JPS6216154B2 - - Google Patents

Description

[Detailed description of the invention]

This invention relates to a method for treating wastewater containing persistent iron-complex cyanide. Generally, wastewater containing cyanide compounds generated in industrial processes (hereinafter referred to as cyanide wastewater) is
Because cyanide is highly toxic, strict treatment is required. Oxidation methods are usually used to treat this cyanide wastewater. An example of an oxidation method is an alkali-chlorine method in which cyanide wastewater is treated with a chlorine-based oxidizing agent under highly alkaline conditions. However, iron complex cyanide compounds have a very stable structure and cannot be easily oxidized and decomposed by the alkali-chlorine method described above. Based on this background, a method is used to treat waste liquid containing iron-complexed cyanide by irradiating it with ultraviolet or visible light to convert the iron-complexed cyanide into a form that can be easily oxidized and decomposed, followed by oxidation treatment. ing. FIG. 1 shows an apparatus for treating wastewater containing iron-complex cyanide such as ferrocyanate and ferrocyanide using a combination of light and ozone. In FIG. 1, 1 is a wastewater feed pump, 2 is an ozone reaction tank, and a light source 3 is installed inside this ozone reaction tank 2. 4 is an ozone generator for producing ozone, 5 is an ozone decomposer for decomposing unreacted ozone discharged from the ozone reaction tank 2, 6 is a raw water inlet, 7 is a treated water outlet, and 8 is the bottom of the ozone reaction tank 2 This is a diffuser plate installed in the In order to treat iron-complexed cyanide wastewater using the apparatus shown in FIG. The iron complex cyanide is irradiated with ultraviolet rays or visible rays from the light source 3 to cause intramolecular excitation. There are various excited states of this iron complex cyanide, and one example is the excitation of the ligand field between a metal atom and a cyanide molecule. As a result, a substitution reaction occurs between water molecules as a solvent and cyan molecules fixed as ligands. Therefore, cyan molecules are liberated and become cyan ions, and the iron complex becomes an aco complex containing water molecules. In addition, this ako complex is
Upon sequential photoexcitation, the cyan molecules of the ligands are replaced with water molecules, and finally become iron ions.
The liberated cyanide ions are usually called free cyanide, and are easily converted to cyanic acid by oxidizing agents.
Furthermore, carbon dioxide and nitrogen are also oxidized. In the apparatus shown in FIG. 1, ozone as an oxidizing agent is supplied to the ozone reaction tank 2 from the ozone generator 4.
The free cyanide is oxidized by being supplied in the form of ozone-containing gas as fine bubbles through a diffuser plate (diffuser) 8 in the inner lower part. Wastewater in which cyanide compounds have been oxidized and decomposed is treated as treated water at treated water outlet 7.
It is being released from Further, the gas discharged from the upper part of the ozone reaction tank 2 contains unreacted ozone, and this ozone is completely decomposed in the ozone decomposer 5 and then discharged to the outside of the system. Note that cyanide, which becomes free cyanide through light irradiation, can be oxidized and decomposed using chlorine-based oxidizing agents, but in order to efficiently treat it down to low concentrations, it is economically advantageous to use ozone, which has strong oxidizing power. It is said that However, when the conventional method for treating iron complex cyanide using light and ozone as described above is applied to water to be treated where a reducing agent coexists or ferrocyan and ferricyan coexist, the treatment takes an extremely long time. It also has the disadvantage that cyan processing up to low density cannot be performed. Curve a in Figure 2 is
This is the result of light irradiation and ozone treatment using a ferrocyan aqueous solution containing 200 ppm of cyanide.
Even if the reaction was carried out for a long time, the cyanide treatment was carried out to only about 10 ppm, and even if the reaction time was extended, the residual cyanide concentration hardly decreased below that level. Furthermore, when 100 mg of sodium thiosulfate is present as a reducing agent in the ferrocyan aqueous solution, the cyanide concentration hardly decreases even in the early stage of the reaction, as shown by curve b in Figure 2, and the ozone introduced is The processing time was increased by the amount of sodium thiosulfate consumed in oxidation, and the cyanide concentration reached was around 15 ppm. In any of the above cases, when the amount of irradiation light was significantly increased, the cyan density reached decreased.
It never went below 1ppm. Therefore, the inventor of the present invention investigated the causes of the decrease in the light irradiation effect and the existence of a limit in the cyanide concentration of the final treated water in the conventional treatment methods described above. As a result, three causes became clear. The first point is that since ferrocyan has a lower light absorption rate than ferricyan, the efficiency of generating freecyan from iron complex cyanide is low even if the same amount of light is irradiated. The second point is that when some iron complex cyanide sequentially releases cyanide molecules as ligands and becomes trivalent iron ions, it reacts with the coexisting ferrocyan to produce a compound called Prussian blue. Prussian blue is a cyanide complex salt that is poorly soluble and stable, but at the same time has the property of easily absorbing ultraviolet rays and visible light.For this reason, most of the irradiated light is absorbed by the Prussian blue and is wasted ineffectively. Ru. Furthermore, the third point is that the coexistence of reducing substances preferentially consumes ozone, and during this time, cyanide ions liberated by light are incorporated into the complex again through a reverse reaction, and the effect of light irradiation is utilized. It disappears. This invention solves the drawback that the conventional treatment method for iron-complex cyanide using a combination of light and ozone, as described above, cannot economically process down to low concentrations. The purpose of the present invention is to provide a processing method that can process iron-complex cyanogen to a lower concentration than conventional processing methods using a light source. To achieve this objective, the method for treating iron-complexed cyanide wastewater according to the present invention comprises treating cyanide wastewater coexisting with reducing substances and/or containing iron-complexed cyanide in the form of ferrocyanide using a combination of light and ozone. Prior to the treatment of oxidizing and decomposing cyanide, an oxidizing agent is added to raw cyanide wastewater to oxidize coexisting reducing substances and/or oxidize ferrocyane, which is a reduced form of iron-complexed cyanide, to ferrocyane. It is something. Embodiments of the present invention will be described below with reference to the drawings. FIG. 3 shows an apparatus for carrying out the method for treating iron-complexed cyanide wastewater according to the present invention. In Figure 3, 1 to 5,
7 and 8 are the same parts as the conventional apparatus shown in FIG. 1, and 9 is a reaction tank for oxidizing reducing substances in raw cyanide wastewater and oxidizing coexisting Ferrocyan to Ferrocyan as a pretreatment; 10 is an oxidizing agent storage tank, 11 is a pump for injecting the oxidizing agent into raw cyanide waste water, 12 is a raw water tank, and 13 is a pump for guiding cyanide waste water from the reaction tank 9 to the ozone reaction tank 2. In order to treat iron-complex cyanide wastewater using the apparatus shown in FIG.
An oxidizing agent is added to the raw cyanide wastewater pumped from the oxidizing agent storage tank 10 using a pump 11, and the mixture is sent to the reaction tank 9. In this reaction tank 9, the reducing substances contained in the raw cyanide wastewater are oxidized, and at the same time, Ferrocyan is oxidized to Ferrocyan. In this way, the raw cyanide wastewater whose iron complex salt form has been completely converted to ferrician is guided into the ozone reaction tank 2 by the pump 13, and is subjected to ozone oxidation treatment while being irradiated with light. Note that the ozone reaction tank 2 and its subsequent operations are the same as those in the conventional treatment method, so a description thereof will be omitted. Next, the effects of this invention will be explained. As mentioned above, the drawbacks of conventional processing methods can be easily overcome by oxidizing the reducing substance and oxidizing ferrocyan to ferrocyan. That is, by converting Ferrocyan to Ferrocyan in advance, the low absorbance of Ferrocyan can be solved, and trivalent iron ions and Ferrocyan will not coexist, thus preventing the formation of Prussian blue. . Furthermore, since the reducing substance has been oxidized in advance, the cyanide ions liberated during the initial stages of light irradiation and ozone treatment will not form a complex again, and will therefore be quickly oxidized by ozone. . The processing result using the above-mentioned processing method of the present invention is shown by curve c in FIG. 3. This curve c
100 ppm of hydrogen peroxide was added as an oxidizing agent to a mixed solution of 200 ppm of ferrocyan as the initial cyanide and 100 ppm of sodium thiosulfate, and then a treatment similar to the conventional treatment method using a combination of light and ozone described above was performed. This is what we did.
As is clear from the curve c in FIG. 3, it has become possible to perform iron complex cyanide treatment down to low concentrations in a short period of time. In this invention, the same effect as described above can be obtained even if a bleaching powder or a hypochlorite-based agent such as sodium hypochlorite is used as the oxidizing agent instead of the hydrogen peroxide. In the present invention, the amount of the oxidizing agent used is an amount necessary and sufficient for oxidizing the ferrocyan complex salt to the ferrocyan complex salt and for oxidizing the coexisting reducing substance. Excessive oxidizing agent causes ozone consumption during ozone treatment, resulting in a decrease in ozone utilization efficiency, which is undesirable. Examples and Comparative Examples Fe(CN) ^4-6 _- In wastewater with a CN concentration of 200 ppm,
100 ppm of Na ₂ S ₂ O ₃ and 100 ppm of H ₂ O ₂ were added, and the mixture was reacted with stirring for 30 minutes, followed by photo-ozone treatment under the following conditions (Example). High pressure mercury lamp 100W Ozone injection 20mg/min semi-batch treatment (continuous ozonated gas injection) For comparison, in the above example, Na ₂ S ₂ O ₃ and H ₂ O ₂ were not added (Comparative Example 1),
The same procedure was carried out for the sample without addition of H ₂ O ₂ (Same 2). Changes in the residual CN concentration of these wastewaters over time were determined and the results are shown in the table below.

