Article pubs.acs.org/IECR
Sustainability and Waste Management in the Resource Industries Hugh Jones† and David V. Boger*,‡,§ †
Golder Associates Pty Ltd. and ‡The University of Melbourne, Melbourne, Australia, and §Faculty of Engineering, Monash University, Clayton Vic 3800, Australia ABSTRACT: Fundamental science and engineering research at Monash University and The University of Melbourne in Australia has enabled the alumina industry worldwide to reduce the volume of waste produced by about fifty percent. Valuable raw materials are recovered, and the risk of storage failure is eliminated. Across the mining industry, there are at least two major storage failures annually worldwide, resulting in deaths and environmental disasters. The failure in Hungary of a storage facility for bauxite residue in October 2011 resulted in eight deaths and a fine of 472 million euro ($US648 million) and the arrest of the CEO (later released). The new technique called dry disposal produces a paste for stacking and drying instead of a water-like suspension to be stored in a dam or pond. Simply removing water from the suspension and reusing and recycling water represents a step toward a more sustainable practice in the industry. As the concentration of such a suspension is increased as a result of dewatering, the materials exhibit non-Newtonian behavior, characterized by shear thinning, a yield stress, and in some instances, thixotropic behavior. Such high concentration, nonideal (dirty) suspensions in the resource industries has meant that new rheological methods and techniques were needed for both shear and compression rheology to measure and interpret the basic flow properties. Also, some older empirical techniques needed to be modified and interpreted in a more fundamental way so that the results could be used in design. The paper reviews these techniques and illustrates how the industry itself has motivated their development. Understanding and exploiting this rheology has resulted in dramatic improvement in the waste disposal strategy for some industries, but many have failed to embrace the available technology. Why? Is regulation the answer? Probably not. The paper concludes that a greater positive change in the waste management practice will occur in the future, motivated by a number of factors, including public perception, water recovery, the necessity to earn the right to operate, and perhaps even by common sense accounting. The paper is a review of thirty years of work with the resource industries on environmental waste minimization. Aspects have been published in the Proceedings of Paste and Thickened Tailings Conferences held annually since 1999 (Jewell, R. J., Fourie, A. B., Eds.; Paste and Thickened Tailings−A Guide, Second ed.; Australian Centre for Geomechanics: Perth, 2006), in Chemical Engineering Science (Boger, D. V. Chem. Eng. Sci. 2009, 64, 4525), and in the Proceedings of the Second International Future Mining Conference in 2011 (Jones, H.; Boger, D. V. Proceedings of the Second International Future Mining Conference 2011, Sydney, 22−23 November, 2011).
■
INTRODUCTION Almost everything used by human beings is either grown or mined. For humanity today, mining is an essential human activity. Without it, we would have virtually no tools and very little conveniently available energy, and to quote Agricola in his treatise on mining published in 1546,4 “If there were no metals, men would pass a horrible and wretched existence in the midst of wild beasts; they would return to the acorns and berries of the forest.” In response to rising concerns about environmental management in mining, the General Assembly of the United Nations (UN) established the World Commission on Environment and Development (Commission) and in late 1983 asked that Commission to formulate “a global agenda for change”. The 21 person Commission was a multicultural organization, chaired by a former Prime Minister of Norway Gro Harlem Brundtland, which conducted a wide ranging investigation into many issues. In 1987, it completed its work and reported to the General Assembly. The report was also published in book form (titled “Our Common Future”5) and became a nonfiction best seller (It is still seen in airport bookshops!). This report can be considered as the starting point of general awareness of the concept of sustainable development. The initial terms of reference for the Commission were directed at developing long-term environmental strategies for © 2012 American Chemical Society
achieving sustainable development by the year 2000 and beyond. During its investigations and preparation of its report, the Commission recognized that social, economic, and environment issues were all very tightly bound together, meaning that changes in one would have real impacts on the other two. The report used a definition of sustainable development that has become the most commonly used definition today. The short version of the definition reads “Humanity has the ability to make development sustainable- to ensure that it meets the needs of the present without compromising the ability of future generations to meet their own needs.” However, we consider the fuller definition that appears slightly later (two paragraphs) in that report to be more applicable to the mining industry. This definition states that “sustainable development is not a fixed state of harmony, but rather a process of change in which the exploitation of resources, the direction of investments, the orientation of technological development, and institutional change are made consistent with Special Issue: APCChE 2012 Received: Revised: Accepted: Published: 10057
December 17, 2011 March 8, 2012 March 19, 2012 March 19, 2012 dx.doi.org/10.1021/ie202963z | Ind. Eng. Chem. Res. 2012, 51, 10057−10065
Industrial & Engineering Chemistry Research
Article
