JPS6133636B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for detoxifying a hydrogen cyanide absorption solution after fumigation for exterminating pests of fruits and vegetables such as bananas. Fumigation treatment using hydrogen cyanide has traditionally been used to exterminate pests of fruits and vegetables such as citrus fruits such as bananas and oranges. In this case, hydrogen cyanide-containing gas after fumigation is usually absorbed and removed by an alkaline aqueous solution containing sodium hydroxide, potassium hydroxide, etc., but sodium cyanide, potassium cyanide, etc. produced by absorbing hydrogen cyanide Since aqueous solutions containing cyanide are highly toxic, they must be thoroughly detoxified. Conventionally, oxidation treatment using an alkali chlorine method has been generally used to detoxify this hydrogen cyanide absorption liquid. This method usually decomposes and detoxifies cyanides such as sodium cyanide and potassium cyanide using sodium chlorate. For example, sodium cyanide is detoxified as shown in the following reaction formula. NaCN+NaClO→NaCNO+NaCl 2NaCNO+3NaClO+H ₂ O→2NaHCO ₃ +3NACl+N ₂ However, this alkali chlorine method has the following various problems. First of all, since it is decomposed by a rigorous two-step chemical reaction as described above, strict monitoring of the reaction process such as pH and reaction progress and accurate adjustment of reaction conditions are essential. Therefore, a reprocessing process is required to keep the residual chlorine in the liquid after decomposition treatment below the regulatory value.
Third, there are drawbacks such as the high cost of sodium chlorate. In order to eliminate the drawbacks of the above-mentioned alkali chlorine method, a treatment method using ozone oxidation has recently been put into practice. This method uses ozone to oxidize and decompose an aqueous solution containing cyanides such as sodium cyanide and potassium cyanide.
This oxidative decomposition process is usually expressed by the following chemical reaction formula (the case of sodium cyanide is shown as an example). NaCN+O ₃ →NaCNO+O ₂ 2NaCNO+3O ₃ +H ₂ O →2NaHCO ₃ +N ₂ +3O _2The reaction to produce the first stage cyanate is a rapid reaction, and the reaction rate is almost 100%. The rate of decomposition reaction of cyanate in the second stage is highly dependent on pH; in the near-neutral pH range of 7 to 9, the rate of decomposition reaction is relatively rapid, but at pH 10 or higher, the rate of decomposition reaction is significantly reduced. , the decomposition reaction hardly progresses at pH 12 or above. On the other hand, in the absorption treatment of hydrogen cyanide after fumigation, a 5 to 10 wt% aqueous sodium hydroxide solution is usually used to increase the removal rate of hydrogen cyanide, and the pH of the aqueous solution after the hydrogen cyanide absorption treatment and before ozone oxidation treatment is generally is 12 or more. In the first stage of ozone oxidation, the pH of the aqueous solution decreases as the oxidation reaction progresses, but is usually 10 or higher. Even if the aqueous solution with a pH of 10 or higher is further subjected to ozone oxidation after the first stage reaction is completed, the rate of the second stage cyanic acid decomposition reaction is extremely slow, and the ozone utilization rate is significantly deteriorated. Therefore, in order to quickly carry out the two-stage chemical reaction described above in the ozone oxidation method, a pH adjustment process is required before the start of the first-stage reaction or after the end of the first-stage reaction. However, there are several problems with this PH adjustment process. First of all, regarding the pH adjustment before the start of the first stage reaction, please be aware that due to incorrect operation etc.
If the pH becomes acidic, highly toxic hydrogen cyanide gas is generated, which poses a high operational risk in normal industrial processes. Adjusting the pH after the first stage reaction requires strict monitoring and accurate adjustment of the pH, the degree of reaction progress, etc., and a large amount of equipment is indispensable. Further, as a method for decomposing cyanate produced by ozone oxidation, there is a hydrolysis method at normal pressure and room temperature using the following chemical reaction formula. NaCNO + 2H ₂ O at room temperature / NaHCO ₃ + NH ₃ However, the speed of this reaction is relatively slow and it takes a long time to complete the reaction, and it takes a long time to complete the decomposition of cyanate, so if it is an emergency. It cannot be applied when decomposition of is required. The present invention has been made in view of the above points, and when detoxifying a hydrogen cyanide absorption solution in which hydrogen cyanide is absorbed with an alkaline aqueous solution containing sodium hydroxide, potassium hydroxide, etc., cyanide produced by absorption of hydrogen cyanide is produced. sodium chloride,
