JP3962855B2 - Recovery method of heavy metals from fly ash - Google Patents

Description

【００２９】
【発明の効果】
本発明の方法によれば、飛灰から銅、カドミウム、亜鉛、鉛等の重金属を効率的に回収することができる。
【図面の簡単な説明】
【図１】実施例による飛灰からの重金属回収方法を示すフローシートである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for electrochemically recovering metals, particularly harmful metals such as copper, lead, cadmium and zinc, from incinerated fly ash and molten fly ash as a waste recycling technology. Usually, scrapped automobiles and waste home appliances are shredded with a shredder to recover the metal, but the remaining shredder dust is incinerated and reduced, and the generated incinerated fly ash or incinerated fly ash is further melted with plasma. . The present invention relates to a method for separating and recovering various metals from incinerated fly ash generated during incineration of shredder dust or from molten fly ash generated during melting of the incinerated fly ash.
[0002]
[Prior art]
The fly ash discharged from garbage incineration facilities, etc. contains high concentrations of harmful metals. For this reason, fly ash is determined to be subjected to intermediate treatment by any one of the following four methods (1) melting method (2) cement solidification (3) chemical treatment (4) acid extraction determined by the Ministry of Health, Labor and Welfare. Intermediate fly ash is landfilled at the final disposal site, but from the viewpoints of removing harmful metals, adverse effects of salts in leachate, reducing the volume of final disposal, or recycling metals. Establishment of technology to recover metals in fly ash is desired. For example, a method of eluting a metal by wet processing, then concentrating it for each type of metal, recovering and refining each metal as a concentrate that can be used as a raw material for nonferrous refining (Japanese Patent Laid-Open No. 7-138630) Have been proposed).
[0003]
However, in this method, an expensive chemical must be used for the metal elution liquid, the process is complicated, and the concentrate is a sludge composed of hydroxide and sulfide. Also, because it is a harmful substance, it has many problems on the transportation surface.
[0004]
[Problems to be solved by the invention]
An object of the present invention is to efficiently and easily make various metals from incineration fly ash generated when incineration of wastes such as shredder dust, or molten fly ash generated when incineration fly ash is further melted with plasma or the like. It is to provide a method that can be separated and recovered.
[0005]
[Means for Solving the Problems]
The present invention was obtained by subjecting fly ash to extraction with a hydrochloric acid aqueous solution or a nitric acid aqueous solution having a pH of 4 or less to elute copper, lead, cadmium, zinc and calcium into the same solution, and solid-liquid separation of the resulting fly ash-containing slurry. Alkali is added to the extract to pH 7-9, copper, lead, cadmium and zinc are precipitated as hydroxides, the resulting precipitate is separated, the liquid containing calcium is removed , then copper, lead, cadmium And an aqueous solution of sulfuric acid to a precipitate containing zinc hydroxide to dissolve copper, cadmium and zinc, precipitate lead as lead sulfate, isolate the resulting precipitate, and then separate copper, cadmium and zinc Electrolysis is performed between the anode and cathode in a sulfuric acid solution, and the cathode potential is gradually or stepwise changed in the negative direction, so that copper, cadmium and zinc are separated and electrolyzed at the cathode. A method for recovering heavy metals from fly ash, characterized in that to out.
