JP2001047024A - Method for treating salts containing toxic metal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for treating salts contg. toxic metals wherein it is unnecessary to perform an operation for increasing vol. of the salts to be treated and constitution of an apparatus for the treatment can be simplified. SOLUTION: A method for removing toxic metals from salt contg. a plurality of metals wherein the salts to be treated are fed in an electrolytic cell 10 and are held under melted condition and electricity is passed through from a DC electric source 11 and a loaded electric voltage is set to a value corresponding to decomposition potential of a metal whose decomposition potential is baser than those for the metals to be removed to perform electrolysis, and the metals being objects to be removed and metals whose decomposition potentials are baser than those for these metals are deposited by this electrolysis, and then, energization of electrocity is stopped to make the deposited metals settle on the bottom part of the electrolytic cell 10 and, after the melted salts from which toxic metals are removed are taken out, salts to be newly treated are fed into the electrolytic cell 10 in which deposited metals remain and the electrolytic treatment is repeated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD 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. Was. Also, among the incineration residues, the ash scattered from the incinerator and collected by the dust collector (incineration fly ash) contains harmful heavy metals such as lead and cadmium,
At the time of its disposal, it must be treated to render heavy metals harmless. For this reason, in recent years, a process of melting incineration residues has been performed as a method capable of simultaneously reducing the volume and detoxifying incineration residues.

[0003] As a processing technology 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. At this time, since the incinerated fly ash contains a large amount of salts such as sodium chloride, potassium chloride, and calcium chloride, in the furnace, the components are separated while the above-mentioned melt is retained, Due to the difference in specific gravity, a layer of molten slag and a layer of molten salt are generated. And
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 detoxified again at the time of disposal.

[0005] As a conventional technique for detoxifying such salts, there is a method described in JP-A-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 metals, and this is filtered to separate and remove precipitates containing harmful metals. 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, there is disclosed a removing method in which salts discharged from a melting furnace are converted into an aqueous solution, and 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 throughput increases and the equipment becomes large. Further, when disposing of the aqueous solution after removing the heavy metals, the salts in the aqueous solution must be removed, so that a crystallization operation must be performed. For this reason, it is necessary to provide devices such as a crystallization device and a filtration device, so that the entire 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, which does not require an operation to increase the amount of salts to be treated and can simplify the structure of the treatment apparatus.

[0009]

The above object is achieved by the following invention.

A first aspect of the present invention is a method for removing harmful metals from salts containing a plurality of metals, wherein the salts are charged into an electrolytic cell and kept in a molten state, and the applied voltage is set higher than that of the metal to be removed. The decomposition potential is also set to a value corresponding to the decomposition potential of the base metal to conduct electrolysis, thereby precipitating a metal whose decomposition potential is more noble than the base metal, and then the molten salt from which the harmful metal has been removed. After extracting, the salt to be newly treated is charged into the electrolytic cell in which the deposited metal remains,
Each of the above operations is repeated in the above order.

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 when depositing harmful metals by electrolyzing the salts discharged from the melting furnace. As shown below, the deposition of metals during the electrolytic treatment is two reactions. Has been obtained.

That is, most of the salts discharged from the melting furnace are made of NaCl, KCl and CaCl ₂ , but also contain various kinds of metals. When electrolyzing the salts in a molten state and depositing a metal to be removed, the applied voltage is set to a value corresponding to the decomposition potential of a metal that is more base than the metal to be removed, and electrolysis is performed. More noble metal ions are electrolyzed together and metallized and deposited. Next, a substitution reduction reaction occurs between a metal that is more noble than the metal to be removed and a noble metal ion in the deposited metal, and the noble metal ion to be removed is reduced by the noble metal. Is deposited as a metal. By this substitution reduction reaction, the above-mentioned base metal is oxidized and ionized again. But,
This metal ion is electrolyzed again, metallized and deposited,
It is involved in the above substitution reduction reaction. In this way, two reactions consisting of reduction by electrolysis and substitution reduction are repeatedly performed, and the target metal is removed.

However, in the treatment of salts discharged from the melting furnace, the above two reactions are not linked efficiently and performed because the content of the base metal is smaller than the metal to be removed.

