Laboratory and Field Performance Assessment of Geocomposite Alternative to Gravel Drainage Overliner in Heap Leach Pads

Transcription

1 Laboratory and Field Performance Assessment of Geocomposite Alternative to Gravel Drainage Overliner in Heap Leach Pads Aigen Zhao, PhD, PE, GSE Environmental, LLC, USA Mark Harris, GSE Environmental, LLC, USA

2 Presentation Outline Introduction Column leach tests with copper and gold ores Large-scale puncture resistance tests Compressive creep tests and thickness-dependent transmissivity tests Full-scale heap leach field tests with copper ores Conclusions 2

3 Geocomposite Alternative to Gravel Drainage Overliner in HLP Geocomposites are used extensively for wastecontainment facilities, they are not commonly used in heap leach pads. In order for a geosynthetic drain to be functionally better or equivalent to drainage gravels in heap leach pads, performance tests must be conducted to demonstrate its long-term performance, including its filter compatibility with the type of ores and irrigation solutions, geonet structural stability under sustained field loads, long-term in-plane flow transmissivity, and interface stability. 3

4 Conventional HLP System 4

5 Dynamic HLP System 5

6 Round Strand (RS) Geonet Bottom layer: Round Strand HDPE Geonet Characteristics Test Method Min. Average Thickness (mm) ASTM D Compressive Strength(MPa) ASTM D Tensile Strength (kn/m) ASTM D Creep Reduction Factor GRI GC8 1.3 under 1.2 MPa Transmissivity 1 ASTM D x 10-3 Density (g/cm 3 ) ASTM D Carbon Black Content (%) ASTM D Geonet Strand Spacing (cm) N/A 1.0 Max 6

7 Conventional Oblong-Shape Geonet (top) vs. High-Strength RS - Geonet (bottom) 7

8 RS-Geocomposite Filter Geotextile Top layer: A permeable nonwoven or woven geotextile filter Characteristics Test Method Nonwoven Filter Woven Filter Grab Tensile (N) ASTM D Apparent Opening Size (mm) ASTM D Permittivity (1/sec) ASTM D Water Flow Rate (l/min/m 2 ) ASTM D UV resistance,% Strength ASTM D

9 RS-Geonet Laminated with a Nonwoven Geotextile 9

10 Tests Performed by Independent Laboratories Test Method Test Sample Function to be assessed Independent Lab Large Scale Puncture RS- Geocomposite Geomembrane Protection Stepped Isothermal Long-term Compression Method (SIM) RS-Geonet Core Creep Geo-Logic Associates Lab (formerly Ausenco Vector) TRI/Environmental Thicknessdependent Transmissivity RS- Geocomposite Long-term Drainage TRI/Environmental Column Leach Tests RS- Geocomposite Long-term Filter and Chemical compatibility Kappes, Cassiday & Associates 10

11 Assessment on RS-Geocomposite Filter Performance by Column Leach Tests Testing materials were from mine sites copper-bearing ores leaching with sulfuric acid; gold-bearing ores leaching with sodium cyanide Irrigation rates: 7 to 12 l/h/m2 Columns with and without RS-Geocomposite for Comparison Testing duration: more than sixty days Tests performed by independent mining laboratories 11

12 Column Leach Tests Results Effluent solution collection and analyses: Effluent quantity Metal recovery Total suspended solids (TSS) Total dissolved solids (TDS) Ore/ RS-Geocomposite interface microscopic analyses Head/Tail material analyses for particle size distribution 12

13 Test Procedure Select ore and prepare desired particle size distribution Prepare 320 mm ID by 2 m column for the RS- Geocomposite test (2.5 m height for control, that includes 0.5m crushed river rock) Place RS-Geocomposite over perforated plate and seal edges Prepare solution to have the same chemistry as desired in the field Start irrigation with the similar rate as in the field Monitor and test effluent 13

14 Cumulative Percent Copper Extraction Copper Extraction Results from Leach Tests 100% 90% 80% 70% 60% 50% 40% Cu-A Cu-B Cu-C 30% 20% 10% 0% Cumulative Tonnes of Solution per Tonne of Ore 14

15 Daily Effluent, Liters Weekly Effluent, Liters Copper Effluent Results from Leach Tests Cu-A, Daily Cu-B, Daily Cu-C, Daily Cu-A, Weekly Cu-B, Weekly Cu-C, Weekly Days of Leach 15

16 TDS, mg/l TDS from Leach Tests on Copper 180, ,000 TDS 140, , ,000 80,000 60,000 40,000 Cu-A Cu-B Cu-C 20, Days of Leach Note TSS was below the detection limit 16

17 Cumulative Weight Percent Passing Head and Tail Screen Analyses Before and After Copper Leach Tests 100% 90% 80% 70% 60% 50% 40% Cu-C, Head Cu-A, Tail Cu-B, Tail Cu-C, Tail 30% 20% 10% 0% Size, mm

18 Microscopic Analyses of Copper Ore/ RS- Geocomposite Interface after Leaching tests Ore material remained at the interface with few particulates entrained in the geotextile. The RS- Geocomposite filter maintained an open and porous structure. 18

19 Cumulative Percent Gold Extraction Gold Extraction Results from Leach Tests 100% 90% 80% 70% 60% 50% 40% 30% Au-A Au-B Au-C 20% 10% 0% Days of Leach 19

20 Daily Effluent, Liters Weekly Effluent, Liters Gold Effluent Results from Leach Tests Au-A, Daily Au-B, Daily Au-C, Daily Au-A, Weekly Au-B, Weekly Au-C, Weekly Days of Leach 20

