Processing method of mercury-containing waste and processing system of mercury-containing waste

Abstract

To provide a processing method of mercury-containing waste and a processing system of mercury-containing waste capable of suppression of mercury adsorption in a catalytic reaction tower and improving removal efficiency of mercury in the mercury adsorption tower.SOLUTION: A processing method of mercury-containing waste includes: a heat treatment process of heat treatment of a mercury-containing waste; a dust collection process for recovery of dusts in an exhaust gas containing mercury generated by the heat treatment process; a dioxin decomposition process of decomposition of dioxins contained in the exhaust gas by bringing the exhaust gas having been through the dust collection process into contact with dioxin decomposition catalyst; and a mercury adsorption process of bringing the exhaust gas having been through the dioxin decomposition process into contact with an adsorbent for absorbing and removing mercury contained in the exhaust gas, wherein an acidic gas concentration adjustment process for adjusting a concentration of the acidic gas contained in the exhaust gas and supplied to the dioxin decomposition process to the objective concentration or over.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for treating mercury-containing waste and a system for treating mercury-containing waste.

Background Art

[0002] Conventionally, heat treatment devices such as incinerators and melting furnaces have been used to reduce the volume of waste. In an incinerator, general waste and the like are incinerated, and main ash and fly ash are recovered. In a melting furnace, main ash and fly ash generated by incineration treatment, as well as surplus sludge generated by wastewater treatment, are melted, and slag and fly ash are recovered.

[0003] In addition, in order to restore the natural environment such as soil, vegetation, the sea, and rivers contaminated with radioactive substances leaked from equipment using nuclear fission reactions such as nuclear power plants, technologies for separating and concentrating radioactive substances from objects to be treated containing radioactive substances have been studied.

[0004] For example, by adopting a radioactive cesium separation and concentration method as described in Patent Document 1, incineration main ash, incineration fly ash, and even soil contaminated with radioactive substances are melted to volatilize radioactive substances, and the remaining substances from which radioactive substances have been removed are reduced in volume as molten slag. Then, the radioactive substances contained in the exhaust gas are separated as dust through a dust collection device by volatilization separation, so that the radioactive substances can be greatly reduced in volume, and the need to expand storage facilities is also reduced.

[0005] Furthermore, Patent Document 2 proposes a method for treating radioactive substance-containing materials that can further reduce the volume of incineration main ash, incineration fly ash, etc. contaminated with radioactive substances and efficiently separate radioactive substances.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

[0007] And, Patent Document 2 is provided with a bag filter that neutralizes and removes acidic gases such as hydrogen chloride by spraying slaked lime, which is a neutralizing agent, into the flue of a melting furnace, and a catalytic reaction tower that is provided with a catalytic metal such as titanium or vanadium that decomposes dioxins contained in the exhaust gas by a catalytic reaction on the downstream side of the bag filter. Further, a mercury adsorption tower filled with pellet-shaped activated carbon for removing mercury contained in the exhaust gas is provided on the downstream side thereof.

[0008] An object of the present invention is to provide a method for treating mercury-containing waste and a system for treating mercury-containing waste that suppress the adsorption of mercury in a catalytic reaction tower and improve the removal efficiency of mercury in a mercury adsorption tower.

MEANS FOR SOLVING THE PROBLEMS

[0009] To achieve the above object, a first characteristic configuration of the method for treating mercury-containing waste according to the present invention is a method for treating mercury-containing waste, including a heat treatment step of heat-treating the mercury-containing waste, a dust collection step of collecting dust in the exhaust gas containing mercury generated by the heat treatment step, a dioxin decomposition step of bringing the exhaust gas that has passed through the dust collection step into contact with a dioxin decomposition catalyst to decompose dioxins contained in the exhaust gas, and a mercury adsorption step of bringing the exhaust gas that has passed through the dioxin decomposition step into contact with an adsorbent to adsorb and remove mercury contained in the exhaust gas, and is characterized in that it includes an acidic gas concentration adjustment step of adjusting the concentration of the acidic gas contained in the exhaust gas supplied to the dioxin decomposition step to a target concentration or higher.

[0010] The exhaust gas generated in the heat treatment process is sent to the dust collection process, and the dust contained in the exhaust gas is recovered. Further, when the exhaust gas containing acidic gas is sent to the dioxin decomposition process, the dioxins are efficiently decomposed in a state where the adsorption of mercury to the dioxin decomposition catalyst is suppressed, and then in the subsequent mercury adsorption process, the mercury contained in the exhaust gas is efficiently adsorbed by the adsorbent. As a result of intensive research and tests by the inventors of the present application, when a predetermined concentration of acidic gas is present in the exhaust gas sent to the dioxin decomposition process, the adsorption of mercury to the dioxin decomposition catalyst is suppressed, and when no acidic gas is contained, it has been found that mercury is adsorbed to the dioxin decomposition catalyst. Therefore, by adjusting the concentration of the acidic gas contained in the exhaust gas supplied to the dioxin decomposition process to the target concentration by the acidic gas concentration adjustment process, the dioxin decomposition process and the mercury adsorption process proceed properly.

[0011] The second characteristic configuration, in addition to the first characteristic configuration described above, is that the target concentration of the acidic gas adjusted in the acidic gas concentration adjustment process is 3 to 100 ppm.

[0012] By adjusting the concentration of the acidic gas in the exhaust gas sent to the dioxin decomposition process to 3 to 100 ppm, the adsorption of mercury to the dioxin decomposition catalyst is effectively suppressed.

