JP4794071B2 - Method for removing mercury contained in exhaust gas - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for easily and effectively removing a mercury component contained in exhaust gas discharged from a smelting furnace for smelting ore containing sulfur.
[0002]
[Prior art]
Conventionally, exhaust gas from boilers fueled with metal smelting and oil and coal contains harmful heavy metals such as mercury, so strict emission standards have been established, and emissions from smelters and power plants have been established. Smoke and wastewater are discharged in a completely controlled manner so as not to exceed this emission standard.
For example, as a method for treating exhaust gas in copper smelting, as shown in FIG. 4, an ore containing sulfur is smelted in a smelting furnace 1 using a fuel mainly containing coal or heavy oil. The smelting furnace 1 generates exhaust gas containing a sulfur component, a mercury component, and the like including SO ₂ gas, SO ₃ , MSO ₄ (M is a heavy metal), and the like. This exhaust gas is sent to the gas cleaning step 2 where it is brought into contact with an adsorbent or gas-liquid in the presence of water, thereby containing SO ₂ gas as a main component, SO ₃ other than SO ₂ gas, MSO _{4 and the} like. Separated into wastewater containing sulfur and mercury components. That is, in the gas cleaning step 2, the cleaning water absorbs a sulfur component and a mercury component containing SO ₃ , MSO _{4 and the} like other than the SO ₂ gas. Of these, SO ₃ reacts with the washing water to become sulfuric acid (H ₂ SO ₄ ). Gas mainly composed of SO ₂ gas separated by the gas washing step 2, the discharge was removed recovering SO ₂ gas as sulfuric acid or gypsum, out of the system in a state in which all of the hazardous materials is regulated to below the emission standard Is done. In addition, the wastewater separated in the gas cleaning step 2 contains a large amount of mercury components, but since the mercury component is contained in a large amount in the solids contained in the wastewater (pH 1 or less), the solids in the solid-liquid separation step 4 Separated into drainage. Specifically, the solid content is separated from the waste water by concentrating the waste water with a thickener or the like, and solid-liquid separation of the concentrated raw water slurry. The separated solid content is raw water sludge, and this raw water sludge is re-supplied to the smelting furnace 1, or a part of the raw water sludge is supplied as a raw water slurry to the waste water secondary treatment step 7 described later, or outside the system. Or bleed off.
[0003]
The waste water from which the solid content has been separated in the solid-liquid separation step 4 is adjusted to a pH of 2 to 4 by adding calcium carbonate (CaCO ₃ ) in the waste water primary treatment step 5 to adjust the sulfuric acid component of the sulfur component. The sulfuric acid component is separated and recovered as gypsum (CaSO ₄ ) by immobilizing only and solid-liquid separation. At this time, mercury dissolved in the waste water is separated together with the sulfuric acid component and transferred to gypsum. The wastewater from which gypsum was separated in the wastewater primary treatment step 5 was separated from the raw water sludge separated in the solid-liquid separation step 4 in the wastewater secondary treatment step 7 (raw water slurry state) with calcium hydroxide (Ca (OH)). ₂ ) By adjusting the pH of the wastewater to 11-12 by adding, the heavy metals mainly contained in the wastewater together with the mercury contained in the raw water sludge (raw water slurry) is fixed, and heavy metals are not contained by solid-liquid separation Get drained. Wastewater is discharged outside the system with all harmful substances regulated below the emission standard. The solid content fixed in the wastewater secondary treatment step 7 is supplied again to the smelting furnace 1 as secondary sludge.
[0004]
As described above, the technology for removing harmful substances contained in the exhaust gas is almost established, but various improved technologies have been proposed (Japanese Patent Laid-Open Nos. 7-308542, 9-308817, and 10- 10). 216476).
