Treatment method for cyanide-containing wastewater

Abstract

To provide a method for treating cyanide-containing wastewater that reduces cyanide compounds in the cyanide-containing wastewater and suppresses the increase in COD when hydrogen peroxide is used in the treatment of cyanide-containing wastewater that contains a reducing agent or to which a reducing agent is added.SOLUTION: A method for treating cyanide-containing wastewater includes a reductant addition step of allowing the wastewater to have a reductant and / or adding a reductant to the wastewater, and further includes a hydrogen peroxide addition step of adding hydrogen peroxide to the wastewater, and a COD measurement step of measuring the COD in the wastewater after the hydrogen peroxide addition step, wherein the reductant is a compound containing sulfur atoms, all of the sulfur atoms haing covalent bonds with oxygen.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for treating cyanide-containing wastewater. [Background technology]

[0002] Because cyanide has a strong adverse effect on ecosystems, cyanide-containing wastewater (hereinafter referred to as "cyanide wastewater") cannot be released directly into the natural environment. Effluent standards for cyanide have been established, and wastewater must be treated to remove cyanide and rendered harmless to meet these standards (1 mg / L or less) before it can be discharged into sewers or other systems. Furthermore, some of the cyanide contained in wastewater disperses into the surrounding area as hydrogen cyanide gas, causing significant damage to the working environment. The Industrial Safety and Health Act stipulates that the working environment concentration of hydrogen cyanide must be 3 ppm or less. Depending on the source of the wastewater, the amount of cyanide varies, but it exists in three forms: persistent cyanide complexes and their ions, readily decomposable cyanide complexes and their ions, and cyanide ions (free cyanide).

[0003] Various methods have been proposed and put into practical use for the removal of cyanide from cyanide-containing wastewater, but each method has its advantages and disadvantages, and the method is used depending on the condition of the wastewater. For example, there are oxidation decomposition methods such as (1) the alkaline chlorine method, in which cyanide-containing wastewater is adjusted to alkaline and then chlorine is injected to oxidize and decompose the cyanide; (2) the ozone oxidation method, in which cyanide is oxidized and decomposed into nitrogen gas and bicarbonate using the strong oxidizing power of ozone; and (3) the electrolytic oxidation method (electrolysis method), in which cyanide is electrolyzed using a non-soluble electrode to cause an oxidation reaction; (4) the Prussian blue method, in which a compound that supplies iron ions, such as ferrous sulfate, is added to cyanide-containing wastewater to generate sparingly soluble ferri / ferrocyanide, which is then precipitated and removed; and (5) the method using zinc chloride and a reducing agent. These include (5) the zinc white method, in which copper salts and a reducing agent are added to precipitate and remove the resulting insoluble complex, and (6) the insoluble complex method, such as the reduced copper salt method, in which copper salts and a reducing agent are added to precipitate and remove the resulting insoluble complex; (7) the biological treatment method, in which microorganisms (cyanide-decomposing bacteria) are used to decompose cyanide; (8) the thermal hydrolysis method, in which cyanide-containing wastewater is heated to high temperatures to hydrolyze cyanide compounds into ammonia and formic acid, precipitating coexisting heavy metals as simple substances or oxides; and (9) hydrothermal reactions such as wet oxidation, which oxidizes and decomposes organic pollutants in addition to decomposing cyanide. Therefore, various chemicals have been used to remove cyanide from cyanide-containing wastewater, depending on the condition of the wastewater.

[0004] Among the above-mentioned methods for removing cyanide from cyanide-containing wastewater, the following disclosures have been made regarding treatment methods using reducing agents. For example, Patent Document 1 discloses a treatment method in which hydrogen peroxide and a reducing agent are added to cyanide-containing wastewater to cause a reaction, a copper salt is then added to cause a reaction, and the reaction solution is subjected to solid-liquid separation to separate and remove sparingly soluble cyanide compounds formed in the reaction solution. The reducing agent is added to the cyanide-containing wastewater in order to make the iron-cyano complex contained in the wastewater sparingly soluble together with the copper salt, and suitable reducing agents include thiosulfate, sulfite, bisulfite, ferrous chloride, and alkali metal sulfides such as sodium sulfide and sodium tetrasulfide. Patent Document 2 describes a treatment method including a step of adding a divalent copper salt and a reducing agent to cyanide-containing wastewater to generate a sparingly soluble salt, which is then separated. These documents disclose that a reducing agent is added to cyanide-containing wastewater together with other chemicals such as copper salts, and is used for the purpose of producing sparingly soluble salts such as sparingly soluble cyanide compounds.

[0005] Furthermore, Patent Document 3 discloses a method for treating cyanogen complexes in wastewater by allowing a copper compound and hydrogen peroxide to coexist in the wastewater. This method allows the cyanide concentration in the wastewater containing cyanogen complexes to be sufficiently reduced without using a reducing agent. Patent Document 3 also describes the use of a reducing agent, but the reducing agent is added to the wastewater containing cyanogen complexes together with a cupric compound for the purpose of reducing the cupric compound in the wastewater to produce a cuprous ion supply compound. The use of such a reducing agent is intended to improve the effectiveness of cyanide compound treatment.

