Mercury removal methods

Abstract

To provide a method of removing mercury from dust collected by an electrostatic precipitator in order to make it possible to efficiently reduces the content of mercury in exhaust gas generated in a cement manufacturing process and to enable the reuse of the dust as a raw material for cement.SOLUTION: The mercury removing method of this invention includes a step S1 of collecting dust collected by an electric dust collector; a step S2 of heating the dust in a heating furnace; a step S3 of vaporizing mercury contained in the dust; a step S4 of cooling the mercury gas removed from the heating furnace in the condenser; a step S5 of removing the condensed mercury from the condenser: and steps of separating (S6) and recovering (S7) water and mercury, respectively; a step (S8) of cooling dust removed from the heating furnace: and a step (S9) of reusing the dust as a cement raw material.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for removing mercury contained in exhaust gas generated in a cement manufacturing process, and more particularly to a mercury removal method for removing mercury adhering to dust collected by an electrostatic precipitator. [Background technology]

[0002] When mercury is contained in natural raw materials such as limestone, fuels such as coal and heavy oil, or waste materials such as sludge and incineration ash, which are used as raw materials or fuels in the cement manufacturing process, mercury and mercury compounds (hereinafter collectively referred to as "mercury") vaporize into mercury gas in the high-temperature sections of cement manufacturing equipment such as rotary kilns. The exhaust gas generated in the rotary kiln is sent to a preheater or raw material dryer to use its residual heat for drying the raw materials, and then dust is collected in an electrostatic precipitator. Mercury gas, which is sent to the electrostatic precipitator via the preheater or raw material dryer along with the exhaust gas, condenses as the temperature drops and adheres to or is adsorbed onto the dust in the exhaust gas (hereinafter collectively referred to as "adhered"). The mercury adsorbed to the dust is then collected together with the dust in the electrostatic precipitator and removed from the exhaust gas. In the present invention, the gas after leaving the rotary kiln is referred to as "exhaust gas", even if the gas is used for preheating or other purposes before being discharged into the atmosphere. However, as mentioned above, if the waste brought into the cement manufacturing facility as a raw material or fuel contains a large amount of mercury, the mercury gas sent to the electrostatic precipitator along with the exhaust gas may not completely adhere to the dust and may be released in that state into the atmosphere from the chimney along with the exhaust gas.

[0003] Here, a cement manufacturing facility will be explained using Figure 7. Figure 7 is a block diagram showing an outline of a cement manufacturing facility. Note that the solid lines in the figure represent the flow of raw materials moving within the cement manufacturing facility, and the dashed lines represent the flow of exhaust gas returned from the rotary kiln to other facilities so that residual heat can be used to dry the raw materials. As shown in Figure 7, the cement manufacturing process is broadly divided into three steps: raw material process, firing process, and finishing process.

[0004] In the raw material process, limestone, clay, silica stone, iron oxide raw materials, etc. are mixed, then sent to raw material dryer 0 where they are dried by contact with exhaust gas, and then sent to raw material mill 1 where they are pulverized. The raw materials pulverized in raw material mill 1 are then sent to blending silo 3 where they are homogeneously blended and stored in raw material storage silo 4. Meanwhile, the gas discharged from raw material dryer 0 is sent to cyclone 2 where large particles in the powdered raw materials or dust are separated, and then sent to electrostatic precipitator 15.

[0005] In the firing process, the powdered raw materials that have been dried, crushed, and mixed in the raw material process are fired in a rotary kiln 6. However, in order to increase firing efficiency, the powdered raw materials are not directly fed into the rotary kiln 6, but are first sent to a preheater 5 that has multiple cyclones. The rotary kiln 6 has a gentle slope, and the powder raw materials preheated in the preheater 5 are fired at a high temperature (approximately 1450°C) while slowly moving through the interior of the rotary kiln 6 due to this slope and rotation. The powder raw materials fired in the rotary kiln 6 are rapidly cooled in the clinker cooler 7 and become a black, lumpy fired product called clinker.

[0006] In the finishing process, clinker stored in clinker silo 8 is sent to pre-crushing mill 10, and then gypsum stored in gypsum yard 9 is added to the pre-crushed clinker, and the mixture is finely ground in finishing mill 11 to an average particle size of about 10 to 20 μm. The powder raw material that has been finely ground in finishing mill 11 and sent to classifier 12 is sent to cement silo 14 either via mixer 13 or directly without passing through mixer 13.

[0007] Next, the main flow of exhaust gas in the cement manufacturing process will be explained. As shown by the dashed line in Figure 7, in the raw material process, the gas discharged from the raw material dryer 0 is sent as exhaust gas through a cyclone 2 to an electrostatic precipitator 15, and after dust is separated, it is released into the atmosphere through a chimney 16. During the firing process, the exhaust gases generated in the rotary kiln 6 and clinker cooler 7 are sent to the lower part of the calciner (such as a vortex chamber) located at the bottom of the preheater 5, so that their residual heat can be used to dry the powdered raw materials. In the preheater 5, the powdered raw materials supplied to the top cyclone descend while passing through the other cyclones in order. Meanwhile, the exhaust gas sent to the calciner ascends inside the preheater 5, counterflowing the flow of the powdered raw materials, and is discharged from the top cyclone. After heat recovery in the boiler 17, this exhaust gas is sent to the raw material dryer 0, so that its residual heat can be used to dry the raw materials.

