British Columbia Mine Reclamation Symposia

Electrochemical cover technology to prevent the formation of acid mine drainage Lin, Mei; Seed, Leonard P.; Yetman, Daren; Fyfe, Joseph; Chesworth, Ward; Shelp, Gene 2001

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Proceedings of the 25th Annual British Columbia Mine Reclamation Symposium in Campbell River, BC, 2001. The Technical and Research Committee on Reclamation ELECTROCHEMICAL COVER TECHNOLOGY TO PREVENT THE FORMATION OF ACID MINE DRAINAGE M. Lina, L. Seedb, D. Yetmanb, J.Fyfec, W. Chesworthd, and G. Shelpb a School of Engineering, University of Guelph Guelph, Ontario, Canada, N1G2W1 b ENPAR Technologies Inc., 449 Laird Rd., Unit 12 Guelph, Ontario, Canada, N1G 4W1 c Falconbridge Ltd., Sudbury Operations Onaping, Ontario, Canada, POM 2RO d Department of Land Resource Science, University of Guelph Guelph, Ontario, Canada, N1G 2W1 ABSTRACT One of the most serious problems facing the mining industry is the oxidation of sulphide rich mine tailings and sulphide rich ore bodies that ultimately results in the release of metal-rich acidic waters. ENPAR Technologies Inc. has developed a patented electrochemical technology to prevent the formation of acid mine/acid rock drainage (AMD/ARD). This new approach complements conventional reclamation strategies. Sulphide minerals exhibit semiconductor properties and consequently are amenable to electrochemical manipulation. Electronegative polarization can be used to prevent the oxidation of sulphide rich tailings deposits in a manner similar to that used in cathodic protection of steel structures. The resulting cathodic polarization leads to in-situ electrochemical reduction of oxygen at the tailings/soil interface and a shift of the redox potential of the sulphide minerals to a stability field where oxidation is notthermodynamically favoured. Field testing of the technology has been completed at a 4 hectare tailings site near Sudbury, Ontario (Canada). Electrochemical treatment successfully removed oxygen from infiltrating water, increased the stability of the tailings (lower redox potential), and polarized the tailings. A modification of the design for high sulphide tailings has been developed for sulphide containing waste rock and low sulphide tailings. 162 Proceedings of the 25th Annual British Columbia Mine Reclamation Symposium in Campbell River, BC, 2001. The Technical and Research Committee on Reclamation INTRODUCTION Acid mine drainage (AMD) resulting from the disposal of mining waste containing sulphide minerals is considered one of the most significant environmental problems facing the mining industry today. AMD is formed as a result of the oxidation of sulphide minerals that ultimately results in the release of metal-rich acidic waters. The oxidation of iron sulphides is catalyzed by bacteria and proceeds according to the following reaction:  The conventional method of dealing with the AMD problem involves treatment by liming. More recently geotextile liners and engineered soil covers have been developed to prevent the infiltration of oxygen to the tailings to reduce the oxidation rate of the tailings. ENPAR Technologies Inc., based in Guelph, Ontario, Canada, has developed and patented a novel electrochemical technology for the prevention of AMD from tailings deposits. This new approach involves negatively polarizing the tailings/electrolyte interface, thus, reducing dissolved O2, in situ, at the surface of the tailings. The end result is to stop the oxidation of the tailings. A modified design (patent pending) has been developed for low-level sulphide mining wastes including acid generating waste rock. SCIENTIFIC BASIS OF THE TECHNOLOGY The scientific principles behind implementation of the technology were establish through a series of laboratory experiments (Lin et. al., 1999). Sulphide minerals are known to behave as semi-conductors (Koch, 1954); high-sulphide bedrock has been shown to act as an electrode in an electrochemical cell (Shelp et. al., 1995). Laboratory studies and field tests conducted by ENPAR demonstrated that high sulphide tailings can act as an electronic conductor. Therefore, large bodies of sulphide rich tailings are amenable to electrochemical manipulation. Basically, the tailings deposit can be converted into the cathode of an electrochemical cell. Figure 1 illustrates a schematic of an electrochemical system utilizing tailings as the cathode. Electrons are transferred to the tailings through the electrical circuit, negatively polarizing the tailings/electrolyte interface, reducing the redox potential of the tailings. 