mines, and mine closure. These are interlinked issues that were considered to be critical to the mining industry’s contribution to sustainable development. The writing of this section of the MMSD report was a complex process that included three full-time Research Officers and a worldwide search for people with expertise and interest in the three fields. A series of expert working papers on the three topics were prepared; a three-day workshop attended by 65 participants was held in Vancouver, Canada, and the final report was drafted (Mining, Minerals and Sustainable Development (MMSD) Project, 2002). The participants were from very diverse fields with different experiences and included a number of nongovernment organizations such as conservation and union groups as well as government representatives, industry personnel, consultants, and academics. While the final report is not a consensus of all the participants, it represents a reasonable snapshot on the perception of environmental management in the mining industry and areas that required attention at that time. The International Council on Mining and Metals (ICMM) was established in 2002 and is responsible for the carriage of the recommendations made by the MMSD project. The ICMM has developed an industrial code of practice for its member organizations. The only specific reference to waste in the code is to rehabilitation of land disturbed or occupied by operations in accordance with appropriate past mining uses and to provide for safe storage of disposed residue wastes and process reagents. The ICMM has developed position statements and guidance in the support of their established code, including a guide on mine closure, although it has yet to produce a specific position statement or guidance document on mine waste management in the nine years since the MMSD recommendations.7
future as well as present needs.” The mining industry is clearly a supplier of resources, definitely makes significant investment, is responsible for significant technological development, and has the ability to change institutions. Using this definition, it is clear that applying the term “sustainable development” to the mining industry is not an oxymoron but a concept that requires serious consideration. In “Our Common Future”, mining is specifically identified in both positive terms, as a contributor to GNP in developing countries and as an energy supplier, and also negatively, as an industry which needs to either prevent environmental damage or invest in restoring unavoidable damage. The UN held a Summit of Heads of Government in Rio de Janeiro in 1991 to address the issues raised by the World Commission. One outcome of the Rio de Janeiro Summit was Agenda 21, a document that set out what needs to be achieved to attain sustainable development on a worldwide basis. Agenda 21 was not limited to requiring government action but is sufficiently broad that various industries, including the mining industry, could clearly identify specific challenges that they needed to address. The next stage in this worldwide process of striving toward sustainable development was a followup to the Rio Summit, held in Johannesburg in 2002.
■
MINING INDUSTRY RESPONSE TO SUSTAINABLE DEVELOPMENT ISSUES In 1999, the World Business Council for Sustainable Development (WBCSD) contracted the International Institute for Environment and Development (IIED) to identify the state of the mining industry in addressing Agenda 21 and to highlight areas where challenges remained and, where possible, develop plans that addressed those challenges. The IIED created the Mining, Minerals and Sustainable Development Project (MMSD) to conduct a participatory analysis of how the mining industry could contribute to the global transition to sustainable development. This project took over three years and was done both at the regional level (Australia was one of the regions) and through addressing nine identified major challenges “across-the-world”. These challenges were seen as nine opportunities to tackle important issues in a broad multistakeholder effort. One of the nine challenges was “How can environmental management in the mining and metals industry be improved?” This challenge concentrated on three aspects of the mining industry namely Large Volume Waste, Abandoned Mines, and Closure. The MMSD project has published its report titled “Breaking New Ground” in 2002, and the report was the main document the mining industry presented to the Johannesburg Summit.6 It covered all nine identified major challenges and outlined various agendas for change that the mining industry needs to adopt as it contributes toward sustainable development. The then President of the WBCSD, Dr Bjorn Stigson, had a very clear understanding of what sustainable development meant in the context of the mining industry. In a meeting held as part of the lead up to the Johannesburg summit, he summed it up as “Leaving a positive legacy while exploiting the resource”. What is particularly interesting about Dr. Stigson’s summary is the order of the two key phrases, “leaving a positive legacy” and “exploiting the resource”. Some Outcomes of the Mining, Minerals, and Sustainable Development Project. The MMSD Project addressed a wide range of issues from the viability of the industry to sector governance. Issues identified of direct engineering interest were the management of large volume waste, abandoned
■
MINING WASTE Background. Since the end of the Second World War, poor management of tailings by the mining industry has caused more deaths in the general community than any other aspect of mining (over 500 and still counting, Mining Journal Research Services, 19968). Major tailings mishaps still happen at an unacceptable rate [e.g., Kingston fossil plant, Harriman, Tennessee, USA (2008), no deaths but significant environmental and infrastructure damage; Karamken, Magadan region, Russia (2009), one death; Kolontár, Hungary (2010), eight deaths]. There have been 43 “documented” failures in the last 20 years.9 ICOLD Bulletin 121 (2001)10 reports the most common causes of reported tailings incidents as lack of control of the water balance, inappropriate site selection, lack of quality assurance/quality control (QA/QC) during embankment construction, and a general lack of understanding of safe operating practice for the facility. Of these identified problem areas, the management of water is a generic challenge that applies to almost every tailings facility and is an area where considerable research has been conducted, particularly over the last 20 years. The mining industry is the world’s largest producer of waste, producing about 65 billion tons annually, based on 2010 production figures.11 Fifty-one billion tons is waste rock (mine overburden), and 14 billion tons is the fine particle tailings where the particle size is generally less than 120 μm. The world’s population continues to increase, and at the same time, the population’s standard of living is rising, which axiomatically results in increasing demands for more metals. As the readily accessed minerals are consumed and less accessible minerals are mined with greater waste being produced for each unit of metal produced, so tailings production will almost certainly expand. 10058
