JP2003027272A - Method for electrochemically recovering heavy metal from fly ash - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method capable of efficiently and easily recovering various kinds of metals from the incineration fly ash produced when wastes of shredder dust, or the like, are incinerated or the molten fly ash produced when the incineration fly ash is further melted by a plasma, or the like. SOLUTION: An aqueous sulfuric acid solution of a pH <=4 is subjected to extraction treatment to elute copper, cadmium and zinc in this solution. An electrolysis is effected between an anode and a cathode in the separated solution obtained by the separation of solid from liquid of a fly ash-containing slurry. The cathode potential is gradually or stepwise changed in a negative direction, by which the copper, cadmium and zinc are discriminated and electrolytically deposited on the cathode. The residue remaining after the separation of the solid from the liquid is thereafter subjected to the extraction treatment with an extraction chemical solution to elute lead in this solution. The lead is electrolytically deposited from the separated solution obtained by the separation of the solid from liquid of the residue-containing slurry.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is an electrochemical technique for recovering metals, particularly harmful metals such as copper, lead, cadmium and zinc, from incineration fly ash and molten fly ash as a technology for recycling waste. On how to do. Usually, scrapped automobiles, discarded home appliances, etc. are crushed by a shredder to recover metal, but the shredder dust remaining at that time is incinerated and reduced in volume, and the generated incinerated fly ash or incinerated fly ash is further melted by plasma etc. . The present invention relates to a method for separating and recovering various metals from incineration fly ash generated during incineration of shredder dust, or molten fly ash generated during melting of the incineration fly ash.

[0002]

2. Description of the Related Art Fly ash discharged from a refuse incineration facility contains a high concentration of harmful metals. Therefore, it is stipulated that fly ash should be subjected to intermediate treatment by one of the following four methods defined by the Ministry of Health, Labor and Welfare, the melting method, cement solidification chemical treatment, and acid extraction. The fly ash after the intermediate treatment is landfilled at the final disposal site, but from the viewpoint of removing harmful metals, adverse effects of salts in leachate, reducing the volume of final disposal, or recycling metals, It is desired to establish a technology for recovering metals in fly ash. For example, a method of eluting a metal by a wet process, then concentrating the metal for each type of metal, recovering each metal as a concentrate enough to be used as a raw material for non-ferrous metal refining, and refining the metal (Japanese Patent Laid-Open No. HEI 7-18753).
No. 138630) is proposed.

However, in this method, an expensive chemical must be used in the liquid for eluting the metal, the process is complicated, and the concentrate is sludge consisting of hydroxide or sulfide. Since it has a large volume and is a harmful substance, it has many problems in terms of transportation.

[0004]

An object of the present invention is to produce incinerated fly ash generated when incinerating waste such as shredder dust, or molten fly ash produced when the incinerated fly ash is further melted by plasma or the like. Therefore, it is to provide a method capable of efficiently and easily separating and recovering various metals.

[0005]

According to the present invention, fly ash is adjusted to pH 4
Copper, cadmium and zinc are eluted in the same solution by extraction with the following sulfuric acid aqueous solution, and electrolysis is performed between the anode and the cathode in the separated liquid obtained by solid-liquid separation of the resulting fly ash-containing slurry, By gradually or stepwise changing the cathode potential in the negative direction, copper, cadmium and zinc are separated and electrolytically deposited on the cathode, and then the residue remaining in the solid-liquid separation is extracted with an extraction chemical solution. Is a method for electrochemically recovering heavy metals from fly ash, characterized in that lead is eluted in the same liquid, and lead is electrolytically deposited from a separated liquid obtained by solid-liquid separation of the resulting residue-containing slurry.

