Treatment method for cyanide-containing wastewater

Abstract

To provide a treatment method for cyanide-containing wastewater capable of reliably removing cyanide from wastewater safely and inexpensively through simple operation by minimizing the amount of chemicals added, reducing generation of chlorine dioxide gas and resulting deterioration of a working environment such as corrosion of equipment.SOLUTION: The above problem is solved by a treatment method for cyanide-containing wastewater, characterized by the use of chlorine dioxide in combination with one or more metal compounds selected from manganese compounds, iron compounds, zinc compounds, and copper compounds to remove cyanide from the wastewater.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a method for treating cyanide-containing wastewater that can suppress the amount of chemicals added as much as possible compared to the prior art, reduce the generation of chlorine dioxide gas and the deterioration of the working environment such as corrosion of equipment caused thereby, and reliably remove cyanide in wastewater safely and at low cost with a simple operation.

Background Art

[0002] Since cyanide has a strong adverse impact on the ecosystem, cyanide-containing wastewater (also referred to as "cyanide wastewater") cannot be directly discharged into nature. Based on the Water Pollution Control Law, drainage standards for cyanide have been established, and cyanide removal treatment must be carried out to meet these standards (1 mg / L or less), and the wastewater can only be discharged into sewage or the like if it has been rendered harmless. In addition, in some regions, additional drainage standards lower than the above drainage standard values have been established by ordinance. There is also a problem that a part of the cyanide contained in the wastewater diffuses into the surroundings as hydrogen cyanide gas, significantly damaging the working environment. The Industrial Safety and Health Law stipulates that the concentration of hydrogen cyanide in the working environment should be 3 ppm or less. Cyanide exists in wastewater in three forms: hardly decomposable cyanide complexes and their ions, easily decomposable cyanide complexes and their ions, and cyanide ions, depending on the origin of the wastewater and with varying contents.

[0003] Conventionally, various methods have been proposed and put into practical use for the removal treatment of cyanide in cyanide-containing wastewater, but each has its advantages and disadvantages and is used appropriately according to the situation of the wastewater. For example, (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 ion supply compounds, such as ferrous sulfate, are added to cyanide-containing wastewater to generate sparingly soluble ferri / ferrocyanides, which are then precipitated and removed; and (5) zinc chloride and a reducing agent (6) Insoluble complex methods such as the zinc white method, which involves adding a zinc oxide and precipitating and removing the resulting insoluble complex; (7) Biological treatment methods in which microorganisms accustomed to cyanide (cyanide-degrading bacteria) decompose cyanide; (8) Thermal hydrolysis methods in which cyanide-containing wastewater is kept at a high temperature to hydrolyze cyanide compounds into ammonia and formic acid, and coexisting heavy metals are precipitated as elements or oxides; and (9) Hot water reactions such as wet oxidation methods that oxidize and decompose organic pollutants in addition to decomposing cyanide.

[0004] Furthermore, Japanese Patent Publication No. 52-123976 (Patent Document 1) discloses a wastewater treatment method in which wastewater containing cyanide compounds and the like, generated from the process of producing coal gas and coke by coal carbonization, is treated with chlorine dioxide.

[0005] However, the above-mentioned prior art requires complicated processes and operations, and may necessitate multiple reaction vessels. Furthermore, depending on the type of wastewater, such as wastewater containing thiocyanate ions or ammonium ions, the cyanide removal effect may not be sufficient, and it may not be possible to bring the cyanide concentration of the treated wastewater down to the discharge standard (1 mg / L or less), making it impossible to discharge the treated wastewater directly into sewage systems. Furthermore, under the Water Pollution Control Act, the effluent standards for hydrogen ion concentration (pH) are set at 5.0 to 9.0 in marine areas and 5.8 to 8.6 outside marine areas. In the prior art described above, if the pH of wastewater is adjusted to be acidic or alkaline, neutralization treatment may be necessary to adjust not only the cyanide concentration but also the pH of the wastewater to within the effluent standard range before discharging it into sewage systems. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 52-123976 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] Therefore, the object of the present invention is to provide a method for treating cyanide-containing wastewater that minimizes the amount of chemicals added compared to conventional methods, reduces the generation of chlorine dioxide gas and the resulting deterioration of the working environment such as corrosion of equipment, and reliably removes cyanide from wastewater safely and inexpensively with simple operation. [Means for solving the problem]

[0008] The inventors of this invention conducted extensive research to solve the above-mentioned problems and, as a result, discovered that by using an effective amount of chlorine dioxide and an effective amount of a specific metal compound in combination with cyanide-containing wastewater, it is possible to reduce the amount of chemicals added compared to conventional methods, minimize the generation of chlorine dioxide gas and the resulting deterioration of the working environment such as corrosion of equipment, and reliably remove cyanide from wastewater safely and inexpensively with simple operation. This led to the completion of the present invention.

