JPH0131925B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for detoxifying exhaust gas containing cyanide components. When exhaust gas containing dust and a small amount of cyanide is washed with water to remove dust, the cyanide will dissolve into the water after dust removal, so from an environmental perspective, it is necessary to remove the cyanide before draining the water. The exhaust gas may contain metals themselves and metal oxides, and when these metals are washed and adsorbed with water, some of the metals dissolve or ionize in the water. These metal ions combine with cyanide ions adsorbed at the same time in the water, and some of these metal ions create cyanide complex ions and salts such as rhodanization, which are difficult to decompose by chemical treatment as described below, thereby reducing cyanide removal efficiency. In other words, these salts are broadly classified into metal salts of hydrogen cyanide with the general formula M(CN)n, and cyanide complex salts, which are metal salts of cyanide complex ions in which a metal is combined with an excess of cyanide ions (CN ^- ). be done. In the former case, it is relatively easy to oxidize, but in the latter case, it is difficult to eradicate dissolved cyanide, and there are even cases where it is impossible to eradicate dissolved cyanide using conventional treatment methods. Generally, the following three methods are adopted as methods for treating cyanide in water. ○A Biological treatment using activated sludge method. ○B Ion exchange treatment method using ion exchange resin. ○C Decomposition, precipitation, and adsorption treatment methods using chemicals. Among the above, the activated sludge method (○a) has unstable factors such as fluctuations in the cyanogen treatment effect when the balance of other salts in the water changes. In addition, in the case of large-scale treatment, the method of ○A above has problems such as the problem of recycled waste liquid and the contamination of replacement resin, and the treatment cost becomes high. For these reasons, the chemical treatment method described in ○C is generally adopted, in which cyanide in water is precipitated as a complex compound, extracted from the system as an insoluble material, and then incinerated or treated with activated sludge. ing. However, this chemical method is also used when processing large volumes,
When obtaining treated water containing a trace amount of cyanide with a permissible value of 1 ppm or less, it is necessary to collect the generated precipitate,
The equipment investment and treatment costs for dehydration, incineration, etc. are enormous, and as mentioned above, cyanide ions coexisting with other metal ions have the disadvantage that it is difficult to decompose them using chemicals. Therefore, an experimental example of changes in cyanide ions in water over time and changes depending on the amount of oxidizing agent added will be described below. Figure 1 shows how much CN is added to sample water containing Fe ions.
It is a diagram showing changes over time when 200 ppm is added.
The solid line shows a state in which the amount of free cyanide decreases, and the broken line shows a state in which ferrocyanide ions generated by the reaction of the CN-containing liquid with Fe ions increase. Figure 2 shows the treatment effect in terms of residual cyanide due to the concentration of the oxidizing agent and the concentration of complex ions in the cyanide salt when an oxidizing agent for cyanogen treatment is added to a CN-containing liquid containing complex cyanide ions such as ferrocyan. .
In this figure, if 95 ppm of the cyanide salt is complex cyanide (i.e. 5 ppm of free cyanide), the residual free cyanide after oxidation treatment is
It shows that 45 ppm was detected, 200 ppm was detected for 2 times the equivalent of B, and 5 ppm was detected for 3 times the equivalent of C. These experiments revealed that when cyanide ions form a complex, it changes into a complex that does not easily oxidize and decompose at room temperature in proportion to the residence time in water. Furthermore, even if cyanide is not a complex, selection of the pH range is generally important in treating cyanide in water. For example, when treating with a chlorine-based oxidizing agent, the reaction of the following formula completes instantaneously at the initial stage regardless of the pH value. CN ^- +HclO→CNcl+OH...(1) However, in the subsequent reaction of formula (2) below, the higher the PH value, the faster the reaction rate, and in that case, the PH is preferably 10 or higher. CNcl+OH ^- →cl ^- +HOCN...(2) Moreover, if the hydrolysis rate is delayed, it becomes highly toxic.
Since CNcl is generated and the water becomes unsuitable for drainage, careful attention must be paid to maintaining the pH. The reaction of equation (3) below, which completely decomposes the cyanic acid into CO ₂ and N ₂ , is delayed when the pH is high;
It is necessary to do the following. 2CNO ^- +30cl ^- +H ₂ O →2CO ₂ +N ₂ +3cl ^- +2OH ^- (3) As described above, extremely complicated operations are required to treat cyanide in water using a chlorine-based oxidizing agent. Other methods include an ozone oxidation method and an oxidation treatment method using iron salts, but all of these methods have problems such as the residual treatment agent itself due to excessive addition of the treatment agent and uneconomical effects. Furthermore, when hydrogen cyanide gas was passed through water in which sulfur ions and zinc ions coexisted, free cyanide was measured immediately after, and it was found to be 105 ppmasCN ^- . When the sample water was left for 3 hours, the free cyanide concentration became 5ppm, then 105ppmasCN ^-
When an equivalent amount of oxidizing agent was also added, 60 ppm of free cyanide was detected. This means that cyanide rhodanized with sulfur ions and cyanide complexed with zinc are