[0006]
As a result of diligent research for separating and recovering metals in fly ash, the present inventors first eluted heavy metals in various fly ash with hydrochloric acid aqueous solution, etc. Utilizing the point that there is a difference in mechanical deposition potential, the cathode potential is gradually or stepwise changed from a noble potential to a base potential to deposit heavy metals such as copper, lead, cadmium, and zinc in the form of metal ingots. (Japanese Patent Application No. 2000-370877). In addition, as an improved invention, after extracting heavy metals such as copper, cadmium, and zinc in fly ash with an aqueous solution having a pH of 1 or higher, the cathode potential can be increased by utilizing the difference in electrochemical deposition potential of various metals. A method of gradually shifting from a low potential to a low potential, electrolytically depositing copper, cadmium and zinc, extracting the lead at a pH of 1 or less, and precipitating the lead by electrolysis of the extract (Japanese Patent Application No. 2001-003384). Further, as an improved invention, after extracting lead and zinc from fly ash with an aqueous solution having a pH of 12 or more, the cathode potential is changed from a noble potential to a base potential by utilizing the difference in electrochemical deposition potential of various metals. After the lead and zinc are electrolytically deposited, the residue containing copper and cadmium that could not be extracted at pH 12 or higher is adjusted to pH 1 to 5, and copper and cadmium are extracted. The present inventors have found a method in which the potential is gradually transferred to the potential and copper and cadmium are electrolytically deposited (Japanese Patent Application No. 2001-037819).
[0007]
Here, among the heavy metals in fly ash, what is important from the viewpoint of disposal regulation is the recovery of lead and zinc, and it is important to separate and recover these.
[0008]
In both the first and second inventions, extraction is carried out with an acidic aqueous solution. However, when sulfuric acid is used as a chemical, insoluble lead sulfate is deposited, and this cannot be used for subsequent electrolytic deposition. Therefore, in this case, hydrochloric acid or nitric acid is inevitably used. In this case, however, electrolysis of the extract causes a problem of generating harmful chlorine gas or nitrous acid gas. In addition, in the third invention, when extracting with an aqueous solution having a pH of 12 or more, a chemical such as caustic soda is consumed in a large amount for the extraction of aluminum and iron, and the cost is increased. In the case of electrolyzing zinc, there is a problem that even if the cathode voltage is controlled, a part of it is electrolytically deposited as an alloy of lead and zinc, and the resource value of the deposit is lowered.
[0009]
The method of the present invention utilizes the phenomenon that lead hydroxide precipitates at pH 7-9 or less, but calcium does not precipitate, and calcium precipitates as slaked lime at pH 9-13.
[0010]
Moreover, since the lead sulfate produced by adding an aqueous sulfuric acid solution to a precipitate containing copper, lead, cadmium and zinc hydroxide does not contain calcium, the amount of a dissolved chemical solution for dissolving lead sulfate can be reduced.
[0011]
As the electrolytic deposition reaction proceeds, the concentration of metal ions in the aqueous solution decreases, and the current efficiency related to the precipitation decreases. Therefore, the standard for reducing the metal ion concentration in the solution by electrolytic deposition is 10 ⁻⁴ to 10. ^{-2 It} is placed at about mol / l, and an aqueous solution containing a low concentration of metal ions is circulated and reused without performing the reaction to a very low concentration. According to this method, electrolytic deposition can be performed efficiently without reducing the current efficiency related to deposition.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
In the first step, fly ash is made into a slurry by adding water in an extraction tank, and a hydrochloric acid aqueous solution or a nitric acid aqueous solution is added while stirring the slurry to make the pH 4 or less. Here, the reason why the pH is lowered to 4 or less by adding an aqueous hydrochloric acid solution or an aqueous nitric acid solution is to dissolve copper, lead, cadmium and zinc. The stirring time for maintaining the pH may be 30 minutes or more, and the temperature may be room temperature, but it is desirable to set the stirring temperature to 50 ° C. or more in order to promote the dissolution rate. Next, the slurry is filtered by a filtration device such as a filtration tank or a filter press to separate into solid and liquid.
[0013]
In the second step, an alkali such as caustic soda and caustic potash or an aqueous solution thereof is added to the hydrochloric acid extract until pH 7-9, and metals other than calcium, that is, copper, lead, cadmium and zinc are precipitated as hydroxides, and the resulting precipitate The waste water containing calcium is removed.
[0014]
In the third step, an aqueous sulfuric acid solution is added to the metal other than calcium generated in the second step, that is, hydroxides of copper, lead, cadmium and zinc to dissolve copper, cadmium and zinc, and lead is precipitated as lead sulfate. The resulting precipitate is separated.
