Treatment method for water to be treated

Abstract

This invention provides a low-cost method for treating water containing cyanide ions. [Solution] A method for treating water to be treated, comprising step (a) of mixing water to be treated containing cyanide ions with hypochlorite, wherein in step (a), the pH of the mixed liquid is maintained at 8.0 or higher and the oxidation-reduction potential at +800mV or higher.

Description

Technical Field

[0006] , ,

[0001] The present invention relates to a method for treating treated water.

Background Art

[0002] Conventionally, as a method for treating cyanide wastewater, an alkaline chlorine method in which chlorine is allowed to act in two stages to decompose cyanide is known. For example, in Patent Document 1, a slurry liquid is made alkaline to pH 10 to 13, heated and stirred for 30 minutes or more, and then hypochlorite is added to the slurry liquid in portions, and reacted with free cyanide, complex cyanide, and poorly soluble cyanide compounds in the above pH range and a temperature range of 80 to 100°C.

[0003] Also, for example, in Patent Document 2, a waste liquid containing at least one of free cyanide, complex cyanide, and a reducing compound showing volatility in an alkaline aqueous solution is heated to a temperature range within room temperature to boiling point and including a high temperature range of 80°C or higher under alkaline conditions, and then the temperature is maintained while detecting the oxidation-reduction potential, and hypochlorite is continuously or intermittently added to the waste liquid until the oxidation-reduction potential of hypochlorite is detected in the waste liquid.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] The methods described in Patent Document 1 and Patent Document 2 require heating to 80°C or higher, which is disadvantageous in terms of energy and results in high treatment costs.

[0006] Therefore, the present invention aims to provide a method for treating water containing cyanide ions at low cost. [Means for solving the problem]

[0007] A first aspect of the present invention is: The process includes a step (a) of mixing the water to be treated, which contains cyanide ions, with hypochlorite. Step (a) described above is a method for treating water to be treated, in which the pH of the mixed liquid is maintained at 8.0 or higher and 11.0 or lower, and the oxidation-reduction potential is maintained at +800mV or higher.

[0008] A second aspect of the present invention is: The process further comprises step (b) preparing a reaction vessel containing a solution having an oxidation-reduction potential of +700mV or higher, The method for treating water to be treated according to the first embodiment is as described above, wherein step (a) involves adding the water to be treated and an aqueous solution of the hypochlorite to the reaction tank and mixing them.

[0009] A third aspect of the present invention is: The method for treating water to be treated according to the first embodiment, wherein step (a) involves mixing the water to be treated with an aqueous solution of the hypochlorite.

[0010] A fourth aspect of the present invention is: The method for treating water to be treated according to any one of the first to third embodiments described above, wherein the hypochlorite is sodium hypochlorite.

[0011] A fifth aspect of the present invention is: The method for treating water to be treated according to any one of the first to fourth embodiments, wherein step (a) maintains the pH of the liquid being mixed at 9.0 or higher and 10.5 or lower.

[0012] A sixth aspect of the present invention is: The method for treating water to be treated according to any one of the first to fifth embodiments, wherein step (a) maintains the temperature of the liquid being mixed at 20°C or higher and 50°C or lower.

[0013] A seventh aspect of the present invention is: The method for treating water to be treated according to any one of the first to sixth embodiments described above, wherein step (a) does not involve external heating of the liquid being mixed.

[0014] An eighth aspect of the present invention is: The method for treating water to be treated according to any one of the first to seventh embodiments described above, wherein the concentration of cyanide ions in the water to be treated is greater than 200 ppm.

[0015] A ninth aspect of the present invention is: The solution prepared in step (b) is the treated water obtained by treating the water to be treated separately using the method for treating water to be treated described in any one of the first to eighth embodiments described above, which is the method for treating water to be treated described in the second embodiment.

[0016] A tenth aspect of the present invention is: The method for treating water to be treated according to the second or ninth embodiment, further comprising step (a) in which the water to be treated and an aqueous solution of the hypochlorite are continuously introduced into the reaction tank, and any liquid exceeding a predetermined proportion of the reaction tank's capacity is recovered into a recovery tank.

