OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY
By PHILIPPE GARNEAU B.Eng., McGill University, 2006
A thesis submitted in partial fulfillment of the requirements for the degree of
MASTER OF SCIENCE in ENVIRONMENT AND MANAGEMENT
We accept this thesis as conforming to the required standard
...... Dr. Bruno Bussière, Thesis Supervisor Université du Québec en Abitibi-Témiscamingue
...... Dr. Tony Boydell, Thesis Coordinator School of Environment and Sustainability
...... Michael-Anne Noble, Director School of Environment and Sustainability
ROYAL ROADS UNIVERSITY
May 2012
© Philippe Garneau, 2012 OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY ii
Abstract
Oxygen consumption from sulphide mine tailings was measured under a partial thermal granular cover in a Northern Canada tailings storage facility. The reclamation concept for the filtered-pressed tailings uses permafrost to maintain the tailings in a permanent frozen state by use of a thermal granular cover. The decrease in the oxygen concentration in a sealed chamber at the tailings – cover interface allowed to calculate the oxygen flux at the tailings surface using fundamental gas diffusion laws. The average measured oxygen flux at the tailings surface was 47 moles / m² / year, an 83% decrease over previous data collected on uncovered tailings. Despite the decrease in the oxidation rate, oxidation still takes place under the partial cover. This initial use of the testing apparatus will allow refinement of the testing method for future measurement campaigns.
OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY iii
Table of Contents
Abstract ...... ii Table of Contents ...... iii List of Figures ...... v List of Tables ...... vi Acknowledgements ...... vii Introduction ...... 1 Literature Review and Case Description ...... 3 Acid Mine Drainage ...... 3 Acid mine drainage generation...... 3 Acid mine drainage prevention...... 5 Mine Site Reclamation in Northern Climates ...... 8 Water covers...... 8 Integration to permafrost...... 9 Effect of climate change...... 12 Raglan Tailings Storage Facility Conditions ...... 13 Site description...... 13 Process description...... 14 Tailings storage facility...... 16 Legal Context ...... 19 Planned reclamation...... 19 Legal requirements...... 20 Rehabilitation and restoration plan submitted to authorities...... 22 Materials and Method ...... 24 Studied Materials ...... 25 0-20mm Cover...... 25 Tailings...... 28 Method Development...... 29 Theory...... 29 OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY iv
Oxygen Consumption Tests ...... 31 Traditional method (without cover), application of Elberling...... 31 Modified method for cover performance assessment...... 32 Results ...... 35 Other Remarks ...... 39 Caveats ...... 39 Oxygen Consumption Testing Under Cover...... 40 Modification of Method for Further Testing ...... 41 Conclusion ...... 42 References ...... 44 Appendix A ...... 53 Details for Tests Under 0-20mm Cover ...... 53 Appendix B ...... 55
Slopes for ln(C/C0) Under 0-20mm Cover ...... 55 Appendix C ...... 57 Details for Tests on Uncovered Tailings ...... 57 Appendix D ...... 59
Slopes for ln(C/C0) for Tests on Uncovered Tailings ...... 59
OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY v
List of Figures
Figure 1. Permafrost temperature profile (MEND, 2009) ...... 11 Figure 2. Xstrata Nickel - Raglan Mine's location ...... 14 Figure 3. Raglan property ...... 14 Figure 4. Raglan's concentrator flow diagram ...... 16 Figure 5. Raglan Mine's Tailings Storage Facility ...... 18 Figure 6: Tailings storage facility insulation cover concept (AMEC, 2008) ...... 18 Figure 7. TSF granular cover construction ...... 20 Figure 8. Particle size curves for 0-20 mm granular cover material ...... 27 Figure 9. Traditional test apparatus ...... 32 Figure 10. Air circulation apparatus ...... 33 Figure 11. Modified apparatus for oxygen consumption test under the granular cover... 34 Figure 12. Cylinder with aluminum sealed cap prior to placement of cover ...... 34
OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY vi
List of Tables
Table 1 : ...... 28 Table 2: ...... 29 Table 3: ...... 36 Table 4 : ...... 38
OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY vii
Acknowledgements
I’d like to thank everybody who supported me through this, my family, friends and colleagues. Special thanks to my thesis supervisor, Bruno, for his everlasting patience. Good luck to Véronique in carrying on the torch!
OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 1
Introduction
Xstrata Nickel, a division of Xstrata, operates the Raglan mine in northern Quebec since
1998. The ore comes from four underground mines, spread over an area of 55 km from east to west. Presently, the ore is processed on site to create a nickel-copper concentrate which is then trucked to the port of Deception Bay. From this port, the concentrate is transported by ship to
Quebec City and from there by rail to Sudbury, where it is processed by the smelter to a matte.
The matte is then transported by rail back to Quebec City and shipped to Xstrata Nickel’s refinery in Norway for final processing.
Like all mining operations involving a concentrator, it is the responsibility of the company to manage the tailings resulting from ore processing. The filtered and pressed tailings
(up to 85% solids) are deposited in layers at the tailings storage facility. Tailings from Raglan
Mine contain an important amount of gangue sulphide minerals such as pyrrhotite (see table 1).
One of the risks associated with the storage of tailings is the release of acid mine drainage
(AMD) from the oxidation reaction of sulphide minerals when exposed to water and atmospheric oxygen (MEND, 1.61.1).
To control AMD production, many mining operations located in arctic climate use the continuous permafrost to keep their tailings frozen and reduce their reactivity (Meldrum,
Jamieson, & Dyke, 2001 and Elberling, 2005). The method used consists in building a cover of non-reactive material thicker than the active zone of the local permafrost. Tailings located under OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 2
the active zone are therefore kept permanently frozen thus reducing sulphide oxidation and the potential movement of contaminated water.
The arctic regions may be the most affected by potential climate change (Pachauri and
Reisinger, 2007). With the potential increase of temperatures in the North and eventual disappearance of the permafrost, the efficiency of the planned reclamation technique is challenged. If the active zone increases with a warming of the atmosphere, it is possible that the non-reactive cover thaws completely in the summer months which could lead to a thawing of the underlying tailings and eventually to AMD generation. Buteau, Fortier & Allard (2010) showed that even a slight increase in permafrost temperature can lead to an increase in unfrozen water content in the permafrost and a weakening of the permafrost’s strength. An increase of the unfrozen water content in the tailings stack may favour transportation of contaminants while a decrease in strength may lead to instabilities in the cover creating paths for water and atmospheric oxygen to react with the sulphide tailings material.
Xstrata Nickel is currently exploring different reclamation methods for the tailings storage facility. It was therefore decided to only place half of the final cover design which consists of the first 1,2m layer of 0-20mm granular cover. The second layer of 1,2m of 0-600mm material has yet to be put in place since the final reclamation concept may change in the near future.
The present research goals are to measure the oxygen consumption under the first layer of the cover. From this measured data, it will be possible to calculate the oxygen flux at the tailings- cover interface and to compare it to the data from uncovered tailings. It will therefore be possible OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 3
to evaluate empirically the efficiency of the first layer of the cover to decrease the oxidation rate of the tailings.
The method used to measure oxygen consumption has never been used under a granular cover in the past. This research will therefore allow testing the proposed method and suggest potential modifications for improvement.
The report is divided in three main sections. The literature review and case description describes the acid mine drainage problematic and mine site reclamations methods in northern climates. It also covers a description of the studied site as well as a brief description of the legal context. The materials and method section describes the studied materials as well as offers a description of the method used to evaluate cover’s performance. The results are presented and discussed in the last section which also presents potential improvements to the testing methods and caveats to the obtained results.