[0008] As mentioned above, mercury contained in the raw materials and fuel in the raw material process and the calcination process is vaporized into mercury gas inside the preheater 5 and rotary kiln 6, and is not contained in the clinker sent to the finishing process. Instead, this mercury gas is sent to the electrostatic precipitator 15 together with the exhaust gas via the preheater 5 and raw material dryer 0, and adheres to the dust as the gas temperature drops along the way. The dust with the mercury attached that is collected in the electrostatic precipitator 15 is then sent to the blending silo 3 and reused as powdered raw material. In this way, mercury brought into cement manufacturing facilities along with raw materials and fuel circulates through the raw material and calcination processes, repeatedly vaporizing and condensing, or adhering to dust. As already mentioned, if the raw materials and fuel brought into the cement manufacturing facility contain a large amount of mercury, the mercury gas sent to the electrostatic precipitator 15 along with the exhaust gas may not completely adhere to the dust and may be released in that state into the atmosphere from the chimney 16 along with the exhaust gas.

[0009] To solve these problems, for example, Patent Document 1 discloses an invention entitled "Method for Removing Mercury from Combustion Exhaust Gas," which relates to a method for easily and at low cost removing mercury contained in combustion exhaust gas generated in cement manufacturing facilities. The invention disclosed in Patent Document 1 is characterized in that the flue gas discharged from the top cyclone of a suspension preheater in a cement manufacturing facility is introduced into a coal dryer and crusher, and mercury contained in the flue gas is adsorbed onto pulverized coal obtained by crushing the coal in the coal dryer and crusher.The flue gas and the pulverized coal are then introduced into a bag filter to capture only the pulverized coal, thereby purifying the flue gas. This mercury removal method, which uses a coal drying and crushing device that is ancillary equipment of a cement manufacturing facility, does not require the installation of a new device for mercury removal, and therefore allows for easy purification of combustion exhaust gas at low cost.

[0010] Furthermore, Patent Document 2 discloses an invention titled "Method for treating exhaust gas from a cement kiln" that relates to a method for removing mercury, organic chlorine compounds, and dust from exhaust gas generated in a cement kiln that uses various types of waste as raw materials or fuel. The invention disclosed in Patent Document 2 is characterized in that exhaust gas is extracted from a dust collector and sent to an adsorption tower, where mercury and organic chlorine compounds contained in the exhaust gas are adsorbed onto activated carbon or pulverized coal, and the activated carbon or pulverized coal is then heated to 400°C or higher in a heating furnace to remove the mercury and organic chlorine compounds, and the activated carbon or pulverized coal obtained in this process is fed into a cement kiln. According to such a method for treating exhaust gas from a cement kiln, it is possible to reduce the size of the heating furnace for removing mercury and organic chlorine compounds, and to reduce the energy required for heating. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-75784 [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-96615 Summary of the Invention [Problem to be solved by the invention]

[0012] According to the invention disclosed in Patent Document 1, mercury contained in combustion exhaust gas can be removed from the combustion exhaust gas by adsorbing the mercury contained in the combustion exhaust gas onto pulverized coal and collecting the pulverized coal using a bag filter. However, if the pulverized coal is reused as a cement raw material, the mercury will be carried into the cement manufacturing process. Therefore, the invention disclosed in Patent Document 1 has the problem that the pulverized coal cannot be reused. Furthermore, the invention disclosed in Patent Document 2 does not have a configuration for removing mercury contained in the dust collected by the dust collector, and therefore has the problem that the dust cannot be reused as a cement raw material.

[0013] The present invention has been made in response to the above-mentioned conventional circumstances, and aims to provide a method for removing mercury from dust collected in an electrostatic precipitator, thereby efficiently reducing the mercury content in exhaust gases generated in the cement production process and enabling the dust to be reused as a cement raw material. [Means for solving the problem]

[0014] To achieve the above-mentioned object, the first invention is a mercury removal method for removing mercury attached to dust contained in exhaust gas generated in a cement manufacturing process, characterized by including a step of recovering a portion of the dust collected in an electrostatic precipitator. Mercury introduced into cement manufacturing facilities vaporizes in high-temperature parts such as rotary kilns and clinker coolers during the firing process. The vaporized mercury travels through preheaters and other equipment along with the exhaust gas, and is eventually collected in an electrostatic precipitator in a state where it adheres to dust. Multi-stage electrostatic precipitators are generally used, and dust extracted from a section closer to the exhaust gas outlet contains a higher concentration of mercury than dust extracted from a section closer to the exhaust gas inlet.

[0015] The second invention is the first invention, in which the fine particle dust collected from the electrostatic precipitator is divided into two types: a first dust and a second dust set to have a smaller mass than the first dust; the second invention comprises a heating process in which the first dust is heated to vaporize the mercury; and a cooling process in which the mercury vaporized in the heating process is cooled while being brought into contact with the second dust; and the second dust having a high concentration of mercury attached thereto is collected in the cooling process. In addition to the effects of the first invention, the second invention has the effect that the mercury content of the dust (second dust) recovered in the heating and cooling process is higher than the mercury content of the dust (first dust) immediately after being collected by the electrostatic precipitator.

[0016] A third invention is characterized in that in the second invention, in the step of vaporizing mercury from the dust, the dust is heated in an oxygen-deficient reducing atmosphere. Normally, when organic matter is burned at low temperatures, dioxins, a type of organic chlorine compound, are produced. However, in an oxygen-deficient state, for example, with an oxygen concentration of 1% or less, dechlorination proceeds and dioxins are decomposed even at low incineration temperatures. Therefore, in the third invention, which includes a step of heating dust collected by an electrostatic precipitator in an oxygen-deficient reducing atmosphere when vaporizing mercury contained in the dust, in addition to the effects of the second invention, there is an effect that there is no risk of dioxins being generated when the dust is heated. Furthermore, there is an effect that mercury oxide or mercury chloride is not produced in the mercury vaporization step, and mercury contained in the exhaust gas is recovered in the form of metallic mercury.

