A review of sources and sinks for nitrate in the mining environment

1. A review of sources and sinks for nitrate in the mining environment Christopher H. Gammons, Ph.D. Professor, Dept. of Geological Engineering Montana Tech Butte, Montana cgammons@mtech.edu

2. Acknowledgements This research was supported by Goldcorp Inc. Thanks to mines and agencies who supplied data and photographs

3. Sources of nitrate in mine settings Mine-related • Explosives • Cyanide breakdown • Sewage Non-mine-related • Atmospheric deposition • Geological background

4. Explosives • ANFO • 96% ammonium-nitrate, 4% fuel oil • Both ammonium and nitrate are highly soluble in water • Very little residual nitrate if explosives are handled carefully and combustion is complete • Emulsions, gels • Also contain ammonium-nitrate, but mixture is contained in a gel or slurry that minimizes contact with water • Leach slower, but over time will also release nitrate and ammonium

5. Nitrate concentrations in mine waters from blasting range: < 1 to > 10 mg/L (NO3-N) Depends on many factors, including: • Blasting efficiency (% detonation) • How much precipitation on site • Contact time of water with mine waste • Evapoconcentration effects (ponds and lakes) • Presence/absence of algae/aquatic plants Ammonium concentrations can also be high, but over time ammonium will oxidize to nitrate

6. Breakdown of cyanide NO3- NOx(g) in air NO2- nitrate cyanate VOLATILIZATION OCN- ammonium NH4++ HCO3- pH < 9 CN- HCN OXIDATION cyanide SCN- SO42- thiocyanate

7. Breakdown of cyanide NO3- NOx(g) in air NO2- nitrate cyanate VOLATILIZATION OCN- ammonium NH4++ HCO3- pH < 9 CN- HCN OXIDATION cyanide SCN- SO42- thiocyanate

8. Breakdown of cyanate OCN-+ 2H2O → NH3 + HCO3- Breakdown of thiocyanate SCN- + 2H2O + 2O2 →SO42- + CO2 + NH4+ Both pathways generate ammonium

9. Concentrations (mg/L) of cyanide and cyanide breakdown products from gold mines in Nevada (from Johnson et al. , 2000).

10. Removal (sinks) of nutrients from mine water • Many technologies exist for removal of nutrients from treated sewage • In theory, these same technologies can be used for mine waters PROBLEMS • Massive volumes of water • e.g., tailings ponds, pit lakes • Low organic carbon in source • Remote settings, extreme climate • Cost

11. Nitrate: Treatment alternatives

12. Nitrate: Treatment alternatives (cont.)

13. Some Examples • Butte, Montana 2) Landusky, Montana 3) Stillwater, Montana

14. 1. Butte • Berkeley Pit lake • pH 2.6 • Very high dissolved metals • No nitrate! • Flooded underground workings • pH 4 to 7 • Low to high dissolved metals • No nitrate!

15. Why no nitrate in Butte mine waters? • All of these waters are anoxic • All of these waters are in contact with pyrite Conclude: pyrite catalyzes denitrification (but only in anoxic waters): 5FeS2 + 14NO3- + 4H+→ 7N2 + 5Fe2+ + 10SO42- + 2H2O (Plenty of published literature on this)

16. Example 2: Zortman-Landusky, Montana 2000: Note very large cyanide heap leach pad in upper left. 2005 Slides courtesy of David Williams, Butte BLM

17. Landusky leach pad treatment system three bioreactors In series Avg. flow ~ 285 L/min

18. Bioreactor performance • At first, great. Denitrifying bacteria NO3 → N2 • More recently, having problems with removal efficiency due to: • Changes in leach pad water chemistry • Drop in pH from neutral to around 4 • Increase in nitrate-N from 200 to > 300 mg/L • Buildup of “organic sludge” in the bioreactors • State is exploring options

19. Example 3: Stillwater Mine, Montana • Large underground platinum-palladium mine • High nitrate (20 to 40 mg/L as N) in mine waters from blasting residues

20. Treatment scheme • Need to treat 100-300 gallons/minute • 15 to 50 kg N per day • Biological treatment • Moving Bed Bioreactors (MBBR) • Anaerobic/Aerobic • Add methanol, SRP • Land Application • Irrigated pasture Land Application site

21. Moving Bed Bioreactors (MBBR) at Stillwater • Full • Empty Photo courtesy Stillwater Mining

22. Stillwater Mine: Summary • Performance has been very good for > 5 years • Optimal temperature ~ 15-20°C • Some problems in winter when water temperature drops below 10°C • Need a heater to keep the MBBR cells warm

23. Summary • N-contamination is a significant problem for the mining industry for which there is very little published information • N- and CN-chemistry is complicated • There are multiple physical, chemical, biological pathways • Some of these pathways can be used to a mine’s advantage to minimize later N-impacts

24. Some references • Akcil, A., and Mudder, T. (2003) Microbial destruction of cyanide wastes in gold mining: process review. Biotechnology Letters, 25(6), 445-450. • Chapman J.T., Coedy W., Schultz S., Rykaart M. (2007) Water treatment and management during the closure of the Colomac Mine. Proc. Mine Closure 2007 Conference, Santiago, Chile. • Ferguson, K.D., and Leask, S.M. (1988) The Export of Nutrients from Surface Coal Mines, Environment Canada Regional Program Report 87-12, March, 1988, 127 p. • Forsyth B., Cameron A., Miller S. (1995) Explosives and water quality. Proc. of Sudbury 1995, Vol. 95, 795-803. • Koren, D. W., Gould, W. D., and Bedard, P. (2000) Biological removal of ammonia and nitrate from simulated mine and mill effluents. Hydrometallurgy, 56(2), 127-144. • Morin K. A. and Hutt N.M. (2009) Mine-water leaching of nitrogen species from explosive residues. Proc. GeoHalifax 2009. • Revey, G.F. (1996) Practical methods to control explosives losses and reduce ammonia and nitrate levels in mine water. Mining Engineering, July, p. 61-64.

25. Questions?

26. Stable isotope composition of different forms of nitrate ANFO