Literature Review and Case Description
Acid Mine Drainage
Acid mine drainage generation. Acid Mine Drainage (AMD) is a phenomenon caused by the oxidation of sulphide minerals contained in mine tailings or waste rock when exposed to both water and oxygen (Kleinmann & Erickson, 1982). Oxidation of some sulphides leads to the production of sulphuric acid (H2SO4) which decreases the pH and increases the concentration of available metals such as iron and nickel. In naturally occurring sulphide minerals, the exposure to water and oxygen is minimized by the overlying soil or groundwater. Mining activities OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 4
disturbs the natural ground and increases the surface area of sulphide minerals exposed to water and atmospheric oxygen which may accelerate the rate of acid generation and metal leaching into runoff water (Roach, Kettley, Kwong, & Roots, 1997).
Pyrite’s (FeS2) natural abundance and low economic value makes it the most common sulphide mineral found in mine waste. Although there isn’t a significant quantity of pyrite in
Raglan’s tailings, it contains an important proportion of pyrrhotite (Fe(1-x)S) which reacts similarly to pyrite for acid mine drainage generation, but at a faster rate (Nicholson & Sharer,
1993).The initial reaction (at pH close to neutrality) responsible for AMD formation in ferrous tailings (such as pyrite) can be written using equation 1, where the sulphide mineral, when reacting with oxygen and water, gives sulphates, ferrous ions and acidity in the form of hydrogen ions.
- [1]
Equation 2 shows the oxidation of ferrous ions to ferric ions. This is an acidity consuming process which is slowed down at pH under 3. The bacteria ferro-oxidans acts as a catalyst for the reaction and greatly influences the reaction rate by accelerating the transformation of Fe2+ ions into Fe3+ (Kleinmann & Erickson, 1982). This second reaction is also highly dependent on the availability of oxygen.
2+ + 3+ Fe + 1/4 O2+ H Fe + 1/2 H2O [2]
Equation 3 presents the hydrolysis of ferric ions in the presence of water. This reaction usually takes place soon after the 2nd reaction. It produces a ferric hydroxide precipitate as well OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 5
as acidity in the form of hydrogen ions. The reaction rate of reaction 3 declines as pH decreases to 4.5 (Kleinmann& Erickson, 1982).
3+ + Fe + 3 H2O Fe(OH)3 + 3 H [3]
Equation 4 shows the oxidation of pyrite by ferric ions. This reaction produces much acidity and perpetuates the formation of acid mine drainage along with reaction 1 as pH decreases.
3+ 2+ 2- + FeS2 + 14 Fe + 8 H2O 15 Fe + 2 SO4 + 16H [4]
The release of acidic runoff water from tailings or waste rock pile can be harmful to the ecosystems through which it runs both in the short and long term. It is therefore necessary to either prevent the creation of AMD or to treat the resulting contaminated water.
Acid mine drainage prevention. The adverse effects of AMD can be eliminated either by preventing its formation or by treating the resulting contaminated water. Although prevention of AMD is more desirable and should be the goal whenever possible, emerging technologies to achieve it are not yet established with proven long-term track records. Contaminated water treatment methods on the other hand are well known and used throughout the mining industry.
Since AMD needs to be controlled in perpetuity, the option to collect and treat contaminated runoff water from tailings and mine waste rock piles is not accepted by many regulatory bodies as a viable long term reclamation option (Price and Errington, 1994).In Quebec, the “Guide sur la restauration minière” (1997) gives broad goals for the adequate reclamation of a mine site. It states that the site must be left in a satisfying state where the production and propagation of potential contaminants are limited. The guide also states that in the longer term, the goal should OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 6
be to eliminate any form of maintenance and monitoring. Therefore, AMD prevention is often the only option for long term mine reclamation and closure.
AMD prevention is done either by eliminating one (or more) of the elements necessary for acid generation or by controlling the parameters of acid generation at the source. As described before, the elements necessary for AMD are sulphide minerals, oxygen and water. The removal of any one of those elements will effectively prevent AMD formation.
Removal of active components. Sulphide removal from mine tailings can be accomplished by gravity separation and flotation as demonstrated by Stuparyk (1995) and
Benzaazoua et al. (2000). The resultant inactive low sulphur material can potentially be used for cover and dam construction for other measures of AMD prevention (Benzaazoua et al., 2008,
Demers et al., 2008). For cost effectiveness, not all tailings need to be de-sulphurized if the resultant material can be used for further AMD prevention measures (Benzaazoua & Kongolo,
2003).
Exclusion of water for AMD prevention is done through the use of impermeable barriers.
These can be synthetic liners or composite covers making use of a low saturated hydraulic conductivity material (such as clay) layer between coarse sand layers (Yanful & St-Arnaud,
1991). The long term efficiency of such covers is dependent on the rates of degradation due to heaving, subsidence and root penetration. In the case of geo-membranes, natural degradation of the membrane also takes place through the depletion of antioxidants (Rowe, Rimal, Arnepalli, &
Bathurst, 2010). Construction of these covers tends to be expensive when compared with other options. OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 7
Exclusion of oxygen can be an effective long-term acid generation control technique.
This approach requires the construction of a cover providing low oxygen diffusion characteristics. This may be achieved with the use of water, low permeability soils and synthetic membranes (or a combination of these materials) and by the use of covers with capillary barrier effect (CCBEs) (Bussière, Aubertin, Mbonimpa, Molson, & Chapuis, 2007, and Molson,
Aubertin, Bussière, & Benzaazoua, 2008).
Control of parameters. The sulphide oxidation process is dependent on temperature, pH and bacterial activity. Chemical and biological oxidation rates both decrease at lower temperatures. Bacterial activity is nearly non-existent below 3°C. Close to 0°C, chemical oxidation rate is only 15% of its value at 25°C (Knapp, 1987). Temperature below the freezing point also reduces the availability of liquid pore water for sulphide oxidation.
Sulphide oxidation rate is dependent on pH for both chemical and biological oxidation.
At an optimal pH range of 2-4, bacterial oxidation is several orders of magnitude greater than chemical oxidation (Otwinowski, 1994, and Lundgren & Silver, 1980). Sulphide minerals chemical oxidation rate is stable at pH < 3 and increases at pH above 3. If the pH of pore water can be maintained in the alkaline range, sulphide oxidation and the resulting acid generation can be decreased at least temporarily. pH may be controlled by blending sulphide minerals with acid consuming wastes or by adding alkaline material such as lime or limestone(Waybrant, Ptacek, &
Blowes, 2002, Cook, Skousen, & Hilton, 2008, and Wilson & Miller, 2011).
Bacterial activity, in appropriate conditions, is a catalyst of the acid generation process from sulphide minerals’ oxidation and can increase the oxidation rate by as much as 5-6 orders OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 8
of magnitude over chemical oxidation alone (Alpers and Nordstrom, 1991). Acid generation can be minimized with the application of bactericides to sulphide minerals, along with pH control.
Most bactericides are water soluble and therefore cannot be considered as a long term solution to sulphide minerals oxidation (Kleinmann, Crerar, & Pacelli, 1981).
Mine Site Reclamation in Northern Climates
In temperate climates, two of the most effective methods of tailings storage facilities reclamation are water and engineered (often called dry) covers (MEND 1.61.1). Engineered covers are made of natural or man-made materials placed over tailings materials. Their goal is to block or reduce the flux of oxygen reaching the sulphide material. Some engineered covers are designed to block or reduce water penetration through the cover. Although the term “dry barrier” is sometimes used, covers with capillary break effect require that one of its layers be kept saturated to be effective (Molson, Aubertin, Bussière, &Benzaazoua, 2008). In permafrost regions, engineered covers can be used as a thermal layer to isolate the underlying tailings from freeze-thaw cycles and keep them frozen.