[0017] An eleventh aspect of the present invention is: The treatment method for water to be treated according to the tenth embodiment described above, wherein the concentration of cyanide ions in the liquid recovered in the recovery tank is 0.5 ppm or less, and the concentration of cyanate ions is 6 ppm or less.

[0018] A twelfth aspect of the present invention is: The method for treating water to be treated according to any one of the second and ninth to eleventh embodiments, wherein the reaction vessel prepared in step (b) contains a solution that is 50% or more of its effective capacity.

[0019] A thirteenth aspect of the present invention is: In the method for treating the water to be treated according to any one of the second and ninth to twelfth aspects, the concentration of cyanide ions in the solution contained in the reaction tank prepared in step (b) is 0.5 ppm or less, and the concentration of cyanate ions is 6 ppm or less.

[0020] A fourteenth aspect of the present invention is In step (a), the rate of introducing the water to be treated into the reaction tank is controlled so that the amount of cyanide ions introduced into the reaction tank per liter of the solution in the reaction tank per minute is 60 mg or less. This is the method for treating the water to be treated according to any one of the second and ninth to thirteenth aspects.

[0021] A fifteenth aspect of the present invention is In step (a), the rates of introducing the water to be treated and the aqueous solution of hypochlorite into the reaction tank are controlled so that the amount of hypochlorite ions introduced into the reaction tank is 2.5 moles or more and 5.0 moles or less per mole of cyanide ions introduced into the reaction tank. This is the method for treating the water to be treated according to any one of the second and ninth to fourteenth aspects.

Advantages of the Invention

[0022] According to the present invention, water to be treated containing cyanide ions can be treated at low cost.

Brief Description of the Drawings

[0023] [Figure 1] FIG. 1 is a flowchart showing an example of a method for treating water to be treated according to a first embodiment of the present invention. [Figure 2] FIG. 2 is a schematic diagram showing an example of a treatment apparatus used in the method for treating water to be treated according to the first embodiment of the present invention.

Embodiments for Carrying Out the Invention

[0024] [Details of Embodiments of the Present Invention] One embodiment of the present invention will be described below with reference to the drawings. However, the present invention is not limited to these examples and is intended to include all modifications within the meaning and scope equivalent to the claims, as shown in the claims.

[0025] In this specification, "A to B" means a numerical range of "A or greater and B or less," where A and B are numbers that satisfy the condition A > B.

[0026] <Overview of the conventional alkaline chlorine method> First, let's explain the outline of the conventional alkaline chlorine method. In the alkaline chlorine method, sodium hypochlorite (NaOCl) is generally used as the chlorine to decompose cyanide. In the alkaline chlorine method, decomposition is carried out in two stages: reaction (1) and reaction (2) below. From these reaction equations, 2.5 moles of sodium hypochlorite are required to decompose 1 mole of cyanide ions. Note that cyanide exists in solution as cyanide ions or complex ions, and cyanide that is strongly bound to complex ions is difficult to decompose with the alkaline chlorine method. NaCN+NaOCl→NaCNO+NaCl ···(1) 2NaCNO+3NaOCl+H2O→N2+3NaCl+2NaHCO3...(2)

[0027] Reaction (1) takes place at a pH of 10 or higher. In reaction (1), sodium cyanide (NaCN) is oxidized by chlorine in an alkaline solution to produce sodium cyanate (NaCNO). The oxidation-reduction potential (ORP) of wastewater containing cyanide ions is low, and as reaction (1) progresses (sodium hypochlorite is added), the ORP increases, and the reaction is complete when it reaches +300 to +350 mV.

[0028] Reaction (2) is carried out at pH 7-8. The reason for adjusting the pH to neutral in reaction (2) is to promote the decomposition of sodium cyanate. As reaction (2) progresses (sodium hypochlorite is added), the ORP will rise further, and the reaction is complete when it reaches +600-+650mV.

[0029] Thus, conventional alkaline chlorination methods involve a two-step decomposition reaction, requiring pH adjustment during the process. The techniques disclosed in Patent Documents 1 and 2 carry out reaction (1) of the alkaline chlorination method at a high temperature of 80°C or higher, thereby allowing reaction (2) to proceed without adjusting the pH to 7-8.

