JP7196575B2 - Method for detoxifying exhaust gas containing sulfur dioxide - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for detoxifying exhaust gas containing sulfur dioxide discharged from a sulfuric acid production facility, and in particular, it is possible to treat the exhaust gas in a detoxification tower and to effectively utilize the waste liquid discharged from the detoxification tower. It relates to possible exhaust gas abatement methods.

As a method for obtaining blister copper by subjecting sulfide concentrate as a raw material to dry processing, a pyrometallurgical method using a smelting furnace such as a flash furnace and a converter is known. In this smelting process, the sulfide concentrate is first smelted in a smelting furnace to separate matte containing copper from the smelting furnace slag. At that time, smelting furnace exhaust gas containing sulfur dioxide (SO ₂ ) generated from part of the sulfur contained in the raw material is discharged from the smelting furnace to the outside of the system.

Next, in a converter, the matte obtained in the smelting furnace is blown to remove impurities contained in the matte as converter slag to produce blister copper with a purity of about 98% by mass. At that time, the converter exhaust gas containing SO ₂ generated from the sulfur contained in the matte as a main component is discharged from the converter to the outside of the system. Thus, most of the sulfur content in the sulfide concentrate is distributed to either the smelting furnace exhaust gas or the converter exhaust gas in the form of SO ₂ and discharged out of the system.

Since SO2 is a harmful air pollutant, the flue gas containing SO2 emitted from the above _- mentioned _{pyrometallurgical} equipment cannot be released into the atmosphere. Therefore, the exhaust gas is supplied to a sulfuric acid manufacturing facility attached to the pyrometallurgical facility, and the SO ₂ in the exhaust gas is recovered there as sulfuric acid. For example, Patent Document 1 discloses a technique for treating exhaust gas discharged from a pyrometallurgical facility consisting of a smelting furnace and a converter to produce sulfuric acid from SO ₂ contained in the exhaust gas. This technology consists of a "gas refining process" in which impurities such as dust contained in the exhaust gas discharged from the _{pyrometallurgical} equipment are removed and purified, and SO2 in the exhaust gas treated in the gas refining process is converted to trioxide. It is mainly composed of the "sulfuric acid production process" that converts to sulfur to produce sulfuric acid. Since the so-called tail gas discharged from this sulfuric acid manufacturing process contains a small amount of sulfur oxides that could not be completely recovered, the tail gas is brought into contact with an alkaline solution before it is released into the atmosphere, and is detoxified by a wet process ( A "tail gas treatment process" (also called detoxification) is usually provided after the sulfuric acid manufacturing process.

In addition to the above _- mentioned wet process, the above _- mentioned detoxification treatment of tail gas containing SO2 includes a dry process in which SO2 is adsorbed by activated carbon, limestone, etc., but the wet process, which has excellent absorption efficiency, is mainly adopted. there is For example, Patent Document 2 discloses a flue gas desulfurization process in which flue gas discharged from a boiler, a heating furnace, or the like is treated in a wet process. This flue gas desulfurization process uses an alkaline chemical solution as an _absorbent for _SO2 gas in the flue gas. (OH) ₂ method) is disclosed.

JP 2018-027870 A JP 2007-181756 A

When the magnesium hydroxide method is adopted for the tail gas treatment step subsequent to the sulfuric acid production step, the tail gas discharged from the sulfuric acid production step and the circulating liquid of the magnesium hydroxide slurry are brought into contact with each other in a countercurrent flow, and residual in the tail gas remains in the tail gas. It is preferable to use a detoxification tower that renders the SO ₂ gas harmless by neutralization. In this detoxification tower, fresh Mg(OH) ₂ is added so that the pH of the circulating liquid of magnesium hydroxide slurry as an absorbent circulating in the system is maintained at a predetermined value, and at the bottom of the detoxification tower A part of the circulating fluid is extracted so that the liquid surface level of the temporarily held circulating fluid is kept constant. Therefore, if unreacted processing agent remains in a part of the extracted circulating fluid, this processing agent is not used for neutralization of SO ₂ gas and is lost, which increases the cost of the chemical. Become. Therefore, from the viewpoint of reducing chemical costs, it is desirable that as little unreacted absorbent as possible remains in a portion of the circulating liquid extracted from the detoxification tower.

