Canadian Geotechnical Journal. Antioxidant depletion in HDPE geomembrane with HALS in low ph heap leach environment. Draft

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1 Antioxidant depletion in HDPE geomembrane with HALS in low ph heap leach environment Journal: Manuscript ID cgj r1 Manuscript Type: Article Date Submitted by the Author: 16-May-2016 Complete List of Authors: Rowe, R. Kerry; Queens University, Abdelaal, Fady; Ain Shams University, Civil Keyword: Geomembranes, HDPE, Antioxidant depletion, Heap leach pads, Mining

2 Page 1 of 58 Antioxidant depletion in HDPE Geomembrane with HALS in low ph heap leach environment. R. Kerry Rowe 1* and Fady B. Abdelaal 2 *Corresponding author 1 Professor and Canada Research Chair in Geotechnical and Geoenvironmental Engineering, GeoEngineering Centre at Queen s-rmc, Queen s University, Ellis Hall, Kingston ON, Canada K7L 3N6. kerry.rowe@queensu.ca., Phone: (613) Fax: (613) Assistant Professor of Geotechnical Engineering, Ain Shams University, Cairo, Egypt. fady.mohamed@eng.asu.edu.eg, Phone: Fax:

3 Page 2 of 58 ABSTRACT Antioxidant depletion from a high density polyethylene geomembrane with hindered amine light stabilizers (HALS) immersed in seven different low ph solutions is examined over a 3-year period. The examined solutions had the range of ph (0.5, 1.25, and 2.0) likely to encompass the ph of the leach solutions found in copper, nickel, and uranium heap leach pads. The metal concentration for these solutions is adopted from copper raffinate solutions. Additional solutions are investigated to examine the effects of field practices such as using surfactants in the leach solutions and pre-curing of the ores used to improve the metallurgical response of the ore. For the antioxidants detected by standard oxidative induction time (Std-OIT), there was a depletion to residual value of about 20% of the initial Std-OIT that varied depending on the incubation temperature and ph of the solution whereas decreasing the ph from 2 to 0.5 did not significantly affect the depletion rates of Std-OIT. The antioxidants detected by high pressure oxidative induction time (HP-OIT) exhibited the fastest depletion in ph=1.25 with the highest residual values followed by ph 2.0 and the slowest HP-OIT depletion was in ph=0.5 but with the lowest residual values. Arrhenius modelling is used to predict the length antioxidant depletion stage for each solution based on both Std-OIT and HP-OIT. KEYWORDS: Geosynthetics, Geomembranes, HDPE, HALS, Antioxidant depletion, Heap leach pads, Mining, Low ph, Copper, Uranium, Nickel. 2

4 Page 3 of 58 INTRODUCTION The primary technologies used to extract metals from ore are (i) milling (crushing and grinding) followed by leaching or flotation, and (ii) heap leaching followed by metal extraction from aqueous phase (Christie and Smith 2013). According to Smith (2014), milling produces higher metal recoveries (85~90% of the contained metal versus 50~75% for heap leaching; Christie and Smith 2013) but with higher per-tonne operating costs whereas heap leaching allows more profitable metallurgical recovery from very low grade ore. Thus, heap leaching technology is used to process low grade deposits that were previously uneconomical to process with traditional milling operations although both types of technologies are used for projects with a wide range of ore grades (Christie and Smith 2013; Smith 2014). According to Breitenbach and Smith (2006) geosynthetics are used in mining applications in heap leaching of mineral-bearing rock, mill tailings disposal and evaporation/solar ponds for recovery of salts. Heap leaching is the largest applications of geomembranes in mining application (Breitenbach and Smith 2006). In nearly all cases, the leach pad area is lined with natural and geosynthetic materials (Lupo 2010). Heap leach operations rely on the performance of geosynthetic products to provide efficient solution recovery and environmental containment (Christie and Smith 2013). Heap leaching now provides 25-40% of the world s copper and gold, compared with ~2-3% in 1990, and consumes approximately 40% of the global geomembrane production (Smith 2014). However, there is a paucity of published research examining the chemical compatibility of high density polyethylene (HDPE) geomembranes with pregnant leach solution (PLS) from low ph heap leach pads applications for anything but very short-term conditions (as discussed in the next section). Thus, the primary objective of this study is to investigate the effect of ph and related metal concentrations found in different low ph heap 3

5 Page 4 of 58 leaching environments on the depletion of antioxidant from a HDPE geomembrane. The secondary objective is to explore the effect on antioxidant depletion of the heap leach field practices of using surfactants and/or pre-curing with a very acidic solution. BACKGROUND Heap leaching technology Heap leaching is one of several methods (in-situ leaching, dump leaching, pressure leaching and tank leaching) whereby metal ores are leached with various chemical solutions that extract valuable minerals (Thiel and Smith 2004). Heap leaching is utilized for the recovery of copper, uranium, gold, silver (at a very large commercial scale), nickel (pilot scale and limited commercial production), nitrate, iodine and other salts (Abdelaal et al. 2011; Christie and Smith 2013). In heap leaching, the ore from a mine (most commonly open pit) is blasted, loaded and transported to the primary crushers to be crushed and screened (Abdelaal et al. 2011). However, in some cases the ore is processed without crushing (run-of-mine) or only with primary crushing (Breitenbach and Theil 2006). To enhance metal recovery and minimize segregation of ore components, crushed ore could be agglomerated (most commonly by pre-wetting and adding chemical binders) prior to mixing in a drum to allow finer particles to adhere to coarse aggregate (Christie and Smith 2013). This reduces short-circuiting of leach solutions in the heap and creates a uniform wetting pattern over the ore (Defilippis 2005). The ore usually is delivered to the leach pads by overland and modular conveyors (Defilippis 2005) and staked in piles over the pad. The ore is then irrigated with solvents such as acids (typically week sulphuric acid for copper and uranium or strong sulphuric acid for nickel ores) or a high ph dilute cyanide solutions for gold and silver bearing ores (Lupo 2010). According to Christie and Smith (2013), 4

