PNNL

Abstract

The effects of six process variables on the hydrothermal growth of iodosodalite, Na8Al6Si6O24I2, were investigated: pH (NaOH concentration), aging time, temperature, Al/Si ratio, the precursors used (i.e., zeolite 4A, kaolinite, meta-kaolin, colloidal silica, and sodium aluminate), and precursor concentration. Powder X-ray diffraction (XRD) was performed to identify phases and Rietveld refinements were used to quantify mass fractions of phases. Iodosodalite yield increased as aging time and pH increased. The crystallization of iodosodalite was favored in the temperature range 140-180 °C whereas, at 200 °C, basic cancrinite formed at the expense of some iodosodalite. Decreasing the Al/Si ratio by half increased the crystallization of basic cancrinite. Lowering the precursor concentration by adding water revealed the crystallization of nepheline hydrate I and a decrease in the sodalite fraction. Among the tested precursors, zeolite 4A yielded the highest mass fraction of iodosodalite in the synthesized powders. From the aging time and temperature variation experiments, the phase transformation of zeolite A ? sodalite ? cancrinite was observed. The crystallization of various minerals suggests that mechanisms for transport of the ions and formation of the aluminosilicate frameworks vary with hydrothermal conditions.