dx.doi.org/10.1021/ie202963z | Ind. Eng. Chem. Res. 2012, 51, 10057−10065
Industrial & Engineering Chemistry Research
Article
The fine particle tailings generally exit the process plant as a fine particle low concentration slurry (suspension). With a few notable exceptions where the approved practice is to directly discharge into rivers or the sea, the tailings are generally pumped to settlement ponds, which can be very large (several hundreds of hectares in area). The main acute risk associated with a tailings storage facility (TSF) is the presence of the supernatant water on top of the settled solids, while the settled solids are generally saturated and susceptible to liquification. Seepage from the TSF both during operations and after closure can be high and generally, potentially harmful to the environment. Such seepage is often a long-term chronic risk that can result in acid or metalliferous drainage (AMD) which pollutes the local groundwater. Removing or at least reducing the free water on the pond would seem to be an obvious step forward in a more sustainable and lower risk disposal strategy. Professor Eli Robinsky12 invented the concept of thickened tailings where water is removed before placing the tailings in the pond, producing a non-Newtonian fluid with a yield stress. Some advantages of moving from a conventional slurry tailings to a thickened tailing are illustrated in Table 1. The concept
Figure 2. A bauxite residue (red mud) dry stack (photograph courtesy of Alcoa W.A.).
Alcan (Rio Tinto-Alcan) brought “deep cone thickening” to the industry and was already dry stacking in Jamaica in 1986 and centrally discharging a paste in Quebec, which is illustrated in Figure 3. Rheological properties (the yield stress) allows one
Table 1. Advantages of Paste and Thickened Tailings reclaim water reclaim process reagents reclaim energy (heat) maximize density of tailings in tailings storage facility minimize tailings storage facility footprint render suitable for mine backfill reduce potential for acid drainage minimize (eliminate?) risk of dam failure
was first put into practice at the Kidd Creek Mine in Ontario, Canada, in 1973. The idea of dewatering the waste stream even further to a paste is a much later development, successfully practiced by the alumina industry for surface disposal.13 Alcoa (W.A.) commissioned the first superthickener (compression thickener) to produce a paste in 1987, one in 1989, and another in 1991, to produce a typical paste product like that shown in Figure 1 which when spread in the disposal area results in a trafficable surface like that shown in Figure 2.
Figure 3. Central discharge of a thickened red mud in Quebec, Canada (photograph courtesy of Alcan, now Rio Tinto-Alcan).
Figure 1. Mineral waste produced with a compression thickener in the alumina industry (photograph courtesy of Alcoa W.A.).
to distinguish between a slurry, thickened tailings, and a paste.2 A slurry tailing is a Newtonian fluid without a yield stress, while a thickened tailing has a yield stress ranging up to about 10 Pa; a paste for surface deposition would have a yield stress, perhaps, of 10−100 Pa, and a mine stope fill material could have a yield stress up to and perhaps even beyond 1000 Pa. There are no fixed rules here, and these boundaries are arbitrary and continually changing. The alumina industry for example handles a material (paste) with a yield stress of about 40−50 Pa. The major technological development, moving from a thickened tailings to a paste, has occurred in the last ten years, largely as a result of the Paste and Thickened Tailings Seminars organized by the Australian Centre for Geomechanics and the examples set by the worldwide alumina industry. There have been huge advances in thickener design, in pumping technology, and in tailored flocculants and flocculation practice. The advancements in thickener design and operation and in pumping of pastes were reviewed by Schoenbrunn14 and Paterson,15 respectively, in 10059
dx.doi.org/10.1021/ie202963z | Ind. Eng. Chem. Res. 2012, 51, 10057−10065
Industrial & Engineering Chemistry Research
Article
measurements of suspensions; again, the problems are associated with sedimentation and slip and analyzing the basic data. Cone and plate torsion flow is good for measuring the properties of polymers and polymer solutions, but again the gap in the instrument has to be very small for the analysis to be correct. Hence, none of the above geometries, which form the basis of some very sophisticated and expensive instruments, are readily suitable for the measurements of the basic viscosity of a paste material. Yield stress measurements with conventional rheometric equipment can be equally as problematic. Yield Stress Measurement. Figure 5 illustrates typical shear stress−shear rate behavior that is observed for a yield
the Paste and Thickened Tailings Seminar held in Perth in 2011. Highly structured thickened products with very high yield stress can now be produced with the concept of shear degradation (shear thinning) used to significantly reduce the yield stress and viscosity prior to pumping. The feed to centrifugal pumps used to be limited to a yield stress of about 30 Pa; now, some manufacturers are talking about a value of about 200 Pa. Also, there are significant economic arguments that in the long term positive displacement pumps can be as economic as centrifugal pumps.
