JPH0957051A - Double decomposition device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently separate the formed magnesium hydroxide and gypsum dihydrate, to enhance the utilization rate of the magnesium hydroxide and to increase the recovery of the gypsum dihydrate by providing a thickener and a regulating tank in addition to an agitated reaction tank and partly circulating a liq. reactant. SOLUTION: An aq. soln. contg. magnesium sulfate and sulfuric acid and a basic calcium liq. are charged in an agitated reaction tank (a) and allowed to react with each other to form a slurry of magnesium hydroxide and gypsum dihydrate. A slurry A rich in gypsum dihydrate is transferred to a gypsum separating stage, and a part of the formed slurry D is charged into a thickener (b). The slurry D is concentrated in the thickener (b), the concd. slurry is drawn off from the lower part and returned to the double decomposition tank (a), and the supernatant liq. is transferred to a regulating tank (c). The supernatant liq. is further separated in the tank (c), a liq. B rich in magnesium hydroxide is extracted from the upper part and returned to a desulfurizing stage, and a part of the regulated liq. F rich in gypsum dihydrate is extracted from the lower part and introduced into the thickener (b).

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a metathesis apparatus for use in a metathesis step, which is one step in a desulfurization method of desulfurizing exhaust gas containing sulfur oxides with a treatment liquid containing a magnesium-based desulfurizing agent. .

[0002]

2. Description of the Related Art As one of various desulfurization methods for exhaust gas,
There is known a method of using a magnesium compound such as magnesium hydroxide or light burned magnesium oxide as a desulfurizing agent.
In this method, first, in the desulfurization process, exhaust gas is brought into contact with the treatment liquid containing the desulfurizing agent so that the sulfur oxide is absorbed in the treatment liquid, and then the treatment liquid is guided to an oxidation step to be oxidized by a gas containing oxygen such as air. Then, an aqueous solution of magnesium sulfate and sulfuric acid is prepared, and this aqueous solution is neutralized with a magnesium compound. If the aqueous solution of magnesium sulfate after neutralization is directly discharged to the sea or river, it will lead to consumption of magnesium source and sulfate radicals, and may be undesirable in view of the effect of the discharge itself on the environment.

In the above-mentioned method, a method in which an aqueous solution of magnesium sulfate is not discharged is desired, but as a conventional technique related to this, the Kawasaki magnesium gypsum method is known (Practical pollution control technology list (1), Kagaku Kogyo Publishing, p.1
4). In this method, a mixed slurry of magnesium hydroxide and calcium hydroxide is used as a desulfurizing agent to absorb sulfur oxides in a desulfurization process, and then the treatment liquid is adjusted to pH 2.0-4.0 with sulfuric acid and air is added thereto. And the like to generate magnesium sulfate and gypsum dihydrate, and then to separate gypsum dihydrate and an aqueous solution of magnesium sulfate by a sedimentation separation step and a centrifuge. The separated magnesium sulfate aqueous solution is circulated and supplied to a raw material adjusting step including a mixed slurry of magnesium hydroxide and calcium hydroxide, in which a part of the calcium hydroxide in the mixed slurry and the metathesis reaction cause magnesium hydroxide and gypsum dihydrate. Is produced, and the mixture containing this and the balance calcium hydroxide is circulated and supplied to the absorption step as a desulfurizing agent. However, this method has a drawback in that desulfurization solution is mixed with gypsum dihydrate, so that scale deposition on the desulfurization process circulation pump and piping is likely to occur.

The following method has been disclosed as another conventional technique in which the magnesium sulfate aqueous solution is not discharged (Japanese Patent Publication No.
7045). The aqueous solution of magnesium sulfate generated in the oxidation step is introduced into a double decomposition tank, and calcium hydroxide, quick lime, etc. are added to it to react, producing gypsum dihydrate and magnesium hydroxide, and the mixture is put into a wet classifier, etc. Then, the slurry is separated into a fine particle slurry mainly containing magnesium hydroxide and a coarse particle slurry mainly containing magnesium hydroxide mainly containing dihydrate gypsum. The separated fine particle slurry of the former is circulated and supplied to the desulfurization process as a desulfurizing agent, and the latter coarse particle slurry contains a small amount of accompanying magnesium hydroxide, so this is led to another process and desulfurized there. By supplying a portion of the treatment liquid that has absorbed sulfur oxides from the process, it is converted to magnesium sulfite by reacting with magnesium hydroxide in the slurry, the liquid containing this magnesium sulfite and dihydrate gypsum by sedimentation separation step etc. It is separated and recycled to the desulfurization process and reused as a desulfurizing agent.