[0030] <First Embodiment of the Invention> Next, the method for treating the water to be treated in this embodiment will be described. The method for treating the water to be treated in this invention is based on the finding that when mixing water to be treated containing cyanide ions with hypochlorite, maintaining the pH of the mixed solution at 8.0 to 11.0 and the oxidation-reduction potential at +800mV or higher allows for the suitable decomposition and removal of cyanide ions without heating.

[0031] In this embodiment, the water to be treated (cyanide wastewater) containing cyanide ions is mixed with a hypochlorite (sodium hypochlorite in this embodiment) to decompose the cyanide ions. Figure 1 is a flowchart showing an example of the water to be treated method in this embodiment. As shown in Figure 1, the water to be treated method in this embodiment includes, for example, a reaction tank preparation step S101 (step (b)), a mixing step S102 (step (a)), and a recovery step S103 (step (c)).

[0032] The water to be treated in this embodiment contains, for example, cyanide ions at a concentration of more than 200 ppm or 3 to 50 g / L. When water with such a high concentration of cyanide ions is treated using the conventional alkaline chlorination method, heat generation and foaming may occur. In the treatment method of this embodiment, by the ingenuity described later, even water with a high concentration of cyanide ions can be treated while suppressing heat generation and foaming.

[0033] Furthermore, it is preferable that the treated water does not contain metal elements that form complex ions with cyanide through strong bonding, such as iron, cobalt, and gold. Specifically, the concentrations of iron, cobalt, and gold are preferably 200 ppm or less each, and more preferably 5 ppm or less each. It is also preferable that the treated water does not contain other metals (except sodium and potassium), and the concentrations of other metals are preferably 200 ppm or less each, and more preferably 50 ppm or less each. Although copper binds with cyanide ions, this bond is easily broken by the alkaline chlorination method and does not inhibit the decomposition of cyanide ions. Sodium and potassium are often found in cyanide wastewater, and like copper, they bind with cyanide ions but do not inhibit the decomposition of cyanide ions.

[0034] In the water to be treated in the water treatment method of the present invention, the cyanide ions are so-called free cyanide, which are not bound to metals such as iron (excluding sodium and potassium). Free cyanide can be quantified by the 4-pyridinecarboxylic acid colorimetric method, as described in the examples below. This method can also quantify cyanide ions bound to sodium and potassium as free cyanide. Furthermore, in this embodiment, since the water to be treated typically contains almost no metals except for sodium and potassium, as described above, the amount of free cyanide is approximately the same as the amount of total cyanide including bound cyanide.

[0035] The pH of the water to be treated is, for example, 10 to 14, in order to keep cyanide ions dissolved in the water.

[0036] (Reaction tank preparation step S101 (step (b))) Figure 2 is a schematic diagram showing an example of a treatment apparatus used in the treatment method for water to be treated according to this embodiment. In the reaction vessel preparation step S101, for example, a reaction vessel 3 is prepared containing a solution having an oxidation-reduction potential (ORP) of +700mV or higher (preferably +750mV or higher, more preferably +780mV or higher). The upper limit of the ORP of the solution is not particularly limited, but for example, the ORP is +1000mV or lower.

[0037] From the viewpoint of ensuring sufficient decomposition of cyanide ions in this embodiment, the concentrations of iron, cobalt, and gold in the solution are preferably 200 ppm or less, and more preferably 5 ppm or less. Similarly, from the viewpoint of ensuring sufficient decomposition of cyanide ions, the concentrations of other metals, excluding sodium and potassium, are preferably 200 ppm or less, and more preferably 50 ppm or less. From the viewpoint of ensuring sufficient decomposition of cyanide ions, the concentration of cyanide ions in the solution is preferably 0.5 ppm or less and the concentration of cyanate ions is preferably 6 ppm or less, and more preferably the concentration of cyanide ions is preferably 0.3 ppm or less and the concentration of cyanate ions is preferably 4 ppm or less.

[0038] The pH of the solution is preferably 8.0 to 11.0, and more preferably 9.0 to 10.5, in order to facilitate the mixing step S102 (step (a)) described below.