This means that it was decomposed by an oxidizing agent and redissolved as free cyanide. It was also observed that this free cyanide coexisted with the oxidizing agent in the liquid. Furthermore, when cyanide ions in water are decomposed into rings using a formaldehyde-based chemical, the reaction is as shown in the following formula, but experiments have shown that as the cyanide complex salt increases, the free cyanide treatment effect decreases. Moreover, cyanide ions are not completely decomposed when added in an amount equivalent to the cyanide component.
If the dose of the drug is increased, the COD value will increase. R.CH ₂ O＋H ₂ O＋CN ^- →NH ₃ ＋R.COOH …(4) As mentioned above, the conventional method of decomposing the cyanide component in the washing water after removing metal-containing dust from the exhaust gas with water requires very little equipment cost. However, various problems remain to be solved in terms of operational complexity, cost, processing efficiency, etc. The present invention has been made to solve the above-mentioned drawbacks, and after decomposing the cyanide component by directly contacting the gas containing dust and cyanide with an oxidizing agent or formaldehyde, the exhaust gas is subjected to wet dust collection. It is characterized by The present invention will be explained below based on experimental examples. It has already been explained that when the cyanide component in the dust-containing exhaust gas comes into contact with water and becomes Rodan salt or complex salt in the water, the amount of salt produced increases in proportion to time. Therefore, in the present invention, an equivalent amount of cyanide removing agent corresponding to the amount of cyanide components in the exhaust gas is directly blown into the exhaust gas to decompose the cyanide components, so that cyanide ions and cyanide salts are added to the treated water during wet dust collection in the subsequent process. is made so that it does not exist at all. The results of a comparative experiment between the conventional method and the present invention using the experimental apparatus shown in FIG. 3 will be described below. In FIG. 3, gas is sucked through a gas inlet 1 and introduced into a container 3 containing a liquid (pH 6 to 8) 2 equivalent to cleaning water to adsorb dust and dissolved components such as cyan components in the gas. 4 is a drain cutter, 5 is a suction flow rate adjustment pot, and 6 is a suction pump, and the gas after suction and cleaning is discharged from an exhaust pipe 7. First, in this experiment, we added cyanide (as500mg/Nm ³ gas), hydrogen sulfide (asH ₂ S5Kg/Nm 3 gas), and iron (asFe1g/Nm ³ gas) to gases mainly composed of N ₂ , O ₂ , _CO 2 , etc.
m ³ of fine powder) is mixed, washed with water in the container 3, and the treated water contains 5 to 10 ppm of cyanide, 40 to 50 ppm of rhodan cyanide, and 40 to 40 ppm of other cyanide complex salts.
50ppm was generated. Five types of chemicals were added to the solution under these conditions using the same chemical treatment method used in conventional equipment.
An equivalent amount was injected relative to the CN salt. This experiment was conducted five times for each chemical, and the residual cyanide values in the treated water were measured. The results are shown in the upper column of Table 1. As is clear from Table 1, it was confirmed that cyanide could not be completely removed by each of the chemicals using this method. Next, based on the present invention, a cyanogen treatment agent was sprayed and injected from the chemical supply pipe 8 while gas under the same conditions as in the conventional experiment was passed into the container 3 from the gas inlet 1 shown in FIG. Note that 9 is Kotsuku. The chemicals and amounts used at this time were the same as those for the conventional method except for the precipitation treatment method, and the experiments were conducted five times each for the four types of chemicals. Shown in the bottom column of the table. According to the above experimental results, when the chemical is brought into direct contact with the gas according to the present invention, no residual cyanide is detected in the treated water no matter which of the four types of chemical is used.
It has been proven that it does not produce cyanide or rhodan salts. This means that cyanide decomposition in gas is completely decomposed before cyanide ions become cyanide complex ions or rhodanization, compared to water. Note that the cyanide oxidizing agent used in the present invention is in liquid form,
It may be in the form of powder, gas, or a mixture thereof, but if it is sprayed into the passing gas at an appropriate pressure, the opportunity for contact with the cyanide component in the gas increases and it can be effectively oxidized. As described above, by carrying out the present invention, it is no longer necessary to decompose cyanide components from exhaust gas dust collection water, and it is possible to eliminate the need for complex processing such as pH adjustment using small-scale equipment and a small amount of oxidizing agent. The cyanide component in the gas can be easily decomposed and removed in a short period of time, resulting in great benefits in terms of equipment costs, operating costs, etc.

【table】

【table】 [Brief explanation of drawings]

Figures 1 and 2 show the experimental results regarding the conventional underwater cyanide decomposition method, of which Figure 1 shows the change over time when CN is added to Fe ion-containing water, and Figure 2 shows a cyanide oxidizing agent added to CN-containing water. This shows the treatment effect when adding . FIG. 3 shows an example of experimental equipment used for comparison of the present invention and the conventional method, in which numerals 1 to 7 are general exhaust gas treatment systems, and 8 to 9 are additional equipment for implementing the present invention. In the figure, 1... Gas inlet, 2... Cleaning liquid, 3... Container, 4... Drain cutter, 6... Suction pump, 7... Exhaust pipe, 8... Drug supply pipe, 5
And 9... Kotuku.

Claims (1)

[Claims] 1 Fluid gas containing dust and cyan components
A method for treating a cyanide component in exhaust gas, which comprises decomposing the cyanide component by contacting it with an oxidizing agent or formaldehyde in the form of powder, gas, or a mixture thereof, and then subjecting the exhaust gas to wet dust collection.