JP2000176414A - Method for detoxifying heavy metal in ash - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for detoxifying heavy metals in ash by which the heavy metals in ash as such incineration ash and fly ash as a general waste are removed at a low cost. SOLUTION: This method consists of a vacuum-heating stage 1 for vacuum- heating ash, an evacuation stage 2 for exhausting the gas generated from the stage 1 and an inert gas supply stage 3 for cooling a vacuum heater after the vacuum heating 1 is finished. The vacuum-heating stage 1 is preferably operated at 1600-1800 deg.C and at 4.8 to 4.8×108 Pa.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for detoxifying heavy metals in ash, such as incinerated ash and fly ash, which are general wastes, and more particularly to detoxification of heavy metals in ash by a physical method such as a vacuum heating method. Regarding the processing method.

[0002]

2. Description of the Related Art Among industrial waste and waste generated from city life, separated combustible materials are incinerated in an incinerator after being collected, and are discarded and landfilled in the form of incinerated ash. . On the other hand, when such waste is burned, various low-boiling components are volatilized to form so-called fly ash,
Since this fly ash contains more heavy metals such as lead, zinc, cadmium, chromium, zinc, arsenic, and mercury than the incinerated ash, it cannot be landfilled as a general waste as it is, but also as flue gas. This is collected by a bag filter carrying slaked lime or the like so as not to diffuse into the external environment. Such fly ash,
As described above, since it contains a lot of heavy metals, strict management is required for environmental hygiene, and heavy metals are insolubilized in water to prevent their elution and then dumped as a concrete molded body with cement etc. at a disposal site. ing.

However, a large amount of unreacted slaked lime remains in the fly ash, and when the pH of the fly ash increases, lead and zinc, which are amphoteric metals among heavy metals contained in the fly ash, are easily eluted into water. This is a particular problem when landfilling.
Therefore, at present, landfills are rendered harmless by using organic chelates or inorganic chemicals that utilize crystallization reactions, etc., but the organic chelates are decomposed by soil bacteria, and the organic lysates are dissolved by humic acid in the soil. Although the processed chemicals have excellent long-term stability, there is a problem that the amount of the processed chemicals increases and the life of the landfill disposal site is shortened. Further, heavy metals contained in the concrete molded body are easily eluted even when the pH is lowered, such as in acid rain. Therefore, there has been a demand for a method for reliably removing heavy metals from fly ash.

[0004] On the other hand, the ash melting method has attracted attention of local governments because it has a great advantage of improving the recycling rate because it can be used as a harmless substance having no volume reduction effect and no elution of heavy metals. However, ash melting furnaces that can be used in combination with existing incinerators include plasma melting furnaces, surface melting furnaces, and electric resistance melting furnaces, all of which have high construction costs. Has become.

[0005]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and can remove heavy metals in ash such as incinerated ash and fly ash, which are general wastes, at low cost. It is an object to provide a method for detoxifying heavy metals in ash.

[0006]

Means for Solving the Problems The gist of the present invention for solving the above-mentioned problems is that a vacuum heating step for heating ash in vacuum and an exhaust gas generated from the vacuum heating step are exhausted. It comprises a vacuum evacuation step and an inert gas supply step for cooling the vacuum heating device after the completion of the vacuum heating.

The gist of the present invention is that the heating temperature in the vacuum heating step is 1600 ° C. to 1800 ° C.
° C, vacuum pressure is 4.8 Pa to 4.8 × 10 ⁻⁸ Pa
The method for detoxifying heavy metals in ash according to claim 1, wherein the method is operated between the above steps.

According to another aspect of the present invention, the vacuum heating step includes a degassing step in which heating is performed at 100 ° C. to 150 ° C. in the first stage, and a detoxification process in which heating is performed at 1600 ° C. to 1800 ° C. in the second stage. In a vacuum heating process consisting of two steps, the pressure of the vacuum is 4.8 Pa to 4.8 × 10 ⁻⁸ P
2. The method for detoxifying heavy metals in ash according to claim 1, wherein the method is operated during a.

Further, the subject matter of claim 4 is that
The gas supplied in the inert gas supply step is at least one gas selected from the group consisting of helium gas, argon gas, nitrogen gas, and carbon dioxide gas. The method for detoxifying heavy metals in ash according to any one of the preceding claims.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A method for detoxifying heavy metals in ash according to the present invention, which is capable of detoxifying heavy metals in ash, will be described with reference to FIG.

The method for detoxifying heavy metals in ash of the present invention comprises:
The main part is composed of a vacuum heating step 1, a vacuum evacuation step 2, and an inert gas supply step 3.

