Lithium, one of the raw materials in various batteries, is the element powering the technological revolution over the past 30 years that has led to people having exponentially more electronic devices in their lives. According to Lopez et al. (2019), lithium prices have changed dramatically in the last 10 years. In 2012, the price for lithium was $9.50 per pound ($4.30/kg). At the end of January 2022, the price reached $110.23 per pound ($50/kg). Projected annual worldwide demand could reach 425,000 tons (386,000 tonnes) by 2025.
The industrial process to mine lithium includes the evaporation of brine in large ponds covered by polyvinyl chloride (PVC) geomembranes (Figure 1), which engineers design to waterproof a site. In South America, construction crews build the ponds in salares (salt flats) where brine can be found underground. Once the infrastructure is built and the ponds are filled, it can take 12 to 24 months to obtain the first lot of lithium salt (Lopez et al. 2019).
Delays during design of the evaporation pond and in pond construction can increase dramatically the time it takes to bring lithium salt from a salar to market. Design engineers must analyze evidence that may indicate failures due to arid climate conditions. Engineers have used the ASTM D7176-06 (ASTM D7176-06 2018) standard specification for buried nonreinforced PVC geomembrane applications since 2006, with the last review occurring in 2018. In 2019, the Fabricated Geomembrane Institute (FGI) updated its Material Specification for New PVC Geomembranes (Fabricated Geomembrane Institute 2019). That specification considers the importance of obtaining a lay-flat geomembrane to avoid wrinkles after installation. Both ASTM and FGI standards list the density as an index property, representing indirect information regarding geomembrane composition and field performance in desert climates, and giving a general idea of geomembrane cost.
Although PVC geomembranes manufactured to these standards have shown good performance in South American salares, not all the project owners choose them, for various reasons that will be analyzed in this article.
The Bolivian, Argentinian and Chilean governments—which have the major salares within the borders of their countries—have sought to protect the activities of lithium mining companies in the region; there are no government-mandated technical regulations for the geosynthetic systems used in waterproofing evaporation ponds at these sites.
An economic overview of the lithium industry
Today, many believe electric cars will eventually be mass-produced in response to CO2 emissions from single combustion engines. Lithium is an essential element for batteries in electric vehicles. In South America, lithium brines are found on the borders between Bolivia, Argentina and Chile, an area known as the Lithium Triangle.
Within the Lithium Triangle, following exploration studies to determine the location of underground lithium deposits and to designate sites for evaporation ponds, construction crews build evaporation ponds that are subsequently covered by PVC geomembranes. The brine is pumped to the pools to promote fractioned crystallization by wind action and the sun (Berube et al. 2007). This evaporation process can take several months to obtain a concentrated brine of lithium salts, which is then purified and dried at an off-site chemical plant.
The importance of PVC geomembrane technical specifications is enormous, considering the projects’ cost and the generally short time (less than 12 months) from the start of construction to the beginning of the lithium production process. When design engineers select an optimal PVC geomembrane for the evaporation pond, brine leakages in any of the crystallization steps and, worse, material failure can be avoided.
Selecting a PVC geomembrane for lithium evaporation ponds
Part of the process for engineers, project owners, manufacturers and contractors is selecting the proper material for the job of containing the brine during the evaporation cycle. But this part is not easy, and each actor in the project will confront challenges.
Controlling cost is one challenge. The quality of the PVC geomembrane is connected to this, because if the geomembrane fails, it can cost the project owner a lot of money. The project owner needs to carefully balance the available resources and the cost of the specified geosynthetic system.
PVC geomembrane density gives an idea of its composition, which defines its price and predicts a relationship with its possible performance in a desertic area. In response to cost sensitivity in the mining industry, PVC geomembrane manufacturers developed technologies to produce large, seamed panels (each 38,750 square feet [3,600 m2]), which reduce the number of seams needed in the field.
Another challenge is the high turnover of personnel in the mining industry, particularly in South America. Usually, some of the technical inspectors and other personnel in the field work only on specific projects, not for the companies. So, after they finish the project, they move on to another project, likely with a different company. This means the technical knowledge and experience they have acquired are lost to the original company. Information regarding optimal technical specifications does not transfer from project to project.
In South America, there is also a lack of geographic stations in the right places to collect data of historical and environmental conditions at mining exploration sites, such as ultraviolet (UV) radiation, rainfall, hailstorms and wind speed. It is ideal to have meteorological stations on-site where the extraction process is carried out (Lopez et al. 2019).
