JPS60125231A - Wet desulfurization process for absorptive removal of sulfurous acid gas in exhaust gas - Google Patents

Abstract

PURPOSE:To bring the titled desulfurization system to a closed system not discharging a MgSO4 solution, by adding a chelate agent to the recirculation solution containing Mg(OH)2 as absorbent of an absorbing tower and keeping the pH thereof low while 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 and to which a chelate agent such as EDTA is added from a pipe 9, to absorb and remove SO2. A part of the recirculation solution is guided to a neutralization/crystallization tank 2 while Mg(OH)2 is added thereto from a pipe 14 to convert Mg(HSO3)2 in the solution to Mg(SO3)2. The deposited MgSO3 crystal is guided to a solid-liquid separator 3 and separated and recovered. The filtrate is refluxed into the absorbing tower 1 through a filtrate tank 16 and a pipe 18.

Description

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

As a means of removing bent sulfuric acid gas contained in flue gas discharged from boilers, etc. or other chemical factories, the flue gas is led to an absorption tower and brought into gas-liquid contact with the flue gas using an aqueous solution containing an alkaline substance as an absorption liquid. A wet desulfurization method that absorbs and removes sulfur dioxide gas from exhaust gas has been implemented.

One of these methods is to suspend magnesium hydroxide in water and use it as an absorbent. That is, it is a wet desulfurization method in which sulfur dioxide gas in the exhaust gas is absorbed and removed as a magnesium compound by bringing the exhaust gas and magnesium hydroxide suspension into gas-liquid contact in an absorption tower, and the conventional method for implementing this method is By continuously injecting magnesium hydroxide suspension water into the absorption tower, the pH of the absorption tower circulation liquid (absorption liquid) is maintained at 6.0 to 7.0, and the exhaust gas and absorption liquid are brought into gas-liquid contact. The sulfur dioxide contained in the sulfur dioxide is absorbed and removed, and the purified exhaust gas is discharged outside the system. At the same time, the magnesium sulfite and bisulfite produced by the absorption reaction, as well as the simultaneously produced magnesium sulfite, are used to remove the oxygen contained in the exhaust gas. A part of the absorption liquid (circulating liquid) containing magnesium sulfate produced by monoxidation is extracted and led to an oxidizer, where magnesium hydroxide is added 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 magnesium sulfate discharged during the process of removing sulfur dioxide gas from waste gas is of great significance.Large amounts of these aqueous magnesium sulfate solutions are discharged into lakes, ponds, etc. As this causes water eutrophication, there is a recent trend in some regions to gradually regulate their emissions.

The present invention has been made in view of the current situation, and according to the present invention, unlike conventional methods, sulfite scum in exhaust gas can be absorbed and removed in a so-called closed system without discharging a magnesium sulfate solution.

In explaining the present invention, first the reaction mechanism of the wet desulfurization method using magnesium hydroxide suspension water as the absorption liquid will be explained. As mentioned above, the absorption reaction product in the absorption tower is composed of magnesium sulfite and magnesium bisulfite. The absorption reaction mechanism is shown by the following equation.

My (OH)g-1-8Ox→Mg 50s 10Hz
OMg Son -(-8ow 1HzO→Mg (H
8Os)iMg (H8Os)g-1-Mg(OH)
2->2Mg5Os+2HzO At the same time, part of the magnesium sulfite in the circulating fluid is oxidized by oxygen contained in the exhaust gas as a side reaction, producing magnesium sulfate. The oxidation reaction mechanism is shown by the following equation.

My SOa +10t → Mg SO4 As a result of various studies, the inventors found that the production ratio of magnesium sulfite and magnesium bisulfite in the above absorption reaction was determined by the P of the circulating fluid.
The difference is that the production ratio of magnesium bisulfite with high pH is large, and as the pH decreases, the production ratio of magnesium bisulfite increases, and that sulfite→gnesium is poorly soluble in water. In contrast, we focused on the fact that magnesium bisulfite is relatively easily soluble, and furthermore, the above oxidation reaction is effective against vanadium, nickel, cobalt, etc. contained in boiler exhaust gas, etc., or calcium, iron, etc. contained as impurities in magnesium hydroxide. It was discovered that heavy metals act as oxidation catalysts, and that the oxidation reaction can be significantly suppressed by adding a chelating agent.The chelating agent was added to the circulating liquid of the absorption tower, and the pH of the liquid was adjusted to 4.0 to 6. 5 and brought into gas-liquid contact with the exhaust gas to absorb and remove sulfur dioxide gas in the exhaust gas, and add magnesium hydroxide to the circulating liquid discharged from the absorption tower to convert the contained magnesium bisulfite into magnesium sulfite. After separating the precipitated magnesium sulfite crystals in a furnace, they succeeded in refluxing the p liquid to an absorption tower and reusing it, completing the present invention.