[0039] The solution prepared in the reaction vessel preparation step S101 described above can be prepared, for example, by adding an oxidizing agent such as sodium hypochlorite to water and adjusting the pH to the aforementioned range with a pH adjusting agent such as an acid or alkali (the order in which sodium hypochlorite and the pH adjusting agent are added is arbitrary).

[0040] The reaction vessel 3 is preferably a sealed structure. The reaction vessel 3 is preferably equipped with a stirrer 5 for stirring the liquid, an ORP sensor 8 for measuring ORP, a pH sensor 9 for measuring pH, and a temperature sensor 10 for measuring temperature. Furthermore, as shown in Figure 2, it is preferable to prepare multiple (for example, two) reaction vessels 3, and to configure them so that any liquid exceeding the predetermined capacity of the first reaction vessel 3a is transferred to the second reaction vessel 3b through piping or the like. Hereinafter, the volume of liquid that can be contained in the reaction vessel 3 will be referred to as the effective capacity of the reaction vessel 3. The effective capacity can also be conceived for each of the reaction vessels 3a and 3b.

[0041] In the processing method of this embodiment, the decomposition reaction of cyanide ions takes place in the reaction vessel 3. Since the solution in the reaction vessel 3 has a high ORP from the beginning, the decomposition of cyanide ions can be made to proceed more easily.

[0042] It is preferable that the reaction vessel prepared in reaction vessel preparation step S101 contains at least 50% of its effective capacity of solution, and more preferably at least 80% of its effective capacity. This reduces the amount of water to be treated introduced into the reaction vessel relative to the solution already contained in the vessel, especially in the initial stages of the cyanide ion decomposition treatment, making it easier to suppress heat generation and foaming. Alternatively, the reaction vessel may be filled with the full effective capacity (approximately 100%) of the solution.

[0043] (Mixing step S102 (step (a))) Mixing step S102 is a step in which the water to be treated, which contains cyanide ions, is mixed with a hypochlorite (aqueous solution). In this embodiment, as shown in Figure 2, the water to be treated, which is contained in the water to be treated storage tank 2, and the sodium hypochlorite aqueous solution, which is contained in the chemical tank 1 are added to the solution in the reaction tank 3 and mixed. The water to be treated and the sodium hypochlorite aqueous solution can be added using pumps 11 and 12 (preferably metering pumps). It is preferable to stir the mixture during mixing using a stirrer 5.

[0044] In mixing step S102, the pH of the mixture (i.e., the liquid in reaction vessel 3) is maintained at 8.0 to 11.0, and the oxidation-reduction potential is maintained at +800mV or higher (preferably +820mV or higher, more preferably +850mV or higher). Specifically, the amount of water to be treated and sodium hypochlorite aqueous solution added should be controlled so that the pH and ORP are within the above ranges. As a result, the two-stage decomposition reaction described above is thought to occur at once, eliminating the need for pH adjustment when moving to the second stage of the reaction, and allowing for simpler treatment of water to be treated than the conventional alkaline chlorination method. Furthermore, as described in Patent Documents 1 and 2, there is no need to heat to 80°C or higher, making it an energy-advantageous and low-cost method. The upper limit of the ORP of the mixture during mixing is not particularly limited, but for example, the ORP can be controlled to +1000mV or lower. Furthermore, regarding the above-mentioned "maintaining the oxidation-reduction potential at +800mV or higher," if the oxidation-reduction potential of the solution prepared in reaction vessel 3 (which is +700mV or higher as mentioned above) is less than 800mV, it is not necessary for the oxidation-reduction potential to be at +800mV or higher from the moment mixing of the water to be treated and the sodium hypochlorite solution begins. Instead, the oxidation-reduction potential can be adjusted to +800mV or higher within, for example, 5 minutes from the start of mixing by controlling the amount of sodium hypochlorite solution added.

