JPS6235840B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to a process in which waste liquid from a wet flue gas desulfurization system of a coal-fired power plant or a dry denitrification system is used in conjunction with a dry denitrification system is treated with a desalination system such as a reverse osmosis system, and then concentrated. The present invention relates to a method for treating liquid.

Recycling and reuse of wastewater is being carried out in various fields for the purpose of saving water or introducing closed wastewater systems, but there are many cases where removal of dissolved salts, or desalination treatment, is necessary as a means of regeneration treatment. Be looked at.

Generally, in desalination treatment using reverse osmosis, ion exchange resin, electrodialysis, evaporation, etc., dissolved salts are removed by a desalination device and treated water that can be reused is recovered. On the other hand, salts separated from treated water are discharged outside the system as concentrated water, but this concentrated water often contains stable substances such as common salt and mirabilite as main components, and sometimes causes problems in raw water. Concentration of harmful components at concentrations that should not be allowed may exceed the allowable discharge concentration, and therefore concentrated water may not be able to be discharged as is. Therefore, concentrated liquids often require some kind of post-treatment.

The concentrated liquid from the waste liquid desalination equipment at the coal-fired power plant mentioned above contains common salt and mirabilite as main components, as well as sulfur compounds such as sulfur oxo acids, and NH ₄ Cl, NaNO ₃ , (NH ₄ ) ₂ Because nitrogen compounds such as SO ₄ are concentrated and contain the main harmful components such as sodium dithionate, nitrates, and free ammonia, the concentrated liquid cannot be discharged as is.
In addition, it was difficult to apply separate waste liquid treatment for sulfur compounds and nitrogen compounds, which was done when the recovered water was not reused.

The applicant has confirmed that a method of oxidizing or thermally decomposing harmful components such as sulfur compounds and nitrogen compounds by combining a vacuum evaporator and a spray incinerator is effective as a treatment method for such concentrated liquid. This has been confirmed by Japanese Patent Application No. 1766-1988. This patent application No. 53-91766 removes harmful components such as sulfur compounds and nitrogen compounds by oxidation and thermal decomposition, and dissolves the molten salt (smelt) of common salt and mirabilite, which are the main components, into the liquid. It is characterized by obtaining a salt solution containing almost no components. In contrast, in this invention, the means for treating harmful components in the concentrated liquid is the same as above, but the operating conditions described below are maintained from the dissolution and collection liquid of the molten salt discharged from the subsequent spray incinerator. In particular, anhydrous sodium sulfate is selectively crystallized, and this can be continuously recovered by solid-liquid separation.

Next, the present invention will be explained based on the attached drawings.

FIG. 1 is a flow sheet showing an embodiment of the present invention, in which 1 is a desalination device consisting of, for example, a reverse osmosis device, 2 is a vacuum evaporation concentration device consisting of a single or multiple-effect evaporator, 3 is a spray incinerator, 4 is a cooling crystallizer, 5 is a solid-liquid separator, 6 is a mirabilite receiver, 7 is a brine receiver, and 8 is a post-treatment device. The raw water is treated in a desalination device 1 to recover the treated water, and then the reverse osmosis concentrate is submerged by the retained heat of the exhaust gas sent from the incinerator 3 through the vacuum evaporation concentration device 2 before being incinerated. water is evaporated and concentrated. In this case, the reverse osmosis concentrate is 100% lower than the limit of salt concentration due to osmotic pressure.
By passing it through the concentrator 2, it can be concentrated to a concentration of, for example, 300 g/ to about 450 g/or more at which salt crystals are precipitated. In particular, when a multi-effect can is used, an improvement in concentration and a reduction in running costs can be expected, but the optimum concentration should be determined by taking into account the increase in equipment costs. Further, since the condensed water obtained by the concentrator 2 contains almost no impurities, it is reused in the same way as the water recovered from the desalination device 1.

Next, this concentrated waste liquid is supplied to the spray incinerator 3,
Sulfur compounds and nitrogen compounds are oxidized or thermally decomposed and removed in a short time in an oxidizing atmosphere at 800-1100°C. At this time, the so-called NO _x such as NO and NO ₂ generated by _the thermal decomposition of _nitrates and the NO ) and (2)) can be reduced to _N2 . Regarding the issue of NH ₃ quantity, if a desulfurization equipment is combined with a denitrification equipment, there will be an excess of NH ₃ and there is no need to replenish it, but if the desulfurization equipment alone is insufficient in NH ₃ , it may be necessary to replenish it. can.

