CS196819B1 - Method of making harmless the nitrous industrial exhalations and device for executing the same - Google Patents

Description

The invention relates to a method and an apparatus for the disposal of industrial exhalations containing nitrogen oxides; The process, which works on the principle of absorption, uses electrotechnical activation of the absorbent and its regeneration becomes waste-free. The new method is particularly suitable for applications in radiochemical processing plants of nuclear fuel and in systems with uneven or sudden and unexpected development of differently concentrated mixtures of nitric oxide and nitrogen dioxide.

Sources of higher concentrations of toxic nitrogen oxides, ie nitric oxide and nitrogen dioxide, commonly referred to as Ν0 _χ , are always associated with human activity. For example, flue gases with an average content of about 300 mg / πΡ Ν0 _χ ; Exhalations from large energy industrial appliances contain about 2000 mg / m ^ Ν0 _χ . In particular, the exhaust gases of internal combustion engines contain nitric oxide in a very variable concentration; depending on the combustion conditions and the type of internal combustion engine, the content Ν0 _{χ is} between 100 and 8000 mg / ra and inhalation. Industrial emissions from nitric acid production, nitrogen fertilizer production, some nitration plants, various metallurgical plants, etc. may contain NO 2 in concentrations of up to * 6000 mg / mi more.

Most of the content of ΝΟ _χ in the JS atmosphere is, according to some data, represented by household flue gas (about 50%) in places of higher population density. Another substantial part of Ν0 _χ contained in the atmosphere (about 40%) is the product of internal combustion engines, and only a relatively small amount (.10%) is waste from industrial production, but industrial waste is one of the most serious problems because of nitrous exhalation toxicity. these are large volumes of undissolved concentrated mixtures which must be conditioned or disposed of unconditionally.

Conventional nitric and nitric acid industries have focused on the thermal or catalytic degradation of Ν0 _χ emissions over the past two decades, using the hydrocarbon or ammonia reducing properties of the toxic emissions. At present, however, pressure equipment capable of processing waste Ν0 _χ into nitric acid is already in operation.

Nitrous gas emissions are also waste from some radiochemical production plants. However, these exhalations generally contain, in addition to nitrogen oxides, some gaseous radioactive impurities, in particular ruthenium dioxide gas and oxides aerosols of a number of radioisotopes; the destruction of such exhalations is always necessary and most often carried out by appropriate fixation of the exhalates and by concentrating the generated solid waste in the designated repositories. Such solid waste is to be stabilized against radiolytic decomposition and it is also sought to occupy as little volume as possible. The current practice is that said emise0 _χ emissions are usually processed to low or medium radioactive nitric acid, which is either returned to the production cycle or neutralized and fixed eg into bitumen; for ha0 _χ exhalations, which are contaminated with dust-free components, they have also been applied technically similar to the conventional steam industry, eg by thermal degradation catalytically or non-catalytically in admixture and reducing gaseous components.

The oldest and at the same time universal way to remove nitrous gases from the exhalation is generally the absorption process. Water or hydroxide solutions are most commonly used for absorption. Absorption processes are particularly useful when objem0 _χ volumes are produced rather smaller, but at concentrations that reach maximum levels in the exhalations. The efficiency of the absorption process can peak to 85-90%, depending on the concentration Ν0 _χ , the absorbent used and the apparatus; however, on average, the absorption efficiency does not exceed 30%.

The advantage of the absorbent process is always the apparatus simplicity, but the disadvantage is, as a rule, the high investment costs, the relatively low efficiency of the device and the necessity of further disposal of the resulting liquid waste or its processing into a by-product.

A particularly effective way to dispose of Ν0 _χ is by alkaline absorption in a strongly reducing or strongly oxidizing environment. These procedures are quite often combined, in different order. Sulfur dioxide or its compounds are most often used as reducing components; there were practices in industry

- 3 of this type applied, for example, according to Japanese Patent Nos. 79,152,947 or No. 7 (), 110,970, or according to US Patent δ. 3,932,585, etc. The oxidation principle is utilized by processes such as those disclosed in Japanese Patent No. 74,106,972, by absorption of Ν0 _χ in alkaline or alkaline earth chlorate solutions, or by Japanese Patent No. 7315,766 by absorbing nitrous gases in aqueous ozone or hydrogen peroxide solutions. or Japanese Patent 75,161,459 in alkali permanganate solutions, etc.

In addition to the demands on auxiliary feedstocks that need to be renewed from time to time in most processes, most processes result in liquid waste for which it is difficult to find a suitable use, and usually only after more expensive processing. However, processes which are important for the regeneration of the absorption medium are also known from the patent literature. For example, according to Japanese Patent No. 7556,375, the ammonium nitrate produced by the absorption of nitrous exhalation in ammonia solution is degraded electrolytically; the electrolyzer results in a gaseous mixture of nitrogen and ammonia, which can be recycled to the process. According to another process known from Dutch Patent No. 7917, 19, a mixture of nitrates and nitrites is reduced electrolytically in an alkaline medium to produce hydroxylamine and in particular ammonia. According to Japanese Patent Nos. 7482,576 or 74,106,972, alternatives to similar procedures can be regenerated and concentrated electrolytically by nitrogen oxides. In practice, however, procedures such as Japanese Patent No. 7543,100 or No. 7562,175, or a similar procedure under DOS 2350,108, and the like, that return the manganese dioxide precipitated from the permanganate solution utilized, have been particularly used in practice; its. regeneration to permanganate is carried out electrolytically from potassium hydroxide medium.