[Table] As shown in the above table, the CN concentration was reduced on the order of several hundred minutes after 10 hours according to the present invention. As explained above, by using the method for treating iron-complexed cyanide wastewater according to the present invention, the irradiated light can be used efficiently, and iron-complexed cyanide treatment can be carried out to low concentrations in a short time even with a small capacity light source. is obtained.

[Brief explanation of the drawing]

Figure 1 is a schematic configuration diagram showing the equipment used in the conventional iron-complex cyanide wastewater treatment method, Figure 2 is a reaction time-cyanide concentration relationship diagram showing the results of treatment with the conventional and present treatment methods, and Figure 3 is a diagram showing the relationship between reaction time and cyanide concentration. 1 is a schematic configuration diagram showing an example of an apparatus used in the method for treating iron-complex cyanide wastewater according to the present invention. 2... Ozone reaction tank, 3... Light source, 4... Ozone generator, 9... Reaction tank, 10... Oxidizing agent storage tank. Note that the same reference numerals in the figures indicate the same or corresponding parts.

Claims (1)

[Scope of Claims] 1. Cyanide wastewater that coexists with reducing substances and/or contains iron-complexed cyanide in the form of ferrocyanide is converted into raw cyanide wastewater prior to treatment of oxidizing and decomposing the cyanide contained using a combination of light and ozone. A method for treating iron-complex cyanide wastewater, which comprises adding an oxidizing agent to oxidize coexisting reducing substances and/or oxidizing ferrocyan, which is a reduced form of iron-complex cyanide, to ferrocyanide. 2. The method for treating iron-complex cyanide wastewater according to claim 1, which uses hydrogen peroxide or a hypochlorite-based oxidizing agent as the oxidizing agent.