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

https://mc06.manuscriptcentral.com/cgj-pubs

Canadian Geotechnical Journal

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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. 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



0 10 20 30 40

HP

-OIT

(m

in)

0

200

400

600

800

1000


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


0 10 20 30 40

Std

-OIT

(m

in)

0

50

100

150

200



0 10 20 30 40

HP

-OIT

(m

in)

0

200

400

600

800

1000


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


0 10 20 30 40

Std

-OIT

(m

in)

0

50

100

150

200

L1: pH=0.5L2: pH=1.25L3: pH=2.0


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.


0 10 20 30 40

Std

-OIT

(m

in)

0

50

100

150

200


0 10 20 30 40

HP

-OIT

(m

in)

0

200

400

600

800

1000

1200

40oC65oC75oC85oC95oC


(a) Std-OIT L1: pH=0.5

(b) HP-OIT L1: pH=0.5

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0 10 20 30 40

Std

-OIT

(m

in)

0

50

100

150

200



0 10 20 30 40

HP

-OIT

(m

in)

0

200

400

600

800

1000

1200



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0 10 20 30 40

Std

-OIT

(m

in)

0

50

100

150

200



0 10 20 30 40

HP

-OIT

(m

in)

0

200

400

600

800

1000

1200



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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


0 5 10 15 20

Std

-OIT

(m

in)

0

50

100

150

200

65oC75oC85oC95oC


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


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


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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Draft

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.


Solution Ea


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


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


Solution Ea


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


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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Draft

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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Draft

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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Draft

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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Draft

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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Draft

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.

Page 56 of 58



Draft

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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Draft

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.

Page 58 of 58



antioxidant depletion in hdpe geomembrane with … copper heap leaching, ... other methods such as...

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