Copper Heap Leaching – A Case for PVC Liners

 

MAY 1997

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This issue of the "Technical Bulletin" is a reprint of a technical paper entitled "Copper Heap Leaching - A Case for PVC Liners" written in 1994 by Mark Smith, Vector Engineering, Marc Orman, D. P. Engineering and Corazon Queja, Precision Laboratories. This paper was given an Award of Merit by the PVC Geomembrane Institute in 1994. In order to receive a copy of this paper, please contact the PVC Geomembrane Institute.
 

Heap leaching of copper bearing mineral ores began large-scale production in the 1970's. Copper heap leaching uses a dilute sulfuric acid solution, and in the first decade of its commercialization the environmental threat was not considered significant. Therefore, it is common to use no liners or very rudimentary systems.

Because of increasing economic and environmental concerns, in the mid 1980's copper heap leaching began improving their containment items. Now, most copper leach pads are lined with good quality geomembrane liner systems.

Chile and the US are the largest producers of copper. The consumption of geomembranes for copper leach pads has been estimated at 3 million square meters annually in Chile (Smith, 1994), and the US consumption probably approaches this level.

Increasing production, and projects being developed on more challenging sites, have led engineers to seek out alternative materials to help solve an array of problems. Further, recent increases in Polyethylene prices have pushed owners to seek out economic alternatives.

The purpose of this paper is to explore the need for and suitability of PVC geomembranes for copper leach pads.
 

Increasing Challenge

With heap leaching in its third decade of large scale commercialization, the fundamentals of designing liner systems for this application are well established the technology transfer from gold, which began using advanced containment systems earlier, to copper is also essentially complete cause of the common pool or designers.
 

Differential Settlement

Leach pad sites are generally selected for their combination of geotechnical and economic considerations. Sites with large settlement potential or slope stability. problems are usually avoided. However, copper leach pads can be enormous, reaching areas in excess of 5 million square meters. This often precludes selection of the optimum site from a geotechnical standpoint. It also can require a liner system to span a wide range of geologic profiles.

Differential settlement typically results from one of three causes: the liner spans soils of differing compressibility, the slope and height of the heap are extreme, or local settlement results from leaks in the liner. The later cause can be significant where the soils contain high concentrations of acid soluble salts such as in northern Chile.

Differential settlement causes multiaxial strains in the geomembrane, and the most commonly used liner, HDPE, is not particularly good at withstanding this stress condition. The allowable multiaxial strain for HDPE is commonly taken as 3 percent. However, strains caused by differential settlement can exceed 20 percent for high salt content soils. Therefore, the engineer needs to either design to avoid this condition which can be economically infeasible - or select a geomembrane that can tolerate these magnitudes of strain.
 

Concurrent Stacking Techniques

A parallel development is a process where the leach pad is stacked just a few meters behind the leading edge of the geomembrane. As the heap advances so does the liner system. This process is discussed in detail in other publications (Smith, 1994) and therefore will not be repeated in this paper. Suffice it to say that economics and metallurgical considerations sometimes dictate that the geomembrane installation be performed on a nearly daily basis by the mine personnel.

The designer must, therefore, specify a geomembrane that can be installed by, a nonprofessional installer and still result in a reliable containment. The preference often becomes a geomembrane with chemically bonded seams that can be prefabricated into project-specific panel sizes.
 

Options and Questions

Polyvinyl Chloride (PVC) and Very Low Density Polyethylene (VLDPE) are the two most common choices when the design criteria includes high multiaxial strains. With multiaxial elongation to-break values in the 50 percent to 100 percent range, both of these materials provide a factor of safety of 2 to 5 when said to accommodate actual strains of up to 20 percent.

The other technical issues facing the designer include: physical properties such as tensile strength, interface friction properties, puncture resistance: durability properties such as resistance to soil burial and UV resistance: and chemical resistance. The first two have been addressed at length by other authors and present no significant difference between copper and gold heap leach or between landfills and heap leach applications.

However, the chemistry of copper leach solution is unique to copper heap leaching. Yet, data on the chemical compatibility of copper leach solutions and PVC or VLDPE is absent in literature.
 

Chemical Compatibility of Geomembrane with Copper PLS

The US Environmental Protection Agency developed the test method EPA 9090 in the early 1980's for determining the compatibility of Geomembranes with liquids. While there is much debate in the industry about this test method, it remains the generally accepted standard.

The test method was used to determine the chemical resistance of HDPE, VLDPE, PVC and Hypalon to actual copper leach solution, commonly called PLS for pregnant leach solution. PLS is the copper-bearing solution produced out of the bottom of a heap.

Methodology

EPA method 9090 testing was performed on four different types of geomembrane in contact with copper leaching solution. The membranes tested were: 60 mil HDPE, 40 mil VLDPE, 30 mil PVC.

