Abstract: High purity silica was synthesized through two-step ammonification method by using waste fluorosilicic acid,a hazardous waste material,by-produced in the production of phosphate fertilizer from phosphorite as raw material.Effects of process conditions,such as reaction time,reaction temperature,mass fraction and addition amount of ammonia on the product quality,were investigated,and optimized parameters of two-step ammoniation technology were obtained.Optimized conditions of the first ammoniation step were:reaction temperature of 45 ℃,reaction time of 10 min,mass fraction of ammonia of 13%,and addition amount of ammonia was 60% of the theoretical amount.The second step were: reaction terminal pH of 9, reaction temperature of 50 ℃,and stirring speed of 260 r/min.The most of indexes of the prepared silica reached HG/T 3061—1999 standard.The tecnology has simple process flow,better use of equipment and low consumption,and reaches the prospective target of taking full advatage of mine resource and lowing the cost of products.