■
RHEOLOGICAL TECHNIQUES FOR PASTE AND THICKENED TAILINGS Basic Rheological Properties. For the suspensions encountered in paste and thickened tailings, non-Newtonian characteristics are generally observed at higher concentrations. The basic measurements required to characterize these materials include the viscosity and how it varies with shear rate (flow rate). The general definition of the viscosity is τ η= γ̇ (1) where τ is the shear stress and γ̇ is the shear rate. In order to make this basic measurement, one must use a characterization device where both the shear stress and the shear rate can be defined. This science is called rheometry. There are basically four flow fields called viscometric flows where the shear stress and the shear rate can be defined and, hence, one can define the viscosity for a non-Newtonian material. These flow fields are shown in Figure 4. Each of these flows has its own significant
Figure 5. Typical shear stress−shear rate behavior illustrating the “yield stress”.
stress material. There are very significant misconceptions associated with yielding. Much of the data obtained for suspensions is not obtained at low enough shear rates to determine a true yield stress by extrapolation. Often, the data are extrapolated from a linear region of behavior at high shear rates to the axis to define what has been called the Bingham yield stress. The Bingham yield stress is a model fitting parameter and has no meaning whatsoever in terms of the true yielding of the material. We have seen many consulting reports in which this yield stress has been used as a basis for design. The yield stress is the value of the shear stress when the material initially flows and in principle can be determined if the measurements of the shear stress as a function of shear rate are made at low enough shear rates. However, there are difficulties in reaching such low shear rates; slip invariably occurs in the concentric cylinder geometry at low shear rates. Thus, extrapolated values of the shear stress−shear rate data are not easy to obtain and are often in significant error. Figure 6 illustrates the potential errors associated with extrapolation, dependent upon the shear rate region in which data are available. The true yield stress for the paste material shown in Figure 6 was obtained with a vane rheometer. The extrapolation of the high shear rate data obtained and reproduced with a capillary and concentric cylinder device is 65 Pa, while the extrapolated value obtained from the lower shear rate data is on the order of 18 Pa. The true yield stress was 250 Pa!! The errors involved are immense, and thus, if the true yield stress is needed, which is the case for rake design in thickeners and for pump restartup and design in a pipeline, then conventional rheometry geometry generally should not be used and a device like the vane is appropriate. The vane and its use for yield stress measurement
Figure 4. Viscometric flows used for viscosity measurement of nonNewtonian fluids.
advantages and disadvantages. Poiseuille flow involves measuring the pressure drop as a function of flow rate in a long straight tube; special capillary rheometers can be designed to make these measurements, but the measurements can be labor intensive and often are associated with significant problems at the wall of the tube where slip can occur. The most common geometry used for measurement of shear stress and shear rate is Couette flow, a cup and bob rheometer. This geometry also has a distinct disadvantage in that the gap has to be large enough so that the particles themselves do not interfere with the measurement. When the gap is large, the analysis of data becomes complex and often is not understood. Slip is a problem, and sedimentation can also occur. For parallel plate torsion flow, almost noone uses this geometry for 10060
dx.doi.org/10.1021/ie202963z | Ind. Eng. Chem. Res. 2012, 51, 10057−10065
Industrial & Engineering Chemistry Research
Article
simplified the geometry to a cylindrical geometry, and established a simple equation for relating the slump to the yield stress. Figure 8 illustrates how the slump is measured while eq 3 is a simple interpretation of the slump measurement. τy ′ =
1 1 + S 2 2
(3)
Figure 6. Flow curve for a paste sample; yield stress = 250 Pa.
in suspensions was developed by Professor Q. D. Nguyen in his PhD thesis in 1983.16,17 The basic vane and the principles involved are illustrated in Figure 7, while the basic equation for analyzing the data is given in eq 2. π ⎛l 1⎞ Tm = d3⎜ + ⎟τy 2 ⎝d 3⎠
Figure 8. The cylindrical slump test.
τy′ is a dimensionless slump (τy′ = (τy′)/(ρgH)), and S is the dimensionless slump which is the actual slump divided by the height of the cylindrical slump vessel, H. Numerous direct comparisons of the cylindrical slump measurement and the vane yield stress measurement are now available in the literature.19,20 The slump measurement was again motivated by the alumina industry and, in particular, from a discussion with Don Glenister at Alcoa. Thus, the alumina industry and the basic properties of the red mud motivated the development of two methods for single point yield stress measurement which are now used industry-wide and extend well beyond the industry into the world as a whole. On the basis of experience gathered with the alumina industry and with the minerals industry as a whole, it was obvious that a simple and perhaps even more effective method for determining the basic shear stress−shear rate data was needed. Shear Stress-Shear Rate Measurements (The Bucket Rheometer). The Couette viscometer is the most commonly used for obtaining the shear stress−shear rate data for many materials and in many industries. The torque, T, on the bob is observed as a function of its rotational speed, Ω. The basic equations used to analyze the data are eq 4 for the shear stress at the two surfaces and eq 5 for the shear rate.
