The disposal of tailings is a major concern for the mining industry.
By Jodie Pritchard, Trina Jensen and David Welsh
The disposal of tailings is a major concern for the mining industry. Processing ore produces a waste stream, or tailings, often discharged as a slurry to a final storage area known as a Tailings Storage Facility (TSF). In recent times, the growing emphasis on ensuring sustainable outcomes from mining has resulted in the need to develop mine closure plans which consider how the TSF will be decommissioned and closed after mining ceases.
The objectives of mine closure planning usually involves some combination of making the site safe and stable and reusable while ensuring the mine closure has no adverse environmental, social or economic impacts.
However the closure challenge around the TSF can be complex because: they occupy significant areas; the tailings must be covered; there is potential for seepage that will affect groundwater water quality downstream of the TSF; there is a potential for dust to be a nuisance and health risk; rehabilitation is prohibitively expensive especially if cash-flow is limited; and there is a limited evidence of successful cost-effective closure strategies.
The last point is particularly important since without effective closure strategies a “ticking time bomb” with the potential to pollute ground and surface water for decades can result.
Effective Tailings Storage Facilities
Most TSFs around the world are approved by regulatory authorities to be designed, operated, decommissioned and closed in compliance with the conditions of approvals and/or legislative requirements for tailings dams and mine closure. Closure objectives for a TSF usually require some combination of legal compliance and commitment toward an Environmental Impact Statement (EIS) submitted as part of the approvals process.
The World Bank Group considers tailings as one of the key areas for governments to regulate to protect vulnerable communities and the environment. Demonstrating leadership in this regard they require all mining projects they support to have a mine closure plan to cover not just closure but also restoration.
Other drivers for consideration include:
• Geotechnical conditions of approvals issued in most jurisdic-tions of the world usually require no evidence of instability on the upstream and downstream TSF embankment slopes and near embankment toe, nor surface subsidence, cracking and slumping of tailings materials near the TSF embankment.
• Conditions relating to decommissioning and closure usually seek to ensure that all plant buildings and equipment are removed and that mineralogical analysis are undertaken to characterize tailings and cover material and identify the presence and nature of potentially acid-producing sulphides, construction is safe and stable and any liquids remaining in tailings ponds are treated prior to discharge or evaporation.
• The EIS will commit operators to make the TSF safe, stable and non-polluting—however often the specific means of achieving these aims is not specified in any great detail.
Tailings Management and Closure
It is generally agreed the planning for the TSF and the mine plan must be aligned so the most cost-effective solution for closure can be developed. The design and operation of a TSF can be optimized to reduce costs and risks, and optimize operation.
Consider, for example:
Reducing costs and risks
• Geochemical characterization of the tailings;
• Selection of the optimal tailings disposal method;
• Containment of tailings and design, and construction of tailings containment wall;
• Seepage control;
• Tailings delivery;
• Water management;
• Dust control; and
• Planning for eventual decommissioning, rehabilitation and clo-sure of the facility.
Operation optimization through design
• Immobilizing the deposited tailings;
• Reducing a capillary action;
• Controlling infiltration;
• Enhancing runoff;
• Diverting uncontaminated surface water;
• Controlling erosion; and
• Ensuring a stable cover at closure.
Research and trials are essential for achieving the required closure objectives in a cost-effective and timely manner. Trials of each design option should be carried out including testing of closure engineering concepts and evaluating cover design for at least three years. In particular, the occurrence of extreme rainfall or higher than average wet years, can provide opportunities for rigorous testing of the design.
Current “best practice” approaches for dealing with groundwater contaminant plumes emanating from TSF fall within three broad categories:
Containment of contaminant without treatment
• Natural attenuation involving assessment of geochemistry;
• Hydraulic containment: whereby the migration of containment plumes are contained by strategically pumping groundwater from or into a series of wells; and
• Physical barriers: slurry walls or grout curtains to prevent or slow groundwater.
Contaminant removal by groundwater extraction
• Pumping: combining groundwater/contaminant removal from a series of extraction points – to create a depression in the water table to expand the area of influence; and
• Groundwater interception trench: involving a trench backfilled with permeable gravel with a central collection sump or series of extraction points.
In-situ groundwater treatment methods
• Permeable reactive barrier wall involving a permeable unit across the flow path of the contaminant plume;
• Ring injection: whereby a reactive material is injected into wells installed in a ring around the contaminant plume;
• In-situ bio-remediation to encourage growth and reproduction of indigenous micro-organisms; and
• Phyto-remediation employing plants to improve water quality.
Some innovative approaches to consider might include: electro-kinetic techniques for immobilizing metals—such processes may be used to desorb and then remove metals; new advances in geochemical technology may result in the possibility of promoting in-situ precipitation of metal sulphates by injection of an electron donor; and mine groundwater contaminants using the groundwater contaminant or products of groundwater remediation as a resource. This has been attempted with some success in making bricks from tailings and provides a mechanism for potentially recovering some costs.
Whichever options are implemented, ongoing monitoring, evaluation and management plans will be required to ensure effectiveness and persistence of remedial technology.
A Roadmap for Action
Selecting the best option for the management and closure of large tailings facilities is the key to successful sustainable outcomes.
Some practical considerations that may guide operators to a solution include:
• Consider the public commitments made at the EIS approvals stage, and in public sustainability reports in relation to returning the TSF to a productive or other acceptable post-mining land use and also to making the TSF safe, stable and non-polluting at mine closure. What are the costs and benefits of achieving these outcomes—environmental, social and economic? What are the risks and how might they be overcome? Is it sufficient to simply aim for compliance? Has the operator indicated (in sus-tainability reports for example) that a beyond-compliance approach is needed in order to achieve stakeholder satisfaction with the outcome, and thereby maintain the reputation of the operator and enhance the likelihood of future access to land?
• Consider the current legislation, regulation and guidelines appli-cable to your site, and also the specific conditions of approvals relating to the TSF. What are your legal requirements in relation to the closure of the TSF? Is it feasible to achieve the legal requirements? If not, why not? What are the risks of non-com-pliance—legally and to the reputation of the operator? What can be done to achieve and/or maintain compliance and to go beyond compliance? What are the costs and benefits? What are the alternative actions?
• Consider the operations of the mine, processing plant, tailings deposition and/or containment methods. What could be done now to change these in ways that may make the closure more cost-effective? What are the costs and benefits of alternative methods—environmental, social and economic? What are the risks and how might they be overcome?
• Consider the alternative methods presented herein for tailings management and closure (and also for groundwater remediation if pollution may be occurring). Which options could be applied at your site? What are the costs and benefits of the options— environmental, social and economic?
• Consider site trials. Is it possible to set aside an area of the TSF to conduct trials of the preferred options, with monitoring to pro-vide high confidence in the technical results? If no trials are possible, what is the plan to demonstrate that effective closure of the TSF can occur, with a high probability that a ‘sign-off’ by both the regulatory authorities and communities in the vicinity of the TSF will be achieved? What are the environmental, social and economic risks of not taking any action soon?
Jodie Pritchard is a hydrogeologist for SKM Consulting based in Adelaide, Australia; Trina Jensen is a senior environmental scientist for SKM Consulting based in Brisbane, Australia; and David Welsh, from SKM Minmetal, is a water and environment operations center manager based in Santiago, Chile. This article is based on a paper they presented at the international forum, enviromine2009, which was held in Santiago, Chile.
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