JP4368133B2 - Advanced treatment method of fluoride ion in waste liquid - Google Patents
Advanced treatment method of fluoride ion in waste liquid Download PDFInfo
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- JP4368133B2 JP4368133B2 JP2003121537A JP2003121537A JP4368133B2 JP 4368133 B2 JP4368133 B2 JP 4368133B2 JP 2003121537 A JP2003121537 A JP 2003121537A JP 2003121537 A JP2003121537 A JP 2003121537A JP 4368133 B2 JP4368133 B2 JP 4368133B2
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Description
【0001】
【発明の属する技術分野】
本発明は、廃液中フッ化物イオンの高度処理方法に関し、更に詳しくは、フッ化カルシウム法を利用した廃液中フッ化物イオンの高度処理方法に関するものである。
【0002】
【従来の技術】
昨今の先端産業の発展と共にフッ素の工業的使用量は増加しており、それに伴い、フッ素を含む廃棄物、廃液の量も増加傾向にある。加えて、フッ素樹脂製品等のフッ素を含む工業製品が広く流通していることにより、都市ゴミを焼却処理した際の洗煙水中にも、高濃度のフッ素が含まれるという現象が起きている。ここで、フッ素化合物を含む廃液の処理においては、そのフッ化物イオン濃度等に関して国が厳しい環境基準を設定しており、これに対応すべく、廃液の様々な高度処理方法が提案されている。この高度処理方法の一つに、フッ化物イオンを含有する廃液中にカルシウム塩溶液を添加して、フッ化物イオンをフッ化カルシウムとして沈殿除去させるフッ化カルシウム法(例えば、特許文献1参照)が知られている。この方法は二次処理を必要とせず、また簡便であるという利点がある。
【0003】
【特許文献1】
特開平11−156355号公報(第5−8段落)
【0004】
【発明が解決しようとする課題】
しかしながら、従来のフッ化カルシウム法による処理方法では処理後の廃液中のフッ化物イオン濃度を数十ppm以下とすることができず、新しい環境基準値である8ppmを達成することはできなかった。そこでフッ化カルシウム法により処理した廃液を更に吸着法やフッ素アパタイト法により二次処理することも試みられているが、フッ化カルシウム法の数十倍に及ぶ大量のスラッジが発生するという問題があった。
【0005】
本発明は上記した従来の問題点を解決するためになされたものであり、フッ化カルシウム法のみで廃水中のフッ化物イオン濃度を基準値である8ppm以下にまで低下させることができ、スラッジの発生量を減少させてミニマムエミッションを達成できる廃液中フッ化物イオンの高度処理方法を提供することを目的とする。
【0006】
【課題を解決するための手段】
本発明者は上記の課題を解決するために、通常のフッ化カルシウム法では処理液中のフッ化物イオン濃度を数十ppm以下とすることができない理由を検討した。フッ化カルシウムの溶解度はフッ化物イオンに関して8ppmであるが、フッ化物イオンとカルシウムイオンとの溶解度平衡から考えると、液中のカルシウムイオンの濃度を高めればフッ化物イオン濃度は8ppm以下にまで低くなるはずである。しかし実際には、カルシウムの添加量を増加させてもスラッジ発生量が増加するだけで、フッ化物イオン濃度は減少しない。本発明者は、その原因が大気中から液中への二酸化炭素の溶け込みにあることを究明した。すなわち、カルシウムは大気中から溶け込む二酸化炭素と反応して炭酸カルシウムを形成することにより消費されてしまい、カルシウムの添加量を増加させても、フッ化物イオンの低減に寄与するカルシウムイオン量は増加しないのである。
【0007】
本発明は上記の知見に基づいて完成されたものであり、フッ化物イオンを含有する廃液にカルシウムイオンを添加し、フッ化物イオンを沈殿させて除去する廃液中フッ化物イオンの処理方法において、廃液と大気との接触を遮断する、または、廃液中に窒素ガスを通気することにより、大気中の二酸化炭素の廃液への溶け込みを防止しながら、カルシウムイオンを添加することを特徴とするものである。なお、廃液の液面を蓋部材で覆うことなどにより廃液と大気との接触を遮断する方法によっても、大気中の二酸化炭素の廃液への溶け込みを防止することができる。
【0008】
本発明の廃液中フッ化物イオンの高度処理方法によれば、廃液への大気中の二酸化炭素の溶解を防止した状態でカルシウムが添加されるので、廃液中に添加したカルシウムイオンが、大気中から溶け込んだ二酸化炭素と反応して炭酸カルシウムとして消費されることを防止できる。このためカルシウムイオンをフッ化カルシウム生成のためのみに積極的に使用することができ、処理効率を向上させることができるとともに、スラッジの発生量を少なくすることができる。
