Study On Purification And Synthesis Of Spherical Silica From Industrial Silica Fume And Its Polishing Slurry

Silica fume（SF）is a main by-product of ferrosilicon and industrial silicon during industrial production.It usually consists of amorphous SiO₂ and shows a grayish black appearance.However,high content of purities in SF lead to low efficiency of utilization,resource waste,environmental pollution and a threat to human health.Based on the physical properties of-SF and the existing purification procedures,this paper performs a systematically study of purification of SF by homogeneous precipitation method.Furthermore,the purified SF product is redissolved to prepare spherical SiO₂ by micelle method.The effects of different process parameters on the particle size of SiO₂ are studied.The precursor made by prepared SiO₂ is used as abrasive to prepare alkaline SiO₂ polishing slurry,and the optimal component of the slurry is determined by surface roughness.Main conclusions are given as follows:（1）The purification of SF by homogeneous precipitation method is mainly di vided into three Steps.Firstly,the SF was dissolved with hydrofluoric acid and filtered to remove the non-metallic impurities in the SF.Then,urea was added to the obtained solution to precipitate SiO₂.Finally,the metal impurities were removed by hydrochloric acid pickling to realize the purification of SF.The results showed that the content of SiO₂ in purified SF increased from 84.00 wt.%to 99.82 wt.%,under the following procedures:raw SF is dissolved with hydrofluoric acid and urea as precipitant with heating reaction in water bath at 95 ℃ f or 8 h,aging for 2 h,and drying at 550℃for 2 h.（2）The main process of preparing spherical SiO₂ by micelle method is to redissolve the purified prorduct and the dispersant and surfactant were added to the solution.The lipophilic group of the surfactant spontaneously combined and agglomerated to form micelles to make the system stable.The electrostatic effect makes the region with high ion concentration near the micelle become the nucleation site.After nucleation,due to the low ion concentration in the solution,the excessive growth of the particles is inhibited.Moreover,the micelles are adsorbed on the surface of the precipitated particles,the vacancy barrier mechanism of the dispersant and the electrostatic stability mechanism of the surfactant effectively avoid the agglomeration between the particles.The results showed that SiO₂ with good dispersibility,average particle size of 250 nm and sphericity of 0.92 was prepared under the conditions of 10 wt.%polyethylene glycol（PEG-4000）solution as dispersant,sodium dodecyl benzene sulfonate（SDBS）as surfactant,reaction concentration of 0.1 mol/L,reaction temperature of 60℃,reaction time of 2 h and aging time of 4 h.（3）The composition of alkaline SiO₂ slurry is determined by single factor experiment:the oxidant,complexing agent,abrasive,corrosion inhibitor and the surfactant are NaClO,ethylenediamine,silica sol,citric acid,and fatty alcohol polyoxyethylene ether and ammonium dodecyl sulfate,respectively.The optimal ratio of polishing solution determined by orthogonal experiment of concentration change of each component on polishing performance as follows:0.3 vol.%fatty alcohol polyoxyethylene ether（AEO-3）,0.3 vol.%ammonium dodecyl sulfate（ADS）,2 vol.%ethylenediamine,5 vol.%silica sol（solid content 40 wt.%）,5 mmol/L citric acid and 5 vol.%NaClO.When polishing pure copper by employing such optimal components,obvious mirror effect can be observed,and the surface roughness is measured to be～2.25 nm.When the precursor silica sol of spherical SiO₂ prepared by micelle method is used as abrasive,the surface roughness of pure copper is～20.75 nm.In this paper,the SF is purified to prepare spherical SiO₂,and it is further applied to the preparation of polishing slurry.It provides a new idea for broadening the application field of SF and improvs its application value.