[0006] Patent Document 4 describes a method for removing cyanide from cyanide-containing wastewater by adding chlorine dioxide and one or more metal compounds selected from manganese compounds, iron compounds, zinc compounds, and copper compounds to the wastewater. It also discloses that the iron compound is an iron compound capable of forming trivalent iron ions in water, and that the copper compound is a cupric compound, and that the reducing agent is added to the wastewater. The reducing agent is added to reduce the co-added iron compound or cupric compound in the wastewater to produce a divalent iron ion supply compound or a cuprous ion supply compound. The use of such a reducing agent is intended to improve the effectiveness of cyanide compound treatment. Furthermore, Patent Document 4 discloses that the COD (chemical oxygen demand) of cyanide-containing wastewater can be removed by adding a reducing agent in addition to chlorine dioxide and the above-mentioned metal compound to the wastewater.

[0007] As described above, there are several methods for adding a reducing agent to the treatment of cyanide-containing wastewater, and in all cases, the reducing agent is added for the purpose of improving the effect of treating cyanide compounds (e.g., Patent Documents 1 to 4). Some methods for adding a reducing agent for the purpose of removing COD from wastewater have also been disclosed (Patent Document 4), but COD (Chemical Oxygen Demand) represents the amount of oxygen consumed when an oxidizing agent is used to oxidize and decompose organic matter, and the reducing agent itself can contribute to increasing the COD in wastewater. Therefore, the COD removal effect using the reducing agent disclosed in Patent Document 4 is merely an effect obtained from the entire invention described in Patent Document 4, and the same effect cannot be obtained in other methods for treating cyanide-containing wastewater.

[0008] In addition to the purpose of improving the treatment effect of cyanide compounds described above, reducing agents used in the treatment of cyanide-containing wastewater generally also have the purpose of removing oxidizing agents. There are various purposes for removing oxidizing agents, such as eliminating concerns about corrosion caused by the oxidizing agent or reactivity in the next process, and satisfying regulations on residual oxidizing agents in treated water when the treated water is discharged. Reducing agents are sometimes used for these purposes in wastewater that is treated for cyanide compounds, and the reducing agent may coexist with the chemicals added for the treatment of cyanide compounds in the wastewater. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Patent No. 6474472 [Patent Document 2] Japanese Patent Application Laid-Open No. 2008-36608 [Patent Document 3] Patent No. 6145682 [Patent Document 4] Patent Publication No. 2021-53620 Summary of the Invention [Problem to be solved by the invention]

[0010] As described above, it is known that reducing agents are used for various purposes in the treatment of cyanide-containing wastewater. However, according to the investigations of the present inventors, when hydrogen peroxide is used in the treatment of cyanide-containing wastewater, the combined use of a reducing agent has been found to pose a problem of a significant increase in the COD of the wastewater. Therefore, when hydrogen peroxide is used in the treatment of cyanide-containing wastewater, depending on whether a reducing agent is present in the wastewater or whether a reducing agent is added, if hydrogen peroxide and a reducing agent coexist in the cyanide-containing wastewater, the COD of the wastewater will rise significantly, and it will become necessary to treat the COD to below the effluent standard value when discharging the treated water, which may require the addition of additional chemicals, dilution, time-consuming discharge, etc. Furthermore, since such additional treatment of the treated water takes time, it is necessary to reduce the operating load of the factory discharging the cyanide-containing wastewater, which poses the problem of a decrease in the factory's operating rate.

[0011] Thus, wastewater treatment methods are required not only to treat specific components contained in the wastewater, but also to ensure that the treated water obtained through the treatment method meets effluent standards. Among the effluent standards, COD in particular is a value that represents the amount of oxidant required for chemical oxidation reactions. Therefore, the value can sometimes rise due to chemicals used to reduce other control items listed in the effluent standards below their standard values, making it difficult to meet the effluent standards for COD by treating it in the same way as other control items.

[0012] Therefore, an object of the present invention is to provide a method for treating cyanide-containing wastewater, which reduces the amount of cyanide compounds in the cyanide-containing wastewater and suppresses an increase in COD when hydrogen peroxide is used in the treatment of cyanide-containing wastewater that contains a reducing agent or to which a reducing agent is added. [Means for solving the problem]

[0013] The present inventors noticed that the COD of treated water obtained by a method for treating cyanide-containing wastewater using hydrogen peroxide did not meet effluent standards. Focusing on the COD of the treated water, they conducted extensive research into a method for treating cyanide-containing wastewater without significantly increasing the COD. As a result, they discovered that, when treating cyanide-containing wastewater using hydrogen peroxide, reducing agents used for the purpose of improving the treatment effect of cyanide compounds in the wastewater and / or reducing agents contained in the wastewater significantly increase the COD of the treated water. They then discovered that, when treating cyanide-containing wastewater, the use of a specific reducing agent that coexists with hydrogen peroxide in the wastewater reduces the amount of cyanide compounds in the cyanide-containing wastewater and significantly suppresses the increase in COD, leading to the completion of the present invention.

[0014] That is, the present invention relates to, but is not limited to, the following method for treating cyanide-containing wastewater.

