Antioxidant depletion in HDPE geomembrane with … copper heap leaching, ... other methods such as...
Transcript of Antioxidant depletion in HDPE geomembrane with … copper heap leaching, ... other methods such as...
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Antioxidant depletion in HDPE geomembrane with HALS in
low pH heap leach environment
Journal: Canadian Geotechnical Journal
Manuscript ID cgj-2016-0026.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
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Antioxidant depletion in HDPE Geomembrane with HALS in low pH heap
leach environment.
R. Kerry Rowe1*
and Fady B. Abdelaal2
*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. E-mail: [email protected]., Phone: (613) 533-3113. Fax: (613) 533-2128.
2 Assistant Professor of Geotechnical Engineering, Ain Shams University, Cairo, Egypt. Email:
[email protected], Phone: +2001000410743. Fax: +20226830947
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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.
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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
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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),
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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,
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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
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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 50oC 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.
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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
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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
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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
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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 50oC to investigate the
geomembrane suitability for direct contact with concentrated H2SO4 in processes involving pre-
curing 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 H2SO4 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%
H2SO4 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
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(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), SO4 (4500 mg/l) and Ca (200
mg/l). The pHs of AMD and acidic water were adjusted using H2SO4. 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 60oC 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
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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 m2
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 -26oC. 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
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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
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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 H2SO4
(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,
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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 + H2SO4; 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 0.936 g/cm3 (ASTM D 1505). The manufactured
geomembrane density was increased to 0.946 g/cm3 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 95oC to allow more
confident extrapolation of the time to antioxidant depletion to lower field temperatures. The
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effect of pre-curing using Solution L4 was investigated at 65 and 85oC while the effect of
surfactant in L1-S was investigated at four different temperatures (65, 75, 85, and 95oC).
Immersion in Solutions L2-Cl was investigated at 75, 85 and 95oC while water-pH=0.5, and
water-pH=7.0 was only at 85oC.
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:
st
t aeOIT −= (1)
where OITt (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 (OITo).
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
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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 (OITo, s, and OITr) exponential
equation (Rowe et al. 2013b), viz:
et r
-stOIT=a +OIT (2)
where OITt (min) is the OIT value at time t, a (min.) is the exponential fit parameter = OITo -
OITr , s (month-1
) is the antioxidant depletion rate, t is the incubation time (month), and OITr
(min) is the is the residual OIT value (i.e., OITt → OITr as t→ ∞). The 3-parameter model given
by Eq. 3 can be transformed into a 2-parameter model:
et o
* * -stOIT =OIT (3)
where OITt* = (OITt – OITr) and OITo
* = (OITo – OITr)
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:
e e1 2-s t -s t
t rOIT= a + b + OIT (4)
where OITt (min) is the OIT value at time t, s1 (month-1
) is the early (first) antioxidant depletion
rate (month-1
), s2 (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
(s1) y-axis (OIT) intercept and b is the second rate (s2) y-axis (OIT) intercept, a + b = (OITo -
OITr), and OITr (min) is the is the residual OIT value.
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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 85oC 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
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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
85oC, 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 85oC 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 0.094 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 85oC
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 0.18
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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 85oC, 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 85oC 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
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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 95oC 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 75oC
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 75oC 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 & 75oC) 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 85oC, 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
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consistently lowest for L1 (pH=0.5). For example, the HP-OIT depletion rates at 85oC 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 H2SO4 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
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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
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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 1.25. However, when prepared
at ambient temperature the high acid content gave rise to a temperature of Solution L4 of
approximately 60oC 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
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comparison between L2 and L4 since the increase in L4 temperature was masked by incubation
at 85oC 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 95oC;
Figs 4-6) and Solution L1-S: pH=0.5+Surfactant at four elevated temperatures (65, 75, 85, and
95oC; 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 95oC (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 20oC was within the limits of the 95% confidence level of the
Arrhenius predictions based on data from temperatures between 40 and 95oC. This suggests that
temperatures as low as 20oC may fall within the appropriate range of temperatures for which the
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current test temperatures combination (i.e., 40, 65, 75, 85, and, 95oC) can be extrapolated,
although caution is always required when extrapolating.
Depletion stage based on Std-OIT
The Std-OIT data of the three low pH solutions (L1: pH= 0.5, L2: pH= 1.25, and L3: pH= 2.0;
Figs. 4a-6a) were modeled using the 3-parameter exponential function (Eq. 3). Both the
depletion rates, s, and residual Std-OIT, OITr, values varied with the incubation temperature
(Table 5) and, hence, must be extrapolated to the temperature of interest to allow the prediction
of the antioxidant depletion time at that temperature. At test temperatures between 65 and 95oC,
the best fits obtained for the Std-OIT data in Figs 4a-6a allowed the direct evaluation of both s
and OITr based on the best fits of the Std-OIT data collected during the 3 years of incubation. At
40oC, 36 months incubation was not sufficient to reach the residual Std-OIT values and hence
did not allow direct estimation of OITr. The estimation of these depletion parameters at any
given temperature based on the available laboratory depletion data are discussed below.
