solidifies, and the heavy metal in soot dust is made into a sparingly water-soluble silicate, and the soot particles are chemically fixed, and the soot particles themselves form a glassy film. The physical immobilization effect due to the covering is also taken into consideration, so that the elution preventing effect of heavy metals becomes particularly excellent.

In the treatment agent of the present invention, an alkali metal silicate may be usually used alone, but if necessary, an organic or inorganic binder, a colorant, a viscosity modifier, a pH.
It may contain a small amount of various additives such as a regulator, an aggregate and a filler.

The amount of the treating agent added is SiO _{2 with} respect to the soot dust.
Is preferably in the range of 1 to 7% by weight. If the amount is too small, a sufficient heavy metal elution preventing effect cannot be obtained. On the contrary, if the amount is too large, there is no performance problem but it is uneconomical. The water to be added together with the soot dust may be at least an amount necessary for uniformly kneading the treating agent and the soot dust, the composition of the treating agent, properties, viscosity,
It may be appropriately set according to the properties of the soot and dust, the properties and the form of the intended treated product, and the like.

[0019]

EXAMPLES Examples of the present invention will be described below in comparison with comparative examples.

Example 1 A method of adding 300 ml of water containing a treating agent to 30 g of soot dust A collected from a dust incinerator of a cleaning plant through a dust collecting facility and shaking elution at room temperature for 6 hours No. 3 water glass (SiO ₂ content 27.5% by weight) was used as the treatment agent, and the addition amount was SiO _{2 and it} was ₂ to the soot and dust.
Four points of treatment, which were set to 4% by weight, 4% by weight, 6% by weight and 8% by weight, respectively, were performed.

Example 2 30 g of the same dust A as in Example 1 was added with water 21 containing a treating agent.
Add ml and knead at room temperature, and leave for 30 hours after leaving for 24 hours.
In a method of adding 0 ml of water and performing shaking elution at room temperature for 6 hours, No. 3 water glass was used as a treating agent, and the addition amount of SiO ₂ was 2% by weight to 4% by weight relative to soot and dust,
Three treatments, each set to 6% by weight, were carried out.

Example 3 In the same manner as in Example 2, sodium metasilicate was used as a treating agent, and the addition amount of SiO ₂ was set to 2% by weight, 4% by weight and 6% by weight based on the soot dust. The points were processed.

Comparative Example 1 In the same manner as in Example 2, a treating agent composed of a polymer having a dithiocarbamic acid group (the above-mentioned chelating agent) was used, and its addition amount was 2% by weight and 4% by weight with respect to dust. % And 6% by weight were set for each of the three treatments.

Comparative Example 2 In the same manner as in Example 2, sodium diethyldithiocarbamate was used as a treating agent, and its addition amount was 1.5% by weight, 2.5% by weight and 3.5% by weight based on the soot dust.
The three treatments set for each of the above were performed.

Example 4 Using 30 g of soot dust B similarly collected from a cleaning plant different from that of Example 1, three points of treatment with No. 3 water glass were performed in the same manner as in Example 2.

Comparative Example 3 No. 3 water glass was replaced by the same treating agent as the one used in Comparative Example 1 and containing a dithiocarbamic acid group-containing polymer.
Three treatments were performed in the same manner as in Example 4.

Each of the treated products of Examples 2 to 4 and Comparative Examples 1 to 3 and the untreated soot dust B used in Example 4 were dispersed in 300 ml of water at room temperature, filtered after stirring for 6 hours. The filtrate was obtained. With respect to these filtrates, the untreated filtrate of the dust A in Example 1, and the liquid after the treatment in Example 1, the pH, the amount of lead and the amount of zinc eluted were measured.
The results are shown in Table 1 below. Further, regarding the untreated filtrates of the dusts A and B and the above-mentioned filtrate of the treated product (3) in which the addition amount of water glass was 6 wt% as SiO ₂ in Examples 2 and 4, cadmium (Cd), chromium (Cr),
The elution amount of arsenic (As) and mercury (Hg) was measured.
The results are shown in Table 2.

[0028]

[Table 1]

From the results shown in Table 1, according to the treatment agent of the present invention, with respect to both the soot dust A having a high elution pH and the soot dust B having a low elution pH, the elution of lead and zinc can be suppressed to an extremely small amount with a small addition amount. Therefore, it is clear that the regulations for specified hazardous industrial waste can be satisfied. In contrast,
It can be seen that the conventional polymer type treating agent having a dithiocarbamic acid group has a good elution preventing effect on lead, but has little effect on zinc. Further, it can be seen that the conventional treating agent composed of sodium dimethyldithiocarbamate has almost no effect on zinc, and if the addition amount is small, the elution preventing effect on lead becomes insufficient.

[0030]

[Table 2]

As is clear from Table 2, with respect to cadmium, chromium, arsenic, and mercury, the amount of elution of both untreated dusts A and B is extremely small, which sufficiently satisfies the reference value. Therefore, in the treatment of soot and dust collected at the cleaning plant,
It is suggested that immobilization of lead and zinc with a large elution amount should be performed. Therefore, the treatment method of the present invention can suppress the elution of lead and zinc to an extremely small amount, and as shown in Table 2, there is no concern of promoting the elution of other heavy metals. It turns out that it is very suitable for.

[0032]

According to the treatment agent for waste incineration soot and dust according to the invention of claim 1 and the treatment method for waste incineration and soot and dust according to the invention of claim 3, heavy metals in soot and dust can be surely fixed and elution of soot and dust. Even when the pH is high, a high elution-preventing effect can be obtained for both lead and zinc, which are amphoteric metals, and since the product becomes non-degradable, the elution-preventing effect can be maintained permanently. There is an advantage that there is no possibility of causing secondary pollution and the treatment cost can be significantly reduced as compared with the case where a conventional chelating agent having a dithiocarbamic acid group is used.

According to the second aspect of the present invention, as the above-mentioned treatment agent for dust incineration and dust, there is provided a treatment agent which is particularly excellent in the elution preventing effect of heavy metals and is inexpensive and has good operability in treatment.

According to the fourth aspect of the present invention, as a method for treating the above-mentioned dust incineration soot and dust, a method capable of reliably exhibiting the elution preventing effect of heavy metals and having a high economical efficiency is provided.

Claims (4)

[Claims] 1. A treatment agent for waste incineration and dust containing an alkali metal silicate as a main component. 2. The treatment agent for waste incineration dust according to claim 1, wherein the alkali metal silicate is present as water glass. 3. A method for treating dust incineration dust, which comprises adding a treatment agent containing an alkali metal silicate as a main component to the dust incineration dust with water and kneading the mixture. 4. A treatment agent containing a silicic acid alkali metal salt as a main component, wherein SiO ₂ is used and 1 to 7% by weight based on dust incinerated dust.
The method for treating refuse incineration dust according to claim 3, wherein the method is added in the proportion of.