JPH0673670B2 - Wastewater treatment method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for treating wastewater, and more particularly to a method for treating wastewater discharged from various places of a wet flue gas desulfurization apparatus and peripheral equipment.

[Conventional technology and its problems] Coal combustion exhaust gas contains SO _X , NO _X , HF, HCl, fly ash, etc., and is used in wet flue gas desulfurization equipment, AH, wet EP, GGH.
The properties of wastewater discharged from various places are extremely complicated and contain many substances that must be removed.

Efficient removal of these substances is of course desirable, but the methods that have been implemented or proposed so far are simpler to implement, processing costs such as drug costs, and maintenance costs associated with scaling. Considering the above, it is not always considered to be efficient.

Examples of substances that must be removed from wastewater include heavy metals such as cadmium and arsenic; fluorine; dithionic acid; nitrogen-containing sulfur compounds. Especially, an efficient method for removing heavy metals, fluorine and dithionate is required. There is.

Regarding the removal of heavy metals by the conventional method, there is a problem that the amount of chemicals used, the amount of sludge produced, and scaling occur because high pH treatment is required. Further, regarding the removal of fluorine, there are problems that the treatment process is complicated and that the amount of chemicals used, the amount of sludge produced, and scaling occur as in the case of removing heavy metals. Especially, the fluorine concentration in the wastewater
When boron is contained at 500 ppm or more, and especially when the boron concentration is 40 ppm or more, the use amount of aluminum sulfate (van sulfate earth) or polyaluminum chloride (PAC) as an aluminum additive is greatly increased, which is economical. The present inventors have found that not only the property but also the problem of scaling occurs, and a stable fluorine removal rate cannot be obtained. Further, with respect to the adsorption and removal of dithionic acid, there is a problem in that sludge and scaling are generated at the stage of the softening operation as a pretreatment for adsorption, and further, the amount of the drug used. In particular, the problem of this softening process operation is a serious negative factor both technically and economically.

[Means for Solving Problems] An object of the present invention is to provide a method for solving these problems and treating wastewater efficiently. That is, the present invention relates to a wastewater treatment method characterized by sequentially treating wastewater containing heavy metals, fluorine and dithionate by the following steps.

a) Hydrochloric acid is added to the wastewater in accordance with the chlorine concentration in the wastewater and the solid slurry generated in the step e) described below, and after dissolution, at least one of calcium hydroxide and sodium hydroxide and a liquid heavy metal scavenger are added, PH while blowing air
The first reaction step of performing the reaction in 5 to 9, b) the step of performing the solid-liquid separation of the reaction mixture in the first step, c) the hydrochloric acid and the aluminum compound are added to the liquid section from the solid-liquid separation step, and the pH is adjusted to 2 to The second reaction step in which the reaction is carried out at 4, d) The second step At least one of calcium hydroxide and sodium hydroxide and a polymer flocculant are added to the reaction mixture, and the reaction is carried out at pH 6-8. 3 reaction steps, e) third step solid-liquid separation of the reaction mixture, f) fourth reaction step of contacting the liquid section from the solid-liquid separation step with a chlorine-type synthetic adsorbent.

Wastewater covered by the present invention is a wet flue gas desulfurizer, AH, wet E
It is discharged from various places such as P and GGH, and the system of the wet flue gas desulfurization device may be either an ash separation system or an ash mixing system.

In the first reaction step of the present invention, hydrochloric acid is added according to the chlorine ion concentration in the waste water to adjust the chlorine ion concentration to 1000 to 30000p.
It is increased to about pm, preferably 5000 to 20000 ppm. As a result, the calcium concentration increases, and efficient wastewater treatment can be performed in the following steps. Furthermore, the addition of hydrochloric acid facilitates the dissolution of the solid slurry generated in step e).

Conventionally, the treatment was carried out in a high pH region of pH 10 or higher to remove heavy metals, but in the present invention, by using a water-soluble chelating agent as a heavy metal scavenger, it is not necessary to operate in a high pH region. became. As a result, the amount of sludge produced by the precipitation of magnesium or the like can be greatly reduced, and most of the fluorine can be removed by adjusting the pH to 5-9. Further, in the first reaction step, the solid matter or the slurry containing the solid matter separated in the step e) described later is added and dissolved. This solid material contains aluminum flocs and has the effect of facilitating the fluorine treatment. Also, by introducing air into the same tank in addition to the heavy metal scavenger, residual SO ₃ , NO _2, etc., which are COD components, can be oxidized or decomposed and released.

Next, in the second reaction step, hydrochloric acid and an aluminum compound are added to the waste water, and the fluorine treatment which is present in the form of borofluoride such as fluoroboric acid at pH 2 to 4 is performed by the main decomposition reaction formula shown below.

_{^{3BF 4 - + 4Al 3+ + 9H}} 2 O → 4AlF 3 + 3B (OH) 3 + 9H + usually aluminum sulfate and PAC as the aluminum compound is used, the use of aluminum chloride has a high solubility Al ion generation rate, and Al Since the ion concentration is also high, the decomposition of borofluoride is fast, and the fluorine treatment can be efficiently performed. Furthermore, aluminum chloride makes it possible to keep the treatment system in a high calcium concentration state by increasing the chloride ion concentration, promotes the generation of calcium fluoride, which is fluorine separated from boron, and enables more efficient removal of fluorine. You can In addition, since it is a chloride, unlike sulphate sulphate, it does not generate sulfate radicals, and its effect as a coagulant is small, so scaling in the COD treatment step and the adhesion of flocs can be reduced. In addition,
When the fluorine concentration is high and also contains boron, for example, the fluorine concentration is 500 ppm or more, and the boron concentration is 40
When it is above ppm, it is particularly suitable to use aluminum chloride as the aluminum compound.

