Heap leaching is a process in which precious metals and copper is extracted from the ore by using different liquids (mainly acids) and then solution will be transported to the lined lagoons for further processing. GCLs and geomembranes are widely used for sealing of heap leach pads and process lagoons because of their resistance to acidic liquids.
Importance of bottom liners in leach pads:
1- Environmental considerations
2- Economical considerations
Based on the experience with leach pads in the world, a combination of geomembrane and CCL (Compacted Clay Liner) is chosen to be a better option over alternatives like asphalt concrete, water tight concrete of clay layer alone, the reasons for such preference are:
• This combination has been performed satisfactory under severely harsh chemical environmental conditions.
• This liner profile is used under very high column of ores (up to 130meter).
• This system has been used in huge projects (up to 10km2 lined area and capacity of 1.5 billion cubic meter)
It should be mentioned that the GCL can suitably take place of CCLs in geomembrane/CCL composite liner.
Current practice for construction of bottom liners in heap leach pads and tailing dams:
For the mining process and disposal applications sometimes a double liner (Geomembrane+geodrain+geomembrane_GCL) is recommended when severe damage to the environment is predicted in case of any leak in bottom liner. This kind of liners are used in developed countries when shallow ground water presents and the leaching liquid contains hazardous concentrations of toxic chemicals.
But in cases, in developing counties or milder environmental risks a single composite liner can be used.
This composite liner consists of:
A layer of geomembrane with GM-13 or BAM standard.
A compacted soil (clay) layer with permeability of 10-8 m/s which can be substituted by a GCL layer.
Double liner profile
Single composite liner profile
The application of a soil layer beneath geomembrane is done for following reasons:
• The risk of damage to geomembrane sealing function due to stress cracking or oxidation under chemical attack.
• high risk of geomembrane puncture during backfilling
Disadvantages of CCL (Compacted Clay Liner)
• Need to heavy equipment on site for construction
• Several construction steps
• Large volume of construction works
• Desiccation and crack forming and required effort for mitigating the phenomena
• Complicated and rigorous task of clay compaction in the slopes
• High construction costs
• slow construction process
• Considerable amount of effort needed on site for quality control of constructed CCL
GCLs can be easily used instead of CCLs. Having in mind all the before mentioned technical advantages, GCLs are cheaper options when there is large hauling distance from clay deposits to the site.
Comparison of costs between GCL and 60cm thick CCL based on different hauling distances
Application of GCLs in Tailing dams
Tailings, also called mine dumps, Culm dumps, slimes, tails, refuse, leach residue or slickens, are the materials left over after the process of separating the valuable fraction from the uneconomic fraction of an ore. Some of the extraction processes like lagoon processes produce great amounts of tailings. If the tailings are not managed in good and efficient way, the wind and surface erosion can pollute water and bodies and corpses while leaking to undergrounds and contaminate groundwater aquifers and soils.
Importance of bottom liner in tailing dams:
contradictory to leach pads, in tailing dams the bottom liner is only used to protect environment so the profile of such liners are totally dependent on governing environmental regulations and sensitivity of the ambience. The profile can include CCL, geomembrane or GCL combined or alone (ANCOLD, 2011) and one layer standing GCL are used in tailing dams in various projects.
As a summary about mining industry sealing applications one can say that:
1. In leach pads and in standard conditions, GCLs can be used under geomembrane in a composite liner as technically/economically preferred option.
2. If local regulations and authorities agree, one layer GCL is far more efficient solution than one layer of geomembrane in leach pads.
3- In tailing dams, GCL can be used in bottom liner and final cover as technically/economically advantageous option.
MOKARRAR Composite Co. also produces a GCL laminated by a geomembrane layer which can be used efficiently as a composite layer in mining applications.