PROCESS FOR TREATING AMMONIUM NITRATE-CONTAINING WASTE WATER

Abstract: This invention provides a process for treating ammo­ nium nitrate-containing waste water which comprises sub­ jecting the waste water to wet thermal decomposition in the presence of a catalyst supported by a carrier and containing as an active component at least one of precious metals and compounds thereof insoluble or sparingly soluble in water while maintaining the waste water at a pH of about 3 to about 11.5 and a temperature of about 100 to about 370°C.

Description: This invention relates to processes for treating ammonium nitrate-containing waste water. In recent years, it has become important from the viewpoint of water-quality control to remove nitrogen components (particularly "ammonia nitrogen") as well as chemically oxidizable substances (hereinafter referred to as "COD components") from waste water. We conducted extensive research for a long time on processes for treating ammonia-containing waste water and developed processes for treating ammonia-containing waste water with ease and with economical feasibility; the processes comprise in general subjecting waste water to wet oxidation in the presence of a specific catalyst and under specified conditions (namely processes disclosed in Japanese Examined Patent Publications Nos. 19757/1984 (corresponding to US-A- 4141 828); 42992/1981; 42391/1982; 27999/1983; 33320/1982, etc.). With greater weight recently attached to nuclear power generation in power generating industry, one of important technological problems before us is the treatment of NH₄NO₃-containing waste water from the processing of uranium ores and reprocessing of the used uranium fuel. We attempted to treat such NH₄NO₃-containing waste water by adapting the foregoing processes for treating ammonia-containing waste water (hereinafter referred to as "prior processes") and found that the prior processes achieve the decomposition of NH₄⁺ ion with a high efficiency but the decomposition of NO₃⁻ ion with an efficiency not invariably satisfactory. This drawback is presumably caused by the presence of NH₄NO₃ in a concentration as high as about 1 % (10,000 ppm) to about 10 % (100,000 ppm). To overcome the foregoing drawback, we carried out further research and discovered that NO₃⁻ ion as well as NH₄⁻ ion is efficiently decomposed when subjecting NH₄NO₃-containing waste water to wet thermal decomposition substantially in the absence of oxygen or in the presence of oxygen in less than the theoretical amount required for the decomposition of the ammonia components, organic substances and inorganic substances present in the waste water instead of subjecting the waste water to the prior processes in which the wet oxidation of waste water is effected using oxygen in more than the theoretical amount required for the decomposition of the ammonia components, organic substances and inorganic substances contained in the waste water. E.g.; US-A- 4141 828 discloses that generally the the oxygen-containing gas is fed in an amount of 1 to 1.5 times, preferrably 1.05 to 1.2 times the theoretical amount of oxygen for treating ammonia-containing waste water which further contains only 300 ppm of nitrite nitrogen and nitrate nitrogen. Our continued research demonstrated that when subjecting to the wet thermal decomposition in the presence of oxygen in less than the theoretical amount NH₄NO₃-containing waste water having incorporated therein ammonia in a mole ratio of 1<NH₃-N/NO₃-N≦5 (which mole ratio means the ratio of the total moles of nitrogen in the forms of (NH₄⁺ and NH₃) per moles of nitrogen in the form of NO₃⁻), the decomposition of components is achieved with a further improved efficiency. Through our more investigations we also found that when subjecting to the wet thermal decomposition in the presence of oxygen in less than the theoretical amount NH₄NO₃-containing waste water having incorporated therein not more than 1 mole of COD component per mole of NO₃⁻ ion, the NH₄⁺ and NO₃⁻ ions decompose with a high efficiency and that when subjecting to the wet thermal decomposition in the presence of oxygen in less than the theoretical amount NH₄NO₃-containing waste water having incorporated therein ammonia in a mole ratio of 1<NH₃-N/NO₃-N≦5 and not more than 1 mole of COD