[0013] The third characteristic configuration, in addition to the first characteristic configuration described above, is that the dust collection process includes a neutralized product recovery process in which a neutralizing agent is sprayed onto the exhaust gas to neutralize the acidic gas and recover the neutralized product, and the acidic gas concentration adjustment process is a process of adjusting the spraying amount of the neutralizing agent based on the acidic gas concentration of the exhaust gas that has passed through the neutralized product recovery process.

[0014] When a neutralized product recovery process is provided as the dust collection process, in which a neutralizing agent is sprayed onto the exhaust gas to neutralize the acidic gas and recover the neutralized product, the process of adjusting the spraying amount of the neutralizing agent in the neutralized product recovery process can be utilized as the acidic gas concentration adjustment process.

[0015] The fourth characteristic configuration, in addition to the first characteristic configuration described above, is that the mercury-containing waste is radioactive substance-containing waste, the heat treatment step is a melting treatment step of melting the radioactive substance-containing material in the melting furnace in the presence of chlorine to volatilize the radioactive substance, the dust collection step includes a dust collection step of recovering dust by using a dust collection device that uses a silica-based auxiliary agent or an alumina-based auxiliary agent as a release agent for the exhaust gas containing the radioactive substance volatilized in the melting treatment step, a dissolution step of dissolving the radioactive substance contained in the dust recovered in the dust collection step in water, a solid-liquid separation step of recovering solids from the aqueous solution that has undergone the dissolution step, and a radioactive substance adsorption step of adsorbing and removing the radioactive substance from the aqueous solution that has undergone the solid-liquid separation step, and further includes a dust treatment step including these steps.

[0016] The exhaust gas containing the radioactive substance volatilized and separated from the molten slag in the melting treatment step passes through the dust collection step and is recovered as dust. The presence of chlorine is a concept that includes cases where chlorine or a chlorine compound is contained in the radioactive substance-containing material, or cases where chlorine or a chlorine compound is added as an auxiliary agent to the radioactive substance-containing material. The dust recovered in the dust collection step includes chlorides of radioactive substances, mineral elements that are oxides such as aluminum and iron, heavy metals such as lead, and a silica-based auxiliary agent or an alumina-based auxiliary agent used as a release agent in the dust collection step. In the dissolution step included in the dust treatment step, the radioactive substance is dissolved in water, the insoluble substances that are not dissolved in water are removed as solids in the solid-liquid separation step, and the radioactive substance dissolved in water is adsorbed and removed in the radioactive substance adsorption step. Since a silica-based auxiliary agent or an alumina-based auxiliary agent is used as a release agent instead of an auxiliary agent such as calcium carbonate in the dust collection step, the occurrence of inconvenient situations such as calcium scale adhering to pipes and blocking is effectively reduced in the dust treatment step.

[0017] The fifth characteristic configuration, in addition to the fourth characteristic configuration described above, is that the solid-liquid separation step is a step of separating mineral elements containing an insoluble silica-based auxiliary agent or alumina-based auxiliary agent as solids by adjusting the pH of the aqueous solution that has undergone the dissolution step, and includes a circulating melting step of melting the solids recovered in the solid-liquid separation step again in the melting furnace.

[0018] For example, if the pH of the aqueous solution is adjusted to acidic in the solid-liquid separation step, insoluble silica-based or alumina-based aids, etc., can be separated as solids from water-soluble heavy metals and radioactive substances. By remelting the recovered solids in a melting furnace, they can be recovered as slag. Also, mercury is removed in the subsequent exhaust gas treatment, and even when the recovered solids are remelted, mercury will not be concentrated.

[0019] The sixth characteristic configuration, in addition to the fourth characteristic configuration described above, further includes a heavy metal removal step of removing water-soluble heavy metals as insoluble heavy metals by adjusting the pH of the aqueous solution between the solid-liquid separation step and the radioactive substance adsorption step or after the radioactive substance adsorption step.

[0020] Heavy metals can be dissolved by performing a dissolution step under acidic conditions. If the pH of the aqueous solution is adjusted to neutral, for example, before the radioactive substance adsorption step after the solid-liquid separation step or after the radioactive substance adsorption step, the dissolved heavy metals will become insoluble and precipitate. For example, a heavy metal fixing agent (chelating agent) can be added to separate as a metal complex, or a flocculant can be added to cause flocculation precipitation.

[0021] The first characteristic configuration of the mercury-containing waste treatment system according to the present invention is a mercury-containing waste treatment system, including a heat treatment device for heat-treating the mercury-containing waste, a dust collection device for collecting dust in the exhaust gas containing mercury generated by the heat treatment device, a dioxin decomposition device for decomposing dioxins contained in the exhaust gas by bringing the exhaust gas passing through the dust collection device into contact with a dioxin decomposition catalyst, and a mercury adsorption device for adsorbing and removing mercury contained in the exhaust gas by bringing the exhaust gas passing through the dioxin decomposition device into contact with an adsorbent, and is characterized by including an acidic gas concentration adjustment device for adjusting the concentration of acidic gas contained in the exhaust gas supplied to the dioxin decomposition device to a target concentration or higher.

[0022] The second characteristic configuration is that, in addition to the first characteristic configuration described above, the target concentration of the acid gas adjusted by the acid gas concentration adjusting device is 3 to 100 ppm.

[0023] The third characteristic configuration is that, in addition to the first characteristic configuration described above, the dust collecting device includes a neutralized product recovery device that sprays a neutralizing agent onto the exhaust gas to neutralize the acid gas and recover the neutralized product, and the acid gas concentration adjusting device is a device that adjusts the spraying amount of the neutralizing agent based on the acid gas concentration of the exhaust gas that has passed through the neutralized product recovery device.