Therefore, in the present invention, the harmful metal removal treatment by electrolysis is performed as follows to increase the content of a metal that is more noble than the metal to be removed. That is, when the predetermined electrolysis is completed, the treated molten salt is extracted, and when the next electrolysis is started, the metal deposited by the previous electrolysis is left in the electrolysis tank, and a new metal is introduced into the electrolysis tank. Is charged with the salt to be treated, and electrolysis is started. When the electrolysis is performed in this manner, the deposited metal accumulates in the electrolytic cell and the content of the base metal increases, so that the removal of the metal to be removed by the two reactions is promoted.

According to a second aspect of the present invention, in the first aspect, a salt containing a plurality of metals is charged into the electrolytic cell, and the salt is charged at 500 ° C. to 110
The method for treating salts containing harmful metals according to claim 1, wherein the electrolysis is performed while maintaining the temperature within a range of 0 ° C.

The molten salt discharged when the municipal waste incineration residue is melted is a mixture mainly composed of chlorides such as NaCl, KCl, CaCl _{2 and the} like, and its melting point is about 500.
８００800 ° C. The heavy metals contained in the molten salt are copper, lead, cadmium, zinc and the like, and the chlorides of these heavy metals have a melting point of about 300 to 600 ° C.
The melting point of the metal that precipitates when this chloride is electrolyzed is about 300 ° C to 1100 ° C.

On the other hand, the main salts NaCl and KC
1, The boiling point of CaCl ₂ is about 1400 ° C. to 1600 ° C. or more. Therefore, if the molten salt is treated in a temperature range of 500 to 1100 ° C., both the salt 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.

According to a third aspect of the present invention, there is provided a method for treating salts containing harmful metals according to the first or second aspect, wherein electrolysis is performed while stirring the molten salts in the electrolytic cell. .

By stirring the melt in the electrolytic cell,
The movement of metal ions is performed quickly, and the metal is deposited efficiently. In addition, the contact efficiency between the ions of the metal to be removed and the base metal is improved, and the reaction is quickly performed.

[0021]

FIG. 1 is a schematic longitudinal sectional view showing an example of an embodiment of an apparatus for carrying out 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. The electrolytic cell 10 has a closed type structure, and its inner wall is formed of a carbon material having corrosion resistance to molten salt. Electrodes 12a, 12
b is also formed of a carbon material. In addition, the electrolytic cell 10 has a heating means 1 on its outer wall surface by electric heating or induction heating.
5 is provided 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. Note that a publishing device 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, which is 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 contact between the metal deposited by electrolysis and the anode 12a. Reference numeral 30 denotes a molten salt, and 31 denotes a deposited molten metal.

The treatment of the molten salt by the apparatus having the above configuration is performed as follows. In this description, for convenience, it is assumed that the metals to be removed in the salts to be treated are cadmium and lead, and the metal that is more base than these metals to be removed is zinc.

A salt containing a harmful metal is charged into the electrolytic cell 10 and heated and melted by the heating means 15,
It is kept at a predetermined temperature within the range of 1100 ° C. Next, power is supplied from the DC power supply 11 while the stirrer 14 is activated to stir the melt, and a DC voltage corresponding to a decomposition potential at which zinc is deposited is applied between the electrodes 12a and 12b. By applying this DC voltage, as shown in the equations (1) to (3), zinc and noble metal ions are converted to the cathode 12b.
Precipitates as a metal on the surface of. Next, as shown in the equations (4) and (5), a substitution reduction reaction occurs between zinc in the precipitate and ions of cadmium and lead which are more noble, and cadmium and lead precipitate, and zinc Dissolves and ionizes. The zinc ions are electrolyzed again to form zinc metal, which acts as a reducing agent in the reactions of the formulas (4) and (5) to precipitate cadmium and lead.

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) Thus, while the metal to be removed is deposited by electrolysis, the replacement is performed. A reduction reaction occurs, and the metal to be removed in two stages is deposited in parallel.

At the time of the above-mentioned 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 by the caustic soda solution.

When the concentration of heavy metal in the molten salt has dropped to a value lower than a predetermined value, the energization is stopped, and the stirrer 14 is stopped to allow the molten salt to stand still. Settle. Thereafter, the molten salt 30 from which the heavy metal has been removed is extracted from the molten salt outlet 17 and solidified. Next, the salt to be newly treated is charged into the electrolytic cell, and the above operation is restarted. When resuming this treatment operation, zinc is added as necessary at the stage where the number of repeated use of the deposited metal is small.

At the stage where the amount of deposited metal has increased, the electrolytic cell 1
The molten metal 31 collected at the bottom of the cylinder 0 is extracted from the metal discharge port 18, received in a mold, and collected as an ingot.

