Method of treating dust collection wastewater

Abstract

To provide a method of treating dust collection wastewater, capable of obtaining treated water of a low cyan residual rate including a cyan component present in a thiocyanate ion that can be formed and the total cyanide concentration.SOLUTION: The method of treating dust collection wastewater includes: adding hydrogen peroxide to dust collection wastewater containing suspended matters, cyanide ions, and reductive sulfur components, obtained by subjecting exhaust to a wet dust collection treatment; causing hydrogen peroxide to react in the dust collection wastewater; adding hydrogen peroxide to apply a solid / liquid separation treatment to the dust collection wastewater having reacted with the same, in a solid / liquid separation facility; and supplying a portion of a separated liquid removed of the suspended matters present in the dust collection wastewater in the solid / liquid separation facility to the wet dust collection treatment using a circulation facility, as washing water. In the present method, the concentration of hydrogen peroxide added to the dust collection wastewater is 1.2 times or more as much as the total cyan concentration (mg-CN / L) in the dust collection wastewater, and the oxidation-reduction potential of the dust collection wastewater immediately after the addition of hydrogen peroxide is 200+59×(7-measured value of pH) mV or higher. Hydrogen peroxide is reacted in the dust collection wastewater in the presence of copper in a concentration of 1 mg-Cu / L or higher.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for treating dust-collected water. [Background technology]

[0002] For example, exhaust gas generated from blast furnaces and converters in steelworks contains dust, and therefore a process (wet dust collection process) is carried out to collect the dust contained in the exhaust gas using a wet dust collector. This wet dust collection process produces collected dust water containing suspended solids and salts derived from the dust contained in the exhaust gas.

[0003] In the dust water treatment system, the dust water is sent from the wet dust collector to a settling tank (thickener), where suspended solids in the dust water are separated and removed by settling. The liquid (separated liquid) from which suspended solids in the dust water have been removed or reduced is sometimes used in a system in which it is circulated to the wet dust collector by a circulation system. Furthermore, the separated liquid is sometimes used as the water to be treated, and water treatment is sometimes performed to remove fine suspended solids and soluble substances such as salts from the water to be treated.

[0004] Some dust collection water contains cyanide components such as cyanide ions, and in order to satisfy the wastewater treatment standards, it is necessary to sufficiently remove the cyanide components before the final treated water is discharged. For example, a technique disclosed in Patent Document 1 has been proposed in which, when removing suspended solids from dust collection water, the cyanide components in the dust collection water are removed together with the suspended solids. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2023-010580 A Summary of the Invention [Problem to be solved by the invention]

[0006] In the above-mentioned circulation system, since the dust collection water is repeatedly used, the present inventors considered that it is preferable to reduce the total cyanide concentration of the dust collection water when the dust collection water contains cyanide ions. Based on this consideration, the present inventors, referring to the technology described in Patent Document 1, investigated adding hydrogen peroxide to the dust collection water containing suspended solids and cyanide ions before solid-liquid separation treatment to oxidize and decompose the cyanide ions in the dust collection water and reduce the total cyanide concentration.

[0007] In the above study, it was found that the total cyanide concentration of the treated water obtained by adding hydrogen peroxide to the dust collection water and reacting the same is lower than the total cyanide concentration of the dust collection water. However, it was also found that the dust collection water may further contain reducing sulfur components in a solid or liquid state, and that adding hydrogen peroxide to the dust collection water containing cyanide ions and reducing sulfur components may generate thiocyanate ions.

[0008] The reduction in the total cyanide concentration of the treated water due to the addition and reaction of hydrogen peroxide with the dust water may be due to the use of cyanide ions in the dust water to generate thiocyanate ions. In addition, since thiocyanate ions are detected as a COD (chemical oxygen demand) component, the COD of the treated water obtained after the addition and reaction of hydrogen peroxide with the dust water may be high. Therefore, when performing treatment to reduce the total cyanide concentration of dust water containing cyanide ions and reductive sulfur components, it is considered better to take into account the generation of thiocyanate ions when evaluating the treated water obtained.

[0009] Therefore, the present invention is intended to provide a method for treating dust water, which can obtain treated water having a low cyanide residual rate, including the cyanide content in thiocyanate ions that may be generated and the total cyanide concentration, from dust water containing cyanide ions and reducible sulfur components. [Means for solving the problem]

[0010] According to the present invention, there is provided a method for treating dust collected water, comprising: adding hydrogen peroxide to dust collected water obtained by subjecting exhaust gas to a wet dust collection treatment and containing suspended solids, cyanide ions, and reducible sulfur components; reacting the hydrogen peroxide in the dust collected water; subjecting the dust collected water to solid-liquid separation treatment in a solid-liquid separation facility to which the hydrogen peroxide has been added and to which the reaction has been carried out; and supplying a part of the separated liquid separated from the suspended solids in the dust collected water in the solid-liquid separation facility as cleaning water to the wet dust collection treatment in a circulation facility; wherein a concentration (mg-H2O2 / L) of the hydrogen peroxide added to the dust collected water is 1.2 times or more the total cyanide concentration (mg-CN / L) of the dust collected water, an oxidation-reduction potential (ORP) of the dust collected water immediately after the addition of the hydrogen peroxide is 200+59×(7-pH measurement value) mV or more, and reacting the hydrogen peroxide in the dust collected water comprises reacting the hydrogen peroxide in the dust collected water in the presence of a copper concentration of 1 mg-Cu / L or more in the dust collected water. Effect of the Invention

[0011] According to the present invention, there can be provided a method for treating dust water which is capable of obtaining treated water having a low cyanide residual rate, including the cyanide content in thiocyanate ions that may be generated and the total cyanide concentration, from dust water containing cyanide ions and reducible sulfur components. [Brief description of the drawings]

[0012] [Figure 1] 1 is a schematic configuration diagram showing a first example of a dust-collected water treatment system that can be used in a dust-collected water treatment method according to one embodiment of the present invention. [Diagram 2] FIG. 2 is a schematic diagram showing a second example of a dust-collected water treatment system that can be used in the dust-collected water treatment method of one embodiment of the present invention. [Diagram 3] FIG. 2 is a schematic diagram showing a third example of a dust-collected water treatment system that can be used in the dust-collected water treatment method of one embodiment of the present invention. [Figure 4] FIG. 11 is a schematic diagram showing a fourth example of a dust-collected water treatment system that can be used in the dust-collected water treatment method of one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The method for treating dust collection water according to one embodiment of the present invention relates to a method for treating dust collection water using a solid-liquid separation facility that performs solid-liquid separation on the dust collection water obtained by wet dust collection of exhaust gas, and a circulation facility that supplies a part of the separated liquid obtained in the solid-liquid separation facility as washing water to the wet dust collection treatment.

[0014] The dust water to be treated in the dust water treatment method of the present embodiment (hereinafter, sometimes referred to as "this method") contains suspended solids, cyanide ions, and reducible sulfur components. This dust water is wastewater obtained by wet dust collection treatment of exhaust gas, and is wastewater before solid-liquid separation treatment for removing suspended solids in the dust water.