An aqueous solution containing a cyanide such as potassium cyanide is brought into contact with an ozone-containing gas and oxidized to convert the cyanide such as sodium cyanide or potassium cyanide into a cyanate such as sodium cyanate or potassium cyanate. By heating the aqueous solution containing it to 60-100°C to perform hydrolysis, hydrogen cyanide can be produced in a very short time using simple equipment and simple operations, without using expensive chemicals or complicated PH adjustment steps. It is an object of the present invention to provide a method that can detoxify an absorbent liquid. Further, in the present invention, after almost all cyanide ions are oxidized to cyanate by contact with an ozone-containing gas, the cyanate-containing alkaline aqueous solution is subjected to ozone treatment by further adding and continuing contact with an ozone-containing gas. We also provide a method to improve the hydrolysis rate and significantly shorten the decomposition time of cyanate by heating the cyanate-containing aqueous solution to 60 to 100°C to perform hydrolysis. That is. Hereinafter, the configuration of the present invention will be explained in more detail based on the drawings. The drawing is a flow sheet showing an example of an apparatus for carrying out the method of the present invention. Reference numeral 1 denotes a fruit and vegetable fumigation chamber, in which fruits and vegetables such as bananas are stored, and hydrogen cyanide is introduced to fumigate the fruits and vegetables. The hydrogen cyanide in the fruit and vegetable fumigation room 1 is sent to the absorption tower 3 by the exhaust fan 2, and in this absorption tower 3 it is absorbed by an alkaline aqueous solution containing sodium hydroxide or (and) potassium hydroxide, and a hydrogen cyanide absorption liquid is produced. 4 is sent to an absorption liquid tank 6 by a pump 5. 7 is an alkaline aqueous solution supply pipe;
8 is an absorption liquid circulation pipe. When using a sodium hydroxide aqueous solution as an example of an alkaline aqueous solution, hydrogen cyanide is absorbed according to the following formula. HCL + NaOH → NaCN + H ₂ O The hydrogen cyanide absorption liquid 4 containing cyanides such as sodium cyanide and potassium cyanide produced by the absorption of hydrogen cyanide is sent from the absorption liquid tank 6 to the oxidation tower 11 by the pump 10. It is brought into contact with a containing gas and oxidized to form cyanates such as sodium cyanate and potassium cyanate. When using sodium hydroxide as an example of the alkaline aqueous solution, cyanide is oxidized according to the following formula. NaCN+O ₃ →NaCNO+O ₂ 12 is an ozone generator for generating ozone-containing gas, and a model that performs silent discharge in dry air or oxygen is used. It is preferable to use an ozone generator that uses air as a raw material. In order to improve the ozone utilization efficiency by performing the oxidation operation by gas-liquid contact between the cyanate-containing aqueous solution and the ozone-containing gas in the oxidation tower 11 as efficiently as possible, it is desirable to provide a suitable gas-liquid contact device. In the drawing, as an example, the oxidation tower 11 is filled with a filler 13 such as a Raschig ring, a liquid is sprayed from the upper part of the filler 13, an ozone-containing gas is supplied from the lower part of the filler 13, and cyanic acid is further supplied. The salt-containing aqueous solution 14 is configured to be circulated by a pump 10. The cyanate-containing aqueous solution 14 is sent by a pump 15 to a heating device 16, where it is heated to 60 to 100°C, preferably 70 to 90°C, to be hydrolyzed and completely detoxified. 17 is a processing liquid tank. When using sodium hydroxide as an example of the alkaline aqueous solution, it is considered that cyanate is hydrolyzed at high temperature according to the following formula. Note that the pressure is preferably normal pressure because the heating device 16 and the like can be manufactured at low cost. 4NaCNO+ _6H2O60 ~100℃/ _{2Na2CO3 +} ( _NH4 ) _{2CO3 +} CO( _NH2 ) _2In the present invention, the amount of ozone required to oxidize cyanide produced by absorption of hydrogen cyanide to cyanate is The stoichiometric amount is 1.78 parts per part of hydrogen cyanide, but when the gas-liquid contact efficiency is good and the ozone utilization rate is almost 100%, the oxidation of cyanide to cyanate is According to the experience of the present inventors, the required amount of ozone is about 1.50 to 1.60 parts per part of hydrogen cyanide. This is because the energy generated during the decomposition of ozone causes the decomposition of other oxygen molecules, and this oxygen participates in the oxidation of cyanide ions. It is believed that oxidation to acid ions is completed. In addition, in order to improve the hydrolysis rate, the amount of ozone that is added and continuously contacted after the oxidation of cyanide ions to cyanate ions is completed is about 0.1 to 0.2 parts per part of cyanide ions before ozone oxidation. is suitable. Note that the cyanate-containing aqueous solution may be hydrolyzed by heating it to a relatively low temperature of less than 60℃ in a heating device, but in this case, the decomposition rate is slow, so it is usually heated at 60 to 100℃ as mentioned above. Hydrolysis is carried out by heating. Next, examples and comparative examples of the present invention will be described. Example A detoxification experiment was conducted using an alkaline aqueous solution that had absorbed hydrogen cyanide by ozone oxidation treatment and high temperature hydrolysis treatment. First, in the ozone oxidation treatment, the decomposition status of hydrogen cyanide by blowing ozone-containing air into the alkaline aqueous solution was as shown in Table 1. In this case, as the ozone oxidation treatment conditions, the amount of sodium hydroxide aqueous solution is 3000ml, the amount of CN ^-
32.1g, and the ozone input amount was 1.5g/HR. In addition, if no ozone is added to liquid No. 1, liquid No. 2 will be used.
Liquid No. 3 is a case in which ozone is added in an amount that almost all of the cyanide ions are oxidized, and liquid No. 3 is a case in which an excessive amount of ozone is added after the cyanide ions are almost completely oxidized.