However, in this method, although the above-mentioned magnesium sulfite has a higher solubility than magnesium hydroxide, a part of the produced magnesium sulfite is separated and discharged from the system together with dihydrate gypsum as an insoluble matter, and the magnesium-based desulfurizing agent is reused. The rate was not enough.

[0006]

The object of the present invention is to efficiently separate magnesium hydroxide and gypsum dihydrate produced in the metathesis step for carrying out the metathesis reaction.
An object of the present invention is to provide a metathesis device capable of increasing the utilization rate of magnesium hydroxide and the recovery rate of gypsum dihydrate.

[0007]

SUMMARY OF THE INVENTION The present invention provides: The following main step of continuously removing the sulfur oxides contained in the exhaust gas by gas-liquid contacting the exhaust gas containing the sulfur oxides with the treatment liquid containing the magnesium-based desulfurizing agent, and (1) the exhaust gas and the desulfurization treatment liquid Desulfurization step of contacting to absorb and remove sulfur oxides (2) Oxidation step of air-oxidizing the desulfurization treatment liquid having absorbed sulfur oxides to an aqueous solution of magnesium sulfate and sulfuric acid (3) Magnesium sulfate obtained in the oxidation step A desulfurization method comprising a metathesis step of adding basic calcium to an aqueous solution of sulfuric acid to form a slurry of magnesium hydroxide and gypsum dihydrate, and (4) a gypsum separation step of taking out the gypsum dihydrate produced in the metathesis step out of the system. The multi-decomposition device used in the multi-decomposition step in 1., which comprises a stirring reaction tank a, a thickener b and an adjusting tank c. An aqueous solution containing magnesium sulfate or magnesium sulfate and sulfuric acid that has undergone the oxidation step, and a basic calcium solution are charged and reacted to produce a slurry of magnesium hydroxide and gypsum dihydrate, and dihydrate The gypsum-rich slurry (A) is discharged and transferred to the gypsum separation step, and a part (D) of the produced slurry is charged into the thickener b, where the produced slurry is concentrated and concentrated. The liquid is extracted from the lower part and returned to the metathesis tank a, and the supernatant liquid is transferred to the adjusting tank c, and in the adjusting tank c, the supernatant liquid is further separated, and a liquid rich in magnesium hydroxide ( B) is extracted from the upper part and returned to the desulfurization step, and a part (F) of the dihydrate-gypsum-rich adjusting liquid is extracted from the lower part and charged into the thickener b, (D).
1. By controlling the amounts of (F) and (F), the sedimentation rate of thickener b can be changed to enable separation and adjustment of magnesium hydroxide and gypsum dihydrate in the metathesis step, and 1. The multi-decomposition device comprises a stirring reaction tank a ′ and an adjusting tank c ′.
The stirring reaction tank a ′ has an inner cylinder whose lower end does not reach the bottom surface of the tank, and a stirrer for flowing the liquid in the inner cylinder downward, and magnesium sulfate or sulfuric acid and magnesium and sulfuric acid which have undergone the oxidation step. The aqueous solution containing the and the basic calcium liquid are charged in the upper part of the inner cylinder, and react with stirring to form a slurry of magnesium hydroxide and gypsum dihydrate, passing through the lower part of the inner cylinder and above the annular part between the inner and outer cylinders. The reaction slurry rises, and the dihydrate-gypsum-rich slurry (A ') is extracted from the bottom of the stirring reaction tank a'and transferred to the gypsum separating step, and the magnesium hydroxide-rich slurry is discharged from the upper part of the annular portion. (C ') is transferred to the adjusting tank c'and the adjusting tank c'
In the above, the slurry is further separated, the magnesium hydroxide-rich liquid (B ') is withdrawn from the upper part of the tank and returned to the desulfurization step, and the dihydrate-gypsum-rich adjusting liquid is withdrawn from the lower part of the tank. It is charged in the upper part of the inner cylinder of the stirring reaction tank a ', and the residence time of the slurry in the stirring reaction tank is adjusted by changing the liquid volume ratio of (A') and (C '), and magnesium hydroxide in the metathesis process is adjusted. It is a multi-decomposition device characterized by enabling separation and adjustment of gypsum and gypsum dihydrate.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION First, an outline of a desulfurization method, which is a technical field to which the present invention belongs, will be described with reference to the drawings (FIG. 3) (however, a device for a metathesis process is a conventional method). In the figure, reference numeral 1 denotes a desulfurization tower which is a main device of a desulfurization process, into which exhaust gas G1 is charged, and a magnesium-based desulfurizing agent (Mg (O
H) ₂ slurry) While the desulfurization liquid charged from the replenishment tank 10 flows down from the upper part of the tower like a shower by a pump, the sulfur oxides in the exhaust gas are absorbed and removed mainly in the form of magnesium sulfite and discharged from the upper part of the tower. .

