JPS6232996B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for efficiently decomposing COD (chemical oxygen demand) components, particularly dithionate ions, in waste water discharged from a flue gas desulfurization process. Generally, when coal or oil is burned, the exhaust gas generated contains sulfur dioxide gas, which is a source of air pollution. In order to remove sulfur dioxide gas from these exhaust gases, the exhaust gases are desulfurized by contacting them with an absorbing solution of alkaline solutions such as NaOH and Ca(OH) ₂ , but the desulfurization wastewater generated during this process contains suspended solids and Because it contains organic matter, it cannot be discharged directly into public waters. Therefore, in the past, pollutants in flue gas desulfurization wastewater were removed using activated sludge method, coagulation sedimentation method, etc., but COD that cannot be removed even with these treatments.
(Chemical oxygen demand) components remained in treated water in considerable amounts, making it difficult to say that this was a sufficient wastewater treatment method. This difficult-to-treat COD is caused by nithionic acid ions (S ₂ O ₆ ^2- ), which are generated when part of the bisulfite ions (HSO ^- ₃ ) in the absorption liquid changes. is a very stable substance and difficult to treat with normal methods such as oxidative decomposition using oxidizing agents such as chlorine, ozone, hydrogen peroxide, and sodium hypochlorite. The method used is to concentrate and separate using an ion exchange resin or the like, and then thermally decompose it with an acidic sulfuric acid solution at 100 to 120°C. Figure 1 shows an example of wet flue gas desulfurization and wastewater treatment using the lime-gypsum method. In Figure 1, A is a cooling process, B is an SO ₂ absorption process, C is an oxidation process, D is a gypsum separation process, E is a coagulation precipitation process, F is a dehydration process, G is a Ca removal process,
H is sand filter process, I is first PH adjustment process, J is ion exchange process, K is sulfuric acid decomposition process, L is second PH adjustment process, a is make-up water, b is wastewater from cooling process (hereinafter referred to as cooling process wastewater) ), c is supernatant water, d is slaked lime, e is polymer flocculant, f is soda carbonate, g is polymer flocculant, h is sulfuric acid, i is caustic soda, j
is sulfuric acid, k is alkaline waste liquid, l is sulfuric acid, m is steam, n is water treated in the sulfuric acid decomposition process, o is caustic soda, and p is treated water. In FIG. 1, combustion flue gas, that is, combustion flue gas fueled by oil or coal, is cooled in a cooling step A using a cooling tower, and then led to an SO ₂ absorption step B, where SO ₂ is removed and then released. At this time, cooling process wastewater containing _SO4 components is discharged from the cooling process A. On the other hand, SO ₂ absorption process B, oxidation process C
Then, the supernatant water c is discharged through a gypsum separation step D. In this way, the waste water discharged from the flue gas desulfurization equipment, that is, the cooling process waste water b and the supernatant water c, are mixed (hereinafter referred to as mixed waste water) and led together to the coagulation-sedimentation process E, where slaked lime and polymer A flocculant is added to remove suspended solids SS, heavy metals, and fluorine from the mixed wastewater. The sludge that has been precipitated and separated in the coagulation-sedimentation step E is then led to a dewatering step F, where it is dehydrated. On the other hand, the wastewater from the coagulation-sedimentation step E is led to the Ca removal step G, where soda carbonate f and polymer flocculant g are added, where Ca ions in the wastewater are precipitated and separated as calcium carbonate. Continuing
The wastewater from the Ca removal process G undergoes sand filtering process H to remove suspended solids, and then undergoes the first PH adjustment process I.
In this step, sulfuric acid, which is a pH adjuster, is added to adjust the pH to a range suitable for ion exchange. The wastewater whose pH has been adjusted in the first pH adjustment step I is passed through an anion exchange resin layer in an ion exchange step J to remove COD components such as dithionate ions in the wastewater. The wastewater from the ion exchange process J continues to the second PH.
