Process Study On Preparation Of High Purity Silica And High Purity Quartz By Ammonification Reaction

Fluorosilicic acid is a byproduct during the production of phosphate fertilize,indiscriminately discharging of it will pollute the environment and cause the waste of resources.Using fluorosilicic acid and converting it into high value-added silicon products or fluorine products has a good market prospects and environmental significance.In this experiment,we use fluorosilicic acid to produce ammonium fluorosilicate solution by the first ammonification reaction,and then we finding suitable ammonium salt and calcium salt as addition agent to remove impurities in solution,so we have purified ammonium fluorosilicate solution.After the second ammonification reaction we can get high purity silica,calcining it in high temperature we will have high purity quartz crystal.We have explored and optimized the condition of the reaction.The optimum conditions of the first ammonification reaction: the final p H is 3.8⁴.0,the dropping speeding of ammonia is 1.5ml·min^-1,the temperature of the reaction is 25? and the stirring speed is 200 r·min^-1.The yield of ammonium fluorosilicate is 93.6% under this optimized conditions.When we put ammonium salt as addition agent into ammonium fluorosilicate solution to change it's p H to 6.5,we find the impurities removal rate of metal Ca is 88.8%,the rate of Mg is 81.2%.And then we add calcium salt as precipitant into the solution to remove Fe?Al impurities,we notice the impurities removal rate of metal Fe is 82.3%,the impurities removal rate of metal Al is 79.8%.And then we explore the optimum conditions.The optimum conditions is as follows: the mass ratio of calcium salt to fluorosilicic acid is 9:100,aging time of 40 minutes,standing time of 9 hours.Under this condition,we removed 97.9% of the metal impurities in the solution and got the highly pure ammonium fluorosilicate solution.The effect of impurities removal achieved the desired results.When the mass ratio of calcium salt to fluorosilicic acid is 9:100,the removal rate of metal impurities in Si O₂ can achieve 98.1%,the impurity content is less than 100?g·g^-1 which has reached the national standard.The final production can be called as high purity silica and the purity of Si O₂ is 99.99%.The optimum conditions to produce high purity silica: the final p H of 8.5,the reaction temperature of 40? and the dropping speeding of ammonia is 4ml·min^-1.Under these conditions,the yield of silica is 94.6%,the particle size is 0.35?m.The TR test shows that the final product is Si O₂,the XRD test shows that the sample is amorphous silica and the SEM test shows Si O₂ is fluffy and has good dispersion.When the calcination temperature is above 1200?,the high purity silica begins to convert to high purity quartz.And when the temperature reaches 1300?,holding time is 2 hours,we found that the high purity amorphous silica transform into cristobalite crystals and the conversion rate is close to 100%.The surface of the sample become more smooth and the structure is more compact.After calcination at high temperature,the particle size of sample reduces and the volume shrinks.When the calcination temperature reached 1300?,the volume shrinkage rate reached 81.89%.