component per mole of NO₃⁻ ion, the NH₄⁺ and NO₃⁻ ions decompose with a higher efficiency. More specifically, this invention provides five kinds of processes for treating waste water. (1) A process for treating waste water containing ammonium nitrate in an amount of 1 to 10% or more which comprises subjecting the waste water to wet thermal decomposition substantially in the absence of oxygen and in the presence of a catalyst supported by a carrier and containing as an active component at least one of precious metals and compounds thereof insoluble or sparingly soluble in water while maintaining the waste water at a pH of 3 to 11.5 and a temperature of 100 to 370°C, (2) A process for treating waste water containing ammonium nitrate in an amount of 1 to 10% or more which comprises subjecting the waste water to wet thermal decomposition in the presence of (i) a catalyst supported by a carrier and containing as an active component at least one of precious metals and compounds thereof insoluble or sparingly soluble in water, and (ii) oxygen in an amount of up to 0,6 time the theoretical amount required for decomposing the ammonia, organic substances and inorganic substances present in the waste water to N₂, H₂O and CO₂ while maintaining the waste water at a pH of 3 to 11.5 and a temperature of 100 to 370°C, (3) A process as defined under (2), wherein prior to said wet thermal decomposition, ammonia is added to the waste water in a mole ratio of 1<NH₃-N/NO₃-N≦5. (4) A process as defined under (2), wherein prior to said wet thermal decomposition, not more than 1 mole of chemically oxidizable substances per mole of NO₃⁻ ion is added to the waste water. (5) A process as defined under (2), wherein prior to said wet thermal decomposition, not more than 1 mole of chemically oxidizable substances per mole of NO₃⁻ ion and ammonia in a mole ratio of 1<NH₃-N/NO₃-N≦5 are added to the waste water. Throughout the specification and the appended claims, the term "theoretical amount" used in respect of oxygen means "theoretical amount required for the decomposition of the ammonia, organic substances and inorganic substances present in the waste water to N₂, H₂O and CO₂, and the term "NH₃-N" and the term "NO₃-N" refer to "ammonia nitrogen" and "nitrate nitrogen", respectively. The waste waters which can be treated by the processes of this invention include all kinds of waste waters containing NH₄NO₃ in a concentration of 1 to 10% or more. The waste waters to be treated by the processes of the invention may contain both organic and inorganic substances. The processes of the invention achieve a high efficency when carried out at a pH of 3 to 11.5, preferably 5 to 11. Therefore, when required, the pH of the waste water may be suitably adjusted with an alkali substance such as sodium hydroxide, sodium carbonate, calcium hydroxide, etc. Examples of active components of catalysts which can be used in this invention are ruthenium, rhodium, palladium, osmium, iridium, platinum and gold, and the compounds thereof insoluble or sparingly soluble in water. These active components can be used singly or at least two of them are usable in mixture. Examples of useful compounds which are insoluble or sparingly soluble in water are ruthenium dichloride, platinum dichloride, ruthenium sulfide, rhodium sulfide, etc. These active components of catalysts are used as supported in a conventional manner by a carrier such as titania, zirconia, alumina, silica, alumina-silica, active carbon, and porous bodies of metals such as nickel, nickel-chromium, nickel-chromium-aluminum, nickel-chrominum-iron and the like. The amount of the active components to be supported by the carrier ranges from about 0.05 to about 25 %, preferably about 0.5 to about 3 %, based on the weight of the carrier. The catalyst can be used in the form of globules, pellets, cylinders, crushed fragments, particles or in any other desired form. When the reaction column used has a fixed bed, the waste water is passed therethrough preferably at a space velocity of about 0.5 to about 10 1/hr, preferably about 1 to about 5 1/hr, based on an empty column. The grains or pieces of the supported catalyst useful for the fixed bed are usually about 3 to about 50 mm, preferably about 5 to about 25 mm in size. In the case of the