JP2001107278A - Method and device for treating harmful metal- contaiining salts - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for treating harmful metal-containing salts capable of simplifying the constitution of the treating device without requiring the execution of increasing the salts to be treated and to provide a device therefor. SOLUTION: A treating device provided with an electrolytic cell 10 having an anode 12a and a cathode 12b, a heating means 15 for charged materials and a stirring means 14 for the melted charged materials, and in which the charged materials are held to the melted state and a DC power source device 11 applying DC voltage to the space between both poles is used, harmful metal- containing salts and a metal having a decomposition potential baser than that of the metal as the object for removal are charged to the electrolytic cell 10 and are held to the melted state, applying voltage is set to the value capable of obtaining the decomposition potential of the base metal, and electrolysis is executed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a treatment technique for detoxifying salts containing harmful metals.

[0002]

2. Description of the Related Art Most of the incineration residues generated when municipal waste and industrial waste are incinerated are disposed of in landfills. However, as it becomes difficult to secure landfill sites, it is required to reduce the volume. It has become. Also, among the incineration residues, ash scattered from the incinerator and collected by the dust collector (incineration fly ash) contains harmful heavy metals such as lead and cadmium. It must be treated to make it harmless. For this reason, in recent years, as a treatment method capable of simultaneously performing volume reduction and detoxification of incineration residues, a process of melting incineration residues has been performed.

As a processing technique for melting this incineration residue, there is a method using an electric resistance type melting furnace or an induction heating type melting furnace. For example, in a method using an electric resistance type melting furnace, a melt of the incineration residue is retained in the furnace, and the melt is charged while heating by supplying electricity to the melt. Let melt. In the melting furnace in which the melting process is performed in this manner, the incinerated fly ash charged contains a large amount of salts such as sodium chloride, potassium chloride, and calcium chloride. During the stagnation, the components separate and a layer of molten slag and a layer of molten salt form due to the difference in specific gravity. In discharging the molten material, the molten salt and the molten slag are separately discharged.

[0004] Of the melt discharged in this way,
After the molten slag is solidified, it is used as an aggregate or landfilled. However, since the molten salt contains heavy metals contained in the incineration fly ash in a water-soluble form, it must be subjected to a deactivation treatment again at the time of disposal.

As a conventional technique for detoxifying the salts, there is a method described in Japanese Patent Application Laid-Open No. 60-61087. In this method, salts discharged from a melting furnace are dissolved in water to prepare an aqueous solution thereof. Then, after adding caustic soda to the aqueous solution to make it alkaline, a chelating agent is also added to precipitate a small amount of heavy metal, and this is filtered to separate and remove a precipitate containing heavy metal. On the other hand, the aqueous solution from which the heavy metals have been removed is subjected to a crystallization treatment by a cooling operation and an evaporation operation, and the dissolved salts are taken out.

In another processing technique, a method is employed in which salts discharged from a melting furnace are converted into an aqueous solution, and then the aqueous solution is electrolyzed to precipitate heavy metals.

[0007]

However, in the above-mentioned prior art, water is added to the salt discharged from the melting furnace to form an aqueous solution, so that the processing amount increases and the equipment becomes large. Further, when disposing of the aqueous solution after removing the heavy metals, the salts must be taken out from the aqueous solution, so that a crystallization operation must be performed. For this reason, in the process, a device such as a crystallization device and a filtration device must be provided, and the entire processing equipment becomes very complicated.
Further, a large amount of power and auxiliary materials are consumed.

An object of the present invention is to provide a method for treating salts containing harmful metals and an apparatus therefor, which does not require an operation to increase the amount of salts to be treated and can simplify the structure of the treatment apparatus. And

[0009]

The above object is achieved by the following invention.

The treatment method according to the first invention is characterized in that a salt containing a harmful metal and a metal whose decomposition potential is lower than that of the metal to be removed and / or a salt thereof are charged into an electrolytic cell and heated to form a molten state. And electrolysis is performed by setting the applied voltage to a value at which the decomposition potential of the base metal is obtained.

