Effect of Quartz on the Preparation of Sodium Stannate from Cassiterite Concentrates by Soda Roasting Process

Sodium stannate (Na2SnO3) has been successfully prepared by a novel process of roasting cassiterite concentrates and sodium carbonate (Na2CO3) under CO−CO2 atmosphere, namely soda roasting-leaching process. However, more than 22 wt. % tin of the cassiterite was not converted into Na2SnO3 and entered the leach residues. Quartz (SiO2) is the predominant gangue in the cassiterite, and phase evolution of SnO2−SiO2−Na2CO3 system roasted under CO−CO2 atmosphere was still uncertain. In this study, the effect of SiO2 in cassiterite concentrates on preparation of Na2SnO3 was clarified. The results indicated that Na8SnSi6O18 was inevitably formed when cassiterite and Na2CO3 were roasted above 775 °C under CO−CO2 atmosphere via the reaction of SnO2 + 6SiO2 + 4Na2CO3 = Na8SnSi6O18 + 4CO2, and formation of Na8SnSi6O18 would be increased with increasing roasting temperature and Si/Sn mole fraction. In addition, it was found that Na8SnSi6O18 was insoluble in the leachate at pH value range of 1−14, which, therefore, was enriched in the leach residues. The silicon content of the cassiterite concentrates should be controlled as lower as possible to obtain a higher conversion ratio of Na2SnO3.