Over the last decade, the mining industry has seen a paradigm shift in the way closure planning occurs. It is well understood that mine closure needs to be more than the final stage in a mine’s lifecycle. Leading miners and industry organisations are recognising mine closure’s important role within the wider community, and the obligation to realise a post closure vision that adequately considers health, safety, social, environmental, legal and reputational aspects.
The road to successfully achieving mine closure remains difficult to navigate. There appears to be a disconnect between mine planning and closure planning, where closure planning is largely compliance and approvals focused at the start of the asset lifecycle. As assets mature, closure plans are only reviewed periodically. In some instance progressive rehabilitation occurs but is never fully integrated into the strategic mine plan as the focus remains on compliance and approvals. As an operation approaches closure, the focus shifts from compliance and approvals to real closure costs and execution methodology. Unfortunately, the common reality is at this point, closure costs, schedules, and methodologies have not been appropriately integrated into the strategic mine plan. As a result, operators are left with closure scenarios that are too expensive, do not meet closure objectives, and will not achieve closure expectations for returning land use.
To understand the disconnect between closure planning and mine planning we will walk through a simple hypothetical life of mine (LOM) example for a project with a 25-year mine life.
At the early stages of the project, the operation has strong initial financing (Yr 0), and a positive year over year cashflow (Yr 0 and Yr10). A preliminary, and manageable estimated closure cost results in a projected positive Net Present Value (NPV) (Figure 1).
At Yr 10 the project may undergo a re-estimate. Traditionally these result in a higher NPV compared to Yr 0 due exclusion of the project sunk costs, and minimal effort in re-estimating closure costs (Figure 1).
At Yr 20 the project undergoes a second project re-estimate. This time, the NPV decreases considerably, since the extraction potential has decreased, and the closure costs approach realisation. During this time, the focus shifts to closure plan optimisation where the true closure costs are calculated, and operators are faced with higher than expected closure costs.
We dub this this point in time the Pivot Point. At this point higher closure costs are due to outdated, unrealistic, and unproven closure plans that no longer meet closure objective resulting in a substantial loss in project NPV (e.g. Yr 20 NPV with Realised Closure Costs; Figure 1).
Figure 1: A Typical Life of Mine
To prevent this scenario from continuing, an assessment of closure plans, their value drivers, and their integration into the mine plan must occur prior to the Pivot Point. Okane creates integrated planning models that combine the mine plan with the closure plan to create a full Life of Asset (LOA) Plan. Integrated closure planning provides the tools to integrate closure into the strategic mine plan, optimising the whole life of asset.
When closure is integrated into the LOM plan, mechanisms that reduce closure liability and cost can be identified through leveraging the mine production schedule. Additionally, through NPV scenario analysis, Okane can identify the best plan for an operation; one that balances closure costs, operation costs, regulatory compliance, and residual risk.
The benefits of integrated closure planning can make material differences over the LOA, within 5-year plans, and 2-year budgets by limiting value destruction and facilitating easier communication between internal and external stakeholders:
This is Okane’s approach to integrated closure planning. The approach locks into an operation’s mine planning system to frame risk and opportunity scenarios against closure compliance, residual risk, and cost. Our interactive value driver tree analysis provides insights into the major cost drivers and decisions that will allow your project to maximise shareholder return by selecting the most optimised closure plans. Okane’s integrated LOA Plans deliver value throughout the entire asset and mine lifecycle.
As a society we often equate the residuals of processes as “waste” whether it be tailings from mining and milling processes, leftover food scraps from our kitchens, or the end-results of our own bodily functions. It is also not uncommon to view these residuals as liabilities, or something that simply has to be managed in a way that minimizes impact to the bottom line of a balance sheet, to stakeholders or to the environment. As our world increasingly focuses on sustainability and developing a Circular Economy, the ability to reframe our thinking of materials from “wastes and liabilities” to “resources and opportunities” has the potential to transform aspects of mine closure and reclamation, providing environmental and economic benefits to a wide array of stakeholders.
Municipal biosolids are the residuals from municipal wastewater and sludge treatment. Solids are settled out and removed from wastewater processes, and subsequently treated to reduce pathogen and bacteria counts such that the materials meet jurisdictional regulatory requirements. Municipal biosolids are an organic material containing many vital nutrients for plant growth including nitrogen and phosphorous.
Cities worldwide grapple with the challenges of accommodating an expanding population: as the number of humans on Earth approaches 8 billion people, the challenges associated with managing natural resources and residuals continues to evolve as well.
Approaches to managing biosolids have historically included landfilling, incineration and sea-dumping incurring costs to the producing municipalities, and creating environmental risks. The use of composting and agricultural application to manage biosolids has allowed for these residuals to be beneficially re-used, providing valuable nutrients to crops and helping to alleviate burdens associated with disposing of a product formerly considered to be ‘waste’.
Revegetation of mine affected lands can require large volumes of organic soils and amendments; one traditional approach has been to source these materials from borrow areas (although borrowing erroneously implies that these materials will eventually be returned). With the contribution of millions of city-dwellers to biosolids production, there exists the potential to develop a consistent and high-quality supply of organic soils and amendments. Mine reclamation and revegetation could provide a year-round disposal option.
