Systems and methods for pre-treatment of acid gas are presented in which ammonium is removed from the acid gas in an absorber that is operated at significantly elevated temperature using dilute phosphoric acid. While seemingly incompatible, absorbing ammonia at high heat in the absorber will allow for production of a diammonium phosphate product that is ultra-low in residual sulfurous compounds and prevent crystallization of phosphate salts due to the increased solubility of the salts in the hot diluted solvent.