JPS60125230A - Wet desulfurization process for absorptive removal of sulfurous acid gas in exhaust gas - Google Patents

Abstract

PURPOSE:To reduce the discharge amount of a discharged aqueous MgSO4 solution, by holding the pH value of the recirculation solution containing an Mg(OH)2 absorbent of an absorbing tower to a low value and depositing, separating and recovering MgSO4 from a waste absorbing solution. CONSTITUTION:Exhaust gas is guided to an absorbing tower 1 and brought into contact with a recirculation solution of which the pH is adjusted to 4.0-6.5 by an Mg(OH)2 slurry to absorb and remove SO2. A part of the recirculation solution is guided to a neutralization/crystallization tank 2 and Mg(OH)2 is added to said tank 2 from a pipe 14 to convert Mg(HSO3)2 in the solution to MgSO3. The deposited MgSO3 is guided to a solid-luquid separator 3, and separated and recovered. The filtrate is guided to an oxidizing tower 4 through a filtrate tank 16 and MgSO3 in the solution is oxidized to MgSO4 by air to be discharge out of the system as an aqueous MgSO4 solution from a pipe 19.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a wet desulfurization method for absorbing and removing sulfur dioxide gas from exhaust gas by bringing exhaust gas containing sulfite residue into gas-liquid contact with magnesium hydroxide suspension.

As a means of removing sulfur dioxide gas contained in exhaust gas discharged from boilers, etc. or other chemical factories, the exhaust gas is led to an absorption tower and an aqueous solution containing an alkaline substance is used as an absorption liquid and brought into gas-liquid contact with the exhaust gas. Many wet desulfurization methods for absorbing and removing bent sulfuric acid gas from exhaust gas have been proposed and implemented.

One promising method is to suspend magnesium hydroxide in water and use it as an absorbent. That is, it is a wet desulfurization method in which the flue gas and magnesium hydroxide suspended water are brought into gas-liquid contact in an absorption tower to absorb and remove bent sulfuric acid gas in the flue gas as a magnesium compound. By continuously injecting magnesium hydroxide suspended water into the absorption tower, the exhaust gas and absorption liquid are brought into gas-liquid contact while maintaining the pH of the absorption tower circulation liquid (absorption system) at 6.0 to 7.0, and the The sulfur dioxide contained in the sulfur dioxide is absorbed and removed, and the purified exhaust gas is discharged to the outside of the system. At the same time, the magnesium sulfite produced by the absorption reaction and the magnesium sulfite produced at the same time remove the oxygen contained in the exhaust gas. A part of the absorption liquid (circulating liquid) containing magnesium sulfate produced by oxidation is extracted, and magnesium hydroxide is added exclusively to the oxidizer to convert the contained magnesium bisulfite into magnesium sulfite. It oxidizes through the process and is discharged and discharged from the system as a harmless aqueous magnesium sulfate solution.

However, although magnesium sulfate itself is harmless, the amount of magnesium sulfate emitted during the process of removing sulfur dioxide gas from exhaust gas is enormous, and large amounts of these aqueous magnesium sulfate solutions are discharged into lakes, ponds, etc. This causes the eutrophication of water and water, so there is a recent trend in some regions to gradually reduce their emissions.

The present invention has been made in view of the current situation. In explaining the present invention, firstly, the reaction mechanism of the wet desulfurization method using suspension of -74gnesium hydroxide as an absorption liquid will be explained. The absorption reaction products are magnesium sulfite and magnesium bisulfite as described above, and the absorption reaction mechanism is shown by the following formula.

Mg (OH) x +802→M9S08-)-H,
0M9SOs-1-8Ch+HtO→My(H8Os)
! My(H5Os)20MIF(OH)g→2Mg50
g+2HzO At the same time, as a side reaction, some of the magnesium sulfite in the circulating fluid is oxidized by oxygen contained in the exhaust gas, producing magnesium sulfate. The oxidation reaction mechanism is shown by the following equation.

Mg505-1-70g÷Mri SO.