JP-A-7-308542 discloses a mixture of lead concentrate or artificially synthesized lead sulfide (PbS) and naturally occurring pyrite (FeS ₂ ) or other sulfide ores or synthesized iron sulfide. A method for removing mercury in exhaust gas by adsorbing and removing mercury by passing an exhaust gas accompanied by gaseous or mist-like mercury through an absorbent made of a material carrier is shown. In this method, the absorbent that is mixed with lead sulfide and iron sulfide and is supported on the porous material carrier can be used stably for a long time, has good mercury adsorption efficiency, and has the ability to remove mercury per unit weight of the absorbent. Since it is high, there is an effect that the filling layer filled with the absorbent can be downsized. Japanese Patent Laid-Open No. 9-308817 discloses an improved technique for removing harmful substances by absorbing, filtering or trapping them with a granular layer on the surface of the bag filter by a bag filter formed by supplying the granular material. Measures one or more of the concentrations of harmful substances in the exhaust gas at the bag filter outlet, and supplies the granular material used to form the granular layer on the bag filter surface based on the increase or decrease of the concentration By increasing or decreasing the amount, it becomes possible to minimize the amount of powder to be supplied to remove harmful substances in the exhaust gas. Japanese Patent Application Laid-Open No. 10-216476 discloses at least one of circulating liquid, absorption liquid used in a desulfurization apparatus, supply water used in a wet electric dust collector, circulating water, water in the dust collector main body, and exhaust gas at the dust collector inlet. A method of adding a mercury removing agent is shown in this figure, and this method is used to remove mercury, especially metallic mercury vapor, in exhaust gas with an extremely low concentration of the order of 10 μg / Nm ³ or less, which is discharged in large quantities from the emission source. Can be removed.
[0005]
[Problems to be solved by the invention]
However, in the technique disclosed in Japanese Patent Laid-Open No. 7-308542, it is shown that mercury is adsorbed and removed by passing an exhaust gas accompanied by gaseous or mist-like mercury through an absorbent. The technology is high because it uses technology to remove mercury components.
Further, the technology disclosed in Japanese Patent Laid-Open No. 9-308817 uses a bag filter for mercury removal, but there is a concern that the dust concentration in the exhaust gas increases when the filter cloth of the bag filter is damaged. The In addition, the method using the bag filter has a low mercury removal efficiency, which may increase the cost.
Furthermore, although the technology shown in Japanese Patent Application Laid-Open No. 10-216476 uses an electrostatic precipitator for removing mercury, the electrostatic precipitator has a problem in dust removal, and the mercury removal effect is simply improved by improving the performance of the precipitator. If raised, the dust collection area becomes excessive, which is not only economical, but also requires a large site area.
[0006]
An object of the present invention is to provide a simple method and a method for removing mercury contained in exhaust gas that can effectively remove the mercury component contained in exhaust gas discharged from a smelting furnace that performs smelting of ore containing sulfur. There is.
[0007]
[Means for Solving the Problems]
The invention according to claim 1 is an improvement of a method for removing mercury contained in exhaust gas discharged from a smelting furnace for smelting ore containing sulfur.
As shown in FIG. 1, the characteristic configuration is that the exhaust gas discharged from the smelting furnace 11 is washed with washing water containing 3 to 5% by weight of hydrochloric acid, and gas and waste water mainly composed of SO ₂ gas, The gas cleaning step 12 for separating the solids into the solid, the solid-liquid separation step 14 for separating the solids contained in the wastewater as raw water sludge, and the mercury components contained in the wastewater removed by contacting the wastewater from which the solids were separated with a mercury removing agent. To the wastewater primary treatment step 18 for separating the sulfuric acid component as gypsum by adding calcium carbonate to the wastewater from which the mercury component has been removed and separating the sulfuric acid component as gypsum. And a waste water secondary treatment step 21 in which a portion of the raw water sludge separated in the solid-liquid separation step 14 and calcium hydroxide are added to separate heavy metals mainly contained in the waste water as secondary sludge.
[0008]
In the invention according to claim 1, since the exhaust gas is cleaned in the gas cleaning step 12 with the cleaning water containing 3 to 5% by weight of hydrochloric acid, the mercury component contained in the exhaust gas is made soluble mercury (II) chloride (HgCl ₂ ). The mercury component removal process 17 is provided between the solid-liquid separation process 14 and the waste water primary treatment process 18, and the mercury component removal process 17 brings the waste water into contact with the mercury removing agent and is contained in the waste water. By removing the mercury component, the mercury component is effectively removed by a simple method.
[0009]
The invention according to claim 2 is the invention according to claim 1, wherein a part of the waste water from which the mercury component is removed by the mercury removing agent in the mercury component removing step 17 is used as cleaning water for SO ₂ gas in the gas cleaning step 12. This is a method of removing mercury to be reused.
In the invention according to claim 2, the concentration of mercury contained in the cleaning water is obtained by reusing a part of the waste water from which the mercury component has been removed in the mercury component removing step 17 as water for cleaning the SO ₂ gas in the gas cleaning step 12. To lower the mercury load in the gas cleaning step 12. Further, the mercury component concentration contained in the gas containing SO ₂ gas as a main component can also be reduced.
