Controlling acid and metalliferous drainage generated by legacy mine wastes in Tasmania using industrial wastes

The adverse environmental impacts of acid and metalliferous drainage (AMD) generated from the oxidation of sulfidic mine wastes are both a global concern and in Tasmania, where 215 historic metal mine sites are producing AMD. Conventional AMD preventive measures targeted at excluding water and oxygen ingress into the mine waste piles typically prove ineffective long-term, with AMD generation inevitable. Widely used chemical treatment methods have high operational costs and generate voluminous secondary wastes that require further processing and disposal. Considering this, there is a global need for an economically and technically viable AMD remediation strategy. One potential solution is the application of alkaline industrial wastes that neutralise acid and immobilise deleterious metal(loid)s. This study used laboratory experiments to investigate the potential long-term use of eight alkaline industrial wastes from Tasmania and Victoria to treat mine wastes from six abandoned sites in Tasmania, comparing the effectiveness of the industrial wastes placed as covers, blended or as layers underlying the mine wastes.
The physical properties, chemical composition, mineralogy, acid-neutralising capacity (ANC), acid-generating potential and leaching behaviour of the individual alkaline industrial wastes and mine wastes in this study were characterised comprehensively. The fine particle size (d₅₀ < 120 µm), high water retention and low hydraulic conductivity (10-⁷ – 10-⁹ m/min) of red mud, green liquor dregs (GLD), coal and wood fly ash make them suitable as sealing covers for limiting water and oxygen infiltration into the mine wastes, unlike the coarse and porous wood bed ash, oyster, mussel, and scallop shells. The ANC of the industrial wastes ranged from 35 to 1,000 kg H₂SO₄/t, proportional to their carbonate and hydroxide fractions, with the shells having the highest ANC because their compositions were almost entirely calcite and aragonite. The industrial wastes leached metal(loid)s in concentrations that were within the ANZECC and ARMCANZ (2000) freshwater guidelines, except for Al, Cr, Se, and V, which are mobile under alkaline pH. The characterisation results suggested that the industrial wastes have the potential to neutralise AMD and pose low metal(loid) environmental risks themselves.
X-ray diffraction, mineral liberation analysis, and reflected light microscopy revealed pyrite, chalcopyrite, and galena as the major sulfides found in the mine wastes together with acid-neutralising minerals including aluminosilicates, chlorite, and biotite. The presence of (oxy)hydroxides and (oxyhydroxy)sulfates of Fe and Pb indicated that partial oxidation of sulfides had occurred at the sites investigated. Negative alkalinity, net acid generation tests (pH of 1.9 to 3.9), and maximum potential acidity of 14 to 1,200 kg H₂SO₄/t qualified five of the mine waste samples as potentially acid-forming. The chemistry of sulfide minerals and their oxidation products mapped by LA-ICPMS showed they were enriched in trace elements. Leach tests indicated some of the mine wastes could liberate acidity, As, Cd, Cr, Cu, Pb, and Zn in concentrations harmful to freshwater systems.
The AMD remediation of all the industrial wastes was first screened using a bench-scale accelerated kinetic leach test developed in this study. The test was conducted on 20 g of milled (< 75 µm) mine and industrial waste combinations over 100 days. The long-term performances of selected industrial wastes were evaluated using two sets of scaled-up kinetic leach tests conducted on 2 kg and 15 kg of crushed materials (-4 mm and -22.4 mm respectively) over 100 weeks. A 7:3 weight ratio of mine to industrial waste was used in all the kinetic tests and the industrial wastes were placed as covers, blended, and as layers with the mine wastes. The GLD showed the greatest capacity for neutralising AMD whilst the wood ash failed to neutralise AMD. The red mud, coal ash and shells all generated lower alkalinity than GLD and maintained near-neutral pH in the columns. All the industrial wastes decreased the drainage volume and pozzolanic reactions in coal ash, fly ash and GLD increased water retention. The application of industrial wastes inhibited the leaching of some of the metal(loids) by as much as ≈ 100 %, and the lowest concentration of metal(loid)s was achieved at pH 6 – 8 via precipitation and sorption mechanisms. Excessively alkaline pH (> 8) increased the mobility of oxyanions such as As, Cr, Mo, Sb, and Se via desorption. The mixed placement and layered placements of industrial wastes generated more alkaline drainages and higher attenuation of metal(loid)s compared to the cover.
The outcomes of this study demonstrated that the in-situ application of alkaline industrial wastes presents a potentially cost-effective and environmentally sustainable long-term option to remediate AMD and contribute towards a zero-waste circular economy for both mine and processing industries. The results from this research can be used to inform remediation decisions at historic and active mine sites and guide future research to enhance efficiency and practicality in environmental management.