[0015] [1] A method for treating cyanide-containing wastewater, wherein the wastewater contains a reducing agent and / or includes a reducing agent addition step of adding a reducing agent to the wastewater, and further includes a hydrogen peroxide addition step of adding hydrogen peroxide to the wastewater, and a COD measurement step of measuring the COD of the wastewater after the hydrogen peroxide addition step, wherein the reducing agent is a compound containing sulfur atoms, and all of the sulfur atoms have a covalent bond to oxygen. [2] The treatment method according to [1] above, further comprising a metal compound addition step of adding a metal compound to the cyanide-containing wastewater. [3] The treatment method according to [2] above, wherein the metal compound is at least one selected from the group consisting of copper compounds, manganese compounds, iron compounds, and zinc compounds. [4] The treatment method according to any one of [1] to [3] above, wherein the reducing agent is at least one selected from the group consisting of sulfites, hydrogen sulfites, and hyposulfites. [5] The treatment method according to any one of [1] to [4], wherein the reducing agent addition step adjusts and adds the reducing agent in an amount based on the COD value in the wastewater obtained in the COD measurement step. [Effects of the Invention]

[0016] According to the present invention, when hydrogen peroxide is used in the treatment of cyanide-containing wastewater containing a reducing agent or to which a reducing agent is added, a method for treating cyanide-containing wastewater can be provided, which reduces the amount of cyanide compounds in the cyanide-containing wastewater and suppresses an increase in COD. DETAILED DESCRIPTION OF THE INVENTION

[0017] The present invention relates to a method for treating cyanide-containing wastewater, wherein the wastewater contains a reducing agent and / or includes a reducing agent addition step of adding a reducing agent to the wastewater, and further includes a hydrogen peroxide addition step of adding hydrogen peroxide to the wastewater, and a COD measurement step of measuring the COD of the wastewater after the hydrogen peroxide addition step, wherein the reducing agent is a compound containing sulfur atoms, and all of the sulfur atoms have a covalent bond to oxygen.

[0018] It is known that the presence of reducing agents themselves has a positive effect on COD in wastewater, based on the reaction principle. However, while studying methods for treating cyanide-containing wastewater, it was confirmed that the COD value in cyanide-containing wastewater is not a value that is solely derived from the inherent reducing properties of the reducing agent, but is also affected by the chemicals added to the cyanide-containing wastewater. The mechanism of action in wastewater of the chemicals (i.e., hydrogen peroxide and specific reducing agent) used in the method for treating cyanide-containing wastewater of the present invention is not clear, but it is thought that the specific reducing agent, i.e., a compound containing sulfur atoms, all of which have covalent bonds to oxygen, suppresses the production of by-products that contribute to an increase in COD when treating cyanide-containing wastewater to which hydrogen peroxide is added, thereby suppressing the generation of high concentrations of COD.

[0019] (Cyanide-containing wastewater) Examples of cyanide-containing wastewater to be treated in the present invention include cyanide-containing wastewater containing metal cyanide compounds, cyanide ions, metal cyano complexes (including cyanide complexes such as iron carbonyl cyano complexes), and cyanide complex ions discharged from steel plants, chemical plants, plating plants, coke plants, metal surface treatment plants, etc.; cyanide-containing wastewater discharged in the treatment process of radioactively contaminated water; and cyanide-containing wastewater discharged from soil treatment devices. The method for treating cyanide-containing wastewater of the present invention is particularly suitable for treating cyanide-containing wastewater discharged from steel plants, chemical plants, coke plants, and metal surface treatment plants. In this specification, the term "cyanide compounds" refers simply to metal cyanide compounds, cyanide ions, metal cyano complexes, and cyanide complex ions contained in cyanide-containing wastewater.

[0020] The cyanide content in the cyanide-containing wastewater to be treated in the present invention is not particularly limited, but examples thereof include cyanide-containing wastewater having a total cyanide concentration of about 1 to 500 mg / L. Preferably, the cyanide-containing wastewater has a total cyanide concentration of 1 to 500 mg / L, more preferably, a total cyanide concentration of 1 to 100 mg / L, and even more preferably, a total cyanide concentration of 1 to 50 mg / L. The cyanide-containing wastewater is only required to contain 1 mg / L or more of cyanide ions, and the ratio of the cyanide ions to other cyanide compounds contained therein is not particularly limited. For example, the ratio of cyanide ions to other cyanide compounds (cyanide ions:other cyanide compounds) may be 1:10 to 10:1, and cyanide compounds other than cyanide ions may not be contained.

[0021] (hydrogen peroxide) The hydrogen peroxide used in the present invention includes aqueous hydrogen peroxide solutions with a concentration of 3 to 60%, which are commercially available mainly for industrial use. It is also possible to use hydrogen peroxide generated from a hydrogen peroxide supplying compound (also called a "hydrogen peroxide generator") or hydrogen peroxide generated by electrolysis of water or an alkaline solution. Examples of hydrogen peroxide-supplying compounds include inorganic peracids such as percarbonate, perborate, and peroxysulfuric acid, and organic peracids such as peracetic acid, which are capable of releasing hydrogen peroxide in water, and salts thereof, such as sodium percarbonate and sodium perborate. The above hydrogen peroxide and hydrogen peroxide-supplying compound may be diluted or dissolved with water to a desired hydrogen peroxide concentration before addition.

[0022] The amount of hydrogen peroxide added in the present invention is affected by the type and concentration of cyanide contained in the cyanide-containing wastewater, as well as the type and concentration of other metal ions contained in the cyanide-containing wastewater, and can be appropriately determined based on these conditions. Specifically, the cyanide concentration of the cyanide-containing wastewater is measured in advance (from immediately before treatment to approximately 3 hours before treatment), and the amount added can be determined based on this measurement. For example, the amount of hydrogen peroxide added can be determined so that the hydrogen peroxide concentration in the cyanide-containing wastewater is 1 to 20 times the cyanide concentration, preferably 2 to 10 times, and more preferably 3 to 5 times. Furthermore, the hydrogen peroxide concentration in the cyanide-containing wastewater can be determined so that the hydrogen peroxide concentration is preferably 2.5 to 1300 mg / L, more preferably 4 to 800 mg / L, even more preferably 5 to 500 mg / L, and particularly preferably 5 to 250 mg / L.