(2)
Figure 7. Illustrating the vane device for yield stress measurement.
Basically, the vane is inserted into the suspension and rotated at a very low speed where the torque is observed as a function of time. The torque increases until reaching a maximum value, Tm, when the material yields. The maximum torque is related to the yield stress by eq 2, where d is the diameter of the vane and l is its height. Equation 2 is valid if end effects are minimized, and this is possible using a large enough l/d vane. The great advantage of the vane is the material yields on itself; slip generally is not an issue, and also, the vane can be inserted into the fluid in different regions. This technique is now used worldwide for measurement of yielding in all matter of materials. The development of the technique was motivated by a need generated by the alumina industry. Another even simpler method for a single point measurement of the yield stress is to exploit slump. The civil engineering community for many years has used a conical device for measuring the slump in concrete. The results of such measurements were reported in inches or centimeters of slump and were not related to any fundamental characteristic. Dr Nick Pashias in his PhD thesis18 took the idea of the slump,
τ1 =
Ω=
T 2πR2
L = ε2τ2
τ f (τ)
∫τ 1 2
2τ
dτ
(4)
(5)
In the equations, τ1 and τ2 are the shear stresses at the surfaces of the bob (r = R) and the cup (r = εR) and f(τ) is the shear rate, γ̇. Note in eq 5 that the shear rate is hidden inside an integral. The fact that the shear rate is not explicitly defined causes some considerable difficulties in analyzing the data. Integration of eq 5 depends on knowing a functional form between the shear stress and shear rate, i.e., substituting a fluid model. Such a model will not be available a priori; hence, approximate techniques are needed to evaluate the shear rate in the cup and bob rheometer, particularly when the gap is wide, which is necessary for paste or suspension-like material that are of 10061
dx.doi.org/10.1021/ie202963z | Ind. Eng. Chem. Res. 2012, 51, 10057−10065
Industrial & Engineering Chemistry Research
Article
Figure 9. Shear stress−shear rate data obtained for a 45.1 w/w limonite slurry illustrating the performance of the bucket rheometer.
The idea of the bucket rheometer has been published.23,24 Figure 9 illustrates some data obtained for a nickel laterite slurry, which is the feed to the extraction process. The figure illustrates the regions in which the data would be required and also illustrates clearly the slip problems associated with the cup and bob instrument and the capillary instrument. Also, note that the vane and cup rotation device extrapolates to the vane yield stress measurement. Other data illustrating the applicability of the bucket rheometer are available.20 Rheological Flow Properties in Compression: The Compressive Yield Stress and the Permeability. Motivated by basic research, by the theoretical work of Buscall and White,25 and by observations at Alcoa on development and use of the super thickeners, where a six meter deep compression zone was used in a 90 m diameter thickener, it was clear that there must be a better way to evaluate how a flocculated suspension would behave in compression. This led to a series of PhD theses at the University of Melbourne starting with the Masters thesis of Nick de Guingand in 1986, the PhD thesis of Matt Green in 1997, continuing through the work of Shane Usher in 2002, and Brendan Gladman in 2006. The overall result of the research carried on at the University of Melbourne has led to the development of a new instrument for measuring the compression rheological properties of flocculated suspensions. This instrument is now available commercially. A review paper published in 200326 summarizes the earlier work. Current research is on understanding the influence of shear on the compression of flocculated suspensions. Professor Peter Scales presented an overview of this work at the Paste and Thickened Tailings seminar in Ireland in 2006, which will not be reviewed here. Recent references are available.27−29 Suffice to say that this is yet another area of research which was motivated initially by the involvement with Alcoa in Western Australia. The impact in the industry has been significant, and active research in this area continues today. Academic research is often driven by curiosity while industrial research is often driven by need. Here, academic research in non-Newtonian fluid mechanics (rheometry) has been motivated and influenced by an industrial need. The outcome has been the development of two techniques for yield stress measurement and a simple and portable technique for viscosity measurement in non-Newtonian fluids. In particular, the techniques are applicable to what we might call “dirty suspensions” or “real suspensions” which could have a significant
interest here. Often, a narrow gap assumption is made in order to define the shear rate simply by eq 6. This definition, often embedded in instrument software, is not valid for the wide gap required when dealing with paste-like systems. γ̇1 = f (τ1) =
2Ω (1 − ε2)
(6)
21
Kreiger and Maron were the first to realize that eq 5 could be differentiated on both sides with respect to τ1 if the outer radius of the cup becomes very large (goes to ∞). The result of this differentiation on both sides yields to the simple result given by eqs 7 and 8. τ1 =
γ̇1 =
T 2πLR2
(7)
2Ω S1
(8)
d ln T d ln Ω
(9)
where S1 =
