Technology of Chemical Precipitation of Struvite from Saturated Water of Acid Scrubbers for Ammonia Absorption

The application of the chemical precipitation method to produce struvite at processing the ammonia-saturated liquid phase of wet acid scrubbers is challenging task, since this phase is an acidic solution of an ammonium salt, and the precipitation of struvite requires a simultaneous introducing salts containing magnesium and phosphate ions into the solution and neutralizing it, which usually leads to the appearance of an admixture of competing phases of newberite (MgHPO4 center dot 3H(2)O) and boberite (Mg-3(PO4)(2)center dot 8H(2)O) in the struvite sediment. The pure struvite phase was obtained by fast neutralizing an acidic solution with the full set of phase-forming ions necessary to form the struvite crystal lattice. Based on the results of the study, conditions were determined that make it possible to achieve a high practical yield of struvite (98% and above) without the admixture of foreign phases at a Mg : N : P molar ratio in the reaction medium of 1.2 : 1 : 1, close to its stoichiometric composition. The conditions for achieving this result consist in limiting the pH to no higher than 3.8 at preparing the reaction medium, maintaining the neutralization stage time from 10 to 45 s, and precipitating struvite at a pH in the range of 8.5-9.5.