JPH06134471A - Method for removing fluorine from waste water containing fluorine - Google Patents

Abstract

PURPOSE:To realize the removal of fluorine from drained water containing fluorine without consequent generation of a large amount of sludge by adding hydrochloric acid to drained water containing fluorine to make it acid based on hydrochloric acid, then adding calcium salt and ferric chloride, and adding an alkali agent and a polymer coagulant sequentially. CONSTITUTION:When removing fluorine from a circulating water (fluorine concentration: 45mg/liter) through a dust collector for a converter gas, as a water to be treated, hydrochloric acid is added to the water in a reaction tank 1 to adjust the pH value. At the same time, calcium chloride and ferric chloride are added to the water and it is left held as it is for about 5 minutes with the concurrent stirring action of the water. Next, the water to be treated is sent to a neutralization tank 2 where calcium hydroxide is added as an alkali agent to adjust the pH value (neutralization). After that, the water is sent to a coagulation tank 3 where a polymer coagulant is added form a flock, then is sent to a sedimentation pond 4 to separated the flock by sedimentation. Consequently, the water to be treated whose fluorine concentration of treated water is reduced to 8mg/liter after separation by sedimentation is obtained and this water to be treated is discharged.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for removing fluorine from dust-collecting wastewater of a converter, or fluorine-containing wastewater generated during hydrofluoric acid treatment of the surface of metal or ceramic.

[0002]

2. Description of the Related Art A conventional method for removing fluorine from fluorine-containing wastewater is to add sulfuric acid to the wastewater to acidify it with sulfuric acid, then add calcium hydroxide to form calcium fluoride, and then add aluminum sulfate or polyaluminum chloride. The coagulation and precipitation method in which the inorganic coagulant is added has become mainstream.

That is, the amount of dissolved calcium hydroxide, that is, the concentration of calcium ions, is increased under the condition of sulfuric acid acidity to promote the formation of calcium fluoride. However, in this method, a part of the dissolved calcium ion reacts with the sulfate ion to form calcium sulfate, and the concentration of calcium ion is reduced by that amount, and the formation of calcium fluoride is slowed down.

Of course, there was an effect of coprecipitating calcium fluoride when calcium sulfate was formed, but in this method, the fluorine concentration of treated water was 10 mg / l.
It was difficult to reduce below. At the same time, there was a problem that the amount of sludge generated increased by the amount of calcium sulfate formed. Furthermore, since an aluminum-based inorganic coagulant is used, the amount of sludge generated is very large, and its treatment has become a major problem.

[0005]

The present invention has been made in view of the current state of the art as described above, and it is possible to reduce the fluorine concentration of treated water to 10 mg / l or less, and The object of the present invention is to provide a method for removing fluorine from a waste water containing fluorine, which generates a small amount of fluorine.

[0006]

The present invention comprises the steps of adding hydrochloric acid to a fluorine-containing wastewater to acidify the hydrochloric acid, then adding a calcium salt and ferric chloride, and neutralizing with an alkaline agent. And a step of performing a coagulation-precipitation treatment with a polymer coagulant.

[0007]

The method of the present invention is characterized in that it is carried out with hydrochloric acid instead of sulfuric acid. Therefore, calcium sulfate is not formed, and there is no decrease in the concentration of calcium ions and no increase in the amount of sludge generated due to the formation of calcium sulfate. The calcium salt added at the same time is added for the formation of calcium fluoride, and calcium chloride is particularly preferably used. However, since it is added under acidic conditions, a mixture of calcium oxide can be used.

At this time, the rate of formation of calcium fluoride is very slow, so 5
A residence time of 10 minutes is recommended. Further, since ferric chloride is added at the same time, iron ions are precipitated in the form of hydroxides in the subsequent neutralization step, which serves as nuclei to improve floc aggregation. Further, a compound of iron and fluorine is also formed, but since this compound has a low solubility in water, it effectively acts to improve the fluorine removal rate.

