Study On Purification Of Silica Sands By Roasting And Acids Leaching

As an important industrial raw material,silica sands was widely used in the field of solar photovoltaics,microelectronics,optical fibers.High purity silica sands has a strictly limited to content of impurity elements such as Fe,Al,and Ti,as well as alkali metals.Most part of iron contamination is contained in hematite,biotite and goethite,external to the quartz,Iron as a kind of harmful element,impairs the transparency and conductivity of sands products.Na,K and Ca may be hosted by fluid inclusions and sometimes mineral within the quartz,excessive content of alkali metal can reduce the performance of high temperature,thus affecting the thermal stability and optical properties.Unfortunately,most of studies were only focused on iron removal.Other impurities including aluminum,lithium and other elements is still an obstacle in achieving high purity silica.Aluminum is the most common and high-concentrated element in quartz(up to a few 1000 ppm),which together with Li,Ti and Ge,are preferentially incorporated into the quartz crystal lattice on substitutional(Al,Ti,Ge)and interstitial(Li)positions,rather than in other minerals as inclusions or adsorbed materials.Therefore,Al,Ti,Ge elements can't be easily removed,however,physical and chemical treatment capable of removing Fe,Na,K and other impurities using crushing,washing,magnetic separation,acidic leaching.At present,there are some methods to purifying quartz sand,such as magnetic separation,acid leaching,flotation,microbiological method and complexing method,etc.As an effective method of removing metal impurity elements,acid leaching purifiy sands efficiently,some factors such as acid types,concentration,temperature,liquid-solid ratio have been researched deeply to increase the removal rate of impurities.Magnetic separation method can only remove magnetite,limonite and other magnetic minerals.Flotation method using anionic and cationic collectors chemically reacted with minerals to removing impurity.Microbiological method using aspergillus niger and penicillium to purify sands especially on the surface of quartz sand.Complexing purification method using agents reacted with the metal impurities.The above methods have their own advantages on purification,but certainly have drawbacks,which only effectively remove impurity elements on the surface of sands particle,but not worked so well for elements inside the particle,gas-liquid inclusion,substitution and interstitial impurity.On these above,following issues are mainly researched in this paper:In order to improve iron impurity removal efficiency of impurities from silica sand,magnetic separation and mixed acid leaching were used,and iron removing kinetics has been analyzed.The results show that iron removal rate is negative correlated with particle size and the removal rate of iron is the highest when the particle size is 150?180 ?m.It is found that mixed acids(HNO3,HCl,HF)has a better performance,and the removal rate can reach 88.3%,the content of iron impurity is reduced to 24.76 ppm under conditions:Vacid:Vwater=1:1,acid leaching temperature is 90?,leaching time is 360 min.This meets the qualification of refined quartz sand.The kinetics analysis reveals that product layer diffusion in heterogeneous reaction system is the rate-control step and the activation energy is 51.73 kJ/mol.(1)A method for removing Fe impurity from quartz by roasting and acid leaching was proposed.The optimum holding time is 180 min under the roasting temperature of 900?.The iron content in quartz sand with roasting is 33.51 ppm,the removal rate is 85%after acid leaching under conditions:acid leaching temperature is 90?,leaching time is 1440 min.The quartz was analyzed by SEM,the corresponding result showed that there are many cracks and corrosion pits on the surface of the quartz particles after roasting,which contributed to removing of Fe impurity.Kinetics analysis reveals that the product layer diffusion in heterogeneous reaction system is the rate-controlling step.Compared to the one without roasting,the acid leaching after roasting reacts more quickly,the iron content and the activation energy are lower.The activation energy is 30.88 kJ/mol after roasting for 900?,holding 180 min compared with 36.18 kJ/mol without roasting,and it reduced to 17.2%after roasting.It showed that change of quartz crystal structure will promote the reaction.(2)Aluminum is one of important elements in silica sands,there are few researches for it at present.Compared to iron impurity,the efficiency of aluminum purification is poor because of content and occurrence states.In this article,a method for removing aluminum and other impurity effectively from quartz by roasting and acid leaching was proposed.The result showed that aluminum content is 40.9 ppm,removal rate is 96.7%,iron content is 27.2 ppm,removal rate is 87.7%,SiO2 content is 99.98%,meeting the qualification of high purity silica sands,under the conditions:roasting temperature 900?;holding time 60 min;mixed acid(VHcl:VHNO3:VHF=3:3:1,Vacid:Vwater=2:1);leaching temperature is 90?.The kinetics analysis reveals that the product layer diffusion in heterogeneous reaction system is the rate-controled step for sands leaching reaction without roasting,however,the reaction for silica sands with roasting was found not meet with heterogeneous reaction system.(3)Acid dissolution and alkali fusion are two methods for preparing silica sands ICP-OES testing samples.The method of acid digestion process for preparing ICP-OES samples is not standardized which is not completely digested and inaccurated,The method of alkali fusion operate tediously and the sample is easy to be polluted.Therefore,it makes sense to find out the impact on testing of two methods.In this paper,two methods including acid wet digestion and alkali-fusion method were used and compared in testing Al,Zr,Mg impurity elements content of sands.By Scanning Electron Spectrometer(SEM/EDX),the undissolved particles are composed of F,Al,Zr,Si and Mg.These results suggested that acid digestion process has some loss of Al,Zr and Mg content,causing lower value of the testing results and experimental bias.The results of ICP-OES show that Al,Zr,Mg content by alkali fusion digestion method is higher than that by acid digestion.The results of recovery experiment show that the method had better accuracy and the recovery rates are in the range of 93.6%?116.3%,which meets detection requirements.But its' detection limit and RSD is higher.