【0009】
【発明の実施の形態】
以下に、本発明の実施形態を説明する。本実施形態では、廃液中のフッ化ナトリウムに塩化カルシウム水溶液を添加して、難溶性のフッ化カルシウムを生成するフッ化カルシウム法について説明する。まず、液中に水素イオン及び水酸化物イオン以外の共存イオンが存在しないという仮定の下でのフッ化カルシウムの溶解平衡について説明する。フッ化カルシウムの溶解平衡は、以下の[化1]で表され、その溶解度積KSP1は、[化2]で与えられる。従って、水溶液中にカルシウムイオンが[Ca2+]Cだけ与えられたとすると、フッ化物イオン濃度は[化3]のように表される。
【0010】
【化1】
【化2】
【化3】
【0011】
ここで、フッ化物イオンF−は、水溶液が弱酸性であると、液中で加水分解反応によりその一部が水素イオンH+とフッ化水素HFを構成する。この平衡反応について[化4]が与えられ、水溶液中の全フッ素濃度は、[化5]のように表される。
【0012】
【化4】
【化5】
【0013】
上記[化5]は、共存するカルシウムイオン濃度が高いほど、共通イオン効果により廃液中の全フッ素濃度が低くなることを示している。即ち、上記した廃液中に水素イオン及び水酸化物イオン以外の共存イオンが存在しないという仮定の下では、廃液中でカルシウムイオンの濃度を高くすれば、フッ素濃度を低くすることができるということになる。前記したように、通常のフッ化カルシウム法を利用した廃液処理においては、大気中の二酸化炭素の溶け込みによってカルシウムイオンの濃度を高くすることができないが、本発明では廃液と大気との接触を遮断する方法、廃液中に窒素ガスを通気する方法、もしくは廃液のpHを7.5以下にすることにより、大気中の二酸化炭素の廃液への溶け込みを防止しながらカルシウムイオンを添加する。その処理方法について以下に詳述する。
【0014】
まず、大気中の二酸化炭素が液中に溶解すると、炭酸水素イオンと水素イオンが生成され、炭酸水素イオンからさらに炭酸イオンと水素イオンとが生成される。これらの平衡反応を以下[化6]、[化7]及び[化8]に示す。
【0015】
【化6】
【化7】
【化8】
【0016】
上記の反応の平衡関係式は、大気中の二酸化炭素分圧をPCO2とすると、以下の[化9]、[化10]及び[化11]のように与えられる。
【0017】
【化9】
【化10】
【化11】
【0018】
ここで液中にカルシウムイオンを添加すると、[化8]で生成された炭酸イオンは、カルシウムイオンと反応して難溶性の炭酸カルシウムを生成することになる。この反応の溶解度積KSP2は[化12]に示すとおりであって、溶液中に存在することができるカルシウムイオンの最大値は、[化13]のように表すことができる。
【0019】
【化12】
【化13】
【0020】
この[化13]は、大気開放系で溶液中にどれだけ多量にカルシウムイオンを添加しても、液中のカルシウムイオン濃度が[Ca2+]maxより高くならないことを示しており、水溶液中にカルシウムイオンが[Ca2+]maxだけ与えられたとすると、この溶液中に存在できる全フッ素濃度の下限値TFminは、[化14]で表すことができる。
【0021】
【化14】
【0022】
この[化14]と、上記[化5]及び[化13]とから、大気中の二酸化炭素の影響を考慮した場合の液中のカルシウムイオンの上限値を計算すると、図1に示すように、pH7以上において、大気中の二酸化炭素がわずかでも溶解すると、液中のカルシウムイオン濃度が急激に低下することが分かる。これは、液中のカルシウムイオンが二酸化炭素の溶解により生成した炭酸イオンと反応して不溶性の炭酸カルシウムを生成して消耗したためであり、カルシウムイオン濃度の低下に伴って、図2に示すように、全フッ素濃度は上昇し、期待される処理効率が得られないことが分かる。なお、上記計算においてカルシウムイオン濃度は、全フッ素濃度を8mg/L以下に低下させられるという条件下で300mg/Lとし、各定数は、それぞれ表1に示す値を用いた。
【0023】
【表1】
【0024】
即ち、大気中でフッ化物イオン含有廃液を処理する際には、いくら過剰にカルシウムイオンを添加しても、それは全て炭酸カルシウムとして沈殿してしまい、廃液処理に際して発生する廃棄物の量が増大するだけであることが分かる。ここで、図1及び図2より、廃液処理を酸性(pH7.5以下)で行うことも効果的であることが見て取れる。しかし、廃液を酸性にした場合、上述したように、フッ化物イオンがフッ化水素となり、処理系外に揮発する可能性がある。従って、大気中でフッ化カルシウム法によりフッ化物イオン含有廃液を効率的に処理するには、pHが5〜7.5程度のごく狭い領域で値をとるように調整を行わなければならない。
【0025】
以上に詳述したように、大気中でのフッ化物イオン含有廃液処理は、理論上でも効率の悪いものであることが分かる。これに対し発明者は、大気開放系と、大気中の二酸化炭素の廃液への溶け込みを防止した系とでフッ化物イオン含有水溶液処理及びフッ化物イオン含有廃液処理の実験を行い、後者の系で、効率のよい処理が行われることを確認した。その実験について以下に詳述する。