Ash is incineration ash (or fly ash) from incineration of garbage, which has been insolubilized with water and then dried. For example, incinerated ash has a particle size of 3 mm to 6 mm, and fly ash has an average particle size of 100 microns.

The vacuum heating step 1 uses molybdenum as a heater material, and converts ash to 1600 ° C. to 1800 ° C.
This is an indirect heating step characterized in that heating is performed at a vacuum pressure of 4.8 Pa to 4.8 × 10 ⁻⁸ Pa. As a material for the heater for high-temperature heating, materials such as tungsten and tantalum can be considered in addition to molybdenum. Of these, molybdenum does not react with the iron in the ash and does not break, nor does it absorb hydrogen gas generated by the decomposition of flammable gas adsorbed on the ash, which may cause an explosion hazard. And is suitable as a heater material. 1600 ° C is
The heating temperature of 1800 ° C., which is inversely determined from the vacuum pressure that is easily created by ordinary vacuum equipment, is the upper limit allowable heating temperature determined by the physical properties of molybdenum.

The evacuation step 2 may be constituted by a single vacuum pump or by combining different types of vacuum pumps. For example, it consists of a combination of an oil rotary pump and an oil diffusion pump. First, the vacuum heating device is roughly roughened by the oil rotary pump, and when the pressure in the system becomes 13 Pa from the atmospheric pressure, the system is switched to the oil diffusion pump to further exhaust the gas. Do. A Pirani vacuum gauge is used as the pressure gauge, and the switching operation between the oil pump and the oil diffusion pump is performed using the contact output of the vacuum gauge.

The inert gas supply step 3 is a step of heating and evaporating, for example, liquefied argon or liquid nitrogen with hot water or an electric heater to supply a gas for cooling the apparatus. The evaporated argon gas or nitrogen gas is introduced to cool the vacuum heating device. Argon gas or nitrogen gas coming out of the vacuum heating device is cooled and dehumidified by a condenser, then is dust-removed and released to the atmosphere. If a large amount of cooling gas is used, it may be circulated. Note that helium gas or carbon dioxide gas can also be used.

The operation of the method for detoxifying heavy metals in ash according to the present invention comprising the above steps will be described. The ash that has been dehydrated and dried by insolubilizing heavy metals in the preceding stage is introduced into the vacuum heating step 1. First, the air in the vacuum heating device is exhausted by the oil rotary pump and the oil diffusion pump in the vacuum evacuation step 2, and when the pressure becomes 4.8 Pa or less, the heating of the ash is started. The heating temperature of the ash is 100 ° C to 1 to perform degassing first.
Perform at 50 ° C. Here, 100 ° C. is a temperature at which moisture in the atmosphere evaporates, and 150 ° C. is not preferable because if the temperature is raised from the beginning, the gas is released at once and the ash soars. This is the preheating temperature for increasing the degassing rate of the gas physically adsorbed on the ash. Next, when the amount of generated gas decreases, the temperature is raised to 1600 ° C.
If the vacuum pressure is between 4.8 Pa and 4.8 × 10 ⁻⁸ Pa, the oxide of silicon and the oxide of aluminum will not be reduced in vacuum, so Certain oxides such as lead and zinc can be reduced by vacuum and suitably removed by evaporation. When the detoxification processing of heavy metals is completed, the amount of gas generated in the vacuum heating device is reduced and the vacuum pressure becomes constant, so that the end point of the detoxification processing can be determined from the reading of the vacuum gauge.

After the detoxification treatment is completed, an inert gas is gradually introduced into the vacuum heating device so as to prevent the powder from rising, and the vacuum heating device is cooled. The gas used in the inert gas supply step 3 is a gas for cooling the vacuum heating device heated to 1600 ° C. or more in the vacuum heating step 1. Since it is a region where a nitride such as silicon nitride is generated, it is preferable to cool with argon gas alone. When nitrogen is used as a cooling gas, it is preferable to first cool to about 1200 ° C. with an argon gas or to naturally cool to 1200 ° C., and then use a nitrogen gas as a cooling gas for cooling below that.

The exhaust gas discharged from the vacuum exhaust step 3 is discharged into the atmosphere through an exhaust gas facility 4 such as an oil mist trap, a liquid nitrogen trap, an activated carbon trap and the like.

After cooling with an inert gas, a vacuum heating step 1
The ash from which heavy metals have been removed is taken out and collected. The harmlessly treated ash is used as an aggregate such as a block tile or a civil engineering material such as a roadbed material.