The present inventors have conducted extensive research to separate and recover metals in fly ash. As a result, the heavy metals in various fly ash were first eluted with hydrochloric acid acidic aqueous solution and then various metals were extracted. By utilizing the difference in the electrochemical deposition potential of, the cathode potential is gradually or gradually changed from a noble potential to a base potential, and heavy metals such as copper, lead, cadmium, and zinc are formed into a metal ingot. It has been found that it is possible to precipitate it by (Japanese Patent Application No. 2000-370877). Also,
As an improved invention, utilizing the fact that after extracting heavy metals such as copper, cadmium, and zinc in fly ash with an aqueous solution of pH 1 or more, there is a difference in the electrochemical deposition potential of various metals,
The cathode potential is gradually shifted from the noble potential to the base potential,
We have found a method of electrolytically depositing copper, cadmium and zinc, then adjusting the pH to 1 or less to extract lead, and electrolyzing the extract to deposit lead (Japanese Patent Application No. 2001-003384). Furthermore, as an improved invention, the pH from fly ash
After extracting lead and zinc with an aqueous solution of 12 or more, utilizing the fact that there are differences in the electrochemical deposition potentials of various metals, the cathode potential is gradually shifted from a noble potential to a base potential, and lead and zinc are extracted. After electrolytically depositing, the residue containing copper and cadmium that could not be extracted at pH 12 or higher was adjusted to pH 1 to 5 to extract copper and cadmium, and the potential was gradually transferred from the noble potential to the base potential, A method for electrolytically depositing copper and cadmium was found (Japanese Patent Application No. 2001-037819).

Here, among the heavy metals in the fly ash, the important one from the viewpoint of waste regulation is the recovery of lead and zinc, and it is important to separate and recover them.

In both the first and second inventions, the extraction is carried out with an acidic aqueous solution. However, when sulfuric acid is used as a chemical, insoluble lead sulfate is deposited, so that it cannot be subjected to subsequent electrolytic deposition. Therefore, in this case, hydrochloric acid or nitric acid is inevitably used, but in this case, when the extract is electrolyzed, harmful chlorine gas or nitrous acid gas is generated. Further, in the third invention, pH12
When extracting with the above aqueous solution, chemicals such as caustic soda are consumed in large amounts for the extraction of aluminum and iron, leading to high costs, and when electrolyzing the extracted lead and zinc, the cathode voltage However, there is a problem that a part of the alloy is electrolytically deposited as an alloy of lead and zinc and the resource value of the deposit is reduced.

In the present invention, since an aqueous sulfuric acid solution having a pH of 4 or less is used as the acid for the fly ash extraction treatment, the extract does not contain lead, and the electrolytically deposited zinc does not contain lead. Further, since the electrolysis is carried out with a sulfuric acid aqueous solution, no harmful chlorine gas or nitrous acid gas is generated during the electrolysis. Furthermore, since only PbSO ₄ , CaSO ₄ , alumina, and silica remain in the residue after extraction with sulfuric acid, the amount of chemicals used for extracting lead can be small.

As the electrolytic deposition reaction proceeds, the concentration of metal ions in the aqueous solution decreases, and the current efficiency for deposition decreases. Therefore, a guideline for reducing the concentration of metal ions in a solution by electrolytic deposition is 10 ^{−. 4} to 10 ^-2 mol /
Place it at about 1 l and do not react to a too low concentration,
An aqueous solution containing a low concentration of metal ions is circulated and reused. According to this method, electrolytic deposition can be performed efficiently without lowering the current efficiency for deposition.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION In the first step, fly ash is made into a slurry by adding water in an extraction tank, and an aqueous sulfuric acid solution is added to the slurry while stirring the slurry to adjust the pH to 4 or less. Here, the reason why the pH is adjusted to 4 or less by adding sulfuric acid is that copper, cadmium and zinc are dissolved but lead is not dissolved. The stirring time while maintaining the above pH may be 30 minutes or longer, and the temperature may be room temperature, but it is preferably 50 ° C. or higher in order to accelerate the dissolution rate. Next, the slurry is filtered with a filtering device such as a filter tank or a filter press to perform solid-liquid separation.