6 Page 5 of 58 leaching cycles could vary between few months (e.g., gold, silver, uranium and oxide copper) to more than a year (e.g., sulfide copper and nickel laterites). The leach solution containing the dissolved mineral [often called the pregnant leach solution (PLS)] is collected from the bottom of the pad to a lined PLS pond. The PLS is subjected to different processes to recover the desired metal and the spent solution is pumped back to a lined raffinate pond to be used in irrigating the next heap. In case of lower tenor pregnant solutions, the PLS is recirculated through the heaps to maximise the metal content before being pumped to the metal recovery plant (Christie and Smith 2013). For copper heap leaching, solvent extraction (SX) process is used to concentrate and purify the copper leach solution so that copper can be recovered at a high electrical current efficiency by electrowinning (EW) cells. This is done by adding a chemical reagent (Lixiviant) to the SX tanks which selectively binds with and extracts the copper (Abdelaal et al. 2011). The concentrated copper solution is then dissolved in sulfuric acid and sent to the electrolytic cells for recovery as copper plates (cathodes). According to Infomine (2007), nickel PLS is initially treated to precipitate the iron by raising the ph level then thickened and filtered in a precipitation plant. The liquor remains after the thickener process is further treated with soda ash to raise its ph to produce a nickel-cobalt hydroxide with a nickel content of above 30% that is filtered and packaged for shipment to refineries. For gold and silver PLS, carbon absorption or zinc precipitation are used to recover the precious metals (Christie and Smith 2013). Chemistry of the low ph heap leach operations The biggest application in terms of both tonnes leached and installed leach pad area is for extracting copper from sulfide and oxide ores (Abdelaal et al. 2011). Table 1 shows the chemistry copper PLS and raffinate solution in contact with the geomembrane liner. For copper, 5

7 Page 6 of 58 a typical (PLS) contains 1-5 g/l copper and up to 5 g/l of iron (Table 1) whereas the copper content is reduced in the raffinate pond after its recovery in the SX/EW process. The ph of copper PLS can range between 0.5 (especially at early times in the leach cycle where concentrated acid is added to the ore; Abdelaal et al. 2011) and 1.7 in a well operated heap (Jergensen 1999). The raffinate solution from the SX plant always contains some organic phase (a solution of copper extractant, diluted with low volatility kerosene based carrier; Defilippis 2005) and with higher acid concentration and hence lower ph than PLS. Pilot testing of mineral extraction from uranium ores with 0.1% uranium by heap leaching in a manner similar to copper is currently in progress (Hornsey et al. 2010) and in this application the PLS typically has a ph similar to those found in copper heap leaching (Abdelaal et al. 2011). Heap leaching is also being applied to nickel laterite and nickel sulfide ores (Steemson 2009; Christie and Smith 2013) as a cheaper alternative to high pressure acid leach plants (Infomine 2007). Acid usage in nickel heap leaching tends to be much higher than for copper or uranium with consumption rates on the order of 500 kg of acid per tonne of ore common (compared to less than 50 acid per tonne of copper ores). Additionally, the process produces a significant quantity of plant filtrate (chemical tailings) that require aggressive management and containment (Christie and Smith 2013). Higher temperatures are expected in nickel heap leaching, with 70ºC measured in pilot facilities and even higher temperatures are possible (Abdelaal et al. 2011). To improve the metallurgical response of the ore, several techniques could be used to enhance metal recovery (Christie and Smith 2013). Modern processes often pre-cure the ores with concentrated sulfuric acid (Thiel and Smith 2004). This is useful to satisfy the non-copper consumption and dissolve the readily soluble copper before the ore is placed on the pad during the agglomerating stage (Abdelaal et al. 2011). This effectively reduces the time required to 6

8 Page 7 of 58 leach the metal and allows a smaller leach pad area relative to the metal production rate. Thus, irrigation of the first lift can result in high (>20 g/l) copper tenor in PLS and may be accompanied by high free acid (10-20 g/l), especially if the operators get over exuberant with the acid addition which can happen at start-up (Abdelaal et al. 2011). Furthermore, temperatures up to 50 o C can be expected in such situations (Theil and Smith 2004). Another technique used by some operators involves adding surfactant with the leach solutions to decrease the surface tension which facilitates the seeping of the leach solution into the ores and hence, results in the increase of copper recovery by 5% (Marigold 1996). Furthermore, other methods such as air injection of sulfide copper ores, physical alteration of the ore by crushing and agglomeration, bio-leaching of sulphide copper are also used to enhance the metal recovery from the ore (Christie and Smith 2013). While these methods generally improve the metallurgical response of the ore, they are expected to change the chemical exposure conditions of the heap leach pad liner such as liner temperature, oxygen content in PLS, metal content, ph etc. For example, chalcopyrite (one of the most important copper minerals) was not amenable to heap leaching (Smith 2014) but with the aid of bio-leaching (forced aeration system that supplies low pressure air to the base of a heap to promote bacterial oxidation reactions in the heap; Defilippis 2005), metal recovery from the chalcopyrite ore is facilitated with optimum ore temperatures in the range of 50 to 70 C (Schrauf et al. 2014). To moderate the impact of seasonal ambient temperature fluctuations, thermal-cover geomembrane material are used in this case (Schrauf et al. 2014). While this technique is beneficial from the metal recovery standpoint, it could also increase the exposure temperature of the pad liner to higher than the ambient temperatures or than liner temperatures in copper oxide leaching operations and hence raise the concerns for durability issues of the geomembrane liners. 7