[0009] Thus, the present invention provides a method for treating cyanide-containing wastewater, characterized by removing cyanide from the wastewater by using chlorine dioxide and one or more metal compounds selected from manganese compounds, iron compounds, zinc compounds, and copper compounds in combination with the cyanide-containing wastewater. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a method for treating cyanide-containing wastewater that minimizes the amount of chemicals added compared to conventional methods, reduces the generation of chlorine dioxide gas and the resulting deterioration of the working environment such as corrosion of equipment, and reliably removes cyanide from wastewater safely and inexpensively with simple operation. In other words, according to the present invention, cyanide contained in wastewater in various forms can be treated with a significantly reduced amount of chemical additives compared to conventional methods, and with simple operations. Furthermore, by ensuring that the cyanide concentration in the wastewater meets the wastewater discharge regulations (1 mg / L or less), the diffusion of hydrogen cyanide gas into the surrounding area can be suppressed, which is expected to improve the working environment. Furthermore, by using a reducing agent in combination with chlorine dioxide and metal compounds, a COD removal effect from wastewater can also be expected. Therefore, even if wastewater treated by the present invention is released directly into the natural environment, the environmental impact is very small, and the amount of suspended solids (waste) generated after treatment is also reduced. Thus, the present invention is extremely useful in industry.

[0011] Furthermore, the cyanide-containing wastewater treatment method of the present invention exhibits the above effects more effectively when any one of the following conditions is met, and in particular, (7) exhibits an additional COD removal effect. (1) The concentration of chlorine dioxide is at least 0.2 times the molar ratio of the cyanide content in the cyanide-containing wastewater, and the concentration of the metal compound in the cyanide-containing wastewater is 0.1 to 1000 mg / L. (2) The cyanide content and metal compound concentration in the cyanide-containing wastewater are measured in advance, and chlorine dioxide or a compound capable of generating chlorine dioxide in the wastewater, corresponding to the measured cyanide content, and a metal compound, corresponding to the measured metal compound concentration, are added to the wastewater simultaneously or separately. (3) The metal compound is a copper compound. (4) The cyanide-containing wastewater has a pH of 6-9. (5) The cyanide-containing wastewater is wastewater containing at least one of the following: cyanide ions, readily decomposable cyanide complexes, and persistently decomposable cyanide complexes. (6) The cyanide-containing wastewater is wastewater that contains one or more coexisting substances selected from thiocyanate ions and their salts and ammonium ions. (7) Chlorine dioxide and metal compounds are used in combination with a reducing agent. [Modes for carrying out the invention]

[0012] The present invention relates to a method for treating cyanide-containing wastewater, characterized by removing cyanide from the wastewater by using chlorine dioxide and one or more metal compounds selected from manganese compounds, iron compounds, zinc compounds, and copper compounds in combination with the cyanide-containing wastewater.

[0013] In this invention, "using chlorine dioxide and a metal compound in combination with cyanide-containing wastewater" means that chlorine dioxide and a metal compound are present in the cyanide-containing wastewater. This combination may be achieved by adding chlorine dioxide and the metal compound, or by generating them in the cyanide-containing wastewater, as described later. In the case of addition, the order is not particularly limited and may be simultaneous or separate. Furthermore, chlorine dioxide may be added to cyanide-containing wastewater that contains a sufficient concentration of metal ions. Moreover, within the facility that treats cyanide-containing wastewater, chlorine dioxide and the metal compound may be added in the same or separate locations.