In the second step, copper, cadmium and zinc are deposited on the filtrate by electrolysis between the anode and the cathode. Here, as the anode, an electrode made of titanium or platinum-plated titanium is used, and as the cathode, a copper electrode is used for electrolytic deposition of copper, an aluminum electrode is used for electrolytic deposition of cadmium, and an electrolytic electrode of zinc is used for electrolytic deposition of zinc. Use titanium electrodes. Furthermore, a guideline for reducing the metal ion concentration in the solution by electrolytic deposition is 10 ⁻⁴ to 10 ⁻² mo.
Place it at about 1 / l so that it does not react to a too low concentration. Because the metal ion concentration in the solution is 10
^{This is because if it is -4} mol / l or less, the current efficiency is reduced to 20% or less.

In the third step, lead is extracted from the residue obtained after the solid-liquid separation in the first step, using a solution containing caustic soda, sodium carbonate and / or sodium thiosulfate.

In the fourth step, lead is electrolytically deposited from the solution extracted in the third step. An electrode made of titanium or platinum-plated titanium is used for the anode, and an electrode made of lead or titanium is used for the cathode. It is desirable to use.

In the fifth step, after the recovery of copper, cadmium and zinc in the second step is completed, the liquid having a metal ion concentration of 10 ^-4 mol / l or less is used as a fly ash in the first step. For the extraction, after the lead recovery was completed in the fourth step, the liquid having a lead ion concentration of 10 ⁻⁴ mol / l or less was circulated and reused in the extraction of the residue in the third step, and a new fly was used. The first to fourth steps are performed on the ash. If such an operation is repeated, aluminum, potassium,
Sodium and the like are gradually concentrated and reach the solubility of themselves, so that they do not spontaneously dissolve in the fourth step. Further, iron and chromium are deposited as alloy plating during cadmium plating.

Next, the present invention will be specifically described based on examples.

Example (First Step) First, molten fly ash 20 was placed in a 500 ml beaker.
g was added, 200 ml of a 1N sulfuric acid aqueous solution was added, and the whole was stirred at 50 ° C. for 1 hour, and the resulting slurry was subjected to solid-liquid separation. The pH of the obtained separated liquid was 1.2.

(Second step) After that, a platinum electrode having a surface area of 50 cm ² and being plated with platinum was used as an anode in the separated liquid.
Put a copper plate with an area of 50 cm ² as a cathode, and
Electrolysis was carried out at a constant current. At that time, the cathode potential was −0.
23 V (vs. standard hydrogen potential) to -0.30 V (v
s. The standard hydrogen potential) gradually decreases, and the copper concentration is shown in Table 1.
As shown in 0.17 (g / l) to 0.012 (g / l)
l).

Then, the cathode was changed to a titanium electrode having an area of 50 cm ² and electrolysis was carried out at a constant current of 2.5 A. At that time, the cathode potential gradually decreased from −1.29 V (vs. standard hydrogen potential) to −1.38 V (vs. standard hydrogen potential), and the zinc concentration was 3.45 V as shown in Table 1.
(G / l) to 0.048 (g / l).

(Third step) 7.8 g of the solid residue after solid-liquid separation in the first step was put into a 500 ml beaker, 170 ml of 1N caustic soda aqueous solution was put therein, and 50 ° C.
The resulting slurry was solid-liquid separated. The pH of the obtained separated liquid was 12.8.

(Fourth step) In the separated liquid obtained in the third step,
The platinum plated titanium electrode with an area of 50 cm ² as the anode, put the copper plate of area 50 cm ² as the cathode,
Electrolysis was performed at a constant current of 2.5A. At that time, the cathode potential was −0.92 V (vs. standard hydrogen potential) to −1.29.
V (vs. standard hydrogen potential) gradually decreased, and the lead concentration was 1.48 (g / l) to 0.21 as shown in Table 1.
(G / l).