Hereinafter, the present invention will be explained based on the drawings.

In the figure, (1) is an absorption tower (packed tower) incorporating a mist catcher (not shown), and (3) in particular shows a solid-liquid separator in the packed tower.

According to the method of the present invention, the sulfur dioxide introduced from the pipe (4) comes into countercurrent contact with the circulating fluid, the contained sulfur dioxide gas is reacted and absorbed, and the purified exhaust gas is sent to the system from the exhaust stack (81). During this time, the chelating agent solution is added to the circulating fluid from the pipe (9), and the pH of the circulating fluid is changed from the pipe (9) to the circulating fluid.
46 by the magnesium hydroxide suspension introduced through the
Adjusted to 0-6.5. If the pH of the circulating fluid exceeds 6.5, the amount of magnesium sulfite precipitated in the neutralization crystallization tank described later will decrease, and if it falls below 4.0, the absorption reaction rate of sulfur dioxide gas will decrease, which is disadvantageous. Preferably PH4.5
It is preferable to keep it at ~6.0.

As the chelating agent to be added, ethylenediaminetetraacetic acid (hereinafter abbreviated as EDTA), diethylenetriaminepentaacetic acid, nitrilotriacetic acid, or the like can be used.

The amount added varies depending on the components of the exhaust gas and the impurities contained in magnesium hydroxide. When using EDTA as a chelating agent, it is usually sufficient to add it so that the concentration in the circulating fluid is 20 to 2000 ppm. ,
The concentration of the chelating agent in the circulating fluid is not particularly limited, and the concentration may be higher than that.

On the other hand, the circulating liquid that has been dispersed from the liquid disperser (6) and flows down the packed bed (7) then passes through the pipe C1,) to the circulation pump (6).
), and a part of the withdrawn circulating liquid passes through the pipe (5) and is dispersed into the column from the disperser (6) as described above, and the remainder passes through the pipe Q3 to the neutralization crystallization tank (2) with a stirrer. will be introduced in Here, the hydroxide (magnesium) introduced through the tube is neutralized, and the magnesium bisulfite in the liquid is converted to magnesium sulfite, and the magnesium sulfite that exceeds the saturation concentration precipitates as crystals.Then, the precipitated sulfite The liquid containing magnesium crystals is guided to the solid-liquid separator (3) by pump a0.

Here, the magnesium sulfite crystals contained in the liquid are separated and recovered, and the p liquid passes through the furnace liquid tank aQ and is refluxed into the absorption tower via the pipe (g) by the pump @. Note that Q* indicates a solid matter outlet.

The present invention is constructed as described above, and the advantages of the present invention over conventional methods will be described next.

As mentioned above, in the wet desulfurization method that uses magnesium hydroxide as an absorbent to absorb and remove sulfur dioxide gas from exhaust gas, the circulating liquid of the absorption tower contains magnesium sulfite produced by the absorption reaction and magnesium bisulfite (such as sulfur dioxide). The salt production ratio is controlled by the pH of the liquid), and magnesium sulfate produced by the oxidation reaction (the production rate is controlled by heavy metal ions in the liquid) are mixed.

However, in the conventional method, the pH of the circulating liquid in the absorption tower is set to 6.0 to 7.
．． Since the sulfur dioxide gas in the exhaust gas is absorbed and removed by adjusting the temperature to 0, the molar ratio (hereinafter referred to as ratio) of magnesium sulfite to the total sulfite in the circulating fluid becomes large. For example, if the pH of the circulating fluid is maintained at 6.5 or higher, the proportion of magnesium fluorosulfate will be 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. Therefore, since the total sulfite concentration contained in the circulating liquid discharged from the absorption tower is low, even in the pretreatment process for oxidizing it to magnesium sulfate (adding magnesium hydroxide to convert magnesium bisulfite to magnesium sulfite), fluorine Since yagnesium sulfate does not precipitate, sulfites are not recovered (the entire amount of the absorption tower effluent is discharged as magnesium sulfate.