[0014] (Chlorine dioxide) The chlorine dioxide used in this invention is an extremely unstable chemical substance, making its storage and transportation very difficult. Therefore, although chlorine dioxide or a compound capable of generating chlorine dioxide in wastewater may be directly added to the wastewater, it is preferable to produce (generate) chlorine dioxide on-site by a known method, or to generate chlorine dioxide by adding a compound capable of generating chlorine dioxide in wastewater to water, and then adjust the concentration to the desired level before use. For example, chlorine dioxide can be produced by the following reaction, and commercially available chlorine dioxide generators (devices) can also be used. (1) Reaction of sodium hypochlorite, hydrochloric acid, and sodium hypochlorite NaOCl+2HCl+2NaClO2→ 2ClO2+3NaCl+H2O (2) Reaction of sodium chlorite with hydrochloric acid 5NaClO2+4HCl → 4ClO2+5NaCl+2H2O (3) Reactions with sodium chlorate, hydrogen peroxide and sulfuric acid 2NaClO3 + H2O2 + H2SO4 → 2ClO2 + Na2SO4 + O2 + 2H2O

[0015] (Metal compound) The metal compounds used in the present invention are one or more selected from manganese compounds, iron compounds, zinc compounds, and copper compounds, and all of these form cyanide and suspended substances in cyanide-containing wastewater. Each metal compound will be described below. Among these metal compounds, copper compounds are particularly preferable in terms of the cyanide removal effect.

[0016] (Manganese compound) The manganese compounds used in the present invention are not particularly limited as long as they are soluble in water and can form manganese ions in water. Examples include manganese chloride, manganese sulfate, manganese nitrate, manganese acetate, etc. Among these, manganese chloride and manganese sulfate are particularly preferable in terms of the removal effect of cyanide compounds, and manganese chloride is particularly preferable in terms of the treatment cost of 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.

[0017] (Iron compound) The iron compounds used in the present invention are not particularly limited as long as they are soluble in water. Examples include compounds that can form divalent iron ions in water, such as ferrous chloride, ferrous sulfate, ferrous nitrate, and ferrous acetate. Among these, ferrous chloride and ferrous sulfate are particularly preferable in terms of the removal effect of cyanide compounds, and ferrous chloride is particularly preferable in terms of the treatment cost of cyanide-containing wastewater. [[ID=]]

[0018] In the method of the present invention, as an iron compound, an iron compound that can form trivalent iron ions in water is added to cyanide-containing wastewater together with a reducing agent, or an iron compound that can form trivalent iron ions in water is added to reducing cyanide-containing wastewater to reduce the iron compound that can form trivalent iron ions in the wastewater, and it contains a divalent iron ion supply compound generated thereby. Examples of the reducing agents mentioned above include sulfites and thiosulfates.

[0019] (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.

[0020] (copper compound) The copper compounds used in the present invention are not particularly limited as long as they are soluble in water or readily dispersible and capable of forming copper ions in water, and include cuprous compounds and cupric compounds, which may be either organocopper compounds or inorganic copper compounds. Examples of organocopper compounds include cupric acetate, cupric benzoate, cupric citrate, copper naphthenate, and cupric oleate.

[0021] Examples of inorganic copper compounds include cuprous compounds that can form monovalent copper ions in water, such as cuprous chloride, cuprous fluoride, cuprous bromide, cuprous iodide, cuprous nitrate, and cuprous sulfate, and cupric compounds that can form divalent copper ions in water, such as cupric chloride, cupric fluoride, cupric bromide, cupric iodide, cupric nitrate, and cupric sulfate. 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.

[0022] The method of the present invention includes, as the copper compound, a cupric compound added to cyanide-containing wastewater together with a reducing agent, 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. Examples of the reducing agents mentioned above include sulfites, divalent iron salts, and thiosulfates.

[0023] (Concentration of chlorine dioxide) The concentration of chlorine dioxide used in combination with cyanide-containing wastewater is affected by factors such as the type and concentration of cyanide contained in the wastewater, as well as the type and concentration of metal ions contained in the wastewater. Therefore, it should be determined appropriately according to these conditions, but it is generally preferable that the concentration is at least 0.2 times the molar ratio of the cyanide content in the wastewater. More preferably, the concentration is at least 1 time the molar ratio of the cyanide content in the wastewater.

[0024] If the molar ratio of chlorine dioxide is less than 0.2, the chlorine dioxide may be consumed (decomposed) depending on the type of wastewater, which may result in insufficient cyanide removal. Specific preferred lower limits for the molar ratio of chlorine dioxide are, for example, 0.2x, 0.4x, 0.5x, 1.0x, 2.0x, 5.0x, and 10x.

