JP2000051819A - Fly ash detoxification method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for effectively decomposing dioxins in molten fly ash containing low melting point substances such as chlorides of heavy metals. SOLUTION: In detoxification of molten fly ash by heat treatment, alkali metal hydroxide or carbonate or alkaline earth metal oxide, hydroxide or carbonate is added to molten fly ash as pretreatment. 1 g of ash and 100 g of water
g so that the pH of the liquid when it is added to g is 7 or more.
The molten fly ash is preferably placed in an inert gas atmosphere at 200 to 3
Heat treatment is performed in a temperature range of 50 ° C. The molten fly ash is heat-treated in an oxygen-containing atmosphere, preferably at a temperature in the range of 250 to 450 ° C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to dust collection for electric dust collectors, bag filters, etc. from waste gas generated when melting incinerated fly ash of general or industrial waste or melting general or industrial waste. The present invention relates to a method for detoxifying fly ash (hereinafter referred to as molten fly ash) collected by an apparatus, and more particularly to dioxins contained in molten fly ash, for example, polychlorinated dibenzo-p-.
The present invention relates to a method for detoxifying harmful organic compounds such as dioxin (hereinafter, referred to as "PCDDs") and polychlorinated dibenzofurans (hereinafter, referred to as "PCDFs").

[0002]

BACKGROUND OF THE INVENTION In recent years, the effect of dioxins contained in exhaust gas discharged from general waste incinerators and industrial waste incinerators on the human body has become a problem, and the emission control of dioxins in the exhaust gas has been implemented. It was decided. Therefore, various technologies have been introduced to satisfy emission standards, for example, quenching the exhaust gas, collecting fly ash with a bag filter or an electric dust collector, and thereby removing fly ash containing dioxins from the exhaust gas. Means for separation and removal are employed.

[0003] As a method for reducing, detoxifying, and reusing the incinerated fly ash, an ash melting method is employed. Then S
The i and Al components are recovered as slag from the lower part of the melting furnace, but dust containing a high concentration of chlorides of heavy metals flows out into the exhaust gas. Further, a method of directly melting general or industrial waste by skipping the incineration process has also been proposed. However, similarly to the ash melting method, dust containing a high concentration of heavy metal chlorides flows out into exhaust gas. This molten fly ash contains a large amount of dioxins other than chlorides of heavy metals.

[0004]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for effectively decomposing dioxins in molten fly ash containing low-melting substances such as chlorides of heavy metals. .

[0005]

Means for Solving the Problems The present inventors firstly obtained fly ash (hereinafter referred to as "incinerated fly ash") obtained by collecting exhaust gas discharged from a general waste incinerator with a bag filter. Elemental analysis was performed on each of the molten fly ash.

Table 1 shows an example of the composition of incinerated fly ash, and Table 2 shows an example of the composition of molten fly ash. The difference between Table 1 and Table 2 is that S
The content of the hardly decomposable oxides such as i, Al, etc. is higher, and the incinerated fly ash contains more of these. Further, the reason why Ca is large in both ashes is that these fly ashes contain Ca (O
H) This is because they were collected in a state where ₂ was blown. Therefore, when desalination is not performed or desalination is performed by another device, the amount of Ca in these fly ash is considerably reduced.

Table 3 shows the pH when 1 g of each fly ash was placed in 100 g of water. These fly ash also contains dioxins. As a method for detoxifying dioxins in fly ash, 200 ° C to 400 ° C in an inert gas
Is adopted. As is clear from Table 3, the molten fly ash (fly ash 4) into which Ca (OH) ₂ was not blown is a weakly acidic substance.

[0008]

[Table 1]

[0009]

[Table 2]

[0010]

[Table 3]

[0011] Then, the present inventors have found that the Ca (OH) ₂ is blown incineration fly ash (fly ash 1), the Ca (OH) ₂ is not blown molten fly ash (fly ash 4) An experiment was conducted to decompose dioxins by heating. An electric heating furnace was used as an experimental apparatus, and each fly ash was heated at a temperature of 250 to 350 ° C. for one hour in a state where the inside of the furnace was in a nitrogen gas atmosphere. After heating, fly ash was taken out and the decomposition rate of dioxins was examined. The result is shown in FIG.

Generally, the concentration of dioxins is T
It is expressed by EQ (equivalent concentration of toxicity), which is considered to be inappropriate when analyzing physicochemical phenomena. Therefore, the total dioxins will be discussed here. Of course, if the total dioxins decrease, the TEQ is considered to decrease unless there is a special reason.

As can be seen from FIG. 1, in fly ash 1, the decomposition rate of dioxins increases as the heating temperature increases. On the other hand, in fly ash 4, as the heating temperature was increased, the decomposition rate became a clearly negative value, indicating that dioxins increased.

FIGS. 2 and 3 show the results of a detailed study of the relationship between the heating temperature of the fly ash 4 and the decomposition rate of dioxins for each type of dioxins. As can be seen from FIG. 2, in the case of PCDDs, tetrachlorodibenzo-p-dioxin (T4CDD
s), pentachlorodibenzo-p-dioxin (P5
CDDs) and hexachlorodibenzo-p-dioxin (H6CDDs) show a positive decomposition rate, but heptachlorodibenzo-p-dioxin (H7CDDs) and octachlorodibenzo-p-dioxin (H7CDDs) having a relatively high chlorine number. O8CDDs) shows a negative decomposition rate. As can be seen from FIG. 3, in the case of PCDFs, when the heating temperature exceeds 300 ° C., tetrachlorodibenzofuran (T4CD
Fs), pentachlorodibenzofuran (P5CDF
s), hexachlorodibenzofuran (H6CDFs),
Both heptachlorodibenzofuran (H7CDFs) and octachlorodibenzofuran (O8CDFs) show a negative decomposition rate. In particular, the higher the chlorine number, the more the decomposition rate at a heating temperature of 350 ° C. is significantly reduced. In other words, in the case of fly ash 4, if this is heated at a temperature exceeding 300 ° C., a polychlorination reaction occurs in which dioxins having a large number of chlorines increase,
This reaction is particularly pronounced for PCDFs.