The ash thus detoxified is added with a chemical such as chlorine gas for converting heavy metals into chloride or acid or alkali for adjusting pH when fixing heavy metals with a heavy metal fixing agent. Since there is no need, there is no danger of environmental secondary pollution such as lead and zinc, which are a large amount of amphoteric metals contained in the ash, being eluted into water due to the effects of residual chemicals.

Next, the principle of the method for detoxifying heavy metals according to the present invention will be described with reference to FIG. FIG. 4 is a diagram of the standard free energy ΔG ⁰ = RTlnpo ₂ of a metal oxide versus temperature. That is, the chemical equilibrium theory on thermodynamics, or go to the oxide producer The more metal oxidation reaction How partial pressure po ₂ oxygen, also, the oxides the less how much the partial pressure po ₂ oxygen on the opposite FIG. 4 is a diagram showing whether the material is reduced and goes to the metal generation side. The straight line indicates the state of chemical reaction equilibrium, the solid line indicates the solid state, and the broken line indicates the molten liquid state. (However, SiO is shown by a broken line, but shows ΔG ⁰ in a gaseous state). A perpendicular line is drawn from the temperature coordinates, an intersection point intersecting with a straight line for each oxidation reaction of each element is connected to point O, and a straight line is drawn. The intersection with the oxygen partial pressure po ₂ coordinate line is the oxygen partial pressure value at the time of chemical reaction equilibrium. is there. As can be seen from FIG. 4, when the heating temperature is 1600 ° C., 1 Pa to 1 × 10 ⁻⁸ Pa
In the case of ash, since silicon (Si) and aluminum (Al), which are the main components of the ash and easily produce complex compounds, are stable as oxides, lead (Pb) which is a heavy metal contained in the ash in a large amount ), Zinc (Zn) and the like can be suitably converted into metal vapor and removed.

[0022]

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

EXAMPLE The apparatus used in the example is a bell jar 5 which is a vacuum vessel in the form of a glass bell as shown in FIG. 2, and a ash is separated as shown in FIG. A boat 10 which is an elongated box made of molybdenum for carrying out, and is a container for powder separation with terminals for energization provided at both ends in the long side direction, and an electrode 6 for energizing the boat 10 with electricity. Oil rotary pump 9 and oil diffusion pump 8 for evacuating bell jar 5
And a liquid nitrogen trap 7 for removing moisture and volatile components in the exhaust gas coming out of the bell jar 5.

Degassing method of incinerated ash or fly ash (pretreatment method) 0.6 L of incinerated ash (or fly ash) is put into a 1 L-volume glass air-collecting bottle, the mouth of the air-collecting bottle is closed with a paper towel, and a bell jar is used. Put in 5. The inside of the bell jar 5 is evacuated from atmospheric pressure to 13 Pa by the oil rotary pump 9 in about 10 minutes. The ash is degassed while heating the air collection bottle to 100 ° C. At this time, when the pressure in the bell jar 5 becomes 800 Pa to 100 Pa, a large amount of gas that can be visually confirmed is generated. In the meantime, since the low-boiling components contained in the ash are gasified, it is necessary to prevent the freezing by heating the entire gas collecting bottle with electric resistance so as to prevent the gas collecting bottle from being cooled by latent heat of vaporization and frozen. When the vacuum pressure reaches 13 Pa, the vacuum pump is switched from the oil rotary pump 9 to the oil diffusion pump 8 for operation, and the operation is started in about 20 minutes.
Exhaust to ^-3 Pa. The end point of the degassing process is the end point when the ultimate pressure becomes constant.

Method for Removing Heavy Metals from Incinerated Ash Using only the molybdenum boat 10 shown in FIG. 3, 7 to 10 g of the degassed incinerated ash is collected. 1 from atmospheric pressure
0 After evacuated to ^-3 Pa, approximately at a current of 10V × 100A 2
Minute heating, a small amount of gas is generated at about 400 ° C. and 10 ⁻² Pa
Until the pressure rises. 10 V x (150 A to 18
Heating is carried out at a temperature of 1600 ° C. by passing a current of 0 A). After the start of heating, the pressure value of the vacuum gauge attached to the pressure gauge port 11 rises to 15 Pa, and then 10 ⁻² Pa in about 5 minutes.
Down to. Vacuum pressure is 8 × 10 ⁻² Pa to 10 × 1
It can be confirmed that lead evaporates from 0 ^-2 Pa. Then 6
It can be confirmed that zinc evaporates from × 10 ⁻² Pa.
Zinc evaporates much more slowly than lead. The operation is continuously performed until the pressure reaches 6.5 × 10 ⁻³ Pa, which is the ultimate pressure of the bell jar. The melting of the incinerated ash starts at a pressure of 1 Pa. When the ultimate pressure becomes constant, the operation is terminated, and a part of the liquid nitrogen in the liquid nitrogen trap 7 is evaporated and introduced into the bell jar 5 to be used as a cooling gas. After cooling, the bell jar 5 was opened and the incinerated ash detoxified was subjected to analysis.