[0015] The solid-liquid separation equipment for performing solid-liquid separation treatment on the dust collection water is for separating and removing suspended matter in the dust collection water. The circulation equipment circulates a part of the separated liquid separated from the suspended matter in the dust collection water in the solid-liquid separation equipment, and supplies it to the wet dust collection treatment as washing water for washing the exhaust gas. Since the separated liquid is a liquid obtained by treating the dust collection water by the present method, in this specification, the separated liquid may be referred to as treated water. In addition, a part of the separated liquid other than the part of the circulated separated liquid may be treated as water to be treated, and may be subjected to a later-stage treatment to further remove soluble substances such as fine suspended matter and salts that may remain in the water to be treated (separated liquid). From this viewpoint, the separated liquid may be referred to as primary treated water by the present method, and may also be referred to as water to be treated as a treatment target in a later-stage treatment.

[0016] In the above-mentioned circulation system, the dust collection water is circulated and repeatedly used as the separated liquid obtained after the solid-liquid separation process, so that the separated liquid obtained after the solid-liquid separation process is preferably used as treated water (primary treated water) with a reduced total cyanide concentration. If the total cyanide concentration is reduced by performing a crude treatment to remove cyanide to some extent in the dust collection water at this stage, it can also contribute to a more advanced removal of cyanide in the subsequent treatment. With this aim, the present method includes adding hydrogen peroxide to the dust collection water before the solid-liquid separation process. This oxidizes and decomposes the cyanide ions in the dust collection water, thereby reducing the total cyanide concentration in the separated liquid (primary treated water) obtained after the solid-liquid separation process.

[0017] By adding hydrogen peroxide to dust-collected water containing cyanide ions and reacting with it, it is possible to obtain treated water whose measured total cyanide concentration is lower than that of the dust-collected water to be treated. On the other hand, when hydrogen peroxide is added to dust-collected water containing cyanide ions and reductive sulfur components and reacted with it, thiocyanate ions may be generated. This is thought to be because cyanide ions react with reductive sulfur in the dust-collected water to generate thiocyanate ions. Therefore, the total cyanide concentration of the treated water may have decreased due to the generation of thiocyanate ions, that is, the use of cyanide ions in the dust-collected water to generate thiocyanate ions. In addition, since thiocyanate ions are detected as a COD (chemical oxygen demand) component, the COD of the treated water obtained after the addition and reaction of hydrogen peroxide with dust-collected water may be high. COD is an index of water pollution that indicates the amount of organic matter in water, and is the amount of oxygen (mg / L) consumed when organic matter, etc. is oxidized and decomposed using an oxidizing agent, and discharge standards are also set for the COD value.

[0018] Therefore, when performing treatment to reduce the total cyanide concentration in dust collection water that contains cyanide ions and reducible sulfur components, it is considered advisable to also take into consideration the generation of thiocyanate ions when evaluating the resulting treated water. Therefore, in this method, the aim is to obtain treated water with a low cyanide residual rate, including the cyanide content in the thiocyanate ions that may be generated and the total cyanide concentration, from dust collection water that contains suspended solids, cyanide ions, and reducible sulfur components. The cyanide residual rate (%) is calculated by dividing the total cyanide concentration in the dust collection water by the thiocyanate ions (SCN - The total cyanide concentration and thiocyanate ion (SCN) concentration in the treated water are compared to the sum of the cyanide (CN) content in the - As in the test example described later, when the dust water to be treated does not contain thiocyanate ions, the above cyanide residual rate (%) is the sum of the total cyanide concentration in the treated water and the thiocyanate ion (SCN - The sum of the cyanide (CN) content in the dust water concentration is divided by the total cyanide concentration in the dust water and expressed as a percentage.

[0019] This method includes reacting hydrogen peroxide in the presence of a copper concentration of 1 mg-Cu / L or more in the dust water, in addition to adding hydrogen peroxide to the dust water before solid-liquid separation treatment, in order to obtain primary treated water with a low residual cyanide rate. Furthermore, when hydrogen peroxide is added to the dust water before solid-liquid separation treatment, the concentration of hydrogen peroxide added to the dust water (mg-H2O2 / L) is set to 1.2 times or more the total cyanide concentration (mg-CN / L) of the dust water.

[0020] Furthermore, it is considered that the amount of hydrogen peroxide required increases as the concentration of reducing substances such as solid or soluble reducing sulfur components in the dust collection water to be treated increases. This is because it is considered that when there are many reducing substances such as reducing sulfur components in the dust collection water, hydrogen peroxide reacts preferentially with the reducing substances rather than with cyanide ions. From this perspective, in this method, the oxidation-reduction potential (ORP) of the dust collection water immediately after adding hydrogen peroxide is also used as an indicator of the concentration of hydrogen peroxide added to the dust collection water.

[0021] Specifically, hydrogen peroxide is added to the dust collection water in an amount such that the oxidation-reduction potential (ORP) of the dust collection water immediately after the addition of hydrogen peroxide is 200+59×(7-pH measurement value)mV or more. The ORP immediately after the addition of hydrogen peroxide to the dust collection water is determined by the balance between the hydrogen peroxide concentration and reducing substances in the dust collection water after the addition of hydrogen peroxide, and it is considered that the copper concentration does not have a large effect, so the concentration of hydrogen peroxide added is determined taking the ORP into consideration. An ORP of "200+59×(7-pH measurement value)mV or more" means that the ORP measurement value is 200mV or more when the pH of the dust collection water immediately after the addition of hydrogen peroxide is 7.0. It is known that the ORP of a water system fluctuates depending on the pH, and that when the pH decreases by 1, the ORP fluctuates by +59mV, and conversely, when the pH increases by 1, the ORP fluctuates by -59mV. Therefore, in this method, taking into consideration the measured pH value of the dust collected water when hydrogen peroxide is added, the ORP of the dust collected water immediately after adding hydrogen peroxide is set to be equal to or higher than "200 + 59 x (7 - measured pH) mV." In addition, the reason why the ORP is measured immediately after adding hydrogen peroxide to the dust collected water is because of the chemical characteristics of reducing substances such as reducing sulfur components, which react with hydrogen peroxide relatively quickly, and because it is easy to measure, in terms of operation.