fluidized bed, it is preferable to use the supported catalyst as suspended in the waste water like a slurry in such an amount that it will form a fluidized bed within the reactor, namely in an amount of usually about 0.5 to about 20 % by weight, preferably about 0.5 to about 10 % by weight, based on the resulting suspension. For the actual operation with the fluidized bed, it is preferred to feed the supported catalyst to the reactor as suspended in the waste water like a slurry, separate the the catalyst from the treated water resulting from the reaction by sedimentation, centrifuging or like suitable method and reuse the separated catalyst again. To facilitate the separation of the catalyst from the treated water, therefore, the supported catalyst useful for the the fluidized bed is advantageously about 0.15 to about 0.5 mm in particle size. When the process of this invention is conducted in the presence of oxygen in less than the theoretical amount, various gases can be used as the oxygen source. Examples of gases useful in this invention are air, oxygen-enriched air, oxygen, oxygen-containing waste gases containing as impurities one or more of hydrogen cyanide, hydrogen sulfide, ammonia, sulfur oxide, organic sulfur compounds, nitrogen oxides, hydrocarbons, etc. The feed rate of the oxygen-containing gases is determinable from the theoretical amount of oxygen required for the decomposition of the ammonia, organic substances and inorganic substances contained in the waste waste. Generally the gas is fed in an amount of about 0.2 to 0.6 time the theoretical amount of the oxygen present in the reaction system. When the oxygen-containing waste gases are used as the oxygen source, the harmful components of the gas can be rendered harmless along with the those contained in the waste water. The oxygen-containing gas may be fed to the reactor at a single level or in two or more levels. The reaction is carried out at a temperature of usually about 100 to about 370°C, preferably about 200 to about 300°C. The higher the reaction temperature, the higher the efficiency of removal of NH₄⁺ ion and NO₃⁻ ion and the shorter the residence time of the waste water within the reactor but the higher the equipment cost. Accordingly the reaction temperature is determined in view of the kind of the waste water, the degree of desired treatment and operation and equipment costs combined. The reaction pressure therefore needs only to be such that the waste water can at least retain its liquid phase at the predetermined temperature. When subjecting to wet thermal decomposition the NH₄NO₃-containing waste water having incorporated therein ammonia in a mole ratio of 1<NH₃-N/NO₃-N≦5, the reaction is conducted under the same conditions as above. When subjecting to wet thermal decomposition the NH₄NO₃-containing waste water having incorporated therein not more than 1 mole of COD component per mole of NO₃⁻ ion, the reaction conditions are the same as those stated above. Preferred amount of COD component to be added is about 0.1 to about 0.5 mole per mole of NO₃⁻ ion contained in the waste water. When subjecting to wet thermal decomposition the NH₄NO₃-containing waste water having incorporated therein COD component and ammonia, the amount of COD component to be present is the same as above, and the amount of ammonia to be incorporated is in a mole ratio of 1<NH₃-N/NO₃-N≦5. The reaction conditions in this case are also the same as above. Examples of sources of COD components and ammonia to be incorporated in the waste water include a wide variety of waste water containing these substances such as gas liquors produced in coke oven plants, coal gasifying plants and coal liquefying plants, waste water from gas cleaning processes employed in these plants, waste water from desulfurization process and from cyanide removal process, oil-containing waste water, waste water from activated sludge process, sedimented acitivated sludge, waste water from chemical plants and oil refineries, municipal effluents, sewage, sewage sludge, etc. This means that the processes of this invention can treat these waste waters, together with the ammonium nitrate-containing waste water. The processes of this invention are capable of treating waste water containing NH₄NO₃ in a high concentration with