The Salar de Atacama is located at the foot of the Andes Mountains (68° 24’ South, 23° 30’ East) at an elevation of 2,130 m (7,000 feet) in Chile and covers an area approximately of 3,000 km2. This area is close to the Atacama Desert, one of the driest, if not the driest, region in the world. The site is situated near the borders of Bolivia and Argentina as shown in photograph-1. One of the most mineral-rich stretches of the desert is known as the Salar de Atacama (see satellite picture in photograph-2). The Salar is an ancient seabed underlain by large reservoirs of liquid brine.
SQM (Sociedad Quimica y Minera de Chile), headquartered in Santiago, has two production facilities at the Salar (see blue squares in Figure-2). To mine the potassium and lithium salts, large amounts of brine are pumped to the ground surface by wells. The pumped brine is conveyed in canals and directed into large lined evaporation ponds. Clouds rarely form or persist over this expanse and the area is extremely windy, which provides an ideal environment to evaporate the large amounts of water that constitute the brine into the ponds.
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| Figure 1. Map of Chile showing location of Salar de Atacama (maps courtesy of Andre Rollin) |
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| Figure 2. Satellite photograph showing location of SQM mining in Salar de Atacama (maps courtesy of Andre Rollin) |
As a first step in the extraction process, a number of large pre-concentration ponds are constructed where, by taking advantage of the evaporation process, a portion of the sodium chloride in the brine is allowed to precipitate as an “undesirable by-product”. After a residence time, the now-concentrated brine is pumped into a number of production ponds where the dry salt-mineral produced is mechanically removed and stockpiled. Potassium and lithium are produced in different ponds which results in a three-stage process. The product is mechanically routed to an on-site chemical processing facility where the desired minerals are extracted. Finally, the extremely concentrated brine is pumped to a fourth-stage pond for boric acid recovery.
Some of the important salts precipitating from the brine are sodium chloride, potassium, which is used for fertilizer, lithium, and boric acid as a by-product. SQM is a leader in production of salts used in fertilizers and provides 35% of the world’s lithium which is used for batteries, pharmaceuticals, and sapphire glasses used in jewelry and aeronautic applications.
The underground brine is recharged, albeit at a reduced rate, by the melting snow caps in the surrounding mountains. As the recharge water flows through the underground bedrock it dissolves the minerals in the sediment of the ancient seabed forming the concentrated brine. The concentrated brine is pumped to the ground surface and contained in ponds lined with PVC geomembranes. Photograph 3 shows one of the ponds filled with brine and undergoing evaporation. A pumping station in a brine filled pond is also shown in Photograph 3. In the background are piles of extracted sodium chloride salt. The Andes Mountains are shown in the far background of Photograph 3.
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| Photograph 3. Brine filled evaporation pond (picture courtesy of Patrick Diebel) |
After the water evaporates, the ponds are carefully mucked out; with the salts acting as a protection layer so the liner system is not damaged. For example, the bottom salt layer protecting the liner is sodium chloride in the ponds where sodium chloride is precipitating, potassium in ponds where potassium is forming, and lithium in the lithium production ponds. After the salts have been partially removed, the pond is refilled and used over and over. Holes in the geomembrane are extremely detrimental because the brine can flow out and return to the subsurface reservoirs. Not having holes in the geomembrane is important because it takes about one year to yield about 1 m of salt, i.e., one year to evaporate a typical pond. Thus, losing brine and having to restart the process after patching a liner hole is time consuming, costly, and reduces the annual production quantity. In addition, holes in the geomembrane are difficult to detect because of the presence of muck so it is imperative that the geomembrane have excellent chemical resistance and resistance to pinholes in manufacturing, fabrication, deployment, and use. The fact that PVC has high elongation and tends to drape around any protrusions on the compacted layer underneath the liner helps minimize the occurrence of small holes and brine loss. Photograph 4 shows the inspection and successful performance of a PVC geomembrane after being in-service for about twelve years. This pond is lined with a PVC.
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| Photograph 4. Inspecting the PVC geomembrane in one of the evaporation pond (picture courtesy of Patrick Diebel) |
PVC geomembranes are a likely choice for this application even though it is a harsh environment. PVC geomembranes are durable and offer excellent chemical resistance to the salts, which is important because of the long-term exposure of the geomembranes to the brine. PVC geomembranes also exhibit much smaller wrinkles than polyethylene geomembranes when installed because of a lower expansion coefficient, higher subgrade/geomembrane interface strength, and the flexibility of PVC geomembranes (see photograph-5 showing a 1 million square meter pond lined with PVC geomembrane and ready to be filled with brine). This is significant in this application because the smaller wrinkles result in substantial intimate contact between the geomembrane and subgrade and the protective salt layer. The benefit of intimate contact is a reduction in the lateral flow from a hole or leak in the geomembrane.
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| Photograph-5: 1 million square meter pond lined with PVC geomembrane and ready to be filled with brine (picture courtesy of Andre Rollin) |
Liner System Design and Installation
The evaporation ponds are large with average dimensions or 3 m (10 ft.) deep, 300 m (1,000 ft.) wide, and 1,000 m (3,000 ft.) long. The liner system of the first ponds consists of compacted soil PVC geomembrane. The current liner system utilizes non woven geotextile over a compacted natural salt layer PVC geomembrane.
