Recycle process for the preparation and processing of a hydroxylammonium salt solution

In a process for producing cyclohexanone oxime in which a buffered, acidic reaction medium comprising an aqueous solution containing phosphoric acid or ammonium bisulphate is cycled from a hydroxylamine synthesis zone to a cyclohexanone oxime synthesis zone and back to the hydroxyl amine synthesis zone, wherein nitrate ions are obtained in said solution by addition of nitric acid to said solution just prior to introduction into the hydroxylamine synthesis zone and molecular hydrogen is added to the hydroxylamine synthesis zone, said nitrate ions being catalytically reduced to hydroxylamine with said molecular hydrogen in the presence of a supported palladium metal catalyst contained in the hydroxylamine synthesis zone, the solution rich in hydroxylamine from the hydroxylamine synthesis zone is fed with cyclohexanone to said cyclohexanone oxime synthesis zone wherein the hydroxylamine and the cyclohexanone react with each other to form cyclohexanone oxime, separating the cyclohexanone oxime and unreacted cyclohexanone from said solution and recycling said solution back to said hydroxylamine synthesis zone, stripping said solution being recycled from the cyclohexanone oxime synthesis zone to the hydroxylamine synthesis zone for a time sufficiently long enough so that residual amount of cyclohexanone and cyclohexanone oxime, present in the solution after removal of the cyclohexanone oxime produced in the cyclohexanone oxime synthesis zone, are substantially reduced to below about 0.02% by weight,

• the improvement of substantially preventing the poisoning of said palladium catalyst consisting in adding nitrate ions to the solution being recycled from the cyclohexanone oxime synthesis zone to the hydroxylamine synthesis zone after the stripping of said recycle solution.