Water Treatment Methods

Abstract

To provide a water treatment method capable of efficiently insolubilizing sulfate ions in treated water with low pH and also neutralizing the treated water.SOLUTION: A water treatment method includes a first step of adding barium chloride to treated water containing sulfate ions and a pH of 2.5 or less, and a second step of adding a basic compound to the treated water to which barium chloride has been added in the first step to raise the pH to 4.0 or higher.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a water treatment method for treating water containing sulfate ions by adding a barium salt, and more particularly to a water treatment method for insolubilizing sulfate ions by adding a barium salt to water containing sulfate ions at a relatively high concentration. [Background technology]

[0002] Conventionally, a method is known in which a barium salt is added to water to be treated that contains sulfate ions, and the sulfate ions are insolubilized as barium sulfate. For example, Patent Document 1 discloses a method in which sulfate ions are reacted with barium chloride to cause precipitation, Patent Document 2 discloses a method in which sulfate ions are reacted with barium hydroxide or barium carbonate to cause precipitation, and Patent Document 3 discloses a method in which sulfate ions are reacted with barium carbonate, barium hydroxide, or barium oxide to cause precipitation. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2006-281067 A [Patent Document 2] JP 2010-053368 A [Patent Document 3] JP 2012-106215 A Summary of the Invention [Problem to be solved by the invention]

[0004] As described above, a method has been known in the past in which a barium salt is added to water to be treated that contains sulfate ions, thereby insolubilizing the sulfate ions as barium sulfate. However, water to be treated that contains a relatively high concentration of sulfate ions often also has a low pH, and when treating such water to be treated, it may be necessary to insolubilize the sulfate ions as well as to perform a neutralization process.

[0005] The present invention has been made in consideration of the above-mentioned circumstances, and its object is to provide a water treatment method that can efficiently insolubilize sulfate ions in water to be treated having a low pH, and can neutralize the water to be treated. [Means for solving the problem]

[0006] The water treatment method of the present invention, which has been able to solve the above problems, is characterized by having a first step of adding barium chloride to water to be treated that contains sulfate ions and has a pH of 2.5 or less, and a second step of adding a basic compound to the water to be treated to which barium chloride has been added in the first step to bring the pH to 4.0 or more. According to the water treatment method of the present invention, by first adding barium chloride to water to be treated that contains sulfate ions and has a pH of 2.5 or less, and then adding a basic compound, sulfate ions contained in the water to be treated can be insolubilized more efficiently than when a basic compound is first added and then barium chloride is added. For example, the sulfate ion concentration of the treated water can be reduced to a desired level with a smaller amount of barium chloride added. In addition, the water to be treated can be neutralized.

[0007] The sulfate ion concentration of the water to be treated is preferably 500 mg / L or more. If the water to be treated contains sulfate ions at such a high concentration, the sulfate ions can be efficiently insolubilized by adding barium chloride. In the second step, an alkali metal hydroxide is preferably added as a basic compound.

[0008] From the viewpoint of adding barium chloride in an amount that efficiently reacts with sulfate ions, the amount of barium chloride added to the water to be treated in the first step is preferably less than the amount of sulfate ions contained in the water to be treated, and more preferably 0.5 times or less the amount of sulfate ions contained in the water to be treated.

[0009] The amount of barium chloride to be added to the water to be treated in the first step can be set based on the amount of basic compound required to bring the same water to a predetermined value A in the range of pH 4.0 to 7.0. This allows the amount of barium chloride to be added to be appropriately set without having to determine the sulfate ion concentration of the water to be treated in advance. For example, prior to the first step, a preliminary step can be performed in which a basic compound is added to the water to be treated in the first step to bring the pH to a predetermined value A (where A is any value in the range of 4.0 to 7.0), and the amount of barium chloride to be added in the first step can be set based on the amount of basic compound added in the preliminary step. When the first step and the second step are repeatedly performed in this order, in the second step, a basic compound can be added to the water to be treated to which barium chloride has been added to bring the pH to a predetermined value A (where A is any value in the range of 4.0 to 7.0), and the amount of barium chloride to be added in the first step can be set based on the amount of basic compound added in the previous second step.

