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.

中文翻译：

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由低浓度溶液形成的氢氧化铁沉淀物脱水行为的温度诱导改性

摘要 采矿活动产生酸性矿山排水 (AMD) 和富含溶解性无机物质的废水流，对水质产生负面影响。AMD 的常规石灰处理导致大量污泥沉淀，主要由氢氧化铁 (Fe-O-OH) 和石膏组成，其基质使其难以脱水。此类沉淀物也显示出较差的沉淀后稳定性，它们会被丢弃到垃圾填埋场，随着时间的推移，金属再动员会发生。本文提出初级沉淀物的温度辅助结构成熟导致老化沉淀物具有改善的脱水行为（在沉降性、过滤比阻力 (SRF) 和过滤性方面）。中和酸性 Fe2(SO4)3（铁浓度为 100，300 和 800 毫克/升）通过亚化学计量的 Ca(OH)2 溶液在室温下在保持在 30、40 和 50 °C 的保温混合罐反应器中连续熟化。分析老化沉淀物的粒度分布、形成的粒子数、质量矩和表面积矩平均粒度、晶体习性、形态和zeta (ζ)-电位。结果表明 (i) 在 [Fe(III)] = 100 mg/L 时，形成了可辨别的球形团聚 Fe-O-OH 颗粒，其表面积矩平均尺寸和过滤速率在 40 °C 时达到最大值，超过颗粒经历重新溶解导致尺寸减小。这些颗粒在所有时效温度下都表现出较差的沉降性，这意味着连续时效对沉降没有明显影响；(ii) 在 [Fe(III)] = 300 毫克/升时，存在过渡区，在 40 °C 时最大沉降率为 2.7 ± 0.2 mm/min，近中性表面电荷（ζ-电位 = +1.1 mV）有利于团聚，最大平均粒径（D4,3 = 26.27 μm, D3,2 = 17.0 μm) 但晶体习性阻碍了过滤性。如果老化时间增加，该 [Fe(III)] 和老化温度 (40 °C) 显示出增强脱水行为的潜力；(iii) 在 [Fe(III)] = 800 mg/L 时，形成紧密堆积的小球晶，其互锁和互连层的程度降低，而粒径随温度增加而增加，从而导致过滤性提高。基于这些结果，石灰中和厂的运营意义在于，需要在温度和 [Fe(III)] 之间进行权衡，以实现更高的污泥吞吐量和改善的沉降特性。然而，需要额外的老化时间才能使结构成熟影响粒度、堆积和形态的变化，从而提高过滤速率——如 40°时 300 mg/L 的 [Fe(III)] 所示C。