Treatment method for cyanide-containing wastewater

Abstract

The present invention aims to provide a method for treating cyanide-containing wastewater that reduces cyanide compounds in the wastewater and suppresses the increase in COD when a reducing agent is used in the treatment of cyanide-containing wastewater to which hydrogen peroxide is added. [Solution] A method for treating cyanide-containing wastewater, wherein hydrogen peroxide is added to the cyanide-containing wastewater, a reducing agent is added to the cyanide-containing wastewater, the oxidation-reduction potential of the wastewater to which the reducing agent has been added is -700 - 59 × (pH measurement value - 7) mV or higher, and the reducing agent is a compound containing sulfur atoms, all of which have covalent bonds with oxygen.

Description

Technical Field

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

Background Art

[0002] Since cyanide has a strong adverse effect on ecosystems, cyanide-containing wastewater (hereinafter also referred to as "cyanide wastewater") cannot be directly discharged into the natural environment. Effluent standards for cyanide have been established, and cyanide removal treatment must be performed to meet this standard (1 mg / l or less), and only detoxified wastewater can be discharged into sewers and the like. In addition, part of the cyanide contained in wastewater diffuses to the surroundings as hydrogen cyanide gas, which causes a problem of significantly impairing the working environment. The Industrial Safety and Health Act stipulates that the working environment concentration of hydrogen cyanide shall be 3 ppm or less. Depending on the origin of the wastewater, cyanide is present in wastewater in three forms, with varying contents: persistent cyanide complexes and their ions, easily decomposable cyanide complexes and their ions, and cyanide ions (free cyanide).

[0003] Various methods have been conventionally proposed and put into practical use as cyanide removal treatments for cyanide-containing wastewater, but all have their respective advantages and disadvantages, and are used properly according to the conditions of the wastewater. For example, oxidative decomposition methods such as (1) the alkaline chlorination method, in which cyanide-containing wastewater is made 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 an insoluble electrode to carry out the oxidation reaction; (4) the Prussian Blue method, in which iron ions are supplied to cyanide-containing wastewater, for example, by adding ferrous sulfate to generate sparingly soluble ferri / ferrocyanides, which are then precipitated and removed; and (5) reduction with zinc chloride. These methods include (6) the zinc white method, which involves adding an agent and precipitating and removing the resulting insoluble complex; (7) the reduced copper salt method, which involves adding a copper salt and a reducing agent and precipitating and removing the resulting insoluble complex; (8) the biological treatment method, which involves using microorganisms (cyanide-degrading bacteria) adapted to cyanide to decompose cyanide; (9) the thermal hydrolysis method, which involves maintaining cyanide-containing wastewater at a high temperature to hydrolyze cyanide compounds into ammonia and formic acid, and precipitating coexisting heavy metals as elements or oxides; and (10) the hot water reaction method, which involves oxidative decomposition of organic pollutants in addition to cyanide decomposition. Therefore, various chemicals were used to remove cyanide from cyanide-containing wastewater, depending on the condition of the wastewater.

[0004] Regarding the treatment methods for removing cyanide from cyanide-containing wastewater as described above, the following disclosures exist 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 and reacted, then a copper salt is added and reacted, and then the sparingly soluble cyanide compounds generated in the reaction solution are separated and removed by solid-liquid separation of the reaction solution. The reducing agent is added to the cyanide-containing wastewater for the purpose of making iron cyano complexes contained in the wastewater sparingly soluble together with the copper salt, and suitable reducing agents include thiosulfates, sulfites, bisulfites, ferrous chloride, and alkali metal sulfides such as sodium sulfide and sodium tetrasulfide. Patent Document 2 describes a treatment method that includes a step of adding a divalent copper salt and a reducing agent to cyanide-containing wastewater to generate a sparingly soluble salt and then separating it. These documents disclose that reducing agents are added to cyanide-containing wastewater along with other chemicals such as copper salts, and are used for the purpose of generating sparingly soluble salts such as sparingly soluble cyanide compounds.

[0005] Furthermore, Patent Document 3 discloses a method for treating cyanide-containing wastewater by coexisting a copper compound and hydrogen peroxide in the wastewater. This method can sufficiently reduce the cyanide concentration in wastewater containing cyanide complexes without using a reducing agent. Although Patent Document 3 also describes the use of a reducing agent, the reducing agent is added to the cyanide-containing wastewater together with a cupric compound, and is added for the purpose of reducing the cupric compound in the wastewater and generating a cuprous ion supply compound. The use of such a reducing agent is for the purpose of improving the cyanide compound treatment effect.

[0006] Furthermore, Patent Document 4 describes a method for removing cyanide from cyanide-containing wastewater by using chlorine dioxide in combination with one or more metal compounds selected from manganese compounds, iron compounds, zinc compounds, and copper compounds. It discloses that, as the iron compound, an iron compound capable of forming trivalent iron ions in water is added to the cyanide-containing wastewater together with a reducing agent, and that, as the copper compound, a cupric compound is added to the cyanide-containing wastewater together with a reducing agent. Thus, the reducing agent is added with the aim of reducing the iron compound or cupric compound added together in the wastewater to generate a divalent iron ion supply compound or a cuprous ion supply compound. The use of such a reducing agent is for the purpose of improving the effectiveness of cyanide compound treatment. Furthermore, Patent Document 4 discloses that by using a reducing agent in addition to chlorine dioxide and the above-mentioned metal compound in cyanide-containing wastewater, a COD (chemical oxygen demand) removal effect can be obtained from the wastewater.