To allow the extrapolation of Std-OITr to any temperature of interest, the Std-OITr data
obtained at temperatures between 65 and 95oC were used to establish an Arrhenius relation
between Std-OITo* (; Eq.3) and the test temperature (Fig. 8a). The Arrhenius equation for the
Std-OITo* values can be written as:
−
=−
RTEar
AeOITStd *
1
o
(5)
where Std-OITo* (month) = Std-OITo - Std-OITr, T (K) = temperature, Ear (J.mol-1
) = activation
energy related to Std-OITo*, A (month-1
) = a constant called a collision factor, R = 8.314 (J.mol-
1.K
-1) is the universal gas constant. Taking the natural logarithm of both sides of Eq. 5 gives:
)1
()()ln(*
1ln
TR
EA
OITStd
ar
o
×−=
− (6)
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Fig. 8a shows the Arrhenius relation obtained for Std-OITo* for low pH solutions L1, L2,
and L3. At the four temperatures plotted in Fig. 8a, Solution L1 had the highest residual Std-OIT
values (lowest Std-OITo*) whereas Solutions L2 and L3 had fairly similar residual Std-OITr
values (Table 5). Immersion in all three low pHs gave activation energies for Std-OITo* that
varied between 3.7 and 5 kJ.mol-1
(Fig. 8a). At 40oC, Table 6 Column [3] shows that the
estimated values of the residual Std-OITr (obtained by subtracting the predicted Std-OITo* values
from the initial Std-OITo) were 60, 44, and 47 min for immersion in Solutions L1, L2, and L3,
respectively. These predicted Std-OITr values at 40oC were used in estimating the best-fit for the
Std-OIT data at 40oC where depletion to residual had not been reached during the 36 months
incubation duration (Figs. 4a-6a). For Solution L5, the Std-OIT data depleted to a constant
residual values (≈ 3 min; Fig. 7a), and hence this value was used for the Std-OIT depletion stage
predictions at all temperatures.
The depletion rates (s) at temperatures of interests (Table 6) were obtained by constructing
an Arrhenius plot for the Std-OIT* depletion rates at the different test temperatures for the three
low pH solutions (Fig. 9). Similar to Eq. 5, the Arrhenius equation for the depletion rates (Hsuan
and Koerner 1998) can be written as:
e-(E /(RT))as = A (7)
where s (month-1
) = antioxidant depletion rate, T (K) = temperature, Ea (J.mol-1
) = activation
energy, A (month-1
) = a constant called a collision factor, R = 8.314 (J.mol-1
.K-1
) is the universal
gas constant. Taking the natural logarithm of both sides of Eq. 7 gives:
E 1aln(s) = ln(A) - ( )( )R T
(8)
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Fig. 9 shows the Arrhenius relation obtained for the Std-OIT depletion rates (s) from the 3-
parameter model for Solutions L1, L2, and L3 and 2-parameter model for Solution L1-S. The
activations energies for Solutions L1, L2, and L3 were fairly similar at around 53 kJ.mol-1
but
increased to 64 kJ.mol-1
for Solution L1-S. Despite the similarity in the depletion rates obtained
for Solutions L1, L2, and L3, the small difference combined with the difference in residual Std-
OITr values resulted in slightly different lengths antioxidant depletion stage based on Std-OIT
(Table 6). For example, it was predicted that the length of Stage I based on Std-OIT at 30oC was
around 54 years for L1, and 48 years for L2 and L3, showing that in pH=0.5 the geomembrane
had a longer length of Stage I than in the other pH solutions (L2 and L3) examined (Table 6;
Columns [3] and [4]). However, the difference was small and generally it would appear that in
solutions L1, L2 or L3 the time to Std-OIT depletion at 30oC for the geomembrane examined
would be a quite respectable approximately 50 years. In contrast, for Solution L1-S, the
predicted time to that at 30oC was three decades shorter at about 20 years compared to about 50
years in Solution L1 with similar chemistry but without the surfactant, highlighting the
significant effect of surfactant on the depletion of the antioxidants detected by Std-OIT.
Depletion stage based on HP-OIT
The HP-OIT depletion in Solutions L1, L2, L3, and L1-S followed similar depletion patterns to
the Std-OIT data and was modeled using the 3-parameter model (Eq.3; Figs 4b-7b). Thus, a
similar procedure to that presented in the previous section for Std-OIT data was followed in
obtaining the residual HP-OIT values and depletion rates at the temperatures of interest. Table 5
shows the variation of the residual HP-OIT values with incubation temperature and the pH of the
solution. An Arrhenius plot of HP-OITo* (HP-OITo - HP-OITr) using Eq. 5 and established using
the experimentally obtained data between temperatures of 65 and 95oC is presented in Fig. 8b.
This shows that the highest residual values were for Solution L1-S then L2: pH=1.25 then L3:
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pH=2.0 and finally the lowest residual value was in Solution L1: pH=0.5. This is an opposite
trend to the residual Std-OIT values where L1-S had the lowest residual Std-OIT and L1 had the
highest values. The activation energies of the Arrhenius plot of HP-OITo* varied from 5 kJ.mol-1
in Solution L1 (i.e., similar to that obtained from Arrhenius plot of Std-OITo*) to 23 kJ.mol-1
in
Solutions L1-S and L3. The predicated HP-OITr values at 40oC (Table 6) were 690, 860, 810,
and 900 min in Solutions L1, L2, L3, and L1-S, respectively. These values were used in
obtaining the best fits for the HP-OIT* data at 40oC since the residual values had not been
reached in the 3-years of incubation reported herein.
Depletion rates based on HP-OIT* were used to establish the Arrhenius plots (Fig. 10) using
Eq. 7 for Solutions L1, L2, L3, and L1-S. The activations energies increased from 61 kJ.mol-1
in
Solution L1 to around 74 kJ.mol-1
in Solutions L2 and L3 while the highest activation energy was
in Solution L1-S (80.4 kJ.mol-1
). These activations energies obtained for the HP-OIT* were
higher than those obtained based on the Std-OIT* data.
Based on the predicted HP-OIT* depletion rates and predicted residual HP-OITr depletion
values, Table 6 shows the predicted length of the antioxidant depletion stage based on the HP-
OIT test. In general, Solution L1 had the slowest HP-OIT* depletion rate and the lowest HP-OIT
residual values and hence had significantly longer predicted antioxidant depletion stage than in
any other solution. Similar to Std-OIT, Solutions L2 and L3 had fairly similar predicated
depletions times while L1-S again had the shortest predicted Stage I based on HP-OIT (Table 6).