However, in the tail gas treatment process, in order to recover _SO2 as much as possible and not release _SO2 gas into the atmosphere, the pH of the circulating liquid circulating through the detoxification tower should be _adjusted to the theoretical value required for neutralization of SO2 gas. It is preferable to set it higher than the value, so that the absorbent is added in excess of the stoichiometric amount required to absorb the SO ₂ contained in the tail gas introduced into the abatement tower. As a result, the waste liquid discharged from the detoxification tower contains a large amount of unreacted absorbent Mg(OH) ₂ .

_In this way, the abatement tower employed in the tail gas treatment process preferably reduces the concentration of SO2 in the exhaust gas after treatment as much as possible, so it is difficult to lower the pH of the absorbent circulating in the abatement tower. , it was difficult to reduce the amount of absorbent used in the abatement tower. The present invention has been made in view of the above-mentioned problems of the tail gas treatment process, and an object of the present invention is to provide a method for reducing the loss of absorbent without lowering the recovery rate of SO ₂ contained in the tail gas. .

To achieve the above object, the tail gas detoxifying method of the present invention is a method for detoxifying tail gas containing SO ₂ discharged from a sulfuric acid production process , wherein the tail gas is placed in a detoxification tower. It is brought into gas-liquid contact with an absorption liquid consisting of a Mg(OH) ₂ slurry containing Mg ₍ OH) ₂ in excess of the stoichiometric amount required to neutralize the SO2 contained in the tail gas, thereby detoxifying it. At the same time, the absorbent containing unreacted Mg(OH) ₂ partially extracted from the detoxification tower is sent to a sedimentation tank for sedimentation separation, and the Mg(OH) ₂ and magnesium sulfate extracted from the sedimentation tank The concentrated slurry on the underflow side containing a mixture of and is used as an absorbent for wet detoxification in other acid gas neutralization equipment.

According to the present invention, it is possible to reduce the loss of the absorbent without lowering the recovery rate of SO ₂ in the detoxification tower.

1 is a block flow diagram of a sulfuric acid manufacturing process including a method for detoxifying exhaust gas according to an embodiment of the present invention; FIG. FIG. 2 is a schematic flow diagram of one specific example of abatement equipment capable of suitably performing the harm-abatement method of FIG. 1;

First, referring to FIG. 1, a sulfuric acid manufacturing process including a method for detoxifying exhaust gas according to an embodiment of the present invention will be described. The sulfuric acid production process shown in FIG. 1 is a process for producing sulfuric acid by treating exhaust gas containing SO ₂ discharged from a copper smelting process, and impurities such as dust mixed in the exhaust gas containing SO ₂ are removed. A gas purification step S1, a sulfuric acid production step S2 in which sulfuric acid is produced from SO ₂ contained in the exhaust gas purified in the gas purification step S1, and a sulfur content remaining in the tail gas discharged from the sulfuric acid production step S2 is removed. and a tail gas processing step S3.

Specifically, each step will be described in detail. First, in the gas refining step S1, the exhaust gas discharged from the copper smelting process is subjected to heat recovery by passing through a waste heat recovery boiler, followed by the humidification tower 11 and the washing tower. 12, where the exhaust gas is cooled by contact with a large amount of water, and the dust in the exhaust gas is captured by the water. The exhaust gas containing mist-like dust that has not been completely captured by the scrubbing tower 12 is then introduced into a mist cottrell (wet type electrostatic precipitator) 13 to remove the dust. The mist cottrell 13 is composed of a linear discharge electrode and a cylindrical collecting electrode surrounding it. By applying a high DC voltage between them, the mist-like dust is charged and the collecting electrode It is removed from the flue gas by collecting The dust-free flue gas exits the Mist Cottrell 13 and is then introduced into a drying tower 14 where it contacts with concentrated sulfuric acid to remove moisture and is discharged as purified flue gas.