As a result of various studies, we found that the production ratio of magnesium sulfite and magnesium bisulfite in the above absorption reaction differs depending on the pH of the circulating fluid. Focusing on the fact that the production ratio of magnesium bisulfite increases as the pH decreases, and that magnesium sulfite is sparingly soluble in water, the pH of the circulating fluid was set to 4.0.
~ 6.5 and brought into gas-liquid contact with the exhaust gas to absorb and remove the sulfur dioxide gas in the exhaust gas, and also to absorb and remove sulfur dioxide from the circulating fluid extracted from the absorption tower. The present invention has been completed by successfully crystallizing and recovering sulfur dioxide as magnesium sulfite.The present invention is a wet desulfurization method in which sulfur dioxide gas in exhaust gas is absorbed and removed using magnesium hydroxide suspension water as a soap collecting agent. In,
While maintaining the pH of the circulating liquid of the absorption tower at 4.0 to 6.5, the extracted absorption tower effluent is led to a crystallization tank, and magnesium hydroxide is added to precipitate magnesium sulfite. It is characterized by separating solid and liquid, leading the liquid to an oxidation tower, and passing air through it to form an aqueous solution of magnesium sulfate.

Hereinafter, the present invention will be explained with reference to the drawings.

In the figure, (1) is an absorption tower (packed tower) with a built-in mist catcher (not shown), but it is not limited to a packed tower and may also be used for other types of absorption towers such as a spray tower or a plate tower. good. (2) shows a neutralization crystallization tank with a stirrer, (3) shows a solid-liquid separator, and (4) shows an oxidation tower.

According to the method of the present invention, the exhaust gas i containing sulfur dioxide introduced from the pipe (5) rises in the absorption tower α) and passes through the pipe (6).
It comes into countercurrent gas-liquid contact with the circulating liquid that is dispersed from the liquid disperser (7) and flows down the packed bed (8), and the contained sulfur dioxide gas is reacted and absorbed, and the purified exhaust gas is sent to the exhaust pipe (9). It is discharged outside. During this period, the pH of the circulating fluid was increased to 4.0 by the magnesium hydroxide suspension introduced through the tube (1G).
Adjusted to ~6.5. When the pH of the circulating fluid exceeds 6.5, the amount of magnesium sulfite precipitated in the neutralization crystallization tank described later decreases, and when it falls below 4.0, the absorption reaction rate of sulfur dioxide gas decreases, which is disadvantageous. Preferably P H4,
It is preferable to set it to 5 to 6.0λ.

On the other hand, the circulating liquid that has been dispersed from the liquid distributor (7) and flowed down the packed bed (8) is then extracted by the circulation pump (2) via the pipe αη. A portion of the circulating fluid that has been extracted is distributed into the tower from the diffuser (7) through the pipe (6) as described above,
The remainder is introduced into a neutralization crystallization tank (2) equipped with a stirrer via a tube. Here, the magnesium bisulfite in the liquid is neutralized by the magnesium hydroxide introduced through the tube (b) and converted to magnesium sulfite, and the magnesium sulfite that exceeds the saturation concentration precipitates as crystals. Next, the liquid containing the precipitated magnesium sulfite crystals is led to the solid-liquid separator (third stage) by a pump (J! The magnesium sulfite content in the liquid is oxidized to magnesium sulfate by the air introduced through the pipe (to), which is then discharged from the pipe through the pipe α as an aqueous magnesium sulfate solution. be done.

(a) shows the solid matter outlet.

The oxidation tower used in the present invention may be of any type, such as a knob type or a perforated plate type.

Next, the advantages of the present invention over the conventional method will be described.

