DE112013001804T5 - Process for treating cyanide-containing wastewater - Google Patents

Abstract

There is provided a process for treating cyanide-containing wastewater which can be used to oxidatively decompose a cyanide sufficiently even though the cyanide-containing waste water contains ammonium ions and an organic substance while preventing the formation of scale. The process for treating cyanide-containing wastewater containing a cyanide includes a step of adding a chlorine source to the cyanide-containing wastewater to decompose the cyanide, the cyanide-containing waste water containing ammonium ions and an organic substance, the pH of the cyanide-containing waste water is adjusted to 11 or more, the chlorine source is added to the waste water such that the free residual chlorine concentration is 0.1 mg / L or more, even after the cyanide decomposition reaction, and a phosphonic acid scale inhibitor is added to the waste water becomes.

Description

Field of the invention

The present invention relates to a process for treating cyanide-containing wastewater, and more particularly to a process for treating cyanide-containing wastewater using an alkaline chlorination process.

Background of the invention

The most widely used process for treating cyanide effluent from industrial plants such as galvanizing factories, ironworks, smelters, power plants and coke-making plants is an alkaline chlorination process. In this method, a chlorine source such as sodium hypochlorite is added to cyanide-containing waste water under alkaline conditions to oxidize a cyanide in the waste water (Patent Literature 1 and 2).

In an alkaline chlorination process described in Patent Literature 1, a cyanide is subjected to oxidative decomposition in a two-step controlled pH reaction and ORP as described below.

First reaction step: pH of 10 or more and ORP regulated in the range of 300 to 350 mV. NaCN + NaOCl → NaCNO + NaCl (1)

Second reaction step: pH in the range of 7 to 8 and ORP, which is controlled in the range of 600 to 650 mV. 2NaCNO + 3NaClO + H ₂ O → N ₂ + 3NaCl + 2NaHCO ₃ (2)

Patent Literature 2 describes a process for treating cyanide-containing wastewater containing ammonia by a two-step reaction in an alkaline chlorination process.

Bibliography

patent literature

• Patent Literature 1: Japanese Patent Publication 2001-269674A ,
• Patent Literature 2: Japanese Patent Publication 2006-334508 A ,

Summary of the invention

Tasks solved by the invention

As a result of investigations, the inventors found that cyanides in cyanide-containing waste water containing ammonium ions and an organic substance are not sufficiently oxidized by the first reaction step in the alkaline chlorination process.

More specifically, when cyanide-containing waste water containing ammonium ions and an organic substance is treated in an ordinary pH range of 10 to 10.5 and in an ORP in the range of 300 to 350 mV by the first reaction stage in the known alkaline chlorination process, the cyanide is insufficiently decomposed. Even adding NaClO and increasing the ORP to 400 mV or more does not decrease the total cyanide concentration. Controlling the ORP in the range of 300 to 350 mV at a pH of 11 or more requires a source with a large amount of chlorine, which increases costs and possibly causes corrosion of steel.

When a cyanide is not sufficiently oxidized by the first reaction step in the alkaline chlorination process, the second reaction step does not proceed and the cyanide can also react with sodium hypochlorite in the second reaction step to form cyanogen chloride (CNCI).

When a chlorine source is added to cyanide-containing waste water containing ammonium ions and an organic substance at a pH of 11 or less, the chlorine source reacts with the ammonium ions to form combined chlorine. This combined chlorine reacts with the organic matter and forms a cyanide. Thus, the concentration of cyanides in the cyanide-containing wastewater is not reduced or sometimes even increased.

An object of the present invention is to solve these problems and to provide a process for treating cyanide-containing waste water which can be used to oxidatively decompose a cyanide sufficiently even though the cyanide-containing waste water contains ammonium ions and an organic substance. In one aspect, the present invention aims to provide a process for treating cyanide-containing waste water while preventing the formation of scale.

Means of solving the tasks

A method for treating cyanide-containing wastewater containing a cyanide according to the present invention includes a step of adding a chlorine source to the cyanide-containing wastewater to decompose the cyanide, the cyanide-containing waste water containing ammonium ions and an organic substance containing pH of the cyanide-containing waste water is adjusted to 11 or more, and the chlorine source is added to the cyanide-containing waste water such that the free residual chlorine concentration is 0.1 mg / L or more, even after the cyanide decomposition reaction.

The chlorine source is preferably at least one of sodium hypochlorite, chlorine and a bleaching powder.

