JP3951041B2

JP3951041B2 - Electrochemical recovery of heavy metals from fly ash

Info

Publication number: JP3951041B2
Application number: JP2001219262A
Authority: JP
Inventors: 近稲住; 淳矢野; 雅芳近藤; 浩史辰己
Original assignee: Hitachi Zosen Corp
Current assignee: Hitachi Zosen Corp
Priority date: 2001-07-19
Filing date: 2001-07-19
Publication date: 2007-08-01
Anticipated expiration: 2021-07-19
Also published as: JP2003027275A

Description

【０００１】
【発明の属する技術分野】
本発明は、廃棄物の再資源化技術として焼却飛灰、溶融飛灰から金属類、とりわけ銅、鉛、カドミウム、亜鉛等の有害な金属類を電気化学的に回収する方法に関する。通常、廃自動車、廃家電製品などはシュレッダーにより破砕して金属を回収するが、その際残ったシュレッダーダストは焼却減容し、発生する焼却飛灰あるいは焼却飛灰をさらにプラズマなどで溶融処理する。本発明は、このようにシュレッダーダストの焼却の際に発生する焼却飛灰、あるいはこの焼却飛灰の溶融の際に発生する溶融飛灰から、各種金属を分離回収する方法に関する。
【０００２】
【従来の技術】
ごみ焼却施設などから排出される飛灰中には有害な金属類類が高濃度で含有されている。そのため、飛灰は厚生労働省の定めるつぎの４方法▲１▼溶融法▲２▼セメント固化▲３▼薬剤処理▲４▼酸抽出のうちいずれかで中間処理することが定められている。中間処理された飛灰は最終処分地で埋め立て処分されるが、有害な金属類の除去、浸出水中の塩類による悪影響、最終処分量の減容化、あるいは金属類の再資源化の観点から、飛灰中の金属類を回収する技術の確立が望まれている。例えば、湿式処理によって金属を溶出し、次いで金属の種類ごとにこれを濃縮し、各金属を非鉄精錬用原料として使用できる程度の濃縮物として回収し、精錬する方法（特開平７−１３８６３０号公報参照）などが提案されている。
【０００３】
しかし、この方法では、金属溶出用の液に高価な薬品を使用しなければならず、工程が複雑であり、加えて濃縮物が水酸化物や硫化物からなるスラッジであるため、容積が大きく、また有害物であることから運搬面上多くの問題を抱えている。
【０００４】
【発明が解決しようとする課題】
本発明の課題は、シュレッダーダスト等の廃棄物を焼却した際に発生する焼却飛灰、あるいは焼却飛灰をさらにプラズマなどで溶融した際に発生する溶融飛灰から、各種金属を効率よくかつ簡単に分離回収することができる方法を提供することである。
【０００５】
【課題を解決するための手段】
本発明は、飛灰をｐＨ４以下の硫酸水溶液で抽出処理して同液に銅、カドミウムおよび亜鉛を溶出させ、生じた飛灰含有スラリーの固液分離により得られた分離液中で陽極と陰極の間で電気分解を行い、陰極電位を徐々にまたは段階的に負の方向に変化させることにより、陰極に銅、カドミウムおよび亜鉛を分別して電解析出させ、その後、上記固液分離で残った残渣をチオ硫酸ソーダやチオ硫酸カリウムのようなチオ硫酸塩の水溶液で抽出処理して同液に鉛を溶出させ、生じた残渣含有スラリーの固液分離により得られた分離液中で陽極と陰極の間に直流電力をかけて、液中の鉛を硫化鉛として沈殿させることを特徴とする飛灰からの重金属の電気化学的回収方法である。
【０００６】