The treatment method according to a second aspect of the present invention is the processing method according to the first aspect, wherein the melt in the electrolytic cell is heated to a temperature of 500 ° C. to 1100 ° C.
It is characterized in that electrolysis is performed while maintaining the temperature in the range of ° C.

A processing method according to a third invention is characterized in that, in the first or second invention, electrolysis is performed while stirring the melt in the electrolytic cell.

An apparatus according to a fourth aspect of the present invention has an anode and a cathode, a means for heating a charge, and a means for stirring a molten charge so that the charge is maintained in a molten state. It is characterized by comprising a configured electrolytic cell and a DC power supply device for applying a DC voltage between the two electrodes.

In the following description, a metal having a base decomposition potential is simply referred to as a base metal. Further, a metal having a noble decomposition potential is simply referred to as a noble metal.

The inventors of the present invention have studied in detail the reaction of depositing harmful metals by electrolyzing salts discharged from a waste melting furnace. I got the knowledge that it is being done.

That is, most of the salts discharged from the waste melting furnace are made of NaCl, KCl and CaCl ₂ , but also contain various types of heavy metals and other metals. This salt is melted and electrolyzed,
When depositing the metal to be removed, if the applied voltage is set to a value corresponding to the decomposition potential of the metal that is more noble than the metal to be removed and electrolysis is performed, the ions of the nobler metals are electrolyzed together. Metallized and precipitated. Next, in the same electrolytic cell, a substitution reduction reaction occurs between the noble metal and the noble metal ion that has been left unelectrolyzed, and the noble metal is removed. A certain metal ion to be removed is reduced and deposited by the base metal. The base metal is oxidized and ionized again by the reduction reaction by the metal substitution. Then, the metal ions are electrolyzed and precipitated, and a reduction reaction by metal replacement is performed again. In this way, the two reactions of reduction by electrolysis and reduction by metal replacement are repeatedly performed, and the metal to be removed is removed.

However, in the treatment of salts discharged from a waste melting furnace, the content of the above-mentioned base metal is small, so that the amount of reaction due to metal substitution is small, and the degree to which the above two reactions are linked and performed is low. Few.

Therefore, in the present invention, a metal that is more base than the metal to be removed is added to increase its content,
Increase the amount of reaction due to metal substitution. Thereby, precipitation of the metal to be removed is promoted. If a metal that is more base than the metal to be removed is added, the amount of the metal to be removed increases by the amount reduced by the added metal. And
Thereafter, the substitution reduction reaction is repeatedly performed, and the metal to be removed is efficiently deposited.

When the above-mentioned base metal is added in the form of a metal, the base metal added is firstly substituted and reduced to precipitate the metal ions to be removed, and the base metal itself is ionized. Next, the ions of the base metal are electrolyzed and deposited, and again participate in the substitution reduction reaction. Therefore, the above two reactions are repeatedly performed.

Examples of the metal to be added include zinc and aluminum, which are base metals for harmful metals such as lead and cadmium contained in salts discharged from a waste melting furnace.

In the present invention, a base metal may be added in the form of a salt. Examples of the salt of the metal to be added include chlorides such as zinc and aluminum, which are base metals with respect to harmful heavy metals. In this case, first, electrolysis of the salt is performed, and then, a substitution reduction reaction is performed between the added metal deposited by the electrolysis and the metal ion to be removed. At this time, since the additional metal deposited in the previous electrolysis is in fine particles, the contact efficiency with the metal ion to be removed is improved, and the reaction is promptly performed.

In the present invention, the salt to be treated is 500
The electrolysis is performed while maintaining the temperature in the range of 1 ° C to 1100 ° C for the following reason. Molten salt discharged when melting municipal waste incineration residue is mainly NaCl, KC
l, a mixture of chlorides such as CaCl ₂ ,
Its melting point is about 500-800 ° C. The heavy metals contained in the molten salt are copper, lead, cadmium, zinc and the like, and chlorides of these heavy metals have melting points of about 300 to
600 ° C. The melting point of the metal deposited when the molten salt is electrolyzed is about 300 ° C to 1100 ° C.