The brine and the climate
Groundwater brine deposits are a result of a combination of geological events across time. The geographic position of salt flats—in Bolivia, Argentina and Chile, as well as in China and some parts of Australia—meets the geological conditions for massive groundwater brines with high content of lithium salts.
Studies carried out in the United States by the U.S. Geological Survey (Bradley et al. 2013) showed that the typical climate for salt flats is between semiarid and hyperarid. In terms of geographic classification, climate categorization is based on temperature, wind speed, latitude and elevation, all of which help determine the UV index and precipitation levels (Christopherson 1994). Table 1 summarizes the characteristics at the Salar de Atacama in Chile, Salar de Uyuni in Bolivia, and Salinas Grandes in Argentina: temperature, wind speed, UV index and precipitation.
In the Lithium Triangle, there is wide variation between high and low temperatures. This means a PVC geomembrane must withstand extreme temperatures once installed. Even if the air temperature is not overly hot compared to that in other deserts around the world, a PVC geomembrane absorbs infrared energy from the sun. The film increases its temperature at noon up to 158˚F (70˚C). In winter, air temperatures can drop as low as 14˚F (-10˚C).
Precipitation levels in the Lithium Triangle are very low, especially in summer. Typically, the evaporation rates are higher than the precipitation level. Because of the low level of precipitation in Salar de Atacama in Chile, there are no expected failures due to plasticizer leaching in geomembranes deployed there and, consequently, the loss of geomembrane elongation. In contrast, the precipitation rates are higher at Salar de Uyuni in Bolivia and Salinas Grandes in Argentina.
Wind is important for intensifying water evaporation. However, wind also promotes plasticizer loss, which increases the risk of a geomembrane failure. If the plasticizer level decreases rapidly because of high wind speed, the elongation will decrease and the geomembrane could rupture easily.
Depending on the geographic area of the Lithium Triangle, the UV index can range from 6 to 12, which is considered quite high. A site’s UV index is an important consideration in PVC geomembrane design to avoid failure due to UV degradation.
Possible failures in PVC geomembranes in arid climates
Once environmental conditions are known, it is important to analyze the varying performance of PVC geomembranes when exposed to a cold, arid climate. Such analysis optimally exposes the PVC geomembrane in three scenarios: when the geomembrane is in storage, when it is installed and, finally, when it is in service.
To save installation time, the geomembrane is delivered in large rolls, which can weigh 2.5 tons (2.3 tonnes) depending on the dimensions. The packaging of the rolls protects the geomembrane from UV degradation.
The rolled geomembrane has numerous folds and, therefore, some wrinkles. So, the pressure on the film is high, especially in the external inferior winding turns. When the temperature rises, the geomembrane becomes softer and dilates in the transverse direction and contracts in the machine direction. The geomembrane must withstand these dimensional changes without breakage and with minimal internal tension. At this point the dimensional stability of the geomembrane is important. If the geomembrane shrinks a lot, internal tensions can occur.
As the temperature decreases at night, the geomembrane must be flexible enough to allow the folding and wrinkled areas to keep the film intact without promoting fracture, particularly in wrinkled areas (Rowe 2018). The lower the nighttime temperature, the higher the performance that is required of the geomembrane. At low temperatures, dimensional stability, elongation and impact resistance play a relevant role in the performance of a PVC geomembrane (Figure 2).
Once the lithium evaporation pond has been covered but is not yet filled, changes in temperature, UV radiation and wind negatively influence the performance of the geomembrane.
During day/night cycles, a wrinkle-free surface is desirable (Berube et al. 2007). Once the geomembrane has been installed, it should be flat with minimal wrinkles. Ideally, the geomembrane must adapt to fluctuations in temperature. It is essential for the geomembrane to maintain contact with the soil without wrinkles, which helps with slope stability (Stark 2018). This characteristic depends on the manufacturing quality control (MQC) process and the geomembrane’s composition—primarily the density. It is optimal for the geomembrane to act like elastic rubber: When sunlight heats up the film, it elongates, but at night as the temperature drops, the tension disappears. If the geomembrane adapts, its service life will lengthen. Density, elongation, tensile strength, tear strength, impact resistance at low temperature and volatile loss are relevant properties to analyze at this second stage (Fabricated Geomembrane Institute 2019; ASTM D7176-06 2018).
Some wrinkles come not from folding but from deviations in the profile during the calendaring process. Wrinkles from strong camber do not readily disappear when the geomembrane’s temperature increases (e2 Technical Textiles 2020). This is one of the reasons FGI included camber as a control property in the most recent specification (Fabricated Geomembrane Institute 2019).