[0045] In this embodiment, since the ORP of the liquid initially in reaction tank 3 is +700mV or higher, it is easy to add the water to be treated and the sodium hypochlorite aqueous solution to achieve an ORP of +800mV or higher in the resulting mixture. In conventional alkaline chlorination methods, including those described in Patent Documents 1 and 2, a sodium hypochlorite aqueous solution is added to cyanide wastewater (whose ORP is usually negative). As a result, although the ORP of the mixture after the aqueous solution is added increases with the addition of the aqueous solution, the first stage reaction ends when the ORP reaches about 300-350mV, and the second stage reaction ends when the ORP reaches about 600-650mV. On the other hand, in this embodiment, the ORP of the liquid in reaction tank 3 at the start is set to a high value of +700mV or higher, and the water to be treated and the sodium hypochlorite aqueous solution are added at a controlled rate so that the ORP of the mixture after addition is maintained at +800mV.

[0046] In this embodiment, although the mechanism by which the second-stage reaction of the alkaline chlorine method occurs without the need for pH adjustment or external heating is unclear, the inventors have considered the following: In this embodiment, the pH of the mixture is maintained in a range of 8.0 to 11.0, which is not far from the pH range in which the first and second-stage reactions proceed favorably. Furthermore, the oxidation-reduction potential of the mixture is maintained at a high value of +800mV or higher, resulting in strong oxidizing power and the occurrence of the first-stage reaction. It is thought that localized heat generation occurs at the site where the first-stage reaction occurs, and that, combined with the strong oxidizing environment and the predetermined pH environment, the second-stage reaction also proceeds locally at that site.

[0047] In mixing step S102, it is preferable to maintain the pH of the mixed liquid at 9.0 to 10.5. This suppresses the gasification of cyanide ions while facilitating their decomposition. The pH can be adjusted to the above range by adding conventionally known pH adjusting agents such as acids or alkalis.

[0048] Regarding the mixing step S102, the cyanide ion decomposition reaction in this embodiment can be carried out at room temperature and does not require external heating as in the technologies of Patent Documents 1 and 2. As mentioned above, the reaction in the alkaline chlorine method is an exothermic reaction, which may cause the temperature of the liquid during mixing to rise slightly. In this way, in this embodiment, the temperature of the liquid during mixing is maintained, for example, between 20°C and 50°C, and does not become excessively high, which is preferable from a safety standpoint. Furthermore, it is preferable not to apply external heating to the liquid during mixing in the mixing step S102. In the processing method of this embodiment, the decomposition of cyanide ions proceeds even within the aforementioned temperature range, so there is no need for heating, which is advantageous in terms of energy costs. Rather, since the cyanide ion decomposition reaction in this embodiment is an exothermic reaction, it is preferable to perform the cyanide ion decomposition treatment while cooling the mixed liquid to maintain the temperature of the mixed liquid within the aforementioned range.

[0049] Furthermore, maintaining the temperature of the liquid during mixing within the above range is preferable from the standpoint of suppressing the gasification of cyanide ions. To confirm that the gasification of cyanide ions is suppressed, for example, a hydrogen cyanide gas sensor 6 can be placed in a duct 7 connected to a scrubber that recovers the generated gas, and the hydrogen cyanide gas flowing through the duct 7 can be measured. If the liquid during mixing needs to be cooled, a cooling means such as a chiller can be provided in the reaction vessel 3.

[0050] In the mixing step S102, it is preferable to control the rate at which the water to be treated is added to the reaction vessel 3 so that the amount of cyanide ions added per minute per liter of solution in the reaction vessel 3 is 60 mg or less. This suppresses heat generation and foaming. From the viewpoint of maintaining the processing speed within a suitable range, it is preferable to control the rate at which the water to be treated is added so that the amount of cyanide ions added per minute per liter of solution in the reaction vessel 3 is 20 mg or more.

[0051] In the mixing step S102, it is preferable to control the rate at which the water to be treated and the aqueous solution of hypochlorite (sodium hypochlorite aqueous solution) are added to the reaction vessel 3 so that the amount of hypochlorite ions added to the reaction vessel 3 is between 2.5 moles and 5.0 moles for every 1 mole of cyanide ions added to the reaction vessel 3. This allows the ORP of the mixed solution in the reaction vessel 3 to be maintained at +800mV or higher, thereby enabling the decomposition of cyanide ions to proceed and be completed effectively. The rate (amount) of adding the aqueous solution of hypochlorite may be controlled by measuring the ORP of the mixed solution over time and ensuring that it does not fall below a predetermined value of +800mV or higher.