[Table] From Table 1, by adding 1.55 parts of ozone to 1 part of CN ^- in the alkaline aqueous solution (converted to hydrogen cyanide, 1.50 parts of ozone to 1 part of hydrogen cyanide), almost all cyanide ions are removed from cyanide. It is confirmed that it is an acid ion. Next, liquids No. 2 and No. 3 in Table 1 were subjected to high-temperature hydrolysis treatment. i.e. No.2 and No.3
The solution was subjected to hydrolysis at 75°C, and the changes in cyanate ion (CNO ^- ) concentration over time were as shown in Table 2.

[Table] From Table 2, cyanate ions are completely decomposed in No. 2 solution after 7 hours of hydrolysis, and ozone is added after the amount of cyanide ions is almost completely oxidized during ozone oxidation treatment. The time required for hydrolysis of cyanate ions in solution No. 3 is:
It can be seen that it is about 1/2 of No.2. Comparative Example 1 A sodium cyanate aqueous solution was prepared using the reagent sodium cyanate (NaCNO) without ozone oxidation, and changes over time in cyanate ion concentration due to hydrolysis treatment in this sodium cyanate aqueous solution were measured. Table 3 shows the change in cyanate ion concentration over time when this sodium cyanate aqueous solution was allowed to stand at room temperature (normal temperature hydrolysis).

[Table] From Table 3, it is clear that the hydrolysis rate of sodium cyanate prepared without ozone oxidation at pH 10 or above at room temperature is extremely small and does not actually proceed in a short period of time. Table 4 shows the change in cyanate ion concentration over time due to hydrolysis at 70°C and 90°C (high-temperature hydrolysis) of a sodium cyanate aqueous solution prepared using the reagent sodium cyanate without ozone oxidation. Ta.