163 Proceedings of the 25th Annual British Columbia Mine Reclamation Symposium in Campbell River, BC, 2001. The Technical and Research Committee on Reclamation  Oxygen is reduced, in situ, at the tailings/electrolyte interface through the electrochemical reduction of O2 as described by the following reaction:  The rate at which the electrochemical reduction reaction proceeds is controlled predominantly by the applied current density which is related to the rate at which oxygen diffuses through the soil cover or overburden to the tailings. As well as removing dissolved oxygen, hydroxyl ions will be generated at the tailings, therefore inhibiting the oxidation of sulphide minerals and associated release of metal ions. The negative polarization of tailings also changes the thermodynamic properties of the iron sulfide minerals in the tailings to more stable conditions. 164 Proceedings of the 25th Annual British Columbia Mine Reclamation Symposium in Campbell River, BC, 2001. The Technical and Research Committee on Reclamation Monitoring Oxygen, In Situ: The aforementioned electrochemical principles can be exploited as a method of determining O2 concentration at the tailings/overburden interface. Since the tailings is acting as a single large conductor, it is possible to survey the impact of the electrochemical treatment on the tailings by conducting half-cell potential surveys. A half-cell potential survey consists of a series of field potential measurements between the conductive tailings body (which behaves as a metallic structure) and a reference electrode placed in contact with the earth directly above the tailings under investigation. The reference electrode commonly used for such surveys is a copper-copper sulphate reference electrode. It consists of a copper rod in a saturated solution of copper sulphate crystals. This reference cell has a stable, constant potential against which the potential of the tailings is measured. A schematic of how the measurement is recorded is presented in Figure 2. The potential that is recorded is a measure of the redox potential of the tailings which is an indirect measure of O2 concentration.  165Proceedings of the 25th Annual British Columbia Mine Reclamation Symposium in Campbell River, BC, 2001. The Technical and Research Committee on Reclamation FIELD TESTING OF ELECTROCHEMICAL SYSTEM Installation: In the spring of 1998 ENPAR installed a pilot-scale electrochemical treatment system at a tailings deposit in the Sudbury basin (Ontario, Canada). The tailings deposit encompasses a 5 ha area and consists primarily of pyrrhotite at a concentration of approximately 60-70%. The tailings is overlain with a 2-2.5 m deep cover comprised of glacial fluvial sands and gravel. Oxidation of the tailings and subsequent contamination of the groundwater with acidic leachate were encountered prior to installation of the treatment system. After initial testing and characterization of the tailings, a 1.2 ha plot was selected for use as a test area for the system. The remaining area of the tailings deposit acted as the control and was also monitored during the pilot test. Anodes were installed in the overburden, contact was made to the tailings body, and then DC power was supplied to the system using a rectifier. Results: A first static half-cell potential survey of the tailings site was performed prior to installation of the electrochemical treatment system in the spring of 1999. The results presented in Figure 3 show that the redox potential (unpolarized potential) of the tailings was in a positive potential range for most of the site. Potential readings shown are vs Cu/CuSO4. 166 Proceedings of the 25th Annual British Columbia Mine Reclamation Symposium in Campbell River, BC, 2001. The Technical and Research Committee on Reclamation After the electrochemical treatment system was energised for approximately one month, half-cell potential measurements were recorded immediately after current interruption "instant off (Figure 4). The potentials in the treatment area (bounded by O to 110 south and 60 west to 40 east) are much more electronegative than the original static potentials and the untreated tailings area. The half-cell potential measurements of the tailings indicated that oxygen has been depleted at the tailings (by greater than 99.9%) and that the tailings are thermodynamically more stable (i.e. less likely to oxidize). Half cell surveys were conducted over the following 12 months (Figures 5 to 8) with similar results observed.  