The shear stress and the shear rate on the inner bob surface rotating in an infinite medium is now directly defined and not dependent on any particular model assumption. The procedure is as follows. The torque is measured as a function of rotational speed and plotted on a log−log graph. Generally, the slope of this graph will be a constant S1. Once Sl is known, the shear rate is defined, as is the shear stress, from the torque measurements. The idea of a bob in an infinite medium has been a relatively obscure fact. Combining this analysis with the idea of the use of the vane itself as a rotational device results in a new rheometer. The vane itself has a distinct advantage that effectively slip is eliminated. Using the vane as the “rotating” bob allows measurements to be made in the absence of slip. Barnes and Carnali22 have shown, while working in the food industry, that the vane in fact can be used as a rotational device and functions as a bob, particularly for shear thinning materials. The advantages of the vane rotating in an infinite medium, i.e., in a bucket of fluid, are obvious. All that is required is a vane and a torque measuring head, and hence, the device is portable. Additionally, the shear stress and the shear rate data are easily determined. It should be noted that eqs 7, 8, and 9 are also valid for yield stress materials. 10062
dx.doi.org/10.1021/ie202963z | Ind. Eng. Chem. Res. 2012, 51, 10057−10065
Industrial & Engineering Chemistry Research
Article
valley. An alternative would be to move to thickened paste, yet the industry continues to deposit unthickened tailings, wasting water, and increasing potential risks. Leakage of these tailing ponds could affect the world’s greatest salmon fishery; failure of the dam (pond) would be a catastrophic disaster,31 and mine drainage is also a potential problem. Reclaiming process reagents has been very important in the alumina industry where a single Australian operation can save as much as ten million dollars a year from caustic recovery. Reclaiming heat is a significant factor in the Canadian tar sands industry. (The Canadian tar sands represent the second largest oil reserve in the world and, as a whole, it is the world’s largest mining operation.) Maximizing the initial placed density of tailings, minimizing the storage footprint, and considerably reducing the potential for pollution through seepage (e.g., acid mine drainage) all represent a movement toward a more sustainable practice. One basic principle in sustainable development for waste management is to reduce the waste and to recycle and reuse. Thickening via water removal represents a significant reduction in the volume of waste (as much as 50%). The alumina industry in Western Australia separates the fine particles from the sand fraction and now is able to find a market for reuse of the sand, while at the same time examining methods to reuse the fine particle fraction in agriculture. The product of a very high yield stress waste mixed with cement is now commonly used for underground mine stope fill.1 The technology is in place and the risk has been minimized for implementation of thickened and/or paste waste disposal. While there is significant advantage and some movements in this direction, what is holding up the change? COST!! Many investment options in the mining industry (including tailings management options) are financially evaluated using the net present value (NPV) approach. This standard approach may be too optimistic when the net cash flow is negative late in the life of the project (e.g., when undertaking cleanup and restoration work) and sufficient funding may not be available. One way to avoid this problem is to include explicit provision for financing any negative cash flow late in the project; that is, explicitly calculate the real cost of financing closure. In the 2011 publication “Guidelines for Preparing Mine Closure Plans”,32 the Western Australia government spells out the type of cost estimating they require from proponents to address this issue. Figure 11 illustrates the cost challenges. Basically, the upfront capital and operating period are well funded as they are supported
variation in particle size and where the interest is primarily in determining the properties at high concentration. The impact in the paste and thickened tailings community has been significant, but with the exception of the alumina industry, the uptake by the more conventional minerals industry (copper, gold, uranium, etc) has been slow. Figure 10 illustrates the deposit of a paste in the gold industry.
Figure 10. Paste placement at a gold mine.
■
THE WAY FORWARD The science and technology is now in place, as is the demonstrated practice to control the water balance by the removal of water, either by moving to a thickened tailing or to the higher yield stress paste. To the layman, the advantages of moving from a slurry to a thickened tailing and/or paste appear obvious. Table 230 illustrates the relative advantages of a thickened Table 2. Properties of a Slurry, a Thickened Tailing, and a Paste in the Disposal Area30 slurry final density
low
segregation supernatant water post placement shrinkage seepage rehabilitation permeability
high high high
application footprint water consumption reagent recovery
high delayed medium/ low above ground medium high low
thickened
paste
medium/ high slight some some
high none none insignificant
some immediate low
insignificant immediate very low
above ground high medium medium
above and under ground low low high
Figure 11. When costing, examine the lifetime picture.