Degradation of the PVC geomembrane can be controlled by the geomembrane’s composition and through the manufacturing process, but these things affect the product’s cost. If the composition and the production process are not optimal, failures can occur in the PVC cover: The geomembrane will lose its mechanical properties faster, especially properties critical at anchorage points and slopes (Stark 2018). Even with a high-quality geomembrane, the evaporation ponds should be covered as soon as possible in arid climates (Berube 2007). Crews use materials such as low-value salts from the same geographic area to cover the geomembrane. Protection against UV degradation and other environmental conditions are relevant design parameters of the geomembrane.
Table 1 shows wind speed: all three salares in Bolivia, Chile and Argentina experience wind velocity of a minimum of 12.4 mph (20 kph). Once the geomembrane is deployed onto the soil, the wind speeds in the Lithium Triangle create a significant risk of geomembrane plasticizer loss. The MQC calendaring process and the geomembrane’s composition are relevant in preventing this loss. When the PVC geomembrane loses plasticizer, mechanical properties—such as tensile strength, tear resistance and puncture resistance—increase, but flexibility and impact resistance decrease. Fractures may occur at night when the temperature decreases. FGI notes the plasticizer average molecular weight for arid areas should be greater than 410 g/mol (Fabricated Geomembrane Institute 2015).
The brine pressure exerted on the watertight liner system is high due to brine density, which typically has a value of 76.2 pounds per cubic foot (1,220 kg/m3) (Garcés 2000). The geomembrane’s mechanical properties—particularly for seams or near them—will maintain optimal performance of the PVC geomembrane under the brine. The geomembrane offers strong chemical resistance for water and, of course, for salt solutions. The water extraction loss can complement a quality index at this stage. If the geomembrane passes through all the described steps, site owners can expect a life expectancy longer than 20 years (Figure 3).
Relevant properties
One of the most relevant properties is the capacity of the PVC geomembrane to dissipate wrinkles after unfolding and, also, to function without tension during the warm-cool temperature cycle (Rowe 2018). Flexibility is important at low temperatures, both in storage and during installation. The PVC resin quality and the type/amount of plasticizer portends such performance.
Another interesting standard introduced in the most-recent version of FGI Specification 9119 (Fabricated Geomembrane Institute 2019) is the control of the lay-flat of the calendar rolls. Having good lay-flat is important to avoid wrinkles along the seams, so suppliers who can comply with this specification will have relatively flat, wrinkle-free, covered evaporation ponds for the lithium mining process.
The geomembrane density gives an idea of the cost and quality the project owner can expect to receive from the PVC geomembrane. FGI Specification 9119 (Fabricated Geomembrane Institute 2019) assigns the density a typical value of 75 pounds per cubic foot (1,200 kg/m3). This value comes from the density of components in the PVC film. PVC resin has a density of 87 pounds per cubic foot (1,400 kg/m3), and plasticizers have densities ranging from 60 to 61 pounds per cubic foot (960 to 980 kg/m3). PVC geomembranes retain flexibility at low temperatures, but because of potential elongation—a minimum decrease in tensile and tear forces is desired—it is necessary to add between 30% and 35% plasticizers. Due to the low density of plasticizers, the compound’s density decreases to values around 76 pounds per cubic foot (1,220 kg/m3). However, depending on the other raw materials in the geomembrane, the density can be higher, up to 81 pounds per cubic foot (1,300 kg/m3). Some minerals—with densities higher than PVC resin—are sometimes incorporated into the film for various reasons. One of them is to provide more volume to the final film, thereby decreasing the cost of the PVC formulation. Unfortunately, such minerals cannot act as an elastic material inside PVC resin. The result is that the geomembrane can partially lose resilience. PVC geomembranes with densities close to 81 pounds per cubic foot (1,300 kg/m3) can be cheaper but will have serious performance issues.
Using a plasticizer with an average molecular weight of 410 g/mol, or even more, helps to maintain the mechanical properties of resilience and impact resistance at low temperatures. The higher the molecular weight of the plasticizer, the lower its volatility.
The mechanical properties specified in both ASTM D7176-06 and FGI 9119 provide reliable benchmarks for project owners, installers and fabricated PVC geomembrane manufacturers.
Myriam Astrid Acevedo Soriano, a chemist at the Universidad Nacional de Colombia in Bogotá, Colombia, worked at Filmtex S.A.S in Bogotá for 30 years, most recently as research and development manager. She retired in April 2022 and works as a technical consultant.
Juan Manuel Cortés is sales manager, decorative films and waterproofing membranes, at Filmtex S.A.S., and an architect at Universidad de Los Andes in Bogotá, Colombia.
All figures courtesy of the authors.
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