[0010] The water to be treated is preferably, for example, a plating waste liquid. The water treatment method of the present invention can efficiently insolubilize sulfate ions from the plating waste liquid by using a plating waste liquid containing sulfate ions as the water to be treated. Effect of the Invention

[0011] According to the water treatment method of the present invention, sulfate ions contained in the water to be treated can be efficiently insolubilized, and the water to be treated can be neutralized. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] The water treatment method of the present invention comprises a first step of adding barium chloride to water to be treated that contains sulfate ions and has a pH of 2.5 or less, and a second step of adding a basic compound to the water to be treated to which barium chloride has been added in step 1 to adjust the pH to 4.0 or more. The water treatment method of the present invention can efficiently insolubilize sulfate ions contained in the water to be treated and neutralize the water to be treated by first adding barium chloride to the water to be treated that contains sulfate ions and has a pH of 2.5 or less in step 1, and then adding a basic compound in step 2.

[0013] The sulfate ion concentration of the water to be treated is not particularly limited, but water to be treated from which sulfate ions need to be removed due to discharge regulations usually contains sulfate ions at a certain high concentration. Therefore, the sulfate ion concentration of the water to be treated is preferably 500 mg / L or more, more preferably 550 mg / L or more, and even more preferably 600 mg / L or more. In such water to be treated, the sulfate ions can be efficiently insolubilized by adding barium chloride. The upper limit of the sulfate ion concentration of the water to be treated is not particularly limited, and may be, for example, 5,000 mg / L or less, 3,000 mg / L or less, or 1,500 mg / L or less. The solvent for the water to be treated is preferably water. The sulfate ion concentration of the water to be treated means the concentration of soluble sulfate ions, and can be determined, for example, by ion chromatography or the like.

[0014] The water to be treated may contain components other than sulfate ions, such as metal ions, chloride ions, sulfite ions, phosphate ions, phosphite ions, nitrate ions, nitrite ions, and cyanide ions.

[0015] The pH of the water to be treated is not particularly limited as long as it is 2.5 or less, but the lower limit of the pH is preferably 1.0 or more, and more preferably 1.5 or more. Examples of wastewater having a low pH and containing a high concentration of sulfate ions include plating wastewater (including plating cleaning wastewater) and metal acid cleaning wastewater.

[0016] In the first step, barium chloride is added to the water to be treated to insolubilize sulfate ions contained in the water to be treated as barium sulfate. In the first step, barium chloride is added to the water to be treated to obtain a first treated water in which at least a portion of the sulfate ions contained in the water to be treated are insolubilized. By using barium chloride, sulfate ions can be efficiently insolubilized as barium sulfate.

[0017] Barium chloride may be added to the water to be treated as a solid (e.g., powder), or as a liquid (aqueous solution) or slurry. The water to be treated to which barium chloride has been added is preferably mixed appropriately by stirring or the like. In the first step, the pH of the water to be treated usually hardly changes before and after the addition of barium chloride.

[0018] In the first step, it is not necessary to insolubilize all of the sulfate ions contained in the water to be treated. Even when the removal of sulfate ions is required due to discharge regulations, it is usually acceptable for sulfate ions to be contained at a certain concentration. Therefore, the amount of barium chloride (molar amount) added to the water to be treated in the first step is preferably less than the amount of sulfate ions (molar amount) contained in the water to be treated. This allows barium chloride to be added in a range that efficiently reacts with sulfate ions. The amount of barium chloride added to the water to be treated in the first step may be less than that, for example, 0.8 times or less, 0.7 times or less, or 0.6 times or less of the amount of sulfate ions contained in the water to be treated. From the viewpoint of more efficiently insolubilizing sulfate ions, the amount of barium chloride added to the water to be treated in the first step is preferably 0.5 times or less, more preferably 0.3 times or less, and even more preferably 0.2 times or less of the amount of sulfate ions contained in the water to be treated. This allows the amount of barium chloride used to be reduced compared to the amount of sulfate ions to be insolubilized, and the amount of insoluble matter to be disposed of as waste can also be reduced. On the other hand, the amount of barium chloride added to the water to be treated in the first step is preferably 0.05 times or more, more preferably 0.1 times or more, the amount of sulfate ions contained in the water to be treated. The amount of barium chloride added to the water to be treated in the first step can be set to a preferred value based on empirical values, for example, by measuring the concentration of sulfate ions contained in the first treated water and setting the ratio of the amount of barium chloride added to the water to be treated in the first step to the amount of sulfate ions contained in the water to be treated.

