United States Patent Office 3,330,620 Patented July 11, 1967 2 3,330,620 aqueous solution is used and this base is selected from METHOD FOR OXEDZNG AMMONUMSULFTE TO AMMONUMSULFATE the group consisting of pyridine, picolines, lutidines, col Angel Vian-Ortino and Vicente Martin-Municio, Madrid, lidines, quinoline, pyridic coal-tar bases, pyridic bases Spain, assignors to Empresa Auxiliar de la Industria, derived from petro-chemical processes, bases containing S.A., Madrid, Spain 5 pyridinic condensed rings and derivatives thereof. No Drawing. Fied Apr. 22, 1963, Ser. No. 274,806 The method of the present invention of converting am Claims priority, application Spain, Apr. 23, 1962, monium sulfite into ammonium sulfate may also be car 276,718 ried out by forming a dispersion of an organic, nitro 15 Claims. (C. 23-119) gen-containing base in an aqueous solution of ammonium 10 sulfate and ammonium sulfite, the organic base being in The present invention relates to a process for the soluble in the aqueous solution and being present in an quantitative oxidation of ammonium sulfite and, more amount equal to between 10% and 80% of the volume particularly, for the oxidation of ammonium sulfite which of the dispersion, passing air through the dispersion while has been formed by absorption of sulfur dioxide in aque the same is maintained at an elevated temperature of be ous ammonia solutions. Thus, the present invention is also 5 tween 40 and 140 C. and at an absolute pressure of be concerned with the recovery of sulfur dioxide from waste tween 1 and 11 atmospheres so as to substantially com gases in the form of ammonium sulfate. pletely oxidize the ammonium sulfite of the solution to The recovery and utilization of small concentrations of ammonium sulfate, separating the base from the thus sulfur dioxide which are contained in waste gases dis formed substantial ammonium sulfite-free solution, and charged, for instance, from sulfuric acid plants, thermal 20 recovering ammonium sulfate from the solution. power stations, certain types of metallurgic foundries and Surprisingly, it has been found that the oxidation of Sul the like poses an important problem. It is desired not only fite to sulfate can be carried out with up to a substantial to prevent contamination of the surrounding atmosphere ly 100% yield by proceeding in accordance with the pres Such as would happen by allowing sulfur dioxide-contain ent invention, namely by oxidizing the aqueous solution ing gases to escape, but also to recover the sulfur diox of ammonium sulfite in the presence of a nitrogenous ide-even though present only in great dilution-and to organic base, and preferably under the conditions de convert the same into valuable products. scribed further below. It is well known to selectively absorb the sulfur dioxide To proceed in this manner has the further advantage of Such waste gases in aqueous dispersions and solutions that the nitrogenous base which is insoluble or immiscible of lime and in aqueous ammonia solutions. However, the 30 in the aqueous sulfite and/or sulfate solution will form a further treatment of the thus-formed sulfites is connected distinct separate layer when the mixture is allowed to with considerable difficulties. stand and thus, upon completion of the oxidation, the If a solution of ammonia in water or in a saline solu nitrogenous base can be easily separated by decantation tion is used as absorbent for the sulfur dioxide, it has from the aqueous ammonium sulafte solution. The thus been proposed to subject the thus-formed sulfur dioxide 35 obtained ammonium sulfate solution which preferably containing solution to contact with oxygen, for instance, will be a substantially Saturated solution maintained at an with the oxygen of the air, so as to transform the am elevated temperature, may then be cooled whereby the monium sulfite of the solution into ammonium sulfate solubility of the ammonium sulfate will be reduced and which then may be recovered as a crystallized product. a portion thereof will crystallize from the supersaturated However, in industrial practice, this direct oxidation poses 40 solution formed by cooling. It is, of course, also possible great difficulties inasmuch as due to the gradual change in to obtain crystallized ammonium sulfate by evaporation of the pH of the solution the yield of the oxidizing reaction the solution. The thus obtained solid product is suitable, diminishes gradually and eventually reaches zero. It has without further processing, for use in and as fertilizer or been attempted to overcome this difficulty by adding cata the like. The working conditions under which the am lysts such as certain cobalt, copper and iron salts as well 45 monium sulfate is recovered from the aqueous solution as by trying to control the pH, however, nevertheless, thereof are less critical than is the case in the separation still.after some time