While these processes economize the process of treating or neutralizing nitrous exhalations by using electrolysis, they cannot avoid greater manipulation of the absorbent solutions used, with occasional removal, and the necessity, to a large extent, of always recovering absorbent raw materials. For the treatment of emissions contaminated by radioactive components, these processes are not applicable at all, especially because of the high demands on handling.

The above-mentioned deficiencies are eliminated by the method of disposing of emissions of 0 _χ , contaminated with possibly radioactive impurities, in particular ruthenium dioxide or radioactive aerosols of solid substances, utilizing the absorption of exhalations in ammonium solutions of complexing acids, especially oxalic, citric, tartaric acid. or phosphorous according to the invention, which is based on the electrolytic activation of said absorbents in an ammoniacal environment. The essence of the device for carrying out the method according to the invention consists in the countercurrent connection of the flow electrolyzer in the recirculation circuit of any multistage absorption apparatus.

The main advantage of the process according to the invention is not only the complete detoxification of nitrous gases, but also the complete removal of the resulting smog and the virtually unlimited use of absorbents. The advantage of the device according to the invention is its simplicity, since an extremely qualitative effect can be achieved in any absorption device in a completely undemanding manner, i.e. simply by incorporating the flow electrolyzer into the recirculation circuit.

The feed mixture of the hitros gas comes into contact with the ammonium salt solution in several absorption stages and the solution is activated anodically in countercurrent. An ammonia-activated medium in the pH range of 8 to 9 is more suitable for nitric oxide predominant emissions mixtures, while a pH of 9 to 10 is preferable for nitrous oxide rich mixtures. The process is effective for particularly high concentrations of nitrous exhalation and is not very sensitive to sudden changes or shocks in gas evolution. If, under extreme conditions, even a rapid and undesirable change in pH could occur, the necessary pH value must be adjusted with ammonia gas and thus eliminate the possibility of an emergency condition.

The above-mentioned absorption environment of ammonium salts captures exactly the same conditions as ruthenium oxide or aerosols of caustic earth oxides or heavy metal oxides, lanthanides and actinides, which in the form of radioactive impurities are part of some nitrous exhalations resulting from eg regeneration or nuclear fuel production.

The absorbent medium of said compositions is capable of further retaining with limited capacity and possibly decomposing other possible constituents of exhalation, such as hydrogen cyanide, carbon monoxide, hydrogen fluoride, iodine and its compounds and the like. However, their use may be limited by other circumstances, such as the permissibility of retained radioactivity.

The apparatus for carrying out the method according to the invention is illustrated in the accompanying drawings, which are examples of three possibilities of absorbent devices according to the invention.

Giant. Fig. 1 shows an absorption system β with two stages of a washer-type abeorbers, Fig. 2 is a diagram of a three-stage column absorber, and Fig. 3 shows an example of a two-stage absorption device ee by self-recirculation.

In all devices, a flow electrolyzer is used upstream of the exhalations to activate the absorption solutions.

According to FIG. 1, an electrolyser 6 is interposed between the absorber 1 and the absorber 2, wherein the inlet of the exhalation device 6 into the absorber X, the inlet of the exhalation device through the passage 6 into the absorber 2 and the outlet of the cleaned gas are oriented As the activated solution 10 enters the absorber 2, it passes through the stream 6 from the absorber 2 to the absorber 1 and flows back to the electrolyzer 6 for activation. Section 2 serves as the degassing line of the electrolyzer 6. The absorption liquid between absorbers 1 and 2 and the electrolyzer 6 is recirculated by forced circulation by means of pumps.

According to FIG. 2, the apparatus operates as a column with three absorption stages X, 2, 2, wherein between the effluent of the spent absorption solution from the base of the column

196819 and the injection absorption stage 2 'is in-flow electrolyzer 2. Exhalation vent 4 to absorption stage 1, transfer of partially detoxified exhalation 2 and 5' between the absorption stages 1, 2 and 2 and the purified gas outlet 6 from the device are countercurrently oriented. the absorbent solution 10 after nastřikovanému electrolytic activation of the pump to the top tray of the column and flows gravitationally přestupujícímu 2 and 9 ^of the second absorption stage 2 and 1 ^and the returning stream 8 back to the activation electrolyzer 2 · Section 2 serves as offgas flow cell.

Referring to FIG. 3, the absorption device operates in a two-stage and countercurrent absorption solution, wherein the absorption solution is recirculated through the device by a pump-less column-free principle. An electrolyzer 2 is interposed between the absorber 1 and the absorber 2, the inlet of the exhalation 4 into the absorber 1, the transfer of the partially purified exhalation 2 from the absorption stage 1 to the absorber 2 and the outlet of the purified gas 6 from the device. the current 2 passing between the absorbers 2 and 1 and the current 8 returning after absorption into the electrolyzer J in a countercurrent fashion. Section 2 presents again the electrolyzer 2 degassing ·

He did

Industrial gas emissions from spent nuclear fuel recovery resulting from denitration and calcination operations in the treatment of radioactive waste solutions were modeled with a mixture of peak composition of up to 8.2 vol% nitrogen dioxide, 63.5 vol% nitric oxide, 11.5 vol%. % nitrogen and 16,8 vol.

