Two-step ammoniation of by-product fluosilicic acid to produce high quality amorphous silica

A two-step ammoniation was applied to neutralize the fluosilicic acid gener ated in the production of super phosphate fertilizer, and to recover high-q uality amorphous silica in the second step. A fluosilicic acid solution at 12.6% or 25.3 wt% H2SiF6 was reacted with aqueous ammonia, precipitating si lica and subsequently leaving NH4F behind in the filtrate. Based on a fixed total of 7.2 moles of ammonia per mole of H2SiF4 (120% stoichiometry), amm onia at a molar ratio of ammonia to H2SiF4 of 2 to 6 was charged in the fir st step and the balance of 5.2-1.2 in the second step in four combination s ets 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 rea dily produced in the second step while a majority of the impurities were pr ecipitated 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 betwee n 1.5 and 2.5 NH3/H2SiF6 molar ratios, indicating the rapid disappearance o f SiF62- ion from the solution.