JPS61222524A - Wet waste gas desulfurization method - Google Patents

Abstract

PURPOSE:To obtain high grade gypsum, by mixing MgSO4-containing waste water and CaCl2-containing waste water to form and separate gypsum ad recoverying water containing Mg(OH)2 and CaCl2 from the separated solution to respectively reutilize Mg(OH)2 and CaCl2. CONSTITUTION:Magnesium sulfate-containing waste water 1 and calcium chloride-containing water 2 are mixed to be introduced into a reaction tank 3 to form gypsum which is, in turn, separated by a filter press while calcium sulfate 8 is added to the separated filtrate 7 to form Mg(OH)2 in a reaction tank 9. The reaction product 10 is separated into magnesium hydroxide 12 and a filtrate by a filter press 11 and Mg(OH)2 is used as the adsorbent in wet desulfurization while the filtrate is sent to a CaCl2-containing water tank 14 and a part 15 of said filtrate is sent to a reverse osmosis unit 16 to obtain conc. CaCl2-containing water 17 which is, in turn, returned to a tank 14 and a part of transmitted water 18 is used in the aqueous Mg(OH)2 slurry in a slurry tank 20.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention is a wet flue gas desulfurization method using magnesium hydroxide, in which magnesium hydroxide is continuously recovered from the waste liquid after treatment. The aim is to reuse the gypsum and produce high-quality gypsum as a by-product.The gypsum that is produced as a by-product is of high purity and has a wide range of uses.

(Prior Art) Conventionally, a wet flue gas desulfurization method using magnesium hydroxide is known, in which flue gas is brought into contact with a water slurry containing magnesium hydroxide to remove sulfur oxides mainly as magnesium sulfite. The reaction formula is as follows.

802 + Mg (0 average z-9MgSOa + H2
Moreover, since magnesium sulfite is dissolvable in water, there is a method of converting magnesium sulfite into highly soluble magnesium sulfate by blowing air into water containing magnesium sulfite and then discharging and collecting it.
It is stated in the No.

In these conventional methods, at least the same number of moles of magnesium hydroxide as the sulfur oxide to be removed is consumed, resulting in a large cost. Moreover, wastewater containing magnesium sulfate is discharged as it is into rivers or the sea, which is not an environmentally preferable method.

[Problem that the invention seeks to solve]

In the wet flue gas desulfurization method using magnesium hydroxide described above, the present invention eliminates external discharge of wastewater containing magnesium sulfate by recovering and reusing magnesium hydroxide from waste liquid containing magnesium sulfate, and further improves the quality of the waste gas. The purpose of the present invention is to provide a flue gas desulfurization method that is highly economical and environmentally preferable by producing gypsum as a by-product.

[Means for solving problems]

In the wet flue gas desulfurization method using magnesium hydroxide, the present invention further provides a method for converting sulfur oxides into magnesium sulfite by contacting the absorption waste liquid after oxidation treatment, that is, the flue gas and the absorption liquid containing magnesium hydroxide. Wet-type absorption liquid that is made by oxidizing the solution and converting it into magnesium sulfate is subjected to a multi-step process to recover magnesium hydroxide and reuse it as a flue gas absorption liquid, as well as producing gypsum as a by-product. Regarding improvement of flue gas desulfurization method.

The present invention consists of the following steps. That is, (1)
The wastewater containing magnesium sulfate discharged in the wet flue gas desulfurization method and the water containing calcium chloride are mixed and reacted to produce and separate gypsum (2) The separated liquid in the step (1) above contains calcium hydroxide, or Calcium oxide is added and reacted to produce magnesium hydroxide.

(3) Magnesium hydroxide, which is a solid content, is separated from the mixture produced in step (2) above by a solid-liquid separation operation, and the separated magnesium hydroxide is reused as an absorbent in the wet flue gas desulfurization method.

(4) After concentrating the separated liquid obtained in step (3) above using a reverse osmosis membrane, the concentrated calcium chloride-containing water is reused in step (1) above.

