JPH08192192A - Method for treating flue gas desulfurization drain - Google Patents

Abstract

PURPOSE: To remove nitrogen compounds in the drain of flue gas desulfurization by catalytic decomposition without causing a lowering of catalyst performance due to the generation of scale of gypsum or metal by a method wherein the flue gas desulfurization drain is softened to provide softened treated water, which is treated in the presence of catalyst to reduce nitric acid, and ammonia is oxidized in the presence of catalyst. CONSTITUTION: Alkali is added to flue gas desulfurization drain in a softening treatment tank 1, while the drain is being agitated, thereby to adjust pH to, for example, 11 or more, and carbonate is added thereto and further flocculant is added. The treated water, in which metal hydroxide and calcium carbonate are deposited and coagulated, is sent to a concentration device 2, where supernatant liquid is sent to a filter 4 by a pump 3, and then pH of the water is adjusted to, for example, 6-8 at a pH adjusting tank 5. Thereafter, water is sent to a catalyst packed tower 9A via a heat exchanger 7 and a heater 8, in which reducing agent is added to water to reduce nitric acid ion. The treated water which leaves the tower A9 and to which an oxidizing agent is added on piping way is sent to a catalyst packed tower B10, where ammonium ion is oxidized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating flue gas desulfurization wastewater. More specifically, the present invention relates to a method for removing nitrogen in wastewater that occurs when absorbing or decomposing exhaust gas generated when fuel such as heavy oil and coal is burned. The present invention relates to a flue gas desulfurization effluent treatment method capable of removing nitrogen efficiently by a catalytic decomposition method without any problem.

[0002]

2. Description of the Related Art Exhaust gas desulfurization is applied to various fields such as exhaust gas from boilers using various fossil fuels, various furnaces, exhaust gas from sintering machines, and exhaust gas from sulfur recovery equipment. For example, boiler exhaust gas is processed through a denitration step, an electrostatic precipitating step, and a desulfurization step. In the denitration process, NO _x is reduced with ammonia in the presence of a catalyst, and in the dust collection process, soot is electrostatically separated. At that time, ammonia is supplied as a neutralizing agent to protect the dust collector. And the desulfurization process,
For example, in the lime gypsum method, SO _x comes into contact with calcium carbonate slurry to be separated as gypsum, and flue gas desulfurization wastewater is discharged. In the wastewater discharged from such a process, ammonium salt is usually contained in an amount of 200 to 1500 pp.
m, nitrate is 10 to 50 ppm, calcium salt and sulfate are saturated as calcium sulfate, and other metal salts such as iron salt, copper salt, manganese salt and magnesium salt are contained. Conventionally, biological treatment methods have been generally adopted for removing nitrogen in flue gas desulfurization wastewater.
(1) Flue gas desulfurization wastewater has large load fluctuations, control is required to maintain conditions that match the fluctuations, and maintenance is difficult. (2) Excess sludge occurs, (3) Startup period That is, there is a problem that the acclimatization period is long. As a method for solving these problems, a method of decomposing and removing a nitrogen compound using a catalyst has been attempted. According to the catalytic decomposition method, the above problems of the biological treatment method are solved, but on the other hand, the desulfurization wastewater is a gypsum saturated solution and contains a large amount of metals, so that scale formation occurs in the heating line. It is not practical because it tends to cause deterioration of catalyst performance. For this reason, it is required to develop a catalytic decomposition method for removing nitrogen from flue gas desulfurization wastewater, which is sufficiently practical.

[0003]

DISCLOSURE OF THE INVENTION The present invention provides a method for removing nitrogen compounds in flue gas desulfurization wastewater by catalytic decomposition without causing scale generation by gypsum and deterioration of catalytic performance by metals. It was made for the purpose.