[0017] The fourth invention is characterized in that, in any one of the first to third inventions, a multi-stage electrostatic precipitator is used, and the dust recovered from the front stage of the electrostatic precipitator is reused as a cement raw material (clinker raw material) without undergoing a process of heating to vaporize the mercury. For example, the dust collected from the upstream stage of the electrostatic precipitator is sent to the raw material process, where it is stored in a raw material storage silo together with the powdered raw material separated from the exhaust gas by a cyclone via a blending silo, and then appropriately reused as cement raw material. The applicant of the present application investigated the concentrations of carbon and mercury contained in dust collected in the front compartment (the compartment close to the exhaust gas supply port) and the rear compartment (the compartment close to the exhaust gas discharge port) of a multi-stage electrostatic precipitator, and found that the concentrations of carbon and mercury contained in the dust collected in the front compartment were low, while the concentrations of carbon and mercury contained in the dust collected in the rear compartment were high. Here, the reason why the carbon concentration in the dust collected in the front compartment is low is thought to be because dust containing highly conductive carbon is difficult to capture by the collecting electrode, so the collecting electrode installed in the front compartment mainly captures dust containing non-conductive particles or particles with lower conductivity than carbon.Furthermore, the carbon concentration in the dust collected in the rear compartment is thought to be high because most of the dust containing non-conductive particles or particles with lower conductivity than carbon is collected in the front compartment, making it easier for carbon to be collected in the rear compartment. Furthermore, because mercury easily adheres to carbon, it is presumed that the mercury concentration in the collected dust was low in the earlier sections where the carbon concentration in the collected dust was low, and that the mercury concentration in the collected dust was high in the later sections where the carbon concentration in the collected dust was high. As described above, in a multi-stage electrostatic precipitator, the mercury concentration in the dust collected in the preceding compartment is low. Therefore, in the fourth invention, the dust is reused as a cement raw material without being subjected to a mercury removal process. In addition to the effects of any of the first to third inventions, this invention also has the effect of reducing the amount of mercury introduced into the cement production process system.

[0018] The fifth invention is the fourth invention, characterized in that the dust collected in the downstream section of a multi-stage electrostatic precipitator and from which mercury has been removed is reused as a cement raw material (clinker raw material). For example, dust collected in the downstream section of the electrostatic precipitator is stored in an ash silo and transported by truck or the like to a mercury removal facility as needed. In the mercury removal facility, the dust from which mercury has been removed by heating in a heating furnace is cooled in a cooler, passed through a blending silo, stored in a raw material storage silo, and then reused as a cement raw material as needed. As mentioned above, the mercury concentration in the dust collected in the later compartments of a multi-stage electrostatic precipitator is high, which means that removing the mercury from the dust is effective in reducing the amount of mercury that is carried into the cement manufacturing process as a raw material or fuel and circulates during the raw material process and calcination process. Therefore, in the fifth invention, which is characterized by the dust being collected in the later compartments of a multi-stage electrostatic precipitator and then having the mercury removed, being reused as a cement raw material, the amount of mercury that is carried into the cement manufacturing process and circulates during the raw material process and calcination process is reduced, thereby achieving the effect of the fourth invention, in addition to the effect of making it less likely that mercury will be emitted to the outside from the chimney together with the exhaust gas, even when the dust collected by the electrostatic precipitator is reused as a cement raw material. As mentioned above, the dust collected in the latter stages of the multi-stage electrostatic precipitator is heated in the heating furnace of the mercury removal equipment and then cooled in a cooler. However, since there is a risk that dioxins that have been decomposed may be resynthesized, it is desirable to quickly lower the temperature of the dust in this cooling process (i.e., to rapidly cool it), which will prevent dioxin resynthesis. [Effects of the Invention]

[0019] According to the first aspect of the present invention, the dust containing high concentrations of mercury captured by the electrostatic precipitator can be easily removed, thereby efficiently reducing the mercury content in the exhaust gas generated in the cement manufacturing process.

[0020] According to the second invention, in addition to the effect of the first invention, the amount of mercury adhering to the dust recovered from the electrostatic precipitator is increased, thereby reducing the amount of second dust to be processed that is recovered after the cooling process.

[0021] According to the third invention, since dioxins are not produced in the process of vaporizing mercury in the second invention, in addition to the effects of the second invention, the third invention has the effect of improving safety during mercury removal work. Furthermore, since mercury oxide or mercury chloride is not produced in the process of vaporizing mercury and the mercury contained in the exhaust gas is recovered in the form of metallic mercury, there is no need to handle highly toxic mercury chloride, and no process of removing mercury from mercury oxide or mercury chloride is required. Therefore, according to the third invention, mercury in exhaust gas can be reduced safely, inexpensively, and efficiently compared to the second invention.

[0022] In the fourth invention, dust with a low mercury concentration that is collected in the upstream section of a multi-stage electrostatic precipitator is reused as a cement raw material without undergoing a mercury removal step. This achieves the same effect as any of the first to third inventions, and also reduces the amount of mercury carried into the cement production system while reducing the amount of dust to be subjected to the mercury removal step, thereby reducing the production costs of cement raw materials.