an improved efficiency and reducing the concentration of NH₄⁺ ion and NO₃⁻ ion to a considerable extent. With this advantage, the processes of the invention can easily treat waste water from the processing of uranium ore into nuclear fuel and reprocessing of the used uranium fuel with use of simple equipment which water may contain NH₄NO₃ in a concentration of 10 % or more. The present invention will be described below in greater detail with reference to the following Examples and Comparison Examples. Example 1 Into a 300 ml-stainless steel autoclave was placed 100 ml of waste water having a pH of 10 and a NH₄NO₃ concentration of 10 % (NH₃-N/NO₃-N=1). The waste water was treated by thermal decomposition at 250°C for 60 minutes. The air supplied for pressurizing the autoclave contained oxygen in about 0.01 time the theoretical amount. The autoclave used above was filled with 10 g of a catalyst composed of ruthenium carried on titania particles having a particle size of 5 mm in an amount of 2 % by weight based on the weight of titania. Table 1 below shows the decomposition ratio of the NH₄⁺ ion, NO₃⁻ ion and the total nitrogen components, along with the results obtained in Example 2. Example 2 The treatment of waste water was conducted by the same procedure of thermal decomposition as in Example 1 with the exception of using as a catalyst palladium carried on titania particles 5 mm in particle size in an amount of 1 % by weight based on the weight of titania. Example 1 Example 2 Active component of catalyst Ru Pd pH of waste water 10 10 NH₄NO₃ concentration (%) 10 10 Decomposition ratio of NH₃ (%) 98 92 Decomposition ratio of NO₃ (%) 75 80 Decomposition ratio of total nitrogen components (%) 87 86 Example 3 Into a 300 ml-stainless steel autoclave was placed 100 ml of waste water having a pH of 10 and NH₄NO₃ concentration of 1 % (NH₃-N/NO₃-N=1). The waste water was treated by thermal decomposition at 250°C for 60 minutes. The air supplied contained oxygen in about 0.2 time the theoretical amount. The autoclave used above was filled with 10 g of a catalyst composed of ruthenium carried on titania particles 5 mm in particle size in an amount of 2 % by weight based on the weight of titania. Table 2 below shows the decomposition ratio of the NH₄⁺ ion, NO₃⁻ ion and the total nitrogen components, along with the results obtained in Examples 4 to 12 and Comparison Examples 1 and 2. Examples 4 to 7 The predetermined amount of NH₄OH was added to the same kind of NH₄NO₃-containing waste water as used in Example 3 to adjust the mole ratio of NH₃-N/NO₃-N and the mixture was treated by the same procedure of thermal decomposition as as in Example 3. Example 8 The treatment of waste water was conducted by the same procedure of thermal decompositon as in Example 3 with the exception of using as a catalyst palladium carried on titania particles 5 mm in particle size in an amount of 2 % by weight based on the weight of titania in place of the ruthenium carried on titania. Examples 9 to 12 NH₄NO₃-containing waste water was treated by the same procedure of thermal decomposition as in Examples 4 to 7 with the exception of using the same kind of the palladium catalyst as used in Example 8 in lieu of the ruthenium catalyst. Comparison Example 1 NH₄NO₃-containing waste water was treated by the same procedure of thermal decomposition as in Example 5 with the exception of not using the catalyst. Comparison Example 2 NH₄NO₃-containing waste water was treated by the same procedure of thermal decomposition as in Example 5 with the exception of using grobules of titania 5 mm in diameter with no active component carried thereon in place of the ruthenium carried on the carrier. Examples 13 to 16 Waste water was treated by the same procedure of thermal decomposition as in Example 3 with the exception of changing the concentration of NH₄NO₃ and the pH with the results shown below in Table 3. Examples 17 to 19 Waste water was treated by the same procedure of thermal decomposition as in Example 8 with the exception of changing the concentration of NH₄NO₃ and the pH with the results shown below in Table 3. Example 20 Waste water having a pH of 10 and a NH₄NO₃ concentration of 10 % (NH₃-N/NO₃-N=1.88) was fed to a lower portion of a cylindrical stainless high nickel steel reactor