% carbon dioxide.

The gaseous mixture was fed in countercurrent to a scrubber-type absorber according to the diagram in Fig. 1, the functional principle of which was described.

A solution containing 2 wt. % ammonium oxalate; the aqueous solution was continuously activated at pH 8.0 electrolytically. The current density at the platinum electrodes was 0.2 A / cm ² and within 60 minutes all of the absorber contents were passed through the flow electrolyzer about four times. The sequential flow rate of the gas mixture to be discharged was 120 mm in diameter in the absorption device. a total height of 700 mm of about 0.1 ms. The absorption temperature was kept between 15 and 50 ° C without clearly affecting the results obtained.

Exhale resultant nitrogen monoxide contained in the limits from 2 to 100 mg.m ^- ^, while carbon dioxide was not detected.

The absorbent solution did not change its efficiency even after 2000 hours of operation. The power consumption consumed did not exceed, on average, 3.6 MJ / kg of nitrogen oxides, calculated as nitric oxide.

- 6 Example 2

Chemical absorption eliminated the emission of the same origin as in Example 1. An aqueous solution of ammonium citrate in water at a concentration of 3 wt. %. The solution was activated electrolytically in the reeylc but with the addition of a catalytically active dicalcium acetate, which was present in an absorption solution at a concentration of 0.1 wt. %. By comparison with Example 1, it was possible to increase the gas flow rate through the absorber almost three times without affecting the quality of the output exhaust gas. However, the pH level had to be kept constant at 8.7 by ejecting ammonia gas into the absorption solution. The results obtained were similar to those of Example 1.

Example 1 a d 3

A mixture of nitrous gases with varying contents and ratio of nitric oxide and due matter in a total concentration of 800 to 6000 mg / m < -1 > was contacted countercurrently with the anolyte in a similar manner as in Example 1. The absorbent solution was prepared by electrolytic activation of a 2 wt.% Aqueous solution of ammonium phosphate. % and pH 9.6-10. The absorption was maintained at 25 ° C and the pH was spontaneously maintained within the specified limits upon absorption. The resulting exhalate showed, after detoxification, 20 to 100 mg / ml of nitric oxide.

He attaches

An inactive experimental model nitrate solution simulating nuclear fuel regeneration waste of 50 g NaNO4, 400 g Al / NO2 / 25 g and 25 g HNO2 in 1000 g aqueous solution was denitrated at 90-100 ° C with formic acid (60 g / 1000, g) from phosphoric acid (50 g, 85% by weight) to form a gaseous mixture having a volume composition of 15% ^Ν 2θ '4% NO, 15% N ₂ and 66% CO ₂ ·

The denitrated solution was concentrated, dried and calcined at 300-700 ° C; calcination exhalates containing drift and aerosols of the solids were combined with the exhalates after solution denitration.

The emitted mixture was accrubbed. countercurrently first in the inlet model solution led to denitration *, this largely eliminated the drift and aerosols and then the emission was conducted to the absorber.

The devices consisted of a three-storey absorption column with a diameter of 120 mm and a total height of 600 mm, with additional wiring and function as described in Fig. 2.

A mixture of solutions of ammonium oxalate with ammonium citrate, at concentrations of 1 wt. % and the pH of the mixture was adjusted to 9.6. The sequential gas velocity of the device was 0.1 ms ^-1 .

in

The output exhalation detoxification was found less than 20 mg.m ^J nitric oxide.

- 7 196819

ExampleJ

The gaseous emission of a completely similar origin and composition as in Example 4 was destroyed by absorption in a 2 wt. % ammonium tartrate in water. The solution also contained a catalytically active copper cation, which was added in the form of 0.1% copper acetate. %. The solution was adjusted to pH 9 with ammonia gas. . . For the absorption, a two-stage absorption apparatus, spontaneously recirculating the absorption solution, was used with the function as described in Fig. 3. The height of each column was 300 mm, the top of the column was 120 mm, and the actual absorber diameter was 50 mm; the gas flow rate was 0.15 ms ^-1 at an absorption temperature of 20-30 ° C.

The conditions of electrolytic regeneration of the absohbent and its activation were approximately the same as in Example 1.

The detoxified effluent contained 20 to 40 mg.m of nitric oxide.

Claims (2)

Object of the invention A process for the disposal of nitrous industrial exhalations by absorption and / or decomposition in aqueous solutions of ammonium salts of citric or tartaric or oxalic or phosphoric acids, preferably in the presence of catalysing cations, in particular cobalt or copper, characterized in that the absorbant is activated electrolytically at ammoniacal pH 8 to 10. 2. A device according to claim 1, characterized in that an electrolytic unit for activating the absorbent is disposed in the absorber recirculation circuit.