Tailings management strategies to meet today's demands.
Nearly 20 years ago, the United Nations Environment Programme (UNEP) commissioned a study into failures and other similar incidents involving tailings dams--now more commonly referred to as tailings storage facilities (TSFs)--between 1980 and 1996. The study reported that tailings dams and other impoundments are essentially a 20th century invention, with common practice before that having been uncontrolled tailings disposal into nearby watercourses or lakes. "In contrast, the modern tailings impoundment is usually of highly sophisticated construction based on firm geotechnical foundations," the report said.
While highly newsworthy at the time, major incidents involving TSFs are infrequent. The UNEP study identified fewer than 10 that had occurred during the 15year review period, a figure that has been borne out by subsequent events. In a paper presented at the 2014 Tailings and Mine Waste conference, organized by Colorado State University and held at Keystone, Colorado, USA, that October, Richard Tocher and others noted that "the incidence of tailings dam failure peaked from the 1960s through the 1980s at a rate of about 50 failures per decade. Since the 1980s, the rate has been about 20 failures per decade.
"The reduction in failures worldwide can be attributed to the improved design, construction and operation of modern facilities," they explained.
So, if that is the case, what is still going wrong? How come the Mount Polleys of this world are still capable of hitting the headlines? In point of fact, of course, the inquiry into that particular incident (See E&MJ, March 2015, p.6) came to the conclusion that the dam wall failure there resulted from the unpredicted behavior of a specific subsoil layer beneath it. "The design did not take into account the complexity of the subglacial and pre-glacial geological environment associated with the perimeter embankment foundation," the inquiry report found, noting that neither human intervention (for which read "people doing things that they shouldn't"), overtopping nor internal piping appeared to have been contributory factors. The loading on the dam wall foundations became excessive, and the foundations failed.
The UNEP report made reference to some TSF incidents that remain fairly clear in the memory, such as Omai, Guyana (1995), and the Merriespruit dam failure in South Africa the previous year. Since then, incidents have included Aznalcollar (Los Frailes) in southern Spain in 1998 and the Kolontar dam failure in Hungary in 2010--although this involved red mud from an alumina refinery rather than mine tailings per se. To these could be added the release of impounded water from the dam serving the former Opemiska copper mine in Quebec in 2008, as an example of the failure of an historic dam rather than one still in active use.
And the hard rock industry is not alone in suffering the effects of TSF incidents. The coal industry has also had its share of problems, one recent event having been the release of slurry and water from a containment facility at the mothballed Obed Mountain mine in Alberta. The subsequent inquiry suggested that the regulatory authorities had failed to inspect the province's coal-mine TSFs for years, and were unaware of the failed structure.
A well-operated TSF is essential to the success of any mine where mineral processing is integral to the operation. Nonetheless, it is clear that until recently, TSFs have often been a poor relation when it comes to investment and management. Speaking at the AusIMM's conference on Tailings and Mine Waste Management, held in Sydney, New South Wales, on July 27-28, Johan Boshoff, principal tailings engineer at Golder Associates, gave his opinion that: "TSFs are often regarded as non-assets and do not receive the attention they deserve in terms of technical expertise and financial provision. A TSF is not a simple structure but a complex and dynamic structure, that changes throughout its operational life."
Focusing on Improvement
A huge amount of work has gone into improving the design of TSFs and reducing the likelihood of failure. Without question, there is a much better understanding of the mechanisms that can contribute to impoundment failure, and the ways in which TSFs should be managed in terms of deposition strategy and water retention.
And, given that water is usually a key factor at some stage in a dam breach, there is increasing interest in reducing the volume of water held in TSFs as well as heading in the direction of dry deposition, in which tailings from the mill are dewatered to a much greater extent than was the case in the past. Dryer tailings means greater material stability, as well as less time being needed before surface restoration can begin.
Add to that the reality that in many areas of the world, water shortage is becoming a problem in its own right, and the benefits of reducing the amount of water that is allowed to evaporate or soak away from TSFs quickly migrate toward the operation's bottom line.