Water covers. Water covers are placed over tailings material to reduce the availability of oxygen for sulphide oxidation. Although dissolved oxygen is present in the water cover, its availability is greatly reduced. Water acts as an oxygen barrier to decrease the rate of oxidation of sulphide rich tailings (Pederson, McNee, Flather, Sahami, Mueller, & Pelletier, 1997, Yanful
&Verma, 1999, and Yanful& Catalan 2002). Remaining oxidation may be manageable with other controls put in place. Water cover is a method widely used in temperate climate to OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 9
effectively mitigate the risk of acid mine drainage. With this reclamation method, the water table is kept above deposited tailings. The efficiency of the method is influenced by water depth above the deposited tailings, oxygen content of the water, wind and precipitation conditions and other factors. In a northern climate, one issue is ice formation. Ice itself may be beneficial by isolating the water column from the effects of wind and precipitations for much of the year (Barica, 1987, and Adams, 1976). On the other hand, a phenomenon known as ice scour can negatively affect the efficiency of the cover. If the water cover is fairly thin (less than 1m) the whole water cover may freeze during the cold period, trapping some of the tailings in the ice block. When spring thaw takes place, trapped suspended tailings are released in the water column and ice blocks, moved by wind and water movement, may disturb the water-tailings interface, causing re- suspension of the tailings, potentially exposing fresh tailings to oxidation and wind action (Ward,
Lawrence & McKinnon, 1994). Mitigation methods to alleviate the downsides of using water covers in a northern climate mainly play on reducing ice thickness or increasing water depth to reduce the likelihood of freezing tailings material at the water-tailings interface.
Integration to permafrost. Permafrost is defined as soil or ground, regardless of composition, that permanently remains under 0°C (Dobinski, 2011). It may be continuous over a region or be found in small scattered islands ranging in surface from a few square meters to several hectares. In Canada, over 50% of the land mass is under permafrost conditions.
Precipitations, temperature and other meteorological factors influence tailings reactivity, the efficiency of reclamation concepts and ultimately, the scale of potential contamination. This holds especially true for mining operations located in the arctic regions. Low temperatures slow OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 10
down most chemical and biological reactions regulating the acid generation process while the frozen ground reduces the migration potential of contaminants. Integration to permafrost is often used as a control method for mine tailings oxidation and AMD production. The goal of this approach is to keep the tailings frozen or to maintain their temperature close to the freezing point for most of the year (Kyhn & Elberling, 2001, and Elberling, 2005). However, acid mine drainage and heavy metal leaching were observed in some sites where this approach was taken.
Several researchers have attempted to determine more accurately the cold conditions at which potentially acid generating mine wastes will not generate contamination. These studies show that the presence of contamination in permafrost region may in part be attributed to the active zone
(soil layer at surface subjected to seasonal thaw) which allows the production of contamination, metal leaching and contaminant transport (Kyhn & Elberling, 2001).The thickness of the active zone depends mainly on the soil’s thermal and hydrogeological properties and meteorological conditions. To take this into account, reclamation concepts involving permafrost are usually done by way of cover construction over the tailings material with a non-reactive material which will act as the active layer (figure 1) going through freeze-thaw cycles as the underlying tailings material remain frozen.
Another aspect to consider is that the oxidation of sulphides still takes place at temperatures as low as -11°C. Freezing limits but does not prevent oxidation altogether
(Elberling, 2005).It is therefore suggested that tailings must be kept at temperatures below the freezing point to prevent contamination of drainage waters. Several authors have attempted to establish a maximum storage temperature of tailings, under which they would be virtually inert
(Godwaldt, 2001; Kyhn and Elberling, 2001; Meldrum, Jamieson and Dyke, 2001). Based on OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 11
field observations and laboratory, it was noted that in general, this technique may be viable for mining sites located in a climate where permafrost temperature is below -2 ° C and the annual average air temperature is below -8 ° C (Holubec, 2004). However, the relationship between air temperature and soil is complex and each site must be specifically assessed (Kyhn and Elberling,
2001; MEND, 2009).
Figure 1. Permafrost temperature profile (MEND, 2009)
In association with the decrease in oxidation rate, contaminant migration is also reduced when tailings are at a temperature below the freezing point. As water becomes solid, contaminants may be effectively trapped if the tailings material remains in frozen conditions.
Even though oxidation may still take place at a lower rate, contaminants do not escape the tailings storage facility and affect the surrounding environment. A caveat of this phenomenon is the depression in freezing point of pore water due to contaminants. Increased water salinity from OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 12
contaminants may depress the freezing point. Pore water pressure in the tailings will also show an effect on the available unfrozen water, with negative pressure (suction) favouring freezing point depression. The freezing point depression phenomenon was observed in tailings in permafrost conditions by Elberling (2001).
Effect of climate change. In addition to considering the climatic characteristics of each site to ensure proper management of tailings, it is becoming increasingly relevant to consider the impact of climate change, particularly in northern conditions where the environment is fragile and the effects of climate change magnified (MEND, 2006; Bussière and Hayley, 2010).
Predictions indicate that the Arctic could be the most affected region of the planet from global warming (Pachauri and Reisinger, 2007). Over the past few decades, temperatures in the Arctic have risen at nearly twice the rate as in the rest of the world. Average winter temperatures in the western Canadian Arctic have increased by up to 4°C (Arctic Climate Impact Assessment, 2004).
For the Nunavik area where the Raglan Mine is located, prediction modelling shows that the ground temperature may increase by 3 to 4°C by 2050 (Sushama, Laprise & Allard, 2006).
Although it may not seem like much, this magnitude of temperature increase is enough to significantly alter the thermal regime of the tailings stack (AMEC, 2007). The current climate change issue brings uncertainty on the perpetuity of the tailings cover concept in the eventuality of a future temperature increase that may cause an increase in the depth of the active layer subject to annual freeze-thaw cycles. In such a case, tailings would no longer be permanently frozen which could lead to AMD production. In the context of potential climatic changes, it is important to know the capacity of the current cover to prevent oxidation in the underlying OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 13
tailings material. This is important to ensure that the tailings stack is chemically stable in the long term and will not produce AMD in the future.
Raglan Tailings Storage Facility Conditions
Site description. Xstrata Nickel’s Raglan Mine has been in operation since 1998. First operated by Falconbridge it is now under the ownership of Xstrata Nickel since 2006. The site is located at 62nd parallel in the Ungava Peninsula of Nunavik, Quebec, Canada (Figure 2). The property extends over 55 kilometres on an east-west axis (Figure 3).
The mining property is divided into nine sectors: Katinniq, Mine 2, Mine 3, Kikialik,
West Boundary, Qakimajurq, Donaldson, Cross Lake and Deception Bay. The main site is
Katinniq where living quarters, the concentrator and several service buildings are located. Active underground mines are Kikialik, Mine 2, Mine 3 and Katinniq. An underground mine is in development at Qakimajurq. Decommissioned open pits are found at Kikialik, Mine 2, Mine 3,
Katinniq, West Boundary (Spoon) and Donaldson. The airport is located at Donaldson while seaport facilities are located at Deception Bay, 100 kilometres north of Katinniq. Cross Lake is at the early exploration phase.