For example, the predicted length of Stage I based on HP-OIT at 30oC was around 90 years for
L1, 11 years for L2, 15 years for L3, and 8 years for L1-S (Table 6 Columns [5] and [6]).
Discussion of Stage I (antioxidant depletion)
Based on the data at five different temperatures (40, 65, 75, 85 and 95oC) and Arrhenius
modelling, the length of Stage I based on Std-OIT was longer than that based on HP-OIT at all
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extrapolated temperatures between 20 and 90oC for immersion in all solutions investigated
except for Solution L1 (pH=0.5) which, in complete contrast, that a longer time to HP-OIT
depletion than Std-OIT at all predicted temperatures (Table 6). This was due to the slow
depletion of HP-OIT to lower residual values in Solution L1 than the other low pH solutions,
thereby increasing the length of Stage I based on HP-OIT. For instance, at 40oC, the predicted
depletion times based on Std-OIT/HP-OIT were 28/45, 25/5, 25/6 and 8.5/4.5 years in Solutions
L1, L2, L3, and L1-S, respectively (Table 6; Columns [4] and [6]).
Some previous studies (e.g. Abdelaal and Rowe 2015; Abdelaal et al. 2015) have shown that
a geomembrane with HALS can experience polymer degradation (i.e., enter Stage III) despite
high residual HP-OIT values and that degradation could begin following the depletion of
antioxidants detected by Std-OIT. However, in these previous studies the Std-OIT depleted to a
very low value (a few minutes) whereas in the current study both Std-OIT and HP-OIT depleted
to high residual values (except for Solution L1-S which behaved similar to previous studies). The
significance of both high residual Std-OITr and HP-OITr values is presently unknown but two
scenarios can be hypothesised for the three degradation stages. Scenario 1 envisages that the
residual antioxidants detected by Std-OIT are continuing to protecting the polymer from
degradation and hence the predictions presented in this papers are more conservative (i.e.,
shorter) than the real case and Stage I would be longer than predicted. Scenario 2 is that the
residual antioxidants are inactive in protecting the geomembrane and that after both residual Std-
OITr and HP-OITr are reached Stage II is entered and following that Stage III. In this case the
predictions in Table 6 represent the actual length of Stage I in immersion tests. Which scenario
represents the actual situation can only be established after sever more years of geomembrane
incubation with monitoring of the changes in the physical properties and will be presented in a
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future publication, however the antioxidant depletion stage is not likely to be shorter than
estimated in the current study.
CONCLUSIONS
The effect of incubation in solutions simulating low pH mining applications on the antioxidant
depletion have been examined for a HDPE geomembrane containing HALS at different
temperatures. Six synthetic solutions were examined with low pH simulating the pH of the
pregnant liquor from copper, nickel, and uranium heap leaching. Solutions L1 (pH=0.5), L2
(pH=1.25), and L3 (pH=2), had the same metal concentrations but a different pH. Case L4
simulated the pre-curing technique adopted in modern heap leaching whereby geomembrane
samples were immersed in a high acid content of 100 ml/L (Solution L4) for two weeks followed
by immersion in L2 with pH of 1.25 for the remaining ten weeks of the pre-curing cycle which
was repeated throughout the three years of incubation. Solution L1-S was similar to L1 (pH=0.5)
but with surfactant to simulate field practices involving the use of surfactant in spraying the ore.
Solution L2-Cl was similar to Solution L2 (pH=1.25) but had a high salt concentration
simulating some mining applications involving low pH and high total dissolved solids content.
Incubation in Water at pH=0.5 was used to isolate the effect of pH from the effect of high metal
concentrations on the antioxidant depletion. Incubation in Water at pH= 7.0 was used as the
reference case. The investigated HDPE geomembrane had an initial Std-OIT (Std-OITo) of 160
min, initial HP-OIT (HP-OITo) of 960 min, and initial stress crack resistance (SCRo) of 800
hours and met the minimum requirements specified in GRI-GM13 (2014). Arrhenius modelling
was used to predict the time to antioxidant depletion (Stage I) based on both Std-OIT and HP-
OIT. For the conditions examined, the following conclusions were reached:
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1. While the HP-OIT depleted with a pH dependant single depletion rate to high residual HP-
OIT values were both solution and temperature dependant, and hence a three-parameter
exponential model was needed to fit the data.
2. The Std-OIT depletion was different depending on the solution examined. With the presence
of a surfactant, there was a single depletion rate to a low residual value and a traditional first-
order (2-parameter) decay function could be used to to fit the data. For the other heap leach
solutions, there was a single depletion rate to relatively high residual Std-OIT values and
hence a three-parameter exponential model was needed to fit the data (i.e., similar to HP-OIT
depletion data).
3. A high concentration of metals in a pH=0.5 solution resulted in slower Std-and HP-OIT
depletion than water at pH=0.5, suggesting that the high concentration of metals in the heap
leach solution may inhibit the diffusion of antioxidants detected by both the Std-and HP-tests
and hence may actually be beneficial in terms of the ageing of the geomembrane at low pH.
4. With the same metal concentration, decreasing the pH from 2.0 to 0.5 resulted in only a small,
largely insignificant, change in the Std-OIT depletion. For HP-OIT, the fastest depletion rate
was for pH =1.25 then pH =2.0 then pH =0.5. This implies that the effect of decreasing the pH
from 2.0 to 0.5 was only evident for the antioxidants detected by HP-OIT (i.e., mainly
HALS). The lower pH=0.5 which may more beneficial for facilitating the metal recovery
from the ore, was also beneficial in terms better (slower) OIT depletion for the geomembrane
examined.
