JPS6211635B2 - - Google Patents

Description

[Detailed description of the invention]

This invention relates to a method for treating wastewater containing iron complex cyanide compounds. 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. A method of oxidatively decomposing cyanide compounds is generally used to treat this cyanide wastewater, and one example is an alkali-chlorine method in which the cyanide wastewater is converted into a highly alkaline state and then treated with a chlorine-based oxidizing agent. However, cyanide compounds with relatively stable structures, such as iron-complex cyanide compounds, cannot be easily oxidized and decomposed by the above-mentioned alkali-chlorine method. When treating cyanide wastewater, a method is used in which the cyanide wastewater is irradiated with ultraviolet rays or visible light to change the iron-complex cyanide compounds into a state where they can be easily oxidized and decomposed, and then treated. FIG. 1 shows an apparatus for treating cyanide wastewater containing stable and persistent cyanide compounds such as iron complex cyanide such as ferricyanide and ferrocyanide using a combination of light and ozone. In Figure 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 making ozone, 5
is an ozone decomposer that decomposes unreacted ozone discharged from the ozone reaction tank 2, 6 is a raw water inlet, and 7 is a treated water outlet. In order to treat iron-complexed cyanide wastewater using the apparatus shown in FIG. The complex cyanide compound is excited by receiving ultraviolet rays or visible rays emitted from the light source 3. There are various excited states of this complex cyanide compound, but one example is the excitation of the metal atom and the ligand field of the cyanide molecule, resulting in the displacement of the water molecule, which is the solvent, and the cyanide molecule, which is the ligand. A reaction occurs. The cyanide molecules liberated from the metal atoms in this way are usually called free cyanide, and are easily oxidized and decomposed by an oxidizing agent. In this way, the cyanide molecules in the complex cyanide compound are converted into free cyanide by light, and the wastewater is simultaneously fed with microscopic bubbles from the ozone generator 4 through the air diffuser plate (not shown) installed at the bottom of the ozone reaction tank 2. By supplying ozone-containing gas as a gas, it is oxidized and decomposed into harmless cyanic acid, and further into carbon dioxide and nitrogen. The wastewater in which the cyanide compounds have been oxidized and decomposed is discharged from the treated water outlet 7 as treated water. 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. The ozone decomposer 5 is usually filled with activated carbon that decomposes ozone, or one that uses a heater to thermally decompose ozone. In addition, cyanide that has become free cyanide due to light irradiation can be oxidized and decomposed using a chlorine-based oxidizing agent, but the efficiency decreases when treating it to less than 1 ppm, so it is economically difficult to oxidize the cyanide to a greater degree. It is advantageous to use ozone with high strength. However, in the conventional treatment method for iron-complexed cyanide that uses both light and ozone as described above, iron ions are also separated as the reaction progresses, resulting in a mixture of iron hydroxide, iron oxide, iron oxyhydroxide, etc. As a result, fine flocs are formed. Furthermore, these flocs absorb light at wavelengths that overlap the optical absorption bands of ferrician and ferrician salts. Therefore, most of the light emitted from the light source 3 with a wavelength effective for dissociating iron-complexed cyanide ions is absorbed by the flocs of iron hydroxide, iron oxide, iron oxyhydroxide, etc. The disadvantage is that the free cyan dissociation reaction may be delayed or stopped. For example, when an iron-complex cyanide solution containing 200 ppm of cyanide is treated with light and ozone, the dissociation of the iron-complex cyanide by light stops at a residual cyanide concentration of 1 to 10 ppm, and higher-level treatments are virtually impossible. be.
Furthermore, if the amount of irradiation light is significantly increased, it is possible to process to a cyan residual density below the above level, but this is not an economical processing method. In view of the above-mentioned circumstances, this invention eliminates the drawbacks of the conventional method of treating iron-complexed cyanide using a combination of light and ozone, and effectively uses light to treat iron-complexed cyanide to a low concentration using a small light source. The purpose of this research is to provide a method for treating iron-complexed cyanide wastewater. That is, the inventor of the present invention has discovered a wavelength effective for the dissociation of iron-complex cyanide by iron hydroxide, iron oxide, iron oxyhydroxide, etc. from iron ions liberated after free cyanide is dissociated by light of iron-complex cyanide. It was experimentally confirmed that the light-induced dissociation reaction of iron-complexed cyanide was delayed and stopped when the light was absorbed. The inventors have found that it is sufficient to prevent the formation of iron oxides and iron oxyhydroxides as long as the above-mentioned stable compounds do not absorb light in the wavelength range effective for dissociation of iron-complexed cyanide. and,
The inventor confirmed that by adding phosphoric acid or soluble phosphate to iron-complexed cyanide wastewater, the free iron ions are converted into iron phosphate, and the effective use of light is not impaired. It is a completed invention. That is, the method for treating iron-complexed cyanide wastewater according to the present invention involves adding phosphoric acid or a soluble phosphate to iron-complexed cyanide wastewater prior to the treatment of decomposing and oxidizing iron-complexed cyanide using a combination of light and ozone. It is characterized by this. Embodiments of the present invention will be described below with reference to the drawings. FIG. 2 shows an apparatus for carrying out the method for treating iron-complexed cyanide wastewater according to the present invention. In FIG. 2, 1 to 7 are the same parts as the conventional apparatus shown in FIG. 1, and 8 is an additive injection pipe for adding phosphoric acid or phosphate to the raw cyanide waste liquid as a pretreatment. In order to treat iron-complexed cyanide wastewater using the apparatus shown in FIG. Add phosphoric acid or soluble phosphate to. In this case, the amount of phosphoric acid or phosphate salt added is equal to or more than the total amount of iron contained in the iron-complexed cyanide wastewater. The cyanide wastewater to which phosphoric acid or phosphate has been added in this manner is led into the ozone reaction tank 2 and 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. When a potassium ferricyanide solution with a cyanide concentration of 200 ppm is treated as iron-complex cyanide wastewater using the treatment method of the present invention described above, the light utilization efficiency is nearly twice as high as that of conventional treatment methods, and the residual cyanide concentration is 0.1. Easily processed down to less than ppm. Furthermore, when cyanide wastewater is treated in batches, the conventional treatment method fails to maintain a balance between light and ozone, and requires a large capacity for both light and ozone. That is, iron-complex cyanide dissociated by light becomes free cyanide and iron ions, but if the liberated free cyanide is not immediately oxidized with ozone, it will return to complex cyanide through a reverse reaction. Therefore, in conventional treatment methods, a large amount of ozone is supplied to oxidize free cyanide, which is produced in large quantities during the period when the complex cyanide concentration is high at the beginning of treatment, and free cyanide, which is dissociated by light at the end of treatment, is supplied. During periods when the amount of cyanide is low, most ozone is not used and is emitted. In this case, although it is possible to reduce the operating cost of the ozone generator by successively reducing the amount of ozone generated, it is impossible to reduce the operating cost to the level of the ozone generator that should be installed. When the treatment method of the present invention is applied to such batch treatment, the free cyanide and iron ions dissociated by light are recomplexed by a reverse reaction to form a stable compound as iron phosphate. Formation of cyan can be prevented. Therefore, it is sufficient to set the capacity of the ozone generator to the required amount of ozone averaged over the entire processing time, and then allow the large amount of free cyan produced at the start of the process to accumulate as it is. It becomes possible to oxidize the free cyanide accumulated during the period of excess ozone at the end of the reaction with ozone. Examples and Comparative Examples Fe(CN) ^3-6 _- For wastewater with a CN concentration of 200 ppm,
K ₂ HPO ₄ -PO ₄ was set at a concentration of 130 ppm, and ozonized gas was continuously injected under the conditions of a high-pressure mercury lamp (100 W) and ozone injection (20 mg/min). The residual CN (ppm) was determined over time and the results are shown in the table below. For comparison, the same procedure was carried out without adding K ₂ HPO ₄ .

