JP2022134987A - Treatment method for incineration fly ash - Google Patents

Abstract

To solve the problem that a COD value of wastewater is high after performing water washing to reduce a chlorine content, being one pretreatment for using fly ash produced in accompany with municipal refuse incineration as a cement raw material, and therefore, that considerable labor is required to decrease the COD value in order to discharge the wastewater.SOLUTION: Temperature at which dioxin removal treatment is carried out prior to fly ash water washing is set to exceed 450°C being a higher temperature than in conventional methods, and to be less than 600°C. In order to prevent sintering and melting of fly ash likely to occur during treatment at this high temperature, stoker fly ash and fluidized bed fly ash are mixed and treated at a mass ratio of preferably 1:2 to 3:1. An organic matter being a factor of increasing a COD value due to high temperature treatment is liable to be decomposed, and is made difficult to sinter by mixing fluidized bed fly ash. In decreasing the COD value by subjecting wastewater after water washing to oxidant treatment or the like, a necessary amount of an oxidant or the like can be reduced.SELECTED DRAWING: None

Description

TECHNICAL FIELD The present invention relates to a novel method for treating fly ash discharged from municipal waste incinerators. More specifically, when fly ash is used as a raw material for cement, it is a fly ash treatment method capable of efficiently and reliably removing problematic inclusions.

More than 80% of municipal waste is incinerated, and incineration ash is generated at that time. Most of the incineration ash used to be landfilled, but in recent years, environmental awareness has increased, and methods for recycling the ash have been investigated. The main components of the incineration ash are compounds of calcium, aluminum and silicon, and it is considered to be reused as a raw material for manufacturing cement, for example.

However, since the incinerated ash contains a large amount of chlorine, it must be removed. As a method for removing such chlorine, washing with water has been proposed. That is, by washing the incinerated ash with water, the chlorine contained as chlorides is removed as soluble matter, and the insoluble matter such as calcium, aluminum and silicon compounds are separated as solid matter, which is used for cement production. Used as raw material.

On the other hand, incinerators that discharge incinerated ash are broadly classified into fluidized bed incinerators (hereinafter referred to as "fluidized bed incinerators") and stoker incinerators (hereinafter referred to as "stoker incinerators"). When more incineration ash is collected, various incineration ash discharged from these furnaces are mixed. That is, the incineration ash in a fluidized bed furnace is discharged together with the exhaust gas of the furnace, and is collected and discharged as fly ash. It is also discharged as bottom ash remaining at the bottom.

Among the incinerated ash, the fly ash contains a large amount of dioxins, containing 80 times as much dioxin as the bottom ash, which poses a problem in handling. Another major problem is that treated wastewater obtained by washing fly ash contains a large amount of COD components.

As a technology for treating such fly ash, the fly ash is heated in an oxygen-free atmosphere to deprocess it, then washed with water, and the wastewater is further contacted with an oxidation catalyst in the presence of an oxidant to remove COD. A method for removing components has been proposed (see, for example, Patent Document 1). It is also proposed that this washing with water is performed together with separately pulverized bottom ash (see, for example, Patent Document 2).

Japanese Patent Application Laid-Open No. 2003-103231 JP-A-2003-103232

However, the fly ash washing waste liquid as described above contains a large amount of organic matter, and as a result, the COD value is high, and the processing cost has been a problem. SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to remove dioxins and reduce the COD value of washing wastewater when using fly ash from a refuse incinerator as a raw material for cement.

As a result of intensive studies in view of the above problems, the present inventors found that by increasing the temperature during dioxin removal treatment, the water-soluble organic matter contained in the fly ash can be reduced by the treatment. The inventors also found that the problem of fly ash melting, which tends to occur when the temperature is high, can be solved by mixing fly ash from a fluidized bed incinerator, and completed the present invention.

That is, the present invention heats fly ash obtained from a municipal waste incinerator in an oxygen-free atmosphere to remove dioxins, and then wash it with water to recover solids with reduced chlorine components, and remove COD components from the resulting wastewater. In a method for treating incineration fly ash that removes
As the fly ash, fly ash generated from a stoker incinerator and fly ash generated from a fluidized bed incinerator are mixed and subjected to dioxin removal treatment, and the heating temperature during the dioxin removal treatment is set to 450 ° C. This is a method for treating incineration fly ash with a temperature of more than 600°C and less than 600°C.