[0004]

Means for Solving the Problems The inventors of the present invention have conducted extensive studies to develop a method capable of catalytically decomposing flue gas desulfurization wastewater without causing the above-mentioned problems in the process. After softening the flue gas desulfurization wastewater, reduction of nitric acid and then oxidation of ammonia are carried out in the presence of a catalyst.
It has been found that removal of nitrogen compounds can be easily and efficiently performed, and the present invention has been completed based on this finding. That is, the present invention includes (1) a first step of softening flue gas desulfurization wastewater, a second step of reducing nitric acid in the softened water in the presence of a catalyst, and a second step of treating water in the presence of a catalyst in ammonia. The present invention provides a method for treating flue gas desulfurization wastewater, which comprises a third step of oxidizing. Further, as a preferred embodiment of the present invention, (2) in the first step, after the pH of the flue gas desulfurization wastewater is set to 11 or more, a carbonate and a coagulant are added to the flue gas desulfurization wastewater. Treatment method, (3) In the second step, the flue gas desulfurization wastewater treatment method according to (1) to (2), wherein hydrazine, hydroxylamine or hydrogen gas is used as a reducing agent, and (4) In the 3 steps, nitrous acid or its derivative or hydrogen peroxide is used as an oxidant.
The method for treating flue gas desulfurization wastewater described in (3) can be mentioned.

In the first step of the method of the present invention, the pH of the flue gas desulfurization effluent is adjusted to 11 or more to add metal as hydroxide, and then carbonate is added to precipitate calcium as calcium carbonate. The pH can be adjusted, for example, by adding an alkali such as sodium hydroxide or potassium hydroxide in the form of an aqueous solution or a solid. By adjusting the pH of the flue gas desulfurization wastewater to 11 or more, metal salts such as iron salts, copper salts, manganese salts and magnesium salts contained in the wastewater are precipitated as hydroxides. Next, a water-soluble carbonate is added to the wastewater to precipitate calcium contained in the wastewater as calcium carbonate. Examples of the water-soluble carbonate used include sodium carbonate, sodium hydrogen carbonate, potassium carbonate, potassium hydrogen carbonate and the like. These water-soluble carbonates can be added as an aqueous solution or can be added in solid form. The amount of these water-soluble carbonates added is preferably 1 to 2 equivalent times the amount of calcium present in the wastewater. The amount of calcium present in the wastewater can be quantified by atomic absorption spectrometry or the like. In the first step, a metal hydroxide is precipitated as alkaline, and a coagulant is added to the wastewater in which calcium is precipitated as calcium carbonate by adding a carbonate, and then a metal hydroxide and calcium carbonate are aggregated. , Promote sedimentation. The aggregating agent used is not particularly limited, and inorganic aggregating agents such as polyaluminum chloride and ferric sulfate, sodium polyacrylate, anionic polymer aggregating agents such as polyacrylamide partial hydrolysates, polyaminoalkyl methacrylate, polyethylene Examples thereof include cationic polymer flocculants such as imines, and nonionic polymer flocculants such as polyacrylamide and polyethylene oxide. After the precipitate is aggregated and settled, the supernatant is left as it is, or if necessary, fine suspended substances not separated by settling are removed by filtration and transferred to a pH adjusting tank. The filter to be used is not particularly limited, and for example, a filter filled with sand or anthracite, a cartridge filter, a membrane separation device or the like can be used. The supernatant or filtrate from which the precipitate has been separated and removed is then adjusted to pH 6-8 in a pH adjusting tank. The acid used for pH adjustment is not particularly limited and, for example, sulfuric acid, hydrochloric acid, etc. can be used.

