Effect of fly ash size fraction on the potential to neutralise acid mine drainage and rheological properties of sludge

Large quantities of fly ash (FA) are generated annually in South Africa and most of it is disposed in landfills and ash dams. Previous studies indicated that FA can be used to treat acid mine drainage (AMD), another waste stream commonly found in the minerals industry. Moreover, encouraging results were obtained from the studies carried out on the sludge (referred as solid residues (SR) hereafter) recovered from FA-AMD treatment as a suitable mine backfill material. Particle size distribution (PSD) of FA plays an important role in the AMD treatment quality and viscosity of the SR that can be used directly for backfill purposes. In this study, we have investigated the effect of PSD of FA on AMD neutralisation and metal removal. This work also looked at the effect of PSD on the rheology of the SR. Neutralisation experiments indicated that the higher fines fraction enhances the treatment of AMD by reducing the time taken to increase the pH to a minimum of 7. Moreover, FA with higher fines fraction has reduced the Mn and SO2 levels better than the normal and higher coarse fraction FA. X-ray fluorescence analysis on SR confirmed the metal removal trends observed during the neutralisation experiments. Laboratory scale studies were carried out on the rheology of SR using a rheometer. It was found that increasing the coarse fraction results in higher viscosity than by increasing the fines fraction. The observed behaviour is contrary to the notion that increase in fines fraction will increase the viscosity of the suspension. This study shows that there may be some truth in the controversial effect of coarse particles on the rheology of suspensions.