21 TDS, mg/l TSS, mg/l TSS and TDS from Gold Leach Tests Au-A Au-B Au-C TSS Days of Leach 7,000 6,000 TDS 5,000 4,000 3,000 2,000 1, Days of Leach Au-A Au-B Au-C 21

22 Cumulative Weight Percent Passing Head and Tail Screen Analyses Before and After Gold Leach Tests 100% 90% 80% 70% 60% 50% 40% Au-C, Head Au-A, Tail Au-B, Tail Au-C, Tail 30% 20% 10% 0% Size, mm

23 Large Scale Puncture Test with Angular Rock from a Mine Site Size: Minus 1 ½ Size: Minus 3/4 (38mm) (19mm) Sand fraction or minus #4 (4.7mm) was removed for both materials Load = 3.1 MPa (450psi) or 170m (550ft) ore rocks with a density 18.2 kn/m3 (120 pcf) 23

24 Large Scale Puncture Test Results at 20 C (68 F) Temperature 2.0 mm LLDPE at 170m simulated ore depth 20 C Temperature, Angular rock size minus 1 ½ (38mm) No Protection With RS-Geocomposite Protection 24

25 Large Scale Puncture Test at Elevated Temperature, 60 o C (140 F) 25

26 Large Scale Puncture Test 60 C (140 F) Temperature 2.0 mm LLDPE at 170 m simulated ore depth 60 C Temperature, Angular rock size minus 1 ½ (38mm) No Protection With RS-Geocomposite Protection 26

27 Long-Term Performance Tests Long-term Compressive Creep & Transmissivity Stepped Isothermal Method(SIM) Creep Test Testing Lab: TRI, Austin, Tx ASTM D7361 Test Targets Geonet creep data: 1.9MPa & 20, 40, & 60 C Test duration, 2 weeks is equivalent to 10 years creep Thickness Dependent Transmissivity(TDT) Test Testing Lab: TRI, Austin, Tx Test Targets 10-year thickness reduction results from SIM creep test Transmissivity test on retained thickness 27

28 Stepped Isothermal Method(SIM) Importance Accelerated compression creep(astm D7361) measures time dependent loss of thickness and porosity thus flow of the geonet in this case under heap leach pad specific conditions Can generate the creep response for several hundred years by using one single test specimen within a reasonable time frame of two weeks or less It is important to find retained thickness values of geonets because geonet thickness reduction will affect drainage capacity Thickness reduction results from SIM test are used in Thickness Dependent Transmissivity (TDT) 28

29 RS-Geonet Master Creep Curve Generated by SIM tests MPa and 60 o C Load (MPa) Temperatu re ( o C) Retained 10 years % % % % % %

30 Thickness Dependent Transmissivity (TDT) Importance TDT is an accelerated flow test for drainage geocomposites to determine the long-term performance transmissivity from 100 hour to 100 year or longer in a short term laboratory flow test. TDT obtains transmissivity readings of RS- Geocomposite after 10 year service life of heap leach pads. 30

31 Thickness-Dependent Trans. Master Curve (generated by TDT test) Load (MPa) Temperature ( o C) Transmissivity (m 2 /sec) Transmissivity years X X X X X X MineDrain transmissivity = 3.21x10-3 m 2 /s (100-m ores, 40- degree temperature and 2% slope) Typical transmissivity of a 50-cm sandy gravel drainage layer = 5x10-4 m 2 /s 31

32 Full-Scale Field Tests in South America 32

33 Full-Scale Field Tests 33

34 Copper Extraction (%) Results from Field Tests Control Pad RS-Geocomposite test pad Leaching Rate (m 3 /Ton) 34

35 RS-Geocomposite Sample Retrieved from the Site RS-geonet core retains structural Integrity; and the geotextile filters fine particles from entering the flow channels. 35

36 Conclusions Results from column leach tests indicated that the measured outflow and metal extraction percentage had no significant difference in the drainage characteristics from the columns with the RS-Geocomposite filter geotextiles and the control column, and the measured flow rate over time remained stable. The trends in TSS, TDS and turbidity were similar for all columns. The ore particle size distribution curves before and after the leaching tests were almost the same. 36

37 Conclusions Column leach tests concluded that no fines were building up in the filter, clogging the pores or reducing the column permeability. Large-scale puncture resistance tests show the RS- Geocomposite provided a separation between the overlying rocks and the geomembrane, and effective protection to the integrity of the geomembrane as compared to without the geocomposite, thus significantly reducing risks for potential damages to the liner system during both construction and long-term service life. 37

38 Conclusions Long-term compressive creep results from the SIM method indicated the RS-Geonet maintained significant geonet thickness during ten-year design life even under extreme normal loads and elevated temperatures. Long-term transmissivity from the TDT method demonstrated the RS-Geocomposite maintained a high flow capacity under simulated field conditions. 38

39 Conclusions When designed properly for unconfined flow by limiting the hydraulic head within the small thickness of the geonet, the geocomposite overliner alternative has the advantage of reducing the head acting on the liner, thus reducing the potential leakage rate. The microscopic pictures show the ores remaining at the ore/filter interface, and the geotextile filter maintained an open and porous structure. 39

40 Conclusions The large-scale field tests show that The RS- Geocomposite pad reached a flow of 0.82 m 3 /ton and yielded 50% copper extraction rate, while the control pad reached 0.27 m 3 /ton and yielded 30% extraction rate. The use of the geocomposite allows lowering the thickness of the drainage layer, which resulted in a significant cost reduction. This performance demonstration provides the heap leach pad community with useful field data of using geocomposite as an alternative to gravel overliner. 40

41 Gracias por su atención 41