JPS6342791A - Treatment of waste water - Google Patents

Abstract

PURPOSE:To decrease the amt. of the chemical to be used with waste water of a particularly high fluorine concn. and to decrease the amt. of the sludge to be generated by executing a treatment of heavy metals in the waste water, fluorine treatment, dithionic acid treatment and further, SS treatment, etc., in the state of combining the treatments in a manner that the respective treatments can be efficiently executed. CONSTITUTION:(a) A hydrochloric acid and the solid slurry to be generated in the stage (d) to be described afterward are added to and dissolved in the waste water according to the concn. of the chlorine in the waste water; thereafter, a Ca hydroxide or Na hydroxide and liquid heavy metal capturing agent are added thereto and while air is blown to the waste water, a reaction is effected at 5-9 pH and the solid-liquid sepn. of the reactive mixture is thereby executed in a 1st reaction stage. (b) The hydrochloric acid and Al compd. are added to the liquid section from the above- mentioned stage and the reaction is caused at 2-4 pH in the 2nd reaction stage. (c) The Ca hydroxide or Na hydroxide and high-polymer flocculating agent are added to the waste water and the reaction is caused at 6-8 pH in the 3rd reaction stage. (d) The solid-liquid sepn. of the above-mentioned reaction mixture is executed. (e) The liquid section from the stage (d) is brought into contact with a chlorine synthesize type adsorbent in the 4th reaction stage.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to the treatment of wastewater, and more particularly to a method for treating wastewater discharged from various parts of a wet flue gas desulfurization device and peripheral equipment.

[Conventional technology and its problems] Coal combustion exhaust gas contains SOx, NOx, and HF.
, HCl2. Contains fly ash etc.
The properties of wastewater discharged from various locations such as wet flue gas desulfurization equipment, AH, wet EP, and GGH are extremely complex, and contain many substances that must be removed.

It is naturally desirable to efficiently remove these substances, but the methods that have been implemented or proposed so far have been limited to simplification of equipment, processing costs such as chemical costs, and maintenance costs associated with scaling. Considering the above, it is not necessarily considered to be efficient.

Substances that must be removed from wastewater include, for example, heavy metals such as cadmium and arsenic; fluorine; dithionic acid; and nitrogen-containing sulfur compounds. In particular, an efficient method for removing heavy metals, fluorine, and dithionic acid is required. There is.

Regarding the removal of heavy metals by conventional methods, since high pH treatment is required, there are problems with the amount of chemicals used, the amount of sludge produced, and the occurrence of scaling. In addition, regarding the removal of fluorine, there are problems such as the amount of chemicals used, the amount of sludge produced, and the occurrence of scaling, as in the case of removing heavy metals, as the treatment process is complicated. In particular, when the fluorine concentration in the wastewater is 500 ppm or more and contains boron, and especially when the boron concentration is 40 ppm or more, the amount of aluminum sulfate (sulfuric acid bread earth) or polyaluminum chloride (PAC) used as an aluminum additive. The present inventors have found that the amount of fluorine increases significantly, causing problems not only in terms of economy but also in the occurrence of scaling, making it impossible to obtain a stable fluorine removal rate. Furthermore, regarding the adsorption and removal of dithionic acid, there are problems with the generation of sludge and scaling during the softening operation as a pretreatment for adsorption, and the amount of chemicals used. In particular, this problem in the operation of the softening process is a major negative factor both technically and economically.

[Means for Solving the Problems] The present invention aims to solve these problems and provide a method for efficiently treating wastewater. That is, the present invention relates to a method for treating wastewater, which is characterized by sequentially treating wastewater containing heavy metals, fluorine, and dithionic acid through the following steps.

a) adding and dissolving hydrochloric acid according to the chlorine concentration in the wastewater and the solid slurry produced in step e) described below to the wastewater, and then adding at least one of calcium hydroxide and sodium hydroxide as well as a liquid heavy metal scavenger; Let's blow some air in
) a first reaction step of carrying out the reaction in steps 15 to 9; b) a step of performing solid-liquid separation of the first step reaction mixture; C) adding hydrochloric acid and an aluminum compound to the liquid fraction from the solid-liquid separation step, and adjusting the pH to 2. d) A second reaction step in which at least one of calcium hydroxide and sodium hydroxide and a polymer flocculant are added to the second step reaction mixture, and the reaction is carried out at pH 6 to 8. 3 reaction steps; e) a step of performing solid-liquid separation of the third step reaction mixture; f) a fourth reaction step of bringing the liquid fraction from the solid-liquid separation step into contact with a chlorine-type synthetic adsorbent.

