Geomembranes in Mining: the State of Research

An interview with Dr. Fady Abdelaal, GeoEngineering Centre at Queen’s-RMC, Queen’s University, Kingston, Canada, by Chris Kelsey
The first conference dedicated exclusively to geosynthetics in mining was held in Vancouver, Canada in September 2014. One of the presenters was Dr. Fady Abdelaal, who for the past 6 years has been part of the long-running geosynthetic barrier systems research being led by Dr. Kerry Rowe at the renowned GeoEngineering Centre at Queen’s-RMC, Queen’s University, in Kingston, Ontario, Canada. The GeoEngineering Centre has contributed a vast amount of insight to the global geosynthetics field, particularly in regards to geomembrane performance in waste management. Dr. Abdelaal has been part of a unique chapter in the Centre’s barrier systems research: their creation of mining-specific barrier studies. Though the mining industry uses up to 40% of the global geomembrane production, there has historically been a gap in research dedicated to better understanding geosynthetic barrier performance in mining. Queen’s University remains at the forefront as its personnel design these experiments, gather and interpret data, and deliver beneficial information to the field. We exchanged thoughts with Dr. Abdelaal via email on the importance of studying geomembranes in mining. – Chris Kelsey
**
GEOSYNTHETICA: Dr. Abdelaal, you’ve had a slightly different path into research. You worked first for a number of years in the field. How has your design engineering and landfill-auditing background influenced your geosynthetic barrier research?
ABDELAAL: My landfill industrial background initially helped me to blend in faster with the barrier research conducted at Queen’s University. It specifically helped me with understanding the big picture of solid waste management and the processes involved before waste is landfilled on site. Understanding the big picture is essential when seeking to investigate the performance of a single component of the barrier system. It is quite imperative to be aware of the possible effects of how the performance of one component could impact other components of the liner system.

GEOSYNTHETICA: Queen’s University has an exceptional, long-term barrier study program to which many international researchers, post-doctoral fellows, and field experts have contributed. The focus has often been on solid waste applications. The study of geomembranes in mining applications has seldom been addressed, at Queen’s or elsewhere. How did mining come to be on your barrier research program? How, if at all, has it altered your approach to setting up investigations?

ABDELAAL: Our investigation of geosynthetics, especially geomembranes, in mining application arose from questions being asked by our industrial partners—especially consultants—involved in the design of mining projects. They were using our landfill liner research for landfill design. They wanted similar information for their mining projects but realized that much of our landfill work was not directly applicable to the very different conditions encountered in mining applications. The several millions of square meters of different geosynthetic materials used in liner systems for different mining applications represent a significant portion of the geosynthetic market. Furthermore, the mining industry is a major contributor to the economy of many countries around the word. For example, according to the mining association of Canada, approximately 400,000 people across Canada work in the mining and mineral processing industries with a value of mineral production of around $47 billion in 2012. However, there is a paucity of information about the longer-term performance of geosynthetics needed to both maximize recovering of minerals and environmental protection that can be achieved with appropriate selection of materials.
The exposure condition for geomembranes in mining applications, especially 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. Since my work is related to chemical durability of geomembranes in such applications, the first objective of this project was to have a good handle of the chemistry of these mining solutions in contact with geomembrane so we could simulate them in the laboratory. This data was not available in the literature and was provided by our industrial partners in the mining field who also checked our final testing matrix. This initial stage took almost a year, including the investigation of the chemistry of the solutions and the preparation of trial immersion solutions to make sure that they are chemically stable and mimicked the target values. We ended up monthly preparing around 300 liters of extremely high and extremely low pH solutions in our laboratory. This required the implementation of new safety measures in our laboratories to be able to conduct these experiments safely over the (at least) 10-year testing period.

GEOSYNTHETICA: You presented in Vancouver at the Geosynthetics Mining Solutions conference on the impact of extremely high pH solutions on LLDPE. This was the fourth of a series of papers you’ve contributed to in regards to geomembranes and mining. The larger barrier research at the GeoEngineering Centre now counts at least 10 papers in its roster of mining-related investigations.
What gaps in knowledge or discussion still exist? What are the next steps?

ABDELAAL: Our mining related investigations at Queen’s can be generally divided into three major areas. The first includes the investigation of the chemical durability of different geomembranes (HDPEs and LLDPEs of different thicknesses). The second involves the investigation of the physical durability of geomembranes under simulated field conditions which involves the quantification of tensile strains in the geomembrane liner. These can be considered as two parallel approaches that complement each other for investigating the long-term performance of geomembrane liners by separately investigating the change in the geomembrane resistance due to chemical aging and the demand on the geomembrane (i.e., strains/stresses) imposed by, for example, the weight of the ore. Ultimately, the gap that needs to be filled is the behavior of the geomembrane liners under the field stresses during the chemical ageing process.
The third area being examined by the Queen’s team is examining the effect of the permeability of the material (e.g., mine tailings) on the leakage through holes in a geomembrane and how this can substantially reduce leakage both without and with a wrinkle.
With regard to our chemical durability studies, although our research program included eight different geomembranes, given the rapid evolution in the geomembrane industry and the variety of resins and additive packages that can be used in geomembranes, the geomembranes tested only represent a relatively small number of those available on the market. While we can provide insight regarding how these specific geomembranes (with their particular resin and antioxidant/stabilizer package) perform, there will be many questions about how others will perform. Unless the other geomembranes have the same resin and antioxidant/stabilizer package, we cannot say how they will perform—ageing under extreme conditions is very specific to the mining fluid chemistry and the geomembrane resin and antioxidant/stabilizer package. Our research shows that this cannot be assessed in short-term (e.g., 3 month) tests.