[0024] The fourth characteristic configuration is that, in addition to the first characteristic configuration described above, the mercury-containing waste is radioactive substance-containing waste, the heat treatment device is a melting furnace that melts the radioactive substance-containing material in the presence of chlorine to volatilize the radioactive substance, the dust collecting device includes a dust collecting device that recovers dust using a silica-based auxiliary agent or an alumina-based auxiliary agent as a peeling agent for the exhaust gas containing the radioactive substance volatilized in the melting furnace, a dissolving device that dissolves the radioactive substance contained in the dust recovered by the dust collecting device in water, a solid-liquid separation device that recovers solids from the aqueous solution in which the radioactive substance is dissolved, and a radioactive substance adsorption device that adsorbs and removes the radioactive substance from the aqueous solution from which solids have been recovered by the solid-liquid separation device.

[0025] The fifth characteristic configuration is that, in addition to the fourth characteristic configuration described above, the solid-liquid separation device is a device that separates mineral elements containing an insoluble silica-based auxiliary agent or alumina-based auxiliary agent as solids by adjusting the pH of the aqueous solution in which the radioactive substance is dissolved, and includes a circulating melting step of melting the solids recovered by the solid-liquid separation device again in the melting furnace.

[0026] The sixth characteristic configuration is that, in addition to the fourth characteristic configuration described above, a heavy metal removal device that removes water-soluble heavy metals as insoluble heavy metals by adjusting the pH of the aqueous solution is further provided between the solid-liquid separation device and the radioactive substance adsorption device or at a subsequent stage of the radioactive substance adsorption device.

Advantages of the Invention

[0027] As described above, according to the present invention, it is possible to provide a mercury-containing waste treatment method and a mercury-containing waste treatment system that suppresses mercury adsorption in a catalytic reaction tower and improves the mercury removal efficiency in a mercury adsorption tower. [Brief description of the drawings]

[0028]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

[0029] Hereinafter, an embodiment of the method for treating mercury-containing waste and the system for treating mercury-containing waste according to the present invention will be described.

[0030] [Configuration of mercury-containing waste treatment system] Figure 1 shows a treatment system 1 for waste containing radioactive substances as an example of mercury-containing waste. The treatment system 1 includes a melting furnace 2, a secondary combustion chamber 3, an air preheater 4, a first waste heat recovery device 5, a desuperheater 6, a first dust collector 7, a second dust collector 8, a neutralization product recovery device 9, a catalytic reaction tower 10, a mercury adsorption tower 11, a second waste heat recovery device 12, an induced draft fan 13, a chimney 15, and a dust treatment device 20 for treating the dust captured by the first dust collector 7 and the second dust collector 8. The neutralization product recovery device 9 is provided with a neutralizing agent supply device 17 and an acidic gas concentration adjustment device 16 for adjusting the supply amount of the neutralizing agent.

[0031] Incineration main ash, incineration fly ash, and other objects to be melted that contain radioactive substances leaked from equipment using nuclear fission reactions, mainly cesium Cs, strontium Sr, etc., and mercury, are put into the melting furnace 2 together with auxiliaries containing chlorine that promote the volatilization of radioactive substances and a melting point depressant. The radioactive substance-containing material input into the melting furnace 2 is not limited to incineration main ash and incineration fly ash generated by incinerating plants, waste wood, or sewage sludge contaminated with radioactive substances in an incinerator. In some cases, it may include fine-grained soil with a particle size of 0.075 mm or less obtained by sieving the soil decontaminated by particle size selection. This is because radioactive substances are taken up at high concentrations in the fine-grained soil.

[0032] As auxiliaries containing chlorine or chlorine compounds, chlorides such as sodium chloride, calcium chloride, potassium chloride, and iron chloride, and organic chlorine compounds such as plastics and resins containing chlorine are preferably used. However, when the object to be melted contains the required amount of chlorine, it is not necessary to add an auxiliary containing chlorine. As a melting point depressant, calcium compounds, alkali metal salts, alkaline earth metal salts, boron compounds, and iron compounds are preferably used. For example, basicity adjusters such as calcium oxide, calcium hydroxide, calcium carbonate, sodium carbonate, lithium carbonate, potassium carbonate, magnesium carbonate, magnesium hydroxide, magnesium oxide, boron oxide, borax, boric acid, ferrous oxide, magnetite, and ferric oxide serve as melting point depressants.

[0033] In the melting furnace 2, waste containing mercury and radioactive substances is melted in a high-temperature environment of 1200°C to 1500°C. At this time, mercury, chlorides of heavy metals, and alkali salts are volatilized, and radioactive substances such as cesium chloride (CsCl) and strontium chloride (SrCl2) that have become chlorides due to an auxiliary agent containing chlorine are volatilized and flow down to the secondary combustion chamber together with the exhaust gas. The remaining solid matter is melted in the melting furnace 2 and recovered as slag. The melting method of the melting furnace 2 is not particularly limited, and a rotary surface melting furnace or the like is preferably used.

[0034] The exhaust gas discharged from the melting furnace 2 has unburned gas components secondarily burned in the secondary combustion chamber 3 to reach about 1100°C, then flows down the flue, passes through the air preheater 4 that preheats the combustion air supplied to the melting furnace 2, the first waste heat recovery device 5 which is a high-temperature heat recovery device, and the desuperheating tower 6, and can suppress the resynthesis of dioxins at the inlet of the dust collection device, and is cooled down to a temperature range of about 160 - 200°C where low-temperature heat recovery can be achieved without condensation.