In one aspect of the present invention, a phosphonic acid scale inhibitor is added to the cyanide-containing effluent.

The phosphonic acid scale inhibitor is preferably at least one selected from 1-hydroxyethylidene-1,1-diphosphonic acid, 2-phosphonobutane-1,2,4-tricarboxylic acid and salts thereof.

In the present invention, the chlorine source is preferably added to the cyanide-containing effluent such that the free residual chlorine concentration ranges from 0.1 to 1 mg / L.

The pH of the cyanide-containing effluent is preferably adjusted in the range of 11 to 12.5 by adding an alkaline agent. In this case, a solution containing the alkaline agent and the scale inhibitor dissolved therein is preferably added to the cyanide-containing wastewater.

The concentration of dissolved iron in the cyanide-containing wastewater is preferably 0.4 mg / L or less.

The temperature of the cyanide-containing waste water is preferably 40 ° C or more, for example, in the range of 40 ° C to 80 ° C, preferably from 50 ° C to 70 ° C.

Advantageous Effects of the Invention

In a method for treating cyanide-containing waste water according to the present invention, the chlorine source is added to the cyanide-containing waste water containing ammonium ions and an organic substance at a pH of 11 or more. At a pH of 11 or more, the reaction between the chlorine source and the ammonium ions is suppressed and the formation of combined chlorine is suppressed. As a result, the formation of cyanide from a reaction between combined chlorine and the organic substance is also suppressed.

The addition of a phosphonic acid scale inhibitor to the cyanide-containing effluent prevents (including suppression) the formation of scale.

The pH of the cyanide-containing waste water is preferably adjusted to 11 or more by the addition of an alkaline agent. The addition of a liquid mixture of an alkaline agent and a scale inhibitor prevents scaling difficulties in chemical injection (delivery) pumps and injection lines.

In the present invention, the pH is preferably 11 or more, even after completion of the cyanide decomposition reaction.

The chlorine source is added to the waste water containing ammonium ions and an organic substance at a pH of 11 or more, and the free residual chlorine concentration is 0.1 mg / L or more even after the reaction. This allows a cyanide to be sufficiently oxidized and sufficiently reduces the concentration of cyanides in the wastewater to be treated.

When the free residual chlorine concentration is 1 mg / L or less after the reaction, the corrosion of parts made of, for example, steel in contact with the waste water is suppressed.

A water temperature of 40 ° C or more in the reaction results in improved efficiency of cyanide decomposition reaction and reduction of cyanide concentration in a short time. A reduced reaction time results in a reduced contact time between the water to be treated containing free residual chlorine and the parts in contact with the water, and suppresses the corrosion of the parts in contact with the water, which are made of steel, for example.

Description of the embodiments

The present invention will be described below in further detail.

Cyanide-containing wastewater to be treated in the present invention can remove cyanide-containing wastewater discharged from industrial plants such as galvanizing plants, power plants, ironworks, smelters and coke-making plants, and a cyanide as a metal cyanide complex, for example, a cyano-complex with a metal such as Ni. Ag, Cu, Zn or Cd, but it is not limited thereto.

Generally, such cyanide-containing wastewater has a cyanide concentration in the range of about 0.1 to 100 mg / L and a pH in the range of about 6 to 10.

In the present invention, cyanide-containing waste water containing ammonium ions and an organic substance is treated. The concentration of the ammonium ions is preferably 5 mg / L or more, for example, in the range of about 5 to 250 mg / L. The organic substance may be coal or it may be derived from coke and the concentration of the organic substance is preferably 1 mg / L or more, for example, in the range of about 1 to 30 mg / L.

Dissolved iron in industrial wastewater containing a cyanide and having a neutral or higher pH is usually in the form of an iron-cyano complex. Iron-cyano complexes are resistant to oxidative decomposition reactions of cyanides in an alkaline chlorination process according to the present invention. Thus, in cyanide-containing wastewater to be treated using a method according to the present invention, the total concentration of cyanides in iron-cyano complexes is 1.0 mg / L or less, and the concentration of dissolved iron is less than 0.4 mg / L.

The chlorine source to be added to the cyanide-containing wastewater may be chlorine, a bleach powder or sodium hypochlorite. The phosphonic acid scale inhibitor to be added to the cyanide-containing wastewater may be at least one selected from 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP), 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC), and salts from that. The salts may be sodium salts and potassium salts. The phosphonic acid scale inhibitor is preferably 1-hydroxyethylidene-1,1-diphosphonic acid.