[0023] According to the fifth invention, in addition to the effects of the fourth invention, the amount of dust that can be reused as a cement raw material can be increased without introducing mercury into the cement production process system, thereby achieving the effect of reducing the production costs of cement raw materials. [Brief explanation of the drawings]

[0024] [Figure 1] 1(a) and 1(b) are flowcharts showing steps in a mercury removal method according to a first embodiment of the present invention. [Figure 2] FIG. 2 is a schematic diagram illustrating the structure of an electric dust collector. [Figure 3] FIG. 1 is a schematic diagram of a mercury removal facility. [Figure 4] FIG. 1 is a schematic diagram showing how dust captured by a multi-stage electrostatic precipitator is recovered. [Figure 5] FIG. 4 is a block diagram showing the configuration of equipment used in a mercury removal method according to a second embodiment of the present invention. [Figure 6] 6 is a flowchart showing each step in the mercury removal method shown in FIG. 5. [Figure 7] FIG. 1 is a block diagram showing an outline of a cement manufacturing facility. DETAILED DESCRIPTION OF THE INVENTION

[0025] The mercury removal method of the present invention aims to reduce the mercury content in exhaust gas generated in the cement production facility shown in Figure 7, and its specific configuration will be described using Figure 1. The treatments performed in each step of the method will be described in detail using Figures 2 to 6. Note that the equipment used in the cement production process that has already been described using Figure 7 will be denoted by the same reference numerals, and the description thereof will be omitted where appropriate. [Example]

[0026] 1(a) and 1(b) are flow charts showing the steps of a mercury removal method according to a first embodiment of the present invention. As shown in Figure 1(a), the mercury removal method according to the first embodiment of the present invention includes the steps of: first, recovering dust collected by an electrostatic precipitator 15 in the cement manufacturing facility already described with reference to Figure 7 (step S1); feeding the recovered dust into a heating furnace and heating it in an oxygen-deficient reducing atmosphere (step S2); removing mercury gas and water vapor generated by evaporation of mercury and moisture contained in the dust from the heating furnace (step S3); and cooling the mercury gas and water vapor in a condenser (step S4). Furthermore, the mercury removal method of the present invention is characterized by comprising the steps of: removing liquid mercury and water produced by condensation of mercury gas and water vapor from the condenser (step S5); separating the liquid mercury from the water by utilizing the difference in specific gravity (step S6); recovering the liquid mercury and water (step S7); removing the dust from which the mercury has been removed from the heating furnace and cooling it (step S8); and reusing the cooled dust as a cement raw material (step S9).

[0027] The structure of the electrostatic precipitator will now be described with reference to Figure 2. Figure 2 is a diagram that schematically illustrates how dust particles are collected by the electrostatic precipitator. In Figure 2, to avoid cluttering the illustration, only one dust particle is labeled. 2, in the electrostatic precipitator 15, a discharge electrode 15b made of a piano wire or the like is installed between a pair of parallel-arranged collecting electrodes 15a, 15a. The electrostatic precipitator 15 is configured such that a DC voltage of, for example, 40 to 60 kV is applied between the collecting electrodes 15a, 15a and the discharge electrode 15b so that the collecting electrodes 15a, 15a are positive and the discharge electrode 15b is negative. When gas containing dust 18 flows through the electrostatic precipitator 15, as indicated by the wide arrow, with a DC voltage applied between the collecting electrodes 15a and the discharge electrode 15b, a corona discharge occurs between the collecting electrodes 15a and the discharge electrode 15b, negatively ionizing countless gas molecules. These gas molecules move from the low-potential discharge electrode 15b to the high-potential collecting electrodes 15a, and dust 18 that collide with them becomes negatively charged. As a result, the negatively charged dust 18 is attracted to the high-potential collecting electrode 15a. Upon contact with the collecting electrode 15a, the dust 18 loses its negative charge and adheres to the surface of the collecting electrode 15a. To collect the dust 18, the collecting electrode 15a is vibrated using a vibrator or other device. This causes the dust 18 to peel off and fall from the surface of the collecting electrode 15a.

[0028] Next, a mercury removal system used to remove mercury contained in the dust 18 collected from the electrostatic precipitator 15 will be described with reference to Fig. 3. Fig. 3 is a diagram showing a schematic diagram of the mercury removal system. Note that, in order to avoid the diagram becoming too complicated, only one of the blades of the first heating furnace, the second heating furnace, and the cooler is designated by a reference numeral. As shown in Figure 3, dust fed into the hopper 19 and discharged through the ash supply pipe 20 is transported by the screw conveyor 32 and supplied to the first heating furnace 21. The first heating furnace 21 has an ash inlet 21a that opens upward at one end of the main body 21c, and an ash outlet 21b that opens downward at the other end of the main body 21c. Therefore, the dust transported by the screw conveyor 32 is fed into the main body 21c from the ash inlet 21a of the first heating furnace 21. The amount of dust fed into the first heating furnace 21 per unit time is adjusted by a rotary valve 20a installed in the ash supply pipe 20. Dust heated inside the main body 21c of the first heating furnace 21 is discharged from the ash outlet 21b and supplied to the second heating furnace 22. The second heating furnace 22 has an ash inlet 22a that opens upward at one end of the main body 22c, and an ash outlet 22b that opens downward at the other end of the main body 22c. A gas outlet 22f is also provided at the top of the other end of the main body 22c.

[0029] Dust discharged from ash outlet 21b of first heating furnace 21 is introduced into main body 22c of second heating furnace 22 through ash inlet 22a. Dust heated inside second heating furnace 22 is then discharged from ash outlet 22b and supplied to cooler 23. The cooler 23 has an ash inlet 23a that opens upward at one end of the main body 23c, and an ash outlet 23b that opens downward at the other end of the main body 23c. Therefore, the dust discharged from the ash outlet 22b of the second heating furnace 22 is introduced into the main body 23c through the ash inlet 23a of the cooler 23.

