Synthesis of zeolites using kaolin in concentrated sodium hydroxide-aluminate solutions

The synthesis of zeolites through the hydrothermal treatment of kaolin presents a cost-effective and sustainable method of production, however, the phases which remain stable in concentrated alkali solution with elevated NaAl(OH)4 are unclear. This study evaluated the equilibrium precipitation behaviour of zeolites from kaolin under a range of NaOH and NaAl(OH)4 concentrations at 523 K. The precipitates contained three types of low-silica (1:1 Si:Al) zeolites: zeolite JBW, two polymorphs of cancrinite (hydroxy- and aluminate-), and (hydroxy-)sodalite, as well as a novel sodium aluminate-intercalated nepheline-carnegieite phase at concentrated alkali-aluminate conditions. The construction of a phase diagram revealed distinct regions of stability and transition zones between phases through changing NaOH and NaAl(OH)4 concentration. The kaolin-to-zeolite phase transformation at early reaction time and Bayer pre-desilication conditions supported the phase transformation series: kaolinite → solution silicate species → aluminate-cancrinite → sodalite or cancrinite or a mixture. Substitution of caged aluminate [Al(OH)4−] for hydroxide [(OH)−] within zeolites was modelled, and revealed distinct deviation from ideal zeolite composition when starting molar Al2O3:SiO2 ≫ 1. As direct kaolin-to-zeolite technology emerges as a low-cost and low-energy alternative to conventional zeolite production, there will only be continued interest in its use in the future.