[0035] As the first dust collection device 7 and the second dust collection device 8, a bag filter with a silica-based auxiliary agent such as diatomaceous earth or glass powder or an alumina-based auxiliary agent spray-coated on the filter cloth is used. Usually, calcium carbonate or the like is often coated on the filter cloth, but in this treatment system 1, as will be described later, a silica-based auxiliary agent such as diatomaceous earth or an alumina-based auxiliary agent is preferably used in relation to the dust treatment device 20.

[0036] When the exhaust gas passes through the desuperheating tower 6 or the like and is cooled down to about 160 - 200°C, salts, mineral elements, and heavy metals containing radioactive substances become solid fine particles, flow into the first dust collection device 7 and the second dust collection device 8 together with the exhaust gas, and are captured as dust. The first dust collection device 7 and the second dust collection device 8 are installed redundantly to ensure reliability in case one fails. The first dust collection device 7 and the second dust collection device 8 may be connected in series or in parallel.

[0037] The waste contains salts of radioactive substances such as cesium chloride (CsCl) and strontium chloride (SrCl2), salts such as sodium chloride (NaCl), potassium chloride (KCl), and ferric chloride (FeCl3), mineral elements such as silicon dioxide (SiO2), aluminum oxide (Al2O3), calcium oxide (CaO), and ferric oxide (Fe2O3), heavy metal compounds containing elements such as lead (Pb), cadmium (Cd), zinc (Zn), and arsenic (As), and silica-based aids or alumina-based aids such as silicon dioxide (SiO2) and aluminum oxide (Al2O3) sprayed on the filter cloth as a release agent.

[0038] The neutralization product recovery device 9 is also composed of a bag filter. Slaked lime, which is a neutralizing agent, is blown into the bag filter by the neutralizing agent supply device 17 together with the exhaust gas, so that corrosive gas components such as hydrogen chloride contained in the exhaust gas are captured on the filter cloth as calcium chloride or the like. The neutralization products such as calcium chloride (CaCl2) captured by the neutralization product recovery device 9 are circulated and supplied to the melting furnace 2 as an aid containing chlorine, and the excess neutralization products are taken out and processed externally. Hereinafter, hydrogen chloride gas will be taken as an example of the acidic gas for explanation.

[0039] The neutralizing agent supply device 17 is equipped with a nozzle for injecting the neutralizing agent and a pump for adjusting the supply amount of the neutralizing agent supplied to the nozzle. Based on the chlorine gas concentration detected by the acidic gas sensor 14 installed on the downstream side of the induced draft fan 13, the acidic gas concentration adjustment device 16, which is a control device, controls the neutralizing agent supply device 17 so that the hydrogen chloride gas concentration contained in the exhaust gas flowing into the catalytic reaction tower 10 is maintained at a predetermined target concentration of 3 to 100 ppm.

[0040] The catalytic reaction tower 10 is equipped with a catalytic metal such as titanium or vanadium, and dioxins contained in the exhaust gas are decomposed by a catalytic reaction with the catalytic metal. At this time, the adsorption of mercury to the catalytic metal is inhibited by the hydrogen chloride gas contained in the exhaust gas, so that dioxins are efficiently decomposed and removed.

[0041] The mercury adsorption tower 11 is filled with pellet-shaped activated carbon, and mercury with a relatively low boiling point is adsorbed and removed by the activated carbon when the exhaust gas passes through. Note that the function of the mercury adsorption tower 11 may be replaced by a dust collection device that sprays activated carbon and removes the activated carbon adsorbed with mercury.

[0042] In the second waste heat recovery device 12, which is a low-temperature heat recovery device, waste heat is recovered from the exhaust gas at about 200°C that has passed through the mercury adsorption tower 11, and is exhausted from the chimney 15 via the induced draft fan 13. Both the first waste heat recovery device 5 and the second waste heat recovery device 12 are composed of heat exchangers that recover the retained heat of the exhaust gas through a heat medium such as water.

[0043] As shown in FIGS. 2 and 3, the dust treatment device 20 is an advanced treatment device for reducing the volume of ash treatment dust, which is molten fly ash captured by the first dust collection device 7 and the second dust collection device 8, and includes a dissolution device 22 that dissolves radioactive substances contained in the dust in water, a solid-liquid separation device 24 that recovers solids remaining without dissolving in the aqueous solution that has passed through the dissolution device 22, and a radioactive substance adsorption device 26 that adsorbs and removes radioactive substances from the aqueous solution from which solids have been separated by the solid-liquid separation device 24.

[0044] The solid-liquid separation device 24 includes a solid-liquid separation device 24A composed of a dehydrator such as a filter press or a screw press, and a solid-liquid separation device 24B equipped with a coagulation sedimentation tank that coagulates and precipitates mineral elements containing a silica-based auxiliary agent or an alumina-based auxiliary agent that has become insoluble by adding a pH adjuster or a coagulant to the aqueous solution.

[0045] The treatment system 1 for the radioactive substance-containing material needs to include at least the above-mentioned melting furnace 2, dust collection devices 7 and 8, catalytic reaction tower 10, mercury adsorption tower 11, and dust treatment device 20, and preferably includes a circulation mechanism 25 for transporting the solids recovered by the solid-liquid separation device 24 to be remelted in the melting furnace 2.

[0046] The dust treatment device 20 further includes a heavy metal removal device 28 that removes water-soluble heavy metals as insoluble heavy metals by adjusting the pH of the aqueous solution, between the solid-liquid separation device 24 and the radioactive substance adsorption device 26 (see FIG. 3), or downstream of the radioactive substance adsorption device 26 (see FIG. 2).