GEOSYNTHETICA: The dialogue between the mining industry and the geosynthetics field has picked up significantly. Many parallels have been drawn with the waste management sector. This is due in part, as you’ve noted, to the historical absence of mining-specific attention in design, research, etc.
So while we move further into this period of recognizing the uniqueness of geomembrane performance and applications in mining, what continues to connect waste management and mining? What can these sectors continue to learn from one another?
ABDELAAL: One of the important lessons learnt from the landfill field is that equal attention should be paid to both the chemical durability of the geomembrane liner and the liner design. The long-term performance of the geomembrane liner could be substantially improved if the tensile strains in the geomembrane are limited with the aid of proper protection layers. For municipal solid waste applications, we have demonstrated such issue of stress cracking of aged geomembranes. (SEE “Brittle Rupture if an aged HDPE geomembrane at local gravel indentations under simulated field conditions” in the reference list.) We showed how a geomembrane can rupture in a brittle manner (and, hence, lose its function as a hydraulic barrier) when it suffers degradation in its mechanical resistance under chemical exposure if the geomembrane protection layer was not adequate in limiting the long-term tensile strains to a level the geomembrane can sustain at such low resistance. We not only raised this issue but through our research attempted to present guidance regarding how a proper protection could ultimately enhance the geomembrane durability. (SEE “Antioxidant depletion of HDPE geomembrane with sand protection layer” in the reference list.)
With the higher overburden pressures found in some mining applications, guidelines could be developed to aid in liner system design similar to those in landfill applications seeking adequate performance of the liner system to ensure proper environmental protection.
We have performed the first longer-term investigation of the durability of LLDPE geomembranes in the mining related applications due to their increasing role in the heap leaching industry and other mining applications. Because of the LLDPE’s molecular architecture, traditional wisdom holds that they are more prone to chemical degradation than HDPE/MDPE, despite their higher flexibility and lower susceptibility to environmental stress cracking. While this may be applicable to the resins, the end-user product (i.e., the geomembrane) includes stabilizer/antioxidant packages that could significantly affect the chemical durability of the geomembrane by delaying the chemical degradation.
For example, we have reported seeing slower depletion of antioxidants used in our investigated LLDPE geomembrane than for the antioxidants used in the investigated HDPE geomembrane; this is more about the choice of antioxidant package rather than the resin. The specific antioxidants/stabilizers used in the LLDPE we tested were better at resisting extraction and/or chemical consumption in the investigated solutions than those used in the HDPE geomembrane. While this cannot be generalized to other HDPE or LLDPE geomembranes, it shows that with a proper high performance antioxidant package and sufficient investigation/verification, appropriate LLDPE geomembranes could be applicable and have good longer-term performance in these extreme environments. However, the fact that LLDPE is more flexible does not mean it should be subjected to high strains.
WE THANK DR. ABDELAAL for sharing his time and insight. A list of papers referenced directly and indirectly in this interview follows. For a full list of papers and research notes from the barrier systems investigations at the GeoEngineering Centre, please visit www.geoeng.ca.

REFERENCES & RECOMMENDED READING
Abdelaal, F.B., Rowe, R.K., Smith, M. and Thiel, R. 2011. OIT depletion in HDPE geomembranes used in contact with solutions having very high and low pH. Proceedings of the 14th Pan-American Conference on Soil Mechanics and Geotechnical Engineering, Toronto, Canada, CD-ROM, 7p.
Abdelaal, F.B., Rowe, R.K., Smith, M., Brachman, R.W.I. and Thiel, R. 2012. Antioxidant depletion from HDPE and LLDPE geomembranes without HALS in and extremely low pH solution. The 2nd Pan American Geosynthetic Conference and Exhibition, GeoAmericas 2012, Lima, Peru, CD-ROM, 9p.
Abdelaal, F.B. and Rowe, R.K. 2013. Antioxidant depletion of HDPE geomembrane without HALS in an extreme environment. Proceedings of Geosynthetics 2013 Conference, Long Beach, CA, USA. CD-ROM, pp. 127-133.
Abdelaal, F.B. and Rowe, R.K. 2014. Antioxidant depletion from a LLDPE geomembrane in an extremely high pH solution. Geosynthetics Mining Solutions 2014. 8-11 September 2014, Vancouver, BC, Canada.
Abdelaal, F.B., Rowe, R.K., Brachman, R.W.I. 2014. “Brittle Rupture if an aged HDPE geomembrane at local gravel indentations under simulated field conditions.” Geosynthetics International, Vol 21 (1), 1 – 23. http://dx.doi.org/10.1680/gein.13.00031.
Brachman, R.W.I., Rowe, R.K. and Irfan, H. 2014. “Short-Term Local Tensile Strains in HDPE Heap Leach Geomembranes from Coarse Overliner Materials”, ASCE Journal of Geotechnical and Geoenvironmental Engineering, 140(5): 04014011, http://dx.doi.org/10.1061/(ASCE)GT.1943-5606.0001087
Joshi, P., Brachman, R.W.I., and Rowe, R.K. 2014. “A new laboratory apparatus for testing geomembrane leakage with mine tailings under large earth and fluid pressures”, Canadian Geotechnical Conference, GeoRegina, Regina. 6p.
Rowe, R.K., Abdelaal, F.B., Brachman, R.W.I. 2013. “Antioxidant depletion of HDPE geomembrane with sand protection layer.” Geosynthetics International, Vol 20 (2), 73 – 89. http://dx.doi.org/10.1680/gein.13.00003
Rowe, R.K, Brachman, R.R.W., Irfan, H., Smith, M.E., and Thiel, R. 2013. “Effect of underliner on geomembrane strains in heap leach applications”, Geotextiles and Geomembranes, 40: 37-47.