In the case where a chlorine source is added to the cyanide-containing waste water, if necessary, the pH of the cyanide-containing waste water is made to be 11 or more, preferably 11 to 12.5, particularly 11 to 12, by Adding an alkaline agent such as NaOH and / or KOH. The addition of the alkaline agent may be made before, after or simultaneously with the addition of the chlorine source. The addition of the alkaline agent is not necessary for cyanide-containing waste water having a pH of 11 or more. The pH of the water after the treatment reaction is preferably 11 or more.

When the alkaline agent and the scale inhibitor are added to the cyanide-containing wastewater, a solution containing both the alkaline agent and the scale inhibitor may be prepared in advance. This can prevent the formation of scale in injection pumps and injection lines. The amount of scale inhibitor to be added is preferably determined experimentally in a manner dependent on the water quality of the cyanide-containing effluent and is generally in the range of 1 to 100 mg / L, preferably approximately 5 to 30 mg / L.

The amount of the chlorine source to be added is controlled so that the residual free chlorine concentration after the reaction is 0.1 mg / L or more, preferably in the range of 0.1 to 1 mg / L, particularly preferably 0.1 to 0.5 mg / L.

In the case of batchwise treatment of cyanide-containing waste water in a container, the concentration of free residual chlorine in the waste water in the container is measured over time and the time at which the rate of decrease in free residual chlorine concentration is zero or less or equal to a predetermined value, is considered to be the end of the reaction. The predetermined value is preferably in the range of 0 to 0.1 mg / L / min.

When cyanide-containing waste water continuously flows into and out of a reaction vessel in which the cyanide decomposition reaction proceeds, the residence time in the vessel is preferably longer than the reaction time, and the residual free chlorine concentration measured at the reaction vessel outlet becomes less than the residual chlorine concentration viewed the reaction.

In the case where cyanide-containing wastewater flowing through a pipe is treated by the addition of a chlorine source and, if necessary, a scale inhibitor and a base in the pipe, the free residual chlorine concentration is measured at several points on the downstream side , If the measured free residual chlorine concentrations at two or more of the points are substantially equal, the reaction at these measurement points or their upstream region may be considered complete. The measuring points are preferably separated by 5 m or more, in particular approximately 10 to 30 m.

In the treatment of cyanide-containing wastewater under such conditions, adjusting the pH to 11 or more suppresses the formation of combined chlorine and the formation of a cyanide by a reaction between combined chlorine and an organic substance. The addition of a scale inhibitor prevents scale deposition and stabilizes the treatment of cyanide-containing wastewater.

The addition of the chlorine source, so that the free residual chlorine concentration after completion of the reaction is 0.1 mg / L or more, enables a cyanide to be sufficiently decomposed. If the residual free chlorine concentration after the completion of the reaction is 1 mg / L or less, it prevents excessive addition of the chlorine source and dampens an increase in the cost of the chlorine source. This also delays the corrosion of metallic materials, such as steel, or parts in contact with the wastewater.

In the present invention, the temperature of the cyanide-containing waste water is preferably 40 ° C or more, for example, in the range of about 40 ° C to 80 ° C, particularly about 50 ° C to 70 ° C, so as to increase the cyanide decomposition reaction rate , A high rate of cyanide decomposition results in a decreased contact time between the water to be treated containing free residual chlorine and parts in contact with the water, which are made of steel, for example, which retards the corrosion of the parts in contact with the water. In order to lower the heating cost, the water temperature is preferably 80 ° C or less, more preferably 70 ° C or less.

Examples

Examples and comparative examples of the present invention will be described below. In the following Examples and Comparative Examples, aqueous NaOH (concentration: 48 wt%) was used as an alkaline agent, aqueous NaClO (concentration: 12 wt%) was used as a chlorine source, and became HEDP, PBTC, an acrylic acid polymer (sodium polyacrylate (weight-average molecular weight: 3500)) or a maleic acid polymer (a sodium salt of an isobutylene-maleic anhydride copolymer (weight-average molecular weight: 15,000)) is used as a scale inhibitor. Analysis of total CN was carried out using 4-pyridine-pyrazolone absorptiometry according to JIS K 0102 without filtration by adding L (+) - ascorbic acid, reducing the residual chlorine and adjusting the pH to 12 with NaOH. The effect of preventing scale was based on the Calcined calcium ion concentration and the presence or absence of scale deposits on a SUS test piece in a reaction vessel.

