JPH0790228B2 - Treatment method for fluorine-containing wastewater - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for treating fluorine-containing wastewater, and more particularly to a method for treating wastewater containing fluorine in a high concentration and containing boron.

[Prior Art] Examples of fluorine-containing wastewater include wastewater discharged from a flue gas desulfurization apparatus that treats coal combustion exhaust gas with an absorbent containing a calcium compound or the like.

Usually, fluorine has a low solubility in waste water and can be removed by reacting with calcium to produce calcium fluoride. However, if the wastewater also contains boron, it reacts with fluorine to form a borofluoride such as fluoroboric acid, so that it is difficult to remove it by an ordinary method.

Therefore, a method of adding an aluminum compound to the treatment of fluorine-containing wastewater has been proposed (Gypsum & Li
me, No.133, 1974). Here, as the aluminum compound, aluminum sulfate (van sulfate earth) or polyaluminum chloride (PAC) is used. Normal fluorine-containing wastewater has a fluorine concentration of 300 ppm or less and a boron concentration of 15 pp
Since it is less than m, the wastewater treatment method using these aluminum compounds has achieved some results.

[Problems to be Solved by the Invention] However, studies by the present inventors have revealed that when the concentration of either fluorine or boron in the wastewater is higher than a normal value, especially when the concentration of boron is high, It becomes difficult to remove it, and it is necessary to drastically increase the addition amount of vanadium sulfate and PAC. If the concentration of fluorine or boron becomes higher, stable treatment cannot be performed even if these aluminum compounds are used. I found out.

Increasing the amount of van sulphate soil and PAC used for wastewater treatment not only increases the treatment cost, but also
This leads to an increase in the amount of NaOH, etc. used on the downstream side, which in turn increases the amount of sludge, which inevitably causes new pollution problems. In addition, an increase in the amount of van sulphate soil used causes an increase in the concentration of sulfate ions in the treated water, which causes the problem of gypsum scaling. In addition,
Since the use of PAC is a coagulant, it is accompanied by the deposition of aluminum hydroxide and the like and scaling, which has a serious drawback that stable wastewater treatment cannot be performed.

Generally, the amount of wastewater is not determined by the concentration of fluorine or boron, but is determined by the water balance of the flue gas desulfurization system such as the concentration of chlorine ions in the liquid, the amount of sealing water, the amount of washing water, etc.

However, due to actual conditions such as tightening of regulations on wastewater and tightening of restrictions on use of industrial water, reduction of wastewater is strongly desired. Therefore, countermeasures for the chlorine ion concentration of the treated water, improvement of the cleaning method, and the like are being carried out, and attempts are being made to reduce the amount of wastewater.

Since the composition of coal, which is a fuel, changes depending on the production site of Soho, the concentrations of fluorine and boron in the wastewater discharged when the coal combustion exhaust gas is treated also change variously. When the concentration of these components in the waste water changes, especially when the concentration is higher than usual, it is not only technically difficult to cope with the above-mentioned conventional techniques, but also economically disadvantageous.

It is difficult to dilute and increase the amount of wastewater in order to facilitate the treatment of wastewater containing a high concentration of fluorine from the standpoint of the above-mentioned wastewater regulation.

[Means for Solving Problems] The present invention has been completed based on the above background.

That is, the present invention is obtained by treating coal combustion exhaust gas with an absorbing solution containing a calcium compound, the fluorine concentration is 500 ppm or more, and after adjusting the pH by adding the calcium compound to the wastewater containing boron, the aluminum concentration is 300 ~. The present invention provides a method for treating fluorine-containing wastewater, which comprises adding aluminum chloride to 600 ppm to cause a reaction, and then treating with a solution containing a calcium compound.

The wastewater to be the subject of the present invention contains fluorine at a high concentration and also contains boron as described above. As such wastewater, for example, coal combustion exhaust gas, a suitable absorbent,
For example, waste water discharged from a flue gas desulfurization apparatus that is treated with an absorbent containing a calcium compound can be used.

The method of the present invention has a fluorine concentration of 500 ppm or more, usually 500 to 3
000ppm or so and 40ppm or more, usually 40-300ppm
It can be preferably applied to wastewater containing a certain amount of boron.

Aluminum chloride is added after adjusting the pH of the wastewater,
The amount used may be an amount capable of removing the fluorine in the wastewater, but it is usually used so that the aluminum concentration is 300 to 600 ppm, and its form is arbitrary such as powdery or liquid, but an aqueous solution is easy to handle. Most preferred.

When aluminum chloride is added, fluorine in the waste water, for example, fluorine that is present as borofluoride such as fluoroboric acid by combining with boron is decomposed and can be removed to the outside of the system by a coprecipitation agglomeration effect. The presence of aluminum ions is required for the decomposition of borofluorides in waste water. Bansulfate and PAC strongly act as a flocculant, and are difficult to dissolve as aluminum ions in the liquid, but aluminum chloride easily dissociates to form aluminum ions. Furthermore, since the concentration of dissolved calcium increases due to an increase in chlorine ions, the production of calcium fluoride having a fluorine removing effect is promoted from the solubility product, and the removal rate of fluorine improves. Therefore, fluorine can be efficiently removed with a small addition amount.

If necessary, hydrochloric acid may be added to adjust the pH. Further, after the reaction with aluminum chloride, it is treated with a solution containing a calcium compound to remove fluorine as calcium fluoride.

[Effects of the Invention] According to the present invention, wastewater containing a high concentration of fluorine together with boron can be efficiently treated. In particular, it is possible to perform the fluorine treatment in the waste water while satisfying the request for reducing the waste water amount. Moreover, by selecting and using aluminum chloride, it has become possible to perform wastewater treatment in a stable state without the generation of gypsum scale due to conventional sulfate ions and the generation of gypsum sludge. Also,
Since aluminum chloride has a small effect as a coagulant, the occurrence of deposition and scaling on pipes and tanks is extremely small.

EXAMPLES Next, the present invention will be described with reference to examples.

Examples and Comparative Examples Wastewater from a flue gas desulfurization unit for treating coal combustion exhaust gas (fluorine concentration 1400ppm, boric acid concentration 150ppm, aluminum concentration 5
(0 ppm) at a flow rate of 3 m ³ / hr, and in the fluorine treatment step shown in FIG. 1, PAC, vanadium sulfate, or aluminum chloride was used as an additive aluminum agent to evaluate the fluorine removal performance and the adhesion and scaling of aluminum flocs. Results are shown in FIG.

As is clear from the figure, the fluorine removal efficiency was highest when aluminum chloride was used, and the amount of scaling and the like was lowest when aluminum chloride was used.

[Brief description of drawings]

FIG. 1 is an explanatory view showing an example of the fluorine treatment process of waste water, and FIG. 2 is a graph showing the results of waste water treatment by using various aluminum agents.

Claims (2)

[Claims] 1. A coal flue gas obtained by treating a coal combustion exhaust gas with an absorption liquid containing a calcium compound, having a fluorine concentration of 500 ppm or more, and adding a calcium compound to wastewater containing boron.
After the pH is adjusted, aluminum chloride is added so that the aluminum concentration becomes 300 to 600 ppm, the reaction is carried out, and then the solution is treated with a solution containing a calcium compound. 2. The method according to claim 1, wherein the waste water has a fluorine concentration of 500 ppm or more and a boron concentration of 40 ppm or more.