167Proceedings of the 25th Annual British Columbia Mine Reclamation Symposium in Campbell River, BC, 2001. The Technical and Research Committee on Reclamation  Another electrochemical indication that we have successfully induced cathodic polarization on the tailings is presented in Figure 9 which shows the difference in potential between the instant off potential and the fully depolarization survey which was recorded 18 hours after current interruption. In the treated area most of the potential has depolarized by more than 50 mV which shows that we have successfully induced cathodic polarization to the tailings and promoted the beneficial reactions discussed in the introduction. Similar results to July 1999 were obtained by subsequent surveys. 168  Proceedings of the 25th Annual British Columbia Mine Reclamation Symposium in Campbell River, BC, 2001. The Technical and Research Committee on Reclamation Pore Water Monitoring: Tensiometers were installed in the unsaturated zone of the tailings throughout the control and treatment area. Initially, changes in pore water chemistry were reported as the contaminated pore water was displaced within the treatment zone. Sampling of the pore water 3 months after start-up indicated an increase in porewater pH within the treatment zone of 0.7 units to a pH of 4.14. Unfortunately, staff were unable to obtain additional pore water samples from the unsaturated tailings during the monitoring period. Power Requirements: The installation operated at a power consumption of approximately 9.6 RW. However, the system is installed at a site with an overburden characterized by high electrical resistivity (gravel and sand) of greater than 500,000 ohm-cm. Future installations at sites undergoing the development of decommissioning plans will include a low resistivity cover (less than 10,000 ohm-cm), such as silt, clay or silt loam. The lower resistivity overburden (i.e. soil) would decrease power costs significantly. An electrochemical system can be designed to utilize solar panels to provide DC power. As an alternative, under certain conditions, a galvanic system requiring no external power source can be designed and implemented. The design of an electrochemical system is site specific and depends primarily on site characteristics. SUMMARY A novel electrochemical treatment system to prevent the formation of acid mine drainage is currently undergoing pilot-scale testing at a tailings deposit. The electrochemical technology has been successful in removing dissolved oxygen and lowering the redox potential at the tailings/electrolyte interface. Also, by negatively polarizing the tailings, a thermodynamically stable environment is created within the tailings. By removing oxygen in situ, the technology tackles the problem at the source, thereby, shutting down the formation of AMD. The technology is robust and modular and can be used in conjunction with a low cost soil cover to remediate and revegetate a tailings deposit.  169 Proceedings of the 25th Annual British Columbia Mine Reclamation Symposium in Campbell River, BC, 2001. The Technical and Research Committee on Reclamation REFERENCES Koch, D.F.A. 1954. Electrochemistry of Sulphide Minerals. ]n Modern Aspects of Electrochemistry - No. 10. Edited by Bockris, J.O.M. and Conway, B.E. (Eds.), pp. 211-237 Lin, M., Seed, L., Fyfe, J., Chesworth, W., Brousseau, R. and Shelp, G., 1999. Electrochemical Treatment of Pyrrhotite-rich Tailings - A Laboratory Study. Jn Conference Proceedings of Sudbury '99 - Mining and the Environment II. September 13-17,1999, Sudbury, Ontario, Canada. Goldsack, D., Belzile, N., Yearwood, P., and Hall. G. (Eds.). v2, pp. 571-579. Shelp, G., Chesworth, W., Spiers, G., and Liu, L.. 1995. Cathodic Protection of a Weathering Orebody. Jn Proceedings of Sudbury '95 - Mining and the Environment. May 28 - June 1, 1995, Sudbury, Ontario, Canada. Hynes, T.P. and Blanchette, M.C. (Eds.). v3, pp. 1035 -1042. 170 

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