The treatment liquid in the lower part of the desulfurization tower is sent to the oxidation tank 2 which is the main unit of the oxidation process and is oxidized by air, so that the magnesium sulfite in the treatment liquid becomes magnesium sulfate and sulfuric acid.

Next, the multi-decomposition tank 3 which is the main device of the multi-decomposition process
Then, calcium hydroxide is charged from the raw material tank 4 of calcium hydroxide, and a metathesis reaction described in detail later occurs to generate a slurry of magnesium hydroxide and gypsum dihydrate.

This slurry is then sent to a settling separator 8 which is the main unit of the gypsum separation process, and the magnesium hydroxide slurry separated here is returned to the desulfurization tower and reused as a desulfurizing agent to obtain gypsum dihydrate. Is discharged from the lower part, further separated by, for example, a centrifuge, and taken out of the system.

The metathesis reaction carried out in the metathesis step described above is a metathesis reaction in which magnesium hydroxide and quicklime are added to an aqueous solution of magnesium sulfate produced in the oxidation step to give metathesis into magnesium hydroxide and gypsum dihydrate. be able to.

MgSO ₄ + Ca (OH) ₂ + 2H ₂ O → Mg (OH) ₂ +
CaSO ₄ · 2H ₂ O Magnesium hydroxide produced in this step is returned to the desulfurization step and reused as a desulfurizing agent as described above,
On the other hand, gypsum dihydrate is separated in the gypsum separation process, taken out of the system, and used for another purpose. Therefore, it is desirable that the slurry of magnesium hydroxide and gypsum dihydrate produced in the metathesis process be efficiently concentrated and separated.

[0014] According to the conventional research, it is known that gypsum dihydrate has a crystal surface growing, and magnesium hydroxide grows by aggregating fine crystal particles in a colloidal form. Gypsum should be retained in the reaction system as long as possible to form large crystals, and should not be destroyed by stirring, transferring, etc., and magnesium hydroxide should be removed from the reaction system as soon as possible. I understand that it is good. The present invention is an apparatus that can realize this finding.
The multi-decomposition device of the present invention will be described below with reference to examples.

[0015]

【Example】

Embodiment 1 FIG. 1 shows a structural example of a multi-decomposition device of the present invention. In this structural example, the apparatus comprises a stirring reaction tank a equipped with a stirrer, a thickener b, and an adjusting tank c. In the figure, the stirring reaction tank a is equipped with an aqueous solution containing magnesium sulfate and sulfuric acid that has undergone an oxidation step (not shown), and a calcium hydroxide aqueous slurry of about 10 to 30 wt% from the calcium hydroxide raw material tank 4. Then, a return liquid from a thickener b, which will be described later, is also added, and while being stirred and mixed, a metathesis reaction between the magnesium sulfate and calcium hydroxide and a reaction between sulfuric acid and calcium hydroxide are carried out, and magnesium hydroxide and dihydrate are added. Produce solid particles of gypsum. The reaction temperature is 80 ° C. or lower, preferably room temperature to 60 ° C.

The amount of the calcium hydroxide aqueous slurry supplied is preferably adjusted so that the pH of the liquid in the stirring reaction tank a is about 10-12. Note that other basic calcium compounds such as quick lime (CaO) and calcium carbonate may be used instead of this calcium hydroxide.

The water slurry containing the two types of solid particles thus obtained is then partially extracted (D) from the lower part of the tank by the pump P2 and supplied to the thickener b, where the slurry is concentrated. The concentrated slurry is extracted from the bottom of the thickener b by the pump P3 and returned to the stirring reaction tank a. The supernatant of the thickener b is transferred to the adjusting tank c, and a part (F) of the liquid in the adjusting tank is returned to the thickener by the pump P4. Since the residence time of the liquid in the thickener b can be adjusted by adjusting the amounts of (D) and (F), the size and concentration of dihydrate gypsum crystals in the liquid extracted from the bottom of the thickener can be adjusted. Can be adjusted,
As a result, the crystal size and concentration of gypsum dihydrate in the liquid (A) transferred from the bottom of the stirring reaction tank to the gypsum separation step (not shown) by the pump P1 can be increased.