[0025] (Metal compound concentration) The metal compounds used in combination with cyanide-containing wastewater are affected not only by the type and concentration of cyanide contained in the wastewater, but also by the type and concentration of the same or different metal compounds (ions) contained in the wastewater. The appropriate metal compounds should be determined according to these conditions, but generally, a concentration of 0.1 to 1000 mg / L is preferred, and a concentration of 2 to 100 mg / L is more preferred.

[0026] If the metal compound concentration is less than 0.1 mg / L, the cyanide removal effect may be insufficient. On the other hand, if the metal compound concentration exceeds 1000 mg / L, excess metal compounds may remain in the treated water, requiring further removal treatment.

[0027] Furthermore, if excess metal compounds remain, treatment with a metal adsorbent may be used. Alternatively, the product may be diluted or dissolved in water, such as industrial water, before use. Examples of the metal adsorbent mentioned above include liquid chelating agents.

[0028] (Methods using compound in combination) In the present invention, it is preferable to measure the cyanide content and metal compound concentration in cyanide-containing wastewater in advance, and to add chlorine dioxide or a compound capable of generating chlorine dioxide in the wastewater according to the measured cyanide content, and the metal compound according to the measured metal compound concentration, simultaneously or separately, to the wastewater, thereby using chlorine dioxide and a metal compound in combination with the cyanide-containing wastewater. Specifically, the cyanide concentration and metal compound concentration in the cyanide-containing wastewater should be measured in advance (from immediately before treatment to approximately 3 hours before treatment), and the amount of each additive to be added should be determined based on these measurements. Furthermore, as will be discussed later, if the cyanide-containing wastewater also contains manganese compounds, iron compounds, zinc compounds, and copper compounds, the amount of metal compound to be added should be determined taking into account their respective content.

[0029] It is preferable to add chlorine dioxide and metal compounds in the form of aqueous solutions. The concentration of each aqueous solution should be determined considering factors such as the ease of adding them to cyanide-containing wastewater and the reactivity between cyanide and the added compounds. Specifically, the molar ratio of chlorine dioxide to cyanide content in wastewater is at least 0.2 times, manganese compounds have a manganese ion concentration of approximately 0.1 to 1000 mg / L, iron compounds have an iron ion concentration of approximately 0.1 to 1000 mg / L, zinc compounds have a zinc ion concentration of approximately 0.1 to 1000 mg / L, and copper compounds have a copper ion concentration of approximately 0.1 to 1000 mg / L. Furthermore, the order in which chlorine dioxide and metal compounds are added to cyanide-containing wastewater is not particularly limited; both compounds may be added simultaneously, or separately, in the order of chlorine dioxide and then the metal compounds, or in the reverse order.

[0030] (Concomitant use of reducing agents) The present invention's method for treating cyanide-containing wastewater preferably involves the combined use of a reducing agent with chlorine dioxide and a metal compound, and this combination provides a COD removal effect from the wastewater. Furthermore, the reducing agent used in combination can be any known reducing agent used in the art, such as sulfites, bisulfites and thiosulfates, iron compounds that can form divalent iron ions, and ascorbic acid. Examples of salts include sodium salts and potassium salts. Among these, sodium bisulfite used in the examples is particularly preferred in terms of its COD removal effect.

[0031] The reducing agent is affected by the type and concentration of cyanide contained in the cyanide-containing wastewater, the COD concentration, the type and concentration of metal ions contained in the cyanide-containing wastewater, and the concentration of chlorine dioxide added. Therefore, it should be determined appropriately according to these conditions, but generally, a concentration of 3.0 to 5000 mg / L is preferred, and a concentration of 6.0 to 2000 mg / L is more preferred in cyanide-containing wastewater.

[0032] (Wastewater containing cyanide) Examples of cyanide-containing wastewater to be treated in this invention include cyanide-containing wastewater containing metal cyanide compounds, cyanide ions, cyanide complexes, and cyano complex ions discharged from steel mills, chemical plants, plating plants, coke manufacturing plants, metal surface treatment plants, etc., cyanide-containing wastewater discharged in the treatment process of radioactive contaminated water, and cyanide-containing wastewater discharged from soil treatment equipment. In particular, the method for treating cyanide-containing wastewater of this invention is suitable for treating cyanide-containing wastewater with strong buffering properties, such as coke oven wastewater, that is, cyanide-containing wastewater containing coexisting substances such as thiocyanate ions and their salts, and ammonium ions. Furthermore, the method for treating cyanide-containing wastewater of the present invention is suitable for treating wastewater containing cyanide ions, readily decomposable cyanide complexes, and persistently decomposable cyanide complexes. Furthermore, the method for treating cyanide-containing wastewater of the present invention is suitable for treating wastewater containing one or more coexisting substances selected from thiocyanate ions and their salts, and ammonium ions.