[0047] The radioactive substance adsorption device 26 is provided with an adsorption tower filled with a radioactive substance adsorbent, and the radioactive substance is adsorbed and removed by the radioactive substance adsorbent. It further includes a concentration device 29 that evaporates and dries the high-concentration brine from which heavy metals and radioactive substances have been removed. The recovered heat sources from the above-described first waste heat recovery device 5 and second waste heat recovery device 12 are used as the heat source of the concentration device 29. Note that the concentration device 29 can also be configured as a temperature reduction tower 6 installed upstream of the dust collection devices 7 and 8, which supplies and sprays a part of the high-concentration brine.

[0048] [Configuration of the method for treating radioactive substance-containing materials] The method for treating radioactive substance-containing materials of the present invention is executed by the above-described radioactive substance-containing material treatment system 1. This will be described in detail below.

[0049] In the above-described melting furnace 2, a melting process is executed to volatilize radioactive substances from the radioactive substance-containing material, and in the first dust collection device 7 and the second dust collection device 8, a dust collection process is executed to recover dust containing radioactive substances from the exhaust gas containing the radioactive substances volatilized in the melting process.

[0050] Also, in the catalytic reaction tower 10, a dioxin decomposition process is executed to remove dioxins contained in the exhaust gas, and in the mercury adsorption tower 11, a mercury adsorption process is executed to remove mercury contained in the exhaust gas.

[0051] Furthermore, in the dust treatment device 20, a dust treatment process is executed to reduce the volume of the dust. In the dissolving device 22, a dissolving step of dissolving the radioactive substances contained in the dust in water is carried out. In the solid-liquid separation device 24, a solid-liquid separation step of recovering solid matter from the aqueous solution that has undergone the dissolving step is carried out. In the radioactive substance adsorption device 26, a radioactive substance adsorption step of adsorbing and removing radioactive substances from the aqueous solution that has undergone the solid-liquid separation step is carried out.

[0052] That is, the exhaust gas containing radioactive substances volatilized and separated from the molten slag in the melting treatment step passes through the dust collection step and is recovered as dust. The exhaust gas from which the dust has been removed is detoxified by removing dioxins in the dioxin decomposition step and further removing mercury in the mercury adsorption step, etc., and then discharged into the atmosphere.

[0053] As described above, the dust recovered in the dust collection step contains chlorides of radioactive substances, mineral elements that are oxides such as aluminum and iron, heavy metals such as lead, and silica-based or alumina-based aids used in the dust collection step. In the dissolving step included in the dust treatment step, the radioactive substances are dissolved in water, the solids that do not dissolve in water are removed in the solid-liquid separation step, and the radioactive substances dissolved in water are adsorbed and removed in the radioactive substance adsorption step.

[0054] In the dust collection step, a silica-based or alumina-based aid is used instead of calcium carbonate or the like as an aid (release agent), so that an inconvenient situation in which calcium scale adheres to pipes or the like and causes blockage in the dust treatment step is effectively reduced.

[0055] In the dissolving step, a pH adjuster is added to water and adjusted to be acidic, so that in addition to radioactive substances, heavy metals are adjusted to a solubilized state. That is, radioactive substances, heavy metals, and some salts are dissolved in water.

[0056] As shown in Fig. 2, in the solid-liquid separation step, an insoluble silica-based auxiliary agent, alumina-based auxiliary agent (release agent), or mineral element is separated from water-soluble heavy metals and radioactive substances by adjusting the pH of the aqueous solution that has undergone the dissolution step. Usually, the pH is adjusted to acidic to solubilize heavy metals and water-soluble substances such as cesium, and the insoluble silica-based auxiliary agent or alumina-based auxiliary agent is separated. Alternatively, the pH may be adjusted to alkaline, such as for lead, to solubilize it together with water-soluble cesium and separate the insoluble silica-based auxiliary agent or alumina-based auxiliary agent.

[0057] In the previous solid-liquid separation device 24A, a dehydrator such as a filter press or a screw press is used, for example, to recover the solids remaining undissolved in the aqueous solution. In the subsequent solid-liquid separation device 24B, by adding a pH adjuster or a flocculant to the aqueous solution, the insoluble silica-based auxiliary agent, alumina-based auxiliary agent (release agent), or mineral element is flocculated and precipitated.

[0058] The solids recovered in the solid-liquid separation step are transported to the melting furnace 2 by the circulation mechanism 25, and a circulation melting step of melting again in the melting furnace 2 is executed. By melting the mineral elements containing the insoluble silica-based auxiliary agent or alumina-based auxiliary agent again in the melting furnace and slagging them, further volume reduction of the waste can be achieved.

[0059] In the radioactive substance adsorption device 26, a radioactive substance adsorption step of adsorbing and removing radioactive substances from the aqueous solution that has undergone the solid-liquid separation step is executed, and the radioactive substances adsorbed on the adsorbing substances such as zeolite, ferrocyanide compounds, and titanate are stored in storage facilities outside the region (outside the prefecture) under strict management.

[0060] As shown in Fig. 2, the aqueous solution from which the radioactive substances have been removed is sent to the heavy metal removal device 28 filled with metal chelate, and by adjusting it to neutral with a pH adjuster, a heavy metal removal step in which the heavy metals are captured by the metal chelate is executed. The heavy metals captured by the metal chelate are reduced at the source and reused, or treated as industrial waste.

