Abstract:
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.