[0010]
The invention according to claim 3 is the invention according to claim 1, wherein before the mercury component is removed in the mercury component removal step 17, the wastewater is passed through the sand layer to filter the remaining solids contained in the wastewater. The method for removing mercury further includes a sand filtration step 16.
In the invention which concerns on Claim 3, clogging by the remainder of the solid content contained in waste_water | drain is prevented in the mercury component removal process 17 which follows by allowing waste_water | drain to pass through the layer by sand in the sand filtration process 16. FIG.
[0011]
The invention according to claim 4 is the invention according to claim 1, wherein the mercury removing agent is activated carbon, and the activated carbon has an ability to adsorb 5 to 20% by weight of a mercury component with respect to 100% by weight of the activated carbon. This is a method for removing mercury.
The invention according to claim 5 is the method according to claim 1, wherein the smelting furnace 11 is a reflection furnace.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Next, an embodiment of the present invention will be described with reference to the drawings, using as an example an exhaust gas generated by copper smelting, as a method for removing mercury contained in the exhaust gas.
As shown in FIG. 1, the exhaust gas generated from the smelting furnace 11 when smelting ore containing sulfur includes sulfur component, mercury component, etc. containing SO ₂ gas, SO ₃ , MSO ₄ (M is heavy metal), etc. Is included. This exhaust gas is sent to the gas cleaning step 12, where it is brought into gas-liquid contact in the presence of cleaning water containing 3 to 5% by weight of hydrochloric acid, so that SO ₂ gas as a main component and SO ₂ gas other than SO ₂ gas are contacted. _3, is separated into a waste water containing sulfur components and mercury components like containing MSO ₄ and the like. That is, in the gas cleaning step 12, the cleaning component absorbs a sulfur component and a mercury component containing SO ₃ and MSO ₄ other than the SO ₂ gas. Of these, SO ₃ reacts with the washing water to become sulfuric acid (H ₂ SO ₄ ). In the gas cleaning step 12, the cleaning water is subdivided into a large number of fine droplets by a nozzle or the like, and these are dispersed in the exhaust gas passing through the empty tower, and solid or liquid such as heavy metals or dust other than mercury floating in the exhaust gas. This is done with a spray-type scrubber that traps mercury particles. When the exhaust gas is washed with washing water containing 3 to 5% by weight of hydrochloric acid, the mercury component contained in the exhaust gas becomes soluble mercury (II) chloride (HgCl ₂ ), and more is transferred to the drainage side. The concentration of the SO ₂ gas cleaning water in this gas cleaning step 12 is adjusted so that it always contains 3 to 5% by weight of hydrochloric acid. The reason why the concentration of hydrochloric acid is specified to be 3 to 5% by weight is that the amount of mercury component transferred to the drainage side does not increase if it is less than the lower limit, and if the upper limit is exceeded, a large amount of hydrochloric acid is used. This is because there is a problem that mercury is not adsorbed on the activated carbon in the mercury component removing step 17 and the amount of calcium carbonate used is increased in the waste water primary treatment step 18. Gas mainly composed of SO ₂ gas separated by the gas washing step 12, the discharge was removed recovering SO ₂ gas as sulfuric acid or gypsum, out of the system in a state in which all of the hazardous materials is regulated to below the emission standard Is done.
[0013]
Solid wastewater containing sulfur components and mercury components including SO ₃ and MSO ₄ other than the SO ₂ gas separated in the gas cleaning step 12 (including hydrofluoric acid, hydrochloric acid and sulfuric acid) has a solid content. Since it is contained, it separates into solid content and waste water in the solid-liquid separation step 14. Specifically, the solid content is separated from the waste water by concentrating the waste water with a thickener or the like, and solid-liquid separation of the concentrated raw water slurry. The separated solid is raw water sludge, and this raw water sludge is resupplied to the smelting furnace 11 or a part of the raw water sludge is supplied as a raw water slurry to the drainage secondary treatment step 21 described later. The wastewater from which the solid content has been separated in the solid-liquid separation step 12 is sent to the sand filtration step 16 where the wastewater is passed through a layer of sand to filter the remainder of the solid content contained in the wastewater. By applying this sand filtration step 16, in the subsequent mercury component removal step 17, clogging due to the remaining solids contained in the waste water is prevented.