[0061] In the heavy metal removal step, the high-concentration brine from which heavy metals have been removed is sent to the concentrator 29, where the concentration step is carried out and it is evaporated to dryness. A first waste heat recovery step of recovering waste heat with the first waste heat recovery device 5 installed upstream of the dust collector 7 from the exhaust gas generated in the melting treatment step, and a second waste heat recovery step of recovering waste heat from the exhaust gas downstream of the mercury adsorption tower are carried out, and the waste heat recovered from the exhaust gas is used as the heat source for the concentration step. Note that a part of the high-concentration brine may be supplied to the temperature reduction tower 6 installed upstream of the dust collector 7 and sprayed for treatment.

[0062] As shown in FIG. 3, as a heavy metal removal step, by adding a pH adjuster to the aqueous solution that has undergone the solid-liquid separation step to adjust it from acidic to neutral, it becomes possible to agglomerate and precipitate the heavy metals dissolved under acidic conditions in the dissolution step. In this case, it is preferable to add a flocculant in addition to the pH adjuster. In this case, a radioactive substance adsorption step is carried out after the heavy metal removal step.

[0063] That is, the heavy metal removal step is a step of removing water-soluble heavy metals as insoluble heavy metals by adjusting the pH of the aqueous solution between the solid-liquid separation step and the radioactive substance adsorption step, or after the radioactive substance adsorption step.

[0064] By carrying out the dissolution step under acidic conditions, heavy metals can be dissolved. If the pH of the aqueous solution is adjusted to, for example, neutral before the radioactive substance adsorption step or after the radioactive substance adsorption step after the solid-liquid separation step, the dissolved heavy metals become insoluble and precipitate. A heavy metal fixing agent (chelating agent) can be added to separate it as a metal complex, or a flocculant can be added to cause flocculation and precipitation.

[0065] FIG. 4 shows another embodiment of the method for treating mercury-containing waste and the mercury-containing waste treatment system according to the present invention. In the above-described embodiment, the case where the waste containing mercury is a waste containing radioactive substances has been described, but the present invention can also be applied to wastes that do not contain radioactive substances.

[0066] As shown in FIG. 4, the mercury-containing waste treatment system 1A includes an incinerator 2A, a secondary combustion chamber 3A, an air preheater 4A, a waste heat recovery device 5A, a desuperheater 6A, a dust collector 7A, a neutralization product recovery device 9A, a catalytic reaction tower 10A, a mercury adsorption tower 11A, an induced draft fan 13A, and a chimney 15A. The dust collector 7A has the function of the neutralization product recovery device 9 of the above-described embodiment, and includes a neutralizing agent supply device 17A and a hydrogen chloride gas concentration adjustment device 16A which is an acidic gas concentration adjustment device 16 for adjusting the supply amount of the neutralizing agent.

[0067] The mercury-containing waste is incinerated in the incinerator 2A, and the generated exhaust gas flows down a flue. The exhaust gas contains corrosive gases such as hydrogen chloride gas, mercury, dioxins, and heavy metals.

[0068] The dust collector 7A is composed of a bag filter, and slaked lime, which is a neutralizing agent, is blown into the bag filter by the neutralizing agent supply device 17A together with the exhaust gas, so that corrosive gas components such as hydrogen chloride contained in the exhaust gas are captured on the filter cloth as calcium chloride or the like.

[0069] The neutralizing agent supply device 17A includes a nozzle for injecting the neutralizing agent and a pump for adjusting the supply amount of the neutralizing agent supplied to the nozzle. Based on the chlorine gas concentration detected by an acidic gas sensor 14A installed downstream of the induced draft fan 13A, the hydrogen chloride gas concentration adjustment device 16A, which is a control device, controls the neutralizing agent supply device 17A so that the hydrogen chloride gas concentration contained in the exhaust gas flowing into the catalytic reaction tower 10A is maintained at a predetermined target concentration of 3 to 100 ppm.

[0070] The catalytic reaction tower 10A is provided with a catalytic metal such as titanium or vanadium, and dioxins contained in the exhaust gas are decomposed by a catalytic reaction with the catalytic metal. At this time, the adsorption of mercury to the catalytic metal is inhibited by the hydrogen chloride gas contained in the exhaust gas, so that dioxins are efficiently decomposed and removed.

[0071] The mercury adsorption tower 11A is filled with pellet-shaped activated carbon, and mercury with a relatively low boiling point is adsorbed and removed by the activated carbon when the exhaust gas passes through. Note that the function of the mercury adsorption tower 11A may be replaced by a dust collection device that sprays activated carbon and removes the activated carbon that has adsorbed mercury.

[0072] That is, the method for treating mercury-containing waste according to the present invention includes a heat treatment step of heat-treating the mercury-containing waste, a dust collection step of collecting dust in the exhaust gas containing mercury generated in the heat treatment step by a dust collector, a dioxin decomposition step of decomposing dioxins contained in the exhaust gas by bringing the exhaust gas containing hydrogen chloride gas that has passed through the dust collection step into contact with a dioxin decomposition catalyst in a catalytic reaction tower, and a mercury adsorption step of adsorbing and removing mercury contained in the exhaust gas by bringing the exhaust gas that has passed through the dioxin decomposition step into contact with an adsorbent in a mercury adsorption tower. The method also includes a hydrogen chloride gas concentration adjustment step of adjusting the concentration of hydrogen chloride gas contained in the exhaust gas supplied to the dioxin decomposition step to a target concentration by a chlorine gas concentration adjustment device.

[0073] The target concentration of hydrogen chloride gas adjusted in the hydrogen chloride gas concentration adjustment step is preferably 3 to 100 ppm. The dust collection step includes a neutralized product recovery step of spraying a neutralizing agent into the exhaust gas to neutralize the acidic gas and recover the neutralized product. The hydrogen chloride gas concentration adjustment step is preferably a step of adjusting the spraying amount of the neutralizing agent based on the hydrogen chloride gas concentration of the exhaust gas that has passed through the neutralized product recovery step.