[0007] As described above, there are several methods for adding reducing agents in the treatment of cyanide-containing wastewater, but in all cases, the reducing agent is added for the purpose of improving the effectiveness of cyanide compound treatment (for example, Patent Documents 1 to 4). In addition, some methods for adding reducing agents for the purpose of removing COD from wastewater have been disclosed (Patent Document 4), but COD (Chemical Oxygen Demand) represents the amount of oxygen consumed when organic matter is oxidized and decomposed using an oxidizing agent, and the reducing agent itself can be a cause of increasing the COD in wastewater. Therefore, the COD removal effect by the reducing agent disclosed in Patent Document 4 is an effect obtained from the entire invention described in Patent Document 4, and a similar effect cannot be obtained in other methods for treating cyanide-containing wastewater.

[0008] Furthermore, in the treatment of cyanide-containing wastewater, reducing agents are used not only to improve the treatment effectiveness of cyanide compounds as described above, but also, for example, to remove oxidizing agents. There are various reasons for removing oxidizing agents, such as eliminating concerns about corrosion caused by oxidizing agents or reactivity in subsequent processes, and meeting regulations regarding residual oxidizing agents in treated water before discharge. Reducing agents may be used for these purposes in wastewater treated with cyanide compounds, and the chemicals added for cyanide compound treatment and the reducing agents may coexist in the wastewater. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Patent No. 6474472 [Patent Document 2] Japanese Patent Publication No. 2008-36608 [Patent Document 3] Patent No. 6145682 [Patent Document 4] Japanese Patent Publication No. 2021-53620 [Overview of the Initiative] [Problems that the invention aims to solve]

[0010] As mentioned above, reducing agents are known to be used for various purposes in the treatment of cyanide-containing wastewater. However, the inventors have found that when hydrogen peroxide is used in the treatment of cyanide-containing wastewater, the combined use of a reducing agent significantly increases the COD in the wastewater. Therefore, when hydrogen peroxide is used in the treatment of cyanide-containing wastewater, if a reducing agent is also used, it becomes necessary to treat the treated water to a COD level below the wastewater discharge standard, which sometimes necessitates the addition of further chemicals, dilution, or delayed discharge. Since such additional treatment of treated water is time-consuming, it forces factories discharging cyanide-containing wastewater to reduce their operational load, leading to a decrease in factory utilization rates.

[0011] Thus, in wastewater treatment methods, it is required not only to treat specific components contained in the wastewater, but also that the treated water obtained through the treatment method meets the discharge standards. Among the discharge standards, COD in particular is a value that represents the amount of oxidizing agent required for a chemical oxidation reaction, and therefore, the value can increase due to chemicals used to reduce other control items listed in the discharge standards to below the standard value, making it difficult to meet the discharge standards with the same treatment as other control items.

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

[0013] The inventors noticed that the COD of treated water obtained by a method of treating cyanide-containing wastewater using hydrogen peroxide did not meet the wastewater discharge standards. Focusing on the COD in the treated water, they diligently investigated methods for treating cyanide-containing wastewater without causing a significant increase in COD. As a result, they found that reducing agents used in the treatment of cyanide-containing wastewater using hydrogen peroxide, for example, to improve the treatment effect of cyanide compounds in the wastewater, significantly increase the COD in the treated water. Furthermore, they discovered that by adding a specific reducing agent to the treatment of cyanide-containing wastewater using hydrogen peroxide so that the oxidation-reduction potential (ORP) of the wastewater exceeds a certain value, the amount of cyanide compounds in the cyanide-containing wastewater after the addition of the reducing agent can be effectively reduced, and the increase in COD can be significantly suppressed. This led to the completion of the present invention.

[0014] In other words, the present invention relates to the following method for treating cyanide-containing wastewater, although it is not limited thereto.

[0015] [1] A method for treating cyanide-containing wastewater, wherein hydrogen peroxide is added to the cyanide-containing wastewater, a reducing agent is added to the cyanide-containing wastewater, the oxidation-reduction potential of the wastewater to which the reducing agent has been added is -700 - 59 × (pH measurement value - 7) mV or higher, and the reducing agent is a compound containing sulfur atoms, all of which are covalently bonded to oxygen. [2] The treatment method according to [1] above, comprising an ORP measurement step of measuring the oxidation-reduction potential (ORP) of the wastewater to which the reducing agent has been added. [3] The treatment method described in [1] or [2] above, wherein the cyanide-containing wastewater is further treated by adding a metal compound. [4] The treatment method according to [3] above, wherein the metal compound is at least one selected from the group consisting of copper compounds, manganese compounds, iron compounds, and zinc compounds. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide a method for treating cyanide-containing wastewater, which reduces cyanide compounds in the cyanide-containing wastewater and suppresses an increase in COD when a reducing agent is used in the treatment of cyanide-containing wastewater to which hydrogen peroxide is added. [Mode for Carrying Out the Invention]

[0017] The present invention relates to a method for treating cyanide-containing wastewater, wherein hydrogen peroxide is added to the cyanide-containing wastewater, a reducing agent is added to the cyanide-containing wastewater, the oxidation-reduction potential of the wastewater to which the reducing agent is added is -700-59×(measured pH value-7) mV or more, and the reducing agent is a compound that contains sulfur atoms, all of which have covalent bonds with oxygen.

