JP2006218407A - Method for treating waste liquid containing hardly decomposable hazardous substance and nitrogen compound - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for treating a waste liquid which can efficiently decompose a hardly decomposable hazardous substance while suppressing the formation of nitric acid derived from a nitrogen compound when the waste liquid containing both the hardly decomposable hazardous substance and the nitrogen compound is treated. <P>SOLUTION: The method for treating the waste liquid containing the hardly decomposable hazardous substance and the nitrogen compound comprises setting a molar ratio (TOC/TN) of organic carbon (TOC) to total nitrogen (TN) in the waste liquid at a fixed value or lower to react at temperature required to decompose the hardly decomposable hazardous substance. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for treating a waste liquid containing both a hardly decomposable hazardous substance typified by PCB (polychlorinated biphenyl) and dioxin and a nitrogen compound, and in particular, hardly decomposed while suppressing nitric acid production from the nitrogen compound. The present invention relates to a method for treating a waste liquid containing a hardly decomposable harmful substance and a nitrogen compound, which can efficiently decompose a harmful substance.

  In recent years, attention has been paid to attempts to decompose and detoxify environmental pollutants using a hydrothermal oxidation reaction in the presence of high-temperature and high-pressure water, particularly supercritical water. In particular, supercritical water oxidation utilizing the high reactivity of supercritical water decomposes harmful and hardly decomposable organic substances, such as PCB, dioxin, and organic chlorinated organic substances, which were difficult to be decomposed by the prior art. Attempts to convert to harmless products such as carbon dioxide, water and inorganic salts are drawing attention. The supercritical water means water in a supercritical state, that is, water in a state exceeding the critical point of water. Specifically, it is at a temperature of 374.1 ° C. or higher and under a pressure of 22.04 MPa or higher. Says water in a state. Supercritical water has a high ability to dissolve organic substances, and can completely dissolve non-polar substances that are abundant in organic compounds. Conversely, the ability to dissolve inorganic substances such as metals and salts is extremely low. Supercritical water can be mixed with a gas such as air, oxygen, or nitrogen at an arbitrary ratio to form a single phase.

  In such supercritical water oxidation of organic matter, when a nitrogen compound is treated using air or oxygen as an oxidant, nitric acid is generated at a high reaction temperature, and there is a concern about apparatus corrosion. Therefore, Patent Document 1 discloses means for suppressing the production of nitric acid by setting the reaction temperature to 580 to 620 ° C.

On the other hand, in the supercritical water oxidation treatment of refractory hazardous substances represented by PCB and dioxin, the reaction temperature is used in order to perform the reliable treatment while taking advantage of the high reaction rate that is the advantage of supercritical water oxidation. There is a case where it is processed at about 630 ° C. (Non-patent Document 1).
Japanese Patent Laid-Open No. 11-226583 Annual Meeting of Chemical Engineering Society 34th Autumn Meeting, 2001, 2001

  However, when supercritical water oxidation treatment is performed on a waste liquid containing nitrogen compounds mixed with hardly decomposable harmful substances, if the reaction temperature is set to about 630 ° C. in order to reliably treat the hardly decomposable harmful substances, the nitrogen compound-derived Nitric acid may be generated, causing equipment corrosion. In this case, the corrosion of the apparatus may be suppressed by adopting a structure and material that give corrosion resistance, but this causes a problem that the equipment cost increases. Further, even when the equipment cost is allowed to increase, the problem that the generated nitric acid must be processed remains. On the other hand, if the reaction temperature is lowered in order to suppress nitric acid production, there arises a problem that undecomposed and hardly decomposable harmful substances remain.

  Accordingly, an object of the present invention is to efficiently decompose a hardly decomposable harmful substance while suppressing the production of nitric acid derived from the nitrogen compound when treating a waste liquid containing both the hardly decomposable harmful substance and the nitrogen compound. Another object of the present invention is to provide a method for treating the waste liquid.

  In order to solve the above problems, in the course of completing the present invention, first, it was examined whether there is a method for suppressing nitric acid production in a supercritical water oxidation reaction at a high temperature. As a result of air oxidation in supercritical water oxidation so far, waste liquid with high concentration, that is, high calorific value, is treated, so the temperature of the measuring part is around 600 ° C, but the temperature of the reaction center is exothermic reaction. It was expected that the temperature was higher than the measured temperature. Therefore, in order to examine the influence of the reaction center temperature, the final control temperature was constant at 630 ° C., and the experiment was performed by changing the heat generation amount of the waste liquid to be treated. The results shown in Table 1 were obtained. It was.

