Co-disposal of wood ashes and mine wastes to control acid and metalliferous drainage

Wood ashes can be an environmentally sustainable option of treating acid and metalliferous drainage (AMD). However, their effectiveness varies according to their type, dosage rate, quantity and composition of AMD, which leads to contradicting results. Furthermore, the different ash placement have not been compared. This study evaluated the potential environmental performance of wood bed (a coarse material from the bottom of boilers) and fly (a fine material collected at the top of boilers) ashes placed with six diverse legacy mine waste samples from Tasmania, Australia. This study also tested the effectiveness of different placement methods, i.e., covers, mixed (thoroughly blended), and layers (i.e., 3 underlayers) at the same dosage rate on the same mine waste sample. The wood ashes (6 g) were added to the mine wastes (14 g) in kinetic leach columns (50 mm diameter) and irrigated with 30 mL of deionised water over 100 days. The bed and fly ash increased leachate pH by 0.7 and 0.1 units, respectively, for two low acid-generating (pH 4.4 and 5.3) samples but maintained or lowered the pH by nearly 0.6 units for the remainder. The increase in pH was from acid buffering by calcite (0.14 % and 0.01 % in bed ash and fly ash, respectively) and reduced the oxidation of sulfides. Long (10-day) dry spells from day 30 onwards enhanced the increase in pH by further reducing the oxidation of sulfides. The wood ash covers reduced the concentrations of PTEs such as Al, As, Cr, Cu, Ni, Pb and Zn by up to 93 % through precipitation and sorption to Al and Fe oxyhydroxides and hydroxysulfates at pH > 3.5 and possibly pozzolanic reactions. The mixed and layered bed ashes increased pH by 0.4 – 0.6 units (i.e., 1.0 units more than the cover system) and immobilised metal(loid)s more than the cover system. The bed ash layers provided extra attenuation of metal(loid)s, possibly through filtration or adsorption. However, the water hazard risk remained high despite the wood ash amelioration; therefore, mixing the wood ashes with more alkaline materials is recommended for the legacy mine wastes.木灰可作为处理酸性金属矿物排水（acid and metalliferous drainage, AMD）的环境可持续方案。然而其处理效果因木灰类型、投加剂量、AMD的体量与组成而异，导致研究结果存在矛盾。此外，目前尚未对不同的灰料投加方式开展对比研究。本研究针对澳大利亚塔斯马尼亚州的6种不同遗留矿山废弃物样品，评估了锅炉底部粗颗粒物料木底灰（wood bed ash）与锅炉顶部细颗粒物料飞灰（fly ash）的潜在环境修复性能。本研究还在相同投加剂量下，测试了不同投加方式——即覆盖层、混合（充分搅拌混匀）、分层（即3层底衬）——对同一种矿山废弃物的处理效果。本实验将6g木灰添加至14g矿山废弃物中，置于直径50mm的动态淋溶柱（kinetic leach columns）内，并在100天内以30mL去离子水（deionised water）进行淋溶。针对2种低产酸（pH分别为4.4和5.3）的样品，底灰与飞灰可使渗滤液pH分别提升0.7与0.1个单位；但对其余样品，渗滤液pH基本维持不变或下降近0.6个单位。pH提升源于方解石（calcite）的酸缓冲作用（底灰与飞灰中方解石含量分别为0.14%与0.01%），同时可抑制硫化物的氧化。自第30天起出现的长达10天的干旱期，通过进一步抑制硫化物氧化，进一步增强了pH的提升效果。木灰覆盖层可通过沉淀作用，以及在pH>3.5时与铝、铁羟基氧化物（oxyhydroxides）和羟基硫酸盐（hydroxysulfates）发生吸附反应，甚至可能通过火山灰反应（pozzolanic reactions），使铝（Al）、砷（As）、铬（Cr）、铜（Cu）、镍（Ni）、铅（Pb）及锌（Zn）等潜在有毒元素（Potentially Toxic Elements, PTEs）的浓度最高降低93%。混合与分层投加的底灰可使pH提升0.4~0.6个单位（较覆盖层系统高出1.0个单位），且对金属与类金属的固定效果优于覆盖层系统。分层投加的底灰可通过过滤或吸附作用，进一步实现金属与类金属的衰减。尽管采用木灰进行了改良，矿山废弃物的水环境危害风险仍处于较高水平；因此建议将木灰与更多碱性物质混合，用于遗留矿山废弃物的修复处理。