The annual average temperature of the air is -10.3 ° C and total annual precipitation reaches 650 mm. Permafrost takes place to a depth of 586 meters under the surface and the active layer is about one meter. Minimum temperature of permafrost is 60 meters below the ground surface at -6.9 ° C (Fortier, 2006). In the first 150 meters below the active layer, the temperature is mostly between this minimum and -6 ° C. OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 14
Figure 2. Xstrata Nickel - Raglan Mine's location
Figure 3. Raglan property
Process description. The ore extracted from the underground mines is transported to the
Katinniq concentrator at a daily average of 4,000 tonnes. Of this material, 12% will come out of the mill as concentrate after being treated to separate non-economical minerals (tailings). Head OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 15
grade to the mill is currently maintained at 2.37% Ni, 0.63% Cu and 16% Fe. The mine produces a nickel – copper concentrate containing significant amounts of palladium, platinum, rhodium and cobalt. A total of 26 000 tonnes of nickel in concentrate is produced annually.
Upon arrival at the concentrator, the ore is stored in a 150-tonnes capacity underground silo. The ore goes through a24ft diameter semi-autogenous (SAG) mill (Figure 4). The output then passes through a vibrating screen to separate larger fragments which are re-circulated to the
SAG. Finer grains are conveyed to a ball mill to obtain a particle size of sands (80μm (potassium amyl xanthate) which attaches to the mineral pentlandite ((Fe, Ni)9S8) and chalcopyrite (CuFeS2) to form a hydrophobic compound. Pentlandite and chalcopyrite covered with xanthate attach to air bubbles generated by mixers and float to the surface. These bubbles overflow from the flotation cells and are recovered for further processing to obtain a concentrate of 10% Ni. The underflow from the “rougher” flotation cells are further processed through another flotation circuit offering increased residence time. The final concentrate is thickened, filtered, dried and stored until it is trucked to the Deception Bay port from which it is shipped to the Sudbury smelter and ultimately, to the Norwegian refinery. In parallel to concentrate production, the minerals that do not end up in the concentrate are recovered, thickened and filtered to produce the tailings material. These tailings are composed of the original rock, crushed and now devoid of nickel and copper (0.3% Ni, 0.15% Cu), but with a proportionally greater amount of iron sulphides (mainly from pyrrhotite), to a OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 16 content of about 18%. The tailings are transported by truck to the tailings storage facility (TSF) located 2 kilometres north-east of the concentrator. Figure 4. Raglan's concentrator flow diagram With an annual throughput of 1.3 Mt of ore processed by the concentrator, 1.1 Mt of tailings are generated and 200 000 tonnes of concentrate are shipped for further processing. Tailings storage facility. Raglan mine’s tailings’ storage facility, in function since the beginning of operations in 1998, is located on a local high ground about 2 km south-east of the Katinniq Complex (figure 5). The tailings material produced by the mine is sandy silt (about 70% silt size) with Sulphide Sulphurs content between 6 % and 8%, mainly contained in pyrrhotite (see section Materials and Methods for further details). The tailings slurry produced in the concentration process is dewatered using a thickener and filter-press to achieve 15% OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 17 gravimetric water content (mass of water / mass of dry solids). Tailings are then trucked to the tailings storage facility and deposited in “dry stacking” conditions (Bussière, 2007). The sulphide minerals contained in the tailings material will oxidize if left exposed to oxygen and water and has the potential to generate acid mine drainage (AMD) (Younger et al., 2002). The current cover concept aims to have the tailings become an integral part of the underlying natural permafrost. With the tailings frozen, the oxidation process is significantly reduced. The purpose of the non-reactive cover is to become the “active layer” that will go through the annual freeze- thaw cycle while the underlying tailings remain frozen even through the summer months. This type of cover is often called “insulation cover” in literature (Coulombe, Bussière, Côté & Garneau, 2012). The current cover concept for the tailings pile consists of two layers (figure 6): Layer 1: 1.2 m thick crushed rock material of minus 20 mm gradation with minimum 6% fines; and, Layer 2: 1.2 m thick coarse rock material of minus 600 mm gradation. The final design of the insulation cover was determined and revised by a consulting firm (AMEC, 2008) using thermal modelling to determine the thickness required for the granular cover. The design objective was to keep the tailings material frozen up to a warm year of 1:100 year recurrence. At the time, it was believed that integration to the permafrost was the best available option since it requires no long term maintenance and is sufficiently robust for the harsh environment. Climate change was taken into account at that time but only taking into OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 18 account the next 100 years. Current climate change prediction models were deemed too uncertain for longer projections into the future. Figure 5. Raglan Mine's Tailings Storage Facility Figure 6: Tailings storage facility insulation cover concept (AMEC, 2008) OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 19 Cover construction is done during the winter months over the areas where tailings deposition has reached the design elevation. Winter construction is favoured in order to promote permafrost propagation through the tailings and granular cover material. Placing the cover while temperatures are well below the freezing point ensures that the tailings material is frozen right under the cover during placement. Legal Context Planned reclamation. Progressive reclamation calls for building the final cover wherever the tailings stack is at its final elevation. Currently, the final cover is built only on a few test locations to monitor the propagation of permafrost underneath. Figure 7 shows the different areas of cover construction progress. The green area has the first 1.2m layer of 0-20mm material built. Five test pads of limited area are also built in this zone. The yellow area has a 0.4m layer of 0-20mm granular material. This cover is built to provide dust control measure and is considered sacrificial at this point (i.e. not part of the final cover design). The blue area is tailings material that reaches the final design elevation but remains uncovered by granular cover material. The red area is tailings material that has yet to reach its final elevation and remains uncovered. The active operational areas where tailings deposition occurs are within this zone. From the start of the mining operation, the plan was to build the final cover as the tailings were reaching their final elevation. However, as production of tailings carried on, it was decided to cover the tailings with the first layer of the granular cover only (1.2m of 0-20mm granular material) and to build test pads to monitor the efficiency of the full 2.4 meters cover before proceeding with the final construction over a larger area. Four test pads are built, one on each of OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 20 the covered slope as well as on the top of the tailings stack. Thermistor strings measuring ground temperatures within the cover and the underlying tailings material are placed in and under the test pads. There is no other instrumentation installed. Figure 7. TSF granular cover construction Following questioning regarding the efficiency of the current cover concept to prevent contaminant release in a context of climate change, it was decided by management to put the progressive reclamation on hold awaiting results from further studies. It was decided that a 0.4 meter layer of 0-20 mm granular cover would be placed over the tailings built to their final elevation. This thin layer is put in place as a dust control measure to prevent the contamination of the cover in place by tailings dust transported by wind. Legal requirements. Like all mining operations in Canada, Raglan falls under various federal, provincial and local laws and regulations. Concerning tailings reclamation at Raglan, the main legal requirements come from the Metal Mining Effluent Regulation (MMER) of the OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 21 Fisheries Act, Quebec’s Environmental Quality Act and Mining Act and the Raglan Agreement (Castrilli, 1999). The Raglan Agreement was signed in 1995 between the Makivik Corporation, the villages of Salluit and Kangiqsujuaq and the SociétéMinière Raglan du Québec Ltée. The Agreement is an integral part of the original Certificate of Authorization delivered