at a space velocity of 0.92 1/hr (based on an empty column) to treat the waste water by thermal decomposition while introducing air into the lower portion of the reactor at a space velocity of 17.7 1/hr (based on an empty column in a standard state). The mass velocity of the liquor was 1.2 t/m² hr. The air supplied contained oxygen in about 0.24 time the theoretical amount. The reactor used above was filled with a globular catalyst composed of palladium carried on titania particles 5 mm in particle size in an amount of 2 % by weight based on the weight of titania. The thermal decomposition was carried out at a temperature of 250°C and pressure of 70 kg/cm². The liquid-gas mixture resulting from the reaction was subjected to heat recovery and led to a liquid-gas separator to separate into a liquid and gas, which were indirectly cooled and taken out of the system respectively. Table 4 shows the decomposition ratio of NH₃, NO₃ and total nitrogen components. None of NO x and SO x were detected in the gas phase. Decomposition ratio of NH₃ (%) 99 Decomposition ratio of NO₃ (%) Above 99 Decomposition ratio of total nitrogen components (%) Above 99 Example 21 To waste water to be treated was added C₆H₅OH as a COD component in a mole ratio of COD component to NO₃-N of 1 : 2. Into a 300 ml-stainless steel autoclave was placed 100 ml of the waste water having a pH of 10 and a NH₄NO₃ concentration of 1 % (NH₃-N/NO₃-N=1). The waste water was treated by thermal decomposition at 250°C for 60 minutes. The air supplied contained oxygen in about 0.2 time the theoretical amount. The autoclave used above was filled with 10 g of a catalyst composed of ruthenium carried on titania particles having a particle size of 5 mm in an amount of 2 % by weight based on the weight of titania. Table 5 below shows the decomposition ratio of the NH₄⁺ ion, NO₃⁻ ion and the total nitrogen components, along with the results obtained in Examples 22 to 26. Examples 22 and 23 The predetermined amount of C₆H₅OH was added to NH₄NO₃-containing waste water which was different in the pH and NH₄NO₃ concentration from the waste water used in Example 21 to adjust the mole ratio of C₆H₅OH/NO₃-N and the mixture was treated by the same procedure of thermal decomposition as in Example 21. Example 24 The treatment of waste water was conducted by the same procedure of thermal decompositon as in Example 21 with the exception of using as a catalyst palladium carried on titania particles 5 mm in particle size in an amount of 2 % by weight based on the weight of titania in place of the ruthenium carried on titania. Examples 25 and 26 The treatment of waste water was conducted by the same procedure of thermal decompositon as in Examples 22 and 23 with the exception of using the same type of palladium catalyst as used in Example 24 in place of the ruthenium catalyst. Examples 27 to 29 To NH₄NO₃-containing waste water were added C₆H₅OH and NH₄OH. The mixture was thermally decomposed under the same conditions as in Example 21. The waste water was adjusted to the specified pH with sodium hydroxide in Examples 27 and 28 while no pH adjustment was performed in Example 29. Table 6 shows the results. Examples 30 to 32 To NH₄NO₃-containing waste water were added C₆H₅OH and NH₄OH. The mixture was thermally decomposed under the same conditions as in Example 24. The waste water was adjusted to the specified pH with sodium hydroxide in Examples 30 and 31 while no pH adjustment was performed in Example 32. Table 6 shows the results. Example 33 To waste water having a NH₄NO₃ concentration of 10 % (NH₃-N/NO₃-N=1.88) was added C₆H₅OH as a COD component in a mole ratio of COD component to NO₃-N of about 1 : 2 and the mixture was adjusted to a pH of 10 with an aqueous solution of NaOH. The resulting mixture was fed to a lower portion of a cylindrical stainless high nickel steel reactor at a space velocity of 0.95 1/hr (based on an empty column) to treat the waste water by thermal decomposition while introducing air into the lower portion of the reactor at a space velocity of 18.5 1/hr (based on an empty column in a standard state). The mass velocity of the liquor was 2.43 t/m² hr. The air supplied contained oxygen in about 0.4 time the theoretical amount. The reactor used above was filled with a globular catalyst