Mercury removal methods

Abstract

To provide a method for removing mercury, capable of efficiently reducing a mercury content in exhaust gas generated in a cement-producing process by removing the mercury from dusts collected by a bag filter, thereby reusing the dusts as a raw material for cement.SOLUTION: A method for removing mercury of the present invention includes: a step (step S2) of collecting dusts caught by a first bag filter and classifying them by a cyclone; a step (step S3) of catching the dusts of fine particles obtained in the step S2 by a second bag filter and then collecting them; a step (step S4) of heating the dusts of fine particles collected in the step S3 in a heating tower to vaporize the mercury; and a step (step S6) of cooling mercury gas generated in the step S4 in a cooling tower.SELECTED DRAWING: Figure 2

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 in a bag filter. [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 dust is then captured using a bag filter or similar device. Mercury gas, which is sent to the bag filter along with the exhaust gas via the preheater or raw material dryer, condenses as the temperature drops and adheres to the dust in the exhaust gas. If mercury does not condense, it is adsorbed by the dust (hereinafter collectively referred to as "adhered"). The mercury adsorbed to the dust is then captured together with the dust in the bag filter 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 raw materials and fuel brought into the cement manufacturing facility contain a large amount of mercury, the mercury gas sent to the bag filter 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, the cement manufacturing facility will be described using Figure 4. Figure 4 is a block diagram showing an outline of the 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. Also, in Figure 4, cyclone 2 and bag filter 15 are labeled "first cyclone" and "first bag filter" in parentheses to distinguish them from cyclone 18 and bag filter 19, which will be described later using Figure 1. As shown in Figure 4, 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 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 (first cyclone) where large particles in the dust are separated, and then sent to bag filter 15 (first bag filter).

[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, to increase firing efficiency, these 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 powdered raw materials preheated in the preheater 5 are fired at a high temperature (approximately 1450°C) as they move slowly inside the rotary kiln 6 due to this slope and rotation. The powdered raw materials fired in the rotary kiln 6 are then rapidly cooled in a clinker cooler 7 to 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 4, in the raw material process, the gas discharged from the raw material dryer 0 is sent as exhaust gas from cyclone 2 (first cyclone) to bag filter 15 (first bag filter), and after dust is separated, it is released into the atmosphere from 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 bag filter 15 (first bag filter) along 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 bag filter 15 (first bag filter) 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 bag filter 15 (first bag filter) along with the exhaust gas may not completely adhere to the dust and may be released into the atmosphere in that state 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 a bag filter, thereby efficiently reducing the mercury content in exhaust gas generated in the cement production process and enabling the dust to be reused as a cement raw material. [Means for solving the problem]

[0014] In order 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 comprising a step of recovering the dust captured by a first bag filter and classifying it using a cyclone, and a step of recovering the fine particle dust to which a high concentration of mercury is attached, obtained in this step and captured by a second bag filter. Mercury introduced into cement manufacturing facilities vaporizes in high-temperature areas such as rotary kilns and preheaters during the firing process. The vaporized mercury travels through exhaust gases, including boilers and raw material dryers, and is eventually collected in the first bag filter as part of dust. When the dust collected by this first bag filter is recovered and classified using a cyclone, the fine dust particles contain high concentrations of mercury.

[0015] The second invention is the first invention, wherein the fine particle dust collected from the second bag filter 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 the second invention, in addition to the effect of the first invention, the mercury content of the dust (second dust) recovered in the heating and cooling process is higher than the mercury content of the fine particle dust (first dust) immediately after being captured by the second bag filter.

[0016] The third invention is characterized in that, in the second invention, the second dust having a high concentration of mercury attached thereto is heated in an oxygen-deficient reducing atmosphere in the cooling process, and the vaporized mercury is cooled and separated and recovered. 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, where the oxygen concentration is 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 the dust captured in the bag filter in an oxygen-deficient reducing atmosphere when vaporizing the 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 produced 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 the mercury contained in the exhaust gas is recovered in the form of metallic mercury. [Effects of the Invention]

[0017] The fine dust particles obtained by classifying the dust captured by the first bag filter contain high concentrations of mercury. Therefore, the first invention, which includes a step of recovering this fine dust particles, can efficiently reduce the mercury content in the exhaust gas generated in the cement manufacturing process.