【0026】
まず実験方法について説明する。大気開放系及び大気中の二酸化炭素の廃液への溶け込みを防止した系において、20mg/Lのフッ化物イオンを含む水溶液400mlに、カルシウム濃度が300mg/Lになるように塩化カルシウム溶液を添加する処理を行った。大気開放系では、水溶液中にエアポンプで空気を送り込むことにより、大気と十分に接触させながら一昼夜反応させた。また、大気中の二酸化炭素の廃液への溶け込みを防止した系では、水溶液中に窒素ガスを通気することにより大気中の二酸化炭素の廃液への溶け込みを防止して同様に反応させた。
【0027】
上記実験によって得られた処理後の水溶液のpHと液中のフッ化物イオン濃度との関係を図3に示す。図3に示すように、大気開放系(図中○で示す)では、pHが7.5以上で液中のフッ化物イオン濃度が上昇し、pH9以上では液中のフッ化物イオンが全く除去されなかった。これに対し、大気中の二酸化炭素の廃液への溶け込みを防止した系(図中●で示す)では、液中のフッ化物イオンが、pHに関わりなくほぼ一定の値を示した。この結果は、通常の大気開放系におけるフッ化カルシウム法によるフッ化物イオン含有水溶液の処理効率が、大気中二酸化炭素の溶解の影響でpH上昇と共に低下するが、本発明ではpHに関わりなく優れた結果が得られることを示している。
【0028】
また図4に、上記両系での実験後溶液中にそれぞれ含まれる沈殿物(廃棄物)の重量を示す。大気開放系(図中○で示す)では、pHが上昇するにつれて廃棄物量が増大しており、大気中の二酸化炭素の廃液への溶け込みを防止した系(図中●で示す)では、廃棄物量は、pHに関わりなく0.01〜0.02gとごく少量であった。大気開放系で行った処理で廃棄物量が増大するのは、炭酸カルシウムが生成されるためであると考えられる。
【0029】
次に、実廃液を用いた実験について説明する。この実験では、フッ化物イオンを含有する実験廃水400mlに種々の量の塩化カルシウムを添加し、pH9.5で窒素ガス通気により大気中の二酸化炭素の廃液への溶け込みを防止して1日間反応させた。反応後の廃液のフッ化物イオン濃度及び発生した廃棄物量を表2に示す。この結果、大気中の二酸化炭素の廃液への溶け込みを防止した本発明の系では、実廃液においても900mg/Lのカルシウムイオンを添加することで、フッ化物イオン濃度を新基準値である8ppm以下にまで低下させることができ、また、発生する廃棄物量も少量にできることが分かる。即ち、本発明に係る廃液中フッ化物イオンの高度処理方法は、実廃液の処理においても十分に効果を奏することが確認できた。
【0030】
【表2】
【0031】
以上に説明した実施形態では、廃液中に窒素ガスを通気することによって大気中の二酸化炭素の廃液への溶け込みを防止し、大気中二酸化炭素の処理への影響を抑制し、平衡論的解析により予想される値に近い高い処理効率を得ることができた。また、処理に伴って発生する廃棄物量を大幅に低減することができた。
【0032】
しかし、実設備では廃液の表面全体を蓋部材で覆うことなどにより、廃液と大気との接触を遮断する方法を取ることもできる。この方法によれば窒素ガスを必要としないので、ランニングコストを引き下げることができる。蓋部材としては非通気性シートを使用し、液面に浮かせる方法が簡便である。
【0033】
【発明の効果】
以上に説明したように、本発明の廃液中フッ化物イオンの高度処理方法では、大気中の二酸化炭素の廃液への溶け込みを防止した状態で処理を行わせるので、廃液中の添加したカルシウムイオンと、大気中の二酸化炭素が廃液中に溶解して生成された炭酸イオンとにより炭酸カルシウムが生成されるのを防止できる。このため、添加したカルシウムイオンをフッ化カルシウム生成のためのみに使用することができ、処理液中のフッ化物イオン濃度を基準値以下にまで低減させることができる。また本発明によれば、スラッジとなる炭酸カルシウムが生成されるのを防止でき、廃棄物の発生を従来よりも少なくすることができる。よって本発明によれば、ミニマムエミッションの概念にかなったフッ化物イオンの高度処理が可能となる。
【図面の簡単な説明】
【図1】溶液のpHと溶液中のカルシウムイオン濃度の上限値との関係を示すグラフである。
【図2】溶液のpHと溶液中の全フッ素濃度の下限値との関係を示すグラフである。
【図3】処理実験後の溶液のpHと溶液中のフッ化物イオン濃度との関係を示すグラフである。
【図4】処理実験後の溶液のpHと溶液中の廃棄物量との関係を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for advanced treatment of fluoride ions in waste liquid, and more particularly to a method for advanced treatment of fluoride ions in waste solution using a calcium fluoride method.