[0023] In addition, in the method for treating cyanide-containing wastewater of the present invention, it is preferable that a metal compound is further added to the cyanide-containing wastewater.

[0024] (metal compound) The metal compound used in the present invention is preferably one or more compounds selected from copper compounds, manganese compounds, iron compounds and zinc compounds. Each metal compound will be described below, but among these metal compounds, one or more selected from copper compounds, manganese compounds and zinc compounds are preferred in terms of the cyanide removal effect, and copper compounds are particularly preferred.

[0025] (copper compound) The copper compound used in the present invention is preferably a cuprous compound capable of forming monovalent copper ions and / or a cupric compound capable of forming divalent copper ions, and may be either an organic copper compound or an inorganic copper compound. Examples of the organic copper compound include copper(II) compounds selected from copper(II) acetate, copper(II) benzoate, copper(II) citrate, copper naphthenate, and copper(II) oleate.

[0026] Examples of inorganic copper compounds include copper (I) compounds capable of forming monovalent copper ions in water selected from copper (I) chloride, copper (I) fluoride, copper (I) bromide, copper (I) iodide, copper (I) nitrate, copper (I) sulfate, and copper (I) acetate; and copper (II) compounds capable of forming divalent copper ions in water selected from copper (II) chloride, copper (II) fluoride, copper (II) bromide, copper (II) iodide, copper (II) nitrate, copper (II) sulfate, and copper (II) acetate. Since organic copper compounds may increase the COD in the cyanide-containing wastewater after treatment, inorganic copper compounds are preferred among the above copper compounds, and from the viewpoints of the cyanide compound removal effect and the cost of treating the cyanide-containing wastewater, inorganic cuprous compounds and / or inorganic cupric compounds are more preferred, and at least one selected from the group consisting of cuprous chloride, cuprous sulfate, cupric chloride and cupric sulfate is even more preferred, with cuprous chloride and / or cupric chloride being particularly preferred.

[0027] The copper compound may be treated with a metal scavenger so that the copper compound has a desired copper concentration when added to the cyanide-containing wastewater, or may be diluted or dissolved in water such as industrial water before use. Here, examples of the metal scavenger include liquid chelating agents. When the cuprous compound is a cuprous salt, it is preferable to prepare a cuprous salt solution using hydrogen chloride water, an aqueous alkali metal halide solution or ethanol as a solvent from the viewpoint of the stability of the cuprous salt. In the method of the present invention, the copper compound includes a cuprous ion-donating compound produced by adding a cupric compound together with a reducing agent to cyanide-containing wastewater, or by adding a cupric compound to reducing cyanide-containing wastewater and reducing the cupric compound in the wastewater. The reducing agent is a compound containing sulfur atoms, all of which have covalent bonds to oxygen. It is preferable to use at least one reducing agent selected from the group consisting of sulfites, hydrogen sulfites, and hyposulfites.

[0028] The copper compound used in the present invention is preferably added so that the copper concentration (copper equivalent) in the cyanide-containing wastewater is about 0.1 to 500 mg / L. If the copper concentration is less than 0.1 mg / L, the effect of promoting cyanide removal may not be sufficient, while if the copper concentration exceeds 500 mg / L, it will not only have a negative impact on the environment but will also be uneconomical. Specific copper concentrations (mg / L) are, for example, 0.1, 0.5, 1.0, 2.0, 5.0, 10, 25, 50, 75, 100, 125, 150, 200, 250, 300, 350, 400, 450, and 500. The copper concentration is preferably 0.1 to 150 mg / L, and more preferably 1 to 50 mg / L. The copper concentration may be determined to be 0.1 times or more the cyan concentration. The copper concentration (copper equivalent) in the cyanide-containing wastewater is not the concentration of copper compounds in the cyanide-containing wastewater, but the copper compound concentration converted into copper.

[0029] (Manganese compounds) The manganese compound used in the present invention is not particularly limited as long as it is a compound that is soluble in water and can form manganese ions in water, and examples thereof include manganese chloride, manganese sulfate, manganese nitrate, manganese acetate, etc. Among these, manganese chloride and manganese sulfate are particularly preferred in terms of the effect of removing cyanide compounds, and manganese chloride is particularly preferred in terms of the cost of treating cyanide-containing wastewater. In the present invention, "soluble in water" means that the compound has a solubility of about 1 g or more per 100 g of water.

[0030] (iron compounds) The iron compound used in the present invention is not particularly limited as long as it is soluble in water, and examples include ferrous compounds capable of forming divalent iron ions in water, such as ferrous chloride, ferrous fluoride, ferrous bromide, ferrous iodide, ferrous nitrate, ferrous sulfate, and ferrous acetate, as well as ferric compounds capable of forming trivalent iron ions in water, such as ferric chloride, ferric fluoride, ferric bromide, ferric iodide, ferric nitrate, ferric sulfate, and ferric acetate. Of these, ferrous chloride and ferrous sulfate are particularly preferred in terms of the effect of removing cyanide compounds, and ferrous chloride is particularly preferred in terms of the cost of treating cyanide-containing wastewater.