[0018] In addition to the effects of the first invention, the second invention has the effect of reducing the amount of dust to be processed in the heating step by increasing the amount of mercury attached to the dust collected from the second bag filter. Furthermore, by cooling the dust (first dust) heated to vaporize the mercury, the dust can be reused as a cement raw material.

[0019] According to the third invention, dioxins are not produced in the process of vaporizing mercury from dust containing high concentrations of mercury obtained by the first or second invention, and therefore, in addition to the effects of the second invention, the third invention has the effect of improving safety during mercury removal work. Furthermore, because mercury oxide or mercury chloride is not produced in the mercury vaporization process 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 for 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 first or second invention. [Brief explanation of the drawings]

[0020] [Figure 1] 1 is a block diagram showing the configuration of equipment used in a mercury removal method according to an embodiment of the present invention. [Figure 2] 2 is a flowchart showing each step in the mercury removal method shown in FIG. 1. [Figure 3] FIG. 1 is a schematic diagram of a mercury removal facility. [Figure 4] FIG. 1 is a block diagram showing an outline of a cement manufacturing facility. DETAILED DESCRIPTION OF THE INVENTION

[0021] 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 4, 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 and 3. Note that the equipment used in the cement production process that has already been described using Figure 4 will be denoted by the same reference numerals, and the description thereof will be omitted where appropriate. [Example]

[0022] FIG. 1 is a block diagram showing the configuration of equipment used in a mercury removal method according to an embodiment of the present invention, and FIG. 2 is a flowchart showing each step in the mercury removal method shown in FIG. 1. In FIG. 1, solid lines represent the flow of dust, and dashed lines represent the flow of gas. Also, in FIG. 1, to distinguish cyclone 18 and bag filter 19 from cyclone 2 and bag filter 15 already described using FIG. 4, cyclone 18 and bag filter 19 are labeled "second cyclone" and "second bag filter," respectively, in parentheses. Furthermore, to distinguish between the dust supplied to heating tower 20 and the dust supplied to cooling tower 21, hereinafter, of the dust recovered from second bag filter 19, the dust supplied to heating tower 20 will be referred to as "first dust," and the dust supplied to cooling tower 21 will be referred to as "second dust." As shown in Figure 1, the mercury removal method of the present invention includes a cyclone 18 (second cyclone) that classifies dust recovered from a bag filter 15 (first bag filter) in the cement manufacturing facility shown in Figure 4, a bag filter 19 (second bag filter) that collects fine dust particles obtained by classification, a heating tower 20 that heats the fine dust particles (first dust) recovered from the bag filter 19 (second bag filter), and a heating concentrator 22 that includes a cooling tower 21 that cools the gas (hereinafter referred to as mercury gas) generated by vaporization of mercury attached to the dust inside the heating tower 20 while bringing it into contact with other dust (second dust) recovered from the bag filter 19 (second bag filter).

[0023] In the mercury removal method of the present invention, first, dust captured by bag filter 15 (first bag filter) in the cement manufacturing facility already described with reference to Fig. 4 is recovered (step S1 in Fig. 2), and then sent to cyclone 18 (second cyclone) as shown in Fig. 1 for classification (step S2 in Fig. 2). Then, fine dust particles obtained by classification are captured by bag filter 19 (second bag filter) and then recovered (step S3 in Fig. 2). After being collected from the bag filter 19 (second bag filter), the dust (first dust) is sent to the heating tower 20 as shown in FIG. 1, where it is heated and maintained at a temperature of 400°C to 600°C (step S4 in FIG. 2). As a result, the mercury adhering to the dust (first dust) vaporizes and becomes mercury gas. This mercury gas is extracted from the heating tower 20 and then sent to the cooling tower 21 as shown in FIG. 1 (step S5 in FIG. 2), while the dust (first dust) remaining in the heating tower 20 is extracted and returned to the cement manufacturing process. The mercury gas sent to the cooling tower 21, which is kept at 140°C or less, is brought into contact with the dust (second dust) collected from the bag filter 19 (second bag filter) and cooled, thereby adhering to the dust (second dust) (step S6 in FIG. 2). This dust (second dust) with mercury adhering thereto is removed from the cooling tower 21 as shown in FIG. 1 (step S7 in FIG. 2) and sent to a mercury removal facility.