[0030] The first heating furnace 21 includes a hollow cylindrical body 21c with both ends closed, and a heater (not shown) attached to the outer periphery of the body 21c. Inside the body 21c, multiple blades 21d and a rotating shaft 21e to which the blades 21d are fixed are rotatably installed. In other words, the first heating furnace 21 is designed so that dust introduced from the ash inlet 21a to one end of the body 21c is transported by the rotating blades 21d toward the side where the ash outlet 21b is provided. The second heating furnace 22 includes a hollow cylindrical body 22c with both ends closed, and a heater (not shown) attached to the outer periphery of the body 22c. Inside the body 22c, multiple blades 22d and a rotating shaft 22e to which the blades 22d are fixed are rotatably installed, and a dust filter 25 is attached to the gas outlet 22f. In other words, the second heating furnace 22 is configured such that dust introduced from the ash inlet 22a to one end of the body 22c is transported by the rotating blades 22d toward the side where the ash outlet 22b is provided.

[0031] The first heating furnace 21 is configured such that when dust is fed into the ash inlet 21a of the first heating furnace 21, nitrogen gas is supplied from the ash supply pipe 20 into the inside of the main body 21c, and this nitrogen gas creates an oxygen-deficient state inside the main body 21c with an oxygen concentration of 0.1% or less. Furthermore, when the dust heated inside the first heating furnace 21 is discharged from the ash outlet 21b and fed into the ash inlet 22a of the second heating furnace 22, oxygen gas or the like does not mix into the inside of the main body 22c from the outside, so the inside of the main body 22c is also in an oxygen-deficient state with an oxygen concentration of 0.1% or less.

[0032] Cooler 23 comprises a hollow cylindrical body 23c with both ends closed, and a water-cooled or air-cooled jacket (not shown) attached to the outer periphery of body 23c. Inside body 23c, multiple blades 23d and a rotating shaft 23e to which blades 23d are fixed are rotatably installed. In other words, cooler 23 is structured so that dust introduced from ash inlet 23a to one end of body 23c is transported by rotating blades 23d toward the other end of body 23c, where ash outlet 23b is provided. The dust cooled inside the cooler 23 is discharged from the ash discharge pipe 24 connected to the ash outlet 23b, and then transported to a predetermined location by the screw conveyor 32. The amount of dust discharged from the cooler 23 per unit time is adjusted by the rotary valve 24a installed in the ash discharge pipe 24.

[0033] The upper part of condenser 26 is connected to dust filter 25 via a gas suction pipe (not shown). The lower part of condenser 26 is connected to adsorption tower 27a via a gas suction pipe (not shown) equipped with a suction pump (not shown), and is also connected to mercury separator 28 via a mercury discharge pipe (not shown). Mercury separator 28 is configured so that mercury, which has a higher specific gravity than water, collects at the bottom, and water, which has a lower specific gravity than mercury, collects above the mercury. A portion of the mercury accumulated at the bottom of the mercury separator 28 is discharged into the mercury reservoir 29 from a drain outlet (not shown) provided at the bottom of the mercury separator 28. On the other hand, the water accumulated above the mercury is drained by overflowing from a water drain pipe (not shown) connected to the side of the mercury separator 28, and then stored in the water tank 30. The water tank 30 is connected to the adsorption tower 27b via a drain pipe (not shown). Therefore, the water accumulated in the water tank 30 is sucked up by a drain pump 31 installed in the drain pipe and sent to the adsorption tower 27b.

[0034] Dust brought into the mercury removal facility with this structure is first heated in a first heating furnace 21 and a second heating furnace 22 at a temperature of 400°C to 600°C for one hour in an oxygen-deficient reducing atmosphere. This vaporizes mercury, which has a boiling point of 356°C, and turns it into mercury gas, facilitating the separation of mercury from the dust. Furthermore, this method does not produce mercury oxide or mercury chloride when heating the dust, and the mercury contained in the exhaust gas is recovered in the form of metallic mercury, eliminating the need to handle highly toxic mercury chloride. Therefore, mercury contained in the exhaust gas can be safely removed. Furthermore, because a process for removing mercury from mercury oxide or mercury chloride is not required, the cost and time required for the process of removing mercury from the exhaust gas are reduced. Therefore, according to the mercury removal method of the present invention including step S2, mercury in exhaust gas can be reduced safely, inexpensively, and efficiently.

[0035] It is known that when organic matter is burned at low temperatures, dioxin precursors such as benzene and phenol are produced, and that dioxins are further produced by chemical reactions between these, and that even if dioxins are decomposed, they are resynthesized during the cooling process of the exhaust gas.On the other hand, when organic matter is incinerated in an oxygen-deficient reducing atmosphere with an oxygen concentration of 1% or less, dechlorination proceeds, and dioxins are decomposed even at incineration temperatures below 800°C. Therefore, in the mercury removal method of the present invention, which includes a step of heating the dust brought into the mercury removal equipment in a first heating furnace 21 and a second heating furnace 22 at a temperature of 400°C to 600°C for one hour in an oxygen-deficient reducing atmosphere (corresponding to step S2 in Figure 1(a)), no dioxins are generated when the dust is heated.