5. Pre-curing with a high acid content of 100 ml/L prior to immersion in a pH=1.25 solution
resulted in slightly faster Std-OIT and HP-OIT depletion rates than when just in the pH=1.25
solution, indicating that this process will have some influence on the geomembrane service
life. This high acid content also gave rise to a solution temperature of approximately 60oC
when prepared at room temperature. Thus, the temperature rise due to pre-curing may have a
more significant effect on service life than the increased chemical interaction.
6. Compared to the base case at pH=1.25, substantially increasing the total dissolved solids
concentration (by the addition of NaCl) at pH=1.25 accelerated the depletion of antioxidants
detected by HP-OIT (predominantly the HALS) while inhibiting the depletion of the
antioxidants detected by the Std-OIT.
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7. Based on Arrhenius modelling of the Std-OIT data for the heap leach solutions at pH of 0.5,
1.25 and 2.0 at five different temperatures (40, 65, 75, 85 and 95oC), the activation energies
were fairly similar (~53 kJ.mol-1
). However, activation energies obtained based on HP-OIT
were 61, 74.9, 73.6 kJ.mol-1
for pH of 0.5, 1.25 and 2.0.
8. For temperatures between 20 and 90oC, the predicted time to antioxidant depletion based on
HP-OIT was longer than based on Std-OIT test for pH = 0.5 while the opposite was true for
pH = 1.25 and pH = 2.0.
9. Predictions of time to antioxidant depletion based on both OIT test were longer for the heap
leach solution at pH=0.5 than at pH=1.25 or 2.0. For example, at 30oC, predications of Std-
OIT depletion were 54 years at pH=0.5 and 48 years at pH=1.25 and 2.0. This difference was
relatively small but it was greater in terms of predictions based on HP-OIT of around 90 years
for pH=0.5 compared to 11 years for pH=1.25 and 15 years for pH=2.0. This implies that the
best performance of the antioxidants stabilizing the examined geomembrane was in the lowest
pH.
10. Both Std-OIT and HP-OIT depleted to high residual values. Extended testing is required to
establish whether these high residual values are providing any protection to the geomembrane
from degradation of its mechanical and physical properties.
This study allows an assessment of how different low pH but high metal concentrations, and
various mineral extraction practices can affect the depletion of antioxidants from a
geomembrane. The results are specific to the particular geomembrane and solutions examined.
These tests reported herein do not directly represent field conditions since, as previously
demonstrated (Rowe and Rimal 2008; Rowe et al. 2010; Rowe et al 2013b), when a
geomembrane is immersed in solutions the depletion of antioxidants is substantially faster than
would be expected in the field and the estimated time to depletion can indicate relative effects
but, in absolute terms, are likely much shorted than they would be in the field.
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ACKNOWLEDGEMENTS
The research presented in this paper was funded by the Natural Science and Engineering
Research Council of Canada (NSERC) grant A1007, and used equipment provided by funding
from the Canada Foundation for Innovation (CFI) and the Ontario Ministry of Research and
Innovation. The authors are grateful to their industrial partners, Solmax International, AMEC
Earth and Environmental, Terrafix Geosynthetics Inc., the Ontario Ministry of Environment, the
Canadian Nuclear Safety Commission, AECOM, Golder Associates Ltd., Knight-Piesold, and
the CTT group for their participation in, and contributions to, the overarching project. The
authors are especially appreciative of the value of discussions with Rod McElroy (Senior
Metallurgist, AMEC Mining and Metals), Richard Thiel (President, Thiel Engineering) and Mark
Smith (President, RRD International Corp.). However, the opinions expressed in this paper are
solely those of the authors.
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Figure Captions
Fig. 1. Antioxidant depletion data at 85oC for Solutions L1, L1-S, Water: pH= 0.5, and Water:
pH=7.0 showing: (a) Std-OIT vs. incubation time; (b) HP-OIT vs. incubation time; (c)
Std-OIT vs. HP-OIT. Data points represent mean values and the error bars represent the ±
1 standard deviation.
Fig. 2. Antioxidant depletion data at 85oC for Solutions L2: pH=1.25, L4-precuring, and L2-Cl
showing: (a) Std-OIT vs. incubation time; (b) HP-OIT vs. incubation time; (c) Std-OIT
vs. HP-OIT. Data points represent mean values and the error bars represent the ± 1
standard deviation.
Fig. 3. Antioxidant depletion data at 85oC for Solutions L1: pH=0.5, L2: pH=1.25, and L3:
pH=2.0 showing: (a) Std-OIT vs. incubation time; (b) HP-OIT vs. incubation time; (c)
Std-OIT vs. HP-OIT. Data points represent mean values and the error bars represent the ±
1 standard deviation.
Fig. 4. Variation with incubation time of: (a) Std-OIT; (b) HP-OIT in L1: pH=0.5 solution at
different temperatures. Data points represent mean values and the error bars represent the
± 1 standard deviation.
Fig. 5. Variation with incubation time of: (a) Std-OIT; (b) HP-OIT in L2: pH=1.25 solution at
different temperatures. Data points represent mean values and the error bars represent the
± 1 standard deviation.
Fig. 6. Variation with incubation time of: (a) Std-OIT; (b) HP-OIT in L3: pH=2.0 solution at
different temperatures. Data points represent mean values and the error bars represent the
± 1 standard deviation.
Fig. 7. Variation with incubation time of: (a) Std-OIT; (b) HP-OIT in L1-S: pH=0.5+Surfactant
solution at different temperatures. Data points represent mean values and the error bars
represent the ± 1 standard deviation.
