In heap leach pad containment, the HDPE geomembrane is specified primarily by resin density, melt flow rate, stress crack resistance, and oxidative stability. A geomembrane manufactured to GRI-GM13 from a bimodal high-density polyethylene with a density of 0.945–0.955 g/cm³ and a melt index of 0.2–0.8 g/10 min at 190 °C/2.16 kg provides the required combination of extrusion processability and slow crack growth resistance. The term “grade” in this context is not a single density value; it includes molecular weight distribution, comonomer type, stabilizer package, and carbon black masterbatch. These variables determine how the liner behaves when exposed to acidic sulfate leachates, alkaline cyanide solutions, and residual organic extractants. The specification GRI-GM13 is the most commonly applied minimum property regime for HDPE geomembranes in heap leach service, but grade selection must be paired with chemical exposure data because density alone does not control environmental stress cracking.

Heap leach pads for copper, gold, and uranium impose continuous contact with aggressive aqueous solutions while the liner is under sustained tensile stress from ore load and thermal movement. HDPE is semi-crystalline and nonpolar; its crystalline lamellae reduce permeation, but its amorphous tie chains govern ductility. Installation damage produces scratches and notches that act as crack initiation sites when surface-active organic compounds lower the energy required for craze formation. Therefore the resin grade is selected for a GRI-GM13 stress crack resistance of at least 500 h under ASTM D5397, not merely for a minimum density.

Does Resin Grade Influence Chemical Exposure More Than Density Alone?

Density and chemical permeation are inversely related, but the relationship is not linear in a highly crystalline polyethylene. A resin with density 0.945 g/cm³ and a bimodal molecular weight distribution can outperform a 0.955 g/cm³ unimodal grade in a surfactant-containing leachate because the bimodal material contains a higher concentration of tie molecules. These tie molecules bridge adjacent crystalline lamellae and resist craze propagation. Chemical exposure in heap leaching therefore includes mechanical stress and wetting agents, not just pH. The ASTM D5397 single-point notched constant tensile load test imposes a 30% yield stress on a notched specimen submerged in 10% Igepal CO-630 at 50 °C. A resin that fails before 500 h may still meet density and tensile specifications but is generally unsuitable for heap leach pad lining.

The practical balance is between lower permeability and higher tie-chain density. Increasing density reduces sorption and diffusion by increasing crystalline tortuosity, but it can reduce the amorphous tie-chain population if achieved through higher crystallinity rather than through molecular design. A bimodal resin avoids this trade-off by combining a low-molecular-weight fraction for melt processing with a high-molecular-weight fraction that increases the probability of load-bearing tie molecules. Chemical exposure is diffusion-limited in HDPE; water, sulfuric acid, and sodium cyanide penetrate primarily through the amorphous phase. Therefore the resin architecture governing that amorphous phase, not the average density alone, determines whether a scratch in contact with pregnant leach solution will craze or remain stable.

Chemical exposure in copper and gold heap leaching spans two distinct pH regimes. Copper oxide and secondary copper sulfide heaps are leached with sulfuric acid at pH 1.5–2.0; gold and silver heaps are leached with sodium cyanide at pH 10.5–11.0. HDPE is resistant to both aqueous solution types because its hydrocarbon backbone is not hydrolytically cleaved under these pH conditions and its crystalline regions limit sorption. However, heap leachate is not a pure acid or alkali. Sulfate salts, dissolved metals, and residual organic extractants such as hydroxyoximes and ketoximes are present in copper raffinate. These organics can lower the surface energy at a scratch tip and accelerate environmental stress cracking. The governing chemical exposure parameter is therefore not pH alone but the combined presence of stress, wetting agent, and temperature.

ASTM D5747 is the standard practice for evaluating chemical resistance of geomembranes to liquids. It is not a simple pass/fail immersion test; it establishes a procedure for exposing coupons to site-specific liquids at controlled temperatures and durations, commonly 30, 60, 90, and 120 days. Evaluations include changes in tensile behavior, tear, puncture, melt index, OIT, and mass. In heap leach design, the test liquid should include pregnant leach solution, raffinate, and, where cyanide destruction is used, the treated effluent. Testing only pH-adjusted water overestimates compatibility because it omits organic extractants and scale constituents that influence stress cracking.

When Sulfuric Acid and Alkaline Cyanide Share the Same Liner History

When a heap leach pad is operated with mixed metal sulfide ores, the liner may encounter sequential acidic and alkaline conditions or localized pH gradients caused by scale and precipitation. HDPE does not undergo rapid chain scission in either environment at ambient heap temperatures. Long-term immersion studies conducted under ASTM D5747 in synthetic sulfuric acid at pH 1.5 and sodium cyanide at pH 10.5 show that tensile property loss at 23 °C is generally limited over 120 days; the more sensitive indicator is antioxidant stabilizer depletion. At 50 °C, the same solutions extract hindered phenolic stabilizers more rapidly, reducing the standard OIT toward or below the 100 min endpoint while tensile break strength may remain above specification. This pattern indicates that thermal oxidation and stabilizer consumption are the primary degradation routes, not acid or alkali hydrolysis. The operational boundary is therefore temperature-dependent: continuous contact with leach solution above 50 °C shortens the antioxidant service reserve and should be evaluated by high-pressure OIT testing under ASTM D5885.