The wastewater that is the object of the present invention is discharged from various places such as wet flue gas desulfurization equipment, AH, wet EP, and GG)I. It may be either.

In the first reaction step of the present invention, hydrochloric acid is added depending on the chloride ion concentration in the wastewater to adjust the chloride ion concentration to 1000 to 3.
About 0000PP■, preferably 5000~2DOOO
ppm. As a result, the calcium concentration increases, making it possible to perform efficient wastewater treatment in the following steps. Furthermore, the addition of hydrochloric acid facilitates the dissolution of the solid slurry produced in step e).

Conventionally, high p of pnto or higher was used to remove heavy metals.
Previously, treatment was carried out in the H region, but in the present invention, by using a water-soluble chelating agent as a heavy metal scavenger, treatment can be performed in the high p region.
Operation in the H region is no longer necessary. As a result, the amount of sludge produced due to precipitation of magnesium and the like can be greatly reduced, and by setting pi to 5 to 9, most of the fluorine can be removed. Furthermore, this first
In the reaction step, the solids separated in step e) below or a slurry containing solids are added and dissolved. This solid material contains aluminum flocs and has the effect of facilitating fluorine treatment. In addition to the heavy metal scavenger, by introducing air into the same tank, residual SO3, which is a COD component, can be removed.
NO2 etc. 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 treatment with fluorine present in the form of fluoroboric acid such as fluoroboric acid is performed at pH 2 to 4 according to the main decomposition reaction formula shown below.

38Fa-+ 4Aff No.3 +8H20→4A4h+3
8(OH)3 +9H・Usually, sulfuric acid sulfate or PAC is used as an aluminum compound, but the use of aluminum chloride has a fast generation rate of dissolved AP ions, and AJ)
Since the ion concentration is also high, the decomposition of the borofluoride is fast and the fluorine treatment can be carried out efficiently.

Furthermore, aluminum chloride makes it possible to maintain a high calcium concentration state in the treatment system due to the increase in chloride ion concentration, which promotes the production of calcium fluoride from fluorine separated from boron, making fluorine removal more efficient. Can be done. In addition, since it is a chloride, unlike on sulfuric acid bread, there is no generation of sulfuric acid groups, and the effect as a flocculant is small, so it can be used for scaling and flocculation in COD treatment processes, etc.
-F can be reduced\. In addition, when the fluorine concentration is high and boron is also included, for example, the fluorine concentration is 5QOppm or more and the boron concentration is 40
When pp+w or more, 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 in L, and the reaction is carried out at a pH of 6 to 8. Through these second and third reaction steps, fluorine in the waste water is removed to a concentration below the regulation value.

Next, solid-liquid separation of the reaction mixture is performed, and the solids containing aluminum hydroxide, gypsum, etc., or the slurry containing the solids, are returned to the first reaction step, thereby making effective use of the aluminum compound, and also returning the solids containing the solids to the first reaction step. used as gypsum seed crystal for prevention of gypsum scaling in

The fourth reaction step of the present invention is COD for removal of dithionic acid.
This is a processing step.

The liquid fraction from the previous step is introduced into a COD removal device using a chlorine-type synthetic adsorbent and brought into contact with it to remove dithionic acid and the like.

In this case, the sulfuric acid type synthetic adsorbent used conventionally increases the SO4 concentration in the liquid at the outlet after adsorption, so if the calcium concentration in the liquid is high, scales such as gypsum will occur, causing trouble. . Therefore, a softening operation was essential as a pretreatment. However, as mentioned above, there are problems such as the generation of sludge and scaling at this stage of the softening operation, and a solution to these problems has been desired.