Examples 1 to 7, Comparative Examples 1 to 9

Dust collecting water of an energy generating device was used as test water. The water quality of the dust collecting water was as follows:

pH value: 8.7,
Total cyanide: 3 mg / L,
Ammonium ion: 120 mg / L,
TOC: 10 mg / L and
dissolved iron: less than 0.1 mg / L.

50 mL of the test water in a glass container with a lid were kept at a temperature of 20 ° C, 40 ° C, 50 ° C or 60 ° C. An alkaline agent and a chlorine source were added to the test water under the conditions shown in Table 1. The reaction time was as follows:

Water temperature of 20 ° C: 120 minutes,
Water temperature of 40 ° C: 90 minutes and
Water temperature of 50 ° C or 60 ° C: 60 minutes.

Table 1 shows the pH values measured 5 minutes after the addition of the chemicals and the times described above, the amount of NaClO added, the residual chlorine concentration after the reaction times described above, the ORP and the total cyanide concentration. In Table 1 and Tables 2 and 3 described below, "free" refers to free residual chlorine.

Table 1 shows that the examples having a pH of ≥ 11 also had a lower total cyanide concentration than the comparative examples having a pH of <11 even after completion of the reaction and that the total cyanide concentration decreased with increasing water temperature.

Examples 8 and 9

Dust collecting water of an energy generating device was used as test water. The water quality of the dust collecting water was as follows:

pH value: 8.2,
Total cyanide: 3 mg / L,
Ammonium ion: 100 mg / L,
TOC: 8 mg / L and
Dissolved iron: less than 0.1 mg / L.

100 mL of the test water was kept in a 1000 mL beaker at a temperature of 60 ° C. An alkaline agent and a chlorine source were added to the test water under the conditions shown in Table 2. An iron test piece was immersed in the test water for 3 days while stirring the test water with a stirrer (number of revolutions: 150 rpm). Table 2 shows the results. In Examples 8 and 9, as described above, the water quality and the corrosion loss were measured, and the corrosion rate was determined after the iron (SPCC) test piece was placed in the beaker for 3 days. Table 2 pH immediately after addition pH after the reaction Amount of added NaClO ORP Total cyanide concentration Residual chlorine concentration corrosion rate total Free (-) (-) (Mg / L) (MV) (Mg / L) (Mg / L) (Mg / L) (mdd ^{* 2} ) Example 8 11.4 11 33.5 263 Less than 0.1 0.3 0.1 290 Example 9 11.5 11 45 327 Less than 0.1 3.2 2.6 446 * 2 mdd: mg / dm ² / day

Table 2 shows that Example 8, which had a lower free residual chlorine concentration than Example 9, had a lower corrosion rate.

Examples 10 to 15

Dust collecting water of an energy generating device was used as test water. The water quality of the dust collecting water was as follows:

pH value: 8,
Total cyanide: 3 mg / L,
Ammonium ion: 130 mg / L,
TOC: 7 mg / L and
dissolved iron: less than 0.1 mg / L.

500 mL of the test water was held in a beaker with a lid at a temperature of 25 ° C, 40 ° C, 50 ° C, 60 ° C or 80 ° C. An alkaline agent and a chlorine source were added to the test water under the conditions shown in Table 3. Table 3 shows the water quality measured after 60 minutes. Table 3 water temperature pH after the reaction Amount of added NaClO Residual chlorine concentration Total cyanide concentration total Free Combined chlorine (° C) (-) (Mg / L) (Mg / L) (Mg / L) (Mg / L) (Mg / L) Example 10 25 11.5 33.5 15.2 12.6 2.6 2.0 Example 11 40 11.4 33.5 5.2 3.8 1.4 0.6 Example 12 50 11.3 33.5 2.5 1.8 0.7 0.2 Example 13 60 11.3 33.5 0.4 0.3 0.1 Less than 0.1 Example 14 70 11.2 33.5 0.3 0.2 0.1 Less than 0.1 Example 15 80 11.2 33.5 0.1 0.1 0.0 Less than 0.1

As shown in Table 3, the cyanide concentration after the reaction decreases with increasing water temperature.