Fig. 8. Arrhenius plot for OITo* = OITo - OITr for: (a) Std-OIT; (b) HP-OIT in different low pH
solutions used in prediction of OITr values at different temperatures. Dotted vertical line
is at 40oC.
Fig. 9. Arrhenius plots of the Std-OIT depletion rates for Solutions L1: pH=0.5, L2: pH=1.25,
L3: pH=2.0; and L1-S:pH=0.5+Surfactant.
Fig. 10. Arrhenius plots of the HP-OIT depletion rates for Solutions L1: pH=0.5, L2: pH=1.25,
L3: pH=2.0; and L1-S:pH=0.5+Surfactant.
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Fig. 1. Antioxidant depletion data at 85oC for Solutions L1, L1-S, Water: pH= 0.5, and Water: pH=7.0 showing: (a) Std-OIT vs. incubation time; (b) HP-OIT vs. incubation time; (c) Std-OIT vs. HP-OIT. Data points represent mean values and the error bars represent the ± 1 standard deviation.
L1: pH=0.5L1-S: pH=0.5+SurfactantWater: pH=0.5Water: pH=7.0
Incubation time (months)
0 10 20 30 40
Std
-OIT
(m
in)
0
50
100
150
200
L1: pH=0.5L1-S: pH=0.5+SurfactantWater: pH=0.5Water: pH=7.0
Incubation time (months)
0 10 20 30 40
HP
-OIT
(m
in)
0
200
400
600
800
1000
L1: pH=0.5L1-S: pH=0.5+SurfactantWater: pH=0.5Water: pH=7.0
ln[Std-OIT (min)]
0 1 2 3 4 5
ln[H
P-O
IT (
min
)]
5
6
7
(b) 85oC HP-OIT
(a) 85oC Std-OIT
(c)
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Fig. 2. Antioxidant depletion data at 85oC for Solutions L2: pH=1.25, L4-precuring, and L2-Cl showing: (a) Std-OIT vs. incubation time; (b) HP-OIT vs. incubation time; (c) Std-OIT vs. HP-OIT. Data points represent mean values and the error bars represent the ± 1 standard deviation.
L2: pH=1.25L4-PrecuringL2-Cl: pH=1.25+Chlorides
Incubation time (months)
0 10 20 30 40
Std
-OIT
(m
in)
0
50
100
150
200
L2: pH=1.25L4-PrecuringL2-Cl: pH=1.25+Chlorides
Incubation time (months)
0 10 20 30 40
HP
-OIT
(m
in)
0
200
400
600
800
1000
L2: pH=1.25L4-PrecuringL2-Cl: pH=1.25+Chlorides
ln[Std-OIT (min)]
2 3 4 5
ln[H
P-O
IT (
min
)]
6
7
(b) 85oC HP-OIT
(c)
(a) 85oC Std-OIT
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Fig. 3. Antioxidant depletion data at 85oC for Solutions L1: pH=0.5, L2: pH=1.25, and L3: pH=2.0 showing: (a) Std-OIT vs. incubation time; (b) HP-OIT vs. incubation time; (c) Std-OIT vs. HP-OIT. Data points represent mean values and the error bars represent the ± 1 standard deviation.
L1: pH=0.5L2: pH=1.25L3: pH=2.0
ln[Std-OIT (min)]
3 4 5
ln[H
P-O
IT (
min
)]
5
6
7
L1: pH=0.5L2: pH=1.25L3: pH=2.0
Incubation time (months)
0 10 20 30 40
Std
-OIT
(m
in)
0
50
100
150
200
L1: pH=0.5L2: pH=1.25L3: pH=2.0
Incubation time (months)
0 10 20 30 40
HP
-OIT
(m
in)
0
200
400
600
800
1000
(b) 85oC HP-OIT
(a) 85oC Std-OIT
(c)
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Fig. 4. Variation with incubation time of: (a) Std-OIT; (b) HP-OIT in L1: pH=0.5 solution at different temperatures. Data points represent mean values and the error bars represent the ± 1 standard deviation.
Incubation time (months)
0 10 20 30 40
Std
-OIT
(m
in)
0
50
100
150
200
Incubation time (months)
0 10 20 30 40
HP
-OIT
(m
in)
0
200
400
600
800
1000
1200
40oC65oC75oC85oC95oC
40oC65oC75oC85oC95oC
(a) Std-OIT L1: pH=0.5
(b) HP-OIT L1: pH=0.5
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Fig. 5. Variation with incubation time of: (a) Std-OIT; (b) HP-OIT in L2: pH=1.25 solution at different temperatures. Data points represent mean values and the error bars represent the ± 1 standard deviation.
40oC65oC75oC85oC95oC
Incubation time (months)
0 10 20 30 40
Std
-OIT
(m
in)
0
50
100
150
200
40oC65oC75oC85oC95oC
Incubation time (months)
0 10 20 30 40
HP
-OIT
(m
in)
0
200
400
600
800
1000
1200
(a) Std-OIT L2: pH=1.25
(b) HP-OIT L2: pH=1.25
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Fig. 6. Variation with incubation time of: (a) Std-OIT; (b) HP-OIT in L3: pH=2.0 solution at different temperatures. Data points represent mean values and the error bars represent the ± 1 standard deviation.
40oC65oC75oC85oC95oC
Incubation time (months)
0 10 20 30 40
Std
-OIT
(m
in)
0
50
100
150
200
40oC65oC75oC85oC95oC
Incubation time (months)
0 10 20 30 40
HP
-OIT
(m
in)
0
200
400
600
800
1000
1200
(a) Std-OIT L3: pH=2.0
(b) HP-OIT L3: pH=2.0
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Fig. 7. Variation with incubation time of: (a) Std-OIT; (b) HP-OIT in L1-S: pH=0.5+Surfactant solution at different temperatures. Data points represent mean values and the error bars represent the ± 1 standard deviation.