[0033] In this invention, the cyanide content in the cyanide-containing wastewater to be treated is not particularly limited, but the above-mentioned cyanide-containing wastewater generally has a total cyanide concentration of about 2 to 500 mg / L. When treating such cyanide-containing wastewater, it is possible to use a combination of chlorine dioxide at a concentration of 2.0 to 2600 mg / L and a metal compound at a concentration of 0.1 to 1000 mg / L in addition to the cyanide-containing wastewater.

[0034] (Metal ions in cyanide-containing wastewater) Cyanide-containing wastewater may contain metal ions corresponding to the metal compounds used in combination, and it is preferable that it originally contains one or more metal ions selected from manganese ions, iron ions, zinc ions, and copper ions. If the cyanide-containing wastewater already contains metal ions equivalent to the metal compound to be added, the reaction with the cyanide in the wastewater will produce suspended metal salts such as manganese salts, iron salts, zinc salts, and copper salts, respectively, thereby promoting the cyanide removal effect of the present invention. The amount of metal compound to be added should be determined considering the content of these metal salts. Metal ions can take on various valencies depending on the type of metal, but in this invention, manganese ions are preferably divalent, iron ions are preferably divalent and trivalent, zinc ions are preferably divalent, and copper ions are preferably monovalent and divalent.

[0035] The manganese ion concentration in cyanide-containing wastewater is approximately 0.1 to 1000 mg / L. If the manganese ion concentration is less than 0.1 mg / L, the effect of promoting cyanide removal may not be sufficiently obtained. On the other hand, if the manganese ion concentration exceeds 1000 mg / L, it not only has adverse effects on the environment but is also economically undesirable. Specific manganese 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, 500, 600, 700, 800, 900, and 1000. The preferred manganese ion concentration is 0.1 to 150 mg / L, and more preferably 5 to 100 mg / L.

[0036] The iron ion concentration in cyanide-containing wastewater is approximately 0.1 to 1000 mg / L. If the iron ion concentration is less than 0.1 mg / L, the effect of promoting cyanide removal may not be sufficiently obtained. On the other hand, if the iron ion concentration exceeds 1000 mg / L, it not only has adverse effects on the environment but is also economically undesirable. Specific iron 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, 500, 600, 700, 800, 900, and 1000. The preferred iron ion concentration is 0.1 to 150 mg / L, and more preferably 2 to 100 mg / L.

[0037] The zinc ion concentration in cyanide-containing wastewater is approximately 0.1 to 1000 mg / L. If the zinc ion concentration is less than 0.1 mg / L, the effect of promoting cyanide removal may not be sufficiently obtained. On the other hand, if the zinc ion concentration exceeds 1000 mg / L, it not only has adverse effects on the environment but is also economically undesirable. Specific zinc 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, 500, 600, 700, 800, 900, and 1000. The preferred zinc ion concentration is 0.1 to 150 mg / L, and more preferably 5 to 100 mg / L.

[0038] The copper ion concentration in cyanide-containing wastewater is approximately 0.1 to 1000 mg / L. If 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, if the copper ion concentration exceeds 1000 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, 500, 600, 700, 800, 900, and 1000. The preferred copper ion concentration is 0.1 to 150 mg / L, and more preferably 2 to 100 mg / L.

[0039] (pH of cyanide-containing wastewater) Cyanide-containing wastewater preferably has a pH of 6 to 9. If cyanide-containing wastewater has a pH below 6 or a pH above 9, the reaction between cyanide and the compounds used in combination may be incomplete, making it difficult to efficiently remove the cyanide. Since the cyanide-containing wastewater to be treated usually has a pH of around 6 to 9, pH adjustment is not necessary. However, if necessary, the pH can be adjusted by adding an acid or alkali that does not interfere with the reaction in the treatment of the present invention, such as sulfuric acid or sodium hydroxide, to the treated wastewater.

