This purification process may be described by the unreacted shrink core model with solid resultant (inert material) and fixed particle size, which is carried out by the action of nitric acid solution on the precipitated silica obtained from yellow phosphorus slag which was leached with phosphoric acid. The study results indicate that the purification process is a chemical reaction controlling step and its apparent activation energy E-a is 30.354 kJ/mol, with reaction order 0.6746.