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 SiF62- ion from the solution.
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