Non-evaporative solid phase ammonium sulfate separation from ammonia-stripped sulfuric acid solution by solvent-driven fractional crystallization

Manifold ammonia separation and recovery processes, such as adsorption, ion exchange, air stripping, membrane stripping, and electrochemical stripping, have been developed to harvest ammonia from waste streams. The separated ammonia was mostly stored in sulfuric acid as aqueous ammonium sulfate, an excellent fertilizer supplying nitrogen and sulfur. This aqueous phase of the ammonium sulfate, however, hindered the direct application of the recovered ammonia, due to low purity and large volume. Evaporative crystallization had been employed to promote its value, but its comprehensive obstacles include high operating costs, slow reaction kinetics, and low-purity end-products. We herein employed solvent-driven fractional crystallization (SDFC), also known as antisolvent crystallization, to crystallize recovered ammonia as sulfate salt (i.e., (NH4)2SO4) from the model ammonia-stripped sulfuric acid solution using methanol, ethanol, and acetone without energy-intensive evaporation. At 20 degrees C, the SDFC technique represented rapid crystallization, which reached equilibrium within only 5 min, and over 80 % of crystallization efficiency when 4-fold of the organic solvents were added. SDFC exclusively produced ammonium sulfate under various ammonium-to-sulfate molar ratios (ASR), whereas the evaporative crystallization produced a substantial amount of an unfavorable product, letovicite (i.e., (NH4)3H (SO4)2) at a low ASR. These features highlight the promise of SDFC as an alternative to the conventional evaporative crystallization. This study demonstrates the SDFC technique to improve the purity and usability of recovered ammonia in an acid stripping solution, thereby improving the potential of existing ammonia recovery technologies and their applicability in the environmental industry.

Automated summary

Various methods for separating and recovering ammonia have been developed, but traditional evaporative crystallization methods have limitations. This study explores the use of solvent-driven fractional crystallization (SDFC) as an alternative technique to improve the purity and usability of recovered ammonia.