NO + NH ₃ + 1/4O ₂ →N ₂ +3/2H ₂ O ……(1) NO ₂ +NH ₃ +1/2O ₂ → 3/2N ₂ +3H ₂ O ……(2) In the above reaction, the excess NH ₃ is 1100 ℃, most of it burns, but at lower temperatures the decomposition rate decreases (see Figure 2). Therefore,
Due to the economic efficiency of NH ₄ ⁺ treatment, a combination of incineration and other ammonia treatments is also a method that can be easily put into practical use.

Next, the sulfur oxide (Na ₂ S ₂ O ₆ ) generates SO _x as it decomposes, but this can be prevented from being discharged outside the system as described below. In other words, since this waste liquid contains ammonia compounds, it generates acids such as HCl or H ₂ SO ₄ as NH ₃ volatilizes, and when this exhaust gas is treated wet, it tends to be extremely acidic. ₄ Cl, (NH ₄ ) ₂ SO ₄ , etc., with an alkali such as NaOH.
Treatment to fix Cl ^- or SO ₄ ^2- is necessary. Therefore, since Na ⁺ is in excess, so-called SO _x such as SO ₂ generated by the decomposition of sulfur oxides is also easily absorbed into the liquid.
It takes the form of Na ₂ SO ₄ . This means that SO _x in the gas
Not only does this method not release sulfur compounds, which are trace components in the waste liquid, it can also be incorporated into the NaCl-Na ₂ SO ₄ system, which is an advantage in terms of the purity of the recovered product.

As mentioned above, not only NO _{x and} SO _x caused by the waste liquid generated during the incineration of this waste liquid, but also NO _x , SO _x , NH ₃ and HCl caused by combustion are also affected.
Another major advantage is that secondary pollution can be prevented by suppressing the amount scattered in exhaust gas.

To explain this invention in more detail with respect to the flow sheet shown in Figure 1, at the same time as the oxidation and thermal decomposition of harmful components, NaCl, the main component in the waste liquid,
Na ₂ SO ₄ is chemically stable in the temperature range of 800 to 1100°C, but physically melts or partially evaporates. The molten salt and combustion exhaust gas are introduced into the cooling crystallizer 4 from the lower part of the spray incinerator 3.
The introduction section contacts the circulating liquid from the cooling crystallizer, the salt is dissolved and transferred to the liquid side, and the exhaust gas is adiabatically cooled.
Therefore, since the liquid constantly evaporates, make-up water 9 is added to maintain the liquid level. However, as the incineration continues, the salt concentration in the circulating fluid gradually increases and finally reaches saturation solubility. During this period, the liquid temperature remains approximately constant within the range of 80 to 100°C because the combustion exhaust gas at 800 to 1100°C is in equilibrium at temperature saturation or very close to saturation due to adiabatic cooling.

Next, Figure 3 shows the solubility equilibrium of the Na ₂ SO ₄ -NaCl-H ₂ O system. In the temperature range of approximately 35°C or higher, the weight ratio of NaCl to Na ₂ SO ₄ is greater than approximately 4:1.
In the Na ₂ SO ₄ rich range, the crystals that precipitate are anhydrous mirabilite alone. However, at temperatures below around 25°C, the salt that precipitates is Na ₂ SO ₄ decahydrate or a mixed salt of this and NaCl. Therefore, in terms of the value of Na ₂ SO ₄ that can be recovered, there is a clear advantage at temperatures above about 35°C. In this regard, the operating conditions of the present invention have the advantage that this temperature condition can be maintained automatically without requiring any external temperature regulating device.

Now, crystals of anhydrous mirabilite are precipitated in the mother liquor, and a part of the circulating slurry is extracted at a slurry concentration range of usually 5 to 20% and supplied to the solid-liquid separator 5.
The crystals of Na ₂ SO ₄ are separated and collected in a mirabilite receiver 6. On the other hand, the crystal separation mother liquor is returned to the cooling crystallizer 4 again, but a part of it is extracted to the brine receiver 7 to adjust the operational balance. .

Note that the mother liquor composition at this time moves to the left (in the NaCl-rich direction) of the dissolution equilibrium line in Figure 3 due to the precipitation of Na ₂ SO ₄ , and under the temperature condition of 80 to 100°C, NaCl
At the point of about 26%, Na ₂ SO ₄ about 5%, finally
Co-deposition of Na ₂ SO ₄ and NaCl begins. Therefore, in order to reduce the loss of Na ₂ SO ₄ , it is preferable to maintain the mother liquor composition within a range that does not reach this point and as close as possible to this point. However, once the operating concentration is reached, it should normally be managed to match the balance of the system, and to be precise, it should be checked by chemical analysis.
You can also roughly check that NaCl does not mix into the crystal using simple methods such as specific quantity and conductivity. Further, since a part of the condensed water recovered by the vacuum evaporation concentrator 2 can be used as the make-up water 9 of the cooling crystallizer 4, no new water is required.