According to the present invention, it is possible to reliably remove dioxins and chlorine from fly ash discharged from a garbage incinerator and use it as a raw material for cement. Problems caused by melting fly ash are unlikely to occur.

The fly ash discharged from the municipal waste incinerator treated in the present invention includes fine powder captured from the exhaust gas of the stoker furnace (hereinafter referred to as stoker fly ash) and fine powder captured from the exhaust gas of the fluidized bed furnace (hereinafter referred to as Fluidized bed fly ash), which generally contain chlorine at a rate of about 5 to 35% (mass).

In the present invention, the fly ash is heated at a temperature of more than 450° C. and less than 600° C. in an oxygen-free atmosphere to remove dioxins. By treating the fly ash under conditions of higher temperature than in the prior art, (1) most of the dioxins in the fly ash can be decomposed, and in the subsequent water washing process, dioxin (2) The decomposition of water-soluble organic matter proceeds steadily, and the organic matter contained in the waste water after washing with water is removed. The amount can be reduced, so the COD value of the waste water can be lowered. This treatment promotes carbonization of not only water-soluble organic compounds but also other organic substances. The higher the treatment temperature is, the more likely it is to obtain the above effects, and the treatment temperature is preferably 460° C., more preferably 475° C. or higher. On the other hand, considering the risk of melting fly ash and the durability of the furnace, the temperature is preferably 575° C. or lower, more preferably 550° C. or lower.

In the present invention, as described above, the treatment is performed at a temperature higher than that of the conventional dioxin removal treatment. Stoker fly ash melts more easily than fluidized bed fly ash, and if treated alone at such a high temperature, problems such as clogging of the furnace may occur. Therefore, in the present invention, stoker fly ash is not handled alone, but is mixed with fluidized bed fly ash for dedioxin treatment. Fluidized-bed fly ash is incinerated while circulating fluidized sand in the furnace when waste is incinerated, and the discharged fly ash contains sand that is difficult to melt. It is difficult to melt even at temperatures higher than fly ash. The mixing ratio is preferably stoker fly ash: fluidized bed fly ash (mass ratio) of 1:2 to 3:1, more preferably stoker fly ash: fluidized bed fly ash (mass ratio) of 1:1 to 2:1.

The oxygen-free atmosphere in the dioxin removal treatment refers to an atmosphere substantially free of oxygen, generally having an oxygen concentration of 0.5% by volume or less, preferably 0.2% by volume or less. A method for forming such an oxygen-free atmosphere is not particularly limited, but, for example, a method of forming a nitrogen atmosphere is common. Moreover, the heating method is generally carried out using a heater.

In order to suitably carry out the above method, it is sufficient to select and use an apparatus capable of carrying out the above method from among known dioxin-removing apparatuses, and there are many commercially available apparatuses. Specifically, a Hagenmeyer furnace, an atmosphere-controllable box-type electric furnace, a horizontal cylindrical heating furnace having an atmosphere-controllable internal stirring function, and the like are suitable.

The above heat treatment time varies slightly depending on the temperature, etc., and cannot be unconditionally limited, but in general, 0.5 to 2 hours is appropriate.

In the present invention, the fly ash from which dioxin has been removed as described above is washed with water. Washing with water can be carried out by mixing fly ash with water to form a slurry, filtering the slurry, and optionally washing and filtering with water.

Specifically, the fly ash is supplied to a slurry tank and mixed with water to form a water slurry. Such water slurry is washed and filtered by a filter typified by a filter press capable of washing the inside, and a method of separating into a solid content and a filtrate can be mentioned.

The solid content obtained in the washing step is mainly composed of silicon, aluminum and calcium compounds, and is therefore used as a raw material for cement production in cement production plants. In this case, since the solid content contains water, it is preferable to mix it with the cement raw material in the raw material preparation process in a cement manufacturing plant, and supply it to the suspension pre-peater through a dryer.