[0024] Next, a mercury removal system used to remove mercury contained in the dust (second dust) recovered from the cooling tower 21 will be described with reference to Fig. 3. Fig. 3 is a schematic diagram of the mercury removal system. Note that, in order to avoid cluttering the diagram, only one of the multiple blades of the first heating furnace, the second heating furnace, and the cooler is labeled with a reference numeral. As shown in Figure 3, dust fed into the hopper 23 and discharged through the ash supply pipe 24 is transported by the screw conveyor 36 and supplied to the first heating furnace 25. The first heating furnace 25 has an ash inlet 25a that opens upward at one end of the main body 25c, and an ash outlet 25b that opens downward at the other end of the main body 25c. Therefore, the dust transported by the screw conveyor 36 is fed into the main body 25c from the ash inlet 25a of the first heating furnace 25. The amount of dust fed into the first heating furnace 25 per unit time is adjusted by the rotary valve 24a installed in the ash supply pipe 24. Dust heated inside the main body 25c of the first heating furnace 25 is discharged from the ash outlet 25b and supplied to the second heating furnace 26. The second heating furnace 26 has an ash inlet 26a that opens upward at one end of the main body 26c, and an ash outlet 26b that opens downward at the other end of the main body 26c. A gas outlet 26f is also provided at the top of the other end of the main body 26c.

[0025] The dust discharged from the ash outlet 25b of the first heating furnace 25 is fed into the main body 26c of the second heating furnace 26 through the ash inlet 26a. The dust heated inside the second heating furnace 26 is then discharged from the ash outlet 26b and supplied to the cooler 27. The cooler 27 has an ash inlet 27a that opens upward at one end of the main body 27c, and an ash outlet 27b that opens downward at the other end of the main body 27c. Therefore, the dust discharged from the ash outlet 26b of the second heating furnace 26 is introduced into the main body 27c through the ash inlet 27a of the cooler 27.

[0026] The first heating furnace 25 includes a hollow cylindrical body 25c with both ends closed, and a heater (not shown) attached to the outer periphery of the body 25c. Inside the body 25c, multiple blades 25d and a rotating shaft 25e to which the blades 25d are fixed are rotatably installed. In other words, the first heating furnace 25 is designed so that dust introduced from the ash inlet 25a to one end of the body 25c is transported by the rotating blades 25d toward the side where the ash outlet 25b is provided. The second heating furnace 26 includes a hollow cylindrical body 26c with both ends closed, and a heater (not shown) attached to the outer periphery of the body 26c. Inside the body 26c, multiple blades 26d and a rotating shaft 26e to which the blades 26d are fixed are rotatably installed, and a dust filter 29 is installed at the gas outlet 26f. In other words, the second heating furnace 26 is configured such that dust introduced from the ash inlet 26a to one end of the body 26c is transported by the rotating blades 26d toward the side where the ash outlet 26b is provided.

[0027] The first heating furnace 25 is configured such that when dust is fed into the ash inlet 25a of the first heating furnace 25, nitrogen gas is supplied from the ash supply pipe 24 into the inside of the main body 25c, and this nitrogen gas creates an oxygen-deficient state inside the main body 25c with an oxygen concentration of 0.1% or less. Furthermore, when the dust heated inside the first heating furnace 25 is discharged from the ash outlet 25b and fed into the ash inlet 26a of the second heating furnace 26, no oxygen gas or the like is mixed into the inside of the main body 26c from the outside, so the inside of the main body 26c is also in an oxygen-deficient state with an oxygen concentration of 0.1% or less.