by the Quebec Government under the Mining Act and as such, is legally binding in order to continue mining operations. The MMER applies to the reclamation of mining sites in that it prescribes the concentration limits for various contaminants released to the effluent (Metal Mining Effluent Regulations, 2012). It is therefore necessary that any runoff water from a reclaimed tailings storage facility meet these regulations. One of the prominent goals of tailings storage facility reclamation is meeting this regulation as well as Quebec’s Ministry of Sustainable Development, Environment and Parks’ Directive 019. The Directive 019 is associated to the Environmental Quality Act and is used whenever a mining project requires a Certificate of Authorization according to the Act (Environmental Quality Act, 2012). Although not yet a Regulation, the Directive is binding when associated with a Certificate of Authorization as is the case for Raglan Mine. Quebec’s Mining Act (2011) and associated regulations details the requirements for the submission of the rehabilitation and restoration plan. This plan must be approved by the authorities before mining operations may take place. The plan is to be reviewed and resubmitted every five years. The plan must contain details about the reclamation methods to be used as well as the associated costs. Once the plan is accepted by the authorities, the mining company must OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 22 provide a financial guarantee for the cost of planned reclamation. This may take the form of cash, gold bullion or a bank bond. The calendar for the deposit of the guarantee is determined by the authorities based on the mineral reserves and planned exploitation schedule of the mine (Ministère des Ressources Naturelles et de la Faune, 1997). The Raglan Agreement is an integral part of the original Certificate of Authorization delivered by the authorities (Société Minière Raglan du Québec Ltée, 1995). The document details all interactions between the mining company and the local communities; from land use to Inuit training and employment. Concerning the rehabilitation of the site after closure, the company agrees to apply mitigation measures to minimize environmental impacts. The company also agrees to “monitor and sample effluents from selected sites after shutdown until such time as effluents meet established criteria for 3 consecutive years” (SMRQ, 1995, Annex 4.2 p11).The revised Mining Act (2011) and Directive 019 now require five year of effluent sampling and monitoring after closure. Raglan Mine will therefore conform to the most stringent of its obligations. Results from required monitoring campaigns and modifications to the agreement go through an approval process with the Raglan Committee which includes representatives from the parties’ signatory of the Raglan Agreement. All new operational zones and expansions must be approved by the Raglan Committee who may require that new environmental impact assessments be performed. The Committee meets regularly and reviews project notices from the mining company. Rehabilitation and restoration plan submitted to authorities. The rehabilitation and restoration plan submitted to authorities details the reclamation activities to take place before, OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 23 during and after site closure. The requirements for the plan are detailed in Quebec’s Mining Act (2012). It should include: (1) the description of the rehabilitation and restoration work relating to the mining activities carried on by the person submitting the plan and intended to restore the affected land to a satisfactory condition; if tailings are present on the site, the required work shall include containment work and, if required, the work necessary for putting in place, operating and maintaining the infrastructure needed to prevent any environmental damage that might be caused by the presence of tailings; (2) if progressive rehabilitation and restoration work is possible, the conditions and phases of completion of the work; (3) the conditions and phases of completion of the work in the event of final cessation of mining activities; (4) an estimate of the expected costs to be incurred for completing the work. (Mining Act, 2012, 232.3) The rehabilitation and restoration plan must be submitted before operations begin and every five years thereafter with appropriate modifications reflecting changes in operations. Whenever significant modifications to the operational methods are made, a new plan must be submitted to the authorities (Mining Act, 2012, 232.6). The company may be released of its obligations when, according to the Minister: […] the rehabilitation and restoration work has been carried out in accordance with the rehabilitation and restoration plan approved by him and no sum of money is due to him OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 24 with respect to the performance of the work and, where there are tailings, they no longer present any risk of acid mine drainage. (Mining Act, 2012, 232.10) In this view, it is hard to imagine in which case a mine site with acid mine drainage generating tailings would be considered as no longer presenting any risk. It is therefore a possibility that despite their best efforts, the company wouldn’t be released of its obligations in the near future. In the plan submitted to the authorities by Raglan Mine, the reclamation concept for the tailings storage facility is described with its two 1.2 meter layers of granular material acting as a thermal barrier. It is stated that progressive reclamation will be performed whenever possible. Monitoring of runoff water will take place during construction and after closure until runoff water quality is satisfactory for a period of five years. The last rehabilitation and restoration plan was submitted in 2007 and a review will be submitted in 2012. The 2012 version will still present the current reclamation concept for the tailings storage facility awaiting the results of the current studies and the potential modification of the concept. If another concept is put forward, it will need to be approved by the authorities and by the Raglan Committee. Materials and Method The goals of this research are to test a newly developed method to measure the oxygen concentration under a granular cover at the tailings-cover interface and to determine the oxygen flux from sulphide minerals oxidation at the interface. This will in turn allow evaluating the OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 25 efficiency of the insulation cover at reducing sulphide minerals oxidation when compared with oxygen flux data from uncovered tailings. Studied Materials The placement of the testing apparatus required the removal of a portion of the 1.2 meter cover of 0-20mm granular material. During this operation, samples were taken of both the cover material and the underlying tailings material. 0-20mm Cover. The 0-20mm granular material used for the cover comes from quarries located within the final footprint of the tailings storage facility. The rock is sampled and tested for its geochemical stability before its use in the construction of the cover. Three criteria are used: the rock must contain less than 0.6% sulphur and less than 0.2% Nickel; and the NP/AP ratio must be greater than 4. The NP/AP ratio refers to the ratio between the neutralisation potential and the acidification potential measured in kg CaCO3/tonne. A NP/AP ratio < 1 usually means a material is acid generating. A NP/AP>3 means a material is non acid generating. A result between 1 and 3 falls into the uncertainty zone (Adam, Kourtis, Gazea and Kontopoulos, 1997). Raglan Mine allows itself a safety factor by using a criteria of NP/AP> 4. The acidification potential (AP) is calculated by the following formula: AP = 31.25 * Ssulph [5] Where AP refers to the acidification potential, 31.25 is the stoichiometric conversion factor and Ssulph is the Sulphur in sulphides content of the sample. OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 26 The neutralisation potential (NP) is measured by titration with chloridric acid after which the remaining acidity is measured and neutralisation potential determined (Plante, Bussière, & Benzaazoua, 2012). In order to meet the required thermal efficiency and perform effectively, the 0-20mm granular material must meet precise particle size characteristics determined by the designer. Particle size will influence water and air penetration through the cover and ultimately determine its efficiency to keep the underlying material frozen and integrated to the permafrost. The sample of 0-20mm granular material was taken from the wall of the excavation in the middle of the cover, about 0.6 meters below the surface. The material was tested for particle size gradation. Figure 8 presents the particle size curves for the material in blue as well as the requirements for particle size from the designer (AMEC) in green and purple. The larger diameter particles (over 425 µm) were measured using sieves