Thermal reactions of alkali-leached aluminosilicates studied by XRD and solid-state Al-27, Si-29 and Na-23 MAS NMR

The interactions of dehydroxylated Zettlitz kaolinite with NaOH solution have been studied using X-ray powder diffraction and solid-state MAS NMR to characterise the crystalline and amorphous products, respectively, and to provide further information about the reaction sequence of kaolinite. Under the present conditions, leaching for very short times (ca. 10 min) preferentially removes uncombined amorphous silica (but not the quartz impurity), and also introduces Na into the remaining amorphous aluminosilicate phase. Leaching for longer times continues to remove Si from the amorphous phase, with a concomitant increase in Na incorporation and an increase in the proportion of available Al. On reheating to 1140 and 1300 degrees C, the resulting crystalline phases reflect the increase in Al and Na content with increased leaching time, forming a-alumina and, in samples leached for > 60 min, a nepheline solid solution at the expense of mullite, residual quartz and cubic spinel-type phase.