3,348,914 vUnited States Patent O??ce Patented Oct. 24, 1967 1 2 3,348,914 Further objectives, features, and advantages of the PROCESS FOR RECOVERING OXIDES OF present process will be better understood from considera NITROGEN tion of the following detailed description. Bernard Quanquin and Honoré Trimbach, Grand-Cou The invention concerns a process in which oxides of ronne, France, assignors to Potasse et Engrais Chl nitrogen, having an average degree of oxidation at least miqnes, Paris, France, a societe anonyme of France No Drawing. Filed Apr. 28, 1964, Ser. No. 363,266 equal to N203, are scrubbed by an aqueous suspension of Claims priority, application France, Jan. 21, 1964, calcium hydroxide (milk of lime) and the nitrite formed 961,015, Patent 1,391,087 during the absorption is oxidized, the process being char 6 Claims. (Cl. 23-161) acterized in that the scrubbing is carried out with a quan tity of calcium oxide (lime) such that the solution ob This invention relates to a process for recovering oxides 10 tained by absorption contains less than about 65 g./l. of nitrogen (nitrous gases) from industrial gaseous efflu (preferably from about 50 to about 60 g./l.) of nitrogen ents containing them. present in form of calcium nitrite and nitrate, that at least In many industries, such as in the manufacture of nitric 2 moles of sulfuric acid for 3 moles of calcium nitrite acid by catalytic oxidation of ammonia, nitration of or 15 are added to the solution of calcium nitrite and nitrate, ganic products, pickling of metals, and so on, the waste and that the nitric oxide evolved is recovered. or tail gases which still contain noticeable quantities of By adding sulfuric acid to the calcium nitrite-nitrate oxides of nitrogen are discharged to the atmosphere; not solution produced by absorption of oxides of nitrogen only are these oxides a source of air pollution injurious in milk of lime, the calcium nitrite present in the solution to the neighborhood, but furthermore they represent cost is oxidized according to a reaction which may be rep ly losses when larger units are considered. 20 resented by the following equation: It is well known that the oxides of nitrogen contained in waste gases can be scrubbed by aqueous alkaline solu tions or suspensions, such as solutions of alkali metal hydroxides or carbonates, suspensions of earth alkali In order to effect oxidation according to reaction (4), metal hydroxides (e.g., milk of lime), or the like. Nitro 25 a stoichiometric quantity of sulfuric acid is used, or pref gen oxide gases comprise mainly two oxides (nitric oxide erably a slight excess, so as to achieve the decomposition NO and nitrogen dioxide N02), and the reactions which of calcium nitrite as quickly as possible. In practice, about occur during their absorption, for example in milk of 1.10 to 1.25 times the stoichiometric quantity (that is to lime, may be represented as follows: say, 2.2 to 2.5 moles of sulfuric acid for 3 moles of 30 nitrite) has been found to give satisfactory results. Nitric oxide evolved during the reaction may be readily recovered, for ‘example, by recycling it to the manufac ture of nitric acid. The solution of calcium nitrate con 35 taining calcium sulfate in suspension no longer contains any poisonous nitrite, and it therefore may be used direct ly; for example, it may be added to complex fertilizers As may be seen from reaction (2), when the average degree of oxidation of the mixture of oxides is equal to during their manufacture or, on the other hand, it may be discharged as sewage without causing any inconvenience. N203 (that is to say when the oxides NO and N02 are in equimolar proportions), calcium nitrite is formed during 4.0 According to a preferred embodiment of the present the absorption process. When nitrogen dioxide N02 is invention, nitric acid may be recovered as well as nitric oxide. In this process embodiment sulfuric acid is added to present in excess to nitric oxide NO, the degree of oxida tion is higher than N203 and a mixture of calcium nitrite the calcium nitrite-nitrate solution in su?icient quantity not only to oxidize all the nitrite present, but also to con and nitrate is obtained according to reaction (3), or ac 45 vert the nitrate formed during absorption of the nitrogen cording to reactions (2) and (3). In most cases, the de oxides and during nitrite oxidation. Thus calcium sulfate gree of oxidation of oxides of nitrogen contained in indus and nitric acid are obtained as shown by the following trial waste gases is higher than N2O3. If, on the contrary, reaction: . the gaseous mixture to be treated contains an excess of nitric oxide, the degree of oxidation must be adjusted by (5 ) adding a su?’icient quantity of oxygen or