Optimization of hydrothermal synthesis of pure phase zeolite Na-P1 from South African coal fly ashes

Abstract

This study was aimed at optimizing the synthesis conditions for pure phase zeolite Na-P1 from three coal fly ashes obtained from power stations in the Mpumalanga province of South Africa. Synthesis variables evaluated were: hydrothermal treatment time (12–48 hours), temperature (100–160°C) and varying molar quantities of water during the hydrothermal treatment step (H₂O:SiO₂ molar ratio ranged between 0–0.49). The optimum synthesis conditions for preparing pure phase zeolite Na-P1 were achieved when the molar regime was 1 SiO₂: 0.36 Al₂O₃: 0.59 NaOH: 0.49 H₂O and ageing was done at 47°C for 48 hours. The optimum hydrothermal treatment time and temperature was 48 hours and 140°C, respectively. Fly ashes sourced from two coal-fired power plants (A, B) were found to produce nearly same high purity zeolite Na-P1 under identical conditions whereas the third fly ash (C) lead to a low quality zeolite Na-P1 under these conditions. The cation exchange capacity for the high pure phase was found to be 4.11 meq/g. These results highlight the fact that adjustment of reactant composition and presynthesis or synthesis parameters, improved quality of zeolite Na-P1 can be achieved and hence an improved potential for application of zeolites prepared from coal fly ash.

Keywords:

• Zeolite Na-P1
• coal fly ash
• hydrothermal synthesis
• ageing step

Acknowledgment

The authors would like to acknowledge and thank Water Research Commission (WRC), National Research Foundation (NRF) and Coaltech 2020 for providing the funds to make this study possible.