9 Page 8 of 58 Role of geomembranes in heap leaching The pad liner is usually either a single geomembrane or geomembrane with clay/geosynthetic clay liner (GCL) to act as a composite liner below a layer of permeable crushed rock drainage layer with a drainage pipe network (Thiel and Smith 2004). A double composite liner system, comprised of two geomembrane liners separated by a leak collection/drainage layer with the secondary geomembrane placed over a compacted liner bedding soil, is normally used in the case of high hydraulic heads (several meters), such as in valley leach facilities (Lupo 2010). Polymeric geomembranes usually used in heap leach pad liner systems are high density polyethylene (HDPE), linear low density polyethylene (LLDPE), polyvinylchloride (PVC), and polypropylene (PP) (Lupo 2010; Abdelaal et al. 2011; Rowe et al. 2013a; Christie and Smith 2013). Based on a survey of the geomembrane liner systems in 88 heap leach projects from 15 countries, Rowe et al. (2013a) reported that HDPE geomembranes were used in 75% of the cases, followed by LLDPE geomembranes in 22% of the cases, and polyvinyl chloride (PVC) in only 3% of the cases. The exposure condition for geomembrane liners in heap leach pads, is very different to that in municipal solid waste landfills where most of the research has previously been directed. These differences arise from the fact that in mining applications the geomembrane is exposed to extreme ph in addition to extremely high vertical pressures. Hence, heap leaching is one of the most aggressive service environments for geomembranes (Scheirs 2009). The stress level on the liner pad generally depends on the type of heap leaching. For static heaps where fresh ores are stacked on leached ores, some ore heaps are over 100 m in height with some approaching 240 m (Lupo 2010). In such cases the geomembrane liner is under overburden pressures exceeding 4 MPa (Lupo 2010; Rowe et al. 2013a). In a dynamic heap, where the leached spent ore is rinsed, removed and disposed in a dump and a lift of fresh ore is 8

10 Page 9 of 58 placed on the pad, the stresses on the liner mainly result from the ore handling equipment (Defilippis 2005; Christie and Smith 2013) with considerable horizontal loading caused by braking and turning (Christie and Smith 2013). The exposure conditions for the geomembrane differs from one place to another within the heap leach pad. According to Defilippis (2005), for the pad area immediately under the heap, in addition to the huge masses of the ore resulting from the successive staking of heaps, the geomembrane liner is exposed to the aggressive solution constantly irrigated throughout the pad (Defilippis 2005). In PLS pond, while the overburden pressures is almost negligible, the geomembrane liner has a continuous exposure to the corrosive acidic solutions that is constantly received by the pond (Defilippis 2005). The raffinate ponds share similar conditions to the PLS pond but with elevated organic content that could lead to swelling of the geomembrane liner. Effect of acidic environments on geomembrane liners Polymeric geomembranes under field conditions may experience degradation with time that ultimately lead to a decrease in their resistance to the sustained stresses imposed by the ore bodies in heap leach pads applications. Even in addition to, or in the absence of over burden pressures (e.g., in PLS and raffinate ponds), stresses also can be induced in the geomembrane due to wind/wave action (in ponds), wrinkles, foundation irregularities, seaming, differential settlement, down-drag on side slopes etc. Failure of the geomembrane liner (i.e., loss of its function as a hydraulic barrier layer) can be expected to occur if the geomembrane suffers sufficient degradation in its mechanical resistance under chemical exposure that it can no longer sustain these stresses. Degradation of polymeric geomembranes depends on the exposure environments. Geomembrane degradation mechanisms includes swelling, UV degradation, degradation by 9

11 Page 10 of 58 extraction, biological degradation, and oxidative degradation (Rowe and Sangam 2002). Conceptually, Hsuan and Koerner (1998) indicated that the chemical aging process of a HDPE geomembrane is divided into three distinct stages. In Stage I, the geomembrane start to deplete its antioxidants due to chemical consumption or physical extraction. In Stage II, while the geomembrane is without effective protection (i.e., antioxidants), it still retains its mechanical and physical properties during this induction period. This is followed by the stage where the geomembrane start to lose its mechanical and physical properties until nominal failure (Stage III). Nominal failure is reached when a selected property degrades to reach 50% of either the initial value (Hsuan and Koerner 1998) or the value specified (Rowe et al. 2009) in GRI-GM13 (2014). Immersion tests conducted according to ASTM D5322, D5747, or EPA 9090 (1992) test methods are used to evaluate the change of the chemical resistance of geomembranes due to exposure to liquid wastes, prepared chemical solutions, and leachates derived from solid wastes (e.g., Sangam and Rowe 2002; Müller and Jacob 2003; Gulec et al. 2004; Rowe et al. 2008; 2009; 2014; Abdelaal et al. 2014). If run for sufficient duration and at several elevated temperatures, they can be used to quantify the three stages of degradation for the geomembrane material at the expected field temperatures of the simulated application (e.g., Rowe et al. 2009; 2014; Abdelaal et al. 2014). However, immersion tests only simulate the chemical exposure of the geomembrane liner and hence could not be used to estimate the geomembrane service life under field conditions that are related to the formation of sufficient number of cracks in the geomembrane jeopardizing its performance as a hydraulic barrier layer. Previous immersion tests considered the evaluation of the chemical compatibility of geomembranes with heap leaching solutions included Smith et al. (1997) who examined the 10