[0022] As described above, the method includes adding hydrogen peroxide to dust collection water containing suspended solids, cyanide ions, and reducible sulfur components obtained by wet dust collection of exhaust gas, and reacting the hydrogen peroxide in the dust collection water. The method also uses the above-mentioned solid-liquid separation equipment and circulation equipment. Therefore, the method includes adding hydrogen peroxide to the dust collection water, subjecting the dust collection water to solid-liquid separation treatment in the solid-liquid separation equipment, and supplying a part of the separated liquid separated from the suspended solids in the dust collection water in the solid-liquid separation equipment as cleaning water to the wet dust collection treatment in the circulation equipment. When hydrogen peroxide is added to the dust collection water, the concentration of hydrogen peroxide added to the dust collection water (mg-H2O2 / L) is set to 1.2 times or more the total cyanide concentration (mg-CN / L) of the dust collection water. The oxidation-reduction potential (ORP) of the dust collection water immediately after adding hydrogen peroxide is set to 200+59×(7-pH measurement value) mV or more. Furthermore, the above-mentioned reaction of hydrogen peroxide in the dust collected water is carried out in the presence of a copper concentration of 1 mg-Cu / L or more in the dust collected water. Under these conditions, a balance can be achieved between the decomposition of cyanide ions in the dust collected water and the inhibition of the generation of thiocyanate ions, making it possible to obtain treated water with a low cyanide residual rate, including the cyanide content in the thiocyanate ions and the total cyanide concentration, specifically, primary treated water with a cyanide residual rate of 50% or less.

[0023] In this method, it is not necessary that the total cyanide concentration of the separated liquid (primary treated water) obtained by the solid-liquid separation process be equal to or lower than the effluent standard. This is because the treatment target is dust collection water in a circulating system, and a cyanide treatment step may be provided in a later stage, rather than completing the treatment in the circulating system alone. Also, since it may be difficult to treat cyanide to a higher degree in the later stage treatment alone, applying this method to perform rough treatment can contribute to treating cyanide to a higher degree in the later stage treatment.

[0024] The present method will be further described below with reference to the drawings, but the present method is not limited to the method described using the drawings. Note that parts common to each figure in the drawings are given the same reference numerals and their description may be omitted. Also, the arrows in the drawings represent the flow of a substance, and the line of the arrow may be shown as the flow path of the substance.

[0025] 1 to 4 are schematic diagrams showing first to fourth examples of dust-collected water treatment systems 11, 12, 13, and 14, respectively, as examples of dust-collected water treatment systems that can be used in the present method. In the present method, a solid-liquid separation facility 40 that performs solid-liquid separation of dust-collected water containing suspended solids obtained by wet dust collection of exhaust gas, and a circulation facility 50 that uses a part of the separated liquid separated from the suspended solids in the dust-collected water by the solid-liquid separation facility 40 as cleaning water for wet dust collection. Hereinafter, the present method will be described by exemplifying a method for treating dust-collected water in the dust-collected water treatment systems 11 to 14 that include the solid-liquid separation facility 40 and the circulation facility 50. Note that the dust-collected water treatment systems 11 to 14 are systems that treat dust-collected water obtained by wet dust collection of exhaust gas, and therefore, in FIGS. 1 to 4, the gas and the wet dust collector 2 are indicated by dashed lines (long dashed lines). The dust-collecting water treatment systems 11 to 14 may be configured as part of an exhaust gas treatment system equipped with the wet dust collector 2.

[0026] In this method, the target of treatment is collected water obtained by wet dust collection of exhaust gas, and containing suspended solids, cyanide ions, and reducible sulfur components. The collected water is obtained by wet dust collection of exhaust gas in a wet dust collector 2. In order to remove suspended solids in the collected water by solid-liquid separation treatment, the collected water can be supplied from the wet dust collector 2 to solid-liquid separation equipment 40 through a flow path (first flow path) 31.

[0027] As the exhaust gas, for example, exhaust gas containing dust containing iron, cyanide components such as hydrogen cyanide, and carbon monoxide is suitable, since dust collection water containing suspended solids, cyanide ions, and reducible sulfur components can be generated by contact with the wash water in the wet dust collector 2. Examples of such exhaust gas include exhaust gas generated from steelworks, exhaust gas generated from metal refining facilities such as melting furnaces (refining furnaces), exhaust gas generated from cement manufacturing facilities, and exhaust gas generated from waste incineration facilities that incinerate waste such as various types of garbage.

[0028] Examples of gas components in the exhaust gas include, but are not limited to, carbon monoxide (CO), cyanide components, carbon dioxide (CO2), and nitrogen (N2). Examples of solid components contained in the dust in the exhaust gas include, but are not limited to, iron (e.g., iron, iron oxide, and iron hydroxide), copper, and solid reducible sulfur components.

[0029] The dust collection water to be treated contains suspended solids (SS) as dust contained in the exhaust gas. The suspended solids (SS) concentration in the dust collection water is preferably 500 mg / L or more, more preferably 500 to 10,000 mg / L, and even more preferably 500 to 5,000 mg / L.

[0030] The dust water to be treated contains cyanide ions (CN - ; also called free cyanide.) is contained. Since the general wastewater standard for cyanide compounds is 1 mg / L or less, dust collected water with a total cyanide concentration of 1 mg / L or more is suitable for treatment by this method. When the total cyanide concentration of dust collected water is relatively high, it is difficult to remove cyanide to a high degree in a single treatment step, so the application of this method is significant. From this viewpoint, dust collected water with a total cyanide concentration of 3 mg-CN / L or more is suitable, dust collected water with a total cyanide concentration of 5 mg-CN / L or more is more suitable, and dust collected water with a total cyanide concentration of 10 mg-CN / L or more is even more suitable. In addition, the total cyanide concentration of dust collected water is preferably 100 mg-CN / L or less.

[0031] The dust collection water to be treated may contain cyanide components other than cyanide ions. Examples of other cyanide components include cyano complexes. Examples of cyano complexes include iron cyano complexes, zinc cyano complexes, nickel cyano complexes, and copper cyano complexes. One or more of these may be contained in the dust collection water. Examples of iron cyano complexes include ferrocyanide ions ([Fe(CN)6] 4- , also known as ferricyanide(II) ion; ferricyanide ion ([Fe(CN)6] 3- , also known as hexacyanoferrate(III); and the tetracyanocarbonyl iron complex ([Fe(CN)4(CO)2] 2- etc.) and pentacyanocarbonyl iron complexes ([Fe II (CN)5(CO)] 3- , [Fe III (CN)5(CO)] 2- One or more of these iron cyano complexes may be contained in the dust collection water.

[0032] The dust collection water to be treated contains reducible sulfur components in a solid or soluble form. The reducible sulfur components in the dust collection water may be in a solid state or may be dissolved in water. The reducible sulfur components are ions having a sulfur atom or compounds having a sulfur atom (reducible sulfur compounds). Examples of the reducible sulfur components include sulfide ions (S 2- ), thiosulfate ion (S2O3 2- ), dithionite ion (S2O4 2- ), sulfite ion (SO3 2- ), bisulfite ion (HSO3 - ), trithionate ion (S3O6 2- ) and tetrathionate ion (S4O6 2- ) containing polythionate ions (S n O6 2-, where n>2) and their salts (compounds), as well as elemental sulfur (S) and hydrogen sulfide (HS). Examples of the above-mentioned salts include alkali metal salts such as potassium and sodium, alkaline earth metal salts such as calcium, and ammonium salts. The dust collection water may contain one type of reducing sulfur component alone, or may contain two or more types of reducing sulfur components.

