Effect of existence form of La3+ on the properties of the Bayan Obo Mine tailing glass ceramics

Abstract

Clarifying the effect of rare earth (RE) elements on the microstructure and properties of glass ceramics is technically and theoretically important for the further development. Thus the glass ceramics of the CaO-Al2O3-MgO-SiO2 with 04 wt% La2O3 are fabricated from Bayan Obo Mine tailing and fly ash by means of the conventional melting method. Effect of the existence form and the concentration variation of La3+ ions on the crystallization behavior, microstructure and properties, such as bending strength, chemical resistance and density of the glass ceramics, are investigated by DTA, XRD, SEM, TEM and EDS. Results show that both the glass transition and crystallization peak temperature of the samples shift to high temperatures with increasing La2O3 content. Augite [Ca(Mg, Al, Fe)(Si, Al)2O6] is the only crystalline phase in all the five samples. Augite crystals in the form of column are distributed uniformly within the residual glass, and their average size is below 100 nm. The crystallinity of augite has been effectively enhanced by the addition of 1 wt% of La2O3. Owing to the similar ion radius of La3+ and Ca2+, Ca2+ ions within augite have been partially substituted by La3+. Such a substitution can serve as one of the key factors to the enhancement of bending strength of the investigated material with 1 wt% of La2O3 because of the stronger bonding energy of La-O than Ca-O. With further increase of La2O3 from 1 to 4 wt%, the Ca3La6 (SiO4)6 secondary phase forms on the boundary between augite grains and residual glass phase in the form of irregular-shaped particles and this in turn hinders the growth of augite crystals. The crystallinity of augite will be decreased gradually since then. Meanwhile, the formation of this La-riched phase (Ca3La6(SiO4)6) may also prevent augite grains from growing through consuming Ca2+ and Si4+ ions which are two key constituent elements of augite grains. Therefore, there are two forms of La3+ ions in the glass ceramics developed from Bayan Obo Mine tailing: one is the substitution of Ca2+ ion by La3+ in augite crystalline phase, and the other is the forming of secondary crystalline phase La-riched Ca3La6 (SiO4)6. The glass ceramic sample with 1 wt% of La2O3 shows the optimum properties. Its density is 3.18 g/cm3, the bending strength is 198 MPa, and the weight loss in 20 wt% NaOH of this sample is lower than 1 wt%.