In the present invention, since a chlorine type synthetic adsorbent is used, S in the liquid is
The Oa concentration does not increase, and a softening operation is not necessary. That is, since the wastewater whose SO4 concentration has been suppressed to a very low level in the previous step 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, SOn 6 degrees can be suppressed to a low concentration based on the gypsum solubility product. Therefore, generation of scaling can be suppressed as much as possible.

In addition, since the acid decomposition liquid of the regeneration liquid discharged from the COD removal device is acidic, it can be effectively used in a desulfurization device, etc., for example, for use in adjusting p)l.

[Effects of the Invention] According to the present invention, heavy metal treatment, fluorine treatment, and
The dithionic acid treatment of the COD component, furthermore the SS treatment, etc. can be carried out in combination so that each can be carried out efficiently. In particular, it is possible to stably treat wastewater with high fluorine concentration and boron content by reducing the amount of chemicals used and reducing the amount of sludge and scale generation.

Moreover, in the present invention, the amount of chemicals such as NaOH and aluminum compounds used is small, and the amount of sludge generated can be suppressed. In addition, since the entire wastewater treatment system is operated while maintaining a high concentration of calcium in the liquid, the efficiency of fluorine treatment is improved, and the problematic softening process can be omitted, simplifying equipment and systems. became the turning section.

[Example] Next, examples of the present invention will be shown.

Example Drainage discharged from a wet flue gas desulfurization equipment for coal combustion flue gas (cadmium concentration 0.2 ppm, arsenic concentration 0.5 ppm)
m, Cr concentration 0.4 ppi+, 7-/element concentration 30
0ppm.

Boron concentration 20pp■, aluminum concentration 50ppm
.

COD concentration 80ppm, magnesium concentration 300ppm
, chlorine concentration 1500 pp) was flowed at a flow rate of 3/hr in two layers according to the flow shown in FIG. In addition, 80% sodium nitrite is added to the wastewater to decompose nitrogen-containing sulfur compounds.
pp+* was added. Moreover, the pH in each reaction step was maintained at the value shown in the figure.

As a result, the concentrations of each component in the treated water released from the fourth reaction step were cadmium 0.002 ppm, arsenic 0, and O5p.
pm or less, Cr concentration 0.01 ppm, fluorine 5 ppm
.

It was con 4PP11. In addition, when the above wastewater is treated by adding sulfuric acid bread soil, the fluorine concentration in the treated water is 1
It was opp ■.

Further, 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 used effectively in the same way as gypsum from the desulfurization equipment. Furthermore, the occurrence of scaling is slight, and the operation-L
There were no problems. It was also confirmed that the treatment performance that satisfies the emission regulation values can be achieved even for wastewater containing 1400 ppm of fluorine and 150 ppm of boron.

[Brief explanation of the drawing]

FIG. 1 is a flow diagram illustrating one embodiment of the method of the present invention.

Claims (5)

[Claims] (1) A method for treating wastewater, which comprises sequentially treating wastewater containing heavy metals, fluorine, and dithionic acid through the following steps. a) adding and dissolving hydrochloric acid according to the chlorine concentration in the wastewater and the solid slurry produced in step e) described below to the wastewater, and then adding at least one of calcium hydroxide and sodium hydroxide as well as a liquid heavy metal scavenger; While blowing air
A first reaction step in which the reaction is carried out at H5-9, b) a step in which solid-liquid separation of the first step reaction mixture is performed, c) hydrochloric acid and an aluminum compound are added to the liquid fraction from the solid-liquid separation step, and the pH is adjusted to pH 2-9. d) A third reaction step in which at least one of calcium hydroxide and sodium hydroxide and a polymer flocculant are added to the second step reaction mixture, and the reaction is carried out at pH 6 to 8. a reaction step; e) a step of performing solid-liquid separation of the third step reaction mixture; f) a fourth reaction step of bringing the liquid fraction from the solid-liquid separation step into contact 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 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.