JPS5889985A - Removing method of fluorine ion in waste water - Google Patents

Abstract

PURPOSE:To prevent the generation of scale troubles in post stage installations by adding water-soluble carbonate, etc. to the liquid from the stage in which Ca compds. are added to waste water contg. fluorine ions and subjecting said liquid to solid-liquid sepn., then adding water-soluble Al salts to the separated liquid. CONSTITUTION:Ca compds. are added to waste water cont. fluroine ions in an amt. larger than reaction equiv. with fluorine, and water-soluble carbonate or bircarbonate is added to the liquid in an amt. larger than reaction equiv. with respect to excessively dissolved Ca to maintain the pH at 9-11. Thereafter, the liquid is subjected to solid-liquid sepn. and after water-soluble Al salts are added to the separated liquid, the liquid is subjected to solid-liquid sepn., whereby the fluorine ions in the waste water are removed. Aluminum sulfate, aluminum chloride, etc. are adequately usable for the above-described water soluble Al salts, and the amt. of the Al salts to be added is generally 1,000-5,000ppm with respect to the separated liquid.

Description

[Detailed description of the invention] The present invention relates to a method for removing fluoride ions from wastewater.

Conventionally, the above method involves adding a calcium compound and a water-soluble carbonate or bicarbonate (hereinafter referred to as carbonate, etc.) to fluoride ion-containing wastewater, maintaining the pH at an appropriate value, and dissolving calcium fluoride. A method of co-precipitating calcium carbonate and calcium carbonate is known. In this method, calcium ions in an amount of 3 to 7 equivalents of the amount of fluoride ions in wastewater are added to form calcilama fluoride, and carbonate is added separately to improve sedimentation properties when precipitating and separating calcium fluoride. etc., and the produced calcium carbonate and the calcium fluoride are coprecipitated. The amount of carbonate, etc. to be added is selected so that the amount of carbonate ions is 1 to 3 equivalents of the amount of fluorine ions in the wastewater. Therefore, in the water treated by this method,
Theoretically, 1 to 3 equivalents of excess calcium ions are contained unreacted relative to the amount of fluorine ions. If the treated water containing excess calcium ions is directly discharged into public waters, there will be no particular problem under current legal regulations. However, in order to further abate this treated water, if a subsequent treatment facility is installed, scale troubles in the subsequent facility due to the excess calcium ions may become a serious problem.

The present invention aims to improve the problems of the conventional methods described above and to provide a method for removing fluorine ions from wastewater that can suppress excess calcium ions in treated water to an extent that does not cause scale trouble in downstream equipment. do.

In order to achieve the above object, the present invention includes a first step of adding a calcium compound to fluorine ion-containing wastewater in an amount equal to or more than the reaction equivalent with fluorine, and dissolving an excessive amount of carbonate, etc. in the liquid from the first step. Add more than the reaction equivalent to calcium,
a second step of maintaining the pH at 9 to 11; a third step of separating the liquid from the second step into solid-liquid; a fourth step of adding a water-soluble aluminum salt to the separated liquid of the third step; It is characterized by comprising a fifth step of separating the liquid from the fourth step into solid-liquid.

In the above configuration, the purpose of the first step is to generate calcium fluoride based on the same concept as the conventional method. As the calcium compound, calcium hydroxide, calcium chloride, calcium carbonate, etc. are preferably used. The amount of calcium compound added is preferably 2 to 7 equivalents of the amount of fluorine ions in the wastewater. If the amount is less than 2 equivalents, the production of calcium fluoride will be insufficient. It is economically meaningless to use more than 7 equivalents. pH in the first step is 7-1
1 is preferred. Calcium fluoride is insufficiently produced in any other pH range.

The addition of carbonate or the like in the second step is completely different from the objective of improving the sedimentation properties of calcium fluoride in the conventional method. That is, although the development of producing calcium carbonate is similar, the purpose of this step is to remove excess calcium from wastewater, not to co-precipitate it with calcium fluoride. The amount of carbonate, etc. to be added is equal to or more than the reaction equivalent amount to the unreacted excess calcium in the first step. Therefore, the concentration of calcium ions in the treatment solution in the second step is minimized, and scale troubles caused by calcium can be prevented in subsequent subsequent steps and other subsequent facilities. However, in this step, since excess calcium ions are no longer present, a problem arises in that part of the calcium fluoride produced in the first step is re-dissolved in the liquid due to the dissociation constant. For this reason, the fluorine concentration of wastewater often does not satisfy the discharge standard value. In order to solve this problem, a fourth step and a fifth step, which will be described later, are provided. As carbonates, sodium carbonate, potassium carbonate, bicarbonate, etc. are preferably used. The amount of carbonate etc. to be added is preferably 1 to 2 equivalents relative to the excess calcium. If the amount is less than 1 equivalent, calcium cannot be removed sufficiently. If the amount is more than 2 equivalents, it is not very meaningful and has the disadvantage of increasing the salt concentration in the treatment liquid. The reason why the pH is limited to 9 to 11 in the second step is that, firstly, outside this pH range, calcium fluoride is redissolved and the fluorine ion concentration in the treatment liquid increases significantly.

