Modifying the dewatering behaviour of iron solids from ferric sulphate solutions during lime treatment

Abstract

In the continuous lime neutralization treatment of iron-laden wastewaters, such as acid mine drainage (AMD), the solid-liquid separation of iron solids formed is difficult to effect. The particle formation mechanisms, the ensuing particle size and size distribution – which in turn shape the dewatering process – are strongly influenced by the OH/Fe molar feed ratios since they dictate the supersaturation level. This work describes tests using a stoichiometric and sub- stoichiometric OH/Fe molar ratio (R = 3 and 2.5 respectively) to establish settleability and filterability variations of a slurry precipitated at steady state in a mixed-suspension-mixed-product-removal (MSMPR) reactor at 15 min residence time. Three initial ferric concentrations ([Fe(III)]) of 100, 300 and 800 mg/L were used. The results revealed that particle-particle repulsion and product dewatering characteristics were dependent on the relationship between supersaturation, pH and ionic strength. The best product performance, as seen in fastest settling rate of 3.3 ± 0.2 mm/min, an increase in the filtration rate from 3.7 ± 0.4 to 5.1 ± 0.5 cm3/cm2 min and a decrease in the specific resistance to filtration (SRF) from 1.42 × 1012 to 6.71 × 1011 m/ kg, was observed for particles formed at R = 2.5 and a [Fe(III)] of 300 mg/L. The product performance was attributed to reduced supersaturation and minimal particle-particle repulsion due to the proximity of the steady state pH and point of zero charge. This favoured a neutral surface charge resulting in the dominance of large (D4,3 of 48.3 ± 0.05 μm and D3,2 of 30.8 ± 0.05 μm), regular shaped agglomerates which had a narrow, unimodal distribution and a notable decrease in the peak number of fine particles from 3.5 × 1013 to 1.7 × 1012 / m3. Sub- stoichiometric molar ratios produced insignificant filterability changes at [Fe(III)] of 100 and 800 mg/L but the latter showed significant settling rate changes (the largest change of threefold - from 0.8 to 2.4 ± 0.2 mm/min). These results suggest that AMD treatment plants that employ lime neutralization should be operated at lower OH concentrations than is the current practice, as this ensures improved iron removal as well as settling and filtration properties of the sludge. This may have the added benefit of decreased neutralization costs as less lime will be required.