[0014]
In the mercury component removal step 17, the wastewater from which the solid content has been separated is brought into contact with the mercury removing agent to remove the mercury component contained in the wastewater. In the mercury component removing step 17, most of the mercury components contained in the waste water are adsorbed by the mercury removing agent. Since the waste water is an acid solution having a pH of 1 or less, activated carbon or fly ash is used as the mercury removing agent. In particular, activated carbon is preferable because of its high adsorption ability. The particle size of the mercury removing agent is 6 to 100 mesh, preferably 10 to 30 mesh. When activated carbon is used as the mercury removing agent, the activated carbon has an ability to adsorb 5 to 20% by weight of a mercury component with respect to 100% by weight of the activated carbon, although it depends on the quality of the activated carbon.
Before sending the wastewater from which the mercury component has been removed to the wastewater primary treatment step 18, a part of the wastewater from which the mercury component has been removed is supplied to the water for washing the SO ₂ gas in the gas washing step 12 and reused. By supplying the wastewater to the water for cleaning the SO ₂ gas, the concentration of the mercury component contained in the washing water is lowered, and the concentration of the mercury component contained in the gas containing the SO ₂ gas as a main component can also be reduced. In the subsequent SO ₂ recovery, for example, the mercury load of the waste gypsum plant can also be reduced. Moreover, since the mercury component density | concentration of the waste_water | drain sent for subsequent waste water treatment also falls, the mercury component density | concentration of the gypsum obtained by neutralizing the sulfuric acid content in waste_water | drain can also be lowered | hung.
[0015]
In the wastewater from which the mercury component has been removed, in the wastewater primary treatment step 18, by adding calcium carbonate (CaCO ₃ ), the pH of the wastewater is adjusted to 2 to 4 to fix only the sulfuric acid component of the sulfur component, The sulfuric acid component is separated as gypsum (CaSO ₄ ) by liquid separation.
[0016]
The wastewater from which the sulfuric acid component has been separated in the wastewater primary treatment step 18 is combined with a portion of the raw water sludge (raw water slurry state) separated in the solid-liquid separation step 14 in the wastewater secondary treatment step 21 together with calcium hydroxide (Ca (OH ) Add ₂ ) to adjust the pH of the wastewater to 11-12, immobilize heavy metals mainly contained in the wastewater together with mercury contained in the raw water sludge (raw water slurry), and solidify the heavy metal by solid-liquid separation. Separate as minutes. Wastewater is discharged outside the system with all harmful substances regulated below the emission standard. The solid content fixed in the wastewater secondary treatment step 21 is supplied again to the smelting furnace 11 as secondary sludge.
[0017]
Next, the distribution ratio of mercury contained in the exhaust gas will be described using the mercury removal method of the present invention and the conventional method.
The case where the exhaust gas generated by smelting is treated by the conventional mercury removal method will be described. As shown in FIG. 3, first, assuming that the ratio of the mercury component contained in the exhaust gas first generated from the smelting furnace is 100%, a part of the mercury component contained in the exhaust gas is combined with selenium contained in the exhaust gas. Insoluble mercury selenide (II) (HgSe) is formed and remains in the gas even after gas cleaning, so all the mercury components are not transferred to the wastewater. Mercury components are distributed at a rate of 10% to the separated SO ₂ gas as a main component, and at a rate of 90% to the waste water. In the solid-liquid separation process, the mercury component contained in the wastewater is transferred to the raw water sludge at a rate of 90% and to the wastewater at a rate of 10%. Accordingly, the mercury component contained in the wastewater is transferred at a rate of 81% on the raw water sludge side and 9% on the wastewater side, out of 90%. The mercury component transferred to the drainage side is almost 9% of the mercury component together with the sulfuric acid component in the gypsum in the primary wastewater treatment process. Of the 81% of the mercury component transferred to the raw water sludge, if a part of the raw water sludge is a and the remainder is 1-a, the distribution ratio of the mercury component supplied to the wastewater secondary treatment process is (1- a) × 81%, and the balance is a × 81%. Mercury components supplied to the wastewater secondary treatment process are transferred to the secondary sludge as solids, and the remainder of the raw water sludge and the secondary sludge are resupplied to the smelting furnace. The rate is 81%.