the oxidation practically comes to a stand and recovery of many other chemical compounds. The organic base which facilitates the oxidation of the It is, therefore, an object of the present invention ammonium sulfite and which should be finely dispersed to overcome the above discussed difficulties in the recov 50 throughout the aqueous solution during oxidation of the ery of Sulfur dioxide in the form of ammonium sulfate. same may be any nitrogenous base which is immiscible It is a further object of the present invention to pro with the aqueous solution and which is weaker than am vide a method by which ammonium sulfite contained in monia so that it will not cause freeing of ammonia from an aqueous solution can be substantially completely con the dissolved ammonium sulfite or sulfate. verted into ammonium sulfate in a simple and economical 55 Thus, heterocyclic aromatic, aliphatic, cycloparaffinic anner. and cycloolefinic amines, the heavy pyridic bases of the Other objects and advantages of the present invention type derived from coal-tar, xylidine, toluidines, quinoline, will become apparent from a further reading of the de dimethylaniline, methylamine, ethanolamines, cyclohexyl scription and of the appended claims. aniline, pyridine, lutidines, diphenylamine, picoline, col With the above and other objects in view, the present 60 lidine, methylethylpyridine and dipyridyl may be used as invention comprises in a method of oxidizing ammonium well as other compounds which are of pyridic character sulfite to ammonium sulfate, the step of contacting with or contain condensed pyridic rings, as well as nitrogen an oxygen-containing gas an aqueous ammonium sulfite containing basic derivatives thereof. containing solution having dispersed therethrough an or Preferably, the solution containing the ammonium sul ganic, nitrogen-containing base so as to oxidize the am 65 fite which is to be oxidized to ammonium sulfate will be monium sulfite of the Solution to ammonium sulfate there a concentrated solution of ammonium sulfate which con by forming an aqueous ammonium sulfate-containing tains between about 10 and 15% of ammonium sulfite. Solution which is substantially free of ammonium sulfite. When feasible it is desirable to control the absorption of According to a preferred embodiment of the present Sulfur dioxide from the waste gases or the like in such a invention, a nitrogen-containing organic base which is 70 manner that thereby a solution is formed which will be a Weaker than ammonium and which is insoluble in the concentrated solution of ammonium sulfate containing be. tween 10 and 15% of ammonium sulfite and the thus 3,330,620 4. t sulfate, sataurated at 20 C., is passed a gas stream which formed solution is then immediately subjected to oxidation contains 1.18 kg. of gaseous ammonia, and simultaneously in accordance with the present invention. Oxidation of the dissolved ammonium sulfite is carried or successively another stream of gas consisting of or con out according to the present invention by contacting the taining 2.22 kg. or SO2. To the resulting solution are ammonium sulfite-containing solution in the presence of added 0.625 kg. of water and 60 kg. of the fraction of a nitrogenous base with gaseous oxygen which may be in pyridic bases boiling between 140 and 170° C. The entire troduced in the form of air, or pure oxygen, or air which mixture is then introduced into a reactor, in the interior has been enriched with oxygen. The best results are ob of which exists an oxygen atmosphere, heated therein to tained if the oxidation is carried out at a somewhat ele 80° C. and vigorously stirred. The oxygen in the reactor is vated temperature such as between 40 and 140° C. and O replenished as it is consumed in the reaction, resulting in preferably at a temperature between 70 and 90° C., and a total consumption of 0.39 standard cubic meter of at an absolute pressure of between 1 and 11 atmospheres. OXygen. It is preferred to operate within the above described tem After one hour the oxidation is complete, and by fur perature and pressure ranges, however, the invention is ther treating the contents of the reactor in the same way not to be considered limited thereto. Generally, somewhat 15 as indicated in Example I, 4.8 kg. of ammonium sulfate higher pressures will be preferred in combination with in form of crystals are obtained, while at the same time somewhat lower temperatures, in other words, the tem a saturated (at (20 C.) ammonium sulfate solution is perature may be inverse to the utilized pressure. formed which is separated from the organic base by de The proportion of the organic base in the liquid mix cantation, or which may be re-used together with the ture containing ammonium sulfite which is to be oxidized 20 organic base for the absorption and oxidation of additional may vary within broad ranges and preferably will be main Sulfur dioxide.