[0019] The sulfate ion concentration of the first treated water obtained by adding barium chloride to the water to be treated in the first step is, for example, preferably less than 500 mg / L, more preferably 400 mg / L or less, and even more preferably 350 mg / L or less. The sulfate ion concentration of the first treated water may basically be set appropriately based on the wastewater standards, etc., and the amount of barium chloride to be added may also be set accordingly.

[0020] In the second step, a basic compound is added to the first treated water obtained in the first step, i.e., the water to be treated to which barium chloride has been added in the first step. This allows neutralization of the water to be treated. In the second step, the pH is adjusted to 4.0 or higher by adding a basic compound. The pH may be higher, for example, 4.5 or higher, 5.0 or higher, 5.5 or higher, 6.0 or higher, 6.5 or higher, or 6.8 or higher. The upper limit of the pH is, for example, preferably 10.0 or lower, more preferably 9.0 or lower, and even more preferably 8.0 or lower.

[0021] The basic compound used in the second step is not particularly limited as long as it acts as an alkali, and known basic compounds can be used. The basic compound may be added to the first treated water as a solid or as a liquid (aqueous solution), but it is preferable to add it to the first treated water as a liquid. That is, it is preferable to add an alkaline solution as the basic compound to the first treated water, which makes it possible to quickly increase the pH of the first treated water in the second step. As the basic compound, it is convenient to use an alkali metal hydroxide, and among them, it is preferable to use sodium hydroxide.

[0022] In the water treatment method of the present invention, barium chloride is first added to water to be treated that contains sulfate ions and has a pH of 2.5 or less, and then a basic compound is added, thereby making it possible to more efficiently insolubilize sulfate ions contained in the water to be treated, compared to the case where a basic compound is first added and then barium chloride is added. For example, the sulfate ion concentration in the treated water can be reduced to a desired level with a smaller amount of barium chloride added.

[0023] The insolubilized barium sulfate can be removed by a known solid-liquid separation means. Examples of the solid-liquid separation means include sedimentation, centrifugation, and filtration (e.g., filter press, belt press, and screw press). By the solid-liquid separation means, the barium sulfate may be recovered as a solid (including a cake-like form), or as a slurry or a concentrated liquid. The removal of the insolubilized barium sulfate may be performed on the first treated water obtained in the first step, or on the second treated water obtained in the second step.

[0024] The amount of barium chloride to be added to the water to be treated in the first step can be set based on the amount of basic compound (i.e., the base equivalent (molar equivalent) of the basic compound, meaning the number of moles of hydroxide ions) required to adjust the pH of the same water to a predetermined value A in the range of 4.0 to 7.0. This allows the amount of barium chloride to be added to be appropriately set without having to determine the sulfate ion concentration of the water to be treated in advance. For example, it is not necessary to insolubilize all of the sulfate ions contained in the water to be treated, and it is sufficient to insolubilize them to a concentration below a standard or other specified concentration, and this allows the amount of barium chloride to be added to be set within a range that efficiently reacts with the sulfate ions.

[0025] The predetermined value A may be any value in the range of pH 4.0 to 7.0. The amount of barium chloride added in the first step is preferably equal to or less than the base equivalent (molar equivalent) of the basic compound required to adjust the pH of the water to the predetermined value A, more preferably 0.8 times or less, even more preferably 0.6 times or less, and preferably 0.1 times or more, more preferably 0.2 times or more, and even more preferably 0.3 times or more. Basically, the amount of barium chloride added in the first step is preferably set to a smaller ratio relative to the base equivalent (molar equivalent) of the basic compound required to adjust the pH of the water to the predetermined value A as the predetermined value A increases.

[0026] When setting the amount of barium chloride to be added as described above, for example, prior to the first step, a preliminary step can be performed in which a basic compound is added to the water to be treated in the first step to adjust the pH to a predetermined value A (wherein A is any value in the range of 4.0 to 7.0), and the amount of barium chloride to be added in the first step can be set based on the amount of basic compound added in the preliminary step.