[0074] The mercury-containing waste is radioactive substance-containing waste, the heat treatment step is a melting treatment step of melting the radioactive substance-containing waste in a melting furnace in the presence of chlorine to volatilize the radioactive substance, the dust collection step includes a dust recovery step of recovering dust from the exhaust gas containing the radioactive substance volatilized in the melting treatment step by a dust collection device using a silica-based auxiliary agent or an alumina-based auxiliary agent as a release agent, a dissolution step of dissolving the radioactive substance contained in the dust recovered in the dust recovery step in water, a solid-liquid separation step of recovering solids from the aqueous solution that has passed through the dissolution step, and a radioactive substance adsorption step of adsorbing and removing the radioactive substance from the aqueous solution that has passed through the solid-liquid separation step. It is further preferably provided with a dust treatment step including the above steps.

[0075] The solid-liquid separation step is a step of separating mineral elements containing an insoluble silica-based auxiliary agent or alumina-based auxiliary agent as solids by adjusting the pH of the aqueous solution that has undergone the dissolution step, and preferably includes a circulating melting step of remelting the solids recovered in the solid-liquid separation step in a melting furnace.

[0076] It is preferable to further include a heavy metal removal step of removing water-soluble heavy metals as insoluble heavy metals by adjusting the pH of the aqueous solution between the solid-liquid separation step and the radioactive substance adsorption step or after the radioactive substance adsorption step.

Example

[0077] Examples will be described below. To confirm the mercury accumulation status in the dioxin decomposition catalyst, a component analysis of the catalyst and a mercury accumulation confirmation test using an indoor experimental apparatus were carried out. The experimental apparatus is a simple device in which a dioxin decomposition catalyst (vanadium titanate) covered with glass wool is filled in two cylindrical containers with adjustable temperature, a simulated gas is supplied to the cylindrical containers by a gas supply device, and the mercury concentration is measured by concentration sensors installed on the inlet side and outlet side of the cylindrical containers.

[0078] The experimental procedure is as follows. The temperature of the cylindrical containers was raised to 180°C, and air gas, nitrogen gas, and carbon dioxide gas were used to prepare the catalyst inlet gas by starting the supply of the HgCl2 solution thereafter. Divalent mercury HgCl2 was generated by heating and vaporizing the HgCl2 solution. Metallic mercury Hg was generated by adding the HgCl2 solution to a stannous chloride solution and reducing it to Hg. The total amount of the simulated gas was exhausted without passing through the catalyst. Using a mercury continuous analyzer, it was confirmed that the mercury concentration of the simulated gas was 1600 μg / m 3 is. After 30 minutes had elapsed since the start of mercury generation, a sample for mercury analysis of the catalyst inlet gas was collected.

[0079] The flow path was changed, and the entire amount of the simulated gas was passed through the cylindrical container filled with the catalyst for 240 minutes. During the passage of the gas through the cylindrical container, samples for mercury analysis (catalyst outlet gas, last flow part gas) were collected at the catalyst outlet, and the concentration was measured using a mercury continuous analyzer. The flow path was changed, and the entire amount of the simulated gas was exhausted without passing through the catalyst. Samples for mercury analysis of the inlet gas were collected (30 min). The catalyst column was taken out from the cylindrical container, allowed to cool, and then the catalyst was taken out. The catalyst was crushed in an agate mortar, a part was separated, and the mercury concentration was measured. For the glass wool, a part was separated and the mercury concentration was measured. The mercury concentrations of the mercury absorption liquids (4 specimens, inlet × 2, outlet, last flow part) that had collected the gas were measured.

[0080] The experimental conditions are shown in FIGS. 5 and 6. In FIG. 5, the simulated gas does not contain acidic gas, and in FIG. 6, the simulated gas contains acidic gas. When the simulated gas does not contain acidic gas, most of the mercury contained in the simulated gas is adsorbed by the dioxin decomposition catalyst. When the simulated gas contains acidic gas, it was confirmed that almost no mercury contained in the simulated gas is adsorbed by the dioxin decomposition catalyst. And it was found that when the gas concentration of the acidic gas is 3 ppm or more, mercury adsorption to the dioxin decomposition catalyst can be effectively avoided. And if the gas concentration of the acidic gas is 3 to 100 ppm, it is more preferable from the viewpoint of environmental protection.

[0081] The above-described embodiments are examples of the present invention, and it is not intended that the scope of the present invention be limited by the description. Needless to say, the configuration can be appropriately changed and designed within the range where the functions and effects of the present invention are achieved.

Explanation of Signs

[0082] 1: Treatment system for mercury-containing waste 2: Melting furnace 3: Secondary combustion chamber 4: Air preheater 5: First waste heat recovery device 6: Desuperheating tower 7: First dust collecting device 8: Second dust collecting device 9: Neutralized product recovery device 10: Catalytic reaction tower 11: Mercury adsorption tower 12: Second waste heat recovery device 14: Hydrogen chloride gas sensor 16: Acid gas concentration adjustment device 17: Neutralizing agent supply device 20: Dust treatment device 22: Dissolving device (water dissolving tank) 24: Solid-liquid separation device 24A: Solid-liquid separation device (dehydrator) 24B: Solid-liquid separation device (coagulation sedimentation tank) 25: Circulation mechanism 26: Radioactive substance adsorption device (cesium adsorption tower) 28: Heavy metal removal device (heavy metal chelate tower) 29: Concentration device

Claims

1. A method for treating mercury-containing waste, comprising a heat treatment step of heat-treating the mercury-containing waste, a dust collection step of collecting dust in the exhaust gas containing mercury generated by the heat treatment step, a dioxin decomposition step of decomposing dioxins contained in the exhaust gas by bringing the exhaust gas passing through the dust collection step into contact with a dioxin decomposition catalyst, a mercury adsorption step of adsorbing and removing mercury contained in the exhaust gas by bringing the exhaust gas passing through the dioxin decomposition step into contact with an adsorbent, and a method for treating mercury-containing waste, comprising an acidic gas concentration adjustment step of adjusting the concentration of acidic gas contained in the exhaust gas supplied to the dioxin decomposition step to a target concentration or higher.