The water is led to the adjustment step L, where caustic soda O, which is a pH regulator, is added to adjust the pH to meet the discharge standards, and then it is discharged as treated water P. On the other hand, the anion exchange resin that has finished passing water in the ion exchange step J is regenerated through the following operations. Clear water is passed through the resin layer and raw water is extruded. Aqueous solution of caustic soda i is passed through to desorb COD components. Clear water is further passed through the resin layer to extrude and wash the remaining caustic soda solution. Sulfuric acid j is passed through the resin layer to turn the resin into a salt form. Clear water is further passed through the resin layer to extrude and wash the sulfuric acid remaining in the resin layer. Furthermore, the alkaline waste liquid k generated during regeneration using the caustic soda solution contains dithionate ions (COD component) and cannot be discharged as is. Therefore, this alkaline waste liquid containing dithionate ions was led to the sulfuric acid decomposition process K and thermally decomposed using sulfuric acid 1 and steam, and the treated water n from the sulfuric acid decomposition process was returned to the coagulation and precipitation process E. However, the conventional method has the following drawbacks. Conventionally, when the supernatant water C from the gypsum separation process D is recycled as it is to the cooling tower of the cooling process A, gypsum scale is generated due to Ca contained in the supernatant water C, which causes operational problems such as blocking the cooling tower nozzles. Therefore, circular reuse was not carried out. Then, the supernatant water c is sent to the coagulation-sedimentation process E of the post-treatment together with the cooling process wastewater b, so the amount of mixed wastewater to be treated from the desulfurization process is large, and the amount of makeup water a supplied is also large. of water was required. As mentioned in the above section, the amount of mixed waste water is large, and direct sulfuric acid decomposition without concentrating and separating the dithionate ions in the mixed waste water consumes a lot of utilities (steam, sulfuric acid) and is inappropriate. The nithionic acid ions in the mixed wastewater were concentrated and separated using an ion exchange resin or the like in the ion exchange step J, and then decomposed with sulfuric acid in the sulfuric acid decomposition step K, making the treatment operation complicated. After concentrating the dithionate ions in the mixed wastewater with an ion exchange resin in the ion exchange step J, adding sulfuric acid L as an acid in the sulfuric acid decomposition step K to the alkaline waste liquid K obtained by desorbing the dithionate ions with an alkaline agent, Decomposed by heating. At this time, the concentration of nitionate in this alkaline waste liquid is higher than that in direct sulfuric acid decomposition.
Since the reaction rate is low at the same temperature and sulfuric acid concentration as for direct sulfuric acid decomposition, it is necessary to increase the sulfuric acid concentration to obtain the same reaction rate as for direct sulfuric acid decomposition. Therefore, sulfuric acid j is required for this reaction, and sulfuric acid l is also required as an acid to neutralize the alkali coexisting in the alkaline waste liquid k.
As a result, it was necessary to add a large amount of sulfuric acid from outside the system. The present invention processes combustion exhaust gas using the lime-gypsum method, removes calcium from the supernatant water discharged from the gypsum separation step of the lime-gypsum method in the calcium carbonate recovery step, and then cools the above-mentioned lime-gypsum method. The feature is that the wastewater from the cooling process, which is circulated to the process and then contains acid components such as sulfuric acid and nithionic acid ions, is acid-decomposed without being concentrated and separated. In addition, the coexistence of an acid such as sulfuric acid is essential in order to thermally decompose the nitionate ions contained in wastewater. The present invention provides a method for extremely efficiently treating wastewater containing nithionic acid ions. In this way, the present invention treats combustion exhaust gas by the lime-gypsum method, and uses the supernatant water discharged from the gypsum separation step of the lime-gypsum method to remove calcium in the calcium carbonate recovery step.