[0040] When adding chlorine dioxide and metal compounds, and when these compounds react with cyanide, it is preferable to stir the mixed solution in order to effectively remove cyanide. This stirring is preferably performed after each compound is added. Furthermore, in order to promote the reaction during stirring, it is preferable that the mixed solution be heated to a certain extent so that the added compounds do not decompose, and the liquid temperature should be around 20-50°C. Furthermore, the reaction time required during stirring varies depending on the amount of cyanide-containing wastewater, the type and concentration of cyanide, the form and scale of the treatment device, etc., but it should be determined appropriately so that the cyanide and the added compound come into sufficient contact. Typically, a stirring time of 10 minutes or more is sufficient.

[0041] (Processing and precipitation / separation) For a series of operations including the addition of compounds used in combination, stirring and mixing, sedimentation and separation, and removal of suspended solids, known equipment such as additive tanks, reaction tanks, thickeners, and turbidity removal and sedimentation tanks can be used, and existing equipment may also be repurposed. In the method for treating cyanide-containing wastewater of the present invention, known agents such as 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. Furthermore, in sedimentation separation, a flocculant may be added to the extent that it does not hinder the effects of the present invention.

[0042] Through the above treatment, the amount of chemicals added can be kept to a minimum compared to conventional methods, reducing deterioration of the working environment such as corrosion of equipment, safely and inexpensively removing cyanide from wastewater with simple operation, and significantly reducing the cyanide concentration (total cyanide content (mg / L)) before treatment to below the discharge standard value. After treatment, the wastewater can be discharged directly into sewage systems or reused without neutralization treatment. In the method of the present invention, if the treated wastewater is discharged as is, it is sufficient to add the amount of compound necessary to reduce the total cyanide concentration to below the wastewater standard value. However, if the treated wastewater is diluted with other wastewater before discharge, it is sufficient to add the compound so that the diluted wastewater is below the above-mentioned wastewater standard value. [Examples]

[0043] The present invention will be specifically described with reference to test examples, but the present invention is not limited to these test examples.

[0044] (Test Example 1) In Test Example 1, cyanide-containing wastewater A (containing 30 mg / L total cyanide and 5 mg / L complex cyanide, pH 8.2) with the water quality shown in Table 1 was used. Specifically, cyanide-containing wastewater A was prepared using potassium hexacyanoferrate(II), potassium cyanide aqueous solution, calcium chloride dihydrate, sodium chloride, sodium sulfate, ammonium chloride, and sodium bicarbonate.

[0045] [Table 1]

[0046] To each 100 mL beaker, 100 mL of cyanide-containing wastewater A was dispensed. Chlorine dioxide was added to achieve the concentrations shown in Table 2, followed by the addition of a copper compound as a metal compound (Examples 1-2 and 4-5), or a copper compound as a metal compound followed by chlorine dioxide (Example 3), or chlorine dioxide alone (Comparative Example 1), or copper compound alone as a metal compound (Comparative Example 2), or sodium hypochlorite was added instead of chlorine dioxide, followed by the addition of a copper compound (Comparative Example 3) to obtain test water. Some of the test waters were adjusted by adding sulfuric acid solution or sodium hydroxide solution to the pH of the test waters to the values ​​shown in Table 2. As copper compounds, cuprous chloride (CuCl) was used as the reagent in Examples 1, 3-5, and Comparative Examples 2-3, while cupric chloride (CuCl2) was used in Example 2.

[0047] For the preparation of the 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 for 30 minutes. When adding the compound in two stages during the preparation of the test water, the remaining compound was added immediately after stirring for 30 minutes, and the mixture was similarly stirred at 200 rpm for 30 minutes (Examples 1-5 and Comparative Example 3). Furthermore, when the compound was added in a single step, the mixture was stirred for 30 minutes, followed by another 30 minutes of stirring at 200 rpm (Comparative Examples 1-2). After a total of 60 minutes of stirring, the suspended solids in the test water were filtered off, and the total cyanide concentration (T-CN) of the filtrate was measured in accordance with JIS K0102 to evaluate the removal effect of cyanide compounds in each test water. In this test, a blank test (Comparative Example 4) without the addition of chlorine dioxide and metal compounds was performed simultaneously. The results obtained are shown in Table 2, along with the added compounds, their amounts, and the methods of addition.