[0002]
[Prior art]
With the recent development of advanced industries, the amount of industrial use of fluorine is increasing, and along with this, the amount of waste and liquid waste containing fluorine is also increasing. In addition, due to the wide distribution of fluorine-containing industrial products such as fluororesin products, there is a phenomenon in which high-concentration fluorine is also contained in the smoke-washed water when municipal waste is incinerated. Here, in the treatment of waste liquids containing fluorine compounds, the country has set strict environmental standards regarding the fluoride ion concentration and the like, and various advanced treatment methods for waste liquids have been proposed in response to this. As one of the advanced treatment methods, there is a calcium fluoride method (for example, see Patent Document 1) in which a calcium salt solution is added to a waste liquid containing fluoride ions, and the fluoride ions are precipitated and removed as calcium fluoride. Are known. This method has the advantage that it does not require a secondary treatment and is simple.
[0003]
[Patent Document 1]
JP 11-156355 A (paragraph 5-8)
[0004]
[Problems to be solved by the invention]
However, the conventional treatment method using the calcium fluoride method cannot reduce the fluoride ion concentration in the waste liquid after the treatment to several tens of ppm or less, and cannot achieve the new environmental standard value of 8 ppm. Therefore, it has been attempted to further treat the waste liquid treated by the calcium fluoride method by the adsorption method or the fluorapatite method, but there is a problem that a large amount of sludge that is several tens of times that of the calcium fluoride method is generated. It was.
[0005]
The present invention has been made in order to solve the above-described conventional problems, and the fluoride ion concentration in wastewater can be reduced to a reference value of 8 ppm or less only by the calcium fluoride method. An object of the present invention is to provide an advanced treatment method of fluoride ions in waste liquid that can achieve minimum emission by reducing the amount of generation.
