JPH0122833B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for treating mercury-containing cleaning liquid discharged from an alkaline liquid cleaning process of incinerator exhaust gas. One of the methods for treating mercury-containing wastewater is the reduction volatilization method, which is characterized by the ability to recover metallic mercury. In this reduction reaction treatment step of the reduction volatilization method,
For example, when hydroxylamine sulfate is used as a reducing agent, the reducing agent reacts with mercury ions 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 sucked 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. Conventional reduction and volatilization treatment methods include, for example, a method in which a reducing agent is added to a concentrated caustic alkaline solution containing mercury, and the mercury in the solution is vaporized and removed by boiling or blowing gas (Special Publication No. 49-23998), or salt water precipitation. A method is known in which mercury in the slurry is vaporized and removed by boiling the slurry in the presence of a reducing agent or by blowing gas into the slurry (Japanese Patent Publication No. 6431/1983). The gases blown into the solution in these methods include hydrogen,
Gases that are inert to the target solution are used, such as nitrogen, air, water vapor, argon, and helium. However, gases other than air require specialized gas equipment and are expensive. When air is used, the added reducing agent is oxidized, so there is a problem that the amount of reducing agent added increases. Furthermore, increasing the liquid temperature is effective in increasing the vaporization efficiency of mercury, but if an electric heater or the like is used, a problem arises with the thermal efficiency of the boiler. The present invention was made to solve these problems, and its purpose is to provide volatilized gas at low cost without requiring special gas equipment, and to prevent oxidation of the reducing agent. Moreover, it is an object of the present invention to provide a method for treating exhaust gas cleaning water in which vaporized gas can assist in heating. That is, the present invention reduces mercury in waste water generated from the process of cleaning incinerator exhaust gas to metal mercury using a reducing agent, and when removing this metal mercury using volatilized gas, the incinerator exhaust gas is removed as volatile gas. It is characterized by using exhaust gas after passing through a dust collection process. The present invention will be described below with reference to illustrative embodiments. The exhaust gas cleaning water generated in the process of cleaning the incinerator exhaust gas with alkaline liquid is put into the reduction reaction tank 1, and is reduced with a reducing agent while being stirred for a certain period of time to convert the mercury ions in the exhaust gas cleaning water into metallic mercury. That is, the exhaust gas generated in the incinerator A is guided to the smoke cleaning process D through the gas cooling process B and the dust collection process C. Here, the exhaust gas is washed with alkaline liquid, the gas is discharged from the chimney E, and the liquid is led from the smoke washing water pit F to the reduction reaction tank 1. The type of reducing agent to be added to the reduction reaction tank 1 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, acetaldehyde, sodium sulfite, hydrosulfide,
One or more reducing agents such as hydroxylamine sulfate and hydroxylamine hydrochloride are used. Further, the amount of the reducing agent added is preferably 10 times or more equivalent to the amount of mercury in the waste water. In addition, the pH during reduction is 7 to 12,
It is desirable that the reaction time be 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, the exhaust gas after passing through the dust collection step C is used. In other words, although the composition and specific heat of the exhaust gas after dust has been collected in the dust collection process vary depending on the incinerator, in general, as shown below, the amount of oxygen is lower than that of air, and the temperature is between 240 and 300°C. expensive.

〔experimental method〕

The smoke washing wastewater was placed in a sealed glass container (No. 5) equipped with a heater, a stirrer, and an aeration tube, a predetermined amount of reducing agent was added, the pH was adjusted, and the reaction was allowed to proceed for 30 minutes. 70 after this
The temperature was raised to ℃, and the flue gas (exhaust gas after passing through the dust collection step) was sucked into the container. At this time, the pH was adjusted to 8 using a pH regulator. The amount of flue gas injected is
The pressure was set at 2.5 N/min (30 times the liquid volume per hour).
Samples for analysis were taken twice hourly. As the reducing agent, hydroxylamine sulfate was used, and the amount added was 300 mg. For comparison, experimental results are also shown for the case where air was blown in as the volatile gas.

[Table] It can be seen that when flue gas is used, the mercury concentration after volatilization can be significantly reduced compared to air. As explained above, according to the present invention, since the exhaust gas after passing through the dust collection process of the incinerator exhaust gas is used as the volatilized gas, the gas cost is not high and expensive equipment is not required. Furthermore, since the oxygen content of the exhaust gas is significantly lower than that of air, the amount of reducing agent consumed by oxidation is reduced, allowing effective reduction treatment.
Furthermore, since the temperature of this exhaust gas is around 240°C to 300°C, this thermal energy can be used as an auxiliary heating source, resulting in significant effects such as reducing heating costs.

[Brief explanation of drawings]

The drawing is a flow sheet diagram showing one embodiment of the present invention. 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. Mercury in waste water generated from the incinerator exhaust gas cleaning process is reduced to metallic mercury using a reducing agent, and when this metallic mercury is removed using volatile gas,
A method for treating exhaust gas cleaning water, characterized in that exhaust gas after passing through a dust collection process of incinerator exhaust gas is used as the volatilized gas.