Temperature-induced modification of the dewatering behaviour of Ferri-Oxyhydroxide precipitates formed from low tenor solutions

Abstract

Mining activities generate Acid Mine Drainage (AMD) and wastewater streams rich in dissolved inorganic species that negatively affect water quality. Conventional lime treatment of AMD leads to the precipitation of a voluminous sludge, comprising mainly of Ferri-Oxyhydroxide (Fe-O-OH) and gypsum, whose matrix renders it difficult to dewater. Such precipitates also show poor post-precipitation stability, and they are disposed of to landfills where metal remobilization can occur over time. This paper proposes that temperature assisted structural maturation of primary precipitates results in aged precipitates with improved dewatering behaviour (in terms of settleability, specific resistance to filtration (SRF) and filterability). A precipitate produced from the neutralization of aqueous acidic Fe2(SO4)3 (ferric concentration of 100, 300 and 800 mg/L) by sub-stoichiometric Ca(OH)2 solutions at room temperature was continuously matured in a holding mixed tank reactor kept at 30, 40 and 50 °C. The aged precipitates were analysed for particle size distribution, number of particles formed, mass moment and surface area moment mean particle size, crystal habit, morphology and zeta (ζ)-potential. The results demonstrate that(i)at [Fe(III)] = 100 mg/L, discernible spherical agglomerated Fe-O-OH particles are formed, whose surface area moment mean size and filtration rate attain a maximum at 40 °C, beyond which the particles experienced re-dissolution leading to size reduction. These particles exhibited poor settleability at all ageing temperatures, implying that continuous ageing had no noticeable effect on settling;(ii)at [Fe(III)] = 300 mg/L, a transition zone exists where there is maximum settleability of 2.7 ± 0.2 mm/min at 40 °C, a near neutral surface charge (ζ-potential = +1.1 mV) that favours agglomeration, the largest mean particle sizes (D4,3 = 26.27 μm, D3,2 = 17.0 μm) but a crystal habit that hindered filterability. This [Fe(III)] and ageing temperature (40 °C) showed potential for enhanced dewatering behaviour, should the ageing time be increased;(iii)at [Fe(III)] = 800 mg/L, formation of small densely packed spherulites whose degree of interlocking and interconnected layers decreased whilst the particle size increased with temperature leading to improved filterability.Based on these results, the operational implications for lime neutralization plants are that there is a temperature and [Fe(III)] trade-off at which higher throughput of sludge with improved settling characteristic is realized. However, additional ageing time is required in order for the structural maturation to effect changes in particle size, packing and morphology that yield increases in the filtration rates – as was shown by the [Fe(III)] of 300 mg/L at 40 °C.