In a keynote address to the AusIMM conference, professor Andy Fourie of the University of Western Australia pointed out that the Mount Polley incident was not alone in 2014--another dam failure (at an iron-ore operation in Brazil) claimed three lives. "The causes of this latter failure may never be as well-investigated as Mount Polley, but the monotonous regularity of failures of this type clearly indicate that significant improvements in tailings management are essential," Fourie went on.
"Over the past decade or two, a great deal of attention has been given to the use of high-density, thickened tailings," he said, before looking at some of the operations that have successfully implemented high-density tailings management systems. Fourie stated that implementation has been more difficult than envisaged in some cases. With the Mount Polley inquiry report concluding that, "the future requires not only an improved adoption of best applicable practices (BAP), but also a migration to best available technology (BAT)," there is potential for implementing even higher levels of dewatering, such as producing filtered tailings, Fourie believes.
Examples of BAT cited in the Mount Polley report include filtered, unsaturated, compacted tailings and a reduction in the use of water covers in a closure setting.
In another presentation to the same meeting, professor David Williams from the University of Queensland noted that "tailings disposal has been based on minimizing short-term capital and operating costs, [which has] led to the widespread adoption of surface TSFs to store slurried tailings delivered by robust and cheap centrifugal pumps and pipelines.
"High rates of rise of dilute tailings to a facility used to store both tailings and supernatant water results in a wet, low-density, soft tailings deposit that not only occupies a large storage volume but also presents rehabilitation difficulties," he went on. "A low tailings density results in high containment wall costs, very high rehabilitation costs, and a low potential for future land use or ecological function.
"Overall, the cost of slurried tailings disposal, thought to be low, is actually high," Williams stated.
Waste or a Resource?
Throughout the history of the mining industry, yesterday's tailings have often turned out to be tomorrow's resource. Advances in technology have made the recovery of residual values not only viable, but also extremely attractive, while the increasing focus on ensuring the long-term stability of TSFs has been another spur toward reworking.
In their presentation to the AusIMM conference, Dr. Dirk van Zyl and others looked at opportunities for reusing tailings and waste rock in other ways, both during and after mine operation. Van Zyl, from the University of British Columbia and coincidentally one of the experts on the panel that undertook the Mount Polley inquiry, noted that not only are there around 75 tailings reworking projects currently active around the world, but there is also a wide range of other uses either in progress or being investigated.
These include the transfer of rock-crushing equipment to local communities, who can then use mine waste for construction aggregate, the installation of solar energy arrays on historical TSFs, new land for agriculture, the creation of new wildlife and recreation facilities, and--where the geochemistry is right--the use of tailings as a carbon-capture sink. And these, of course, do not take into account the growing use of tailings for underground backfill where the mining method requires this.
All of which is fine in theory, but does require long-term management to ensure that any materials for reuse later on are in fact suitable, and that there is ongoing communication with potential end-users to ensure that community expectations are fulfilled.
In terms of their potential as secondary sources of minerals, Eugene Louwens and others at the University of Queensland described their work on applying geometallurgical principles and methodologies to TSFs. As described in E&MJ's report on the topic (September 2014, pp. 74-77), geometallurgy encompasses bringing together geology, mining and mineral processing to optimize mineral recovery. Louwens and his co-authors argued that fundamental geometallurgical characterization, as applied to ore deposits, should also be applied to planning the retreatment of TSFs.
Using the TSF at Glencore's Ernest Henry operation as their test bed, the authors developed an approach that takes processing performance into account. This, they said, permits a more realistic economic evaluation of the potential of reprocessing the tailings material, which, in the case of Ernest Henry, contains magnetite, copper, cobalt and gold resources. Furthermore, they believe the approach to be generic, and that it can be applied to other TSFs that are being considered for reprocessing.
Mixing in Waste
One of the biggest handicaps to maintaining long-term stability in conventional TSFs remains the nature of the tailings themselves. Fine-grained, their high water-retention capability means that it often takes decades for the tailings to become dry enough that reclamation can be successful. Add to that the potential for liquefaction under shock loading or if a dam breach occurs, and it is clear that conventional TSF operation can be far from ideal.
One approach now being addressed is the possibility of co-depositing fine tailings with much coarser mine waste. While it has been regular practice in the past to use mine waste in dam wall construction, this takes the concept a stage further.
