Mechanism Of Removing Muscovite And Lattice Imputiry Elements From Vein Quartz

Vein quartz is a kind of high-quality silicon resource,an indispensable raw mineral material for preparing high-quality quartz,which is widely used in semiconductor,high-performance glass,electronic information,photovoltaic power generation,aerospace,defense industry and other modern high-tech fields.Because of complex occurrences of gangue minerals within vein quartz,a combination process of physical（gravity separation,magnetic separation,flotation）and chemical methods is ofen used to purify quartz,which commonly leads to unstable product quality,high energy consumption and serious environment pollution from fluorides and acids.Different from the previous research,this study uses vein quartz from Qichun County in Hubei province to prepare high-purity quartz sand,aims at developing a chlorination leaching technology to separate muscovite in surface and interface of quartz,and explores technology and mechanism of removing lattice impurities during chloridizing calcination of quartz so as to provide a novel technical way for preparing high-purity quartz using vein quartz.Based on the analyses of mineral characteristics in the vein quartz,the objectives of the study are to investigate structures of mineral inclusions and occurrences of impurity elements in the vein quartz,explore a novel fluorine-free pressure leaching technology of fine muscovite occurred in quartz interfaces,discuss technology and mechanism of activating and separating trace lattice impurity elements during chloridizing calcination of quartz,and focus on analyzing activation mechanisms（thermal permeation and phase-transition thermal）of lattice impurities during KCl-doping calcination of quartz.The main results are as follows:（1）Main gangue minerals occurred in the vein quartz from Qichun County are muscovite,hematite,phosphorite and salts.Main impurity elements are Al,K,Fe,Mg,Na,Ti,Ca,Zr B,P,and S,of which mass percent of Al in all impuritir elements is56.98wt%.（2）Quartz sand is calcined at 900°C for 5 h,and respectively leached by NH₄Cl-HCl（0.8 mol/L HCl,0.8 mol/L NH₄Cl,280°C,6 h,liquid-solid ratio of 10 cm³/g）and quartz NH₄Cl-H₂SO₄（0.3 mol/L H₂SO₄,0.45 mol/L NH₄Cl,250°C,7 h,liquid-solid ratio of 4 cm³/g）solutions.At optimum leaching conditions,total contents of impurity elements are respectively reduced to 91.89μg/g and 99.21μg/g from 619.0μg/g.Contents of SiO₂ in quartz concentrates are 99.991wt%99.990wt%,respectively,and total removal rates of impurity elements are 85.2%and 84%,respectively.（3）Quartz sand purified by NH₄Cl-H₂SO₄ solution is used for KCl-doping calcination experiment.The quartz sand mixed with 2wt‰of KCl is calcined at 900°C for 45 h,and then leached by H₂SO₄-HF solution（0.30 mol/L H₂SO₄,0.5 mol/L HF,200°C,4 h,liquid-solid ratio of 5 cm³/g）.At optimum leaching conditions,total content of impurity elements is reduced from 99.21μg/g to 29.40μg/g.Content of SiO₂ in quartz concentrate is 99.997wt%,and total removal rate of impurity elements is 70.4%.（4）Thermodynamic analysis shows that Gibbs free energy of chemical reaction between mineral muscovite or its decomposition products of oxidation calcination and hydrochloric acid or sulphuric acid at 200°C-300°C is always less than zero.Hence the trace muscovite occurred in quartz can be dissolved by oxidizing calcination and pressure acid leaching.NH₄⁺is added into leaching solutions to promote the hydrolysis of Si-O^-,passivate Si-O^-on surface of silicate minerals,and inhibit hydrolysis of Al³⁺so as to promote a stable migration of dissolved Al³⁺to diffusion layer.（5）Oxidizing calcination tiggers a dehydroxylation of muscovite which leads to removal of interlayer water,increase of cleavage plane space and structural damage of Al-O octahedron close to interlayer.Directional micro-cracks are formed in surface and inner layers due to calcination.The mineral muscovite is transformed into active structures including Si-O-Al,Si-O-K,Al-O-K.The directional micro-cracks are developed into crossed cracks during pressure leaching when the leaching agents dissolved surface,edge,interlayer and fracture of active structures.The kinetic study shows that apparent activation energy of the leaching process of Al is 52.18 kJ/mol,which is mainly controlled by chemical reaction.Combined oxidizing calcination pretreatment with chloridizing pressure leaching,dissolution of the muscovite（reaction-diffusion-reaction）is converted into a simple chemical reaction.Meanwhile,hydrolysis of Al³⁺caused by Si-O^-structure is inhibited by NH₄⁺so as to achieve an constant exchange of H⁺and Al³⁺.Hence Al³⁺can steadily enter the diffusion layer.（6）Trace elements bound in quartz are activated and enriched on quartz surface by high-temperature calcination.Contents of lattice impurity elements including B,P,Al,Fe,Li,Na,Ca,Mg and Z are apparently reduced due to activation and release.The presence of K⁺decreased phase-transition temperature of quartz,promoted the transformation ofα-quartz to cristobalite so as to produce abundant micro-cracks on surface and interior of quartz,which provide more crystal defects and activation interfaces.Phase transformation ofα-quartz to cristobalite reduced activation energy of lattice impurities,promoted a directional outward diffusion of lattice impurities.Some impurity elements remained on quartz surface can be separated by enhanced pressure leaching.