[0033] The concentration of the reducing sulfur components in the dust collection water is not particularly limited. Since it is not practical to identify all the types of reducing sulfur components in the dust collection water, the reducing sulfur component concentration is preferably 0.2 to 100 mg-S / L, more preferably 0.5 to 50 mg-S / L, and even more preferably 1.0 to 50 mg-S / L, in terms of sulfur (S),.

[0034] The dust collection water to be treated is further treated with ammonia nitrogen (NH3 + -N, NH4 + The concentration of ammonia nitrogen in the dust collection water is preferably 10 to 1000 mg-N / L, more preferably 10 to 500 mg-N / L, and even more preferably 20 to 500 mg-N / L.

[0035] The pH of the dust collection water is not particularly limited, but is preferably 6.0 to 9.0 when hydrogen peroxide is added to the dust collection water and reacted. The pH of the dust collection water may be adjusted to within the above range by adding a pH adjuster to the dust collection water. The pH adjuster is not particularly limited, and known pH adjusters such as acids such as hydrochloric acid and sulfuric acid, and bases such as sodium hydroxide, calcium hydroxide, and sodium carbonate may be appropriately used.

[0036] The temperature of the dust collecting water is not particularly limited. The above-mentioned suitable exhaust gas is often in a relatively high temperature state, and from the viewpoint that the dust collecting water obtained by subjecting such exhaust gas to a wet dust collecting treatment is suitable, the temperature of the dust collecting water is preferably 20 to 80°C, and more preferably 35 to 80°C, when hydrogen peroxide is added to the dust collecting water.

[0037] This method includes adding hydrogen peroxide to the dust-collecting water before the solid-liquid separation process. When hydrogen peroxide is added to the dust-collecting water, an aqueous solution of hydrogen peroxide (hydrogen peroxide solution) can usually be used.

[0038] The location where hydrogen peroxide is added to the dust collecting water may be any location before the solid-liquid separation equipment 40 so that the dust collecting water that has been reacted with hydrogen peroxide is the target of solid-liquid separation processing in the solid-liquid separation equipment 40. The location before the solid-liquid separation equipment 40 may be any location between the wet dust collector 2 and the solid-liquid separation equipment 40, and for example, a hydrogen peroxide addition position may be provided midway through the first flow path 31, as in the hydrogen peroxide addition device 62 shown in FIG.

[0039] Furthermore, although not shown, a receiving tank for receiving the dust-collected water flowing from the wet dust collector 2 through the first flow path 31, a flow path for sending the dust-collected water from the receiving tank to the solid-liquid separation equipment 40, or the like may be provided between the wet dust collector 2 and the solid-liquid separation equipment 40. The receiving tank, the flow path from the receiving tank to the solid-liquid separation equipment 40, or the like may be the position for adding hydrogen peroxide.

[0040] 2 to 4, the solid-liquid separation equipment 40 is preferably provided with a flow path (second flow path) 32 such as a gutter or a pipe for sending the dust-collected water from the wet dust collector 2 to the solid-liquid separation equipment 40. In the flow path (second flow path) 32 for supplying the dust-collected water to the solid-liquid separation equipment, it is preferable to add hydrogen peroxide to the dust-collected water flowing through the second flow path 32. By setting the second flow path 32 as the position for adding hydrogen peroxide, the dust-collected water to which hydrogen peroxide has been added can be sent to the solid-liquid separation equipment 40 by flowing through the second flow path 32. The second flow path 32 may be configured as the same flow path as the first flow path 31, or may be connected to the first flow path 31.

[0041] 1 to 4, a device for adding hydrogen peroxide to dust collecting water (hydrogen peroxide adding device) 62 can be provided in first flow path 31, second flow path 32, etc. Hydrogen peroxide adding device 62 can include, for example, a tank for storing hydrogen peroxide, and a pump and supply pipe for supplying hydrogen peroxide, etc.

[0042] In this method, the concentration of hydrogen peroxide added to the dust collection water (mg-H2O2 / L) is 1.2 times or more the total cyanide concentration (mg-CN / L) of the dust collection water so that the cyanide ions in the dust collection water can be oxidized and decomposed. The concentration of hydrogen peroxide added to the dust collection water (mg-H2O2 / L) is preferably 1.3 times or more the total cyanide concentration (mg-CN / L) of the dust collection water, more preferably 1.5 times or more, and even more preferably 2.0 times or more.

[0043] On the other hand, as a result of the experimental investigation by the present inventors, it was found that the amount of thiocyanate ions generated tends to increase with an increase in the amount of hydrogen peroxide added. Therefore, from the viewpoint of a balance with the amount of thiocyanate ions generated, i.e., a reduction in the residual rate of cyanide, the concentration of hydrogen peroxide added to the dust collecting water (mg-H2O2 / L) is preferably 7.0 times or less, more preferably 6.7 times or less, and even more preferably 6.0 times or less, relative to the total cyanide concentration of the dust collecting water (mg-CN / L).

[0044] As described above, when the concentration of reducing substances such as reducing sulfur components in the dust collection water to be treated is relatively high, hydrogen peroxide may preferentially react with reducing substances such as reducing sulfur components rather than with cyanide ions. Therefore, the concentration of hydrogen peroxide added to the dust collection water is set to an amount that satisfies the oxidation-reduction potential (ORP) of the dust collection water (in other words, the mixed solution of hydrogen peroxide and dust collection water) immediately after the addition of hydrogen peroxide ≧200+59×(7−pH measurement value) mV. The pH measurement value in this formula is the pH measurement value of the dust collection water (the mixed solution) immediately after the addition of hydrogen peroxide, and the ORP (mV) value is the ORP measurement value of the dust collection water (the mixed solution) at the pH measurement value immediately after the addition of hydrogen peroxide. For example, when the pH measurement value is 6.0, the ORP is 259 mV or more, when the pH measurement value is 8.0, the ORP is 141 mV or more, and when the pH measurement value is 9.0, the ORP is 82 mV or more. In a preferred embodiment of the present method, the concentration of hydrogen peroxide added to the dust collection water can be controlled based on the ORP measurement value.

[0045] The ORP of the dust collection water immediately after the addition of hydrogen peroxide is preferably 210+59×(7-pH measurement value)mV or more, more preferably 220+59×(7-pH measurement value)mV or more, and even more preferably 230+59×(7-pH measurement value)mV or more. The ORP of the dust collection water immediately after the addition of hydrogen peroxide is preferably 350+59×(7-pH measurement value)mV or less, and more preferably 320+59×(7-pH measurement value)mV or less.