[0018] From the reaction principle, it is known that the presence of the reducing agent itself has a positive effect on the COD in wastewater, but during the investigation of the method for treating cyanide-containing wastewater, it was confirmed that the COD value in the cyanide-containing wastewater is not a value derived only from the inherent reducibility of the reducing agent, but is affected by the chemicals added to the cyanide-containing wastewater. Although the action mechanism of the chemicals (i.e., hydrogen peroxide and reducing agent) used in the method for treating cyanide-containing wastewater of the present invention in wastewater is not clear, the addition of the specific reducing agent to the cyanide-containing wastewater to which hydrogen peroxide is added is adjusted based on the oxidation-reduction potential (ORP) in the wastewater, and the oxidation-reduction potential of the wastewater to which the reducing agent is added is -700-59×(measured pH value-7) mV or more, whereby in the treatment of cyanide-containing wastewater, it is considered that the total cyanide concentration is reduced to an extremely low level, the production of by-products contributing to the increase of COD is suppressed, and the generation of high-concentration COD is inhibited.

[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), cyano complex ions, etc. discharged from ironworks, chemical plants, plating plants, coke manufacturing 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 apparatuses. In particular, the method for treating cyanide-containing wastewater of the present invention is suitable for treating cyanide-containing wastewater discharged from ironworks, chemical plants, coke manufacturing plants, and metal surface treatment plants. In the present specification, the simple reference to "cyanide compound" is a general term for metal cyanide compounds, cyanide ions, metal cyano complexes, cyano complex ions, and the like 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, and examples thereof include cyanide-containing wastewater having a total cyanide concentration of about 2 to 500 mg / L. The cyanide-containing wastewater preferably has a total cyanide concentration of 2 to 100 mg / L, and more preferably a total cyanide concentration of 2 to 50 mg / L. For example, as the cyanide-containing wastewater to be treated in the present invention, blast furnace dust collection water is preferred, which may be blast furnace dust collection water before solid-liquid separation or after solid-liquid separation, and more preferably is blast furnace dust collection water after solid-liquid separation. The cyanide-containing wastewater to be treated in the present invention is not limited to the above-mentioned blast furnace dust collection water, and may be any wastewater containing cyanide, and any cyanide-containing wastewater before or after solid-liquid separation can be targeted. The cyanide-containing wastewater to be treated in the present invention is preferably cyanide-containing wastewater after solid-liquid separation.

[0021] (Hydrogen peroxide) Examples of the hydrogen peroxide used in the present invention include aqueous hydrogen peroxide solutions with a concentration of 3 to 60% that are mainly commercially available for industrial use. It is also possible to use hydrogen peroxide generated from a hydrogen peroxide supply compound (also referred to as "hydrogen peroxide generating agent"), or hydrogen peroxide generated by electrolysis of service water or an alkali solution, etc. Examples of hydrogen peroxide-releasing compounds include inorganic peracids such as percarbonate, perboric acid, and peroxysulfuric acid, which can release hydrogen peroxide in water, organic peracids such as peracetic acid, and their salts. Examples of these salts include sodium percarbonate and sodium perborate. The hydrogen peroxide and hydrogen peroxide supply compound described above may be diluted or dissolved in water to achieve the desired hydrogen peroxide concentration before use.

[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 types and concentrations of other metal ions contained in the cyanide-containing wastewater. Therefore, it should be determined appropriately according to these conditions. Specifically, the cyanide concentration of the cyanide-containing wastewater should be measured in advance (from immediately before treatment to about 3 hours before treatment), and the amount to be added should be determined based on these measured values. For example, the amount of hydrogen peroxide added may be determined so that the hydrogen peroxide concentration in the cyanide-containing wastewater is preferably 2.5 to 1300 mg / L, and more preferably 5 to 1300 mg / L.

[0023] (Addition of reducing agent) The present invention relates to a method for treating cyanide-containing wastewater, in which a reducing agent is added to the cyanide-containing wastewater. The addition of the reducing agent may occur before or after the addition of hydrogen peroxide to the cyanide-containing wastewater, or at the same location.

[0024] (Reducing agent) The reducing agent used in the present invention is a compound containing sulfur atoms, all of which are covalently bonded to oxygen. Specifically, it is preferably at least one selected from the group consisting of sulfites, bisulfites, and hyposulfites, and more preferably at least one selected from the group consisting of sodium sulfite, sodium bisulfite, and sodium hyposulfite.

[0025] Here, the quantitative change that pH has on ORP can be derived from thermodynamic theory using the following equation (Equation 1). ΔEh(mV) = -59 × ΔpH (ΔpH = pH measurement value - 7) (Equation 1) In the present invention, the addition of a reducing agent to wastewater is sufficient if the reducing agent is added so that the oxidation-reduction potential (ORP) of the cyanide-containing wastewater to which the reducing agent has been added is -700 - 59 × (pH measurement value - 7) mV or higher, using the above formula (1). Preferably, the amount of reducing agent added is controlled so that the oxidation-reduction potential (ORP) of the cyanide-containing wastewater to which the reducing agent has been added is -700 - 59 × (pH measurement value - 7) mV or higher, using the above formula (1). For example, if the pH of the cyanide-containing wastewater to which the reducing agent has been added is 7, the reducing agent is added so that the oxidation-reduction potential (ORP) of the cyanide-containing wastewater to which the reducing agent has been added is -700 mV or higher. Also, for example, if the pH of the cyanide-containing wastewater to which the reducing agent has been added is 8, the reducing agent is added so that the oxidation-reduction potential (ORP) of the cyanide-containing wastewater to which the reducing agent has been added is -759 (i.e., -700 - 59) mV or higher. The daily average COD standard for wastewater is 120 mg / L. However, if the oxidation-reduction potential (ORP) of cyanide-containing wastewater to which a reducing agent has been added falls below -700 mV (pH 7), it may not meet the COD standard.