12 Page 11 of 58 suitability of several geomembranes for copper leach pads. The study used the test methodology of EPA 9090 (1992) to examine the compatibility of HDPE, very low density polyethylene (VLDPE) and PVC with actual copper PLS provided by an operating SX/EW facility in Arizona. It was concluded that through this short-term testing, both HDPE and PVC are compatible with PLS used in this study while VLDPE exhibited a significant loss of physical properties. Using ASTM D 5322 method, Thiel and Smith (2004) immersed a 1.5 mm HDPE, a 1.5 mm LLDPE, and a 0.75 mm PVC geomembranes in 96% sulfuric acid for 120 days at 50 o C to investigate the geomembrane suitability for direct contact with concentrated H 2 SO 4 in processes involving precuring of the ores. The issue was raised based on a field case that showed a significant softening of the HDPE geomembrane with a 3% loss in tensile properties after very short term exposure to concentrated H 2 SO 4 and the concerns of geomembrane additive package and resin suppliers for exposure to such severe conditions. The results showed a loss of 64% and 73% of the initial standard (Std) oxidative induction time (OIT; ASTM D3895) after 120 days incubation for the HDPE and LLDPE geomembranes examined, respectively. Both the LLDPE and HDPE geomembranes exhibited less than 10% loss of the tensile strength and elongation at break within the 120 days of incubation. It was concluded that both types of the PE geomembranes examined performed better than expected. However, for the 0.75 mm PVC examined, there was a dramatic loss of flexibly even after one month of incubation. Upon immersion of the PVC in the 96% H 2 SO 4 during the first day of incubation, the solutions turned very dark singling a rapid loss of the plasticizers. After one month of incubation, there was a 74% loss of the tensile elongation at break indicating that the material has become brittle. It was concluded that apparently the PVC examined was not suitable for use in concentrated acid for pre-curing operations, even for relatively short exposure periods. However, the simulated test conditions by Thiel and Smith 11

13 Page 12 of 58 (2004) for the pre-curing were intentionally more aggressive than those experienced in the field. While the simulated acid concentration represented an upper limit of the acid concentration that may be added during pre-curing processes, it was maintained constant during the 4 months incubation duration which does not simulate the field conditions where the acid concentration are expected to decrease with time. The experiments described by Smith et al. (1997) and Thiel and Smith (2004) presented the short term performance of different types of geomembranes (HDPE, LLDPE, VLDPE, PVC) in acidic environments, however their results cannot be generalized for other geomembranes in the same groupings without addition study and the results should be regarded as specific to the products (i.e., their formulations and additive packages) tested. A study conducted by Gulec et al. (2004) involved a 1.5 mm HDPE geomembrane with a Std-OIT of 208 min and high pressure (HP) OIT (ASTM D 5885) of 484 min incubated in synthetic acid mine drainage (AMD), acidic water with ph =2.1, and deionized (DI) water. The AMD contained Fe (1500 mg/l), Zn (350 mg/l), Cu (35 mg/l), SO 4 (4500 mg/l) and Ca (200 mg/l). The phs of AMD and acidic water were adjusted using H 2 SO 4. The acidic water was used to distinguish the effects of metals and low ph on the geomembrane degradation while the DI water was used as the reference solution. The geomembrane ageing was conducted using stainless steel tanks for immersion tests at 20, 40, and 60 o C for an incubation period of 2 years. Their results showed a faster antioxidant depletion rate in synthetic AMD than in acidic and deionized water but slower than synthetic municipal solid waste leachate. The estimated antioxidant depletion time range between 46 and 426 years based on the field temperatures and whether the geomembrane is exposed from one side or two sides to the AMD. During the 2 years incubation, the melt index (MI; ASTM D 1238) and the Fourier transform infrared spectrum (FTIR) did not show consistent changes in polymer due to degradation. Although this 12

14 Page 13 of 58 study presented a longer-term incubation (2 years) of a HDPE geomembrane in an acidic media, it was relevant to acid mine drainage containment and not to low ph heap leaching where the exposure conditions is more aggressive in terms of the lower ph and the higher metal concentrations of the leach solutions. GSE (2014) previewed a case study for a 2.0 mm thick geomembrane used for a bottom liner of a copper dump leach pad and 8 attached solution ponds in Mongolia. The 100,000 m 2 lined site included a 56-m-high dump leach pad, four pregnant solution ponds connecting with geomembrane lined ditches, two raffinate ponds, and a waste impoundment. No information was given about the chemistry of copper PLS/raffinate solutions. The average yearly temperature in the site area can range from 21 to -26 o C. Due to different seasons, the water level of the ponds varied and a large portion of the geomembrane was exposed to weather conditions and UV radiation over long periods of time. After 16 years of exposure, samples of the liner were exhumed to be evaluated against the minimum specified properties by GRI-GM13 specifications. The exhumed samples showed no significant reduction in the physical and mechanical properties (density, tensile, tear, puncture, carbon black content and dispersion). However, these samples showed a reduction in (OIT) values due to depletion of the antioxidant over time but are still at relatively high and well within the specification of GRI-GM13. Based on calculations, this geomembrane was expected to continue working in its desired function for another 141 years. However, it was not mentioned whether the exhumed samples were below or above the solution levels. Another case was reviewed by Defilippis (2005) for a 2 mm HDPE geomembrane lining a PLS pond after about 4 years in service. The liner was evaluated due to continuous leaks that existed over time due to defects found in extrusion welds between the pond liner and the pump 13