In the conventional method, in order to absorb and remove sulfur dioxide gas in the exhaust gas by adjusting the pH of the circulating liquid of the absorption tower to 6.0 to 7.0, the molar ratio of magnesium sulfite to the total sulfite in the circulating liquid (hereinafter referred to as ratio) becomes thicker. For example, when the pH of the circulating fluid is maintained at 6.5 or higher, the ratio of magnesium sulfite becomes 80% or higher. In addition, since magnesium sulfite is poorly soluble, there is a risk that it will precipitate and adhere to equipment and form scales, so it must be operated at a low total sulfite concentration of 0. Therefore, since the total sulfite concentration contained in the circulating liquid discharged from the absorption tower is low, a pretreatment step for oxidizing it to magnesium sulfate (adding magnesium hydroxide to convert magnesium bisulfite to magnesium sulfite) Also, the magnesium sulfite is not precipitated, so the entire amount is discharged as magnesium sulfate to the absorption tower circulating liquid without recovering the sulfites.

In order to absorb and remove flowing acid gas, the ratio of magnesium sulfite in the circulating fluid decreases, and the ratio of magnesium bisulfite increases to 4. For example, if the PI of the circulating fluid is maintained at 5.5, the ratio of magnesium sulfite decreases to 20-50%, and the ratio of magnesium bisulfite increases to 50-80%.Furthermore, MJh magnesium sulfate is easily soluble; ``Since there is little risk of precipitation, adhesion to equipment, and scale formation, the concentration of total sulfate in the circulating fluid can be increased.Therefore, the concentration of total sulfite contained in the circulating fluid discharged from the absorption tower can be increased. When the salt concentration increases, the effluent from the absorption tower is sent to the neutralization crystallization tank installed before the oxidation tower, and magnesium hydroxide is added to convert the contained magnesium bisulfite into 41!magnesium sulfate. Since magnesium sulfite is water-soluble, magnesium sulfite that exceeds the saturation concentration precipitates as crystals.

The precipitated magnesium sulfite is separated and collected, and the filtrate is oxidized and discharged as an aqueous magnesium sulfate solution.

Therefore, according to the method of the present invention, compared to the conventional method, the amount of magnesium sulfate discharged can be reduced by the equivalent amount of separated and recovered magnesium sulfite.

Example A test was conducted using an apparatus configured as shown in FIG. 1, in which boiler exhaust gas containing 950 PPm of SOg was introduced into the absorption tower (1) at a rate of 600 Nm''/Hr. The contained SO2 gas was absorbed and removed.

During the test, the pH of the absorption tower circulating liquid was adjusted to 5.5.

The liquid temperature was 50°C, and the composition of the liquid was as follows.

Composition Total 50g--31,569/1My
S O4...15.809# The circulating fluid is extracted at a ratio of 38.11/HT and sent to the neutralization crystallization tank (
2), Mg (OH)! When adding 110 wt% slurry and adjusting the pH to 7.0, a large amount of M g
Crystals of SOs.3H20 were precipitated. The residence time in this neutralization crystallization tank was 1.0 hours, and the suspension containing the crystals was then sent to the separator (3) for solid-liquid separation, resulting in 1.3 hours.
Solids were collected at a rate of 36 kg/Hr. The composition of this solid was as follows.

Composition Mg501・3H20...1.136#Adhesive liquid・
...0.204 The p liquid is sent to the oxidation tower (4) and oxidized through air to obtain M.

It was discharged as a 804 aqueous solution.

Note that the above operations were performed continuously.

42wt% of MgS04 to be flowed was replaced with Mg5O11.
It was separated and recovered as

[Brief explanation of the drawing]

FIG. 1 shows a flow sheet of one embodiment of the apparatus used in the present invention. 1... Absorption tower (packed tower) 2... Neutralization crystallization tank 3...
・Separator 4... Oxidation tower

Claims (1)

[Claims] In a wet desulfurization method in which sulfur dioxide gas in exhaust gas is absorbed and removed using magnesium hydroxide as an absorbent, the pH of the circulating fluid in the absorption tower is maintained at 4.0 to 6.5, and the extracted absorption tower effluent is A crystallization tank Cζ is introduced, magnesium hydroxide is added to precipitate magnesium sulfite, solid-liquid separation is performed, and the liquid is oxidized to form an aqueous magnesium sulfate solution. A wet desulfurization method that uses absorption and removal.