The kinetics and mechanisms of the oxidation and precipitation of iron : the high density sludge (HDS) process
The oxidation and precipitation of iron are key reactions in the treatment of hydrometallurgical waste water and acid mine drainage (AMD) by the high density sludge process (HDS). The removal of other pollutants by adsorption and coprecipitation using this technique depends on the nature and the process by which iron is removed. Although this technique is cheap and easy to operate and therefore the most common method by which hydrometallurgical waste water is treated, there are some problems inherent in this technique. Such problems include; the formation of gelatinous sludge which is difficult to dewater, the low solids density resulting in considerable water losses and a large land area for sludge disposal, the instability of the sludge thus requiring post neutralisation stabilisation and the formation of the solids as mixed sludge, thus diminishing chances for waste material recovery, reuse or recycle. In view of the above problems, the objectives of the present study were; to gain more understanding of the oxidation and precipitation reactions of iron through investigating the effect of pH on the oxidation rate and on the chemical and physical characteristics of the iron precipitates, to make an inference on the mechanisms by which iron precipitates are formed and finally to investigate the effect of seeding on the oxidation and precipitation of iron. To achieve the set objectives, experiments were carried out in a 4 L perspex batch reactor. The reactor was fitted with four equally spaced buffles and a draft tube to maximise mixing. Ferrous iron (Fe2+) was added as FeSO4.7H2O at initial concentrations of 200 mg/L and oxidised using air. pH control was achieved by titration with either 0.1 M NaOH or 0.1 M H2SO4 through a custom made Hitech Micro Systems controller. The reactions were carried out at pH levels 6.0, 8.0, 9.0 and 10.0. Three types of materials (recycled solids, natural haematite and natural goethite) at three concentration levels (Cs = 1.0, 0.5 and 0.1) were used as seed material in the seeded experiments.
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