JPH0148832B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention deals with mercury-containing wastewater (for example, cleaning water discharged from an alkaline solution cleaning process of incinerator exhaust gas, etc.)
Regarding the processing method. A conventional mercury treatment method is the sulfur coagulation sedimentation method. In this method, mercury is incorporated into the coagulation sedimentation sludge as mercury sulfide, and when this is dehydrated and landfilled, methylmercury may be generated in the landfill. It has been pointed out that there is. Therefore, it can be said that a desirable treatment method is to reduce the amount of mercury-containing sludge as much as possible. One of the methods for treating mercury-containing wastewater that can achieve this purpose is the reduction volatilization method, which is characterized by the ability to recover metallic mercury. In the reduction reaction treatment step of this reduction volatilization method, for example, when hydroxylamine sulfate is used as the reducing agent, the reducing agent reacts with ionic mercury according to the following reaction formula. Hg ²⁺ + (NH ₂ OH) ₂ H ₂ SO ₄ +4OH ^- →Hg+N ₂ +6H ₂ O
+SO ₄ ^2- Thus, in the reduction reaction treatment process, the ionic mercury contained in the wastewater is reduced by the reducing agent,
becomes metallic mercury. When a gas such as air or an inert gas is blown into the wastewater containing metallic mercury as a volatile gas, the metallic mercury easily migrates into the volatile gas because of its high vapor pressure. This phenomenon is further accelerated by heating the wastewater. A portion of the metallic mercury that has migrated into the vaporized gas is condensed by cooling the vaporized gas. In this way, by subjecting mercury-containing wastewater to reduction reaction and volatilization treatment, the mercury contained therein can be removed. However, mercury may still remain in the wastewater that has undergone this reduction reaction and volatilization treatment process. In particular, the exhaust gas cleaning water discharged from the alkaline solution cleaning process for incinerator exhaust gas contains not only ionic mercury but also non-ionic mercury (referring to sparingly soluble mercury and mercury that cannot be reduced to metallic mercury by reducing agents). (including complex ions) are also included. Therefore, nonionic mercury may remain and the treated water after treatment may contain mercury exceeding water quality standards. The present invention has been made in view of the above circumstances, and its purpose is to provide a method for treating mercury-containing wastewater that can sufficiently remove mercury and keep the mercury content of treated water below the standard value. This is what we do. That is, the present invention subjects mercury-containing wastewater to a reduction reaction treatment to reduce ionic mercury in the wastewater to metallic mercury, performs volatilization treatment to entrain metallic mercury in the volatile gas, and cools and condenses it to remove metallic mercury. The wastewater that has been removed, collected, and volatilized is oxidized to neutralize the excess reducing agent, and the nonionic mercury in the wastewater is converted to ionic mercury, and then the excess oxidizing agent is neutralized by reduction treatment. Then, a portion of the reduced wastewater is returned to the reduction reaction treatment step, and the remainder is sent to a mercury recovery step consisting of a combination of one or more steps selected from the sulfur coagulation precipitation treatment step and the chelate treatment step. This is a method for treating wastewater containing wastewater. The present invention will be described below with reference to illustrative embodiments. As shown in Figure 1, mercury-containing wastewater (for example, exhaust gas cleaning water generated in the alkaline solution cleaning process of incinerator exhaust gas) is placed in a reduction reaction tank 1, and the ionic mercury in the wastewater is reduced to metal by a reducing agent. Mercury. The type of reducing agent is selected from the viewpoints of the relationship between the amount added and the volatilization effect in the volatilization treatment, and from the viewpoint of not contaminating the volatilization treatment liquid. For example, hydrazine hydrate, hydrazine sulfate, stannous chloride, silver sulfate,
One or more reducing agents such as acetaldehyde, sodium sulfite, hydrosulfite, hydroxylamine sulfate, and hydroxylamine hydrochloride are used. The amount of the reducing agent added is preferably at least 10 times the amount of mercury in the waste water. Also, at the time of reduction
It is desirable that the pH is 7 to 12 and the reaction time is 30 minutes or more. Further, the reduction treatment can be performed either batchwise or continuously. The exhaust gas cleaning water subjected to the reduction treatment in this manner is led to the volatilization device 2. In the volatilization device 2, the volatilization gas is blown into the gas and brought into gas-liquid contact, and metallic mercury is entrained in the volatilization gas. As the volatilization gas blown into the volatilization device 2, a gas that does not react with wastewater, such as air, nitrogen, or argon, is used. As the volatilization device 2 for bringing into contact with gas and liquid, one equipped with a suitable gas dispersion device or a gas-liquid contact device such as a packed tower is used. The volatilization conditions vary slightly depending on the type of volatilization gas to be blown in, the type of volatilization device 2, the quality of waste water, etc., but in general, the pH is 8 to 12, the volatilization time is 0.5 hours or more, and the amount of volatilization gas is at least 5 times the amount of inflow water. is preferable. Further, it is desirable to use steam, a heater, etc. to set the volatilization temperature to about room temperature to 100°C. Note that it is also possible to boil the exhaust gas cleaning water in the volatilization device 2 without blowing in the volatilization gas. Further, the volatilization treatment can be performed either batchwise or continuously. Further, it is also possible to perform the reduction reaction treatment and the volatilization treatment in the same tank. Next, the vaporized gas accompanied by metallic mercury is transferred to a cooler 3.
