The waste generated from mining operations is one of the largest waste streams in the world, and there can be catastrophic consequences if it’s not handled properly. Knowing this, a team at Flinders University is sifting through tonnes of mining waste to address this problem head-on.
Acid and metalliferous drainage, also known as acid mine drainage or AMD, is the outflow of acidic and/or metalliferous water into the environment. This occurs when rock containing sulphide minerals (predominantly pyrite) is exposed to water and oxygen, causing them to break down and oxidise, triggering a chemical reaction which produces sulphuric acid. This usually occurs when earth is disturbed and rock containing pyrite is brought to the surface.
This is especially problematic at mine sites, when the smaller grain size of crushed waste rock and tailings and high pyrite content produce more acid than the natural environment can neutralise.
This outflow can result in the release of acid into surrounding water systems which can, in turn, dissolve rock and leach metals and acid into the environment. This damage is not only long-term, but can be costly to remedy. That is why it’s essential to assess the potential of AMD across a mine’s lifespan during the planning stage of a project.
A major $10 million dollar, five-year initiative called ‘Improved Prediction, Remediation and Closure of Acid and Neutral Metalliferous Drainage (AMD/NMD) Sites by Examination of Mine Waste Behaviour at the Meso-scale’ is taking place at Flinders University, with Professor Sarah Harmer at the helm.
The project is funded through Cooperative Research Centre Transformations in Mining Economies and involves Adjunct Professor Andrea Gerson from Blue Minerals Consultancy, the University of Queensland, the University of Windsor, eight mining and METS companies, Minerals Research Institute of Western Australia (MRIWA), the Department of Agriculture, Water and the Environment (DAWE) and International Network for Acid Prevention (INAP).
The project is centred on scaling-up innovative methods for acid mine drainage assessment and remediation and water and ecosystem decontamination. Professor Harmer’s team is also looking at other methods to improve both mine site closures and mining’s impact on the landscape.
Acknowledging AMD at an early stage
Professor Harmer stressed that a critical element of AMD management is that it should take place proactively, not retrospectively – meaning that management should start during greenfield mine development based on geochemical testing of waste rock and tailings.
“Although at this stage there is no mine or mill, waste rock geochemistry can be determined from exploration core samples obtained to forecast the deposit value elements. This can then enable block modelling of potentially acid forming and non-acid forming strata and lithography.”
A geochemical understanding of the tailings can be undertaken through test work on pilot plant process residues.
“Understanding how these mine wastes will react to prolonged weathering, in terms of both acid and metalliferous drainage, enables planning for appropriate segregation and deposition during operations. This has high potential to reduce water treatment costs prior to site discharge during operations, enable effective emergency closure if required and greatly reduce the cost on final closure.”
Impeding mine closure
The consequences of improper AMD management can be far-reaching, extending beyond just environmental to also impact companies financially, with mine waste materials often getting in the way of mine closures.
“AMD can significantly restrict a company’s capacity to relinquish a site, whether to a private party or government agency, after mine closure due to residual liabilities and risk,” Professor Harmer said.
Alternatives for companies are either to go through an approved closure process while maintaining ownership and liability, or to place the mine in care and maintenance with the option for future re-opening. Adjunct Professor Gerson said that these options, however, come with varying degrees of environmental and legal responsibility, and all require appropriate judicial approvals.
“They also all come with some degree of cost with relinquishment frequently requiring the greatest capital expenditure particularly where insufficient planning has taken place.”
Adjunct Professor Gerson explained that, as an example, a roadblock to successful closure and relinquishment of a mine site can be the lack of planning of final landforms, such that potentially AMD materials are not sufficiently encapsulated within benign wastes.(1)
Confronting industry challenges
“There are numerous challenges faced by industry in its ability to manage AMD, which stem from the complexity of understanding AMD processes, coupled with evolving regulatory requirements.”
Professor Harmer attributes a lack of consistent site AMD management/investment, constant tension between short-term profit and long-term remediation costs with less clear financial benefits, everchanging mine personnel and a lack of effective record keeping as having hampered effective AMD management.
Another challenge lies in correctly forecasting acid and metalliferous drainage. Accurate predictions of AMD enable effective mitigation strategies that result in minimal downstream impact on flora, fauna and human utilisation. On the other hand, the incorrect prediction of AMD can mean the associated company has to undertake water treatment prior to discharge, resulting in a liability of potentially hundreds of millions of dollars.