[0048] [Table 2]

[0049] The following can be seen from the test results in Table 2. (1) In the combined treatment of chlorine dioxide and metal compounds at pH 6.5 (Examples 1-3), a sufficient cyanide removal effect was obtained regardless of the order in which the chlorine dioxide and metal compounds were added. (2) In contrast, treatment using only chlorine dioxide or metal compounds at pH 6.5 (Comparative Examples 1-2) does not provide sufficient cyanide removal. (3) In the combined treatment of chlorine dioxide and metal compounds (Examples 4-5), a sufficient cyanide removal effect can be obtained in the pH range of 6-9. (4) The combined treatment of sodium hypochlorite and copper compounds at pH 6.5 (Comparative Example 3) did not provide sufficient cyanide removal effect at lower addition amounts compared to the combined treatment of chlorine dioxide and metal compounds (Example 1).

[0050] (Test Example 2) In Test Example 2, cyanide-containing wastewater B (containing 30 mg / L total cyanide, 5 mg / L complex cyanide, and 30 mg / L thiocyanate ions, pH 7.7) with the water quality shown in Table 3 was used. Specifically, cyanide-containing wastewater B was prepared using potassium hexacyanoferrate(II), potassium cyanide aqueous solution, calcium chloride dihydrate, sodium chloride, sodium sulfate, ammonium chloride, sodium bicarbonate, and potassium thiocyanate aqueous solution.

[0051] [Table 3]

[0052] 100 mL of cyanide-containing wastewater B was dispensed into each 100 mL beaker, and chlorine dioxide was added to achieve the concentrations shown in Table 4. Then, either cuprous chloride (CuCl) was added as a metal compound (Example 1), only chlorine dioxide was added (Comparative Example 1), or only cuprous chloride (CuCl) was added as a metal compound (Comparative Example 2) to obtain test water. Some of the test waters were adjusted by adding sulfuric acid solution or sodium hydroxide solution to the pH of the test waters to the values ​​shown in Table 4.

[0053] For the preparation of the 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 for 30 minutes. When adding the compound in two stages during the preparation of the test water, the remaining compound was added immediately after stirring for 30 minutes, and the mixture was similarly stirred at 200 rpm for 30 minutes (Example 1). Furthermore, when the compound was added in a single step, the mixture was stirred for 30 minutes, followed by another 30 minutes of stirring at 200 rpm (Comparative Examples 1-2). After a total of 60 minutes of stirring, water-insoluble products were filtered off from the test water, and the total cyanide concentration (T-CN) in the filtrate was measured in accordance with JIS K0102 to evaluate the removal effect of cyanide compounds in each test water. Furthermore, the thiocyanate ion concentration (SCN) in the filtrate was measured using an ion chromatography system (Thermo Fisher Scientific, model: Dionex ICS-2100) to evaluate the thiocyanate ion removal effect in each test water sample. In this test, a blank test (Comparative Example 3) without the addition of chlorine dioxide and metal compounds was performed simultaneously. The results obtained are shown in Table 4, along with the added compound, its amount added, and the method of addition.

[0054] [Table 4]

[0055] The following can be seen from the test results in Table 4. (1) In the combined treatment of chlorine dioxide and cuprous chloride at pH 6.5 (Example 1), sufficient removal effect of cyanide and thiocyanate ions was obtained. (2) In contrast, while a single-agent treatment using chlorine dioxide at pH 6.5 (Comparative Example 1) can remove thiocyanate ions, it cannot remove cyanide sufficiently. (3) Furthermore, in the single-agent treatment using cuprous chloride at pH 6.5 (Comparative Example 2), sufficient removal of cyanide and thiocyanate ions was not obtained.

[0056] (Test Example 3) In Test Example 3, cyanide-containing wastewater C (containing 30 mg / L total cyanide, 1 mg / L complex cyanide, and 30 mg / L thiocyanate ions, pH 8.2) with the water quality shown in Table 5 was used. Specifically, cyanide-containing wastewater C was prepared using potassium hexacyanoferrate(II), potassium cyanide aqueous solution, calcium chloride dihydrate, sodium chloride, sodium sulfate, ammonium chloride, sodium bicarbonate, and potassium thiocyanate aqueous solution.