After cooling to about 0 to 10° C. and transferring mercury into the condensate, the condensate is placed in a solid-liquid separation tank 4 for solid-liquid separation to recover metallic mercury. The condensate from which metallic mercury has been removed is returned to the reduction reaction tank 1. On the other hand, on the gas side (volatized gas after cooling), residual mercury is treated with a gas chelator 5 or the like and then diffused. Alternatively, it is allowed to flow into the flue upstream of the smoke cleaning process. Next, the treated liquid after removing metallic mercury with a volatilized gas (hereinafter referred to as volatilized treated water) is subjected to oxidation treatment and reduction treatment as a pre-process for sulfidation coagulation and precipitation treatment. In other words, for volatilized water, 0.0005 to 0.2
It contains mercury in the amount of 0.005 mg/mg/ml, and if left as is, there is a risk that it will contain mercury in excess of the emission standard (0.005 mg/ml). Therefore, in the present invention, mercury is removed by subjecting the volatilized water to sulfidation coagulation and precipitation treatment, but if the volatilization treatment water contains a reducing agent, the efficiency of this sulfidation coagulation and precipitation treatment will decrease. For this reason, in the present invention, the volatilized water from the volatilization device 2 is first led to the oxidation tank 6, and an oxidizing agent is added thereto for oxidation treatment to neutralize the reducing agent remaining in the volatilized water. Use sodium diachloride as the oxidizing agent, and the amount added is sufficient or excessive to neutralize the reducing agent, for example, in the case of sodium hypochlorite, the concentration is 0 to 500 (mg/). It is desirable to do so. The pH value in the oxidation tank 6 is preferably 2-6, particularly preferably 3-4.
Next, the volatilized water containing the oxidizing agent is led to the reduction tank 7 and subjected to reduction treatment with the reducing agent to neutralize the oxidizing agent.
As the reducing agent, sodium sulfite or the like is used, and the amount added is such that no reducing agent remains. Reduction tank 7 can be operated under ORP control; in this case,
Preferably, the ORP is between 300 and 600 mV. A portion of the wastewater subjected to the reduction treatment in the reduction tank 7 is returned to the reduction reaction tank 1. The reason for returning the mercury is that if the mercury in the mercury-containing wastewater is contained in a form that is difficult to reduce, such as HgI ₂ , HgS, chelate compounds, etc., it cannot be reduced in the reduction reaction tank 1, and therefore the volatilization equipment 2 means that it cannot be volatilized. Therefore, in the present invention, the wastewater after the volatilization treatment is sent to the oxidation tank 6, where the residual reducing agent in the wastewater is neutralized and, at the same time, the hard-to-reduced form of mercury is converted into the easily-reduced form of mercury. Then, after neutralizing the excess oxidizing agent in the reduction tank 7, it is returned to the reduction reaction tank 1. Note that in the reduction tank 7, an inexpensive and relatively weak reducing agent such as sodium sulfite is usually used, but in this case, the reducing effect on mercury ions is low, and therefore the mercury is not directly returned to the volatilization process, and the mercury is reduced. In this example, the reduction reaction tank 1
I am sending it back to . This makes it possible to increase the recovery rate of metallic mercury from mercury-containing wastewater and to reduce the mercury load on subsequent processes (sulfidation coagulation precipitation treatment process, etc.). However, if the returned amount is too large, the reduction reaction tank 1 and the volatilization device 2 will become large, and equipment costs, operating costs, etc. will increase, so the returned amount is appropriately set in consideration of economic efficiency. On the other hand, the remainder of the wastewater that has been reduced in the reduction tank 7 is led to the sulfurization reaction tank 8 and subjected to sulfidation coagulation and precipitation treatment. In this treatment, sodium sulfide is added according to the amount of mercury contained in the treated water to convert the mercury into mercury sulfide, and then ferric chloride, sodium hydroxide, etc. are added to coagulate the mercury sulfide. The sulfurization reaction takes place at a pH of 5 to 7.
The first flocculation tank 9, in which ferric chloride is added, conducts the reaction at 2 to 20 mg of sodium sulfide and a reaction time of 5 to 30 minutes.
It is preferable to carry out the reaction at pH 5-7, ferric chloride 100-1000 mg/reaction time, and reaction time of 15 minutes or more. The treated water treated in this way is put into a first flocculating tank 9 to which an inorganic flocculant such as sulfuric acid is added if necessary, then passes through a second flocculating tank 10 to which a polymer flocculant is added, and then to a settling tank 11. to separate solid and liquid and remove sludge. The treatment liquid can have a mercury concentration of 0.005 mg/or less, and can be discharged. Note that, if necessary, the treatment liquid may be passed through the sand filter tower 12 and the chelate resin tower 13 to further remove mercury before being discharged. In addition, instead of performing sulfur coagulation and precipitation treatment, sand filter tower 1
4 and chelate resin column 15 to remove the mercury and discharge it. Next, specific examples of the present invention will be explained together with comparative examples. Example: Wastewater generated in the smoke washing process of a garbage incinerator (PH7~
7.5, Hg4.5-7.1mg/, evaporation residue 5-8%)
was used as raw water and subjected to reduction reaction, volatilization, oxidation, reduction, and sulfidation condensation and precipitation treatments. The processing conditions are shown in Tables 1 and 2. Furthermore, the processing results are shown in FIG.
Note that 1/2 of the wastewater that had been reduced in the reduction tank 7 was returned to the reduction reaction tank.