[Table] From Table 4, the hydrolysis rate of sodium cyanate prepared without ozone oxidation is extremely low even at high temperatures. From the above, the hydrolysis rate of cyanate produced without ozone oxidation is extremely low at room temperature or at high temperature (60 to 100°C), and it takes a long time to decompose cyanic acid. is clear. Comparative Example 2 Changes in cyanate ion concentration over time were measured by normal temperature (room temperature) hydrolysis of a sodium cyanate aqueous solution prepared by ozone oxidation treatment of an alkaline aqueous solution that had absorbed hydrogen cyanide. Ozone oxidation conditions are sodium hydroxide aqueous solution volume 3000ml, CN ^- volume
30.0g, and the ozone input amount was 1.5g/HR. The results were as shown in Table 5. In Table 5, liquid
If no ozone is added to No. 4, liquid No. 5 will be
When ozone was added in such an amount that almost all of the CN ^- ions were oxidized, solution No. 6 was obtained by adding an excessive amount of ozone after almost all of the cyanide ions were oxidized.

[Table] Next, the change over time in the cyanate ion (CNO ^- ) concentration during normal temperature (room temperature) hydrolysis of solutions No. 5 and No. 6 in Table 5 was measured. The results are in Table 6.
It was as shown below. Hydrolysis was carried out at room temperature of 15-20°C.

[Table] From Table 6, cyanate ions in No. 5 solution became significantly smaller after 15 days of hydrolysis, and no. It can be seen that the cyanate ion concentration in the solution No. 6 becomes significantly smaller after 11 days of hydrolysis, and the cyanate ions are completely decomposed after 15 days. From the above, in detoxifying alkaline aqueous solutions that have absorbed hydrogen cyanide, the rate of hydrolysis of cyanate produced by ozone oxidation of cyanide ions is high even at room temperature (or room temperature), and at temperatures of 60 to 100℃.
At high temperatures, the rate of hydrolysis becomes even greater.
Further, by further introducing ozone and performing ozone treatment after the cyanide ions have been almost completely oxidized, the hydrolysis rate can be further increased, and cyanate can be decomposed in an extremely short time. As explained above, the method of the present invention does not require the use of expensive chemicals or strict monitoring of the reaction process, and does not require accurate adjustment operations.
The hydrogen cyanide absorption liquid can be detoxified in an extremely short time using simple equipment and simple operations without the need for a PH adjustment process.This makes the equipment inexpensive, economically advantageous, and easy to operate. It has the advantage of being large and safe.

[Brief explanation of the drawing]

The drawing is a flow sheet showing an example of an apparatus for carrying out the method of the present invention. 1... Fruit and vegetable fumigation chamber, 2... Exhaust fan, 3... Absorption tower, 4... Hydrogen cyanide absorption liquid, 5... Pump,
6...Absorption liquid tank, 7...Alkaline aqueous solution supply pipe, 8...Absorption liquid circulation pipe, 10...Pump, 1
1... Oxidation tower, 12... Ozone generator, 13...
Filler, 14...Cyanate-containing aqueous solution, 15...
... pump, 16 ... heating device, 17 ... processing liquid tank.

Claims (1)

[Scope of Claims] 1. An aqueous solution containing sodium cyanide and potassium cyanide produced by absorption of hydrogen cyanide when detoxifying a hydrogen cyanide absorption solution in which hydrogen cyanide was absorbed with an alkaline aqueous solution containing sodium hydroxide and potassium hydroxide. is brought into contact with an ozone-containing gas and oxidized to convert sodium cyanide and potassium cyanide into sodium cyanate and potassium cyanate, and then the aqueous solution containing this sodium cyanate and potassium cyanate is heated to 60 to 100 g.
A method for detoxifying a hydrogen cyanide absorption liquid, which is characterized by heating to ℃ to perform hydrolysis. 2. When detoxifying a hydrogen cyanide absorption solution in which hydrogen cyanide has been absorbed with an alkaline aqueous solution containing sodium hydroxide and potassium hydroxide, an aqueous solution containing sodium cyanide and potassium cyanide produced by the absorption of hydrogen cyanide and an ozone-containing gas are used. After contacting and oxidizing almost all of the sodium cyanide and potassium cyanide into sodium cyanate and potassium cyanate, this cyanate-containing alkaline aqueous solution is further brought into contact with an ozone-containing gas, and then this cyanate-containing aqueous solution is A method for detoxifying a hydrogen cyanide absorption liquid, which is characterized by heating to 60 to 100°C to perform hydrolysis.