Fundamental study of a one-step ambient temperature ferrite process for treatment of acid mine drainage waters

A novel approach towards the removal of iron and heavy metals from South Af rican acid mine drainage (AMD) waters is presented. The approach involves t he controlled oxidation of ferrous-containing AMD water at ambient temperat ures in the presence of magnetite seed. The resulting oxidation product is the ferrite (Ml(2)(3+)M2(2+)O(4)) magnetite (Fe3O4), which has the capacity for non-ferrous metal removal, and which forms a stable sludge that is eas ily separated from the effluent. Sludge characterisation studies (XRD, SEM and dissolution tests) show that oxidation of ferrous solutions under contr olled pH and oxidation conditions (pH 10.5, air flow rate = 0.05 l/min) in the presence of magnetite seed (initial seed : ferrous ratio = 7:1) yields almost pure magnetite at ambient temperature. It was found that magnetite s eed channels the end products of the AMD oxidation reaction towards magneti te. Under identical conditions, but in the absence of magnetite seed, a poo rly characterised mixture of largely amorphous iron oxides are formed with magnetite comprising not more than 17% of the total iron. The kinetics of t he reaction under the investigated conditions were found to be very favoura ble, with magnetite forming at a rate of 12.8 mg Fe/l/min. The total iron c oncentration in the affluent was always less than 1 mg/l representing an ir on removal efficiency of 99.9%. The precipitant settled well (SVI 8 ml/g) a nd showed substantial stability at pH 3 (dissolution of 1.1% after 120 h). An outline for a one-step ambient temperature ferrite process is presented.