15 Page 14 of 58 station liner. A laboratory test analysis was conducted for the geomembrane liner and showed that the geomembrane still retaining almost 90% of the mechanical resistance and flexibility. None of the previous literature addresses the question of how long it will take to deplete the antioxidant form an HDPE geomembrane used in acidic heap leach applications. The reminder of this paper addresses this question. EXPERIMENTAL INVESTIGATION ph solutions investigated Seven different synthetic solutions were examined in the current study. The solutions were prepared by mixing de-ionized water (ph 7.0) with different inorganic salts (Table 2). To adjust the ph, concentrated sulphuric acid (98%) was titrated until the target phs were achieved. To ensure a constant ph and prevent the build-up of antioxidant concentrations in the solution, the solutions were changed about every 1.3 months during the 36 months (3 years) of incubation. The ph of each fresh solution was checked and was in good agreement with the target ph (Table 3). The solutions also were analyzed during the experiments to ensure consistent concentrations of the different components throughout the testing duration and good agreement was obtained between the observed and target concentrations (Table 3). Solutions L1 (ph=0.5), L2 (ph=1.25), and L3 (ph=2.0) were the base-case solutions (Tables 2 & 3) investigated since they address the typical chemical composition and ph range relevant to copper PLS above the liner and raffinate solution (Queja et al. 1995; Jergensen 1999). In addition, the simulated range of ph encompass those found in uranium and nickel PLS solutions. While pre-curing of the ore has become an almost universal practice in copper heap leach projects and is adopted in many nickel and some uranium projects (Smith, personal 14

16 Page 15 of 58 communication), this practice raises the issue of HDPE compatibility with concentrated sulfuric acid (Thiel and Smith 2004). The exposure to concentrated acids in dynamic leach pads is more aggressive than in static heap leach pads (Thiel and Smith 2004). This is because in static heap leach pads, the acid content at the liner level would be diluted with time as the solution percolates down the lifts and the geomembrane would be only exposed to such high acid concentration during the first lift (Thiel and Smith 2004). For dynamic heap leach pads, the exposure of the geomembrane liner to high acid concentrations is repeated with each fresh charge of the ore on the liner for a certain period of time (depending on the ore type, leach cycles duration, etc) and then will be diluted with time (Thiel and Smith 2004). Thus, the geomembrane liner would be exposed to cyclic spikes in acidity for a certain period of time and between those spikes the geomembrane would be exposed to the "normal" PLS acidity. To simulate this exposure condition, Solution L4 (Table 2) was prepared with an acid content of 100 g/l of H 2 SO 4 (ph < 0) and the geomembrane was incubated in this solution for two weeks before being removed and incubated in the Solution L2: ph= 1.25 for ten weeks to simulate such cyclic exposure to concentrated acids. This incubation cycle is repeated every three months. It is known that the presence of a surfactant accelerates antioxidant depletion from HDPE geomembranes, (e.g., Rowe et al. 2008; 2014; Abdelaal et al. 2014; Abdelaal and Rowe 2014; 2015). Thus, the effect of Solution L1-S (ph=0.5+surfactant; Table 2) on antioxidant depletion was investigated to address the combined effect of surfactant that is sometimes added to the leach solution to enhance the permeability of the ore combined with low ph. Water with a ph = 0.5 (Table 2) was used as a control test to separate the effect of metals in copper PLS and low ph on the antioxidant depletion by allowing a comparison with the results obtained for Solution L1 which was also at ph = 0.5 but with the metals (Table 2). In addition, 15

17 Page 16 of 58 the sole effect of ph was investigated by also considering antioxidant depletion in di-ionised water with ph 7.0 to water with a ph = 0.5 (i.e., water + H 2 SO 4 ; Table 2). Solution L2-Cl is similar to solution L2 with the same ph of 1.25 but with high chloride concentration (boosted almost 15 times) to investigate the combined effect of low ph and extremely high chloride concentration. Geomembrane A 1.5 mm thick high density polyethylene manufactured by Solmax International, Varennes, Quebec in 2008 was investigated (Table 4). The initial standard oxidative induction time (Std- OIT; ASTM D3895) was 160 min predominantly due to a phosphite and phenol-based antioxidant package whereas the initial high pressure oxidative induction time (HP-OIT; ASTM D5885) of 960 min is associated with the presence of hindered amine light stabilizers (HALS) as part of the antioxidant package. The resin was a medium density, high molecular weight hexene copolymer with a resin density of g/cm 3 (ASTM D 1505). The manufactured geomembrane density was increased to g/cm 3 by the addition of the 2.5% (by mass) carbon black. The geomembrane met all the minimum requirements specified by GRI-GM13 (2014). Accelerating ageing and index testing for geomembrane Testing involved placing geomembrane coupons (190 mm x 100 mm) in 4-liter glass containers. The coupons were separated using 5 mm glass rods to ensure that the immersion solution was in contact with all surfaces of the coupons. The jars filled with the three primary low ph solutions (L1, L2, and L3) were incubated at temperatures of 40, 65, 75, 85, and 95 o C to allow more confident extrapolation of the time to antioxidant depletion to lower field temperatures. The 16

18 Page 17 of 58 effect of pre-curing using Solution L4 was investigated at 65 and 85 o C while the effect of surfactant in L1-S was investigated at four different temperatures (65, 75, 85, and 95 o C). Immersion in Solutions L2-Cl was investigated at 75, 85 and 95 o C while water-ph=0.5, and water-ph=7.0 was only at 85 o C. Coupons were periodically removed from the jars to allow specimens to be taken and then placed back in the jars. Standard oxidative induction time (Std-OIT; ASTM D3895) and high pressure oxidative induction time (HP-OIT; ASTM D5885) tests were performed on the specimens to allow monitoring of antioxidant depletion with time for the different test conditions examined. RESULTS Modeling of antioxidant depletion Previous investigators (e.g., Hsuan and Koerner 1998; Sangam and Rowe 2002; Müller and Jacob 2003; Gulec et al. 2004; Rowe et al. 2009; 2014; Abdelaal et al. 2014) modeled the antioxidant depletion in terms of Std-OIT by a first-order (2-parameter exponential model) decay function. In such case, the two parameters (initial OIT and depletion rate) were used to describe the change in the Std-OIT with time viz: OIT t ae st = (1) where OIT t (min) is the OIT value at time t, s (month -1 ) is the antioxidant depletion rate (month - 1 ), and a (min) is the initial OIT value (OIT o ). Among the different Std-OIT depletion curves presented in Fig. 1a, only Solution L1-S (with surfactant) followed the depletion pattern described by Eq. 1. The depletion of Std-OIT in Solution L1 followed a pattern of depletion to a high residual value (Fig. 1a) which has only 17