[0027] Alternatively, when the first and second steps are repeated in this order, i.e., when the water treatment method of the present invention is performed batchwise many times, the amount of barium chloride added in the first step can be set based on the amount of basic compound added in the immediately preceding second step (i.e., the base equivalent of the basic compound required to bring the pH of the first treated water obtained in the immediately preceding first step to the predetermined value A). In this case, it is preferable to install a raw water tank for storing the water to be treated, extract a portion of the water to be treated from this raw water tank to perform the first and second steps, and then extract a portion of the water to be treated from the same raw water tank to perform the next first and second steps. Note that new water to be treated may be added to the raw water tank. EXAMPLES

[0028] The present invention will be described in more detail below by showing examples, but the scope of the present invention is not limited to these examples.

[0029] (1) Treatment method Barium chloride and sodium hydroxide were added to the water to be treated, which contained 864-1221 mg / L of sulfate ions and had a pH of 2.0-2.1, to insolubilize the sulfate ions. The treatment conditions are shown in Table 1. In treatments 1-3, barium chloride was added first, and then sodium hydroxide was added to adjust the pH to 4.5. In treatments 4 and 5, sodium hydroxide was added first to adjust the pH to 4.5 or 6.5, and then barium chloride was added. Barium chloride was added in an amount of 0.2 or 0.5 times the molar amount of sulfate ions contained in the water to be treated. The concentrations of soluble sulfate ions in the water to be treated and the treated water were measured, and the insolubilization rate of sulfate ions and the reaction efficiency of barium chloride were obtained. The reaction efficiency of barium chloride was obtained by dividing the number of moles of insolubilized sulfate ions by the number of moles of barium chloride added.

[0030] [Table 1]

[0031] (2) Processing results The treatment results are shown in Table 1. Treatments 1 to 3, in which barium chloride was added first and then sodium hydroxide, were able to insolubilize sulfate ions more efficiently than treatments 4 and 5, in which sodium hydroxide was added first and then barium chloride. In treatments 1 to 3, barium chloride was added at pH 2.0 to 2.1, in treatment 4, barium chloride was added at pH 4.5, and in treatment 5, barium chloride was added at pH 6.5. The lower the pH when barium chloride was added, the more efficiently sulfate ions were insolubilized. Therefore, it was found that neutralization by adding an alkaline solution is preferably performed after the addition of barium chloride. [Industrial Applicability]

[0032] The present invention can be used to remove sulfate ions from water to be treated that contains a relatively high concentration of sulfate ions, and can be used to treat, for example, plating wastewater, metal pickling wastewater, and flue gas desulfurization wastewater.

Claims

1. A water treatment method for insolubilizing sulfate ions as barium sulfate, comprising the steps of: A first step of adding barium chloride to water to be treated, the water containing sulfate ions and having a pH of 2.5 or less; A second step of adding a basic compound to the water to be treated to which barium chloride has been added in the first step to adjust the pH to 4.0 or more, A water treatment method characterized in that the amount of barium chloride added to the water to be treated in the first step is 0.5 times or less the amount of sulfate ions contained in the water to be treated.

2. 2. The water treatment method according to claim 1, wherein the sulfate ion concentration in the water to be treated is 500 mg / L or more.

3. 3. The water treatment method according to claim 1, wherein an alkali metal hydroxide is added as the basic compound.

4. 4. The water treatment method according to claim 1, wherein the amount of barium chloride added to the water to be treated in the first step is 0.3 times or less the amount of sulfate ions contained in the water to be treated.

5. Prior to the first step, a preliminary step is carried out in which a portion of the water to be treated is extracted and a basic compound is added thereto to adjust the pH to a predetermined value A (where A is any value in the range of 4.0 to 7.0); The water treatment method according to any one of claims 1 to 4, wherein the amount of barium chloride added in the first step is set based on the amount of the basic compound added in the preliminary step.

6. The water treatment method is carried out in a batch manner in which the first step and the second step are carried out and then the first step and the second step are carried out with new treated water, In a second step, a basic compound is added to the water to be treated to which the barium chloride has been added, so that the pH of the water is adjusted to a predetermined value A (wherein A is any value in the range of 4.0 to 7.0); The water treatment method according to any one of claims 1 to 4, wherein the amount of barium chloride added in the first step is set based on the amount of the basic compound added in the immediately preceding second step.

7. The water treatment method according to any one of claims 1 to 6, wherein the water to be treated is a plating wastewater.

Images