air, because 50 Without this addition the excess of nitric oxide would not When operating according to the above embodiment of be absorbed. the process, a substantially stoichiometric quantity of Many methods have already been proposed for remov sulfuric acid is preferably added (that isto say, about 1 ing oxides of nitrogen from industrial gaseous e?’luent by 55 mole of sulfuric acid for 1 mole of calcium nitrite), in using an alkali metal or earth alkali metal hydroxide or order to obtain a ?nal nitric acid solution substantially carbonate. However the solutions of nitrite-nitrate so ob free of sulfuric acid. In either case, whether the recovery tained have no direct industrial use and furthermore can of nitric acid is desired or not, the calcium sulfate ob not be discharged as sewage because they are strongly tained by the’ process is easily ?ltrable so long as the poisonous to plants. Usually the nitrite-nitrate solutions 60 concentration of the solution does not exceed 65 g./l. of are treated with an oxidizing agent, particularly with nitric nitrogen in the form of calcium nitrite and nitrate. acid, so as to convert nitrite into nitrate with formation When the recovery of nitric acid is desired, ‘the con of nitric oxide which can be re-used. The solution obtained centration of the calcium nitrite-nitrate solution is ad is concentrated by evaporation, and/or cooled to precip vantageously maintained between about 50 and 60 g./l. of itate the nitrate contained therein. All these prior art 65 nitrogen. Under these conditions, a nitric acid solution methods have had practically no industrial application due containing about 10 to 20% HNO3 is obtained, such a to the di?iculties encountered in recovering the nitrate and concentration being satisfactory for subsequent industrial use. In particular, this solution can be recycled to the the cost of such recovery. In contrast, the present inven manufacture of nitric acid for the absorption of the nitro tion relates to a novel recovery process which makes it 70 gen oxides. On the other hand, when the recovery of nitric possible to recover the oxides of nitrogen from gaseous acid is not desired, the nitrite-nitrate solution may con etliuents in a satisfactory economical manner. tain less than 50 g./l. of nitrogen, and if the solution is to 8,348,914 3 4 be discharged as sewage after oxidation of nitrite into the temperature rises markedly——a condition which is nitrate, there is no objection to the nitrogen concentration known to facilitate the separation of calcium sulfate. ‘being lower still. In industrial practice, with nitrite-nitrate solutions con It has been found that the absorption yield of nitrogen taining about 50 to 65 g./l. of nitrogen, the heat evolved oxides decreases when the calcium nitrite-nitrate concen through the addition of sulfuric acid is generally sufficient tration of the solution increases. It is therefore particu to permit a good separation of the calcium sulfate pre larly advantageous in industrial practice to employ several cipitate without the need forrany external heating. When scrubbers disposed in series wherein the gases to be treated the solutions contain less than 50 g./l. of nitrogen, a and the scrubbing solution circulate in counter-current smaller quantity of sulfuric acid is required, and thus flow. In the ?rst scrubber, which is fed with the suspen 10 less heat is evolved in the process. Nevertheless, the sepa sion having the lowest CaO content and with the gases ration of the calcium sulfate, due to the low concentra having the highest oxides of nitrogen content, the major tions involved, remains easy and rapid even at ordinary part of the nitrogen oxides is absorbed with a relatively temperature. low yield; in the last scrubber, which is fed with milk of Although the novel features which are believed to be lime substantially free of calcium nitrite-nitrate and with 15 characteristic of the process of the. present invention will gases, very poor in nitrogen oxides, the absorption is com be pointed out in the annexed claims, the invention itself, pleted with a very high yield. as to its objects and advantages and the manner in which In order to avoid a loss of sulfuric acid during nitrite it may be practiced, may be better understood by‘ reference decomposition, the added quantity of milk of lime may to the following examples taken in conjunction with the be advantageously controlled so that the CaO concentra foregoing description. tion of the solution discharged from the single scrubber Example 1 (or from the ?rst scrubber, it several are disposed in series) does not exceed about 5 g./l., and is preferably The scrubber comprised a column packed with Raschig maintained between about 3 and 5 g./l.