  In Table 1, the reaction center assumed temperature is the fluid temperature reached by the amount of heat generated when the supercritical water oxidation reaction starts at 400 ° C. and the entire amount is oxidized. Table 1 also shows the relationship between the amount of heat generated and the expected reaction center temperature. As described above, it was confirmed that the nitric acid conversion rate greatly changed due to the different calorific value of the waste liquid. As a result, it was considered that the temperature reached at the reaction center was high, affecting the nitric acid production.

Here, in each of the experiments described above, the final reached temperature was 630 ° C., and although it was higher than the expected reaction center portion reached temperature in Experiment No. 1-2 where nitric acid was generated, Experiment No. 1- In 1, no nitric acid was substantially generated. This was considered as follows.
(1) The general TOC (total organic carbon) component disappears at the reaction center expected temperature because the reaction occurs relatively quickly, but the nitrogen compound remains when the temperature is low. There is.
(2) Nitric acid is generated not by a single reaction of the nitrogen compound at a high temperature but by a combined reaction of the nitrogen compound and the TOC component.
Then, in order to confirm whether or not the coexisting TOC component has an influence on the nitric acid production, an experiment was conducted by changing the value of TOC / TN (total nitrogen), and the results shown in Table 2 were obtained. It was.

  As shown in Table 2, the TOC / TN value (shown as C / N molar ratio in Table 2) is low even though the reaction center temperature is higher than the expected temperature at which the nitric acid is generated. In numbers 2-1 to 2-3, nitric acid was not substantially produced, and it was confirmed that the presence of the TOC component was involved in the nitric acid production.

From the above results, it was concluded that nitric acid generation from nitrogen compounds in the supercritical water oxidation reaction occurs when the following requirements overlap.
(1) A large amount of organic carbon exists in the presence of a nitrogen compound.
(2) The temperature is high.
That is, the amount of organic carbon is larger in a normal waste liquid than in a nitrogen compound. Therefore, nitric acid is produced when treated at high temperatures. However, for example, non-oxidized nitrogen such as ammonia salts and urea such as ammonia salts and urea in which the number of nitrogen atoms in the compound is larger than the number of carbon atoms, excluding nitric acid and nitrite nitrogen (oxidized nitrogen). ) Reduces the amount of organic carbon compared to nitrogen compounds. As a result, it has been found that the condition (1) is not satisfied and nitric acid production can be suppressed, and the present invention has been completed.

  That is, according to the present invention, the method for treating a waste liquid containing a hardly decomposable harmful substance and a nitrogen compound sets the molar ratio of organic carbon (TOC) to total nitrogen (TN) in the waste liquid, and TOC / TN to a certain value or less. The method comprises reacting at a temperature required to decompose a hardly decomposable harmful substance.

  In this waste liquid treatment method, the amount of apparent non-oxidized nitrogen in which the TOC / TN was a nitrogen amount obtained by subtracting twice the amount of oxidized nitrogen from total nitrogen (TN) was used. (TOC / apparent non-oxidized nitrogen) is preferably 2 or less in terms of molar ratio. The TOC / TN can be reduced to a certain value or less by adding a nitrogen compound (for example, ammonia salt, urea, etc.) having a larger number of nitrogen atoms than the number of organic carbon atoms.

  The temperature required for decomposition of the hardly decomposable harmful substance is preferably 600 ° C. or higher.

  According to the method for treating a waste liquid containing a hardly decomposable hazardous substance and a nitrogen compound according to the present invention, the supercritical water oxidation reaction is performed at a high temperature necessary for decomposing the hardly decomposable harmful substance with the TOC / TN of the waste liquid being a predetermined value or less. Therefore, the hardly decomposable harmful substance can be completely decomposed, and the production of nitric acid can be suppressed, whereby the corrosion of the apparatus can be suppressed.

Hereinafter, the present invention will be described in detail together with preferred embodiments.
In the conventional hydrothermal oxidation reaction, it was thought that nitric acid was produced when a nitrogen compound was oxidized with air at a high temperature (around 600 ° C.). However, as described above, it was actually confirmed that nitric acid was generated only when organic carbon and a nitrogen compound coexist at a high temperature (around 600 ° C.).