[0026] In the present invention, the oxidation-reduction potential (ORP) of cyanide-containing wastewater to which the reducing agent has been added should be -700 - 59 × (pH measurement value - 7) mV or higher, and preferably -650 - 59 × (pH measurement value - 7) mV or higher. Furthermore, the oxidation-reduction potential (ORP) of cyanide-containing wastewater to which the reducing agent has been added is preferably 230-59 × (pH measurement value - 7) mV or less, and more preferably 200-59 × (pH measurement value - 7) mV or less. The oxidation-reduction potential (ORP) of cyanide-containing wastewater to which the reducing agent has been added fluctuates gradually after the reducing agent is added, but the oxidation-reduction potential (ORP) of the treated water of cyanide-containing wastewater obtained by the present invention is preferably 230-59 × (pH measurement value - 7) mV or less, and more preferably 200-59 × (pH measurement value - 7) mV or less.

[0027] In the present invention, the amount (concentration) of the reducing agent added to cyanide-containing wastewater is preferably 1.0 to 5000 mg / L, more preferably 1.0 to 2000 mg / L, and even more preferably 3.0 to 1000 mg / L. If the amount (concentration) of the reducing agent added to 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, even if the amount of the reducing agent added is adjusted based on the oxidation-reduction potential (ORP) of the cyanide-containing wastewater to which the reducing agent has been added, the COD reduction suppression effect may be low.

[0028] The present invention's method for treating cyanide-containing wastewater preferably includes an ORP measurement step for measuring the oxidation-reduction potential (ORP) of cyanide-containing wastewater to which a reducing agent has been added. The ORP measurement step can be performed either in the same step as the reducing agent addition step, or in a subsequent step, and is preferably located after the reducing agent addition step. Furthermore, the ORP measurement step is preferably performed at one or more arbitrary locations. The ORP measurement in the ORP measurement step can be performed using known methods and equipment; for example, it can be measured using a portable ORP meter. In the present invention, the ORP measurement step is preferably installed from immediately after the reducing agent is added to the cyanide-containing wastewater in the reducing agent addition step until 120 minutes later, more preferably until 90 minutes later, even more preferably until 60 minutes later, and even more preferably until 30 minutes later. As described above, the oxidation-reduction potential (ORP) of cyanide-containing wastewater to which the reducing agent has been added fluctuates gradually after the addition of the reducing agent, but by measuring the oxidation-reduction potential (ORP) during the period from the addition of the reducing agent to 120 minutes later, the rise in COD after the reducing agent addition step can be suppressed more effectively.

[0029] In the treatment method of the present invention, if the addition of a reducing agent causes a pH change in cyanide-containing wastewater, pH adjustment may be performed. If the treatment method of the present invention includes a reducing agent addition step and an ORP measurement step, the above pH adjustment may be performed before, after, or simultaneously with the reducing agent addition step, and is preferably performed before the ORP measurement step. The above pH adjustment can be performed using a known pH adjusting agent, for example, sodium hydroxide can be used.

[0030] Furthermore, it is preferable that the method for treating cyanide-containing wastewater of the present invention further involves the addition of a metal compound to the cyanide-containing wastewater.

[0031] (metal compound) The metal compound used in the present invention is preferably one or more selected from copper compounds, manganese compounds, iron compounds, and zinc compounds. The following describes each metal compound, but among these metal compounds, at least one selected from the group consisting of copper compounds, manganese compounds, iron compounds, and zinc compounds is preferred in terms of cyanide removal effect, and copper compounds are particularly preferred.

[0032] (copper compound) The copper compound used in the present invention is preferably a cuprous compound capable of forming a monovalent copper ion and / or a cupric compound capable of forming a divalent copper ion, and may be either an organocopper compound or an inorganic copper compound. Examples of organocopper compounds include cupric compounds selected from cupric acetate, cupric benzoate, cupric citrate, copper naphthenate, and cupric oleate.

[0033] Examples of inorganic copper compounds include cuprous compounds that can form monovalent copper ions in water, selected from cuprous chloride, cuprous fluoride, cuprous bromide, cuprous iodide, cuprous nitrate, cuprous sulfate, and cuprous acetate, and cupric compounds that can form divalent copper ions in water, selected from cupric chloride, cupric fluoride, cupric bromide, cupric iodide, cupric nitrate, cupric sulfate, and cupric acetate. Since organocopper compounds can increase the COD in cyanide-containing wastewater after treatment, inorganic copper compounds are preferred among the above copper compounds, inorganic cuprous compounds are more preferred in terms of cyanide removal effect and treatment cost of cyanide-containing wastewater, cuprous chloride and cuprous sulfate are even more preferred, and cuprous chloride is particularly preferred.

[0034] The above copper compound may be treated with a metal adsorbent to achieve the desired copper equivalent concentration when added to cyanide-containing wastewater. Alternatively, it may be diluted or dissolved in water such as industrial water before use. Examples of metal adsorbents include liquid chelating agents. Furthermore, when the cuprous compound is a cuprous salt, it is preferable to use a cuprous salt solution with hydrogen chloride solution, alkali metal halide aqueous solution, or ethanol as the solvent, from the viewpoint of the stability of the cuprous salt. The method of the present invention includes, as the copper compound, a cupric compound added together with a reducing agent to cyanide-containing wastewater, or a cupric compound added to reducing cyanide-containing wastewater, and a cuprous ion supplying compound produced by reducing the cupric compound in the wastewater. The reducing agent mentioned above includes compounds containing sulfur atoms, all of which are covalently bonded to oxygen. It is preferable to use at least one reducing agent selected from the group consisting of sulfites, bisulfites, and hyposulfites.