Finally, the brine containing NaCl as its main component extracted from the system is subjected to necessary normal treatments such as turbidity and pH in a post-processing device 8, and is used for purposes such as discharge or salt recovery.

Example 700m ³ /day of thermal power plant desulfurization equipment wastewater was subjected to reverse osmosis desalination and evaporation treatment, and then the concentrated liquid was
When processed using the method shown in the flow sheet in the figure,
The income and expenditure was as follows.

(A) Incineration stock NaCl 3.2T/D Na ₂ SO ₄ 6.7T/D H ₂ O 23.6T/D Total 33.5T/D (B) Make-up water H ₂ O 66.3T/D (C) Recovered mirabilite Na ₂ SO ₄ 5.9T/D (D) Brine extracted NaCl 3.2T/D Na ₂ SO ₄ 0.8T/D H ₂ O 9.3T/D Total 13.3T/D (E) Exhaust gas H ₂ O 80.6T/D Gas Omitted total 80.6T/D The raw water composition before reverse osmosis desalination is as follows.

Na ⁺ : 5000ppm Cl ^- : 4000ppm SO _-4 ^2- : 6500ppm Based on the above ionic composition, the amounts of each salt are as follows.

NaCl: 3.2T/D Na ₂ SO ₄ : 6.7T/D As a result of the above, 5.9T/D of anhydrous mirabilite could be recovered.
The extracted salt water is NaCl3.2T/D, Na ₂ SO ₄ 0.8T/
D, H ₂ O9.3T/D, total 13.3T/D,
_{The Na2SO4} recovery rate is 86%.

In this example, the loss of solute permeation in the reverse osmosis device, the loss of the mother liquor to which the separated crystals adhered in the solid-liquid separator, and the loss due to the mixing of crystals into the extracted liquid were ignored.

[Brief explanation of the drawing]

Figure 1 is a flow sheet showing an embodiment of the present invention, Figure 2 is a curve diagram showing the relationship between the incineration temperature of concentrated waste liquid and the decomposition rate of each component, and Figure ₃ is a flow sheet showing an embodiment of the _present invention.
It is a dissolution equilibrium diagram of the NaCl-H ₂ O system. 1... Desalting device, 2... Vacuum evaporation concentration device, 3... Spray incinerator, 4... Cooling crystallizer, 5... Solid-liquid separator, 6
... Glauber's salt receiver, 7... Salt water receiver, 8... Post-processing device, 9
...supply water.

Claims (1)

[Claims] 1 Wastewater containing sulfur compounds and/or nitrogen compounds and containing NaCl and Na ₂ SO ₄ as main components is treated with a desalting device, and then the concentrated liquid is further concentrated with an evaporator. Then, the concentrated liquid is incinerated to oxidize or thermally decompose the sulfur compounds and/or nitrogen compounds in the liquid, and the dissolved salts of NaCl and Na ₂ SO ₄ discharged along with the exhaust gas from the incineration process are removed. The solution was dissolved in a liquid in a cooling crystallizer, and part of the condensed water from the evaporator was used as the dissolution water, and new conditions were established at a temperature of about 35°C or higher to prevent the precipitation of Na ₂ SO ₄ 10H ₂ O. Only anhydrous sodium sulfate is selectively precipitated without using a heating source and recovered by solid-liquid separation.Then, from the separated mother liquor, an amount of liquid corresponding to the absolute amount of NaCl input into the system is continuously poured out of the system. A method for treating a concentrated liquid from a wastewater desalting apparatus, characterized in that the remaining mother liquor is returned to the cooling crystallizer. 2. The treatment method according to claim 1, wherein the waste heat from the incineration process of the concentrated liquid is used for concentrating the waste liquid in the evaporator and for maintaining temperature conditions effective for precipitation of anhydrous sodium sulfate in the cooling crystallizer. . 3. Claim 1 or 2, wherein the weight ratio of NaCl to Na ₂ SO ₄ in the concentrated solution to be subjected to incineration is ₁ part or more of Na ₂ SO _{4 to} 4 parts of NaCl. Processing method.