The purpose of the third step is to separate solid-liquid calcium fluoride and calcium carbonate present in the O liquid from the second step. Precipitation separation is usually employed as a means of solid-liquid separation. At this time, it is preferable to add a small amount of a polymer flocculant and then perform flocculation and precipitation separation.

The fourth step is the most characteristic component of the present invention. That is, the present inventors have found that by adding a water-soluble aluminum salt, the fluorine ions in the processing solution that could not be removed or were redissolved in the first and second steps were very effectively precipitated. Through a series of experiments, we discovered that fluorine ions can be separated, and this process was incorporated as a finishing process for removing fluorine ions. Water-soluble aluminum salts include aluminum sulfate, aluminum chloride,
Polysalt aluminum and the like are preferred. Although the action in this process has not been clearly elucidated, it is speculated that it is a type of chemical reaction with the addition of a flocculation-sedimentation action. The amount of aluminum salt added is usually in the range of 1,000 to 5.00 f:) ppm relative to the separated liquid in the third step, but depending on the processing target, a small amount outside this range may be used.
Or you may need more. pH in this process
The conditions are preferably 5 to 9. At a pH other than this, the fluorine ion concentration in the treatment liquid increases significantly, or it is necessary to significantly increase the amount of aluminum salt added.

In the fifth step, like the third step, a coagulation-precipitation separation operation is usually performed, but other solid-liquid separation means may be used. Since the separated liquid (treated water) in the fifth step has passed through the first to fourth steps, the concentration of fluorine ions is low, and the concentration of calcium ions is also low, so it may cause scale trouble in the subsequent equipment. There isn't.

Example 1 Calcium hydroxide was added to wastewater having a fluorine ion concentration of 600 ppm and a pH of 2 so that the amount of calcium ions was 3 equivalents to the amount of fluorine ions, and the mixture was sufficiently stirred while maintaining the pH at 9. Next, 1. Add sodium carbonate in an amount of 1.5 to the amount of excess calcium ions,
After maintaining the pH at 10 and stirring thoroughly, add the polymer flocculant t.
5 ppm was added and subjected to coagulation and precipitation treatment. The fluorine ion concentration in this separated liquid was 31.2 P7)FW, and the calcium ion concentration was 39.6 pp'llL. 3000 ppm of aluminum sulfate was added to the above separated liquid, and the PH
After sufficient stirring while maintaining the temperature at 7, coagulation and precipitation treatment was performed. The fluorine ion concentration in this treated water was 9. lppm, and the calcium ion concentration was 32.7 pprn.

Example 2 To the wastewater used in Example 1, calcium chloride, sodium bicarbonate, and polyaluminum chloride were added in the same reaction equivalent or addition amount in place of the added calcium hydroxide, sodium carbonate, and aluminum sulfate, respectively. The process was carried out under the same conditions as in Example 1, except that part or all of it was replaced. The results are the separated liquid in the third step and the separated liquid (treated water) in the fifth step.
The fluorine ion concentration and calcium ion concentration of the tomato were not significantly different from those of Example 1.

Comparative Example Calcium hydroxide was added to the wastewater used in Example 1 so that the amount of calcium ions was 3 equivalents to the amount of fluorine ions, and sodium carbonate was added so that the amount of carbonate ions was excessive in the above calcium fluoride production reaction. 05 equivalent to the amount of calcium ions calculated as PH1
After maintaining the temperature at 0 and stirring thoroughly, add 5 PPm of polymer flocculant.
and flocculation and precipitation treatment. The fluorine ion concentration in this separated liquid (treated water) was relatively low at 23.8 ppm, but the calcium ion concentration was extremely high at 1140 ppm.

Claims (1)

[Claims] A first step in which a calcium compound is added to the fluorine ion-containing wastewater in an amount equal to or more than the reaction equivalent with fluorine, and a water-soluble carbonate or bicarbonate is added to the liquid from the first step in an amount equal to or more than the reaction equivalent to calcium dissolved in excess. a second step in which the pH is maintained at 9 to 11; a third step in which the liquid from the second step is separated into solid-liquid; and a fourth step in which a water-soluble aluminum salt is added to the separated liquid in the third step. and a fifth step of separating the liquid from the fourth step into solid-liquid.