【table】

[Table] As shown in the above table, the above-mentioned effects of the present invention are extremely excellent. As explained above, the method for treating iron-complexed cyanide wastewater according to the present invention involves adding phosphoric acid or phosphate salts to cyanide wastewater prior to oxidation treatment using a combination of light and ozone, thereby liberating the cyanide wastewater during the treatment. By forming a stable compound from iron ions as iron phosphate, it is possible to use light effectively, and the cyanide concentration can be reduced to 0.1ppm with about half the amount of light compared to conventional methods. In the formation of , the reverse reaction of complex cyanide dissociation can be prevented by light, and the imbalance between the amount of light and the amount of ozone generated when applying the conventional batch treatment method is eliminated, and the ozone with the minimum ozone generation capacity is eliminated. It has the effect of being able to use a generator.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing an apparatus used in a conventional method for treating iron-complexed cyanide wastewater, and FIG. 2 is a schematic diagram showing an example of an apparatus used in a method for treating iron-complexed cyanide wastewater according to the present invention. 2...Ozone reaction tank, 3...Light source, 4...Ozone generator, 8...Additive injection pipe. Note that the same reference numerals in the figures indicate the same or corresponding parts.

Claims (1)

[Claims] 1. An iron complex characterized in that phosphoric acid or a soluble phosphate is added to iron complex cyanide wastewater prior to treatment of decomposing and oxidizing iron complex cyanide using a combination of light and ozone. How to treat cyanide wastewater. 2. The method for treating iron-complexed cyanide wastewater according to claim 1, wherein the amount of phosphoric acid or phosphate salt added is set to be equal to or more than the total amount of iron contained in the iron-complexed cyanide wastewater.