19 Page 18 of 58 previously been reported for the depletion of the HP-OIT data (e.g., Rowe et al. 2013b). Cases with high residual OIT can be modelled using a 3-parameter (OIT o, s, and OIT r ) exponential equation (Rowe et al. 2013b), viz: OIT =a e -st +OIT (2) t r where OIT t (min) is the OIT value at time t, a (min.) is the exponential fit parameter = OIT o - OIT r, s (month -1 ) is the antioxidant depletion rate, t is the incubation time (month), and OIT r (min) is the is the residual OIT value (i.e., OIT t OIT r as t ). The 3-parameter model given by Eq. 3 can be transformed into a 2-parameter model: OIT * =OIT * e -st t o (3) where OIT t * = (OIT t OIT r ) and OIT o * = (OIT o OIT r ) The third depletion pattern of the Std-OIT data in Fig. 1a was exhibited in Water ph=7 and Water ph=0.5. In this case, there was a clear difference in the early-time and later-time depletion rates. For this case, the Std-OIT data can be modeled (Fig. 1a) by superposition of two exponential decay functions with 4-parameters (Abdelaal and Rowe 2014) or for similar cases with a high residual values using 5-parameter, viz: OIT = a e -s1t + b e -s2t + OIT (4) t r where OIT t (min) is the OIT value at time t, s 1 (month -1 ) is the early (first) antioxidant depletion rate (month -1 ), s 2 (month -1 ) is the late (second) antioxidant depletion rate (month -1 ), t (month) is the incubation time, a and b are the exponential fit parameters where in this case a is the first rate (s 1 ) y-axis (OIT) intercept and b is the second rate (s 2 ) y-axis (OIT) intercept, a + b = (OIT o - OIT r ), and OITr (min) is the is the residual OIT value. 18

20 Page 19 of 58 Considering the HP-OIT data (Fig. 1b), the slow depletion of HP-OIT in Water ph=7 can be reasonable approximated using the 2-parameter model (Eq. 1) while the depletion in Solutions L1, L1-S, and Water: ph=0.5 can be modelled using the 3-paramater model (Eq. 2). OIT depletion in different low ph solutions examined Effect of low ph and metals found in heap leaching solutions Adding sulphuric acid to di-ionised water to decrease the ph from 7 to 0.5 increased the Std-OIT early depletion rate at 85 o C from 1.58 to 2.5 month -1 and increased the late time depletion rate from 0.03 to 0.35 month -1 (Fig. 1a and Table 5). Due to the faster depletion in Water at ph=0.5 than at ph=7, at ph=0.5 a Std-OIT residual value of around 11 min was reached after approximately 13 months and remained at this value for the following 17 months of monitoring. During the 13 months of Std-OIT depletion, HP-OIT data for water at ph=0.5 followed Eq. 3 and depleted with a rate of 0.27 month -1 until reaching a residual value of 500 min (Fig. 1b). Thus, during the 13 months of depletion to a residual value, there was linear relation between Std-OIT and HP-OIT depletion (Fig. 1c). In Water with ph=7, both the Std-OIT and HP-OIT were still depleting without reaching a residual value at the time of writing (30 months; Figs. 1a and b). In this case, the HP-OIT depleted linearly with the Std-OIT during the early time depletion of the Std-OIT then there was change in the depletion rate to the late time Std-OIT depletion (Fig. 1c). The above results showing the faster depletion of both Std-OIT and HP-OIT when decreasing the ph of the de-ionised water from ph 7 to 0.5 highlights the effect of acidic environments on the depletion of the antioxidants stabilizing the tested geomembrane and detected by both OIT tests. The effect of the high metals concentrations generally found in copper PLS and simulated in the current study in the synthetic solutions L1-L4 (Table 3) on antioxidant depletion can be 19

21 Page 20 of 58 inferred by comparing the OIT depletion of the geomembrane in Solutions L1: ph=0.5 and Water: ph=0.5 which differ in the high metal concentration available in L1. In Solution L1 at 85 o C, the Std-OIT and HP-OIT depleted with a single rate to relatively high residual values of 30 and 357 min after 27 and 36 months, respectively, and hence were modelled using Eq. 3 (Figs 1a and b). Thus, there was a linear relationship between the depletion of Std-OIT and HP-OIT during the 27 months of incubation until Std-OIT reached the residual value (Fig. 1c). Comparing the OIT depletion at 85 o C for L1 and Water at ph=0.5 (Fig. 1a and Table 5), it was found that adding the metals (i.e., Solution L1: ph=0.5) resulted in a decrease in Std-OIT depletion rate from 2.5 month -1 (Water) to 0.18 month -1 (L1) while the residual Std-OIT value increased from 11 min (Water) to 30 min (L1). Similarly, the HP-OIT depletion rate decreased from 0.27 month -1 (Water) to month -1 (L1). Thus, a high concentration of metals in a ph=0.5 Solution (L1) resulted in slower Std-and HP-OIT depletion rates than in Water ph=0.5, implying that the high concentration of metals simulating copper PLS used in this study was actually beneficial and reduced the rate of depletion of antioxidants and hence increased the time for antioxidant depletion to residual values (i.e., increased Stage 1 of the geomembranes degradation stages compared to just water at ph=0.5). The results suggest a synergetic effect of the metals and low ph on the antioxidants/stabilizers in the geomembrane. Effect of surfactant addition on OIT depletion Surfactant that is sometimes added during the irrigation of the ore was found to significantly increase the rate of antioxidant depletion from the HDPE geomembrane as is evident from the rate of Std-OIT depletion in Solution L1-S (ph=0.5 + surfactant) compared to that for L1 without surfactant (e.g., Fig. 1a) at all comparable temperatures (Table 5). For example, at 85 o C the Std-OIT depletion in Solution L1-S followed a single depletion rate (1.2 month -1 ) to a low residual value of about 3 min after only 4.3 months of incubation as compared to a rate of