“Accurate prediction requires not just the bulk assessment of detrimental metals and/or acidification and neutralisation capacities, but an understanding of their rates of release. Static testing may indicate low acid drainage risk, but acid drainage may still occur due to greater rate of release of acidity than neutralisation. Relinquishment is only viable if the acid generation rate is matched by the neutralisation rate in the long-term.”
Professor Harmer said that there has been a long record of incorrect AMD forecasting in the industry to date.
“Much of this is not formally documented; however a survey undertaken to determine the accuracy of prediction of water quality at hard rock mines indicated a low level of predictive capability.(2) Where the mines were close to water resources and had significant potential for acid and/or metalliferous drainage about 75 per cent underestimated the impacts of operational water quality in the environmental impact statement predictions.”
Taking on AMD
According to Professor Harmer, the Flinders University AMD meso-scale project has several themes that merge to a common purpose for improving the assessment and remediation of mine waste for mine closure.
“The project aims to provide links between prediction, scale-up and residual risk. For prediction of waste management, there are potential improvements, utilising both detailed mineralogy and microbiological assessment of waste for acid generation capacity and remediation of mine waste.
“The project aims to bridge the gap between lab-scale standardised tests and mine site implementation through emphasis on meso-scale testing of mine waste at more than one tonne.
“A more complete focus on control at source can influence early changes to process flowsheet and mine planning with the potential to reduce residual risk through improved mine and process design. The link to prediction, scale up and residual risk is key to the project success.”
The value of this project lies in:
- Improved forecasting of AMD risk
- Identification of resources for AMD prevention, reduction, and control at source enabling a progressive shift away from downstream water treatment approaches
- Environmentally sustainable, reduced rates of AMD release through implementation of source control strategies, both microbial and geochemical
- Testing of prediction and control strategies through scale-up from laboratory to mesoscale and for potential demonstration in sponsor mine site trials
- Improved block modelling of non-acid forming (NAF) and potential acid forming (PAF) rock zones for green and brown field sites
- Better waste rock dump planning and construction, and improved flowsheet and process planning for reduced residual risk and AMD prevention planning in operation and closure
Project in progress
Professor Harmer said that project partners have identified multiple sites of interest in Australia, the US, Canada and Mongolia – covering a range of climatic conditions from the desert to cool and wet to sub-arctic.
“These sites cover the evolutionary range for closure planning from green field, working mine to legacy. To date, 12 sites have been chosen in collaboration with industry partners and detailed scopes of work for each site have been established and approved.
“Project sampling techniques for mineralogical and microbial analysis have been developed and are being added to a new AMD/NMD Test Handbook. Mineralogical and microbiological characterisation of principal nominated wastes is well underway and has provided insight into the complexity of microbial life on mine sites.
“The design of both laboratory based and mesoscale tests has been completed. Kinetic column testing of principal nominated waste with agreed weathering parameters has begun. A new mesoscale facility has been completed at Flinders University where mesoscale kinetic leach testing has started. The initial studies are defining potential remediation strategies for each site.”
Results informing industry
Professor Harmer expects that the results generated from the study will be high-quality and will help produce transformative change for the utilisation of post-mining land.
“The results will impact policy and community practice across Australia and internationally through industry and government collaboration, while widespread dissemination of state-of-theart practice will inform ecosystem restoration and mine-site closure standards.”
Professor Harmer said that dissemination of project outcomes will be provided through the new AMD/NMD Handbook, with collaboration with government bodies ensuring that the results will be disseminated broadly.
“Further utilisation of the research findings will be carried out through engineered large-scale trials on mine sites of the companies involved in the project. This will ensure that the project’s outcomes reach the target audience and are commercialised for productive usage and greater impact.”
Footnotes:
- Tiemann C. D., McDonald M. C., Middle G. and Dixon K. W., (2019). Mine relinquishment policy in Australia, in: Fourie, A.B., Tibbett, M. (Eds.), Mine Closure. Australian Centre for Geomechanics, Perth, Perth, Australia, pp. 1451-1460
- Maest A., (2006). Predicted versus actual water quality at hardrock mine sites: Effect of inherent geochemical an hydrological characteristics, in: Barnhisel, R.I. (Ed.), 7th International Conference on Acid Rock Drainage (ICARD). American Society of Mining and Reclamation (ASMR), 3134 Montavesta Road, Lexington, KY 40502 St. Louis MO, pp. 1122-1141