65oC75oC85oC95oC
Incubation time (months)
0 5 10 15 20
Std
-OIT
(m
in)
0
50
100
150
200
65oC75oC85oC95oC
Incubation time (months)
0 10 20 30 40
HP
-OIT
(m
in)
0
200
400
600
800
1000
1200
(a) Std-OIT L1-S
(b) HP-OIT L1-S
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Fig. 8. Arrhenius plot for OITo* = OITo - OITr for: (a) Std-OIT; (b) HP-OIT in different low pH solutions used in prediction of OITr values at different temperatures. Dotted vertical line is at 40oC.
L1: pH=0.5L2: pH=1.25L3: pH=2.0
Solution Ea
(kJ/mol) Arrhenius Equation R2
L1 5 ln(1/Std-OIT*)=4.1+644/T 0.99 L2 3.7 ln(1/Std-OIT*)=4.6+412/T 0.98 L3 4.4 ln(1/Std-OIT*)=4.5+468/T 0.98
1/Temperaturex10-3 (K-1)
2.6 2.8 3.0 3.2
ln[1
/std
-OIT
* (
mo
nth-1
)]
5.4
5.5
5.6
5.7
5.8
5.9
6.0
6.1
6.2
L1: pH=0.5L2: pH=1.25L3: pH=2.0L1-S: pH=0.5+Surfactant
Solution Ea
(kJ/mol) Arrhenius Equation R2
L1 5 ln(1/HP-OIT*)=-1.3+1984/T 0.99 L2 12 ln(1/HP-OIT*)=-2.2+2544/T 0.90 L3 23 ln(1/HP-OIT*)=-1.3-2179/T 0.90
L1-S 23 ln(1/HP-OIT*)=-2.6+2771/T 0.96
1/Temperaturex10-3 (K-1)
2.6 2.8 3.0 3.2
ln[1
/HP
-OIT
* (
mo
nth
-1)]
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
(a) Std-OIT
(b) HP-OIT
o o
o
o
o
o
o
o
o
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Fig. 9. Arrhenius plots of the Std-OIT depletion rates for Solutions L1: pH=0.5, L2: pH=1.25, L3: pH=2.0; and L1-S:pH=0.5+Surfactant.
Fig. 10. Arrhenius plots of the HP-OIT depletion rates for Solutions L1: pH=0.5, L2: pH=1.25, L3: pH=2.0; and L1-S:pH=0.5+Surfactant.
L1: pH=0.5L2: pH=1.25L3: pH=2.0L1-S: pH=0.5+Surfactant
Solution Ea
(kJ/mol) Arrhenius Equation R2
L1 52.8 ln(s)=15.9-6351/T 0.97 L2 52.7 ln(s)=16.1-6339/T 0.99 L3 53.6 ln(s)=16.2-6445/T 0.98
L1-S 64.3 ln(s)=21.5-7743/T 0.96
1/Temperaturex10-3 (K-1)
2.6 2.8 3.0 3.2
ln[s
(m
onth
-1)]
-8
-7
-6
-5
-4
-3
-2
-1
0
1
2
3
Std-OIT
L1: pH=0.5L2: pH=1.25L3: pH=2.0L1-S: pH=0.5+Surfactant
Solution Ea
(kJ/mol) Arrhenius Equation R2
L1 61 ln(s)=18.1-7328/T 0.96 L2 74.9 ln(s)=25.2-9013/T 0.98 L3 73.6 ln(s)=24.3-8849/T 0.99
L1-S 80.4 ln(s)=26.9-9670/T 0.99
1/Temperaturex10-3 (K-1)
2.6 2.8 3.0 3.2
ln[s
(m
onth
-1)]
-10-9-8-7-6-5-4-3-2-101234
HP-OIT
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Table 1. Chemical composition of the different solutions found in copper heap leach pad
operations concentrations are in mg/L unless otherwise note.
Metal
Copper pregnant
leach solution
(Queja et al. 1995)
Copper pregnant leach
solution
(AMEC Mining and Metals,
personal communication)
Copper raffinate
solution
(Jergensen, 1999)
Aluminum -- 4500 4500
Arsenic 3.8 -- 0.25
Cadmium 0.33 -- 1.7
Calcium 58 -- --
Chromium 4.2 -- 0.32
cobalt -- -- 20
Copper 1700 1000-5000 87
Iron 1400 up to 5000 1300
Lead 1.3 -- 1.4
Lithium -- 1000 1000
Magnesium 4600 3300 3300
Manganese 600 -- 750
Mercury 0.002 -- <0.0002
Nickel -- -- 7.6
Potassium 210 -- <2.0
Sodium 250 -- 11
Zinc -- -- 110
Sulfates 71400 -- 46000
Surfactant (ml/L) 8 -- --
pH 1.8 0.5-1.8 1.7
-- = unknown concentration
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Table 2. Composition of different synthetic solutions used in current study.