In addition, the wastewater obtained by separating the solid content is subjected to removal of COD components to reduce the COD (chemical oxygen demand) value to a level that meets environmental regulations, etc., and if necessary, other detoxification Discharged after processing. In the present invention, the waste water collected after the water washing has a reduced COD component compared to the conventional one, and it is relatively easy to reduce the COD component to a level that can be discharged into the environment.

For the removal of COD components in the waste water, known methods can be employed without particular limitation, and examples thereof include methods such as oxidizing agent treatment and activated carbon treatment. In the present invention, the above-mentioned "removal" is a concept including decomposition and the like, and applies to any treatment as long as it is a treatment for lowering the COD value of wastewater (however, simple dilution is excluded).

The oxidizing agent used for the oxidizing agent treatment is not particularly limited as long as it can provide a strong oxidizing action to the COD component. Specifically, sodium hypochlorite, potassium hypochlorite, hypochlorites such as calcium hypochlorite, perchlorates such as sodium perchlorate, and peroxates such as potassium peroxodisulfate. Furthermore, oxidizing agents such as hydrogen peroxide and the like are included. Among these oxidizing agents, hypochlorite, especially sodium hypochlorite, is most effective in removing (decomposing) COD components and is preferably used.

If an oxidation catalyst is used in combination with the oxidant treatment, COD components can be removed (oxidative decomposition) more strongly.

Although the shape of the oxidation catalyst is not particularly limited, it is preferably granular because it is easily separated from water after treatment. The particulate oxidation catalyst generally consists of an oxidation catalyst component and a binder for granulating it. As the oxidation catalyst component, known ones are used without particular limitation, and examples thereof include oxides and peroxides of metals such as titanium, nickel, iron, copper and cobalt, oxides and peroxides of transition metals, and metals such as platinum and silver. . Of these, nickel peroxide is particularly effective and is most preferably used.

In addition, the binder for granulating the oxidation catalyst component should be made of a material that allows the specific gravity of the resulting granular oxidation catalyst to be appropriately adjusted depending on the method of use or the conditions of use, and that is resistant to the environment in which it is used. is not particularly limited. Examples thereof include various cements, silicon compounds, clay minerals, resins, and the like.

The size and shape of the particulate oxidation catalyst may be appropriately determined depending on the method of use or the conditions of use, taking into account the efficiency of contact with and separation from waste water.

In the treatment for removing the COD component, the concentration of the oxidizing agent present in the wastewater varies depending on the type of the oxidation catalyst component, etc., and it is difficult to determine it unconditionally. It is about 10 times the amount, preferably about the same amount to 5 times the amount.

As a method for removing COD components by using an oxidizing agent and an oxidation catalyst in combination, known methods such as a catalyst packed tower method and a fluidized bed method can be used without particular limitation.

Specifically, the catalyst used in this method is preferably a spherical body containing 5% by weight or less of particles of less than 100 μm and having an average particle size of 150 μm to 10 mm, preferably 600 μm to 5 mm.

Moreover, the concentration of the oxidation catalyst in the waste water is 0.2 to 50% by weight, preferably 0.5 to 10% by weight.

As a method for removing COD components by treating with activated carbon, known methods such as activated carbon packed tower method and fluidized bed method can be used without particular limitation.

Specifically, the activated carbon used in this method is preferably a spherical body containing 5% by weight or less of particles less than 100 μm in diameter and having an average particle size of 150 μm to 10 mm, preferably 600 μm to 5 mm.

Also, the concentration of the activated carbon in the waste water is 0.2 to 50% by weight, preferably 0.5 to 10% by weight.

In the wastewater obtained by washing fly ash, heavy metal components are dissolved in addition to inorganic chlorides such as sodium chloride and calcium chloride, which are the main components. Treatment is desirable.

That is, the COD component removal treatment can be applied as long as there is no solid content in the wastewater, and can be applied to wastewater before the metal compounds are removed. The removal efficiency of the COD component may be lowered due to the deterioration of the particulate oxidation catalyst or activated carbon due to the heat and the formation of insolubilized substances during the reaction.

The method of removing dissolved metal compounds from waste water obtained by washing fly ash with water is to add acid to the waste water to adjust the pH of the liquid to about 6 to 10 to make these dissolved substances insoluble. A preferred method is to add a flocculant if necessary and separate the above insolubilized matter with a filter.