This is an improved method of wet flue gas desulfurization using magnesium hydroxide, which consists of a series of steps.

The concentration of magnesium sulfate in the magnesium sulfate-containing wastewater produced as a by-product from the conventional wet flue gas desulfurization method is usually 5 to 15% by weight.
The calcium chloride concentration of the calcium chloride-containing water obtained from step (4) to be reused in step (1) above is preferably 1 to 6 weight. Further, the concentration of calcium chloride is preferably set in consideration of the operability and economic efficiency of the concentration operation using the reverse osmosis membrane in step (4). The molar ratio of calcium chloride and magnesium sulfate is preferably in the range of 1.1 to 1.3. The reaction temperature is in the range from room temperature to the boiling point, and the reaction pressure is normal pressure, and each condition is set depending on the operability, economic efficiency, etc. when separating gypsum.

The solid-liquid separation operations in steps (1) and (3) may be carried out by ordinary unit operations such as filtration, centrifugation, etc., and preferred methods include methods using a filter press, screw den canter, etc. In the above step (2), it is preferable in terms of operation that calcium oxide or calcium hydroxide be added in the form of an aqueous slurry.

As the reverse osmosis membrane in step (4), a conventional reverse osmosis membrane having performance applicable to seawater desalination can be used. still,
When attempting to concentrate a water containing calcium chloride using an evaporation method with excellent thermal efficiency, such as multi-stage flash, the operating costs are several times higher and the required equipment increases compared to the powder method, making it undesirable. It can not be said.

(Example) Next, an example of an embodiment of the present invention will be described with reference to a flow sheet shown in the drawings.

Wastewater containing magnesium sulfate 1 and water containing calcium chloride 2
are mixed and introduced into reaction tank 3, and most of the sulfate groups are
Reacts with calcium chloride to form gypsum.

The reaction product 4 is separated into gypsum 6 and filtrate 7 by a filter press 5. Calcium oxide 8 is added to the filtrate 7 and introduced into the reaction tank 9. In reaction tank 9, most of the magnesium reacts with hydroxyl groups to become magnesium hydroxide.

The reaction product 1o is separated into magnesium hydroxide 12 and a filtrate by a filter press 11. The filtered magnesium hydroxide is added with a reverse osmosis membrane permeation liquid 19 in a slurry tank 20 to form an aqueous slurry, which is sent to the sulfur oxide absorption section through a pipe 21.

The filtrate is sent to a calcium chloride-containing water tank 14. A portion 16 of the tank contents is sent to the reverse osmosis membrane unit 16 and concentrated calcium chloride containing water 17 is returned to the tank 14. A portion of the permeated water 18 is added to the slurry tank 2.
o is used for magnesium hydroxide aqueous slurry, and the rest is discharged outside the system.

The reverse osmosis membrane unit comes with a pH adjustment device and a filter to protect the membrane. The concentration of the aqueous solution dissolved in the tank 14 is kept approximately constant by the operation of the reverse osmosis membrane unit.

Next, the effects obtained by implementing the present invention will be explained by giving examples for each step.

Example (11) Recovery of gypsum Magnesium sulfate-containing wastewater from a wet flue gas desulfurization device using magnesium hydroxide installed in a petroleum-based pitch fine powder co-firing boiler was filtered and diluted with water to produce magnesium sulfate-containing water with different concentrations. I got it.

500 d of magnesium sulfate-containing liquid with different concentrations,
Calcium chloride was added at a molar ratio of 1.2 to magnesium sulfate at room temperature while stirring, and the mixture was left for 5 minutes. The reaction mixture was then filtered through a glass filter. The filtered solid content was washed with water, the weight of the dried solid content was measured, and Ca in the solid content was determined by atomic absorption spectrometry.
Assuming that V2H20 has purity as gypsum and that an equimolar amount of magnesium sulfate becomes gypsum, the recovery rate was determined. The results are shown in Table 1.