Component Formula Unit Water
pH=0.5 L1 L2 L3 L4 L1-S L2-Cl
Inorganic salts
Aluminum Sulfate Hydrate Al2S3O12•20H2O mg/L -- 51,200 51,200 51,200 51,200 51,200 51,200
Cadmium Sulfate 8/3hrdrate CdSO4•2.7H2O mg/L -- 3.2 3.2 3.2 3.2 3.2 3.2
Calcium Sulfate Hemihydrate Ca SO4•0.5 H2O mg/L -- 1,900 1,900 1,900 1,900 1,900 1,900
Cobaltous Sulfate Heptahydrate CoSO4•7H2O mg/L -- 96 96 96 96 96 96
Copper sulphate Anhydrous CuSO4 mg/L -- 220 220 220 220 220 220
Ferrous Sulfate Heptahydrate FeSO4•7H2O mg/L -- 3,600 3,600 3,600 3,600 3,600 3,600
lead Sulfate PbSO4 PbSO4 mg/L -- 2.1 2.1 2.1 2.1 2.1 2.1
Lithium Chloride LiCL LiCL mg/L -- 6,100 6,100 6,100 6,100 6,100 6,100
Magnesium Sulfate Anhydrous MgSO4 mg/L -- 16,340 16,340 16,340 16,340 16,340 16,340
Manganese Sulphate MnSO4 mg/L -- 2,060 2,060 2,060 2,060 2,060 2,060
Nickel Sulfate Hexahydrate NiSO4•6H2O mg/L -- 34 34 34 34 34 34
Sodium Sulfate Anhydrous Na2SO4 mg/L -- 68 68 68 68 68 68
Zinc Sulfate Heptahydrate ZnSO4•7H2O mg/L -- 270 270 270 270 270 270
Others
Sodium chloride NaCl mg/L -- -- -- -- -- -- 114,400
Surfactant IGEPAL® CA720
(C2H4O)n.C14H22O
, n~12.5 ml/L -- -- -- -- -- 5 --
pH adjustment
Sulphuric acid H2SO4 ml/L 20 15 3.7 1.1 100 15 1.7
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Table 3. Composition of different solutions used in current study (mg/L unless noted).
aMetal ions were analyzed using inductively coupled plasma-mass spectrometer (ICP-MS), while the anions were analyzed using Ion chromatography (IC). bAverage pH (average of 18 values) measured at the times of incubation solution replacement every 2 months during the 3 years of incubation . 98%
concentrated sulfuric acid was used for pH adjustment. c(C2H4O)n.C14H22O, n~12.5. dReverse osmosis (RO) water prepared by by filtering tap water through a semi permeable membrane under sufficient pressure allowing the passage of water but
not ions such as Ca2+, Na
2+ and Cl
-,etc and was used in preparation of all solutions.
eSulfuric acid was added to RO water to adjust pH to ~ 0.5. fThe incubation solution in the first 2 weeks of every precuring cycle (3 months) with acid content of 100 g/l. The geomembrane is immersed in L2 during the
remaining 10 weeks of the precuring cycle. gL2 but with boosted chloride content by adding NaCl.
Componenta
Water
pH=7.0d
Water
pH=0.5e
L1 L2 L3 L4f L1-S L2-Cl
g
Nominal pH 7 0.5 0.5 1.25 2.0 <0 0.5 1.25
Average pHb 6.5 ± 0.2 0.51 ± 0.03 0.53 ± 0.07 1.31 ± 0.12 2.11 ± 0.25 <0 0.51 ± 0.12 1.28 ± 0.16
Al3+ <1.0 <1.0 5,000 5,000 5,000 5,000 5,000 5,000
Cd2+ <0.025 <0.025 1.7 1.7 1.7 1.7 1.7 1.7
Ca2+ <0.05 <0.05 515 515 515 515 515 515
Co2+ <0.02 <0.02 20 20 20 20 20 20
Cu2+ <0.2 <0.2 87 87 87 87 87 87
Fe2+ <0.05 <0.05 710 710 710 710 710 710
Li+ <0.05 <0.05 1,000 1,000 1,000 1,000 1,0000 1,000
Mg2+ <0.05 <0.05 3,300 3,300 3,300 3,300 3,300 3,300
Mn2+ <1.0 <1.0 620 620 620 620 620 620
Na+ <1.0 <1.0 50 50 50 50 50 42,500
Ni2+ <1.0 <1.0 7.6 7.6 7.6 7.6 7.6 7.6
Pb2+ <0.03 <0.03 1.4 1.4 1.4 1.4 1.4 1.4
S6+ <1.0 11970 2,250 1,580 1.,420 77,770 23,100 14,100
Zn2+ <0.01 <0.01 62 62 62 62 62 62
Cl- <0.5 -- 5,100 5,100 5,100 5,100 5,100 74,500
SO42- <0.1 36,000 68,000 48,000 43,000 220,000 56,000 43,000
IGEPAL® Ca-720(ml/l)
c 0 0 0 0 0 0 5 0
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Table 4. Geomembrane properties
Propertiesa Method Unit Mean ± SD
Nominal thickness ASTM D5199 mm 1.5
Geomembrane designator --- --- xC
Manufacturing date --- --- May 2008
Geomembrane Density ASTM D1505 g/cc 0.946
Carbon black ASTM D 4218 % 2.5
Standard oxidative induction time
(Std-OIT; 200oC/35 kPa)
ASTM D3895 min 160 ± 1.5
High-pressure oxidative induction Time
(HP-OIT; 150oC/3500 kPa)
ASTM D5885 min 960 ± 17
Crystallinity ASTM D3418 % 50.5 ± 0.7
HLMI (21.6 kg/190oC)b
ASTM D1238 g/10min 12.9 ± 0.4
LLMI (2.16 kg/190oC)c 0.115 ± 0.001
Melt flow ratio (MFR) = (HLMI/LLMI) --- --- 111
Single point stress-crack resistance
(NCTL-SCR)SCR) ASTM D5397 hours 800 ± 90
Tensile properties (machine direction)
Strength at yield ASTM D6693 kN/m 27.8 ± 1.2
Strength at break Type (IV) kN/m 49.8 ± 2.7
Strain at yield % 20.6 ± 0.7
Strain at break % 818 ± 18
Tensile properties (cross-machine direction)
Strength at yield ASTM D6693 kN/m 29.1 ± 1.0
Strength at break Type (IV) kN/m 50.7 ± 2.7
Strain at yield % 18.3 ± 0.7
Strain at break % 857 ± 23 aGeomembrane initial properties are subjected to small changes with time due to storage of the roll in room
temperature for long period, variability of the material within the same roll (e.g., distribution of additives; resin
imperfections), and periodic calibration of the testing equipment. The initial values reported in the current study are
at 2010 that may be different from initial properties reported previously for the same geomembrane when roll was
received or for studies will be initiated in future. bHigh load Melt Index. cLow load Melt Index.