Municipal waste may contain mercury derived from button batteries and the like, so stoker fly ash and fluidized bed fly ash may contain mercury. Since the fly ash contains a large amount of unburned carbon, the mercury (reduced depending on the form of existence in the fly ash) volatilizes as metallic mercury by heating in an oxygen-free atmosphere in the above-described dioxin removal treatment. It will be contained in the exhaust gas generated during the dioxin removal process.

The exhaust gas containing such mercury gas can be treated by a known method. Preferably, the exhaust gas is passed through a condenser (gas cooler) to condense gaseous mercury as metallic mercury and collect it. . Any known condenser may be appropriately selected and used as long as the condenser can cool the exhaust gas to a temperature at which metallic mercury liquefies. Liquefied metallic mercury may be recovered and treated according to a known method.

Furthermore, the exhaust gas collected by condensing the metallic mercury can be released into the atmosphere after further detoxification treatment if necessary.

The present invention will be described in more detail below using examples, but the present invention is not limited to these examples.

The chemical oxygen demand (COD value) in Examples and Comparative Examples was measured by the following method.

The COD value was measured according to the measuring method of oxygen demand (COD _Mn ) by potassium permanganate at 100° C. in the JIS K0102 factory wastewater test method.

Example 1
As municipal refuse incineration fly ash to be treated, stoker fly ash and fluidized bed fly ash were mixed at a ratio of 1:1. This mixed fly ash contained 8.8% chlorine, 0.72 ng-TEQ/g dioxins, 2.7% C, and 10.7 ppm mercury. The mixed fly ash was heat-treated at 475° C. for 1 hour in a nitrogen atmosphere in an atmosphere-controllable electric furnace. No melting or sintering of the ash was observed after treatment. The dioxin concentration after heat treatment was 0.0015 ng-TEQ/g, and the dioxin removal rate was 99.8%. Also, the Hg concentration was 0.34 ppm, and the mercury removal rate was 96.8%.

Then, the incinerated fly ash subjected to the above heat treatment was washed with water. This water washing was performed at a water/ash weight ratio of about 10, and filtered to obtain a solid content (dehydrated cake) and waste water. The chlorine concentration in the obtained solid content was 0.33%, and the chlorine component removal rate was 96.9%. On the other hand, the obtained waste water was pH-adjusted with an acid to precipitate heavy metals, which were then removed by solid-liquid separation. Table 1 shows the results of measuring the COD _Mn of the wastewater after the separation treatment.

Example 2
The same treatment operation as in Example 1 was performed, except that the heat treatment temperature was 525°C. No melting or sintering of the ash was observed during the heat treatment. Table 1 shows the COD _Mn of wastewater after metal deposition and separation treatment.

Comparative Examples 1 and 2
The same treatment operation as in Example 1 was performed except that the heat treatment temperature was changed to 375° C. (Comparative Example 1) or 425° C. (Comparative Example 2). No melting or sintering of the ash was observed during the heat treatment. Table 1 shows the COD _Mn of wastewater after metal deposition and separation treatment.

Comparative example 3
When the stoker fly ash was treated alone under the conditions of Example 1, it was sintered in an electric furnace.

Claims (3)

Fly ash obtained from municipal waste incinerators is heated in an oxygen-free atmosphere to remove dioxins, then washed with water to recover solids with reduced chlorine components, and to remove COD components from the resulting wastewater. In the ash disposal method,
As the fly ash, fly ash generated from a stoker incinerator and fly ash generated from a fluidized bed incinerator are mixed and subjected to dioxin removal treatment, and the heating temperature during the dioxin removal treatment is set to 450 ° C. A method of treating incineration fly ash with a temperature of more than 600°C. 2. The method for treating incineration fly ash according to claim 1, wherein the mixing ratio (mass) of the fly ash generated from the stoker type incinerator and the fly ash generated from the fluidized bed type incinerator is 1:2 to 3:1. 3. The method for treating incineration fly ash according to claim 1 or 2, wherein exhaust gas generated during the dioxin removal treatment is passed through a condenser to recover metallic mercury as a liquid.