[0031] In the method of the present invention, the iron compound includes a divalent iron ion supply compound produced by adding an iron compound capable of forming trivalent iron ions in water together with a reducing agent to cyanide-containing wastewater, or by adding an iron compound capable of forming trivalent iron ions in water to reducible cyanide-containing wastewater and reducing the iron compound capable of forming trivalent iron ions in the wastewater. The reducing agent is a compound containing sulfur atoms, all of which have covalent bonds to oxygen. It is preferable to use at least one reducing agent selected from the group consisting of sulfites, hydrogen sulfites, and hyposulfites.

[0032] (Zinc compounds) The zinc compound used in the present invention is not particularly limited as long as it is a compound that is soluble in water and can form zinc ions in water, and examples thereof include zinc chloride, zinc oxide, zinc hydroxide, zinc carbonate, zinc peroxide, zinc sulfate, zinc nitrate, etc. Among these, zinc chloride and zinc sulfate are particularly preferred in terms of the effect of removing cyanide compounds, and zinc chloride is particularly preferred in terms of the cost of treating cyanide-containing wastewater.

[0033] (reducing agent) In the treatment method of the present invention, the cyanide-containing wastewater to be treated contains a reducing agent, and / or the treatment method of the present invention includes a reducing agent addition step of adding a reducing agent to the cyanide-containing wastewater to be treated. This means that the cyanide-containing wastewater to be treated (hereinafter also simply referred to as wastewater to be treated) contains a specific reducing agent described below, or includes a reducing agent addition step of adding a specific reducing agent to the wastewater to be treated, or includes a reducing agent addition step of adding a specific reducing agent to cyanide-containing wastewater that contains a specific reducing agent and / or another reducing agent (i.e., a reducing agent different from the specific reducing agent described below). In the present invention, the reducing agent contained in the wastewater to be treated and / or the reducing agent added to the wastewater to be treated is a compound containing sulfur atoms, all of which have covalent bonds to oxygen, and is preferably at least one selected from the group consisting of sulfites, hydrogen sulfites, and hyposulfites, and more preferably at least one selected from the group consisting of sodium sulfite, sodium hydrogen sulfite, and sodium hyposulfite. The specific reducing agent is preferably present in / added to the cyanide-containing wastewater at a concentration of 1.0 to 5000 mg / L, more preferably present in / added to the cyanide-containing wastewater at a concentration of 3.0 to 2000 mg / L, and even more preferably present in / added to the cyanide-containing wastewater at a concentration of 3.0 to 1000 mg / L. If the (added) concentration of the reducing agent to the cyanide-containing wastewater is less than 1.0 mg / L, the reduction of poorly soluble cyanide compounds among the cyanide compounds in the cyanide-containing wastewater may be insufficient, and if it exceeds 5000 mg / L, the effect of inhibiting the reduction of COD may be reduced.

[0034] In the treatment method of the present invention, the hydrogen peroxide addition step may be a single step or may be divided into multiple steps. Furthermore, the hydrogen peroxide addition step in the treatment method of the present invention may be located in a position appropriate for the intended use of the hydrogen peroxide, and may be the same step as or different from other chemical addition steps. For example, when a reducing agent addition step and / or a metal compound addition step are included, the hydrogen peroxide addition step may be located in the same step as the reducing agent addition step and / or the metal compound addition step, or may be located in a separate step. In the treatment method of the present invention, the hydrogen peroxide addition step may be any step in which the above-described hydrogen peroxide is added to the wastewater to be treated, and the means for addition is not limited.

[0035] When the treatment method of the present invention has multiple hydrogen peroxide addition steps, the COD measurement step described below is preferably provided after the most downstream hydrogen peroxide addition step, because this allows appropriate confirmation of the effect on the COD value of the target wastewater caused by the coexistence of hydrogen peroxide added in the hydrogen peroxide addition step and a reducing agent in the wastewater.

[0036] The COD measurement step in the treatment method of the present invention measures the COD in the cyanide-containing wastewater after the hydrogen peroxide addition step, and the COD may be measured using a known method. In one embodiment, the COD can be measured using an analytical method in accordance with JIS K010217 or an ultraviolet absorption method. The COD measurement step in the treatment method of the present invention may be performed after the hydrogen peroxide addition step of adding hydrogen peroxide to cyanide-containing wastewater. The stage of treatment of cyanide compounds in wastewater in the COD measurement step is not particularly limited. That is, the cyanide concentration in the cyanide-containing wastewater in the COD measurement step may be either unreduced or reduced, or may be wastewater in which the cyanide compounds have been treated to a cyanide concentration below the lower analytical limit.

[0037] The COD measurement step in the treatment method of the present invention may be a step of measuring the COD continuously, periodically, or irregularly in a system through which the cyanide-containing wastewater to be treated in the present invention flows, or may be a step of measuring the COD of wastewater sampled from the system through which the cyanide-containing wastewater flows, and the measurement means is not particularly limited. For example, when the COD measurement step is a continuous measurement, an automatic measurement device may be used, and when the COD measurement step is a periodic or irregular measurement, manual measurement using a simple COD meter may be used.

[0038] When the treatment method of the present invention includes a reducing agent addition step, the reducing agent addition step preferably involves adjusting and adding the specific reducing agent in an amount based on the COD value of the wastewater obtained in the COD measurement step. By adjusting the amount of reducing agent added to the cyanide-containing wastewater using the COD value obtained in the COD measurement step as an indicator, treated water with reduced cyanide compounds can be obtained without increasing the COD.