Table 1 (2) Recovery of magnesium hydroxide The magnesium chloride-containing water after filtering off the gypsum as described above was diluted with water to obtain magnesium chloride-containing waters with different concentrations. Calcium hydroxide thriller was added to 500 WL of magnesium chloride-containing water at room temperature with stirring and left for 5 minutes. The reaction mixture was then filtered through a glass filter. After washing the filtered solid content with water, the weight of the solid content was measured, and the Mg in the solid content was analyzed by atomic absorption spectrometry and calculated as M g (OH) 2. The recovery rate of magnesium oxide was determined. The results are shown in Table 2.

Table 2 (3) Calcium chloride concentration discharged outside the system Among the filtrate after separating magnesium hydroxide as a solid content as described above, the filtrate with a high calcium chloride content was diluted with water to determine the concentration. Electrified calcium-containing water was obtained. Commercially available reverse osmosis membrane elements (manufactured by Toshi Co., Ltd.).

A concentration test of calcium chloride-containing water was conducted using 5P-120, 201 Dragon 9b x 1016 inches). While adjusting the flow rate of the aqueous solution supplied by one reverse osmosis membrane element constant during the positive pressure test, the amount of permeated liquid and the concentration of calcium chloride in the permeated liquid and concentrated liquid were measured. The results were as shown in Table 3.
1z is 99.8% by weight, and the C recovered in the concentrate is
The aCl2 could be recycled for gypsum recovery.

〔Effect of the invention〕

From the results of (1) in Example, the yield of gypsum is 99% of the theoretical value.
% or more, and the purity of the obtained gypsum is at least 97%.
~98 weight stroke or higher. If it can be said that there is a surplus of free water in the gypsum after drying, then the by-product 2 gypsum is considered to be of high purity.

From the results in Example (2), it is presumed that it is easy to maintain the recovery rate of magnesium hydroxide at 95% or higher. In addition, the amount of magnesium hydroxide used is at least 9
From the results in Example (3), it is easy to maintain the calcium chloride concentration in the permeated water to about several hundred ppM by weight, and the concentration of calcium chloride in the permeated water can be reduced by about 5%. It can be said that the amount of salt inside is minute. It is also understood that concentration using a reverse osmosis membrane can be easily performed and the required membrane area does not become large. Moreover, it can be said that the loss of calcium chloride is extremely small.

As described above, according to the method of the present invention, magnesium hydroxide and calcium chloride are recovered from the treatment solution and recycled back into the system, and high-quality gypsum is produced as a by-product, thereby improving economic efficiency and improving the system efficiency. Since the wastewater discharged outside contains only a trace amount of calcium chloride, this method does not cause any environmental pollution problems and is an industrially excellent method.

[Brief explanation of the drawing]

The drawing is a flow sheet showing an example of implementing the method of the present invention. 1...Wastewater containing magnesium sulfate. 2... Calcium chloride-containing water. 3...Reaction tank, 6...Gypsum. 8... Calcium oxide, 9... Reaction tank. 12...Magnesium hydroxide. 16... Reverse osmosis membrane unit. 2o...One tank of slurry.

Claims (4)

[Claims] (1) In the wet flue gas desulfurization method using magnesium hydroxide, the discharged wastewater containing magnesium sulfate and water containing calcium chloride are mixed and reacted, and the generated gypsum is separated, (2) adding calcium hydroxide or calcium oxide to the separated liquid in the step (1) above and reacting it to produce magnesium hydroxide; (3) Separating magnesium hydroxide from the liquid produced in step (2) above by solid-liquid separation operation and reusing it as an absorbent in the wet flue gas desulfurization process; (4) After concentrating the separated liquid obtained in step (3) above using a reverse osmosis membrane, the concentrated calcium chloride-containing water is used as described in step (1) above.
A wet flue gas desulfurization method that involves reuse in the process.