Further, when the state of each fly ash after heating was observed with the naked eye and a microscope, no change was observed in fly ash 1 at any heating temperature compared to the state before heating. On the other hand, with respect to fly ash 4, when the heating temperature was 300 ° C. or more, it appeared that the fly ash had contracted when observed with the naked eye, and the fly ash particles were in a fused state when observed with a microscope.

When the above experimental results are combined, the process of polychlorination of dioxins in fly ash 4 can be considered as follows. First, in the fly ash 4, a base material such as sodium chloride or potassium chloride, which is mostly composed of chloride, a low-melting substance such as PbCl ₂ or ZnCl ₂ , and dioxins coexist. When the fly ash 4 having such a composition is heated to a temperature equal to or higher than the eutectic point (260 ° C.) of the base material and the low-melting substance, the periphery of the low-melting substance is locally melted to be in a molten state. The eutectic point depends on the composition of the fly ash.
When the pH of the aqueous solution dissolved in g becomes about 7,
It can be estimated that the eutectic point will also be 300 ° C. Dioxins coexist in the melt, and chlorine partially ionized in the melt is added to the dioxins or replaced with hydrogen ions, thereby causing a polychlorination reaction. Thus, it is considered that polychlorination of dioxins progresses. Fly ash 2 shown in Table 3 is in a weakly acidic state.
Because of the large amount of the l component, the eutectic point is high and the polychlorination of dioxins does not occur. Since fly ash 3 is in an alkaline state, the eutectic point is high, and polychlorination of dioxins does not occur as in fly ash 1 and 2.

As a result of intensive studies based on the above experimental facts, the present invention has been completed.

[0018]

[Function] Since organic chlorides in fly ash containing low-melting substances such as chlorides of zinc and lead are eutectic and polychlorination of dioxins occurs, at the stage when the low-melting substances are melted By adding a substance that takes in chlorine in preference to chlorine addition to dioxins, i.e., alkali metal hydroxides, carbonates, alkaline earth metal oxides, hydroxides, and carbonates, Transfer of chlorine from the melting point substance to the dioxins can be prevented.

[0019]

Next, the present invention will be specifically described with reference to examples of the present invention.

Example 1 In heating the fly ash 4 described above, 2.5 kg of slaked lime was added to 50 kg of the fly ash, and these were mixed well. When the pH of an aqueous solution in which 1 g of the obtained mixture was dissolved in 100 g of water was measured, it was 12. The above mixture was charged into an electric heating furnace and heated at 300 ° C. and 3 ° C. in a nitrogen gas flow.
Heated at 50 ° C. for 1-3 hours. Thereafter, fly ash 4 was taken out of the electric heating furnace, and the decomposition rate of dioxins was examined. The results are shown in Table 4 below.

[0021]

[Table 4]

As is apparent from Table 4, a positive decomposition rate was obtained for both the heating temperatures of 300 ° C. and 350 ° C.
That is, it indicates a decrease in dioxins. In addition, even when the heating time was 1 hour or 2 hours, the decomposition rate was higher than that in Example 1, and when the heating time was 3 hours, the decomposition rate was 99.9%, and almost completely dioxin was obtained. Kind had been decomposed.

[0023]

According to the method of the present invention, dioxins in molten fly ash containing low-melting substances such as zinc and lead chlorides can be effectively decomposed.

[Brief description of the drawings]

FIG. 1 is a diagram showing the results of a thermal decomposition experiment of dioxins of fly ash 1 and fly ash 4;

FIG. 2 is a diagram showing the results of a thermal decomposition experiment of dioxins on fly ash 4 according to the number of chlorine atoms added to PCDDs.

FIG. 3 is a diagram showing the results of a dioxin thermal decomposition experiment on fly ash 4 according to the number of chlorine added to PCDFs.

Continuation of the front page (72) Inventor Kiichi Nagaya 1-7-89 Minami Kohoku, Suminoe-ku, Osaka F-term in Tachibai Shipbuilding Co., Ltd. (reference) 2E191 BA12 BC01 BD11

Claims (3)

[Claims] In detoxifying molten fly ash by heat treatment, alkali metal hydroxide or carbonate, or alkaline earth metal oxide, hydroxide or carbonate is added to the molten fly ash as a pretreatment. Add salt and add 1 g of ash to 100
g. A fly ash detoxification treatment method characterized in that the pH of the liquid when charged into g is 7 or more. 2. Melting fly ash in an inert gas atmosphere for 200 hours
The fly ash detoxification treatment method according to claim 1, wherein the heat treatment is performed in a temperature range of -350 ° C. 3. A method for melting fly ash in an oxygen-containing atmosphere at 250 to
The fly ash detoxification treatment method according to claim 1, wherein the heat treatment is performed in a temperature range of 450 ° C.