with the ASTM E 11-87 standard. Smaller particles were measured using a Microtrac laser particle size analyzer (Lee Black, McKay et al., 1996). Under the Unified Soil Classification System (USCS) the granular 0-20 mm cover is classified as well graded gravel (GW). Figure 8 shows that the particle size curve for the material is outside of (but close to) the required parameters, a too large proportion of particles being over 200 µm in size. This may be due to particle migration within the cover where the finer particles migrate through the cover due to water percolation or wind erosion. An alternative is that the material was not respecting the size criteria when put in place. Although size analyses are performed when the granular cover material is produced, the results are currently unavailable. OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 27 Figure 8. Particle size curves for 0-20 mm granular cover material Table 1 presents some additional parameters for the studied materials such as specific gravity, sulfur content, saturated hydraulic conductivity, thermal conductivity and frozen thermal conductivity. The data from this table is from Coulombe et al. (2012) except for the specific gravity which was measured from samples taken during the installation of the testing apparatus described below. The specific gravity was measured with a helium pycnometer (Micrometrics Accupyc 1330) having an absolute precision of ± 0.01. Following the ASTM D854-91 standard, the specific gravity measured is 2.938 for the tailings samples and 2.936 for the 0-20mm cover. OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 28 Table 1 : Measured and calculated parameters for studied materials (adapted from Coulombe et al. 2012) Properties 0-20mm cover Tailings Specific gravity 2.936 2.938 Sulphur content (% 0.06 4.93 w/w) Saturated hydraulic 3.4 * 10-1 5.7 * 10-6 conductivity (cm/s) Thermal conductivity 1.45 1.43 (W / m °C) Frozen thermal conductivity (W / m 1.51 1.68 °C) Tailings. The tailings material underlying the cover was also tested for particle size as well as mineralogy. The particle size distribution is presented in pale blue in figure 8 above. Under the USCS, the tailings material qualifies as silt with low plasticity (ML). At the time of sampling, the material was frozen and ice lenses could be seen through the material. The material was kept in a sealed plastic bag to preserve its water content until it could be measured. Once above the freezing point, it was obvious that the water content was high as the material was akin to a wet mud. Table 1 presents the mineralogy of the tailings material studied. Mineralogical analysis was performed by X-Ray diffraction using a Bruker A.X.S. Advance D8 device. The Eva and Topas software was used to quantify (+/- 1.0%) and identify minerals. The analysis shows a significant amount of pyrrhotite with trace amounts of pentlandite, chalcopyrite and pyrite. Those minerals are the sources of sulphur required for acid mine drainage. Pentlandite is also the extracted ore mineral. Other minerals include chlorite, lizardite, actinolite, magnetite, and quartz. OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 29 Table 2: Mineralogy of tailings material Mineral Formula % − Chlorite ClO2 44,7% Lizardite 1T ((Mg, Fe)3Si2O5(OH)4) 16,9% Pyrrhotite 3T Fe(1-x)S 14,7% Actinolite Ca2(Mg,Fe)5Si8O22(OH)2 9,6% Magnetite Fe3O4 8,2% Pentlandite (Fe,Ni)9S8 3,1% Quartz SiO4 2,5% Chalcopyrite CuFeS2 0,4% Pyrite FeS2 0,1% Method Development Theory. As discussed previously, although low temperatures decrease the oxidation rate of sulphide tailings, oxidation still take place at temperatures as low as -11°C (Elberling, 2005). Since oxidation still takes place it is necessary to measure it by means other than relying solely on temperature data. The chemical reaction for the oxidation of sulphide minerals presented previously is oxygen consuming (see equation 1, p 4). Thus, measuring the oxygen concentration in a closed vessel where the reaction is taking place allows the measuring of the oxidation rate. The oxygen consumption test used to quantify the reactivity of acid generating tailings is based on Fick's law (defining gas diffusion in porous media) and the conservation law (Mbonimpa et al., 2003). The rate of acid production is controlled by oxygen availability; a decrease in the amount of oxygen in a known volume of air is a sign of an oxidation reaction. The reaction is controlled by the reactivity of the tailings material and by the diffusion rate of oxygen through the material (Demers, Bussière, Mbonimpa, & Benzaazoua, 2009). OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 30 Assuming a stable regime prior to the start of the experiment and that the sulphurs oxidation reaction is of the first order, Fick’s second law can be expressed as (Smith, 2004): [6] Where Deffisthe effective diffusion coefficient in the ground, C is the oxygen concentration, z is the distance in the flux direction and k is the Sulphur reaction rate (s-1) The conservation of mass equation for a finite volume air chamber placed above a reactive material is expressed by: [7] where V is the volume of the air chamber and A is the surface area. Assuming initial conditions C=C0 at time t0, the equation can be written to isolate unknown parameters of the studied material: [8] For a known volume and area, the slope of the ln(C/C0) vs time graph multiplied by the 0,5 ratio V/A is equal to (kDeff) . Parameters m and b from the first order equation y=mx+b describing the tangent to the curve of the graphs obtained from experimentation are derived by linear regression. Supposing a permanent regime, an oxygen concentration at tailings surface equal to oxygen concentration in the air and an oxygen concentration of 0 as depth z goes to infinite, the oxygen flux through the tailings surface area can be expressed as: [9] OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 31 2 2 2 The oxygen flux FL is expressed in moles of O /m /year or moles of O2/m /day (Elberling, & Nicholson 1994). This analytical interpretation is valid for short-duration tests assuming a steady-state condition prior to and during the test. It also requires relatively small headspace; the test is valid with a reduction of 2 to 3% of the oxygen concentration in the reservoir, from 20.9% to 17.9% (Mbonimpa et al., 2011). The method always underestimates the real oxygen flux acrosse tailings as demonstrated by Mbonimpa et al. (2011). Headspace, test duration and tailings degree of saturation also influence the precision of the oxygen consumption test. The work of Mbonimpa et al., 2011is used to estimate the precision of the oxygen consumption test for specific conditions and to determine if the analytical interpretation procedure is appropriate. The test is considered appropriate in this case, allowing comparing, on a relative basis, the performance of the cover to reduce oxygen flux against the oxygen flux previously measured at the surface of uncovered tailings. Oxygen Consumption Tests Traditional method (without cover), application of Elberling. The apparatus is set to create a confined air volume at the tailings surface. Oxygen and temperature sensors are placed in the confined air chamber and measure oxygen concentration and temperature as the oxidation reaction takes place. From the known volume of air through the experiment, the oxygen consumption is calculated (Bussière, Dagenais, Mbonimpa, & Aubertin, 2002).At ambient temperature, concentration of oxygen in the air is 20.9% v/v (or 8.9 mole/m³). The initial voltage reading on the oxygen sensor is assumed to be for that oxygen concentration. The oxygen sensor OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 32 reading for 0% oxygen concentration is 0. Based on that, a simple ratio allows calculating the oxygen concentration in the enclosed cell at the end of the experiment. Oxygen consumption is derived from those numbers. Non-covered oxygen consumption tests are performed with 30 cm long stainless steel cylinders of 15 cm diameter. The cylinder is inserted in tailings to leave an empty space (height “h”, as presented in Figure 9). An aluminum sealed cover equipped with an oxygen sensor and two valves is placed on the cylinder to create the sealed chamber. The oxygen sensor used is a Teledyne Class R – 17A. An insulated box protects the setup from the sun during testing to reduce fluctuations from the sensor. A control oxygen sensor is placed inside the insulated box but outside the sealed cylinder in order to monitor fluctuations of the oxygen concentration in ambient air. This data is then used to adjust the results from the oxygen sensor placed under the sealed chamber. Isolated box Oxygen sensor Valve Control oxygen Aluminum cap sensor h Air filled chamber Tailings Cylinder d Figure 9. Traditional test apparatus Modified method for cover performance assessment. To evaluate oxygen consumption at the tailings surface under the first layer of granular cover, the granular cover needs to be OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 33 