On the other hand, the boiling point of each of the above substances is about 1400 ° C. to 16 ° C. for the main salts NaCl, KCl and CaCl _2.
Since the temperature is not lower than 00 ° C., if the molten salt is treated in a temperature range of 500 to 1100 ° C., both the salts and the precipitated heavy metal can be kept in a molten state. In this way, the salts and the precipitated heavy metal are kept in a molten state, so that the heavy metal and the salts can be easily separated.

Further, in the present invention, it is preferable to stir the melt in the electrolytic cell, and this stirring causes the metal ions to move quickly and the metal to be deposited efficiently. Further, the contact efficiency between the metal ions to be removed and the base metal is improved, and the reaction is carried out quickly.

[0025]

FIG. 1 shows an embodiment of the apparatus of the present invention and a processing method of the present invention using the apparatus.
1 is a schematic longitudinal sectional view showing an example in an embodiment according to the present invention. In FIG. 1, 10 is an electrolytic cell, 11 is a DC power supply, 12a is an anode, and 12b is a cathode. Reference numeral 30 denotes a molten salt, and 31 denotes a deposited molten metal.

The electrolytic cell 10 has a closed type structure, and its inner wall has a corrosion resistance to molten salt, for example,
It is made of carbon material. The electrodes 12a and 12b are also formed of a carbon material. The electrolyzing tank 10 is provided with a heating means 15 on the outer wall surface by electric heating or induction heating, so that the molten salt in the tank can be heated and maintained at a predetermined temperature. The electrolytic cell 10 is provided with a stirrer 14 as a means for stirring the melt in the cell. In addition, a publishing apparatus using a non-oxidizing gas such as a nitrogen gas may be used as the stirring means. In the figure, reference numeral 13 denotes a partition wall disposed so as to surround the anode 12a, and is formed of a porous material such as ceramics. The partition 13 is provided to prevent the deposited metal from being ionized again by the contact between the metal deposited by electrolysis and the anode 12a.

The treatment of the molten salt by the apparatus having the above configuration is performed as follows. In this description, for the sake of convenience, it is assumed that the metals to be removed in the salts to be treated are cadmium and lead, and a case where zinc is added as a metal which is lower than these harmful metals will be described.

A salt containing cadmium and lead, which are harmful metals, is charged into the electrolytic cell 10, heated and melted by the heating means 15, and maintained at a predetermined temperature in the range of 500 ° C. to 1100 ° C. Then, zinc, which is more base than the harmful metal, is charged, and while the melt is stirred to activate the stirrer 14, power is supplied from the DC power supply 11 to a decomposition potential at which zinc is deposited between the electrodes 12 a and 12 b. Apply the corresponding DC voltage.

By applying the DC voltage, (1) to
The reaction represented by the formula (3) proceeds, and zinc contained in the salts and ions of cadmium and lead, which are lower than the zinc, are electrolyzed and deposited as a metal on the surface of the cathode 12b. or,
At the same time, the substitution reduction reaction shown in the formulas (4) and (5) occurs between the added zinc and cadmium, which is lower than that, and unreacted ions of lead, whereby cadmium and lead are precipitated and zinc is dissolved. And ionize. Next, a substitution reduction reaction represented by the formulas (4) and (5) occurs between the zinc deposited by electrolysis and unreacted ions of cadmium and lead, which are lower than that, and cadmium and lead precipitate. , Zinc dissolves and ionizes. These zinc ions are electrolyzed again into zinc metal.