本発明者らは、飛灰中の金属類の分離回収を行うために鋭意研究を進めた結果、先に、各種飛灰中の重金属を塩酸酸性水溶液などで溶出した後、各種金属の電気化学的析出電位に差異がある点を利用し、陰極電位を貴な電位から卑な電位へ徐々にまたは段階的に変化させ、銅、鉛、カドミウム、亜鉛等の重金属を金属インゴットの形態で析出させることが可能であることを知見した（特願２０００−３７０８７７号明細書）。また、その改良発明として、ｐＨ１以上の水溶液で飛灰中の銅、カドミウム、亜鉛などの重金属を抽出した後に、各種金属の電気化学的析出電位に差異がある点を利用し、陰極電位を貴な電位から卑な電位へ段階的に移行させ、銅、カドミウムおよび亜鉛を電解析出させた後、ｐＨを１以下にして鉛を抽出し、抽出液の電気分解で鉛を析出させる方法を見出した（特願２００１−００３３８４号明細書）。さらに、その改良発明として、飛灰からｐＨ１２以上の水溶液で鉛と亜鉛を抽出した後に、各種金属の電気化学的析出電位に差異がある点を利用し、陰極電位を貴な電位から卑な電位へ段階的に移行させ、鉛、亜鉛を電解析出させた後、ｐＨ１２以上で抽出できなかった銅、カドミウムを含む残渣をｐＨ１〜５にして銅、カドミウムを抽出させ、貴な電位から卑な電位へ電位を段階的に移行させ、銅、カドミウムを電解析出させる方法を見出した（特願２００１−０３７８１９号明細書）。
【０００７】
ここで、飛灰中の重金属の内で廃棄規制の観点から重要なものは、鉛と亜鉛の回収であり、これらを分別して回収することが重要である。
【０００８】
一番目と二番目の発明では、いずれも酸性水溶液で抽出を行うが、薬剤として硫酸を使用すると不溶性の硫酸鉛が析出するため、これを以後の電解析出へ供することができない。従ってこの場合には必然的に塩酸あるいは硝酸を使用することになるが、この場合は抽出液を電解すると有害な塩素ガスや、亜硝酸ガスが発生する問題が生ずる。また、三番目の発明では、ｐＨ１２以上の水溶液で抽出をする場合に、苛性ソーダなどの薬剤がアルミニウムや鉄の抽出のためにも多量に消費されてしまいコスト高を招く上に、抽出した鉛と亜鉛を電解する場合に、陰極電圧を制御しても一部は鉛と亜鉛の合金として電解析出し、析出物の資源価値が落ちるという問題があった。
【０００９】
本発明においては、飛灰抽出処理用の酸としてｐＨ４以下の硫酸水溶液を使用するので、抽出液には鉛が含まれず、電解析出した亜鉛は鉛を含まない。また、電解を硫酸水溶液で行うので、電解中には有害な塩素ガスや亜硝酸ガスが発生しない。さらに、硫酸抽出後の残渣にはＰｂＳＯ_４、ＣａＳＯ_４、アルミナ、シリカしか残っていないので、鉛を抽出する場合の薬剤使用量が少なくてすむ。
【００１０】
本発明においては、また、固液分離で残った残渣をチオ硫酸塩水溶液で抽出処理して同液に鉛を溶出させ、生じた残渣含有スラリーの固液分離により得られた分離液中で陽極と陰極の間に直流電力をかけて、液中の鉛を硫化鉛として沈殿させるので、錯体の形態で溶解していた鉛イオンがチオ硫酸塩中のイオウと非常に高い効率で反応を起こして硫化鉛を生じる。この反応は水溶液中の鉛イオン濃度が１０^−５ｍｏｌ／ｌ以下まで高い効率で進行する。
また、従来の発明と同様に、硫酸水溶液中で銅、亜鉛およびカドミウムの電解析出を行うと、電解析出反応が進行するにつれて水溶液中の金属イオンの濃度が薄くなり、析出に関する電流効率が低下するので、電解析出により溶液中の金属イオン濃度を減少させる際の目安を１０^−４〜１０^−２ｍｏｌ／ｌ程度に置き、あまり低い濃度まで反応を行わないで、低濃度の金属イオンを含有する水溶液を循環再利用する。
【００１１】
また、当然ながら硫化鉛の析出後に濾過した残液（チオ硫酸ソーダ、チオ硫酸カリウムなどのチオ硫酸塩を含む液）も循環再使用することができる。
【００１２】
本方法によると、本方法によると、析出に関する電流効率を下げずに効率よく電解析出を行うことができる。
【００１３】
【発明の実施の形態】
第１工程で、抽出槽で飛灰を水の添加によりスラリーとし、このスラリーを攪拌しながら硫酸水溶液を添加してｐＨを４以下にする。ここで、硫酸の添加でｐＨを４以下にするのは、銅、カドミウムおよび亜鉛を溶解させるが、鉛は溶解させないためである。上記ｐＨを維持しての攪拌時間は、３０分以上であればよく、温度は室温でもよいが溶解速度を促進するためには５０℃以上にするのが望ましい。次いで、上記スラリーを濾過槽やフィルタープレスなどの濾過装置で濾過して固液分離する。
【００１４】
第２工程では、濾液を陽極と陰極との間で電気分解により銅、カドミウムおよび亜鉛を析出させる。ここで、陽極としては、チタンあるいは白金めっきしたチタンからなる電極を使用し、陰極としては、銅の電解析出時には銅電極を、カドミウムの電解析出時にはアルミニウム電極を、亜鉛の電解析出時にはチタン電極を使用する。さらに、電解析出により溶液中の金属イオン濃度を減少させる際の目安を１０^−４〜１０^−２ｍｏｌ／ｌ程度に置き、あまり低い濃度まで反応させないようにする。なぜなら、溶液中の金属イオン濃度が１０^−４ｍｏｌ／ｌ以下になると、電流効率が２０％以下と小さくなるからである。