Examples 16 to 19

Test water having a dissolved iron concentration of 0.1, 0.3, 0.4 or 0.5 mg / L was prepared by adding aqueous ferric chloride to the dust collecting water of the power plant as described in Example 1. 500 mL of the test water was held in a glass container with a lid at a temperature of 60 ° C. An alkaline agent was added to the test water so that the test water after the reaction had a pH of 11, and a chlorine source was added to the test water so that the concentration immediately after the addition was 33.5 mg / L. The reaction was carried out for 60 minutes. Table 4 shows the water quality measured after 60 minutes. Table 4 Concentration of dissolved iron Total cyanide (Mg / L) (Mg / L) Example 16 0.1 0.1 Example 17 0.3 0.5 Example 18 0.4 1.1 Example 19 0.5 1.3

As shown in Table 4, the cyanide concentration after the reaction increased with increasing the concentration of dissolved iron.

Examples 20 to 22, Comparative Examples 10 to 13

Dust collecting water from an energy generating device was used as the test water. The water quality of the dust collecting water was as follows:

pH value: 8.7,
Total cyanide: 3 mg / L,
Ammonium ion: 120 mg / L,
TOC: 10 mg / L and
dissolved iron: less than 0.1 mg / L.

500 mL of the test water was held in a glass container with a lid at a temperature of 60 ° C. A scale inhibitor, an alkaline agent and a chlorine source were added to the test water under the conditions shown in Table 1. An SUS test piece was placed in the container. The reaction time was 60 minutes.

Table 5 shows the pH values measured 5 minutes after the chemicals were added and after 60 minutes as described above, the amount of NaClO added, the calcium ion concentration after the reaction time described above, the presence or absence of scale deposits on the Test piece and the total cyanide concentration.

As shown in Table 5, in accordance with the present invention, cyanide can be sufficiently decomposed while preventing the formation of scale. In Comparative Example 10, which had a pH of less than 11, the residual cyanide concentration was high. In Comparative Example 11, in which no scale inhibitor was added, scale was formed. In Comparative Examples 12 and 13, the scale inhibitor was not a phosphonic acid scale inhibitor, and although a scale inhibitor was added, scale has settled.

Although the present invention has been described in detail with reference to particular embodiments, it will be obvious to those skilled in the art that various modifications can be made therein without departing from the spirit and scope of the present invention.

The present application is based on Japanese Patent Application 2012-080437 filed on March 30, 2012, and the Japanese Patent Application 2012-080438 filed on Mar. 30, 2012, which is incorporated herein by reference in its entirety.

Claims (10)

A process for treating cyanide-containing wastewater containing a cyanide, comprising a step of adding a chlorine source to the cyanide-containing wastewater to decompose the cyanide, wherein the cyanide-containing wastewater contains ammonium ions and an organic substance, the cyanide-containing wastewater is adjusted to a pH of 11 or more, and the chlorine source is added to the cyanide-containing waste water so that the free residual chlorine concentration becomes 0.1 mg / L or more, even after a reaction in which the cyanide is decomposed. A process for treating cyanide-containing wastewater according to claim 1, wherein a phosphonic acid scale inhibitor is added to the cyanide-containing wastewater. The process for treating cyanide-containing waste water according to claim 2, wherein the phosphonic acid scale inhibitor is at least one selected from the group consisting of 1-hydroxyethylidene-1,1-diphosphonic acid, 2-phosphonobutane-1,2,4-tricarboxylic acid and salts thereof becomes. A process for treating cyanide-containing waste water according to claim 2 or 3, wherein a solution containing the alkaline agent and the scale inhibitor is added to the cyanide-containing waste water. A process for treating cyanide-containing waste water according to claim 1 or 2, wherein the chlorine source is added to the cyanide-containing waste water so that the free residual chlorine concentration is in the range of 0.1 to 1 mg / L. A process for treating cyanide-containing waste water according to claim 1 or 2, wherein the pH of the cyanide-containing waste water is adjusted in the range of 11 to 12.5 by adding an alkaline agent. The method for treating cyanide-containing waste water according to claim 6, wherein the alkaline agent is added to the cyanide-containing waste water so that the cyanide-containing waste water has a pH of 11 or more after the cyanide decomposition reaction. A process for treating cyanide-containing waste water according to claim 1 or 2, wherein the cyanide-containing wastewater has a dissolved iron concentration of 0.4 mg / L or less. A process for treating cyanide-containing waste water according to claim 1 or 2, wherein the cyanide-containing waste water has a temperature of 40 ° C or more. A process for treating cyanide-containing wastewater according to claim 1 or 2, wherein the chlorine source is at least one of. the group consisting of sodium hypochlorite, chlorine and a bleaching powder is selected.