Liquid purification or separation – Processes – Chemical treatment
Reexamination Certificate
1999-12-10
2001-09-25
Hruskoci, Peter A. (Department: 1724)
Liquid purification or separation
Processes
Chemical treatment
C210S757000, C210S758000, C210S766000, C210S903000, C423S237000
Reexamination Certificate
active
06294097
ABSTRACT:
FIELD OF ART
The present invention relates to a method for treating, by decomposing, waste water containing nitrate ions such as ammonium nitrate or sodium nitrate, that is generated in various industries including non-ferrous metal refining and petrochemical industry, and in particular, to a method for treating waste water containing nitrate ions that is capable of rendering decomposition product gases generated during the decomposition harmless in an effective, stable manner.
BACKGROUND ART
Waste water containing nitrate ions such as ammonium nitrate or sodium nitrate is generated in various kinds of industries including non-ferrous metal refining and petrochemical industry. Such waste water containing nitrate ions has been treated by conventional methods known in the art including activated sludge process and thermal decomposition method.
The activated sludge process has been put into practice in sewage treatment plants. In this method, appropriate activation of the function of microorganisms requires control of the nitrogen compound concentration in the incoming waste water containing nitrate ions not to exceed 0.3% by weight in terms of ammonium nitrate. Thus, treatment of a waste water containing high concentration of nitrate ions necessitates a large amount of dilution water, vast area for treatment, expensive plants, and considerable treatment cost. Moreover, treatment of a large volume of sludge which has been generated poses another problem.
Ammonium nitrate, as an example of a nitrate ion source, is known to be thermally decomposed in accordance with the following formulae (Chemical Handbook (Kagaku Binran), Application, Third Edition, p115):
NH
4
NO
3
→ NH
3
+ HNO
3
−41 kcal/mol (180° C.)
(1)
NH
4
NO
3
→ N
2
O + 2H
2
O
+10 kcal/mol (250° C.)
(2)
2NH
4
NO
3
→ 2N
2
+ 4H
2
O + O
2
+28 kcal/mol (300° C.)
(3)
4NH
4
NO
3
→ 2NO
2
+ 8H
2
O + 3N
2
+27 kcal/mol
(4)
Heating a waste water containing ammonium nitrate induces decomposition of ammonium nitrate through the reactions represented by the formulae (1) and (2) stepwise depending on the temperature of the ammonium nitrate, thereby generating HNO
3
and N
2
O, which cause formation of NOx. When the temperature for decomposition is lower, however, the reaction of the formula (1) proceeds in the reverse direction, resulting in recombined ammonium nitrate. In order to suppress the generation of the recombined ammonium nitrate and to decrease the decomposition gases, the temperature of ammonium nitrate should be rapidly raised to 300° C. or higher so that the reaction represented by the formula (3) proceeds. Thus, the conventional thermal decomposition method requires supply of the waste water by spraying into a decomposition furnace, while the furnace is kept at a temperature of not lower than 1000° C. to facilitate increase in temperature of the waste water. However, at a temperature of as high as not lower than 1000° C., nitrogen gas and oxygen react to form thermal NOx.
For overcoming such drawbacks, there has been proposed a two-stage decomposition method including the steps of decomposing ammonium nitrate at a lower temperature, and then further decomposing the resulting decomposition gas at a higher temperature.
One two-stage decomposition method known in the art follows the steps of spraying a concentrated ammonium nitrate solution into a heating furnace held at 180 to 210° C. to cause thermal decomposition, and then introducing the resulting decomposition gas into a decomposition furnace previously heated to 600 to 1000° C. for further decomposition (JP-52-22751A). This method, however, requires a decomposition furnace equipment that is resistant to a temperature of as high as 1000° C., and generates an exhaust gas containing as much as 0.4% NOx.
Another two-stage decomposition method known in the art for treatment of an ammonium nitrate waste water containing radioactive residue includes the steps of thermal-decomposing the waste water contained in a vessel in a primary decomposition furnace previously heated to 250 to 310° C., and then further decomposing the resulting recombined ammonium nitrate in a secondary decomposition furnace previously heated to 350 to 600° C. (JP-62-52277B). However, this method results in undesired generation of as much as 1.9% NOx since the decomposition temperature in the secondary decomposition furnace is maintained at a temperature of as low as 350 to 600° C. for preventing gassification and dispersion of the radioactive residue.
DISCLOSURE OF THE INVENTION
It is an object of the present invention to provide a method for treating waste water containing nitrate ions which is capable of effecting the treatment at a lower temperature than the conventional thermal decomposition method, and which is capable of suppressing the generation of harmful. gases such as NOx at a lower level and rendering the gases harmless.
The present inventors have made intensive studies, seeking a method for decomposing at a lower temperature HNO
3
and N
2
O that have been generated as a result of decomposition of ammonium nitrate through the reactions according to the formulae (1) and (2). They have taken notice of reactions of HNO
3
and N
2
O with alcohol, and found out that the above problems may be solved even at a temperature lower than the conventional decomposition temperature by adding a particular lower alcohol to a waste water of a particular concentration in terms of ammonium nitrate, and subjecting the resulting mixture to a treatment at a temperature exceeding a particular level, thereby completing the present invention.
According to the present invention, there is provided a method for treating waste water containing nitrate ions by subjecting the waste water to decomposition and oxidation to eliminate a harmful gas, the method comprising the steps of:
(a) mixing a waste water containing 10 to 90% by weight nitrate ions in terms of ammonium nitrate with a lower alcohol having 1 to 4 carbon atoms to prepare a mixture,
(b) spraying said mixture into a decomposition furnace held at a temperature of 250° C. or higher to decompose said mixture to generate a decomposition gas, and
(c) reacting said decomposition gas with a gas containing oxygen at a temperature of 250° C. or higher to render said decomposition gas harmless.
REFERENCES:
patent: 4990266 (1991-02-01), Vorlop et al.
patent: 5362405 (1994-11-01), Birbara et al.
patent: 5460734 (1995-10-01), Birbara et al.
patent: 52-007369 (1977-01-01), None
patent: 62-052277 (1987-11-01), None
patent: 01-040318 (1989-08-01), None
patent: 08-309370 (1996-11-01), None
patent: 52-022751 (1997-06-01), None
Katayama Yoshimasa
Ohashi Michiya
Anan Kasei Co., Ltd.
Darby & Darby
Hruskoci Peter A.
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