[0039] When the treatment method of the present invention includes a reducing agent addition step, the reducing agent addition step may be a single step or may be divided into multiple steps. Furthermore, the reducing agent addition step in the treatment method of the present invention may be installed at a location appropriate for the purpose of using the reducing agent, and may be the same step as or different from other chemical addition steps. For example, the reducing agent may be added in the same process as the hydrogen peroxide addition step, or they may be added in different processes. Furthermore, a reducing agent may also be added when a metal compound is added to cyanide-containing wastewater, but the metal compound and the reducing agent may be added in the same process, or they may be added in different processes. In the treatment method of the present invention, as described above, the reducing agent addition step may be a step in which a compound in which all sulfur atoms have covalent bonds to oxygen is added as a reducing agent, and is preferably a step in which at least one reducing agent selected from the group consisting of sulfites, hydrogen sulfites, and hyposulfites is added. Furthermore, when there are multiple reducing agent addition steps, the same type of the specific reducing agent may be used, or different types of the specific reducing agents may be used, but it is preferable to use the same type of the specific reducing agent, because this makes it easier to adjust the amount of reducing agent added based on the COD value of the cyanide-containing wastewater obtained in the COD measurement step.

[0040] In addition, when the treatment method of the present invention has a plurality of reducing agent addition steps, the COD measurement step is preferably provided after the most downstream reducing agent addition step, because this allows appropriate confirmation of the effect of the reducing agent added in the reducing agent addition step on the COD value of the target wastewater.

[0041] Furthermore, the treatment method of the present invention preferably further comprises a cyanide concentration measurement step of measuring the cyanide concentration in the cyanide-containing wastewater after the hydrogen peroxide addition step. The cyanide concentration can be measured using a known method, and in one embodiment, it can be measured using an analytical method in accordance with JIS K0102, Section 38. Furthermore, in the treatment method of the present invention, the hydrogen peroxide addition step preferably involves adjusting the amount of hydrogen peroxide to be added based on the cyanide concentration in the cyanide-containing wastewater obtained in the cyanide concentration measurement step. The cyanide concentration measuring step in the treatment method of the present invention can be installed regardless of the position of the hydrogen peroxide adding step of adding hydrogen peroxide to cyanide-containing wastewater, and the treatment stage of the cyanide compounds in the cyanide-containing wastewater to be measured in the cyanide concentration measuring step is not particularly limited. That is, the cyanide concentration in the cyanide-containing wastewater in the cyanide concentration measuring step may be either not reduced or reduced, or may be wastewater in which the cyanide compounds have been treated to a cyanide concentration equal to or lower than the lower analytical limit.

[0042] In the treatment method of the present invention, it is more preferable that the hydrogen peroxide addition step adjusts and adds the amount of hydrogen peroxide based on the COD value of the cyanide-containing wastewater obtained in the COD measurement step and the cyanide concentration of the cyanide-containing wastewater obtained in the cyanide concentration measurement step. By adjusting the amount of hydrogen peroxide to be added to the cyanide-containing wastewater using the COD value of the cyanide-containing wastewater obtained in the COD measurement step and the cyanide concentration of the cyanide-containing wastewater obtained in the cyanide concentration measurement step as indicators, it is possible to obtain treated water in which the cyanide compounds have been sufficiently reduced while suppressing an increase in COD.

[0043] When a metal compound is further added to the cyanide-containing wastewater, the treatment method of the present invention preferably further comprises a metal scavenger addition step of adding a metal scavenger to the cyanide-containing wastewater after the hydrogen peroxide addition step and after the metal compound has been added, because this allows for a high level of treatment of the metal compound added to reduce the cyanide compound in the cyanide-containing wastewater.

[0044] Examples of the metal scavenger include dithiocarbamates and compounds having a dithiocarbamic acid group, ferrous salts, ferric salts, dimethyldithiocarbamate, diethyldithiocarbamate, dipropyldithiocarbamate, dibutyldithiocarbamate, piperazinebisdithiocarbamate, tetraethylenepentaminedithiocarbamate, polymeric compounds containing polyalkylene polyamines having a molecular weight of 500 to 100,000 and modified with 5 to 80 mol % of dithiocarbamic acid relative to the polyalkylene (having 2 to 4 carbon atoms) polyamine, and polymeric compounds containing polyalkylene polyimines having a molecular weight of 500 to 1,000,000 and modified with 2 to 80 mol % of dithiocarbamic acid relative to the polyalkylene (having 2 to 4 carbon atoms) polyimines. Among these, dithiocarbamates, compounds having a dithiocarbamic acid group, ferrous salts, and ferric salts are preferred in terms of the effect of removing cyanide compounds and the cost of treating cyanide-containing wastewater.