(Fifth Step) After the completion of the recovery of copper, cadmium and zinc by the second step, the liquid containing these metal ions at a concentration of 10 ⁻⁴ mol / l or less is used as a fly ash in the first step. After extraction of lead in the fourth step,
The liquids having a lead ion concentration of 10 ⁻⁴ mol / l or less were circulated and reused for extraction of the residue in the third step, and the first to fourth steps were performed on the new fly ash. In that case, the second eluate and the second post-electrolysis solution could be eluted and electrolyzed in the same manner as the first. When such an operation was repeated, the cadmium concentration in the eluate increased after the fifth time, and the electrolysis of cadmium became possible. Therefore, during the electrolysis of copper, -0.42 V (vs. standard hydrogen potential) was obtained. At that point, the cathode was changed to an aluminum plate having an area of 50 cm ² and 2.5 was applied until it became −1.03 V (vs. standard hydrogen potential).
Electrolysis was performed at the current of A. 90% yield in each elution operation
As described above, the efficiency during the electrolysis operation was 60% or more, and it was revealed that heavy metals can be efficiently recovered from fly ash.

[0023]

[Table 1]

[0024]

According to the method of the present invention, heavy metals such as copper, cadmium, zinc and lead can be efficiently recovered from fly ash.

[Brief description of drawings]

FIG. 1 is a flow sheet showing a method for recovering heavy metals from fly ash according to an embodiment.

─────────────────────────────────────────────────── ─── Continued Front Page (51) Int.Cl. ⁷ Identification Code FI Theme Coat (Reference) C22B 7/02 C25C 1/16 A 13/00 B 15/00 1/18 17/00 7/02 306 19 / 30 B09B 3/00 304G C25C 1/12 ZAB 1/16 C22B 3/00 A 15/08 1/18 17/04 7/02 306 13/04 (72) Inventor Masayoshi Kondo 1 Minami Kohoku, Suminoe-ku, Osaka 7-89, Nitate Shipbuilding Co., Ltd. (72) Inventor Hiroshi Tatsumi 1-89, Minami Kohoku, Suminoe-ku, Osaka City F-Term (Reference) 4-9 4D056 AB03 AB04 AB08 AC21 AC22 DA10 4K001 AA06 AA09 AA20 AA30 BA22 DB03 4K058 AA21 BA21 BA24 BA25 BA27 BB04 CA04 CA07 CA13 CA20 EB13 EB15 EB16 FB03 FB08 FC07 FC14 FC15

Claims (4)

[Claims] 1. A fly ash is subjected to extraction treatment with a sulfuric acid aqueous solution having a pH of 4 or less to elute copper, cadmium and zinc in the same solution, and an anode is used in a separation liquid obtained by solid-liquid separation of the resulting fly ash-containing slurry. By performing electrolysis between the cathodes and gradually or gradually changing the cathode potential in the negative direction, copper, cadmium and zinc are separated and electrolytically deposited on the cathodes, and then the solid-liquid separation is performed. Heavy metal from fly ash, characterized in that the residue is extracted with an extract chemical to elute lead in the same, and lead is electrolytically deposited from the separated liquid obtained by solid-liquid separation of the resulting residue-containing slurry. Electrochemical recovery method of. 2. The liquid after recovery of copper, cadmium and zinc by the method according to claim 1 is recycled for extraction of fly ash, and the liquid after recovery of lead is recycled for extraction of residues, respectively. Then, a method for electrochemically recovering heavy metals from fly ash, which comprises subjecting new fly ash to metal extraction and electrolytic deposition according to the method of claim 1. 3. The method for electrochemically recovering heavy metals from fly ash according to claim 1, wherein a liquid containing caustic soda, sodium carbonate and / or sodium thiosulfate is used as an extracting chemical liquid for extracting lead. 4. The invention according to claim 1 or 2,
An electrode made of titanium or platinum-plated titanium for the anode, a cathode for the electrolytic deposition of copper, an aluminum electrode for the electrolytic deposition of cadmium, a titanium electrode for the electrolytic deposition of zinc, and an electrical analysis for lead. A method for electrochemically recovering heavy metals from fly ash, characterized in that the lead electrode or the titanium electrode is sequentially replaced at the time of release.