To reduce field seaming in this harsh environment, the PVC geomembrane is fabricated into panels in a controlled factory environment which is much more suitable to high quality seaming than the Salar. The panels are typically about 15 m (50 ft.) wide and 300 m (1,000 ft.) long and shipped to the site. Thus, the only field seaming required is the seaming of
the panels. The panel size is usually limited by an allowable field handling weight so a typical panel weighs about 3,000 kg (6,600 lbs). The PVC geomembrane is field seamed using a solvent or thermal fusion. If thermal fusion, hot wedge or hot air welder, is used. Thermal fusion is now the recommended technique because the produced seam can be air-channel tested if a dual track weld is performed.
Testing of Field Seams and Completed Liner
A dual-track field seam was specified by SQM as the primary seaming method for the ponds installed in 2004. Given the high cost of pumping and storing the brine, a seaming process that allowed the testing of the entire length of the field seams, instead of isolated areas with destructive samples, was sought. This resulted in the use of dual-track welds and air-channel testing of the field seam. The air channel testing of PVC field seams has gained popularity and provides a number of advantages over destructive testing of PVC seams. One of the advantages is the air-channel pressure can be used to verify the PGI specified seam peel strength (PGI 2004) of 2.6 N/mm (15 lbs/inch) using the sheet temperature and a relationship presented by Stark et al. (2004) and shown in Figure 6. This relationship has been incorporated into the new ASTM Standard Test Method D7177 (ASTM 2005) for air-channel testing of PVC field seams. Thus, if the air-channel holds the required pressure, the frequency of destructive sampling and testing is less. Air-channel testing was challenging on this project because the sheet temperatures usually exceeded 70oC (158oF) because of the harsh desert environment. A sheet temperature greater than 70oC (158oF) is challenging because the relationship between air-channel pressure and geomembrane sheet temperature for the PGI specified seam peel strength of 2.6 N/mm (15 lbs/inch) in ASTM D7177 extends to a sheet temperature of 48oC 120oF (see Figure 6). Testing is currently being conducted to overcome this limitation. In the interim, the relationship shown in Figure 6, i.e., the relationship between air-channel pressure and geomembrane sheet temperature included in ASTM D7177, is extended to cover the range of sheet temperatures encountered on this project. Thus, the air-channel pressure required for the PGI specified seam peel strength of 2.6 N/mm (15 lbs/inch) as about 60 kPa (9 psi) for a sheet temperature of 70oC (158oF).
Another advantage of air-channel testing of field PVC geomembrane seams is the flexible nature of PVC geomembranes allows the inflated air-channel to expand like an inflated bicycle tube. This allows a visual examination of the entire inflated seam and identification of any seam defects even though the seam may pass the required air-channel pressure. These defects are usually visible on the outside of the air channel in the form of an aneurysm. The flexible nature permits the inspection of the air-channel as the air pressure migrates along the entire seam. If a defect is encountered, the inflation process will usually cease in the vicinity of the defect. This allows the entire length of field seam to be inspected and tested using the air-channel test procedure
The project specifications initially required destructive field seam tests every 300 m (1,000 ft.) of field seam, but allowed the destructive samples to be obtained from the anchor trench and not on the production liner based on successful air channel test results. This destructive sampling is significantly less frequent than traditional destructive tests that are conducted every 150 lineal meters (500 lineal feet) of field PVC geomembrane seam. The elimination of destructive samples from the production liner is noteworthy and should be adopted in other applications.
After the field seams are tested and approved, the integrity of the PVC geomembrane was also tested using electrical leak location methods to ensure the exposed geomembrane is defect free to protect the pumped brine. Electrical leak location methods are readily used for PVC geomembrane and can locate extremely small defects.
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| Figure 6. Relationship between sheet temperature and required air-channel pressure to achieve seam peel strength of 2.6 N/mm (15 lbs/inch) from Stark et al. (2004) |
Summary
The evaporation ponds in the Salar of Atacama are lined with PVC geomembranes and the geomembrane are performing excellently in this harsh environment. The use of a PVC geomembrane facilitated installation of a liner system in this dry and windy environment because of the large reduction in field seams due to the use of prefabricated panels. The use of dual-track thermal fusion welds to create the field seams facilitated testing of the entire-length of the field seam and omission of destructive tests on the completed liner with air-channel testing. In addition, the use of prefabricated panels and fewer field seams resulted in the liners being completed much quicker than if geomembrane sheets of 7 m width was used which facilitated the initiation of the evaporation process and revenue generation. A average of 30,000 square meters (325,000 square feet) of PVC geomembrane was deployed, welded and tested on a daily basis.
SQM’s Atacama Salar evaporation ponds represent the largest PVC geomembrane installation in the world to date with more than 16 million square feet of PVC geomembrane being successfully installed and utilized since 1996.
ACKNOWLEDGMENTS
PVC manufacturer: Canadian General-Tower Limited
Panels fabricator and installer: Solmax International Inc.
QA/QC and electrical leak detection: Solmers International Inc.
REFERENCES
ASTM D 7177, 2005, “Standard test method for air-channel testing of field PVC Geomembrane Seams”, American Society for Testing and Materials, West Conshohocken, Pennsylvania, USA.
PVC Geomembrane Institute (PGI), 2004, “PVC Geomembrane Material Specification 1104”, University of Illinois, Urbana, IL, www.pvcgeomembrane.com, January 1, 2004.
Stark, T.D., Choi, H., and Thomas, R.W., 2004, "Low Temperature Air Channel Testing of Thermally Bonded PVC Seams," Geosynthetics International Journal, Industrial Fabrics Association International (IFAI), Vol. 11, No. 6, December, pp. 481-490.