composed of palladium carried on titania particles 5 mm in particle size in an amount of 2 % by weight based on the weight of titania. The thermal decomposition was carried out at a temperature of 250°C and pressure of 70 kg/cm². The liquid-gas mixture resulting from the reaction was subjected to heat recovery and led to a liquid-gas separator to separate into a liquid and gas, which were indirectly cooled and taken out of the system respectively. Table 7 shows the decomposition ratio of NH₃, NO₃ and total nitrogen components. None of NO x and SO x were detected in the gas phase. Decomposition ratio of NH₃ (%) Above 99 Decomposition ratio of NO₃ (%) Above 99 Decomposition ratio of total nitrogen components (%) Above 99 Decomposition ratio of COD component (%) Above 99

Claims: A process for treating waste water containing ammonium nitrate in an amount of 1 to 10 % or more which comprises subjecting the waste water to wet thermal decomposition substantially in the absence of oxygen and in the presence of a catalyst supported by a carrier and containing as an active component at least one of precious metals and compounds thereof insoluble or sparingly soluble in water while maintaining the waste water at a pH of 3 to 11.5 and a temperature of 100 to 370°C. A process for treating waste water containing ammonium nitrate in an amount of 1 to 10% or more which comprises subjecting the waste water to wet thermal decomposition in the presence of (i) a catalyst supported by a carrier and containing as an active component at least one of precious metals and compounds thereof insoluble or sparingly soluble in water and (ii) oxygen in an amount of up to 0,6 times the theoretical amount required for decomposing the ammonia, organic substances and inorganic substances present in the waste water to N₂, H₂O and CO₂ while maintaining the waste water at a pH of 3 to 11.5 and a temperature of 100 to 370°C. A process as defined in claim 2, wherein the oxygen-containing gas is fed in an amount of 0.2 to 0.6 times the theoretical amount of oxygen. A process as defined in claim 2 or 3, wherein prior to subjecting the waste water to said wet thermal decomposition, ammonia is added to the waste water ammonia in a mole ratio of 1<NH₃-N/NO₃-N≦5. A process as defined in claim 2 or 3, wherein prior to subjecting the waste water to said wet thermal decomposition not more than 1 mole of chemically oxidizable substances per mole of NO₃⁻ ion is added to the waste water. A process as defined in claim 2 or 3, wherein prior to subjecting the waste water to said wet thermal decomposition, not more than 1 mole of chemically oxidizable substances per mole of NO₃⁻ ion and ammonia in a mole ratio of 1<NH₃-N/NO₃-N≦5 are added to the waste water. A process as defined in claim 5 or 6, wherein the amount of chemically oxidizable substances to be added is 0.1 to 0.5 mole per mole of NO₃⁻ ion. A process as defined in anyone of claims 1, 2, 4, 5, 6, wherein the waste water is subjected to the wet thermal decomposition at a pH of 5 to 11. A process as defined in anyone of claims 1, 2, 4, 5, 6, wherein the active component of the catalyst comprises at least one of ruthenium, rhodium, palladium, osmium, iridium, platinum and gold. A process as defined in anyone of claims 1, 2, 4, 5, 6, wherein the active component of the catalyst comprises at least one of compounds insoluble or sparingly soluble in water of ruthenium, rhodium, palladium, osmium, iridium, platinum and gold. A process as defined in claim 10, wherein the active component of the catalyst comprises at least one of ruthenium dichloride, platinum dichloride, ruthenium sulfide and rhodium sulfide. A process as defined in anyone of claims 1, 2, 4, 5, 6, wherein the wet thermal decomposition is carried out in a reactor of the fixed bed type. A process as defined in anyone of claims 1, 2, 4, 5, 6, wherein the wet thermal decomposition is carried out in a reactor of the fluidized bed type. A process as defined in anyone of claims 1, 2, 4, 5, 6, wherein the wet thermal decomposition is carried out at a temperature of 200 to 300°C.

Inventors: DOI, YASUHUMI, HARADA, YOSHIAKI, MATHURA, HIROYUKI, OKINO, TEIZOU, YAMASAKI, KENICHI
Applicants: OSAKA GAS CO., LTD
Application year: 1986
Main classification: C02F1
IPC codes: B01J27, C02F1, C02F11