Application of municipal biosolids for mine reclamation is not a new concept, several studies have evaluated the use of biosolids for revegetating mine impacted areas and have examined the impact of biosolids on metal leaching and acid rock drainage. Despite encouraging findings, the use of biosolids in mine reclamation has not been widely adopted.
Our team of soil scientists, plant ecologist, geologists, engineers, agrologists, and geochemists are constantly working with clients to develop effective solutions to unique challenges. Our experience examining the physical and chemical characteristics of tailings and mine waste at the laboratory scale using tools like our advanced customizable leach columns, allows Okane to characterize site-specific materials and their suitability for reclamation. Our expertise in designing and overseeing the construction of cover systems allows Okane to efficiently scale-up from the lab to the field.
As practitioners who seek to develop mine closure solutions which provide our clients and stakeholders with achievable, effective, and holistic closure planning services, we endeavour to continue advancing the research and implementation of new technologies and solutions.
Pit lakes are often perceived as liabilities when planning for mine closure. However, when closure planning is integrated into mine operations, opportunities can be realized to leverage pit lakes as waste containment and treatment. Consider the following potential advantages of using your pit lake for waste treatment.
When mine wastes are confined within the pit, they do not rely on elevated impoundment structure. Pit lakes can also provide general water treatment includes flow equalization and sediment trapping. To achieve this goal, the water layer needs to be sufficiently deep to prevent wave erosion and resuspension of fine particles. Pit lakes are also a location to discharge water treatment effluent and by-products such as lime sludge. This is an economically beneficial practice because no extra storage facility needs to be constructed (Morgenstern et al. 2015, Verburg et al. 2009).
The water cover above waste rock can effectively restrict exposure of mine waste to atmospheric oxygen and thus limit the formation of acid and metalliferous drainage (AMD). A water cover is considered one of the best practices for permanent storage of saturating potentially acid generating (PAG) materials.
A stratified pit lake can provide some opportunities for mining companies to treat the water: Algae growing in the surface layer of pit lake can provide a surface adsorption site for metals. When algae die in the winter, those absorbed metals along with algae bodies will sink to the bottom layer of the lake and store there permanently.
Saturated Rock Fills (SRFs) occur when pits or voids at mine operations are backfilled with mine rock and the backfilled rock is allowed to saturate or flood with water. In saturated environments, it is not uncommon for low oxygen conditions to develop. These low oxygens, or suboxic, conditions can create conditions suitable for the geochemical reduction of some oxidized species within mine impacted water. The geochemically reduced forms are typically much less mobile and/or pose little to no water quality risk. For example, nitrate can be reduced to form inert nitrogen gas, or selenate (oxidized form of selenium) can be reduced to form selenite, which easily adsorbs, or ‘sticks’, to minerals and is immobilized from transport to receiving environments. The high hydraulic conductivity of backfilled mine rock, coupled with sufficiently low oxygen, or geochemically reducing, conditions can result in powerful opportunity for low cost and effective water quality management. Mine impacted water can be diverted to SRFs and given sufficient residence time and geochemical conditions within the SRF resulting in effective treatment of water quality issues.
Some solid mining wastes are not acid generating but contain degradable organic contaminants (e.g. oil sands tailings). If oxygen is available, the organic contaminants existing in solid tailings can be slowly degraded below the water cap. In this situation, the water column of a pit lake needs to be fully mixed to create an aerobic condition in the lake bottom. If pit lakes become stratified for a while, the bottom portion of the lakes and sediments are likely to become anaerobic and produce hydrogen sulphide, carbon dioxide and methane (Schultze and Boehre, 2009). Some tailings ponds from oil sands operations produce significant quantities of methane. In some oil sands operations, oil sands tailings have also been treated with gypsum (CaSO4·2H2O) or aluminum sulphate (e.g., alum: Al2(SO4)3) to accelerate the consolidation. If these sulphate-rich tails are stored under a stratified water column, sulphate reduction may occur in the presence of organic carbon source, which is rich in oil sands tailings. When water column turnover occurs under an extreme weather event, the sudden release of these gas may damage the biologic system that had evolved in the pit lake.
The hydrogeology of the pit is often the most critical factor for determining the sustainability of a pit lake for waste disposal. The main purpose of hydrogeologic control is to minimize the outflow seepage which may transport the contaminants to the downstream aquafer. The ideal pit for mine waste disposal has a minimal groundwater gradient across the pit so that contaminant transportation is minimized.
Okane ensures the geologic and hydrogeologic condition of the project site are thoroughly investigated. Some of the factors we evaluate include the permeability of rock around the pit, gradient of the groundwater table, pit lake water balance, hydrology of the downgradient receiving water body, presence of any faults or fractures which increase the hydraulic transportation.
Okane supports mine sites in the realization of opportunities associate with pit lake waste management. By integrating active closure planning into the strategic mine planning phases, your pit could go from being a liability to being your best asset at closure.
Morgenstern NR, Vick SG, Van Zyl D. 2015. Independent expert engineering investigation and review panel report on Mount Polley tailings storage facility breach. Available at: https://www.mountpolleyreviewpanel.ca/
Schultze, B and M. Boehrer, (2009), Induced meromixis, in Castendyk, D.N., and Eary, L.E.,eds., Mine Pit Lakes: Characteristics, Predictive Modelling, and Sustainability: Society for Mining, Metallurgy, and Exploration, Inc., Littleton, Colorado, p. 239-246.