[0035] In the present invention, the copper compound is preferably added in such a way that the copper ion concentration in cyanide-containing wastewater is approximately 0.1 to 500 mg / L. When the copper ion concentration is less than 0.1 mg / L, the effect of promoting cyanide removal may not be sufficiently obtained. On the other hand, when the copper ion concentration exceeds 500 mg / L, it not only has adverse effects on the environment but is also economically undesirable. Specific copper ion 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 preferred copper ion concentration is 0.1 to 250 mg / L, and more preferably 1 to 150 mg / L.

[0036] (Manganese compounds) The manganese compound used in the present invention is not particularly limited as long as it is soluble in water and capable of forming manganese ions in water, and examples include manganese chloride, manganese sulfate, manganese nitrate, and manganese acetate. Among these, manganese chloride and manganese sulfate are particularly preferred in terms of their effectiveness in removing cyanide compounds, and manganese chloride is particularly preferred in terms of the treatment cost of cyanide-containing wastewater. In this invention, "soluble in water" means that the compound has a solubility of about 1 g or more per 100 g of water.

[0037] (Iron compounds) The iron compounds used in the present invention are not particularly limited as long as they are soluble in water. Examples include ferrous compounds that can form 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 that can form trivalent iron ions in water, such as ferric chloride, ferric fluoride, ferric bromide, ferric iodide, ferric nitrate, ferric sulfate, and ferric acetate. Among these, ferrous chloride and ferrous sulfate are particularly preferred in terms of their effectiveness in removing cyanide compounds, and ferrous chloride is particularly preferred in terms of the treatment cost of cyanide-containing wastewater.

[0038] The method of the present invention includes, as the iron compound, an iron compound capable of forming trivalent iron ions in water, which is added to cyanide-containing wastewater together with a reducing agent, or an iron compound capable of forming trivalent iron ions in water, which is added to reducing cyanide-containing wastewater, and a divalent iron ion supplying compound produced by reducing the iron compound capable of forming trivalent iron ions in the wastewater. The reducing agent mentioned above includes compounds containing sulfur atoms, all of which are covalently bonded to oxygen. It is preferable to use at least one reducing agent selected from the group consisting of sulfites, bisulfites, and hyposulfites.

[0039] (Zinc compounds) The zinc compound used in the present invention is not particularly limited as long as it is soluble in water and capable of forming zinc ions in water. Examples include zinc chloride, zinc oxide, zinc hydroxide, zinc carbonate, zinc peroxide, zinc sulfate, and zinc nitrate. Among these, zinc chloride and zinc sulfate are particularly preferred in terms of their effectiveness in removing cyanide compounds, and zinc chloride is particularly preferred in terms of the treatment cost of cyanide-containing wastewater.

[0040] Furthermore, the treatment method of the present invention preferably further includes a cyanide concentration measurement step for measuring the cyanide concentration in cyanide-containing wastewater to which a reducing agent has been added. The cyanide concentration can be measured using a known method, and in one embodiment, it can be measured using an analytical method compliant with JIS K0102 Section 38. Moreover, in the treatment method of the present invention, it is preferable that the amount of reducing agent added is adjusted and added based on the cyanide concentration in the cyanide-containing wastewater obtained in the cyanide concentration measurement step. The cyanide concentration measurement step in the treatment method of the present invention can be installed regardless of the location of the reducing agent addition step in which a reducing agent is added to the cyanide-containing wastewater, and the treatment stage of the cyanide compounds in the cyanide-containing wastewater measured in the cyanide concentration measurement step is not particularly limited. That is, the cyanide concentration in the cyanide-containing wastewater measured in the cyanide concentration measurement step may be that of wastewater that has not been reduced, that has been reduced, or that the cyanide compounds have been treated to a level below the analytical lower limit of cyanide concentration.

[0041] In the treatment method of the present invention, the addition of a reducing agent to cyanide-containing wastewater is sufficient if the reducing agent is added such that the ORP value of the cyanide-containing wastewater to which the reducing agent has been added is "-700 - 59 × (pH measurement value - 7) mV or higher". Furthermore, it is more preferable that the amount of reducing agent added is adjusted based on the cyanide concentration in the cyanide-containing wastewater obtained in the cyanide concentration measurement step. By adjusting the amount of reducing agent added to the cyanide-containing wastewater using the ORP value of the cyanide-containing wastewater to which the reducing agent has been added and the cyanide concentration in the cyanide-containing wastewater obtained in the cyanide concentration measurement step as indicators, it is possible to obtain treated water in which cyanide compounds are sufficiently reduced while suppressing the rise in COD.

[0042] In the treatment method of the present invention, if a metal compound is further added to the cyanide-containing wastewater, it is preferable that the metal compound be added after the addition of the reducing agent. Furthermore, if a metal adsorbent is further added to the cyanide-containing wastewater, it is preferable that the metal adsorbent be added after the addition of the metal compound. This is because the metal compound added to reduce the cyanide compounds in the cyanide-containing wastewater can be treated to a high degree. Examples of metal adsorbents include dithiocarbamates and compounds having a dithiocarbamate group, ferrous salts, ferric salts, and the like.

