Industrial Waste and Mining Applications

Geosynthetic liners have had a long history in mining applications. HDPE has been used extensively as the lining medium for Heap Leach Pads and lining of tailings dams. Other geosynthetic applications have included evaporation ponds for the reduction of liquid waste. The complexity of the lining system for a particular application depends on the nature of the material being stored or treated.

Heap leach pads generally consist of only a single liner, which is contoured to provide flow to a sump or collection point. The liner is installed under a strict quality assurance specification. Heap leach pads with an area of up to 500,000m2 have been constructed.

The subgrade for the heap leach pad is required to be smooth and free of oversize (> 10mm) material. The liner is then installed over the subgrade and an overlying layer of sand or other fine grained material is then placed to allow traffic over the pad without damage to the liner. Ore is then stacked on the pad by heavy haul trucks. The stockpile is then irrigated with a reactive liquid which reduces the metal from the ore. The liquid is allowed to flow to a collection point from where it is taken to a treatment plant.

Evaporation Ponds. The disposal of contaminated treatment water is a problem for ore processing plants across the mining industry. Where the plant is located in an area where evaporation is greater than precipitation it is possible to reduce the volume of water by natural evaporation. The proportion of contaminants in the polluted water is often very small being measured in parts per million. The problem of sludge buildup is not great. Where liquid pollutants are stored a more involved, multi-layered lining system is necessary. The system may incorporate a primary and secondary liner and a leak detection layer. The primary and secondary liner may be formed from HDPE and the leak detection layer from a combination of geotextile and flow mesh. Evaporation ponds are usually very shallow and the liner may be exposed to sunlight and heat. The movement of the lining system with diurnal variations in temperature may be a problem in that cyclical movement, over a long period of time, may result in stress cracking of the liner. The use of a reinforced top liner will reduce temperature expansion and contraction and may reduce the potential for stress cracking.

Where a pond is located in an area which has a well defined wet season it may be possible to install a removable cover to collect and divert rain water. These systems are not common and require further development work.

Tailings Dams. Where ball mills are used for grinding ores, a slurry is formed. Following the extraction of the metal the residual material is usually pumped to a disposal dam where the solids eventually settle out and the liquid is partially evaporated. The seepage of liquid through the storage dam base and walls is an important consideration. Following the filling of the dam, it is required to cap the structure and rehabilitate the site. Natural clays are the most common forms of lining and capping materials for tailings dams. In areas where clays are not common or where the transport and placement costs are high Geosynthetics materials may be used. When considering clays as lining and capping materials the following needs to be considered:

• Will the clay react with the stored material and will this affect the permeability of the clay?

• Will the clay when used as a capping material desiccate under dry conditions?

Geosynthetic materials eliminate such potential problems.