by cash flow from the operation. However, closure, rehabilitation, and long-term maintenance (of a tailings pond facility) happen after the cash flow stops and are often not properly costed. These costs often become an unfunded liability, a liability sometimes escaped through bankruptcy and other means. The authors have been told by those who will not be referenced that deferring the liability is good business practice, while a large waste stream produces no cash contribution to profits so expenditure of up front capital to improve waste
and/or paste in the disposal area, while Table 1 illustrated the obvious advantages of removing water. In fact, maximizing reclaim water (and reagents) now is a significant driving force to move away from a traditional tailing pond, where losses to seepage and evaporation can be up to 60%. However, we continue to build tailing dams. At the controversial proposed Pebble Mine in Alaska, one of the proposed tailing ponds is to be 760 feet high and extend back 4.5 miles, flooding an entire 10063
dx.doi.org/10.1021/ie202963z | Ind. Eng. Chem. Res. 2012, 51, 10057−10065
Industrial & Engineering Chemistry Research
■
Article
CONCLUSION The short and long-term risks relating to tailings management can be significantly reduced by reducing the amount of water in the tailings storage facility. The adoption of modern water management using the now available technology can assist in attaining safer tailings facilities with minimal cost implications, if lifetime accounting methods are employed. The alumina industry is now approaching this ideal. While recognizing that each case can be different, the authors believe that most tailings ponds can be eliminated (or at least significantly reduced in size) by implementation of paste and/ or thickened tailings. Producing and stacking a designer paste or thickened tailings will allow concurrent reclamation.
management is usually not encouraged and generally avoided, if possible. There are many examples of operations that are poorly funded for closure. Two that come to mind in the personal experience of one of the authors are in Florida, in the Florida phosphate industry, and in the Pennsylvania coal industry. In each case, governments have been left with a significant financial liability for cleanup. There are many other examples worldwide. Yes, performance bonds and/or environmental sureties have become a requirement in many countries as a means of protecting governments and taxpayers from the rehabilitation and long-term maintenance costs, but these sureties are often never enough. The tide is turning.3 A social license to operate for the mining industry is becoming more difficult to obtain as a result of the environmental record of the industry. To use an analogy expressed by Jones in the unpublished opening remarks in the Paste and Thickened Tailings Seminar in 2000, “...The rubbish end of our business still has far too many bins being spilt over our neighbours’ front gardens”. This observation was made ten years ago and is still true today. In paste and thickened tailings, we are not talking about rocket science; we are talking about accounting practices which discourage protection of the environment and regulators who have not had the political support to implement the latest technology. This is changing. The recent ERCB Directive 074 (2009) by the Energy Resource Conservation Board, Alberta, Canada, requires that a trafficable surface of the tar sands tailings has to be established within ten years for final closure. This step forward in regulation will have an impact worldwide in a very positive way.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
REFERENCES
(1) Jewell, R. J., Fourie, A. B., Eds. Paste and Thickened Tailings−A Guide, Second ed.; Australian Centre for Geomechanics: Perth, 2006. (2) Boger, D. V. Rheology and the Resource Industries. Chem. Eng. Sci. 2009, 64, 4525. (3) Jones, H.; Boger, D. V. Sustainable Development and Tailings, Proceedings Second International Future Mining Conference 2011; Sydney, 22−23 November, 2011; Australian Institute of Mining and Metallurgy: Australia (in press). (4) Agricola, G. De Re Metalica, 1556 (1950 Translation by Hoover, H. C. and Hoover, L. H.); Dover Publications Inc. Ltd.: New York, 1950. (5) The World Commission on Environment and Development. Our Common Future; Oxford University Press: Oxford, England, 1987. (6) Mining, Minerals and Sustainable Development Project. Breaking New Ground; Earthscan for IIED and WBCSD, 2002 (www.iied.org/ mmsd). (7) Franks, D. M.; Boger, D. V.; Cote, C. M.; Mulligan, D. R.. Sustainable development principles for the disposal of mining wastes. Resour. Policy 2011, 36, 114. (8) Mining Research Services. Tailings Dam Incidents 1980−1996, report prepared for UNEP, 1996. (9) www.wise.uranium.org/mdaf.hlmf. (10) Tailings dams risk of dangerous occurrences−Lessons learnt from practical experiences. ICOLD Bulletin 121; 2001. Available through: www.icold-cigb.net/listepublications.aspx#bull. (11) Mudd, G E. Private communication. 2011. (12) Robinsky, E. Thickened tailings disposal in the mining industry; Eli Robinsky and Associates Ltd: Toronto, Canada, 1999. (13) Cooling, D. Improving the sustainability of bauxite residue management practices, Alcoa World Alumina. Paste 2007; Jewell, R. , Fourie, A., Eds.; Australian Centre for Geomechanics: Perth, 2007; p. 3. (14) Schoenbrunn, F. Dewatering to higher densities−an industry review. Proceedings 14th International Seminar on Paste and Thickened Tailings; Jewell, R., Fourie, A., Eds.; Australian Centre for Geomechanics: Perth, 2011; p 19. (15) Paterson, A. J. C. The pipeline transport of high density slurries−a historical review of past mistakes, lessons learned and current technologies. Proceedings 14th International Seminar on Paste and Thickened Tailings; Jewell, R., Fourie, A., Eds.; Australian Centre for Geomechanics: Perth, 2011; p 351. (16) Nguyen, Q. D.; Boger, D. V. Yield stress measurement for concentrated suspensions. J. Rheol. 1983, 27, 321. (17) Nguyen, Q. D.; Boger, D. V. Direct yield stress measurement with the vane method. J. Rheol. 1985, 29, 335.