【００１５】
第３工程では、第１工程で固液分離した後の残査から、チオ硫酸ソーダ、チオ硫酸カリウムなどのチオ硫酸塩を含む液を用いて鉛を抽出する。
【００１６】
第４工程では、第３工程で抽出により生じた残渣含有スラリーの固液分離により得られた分離液中で陽極と陰極の間に直流電力をかけて、液中の鉛とイオウを反応させて、硫化鉛を析出させる。直流電力をかける際に、陽極にはチタンあるいは白金めっきしたチタンからなる電極を使用し、陰極にはチタン、ステンレス鋼または炭素鋼からなる電極を使用するのが望ましい。本反応は、錯イオンであるＰｂ（Ｓ_２Ｏ_３）_３ ^４＋イオンが陰極上で分解反応してＰｂＳに変化するものと考えられる。
【００１７】
第５工程では、第２工程により銅、カドミウムおよび亜鉛の回収を終えた後の、これらの金属イオン濃度が１０^−４ｍｏｌ／ｌ以下になった液を第１工程の飛灰の抽出に、第４工程により鉛の回収を終えた後の、鉛イオン濃度が１０^−５ｍｏｌ／ｌ以下になった液を第３工程の残渣の抽出にそれぞれ循環再使用して、新たな飛灰に対し、第１〜第４工程を行う。なお、このような操作を繰り返し行うと、アルミニウム、カリウム、ナトリウムなどは次第に濃縮してこれら自身の溶解度まで達して、第４工程において自然に溶解しなくなる。また、鉄、クロムはカドミウムめっき時に合金鍍金として析出する。
【００１８】
つぎに、本発明を実施例に基づいて具体的に説明する。
【００１９】
実施例
（第１工程）
まず、５００ｍｌビーカーに溶融飛灰２０ｇを入れ、その中に１Ｎ硫酸水溶液を２００ｍｌ入れて全体を５０℃で１時間攪拌し、生じたスラリーを固液分離した。得られた分離液のｐＨは１．２であった。
【００２０】
（第２工程）
その後、分離液中に、陽極として面積５０ｃｍ^２の白金めっきしたチタン電極を、陰極として面積５０ｃｍ^２の銅板を入れて、２．５Ａの一定電流で電解を行った。その時、陰極電位は−０．２３Ｖ（ｖｓ．標準水素電位）から−０．３０Ｖ（ｖｓ．標準水素電位）まで徐々に低下し、銅の濃度は表１に示すように０．１７（ｇ／ｌ）から０．０１２（ｇ／ｌ）まで減少した。
【００２１】
その後、陰極を面積５０ｃｍ^２のチタン電極に変えて同じく２．５Ａの一定電流で電解を行った。その時、陰極電位は−１．２９Ｖ（ｖｓ．標準水素電位）から−１．３８Ｖ（ｖｓ．標準水素電位）まで徐々に低下し、亜鉛の濃度は表１に示すように３．４５（ｇ／ｌ）から０．０４８（ｇ／ｌ）まで減少した。
【００２２】
（第３工程）
第１工程で固液分離した後の固体残渣７．８ｇを５００ｍｌのビーカーに入れ、その中に、１Ｎチオ硫酸ソーダ水溶液を１７０ｍｌ入れて５０℃で１時間攪拌し、生じたスラリーを固液分離した。得られた分離液のｐＨは５．４であった。
【００２３】
（第４工程）
第３工程で得た分離液中に、陽極として面積５０ｃｍ^２の白金めっきしたチタン電極を、陰極として面積５０ｃｍ^２のチタン電極を入れて、２．５Ａの一定電流で電解を行った。その時、陰極電位は−１．２〜１．３Ｖ（ｖｓ．標準水素電位）まで徐々に低下し、鉛の濃度は表１に示すように１．４８（ｇ／ｌ）から０．００５（ｇ／ｌ）まで減少した。また、電流効率も９５％以上を示し、鉛の苛性ソーダ水溶液から電解析出させる場合の電流効率５０％と比較してかなりの効率向上が見られた。
【００２４】
（第５工程）
第２工程により銅、カドミウムおよび亜鉛の回収を終えた後の、これらの金属イオン濃度が１０^−４ｍｏｌ／ｌ以下になった液を第１工程の飛灰の抽出に、第４工程により鉛の回収を終えた後の、鉛イオン濃度が１０^−５ｍｏｌ／ｌ以下になった液を第３工程の残渣の抽出にそれぞれ循環再使用して、新たな飛灰に対し、第１〜第４工程を行った。その場合、２回目溶出液、２回目電解後液も１回目と同様に溶出および電解ができた。このような操作を繰り返し行ったところ、５回目以降では、溶出液中のカドミウム濃度が高くなり、カドミウムの電解が可能となったので、銅の電解時に−０．４２Ｖ（ｖｓ．標準水素電位）になった時点で陰極を面積５０ｃｍ^２のアルミニウム板に変更して−１．０３Ｖ（ｖｓ．標準水素電位）になるまで２．５Ａの電流で電解を行った。各溶出操作時の収率は９０％以上、電解操作時の効率は６０％以上であり、飛灰から効率的に重金属を回収できることが明らかになった。
【００２５】
【表１】