FIG. 2 is a schematic longitudinal sectional view showing another example of the embodiment of the apparatus for carrying out 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 1) A test in which a treatment apparatus having the same configuration as that shown in FIG. The electrolytic cell used was made of carbon and had a cylindrical shape with an inner diameter of 150 mm. The electrode was used as an anode by inserting a 10 mm carbon rod, and the inner wall of the electrolytic cell was used as a cathode.

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. This salt is mainly composed of NaC
1, KCl, and CaCl ₂ , and contained Zn and Pb, which is a harmful heavy metal.

[0032]

[Table 1]

The test was performed as follows. About 3.0 kg of the above salts were charged into the electrolytic cell, and heated to 700 ° C. with a heater to be melted. Next, a voltage of 2 V was applied with stirring to perform electrolysis, so that Zn and Pb, which is a harmful metal, were deposited together. Then, when the concentration of Pb was reduced to a value lower than the predetermined value, the energization was stopped, and the deposited metal was settled. Next, after extracting the molten salt from which the harmful heavy metals have been removed, the salt to be newly treated is charged into the electrolytic cell in which the metal deposited at the bottom remains, and the same operation as above is performed. The resultant was subjected to an electrolytic treatment to precipitate a metal.

In this way, the process of charging a salt to be newly treated into the electrolytic cell in which the metal deposited on the bottom remains and electrolyzing was repeated 10 times. FIG. 3 shows the transition of the Pb concentration when a new electrolytic treatment is sequentially repeated while the metal deposited by the previous electrolysis remains. As described above, when the electrolytic treatment is repeated with the deposited metal remaining, the removal of Pb is remarkably promoted.

In this test, the processing time required for the concentration of Pb in the molten salt to drop to 1 mg / kg or less was 50 minutes in the first test, and 25 minutes in the tenth test. Minutes, and the processing time has been greatly reduced.

[0036]

According to the present invention, the molten salt is treated as it is by applying a DC voltage to the molten salt to precipitate the metal to be removed, so that the throughput is not increased. The size of the processing device can be reduced. Furthermore, since the salt to be newly treated is charged into the electrolytic cell in which the deposited metal remains, and the electrolytic treatment is repeated, the amount of the base metal is larger than the metal to be removed, and the amount of the metal to be removed by the substitution reduction reaction is increased. The amount of metal deposition increases. For this reason, two reduction reactions, ie, reduction by electrolysis and substitution reduction, are performed in parallel, and the processing apparatus can be reduced in size and simplified.

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.

Further, by performing electrolysis while stirring the melt in the electrolytic cell, the movement of metal ions is accelerated, and the contact efficiency between the base metal and the ions of the metal to be removed is improved. The precipitation of heavy metals by displacement reduction is promoted.

[Brief description of the drawings]

FIG. 1 is a schematic longitudinal sectional view showing an example of an embodiment of an apparatus for carrying out the present invention.

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

FIG. 3 shows the transition of the Pb concentration when new electrolysis is performed repeatedly while the metal deposited by the previous electrolysis remains.

[Description of Signs] 10 Electrolyzer 11 DC power supply 12a Anode 12b Cathode 13 Partition wall 14 Stirrer 15 Heating means 17 Molten salt outlet 18 Metal outlet 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 AA16 AB03 AC08 BA05 CA15 CA29 CA37 CA44 CB04 CB32 CB33 CB50 DA02 DA03 DA06 DA20

Claims (3)

[Claims] 1. A method for removing harmful metals from salts containing a plurality of metals, wherein the salts are charged into an electrolytic cell and kept in a molten state, and an applied voltage is set to have a decomposition potential lower than that of the metal to be removed. By performing electrolysis at a value corresponding to the decomposition potential of the base metal, a metal whose decomposition potential is more noble than the base metal is deposited, and then the molten salt from which the harmful metal has been removed is extracted. A method of treating a salt containing a harmful metal, wherein the salt to be treated is newly charged into an electrolytic cell in which the deposited metal remains, and each of the above operations is repeated in the order described above. 2. The method according to claim 1, wherein a salt containing a plurality of metals is charged into the electrolytic cell and electrolysis is performed while maintaining the temperature within a range of 500 ° C. to 1100 ° C. Treatment method for the salt containing. 3. The method for treating harmful metal-containing salts according to claim 1, wherein the electrolysis is performed while stirring the molten salts in the electrolytic cell.