At the AusIMM meeting, a paper by Rob Longey and several others described the construction of a new TSF at Grange Resources' Savage River iron-ore mine in Tasmania, which will provide storage for some 37 million m3 of tailings over a 20year life. They described how the embankment is being constructed entirely out of waste rock from mining, and includes a flow-through drain built from non-acid forming rock. This approach provides economical storage for tailings and waste rock while maximizing the available space on site.
Another benefit of this concept, they said, is that the run-off from the TSF will provide a long-term source of alkalinity to the watercourses below, with the aim of helping to redress some of the environmental degradation there caused by acidic flows from historical mining operations in the district.
The idea of using alkaline waste to neutralize outflows from acid-forming mineralization is nothing new, but requires careful planning and management to ensure that the appropriate material is used in the right places. Speaking at last year's Colorado conference, Golder Associates' Kebreab Habte and others looked at the co-deposition TSF designed for Fortune Minerals' proposed NICO gold-cobalt-bismuth-copper mine in Canada's Northwest Territories. The TSF has been designed as a permanent storage facility for the 97 million mt of waste rock and 30 million mt of tailings that the mine will produce over its life.
"Co-disposal provides an alternative to the most frequently used method of separate disposal of tailings and mine rock," they said. "Although the co-disposal concept will increase the complexity and cost of operations, it will reduce the footprint area, increase the rate of consolidation of the tailings, improve the stability, reduce the potential for freeze drying and dusting of the tailings, and reduce the metal leaching and acid mine drainage from the mine rock."
Thickening Gains Ground
As Fourie mentioned in his keynote address to the AusIMM conference, the practicalities of thickening tailings before their deposition in a TSF are now occupying the engineering skills of plenty of mining companies around the world--and it has not all been smooth sailing.
Delegates to the Colorado conference heard a presentation from Jeremy Boswell and colleagues at Thurber Engineering on some of the operational challenges experienced, and remedies found, where using thickened tailing technology. "The deposition of conventional thickened tailings has encountered many operational challenges over the years," they pointed out. "These include variation in feed, solids content, beach establishment, stability and capacity constraints, drainage, consolidation, supernatant and fines.
"High-quality design and operation of thickeners can prevent many of the problems encountered in tailings deposition," the authors suggested, while listing some of the remedies that can be employed as being control of the slurry relative density, the elimination of excess water, development of a conservative deposition cycle which takes account of the weather, discharge control, drainage and decanting techniques, and planning for off-specification events.
"Many tailings challenges do not arise overnight," they stated. "They are often the result of a combination of factors, which, if unaddressed, may eventually spiral out of control. Similarly, the prevention and remediation of tailings operational challenges are the result of an intentional, integrated team plan that systematically addresses each aspect of the operation."
Speaking at the AusIMM conference, professor David Boger from Monash University looked at thickening tailings from his own specialist perspective: slurry rheology. The initial stages of dewatering tailings changes the material's behavior from that of a Newtonian fluid to non-Newtonian, he said, while further dewatering leads to the formation of paste that requires different concepts for transport and storage. "Dry stacking has been made possible by significant advances in thickener design through the development of compression thickeners and advances in pump technology," he added.
More recent work has focused on the use of in-line polymer flocculation which, he pointed out, has some advantages over using expensive compression thickeners. With the flocculant being added to the discharge pipeline, the tailings can be pumped as a low-concentration fluid that segregates when it reaches the TSF, giving lower pumping costs than for pastes and improving settling characteristics.
Oil Sands: A Special Case
In this year's review of Canada's oil sands industry (E&MJ, August 2015, pp. 32-40), mention was made of Syncrude's C$1.9 billion tailings centrifuge project. Designed to remove water from the tailings resulting from bitumen extraction, the plant is based on pilot work the company undertook in 2007, specifically to address the problems caused by the water-retention characteristics of mature fine tailings (MFT). Shell subsequently adopted the technology, modifying it for use at its own Jackpine operation. The use of centrifuging as a means of dewatering MFT has the potential to reduce the amount of space needed for drying and storing MFT by as much as 50%, according to COSIA, Canada's Oil Sands Innovation Alliance.