Further, since the liquid (B) is extracted from the upper part of the adjusting tank c and returned to the desulfurization step (not shown) via the pump P5, the content of dihydrate gypsum in the liquid (B) becomes extremely low. For example, in a conventional metathesis tank, the average particle size (major axis) generally grows into coarse particles having a particle size of 70 μm or more and usually 200 μm, while magnesium hydroxide has a particle size of 1 μm or less, usually 0.3 to 20 μm. Although it apparently aggregates to a size of about 10 to 20 μm, the average amount of gypsum dihydrate in the liquid (A) is 1 in this device.
It grows to coarse particles up to 00 μm, magnesium hydroxide is 15 μm. Embodiment 2 FIG. 2 shows another structural example of the multi-decomposition device of the present invention. In this structure, it comprises a stirring reaction tank a'and an adjusting tank c '. The adjusting tank c ′ is substantially the same as the adjusting tank of the first embodiment. In addition to the stirrer, the stirring reaction tank a ′ is provided with an inner cylinder 5 that does not reach the bottom of the tank, and the liquid supplied to the upper part of the inner cylinder is stirred and moved from the upper part of the inner cylinder to the lower part of the inner cylinder and the bottom of the tank. The structure is such that the annular portion between the inner and outer cylinders is lifted through the gap between and.

In the figure, an aqueous solution containing magnesium sulfate and sulfuric acid, which has been subjected to an oxidation process (not shown), is provided at the upper part of the inner cylinder of the stirring reaction tank a ', and 1 from the raw material tank 4 for calcium hydroxide.
About 0 to 30% by weight of calcium hydroxide aqueous slurry was charged, and the return liquid from the adjusting tank c ′ described later was also added. While moving in the above-described flow, the stirring reaction tank of Example 1 The same reaction as in a occurs. At this time, the amount of water slurry of calcium hydroxide supplied and the reaction conditions are the same as in Example 1.

In this agitation reaction tank a ', since the flow path of the reaction solution in the tank is long, the crystals of gypsum dihydrate formed grow and settle. Since the growth of gypsum progresses as the residence time in the stirring reaction tank becomes longer, it is preferable to make the residence time longer in this portion. From the upper part of the annular portion of the inner and outer cylinders of this reaction tank, a slurry (C ') containing a little gypsum dihydrate and rich in magnesium hydroxide is sent to the adjusting tank c'. In the adjusting tank c ′, the solid component in the slurry is further separated, and the liquid (B ′) containing almost no gypsum dihydrate and rich in magnesium hydroxide is withdrawn from the upper part of the adjusting tank c ′ and is not illustrated by the pump P5. Is returned to the desulfurization step as desulfurization liquid. From the lower part of the adjusting tank, a slurry containing almost no magnesium hydroxide and rich in gypsum dihydrate is returned to the upper part of the inner cylinder of the stirring reaction tank a'by a pump P4 '. Therefore, it is withdrawn by the pump P1 from the bottom of the stirring reaction tank a ′,
The liquid (A ') sent to the gypsum separation step (not shown) contains dihydrate gypsum containing almost no magnesium hydroxide but having large crystals and a high concentration.

This embodiment is characterized in that the residence time of the slurry in the stirring tank can be adjusted by changing the liquid volume ratio of (A ') and (C').

As a result, it is possible to sufficiently reduce the mixing ratio of the components (A ') and (B') other than the components of interest to 10% or less. As is clear from the above description, in the second embodiment, the function of the thickener b in the first embodiment is provided by providing the inner cylinder 5 in the stirring reaction tank a '. As an example, the processing liquid amount (L) in the stirred reaction tank is 50 t /
hr, residence time (θ) 2.5 hr, liquid rising speed (U
_t ) The capacity (L _v ) is 125 m under the condition of 1.5 m / hr.
It becomes ³ . Under these conditions, the diameter of the tank of Example 1 (a) was 5.
6m, tank height 6m (effective liquid height 5m), thickener diameter 6.5m, depth 2.5m, in Example 2 (a '), the tank height is the same and diameter 8.6m. In addition to eliminating the 6.5 mφ thickener,
The rake in the lower portion of the thickener, the crushing of gypsum dihydrate crystals by the pump P3, etc. are reduced, and the configuration of Example 1 can be said to be more preferable.

[0023]

By applying the metathesis apparatus of the present invention to a wet exhaust gas desulfurization method using a magnesium-based desulfurizing agent, the utilization rate of the desulfurizing agent can be increased and dihydrate gypsum with large crystals can be efficiently recovered.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an example of a configuration of a double decomposition apparatus of the present invention.

FIG. 2 is a schematic diagram showing another example of the configuration of the double decomposition apparatus of the present invention.

FIG. 3 is a schematic diagram illustrating steps of a desulfurization method to which a conventional metathesis device is applied.