Method for removing heavy metals from fly ash 2 g of the above-mentioned degassed fly ash was put into a boat 10 and FIG.
The cover 12 having a circular hole as shown in FIG. 1 is covered with a boat 10 made of molybdenum, and 10 V × 1 from 5 × 10 ⁻² Pa.
After heating at a current of 25 A, a second degassing process is performed following the degassing process. Fly ash has a large amount of adsorbed gas, unlike incinerated ash. Therefore, unless the powder is pressed down by the lid 12 having a circular hole, the powder scatters in the bell jar 5. When gas is generated, the pressure initially rises to 10 Pa, and then 10
It returns to 5 × 10 ⁻² Pa in about a minute. The sample which has been subjected to the second degassing treatment is subjected to a heat treatment at 1600 ° C. by removing the lid 12 having the circular hole formed in the same manner as in the incineration ash. The operation is 6.5 × 1 which is the ultimate pressure of the bell jar 5.
^Operate continuously until it becomes 0 ^-3 Pa. When the ultimate pressure becomes constant, the operation is terminated, a part of the liquid nitrogen in the liquid nitrogen trap 7 is evaporated and introduced into the bell jar 5, and used as a cooling gas. After cooling, the bell jar 5 was opened and the fly ash detoxified was subjected to analysis.

Tables 1 and 2 show the results of the detoxification treatment of the incinerated ash and fly ash treated as described above. As can be seen from the results, all heavy metal components cleared the municipal target.

[0028]

[Table 1]

[0029]

[Table 2]

[0030]

According to the present invention having the above configuration and operation, 1) Since it is a treatment method in which only physical operation is performed and a chemical does not need to be added to ash, elution of heavy metals by residual chemicals and the like are performed.
The risk of secondary environmental pollution is eliminated. 2) Compared with the plasma melting method, the ash can be rendered harmless without melting, so that the heat treatment temperature is lowered and energy is saved.

[Brief description of the drawings]

FIG. 1 is an overall flowchart of a method for detoxifying heavy metals in ash according to the present invention.

FIG. 2 is an overall view of equipment of equipment used in an embodiment of the present invention.

FIG. 3 (a) is a perspective view showing a state in which a molybdenum boat for vacuum heating used in an embodiment of the present invention is not covered with a lid having a circular hole. (B) It is the perspective view which showed the state at the time of covering with the lid | cover which provided the circular hole in the boat made of molybdenum for vacuum heating used for the Example of this invention.

FIG. 4 is a diagram illustrating the principle of the method for detoxifying heavy metals in ash according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Vacuum heating process 2 Vacuum exhaust process 3 Inert gas supply process 4 Exhaust gas treatment equipment 5 Bell jar 6 Electrode 7 Liquid nitrogen trap 8 Oil diffusion pump 9 Oil rotary pump 10 Boat 11 Pressure gauge port 12 Lid with a circular hole

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4D004 AA36 AA37 AB03 BA02 CA27 CA32 CA50 CB04 CB31 CC01 DA02 DA03 DA06 DA07

Claims (4)

[Claims] A vacuum heating step of vacuum-heating the ash; a vacuum evacuation step for exhausting gas generated from the vacuum heating step; and an inert gas for cooling a vacuum heating apparatus after the vacuum heating is completed. A method for detoxifying heavy metals in ash, comprising a supply step. 2. A heating temperature in the vacuum heating step is 1600.
° C to 1800 ° C, vacuum pressure is 4.8 Pa to 4.8
The method for detoxifying heavy metals in ash according to claim 1, wherein the method is operated at a pressure of ^10-8 Pa. 3. The vacuum heating step is a degassing step in which heating is performed at 100 ° C. to 150 ° C. in the first stage, and 1600 ° C. in the second stage.
2. A vacuum heating step comprising two stages of a detoxification treatment step of heating at a temperature of from 1 to 1800 [deg.] C., wherein the operation is performed at a vacuum pressure of 4.8 Pa to 4.8 * 10 < ^-8 > Pa. 3. The method for detoxifying heavy metals in ash described in 4. above. 4. The gas supplied in the inert gas supply step is at least one gas selected from the group consisting of helium gas, argon gas, nitrogen gas, and carbon dioxide gas. The method for detoxifying heavy metals in ash according to any one of claims 1 to 3.