[0036] The dust heated inside the first heating furnace 21 and the second heating furnace 22 is removed from the second heating furnace 22 and cooled to 70°C in the cooler 23 (corresponding to step S8 in FIG. 1(a)). If the cooling rate is slow, the dioxins that have been decomposed will be re-synthesized at around 250°C. Therefore, in this step, it is desirable to rapidly cool the dust heated to 400°C to 600°C (for example, by rapidly lowering the temperature to 70°C in about 60 minutes). This prevents the re-synthesis of dioxins. The dust, from which the mercury has been removed, is discharged from the ash discharge pipe 24 connected to the ash outlet 23b of the cooler 23 and stored in the raw material storage silo 4 (see FIG. 7) via the blending silo 3 (see FIG. 7), and is then reused as a cement raw material as appropriate (corresponding to step S9 in FIG. 1(a)). Of course, the dust is not limited to being reused as a cement raw material, and may be used for other purposes, such as a ceramic raw material other than cement, or may be disposed of by landfilling, etc.

[0037] The water and mercury heated and vaporized inside the first heating furnace 21 and the second heating furnace 22 become water vapor and mercury gas, which are discharged from the gas outlet 22f of the second heating furnace 22 (corresponding to step S3 in FIG. 1(a)), filtered by the dust filter 25, and then sent to the condenser 26 where they are cooled (corresponding to step S4 in FIG. 1(a)). The water vapor and mercury gas cooled and condensed inside the condenser 26 become water and liquid mercury, respectively. This makes it easy to recover the mercury. On the other hand, the gas that is not condensed inside the condenser 26 is sent to the adsorption tower 27a, where predetermined components are removed, and then the gas is released as exhaust gas. The water and liquid mercury are extracted from the condenser 26 and sent to the mercury separator 28 where they are separated (corresponding to steps S5 and S6 in FIG. 1(a)). The mercury separated from the water is then led from the mercury separator 28 to the mercury reservoir 29, from which it is appropriately recovered. Meanwhile, the water from which the mercury has been removed is sent to the adsorption tower 27b, where predetermined components are removed and the water is then recovered (corresponding to step S7 in FIG. 1(a)). Many devices having the above-described structure and functions for heating in an oxygen-deficient reducing atmosphere with an oxygen concentration of 0.1% or less are known. Many manufacturers also offer so-called Hagenmeyer furnaces. Therefore, any of these can be appropriately selected and used in carrying out the present invention. Furthermore, simply heating the dust sufficiently will volatilize the mercury contained in the dust as mercury chloride or mercury oxide, depending on the atmosphere and other coexisting elements. Therefore, dust from which mercury has been removed can also be obtained by volatilizing mercury using a known method other than the above-mentioned method of heating in an oxygen-deficient reducing atmosphere, followed by cooling and solidifying the mercury and separately recovering it. Such an embodiment is also included in the present invention.

[0038] Multi-stage electrostatic precipitators are usually used in places such as factories where a large amount of exhaust gas needs to be treated at once. Here, we will explain how to recover the dust captured by a multi-stage electrostatic precipitator using Figure 4. Figure 4 is a schematic diagram showing how dust captured by a multi-stage electrostatic precipitator, the interior of which is divided into five compartments, is recovered. The numbers in the diagram represent the compartment numbers provided within the electrostatic precipitator. That is, in the electrostatic precipitator shown in Figure 4, the compartments closest to the exhaust gas supply inlet and exhaust outlet are numbered 1 and 5, respectively, and the compartments in between are numbered higher the closer they are to the exhaust gas outlet. In Figure 4, the flow of raw material is indicated by a solid line, and the flow of exhaust gas is indicated by a dashed line. As already explained using Figure 7, a blend of raw materials such as limestone, clay, silica, and iron oxide raw materials is sent to raw material dryer 0, where it is dried by contact with exhaust gas, and then sent to raw material mill 1 where it is pulverized. The raw materials pulverized in raw material mill 1 are sent to blending silo 3, where they are homogeneously blended, and then stored in raw material storage silo 4. Meanwhile, the gas discharged from raw material dryer 0 is sent to cyclone 2, where large particles in the powder raw materials and dust are separated, and then sent to electrostatic precipitator 15. Furthermore, after the dust from this gas is collected in electrostatic precipitator 15, it is released into the atmosphere from chimney 16.

[0039] Table 1 shows examples of the "concentration of carbon and mercury contained in the dust collected in each section (however, the values ​​for the fourth and fifth sections are the combined values ​​of the dust)" in the electrostatic precipitator 15. Table 1 shows that the concentrations of carbon and mercury contained in the collected dust are low in the compartments closer to the exhaust gas supply port. The reason why the carbon concentration in the dust collected in the compartments in the front stage (closer to the exhaust gas supply port) is low is thought to be because dust containing highly conductive carbon is difficult to capture on the collection electrode 15a, and so the collection electrode 15a installed in the compartments in the front stage captures mainly dust containing particles that are not conductive or particles with lower conductivity than carbon.

[0040] In contrast, the concentrations of carbon and mercury in the collected dust were higher in the later compartments (closer to the exhaust gas outlet). This high carbon concentration in the dust collected in the later compartments is thought to be due to the fact that most of the dust, which contains particles that are not conductive or have lower conductivity than carbon, is collected in the earlier compartments, making it easier for carbon to be collected in the later compartments. Furthermore, because mercury easily adheres to carbon, it is presumed that the mercury concentration in the collected dust was low in the earlier sections where the carbon concentration in the collected dust was low, and that the mercury concentration in the collected dust was high in the later sections where the carbon concentration in the collected dust was high.

[0041] [Table 1]

[0042] As shown in Table 1, the mercury concentrations in the dust collected in sections 3 to 5 are significantly higher than those in the dust collected in sections 1 and 2. This means that removing the mercury from the dust collected in sections 3 to 5 is effective in reducing the amount of mercury contained in waste brought into the cement manufacturing process as raw materials or fuel that circulates between the raw material and calcination processes. Therefore, in the mercury removal method according to the first embodiment of the present invention, when the electrostatic precipitator is a multi-stage type having five compartments as shown in FIG. 4, the process of recovering the dust collected in the later compartments (the third to fifth compartments in this embodiment) (step S1-2 in FIG. 1(b)) is set as the dust recovery process shown in step S1 in FIG. 1(a), and the dust collected in the earlier compartments (the first and second compartments in this embodiment) is reused as cement raw material without undergoing the mercury removal process as described above.

