1. Introduction
Acid and metalliferous drainage (AMD) from mine wastes is a global environmental issue and remains a costly economic and social challenge for the international mining sector. AMD is primarily caused by the oxidation of sulphide minerals, particularly pyrite (Equations (1) and (2)), and can also contain toxic metal/metalloid ions (e.g., As, Cd, Zn, Pb) [
1,
2].
In principle, a sustainable approach to the management of AMD in pre-planning or during operation should be to exploit, where possible, readily-available, on-site, geochemical and microbiological resources. The approach proposed is to sufficiently reduce the acid generation rate (AGR) so that the neutralisation from on-site non-acid-forming (NAF) materials can be used to match the AGR from waste rock and/or tailings oxidation before final rehabilitation. This matching of AGR to ANR (acid neutralising rate) in kinetically-controlled processes is, in principle, the only viable and sustainable option to achieve long-term mine closure and site relinquishment [
3].
Our previous studies have shown that pyrite oxidation rates can be reduced by 50%–95% by the formation and maintenance of continuous amorphous silicate-stabilised iron (oxy)hydroxide pyrite surface layers [
4,
5]. The formation of these layers is achieved by maintaining circum-neutral pH conditions in the pore water surrounding the pyrite particles in the presence of dissolved silicates, leading to surface layer formation as pyrite oxidation proceeds. These passivating layers can be preserved in a continuous, coherent, and stable form at pH ≥ 6. In some cases, the establishment of these stable surface layers may require initial short-term treatment with greater concentrations of alkalinity than those obtained from limestone covers. This is especially the case for semi-arid regions, such as Western Australia’s Pilbara region, where alkaline covers are unlikely to succeed as a stand-alone remedy due to the relatively low moisture content and insufficient soluble alkalinity in waste materials. After passivation layers are established, the AGR can be reduced significantly, enabling carbonates (e.g., dolomite, limestone) and some reactive silicates commonly found in waste rocks (e.g., anorthite, feldspar, hornblende) to provide matching ANR [
6].
The development of surface passivation, and thus matching of AGR by ANR, forms the fundamental hypothesis of our experimental approach. In this study, we designed two sets of kinetic leach column (KLC) tests using both synthetic and natural mine wastes. This type of KLC setup (
Figure 1) or other similar leach columns have been widely used in laboratory AMD studies [
3,
7,
8,
9,
10]. The synthetic waste has a relatively low peak acidity (≈2000 mg·CaCO
3·L
−1), while the natural, potentially acid forming (PAF) waste has a much greater peak acidity (≈20,000 mg·CaCO
3·L
−1) as determined from previous KLC tests. For the KLC tests conducted with the synthetic mine waste, our aims were: (1) to verify that initial flushing with lime-saturated water results in circum-neutral pH, which is favourable for the formation of surface passivating layers on pyrite; and (2) to determine the requirement for ongoing alkalinity input (calcite-saturated water) for maintaining circum-neutral pH conditions in the longer term.
For the natural PAF waste, which had a significantly greater peak acidity than the equivalent alkalinities available from calcite- or even lime-saturated water, we chose to add lime via blending (≈2 wt %). While lime addition would ideally be much smaller (e.g., 1 t·lime·kt−1 waste, or 0.1 wt %), addition at such small dosages here would provide no more alkalinity than would the addition of lime-saturated water. Therefore, we chose to blend lime at a significantly greater, but likely uneconomic, concentration. The specific aim of our KLC tests using the natural mine wastes was to determine whether blending of lime with and without application of natural NAF top covers could maintain circum-neutral pH conditions as required for the formation and maintenance of pyrite surface passivation. The formation of these layers would enable the AGR of natural highly-reactive PAF waste materials, such as this natural iron ore PAF waste from the Pilbara region, to be sufficiently attenuated so as to be controlled by alkalinity available from site-derived NAF wastes.
4. Conclusions and Implications
We have investigated the application of added alkalinity in the form of saturated lime and calcite water, and lime and limestone (calcite and dolomite) to reduce the acid generation rate, through pyrite surface passivation, of both synthetic and natural mine wastes using laboratory-scale KLC tests. The results suggest that under these conditions, pore water pH needs to be kept near neutral with a slight excess of alkalinity for successful pyrite passivation.
For the synthetic mine waste (peak acidity ≈2000 mg·CaCO3·L−1), the initial application of lime-saturated water alone (watering/flushing with Milli-Q water, thereafter) resulted in the maintenance of circum-neutral pH (favourable for the formation and maintenance of pyrite surface passivation) up to 60 weeks. Circum-neutral pH was achieved and maintained up to 100 weeks (to date) through initial application of lime-saturated water and ongoing treatment with calcite-saturated water (low alkalinity input). These results suggest that for a moderately reactive PAF waste such as our synthetic waste, treatment with low levels of lime at the time of waste rock emplacement (i.e., equivalent to lime-saturated water treatment), combined with capping of the waste using limestone-containing materials (i.e., equivalent to calcite-saturated water treatment), may provide the conditions to both establish and maintain surface passivating layers on pyrite. This would greatly diminish the rate of acid generation and metal dissolution from within such wastes.
For the highly reactive natural PAF waste (peak acidity ≈20,000 mg·CaCO
3·L
−1), treatment with lime blending (2 wt %) into the PAF waste and addition of limestone-containing wastes as top covers were inadequate for maintenance of pyrite passivation, but did successfully delay the onset and extent of acid generation (and toxic/heavy metals release;
Table S2, Supplementary Materials). Based on these results, the amount of lime required to maintain the pH at or above 7 in these types of wastes until pyrite passivation can be achieved is likely to be cost prohibitive. Nevertheless, the addition of lime combined with a cover containing up to 80% dolomite, substantially reduced KLC leachate acidity by approximately 300-fold relative to lime addition alone. Results suggest that the observed reduction in acidity generation is due to elimination of ferric iron, resulting in much slower pyrite oxidation and commensurate reduction in acid generation rate.
These outcomes, if maintained in the longer term, may mean that treatment of reactive PAF wastes in this way, combined with other control measures (e.g., construction of waste rock emplacements using layered and compacted methods), could substantially reduce or eliminate acid and metal loads in mine drainage. This highlights to industry the potential for the beneficial use of on-site neutralising waste materials and/or lithologies for cost-effective AMD mitigation, and reiterates the importance of comprehensive mineralogical surveys of these materials during dump planning and construction to assess their potential to offset requirements for sourcing off-site materials at high cost. Even incremental reductions in leachate pH and metal loads will translate to significant cost savings for AMD management at impacted mine sites worldwide.
While this study focused on the effect of extrinsic parameters/applications (e.g., lime blending, initial flushing with lime solution, NAF cover addition) on AMD generation from synthetic and natural mine wastes, the presence of microbes in these wastes may also play a role in AMD dynamics. For example, sulphur/sulphide-oxidising (e.g.,
Thiomicrospira and
Sulfolobus [
28,
29]) and iron(II)-oxidising bacteria (e.g.,
Acidithiobacillus ferrooxidans [
2,
30]) are known to increase the oxidation of sulphide minerals, thus accelerating the AMD generation, while iron- and sulphate-reducing bacteria [
2,
30,
31] can potentially slow the AMD process. Investigation of the microbial communities in synthetic and natural waste KLC tests, treated with a range of organic amendments, is underway to more fully define the interplay of geochemistry and microbial actions and how this may be manipulated for more effective AMD control.