removed until the tailings surface is reached. The apparatus is then set on the tailings surface and the cover material put back in place. Two flexible rubber tubes are used to allow replenishing the air at the tailings interface. A compressor was used to push air down one tube while the other one is left open to allow circulation (Figure 10). Readings from the oxygen sensor were monitored and air circulation was stopped once it reached the original open air reading, indicating that the oxygen concentration in the cylinder was back at 20.9% v/v. The tubes were then closed and the data recording started. A temperature sensor was placed in the tailings next to the apparatus to record temperatures during the tests. A rigid PVC pipe was used to protect the rubber tubes, oxygen sensor cables and temperature sensor cables from the cover material. A rigid plastic protection device was placed over the apparatus to prevent damage from the granular material cover. Figures 10 and 12 show the apparatus. Figure 10. Air circulation apparatus OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 34 PVC Protector pipe Pipe reducer Connections ABS pipe Flexible pipe Connection Protection device Crushed rockfill Tailings Figure 11. Modified apparatus for oxygen consumption test under the granular cover Figure 12. Cylinder with aluminum sealed cap prior to placement of cover This is the first time that oxygen consumption tests are performed under a granular cover at the tailings – cover interface. Previous testing protocols to assess the performance of particular covers were done from the surface of the cover (Tibble & Nicholson, 1997). From the results obtained and the problems encountered, modifications to the testing protocol are suggested to improve the testing method for future data gathering campaigns. OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 35 Results Oxygen consumption tests were performed in late August 2010 on the tailings storage facility south-eastern face. The cover had to be removed in four locations before a suitable tailings-cover interface was found. The first three locations showed water accumulating at the interface. The presence of water made it impossible to install the testing set up since water over the tailings would prevent or impede the oxidation reaction from taking place as described in the previous section on water covers. Once a suitable location was found, the stainless steel cylinder of 15cm length was installed in the tailings. Ice lenses were present in the tailings material. With the installation complete and the wires and tubes protected in a PVC pipe, the granular cover material was put back in place and testing began. The original reading on the oxygen sensor under the cover was lower by 2,0 mV from the readings in open air. It was decided to force air through the flexible tubing by way of a compressor until the oxygen sensor showed a reading similar to the open-air readings. Once that was the case, the tubes were blocked to close the system and the data acquisition system was started. The system used was Omega’s OM-CP Data Logger. Temperature readings were taken manually using a multi-meter to read the resistance of the thermistor. Temperature at the tailings surface under the cover was constant through the tests. The testing procedure was repeated four times at the same location over two days following testing of the apparatus. Fresh air was circulated in the cylinder between each test until the voltage reading was back to the original open air readings. Results are presented in Table 3. OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 36 Table 3: Oxygen consumption tests results, under 1,2m 0-20mm granular material cover Test # 1 2 3 4 2010-08-23 2010-08-23 2010-08-24 2010-08-24 Start Date & Time 08:59 14:53 09:30 15:40 Time length 343 620.5 353.5 993.5 (minutes) Temperature (°C) -0,2 -0,2 -0,2 -0,2 Start Voltage (mV) 7,835 7,795 7,745 7,720 Start Oxygen 20,9 20,9 20,9 20,9 Concentration (%) End Voltage (mV) 7,095 6,655 7,105 6,595 End Oxygen 18,9 17,9 19,2 17,9 Concentration (%) Slope of ln(C/C0) -2,84E-04 -2,39E-04 -2,42E-04 -1,49E-04 Regression r² 0,998 0,974 0,998 0,956 O Flux 2 58,43 49,22 49,76 30,72 (moles / m² / year) The tests varied in length from 343 minutes for Test #1 to 993 minutes for test #4. Tests #1 and #3 were interrupted before the oxygen concentration had dropped by a full 3%. The oxygen concentration had dropped by 2% and 1,7% respectively. Since it is the rate of decrease of oxygen concentration that influences the calculated oxygen flux, this shouldn’t have an incidence on results. Tests #1 and #3 were started in the morning and interrupted in the afternoon to circulate fresh air and start a new test. Tests #2 and #4 were left overnight and recorded data for longer periods. Only the data down to the 3% drop in oxygen concentration was kept for OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 37 calculations (Mbonimpa & Aubertin, 2011). The calculated regression coefficient r² between the measured data ln(C/C0) and the calculated regression points vary from 0,956 to 0,998 showing a good correlation between the data points and the calculated data. The graphical representations of the data for ln(C/Co) for the oxygen flux calculations for the tests under the cover are presented in Appendix B. The oxygen flux measured at the tailings surface under the cover varied from 30,72 moles / m² / year for test #4 to 58,43 moles / m² / year for the test #1 for an average oxygen flux of 47,03 moles /m² / year. From these results, we see that there is some oxidation taking place at the tailings surface under the cover, even at temperatures slightly under the freezing point as suggested by Elberling (2005) and discussed above. The results of previous tests performed on non-covered tailings are presented in table 4. These tests were performed in late June 2009 as a final project by a BEng candidate. Data from test station S03, test series 06, were adapted to respect the maximum 3% decrease in oxygen criteria (Mbonimpa &Aubertin, 2011). Data from that station closely match the test conditions of the 2010 tests under the cover. The location was similar as well as the length of the stainless tube used (15 cm). The length of the tests was much shorter than under the cover, at roughly 100 minutes. The calculated regression coefficient r² between the measured data ln(C/C0) and the calculated regression points vary from 0,993 to 0,999 showing a good correlation between the data points and the calculated data. For comparison purposes, it is assumed that testing conditions in June and August are similar and comparable and that it is therefore possible to OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 38 contrast the two data sets. Ideally, testing on the uncovered tailings surface and under the partial cover would have been run in parallel to ensure comparability of the data. Table 4 : Test # 1 2 3 4 2009-06-24 2009-06-25 2009-06-27 2009-06-28 Start Date & Time 14:32 19:16 08:49 12:44 Time length 125 105 110 85 (minutes) Temperature (°C) 3,16 1,12 10,31 11,22 Start Voltage (mV) 7,075 7,365 7,215 7,125 Start Oxygen 20,9 20,9 20,9 20,9 Concentration (%) End Voltage (mV) 6,065 6,355 6,205 6,090 End Oxygen 17,9 18,0 18,0 17,9 Concentration (%) Slope of ln(C/C0) -1,26E-03 -1,49E-03 -1,28E-03 -1,73E-03 Regression r² 0,999 0,996 0,993 0,993 O Flux 2 243,42 291,29 241,53 326,07 (moles / m² / year) Oxygen consumption tests results, no cover (adapted from Archambault, 2009) The calculated flux at the tailings uncovered surface varied from 241,3 moles / m² / year for test #3 to 326,07 moles /m² / year for test #4 for an average oxygen flux of 275,58 moles / m² / year. The graphical representations of the data for ln(C/Co) for the oxygen flux calculations for the tests without cover are presented in Appendix D. OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 39 The partial cover installed on the tailings storage facility appear to lead to a decrease in the oxygen flux measured at the tailings surface when compared to uncovered tailings, from an average of 275,6 moles / m² / year to 47,0 moles / m² / year or about a 5-fold decrease. The efficiency of the partial cover at reducing oxygen flow can be calculated as: [10] Where Eff is the efficiency of the cover at reducing oxygen flux, F cover is the average oxygen flux measured under the cover and F no cover is the average oxygen flux measured on the uncovered tailings. Using this approach, the calculated efficiency of the cover is 82.9%. Other Remarks Caveats The location chosen to perform the test was the fourth one dug out. Water was found at the interface between the cover and the tailings at the first three locations. The fourth location was dry, but this may be due to the temperature (see table 3) keeping the material frozen. Ice lenses were also observed, confirming the presence of water. Water is problematic for oxygen consumption testing since it may decrease or impede the oxidation rate that would be taking place if it was not present. It is therefore