CONTAINMENT AND DEWATERING OF MINE TAILINGS WITH GEOTEXTILE TUBES AT A SILVER MINE IN SAN LUIZ POTOSI, MEXICO Tom Stephens, TenCate Geosynthetics, Pendergrass, Georgia USA Angel Diaz Sobrino, Geomembranas y Geosinteticos, Mexico City, Mexico ABSTRACT A pipeline connecting the mine tailings impoundment to a secondary settling pond ruptured causing water and tailings material to be discharged outside of the tailings treatment system. Fast action by the mine personnel prevented any contamination of the water supply of the local community. Mexican Federal Authorities established the necessary steps for resuming mining operations by the mining company. These steps included completing a full remediation of the spill area and incorporating an alternative tailing storage facility. The technically acceptable and most economical alternative solution accepted by the federal authorities was geotextile tube containment and dewatering technology to receive all of the tailings generated from the mining operation. This paper will detail the testing, design, installation, and operation of the geotextile tube system and analyze the economic impact by allowing the silver mine to restore full operation in 60 days of the rupture. 1. BACKGROUND OF MINING OPERATION The Mexican silver mine is an underground mine located in the north central region of Mexico which is a mountainous, semi desert, arid region. The mine is designed to be a 700 tons per day (tpd) ore mining and milling operation that when at full capacity will be producing approximately 1.8 million equivalent oz. of silver per year according to industry analyst. Operations began in second quarter 2014 and achieved a rate of 270 tpd producing 168,300 equivalent oz of silver. By the end of the fourth quarter 2014, the mine had increased operations to 450 tpd. The processing operation was generating 950 cubic meters per day tailings that were being discharged into the onsite tailings lagoon.
Figure 1 Mexico Silver Mine Location in Central Mexico. 2. THE EMERGENCY CHALLENGE On January 2, 2015 the mine experienced a leak in the decant structure and 2,000 cubic meters of tailings discharged outside of the tailings impoundment area. Mine personel were able to stabilize the tailings structure and prevent any of the tailings from contaminating the ground water of pollute any stream in the area. Next, they focused their efforts on sealing the decant system. PROFEPA (Mexican Environmental Protection Agency) was notified and mine operations were suspended until the existing tailings facility could be safely recommissioned or an alternative temporary tailings containment system could be brought online that satisfied PROFEPA regulations. 3. GEOTEXTILE TUBE TAILINGS AND CONTAINMNET DEWATERING SOLUTION The mine hired an internationally recognized mining engineering company to determine how to repair the tailing storage facility and the time that it would take. They determined that the repair would take a minimum of six month and probably up to nine months to make the necessary repairs required by PROFEPA to issue a revised operating permit and to reopen the mine using the original tailings facility. The downtime would cost the mine up to $34 million in revenue for this downtime. Therefore, an interim tailings management alternative must be brought online as soon as possible. The alternative solution that the engineering firm, recommended was geotextile tubes containment and dewatering technology. 3.1 The Geotextile Tube System Requirements The Geotextile tube containment and dewatering system would include; PVC lined dewatering cells in which to install the geotextile tubes
A piping and manifold system to transfer tailings from the mining operation to the dewatering cells A polymer injection system to make down and inject organic polymers inline to flocculate the tailings and initiate liquid/solid separation of the tailings before the tailings enter the geotextile tubes. A series of large volume geotextile tubes, installed in the dewatering cells, that are capable of receiving uninterrupted 24 hour flow rates of up to 900 cubic meters per day. Effluent that is pure enough to meet the PROFEPA permitted allowable discharge After receiving the geotextile tube conceptual design from the mine engineer and reviewing geotextile tube case histories of similar scope and size, PROFEPA authorized the mine to proceed in developing a final design of the geotextile tube system. 3.2 Polymer Testing and Qualification As the final design was being developed, 200 liters of tailing samples were sent to a lab at Federal Technical University in Mexico City. The samples were representative of the tailings flow direct from the mining and milling operation. When weighed, the bulk density was 1.23 tons/m3and when a sample was oven dried, it was determined that they were 30% dry solids by weight with the solid particles having a specific gravity of 2.6 tons/m3. Several tests were conducted using the cone method without any polymer and the tailings did not dewater. Next, the 200 liters were diluted to 10% solids and 18 different polymers were tested using at a concentration of 0.1% and a dosage of 30 ml per 1 liter sample of tailings. Figure 2 below provides an example of the 1 liter jar test. The test measured flocc size, separation time, and clarity of the water. It was determined that polymer #14 provided the best results. Figure 2 Example of 1 liter tailings/polymer testing The next set of four 1 liter tailings jar tests were conducted but with all four being dosed with 30 ml of 0.1% concentration of polymer #14. The 1 liter samples were allowed to settle for 30 seconds and then the entire dosed tailings sample was poured thru a geotextile Cone Test. The retained solids were retained as displayed in Figure 3
