Improvements in and relating to the oxidation of ammonium sulphite in aqueous solution

摘要

In oxidizing a solution of ammonium sulphite by means of oxygen-containing gases such as air, the solution contains a ferrous salt and free acid in quantity sufficient to keep the ferrous salt in solution, but insufficient to cause loss of sulphur dioxide, the temperature being maintained below 50 DEG C., and preferably below 40 DEG C. The acidity of the solution must be within the limits defined by a pH value of 5,5-6. The quantity of iron required need not exceed 1 part by weight per 50 parts of ammonium sulphite to be oxidized. The ammonium sulphite solution may be produced by reacting sulphur dioxide, such as obtained from combustion gases or by burning spent oxide used in gas purification, with ammonia, for example, synthetic ammonia, or as crude ammoniacal liquor or ammonia obtained by distillation of this latter with lime. The solution to be oxidized may contain ammonium sulphate, and the iron salt may be passed continuously into solution, as by bringing the solution into contact with scrap iron, or by effecting the reaction in an iron vessel. The iron may be precipitated from the final solution by means of hydrogen sulphide or ammonium sulphide, and the ferrous sulphide returned directly to the oxidizing vessel wherein it is re-dissolved. The oxidation process may be effected by passing the solution down a tower provided with baffles into which the oxygen-containing gas is passed. The solution obtained may be concentrated by means of heated gases, and crystallized. According to the Provisional Specification, an iron salt, preferably in the ferrous state, is employed, and no specific limitation is imposed as to the minimum pH value of the solution to be oxidized. Specifications 356,283 and 360,574 are referred to.