[0046] The ORP can be measured using an oxidation-reduction potentiometer (ORP meter) 64. When using a precipitator (thickener) 40 suitable as the solid-liquid separation equipment 40 described below, it is preferable to provide the ORP meter 64 inside the center well 42 of the precipitator 40, as shown in Figures 2 to 4. As described below, the center well 42 is the inflow site of the dust collection water, and therefore, by providing the ORP meter 64 inside the center well 42, it is easy to measure the ORP of the dust collection water immediately after the addition of hydrogen peroxide.

[0047] This method includes reacting hydrogen peroxide in the dust water in the presence of a copper concentration of 1 mg-Cu / L or more. When hydrogen peroxide is reacted in the dust water, the presence of a copper concentration of 1 mg-Cu / L or more promotes the decomposition of cyanide ions and makes it easier to suppress the generation of thiocyanate ions.

[0048] The reaction time when hydrogen peroxide is reacted in the dust collection water in the presence of a copper concentration of 1 mg-Cu / L or more is preferably 30 to 300 minutes, more preferably 30 to 240 minutes. During the reaction, it is preferable to stir the mixed liquid having a copper concentration of 1 mg-Cu / L or more after adding hydrogen peroxide to the dust collection water. Furthermore, during the reaction, it is preferable to appropriately use the above-mentioned pH adjuster to maintain the pH of the reaction system, and the pH at that time is preferably 6.0 to 9.0, more preferably 6.0 to 8.0.

[0049] Copper present in the dust collection water may be in the form of monovalent copper compounds, divalent copper compounds, or in the form of solids such as fine powder, or in the form of ions (Cu + , Cu 2+ ) is also acceptable. The copper concentration in the dust collection water needs to be 1 mg-Cu / L or more as long as it is present in the dust collection water when hydrogen peroxide is reacted with it. Therefore, if the dust collection water obtained by wet dust collection of exhaust gas already has a copper concentration of 1 mg-Cu / L or more, there is no need to add a copper compound to the dust collection water.

[0050] On the other hand, in one aspect of the present method, it is preferable that the method further comprises adding a copper compound to the dust collection water at a copper concentration of 1 mg-Cu / L or more when reacting hydrogen peroxide in the dust collection water, which makes it possible to omit an analysis step for confirming the presence of a copper concentration of 1 mg-Cu / L or more in the dust collection water at an actual dust collection water treatment site.

[0051] The copper concentration or the concentration of the copper compound added when reacting hydrogen peroxide in the dust collecting water is preferably 2 mg-Cu / L or more, more preferably 5 mg / L or more, and is preferably 20 mg-Cu / L or less.

[0052] As the copper compound, at least one selected from the group consisting of copper (I) compounds and copper (II) compounds can be used. Suitable copper (I) compounds include, for example, copper chloride (I), copper oxide (I) (cuprous oxide), copper bromide (I), copper acetate (I), and copper sulfide (I). Suitable copper (II) compounds include, for example, copper chloride (II), copper sulfate (II), copper acetate (II), copper oxide (II), and copper nitrate (II). Among these, copper oxide (I) and copper sulfate (II) are preferred.

[0053] When adding a copper compound to the dust collection water, it is preferable to use a copper compound capable of supplying copper ions (copper (I) ions or copper (II) ions) to the dust collection water. As the form of such a copper compound, it is more preferable to use the copper compound in at least one of the following forms: a copper ion-containing solution containing copper ions generated from the copper compound by dissolving the copper compound in a solvent; and a solid copper compound such as a powder that generates copper ions when added to the dust collection water. Among these, it is more preferable to use a copper ion-containing solution from the viewpoint of ease of use and ease of supplying a copper concentration of 1 mg-Cu / L or more to the dust collection water. Examples of the solvent used for the copper ion-containing solution include water; acids such as dilute hydrochloric acid and dilute sulfuric acid; and bases such as aqueous ammonia.

[0054] When a copper compound is added to the dust collection water, the location of the copper compound addition may be the solid-liquid separation equipment 40 or before the solid-liquid separation equipment 40, since the copper concentration is required to be 1 mg-Cu / L or more when hydrogen peroxide is reacted in the dust collection water before the solid-liquid separation treatment. As the location before the solid-liquid separation equipment 40, similar to the location of addition of hydrogen peroxide described above, it may be between the wet dust collector 2 and the solid-liquid separation equipment 40. For example, the copper compound may be added to a receiving tank that receives the dust collection water flowing from the wet dust collector 2 through the first flow path 31, a flow path from the receiving tank to the solid-liquid separation equipment 40, a second flow path 32 for sending the dust collection water from the wet dust collector 2 to the solid-liquid separation equipment 40, or the like in the middle of the first flow path 31.

[0055] When a copper compound is added to the dust collection water, it is preferable to provide a device for adding a copper compound to the dust collection water (copper compound adding device) 66 in the solid-liquid separation equipment 40, the second flow path 32, or the like. As shown in Figures 3 and 4, it is more preferable to provide the copper compound adding device 66 so that the second flow path 32 is the addition position of the copper compound. The copper compound adding device 66 can include, for example, a tank for storing the copper compound, and a pump and supply pipe for supplying the copper compound.

[0056] As described above, this method treats dust collection water using solid-liquid separation equipment 40 and circulation equipment 50. Therefore, this method includes subjecting the dust collection water, which has been reacted with hydrogen peroxide, to solid-liquid separation treatment in solid-liquid separation equipment 40, and supplying a part of the separated liquid, which has been separated from the suspended solids in the dust collection water in solid-liquid separation equipment 40, as wash water to wet dust collection treatment in circulation equipment 50.

[0057] It is possible to reduce the suspended solids concentration in the dust collection water by adding hydrogen peroxide, reacting the dust collection water, and subjecting the resulting water to solid-liquid separation treatment in solid-liquid separation equipment 40. The suspended solids concentration in the treated water (primary treated water, separated liquid) obtained after solid-liquid separation treatment is preferably less than 500 mg / L, more preferably less than 300 mg / L, and even more preferably less than 100 mg / L.

[0058] Examples of the solid-liquid separation equipment 40 include a precipitation device such as a thickener capable of carrying out precipitation treatment, a membrane separation device capable of carrying out membrane separation treatment, and a filtration device capable of carrying out filtration treatment. Among these, precipitation treatment using a precipitation device is preferred. The solid-liquid separation equipment 40 may be provided with a stirring mechanism 44 (see Figs. 2 to 4). The stirring mechanism 44 can also be used to stir the mixed liquid of the reaction system when reacting hydrogen peroxide in the presence of copper concentration of 1 mg-Cu / L or more in the dust collection water.

[0059] When a precipitation device 40 is used as the solid-liquid separation equipment 40, it is more preferable to use a precipitation device 40 including a center well 42, as shown in Figs. 2 to 4. The center well 42 is a portion formed in a bottomless cylindrical shape installed inside the precipitation device 40, and is a portion into which the dust-collected water flows in through the flow path (second flow path) 32. The center well 42 can rectify the dust-collected water that flows in through the flow path 32. As described above, by providing the center well 42, an ORP meter 64 and a pH meter (not shown) can be provided inside the center well 42, and thus the ORP and pH of the dust-collected water immediately after the addition of hydrogen peroxide can be easily measured.