[0028] Cooler 27 includes a hollow cylindrical body 27c with both ends closed, and a water-cooled or air-cooled jacket (not shown) attached to the outer periphery of body 27c. Inside body 27c, multiple blades 27d and a rotating shaft 27e to which blades 27d are fixed are rotatably installed. In other words, cooler 27 is structured so that dust introduced from ash inlet 27a to one end of body 27c is transported by rotating blades 27d toward the other end of body 27c where ash outlet 27b is provided. The dust cooled inside the cooler 27 is discharged from an ash discharge pipe 28 connected to an ash outlet 27b, and then transported to a predetermined location by a screw conveyor 36. The amount of dust discharged from the cooler 27 per unit time is adjusted by a rotary valve 28a installed in the ash discharge pipe 28.

[0029] The upper part of the condenser 30 is connected to the dust filter 29 via a gas suction pipe (not shown). The lower part of the condenser 30 is connected to an adsorption tower 31a via a gas suction pipe (not shown) equipped with a suction pump (not shown), and is also connected to a mercury separator 32 via a mercury discharge pipe (not shown). The mercury separator 32 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 32 is discharged into the mercury reservoir 33 from a drain outlet (not shown) provided at the bottom of the mercury separator 32. 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 32, and then stored in a water tank 34. The water tank 34 is connected to the adsorption tower 31b via a drain pipe (not shown). Therefore, the water accumulated in the water tank 34 is sucked up by a drain pump 35 installed in the drain pipe and sent to the adsorption tower 31b.

[0030] Dust brought into the mercury removal facility with this structure is first heated in the first heating furnace 25 and the second heating furnace 26 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 S8, mercury in exhaust gas can be reduced safely, inexpensively, and efficiently.

[0031] 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 the first heating furnace 25 and the second heating furnace 26 at a temperature of 400°C to 600°C for one hour in an oxygen-deficient reducing atmosphere (corresponding to step S8 in Figure 2), no dioxins are generated when the dust is heated.

[0032] The dust heated inside the first heating furnace 25 and the second heating furnace 26 is removed from the second heating furnace 26 and cooled to 70°C in the cooler 27 (corresponding to step S14 in FIG. 2). 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 ash discharge pipe 28 connected to ash outlet 27b of cooler 27 and stored in raw material storage silo 4 (see FIG. 4) via blending silo 3 (see FIG. 4), and then reused as cement raw material as appropriate (corresponding to step S15 in FIG. 2). Of course, it is not limited to being reused as cement raw material, and may be used for other purposes, such as ceramic raw material other than cement, or may be disposed of by landfill, etc.

[0033] The water and mercury heated and vaporized inside the first heating furnace 25 and the second heating furnace 26 become water vapor and mercury gas, which are discharged from the gas outlet 26f of the second heating furnace 26 (corresponding to step S9 in FIG. 2), filtered by the dust filter 29, and then sent to the condenser 30 where they are cooled (corresponding to step S10 in FIG. 2). The water vapor and mercury gas cooled and condensed inside the condenser 30 become water and liquid mercury, respectively. This facilitates the recovery of mercury. On the other hand, the gas that is not condensed inside the condenser 30 is sent to the adsorption tower 31a, where predetermined components are removed, and then the gas is released as exhaust gas. The water and liquid mercury are extracted from the condenser 30 and sent to the mercury separator 32 where they are separated (corresponding to steps S11 and S12 in FIG. 2). The mercury separated from the water is then led from the mercury separator 32 to the mercury reservoir 33 and appropriately recovered from the mercury reservoir 33. Meanwhile, the water from which the mercury has been removed is sent to the adsorption tower 31b, where predetermined components are removed and then recovered (corresponding to step S13 in FIG. 2). Many devices having the above-described structure and function for heating in an oxygen-deficient reducing atmosphere with an oxygen concentration of 0.1% or less are known. These devices are known as Hagenmeyer furnaces and are offered by many manufacturers. Therefore, any of these devices 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.

