Transformation of a low-grade Korean natural zeolite to high cation exchanger by hydrothermal reaction with or without fusion with sodium hydroxide

A low-grade Korean natural zeolite in which clinoptilolite coexists with quartz and feldspars was treated hydrothermally with 3, 4 and 5 M NaOH solutions during different times ranging from 4 to 40 h, either with or without fusion with NaOH powder at 550°C during 2 h as a pretreatment. Zeolite Na-P, zeolite Na-X and hydroxysodalite were identified as the reacted products, depending on the reaction conditions such as NaOH concentration and reaction time. The fusion with NaOH powder prior to the hydrothermal reaction brought about zeolitization of feldspars in the starting material, whereas it hardly occurred in the hydrothermal reaction without the fusion treatment. Cation exchange capacity (CEC) of the products with the fusion treatment was 410 cmol(+)/kg for zeolite Na-P at room temperature and 400 cmol(+)/kg for zeolite Na-X and hydroxysodalite at 80°C. These values were strikingly high in comparison with the CEC of 69 cmol(+)/kg of the starting low-grade natural zeolite.