In the second step of the method of the present invention, a reducing agent is added to the treated water to reduce nitrate ions. Reduction is
It is preferable to carry out heating in the presence of a catalyst. The reducing agent used is not particularly limited, and hydrazine, hydroxylamine, hydrogen gas and the like can be preferably used.
When the reducing agent is hydrazine or hydroxylamine, the amount of the reducing agent used is preferably 0.5 to 2 mol times the nitrate ion present in the treated water. When the reducing agent is hydrogen gas, it is preferable to add hydrogen gas in an amount exceeding the solubility in treated water. As the catalyst used in the second step, as a catalyst effective component,
Platinum, palladium, ruthenium, rhodium, indium, iridium, silver, gold, cobalt, nickel and tungsten, and water-insoluble or sparingly water-soluble compounds of these metals, such as cobalt monoxide, nickel monoxide, ruthenium dioxide, and trioxide. One selected from the group consisting of oxides such as rhodium, palladium monoxide, iridium dioxide, and tungsten dioxide, and further chlorides such as ruthenium dichloride and platinum dichloride; ruthenium sulfide; sulfides such as rhodium sulfide; Α-
Examples thereof include those supported on a carrier such as alumina, γ-alumina, activated carbon, titania, zirconia, zeolite, glass, silica, silica-alumina and ion exchange resin.
The supported amount of the metal or the compound thereof in such a supported catalyst is usually 0.05 to 25% by weight, preferably 0.5 to 3% by weight based on the weight of the carrier. Such a supported catalyst can be used in various forms such as a sphere, a pellet, a column, a crushed piece, a honeycomb, and a powder.
In particular, a catalyst in which platinum is supported on a granular carrier such as titania or γ-alumina is preferable. These catalysts are preferably packed in a column and the reaction is carried out by passing the nitrate ion-containing waste water containing a reducing agent under heating, and in this case, an upward flowing liquid is desirable. The reaction condition in the second step is a decomposition treatment temperature of 70 to 300 ° C, preferably 80 to 250 ° C.
And more preferably 140 to 180 ° C. The SV is 0.1-5 hr ^-1 , more preferably 0.5-3 hr.
^-1 is desirable. By this reduction treatment, most of nitrate ions become nitrogen gas according to the following formula. _{^{2NH 4 + + 2NO 3 - +}} N 2 H 4 → 3N 2 + 6H 2 O NH 4 + + NO 3 - + H 2 → N 2 + 3H 2 O

In the third step of the method of the present invention, an oxidant is added to the treated water to oxidize ammonium ions.
The oxidation is preferably carried out by heating in the presence of a catalyst.
The oxidizing agent used is not particularly limited, and nitrite, hydrogen peroxide and the like can be preferably used. Even if nitrite ions are generated in the second step, they act as an oxidant and are consumed up in this step. When the oxidant is nitrite, the amount of nitrite used is preferably 0.8 to 1.0 mol times the ammonium ion present in the treated water. When the amount of nitrite used is less than 0.8 times the molar amount of ammonium ions, the amount of ammonium ions remaining without being decomposed by nitrite increases. When the amount of nitrite used exceeds 1.0 mol times that of ammonium ions, excess nitrite remains in the waste water. When the oxidizing agent is hydrogen peroxide, the amount of hydrogen peroxide used is preferably 1.0 to 3.0 times the molar amount of ammonium ions present in the treated water. When the amount of hydrogen peroxide used is less than 1.0 mol times that of ammonium ions, the ammonium ions may not be completely decomposed by hydrogen peroxide and may remain in the waste water. Even when the amount of hydrogen peroxide used exceeds 3.0 mol times that of ammonium ions, the removal efficiency of the remaining ammonium ions does not improve in proportion to the increase in the amount of hydrogen peroxide added. In the method of the present invention, it is also possible to use nitrite and hydrogen peroxide together as an oxidizing agent, first to decompose ammonium ions using nitrite, and then to add hydrogen peroxide to decompose the remaining ammonium ions. is there. As the catalyst used in the third step, platinum, palladium, ruthenium, rhodium, indium, iridium, silver, gold, cobalt, nickel and tungsten, and a water-insoluble or sparingly water-soluble compound of these metals, as a catalyst active ingredient, For example, cobalt monoxide,
From oxides such as nickel monoxide, ruthenium dioxide, rhodium trioxide, palladium monoxide, iridium dioxide, and tungsten dioxide, as well as chlorides such as ruthenium dichloride and platinum dichloride, and sulfides such as ruthenium sulfide and rhodium sulfide. 1 or 2 selected from the group
Examples thereof include those in which the above-described species are supported on a carrier such as α-alumina, γ-alumina, activated carbon, titania, zirconia, zeolite, glass, silica, silica-alumina, and ion exchange resin. The supported amount of the metal or the compound thereof in such a supported catalyst is usually 0.05 to 25% by weight, preferably 0.5 to 3% by weight based on the weight of the carrier. Such a supported catalyst can be used in various forms such as a sphere, a pellet, a column, a crushed piece, a honeycomb, and a powder. In particular, a catalyst in which platinum is supported on a granular carrier such as titania or γ-alumina is preferable. It is preferable that these catalysts are packed in a column and the reaction is carried out by passing ammonium ion-containing wastewater containing an oxidizing agent under heating, and in this case, an upward flowing liquid is desirable. In the third step, the pH of the treated water after the second step is 5 to 8,
It is preferably 6 to 7. If the pH of the treated water after the second step is out of this range, use an acid such as sulfuric acid or hydrochloric acid or an alkali such as sodium hydroxide, potassium hydroxide or sodium carbonate.
Readjust. If the pH of the treated water is less than 5, a part of the nitrite ion may be changed to the nitrate ion again. If the pH of the treated water exceeds 8, the reaction rate of ammonium ion and the oxidant may decrease. The decomposition treatment temperature is 70 to 300 ° C, preferably 80 to 250 ° C, and more preferably 140 to 180 ° C. Also,
The SV is 0.5 to 20 hr ^-1 , preferably 2 to 5 hr ^-1 . By this oxidation treatment, ammonium ions become nitrogen gas according to the following formula. _{^{NH 4 + + NO 2 - →}} N 2 + 2H 2 O 2NH 4 + + 3H 2 O 2 → N 2 + 2H + + 6H 2 O