[0060] Furthermore, when a settling device 40 is used as the solid-liquid separation facility 40, it is more preferable to use a settling device 40 equipped with an overflow weir section 46, as shown in Figures 2 to 4. The overflow weir section 46 is a section in the tank of the settling device 40 where the supernatant liquid (part of the separated liquid) that has been separated from the settled suspended solids overflows from the tank of the settling device 40 and is stored. The supernatant liquid that has overflowed into the overflow weir section 46 can be supplied to the wet dust collection process by the circulation facility 50 as wash water.

[0061] In addition, a flocculant may be used during the solid-liquid separation treatment. The type of flocculant is not particularly limited, and examples thereof include inorganic flocculants such as polyaluminum chloride, aluminum sulfate, polyferric sulfate, and ferric chloride, and polymer flocculants such as anionic polymer flocculants, nonionic polymer flocculants, and cationic polymer flocculants. One or more of these flocculants may be used.

[0062] In the solid-liquid separation equipment 40, a part of the separated liquid separated from solid contents such as suspended matter is supplied to the wet dust collection process (wet dust collector 2) as washing water for the exhaust gas by the circulation equipment 50. Since the separated liquid supplied to the wet dust collection process as washing water for the exhaust gas is treated water (primary treated water) that has been treated by this method, the separated liquid is circulated, but this method makes it possible to suppress a cumulative increase in the total cyanide concentration in the collected water in the circulation system.

[0063] A portion of the separated liquid separated from the solid content in the solid-liquid separation equipment 40 can be supplied to the circulation equipment 50 through a flow path (third flow path) 33 such as piping. The circulation equipment 50 can include a storage tank 52 for storing the separated liquid obtained in the solid-liquid separation equipment 40, and a flow path (fourth flow path) 54 such as piping and a circulation pump 56 for sending a portion of the separated liquid in the storage tank 52 to a wet dust collection process.

[0064] In addition, in this method, a part of the separated liquid obtained by the solid-liquid separation process (solid-liquid separation equipment 40) can be sent to the outside of the circulation equipment as primary treated water separately from the washing water to be used for the wet dust collection process. For example, the separated liquid obtained by the solid-liquid separation process can be sent to the storage tank 52 of the circulation equipment 50 through the third flow path 33, and a part of the separated liquid in the storage tank 52 can be sent to the outside of the circulation equipment as primary treated water through a flow path (fifth flow path) 35 such as a pipe. Although not shown, the solid-liquid separation equipment 40 and the fifth flow path (35) may be connected, and the primary treated water may be sent from the solid-liquid separation equipment 40 to the outside of the circulation equipment through the fifth flow path (35) separately from the separated liquid sent to the storage tank 52 of the circulation equipment 50. The primary treated water may be treated as water to be treated and may be subjected to a subsequent treatment step to further remove substances to be treated such as cyanide ions that may remain in the water to be treated.

[0065] The slurry containing solids such as suspended solids separated from the liquid in the solid-liquid separation equipment 40 is preferably subjected to either or both of a concentration treatment and a dehydration treatment to obtain a dehydrated cake and separated water. In addition, the obtained separated water is preferably sent to a solid-liquid separation treatment (solid-liquid separation equipment 40).

[0066] For example, as shown in FIG. 4, the slurry containing the solid content separated from the liquid content in the solid-liquid separation equipment 40 is sent to the dehydrator 80 through a flow path (sixth flow path) 36 such as a pipe, and dehydrated to obtain a dehydrated cake and separated water. Instead of the dehydrator 80, a concentration process using a concentration tank (not shown) may be used, or the concentration process using the concentration tank and the dehydrator 80 may be used in combination. The separated water (dehydrated filtrate) obtained in the dehydrator 80 is preferably sent to the solid-liquid separation equipment 40 through a flow path (seventh flow path) 37 such as a pipe, and more preferably through the second flow path 32. In this case, it is preferable that the seventh flow path 37 and the second flow path 32 are connected. In addition to the separated water (dehydrated filtrate) obtained in the dehydrator 80, the separated water obtained in the concentration tank may be sent to the solid-liquid separation equipment 40, and excess slurry may be sent thereto.

[0067] As described above in detail, the dust collection water treatment method according to one embodiment of the present invention can have the following configuration. [1] Adding hydrogen peroxide to collected water containing suspended solids, cyanide ions, and reducible sulfur components obtained by wet dust collection of exhaust gas; reacting said hydrogen peroxide in said dust collection water; adding the hydrogen peroxide to the reacted dust collection water and subjecting the resulting dust collection water to solid-liquid separation treatment in a solid-liquid separation facility; supplying a part of the separated liquid from the suspended solids in the dust collection water in the solid-liquid separation equipment as washing water to the wet dust collection treatment in a circulation equipment; The concentration (mg-H2O2 / L) of the hydrogen peroxide added to the dust collecting water is 1.2 times or more the total cyanide concentration (mg-CN / L) of the dust collecting water; The oxidation-reduction potential (ORP) of the dust collection water immediately after the addition of the hydrogen peroxide is 200 + 59 × (7-pH measurement value) mV or more; The method for treating dust collected water, wherein reacting the hydrogen peroxide in the dust collected water comprises reacting the hydrogen peroxide in the dust collected water in the presence of copper at a concentration of 1 mg-Cu / L or more. [2] The method for treating dust collected water according to the above [1], further comprising adding a copper compound to the dust collected water in a copper concentration of 1 mg-Cu / L or more when reacting the hydrogen peroxide in the dust collected water. [3] The method for treating dust collected water according to the above [1] or [2], wherein the concentration (mg-H2O2 / L) of the hydrogen peroxide added to the dust collected water is 7.0 times or less the total cyanide concentration (mg-CN / L) of the dust collected water. [4] The method for treating dust collected water according to any one of the above [1] to [3], wherein the dust collected water has a suspended solids concentration of 500 mg / L or more. [5] The method for treating dust collection water according to any one of the above [1] to [4], wherein the hydrogen peroxide is added to the dust collection water flowing through a flow path that supplies the dust collection water to the solid-liquid separation facility. [6] The method for treating dust collected water according to any one of the above [1] to [5], wherein the total cyanide concentration of the dust collected water is 3 mg-CN / L or more. EXAMPLES

[0068] The method for treating dust collection water according to one embodiment of the present invention will be described in more detail below with reference to preliminary experimental examples and test examples, but the present invention is not limited to the following test examples. Note that the preliminary experimental examples and test examples described below are experiments in which the circulation equipment described above is omitted, but the objective is to obtain treated water with a low residual cyanide rate, assuming that the treated water obtained in each experimental example is circulated and supplied as cleaning water for use in wet dust collection treatment of exhaust gas.

[0069] <Analysis method> In this test example, the analysis was carried out by the method described below.