[0045] (Order of adding each drug) In the treatment method of the present invention, the cyanide-containing wastewater may be treated in any manner as long as it includes a hydrogen peroxide addition step, and preferably further includes a metal compound addition step. When hydrogen peroxide and a metal compound are added to the cyanide-containing wastewater, the order of the hydrogen peroxide addition step and the metal compound addition step is not particularly limited, and the hydrogen peroxide addition step and the metal compound addition step may be the same step, or the metal compound addition step may be provided after the hydrogen peroxide addition step, or the steps may be provided in the reverse order. Furthermore, when the present invention includes a reducing agent addition step, the order of the hydrogen peroxide addition step and the reducing agent addition step is not limited. For example, the hydrogen peroxide addition step and the reducing agent addition step may be performed in the same step to the cyanide-containing wastewater, or the reducing agent addition step may be performed after the hydrogen peroxide addition step, or the steps may be performed in the reverse order. Furthermore, in the treatment method of the present invention, for example, when hydrogen peroxide, a metal compound, and a reducing agent are added to cyanide-containing wastewater, the order of the hydrogen peroxide addition step, metal compound addition step, and reducing agent addition step is not particularly limited, and the method may be one in which (a) hydrogen peroxide and a metal compound are added in any order as described above, followed by a reducing agent addition step, (b) hydrogen peroxide and a metal compound are added after the reducing agent addition step, without any order being limited as described above, (c) hydrogen peroxide, a reducing agent, and a metal compound are added separately in this order or in the reverse order, or (d) a reducing agent, hydrogen peroxide, and a metal compound are added simultaneously. When the treatment method of the present invention further includes a metal scavenger addition step of adding a metal scavenger, the metal scavenger addition step is preferably carried out after the hydrogen peroxide addition step, the metal compound addition step, and the reducing agent addition step, i.e., the metal scavenger is added in a state where the hydrogen peroxide, the metal compound, and the reducing agent are contained in the wastewater to be treated.

[0046] In the method for treating cyanide-containing wastewater of the present invention, known agents such as antifoaming agents, polymer flocculants, rust inhibitors, corrosion inhibitors, scale dispersants, slime control agents, etc. may be used in combination, as long as they do not impair the effects of the present invention. For example, cationic polymer compounds and the like can be used as flocculants.

[0047] (Cyanide-containing wastewater after treatment) According to the treatment method of the present invention, the cyanide concentration of cyanide-containing wastewater containing cyanide compounds before treatment can be sufficiently reduced, and an increase in COD can be suppressed. Furthermore, according to the treatment method of the present invention, the cyanide concentration of cyanide-containing wastewater containing cyanide compounds before treatment can be sufficiently reduced to less than the general wastewater standard concentration (1 mgCN / L), and the COD can be sufficiently reduced to less than the general wastewater standard concentration (160 mg / L (daily average 120 mg / L)). Therefore, the treated water after treatment can be directly discharged into sewage or reused without dilution.

[0048] (Treatment agent for cyanide-containing wastewater) According to the present invention, there is provided an agent for treating cyanide-containing wastewater, which comprises, as hydrogen peroxide, an aqueous solution of hydrogen peroxide or an aqueous solution of a hydrogen peroxide-supplying compound, and, as a reducing agent, a compound containing sulfur atoms, all of the sulfur atoms having covalent bonds to oxygen. The present invention also provides an agent for treating cyanide-containing wastewater, which comprises an aqueous solution of hydrogen peroxide or an aqueous solution of a hydrogen peroxide-supplying compound as hydrogen peroxide, an aqueous solution of a metal compound, and a compound as a reducing agent containing sulfur atoms, all of the sulfur atoms having covalent bonds to oxygen. The hydrogen peroxide supply compound, metal compound, and reducing agent used in the agent for treating cyanide-containing wastewater, which is one embodiment of the present invention, can be the same compounds as the hydrogen peroxide supply compound, metal compound, and reducing agent used in the method for treating cyanide-containing wastewater, which is another embodiment of the present invention, and preferred embodiments of these can also be used as preferred embodiments. [Example]

[0049] The present invention will be further described below using examples, but the present invention is not limited to the following examples.

[0050] [Test Example 1: Hydrogen peroxide, a reducing agent, and a copper compound were added in this order to cyanide-containing water] In Test Example 1, cyanide-containing wastewater A (pH 6.2) having the water quality shown in Table 1 was used. Specifically, a cyanide-containing wastewater A having a cyanide concentration of 50 mg / L was prepared using an aqueous potassium cyanide solution.

[0051] [Table 1]

[0052] 550 mL of cyanide-containing wastewater A was dispensed into each of 550 mL sealable plastic containers, and hydrogen peroxide was added to the concentration shown in Table 2. The mixture was stirred at 40°C for 30 minutes, after which a reducing agent shown in Table 2 was added to the concentration shown in Table 2 and stirred for 60 minutes. Next, after confirming that no hydrogen peroxide concentration was detected in the wastewater, a cuprous chloride solution or cupric chloride solution was added as a copper compound to the copper concentration shown in Table 2, and the mixture was stirred for 30 minutes to obtain test water. In preparing the test water, the water was stirred at 200 rpm using a stirring device (Magnetic Stirrer REXIM, product number RS-4AR, manufactured by AS ONE Corporation).

[0053] After adding each additive compound and stirring the test water as described above, the water was filtered using a 5C filter to obtain treated water. The total cyanide concentration (T-CN) and COD in each treated water were measured in accordance with JIS K0102, and the removal effect of cyanide compounds and COD in each treated water was evaluated. In this test, a blank test (Reference Example 1) in which no reducing agent was added was also carried out at the same time. The results obtained are shown in Table 2 together with the compounds added and their amounts.

[0054] [Table 2]

[0055] The treated water from Comparative Example 1, which used sodium thiosulfate Na2S2O3 as a reducing agent, showed a large increase in COD, which significantly exceeded the effluent standard (160 mg / L, daily average 120 mg / L) compared to Reference Example 1, which did not use a reducing agent. Furthermore, the treated water from Comparative Example 2, which used sodium sulfide Na2S as a reducing agent, showed a large increase in T-CN, as well as a large increase in COD, which significantly exceeded the effluent standard (1 mg / L) compared to Reference Example 1, which did not use a reducing agent. On the other hand, the treated water in Examples 1 and 2, which used sodium hydrogen sulfite NaHSO3 as a reducing agent, showed a sufficient cyanide removal effect and fully met the COD wastewater standard (160 mg / L, daily average 120 mg / L). The results in Table 2 show that the use of a specific reducing agent (in which all sulfur atoms have covalent bonds with oxygen) was able to suppress the increase in COD.