[0052] (Recovery process S103 (process (c))) The recovery step S103 is a step in which, for example, water to be treated and an aqueous solution of hypochlorite are continuously fed into the reaction tank 3, and the liquid exceeding a predetermined percentage (e.g., effective capacity) of the capacity of the reaction tank 3 is recovered into the recovery tank 4. In the example shown in Figure 2, the liquid exceeding a predetermined capacity of the reaction tank 3, or more specifically, the reaction tank 3b, is transported to the recovery tank 4 as post-treatment liquid through piping, etc. In this embodiment, the decomposition reaction of cyanide ions proceeds in a short time, and it is thought that most of the cyanide ions contained in the water to be treated are decomposed while the water to be treated is being fed into the reaction tank 3a and remaining there. By installing the reaction tank 3b and allowing the liquid transported from the reaction tank 3a to remain there, the decomposition of cyanide ions is made even more sufficient. Furthermore, by configuring the system to transport the liquid exceeding a predetermined percentage (e.g., effective capacity) of the reaction tank 3b to the recovery tank 4, it becomes easier to continuously feed water to be treated and an aqueous solution of hypochlorite into the reaction tank 3 (3a) and to continuously treat the water to be treated.

[0053] In the treated liquid recovered in the recovery tank 4, the decomposition of cyanide ions present in the treated water has progressed sufficiently, and the concentration of cyanide ions is, for example, 0.5 ppm or less, and the concentration of cyanate ions is, for example, 6 ppm or less. Preferably, the concentration of cyanide ions in the treated liquid is 0.3 ppm or less, and the concentration of cyanate ions is 4 ppm or less.

[0054] The liquid remaining in reaction tank 3 (post-treatment liquid), like the liquid recovered in recovery tank 4, is a liquid in which cyanide ions have been decomposed and the treatment is complete, and it has a high ORP. Therefore, the liquid remaining in reaction tank 3 is suitable as a solution to be prepared in advance in reaction tank preparation step S101 when separately carrying out the treatment method of the water to be treated according to the present invention. In other words, it is preferable that the solution prepared in advance in reaction tank preparation step S101 is post-treatment water obtained by treating the water to be treated according to the treatment method of the water to be treated according to the present invention.

[0055] Through the above process, it is possible to treat water containing cyanide ions simply and at low cost.

[0056] <Other embodiments of the present invention> Although embodiments of the present invention have been specifically described above, the present invention is not limited to the embodiments described above, and various modifications are possible without departing from the spirit of the invention.

[0057] For example, in the above embodiment, a reaction vessel 3 containing a solution with an ORP of +700mV or higher was prepared, and the water to be treated and an aqueous sodium hypochlorite solution were added to the reaction vessel 3 and mixed. However, the reaction vessel preparation step S101 may be omitted. Even if the reaction vessel preparation step S101 is omitted, the water to be treated can be treated at low cost by maintaining the pH of the mixed liquid at 8.0 to 11.0 and the oxidation-reduction potential at +800mV or higher during the mixing step S102. [Examples]

[0058] Next, embodiments of the present invention will be described. These embodiments are examples of the present invention, and the present invention is not limited to these embodiments.

[0059] <Example 1> For the treated water (concentrated cyanide wastewater), pH, ORP, free cyanide concentration, cyanate concentration, and the concentrations of various metals (ions) were measured. The results are shown in Table 1. In this example, the 4-pyridinecarboxylic acid colorimetric method (Kyoritsu Chemical Laboratory, Pack Test Free Cyanide WAK-CN-2) was used to measure the free cyanide concentration. The absorbance at 603 nm of the sample solution colored by this method was measured using the absorbance spectroscopy method (Ocean Photonics, FLAME-S) to determine the free cyanide concentration. An ion chromatograph (Tosoh, IC-8100EX) was used to measure the cyanate concentration. A pH meter and electrode (HORIBA, D-73) was used to measure pH, ORP, and liquid temperature. An ICP emission spectrometer (Hitachi High-Tech Science, SPS-5100) was used to measure the concentrations of various metals (ions). A hydrogen cyanide meter (GAXT-Z-DL, manufactured by Ichinen Jiko) was used to measure hydrogen cyanide (HCN).