Here, the nitric acid production mechanism is considered as follows.
(1) For example, the rate at which nitric acid is produced from ammonia alone is much smaller than the rate at which nitrogen gas is produced by the reaction between nitric acid and ammonia.
(2) At high temperatures, ammonia and nitrogen compounds form an intermediate product with organic carbon, and the overall reaction rate until nitric acid is generated via the intermediate product is determined by the reaction of nitrogen gas due to the reaction of nitric acid and ammonia. Greater than the production rate. The high temperature here is considered to be approximately 550 ° C. or higher although it depends on the type of nitrogen compound. Furthermore, in the above (2), it has also been found that in order to form an intermediate product such that nitric acid remains, there must be a certain amount or more of organic carbon relative to the amount of nitrogen compound.

  As a result of obtaining such knowledge, in order to suppress nitric acid production, the molar ratio of organic carbon (TOC) and total nitrogen (TN) is set to a certain value or less, thereby suppressing the production of nitric acid. The present invention has been completed in which the decomposable harmful substances can be satisfactorily decomposed. More specifically, although depending on the waste liquid to be treated, in the amount of nitrogen (apparent non-oxidized nitrogen) obtained by subtracting twice the amount of oxidized nitrogen from total nitrogen (TN), (TOC / apparent non-oxidized nitrogen) is It is necessary that the molar ratio is 2 or less, and the molar ratio is preferably 1.5 or less. That is, the oxidized nitrogen reacts quickly with the non-oxidized nitrogen compound to generate nitrogen gas, so that not only the amount of oxidized nitrogen is subtracted from the total nitrogen (TN) but also oxidized. By examining the amount of non-oxidized nitrogen that reacts with the nitrogen and disappears rapidly, the suppression of nitric acid production is examined at the ratio of the real TOC that contributes to nitric acid production and the apparent non-oxidized nitrogen in the reaction field. Is considered necessary.

  In ordinary waste liquid, TOC is often present in a larger amount than TN. Therefore, as described above, in order to keep the molar ratio of TOC and TN below a certain value, it is effective to add a compound that does not contain organic carbon or has a ratio of organic carbon smaller than that of nitrogen to the waste liquid. is there. Examples of these TOC / TN adjusting agents include aqueous ammonia, ammonium carbonate, urea and the like. In particular, ammonium carbonate is preferable because it does not contain organic carbon, can be added to the waste liquid in a solid state, does not increase the amount of treated water, and can suppress fluctuations in pH.

  Table 3 shows the results of the hydrothermal oxidation reaction by changing the ratio of TOC and TN. Here, ammonia water was used as a TN source for adjusting the molar ratio of TOC and TN. As shown in Table 3, when the molar ratio, TOC / TN is about 1.6, nitric acid is hardly generated. However, when TOC / TN was about 3, the pH of the treated water decreased to about 1.4, and the risk of equipment corrosion increased. In Table 3, TOC / TN indicates the molar ratio of (TOC / apparent non-oxidized nitrogen) described above.

  As shown in Table 3, it is clear that by setting the TOC / TN of the waste liquid to a certain value or less, particularly 2 or less, the nitric acid concentration can be suppressed to a level that does not cause practical problems, and the risk of corrosion of the apparatus can be eliminated. Became.

  The present invention can be applied to treatment in all fields as long as treatment of waste liquid containing a hardly decomposable harmful substance and a nitrogen compound is required.

Claims (4)

  Refractory hazardous substance, characterized by reacting at the temperature required for decomposition of refractory hazardous substances, with a molar ratio of organic carbon (TOC) to total nitrogen (TN) in the waste liquid, TOC / TN not exceeding a certain value And a method for treating a waste liquid containing a nitrogen compound.   The amount of apparent non-oxidized nitrogen in which the TOC / TN is a nitrogen amount obtained by subtracting twice the amount of oxidized nitrogen from total nitrogen (TN) was used (TOC / apparent non-oxidized nitrogen) The processing method of the waste liquid of Claim 1 whose is 2 or less in molar ratio.   The waste liquid treatment method according to claim 1 or 2, wherein the TOC / TN is made equal to or less than the predetermined value by adding a nitrogen compound having a larger number of nitrogen atoms than the number of organic carbon atoms.   The method for treating a waste liquid according to any one of claims 1 to 3, wherein the temperature required for decomposition of the hardly decomposable harmful substance is 600 ° C or higher.