A two-step ammoniation was applied to neutralize the fluosilicic acid generated in the production of super-phosphate fertilizer, and to recover high-quality amorphous silica in the second step. A fluosilicic acid solution at 12.6% or 25.3 wt% H₂SiF₆ was reacted with aqueous ammonia, precipitating silica and subsequently leaving NH₄F behind in the filtrate. Based on a fixed total of 7.2 moles of ammonia per mole of H₂SiF₆ (120% stoichiometry) ammonia at a molar ration of ammonia to H₂SiF₆ of 2 to 6 was charged in the first step and the balance of 5.2-1.2 in the second step in four combination sets of ammonia addition. At a combination of 3 and 4.2 molar ratios in the respective first and second steps, amorphous silica of 99.7% purity was readily produced in the second step while a majority of the impurities were precipitated in the first step. The two-step ammoniation was near completion despite a non-equilibrium reaction. However, rapid ammoniation favored the crystallization of ammonium fluosilicate. A sharp pH change occurred between 1.5 and 2.5 NH₃/H₂SiF₆ molar ratios, indicating the rapid disappearance of SiF₆^2- ion from the solution.