In the next sulfuric acid production step S2, the purified exhaust gas purified in the gas purification step S1 is pressurized by the blower 21 and then introduced into the first converter 22, where SO ₂ in the purified exhaust gas is converted to sulfur trioxide. be converted. The gas converted in the first converter 22 is then temperature-controlled (cooled) and then introduced into the first absorption tower 23, where sulfur trioxide is absorbed into 98% concentrated sulfuric acid. After that, it is diluted with 95% sulfuric acid and water generated in the drying tower 14 to produce 98% concentrated sulfuric acid.

The treated gas after being treated in the first converter 22 and the first absorption tower 23 may be treated as tail gas in the next tail gas treatment step S3. In the sulfuric acid production process shown in , the second converter 24 and the second absorption tower 25 are provided in the rear stage of the first absorption tower 23, and the same treatment as the first converter 22 and the first absorption tower 23 in the front stage is performed. performed to produce 98% concentrated sulfuric acid. By providing a bypass line 27 equipped with a regulating valve 26 that bypasses the first absorption tower 23, it is possible to flexibly respond to the start-up of the copper refining process or temporary reduction in throughput. Become. Since the tail gas discharged from the second absorption tower 25 contains a small amount of sulfur trioxide and unconverted SO ₂ , it is treated in the tail gas treatment step S3.

In the tail gas treatment step S3, the tail gas is introduced into the detoxification tower 31, where trace amounts of sulfur trioxide and unconverted SO ₂ contained in the tail gas are detoxified by being absorbed and fixed with alkali. , Mist Cottrell 32 into the atmosphere. On the other hand, in order to reduce the cost of the neutralizing agent, the absorbent partially extracted from the detoxification tower 31 is introduced into the sedimentation separation device 33, where it contains Mg(OH) ₂ obtained by sedimentation separation. The concentrated slurry on the underflow side is transferred to the acid gas neutralization facility and effectively used as an alkali absorbent.

More specifically, with reference to the flow diagram of FIG. 2, the abatement tower for removing SO ₂ contained in the tail gas by absorption of an alkaline liquid preferably has a vertical cylindrical shape as shown in FIG. A container body 1 is provided with a filling portion 2 of a filler in the central portion in the height direction, and a storage portion 3 for temporarily storing the absorbent is provided in the bottom portion. The absorbent extracted from the reservoir 3 is pressurized by a pump 4 and then passed through a circulation line 5 and sprayed toward the upper portion of the filling section 2 from a spray nozzle 5a provided at the tip thereof.

With such a configuration, the tail gas introduced from the side of the container body 1 rises in the packed section 2 toward the top outlet of the container body 1, and mixes with the absorbent flowing down in the packed section 2. Countercurrent gas-liquid contact occurs, and sulfur compounds such as SO ₂ contained in the tail gas are absorbed by the absorbent. A demister 6 is provided on the upper portion of the container body 1 in order to remove droplets of absorbent accompanying the gas that has exited from the upper portion of the filling portion 2 . Further, a nozzle 7 such as a sparger for blowing gas containing an oxidant such as air into the absorbing liquid is provided in the reservoir 3 for the purpose of oxidizing the absorbing liquid, which will be described later.

Since the gas discharged from the tower top outlet of the abatement tower 31 contains fine mist that has not been removed by the demister 6, the Mist Cottrell (MC) 32 similar to the Mist Cottrell in the gas purification step S1 described above contains After the fine mist is removed here, it is discharged to the atmosphere. On the other hand, the absorption liquid circulating in the harm removal tower 31 is partly circulated from the discharge side of the pump 4 so that the liquid level measured by the level gauge provided in the reservoir 3 at the bottom of the harm removal tower 31 is constant. After being extracted to the outside, the liquid is sent to a sedimentation tank, which will be described later.