possible that the oxidation rates measured are underestimated. The differences in temperatures between the two series of tests (uncovered and covered) also make it problematic to perform a direct comparison. As discussed before, temperature is an important factor influencing the rate of oxidation and therefore the oxygen flux measured, at the tailings surface. Furthermore, the water content and saturation rate for the two OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 40 series of tests were not measured. These factors will also have an effect on the oxidation reaction rate. Future tests should take this into consideration and ensure that these parameters are measured and accounted for. When analyzing the cover material for size, it was discovered that the material was “out of specs” compared to the design. It appears that there might have been a migration of the finer material. This makes the remaining material more porous and modifies its thermal properties which may in turn affect the efficiency of the cover. The coarser material leads to greater hydraulic conductivity which could explain the water found in various locations at the tailings- cover interface. The tests performed were only done under the partial cover. In order to evaluate the performance of the complete 2,4 meter cover, further testing is required under the full cover. Oxygen Consumption Testing Under Cover The tests performed under the cover were the first of their kind in such conditions. As such, the testing apparatus was untested in such conditions and the tests should be seen as more of a “proof-of-concept” of the testing apparatus and the results from said testing should be viewed in that light. Regardless of that warning, the testing procedure did allow collecting some valuable data and allowed for comparison to previous collected data from uncovered tailings. Some drawbacks from the testing method and apparatus were identified and will need to be addressed for further testing. OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 41 Modification of Method for Further Testing One issue with the testing apparatus used was the lack of a data acquisition system for the temperature sensor. The sensor readings were taken manually but a data acquisition system would allow developing a better understanding of potential correlation between temperature and rate of oxygen depletion under the sealed cover. A second oxygen sensor could also be installed outside of the sealed steel cylinder as a control sensor. This sensor could be used to measure the ambient oxygen concentration and therefore adjust the data from inside the sealed cylinder accordingly. The flexible rubber tubing used to circulate fresh air inside the cylinder between tests was too flexible and collapsing and blockage of the tube was a possibility. The use of a rigid non- collapsible type of tubing is recommended. This would also facilitate fresh air circulation under the sealed cylinder. Fresh air circulation was problematic with the flexible tubing as there was no proper connection between the tubing and the compressor. No air could be felt from the outlet tubing either during circulation. The connection between the sealed cap and tubing was also problematic, relying on silicone caulk to ensure an airtight seal. In order to better understand and analyze the causes for the reduction in sulphide oxidation and therefore oxygen flux at the tailings interface under the cover, it is suggested that volumetric water content be measured during the tests for both covered and uncovered tailings. 0.5 As shown in equation [8], the oxygen flux is dependent on (kDeff) , where k is the sulphur reaction rate and Deff is the effective diffusion coefficient. From the measured value for volumetric water content, it is possible to calculate the saturation rate Sr (when the porosity is OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 42 known) which is directly related to Deff. From these measured values it would be possible to evaluate wether the decrease in the oxygen flux at the tailings interface under the cover compared to uncovered tailings is due to a decrease in the sulphur reaction rate k (from lower temperatures), from a decrease in Effective Diffusion Coefficient Deff caused by an increase in the degree of saturation Sr, or from both phenomenon. Conclusion Sulphide minerals oxidation causes acid mine drainage, a major issue with mining throughout the world. By measuring the oxygen consumption of sulphide minerals such as mine tailings, we can evaluate the potential for acid mine drainage. Oxygen consumption under the first layer of a thermal cover was measured in the tailings storage facility at Xstrata Nickel’s Raglan Mine. The previously untested method allowed measuring the oxygen concentration at the tailings – cover interface. The cover consists of a 1,2 meter layer of compacted 0-20mm non-reactive crushed rock fill. The data collected was then used to calculate the oxygen flux at the tailings surface. The oxygen flux under the cover could then be compared to the oxygen flux at the surface of uncovered tailings. Measurements lead to an average calculated oxygen flux at the tailings – cover interface under the cover of 47 moles / m² / year. An adaptation of data previously collected for uncovered tailings gives an average oxygen flux at the uncovered tailings surface of 276 moles / m² / year. The placement of the partial cover, half of the final design, lead to a 5-fold decrease of the oxygen flux at the tailings surface. These results show an efficiency of 83% for the partial cover in reducing the oxygen OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 43 flux at the tailings surface. However, presently, no minimum value of oxygen consumption is known to guarantee the control of AMD at the tested site. Although oxygen consumption is an indicator of possible AMD generation, it doesn’t prove that water contamination is taking place. Although there is a decrease of the oxygen flux with the placement of the 1,2m cover, there is still oxidation taking place as shown by the decrease in oxygen concentration under the sealed cylinder. The partial cover is therefore insufficient to completely prevent oxidation at the tailings interface. This information was not yet known from tests performed in the past. With this information, the company may now re-evaluate their tailings management and progressive reclamation strategies. In order to facilitate future oxygen consumption tests under a thermal granular cover, it is suggested that the testing apparatus be modified to include data acquisition for temperature sensors, a control oxygen sensor placed outside the sealed cylinder and rigid air circulation tubing. 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OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 53 Appendix A Details for Tests Under 0-20mm Cover OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 54 OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 55 Appendix B Slopes for ln(C/C0) Under 0-20mm Cover ln(C/C0) vs time, Test 1 Régression Points 0,0200 0,0000 0 50 100 150 200 250 300 350 400 -0,0200 ) 0 -0,0400 -0,0600 ln(C/C -0,0800 -0,1000 -0,1200 Time from t0 in minutes ln(C/C0) vs time, Test 2 Régression Points 0,0200 0,0000 -0,0200 0 100 200 300 400 500 600 700 -0,0400 ) -0,0600 0 -0,0800 -0,1000 ln(C/C -0,1200 -0,1400 -0,1600 -0,1800 -0,2000 Time from t0 in minutes OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 56 ln(C/C0) vs time, Test 3 Régression Points 0,0000 -0,0100 0 50 100 150 200 250 300 350 400 -0,0200 -0,0300 ) 0 -0,0400 -0,0500 -0,0600 ln(C/C -0,0700 -0,0800 -0,0900 -0,1000 Time from t0 in minutes ln(C/C0) vs time, Test 4 Régression Points 0,0000 -0,0200 0 100 200 300 400 500 600 700 800 900 1000 -0,0400 -0,0600 ) 0 -0,0800 -0,1000 -0,1200 ln(C/C -0,1400 -0,1600 -0,1800 -0,2000 Time from t0 in minutes OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 57 Appendix C Details for Tests on Uncovered Tailings OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 58 OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 59 Appendix D Slopes for ln(C/C0) for Tests on Uncovered Tailings ln(C/C0) vs time, Uncovered - S03 - 6in - Test #1 Régression Points 0,0000 -0,0200 0 20 40 60 80 100 120 140 160 -0,0400 -0,0600 ) 0 -0,0800 -0,1000 -0,1200 ln(C/C -0,1400 -0,1600 -0,1800 -0,2000 Time from t0 in minutes ln(C/C0) vs time, Uncovered - S03 - 6in - Test #2 Régression Points 0,0000 -0,0200 0 20 40 60 80 100 120 -0,0400 ) -0,0600 0 -0,0800 -0,1000 ln(C/C -0,1200 -0,1400 -0,1600 -0,1800 Time from t0 in minutes OXYGEN CONSUMPTION IN A NORTHERN CANADA TAILINGS STORAGE FACILITY 60 ln(C/C0) vs time, Uncovered - S03 - 6in - Test #3 Régression Points 0,0400 0,0200 0,0000 -0,0200 0 20 40 60 80 100 120 ) 0 -0,0400 -0,0600 -0,0800 ln(C/C -0,1000 -0,1200 -0,1400 -0,1600 Time from t0 in minutes ln(C/C0) vs time, Uncovered - S03 - 6in - Test #4 Régression Points 0,0000 -0,0200 0 10 20 30 40 50 60 70 80 90 -0,0400 ) -0,0600 0 -0,0800 -0,1000 ln(C/C -0,1200 -0,1400 -0,1600 -0,1800 Time from t0 in minutes