In contrast, in the method of the present invention, the pH of the circulating liquid in the absorption tower is adjusted to 4.0 to 6.5 to absorb and remove sulfite residue in the exhaust gas, so the ratio of magnesium sulfite in the circulating liquid is reduced. The proportion of magnesium bisulfite increases. For example, when the pH of the circulating fluid is maintained at 5.5, the proportion of magnesium sulfite decreases to 20-50%, and the proportion of magnesium bisulfite increases to 50-80%. Furthermore, since magnesium bisulfite is easily soluble and there is little risk of it precipitating and adhering to equipment or forming scales, it is possible to increase the total concentration of sulfites in the circulating fluid. 'Therefore, the concentration of total sulfites contained in the circulating liquid discharged from the absorption tower increases, and the discharged liquid from the absorption tower is sent to the Shubin Temple M Appanxia → Kui medium phase crystallization tank, where magnesium hydroxide is extracted. Contains additives
If you convert magnesium bisulfite into magnesium sulfite,! ! II! Since magnesium sulfate is poorly soluble, magnesium sulfite that exceeds the saturation concentration precipitates as crystals. The precipitated magnesium sulfite is separated and recovered, and the furnace liquid (saturated magnesium sulfite solution) is refluxed to the absorption tower.

In the method of the present invention, unlike the conventional method, since a chelating agent is added to the circulating liquid, the oxidation rate in the absorption tower is significantly suppressed, and the amount of magnesium sulfate produced is extremely small. Therefore, there is no problem in reusing this filtrate by refluxing it to the absorption tower. Of course, by circulating this furnace fluid, the concentration of magnesium sulfite in the circulating fluid increases, but on the other hand, when the magnesium sulfite crystals are separated, magnesium sulfate is entrained and separated as a deposited liquid and taken out of the system. , magnesium sulfate in the circulating fluid has no effect on the absorption effect at a constant concentration and the equilibrium book.

Therefore, according to the method of the present invention, unlike the conventional method, sulfite scum in exhaust gas can be absorbed and removed in a so-called closed system without discharging an aqueous magnesium sulfate solution.

In carrying out the present invention, it goes without saying that a very small amount of furnace liquid may be discharged depending on the case.

Example A test was conducted using an apparatus configured as shown in Fig. 1, in which boiler exhaust gas containing 801,950 ppm was introduced into the absorption tower (1) at a rate of 60 ONml/Hr. Soz gas was absorbed and removed.

During the test, 2 N m of EDTA was added to reduce the E of the circulating fluid.
The concentration of DTA・2Nm is 2000PPm1PH is 5.5
Adjusted to °. After about 20 hours of operation at a liquid temperature of 50°C, the composition of the circulating liquid reached equilibrium. Its composition was as follows.

Composition Total SOs -31,569/lMgS
O4...98.8971 The circulating fluid is extracted at a rate of 48.31/Hr and sent to the neutralization crystallization tank (
21 and added a 210wt% Mg(OH) slurry to adjust the pH to 7.0, a large amount of Mg SO
Crystals of s.3HxO were precipitated. The residence time in this neutralization crystallization tank was 1.0 hours, and the liquid containing the crystals was then sent to the separator (3) for solid-liquid separation, resulting in the separation of solids at a rate of 8.139 #/Hr. Recovered.

The composition of this solid material is as follows.

Composition Mg5Os・3H20...2.669# Adhering liquid...047# The filtrate was refluxed to the absorption tower and reused. Note that the above operations were performed continuously.

As a result, the method of the present invention is different from the conventional method, and M
The fluorosulfuric acid gas in the exhaust gas could be removed without discharging wastewater containing gS04.

[Brief explanation of the drawing]

FIG. 1 shows a flow sheet of one embodiment of the apparatus used in the present invention. l...Absorption tower (packed tower)-2...Neutralization crystallization tank 3...
・Solid-liquid separator

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, a chelating agent is added to the circulating fluid of the absorption tower, and the pH of the circulating fluid is adjusted to 4.
．． 0 to 6.5 and bring it into gas-liquid contact with the exhaust gas to absorb and remove sulfur dioxide gas in the exhaust gas, and add magnesium hydroxide to the circulating fluid discharged from the absorption tower to convert the contained magnesium bisulfite into sulfite. A wet desulfurization method for absorbing and removing sulfite scum in exhaust gas, which is characterized by precipitating it as magnesium, followed by solid-liquid separation, and refluxing the p-liquid to an absorption tower.