[0043] (Order of adding each drug) In the present invention, the order of addition of the reducing agent is not limited to that of the hydrogen peroxide. For example, when hydrogen peroxide and a reducing agent are added to cyanide-containing wastewater, the order of addition of hydrogen peroxide and the reducing agent is not particularly limited. Hydrogen peroxide and the reducing agent may be added simultaneously, or separately in the order of hydrogen peroxide and the reducing agent, or in the reverse order. Furthermore, in the treatment method of the present invention, if a metal compound is further added, the order in which hydrogen peroxide, reducing agent, and metal compound are added to cyanide-containing wastewater is not particularly limited, and (a) hydrogen peroxide and metal compound may be added in any order, followed by the addition of the reducing agent; (b) hydrogen peroxide and metal compound may be added after the addition of the reducing agent, in any order; (c) hydrogen peroxide, reducing agent, and metal compound may be added separately in either order or in reverse order; or (d) the reducing agent, hydrogen peroxide, and metal compound may be added simultaneously. Furthermore, if a metal adsorbent is to be added, it is preferable that the metal adsorbent be added after hydrogen peroxide, a metal compound, and a reducing agent have been added to the cyanide-containing wastewater.

[0044] In the cyanide-containing wastewater treatment method of the present invention, known agents such as defoamers, polymer flocculants, rust inhibitors, corrosion inhibitors, scale dispersants, and slime control agents may be used in combination, to the extent that they do not hinder the effects of the present invention. For example, cationic polymer compounds can be used as flocculants.

[0045] In the treatment method of the present invention, the addition of the reducing agent may be a single step or may be divided into multiple steps. Furthermore, the addition of the reducing agent in the treatment method of the present invention may be performed at a position appropriate to the purpose of use of the reducing agent, and may be the same step as other chemical addition steps or may be different. For example, when a metal compound is added to cyanide-containing wastewater, a reducing agent may also be added, but the metal compound and the reducing agent may be added in the same step, or they may be added in different steps. In the processing method of the present invention, the addition of a reducing agent may be any step in which a compound having all of its sulfur atoms covalently bonded to oxygen is added as the reducing agent, as described above, and it is more preferable that the reducing agent be at least one selected from the group consisting of sulfites, bisulfites, and hyposulfites. Furthermore, when reducing agents are added at multiple locations, the same type of the above-mentioned specific reducing agent may be used, or different types of the above-mentioned specific reducing agent may be used, but it is preferable to use the same type of the above-mentioned specific reducing agent. This is because it is easier to adjust the amount of reducing agent added based on the ORP value of the cyanide-containing wastewater to which the reducing agent has been added.

[0046] Furthermore, if there are multiple additions of reducing agents in the treatment method of the present invention, it is preferable that the ORP value in the cyanide-containing wastewater to which the reducing agent has been added is the ORP value after the reducing agent addition step located at the furthest downstream.

[0047] The processing method of the present invention may further include a COD measurement step after the addition of a reducing agent. The COD measurement process can be any process that measures COD using a known method, and as one embodiment, COD can be measured using an analytical method or ultraviolet absorption method in accordance with JIS K0102 Section 17. Furthermore, the COD measurement step described above may be provided after the addition of the reducing agent, and if the cyanide-containing wastewater is treated in a single pass, it may be located downstream of the addition of the reducing agent, and if the cyanide-containing wastewater is circulated, it may be measured at any point within the circulation system. The treatment stage of cyanide compounds in the 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 that of wastewater that has not been reduced, that has been reduced, or that has been treated to a level below the analytical lower limit of cyanide concentration.

[0048] In the treatment method of the present invention, the addition of the reducing agent may be adjusted based on the oxidation-reduction potential (ORP) of the cyanide-containing wastewater to which the reducing agent has been added, as well as the cyanide concentration in the wastewater obtained in the cyanide concentration measurement step and / or the COD value in the wastewater obtained in the COD measurement step. By adding the cyanide concentration and / or COD value to the oxidation-reduction potential (ORP) of the wastewater to which the reducing agent has been added as indicators for adjusting the amount (concentration) of the reducing agent to be added to the cyanide-containing wastewater, the amount of reducing agent to be added to the cyanide-containing wastewater can be made even more appropriate.

[0049] If the present invention includes a COD measurement step, the COD value in the wastewater obtained in the COD measurement step should be less than the general wastewater standard of 120 mg / L on a daily average, and preferably 100 mg / L or less.

[0050] (Wastewater containing cyanide after treatment) The treatment method of the present invention can sufficiently reduce the cyanide concentration in cyanide-containing wastewater containing cyanide compounds before treatment, and suppress the increase in COD. Furthermore, the treatment method of the present invention can sufficiently reduce the cyanide concentration in cyanide-containing wastewater containing cyanide compounds before treatment to below the general wastewater standard concentration (1 mg C / L), and sufficiently reduce the COD to below the general wastewater standard concentration (160 mg / L (daily average 120 mg / L)). Therefore, the treated water after treatment can be discharged directly into sewage systems or reused without dilution.

[0051] (Wastewater treatment agent containing cyanide) The present invention provides a treatment agent for cyanide-containing wastewater, characterized by comprising an aqueous solution of hydrogen peroxide or an aqueous solution of a hydrogen peroxide supply compound as hydrogen peroxide, and a compound containing sulfur atoms, all of which are covalently bonded to oxygen, as a reducing agent. Furthermore, the present invention provides a treatment agent for cyanide-containing wastewater, characterized by comprising an aqueous solution of hydrogen peroxide or an aqueous solution of a hydrogen peroxide supply compound as hydrogen peroxide, an aqueous solution of a metal compound, and a compound containing sulfur atoms as a reducing agent, all of which are covalently bonded to oxygen. The hydrogen peroxide supply compound, metal compound, and reducing agent used in the treatment agent for cyanide-containing wastewater, which is one embodiment of the present invention, can be the same compounds as those used in the hydrogen peroxide supply compound, metal compound, and reducing agent used in the treatment method for cyanide-containing wastewater, which is another embodiment of the present invention, and preferred embodiments of these can be used in similarly preferred embodiments. [Examples]

[0052] The present invention will be further described below using examples. However, the present invention is not limited to the following examples.