For several years, the companies involved in mined oil-sands production have been working toward compliance in tailings management with the requirements of the Alberta Energy Regulator's Directive 074, issued in 2009. In March, the Alberta provincial government released a new draft Tailings Management Framework, which aims to manage fluid tailings volumes in such as way as to minimize environmental risk and liability from their impact on the northern Alberta landscape.
By way of illustration, the draft framework document suggests that the potential aggregate impact of tailings could reach more than 7,000 million m3 by 2060 under a "business-as-usual" scenario. This would have been reduced to some 2,600 million m3 of new tailings under the previous directive, while the new framework is targeting a combined total of both historic and new tailings of around 600-700 million m3 depending, of course, on the level of policy implementation in the future. Whatever the outcome, there is clearly substantial opportunity for a radical rethink on how oil sands tailings will be managed in the future, with the new framework maintaining the earlier criterion that tailings have to be in a ready-to-reclaim state within 10 years of the end of a mine's life.
Looking Long Term
Longevity is just one of the factors that needs to be built in to a sustainable TSF plan. Natural degradation processes may not be of significance 10 years from now, but in 100 or 500 years, the situation could be markedly different.
At the Colorado conference, Franco Oboni and others presented their views on the risk assessment of the long-term performance of closed tailings facilities, showing that the design of closure works for different time frames may result in a significant cost differences and perceptions of what constitutes a good closure approach. Rather than relying on past practice to satisfy societal concerns, what is needed is "a transparent procedure that enables us to rationally compare designs, foster an understanding of what we know and what we do not know, and measures chances of success in comparable-reproducible manner which, together with our good engineering sense, will give the necessary comfort to our proposed ideas in a way that can be explained to the public and regulators," they stated.
Public opinion and public awareness are much more focused than they used to be. "In a little more than 100 years, we have gone from 'throwing everything into the river' to spending hundreds of millions of dollars for TSFs and then spending one billion (or more) for the environmental rehabilitation of facilities 'gone wrong,'" Oboni went on. "Contamination is persecuted and companies lose large parts of their market capitalization because of tailings accidents."
The integrity of geostructures such as tailings dams will inevitably decay over time, often as a result of repeated incidents or events such as settlement, flooding, ice damage or, in susceptible areas of the world, earthquakes. The only way to reduce the increase of the probability of failure is to repair the damage that occurs as a result of each hazard hit, Oboni said. "Risks, especially long-term ones, can never be reduced to nil. Engineering skills and good sense enable us to imagine robust solutions that, in an economically sustainable way, will deliver the best imaginable results."
As Johan Boshoff pointed out in his Sydney conference address, "The operational life of a TSF is the period in which there is the greatest opportunity for risks to occur. The failure of a TSF is normally not as a result of a single event but as a result of a consequence of events. These events are often not recognized in time.
"The mining industry [must] take a longer-term approach to the planning of TSFs," he stated. "One of the factors critical to the final rehabilitation of a TSF is the management of tailings deposition during the mining operation. Without systematic tailings deposition and careful water management, the final rehabilitation could be very costly, at a time when cash flow is limited or non-existent.
"Much of this challenge can be overcome by adequate planning, associated with good tailings management and the use of a sound technical approach early in the life of the facility," he concluded.
And one such sound technical approach is already available, said professor Boger, quoting a reference source: "Implementation of paste and thickened tailings can result in reduced cost and financial risk, based on full life cycle costs, improved environmental benefits and improved public perception and safety.
"The problem is that full life cycle accounting methods are generally not used or imposed," he added.
E&MJ would like to thank Linda Hinshaw at Colorado State University and Eliza Sanneman and Kelly Steele at AusIMM for providing access to the papers cited in this article. The 2015 Tailings and Mine Waste conference took place on October 25-28 in Vancouver, and was organized by The Norman B. Keevil Institute of Mining Engineering at the University of British Columbia, Colorado State University and the University of Alberta.
By Simon Walker, European Editor
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|Title Annotation:||TAILINGS MANAGEMENT|
|Publication:||E&MJ - Engineering & Mining Journal|
|Date:||Nov 1, 2015|
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