【００２６】
【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]
In the present invention, the fly ash is extracted with a sulfuric acid aqueous solution having a pH of 4 or less to elute copper, cadmium and zinc into the same solution, and the anode and cathode are separated in the separated liquid obtained by solid-liquid separation of the resulting fly ash-containing slurry. The cathode potential was gradually or stepwise changed in the negative direction to separate copper, cadmium and zinc on the cathode for electrolytic deposition, and then remained in the solid-liquid separation. The residue was extracted with an aqueous solution of thiosulfate such as sodium thiosulfate or potassium thiosulfate to elute lead into the same solution, and the anode and cathode were separated in the separated liquid obtained by solid-liquid separation of the resulting slurry containing the residue. The method of electrochemical recovery of heavy metals from fly ash is characterized in that direct current power is applied between the two to precipitate lead in the liquid as lead sulfide.
[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]
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 electrodeposited zinc does not contain lead. Further, since electrolysis is performed with a sulfuric acid aqueous solution, no harmful chlorine gas or nitrous acid gas is generated during electrolysis. Furthermore, since only PbSO ₄ , CaSO ₄ , alumina, and silica remain in the residue after the sulfuric acid extraction, the amount of chemical used when extracting lead can be reduced.
[0010]
In the present invention, the residue remaining after the solid-liquid separation is extracted with an aqueous thiosulfate solution to elute lead into the same liquid, and the anode is separated in the separated liquid obtained by solid-liquid separation of the resulting residue-containing slurry. DC power is applied between the cathode and the cathode to precipitate lead in the liquid as lead sulfide, so that the lead ions dissolved in the complex form react with sulfur in the thiosulfate with very high efficiency. This produces lead sulfide. This reaction proceeds with high efficiency until the lead ion concentration in the aqueous solution is 10 ⁻⁵ mol / l or less.
Similarly to the conventional invention, when electrolytic deposition of copper, zinc and cadmium is carried out in an aqueous sulfuric acid solution, the concentration of metal ions in the aqueous solution decreases as the electrolytic deposition reaction proceeds, and the current efficiency related to the precipitation is reduced. Therefore, the standard for reducing the metal ion concentration in the solution by electrolytic deposition is set to about 10 ⁻⁴ to 10 ⁻² mol / l, and the reaction is not performed to a very low concentration. The aqueous solution containing is recycled.
[0011]
Naturally, the residual liquid (liquid containing thiosulfate such as sodium thiosulfate and potassium thiosulfate) filtered after the precipitation of lead sulfide can be circulated and reused.
[0012]
According to this method, according to this method, it is possible to efficiently perform electrolytic deposition without lowering the current efficiency related to deposition.
[0013]
DETAILED DESCRIPTION OF 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 pH of 4 or less while stirring the slurry. Here, the reason why the pH is lowered to 4 or less by the addition of sulfuric acid is to dissolve copper, cadmium and zinc, but not lead. 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.
[0014]
In the second step, copper, cadmium and zinc are deposited by electrolysis of the filtrate between the anode and the cathode. Here, an electrode made of titanium or platinum-plated titanium is used as the anode, and a copper electrode is used for the electrolytic deposition of copper, an aluminum electrode is used for the electrolytic deposition of cadmium, and an electrolytic electrode is used for the zinc. Use titanium 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.
[0015]
In the third step, lead is extracted from the residue after the solid-liquid separation in the first step, using a liquid containing a thiosulfate such as sodium thiosulfate and potassium thiosulfate.
[0016]
In the fourth step, DC power is applied between the anode and the cathode in the separated liquid obtained by solid-liquid separation of the residue-containing slurry generated by the extraction in the third step to react lead and sulfur in the liquid. , Lead sulfide is deposited. When applying DC power, it is desirable to use an electrode made of titanium or platinum-plated titanium for the anode and an electrode made of titanium, stainless steel or carbon steel for the cathode. In this reaction, it is considered that Pb (S ₂ O ₃ ) ₃ ⁴⁺ ions, which are complex ions, are decomposed on the cathode and changed to PbS.