[0043] Specifically, as shown in Fig. 4, the dust collected in the third to fifth compartments of the electrostatic precipitator 15 is sent to an ash storage silo 33 having a vent filter 35 installed on top. The dust stored in the ash storage silo 33 is transported by truck 34 to the mercury removal facility (see Fig. 3) as appropriate. As already explained with reference to Fig. 3, the dust from which mercury has been removed in the first heating furnace 21 and the second heating furnace 22 of the mercury removal facility is cooled in a cooler 23, sent to the blending silo 3 and homogeneously blended, and then stored in the raw material storage silo 4 and reused as a cement raw material as appropriate. Meanwhile, the dust collected in the first and second compartments of the electrostatic precipitator 15, together with the powdered raw material separated from the exhaust gas in the cyclone 2, is sent to the blending silo 3 where it is homogeneously blended, and then stored in the raw material storage silo 4 and reused as cement raw material as appropriate.

[0044] Thus, according to the mercury removal method of the present invention, which includes a step of directly using the dust recovered in step S1-1 as a cement raw material, the amount of mercury contained in the dust reused as a cement raw material can be reduced, and the amount of dust subjected to mercury treatment externally can be reduced. This allows for a reduction in the amount of mercury brought into the cement production facility while also reducing the production costs of cement raw materials. Regarding the "front stage" and "rear stage" of the above-mentioned multi-stage electrostatic precipitator, the section closest to the supply port is the "front stage," and the section closest to the discharge port is the "rear stage." The remaining intermediate sections can be appropriately allocated depending on the characteristics of the electrostatic precipitator used and the properties of the exhaust gas and dust. Note that, typically, if the number of stages is even, the division is made in the middle, and if the number is odd, the middle section is allocated to the "rear stage." [Example]

[0045] FIG. 5 is a block diagram showing the configuration of equipment used in a mercury removal method according to a second embodiment of the present invention, and FIG. 6 is a flowchart showing each step in the mercury removal method shown in FIG. 5. In FIG. 5, solid lines represent the flow of dust, and dashed lines represent the flow of gas. Furthermore, step S1 shown in FIG. 6 is the same as the dust recovery step (step S1 shown in FIGS. 1(a) and 1(b)) in the mercury removal method of Example 1 already described using FIGS. 1(a) and 1(b), and therefore its description will be omitted. Furthermore, steps S6 to S13 shown in FIG. 6 are the same as steps S2 to S9 shown in FIG. 1(a), respectively, and therefore their description will also be omitted.

[0046] As shown in Figure 5, the mercury removal method according to the second embodiment of the present invention includes a heating tower 36 that heats the dust recovered from the electrostatic precipitator 15 in the cement manufacturing facility shown in Figure 7, and a heating concentrator 38 that includes a cooling tower 37 that cools the gas (hereinafter referred to as mercury gas) generated by vaporizing mercury attached to the dust inside the heating tower 36 while bringing the gas into contact with other dust recovered from the electrostatic precipitator 15. In the present specification and claims, in order to distinguish between the dust supplied to the heating tower 36 and the dust supplied to the cooling tower 37, for convenience, of the dust recovered from the electrostatic precipitator 15, the dust supplied to the heating tower 36 will be referred to as the first dust, and the dust supplied to the cooling tower 37 will be referred to as the second dust.

[0047] In the mercury removal method according to the second embodiment of the present invention, first, the dust collected in the third to fifth sections of the electrostatic precipitator 15 is recovered (step S1 in FIG. 6) and sent to the heating tower 36 as shown in FIG. 5. This dust (first dust) is heated in the heating tower 36, which is maintained at 450°C to 600°C (step S2 in FIG. 6), and the mercury attached to the dust (first dust) is vaporized to become mercury gas. This mercury gas is extracted from the heating tower 36 and sent to the cooling tower 37 (step S3 in FIG. 6), while the dust (first dust) remaining in the heating tower 36 is extracted and returned to the cement production process. The mercury gas sent to the cooling tower 37, which is kept at 140°C or less, is cooled while in contact with the dust (second dust) collected from the third to fifth sections of the electrostatic precipitator 15, causing the mercury gas to adhere to the dust (second dust) (step S4 in FIG. 6). The dust (second dust) to which mercury has adhered is extracted from the cooling tower 37 as shown in FIG. 5 (step S5 in FIG. 6) and sent to the mercury removal facility (see FIG. 3). This dust (second dust) is heated in the first heating furnace 21 and the second heating furnace 22 already described with reference to FIG. 3 (step S6 in FIG. 6), and the gas from which the mercury has been removed is extracted from the cooling tower 37 and returned to the cement manufacturing process.

[0048] Table 2 shows the results of a test in which dust collected in the electrostatic precipitator 15 was separated into a first dust and a second dust, the first dust was supplied to a heating tower 36 and heated to approximately 500°C to gasify the mercury, and the gas was brought into contact with the second dust in a cooling tower 37 at approximately 100°C to recover the second dust containing a high concentration of mercury. Table 2 shows that the mercury concentration in the first and second dusts before feeding was 58.4 ppm, whereas after removal, the mercury concentration in the first dust was 0.4 ppm and the mercury concentration in the second dust was 827.7 ppm, a concentration ratio of approximately 14 times. Furthermore, the mass of the removed second dust was 75 g, which is a volume reduction of approximately 6.8% based on the combined weight of the first and second dusts. This indicates that the mercury removal method is effective, as mercury is concentrated in the second dust obtained by heating and cooling the dust recovered from the electrostatic precipitator 15 and the volume of the dust is reduced.