[0018]
Next, when the distribution ratio of the mercury component contained in the continuously generated exhaust gas is [+ α], the ratio of the mercury component sent to the gas cleaning step is [81% + α]. In this gas cleaning step, as described above, the transition is performed at a rate of 10% for the gas mainly composed of SO ₂ gas and 90% for the waste water, and therefore [81% + α on the gas side mainly composed of SO ₂ gas. ] × 0.1, and [81% + α] × 0.9 at the drainage side. In the solid-liquid separation process, the mercury component contained in the wastewater is transferred to the raw water sludge at a rate of 81% and 9% to the wastewater, respectively, so out of the mercury component [81% + α] × 0.9 contained in the wastewater, It moves at a rate of [81% + α] × 0.81 on the raw water sludge side and [81% + α] × 0.09 on the drainage side. In the primary wastewater treatment process, the mercury component that has moved to the wastewater side is [81% + α] × 0.09, and almost all of the mercury component is separated as a solid content together with the sulfuric acid component and contained in the gypsum. Of the mercury component [81% + α] × 0.81 transferred to the raw water sludge, assuming that a part of the raw water sludge is a and the remainder is 1-a, the mercury component supplied to the wastewater secondary treatment process The distribution ratio is (1−a) × [81% + α] × 0.81, and the remainder is a × [81% + α] × 0.81. The mercury component supplied to the wastewater secondary treatment process is transferred to the secondary sludge as a solid content, and the remainder of the raw water sludge and the secondary sludge are re-supplied to the smelting furnace. The distribution rate is [81% + α] × 0.81.
If the mercury distribution ratio [+ α] of the exhaust gas generated continuously is 100%, which is the same ratio as the mercury distribution ratio of the first exhaust gas generated, the mercury component returning to the smelting furnace again exceeds 100%. Therefore, every time the mercury component is resupplied to the smelting furnace, there arises a problem that the concentration of the mercury component increases.
[0019]
The case where the exhaust gas generated by smelting is treated by the mercury removal method of the present invention will be described. As shown in FIG. 2, first, assuming that the ratio of the mercury component contained in the exhaust gas first generated from the smelting furnace is 100%, when the conventional mercury removal method is used, one of the mercury components contained in the exhaust gas is obtained. The part combines with selenium contained in the exhaust gas to form water-insoluble mercury selenide (II), which remains in the gas even after gas cleaning. It does not migrate, but migrates at a rate of 10% on the gas side mainly composed of SO ₂ gas separated in the gas washing step and 90% on the drain side. Since the exhaust gas is washed with wash water containing 5% by weight of hydrochloric acid, the mercury component contained in the exhaust gas becomes soluble mercury (II) chloride, and is largely transferred to the waste water side. Therefore, the mercury component contained in the exhaust gas can be transferred to the drain side more than the conventional method, and can be transferred at a rate of 5% on the gas side mainly composed of SO ₂ gas and 95% on the drain side. . In the solid-liquid separation process, the mercury component contained in the wastewater is transferred to the raw water sludge at a rate of 10% and to the wastewater at a rate of 90%. Accordingly, the mercury component contained in the wastewater moves out of 95% at a ratio of 9.5% to the raw water sludge side and 85.5% to the wastewater side. The mercury component transferred to the drainage side is 85.5% in the mercury component removal process, and almost all the mercury component is removed by the mercury removing agent. Assuming that 9.5% of the mercury component transferred to the raw water sludge is a part of the raw water sludge and that the remainder is 1-a, the distribution ratio of the mercury component supplied to the wastewater secondary treatment process is ( 1-a) × 9.5%, and the balance is a × 9.5%. Mercury components supplied to the wastewater secondary treatment process are transferred to the secondary sludge as solids, and the remainder of the raw water sludge and the secondary sludge are resupplied to the smelting furnace. The rate is 9.5%.
[0020]
Next, when the distribution ratio of the mercury component contained in the continuously generated exhaust gas is [+ α], the ratio of the mercury component sent to the gas cleaning step is [9.5% + α]. 5% to the gas side of the main component as SO ₂ gas described above in the gas washing step, the process proceeds at a rate of 95% in the waste water side, the gas side composed mainly of SO ₂ gas [9 .5% + α] × 0.05, and on the drain side, [9.5% + α] × 0.95. In the solid-liquid separation process, the mercury component contained in the wastewater is transferred to the raw water sludge at a rate of 10% and 90% to the wastewater, respectively. Therefore, the mercury component contained in the wastewater [9.5% + α] × 0.95 Of these, [9.5% + α] × 0.095 is transferred to the raw water sludge side, and [9.5% + α] × 0.855 is transferred to the drainage side. In the mercury component removal process, almost all mercury components [9.5% + α] × 0.855 are removed by the mercury removing agent. Of the mercury component [9.5% + α] × 0.095 transferred to the raw water sludge, assuming that a part of the raw water sludge is a and the remainder is 1-a, it is supplied to the secondary wastewater treatment process. The distribution ratio of the mercury component is (1−a) × [9.5% + α] × 0.095, and the balance is a × [9.5% + α] × 0.095. The mercury component supplied to the wastewater secondary treatment process is transferred to the secondary sludge as a solid content, and the remainder of the raw water sludge and the secondary sludge are re-supplied to the smelting furnace. The distribution rate is [9.5% + α] × 0.095.