[0070] (Total Cyan Density) The total cyanide concentration of each raw water used in this test example and each treated water obtained in this test example was measured by the total cyanide measurement method in JIS K0102: 2019. Specifically, the water was pretreated with "hydrogen cyanide generated at pH 2 or less" as specified in JIS K0102, 38.1.2, and the total cyanide (hereinafter sometimes abbreviated as "T-CN") concentration (mg-CN / L) was measured using a method based on the "4-pyridinecarboxylic acid-pyrazolone absorptiometry method" as specified in JIS K0102, 38.3.

[0071] (cyanide ion concentration) For each raw water used in this test example, the cyanide ion (hereinafter sometimes abbreviated as "F-CN") concentration (mg-CN / L) was measured using the thermal distillation method (hydrogen cyanide generated in the presence of zinc acetate at pH 5.5) among the cyanide measurement methods in JIS K0102:2019.

[0072] (Ammonia nitrogen concentration) For each raw water used in this test, ammonia nitrogen (hereinafter referred to as "NH4 + The concentration of EDTA (sometimes abbreviated as "-N") (mg-N / L) was measured.

[0073] (Thiocyanate ion concentration) The thiocyanate ion (hereinafter sometimes abbreviated as "SCN") concentration (mg-SCN / L) was measured for each raw water used in this test example and each treated water obtained in this test example by ion chromatography (reference: Hiraki et al., Analytical Chemistry, 52(11), 1019-1024). In addition, the concentration (mg-CN / L) of cyanide in thiocyanate ion (hereinafter sometimes abbreviated as "CN in SCN") was calculated for each treated water. Specifically, the molecular weight of SCN was set to 58g / mol, the molecular weight of CN was set to 26g / mol, and the concentration of CN in SCN was calculated by multiplying the measured SCN concentration by 26 / 58. Furthermore, for each treated water, the cyanide residual rate (hereinafter sometimes referred to as "CN residual rate") was calculated as a percentage (%) by dividing the sum of the T-CN concentration of the treated water and the CN concentration in SCN by the T-CN concentration of the simulated dust-collected water (raw water) to be treated.

[0074] (pH) The pH was measured using a pH meter (product name "D-71", manufactured by Horiba, Ltd.).

[0075] (Oxidation-Reduction Potential) The oxidation-reduction potential (ORP) was measured using an ORP meter (product name "RM-30P", manufactured by DKK-TOA Corporation; the ORP electrode connected to this was product name "PST-2729C", manufactured by DKK-TOA Corporation).

[0076] <Simulated dust collection water> In this test example, simulated dust collection water prepared by mixing raw water containing cyanide ions and the like with suspended solids, as described below, was used as the dust collection water to be treated.

[0077] (raw water) Wastewater containing suspended solids (SS) including unburned carbon, iron, and zinc, discharged from an exhaust gas treatment device that cleans exhaust gas at a specified factory, was treated by sedimentation to remove the suspended solids, and supernatant water (SS concentration less than 1 mg / L) was obtained. This supernatant water was used as raw water, and two types of raw water (raw water 1 and raw water 2) were prepared, collected on different days. The T-CN concentration, F-CN concentration, SCN concentration, and ammonia nitrogen concentration of each raw water are shown in Table 1.

[0078] TIFF2025025088000001.tif32170

[0079] (Suspended solids) Suspended solids (SS1) containing unburned carbon, iron, zinc, etc., were prepared and discharged from an exhaust gas treatment device that cleans exhaust gas at a specified factory. The elemental composition (unit: mass%) of these suspended solids (SS) was determined by X-ray fluorescence analysis and is shown in Table 2. In addition, the thiosulfate ion (SO3) contained in the total sulfur (S) in the suspended solids was also determined. 2- The sulfur content (mass%) was determined by iodometric titration (reference: "Method for Analyzing Sulfur Compounds in Blast Furnace Slag by Form", Iron and Steel, No. 9, 72nd year (1986). As shown in Table 2, suspended solids SS2 was prepared, which has an elemental composition very similar to that of suspended solids SS1 and does not contain sulfur. Specifically, suspended solids SS2 was prepared by mixing iron (III) oxide, silicon dioxide, aluminum oxide, magnesium oxide, zinc oxide, calcium carbonate, and graphite in a specified ratio.

[0080] TIFF2025025088000002.tif36170

[0081] <Preliminary experiment example> The raw water and SS shown in the "Simulated dust water" column of Table 3 were used. 200 mL of raw water and SS in an amount that would result in the SS concentration (mg / L) shown in the "Simulated dust water" column of Table 3 were mixed in a beaker to prepare simulated dust water. At this time, in preliminary experimental examples 1 to 5, an aqueous solution of 1 mass % of sodium thiosulfate, which is a soluble reducing sulfur compound, was further added to the simulated dust water to add thiosulfate ions (SO3 2-) was added to the simulated dust water adjusted to a temperature of 60°C and a pH of 7.0. Hydrogen peroxide solution with a concentration of 35% by mass, which corresponds to the amount of H2O2 shown in the "H2O2 added concentration" column in Table 3, was added to the simulated dust water, and the pH was checked and maintained at 7.0 with hydrochloric acid while stirring for 60 minutes. The liquid after the 60-minute reaction was then filtered with filter paper (Type 5 C) to remove SS, and treated water was obtained as a filtrate. Table 3 shows the measured values ​​of the T-CN concentration (soluble T-CN; mg-CN / L) and SCN concentration (soluble SCN; mg-SCN / L) of the obtained treated water, as well as the calculated values ​​of CN in SCN (mg / L) and CN residual rate (%).

[0082] TIFF2025025088000003.tif74170

[0083] From the results of the preliminary experiment examples, when hydrogen peroxide was added to dust collection water containing suspended solids, cyanide ions, and reductive sulfur components and reacted, the measured total cyanide concentration seemed to decrease somewhat. However, it was confirmed that thiocyanate ions were generated, and as a result, it was confirmed that the CN residual rate exceeded 50% (Preliminary Experiment Examples 1 to 7). In addition, from the results of preliminary experiment examples 1 to 3, it was confirmed that the amount of thiocyanate ions generated tends to increase with an increase in the content of SS in the simulated dust collection water, that is, an increase in the content of reductive sulfur components. Furthermore, from the results of preliminary experiment examples 2, 4, and 5 and the results of preliminary experiment examples 6 and 7, it was confirmed that the amount of thiocyanate ions generated tends to increase with an increase in the amount of hydrogen peroxide added. On the other hand, from the results of preliminary experiment example 8, it was confirmed that when the dust collection water containing suspended solids and cyanide ions does not contain reductive sulfur components, thiocyanate ions are hardly generated even if hydrogen peroxide is added to the dust collection water.