Sulfuric acid at pH 1.5 is a non-oxidizing acid under heap leach conditions; it does not attack the carbon-carbon or carbon-hydrogen bonds of polyethylene. Sodium cyanide at pH 10.5 is an alkaline leach solution; it also does not hydrolyze HDPE at typical heap temperatures. The risk arises when cyanide destruction uses sodium hypochlorite or hydrogen peroxide. If hypochlorite solution is discharged or stored in contact with the liner, the oxidizer can abstract hydrogen atoms and cause chain scission, reducing elongation at break rapidly, often before visual cracking appears. Therefore chemical exposure evaluation must include the entire process stream, not only the leach pad solution.

Strong oxidizing agents are incompatible with HDPE geomembranes. Sodium hypochlorite used for cyanide destruction, hydrogen peroxide, potassium permanganate, and nitric acid above 20% cause oxidative embrittlement of the sheet core. Organic solvents such as toluene, xylene, methyl ethyl ketone, and acetone swell the amorphous phase and accelerate stress cracking. Published data for these specific immersion configurations in heap leach raffinate is limited; therefore, site-specific chemical compatibility testing under ASTM D5747 is required when solvent extraction reagents or oxidizers are present. The absence of uniform chemical attack does not imply safe service; stress cracking from organic extractants and leachates can occur without measurable mass loss or thickness reduction.

Oxidative Induction Time, Carbon Black Dispersion, and Weld Heat History

Oxidative induction time under ASTM D3895 is measured at 200 °C with oxygen at 1 atm; the 100 min minimum confirms the presence of a hindered phenolic antioxidant package but does not predict field lifetime. High-pressure OIT under ASTM D5885 is measured at 150 °C and 3.4 MPa oxygen; the 400 min minimum provides a more sensitive indicator of long-term thermo-oxidative stability. Carbon black dispersion tested under ASTM D5596 must be Category 1 or 2, because agglomerates act as stress concentrations and can reduce SP-NCTL failure time. On a production flat-die extrusion line with a 30:1 to 36:1 L/D extruder, melt temperatures above 240 °C consume hindered phenolic antioxidants before the sheet reaches the roll stack. Weld heat history introduces a second thermal exposure at the seam; the heat-affected zone often shows lower OIT than the parent sheet, making seam integrity a controlling factor in long-term chemical exposure.

OIT values are kinetic indicators, not additive quantities. A sheet with standard OIT of 100 min and high-pressure OIT of 400 min at installation may lose a substantial fraction of its high-pressure OIT within the first decade of service depending on temperature; published long-term data for this specific heap leach configuration is limited. For heap leach pads with expected service life of 10–20 years, the stabilization package must be assessed using Arrhenius extrapolation from high-pressure OIT values at multiple temperatures, not only the 150 °C single point. Carbon black dispersion is not only a ultraviolet concern; it affects the local stress field around particles. A poorly dispersed carbon black agglomerate produces a local modulus mismatch and can initiate microvoids during sheet bending and welding.

Stress Crack Resistance Is the Governing Mechanical Limitation in Heap Leach Service

Field failures on operating heap leach facilities are rarely uniform chemical attack; they are more commonly stress cracks initiated at notches, rock punctures, or weld defects. At the ore-liner interface, angular crushed rock creates point loads. GRI-GM13 specifies puncture resistance under ASTM D4833 of 400 N for a 1.5 mm geomembrane and 534 N for a 2.0 mm geomembrane. Tear resistance under ASTM D1004 limits propagation of installation damage. Tensile yield and break properties under ASTM D6693 are specified at 15 kN/m and 28 kN/m for a 1.5 mm sheet, with elongation at break of at least 700%. These index values do not guarantee stress crack resistance; only ASTM D5397 directly evaluates slow crack growth under a constant load in a surfactant environment. A liner that exceeds tensile and puncture specifications but falls below 500 h in SP-NCTL is not considered robust for heap leach contact with organic-bearing leachate.

At the heap leach pad, liner stress is generated by overliner drainage gravel, ore stack height, and equipment loads. Compressive loads on the side slopes create tensile strain in the geomembrane, especially at toe anchorage and pipe penetrations. The yield elongation of HDPE is about 12% under ASTM D6693; however, local strains at a notch tip can exceed the yield strain even when global strain is below 5%. In a leachate with organic extractants, the crack propagation rate can be several times higher than in air. Therefore the usual engineering design limit is not yield strength but the stress crack threshold under ASTM D5397. Weld seams are tested under ASTM D6392 for peel and shear; seam failure in the heat-affected zone is an early indicator of antioxidant depletion from double thermal exposure.