[0060] Using the treatment apparatus shown in Figure 2, cyanide ions were decomposed in treated water (concentrated cyanide wastewater) with the composition shown in Table 1 below. Based on the origin of the concentrated cyanide wastewater, the amounts of all metal elements, excluding sodium and potassium, that could be contained in the wastewater were measured.

[0061] [Table 1]

[0062] The two reaction tanks 3 (3a, 3b) were pre-filled with the post-treatment liquid, which had been obtained by separately carrying out the treatment method of the present invention, to the maximum effective capacity (5L) of each reaction tank 3 (the liquid was filled to the very bottom of the opening of the piping connecting the two reaction tanks 3). The pH of the post-treatment liquid was 9.64, and the ORP was +789mV. Free cyanide and cyanic acid were not detected in the post-treatment liquid, and the concentrations of each metal (ion) were approximately the same as those of the treated water shown in Table 1.

[0063] The water to be treated, contained in the water storage tank 2, was quantitatively supplied to the reaction tank 3 at a rate of 10 mL / min using pump 12. The 12% by mass sodium hypochlorite aqueous solution contained in the chemical tank 1 was continuously added using pump 11, adjusting the supply rate so that the ORP value in reaction tank 3 was +850 mV or higher (the ORP value exceeded +800 mV within 1 minute of the start of mixing of the water to be treated and the sodium hypochlorite aqueous solution). Specifically, the supply rate of the sodium hypochlorite aqueous solution was approximately 35 mL / min. Since the treated solution was continuously discharged into the recovery tank 4 at approximately 45 mL / min, the reaction time in each reaction tank (effective capacity 5 L) is estimated to be approximately 111 min. In addition, the amount of cyanide ions added per minute per liter of solution in reaction tank 3 was 44.7 mg. For every 1 mole of cyanide ions added to reaction tank 3, 3.8 moles of hypochlorite ions were added to reaction tank 3.

[0064] Reaction vessel 3 was cooled by placing it in a water tank. While the water to be treated and sodium hypochlorite were being supplied, the pH and temperature of the mixture in reaction vessel 3 were continuously measured. The pH was in the range of 9.49 to 9.66, and the liquid temperature was in the range of 22.4 to 33.4°C. In addition, the gas discharged to the scrubber was continuously measured using a hydrogen cyanide gas sensor 6, and it was confirmed that hydrogen cyanide was always undetectable (less than 0.1 ppm). No foaming occurred in reaction vessel 3. After treatment, the solution was sampled every 15 to 40 minutes from the start of treatment, and the concentrations of free cyanide and cyanic acid were measured. The results are shown in Table 2 below. Note that in Table 2, the concentration value at 0 min of treatment time is the value for the solution prepared in advance in reaction vessel 3.

[0065] [Table 2]

[0066] As shown in Table 2, virtually no free cyanide or cyanic acid was detected in the treated solution. This suggests that the cyanide ions in the treated water were decomposed in a short time as soon as the water was supplied to the reaction tank 3. Furthermore, since the amount of metals (other than sodium and potassium) in the treated solution is trace, the amount of cyanide forming a complex (amount of bound cyanide) is estimated to be only a few ppm.

[0067] Based on the above, it was confirmed that this treatment method can treat water containing cyanide ions at a low cost.

[0068] <Comparative Example 1> Except for the fact that the pH of the solution prepared in advance in reaction vessel 3 was 9.45 and the ORP was +770mV, and that an aqueous sodium hypochlorite solution was supplied so that the ORP value of the liquid in reaction vessel 3 was between +700mV and +770mV, the water to be treated was treated in the same manner as in Example 1, as shown in Table 1. After treatment, the treated liquid was sampled every 10 minutes from the start of treatment, and the concentrations of free cyanide and cyanic acid were measured. The results are shown in Table 3.

