The Dewatering Behaviour of Transformed Ferri-Oxyhydroxide Precipitates Formed Under Moderate Temperature and Varying Fe(III) Concentrations

Mining operations generate Acid Mine Drainage (AMD) that poses a significant threat to the natural environment. AMD treatment using Ca(OH)₂ leads to the precipitation of a sludge dominated by a mixture of ferri-oxyhydroxides. This sludge has poor dewatering tendencies and is deposited in landfills with potential for metal remobilization. This study investigated the dewatering behaviour of a precipitate formed during elevated temperature treatment of a primary, pre-settled sludge from the neutralisation of aqueous acidic Fe₂(SO₄)₃ and Ca(OH)₂ solutions in an MSMPR reactor. The resulting treated precipitates were analysed for micro-properties, with results showing that an increase in the secondary reactor temperature from 25 °C up to 50 °C led to an increase in mean particle size, a decrease in the number of particles and improved dewatering behaviour at a ferric concentration of 300 mg/L. This was ascribed to the attainment of a circum-neutral surface charge that favoured agglomeration and a change in micro and nano-structure that allowed for better water passage.