[Explanation of symbols]

 1 Desulfurization tower 2 Oxidation tank 3 Double decomposition tank 4 Calcium hydroxide raw material tank 5 Inner cylinder 8 Sedimentation separator 10 Magnesium-based desulfurizing agent supply tank a, a'Double decomposition tank b Thickener c, c 'Adjustment tank

Claims (2)

[Claims] 1. The following main step of adsorbing and removing sulfur oxides contained in exhaust gas by continuously gas-liquid contacting exhaust gas containing sulfur oxides with a treatment liquid containing a magnesium-based desulfurizing agent (1) And a desulfurization treatment liquid are brought into contact with each other to absorb and remove the sulfur oxide (2) An oxidation process in which the desulfurization treatment liquid absorbing the sulfur oxide is converted into an aqueous solution of magnesium sulfate and sulfuric acid by air oxidation (3) A metathesis step of adding basic calcium to the aqueous solution of magnesium sulfate and sulfuric acid to form a slurry of magnesium hydroxide and gypsum dihydrate; and (4) a gypsum separation step of taking out the gypsum dihydrate produced in the metathesis step to the outside of the system. A metathesis device used in the metathesis step of the desulfurization method comprising the following: the device comprising a stirring reaction tank a, a thickener b and an adjusting tank c. The reaction vessel a is equipped with a stirrer, and magnesium sulfate or magnesium sulfate that has undergone the oxidation step is charged with an aqueous solution containing sulfuric acid and a basic calcium solution to react with each other.
Produces a slurry of magnesium hydroxide and gypsum dihydrate,
The slurry (A) rich in gypsum dihydrate is discharged and transferred to the gypsum separation step, and a part (D) of the produced slurry is charged into the thickener b, and in the thickener b, the produced slurry is concentrated. The concentrated slurry is withdrawn from the lower part and returned to the metathesis tank a, and the supernatant liquid is transferred to the adjusting tank c. In the adjusting tank c, the above supernatant liquid is further separated and a liquid rich in magnesium hydroxide. (B) is extracted from the upper part and returned to the desulfurization step, and a part (F) of the dihydrate-gypsum-rich adjusting liquid is extracted from the lower part and charged into the thickener b, and (D) and (F). ) Thickener b
The multi-decomposition apparatus characterized in that the settling speed of is changed to enable separation and adjustment of magnesium hydroxide and gypsum dihydrate in the multi-decomposition process. 2. The following main step of absorbing and removing the sulfur oxides contained in the exhaust gas by continuously bringing the exhaust gas containing the sulfur oxides into gas-liquid contact with a treatment liquid containing a magnesium-based desulfurizing agent (1) And a desulfurization treatment liquid are brought into contact with each other to absorb and remove the sulfur oxide (2) An oxidation process in which the desulfurization treatment liquid absorbing the sulfur oxide is converted into an aqueous solution of magnesium sulfate and sulfuric acid by air oxidation (3) A metathesis step of adding basic calcium to the aqueous solution of magnesium sulfate and sulfuric acid to form a slurry of magnesium hydroxide and gypsum dihydrate; and (4) a gypsum separation step of taking out the gypsum dihydrate produced in the metathesis step to the outside of the system. A metathesis apparatus for use in the metathesis step in the desulfurization method, which comprises an agitation reaction tank a ′ and an adjusting tank c ′, wherein the agitation reaction tank a ′ is , An inner cylinder whose lower end does not reach the bottom of the tank, and an agitator for causing the liquid in the inner cylinder to flow downward, and an aqueous solution containing magnesium sulfate or magnesium sulfate and sulfuric acid that has undergone the oxidation step and a basic calcium solution, It is charged into the upper part of the inner cylinder and reacts while stirring to form a slurry of magnesium hydroxide and gypsum dihydrate, and the reaction slurry rises above the annular portion between the inner and outer cylinders from the lower part of the inner cylinder, and the stirring reaction tank a ' Slurry rich in gypsum dihydrate (A ') from the bottom of
Is extracted and transferred to the gypsum separating step, and a slurry (C ') rich in magnesium hydroxide is introduced from the upper part of the annular portion.
Is transferred to the adjusting tank c ′, and in the adjusting tank c ′, the slurry is further separated, and the magnesium hydroxide-rich liquid (B ′) is extracted from the upper part of the tank and returned to the desulfurization step to obtain gypsum dihydrate. The rich adjustment liquid is withdrawn from the lower part of the tank and charged into the upper part of the inner cylinder of the stirring reaction tank a ', and the slurry in the stirring reaction tank is changed by changing the liquid volume ratio of (A') and (C '). A multi-decomposition apparatus characterized in that the separation time of magnesium hydroxide and gypsum dihydrate can be adjusted in the multi-decomposition process by adjusting the residence time of.