In the detoxification method of the embodiment of the present invention, SO2 in the tail gas is removed by the Mg ₍ OH) ₂ method using Mg(OH) ₂ slurry as the absorbent circulating inside the detoxification tower 31. It is carried out. That is, in this Mg(OH) ₂ method, SO ₂ is absorbed into the Mg(OH) ₂ slurry by the absorption reactions of formulas 1 and 2 below.
[Formula 1]
Mg(OH) ₂ +SO2→ _{MgSO3 + H2O}
[Formula 2]
_MgSO3 +SO2 _{+ H2O} →Mg( _HSO3 ) ₂

As the absorption reaction between Mg(OH) ₂ and SO ₂ shown in the above formulas 1 and 2 progresses, the ability of the absorption liquid to absorb SO ₂ and desulfurize it, which it originally had, gradually decreases. Therefore, in order to maintain this desulfurization capability, fresh Mg(OH) ₂ slurry is added to the detoxification tower 31 as shown in FIG. The amount of Mg(OH) ₂ slurry to be added is preferably adjusted based on the amount of tail gas supplied to the detoxification tower 31, the concentration of SO ₂ contained therein, the pH and specific gravity of the circulating absorbent, etc. Furthermore, it is preferable that the concentration of sulfur in the gas discharged from the detoxification tower 31 after detoxification treatment is adjusted to a predetermined value or less.

For example, in FIG. 2, the valve opening degree of the supply line of Mg(OH) ₂ slurry supplied to the detoxification tower 31 is controlled so that the pH value of the circulating absorbent reaches the set value, and this set value is used for detoxification. An example of control by a cascade control system that adjusts based on the concentration of sulfur in the gas discharged from the top of the tower 31 is shown. In this case, in the detoxification method of the embodiment of the present invention, Mg(OH) ₂ is added in excess of the stoichiometric amount necessary for neutralizing SO ₂ contained in the tail gas introduced into the detoxification tower 31. The slurry of the absorbent that is added and drawn out of the circulation system from the discharge side of the pump 4 contains 2 to 10%, preferably 3 to 5%, of the added Mg(OH) ₂ . make it

In order to adjust the amount of Mg(OH) ₂ to be added in excess of the stoichiometric amount in this way, the absorption liquid discharged from the discharge side of the pump 4 to the outside of the circulation system is sampled and Mg(OH) 2 is added. ) ₂ content may be analyzed, but as shown in FIG. It is preferable to control it to be about 8.4 to 8.6. By the control system described above, the pH of the absorption liquid that has absorbed SO ₂ is adjusted, causing the reaction of formula 3 below, and the blowing of the oxidizing agent described above causes the reaction of formula 4 below.
[Formula 3]
Mg( _HSO3 ) ₂ +Mg(OH) _2- > _{2MgSO3 +} 2H2O
[Formula 4]
_MgSO3 + _½O2 → _MgSO4

As described above, the slurry-like absorbent discharged from the discharge side of the pump 4 of the detoxification tower 31 is sent to the sedimentation device 33 such as a thickener, where it undergoes gravity sedimentation separation. The sedimentation separation device 33 is composed of, for example, a substantially cylindrical sedimentation tank 8 having a bottom surface that gradually becomes deeper toward the center, and a rake 9 that rotates along the bottom surface. The removed clarified liquid overflows the upper end of the sedimentation tank 8 and is discharged, and the Mg(OH) ₂ and magnesium sulfate as the solid content become aggregates and settle at the bottom of the sedimentation tank 8, It is raked up by the rake 9 and withdrawn from the bottom of the sedimentation tank 8 as underflow in the form of a concentrated slurry. Note that the sedimentation separation device 33 may be one without a rake.