Since it is circulated to the cooling process of the gypsum method, there is no trouble associated with the generation of gypsum scale in the cooling tower in the above cooling process, and therefore the amount of makeup water supplied is small, and the amount of wastewater to be treated is reduced accordingly. The amount will also be very small. Furthermore, the combustion exhaust gas is treated by the lime-gypsum method, and the supernatant water discharged from the gypsum separation step of the lime-gypsum method is subjected to calcium carbonate recovery step, after which calcium is removed from the supernatant water, which is then subjected to the cooling step of the lime-gypsum method. Since the nithionic acid ions contained in the supernatant water are circulated to This eliminates the need for the complicated process of concentrating and separating the nithionic acid ions using a sulfuric acid solution and then thermally decomposing them with sulfuric acid. Furthermore, the wastewater discharged from the cooling process of the lime-gypsum method contains concentrated acid components such as sulfuric acid and nithionic acid ions, and the acid components are converted into the acid necessary for the decomposition reaction of the nithionic acid ions. Since it can be used as a catalyst, it is not necessary to supply a large amount of sulfuric acid from outside the system as in the past, or it is almost unnecessary, making it extremely economical. In addition, combustion exhaust gas is treated using the lime-gypsum method,
The supernatant water discharged from the gypsum separation step of the lime-gypsum method is recycled to the cooling step of the lime-gypsum method after calcium is removed in the calcium carbonate recovery step, and is then treated with acid components such as sulfuric acid and nitionate ions. The wastewater from the above cooling process containing
Since acid decomposition is performed without concentration separation, ion exchange resin is not required. The most preferred embodiment of the present invention will be described below with reference to the embodiment shown in FIG. In Figure 2, A is the cooling process, B is the SO ₂ absorption process, C is the oxidation process, D is the gypsum separation process, and A is the gypsum separation process.
Ca removal process, B is the first coagulation precipitation process, C is the sulfuric acid decomposition process, D is the second coagulation precipitation process, E is the dehydration process, F is the recooling process, G is the dilution process, H is the PH adjustment process, Activated carbon treatment process, 1 is makeup water, 2 is wastewater from the cooling process (hereinafter referred to as cooling process wastewater), 3 is supernatant water, 4 is soda carbonate, 5 is polymer flocculant, 6 is circulating water, 7 is calcium carbonate slurry, 8 is polymer flocculant, 9 is sulfuric acid, 10 is steam,
11 is slaked lime, 12 is a polymer flocculant, 13 is re-cooling water, 14 is dilution water, 15 is sulfuric acid, 16 is treated water, the desulfurization process is cooling process A, SO ₂ absorption process B,
Consisting of oxidation process C and gypsum separation process D, the wastewater treatment process includes Ca removal process (a), first coagulation and precipitation process (b), sulfuric acid decomposition process (c), second coagulation and precipitation process (d), dehydration process (e), recooling process (d), and dilution process. The process consists of (1), PH adjustment process (1), and activated carbon treatment process (1). In FIG. 2, combustion flue gas, i.e., exhaust gas, is cooled by a portion of make-up water 1a and circulating water 6 from Ca removal step A in a cooling step A using a cooling tower, not shown. , fly heat (SS), heavy metals, F, Ca and
A part of the COD is removed and then led to the SO ₂ absorption step B, where the SO ₂ component is absorbed and removed by the remaining portion 1b of the make-up water 1 and then released. On the other hand, the water that has absorbed the SO ₂ component in the SO ₂ absorption step B passes through the oxidation step C and the gypsum separation step D, and is discharged as supernatant water 3. In this way, the supernatant water 3 from the gypsum separation step D is led to the Ca removal step A, and then the supernatant water 3
Soda carbonate 4 and a polymer flocculant 5 are added to the supernatant water 3, and Ca ions in the supernatant water 3 are separated as insoluble calcium carbonate precipitates. Further, the circulating water 6 from which Ca ions have been removed is circulated to the cooling tower of the cooling process A, and is used for cooling exhaust gas, removing dust, etc. as described above. On the other hand, the calcium carbonate slurry 7 containing the calcium carbonate precipitate is either supplied to the second coagulation-sedimentation step (d) and used as an alkali agent, or led to the dehydration step (e) and discharged after dehydration. Also,
