South African coal combustion power utilities generate huge amounts of coal fly ash that can be beneficiated into zeolitic products. This chapter reports on the optimization of the presynthesis and synthesis conditions for a pure‐phase zeolite Na‐P1 from selected South African coal fly ashes. The hydrothermal treatment time, temperature, and molar quantities of water during the hydrothermal treatment step were successfully optimized. The optimum hydrothermal treatment time and temperature were 48 h and 140°C, respectively. Pure‐phase zeolite Na‐P1 was obtained with a molar regime of 1 SiO2:0.36 Al2O3:0.59 NaOH:0.49 H2O at an aging temperature of 47°C for 48 h. The optimized conditions were applied to two fly ashes from two coal‐fired power utilities, and high‐purity zeolite Na‐P1 was obtained. The third coal fly ash with a different chemical composition gave a low‐quality Na‐P1 under the optimized conditions. The cation exchange capacity for the high‐purity zeolite phase was 4.11 mEq/g, indicating that the adjustment of reactant composition and presynthesis or synthesis parameters leads to yields of high‐quality zeolite Na‐P1. The results also show that conversion of the coal fly ash into high‐purity zeolite also depends on the chemical and mineralogical composition of the coal fly ash.
Part of the book: Zeolites
A treatment process for Acid mine drainage (AMD) using coal fly ash (CFA) was developed. AMD was treated with CFA as the alkaline agent at different CFA: AMD ratios and pH, electrical conductivity (EC) evolution monitored over time. In a separate experiment two AMD sources with differing chemistry were treated with the same CFA to evaluate the impact of AMD chemistry on the treatment process and product water quality. Various CFA: AMD ratios were stirred in a beaker for a pre-set time and the process water chemistry determined. pH was observed to increase in a stepwise manner with buffer zones observed at 4-4.5, 4.5-7 and 6-8. AMD with low concentration of Al3+, Fe2+, Fe3+ and Mn2+ didn’t exhibit these buffer zones. Two competing processes were observed to control the evolving pH of process water: dissolution of basic oxides (CaO, MgO) from CFA led to pH increase and hydrolysis of AMD species such as Al3+, Fe2+, Fe3+ and Mn2+ led to pH decrease. These processes initiated mechanisms such as precipitation, adsorption and ion exchange that led to decrease in inorganic contaminants as the treatment progressed. Inorganic contaminants removal was directly related to amount of CFA in reaction media. Precipitation of insoluble hydroxides and Al, Fe-oxyhydroxysulphates contributed to removal of major and minor chemical species. Precipitation of gypsum contributed to removal of sulphate. Na, K and Mg remained largely in solution after initial decrease. Significant leaching of B, Sr, Ba, and Mo from CFA was observed and was directly linked to amount of CFA in the reaction media. This will be a shortcoming of the treatment process since other processes may be required to polish up the product water. The treatment of AMD with CFA was observed to depend on CFA, AMD chemistry, treatment time and might therefore be site specific.
Part of the book: Coal Fly Ash Beneficiation