[0084] From the results of the above preliminary experiments, it was thought that when hydrogen peroxide is added to dust collection water containing suspended solids, cyanide ions, and reduced sulfur components, the cyanide ions react with the reduced sulfur to produce thiocyanate ions. The measured total cyanide concentration may have decreased by the amount of thiocyanate ions produced, and since thiocyanate ions are a type of COD component, it was thought that it would be more appropriate to evaluate the cyanide residual rate, which also takes into account the production of thiocyanate ions.

[0085] <Test Example> (Test Examples A to E) In a beaker, 200 mL of raw water shown in the "Simulated dust water" column in Table 4 and the amount of suspended solids SS1 that would result in the SS concentration (mg / L) shown in the "Simulated dust water" column were mixed to prepare the simulated dust water. At this time, in Test Examples D1 to D6, a 1 mass% aqueous solution of sodium thiosulfate, a soluble reducing sulfur compound, was further added to the simulated dust water to add thiosulfate ions (SO3 2- ) was contained at 50 mg / L.

[0086] The temperature of the above simulated dust-collected water was adjusted to 60°C, and the pH was adjusted to the value (7.0, 6.0, 8.0, or 9.0) shown in the "pH" column in the "Treatment Conditions" column of Table 4. To this simulated dust-collected water, a 35% by mass concentration of hydrogen peroxide solution corresponding to the H2O2 amount shown in the "H2O2 Addition Concentration" column of Table 4, and a 1% by mass concentration of copper sulfate (II) pentahydrate solution corresponding to the copper concentration shown in the "CuSO4 Addition Concentration" column of Table 4 were added to obtain a mixed solution (however, "0" in the above-mentioned added concentration column indicates that the corresponding agent was not added). In addition, the oxidation-reduction potential (ORP) of the simulated dust-collected water (mixed solution) at the above pH immediately after adding hydrogen peroxide was measured. In the above treatment process, while checking the pH of the mixed solution, the pH (7.0, 6.0, 8.0, or 9.0) of the simulated dust-collected water adjusted at the beginning was maintained with hydrochloric acid and sodium hydroxide, and the reaction was allowed to proceed for 60 minutes under stirring. The mixture was then filtered through filter paper (Type 5 C) to remove suspended solids, and treated water was obtained as the filtrate. The measured values ​​of the T-CN concentration (soluble T-CN; mg-CN / L) and SCN concentration (soluble SCN; mg-SCN / L) for the treated water obtained, as well as the calculated values ​​of CN in SCN (mg / L) and CN residual rate (%) are shown in Table 4.

[0087] TIFF2025025088000004.tif220170

[0088] (Test Example F) In Test Examples F1 to F5, the "CuSO4 additive concentration" in Test Example C7 was changed to the value shown in the "CuSO4 additive concentration" column in Table 5, but the test was performed in the same manner as Test Example C7 to obtain treated water. In Test Examples F6 to F10, the "1 mass% copper sulfate (II) pentahydrate aqueous solution" used in Test Example C7 was changed to a (1+1) hydrochloric acid solution of copper (I) oxide with a copper concentration shown in the "Cu2O additive concentration" column in Table 5, but the test was performed in the same manner as Test Example C7 to obtain treated water. Table 5 shows the measured values ​​of T-CN concentration (soluble T-CN; mg-CN / L) and SCN concentration (soluble SCN; mg-SCN / L) for the obtained treated water, as well as the calculated values ​​of CN in SCN (mg / L) and CN residual rate (%).

[0089] TIFF2025025088000005.tif89170

[0090] In the test examples A5 and A6, B5 and B6, C7-C10, D6, E2, and F1-F10, the amount of thiocyanate ions produced could be suppressed to a degree corresponding to the content of reductive sulfur components in the dust collection water, and the CN residual rate of the treated water could be reduced to 50% or less. To achieve this, it was confirmed that it is necessary to add hydrogen peroxide to the dust collection water containing suspended solids, cyanide ions, and reductive sulfur components at a concentration of 1.2 times or more the total cyanide concentration in the dust collection water. Regarding the concentration of hydrogen peroxide added to the dust collection water, it was considered that in order to reduce the CN residual rate of the treated water, the concentration is preferably 1.3 times or more the total cyanide concentration in the dust collection water, and more preferably 2.0 times or more. In addition, it was confirmed that the concentration of hydrogen peroxide added to the dust collection water needs to be 200+59×(7-pH measurement value) mV or more in the ORP of the dust collection water (mixed liquid) immediately after adding hydrogen peroxide. Furthermore, it was confirmed that the presence of copper in the dust water at a concentration of 1 mg-Cu / L or more is necessary when hydrogen peroxide is reacted in the dust water. [Explanation of symbols]

[0091] 11, 12, 13, 14: Dust collection and water treatment system 2: Wet type dust collector 31, 32, 33, 35, 36, 37: Flow path 40:Solid-liquid separation equipment 62: Hydrogen peroxide addition device 64:ORP meter 66:Copper compound addition device 50:Circulation equipment 52:Storage tank 54: Flow path 56: Circulation pump

Claims

1. Adding hydrogen peroxide to dust collection water containing suspended solids, cyanide ions, and reducible sulfur components obtained by wet dust collection of exhaust gas; reacting said hydrogen peroxide in said dust collection water; adding hydrogen peroxide to the collected dust water and subjecting the collected dust water to a solid-liquid separation treatment in a solid-liquid separation facility; supplying a part of the separated liquid from the suspended solids in the dust collection water in the solid-liquid separation equipment as washing water to the wet dust collection treatment in a circulation equipment; The concentration of hydrogen peroxide added to the dust collection water (mg-H ２ O ２ / L) is 1.2 times or more the total cyanide concentration (mg-CN / L) of the dust collection water, The oxidation-reduction potential (ORP) of the dust collecting water immediately after the addition of the hydrogen peroxide is 200 + 59 × (7 - pH measurement value) mV or more; The method for treating dust collected water, wherein reacting the hydrogen peroxide in the dust collected water comprises reacting the hydrogen peroxide in the dust collected water in the presence of copper at a concentration of 1 mg-Cu / L or more.

2. 2. The method for treating dust collected water according to claim 1, further comprising the step of adding a copper compound to the dust collected water in a copper concentration of 1 mg-Cu / L or more when reacting the hydrogen peroxide in the dust collected water.

3. The concentration of hydrogen peroxide added to the dust collection water (mg-H ２ O ２ 2. The method for treating dust collected water according to claim 1, wherein the cyanide concentration (mg-CN / L) of the dust collected water is 7.0 times or less relative to the total cyanide concentration (mg-CN / L) of the dust collected water.

4. 2. The method for treating dust-collected water according to claim 1, wherein the concentration of suspended solids in the dust-collected water is 500 mg / L or more.

5. 2. The method for treating dust collected water according to claim 1, further comprising the step of adding the hydrogen peroxide to the dust collected water flowing through a flow path that supplies the dust collected water to the solid-liquid separation facility.

6. 2. The method for treating dust-collected water according to claim 1, wherein the total cyanide concentration in the dust-collected water is 3 mg-CN / L or more.

Images