In the third reaction step, at least one of calcium hydroxide and sodium hydroxide and a polymer flocculant are added to the reaction mixture of the second reaction step, and the reaction is carried out at a pH of 6-8. In the second and third reaction steps, the fluorine in the waste water is removed below the regulated concentration.

Next, solid-liquid separation of the reaction mixture is performed, and a solid containing aluminum hydroxide, gypsum or the like or a slurry containing solid is returned to the first reaction step to effectively utilize the aluminum compound, and at the same time, to perform the first reaction step. Used as a gypsum seed crystal to prevent gypsum scaling in.

The fourth reaction step of the present invention is a COD treatment step for removing dithionic acid.

The liquid section from the previous step is guided to and brought into contact with a COD removal device that uses a chlorine-type synthetic adsorbent to remove dithionic acid, etc.

In this case, the conventionally used sulfuric acid type synthetic adsorbent increases the SO ₄ concentration in the liquid at the outlet after adsorption,
If the calcium concentration in the liquid is high, scales such as gypsum will occur, causing troubles. Therefore, a softening operation was essential as a pretreatment. However, at the stage of this softening operation, there are problems such as the generation of sludge and scaling as described above, and the solution thereof has been desired.

Since chlorine-based synthetic adsorbent is used in the present invention,
₄ Concentration does not increase and no softening operation is required.
In other words, in the previous step, since the wastewater with a very low SO ₄ concentration is passed through the COD removal device, no scale is generated. In the present invention, since the calcium concentration in the liquid is maintained at a high concentration in the entire wastewater treatment system, the SO ₄ concentration can be suppressed to a low concentration from the gypsum solubility product. Therefore, generation of scaling can be suppressed as much as possible.

Since the acid decomposition liquid of the regenerated liquid discharged from the COD removal device is acidic, it can be effectively used in a desulfurization device or the like, for example, used for pH adjustment.

[Effects of the Invention] According to the present invention, heavy metal treatment in wastewater, fluorine treatment, COD
It can be carried out in a combined state such that the dithionic acid treatment of the components, and further the SS treatment and the like can be performed efficiently. In particular, even for wastewater containing a high fluorine concentration and containing boron, the amount of chemicals used can be suppressed and stable treatment with reduced sludge volume and scale generation can be performed.

Moreover, in the present invention, the amount of chemicals such as NaOH and aluminum compound used is small, and the amount of sludge generated can be suppressed. Moreover, since the calcium concentration in the liquid is maintained at a high concentration in the entire wastewater treatment system for operation, the efficiency of the fluorine treatment is improved, and the softening step, which has many problems, can be omitted.
It has become possible to simplify the equipment and system.

[Examples] Next, examples of the present invention will be described.

Example Wastewater discharged from wet flue gas desulfurization equipment of coal combustion exhaust gas (cadmium concentration 0.2ppm, arsenic concentration 0.5ppm, cr concentration 0.4pp
m, fluorine concentration 300 ppm, boron concentration 20 ppm, aluminum concentration 50 ppm, COD concentration 60 ppm, magnesium concentration 300 ppm, chlorine concentration 1500 ppm) were treated by flowing at a flow rate of 2 m ³ / hr according to the flow shown in FIG. In addition, 80 ppm of sodium nitrite was added to the wastewater to decompose the nitrogen-containing sulfur compounds. Also,
The pH in each reaction step was maintained at the indicated value.

As a result, the concentration of each component of the treated water released from the 4th reaction step was 0.002pm for cadmium, 0.05ppm for arsenic, 0.01ppm for Cr,
Fluorine was 5 ppm and COD was 4 ppm. The concentration of fluorine in the treated water when the wastewater was treated with van sulphate was 10%.
It was ppm.

In addition, as a result of component analysis, it was confirmed that the sludge containing gypsum obtained by solid-liquid separation of the reaction mixture from the first reaction step can be effectively used similarly to gypsum from the desulfurization device. Furthermore, the occurrence of scaling was minute, and there was no hindrance in operation. Also, fluorine concentration 1400ppm, boron concentration 15
It was confirmed that the treatment performance that satisfies the emission control value can be exhibited even for wastewater containing 0 ppm.

[Brief description of drawings]

FIG. 1 is a flow chart showing one embodiment of the method of the present invention.

Claims (5)

[Claims] 1. A method for treating wastewater, which comprises successively treating wastewater containing heavy metals, fluorine and dithionic acid by the following steps. a) Hydrochloric acid is added to the wastewater in accordance with the chlorine concentration in the wastewater and the solid slurry generated in the step e) described below, and after dissolution, at least one of calcium hydroxide and sodium hydroxide and a liquid heavy metal scavenger are added, PH while blowing air
The first reaction step of performing the reaction in 5 to 9, b) the step of performing the solid-liquid separation of the reaction mixture in the first step, c) the hydrochloric acid and the aluminum compound are added to the liquid section from the solid-liquid separation step, and the pH is adjusted to 2 to Second reaction step in which the reaction is carried out in 4, d) Second step At least one of calcium hydroxide and sodium hydroxide and a polymer flocculant are added to the reaction mixture, and the reaction is carried out at pH 6 to 8 Reaction step, e) third step solid-liquid separation of the reaction mixture, f) fourth reaction step of contacting the liquid section from the solid-liquid separation step with a chlorine-type synthetic adsorbent. 2. The method according to claim 1, wherein the waste water is discharged from a wet flue gas desulfurization device. 3. The method according to claim 1, wherein the aluminum compound used in c) the second reaction step is aluminum chloride. 4. The method according to claim 3, wherein the waste water has a fluorine concentration of 500 ppm or more and contains boron. 5. The method according to claim 3, wherein the waste water has a fluorine concentration of 500 ppm or more and a boron concentration of 40 ppm or more.