[0017]
In the fifth step, after the recovery of copper, cadmium and zinc in the second step, the liquid in which the concentration of these metal ions is 10 ⁻⁴ mol / l or less is used for the extraction of fly ash in the first step. After the recovery of lead in the fourth step, the liquid with a lead ion concentration of 10 ⁻⁵ mol / l or less is circulated and reused for the extraction of the residue in the third step. First to fourth 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 fourth step. Iron and chromium are deposited as alloy plating during cadmium plating.
[0018]
Next, the present invention will be specifically described based on examples.
[0019]
Example (first step)
First, 20 g of molten fly ash was put into a 500 ml beaker, 200 ml of 1N sulfuric acid aqueous solution was put therein, 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 separation liquid was 1.2.
[0020]
(Second step)
Thereafter, a 50-cm ² platinum-plated titanium electrode as an anode and a 50-cm ² copper plate as a cathode were placed in the separation liquid, and electrolysis was performed at a constant current of 2.5 A. 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).
[0021]
Thereafter, the cathode was changed to 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).
[0022]
(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 sodium thiosulfate aqueous solution was put therein, and stirred at 50 ° C. for 1 hour, and the resulting slurry was subjected to solid-liquid separation. did. The pH of the obtained separation liquid was 5.4.
[0023]
(4th process)
During the separation liquid obtained in the third step, a platinum plated titanium electrode with an area of 50 cm ² as the anode, put the titanium electrode with an area of 50 cm ² as a cathode, subjected to electrolysis at a constant current of 2.5A. At that time, the cathode potential gradually decreases from −1.2 to 1.3 V (vs. standard hydrogen potential), and the lead concentration is from 1.48 (g / l) to 0.005 (g / L). In addition, the current efficiency was 95% or more, and a considerable improvement in efficiency was seen compared to the current efficiency of 50% when electrolytically deposited from a lead caustic soda solution.
[0024]
(5th process)
After the recovery of copper, cadmium and zinc in the second step, the solution in which the metal ion concentration is 10 ⁻⁴ mol / l or less is used for the extraction of fly ash in the first step, and the lead in the fourth step. After the recovery of the solution, the liquid with a lead ion concentration of 10 ⁻⁵ mol / l or less is circulated and reused for the extraction of the residue in the third step. Four steps were performed. In that case, the second elution solution and the second electrolysis solution were eluted and electrolyzed in the same manner as in the first time. When such an operation was repeated, the cadmium concentration in the eluate became high and the cadmium electrolysis became possible after the fifth time, so that -0.42 V (vs. standard hydrogen potential) was obtained during the electrolysis of copper. 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). The yield during each elution 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.
[0025]
[Table 1]

[0026]
【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

The fly ash was extracted with an aqueous sulfuric acid solution having a pH of 4 or less to elute copper, cadmium and zinc into the same solution. By performing decomposition and gradually changing the cathode potential in a negative direction stepwise or stepwise, copper, cadmium and zinc are separated and electrolytically deposited on the cathode, and then the residue remaining in the solid-liquid separation is thiosulfuric acid. Extract the lead with the soda salt aqueous solution and elute lead into the same liquid. In the separated liquid obtained by solid-liquid separation of the resulting slurry containing residue, apply direct current power between the anode and the cathode to A method for electrochemical recovery of heavy metals from fly ash, which is precipitated as lead sulfide.

The liquid after completion of the recovery of copper, cadmium and zinc by the method of claim 1 is reused for extraction of fly ash, and the liquid after completion of recovery of lead is reused for extraction of the residue. A method for electrochemical recovery of heavy metals from fly ash, comprising repeating the method according to claim 1 for fly ash.

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 zinc is electroanalyzed. Titanium electrodes are replaced and used in order at the time of discharge, and when direct current is applied to precipitate lead as lead sulfide, an electrode made of titanium, stainless steel or carbon steel is used as a cathode. Electrochemical recovery of heavy metals.