[0056] [Test Example 2: Hydrogen peroxide, a reducing agent, and a copper compound were added in this order to cyanide-containing water] In Test Example 2, cyanide-containing wastewater B (pH 6.2) was used, which had the water quality shown in Table 3. Specifically, a potassium cyanide aqueous solution and a potassium ferrocyanide aqueous solution were added to wastewater from a certain steel mill to prepare cyanide-containing wastewater B with a cyanide concentration of 25 mg / L.

[0057] [Table 3]

[0058] Using cyanide-containing wastewater B, the same test as in Test Example 1 was carried out, except that the concentrations of each additive listed in Table 4 were adjusted to the concentrations shown in Table 4, to evaluate the effect of removing cyanide compounds and COD in each treated water. The results obtained are shown in Table 4, along with the added compounds and their amounts.

[0059] [Table 4]

[0060] The treated water from Comparative Example 3, which used sodium sulfide (Na2S) as a reducing agent, showed significantly higher COD and T-CN values ​​than Reference Example 2, which did not use any reducing agent, and these values ​​significantly exceeded the effluent standards. On the other hand, the treated water from Example 3, which used sodium hydrogen sulfite (NaHSO3) as a reducing agent, showed a sufficient cyanide removal effect and fully met the effluent COD standard (160 mg / L, daily average 120 mg / L). The results in Table 4 show that the use of a specific reducing agent (in which all sulfur atoms are covalently bonded to oxygen) was able to suppress the increase in COD.

[0061] [Test Example 3: Hydrogen peroxide, copper compound, and reducing agent added in this order to cyanide-containing water] In Test Example 3, cyanide-containing wastewater C (pH 6.2) having the water quality shown in Table 5 was used. Specifically, a cyanide-containing wastewater C with a cyanide concentration of 50 mg / L was prepared using an aqueous potassium cyanide solution.

[0062] [Table 5]

[0063] 550 mL of cyanide-containing wastewater C was dispensed into each of 550 mL sealable plastic containers, and hydrogen peroxide was added to the solution to the concentration shown in Table 6. After stirring at 40°C for 30 minutes, a cuprous compound was added as a copper compound to the concentration shown in Table 6 and stirred for 30 minutes. Next, a reducing agent shown in Table 6 was added to the solution to the concentration shown in Table 6 and stirred for 60 minutes to obtain test water. In preparing the test water, the water was stirred at 200 rpm using a stirring device (Magnetic Stirrer REXIM, product number RS-4AR, manufactured by AS ONE Corporation).

[0064] To each of the obtained test waters, a compound having a dithiocarbamic acid group was further added as a heavy metal scavenger at 300 mg / L relative to the test water, and the mixture was stirred for 15 minutes to obtain treated water.

[0065] The total cyanide concentration (T-CN) and COD in each treated water were measured in accordance with JIS K0102, and the removal effect of cyanide compounds and COD in each treated water was evaluated. In this test, a blank test (Reference Example 3) in which no reducing agent was added was also carried out at the same time. The results obtained are shown in Table 6 together with the compounds added and their amounts added.

[0066] [Table 6]

[0067] The COD of the treated water from Comparative Examples 4 and 5, which used sodium thiosulfate Na2S2O3 and sodium sulfide Na2S as reducing agents, significantly exceeded the effluent standard (160 mg / L, daily average 120 mg / L) and increased significantly compared to Reference Example 3, which did not use any reducing agent. On the other hand, the treated water in Examples 4 to 6, which used sodium hydrogen sulfite NaHSO3 and sodium hyposulfite Na2S2O4 as reducing agents, showed sufficient cyanide removal effect and fully met the COD wastewater standard (160 mg / L, daily average 120 mg / L). The results in Table 6 confirm that by using a specific reducing agent (in which all sulfur atoms have covalent bonds with oxygen), it is possible to achieve both sufficient cyanide removal and suppression of an increase in COD.

Claims

1. A method for treating cyanide-containing wastewater, comprising: The wastewater contains a reducing agent, and / or the process includes a reducing agent addition step of adding a reducing agent to the wastewater; Furthermore, a hydrogen peroxide addition step of adding hydrogen peroxide to the wastewater; a COD measurement step of measuring the COD in the wastewater after the hydrogen peroxide addition step, A method for treating cyanide-containing wastewater, wherein the reducing agent is a compound containing sulfur atoms, all of the sulfur atoms having covalent bonds with oxygen.

2. 2. The treatment method according to claim 1, further comprising a metal compound addition step of adding a metal compound to the cyanide-containing wastewater.

3. 3. The method according to claim 2, wherein the metal compound is at least one selected from the group consisting of copper compounds, manganese compounds, iron compounds, and zinc compounds.

4. 4. The treatment method according to claim 1, wherein the reducing agent is at least one selected from the group consisting of sulfites, hydrogen sulfites, and hyposulfites.

5. 4. The treatment method according to claim 1, 2 or 3, wherein the reducing agent addition step adjusts the amount of the reducing agent to be added based on the COD value of the wastewater obtained in the COD measurement step.

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