That is, ferric chloride functions as a coagulant aid and directly as a fluorine scavenger.
Calcium hydroxide is suitable as the alkaline agent as the neutralizing agent. As the coagulant, a high molecular coagulant is used instead of an aluminum-based inorganic coagulant from the viewpoint of sludge generation amount. As the polymer flocculant, a copolymer of acrylamide and sodium acrylate, a polymer of acrylamide, and the like can be considered, but the former is preferable.

By treating according to the above steps, flocs with good cohesiveness are formed, the fluorine concentration of treated water is reduced to 10 mg / l or less, and at the same time, the amount of sludge generated is minimized. You can

[0011]

EXAMPLE FIG. 1 is a flow chart of an example of the present invention, 1 is a reaction tank, 2 is a neutralization tank, 3 is a coagulation tank, and 4 is a sedimentation tank.
In the example, according to the flow chart of FIG. 1, circulating water (fluorine concentration: 45 mg / l) of the converter gas dust collector was used as the water to be treated.

That is, hydrochloric acid was added to the water to be treated in the reaction tank 1 to adjust the pH to 2. At the same time, calcium chloride was added as a calcium salt so as to have a concentration of 100 mg / l, and ferric chloride was further added so as to have a concentration of 100 mg / l, and the mixture was retained for 5 minutes while stirring. As the hydrochloric acid used here, if waste hydrochloric acid generated in the surface treatment process (descaling, etc.) of the steel sheet is used, ferric chloride is already contained in the waste hydrochloric acid, so that the present invention can be carried out more effectively. can do.

Then, the water to be treated was sent to the neutralization tank 2, and calcium hydroxide was added as an alkaline agent to adjust the pH to 7. In the neutralization tank, it was retained for 10 minutes while stirring. Next, the water to be treated was sent to the flocculation tank 3, a polymer flocculant was added to form flocs, and the flocs were sent to the settling tank 4 to separate the flocs by sedimentation. As the polymer flocculant, a copolymer of acrylamide and sodium acrylate (trade name: Sampori AS2
0, manufactured by Sankyo Kasei Co., Ltd.) was used. Floc sedimentation is very good and the concentration of fluorine in treated water after sedimentation is 8 mg
It was / l.

The solubility of calcium fluoride in water is 1
Since this is 6 mg / l or about 8 mg / l in terms of fluorine, this value is considered to be the theoretical limit value of fluorine removal. Next, as a comparative experiment, using the same water to be treated as in the example, a conventional method of adding sulfuric acid to acidify sulfuric acid and adding calcium hydroxide. An improved method of the conventional method in which hydrochloric acid is added to make the hydrochloric acid acidic and calcium hydroxide is added. A comparative experiment with the example was performed for the two methods. At this time, the pH conditions, the amount of calcium hydroxide added, and the residence time in each tank were the same. The fluorine concentration of the treated water after sedimentation and separation of flocs is 15 mg / l
However, it was reduced to 12 mg / l, indicating that a better result can be obtained by acidifying with hydrochloric acid.

[0015]

The effects of the present invention are listed below. The removal rate of fluorine is improved. Along with the fact that calcium sulfate is not formed, the amount of sludge generated is reduced because an aluminum-based inorganic coagulant is not used. Since the treated water after removing the fluorine is neutral, it can be discharged as it is. It is possible to effectively use the waste hydrochloric acid generated in the surface treatment process (descaling, etc.) of the steel sheet.

[Brief description of drawings]

FIG. 1 is a flow chart of a method for removing fluorine from fluorine-containing wastewater according to an embodiment.

[Explanation of symbols]

 1 Reaction tank 2 Neutralization tank 3 Coagulation tank 4 Sedimentation tank

Claims (1)

[Claims] 1. A step of adding calcium salt and ferric chloride after adding hydrochloric acid to the fluorine-containing wastewater to acidify the hydrochloric acid, a step of adding an alkaline agent for neutralization, and a polymer flocculant. And a step of coagulating and precipitating the fluorinated waste water.