【table】

【table】

[Table] Comparative example Figure 3 shows a comparative method, that is, a simple combination of the reduction volatilization method and the sulfurization condensation precipitation method, and the oxidation of the volatilization treated water.
This method omits both the reduction treatment and the step of returning the treatment liquid after the reduction treatment to the reduction reaction tank. The quality of the raw water in this method is the same as in the above example, and the treatment conditions are as shown in Table 1 above. The processing results are shown in FIG. Figure 5 shows the relationship between the concentration of the residual reducing agent in the volatilized water and the mercury concentration in the final treated water.From this figure, as the residual reducing agent concentration in the volatilized water increases, the mercury concentration in the final treated water increases. There was a tendency for the number to increase. As is clear from the above results, the comparative method can recover some of the various forms of mercury contained in mercury-containing wastewater as metallic mercury (recovery rate, e.g., approximately 88.5%), and also Concentration can be reduced. However, due to the residual reducing agent in the volatilized water and nonionic mercury that does not become metallic mercury in the reduction process, it may not be possible to continuously and stably reduce the mercury concentration below the regulatory value (0.005 mg/). On the other hand, it can be seen that in the examples, the recovery rate of metallic mercury is increased and the mercury concentration of the final treated water can be reliably kept below the regulation value. As explained above, according to the present invention, mercury-containing wastewater is subjected to reduction reaction and volatilization treatment, followed by oxidation and reduction treatment, and a part of the wastewater is returned to the reduction reaction treatment process, so that the recovery rate of metal mercury is increased and It is possible to reliably reduce the mercury concentration of the final treated water to below the regulatory value.

[Brief explanation of drawings]

FIG. 1 is a flow sheet diagram showing an example of the present invention, FIG. 2 is an explanatory diagram showing experimental results of the same example,
Figure 3 is a flow sheet diagram showing a comparative example, Figure 4 is an explanatory diagram showing the experimental results of the same comparative example, and Figure 5 shows the relationship between the residual reducing agent concentration in the volatilized water and the mercury concentration in the final treated water. FIG. 1...Reduction reaction tank, 2...Volatilization device, 3...Cooler, 4...Solid-liquid separation tank, 5...Gas chelator, 6...Oxidation tank, 7...Reduction tank, 8...Sulfurization Reaction tank, 9...first flocculation tank, 10...second flocculation tank,
11... Sedimentation tank, 12, 14... Sand filter tower, 1
3,15...Chelate resin tower.

Claims (1)

[Claims] 1 Reduction reaction treatment of mercury-containing wastewater to reduce ionic mercury in the wastewater to metallic mercury, and volatilization treatment to entrain metallic mercury in the volatile gas, which is cooled and condensed to remove and recover metallic mercury, On the other hand, the wastewater that has been volatilized is oxidized to neutralize the excess reducing agent, and the nonionic mercury in the wastewater is converted to ionic mercury. Treatment of mercury-containing wastewater in which a part of the wastewater is returned to the reduction reaction treatment step, and the remainder is sent to a mercury recovery step consisting of a combination of one or more steps selected from the sulfurization coagulation precipitation treatment step and the chelation treatment step. Method.