Suspended substances such as fly ash, sulfuric acid (SO ₄ ^2- ),
The cooling process wastewater 2 containing dithionate ions (SO ₆ ^2- ), etc. (the pH of the cooling process wastewater is 1 to 2) is led to the first coagulation and precipitation process B, where a cationic polymer flocculant 8 is added. Added suspended solids (fly ash)
is removed. In this case, the concentration of the polymer flocculant added is about 2 ppm. In addition, nithionic acid ions are mainly produced in the above SO ₂ absorption process B and oxidation process C, but since the supernatant water containing them is recycled and used in the cooling process A, they are included in the cooling process wastewater 2 and come out. be. The waste water from the first coagulation-sedimentation process B is led to the sulfuric acid decomposition process C, where it is heated by exchanging heat with the waste water of a sulfuric acid decomposition tank (not shown), and then sulfuric acid 9 is added as needed and heated by steam 10. Nithionate ions in wastewater are decomposed as shown in the first equation. This decomposition reaction is preferably carried out at a temperature of 130°C, a pressure of 1.7 Kg/cm ² G, and a sulfuric acid concentration of 0.5 to 1.0 wt%. Usually, the sulfuric acid produced by absorption of SO ₂ in the exhaust gas in the cooling process wastewater 2 is sufficient. , replenishment from outside the system is generally not necessary. Note that in the first formula, sulfuric acid exhibits a catalytic action. The wastewater from the sulfuric acid decomposition step C is led to the second coagulation-sedimentation step D, where the calcium carbonate slurry 7 supplied from the Ca removal step A is combined with the slaked lime 12 and polymer flocculant 13 added if necessary. 2
As shown in the formula and the third formula, heavy metals are precipitated and separated as sparingly soluble hydroxides, fluorine is precipitated and separated as insoluble calcium fluoride, and suspended substances are separated by coagulation and precipitation. In this case, it is preferable to carry out the treatment under the condition that the concentration of the anionic polymer flocculant is 5 ppmPH>4 or more, and in this example, the calcium carbonate slurry is supplied from the Ca removal step, but this is not necessarily necessary. The sludge, which is the precipitate from the second coagulation-sedimentation step (d), is dehydrated and discharged to reduce its volume in the dehydration step (e). On the other hand, the high-temperature supernatant water from the second coagulation-sedimentation step 2 is led to the cooling step, where it exchanges heat with the cooling water 13 and is cooled from 80° C. to 40° C. Then, in order to avoid the formation of gypsum scale in subsequent equipment, dilution water 14 is added to the wastewater from the recooling process in a dilution process to make the wastewater unsaturated with gypsum. In this case, the risk of scale generation differs depending on the condition (concentration, composition) of Ca salt in the waste water from the re-cooling process, water temperature, etc. The necessity of supply and the amount of dilution water differ. Next, in the PH adjustment step 1, sulfuric acid 15, which is a PH adjusting agent, is added to the wastewater to adjust it to a discharge standard of PH=7, and the wastewater is sent to the activated carbon treatment step 2. In the activated carbon treatment process, COD components originating from organic matter in the wastewater are adsorbed and removed by the activated carbon and discharged as treated water 16. Incidentally, since secondary decomposition of organic matter is expected in the sulfuric acid decomposition step 11, the activated carbon treatment step 11 is not necessarily necessary. Next, an experimental example based on the flow shown in this embodiment will be shown. Experimental example When wastewater generated when desulfurizing exhaust gas (1,860,000Nm ³ /H) discharged from a coal-fired power generation facility with an output of 600MW was treated in the manner shown in Figure 2, the amount and quality of water in the experiment was as follows. Ta. Water quantity and water quality of supernatant water 3 from gypsum separation process B 5 m ³ /h PH 3 to 4 SS 200ppm S ₂ O ₆ 9600ppm Ca 700ppm SO ₄ 1200ppm After removing Ca from the above supernatant water 3 in Ca removal process A Quantity and quality of circulating water 7 4.9m ³ /h PH 8-9 SS 20ppm S ₂ O ₆ 9600ppm Ca 200ppm SO ₄ 1200ppm Quantity and quality of cooling process wastewater 2