A mixture of Mg(OH) ₂ and magnesium sulfate is thus recovered from the bottom of the settling tank 8 in the form of a concentrated slurry, which can be used as an absorbent in other acid gas neutralization installations. Other _acid gas neutralization facilities include, for example, an environmental smoke collection facility that detoxifies the _SO2 contained in the leaked gas from the smelting building. At least part of this absorbent is Mg(OH) ₂ of the above mixture, and the leaking gas can be treated in the same manner as in the above harm removal tower.

As described above, in the exhaust gas detoxification method of the embodiment of the present invention, unused Mg(OH) ₂ remaining in the circulation system of the absorption liquid of the detoxification tower is similarly treated with an acidic solution such as SO ₂ . To effectively reduce the cost of the neutralizing agent for the entire plant because it can be effectively used in an acid gas neutralizing facility using an alkaline agent such as Mg(OH) ₂ for gas neutralization. can be done. EXAMPLES Next, the present invention will be described in more detail with reference to examples, but the present invention is not limited to the following examples.

<Example 1>
The tail gas discharged from the sulfuric acid production step S2 of the sulfuric acid production process as shown in FIG. 1 is treated with the detoxification equipment shown in FIG. The slurry was used as an absorbent in a detoxification tower for detoxification of environmental flue gases installed in the smelting building as another acid gas neutralization facility. As a result, it was possible to reduce the conventional Mg(OH) ₂ consumption in the other acid gas neutralization equipment. This reduction amount corresponds to 4.25% of the Mg(OH) ₂ added to the abatement tower 31 . The concentration of solids in the above concentrated slurry was about 2.5% by mass, most of which was Mg(OH) ₂ , and a small amount of magnesium sulfate was also found. Further, the pH of the absorbent circulating through the harm removing tower 31 was 8.4.

<Comparative example>
For comparison, the slurry-like absorbent extracted from the discharge side of the pump 4 of the detoxification tower 31 was not introduced into the sedimentation separation device 33, but was sent as it was to the other acidic gas neutralization equipment. As a result, the liquid balance of the other acid gas neutralization equipment was adversely affected. Specifically, the amount of wastewater from the other acid gas neutralization equipment increased, and more wastewater was generated than the pump capacity to send this wastewater to another wastewater treatment process. It became necessary to stop the operation of the neutralization equipment.

1 Container Body 2 Filling Part 3 Storage Part 4 Pump 5 Circulation Line 5a Spray Nozzle 6 Demister 7 Sparger Nozzle 8 Sedimentation Tank 9 Rake 11 Humidification Tower 12 Washing Tower 13 Mist Cottrell 14 Drying Tower 21 Blower 22 First Converter 23 Second 1 absorption tower 24 second converter 25 second absorption tower 26 regulating valve 31 harm removal tower 32 Mist Cottrell 33 sedimentation separation device S1 gas purification process S2 sulfuric acid production process S3 tail gas treatment process

Claims (2)

A method for detoxification of tail gas containing SO ₂ discharged from a sulfuric acid production process , wherein the tail gas is introduced into a detoxification tower and the stoichiometry required for neutralization of SO ₂ contained in the tail gas The unreacted Mg(OH) 2 is detoxified by gas-liquid contact with an absorption liquid consisting of a Mg(OH) ₂ slurry containing Mg(OH) ₂ in excess of the amount, and a part of the unreacted Mg(OH) ₂ is extracted from the detoxification tower. is sent to a sedimentation tank for sedimentation separation, and the concentrated slurry on the underflow side containing a mixture of Mg(OH) ₂ and magnesium sulfate extracted from the sedimentation tank is neutralized with another acid gas A method for detoxification of tail gas containing SO ₂ , characterized in that it is used as an absorbent for wet detoxification in a facility. The method of claim 1, wherein the Mg(OH) ₂ slurry has a pH value of _8.0-9.0 .

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