[0006]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, the present inventor examined the reason why the fluoride ion concentration in the treatment liquid cannot be reduced to several tens of ppm or less by the normal calcium fluoride method. The solubility of calcium fluoride is 8 ppm with respect to fluoride ions, but considering the solubility equilibrium between fluoride ions and calcium ions, if the concentration of calcium ions in the liquid is increased, the fluoride ion concentration is lowered to 8 ppm or less. It should be. However, in actuality, increasing the amount of calcium added only increases the amount of sludge generated and does not decrease the fluoride ion concentration. The present inventor has found that the cause is the dissolution of carbon dioxide from the atmosphere into the liquid. That is, calcium is consumed by reacting with carbon dioxide dissolved from the atmosphere to form calcium carbonate, and increasing the amount of calcium added does not increase the amount of calcium ions that contribute to the reduction of fluoride ions. It is.
[0007]
The present invention has been completed on the basis of the above knowledge, and in a method for treating fluoride ions in a waste liquid by adding calcium ions to a waste liquid containing fluoride ions and precipitating and removing the fluoride ions, the waste liquid It is characterized by adding calcium ions while preventing the carbon dioxide in the atmosphere from dissolving into the waste liquid by blocking the contact between the atmosphere and the atmosphere or by passing nitrogen gas through the waste liquid. . It is also possible to prevent the carbon dioxide in the atmosphere from dissolving into the waste liquid by a method of blocking the contact between the waste liquid and the atmosphere by covering the liquid surface of the waste liquid with a lid member.
[0008]
According to the advanced treatment method of fluoride ions in waste liquid of the present invention, calcium is added in a state in which dissolution of carbon dioxide in the atmosphere into the waste liquid is prevented, so that the calcium ions added in the waste liquid are removed from the atmosphere. It can be prevented from reacting with the dissolved carbon dioxide and being consumed as calcium carbonate. For this reason, calcium ions can be actively used only for the production of calcium fluoride, the processing efficiency can be improved, and the amount of sludge generated can be reduced.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described. In the present embodiment, a calcium fluoride method will be described in which a calcium chloride aqueous solution is added to sodium fluoride in a waste liquid to generate poorly soluble calcium fluoride. First, the dissolution equilibrium of calcium fluoride under the assumption that coexisting ions other than hydrogen ions and hydroxide ions do not exist in the liquid will be described. The dissolution equilibrium of calcium fluoride is expressed by the following [Chemical Formula 1], and the solubility product K SP1 is given by [Chemical Formula 2]. Accordingly, if only [Ca 2+ ] C is given to the aqueous solution, the fluoride ion concentration is expressed as [Chemical Formula 3].
[0010]
[Chemical 1]
[Chemical formula 2]
[Chemical 3]
[0011]
Here, if the aqueous solution is weakly acidic, a part of the fluoride ions F − constitutes hydrogen ions H + and hydrogen fluoride HF by a hydrolysis reaction in the liquid. For this equilibrium reaction, [Chemical Formula 4] is given, and the total fluorine concentration in the aqueous solution is expressed as [Chemical Formula 5].
[0012]
[Formula 4]
[Chemical formula 5]
[0013]
The above [Chemical Formula 5] shows that the higher the coexisting calcium ion concentration, the lower the total fluorine concentration in the waste liquid due to the common ion effect. That is, under the assumption that there are no coexisting ions other than hydrogen ions and hydroxide ions in the waste liquid, the fluorine concentration can be lowered by increasing the calcium ion concentration in the waste liquid. Become. As described above, in the waste liquid treatment using the normal calcium fluoride method, the concentration of calcium ions cannot be increased by the dissolution of carbon dioxide in the atmosphere, but in the present invention, the contact between the waste liquid and the atmosphere is blocked. Calcium ions are added while preventing the infiltration of carbon dioxide in the atmosphere into the waste liquid by the method of performing nitrogen gas in the waste liquid or by adjusting the pH of the waste liquid to 7.5 or lower. The processing method will be described in detail below.
[0014]
First, when carbon dioxide in the atmosphere is dissolved in the liquid, bicarbonate ions and hydrogen ions are generated, and carbonate ions and hydrogen ions are further generated from the bicarbonate ions. These equilibrium reactions are shown in [Chemical 6], [Chemical 7] and [Chemical 8] below.
[0015]
[Chemical 6]
[Chemical 7]
[Chemical 8]
[0016]
The equilibrium relational expression of the above reaction is given by the following [Chemical 9], [Chemical 10] and [Chemical 11] when the partial pressure of carbon dioxide in the atmosphere is PCO2 .
