Method for treating cyanide-containing wastewater

Abstract

To provide a treatment method capable of converting cyan-containing effluent to a safer treated liquid by decomposing cyan, not by adsorbing nor by filtering cyan, without using a toxic / deleterious substance, among methods of treating cyan-containing effluent including a toxic cyan component generated when a small amount of sodium cyanide or potassium cyanide is used in an experimentation facility, an analysis facility or the like.SOLUTION: The method of treating cyan-containing effluent, comprises the steps of: adding reducing sugar to cyan-containing effluent, and reacting the reducing sugar and cyan present in the effluent under the condition of pH 7 to 11 to produce cyanohydrin; subsequently adding an acid to the effluent to hydrolyze the cyanohydrin under an acidic condition; and subsequently neutralizing the effluent.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for treating cyanide-containing waste liquid, and specifically to a novel method for simply and safely treating cyanide-containing waste liquid without requiring harmful treatment agents, adsorbents or filtration steps. [Background Art]

[0002] Conventionally, as treatment methods for cyanide-containing waste liquid, the alkali chlorine method and a treatment method using a treatment agent containing formaldehyde are known, but both methods have drawbacks in terms of toxicity and economic efficiency. In addition, when the amount of waste liquid is relatively small, it is common practice to detoxify it within the facility or alkalinize and store the waste liquid before transporting it to a detoxification facility. However, even if the waste liquid is alkalinized, the generation of cyanide gas cannot be completely eliminated, and there is a risk of cyanide gas leakage during storage. Detoxification in facilities generally uses a method employing hypochlorous acid compounds, but there is a risk of generation of chlorine gas and cyanogen chloride gas during detoxification. Accordingly, as a method for preventing cyanide components from remaining, Patent Document 1 discloses that at least one compound selected from the group consisting of a compound having an aldehyde group or keto group and a hydrophobic group, a compound having a carboxy group and a hydrophobic group, glyoxylic acid or an ester of glyoxylic acid, ascorbic acid or an optical isomer thereof, lignin or a derivative thereof, humic acid or a derivative thereof, and a reducing sugar is added to a cyanide-containing waste liquid, the waste liquid is maintained at a pH of 5 to 12, the above compound is reacted with cyanide in the waste liquid to form cyanohydrin, the cyanohydrin is adsorbed on a suspending substance, and then the adsorbate is subjected to solid-liquid separation. A disadvantage of adsorption removal in prior art documents is that the presence of a suspending substance is required. When treating a waste liquid that does not contain coal, coke, or the like, it is necessary to add activated carbon, zeolite, or the like. Furthermore, after adsorption removal, it is also necessary to safely discharge and dispose of the adsorbate. [Prior Art Documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2012-239955 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] The object of the present invention is to provide a method for treating cyanide-containing wastewater containing toxic cyanide components, which is generated when small amounts of potassium cyanide or sodium cyanide are used in experimental facilities, analytical facilities, etc., and which does not use toxic or deleterious substances, and which converts the wastewater into a safer treatment solution by decomposing the cyanide rather than by adsorption or filtration as described in Patent Document 1. [Means for solving the problem]

[0005] This invention relates to the following invention. (1) A method for treating cyanide-containing wastewater, characterized by adding reducing sugar to cyanide-containing wastewater, reacting the reducing sugar with the cyanide in the wastewater at a pH of 7 to 11 to form cyanohydrin, then hydrolyzing the cyanohydrin in the wastewater under acidic conditions, and then neutralizing the wastewater. (2) The method for treating cyanide-containing wastewater as described in (1), wherein the reaction conditions between the reducing sugar and cyanide are set to a pH of 8 to 10. (3) The method for treating cyanide-containing wastewater according to (1) or (2), wherein the acidic conditions during the hydrolysis are pH 3 to 4. [Effects of the Invention]

[0006] According to the present invention, a method is provided that is safe during the processing operation and can ultimately render all cyanide compounds in cyanide-containing wastewater harmless. Furthermore, the method of the present invention can safely and reliably decompose and remove cyanide components from wastewater efficiently by extremely simple means, without the need for special equipment. [Modes for carrying out the invention]

[0007] The present invention's method for treating cyanide-containing wastewater consists of the following steps. 1st process Reducing sugars are added to cyanide-containing wastewater, and the wastewater is reacted with the cyanide under pH conditions of 7 to 11 to produce cyanohydrins. 2nd process The cyanohydrin is hydrolyzed under acidic conditions. 3rd process Neutralize the waste liquid.