The method of the present invention will be described below with reference to the drawings.
FIG. 1 is a schematic view of an example of an apparatus for carrying out the method of the present invention. First, the flue gas desulfurization wastewater is introduced into the softening treatment adjusting tank 1, and the pH is adjusted to 11 or more by adding an alkali under stirring in this tank, and then a carbonate is added and a coagulant is further added. The treated water in which the metal hydroxide and calcium carbonate are deposited and aggregated is sent to the concentrating device 2 to settle the aggregate. The supernatant liquid is sent to the filter 4 by the pump 3 and the fine suspended substance is removed by filtration. The filtrate that has passed through the filter is guided to the pH adjusting tank 5, and the pH is adjusted to 6 to 8 in this tank. The treated water after the pH adjustment is then pumped by the pump 6.
A reducing agent is added in the middle of the piping via the heat exchanger 7 and the heater 8 and sent to the catalyst packed column A9 to carry out a reduction treatment of nitrate ions. The treated water discharged from the catalyst-packed tower A is added with an oxidant in the middle of the pipe, and is introduced into the catalyst-packed tower B10 to oxidize ammonium ions. The treated water that has left the catalyst-filled tower B uses residual heat in a heat exchanger, and then is sent to a gas-liquid separation tower 12 via a pressure regulating valve 11 and separated into nitrogen gas and treated water. According to the method of the present invention, nitrogen in wastewater generated when absorbing or decomposing exhaust gas generated when burning fossil fuel is efficiently removed without deteriorating the treatment performance of the catalyst and clogging of piping and the like. Is possible.