[0069] [Table 3]

[0070] As shown in Table 3, in Comparative Example 1, unlike Example 1, the treated solution contained a large amount of undecomposed cyanate. Furthermore, the cyanate concentration increased over time, indicating that the second stage of the alkaline chlorination method had not progressed sufficiently. When the cyanate concentration exceeds 200 ppm, it generally falls into a range unsuitable for the alkaline chlorination method, making it difficult to apply the conventional alkaline chlorination method again to the treated solution (even if it could be applied, the treatment cost of the treated water would increase), and it is expected that the decomposition of cyanide ions will become even more difficult. [Explanation of Symbols]

[0071] 1. Chemical storage tank 2. Water storage tank for treated water 3 Reaction vessels 4. Recovery tank 5 Stirrer 6. Hydrogen cyanide gas sensor 7 ducts 8 ORP sensors 9 pH sensor 10 Temperature Sensor 11 pumps 12 pumps S101 Reaction vessel preparation process S102 Mixing process S103 Recovery Process

Claims

1. The process includes a step (a) of mixing the water to be treated, which contains cyanide ions, with hypochlorite. A method for treating water to be treated, wherein in step (a), the pH of the mixed liquid is maintained at 8.0 or higher and the oxidation-reduction potential is maintained at +800 mV or higher.

2. The process further includes step (b) of preparing a reaction vessel containing a solution having an oxidation-reduction potential of +700 mV or higher, The method for treating water to be treated according to claim 1, wherein in step (a), the water to be treated and an aqueous solution of the hypochlorite are added to the reaction tank and mixed.

3. The method for treating water to be treated according to claim 1, wherein in step (a), the water to be treated is mixed with an aqueous solution of the hypochlorite.

4. The method for treating water to be treated according to claim 1 or 2, wherein the hypochlorite is sodium hypochlorite.

5. The method for treating water to be treated according to claim 1 or 2, wherein in step (a), the pH of the liquid being mixed is maintained at 9.0 or higher and 10.5 or lower.

6. The method for treating water to be treated according to claim 1 or 2, wherein in step (a), the temperature of the liquid being mixed is maintained at 20°C or higher and 50°C or lower.

7. The method for treating water to be treated according to claim 1 or 2, wherein in step (a), no external heating is applied to the liquid being mixed.

8. The method for treating water to be treated according to claim 1 or 2, wherein the concentration of cyanide ions in the water to be treated is greater than 200 ppm.

9. The method for treating water to be treated according to claim 2, wherein the solution prepared in step (b) is treated water obtained by treating the water to be treated according to the method for treating water to be treated according to claim 1 or 2.

10. The method for treating water to be treated according to claim 2, further comprising step (c) in step (a) above, in which the water to be treated and the aqueous solution of the hypochlorite are continuously introduced into the reaction tank, and any liquid exceeding a predetermined proportion of the capacity of the reaction tank is recovered into a recovery tank.

11. The method for treating water to be treated according to claim 10, wherein the concentration of cyanide ions in the liquid recovered in the recovery tank is 0.5 ppm or less, and the concentration of cyanate ions is 6 ppm or less.

12. The method for treating water to be treated according to claim 2, wherein the reaction vessel prepared in step (b) contains a solution that is 50% or more of its effective capacity.

13. The method for treating water to be treated according to claim 2, wherein the concentration of cyanide ions in the solution contained in the reaction vessel prepared in step (b) is 0.5 ppm or less, and the concentration of cyanate ions is 6 ppm or less.

14. The method for treating water to be treated according to claim 2, wherein in step (a), the rate at which the water to be treated is introduced into the reaction vessel is controlled so that the amount of cyanide ions introduced per minute per liter of solution in the reaction vessel is 60 mg or less.

15. The method for treating water to be treated according to claim 2 or 14, wherein in step (a), the rate at which the water to be treated and the aqueous solution of the hypochlorite salt are introduced into the reaction vessel is controlled so that the amount of hypochlorite ions introduced into the reaction vessel is 2.5 moles or more and 5.0 moles or less for every 1 mole of cyanide ions introduced into the reaction vessel.

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