[0053] [Test Example 1: Hydrogen peroxide, reducing agent, and copper compound are added to cyanide-containing water in that order.] In Test Example 1, cyanide-containing wastewater A (pH 6.2) with the water quality shown in Table 1 was used. Specifically, cyanide-containing wastewater A with a cyanide concentration of 50 mg / L was prepared using an aqueous potassium cyanide solution.

[0054] [Table 1]

[0055] 550 mL of cyanide-containing wastewater A was dispensed into each of 550 mL resealable plastic containers. Hydrogen peroxide was added to the concentrations shown in Table 2, and the mixture was stirred at 40°C for 30 minutes. Then, the reducing agents shown in Table 2 were added to the concentrations shown in Table 2, and the mixture was stirred for 60 minutes. After confirming that no hydrogen peroxide was detected in the wastewater, cuprous compounds were added as copper compounds to the concentrations shown in Table 2, and the mixture was stirred for 30 minutes to obtain each test water. The pH and ORP of each obtained test water were measured. For the preparation of each test water, a stirring device (AS ONE Corporation, Magnetic Stirrer REXIM, Part Number: RS-4AR) was used to stir the water at a rotation speed of 200 rpm.

[0056] As described above, each test water sample was mixed with the added compound and then filtered using a 5C filter to obtain treated water. The total cyanide concentration (T-CN) and COD of each treated water sample were measured in accordance with JIS K0102, and the removal effect of cyanide compounds and COD in each treated water sample was evaluated. In this study, a blank test (Reference Example 1) without the addition of a reducing agent was also conducted simultaneously. The results obtained, along with the added compounds and their amounts, are shown in Table 2.

[0057] [Table 2]

[0058] In Example 1, the treated water to which sodium bisulfite was added as a reducing agent had an ORP of 265 mV, a total cyanide concentration (T-CN concentration) below 0.1 mg-CN / L, demonstrating excellent cyanide removal effectiveness, and a COD value that was almost the same as that of Reference Example 1 (blank), to which only the reducing agent was not added. Therefore, it was confirmed that the amount of cyanide compounds in the treated water was reduced while suppressing the increase in the COD value. It was also confirmed that the ORP in the treated water was -700 - 59 × (5.9 - 7) = -735.1 mV or higher. Furthermore, in Comparative Example 1, where sodium thiosulfate was added as a reducing agent, the treated water had an ORP of -3mV, a total cyanide concentration (T-CN concentration) of 0.28 mg-CN / L, and a COD value of 346 mg / L. These values ​​were higher than the total cyanide concentration (T-CN concentration) of the treated water in Reference Example 1 (blank), where no reducing agent was added, and the COD value was significantly more than 100 times higher. Therefore, a significant increase in COD due to the addition of a reducing agent different from the specific reducing agent was confirmed, and it was confirmed that this significant increase in COD significantly exceeds the COD wastewater standards (maximum: 160 mg / L, daily average: 120 mg / L).

[0059] [Test Example 2: Hydrogen peroxide, copper compound, and reducing agent were added to cyanide-containing water in that order.] In Test Example 2, cyanide-containing wastewater B (pH 6.2) with the water quality shown in Table 3 was used. Specifically, cyanide-containing wastewater B with a cyanide concentration of 50 mg / L was prepared using an aqueous potassium cyanide solution.

[0060] [Table 3]

[0061] 550 mL of cyanide-containing wastewater B was dispensed into each 550 mL resealable plastic container. Hydrogen peroxide was added to the concentrations shown in Table 4 and stirred at 40°C for 30 minutes. Then, a copper compound, either a cuprous compound or a cupric compound, was added to the concentrations shown in Table 4 and stirred for 30 minutes. Next, the reducing agent shown in Table 4 was added to the concentrations shown in Table 4 and stirred for 60 minutes to obtain each test water. The pH and ORP of each test water after the addition of the reducing agent were measured. In Examples 4 and 5, the pH of each test water during the test was adjusted to near neutral using an aqueous sodium hydroxide solution immediately after the addition of the reducing agent, and the ORP of the test water after pH adjustment was measured. For the preparation of each test water, a stirring device (AS ONE Corporation, Magnetic Stirrer REXIM, Part Number: RS-4AR) was used to stir the water at a rotation speed of 200 rpm.

[0062] To each of the obtained test waters, 300 mg / L of "Floclan 7000" was added as a heavy metal adsorbent, and the mixture was stirred for 15 minutes to obtain each of the treated waters.

[0063] The total cyanide concentration (T-CN) and COD of 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. The results obtained, along with the added compounds and their amounts, are shown in Table 4.