[0017]
[Chemical 9]
Embedded image
Embedded image
[0018]
Here, when calcium ions are added to the liquid, the carbonate ions generated in [Chemical Formula 8] react with the calcium ions to form hardly soluble calcium carbonate. The solubility product K SP2 of this reaction is as shown in [Chemical Formula 12], and the maximum value of calcium ions that can exist in the solution can be expressed as [Chemical Formula 13].
[0019]
Embedded image
Embedded image
[0020]
This [Chemical Formula 13] shows that no matter how much calcium ion is added to the solution in the open air system, the calcium ion concentration in the liquid does not become higher than [Ca 2+ ] max. If calcium ions are given by [Ca 2+ ] max, the lower limit value T Fmin of the total fluorine concentration that can exist in this solution can be expressed by [Chemical Formula 14].
[0021]
Embedded image
[0022]
From this [Chemical Formula 14] and the above [Chemical Formula 5] and [Chemical Formula 13], when calculating the upper limit value of calcium ions in the liquid when the influence of carbon dioxide in the atmosphere is taken into account, as shown in FIG. It can be seen that at a pH of 7 or higher, even a slight amount of carbon dioxide in the atmosphere dissolves, the calcium ion concentration in the liquid rapidly decreases. This is because calcium ions in the liquid react with carbonate ions generated by dissolution of carbon dioxide to produce insoluble calcium carbonate and are consumed. As the calcium ion concentration decreases, as shown in FIG. It can be seen that the total fluorine concentration increases and the expected treatment efficiency cannot be obtained. In the above calculation, the calcium ion concentration was set to 300 mg / L under the condition that the total fluorine concentration could be lowered to 8 mg / L or less, and the values shown in Table 1 were used for each constant.
[0023]
[Table 1]
[0024]
In other words, when processing fluoride ion-containing waste liquid in the atmosphere, no matter how much calcium ions are added, it all precipitates as calcium carbonate, increasing the amount of waste generated during waste liquid processing. It turns out that it is only. Here, it can be seen from FIGS. 1 and 2 that it is also effective to perform the waste liquid treatment in an acidic state (pH 7.5 or lower). However, when the waste liquid is acidified, as described above, fluoride ions may become hydrogen fluoride and volatilize outside the processing system. Therefore, in order to efficiently treat the fluoride ion-containing waste liquid by the calcium fluoride method in the atmosphere, adjustment must be made so that the value is taken in a very narrow region where the pH is about 5 to 7.5.
[0025]
As described in detail above, it can be seen that the fluoride ion-containing waste liquid treatment in the atmosphere is theoretically inefficient. On the other hand, the inventor conducted experiments on fluoride ion-containing aqueous solution treatment and fluoride ion-containing waste liquid treatment in an atmosphere open system and a system that prevented the dissolution of carbon dioxide in the atmosphere into the waste liquid. It was confirmed that efficient processing was performed. The experiment will be described in detail below.
[0026]
First, the experimental method will be described. Treatment in which calcium chloride solution is added to 400 ml of an aqueous solution containing 20 mg / L fluoride ions so that the calcium concentration is 300 mg / L in an open air system and a system in which carbon dioxide in the atmosphere is prevented from dissolving in waste liquid. Went. In the open air system, air was sent into the aqueous solution with an air pump to react all day and night while being in sufficient contact with the atmosphere. Further, in a system in which the carbon dioxide in the atmosphere was prevented from dissolving in the waste liquid, nitrogen gas was passed through the aqueous solution to prevent the carbon dioxide in the atmosphere from dissolving in the waste liquid and reacted in the same manner.
[0027]
FIG. 3 shows the relationship between the pH of the treated aqueous solution obtained by the above experiment and the fluoride ion concentration in the solution. As shown in FIG. 3, in the open air system (indicated by a circle in the figure), the fluoride ion concentration in the liquid rises when the pH is 7.5 or higher, and the fluoride ion in the liquid is completely removed when the pH is 9 or higher. There wasn't. On the other hand, in the system (indicated by ● in the figure) in which carbon dioxide in the atmosphere was prevented from dissolving in the waste liquid, the fluoride ions in the liquid showed a substantially constant value regardless of the pH. This result shows that the treatment efficiency of the fluoride ion-containing aqueous solution by the calcium fluoride method in a normal open air system decreases with increasing pH due to the effect of dissolution of atmospheric carbon dioxide, but the present invention is excellent regardless of the pH. It shows that the result is obtained.