[0049] [Table 2]

[0050] As described above, the mercury removal method according to the second embodiment of the present invention is characterized in that the process of vaporizing mercury attached to dust comprises a first heating process (step S2 in FIG. 6) of heating the first dust and a second heating process (step S6 in FIG. 6) of heating the second dust, and further comprises a cooling process (step S4 in FIG. 6) of cooling the mercury separated from the first dust by vaporization in the first heating process while bringing it into contact with the second dust. The mercury concentration adhering to the first dust supplied from the electrostatic precipitator 15 to the heating tower 36 is, for example, 40 ppm to 100 ppm, whereas the mercury concentration adhering to the second dust extracted from the cooling tower 37 is, for example, 200 ppm to 5000 ppm. That is, the mercury content of the dust collected in the third to fifth compartments of the electrostatic precipitator 15 is increased 5 to 50 times by processing in the heated concentrator 38. Thus, according to the mercury removal method of the second embodiment of the present invention, the mercury adhering to the dust collected in the third to fifth compartments of the electrostatic precipitator 15 is concentrated in the heated concentrator 38. Therefore, dust with a higher mercury content can be supplied to the mercury removal equipment (see FIG. 3 ) compared to the mercury removal method of the first embodiment of the present invention. This significantly reduces the amount of dust treated by the mercury removal equipment. [Industrial Applicability]

[0051] The mercury removal method according to the present invention can be applied to the removal of mercury contained in exhaust gas in the cement manufacturing process. [Explanation of symbols]

[0052] 0...Raw material dryer 1...Raw material mill 2...Cyclone 3...Blending silo 4...Raw material storage silo 5...Preheater 6...Rotary kiln 7...Clinker cooler 8...Clinker silo 9...Gypsum yard 10...Preliminary crusher 11...Finishing mill 12...Classifier 13...Mixer 14...Cement silo 15...Electrostatic precipitator 15a...Dust collecting electrode 15b...Discharge electrode 16...Chimney 17...Boiler 18...Dust 19...Hopper 20...Ash supply pipe 20a...Rotary valve 21...First heating furnace 21a...Ash inlet 21b...Ash outlet 21c...Main body 21d...Blade 21e...Rotary shaft 22...Second heating furnace 22a...Ash inlet 22b...Ash outlet 22c...Main body 22d...Blade 22e...Rotating shaft 22f...Gas outlet 23...Cooler 23a...Ash inlet 23b...Ash outlet 23c...Main body 23d...Blade 23e...Rotating shaft 24...Ash discharge pipe 24a...Rotary valve 25...Dust filter 26...Condenser 27a, 27b...Adsorption tower 28...Mercury separator 29...Mercury reservoir 30...Water tank 31...Drainage pump 32...Screw conveyor 33...Ash storage silo 34...Truck 35...Vent filter 36...Heating tower 37...Cooling tower 38...Heated concentrator

Claims

1. A mercury removal method for removing mercury attached to dust contained in exhaust gas generated in a cement manufacturing process, comprising the steps of: a dust collection step of collecting the dust having the mercury attached thereto from the electrostatic precipitator after the dust has been collected by the electrostatic precipitator, The dust collected in the dust collection step is divided into two parts: a first dust and a second dust; a mass ratio of the second dust to the total mass of the first dust and the second dust being 100% is less than 50%; a heating step of heating the first dust in a heating tower to vaporize the mercury; a cooling step of cooling the second dust in a cooling tower while bringing the second dust into contact with the mercury vaporized in the heating step, thereby causing the mercury vaporized in the heating step to adhere to the second dust, After the mercury is vaporized in the heating step, the first dust is returned to the cement manufacturing process; a cooling tower for removing the second dust having a high concentration of mercury attached thereto in the cooling step, and heating the second dust to remove the mercury;

2. A mercury removal method for removing mercury attached to dust contained in exhaust gas generated in a cement manufacturing process, comprising the steps of: a dust collection step of collecting the dust having the mercury attached thereto from the electrostatic precipitator after the dust has been collected by the electrostatic precipitator, the electrostatic precipitator is a multi-stage type, and the dust collected in the dust recovery step is composed of front-stage recovered dust and rear-stage recovered dust recovered from the front and rear stages of the electrostatic precipitator, respectively, and the rear-stage recovered dust is divided into two types, i.e., first dust and second dust; a mass ratio of the second dust to the total mass of the first dust and the second dust being 100% is less than 50%; a heating step of heating the first dust in a heating tower to vaporize the mercury; a cooling step of cooling the second dust in a cooling tower while bringing the second dust into contact with the mercury vaporized in the heating step, After the mercury is vaporized in the heating step, the first dust is returned to the cement manufacturing process; the second dust having the high concentration of mercury attached thereto in the cooling step is removed from the cooling tower and heated to remove the mercury; The method for removing mercury, wherein the pre-stage recovered dust is reused as a cement raw material without the mercury being removed.

3. 3. The mercury removal method according to claim 2, wherein the first dust and the second dust, which are the post-stage recovered dust, are reused as the cement raw material after the mercury is removed from the first dust and the second dust.

4. 4. The mercury removal method according to claim 1, wherein the first dust is heated in an oxygen-deficient reducing atmosphere in the heating step.

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