Reinforced chloro-sulfinated polyethylene (CSPE-R, or Hypalon) in 36 mil thickness was also tested because of Hypalon's use as a liner for ditches, tank farms and for floating covers.

The thickness’ were selected as being the most commonly used thickness’ for each material in the mining industry. HDPE was selected because it is the most commonly used; PVC and VLDPE because they are the best choices for differential settlement applications.

Square test samples or coupons were immersed in PLS in a tank: one maintained at 23 degrees Celsius and the other at 50 degrees Celsius. The liner coupons were suspended in the tanks from quartz glass frames using nylon ties and were weighted on the bottom with stainless steel weights to keep them from floating.

The PLS was provided by an operating solvent extraction/electrowinning facility in Arizona. The PLS was maintained at pH I.9 through the duration of the test.

For each test interval the coupons were removed from the tank, blotted dry, and then smaller coupons were cut for testing.

The physical property tests listed in Table I (above) were performed on samples of each geomembrane at 30 day intervals for a total test period of 120 days. All tests were performed in accordance with the American Society for Testing and Materials (ASTM), EPA methods or the Federal Test Method Standard (FTMS).
 

Results

All of the materials tested showed changes in some of their properties over the 120 day testing period. Most of the changes were minor and could be considered to be due to sample variation or test method precision. However, some results were more significant.

Figures 1, 2 and 3 present graphical representations of changes after 120 days of select properties. Results for both 23 degrees and 50 degrees Celsius tests are shown, along with machine direction (MD) and transverse direction (TD) where appropriate.

Only VLDPE (in tensile strength and elongation at break) exceeded the suggested limits. It must be noted, however, that the suggested limits for these properties are for the yield points, and not for breaking conditions. Since VLDPE has no clear yield point, direct application of the suggested limits may not be appropriate. Further, the test results at the 30-day increments show considerable scatter. Therefore, the VLDPE data must be considered inconclusive.
 

Interpretation

The test duration is relatively short at 120-days. Therefore, the results of the temperature testing does not ensure accurate long-term prediction, it should be more reliable. No attempt to project long term performance using time-temperature superposition was made, but this would be a valuable exercise if the data were extended to at least a dozen readings per material.

In addition, the tests were not performed in duplicate (except for PVC) and the statistical significance of the data is limited. In some cases the data is internally contradictory (for PVC, both tensile strength, and elongation increased) or show no clear trend over time (VLDPE elongation at break).

Further, copper leaching solutions will vary in chemical composition, including sulfuric acid concentration and organic constituent composition. This change will occur between mines, but also over time at any given site. This test program considered only a single source of PLS. However, not withstanding these limitations, the following preliminary conclusions have been reached regarding chemical compatibility, with PLS:

• HDPE is compatible and performed better than the other materials used.
• VLDPE requires more testing before conclusions can be drawn.
• PVC is compatible, but some of the results are contradictory and indicate the need for more testing.
• CSPE-R is not compatible unless addition testing so indicates.

Until a larger database is developed the authors recommend project-specific compatibility testing for both VLDPE and PVC using actual or synthetic PLS solutions representative of the solution that will contact the geomembrane.
 

Additional Research Needed

The EPA 9090 test has been criticized for its relatively short-time frame, even at 50 degrees C. Interpretation of these results must include consideration of short-term indicators of long-term affects.

For example, PVC exhibited increased tensile strength, which is likely an indication of plasticizer loss. On the other hand, this could be anomaly since the elevated temperature tests exhibited less change and there was a large variability between samples. At the same time an increase in elongation at break was found, which indicates the softening of the material and is inconsistent with the changes in tensile strength and with the typical performance of PVC, indicative of hardening during aging.

VLDPE exhibited major decreases in elongation at break and tensile strength at break, both exceeded suggested limits. This may indicate important changes in molecular properties and, therefore, more significant long-term changes.

Additional research is needed in two areas: long term elevated temperature testing with more data points to allow reliable time-temperature superposition for at least I0 years (typical life of a leach pad); and determination of the polymeric changes to determine the cause of the deterioration, which will aid in long-term predictions, as well as in extrapolating this data for PLS with different chemistry. All this work should be done with a larger sample population and with a range of PLS chemistries.
 

References:

Koerner, Robert, 1994, 'Designing with Geosynthetics,' 2nd ed., Prentice Hall, pp 392.

Little, AD, Oct. 1985, 'Resistance of Flexible Membrane Liners to Chemicals and Wastes, US EPA Report, PB86- I 19955.

Smith, Mark E., 1994, 'The First PVC-Lined Leach Pads' draft manuscript pending publication.

 

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