[0021]
In this way, the distribution ratio of the mercury component that returns to the smelting furnace decreases in the amount of mercury component each time it is re-supplied, so it is included in SO ₂ separated in the gas cleaning process and gypsum separated in the wastewater primary treatment process. Mercury component concentration is also reduced. Although not shown in FIG. 2, a part of the waste water from which the mercury component was removed in the mercury component removal step is reused as water for washing SO ₂ in the gas washing step, and thus separated in the gas washing step. The distribution rate of mercury contained in the wastewater is smaller than the value described in FIG.
[0022]
【The invention's effect】
As described above, according to the present invention, in the method for removing mercury contained in exhaust gas generated by smelting, exhaust gas is washed in the gas washing step with washing water containing 3 to 5% by weight hydrochloric acid. The mercury component contained in is converted to soluble mercury chloride and transferred to the drainage side more, and a mercury component removal step is provided between the solid-liquid separation step and the wastewater primary treatment step. In this mercury component removal step, the wastewater is converted into a mercury remover. By removing the mercury component contained in the wastewater by contacting it, the gypsum separated in the subsequent wastewater primary treatment step contains almost no mercury component.
In addition, part of the waste water from which mercury components have been removed in the mercury component removal process is reused in the water used to wash SO ₂ in the gas cleaning process, so it is included in the gas containing SO ₂ as the main component separated in the gas cleaning process. The mercury content in the gypsum produced using the recovered sulfur component can be minimized by reducing the distribution rate of the mercury component and the mercury distribution rate in the wastewater separated in the gas cleaning process. be able to.
[Brief description of the drawings]
FIG. 1 is a diagram showing a method for removing mercury contained in exhaust gas of the present invention.
FIG. 2 is a graph showing a distribution ratio of mercury when the mercury removal method of the present invention is used for processing.
FIG. 3 is a diagram showing a distribution ratio of mercury when processing is performed using a conventional mercury removal method.
FIG. 4 is a view showing a conventional method for removing mercury contained in exhaust gas.
[Explanation of symbols]
11 Smelting furnace 12 Gas cleaning process 14 Solid-liquid separation process 17 Mercury component removal process 18 Wastewater primary treatment process 21 Wastewater secondary treatment process

Claims (5)

In a method for removing mercury contained in exhaust gas discharged from a smelting furnace (11) for smelting ore containing sulfur,
A gas cleaning step (12) for cleaning the exhaust gas discharged from the smelting furnace (11) with a cleaning water containing 3 to 5% by weight of hydrochloric acid and separating the exhaust gas into a gas mainly composed of SO ₂ gas and waste water; ,
A solid-liquid separation step (14) for separating solids contained in the wastewater as raw water sludge;
A mercury component removing step (17) for removing the mercury component contained in the waste water by contacting the waste water from which the solid content has been separated with a mercury removing agent,
Waste water primary treatment step (18) for separating the sulfuric acid component as gypsum from the sulfur component contained in the waste water by adding calcium carbonate to the waste water from which the mercury component has been removed,
Waste water secondary treatment for separating heavy metals mainly contained in the waste water as secondary sludge by adding a part of the raw water sludge separated in the solid-liquid separation step (14) and calcium hydroxide to the waste water from which the sulfuric acid component has been separated. And a step (21) of removing mercury contained in the exhaust gas. The method for removing mercury according to claim 1, wherein a part of the waste water from which the mercury component has been removed by the mercury removing agent in the mercury component removing step (17) is reused as the SO ₂ gas washing water in the gas washing step (12). The sand filtration step (16) according to claim 1, further comprising a sand filtration step (16) for filtering the remainder of the solids contained in the wastewater by passing the wastewater through a layer of sand before removing the mercury component in the mercury component removal step (17). How to remove mercury. The mercury removal method according to claim 1, wherein the mercury removing agent is activated carbon, and the activated carbon has an ability to adsorb 5 to 20% by weight of a mercury component with respect to 100% by weight of the activated carbon. The method for removing mercury according to claim 1, wherein the smelting furnace (11) is a reflection furnace.

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