[0034] Table 1 shows the results of classifying dust collected in the bag filter 15 (first bag filter) and examining the relationship between particle size and mercury concentration. Note that "D10," "D50," and "D90" in the table refer to the particle sizes at which the cumulative frequency is 10%, 50%, and 90%, respectively. Table 1 shows that the mercury concentration in the dust before classification was 22.3 ppm, while the mercury concentration in the dust after classification was approximately 0.9 times higher for coarse powder (D50 = 7.77 μm) and approximately 1.7 times higher for fine powder (D50 = 1.77 μm).This means that an effective way to reduce the amount of mercury contained in the raw materials and fuel brought into the cement manufacturing process that circulates during the raw material and firing processes is to remove the mercury contained in the fine particle dust (equivalent to the fine powder shown in Table 1) obtained by classifying the dust recovered from bag filter 15 (first bag filter). Table 2 also shows the test results in which the dust collected in the bag filter 19 (second bag filter) was separated into a first dust and a second dust, the first dust was supplied to a heating tower 20 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 21 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 36.5 ppm, whereas after removal, the mercury concentration in the first dust was 0.3 ppm and the mercury concentration in the second dust was 516.6 ppm, a concentration ratio of approximately 14 times. Furthermore, the mass of the removed second dust was 72 g, which is a volume reduction of approximately 6.8% of the combined amount of the first and second dusts fed. 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 bag filter 15 (first bag filter), and the volume of the dust is also reduced. Therefore, as already explained with reference to Figures 1 and 2, the mercury removal method of the present invention is characterized by comprising a step of recovering the dust captured by the bag filter 15 (first bag filter) and classifying it using a cyclone 18 (second cyclone) (step S2 in Figure 2), a step of capturing the fine dust obtained in this step using a bag filter 19 (second bag filter) and then recovering it (step S3 in Figure 2), a step of heating the fine dust recovered in this step in a heating tower 20 to vaporize the mercury (step S4 in Figure 2), and a step of cooling the mercury gas generated in this step in a cooling tower 21 (step S6 in Figure 2).

[0035] [Table 1]

[0036] [Table 2]

[0037] In the present invention, which includes the above-described steps, mercury adhering to dust collected by the bag filter 15 (first bag filter) is concentrated in a heating concentrator 22 consisting of a heating tower 20 and a cooling tower 21, and dust with a high mercury content is supplied to a mercury removal facility (see FIG. 3). This significantly reduces the amount of dust to be treated in the mercury removal facility (volume reduction). In other words, according to the present invention, the load on the mercury removal facility is reduced, making it possible to efficiently reduce the mercury content in the exhaust gas generated in the cement production process. Furthermore, by removing mercury, the dust can be reused as a cement raw material. [Industrial Applicability]

[0038] 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]

[0039] 0...Raw material dryer 1...Raw material mill 2...Cyclone (first 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...Bag filter (first bag filter) 16...Chimney 17...Boiler 18...Cyclone (second cyclone) 19...Bag filter (second bag filter) 20...Heating tower 21...Cooling tower 22...Heated concentrator 23...Hopper 24...Ash supply pipe 24a...Rotary valve 25...First heating furnace 25a...Ash inlet 25b...Ash outlet 25c...Main body 25d...Blade 25e...Rotating shaft 26... Second heating furnace 26a... Ash inlet 26b... Ash outlet 26c... Main body 26d... Blade 26e... Rotating shaft 26f... Gas outlet 27... Cooler 27a... Ash inlet 27b... Ash outlet 27c... Main body 27d... Blade 27e... Rotating shaft 28... Ash discharge pipe 28a... Rotary valve 29... Dust filter 30... Condenser 31a, 31b... Adsorption tower 32... Mercury separator 33... Mercury reservoir 34... Water tank 35... Drainage pump 36... Screw conveyor

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 classification step of collecting the dust having the mercury attached thereto by a first bag filter and then recovering the dust from the first bag filter and classifying the dust using a cyclone; and a dust recovery step of collecting and recovering the dust obtained in the classification step using a second bag filter, 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. 2. The mercury removal method according to claim 1, wherein the second dust is heated after being removed from the cooling tower in an oxygen-deficient reducing atmosphere.

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