[0008] The details are explained below. (1st step) The cyanide wastewater targeted by this invention is cyanide-containing wastewater containing toxic cyanide components, which is generated when small amounts of cyanide compounds such as potassium cyanide and sodium cyanide are used in laboratory facilities, analytical facilities, etc. In this invention, cyanide refers to the cyanide ion (CN - ) means that cyanide gas refers to HCN gas. Specifically, the reducing sugars used in the present invention include monosaccharides such as glucose, mannose, galactose, arabinose, xylose, ribose, fructose, and glyceryl aldehyde; disaccharides such as maltose, lactose, and cellobiose; trisaccharides such as maltotriose; and oligosaccharides such as maltooligosaccharides. One or more of these can be used. There are no particular restrictions on reducing sugars, but monosaccharides or disaccharides are preferred, disaccharides are more preferred because they react somewhat faster, and maltose is the most preferred because it reacts most efficiently. For monosaccharides, the amount added is 8 times or more in molar ratio to cyanide, preferably 30 to 90 times. For disaccharides, the amount added is 4 times or more in molar ratio to cyanide, preferably 15 to 45 times. Based on the principle of the treatment, it is presumed that the reaction is due to the functional groups of reducing sugars, so the amount of reducing sugars required for the reaction can be theoretically calculated. When carried out at the above molar ratios, more than 80% of the cyanide can be decomposed after 48 hours. The more reducing sugar added, and the higher the concentration, the shorter the reaction time. The waste liquid during the reaction can be most efficiently reacted by maintaining a pH of 7-11, preferably 8-10. If the pH during the reaction exceeds 12, the reaction rate between reducing sugars and cyanide decreases, and on the acidic side, there is a risk of deterioration of reducing sugars and generation of cyanide gas. Acids such as sulfuric acid and hydrochloric acid, or alkalis such as sodium hydroxide, potassium hydroxide, and sodium bicarbonate can be used to adjust the pH. The reaction is faster at higher temperatures, and cyanide can be decomposed at 4-80°C, but for safety reasons, 15-30°C is preferable, and the reaction can usually be carried out at room temperature. At room temperature, the reaction time varies depending on the amount of reducing sugar added, but under conditions where a molar ratio of 45 times the amount of reducing sugar is added, the reaction time is 5 hours or more, preferably 5-30 hours. In this processing method, cyanide ions (CN) in the solution - Since the target of the detoxification is [unspecified substance], and the reaction does not involve the generation of gas, it can be carried out in a tightly sealed container.

[0009] Some of the cyanide in the wastewater vaporizes as cyanide gas, but after a certain period of time, the vaporized cyanide gas is reabsorbed into the wastewater and neutralized, so ultimately all cyanide can be rendered harmless. For example, if the amount of reducing sugar added is 45 times the molar ratio of the cyanide in the wastewater, cyanide gas generation into the gas phase ceases after about 2.5 hours at room temperature, and after 4 hours, 99% of the cyanide in the liquid is converted into cyanohydrins. This is presumably because, due to the effect of vapor-liquid equilibrium, when cyanide in the wastewater is decomposed, the resulting cyanide gas is reabsorbed into the wastewater, ultimately leading to the decomposition of almost all of the cyanide. When determining the amount of reducing sugar to add during actual processing, it is sufficient to measure the cyanide concentration in the wastewater beforehand and add an amount that can completely eliminate the cyanide, taking into account the volume of wastewater. Cyanohydrins are compounds that contain both a cyano group and a hydroxyl group in their molecule, produced by the addition of cyanide to the aldehyde or ketone group of a reducing sugar.

[0010] (2nd process) Thereafter, an acid is added to the waste liquid to hydrolyze cyanohydrin in the waste liquid under acidic conditions. It has been confirmed that cyanohydrin generated in the first step reverts to cyanide under strong alkali conditions. To prevent this, cyanohydrin is hydrolyzed under acidic conditions to convert it into a carboxylic acid compound. This treatment prevents reversion to cyanide even when the waste liquid becomes strongly alkaline. Acidic conditions can be achieved using one or more selected from inorganic acids such as hydrochloric acid and sulfuric acid, and organic acids such as acetic acid and trifluoroacetic acid. Hydrolysis is usually performed at pH 1 to 5, preferably pH 2 to 4. Hydrolysis is usually performed at 15 to 30°C, preferably 20 to 25°C. Although the hydrolysis time depends on the treatment apparatus and the scale of treatment, it can usually be completed in 15 minutes to 1 hour. Cyanohydrin is converted into α-hydroxy acid via hydrolysis.

[0011] (Step 3) After hydrolysis, a neutralizing agent is added to neutralize the waste liquid to pH 5 to 7. Since the waste liquid after hydrolysis is acidic, the neutralizing agent is not particularly limited as long as it is an alkali, and alkalis such as sodium hydroxide, calcium hydroxide, sodium carbonate and cobalt carbonate can be used. Neutralization is usually performed at 15 to 30°C, preferably 20 to 25°C. Although the neutralization time depends on the treatment apparatus and the scale of treatment, it can usually be completed in 15 minutes to 1 hour. As described above, as long as the required conditions are satisfied, reliable removal of cyanide components can be achieved simply by allowing the waste liquid to stand at room temperature.