■
CLOSURE Closure is the only certainty for any mine. Closure planning should begin during the prefeasibility phase for any proposed mines and as early as practicable in the mine life cycle for existing mines. It commences with a clear definition of the final land use objectives for the site, which often involves the mining company relinquishing responsibility for the site when mining has been completed and the mine site rehabilitated to the satisfaction of the many stakeholders. The overall design objective for tailings is to place the tailings in a facility (or facilities) that will retain the tailings at that site in the very long-term (millennia rather than decades!). This includes minimizing the leakage of leachates as well as the retention of the solid particles. Many closure designs require the tailings be capped with a suitable cover to reduce the erosion potential at the site and reduce rainwater infiltration to minimize seepage from the facility. The physical properties of conventional slurry deposited tailings mean that the facility often has to be left for several years before the upper surface can be worked on to construct the designed cap. By reducing the water placed in the facility during tailings deposition, this consolidation time is considerably reduced, usually reducing the cost of constructing the cap. In describing the closure of the Pillara tailings storage facility, a base metal mine in the Kimberley Region of Western Australia, Prasad32 notes that a part of the TSF was identified as poorly consolidated. This was “traced back to a time during operation when the processing plant was producing poorly thickened tailings. The high slimes content in the tailings meant that the area was unlikely to dry within the rehabilitation timeline allowance. The depth of the rock layer was instead increased to 400 mm to accommodate the ongoing settlement process.” 10064
dx.doi.org/10.1021/ie202963z | Ind. Eng. Chem. Res. 2012, 51, 10057−10065
Industrial & Engineering Chemistry Research
Article
(18) (a) Pashias, N. The characterization of bauxite residue suspensions in shear and compression, PhD Thesis, The University of Melbourne, 1997. (b) Pashias, N.; Summers, J.; Glenister, D. J.; Boger, D. V. A Fifty Cent Rheometer. J. Rheol. 1996, 90 (6), 1179. (19) Piau, J.-M. Axisymmetric slump in spreading of cohesive plastic soft materials: a yield stress measurement by consisto-rheometry. J. Rheol. 2005, 49 (6), 1253. (20) Boger, D. V.; Scales, P. J.; Sofra, F. Rheological concepts, Paste and Thickened Tailings−A Guide; Jewell, R. J., Fourie, A. B., Eds.; Australian Centre for Geomechanics: Nedlands, W.A., 2006; p 25. (21) Kreiger, I. M.; Maron, S. H. Direct determination of the flow curves of non-Newtonian fluids, III Standardised treatment of viscometric data. J. Appl. Phys. 1954, 25 (1), 72. (22) Barnes, H. A.; Carnali, J. O. The vane-in-cup as a novel rheometer geometry for shear thinning and thixotropic materials. J. Rheol. 1990, 34 (6), 841. (23) Fisher, D. T.; Scales, P. J.; Boger, D. V. The bucket rheometer for the viscosity characterization of yield stress suspensions. J. Rheol. 2007, 51 (5), 82. (24) Sofra, F.; Fisher, D. T.; Boger, D. V. The bucket rheometer for thickened tailings and paste flow curve determination. Paste 2007: Proceedings of the Tenth International Seminar on Paste and Thickened Tailings; Perth, 2007; Fourie, A. B., Jewell, R. J., Eds.; Australian Centre of Geomechanics: Nedlands, W.A., Australia; p 249. (25) Buscall, R.; White, L. R. The consolidation of concentrated suspensions, Part I: The theory of sedimentation. Chem. Soc. Faraday Trans I. 1987, 83, 873. (26) de Kretser, R. G.; Boger, D. V.; Scales, P. J. Compressive Rheology: An overview. Rheological Reviews 2003; Binding, Walters, Eds.; The British Society of Rheology: U.K., 2003; p 1255. (27) Gladman, B.; Usher, S. P.; Scales, P. J. Compressive rheology of aggregated particulate suspensions. Korea-Australia Rheol. J. 2006, 18, 191. (28) Usher, S. P.; Scales, P. J. Steady state thickener modeling from the compressive yield stress and hindered settling function. Chem. Eng. J. 2005, 111, 253. (29) Gladman, B.; de Kretser, R. G.; Rudman, M.; Scales, P. J. Effect of shear on particulate suspension dewatering. Chem. Eng. Res. Des. 2005, 83, 933. (30) Jones, H. Designer Waste. UNEP Ind. Environ. 2000, 23 (Special Issue), 76. (31) Sherwonit, B. Alaska’s Pebble Mine: fish versus gold, 2008; Yale Environment 360, http://e360.yale.edy/content/feature.msp?id=2062. (32) Prasad, A. Pillara mine closure and rehabilitation. Closure 2011, in press.
10065
dx.doi.org/10.1021/ie202963z | Ind. Eng. Chem. Res. 2012, 51, 10057−10065