[0064] [Table 4]

[0065] In Table 4 above, the reference value for ORP using pH is calculated as "-700 - 59 × (measured pH value - 7)". In Example 2, where sodium bisulfite was added as a reducing agent, the ORP of the test water after the addition of the reducing agent was 250 mV, significantly exceeding the standard value of -499. The total cyanide concentration (T-CN concentration) of the treated water in Example 2 was less than 0.05 mg-CN / L, demonstrating an extremely excellent cyanide removal effect, and the COD value was 5 mg / L. Therefore, it was confirmed that the amount of cyanide compounds in the treated water was reduced while suppressing the increase in the COD value. In Example 3, where sodium hyposulfite was added as a reducing agent, the ORP of the test water after the addition of the reducing agent was -450 mV, which was above the standard value of -482 mV. The total cyanide concentration (T-CN concentration) of the treated water in Example 3 was less than 0.05 mg-CN / L, and the COD value was 70 mg / L. Therefore, the results for both the total cyanide concentration and the COD value were well below the general wastewater standards. Furthermore, in Comparative Example 2, where sodium thiosulfate was added as a reducing agent, the ORP of the test water after the addition of the reducing agent was 130 mV, exceeding the standard value of -741 mV. The total cyanide concentration (T-CN concentration) of the treated water in Comparative Example 2 was less than 0.05 mg-CN / L, and the COD value was 420 mg / L. Therefore, although the total cyanide concentration was well below the general wastewater standard, the COD value was significantly above the general wastewater standard. Thus, a remarkable increase in COD far exceeding the general wastewater standard was confirmed due to the selection of the reducing agent. Furthermore, in Comparative Example 3, where sodium hyposulfite was added as a reducing agent, the ORP of the test water after the addition of the reducing agent was -489 mV, which was below the standard value of -464 mV. The total cyanide concentration (T-CN concentration) of the treated water in Comparative Example 3 was less than 0.05 mg-CN / L, and the COD value was 120 mg / L. Therefore, although the total cyanide concentration was well below the general wastewater standard, the COD value overlapped with the general wastewater standard value (daily average of 120 mg / L). Thus, the increase in COD due to the amount of reducing agent added was confirmed. Furthermore, in Example 4, where sodium bisulfite was added as a reducing agent, similar to Example 2, the pH of the test water was adjusted to 7 by adding an aqueous sodium hydroxide solution. However, the ORP of the test water after the addition of the reducing agent was 85 mV, exceeding the standard value of -700. The total cyanide concentration (T-CN concentration) of the treated water in Example 4 was less than 0.05 mg-CN / L, demonstrating an extremely excellent cyanide removal effect, and the COD value was 27 mg / L. Therefore, it was confirmed that the amount of cyanide compounds in the treated water was reduced while suppressing the increase in the COD value. Furthermore, in Example 5, similar to Example 2, sodium bisulfite was added as a reducing agent. The pH of the test water was adjusted to 7 by adding an aqueous sodium hydroxide solution, and a divalent copper compound was used as the copper compound. However, the ORP of the test water after the addition of the reducing agent was 105 mV, which was above the standard value of -700 mV. The total cyanide concentration (T-CN concentration) of the treated water in Example 5 was less than 0.05 mg-CN / L, demonstrating an extremely excellent cyanide removal effect, and the COD value was 30 mg / L. Therefore, it was confirmed that the cyanide compounds in the treated water were reduced while suppressing the increase in the COD value. The results of Test Example 2 confirmed that in all of the test examples related to Examples 2 to 5, the ORP of the treated water after the addition of the reducing agent exceeded the standard value calculated using pH.

[0066] [Test Example 3: Hydrogen peroxide, copper compound, and reducing agent are added simultaneously to cyanide-containing water] In Test Example 3, cyanide-containing wastewater C (pH 6.2) with the water quality shown in Table 5 was used. Specifically, a cyanide-containing wastewater sample C with a cyanide concentration of 50 mg / L was prepared using an aqueous potassium cyanide solution.

[0067] [Table 5]

[0068] Each 550 mL of cyanide-containing wastewater C was dispensed into a 550 mL resealable plastic container. Hydrogen peroxide, a cuprous or cupric compound as a copper compound, and a reducing agent were added to achieve the concentrations shown in Table 6, and the mixture was stirred at 40°C for 5 minutes to obtain each test water. The pH of each test water was adjusted to near neutral using an aqueous sodium hydroxide solution, and then the ORP was measured. For the preparation of each test water, a stirring device (AS ONE Corporation, Magnetic Stirrer REXIM, Part Number: RS-4AR) was used to stir the water at a rotation speed of 200 rpm.

[0069] After stirring each of the pH-adjusted test waters for another 120 minutes, the waters were filtered using a 5C filter to obtain the treated waters. The total cyanide concentration (T-CN) and COD of 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. The results obtained, along with the added compounds and their amounts, are shown in Table 6. [Table 6] In Table 6 above, the reference value for ORP using pH is calculated as "-700 - 59 × (measured pH value - 7)". In Examples 6 and 7, where sodium bisulfite was added as a reducing agent, the ORP of the test water after the addition of the reducing agent significantly exceeded the standard value. Furthermore, the total cyanide concentration (T-CN concentration) of the treated water in Examples 6 and 7 was less than 0.05 mg-CN / L, demonstrating an extremely excellent cyanide removal effect, and the COD value was 16 mg / L. Therefore, it was confirmed that cyanide compounds in the treated water were reduced while suppressing the increase in COD value.

Claims

1. A method for treating cyanide-containing wastewater, The aforementioned cyanide-containing wastewater is to which hydrogen peroxide is added. A reducing agent is added to the aforementioned cyanide-containing wastewater. The oxidation-reduction potential of the wastewater to which the reducing agent has been added is -700 - 59 × (pH measurement value - 7) mV or higher. A method for treating cyanide-containing wastewater, wherein the reducing agent is a compound containing sulfur atoms, all of which are covalently bonded to oxygen.

2. The treatment method according to claim 1, further comprising an ORP measurement step of measuring the oxidation-reduction potential (ORP) of the wastewater to which the reducing agent has been added.

3. The treatment method according to claim 1 or 2, wherein the cyanide-containing wastewater is further treated by adding a metal compound.

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

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