Preparation And Study On Low Fluoride Opal Glass

Currently,fluorides are used for producing opal glasses as opacifiers in China, but fluorine is harmful to human being,animals and plants.In many countries,the content of fluorine in atmosphere is restricted strictly.In recent years,low-alkali phosphate opal glass has been used extensively as mosaic glass,but because of its insufficient whiteness and difficulties in forming its development is subjected to restriction.In order to get a better opacifying result by the mixed opacifier,in the present paper,based on studying the crystalline properties of fluoride and phosphorus opal glasses,the mixed opacifier was applied to achieve better performance and lower cost.In the present paper,the F-Li₂O-Al₂O₃-SiO₂ glass system was studied.Its melting temperature was 1480℃,and the melting time was less than three hours.The influence of different Al₂O₃ contents in samples was studied.The results show that [AlO₄]can adjust network linkage.In addition,the thermal stability,mechanical strength and chemical durability were also improved.Besides,the corosion of refractories and volatilization of fluoride were reduced.When 8wt% of Al₂O₃ was added,the volatilization of fluoride could be fully controlled,but the viscosity of molten glass was increased,which accounts for melting difficulty.Therefore,the content of Al₂O₃ has to be controlled within 6～7wt%.By XRD and SEM analyses,the results showed the crystalline phase was CaF₂.According to Scherrer formula we could calculate the average diameters of particles,which were close to the results of SEM observations.Small amount of Li₂O was helpful to melting glass,but usually when the ratio of Na₂O;Li₂O was less than 7%,the opal glass would lead to "lose porcelain" because of their own strong electric field,which would play the key role in the change of Al₂O₃ configuration from[AlO₄]to octahedral crystal structure. In the chapter four of this paper,opacifying performance of P₂O₅-Al₂O₃-SiO₂-CaO system was studied,and the influence of the ratio of CaO;P₂O₅ on crystallization of glass was discussed especially.The XRD analysis indicated that Ca₃(PO₄)₂ was separated out from network.Before the separation of Ca₃(PO₄)₂,the liquid droplets were separated out first.With increasing content of CaO the size of the droplets became bigger.When it reached to 3wt%,the second separating phase appeared.The initial droplets formed during the first separation split into smaller droplets.When the contents of CaO reached to 4.5wt%,the bigger droplets formed during the first separation would crystallize as apatite,and the small droplets formed during the second separation would keep in their own liquid phase.The solubility of P₂O₅ in silicate glass is not more than 0.2 mol/l,when the content was over this value, P₂O₅ could resolve at higher temperature but at lower temperature it would be precipitated as multiphase particles because of supersaturation.Composite opacifiers in different ratios of phosphate to fluoride were studied. Addition of fluoride could reduce the activation energy of crystallization,and the effective frequency factor K₀ can be increased by the addition of phosphate.When(?)= 5℃/min,the crystallization kinetic parameters K in two samples were 0.144 and 0.213,respectively.The results showed fluoride played an important role in reducing the crystallization energy.The time of opacification could be reduced by phosphate, and the crystallization kinetics coefficient(K)could be reasonably strengthened. Moreover,the melting temperature and time could be also reduced.All of these would be beneficial to the environment.