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.