and the effluent collected as displayed in Figure 4. After testing the effluent, it was determined that the geotextile used in the cone test captured 99% of the solids as exhibited in Table 1. Figure 3 Contained solids fro Cone Test Figure 4 Effluent drained and collected from Cone Test Table 1 Result of 1 Liter Jar Testing Using this data generated in the lab, GDT test were conducted at the mine site scaling up the polymer #14 dosage to the quantity of tailings used in the test. It was determined that the same or better solids retention and effluent quality results were achievable. The GDT indicated that with proper polymer dosage and inline mixing utilizing the GDT geotextile, 70% dewatered solids could be achieved. Polymer #14 is characterized as a Super High Molecular Weight Cationic organic polymer. The geotextile GT500 used in the Cone and GDT test has the mechanical and filtration properties that are detailed in the Table 2. As a
result of the testing, the mine engineers selected Polymer #14 and selected the geotextile GT500 be the geotextile tube material. Table 2 Geotextile Tube properties 3.3 Geotextile Tube Design The mine engineers wanted the most economical geotextile dimension that could receive a minimum of 1,200 cubic meters per 24 hours without interruption. Therefore, after consulting with the geotextile tube supplier, they selected a product that was a standard product that could be shipped form inventory at any time. The geotextile tube selected was a 36.5m circumference x 30 meter long tube with the properties detailed in Table 2 and with the capability to contain 35 cubic meters per linear meter and provide a factor of safety equal to or greater than 3.5 against rupture for all critical forces. Figure 5 provides the details of the geotextile tube cross section, filling height, volume per linear meter and the factors against rupture when filled with dewatered tailings with a relative density of 1.76 ton/m3. 3.4 Dewatering Cell Design Because of the terrain of the mine site and to reduce the site work construction time, the mine engineering company designed two dewatering cells to contain the geotextile tubes. The cells would be lined with a 1.0 mm HDPE liner covered with a 200 grams per square meter non woven geotextile over which 30 cm of drainage gravel is placed. Each dewatering cell has a collection sump into which the effluent water that drains from the geotextile tubes is collected. Submersible pumps will pump the effluent water back to the milling operation for recycling. Figure 6 detail the dewatering cell design.
Figure 5 Geotextile Tube Cross Section with Volume, Dimensions, and Factors of Safety Figure 6 Geotextile Tube Dewatering Cell Using a geotextile tube estimator program provided by the geotextile tube manufacturer, and imputing all of the mine tailings daily volume, and the lab and GDT test data, it was calculated that 10 linear meters of the 36.5m circumference geotextile tube would be required to confine and dewater the total mine tailings generated per day of operation. Figure 7 provides the input and output date of the estimator program.
Figure 7 Geotextile Tube Estimator Program Input and Output Data 4. CONSTRUCTION AND OPERATION On March 26, 2015 construction of the dewatering cells were completed and the mine resumed mining and milling operations with a PROFEPA and SEMARNAT approved permit utilizing geotextile tube containment and dewatering technology. Production resumed at a rate of approximately 350 tons per day and increased to 450 tons per day in April which was the rate when operations were halted in January. 5. CONCLUSION Geotextile tube technology has proven to be an economical alternative to tailings lagoons as a method to receive contain and dewater high volumes of tailings on a daily basis for extended periods of time for regular operations or in response to emergency situations. Engineers now have testing methods and analytical tools developed by geotextile tube manufactures that can assist engineers to calculate the actual field results of full scale projects. The geotextile tube technology in this real emergency case enabled the owner to bring the mine back online utilizing geotextile tube technology with permits issued by the regulating authorities and achieving a time savings of 3 to 6 months saving millions of dollars.
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