22 Page 21 of 58 month -1 and a residual of 30 min after 27 months for L1. Similarly, there was also faster depletion of HP-OIT to residual in Solution L1-S (with surfactant) than in Solution L1 (e.g., Fig. 1b and Table 5). At 85 o C, the HP-OIT depletion rate in L1-S was almost an order of magnitude greater than in L1 and the HP-OIT depleted to a residual value of 717 min after only 4.3 months in L1-S compared to residual value of 360 min after 36 months in L1. Thus, adding surfactant to the leach solution greatly accelerated the depletion rates of the antioxidants detected by both the Std- and HP-OIT tests in presence of the high metal concentration at ph= 0.5 and can be expected to corresponding greatly reduce the length of Stage 1 of geomembrane degradation. Interestingly, the combination of surfactant and ph=0.5 (i.e., L1-S) may have reduced the absolute removal of some of the antioxidants only detected by HP-OIT (compared to Water and L1 at ph=0.5) resulting in such high residual HP-OIT value in L1-S. The implications of the high residual value are presently unknown. Effect of pre-curing on OIT depletion The effect of pre-curing the ore was investigated by immersing the geomembrane in Solution L4 (Table 2) for two weeks before incubating it in Solution L2 (ph= 1.25) for ten weeks to simulate the cyclic exposure to concentrated acids that may occur in the field. This immersion history is referred to herein as L4-Precuring. With L4-Precuring, at 85 o C there was a higher rate of depletion (0.27 month -1 ) than with simple immersion in L2 (0.2 month -1 ). The residual Std-OIT values were similar but slightly lower for L4-Precuring (20 min) than for L2 (23 min: Fig.2a and Table 5). Similarly, the HP-OIT depletion was slightly faster with L4-Precuring (1.6 month -1 ) than in L2 (1.0 month -1 ; Fig. 2b and Table 5). The residual HP-OIT values were very similar but slightly higher (700 min) for L4-Precuring than in L2 (664 min). Although the depletion pattern in both Std-OIT and HP-OIT was fairly similar, the exposure of the geomembrane to L4 for a 21

23 Page 22 of 58 two weeks cycle every three months did increase the Std-OIT and HP-OIT depletion rates compared to Solution L2 with a constant ph of 1.25 and can therefore be expected to slightly decrease Stage 1 of geomembrane degradation. Effect of a high chloride content on OIT depletion Substantially increasing the sodium chloride content in Solution L2 (to by more than three times that in sea water) to give Solution L2-Cl considerably complicated the overall response of the geomembrane to the solution compared to L2. At 95 o C the extra salt resulted in a higher Std- OIT depletion and a much lower residual value than in L2. However, at 85 (Fig. 2b) and 75 o C there was a slightly slower Std-OIT depletion rate and similar but slightly higher residual OIT values in L2-Cl than in L2 (Table 5). Thus at 85 and 75 o C the extra salt in L2-Cl had a mildly beneficial effect on depletion of Std-OIT. With respect to HP-OIT, substantially increasing the salt content increased the depletion rate (relative to L2) to 1.63 month -1 (L2-Cl) but there was no clear effect of temperature in the HP- OIT depletion at all three temperatures (95, 85 & 75 o C) examined (Table 5). Effect of different low phs on the OIT depletion Decreasing the ph from 2.0 to 0.5 resulted in a very small change in the Std-OIT depletion. Although the differences were small, the depletion rate was fastest for L2 with ph = 1.25 at all temperatures examined (Fig. 3a and Table 5). For instance, at 85 o C, the Std-OIT depletion rates were 0.18, 0.20, 0.18, month -1 for immersion in Solutions L1 (ph=0.5), L2 (ph=1.25), and L3 (ph=2.0), respectively with residual Std-OIT values of 30, 23, and 26 min, respectively (Table 5). The depletion rates and the residual values for the antioxidants detected by the HP-OIT varied significantly between the three solutions (Fig. 3b and Table 5), with the depletion rate being highest for L2 (ph=1.25) for all temperatures examined. The HP-OIT depletion rate was 22

24 Page 23 of 58 consistently lowest for L1 (ph=0.5). For example, the HP-OIT depletion rates at 85 o C were 0.094, 1.0, and 0.7 month -1 to residual HP-OIT values of 357, 664, and 590 min for phs 0.5, 1.25, and 2.0, respectively (Fig. 3b and Table 5). The variation in the HP-OIT depletion between the three solutions resulted in different patterns of the relationship between the Std-OIT and HP- OIT depletion (Fig. 3c). DISCUSSION The Std-OIT depletion pattern was different among the examined solution. Std-OIT depletion in Solution L1-S: ph=0.5+surfactant followed a single depletion rate to a low residual value (~ 3 min) and hence a first-order (2-parameter) decay function was used to to fit the data. In absence of surfactant, for solutions L1, L2, L3, L4, and L2-Cl the Std-OIT depleted according to a 3- parmeter model with a single depletion rate to relatively high residual Std-OIT values that varied depending on the solution. In water with ph=7 and ph=0.5, Std-OIT data exhibited quite different early-time (faster) and later-time (slower) depletion rates and hence a four-parameter exponential model was needed to fit the data. This demonstrates, how for the same geomembrane, there can be substantially different interactions between the antioxidants detected by the Std-OIT test and the different solution chemistries examined. This observation was also true for the antioxidants detected by the HP-OIT test although the nature of the interaction could be different for the antioxidants detected by the Std-OIT and HP-OIT tests. The faster depletion of both Std-OIT and HP-OIT when adding H 2 SO 4 to de-ionised water to decrease the ph from 7 to 0.5 indicates that acidic environments affected the depletion of the vast majority of antioxidants stabilizing the tested geomembrane. The faster depletion of Std- OIT at ph 0.5 could be attributed to the faster depletion of phosphites detected by the Std-OIT 23

