The effect of Fe(III) concentration on the dewatering behaviour of Fe(III) oxyhydroxide precipitates from low-tenor solutions

Abstract

The settling and dewatering of sludge are critical steps in the lime precipitation treatment of iron rich wastewaters, such as acid mine drainage (AMD). This fundamental study describes the effect of different ferric feed concentrations ([Fe(III)]: 50 to 800 mg/L) on the dewatering behaviour of the sludge precipitated under steady state conditions and high pH (9 to 11), as measured through filterability and settleability. Experiments were conducted in an agitated mixed-suspension-mixed-product-removal (MSMPR) reactor in which Fe2(SO4)3 and Ca(OH)2 were added as the Fe(III) and OH– sources, respectively, and the product was collected for analysis from the overflow. Reaction conditions were such that no gypsum was precipitated. The results indicate that the Fe(III) in the feed influenced the dewatering behaviour of the precipitation product, via particle size, particle size distribution (PSD), surface charge, morphology and structure. Particles formed at 300 mg/L exhibited increased size with a maximum D4,3 of 27 μm, a narrow PSD characterized by a span of 1.65, a specific resistance to filtration (SRF) of 1.42 × 1012 m/kg and a zeta potential of approximately 0 mV. These conditions resulted in enhanced agglomeration and better settling behaviour, although the filtering behaviour was not improved. This dewatering behaviour was attributed to a dense network of interlocked and interconnected bridges in the bulk morphology of the particles as evidenced by image analysis results. The steady state pH and Fe(III) conversion in the reactor were also influenced by the feed [Fe(III)] changes. The results show scope for potentially better solid-liquid separation in iron precipitation processes through controlling feed [Fe(III)].