[0015]
In the fourth step, electrolysis is performed between the anode and the cathode in a sulfuric acid solution of copper, cadmium and zinc, and the cathode potential is gradually or stepwise changed in the negative direction, whereby copper, cadmium and Zinc is fractionated and electrolytically deposited. Here, as the anode, an electrode made of titanium or platinum-plated titanium is used. As the cathode, a copper electrode is used for electrolytic deposition of copper, an aluminum electrode is used for electrolytic deposition of cadmium, and a titanium electrode is used for electrolytic deposition of zinc. Use electrodes. Furthermore, the standard at the time of reducing the metal ion concentration in the solution by electrolytic deposition is set to about 10 ⁻⁴ to 10 ⁻² mol / l so as not to react to a very low concentration. This is because when the metal ion concentration in the solution is 10 ⁻⁴ mol / l or less, the current efficiency is as small as 20% or less.
[0016]
In the fifth step, the lead sulfate separated in the third step is dissolved in a dissolving chemical solution, and lead is electrolytically deposited from the obtained lead dissolving solution. A solution containing caustic soda, sodium carbonate and / or sodium thiosulfate is preferably used as a dissolving chemical for dissolving lead.
[0017]
In the sixth step, lead is electrolytically deposited from the solution extracted in the fifth 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. Is desirable.
[0018]
In the seventh step, the recovery of copper, cadmium, and zinc in the fourth step is completed and the liquid in which the metal ion concentration is 10 ⁻⁴ mol / l or less is circulated and reused as the acid-containing liquid in the first step. The liquid whose lead concentration is 10 ⁻⁴ mol / l or less after the recovery of lead in the process is recycled and reused as a solution for dissolving lead sulfate in the fifth process. Thus, the first to sixth steps are performed. When such an operation is repeated, aluminum, potassium, sodium and the like are gradually concentrated to reach their own solubility and are not naturally dissolved in the seventh step. Iron and chromium are deposited as alloy plating during cadmium plating.
[0019]
Next, the present invention will be specifically described based on examples.
[0020]
Example (first step)
First, 20 g of molten fly ash was put into a 500 ml beaker, 200 ml of 1N hydrochloric acid aqueous solution was put therein, the whole was stirred at 50 ° C. for 1 hour, and copper, lead, cadmium, zinc and calcium were eluted in the same solution. The fly ash-containing slurry was subjected to solid-liquid separation. The pH of the obtained separation liquid was 1.0.
[0021]
(Second step)
Thereafter, a 1N aqueous solution of sodium hydroxide was added to the separated solution until pH 9, and copper, lead, cadmium and zinc were precipitated as hydroxides. The resulting precipitate was separated and the waste water containing calcium was removed.
[0022]
(Third step)
To the precipitate containing hydroxide precipitated in the second step, 200 ml of 1N sulfuric acid was added to dissolve copper, cadmium and zinc, and lead was precipitated as lead sulfate, and the resulting precipitate was separated.
[0023]
(4th process)
The sulfuric acid solution separated in the third step, a platinum plated titanium electrode with an area of 50 cm ² as the anode, put the copper plate of area 50 cm ² as a cathode, subjected to electrolysis at a constant current of 2.5A. At that time, the cathode potential gradually decreased from −0.23 V (vs. standard hydrogen potential) to −0.30 V (vs. standard hydrogen potential), and the copper concentration was 0.17 (g / kg) as shown in Table 1. from 1) to 0.012 (g / l).
[0024]
Thereafter, the cathode was replaced with a titanium electrode having an area of 50 cm ² and electrolysis was performed 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 (g / g) as shown in Table 1. from 1) to 0.048 (g / l).
[0025]
(5th process)
Into a 500 ml beaker, 0.2 g of lead sulfate separated in the third step was added, and 70 ml of 1N sodium hydroxide aqueous solution was added and stirred at 50 ° C. for 1 hour to dissolve lead as sodium lead acid. The pH of this solution was 13.8.
