• acid mine drainage;
  • high-density sludge process;
  • iron oxidation precipitation


The oxidation and precipitation of iron from ferrous sulfate solutions was studied at high Fe2+ concentrations, as found in typical South African acid mine drainage, and at high pH, as practised in the removal of iron from acid mine drainage (AMD) by the high-density sludge process. The results obtained showed that the nature of the precipitates formed was largely determined by the rate of Fe2+ oxidation. High pH promoted higher-oxidation rates resulting in high-nucleation rates, and the consequent formation of finer particles. The precipitation process in all cases was found to produce nanosized, growth limited primary particles. These primary particles later aggregated, producing particles several hundreds of nanometres in size. At pH 8.0, these fine particles aggregated due to Van der Waals forces, while at pH 10.0, ferrihydrite was found to transform into goethite. The settling rates of the precipitates showed little dependence on the pH. The precipitate formed at pH 6.0 contained consistently higher proportions of Fe2+ compared to the precipitates formed at higher pHs. Interestingly, particles formed at higher pH gave lower final BET surface areas, contrary to the trend that would be predicted from the evolution of the population based average particle size. © 2007 American Institute of Chemical Engineers AIChE J, 2007.