[0009]

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. Example 1 Ammoniacal nitrogen 514 mg / liter, Nitrate nitrogen 52
To the flue gas desulfurization wastewater containing mg / l, calcium 320 mg / l, magnesium 450 mg / l, iron 3 mg / l and having a pH of 5.4, sodium hydroxide 3,400 mg / l was added to obtain a pH of 11.5. did. Then, 850 mg / liter of sodium carbonate was added with vigorous stirring. After 30 minutes, 2 mg / liter of an anionic flocculant (polyacrylamide partial hydrolyzate) was added, and the mixture was gently stirred for 3 minutes and then allowed to stand for 1 hour. The supernatant is filtered using No.5A filter paper, sulfuric acid is added to the filtrate, and
The H was adjusted to 6.5. The metals in this treated water are calcium 3 mg / liter, magnesium 1 mg / liter or less,
The iron content was 1 mg / liter or less. This treated water was continuously heated to 160 ° C. by a heater using a pump, and then, in a catalyst packed tower A filled with titania spheres having a diameter of 1.5 mm and carrying 0.5% by weight of platinum, per 1 liter of treated water While adding 3.7 × 10 ^-2 mol of hydrogen gas, the solution was passed through at SV = 0.8 hr ^-1 and reduction treatment was performed at 160 ° C. Further, as a catalyst, in a catalyst packed tower B packed with titania spheres having a diameter of 1.5 mm supporting 0.5% by weight of platinum, treated water flowing out from the catalyst packed tower A was treated with an aqueous sodium nitrite solution as nitrite nitrogen. While adding so that 470 mg / liter, SV =
The solution was passed at 2 hr ^-1 , and an oxidation treatment was performed at 160 ° C. The nitrogen content in the treated water flowing out from the catalyst packed column B is 2 mg / liter of ammonia nitrogen, 3 mg / liter of nitrate nitrogen and 1 mg / liter of nitrite nitrogen or less, and the nitrogen in the flue gas desulfurization wastewater is sufficient. Had been removed. Further, no scale was generated in the heating part and the like, and the operation could be continued stably. Comparative Example 1 The same treatment as in Example 1 was performed except that the same flue gas desulfurization wastewater as that used in Example 1 was used and the softening treatment was not performed by adding sodium hydroxide, sodium carbonate and an anionic flocculant. Tried to do. However, before a stable quality of treated water was obtained, scales were generated in the heating part and liquid could not be passed through, and the test had to be stopped.

[0010]

According to the method of the present invention, by incorporating the softening treatment step into the nitrogen removal treatment of the flue gas desulfurization wastewater, the scale of the wastewater desulfurization wastewater containing many coexisting substances can be reduced during the process. It is possible to remove nitrogen by the catalytic decomposition method without the generation of hydrogen and the deterioration of catalyst performance.

[Brief description of drawings]

FIG. 1 is a schematic view of an example of an apparatus for carrying out the method of the present invention.

[Explanation of symbols]

 1 Softening Treatment Control Tank 2 Concentrator 3 Pump 4 Filter 5 pH Control Tank 6 Pump 7 Heat Exchanger 8 Heater 9 Catalyst Packing Tower A 10 Catalyst Packing Tower B 11 Pressure Control Valve 12 Gas-Liquid Separation Tower

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication location C02F 9/00 504 BE E01D 53/50 53/77 C02F 1/58 ZAB H (72) Inventor Tomoyuki Asada, 3-3-22 Nakanoshima, Kita-ku, Osaka City, Kansai Electric Power Co., Inc. (72) Inventor, Hiroshi Kimoto 3-32-22, Nakanoshima, Kita-ku, Osaka City, Kansai Electric Power Co., Ltd. (72) Inventor, Nakahara Toshiji, 3-4-7 Nishishinjuku, Shinjuku-ku, Tokyo Kurita Industry Co., Ltd. (72) Inventor Yasuhiko Takabayashi 3-4-7 Nishishinjuku, Shinjuku-ku, Tokyo Kurita Industry Co., Ltd. (72) Yuko Kitami 3-4-7 Nishi-Shinjuku, Shinjuku-ku, Tokyo Kurita Industry Co., Ltd.

Claims (1)

[Claims] 1. A first step of softening flue gas desulfurization wastewater, a second step of reducing nitric acid from softened water in the presence of a catalyst, and a second step of oxidizing ammonia in the treated water of the second step in the presence of a catalyst. A flue gas desulfurization wastewater treatment method consisting of three steps.