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Table 5. Antioxidant depletion rates and residual OIT values at five different temperatures
in low pH solutions.
Temperature
(oC)
Solution
Std-OIT HP-OIT
s
(month-1)
Std-OITr (min)
s
(month-1)
HP-OITr (min)
95
L1: pH=0.5 0.2 25 0.11 268
L2: pH=1.25 0.25 19 1.47 624
L3: pH=2.0 0.22 18 1.18 490
L1-S: pH=0.5+surfactant 2.5 ~3 2.0 638
L2-Cl: pH=1.25+chlorides 0.3 7.6 1.63 665
85
L1: pH=0.5 0.18 30 0.094 357
L2: pH=1.25 0.2 23 1.0 664
L3: pH=2.0 0.18 26 0.70 590
L4-Precuring 0.27 20 1.6 700
L1-S: pH=0.5+surfactant 1.2 ~3 1.0 717
L2-Cl: pH=1.25+chlorides 0.16 25 1.63 665
Water: pH=0.5 2.5/0.35 11 0.27 500
Water: pH=7 1.58/0.03 NR 0.0136 NR
75
L1: pH=0.5 0.11 38 0.064 439
L2: pH=1.25 0.12 30 0.60 692
L3: pH=2.0 0.11 31 0.30 693
L1-S: pH=0.5+surfactant 0.38 ~3 0.42 765
L2-Cl: pH=1.25+chlorides 0.11 34 1.63 665
65
L1: pH=0.5 0.07 42 0.038 525
L2: pH=1.25 0.07 33 0.30 760
L3: pH=2.0 0.07 35 0.20 710
L4-Precuring 0.09 40 NA NA
L1-S: pH=0.5+surfactant 0.19 ~3 0.20 825
40
L1: pH=0.5 0.010 NR 0.0038 NR
L2: pH=1.25 0.013 NR 0.021 NR
L3: pH=2.0 0.011 NR 0.017 NR NR= Not reached
All HP-OIT depletion rates were modeled using a 3-parameter model.
For Std-OIT data, A 2-parameter exponential function was used to model the depletion in L1-S, 3-parameter
exponential function was used for L1, L2, L3, L4, and L2-Cl, and 4-parameter exponential function was used for
Water and Water pH=0.5.
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Table 6. Predicted antioxidant depletion times at different temperatures for the different
low pH solutions (rounded to two significant digits).
[1] [2] [3] [4] [5] [6]
Temperature
(oC)
Solution
Predictionsa
Std-OIT HP-OIT
Std-OITrb
(min)
time to Std-OITrc
(years)
HP-OITrd
(min) time to HP-OITr
e
(years )
20
L1: pH=0.5 72 110 780 190
L2: pH=1.25 54 97 900 24
L3: pH=2.0 58 110 870 47
L1-S: pH=0.5+surfactant 3 46 930 10
30
L1: pH=0.5 66 54 740 89
L2: pH=1.25 49 48 880 11
L3: pH=2.0 53 48 840 15
L1-S: pH=0.5+surfactant 3 19 920 8
40
L1: pH=0.5 60 28 690 45
L2: pH=1.25 44 25 860 5
L3: pH=2.0 47 25 810 6
L1-S: pH=0.5+surfactant 3 8.5 900 4.5
50
L1: pH=0.5 53 15 630 23
L2: pH=1.25 40 13 820 2.5
L3: pH=2.0 42 13 770 2.9
L1-S: pH=0.5+surfactant 3 3.9 880 2.3
55
L1: pH=0.5 50 11 600 17
L2: pH=1.25 37 10 810 1.7
L3: pH=2.0 40 10 750 2.0
L1-S: pH=0.5+surfactant 3 2.7 860 1.6
60
L1: pH=0.5 47 8.5 560 13
L2: pH=1.25 35 7.5 790 1.2
L3: pH=2.0 37 7.5 730 1.4
L1-S: pH=0.5+surfactant 3 1.9 840 1.2
70
L1: pH=0.5 40 4.9 490 7
L2: pH=1.25 31 4.3 740 0.6
L3: pH=2.0 32 4.3 680 0.7
L1-S: pH=0.5+surfactant 3 1.0 800 0.6
80
L1: pH=0.5 34 3.0 400 4.0
L2: pH=1.25 26 2.6 690 0.3
L3: pH=2.0 27 2.6 630 0.3
L1-S: pH=0.5+surfactant 3 0.5 740 0.3
90
L1: pH=0.5 27 1.8 310 2.4
L2: pH=1.25 22 1.6 630 0.2
L3: pH=2.0 22 1.6 563 0.2
L1-S: pH=0.5+surfactant 3 0.3 672 0.16
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a Predictions are in years unless otherwise noted. b Residual Std-OIT (Std-OITr) values estimated based on the Arrhenius plot of Std-OIT* in Fig. 8a. c Prediction of the antioxidant depletion stage based on Std-OIT using the Arrhenius equation presented in
Fig.9. d Residual HP-OIT (HP-OITr) values estimated based on the Arrhenius plot of HP-OIT* in Fig. 8b. e Prediction of the antioxidant depletion stage based on HP-OIT using the Arrhenius equation presented in
Fig.10.
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