[0057] [Table 5]

[0058] In each 100 mL beaker, 100 mL of cyanide-containing wastewater C was dispensed, and chlorine dioxide was added to achieve the concentrations shown in Table 6, followed by the addition of cuprous chloride (CuCl) as a metal compound, and then sodium bisulfite as a reducing agent (Example 1); or chlorine dioxide was added, followed by cuprous chloride (CuCl) as a metal compound (Example 2); or chlorine dioxide was added, followed by sodium bisulfite as a reducing agent (Comparative Example 1); or cuprous chloride (CuCl) was added as a metal compound, followed by sodium bisulfite as a reducing agent (Comparative Example 2); or only chlorine dioxide was added (Comparative Example 3); or only sodium bisulfite was added as a reducing agent (Comparative Example 4) to obtain test water. Some of the test waters were adjusted by adding sulfuric acid solution or sodium hydroxide solution to the pH of the test waters to the values ​​shown in Table 6.

[0059] For the preparation of the 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 for 20 minutes. When adding the compounds in three stages during the preparation of the test water, one of the remaining compounds was added immediately after stirring for 20 minutes, and the mixture was similarly stirred at 200 rpm for 20 minutes. Then, one of the remaining compounds was added immediately after stirring for another 20 minutes, and the mixture was similarly stirred at 200 rpm for 20 minutes (Example 1). In addition, for the preparation of the other test waters, 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 for 30 minutes. When adding the compound in two stages during the preparation of the test water, the remaining compound was added immediately after stirring for 30 minutes, and the mixture was similarly stirred at 200 rpm for 30 minutes (Example 2 and Comparative Examples 1-2). When the compound was added in a single step, the mixture was stirred for 30 minutes, followed by another 30 minutes of stirring at 200 rpm (Comparative Examples 3-4).

[0060] After a total of 60 minutes of stirring, the water-insoluble products in the test water were filtered off, and the total cyanide concentration (T-CN) and COD in the filtrate were measured. Mn The concentrations were measured in accordance with JIS K0102, and the removal effect of cyanide compounds and COD in each test water sample was evaluated. In this test, a blank test (Comparative Example 5) was simultaneously conducted without the addition of chlorine dioxide, cuprous chloride, and sodium bisulfite. The results obtained are shown in Table 6, along with the added compounds, their amounts, and the methods of addition.

[0061] [Table 6]

[0062] The following can be seen from the test results in Table 6. (1) In the combined treatment of chlorine dioxide, metal compounds, and a reducing agent (Example 1), sufficient COD removal effect can be obtained along with the cyanide removal effect. (2) In the combined treatment of chlorine dioxide and metal compounds (Example 2), although a cyanide removal effect is obtained, a sufficient COD removal effect is not obtained. (3) In contrast, the combined treatment of chlorine dioxide and a reducing agent, the combined treatment of a metal compound and a reducing agent, and the treatment using only chlorine dioxide or a reducing agent (Comparative Examples 1-4) did not provide sufficient cyanide removal and COD removal effects.

Claims

1. The method is characterized by removing cyanide from cyanide-containing wastewater by using chlorine dioxide in combination with one or more metal compounds selected from manganese compounds that can form manganese ions in water, iron compounds that can form iron ions in water, zinc compounds that can form zinc ions in water, and copper compounds that can form copper ions in water (except when hydrogen peroxide is also used in combination). The chlorine dioxide is at least 0.2 times the molar ratio of the cyanide content in the cyanide-containing wastewater, and the metal compound is at a concentration of 0.1 to 1000 mg / L in the cyanide-containing wastewater. The cyanide content and metal compound concentration in the cyanide-containing wastewater are measured in advance, and the chlorine dioxide or a compound capable of generating chlorine dioxide in the wastewater, corresponding to the measured cyanide content, and the metal compound, corresponding to the measured metal compound concentration, are added to the wastewater simultaneously or separately. The aforementioned cyanide-containing wastewater has a pH of 6 to 9. Methods for treating cyanide-containing wastewater.

2. The method for treating cyanide-containing wastewater according to claim 1, wherein the metal compound is a copper compound capable of forming copper ions in water.

3. The method for treating cyanide-containing wastewater according to claim 1 or 2, wherein the cyanide-containing wastewater contains at least one of cyanide ions, easily decomposable cyanide complexes, and persistent cyanide complexes.

4. The method for treating cyanide-containing wastewater according to any one of claims 1 to 3, wherein the cyanide-containing wastewater contains one or more coexisting substances selected from thiocyanate ions and their salts and ammonium ions.

5. A method for treating cyanide-containing wastewater according to any one of claims 1 to 4, further comprising using a reducing agent in combination with the chlorine dioxide and metal compound.

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