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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 African acid mine drainage (AMD) waters is presented. The approach involves the controlled oxidation of ferrous-containing AMD water at ambient temperatures in the presence of magnetite seed. The resulting oxidation product is the ferrite (M1 3+ 2 M2 2+ O 4 ) magnetite (Fe 3 O 4 ), which has the capacity for nonferrous metal removal, and which forms a stable sludge that is easily separated from the effluent. Sludge characterisation stu dies (XRD, SEM and dissolution tests) show that oxidation of ferrous solutions under controlled 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 seed channels the end products of the AMD oxidation reaction towards magnetite. Under identical conditions, but in the absence of magnetite seed, a poorly characterised mixture of largely amorphous iron oxides are formed with magnetite comprising not more than 17% of the total iron. The kinetics of the reaction under the investigated condi tions were found to be very favourable, with magnetite forming at a rate of 12.8 mg Fe/ l/min. The total iron concentration in the effluent was always less than 1 mg/l representing an iron removal efficiency of 99.9%. The precipitant settled well (SVI 8 m l/g) and 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.