2. The method for treating mercury-containing waste according to claim 1, wherein the target concentration of the acidic gas adjusted in the acidic gas concentration adjustment step is 3 to 100 ppm.

3. The dust collection step includes a neutralized product recovery step of spraying a neutralizing agent onto the exhaust gas to neutralize the acidic gas and recover the neutralized product, The method for treating mercury-containing waste according to claim 1, wherein the acidic gas concentration adjustment step is a step of adjusting the spraying amount of the neutralizing agent based on the acidic gas concentration of the exhaust gas passing through the neutralized product recovery step.

4. The mercury-containing waste is radioactive substance-containing waste, the heat treatment step is a melting treatment step of melting the radioactive substance-containing waste in the melting furnace in the presence of chlorine to volatilize the radioactive substance, the dust collection step includes a dust collection step of collecting dust in the exhaust gas containing the radioactive substance volatilized in the melting treatment step by using a dust collection device using a silica-based auxiliary agent or an alumina-based auxiliary agent as a release agent, A dust treatment step including a dissolution step of dissolving radioactive substances contained in the dust collected in the dust collection step in water, a solid-liquid separation step of recovering solid matter from the aqueous solution that has undergone the dissolution step, and a radioactive substance adsorption step of adsorbing and removing radioactive substances from the aqueous solution that has undergone the solid-liquid separation step. The method for treating mercury-containing waste according to claim 1, further comprising .

5. The solid-liquid separation step is a step of separating mineral elements including an insoluble silica-based auxiliary agent or alumina-based auxiliary agent as solid matter by adjusting the pH of the aqueous solution that has undergone the dissolution step. The method for treating mercury-containing waste according to claim 4, including a circulation melting step of melting again the solid matter recovered in the solid-liquid separation step in the melting furnace.

6. The method for treating mercury-containing waste according to claim 4, further comprising a heavy metal removal step of removing water-soluble heavy metals as insoluble heavy metals by adjusting the pH of the aqueous solution between the solid-liquid separation step and the radioactive substance adsorption step, or after the radioactive substance adsorption step.

7. A mercury-containing waste treatment system, A heat treatment device for heat-treating the mercury-containing waste, A dust collection device for collecting dust in the exhaust gas containing mercury generated by the heat treatment device, A dioxin decomposition device for decomposing dioxins contained in the exhaust gas by bringing the exhaust gas that has passed through the dust collection device into contact with a dioxin decomposition catalyst, A mercury adsorption device for adsorbing and removing mercury contained in the exhaust gas by bringing the exhaust gas that has passed through the dioxin decomposition device into contact with an adsorbent, including A mercury-containing waste treatment system provided with an acidic gas concentration adjustment device for adjusting the concentration of acidic gas contained in the exhaust gas supplied to the dioxin decomposition device to a target concentration or higher.

8. The treatment system for mercury-containing waste according to claim 7, wherein the target concentration of the acid gas adjusted by the acid gas concentration adjusting device is 3 to 100 ppm.

9. The dust collecting device includes a neutralized product recovery device that sprays a neutralizing agent onto the exhaust gas to neutralize the acid gas and recover the neutralized product. The acid gas concentration adjusting device is a device that adjusts the spraying amount of the neutralizing agent based on the acid gas concentration of the exhaust gas that has passed through the neutralized product recovery device. The treatment method for mercury-containing waste according to claim 7.

10. The mercury-containing waste is radioactive substance-containing waste. The heat treatment device is a melting furnace that melts the radioactive substance-containing material in the presence of chlorine to volatilize the radioactive substance. The dust collecting device includes a dust collecting device that recovers dust using a silica-based auxiliary agent or an alumina-based auxiliary agent as a peeling agent for the exhaust gas containing the radioactive substance volatilized in the melting furnace. A dissolving device that dissolves the radioactive substance contained in the dust recovered by the dust collecting device in water, a solid-liquid separation device that recovers solids from the aqueous solution in which the radioactive substance is dissolved, and a radioactive substance adsorption device that adsorbs and removes the radioactive substance from the aqueous solution from which solids have been recovered by the solid-liquid separation device. A dust treatment device including. The treatment system for mercury-containing waste according to claim 7, further comprising.

11. The solid-liquid separation device is a device that separates mineral elements containing an insoluble silica-based auxiliary agent or alumina-based auxiliary agent as solids by adjusting the pH of the aqueous solution in which the radioactive substance is dissolved. The treatment system for mercury-containing waste according to claim 10, including a circulating melting step of melting the solids recovered by the solid-liquid separation device again in the melting furnace.

12. The mercury-containing waste treatment system according to claim 10, further comprising a heavy metal removal device that removes water-soluble heavy metals as insoluble heavy metals by adjusting the pH of an aqueous solution, between the solid-liquid separation device and the radioactive substance adsorption device, or downstream of the radioactive substance adsorption device.

Images