The invention relates to a process for converting ammonium formed in a hydroxylamine phosphate oxime process into molecular nitrogen in an ammonium destruction zone, comprising—preparing a vapour stream comprising nitrogen oxide from ammonia, in an ammonia combustion zone;—bringing into contact by feeding to the ammonium destruction zone, individually and/or as pre-mixed combinations, at least part of said vapour stream, and a first liquid stream, comprising ammonium formed in the hydroxylamine phosphate oxime process, and a second liquid stream, comprising at least one acid selected from nitric acid and nitrous acid in a total nitric+nitrous acid concentration of at least 30 wt. %, thereby forming in the ammonium destruction zone a fluid mixture; and—reacting ammonium ions in the fluid mixture with nitrogen oxide under formation of molecular nitrogen, in the ammonium destruction zone. The invention further relates to an installation for converting ammonium formed in a hydroxylamine phosphate oxime process.