25 Page 24 of 58 test by catalyzing their hydrolysis under such acidic conditions (Bauer et al. 1998; Papanastasiou et al. 2006; Ortuoste et al. 2006; Kriston 2010). For the HALS detected only by HP-OIT, Scheirs (2009) indicated that strong acids can interact with the basic HALS by neutralizing them to form non-active salts and hence HALS can be deactivated and suffer significant reduction in their effectiveness in such acidic environments. Furthermore, the extent of depletion of HALS by hydrolysis mainly depend on the type of HALS. For HALS based on a polyester structure such as Tinuvin 770 and Tinuvin 622, the polyester backbone is prone to hydrolytic and photolytic cleavage that can be accelerated by the presence of acids, whereas Chimassorb 944 is not prone to hydrolytic breakdown due to the absence of ester groups (Scheirs 2009). Thus, for the examined geomembrane, the specific depletion mechanism is unknown since the type of HALS used in stabilizing the tested geomembrane is a trade secret kept by the resin manufacturer and unknown to the user (or even to the geomembrane manufacturer in this case). What is known is the effect of acidic environments on the HALS that reduces its effectiveness and was demonstrated in the current study by comparing the depletion of the HP-OIT in water ph=7 to ph=0.5 (Fig. 1b). In acidic environments where HALS can be readily deactivated and decomposed, Scheirs (2009) indicated that polyolefin geomembranes would rely solely on the hindered phenolic antioxidants for oxidative stability. However, low basicity methylated HALS could be used to overcome such problem by offering higher stabilization of polyolefin geomembranes in such acidic environments than the commonly used more basic HALS (Scheirs 2009). Combing the effect of high metal concentration found in copper leach solutions with ph=0.5 to give Solution L1 substantially reduced the rate of Std-OIT depletion compared to water at both ph=0.5 and 7 (Table 5). The effect of ph and metals on the depletion of the antioxidants 24

26 Page 25 of 58 detected by HP-OIT was more complicated. Adding the high metal concentrations to ph=0.5 water decreased the rate of HP-OIT depletion compared to Water at ph=0.5 although it was still higher than for Water at ph=7. These results suggest that at ph=0.5, the presence of a high metal content in the solution inhibited the diffusion of antioxidants from the geomembrane to the solution (i.e., they had a beneficial effect in terms of rate of depletion) for the antioxidants represented by both the Std-OIT and HP-OIT tests. In presence of a high metal concentration at different phs, the chemistry of the solutions become complex and the reactions varied between the three low ph solutions (L1, L2, and L3). As a result, the intermediate ph (i.e., ph 1.25) was the most aggressive environment with respect to both Std-OIT and HP-OIT depletion. Reducing the ph to 0.5 seemed beneficial with respect to the depletion of the antioxidants detected by both Std and HP-OIT tests for this geomembrane. The very different response of the antioxidants detected by the Std-OIT test to those detected by the HP-OIT tests to a change in ph from 2.0 to 1.25 to 0.5 (other things being constant) highlighted the complex interactions that can occur between the different components of an antioxidant package and the chemical characteristics of the fluid with which it is in contact and that one can not infer the performance at one ph for that observed at a quite different ph for a given geomembrane. Pre-curing the geomembrane in the high concentration of acid (Solution L4) every three months and immersion in Solution L2 the rest of the time did not seem to significantly affect the OIT depletion compared to samples consistently incubated at ph However, when prepared at ambient temperature the high acid content gave rise to a temperature of Solution L4 of approximately 60 o C indicating that exothermic effects associated with the use of this solution could have an additional effect in accelerating the depletion of antioxidants not captured in the 25

27 Page 26 of 58 comparison between L2 and L4 since the increase in L4 temperature was masked by incubation at 85 o C for the test reported herein. The addition of considerable salt (Solution L2-Cl) demonstrated the potentially complex interactions that can occur between a solution chemistry and the depletion of antioxidants from a geomembrane. PREDICTIONS OF ANTIOXIDANT DEPLETION The OIT data obtained by incubating the tested geomembrane in three low ph solutions (L1: ph= 0.5, L2: ph= 1.25, and ph= 2.0) at five elevated temperatures (40, 65, 75, 85, and 95 o C; Figs 4-6) and Solution L1-S: ph=0.5+surfactant at four elevated temperatures (65, 75, 85, and 95 o C; Fig. 7) were used to obtain the Std-OIT and HP-OIT depletion rates (Table 5). These can be used to estimate the antioxidant depletion stage at lower temperatures than those used in the study by means of Arrhenius modeling. According to Koerner et al. (1992), if the activation energy (i.e., the slope of the linear relation between the natural logarithms of the laboratory depletion rates versus the temperature at which they were obtained) remains constant over the range of temperatures to be extrapolated, then the depletion rate can be predicted at these temperatures. Many researchers have adopted this approach based on the incubation of HDPE geomembranes in different media at temperatures between room temperature and 95 o C (e.g., Hsuan and Koerner 1998, Müller and Jacob 2003; Rowe et al. 2009, 2014; Abdelaal and Rowe 2014). In particular, Abdelaal and Rowe (2014) showed that for the case they considered the observed antioxidant depletion at 20 o C was within the limits of the 95% confidence level of the Arrhenius predictions based on data from temperatures between 40 and 95 o C. This suggests that temperatures as low as 20 o C may fall within the appropriate range of temperatures for which the 26