[Table] Sulfuric acid decomposition conditions and decomposition rate In this case, sulfuric acid from outside the system was not added. H ₂ SO ₄ concentration in wastewater 0.5wt% Temperature 130℃ Pressure 1.7Kg/cm ² G Reaction time 1hr S ₂ O ₆ decomposition rate 96% Water volume and quality of wastewater from sulfuric acid decomposition process 20.6m ³ /h PH 2 SS 500ppm S ₂ O ₆ 194ppm Organic matter 9.7ppm F 970ppm FeAlMg 837ppm In this example, the combustion exhaust gas is treated by the lime-gypsum method, and is discharged from the gypsum separation step D of the lime-gypsum method. Since the supernatant water 3 is circulated to the cooling process A of the lime-gypsum method after removing calcium in the calcium carbonate recovery process A, there is no trouble associated with the generation of gypsum scale in the cooling tower in the cooling process A, and the drainage is can be reused as circulating water 6, so the supply amount of make-up water 1 can be reduced, and the amount of cooling process waste water 2 can be reduced accordingly, making post-treatment easier. Furthermore, the combustion exhaust gas is treated by the lime-gypsum method, and the supernatant water 3 discharged from the gypsum separation step D of the lime-gypsum method is removed from calcium in the calcium carbonate recovery step A, and then calcium is removed by the lime-gypsum method. Since it is circulated to the cooling process A, the nithionic acid ions S ₂ O ₆ ^2- contained in the supernatant water 3 are sequentially concentrated, and thus a small amount of the cooling process wastewater 2 with concentrated nithionic acid ions is directly acid-decomposed. Therefore, the complicated process of concentrating and separating nithionic acid ions using a conventional ion exchange resin or the like and then thermally decomposing them with sulfuric acid becomes unnecessary. Furthermore, the cooling process wastewater 2 discharged from the cooling process of the lime-gypsum method contains concentrated acid components such as sulfuric acid and nithionic acid ions, and the acid components are used in the decomposition reaction of the nithionic acid ions. Since it can be used as a necessary acid catalyst, it becomes unnecessary to supply a large amount of sulfuric acid 9 from outside the system as in the past, and generally there is no need to supply sulfuric acid 9 from outside the system, making it extremely economical. In addition, combustion exhaust gas is treated using the lime-gypsum method,
The supernatant water 3 discharged from the gypsum separation step D of the lime-gypsum method is circulated to the cooling step A of the lime-gypsum method after removing calcium in the calcium carbonate recovery step A, and then treated with acid components such as sulfuric acid. The above cooling process wastewater 2 containing nitionate ions
Since it performs acid decomposition without concentrating and separating, ion exchange resin is not required. Note that the acid that acts as a catalyst for the decomposition of nithionic acid ions is not limited to sulfuric acid, and acids such as hydrochloric acid may also be used as a catalyst. In the future, coal-fired power generation will be in the spotlight, and needs will tend to increase, and the present invention efficiently treats desulfurization wastewater containing nitionate ions in order to promote this trend. Furthermore, the present invention can be applied not only to coal fuel flue gas, but also to a method for treating gas generated by carbonizing or burning petroleum, or flue gas containing sulfur dioxide gas, and to a method for treating wastewater associated therewith.

[Brief explanation of the drawing]

FIG. 1 is a system explanatory diagram showing a conventional method for treating wastewater discharged from a flue gas desulfurization process, and FIG. 2 is a system explanatory diagram showing an embodiment of the present invention. A... Cooling process, B... SO ₂ absorption process, C... Oxidation process, D... Gypsum separation process, A... Ca removal process, H...
Sulfuric acid decomposition process, 1... make-up water, 2... cooling process wastewater,
3...supernatant water,...desulfurization process,...wastewater treatment process.

Claims (1)

[Claims] 1 The combustion exhaust gas is treated by the lime-gypsum method, and the supernatant water discharged from the gypsum separation step of the lime-gypsum method is recycled to the cooling step of the lime-gypsum method after removing calcium in the calcium carbonate recovery step. A method for treating wastewater, characterized in that the wastewater from the cooling step containing acid components such as sulfuric acid and nithionic acid ions is then subjected to acid decomposition without being concentrated and separated.