As described above, in the same electrolytic cell, the reduction reaction by metal substitution and the reduction reaction by electrolysis are performed in two stages and in parallel, and the deposition of the metal to be removed is promoted. Zn ²⁺ + 2e ⁻ → Zn (1) Cd ²⁺ + 2e ⁻ → Cd (2) Pb ²⁺ + 2e ⁻ → Pb (3) Cd ²⁺ + Zn → Cd + Zn ²⁺ ... (4) Pb 2 ⁺ + Zn → Pb + Zn 2 ⁺ ... (5)

Then, when the concentration of the metal to be removed remaining in the molten salt falls below a predetermined value, the energization is stopped, the stirrer 14 is stopped, and the molten salt is allowed to stand still. Then, the deposited metal is settled at the bottom of the electrolytic cell 10 and separated. Next, the molten salt 30 from which the harmful metal has been separated is extracted from the molten salt outlet 17 and solidified.

After the molten salt 30 has been extracted, a salt to be newly treated is charged with the molten metal 31 remaining at the bottom of the electrolytic cell 10, and the above-described treatment operation is restarted. Upon resumption of this processing operation, zinc in the molten metal 31 accumulated at the bottom of the electrolytic cell 10 acts as a reducing agent in the above-described substitution reduction reaction similarly to the added zinc. No addition is made.

Then, when the amount of deposited metal increases due to the repetition of the processing operation, the molten metal 31 collected at the bottom of the electrolytic cell 10 is extracted from the metal discharge port 18, received in a mold, and collected as an ingot. .

At the time of the above electrolysis, chlorine gas is generated from the surface of the anode 12a, so that the exhaust gas is extracted and sent to an exhaust gas treatment device, where the chlorine gas is absorbed in a caustic soda solution.

In the above description, zinc is taken as an example of the metal to be added. However, the metal to be added is not limited to zinc, and may be any metal that is more base than the metal to be removed. Generally, among the heavy metals contained in the salts discharged from the waste melting furnace, zinc is a metal with a higher content ratio than other metals, but the content itself is very small, so zinc is added. The case where the content is increased is taken as an example. And other additive metal may be aluminum, which is even lower than zinc.

The form of the metal to be added may be a salt such as a chloride. When added in the form of a salt, the substitution reduction reaction is efficiently performed as described above.

FIG. 2 is a schematic longitudinal sectional view showing another example of the embodiment of the apparatus of the present invention. In FIG. 2, portions having the same configuration as in FIG. 1 are denoted by the same reference numerals, and description thereof is omitted. In this processing apparatus, the inner wall of the electrolytic cell 10 serves as the cathode 12b. If this electrolytic cell is used, the metal can be deposited quickly because the surface area of the cathode on which the metal is deposited is very large. Therefore, the electrolysis of the molten salt can be performed efficiently.

Example A test apparatus having the same configuration as that shown in FIG. 2 was used to detoxify salts containing harmful metals. The electrolytic cell was made of carbon and had a cylindrical shape with an inner diameter of 150 mm. The electrode was an anode by inserting a carbon rod having an outer diameter of 10 mm, and the inner wall of the electrolytic cell was a cathode. Metal zinc powder was used as the added metal. The salts used in this test were obtained by collecting the salts generated in the melting furnace when the incineration residue of municipal waste (mixed incineration ash and fly ash) was melted. The analytical values of the salts are shown in Table 1. The salts are mainly composed of NaCl, KCl, CaCl
₂ , and Pb was contained as a harmful heavy metal.

[0039]

[Table 1]

The test was conducted as follows. About 3.0 kg of the above salts were charged into an electrolytic cell, and heated to 700 ° C. with a heater to be melted. Next, 30 g of zinc metal powder was charged. Then, while stirring, a voltage of 2.5 V was applied to perform electrolysis, so that Zn and Pb were precipitated together. Then, when the concentration of Pb dissolved in the molten salt was reduced to a value lower than a predetermined value, the energization was stopped, and the precipitated metal was settled. Then, the molten salt from which Pb was removed was extracted.

As a comparative example, a treatment for electrolyzing only the above salts without adding zinc metal powder was performed.
The transition of the Pb concentration during each of the above electrolysis treatments was as shown in FIG.

