EFFICACITÉ D UN RECOUVREMENT MONOCOUCHE PLACÉ SUR DES REJETS MINIERS GÉNÉRATEUR D EAUX ACIDES

Transcription

1 Réunion du comité scientifique Chaire CRSNG Polytechnique-UQAT Environnement et gestion des rejets miniers EFFICACITÉ D UN RECOUVREMENT MONOCOUCHE PLACÉ SUR DES REJETS MINIERS GÉNÉRATEUR D EAUX ACIDES Géraldine Cosset Directeur : Michel Aubertin 12 novembre

2 Plan of the Presentation 1. Introduction 2. Laboratory test 3. Column tests results 4. Numerical modelling 5. Sensitivity analysis 6. Closing remarks 2

3 INTRODUCTION Reclamation solution Most effective reclamation solution (Aubertin et al., 2002) : Covers with capillary barrier effects Water covers The objectives of these covers : reduce oxygen flux diffusion in water less than in air inhibit oxidation reaction Effective oxygen barrier (Mbonimpa et al, 2003; Aachib et al, 2004) High degree of saturation in the cover S r 85% Typical target for diffusion flux through the cover F D 1mol/m 2 /yr 3

4 INTRODUCTION Elevated water table techniques EWT technique (MEND, 1996; Aubertin et al. 1999) : Based on the water cover concept -> lower cost water table kept close to the tailings surface in the impoundment reactive material maintained highly saturated capillary fringe above the water table is kept saturated by capillarity. Capillary fringe height is a function of material air entry value (AEV) (Dagenais et al., 2006 and Ouangrawa et al., 2009) (adapted from MEND, 1996) 4

5 INTRODUCTION EWT and monolayer covers EWT implemented in conjunction monolayer covers : Favour infiltration and limit evaporation (Aubertin et al., 1999) Coarse grain material : small AEV and large k sat (Aubertin et al., 1999) 5

6 INTRODUCTION Case studied here EWT technique implemented in conjunction with monolayer cover Water table in the tailings impoundment is at variable depth (0 to 12 m depth on site) Monolayer cover is made of till Cover : saturated in certain areas Favour evaporation May act as an oxygen barrier Efficiency of a single-layer cover Effect of influence factors 6

7 Plan of the Presentation 1. Introduction 2. Laboratory test 3. Column tests results 4. Numerical modelling 5. Sensitivity analysis 6. Closing remarks 7

8 LABORATORY TESTS Laboratory tests Laboratory characterization tests : index and hydrogeological properties of the material column tests : hydrogeological behaviour of the cover-tailings system (large) behaviour of an unrestored site (small) 8

9 LABORATORY TESTS Columns tests 2 large : 170 cm tailings + 30 cm till (a) Five drainage cycle were realised by adding water on the top of each column Suctions were applied at the bottom using a valve and a u-shaped capillary tube. Column instrumentation : Tensiometers : positive and negative water pressure TDR probes : water content 9

10 Plan of the Presentation 1. Introduction 2. Laboratory test 3. Column tests results 4. Numerical modelling 5. Sensitivity analysis 6. Closing remarks 10

11 COLUMN TEST RESULTS Water content profiles -Fluctuation of the water content at the top of the large column -Stabilisation of the water content in the tailings Tailings are kept saturated by capillary retention Cover is not kept saturated can not act as an oxygen barrier 11

12 Plan of the Presentation 1. Introduction 2. Laboratory test 3. Column tests results 4. Numerical modelling 5. Sensitivity analysis 6. Closing remarks 12

13 NUMERICAL MODELLING Model validation Characterization tests results properties of the material Column tests results validation of the model (dots) Numerical model code SEEP/W 2007 (lines) The model depicts well the hydrogeological behaviour of the system 13

14 1. Introduction Plan of the Presentation 2. Laboratory test 3. Column tests results 4. Numerical modelling 5. Sensitivity analysis cover thickness depth of the water table 6. Closing remarks 14

15 SENSITIVITY ANALYSIS Cover thickness S r final 1m S r final 2m 60 % S r final 1m S r final 2m 75 % Little impact on the cover water content No impact on the tailings water content 15

16 SENSITIVITY ANALYSIS Water table depth S r final 1m S r final 11.7m 60 % S r final 1m 98 % S r final 11.7m 55 %. Little impact on the cover water content Great impact on the tailings water content 16

17 Plan of the Presentation 1. Introduction 2. Laboratory test 3. Column tests results 4. Numerical modelling 5. Sensitivity analysis 6. Closing remarks 17

18 DISCUSSION AND CONCLUSION Position of the water table Shallow water table : -tailings are kept highly saturated -Cover should reduce moisture loss upward (evaporation barrier) COARSE MATERIAL Deep water table : -tailings naturally tend to desaturate -cover should prevent oxidation of the tailings (oxygen barrier ) LAYER OF FINE MATERIAL Conclusion Water table depth major influence on the solution Cover material must be selected rigorously on 18

19 Travaux complémentaires présentés dans mon mémoire 19

20 QUESTIONS 20

21 Height (m) FURTHER RESULTS Effect of the grain size distribution of the cover on the system : -Major impact on the degree of saturation in the cover -No impact on the degree of saturation in the tailings 4 3,5 3 2,5 2 1,5 1 0, ,05 0,1 0,15 0,2 0,25 0,3 0,35 0,4 water content CI fine moyenne grossier 21

22 FURTHER RESULTS -Fine grain material presents good capillary retention and stays saturated in the presence of a deep water table GOOD OXYGEN BARRIER -Coarse grain material presents low capillary retention and desaturates rapidly in the presence of a shallow water table GOOD EVAPORATION BARRIER 22

23 LABORATORY RESULTS log(k) log (k) Saturated hydraulic conductivity Saturated hydraulic conductivity obtained through laboratory test and with Kozeny Carman (KC) and modified Kozeny Carman (KCM) predictive model ,5 1 Void ratio KCM KC K MESURED ,5 1 Void ratio KCM KC k MESURED 23

24 LABORATORY RESULTS water content water content Water retention curve Water retention curve obtained in TEMPE cell and with Kovacs modified (KM) predictive model 0,35 0,35 0,3 0,25 0,2 0,15 0,1 0,3 0,25 0,2 0,15 0,1 0, Pressure kpa RETC laboratory prediction AEV calculated 0, RETC Pressure kpa laboratory prediction AEV calculated 24

25 LABORATORY RESULTS (m 2 /s) Oxygen Diffusion coefficient Diffusion coefficient D e obtained through laboratory experiment and with Aachib et al., (2004) predictive model Oxygen diffusion coefficient D e 1E-04 1E-05 1E-06 1E-07 1E-08 1E-09 1E-10 1E-11 prediction lab Degree of saturation S r 25