[0012] (Waste Liquid After The Above Treatment) The treated liquid obtained after neutralizing the waste liquid contains no residual cyanide, and only contains cyanide-derived neutral salts and residual reducing agent that has been added. Therefore, the finally discharged treated liquid can be handled safely, and can also be disposed of as wastewater as it is.

Examples

[0013] Hereinafter, the present invention will be described in further detail with reference to reference examples and working examples. (Reference Example) Cyanide waste liquid is an aqueous solution discharged from manufacturing processes in facilities such as factories, which contains by-products and impurities in addition to cyanide. Accordingly, in the present example, treatment was performed using a cyanide-containing aqueous solution simulating a waste liquid. Cyanide-containing aqueous solution 1: 0.005 g of NaCN + 50 mL of 0.4% NaOH, pH 12.9 Cyanide-containing aqueous solution 1 under strong alkaline conditions was used as sample water, prepared in a stoppered colorimetric tube with a volume of 50 mL. Various saccharides shown in Table 1 were used to investigate whether cyanide components can be removed from the solution . 1) 5 g each of reducing sugars (maltose, lactose, glucose) and sucrose were added to the sample water, stirred, and then allowed to react for 24 hours. 2) After completion of the reaction, the residual cyanide content in the treated water was confirmed by analysis using the cyanide distillation method.

[0014]

Table 1

[0015] [Table 2] From these results, it was found that the optimal pH for the treatment is 7-11, more preferably 8-10.

[0016] (Example 1) We investigated the appropriate amount of maltose added to control cyanide levels. (1) A 500 ppm cyanide aqueous solution was prepared. 5000ppm NaCN 1 g + pure water 100ml pH11.4 500ppm 5000ppm NaCN aqueous solution 10ml + pure water 90ml pH10.9 (2) A specified amount of maltose (Table 3) was added to the cyanide in the sample solution and the reaction was carried out at room temperature. (3) After 2 hours and 30 minutes, the amount of cyanide gas generated in the gas phase was checked using a cyanide gas detector. (4) The residual cyanide content was checked after 4 hours and 29 hours. The cyanide removal rate was calculated by analyzing and comparing samples under the same conditions without the addition of reducing sugars.

[0017] [Table 3] Based on the above, we confirmed that, regardless of the amount of maltose added, cyanide was almost completely removed after 29 hours, and that there was virtually no generation of cyanide gas into the gas phase. (Example 2) The ratio of cyanide solution to maltose, and the decomposition of cyanide after neutralization were confirmed. (1) In addition to (Example 1), a 250 ppm cyanide aqueous solution was prepared. 250ppm 500ppm NaCN aqueous solution 50ml + pure water 50ml pH10.7 (2) A specified amount of maltose (Table 4) was added to the cyanide in the sample solution and the reaction was carried out at room temperature.

[0018] [Table 4] (3) A 0.05 mol / l sulfuric acid solution was added to the three samples above while stirring to adjust the pH to approximately 3. Since cyanohydrins were hydrolyzed under acidic conditions, the absence of cyanide was confirmed by the following procedure. (4) After standing for 15 minutes, a 0.1 mol / l sodium hydroxide solution was added while stirring to adjust the pH to approximately 7. (5) A 20% sodium hydroxide solution was added to each sample to create a strongly alkaline pH of approximately 13.8, and the generation of cyanide gas was confirmed using a constant potential electrolytic cyanide gas detector. As a result, regardless of the ratio of cyanide solution to maltose, no cyanide gas was detected in any of the three samples even when the conditions were returned to strongly alkaline. Therefore, it was determined that the cyanide decomposition in the cyanide wastewater was successful.

Claims

1. Disaccharides, which are reducing sugars, are added to cyanide-containing wastewater, and this wastewater is reacted with the cyanide in the wastewater under conditions of pH 7 to 11 to produce cyanohydrins. Subsequently, the cyanohydrin in the waste liquid is hydrolyzed under acidic conditions. Subsequently, the waste liquid is neutralized, which is a characteristic feature. Method for treating cyanide-containing wastewater.

2. The method for treating cyanide-containing wastewater according to claim 1, wherein the reaction conditions between the disaccharide, which is a reducing sugar, and cyanide are set to pH 8 to 10.

3. The method for treating cyanide-containing wastewater according to claim 1 or 2, wherein the acidic conditions during the hydrolysis are pH 3 to 4.

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