(Comparative Example) Using the same apparatus as in the example, a test for detoxifying the same salts as in the example was performed. However, no metal zinc was added. Pb during electrolytic treatment
The transition of the density is shown in FIG. 3 together with the data of the example.

Referring to FIG. 3, comparing the results of the embodiment and the comparative example, the removal of Pb is remarkably promoted in the embodiment. It is clear that this good result was obtained by adding metallic zinc. In the tests of the above Examples and Comparative Examples, the processing time required for reducing the concentration of Pb in the molten salt to less than 1 mg / kg was 50 minutes in the Comparative Example, whereas the processing time in the Example was 2
It was 0 minutes. As described above, in the example, the processing time was significantly reduced.

[0044]

According to the treatment method of the present invention, the salt containing a harmful metal and the metal which is more noble than the harmful metal are charged into the electrolytic cell and a direct current voltage is applied, and the reduction by electrolysis can be achieved. Since the process of causing two reactions consisting of reduction by metal substitution to precipitate harmful metals is performed, removal of the harmful metals to be removed is promoted, and downsizing and simplification of the processing apparatus can be achieved.

When a metal which is more base than the above-mentioned metal to be removed is added, if the metal is added in the form of a salt, the base metal is precipitated as fine particles by electrolysis, so that the base metal and harmful metal ions are added. The efficiency of contact with the metal is improved, and the deposition of harmful metals by this reaction is promoted.

By conducting the electrolysis while maintaining the salt at a temperature of 500 ° C. to 1100 ° C., the salt and the precipitated metal are kept in a molten state, so that the metal and the salt can be easily separated.

When the electrolysis is performed while stirring the melt in the electrolytic cell, the movement of the metal ions is accelerated, and the contact efficiency between the base metal and the harmful metal ions is improved. Precipitation of harmful metals is promoted.

[Brief description of the drawings]

FIG. 1 is a schematic longitudinal sectional view showing an example of an embodiment according to the apparatus of the present invention.

FIG. 2 is a schematic longitudinal sectional view showing another example of the embodiment of the device of the present invention.

FIG. 3 is a diagram showing a transition of Pb concentration during an electrolytic treatment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Electrolysis tank 11 DC power supply 12a Anode 12b Cathode 13 Partition wall 14 Stirrer 15 Heating means 16 Exhaust gas discharge port 17 Molten salt discharge port 18 Metal discharge port 30 Molten salt 31 Molten metal

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Hiroshi Yamamoto 1-2-1 Marunouchi, Chiyoda-ku, Tokyo F-term in Nihon Kokan Co., Ltd. 4D004 AB03 CA15 CA29 CA37 CA44 CB04 CB32 CC20 DA02 DA03 DA06 4K058 AA21 BA24 BA27 BB05 CB04 CB05 CB17 CB26 DD01 DD02 DD13 DD17 FC01

Claims (4)

[Claims] 1. A salt containing a harmful metal and a metal whose decomposition potential is lower than that of a metal to be removed and / or a salt thereof are charged into an electrolytic cell and heated to maintain a molten state. A method for treating salts containing harmful metals, characterized in that electrolysis is carried out at a value at which a decomposition potential of a base metal is obtained. 2. The method for treating a salt containing a harmful metal according to claim 1, wherein the molten material in the electrolytic cell is heated to 500 ° C. to 11 ° C.
A method for treating salts containing harmful metals, comprising performing electrolysis while maintaining the temperature within a range of 00 ° C. 3. The method for treating a salt containing a harmful metal according to claim 1 or 2, wherein the electrolysis is performed while stirring the melt in the electrolytic cell. Method. 4. An anode and a cathode, means for heating the charge,
Having an agitation means for the melted charge, an electrolytic cell configured to hold the charge in a molten state, and a DC power supply for applying a DC voltage between the two electrodes. Characteristic treatment equipment for salts containing harmful metals.