[0026]
(6th process)
In the solution obtained in the fifth 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 a cathode, subjected to electrolysis at a constant current of 2.5A. At that time, the cathode potential gradually decreased from −0.92 V (vs. standard hydrogen potential) to −1.29 V (vs. standard hydrogen potential), and the lead concentration was 3.59 g / g as shown in Table 1. from 1) to 0.21 (g / l).
[0027]
(Seventh step)
After the recovery of copper, cadmium and zinc in the fourth step, the solution in which the metal ion concentration is 10 ⁻⁴ mol / l or less is recycled and used as the acid-containing solution in the first step, and the recovery of lead is performed in the sixth step. Then, the solution whose ion concentration became 10 ⁻⁴ mol / l or less was recycled and reused as a solution for dissolving lead sulfate in the fifth step. Thus, the first to sixth steps were performed. In that case, the second extract and the second electrolyzed solution were extracted and electrolyzed in the same manner as in the first. When such an operation was repeated, the cadmium concentration in the extract became high and the cadmium electrolysis became possible after the fifth time, so that -0.42 V (vs. standard hydrogen potential) during copper electrolysis. At that time, the cathode was changed to an aluminum plate with an area of 50 cm ² and electrolysis was performed at a current of 2.5 A until it reached −1.03 V (vs. standard hydrogen potential). After this, the cathode was changed to a galvanized steel sheet in the same manner up to the fourth time, and zinc plating was performed. The yield during each extraction operation was 90% or more, and the efficiency during the electrolysis operation was 60% or more. It was revealed that heavy metals can be efficiently recovered from fly ash.
[0028]
[Table 1]

[0029]
【The invention's effect】
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 the drawings]
FIG. 1 is a flow sheet showing a heavy metal recovery method from fly ash according to an embodiment.

Claims (5)

The fly ash is extracted with a hydrochloric acid aqueous solution or a nitric acid aqueous solution having a pH of 4 or less to elute copper, lead, cadmium, zinc and calcium into the same solution, and the extract obtained by solid-liquid separation of the resulting fly ash-containing slurry is alkaline. Is added to pH 7-9, copper, lead, cadmium and zinc are precipitated as hydroxides, the resulting precipitate is separated, the liquid containing calcium is removed ,
Next, an aqueous sulfuric acid solution is added to the precipitate containing copper, lead, cadmium and zinc hydroxide to dissolve copper, cadmium and zinc, lead is precipitated as lead sulfate, and the resulting precipitate is separated,
Next, electrolysis is performed between the anode and the cathode in a sulfuric acid solution of copper, cadmium and zinc, and the cathode potential is gradually or stepwise changed in the negative direction to separate copper, cadmium and zinc into the cathode. A method for recovering heavy metals from fly ash, which is separately electrolytically deposited . Lead sulfate separated in claim 1 is dissolved in dissolved chemical, resulting method for recovering heavy metals from fly ash as claimed in claim 1, in which the lead solution, characterized in that to electrolytic deposition of lead. 3. The method for recovering heavy metals from fly ash according to claim 2 , wherein a solution containing caustic soda, sodium carbonate and / or sodium thiosulfate is used as a dissolving chemical for dissolving lead. 3. The invention according to claim 1 or 2 , wherein an electrode made of titanium or platinum plated titanium is used for the anode, a copper electrode is used for the electrolytic deposition of copper, an aluminum electrode is used for the electrolytic deposition of cadmium, and an electroanalysis of zinc. A method for recovering heavy metals from fly ash, wherein a titanium electrode is used at the time of discharge, and a lead electrode or a titanium electrode is sequentially replaced at the time of electrolytic deposition of lead. Copper by the process of claim 1, the liquid after completing the recovery of cadmium and zinc circulates reused as an acid-containing solution of the process of claim 1 wherein, finishing the recovery of lead by the method of claim 2 wherein The method for recovering heavy metals from fly ash according to claim 1 , wherein the subsequent liquid is circulated and reused as a solution for dissolving the lead sulfate separated in claim 1.

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