[0028]
FIG. 4 shows the weight of the precipitate (waste) contained in the solution after the experiment in both systems. In the open air system (indicated by a circle in the figure), the amount of waste increases as the pH increases. In the system that prevents the atmospheric carbon dioxide from dissolving into the waste liquid (indicated by the black circle in the figure), the amount of waste Was a very small amount of 0.01 to 0.02 g regardless of pH. It is considered that the amount of waste increases in the treatment performed in the open air system because calcium carbonate is generated.
[0029]
Next, an experiment using actual waste liquid will be described. In this experiment, various amounts of calcium chloride were added to 400 ml of experimental wastewater containing fluoride ions, and the reaction was carried out for 1 day at pH 9.5 to prevent the carbon dioxide in the atmosphere from dissolving into the waste liquid by aeration of nitrogen gas. It was. Table 2 shows the fluoride ion concentration of the waste liquid after the reaction and the amount of waste generated. As a result, in the system of the present invention in which the carbon dioxide in the atmosphere is prevented from dissolving in the waste liquid, the fluoride ion concentration is 8 ppm or less, which is a new reference value, by adding 900 mg / L of calcium ions in the actual waste liquid. It can be seen that the amount of waste generated can be reduced to a small amount. That is, it has been confirmed that the advanced treatment method for fluoride ions in waste liquid according to the present invention is sufficiently effective in the treatment of actual waste liquid.
[0030]
[Table 2]
[0031]
In the embodiment described above, nitrogen gas is introduced into the waste liquid to prevent the carbon dioxide in the atmosphere from dissolving into the waste liquid, and the influence on the treatment of atmospheric carbon dioxide is suppressed. A high processing efficiency close to the expected value could be obtained. In addition, the amount of waste generated during the treatment could be greatly reduced.
[0032]
However, in actual equipment, it is possible to take a method of blocking the contact between the waste liquid and the atmosphere by covering the entire surface of the waste liquid with a lid member. According to this method, since no nitrogen gas is required, the running cost can be reduced. A simple method is to use a non-breathable sheet as the lid member and float it on the liquid surface.
[0033]
【The invention's effect】
As described above, in the advanced treatment method of fluoride ions in waste liquid of the present invention, the treatment is performed in a state in which the carbon dioxide in the atmosphere is prevented from being dissolved into the waste liquid. In addition, it is possible to prevent calcium carbonate from being generated by carbonate ions generated by dissolving carbon dioxide in the atmosphere in the waste liquid. For this reason, the added calcium ion can be used only for the production of calcium fluoride, and the fluoride ion concentration in the treatment liquid can be reduced to a reference value or less. Moreover, according to this invention, it can prevent that the calcium carbonate used as sludge is produced | generated, and can reduce generation | occurrence | production of a waste compared with the past. Therefore, according to the present invention, it is possible to perform advanced processing of fluoride ions that conforms to the concept of minimum emission.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between the pH of a solution and the upper limit value of the calcium ion concentration in the solution.
FIG. 2 is a graph showing the relationship between the pH of a solution and the lower limit value of the total fluorine concentration in the solution.
FIG. 3 is a graph showing the relationship between the pH of the solution after the treatment experiment and the fluoride ion concentration in the solution.
FIG. 4 is a graph showing the relationship between the pH of a solution after a treatment experiment and the amount of waste in the solution.
Claims (2)
廃液と大気との接触を遮断する、または、廃液中に窒素ガスを通気することにより、大気中の二酸化炭素の廃液への溶け込みを防止しながら、カルシウムイオンを添加することを特徴とする廃液中フッ化物イオンの高度処理方法。In the method for treating fluoride ions in waste liquid, calcium ions are added to the waste liquid containing fluoride ions to precipitate and remove fluoride ions.
In the waste liquid characterized by adding calcium ions while preventing the carbon dioxide in the atmosphere from dissolving into the waste liquid by blocking the contact between the waste liquid and the atmosphere, or by passing nitrogen gas through the waste liquid. Advanced treatment method of fluoride ion.
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